Pleural-buccal interneurons in the pteropod mollusc Clione limacina

Paired, Phe-Met-Arg-Phe-NH₂-ergic pleural-to-buccal projecting neurons of the pleural ganglia were suggested to be responsible for feeding arrest associated with defensive withdrawal in freshwater and terrestrial pulmonate molluscs. In the present study, the pleural-to-buccal projecting cells were, for the first time, identified in a representative opisthobranch, the carnivorous marine pteropod Clione limacina. Two symmetric neurons of its pleural ganglia were found to be similar to the pulmonate pleural-to-buccal projecting neurons in the number of neurons, positions of their cell bodies in the central nervous system, a unique, indirect route of their axon, electrotonic coupling of the left and right cells, and expression of Phe-Met-Arg-Phe-NH₂-like immunoreactivity and inhibitory action on neurons participating in the motor program for feeding. In their turn, pleural-to-buccal projecting neurons receive excitatory inputs from the protractor interneurons involved in the feeding rhythm generation. Also, it was demonstrated that the pleural-to-buccal projecting cells activity positively correlates with spontaneous and induced acceleration of the locomotor rhythm. Accordingly, stimulation of the cerebral command neuron for locomotion, cell CPA1, excited pleural-to-buccal projecting neurons. We conclude that the neuronal network underlying feeding behavior in both pulmonate and opisthobranch molluscs is similarly linked to defensive behavior by pleural Phe-Met-Arg-Phe-NH₂-ergic neurons, thus indicating evolutionary conservation of these pleural-buccal projections. Accepted: 22 June 1999