Understanding how the brain perceives alcohol: neurobiological basis of ethanol discrimination

mGlu2 and mGlu3 receptor negative allosteric modulators attenuate the interoceptive effects of alcohol in male and female rats

RATIONALE

The subjective effects of alcohol are associated with alcohol use disorder (AUD) vulnerability and treatment outcomes. The interoceptive effects of alcohol are part of these subjective effects and can be measured in animal models using drug discrimination procedures. The newly developed mGlu2 and mGlu3 negative allosteric modulators (NAMs) are potential therapeutics for AUD and may alter interoceptive sensitivity to alcohol.

OBJECTIVES

To determine the effects of mGlu2 and mGlu3 NAMs on the interoceptive effects of alcohol in rats.

METHODS

Long-Evans rats were trained to discriminate the interoceptive stimulus effects of alcohol (2.0 g/kg, i.g.) from water using both operant (males only) and Pavlovian (male and female) drug discrimination techniques. Following acquisition training, an alcohol dose-response (0, 0.5, 1.0, 2.0 g/kg) experiment was conducted to confirm stimulus control over behavior. Next, to test the involvement of mGlu2 and mGlu3, rats were pretreated with the mGlu2-NAM (VU6001966; 0, 3, 6, 12 mg/kg, i.p.) or the mGlu3-NAM (VU6010572; 0, 3, 6, 12 mg/kg, i.p.) before alcohol administration (2.0 g/kg, i.g.).

RESULTS

In Pavlovian discrimination, male rats showed greater interoceptive sensitivity to 1.0 and 2.0 g/kg alcohol compared to female rats. Both mGlu2-NAM and mGlu3-NAM attenuated the interoceptive effects of alcohol in male and female rats using Pavlovian and operant discrimination. There may be a potential sex difference in response to the mGlu2-NAM at the highest dose tested.

CONCLUSIONS

Male rats may be more sensitive to the interoceptive effects of the 2.0 g/kg alcohol training dose compared to female rats. Both mGlu2-and mGlu3-NAM attenuate the interoceptive effects of alcohol in male and female rats. These drugs may have potential for treatment of AUD in part by blunting the subjective effects of alcohol.

Predator odor (TMT) exposure potentiates interoceptive sensitivity to alcohol and increases GABAergic gene expression in the anterior insular cortex and nucleus accumbens in male rats

Post-traumatic stress disorder (PTSD) confers enhanced vulnerability to developing comorbid alcohol use disorder (AUD). Exposure to the scent of a predator, such as the fox odor TMT, has been used to model a traumatic stressor with relevance to PTSD symptomatology. Alcohol produces distinct interoceptive (subjective) effects that may influence vulnerability to problem drinking and AUD. As such, understanding the lasting impact of stressors on sensitivity to the interoceptive effects of alcohol is clinically relevant. The present study used a 2-lever, operant drug discrimination procedure to train male Long-Evans rats to discriminate the interoceptive effects of alcohol (2 g/kg, i.g. [intragastrically]) from water. Upon stable performance, rats underwent a 15-min exposure to TMT. Two weeks later, an alcohol dose-response curve was conducted to evaluate the lasting effects of the TMT stressor on the interoceptive effects of alcohol. The TMT group showed a leftward shift in the effective dose (ED50) of the dose-response curve compared to controls, reflecting potentiated interoceptive sensitivity to alcohol. TMT exposure did not affect response rate. GABAergic signaling in both the anterior insular cortex (aIC) and the nucleus accumbens (Acb) is involved in the interoceptive effects of alcohol and stressor-induced adaptations. As such, follow-up experiments in alcohol-naïve rats examined neuronal activation (as measured by c-Fos immunoreactivity) following TMT and showed that TMT exposure increased c-Fos expression in the aIC and the nucleus accumbens core (AcbC). Two weeks after TMT exposure, Gad-1 gene expression was elevated in the aIC and Gat-1 was increased in the Acb, compared to controls. Lastly, the alcohol discrimination and alcohol-naïve groups displayed dramatic differences in stress reactive behaviors during the TMT exposure, suggesting that alcohol exposure may alter the behavioral response to predator odor. Together, these data suggest that predator odor stressor results in potentiated sensitivity to alcohol, possibly through GABAergic adaptations in the aIC and Acb, which may be relevant to understanding PTSD-AUD comorbidity.

Interoception and alcohol: Mechanisms, networks, and implications

Interoception refers to the perception of the internal state of the body and is increasingly being recognized as an important factor in mental health disorders. Drugs of abuse produce powerful interoceptive states that are upstream of behaviors that drive and influence drug intake, and addiction pathology is impacted by interoceptive processes. The goal of the present review is to discuss interoceptive processes related to alcohol. We will cover physiological responses to alcohol, how interoceptive states can impact drinking, and the recruitment of brain networks as informed by clinical research. We also review the molecular and brain circuitry mechanisms of alcohol interoceptive effects as informed by preclinical studies. Finally, we will discuss emerging treatments with consideration of interoception processes. As our understanding of the role of interoception in drug and alcohol use grows, we suggest that the convergence of information provided by clinical and preclinical studies will be increasingly important. Given the complexity of interoceptive processing and the multitude of brain regions involved, an overarching network-based framework can provide context for how focused manipulations modulate interoceptive processing as a whole. In turn, preclinical studies can systematically determine the roles of individual nodes and their molecular underpinnings in a given network, potentially suggesting new therapeutic targets and directions. As interoceptive processing drives and influences motivation, emotion, and subsequent behavior, consideration of interoception is important for our understanding of processes that drive ongoing drinking and relapse.

Comorbid HIV infection and alcohol use disorders: Converging glutamatergic and dopaminergic mechanisms underlying neurocognitive dysfunction

Alcohol use disorders (AUDs) are highly comorbid with human immunodeficiency virus (HIV) infection, occurring at nearly twice the rate in HIV positive individuals as in the general population. Individuals with HIV who consume alcohol show worse long-term prognoses and may be at elevated risk for the development of HIV-associated neurocognitive disorders. The direction of this relationship is unclear, and likely multifactorial. Chronic alcohol exposure and HIV infection independently promote cognitive dysfunction and further may interact to exacerbate neurocognitive deficits through effects on common targets, including corticostriatal glutamate and dopamine neurotransmission. Additionally, drug and alcohol use is likely to reduce treatment adherence, potentially resulting in accelerated disease progression and subsequent neurocognitive impairment. The development of neurocognitive impairments may further reduce cognitive control over behavior, resulting in escalating alcohol use. This review will examine the complex relationship between HIV infection and alcohol use, highlighting impacts on dopamine and glutamate systems by which alcohol use and HIV act independently and in tandem to alter corticostriatal circuit structure and function to dysregulate cognitive function.

Ethanol→Nicotine & Nicotine→Ethanol drug-sequence discriminations: Conditional stimulus control with two interoceptive drug elements in rats

Self-administration of ethanol (E) and nicotine (N) occurs frequently in tandem orders (i.e., N→E vs. E→N) and thereby produces differing interoceptive profiles of subjective effects in humans. If the interoceptive stimulus characteristics of N→E differ from E→N, it is possible that such differences contribute to their co-dependence. The rationale for the present investigation was to determine whether ethanol, when preceded or followed by nicotine, produces different discriminative stimulus effects in rats. In two experiments, using a one-manipulandum operant drug discrimination procedure, rats were trained to discriminate temporal sequential administrations of ethanol (1.0 g/kg) that was followed or preceded by nicotine (0.3 mg/kg). Sessions alternated between food-reinforcement sessions on a variable interval 30-sec schedule (i.e., SD) and non-reinforcement sessions (i.e., SΔ). In Experiment 1, administrations of ethanol were followed or preceded by a 10-min interval of nicotine. Training sessions took place 10 min following the second drug injection. Four groups of rats were trained to discriminate only one sequence from sequential administrations of saline, and each drug sequence was counterbalanced across groups for their roles as SD or SΔ. There was robust stimulus control. N→E and E→N functioned equally well as SD or SΔ. Experiment 2 used two groups of rats. For one group, the E→N sequence functioned as the SD and the N→E sequence functioned as the SΔ. The drug sequences were counterbalanced for the other group. Brief non-reinforcement tests revealed significantly greater responding during the SD sequence compared to the SΔ sequence for both groups. These results suggest that different drug sequences of ethanol followed or preceded by nicotine established reliable discriminative stimulus control over operant responding, potentially because of characteristic differences in the overlapping pharmacokinetic profiles of the NE compound. The results are discussed in terms of: 1) conditional stimulus control among two interoceptive drug states; and 2) the clinical modulation of human alcohol consumption and tobacco smoking.

Cross-Species Translational Findings in the Discriminative Stimulus Effects of Ethanol

The progress on understanding the pharmacological basis of ethanol's discriminative stimulus effects has been substantial, but appears to have plateaued in the past decade. Further, the cross-species translational efforts are clear in laboratory animals, but have been minimal in human subject studies. Research findings clearly demonstrate that ethanol produces a compound stimulus with primary activity through GABA and glutamate receptor systems, particularly ionotropic receptors, with additional contribution from serotonergic mechanisms. Further progress should capitalize on chemogenetic and optogenetic techniques in laboratory animals to identify the neural circuitry involved in mediating the discriminative stimulus effects of ethanol. These infrahuman studies can be guided by in vivo imaging of human brain circuitry mediating ethanol's subjective effects. Ultimately, identifying receptors systems, as well as where they are located within brain circuitry, will transform the use of drug discrimination procedures to help identify possible treatment or prevention strategies for alcohol use disorder.

Midbrain circuit regulation of individual alcohol drinking behaviors in mice

Alcohol-use disorder (AUD) is the most prevalent substance-use disorder worldwide. There is substantial individual variability in alcohol drinking behaviors in the population, the neural circuit mechanisms of which remain elusive. Utilizing in vivo electrophysiological techniques, we find that low alcohol drinking (LAD) mice have dramatically higher ventral tegmental area (VTA) dopamine neuron firing and burst activity. Unexpectedly, VTA dopamine neuron activity in high alcohol drinking (HAD) mice does not differ from alcohol naive mice. Optogenetically enhancing VTA dopamine neuron burst activity in HAD mice decreases alcohol drinking behaviors. Circuit-specific recordings reveal that spontaneous activity of nucleus accumbens-projecting VTA (VTA-NAc) neurons is selectively higher in LAD mice. Specifically activating this projection is sufficient to reduce alcohol consumption in HAD mice. Furthermore, we uncover ionic and cellular mechanisms that suggest unique neuroadaptations between the alcohol drinking groups. Together, these data identify a neural circuit responsible for individual alcohol drinking behaviors.

Mice exposed to a two-bottle alcohol choice paradigm can be divided into high and low drinking groups. Here, the authors show that stimulating VTA neurons to induce higher phasic activity patterns that are observed in low alcohol drinking mice, suppresses alcohol drinking in mice that are high alcohol drinking.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

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.

Frequency of alcohol consumption in humans; the role of metabotropic glutamate receptors and downstream signaling pathways

Rodent models implicate metabotropic glutamate receptors (mGluRs) and downstream signaling pathways in addictive behaviors through metaplasticity. One way mGluRs can influence synaptic plasticity is by regulating the local translation of AMPA receptor trafficking proteins via eukaryotic elongation factor 2 (eEF2). However, genetic variation in this pathway has not been examined with human alcohol use phenotypes. Among a sample of adults living in Detroit, Michigan (Detroit Neighborhood Health Study; n = 788; 83% African American), 206 genetic variants across the mGluR-eEF2-AMPAR pathway (including GRM1, GRM5, HOMER1, HOMER2, EEF2K, MTOR, EIF4E, EEF2, CAMK2A, ARC, GRIA1 and GRIA4) were found to predict number of drinking days per month (corrected P-value < 0.01) when considered as a set (set-based linear regression conducted in PLINK). In addition, a CpG site located in the 3'-untranslated region on the north shore of EEF2 (cg12255298) was hypermethylated in those who drank more frequently (P < 0.05). Importantly, the association between several genetic variants within the mGluR-eEF2-AMPAR pathway and alcohol use behavior (i.e., consumption and alcohol-related problems) replicated in the Grady Trauma Project (GTP), an independent sample of adults living in Atlanta, Georgia (n = 1034; 95% African American), including individual variants in GRM1, GRM5, EEF2, MTOR, GRIA1, GRIA4 and HOMER2 (P < 0.05). Gene-based analyses conducted in the GTP indicated that GRM1 (empirical P < 0.05) and EEF2 (empirical P < 0.01) withstood multiple test corrections and predicted increased alcohol consumption and related problems. In conclusion, insights from rodent studies enabled the identification of novel human alcohol candidate genes within the mGluR-eEF2-AMPAR pathway.

Increased extracellular glutamate in the nucleus accumbens promotes excessive ethanol drinking in ethanol dependent mice

Using a well-established model of ethanol dependence and relapse, this study examined adaptations in glutamatergic transmission in the nucleus accumbens (NAc) and their role in regulating voluntary ethanol drinking. Mice were first trained to drink ethanol in a free-choice, limited access (2 h/day) paradigm. One group (EtOH mice) received repeated weekly cycles of chronic intermittent ethanol (CIE) exposure with intervening weeks of test drinking sessions, whereas the remaining mice (CTL mice) were similarly treated but did not receive CIE treatment. Over repeated cycles of CIE exposure, EtOH mice exhibited significant escalation in drinking (up to ∼3.5 g/kg), whereas drinking remained relatively stable at baseline levels (2-2.5 g/kg) in CTL mice. Using in vivo microdialysis procedures, extracellular glutamate (GLUEX) levels in the NAc were increased approximately twofold in EtOH mice compared with CTL mice, and this difference was observed 7 days after final CIE exposure, indicating that this hyperglutamatergic state persisted beyond acute withdrawal. This finding prompted additional studies examining the effects of pharmacologically manipulating GLUEX in the NAc on ethanol drinking in the CIE model. The non-selective glutamate reuptake antagonist, threo-β-benzyloxyaspartate (TBOA), was bilaterally microinjected into the NAc and found to dose-dependently increase drinking in nondependent (CTL) mice to levels attained by dependent (EtOH) mice. TBOA also further increased drinking in EtOH mice. In contrast, reducing glutamatergic transmission in the NAc via bilateral injections of the metabotropic glutamate receptor-2/3 agonist LY379268 reduced drinking in dependent (EtOH) mice to nondependent (CTL) levels, whereas having a more modest effect in decreasing ethanol consumption in CTL mice. Taken together, these data support an important role of glutamatergic transmission in the NAc in regulating ethanol drinking. Additionally, these results indicate that ethanol dependence produces adaptations that favor elevated glutamate activity in the NAc which, in turn, promote excessive levels of ethanol consumption associated with dependence.

Age differences in ethanol discrimination: acquisition and ethanol dose generalization curves following multiple training conditions in adolescent and adult rats

BACKGROUND

Adolescents and adults vary in sensitivity to many effects of ethanol (EtOH), although it is unknown whether they also differ in their perception of EtOH's subjective cues. This study characterized EtOH discrimination in adolescent and adult male rats using a rapidly acquired Pavlovian conditioned approach procedure.

METHODS

EtOH at 1 of the 3 training doses (0.75, 1.0, or 1.25 g/kg) served as either a positive (POS) or negative (NOS) occasion setter. Each 20-minute training session consisted of eight 15-second presentations of 2 cue lights located on either side of a dipper delivering chocolate Boost(®) . For POS-trained rats, the cue lights reliably predicted 5-second presentations of chocolate Boost during EtOH but not saline sessions, with the opposite contingencies used for NOS-trained rats. Anticipatory approach behavior (head entries into the reward delivery area) in the presence and absence of the cue lights was used to calculate discrimination scores on EtOH and saline sessions. Following acquisition, various doses of EtOH (0 to 1.5 g/kg) were administered to establish generalization curves.

RESULTS

Although animals of both ages responded differentially on EtOH and saline sessions by the end of acquisition, adults met criterion more quickly and had higher discrimination scores during reinforced sessions than adolescents. Whereas adolescents failed to demonstrate any dose-dependent responding during testing when trained with the 0.75 or 1.25 g/kg EtOH doses, adults demonstrated broader EtOH generalization during testing sessions following training with all 3 EtOH doses. Among adolescents trained with 1.0 g/kg EtOH, less generalization occurred relative to adults.

CONCLUSIONS

Adolescents were less sensitive to EtOH's interoceptive effects, indicating that EtOH is likely a more salient cue for adults than for adolescents. These findings contribute to evidence that suggests adolescent-typical insensitivity to internal cues that typically limit EtOH consumption may contribute to the elevated intake commonly reported during this developmental period.

Ovarian hormones and the heterogeneous receptor mechanisms mediating the discriminative stimulus effects of ethanol in female rats

Past studies have suggested that progesterone-derived ovarian hormones contribute to the discriminative stimulus effects of ethanol, particularly via progesterone metabolites that act at γ-aminobutyric acid type A (GABA(A)) receptors. It is unknown whether loss of ovarian hormones in women, for example, after menopause, may be associated with altered receptor mediation of the effects of ethanol. The current study measured the substitution of allopregnanolone, pregnanolone, pentobarbital, midazolam, dizocilpine, TFMPP, and RU 24969 in female sham and ovariectomized rats trained to discriminate 1.0 g/kg ethanol from water. The groups did not differ in the substitution of GABA(A)-positive modulators (barbiturates, benzodiazepines, neuroactive steroids) or the N-methyl-D-aspartate receptor antagonist dizocilpine. Similarly, blood-ethanol concentration did not differ between the groups, and plasma adrenocorticotropic hormone, progesterone, pregnenolone, and deoxycorticosterone were unchanged 30 min after administration of 1.0 g/kg ethanol or water. However, substitution of neuroactive steroids and RU 24969, a 5-hydroxytryptamine (5-HT)(1A/1B) receptor agonist, was lower than observed in previous studies of male rats, and TFMPP substitution was decreased in ovariectomized rats. Ovarian hormones appear to contribute to 5-HT receptor mediation of the discriminative stimulus effects of ethanol in rats.

Assessment of the interoceptive effects of alcohol in rats using short-term training procedures

In the present study, we sought to determine whether the interoceptive effects of alcohol (1 g/kg, IG) could be assessed using a Pavlovian discrimination method, in which the alcohol drug state sets the occasion for which an environmental stimulus (e.g., light) will be followed by a sucrose reward. This procedure takes advantage of a naturally occurring behavior (i.e., food-seeking) which can be trained rapidly prior to the initiation of discrimination training. Given that the interoceptive effects of alcohol are routinely assessed using operant drug discrimination methods, another group of rats was trained using standard two-lever operant drug discrimination procedures in an effort to compare the Pavlovian procedure to a known behavioral benchmark. The results from this work show that, in addition to operant discrimination procedures, a Pavlovian discrimination task can be used to evaluate the interoceptive effects of alcohol. In addition to the brief behavioral sucrose access training (3 days) required prior to the initiation of the Pavlovian discrimination, the alcohol discrimination was acquired relatively rapidly (i.e., 8 training sessions), shortening the overall duration of the experiment. These features of the Pavlovian procedure make it a valuable method by which to assess the interoceptive effects of alcohol if a short experimental time frame is required, such as assessing the interoceptive effects of alcohol during a brief developmental window (e.g., adolescence) or determining the effects of a pretreatment (i.e., chronic stress, chronic drug pretreatment) on the acquisition of the alcohol discrimination. As such, this initial characterization confirms the feasibility of using this Pavlovian discrimination training method as an additional tool by which to assess the interoceptive effects of alcohol, as there may be experimental situations that necessitate short term discrimination training.

Effects of vigabatrin, an irreversible GABA transaminase inhibitor, on ethanol reinforcement and ethanol discriminative stimuli in mice

We tested the hypothesis that the irreversible γ-amino butyric acid transaminase inhibitor, γ-vinyl γ-amino butyric acid [vigabatrin (VGB)], would reduce ethanol reinforcement and enhance the discriminative-stimulus effect of ethanol, effectively reducing ethanol intake. The present studies used adult C57BL/6J (B6) mice in well-established operant, two-bottle choice consumption, locomotor activity, and ethanol discrimination procedures to comprehensively examine the effects of VGB on ethanol-supported behaviors. VGB dose-dependently reduced operant responding for ethanol and ethanol consumption for long periods of time. Importantly, a low dose (200 mg/kg) of VGB was selective for reducing ethanol responding without altering the intake of food or water reinforcement. Higher VGB doses (>200mg/kg) reduced ethanol intake, but also significantly increased water consumption and, more modestly, increased food consumption. Although not affecting locomotor activity on its own, VGB interacted with ethanol to reduce the stimulatory effects of ethanol on locomotion. Finally, VGB (200 mg/kg) significantly enhanced the discriminative-stimulus effects of ethanol as evidenced by significant leftward and upward shifts in ethanol generalization curves. Interestingly, VGB treatment was associated with slight increases in blood ethanol concentrations. The reduction in ethanol intake by VGB appears to be related to the ability of VGB to potentiate the pharmacological effects of ethanol.

Cannabinergic aminoalkylindoles, including AM678=JWH018 found in 'Spice', examined using drug (Δ(9)-tetrahydrocannabinol) discrimination for rats

We examined four different cannabinergic aminoalkylindole ligands, including one drug (AM678=JWH018) found in herbal 'Spice' concoctions, for their ability to substitute for Δ(9)-tetrahydrocannabinol (THC), and the ability of the cannabinoid receptor 1-selective antagonist/inverse agonist rimonabant to block the substitution, 30 and 90 min after intraperitoneal injection. Rats trained to discriminate the effects of vehicle from those produced by 3 mg/kg of THC were used. The order of potency was: AM5983≥AM678>AM2233>WIN55212-2 at both test intervals. AM5983 and AM678 appeared eight times more potent than THC, followed by AM2233 (about twice as potent as THC), and WIN55212-2 approximately THC at the 30-min test interval. The aminoalkylindoles showed reduced potency (i.e. an increased ED50 value) at the longer injection-to-test interval of 90 min compared with testing at 30 min. The rightward shifts by coadministration of rimonabant were approximately 8-fold to 12-fold for AM5983 and AM678, compared with an approximately 3-fold rightward shift for the WIN55212-2 curve. AM2233 (1.8 mg/kg) substitution was also blocked by 1 mg/kg of rimonabant. In conclusion, AM5983 and AM678=JWH018 are potent cannabimimetics derived from an aminoalkylindole template. WIN55212-2 seemed to interact differently with rimonabant, compared with either AM5983 or AM678, indicating potential differences in the mechanism(s) of action among cannabinergic aminoalkylindoles.

To use or not to use: Expanding the view on non-addictive psychoactive drug consumption and its implications

Proposing a change to the view on psychoactive drug use in non-addicts touches a sensitive issue because of its potential implications to addiction prevention, therapeutic practice, and drug policy. Commentators raised nine questions that ranged from clarifications, suggested extensions of the model to supporting data previously not regarded, to assumptions on the implications of the model. Here, we take up the suggestions of the commentators to expand the model to behavioral addictions, discuss additional instrumentalization goals, and review the evidence from laboratory animal studies on drug instrumentalization. We consider further the role of sociocultural factors and individual development in the establishment in drug instrumentalization and addiction. Finally, we clarify which implications we think this model may have. We conclude that drug instrumentalization theory can be further applied to other behaviors but will require a sensitive debate when used for drug and addiction policy that directly affects prevention and treatment.

The discriminative stimulus properties of methylphenidate in C57BL/6J mice

Methylphenidate (MPH) therapy for attention-deficit/hyperactivity disorder is common in children and adults. Concerns regarding abuse of MPH prompted studies to better understand its pharmacology. We used an established drug discrimination task to determine whether MPH could be discriminated by C57BL/6J (B6) mice. B6 mice learned to discriminate cues produced by racemic MPH (dl-MPH 5.0 mg/kg) or half the dose of pure d-isomer (2.5 mg/kg), and dose-response tests established appropriate reductions in discrimination with declining dose. Importantly, the two drug forms generalized to each other completely in substitution tests; consistent with reports that the l-isomer is pharmacodynamically inactive. An additional experiment indicated that lower doses (1 and 2 mg/kg) of dl-MPH did not support acquisition of MPH discrimination despite extensive training. Mice acquired discrimination of dl-MPH only when the dose was increased to 4 mg/kg. Thus, although these lower doses increased drug lever responding in mice trained on the higher dose, their stimuli were not sufficient to support acquisition of the discrimination task. These findings correspond to earlier studies conducted in our laboratory on threshold doses needed to produce stimulatory effects of motor activity in B6 mice. These preclinical findings provide insight into the relative potency, and by extension, efficacy of dl-MPH versus d-MPH doses.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Alcoholism: A Systems Approach From Molecular Physiology to Addictive Behavior

Alcohol consumption is an integral part of daily life in many societies. The benefits associated with the production, sale, and use of alcoholic beverages come at an enormous cost to these societies. The World Health Organization ranks alcohol as one of the primary causes of the global burden of disease in industrialized countries. Alcohol-related diseases, especially alcoholism, are the result of cumulative responses to alcohol exposure, the genetic make-up of an individual, and the environmental perturbations over time. This complex gene × environment interaction, which has to be seen in a life-span perspective, leads to a large heterogeneity among alcohol-dependent patients, in terms of both the symptom dimensions and the severity of this disorder. Therefore, a reductionistic approach is not very practical if a better understanding of the pathological processes leading to an addictive behavior is to be achieved. Instead, a systems-oriented perspective in which the interactions and dynamics of all endogenous and environmental factors involved are centrally integrated, will lead to further progress in alcohol research. This review adheres to a systems biology perspective such that the interaction of alcohol with primary and secondary targets within the brain is described in relation to the behavioral consequences. As a result of the interaction of alcohol with these targets, alterations in gene expression and synaptic plasticity take place that lead to long-lasting alteration in neuronal network activity. As a subsequent consequence, alcohol-seeking responses ensue that can finally lead via complex environmental interactions to an addictive behavior.

The role of 5-HT3 receptors in drug abuse and as a target for pharmacotherapy

Alcohol and drug abuse continue to be a major public health problem in the United States and other industrialized nations. Extensive preclinical research indicates the mesolimbic dopamine (DA) pathway and associated regions mediate the rewarding and reinforcing effects of drugs of abuse and natural rewards, such as food and sex. The serotonergic (5-HT) system, in concert with others neurotransmitter systems, plays a key role in modulating neuronal systems within the mesolimbic pathway. A substantial portion of this modulation is mediated by activity at the 5-HT3 receptor. The 5-HT3 receptor is unique among the 5-HT receptors in that it directly gates an ion channel inducing rapid depolarization that, in turn, causes the release of neurotransmitters and/or peptides. Preclinical findings indicate that antagonism of the 5-HT3 receptor in the ventral tegmental area, nucleus accumbens or amygdala reduces alcohol self-administration and/or alcohol-associated effects. Less is known about the effects of 5-HT3 receptor activity on the self-administration of other drugs of abuse or their associated effects. Clinical findings parallel the preclinical findings such that antagonism of the 5-HT3 receptor reduces alcohol consumption and some of its subjective effects. This review provides an overview of the structure, function, and pharmacology of 5-HT3 receptors, the role of these receptors in regulating DA neurotransmission in mesolimbic brain areas, and discusses data from animal and human studies implicating 5-HT3 receptors as targets for the development of new pharmacological agents to treat addictions.

Ethanol intake patterns in female mice: influence of allopregnanolone and the inhibition of its synthesis

The neurosteroid allopregnanolone (ALLO) is a positive modulator of GABA(A) receptors that exhibits a psychopharmacological profile similar to ethanol (i.e., anxiolytic, sedative-hypnotic). Based on research suggesting that manipulation of ALLO levels altered ethanol self-administration in male rodents, the current studies determined whether exogenous ALLO administration or the inhibition of its synthesis in vivo modulated ethanol intake patterns in female C57BL/6J mice. Lickometer circuits collected temporal lick records of ethanol (10%, v/v) and water consumption during daily 2h limited access sessions. Following the establishment of stable ethanol intake, studies examined the effect of an acute ALLO challenge (3.2-24.0 mg/kg) or a 7-day blockade of ALLO production with finasteride (FIN; 50 or 100 mg/kg) on ethanol intake in a within-subjects design. In contrast to results in male mice, ethanol dose (g/kg), ethanol preference and most of the bout parameters were unaltered by ALLO pretreatment in female mice. Ethanol intake in females also was recalcitrant to 7-day treatment with 50 mg/kg FIN, whereas 100 mg/kg FIN significantly reduced the ethanol dose consumed by 35%. The FIN-attenuated ethanol intake was attributable to a significant decrease in bout frequency (up to 45%), with lick patterns indicating reduced maintenance of consumption throughout the 2-h session. FIN also produced a dose-dependent decrease in brain ALLO levels. In conjunction with data in male mice, the present findings indicate that there are sex differences in the physiological regulation of ethanol intake patterns by GABAergic neurosteroids.

Role of major NMDA or AMPA receptor subunits in MK-801 potentiation of ethanol intoxication

BACKGROUND

The glutamate system plays a major role in mediating EtOH's effects on brain and behavior, and is implicated in the pathophysiology of alcohol-related disorders. N-methyl-D-aspartate receptor (NMDAR) antagonists such as MK-801 (dizocilpine) interact with EtOH at the behavioral level, but the molecular basis of this interaction is unclear.

METHODS

We first characterized the effects of MK-801 treatment on responses to the ataxic (accelerating rotarod), hypothermic and sedative/hypnotic effects of acute EtOH administration in C57BL/6J and 129/SvImJ inbred mice. Effects of another NMDAR antagonist, phencyclidine, on EtOH-induced sedation/hypnosis were also assessed. Gene knockout of the NMDAR subunit NR2A or l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate GluR1 or pharmacological antagonism of the NMDAR subunit NR2B (via Ro 25-6981) was employed to examine whether inactivating any one of these glutamate signaling molecules modified MK-801's effect on EtOH-related behaviors.

RESULTS

MK-801 markedly potentiated the ataxic effects of 1.75 g/kg EtOH and the sedative/hypnotic effects of 3.0 g/kg EtOH, but not the hypothermic effects of 3.0 g/kg EtOH, in C57BL/6J and 129/SvImJ mice. Phencyclidine potentiated EtOH-induced sedation/hypnosis in both inbred strains. Neither NR2A nor GluR1 KO significantly altered basal EtOH-induced ataxia, hypothermia, or sedation/hypnosis. Ro 25-6981 modestly increased EtOH-induced sedation/hypnosis. The ability of MK-801 to potentiate EtOH-induced ataxia and sedation/hypnosis was unaffected by GluR1 KO or NR2B antagonism. NR2A KO partially reduced MK-801 + EtOH-induced sedation/hypnosis, but not ataxia or hypothermia.

CONCLUSIONS

Data confirm a robust and response-specific potentiating effect of MK-801 on sensitivity to EtOH's intoxicating effects. Inactivation of three major components of glutamate signaling had no or only partial impact on the ability of MK-801 to potentiate behavioral sensitivity to EtOH. Further work to elucidate the mechanisms underlying NMDAR x EtOH interactions could ultimately provide novel insight into the role of NMDARs in alcoholism and its treatment.

Effect of duration and pattern of chronic ethanol exposure on tolerance to the discriminative stimulus effects of ethanol in C57BL/6J mice

This study was conducted to examine whether amount and/or pattern (intermittent or continuous) of chronic ethanol exposure subsequently alters sensitivity to the discriminative stimulus effects of ethanol. Adult male C57BL/6J mice were trained to discriminate between 1.5 g/kg ethanol and saline in a two-lever food-reinforced operant procedure. Once ethanol discrimination was successfully acquired, generalization testing was conducted using a cumulative dosing procedure to generate a baseline dose-response function (0-2.5 g/kg ethanol). Discrimination training was then suspended while mice received chronic ethanol vapor or air exposure in inhalation chambers. The total amount of ethanol exposure was systematically increased, but it was delivered in an intermittent or continuous manner. At 24 or 16 h after inhalation treatment, ethanol discriminability was reassessed using the same generalization testing procedures. Results indicated that discrimination performance in control (air-exposed) mice was similar to baseline. However, sensitivity to the discriminative cue of ethanol following chronic ethanol treatment was reduced (as evidenced by rightward shifts in the dose-response functions and increased ED(50) values). The magnitude of this tolerance effect increased as a function of the number of chronic ethanol exposures as well as the total duration of ethanol exposure. In addition, tolerance was more robust when generalization testing was conducted earlier (16 versus 24 h) after chronic ethanol treatment was halted (2- to 3-fold increase in ED(50) values). These results may have important clinical implications, because blunted sensitivity to the discriminative cue of ethanol may contribute to enhanced ethanol self-administration behavior observed in these mice following similar chronic ethanol treatment.