Modulation of synapse RC1-R15 of Aplysia californica by fiber(s) of the right connective.

A synapse or a synaptic cluster, here called synapse RC1-R15, can be activated by threshold stimulation of the right visceropleural connective (RC) of Aplysia californica. The activity of this synapse is recorded in cell R15 of the abdominal ganglion. Repeated threshold stimulation of this synapse at 1-2 Hz generally produces a reduction of the RC1-R15 EPSPs (synaptic depression), followed by an increase to a sustained plateau (frequency facilitation). When the rate of stimulation is reduced, a rapid increase of EPSP size (posttetanic potentiation) is seen followed by an exponential decay. In this paper, we show that suprathreshold stimulation of RC accelerates this decay. It does not modify the synaptic depression and the frequency facilitation phenomena, nor does it affect the membrane resistance and potential of cell R15. In some preparation, subthreshold stimulation of RC also reduces the PTP generated by a preceding rapid stimulation of synapse RC1-R15. High frequency (5-Hz) subthreshold stimulation of RC also reduces the synaptic depression of synapse RC1-R15. However, the subthreshold stimulation has no effect on frequency facilitation of RC1-R15. The effect of subthreshold stimulation on PTP is often correlated with the recording of a very small EPSP (sEPSP) in cell R15. This sEPSP is barely detected at the beginning of the subthreshold train of stimuli but increases with repeated stimulation. The effects obtained with subthreshold and suprathreshold stimulation of the RC are abolished in sea water with a high concentration of Ca2+ and of Mg2+. Both types of effects are attributed to the activation of fibers in the RC (called modulating fibers) which by the intermediary of one or several postulated interneurons modulate presynaptically the transmission of synapse RC1-R15.