d-[3H]Lysergic acid diethylamide binding to serotonin receptors in the molluscan nervous system.

d-[3H]Lysergic acid diethylamide (LSD) bound to both dopamine- and serotonin (5HT)-sensitive sites in a particulate fraction derived from the central nervous system of the snail Helix pomatia. Conditions were found which enabled the two sites to be studied independently. [3H]LSD appeared to have a slightly higher affinity for dopamine-sensitive binding (Kd = 0.5 nM) than for 5HT-sensitive binding (Kd = 1.2 nM). A pharmacological analysis of the binding indicated that while dopamine- and 5HT-related agonists clearly discriminated between the two sites, putative antagonists showed little specificity for dopamine- or 5HT-sensitive binding. The pharmacology of 5HR-sensitive [3H]LSD binding was studied in relation to a 5HT-sensitive adenylate cyclase present in a particulate fraction derived from the same tissue. There was a very good correlation between the abilities of a range of agents to act as agonists or antagonists in the 5HT-sensitive adenylate cyclase assay and their abilities to displace 5HT-sensitive [3H]LSD binding (r = 0.94; p less than 0.001). In particular, d-LSD and a number of neurologic drugs were inhibitory in both assays in a stereoselective manner. These data suggest that in molluscan tissues, 5HT-sensitive [3H]LSD binding is related to the 5HT receptor which is coupled to adenylate cyclase.