Peptidergic and serotonergic facilitation of a neuromuscular synapse in Aplysia

Abstract

(1) A buccal muscle motor neuron which synthesizes the neuromodulatory small cardioactive peptides (SCPs) was identified in the buccal ganglion of Aplysia by using a combination of electrophysiological and single cell biochemical experiments. This neuron was designated B38. (2) Exogenous SCP B enhanced B38-induced contractions when perfused over the target muscle, the rostral portion of the buccal I3 muscle. SCP B potentiation of muscle contraction was associated with an increase in the excitatory junction potential (EJP) amplitude recorded from the muscle fibers, increased muscle cyclic AMP (cAMP) content, hyperpolarization of the muscle fibers, and an increase in the muscle fiber membrane conductance. Exogenous SCP B also depolarized the cell body of B38 and increased electrical coupling between the symmetrically paired B38 neurons. (3) These results suggest that the SCPs may be co-released from B38 along with an unidentified conventional neurotransmitter to homosynaptically facilitate B38 synaptic transmission by modulating presynaptic and postsynaptic components. (4) Stimulation of the identified serotonergic metacerebral neuron or perfusion of exogenous serotonin (5-HT) over the I3 muscle also potentiated B38-induced muscle contractions and EJP amplitude. Thus the B38 neuromuscular synapse represents a peripheral site of serotonergic heterosynaptic facilitation in Aplysia. (5) Presynaptic and postsynaptic serotonergic effects were qualitatively similar to those of SCP B. Serotonergic effects on muscle fiber hyperpolarization and increase in muscle fiber membrane conductance were similar in magnitude to those of SCP B but 5-HT induced a much larger increase in the EJP amplitude which was additive with that of SCP B. (6) The effect of 5-HT on the EJP amplitude was associated with inhibition of a slowly decaying component of synaptic facilitation. Concentrations of SCP B that increased the EJP were much less effective at inhibiting the slow component of facilitation. These observations indicate that 5-HT also exerted a presynaptic effect on B38 transmitter release. (7) Both 5-HT and SCP B increased muscle cAMP levels and application of forskolin mimicked many of their effects, suggesting that at least some of the postsynaptic effects were mediated by increased cAMP levels in the I3 muscle.