The alcohol-preferring AA and alcohol-avoiding ANA rats: neurobiology of the regulation of alcohol drinking

Coordination among frequent genetic variants imparts substance use susceptibility and pathogenesis

Determining the key genetic variants is a crucial step to comprehensively understand substance use disorders (SUDs). In this study, utilizing whole exome sequences of five multi-generational pedigrees with SUDs, we used an integrative omics-based approach to uncover candidate genetic variants that impart susceptibility to SUDs and influence addition traits. We identified several SNPs and rare, protein-function altering variants in genes, GRIA3, NCOR1, and SHANK1; compound heterozygous variants in LNPEP, LRP1, and TBX2, that play a significant role in the neurotransmitter-neuropeptide axis, specifically in the dopaminergic circuits. We also noted a greater frequency of heterozygous and recessive variants in genes involved in the structural and functional integrity of synapse receptors, CHRNA4, CNR2, GABBR1, DRD4, NPAS4, ADH1B, ADH1C, OPRM1, and GABBR2. Variant analysis in upstream promoter regions revealed regulatory variants in NEK9, PRRX1, PRPF4B, CELA2A, RABGEF1, and CRBN, crucial for dopamine regulation. Using family-and pedigree-based data, we identified heterozygous recessive alleles in LNPEP, LRP1 (4 frameshift deletions), and TBX2 (2 frameshift deletions) linked to SUDs. GWAS overlap identified several SNPs associated with SUD susceptibility, including rs324420 and rs1229984. Furthermore, miRNA variant analysis revealed notable variants in mir-548 U and mir-532. Pathway studies identified the presence of extensive coordination among these genetic variants to impart substance use susceptibility and pathogenesis. This study identified variants that were found to be overrepresented among genes of dopaminergic circuits participating in the neurotransmitter-neuropeptide axis, suggesting pleiotropic influences in the development and sustenance of chronic substance use. The presence of a diverse set of haploinsufficient variants in varying frequencies demonstrates the existence of extraordinary coordination among them in attributing risk and modulating severity to SUDs.

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.

Modeling Brain Gene Expression in Alcohol Use Disorder with Genetic Animal Models

Animal genetic models have and will continue to provide important new information about the behavioral and physiological adaptations associated with alcohol use disorder (AUD). This chapter focuses on two models, ethanol preference and drinking in the dark (DID), their usefulness in interrogating brain gene expression data and the relevance of the data obtained to interpret AUD-related GWAS and TWAS studies. Both the animal and human data point to the importance for AUD of changes in synaptic transmission (particularly glutamate and GABA transmission), of changes in the extracellular matrix (specifically including collagens, cadherins and protocadherins) and of changes in neuroimmune processes. The implementation of new technologies (e.g., cell type-specific gene expression) is expected to further enhance the value of genetic animal models in understanding AUD.

Ethanol-induced transcriptional and translational changes in Aldh1l1-Egfp/Rpl10a cortical astrocyte cultures

The role astrocytes play in brain development and function has garnered greater attention as the diversity of roles they are involved in has become apparent. We have previously shown that ethanol-exposed astrocytes alter neuronal neurite outgrowth in an in vitro co-culture system and that ethanol alters the astrocyte-produced extracellular matrix (ECM) in vitro, with similar alterations in vivo. In this study, we utilized the translating ribosome affinity purification (TRAP) procedure in Aldh1l1-EGFP/Rpl10a transgenic mouse primary cortical astrocyte cultures to transcriptionally and translationally profile the astrocyte response to ethanol. We found a large number of differences between the total RNA pool and the translating RNA pool, indicating that the transcriptional state of astrocytes may not always reflect the translational state of astrocytes. In addition, there was a considerable overlap between ethanol-dysregulated genes in the total RNA pool and the translating RNA pool. Comparisons to published datasets indicate the in vitro model used here is most similar to PD1 or PD7 in vivo cortical astrocytes, and the ethanol-regulated genes showed a significant overlap with models of chronic ethanol exposure in astrocytes, a model of third-trimester ethanol exposure in the hippocampus and cerebellum, and an acute model of ethanol exposure in the hippocampus. These findings will further our understanding of the effects of ethanol on astrocyte gene expression and protein translation and how these changes may alter brain development and support the use of in vitro astrocyte cultures as models of neonatal astrocytes.

Role of Metabolism on Alcohol Preference, Addiction, and Treatment

Studies presented in this chapter show that: (1) in the brain, ethanol is metabolized by catalase to acetaldehyde, which condenses with dopamine forming salsolinol; (2) acetaldehyde-derived salsolinol increases the release of dopamine mediating, via opioid receptors, the reinforcing effects of ethanol during the acquisition of ethanol consumption, while (3) brain acetaldehyde does not influence the maintenance of chronic ethanol intake, it is suggested that a learned cue-induced hyperglutamatergic system takes precedence over the dopaminergic system. However, (4) following a prolonged ethanol deprivation, the generation of acetaldehyde in the brain again plays a role, contributing to the increase in ethanol intake observed during ethanol re-access, called the alcohol deprivation effect (ADE), a model of relapse behavior; (5) naltrexone inhibits the high ethanol intake seen in the ADE condition, suggesting that acetaldehyde-derived salsolinol via opioid receptors also contributes to the relapse-like drinking behavior. The reader is referred to glutamate-mediated mechanisms that trigger the cue-associated alcohol-seeking and that also contribute to triggering relapse.

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.

From a systems view to spotting a hidden island: A narrative review implicating insula function in alcoholism

Excessive use of alcohol promotes the development of alcohol addiction, but the understanding of how alcohol-induced brain alterations lead to addiction remains limited. To further this understanding, we adopted an unbiased discovery strategy based on the principles of systems medicine. We used functional magnetic resonance imaging data from patients and animal models of alcohol addiction-like behaviors, and developed mathematical models of the 'relapse-prone' network states to identify brain sites and functional networks that can be selectively targeted by therapeutic interventions. Our systems level, non-local, and largely unbiased analyses converged on a few well-defined brain regions, with the insula emerging as one of the most consistent finding across studies. In proof-of-concept experiments we were able to demonstrate that it is possible to guide network dynamics towards increased resilience in animals but an initial translation into a clinical trial targeting the insula failed. Here, in a narrative review, we summarize the key experiments, methodological developments and knowledge gained from this completed round of a discovery cycle moving from identification of 'relapse-prone' network states in humans and animals to target validation and intervention trial. Future concerted efforts are necessary to gain a deeper understanding of insula function a in a state-dependent, circuit-specific and cell population perspective, and to develop the means for insula-directed interventions, before therapeutic targeting of this structure may become possible.

Behavioral Profiles of Adolescent Alcohol-Preferring/Non-preferring (P/NP) and High/Low Alcohol-Drinking (HAD/LAD) Rats Are Dependent on Line but Not Sex

Initial contact with alcohol generally occurs during adolescence, and high consumption during this period is associated with increased risk for later alcohol (AUDs) and/or substance use disorders (SUDs). Rodents selectively bred for high or low alcohol consumption are used to identify behavioral characteristics associated with a propensity for high or low voluntary alcohol intake. The multivariate concentric square field™ (MCSF) is a behavioral test developed to study rodents in a semi-naturalistic setting. Testing in the MCSF creates a comprehensive behavioral profile in a single trial. The current aim was to examine the behavioral profiles of adolescent, bidirectionally selectively bred male and female high alcohol-consuming (P and HAD1/2) and low alcohol-consuming (NP and LAD1/2) rat lines, and outbred Wistar rats. Alcohol-naïve rats were tested once in the MCSF at an age between postnatal days 30 and 35. No common behavioral profile was found for either high or low alcohol-consuming rat lines, and the effect of sex was small. The P/NP and HAD2/LAD2 lines showed within pair-dependent differences, while the HAD1/LAD1 lines were highly similar. The P rats displayed high activity and risk-associated behaviors, whereas HAD2 rats displayed low activity, high shelter-seeking behavior, and open area avoidance. The results from P rats parallel clinical findings that denser family history and risk-taking behavior are strong predictors of future AUDs, often with early onset. Contrarily, the HAD2 behavioral profile was similar to individuals experiencing negative emotionality, which also is associated with a vulnerability to develop, often with a later onset, AUDs and/or SUDs.

Repeated Ethanol Exposure Alters DNA Methylation Status and Dynorphin/Kappa-Opioid Receptor Expression in Nucleus Accumbens of Alcohol-Preferring AA Rats

Growing evidence suggests that epigenetic mechanisms, such as DNA methylation and demethylation, and histone modifications, are involved in the development of alcohol and drug addiction. However, studies of alcohol use disorder (AUD) that are focused on epigenetic DNA modifications and gene expression changes remain conflicting. Our aim was to study the effect of repeated ethanol consumption on epigenetic regulatory enzymes such as DNA methyltransferase and demethylase enzymes and whether those changes affected dynorphin/kappa-opioid receptor system in the Nucleus Accumbens (NAc). Two groups of male alcohol-preferring Alko Alcohol (AA) rats, rats which are selectively bred for high voluntary alcohol consumption and one group of male Wistar rats were used. The first group of AA rats had access to alcohol (10% ethanol solution) for 90 min on Mondays, Wednesdays and Fridays over a period of 3 weeks to establish a stable baseline of ethanol intake (AA-ethanol). The second group of AA rats (AA-water) and the Wistar rats (Wistar-water) were provided with water. Using qPCR, we found that voluntary alcohol drinking increased Dnmt1, -3a, and -3b mRNA levels and did not affect Tet family transcripts in the AA-ethanol group when compared with AA- and Wistar-water rats. DNMT and TET enzymatic activity measurements showed similar results to qPCR, where DNMT activity was increased in AA-ethanol group compared with AA-water and Wistar-water groups, with no statistically significant difference between groups in TET enzyme activity. In line with previous data, we found an increased percentage of global DNA methylation and hydroxymethylation in the AA-ethanol group compared with control rats. Finally, we investigated changes of selected candidate genes from dynorphin/kappa-opioid receptor system (Pdyn, Kor) and Dnmt3a genes that might be important in AUD-related behaviour. Our gene expression and promoter methylation analysis revealed a significant increase in the mRNA levels of Pdyn, Kor, and Dnmt3a in the AA-ethanol group, however, these changes can only be partially associate with the aberrant DNA methylation in promoter areas of the selected candidate genes. Thus, our findings suggest that the aberrant DNA methylation is rather one of the several mechanisms involved in gene expression regulation in AA rat model.

Deficit of β-endorphin neurons in the hypothalamus and high expression of MOR in mesolimbic structures are related to high alcohol consumption in outbred rats

Clinical studies have postulated that β-endorphin deficiency generates excessive alcohol consumption, and it has been shown that the reduction of β-endorphin neurons increases alcohol intake in animal models. The β-endorphin produce their rewarding effect when they act mainly on the μ-opioid receptors (MOR) located in mesolimbic structures. Thus, it is possible that individual differences in these components of the endogenous opioid system are related to different levels of alcohol consumption. The present study thus examines the relation between two levels of alcohol consumption and intrinsic characteristics of the components of the opioid system in outbred Wistar rats that were not genetically selected. We analyzed the number of β-endorphin-positive neurons in the arcuate nucleus (ArN) and the expression of μ-opioid receptors (MOR) in regions of the reward system, such as the nucleus accumbens (NAc), amygdala (Amy), and ventral tegmental area (VTA) in outbred rats with low (LC) or high (HC) voluntary alcohol consumption. Findings showed that the HC rats had a lower number of β-endorphin-positive neurons in the hypothalamic ArN and a higher expression of MOR in the NAc and VTA, compared to the LC rats. No changes in the expression of MOR in the Amy were observed between the two groups. Results suggest that intrinsic variability in the number of β-endorphin neurons and in the expression of MOR in the LC and HC rats could explain their different patterns for alcohol intake.

Voluntary Adolescent-Onset Alcohol Drinking Fails to Influence Alcohol Consumption or Anxiety-Like Behaviour in Adulthood in Female Alcohol-Preferring Rats

AIMS

Alcohol exposure during adolescence is associated with both increased risk for alcohol use disorders and anxiety in adulthood. Our present experiments examined this association using alcohol-preferring AA (Alko Alcohol) rats selected for high voluntary alcohol drinking.

METHODS

Two groups of female AA rats acquired alcohol drinking at different ages. We gave the adolescent-onset group free choice to 10% alcohol and water for seven weeks, starting on post-natal day 42 (PND 42), whereas the adult-onset group started drinking alcohol on PND 112. After the 7-week drinking, we withdrew the adolescent group from alcohol for two weeks, followed by another voluntary 7-week drinking period, started at the same age as the adult-onset group. We assessed anxiety-like behaviour repeatedly during alcohol drinking with open field and elevated plus maze tests. At the end of alcohol drinking, we also tested the rats using the light/dark box, stress-induced body temperature test and social dominance test.

RESULTS

During the first 7-week alcohol drinking, adolescent rats exhibited significantly slower acquisition of alcohol drinking and lower alcohol preference than the adult-onset group. However, when tested at the same age as the adult-onset rats, they displayed identical alcohol intake and preference. We found no alcohol-induced effects on anxiety- or stress-related behaviour in the experimental groups at any time points.

CONCLUSIONS

These data show that the genetically determined phenotype of high alcohol drinking of the female alcohol-preferring AA rats is not associated with a predisposition to develop anxiety-like behaviour following voluntary alcohol exposure, even when initiated during adolescence.

Pre-training naltrexone increases conditioned fear learning independent of adolescent alcohol consumption history

Our previous research has shown a relationship between low voluntary alcohol consumption and high conditioned fear in male Long Evans rats. Here, we examined whether differences in the endogenous opioid systems might be responsible for these differences. Rats received 6 weeks of chronic intermittent to 20% alcohol (v/v) or water-only from PND 26-66. Based on their consumption during the last 2 weeks of alcohol access, the alcohol-access rats were divided into high drinking (>2.5 g/kg/24-h) or low drinking (<2 g/kg/24-h). Rats were then given injections of the preferential mu opioid receptor antagonist naltrexone (1 mg/kg, s.c.) or the selective kappa opioid receptor antagonist LY2456302 (10 mg/kg, s.c.) prior to fear conditioning and were then tested for conditioned fear 2 days later. Pre-training naltrexone increased conditioned suppression of lever-pressing during training and testing, with no differences between high versus low alcohol drinkers or between water-only versus alcohol access groups (averaged across drinking levels). There was no effect of LY2456302 on conditioned fear in any comparison. We also found no differences between high and low alcohol drinkers and no reliable effect of prior alcohol access (averaged across drinking levels) on conditioned fear. Our experiment replicates and extends previous demonstrations that a preferential MOR antagonist can increase fear learning using conditioned suppression of lever-pressing as a fear measure. However, additional research is needed to determine the cause of the differences in conditioned fear that we previously observed (as they were not observed in the current experiments).

Synaptic Plasticity and its Modulation by Alcohol

Alcohol is one of the oldest pharmacological agents used for its sedative/hypnotic effects, and alcohol abuse and alcohol use disorder (AUD) continues to be major public health issue. AUD is strongly indicated to be a brain disorder, and the molecular and cellular mechanism/s by which alcohol produces its effects in the brain are only now beginning to be understood. In the brain, synaptic plasticity or strengthening or weakening of synapses, can be enhanced or reduced by a variety of stimulation paradigms. Synaptic plasticity is thought to be responsible for important processes involved in the cellular mechanisms of learning and memory. Long-term potentiation (LTP) is a form of synaptic plasticity, and occurs via N-methyl-D-aspartate type glutamate receptor (NMDAR or GluN) dependent and independent mechanisms. In particular, NMDARs are a major target of alcohol, and are implicated in different types of learning and memory. Therefore, understanding the effect of alcohol on synaptic plasticity and transmission mediated by glutamatergic signaling is becoming important, and this will help us understand the significant contribution of the glutamatergic system in AUD. In the first part of this review, we will briefly discuss the mechanisms underlying long term synaptic plasticity in the dorsal striatum, neocortex and the hippocampus. In the second part we will discuss how alcohol (ethanol, EtOH) can modulate long term synaptic plasticity in these three brain regions, mainly from neurophysiological and electrophysiological studies. Taken together, understanding the mechanism(s) underlying alcohol induced changes in brain function may lead to the development of more effective therapeutic agents to reduce AUDs.

The role of opioidergic system in modulating cost/benefit decision-making in alcohol-preferring AA rats and Wistar rats

Research has highlighted the association of a positive family history of alcoholism with a positive treatment response to opioid antagonists in those with a gambling disorder. However, the role of the opioidergic system in gambling behavior is not well understood, and preclinical studies are needed to clarify this. In this study, Alko Alcohol (AA) and Wistar rats went through operant lever pressing training where the task was to choose the more profitable of two options. Different sized sucrose rewards guided the lever choices, and the probability of gaining rewards changed slowly to a level where choosing the smaller reward was the most profitable option. After training, rats were administered subcutaneously with opioid agonist morphine or opioid antagonist naltrexone to study the impact of opioidergic mechanisms on cost/benefit decisions. No difference was found in the decision-making between AA rats or Wistar rats after the morphine administration, but control data revealed a minor decision enhancing effect in AA rats. Naltrexone had no impact on the decisions in AA rats but promoted unprofitable decisions in Wistar rats. Supporting behavioral data showed that in both rat strains morphine increased, and naltrexone decreased, sucrose consumption. Naltrexone also increased the time to accomplish the operant task. The results suggest that opioid agonists could improve decision-making in cost-benefit settings in rats that are naturally prone to high alcohol drinking. The naltrexone results are ambiguous but may partly explain why opioid antagonists lack a positive pharmacotherapeutic effect in some subgroups of gamblers.

Chemogenetic Stimulation and Silencing of the Insula, Amygdala, Nucleus Accumbens, and Their Connections Differentially Modulate Alcohol Drinking in Rats

The anterior insular cortex is hypothesized to represent interoceptive effects of drug reward in the service of goal-directed behavior. The insula is richly connected, but the insula circuitry in addiction remains poorly characterized. We examined the involvement of the anterior insula, amygdala, and nucleus accumbens, as well as the projections of the anterior insula to the central amygdala, basolateral amygdala (BLA), and nucleus accumbens core in voluntary alcohol drinking. We trained alcohol-preferring Alko Alcohol (AA) rats to drink alcohol during intermittent 2-h sessions. We then expressed excitatory or inhibitory designer receptors [designer receptors exclusively activated by designer drugs (DREADDs)] in the anterior insula, nucleus accumbens, or amygdala by means of adenovirus-mediated gene transfer and activated the DREADDs with clozapine-N-oxide (CNO) prior to the drinking sessions. Next, to examine the role of specific insula projections, we expressed FLEX-DREADDs in the efferent insula → nucleus accumbens core, insula → central amygdala, and insula → BLA projections by means of a retrograde AAV-Cre vector injected into the insula projection areas. In the anterior insula and amygdala, excitatory Gq-DREADDs significantly attenuated alcohol consumption. In contrast, in the nucleus accumbens, the Gq-DREADD stimulation increased alcohol drinking, and the inhibitory Gi-DREADDs suppressed it. The Gq-DREADDs expressed in the insula → nucleus accumbens core and insula → central amygdala projections increased alcohol intake, whereas inhibition of these projections had no effect. These data demonstrate that the anterior insula, along with the amygdala and nucleus accumbens, has a key role in controlling alcohol drinking by providing excitatory input to the central amygdala and nucleus accumbens to enhance alcohol reward.

Alpha-Synuclein in Alcohol Use Disorder, Connections with Parkinson's Disease and Potential Therapeutic Role of 5' Untranslated Region-Directed Small Molecules

Alpha-synuclein (α-Syn) is a 140-amino acid (aa) protein encoded by the Synuclein alpha SNCA gene. It is the synaptic protein associated with Parkinson's disease (PD) and is the most highly expressed protein in the Lewy bodies associated with PD and other alpha synucleopathies, including Lewy body dementia (LBD) and multiple system atrophy (MSA). Iron deposits are present in the core of Lewy bodies, and there are reports suggesting that divalent metal ions including Cu2+ and Fe2+ enhance the aggregation of α-Syn. Differential expression of α-Syn is associated with alcohol use disorder (AUD), and specific genetic variants contribute to the risk for alcoholism, including alcohol craving. Spliced variants of α-Syn, leading to the expression of several shorter forms which are more prone to aggregation, are associated with both PD and AUD, and common transcript variants may be able to predict at-risk populations for some movement disorders or subtypes of PD, including secondary Parkinsonism. Both PD and AUD are associated with liver and brain iron dyshomeostasis. Research over the past decade has shown that α-Syn has iron import functions with an ability to oxidize the Fe3+ form of iron to Fe2+ to facilitate its entry into cells. Our prior research has identified an iron-responsive element (IRE) in the 5' untranslated region (5'UTR) of α-Syn mRNA, and we have used the α-Syn 5'UTR to screen for small molecules that modulate its expression in the H4 neuronal cell line. These screens have led us to identify several interesting small molecules capable of both decreasing and increasing α-Syn expression and that may have the potential, together with the recently described mesenchymal stem cell therapies, to normalize α-Syn expression in different regions of the alcoholic and PD brain.

Neuroimaging reveals functionally distinct neuronal networks associated with high-level alcohol consumption in two genetic rat models

Human imaging data suggest that the motivational processes associated with alcohol reward are reflected in the patterns of neural activation after alcohol or alcohol-related cues. In animal models of alcohol drinking, however, the changes in brain activation during voluntary alcohol ingestion are poorly known. In order to improve the translational utility of animal models, we examined alcohol-induced functional brain activation in Alko Alcohol (AA) and Marchigian-Sardinian alcohol-preferring (msP) rats that drink voluntarily high levels of alcohol, but exhibit widely different neurochemical and behavioral traits cosegregated with alcohol preference. Brain imaging was performed using manganese-enhanced MRI (MEMRI), which is based on accumulation of Mn2+ ions in activated neurons, allowing the identification of functional neuronal networks recruited during specific behaviors in awake animals during a subsequent imaging session under anesthesia. MEMRI was performed following 4 weeks of voluntary alcohol drinking, using water drinking as the control. Despite similar levels of alcohol drinking, strikingly different alcohol-induced neuronal activity patterns were observed in AA and msP rats. Overall, functional activation in the AA rats was more widespread, involving large cortical areas and subcortical structures, such as the bed nucleus of the stria terminalis, preoptic area, hypothalamus, periaqueductal grey, and substantia nigra. In the msP rats, however, alcohol-related activation was largely confined to prefrontal cortical regions and insular cortex, and olfactory areas. Overlapping areas of activation found in both rat lines included the nucleus accumbens, prelimbic, orbital, and insular cortex. In conclusion, our data reveal strikingly different brain circuits associated with alcohol drinking in two genetically different rat lines and suggest innately different motivational and behavioral processes driving alcohol drinking. These findings have important implications for the use of these lines in translational alcohol research.

Anterior insula stimulation suppresses appetitive behavior while inducing forebrain activation in alcohol-preferring rats

The anterior insular cortex plays a key role in the representation of interoceptive effects of drug and natural rewards and their integration with attention, executive function, and emotions, making it a potential target region for intervention to control appetitive behaviors. Here, we investigated the effects of chemogenetic stimulation or inhibition of the anterior insula on alcohol and sucrose consumption. Excitatory or inhibitory designer receptors (DREADDs) were expressed in the anterior insula of alcohol-preferring rats by means of adenovirus-mediated gene transfer. Rats had access to either alcohol or sucrose solution during intermittent sessions. To characterize the brain network recruited by chemogenetic insula stimulation we measured brain-wide activation patterns using pharmacological magnetic resonance imaging (phMRI) and c-Fos immunohistochemistry. Anterior insula stimulation by the excitatory Gq-DREADDs significantly attenuated both alcohol and sucrose consumption, whereas the inhibitory Gi-DREADDs had no effects. In contrast, anterior insula stimulation failed to alter locomotor activity or deprivation-induced water drinking. phMRI and c-Fos immunohistochemistry revealed downstream activation of the posterior insula and medial prefrontal cortex, as well as of the mediodorsal thalamus and amygdala. Our results show the critical role of the anterior insula in regulating reward-directed behavior and delineate an insula-centered functional network associated with the effects of insula stimulation. From a translational perspective, our data demonstrate the therapeutic potential of circuit-based interventions and suggest that potentiation of insula excitability with neuromodulatory methods, such as repetitive transcranial magnetic stimulation (rTMS), could be useful in the treatment of alcohol use disorders.

Differential expression of μ-opioid receptors in the nucleus accumbens, amygdala and VTA depends on liking for alcohol, chronic alcohol intake and estradiol treatment

Affectations of the opioid system have been related to exacerbated alcohol consumption. The objectives of this work were to assess whether a deficit of β-endorphinergic neurons differentially affects alcohol intake in female rats with low (LC) and high alcohol consumption (HC), and to determine changes in the μ-opioid receptors (MOR) related to alcohol consumption and chronic exposure to alcohol in structures of the mesolimbic system. Female wild-type rats were selected according to their baseline alcohol intake levels and then exposed to chronic voluntary alcohol consumption after a single injection of either the vehicle or estradiol valerate (EV) to produce a β-endorphin neuronal deficit. Changes in alcohol consumption and MOR expression levels were assessed in the nucleus accumbens (NAc), amygdala (Amy) and ventral tegmental area (VTA) at 5 and 10 weeks after EV treatment. The LC rats increased alcohol intake from baseline to the initial weeks after EV treatment and this consumption remained stable throughout the studied period. In contrast, alcohol consumption increased steadily over time in the HC rats. The HC vehicle rats had a 38% higher MOR protein expression in the NAc than the LC vehicle rats. In addition, chronic alcohol consumption increased MOR expression in the Amy regardless of consumption level, whereas EV treatment produced a decrease in MOR expression in the VTA in all groups. These results suggest intrinsic differences in MOR expression related to alcohol consumption levels. Also, the EV treatment and chronic exposure to alcohol produced adaptive changes in MOR expression.

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.

Animal models of addiction

In recent years, animal models in psychiatric research have been criticized for their limited translational value to the clinical situation. Failures in clinical trials have thus often been attributed to the lack of predictive power of preclinical animal models. Here, I argue that animal models of voluntary drug intake—under nonoperant and operant conditions—and addiction models based on the Diagnostic and Statistical Manual of Mental Disorders are crucial and informative tools for the identification of pathological mechanisms, target identification, and drug development. These models provide excellent face validity, and it is assumed that the neurochemical and neuroanatomical substrates involved in drug-intake behavior are similar in laboratory rodents and humans. Consequently, animal models of drug consumption and addiction provide predictive validity. This predictive power is best illustrated in alcohol research, in which three approved medications—acamprosate, naltrexone, and nalmefene—were developed by means of animal models and then successfully translated into the clinical situation.

The κ-opioid receptor antagonist JDTic decreases ethanol intake in alcohol-preferring AA rats

RATIONALE

Studies suggest that the κ-opioidergic system becomes overactivated as ethanol use disorders develop. Nalmefene, a currently approved treatment for ethanol use disorders, may also elicit some of its main effects via the κ-opioidergic system. However, the exact role of κ-opioid receptors on regulating ethanol intake and contribution to the development of ethanol addiction remains to be elucidated.

OBJECTIVES

The aim of the present study was to clarify the role of accumbal κ-opioid receptors in controlling ethanol intake in alcohol-preferring Alko Alcohol (AA) rats.

METHODS

Microinfusions of the long-acting and selective κ-opioid receptor antagonist JDTic (1-15 μg/site) were administered bilaterally into the nucleus accumbens shell of AA rats voluntarily consuming 10% ethanol solution in the intermittent, time-restricted two-bottle choice access paradigm. JDTic (10 mg/kg) was also administered subcutaneously. Both the acute and long-term effects of the treatment on ethanol intake were examined. As a reference, nor-BNI (3 μg/site) was administered intra-accumbally.

RESULTS

Systemically administered JDTic decreased ethanol intake significantly 2 days and showed a similar trend 4 days after administration. Furthermore, intra-accumbally administered JDTic showed a weak decreasing effect on ethanol intake long-term but had no acute effects. Intra-accumbal administration of nor-BNI tended to decrease ethanol intake.

CONCLUSIONS

The results provide further evidence that κ-opioid receptors play a role in controlling ethanol intake and that accumbal κ-opioid receptors participate in the modulation of the reinforcing effects of ethanol. Furthermore, the results suggest that κ-opioid receptor antagonists may be a valuable adjunct in the pharmacotherapy of ethanol use disorders.

Amphetamine primes enhanced motivation toward uncertain choices in rats with genetic alcohol preference

RATIONALE

Comorbidity with gambling disorder (GD) and alcohol use disorder (AUD) is well documented.

OBJECTIVE

The purpose of our study was to examine the influence of genetic alcohol drinking tendency on reward-guided decision making behavior of rats and the impact of dopamine releaser D-amphetamine on this behavior.

METHODS

In this study, Alko alcohol (AA) and Wistar rats went through long periods of operant lever pressing training where the task was to choose the profitable of two options. The lever choices were guided by different-sized sucrose rewards (one or three pellets), and the probability of gaining the larger reward was slowly changed to a level where choosing the smaller reward would be the most profitable in the long run. After training, rats were injected (s.c.) with dopamine releaser D-amphetamine (0.3, 1.0 mg/kg) to study the impact of rapid dopamine release on this learned decision making behavior.

RESULTS

Administration of D-amphetamine promoted unprofitable decision making of AA rats more robustly when compared to Wistar rats. At the same time, D-amphetamine reduced lever pressing responses. Interestingly, we found that this reduction in lever pressing was significantly greater in Wistar rats than in AA rats and it was not linked to motivation to consume sucrose.

CONCLUSIONS

Our results indicate that conditioning to the lever pressing in uncertain environments is more pronounced in AA than in Wistar rats and indicate that the reinforcing effects of a gambling-like environment act as a stronger conditioning factor for rats that exhibit a genetic tendency for high alcohol drinking.

Reduced Levels of mGlu2 Receptors within the Prelimbic Cortex Are Not Associated with Elevated Glutamate Transmission or High Alcohol Drinking

BACKGROUND

A Grm2 cys407* stop codon mutation, which results in a loss of the metabotropic glutamate 2 (mGlu2) receptor protein, was identified as being associated with high alcohol drinking by alcohol-preferring (P) rats. The objectives of the current study were to characterize the effects of reduced levels of mGlu2 receptors on glutamate transmission and alcohol drinking.

METHODS

Quantitative no-net-flux microdialysis was used to test the hypothesis that basal extracellular glutamate levels in the prelimbic (PL) cortex and nucleus accumbens shell (NACsh) will be higher in P than Wistar rats. A lentiviral-delivered short-hairpin RNA (shRNA)-mediated knockdown was used to test the hypothesis that reduced levels of mGlu2 receptors within the PL cortex will increase voluntary alcohol drinking by Wistar rats. A linear regression analysis was used to test the hypothesis that there will be a significant correlation between the Grm2 cys407* mutation and level of alcohol intake.

RESULTS

Extracellular glutamate concentrations within the PL cortex (3.6 ± 0.6 vs. 6.4 ± 0.6 μM) and NACsh (3.2 ± 0.4 vs. 6.6 ± 0.6 μM) were significantly lower in female P than female Wistar rats. Western blot detected the presence of mGlu2 receptors in these regions of female Wistar rats, but not female P rats. Micro-infusion of shRNAs into the PL cortex significantly reduced local mGlu2 receptor levels (by 40%), but did not alter voluntary alcohol drinking in male Wistar rats. In addition, there was no significant correlation between the Grm2 mutation and alcohol intake in 36 rodent lines (r = 0.29, p > 0.05).

CONCLUSIONS

Collectively, these results suggest a lack of association between the loss of mGlu2 receptors and glutamate transmission in the NACsh and PL cortex of female P rats, and between the level of mGlu2 receptors in the PL cortex and alcohol drinking of male Wistar rats.

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

Loss of control over the ethanol consumption: differential transcriptional regulation in prefrontal cortex

Alcohol use disorder (AUD) is a complex multifactorial disease with heritability of ∼50% and corresponds to the state in which the body triggers a reinforcement or reward compulsive behavior due to ethanol consumption, even when faced with negative consequences. Although several studies have shown the impact of high ethanol intake on the prefrontal cortex (PFC) gene expression, few have addressed the relationship between the patterns of gene expression underlying the compulsive behaviour associated with relapsing. In this study, we used a chronic three-bottle free-choice mouse model to investigate the PFC transcriptome in three different groups of mice drinkers: 'Light drinkers' (preference for water throughout the experiment); 'Heavy drinkers' (preference for ethanol with a non-compulsive intake), and 'Inflexible drinkers' (preference for ethanol with a compulsive drinking component). Our aim was to correlate the intake patterns observed in this model with gene expression changes in the PFC, a brain region critical for the development and maintenance of alcohol addiction. We found that the Camk2a gene showed a downregulated profile only in the Inflexible when compared to the Light drinkers group, the Camk2n1 and Pkp2 genes showed an upregulated profile only in the Inflexible drinkers when compared to the Control group, and the Gja1 gene showed an upregulated profile in the Light and Inflexible drinkers when compared to the Control group. These different transcription patterns have been associated to the presence of alcohol, in the Camk2n1 and Gja1 genes; to the amount of ethanol consumed, in the Camk2a gene; and to the loss of control in the alcohol consumption, in the Pkp2 gene. Here, we provide, for the first time, the potential involvement of the Pkp2 gene in the compulsivity and loss of control over the voluntary ethanol consumption.

Voluntary induction and maintenance of alcohol dependence in rats using alcohol vapor self-administration

RATIONALE

A major issue in the addiction field is the limited number of animal models of the voluntary induction and maintenance of alcohol dependence in outbred rats.

OBJECTIVES

To address this issue, we developed a novel apparatus that vaporizes alcohol for 2-10 min after an active nosepoke response.

METHODS

Male Wistar rats were allowed to self-administer alcohol vapor for 8 h/day every other day for 24 sessions (escalated) or eight sessions (non-escalated). Escalated and non-escalated rats were then tested for progressive ratio responding. Anxiety-like behavior, somatic signs of withdrawal, and hyperalgesia were assessed during acute withdrawal.

RESULTS

The results showed that rats exhibited excellent discrimination between the active and inactive operanda (>85%), and the escalated rats quickly increased their blood alcohol levels from ~50 to >200 mg% in ~6 weeks. Compared with non-escalated rats, escalated rats exhibited severe addiction-like behavior, including somatic signs of withdrawal, anxiety-like behavior, hyperalgesia, and higher responding on a progressive ratio schedule of reinforcement.

CONCLUSIONS

These results demonstrate that outbred rats will voluntarily self-administer alcohol vapor to the point of dependence without the use of forced alcohol administration, sweeteners, food/water restriction, operant pretraining, or behavioral/genetic selection. This novel animal model may be particularly useful for medication development to help unveil the neuronal circuitry that underlies the voluntary induction of alcohol addiction and identify novel molecular targets that are specifically recruited after the voluntary induction and maintenance of alcohol dependence.

Further characterization of the GlyT-1 inhibitor Org25935: anti-alcohol, neurobehavioral, and gene expression effects

The glycine transporter-1 inhibitor Org25935 is a promising candidate in a treatment concept for alcohol use disorder targeting the glycine system. Org25935 inhibits ethanol-induced dopamine elevation in brain reward regions and reduces ethanol intake in Wistar rats. This study aimed to further characterise the compound and used ethanol consumption, behavioral measures, and gene expression as parameters to investigate the effects in Wistar rats and, as pharmacogenetic comparison, Alko-Alcohol (AA) rats. Animals were provided limited access to ethanol in a two-bottle free-choice paradigm with daily drug administration. Acute effects of Org25935 were estimated using locomotor activity and neurobehavioral status. Effects on gene expression in Wistar rats were measured with qPCR. The higher but not the lower dose of Org25935 reduced alcohol intake in Wistar rats. Unexpectedly, Org25935 reduced both ethanol and water intake and induced strong CNS-depressive effects in AA-rats (withdrawn from further studies). Neurobehavioral effects by Org25935 differed between the strains (AA-rats towards sedation). Org25935 did not affect gene expression at the mRNA level in the glycine system of Wistar rats. The data indicate a small therapeutic range for the anti-alcohol properties of Org25935, a finding that may guide further evaluations of the clinical utility of GlyT-1 inhibitors. The results point to the importance of pharmacogenetic considerations when developing drugs for alcohol-related medical concerns. Despite the lack of successful clinical outcomes, to date, the heterogeneity of drug action of Org25935 and similar agents and the unmet medical need justify further studies of glycinergic compounds in alcohol use disorder.

Reprogramming of mPFC transcriptome and function in alcohol dependence

Despite its limited immediate reinforcement value, alcohol has a potent ability to induce neuroadaptations that promote its incentive salience, escalation of voluntary alcohol intake and aversion-resistant alcohol seeking. A constellation of these traits, collectively called 'post-dependent', emerges following brain exposure to repeated cycles of intoxication and withdrawal. The medial prefrontal cortex (mPFC) and its subdivisions exert top-down regulation of approach and avoidance behaviors, including those that lead to alcohol intake. Here, we review an emerging literature which indicates that a reprogramming of mPFC function occurs with prolonged exposure of the brain to cycles of alcohol intoxication and withdrawal. This reprogramming results in molecular dysregulations that contribute to the post-dependent syndrome. Convergent evidence has identified neuroadaptations resulting in altered glutamatergic and BDNF-mediated signaling, and for these pathways, direct evidence for a mechanistic role has been obtained. Additional evidence points to a dysregulation of pathways involving calcium homeostasis and neurotransmitter release. Recent findings indicate that global DNA hypermethylation is a key factor in reprogramming the mPFC genome after a history of dependence. As one of the results of this epigenetic remodeling, several histone modifying epigenetic enzymes are repressed. Among these, PR-domain zinc-finger protein 2, a methyltransferase that selectively mono-methylates histone H3 at lysine 9 has been functionally validated to drive several of the molecular and behavioral long-term consequences of alcohol dependence. Information processing within the mPFC involves formation of dynamic neuronal networks, or functional ensembles that are shaped by transcriptional responses. The epigenetic dysregulations identified by our molecular studies are likely to alter this dynamic processing in multiple ways. In summary, epigenetic molecular switches in the mPFC appear to be turned on as alcoholism develops. Strategies to reverse these processes may offer targets for disease-modifying treatments.

Social stress in adolescents induces depression and brain-region-specific modulation of the transcription factor MAX

MAX is a conserved constitutive small phosphoprotein from a network of transcription factors that are extensively studied in tumorigenesis and whose functions affect cell proliferation, differentiation and death. Inspired by its higher expression during development and in regions involved in emotional behaviors, we hypothesized its involvement in cerebral changes caused by early-life stress. We studied the effects of repeated social stress during adolescence on behaviors and on MAX and its putative partner MYC. Thirty-day-old C57BL/6 male mice underwent brief daily social defeat stress from an adult aggressor for 21 days. Following social stress episodes and housing in social groups after each defeat, adolescent mice exhibit depressive-like, but not anxiety-like behaviors and show higher MAX nuclear immunoreactivity in hippocampal (HC) but not prefrontal cortical (PFC) neurons. Conversely, MAX immunoreactivity is lower in the striatum (ST) of defeated adolescents. The positive correlation between MAX and MYC levels in the PFC revealed disruptions in both the HC and ST. The changes in MAX protein levels are not due to differential gene expression or protein degradation in those regions, suggesting that posttranscriptional modifications occurred. These findings indicate that repeated, brief social defeat in adolescent male mice, combined with group housing, is a useful protocol to study a subtype of depression that is dissociated from generalized (non-social) anxiety. To our knowledge, this is the first report of an association between dysregulation of the MAX-MYC network in the brain and a behavior, suggesting a novel approach for exploiting the neuroplasticity associated with depression.

Alcohol preference and consumption are controlled by the caudal linear nucleus in alcohol-preferring rats

The neuroanatomical and neurochemical basis of alcohol drinking has been extensively studied, but the neural circuitry mediating alcohol reinforcement has not been fully delineated. In the present experiments, we used both neuroimaging and pharmacological tools to identify neural systems associated with alcohol preference and high voluntary alcohol drinking in alcohol-preferring AA (Alko Alcohol) rats. First, we compared the basal brain activity of AA rats with that of heterogeneous Wistar rats with manganese-enhanced magnetic resonance imaging (MEMRI). Briefly, alcohol-naïve rats were implanted with subcutaneous osmotic minipumps delivering 120 mg/kg MnCl2 over a 7-day period, and were then imaged using a three-dimensional rapid acquisition-relaxation enhanced pulse sequence. MEMRI analysis revealed that the most conspicuous subcortical activation difference was located in the caudal linear nucleus of raphe (CLi), with AA rats displaying significantly lower T1 signal in this region compared to Wistar rats. However, following long-term alcohol drinking, CLi activity was increased in AA rats. In the second experiment, the CLi was targeted with pharmacological tools. AA rats trained to drink 10% alcohol during 2-h sessions were implanted with guide cannulas aimed at the CLi and were given injections of the GABAA receptor agonist muscimol into the CLi before drinking sessions. Muscimol dose-dependently increased alcohol drinking, and co-administration of the gamma aminobutyric acid (GABA)A antagonist bicuculline blocked muscimol's effect. These findings suggest that the mediocaudal region of the ventral tegmental area, particularly the CLi, is important for the propensity for high alcohol drinking and controls alcohol reward via GABAergic transmission.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

Prevalence and influence of cys407* Grm2 mutation in Hannover-derived Wistar rats: mGlu2 receptor loss links to alcohol intake, risk taking and emotional behaviour

Modulation of metabotropic glutamate 2 (mGlu2) receptor function has huge potential for treating psychiatric and neurological diseases. Development of drugs acting on mGlu2 receptors depends on the development and use of translatable animal models of disease. We report here a stop codon mutation at cysteine 407 in Grm2 (cys407*) that is common in some Wistar rats. Therefore, researchers in this field need to be aware of strains with this mutation. Our genotypic survey found widespread prevalence of the mutation in commercial Wistar strains, particularly those known as Han Wistar. Such Han Wistar rats are ideal for research into the separate roles of mGlu2 and mGlu3 receptors in CNS function. Previous investigations, unknowingly using such mGlu2 receptor-lacking rats, provide insights into the role of mGlu2 receptors in behaviour. The Grm2 mutant rats, which dominate some selectively bred lines, display characteristics of altered emotionality, impulsivity and risk-related behaviours and increased voluntary alcohol intake compared with their mGlu2 receptor-competent counterparts. In addition, the data further emphasize the potential therapeutic role of mGlu2 receptors in psychiatric and neurological disease, and indicate novel methods of studying the role of mGlu2 and mGlu3 receptors. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Modulation of nucleus accumbens connectivity by alcohol drinking and naltrexone in alcohol-preferring rats: A manganese-enhanced magnetic resonance imaging study

The nonselective opioid receptor antagonist naltrexone is now used for the treatment of alcoholism, yet naltrexone's central mechanism of action remains poorly understood. One line of evidence suggests that opioid antagonists regulate alcohol drinking through interaction with the mesolimbic dopamine system. Hence, our goal here was to examine the role of the nucleus accumbens connectivity in alcohol reinforcement and naltrexone's actions using manganese-enhanced magnetic resonance imaging (MEMRI). Following long-term free-choice drinking of alcohol and water, AA (Alko Alcohol) rats received injections of MnCl2 into the nucleus accumbens for activity-dependent tracing of accumbal connections. Immediately after the accumbal injections, rats were imaged using MEMRI, and then allowed to drink either alcohol or water for the next 24h. Naltrexone was administered prior to the active dark period, and the second MEMRI was performed 24h after the first scan. Comparison of signal intensity at 1 and 24h after accumbal MnCl2 injections revealed an ipsilateral continuum through the ventral pallidum, bed nucleus of the stria terminalis, globus pallidus, and lateral hypothalamus to the substantia nigra and ventral tegmental area. Activation was also seen in the rostral part of the insular cortex and regions of the prefrontal cortex. Alcohol drinking resulted in enhanced activation of these connections, whereas naltrexone suppressed alcohol-induced activity. These data support the involvement of the accumbal connections in alcohol reinforcement and mediation of naltrexone's suppressive effects on alcohol drinking through their deactivation.

Ethanol-induced conditioned taste aversion in Warsaw Alcohol High-Preferring (WHP) and Warsaw Alcohol Low-Preferring (WLP) rats

The aversive action of the pharmacological properties of ethanol was studied in selectively bred Warsaw Alcohol High-Preferring (WHP) and Warsaw Alcohol Low-Preferring (WLP) rats. For this study, a conditioned-taste aversion test was used. Male WHP and WLP rats were submitted to daily 20-min sessions for 5 days, in which a saccharin solution (1.0 g/L) was available (pre-conditioning phase). Next, this drinking was paired with the injection of ethanol (0, 0.5, 1.0 g/kg), intraperitoneally [i.p.] immediately after removal of the saccharin bottle (conditioning phase). Afterward, the choice between the saccharin solution and water was extended for 18 subsequent days for 20-min daily sessions (post-conditioning phase). Both doses of ethanol did not produce an aversion to saccharin in WLP and WHP rats in the conditioning phase. However, injection of the 1.0 g/kg dose of ethanol produced an aversion in WLP rats that was detected by a decrease in saccharin intake at days 1, 3, 7, and 10 of the post-conditioning phase, with a decrease in saccharin preference for 16 days of the post-conditioning phase. Conditioned taste aversion, measured as a decrease in saccharin intake and saccharin preference, was only visible in WHP rats at day 1 and day 3 of the post-conditioning phase. This difference between WLP and WHP rats was apparent despite similar blood ethanol levels in both rat lines following injection of 0.5 and 1.0 g/kg of ethanol. These results may suggest differing levels of aversion to the post-ingestional effects of ethanol between WLP and WHP rats. These differing levels of aversion may contribute to the selected line difference in ethanol preference in WHP and WLP rats.

Transgenerational Transmission of the Effect of Gestational Ethanol Exposure on Ethanol Use-Related Behavior

BACKGROUND

Prenatal alcohol exposure (PAE) enhances the risk for alcoholism by increasing the propensity to consume alcohol and altering neurophysiological response to alcohol challenge. Trans-generationally transmittable genetic alterations have been implicated in these behavioral changes. To date, transgenerational transmission of PAE-induced behavioral responses to alcohol has never been experimentally investigated. Therefore, we explored the transgenerational transmission of PAE-induced behavioral effects across 3 generations.

METHODS

Pregnant Sprague Dawley dams received 1 g/kg ethanol (EtOH) or water daily on gestational days 17 through 20 via gavage, or remained untreated in their home cages. To produce second filial (F2) or F3 generations, similarly treated adult F1 or F2 offspring were mated and left undisturbed through gestation. On postnatal day (PND) 14, male and female F1, F2, and F3 offspring were tested for consumption of 5% (w/v) EtOH (in water), or water. Using the loss of righting reflex (LORR) paradigm on PND 42, F1 and F2 adolescent male offspring were tested for sensitivity to acute EtOH-induced sedation-hypnosis at 3.5 or 4.5 g/kg dose. F3 male adolescents were similarly tested at 3.5 g/kg dose. Blood EtOH concentration (BEC) was measured at waking.

RESULTS

EtOH exposure increased EtOH consumption compared to both water and untreated control groups in all generations. EtOH-treated group F1 and F2 adolescents displayed attenuated LORR duration compared to the water group. No attenuated LORR was observed in the F3 generation. BEC at waking corroborated with the significant LORR duration differences while also revealing differences between untreated control and water groups in F1 and F2 generations.

CONCLUSIONS

Our results provide novel behavioral evidence attesting that late gestational moderate EtOH exposure increases EtOH intake across 3 generations and may alter sensitivity to EtOH-induced sedation-hypnosis across 2 generations.

Different Hypothalamic Nicotinic α7 Receptor Expression and Response to Low Nicotine Dose in Alcohol-Preferring and Alcohol-Avoiding Rats

BACKGROUND

The aim of this study was to examine possible differences in nicotinic acetylcholine receptors and responses in rats with genetic preference or avoidance for alcohol. This was done by using 2 rat lines with high alcohol preference (Alko Alcohol [AA]) or alcohol avoidance (Alko Non-Alcohol [ANA]).

METHODS

Locomotor activity was measured following nicotine and histamine H3 receptor (H3R) antagonist treatment. In situ hybridization and receptor ligand binding experiments were used in drug-naïve animals to examine the expression of different α nicotinic receptor subunits.

RESULTS

The AA rats were found to be more sensitive to the stimulatory effect of a low dose of nicotine than ANA rats, which were not significantly activated. Combination of histamine H3R antagonist, JNJ-39220675, and nicotine resulted to similar locomotor activation as nicotine alone. To further understand the mechanism underlying the difference in nicotine response in AA and ANA rats, we studied the expression of α5, α6, and α7 nicotinic receptor subunits in specific brain areas of AA and ANA rats. We found no differences in the expression of α5 nicotinic receptor subunits in the medial habenula and hippocampus or in α6 subunit in the ventral tegmental area and substantia nigra. However, the level of α7 nicotinic receptor subunit mRNA was significantly lower in the tuberomamillary nucleus of posterior hypothalamus of alcohol-preferring AA rats than in alcohol-avoiding ANA rats. Also the hypothalamic [125I-α-bungarotoxin binding was lower in AA rats indicating lower levels of α7 nicotinic receptors.

CONCLUSIONS

The lower expression and receptor binding of α7 nicotinic receptors in the tuberomamillary nucleus of AA rats suggest a difference in the regulation of brain histamine neurons between the rat lines since the α7 nicotinic receptors are located in histaminergic neurons. Stronger nicotine-induced locomotor response, mediated partially via α7 receptors, and previously described high alcohol consumption in AA rats could be explained by the found difference in tuberomamillary α7 receptor levels.

Brain activation induced by voluntary alcohol and saccharin drinking in rats assessed with manganese-enhanced magnetic resonance imaging

The neuroanatomical and neurochemical basis of alcohol reward has been studied extensively, but global alterations of neural activity in reward circuits during chronic alcohol use remain poorly described. Here, we measured brain activity changes produced by long-term voluntary alcohol drinking in the alcohol-preferring AA (Alko alcohol) rats using manganese-enhanced magnetic resonance imaging (MEMRI). MEMRI is based on the ability of paramagnetic manganese ions to accumulate in excitable neurons and thereby enhance the T1-weighted signal in activated brain areas. Following 6 weeks of voluntary alcohol drinking, AA rats were allowed to drink alcohol for an additional week, during which they were administered manganese chloride (MnCl2 ) with subcutaneous osmotic minipumps before MEMRI. A second group with an identical alcohol drinking history received MnCl2 during the abstinence week following alcohol drinking. For comparing alcohol with a natural reinforcer, MEMRI was also performed in saccharin-drinking rats. A water-drinking group receiving MnCl2 served as a control. We found that alcohol drinking increased brain activity extensively in cortical and subcortical areas, including the mesocorticolimbic and nigrostriatal dopamine pathways and their afferents. Remarkably similar activation maps were seen after saccharin ingestion. Particularly in the prelimbic cortex, ventral hippocampus and subthalamic nucleus, activation persisted into early abstinence. These data show that voluntary alcohol recruits an extensive network that includes the ascending dopamine systems and their afferent connections, and that this network is largely shared with saccharin reward. The regions displaying persistent alterations after alcohol drinking could participate in brain networks underlying alcohol seeking and relapse.

Dual-trait selection for ethanol consumption and withdrawal: genetic and transcriptional network effects

BACKGROUND

Data from C57BL/6J (B6) × DBA/2J (D2) F2 intercrosses (B6xD2 F2 ), standard and recombinant inbred strains, and heterogeneous stock mice indicate that a reciprocal (or inverse) genetic relationship exists between alcohol consumption and withdrawal severity. Furthermore, some genetic studies have detected reciprocal quantitative trait loci (QTLs) for these traits. We used a novel mouse model developed by simultaneous selection for both high alcohol consumption/low withdrawal and low alcohol consumption/high withdrawal and analyzed the gene expression and genome-wide genotypic differences.

METHODS

Randomly chosen third selected generation (S3 ) mice (N = 24/sex/line), bred from a B6xD2 F2 , were genotyped using the Mouse Universal Genotyping Array, which provided 2,760 informative markers. QTL analysis used a marker-by-marker strategy with the threshold for a significant log of the odds (LOD) set at 10. Gene expression in the ventral striatum was measured using the Illumina Mouse 8.2 array. Differential gene expression and the weighted gene co-expression network analysis (WGCNA) were implemented.

RESULTS

Significant QTLs for consumption/withdrawal were detected on chromosomes (Chr) 2, 4, 9, and 12. A suggestive QTL mapped to Chr 6. Some of the QTLs overlapped with known QTLs mapped for 1 of the traits individually. One thousand seven hundred and forty-five transcripts were detected as being differentially expressed between the lines; there was some overlap with known withdrawal genes (e.g., Mpdz) located within QTL regions. WGCNA revealed several modules of co-expressed genes showing significant effects in both differential expression and intramodular connectivity; a module richly annotated with kinase-related annotations was most affected.

CONCLUSIONS

Marked effects of selection on expression and network structure were detected. QTLs overlapping with differentially expressed genes on Chr 2 (distal) and 4 suggest that these are cis-eQTLs (Chr 2: Kif3b, Kcnq2; Chr 4: Mpdz, Snapc3). Other QTLs identified were on Chr 2 (proximal), 9, and 12. Network results point to involvement of kinase-related mechanisms and outline the need for further efforts such as interrogation of noncoding RNAs.

The neurometabolic fingerprint of excessive alcohol drinking

'Omics' techniques are widely used to identify novel mechanisms underlying brain function and pathology. Here we applied a novel metabolomics approach to further ascertain the role of frontostriatal brain regions for the expression of addiction-like behaviors in rat models of alcoholism. Rats were made alcohol dependent via chronic intermittent alcohol vapor exposure. Following a 3-week abstinence period, rats had continuous access to alcohol in a two-bottle, free-choice paradigm for 7 weeks. Nontargeted flow injection time-of-flight mass spectrometry was used to assess global metabolic profiles of two cortical (prelimbic and infralimbic) and two striatal (accumbens core and shell) brain regions. Alcohol consumption produces pronounced global effects on neurometabolomic profiles leading to a clear separation of metabolic phenotypes between treatment groups, particularly. Further comparisons of regional tissue levels of various metabolites, most notably dopamine and Met-enkephalin, allow the extrapolation of alcohol consumption history. Finally, a high-drinking metabolic fingerprint was identified indicating a distinct alteration of central energy metabolism in the accumbens shell of excessively drinking rats that could indicate a so far unrecognized pathophysiological mechanism in alcohol addiction. In conclusion, global metabolic profiling from distinct brain regions by mass spectrometry identifies profiles reflective of an animal's drinking history and provides a versatile tool to further investigate pathophysiological mechanisms in alcohol dependence.

Low copulatory activity in selectively bred Sardinian alcohol-nonpreferring (sNP) relative to alcohol-preferring (sP) rats

BACKGROUND

There is a growing consensus that similar neural mechanisms are involved in the reinforcing properties of natural rewards, like food and sex, and drugs of abuse. Rat lines selectively bred for high and low oral alcohol intake and preference have been useful for understanding factors contributing to excessive alcohol intake and may constitute proper animal models for investigating the neurobiological basis of natural rewarding stimuli.

METHODS

The present study evaluated copulatory behavior in alcohol and sexually naïve Sardinian alcohol-preferring (sP) and -nonpreferring (sNP) male rats in three consecutive copulatory behavior tests.

RESULTS

The main finding was that, under the conditions used in this study, sNP rats were sexually inactive relative to sP rats. To gain more information about the sexual behavior in sP rats, Wistar rats were included as an external reference strain. Only minor differences between sP and Wistar rats were revealed.

CONCLUSIONS

The reason behind the low copulatory activity of sNP rats remains to be elucidated, but may in part be mediated by innate differences in brain transmitter systems. The comparison between sP and Wistar rats may also suggest that the inherent proclivity to excessive alcohol drinking in sP rats may mainly be dependent on its anxiolytic properties, as previously proposed, and not changes in the reward system.

Enhanced Extracellular Glutamate and Dopamine in the Ventral Pallidum of Alcohol-Preferring AA and Alcohol-Avoiding ANA Rats after Morphine

The purpose of the present study was to investigate the role of ventral pallidal opioidergic mechanisms in the control of ethanol intake by studying the effects of acute administration of morphine on the levels of GABA, glutamate, and dopamine in the ventral pallidum. The study was conducted using the alcohol-preferring Alko Alcohol (AA) and alcohol-avoiding Alko Non-Alcohol (ANA) rat lines that have well-documented differences in their voluntary ethanol intake and brain opioidergic systems. Therefore, examination of neurobiological differences between the lines is supposed to help to identify the neuronal mechanisms underlying ethanol intake, since selection pressure is assumed gradually to lead to enrichment of alleles promoting high or low ethanol intake, respectively. The effects of an acute dose of morphine (1 or 10 mg/kg s.c.) on the extracellular levels of GABA and glutamate in the ventral pallidum were monitored with in vivo microdialysis. The concentrations of GABA and glutamate in the dialyzates were determined with a high performance liquid chromatography system using fluorescent detection, while electrochemical detection was used for dopamine. The levels of glutamate in the rats injected with morphine 1 mg/kg were significantly above the levels found in the controls and in the rats receiving morphine 10 mg/kg. Morphine 10 mg/kg also increased the levels of dopamine. Morphine could not, however, modify the levels of GABA. The rat lines did not differ in any of the effects of morphine. The data suggest that the glutamatergic and dopaminergic systems in the ventral pallidum may mediate some effects of morphine. Since there were no differences between the AA and ANA lines, the basic hypothesis underlying the use of the genetic animal model suggests that the effects of morphine detected probably do not underlie the different intake of ethanol by the lines and contribute to the control of ethanol intake in these animals.

Postdependent state in rats as a model for medication development in alcoholism

Rational development of novel therapeutic strategies for alcoholism requires understanding of its underlying neurobiology and pathophysiology. Obtaining this knowledge largely relies on animal studies. Thus, choosing the appropriate animal model is one of the most critical steps in pre-clinical medication development. Among the range of animal models that have been used to investigate excessive alcohol consumption in rodents, the postdependent model stands out. It was specifically developed to test the role of negative affect as a key driving force in a perpetuating addiction cycle for alcoholism. Here, we will describe our approach to make rats dependent via chronic intermittent exposure to alcohol, discuss the validity of this model, and compare it with other commonly used animal models of alcoholism. We will summarize evidence that postdependent rats fulfill several criteria of a 'Diagnostic and Statistical Manual of Mental Disorders IV/V-like' diagnostic system. Importantly, these animals show long-lasting excessive consumption of and increased motivation for alcohol, and evidence for loss of control over alcohol intake. Our conclusion that postdependent rats are an excellent model for medication development for alcoholism is underscored by a summary of more than two dozen pharmacological tests aimed at reversing these abnormal alcohol responses. We will end with open questions on the use of this model. In the tradition of the Sanchis-Segura and Spanagel review, we provide comic strips that illustrate the postdependent procedure and relevant phenotypes in this review.

Innate BDNF expression is associated with ethanol intake in alcohol-preferring AA and alcohol-avoiding ANA rats

We have shown recently that acute administration of ethanol modulates the expression of brain-derived neurotrophic factor (BDNF) in several rat brain areas known to be involved in the development of addiction to ethanol and other drugs of abuse, suggesting that BDNF may be a factor contributing to the neuroadaptive changes set in motion by ethanol exposure. The purpose of the present study was to further clarify the role of BDNF in reinforcement from ethanol and in the development of addiction to ethanol by specifying the effect of acute administration of ethanol (1.5 or 3.0 g/kg i.p.) on the expression profile of BDNF mRNA in the ventral tegmental area and in the terminal areas of the mesolimbic dopamine pathway in the brain of alcohol-preferring AA and alcohol-avoiding ANA rats, selected for high and low voluntary ethanol intake, respectively. The level of BDNF mRNA expression was higher in the amygdala and ventral tegmental area of AA than in those of ANA rats, and there was a trend for a higher level in the nucleus accumbens. In the amygdala and hippocampus, a biphasic change in the BDNF mRNA levels was detected: the levels were decreased at 3 and 6h but increased above the basal levels at 24h. Furthermore, there was a difference between the AA and ANA lines in the effect of ethanol, the ANA rats showing an increase in BDNF mRNA levels while such a change was not seen in AA rats. These findings suggest that the innate levels of BDNF expression may play a role in the mediation of the reinforcing effects of ethanol and in the control of ethanol intake.

Is the alcohol deprivation effect genetically mediated? Studies with HXB/BXH recombinant inbred rat strains

BACKGROUND

Two features of alcohol addiction that have been widely studied in animal models are relapse drinking following periods of alcohol abstinence and the escalation of alcohol consumption after chronic continuous or intermittent alcohol exposure. The genetic contribution to these phenotypes has not been systematically investigated.

METHODS

HXB/BXH recombinant inbred (RI) rat strains were given access to alcohol sequentially as follows: alcohol (10%) as the only fluid for 1 week; alcohol (10%) and water in a 2-bottle choice paradigm for 7 weeks ("pre-alcohol deprivation effect [ADE] alcohol consumption"); 2 weeks of access to water only (alcohol deprivation); and 2 weeks of reaccess to 10% alcohol and water ("post-ADE alcohol consumption"). The periods of deprivation and reaccess to alcohol were repeated 3 times. The ADE was defined as the amount of alcohol consumed in the first 24 hours after deprivation minus the average daily amount of alcohol consumed in the week prior to deprivation. Heritability of the phenotypes was determined by analysis of variance, and quantitative trait loci (QTLs) were identified.

RESULTS

All strains showed increased alcohol consumption, compared to the predeprivation period, in the first 24 hours after each deprivation (ADE). Broad-sense heritability of the ADEs was low (ADE1, 9.1%; ADE2, 26.2%; ADE3, 16.3%). Alcohol consumption levels were relatively stable over weeks 2 to 7. Post-ADE alcohol consumption levels consistently increased in some strains and were decreased or unchanged in others. Heritability of pre- and post-ADE alcohol consumption was high and increased over time (week 2, 38.5%; week 7, 51.1%; week 11, 56.8%; week 15, 63.3%). QTLs for pre- and post-ADE alcohol consumption were similar, but the strength of the QTL association with the phenotype decreased over time.

CONCLUSIONS

In the HXB/BXH RI rat strains, genotypic variance does not account for a large proportion of phenotypic variance in the ADE phenotype (low heritability), suggesting a role of environmental factors. In contrast, a large proportion of the variance across the RI strains in pre- and post-ADE alcohol consumption is due to genetically determined variance (high heritability).

A model of alcohol drinking under an intermittent access schedule using group-housed mice

Here, we describe a new model of voluntary alcohol drinking by group-housed mice. The model employs sensor-equipped cages that track the behaviors of the individual animals via implanted radio chips. After the animals were allowed intermittent access to alcohol (three 24 h intervals every week) for 4 weeks, the proportions of licks directed toward bottles containing alcohol were 50.9% and 39.6% for the male and female mice, respectively. We used three approaches (i.e., quinine adulteration, a progressive ratio schedule and a schedule involving a risk of punishment) to test for symptoms of compulsive alcohol drinking. The addition of 0.01% quinine to the alcohol solution did not significantly affect intake, but 0.03% quinine induced a greater than 5-fold reduction in the number of licks on the alcohol bottles. When the animals were required to perform increasing numbers of instrumental responses to obtain access to the bottle with alcohol (i.e., a progressive ratio schedule), they frequently reached a maximum of 21 responses irrespective of the available reward. Although the mice rarely achieved higher response criteria, the number of attempts was ∼10 times greater in case of alcohol than water. We have developed an approach for mapping social interactions among animals that is based on analysis of the sequences of entries into the cage corners. This approach allowed us to identify the mice that followed other animals in non-random fashions. Approximately half of the mice displayed at least one interaction of this type. We have not yet found a clear correlation between imitative behavior and relative alcohol preference. In conclusion, the model we describe avoids the limitations associated with testing isolated animals and reliably leads to stable alcohol drinking. Therefore, this model may be well suited to screening for the effects of genetic mutations or pharmacological treatments on alcohol-induced behaviors.