NEUROSCIENCE POSTER DAY - April 26, 2001

SCHEDULE 1:00 - 2:00pm: Registration (free), Coffee/ Cookies, Poster set up and viewing Medical Sciences Building, Stone Lobby 2:00 - 4:00pm: Poster viewing and judging

Medical Sciences Building, Stone Lobby 4:00 -5:30pm: GRAHAM COLLINGRIDGE, University of Bristol, UK

"Molecular mechanisms of LTP and LTD at hippocampal synapses" Medical Sciences Building, Rm 3153

Poster Day Organizers: Jonathan Dostrovsky, Program in Neuroscience Director Elena Antoniadis, Graduate Student Representative Pat Reed, PIN Secretary

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POSTERS

1 Bajec, M.R.Department of Physiology

* and Atwood, H.L.

DIFFERENTIAL LOCALISATION OF FREQUENIN AT CRAYFISH NERVE TERMINALS

2 Bush, D.E.A.

1Dept. of Psychology, University of Toronto, 2Dept. of Psychiatry, University of Toronto, 2Centre for Addiction and Mental Health

1 *, De Cristofaro, A.1, Presta, C.1, and Vaccarino, F.J. 1,2.

INDIVIDUAL DIFFERENCES IN ANXIETY ON THE ELEVATED PLUS-MAZE ARE PREDICTIVE OF PROGRESSIVE RATIO INTRAVENOUS COCAINE SELF-ADMINISTRATION

3 Cheung, Una S.

Departments of Zoology and Physiology, University of Toronto * and Atwood, Harold L.

REDUCED SPONTANEOUS TRANSMITTER RELEASE AT NEUROMUSCULAR JUNCTIONS OF TRANSGENIC DUNCE DROSOPHILA LARVAE DEFICIENT IN CYCLIC AMP

4 Chow, Brenda M

1Departments of Medical Genetics and 2Zoology, University of Toronto, and 3Program in Developmental Biology, Hospital for Sick Children, Toronto, ON, Canada M5R 1X8

1,3*, Leventis, Peter A2,3, Stewart, Bryan A3 and Boulianne, Gabrielle L.1,2,3

FUNCTIONAL CHARACTERIZATION OF AMPHIPHYSIN IN DROSOPHILA MELANOGASTER

5 Corson, Tim1,4,5,*

1Section of Biochemical Psychiatry, Centre for Addiction and Mental Health; Departments of 2Pharmacology and 3Psychiatry, 4Institute of Medical Science and 5Program in Neuroscience, University of Toronto

, Li, Peter P.1,2,3 and Warsh, Jerry J.1,2,3,4,5

EFFECTS OF ANTIBIPOLAR DRUGS ON CALRETICULIN AND BCL-2 EXPRESSION IN HUMAN NEURONAL, GLIAL AND B LYMPHOBLAST CELL LINES

6 Estrada, J.,1*

1 Institute of Medical Sciences, University of Toronto, Toronto, ON, Canada M5S 1A8. 2Division of Life Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, ON, Canada M1C 1A4. 3Institute for Brain Aging and Dementia, University of California, Irvine, CA 92717-4550, USA. 4Lovelace Respiratory Research Institute, Albuquerque, NM 87185, USA. 5 Hill's Pet Nutrition, Topeka, KS, USA

Ikeda-Douglas, C.; Milgram, N.W, 2 Head, E.; Cotman, C.W.,3 Murphey, H.; Muggenburg, B.,4 Zicker, S.5

THE EFFECTS OF DIET AND AGE ON THE PERFORMANCE OF THE LANDMARK DISCRIMINATION LEARNING TASK

3

7 Faerman, Paul

Departments of Zoology and Psychology, University of Toronto *, Steidl, Stephan, Li, Liang, Yeomans, John

EFFECTS OF PHACLOFEN AND SCOPOLAMINE ON PREPULSE INHIBITION OF THE STARTLE REFLEX IN RATS

8 Farrow, Karl

Department of Physiology, University of Toronto; Toronto Western Research Institute, Toronto Western Hospital

*, Broussard, Dianne M.

THE NEURAL BASIS OF RECOVERY FROM UNILATERAL VESTIBULAR DAMAGE: ROLE OF THE EXCITATORY VESTIBULAR COMMISSURE

9 Forster, G.L., Yeomans, J.S.

Department of Psychology, University of Toronto and , Takeuchi, J., and Blaha, C.D.

Department of Psychology, Macquarie University, Australia M5 MUSCARINIC RECEPTORS ARE REQUIRED FOR PROLONGED DOPAMINE RELEASE FOLLOWING MESOPONTINE STIMULATION IN MICE

10 Gordon, K.A.* 1,3, Papsin, B.C.1, Harrison, R.V.1,2,3

1 Department of Otolaryngology and the Cochlear Implant Program, The Hospital for Sick Children and University of Toronto, 2 Department of Physiology, The University of Toronto, 3 Institute of Medical Sciences, The University of Toronto DEVELOPMENT OF THE CENTRAL AUDITORY SYSTEM IN CHILDREN WHO USE A COCHLEAR IMPLANT

11 Govinda, Nirmala*, Zhu, Zhiping, & Mills, Linda R.

Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Department of Physiology, University of Toronto, Toronto, Ontario, Canada. DIFFERENTIATED PC12 CELLS: AN IN VITRO MODEL FOR ISCHEMIA

12 Honsa, Katya

University of Toronto, Department of Zoology *, Pearce, Joanne, Govind, CK

NEUROMUSCULAR AND AXO-AXONAL SYNAPSES IN THE SWIMMING PADDLE OPENER MUSCLE OF BLUE CRABS

13 Huber-Okrainec, Joelene

& Spiegle, Brenda J.1

*1,4

, Dennis, Maureen,1,3,4

Brettschneider, Jane 2,

Brain and Behaviour Program1 and Speech Pathology

2, The Hospital for Sick Children

Department of Surgery,3 Institute of Medical Sciences,

4 University of Toronto

NEUROMOTOR SPEECH DEFICITS IN CHILDREN AND ADULTS WITH SPINA BIFIDA

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14 Jazayeri, MehrdadToronto Western Research Institute, University Health Network, Institute of Biomaterials and Biomedical Engineering, Departments of Medicine (Neurology) and Physiology,

*, Zhang, Liang, Skinner, Frances K.

Program in Neuroscience, University of Toronto SYNAPTIC CONTROL OF NETWORK FREQUENCY

15 Jeromin, A. ∗,

Mt. Sinai Hospital, Samuel Lunenfeld Research Institute, Toronto, ON M5G 1X5 Schutz, G., Schindler, H. and Roder, J.C.

IMAGING TRANSPORT PATHWAYS OF SINGLE VESICLES ALONG NEURITES OF LIVING NEUROENDOCRINE CELLS

16 Kwan, C.L. *

Inst. of Medical Science1, Depts. of Surgery2 and Medicine3, Univ. of Toronto, and the Toronto Western Research Institute4, Toronto, Canada.

1,4, Mikula, K. 3, Diamant, N.E. 1,3,4 and Davis, K.D. 1,2,3,4

A STUDY OF RECTAL SENSATIONS AND PHYSIOLOGY IN NORMALS AND PATIENTS WITH IRRITABLE BOWEL SYNDROME

17 Lee1,*, H. Jin

1Department of Speech-Language Pathology, 2Institute of Medical Science, * Program in Neuroscience, University of Toronto, Toronto, Ontario, Canada

, Wallani, Tasneem2,*, Mendelson, Julie R.1,2

THE NEUROPHYSIOLOGICAL EFFECTS OF AGING ON TEMPORAL PROCESSING SPEED IN THE RAT INFERIOR COLLICULUS

18 Li, Ming*, Kapur, Shitij and Fleming, Alison

Department of Psychology ANTIPSYCHOTICS INTERFERE WITH MATERNAL AND SOCIAL BEHAVIORS IN RATS: DIFFERENTIAL EFFECTS OF HALOPERIDOL AND CLOZAPINE

19 Louis, R1,

1Dept. of Physiology, and 2Dept. of Surgery, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.

Tang, J.T1, Hutchison B.1,2, Davis, K2, Lozano, A.2, Dostrovsky, J.O.1

LOCATION OF MICROSTIMULATION-INDUCED TREMOR REDUCTION IN THALAMUS OF ESSENTIAL TREMOR AND PARKINSON'S DISEASE PATIENTS

20 Luthra, Anchla1,2

Vision Science Research Program, University of Toronto1, Department of Laboratory Medicine and Pathobiology, Faculty of Medicine, University of Toronto2; Department of Ophthalmology, University of Toronto3; Health Sciences Research Center, St. Michael's Hospital, Toronto4; Department of Ophthalmology & Visual Sciences, University of Wisconsin, Madison5; Glaucoma Center, University of California, San Diego6.

, Gupta, Neeru1,2,3,4, Kaufman, Paul L. 5, Weinreb, Robert N. 6, and Yucel, Yeni H. 1,2,3,4

OXIDATIVE CELLULAR INJURY MEDIATED BY PEROXYNITRITE IN THE LATERAL GENICULATE NUCLEUS IN GLAUCOMA

5

21 MacMillan, M.L.,2 Hutchison, W.D. 1,2,4, Lang, A.E. 3, Lozano, A.M. 1,4

1Departments of Surgery, 2Physiology & 3Medicine, University of Toronto, and 4Div. of Neurosurgery, Toronto Western Hospital, Toronto, On Canada M5T 2S8

RESPONSES OF HUMAN THALAMIC AND CINGULATE CORTICAL NEURONS DURING SEQUENTIAL REACHING

22 Muinuddin, Ahmad

Departments of Physiology1 and Medicine2, University of Toronto, and Playfair Neurosciences Unit3, University Health Network

*1,3, Junzhi, Ji3, Diamant, N.E., Supervisor1,2,3

WHOLE CELL OUTWARD POTASSIUM CURRENTS OF FELINE ESOPHAGEAL CIRCULAR SMOOTH MUSCLE CELLS DEMONSTRATE A BIPHASIC RESPONSE TO CHOLINERGIC STIMULATION

23 Nguyen, Thomas A.

Structural Biology and Biochemistry, The Hospital for Sick Children, and the Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada.

* and Moscarello, Mario A.

MECHANISMS INVOLVED IN THE THERAPEUTIC TREATMENT OF A SPONTANEOUSLY DEMYELINATING MOUSE MODEL, THE ND4 TRANSGENIC MOUSE

24 Patra, S.E.*

1 Dept. of Physiology, 2 Dept. of Surgery, and 3 Institute of Medical Science, University of Toronto, Toronto, Ontario, M5S 1A8, Canada

1, Hutchison, W.D. 1,2, Davis, K. D. 2,3, Lozano, A.M. 2,3, Dostrovsky, J.O. 1

RESPONSE PROPERTIES OF HUMAN THALAMIC NEURONS TO DEEP BRAIN STIMULATION (DBS)

25 Saraga, F.

1Toronto Western Research Institute, University Health Network, Toronto, ON, Canada, 2Department of Medicine (Neurology), 3Department of Physiology, Institute of Biomaterials and Biomedical Engineering, University of Toronto

1,3*; Zhang, L.1,2; Wu, C. 1; Skinner, F.K. 1,2

A COMPUTATIONAL STUDY OF THE HETEROGENEOUS OUTPUT OF O/A INTERNEURONS

26 Schreiber, K.1*

1 Dept. of Physiology and Medicine, University of Toronto, 2 Centre for Vision Research, York University, 4700 Keele Street, M3J 1P3 Toronto, Canada & Department of Neurology, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72072 Tübingen, Germany.

, Crawford, J. D. 2, Fetter, M. & Tweed, D. 1,2.

THE MOTOR SIDE OF DEPTH VISION 27 Smith, Shannon A. M.*1,3

1Department of Psychology 2Department of Psychiatry 3Centre for Addiction and Mental Health (CAMH)

, Korth, Karin M.*1,3, & Fletcher, Paul J.1,2,3

HEROIN SELF-ADMINISTRATION IN THE RAT FOLLOWING NEONATAL 6-OHDA INDUCED DEPLETION OF DOPAMINE

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28 Snyder, Jason

Department of Physiology, University of Toronto * & Wojtowicz, J. Martin

CONSEQUENCES OF NEUROGENESIS ON SYNAPTIC PLASTICITY IN THE ADULT DENTATE GYRUS OF THE RAT

29 Steele, Catriona M.*

Oral Dynamics Laboratory, Graduate Department of Speech-Language Pathology, University of Toronto

& Van Lieshout, Pascal H.H.M.

SENSORY-MOTOR INTERACTIONS REVEALED IN ORAL MOVEMENTS DURING SWALLOWING OF DIFFERENT TEXTURES

30 Till, Christine*

Departments of Psychology,1 and Pediatrics2 at the University of Toronto, Brain and Behaviour Programs3, Division of Clinical Pharmacology and Toxicology4 at The Hospital for Sick Children, Toronto, Canada

1,3, Koren, Gideon 2,4, & Rovet, Joanne F. 2,3

PRENATAL EXPOSURE TO ORGANIC SOLVENTS AND CHILD NEURODEVELOPMENT

31 Wallani, Tasneem1,*,

1 Institute of Medical Science, 2 Department of Speech-Language Pathology, * Program in Neuroscience, University of Toronto, Toronto, Ontario, Canada

Lee, H. Jin2,*, Mendelson, Julie R. 1,2

THE NEUROPHYSIOLOGICAL EFFECTS OF AGING ON AM SPEED PROCESSING IN THE RAT AUDITORY CORTEX

32 Wong, Agnes. M.F.

Division of Neurology1, and Departments of Ophthalmology2 and Physiology3, University of Toronto, Canada

* 1,2 , Tweed, Douglas 1,3 and Sharpe, James A. 1,2

VERTICAL MISALIGNMENT IN UNIATERAL SIXTH NERVE PALSY 33 Wong, Guin-Ting *

Hampson, David R. 1,3 1, Scherer, Stephen W. 2,4, Snead, O. Carter III1,4, and

Departments of Pharmacology 1, Medical Genetics 2, and the Faculty of Pharmacy 3, University of Toronto, and the 4 Hospital for Sick Children, Toronto, Ontario, Canada LOCALIZATION OF THE HUMAN MGLUR4 GENE WITHIN AN EPILEPSY SUSCEPTIBILITY LOCUS

34 Yao, D.Y. Faculty of Dentistry, Univ. of Toronto, Toronto, Ont., M5G 1G6, Canada

, Narita, N., Yamamura, K., Masuda, Y. and Sessle, B.J.

INVOLVEMENT OF FACE PRIMARY MOTOR CORTEX (MI) IN PRIMATE SEMI-AUTOMATIC MOVEMENTS

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35 Yeung, J.Y.T. 1*

Institute of Medical Science1, Depts. of Physiology2, Pharmacology3, Anesthesia4, Univ. of Toronto, Ontario, Canada; Department of Anesthesia5, Sunnybrook & Women's College HSC, Toronto, Ontario, Canada

, Macdonald, J.F. 2,3, Orser, B.A. 1,2,4,5

TONIC AND SYNAPTIC INHIBITORY GABAERGIC CURRENT ARE DIFFERENTIALLY MODULATED BY NON-COMPETITIVE GABAA ANTAGONISTS

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POSTER ABSTRACTS 1. DIFFERENTIAL LOCALISATION OF FREQUENIN AT CRAYFISH NERVE TERMINALS M.R. BajecDepartment of Physiology

* and H.L. Atwood

Crustacean motoneurons have provided valuable insight into the mechanisms governing the strength of synaptic transmission. In crayfish, phasic nerve terminals release an average of 100 to 1000 times more neurotransmitter per synapse than do tonic terminals. Tonic and phasic terminals differ in both morphology and physiology. Tonic terminals are large and varicose, contain a greater volume of mitochondria and neurotransmitter, and exhibit facilitation with repeated stimulation. Conversely, phasic terminals are small, filiform, and display considerable depression with repetitive stimulation. It has been demonstrated that neither differences in terminal ultrastructure, nor differences in stimulation evoked calcium entry per synapse can account for the large differences in transmitter release between the two types of axons. However, a molecular difference has recently been identified. Frequenin (frq) is a calcium binding protein with four EF-hand calcium binding motifs and an N-terminal myristoylation site. The finding that in both Drosophila and Xenopus frq is involved with regulation of neurotransmitter release led to the hypothesis that the differential expression of frq may bear responsibility for the differences between the phasic and tonic axons in crayfish. Using an anti-Drosophila antibody, frq was shown to be expressed prominently in phasic, but not tonic terminals. Subsequently, an anti-crayfish frq antibody has been synthesised. Western blots of crayfish muscle probed with the anti-crayfish antibody indicate that the antibody recognizes a single band of approximately 32 kDa, while in blots of crayfish nerve cord a band is seen at 21 kDa. Immunocytochemical studies using the anti-crayfish antibody confirm the results obtained with the anti-Drosophila antibody, namely that frq is localised to phasic terminals, not tonic in the leg extensor muscle of the crayfish.

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2. INDIVIDUAL DIFFERENCES IN ANXIETY ON THE ELEVATED PLUS-MAZE ARE PREDICTIVE OF PROGRESSIVE RATIO INTRAVENOUS COCAINE SELF-ADMINISTRATION D.E.A. Bush1Dept. of Psychology, University of Toronto, 2Dept. of Psychiatry, University of Toronto, 2Centre for Addiction and Mental Health

1 *, A. De Cristofaro1, C. Presta, 1, and F.J. Vaccarino 1,2

Rats show marked individual variability in the propensity to self-administer cocaine intravenously. Despite its reinforcing properties, cocaine is also reported to induce symptoms of anxiety in humans and animals. As such, individual differences in the motivation to self-administer cocaine may be inversely related to individual differences in anxiety. To test this hypothesis, anxiety levels were measured in male Wistar rats prior to cocaine exposure using the elevated plus-maze paradigm. Based on a median split of the percent time spent exploring the open arms of the maze, animals were divided into high explorers (HE) and low explorers (LE). All animals were then trained to self-administer intravenous (IV) 100 (L infusions of cocaine on a progressive ratio schedule of reinforcement. Stable break points were evaluated across several cocaine doses (0, 0.1, 0.3, 0.6, 0.9, 1.2, & 1.5 mg/kg/infusion). Results showed that relative to LE, break points in HE were significantly higher across the cocaine dose-response, with no observable differences between LE and HE for saline alone. These data suggest that individual differences in anxiety are predictive of the motivation to self-administer IV cocaine. Higher levels of anxiety may suppress the rewarding effects of cocaine self-administration. Supported by: Medical Research Council of Canada grant to FJV.

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3. REDUCED SPONTANEOUS TRANSMITTER RELEASE AT NEUROMUSCULAR JUNCTIONS OF TRANSGENIC DUNCE DROSOPHILA LARVAE DEFICIENT IN CYCLIC AMP Una S. CheungDepartments of Zoology and Physiology, University of Toronto

* and Harold L. Atwood

We measured spontaneous transmitter release at neuromuscular junctions of transgenic Drosophila larvae overexpressing the learning gene dunce (dnc). This gene codes for a phosphodiesterase enzyme; consequently, these larvae exhibit lower-than-normal cyclic adenosine 3',5'-monophosphate(cAMP) concentrations in the nervous system. We tested whether altering cAMP chronically affects the sensitivity of spontaneous transmitter release to exogenously applied Dibutyryl cAMP (db-cAMP). Comparative analyses of relations between [cAMP] and spontaneous excitatory junctional potential (mEJP) frequencies reveal that the threshold to [db-cAMP] is much lower in transgenic dnc larvae than in wild types. Surprisingly, the maximum rate of release elicited from transgenic larvae was smaller in magnitude than baseline levels of wild types, and rescue of mEJP frequencies to the level of wild types could not be achieved. These findings were obtained from whole cell recordings. At the individual bouton level, there was no significant change in evoked excitatory junctional current (EJC) amplitudes in response to increased [db-cAMP] in transgenic dnc larvae. This relative stability (in comparison to the response elicited in wild types) might be due to the abnormally large EJC amplitudes that we have observed in transgenic dnc larvae.

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4. FUNCTIONAL CHARACTERIZATION OF AMPHIPHYSIN IN DROSOPHILA MELANOGASTER Brenda M. Chow1Departments of Medical Genetics and 2Zoology, University of Toronto, and 3Program in Developmental Biology, Hospital for Sick Children, Toronto, ON, Canada M5R 1X8

1,3*, Peter A. Leventis2,3, Bryan A. Stewart3, Gabrielle L. Boulianne1,2,3

First discovered in association with synaptic vesicles (SVs) in chick brain, amphiphysin (amp) is believed to be important in SV endocytosis. Biochemical assays have demonstrated that amp binds to proteins involved in SV endocytosis, such as dynamin and clathrin. One model suggests that amp targets proteins to clathrin-coated vesicles. However, amp function in vivo is still unknown. We propose to define amp’s role in endocytosis in vivo, using Drosophila as a model organism. Molecular characterization of Drosophila amphiphysin (Damp) reveals multiple isoforms of the protein that are highly related to the vertebrate homologue at the N and C termini. However, consensus sequences for binding to other endocytic proteins, such as clathrin, are absent. To study Damp function, we have taken several approaches, including the production of antibodies for biochemical studies, and the generation of mutant and transgenic flies. Using an antibody generated against full-length Damp, we have found that several Damp isoforms are widely expressed throughout development. Damp is also expressed in a broad range of tissues, including the CNS, discs and body wall of larvae. Immunofluorescent staining with pre- and postsynaptic markers at the neuromuscular junction (NMJ) indicate that Damp is localized postsynaptically. To explore Damp function in vivo, we have generated mutant and transgenic flies. Null mutants are viable, but sluggish, and show no defects in synaptic morphology or synaptic physiology at the NMJ. Damp mutants, however, do demonstrate deficiencies in locomotion compared with controls of the same genetic background. These surprising results indicate that Damp is not required for viability or SV endocytosis at the larval NMJ. We are currently undertaking several screens to test for Damp interactions with known endocytic proteins, as well as novel proteins. The discovery of novel protein partners for Damp may reveal functions for Damp beyond its putative role at the synapse.

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5. EFFECTS OF ANTIBIPOLAR DRUGS ON CALRETICULIN AND BCL-2 EXPRESSION IN HUMAN NEURONAL, GLIAL AND B LYMPHOBLAST CELL LINES Tim Corson1,4,5,*1Section of Biochemical Psychiatry, Centre for Addiction and Mental Health; Departments of 2Pharmacology and 3Psychiatry, 4Institute of Medical Science and 5Program in Neuroscience, University of Toronto

, Peter P. Li1,2,3 and Jerry J.Warsh1,2,3,4,5

Bipolar disorder (BD) is a chronic, debilitating mood disorder, characterized by recurrent episodes of depression and mania (BD-I) or hypomania (BD-II). Signal transduction disturbances affecting intracellular Ca2+ homeostasis in BD pathophysiology may modify apoptotic signaling and compromise neuronal or glial integrity. Since certain Ca2+ abnormalities are manifested in B lymphoblast cell lines (BLCLs) from BD-I patients, we asked whether candidate Ca2+ signal transducer genes, implicated by biochemistry and/or genetic linkage, are differentially expressed in response to the antibipolar drugs, lithium and valproate, in human hNT neuronal and SVG glial cells, and in BLCLs from a subgroup of BD-I patients showing high [Ca2+]i. mRNA levels of candidates were determined by semi-quantitative RT-PCR. Expression of Bcl-2, a major anti-apoptotic protein upregulated by lithium and valproate in rat brain, did not differ among BLCLs from BD patients stratified on [Ca2+]i (high vs. normal) and healthy subjects, nor in response to one-week 0.75 mM lithium or 0.35 mM valproate treatment in these cells. However, in SVG cells, valproate (1 mM), but not lithium (0-2 mM), reduced Bcl-2 mRNA (p=0.058, Dunnett). mRNA levels of calreticulin, a Ca2+-binding chaperone upregulated by valproate in rat brain and downregulated by Ca2+, were reduced in high [Ca2+]i compared with normal [Ca2+]i BD-I patient BLCLs (p=0.030, Tukey). Calreticulin mRNA levels also correlated negatively with patient BLCL [Ca2+]i (R2=0.19, p=0.059), but were not significantly modulated by drug treatment. Valproate (1 mM) increased calreticulin mRNA in SVG cells (p=0.019, Dunnett), but neither valproate nor lithium had an observed effect on calreticulin in hNT neurons. These results suggest that Bcl-2, although an interesting pharmacological target, may not be affected in BD disease processes. However, altered calreticulin expression could be involved in Ca2+ abnormalities in BD and in antibipolar drug effects, warranting further investigation into calreticulin’s role in BD Ca2+ compartmentalization.

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6. THE EFFECTS OF DIET AND AGE ON THE PERFORMANCE OF THE LANDMARK DISCRIMINATION LEARNING TASK J. Estrada 1*,

1 Institute of Medical Sciences, University of Toronto, Toronto, ON, Canada M5S 1A8. 2Division of Life Sciences, University of Toronto at Scarborough, 1265

C. Ikeda-Douglas, N.W. Milgram, 2 E. Head, C.W. Cotman,3 H. Murphey, B. Muggenburg,4 S. Zicker 5

Military Trail, Scarborough, ON, Canada M1C 1A4. 3 Institute for Brain Aging and Dementia, University of California, Irvine, CA 92717-4550, USA. 4 Lovelace Respiratory Research Institute, Albuquerque, NM 87185, USA. 5 Hill's Pet Nutrition, Topeka, KS, USA Free radicals, which are byproducts of oxidative reactions, are an essential factor in the degenerative neural changes that accompany aging. We fed young and old beagle dogs a diet consisting of a broad spectrum of antioxidants and tested the effects on series of landmark discrimination problems. Young and aged beagle dogs were each divided into groups receiving either control or antioxidant-enriched diets. The task required the animals to respond selectively to the object closest to an external landmark. Aged animals on the enriched diet learned the initial landmark task more rapidly than aged dogs on the control diet. The young animals, by contrast, showed no effect of diet. We also found dramatic age differences in learning the landmark discrimination task, with young dogs learning more rapidly and having fewer errors than old dogs. These results showed first that allocentric visuospatial function deteriorates as a function of age and second that performance of allocentric task can be improved by the administration of an antioxidant enriched diet. Key words: antioxidants, dogs, age, landmark discrimination.

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7. EFFECTS OF PHACLOFEN AND SCOPOLAMINE ON PREPULSE INHIBITION OF THE STARTLE REFLEX IN RATS Paul FaermanDepartments of Zoology and Psychology, University of Toronto

*, Stephan Steidl, Liang Li, John Yeomans

The startle reflex is elicited by tactile, vestibular, or acoustic stimuli via the trigeminal (Pr5), vestibular (VN) and cochlear (CN) nuclei. The three nuclei send projections to the caudal pontine reticular nucleus (PnC) which, in turn, elicits startle via the reticulospinal pathway. Electrodes were placed in chloral hydrate-anaesthetized rats in the left inferior colliculus (IC), a startle-inhibiting nucleus, and the right Pr5, respectively. Two cannulae were placed in the right and left PnC. EMG was recorded in the left bicep femoris, contralateral to the stimulation site in the right Pr5. Stimulations of the IC was followed by Pr5 stimulation at varying interstimulus intervals (ISIs) of 0-1000ms. Prepulse inhibition (PPI) occurred most strongly, at approximately 50% of the startle baseline, at ISI of 30-100ms, peaking at the 100ms ISI. Further, summation was evident when the IC and Pr5 were stimulated simultaneously. Injection of phaclofen, a GABA-B antagonist, into the PnC resulted in 25-30% inhibition of PPI. We also inhibited PPI by 50% with PnC injection of scopolamine (100µg), a muscarinic antagonist. Inhibition of the startle reflex at the PnC may result from projections of laterodorsal tegmental nucleus and pedunculoponitne tegmental nucleus cholinergic neurons to the PnC.

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8. THE NEURAL BASIS OF RECOVERY FROM UNILATERAL VESTIBULAR DAMAGE: ROLE OF THE EXCITATORY VESTIBULAR COMMISSURE Karl Farrow Department of Physiology, University of Toronto; Toronto Western Research Institute, Toronto Western Hospital

*, Dianne M. Broussard

The vestibulo-ocular reflex (VOR) produces eye movements equal in magnitude and opposite in direction to head movements. In its normal operation, the VOR operates in an open loop fashion and thus is extremely sensitive to injury. In particular the performance of the VOR is severely compromised after peripheral vestibular damage. After damage to one of the horizontal vestibular canals, the gain of the VOR is reduced and asymmetric and although there is recovery it is only partial. Changes in the efficacy of commissure transmission have long been thought to contribute to the recovery of the VOR after peripheral damage, but evidence is lacking. The vestibular commissure connects the vestibular nuclei on the two sides of the brainstem. Here the contribution of the contralesional horizontal canal is assessed via the response of ipsilesional secondary vestibular neurons to single shock electrical stimulation of the intact vestibular labyrinth. This investigation attempted to identify pathways in the VOR circuitry that mediate the recovery of the dynamic symptoms associated with unilateral peripheral vestibular lesions. A significant decrease in the efficacy of the excitatory commissure was observed connecting the contralesional labyrinth to secondary neurons in the ipsilateral vestibular nucleus. This change may be functionally compensatory for recovery of the dynamic symptoms associated with unilateral damage to the vestibular labyrinth.

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9. M5 MUSCARINIC RECEPTORS ARE REQUIRED FOR PROLONGED DOPAMINE RELEASE FOLLOWING MESOPONTINE STIMULATION IN MICE G.L. Forster, J.S. YeomansDepartment of Psychology, University of Toronto and

, J. Takeuchi, and C.D. Blaha

Department of Psychology, Macquarie University, Australia Midbrain dopamine neurons are directly activated by cholinergic neurons of the laterodorsal tegmental nucleus (LDT). In urethane anestheized mice, brief electrical stimulation of the LDT resulted in two phases of increased dopamine release in the nucleus accumbens, one lasting for 2 min, and the second lasting from 8-40 min after LDT stimulation. The prolonged phase was blocked by systemic scopolamine (5 mg/kg), a muscarinic antagonist. The M5 muscarinic receptor is expressed near dopamine neurons, and is removed by 6-hydroxydopamine. In homozygous mice with truncated M5 receptor genes, the prolonged phase of dopamine release was absent, while the initial phase was normal. Therefore, M5 muscarinic receptors mediate a prolonged facilitation of dopamine release.

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10. DEVELOPMENT OF THE CENTRAL AUDITORY SYSTEM IN CHILDREN WHO USE A COCHLEAR IMPLANT K.A. Gordon1 Department of Otolaryngology and the Cochlear Implant Program, The Hospital for Sick Children and University of Toronto 2 Department of Physiology, The University of Toronto

* 1,3, B.C. Papsin1, R.V Harrison 1,2,3

3 Institute of Medical Sciences, The University of Toronto Normal development of the human central auditory pathways has been characterized through studies of evoked potentials. Changes in wave morphology and decreases in latencies reflect underlying mechanisms including myelination and improved synaptic efficacy. Developmental processes are dependent upon auditory input and are thus hindered by hearing loss. Input to the central auditory system through a cochlear implant promotes psychophysical improvements of auditory skills. However, developmental changes along the pathways have never been characterized in children with hearing loss who use implants. We aimed to measure developmental changes in such children using the electrically evoked auditory brainstem response (EABR). We made repeated measures of EABR in n=41 non-sedated children beginning at initial stimulation and after two, six, and twelve months of consistent implant use. We find that EABRs are present in all children even with only initial auditory stimulation and that EABR wave latencies decrease from the time of initial activation throughout the first year of cochlear implant use. These findings suggest that: 1) the auditory pathways develop to some degree in the absence of stimulation and 2) developmental plasticity of the auditory brainstem and midbrain is retained despite auditory deprivation in childhood. Developmental changes promoted by cochlear implant use include decreased neural conduction times and increased synchrony. Underlying mechanisms include activity dependent myelination and enhanced synaptic efficacy.

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11. DIFFERENTIATED PC12 CELLS: AN IN VITRO MODEL FOR ISCHEMIA Nirmala Govinda*Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Department of Physiology, University of Toronto, Toronto, Ontario, Canada

, Ziping Zhu & Linda R. Mills

Ischemic cell death is initiated by changes that result from the inhibition of oxidative phosphorylation. An initial decrease in ATP and pH, and an increase in free radicals, leads to secondary changes e.g. increases in intracellular Na2+ and Ca2+ which in turn activate proteases and phospholipases further increasing free radical generation and ultimately cell death. We have examined the effects of Ischemia, Hypoxia, and Hypoglycemia using an in vitro model, the oxygen sensitive, differentiated PC12 cell line. Typically, cell cultures are given an oxygen-glucose deprivation (OGD) insult in Balanced Salt Solution (BSS), considering culture media to be a more physiological medium to expose cells to OGD, we used Dulbecco’s Modified Eagle Medium (DMEM). Cultures exposed to OGD for up to 3 hours in BSS plus NGF showed minimal cell death both immediately post-insult, and 24 hours post-insult, compared to controls. Surprisingly, cultures exposed to OGD in DMEM, which contains amino acids and vitamins, plus NGF, showed significant and dose dependent increases in cell death both immediately and 24 hours post-insult. Cell death was analysed using the Lactate Dehydrogenase (LDH) assay and Propidium Iodide (PI) labelling. Deep hypothermia at 20oC decreased cell death immediately and 24 hours subsequent to an ischemic or hypoxic exposure. Our experiments using Flow-Cytometry and Confocal-Microscopy with 5’6’ CM-H2DCF-DA (DCF) to assay free radicals, suggest that immediately following a one and two hour OGD insult, free radical generation was elevated in cultures exposed to OGD in DMEM compared to sister cultures exposed to OGD in BSS. Flow-Cytometric analysis of the mitochondrial membrane potential using Rhodamine 123 was however not significantly reduced in cells in BSS or DMEM as compared to controls.

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12. NEUROMUSCULAR AND AXO-AXONAL SYNAPSES IN THE SWIMMING PADDLE OPENER MUSCLE OF BLUE CRABS Katya HonsaUniversity of Toronto, Department of Zoology

*, Joanne Pearce, CK Govind

Crustacean neuromuscular systems model the mammalian central nervous system in that both have excitatory and inhibitory axons that interact with each other as well as with the target. To better understand these synaptic contacts we examined the opener muscle in the highly specialized swimming paddle of blue crabs. The opener muscle is innervated by an excitor axon and a private inhibitor axon (a common inhibitor axon innervates a small restricted region of the muscle). Light microscopy, enzyme histochemistry, and electron microscopy were performed on the muscle in order to develop a viable model of this system. The opener muscle was determined to be a slow muscle (sarcomere lengths longer than 6µm), and was found to be profusely innervated. Synapse size, terminal length and terminal content were analyzed and comparisons were made between the specific inhibitor and the excitor nerve terminals. It was found that the excitor terminal length was three times the inhibitor length, but that the two shared very similar content distribution. Clear vesicles were the most prominent in the terminals, whereas dense core vesicles were the least common. High levels of energy substrates were comprised of equal proportions of glycogen granules and mitochondria, the former providing a more rapid source of energy than the latter. Consequently, energy stores appear to be adapted to swimming which is both rapid and sustained. The overwhelming majority of synapses formed by the excitor were neuromuscular, in keeping with its primary function of making the muscle contract. However, there were a few axo-axonal synapses from the excitor onto the inhibitor axon, suggesting putative presynaptic excitation of the inhibitory nerve terminals, an unusual phenomenon. The inhibitor axon, with far fewer synapses than the excitor, originated synapses that were two thirds neuromuscular, inhibiting the muscle directly, and one third axo-axonal, inhibiting the excitor axon itself. This is an unusually high degree of presynaptic inhibition possibly associated with the quick action of the swimming stroke.

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13. NEUROMOTOR SPEECH DEFICITS IN CHILDREN AND ADULTS WITH SPINA BIFIDA Joelene Huber-Okrainec& Brenda J. Spiegler

1 *

1,4, Maureen Dennis,

1,3,4 Jane Brettschneider

2,

Brain and Behaviour Program1 and Speech Pathology

2, The Hospital for Sick Children

Department of Surgery,3 Institute of Medical Sciences,

4 University of Toronto

Acquired cerebellar lesions are associated with neuromotor speech deficits. Spina bifida with hydrocephalus (SBH) is a neurodevelopmental disorder that involves the Arnold-Chiari II malformation and thus, significant dysmorphology of the cerebellum. Until now, there has been a paucity of information on the effects of congenital cerebellar dysmorphology on neuromotor speech. Videotaped narrative speech samples produced by 40 children and adults with SBH and their 40 age matched controls were coded for three motor speech deficits: Dysfluency, Ataxic Dysarthria (articulatory inaccuracy, prosodic excess, and phonatory-prosodic insufficiency), and Speech Rate. Individuals with SBH had more motor speech deficits than controls. Dysfluency was related to an interaction between chronological age and SBH. Speech rate was related independently to chronological age and SBH. Ataxic dysarthria was related to the biology of SBH, and was associated with both physical phenotype (level of spinal cord lesion) and medical history (number of shunt revisions). The data show that, similar to acquired lesions of the cerebellum, congenital dysmorphology of the cerebellum disrupts motor speech. Further, the results of this study provide new information about the developmental role of the cerebellum in the automatization of motor skills, including motor speech.

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14. SYNAPTIC CONTROL OF NETWORK FREQUENCY Mehrdad JazayeriToronto Western Research Institute, University Health Network,

*, Liang Zhang, Frances K. Skinner,

Institute of Biomaterials and Biomedical Engineering, Departments of Medicine (Neurology) and Physiology, Program in Neuroscience, University of Toronto Different rhythmic activities in CA1 characterize the neuronal correlate of several behavioral states. Recently, in an in vitro preparation of the whole hippocampus, spontaneous delta rhythms similar to hippocampal EEG seen in behaving animals, have been recorded. Based on the experimental data and by using numerical simulations, we suggest a mechanism in which a dynamical feedback from populations of synchronized interneurons entrain an increasing and/or more synchronized activity in the pyramidal cell population. In this scenario, the number of the excitatory cells as well as the excitability of the interneurons determine the frequency and robustness of these slow rhythms.

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15. IMAGING TRANSPORT PATHWAYS OF SINGLE VESICLES ALONG NEURITES OF LIVING NEUROENDOCRINE CELLS A. Jeromin∗, Mt. Sinai Hospital, Samuel Lunenfeld Research Institute, Toronto, ON M5G 1X5

G. Schutz, H. Schindler and J.C. Roder

IMAGING TRANSPORT PATHWAYS OF SINGLE VESICLES ALONG NEURITES OF LIVING NEUROENDOCRINE CELLS The trafficking of single synaptic-like microvesicles in living neuroendocrine cells was monitored along neurites into growth cones. Ultra-sensitive fluorescence microscopy permitted real-time 3-dimensional imaging of single vesicle motion. From the pathways of vesicles, the arrangement of individual microtubules was reconstructed at a resolution of 50nm. During directional transport vesicles were seen to change between distinct microtubule pathways. Such crossing events were either instantaneous or mediated via transient trapping of the vesicle. Trapping also facilitated interactions between vesicles, allowing coordinated cargo transport.

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16. A STUDY OF RECTAL SENSATIONS AND PHYSIOLOGY IN NORMALS AND PATIENTS WITH IRRITABLE BOWEL SYNDROME. C.L. Kwan *Inst. of Medical Science1, Depts. of Surgery2 and Medicine3, Univ. of Toronto, and the Toronto Western Research Institute4, Toronto, Canada

1,4, K. Mikula 3, N.E. Diamant 1,3,4 and K.D. Davis 1,2,3,4

Aim: The mechanism underlying irritable bowel syndrome (IBS), a disorder characterized by a decrease in rectal pain threshold and an increase in rectal pain response, is unknown. The etiology may involve the rectum itself and/or the neural pathways mediating rectal sensations. The aim of this study was to compare the rectal physiology and evoked sensations in normal subjects versus IBS patients. Methods: A computer-controlled barostat was used to inflate a polystyrene rectal balloon in normal healthy subjects and IBS patients. In each subject, the pressure required to initially inflate the balloon by 20 ml was considered the baseline pressure. Phasic trials consisted of five 30s distentions at pressures eliciting either a sensation of moderate urge to defecate or moderate pain interleaved with deflation to baseline pressure for 30s. Analogous pressures were used in tonic trials where a single 5min distension was delivered following a 30s distention at baseline pressure. Subjects continuously rated urge, unpleasantness or pain intensity on a computerized visual analogue scale throughout the duration of all trials. Results: Compared to normals, IBS patients had significantly (P<0.05) lower baseline and destination compliance (volume/pressure). Phasic distention in normals evoked sensations of urge, pain intensity and unpleasantness which faithfully followed the temporal profile of each distention. In IBS patients, the sensations were infrequently matched to the temporal profile of isobaric distentions. Sensations evoked during tonic distentions were highly variable in magnitude in normals, but IBS patients showed consistently elevated magnitudes of urge, pain and unpleasantness. Also, the diminution of sensations after the termination of rectal distentions occurred at a significantly (P<0.05) longer delay in IBS patients versus normals. Conclusions: These findings demonstrate that: 1) rectal physiology is altered in IBS patients, and 2) temporal characteristics of urge, unpleasantness and pain perception are significantly different in IBS patients. These data also support the utility of continuous monitoring of rectal sensations in studies of rectal physiology. This study was supported by funding from NSERC and CIHR.

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17. THE NEUROPHYSIOLOGICAL EFFECTS OF AGING ON TEMPORAL PROCESSING SPEED IN THE RAT INFERIOR COLLICULUS H. Jin Lee1,*,1Department of Speech-Language Pathology, 2Institute of Medical Science, * Program in Neuroscience, University of Toronto, Toronto, Ontario, Canada

Tasneem Wallani2,*, Julie R. Mendelson 1,2

Difficulty in understanding speech is one of the most common problems afflicting the elderly population. One potential factor in this problem is a deterioration in the ability to process dynamic aspects of speech such as the formant transitions (components of speech in which frequency and amplitude change over time). Underlying this deterioration may be an age-related decline in temporal processing speed in the central auditory system. In order to study temporal processing speed, a stimulus that shares common features with formant transitions found in animal communication signals and human speech is necessary, and this is the frequency-modulated (FM) sweep. In this study, age-related temporal processing speed was examined in the inferior colliculus, an important auditory integration centre in the central auditory system. Extracellular single-unit recordings were made from the inferior colliculus of young (2-4 months) and old (24-30 months) Long-Evans Hooded rats in response to linear FM sweeps that changed in speed and direction. Results showed that the majority of neurons recorded from young and old rats responded best to fast and medium FM sweeps. For preferred direction of FM sweep, most of the cells in both young and old rats were non-direction selective. The results suggest that processing of dynamic aspects of complex acoustic stimuli such as the FM sweep may be reserved for the auditory cortex.

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18. ANTIPSYCHOTICS INTERFERE WITH MATERNAL AND SOCIAL BEHAVIORS IN RATS: DIFFERENTIAL EFFECTS OF HALOPERIDOL AND CLOZAPINE Ming Li *, Shitij Kapur and Alison Fleming Department of Psychology Maternal behavior in rats is a highly-motivated, well-organized social behavior that is in some respects analogous to human maternal behavior. We investigated the effects of a typical antipsychotic haloperidol (HAL, 0.2 mg/kg, n=12), an atypical antipsychotic clozapine (CLZ, 10 mg/kg, n=12) and vehicle (n=12) on this model in lactating postpartum female rats. The maternal and social behaviors were observed before and 0.5, 1, 2, 4, 6 and 24 hours after drug administration. Both HAL and CLZ significantly impaired pup approach, pup retrieval, nursing, and nest-building behaviors, as well as social interactions with juvenile or adult conspecifics (all p<0.001 vs. vehicle), although the temporal course was different. CLZ caused a rapid and transient disruption, whereas HAL elicited a slower-onset but prolonged disruption. These behavioral deficits are unlikely to be due to motor deficits since they were observed in CLZ animals and in the absence of catalepsy. We interpret these findings as reflecting the adverse impact of antipsychotics on mesolimbic-mediated motivated social/interpersonal behaviors, an analog of drug-induced negative/deficit symptoms. The implications of these findings in patients need to be investigated.

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19. LOCATION OF MICROSTIMULATION-INDUCED TREMOR REDUCTION IN THALAMUS OF ESSENTIAL TREMOR AND PARKINSON'S DISEASE PATIENTS R. Louis1,1Dept. of Physiology, and 2Dept. of Surgery, University of Toronto, Toronto, Ontario, M5S 1A8, Canada

J.T. Tang1, B. Hutchison1,2, K. Davis2, A. Lozano2, J.O. Dostrovsky1

High frequency (HF) stimulation through chronically implanted deep brain stimulation (DBS) electrodes in thalamus is an effective treatment for tremor in essential tremor (ET) and Parkinson's disease (PD) patients. It is assumed that the effective region is in Vim (ventralis intermedius nucleus), the presumed cerebellar receiving area of motor thalamus, although there has been no direct verification of this. Stimulation through microelectrodes during functional stereotactic surgery can also decrease or block tremor and this is used as a good indication for placement of the DBS electrode. The aim of this study was to analyze the locations of these effective microstimulation sites and of tremor cells in relation to the location of the anterior border of the tactile relay nucleus, Vc (ventrocaudal nucleus). Data were obtained from 11 electrode tracks in 9 ET patients, and 26 electrode tracks in 17 PD patients. The effectiveness of microstimulation-induced tremor reduction in both patient groups was found to be maximal in the region 1.5-2.0 mm anterior to Vc, in the Vim region. The locations of tremor cells was not highly correlated (r2=0.49) with the locations of tremor reduction sites in ET, but the correlation was higher (r2=0.88) in the PD group. These findings indicate that the therapeutic effects of DBS are probably mediated by an action on cells in the rostral Vim region. They also suggest that the presence of tremor cells may not be a very useful indicator for localizing the optimal site of DBS electrode implantation for ET, but may hold promise for PD.

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20. OXIDATIVE CELLULAR INJURY MEDIATED BY PEROXYNITRITE IN THE LATERAL GENICULATE NUCLEUS IN GLAUCOMA Anchla Luthra1

,2, Neeru Gupta1,2,3,4, Paul L. Kaufman5, Robert N.Weinreb6, Yeni H. Yucel1,2,3,4 Vision Science Research Program, University of Toronto1, Department of Laboratory Medicine and Pathobiology, Faculty of Medicine, University of Toronto2; Department of Ophthalmology, University of Toronto3; Health Sciences Research Center, St. Michael's Hospital, Toronto4; Department of Ophthalmology & Visual Sciences, University of Wisconsin, Madison5; Glaucoma Center, University of California, San Diego6

Background: Neuronal atrophy, reduced neuronal metabolic activity, and neuronal loss has been demonstrated in the lateral geniculate nucleus (LGN) in glaucoma. Purpose: To determine whether oxidative damage mediated via peroxynitrite, a reaction product of nitric oxide and superoxide radicals, plays a role in cell degeneration in the LGN in glaucoma. Methods: The right LGNs of 5 cynomolgus monkeys with experimentally induced glaucoma in the right eye, and of 5 normal control monkeys were studied. Mean intraocular pressure in the right eye ranged from 30 mmHg to 52 mmHg in experimental monkeys and from 14 mmHg to 21 mmHg in control monkeys. In the experimental monkeys, survival time following laser trabeculoplasty was 1, 4, 5, 6, and 14 months. Nitrotyrosine, an oxidative protein modification mediated by peroxynitrite, was detected with immunohistochemistry using nitrotyrosine antibody. Results: Intense nitrotyrosine immunoreactivity in cytoplasmic and nuclear components of a few isolated cells in the LGN was observed in all glaucoma monkeys. No difference among monkeys with different survival periods was seen. No such immunoreactivity was seen in the LGN from normal controls. Conclusions: Nitrotyrosine is present in the LGN in experimental glaucoma. This suggests that peroxynitrite mediated oxidative cell injury occurs in the LGN in glaucoma. Significance

: Understanding mechanisms of oxidative cellular injury in the LGN may help the design of new therapeutic strategies for glaucoma.

Supported in part by the Vision Science Research Program, University of Toronto (AL), Glaucoma Research Foundation (YY), E.A. Baker Foundation of the Canadian National Institute of the Blind (NG, YY), Glaucoma Research Society of Canada (YY), NEI (EY02698, PLK), Joseph Drown Foundation (RNW).

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21. RESPONSES OF HUMAN THALAMIC AND CINGULATE CORTICAL NEURONS DURING SEQUENTIAL REACHING M.L. MacMillan2, W.D. Hutchison1,2,4, A.E. Lang3, A.M. Lozano1,4. 1Departments of Surgery, 2Physiology & 3Medicine, University of Toronto, and 4Div. of Neurosurgery, Toronto Western Hospital, Toronto, ON Canada M5T 2S8

Several anatomical studies have shown evidence for neural pathways from the motor cortex to the thalamus via the basal ganglia and back to the cortex. Neuronal responses have been recorded in the motor and premotor monkey cortex that implicate these regions in coding specific sequences (Shima and Tanji 1999) as well as serial order (Carpenter 1999). However, few studies have examined neurons at the thalamic and cingulate level of this pathway. The aim of the current study is to examine the neuronal activity in the thalamus and cingulate cortex and whether there is evidence of serial coding of order within a sequence. In Parkinsonian patients off drugs undergoing bilateral implantation of stimulating electrodes in stn, and patients undergoing cingulotomies for chronic depression or obsessive compulsive disorder, single and dual microelectrode recordings were carried out during a sequential reaching. Patients pressed a variable sequence of 3 of 5 buttons (p1-p3) over 8-16 trials in each of 2 visually-cued tasks: an internally generated task with a memorized sequence (mem) and an externally driven task with the sequence given during task performance (follow). Average histograms of neuronal firing were constructed for each task and aligned with respect to visual cues (ready, go) or button presses (p1, p2, p3). Reaching movements were monitored with surface emg and hand accelerometry, and cell responses were divided into movement-defined epochs for analysis of variance (anova) and post-hoc means testing. Three neurons were analyzed from the cingulate region, two with inhibitory and one with excitatory responses in mean discharge rate during the entire movement sequence. Three thalamic neurons were analyzed; two had excitatory and one had inhibitory responses with mean discharge rates that decreased significantly with subsequent button presses (p1>p2>p3). In contrast to the cortical neurons the thalamic neurons showed evidence of coding for serial order during movement sequences. Supported by NSERC and The Parkinson Foundation of Canada

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22. WHOLE CELL OUTWARD POTASSIUM CURRENTS OF FELINE ESOPHAGEAL CIRCULAR SMOOTH MUSCLE CELLS DEMONSTRATE A BIPHASIC RESPONSE TO CHOLINERGIC STIMULATION Ahmad MuinuddinDepartments of Physiology1 and Medicine2, University of Toronto, and Playfair Neurosciences Unit3, University Health Network

*1,3, Ji Junzhi, 3 N.E. Diamant, Supervisor1,2,3

Peristalsis in the smooth muscle esophagus has in part been attributed to regional differences in the excitatory (cholinergic) influence with shorter delay to onset of the contraction proximally. However, there are no objective differences in neural elements to explain the differing neural influence along the esophagus. Hypothesis: The responses of the muscle ion channel currents to acetylcholine (ACh) hold the potential to impact on the responses of the muscle to its innervation. Methods: The whole cell patch-clamp technique was used to investigate the effect of cholinergic stimulation on outward potassium currents of freshly dissociated single circular esophageal smooth muscle cells. Viability was established by cell contraction in response to ACh. With a micropipette positioned 50mm from the cell, ACh (100uM) was delivered for 20s at a pressure of 5 psi by picospritzer. Cells were held at -50mV and a single step protocol to +30mV for 250ms was applied every 2s for 30s to study the time course of outward current response to ACh. Results: The outward current was inhibited when ACh was added manually to the whole cell chamber (10uM-100uM). When ACh was delivered directly on the cell there was a transient increase in outward current of between 25% to 50% of control values within 10s. This was followed by decline in outward current with maximal inhibition of between 30% to 60% of control values within 30s. The outward current returned to normal within 180s. Conclusion: The initial increase in whole cell current should be associated with hyperpolarization and presumably delay to onset of the depolarization (which normally is associated with contraction) with inhibition of the outward current. Therefore the functional significance of a biphasic response to ACh may relate to the timing of the peristaltic contraction. Whether regional differences exist in this response remains to be determined. (The authors gratefully acknowledge the support of the CIHR)

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23. MECHANISMS INVOLVED IN THE THERAPEUTIC TREATMENT OF A SPONTANEOUSLY DEMYELINATING MOUSE MODEL, THE ND4 TRANSGENIC MOUSE Thomas A. NguyenStructural Biology and Biochemistry, The Hospital for Sick Children, and the Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada

* and Mario A. Moscarello

Demyelination in the Central Nervous System (CNS) is a complex process involving many cellular and biochemical events. The expression of certain myelin proteins reflects an immature form of myelin, causing destabilization and susceptibility to enzymatic attack. Using the ND4 transgenic mouse, a spontaneously demyelinating mouse model, we are investigating potential therapeutic treatments using various compounds to prevent or delay the on-set of pathological and clinical signs. A peptide mimetic of the receptor for hyaluronan mediated motility (RHAMM) was shown to be a successful in the attenuation of clinical signs. Treatment with the recombinant fusion protein RHAMM v2 also showed comparable improvement. Interferon-β (IFN-β), an established and important treatment for relapse-remitting Multiple Sclerosis, was used as a basis for clinical comparison. Histopathological examination showed improvement in myelination (using Luxol fast blue stain) and a possible decrease in astrogliosis (using the astrocytic marker glial fibrillary acidic protein, GFAP).

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24. RESPONSE PROPERTIES OF HUMAN THALAMIC NEURONS TO DEEP BRAIN STIMULATION (DBS) S.E. Patra *1 Dept. of Physiology, 2 Dept. of Surgery, and 3 Institute of Medical Science University of Toronto, Toronto, Ontario, M5S 1A8, Canada

1, W.D. Hutchison 1,2, K. D. Davis 2,3, A.M. Lozano 2,3, J.O. Dostrovsky 1

The ventral lateral region of the thalamus plays a major role in normal motor and sensory function, as well as in pathological conditions such as Parkinson’s disease (PD). Both thalamotomy and high frequency deep brain stimulation (DBS) in motor thalamus have been employed as techniques to alleviate the tremor associated with PD and other movement disorders. It is thought that the DBS effects result from blocking thalamic neuronal activity. However there has been no direct confirmation of this and the mechanisms are not known. The present study directly examined the effects of electrical stimulation in the ventrolateral thalamus on the firing of nearby neurons. Low frequency (less than 20 Hz) stimulation through one electrode while recording from a nearby electrode (~250um) produced short latency and duration excitation following each stimulus pulse (currents typically <10uA) in 29 of 47 cells tested (62%). In contrast, high frequency stimulation (100-333Hz; current 5-40uA) from either a nearby stimulating electrode (~250um) or directly from the recording electrode was found to produce a prolonged inhibition lasting up to 10s. This inhibition was usually preceded by a short period of bursting activity and was observed in 22 of 29 neurons that had a bursting firing pattern and in 9 of 31 non-bursting neurons. These preliminary findings indicate that high frequency stimulation in thalamus can directly inhibit the firing of thalamic neurons and suggest that this may be responsible for the therapeutic benefit of DBS.

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25. A COMPUTATIONAL STUDY OF THE HETEROGENEOUS OUTPUT OF O/A INTERNEURONS F. Saraga 1,3*, L. Zhang1,2, C. Wu 1, F.K. Skinner1,2 1Toronto Western Research Institute, University Health Network, Toronto, ON, Canada, 2Department of Medicine (Neurology), 3Department of Physiology, Institute of Biomaterials and Biomedical Engineering, University of Toronto Interneurons constitute approx. 10% of the total neuronal population in the hippocampus but can exert widespread control over brain output due to their interactions with hundreds of principal cells. Interneurons are extremely heterogeneous with regard to their morphologies, biophysical properties and pharmacological sensitivities. Recently, slow field rhythms (0.5-4 Hz) have been found to occur spontaneously in an intact hippocampal preparation. It is not clear how these oscillations are generated but simultaneous extracellular and patch clamp recordings reveal that summated IPSPs from coherent interneuronal networks are critically involved. Given this and their known heterogeneous nature, we developed models to help gain insight into possible generation mechanisms. Using a CA1 O/A horizontal interneuron, we built an 84-compartment model using NEURON. Appropriate passive properties were used and minimal active currents inserted. Since spikes are still measured in the soma when it is held at a hyperpolarized voltage, active dendrites are likely responsible for spike generation. Simulations using this model show that the signal shape, pattern, and frequency is very dependent on the distribution of active channels along the cell body. This suggests that a critical aspect in interneuronal heterogeneity might lie in the amount of dendritic active conductances which thus allow spike generation and network coherence. Supported by: OGSST, MRC, NSERC, DCIEM of Canada.

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26. THE MOTOR SIDE OF DEPTH VISION K. Schreiber1*1 Dept. of Physiology and Medicine, University of Toronto 2 Centre for Vision Research, York University, 4700 Keele Street, M3J 1P3 Toronto, Canada & Department of Neurology, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72072 Tübingen, Germany

, J. D. Crawford2, M. Fetter &, D. Tweed1,2

This study (Schreiber et al., Nature, in press) shows that current theories of stereo vision must be revised to take into account the role played by eye movements. In all current models, the brain first searches for corresponding image features on the two retinas. As long as the eyes stay still, the search is relatively simple because the optical geometry of the two eyes confines the corresponding features to narrow retinal bands called epipolar lines. But when the eyes change position, the epipolar lines migrate on the retinas. To find the matching features, then, the brain must either search different retinal bands depending on current eye position, or search much larger retina-fixed zones, large enough to cover all usual locations of the epipolar lines. The key prediction, we show, is that if the brain searches retina-fixed zones, then it should be possible to construct a new type of stereogram in which the depth image is visible only at certain gaze elevations, only looking up, for instance, or only looking down. We construct these stereograms and show that people do perceive them only in the predicted eye positions, confirming that the stereoptic search zones are indeed eye-fixed. This being the case, it follows that motor control could theoretically play a crucial part in depth vision. We explain this stereomotor theory and confirm it with eye-movement recordings, showing that the eyes twist about their lines of sight in a way that reduces the motion of the epipolar lines, allowing stereopsis to get by with smaller search zones and thereby lightening its computational load. This study has been supported by the Canadian Institutes of Health Research and the Deutscher Akademischer Austauschdienst.

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27. HEROIN SELF-ADMINISTRATION IN THE RAT FOLLOWING NEONATAL 6-OHDA INDUCED DEPLETION OF DOPAMINE Smith, Shannon A. M.*1,3

1Department of Psychology 2Department of Psychiatry 3Centre for Addiction and Mental Health (CAMH)

, Korth, Karin M.*1,3, & Fletcher, Paul J.1,2,3

Opioid agonists such as heroin activate the mesolimbic dopamine system which plays a central role in reward-related behaviour. However, the role of dopamine in mediating the rewarding effects of opioids is controversial. The primary goal of this study was to investigate how a complete depletion of neural dopamine affects heroin self-administration behaviour in rats. Three days after birth (PND 3), the experimental pups were administered. 6-OHDA (150µg in 5µl vehicle) and the control pups were given vehicle (1% ascorbic acid) infused into the lateral ventricle. Once the animals reached 120 days, they were anaesthetised and a chronic silastic catheter was then inserted into the right external jugular vein. In experiment one, animals learned to self-administer heroin (30µg/kg) by nose poking during 1 hour sessions. After baseline was established, animals were exposed, in a counterbalanced design to an additional three heroin doses (7.5, 15, and 60 µg/kg). They were then returned to the baseline dose and challenged with an IP injection of the opioid receptor antagonist, naloxone. Finally heroin was replaced with saline and behaviour was extinguished. In experiment two, animals learned to nose poke for sucrose water (0.1 ml) over a 5 day period. Sucrose was delivered on a progressive ratio (PR) schedule where response requirements increased exponentially for successive reinforcers. Sessions terminated when a period of 20 minutes elapsed without having earned a reinforcer. Next sucrose was replaced with water and behaviour was extinguished over a four day period. In experiment one, sham- and 6-OHDA-lesioned rats responded for approximately 10 heroin infusions per session. For sham control animals, the number of heroin infusions increased as the dose was lowered, and decreased as the dose was raised. Lesioned animals did not change their behaviour in response to changes in dose. Naloxone increased heroin intake in sham- but not in 6-OHDA-lesioned rats. Finally sham-lesioned, but not 6-OHDA-lesioned animals, showed an extinction induced increase in responding when heroin was replaced with saline. This declined over the following days. In experiment two, both groups responded for sucrose, and for both groups responding decreased when sucrose was replaced with water. Our results indicate that animals with an almost complete reduction of dopamine appeared to be capable of self-administering heroin. However they were unable to regulate the dose of heroin in response to several challenges including changes in dose, administration of naloxone, and saline extinction. Animals were also able to respond for a sucrose reward, and in that case reduced responding when water was substituted for sucrose. It appears that dopamine may be necessary for regulation of heroin self-administration behaviour.

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28. CONSEQUENCES OF NEUROGENESIS ON SYNAPTIC PLASTICITY IN THE ADULT DENTATE GYRUS OF THE RAT Jason SnyderDepartment of Physiology, University of Toronto

* & J. Martin Wojtowicz

Neurogenesis in the dentate gyrus of the adult brain generates a substantial population of neurons during postnatal life. This study examined the physiological differences between young and mature dentate granule neurons. Since proliferating cells are susceptible to irradiation induced death we utilized low dose irradiation to eliminate and thereby differentiate the physiology of the young neuronal population from the mature population. Using hippocampal slices and extracellular field recordings young neurons displayed greater synaptic plasticity than mature neurons and also possessed a unique pharmacology. In standard artificial cerebrospinal fluid (ACSF) the granule cell population produced 10% long term potentiation (LTP) of the field response after high frequency stimulation of the perforant path. The same stimulation in the presence of picrotoxin, a GABA blocker, produced an additional 40-50% LTP. In slices containing a reduced young neuronal population as a result of irradiation induced blockade of neurogenesis, ACSF-LTP was absent but PICRO-LTP remained. This young neuronal LTP (ie. ACSF-LTP) was also found to be insensitive to the NMDA blocker APV but was prevented by the NMDA-NR2B subunit blocker ifenprodil. In contrast, the mature neuronal LTP (ie. PICRO-LTP) was blocked by APV but not ifenprodil. The young neuronal LTP (ACSF-LTP) also could be induced with fewer high frequency stimuli and with stimuli of a lower frequency indicating a lower LTP induction threshold. These results are consistent with single cell recordings describing the synaptic plasticity and electrophysiology of putative young dentate granule neurons (J Neurobiol 42: 248-257). Since the dentate gyrus and associated hippocampal stuctures are responsible for certain types of memory formation it is proposed that the robust synaptic plasticity of these young neurons may contribute significantly to learning in adult animals.

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29. SENSORY-MOTOR INTERACTIONS REVEALED IN ORAL MOVEMENTS DURING SWALLOWING OF DIFFERENT TEXTURES Catriona M. Steele*Oral Dynamics Laboratory, Graduate Department of Speech-Language Pathology, University of Toronto

& Pascal H.H.M.Van Lieshout

During the oral stage of swallowing, the tongue positions the bolus and propels it to the pharynx. Videofluoroscopic investigations (Hiiemae & Palmer, 1999) have examined tongue-jaw linkages during mastication and swallowing of chewable foods; ultrasound studies (Chi-Fishman, Stone & McCall, 1998) have examined tongue-palate contact during water swallows. Little is known, however, about tongue movement patterns related to sensory properties of food texture. We propose that tongue kinematics should vary according to perceived rheological bolus properties. Propulsion of higher viscosity boluses should demonstrate greater movement accelerations. Oral preparation of fast-flowing liquids should show greater posterior tongue recruitment to prevent accidental bolus spillage into the throat. Coupling relationships between anatomically different tongue regions should be more complex for two-phase food items, which separate into solid and liquid components under force, than for cohesive items. We use electromagnetic midsagittal articulography (EMMA) to measure movements of three tongue regions (blade, mid and back) during swallowing of 5 rheologically different liquids. Movement data will be reported for 168 swallows from 2 normal subjects. Preliminary results show that coupling relationships across tongue regions exhibit greatest stability for cohesive fast-flowing liquids, and decrease as viscosity increases. Coupling strength, expressed by frequency entrainment and phase-relationship variability, differs significantly for non-Newtonian fluids (in which bolus resistance yields flow disproportional to tongue force) in comparison to Newtonian fluids. We interpret these observations of task-related synergistic or decoupled tongue behaviours as evidence of active behaviour control, operating in response to perceived rheological affordances of the bolus. References: Chi-Fishman, G.; Stone, M.; McCall, G.N., (1998). Lingual action in normal sequential swallowing. Journal of Speech, Language and Hearing Research, 41, 771-785. Hiiemae, K. M., & Palmer, J. B. (1999). Food transport and bolus formation during complete sequences on foods of different initial consistency. Dysphagia, 14(1), 31-42.

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30. PRENATAL EXPOSURE TO ORGANIC SOLVENTS AND CHILD NEURODEVELOPMENT Christine Till1,3*Departments of Psychology,1 and Pediatrics2 at the University of Toronto,

, Gideon Koren2,4 & Joanne F. Rovet2,3

Brain and Behaviour Programs3, Division of Clinical Pharmacology and Toxicology4

at The Hospital for Sick Children, Toronto, Canada Although many pregnant women are exposed to organic solvents in workplace settings, the impact of prenatal exposure to organic solvents on brain development is not known. To address this issue, we conducted a comprehensive evaluation in 33 children, 3 to 7 years of age, born to prospectively identified women who were occupationally exposed to organic solvents during pregnancy. The exposed group was compared with 28 referent children matched on age, gender, ethnicity, and socioeconomic status. Groups were compared on neuropsychological tasks, including subtests of the NEPSY, and on tasks of color vision and visual acuity. Individual tests were collapsed within domains to derive composite scores. An exposure index was estimated using questionnaire data obtained at time of initial contact. Multiple regression analyses revealed that the exposed group performed more poorly than did controls on expressive and receptive language and graphomotor ability. Attention, visuo-spatial and fine-motor abilities did not significantly differ between the groups. Evidence of a dose-response relationship was demonstrated, with increasing exposure level associated with poorer performance on language and graphomotor abilities. Observed visual deficits included problems in red and blue-yellow color discrimination as well as poorer visual acuity in the solvent-exposed children. On a pre-school continuous performance task, the exposed group committed more false alarms than controls but did not differ on omission errors, response latency or signal detection. Overall, these findings suggest that chronic, low-level maternal occupational exposure to organic solvents during pregnancy is associated with selective neuropsychological and sensory deficits in the offspring.

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31. THE NEUROPHYSIOLOGICAL EFFECTS OF AGING ON AM SPEED PROCESSING IN THE RAT AUDITORY CORTEX Tasneem Wallani1 Institute of Medical Science, 2 Department of Speech-Language Pathology, * Program in Neuroscience

1,*, H. Jin Lee2,* and Julie R. Mendelson1,2

University of Toronto, Toronto, Ontario, Canada A common complaint among the elderly is a difficulty in discriminating speech. This difficulty is due in part to an age-related change in temporal processing speed within the central auditory system. The present study explores the neural mechanisms underlying the effects of aging on temporal processing speed. Using the sinusoidal amplitude-modulated (AM) stimuli, temporal processing in auditory cortical neurons is compared between the young (2-4 months) and old (24-30 months) Long Evans Hooded rats. Extracellular single units were recorded from the animals while changing the rate of amplitude over time. It was hypothesized that the young animals would prefer the faster rate of amplitude change while a decline in cortical temporal processing speed would result in the old animals preferring the slower rate of amplitude change. Preliminary results indicate a tendency for the old animals to favour the slow to medium speeds, while the young animals prefer the medium to fast speeds. Thus far, these results are in support of those of Mendelson and Ricketts (2000) whose study provided the first indication of a neurophysiological difference in temporal processing speed in the aging auditory cortex.

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32. VERTICAL MISALIGNMENT IN UNIATERAL SIXTH NERVE PALSY Agnes M.F. WongDivision of Neurology1, and Departments of Ophthalmology2 and Physiology3, University of Toronto, Canada

* 1,2, D. Tweed1,3 and J.A. Sharpe1,2

Purpose: To detect and determine the magnitude of vertical deviation in patients with unilateral sixth nerve palsy. Methods: Twenty patients with peripheral palsy, 7 patients with central palsy caused by brainstem lesions, and 10 normal subjects were studied. Eye alignments were recorded using the prism-cover test, the Maddox rod and prism test, and magnetic search coil recordings in 9 diagnostic eye positions. Results: All patients had an abduction deficit and incomitant esodeviation that increased in the field of action of the paretic muscle, indicating sixth nerve palsy. Mean vertical deviations in peripheral palsy were: 0.33 prism diopter (PD) by prism-cover test, 1.29 PD by Maddox test, and 1.98 PD by coil recordings. Mean vertical deviations in normal subjects were: 0.00 PD by prism-cover test, 0.99 PD by Maddox test, and 1.94 PD by coil recordings. Peripheral palsy did not cause abnormal vertical deviation. Mean vertical deviations in central palsy were: 0.89 PD by prism-cover test, 1.41 PD by Maddox test, and 2.51 PD by coil recordings. Although mean vertical deviations in central palsy were significantly greater than normal (p < 0.05), individual patients did not have abnormal vertical deviations when compared with normal subjects (Z-test). During static head roll, patients with peripheral palsy had a right hyperdeviation on right head tilt, and a left hyperdeviation on left head tilt, regardless of the side of the palsy. In contrast, the vertical strabismus in central palsy remained on the same side during static head tilt to either side. Conclusions: Small vertical deviations in peripheral and central sixth nerve palsy are consistent with normal hyperphorias that become manifest in the presence of esotropia. In peripheral sixth nerve palsy, static head roll to either side induces hyperdeviation in the eye on the side of the head tilt. Hyperdeviation of the same eye induced by head tilt to either direction implicates a brainstem lesion as the cause of paretic abduction. Quantitative study of sixth nerve palsy demonstrates that if a vertical deviation falls within the range of normal hyperphoria, investigation for vertical muscle palsies or skew deviation is not required.

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33. LOCALIZATION OF THE HUMAN MGLUR4 GENE WITHIN AN EPILEPSY SUSCEPTIBILITY LOCUS Guin-Ting Wong*David R. Hampson 1,3

1, Stephen W. Scherer 2,4, O. Carter Snead III1,4, and

Departments of Pharmacology 1, Medical Genetics 2, and the Faculty of Pharmacy 3, University of Toronto, and the 4 Hospital for Sick Children, Toronto, Ontario, Canada The family of metabotropic glutamate receptors (mGluRs) consists of eight homologous G-protein coupled receptors. Several of the mGluRs, including the mGluR4 receptor subtype, are localized presynaptically; activation of this receptor induces an inhibition of neurotransmitter release from nerve terminals. Disruption of the mGluR4 gene in mice results in impaired motor and spatial learning, and alterations in seizure susceptibility. In this study, we have determined the structure of the human mGluR4 gene, as well as its chromosomal localization. A comparison of the gene structure of mGluR4 with the highly homologous mGluR6 receptor subtype reveals that both of the genes contain 10 exons with similar exon/intron boundaries. A refined localization of mGluR4 was carried out by constructing a bacterial artificial chromosome clone contig of the region surrounding the gene. Thirteen sequence tagged sites (STSs) were identified within this contig. The gene was localized to chromosome 6 band p21.3 by fluorescence in situ hybridization (FISH). The mapping of the mGluR4 gene indicates that it is approximately 1 megabases centromeric of the major histocompatibility complex and 5 megabases from the GABABR1 gene. The mGluR4 gene also falls within a susceptibility locus for juvenile myoclonic epilepsy suggesting a potential link to this form of epilepsy.

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34. INVOLVEMENT OF FACE PRIMARY MOTOR CORTEX (MI) IN PRIMATE SEMI-AUTOMATIC MOVEMENTS. D.Y. Yao

, N. Narita, K. Yamamura, Y. Masuda and B.J. Sessle. Faculty of Dentistry, Univ. of Toronto, Toronto, Ont., M5G 1G6, Canada

The role of MI in trained motor behaviour is well documented, but we have shown that intracortical microstimulation (ICMS) in face MI can evoke semi-automatic movements such as chewing and swallowing, and that swallowing-related or chewing-related activity may be recorded in MI (e.g. Huang et al, J. Neurophysiol. 1989; Martin et al, J. Neurophysiol. 1997, 1999). The aim of this study was to test further the possible involvement of MI in chewing. Bilateral reversible cold block of the ICMS-defined face MI or recording of the activity of single face MI neurones was carried out in awake monkeys during chewing. Face MI cold block significantly lengthened (p<0.05) food preparatory and preswallow periods, and masticatory cycle duration, from 1.36 ± 0.54 s, 1.68 ± 0.33 s, and 0.31 ± 0.05 (mean ± SD; precool) to 3.56 ± 1.45 s, 2.34 ± 0.66 s, and 0.43 ± 0.09 s (cold block), respectively. Of 73 face MI neurones studied during chewing, 7 had tonic activity and 4 decreased activity but most (n = 62) showed a burst of activity during the jaw-opening or jaw-closing phase of each chewing cycle and/or during the food preparatory period. Most (69%) chewing-related neurones also were active in a trained tongue protrusion task. An orofacial mechanoreceptive field was detected in 81% of the neurones. These data provide further evidence for a role for MI in semi-automatic movements as well as trained motor behaviour. Supported by Canadian MRC grant MT-4918.

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35. TONIC AND SYNAPTIC INHIBITORY GABAERGIC CURRENT ARE DIFFERENTIALLY MODULATED BY NON-COMPETITIVE GABAA ANTAGONISTS J.Y.T. Yeung1*

Institute of Medical Science1, Depts. of Physiology2, Pharmacology3, Anesthesia4, Univ. of Toronto, Ontario, Canada; Department of Anesthesia5, Sunnybrook & Women's College HSC, Toronto, Ontario, Canada. *PIN student

, J.F. Macdonald2,3, B.A. Orser1,2,4,5

γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system (CNS). Traditionally, GABA is thought to activate GABAA receptors (GABAARs) clustered at postsynaptic termini, leading to increased chloride conductance and the generation of inhibitory postsynaptic currents (IPSCs). However, recent studies revealed the existence of a tonic or continuous inhibitory current in several brain regions including the hippocampus and cerebral cortex. Our laboratory provided the first evidence that GABAARs mediating the tonic current in hippocampal neurons are pharmacologically and biophysically distinct from synaptic receptors. Specifically, the GABAA receptor antagonist gabazine (SR 95531) blocked the synaptic current but not the tonic current. In contrast, conventional GABAAR antagonists, picrotoxin and bicuculline, blocked both the tonic and synaptic current. In the present study, we now hypothesize that GABAARs mediating the tonic current are of extrasynaptic origin and they are differentially sensitive to use-dependent blockers. To test this hypothesis, we used whole cell patch clamp technique to characterize the effect of two use-dependent GABAAR blockers, penicillin and t-butyl-bicyclo-phosphorothionate (TBPS), on the tonic and synaptic current. We discovered that penicillin selectively inhibits the synaptic current while leaving the tonic current unaffected. Conversely, TBPS inhibited both the tonic and synaptic GABAARs. In future experiments, we will take advantage of the interaction between GABAAR blockers with different affinities and mechanisms of action to map the tonic and synaptic receptors on the surface of the neurons using microscopic techniques. The results from our experiments will offer a novel paradigm regarding a population of GABAAR that is involved in areas of anesthesiology, epilepsy and inhibitory network activity.

(This work was supported by the First IARS Frontiers in Anesthesia Research Award, the Ontario Ministry of Health and the CIHR to B.A.O., the CIHR to J.F.M. and NSERC to J.Y.T.Y.)