Peripheral Actions of the SCPs inAplysiaand Other Gastropod Molluscs

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Abstract
The SCPs are a family of neuropeptides found in many gastropod species. Two SCPs with similar sequences have been characterized in Aplysia. These peptides are potent modulators of central and peripheral synapses. They also enhance ongoing contractile activity in spontaneously active tissues such as heart and gut. Their distribution in central ganglia suggests that their predominant role is in the regulation of feeding behavior. There is good evidence that the identified SCP-containing neurons, B1 and B2, provide the major central regulation of gut motility during feeding through the release of the SCPs from their terminals in gut. The SCPs have also been localized to motor neurons that innervate buccal muscles which generate biting and swallowing movements. In many of these neurons, the SCPs have been shown to coexist with conventional transmitters such as ACh, or other peptides such as FMRFamide. The SCPs appear to be released along with conventional transmitters from these neurons to modulate the effectiveness of the conventional transmitter. In all cases, the SCPs cause an enhancement of the amplitude of contractions produced by motor neuron stimulation. The precise mechanisms underlying this effect vary from muscle to muscle. All of the effects of the SCPs are mediated by increased cAMP levels in target tissue. At many sites of action, serotonin produces actions that are qualitatively similar to those of the SCPs. This is likely to involve a convergence at the level of the adenylyl cyclase. In addition to these peripheral effects, the SCPs also have multiple central effects on feeding and other behaviors in gastropods.

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