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Abstract
Many radiolabelled receptors coupled to intracellular adenylate cyclase activity have been found to be modulated by physiological modulators such as GTP (guanosine triphosphate) and Gpp(NH)p (guanosine-imido-diphosphate). In particular, the apparent affinity of agonists competing for the binding of 3H-antagonist-labelled receptors is reduced in the presence of GTP and Gpp(NH)p. We report herein the agonist-specific effects of GTP and Gpp(NH)p on rat brain cortical S2 serotonin receptors. The agonists serotonin, 5-methoxytryptamine, bufotenine, and tryptamine display threefold lower affinities for S2 serotonin receptors in the presence of 10-4M GTP or Gpp(NH)p than in the absence of the nucleotides. The antagonists spiperone, cinanserin, cyproheptadine and methysergide are unaffected by the guanine nucleotides. The Hill coefficients of the agonists increase from between 0.70–0.80 to 0.90–1.00 due to guanine nucleotides. ATP, ADP, and GDP have little or no effect. This pattern of guanine nucleotide effects has been found with receptors which are modulated by a guanine nucleotide regulatory protein and may indicate that the S2 serotonin receptor may be coupled to intracellular adenylate cyclase activity.