Effect of sex on ethanol consumption and conditioned taste aversion in adolescent and adult rats

Sugar consumption induces the consummatory suppression of sugary ethanol: Differential effects of sugar restriction according to sex and age

Sweet foods activate the reward system that is essential in processing natural reinforcers. Maturation changes in this system during adolescence are linked to heightened impulsivity and risk-seeking behavior, including the use of drugs like ethanol. This usually starts with the consumption of sugary mixtures. However, the influence of sugar exposure on ethanol consumption remains inconclusive. The present research examines the effect of long-term sugar exposure on sugary ethanol (S-EtOH) preference and net intake, exploring the implications of sex, age, accessor restriction of sugar, and its effect during the transition into adulthood. Wistar rats of both sexes were given 24-hour access to a sugar solution for 21 days during adolescence or adulthood. Subsequently, four preference tests of S-EtOH vs. water were carried out every other day, with or without sugar access between each preference test. Our results demonstrate that continuous acute and long-term sugar access induces a consummatory suppression effect on S-EtOH intake, particularly in adult rats, irrespective of sex. This effect becomes more pronounced with more extended periods of exposure to sugar, leading to a higher prevalence of low consumers. Notably, when sugar access was restricted after high familiarization, the suppression effect in adolescent male rats was reduced. Under these conditions, the rats appeared to be more susceptible to developing a preference for S-EtOH consumption. Furthermore, our longitudinal observations reveal that sugar access or restriction conditions during the transition from adolescence to adulthood play a crucial role in shaping S-EtOH consumption patterns in adulthood.

Attenuated incubation of ethanol-induced conditioned taste aversion in a model of dependence

RATIONALE

Preclinical studies report attenuated ethanol-induced conditioned taste aversion (CTA) following chronic ethanol exposure, suggesting that tolerance develops to the aversive properties of ethanol. However, these studies are confounded by pre-exposure to the unconditioned stimulus (US; ethanol), which is well known to hinder conditioning.

OBJECTIVES

This study was designed to determine whether chronic ethanol exposure produces tolerance to the aversive properties of ethanol in the absence of a US pre-exposure confound.

METHODS

CTA was performed in adult male and female Long-Evans rats by pairing 0.1% ingested saccharin with an intraperitoneal injection of ethanol (1.5 or 2.0 g/kg) or saline. Rats were then rendered ethanol dependent using chronic intermittent ethanol (CIE) vapor exposure. Controls were exposed to room air (AIR). The effect of chronic ethanol on CTA expression and reconditioning were examined following vapor exposure.

RESULTS

Prior to vapor exposure, both sexes developed CTA to a comparable degree with 2.0 g/kg producing greater CTA than 1.5 g/kg ethanol. Following vapor exposure, AIR controls exhibited an increase in CTA magnitude compared to pre-vapor levels. This effect was largely absent in CIE-exposed rats. Re-conditioning after vapor exposure facilitated increased CTA magnitude to a similar degree in AIR- and CIE-exposed males. In contrast, CTA magnitude was unchanged by re-conditioning in females.

CONCLUSIONS

These data suggest that chronic ethanol does not facilitate tolerance to the aversive properties of ethanol but rather attenuates incubation of ethanol-induced CTA. Loss of CTA incubation suggests that CIE exposure disrupts circuits encoding aversion.

Ceftriaxone alters the gut microbiome composition and reduces alcohol intake in male and female Sprague-Dawley rats

Ceftriaxone is an antibiotic that increases central nervous system (CNS) protein expression of the glutamate transporters GLT-1 and xCT and ameliorates pathological behaviors in rodent models of neurological disease and substance use disorder. However, little ceftriaxone passes through the blood-brain-barrier, the CNS binding partner of ceftriaxone is unknown, and ceftriaxone does not consistently upregulate GLT-1 and xCT in cell culture. Ceftriaxone alters the gut microbiome composition in rodents and humans, and the microbiome-gut-brain axis regulates drug-seeking. Thus, here we test the hypothesis that ceftriaxone reduces alcohol intake while ameliorating alcohol-induced disruption of the gut microbiome composition. Male and female Sprague-Dawley rats received intermittent access to alcohol (IAA) while controls received access to only water. Following 17 IAA sessions, ceftriaxone/vehicle treatment was given for 5 days. Analysis of the gut microbiome composition was assessed by 16S rRNA gene amplicon sequencing conducted on fecal pellets collected prior to and after alcohol consumption and following ceftriaxone treatment. Male rats displayed escalated alcohol intake and preference over the course of the 17 sessions; however, total alcohol intake did not differ between the sexes. Ceftriaxone reduced alcohol intake and preference in male and female rats. While alcohol affected a diverse set of amplicon sequencing variants (ASV), ceftriaxone markedly reduced the diversity of microbial communities reflected by a blooming of the Enterococcaceae family. The remaining effects of ceftriaxone, however, encompassed families both affected and unaffected by prior alcohol drinking and highlight the Ruminococcaceae and Muribaculaceae families as bidirectionally modulated by alcohol and ceftriaxone. Altogether, our study confirms that ceftriaxone reduces alcohol intake in rats and partially reverses alcohol-induced dysbiosis.

Cannabidiol tempers alcohol intake and neuroendocrine and behavioural correlates in alcohol binge drinking adolescent rats. Focus on calcitonin gene-related peptide's brain levels

Alcohol binge drinking is common among adolescents and may challenge the signalling systems that process affective stimuli, including calcitonin gene-related peptide (CGRP) signalling. Here, we employed a rat model of adolescent binge drinking to evaluate reward-, social- and aversion-related behaviour, glucocorticoid output and CGRP levels in affect-related brain regions. As a potential rescue, the effect of the phytocannabinoid cannabidiol was explored. Adolescent male rats underwent the intermittent 20% alcohol two-bottle choice paradigm; at the binge day (BD) and the 24 h withdrawal day (WD), we assessed CGRP expression in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), amygdala, hypothalamus and brainstem; in addition, we evaluated sucrose preference, social motivation and drive, nociceptive response, and serum corticosterone levels. Cannabidiol (40 mg/kg, i.p.) was administered before each drinking session, and its effect was measured on the above-mentioned readouts. At BD and WD, rats displayed decreased CGRP expression in mPFC, NAc and amygdala; increased CGRP levels in the brainstem; increased response to rewarding- and nociceptive stimuli and decreased social drive; reduced serum corticosterone levels. Cannabidiol reduced alcohol consumption and preference; normalised the abnormal corticolimbic CGRP expression, and the reward and aversion-related hyper-responsivity, as well as glucocorticoid levels in alcohol binge-like drinking rats. Overall, CGRP can represent both a mediator and a target of alcohol binge-like drinking and provides a further piece in the intricate puzzle of alcohol-induced behavioural and neuroendocrine sequelae. CBD shows promising effects in limiting adolescent alcohol binge drinking and rebalancing the bio-behavioural abnormalities.

Attenuated incubation of ethanol-induced conditioned taste aversion in a model of dependence

Rationale

Preclinical studies report attenuated ethanol-induced conditioned taste aversion (CTA) following chronic ethanol exposure, suggesting that tolerance develops to the aversive properties of ethanol. However, these studies are confounded by pre-exposure to the unconditioned stimulus (US; ethanol), which is well known to hinder conditioning.

Objectives

This study was designed to determine whether chronic ethanol exposure produces tolerance to the aversive properties of ethanol in the absence of a US pre-exposure confound.

Methods

CTA was performed in adult male and female Long-Evans rats by pairing 0.1% ingested saccharin with an intraperitoneal injection of ethanol (1.5 or 2.0 g/kg) or saline. Rats were then rendered ethanol dependent using chronic intermittent ethanol (CIE) vapor exposure. Controls were exposed to room air (AIR). The effect of chronic ethanol on CTA expression and reconditioning were examined following vapor exposure.

Results

Prior to vapor exposure, both sexes developed CTA to a comparable degree with 2.0 g/kg producing greater CTA than 1.5 g/kg ethanol. Following vapor exposure, AIR controls exhibited an increase in CTA magnitude compared to pre-vapor levels. This effect was absent in CIE-exposed rats. These group differences were eliminated upon re-conditioning after vapor exposure.

Conclusions

These data suggest that chronic ethanol does not facilitate tolerance to the aversive properties of ethanol but rather, attenuates incubation of ethanol-induced CTA. Loss of CTA incubation suggests that CIE exposure disrupts circuits encoding aversion.

Impacts of maternal separation stress on ethanol-related responses, anxiety- and depressive-like behaviors in adolescent mice

The present work evaluated the consequences of chronic maternal separation (MS), an animal model of early-life stress, on ethanol intake and striatal Fos expression induced by ethanol consumption. Furthermore, we analyzed MS impacts on anxiety- and depressive-like behaviors and on locomotor and plasma corticosterone responses to intraperitoneal treatment with ethanol in adolescent mice. For that, male and female C57BL/6J mice were exposed or not to MS stress, for 3 h per day, from postnatal day (PND) 1 to 14, and submitted to behavioral tests from PND 28. In Experiment 1, MS and control groups of mice were submitted to an involuntary ethanol intake protocol, and striatal Fos expression following ethanol exposure was analyzed. In Experiment 2, mice behavior was assessed in elevated plus-maze, sucrose splash, saccharin preference, and open field tests. Locomotor and plasma corticosterone responses induced by a systemic dose of ethanol (1.75 g/kg) were also evaluated. Our results demonstrated that MS increased ethanol intake only in an acute manner and did not impact ethanol-induced Fos expression in the dorsal striatum and nucleus accumbens (NAc) core and shell subregions. MS did not change the parameters analyzed during elevated plus-maze, sucrose splash, preference for saccharin, and open field tests. MS did not affect locomotor activity following ethanol injection nor plasma corticosterone response to the drug. Thus, our data showed that MS transiently increased ethanol intake. However, early-life stress did not impact Fos, locomotor, or plasma corticosterone responses to the drug. In addition, MS did not affect anxiety- and depressive-like behaviors in adolescent mice.

Sex- and age-specific effects on the development of addiction and compulsive-like drinking in rats

BACKGROUND

Biological factors are known to influence disease trajectories and treatment effectiveness in alcohol addiction and preclinical and clinical evidence suggests that sex is an important factor influencing disease dynamics in alcohol dependence. Another critical factor is age at first intoxicating drink, which has been identified as a risk factor for later alcohol binging. Preclinical research allows prospective monitoring of rodents throughout the lifespan, providing very detailed information that cannot be acquired in humans. Lifetime monitoring in rodents can be conducted under highly controlled conditions, during which one can systematically introduce multiple biological and environmental factors that impact behaviors of interest.

METHODS

Here, we used the alcohol deprivation effect (ADE) rat model of alcohol addiction in a computerized drinkometer system, acquiring high-resolution data to study changes over the course of addictive behavior as well as compulsive-like drinking in cohorts of adolescent vs. adult as well as male vs. female rats.

RESULTS

Female rats drank more alcohol than male rats during the whole experiment, drinking much more weak alcohol (5%) and similar amounts of stronger alcohol solutions (10%, 20%); female rats also consumed more alcohol than male rats during quinine taste adulteration. Increased consumption in females compared to males was driven by larger access sizes of alcohol. Differences in circadian patterns of movement were observed between groups. Early age of onset of drinking (postnatal day 40) in male rats had surprisingly little impact on the development of drinking behavior and compulsivity (quinine taste adulteration) when compared to rats that started drinking during early adulthood (postnatal day 72).

CONCLUSIONS

Our results suggest that there are sex-specific drinking patterns, not only in terms of total amount consumed, but specifically in terms of solution preference and access size. These findings provide a better understanding of sex and age factors involved in the development of drinking behavior, and can inform the preclinical development of models of addiction, drug development and exploration of options for new treatments.

A translational rodent model of individual differences in sensitivity to the aversive properties of ethanol

Background

A strong relationship exists between individual sensitivity to the aversive properties of ethanol and risk for alcohol use disorder (AUD). Despite this, our understanding of the neurobiological mechanisms underlying subjective response to ethanol is relatively poor. A major contributor to this is the absence of preclinical models that enable exploration of this individual variability similar to studies performed in humans.

Methods

Adult male and female Long-Evans rats were trained to associate a novel tastant (saccharin) with acute exposure to either saline or ethanol (1.5 g/kg or 2.0 g/kg i.p.) over three conditioning days using a standard conditioned taste aversion (CTA) procedure. Variability in sensitivity to ethanol-induced CTA was phenotypically characterized using a median split across the populations studied.

Results

When examining group averages, both male and female rats that had saccharin paired with either dose of ethanol exhibited reduced saccharin intake relative to saline controls of ethanol-induced CTA. Examination of individual data revealed a bimodal distribution of responses uncovering two distinct phenotypes present in both sexes. CTA-sensitive rats exhibited a rapid and progressive reduction in saccharin intake with each successive ethanol pairing. In contrast, saccharin intake was unchanged or maintained after an initial decrease from baseline levels in CTA-resistant rats. While CTA magnitude was similar between male and female CTA-sensitive rats, CTA-resistant females were more resistant to the development of ethanol-induced CTA than their male counterparts. Phenotypic differences were not driven by differences in baseline saccharin intake. CTA sensitivity correlated with behavioral signs of intoxication in only a subset of rats.

Conclusions

These data parallel work in humans by revealing individual differences in sensitivity to the aversive properties of ethanol that emerge immediately after initial exposure to ethanol in both sexes. This model can be leveraged in future studies to investigate the neurobiological mechanisms that confer risk for AUD.

Ethanol-induced conditioned taste aversion and associated neural activation in male rats: Impact of age and adolescent intermittent ethanol exposure

Individuals that initiate alcohol use at younger ages and binge drink during adolescence are more susceptible to developing alcohol use disorder. Adolescents are relatively insensitive to the aversive effects of alcohol and tend to consume significantly more alcohol per occasion than adults, an effect that is conserved in rodent models. Adolescent typical insensitivity to the aversive effects of alcohol may promote greater alcohol intake by attenuating internal cues that curb its consumption. Attenuated sensitivity to the aversive effects of alcohol is also retained into adulthood following protracted abstinence from adolescent intermittent ethanol (AIE) exposure. Despite these effects, much remains unknown regarding the neural contributors. In the present study, we used a conditioned taste aversion (CTA) paradigm to investigate neuronal activation in late-developing forebrain structures of male adolescents and adult cFos-LacZ transgenic rats as well as in AIE adults following consumption of 0.9% sodium chloride previously paired with an intraperitoneal injection of 0, 1.5 or 2.5 g/kg of ethanol. Adults that were non-manipulated or received water exposure during adolescence showed CTA to both ethanol doses, whereas adolescents displayed CTA only to the 2.5 g/kg ethanol dose. Adults who experienced AIE did not show CTA. Adults displayed increased neuronal activation indexed via number of β-galactosidase positive (β-gal+) cells in the prefrontal and insular cortex that was absent in adolescents, whereas adolescents but not adults had a reduced number of β-gal+ cells in the central amygdala. Adults also displayed greater cortical-insular functional connectivity than adolescents as well as insular-amygdalar and prefrontal cortex-accumbens core functional connectivity. Like adolescents, adults previously exposed to AIE displayed reduced prefrontal-insular cortex and prefrontal-accumbal core functional connectivity. Taken together, these results suggest that attenuated sensitivity to the aversive effects of ethanol is related to a loss of an insular-prefrontal cortex-accumbens core circuit.

Desformylflustrabromine (dFBr), a positive allosteric modulator of α4β2 nicotinic acetylcholine receptors decreases voluntary ethanol consumption and preference in male and female Sprague-Dawley rats

Alcohol use disorder is a medical condition that impacts millions of individuals worldwide. Although there are a few pharmacotherapeutic options for alcohol-dependent individuals; there is a need for the development of novel and more effective therapeutic approaches. Alcohol and nicotine are commonly co-abused, and there is evidence that neuronal nicotinic acetylcholine receptors (nAChRs) play a role in both alcohol and nicotine dependence. Desformylflustrabromine (dFBr), a positive allosteric modulator of the α4β2 nAChRs has been shown to reduce nicotine intake, compulsive-like behavior and neuropathic pain in animal models. dFBr has also been previously shown to cross the blood-brain-barrier. We have recently shown that dFBr can attenuate the response to an acute, hypnotic dose of ethanol, via β2 nAchR. Here, we have investigated the effect of dFBr in modulating ethanol consumption using the intermittent access two-bottle choice (IA2BC) model of voluntary ethanol consumption in male and female Sprague Dawley rats. We show that dFBr selectively reduced ethanol but not sucrose consumption in the IA2BC model. Furthermore, dFBr decreased preference for ethanol in both male and female rats. No rebound increase in ethanol intake was observed after the washout period after dFBr treatment. The ability of dFBr to decrease ethanol consumption, along with its previously demonstrated ability to decrease nicotine self-administration in rodents, suggest that dFBr is an attractive therapeutic candidate to target both nicotine and alcohol abuse.

Sex-related differences in pattern of ethanol drinking under the intermittent-access model and its impact on exploratory and anxiety-like behavior in Long-Evans rats

BACKGROUND

While men in the United States consume more alcohol than women, rates of drinking are converging. Nevertheless, females remain underrepresented in preclinical alcohol research. Here, we examined rats' sex-related differences in patterns of ethanol (EtOH) drinking and the effects of this drinking on exploratory and anxiety-like behavior.

METHODS

Adult male and female Long-Evans rats were given 20% ethanol under the intermittent-access two-bottle-choice paradigm. Their intake was measured daily for the first 7 weeks. During the eighth week, intake was measured over the 24 h of daily access. During the ninth week, they, along with EtOH-naive controls, were tested prior to daily access in a novel chamber, light-dark box, and hole board apparatus. During the tenth week, blood ethanol concentration (BEC) was assessed after 30 to 40 min of access.

RESULTS

Females overall demonstrated higher ethanol intake and preference across all access weeks than males, although only half of females drank significantly more than males. Across 24 h of daily access, both sexes had their highest intake in the first 30 min and their lowest in the middle of the light phase of the light/dark cycle. Despite their greater ethanol intake, females did not show significantly different BECs than males. In behavioral tests, females showed less vertical time in a novel activity chamber, more movement between chambers in a light-dark box, and more nose pokes in a hole-board apparatus than males. While a history of ethanol drinking led to a trend for lower vertical time in the activity chamber and greater chamber entries in the light-dark box, the effects were not sex-dependent.

CONCLUSIONS

These results suggest that female and male rats could both be tested for acute effects of ethanol after 30 min of daily access, but that nuanced considerations are needed in the design of these experiments and the interpretation of their findings.

The influence of chromosome 4 on high ethanol consumption and blood pressure

The Spontaneously Hypertensive Rats (SHR) strain was developed through selective breeding for high systolic blood pressure. In our laboratory, we established a congenic rat strain named SHR.Lewis-Anxrr16 (SLA16). The SLA16 rat strain is genetically identical to the SHR except for the inserted Anxrr16 region in chromosome 4. Our objective was to evaluate the influence of this genomic region on ethanol consumption and blood pressure. First, we exposed SHR and SLA16 male and female rats to ethanol consumption. Results showed that, regardless of strain, females consumed more ethanol than males during forced (10% v/v) and spontaneous ethanol consumption (SEC; 2.5-20% v/v). Then, females from both strains were used to evaluate sensitivity to ethanol. No strain differences in the loss of righting reflex were observed after ethanol treatment (3 g/kg, 20% w/v, intraperitoneal [i.p.]). But, in the triple test, female SHR rats presented lower sensitivity to the ethanol (1.2 g/kg, 14% w/v, i.p.). Surprisingly, female SHR rats also presented higher blood pressure after SEC (10% v/v). Finally, losartan treatment was effective in decreasing the blood pressure of female rats of both strains, but had specific effects on SHR ethanol consumption. Our data suggest that SLA16 female rats consume less ethanol (10%), are more sensitive to its effects, and present lower blood pressure than SHR female rats. We demonstrated that the Anxrr16 locus in chromosome 4 is a genetic candidate to explain high ethanol consumption and blood pressure, at least in females.

Sex differences in affective states and association with voluntary ethanol intake in Sprague-Dawley rats

Alcohol use disorders (AUDs) are a major problem across the USA. While AUD remains a complex human condition, it is difficult to isolate the directionality of anxiety and ethanol (EtOH) drinking from outside influences. The present study sought to investigate the relationship between affective states and EtOH intake using male and female Sprague-Dawley rats. Using complementary tests of anxiety- and depressive-like behavior, we found sex- and test-specific differences in basal affective behavior such that females displayed enhanced anxiety-like behavior in the splash test and males displayed enhanced anxiety-like behavior in the novelty-suppressed feeding test. Although, there were no sex differences in EtOH intake and no correlation between baseline anxiety-like behavior and subsequent EtOH intake, we did find that depressive-like behavior predicted future EtOH intake in female rats only. In addition, we observed an increase in depressive-like behavior is male rats in both the water and EtOH drinking groups (compared to baseline levels). Furthermore, post-drinking anxiety-like behavior, but not depressive-like behavior predicted subsequent EtOH intake in female rats. Lastly, we found a history of EtOH intake decreased pain thresholds in male and female rats, but increased anxiety-like and depressive-like behavior was associated with decreased thermal sensitivity only in EtOH-drinking males. Together, these experiments provide important information on the complex interaction between negative affect and EtOH intake and how these two contexts reciprocally do, or do not, influence each other in a sex-specific manner.

Behavior and Fos activation reveal that male and female rats differentially assess affective valence during CTA learning and expression

Females are more affected by psychiatric illnesses including eating disorders, depression, and post-traumatic stress disorder than males. However, the neural mechanisms mediating these sex differences are poorly understood. Animal models can be useful in exploring such neural mechanisms. Conditioned taste aversion (CTA) is a behavioral task that assesses how animals process the competition between associated reinforcing and aversive stimuli in subsequent task performance, a process critical to healthy behavior in many domains. The purpose of the present study was to identify sex differences in this behavior and associated neural responses. We hypothesized that females would value the rewarding stimulus (Boost®) relative to the aversive stimulus (LiCl) more than males in performing CTA. We evaluated behavior (Boost® intake, LiCl-induced behaviors, ultrasonic vocalizations (USVs), CTA performance) and Fos activation in relevant brain regions after the acute stimuli [acute Boost® (AB), acute LiCl (AL)] and the context-only task control (COT), Boost® only task (BOT) and Boost®-LiCl task (BLT). Acutely, females drank more Boost® than males but showed similar aversive behaviors after LiCl. Females and males performed CTA similarly. Both sexes produced 55 kHz USVs anticipating BOT and inhibited these calls in the BLT. However, more females emitted both 22 kHz and 55 kHz USVs in the BLT than males: the latter correlated with less CTA. Estrous cycle stage also influenced 55 kHz USVs. Fos responses were similar in males and females after AB or AL. Females engaged the gustatory cortex and ventral tegmental area (VTA) more than males during the BOT and males engaged the amygdala more than females in both the BOT and BLT. Network analysis of correlated Fos responses across brain regions identified two unique networks characterizing the BOT and BLT, in both of which the VTA played a central role. In situ hybridization with RNAscope identified a population of D1-receptor expressing cells in the CeA that responded to Boost® and D2 receptor-expressing cells that responded to LiCl. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior.

The effects of voluntary adolescent alcohol consumption on alcohol taste reactivity in Long Evans rats

RATIONALE

The relationship between age, ethanol intake, and the hedonic value of ethanol is key to understanding the motivation to consume ethanol.

OBJECTIVE

It is uncertain whether ethanol drinking during adolescence changes ethanol's hedonic value into adulthood.

METHODS

The current study compared voluntary intermittent ethanol consumption (IAE; 2-bottle choice; 20%v/v) among adolescent and adult Long-Evans rats to examine the effects of age and IAE on taste reactivity in adulthood. For taste reactivity, orally infused fluids included water, ethanol (5, 20, and 40%v/v), and sucrose (0.01, 0.1, 1M).

RESULTS

IAE results indicate that adolescents drank more ethanol during IAE but had a lower rate of change in ethanol consumption across time than adults due to initially high adolescent drinking. During taste reactivity testing for ethanol, IAE rats had greater hedonic responding, less aversive responding, and a more positive relationship between hedonic responses and ethanol concentration than water-receiving control rats. Hedonic responses had positive, while aversive responses had negative relationships with ethanol concentration and total ethanol consumed during IAE. Adolescent+IAE rats displayed less hedonic and more aversive responses to ethanol than Adult+IAE rats. Sucrose responding was unrelated to ethanol consumption.

CONCLUSIONS

These results suggest that ethanol consumption influences the future hedonic and aversive value of ethanol in a way that makes ethanol more palatable with greater prior consumption. However, it appears that those drinking ethanol as adolescents may be more resistant to this palatability shift than those first drinking as adults, suggesting different mechanisms of vulnerability to consumption escalation for adolescents and adults.

Sex Differences in the Brain Transcriptome Related to Alcohol Effects and Alcohol Use Disorder

There is compelling evidence that sex and gender have crucial roles in excessive alcohol (ethanol) consumption. Here, we review some of the data from the perspective of brain transcriptional differences between males and females, focusing on rodent animal models. A key emerging transcriptional feature is the role of neuroimmune processes. Microglia are the resident neuroimmune cells in the brain and exhibit substantial functional differences between males and females. Selective breeding for binge ethanol consumption and the impacts of chronic ethanol consumption and withdrawal from chronic ethanol exposure all demonstrate sex-dependent neuroimmune signatures. A focus is on resolving sex-dependent differences in transcriptional responses to ethanol at the neurocircuitry level. Sex-dependent transcriptional differences are found in the extended amygdala and the nucleus accumbens. Telescoping of ethanol consumption is found in some, but not all, studies to be more prevalent in females. Recent transcriptional studies suggest that some sex differences may be due to female-dependent remodeling of the primary cilium. An interesting theme appears to be developing: at least from the animal model perspective, even when males and females are phenotypically similar, they differ significantly at the level of the transcriptome.

Recent Perspectives on Sex Differences in Compulsion-Like and Binge Alcohol Drinking

Alcohol use disorder remains a substantial social, health, and economic problem and problem drinking levels in women have been increasing in recent years. Understanding whether and how the underlying mechanisms that drive drinking vary by sex is critical and could provide novel, more targeted therapeutic treatments. Here, we examine recent results from our laboratories and others which we believe provide useful insights into similarities and differences in alcohol drinking patterns across the sexes. Findings for binge intake and aversion-resistant, compulsion-like alcohol drinking are considered, since both are likely significant contributors to alcohol problems in humans. We also describe studies regarding mechanisms that may underlie sex differences in maladaptive alcohol drinking, with some focus on the importance of nucleus accumbens (NAcb) core and shell regions, several receptor types (dopamine, orexin, AMPA-type glutamate), and possible contributions of sex hormones. Finally, we discuss how stressors such as early life stress and anxiety-like states may interact with sex differences to contribute to alcohol drinking. Together, these findings underscore the importance and critical relevance of studying female and male mechanisms for alcohol and co-morbid conditions to gain a true and clinically useful understanding of addiction and neuropsychiatric mechanisms and treatment.

Studying Sex Differences in Rodent Models of Addictive Behavior

Animal models of addictive behaviors are useful for uncovering neural mechanisms involved in the development of dependence and for identifying risk factors for drug abuse. One such risk factor is biological sex, which strongly moderates drug self-administration behavior in rodents. Female rodents are more likely to acquire drug self-administration behaviors, consume higher amounts of drug, and reinstate drug-seeking behavior more readily. Despite this female vulnerability, preclinical addiction research has largely been done in male animals. The study of sex differences in rodent models of addictive behavior is increasing, however, as more investigators are choosing to include both male and female animals in experiments. This commentary is meant to serve as an introductory guide for preclinical investigators new to the study of sex differences in addiction. We provide an overview of self-administration models, a broad view of female versus male self-administration behaviors, and suggestions for study design and implementation. Inclusion of female subjects in preclinical addiction research is timely, as problem drug and alcohol use in women is increasing. With proper attention, design, and analysis, the study of sex differences in addiction has the potential to uncover novel neural mechanisms and lead to greater translational success for addiction research. © 2021 Wiley Periodicals LLC.

Adolescent intermittent ethanol exposure induces sex-dependent divergent changes in ethanol drinking and motor activity in adulthood in C57BL/6J mice

With alcohol readily accessible to adolescents, its consumption leads to many adverse effects, including impaired learning, attention, and behavior. Adolescents report higher rates of binge drinking compared to adults. They are also more prone to substance use disorder in adulthood due to physiological changes during the adolescent developmental period. We used C57BL/6J male and female mice to investigate the long-lasting impact of binge ethanol exposure during adolescence on voluntary ethanol intake and open field behavior during later adolescence (Experiment 1) and during emerging adulthood (Experiment 2). The present set of experiments were divided into four stages: (1) adolescent intermittent vapor inhalation exposure, (2) abstinence, (3) voluntary ethanol intake, and (4) open field behavioral testing. During adolescence, male and female mice were exposed to air or ethanol using intermittent vapor inhalation from postnatal day (PND) 28-42. Following this, mice underwent short-term abstinence from PND 43-49 (Experiment 1) or protracted abstinence from PND 43-69 (Experiment 2). Beginning on PND 50-76 or PND 70-97, mice were assessed for intermittent voluntary ethanol consumption using a two-bottle choice drinking procedure over 28 days. Male adolescent ethanol-exposed mice showed increased ethanol consumption following short-term abstinence and following protracted abstinence. In contrast, female mice showed no changes in ethanol consumption following short-term abstinence and decreased ethanol consumption following protracted abstinence. There were modest changes in open field behavior following voluntary ethanol consumption in both experiments. These data demonstrate a sexually divergent shift in ethanol consumption following binge ethanol exposure during adolescence and differences in open field behavior. These results highlight sex-dependent vulnerability to developing substance use disorders in adulthood.

Sex differences in cholinergic circuits and behavioral disruptions following chronic ethanol exposure with and without thiamine deficiency

BACKGROUND

Few studies have investigated differences in the vulnerabilities of males and females to alcohol use disorder and alcohol-related brain damage (ARBD). According to epidemiological and clinical findings, females appear to be more sensitive to the effects of alcohol and thiamine deficiency and have a worse prognosis in recovery from neurocognitive deficits compared with males. This study aimed to characterize the effects of chronic ethanol (EtOH) toxicity and thiamine deficiency across the sexes using rodent models.

METHODS

Male and female Sprague Dawley rats were assigned to chronic forced EtOH treatment (CET), pyrithiamine-induced thiamine deficiency (PTD), combined CET-PTD, or pair-fed (PF) control treatment conditions. Following treatments, spatial working memory was assessed during a spontaneous alternation task while measuring acetylcholine (ACh) in the prefrontal cortex (PFC) and the hippocampus (HPC). The animals also underwent an operant-based attentional set-shifting task (ASST) for the analysis of behavioral flexibility.

RESULTS

Female and male rats did not differ in terms of EtOH consumption; however, the CET and CET-PTD-treated female rats had lower BECs than male rats. Compared with the PF group, the CET, PTD, and CET-PTD groups exhibited spatial working memory impairments with corresponding reductions in ACh efflux in the PFC and HPC. The ASST revealed that CET-PTD-treated males and females displayed impairments marked by increased latency to make decisions. Thalamic shrinkage was prominent only in the CET-PTD and PTD treatment conditions, but no sex-specific effects were observed.

CONCLUSIONS

Although the CET and CET-PTD-treated females had lower BECs than the males, they demonstrated similar cognitive impairments. These results provide evidence that female rats experience behavioral and neurochemical disruptions at lower levels of alcohol exposure than males and that chronic EtOH and thiamine deficiencies produce a unique behavioral profile.

Housing Condition Differentially Impacts Escalation of Alcohol Intake, Relapse-Like Drinking, Anxiety-Like Behavior, and Stress History Effects by Sex

BACKGROUND

Stress triggers alcohol use and relapse to drinking, with different effects by sex. Women are more susceptible to stress-related alcohol misuse, and most stressors in rodents produce sexually divergent effects. Female rodents are particularly sensitive to the stress produced by solitary housing, yet the impact of housing conditions on the establishment, escalation, and post-abstinence potentiation of intermittent access alcohol drinking in male and female rats, and the interaction of these factors with stress history are not well described.

METHODS

Male (n = 62) and female (n = 64) Wistar rats were housed individually or in pairs separated by a perforated divider. Rats were exposed to light-cued footshock stress (stress history), or cues alone (control), once daily for 3 days, followed by 8 weeks' drinking under intermittent access to a 2-bottle choice (IA2BC), with 20% alcohol (v/v in water) available in addition to water for 24 hours on alternate days. After a 2-week forced abstinence, anxiety-like behavior was assessed via defensive withdrawal testing; then, IA2BC alcohol access was renewed for 2 weeks to model relapse-like behavior.

RESULTS

Pair-housed female rats did not increase their alcohol intake across the 8-week drinking period, unlike all other groups, and stress history did not significantly change alcohol consumption. After abstinence, anxiety-like behavior was greatest in pair-housed stress history males, whereas alcohol intake was significantly elevated only in female rats, particularly those in solitary housing.

CONCLUSIONS

Together, these findings suggest that paired housing differentially contributes to behavior in male and female rats, blunting alcohol intake in females, and unmasking stress history effects on anxiety-like behavior in males.

Age differences in brain structural and metabolic responses to binge ethanol exposure in fisher 344 rats

An overarching goal of our research has been to develop a valid animal model of alcoholism with similar imaging phenotypes as those observed in humans with the ultimate objective of assessing the effectiveness of pharmacological agents. In contrast to our findings in humans with alcohol use disorders (AUD), our animal model experiments have not demonstrated enduring brain pathology despite chronic, high ethanol (EtOH) exposure protocols. Relative to healthy controls, older individuals with AUD demonstrate accelerating brain tissue loss with advanced age. Thus, this longitudinally controlled study was conducted in 4-month old (equivalent to ~16-year-old humans) and 17-month old (equivalent to ~45-year-old humans) male and female Fisher 344 rats to test the hypothesis that following equivalent alcohol exposure protocols, older relative to younger animals would exhibit more brain changes as evaluated using in vivo structural magnetic resonance imaging (MRI) and MR spectroscopy (MRS). At baseline, total brain volume as well as the volumes of each of the three constituent tissue types (i.e., cerebral spinal fluid (CSF), gray matter, white matter) were greater in old relative to young rats. Baseline metabolite levels (except for glutathione) were higher in older than younger animals. Effects of binge EtOH exposure on brain volumes and neurometabolites replicated our previous findings in Wistar rats and included ventricular enlargement and reduced MRS-derived creatine levels. Brain changes in response to binge EtOH treatment were more pronounced in young relative to older animals, negating our hypothesis. Higher baseline glutathione levels in female than male rats suggest that female rats are perhaps protected against the more pronounced changes in CSF and gray matter volumes observed in male rats due to superior metabolic homeostasis mechanisms. Additional metabolite changes including low inositol levels in response to high blood alcohol levels support a mechanism of reversible osmolarity disturbances due to temporarily altered brain energy metabolism.

Additive influences of acute early life stress and sex on vulnerability for aversion-resistant alcohol drinking

Acute early life stress (ELS) alters stress system functioning in adulthood and increases susceptibility to posttraumatic stress disorder (PTSD) and alcohol use disorder (AUD). The current study assessed the effects of acute, infant ELS on alcohol drinking, including aversion-resistant drinking, in male and female Long Evans rats. Acute ELS was induced using a stress-enhanced fear learning (SEFL) protocol that consisted of 15 footshocks delivered on postnatal day (PND) 17. Alcohol drinking during adolescence and adulthood was measured with a two-bottle choice intermittent alcohol access paradigm. Aversion-resistant drinking was assessed in adulthood by adding quinine (0.01, 0.1, and 1.0 g/L) to the alcohol bottle after 5 to 6 weeks and 11 to 12 weeks of drinking. ELS had minimal influences on adolescent and adult alcohol consumption and preference. However, ELS, sex, and alcohol exposure history all influenced aversion-resistant alcohol drinking in an additive fashion. Higher concentrations of quinine were tolerated in females, ELS-exposed rats, and after 11 to 12 weeks of drinking. Tests of quinine sensitivity in a separate cohort of animals found that rats can detect concentrations of quinine as low as 0.001 g/L in water and that quinine sensitivity is not influenced by sex or ELS exposure. These results agree with reports of sex differences in aversion-resistant drinking and are the first to demonstrate an influence of ELS on this behavior. Our results also suggest that a single traumatic stress exposure in infancy may be a promising model of comorbid PTSD and AUD and useful in studying the interactions between ELS, sex, and alcohol dependence.

Alcohol Sensitivity as an Endophenotype of Alcohol Use Disorder: Exploring Its Translational Utility between Rodents and Humans

Alcohol use disorder (AUD) is a complex, chronic, relapsing disorder with multiple interacting genetic and environmental influences. Numerous studies have verified the influence of genetics on AUD, yet the underlying biological pathways remain unknown. One strategy to interrogate complex diseases is the use of endophenotypes, which deconstruct current diagnostic categories into component traits that may be more amenable to genetic research. In this review, we explore how an endophenotype such as sensitivity to alcohol can be used in conjunction with rodent models to provide mechanistic insights into AUD. We evaluate three alcohol sensitivity endophenotypes (stimulation, intoxication, and aversion) for their translatability across human and rodent research by examining the underlying neurobiology and its relationship to consumption and AUD. We show examples in which results gleaned from rodents are successfully integrated with information from human studies to gain insight in the genetic underpinnings of AUD and AUD-related endophenotypes. Finally, we identify areas for future translational research that could greatly expand our knowledge of the biological and molecular aspects of the transition to AUD with the broad hope of finding better ways to treat this devastating disorder.

Age, sex and pre-exposure effects on acquisition and generalization of conditioned taste aversion in rats

The main aim of the present study was to assess the effect of sex and aging in two pre-exposure learning effects, latent inhibition (LI) and perceptual learning (PL), with a conditioned taste aversion paradigm. Young adult (90 days) and aged (more than 18 months) males and females received 8 pre-exposure trials either with stimulus AX (LI conditions) or BX (PL conditions). Then, all animals received a conditioning trial with AX and two test trials, one with AX and other with BX. The level of generalization between AX and BX was assessed by means of the absolute level of consumption of BX and by the difference in consumption between both stimuli. The results showed an attenuation of latent inhibition as well a stronger generalization of conditioned taste aversion in females when generalization is inferred from the BX consumption. A facilitation of conditioning for the aged animals was also found regardless of the pre-exposed stimulus. Pre-exposures to BX resulted in little generalization, but pre-exposures to AX resulted in a very similar consumption of both compounds, indicating a strong generalization between them. Overall, the study provided novel evidence about the effect of sex and aging on taste aversion, raising at the same time some relevant questions about perceptual learning and how such pre-exposure effect has been typically assessed.

Alcohol intake in Carioca High- and Low-conditioned Freezing rats

Evidence from clinical and epidemiological studies point towards an association between generalized anxiety disorder (GAD) and alcohol abuse. In the present study we investigated whether a similar relationship could be observed in an animal model of GAD. Specifically, we evaluated the alcohol intake of Carioca High- and Low-conditioned Freezing rats (CHF and CLF, respectively). Sex differences in alcohol drinking behavior were also studied. Male and female rats from randomized crossbreeding populations served as controls (CTL). Free- and forced-choice protocols were used to measure alcohol consumption, and quinine and saccharin were used as taste control solutions. Our results indicate that CHF rats consumed more alcohol than CLF and CTL ones in both the free-choice (6 and 10% concentrations) and the forced-choice (10% concentration) conditions. CHF female rats exhibited the highest amount of alcohol intake in the forced-choice condition. CHF females also consumed more quinine than CHF male rats. Finally, CHF rats exhibited lower saccharin consumption compared to CLF and CTL animals. Altogether, these results support the hypothesis that there is a positive relationship between anxiety and alcohol intake, and provide further evidence for the use of CHF rats as a model of GAD.

Effects of pair housing on voluntary alcohol intake in male and female Wistar rats

A number of different voluntary alcohol intake paradigms are available for home cage drinking studies. Traditionally, these paradigms involve single housing in order for individual intake to be measured. This study aimed at investigating the effects of pair housing on voluntary alcohol intake. Male and female Wistar rats were housed in pairs or individually for studies of voluntary alcohol intake using the modified intermittent access paradigm with alcohol access during three consecutive days per week followed by four days of water only. Individual intake of 20% alcohol solution and water was measured during 12 sessions, i.e., 4 weeks. Pair-housed animals could interact freely with their cage mate for four consecutive days each week and were then separated by an inserted mesh divider for three consecutive days each week during alcohol intake sessions. Alcohol intake and preference were compared between pair-housed and individually housed rats. The results revealed higher alcohol intake in females than in males. Pair-housed males had a higher alcohol intake and preference during the first 3 weeks, but not during the fourth week, compared to individually housed males No effect of housing condition was observed in female rats. The alcohol intake was higher on the first day of access relative to the two consecutive days in pair-housed males and higher on the first two days relative to the third day in female rats. Social rank or female estrus cycle had no effect on alcohol intake or preference. Taken together, the use of a divider during alcohol intake sessions had no impact on alcohol intake in female rats and may not exert long-term influences in male rats. Future studies are needed in order to elucidate whether the use of a divider can constitute an experimental refinement as an alternative to individual housing in studies of voluntary alcohol intake using the limited access and/or intermittent access paradigms.

Sex-dependent effects of chronic intermittent voluntary alcohol consumption on attentional, not motivational, measures during probabilistic learning and reversal

Background

Forced alcohol (ethanol, EtOH) exposure has been shown to cause significant impairments on reversal learning, a widely-used assay of cognitive flexibility, specifically on fully-predictive, deterministic versions of this task. However, previous studies have not adequately considered voluntary EtOH consumption and sex effects on probabilistic reversal learning. The present study aimed to fill this gap in the literature.

Methods

Male and female Long-Evans rats underwent either 10 weeks of voluntary intermittent 20% EtOH access or water only (H2O) access. Rats were then pretrained to initiate trials and learn stimulus-reward associations via touchscreen response, and subsequently required to select between two visual stimuli, rewarded with probability 0.70 or 0.30. In the final phase, reinforcement contingencies were reversed.

Results

We found significant sex differences on several EtOH-drinking variables, with females reaching a higher maximum EtOH consumption, exhibiting more high-drinking days, and escalating their EtOH at a quicker rate compared to males. During early abstinence, EtOH drinkers (and particularly EtOH-drinking females) made more initiation omissions and were slower to initiate trials than H2O drinking controls, especially during pretraining. A similar pattern in trial initiations was also observed in discrimination, but not in reversal learning. EtOH drinking rats were unaffected in their reward collection and stimulus response times, indicating intact motivation and motor responding. Although there were sex differences in discrimination and reversal phases, performance improved over time. We also observed sex-independent drinking group differences in win-stay and lose-shift strategies specific to the reversal phase.

Conclusions

Females exhibit increased vulnerability to EtOH effects in early learning: there were sex-dependent EtOH effects on attentional measures during pretraining and discrimination phases. We also found sex-independent EtOH effects on exploration strategies during reversal. Future studies should aim to uncover the neural mechanisms for changes in attention and exploration in both acute and prolonged EtOH withdrawal.

Adolescent Ethanol Exposure: Anxiety-Like Behavioral Alterations, Ethanol Intake, and Sensitivity

Adolescence is a developmental period associated with rapid age-specific physiological, neural, and hormonal changes. Behaviorally, human adolescents are characterized by age-typical increases in novelty-seeking and risk-taking, including the frequent initiation of alcohol and drug use. Alcohol use typically begins during early adolescence, and older adolescents often report high levels of alcohol consumption, commonly referred to as high-intensity drinking. Early-onset and heavy drinking during adolescence are associated with an increased risk of developing alcohol use disorders later in life. Yet, long-term behavioral consequences of adolescent alcohol use that might contribute to excessive drinking in adulthood are still not well understood. Recent animal research, however, using different exposure regimens and routes of ethanol administration, has made substantial progress in identifying the consequences of adolescent ethanol exposure that last into adulthood. Alterations associated with adolescent ethanol exposure include increases in anxiety-like behavior, impulsivity, risk-taking, and ethanol intake, although the observed alterations differ as a function of exposure regimens and routes of ethanol administration. Rodent studies have also shown that adolescent ethanol exposure produces alterations in sensitivity to ethanol, with these alterations reminiscent of adolescent-typical ethanol responsiveness. The goal of this mini-review article is to summarize the current state of animal research, focusing on the long-term consequences related to adolescent ethanol exposure, with a special emphasis on the behavioral alterations and changes to ethanol sensitivity that can foster high levels of drinking in adulthood.

The effects of social instability stress and subsequent ethanol consumption in adolescence on brain and behavioral development in male rats

Excessive drinking in adolescence continues to be a problem, and almost a quarter of young Canadians have reported consuming five or more alcoholic drinks in one occasion in recent surveys. The consequences of such drinking may be more pronounced when commenced in adolescence, given the ongoing brain development during this period of life. Here, we investigated the consequences of 3 weeks' intermittent access to ethanol in mid-adolescence to early adulthood in rats, and the extent to which a stress history moderated the negative consequences of ethanol access. In experiment 1, male rats that underwent adolescent social instability stress (SS; daily 1 h isolation + return to unfamiliar cage partner every day from postnatal day [PND] 30-45) did not differ from control (CTL) rats in intake of 10% ethanol sweetened with 0.1% saccharin (access period; PND 47-66). Ethanol drinking reduced proteins relevant for synaptic plasticity (αCaMKII, βCaMKII, and PSD-95) in the dorsal hippocampus, and in CTL rats only in the prefrontal cortex (αCaMKII and PSD 95), attenuating the difference between CTL and SS rats in the water-drinking group. In experiment 2, ethanol also attenuated the difference between SS and CTL rats in a social interaction test by reducing social interaction in SS rats; CTL rats, however, had a higher intake of ethanol than did SS rats during the access period. Ethanol drinking reduced baseline and fear recall recovery concentrations of corticosterone relative to those exposed only to water, although there was no effect of either ethanol or stress history on fear conditioning. Ethanol drinking did not influence intake after 9 days of withdrawal; however, ethanol-naïve SS rats drank more than did CTL rats when given a 24-h access in adulthood. These results reveal a complex relationship between stress history and ethanol intake in adolescence on outcomes in adulthood.

Sex Differences in Aversion-Resistant Ethanol Intake in Mice

AIMS

Compulsive ethanol intake, characterized by persistent consumption despite negative consequences, is an addictive behavior identified by the DSM-5 as a central criterion in diagnosing alcohol use disorders (AUD). Epidemiological data suggest that females transition from recreational alcohol use to AUD more rapidly than males. Because of this potential sex difference in the etiology of AUD, it is critical to assess addictive behaviors such as compulsive intake in both males and females in preclinical studies.

METHODS

We used the model of aversion-resistant ethanol consumption to assess compulsive-like ethanol intake. In these experiments, C57BL6/J mice were first provided with continuous access two-bottle choice between water and ethanol to establish baseline intake. Ethanol solution was then adulterated with increasing concentrations of the bitter tastant quinine hydrochloride. Animals that consume ethanol solution despite its pairing with this negative stimulus are thought to be exhibiting compulsive-like behavior.

RESULTS

We found that higher concentrations of quinine were required to suppress ethanol consumption in female mice relative to males. We found no effect of estrous cycle phase on baseline ethanol intake or on quinine-adulterated ethanol intake in females.

CONCLUSIONS

Collectively, these data suggest that females exhibit a higher degree of aversion-resistance than male mice. Because we observed no effect of estrous cycle phase, it is likely that the presence of threshold levels of estradiol or progesterone, as opposed to their natural fluctuation across the estrous cycle, mediates increased aversion-resistance in females. Alternatively, or in combination, developmental effects of sex hormones could contribute to aversion-resistant ethanol intake.

Adolescent Vulnerability to Alcohol Use Disorder: Neurophysiological Mechanisms from Preclinical Studies

Adolescent alcohol use in human populations dramatically increases the likelihood of adult alcohol use disorder. This adolescent vulnerability is recapitulated in preclinical models which provide important opportunities to understand basic neurobiological mechanisms. We provide here an overview of GABAergic and glutamatergic neurotransmission and our current understanding of the sensitivity of these systems to adolescent ethanol exposure. As a whole, the preclinical literature suggests that adolescent vulnerability may be directly related to region-specific neurobiological processes that continue to develop during adolescence. These processes include the activity of intrinsic circuits within diverse brain regions (primarily represented by GABAergic neurotransmission) and activity-dependent regulation of synaptic strength at glutamatergic synapses. Furthermore, GABAergic and glutamatergic neurotransmission within regions/circuits that regulate cognitive function, emotion, and their integration appears to be the most vulnerable to adolescent ethanol exposure. Finally, using documented behavioral differences between adolescents and adults with respect to acute ethanol, we highlight additional circuits and regions for future study.

Excitatory/inhibitory balance across ontogeny contributes to age-specific behavioral outcomes of ethanol-like challenge in conditioned taste aversion

Adolescent-typical sensitivities to ethanol (EtOH) are characterized in part by reduced sensitivity to EtOH's aversive effects. Rodent studies have shown that adolescents are less sensitive than adults to aversive properties of EtOH in a conditioned taste aversion (CTA) paradigm. To the extent that EtOH exerts antagonist-like actions upon glutamate receptors and/or agonist-like actions upon γ-aminobutyric acid (GABA) receptors, age differences in excitatory/inhibitory balance may regulate age-specific EtOH sensitivities, such as attenuated sensitivity of adolescents to EtOH aversion. In our experiments, adolescent and adult Sprague-Dawley rats were tested for CTA following challenge with one of the following pharmacological agents: glutamatergic AMPA1 receptor antagonist NBQX, glutamatergic N-methyl-d-aspartate NR2B receptor antagonist ifenprodil, and extrasynaptic GABAA receptor agonist THIP to determine whether these induced age-specific aversive sensitivities like those seen with EtOH. NBQX administration did not induce CTA. The highest dose of extrasynaptic GABAA agonist THIP induced CTA in adolescents but not adults, an opposite ontogenetic profile as seen following EtOH. Ifenprodil exerted an age-specific pattern of CTA similar to that seen with EtOH in males, with adolescents being insensitive to ifenprodil's aversive effects relative to adults. Thus, only antagonism of NR2B receptors in male rats mimicked age-specific sensitivities to the aversive effects of EtOH.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Non-nicotine constituents in e-cigarette aerosol extract attenuate nicotine's aversive effects in adolescent rats

BACKGROUND

Development of preclinical methodology for evaluating the abuse liability of electronic cigarettes (ECs) in adolescents is urgently needed to inform FDA regulation of these products. We previously reported reduced aversive effects of EC liquids containing nicotine and a range of non-nicotine constituents (e.g., propylene glycol, minor tobacco alkaloids) compared to nicotine alone in adult rats as measured using intracranial self-stimulation. The goal of this study was to compare the aversive effects of nicotine alone and EC aerosol extracts in adolescent rats as measured using conditioned taste aversion (CTA), which can be conducted during the brief adolescent period.

METHODS AND RESULTS

In Experiment 1, nicotine alone (1.0 or 1.5 mg/kg, s.c.) produced significant CTA in adolescent rats in a two-bottle procedure, thereby establishing a model to study the effects of EC extracts. At a nicotine dose of 1.0 mg/kg, CTA to Vuse Menthol EC extract, but not Aroma E-Juice EC extract, was attenuated compared to nicotine alone during repeated two-bottle CTA tests (Experiment 2a). At a nicotine dose of 0.5 mg/kg, CTA to Vuse Menthol EC extract did not differ from nicotine alone during the first two-bottle CTA test but extinguished more rapidly across repeated two-bottle tests (Experiment 2b).

CONCLUSIONS

Non-nicotine constituents in Vuse Menthol EC extracts attenuated CTA in a two-bottle procedure in adolescents. This model may be useful for anticipating the abuse liability of ECs in adolescents and for modeling FDA-mandated changes in product standards for nicotine or other constituents in ECs.

Prenatal ethanol exposure attenuates sensitivity to the aversive effects of ethanol in adolescence and increases adult preference for a 5% ethanol solution in males, but not females

The present set of experiments investigated the effects of a moderate dose of ethanol (2 g/kg; 20% v/v intragastrically) during late gestation (G17-20 [gestational day]) on ethanol-induced conditioned taste aversion (CTA) in adolescence, and on ethanol consumption during adolescence and early adulthood. In experiment 1, male and female Sprague-Dawley rats were given 30-min access to a sweetened "supersaccharin" (SS) solution or sodium chloride (NaCl), followed by an intraperitoneal injection of 20% ethanol (0, 1, 1.25, or 1.5 g/kg) for three conditioning/test sessions. Among animals conditioned with SS, prenatally ethanol-exposed males exhibited attenuated ethanol-induced CTA relative to males prenatally gavaged with water or non-manipulated, whereas prenatal treatment had no effect on CTA in females. Among animals conditioned with NaCl, there were no exposure group differences in males, with modest evidence for attenuated CTA in prenatally ethanol-exposed females. In experiment 2, the effects of prenatal ethanol exposure on ethanol consumption in adolescents (P35 ± 1 day [postnatal day]) and adults (P56-60) were explored. At the beginning of the dark cycle, pair-housed rats were given three bottles containing 0, 5, and 10% ethanol for 18 h every other day (i.e., Monday, Wednesday, Friday) for 3 weeks. Relative to water controls, adult males prenatally exposed to ethanol showed greater preference and more intake (g/kg) of 5% ethanol, while showing lower intake of 10% ethanol. These intake and preference differences were not evident in adolescent males. Among females at both ages, ethanol-exposed animals showed lower preference and intake (g/kg) of 5% ethanol than their water-exposed controls. Thus, moderate ethanol exposure during late gestation produced a largely male-specific attenuation in the aversive effects of ethanol during adolescence that could contribute to later increases in preference and intake of a 5% ethanol solution, although this emergent effect was not evident in adolescence (or in females), but only manifested in adulthood.

Operant, oral alcohol self-administration: Sex differences in Sardinian alcohol-preferring rats

Sardinian alcohol-preferring (sP) rats have been selectively bred, over almost 40 years, for high alcohol preference and consumption. sP rats have served as an animal model for more than 120 published studies. With very few exceptions, however, these studies have always employed male sP rats, and little is known about alcohol-related behaviors in female sP rats. The present study was designed to fill, at least in part, this gap. Accordingly, alcohol self-administration under the fixed ratio 4 schedule of reinforcement was compared among male, intact female, and ovariectomized female sP rats. Additionally, it was investigated whether i) estrous cycle influenced alcohol self-administration, and ii) alcohol self-administration in the three sP rat groups differed in sensitivity to pharmacological manipulation. Lever-responding for alcohol was steadily higher in male than intact and ovariectomized female sP rats; conversely, because of large sex differences in rat body weight, estimated amount of self-administered alcohol (in g/kg) did not differ among the three sP rat groups or occasionally was higher in intact female than male and ovariectomized female sP rats. Blood alcohol levels derived from self-administered alcohol i) did not differ among the three sP rat groups and ii) were positively correlated with the number of lever-responses for alcohol and the estimated amount of self-administered alcohol. Treatment with the opioid receptor antagonist, naloxone (0, 0.3, 1, and 3 mg/kg, i.p. [intraperitoneally]), and the positive allosteric modulator of the GABA_B receptor, GS39783 (0, 25, 50, and 100 mg/kg, i.g. [intragastrically]), reduced alcohol self-administration with comparable potency and efficacy in the three sP rat groups. The impact of the estrous cycle on alcohol self-administration was relatively modest, limited to a tendency toward a reduction in the number of lever-responses for alcohol and the estimated amount of self-administered alcohol in estrus and metestrus. Together, these results provide the first characterization of alcohol-seeking and -taking behavior in female sP rats.

Sex differences in α-pyrrolidinopentiophenone (α-PVP)-induced taste avoidance, place preference, hyperthermia and locomotor activity in rats

RATIONALE

The majority of synthetic cathinone research has used only male subjects, and as a result there are few studies assessing the impact of biological sex on their effects.

OBJECTIVES

The current work extends the characterization of the second-generation synthetic cathinone, α-PVP, by investigating how biological sex impacts α-PVP's aversive and rewarding effects important to its use and potential abuse.

METHODS

A combined conditioned taste avoidance/conditioned place preference preparation was utilized in which adult male and female Sprague Dawley rats were injected with 1.5, 3 or 6 mg/kg of racemic α-PVP or vehicle (saline) (IP). Following a 24-day washout period, rats were then tested for thermoregulatory effects of α-PVP using subcutaneous microchips to measure body temperature changes over the course of 8 h. This was followed 21 days later by assessments for α-PVP-induced locomotor activity and stereotypies over a 1-h session.

RESULTS

Dose-dependent conditioned taste avoidance was evident in both males and females, although females displayed weaker avoidance at 3 mg/kg compared to males. Males displayed a dose-dependent conditioned place preference, while females did not form a place preference at any dose. α-PVP elicited dose- and time-dependent hyperthermia, with males displaying a faster on-set and delayed off-set compared to females. α-PVP also produced dose- and time-dependent increases in locomotor activity (F > M) and stereotypies (M > F).

CONCLUSIONS

As described, males displayed greater rewarding (as indexed by place preference conditioning) and aversive (as indexed by taste avoidance, hyperthermia and stereotypies) effects of α-PVP. Although comparisons between males and females in α-PVP self-administration have not been reported, these data suggest that males may be more likely to use the drug. The implications for sex differences in human use of α-PVP were discussed.

Chronic inflammatory pain drives alcohol drinking in a sex-dependent manner for C57BL/6J mice

Sex differences in chronic pain and alcohol abuse are not well understood. The development of rodent models is imperative for investigating the underlying changes behind these pathological states. In the present study, we investigated whether hind paw treatment with the inflammatory agent Complete Freund's Adjuvant (CFA) could generate hyperalgesia and alter alcohol consumption in male and female C57BL/6J mice. CFA treatment led to greater nociceptive sensitivity for both sexes in the Hargreaves test, and increased alcohol drinking for males in a continuous-access two-bottle choice (CA2BC) paradigm. Regardless of treatment, female mice exhibited greater alcohol drinking than males. Following a 2-h terminal drinking session, CFA treatment failed to produce changes in alcohol drinking, blood ethanol concentration (BEC), and plasma corticosterone (CORT) for both sexes. Two-hour alcohol consumption and CORT was higher in females than males, regardless of CFA treatment. Taken together, these findings have established that male mice are more susceptible to escalations in alcohol drinking when undergoing pain, despite higher levels of total alcohol drinking and CORT in females. Furthermore, the exposure of CFA-treated C57BL/6J mice to the CA2BC drinking paradigm has proven to be a useful model for studying the relationship between chronic pain and alcohol abuse. Future applications of the CFA/CA2BC model should incorporate manipulations of stress signaling and other related biological systems to improve our mechanistic understanding of pain and alcohol interactions.

Voluntary ethanol consumption during early social isolation and responding for ethanol in adulthood

Little is known about the influence of rearing environments concurrent with voluntary intermittent access to ethanol on subsequent adult ethanol-related behaviors. Previous research has shown that adult rats reared in post-weaning, social isolation conditions (IC) respond more for operant ethanol compared to laboratory standard conditions (SC). Ethanol-exposed adolescents tend to consume more ethanol in adulthood than rats exposed as adults. The current study examined voluntary ethanol consumption during adolescence between IC and SC rats, subsequent operant responding for ethanol, and extinction of responding in the same rats as adults. Differences in ethanol metabolism may alter the amount of reward value per unit of ethanol consumed. Therefore, the current study also examined blood ethanol concentrations (BEC) between IC rats and SC rats. Ethanol-naïve Long-Evans rats arrived in the lab at postnatal day (PND) 21 and were separated into either IC or SC where they remained for the duration of the experiments. On PND 27, rats received intermittent access to 20% ethanol (3 days/week) for 4 or 6 weeks. Rats in the 6-week cohort were then trained to lever press for 20% ethanol in 30-min sessions followed by extinction. A separate cohort was reared in IC or SC, injected with 1.5 or 3.0 g/kg of ethanol (intraperitoneally [i.p.]), followed by BEC measurement. Overall, IC rats had higher ethanol preference and consumption during adolescence/early adulthood. IC and SC rats did not differ in their rates of operant responding for ethanol, and SC rats responded more than IC rats during extinction. There were no differences in BEC between IC and SC rats. These findings highlight the importance of the environment during rat adolescent development with isolation conditions increasing binge-like drinking and ethanol preference after 3-4 weeks without differences in metabolism as a potential factor. Additionally, the findings indicate that intermittent adolescent access to ethanol may change typical differences in operant responding patterns between IC and SC rats in adulthood.

Ethanol Conditioned Taste Aversion in High Drinking in the Dark Mice

Two independent lines of High Drinking in the Dark (HDID-1, HDID-2) mice have been bred to reach high blood alcohol levels after a short period of binge-like ethanol drinking. Male mice of both lines were shown to have reduced sensitivity to develop a taste aversion to a novel flavor conditioned by ethanol injections as compared with their unselected HS/NPT founder stock. We have subsequently developed inbred variants of each line. The current experiments established that reduced ethanol-conditioned taste aversion is also seen in the inbred variants, in both males and females. In other experiments, we asked whether HDID mice would ingest sufficient doses of ethanol to lead to a conditioned taste aversion upon retest. Different manipulations were used to elevate consumption of ethanol on initial exposure. Access to increased ethanol concentrations, to multiple tubes of ethanol, and fluid restriction to increase thirst motivation all enhanced initial drinking of ethanol. Each condition led to reduced intake the next day, consistent with a mild conditioned taste aversion. These experiments support the conclusion that one reason contributing to the willingness of HDID mice to drink to the point of intoxication is a genetic insensitivity to the aversive effects of ethanol.

Withdrawal from Brief Repeated Alcohol Treatment in Adolescent and Adult Male and Female Rats

BACKGROUND

Early initiation of alcohol drinking has been associated with increased risk of alcohol dependence in adulthood. Although negative affect mediated in part by corticotropin-releasing factor (CRF) is a strong motivator for alcohol consumption in adults, comparisons of alcohol withdrawal in adolescents and adults generally have not included CRF-related measures such as anxiety. The purpose of the present study was to compare withdrawal signs including anxiety-like behavior after a brief multiday alcohol treatment in adolescent and adult male and female rats.

METHODS

Animals were treated with a 5-day regimen of alcohol injections (3 daily intraperitoneal injections of 1.5 g/kg at 3-hour intervals, total of 15) starting on postnatal day (PN) 28 or PN 70. Spontaneous withdrawal signs and anxiety-like behavior (light/dark box) were assessed 18 hours after the last injection as described. One cohort of rats was treated with alcohol, killed 18 hours after the last injection, and blood was collected to assess corticosterone. Another cohort of rats was treated with alcohol or vehicle, given 1, 2, or 3 alcohol injections (1.5 g/kg), and killed 1 hour after final injection to determine blood alcohol concentration (BAC). Finally, adult and adolescent males and females received 5 days of alcohol or vehicle treatment followed by a final challenge with alcohol (3 g/kg), and blood was collected for corticosterone.

RESULTS

BAC was comparable in adolescents and adults. Spontaneous withdrawal signs were comparable in adolescents and adults, and no sex differences were observed. Anxiety-like behaviors (time and distance in light, latency to emerge, and light entries) differed in alcohol- and vehicle-treated adults but not adolescents. Corticosterone was not elevated at withdrawal. Alcohol increased corticosterone significantly in vehicle-treated animals, but both adolescents and adults were tolerant to alcohol-induced elevation of corticosterone after 5 days of alcohol treatment.

CONCLUSIONS

These findings suggest that adolescents experience milder negative affect during withdrawal from brief alcohol exposures relative to adults but comparable suppression of hypothalamic-pituitary-adrenal axis function.

The role of the orbitofrontal cortex in alcohol use, abuse, and dependence

One of the major functions of the orbitofrontal cortex (OFC) is to promote flexible motivated behavior. It is no surprise, therefore, that recent work has demonstrated a prominent impact of chronic drug use on the OFC and a potential role for OFC disruption in drug abuse and addiction. Among drugs of abuse, the use of alcohol is particularly salient with respect to OFC function. Although a number of studies in humans have implicated OFC dysregulation in alcohol use disorders, animal models investigating the association between OFC and alcohol use are only beginning to be developed, and there is still a great deal to be revealed. The goal of this review is to consider what is currently known regarding the role of the OFC in alcohol use and dependence. I will first provide a brief, general overview of current views of OFC function and its contributions to drug seeking and addiction. I will then discuss research to date related to the OFC and alcohol use, both in human clinical populations and in non-human models. Finally I will consider issues and strategies to guide future study that may identify this brain region as a key player in the transition from moderated to problematic alcohol use and dependence.

Oral Binge-Like Ethanol Pre-Exposure During Juvenile/Adolescent Period Attenuates Ethanol-Induced Conditioned Place Aversion in Rats

Aims

To determine if oral ethanol self-administration produces a conditioned place preference (CPP) and to determine if ethanol pre-exposure conditions during the juvenile/adolescent period alter the conditioned effects of ethanol and subsequent ethanol self-administration.

Short summary

Modified conditioned place preference paradigm allowed rats to orally self-administer ethanol followed by short duration exposure to conditioning chambers. Ethanol produced a conditioned place aversion even though rats self-administered ethanol following the final conditioning test. Juvenile/adolescent pre-exposure to ethanol decreased the place aversion but did not produce place preference.

Methods

Juvenile/adolescent rats consumed sweetened 5% ethanol in the home-cage either during continuous access or intermittent access with water restriction that promoted binge-like consumption. A control group had water access during the 4-week period. Adult rats were conditioned using a modified CPP paradigm wherein rats were water-restricted overnight before being placed in operant chambers to respond for 5% ethanol for 7 min. Following the operant session, rats were placed in the conditioning chamber for 8 min. After the conditioning post-test, rats self-administered ethanol during daily operant sessions.

Results

Ethanol produced a conditioned place aversion in water access rats and the continuous access rats. Binge-like ethanol consumption induced by intermittent access with water restriction abolished the place aversion, but did not allow place preference to develop. After conditioning, continuous access rats self-administered ethanol above ~0.6 g/kg which was similar to rats with binge-like experience via intermittent access.

Conclusions

Results suggest that oral ethanol self-administration elicits aversive properties in this model even though ethanol continues to maintain self-administration. Pre-exposure to ethanol during the juvenile/adolescent period may produce tolerance to ethanol's aversive properties only when consumed in a binge-like manner with water restriction. More exploration is needed to understand how behavioral history can influence sensitivity to ethanol's rewarding and aversive properties and subsequent ethanol consumption or self-administration.

Sex Differences in Binge-Like and Aversion-Resistant Alcohol Drinking in C57BL/6J Mice

BACKGROUND

Alcohol use disorder is characterized by compulsive alcohol intake, or drinking despite negative consequences. Previous studies have shown that female rodents have a heightened vulnerability to drug use across different stages of the addictive cycle, but no previous studies have studied females in a model of aversion-resistant alcohol intake. Here, we investigated sex differences in binge-like and aversion-resistant alcohol drinking in C57BL/6J mice using a modified drinking-in-the-dark (DID) paradigm.

METHODS

In Experiment 1, 24-hour aversion to quinine (0, 100, or 250 μM) was assessed. In Experiment 2, male and female adult C57BL/6J mice consumed 15% ethanol (EtOH) or water in a 2-bottle limited-access DID paradigm for 2 h/d for 15 days. The EtOH was next adulterated with quinine (0, 100, or 250 μM) over 3 consecutive drinking sessions to test aversion-resistant intake. In Experiment 3, intake of quinine-adulterated (100 μM) EtOH was assessed across all 15 drinking sessions.

RESULTS

Quinine was equally aversive to both sexes in Experiment 1. In Experiment 2, female mice consumed significantly more alcohol than male mice during the final 6 drinking sessions. Levels of aversion-resistant intake did not differ between the sexes. In Experiment 3, quinine suppressed consumption in all mice, though females drank significantly more on the final 2 sessions.

CONCLUSIONS

The results of this study demonstrate that while female mice escalate and consume more EtOH than males, both sexes exhibit similar levels of aversion-resistant drinking. These results inform our understanding of how sex interacts with vulnerability for addiction and argue for the inclusion of females in more studies of aversion-resistant alcohol drinking.

Exploring Sex Differences in the Attenuation of Ethanol Drinking by Naltrexone in Dependent Rats During Early and Protracted Abstinence

BACKGROUND

Despite considerable efforts, few drugs are available for the treatment of alcohol (ethanol [EtOH]) use disorder (AUD). EtOH directly or indirectly modulates several aspects of the central nervous system, including neurotransmitter/neuromodulator systems. Relapse vulnerability is a challenge for the treatment of EtOH addiction. EtOH withdrawal symptoms create motivational states that lead to compulsive EtOH drinking and relapse even after long periods of abstinence. Among the therapeutics to treat AUD, naltrexone (NTX) is a pharmacological treatment for relapse. The present study evaluated the effect of NTX on EtOH drinking in male and female EtOH-dependent rats during abstinence.

METHODS

Wistar rats (males and females) were first trained to orally self-administer 10% EtOH. Half of the rats were then made dependent by chronic intermittent EtOH (CIE) vapor exposure, and the other half were exposed to air. Using this model, rats exhibit somatic and motivational signs of withdrawal. At the end of EtOH vapor (or air) exposure, the rats were tested for the effects of NTX (10 mg/kg, oral) on EtOH self-administration at 3 abstinence time points: acute abstinence (A-Abst, 8 hours), late abstinence (L-Abst, 2 weeks), and protracted abstinence (P-Abst, 6 weeks).

RESULTS

NTX decreased EtOH intake in nondependent rats, regardless of sex and abstinence time point. In postdependent rats, NTX decreased EtOH intake only at a delayed abstinence time point (P-Abst) in males, whereas it similarly reduced EtOH drinking in females at all abstinence time points.

CONCLUSIONS

The therapeutic efficacy of NTX depends on the time of intervention during abstinence and is different between males and females. The data further suggest that EtOH dependence causes different neuroadaptations in male and female rats, reflected by differential effects of NTX. The results underscore the significance of considering the duration of EtOH abstinence and sex as a biological variable as important factors when developing pharmacotherapies for AUD.

Sex Differences in Photic Entrainment and Sensitivity to Ethanol-Induced Chronodisruption in Adult Mice After Adolescent Intermittent Ethanol Exposure

BACKGROUND

Evidence supports a role for the circadian system in alcohol use disorders, but the impact of adolescent alcohol exposure on circadian timing later in life is unknown. Acute ethanol (EtOH) attenuates circadian photic phase-resetting in adult, but not adolescent, rodents. However, nearly all studies have focused on males and it is unknown whether this adolescent-typical insensitivity to EtOH persists into adulthood after adolescent drinking.

METHODS

Circadian activity was monitored in C57BL/6J mice receiving adolescent intermittent EtOH (AIE) exposure (15% EtOH and water every other day throughout adolescence) or water alone followed by 24 days wherein EtOH was not available (washout). Mice then received a challenge dose of EtOH (1.5 g/kg, intraperitoneal) or saline 15 minutes prior to a 30-minute phase-delaying light pulse and then were released into constant darkness (DD). To control for possible phase-shifting by EtOH challenge alone, a separate group of mice underwent AIE exposure (or water-only) and washout and then received an EtOH or saline injection, but did not receive a light pulse prior to DD.

RESULTS

Striking sex differences in nearly all measures of circadian photic entrainment were observed during adolescence but AIE effects were subtle and few. Only EtOH-naïve adult male mice showed attenuated photic phase-shifts with EtOH challenge, while all other groups showed normal phase-resetting responses to light. AIE-exposed females showed a persistent delay in activity offset.

CONCLUSIONS

Adult male AIE-exposed mice retained adolescent-like insensitivity to EtOH-induced suppression of photic phase-resetting, suggesting AIE-induced "lock-in" of an adolescent behavioral phenotype. Adult AIE-exposed females showed delayed initiation of the rest phase. Our results also indicate that intermittent EtOH drinking has subtle effects on circadian activity in mice during adolescence that differ from previously reported effects on adult males. The observed sex differences in circadian activity, EtOH consumption and preference, and responses to EtOH challenge merit future mechanistic study.

Studying Sex Differences in Animal Models of Addiction: An Emphasis on Alcohol-Related Behaviors

Animal models are essential for understanding the biological factors that contribute to drug and alcohol addiction and discovering new pharmacotherapies to treat these disorders. Alcohol (ethanol) is the most commonly abused drug in the world, and as the prevalence of alcohol use disorder (AUD) increases, so does the need for effective pharmacotherapies. In particular, treatments with high efficacy in the growing number of female AUD sufferers are needed. Female animals remain underrepresented in biomedical research and sex differences in the brain's response to alcohol are poorly understood. To help bridge the gender gap in addiction research, this Review discusses strategies that researchers can use to examine sex differences in the context of several common animal models of AUD. Self-administration, two-bottle choice, drinking in the dark, and conditioned place preference are discussed, with a focus on the role of estrogen as a mediator of sex differences in alcohol-related behaviors.