Two neuronal populations in the head ganglia of Aplysia californica with egg-laying hormone-like immunoreactivity.

The egg-laying hormone (ELH) is a polypeptide of known structure, which is synthesized and released by the neurosecretory bag cells in the abdominal ganglion of the mollusc, Aplysia californica. We have used immunohistochemical procedures to reveal organized groups of specific cells and fiber tracts within the pleural and cerebral ganglia in adult Aplysia which are immunopositive when reacted with antibodies generated against ELH. These antibodies are highly specific in that they only stain the bag cells within the abdominal ganglion. The ELH+ systems in the head ganglia persisted in two animals 43 days and 67 days after surgical removal of their abdominal ganglia and pleurovisceral connectives. It is therefore likely that these immunoreactive neurons and their processes are independent of the bag cell system. The pleural ganglia neurons, with ELH-like immunoreactivity, bear a striking resemblance to bag cells in somal and nuclear dimensions, neuritic morphology and association with the pleurovisceral connectives. This suggests that both populations of cells may have descended from a common precursor pool during embryogenesis. In the cerebral ganglion, a pair of bilateral clusters of 6-10 small immunopositive neurons are located on the dorsal surface in the vicinity of the C clusters, and send their processes into the neuropil. Intensely stained tracts of ELH+ fibers are prominent at all levels of section in the cerebral neuropil; stained fibers can also be traced into most of the nerves emanating from the cerebral ganglion. Although the functions of these systems, as well as the specific nature of the immunoreactive molecule(s) they contain, remain unknown, 5 ELH-like genes of Aplysia have now been cloned and 3 of these have been sequenced. Our results suggest that the immunoreactive molecules in the pleural and cerebral neurons are due to peptides controlled by one or more of these genes. ELH is known to markedly change the electrical activity of neurons in the head ganglia in vitro. This demonstration of the presence and distribution of ELH-like molecules endogenous to the head ganglia raises the possibility that ELH target neurons in the head ganglia may be activated by local sources of ELH-like neuroactive peptides in vivo.