Depression and frequency facilitation at a synapse in Aplysia californica: Evidence for regulation by availability of transmitter

Abstract

Summary Synaptic depression and frequency facilitation were observed in the monosynaptic and uritary excitatory postsynaptic potential (EPSP) obtained in the ‘parabolic burster cell’ (R15) by stimulating the right visceropleural connective in Aplysia californica . During trains of stimulation at 0.5–2 c/sec the size of the EPSP was initially depressed during the first few stimuli and subsequently slowly increased to reach a potentiated steady state after a few hundred stimuli. The relative amount of depression between the first two EPSPs depended on (a) the inter-stimulus interval, and (b) the size of the first EPSP. The larger the first EPSP the larger the percentage depression whether variations in the size of the first EPSP were due to natural interanimal variability or experimental manipulation of the concentrations of Ca 2+ , Mg 2+ and Co 2+ . The amount of frequency facilitation (expressed as the ratio of the 100th EPSP to the first EPSP of a train) increased with increasing frequency of stimulation and was inversely related to the log of the size of the first EPSP both for natural variations in EPSP size and manipulations of the EPSP size by changing the divalent cation concentrations. Neither the level of hyperpolarization of R 15 nor partial curarization affected the relative depression and potentiation. We suggest that the initial depression is largely due to depletion of a rather small pool of transmitter available for release, while the frequency facilitation is the result of a Ca 2+ and frequency dependent increase in the net rate of supply of transmitter into the available pool.