Abstract

Alcoholism and alcoholic cirrhosis are the fourth and fifth causes of mortality in Chile. When combined, these have a mortality rate comparable to that of cardiovascular diseases or cancer (Instituto Nacional de Estadísticas, 1989) Studies in the last decade have shown that 50 to 60% of the risk of developing alcoholism is genetic. It has been demonstrated that there are genetic factors that predispose to the development of alcoholism as well genetic factors that protect against this condition. The protecting genes have been well defined. Upon alcohol ingestion, some gene variants lead to a marked elevation in blood acetaldehyde, the first product of ethanol metabolism in the liver. Such an elevation results in a number of dysphoric reactions, which include profuse vasodilation, tachycardia and nausea, which lead to an aversion to ethanol. Thus, the genetic protection against alcoholism is generated by a prodrug action of ethanol. The predisposition genes, responsible for the pleasant (motivational) and rewarding effects of ethanol, which lead a person or animal to repeat and increase its consumption, have not been defined. Although it is clear that some genes mediate the hypnotic and anesthetic actions of ethanol; namely genes that code for the gamma amino butyric acid (GABA) and the glutamate (NMDA) receptors, the action of ethanol on these receptors does not correlate with its motivational and rewarding effects. Searching for a possible window into the mechanism of reward, a number of studies have shown that the administration of ethanol increases the release of dopamine in the nucleus accumbens, area of the brain mesolimbic system innervated by axonal projections of dopaminergic neurons of the ventral tegmental area. It has also been demonstrated that naïve rats bred for their high alcohol intake, as well as humans who are genetically prone to develop alcoholism, have a reduced dopaminergic innervation, as well as low dopamine receptor densities. Thus, alcohol is likely consumed with the aim of activating a deficient dopaminergic system, generating a rewarding effect similar to that of cocaine and of other drugs of abuse. Recent studies have shown that alcohol-drinker rat strains will self administer ethanol directly into the ventral tegmental area, thus showing a localized rewarding action of this drug in the central nervous system. Animals will also readily self administer acetaldehyde, a likely product of ethanol oxidation by brain catalase. Since there are no specific inhibitors of catalase nor knockout animals for this enzyme which in their wild-type state would be high ethanol-drinkers, it has not been possible to test the possibility that acetaldehyde generated by the action of catalase is responsible for the self administration of ethanol into the brain. Further, other studies have shown that a secondary metabolite of ethanol may also be formed. Indeed, acetaldehyde is known to condense with dopamine forming salsolinol in a reaction catalyzed by brain salsolinol synthase. The generation of salsolinol from acetaldehyde is relevant as it has been recently shown that alcohol-drinker rats will also self administer salsolinol directly into the ventral tegmental area, administration that is blocked by dopaminergic inhibitors. It is, however, not clear if these two metabolites of ethanol: acetaldehyde and salsolinol, are actually formed in the brain at levels that could account for the motivational and rewarding effects of ingested ethanol. The present project aims at conducting genetic modification studies to test the hypothesis that in the central nervous system ethanol acts as a prodrug which (a) by the action of catalase generates acetaldehyde, which (b) condenses enzymatically with dopamine forming salsolinol, leading to (c) the activation of the dopaminergic mesolimbic system, responsible for the motivational and rewarding effects of ethanol. We postulate that the stereotaxic brain microinjection of lentiviral gene delivery vectors that specifically increase the levels of acetaldehyde and salsolinol in the ventral tegmental area will enhance the rewarding and motivational effects of ethanol, measured as (i) chronic voluntary ethanol intake, (ii) return to the place of preference conditioned by the chronic administration of ethanol and (iii) release of dopamine in the nucleus accumbens. Conversely, we postulate that lentiviral vectors carrying genes that specifically reduce the levels of acetaldehyde and salsolinol in the ventral tegmental area will inhibit the rewarding and motivational effects of ethanol. Overall, the studies proposed will define the possible mechanism of action of ethanol in the central nervous system, acting as a prodrug, will examine the role of its mediators, and will develop means to block their effects. The studies may be relevant to the development of novel therapeutic strategies against alcoholism