Convergent effects of peptides on the initiation of feeding motor programs in the mollusk Aplysia.

Neuropeptides configure the feeding network of Aplysia. For example, egestive activity is promoted by small cardioactive peptide (SCP), and ingestive activity is promoted by a combination of feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP-2). Additionally, SCP and FCAP/CP-2 have a common network effect that does not contribute to motor program specification. They increase the excitability of an interneuron, B63. In this report we further characterized effects of peptides on B63. We performed voltage clamp experiments and used a step protocol to look at steady state currents. We found that SCP and FCAP/CP-2 both induced an inward current that was virtually absent in low sodium saline. Previous work has established that B63 is unusual in the feeding circuit in that subthreshold depolarizations are autonomously generated that can trigger motor programs. Here we show that this autonomous activity is more frequent in the presence of peptides. Previous studies have also shown that activity of the feeding central pattern generator (CPG) can be initiated by neurons that excite B63, e.g., by cerebral buccal interneuron 2 (CBI-2), a projection neuron that triggers biting-like motor programs. Here we show that the latency of CBI-2 induced activity is decreased by stimulation of the esophageal nerve (EN) (which releases endogenous SCP). These results taken together with previous results indicate that peptides that act divergently to configure network activity additionally act convergently to promote motor program induction. We present data that suggest that this arrangement facilitates brief switches between ingestive and egestive motor activity.