Abstract

[³H]Serotonin (5-HT) binds to membrane preparations of rat brain in a saturable fashion and with substrate specificity and regional variations consistent with its binding to the postsynaptic serotonin receptor. The dissociation constant for [³H]5-HT binding is about 8 nM, and the total number of 5-HT binding sites in the brain is 16 pmoles/g of tissue, wet weight. There is considerable structural specificity in the affinity of various tryptamines for the [³H]5-HT binding sites, with a crucial role played by the 5-hydroxy substituent. d-[³H]Lysergic acid diethylamide (LSD) binding sites have substrate specificity requirements similar to the [³H]5-HT binding sites, but the 5-hydroxy substituent is less critical. 5-HT and related agonists have about 100 times more affinity for 5-HT than LSD binding sites, while classical 5-HT antagonists have 4-100 times greater affinity for LSD binding sites. LSD itself has a similar affinity for 5-HT and LSD binding sites. Raphe lesions which result in degeneration of 5-HT neurons do not lower [³H]5-HT binding, indicating that binding does not take place to presynaptic 5-HT neurons. Regional variations in serotonin and LSD binding are fairly similar. Highest binding occurs in the corpus striatum, hippocampus, and cerebral cortex, with lowest binding in the cerebellum. The ontogeny of 5-HT and LSD binding sites is nearly identical and does not appear to depend on functionally intact presynaptic 5-HT neuronal input.