Different chronic ethanol exposure regimens in adolescent and adult male rats: effects on tolerance to ethanol-induced motor impairment

Abuse-like toluene exposure during early adolescence alters subsequent ethanol and cocaine behavioral effects and brain monoamines in male mice

Currently, there is a gap in understanding the neurobiological impact early adolescent toluene exposure has on subsequent actions of other drugs. Adolescent (PND 28-32) male Swiss-Webster mice (N = 210) were exposed to 0, 2000, or 4000 ppm of toluene vapor for 30 min/day for 5 days. Immediately following the last toluene exposure (PND 32; n = 15) or after a short delay (PND 35; n = 15), a subset of subjects' brains was collected for monoamine analysis. Remaining mice were assigned to one of two abstinence periods: a short 4-day (PND 36) or long 12-day (PND 44) delay after toluene exposure. Mice were then subjected to a cumulative dose response assessment of either cocaine (0, 2.5, 5, 10, 20 mg/kg; n = 60), ethanol (0, 0.5, 1, 2, 4 g/kg; n = 60), or saline (5 control injections; n = 60). Toluene concentration-dependently increased locomotor activity during exposure. When later challenged, mice exposed previously to toluene were significantly less active after cocaine (10 and 20 mg/kg) compared to air-exposed controls. Animals were also less active at the highest dose of alcohol (4 g/kg) following prior exposure to 4000 ppm when compared to air-exposed controls. Analysis of monoamines and their metabolites using High Pressure Liquid Chromatography (HPLC) within the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum (dSTR), and ventral tegmental area (VTA) revealed subtle effects on monoamine or metabolite levels following cumulative dosing that varied by drug (cocaine and ethanol) and abstinence duration. Our results suggest that early adolescent toluene exposure produces behavioral desensitization to subsequent cocaine-induced locomotor activity with subtle enhancement of ethanol's depressive effects and less clear impacts on levels of monoamines.

Impact of adolescent intermittent ethanol exposure in male and female rats on social drinking and neuropeptide gene expression

BACKGROUND

Alcohol use during adolescence can alter maturational changes that occur in brain regions associated with social and emotional responding. Our previous studies have shown that adult male, but not female rats demonstrate social anxiety-like alterations and enhanced sensitivity to ethanol-induced social facilitation following adolescent intermittent ethanol exposure (AIE). These consequences of AIE may influence adult social drinking in a sex-specific manner.

METHODS

To test the effects of AIE on social drinking, male and female Sprague-Dawley rats exposed to water or ethanol (0 or 4 g/kg, intragastrically, every other day, between postnatal day [P] 25 and 45) were tested as adults (P72-83) in a social drinking paradigm (30-minute access to a 10% ethanol solution in supersac or supersac alone in groups of three same-sex littermates across two 4-day cycles separated by 4 days off). Social behavior was assessed during the last drinking session, along with assessment of oxytocin (OXT), oxytocin receptor (OXTR), vasopressin (AVP), and vasopressin receptors 1a and 1b (AVPR1a, AVPR1b) in the hypothalamus and lateral septum.

RESULTS

Males exposed to AIE consumed more ethanol than water-exposed controls during the second drinking cycle, whereas AIE did not affect supersac intake in males. AIE-exposed females consumed less ethanol and more supersac than water-exposed controls. Water-exposed females drinking ethanol showed more social investigation and significantly higher hypothalamic OXTR, AVP, and AVPR1b gene expression than their counterparts ingesting supersac and AIE females drinking ethanol. In males, hypothalamic AVPR1b gene expression was affected by drinking solution, with significantly higher expression evident in males drinking ethanol than those consuming supersac.

CONCLUSIONS

Collectively, these findings provide new evidence regarding sex-specific effects of AIE on social drinking and suggest that the hypothalamic OXT and AVP systems are implicated in the effects of ingested ethanol on social behavior in a sex- and adolescent-exposure-dependent manner.

How to study sex differences in addiction using animal models

The importance of studying sex as a biological variable in biomedical research is becoming increasingly apparent. There is a particular need in preclinical studies of addiction to include both sexes, as female animals are often excluded from studies, leaving large gaps in our knowledge of not only sex differences and potential prevention and treatment strategies but also with regard to the basic neurobiology of addiction. This review focuses on methodology that has been developed in preclinical studies to examine sex differences in the behavioral aspects and neurobiological mechanisms related to addiction across the full range of the addiction process, including initiation (acquisition), maintenance, escalation, withdrawal, relapse to drug seeking and treatment. This review also discusses strategic and technical issues that need to be considered when comparing females and males, including the role of ovarian hormones and how sex differences interact with other major vulnerability factors in addiction, such as impulsivity, compulsivity and age (adolescent versus adult). Novel treatments for addiction are also discussed, such as competing non-drug rewards, repurposed medications such as progesterone and treatment combinations. Practical aspects of conducting research comparing female and male animals are also considered. Making sex differences a point of examination requires additional effort and consideration; however, such studies are necessary given mounting evidence demonstrating that the addiction process occurs differently in males and females. These studies should lead to a better understanding of individual differences in the development of addiction and effective treatments for males and females.

Group 1 mGlu-family proteins promote neuroadaptation to ethanol and withdrawal-associated hippocampal damage

BACKGROUND

Group 1 mGlu-family proteins (i.e., mGlu) consist of mGlu1 and mGlu5 and their activity may influence voluntary ethanol intake. The present studies sought to examine the influence of these receptors on the development of ethanol dependence using in vitro and in vivo models of chronic, intermittent ethanol (CIE).

METHODS

Rat hippocampal explants were exposed to CIE with or without the addition of mGlu1 antagonist (7-hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt; 0.5, 1, and 3μM) or mGlu5 antagonist (E)-2-methyl-6-styryl-pyridine (SIB-1893; 20, 100, and 200μM) to assess sparing of withdrawal-induced cytotoxicity. In a separate study, adult male rats were administered CIE with or without the addition of oral administration of group 1 mGlu antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 3mg/kg). Blood ethanol levels (BELs) were determined at 0930h on Day 2 of Weeks 1, 2, and 3. Withdrawal behavior was monitored during Day 6 of the third consecutive withdrawal.

RESULTS

CIE produced significant hippocampal cytotoxicity. These effects were attenuated by co-exposure to CPCCOEt (3μM) with ethanol in the CA3. By contrast, these effects were blocked by SIB-1893 (20μM) in each primary cell layer. Oral administration of MPEP with ethanol significantly attenuated behavioral effects of subsequent withdrawal and reduced BELs.

CONCLUSIONS

These data demonstrate that ethanol activates group 1 mGlu-family proteins to promote withdrawal-associated cytotoxicity in vitro and physical dependence in vivo. These findings suggest that group 1 mGlu-family proteins may be therapeutic targets for treatment of alcohol use disorders.

Alcohol use across the lifespan: An analysis of adolescent and aged rodents and humans

Adolescence and old age are unique periods of the lifespan characterized by differential sensitivity to the effects of alcohol. Adolescents and the elderly appear to be more vulnerable to many of alcohol's physiological and behavioral effects compared to adults. The current review explores the differential effects of acute alcohol, predominantly in terms of motor function and cognition, in adolescent and aged humans and rodents. Adolescents are less sensitive to the sedative-hypnotic, anxiolytic, and motor-impairing effects of acute alcohol, but research results are less consistent as it relates to alcohol's effects on cognition. Specifically, previous research has shown adolescents to be more, less, and similarly sensitive to alcohol-induced cognitive deficits compared to adults. These equivocal findings suggest that learning acquisition may be differentially affected by ethanol compared to memory, or that ethanol-induced cognitive deficits are task-dependent. Older rodents appear to be particularly vulnerable to the motor- and cognitive-impairing effects of acute alcohol relative to younger adults. Given that alcohol consumption and abuse is prevalent throughout the lifespan, it is important to recognize age-related differences in response to acute and long-term alcohol. Unfortunately, diagnostic measures and treatment options for alcohol dependence are rarely dedicated to adolescent and aging populations. As discussed, although much scientific advancement has been made regarding the differential effects of alcohol between adolescents and adults, research with the aged is underrepresented. Future researchers should be aware that adolescents and the aged are uniquely affected by alcohol and should continue to investigate alcohol's effects at different stages of maturation.

GABAergic contributions to alcohol responsivity during adolescence: insights from preclinical and clinical studies

There is a considerable body of literature demonstrating that adolescence is a unique age period, which includes rapid and dramatic maturation of behavioral, cognitive, hormonal and neurobiological systems. Most notably, adolescence is also a period of unique responsiveness to alcohol effects, with both hyposensitivity and hypersensitivity observed to the various effects of alcohol. Multiple neurotransmitter systems are undergoing fine-tuning during this critical period of brain development, including those that contribute to the rewarding effects of drugs of abuse. The role of developmental maturation of the γ-amino-butyric acid (GABA) system, however, has received less attention in contributing to age-specific alcohol sensitivities. This review integrates GABA findings from human magnetic resonance spectroscopy studies as they may translate to understanding adolescent-specific responsiveness to alcohol effects. Better understanding of the vulnerability of the GABA system both during adolescent development, and in psychiatric conditions that include alcohol dependence, could point to a putative mechanism, boosting brain GABA, that may have increased effectiveness for treating alcohol use disorders.

Consequences of adolescent or adult ethanol exposure on tone and context fear retention: effects of an acute ethanol challenge during conditioning

BACKGROUND

An acute ethanol (EtOH) challenge prior to fear conditioning typically disrupts fear retention to contextual cues to a greater degree than fear retention to a discrete tone cue, and adolescent rats are less sensitive than adults to these EtOH-induced disruptions of context fear memory. Given that some research suggests that repeated EtOH exposure during adolescence may "lock-in" adolescent-typical EtOH sensitivity into adulthood, the purpose of this study was to determine whether adults exposed to EtOH as adolescents would be less sensitive to EtOH-induced disruptions of context fear.

METHODS

Male Sprague-Dawley rats were given 4 g/kg intragastric EtOH (25% v/v) or water every 48 hours for a total of 11 exposures during adolescence (postnatal day [P] 28 to 48) or adulthood (P70-90). After a 22-day non-EtOH period, animals were acutely challenged with 1 g/kg intraperitoneal EtOH or saline 10 minutes prior to tone or context (noncued) fear conditioning. Tone and context fear retention was subsequently examined.

RESULTS

Regardless of age or exposure history, typical deficits in context fear retention were evident after EtOH challenge during conditioning. Similarly, tone fear retention was disrupted in all animals that were trained in the presence of EtOH, which was somewhat surprising given the relative resistance of tone fear retention to an acute EtOH challenge.

CONCLUSIONS

These results do not support the notion of a "lock-in" of adolescent-typical EtOH sensitivity as there was no influence of exposure age on sensitivity to the disruptive effects of an acute EtOH challenge. Thus, it appears that not all adolescent-like EtOH sensitivities persist into adulthood after prior EtOH exposure during adolescence.

Age differences in ethanol discrimination: acquisition and ethanol dose generalization curves following multiple training conditions in adolescent and adult rats

BACKGROUND

Adolescents and adults vary in sensitivity to many effects of ethanol (EtOH), although it is unknown whether they also differ in their perception of EtOH's subjective cues. This study characterized EtOH discrimination in adolescent and adult male rats using a rapidly acquired Pavlovian conditioned approach procedure.

METHODS

EtOH at 1 of the 3 training doses (0.75, 1.0, or 1.25 g/kg) served as either a positive (POS) or negative (NOS) occasion setter. Each 20-minute training session consisted of eight 15-second presentations of 2 cue lights located on either side of a dipper delivering chocolate Boost(®) . For POS-trained rats, the cue lights reliably predicted 5-second presentations of chocolate Boost during EtOH but not saline sessions, with the opposite contingencies used for NOS-trained rats. Anticipatory approach behavior (head entries into the reward delivery area) in the presence and absence of the cue lights was used to calculate discrimination scores on EtOH and saline sessions. Following acquisition, various doses of EtOH (0 to 1.5 g/kg) were administered to establish generalization curves.

RESULTS

Although animals of both ages responded differentially on EtOH and saline sessions by the end of acquisition, adults met criterion more quickly and had higher discrimination scores during reinforced sessions than adolescents. Whereas adolescents failed to demonstrate any dose-dependent responding during testing when trained with the 0.75 or 1.25 g/kg EtOH doses, adults demonstrated broader EtOH generalization during testing sessions following training with all 3 EtOH doses. Among adolescents trained with 1.0 g/kg EtOH, less generalization occurred relative to adults.

CONCLUSIONS

Adolescents were less sensitive to EtOH's interoceptive effects, indicating that EtOH is likely a more salient cue for adults than for adolescents. These findings contribute to evidence that suggests adolescent-typical insensitivity to internal cues that typically limit EtOH consumption may contribute to the elevated intake commonly reported during this developmental period.

Genetic studies of acute tolerance, rapid tolerance, and drinking in the dark in the LXS recombinant inbred strains

BACKGROUND

We hypothesized that rapid tolerance (1-day tolerance) for the duration of the loss of righting reflex ("sleep time" [ST]) was mediated by an increase in acute functional tolerance (AFT). We also hypothesized that increased AFT would correspond to increased drinking. These questions were addressed using the LXS recombinant inbred mouse strain panel.

METHODS

Mice were given a pretreatment dose of either saline or 5 g/kg alcohol on day 1. On day 2, mice were tested for ST (4.1 g/kg) using a method with which it is possible to accurately assess AFT. Genetic correlation analysis was conducted among the ST-related variables and also with "drinking in the dark" (DID) which was previously measured by Saba and colleagues (2011).

RESULTS

Saline-pretreated mice showed a continuous distribution of ST ranging from ~40 minutes to over 3 hours. Of the 43 strains tested, 9 showed significantly decreased ST after alcohol pretreatment, while in 3 strains, ST was significantly increased. AFT scores ranged from 0 to over 200 mg% in the saline group, and in the alcohol group, 8 strains showed a significant increase in AFT and 2 strains showed significant decrease in AFT. In the saline group, AFT was significantly correlated with ST (r = -0.47), but not in the alcohol group (r = -0.22). DID was significantly correlated with only AFT in the alcohol pretreated group (r = 0.64).

CONCLUSIONS

The results suggest that AFT is an important component of the overall ST response, but that the alcohol pretreatment-induced change in AFT does not contribute to rapid ST tolerance. The significant correlation between DID and AFT in the alcohol group suggests that AFT may be a more relevant predictor of drinking behavior than the static measurement of ST. Moreover, preexposure to alcohol seems to change AFT in a way that makes it an even stronger predictor of drinking behavior.

Sex and age differences in heavy binge drinking and its effects on alcohol responsivity following abstinence

Binge drinking during adolescence may perturb the maturing neuroenvironment and increase susceptibility of developing an alcohol use disorder later in life. In the present series of experiments, we utilized a modified version of the drinking in the dark-multiple scheduled access (DID-MSA) procedure to study how heavy binge drinking during adolescence alters responsivity to ethanol later in adulthood. Adult and adolescent C57BL/6J (B6) and DBA/2J (D2) males and females were given access to a 20% ethanol solution for 3 hourly periods, each separated by 2h of free water access. B6 adults and adolescents consumed 2 to 3.5 g/kg ethanol an hour and displayed significant intoxication and binge-like blood ethanol concentrations. There was an interaction of sex and age, however, driven by high intakes in adult B6 females, who peaked at 11.01 g/kg. Adolescents of both sexes and adult males never consumed more than 9.3 g/kg. D2 mice consumed negligible amounts of alcohol and showed no evidence of intoxication. B6 mice were abstinent for one month and were retested on the balance beam 10 min following 1.75 g/kg ethanol challenge (20%v/v; i.p). They were also tested for changes in home cage locomotion immediately following the 1.75 g/kg dose (for 10 min prior to balance beam). Although there was no effect of age of exposure, all mice with a binge drinking history demonstrated a significantly dampened ataxic response to an ethanol challenge. Female mice that binge drank during adulthood showed a significantly augmented locomotor response to ethanol when compared to their water drinking controls. This alteration was not noted for males or for females that binge drank during adolescence. These results highlight the importance of biological sex, and its interaction with age, in the development of behavioral adaptation following binge drinking.

Adolescent brain development and underage drinking in the United States: identifying risks of alcohol use in college populations

Alcohol use typically is initiated during adolescence, a period that coincides with critical structural and functional maturation of the brain. Brain maturation and associated improvements in decision making continue into the third decade of life, reaching a plateau within the period referred to as emerging adulthood (18-24 years). This particular period covers that of traditionally aged college students, and includes the age (21 years) when alcohol consumption becomes legal in the United States. This review highlights neurobiological evidence indicating the vulnerabilities of the emerging-adult brain to the effects of alcohol. Factors increasing the risks associated with underage alcohol use include the age group's reduced sensitivity to alcohol sedation and increased sensitivity to alcohol-related disruptions in memory. On the individual level, factors increasing those risks are a positive family history of alcoholism, which has a demonstrated effect on brain structure and function, and emerging comorbid psychiatric conditions. These vulnerabilities-of the age group, in general, as well as of particular individuals-likely contribute to excessive and unsupervised drinking in college students. Discouraging alcohol consumption until neurobiological adulthood is reached is important for minimizing alcohol-related disruptions in brain development and decision-making capacity, and for reducing the negative behavioral consequences associated with underage alcohol use.