Mutation of Putative Phosphorylation Sites in the 5-Hydroxytryptarnine3 Receptor Does Not Eliminate Its Modulation by Ethanol.

The function of the 5-hydroxytryptamine₃ (5-HT₃) receptor is enhanced by ethanol, but the amino acid residue(s) that confers sensitivity to ethanol remains to be identified. Phosphorylation of the related GABA_A receptor has been implicated in conferring its sensitivity to ethanol. In common with the GABA_A receptor, the 5-HT₃ receptor contains multiple consensus sites for protein kinases. To evaluate the possibility that phosphotylation of the 5-HT₃ receptor underlies its ethanol sensitivity, we examined the ability of ethanol to enhance 5-HT-mediated currents in a mutant 5-HT₃ receptor containing no intracellular serines, threonines, or tyrosines. Mutation of these 13 residues in the intracellular loops produced a modest leftward shift in the 5-HT concentration response curve, with the EC₅₀'s for 5-HT decreasing from 0.839 ± 0.03 μM in the wild-type receptor to 0.713 ± 0.03 μM in the mutant receptor. Cooperativity of the 5-HT binding sites was enhanced by the mutations, with Hill coefficients of 2.92 for the wild-type receptor and 3.74 for the mutant receptor, respectively. In oocytes expressing mutant receptors, ethanol (50 to 200 mM) enhanced the currents produced by low concentrations of 5-HT by ∼5 to 45%, which was not statistically different from the potentiation produced by ethanol in wild-type receptors. These results suggest that ethanol enhancement of the 5-HT₃ receptor function does not require receptor phosphorylation. [ABSTRACT FROM AUTHOR]