Polycyclic neuromodulation of the feeding rhythm of the pond snail Lymnaea stagnalis by the intrinsic octopaminergic interneuron, OC.

We have examined the role of the octopamine-containing buccal OC interneuron in the fictive feeding rhythm generated by depolarizing a modulatory interneuron, SO, in the isolated central nervous system (CNS) of Lymnaea stagnalis. Before stimulating the SO, the initial fictive feeding rate was 2.0+/-0.37 bites/min (mean+/-S.E.). When the SO was stimulated, the fictive feeding rate more than doubled, increasing by 5.4+/-2.6 bites/min. Prestimulation of OC facilitates the ability of the modulatory neuron SO to drive fictive feeding 4 s later. Following OC stimulation, the increase in SO-driven feeding rate was 10.8+/-1.6 bites/min, significantly more than when only the SO was stimulated (P<0.02, paired t-test on five preparations). OC activity is not required during the SO stimulation for this enhancement. The maximum of the SO driven rhythm occurs between 6 and 12 s after the end of the OC stimulation at 20 bites/min. This is the maximum feeding rate of intact Lymnaea in sucrose. Facilitation is mimicked by bath applied octopamine at 5 microM. Facilitation is specific to OC interneurons, as the same prestimulation of the electrically coupled neuron N3P (central pattern generator) interneurons does not affect the feeding rhythm. The OC interneuron acts as a long term, polycyclic modulator, which peaks several feeding cycles after the OC activity.