Heterosynaptic stimulation modulates the duration of post-tetanic potentiation at an Aplysia synapse without affecting other aspects of synaptic transmission.

Repetitive stimulation of an axon in the right visceropleural connective to the abdominal ganglion of Aplysia californica produces post-tetanic potentiation (PTP) of a unitary monosynaptic EPSP recorded from cell R15. PTP decays within one half hour following cessation of repatitive stimulation. Stimulation of the left visceropleural connective speeds the rate of decay of PTP, but does not affect the amount of potentiation which is developed or the size of the non-potentiated EPSP. The effect of heterosynaptic left connective stimulation is similar to the previously reported effects of low doses of serotonin but different from the effects of heterosynaptic branchial nerve stimulation. As previously reported, branchial nerve stimulation causes a reduction in the size of the EPSP, and can also speed the rate of decay of PTP. Effects of branchial nerve and left connective stimulation are blocked by SQ10,631, a serotonin antagonist. The left connective effect on the rate of decay of PTP, like the serotonin effect, appears to be mediated by a cyclic nucleotide. The data are consistent with the existence of two separable serotonergic receptors presynaptic to the homosynaptic pathway; one receptor mediates a change in the rate of decay of PTP, one mediates a change in the size of the EPSP. The possible physiological significance of heterosynaptic control of PTP is considered.