Mechanisms that create a sequence of feeding-related behaviors in the mollusk Aplysia

Neural mechanisms that create action sequences are not well understood, particularly when feedforward excitation will not suffice. This is likely to be the case when one behavior is often but not always followed by another. This report studies transitions between ingestion and digestion in the mollusk Aplysia. Previous studies demonstrated that ingestive responses are triggered by higher-order projection neurons, the cerebral buccal interneurons (CBIs). Here we record from intact animals and show that CBI activity declines as food moves through the gastrointestinal (GI) tract. In addition to input from the CBIs, the feeding circuitry also receives excitatory input from GI afferents. We show that GI afferent input is responsible for triggering post-CBI motor programs. When the CBIs are active, they release peptides that produce a persistent increase in the excitability of B63, a neuron that is part of the feeding central pattern generator (CPG). Our data strongly suggest that this increased excitability facilitates afferent-induced motor program induction. To summarize, our data suggest that the initial food intake results from CBI activity. While the CBIs are active, they release peptides that increase the likelihood that subsequent motor activity will follow. The induction of this activity is, however, conditional, i.e., it requires afferent feedback. We suggest that this arrangement is advantageous because it permits flexibility. Digestion will follow ingestion but only if food has been successfully pulled into the buccal cavity. If ingestion fails, presumably a different type of motor activity will be triggered.NEW & NOTEWORTHY Behaviors are often executed in preferred sequences that achieve a particular goal. Neural mechanisms that create these sequences are not well understood, particularly when it is important to retain the ability to be flexible. In this report we ask how this is achieved during transitions between two types of feeding-related behaviors in the mollusk Aplysia.