Rodent models of genetic contributions to motivation to abuse alcohol

Yohimbine as a pharmacological probe for alcohol research: a systematic review of rodent and human studies

Alcohol use disorder (AUD) is a significant public health concern, contributing to a myriad of social, psychological, and physiological issues. Despite substantial efforts within the alcohol research field, promising preclinical findings have failed to translate to clinical use, highlighting the necessity to develop safe and effective pharmacological probes with the ability to be used in preclinical and clinical research. Yohimbine, an α2 adrenergic receptor antagonist, is a well-validated pharmacological tool that has been widely employed in alcohol studies to evaluate noradrenergic activation. This scoping systematic review examines published literature in rodent and human studies involving the use of yohimbine relevant to alcohol research. We conducted a systematic literature review of MEDLINE, Embase, Web of Science Core Collection, CINAHL, PsycInfo, and Cochrane Central Register of Controlled Trials to identify: (1) Experimental Characteristics and Methodology, (2) Sex Differences, (3) Neurochemical Systems and Brain Regions, and (4) Discussion of Applications for Medication Development. Sixty-seven (62 preclinical and 5 clinical) studies were identified meeting the stated criteria, comprising extensive evidence supporting the use of yohimbine as a safe, titratable pharmacological agent for translational alcohol research. Support for the use of yohimbine as a fully translational tool, however, is hindered by limited available findings from human laboratory studies, as well as a dearth of studies examining sex differences in yohimbine's mechanistic actions. Additional consideration should be given to further translational modeling, ideally allowing for parallel preclinical and clinical assessment of yohimbine, methodological assessment of neurochemical systems and brain regions.

Assessing negative affect in mice during abstinence from alcohol drinking: Limitations and future challenges

Alcohol use disorder (AUD) is frequently comorbid with mood disorders, and these co-occurring neuropsychiatric disorders contribute to the development and maintenance of alcohol dependence and relapse. In preclinical models, mice chronically exposed to alcohol display anxiety-like and depressive-like behaviors during acute withdrawal and protracted abstinence. However, in total, results from studies using voluntary alcohol-drinking paradigms show variable behavioral outcomes in assays measuring negative affective behaviors. Thus, the main objective of this review is to summarize the literature on the variability of negative affective behaviors in mice after chronic alcohol exposure. We compare the behavioral phenotypes that emerge during abstinence across different exposure models, including models of alcohol and stress interactions. The complicated outcomes from these studies highlight the difficulties of assessing negative affective behaviors in mouse models designed for the study of AUD. We discuss new behavioral assays, comprehensive platforms, and unbiased machine-learning algorithms as promising approaches to better understand the interaction between alcohol and negative affect in mice. New data-driven approaches in the understanding of mouse behavior hold promise for improving the identification of mechanisms, cell subtypes, and neurocircuits that mediate negative affect. In turn, improving our understanding of the neurobehavioral basis of alcohol-associated negative affect will provide a platform to test hypotheses in mouse models that aim to improve the development of more effective strategies for treating individuals with AUD and co-occurring mood disorders.

Pituitary adenylate cyclase-activating polypeptide type 1 receptor within the nucleus accumbens core mediates excessive alcohol drinking in alcohol-preferring rats

Alcohol use disorders (AUD) have a strong component of heritability; however, the neurobiological mechanisms mediating the propensity to consume excessive amounts of alcohol are still not well understood. Pituitary adenylate cyclase-activating polypeptide (PACAP), a highly conserved neuropeptide which exerts its effects mainly through the PAC1 receptor (PAC1R), has been suggested to be one of the mediators of the effects of drugs of abuse and alcohol. Here, we investigated the role of the PACAP/PAC1R system in excessive alcohol drinking in alcohol-preferring rats, an established animal model of AUD. Intracerebroventricular (i.c.v.) administration of the PAC1R antagonist PACAP(6-38) blocked excessive alcohol drinking and motivation to drink in Sardinian alcohol-preferring (Scr:sP) rats, without affecting water, saccharin, or sucrose intake. Notably, PACAP(6-38) did not affect ethanol responding in outbred Wistar rats. PACAP(6-38) also significantly reduced alcohol-seeking behavior under a second-order schedule of reinforcement. Using immunohistochemistry, a significant increase in the number of PAC1R positive cells was observed selectively in the nucleus accumbens (NAcc) Core of Scr:sP rats, compared to Wistar rats following alcohol drinking. Finally, excessive drinking in Scr:sP rats was suppressed by intra-NAcc Core, but not intra-NAcc Shell, PACAP(6-38), as well as by virally-mediated PAC1R knockdown in the NAcc Core. The present study shows that hyperactivity of the PACAP/PAC1R system specifically in the NAcc Core mediates excessive drinking of alcohol-preferring rats, and indicates that this system may represent a novel target for the treatment of 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.

Viral-Mediated Knockdown of Nucleus Accumbens Shell PAC1 Receptor Promotes Excessive Alcohol Drinking in Alcohol-Preferring Rats

Alcohol use disorder (AUD) is a chronic, relapsing disorder whose genetic and environmental susceptibility components are not fully understood. Neuropeptidergic signaling has been repeatedly implicated in modulating excessive alcohol drinking, especially within sub-regions of the striatum. Here, we investigated the potential involvement of the selective receptor for pituitary adenylate cyclase-activating polypeptide (PACAP), PAC1R, in the nucleus accumbens shell (NAcc Shell) in excessive alcohol drinking in alcohol-preferring rats, an established animal model of the genetic propensity for alcoholism. Scr:sP alcohol-preferring rats were trained to operantly self-administer alcohol and then either an AAV virus short-hairpin RNA (shRNA) targeted to knockdown PAC1R, or an AAV control virus were microinfused into the NAcc Shell. NAcc Shell PAC1R shRNA knockdown virus was confirmed to significantly decrease PAC1R levels in the NAcc Shell. The effects of NAcc Shell PAC1R shRNA knockdown on ethanol self-administration were investigated using a Fixed Ratio (FR) 1 and a Progressive Ratio (PR) schedule of reinforcement. The effect of PAC1R knockdown on self-administration of an alternative reinforcer, saccharin, was also assessed. The results showed that the reduction in PAC1R in the NAcc Shell led to excessive ethanol drinking, increased preference for ethanol, and higher motivation to drink. NAcc Shell PAC1R shRNA knockdown did not comparably increase saccharin self-administration, suggesting selectivity of action. These data suggest that NAcc Shell PAC1R may serves as a "brake" on alcohol drinking, and thereby the loss of function of PAC1R leads to excessive alcohol consumption. Therefore, the PACAP/PAC1R system may represent a novel target for the treatment of AUD.

Heritability of ethanol consumption and pharmacokinetics in a genetically diverse panel of collaborative cross mouse strains and their inbred founders

BACKGROUND

Interindividual variation in voluntary ethanol consumption and ethanol response is partially influenced by genetic variation. Discovery of the genes and allelic variants that affect these phenotypes may clarify the etiology and pathophysiology of problematic alcohol use, including alcohol use disorder. Genetically diverse mouse populations, which demonstrate heritable variation in ethanol consumption, can be utilized to discover the genes and gene networks that influence this trait. The Collaborative Cross (CC) recombinant inbred strains, Diversity Outbred (DO) population and their 8 founder strains are complementary mouse resources that capture substantial genetic diversity and can demonstrate expansive phenotypic variation in heritable traits. These populations may be utilized to discover candidate genes and gene networks that moderate ethanol consumption and other ethanol-related traits.

METHODS

We characterized ethanol consumption, preference, and pharmacokinetics in the 8 founder strains and 10 CC strains in 12-hour drinking sessions during the dark phase of the circadian cycle.

RESULTS

Ethanol consumption was substantially heritable, both early in ethanol access and over a chronic intermittent access schedule. Ethanol pharmacokinetics were also heritable; however, no association between strain-level ethanol consumption and pharmacokinetics was detected. The PWK/PhJ strain was the highest drinking strain, with consumption substantially exceeding that of the C57BL/6J strain, which is commonly used as a model of "high" or "binge" drinking. Notably, we found strong evidence that sex moderated genetic effects on voluntary ethanol drinking.

CONCLUSIONS

Collectively, this research serves as a foundation for expanded genetic study of ethanol consumption in the CC/DO and related populations. Moreover, we identified reference strains with extreme consumption phenotypes that effectively represent polygenic models of excessive ethanol use.

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.

Photic Regulation of Circadian Rhythms and Voluntary Ethanol Intake: Role of Melanopsin-expressing Intrinsically Photosensitive Retinal Ganglion Cells

"Non-image-forming" (NIF) effects of light are mediated primarily by a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment, melanopsin (OPN4). These NIF functions include circadian entrainment, pupillary reflexes, and photic effects on sleep, mood, and cognition. We recently reported that mice of multiple genotypes exhibit reduced voluntary ethanol intake under both constant darkness (DD) and constant light (LL) relative to standard light-dark (LD) conditions. In the present study, we sought to determine whether these effects are mediated by melanopsin-expressing ipRGCs and their potential relationship to photic effects on the circadian system. To this end, we examined the effects of environmental lighting regimen on both ethanol intake and circadian activity rhythms in a genetically engineered mouse model (Opn4^aDTA/aDTA) in which melanopsin expression is completely blocked while ipRGCs are progressively ablated due to activation of attenuated diphtheria toxin A (aDTA) transgene under the control of the Opn4 promoter. As expected from previous studies, Opn4^aDTA/aDTA mice displayed dramatic attenuation of circadian photosensitivity, but surprisingly, showed identical suppression of ethanol intake under both DD and LL as that seen in controls. These results demonstrate that the effects of lighting regimen on voluntary ethanol intake are independent of melanopsin-expressing ipRGCs and ipRGC-mediated photic effects on the circadian system. Rather, these effects are likely mediated by classical retinal photoreceptors and central pathways.

Preclinical methodological approaches investigating of the effects of alcohol on perinatal and adolescent neurodevelopment

Despite much evidence of its economic and social costs, alcohol use continues to increase. Much remains to be known as to the effects of alcohol on neurodevelopment across the lifespan and in both sexes. We provide a comprehensive overview of the methodological approaches to ethanol administration when using animal models (primarily rodent models) and their translational relevance, as well as some of the advantages and disadvantages of each approach. Special consideration is given to early developmental periods (prenatal through adolescence), as well as to the types of research questions that are best addressed by specific methodologies. The zebrafish is used increasingly in alcohol research, and how to use this model effectively as a preclinical model is reviewed as well.

Convergent Evidence From Humans and Drosophila melanogaster Implicates the Transcription Factor MEF2B/Mef2 in Alcohol Sensitivity

BACKGROUND

Self-Rating of the Effects of Alcohol (SRE) measures level of response to ethanol (EtOH) in humans. Interestingly, there is a positive relationship between the SRE and risk for abusing alcohol, suggesting mechanistic connections between SRE and alcohol abuse.

METHODS

To identify candidate genes with a role in SRE and alcohol-related behavior more generally, we coupled human genetic analyses with studies in Drosophila melanogaster. We first performed a gene-based analysis of Genomewide association studies (GWAS) summary statistics for SRE in the Avon Longitudinal Study of Parents and Children sample. Based on prior findings in humans, orthology to fly genes, and the availability of genetic reagents, we selected a subset of these genes for studies on EtOH behavior in Drosophila.

RESULTS

We found 37 genes with nominal associations in our SRE GWAS. We explored the role of 6 orthologous genes in Drosophila EtOH sedation and rapid tolerance. We found that the transcription factor Mef2 is required for normal EtOH sedation in flies. Pan-neuronal expression of 2 independent Mef2 RNAi transgenes significantly reduced Mef2 expression and made flies resistant to EtOH sedation. Additionally, flies with multiple independent mutant alleles of Mef2 were also resistant to EtOH sedation, confirming a role for Mef2 in this behavior. Altered expression of Mef2 did not change EtOH rapid tolerance or cause a net change in internal EtOH concentrations.

CONCLUSIONS

Our studies indicate that MEF2B influences SRE in humans and that Mef2 impacts EtOH sedation in Drosophila.

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

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

Sweetened ethanol drinking during social isolation: enhanced intake, resistance to genetic heterogeneity and the emergence of a distinctive drinking pattern in adolescent mice

With its ease of availability during adolescence, sweetened ethanol ('alcopops') is consumed within many contexts. We asked here whether genetically based differences in social motivation are associated with how the adolescent social environment impacts voluntary ethanol intake. Mice with previously described differences in sociability (BALB/cJ, C57BL/6J, FVB/NJ and MSM/MsJ strains) were weaned into isolation or same-sex pairs (postnatal day, PD, 21), and then given continuous access to two fluids on PDs 34-45: one containing water and the other containing an ascending series of saccharin-sweetened ethanol (3-6-10%). Prior to the introduction of ethanol (PDs 30-33), increased water and food intake was detected in some of the isolation-reared groups, and controls indicated that isolated mice also consumed more 'saccharin-only' solution. Voluntary drinking of 'ethanol-only' was also higher in a subset of the isolated groups on PDs 46-49. However, sweetened ethanol intake was increased in all isolated strain × sex combinations irrespective of genotype. Surprisingly, blood ethanol concentration (BEC) was not different between these isolate and socially housed groups 4 h into the dark phase. Using lickometer-based measures of intake in FVB mice, we identified that a predominance of increased drinking during isolation transpired outside of the typical circadian consumption peak, occurring ≈8.5 h into the dark phase, with an associated difference in BEC. These findings collectively indicate that isolate housing leads to increased consumption of rewarding substances in adolescent mice independent of their genotype, and that for ethanol this may be because of when individuals drink during the circadian cycle.

C57BL/6J mouse apolipoprotein A2 gene is deterministic for apnea

RATIONALE

Brainstem apolipoprotein AII (apoa2) mRNA expression correlates with apnea in breathing present in the adult C57Bl/6J (B6) sleep apnea model.

OBJECTIVES

To test the hypothesis that the B6 apoa2 gene contributes to the trait, we performed plethysmographic testing in apoa2 knock out (KO: -/-) mice, an in situ brainstem-spinal cord preparation comparing KO to WT (+/+) mice, and B6xDBA recombinant inbred strains (RISs).

MEASUREMENTS AND MAIN RESULTS

Apoa2 WT do, but KO and heterozygote (+/-) mice do not exhibit apnea during post-hypoxic breathing, measured in vivo. In the in situ model, pauses and instability in fictive phrenic bursting are substantially reduced in KO vs. WT preparations. In 24 RISs, apnea number in vivo was higher in strains with B6 apoa2 than with DBA apoa2 alleles.

CONCLUSIONS

The B6 apoa2 polymorphism is directly involved in breath production, and its identification suggests a novel pathway influencing risk for adult sleep apnea.

Phosphodiesterase 4 inhibitors and drugs of abuse: current knowledge and therapeutic opportunities

BACKGROUND

Long-term exposure to drugs of abuse causes an up-regulation of the cAMP-signaling pathway in the nucleus accumbens and other forebrain regions, this common neuroadaptation is thought to underlie aspects of drug tolerance and dependence. Phosphodiesterase 4 (PDE4) is an enzyme that the selective hydrolyzes intracellular cAMP. It is expressed in several brain regions that regulate the reinforcing effects of drugs of abuse.

OBJECTIVE

Here, we review the current knowledge about central nervous system (CNS) distribution of PDE4 isoforms and the effects of systemic and brain-region specific inhibition of PDE4 on behavioral models of drug addiction.

METHODS

A systematic literature search was performed using the Pubmed.

RESULTS

Using behavioral sensitization, conditioned place preference and drug self-administration as behavioral models, a large number of studies have shown that local or systemic administration of PDE4 inhibitors reduce drug intake and/or drug seeking for psychostimulants, alcohol, and opioids in rats or mice.

CONCLUSIONS

Preclinical studies suggest that PDE4 could be a therapeutic target for several classes of substance use disorder. We conclude by identifying opportunities for the development of subtype-selective PDE4 inhibitors that may reduce addiction liability and minimize the side effects that limit the clinical potential of non-selective PDE4 inhibitors. Several PDE4 inhibitors have been clinically approved for other diseases. There is a promising possibility to repurpose these PDE4 inhibitors for the treatment of drug addiction as they are safe and well-tolerated in patients.

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.

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.