Properties of glycine receptors underlying synaptic currents in presynaptic axon terminals of rod bipolar cells in the rat retina.

The excitability of presynaptic terminals can be controlled by synaptic input that directly targets the terminals. Retinal rod bipolar axon terminals receive presynaptic input from different types of amacrine cells, some of which are glycinergic. Here, we have performed patch-clamp recordings from rod bipolar axon terminals in rat retinal slices. We used whole-cell recordings to study glycinergic inhibitory postsynaptic currents (IPSCs) under conditions of adequate local voltage clamp and outside-out patch recordings to study biophysical and pharmacological properties of the glycine receptors with ultrafast application. Glycinergic IPSCs, recorded in both intact cells and isolated terminals, were strychnine sensitive and displayed fast kinetics with a double-exponential decay. Ultrafast application of brief (approximately 1 ms) pulses of glycine (3 mM) to patches evoked responses with fast, double-exponential deactivation kinetics, no evidence of desensitization in double-pulse experiments, relatively low apparent affinity (EC(50) approximately 100 microM), and high maximum open probability (0.9). Longer pulses evoked slow, double-exponential desensitization and double-pulse experiments indicated slow, double-exponential recovery from desensitization. Non-stationary noise analysis of IPSCs and patch responses yielded single-channel conductances of approximately 41 pS and approximately 64 pS, respectively. Directly observed single-channel gating occurred at approximately 40-50 pS and approximately 80-90 pS in both types of responses, suggesting a mixture of heteromeric and homomeric receptors. Synaptic release of glycine leads to transient receptor activation, with about eight receptors available to bind transmitter after release of a single vesicle. With a low intracellular chloride concentration, this leads to either hyperpolarizing or shunting inhibition that will counteract passive and regenerative depolarization and depolarization-evoked transmitter release.