THE EFFECT OF ETHANOL ON MUSCARINIC RECEPTOR-G PROTEIN COUPLING IN THE RAT CORTEX

To understand the mechanism of ethanol action on G protein-mediated si gnal transduction pathway, the effect of ethanol on muscarinic recepto r-G protein coupling in the rat cerebral cortex was examined. Acetylch oline (ACh)stimulated G protein GTPase activity was used as an index o f receptor-G protein coupling. ACh stimulation of G protein GTPase act ivity was time-and concentration-dependent, and atropine-sensitive. Ra ts injected with ethanol (3 g/kg body weight) were sacrificed after 4 hr, and the cerebral cortices removed. The ability of ACh to stimulate GTPase activity was similar in cortical cell membranes obtained from control and ethanol-treated rats; ACh maximally stimulated the enzymat ic activity by 22% in membranes from both groups of rats. Next, in cor tical cell membranes obtained from control rats (i.e., not injected wi th ethanol) the ability of ACh to stimulate GTPase activity in the pre sence of ethanol was examined. ACh stimulated GTPase activity in a con centration-dependent manner; the activity was 12.3 +/-0.1, 14.5+/-0.64 , 15.7+/-0.54, and 16.1+/-0.33 Pi pmol/min./mg protein, at 0, 0.01, 0. 1, and 1 mM ACh, respectively (P<0.05). In the presence of 100 mM etha nol ACh-stimulated GTPase activity was significantly inhibited. The IC 50 value of ethanol inhibition of ACh-stimulated GTPase activity was s imilar to 50 mM. These results suggest that: 1) in vitro, ethanol impa irs ACh-stimulated G protein GTPase activity in the rat cortical cell membranes, and 2) in vivo, the acute effects of alcohol on G protein f unction may be transient and reversible.