CHANGE OF TRANSMITTER RELEASE KINETICS DURING FACILITATION REVEALED BY PROLONGED TEST PULSES AT THE INHIBITOR OF THE CRAYFISH OPENER MUSCLE

A presynaptic voltage-control method was used to study synaptic facili tation at the inhibitory neuromuscular synapse of the crayfish opener muscle. The expression of the F-2 component of facilitation, monitored 150 ms after a conditioning stimulus, was examined by systematically changing the duration of the presynaptic test pulse. (Test pulses in a ll experiments were depolarized to 0 mV.) Control and facilitated inhi bitory postsynaptic potentials (IPSPs) exhibited identical time course s when test pulse duration was brief (similar to 2 ms). When the durat ion of the test pulse was increased beyond 2 ms, the transmitter relea se time course shifted to an earlier point in time during facilitation . Meanwhile, the increase in total transmitter release, measured from inhibitory postsynaptic conductance (IPSG) area (total release), becam e less pronounced with increasing duration of the test pulse. With a 2 0-ms test pulse, facilitation did not cause any detectable change in t otal release but the half-maximal point of the facilitated IPSG shifte d by 3 ms (release shift). This change in release kinetics was not ass ociated with a decrease in minimal synaptic delay. Furthermore, the re lationship between total release and presynaptic pulse duration sugges ted that the transmitter release activated by a 20-ms pulse can be def ined as a distinct component of continuous transmitter release (early component). The facilitation process accelerated the release kinetics of the early component but did not modify its total transmitter conten t. To test the hypothesis that the release shift is indeed mediated by the same mechanism that increases IPSP amplitude during facilitation, we investigated the correlation between the release shift and IPSP am plitude change. The two parameters were significantly correlated when the magnitude of facilitation was changed I) during the decay of facil itation and 2) by varying the strength of the conditioning stimulus. T he experimental approach reported here provides two new physiological parameters, release shift and total release, for the analysis of synap tic facilitation.