An immunohistochemical study of the neuropeptidergic bag cells of Aplysia.

The bag cell neurosecretory system of Aplysia californica synthesizes and releases a neuropeptide, the egg-laying hormone (ELH), which can induce egg-laying behavior. While ELH acts as a neurohormone on distant neuronal and non-neuronal targets, it also can moderate the electrical activity of neurons within the abdominal ganglion. Using antibodies generated against pure ELH, our immunohistochemical study provides direct morphological support for this dual function of ELH by defining the distribution of this neuropeptide within the abdominal ganglion of A. californica. We also find ELH cross-reactivity in the bag cell systems of three other species of Aplysia. In sections of abdominal ganglia stained for ELH by the peroxidase-antiperoxidase method, bag cell clusters were found to be homogeneous populations of ELH-immunoreactive neurons, extending processes in a complex array within the connective tissue sheath overlying the ganglion and the proximal regions of neighboring nerves. This association of neurites with the sheath is the neurohemal organ which facilitates release of ELH into circulating hemolymph. Stained processes also invade the interior of the ganglion in association with septal sheets of connective tissue and as scattered fibers in the neuropil. Local modulation by ELH probably occurs through release from these intraganglionic sites. A few ELH-positive ectopic cells were seen outside of the confines of the bag cell clusters as far displaced as the base of the branchial nerve; they appear to be bag cells which have mis-migrated during development. Specificity of the antiserum was confirmed by the total lack of staining of all other neurons within the abdominal ganglion as well as the absence of staining of bag cells with pre-immune serum. Although th ELHs of Aplysia vaccaria, Aplysia braziliana, and Aplysia dactylomela have not been characterized biochemically, it is known that abdominal ganglion extracts from A. californica can induce egg laying in all three species. We find that antiserum against A. californica ELH stains the bag cell systems of A. vaccaria, A. braziliana, and A. dactylomela and not other parts of their abdominal ganglia. It is likely that this staining represents cross-immunoreactivity of the egg-laying neurohormones from these species which also share cross-bioactivity.