The amplitude of post-tetanic potentiation of the EPSP RC1-R15 in Aplysia is modulated by environmental parameters.

Evidence is presented that the EPSP called RC1-R15, which is recorded from cell R15 of the abdominal ganglion of Aplysia californica upon appropriate stimulation of the right connective, is endogenously active. In previous studies we showed that after repetitive stimulation the amplitude of this EPSP increases and then slowly decays over many minutes, a phenomenon called post-tetanic potentiation (PTP). The rate of endogenous firing of this EPSP varies with time of day and tonicity of the animal's external environment. It is shown that this endogenous firing increases the magnitude of the PTP component of the amplitude of the EPSP. The degree of this potentiation varies with the endogenous firing rate. In general, daytime or a hypertonic external environment increases the firing rate of the EPSP and its degree of potentiation. Since the degree of this PTP reflects time of day and environmental tonicity, it is inferred that it conveys physiologically significant information to the postsynaptic cell R15.