Neuroendocrine control of egg-laying behavior in the nudibranch, Archidoris montereyensis

We describe a group of neurons with egg-laying bioactivity in the cerebral ganglia of an opisthobranch mollusc, the nudibranch Archidoris montereyensis. These cells, the intercerebral white cells (IWCs), share morphological, biochemical, and electrophysiological characteristics with the egg-laying neuroendocrine cells of two other molluscs, Aplysia californica (bag cells) and Lymnaea stagnalis (caudodorsal cells). The IWCs, comprising two superficial clusters of about 100 neurons each, were located immediately posterior to the intercerebral commissure in the cerebral ganglia. The somata of these cells were small (< 20 microns) and possessed varicose, bifurcating unipolar processes that collectively formed a loop within the commissure and bilateral extensions into the cerebral ganglia. The IWC clusters and commissural processes were enveloped by a large ganglionic vascular sinus, forming a potential neurohemal release site. Homogenates of whole cerebral ganglia or isolated IWC clusters induced egg-laying behavior within hours of injection into the hemocoel of quiescent animals. The IWCs were immunoreactive for alpha bag-cell peptide, one of the neuropeptide transmitters encoded by the egg-laying hormone gene of Aplysia. Electrophysiologically, the IWCs were silent neurons with large resting potentials and appeared to be highly refractory to electrical stimulation. The similarities of the IWCs to the egg-laying neuroendocrine cells in Aplysia and Lymnaea suggest that they are members of a homologous group of neurons controlling egg-laying behavior in gastropod molluscs.