Hormonal Tolerance to Ethanol is Associated with Decreased Expression of the GTP‐Binding Protein, Gsα, and Adenylyl Cyclase Activity in Ethanol‐Treated LS Mice

Abstract

Using the proopiomelanocortin (POMC) system as a marker, long sleep (LS) and short sleep (SS) lines of mice were investigated to explore the cellular events that occur during the acquisition of hormonal tolerance to ethanol. Four-day ethanol exposure (1.8 g/ kg/24 hr) increased anterior pituitary POMC mRNA levels 4-fold in the LS line and 2-fold in the SS line. Following 7 days of ethanol exposure (1.8 g/kg/24 hr), anterior pituitary POMC levels returned to basal values in the LS line but remained elevated (2-fold) in the SS line. In this setting, the loss of ethanol's ability to sustain elevated POMC mRNA levels in the LS line is defined as hormonal tolerance. Since POMC biosynthesis is primarily regulated through adenylyl cyclase, ethanol-induced alterations in this signal transduction system were explored. Paralleling the effects of ethanol on POMC mRNA levels, ethanol exposure reduced GTP-γ-S, AIF₃-, and MnCl₂-stimulated adenylyl cyclase activity by 35%, 21%, and 24%, respectively, in the LS line without effecting adenylyl cyclase activity in the SS line. To determine whether ethanol-induced changes in adenylyl cyclase activity in LS mice could result from alterations in G proteins, protein levels of G_sα and G_iα were determined by western analysis before and after ethanol exposure. Paralleling the effect on POMC mRNA levels and adenylyl cyclase activity, ethanol induced a 35% reduction in G_sα protein levels in LS mice but did not alter G_iα levels. Neither G_sα nor G_iα levels were altered in the SS line. The genetic specificity of ethanol-induced alterations in G_sα protein levels in LS mice was not limited to the anterior pituitary. Ethanol exposure reduced G_sα protein levels in cerebellar membranes in the LS line with no effect on cerebellar G G_sα levels in the SS line. Differential regulation of G proteins could be a critical molecular event in the pathogenesis of ethanol tolerance. These observations underscore that the threshold for tolerance to ethanol is in part genetically determined.