5-HYDROXYTRYPTAMINE(1A) RECEPTOR SYNTHETIC PEPTIDES - MECHANISMS OF ADENYLYL-CYCLASE INHIBITION

The 5-hydroxytryptamine(1A) receptor (5-HT(1A)R) is a G-protein-coupled receptor negatively coupled to adenylyl cyclase (AC). We have studied the functional domains of 5-HT(1A)R using synthetic peptides to block or mimic receptor function. The entire second intracellular loop (5-HT(1A)R-i(2)) and the carboxyl end of the third intracellular loop (5-HT(1A)R-i(3)-C) strongly inhibited forskolin-stimulated AC activity. These effects were not additive with those of 5-HT. Like 5-HT, the peptides 5-HT(1A)R-i(3)-C and -i(2) weakly inhibited AlF4- and Mn2+ stimulated AC activity. 5-HT(1A)R binding assays indicated that peptides could interact with the same G-protein pool as the 5-HT(1A)R. 5-HT(1A)R-i(3)-C- and -i(2)-stimulated [S-35]guanosine 5'-O-(thiotriphosphate) binding on G(o)/G(i) proteins. Only 5-HT(1A)R-i(3)-C partially adopted an alpha-helical conformation in solution. These data show that different domains in the 5-HT(1A)R second and third intracellular loops can couple to and activate G(i) proteins in order to mediate AC inhibition. Peptide-induced AC inhibition was not sensitive to pertussis toxin as opposed to the 5-HT(1A)R-mediated effect. Our data show that the 5-HT(1A)R and the 5-HT(1A)R peptides activate G(i) proteins in a slightly different manner.