Ethanol inhibits the function of 5-hydroxytryptamine type 1c and muscarinic M1 G protein-linked receptors in Xenopus oocytes expressing brain mRNA: role of protein kinase C.

Effects of ethanol on the function of Ca(2+)-activated Cl- channels activated by G protein-coupled serotonin (5-hydroxytryptamine, (5-HT)1c) and muscarinic M1 cholinergic receptors were studied in Xenopus oocytes expressing mouse whole-brain mRNA. Ethanol (25-200 mM) inhibited currents evoked by both 5-HT and acetylcholine (ACh), in a concentration-dependent manner. The maximal effect was obtained with 150 mM ethanol, which produced 65 and 49% inhibition of 5-HT and ACh responses, respectively. In the presence of 100 mM ethanol, the EC50 values for both 5-HT and ACh were increased about 4-fold. In contrast, in oocytes expressing rat cerebellar mRNA, metabotropic glutamate receptor responses were much less sensitive to ethanol. To examine potential postreceptor sites for ethanol inhibition, guanosine-5'-O-(3-thio)triphosphate and myo-inositol-1,4,5-trisphosphate were injected intracellularly. Ethanol (100 mM) did not significantly inhibit the currents produced by either guanosine-5'-O-(3-thio)triphosphate or myo-inositol-1,4,5-trisphosphate. Activation of protein kinase C (PKC) by phorbol-12-myristate-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-myristate-13-acetate and opposite to PKC inhibitors, enhanced the rate of Ca(2+)-activated Cl- current desensitization induced by repeated applications 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, which phosphorylates the receptors, resulting in inhibition of the responses.