Frequency facilitation and post-tetanic potentiation of a unitary synaptic potential in Aplysia californica are limited by different processes.

Post-tetanic potentiation (PTP) of the monosynaptic and unitary excitatory postsynaptic potential (EPSP) recorded in cell R15 of the abdominal ganglion of Aplysia californica was observed after repetitive stimulation of the right visceropleural connective. PTP at this synapse developed after a few pulses (about 20) and after trains of low frequency stimulation (1/2 sec) under normal physiological conditions of media and temperature. No phase of post-tetanic depression was observed. Evidence is presented that the PTP is due to an increase in transmitter release. The amplitude of the PTP was a function of the frequency and number of stimuli in the preceding train. The PTP was observed to decay, with a single exponential time course, to the size of an isolated EPSP. The rate constant of PTP decay depended upon both the frequency and number of stimuli in the preceding train. The magnitude of the various types of synaptic plasticities seen at this junction, i.e., synaptic depression, frequency facilitation and PTP, correlated with the size of an isolated EPSP as well as with each other. Based on the analysis of the data in terms of a flow model of transmitter release, it is concluded that: (a) during a train of repetitive stimulation the net rate of transmitter supply into the immediately available pool (net transmitter mobilization) increases, the efficiency of the release mechanism (fractional release) increases, and the pool of immediately available transmitter depletes; (b) upon the cessation of the train, as the peak amplitude of PTP is approached, the increased but diminishing rate of net transmitter mobilization refills the available pool to its equilibrium size, while the fractional release is still elevated; (c) during the PTP period after the peak potentiation, the elevated fractional release slowly decays with a single exponential time course; (d) the size of the facilitated EPSPs during the train is limited by the net rate of transmitter supply, although the efficiency of release is also increased; while the size of the EPSPs during the falling phase of the PTP period is determined solely by an increased efficiency of the release mechanism; and (e) the rising phase of the PTP observed in the period shortly after termination of the train is produced by the refilling of the depleted pool of available transmitter in the presence of an elevated release efficiency.