Ethanol inhibits kainate responses of glutamate receptors expressed in Xenopus oocytes: role of calcium and protein kinase C.

Recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors expressed in oocytes are inhibited by ethanol and the sensitivity to ethanol depends on the kainate concentration and the subunit(s) expressed. For example, GluR3 kainate channels are more sensitive to inhibition by ethanol than GluR6 channels in the presence of maximally effective kainate concentrations. To determine if the ethanol inhibition was influenced by the cation permeability (Na+ vs Na+ and Ca2+) of the channels expressed, we compared ethanol inhibition of Ca(2+)-permeable glutamate receptors (GluRs) in oocytes perfused with normal- and high-Ca2+ buffers. The ethanol inhibition was much greater when Ca2+ was the only permeant cation. When Ba2+ was substituted for Ca2+, the ethanol inhibition was reduced, although it was still greater than with normal buffer. The enhanced ethanol inhibition of kainate-stimulated Ca2+ currents was reduced in oocytes injected with the Ca2+ chelator BAPTA, suggesting a role for intracellular Ca2+ in mediating enhanced ethanol sensitivity of kainate channels. The enhanced ethanol inhibition of Ca2+ currents was not due to a direct ethanol inhibition of Ca(2+)-stimulated Cl- currents in the oocyte because ethanol produced no effect on Ca(2+)-stimulated Cl-currents induced by injection of myo-inositol-1,4,5-trisphosphate. Because Ca2+ activates protein kinase C (PKC) and because we found that the PKC activator phorbol 12-myristate 13-acetate inhibits kainate responses (Dildy-Mayfield and Harris, 1994), we examined the role of PKC in mediating the enhanced ethanol inhibition of kainate responses produced by increased Ca2+. Inhibition of PKC by injection of the PKC inhibitor peptide or calphostin C prevented the enhanced ethanol inhibition of kainate-induced Ca2+ responses without altering ethanol inhibition in normal buffer. Thus, ethanol inhibition of kainate channels may involve two mechanisms, one that is independent of PKC and a second type that is due to activation of PKC under conditions of elevated Ca2+, resulting in enhanced inhibition of kainate responses.