Comparative study of histamine immunoreactivity in nervous systems of Aplysia and Pleurobranchaea

The distribution of histamine in the nervous system of the marine molluscs Aplysia californica and Pleurobranchaea californica was studied by using a newly available immunohistochemical localization technique and specific antiserum against histamine-protein conjugate. We examined several sets of complete histological sections through the major ganglia of both animals, as well as all nerve roots of the buccal and cerebral ganglia and the corresponding target tissues. The results indicate that histamine is present in several neurons and/or nerve fibers of all major ganglia. An especially dense histamine fiber network in the buccal ganglion of both species suggests a major role for histamine in regulation of buccal-oral behaviors. Histamine was also observed in several identified nerve roots of the buccal and cerebral ganglia, as well as in the corresponding target tissues. Its localization in the Aplysia radular sac and in the statocyst neurons of both species suggest a role for histamine in sensory functions. Our study revealed many previously unknown histamine cells or cell clusters, some of which may be identifiable by electrophysiological methods. The findings also point to possible reinterpretation of previous findings, indicating that histamine may be a cotransmitter in identified cells, whereas the methodology itself suggests that special precautions must be taken to avoid spurious interpretations of specificity. As has now been observed in studies of serotonergic immunohistochemistry and in our own findings on VIP, histamine terminals were observed to lie in close contact with somata and axon hillocks, all of which suggest that axo-somatic connections in molluscs may be more prevalent than previously considered.