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Release probability modulates short-term plasticity at a rat giant terminal

Sharon Oleskevich, John Clements * and Bruce Walmsley

1. Modulation of release probability is a major factor underlying short-term synaptic plasticity in the central nervous system. We have investigated the relationship between release probability ((P_r) and paired-pulse modulation at a large auditory calyceal synapse containing many transmitter release sites. Whole-cell patch electrode recordings were made of excitatory postsynaptic currents (EPSCs), evoked by stimulation of auditory nerve fibres giving rise to the endbulbs of Held.

2. Quantitative estimates of P_r and quantal amplitude were obtained using the recently developed variance-mean analysis technique. Release probability conditions were modulated by bath application of cadmium, elevated calcium and protein kinase C activation by phorbol esters.

3. Our results show that, under physiological conditions, most sites released neurotransmitter following a single presynaptic nerve impulse, with a mean P_r of 0·6. The mean quantal amplitude was 44 pA, which was consistent with the mean amplitude of miniature EPSCs (47 pA).

4. Under high release probability conditions with elevated calcium or phorbol esters, P_r at all sites approached 1·0. At these high P_r values, variance-mean analysis indicated a significant postsynaptic contribution to paired-pulse depression. The miniature EPSC amplitudes were decreased following stimulation in elevated calcium, confirming a postsynaptic component of paired-pulse depression at this glutamatergic connection.

5. A notable feature was the large variability between neurons in the relationship between paired-pulse ratio and P_r. Based on current models of vesicle release and ultrastructural evidence, we suggest that this variability may be partly due to morphological differences between endbulb specializations, particularly in the ratio of fusion-ready to reserve populations of vesicles at endbulb release sites.

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