We have previously reported that approximately 20% of socially-housed vervet monkeys monkeys (C. aethiops St Kitts) will voluntarily consume beverage alcohol to excess (>8 g/kg/day) and that nearly 80% of animals will drink smaller amounts of alcohol in the absence of behavioral or dietary coercion. In the present project, we probed the patterns of consumption in a population sample of over 600 animals tested under alternative conditions. For quantitative evaluation of alcohol consumption, animals are individually housed and tested in a two-bottle choice paradigm (10% ethanol in vehicle vs. vehicle alone), with variation of the duration of ethanol exposure, and/or the time of day of exposure, and/or the vehicle (tap water vs. sweetened tap water). The period of ethanol exposure ranges from one to four weeks for a given condition. Data reduction produced three measures for each condition: g ethanol consumed/kg body weight/day, proportion of fluid consumed as alcohol, and proportional change in alcohol consumed over the test period.

Using this repeated measure paradigm, alcohol consumption is remarkably stable in adult animals (r > 0.85) tested two or more times under standard conditions (tap water, 24 h access in week 1, 4 h access in week 2) at intervals of 6-12 months. There is substantial individual variability with respect to consumption of alcohol vs. sweet, ratio of alcohol to vehicle, preferred vehicle, time of day, etc. Patterns of alcohol consumption were extracted from a principle components analysis of over 2000 trials performed under standardized conditions, with gender, age and 3 dependent measures for each test condition entered into the equation. Four factors were extracted, corresponding to (1) abusive binge drinking, (2) heavy steady drinking, (3) moderate drinking, and (4) avoidance of drinking. Both abusive and heavy steady drinkers consume > 5 g/kg/day, but abusive drinkers drink rapidly to intoxication or coma, sometimes repeatedly within a single 24 h period. Abusive drinkers also exhibit a significantly greater alcohol:water preference ratio under scheduled access drinking and can be distinguished pharmacologically from heavy social drinkers. Moderate drinkers are more likely to be female, to prefer alcohol in sweetened fluid, and to drink little in the morning. The similarity of this solution to the human condition is striking.

More recently, we have explored the biochemical and pharmacological correlates which might distinguish abusive drinkers from heavy social drinkers. Heavy social drinkers, as a group, show exaggerated lymphocyte signal transduction, as compared to non-drinkers, while abusive drinkers show a suppressed pattern of signal transduction. Acute exposure to beverage alcohol elevates CSF levels of the serotonin metabolite 5-HIAA and the dopamine metabolite HVA in all individuals, but the magnitude of increase does not predict alcohol preference. Basal levels of 5-HIAA are not correlated with the quantity of alcohol consumed, but abusive drinkers typically display 5-HIAA levels in the lower 2 deciles of the population distribution. Naltrexone reduces alcohol consumption in heavy social drinkers, but is marginally effective in abusive drinkers. Although our own primary interest is to determine the genetic factors underlying these different patterns of alcohol consumption in vervet monkeys, the potential utility of this population to investigations of mechanisms of the gene-environment interactions in this and other addictive disorders is obvious.

Supported by the Medical Research Council of Canada and Behavioral Sciences Foundation, St. Kitts.