Rodent models and mechanisms of voluntary binge-like ethanol consumption: Examples, opportunities, and strategies for preclinical research

Binge ethanol consumption has widespread negative consequences for global public health. Rodent models offer exceptional power to explore the neurobiology underlying and affected by binge-like drinking as well as target potential prevention, intervention, and treatment strategies. An important characteristic of these models is their ability to consistently produce pharmacologically-relevant blood ethanol concentration. This review examines the current available rodent models of voluntary, pre-dependent binge-like ethanol consumption and their utility in various research strategies. Studies have demonstrated that a diverse array of neurotransmitters regulate binge-like drinking, resembling some findings from other drinking models. Furthermore, repeated binge-like drinking recruits neuroadaptive mechanisms in mesolimbocortical reward circuitry. New opportunities that these models offer in the current context of mechanistic research are also discussed

Adenosinergic regulation of binge-like ethanol drinking and associated locomotor effects in male C57BL/6J mice

We recently observed that the addition of caffeine (a nonselective adenosine receptor antagonist) to a 20% ethanol solution significantly altered the intoxication profile of male C57BL/6J (B6) mice induced by voluntary binge-like consumption in the 'Drinking-in-the-Dark' (DID) paradigm. In the current study, the roles of A1 and A2A adenosine receptor subtypes, specifically, in binge-like ethanol consumption and associated locomotor effects were explored. Adult male B6 mice (PND 60-70) were allowed to consume 20% ethanol (v/v) or 2% sucrose (w/v) for 6days via DID. On day 7, mice received a systemic administration (i.p.) of the A1 antagonist DPCPX (1, 3, 6mg/kg), the A2A antagonist MSX-3 (1, 2, 4mg/kg), or vehicle immediately prior to fluid access in DID. Antagonism of the A1 receptor via DPCPX was found to dose-dependently decrease binge-like ethanol intake and associated blood ethanol concentrations (p's<0.05), although no effect was observed on sucrose intake. Antagonism of A2A had no effect on ethanol or sucrose consumption, however, MSX-3 elicited robust locomotor stimulation in mice consuming either solution (p's<0.05). Together, these findings suggest unique roles for the A1 and A2A adenosine receptor subtypes in binge-like ethanol intake and its associated locomotor effects

Intra-nucleus accumbens shell injections of R(+)- and S(-)-baclofen bidirectionally alter binge-like ethanol, but not saccharin, intake in C57Bl/6J mice

The GABAB agonist baclofen has been widely researched clinically and preclinically as a treatment of alcohol use disorders (AUDs). However, the efficacy of baclofen remains uncertain. The clinically used racemic compound can be separated into separate enantiomers. These enantiomers have produced different profiles in behavioral assays, with the S- compound often being ineffective compared to the R- compound, or the S- compound antagonizing the effects of the R- compound. We have previously demonstrated that the R(+)-baclofen enantiomer decreases binge-like ethanol intake in the Drinking-in-the-Dark (DID) paradigm, whereas the S(-)-baclofen enantiomer increases ethanol intake. One area implicated in drug abuse is the nucleus accumbens shell (NACsh).The current study sought to define the role of the NACsh in the enantioselective effects of baclofen on binge-like ethanol consumption by directly microinjecting each enantiomer into the structure. Following bilateral cannulation of the NACsh, C57Bl/6J mice were given 5 days of access to ethanol or saccharin for 2h, 3h into the dark cycle. On Day 5 mice were given an injection of aCSF, 0.02 R(+)-, 0.04R(+)-, 0.08 S(-)-, or 0.16 S(-)-baclofen (μg/side dissolved in 200nl of aCSF). It was found that the R(+)-baclofen dose-dependently decreased ethanol consumption, whereas the high S(-)-baclofen dose increased ethanol consumption, compared to the aCSF group. Saccharin consumption was not affected. These results further confirm that GABAB receptors and the NACsh shell are integral in mediating ethanol intake. They also demonstrate that baclofen displays bidirectional, enantioselective effects which are important when considering therapeutic uses of the drug

Repeated Daily Drinking-in-the-Dark Results in Inflexible Ethanol Drinking in C57BL/6J Mice

poster abstractWe recently demonstrated that repeated daily binge ethanol (EtOH) intake (Drinking-in-the-Dark; DID) alters the pattern of subsequent binge drinking, suggesting neuroadaptation as a consequence of repeated binge drinking. The current objective was to determine whether repeated binge drinking behavior using DID procedures produces a compulsive pattern of drinking. We took advantage of a paradigm in which the aversive stimulus quinine is used to adulterate the EtOH solution after a history of EtOH drinking. Using this approach, Lesscher et al. (2010) demonstrated that C57BL/6J (B6) mice develop inflexible drinking after repeated binge-like EtOH drinking; that is, mice with longer drinking histories become insensitive to the quinine addition, “compulsively” consuming the EtOH solution despite the aversive stimulus. Seventy male B6 mice (PND 60) from Jackson Laboratory were randomly assigned to one of seven fluid consumption groups using DID procedures (daily 2hr EtOH access, 3 hrs into the dark cycle). The first group received access to a 20% (v/v) EtOH in tap water for 6 weeks. The second group also received access to the EtOH solution for six weeks, but with the addition of a low concentration of quinine (0.35mM) that produces minimal avoidance in naïve B6 mice in the sixth week. The third group was treated similarly; however, on the sixth week a higher concentration of quinine (0.45mM) that produces clear avoidance in naïve B6 mice was added to the EtOH solution. The fourth and fifth groups received access to the EtOH solution two weeks prior to adulteration with the low or high quinine concentration, respectively, for one week. Groups six and seven were allowed access to the EtOH solution for one week before the lower or higher concentrations of quinine were added, respectively, for one week. Two naïve groups with no previous DID exposure were also presented with low or high concentrations of quinine adulterated EtOH solution for comparison. We predicted that daily binge EtOH drinking for two and five weeks (but not one week) would result in resistance to the aversive quinine (inflexible drinking). However, results showed that even one week of DID produced inflexible drinking. Future studies are planned to examine binge drinking durations within the first week of DID to determine precisely when this shift to more compulsive drinking occurs. This work was supported by NIH grants AA007611 (SLB) and GM109432 (MAC)

Site-specific microinjection of Gaboxadol into the infralimbic cortex modulates ethanol intake in male C57BL/6J mice

Extrasynaptic GABAA receptors, often identified as those containing both α4 and δ subunits, demonstrate super-sensitivity to GABA and are involved in tonic inhibitory processes regulating activity within mesolimbocortical circuitry. Rodent studies testing the effects of the δ-subunit selective agonist Gaboxadol (THIP) on alcohol consumption have produced mixed results. The goal of this study was to determine the role of extrasynaptic GABAA receptors located in the infralimbic cortex (ILC) in the alcohol consumption of male C57BL/6J (B6) mice. The ILC is of interest due to its demonstrated involvement in stress reactivity. Furthermore, alcohol exposure has been shown to interfere with extinction learning; impairments of which may be related to inflexible behavior (i.e., problematic alcohol consumption). Adult male B6 mice were bilaterally implanted with guide cannulas aimed at the ILC and were subsequently offered daily limited access to 20% ethanol or 5% sucrose for 7 days. Immediately prior to ethanol or sucrose access on day 7, mice were bilaterally injected with 50 or 100ng THIP (25 or 50ng per side respectively) or saline vehicle into the ILC. The highest dose of intra-ILC THIP (100ng/mouse) increased alcohol intake relative to vehicle controls, although control animals consumed relatively little ethanol following infusion. Intra-ILC THIP had no effect on sucrose consumption (p>0.05), suggesting that the effect of THIP was selective for ethanol consumption. Together, these findings suggest that THIP may have effectively prevented the decrease in ethanol intake on day 7 induced by the microinjection process, perhaps supporting a suggested role for the ILC in adaptive learning processes and behavioral flexibility

Self-administration of edible Δ9-tetrahydrocannabinol and associated behavioral effects in mice

Background With increasing access to legal cannabis across the globe, it is imperative to more closely study its behavioral and physiological effects. Furthermore, with the proliferation of cannabis use, modes of consumption are changing, with edible formulations becoming increasingly popular. Nevertheless, there are relatively few animal models of self-administration of the primary psychoactive component of cannabis, Δ9-tetrahydrocannabinol (THC), and almost all incorporate routes of administration other than those used by humans. The aim of the current study was to develop a model of edible THC self-administration and assess its impact on CB1 receptor-mediated behaviors in female and male mice. Methods Mice were given limited access to a palatable dough which occasionally contained THC in doses ranging from 1 to 10 mg/kg. Following dough consumption, mice were assessed for home cage locomotor activity, body temperature, or analgesia. Locomotor activity was also assessed in conjunction with the CB1 receptor antagonist SR141716A. Results Dough was well-consumed, but consumption decreased at the highest THC concentrations. Edible THC produced dose-dependent decreases in locomotor activity and body temperature in both sexes, and these effects were more pronounced in male mice. Hypolocomotion induced by edible THC was attenuated by SR141716A, indicating mediation by CB1 receptor activation. Conclusions In contrast to other cannabinoid self-administration models, edible THC is relatively low in stress and uses a route of administration analogous to one used by humans. Potential applications include chronic THC self-administration, determining THC reward/reinforcement, and investigating consequences of oral THC use

GABAA Receptor Subunit Expression in Early Adolescent Mice

poster abstractGamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system (CNS). Of the known receptors for GABA, the pentameric GABAA receptor appears to mediate fast GABA neurotransmission in the CNS. A number of different GABAA receptor subunits have been described to date, including α1-6, β1-3, γ1-2, and δ, with the specific functionality of any one receptor dictated by the combination of subunits. Although much is known about the pattern of GABAA subunit expression across the adult brain, the pattern of such expression during the critical developmental period of adolescence, which is a time of rapid neurobiological, hormonal, and behavioral change, remains largely unknown. GABAA receptor systems play a key role in normal neural, hormonal, and behavioral function, warranting a basic understanding of adolescent-specific alterations in the pattern of subunit expression. The current project focuses on determining the pattern of mRNA expression of GABAA receptor subunits in early adolescent (postnatal day 30) versus adult (postnatal day 65) mice. The prefrontal cortex, hippocampus, nucleus accumbens, midbrain, amygdala and cerebellum were harvested at these two ages; thus far the midbrain has been the focus due to the presence of the ventral tegmental area and its well-known role in motivation, reward and drug addiction. Midbrain tissue was processed for determination of GABAA receptor subunit mRNA expression using RT-PCR. We predict that early adolescence will be associated with a unique pattern of GABAA receptor subunit expression, suggesting an important role for GABAA receptors in the neurobiological, hormonal, and behavioral profile of this developmental period in mice. Preliminary findings indicate an increase in GABAA delta subunit expression in the adult midbrain

Determining the heritability of ethanol-induced locomotor sensitization in mice using short-term behavioral selection

RATIONALE: Sensitization to the locomotor stimulant effects of alcohol (ethanol) is thought to be a heritable risk factor for the development of alcoholism that reflects progressive increases in the positive motivational effects of this substance. However, very little is known about the degree to which genes influence this complex behavioral phenomenon. OBJECTIVES: The primary goal of this work was to determine the heritability of ethanol-induced locomotor sensitization in mice using short-term behavioral selection. METHODS: Genetically heterogeneous C57BL/6J (B6) × DBA/2J (D2) F2 mice were generated from B6D2F1 progenitors, phenotyped for the expression of locomotor sensitization, and bred for high (HLS) and low (LLS) expression of this behavior. Selective breeding was conducted in two independently generated replicate sets to increase the confidence of our heritability estimates and for future correlated trait analyses. RESULTS: Large and significant differences in locomotor sensitization between HLS and LLS lines were evident by the fourth generation. Twenty-two percent of the observed line difference(s) were attributable to genes (h² = .22). Interestingly, locomotor activity in the absence of ethanol was genetically correlated with ethanol sensitization; high activity was associated with high sensitization. CONCLUSIONS: That changes in ethanol sensitivity following repeated exposures are genetically regulated highlights the relevance of studies aimed at determining how genes regulate susceptibility to ethanol-induced behavioral and neural adaptations. As alcohol use and abuse disorders develop following many repeated alcohol exposures, these data emphasize the need for future studies determining the genetic basis by which changes in response to alcohol occur

"Drinking in the Dark" (DID): a simple mouse model of binge-like alcohol intake

One of the greatest challenges that scientists face when studying the neurobiology and/or genetics of alcohol (ethanol) consumption is that most preclinical animal models do not voluntarily consume enough ethanol to achieve pharmacologically meaningful blood ethanol concentrations (BECs). Recent rodent models have been developed that promote binge-like levels of ethanol consumption associated with high BECs (i.e., ≥100 mg/dl). This unit describes procedures for an animal model of binge-like ethanol drinking which has come to be called "drinking in the dark" (DID). The "basic" variation of DID involves replacing the water bottle with a bottle containing 20% ethanol for 2 to 4 hr, beginning 3 hr into the dark cycle, on cages of singly-housed C57BL/6J mice. Using this procedure, mice typically consume enough ethanol to achieve BECs >100 mg/dl and to exhibit behavioral evidence of intoxication. An alternative two-bottle (ethanol and water) procedure is also described