PRESSURE EXPOSURE UNMASKS DIFFERENCES IN RELEASE PROPERTIES BETWEEN HIGH AND LOW-YIELD EXCITATORY SYNAPSES OF A SINGLE CRUSTACEAN AXON

The cellular mechanisms underlying the effect of high pressure on syna ptic transmission at two types of synapses were studied in the opener muscle of the lobster walking leg. Excitatory postsynaptic currents (E PSCs) were recorded using a loose macropatch clamp technique at normal pressure and 3.5, 6.9 MPa helium pressure. Responses of the single ex citatory axon could be grouped into two types: low yield (L) synapse e xhibiting a small mean EPSC with a considerable number of failures, an d high yield (H) synapse having a larger mean EPSC with very few failu res. The change in several synaptic transmission parameters indicated that high pressure similarly reduced presynaptic evoked release in bot h L and H synapses. However, some differences in the kinetics and prob ability of release could be detected. A major difference was the spont aneous miniature EPSCs (mEPSCs) activity. Many of the mEPSCs, observed only in L synapses, were 'giant' (size of 2-5 q). High pressure selec tively increased the frequency of the giant mEPSCs in the L synapse bu t had little effect on their amplitude histogram. High pressure depres sed evoked synaptic transmission in both synapses by modulating the pr esynaptic quantal release parameters, but concomitantly enhanced spont aneous quantal release in L synapses by an unknown mechanism.