Rewarding and aversive effects of ethanol in High Drinking in the Dark selectively bred mice

Retrieval of an ethanol-conditioned taste aversion promotes GABAergic plasticity in the insular cortex

Blunted sensitivity to ethanol's aversive effects can increase motivation to consume ethanol; yet, the neurobiological circuits responsible for encoding these aversive properties are not fully understood. Plasticity in cells projecting from the insular cortex (IC) to the basolateral amygdala (BLA) is critical for taste aversion learning and retrieval, suggesting this circuit's potential involvement in modulating the aversive properties of ethanol. Here, we tested the hypothesis that GABAergic activity onto IC-BLA projections would be facilitated following the retrieval of an ethanol-conditioned taste aversion (CTA). Consistent with this hypothesis, frequency of mIPSCs was increased following retrieval of an ethanol-CTA across cell layers in IC-BLA projection neurons. This increase in GABAergic plasticity occurred in both a circuit-specific and learning-dependent manner. Additionally, local inhibitory inputs onto layer 2/3 IC-BLA projection neurons were greater in number and strength following ethanol-CTA. Finally, DREADD-mediated inhibition of IC parvalbumin-expressing cells blunted the retrieval of ethanol-CTA in male, but not female, mice. Collectively, this work implicates a circuit-specific and learning-dependent increase in GABAergic tone following retrieval of an ethanol-CTA, thereby advancing our understanding of how the aversive effects of ethanol are encoded in the brain.

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 the subjective response to ethanol is limited. A major contributor to this lack of knowledge is the absence of preclinical models that enable exploration of this individual variability such as is possible in studies of 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 exposed to saccharin paired with either dose of ethanol exhibited lower saccharin intake relative to saline controls indicative 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, among CTA-resistant animals females were more resistant to the development of ethanol-induced CTA than males. Phenotypic differences were not driven by differences in baseline saccharin intake.

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 used in future studies to investigate the neurobiological mechanisms that confer risk for AUD.

Chemogenetic inhibition of locus coeruleus to rostromedial tegmental nucleus noradrenergic pathway increases light cycle ethanol drinking in male and female mice and blunts ethanol-induced CTA

We recently showed that chemogenetic activation of the locus coeruleus (LC) to the rostromedial tegmental nucleus (RMTg) noradrenergic (NE) pathway significantly blunted binge-like ethanol drinking and induced aversive-like behaviors in mice. The aim of the present study is to determine if silencing this TH + LC → RMTg noradrenergic pathway promotes increased levels of binge-like ethanol intake and reduced ethanol-induced conditioned taste aversion (CTA). To this end, both male and female TH-ires-cre mice on a C57BL/6 J background were cannulated in the RMTg and injected in the LC with rAVV viruses that encode cre-dependent Gi-expressing designer receptor exclusively activated by designer drugs (DREADDs), or its control, to directly control the activity of NE neurons. Inhibition of the LC to RMTg pathway had no effect on the binge-ethanol drinking in a "drinking-in-the-dark" (DID) paradigm. However, when using this paradigm during the light cycle, silencing of this circuit significantly increased ethanol intake without altering sucrose drinking. Moreover, we found that inhibition of this circuit significantly attenuated an ethanol-induced CTA. In addition, when compared to control animals, pairing RMTg-directed Clozapine N-oxide (CNO) with an i.p. injection of 1.5 g/kg ethanol reduced c-Fos activation in the LC, and increased c-Fos expression in the ventral tegmental area (VTA) in Gi-expressing mice. Our data show that inhibition of the TH + LC to the RMTg pathway significantly increased ethanol drinking as well as attenuated ethanol-induced CTA, supporting the involvement of the LC to RMTg noradrenergic circuit as an important protective mechanism against excessive ethanol consumption.

Animal Models of Excessive Alcohol Consumption in Rodents

The development of animal models that demonstrate excessive levels of alcohol consumption has played an important role in advancing our knowledge about neurobiological underpinnings and environmental circumstances that engender such maladaptive behavior. The use of these preclinical models has also provided valuable opportunities for discovering new and novel therapeutic targets that may be useful in the treatment of alcohol use disorder (AUD). While no single model can fully capture the complexities of AUD, the goal is to develop animal models that closely approximate characteristics of heavy alcohol drinking in humans to enhance their translational value and utility. A variety of experimental approaches have been employed to produce the desired phenotype of interest-robust and reliable excessive levels of alcohol drinking. Here we provide an updated review of five animal models that are commonly used. The models entail procedural manipulations of scheduled access to alcohol (time of day, duration, frequency), periods of time when access to alcohol is withheld, and history of alcohol exposure. Specially, the models involve (a) scheduled access to alcohol, (b) scheduled periods of alcohol deprivation, (c) scheduled intermittent access to alcohol, (d) scheduled-induced polydipsia, and (e) chronic alcohol (dependence) and withdrawal experience. Each of the animal models possesses unique experimental features that engender excessive levels of alcohol consumption. Both advantages and disadvantages of each model are described along with discussion of future work to be considered in developing more optimal models. Ultimately, the validity and utility of these models will lie in their ability to aid in the discovery of new and novel potential therapeutic targets as well as serve as a platform to evaluate treatment strategies that effectively reduce excessive levels of alcohol consumption associated with AUD.

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.

Long-term alcohol drinking in High Drinking in the Dark mice is stable for many months and does not show alcohol deprivation effects

We have modelled genetic risk for binge-like drinking by selectively breeding High Drinking in the Dark-1 and -2 (HDID-1 and HDID-2) mice for their propensity to reach intoxicating blood alcohol levels (BALs) after binge-like drinking in a single bottle, limited access paradigm. Interestingly, in standard two-bottle choice (2BC) tests for continuously available alcohol versus water, HDID mice show modest levels of preference. This indicates some degree of independence of the genetic contributions to risk for binge-like and sustained, continuous access drinking. We had few data where the drinking in the dark (DID) tests of binge-like drinking had been repeatedly performed, so we serially offered multiple DID tests to see whether binge-like drinking escalated. It did not. We also asked whether HDID mice would escalate their voluntary intake with prolonged exposure to alcohol 2BC. They did not. Lastly, we assessed whether an alcohol deprivation effect (ADE) developed. ADE is a temporary elevation in drinking typically observed after a period of abstinence from sustained access to alcohol choice. With repetition, these periods of ADE sometimes have led to more sustained elevations in drinking. We therefore asked whether repeated ADE episodes would elevate choice drinking in HDID mice. They did not. After nearly 500 days of alcohol access, the intake of HDID mice remained stable. We conclude that a genetically-enhanced high risk for binge-like drinking is not sufficient to yield alterations in long-term alcohol intake.

Sex differences in the development of conditioned place preference induced by intragastric alcohol administration in mice

BACKGROUND

The present study aimed to identify for the first time sex differences in the development of CPP induced by intragastric alcohol administration in mice.

METHODS

Male and female adult Swiss mice were submitted to 16 days of conditioning with alcohol (0.5-3.0 g/kg, N = 8/dose/sex), with 2 post-conditioning tests (after 8 and 16 sessions) during the protocol.

RESULTS

8 days of conditioning (4 alcohol sessions, 4 saline sessions) with intragastric alcohol administration were sufficient to induce CPP in male mice at the doses of 1.0, 1.5 and 2.0 g/kg. However, only higher doses (2.0, 2.5 and 3.0 g/kg) induced CPP in female mice using an 8-day conditioning protocol, while a 16-day conditioning protocol was necessary for the development of intragastric alcohol-induced CPP at the doses of 1.0 and 1.5 g/kg. Regardless of the conditioning protocol, higher doses or alcohol that had rewarding effects in females (2.5 and 3.0 g/kg) did not induce CPP in males, with a significant difference between males and females at those doses. Analysis of the potency (EC₅₀) and efficacy (E_max) of alcohol in inducing CPP when administered intragastrically in male and female mice showed significant sex differences with 8 conditioning sessions.

CONCLUSIONS

Our data show a clear protocol (8 vs 16 days) and dose difference between male and female Swiss mice regarding the development of CPP induced by intragastric alcohol administration. Intragastric alcohol administration is closer to human drinking, and our protocol provides a more translational approach to studying the rewarding effects of alcohol in mice.

Activation of locus coeruleus to rostromedial tegmental nucleus (RMTg) noradrenergic pathway blunts binge-like ethanol drinking and induces aversive responses in mice

There is strong evidence that ethanol entails aversive effects that can act as a deterrent to overconsumption. We have found that in doses that support the development of a conditioned taste aversion ethanol increases the activity of tyrosine hydroxylase (TH) positive neurons in the locus coeruleus (LC), a primary source of norepinephrine (NE). Using cre-inducible AAV8-ChR2 viruses in TH-ires-cre mice we found that the LC provides NE projections that innervate the rostromedial tegmental nucleus (RMTg), a brain region that has been implicated in the aversive properties of drugs. Because the neurocircuitry underlying the aversive effects of ethanol is poorly understood, we characterized the role of the LC to RMTg circuit in modulating aversive unconditioned responses and binge-like ethanol intake. Here, both male and female TH-ires-cre mice were cannulated in the RMTg and injected in the LC with rAVV viruses that encode for a Gq-expressing designer receptor exclusively activated by designer drugs (DREADDs) virus, or its control virus, to directly control the activity of NE neurons. A Latin Square paradigm was used to analyze both 20% ethanol and 3% sucrose consumption using the "drinking-in-the-dark" (DID) paradigm. Chemogenetic activation of the LC to RMTg pathway significantly blunted the binge-ethanol drinking, with no effect on the sucrose consumption, increased the emission of mid-frequency vocalizations and induced malaise-like behaviors in mice. The present findings indicate an important involvement of the LC to RMTg pathway in reducing ethanol consumption, and characterize unconditioned aversive reactions induced by activation of this noradrenergic pathway.

Acetaldehyde Excitation of Lateral Habenular Neurons via Multiple Cellular Mechanisms

Acetaldehyde (ACD), the first metabolite of ethanol, is implicated in several of ethanol's actions, including the reinforcing and aversive effects. The neuronal mechanisms underlying ACD's aversive effect, however, are poorly understood. The lateral habenula (LHb), a regulator of midbrain monoaminergic centers, is activated by negative valence events. Although the LHb has been linked to the aversive responses of several abused drugs, including ethanol, little is known about ACD. We, therefore, assessed ACD's action on LHb neurons in rats. The results showed that intraperitoneal injection of ACD increased cFos protein expression within the LHb and that intra-LHb infusion of ACD induced conditioned place aversion in male rats. Furthermore, electrophysiological recording in brain slices of male and female rats showed that bath application of ACD facilitated spontaneous firing and glutamatergic transmission. This effect of ACD was potentiated by an aldehyde dehydrogenase (ALDH) inhibitor, disulfiram (DS), but attenuated by the antagonists of dopamine (DA) receptor (DAR) subtype 1 (SCH23390) and subtype 2 (raclopride), and partly abolished by the pretreatment of DA or DA reuptake blocker (GBR12935; GBR). Moreover, application of ACD initiated a depolarizing inward current (I _ACD) and enhanced the hyperpolarizing-activated currents in LHb neurons. Bath application of Rp-cAMPs, a selective cAMP-PKA inhibitor, attenuated ACD-induced potentiation of EPSCs and I _ACD Finally, bath application of ZD7288, a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channels, attenuated ACD-induced potentiation of firing, EPSCs, and I _ACD These results show that ACD exerts its aversive property by exciting LHb neurons via multiple cellular mechanisms, and new treatments targeting the LHb may be beneficial for alcoholism.SIGNIFICANCE STATEMENT Acetaldehyde (ACD) has been considered aversive peripherally and rewarding centrally. However, whether ACD has a central aversive property is unclear. Here, we report that ACD excites the lateral habenula (LHb), a brain region associated with aversion and negative valence, through multiple cellular and molecular mechanisms. Intra-LHb ACD produces significant conditioned place aversion. These results suggest that ACD's actions on the LHb neurons might contribute to its central aversive property and new treatments targeting the LHb may be beneficial for alcoholism.

Relationship between voluntary ethanol drinking and approach-avoidance biases in the face of motivational conflict: novel sex-dependent associations in rats

RATIONALE

Aberrant approach-avoidance conflict processing may contribute to compulsive seeking that characterizes addiction. Exploration of the relationship between drugs of abuse and approach-avoidance behavior remains limited, especially with ethanol.

OBJECTIVES

To investigate the effects of voluntary ethanol consumption on approach-avoidance conflict behavior and to examine the potential approach/avoidance bias to predict drinking in male and female rats.

METHODS

Long-Evans rats consumed ethanol for 5 weeks under the intermittent access two-bottle choice (IA2BC) paradigm. Approach-avoidance tendencies were assessed before and after IA2BC drinking using a previously established cued approach-avoidance conflict maze task and the elevated plus maze (EPM).

RESULTS

Female rats displayed higher consumption of and preference for ethanol than males. In the conflict task, males showed greater approach bias towards cues predicting conflict than females. In females only, a median split and regression analysis of cued-conflict preference scores revealed that the more conflict-avoidant group displayed higher intake and preference for ethanol in the first few weeks of drinking. In both sexes, ethanol drinking did not affect cued-conflict preference, but ethanol exposure led to increased time spent in the central hub in the males only. Finally, anxiety levels in EPM predicted subsequent onset of ethanol drinking in males only.

CONCLUSIONS

Our results highlight sex and individual differences in both drinking and approach-avoidance bias in the face of cued conflict and further suggest that cued-conflict preference should be examined as a potential predictor of ethanol drinking. Ethanol exposure may also affect the timing of decision-making in the face of conflict.

The impact of Drinking in the Dark (DID) procedural manipulations on ethanol intake in High Drinking in the Dark (HDID) mice

The High Drinking in the Dark mouse lines (HDID-1 and HDID-2) were selectively bred to achieve high blood ethanol concentrations (BECs) in the Drinking in the Dark (DID) task, a widely used model of binge-like intake of 20% ethanol. There are several components that differentiate DID from other animal models of ethanol intake: time of day of testing, length of ethanol access, single-bottle access, and individual housing. Here, we sought to determine how some of these individual factors contribute to the high ethanol intake observed in HDID mice. HDID-1, HDID-2, and non-selected HS/NPT mice were tested in a series of DID experiments where one of the following factors was manipulated: length of ethanol access, fluid choice, number of ethanol bottles, and housing condition. We observed that 1) HDID mice achieve intoxicating BECs in DID, even when they are group-housed; 2) HDID mice continue to show elevated ethanol intake relative to HS/NPT mice during an extended access session, but this is most apparent during the first 4 h of access; and 3) offering a water choice during DID prevents elevated intake in the HDID-1 mice, but not necessarily in HDID-2 mice. Together, these results suggest that the lack of choice in the DID paradigm, together with the length of ethanol access, are important factors contributing to elevated ethanol intake in the HDID mice. These results further suggest important differences between the HDID lines in response to procedural manipulations of housing condition and ethanol bottle number in the DID paradigm, highlighting the distinct characteristics that each of these lines possess, despite being selectively bred for the same phenotype.

Ethanol-Related Behaviors in Mouse Lines Selectively Bred for Drinking to Intoxication

Alcohol use disorder (AUD) is a devastating psychiatric disorder that has significant wide-reaching effects on individuals and society. Selectively bred mouse lines are an effective means of exploring the genetic and neuronal mechanisms underlying AUD and such studies are translationally important for identifying treatment options. Here, we report on behavioral characterization of two replicate lines of mice that drink to intoxication, the High Drinking in the Dark (HDID)-1 and -2 mice, which have been selectively bred (20+ generations) for the primary phenotype of reaching high blood alcohol levels (BALs) during the drinking in the dark (DID) task, a binge-like drinking assay. Along with their genetically heterogenous progenitor line, Hs/Npt, we tested these mice on: DID and drinking in the light (DIL); temporal drinking patterns; ethanol sensitivity, through loss of righting reflex (LORR); and operant self-administration, including fixed ratio (FR1), fixed ratio 3:1 (FR3), extinction/reinstatement, and progressive ratio (PR). All mice consumed more ethanol during the dark than the light and both HDID lines consumed more ethanol than Hs/Npt during DIL and DID. In the dark, we found that the HDID lines achieved high blood alcohol levels early into a drinking session, suggesting that they exhibit front loading like drinking behavior in the absence of the chronicity usually required for such behavior. Surprisingly, HDID-1 (female and male) and HDID-2 (male) mice were more sensitive to the intoxicating effects of ethanol during the dark (as determined by LORR), while Hs/Npt (female and male) and HDID-2 (female) mice appeared less sensitive. We observed lower HDID-1 ethanol intake compared to either HDID-2 or Hs/Npt during operant ethanol self-administration. There were no genotype differences for either progressive ratio responding, or cue-induced ethanol reinstatement, though the latter is complicated by a lack of extinguished responding behavior. Taken together, these findings suggest that genes affecting one AUD-related behavior do not necessarily affect other AUD-related behaviors. Moreover, these findings highlight that alcohol-related behaviors can also differ between lines selectively bred for the same phenotype, and even between sexes within those same line.

Confirmation of a Causal Taar1 Allelic Variant in Addiction-Relevant Methamphetamine Behaviors

Sensitivity to rewarding and reinforcing drug effects has a critical role in initial use, but the role of initial aversive drug effects has received less attention. Methamphetamine effects on dopamine re-uptake and efflux are associated with its addiction potential. However, methamphetamine also serves as a substrate for the trace amine-associated receptor 1 (TAAR1). Growing evidence in animal models indicates that increasing TAAR1 function reduces drug self-administration and intake. We previously determined that a non-synonymous single nucleotide polymorphism (SNP) in Taar1 predicts a conformational change in the receptor that has functional consequences. A Taar1 ^m1J mutant allele existing in DBA/2J mice expresses a non-functional receptor. In comparison to mice that possess one or more copies of the reference Taar1 allele (Taar1 ^+/+ or Taar1 ^+/m1J ), mice with the Taar1 ^m1J/m1J genotype readily consume methamphetamine, express low sensitivity to aversive effects of methamphetamine, and lack sensitivity to acute methamphetamine-induced hypothermia. We used three sets of knock-in and control mice in which one Taar1 allele was exchanged with the alternative allele to determine if other methamphetamine-related traits and an opioid trait are impacted by the same Taar1 SNP proven to affect MA consumption and hypothermia. First, we measured sensitivity to conditioned rewarding and aversive effects of methamphetamine to determine if an impact of the Taar1 SNP on these traits could be proven. Next, we used multiple genetic backgrounds to study the consistency of Taar1 allelic effects on methamphetamine intake and hypothermia. Finally, we studied morphine-induced hypothermia to confirm prior data suggesting that a gene in linkage disequilibrium with Taar1, rather than Taar1, accounts for prior observed differences in sensitivity. We found that a single SNP exchange reduced sensitivity to methamphetamine conditioned reward and increased sensitivity to conditioned aversion. Profound differences in methamphetamine intake and hypothermia consistently corresponded with genotype at the SNP location, with only slight variation in magnitude across genetic backgrounds. Morphine-induced hypothermia was not dependent on Taar1 genotype. Thus, Taar1 genotype and TAAR1 function impact multiple methamphetamine-related effects that likely predict the potential for methamphetamine use. These data support further investigation of their potential roles in risk for methamphetamine addiction and therapeutic development.

Distinct and Overlapping Patterns of Acute Ethanol-Induced C-Fos Activation in Two Inbred Replicate Lines of Mice Selected for Drinking to High Blood Ethanol Concentrations

The inbred high drinking in the dark (iHDID1 and iHDID2) strains are two replicate lines bred from the parent HS/Npt (HS) line for achieving binge levels of blood ethanol concentration (≥80 mg/dL BEC) in a four-hour period. In this work, we sought to evaluate differences in baseline and ethanol-induced c-Fos activation between the HS, iHDID1, and iHDID2 genetic lines in brain regions known to process the aversive properties of ethanol. Methods: Male and female HS, iHDID1, and iHDID2 mice underwent an IP saline 2 3 g/kg ethanol injection. Brain sections were then stained for c-Fos expression in the basolateral/central amygdala (BLA/CeA), bed nucleus of the stria terminals (BNST), A2, locus coeruleus (LC), parabrachial nucleus (PBN), lateral/medial habenula (LHb/MHb), paraventricular nucleus of the thalamus (PVT), periaqueductal gray (PAG), Edinger–Westphal nuclei (EW), and rostromedial tegmental nucleus (RMTg). Results: The iHDID1 and iHDID2 lines showed similar and distinct patterns of regional c-Fos; however, in no region did the two both significantly differ from the HS line together. Conclusions: Our findings lend further support to the hypothesis the iHDID1 and the iHDID2 lines arrive at a similar behavior phenotype through divergent genetic mechanisms.

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.

Targeting the Glucocorticoid Receptor Reduces Binge-Like Drinking in High Drinking in the Dark (HDID-1) Mice

BACKGROUND

Chronic alcohol exposure can alter glucocorticoid receptor (GR) function in some brain areas that promotes escalated and compulsive-like alcohol intake. GR antagonism can prevent dependence-induced escalation in drinking, but very little is known about the role of GR in regulating high-risk nondependent alcohol intake. Here, we investigate the role of GR in regulating binge-like drinking and aversive responses to alcohol in the High Drinking in the Dark (HDID-1) mice, which have been selectively bred for high blood ethanol (EtOH) concentrations (BECs) in the Drinking in the Dark (DID) test, and in their founder line, the HS/NPT.

METHODS

In separate experiments, male and female HDID-1 mice were administered one of several compounds that inhibited GR or its negative regulator, FKBP51 (mifepristone [12.5, 25, 50, 100 mg/kg], CORT113176 [20, 40, 80 mg/kg], and SAFit2 [10, 20, 40 mg/kg]) during a 2-day DID task. EtOH consumption and BECs were measured. EtOH conditioned taste and place aversion (CTA and CPA, respectively) were measured in separate HDID-1 mice after mifepristone administration to assess GR's role in regulating the conditioned aversive effects of EtOH. Lastly, HS/NPT mice were administered CORT113176 during DID to assess whether dissimilar effects from those of HDID-1 would be observed, which could suggest that selective breeding had altered sensitivity to the effects of GR antagonism on binge-like drinking.

RESULTS

GR antagonism (with both mifepristone and CORT113176) selectively reduced binge-like EtOH intake and BECs in the HDID-1 mice, while inhibition of FKBP51 did not alter intake or BECs. In contrast, GR antagonism had no effect on EtOH intake or BECs in the HS/NPT mice. Although HDID-1 mice exhibit attenuated EtOH CTA, mifepristone administration did not enhance the aversive effects of EtOH in either a CTA or CPA task.

CONCLUSION

These data suggest that the selection process increased sensitivity to GR antagonism on EtOH intake in the HDID-1 mice, and support a role for the GR as a genetic risk factor for high-risk alcohol intake.

Chronic Chemogenetic Stimulation of the Nucleus Accumbens Produces Lasting Reductions in Binge Drinking and Ameliorates Alcohol-Related Morphological and Transcriptional Changes

Binge drinking is a dangerous pattern of behavior. We tested whether chronically manipulating nucleus accumbens (NAc) activity (via clozapine-N-oxide (CNO) and Designer Receptors Exclusively Activated by Designer Drugs (DREADD)) could produce lasting effects on ethanol binge-like drinking in mice selectively bred to drink to intoxication. We found chronically increasing NAc activity (4 weeks, via CNO and the excitatory DREADD, hM3Dq) decreased binge-like drinking, but did not observe CNO-induced changes in drinking with the inhibitory DREADD, hM4Di. The CNO/hM3Dq-induced reduction in ethanol drinking persisted for at least one week, suggesting adaptive neuroplasticity via transcriptional and epigenetic mechanisms. Therefore, we defined this plasticity at the morphological and transcriptomic levels. We found that chronic binge drinking (6 weeks) altered neuronal morphology in the NAc, an effect that was ameliorated with CNO/hM3Dq. Moreover, we detected significant changes in expression of several plasticity-related genes with binge drinking that were ameliorated with CNO treatment (e.g., Hdac4). Lastly, we found that LMK235, an HDAC4/5 inhibitor, reduced binge-like drinking. Thus, we were able to target specific molecular pathways using pharmacology to mimic the behavioral effects of DREADDs.

Effects of Pharmacologically Targeting Neuroimmune Pathways on Alcohol Drinking in Mice Selectively Bred to Drink to Intoxication

BACKGROUND

Rodent models of high alcohol drinking offer opportunities to better understand factors for alcohol use disorders (AUD) and test potential treatments. Selective breeding was carried out to create 2 unique High Drinking in the Dark (HDID-1, HDID-2) mouse lines that represent models of genetic risk for binge-like drinking. A number of studies have indicated that neuroimmune genes are important for regulation of alcohol drinking. We tested whether compounds shown to reduce drinking in other models also reduce alcohol intake in these unique genetic lines.

METHODS

We report tests of gabapentin, tesaglitazar, fenofibrate, caffeic acid phenethyl ester (CAPE), ibrutinib, and rolipram. Although these compounds have different mechanisms of action, they have all been shown to reduce inflammatory responses. We evaluated effects of these compounds on alcohol intake. In order to facilitate comparison with previously published findings for some compounds, we employed similar schedules that were previously used for that compound.

RESULTS

Gabapentin increased ethanol (EtOH) binge-like alcohol drinking in female HDID-1 and HS/NPT mice. Tesaglitazar and fenofibrate did not alter 2-bottle choice (2BC) drinking in male HDID-1 or HS/NPT mice. However, tesaglitazar had no effect on DID EtOH intake but reduced blood alcohol levels (BAL), and fenofibrate increased DID intake with no effects on BAL. CAPE had no effect on EtOH intake. Ibrutinib reduced intake in female HDID-1 in initial testing, but did not reduce intake in a second week of testing. Rolipram reduced DID intake and BALs in male and female HDID-1, HDID-2, and HS/NPT mice.

CONCLUSIONS

A number of compounds shown to reduce EtOH drinking in other models, and genotypes are not effective in HDID mice or their genetically heterogeneous founders, HS/NPT. The most promising compound was the PDE4 inhibitor, rolipram. These results highlight the importance of assessing generalizability when rigorously testing compounds for therapeutic development.

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.

Effects of sex on ethanol conditioned place preference, activity and variability in C57BL/6J and DBA/2J mice

Previous studies of ethanol drinking in rodents have shown greater intake in females than in males, but the reasons behind this difference are unknown. To address one possible interpretation of the drinking difference, these studies tested the hypothesis that female and male mice differ in sensitivity to the rewarding effects of ethanol using the conditioned place preference (CPP) procedure. To increase the generalizability of the results, sex differences were examined in two inbred mouse strains known to differ in their sensitivity to ethanol reward: C57BL/6J (B6) and DBA/2J (D2). Mice were conditioned in an unbiased CPP procedure using either 1 or 2 g/kg ethanol. To detect possible differences in learning rate, they were tested once at the midpoint of conditioning and again after conditioning ended. As expected, CPP was stronger with 2 g/kg than with 1 g/kg, and D2 mice generally showed stronger CPP than B6 mice. However, there were no sex differences in the rate of CPP acquisition or in CPP magnitude, suggesting no sex difference in ethanol reward sensitivity as indexed by CPP. Nevertheless, there were sex differences in locomotor activity. B6 females were generally more active than B6 males during CPP acquisition whereas D2 females were slightly less active than D2 males during both CPP acquisition and preference testing. Unexpectedly, female mice showed more variability than males in the behavioral measures recorded in these studies, encouraging greater attention to variability in the design, analysis and interpretation of future studies of sex differences in mice.

Alcohol operant self-administration: Investigating how alcohol-seeking behaviors predict drinking in mice using two operant approaches

Alcohol operant self-administration paradigms are critical tools for studying the neural circuits implicated in both alcohol-seeking and consummatory behaviors and for understanding the neural basis underlying alcohol-use disorders. In this study, we investigate the predictive value of two operant models of oral alcohol self-administration in mice, one in which alcohol is delivered into a cup following nose-poke responses with no accurate measurement of consumed alcohol solution, and another paradigm that provides access to alcohol via a sipper tube following lever presses and where lick rate and consumed alcohol volume can be measured. The goal was to identify a paradigm where operant behaviors such as lever presses and nose pokes, as well as other tracked behavior such as licks and head entries, can be used to reliably predict blood alcohol concentration (BAC). All mice were first exposed to alcohol in the home cage using the "drinking in the dark" (DID) procedure for 3 weeks and then were trained in alcohol self-administration using either of the operant paradigms for several weeks. Even without sucrose fading or food pre-training, mice acquired alcohol self-administration with both paradigms. However, neither lever press nor nose-poke rates were good predictors of alcohol intake or BAC. Only the lick rate and consumed alcohol were consistently and significantly correlated with BAC. Using this paradigm that accurately measures alcohol intake, unsupervised cluster analysis revealed three groups of mice: high-drinking (43%), low-drinking (37%), and non-drinking mice (20%). High-drinking mice showed faster acquisition of operant responding and achieved higher BACs than low-drinking mice. Lick rate and volume consumed varied with the alcohol concentration made available only for high- and low-drinking mice, but not for non-drinking mice. In addition, high- and low-drinking mice showed similar patterns during extinction and significant cue-induced reinstatement of seeking. Only high-drinking mice showed insensitivity to quinine adulteration, indicating a willingness to drink alcohol despite pairing with aversive stimuli. Thus, this study shows that relying on active presses is not an accurate determination of drinking behavior in mice. Only paradigms that allow for accurate measurements of consumed alcohol and/or lick rate are valid models of operant alcohol self-administration, where compulsive-like drinking could be accurately determined based on changes in alcohol intake when paired with bitter-tasting stimuli.

High Drinking in the Dark (HDID) mice are sensitive to the effects of some clinically relevant drugs to reduce binge-like drinking

BACKGROUND

There is a serious public health need for better understanding of alcohol use disorder disease mechanisms and for improved treatments. At this writing, only three drugs are approved by the Food and Drug Administration as medications to treat alcohol use disorders - disulfiram, naltrexone, and acamprosate. Binge drinking is a form of abusive alcohol drinking defined by the NIAAA as a drinking to blood alcohol levels (BALs)>0.08% during a period of approximately 2h. To model genetic risk for binge-like drinking, we have used selective breeding to create a unique animal model, High Drinking in the Dark (HDID) mice. Behavioral characterization of HDID mice has revealed that HDID mice exhibit behavioral impairment after drinking, withdrawal after a single binge-drinking session, and escalate their intake in response to induction of successive cycles of dependence. Notably, HDID mice do not exhibit altered tastant preference or alcohol clearance rates. We therefore asked whether drugs of known clinical relevance could modulate binge-like ethanol drinking in HDID mice, reasoning that this characterization of HDID responses should inform future use of this genetic animal model for screening and development of novel potential therapeutics.

METHODS

We tested the efficacy of acamprosate and naltrexone to reduce binge-like drinking in HDID mice. Additionally, we tested the GABAB receptor agonist, baclofen, based on recent pre-clinical and clinical studies demonstrating that it reduces alcohol drinking. We elected not to include disulfiram due to its more limited clinical usage. Mice were tested after acute doses of drugs in the limited-access Drinking in the Dark (DID) paradigm.

RESULTS

HDID mice were sensitive to the effects of acamprosate and baclofen, but not naltrexone. Both drugs reduced binge-like drinking. However, naltrexone failed to reduce drinking in HDID mice. Thus, HDID mice may represent a useful model for screening novel compounds.

Fear conditioning in mouse lines genetically selected for binge-like ethanol drinking

The comorbidity of substance- and alcohol-use disorders (AUD) with other psychiatric conditions, especially those related to stress such as post-traumatic stress disorder (PTSD), is well-established. Binge-like intoxication is thought to be a crucial stage in the development of the chronic relapsing nature of the addictions, and self-medication through binge-like drinking is commonly seen in PTSD patients. We have selectively bred two separate High Drinking in the Dark (HDID-1 and HDID-2) mouse lines to reach high blood ethanol concentrations (BECs) after a 4-h period of access to 20% ethanol starting shortly after the onset of circadian dark. As an initial step toward the eventual goal of employing binge-prone HDID mice to study PTSD-like behavior including alcohol binge drinking, we sought first to determine their ability to acquire conditioned fear. We asked whether these mice acquired, generalized, or extinguished conditioned freezing to a greater or lesser extent than unselected control HS/Npt mice. In two experiments, we trained groups of 16 adult male mice in a standard conditioned fear protocol. Mice were tested for context-elicited freezing, and then, in a novel context, for cue-induced freezing. After extinction tests, renewal of conditioned fear was tested in the original context. Mice of all three genotypes showed typical fear responding. Context paired with shock elicited freezing behavior in a control experiment, but cue unpaired with shock did not. These studies indicate that fear learning per se does not appear to be influenced by genes causing predisposition to binge drinking, suggesting distinct neural mechanisms. However, HDID mice are shown to be a suitable model for studying the role of conditioned fear specifically in binge-like drinking.

Cocaine self-administration punished by intravenous histamine in adolescent and adult rats

Adolescence is a transitional phase marked by a heightened vulnerability to substances of abuse. It has been hypothesized that both increased sensitivity to reward and decreased sensitivity to aversive events may drive drug-use liability during this phase. To investigate possible age-related differences in sensitivity to the aversive consequences of drug use, adolescent and adult rats were compared on self-administration of cocaine before, during, and after a 10-day period in which an aversive agent, histamine, was added to the cocaine solution. Adult and adolescent female rats were trained to self-administer intravenous cocaine (0.4 mg/kg/infusion) over 10 sessions (2 h/session; 2 sessions/day). Histamine (4 mg/kg/infusion) was then added directly into the cocaine solution for the next 10 sessions. Finally, the cocaine/histamine solution was replaced with a cocaine-only solution, and rats continued to self-administer cocaine (0.4 mg/kg) for 20 sessions. Compared with adolescent rats, adult rats showed a greater decrease in cocaine self-administration when it was punished with intravenous histamine compared with their baseline cocaine self-administration rates. These results suggest that differences in the sensitivity to negative consequences of drug use may partially explain developmental differences in drug use vulnerability.

Neuropeptide Y response to alcohol is altered in nucleus accumbens of mice selectively bred for drinking to intoxication

The High Drinking in the Dark (HDID) mice have been selectively bred for drinking to intoxicating blood alcohol levels and represent a genetic model of risk for binge-like drinking. Presently, little is known about the specific genetic factors that promote excessive intake in these mice. Previous studies have identified neuropeptide Y (NPY) as a potential target for modulating alcohol intake. NPY expression differs in some rodent lines that have been selected for high and low alcohol drinking phenotypes, as well as inbred mouse strains that differ in alcohol preference. Alcohol drinking and alcohol withdrawal also produce differential effects on NPY expression in the brain. Here, we assessed brain NPY protein levels in HDID mice of two replicates of selection and control heterogeneous stock (HS) mice at baseline (water drinking) and after binge-like alcohol drinking to determine whether selection is associated with differences in NPY expression and its sensitivity to alcohol. NPY levels did not differ between HDID and HS mice in any brain region in the water-drinking animals. HS mice showed a reduction in NPY levels in the nucleus accumbens (NAc) - especially in the shell - in ethanol-drinking animals vs. water-drinking controls. However, HDID mice showed a blunted NPY response to alcohol in the NAc core and shell compared to HS mice. These findings suggest that the NPY response to alcohol has been altered by selection for drinking to intoxication in a region-specific manner. Thus, the NPY system may represent a potential target for altering binge-like alcohol drinking in these mice.

An animal model of differential genetic risk for methamphetamine intake

The question of whether genetic factors contribute to risk for methamphetamine (MA) use and dependence has not been intensively investigated. Compared to human populations, genetic animal models offer the advantages of control over genetic family history and drug exposure. Using selective breeding, we created lines of mice that differ in genetic risk for voluntary MA intake and identified the chromosomal addresses of contributory genes. A quantitative trait locus was identified on chromosome 10 that accounts for more than 50% of the genetic variance in MA intake in the selected mouse lines. In addition, behavioral and physiological screening identified differences corresponding with risk for MA intake that have generated hypotheses that are testable in humans. Heightened sensitivity to aversive and certain physiological effects of MA, such as MA-induced reduction in body temperature, are hallmarks of mice bred for low MA intake. Furthermore, unlike MA-avoiding mice, MA-preferring mice are sensitive to rewarding and reinforcing MA effects, and to MA-induced increases in brain extracellular dopamine levels. Gene expression analyses implicate the importance of a network enriched in transcription factor genes, some of which regulate the mu opioid receptor gene, Oprm1, in risk for MA use. Neuroimmune factors appear to play a role in differential response to MA between the mice bred for high and low intake. In addition, chromosome 10 candidate gene studies provide strong support for a trace amine-associated receptor 1 gene, Taar1, polymorphism in risk for MA intake. MA is a trace amine-associated receptor 1 (TAAR1) agonist, and a non-functional Taar1 allele segregates with high MA consumption. Thus, reduced TAAR1 function has the potential to increase risk for MA use. Overall, existing findings support the MA drinking lines as a powerful model for identifying genetic factors involved in determining risk for harmful MA use. Future directions include the development of a binge model of MA intake, examining the effect of withdrawal from chronic MA on MA intake, and studying potential Taar1 gene × gene and gene × environment interactions. These and other studies are intended to improve our genetic model with regard to its translational value to human addiction.

Genotypic and sex differences in anxiety-like behavior and alcohol-induced anxiolysis in High Drinking in the Dark selected mice

Alcohol use disorders and anxiety disorders are highly comorbid in humans. In rodent lines selected for alcohol drinking, differences in anxiety-like behavior are also seen. The High Drinking in the Dark (HDID) lines of mice are selectively bred for drinking to intoxication during limited access to alcohol, and these mice represent a genetic model of risk for binge-like drinking. The present studies investigated whether these selected lines differ from control (HS) mice in basal anxiety behavior or in anxiolytic response to alcohol. We also assessed the genetic correlation between alcohol drinking in the dark (DID) and basal anxiety-like behavior using existing inbred strain data. Mice of both sexes and HDID replicates (HDID-1 and HDID-2) were tested on an elevated zero maze immediately following a DID test. In general, HDID mice showed more time spent in the open arms after drinking alcohol than HS mice, and open-arm time was significantly correlated with blood alcohol concentration. HDID-1 male mice also showed less anxiety-like behavior at baseline (water-drinking controls). In a separate experiment, HDID-1 and HS mice were tested for anxiolytic dose-response to acute alcohol injections. Both genotypes showed increasing time spent in the open arms with increasing alcohol doses, and HDID-1 and female mice had greater open-arm time across all doses. HDID-1 control males showed lower anxiety-like behavior than the HS control males. Inbred strain data analysis also showed no significant genetic relationship between alcohol DID and anxiety. These findings suggest that HDID selection has not produced systematic changes in anxiety-like behavior or sensitivity to alcohol-induced anxiolysis, though there is a tendency in the male mice of the first replicate toward reduced basal anxiety-like behavior. Therefore, anxiety state and sensitivity to alcohol's anxiolytic effects do not appear to contribute significantly to the high drinking behavior of the HDID mice.

High drinking in the dark mice: a genetic model of drinking to intoxication

Drinking to intoxication is a critical component of risky drinking behaviors in humans, such as binge drinking. Previous rodent models of alcohol consumption largely failed to demonstrate that animals were patterning drinking in such a way as to experience intoxication. Therefore, few rodent models of binge-like drinking and no specifically genetic models were available to study possible predisposing genes. The High Drinking in the Dark (HDID) selective breeding project was started to help fill this void, with HDID mice selected for reaching high blood alcohol levels in a limited access procedure. HDID mice now represent a genetic model of drinking to intoxication and can be used to help answer questions regarding predisposition toward this trait as well as potential correlated responses. They should also prove useful for the eventual development of better therapeutic strategies.

Behavioral disinhibition in mice bred for high drinking in the dark (HDID) and HS controls increases following ethanol

BACKGROUND

Alcohol consumption and behavioral inhibition share some common underlying genetic mechanisms. The current study examined whether lines of mice selected for high blood ethanol concentrations, attained by heavy drinking in the dark period (DID) of the light-dark cycle that models binge drinking, also exhibit higher levels of drug-naïve inhibition. It also examined whether the administration of ethanol would result in higher levels of disinhibition in these selected lines compared to the founder stock (HS).

METHODS

A Go/No-Go task was used to assess baseline inhibition and the effects of acute ethanol on disinhibition (response to a No-Go cue) in the HS line and in mice selected for high levels of DID (HDID-1 and HDID-2).

RESULTS

Lines did not differ in inhibition at baseline and all lines showed increased disinhibition following moderate doses of ethanol. Ethanol decreased responding to Go cues for HDID-2 and HS lines at high doses but not HDID-1 mice.

CONCLUSIONS

These data corroborate previous work showing ethanol-induced increases in behavioral disinhibition. The selection paradigm did not result in differential sensitivity to the disinhibiting effects of ethanol, but did result in differential sensitivity to the suppressant effects of ethanol on operant behavior between the two HDID lines.

Genetic relationship between predisposition for binge alcohol consumption and blunted sensitivity to adverse effects of alcohol in mice

BACKGROUND

Initial sensitivity to ethanol (EtOH) and the capacity to develop acute functional tolerance (AFT) to its adverse effects may influence the amount of alcohol consumed and may also predict future alcohol use patterns. The current study assessed sensitivity and AFT to the ataxic and hypnotic effects of EtOH in the first replicate of mice (HDID-1) selectively bred for high blood EtOH concentrations (BECs) following limited access to EtOH in the Drinking in the Dark (DID) paradigm.

METHODS

Naïve male and female HDID-1 and HS/Npt mice from the progenitor stock were evaluated in 3 separate experiments. In Experiments 1 and 2, EtOH-induced ataxia was assessed using the static dowel task. In Experiment 3, EtOH-induced hypnosis was assessed by using modified restraint tubes to measure the loss of righting reflex (LORR).

RESULTS

HDID-1 mice exhibited reduced initial sensitivity to both EtOH-induced ataxia (p < 0.001) and hypnosis (p < 0.05) relative to HS/Npt mice. AFT was calculated by subtracting the BEC at loss of function from the BEC at recovery (Experiments 1 and 3) or by subtracting BEC at an initial recovery from the BEC at a second recovery following an additional alcohol dose (Experiment 2). The dowel test yielded no line differences in AFT, but HS/Npt mice developed slightly greater AFT to EtOH-induced LORR than HDID-1 (p < 0.05).

CONCLUSIONS

These results suggest that HDID-1 mice exhibit aspects of blunted ataxic and hypnotic sensitivity to EtOH which may influence their high EtOH intake via DID, but do not display widely different development of AFT. These findings differ from previous findings with the high alcohol-preferring (HAP) selected mouse lines, suggesting that genetic predisposition for binge, versus other forms of excessive alcohol consumption, is associated with unique responses to EtOH-induced motor incoordination.

Progress in a replicated selection for elevated blood ethanol concentrations in HDID mice

Drinking in the dark (DID) is a limited access ethanol-drinking phenotype in mice. High Drinking in the Dark (HDID-1) mice have been bred for 27 selected generations (S27) for elevated blood ethanol concentrations (BECs) after a 4-h period of access to 20% ethanol. A second replicate line (HDID-2) was started later from the same founder population and is currently in S20. An initial report of response to selection in HDID-1 was published after S11. This article reports genetic and behavioral characteristics of both lines in comparison with the HS controls. Heritability is low in both replicates (h(2)  = 0.09) but the lines have shown 4-5 fold increases in BEC since S0; 80% of HDID-1 and 60% of HDID-2 mice reach BECs greater than 1.0 mg/ml. Several hours after a DID test, HDID mice show mild signs of withdrawal. Although not considered during selection, intake of ethanol (g/kg) during the DID test increased by approximately 80% in HDID-1 and 60% in HDID-2. Common genetic influences were more important than environmental influences in determining the similarity between BEC and intake for HDID mice. Analysis of the partitioning of intake showed that 60% of intake is concentrated in the last 2 h of the 4 h session. However, this has not changed during selection. Hourly BECs during the DID test reach peak levels after 3 or 4 h of drinking. HDID mice do not differ from HS mice in their rate of elimination of an acute dose of alcohol.