Controlled and behaviorally relevant levels of oral ethanol intake in rhesus macaques using a flavorant-fade procedure.

Forty four percent of the adult U.S. population (age 18 and over) drink alcohol and consume at least 12 drinks per year (Dawson et al., 1995). Although most individuals drink alcohol in a responsible manner, approximately 14 million Americans (17.4 percent) meet the criteria for alcohol abuse and alcoholism (Grant et al., 1994) and more than one-half of American adults have a close family member who has or has had alcoholism (Dawson and Grant, 1998). It is difficult to determine conclusively that reported perturbations in behavior, and perhaps physiology, in alcohol abusing populations are a direct result of exposure to alcohol. In many cases for example, individuals at risk for alcoholism and/or alcohol abuse may exhibit preexisting differences on the measure in question. Animal models are useful in controlling variables such as the amount of ethyl alcohol (ethanol) consumed, the duration of exposure and concurrent use of other illicit drugs. Nonhuman primates are particularly useful for evaluating the neurobiological consequences of alcoholism and alcohol abuse because many of the physiological, neuroanatomical and behavioral systems potentially affected by ethanol are more similar to humans in nonhuman primates in comparison to commonly employed rodent species. In addition, the protracted life span of nonhuman primates makes it possible to perform more extensive and elaborate studies to determine the long-term consequences of ethanol exposure. Previous experiments have shown that rhesus monkeys will readily consume low concentrations of ethanol (1 and 2% (w/v)) in tap water in 3 hr sessions without any training history (Stewart et al., 1996), however consumption under such conditions is typically under 0.5 grams of ethanol per kilogram of bodyweight (g/kg). This level of intake produces blood alcohol levels (BALs) of substantially less than the 0.08 BAC (i.e., 80 mg%) level which constitutes the legal limit to operate an automobile in most jurisdictions in the United States. However, oral ethanol consumption in nonhuman primates can be greatly enhanced by a variety of induction techniques. One such technique used to induce oral ethanol consumption in nonhuman primates is to gradually increase the ethanol concentration across daily 2 to 3 hr sessions until the desired concentration (e.g., 4-8%) is reached (Meisch, 1995; Stewart et al., 1996). Using such procedures, monkeys will consume an average of 1.2 to 1.5 g/kg of ethanol (Pakarinen et al., 1999; Stewart et al., 1996; Williams et al., 1998; Williams and Woods, 1999b). These studies also illustrate two findings consistent across most of the methodological techniques reviewed here. First, individual macaque consumption preferences vary widely, with some monkeys (broadly approximating 25-33%) failing to consume significant amounts ( 2.0 g/kg). Second, while intake patterns within individuals are reasonably stable from day to day, it is possible to observe differences on the order of 0.5-1.5 g/kg from one session to another. Thus, the presentation of gradually increasing concentrations of ethanol to nonhuman primates is effective in inducing higher ethanol intake, however, these levels are below behaviorally relevant levels for significant fractions of the sample. Another procedure which may increase ethanol consumption in nonhuman primates is to make the ethanol available in combination with a meal. Such postprandial availability techniques are an effective way to induce ethanol consumption in both rhesus monkeys and baboons (Ator and Griffiths, 1983; Henningfield et al., 1981; Meisch and Henningfield, 1977). It has also been shown that food deprivation increases ethanol consumption in rhesus monkeys (Macenski and Meisch, 1992; Meisch and Lemaire, 1991), i.e., the majority of monkeys increased their ethanol intake when they were food restricted compared to when they were food satiated, although the average ethanol intake was only 0.25 g/kg (Meisch and Lemaire, 1991). However, in a similar study, rhesus monkeys that were slightly food restricted drank the same amount of ethanol as those that were not food restricted with an average ethanol intake of 1.5 g/kg (Pakarinen et al., 1999). These studies indicate that the presentation of gradually increasing concentrations of ethanol and post-prandial drinking can produce significant levels of ethanol intake, however, food restriction does not always enhance ethanol consumption in nonhuman primates. Much as with humans, the addition of a flavorant and/or sweetener to ethanol can initiate and maintain significant levels of ethanol consumption in monkeys, especially at ethanol concentrations above 2% (w/v) (Crowley et al., 1983; Crowley et al., 1990; Erwin et al., 1979; Fincham et al., 1986; Fitz-Gerald et al., 1968; Grant and Johanson, 1988; Higley et al., 1996; Shelton and Grant, 2001; Vivian et al., 1999; Williams and Woods, 1999a). Rhesus monkeys have been shown to consume 0.8 g/kg of ethanol sweetened with aspartame in 60 min sessions (Higley et al., 1996), 1.1 g/kg of ethanol in orange juice in 40 min sessions (Cadell and Cressman, 1972) and 1.2 g/kg of ethanol in grape drink in 100 min sessions (Cressman and Cadell, 1971). A group of pigtail macaques consumed 1.4 g/kg of 5% ethanol in grape-flavored, saccharin sweetened Kool-Aid over 2 hrs (Crowley et al., 1983) and Japanese snow monkeys consumed 0.5 to 2.0 g/kg of ethanol, attaining BALs sometimes in excess of 100 mg% with a similar protocol (Crowley et al., 1990). The initial presentation of ascending concentrations of ethanol sweetened with aspartame later produced an average unsweetened ethanol intake of 1.9 g/kg in rhesus monkeys versus 0.9 g/kg ethanol in a second group of oral methadone experienced rhesus monkeys (Vivian et al., 1999) in a post-prandial procedure. These findings suggest that sweetened and/or flavored ethanol solutions facilitate oral ethanol intake in several different species of nonhuman primates, presumably by masking the aversive taste of ethanol (for review see Meisch and Stewart, 1994), however, it has also been proposed that animals may consume sweetened ethanol solutions as a result of the intrinsic reinforcing properties of sweet solutions. See Grant and Bennett (2003) for a recent exhaustive review of alcohol self-administration in nonhuman primates. The major hypothesis under investigation in the present study was that a flavorant-fading procedure will maintain relatively high, stable levels of ethanol intake in rhesus monkeys. Similar procedures (i.e., initially presenting the ethanol at very low concentrations in a palatable sucrose or saccharin solution and then gradually fading in greater concentrations of ethanol across a number of sessions) have been used extensively to induce oral ethanol intake in rats, including in strains which will not otherwise consume pharmacologically relevant amounts of ethanol (Katner et al., 1999; Katner and Weiss, 1999; Samson et al., 1998; Samson et al., 1989), however this procedure has less frequently been applied to nonhuman primates. The present study is innovative in adapting several techniques from previous nonhuman primate oral ethanol self-administration studies and incorporated a procedure which has not been well characterized in monkeys, a sweet solution ethanol fading procedure, to produce significant levels of ethanol intake. Since the goal was to induce and maintain consistent high levels of drinking, rather than to demonstrate evidence of ethanol-seeking, the flavorant was not faded out of the solutions. Another goal of this study was to use alternative induction techniques in animals that fail to achieve or maintain sufficiently high levels of ethanol intake using the fading procedure. Finally, this study sought to determine the behaviorally impairing effects of different amounts of orally consumed ethanol (g/kg) (i.e. dose-dependent effects) on fine motor coordination using a bimanual motor skill test.