Cellular NeurobiologyBIPN140

1st Midterm Exam Ready for PickupBy the elevator on the 3rd Floor of Pacific Hall (waiver)

Exam Depot Window at the north entrance to Pacific Hall (no waiver)Mon-Fri, 10:00 AM to 4:00 PM

PS 4 will be posted on October 24th

All Sections and Office Hours will resume.

Chih-Ying’s Office Hour: Monday, 1:00-2:00 PM, Bonner Hall 4146

BIPN140 Lecture 8: Synaptic Transmission II

1. Postsynaptic Receptors: Metabotropic & Ionotropic

2. Postsynaptic Responses (Postsynaptic Potentials, PSPs)

3. Neurotransmitters

Su (FA16)

Chemical Synapse: an Overview (Fig. 5.3)

Two Types of Neurotransmitter Receptors (Fig. 5.16)

Postsynaptic Responses: ACh Receptors at the NMJ (Fig. 5.17)

microscopic current

macroscopic inward current

The Influence of PSP on End Plate Currents (Fig. 5.18)

Reversal Potential (Figs. 5.18 & 5.19)

Na+ and K+ Movements during EPCs and EPPs (Fig. 5.20)

Reversal Potentials and Threshold Potentials Determine Postsynaptic Excitation and Inhibition (Fig. 5.21)

ECl = -70 mV

EPSP; depolarizing IPSP; hyperpolarizing IPSP; depolarizing

ECl = -50 mV

Summation of Postsynaptic Potentials (Fig. 5.22)

Location of Synapses Matters

If the neuron responds to two identical GABAergic inputs at location (1) and (2) with two identical IPSCs, which of the following is true?

A. IPSC at location 1 is more effective in inhibiting action potentials than location 2.

B. IPSC at location 1 is less effective in inhibiting action potentials than location 2.

C. IPSC at location 1 is as effective in inhibiting action potentials as location 2.

D. None of the above. It depends on the threshold of the neuron.

Size of Neurons Matters

If neuron (a) and neuron (b) respond to glutamatergic inputs with EPSCs of identical amplitude, which of the following is true? (Remember Vm= i x Rinput)

A. The EPSC at neuron (a) is more effective in generating action potentials than at neuron (b).

B. The EPSC at neuron (a) is less effective in generating action potentials than at neuron (b).

C. The EPSC at neuron (a) is equally effective in generating action potentials as at neuron (b).

a b

From NT Release to Postsynaptic Excitation or Inhibition (Fig. 5.23)

NT release at all presynaptic terminals on a cell results in receptor binding, which causes the opening or closing of specific ion channels.

The resulting conductance change (g; I = g x V) causes current (I) to flow, which may change the membrane potential ( V = I x R).

The postsynaptic cell integrates all of the EPSPs and IPSPs, resulting in moment-to-moment control of action potential generation (neuronal output, signal that can be propagated to another neuron).

Major Neurotransmitters (Fig 6.1)

Rate-limiting Rate-limiting

Biogenic Amines (Figs. 6.10 & 6.14)

Unconventional NTs: Endocannabinoids (Fig. 6.18, Box 6G)

endocannabinoid

endocannabinoid

agonist

antagonist

Unconventional NTs: Endocannabinoids (Fig. 6.19)

CB1 antagonist: blocks the reduction of IPSCs

Depolarization-induced Suppression of Inhibition(DSI)

Background: Endogenous cannabinoids, e.g. anandamide (as well as THC from marijuana), diffuse across cell membranes and activate receptors in hippocampal interneurons that depress GABA release. Depolarization of hippocampal pyramidal neurons suppresses GABAergic IPSPs the neurons receive, a phenomenon called “Depolarization-induced Suppression of Inhibition” or DSI. DSI was thought to represent a retrograde effect of the postsynaptic neuron onto presynaptic inhibitory terminals, but the mechanism was unknown.

Experiments: Induce DSI by depolarizing a hippocampal pyramidal neuron with a patch pipette, and record the changes that occur in IPSPs elicited by stimulating interneuron input to the neuron. (1) Test the effects of compounds that block cannabinoid receptors to see if they block DSI. (2) Similarly, test compounds that activate cannabinoid receptors to see if they mimic DSI. (3) Determine if the cannabinoid effect is likely to be presynaptic, e.g. affects mEPSC frequency (how frequent are SVs released by the presynaptic neuron) but not mEPSC amplitude (unitary activation of postsynaptic receptors).

Fig. 1. DSI requires endogenous cannabinoids

eIPSC: evoked inhibitory

postsynaptic current

CB1 antagonist

CB1 agonist: mimics DSI(occlusion)

CB1 antagonist: prevents DSI

Control experiment: DSI after 30 min of recording

Fig. 3. DSI and a CB1 agonist suppress IPSCs by the same mechanism

Paired-Pulse Ratio: the ratio of two PSPs or PSCs evoked in close succession. Changes in PPR are usually interpreted as presynaptic changes, reflecting the differences in the availability of readily releasable SVs (the first pulse, or stimulus, may deplete a lot of SVs)

Markedly reduced the first PSC (i.e. not so many SVs were released due to CB1 antagonist or DSI)

Cd2+: blocks VGCC, thereby blocking SV release (positive control)

mIPSC frequency: miniature IPSC frequency,(reflecting how often a SV is released)

TTX: block voltage gated Na+ channel to block action potentials.

KCl: to depolarize presynaptic terminal to favor SV release

Recovery: due to removal of endogenous ligand (e.g. 2-AG) by a transporter

CB1 agonist

Results: Blockers of cannabinoid receptors blocked DSI expression; agonist of the receptors mimicked DSI. Both the agonists and DSI acted on presynaptic machinery in GABAergic (inhibitory) terminals innervating the pyramidal neuron. Finally, DSI was shown to “spread” locally, affecting IPSP amplitude in nearby neurons, exactly as predicted for a diffusible membrane-permeant regulator like cannabinoids.

Take-home message: These DSI relies on the retrograde (back across the synapse) transfer of cannabinoids which inhibit GABA release from presynaptic terminals, thereby reducing the evoked IPSC seen in the postsynaptic pyramidal neuron. Globally applied cannabinoids, e.g. THC, can be expected to globally diminish inhibitory activity and alter information processing and system output.