Age‐ and Sex‐Dependent Effects of Footshock Stress on Subsequent Alcohol Drinking and Acoustic Startle Behavior in Mice Selectively Bred for High‐Alcohol Preference

A dolescence is a period during which exposure to environmental stimuli can result in enduring changes in physiological and psychological functioning (Spear, 2000). Exposure to stressful experiences, such as physical or emotional abuse, during adolescence is known to be a risk factor for various psychiatric disorders, including anxiety and alcohol-use disorders (see reviews by Arnsten and Shansky, 2004; Enoch, 2006; Heim and Nemeroff, 1999; Koehl et al., 2002). Encouraging progress has been made toward understanding how environmental and genetic factors may interact to influence the risk for alcohol-use disorders in humans (see review by Zimmermann et al., 2007), yet this endeavor has proved to be quite challenging. For instance, genetic factors are known to increase risk for developing alcohol-use disorders (Enoch, 2006) but familial alcoholism is also linked with adolescent alcohol use, which in turn appears to be a risk factor for later alcohol-use disorders regardless of family history (Grant and Dawson, 1998; Grant et al., 2006; King and Chassin, 2007). Exposure to stress during adolescence may further complicate these relationships. For instance, a link exists between degree of perceived stress and adolescent alcohol use (Wagner, 1993), and adolescents with a family history of alcoholism may be exposed to more stressful life experiences due to both genetic and environmental causes (see reviews by Johnson and Leff, 1999; Zimmermann et al., 2007). Potentially confounded factors in human studies, such as history of stress and/or alcohol exposure, can be better controlled in studies using animal models. These models have provided important information on how stress exposure during development may influence risk for psychiatric disorders in humans (see reviews by Heim and Nemeroff, 1999; Koehl et al., 2002). Although there are abundant reports in the animal literature demonstrating effects of early life stress on anxiety- and alcohol-related behavior (e.g., Huot et al., 2001; Ladd et al., 2000; Roman and Nylander, 2005), relatively fewer studies have been conducted in adolescent rodent models. Adolescence in the rodent has been defined as the developmental time period after weaning that encompasses an inexact range of postnatal day (PND) 24-55, with PND 37-48 approximating middle adolescence (Adriani et al., 2002; Hefner and Holmes, 2007; Spear, 2000). Spear et al. have reported that isolate housing, a form of chronic stress that adolescent rodents seem to be particularly vulnerable to (Hall, 1998), did not alter home-cage alcohol consumption in adolescent versus adult rats (Doremus et al., 2005). In another study, adolescent rats exposed to daily footshock stress, during a period of concurrent access to alcohol in the home cage, showed reduced alcohol consumption relative to their adult counterparts exposed to the same stress procedure (Brunell and Spear, 2005). To our knowledge, there are no reports of adolescent stress effects on alcohol drinking behavior in mice. The primary goal of this study was to compare the effects of a stress manipulation during middle adolescence versus adulthood on subsequent alcohol drinking behavior in male and female mice selectively bred for high-alcohol preference. Selectively bred rodent lines provide a useful animal model to examine how environmental influences, such as stress, may interact with genetic vulnerability to influence high alcohol drinking behavior. The stress manipulation in this study consisted of 10 consecutive days of exposure to footshock. Another goal of this study was to determine whether a prior history of footshock stress during adolescence would produce enduring effects on anxiety-related behavior and sensorimotor gating. Anxiety-related behavior and sensorimotor gating were quantified using acoustic startle responses and prepulse inhibition (PPI) of the acoustic startle response both in the absence of, and immediately following, re-exposure to foot-shock stress in adulthood. Enhanced acoustic startle responses have been used as a sensitive index of acute and persistent arousal or anxiety produced by stress in rodents (Conti and Printz, 2003; Davis, 1989, 1990; Dubovicky et al., 2007; Faraday, 2002; Garrick et al., 2001; Servatius et al., 1994). PPI is a cross-species phenotype that reflects sensori-motor gating mechanisms. Disrupted PPI is often used as an index of cognitive deficits that occur with several psychiatric disorders, most notably, schizophrenia (Braff et al., 1978). In both rats and mice, exposure to various forms of stress during development and adulthood have been shown to produce deficits in PPI (e.g., Bakshi and Geyer, 1999; Dubovicky et al., 2007; Ellenbroek et al., 1998; Varty et al., 2006; but see Millstein et al., 2006; Pijlman et al., 2003). It was hypothe-sized that exposure to footshock stress during adolescence would increase subsequent alcohol drinking behavior in HAP1 mice. Further, adolescent mice exposed to footshock stress were expected to show greater behavioral reactivity to subsequent footshock stress exposures in adulthood, as measured by enhanced startle and disrupted PPI. Finally, based on prior studies in rats (Roman et al., 2004) and in HAP1 mice (Chester et al., 2006), we posited that females might be more resistant to the stress manipulation as measured by effects on alcohol drinking, startle responses, and PPI.