Distribution of serotonin and dopamine receptors in Aplysia tissues: analysis by [3H]LSD binding and adenylate cyclase stimulation.

The distribution of receptors for serotonin and dopamine has been studied in various neuronal and non-neuronal tissues from Aplysia californica using: (1) a [3H]LSD binding assay; and (2) stimulation of adenylate cyclase activity. High levels of specific [3H]LSD binding were found in all ganglia and nerves examined. Lower levels of binding were present in a number of muscle tissues and in the sheath surrounding the central ganglia. The ability of serotonin and dopamine to inhibit [3H]LSD binding depended upon the tissue examined. In muscle tissue, most of the binding was sensitive to serotonin. In contrast, a number of ganglia (e.g. the pleural, abdominal or cerebral) contained a considerable proportion of dopamine-sensitive binding. A limited pharmacological analysis of serotonin-sensitive [3H]LSD binding indicated that Aplysia serotonin receptors are closely related to those found in the snail, Helix pomatia, and in rat brain. Adenylate cyclase activity in membranes from Aplysia ganglia, muscles and connective nerves was stimulated by serotonin (but not by dopamine). The amount of serotonin-sensitive adenylate cyclase correlated well with the amount of serotonin-sensitive [3H]LSD binding in most tissues. D-LSD was a partial agonist on the serotonin-sensitive adenylate cyclase, whereas the pharmacologically inactive stereoisomer L-LSD was without effect. The high density of serotonin receptors in pleuro-abdominal connective nerves, and their presence in the connective tissue sheaths surrounding the ganglia, suggests that not all of these receptors are located at synapses. On the other hand, the tissue distribution of dopamine and serotonin receptors, as measured by these techniques, is consistent with that expected from electrophysiological data.