Ultrastructural demonstration of serotonin-immunoreactivity in the nervous system of an insect (Calliphora erythrocephala).

Serotonin (5-HT) immunocytochemistry, was performed on the whole dissected nervous system of the blowfly. Employing the peroxidase-antiperoxidase technique and osmium postfixation, it was possible to examine 5-HT-immunoreactive neuronal elements first light microscopically in 25 microns sections, and, after re-embedding, to analyze the same sections electron microscopically in ultrathin sections. We describe the ultrastructure of 5-HT-positive terminals in the neural sheath of peripheral nerves and in the optic lobes. The immunoreactivity was observed in large (100 nm) granular vesicles, on membranes of clear vesicles, along neurotubules, and along the internal surface of the plasmalemma.