Catecholamine neurons in Aplysia: Improved light‐microscopic resolution and ultrastructural study using paraformaldehyde and glutaraldehyde (FaGlu) cytochemistry

We have modified the formaldehyde‐glutaraldehyde (FaGlu) histofluorescence method of Furness, Costa, and Blessing (1977a) and Furness, Costa, and Wilson (1977b) to examine wholemounts and sections of both juvenile and adult ganglia as well as peripheral tissues of Aplysia californica. FaGlu fluorescence is the result of a reaction between formaldehyde and tissue catecholamines to produce water‐insoluble (fixed) fluorophores. In serially sectioned cerebral ganglia, 70–80 positive neurons were observed (many in clusters of 10–20 cells), many more than were found using the glyoxylic acid technique. Catecholamine‐containing varicosities were densely packed in localized portions of the neuropil of all central ganglia. Exclusive localization in the neuropil of presumed dopamine release sites is similar to that previously found for the neuropeptide SCP but differs form the widespread ramification of varicose neurites containing 5‐HT, FMRFamide, and ELH. The FaGlu technique also enabled us to study the ultrastructure of catecholamine‐containing neurons. In contrast to the larger vesicles found in serotonergic and histaminergic neurons, these dopaminergic neurons contain 70 nm dense‐cored vesicles.