Lesions of the Basal Forebrain Cholinergic System Impair Task Acquisition and Abolish

Cortical Plasticity Associated with Motor Skill Learning

James M. Conner, Andrew Culberson,Christine Packowski, Andrea A. Chiba,

and Mark H. Tuszynski

Plasticity and learningPlasticity and learning

• Molecular modifications

• Synaptic modifications

• Cellular modifications

• System/Circuit modifications

Learning

Reorganization of Cortical Representations

• Plasticity of adult motor representations following both peripheral (Cohen et al., 1991; Donoghue et al.,

1990; Sanes, et al., 1988; Schieber and Deuet, 1997; Wu and Kaas, 1999) and central (Chen et al., 2002; Hallett, 2001; Nudo et al., 1996) lesions and following motor skill learning (Kleim et al., 1998; Pascual-Leone et al., 1995).

• Reorganization of motor maps may therefore be a substrate enabling normal motor learning and functional recovery following a lesion.

• But, is it required?

The basal forebrain

• The basal forebrain cholinergic system may play a role in mediating lesion-induced plasticity of sensory representations.– Excitotoxic lesions of the nucleus basalis

magnocellularis (NBM) in rats (Juliano et al., 1991; Webster et al., 1991).

– Immunotoxin specific for cholinergic neurons in basal forebrain (Baskerville et al., 1997; Sachdev et al., 1998; Zhu and

Waite, 1998) and selective blockade of cholinergic signaling within barrel cortex (Maalouf et al., 1998).

– Selective removal of cholinergic projections to olfactory bulb (Linster et al., 2001).

More from the basal forebrain

• The basal forebrain and learning– Nonspecific NBM lesions and learning

deficits (Olton and Wenk, 1987, review).

– Highly selective lesions of cholinergic neurons in basal forebrain and no (or modest) deficits (Baxter and Chiba; 1999; Wrenn and Wiley, 1998, reviews).

?

Now, for this study

– Postulated that a key physiological role of the basal forebrain cholinergic (BFC) system is to modulate plasticity associated with cortical representations

– Learning paradigms relying on cortical reorganizations will be especially susceptible to alterations to BFC function.

• What are the behavioral consequences of BFC lesions on learning a skilled motor task?

• Group 1 – Behavioral Consequences of NBM lesions

– Bilateral NBM lesions (11) or sham (17) prior to learning skilled reaching task.

• Group 2 – NBM lesions and memory retention

– Trained on skilled reaching task for 3 weeks prior to receiving bilateral NBM lesions (12), bilateral injections of vehicle (3) or sham (9).

• Group 3 – NBM + medial septum lesions

– Bilateral lesions of both NBM and medial septum (6), bilateral injections of vehicle in NBM and medial septum (3) and sham (3).

• Lesions induced by intraparenchymal injections of 192-IgG-saporin (SAP) in artificial cerebrospinal fluid.– 2 sites for NBM and 2 sites for medial septum

• Vehicle was artificial cerebrospinal fluid.

http://www.bol.ucla.edu/~nwoolf/

http://www.bol.ucla.edu/~nwoolf/

Miranda and Bernudez-Rattoni, 1999.http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=26907

Skilled Reach Training and Skilled Reach Training and HistologyHistology

• Single-pellet retrieval task– 15 days (first 3-4 days as shaping)– 60 trials or 10 minutes– 2 cm distance by d5– Animals reached with preferred limb

• Histology– Sections processed for AChE, receptor p75

and parvalbumin

Average reaching accuracy: Intact=47.4%±3.7%; NBM lesion=21.2%±4.1%

Group 3

• Is residual learning in NBM animals a result of unlesioned BFC neurons from the medial septum?– Combined lesions of NBM and medial septum

resulted in impaired acquisition of the single-pellet retrieval task relative to vehicle treated animals and intact animals (p<.05), but were not found to result in significant differences in reaching performance compared to NBM lesions alone (p>.05).

Motor Learning Impairments• Were motor learning impairments in NBM and NBM+medial

septum lesioned animals a result of attention deficits?

Are NMB lesions specific to motor learning?

No differences were observed between groups in activitylevel during training or testing. Both groups had significantreductions in activity level during testing relative to baseline.

NBM lesions and overall sensorimotor deficits

No difference between groupson footfalls per trial with either the hindpaw or forepaw.

Group 2

P>.05 for prelesion performancebetween groups,postlesion performancebetween groups and from pre- to post-lesionin SAP lesioned animals.

50μm

AChE fibers in primary motor cortex

Control SAP

250μm

Control SAP

250μm

SAP lesions did not damage GABAergic neurons within the basal forebrain

Godde et al., 2002

Conclusions

• BFC system is necessary for mediating cortical plasticity associated with skilled motor learning

• Cortical map reorganization is a key substrate for enabling an animal to effectively learn a skilled motor behavior– What do the cortical maps look like in the

animals with NBM + medial septum lesions after learning?

More Conclusions

• Rats with NBM lesions had impaired, but not eliminated, learning of the skilled motor task– Forelimb area was not totally abolished, but

plasticity of the motor maps was blocked– Some cortical control of movement

• BFC system increases efficiency of learning