Neurotransmitter release and its facilitation in crayfish

Quantal postsynaptic currents were recorded by a macro-patch-clamp electrode from synaptic spots on the opener muscle of the walking leg of large crayfish at 0° to 4°C. Through the same electrode, current pulses were applied which de- or hyperpolarized the nerve terminal. The depolarizing test pulse of fixed amplitude and duration elicited EPSCs with an average quantum contentm₁. If the test pulse was preceded or followed immediately by a modulatory hyperpolarizing pulse, the quantum contentm_1p of the EPSC was reduced by factorsm₁/m_1p up to 10. This modulation of release increased with amplitude and duration of hyperpolarization, reaching a saturation level for durations >3 ms. If an interval was interposed between modulatory hyperpolarization and test pulse, the reduction of release decayed with increasing interval with a time constant of about 2 ms. The releasem₂ due to a second test pulse following the first one with 10 ms interval was facilitated,m₂/m₁>1. If release by the first test pulse was modulated by associated hyperpolarizing pre- or postpulses, facilitation of the second test EPSC was not reduced in comparison to facilitation without modulation. The time course of phasic release (distribution of delays of quanta after test pulse) was not appreciably affected by hyperpolarizing pre-pulses, but such post-pulses shortened the delay of the maximum of release.The effects of modulatory pre- and post-pulses could be described by an extension of the model given in H. Parnas et al. (1986). Release sites,S, are assumed to have two inactive states,T₁ andT. In the reactionT₁TS the rate constants are potential dependent, depolarization promoting the stateS and hyperpolarization the stateT₁. At rest,S is very low andT andT₁ both have high values. These reactions are essentially independent of [Ca]_i. The kinetics of the system were stimulated for an assumed set of parameters. The simulated responses showed all essential features of the experimentally observed modulation.This study was supported by a grant of the Deutsche Forschungsgemeinschaft