Localization of immunoreactive alpha‐bag‐cell peptide in the central nervous system of Aplysia

The bag cells of the marine mollusc Aplysia are well‐characterized neuroendocrine cells that initiate egg laying, but the natural stimulus triggering bag‐cell activity has not been determined. As a first step toward identifying central neurons that might provide synaptic or neurohormonal input onto the bag‐cell network, antibodies specific for alpha‐bag‐cell peptide (α‐BCP) were generated. This peptide belongs to a small family of structurally related peptides that can elicit bag‐cell activity in vitro. Antibody specificity was established by immunodot assay and preabsorption studies: immunocytochemical labeling was abolished in each ganglion when the antibodies were preincubated with either α‐BCP‐thyroglobulin conjugate or α‐BCP‐(1–8) but was not affected by preincubation with thyroglobulin or thyroglobulin‐thyroglobulin conjugate. The antibodies specifically labeled the bag cells in the abdominal ganglion and ectopic bag cells in both the abdominal and right pleural ganglia. The ectopic bag cells were similar to conventional bag cells in size and morphology, but varied in number and location among preparations. In the cerebral ganglion, the antibodies labeled a bilaterally symmetrical pair of cell clusters, containing approximately ten cells each, on the dorsal surface of the ganglion. The cerebral cells were smaller than bag cells, were constant in location, and sent their processes into the neuropil rather than the connective tissue sheath. Immunoreactive processes were observed in the neuropils of the cerebral, pleural, and pedal ganglia and among the axons of the cerebropedal, cerebropleural, and pleurovisceral connectives. No immunoreactive cell bodies were observed in the buccal or pedal ganglia. Identical patterns of labeling were observed in Aplysia californica, A. Brasiliana, and A. Dactylomela. The distribution of immunoreactive cell bodies within the circumesophageal ganglia of all three species thus parallels the distribution of receptive sites for the in vitro induction of bag‐cell activity by atrial gland peptide B, a peptide structurally related to α‐BCP. These observations suggest that the immunoreactive cells identified in these studies, or a subset of them, may be involved in the physiological induction of bag‐cell activity. Since low doses of α‐BCP have additional inhibitory actions on the bag cells, however, it is possible that the identified cells could play a more complex role in the regulation of bag‐cell activity.