Octopamine-containing (OC) interneurons enhance central pattern generator activity in sucrose-induced feeding in the snail Lymnaea.

In the pond snail Lymnaea stagnalis octopamine-containing (OC) interneurons trigger and reconfigure the feeding pattern in isolated CNS by excitation of the central pattern generator. In semi-intact (lip-mouth-CNS) preparations, this central pattern generator is activated by chemosensory inputs. We now test if sucrose application to the lips activates the OC neurons independently of the rest of the feeding central pattern generator, or if the OC interneuron is activated by inputs from the feeding network. In 66% of experiments, sucrose stimulated feeding rhythms and OC interneurons received regular synaptic inputs. Only rarely (14%) did the OC interneuron fire action potentials, proving that firing of OC interneurons is not necessary for the sucrose-induced feeding. Prestimulation of OC neurons increased the intensity and duration of the feeding rhythm evoked by subsequent sucrose presentations. One micromolar octopamine in the CNS bath mimicked the effect of OC interneuron stimulation, enhancing the feeding response when sucrose is applied to the lips. We conclude that the modulatory OC neurons are not independently excited by chemosensory inputs to the lips, but rather from the buccal central pattern generator network. However, when OC neurons fire, they release modulatory octopamine, which provides a positive feedback to the network to enhance the sucrose-activated central pattern generator rhythm.