Effect of mu opioid receptor blockade on alcohol intake in rats bred for high alcohol drinking

Sigma receptor-induced heavy drinking in rats: Modulation by the opioid receptor system

Alcohol use disorder (AUD) is a major cause of morbidity and mortality worldwide, for which new efficacious treatments are necessary. The opioid receptor system is a mediator of the rewarding effects of alcohol; in particular, while activation of μ opioid receptors enhances ethanol intake in rodents, opioid-receptor antagonists, such as naloxone and naltrexone, reduce its pleasurable and reinforcing effects, thereby decreasing alcohol. Sigma receptors (Sig-Rs) have been proposed as modulators of the effects of alcohol and, therefore, as a potential new pharmacological target for AUD. Somewhat analogously to μ opioid ligands, SigR agonists increase, while SigR antagonists decrease alcohol intake in animal models of excessive alcohol drinking. However, a potential cross-talk between these two receptor systems in relation to alcohol consumption has so far not been investigated. Here, we addressed this question pharmacologically, by testing the effects of either activating or inhibiting opioid receptors on the heavy alcohol drinking induced by chronic stimulation of SigR in alcohol-preferring rats. We found that the opioid receptor agonist morphine, which per se increases ethanol intake, at a sub-threshold dose reduces the binge-like drinking induced by the repeated treatment with the SigR agonist 1,3-di-o-tolylguanidine (DTG); conversely, the opioid receptor antagonist naltrexone, which per se reduces ethanol intake, at a sub-threshold dose potentiates the DTG-induced binge-like drinking. Our data show a cross-talk between the opioid and SigR systems relevant to the modulation of alcohol drinking, which provides important insights into the neurobiology of AUD and may lead to the development of novel therapies, either standalone or in combination.

Building better strategies to develop new medications in Alcohol Use Disorder: Learning from past success and failure to shape a brighter future

Alcohol Use Disorder (AUD) is a chronic disease that develops over the years. The complexity of the neurobiological processes contributing to the emergence of AUD and the neuroadaptive changes occurring during disease progression make it difficult to improve treatments. On the other hand, this complexity offers researchers the possibility to explore new targets. Over years of intense research several molecules were tested in AUD; in most cases, despite promising preclinical data, the clinical efficacy appeared insufficient to justify futher development. A prototypical example is that of corticotropin releasing factor type 1 receptor (CRF1R) antagonists that showed significant effectiveness in animal models of AUD but were largely ineffective in humans. The present article attempts to analyze the most recent venues in the development of new medications in AUD with a focus on the most promising drug targets under current exploration. Moreover, we delineate the importance of using a more integrated translational framework approach to correlate preclinical findings and early clinical data to enhance the probability to validate biological targets of interest.

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".

Kappa-opioid receptors differentially regulate low and high levels of ethanol intake in female mice

INTRODUCTION

Studies in laboratory animals and humans indicate that endogenous opioids play an important role in regulating the rewarding value of various drugs, including ethanol (EtOH). Indeed, opioid antagonists are currently a front-line treatment for alcoholism in humans. Although roles for mu- and delta-opioid receptors have been characterized, the contribution of kappa-opioid receptors (KORs) is less clear. There is evidence that changes in KOR system function can decrease or increase EtOH drinking, depending on test conditions. For example, female mice lacking preprodynorphin - the precursor to the endogenous KOR ligand dynorphin - have reduced EtOH intake. Considering that KORs can regulate dopamine (DA) transmission, we hypothesized that KORs expressed on DA neurons would play a prominent role in EtOH intake in females.

METHODS

We used a Cre/loxP recombination strategy to ablate KORs throughout the body or specifically on dopamine uptake transporter (DAT)-expressing neurons to investigate the role of KORs on preference for and intake of EtOH (2-bottle choice), the transition from moderate to excessive EtOH drinking (intermittent EtOH access), and binge EtOH drinking (drinking in the dark [DID]).

RESULTS

KOR deletion decreased preference for EtOH, although this effect was less pronounced when EtOH intake increased beyond relatively low levels.

DISCUSSION

Our findings indicate that KOR activation increases EtOH drinking via effects mediated, at least in part, by KORs on DA neurons. While the mechanisms of this regulation remain unknown, previous work suggests that alterations in negative reinforcement processes or sensitivity to the sensory properties of EtOH can affect preference and intake.

The Novel μ-Opioid Receptor Antagonist GSK1521498 Decreases Both Alcohol Seeking and Drinking: Evidence from a New Preclinical Model of Alcohol Seeking

Distinct environmental and conditioned stimuli influencing ethanol-associated appetitive and consummatory behaviors may jointly contribute to alcohol addiction. To develop an effective translational animal model that illuminates this interaction, daily seeking responses, maintained by alcohol-associated conditioned stimuli (CSs), need to be dissociated from alcohol drinking behavior. For this, we established a procedure whereby alcohol seeking maintained by alcohol-associated CSs is followed by a period during which rats have the opportunity to drink alcohol. This cue-controlled alcohol-seeking procedure was used to compare the effects of naltrexone and GSK1521498, a novel selective μ-opioid receptor antagonist, on both voluntary alcohol-intake and alcohol-seeking behaviors. Rederived alcohol-preferring, alcohol-nonpreferring, and high-alcohol-drinking replicate 1 line of rats (Indiana University) first received 18 sessions of 24 h home cage access to 10% alcohol and water under a 2-bottle choice procedure. They were trained subsequently to respond instrumentally for access to 15% alcohol under a second-order schedule of reinforcement, in which a prolonged period of alcohol-seeking behavior was maintained by contingent presentations of an alcohol-associated CS acting as a conditioned reinforcer. This seeking period was terminated by 20 min of free alcohol drinking access that achieved significant blood alcohol concentrations. The influence of pretreatment with either naltrexone (0.1-1-3 mg/kg) or GSK1521498 (0.1-1-3 mg/kg) before instrumental sessions was measured on both seeking and drinking behaviors, as well as on drinking in the 2-bottle choice procedure. Naltrexone and GSK1521498 dose-dependently reduced both cue-controlled alcohol seeking and alcohol intake in the instrumental context as well as alcohol intake in the choice procedure. However, GSK1521498 showed significantly greater effectiveness than naltrexone, supporting its potential use for promoting abstinence and preventing relapse in alcohol addiction.

Effects of systemic opioid receptor ligands on ethanol- and sucrose seeking and drinking in alcohol-preferring (P) and Long Evans rats

The endogenous opioid system has been implicated in mediating the reinforcing effects of ethanol (EtOH). Naltrexone (NTX), an opioid antagonist with concentration-dependent selectivity for the mu receptor, naltrindole (NTI), a selective delta receptor antagonist, and U50,488H, a selective kappa receptor agonist were examined in both alcohol-preferring (P) and nonselected (Long Evans (LE)) rats to determine whether they differentially affected the seeking and consumption of EtOH and sucrose. Using the sipper-tube model, rats reinforced with either 2% sucrose or 10% EtOH were injected with vehicle and either NTI (2.5, 5.0, or 10.0 mg/kg), U50 (2.5, 5.0, or 10.0 mg/kg), low-dose NTX (0.1, 0.3, or 1.0 mg/kg), or high-dose NTX (1.0, 3.0, or 10.0 mg/kg). Subsequent intakes (consummatory) or lever responses (seeking) were assessed. Overall, NTI, U50, and NTX attenuated intake and responding for sucrose and EtOH, with EtOH-reinforced P rats being the most sensitive to the effects of NTI on intake and seeking. U50 treatment decreased intake and seeking in both P and LE rats but did not selectively reduce EtOH intake or seeking in either line. P rats were more sensitive than LE rats to lower doses of NTX, and these doses more selectively attenuated responding for EtOH than sucrose. Higher doses of NTX suppressed intake and responding across both lines and reinforcers. These results suggest that drugs selective for the opioid receptors may be good pharmacotherapeutic targets, particularly in those with an underlying genetic predisposition for greater EtOH preference/intake.

Operant self-administration of alcohol and nicotine in a preclinical model of co-abuse

RATIONALE AND OBJECTIVES

Alcohol and nicotine are often taken together. In humans, intake of nicotine, via smoked tobacco, increases alcohol drinking, and alcohol increases smoking. Chronic nicotine treatment increases alcohol self-administration (SA) in laboratory animals; the reverse relationship is less clear. Most animal work modeling this has used passive administration, which lacks relevance to human co-abuse. Here, we describe a model based on sequential operant SA of alcohol and nicotine.

METHODS

Animals are first trained on alcohol SA (0.19 ml of 12 % alcohol (w/v)/delivery) and then receive separate alcohol (8 %, w/v) and nicotine (15 μg/kg/infusion) SA sessions on the same day ("daily dual access"). Animals then receive access to alcohol and then to nicotine (or in the reverse order) in alternating 5-min periods in 2-h sessions ("alternating access"). We then determine if alternating access modifies the effects of naltrexone on responding for alcohol and nicotine.

RESULTS

We found that with daily dual access, nicotine significantly increased alcohol SA when alcohol access occurred prior to nicotine access and that nicotine SA significantly decreased when the alcohol SA session preceded it. During alternating access, nicotine also significantly increased alcohol intake. Naltrexone (0.3 or 1 mg/kg) significantly reduced alcohol SA during these alternating access sessions in animals that also received nicotine SA, but had minimal effects on animals receiving alcohol SA alone. Naltrexone did not affect nicotine SA under any condition.

CONCLUSIONS

This sequential access procedure effectively models the effects of nicotine on alcohol intake noted in humans.

μ-Opioid receptors mediate the effects of chronic ethanol binge drinking on the hippocampal neurogenic niche

Ethanol exposure and withdrawal alter the generation of new neurons in the adult hippocampus. The endogenous opioid system, particularly the μ-opioid receptor (MOR), can modulate neural progenitors and also plays a critical role in ethanol drinking and dependence. In the present study, we sought to determine whether MOR contributes to the effects of ethanol on the dentate gyrus (DG) neurogenic niche. MOR wild-type (WT), heterozygous (Het) and knockout (KO) littermates were subjected to voluntary ethanol drinking in repeated limited-access two-bottle choice (2BC) sessions. MOR deficiency did not alter progenitor proliferation, neuronal differentiation and maturation, apoptosis or microglia in ethanol-naïve mice. When exposed to five consecutive weeks of 2BC, MOR mutant mice exhibited a gene-dosage-dependent reduction of ethanol consumption compared with WT mice. Introducing a week of ethanol deprivation between each week of 2BC increased ethanol consumption in all genotypes and produced equivalent intakes in WT, Het and KO mice. Under the latter paradigm, ethanol drinking decreased progenitor proliferation and neuronal differentiation in the DG of WT mice. Interestingly, WT mice exhibited a strong negative correlation between ethanol intake and proliferation, which was disrupted in Het and KO mice. Moreover, MOR deficiency blocked the effect of ethanol on neuronal differentiation. MOR deficiency also protected against the neuroimmune response to ethanol drinking. Finally, chronic binge drinking induced a paradoxical decrease in apoptosis, which was independent of MOR. Altogether, our data suggest that MOR is implicated in some of the neuroplastic changes produced by chronic ethanol exposure in the DG.

The role of the opioid system in alcohol dependence

The role of the brain opioid system in alcohol dependence has been the subject of much research for over 25 years. This review explores the evidence: firstly describing the opioid receptors in terms of their individual subtypes, neuroanatomy, neurophysiology and ligands; secondly, summarising emerging data from specific neurochemical, behavioural and neuroimaging studies, explaining the characteristics of addiction with a focus on alcohol dependence and connecting the opioid system with alcohol dependence; and finally reviewing the known literature regarding opioid antagonists in clinical use for alcohol dependence. Further interrogation of how modulation of the opioid system, via use of MOP (mu), DOP (delta) and KOP (kappa) agents, restores the balance of a dysregulated system in alcohol dependence should increase our insight into this disease process and therefore guide better methods for understanding and treating alcohol dependence in the future.

Combining naltrexone and prazosin in a single oral medication decreases alcohol drinking more effectively than does either drug alone

BACKGROUND

Naltrexone (NTX) is underutilized in clinical treatment settings because its efficacy is modest, and it is not effective for all alcoholics and, when it is effective, a significant number of alcoholics fail to maintain initial treatment gains and subsequently relapse to heavy drinking. This has slowed acceptance of NTX by the treatment community, and there is a clear need for additional treatments for alcoholism and alcohol use disorders. Given that NTX and prazosin can each reduce alcohol drinking in rats selectively bred for alcohol preference and high voluntary alcohol drinking (alcohol-preferring "P" rats), we tested whether a combination of NTX + prazosin is more effective in decreasing alcohol drinking than is either drug alone.

METHODS

P rats were given access to a 15% (v/v) alcohol solution for 2 hours daily. Rats were fed NTX and prazosin, alone or in combination, prior to onset of the daily 2-hour alcohol access period for 4 weeks and the effect of drug treatment on alcohol and water intake was assessed.

RESULTS

During the first week of treatment, neither a low dose of NTX, nor prazosin, was effective in decreasing alcohol intake when each drug was administered alone, but combining the 2 drugs in a single medication significantly reduced alcohol intake. The combination was as effective as was a higher dose of NTX. Using a low dose of NTX in combination with prazosin may reduce the potential for undesirable side effects early in treatment which, in turn, may improve patient compliance and result in a more successful outcome when NTX is used for treating alcoholism and alcohol use disorders.

CONCLUSIONS

Combining low-dose NTX and prazosin in a single medication may be more useful than is either drug alone for treating both inpatient and outpatient alcoholics and heavy drinkers early in the treatment process.

μ-opioid receptors in the stimulation of mesolimbic dopamine activity by ethanol and morphine in Long-Evans rats: a delayed effect of ethanol

RATIONALE

Naltrexone, a non-selective opioid antagonist, decreases the euphoria and positive subjective responses to alcohol in heavy drinkers. It has been proposed that the μ-opioid receptor plays a role in ethanol reinforcement through modulation of ethanol-stimulated mesolimbic dopamine release.

OBJECTIVES

To investigate the ability of naltrexone and β-funaltrexamine, an irreversible μ-opioid specific antagonist, to inhibit ethanol-stimulated and morphine-stimulated mesolimbic dopamine release, and to determine whether opioid receptors on mesolimbic neurons contribute to these mechanisms.

METHODS

Ethanol-naïve male Long Evans rats were given opioid receptor antagonists either intravenously, subcutaneously, or intracranially into the ventral tegmental area (VTA), followed by intravenous administration of ethanol or morphine. We measured extracellular dopamine in vivo using microdialysis probes inserted into the nucleus accumbens shell (n = 114).

RESULTS

Administration of naltrexone (intravenously) and β-funaltrexamine (subcutaneously), as well as intracranial injection of naltrexone into the VTA did not prevent the initiation of dopamine release by intravenous ethanol administration, but prevented it from being as prolonged. In contrast, morphine-stimulated mesolimbic dopamine release was effectively suppressed.

CONCLUSIONS

Our results provide novel evidence that there are two distinct mechanisms that mediate ethanol-stimulated mesolimbic dopamine release (an initial phase and a delayed phase), and that opioid receptor activation is required to maintain the delayed-phase dopamine release. Moreover, μ-opioid receptors account for this delayed-phase dopamine response, and the VTA is potentially the site of action of this mechanism. We conclude that μ-opioid receptors play different roles in the mechanisms of stimulation of mesolimbic dopamine activity by ethanol and morphine.

Genetically selected alcohol preferring rats to model human alcoholism

Animal models have been successfully developed to mimic and study alcoholism. These models have the unique feature of allowing the researcher to control for the genetic characteristics of the animal, alcohol exposure and environment. Moreover, these animal models allow pharmacological, neurochemical and behavioral manipulations otherwise impossible. Unquestionably, one of the major contributions to the understanding of the neurobiological basis of alcoholism comes from data that have been obtained from the study of genetically selected alcohol preferring rat lines and from the consequences that alcohol drinking and environmental manipulations, (i.e., protracted alcohol drinking, intoxication, exposure to stress, etc.) have on them. In fact, if on the one hand genetic factors may account for about 50-60% of the risk of developing alcohol dependence, on the other hand protracted alcohol exposure is a necessary precondition to actually develop the disease, while environmental vulnerability factors may be crucial for disease progression. The present article will offer an overview of the different genetically selected alcohol preferring rat lines developed and used to study alcoholism. The predictive, face and construct validity of these animal models and the translational significance of findings achieved through their use will be critically discussed.

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

One major cause for discrepancies in results from animal experimental studies is the use of different animal strains and suppliers. We have previously reported that Wistar rats from five different suppliers display profound differences in ethanol intake and behavior. One of the neurobiological processes that could be underlying these differences is the endogenous opioid system, which has been implicated in the rewarding and reinforcing effects of alcohol. We therefore hypothesized that the differences between the supplier groups would also be evident in the endogenous opioid system. Radioimmunoassay was used to determine the levels of the opioid peptides Met-enkephalin-Arg(6)Phe(7) and dynorphin B in several brain areas of ethanol-drinking and ethanol naïve Wistar rats from five different suppliers. In the ethanol naïve animals, differences between the supplier groups were found in the pituitary gland, hypothalamus, frontal cortex, dorsal striatum and hippocampus. In the ethanol-drinking rats, differences were found in the same structures, with the addition of medial prefrontal cortex and substantia nigra. Correlations between ethanol intake and peptide levels were also found in several of the areas examined. The structures in which differences were found have all been implicated in the transition from drug use to addiction and these differences may lead to different propensities and vulnerability to this transition. Because the endogenous opioids have been suggested to be involved in a number of neurobiological disorders the results do not only have implications for research on alcohol or drug addiction, but many other fields as well.

Acute ethanol administration differentially alters enkephalinase and aminopeptidase N activity and mRNA levels in regions of the nigrostriatal pathway

Opioid peptides play a key role in ethanol reinforcement and may also represent important determinants in brain sensitivity to ethanol through modulation of nigrostriatal dopaminergic activity. Regulation of opioid levels by peptidase-degrading enzymes could be relevant in ethanol's actions. The aim of this work was to study the acute ethanol (2.5 g/kg) effects on the activity and mRNA expression of enkephalinase (NEP) and aminopeptidase N (APN) in the rat substantia nigra (SN) and the anterior-medial (amCP) and medial-posterior (mpCP) regions of the caudate-putamen (CP). Enzymatic activities were measured by fluorometric assays and mRNA expression by reverse transcriptase polymerase chain reaction. Acute ethanol administration differentially altered peptidase activities and mRNA expression with different kinetics. Ethanol increased and decreased NEP mRNA levels in the SN and amCP, respectively, but produced biphasic effects in the mpCP. APN mRNA levels were increased by ethanol in all brain regions. Ethanol induced a transient and long-lasting increase in NEP (mpCP) and APN (amCP) activities, respectively. Peptidase activities were not changed by ethanol in the SN. Our results indicate that striatal NEP and APN are important ethanol targets. Ethanol-induced changes in these neuropeptidases in the CP could contribute to the mechanisms involved in brain sensitivity to ethanol.

Ontogenetic differences in ethanol's motivational properties during infancy

Pairing a conditioned stimulus (CS) with ethanol generally produces aversion for that CS in adult rodents. However, infant rats (PD1-PD3) exposed to ethanol demonstrate appetitive reinforcement to ethanol (Nizhnikov, Varlinskaya, Petrov, & Spear, 2006; Petrov, Varlinskaya, & Spear, 2003). This sensitivity to the appetitive properties of ethanol during infancy may be transient, as during the second postnatal week rat pups tend to exhibit conditioned aversions to flavors paired with ethanol. The present study examined changes in the motivation properties of ethanol through ontogeny and the neurobiology underlying these changes. Rat pups were exposed to a taste conditioning procedure on PD4 or PD12. Rat pups were intraorally infused with 2.5% of their body weight of saccharin solution (0.1%) and immediately after injected intraperitoneolly (i.p.) with one of six doses of ethanol (0.0-2.0 g/kg). A day later pups were given saccharine infusions and percent body weight gain was used as an index of ethanol's reinforcing effects. PD4 pups expressed appetitive reinforcement to ethanol, as indicated by greater saccharin intake, as compared to control counterparts and to the older PD12 pups. Subsequent experiments revealed that PD4 pups were less sensitive to the aversive properties of the drug than PD12 pups. The older pups found high doses of ethanol aversive while PD4 rat pups did not condition aversions to this dose of ethanol after a single trial. A similar pattern of results was observed between the low doses of ethanol and the highest doses of a kappa opioid agonist. The PD12 animals did not condition to the kappa opioid agonist, while the younger rats expressed an appetitive response. These results illustrate an ontogenetic change in the motivational properties of ethanol, with sensitivity to its appetitive properties declining and responsiveness to the aversive properties increasing with age during early infancy.

Sex hormone activity in alcohol addiction: integrating organizational and activational effects

There are well-known sex differences in the epidemiology and etiopathology of alcohol dependence. Male gender is a crucial risk factor for the onset of alcohol addiction. A directly modifying role of testosterone in alcohol addiction-related behavior is well established. Sex hormones exert both permanent (organizational) and transient (activational) effects on the human brain. The sensitive period for these effects lasts throughout life. In this article, we present a novel early sex hormone activity model of alcohol addiction. We propose that early exposure to sex hormones triggers structural (organizational) neuroadaptations. These neuroadaptations affect cellular and behavioral responses to adult sex hormones, sensitize the brain's reward system to the reinforcing properties of alcohol and modulate alcohol addictive behavior later in life. This review outlines clinical findings related to the early sex hormone activity model of alcohol addiction (handedness, the second-to-fourth-finger length ratio, and the androgen receptor and aromatase) and includes clinical and preclinical literature regarding the activational effects of sex hormones in alcohol drinking behavior. Furthermore, we discuss the role of the hypothalamic-pituitary-adrenal and -gonadal axes and the opioid system in mediating the relationship between sex hormone activity and alcohol dependence. We conclude that a combination of exposure to sex hormones in utero and during early development contributes to the risk of alcohol addiction later in life. The early sex hormone activity model of alcohol addiction may prove to be a valuable tool in the development of preventive and therapeutic strategies.

Critical thoughts on current rodent models for evaluating potential treatments of alcohol addiction and withdrawal

Despite years of neurobiological research that have helped to identify potential therapeutic targets, we do not have a reliable pharmacological treatment for alcoholism. There are a range of possible explanations for this failure, including arguments that alcoholism is a spectrum disorder and that different population subtypes may respond to different treatments. This view is supported by categorisations such as early- and late-onset alcoholism, whilst multifactorial genetic factors may also alter responsivity to pharmacological agents. Furthermore, experience of alcohol withdrawal may play a role in future drinking in a way that may distinguish alcoholism from other forms of addiction. Additionally, our neurobiological models, based largely upon results from rodent studies, may not mimic specific aspects of the human condition and may reflect different underlying phenomena and biological processes from the clinical pattern. As a result, potential treatments may be targeting inappropriate aspects of alcohol-related behaviours. Instead, we suggest a more profitable approach is (a) to identify well-defined intermediate behavioural phenotypes in human experimental models that reflect defined aspects of the human clinical disorder and (b) to develop animal models that are homologous with those phenotypes in terms of psychological processes and underlying neurobiological mechanisms. This review describes an array of animal models currently used in the addiction field and what they tell us about alcoholism. We will then examine how established pharmacological agents have been developed using only a limited number of these models, before describing some alternative novel approaches to achieving homology between animal and human experimental measures.

Positron emission tomography imaging of mu- and delta-opioid receptor binding in alcohol-dependent and healthy control subjects

BACKGROUND

The endogenous opioid system plays a significant role in alcohol dependence. The goal of the current study was to investigate regional brain mu-opioid receptor (MOR) and delta-opioid receptor (DOR) availability in recently abstinent alcohol-dependent and age-matched healthy control men and women with positron emission tomography (PET) imaging.

METHODS

Alcohol-dependent subjects completed an inpatient protocol, which included medically supervised withdrawal and PET imaging on day 5 of abstinence. Control subjects completed PET imaging following an overnight stay. PET scans with the MOR-selective ligand [(11)C]carfentanil (CFN) were completed in 25 alcohol-dependent and 30 control subjects. Most of these same subjects (20 alcohol-dependent subjects and 18 controls) also completed PET scans with the DOR-selective ligand [(11)C]methylnaltrindole (MeNTL).

RESULTS

Volumes of interest and statistical parametric mapping analyses indicated that alcohol-dependent subjects had significantly higher [(11)C]CFN binding potential (BP(ND) ) than healthy controls in multiple brain regions including the ventral striatum when adjusting for age, gender, and smoking status. There was an inverse relationship between [(11)C]CFN BP(ND) and craving in several brain regions in alcohol-dependent subjects. Groups did not differ in [(11)C]MeNTL BP(ND) ; however, [(11)C]MeNTL BP(ND) in caudate was positively correlated with recent alcohol drinking in alcohol-dependent subjects.

CONCLUSIONS

Our observation of higher [(11)C]CFN BP(ND) in alcohol-dependent subjects can result from up-regulation of MOR and/or reduction in endogenous opioid peptides following long-term alcohol consumption, dependence, and/or withdrawal. Alternatively, the higher [(11)C]CFN BP(ND) in alcohol-dependent subjects may be an etiological difference that predisposed these individuals to alcohol dependence or may have developed as a result of increased exposure to childhood adversity, stress, and other environmental factors known to increase MOR. Although the direction of group differences in [(11)C]MeNTL BP(ND) was similar in many brain regions, differences did not achieve statistical significance, perhaps as a result of our limited sample size. Additional research is needed to further clarify these relationships. The finding that alcohol-dependent subjects had higher [(11)C]CFN BP(ND) is consistent with a prominent role of the MOR in alcohol dependence.

The μ opioid receptor is not involved in ethanol-stimulated dopamine release in the ventral striatum of C57BL/6J mice

BACKGROUND

The mu opioid receptor (MOR) has previously been found to regulate ethanol-stimulated dopamine release under some, but not all, conditions. A difference in ethanol-evoked dopamine release between male and female mixed background C57BL/6J-129SvEv mice led to questions about its ubiquitous role in these effects of ethanol. Using congenic C57BL/6J MOR knockout (KO) mice and C57BL/6J mice pretreated with an irreversible MOR antagonist, we investigated the function of this receptor in ethanol-stimulated dopamine release.

METHODS

Microdialysis was used to monitor dopamine release and ethanol clearance in MOR -/-, +/+, and +/- . male and female mice after intraperitoneal (i.p.) injections of 1.0, 2.0, and 3.0 g/kg ethanol (or saline). We also measured the increase in dopamine release after 5 mg/kg morphine (i.p.) in male and female MOR+/+ and -/- mice. In a separate experiment, male C57BL/6J mice were pretreated with either the irreversible MOR antagonist beta funaltrexamine (BFNA) or vehicle, and dopamine levels were monitored after administration of 2 g/kg ethanol or 5 mg/kg morphine.

RESULTS

Although ethanol-stimulated dopamine release at all the 3 doses of alcohol tested, there were no differences between MOR+/+, -/-, and +/- mice in these effects. Female mice had a more prolonged effect compared to males at the 1 g/kg dose. Administration of 2 g/kg ethanol also caused a similar increase in dopamine levels in both saline-pretreated and BFNA-pretreated mice. Five mg/kg morphine caused a significant increase in dopamine levels in MOR+/+ mice but not in MOR-/- mice and in saline-pretreated mice but not in BFNA-pretreated mice. Intraperitoneal saline injections had a significant, albeit small and transient, effect on dopamine release when given in a volume equivalent to the ethanol doses, but not in a volume equivalent to the 5 mg/kg morphine dose. Ethanol pharmacokinetics were similar in all genotypes and both sexes at each dose and in both pretreatment groups.

CONCLUSIONS

MOR is not involved in ethanol-stimulated dopamine release in the ventral striatum of C57BL/6J mice.

Opioid receptors and the discriminative stimulus effects of ethanol in squirrel monkeys: Mu and delta opioid receptor mechanisms

Mu and delta opioid receptors modulate the reinforcing effects of ethanol, however, their role in the subjective effects of ethanol is not well understood. This study evaluated the contribution of mu and delta opioid receptors to the subjective effects of ethanol using drug discrimination procedures. Monkeys were trained to discriminate ethanol from saline under a schedule of food delivery. In tests, ethanol engendered increases in drug-lever responding, reaching a maximum of >80%. The mu opioid receptor agonists fentanyl and buprenorphine and the delta opioid receptor agonists SNC 80 and SNC 162 did not substitute for the discriminative stimulus effects of ethanol. As pretreatments, the full agonists fentanyl and SNC 80 enhanced the effects of low doses of ethanol and fentanyl attenuated the effects of the ethanol training dose. Although the possibility of pharmacological antagonism of the effects of ethanol cannot be ruled out, a more likely alternative is that the diminished effects of ethanol were due to perceptual masking of the ethanol stimulus. In contrast, the partial agonists buprenorphine and SNC 162 did not alter ethanol's effects. Finally, the discriminative stimulus effects of ethanol were attenuated following administration of presumably mu-selective doses of the antagonist naltrexone, but not after administration of the delta opioid receptor antagonist naltrindole. The ability of naltrexone to block the discriminative stimulus effects of ethanol likely reflects its capacity to attenuate ethanol-induced increases in endogenous opioids, in particular beta-endorphin, because attenuation of the ethanol stimulus was not accompanied by significant suppression of response rate.

Effects of acute ethanol administration on methionine-enkephalin expression and release in regions of the rat brain

The dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of ethanol and other drugs of abuse. Ethanol reinforcement and high alcohol drinking behaviour have been suggested to involve the ethanol-induced activation of endogenous opioid systems. Ethanol may alter opioidergic transmission at different levels, including opioid peptide biosynthesis and release, as well as binding to opioid receptors. The aim of this work was to investigate the effects of different ethanol doses on methionine-enkephalin (Met-enk) release from the rat nucleus accumbens (NAcc). Ethanol effects were also studied on Met-enk content in the NAcc, prefrontal cortex (PFC) and caudate-putamen (CP). Met-enk release was studied by microdialysis in Wistar anesthetized rats and peptide concentrations were quantitated by radioimmunoassay. Ethanol was administered by intraperitoneal injection after a 2-h basal release period. Ethanol doses of 0.5, 1 and 2.5 g/kg induced a 2.7-, 4.9- and 3.4-fold increase in Met-enk release from the NAcc. However, ethanol responses followed different kinetics, with earliest effects observed with the highest ethanol dose. In comparison, a 2.5-fold increase in peptide release was produced by 100 mM KCl. Ethanol, at a dose of 2.5 g/kg, induced a significant 66.7% decrease in Met-enk content in the NAcc, as well as a 76.4% reduction in peptide levels in the CP. Lower ethanol doses did not alter Met-enk content in these regions. On the other hand, an ethanol dose of 0.5 g/kg produced a non-significant decrease in Met-enk levels in the PFC. Our results suggest that ethanol-induced changes in enkephalin expression and release in regions of the mesocorticolimbic and nigrostriatal pathways could be involved in ethanol central effects. Released enkephalins by ethanol may modulate the dopaminergic activity of mesolimbic neurons and play a critical role in ethanol reinforcement mechanisms.

What aspects of human alcohol use disorders can be modeled using selectively bred rat lines?

The use of selective breeding to produce animal models for the study of alcohol abuse and alcoholism represents one of the major advances in the field of alcohol research. Rats selectively bred for alcohol preference and alcohol nonpreference have been useful to both preclinical and clinical investigators in the alcohol research community for studying the behavioral, neurobiological, and molecular basis of alcohol drinking, for identifying the genes that may contribute to the development of alcohol abuse and alcoholism, and for evaluating the utility of drugs aimed at reducing alcohol intake and preventing alcohol relapse. Rats selectively bred for alcohol preference (alcohol preferring or "P" line) have enhanced responsiveness to the low dose reinforcing effects of alcohol, less aversion to moderate/high doses of alcohol, and are able to develop tolerance to the aversive effects of alcohol more rapidly and to maintain tolerance longer than rats selectively bred for alcohol nonpreference (alcohol nonpreferring or "NP" line). The increased potency of low-dose alcohol as a reinforcer for P rats might be expected to foster and maintain alcohol drinking. Weaker aversion to the pharmacological effects of moderate/high doses of alcohol in the P line would allow P rats to drink more alcohol than NP rats before the postingestional effects become aversive. Rapid induction of tolerance to the aversive effects of alcohol with repeated bouts of voluntary alcohol drinking, as well as persistence of alcohol tolerance in rats of the P line might serve to maintain alcohol drinking. These are powerful mechanisms that may serve to promote and maintain a high alcohol drinking behavior. Although these rat lines have been used to address several characteristics of excessive alcohol consumption in humans, they have not yet been used to model several aspects of human alcohol use disorders. New applications of these selectively bred rat lines are discussed which may further our understanding of the factors contributing to alcohol abuse and alcoholism.

Ethanol blocks long-term potentiation of GABAergic synapses in the ventral tegmental area involving mu-opioid receptors

It is well documented that ethanol exposure alters GABA (gamma-aminobutyric acid)-releasing synapses, and ethanol addiction is associated with endogenous opioid system. Emerging evidence indicates that opioids block long-term potentiation in the fast inhibitory GABA(A) receptor synapses (LTP(GABA)) onto dopamine-containing neurons in the ventral tegmental area (VTA), a brain region essential for reward-seeking behavior. However, how ethanol affects LTP(GABA) is not known. We report here that in acute midbrain slices from rats, clinically relevant concentrations of ethanol applied both in vitro and in vivo prevents LTP(GABA), which is reversed, respectively, by in vitro and in vivo administration of naloxone, a mu-opioid receptor (MOR) antagonist. Furthermore, the blockade of LTP(GABA) induced by a brief in vitro ethanol treatment is mimicked by DAMGO ([D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin), a MOR agonist. Paired-pulse ratios are similar in slices, 24 h after in vivo injection with either saline or ethanol. Sp-cAMPS, a stable cAMP analog, and pCPT-cGMP, a cGMP analog, potentiates GABA(A)-mediated inhibitory postsynaptic currents in slices from ethanol-treated rats, indicating that a single in vivo ethanol exposure does not maximally increase GABA release, instead, ethanol produces a long-lasting inability to generate LTP(GABA). These neuroadaptations to ethanol might contribute to early stage of addiction.

Alcohol response and consumption in adolescent rhesus macaques: life history and genetic influences

The use of alcohol by adolescents is a growing problem and has become an important research topic in the etiology of the alcohol use disorders. A key component of this research has been the development of animal models of adolescent alcohol consumption and alcohol response. Because of their extended period of adolescence, rhesus macaques are especially well suited for modeling alcohol-related phenotypes that contribute to the adolescent propensity for alcohol consumption. In this review, we discuss studies from our laboratory that have investigated both the initial response to acute alcohol administration and the consumption of alcohol in voluntary self-administration paradigms in adolescent rhesus macaques. These studies confirm that adolescence is a time of dynamic change both behaviorally and physiologically, and that alcohol response and alcohol consumption are influenced by life history variables, such as age, sex, and adverse early experience in the form of peer-rearing. Furthermore, genetic variants that alter functioning of the serotonin, endogenous opioid, and corticotropin-releasing hormone systems are shown to influence both physiological and behavioral outcomes, in some cases interacting with early experience to indicate gene by environment interactions. These findings highlight several of the pathways involved in alcohol response and consumption, namely reward, behavioral dyscontrol, and vulnerability to stress, and demonstrate a role for these pathways during the early stages of alcohol exposure in adolescence.

Roles of opioid receptor subtypes in mediating alcohol-seeking induced by discrete cues and context

The aim of this study was to assess the effects of selective blockade of the delta (DOP) or mu (MOP) opioid receptors on alcohol-seeking induced by discrete cues and context. In Experiment 1, rats were trained to self-administer alcohol in an environment with distinct sensory properties. After extinction in a different context with separate sensory properties, rats were tested for context-induced renewal in the original context following treatment with the DOP receptor antagonist naltrindole (0-15 mg/kg, i.p.) or the MOP receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) (0-3 microg/4 microL, i.c.v.). In Experiment 2, reinstatement was tested with the presentation of a discrete light + tone cue previously associated with alcohol delivery, following extinction without the cue. The effects of naltrindole (0-5 mg/kg, i.p.) or CTOP (0-3 microg/4 microL, i.c.v.) were assessed. For context-induced renewal, 7.5 mg/kg naltrindole reduced responding without affecting locomotor activity. Both doses of CTOP attenuated responding in the first 15 min of the renewal test session; however, total responses did not differ at the end of the session. For discrete-cue-induced reinstatement, 1 and 5 mg/kg naltrindole attenuated responding but CTOP had no effect. We conclude that whereas DOP receptors mediate alcohol-seeking induced by discrete cues and context, MOP receptors may play a modest role only in context-induced renewal. These findings point to a differential involvement of opioid receptor subtypes in the effects of different kinds of conditioned stimuli on alcohol-seeking and support a more prominent role for DOP receptors.

Potency of naltrexone to reduce ethanol self-administration in rats is greater for subcutaneous versus intraperitoneal injection

The opioid antagonist naltrexone (NTX) is used to treat alcohol dependence and may reduce alcohol consumption by selectively blocking opioid receptors. In rat experiments, discrepancy exists across studies regarding the potency of NTX to reduce ethanol consumption. One cause of this discrepancy may be the use of different routes of NTX administration (e.g., intraperitoneal vs. subcutaneous). The purpose of this study was to directly compare the effects of intraperitoneal and subcutaneous injections of NTX on ethanol self-administration. Rats pressed a lever for a sweetened ethanol solution (10% wt/vol in 0.1% saccharin) during 20 min daily sessions. One group received intraperitoneal injections of 1, 3, 10, and 30 mg/kg NTX before the sessions. Another group received subcutaneous injections of 0.03, 0.1, 0.3, and 1 mg/kg NTX before the sessions. The group that received subcutaneous NTX was also tested with a single intraperitoneal injection of 0.3 mg/kg NTX. Naltrexone significantly reduced ethanol self-administration, and NTX was more potent when administered via subcutaneous injection versus intraperitoneal injection. Ethanol intake (g/kg) was significantly reduced after subcutaneous injection of NTX 0.1 mg/kg and higher. In contrast, ethanol intake was significantly reduced after intraperitoneal injection of NTX 3 mg/kg and higher. A comparison of the NTX ED(50) values showed that subcutaneous NTX was approximately 30-fold more potent than intraperitoneal NTX. For the subcutaneous 0.3 mg/kg NTX dose, a detailed bin analysis showed that responding during the first 2 min after injection was similar to that during the first 2 min after a saline injection while responding after NTX decreased in subsequent bins. These findings suggest that researchers should carefully consider the route of NTX administration when discussing potency and selectivity of NTX's effects on ethanol-related behaviors in rats. These findings further support the notion that NTX acts by terminating responding early rather than reducing the initial responding.

Delta-opioid receptor expression in the ventral tegmental area protects against elevated alcohol consumption

Alcoholism is a complex and debilitating syndrome affecting approximately 140 million people worldwide. However, not everyone who consumes ethanol develops abuse, raising the possibility that some individuals have a protective mechanism that inhibits elevated alcohol consumption. We tested the hypothesis that the delta-opioid receptor (DOR) plays such a protective role. Here we show that DOR activity in the ventral tegmental area (VTA) robustly decreases ethanol consumption in rats and that these effects depend on baseline ethanol consumption. Intra-VTA microinjection of the DOR agonist DPDPE decreases drinking, particularly in low-drinking animals. Furthermore, VTA microinjection of the DOR selective antagonist TIPP-Psi increases drinking in low, but not high, drinkers and this increase is blocked by comicroinjection of the GABA(A) antagonist bicuculline. Using electrophysiological techniques we found that in VTA brain slices from drinking rats DPDPE presynaptically inhibits GABA(A) receptor mediated IPSCs in low drinkers, but not in high drinkers or naive animals, most likely through activation of DORs on GABA terminals. This DOR-mediated inhibition of IPSCs also correlates inversely with behavioral correlates of anxiety measured in the elevated plus maze. In contrast, presynaptic inhibition of VTA GABA(A) IPSCs by the mu-opioid receptor agonist DAMGO is significantly reduced in both high- and low-drinking rats (<30%) compared with age-matched nondrinking controls (>70%). Together, our findings demonstrate the protective nature of VTA DORs and identify an important new target for therapeutic intervention for alcoholism.

Effects of CRF1-receptor and opioid-receptor antagonists on dependence-induced increases in alcohol drinking by alcohol-preferring (P) rats

BACKGROUND

Selective breeding of rats over generations and induction of alcohol dependence via chronic vapor inhalation both enhance alcohol consumption in animal models. The purpose of this study was to determine whether dependence-induced increases in alcohol consumption by P rats is sensitive to naltrexone, a general opioid receptor antagonist (but with highest affinity at the mu-opioid receptor at low doses), and the recently characterized small molecule CRF(1)-receptor antagonist MPZP (N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo[1,5-a]pyrimidin-7-amine).

METHODS

P rats (n = 20) were trained to respond for alcohol and water in a 2-lever operant situation during daily 30-minute sessions. P rats were then matched for alcohol intake and exposed to chronic intermittent alcohol vapor (n = 10) or ambient air (n = 10) for approximately 10 weeks. All rats were then administered MPZP and naltrexone in 2 separate and consecutive Latin-square designs.

RESULTS

MPZP attenuated dependence-induced increases in alcohol intake by P rats while having no effect on alcohol consumption by nondependent controls. Conversely, operant alcohol responding was reduced similarly in dependent and nondependent P rats by naltrexone.

CONCLUSIONS

These results confirm a role for brain CRF(1)-receptor systems in dependence-induced changes in the reinforcing properties of alcohol, and CRF(1)-receptor blockade appears to suppress dependence-induced drinking at lower doses in P rats relative to other rat lines. Therefore, brain CRF(1)-receptor systems are important in the regulation of dependence-induced alcohol consumption, whereas brain opioid systems are important in the regulation of basal alcohol consumption by rats.

Time dependent alterations on tyrosine hydroxylase, opioid and cannabinoid CB1 receptor gene expressions after acute ethanol administration in the rat brain

The aim of this study was to examine the differential regulation after acute ethanol administration on tyrosine hydroxylase, proenkephalin and cannabinoid CB(1) receptor gene expressions in selected areas of the rat brain. Rats received an intragastric administration of 3 g/kg ethanol and were killed by decapitation at 1, 2, 4, 8 and 24 h. The results showed an activation of tyrosine hydroxylase gene expression in the ventral tegmental area and the substantia nigra, increased proenkephalin gene expression in the caudate-putamen, nucleus accumbens core and shell, central and medial amygdala, ventromedial hypothalamic nucleus and the paraventricular hypothalamic nucleus. In contrast, a significant decrease in the cannabinoid CB1 receptor gene expression was found in caudate-putamen, central amygdala and ventromedial hypothalamic nucleus. In conclusion, the results suggest that an acute dose of ethanol induces neuroplastic alterations in proenkephalin, tyrosine hydroxylase and cannabinoid CB1 receptor gene expressions that may contribute to trigger the rewarding effects of ethanol consumption.

Differences in delta- and mu-opioid receptor blockade measured by positron emission tomography in naltrexone-treated recently abstinent alcohol-dependent subjects

Blockade of brain mu-opioid receptor (mu-OR) and delta-opioid receptor (delta-OR) was investigated in recently abstinent alcohol-dependent subjects (N=21) maintained on naltrexone. Subjects completed a 19-day inpatient protocol, which included alcohol abstinence followed by naltrexone treatment (50 mg) on days 15-19. Blood samples were collected after the first administration of naltrexone to evaluate serum levels of naltrexone and 6-beta-naltrexol. Regional brain mu-OR binding potential (BP) and delta-OR Ki was measured using [11C]carfentanil (CAR) positron emission tomography (PET) and [11C]methyl naltrindole ([11C]MeNTI) PET, respectively, before (day 5) and during naltrexone treatment (day 18). Naltrexone inhibition of [11C]CAR BP was near maximal across all brain regions of interest with little variability across subjects (mean+SD% inhibition=94.9+4.9%). Naltrexone only partially inhibited the [11C]MeNTI Ki and there was more variability across subjects (mean+SD% inhibition=21.1+14.49%). Peak serum levels of naltrexone were positively correlated with % inhibition of delta-OR Ki in neocortex and basal ganglia. Peak serum levels of naltrexone were not correlated with % inhibition of mu-OR BP. Peak levels of 6-beta-naltrexol were not significantly correlated with % inhibition of mu-OR BP or delta-OR Ki. Thus, the FDA recommended therapeutic dose of naltrexone was sufficient to produce near complete inhibition of the mu-OR in recently abstinent alcohol dependent subjects. The lower percent inhibition of delta-OR and greater variability in delta-OR blockade by naltrexone across subjects may contribute to individual differences in treatment outcomes to naltrexone. Further investigations on the relationship between individual differences in delta-OR blockade by naltrexone and clinical outcomes should be explored.

Pharmacological evidence for a motivational role of kappa-opioid systems in ethanol dependence

The purpose of this study was to test the hypothesis that activation of the dynorphin/kappa (kappa)-opioid system has a role in the increased consumption of ethanol in dependent animals. The effects of three opioid receptor antagonists with different effects on opioid receptors, naltrexone, nalmefene, and nor-binaltorphimine (nor-BNI), were compared in their ability to decrease ethanol self-administration in nondependent and ethanol-dependent male Wistar rats. Nalmefene and naltrexone are both opioid receptor ligands with comparable molecular weights and pharmacokinetic profiles, but differing specificity for the three opioid receptor subtypes at low doses, while nor-BNI is a selective kappa-opioid receptor antagonist. Dependence was induced in half the animals by subjecting them to a 4-week intermittent vapor exposure period in which animals were exposed to ethanol vapor for 14 h per day. Subsequent to dependence induction, nalmefene, naltrexone, and nor-BNI were tested for their ability to modulate self-administration of ethanol in vapor-exposed and control rats. The results indicated that both nalmefene and naltrexone induced a significant dose-dependent decrease in the number of lever presses for ethanol in both groups of animals. However, in ethanol-dependent animals, nalmefene was significantly more effective in suppressing ethanol intake than naltrexone. Nor-BNI selectively attenuated ethanol-dependent self-administration while leaving nondependent ethanol self-administration intact. Because naltrexone is primarily selective for the mu-opioid receptor, and nalmefene is primarily selective for the mu- and kappa-opioid receptor subtypes, the fact that nalmefene demonstrates more suppression in dependent animals suggests that opioid systems distinct from the mu-regulated portion may be involved in the increased drinking seen during withdrawal in dependent animals. The results with nor-BNI confirm that kappa-opioid receptor antagonism selectively decreases dependence-induced ethanol self-administration, which supports the hypothesis that dynorphin/kappa-opioid systems are dysregulated in dependence and contribute to the increased drinking seen during acute withdrawal in dependent rats.

Ethanol-induced changes in proenkephalin mRNA expression in the rat nigrostriatal pathway

Endogenous opioid systems have been suggested to play a key role in ethanol reinforcement mechanisms and alcohol-drinking behavior. Ethanol induces differential alterations in opioid peptide expression in brain areas of the reward circuits, which may be linked to the reinforcing effects of ethanol. In addition, ethanol-induced alterations in opioidergic nigrostriatal transmission could be involved in brain sensitivity to ethanol and play a role in addictive processes. The aim of this work was to study the effects of acute ethanol administration on proenkephalin (proenk) mRNA expression in the rat substantia nigra and caudate-putamen (CP) for up to 24 h post treatment. Male Wistar rats received ethanol (2.5 g/kg) or distilled water by intragastric administration, and proenk mRNA expression was studied by in situ hybridization and densitometry. Ethanol transiently increased proenk mRNA expression in the CP 1 h after drug administration. Proenk mRNA levels remained elevated 2 h post treatment in the anterior-medial and medial-posterior regions of the CP. In contrast, ethanol decreased proenk mRNA expression in the substantia nigra pars compacta and pars reticulata 2 h after drug exposure. Alterations in enkephalin expression in the substantia nigra and CP in response to ethanol exposure could be involved in the mechanisms underlying brain sensitivity to the drug.

Polymorphisms in the µ-opioid receptor gene (OPRM1) and the implications for alcohol dependence in humans

Twin and adoption studies have shown that alcohol dependence contains a substantial genetic component. In attempts to identify the genetic factors involved, association studies have linked the opioid system to alcohol dependence, with a main focus on the OPRM1 gene encoding the μ-opioid receptor. Our aim was to conduct a systematic review of the literature on the associations between polymorphisms in OPRM1 and alcohol dependence. We addressed findings of 12 studies that met our inclusion criteria. All studies employed a case–control design and included alcohol dependence as a dependent outcome measure. Our review showed that clinical studies do not unequivocally support an association between polymorphisms in OPRM1 and alcohol dependence. Factors that complicate genetic research on alcohol dependence, such as gene–environment interaction, and genetic and clinical heterogeneity, are discussed.

Mu (mu) opioid receptor regulation of ethanol-induced dopamine response in the ventral striatum: evidence of genotype specific sexual dimorphic epistasis

BACKGROUND

Ethanol stimulates the dopaminergic mesoaccumbal pathway, which is thought to play a role in ethanol reinforcement. Mu (mu)-opioid (MOP) receptors modulate accumbal dopamine activity, but it is not clear whether MOP receptors are involved in the mechanism of ethanol-stimulated accumbal dopamine release.

METHODS

We investigated the role that MOP receptors play in ethanol (2.0 g/kg)-stimulated accumbal dopamine release by using MOP receptor knockout mice (C57BL/6J-129SvEv and congenic C57BL/6J genotypes) along with blockade of MOP receptors with a mu1 selective antagonist (naloxonazine).

RESULTS

Both gene deletion and pharmacological antagonism of the MOP receptor decreased ethanol-stimulated accumbal dopamine release compared with controls with female mice showing a larger effect in the C57BL/6J-129SvEv genotype. However, both male and female mice showed reduced ethanol-stimulated dopamine release in the congenic MOP receptor knockout mice (C57BL/6J). No differences in the time course of dialysate ethanol concentration were found in any of the experiments.

CONCLUSIONS

The data demonstrate the existence of a novel interaction between genotype and sex in the regulation of ethanol-stimulated mesolimbic dopamine release by the MOP receptor. This implies that a more complete understanding of the epistatic influences on the MOP receptor and mesolimbic dopamine function may provide more effective pharmacotherapeutic interventions in the treatment of alcoholism.

14-Methoxymetopon, a highly potent mu opioid agonist, biphasically affects ethanol intake in Sardinian alcohol-preferring rats

RATIONALE

Increased opioidergic activity is thought to increase the propensity to consume ethanol. However, the dose monotonicity and receptor subtype for this effect remain uncertain. 14-methoxymetopon is a centrally acting, selective micro opioid receptor agonist with greater systemic antinociceptive potency than morphine and a putatively improved therapeutic index.

OBJECTIVE

To determine whether 14-methoxymetopon influenced voluntary ethanol intake in Sardinian alcohol-preferring (sP) rats.

METHODS

Male sP rats with continuous 2-bottle choice access to ethanol (10% v/v) or water were subjects. The effects of systemic 14-methoxymetopon administration (2, 5, 12.25, 30 micro/kg, s.c.) on 4-h ethanol intake were determined. The ability of naltrexone (50 micro/kg, s.c.), an opioid antagonist, to block actions of 14-methoxymetopon (12.25, 30 micro/kg, s.c.) was examined as were the effects of 14-methoxymetopon (12.25 micro/kg, s.c.) on self-administered blood alcohol levels (BALs) and clearance of a passive ethanol bolus (1 g/kg). Finally, the effects of central 14-methoxymetopon administration (0.0003-100 ng, i.c.v.) on 4-h ethanol intake were evaluated.

RESULTS

Systemic 14-methoxymetopon very potently and dose-dependently suppressed ethanol and food intake for 30 min, followed by a greater, longer-lasting, and behaviorally specific increase in ethanol intake. The increased ethanol intake led to threefold higher BALs, was naltrexone-reversible, and not due to altered ethanol clearance. Intracerebroventricular 14-methoxymetopon administration rapidly altered ethanol intake per an inverted U-shaped dose-response function, increasing it at a 10 pg dose, while suppressing it at a 10,000-fold higher dose.

CONCLUSIONS

The novel mu analgesic increases ethanol intake, a potential therapeutic liability, and results suggest a non-monotonic influence of brain mu opioid receptor stimulation on ethanol intake.

Evidence for the role of dopamine D3 receptors in oral operant alcohol self-administration and reinstatement of alcohol-seeking behavior in mice

The present study examined the effects of the acute intraperitoneal (i.p.) administration of the selective dopamine (DA) D(3) receptor antagonist SB-277011A (10, 20 or 30 mg/kg i.p.) on the oral operant self-administration of alcohol in male C57BL/6N mice. These effects were compared with those of naltrexone (0.5, 1 and 2 mg/kg i.p.) and acamprosate (100, 200 and 400 mg/kg i.p.). Compared with vehicle, the acute administration of SB-277011A (10 or 20 mg/kg) did not significantly alter the operant self-administration of alcohol, whereas the 30 mg/kg dose significantly reduced alcohol intake (g/kg), the number of reinforcers, and the number of active lever presses. The oral self-administration of alcohol was not significantly altered by the acute administration of either naltrexone or acamprosate, compared with vehicle-treated mice. SB-277011A, naltrexone and acamprosate were also tested in a model of drug/cue-triggered reinstatement of alcohol-seeking behavior. In this model, neither naltrexone (2 mg/kg) nor acamprosate (400 mg/kg) prevented relapse to alcohol-seeking behavior. In contrast, SB-277011A significantly reduced reinstatement of alcohol seeking in a dose-dependent manner. Provided these results can be extrapolated to humans, they suggest that selective DA D(3) receptor antagonists may be useful in the pharmacotherapeutic management of alcohol intake and prevention of relapse to alcohol-seeking behavior.

Reinforcing properties of ethanol in neonatal rats: involvement of the opioid system

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanol's gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanol's reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

Differential regulation of proopiomelanocortin (POMC) mRNA expression in hypothalamus and anterior pituitary following repeated cyanamide with ethanol administration

BACKGROUND/AIM

We have investigated proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus of the hypothalamus (ARC) and the anterior lobe of the pituitary (AL) following repeated cyanamide-ethanol reaction (CER).

METHODS

Adult male Sprague-Dawley rats (250-290 gr) were housed in a temperature and humidity controlled environment with free access to food and water. Four experimental groups were used as follows: saline (as control), cyanamide alone, ethanol alone and ethanol with cyanamide. The animals recived daily intraperitoneal injections (i.p.) of cyanamide (10 mg/kg, 60 min before ethanol dosing) with or without ethanol (1g/kg) for 5 consecutive days, and were sacrificed 60 min after the last dosing of ethanol. The results were presented as the mean +/- SEM for each group. All groups within each deta set were compared by one-way ANOVA followed by Fisher PLSD test for multiple comparisons. A value of p < 0.05 was considered significant.

RESULTS

The POMC mRNA levels in ARC were significantly decreased with cyanamide compared to the control and ethanol alone (p < 0.05 and p < 0.05 respectively), but increased in AL following repeated CER.

CONCLUSION

We speculate that this differential regulation of POMC mRNA expression may be partially involved in the preventive effects on alcohol intake in response to CER.

Reduction of ethanol intake by chronic treatment with Hypericum perforatum, alone or combined with naltrexone in rats

Acute treatment with extracts of Hypericum perforatum, the common plant usually called St. John's Wort, reduces voluntary ethanol intake in Marchigian Sardinian alcohol-preferring (msP) rats and acts synergistically with opioid receptor antagonists to further attenuate ethanol consumption. The present study evaluated the effect of chronic (once a day for 12 days) intragastric administration of a CO2 Hypericum perforatum extract (HPCO2), given alone or combined with naltrexone (NTX), on ethanol intake offered 2h/day in msP rats. Chronic treatment with HPCO2 markedly reduced ethanol intake at the dose of 125, but not at 7 mg/kg; the effect of 125 mg/kg was observed since the first day of treatment and remained constant across the 12 days. The same dose of HPCO2 slightly reduced the simultaneous intake of food only on day 3 and day 11 of treatment. Treated rats promptly recovered baseline ethanol intake when treatment did not precede access to ethanol (on day 8) or after the end of treatment (day 13 and day 14), suggesting that HPCO2 administrations did not induce conditioned aversion to alcohol. Chronic intraperitoneal treatment with NTX reduced ethanol intake at 3, but not at 0.5mg/kg. The synergistic effect on ethanol intake of HPCO2 and NTX was evident also in conditions of chronic treatment. HPCO2, 7 mg/kg, and NTX, 0.5mg/kg, evoked a pronounced and statistically significant reduction of ethanol intake, while being inactive. The effect on ethanol intake of the combined treatment remained stable over the 12 days of treatment; food intake was slightly reduced only on day 3 and on day 7 in response to 125 mg/kg of HPCO2 combined with NTX 0.5mg/kg, but no difference in body weight between controls and treated rats was observed at the end of treatment. Following 12-day treatment with 125 mg/kg of HPCO2, no difference was observed in the responsivity of msP rats to the effect on ethanol intake of several doses of the extract. In conclusion, the present results provide evidence for a selective and pronounced effect of HPCO2, alone or combined with naltrexone, on ethanol intake in conditions of chronic treatment, without development of tolerance. These findings further support the view that clinical trials for extracts of Hypericum perforatum in the treatment of alcoholism should be considered.

Development of medications for alcohol use disorders: recent advances and ongoing challenges

During the past decade, efforts to develop medications for alcoholism have burgeoned. Three agents, disulfiram, naltrexone and acamprosate, are now approved in a large number of countries. Although many patients have benefited from existing medications, their effects are moderate, and some alcoholics fail to respond to them. A host of new agents are currently under active investigation. Critical barriers must be overcome to ensure that future efforts in the development of medications for alcohol use disorders reach full fruition. These challenges include: establishing key targets for drug discovery; validating animal and human screening models; and developing biomarkers to help predict treatment success. In addition, it is important to formulate methodological and statistical strategies to efficiently conduct alcohol pharmacotherapy trials; to specify genetic and phenotypic patient characteristics associated with efficacy and safety for lead compounds; to forge productive alliances among governmental agencies, the pharmaceutical industry and academic researchers to further drug development; and, ultimately and perhaps most difficult, to engage the practitioner community to incorporate medications into the alcohol treatment process.

Effect of Hypericum perforatum CO2 extract on the motivational properties of ethanol in alcohol-preferring rats

AIMS

Extracts of Hypericum perforatum (HPE) attenuate voluntary ethanol intake in different lines of alcohol-preferring rats. The present study evaluated the effect of the intragastric (IG) administration of a CO(2) Hypericum perforatum extract (HPCO(2)) on operant ethanol self-administration, as well as on voluntary ethanol intake, after a period of ethanol deprivation in genetically selected Marchigian Sardinian alcohol-preferring rats.

METHODS

HPCO2 was administered by means of an indwelling IG catheter, 1 h before the tests. For the self-administration experiments, the rats were trained to self-administer 10% (v/v) ethanol in 30-min daily sessions under a fixed ratio 1 schedule of reinforcement. HPCO2 was also tested on 0.2% w/v saccharin self-administration. For the ethanol deprivation experiments, rats that had a previous experience with voluntary ethanol drinking were deprived of ethanol for 9 days, whereas water and food were freely available; HPCO2 was given by IG injection 1 h before the ethanol re-presentation.

RESULTS

HPCO2 in doses of 31 or 125 mg/kg but not 7 mg/kg, significantly reduced ethanol self-administration, while it did not modify saccharin self-administration. The same doses of the extract abolished the increased ethanol intake following ethanol deprivation.

CONCLUSIONS

These findings provide evidence that HPCO2 markedly reduces the reinforcing properties of ethanol in the self-administration paradigm, as well as the increase of ethanol intake following ethanol deprivation. These findings further support the view that the use of HPE may represent an interesting pharmacological approach in the treatment of alcohol abuse and alcoholism.

Deficit in beta-endorphin peptide and tendency to alcohol abuse

Human and animal studies suggest that there is a correlation between endogenous opioid peptides, especially beta-endorphin, and alcohol abuse. It has been proven that the consumption of alcohol activates the endogenous opioid system. Consumption of alcohol results in an increase in beta-endorphin level in those regions of the human brain, which are associated with a reward system. However, it has also been observed that habitual alcohol consumption leads to a beta-endorphin deficiency. It is a well-documented phenomenon that people with a genetic deficit of beta-endorphin peptide are particularly susceptible to alcoholism. The plasma level of beta-endorphin in subjects genetically at high risk of excessive alcohol consumption shows lower basal activity of this peptide. Its release increases significantly after alcohol consumption. Clinical and laboratory studies confirm that certain genetically determined factors might increase the individual's vulnerability to alcohol abuse.

[3H]DPDPE binding to delta opioid receptors in the rat mesocorticolimbic and nigrostriatal pathways is transiently increased by acute ethanol administration

Dopaminergic transmission in the mesolimbic and nigrostriatal pathways plays a key role in the reinforcement mechanisms and brain sensitivity to ethanol, respectively. Ethanol reinforcement and high alcohol drinking behaviour have been postulated to be partially mediated by a neurobiological mechanism involving the ethanol-induced activation of the endogenous opioid system. Activation of opioid neural pathways by ethanol may include alterations in the processing, release and/or the receptor binding of opioid peptides. The aim of this work was to investigate the effects of acute ethanol administration on delta opioid receptors in the rat mesocortical, meso-accumbens and nigrostriatal pathways by quantitative receptor autoradiography, using [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin as radioligand. A significant increase in [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin binding was observed in the substantia nigra pars reticulata 1 h after ethanol treatment. Two hours after drug exposure, ligand binding was significantly increased in the frontal and prefrontal cortices, the core and shell regions of the nucleus accumbens, and in the anterior-medial and medial-posterior regions of the caudate-putamen. In contrast, ligand binding was significantly decreased in the posterior region of the caudate-putamen 30 min after ethanol administration. The observed effects may reflect ethanol-induced changes in ligand binding affinity and/or in receptor density. Our results suggest that transitory changes in delta opioid receptors with different kinetic patterns may be involved in ethanol reinforcement and brain sensitivity to the drug. Ethanol-induced delta receptor up- and down-regulation mechanisms may participate in modulation of dopaminergic transmission in the mesocorticolimbic and nigrostriatal pathways.

Alcohol and gene expression in the central nervous system

AIMS

To describe recent research focusing on the analysis of gene and protein expression relevant to understanding ethanol consumption, dependence and effects, in order to identify common themes.

METHODS

A selective literature search was used to collate the relevant data.

RESULTS

Over 160 genes have been individually assessed before or after ethanol administration, as well as in genetically selected lines. Techniques for studying gene expression include northern blots, differential display, real time reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. More recently, high throughput functional genomic technology, such as DNA microarrays, has been used to examine gene expression. Recent gene expression analyses have dramatically increased the number of candidate genes (nine array papers have illuminated 600 novel gene transcripts that may contribute to alcohol abuse and alcoholism).

CONCLUSIONS

Although functional genomic experiments (transcriptome analysis) have failed to identify a single alcoholism gene, they have illuminated important pathways and gene products that may contribute to the risk of alcohol abuse and alcoholism.

INTERACTIONS BETWEEN CANNABINOID AND OPIOID RECEPTOR SYSTEMS IN THE MEDIATION OF ETHANOL EFFECTS

Over the past few years, advances in the investigation of the neurochemical circuits involved in the development and treatment of alcohol dependence have identified peptides and receptors as potential key targets in the treatment of problems related to alcohol consumption. The endogenous opioid system is modified by alcohol intake in areas of the brain related to reward systems, and differential basal levels of opioid gene expression are found in rodents with a high preference for ethanol. This suggests a greater vulnerability to alcohol consumption in relation to differences in genetic background. Further evidence of the involvement of opioid peptides in alcohol dependence is the ability of the opioid antagonist naltrexone to reduce alcohol intake in animal models of dependence and in alcohol-dependent patients. Abundant evidence indicates that the activation of cannabinoid receptors stimulates the release of opioid peptides, therefore the cannabinoid receptor antagonists may presumably alter opioid peptide release, thus facilitating the reduction of ethanol consumption. However, little is known about the effects of ethanol on the endogenous cannabinoid system, the vulnerability of cannabinoid receptors to alcohol intake or their neurochemical implications in reducing consumption of alcohol. In this paper, we review the role of opioid and cannabinoid receptor systems, their vulnerability to alcohol intake and the development of dependence, and the targeting of these systems in the treatment of alcoholism.

Effects of stress on alcohol consumption in rats selectively bred for high or low alcohol drinking

BACKGROUND

Stress has long been thought to influence the initiation and maintenance of alcohol drinking in humans. However, results of studies in animals suggest that the relationship between stress and alcohol drinking is not well understood. The purpose of this study was to examine the effect of unpredictable and uncontrollable restraint stress on alcohol consumption in two sets of rat lines selectively bred for alcohol preference (P) and high alcohol drinking (HAD1) and for alcohol nonpreference (NP) and low alcohol drinking (LAD1).

METHODS

Male P (n = 26) and NP (n = 26) and HAD1 (n = 17) and LAD1 (n = 20) rats were counterbalanced on the basis of alcohol intake and assigned, in matched pairs, to either a stress (Stress) or a no-stress (Control) group. All rats were given a free choice between a 10% v/v alcohol solution and water, with food freely available. Unpredictable, uncontrollable stress, which consisted of immobilization in a nylon restraint sleeve for 30 to 120 min/day, was applied for 10 consecutive days.

RESULTS

Stress moderately reduced alcohol intake in both P and HAD1 rats versus controls and had no effect on alcohol intake in either the NP or the LAD1 rats during the 10 days of stress application. Alcohol intake was increased for the first 5 days after stress termination in P rats but not in HAD1 rats. Alcohol intake remained stable for several weeks in both the NP and LAD1 lines after stress termination and then increased during the last 15 days of the 35-day poststress period in NP rats, but not in LAD1 rats.

CONCLUSIONS

A reduction in alcohol intake during stress in rats with a genetic predisposition toward high alcohol intake seems to be a moderate but consistent finding, whereas an increase in alcohol intake after stress termination is less consistent and may be influenced by genetic background.