The neuropeptide designated SCPB (small cardioactive peptide B), the sequence of which has recently been determined, was found in the accessory radula closer muscle, a muscle involved in biting movements. The ganglia and nerves that innervate the accessory radula closer muscle also contain SCPB. At nanomolar concentrations, it enhances the contractions of the muscle. The effect of SCPB on the muscle resembles the effect of an identified serotonergic neuron that previously was shown to mediate behavioral effects that reflect a food arousal state in Aplysia. Like serotonin, SCPB enhances contractions by a postsynaptic action, which appears to involve an increase in cAMP levels in the muscle. Our findings suggest that parallel peptidergic and serotonergic pathways may mediate similar aspects of arousal in Aplysia.Gastropod molluscs contain a number of neuropeptides that have been identified on the basis of their powerful cardioactive actions (1). One of these peptides, termed small cardioactive peptide B (SCPB) was recently purified from the neural tissue of Aplysia (2) and its sequence has been determined (Met-Asn-Tyr-Leu-Ala-Phe-Pro-Arg-Met-NH2). The present results describe our first functional study utilizing synthetic SCPB.We have found that SCPB is present in particularly high levels in the buccal ganglion of Aplysia, the ganglion that controls the muscles that effect biting and swallowing (3,4). Therefore, we have examined whether SCPB might be involved in the regulation of these muscles. The accessory radula closer (ARC) muscle was chosen for study because this muscle has been examined physiologically and behaviorally (4,5). The ARC muscle has been used as a model system for studying the peripheral aspects of a behavioral arousal state elicited by food. Previous research has shown that food-induced arousal in Aplysia is associated with a serotonergic input to the ARC muscle and that serotonin enhances the force of contraction of this muscle. We now report that SCPB enhances the contractile response of this muscle to motor neuron stimulation in a manner similar to that of serotonin.We also determined that SCPB is present in neural elements in the ARC muscle and in the nerve that innervates it. These findings suggest that a peptidergic system using SCPB may function in parallel to the serotonergic system to mediate aspects of food-induced arousal.
METHODSTo determine the levels of SCPB in muscle, ganglia, and nerves, the tissues were homogenized and purified with HPLC, and the levels of SCPB were measured by bioassay. Fresh or frozen tissue was placed in at least a 20-fold excess of 0.1 M acetic acid, heated to 100°C for 10 min, and homogenized. The supernatant of a 10-min, 10,000 x g centrifugation of the homogenate was lyophilized, taken up in a measured volume of 0.1 M acetic acid, and subjected to reversephase HPLC on a Zorbax C8 column (DuPont) using a DuPont 850 pump and gradient controller. The column was developed with an acetonitrile gradient from 25% to 50% at 1%/min at a flow rate of 1 ml/m...