Research reportImpulsive choice and anxiety-like behavior in adult rats exposed to chronic intermittent ethanol during adolescence and adulthood
Introduction
The high level of alcohol binge drinking early in life (17% among adolescents aged 12–20 years and 40% among young adults aged 18–25 years) remains an important public health concern [1]. Younger drinkers (<25 years), categorized as type 2 alcoholics based on age of drinking onset and personality type [2], are characterized by high levels of impulsivity and novelty seeking and low levels of harm avoidance. In contrast, type 1 alcoholics (>25 years) start heavy drinking later in life and show low levels of novelty seeking and high levels of harm avoidance. Anxiety-like behavior is another behavioral trait that may be affected by drinking (for review, see [3]). Differences in personality traits, including impulsivity and anxiety, between type 1 and type 2 alcoholics may precede drinking or may result from heavy alcohol use during either adolescence or adulthood. However, studies investigating the long-term consequences of drinking during adolescence or adulthood on impulsivity and anxiety-related behaviors are very limited.
High self-reported impulsivity has been documented in alcohol-dependent adults [4], adolescents [5], and college students [6]. A small number of studies have investigated the impact of drinking on multiple aspects of impulsive behavior in laboratory settings. The delay discounting task (DDT) is commonly used in both human and animal studies to measure impulsive choice [7]. In this task, impulsivity is defined and measured as the preference for a smaller immediate reward over a larger delayed reward [8], [9]. Increased impulsive choice has been reported in adult abstinent alcoholics and heavy drinkers compared with light drinkers and control subjects [10], [11], [12]. Acute alcohol intoxication decreased impulsive choice in healthy undergraduate students [13] but increased impulsive choice in healthy adults [14] and non-dependent alcohol drinkers in a laboratory [15], [16] or bar setting [17] suggesting age-dependent effects of acute ethanol on impulsive choice. In experimental animals without a history of ethanol exposure, an acute ethanol challenge increased impulsive choice in non-selected adult rats [18], [19], [20], [21] and in rats and mice bred for high but not low ethanol drinking [22], [23]; but see [24]. However, the long-term effects of binge ethanol exposure during adolescence or adulthood on impulsive choice during adulthood have been largely unexplored.
Increased anxiety and an enhanced startle response are often associated with ethanol withdrawal [25], [26], [27], [28]. In contrast to the majority of physical ethanol withdrawal symptoms that usually disappear within a few days, increased anxiety may last for months and even years, resulting in relapse to drinking [26], [29]. In both humans and animals, anxiety-like behavior can be assessed in the light-potentiated startle (LPS) procedure, in which startle responses are measured in successive sessions, during which the startle chambers are either dark or brightly lit. The startle response is potentiated by the aversive bright light in rodents. The degree to which light enhances startle reactivity is used as an operational measure of anxiety, and this response is selectively reduced by anxiolytic compounds [30], [31]. Work in our laboratory has shown that LPS was increased during spontaneous nicotine withdrawal in rats [32]. To the best of our knowledge, no studies have assessed LPS during ethanol withdrawal in rats with or without a history of previous ethanol exposure.
The aim of the present work was to investigate impulsive choice and anxiety-like behavior in rats exposed to chronic intermittent ethanol (CIE) during either adolescence or adulthood. To ensure ethanol exposure during the developmentally sensitive adolescent period, male Wistar rats were exposed to CIE throughout adolescence (PND28–53), broadly defined from postnatal day (PND) 28 to PND42 or PND60 in males [33]. Adult rats were exposed to an identical CIE regimen during adulthood (PND146–171) to determine whether the effects of CIE exposure on impulsivity and anxiety are specific to ethanol exposure during adolescence. CIE exposure consisted of ethanol binges for two consecutive days at 48-h intervals of abstinence for 25 days. Similar intermittent ethanol administration regimens produced inflammatory brain damage and long-term alterations in cognitive and motor function [34] and resulted in tolerance to the hypnotic effects of an ethanol challenge in rats during adulthood [35]. Thus, we hypothesized that CIE exposure during adolescence or adulthood would have long-lasting effects on impulsivity and anxiety. The effects of CIE exposure on impulsivity were assessed using the DDT. We assessed impulsive choice under baseline conditions and after acute ethanol challenges in adult rats exposed to CIE during adolescence or adulthood. Anxiety-like behavior was assessed in the LPS procedure. This procedure also provides measures of the acoustic startle response (ASR). The ASR has been used to characterize arousal during ethanol withdrawal in both humans [25], [36] and rodents [37], [38]. LPS and the ASR were assessed in both adult and adolescent rats during withdrawal from CIE exposure as well as during withdrawal from a 4-day ethanol binge in adulthood. Both CIE and 4-day ethanol binge exposures produced an average blood ethanol concentration (BEC) of 300 mg/dl, mimicking heavy alcohol use in humans [39].
Section snippets
Animals
Two cohorts of 12 pregnant female Wistar rats (Charles River, Raleigh, NC, USA) arrived in the laboratory on gestational day 13. Male rats were weaned from each litter at 21 days of age (PND21; average body weight, 83.7 ± 1.8 g), assigned to the adolescent experimental groups (water and CIE), and tested in two cohorts (n = 50 total). Another 26 adult male Wistar rats (Charles River, Raleigh, NC, USA) arrived in the laboratory on PND 134 (average body weight, 361.4 ± 4.2 g) and were assigned to the
Ethanol dose, body weights, behavioral intoxication scores, and blood ethanol concentrations during chronic intermittent ethanol exposure
During CIE exposure, two rats from the adolescent group and three rats from the adult group died before the completion of ethanol exposure, resulting in 48 rats in the adolescent group (Cohort 1 and Cohort 2 combined) and 23 rats in the adult group that were included in the statistical analyses. In the analyses reported in this section, the data from both adolescent cohorts were combined (see rationale in the Supplementary Materials).
The ANOVAs of the ethanol dose administered revealed
Discussion
Our findings demonstrate that withdrawal from chronic intermittent ethanol exposure either during adolescence or adulthood decreased both the ASR and LPS, indicating decreased arousal and anxiety-like behavior, respectively. In contrast, withdrawal from exposure to the 4-day ethanol binge during adulthood increased LPS in older adult rats with previous CIE/water exposure during adulthood, indicating increased anxiety-like behavior. Exposure to a 4-day ethanol binge had no effect on LPS in
Conclusions
Our findings indicate that withdrawal from exposure to CIE decreased anxiety-like behavior in both adult and adolescent rats. Exposure to a single ethanol binge in adulthood increased anxiety-like behavior in rats previously exposed to CIE or water during adulthood, whereas it had no effect in rats previously exposed to CIE/water during adolescence. These findings suggest that drinking during adulthood may be promoted by increased anxiety in rats exposed to ethanol during adulthood and by
Conflict of interest
SS, NB and JMT, have nothing to disclose. AM has received contract research support from Bristol-Myers Squibb Co., Forest Laboratories, and Astra-Zeneca and honoraria/consulting fees from AbbVie during the past 3 years. There are no actual or potential financial conflicts of interest.
Acknowledgments
This work was supported by National Institutes of Health grant U01-AA019970-NADIA Project to AM and a UC MEXUS-CONACYT fellowship to JMT. The NIH and UC MEXUS-CONACYT had no role in the study design, data collection, data analysis, data interpretation, writing of the report, or decision to submit the article for publication.
The authors would like to thank Dr. Xia Li and Mrs. Jessica Benedict for excellent technical assistance with LPS data collection and analyses and Mr. Michael Arends for
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