title: Disruption of potential alpha-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 PI hydrolysis, 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 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 alpha-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 PI 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.
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis.
authors.forename: S W
authors.initials: SW
authors.lastname: Watts
authors.forename: M L
authors.initials: ML
authors.lastname: Cohen
authors.forename: P Q
authors.initials: PQ
authors.lastname: Mooney
authors.forename: B G
authors.initials: BG
authors.lastname: Johnson
authors.forename: D D
authors.initials: DD
authors.lastname: Schoepp
authors.forename: M
authors.initials: M
authors.lastname: Baez
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002540:Cerebral Cortex / Q000378:metabolism; D004247:DNA / Q000235:genetics; D006168:Guinea Pigs; D006868:Hydrolysis; D066298:In Vitro Techniques; D008297:Male; D015335:Molecular Probes / Q000235:genetics; D008969:Molecular Sequence Data; D009693:Nucleic Acid Hybridization; D010716:Phosphatidylinositols / Q000378:metabolism*; D016133:Polymerase Chain Reaction; D051381:Rats; D011985:Receptors, Serotonin / Q000235:genetics* / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology
chemical_list: D015335:Molecular Probes; D010716:Phosphatidylinositols; D011985:Receptors, Serotonin; D012701:Serotonin; D004247:DNA