Disruption of Potential α-Helix in the G Loop of the Guinea: Pig 5-Hydroxytryptamine2 Receptor Does Not Prevent Receptor Coupling to Phosphoinositide Hydrolysis

Abstract: Heterogeneity of the 5-hydroxytryptamine₂ (5-HT₂) receptor across species has been implicated in several pharmacological and physiological studies. Although 5-HT₂ receptors in the rat have been linked to increases in Phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT₂ receptors with Pl hydrolysis, the second messenger generally linked with 5-HT₂receptors. 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-HT_a 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 theoretically could alter the intracellular coupling of the guinea pig cortical 5-HT₂ receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5-HT₂ receptor from guinea pig. 5-HT and the 5-HT₂ receptor agonist α-methyl-5-HT increased PI hydrolysis in guinea pig cortical slices whereas the 5-HT_1c receptor agonist 5-methyltryptamine was significantly less potent. In addition, the 5-HT₂ receptor antagonists LY53857, ketanserin, and spiperone blocked 5-HT-stimulated Pl hydrolysis. These pharmacological data suggested that activation of the 5-HT₂ receptor in guinea pig cortical slices was associated with PI 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 critical to receptor-effector coupling because, as in the rat, guinea pig brain 5-HT₂ receptors were also coupled to PI hydrolysis.