Primates in Alcohol Research

Because of their close genetic similarity to humans and their complex social behaviors, nonhuman primates have been widely used to study a number of psychiatric syndromes (for a review, see McKinney 1988). Although they have been used less often in alcohol research, nonhuman primates are ideally suited as subjects for psychobiological studies of excessive alcohol consumption, because the similarities in human and other primate DNA result in their sharing many physiological and behavioral processes. In addition, nonhuman primates have genetic and rearing backgrounds that can be carefully planned so as to allow for observation in closely controlled settings. These two attributes allow researchers to extrapolate results from studies of nonhuman primates to humans more readily than the results from less closely related animals. This article reviews examples of ongoing research that uses nonhuman primates to investigate how genetic predispositions influence alcohol consumption. It also examines how a genetic predisposition may interact with other factors, such as environment and exposure to stress, to produce greater voluntary alcohol consumption. PRIMATE DRINKING One reason nonhuman primates are not used frequently in alcohol research is that they rarely consume alcohol in the amounts necessary to create models of human alcoholism. Some researchers have suggested that this is because the animals find the initial taste of alcohol aversive (Higley et al. in press b). Most humans also initially find the taste of alcohol unpleasant, particularly when the alcohol concentration in a solution is high. Consequently, alcohol is rarely consumed in its pure state. Instead, it is consumed in solutions with low concentrations; the taste often is disguised using colas, fruit juices, and other flavorings. Similarly, in at least some species of nonhuman primates, when the solution is palatable and the concentration of alcohol is under 15 to 20 percent, most animals will consume alcohol at rates producing pharmacological effects. Only about 10 to 20 percent of subjects who have not been subjected to stressful environmental conditions that induce drinking, however, will freely consume palatable alcohol solutions at rates that consistently produce blood alcohol levels greater than the "legal" level of intoxication for most States (Ervin et al. 1990; Higley et al. in press by Kraemer and McKinney 1985). Because few animals consume alcohol at rates higher than the legal intoxication levels, when using nonhuman primates, such as monkeys, researchers must have access to a sizable population of subjects. The cost of maintaining nonhuman primates prevents most research facilities from supporting populations large enough to study excessive alcohol consumption. Most nonhuman primates that have been tested for alcohol preference are Old World species (e.g., baboons, vervet monkeys, and many macaques), although at least one study investigated squirrel monkeys, a New World species that frequently is used in the laboratory (Kaplan et al. 1981). Although consumption rates vary somewhat between species, when an alcohol solution is palatable and freely available, most Old World species consume alcohol in amounts that produce pharmacological effects. Nevertheless, wide differences exist between individuals within a species. However, as discussed below, other factors can increase the amount of alcohol that individual subjects consume. STUDIES OF INDIVIDUAL DIFFERENCES Researchers studying alcohol consumption in nonhuman primates have made significant progress in developing an animal model of alcohol abuse(1) by focusing research on subjects that show high rates of voluntary alcohol consumption. Studies show that once primates' rates of consumption stabilize, average differences in consumption between subjects are markedly consistent over time (Higley et al. 1991; Kraemer and McKinney 1985). A similar phenomenon has been observed in humans. …