ANALYSIS OF EVOKED AND SPONTANEOUS QUANTAL RELEASE AT HIGH-PRESSURE IN CRUSTACEAN EXCITATORY SYNAPSES

The cellular mechanisms underlying the effect of high pressure on syna ptic transmission were studied in the opener muscle of the lobster wal king leg. Excitatory postsynaptic currents (EPSCs) were recorded using a loose macropatch-clamp technique at normal pressure and 3.5, 6.9 MP a helium pressure. Responses of the single excitatory axon could be gr ouped into two types: low-yield (L) synapses exhibiting small EPSCs wi th a considerable number of failures, and high-yield (H) synapses havi ng larger EPSCs with very few failures. High pressure reduced the aver age EPSC amplitude in all synapses and shifted their amplitude histogr ams to the left by decreasing the quantal content (m) without changing their quantum current (q). A binomial distribution fit of EPSC amplit udes revealed that high pressure greatly decreased n, the number of av ailable active zones, but the effect on p, the probability of release for each zone, was not consistent. Many of the spontaneous miniature E PSCs (mEPSCs), observed only in L-type synapses, were ''giant'' (size = 2-5 q). High pressure increased the frequency of the giant mEPSCs bu t had little effect on their amplitude histogram. High pressure depres sed evoked synaptic transmission by modulating the presynaptic quantal release parameters, but concomitantly enhanced spontaneous quantal re lease by an unknown mechanism.