Localization and identification of neurons with cholecystokinin and gastrin-like immunoreactivity in wholemounts of Aplysia ganglia.

Immunohistochemical procedures were applied to wholemounts of the central nervous system and posterior intestine of the mollusc, Aplysia californica, to facilitate localization of cells that were immunoreactive to several antisera recognizing various epitopes of the peptides cholecystokinin and gastrin. Only antisera that recognized the carboxyl terminal sequence common to cholecystokinin and gastrin reacted with the Aplysia tissues tested. Intracellular electrophysiological studies of identified postsynaptic targets of immunoreactive neurons in the cerebral ganglia indicated that mammalian forms of gastrin 1-17, several cholecystokinin fragments, and the related peptide, amphibian caerulein, did not mimick the synaptic response mediated by the immunoreactive presynaptic neurons. Combinations of electrophysiological, immunohistochemical, and biochemical studies of several neurons in the buccal ganglia indicated that neurons B7 and B13 were immunoreactive to antisera against cholecystokinin and gastrin and that neuron B13 also contained a concentration of the neurotransmitter acetylcholine as high as in the identified cholinergic buccal neurons, B4 and B5. Several differences in the immunoreactivity of the various antisera were observed. Only one of the antisera was effective in staining neurons in the abdominal ganglia and another antiserum stained subsets of neurons that were immunoreactive to most of the other antisera recognizing the carboxyl terminus common to cholecystokinin and gastrin. The giant serotoninergic metacerebral neurons in Aplysia were not immunoreactive to the cholecystokinin/gastrin antisera even though it has been reported that the homologous neurons in a pulmonate mollusc contain cholecystokinin-like immunoreactivity. These studies demonstrated that there are many neurons with cholecystokinin/gastrin-like immunoreactivity in the Aplysia central and peripheral nervous system and suggested that the peptide may differ from vertebrate forms of cholecystokinin and gastrin. The identification of immunoreactive neurons with known postsynaptic target neurons and buccal neurons with acetylcholine co-localized with a cholecystokinin/gastrin-like peptide will facilitate elucidation of the functions of peptides in the nervous system since the Aplysia preparation is well known to be amenable to multidisciplinary studies.