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‐HTa 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‐HT1c 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.