Serotonin-sensitive adenylate cyclase and [3]serotonin binding sites in the CNS of the rat—I: Kinetic parameters and pharmacological properties

The 5-HT receptor linked to adenylate cyclase and the high affinity binding site for ³H]-5-HT were compared on the basis of their kinetic and pharmacological properties in the CNS of new born rats. Under normal assay conditions, the apparent affinity of 5-HT for its specific binding sites (K_d = 1?2 nM) was much higher than that for the receptor coupled to adenylate cyclase (K_A app = 0.5?1.0 μM). When measured under the conditions of the cyclase assay, the apparent K_d for the binding was increased to 11.9 nM, a value which is still more than 40 times lower than the K_A app characterizing the activation of adenylate cyclase by 5-HT. GTP affected both the binding of ³H]-5-HT and the 5-HT-sensitive adenylate cyclase. Guanyl nucleotides appeared to be essential for the activation of adenylate cyclase by 5-HT as 5-HT was inactive in a preparation of washed membranes unless added in the presence of GTP or GppNHp. In whole homogenates, GTP increased the affinity of 5-HT for the receptor-adenylate cyclase complex (K_A app = 0.33μM in the presence of 10μM GTP). The specific binding of ³H]-5-HT was reduced by GTP and GppNHp but not GMP or ATP. However, the range of concentrations inducing a significant effect (?0.10mM GTP) was far higher than those which increased the 5-HT-induced activation of adenylate cyclase.There was little in common between the pharmacological profiles of the two systems. A group of 5-HT agonists containing a piperazine heterocycle 1- (m-trifluoromethylphenyl) piperazine, quipazine and MK-212] effectively displaced ³H]-5-HT from its binding sites but exerted no action on the 5-HT-sensitive cyclase, affecting neither the basal nor the 5-HT-stimulated cAMP production. Likewise, there was no correlation between the respective potencies of a series of 5-HT antagonists for inhibiting the binding of ³H]-5-HT and the 5-HT-induced cAMP production. These data suggest that the 5-HT receptor linked to adenylate cyclase is not identical with that which is measured by the binding of ³H]-5-HT and, thus, provide evidence for the possible existence of multiple receptors for 5-HT in the rat brain.