ETHANOL INHIBITS THE FUNCTION OF 5-HYDROXYTRYPTAMINE TYPE-1C AND MUSCARINIC-M(1), G-PROTEIN-LINKED RECEPTORS IN XENOPUS-OOCYTES EXPRESSING BRAIN MESSENGER-RNA - ROLE OF PROTEIN-KINASE-C

Effects of ethanol on the function of Ca2+-activated Cl- channels acti vated by G protein-coupled serotonin (5-hydroxytryptamine, (5-HT)(1c)) and muscarinic M(1) cholinergic receptors were studied in Xenopus ooc ytes expressing mouse whole-brain mRNA. Ethanol (25-200 mM) inhibited currents evoked by both 5-HT and acetylcholine (ACh), in a concentrati on-dependent manner. The maximal effect was obtained with 150 mM ethan ol, which produced 65 and 49% inhibition of 5-HT and ACh responses, re spectively. In the presence of 100 mM ethanol, the EC(50) values for b oth 5-HT and ACh were increased about 4-fold. In contrast, in oocytes expressing rat cerebellar mRNA, metabotropic glutamate receptor respon ses were much less sensitive to ethanol. To examine potential postrece ptor sites for ethanol inhibition, guanosine-5'-O-(3-thio)triphosphate and myo-inositol-1,4,5-tris-phosphate were injected intracellularly. Ethanol (100 mM) did not significantly inhibit the currents produced b y either guanosine-5'-O-(3-thio)triphosphate or myo-inositol-1,4,5-tri sphosphate. Activation of protein kinase C (PKC) by phorbol-12-myrista te-13-acetate markedly inhibited 5-HT-induced responses. Both the PKC inhibitor peptide and staurosporine prevented ethanol inhibition of 5- HT-induced responses. Moreover, ethanol, similarly to phorbol-12-myris tate-13-acetate and opposite to PKC inhibitors, enhanced the rate of C a2+-activated Cl- current desensitization induced by repeated applicat ions of 5-HT. These results indicate that certain types of receptor-G protein interactions are more susceptible than others to uncoupling by ethanol and that ethanol inhibition of 5-HT1c receptors requires PKC- mediated phosphorylation. We suggest that ethanol may activate PKC, wh ich phosphorylates the receptors, resulting in inhibition of the respo nses.