Ultrastructural evidence for the uptake of 5,6-dihydroxytryptamine in the frog's gustatory cell

The fungiform papillae of the frog were examined electron-microscopically after a single injection of 5,6-dihydroxytryptamine (5,6-DHT). Thirty minutes after 5.6-DHT injection, numerous vesicles partially occupied with dark material were found in the cytoplasm of gustatory cell processes. Three days after the drug treatment, the majority of vesicles were completely filled with dark material so that their limiting membrane became obscure, while the terminal portions of gustatory cells showed no evidence for fine structural alteration indicative of drug retention. In addition, clear vacuoles, suggesting a degenerative change induced by the uptake of the neurotoxic agent, occurred in the cytoplasm of gustatory cell processes. It is suggested that the uptake of 5.6-DHT in gustatory cells may reflect the presence of an indoleamine, probably 5-hydroxytryptamine (serotonin).