Sex differences in alcohol self-administration and relapse-like behavior in Long-Evans rats

Individual and sex differences in frontloading behavior and approach- avoidance conflict preference predict addiction-like ethanol seeking in rats

Recent research has identified sex-dependent links between risk taking behaviors, approach-avoidance bias and alcohol intake. However, preclinical studies have typically assessed alcohol drinking using a singular dimension of intake (i.e. drinking level), failing to capture the multidimensional pattern of aberrant alcohol-seeking observed in alcohol use disorder. In this study, we sought to further explore individual and sex differences in the relationship between approach-avoidance bias, frontloading (bingeing and onset skew) and multiple addiction-like indices of ethanol seeking that included motivation for ethanol, persistence despite its absence (extinction), and ethanol-taking in the face of mild footshock. We found that female rats displayed more addiction-like phenotypes than males overall, and that frontloading patterns differed by sex, with females outdrinking males in the early part of access sessions (bingeing), but males strongly concentrating their lever pressing for ethanol in that period (onset skew). Multiple regression analyses revealed that bingeing was a strong positive predictor and onset skew a negative predictor of motivational breakpoint. Cued-conflict preference - a measure of approach-avoidance bias towards a mixed-valence conflict cue - was predictive of both extinction and footshock in males, but not females. Our data highlight key sex differences, and the relevance of both frontloading patterns and conflict preference in predicting future addiction-like phenotypes.

Sex-associated differences in incentive salience and drinking behaviour in a rodent model of alcohol relapse

The ability of environmental cues to trigger alcohol-seeking behaviours is thought to facilitate problematic alcohol use. Individuals' tendency to attribute incentive salience to cues may increase the risk of addiction. We sought to study the relationship between incentive salience and alcohol addiction using non-preferring rats to model the heterogeneity of human alcohol consumption, investigating both males and females. Adult rats were subjected to the alcohol deprivation effect (ADE) paradigm, where they were given voluntary access to different alcohol solutions with repeated interruptions by deprivation and reintroduction phases over a protracted period (five Alcohol Deprivation Cycles). Before each Alcohol Deprivation Cycle, rats were tested in the Pavlovian Conditioned Approach (PCA) paradigm, which quantifies the individual salience toward a conditional cue and the reward, thus allowing us to trace the process of attributing incentive salience to reward cues. During the final Alcohol Deprivation Cycle (ADE5), animals were tested for compulsive-like behaviour using quinine taste adulteration. We investigated sex differences in drinking behaviour and PCA performance. We observed thatb females drank significantly more alcohol than males and displayed more sign-tracking (ST) behaviour in the PCA, whereas males showed goal-tracking (GT) behaviour. Furthermore, we found that high drinkers exhibited more ST behaviour. The initial PCA phenotype was correlated with later alcohol consumption. Our findings indicate a complex relationship between incentive salience and alcohol addiction and emphasize the importance of considering both sexes in preclinical research.

A novel alcohol+nicotine co-use self-administration procedure reveals sex differences and differential alteration of mesocorticolimbic TLR- and cholinergic-related neuroimmune gene expression in rats

Although alcohol and nicotine are two of the most commonly co-used drugs with upwards of 90% of adults with an alcohol use disorder (AUD) in the US also smoking, we don't tend to study alcohol and nicotine use this way. The current studies sought to develop and assess a novel alcohol + nicotine co-access self-administration (SA) model in adult male and female Long-Evans rats. Further, both drugs are implicated in neuroimmune function, albeit in largely opposing ways. Chronic alcohol use increases neuroinflammation via toll-like receptors (TLRs) which in turn increases alcohol intake. By contrast, nicotine produces anti-inflammatory effects, in part, through the monomeric alpha7 receptor (ChRNa7). Following long-term co-access (6 months), rats reliably administered both drugs during daily sessions, however males generally responded for more alcohol and females for nicotine. This was reflected in plasma analysis with translationally relevant intake levels of both alcohol and nicotine, making it invaluable in studying the effects of co-use on behavior and CNS function. Moreover, male rats show sensitivity to alterations in alcohol concentration whereas females show sensitivity to alterations in nicotine concentration. Rats trained on this procedure also developed an anxiogenic phenotype. Finally, we assessed alterations in neuroimmune-related gene expression in the medial prefrontal cortex - prelimbic, (mPFC-PL), nucleus accumbens core (AcbC), and ventral tegmental area (VTA). In the AcbC, where α7 expression was increased and β2 was decreased, markers of pro-inflammatory activity were decreased, despite increases in TLR gene expression suggesting that co-use with nicotine modulates inflammatory state downstream from the receptor level. By contrast, in mPFC-PL where α7 was not increased, both TLRs and downstream proinflammatory markers were increased. Taken together, these findings support that there are brain regional and sex differences with co-use of alcohol + nicotine SA and suggest that targeting nicotinic α7 may represent a novel strategy for treating alcohol + nicotine co-dependence.

Heart rate variability: A primer for alcohol researchers

Problem alcohol drinking remains a major cost and burden for society. Also, rates of problem drinking in women have dramatically increased in recent decades, and women are at risk for more alcohol problems and comorbidities. The purpose of this commentary is to discuss the potential utility of cardiac measures, including heart rate (HR) and HR variability (HRV), as markers of individual and sex differences in the drive to drink alcohol. We recently used cardiac telemetry in female and male adult rats to determine whether different cardiac markers, including HR and HRV, would differently predict alcohol and anxiety-like behavior across the sexes. Indeed, female behaviors related to HRV measures that indicate more parasympathetic (PNS) influence (the "rest and digest" system). In contrast, male behaviors are associated more with sympathetic (SNS) indicators (the activation system). Remarkably, similar sex differences in PNS versus SNS engagement under challenge are seen in several human studies, suggesting strong cross-species convergence in differential autonomic regulation in females and males. Here, we describe the larger challenges that alcohol addiction presents, and how HRV measures may provide new biomarkers to help enhance development of more individualized and sex-specific treatments. We briefly explain the physiological systems underlying cardiac PNS and SNS states, and how specific HRV metrics are defined and validated, especially why particular HRV measures are considered to reflect more PNS versus SNS influence. Finally, we describe hormonal influences and sex differences in brain circuits related to cardiac autonomic regulation. Together, these findings show that HR and HRV have potential for uncovering key underlying mechanisms of sex and individual differences in autonomic drivers, which could guide more personalized treatment.

Operant ethanol self-administration behaviors do not predict sex differences in continuous access home cage drinking

Understanding sex differences in disease prevalence is critical to public health, particularly in the context of alcohol use disorder (AUD). The goal of this study was to understand sex differences in ethanol drinking behavior and define the precise conditions under which sex differences emerge. Consistent with prior work, C57BL/6J females drank more than males under continuous access two-bottle choice conditions. However, using ethanol self-administration - where an operant response results in access to an ethanol sipper for a fixed time period - we found no sex differences in operant response rates or ethanol consumption (volume per body weight consumed, as well as lick behavior). This remained true across a wide range of parameters including acquisition, when the ethanol sipper access period was manipulated, and when the concentration of the ethanol available was scaled. The only sex differences observed were in total ethanol consumption, which was explained by differences in body weight between males and females, rather than by sex differences in motivation to drink. Using dimensionality reduction approaches, we found that drinking behavior in the operant context did not cluster by sex, but rather clustered by high and low drinking phenotypes. Interestingly, these high and low drinking phenotypes in the operant context showed no correlation with those same categorizations in the home cage context within the same animals. These data underscore the complexity of sex differences in ethanol consumption, highlighting the important role that drinking conditions/context plays in the expression of these differences.

Sex- and estrous-related response patterns for alcohol depend critically on the level of compulsion-like challenge

Alcohol use disorder is a substantial social and economic burden. During the last years, the number of women with drinking problems has been increasing, and one main concern is that they are particularly more vulnerable to negative consequences of alcohol. However, little is known about female-specific response patterns for alcohol, and potential underlying differences in brain mechanisms, including for compulsion-like alcohol drinking (when intake persists despite adverse consequences). We used lickometry to assess behavioral microstructure in adult Wistar male and female rats (n = 28-30) during alcohol-only drinking or moderate- or higher-challenge alcohol compulsion (10 or 60 mg/l quinine in alcohol, respectively). Estrous stages were determined and related to drinking levels and patterns of responding to alcohol, as was ovariectomy. Our findings showed that females (where we didn't determine estrus stage) had similar total licks in a session as males, but significantly longer licking bouts under alcohol-only and moderate-challenge, suggesting greater persistence. Further, greater intake under alcohol-only and moderate-challenge was related to faster licking in males, while female consumption was not related to licking speed. Thus, females could have increased persistence without greater vigor, unlike males. However, under higher-challenge, faster licking did predict higher intake in females, similar to males. To better understand female higher-challenge responding, we examined drinking in relation to phases of the estrous cycle. Higher-challenge had longer bouts only in late diestrus. In addition, ovariectomy led to longer bouts only under higher-challenge, suggesting that conditions with reduced hormone levels could increase female persistence for alcohol under higher-challenge. However, ovariectomy also reduced alcohol-only and moderate-challenge drinking but did not reduce bout length. Thus, intake level and response strategy could be regulated somewhat differently by ovarian hormones. Finally, moderate-challenge licking speed was less variable during early diestrus, and we previously showed more stereotyped responding specifically under moderate-challenge in males. By combining behavioral microstructure and sex- and estrus-related changes in drinking patterns, our results suggest that females have greater persistence for alcohol under lower-challenge drinking, while late diestrus and ovariectomy unmasked greater persistence under higher-challenge. Together, our novel insights could help develop more effective and personalized treatments for problematic alcohol use.

Greater inhibition of female rat binge alcohol intake by adrenergic receptor blockers using a novel Two-Shot rat binge drinking model

Binge drinking (BD) contributes strongly to the harms of alcohol use disorder. Most rodent models do not result in binge-level blood alcohol concentrations (BACs), and to better understand individual and sex differences in neurobiological mechanisms related to BD, the use of outbred rat strains would be valuable. Here, we developed a novel BD model where after 3+ months of intermittent access to 20% alcohol Wistar rats drank, twice a week, with two 5-min intake (what we called Two-shot) separated by a 10-min break. Our findings showed during Two-Shot that most animals reached ≥ 80 mg% BAC levels (when briefly food-restricted). However, when increasing alcohol concentrations from 20 to 30%, 40%, or 50%, rats titrated to similar intake levels, suggesting rapid sensing of alcohol effects even when front-loading. Two-Shot drinking was reduced in both sexes by naltrexone (1 mg/kg), validating intake suppression by a clinical therapeutic agent for human problem drinking. Further, both propranolol (β-adrenergic receptor antagonist) and prazosin (α1-adrenergic receptor antagonist) reduced female but not male BD at the lower dose. Thus, our results provide a novel model for BD in outbred rats and suggest that female binging is more sensitive to adrenergic modulation than males, perhaps providing a novel sex-related therapy.

Greater inhibition of female rat binge alcohol intake by adrenergic receptor blockers using a novel Two-Shot rat binge drinking model

Binge drinking (BD) contributes strongly to the harms of alcohol use disorder. Most rodent models do not result in binge-level blood alcohol concentrations (BACs), and to better understand individual and sex differences in neurobiological mechanisms related to BD, the use of outbred rat strains would be valuable. Here, we developed a novel BD model where after 3+ months of intermittent access to 20% alcohol Wistar rats drank, twice a week, with two 5-minute intake (what we called Two-shot) separated by a 10-minute break. Our findings showed during Two-Shot that most animals reached ≥ 80mg% BAC levels (when briefly food-restricted). However, when increasing alcohol concentrations from 20% to 30%, 40%, or 50%, rats titrated to similar intake levels, suggesting rapid sensing of alcohol effects even when front-loading. Two-Shot drinking was reduced in both sexes by naltrexone (1mg/kg), validating intake suppression by a clinical therapeutic agent. Further, both propranolol (β adrenergic receptor antagonist) and prazosin (α1 adrenergic receptor antagonist) reduced female but not male BD at the lower dose. Thus, our results provide a novel model for BD in outbred rats and suggest that female binging is more sensitive to adrenergic modulation than males, perhaps providing a novel sex-related therapy.

Validation of the extinction probe trial as a measure of motivation in male and female Long Evans rats

BACKGROUND

Ethanol self-administration is governed by appetitive and consummatory behaviors. The sipper model procedurally separates these behaviors by training rats to meet a response requirement within 20 min to obtain continuous access to a sipper tube for an additional 20 min. Variations of this paradigm have been developed to quantify appetitive strength by evaluating lever presses during an extinction probe trial (EPT) or by deriving a break point (BP) from a progressive ratio (PR) schedule of reinforcement. However, no study has assessed the relationship between these tasks, within subjects, in both sexes.

METHODS

Male and female rats (n = 16) were trained to meet a response requirement of 20 to access a slightly sweetened ethanol solution (10% ethanol + 1% sucrose). Two EPTs, during which no operant behavior was reinforced, were interleaved between 18 reinforced sessions. Next, rats completed an across-session PR schedule, where the response requirement increased each session. BP was defined as the highest completed response requirement. We then replicated the methodology in the same subjects responding for a 3% sucrose solution. Finally, the experiment was replicated in a separate cohort of rats (n = 24) trained to a response requirement of 4 to earn access to the ethanol solution and paradigm order (EPT vs. PR) was counterbalanced.

RESULTS

We report strong, positive correlations between average EPT lever presses and BP across all experiments. No sex differences were observed in appetitive behaviors. However, the two cohorts revealed mixed results when assessing sex differences in consummatory measures.

CONCLUSIONS

This study further validates the EPT as a measure of motivation and suggests that similar levels of motivation exist to procure alcohol in males and females. The findings complement the literature showing that appetitive and consummatory processes are distinct and thus should be independently assessed in self-administration paradigms.

Resurgence of ethanol seeking following voluntary abstinence produced by nondrug differential reinforcement of other behavior

Resurgence refers to the relapse of a target behavior following the worsening of a source of alternative reinforcement that was made available during response elimination. Most laboratory analyses of resurgence have used a combination of extinction and alternative reinforcement to reduce target behavior. In contingency-management treatments for alcohol use disorder, however, alcohol use is not placed on extinction. Instead, participants voluntarily abstain from alcohol use to access nondrug alternative reinforcers. Inasmuch, additional laboratory research on resurgence following voluntary abstinence is warranted. The present experiment evaluated resurgence of rats' ethanol seeking following voluntary abstinence produced by differential reinforcement of other behavior (DRO). Lever pressing produced ethanol reinforcers during baseline phases. During DRO phases, lever pressing continued to produce ethanol and food reinforcers were delivered according to resetting DRO schedules. Ethanol and food reinforcers were suspended during resurgence test phases to evaluate resurgence following voluntary abstinence. Lever pressing was elevated during baseline phases and occurred at near-zero rates during DRO phases. During the resurgence test phases, lever pressing increased, despite that it no longer produced ethanol. The procedure introduced here may help researchers better understand the variables that affect voluntary abstinence from ethanol seeking and resurgence following voluntary abstinence.

Sex differences and sex-specific regulation of motivated behavior by Melanin-concentrating hormone: a short review

Recent preclinical research exploring how neuropeptide transmitter systems regulate motivated behavior reveal the increasing importance of sex as a critical biological variable. Neuropeptide systems and their central circuits both contribute to sex differences in a range of motivated behaviors and regulate sex-specific behaviors. In this short review, we explore the current research of how sex as a biological variable influences several distinct motivated behaviors that are modulated by the melanin-concentrating hormone (MCH) neuropeptide system. First, we review how MCH regulates feeding behavior within the context of energy homeostasis differently between male and female rodents. Then, we focus on MCH's role in lactation as a sex-specific process within the context of energy homeostasis. Next, we discuss the sex-specific effects of MCH on maternal behavior. Finally, we summarize the role of MCH in drug-motivated behaviors. While these topics are traditionally investigated from different scientific perspectives, in this short review we discuss how these behaviors share commonalities within the larger context of motivated behaviors, and that sex differences discovered in one area of research may impact our understanding in another. Overall, our review highlights the need for further research into how sex differences in energy regulation associated with reproduction and parental care contribute to regulating motivated behaviors.

Different mechanisms underlie compulsive alcohol self-administration in male and female rats

BACKGROUND

Sex is an important factor in the progression and treatment of alcohol addiction, and therapeutic approaches may have to be tailored to potential sex differences. This highlights the importance of understanding sex differences in behaviors that reflect key elements of clinical alcohol addiction, such as continued use despite negative consequences ("compulsive use"). Studies in experimental animals can help provide an understanding of the role sex plays to influence these behaviors.

METHODS

Large populations of genetically heterogeneous male and female Wistar rats were tested in an established model of compulsive alcohol self-administration, operationalized as alcohol responding despite contingent foot shock punishment. We also tested baseline (fixed ratio, unpunished) operant alcohol self-administration, motivation to self-administer alcohol (progressive ratio), and temporal discounting for alcohol reward. In search of predictors of compulsivity, animals were screened for novelty-induced place preference, anxiety-like behavior, pain sensitivity and corticosterone levels. The estrous cycle was monitored throughout the study.

RESULTS

Unpunished self-administration of alcohol did not differ between males and females when alcohol intake was corrected for body weight. Overall, females showed higher levels of compulsive responding for alcohol. Compulsive response rates showed bimodal distributions in male but not in female rats when intermediate shock intensities were used (0.2 and 0.25 mA); at higher shock intensities, responding was uniformly suppressed in both males and females. We also found less steep discounting in females when alcohol was devalued by delaying its delivery. Males exhibited a stronger motivation to obtain alcohol under unpunished conditions, while females showed higher corticosterone levels at baseline. Factor analysis showed that an underlying dimension related to stress and pain predicted compulsivity in females, while compulsivity in males was predicted by a reward factor. We did not find differences in alcohol-related behaviors throughout the various stages of the estrous cycle.

CONCLUSIONS

Our results suggest that mechanisms promoting compulsivity, a key feature of alcohol addiction, likely differ between males and females. This underscores the importance of considering sex as a biological variable in both preclinical and clinical research, and has potential treatment implications in alcohol addiction.

Acute Ethanol Modulates Synaptic Inhibition in the Basolateral Amygdala via Rapid NLRP3 Inflammasome Activation and Regulates Anxiety-Like Behavior in Rats

Chronic alcohol exposure leads to a neuroinflammatory response involving activation of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome and proinflammatory cytokine production. Acute ethanol (EtOH) exposure activates GABAergic synapses in the central and basolateral amygdala (BLA) ex vivo, but whether this rapid modulation of synaptic inhibition is because of an acute inflammatory response and alters anxiety-like behavior in male and female animals is not known. Here, we tested the hypotheses that acute EtOH facilitates inhibitory synaptic transmission in the BLA by activating the NLRP3 inflammasome-dependent acute inflammatory response, that the alcohol-induced increase in inhibition is cell type and sex dependent, and that acute EtOH in the BLA reduces anxiety-like behavior. Acute EtOH application at a binge-like concentration (22-44 mm) stimulated synaptic GABA release from putative parvalbumin (PV) interneurons onto BLA principal neurons in ex vivo brain slices from male, but not female, rats. The EtOH facilitation of synaptic inhibition was blocked by antagonists of the Toll-like receptor 4 (TLR4), the NLRP3 inflammasome, and interleukin-1 receptors, suggesting it was mediated by a rapid local neuroinflammatory response in the BLA. In vivo, bilateral injection of EtOH directly into the BLA produced an acute concentration-dependent reduction in anxiety-like behavior in male but not female rats. These findings demonstrate that acute EtOH in the BLA regulates anxiety-like behavior in a sex-dependent manner and suggest that this effect is associated with presynaptic facilitation of parvalbumin-expressing interneuron inputs to BLA principal neurons via a local NLRP3 inflammasome-dependent neuroimmune response.SIGNIFICANCE STATEMENT Chronic alcohol exposure produces a neuroinflammatory response, which contributes to alcohol-associated pathologies. Acute alcohol administration increases inhibitory synaptic signaling in the brain, but the mechanism for the rapid alcohol facilitation of inhibitory circuits is unknown. We found that acute ethanol at binge-like concentrations in the basolateral amygdala (BLA) facilitates GABA release from parvalbumin-expressing (PV) interneuron synapses onto principal neurons in ex vivo brain slices from male rats and that intra-BLA ethanol reduces anxiety-like behavior in vivo in male rats, but not female rats. The ethanol (EtOH) facilitation of inhibition in the BLA is mediated by Toll-like receptor 4 (TLR4) and nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome activation and proinflammatory IL-1β signaling, which suggests a rapid NLRP3 inflammasome-dependent neuroimmune cascade that plays a critical role in acute alcohol intoxication.

Estradiol mediates sex differences in aversion-resistant alcohol intake

Introduction

Alcohol consumption despite negative consequences is a core symptom of alcohol use disorder. This can be modeled in mice by pairing aversive stimuli with alcohol consumption, such as adding the bitter tastant quinine to the alcohol solution. If an animal continues to drink alcohol despite such negative stimuli, this is typically considered aversion-resistant, or inflexible, drinking behavior. Previous studies in our lab have found that females are more aversion-resistant than males in that they tolerate higher concentrations of quinine before they suppress their alcohol intake. Interestingly, we did not observe any differences in intake across the estrous cycle. In regards to neuronal activation patterns during quinine-alcohol intake, we have found that male mice show higher levels of activation in the ventromedial prefrontal cortex and posterior insular cortex, while females show higher levels of activation in the ventral tegmental area.

Methods

In the experiments presented here, we conducted ovariectomies to further examine the role of circulating sex hormones in aversion-resistant alcohol intake and neuronal activation patterns. Furthermore, we used hormonal addback of estradiol or progesterone to determine which ovarian sex hormone mediates aversion-resistant consumption.

Results

We found that ovariectomy reduced quinine-adulterated alcohol intake, demonstrating that circulating sex hormones play a role in this behavior. We also observed reduced neuronal activation in the VTA of ovariectomized mice compared to sham females, and that estradiol supplementation reversed the effect of ovariectomy on quinine-alcohol intake.

Discussion

Taken together with our prior data, these findings suggest that circulating estradiol contributes to the expression of aversion-resistant alcohol intake and neuronal activity in the VTA. However, since this behavior is not affected by the estrous cycle, we believe this is due to a threshold level of this hormone, as opposed to fluctuations that occur across the estrous cycle.

Novel RXFP3 negative allosteric modulator RLX-33 reduces alcohol self-administration in rats

There is much interest in identifying novel pharmacotherapeutic targets that improve clinical outcomes for the treatment of alcohol use disorder (AUD). One promising target for therapeutic intervention is the relaxin family peptide 3 (RXFP3) receptor, a cognate receptor for neuropeptide relaxin-3, which has previously been implicated in regulating alcohol drinking behavior. Recently, we developed the first small-molecule RXFP3-selective negative allosteric modulator (NAM) RLX-33. Therefore, the goal of the present work was to characterize the impact of this novel NAM on affective-related behaviors and alcohol self-administration in rats. First, the effects of RLX-33 were tested on alcohol and sucrose self-administration in Wistar and alcohol-preferring P rats to determine the dose-response profile and specificity for alcohol. Then, we assessed the effects of systemic RLX-33 injection in Wistar rats in a battery of behavioral assays (open-field test, elevated zero maze, acoustic startle response test, and prepulse inhibition) and tested for alcohol clearance. We found that the lowest effective dose (5 mg/kg) reduced alcohol self-administration in both male and female Wistar rats, while in alcohol-preferring P rats, this effect was restricted to males, and there were no effects on sucrose self-administration or general locomotor activity. The characterization of affective and metabolic effects in Wistar rats generally found few locomotor, affective, or alcohol clearance changes, particularly at the 5 mg/kg dose. Overall, these findings are promising and suggest that RXFP3 NAM has potential as a pharmacological target for treating AUD.

Blocking μ-opioid receptors attenuates reinstatement of responding to an alcohol-predictive conditioned stimulus through actions in the ventral hippocampus

The µ-opioid system is involved in the reinstatement of responding that is immediately evoked by alcohol-predictive cues. The extent of its involvement in reinstatement observed in a new model that evaluates the delayed effects of re-exposure to alcohol, however, is unclear. The current study investigated the role of µ-opioid receptors (MORs) in the delayed reinstatement of an extinguished, Pavlovian conditioned response that was evoked 24 h after alcohol re-exposure. Female and male Long-Evans rats received Pavlovian conditioning in which a conditioned stimulus (CS) was paired with the delivery of an appetitive unconditioned stimulus (US; Experiments 1, 2, 4: 15% v/v alcohol; Experiment 3: 10% w/v sucrose) that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before but without the US. Next, the US was delivered but without the CS. A reinstatement test was conducted 24 h later, during which the CS was presented in the absence of the US. Silencing MORs via systemic naltrexone (0.3 or 1.0 mg/kg) attenuated reinstatement of port entries elicited by an alcohol-CS, but not those elicited by a sucrose-CS. Finally, blocking MORs in the ventral hippocampus via bilateral microinfusion of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 2.5 or 5.0 µg/hemisphere) prevented reinstatement of port alcohol-CS port entries. These data show that MORs are involved in the delayed reinstatement of a Pavlovian conditioned response in an alcohol-specific manner. Importantly, these data illustrate, for the first time, that MORs in the ventral hippocampus are necessary for responding to an alcohol-predictive cue.

Paternal alcohol exposure attenuates maintenance and reinstated operant responding for alcohol in the offspring of rats

BACKGROUND

The heritability of alcohol use disorder is close to 50%, yet common genetic variants account for less than 5% of risk. The missing heritability may reflect environmental exposure in the parents prior to conception. Indeed, paternal alcohol exposure has many behavioral and biological consequences for rodent offspring. We recently found that paternal alcohol exposure attenuated the acquisition of operant alcohol self-administration in offspring of rats of both sexes. Here we test whether this effect extends to other phases of operant self-administration thought to model motivation, craving, and relapse.

METHODS

Wistar male rats exposed to alcohol vapors or air for 6 weeks were mated with alcohol-naïve females 8 weeks later. The adult offspring were trained to lever press for alcohol and tested under several conditions: (1) maintenance responding under a progressive ratio schedule, (2) extinction responding due to removal of the alcohol delivery contingency, (3) reinstatement of extinguished responding in the presence of alcohol-associated cues, and (4) reinitiation of lever press responding for alcohol delivery under fixed and progressive ratio schedules.

RESULTS

Alcohol-sired offspring showed reduced responding under the progressive ratio schedule and blunted cue-induced reinstatement of extinguished responding. Alcohol-sired offspring also emitted fewer responses during extinction sessions and did not reinitiate responding to the same extent as control-sired rats after alcohol delivery was restored.

CONCLUSIONS

Across all conditions, paternal alcohol exposure led to a reduction in the reinforcing effects of alcohol in offspring. These results are consistent with studies conducted with paternal cocaine exposure except that here we find effects in rats of both sexes.

Inflammatory pain affects alcohol intake in a dose-dependent manner in male rats in the intermittent access model

Introduction

Epidemiological studies have shown that there is a relation between pain and alcohol use disorder (AUD). Persistent pain is directly correlated with an increment in alcohol consumption and an increased risk of developing an AUD. Greater levels of pain intensity and unpleasantness are associated with higher levels of relapse, an increase in alcohol consumption, rates of hazardous drinking, and delay to seek for treatment. However, this interaction has not been deeply studied in the preclinical setting.

Methods

Here, we aim to evaluate how inflammatory pain affects levels of alcohol drinking in male and female rats with a history of alcohol. For that, we used an intermittent access 2-bottle choice paradigm combined with the complete Freund Adjuvant (CFA) model of inflammatory pain.

Results

Our results show that CFA-induced inflammatory pain does not alter total intake of 20% alcohol in male or female rats. Interestingly, in males, the presence of CFA-induced inflammatory pain blunts the decrease of alcohol intake when higher concentrations of alcohol are available, whereas it does not have an effect on intake at any concentration in female rats.

Conclusion

Altogether, this study provides relevant data and constitutes an important contribution to the study of pain and AUD and it highlights the necessity to design better behavioral paradigms in animal models that are more translational and reflect current epidemiological findings.

FGF2 activity regulates operant alcohol self-administration and mesolimbic dopamine transmission

Fibroblast growth factor 2 (FGF2) is involved in the development and maintenance of the brain dopamine system. We previously showed that alcohol exposure alters the expression of FGF2 and its receptor, FGF receptor 1 (FGFR1) in mesolimbic and nigrostriatal brain regions, and that FGF2 is a positive regulator of alcohol drinking. Here, we determined the effects of FGF2 and of FGFR1 inhibition on alcohol consumption, seeking and relapse, using a rat operant self-administration paradigm. In addition, we characterized the effects of FGF2-FGFR1 activation and inhibition on mesolimbic and nigrostriatal dopamine neuron activation using in vivo electrophysiology. We found that recombinant FGF2 (rFGF2) increased the firing rate and burst firing activity of dopaminergic neurons in the mesolimbic and nigrostriatal systems and led to increased operant alcohol self-administration. In contrast, the FGFR1 inhibitor PD173074 suppressed the firing rate of these dopaminergic neurons, and reduced operant alcohol self-administration. Alcohol seeking behavior was not affected by PD173074, but this FGFR1 inhibitor reduced post-abstinence relapse to alcohol consumption, albeit only in male rats. The latter was paralleled by the increased potency and efficacy of PD173074 in inhibiting dopamine neuron firing. Together, our findings suggest that targeting the FGF2-FGFR1 pathway can reduce alcohol consumption, possibly via altering mesolimbic and nigrostriatal neuronal activity.

INVOLVEMENT OF CORTICAL PROJECTIONS TO BASOLATERAL AMYGDALA IN CONTEXT-INDUCED REINSTATEMENT OF ETHANOL-SEEKING IN RATS

Ethanol is the most consumed substance of abuse in the world, and its misuse may lead to the development of alcohol use disorder (AUD). High relapse rates remain a relevant problem in the treatment of AUD. Exposure to environmental cues previously associated with ethanol intake could trigger ethanol-seeking behavior. However, the neural mechanisms involved in this phenomenon are not entirely clear. In this context, cortical projections to the basolateral amygdala (BLA) play a role in appetitive and aversive learned behaviors. Therefore, we aimed to evaluate the activation of the cortical projections from the prelimbic (PL), orbitofrontal (OFC), and infralimbic (IL), to the BLA in the context-induced reinstatement of ethanol-seeking. Male Long-Evans rats were trained to self-administer 10% ethanol in Context A. Subsequently, lever pressing in the presence of the discrete cue was extinguished in Context B. After nine extinction sessions, rats underwent intracranial surgery for the unilateral injection of red fluorescent retrograde tracer into the BLA. The context-induced reinstatement of ethanol-seeking was assessed by re-exposing the rats to Context A or B under extinction conditions. Finally, we combined retrograde neuronal tracing with Fos to identify activated cortical inputs to BLA during the reinstatement of ethanol-seeking behavior. We found that PL, but not OFC or IL, retrogradely-labeled neurons from BLA presented increased Fos expression during the re-exposure to the ethanol-associated context, suggesting that PL projection to BLA is involved in the context-induced reinstatement of ethanol-seeking behavior.

Learning processes in relapse to alcohol use: lessons from animal models

RATIONALE

Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use.

OBJECTIVE

We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies.

KEY FINDINGS

Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats.

CONCLUSION

Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

Drinking history dependent functionality of the dorsolateral striatum on gating alcohol and quinine-adulterated alcohol front-loading and binge drinking

After an extended alcohol-drinking history, alcohol use can transition from controlled to compulsive, causing deleterious consequences. Alcohol use can be segregated into two distinct behaviors, alcohol seeking and alcohol taking. Expression of habitual and compulsive alcohol seeking depends on the dorsolateral striatum (DLS), a brain region thought to engage after extended alcohol access. However, it is unknown whether the DLS is also involved in compulsive-like alcohol taking. The purpose of this experiment was to identify whether the DLS gates compulsive-like binge alcohol drinking. To ask this question, we gave adult male and female C57BL/6J mice a binge-like alcohol-drinking history, which we have previously demonstrated to produce compulsive-like alcohol drinking (Bauer, McVey, & Boehm, 2021), or a water-drinking history. We then tested the involvement of the DLS on gating binge-like alcohol drinking and compulsive-like quinine-adulterated alcohol drinking via intra-DLS AMPA receptor antagonism. We hypothesized that pharmacological lesioning of the DLS would reduce compulsive-like quinine-adulterated alcohol (QuA) drinking, but not non-adulterated alcohol drinking, in male and female C57BL/6J mice. Three important findings were made. First, compulsive-like alcohol drinking is significantly blunted in cannulated mice. Because of this, we conclude that we were not able to adequately assess the effect of intra-DLS lesioning on compulsive-like alcohol drinking. Second, we found that the DLS gates binge-like alcohol drinking initially, which replicates findings in our previous work (Bauer, McVey, Germano, Zhang, & Boehm, 2022). However, following an extended alcohol history, the DLS no longer drives this behavior. Finally, alcohol and QuA front-loading is DLS-dependent in alcohol-history mice. Intra-DLS NBQX altered these drinking behaviors without altering ambulatory locomotor activity. These data demonstrate the necessity of the DLS in binge-like alcohol drinking before, but not following, an extended binge-like alcohol-drinking history and in alcohol front-loading in alcohol-history mice.

Alcohol availability during withdrawal gates the impact of alcohol vapor exposure on responses to alcohol cues

RATIONALE

Chronic intermittent ethanol (CIE) vapor inhalation is a widely used model of alcohol dependence, but the impact of CIE on cue-elicited alcohol seeking is poorly understood.

OBJECTIVE

Here, we assessed the effects of CIE on alcohol-seeking elicited by cues paired with alcohol before or after CIE vapor inhalation.

METHODS

In experiment 1, male and female Long-Evans rats were trained in a discriminative stimulus (DS) task, in which one auditory cue (the DS) predicts the availability of 15% ethanol and a control cue (the NS) predicts no ethanol. Rats then underwent CIE or served as controls. Subsets of each group received access to oral ethanol twice a week during acute withdrawal. After CIE, rats were presented with the DS and NS cues under extinction and retraining conditions to determine whether they would alter their responses to these cues. In experiment 2, rats underwent CIE prior to training in the DS task.

RESULTS

CIE enhanced behavioral responses to cues previously paired with alcohol, but only in rats that received access to alcohol during acute withdrawal. When CIE occurred before task training, male rats were slower to develop cue responses and less likely to enter the alcohol port, even though they had received alcohol during acute withdrawal.

CONCLUSIONS

These results suggest that CIE vapor inhalation alone does not potentiate the motivational value of alcohol cues but that an increase in cue responses requires alcohol experience during acute withdrawal. Furthermore, under some conditions, CIE may disrupt responses to alcohol-paired cues.

Modeling Aversion Resistant Alcohol Intake in Indiana Alcohol-Preferring (P) Rats

With the substantial social and medical burden of addiction, there is considerable interest in understanding risk factors that increase the development of addiction. A key feature of alcohol use disorder (AUD) is compulsive alcohol (EtOH) drinking, where EtOH drinking becomes “inflexible” after chronic intake, and animals, such as humans with AUD, continue drinking despite aversive consequences. Further, since there is a heritable component to AUD risk, some work has focused on genetically-selected, EtOH-preferring rodents, which could help uncover critical mechanisms driving pathological intake. In this regard, aversion-resistant drinking (ARD) takes >1 month to develop in outbred Wistar rats (and perhaps Sardinian-P EtOH-preferring rats). However, ARD has received limited study in Indiana P-rats, which were selected for high EtOH preference and exhibit factors that could parallel human AUD (including front-loading and impulsivity). Here, we show that P-rats rapidly developed compulsion-like responses for EtOH; 0.4 g/L quinine in EtOH significantly reduced female and male intake on the first day of exposure but had no effect after one week of EtOH drinking (15% EtOH, 24 h free-choice paradigm). Further, after 4−5 weeks of EtOH drinking, males but not females showed resistance to even higher quinine (0.5 g/L). Thus, P-rats rapidly developed ARD for EtOH, but only males developed even stronger ARD with further intake. Finally, rats strongly reduced intake of quinine-adulterated water after 1 or 5 weeks of EtOH drinking, suggesting no changes in basic quinine sensitivity. Thus, modeling ARD in P-rats may provide insight into mechanisms underlying genetic predispositions for compulsive drinking and lead to new treatments for AUDs.

Alcohol Deprivation Differentially Changes Alcohol Intake in Female and Male Rats Depending on Early-Life Stressful Experience

Experiencing early-life adverse events has enduring effects on individual vulnerability to alcohol abuse and the development of addiction-related behaviors. In rodents, it can be studied using maternal separation (MS) stress. Studies have shown that, depending on the protocol used, MS can affect the mother and pups' behavior and are associated with behavioral alterations later in adulthood, associated with both positive or negative outcomes. However, it is not fully elucidated how MS affects relapse-like behaviors when experienced by female or male individuals. Therefore, the aim of our study was to evaluate the effects of brief and prolonged MS on the alcohol deprivation effect (ADE) in female and male rats. Female and male Wistar rats were exposed to brief (15 min/day) or prolonged (180 min/day) MS from postnatal day (PND) 2 to 10. Later, during adulthood (PND 70), animals were submitted to an ADE protocol. Brief MS exposure prevented the ADE in both females and males, while prolonged MS exposure also prevented the ADE in female rats. Moreover, the ADE was more robust in females when compared to males. In conclusion, we showed that male and female rats are differentially affected by alcohol deprivation periods depending on their early-life experiences.

The Toll-like receptor 7 agonist imiquimod increases ethanol self-administration and induces expression of Toll-like receptor related genes

There is growing evidence that immune signalling may be involved in both the causes and consequences of alcohol abuse. Toll-like receptor (TLR) expression is increased by alcohol consumption and is implicated in AUD, and specifically TLR7 may play an important role in ethanol consumption. We administered the TLR7-specific agonist imiquimod in male and female Long-Evans rats to determine (1) gene expression changes in brain regions involved in alcohol reinforcement, the nucleus accumbens core and anterior insular cortex, in rats with and without an alcohol history, and (2) whether TLR7 activation could modulate operant alcohol self-administration. Interferon regulatory factor 7 (IRF7) was dramatically increased in both sexes at both 2- and 24-h post-injection regardless of alcohol history and TLR3 and 7 gene expression was increased as well. The proinflammatory cytokine TNFα was increased 24-h post-injection in rats with an alcohol self-administration history, but this effect did not persist after four injections, suggesting molecular tolerance. Ethanol consumption was increased 24 h after imiquimod injections but did not occur until the third injection, suggesting adaptation to repeated TLR7 activation is necessary for increased drinking to occur. Notably, imiquimod reliably induced weight loss, indicating that sickness behaviour persisted across repeated injections. These findings show that TLR7 activation can modulate alcohol drinking in an operant self-administration paradigm and suggest that TLR7 and IRF7 signalling pathways may be a viable druggable target for treatment of AUD.

Increased alcohol self-administration following repeated Toll-like receptor 3 agonist treatment in male and female rats

Toll-like receptor (TLR) signaling may play an important role in the neuroimmune system's involvement in the development and maintenance of alcohol use disorder (AUD). In the present study we administered the TLR3 agonist poly(I:C) in male and female Long-Evans rats to determine whether TLR3 agonism can increase alcohol consumption on a daily 15% alcohol operant self-administration paradigm. We found few effects when poly(I:C) was given every-other-day at 0.3 or 1.0 mg/kg. However, when 1.0 mg/kg was given on consecutive days, alcohol intake increased in the days following injections specifically in females. In a second experiment, we found that this effect only emerged when rats had a history of multiple poly(I:C) injections. In the final experiment the poly(I:C) dose was increased to 3.0 mg/kg on consecutive days which resulted in significant reductions in alcohol intake on injection days in females that were not accompanied by subsequent increases. The poly(I:C) dose was increased to 9.0 mg/kg for one final pair of injections which led to reductions in intake in both males and females followed by a male specific delayed increase in alcohol intake. Overall, repeated poly(I:C) administration was able to increase subsequent alcohol consumption in both sexes, with females showing an increase at a lower dose than males. These findings support TLR3 agonism in contributing to increased alcohol consumption and add to the body of work identifying the neuroimmune system as a potential therapeutic target for AUD.

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.

RTICBM-74 Is a Brain-Penetrant Cannabinoid Receptor Subtype 1 Allosteric Modulator that Reduces Alcohol Intake in Rats

The endocannabinoid system is implicated in the neuronal mechanisms of alcohol use disorder (AUD), with the cannabinoid receptor subtype 1 (CB1) representing a promising target for AUD therapeutic interventions. We have previously shown negative allosteric modulators (NAMs) of the CB1 receptor attenuated the reinstatement of other drugs of abuse including cocaine and methamphetamine in rats; however, their effects on alcohol-related behaviors have not been investigated. Here, we tested the pharmacokinetic properties of one such CB1 NAM, RTICBM-74, and its effects on alcohol self-administration in rats. RTICBM-74 showed low aqueous solubility and high protein binding but had excellent half-life and low clearance against rat liver microsomes and hepatocytes, and excellent brain penetrance in rats. RTICBM-74 pretreatment specifically reduced alcohol intake across a range of doses in male or female Wistar or Long-Evans rats that were trained to self-administer alcohol. These effects were similar to the CB1 antagonist/inverse agonist rimonabant, which was tested as a positive control. Importantly, RTICBM-74 was effective at reducing alcohol intake at doses that did not affect locomotion or sucrose self-administration. Our findings suggest that CB1 NAMs such as RTICBM-74 have promising therapeutic potential in treatment of AUD. SIGNIFICANCE STATEMENT: The present work shows that a metabolically stable and brain-penetrant cannabinoid receptor subtype 1 negative allosteric modulator reduces alcohol self-administration in rats without affecting locomotion or sucrose self-administration, suggesting potential therapeutic relevance for the treatment of alcohol use disorder.

Structural, functional, and behavioral significance of sex and gonadal hormones in the basolateral amygdala: A review of preclinical literature

The basolateral amygdala (BLA) is intimately involved in the development of neuropsychiatric disorders such as anxiety and alcohol use disorder (AUD). These disorders have clear sex biases, with women more likely to develop an anxiety disorder and men more likely to develop AUD. Preclinical models have largely confirmed these sex-specific vulnerabilities and emphasize the effects of sex hormones on behaviors influenced by the BLA. This review will discuss sex differences in BLA-related behaviors and highlight potential mechanisms mediated by altered BLA structure and function, including the composition of GABAergic interneuron subpopulations, glutamatergic pyramidal neuron morphology, glutamate/GABA neurotransmission, and neuromodulators. Further, sex hormones differentially organize dimorphic circuits during sensitive developmental periods (organizational effects) and initiate more transient effects throughout adulthood (activational effects). Current literature indicates that estradiol and allopregnanolone, a neuroactive progestogen, generally reduce BLA-related behaviors through a variety of mechanisms, including activation of estrogen receptors or facilitation of GABA_A-mediated inhibition, respectively. This enhanced GABAergic inhibition may protect BLA pyramidal neurons from the excitability associated with anxiety and alcohol withdrawal. Understanding sex differences and the effects of sex hormones on BLA structure and function may help explain sex-specific vulnerabilities in BLA-related behaviors and ultimately improve treatments for anxiety and AUD.

Maternal Separation Stress Affects Voluntary Ethanol Intake in a Sex Dependent Manner

Maternal separation (MS) stress is a predictive animal model for evaluating the effects of early stress exposure on alcohol use disorders (AUD). The extended amygdala (AMY) is a complex circuit involved in both stress- and ethanol-related responses. We hypothesized that MS stress may increase ethanol consumption in adulthood, as well as augment neuronal activity in extended AMY, in a sex-dependent manner. We aimed to investigate the influence of MS stress on the ethanol consumption of male and female mice, and the involvement of extended amygdala sub-nuclei in this process. The C57BL/6J pups were subjected to 180min of MS, from postnatal day (PND) 1 to 14. The control group was left undisturbed. On PND 45, mice (n=28) in cages were exposed to a bottle containing 20% ethanol (w/v) for 4h during the dark period of the light-dark cycle, for 3weeks. Afterward, mice underwent ethanol self-administration training in operant chambers under fixed ratio (FR) schedule. Then, subjects were tested under 2h sessions of a progressive-ratio (PR) schedule of reinforcement (the last ratio achieved was considered the breaking point), and at the end, a 4h session of FR schedule (binge-intake). An immunohistochemistry assay for Fos protein was performed in Nucleus Accumbens (NAcc), Bed Nucleus of Stria Terminalis (BNST), and AMY. Our results showed that in the third week of training, the female MS group consumed more ethanol than the respective control group. The MS group presented increased breakpoint parameters. Female control group and male MS group were more resistant to bitter quinine taste. Increased Fos-immunoreactive neurons (Fos-IR) were observed in the central nucleus of AMY, but not in NAcc nor BNST in male maternal-separated mice. Maternal separation stress may influence ethanol intake in adulthood, and it is dependent on the sex and reinforcement protocol.

Shortening time for access to alcohol drives up front-loading behavior, bringing consumption in male rats to the level of females

Background

Incentives to promote drinking (“happy hour”) can encourage faster rates of alcohol consumption, especially in women. Sex differences in drinking dynamics may underlie differential health vulnerabilities relating to alcohol in women versus men. Herein, we used operant procedures to model the happy hour effect and gain insight into the alcohol drinking dynamics of male and female rats.

Methods

Adult male and female Wistar rats underwent operant training to promote voluntary drinking of 10% (w/v) alcohol (8 rats/sex). We tested how drinking patterns changed after manipulating the effort required for alcohol (fixed ratio, FR), as well as the length of time in which rats had access to alcohol (self-administration session length). Rats were tested twice within the 12 h of the dark cycle, first at 2 h (early phase of the dark cycle, “early sessions”) and then again at 10 h into the dark cycle (late phase of the dark cycle, “late sessions”) with an 8-h break between the two sessions in the home cage.

Results

Adult females consumed significantly more alcohol (g/kg) than males in the 30-min sessions with the FR1 schedule of reinforcement when tested late in the dark cycle. Front-loading of alcohol was the primary factor driving higher consumption in females. Changing the schedule of reinforcement from FR1 to FR3 reduced total consumption. Notably, this manipulation had minimal effect on front-loading behavior in females, whereas front-loading behavior was significantly reduced in males when more effort was required to access alcohol. Compressing drinking access to 15 min to model a happy hour drove up front-loading behavior, generating alcohol drinking patterns in males that were similar to patterns in females (faster drinking and higher intake).

Conclusions

This strategy could be useful for exploring sex differences in the neural mechanisms underlying alcohol drinking and related health vulnerabilities. Our findings also highlight the importance of the time of testing for detecting sex differences in drinking behavior.

Voluntary alcohol drinking is higher in adult female rats compared to adult male rats. This sex difference is most pronounced in the later phase of the dark cycle, and when the operant effort is minimal (when 1 lever press gives 1 reward: fixed ratio 1, FR1).

Higher alcohol intake in females is primarily due to “front-loading”, or the rapid consumption of alcohol within the first 5 min of access.

Increasing the effort required to obtain alcohol from FR1 to FR3 dampens front-loading drinking behavior, resulting in similar levels of total intake in males and females.

Compressing the time of access to 15 min drives up front-loading to such a degree that rats end up consuming more alcohol in total than they do in 30-min sessions. In males, this increase in drinking is large enough that it eliminates the sex difference in total alcohol intake.

The GABAB receptor positive allosteric modulator ASP8062 reduces operant alcohol self-administration in male and female Sprague Dawley rats

RATIONALE

Pre-clinical evidence implicates the GABAergic system in mediating the reinforcing effects of alcohol and offers a therapeutic target for alcohol use disorder (AUD). The orthosteric GABA_B receptor agonist baclofen decreases alcohol self-administration in animals and alcohol use in humans; however side effects limit its utility. Pre-clinical evidence shows positive allosteric GABA_B receptor modulators also decrease alcohol self-administration without untoward side effects.

OBJECTIVES

We assessed the impact of the novel GABA_B-positive allosteric modulator ASP8062 and baclofen on operant alcohol self-administration and their potential non-specific effects.

METHODS

The effects of ASP8062 (1 - 10 mg/kg, PO) and baclofen (0.3 - 3 mg/kg, IP) were evaluated in male and female rats lever pressing for alcohol (10%, w/v) under a fixed ratio 2 schedule of reinforcement. On the fourth consecutive day of vehicle, ASP8062 or baclofen administration, active and inactive lever presses, reinforcers earned, head entries, and estimated alcohol consumed were analyzed. Locomotor activity was assessed in separate groups of rats following dosing.

RESULTS

Both ASP8062 and baclofen decreased alcohol self-administration and amount consumed (g/kg) in male and female rats. ASP8062 decreased operant alcohol self-administration to a greater extent in male rats, whereas baclofen was more efficacious in female rats. ASP8062 did not alter locomotor activity in either sex, whereas baclofen (3.0 mg/kg) decreased activity in male rats yet (1.0 mg/kg) increased activity in female rats.

CONCLUSIONS

ASP8062 decreases alcohol reinforcement like baclofen but without non-specific effects which are influenced by sex. Results support further development of ASP8062 as a potential treatment for AUD in humans.

Pairing Binge Drinking and a High-Fat Diet in Adolescence Modulates the Inflammatory Effects of Subsequent Alcohol Consumption in Mice

Alcohol binge drinking (BD) and poor nutritional habits are two frequent behaviors among many adolescents that alter gut microbiota in a pro-inflammatory direction. Dysbiotic changes in the gut microbiome are observed after alcohol and high-fat diet (HFD) consumption, even before obesity onset. In this study, we investigate the neuroinflammatory response of adolescent BD when combined with a continuous or intermittent HFD and its effects on adult ethanol consumption by using a self-administration (SA) paradigm in mice. The inflammatory biomarkers IL-6 and CX3CL1 were measured in the striatum 24 h after BD, 3 weeks later and after the ethanol (EtOH) SA. Adolescent BD increased alcohol consumption in the oral SA and caused a greater motivation to seek the substance. Likewise, mice with intermittent access to HFD exhibited higher EtOH consumption, while the opposite effect was found in mice with continuous HFD access. Biochemical analyses showed that after BD and three weeks later, striatal levels of IL-6 and CX3CL1 were increased. In addition, in saline-treated mice, CX3CL1 was increased after continuous access to HFD. After oral SA procedure, striatal IL-6 was increased only in animals exposed to BD and HFD. In addition, striatal CX3CL1 levels were increased in all BD- and HFD-exposed groups. Overall, our findings show that adolescent BD and intermittent HFD increase adult alcohol intake and point to neuroinflammation as an important mechanism modulating this interaction.

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.

Differential importance of nucleus accumbens Ox1Rs and AMPARs for female and male mouse binge alcohol drinking

Alcohol use disorder exhausts substantial social and economic costs, with recent dramatic increases in female problem drinking. Thus, it is critically important to understand signaling differences underlying alcohol consumption across the sexes. Orexin-1 receptors (Ox1Rs) can strongly promote motivated behavior, and we previously identified Ox1Rs within nucleus accumbens shell (shell) as crucial for driving binge intake in higher-drinking male mice. Here, shell Ox1R inhibition did not alter female mouse alcohol drinking, unlike in males. Also, lower dose systemic Ox1R inhibition reduced compulsion-like alcohol intake in both sexes, indicating that female Ox1Rs can drive some aspects of pathological consumption, and higher doses of systemic Ox1R inhibition (which might have more off-target effects) reduced binge drinking in both sexes. In contrast to shell Ox1Rs, inhibiting shell calcium-permeable AMPA receptors (CP-AMPARs) strongly reduced alcohol drinking in both sexes, which was specific to alcohol since this did not reduce saccharin intake in either sex. Our results together suggest that the shell critically regulates binge drinking in both sexes, with shell CP-AMPARs supporting intake in both sexes, while shell Ox1Rs drove drinking only in males. Our findings provide important new information about sex-specific and -general mechanisms that promote binge alcohol intake and possible targeted therapeutic interventions.

The synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline increases alcohol self-administration and alters basolateral amygdala response to alcohol in rats

Post-traumatic stress disorder (PTSD) is a psychiatric illness that can increase the risk for developing an alcohol use disorder (AUD). While clinical data has been useful in identifying similarities in the neurobiological bases of these disorders, preclinical models are essential for understanding the mechanism(s) by which stressors increase the risk for escalated alcohol consumption. The purpose of these studies was to examine if exposure of male Long-Evans rats to the synthetically derived predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT; a component of fox feces) would increase sweetened alcohol self-administration, potentially by facilitating transfer of salience towards cues, and alter neuronal response to alcohol as measured by the immediate early gene c-Fos. In experiment 1, rats exposed to repeated (4×) TMT showed reductions in port entries in Pavlovian conditioned approach and increases in sweetened alcohol self-administration. In experiment 2, rats exposed to repeated TMT showed blunted basolateral amygdala c-Fos response to alcohol. In experiment 3, rats exposed to single, but not repeated TMT, showed increases in sweetened alcohol self-administration, and no change in anxiety-like behavior or hyperarousal. In experiment 4, rats continued to show a significant corticosterone response to TMT after repeated exposures. In summary, exposure of male rats to TMT can cause escalations in sweetened alcohol self-administration and reduction in BLA response to alcohol. These studies outline and utilize a novel preclinical model that can be used to further neurobiological understanding of the emergence of escalated alcohol consumption following stress exposure.

Central amygdala mineralocorticoid receptors modulate alcohol self-administration

The mineralocorticoid receptor (MR) is an emerging target in the field of alcohol research. The MR is a steroid receptor in the same family as the glucocorticoid receptor, with which it shares the ligand corticosterone in addition to the MR selective ligand aldosterone. Recent studies have shown correlations between central amygdala (CeA) MR expression and alcohol drinking in rats and macaques, as well as correlations between aldosterone and alcohol craving in individuals with alcohol use disorder (AUD). Additionally, our previous work demonstrated that systemic treatment with the MR antagonist spironolactone reduced alcohol self-administration and response persistence in both male and female rats. This study examined if reductions in self-administration following MR antagonist treatment were related to dysregulation of MR-mediated corticosterone negative feedback. Female rats treated with spironolactone (50 mg/kg; IP) showed increased plasma corticosterone following self-administration, which correlated with reduced alcohol self-administration. Next, local microinjection of the MR-selective antagonist eplerenone was used to identify the brain-regional locus of MR action on alcohol self-administration. Eplerenone infusion produced dose-dependent reductions in alcohol self-administration in the CeA, but had no effect in the dorsal hippocampus. Finally, to assay the functional role of CeA MR expression in alcohol self-administration, CeA MR was knocked down by antisense oligonucleotide (ASO) infusion prior to alcohol self-administration. Rats showed a transient reduction in alcohol self-administration 1 day after ASO infusion. Together these studies demonstrate a functional role of CeA MR in modulating alcohol self-administration and make a case for studying MR antagonists as a novel treatment for AUD.

Sex Differences in the Neurobiology of Alcohol Use Disorder

Sex differences may play a critical role in modulating how chronic or heavy alcohol use impacts the brain to cause the development of alcohol use disorder (AUD). AUD is a multifaceted and complex disorder driven by changes in key neurobiological structures that regulate executive function, memory, and stress. A three-stage framework of addiction (binge/intoxication; withdrawal/negative affect; preoccupation/anticipation) has been useful for conceptualizing the complexities of AUD and other addictions. Initially, alcohol drinking causes short-term effects that involve signaling mediated by several neurotransmitter systems such as dopamine, corticotropin releasing factor, and glutamate. With continued intoxication, alcohol leads to dysfunctional behaviors that are thought to be due in part to alterations of these and other neurotransmitter systems, along with alterations in neural pathways connecting prefrontal and limbic structures. Using the three-stage framework, this review highlights examples of research examining sex differences in drinking and differential modulation of neural systems contributing to the development of AUD. New insights addressing the role of sex differences in AUD are advancing the field forward by uncovering the complex interactions that mediate vulnerability.

Chemogenetic Inactivation of Orbitofrontal Cortex Decreases Cue-induced Reinstatement of Ethanol and Sucrose Seeking in Male and Female Wistar Rats

BACKGROUND

The orbitofrontal cortex (OFC) encodes internal representations of outcomes and subjective value to facilitate flexible reward seeking. OFC activation is associated with drug seeking in both human subjects and animal models. OFC plays a role in alcohol use, but studies in animal models have produced conflicting results with some showing decreased seeking after OFC inactivation but others showing increased seeking or no changes. In part, this may be due to the different measures of alcohol seeking used (e.g., homecage drinking vs. operant seeking).

METHODS

We characterized the impact of transient inactivation of OFC (primarily lateral and, to a lesser extent, ventral subregions) using inhibitory hM4Di designer receptors exclusively activated by designer drugs (DREADDs). OFC neurons were transiently inhibited during 10% and 20% alcohol (ethanol, EtOH) and sucrose homecage consumption, fixed ratio (FR1) operant self-administration, and cue-induced reinstatement of either 10% EtOH or sucrose in male and female rats.

RESULTS

OFC inactivation did not affect sucrose or EtOH consumption in the homecage, nor did it influence seeking or consumption under FR1 operant conditions. In contrast, OFC inactivation suppressed cued-induced reinstatement for both EtOH and sucrose in both male and female rats.

CONCLUSIONS

Our results are aligned with previous work indicating a selective suppressive effect of OFC inactivation on reinstatement for alcohol and other drugs of abuse. They extend these findings to demonstrate no effect on homecage consumption or FR1 seeking as well as showing an impact of sucrose reinstatement. These data indicate that OFC plays a uniquely important role when reward seeking is driven by associations between external stimuli and internal representations of reward value, both for natural and drug rewards. They further implicate the OFC as a key structure driving relapse-associated seeking and potentially contributing to alcohol use disorder and other diseases of compulsive reward seeking.

Increased alcohol preference and intake in nicotine-preferring rats

BACKGROUND

Alcohol and tobacco are among the leading substances that are misused together and shared genetic vulnerability is likely. Increased susceptibility to nicotine self-administration has been shown in alcohol-preferring rat-lines. However, a nicotine-preferring (nP) rat-line has not been studied for alcohol preference.

OBJECTIVES

To evaluate alcohol preference and intake in male and female nP rats. We hypothesized that nP rats and females would drink more ethanol than control rats and males, respectively.

METHODS

nP rats are being selectively outbred for high oral nicotine intake at Ege University. Seventeen nP (18^th generation) and 20 naïve female and male SD rats, not previously exposed to alcohol or nicotine, were used. Twelve-week-old rats were given intermittent access to 20% ethanol in a 2-bottle-choice-procedure for six weeks. After one week withdrawal, six weeks of oral nicotine self-administration was applied.

RESULTS

nP rats drank significantly more ethanol than controls and their preference for ethanol over water was higher. Female rats' ethanol intake was higher than males'. The nP rats' nicotine preference and intake were higher than controls, and they gained less weight.

CONCLUSION

We have shown for the first time that nP rats also have high alcohol intake. Our results support the hypothesis that shared genetic factors may underlie concurrent addiction to nicotine and alcohol and have translational value in understanding their misuse. Considering the increased vulnerability for alcohol use disorder in smokers and sex differences observed, early preventive measures in families with a history of tobacco addiction, specifically targeting female members, could have public health benefits.

Effects of ethanol, naltrexone, nicotine and varenicline in an ethanol and nicotine co-use model in Sprague-Dawley rats

BACKGROUND

As alcohol and nicotine use disorders are entwined, it may be possible to develop a single medication to treat both. We previously developed a model for ethanol (EtOH) and nicotine co-use in female selectively bred alcohol-preferring (P) rats. To model co-use in a genetically diverse population, we adapted the model to outbred Sprague-Dawley rats of both sexes and assessed the effect of drug pretreatments.

METHODS

In phase 1, rats were trained in a 2-bottle choice between water and a sweetened or unsweetened EtOH solution in operant chambers. In phase 2, rats were trained in nicotine self-administration under an increasing fixed ratio (FR) schedule with 2 bottles containing water or saccharin-sweetened EtOH also available. In phase 3, rats were pretreated with EtOH (0.5, 1.5 g/kg), naltrexone (0.3 mg/kg), nicotine (0.2, 0.6 mg/kg), varenicline (3.0 mg/kg) or vehicle before the session.

RESULTS

Sweetening the EtOH solution was required to obtain pharmacologically relevant levels of consumption in Phase 1, with males showing increased sweetened EtOH preference compared to females. In Phase 2, increasing the FR requirement for nicotine decreased nicotine infusions, but increased EtOH consumption. In Phase 3, EtOH, naltrexone, and nicotine failed to alter EtOH consumption; however, varenicline decreased both EtOH and nicotine intake.

CONCLUSIONS

The co-use model was successfully adapted to Sprague-Dawley rats by adding saccharin to the EtOH solution. In contrast to previous results in P rats, varenicline reduced both EtOH and nicotine intake, indicating it may be a useful monotherapy for co-use in a genetically diverse population.

7,8-Dihydroxyflavone Enhances Cue-Conditioned Alcohol Reinstatement in Rats

Alcohol use disorder (AUD) is a detrimental disease that develops through chronic ethanol exposure. Reduced brain-derived neurotrophic factor (BDNF) expression has been associated with AUD and alcohol addiction, however the effects of activation of BDNF signalling in the brain on voluntary alcohol intake reinstatement and relapse are unknown. We therefore trained male and female Sprague Dawley rats in operant chambers to self-administer a 10% ethanol solution. Following baseline acquisition and progressive ratio (PR) analysis, rats were split into drug and vehicle groups during alcohol lever extinction. The animals received two weeks of daily IP injection of either the BDNF receptor, TrkB, agonist, 7,8-dihydroxyflavone (7,8-DHF), or vehicle. During acquisition of alcohol self-administration, males had significantly higher absolute numbers of alcohol-paired lever presses and a higher PR breakpoint. However, after adjusting for body weight, the amount of ethanol was not different between the sexes and the PR breakpoint was higher in females than males. Following extinction, alcohol-primed reinstatement in male rats was not altered by pretreatment with 7,8-DHF when adjusted for body weight. In contrast, in female rats, the weight-adjusted potential amount of ethanol, but not absolute numbers of active lever presses, was significantly enhanced by 7,8-DHF treatment during reinstatement. Analysis of spontaneous locomotor activity in automated photocell cages suggested that the effect of 7,8-DHF was not associated with hyperactivity. These results suggest that stimulation of the TrkB receptor may contribute to reward craving and relapse in AUD, particularly in females.

Sex differences in the glutamate system: Implications for addiction

Clinical and preclinical research have identified sex differences in substance use and addiction-related behaviors. Historically, substance use disorders are more prevalent in men than women, though this gap is closing. Despite this difference, women appear to be more susceptible to the effects of many drugs and progress to substance abuse treatment more quickly than men. While the glutamate system is a key regulator of addiction-related behaviors, much of the work implicating glutamate signaling and glutamatergic circuits has been conducted in men and male rodents. An increasing number of studies have identified sex differences in drug-induced glutamate alterations as well as sex and estrous cycle differences in drug seeking behaviors. This review will describe sex differences in the glutamate system with an emphasis on implications for substance use disorders, highlighting the gaps in our current understanding of how innate and drug-induced alterations in the glutamate system may contribute to sex differences in addiction-related behaviors.

Biological intersection of sex, age, and environment in the corticotropin releasing factor (CRF) system and alcohol

The neuropeptide corticotropin-releasing factor (CRF) is critical in neural circuit function and behavior, particularly in the context of stress, anxiety, and addiction. Despite a wealth of preclinical evidence for the efficacy of CRF receptor 1 antagonists in reducing behavioral pathology associated with alcohol exposure, several clinical trials have had disappointing outcomes, possibly due to an underappreciation of the role of biological variables. Although he National Institutes of Health (NIH) now mandate the inclusion of sex as a biological variable in all clinical and preclinical research, the current state of knowledge in this area is based almost entirely on evidence from male subjects. Additionally, the influence of biological variables other than sex has received even less attention in the context of neuropeptide signaling. Age (particularly adolescent development) and housing conditions have been shown to affect CRF signaling and voluntary alcohol intake, and the interaction between these biological variables is particularly relevant to the role of the CRF system in the vulnerability or resilience to the development of alcohol use disorder (AUD). Going forward, it will be important to include careful consideration of biological variables in experimental design, reporting, and interpretation. As new research uncovers conditions in which sex, age, and environment play major roles in physiological and/or pathological processes, our understanding of the complex interaction between relevant biological variables and critical signaling pathways like the CRF system in the cellular and behavioral consequences of alcohol exposure will continue to expand ultimately improving the ability of preclinical research to translate to the clinic. This article is part of the special issue on Neuropeptides.

Chronic ethanol exposure alters prelimbic prefrontal cortical Fast-Spiking and Martinotti interneuron function with differential sex specificity in rat brain

Chronic ethanol exposure results in numerous neurobiological adaptations that promote deficits in medial prefrontal cortical (mPFC) function associated with blunted inhibitory control and elevated anxiety during withdrawal. Studies exploring alcohol dependence-related changes in this region have largely investigated adaptations in glutamatergic signaling, with inhibitory neurotransmission remaining relatively understudied. To address this, we used biochemical and electrophysiological methods to evaluate the effects of ethanol on the activity of deep-layer prelimbic mPFC Fast-Spiking (FS) and Martinotti interneurons after chronic ethanol exposure in male and female rats. We report that chronic alcohol exposure significantly impairs FS neuron excitability in both males and females. Interestingly, we observed a marked sex difference in the baseline activity of Martinotti cells that furthermore displayed differential sex-specific responses to alcohol exposure. In addition, alcohol effects on Martinotti neuron excitability negatively correlated with hyperpolarization-activated currents mediated by hyperpolarization-activated cyclic nucleotide gated (HCN) channels, indicative of a causal relationship. Analysis of HCN1 protein expression also revealed a substantial sex difference, although no effect of ethanol on HCN1 protein expression was observed. Taken together, these findings further elucidate the complex adaptations that occur in the mPFC after chronic ethanol exposure and reveal fundamental differences in interneuron activity between sexes. Furthermore, this disparity may reflect innate differences in intracortical microcircuit function between male and female rats, and offers a tenable circuit-level explanation for sex-dependent behavioral responses to alcohol.

Evidence for incentive salience sensitization as a pathway to alcohol use disorder

The incentive salience sensitization (ISS) theory of addiction holds that addictive behavior stems from the ability of drugs to progressively sensitize the brain circuitry that mediates attribution of incentive salience (IS) to reward-predictive cues and its behavioral manifestations. In this article, we establish the plausibility of ISS as an etiological pathway to alcohol use disorder (AUD). We provide a comprehensive and critical review of evidence for: (1) the ability of alcohol to sensitize the brain circuitry of IS attribution and expression; and (2) attribution of IS to alcohol-predictive cues and its sensitization in humans and non-human animals. We point out gaps in the literature and how these might be addressed. We also highlight how individuals with different alcohol subjective response phenotypes may differ in susceptibility to ISS as a pathway to AUD. Finally, we discuss important implications of this neuropsychological mechanism in AUD for psychological and pharmacological interventions attempting to attenuate alcohol craving and cue reactivity.

Cue-alcohol associative learning in female rats

The ability of environmental cues to trigger alcohol-seeking behaviors is believed to facilitate problematic alcohol use. We previously showed that the development of this cue-evoked alcohol approach reflects cue-alcohol learning and memory in the adult male rat; however, we do not know whether the same is true for similarly aged female rats. Consequently, adult Long-Evans female rats were allowed to drink unsweetened alcohol in the home cage (Monday, Wednesday, Friday; 24-h two-bottle choice; 5 weeks) and were subsequently split into two experimental groups: Paired and Unpaired. Groups were matched for ingested doses and alcohol bottle preference across the pre-conditioning home cage period. Both groups were trained in conditioning chambers using a Pavlovian procedure. For the Paired group, the chamber houselight was illuminated to signal access to an alcohol sipper. Houselight onset was yoked for the Unpaired group, but access to the alcohol sipper was scheduled to occur only during the intervening periods (in the absence of light). We found that in the Paired, but not Unpaired group, an alcohol approach reaction was conditioned to houselight illumination, and the level of cue-conditioned reactivity predicted drinking behavior within trials. Groups experienced equivalently low but non-negligible blood alcohol concentrations over the course of conditioning sessions. We conclude that cue-triggered alcohol-seeking behavior in adult female rats reflects associative learning about the relationship between alcohol availability and houselight illumination.