Ethanol-induced conditioned taste aversion in male sprague-dawley rats: impact of age and stress.

Adolescence is a developmental period characterized by considerable transformations that are highly conserved across species. In addition to neural and hormonal changes, increases in social activity, novelty seeking, impulsivity and risk-taking are frequently reported during this transitional phase (see Spear, 2000; 2010). In line with these behavioral changes, alcohol use is commonly initiated during adolescence. Survey results from the 2008 Monitoring the Future study indicate that as many as 72% of high school seniors have tried alcohol at least once and that nearly 30% have been drunk in the past 30 days (Johnston et al., 2009). Elevated ethanol intake is characteristic of rodent models of adolescence as well, using a conservative age range (postnatal days [P] 28 to 42) to typify the developmental period during which adolescent-characteristic neural and behavioral features are evident in both males and females (Spear, 2000). Adolescent rats often voluntarily consume 2–3 times more ethanol than do adults (Brunell and Spear, 2005; Doremus et al., 2005; Vetter et al., 2007). For this reason, animal models of adolescence are particularly useful in examining factors that may contribute to the age-related elevation of ethanol consumption, given ethical constraints limiting this line of research in human subjects. There are several age-specific characteristics that may lead to elevated ethanol consumption during adolescence. For example, adolescents appear to be more sensitive to the social-facilitating effects of ethanol (Varlinskaya and Spear, 2002) and less sensitive than adults to many of the aversive consequences of ethanol that may serve as cues to curb intake, such as motor impairment (Hollstedt et al., 1980; Silveri and Spear, 2001; White et al., 2002), suppression of locomotor activity (Little et al., 1996), social impairment (Varlinskaya and Spear, 2002), and sedation (Draski et al., 2001; Moy et al., 1998; Silveri and Spear, 1998; 1999; 2002; 2004). Additionally, adolescent rats experience attenuated anxiogenesis compared to adults following acute ethanol withdrawal (Doremus et al., 2003; Varlinskaya and Spear, 2004). Together these age-related differences in sensitivity to various effects of ethanol may permit or even encourage higher ethanol consumption by adolescents relative to adults. Conditioned taste aversion (CTA) is one method used to assess the dysphoric effects of alcohol and other drugs. This procedure involves the ingestion of a novel-flavored solution (conditioned stimulus; CS) paired with the effects of a particular drug (unconditioned stimulus; US). When given a subsequent opportunity to consume the drug-paired flavor, the degree to which an animal avoids the solution provides an index of the relative dysphoria experienced following the previous encounter with the drug. Supporting this interpretation, Green and Grahame (2008) report that CTA negatively correlates with ethanol intake across a wide variety of strains and selected lines of rats and mice, suggesting that low sensitivity to the aversive properties of ethanol may facilitate high ethanol intake. A recent study in our laboratory (Vetter-O'Hagen et al., 2009) demonstrated an age-related insensitivity to ethanol-induced taste aversion using saccharin as a CS, with both male and female adolescents requiring higher doses of ethanol than their adult counterparts to elicit a CTA. This initial study exploring the role of social context in sensitivity to the dysphonic effects of ethanol reported that adolescent males (but not females) who were exposed to an unfamiliar social partner for 24 hr following the pairing of ethanol with a saccharin solution CS demonstrated an attenuated sensitivity to the aversive properties of ethanol relative to males that remained isolated for this period. While the previously reported reduction in sensitivity to ethanol’s effect in the presence of a partner among adolescent males was interpreted as an effect of social experience during intoxication (Vetter-O’Hagen et al., 2009), we cannot preclude that exposure to an unfamiliar conspecific may have been stressful for a test subject. Therefore, the decreases in sensitivity to the aversive properties of ethanol seen in that CTA paradigm might have been stress-induced. Human adolescents are confronted with a number of social and environmental challenges that are potentially stressful (Buchanan et al., 1992; Spear, 2000), and drinking to cope with problems is associated with extensive alcohol use in adolescence (Cooper et al., 2000). Research conducted in laboratory animals, although still limited, has revealed age-related differences in responsiveness to stressors, with adolescent rodents sometimes displaying increased sensitivity to stressors when indexed via hormonal responses as well as behavioral alterations (Brunell and Spear, 2005; Doremus-Fitzwater et al., 2009; Romeo et al., 2006; Stone and Quartermain, 1997). Given these ontogenetic differences in stress responses, it is possible that exposure to stressors might differentially impact sensitivity to the aversive effects of ethanol in adolescents compared to adults. Some experimental evidence suggests that exposure to stressors either immediately before or after ingestion of the CS may attenuate CTA (Misanin et al., 2006; Bourne et al., 1991; Revusky & Reilly, 1989), likely via an interruption in the learned association between the CS and US. However, no attempts have been made to assess age-related differences in stress-induced alterations using ethanol-induced CTA. Therefore, the effects of two different stressors on ethanol-induced CTA were examined in adolescent and adult animals in the present experiments: Repeated restraint (Experiment 1) and isolate-housing (Experiment 2). An additional explanation for the elevated ethanol consumption commonly reported during adolescence involves possible age-related differences in hedonic sensitivity. In general, consummatory behaviors toward appetitive stimuli (such as food and other natural rewards) increase during adolescence. This change may be due to alterations in the hedonic value of the stimuli; that is, individuals at this age may consume more of a particular reinforcer either because they find it more rewarding or because they find it less rewarding and must consume more in order to achieve the positive consequences. If adolescents do indeed exhibit a partial anhedonia, they may seek out more natural and drug rewards to compensate for this age-related insensitivity to natural reinforcers. Sucrose consumption and preference are measures commonly used as a presumptive index of anhedonia (Katz, 1982; Konkle et al., 2003; Matthews et al., 1995; Willner et al., 1987), given evidence that animals subjected to chronic mild stress (an animal model of depression) typically reduce their intake of sweetened solutions (Gronli et al., 2004; Willner et al., 1987). There is some evidence that sucrose intake is generally more affected by prior stress exposure than that of saccharin (Harris et al., 1998; Gronli et al., 2005). In order to allow for assessment of hedonic sensitivity at both ages, sucrose, but not saccharin (Vetter-O’Hagen et al., 2009) was chosen as the CS in the present study, with the baseline sucrose consumption (prior to ethanol exposure) serving as an index of relative of stress-induced anhedonia. Given that our previous study revealed no sex differences in ethanol-induced CTA in adult animals and decreased sensitivity to the aversive properties of ethanol when intoxication occurred in the presence of the peer in adolescent male but not female rats (Vetter-O’Hagen et al., 2009), only male subjects were used in the present experiments.