DISRUPTION OF POTENTIAL ALPHA-HELIX IN THE G-LOOP OF THE GUINEA-PIG 5-HYDROXYTRYPTAMINE(2) RECEPTOR DOES NOT PREVENT RECEPTOR COUPLING TO PHOSPHOINOSITIDE HYDROLYSIS

Heterogeneity of the 5-hydroxytryptamine(2) (5HT(2)) receptor across s pecies has been implicated in several pharmacological and physiologica l studies. Although 5-HT2 receptors in the rat have been linked to inc reases in phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT2 receptors with PI hydrolys is, the second messenger generally linked with 5-HT2 receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5-HT2 receptors exist between rat and guinea pig. First, we isolated partial cortical 5-HT2 receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor-effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5-HT2 receptor were 97% homologous. However, the guinea pig 5-HT2 receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theore tically could alter the intracellular coupling of the guinea pig corti cal 5-HT2 receptor. Because of these molecular differences, we examine d further the pharmacological activation of the brain 5HT, receptor fr om guinea pig. 5-HT and the 5-HT2 receptor agonist alpha-methyl-5-HT i ncreased PI hydrolysis in guinea pig cortical slices whereas the 5-HT1 C-receptor agonist 5-methyltryptamine was significantly less potent. I n addition, the 5-HT2 receptor antagonists LY53857, ketanserin, and sp iperone blocked 5-HT-stimulated PI hydrolysis. These pharmacological d ata suggested that activation of the 5-HT2 receptor in guinea pig cort ical slices was associated with Pl hydrolysis. Thus, although areas of the guinea pig brain 5-HT2 receptor that influence receptor-effector coupling were different from the rat, such differences were not critic al to receptor-effector coupling because, as in the rat, guinea pig br ain 5-HT2 receptors were also coupled to PI hydrolysis.