The Distribution and Possible Roles of Small Cardioactive Peptide in the Nudibranch Melibe leonina

The neuropeptide small cardioactive peptide (SCP) plays an integrative role in exciting various motor programs involved in feeding and locomotion in a number of gastropod species. In this study, immunohistochemistry, using monoclonal antibodies against SCP(B), was used to localize SCP(B)-like-immunoreactive neurons in the central nervous system, and map their connections to various tissues, in the nudibranch, Melibe leonina. Approximately 28–36 SCP(B)-like-immunoreactive neurons were identified in the M. leonina brain, as well as one large neuron in each of the buccal ganglia. The neuropil of the pedal ganglia contained the most SCP(B)-like-immunoreactive varicosities, although only a small portion of these were due to SCP(B)-like-immunoreactive neurons in the same ganglion. This suggests that much of the SCP(B)-like immunoreactivity in the neuropil of the pedal ganglia was from neurons in other ganglia that projected through the pedal–pedal connectives or the connectives from the cerebral and pleural ganglia. We also observed extensive SCP(B) innervation along the length of the esophagus. Therefore, we investigated the impact of SCP(B) on locomotion in intact animals, as well as peristaltic contractions of the isolated esophagus. Injection of intact animals with SCP(B) at night led to a significant increase in crawling and swimming, compared to control animals injected with saline. Furthermore, perfusion of isolated brains with SCP(B) initiated expression of the swim motor program. Application of SCP(B) to the isolated quiescent esophagus initiated rhythmic peristaltic contractions, and this occurred in preparations both with and without the buccal ganglia being attached. All these data, taken together, suggest that SCP(B) could be released at night to arouse animals and enhance the expression of both feeding and swimming motor programs in M. leonina.