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We have examined synaptic transmission between isolated pairs of chick GABAergic amacrine cells, maintained in sparse culture and identified by their binding of an amacrine cell-selective antibody. Using the perforated- patch method to whole-cell clamp both cells of a pair, postsynaptic currents were examined for step depolarizations of the 'presynaptic' cell. Synaptic transmission, frequently reciprocal, was calcium dependent and reversibly blocked by bicuculline. Postsynaptic currents, excluding those due to ohmic electrical coupling, were elicited only for presynaptic voltage steps positive to about -40 mV and were always very noisy, suggesting that they were summed from relatively small numbers of quanta. Postsynaptic currents continued well after the termination of the 100 msec presynaptic voltage step when the step was to -10 mV, or positive to this value. This result is interpreted to imply that presynaptic calcium concentration remains elevated after the membrane is returned to its holding potential. When presynaptic voltages were kept low or else presynaptic voltage was uncontrolled, spontaneous quantal events mediated by GABA(A) receptors could often be seen. Quanta rose quickly (less than 4 msec) and decayed with a mean time constant of 19.3 msec. The amplitude distributions of quantal currents were positively skewed, sometimes showing rare quanta of exceptionally large amplitude. Peak conductance per quantum was about 300 pS, corresponding to the simultaneous opening of only 17 GABA(A) channels and corresponding to a net flux of only 32 x 103 Cl- ions per millivolt of driving force. Estimates of the maximum sustained release rate at individual release sites suggest an upper bound of between 19 and 42 quanta per second.

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Journal of Neuroscience

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