Mechanisms of ethanol-induced steroidogenesis following acute and chronic ethanol exposure

The 3alpha,5alpha-reduced pregnan steroids, including (3alpha,5alpha)-3-hydroxypregnan-20-one (3alpha,5alpha-THP) and (3alpha,5alpha)-3,21-dihydroxypregnan-20-one (3alpha,5alpha-THDOC), are potent allosteric modulators of gamma-aminobutyric acid type A (GABAA) receptor activity. These neuroactive steroid levels are increased by acute ethanol administration at doses ≥ 1.5 g/kg in rats and mediate specific actions of ethanol in rodents and subjective effects of ethanol in humans. Acetaldehyde, a metabolite of ethanol, may also play a role in some of ethanol's actions. The first aim of this project examined if acetaldehyde plays a role in ethanol-induced increases in neuroactive steroids. We found that acetaldehyde is capable of increasing neuroactive steroid levels at high doses but does not seem to have effects when administered at doses observed after acute ethanol administration (2 g/kg). Manipulation of various ethanol and acetaldehyde metabolizing enzymes to alter acetaldehyde concentrations confirmed that the increases in neuroactive steroids observed after ethanol administration are not directly resulting from acetaldehyde. I also examined mechanisms by which acute ethanol administration elicits increases in neuroactive steroids. Focusing on key enzymes and signaling molecules involved in the steroid biosynthetic pathway, I found that pituitary adrenocorticotrophic hormone (ACTH) release and de novo adrenal steroidogenic acute regulatory (StAR) protein synthesis are each necessary, but not sufficient for ethanol-induced steroidogenesis. Furthermore, phosphorylation of StAR is markedly increased by acute ethanol administration and may be involved in StAR activity. Interestingly, tolerance to ethanol-induced increases in neuroactive steroids occurs following chronic ethanol exposure. Thus, I investigated the biosynthetic enzymes and signaling molecules found to be important for ethanol-induced steroidogenesis to see if any were altered by chronic ethanol exposure. Indeed, chronic ethanol exposure elicited tolerance to ethanol-induced ACTH as well as plasma and brain steroids. StAR remained elevated following chronic ethanol exposure; however phosphorylation of StAR was no longer observed. ACTH replacement restored the neuroactive steroid response and enhanced phosphorylation of StAR protein following chronic ethanol exposure. Thus, the dysregulation of ethanol-induced ACTH release may lead to tolerance to ethanol-induced increases in neuroactive steroid levels and contribute to behavioral tolerance to ethanol. These mechanisms may provide a better understanding of ethanol sensitivity and factors that influence the progression towards alcoholism.