Peptidergic modulation of neuronal circuitry controlling feeding in Aplysia

We examined the effects of 3 neuropeptides and the bioactive amine 5-HT on identified motoneurons (B15 and B16) and interneurons (B4, B5) involved in the control of feeding behavior in Aplysia californica. The application of egg-laying hormone (ELH), small cardioactive peptide b (SCPb), and 5-HT elicits distinct patterns of synaptically induced bursting in the neurons, while PheMetArgPheamide (FMRFamide) inhibits firing due to synaptic activity. Repetitive IPSPs recorded in B15 and B16 are induced by 5-HT and SCPb and inhibited by FMRFamide. The substances also may act directly: In solutions that block synaptic transmission SCPb excites B15, ELH excites B16, 5-HT excites B15, B16, and B4, and FMRFamide both inhibits B15 and B16 and excites B4. We suggest that the output of a buccal ganglion central pattern generator may be modulated to produce distinct patterns of motoneuron activity by these candidate transmitters. We also noted differences in the intrinsic properties of the 2 motoneurons. B15 contains SCPb immunoreactivity while B16 does not. This finding suggests that B15 may be the source for the SCPb immunoreactivity previously found at the ARC muscle and that SCPb may be acting in an autocrine mode. Also, B15 has a significantly lower resting potential than B16 and contains a large transient outward (Ia-like) current. The candidate transmitters act by exciting or inhibiting elements at every level within the hierarchically organized motor system that controls feeding. This expands the diversity of behavioral repertoires that may be elicited from a particular neural circuit.