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The neuroaftive peptides SCPB (small cardioactive peptide B) and FMRFamide (Phe-Met-Arg-Phe-NH2), both originally isolated from molluscs, have potent modulatory effects upon the production of patterned motor activity in identified neurons (e.g., B5 and B19) in the buccal ganglia of the snail Helisoma. Such patterned motor activity has previously been shown to underlie feeding behavior. (7,8).Several aspects of the neural basis of feeding in the snail Helisoma trivolvis have been described. Cyclical activity of many identified effector neurons and elements of the central pattern generator has been characterized (9-11). In addition, the monoamines serotonin (12) and dopamine (13) have been shown to activate or modulate such patterned motor activity (PMA) associated with feeding and generated by neurons in the buccal ganglia. As a first step in characterizing the potential roles of neuroactive peptides in regulating feeding behavior in Helisoma, we report here that two neuroactive peptides, originally discovered in molluscs, have profound effects on the production of PMA (which apparently represents "fictive feeding") by buccal ganglion neurons. Small cardioactive peptide B (SCPB, Met-Asn-Tyr-Leu-Ala-PhePro-Arg-Met-NH2; ref. 14) has stimulatory effects upon the generation of PMA that resemble, at least superficially, those described for serotonin (5-hydroxytryptamine) and dopamine. In contrast, FMRFamide (Phe-Met-Arg-Phe-NH2; ref.15) inhibits and, at a sufficient concentration (10-5 M), effectively suppresses PMA in the buccal ganglia. SCP has been indicated, by chromatographic techniques, to occur in Helisoma central nervous system (see table 1 in ref. 14). We show here that the buccal ganglion ofHelisoma contains both neurons with SCPB-like immunoreactivity and neurons with anti-FMRFamide immunoreactivity.MATERIALS AND METHODS Albino specimens of Helisoma trivolvis were used; they were obtained from stocks maintained in the laboratory of A. G. M. Bulloch (University of Calgary) and derived from the Oregon Red stock of S. B. Kater's laboratory (University of Iowa). The dissection, the physiological saline, and the electrophysiological recording and display techniques have been described (16,17). For these experiments, either isolated buccal ganglia or buccal ganglia attached to the central ring of ganglia via the cerebrobuccal connectives were used. There was no detectable difference in the effects of peptides upon PMA whether or not the central ganglia were attached, but the rate of spontaneous PMA varies considerably and tends to be higher in isolated buccal ganglia. Once it had been determined that SCPB was excitatory and FMRFamide was inhibitory, relatively quiescent preparations were used for SCPB experiments and relatively active preparations were used for FMRFamide experiments. The synthetic peptides, SCPB (Peninsula Laboratories, San Carlos, CA) and FMRFamide (Sigma), were used. Saline volume (<0.5 ml) in the recording chamber was maintained by suction from the surface, and solutions were changed ...

The neurotransmitter dopamine is shown to play a fundamental role in the generation of the feeding motor pattern and resultant feeding behavior in Helisoma. Application of exogenous dopamine triggered the fictive feeding motor pattern in the isolated CNS and triggered feeding movements in semi-intact preparations. Application of feeding stimulants to the oral cavity excited the putatively dopaminergic buccal interneuron N1a, and depolarization of interneuron N1a triggered the production of the fictive feeding motor pattern. The ability of dopamine superfusion and of interneuron N1a stimulation to activate the fictive feeding motor pattern was blocked by the dopamine antagonist sulpiride. The phase of the fictive feeding motor pattern was reset by brief hyperpolarization of interneuron N1a, demonstrating that interneuron N1a is an integral component of the buccal central pattern generator (CPG). During spontaneous fictive feeding patterns, prolonged hyperpolarizations of interneuron N1a inhibited the production of patterned activity. Exogenous dopamine maintained the fictive feeding motor pattern in the absence of interneuron N1a activity. Interneuron N1a was labeled by the formaldehyde-glutaraldehyde histochemical technique, which is indicative of the presence of dopamine in mollusks. These data suggest that interneuron N1a is an endogenous source of the neuromodulator dopamine, intrinsic to the buccal CPG, and that interneuron N1a has a prominent role in the sensory-motor integration triggering the consummatory response.

The electrically coupled buccal ganglion neurons 4R and 4L of the snail, Helisoma, display predictable plasticity. The strength of the electronic synapse between them increases significantly following axotomy. Synaptic strength was assayed by measurements of electrical coupling coefficients and by assessment of dye coupling (passage of dye into the uninjected neuron) following injection of Lucifer Yellow CH into one neuron. Within 3 to 5 days, axotomy induced an increase in electrical coupling coefficients between neurons 4R and 4L from 0.54 +/- 0.11 (n = 13) in normal preparations to 0.72 +/- 0.14 (n = 24). A parallel axotomy-induced increase in the probability of dye coupling occurred. Only 27% (n = 27) of normal neuron 4 pairs were dye coupled, compared with 87% (n = 15) of axotomized neuronal pairs. Irrespective of treatment, electrical and dye measurements in the same neuron 4 pairs showed a consistent correlation between the magnitude of the electrical coupling coefficients and the probability of detectable dye coupling. No dye coupling was observed in neuronal pairs with electrical coupling coefficients less than 0.50. Dye coupling always occurred when coupling coefficients were greater than 0.70. Sixty-seven percent of neuronal pairs with intermediate coupling (0.50 to 0.70) coefficients displayed dye coupling. The results show that axotomy evokes a predictable enhancement of communication at an "identified" electronic synapse and suggests that electrical and dye coupling are mediated by similar mechanisms.