Chronic intermittent ethanol exposure during adolescence: Effects on social behavior and ethanol sensitivity in adulthood

Abstract

This study assessed long-lasting consequences of repeated ethanol exposure during two different periods of adolescence on 1) baseline levels of social investigation, play fighting, and social preference and 2) sensitivity to the social consequences of acute ethanol challenge. Adult male and female Sprague-Dawley rats were tested 25 days after repeated exposure to ethanol (3.5 g/kg intragastrically [i.g.], every other day for a total of 11 exposures) in a modified social interaction test. Early-mid adolescent intermittent exposure (e-AIE) occurred between postnatal days (P) 25 and 45, whereas late adolescent intermittent exposure (l-AIE) was conducted between P45 and P65. Significant decreases in social investigation and social preference were evident in adult male rats, but not their female counterparts following e-AIE, whereas neither males nor females demonstrated these alterations following l-AIE. In contrast, both e-AIE and l-AIE produced alterations in sensitivity to acute ethanol challenge in males tested 25 days after adolescent exposure. Ethanol-induced facilitation of social investigation and play fighting, reminiscent of that normally seen during adolescence, was evident in adult males after e-AIE, whereas control males showed an age-typical inhibition of social behavior. Males after l-AIE were found to be insensitive to the socially suppressing effects of acute ethanol challenge, suggesting the development of chronic tolerance in these animals. In contrast, females showed little evidence for alterations in sensitivity to acute ethanol challenge following either early or late AIE. The results of the present study demonstrate a particular vulnerability of young adolescent males to long-lasting detrimental effects of repeated ethanol. Retention of adolescent-typical sensitivity to the socially facilitating effects of ethanol could potentially make ethanol especially appealing to these males, therefore promoting relatively high levels of ethanol intake later in life.

Introduction

In humans, initiation of alcohol use occurs predominantly during early adolescence (Faden, 2006), with high drinking levels seen in some individuals. For instance, approximately 5.1% of 8th graders, 15.6% of 10th graders, and 23.7% of high school seniors in the United States reported a binge pattern of drinking (5+ drinks in a row) in the previous 2 weeks (Johnston, O'Malley, Bachman, & Schulenberg, 2013). Adolescents who engage in even episodic heavy drinking at an early age are more likely to develop alcohol use disorders later in life (Bonomo et al., 2004, Grant et al., 2001, Hingson et al., 2006) and to experience a number of long-lasting adverse psychosocial consequences (Wells, Horwood, & Fergusson, 2004). Although causal relationships remain to be firmly established, binge patterns of alcohol consumption that result in blood alcohol levels of 80 mg/dL and higher are thought to be harmful to the developing adolescent brain (Bava and Tapert, 2010, Silveri, 2012). According to general principles of neurobehavioral toxicology, the brain is often especially vulnerable to long-lasting effects of drugs during its development, with the periods of pronounced developmental changes in brain regions sensitive to a certain drug defining the critical periods for induction of lasting consequences of that drug (Adams et al., 2000). During adolescence, marked changes occur in brain circuits implicated in responsiveness to social and emotional stimuli (Blakemore, 2012), with these brain circuits being sensitive to alcohol as well (Vilpoux, Warnault, Pierrefiche, Daoust, & Naassila, 2009).

It is not surprising, therefore, that alcohol-dependent adolescents have been found to exhibit enhanced negative emotionality, characterized by depression, anxiety, and enhanced stress reactivity (Martin, Lynch, Pollock, & Clark, 2000). Alcohol-related problems are particularly common in adolescents with social anxiety (Buckner et al., 2006, Gilles et al., 2006). Although social anxiety often precedes alcohol use and abuse (Buckner et al., 2008), causal relationships are still not well understood (e.g., Morris, Stewart, & Ham, 2005). Indeed, it is not clear whether chronic alcohol use during adolescence can enhance or even elicit social anxiety or, in contrast, whether alcohol is more appealing for socially anxious adolescents due to its anxiolytic effects (Carrigan & Randall, 2003). Human studies of underage drinking, however, do not permit systematic manipulation of critical variables, due to ethical considerations that preclude administration of alcohol to underage adolescents. Similarities found between human adolescents and adolescents of various mammalian species in terms of developmental history and behavioral changes, as well as neural and hormonal alterations (Spear, 2000, Spear, 2011, Spear and Varlinskaya, 2010), provide reasonable justification for the use of animal models for the assessments of consequences of repeated ethanol exposure during adolescence on anxiety-related behavior under social circumstances, as well as sensitivity to the socially anxiolytic effects of ethanol.

The social interaction test has been used extensively for the assessment of anxiety-like behavior in laboratory rodents (File, 1980, File and Hyde, 1978, File and Seth, 2003). In the conventional social interaction test, a pair of rats is placed into a testing chamber, and overall time spent in social interactions is used as a dependent variable (File, 1980). Yet, the discrete behavioral acts summed together for these assessments (e.g., social investigation and play fighting) reflect behaviorally distinctive and differentially regulated forms of interactive social behaviors with separable ontogenetic patterns (Vanderschuren et al., 1997, Varlinskaya and Spear, 2008, Varlinskaya et al., 1999) and differential responsiveness to seemingly anxiogenic manipulations (Doremus-Fitzwater, Varlinskaya, & Spear, 2009). For instance, play fighting exhibits an inverted U-shaped ontogenetic pattern that peaks around P30–35, whereas social investigation increases ontogenetically and represents a more adult-typical form of social interactions (Panksepp, 1981, Vanderschuren et al., 1997, Varlinskaya et al., 1999). Play fighting, but not social investigation, is drastically increased by deprivation from social contact via isolate housing throughout the entire adolescent period (Vanderschuren et al., 1997, Varlinskaya and Spear, 2008), whereas social investigation is exclusively decreased by prior history of exposure to non-social stressors (Doremus-Fitzwater et al., 2009, Varlinskaya et al., 2010). Taken together, these findings suggest that play fighting and social investigation may be mediated via different neural systems, and hence may be differentially vulnerable to alterations induced by intermittent ethanol exposure during adolescence. Modification of the social interaction test, allowing an experimental animal to freely move toward or away from a non-manipulated social partner in a 2-compartment testing apparatus, permits assessment of social motivation via a preference/avoidance coefficient in addition to measuring the frequency of play fighting and social investigation (Varlinskaya et al., 1999). Using this modified social interaction test, we have found decreases in social preference and/or social investigation to reflect anxiety-like alterations in social interactions (Doremus-Fitzwater et al., 2009, Morales et al., 2013a, Varlinskaya et al., 2010, Varlinskaya and Spear, 2012).

Using a rat model of adolescence and the modified social interaction test, we have shown that repeated ethanol exposure (1.0 g/kg intraperitoneally [i.p.] for 7 days) increases anxiety-like behavior under social circumstances in adolescent but not adult animals, as indexed by decreases in social preference (Varlinskaya & Spear, 2007). Furthermore, these adolescent animals became unusually sensitive to the anxiolytic effects of ethanol, with the decreases in social preference effectively reversed by acute ethanol challenge. Taken together, these findings suggest that increases in anxiety-like behavior and sensitivity to the socially anxiolytic effects of ethanol associated with repeated ethanol exposure are typical for the adolescent developmental period, with no such alterations evident following adult ethanol exposure. In that study, however, animals were tested 48 h following the last ethanol exposure, and hence it is not clear whether the social anxiety-like behavior and changes in ethanol sensitivity induced by repeated ethanol in adolescents are short-lasting and associated merely with ethanol withdrawal or whether these effects can persist for a long time and be detected later in life. In order to assess whether social consequences of adolescent intermittent exposure to ethanol (AIE) are persistent and continue into adulthood (i.e., 25 days after repeated exposure to ethanol), the present study investigated consequences of AIE on 1) baseline levels of play fighting, social investigation, and social preference and 2) sensitivity to the social consequences of acute ethanol challenge in adult male and female rats. Plasma corticosterone (CORT) levels were measured in order to assess possible relationships between anxiety-like behavioral alterations, sensitivity to acute ethanol challenge, and responsiveness of the hypothalamus-pituitary-adrenal (HPA) axis following AIE.

Animals were exposed to ethanol either during early-mid adolescence [postnatal days (P) 25–45; Experiment 1] or late adolescence (P45–65; Experiment 2) and tested in adulthood, 25 days following exposure, in order to assess whether exposure relatively early in adolescence has more detrimental effects than exposure later in adolescence. Rats were exposed to 3.5 g/kg ethanol i.g. every 48 h for a total of 11 exposures, with control animals either receiving water i.g. or left non-exposed. Both of these control groups were included, given that the repeated gavage process may be relatively stressful and might influence the target measures. Indeed, in prior work we have found that repeated stress alters both social behavior and responsiveness to acute ethanol challenge in adolescents and adults (Varlinskaya et al., 2010), with stressed animals demonstrating anxiety-like behavioral alterations, indexed via significant decreases in social investigation and/or social preference, and enhanced sensitivity to socially anxiolytic effects of ethanol.