Quantitative autoradiography of mu-opioid receptors in the CNS of alcohol-naive alcohol-preferring P and -nonpreferring NP rats

Perigestational Opioid Exposure Alters Alcohol-Driven Reward Behaviors in Adolescent Rats

Every fifteen minutes, a baby is born in the U.S. experiencing neonatal opioid withdrawal syndrome (NOWS). Since 2004, the rate of NOWS has increased 7-fold. Clinical studies have established intrauterine exposure to drugs of abuse as a risk factor for adverse health outcomes in adult life, including the propensity for future illicit drug use. Despite extensive knowledge about common mechanisms of action in the neural circuitry that drives opioid and alcohol reward, there is little data on the risks that those born with NOWS face regarding alcohol use later in life. Here, we investigate the impact of perigestational opioid exposure (POE) on the mesolimbic reward system of male and female Sprague Dawley rats at postnatal and adolescent ages. Our laboratory has developed a clinically relevant model for morphine exposure spanning pre-conception to the first week of life. Using this model, we found that POE increased alcohol consumption in female rats under noncontingent conditions, and inversely, reduced alcohol consumption in both male and female rats during operant conditioning sessions. Operant responding was also reduced for sucrose, suggesting that the impact of POE on reward-seeking behaviors is not limited to drugs of abuse. Expression of µ-opioid receptors was also significantly altered in the nucleus accumbens and medial habenula, regions previously shown to play a significant role in reward/aversion circuitry.

Significance Statement

Early life exposure to opioids is known to alter future drug behavior in rats. In the present study, female rats exposed to morphine via their mothers throughout and after pregnancy exhibited increased alcohol consumption when allowed to consume freely. During operant conditioning, however, male and female rats exposed to gestational morphine decreased consumption of alcohol as well as sucrose. We also observed that gestational morphine exposure altered µ-opioid receptor expression in reward-related brain regions. Our study provides the first evidence of changes in alcohol-directed reward behavior in a gestational opioid exposure rat model.

Blocking μ-opioid receptors attenuates reinstatement of responding to an alcohol-predictive conditioned stimulus through actions in the ventral hippocampus

The µ-opioid system is involved in the reinstatement of responding that is immediately evoked by alcohol-predictive cues. The extent of its involvement in reinstatement observed in a new model that evaluates the delayed effects of re-exposure to alcohol, however, is unclear. The current study investigated the role of µ-opioid receptors (MORs) in the delayed reinstatement of an extinguished, Pavlovian conditioned response that was evoked 24 h after alcohol re-exposure. Female and male Long-Evans rats received Pavlovian conditioning in which a conditioned stimulus (CS) was paired with the delivery of an appetitive unconditioned stimulus (US; Experiments 1, 2, 4: 15% v/v alcohol; Experiment 3: 10% w/v sucrose) that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before but without the US. Next, the US was delivered but without the CS. A reinstatement test was conducted 24 h later, during which the CS was presented in the absence of the US. Silencing MORs via systemic naltrexone (0.3 or 1.0 mg/kg) attenuated reinstatement of port entries elicited by an alcohol-CS, but not those elicited by a sucrose-CS. Finally, blocking MORs in the ventral hippocampus via bilateral microinfusion of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 2.5 or 5.0 µg/hemisphere) prevented reinstatement of port alcohol-CS port entries. These data show that MORs are involved in the delayed reinstatement of a Pavlovian conditioned response in an alcohol-specific manner. Importantly, these data illustrate, for the first time, that MORs in the ventral hippocampus are necessary for responding to an alcohol-predictive cue.

Multi-animal-model study reveals mutations in neural plasticity and nociception genes linked to excessive alcohol drinking

BACKGROUND

The basis for familial alcohol use disorder (AUD) remains an enigma due to various biological and societal confounds. The present study used three of the most adopted and documented rat models, combining the alcohol-preferring/non-alcohol-preferring (P/NP) lines and high alcohol-drinking/low alcohol-drinking (HAD/LAD) replicated lines, of AUD as examined through the lens of whole genomic analyses.

METHODS

We used complete genome sequencing of the P/NP lines and previously published sequences of the HAD/LAD replicates to enhance the discovery of variants associated with AUD and to remove confounding with genetic background and random genetic drift. Specifically, we used high-order statistical methods to search for genetic variants whose frequency changes in whole sets of gene ontologies corresponded with phenotypic changes in the direction of selection, that is, ethanol-drinking preference.

RESULTS

Our first finding was that in addition to variants causing translational changes, the principal genetic changes associated with drinking predisposition were silent mutations and mutations in the 3' untranslated regions (3'UTR) of genes. Neither of these types of mutations alters the amino acid sequence of the translated protein but they influence both the rate and conformation of gene transcription, including its stability and posttranslational events that alter gene efficacy. This finding argues for refocusing human genomic studies on changes in gene efficacy. Among the key ontologies identified were the central genes associated with the Na+ voltage-gated channels of neurons and glia (including the Scn1a, Scn2a, Scn2b, Scn3a, Scn7a, and Scn9a subtypes) and excitatory glutamatergic secretion (including Grm2 and Myo6), both of which are essential in neuroplasticity. In addition, we identified "Nociception or Sensory Perception of Pain," which contained variants in nociception (Arrb1, Ccl3, Ephb1) and enlist sodium (Scn1a, Scn2a, Scn2b, Scn3a, Scn7a), pain activation (Scn9a), and potassium channel (Kcna1) genes.

CONCLUSION

The multi-model analyses used herein reduced the confounding effects of random drift and the "founders" genetic background. The most differentiated bidirectionally selected genes across all three animal models were Scn9a, Scn1a, and Kcna, all of which are annotated in the nociception ontology. The complexity of neuroplasticity and nociception adds strength to the hypothesis that neuroplasticity and pain (physical or psychological) are prominent phenotypes genetically linked to the development of AUD.

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.

Baclofen and naltrexone, but not N-acetylcysteine, affect voluntary alcohol drinking in rats regardless of individual levels of alcohol intake

In humans, there is profound individual variation in the risk of alcohol use disorder (AUD). Because GABA, opioid and glutamate neurotransmission have been implicated in AUD, functional differences in these neural systems may underlie the individual vulnerability to AUD. We therefore determined the effects of drugs affecting GABA, opioid and glutamatergic neurotransmission on alcohol consumption in rats that differed in baseline alcohol intake. Subgroups of low-, medium- and high-alcohol-drinking rats were selected on the basis of alcohol consumption using an intermittent alcohol access procedure. The subgroups were treated with the GABAB receptor agonist baclofen, the opioid receptor antagonist naltrexone and the cysteine precursor N-acetylcysteine, and the effects on alcohol intake and preference were determined. Both baclofen and naltrexone reduced alcohol consumption, but N-acetylcysteine did not. These effects were comparable for low-, medium- and high-alcohol-drinking rats. However, there was a substantial degree of individual variation in the responsivity to baclofen and naltrexone, across the subgroups. Taken together, these results suggest that variation in alcohol consumption does not predict the responsivity to baclofen and naltrexone. This implies that individual variability in alcohol consumption on the one hand and sensitivity to treatment with these drugs on the other hand represent separate processes that likely involve distinct biological mechanisms.

Sex and Age Effects on Neurobehavioral Toxicity Induced by Binge Alcohol

Historically, most alcohol neurotoxicity studies were conducted in young adult males and focused on chronic intake. There has been a shift towards studying the effects of alcohol on the adolescent brain, due to alcohol consumption during this formative period disrupting the brain's developmental trajectory. Because the most typical pattern of adolescent alcohol intake is heavy episodic (binge) drinking, there has also been a shift towards the study of binge alcohol-induced neurobehavioral toxicity. It has thus become apparent that binge alcohol damages the adolescent brain and there is increasing attention to sex-dependent effects. Significant knowledge gaps remain in our understanding of the effects of binge alcohol on the female brain, however. Moreover, it is unsettling that population-level studies indicate that the prevalence of binge drinking is increasing among American women, particularly those in older age groups. Although study of adolescents has made it apparent that binge alcohol disrupts ongoing brain maturational processes, we know almost nothing about how it impacts the aging brain, as studies of its effects on the aged brain are relatively scarce, and the study of sex-dependent effects is just beginning. Given the rapidly increasing population of older Americans, it is crucial that studies address age-dependent effects of binge alcohol, and given the increase in binge drinking in older women who are at higher risk for cognitive decline relative to men, studies must encompass both sexes. Because adolescence and older age are both characterized by age-typical brain changes, and because binge drinking is the most common pattern of alcohol intake in both age groups, the knowledge that we have amassed on binge alcohol effects on the adolescent brain can inform our study of its effects on the aging brain. In this review, we therefore cover the current state of knowledge of sex and age-dependent effects of binge alcohol, as well as statistical and methodological considerations for studies aimed at addressing them.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

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.

Short-term dynamics of input and output of CA1 network greatly differ between the dorsal and ventral rat hippocampus

Background

The functional heterogeneity of the hippocampus along its longitudinal axis at the level of behavior is an established concept; however, the neurobiological mechanisms are still unknown. Diversifications in the functioning of intrinsic hippocampal circuitry including short-term dynamics of synaptic inputs and neuronal output, that are important determinants of information processing in the brain, may profoundly contribute to functional specializations along the hippocampus. The objectives of the present study were the examination of the role of the GABA_A receptor-mediated inhibition, the μ-opioid receptors and the effect of stimulation intensity on the dynamics of both synaptic input and neuronal output of CA1 region in the dorsal and ventral hippocampus. We used recordings of field potentials from adult rat hippocampal slices evoked by brief repetitive activation of Schaffer collaterals.

Results

We find that the local CA1 circuit of the dorsal hippocampus presents a remarkably increased dynamic range of frequency-dependent short-term changes in both input and output, ranging from strong facilitation to intense depression at low and high stimulation frequencies respectively. Furthermore, the input–output relationship in the dorsal CA1 circuit is profoundly influenced by frequency and time of presynaptic activation. Strikingly, the ventral hippocampus responds mostly with depression, displaying a rather monotonous input–output relationship over frequency and time. Partial blockade of GABA_A receptor-mediated transmission (by 5 μM picrotoxin) profoundly influences input and output dynamics in the dorsal hippocampus but affected only the neuronal output in the ventral hippocampus. M-opioid receptors control short-term dynamics of input and output in the dorsal hippocampus but they play no role in the ventral hippocampus.

Conclusion

The results demonstrate that information processing by CA1 local network is highly diversified between the dorsal and ventral hippocampus. Transient detection of incoming patterns of activity and frequency-dependent sustained signaling of amplified neuronal information may be assigned to the ventral and dorsal hippocampal circuitry respectively. This disparity should have profound implications for the functional roles ascribed to distinct segments along the long axis of the hippocampus.

Electronic supplementary material

The online version of this article (10.1186/s12868-019-0517-5) contains supplementary material, which is available to authorized users.

Effects of ceftriaxone on hydrocodone seeking behavior and glial glutamate transporters in P rats

Hydrocodone (HYD) is one of the most widely prescribed opioid analgesic drugs. Several neurotransmitters are involved in opioids relapse. Among these neurotransmitters, glutamate is suggested to be involved in opioid dependence and relapse. Glutamate is regulated by several glutamate transporters, including glutamate transporter 1 (GLT-1) and cystine/glutamate transporter (xCT). In this study, we investigated the effects of ceftriaxone (CEF) (200 mg/kg, i.p.), known to upregulate GLT-1 and xCT, on reinstatement to HYD (5 mg/kg, i.p.) using the conditioned place preference (CPP) paradigm in alcohol-preferring (P) rats. Animals were divided into three groups: 1) saline-saline group (SAL-SAL); 2) HYD-SAL group; and 3) HYD-CEF group. The CPP was conducted as follows: habituation phase, conditioning phase with HYD (i.p.) injections every other day for four sessions, extinction phase with CEF (i.p.) injections every other day for four sessions, and reinstatement phase with one priming dose of HYD. Time spent in the HYD-paired chamber after conditioning training was increased as compared to pre-conditioning. There was an increase in time spent in the HYD-paired chamber with one priming dose of HYD in the reinstatement test. HYD exposure downregulated xCT expression in the nucleus accumbens and hippocampus, but no effects were observed in the dorsomedial prefrontal cortex and amygdala. Importantly, CEF treatment attenuated the reinstatement effect of HYD and normalized xCT expression in the affected brain regions. These findings demonstrate that the attenuating effect of HYD reinstatement with CEF might be mediated through xCT.

Role of cannabinoid CB2 receptor in the reinforcing actions of ethanol

This study examines the role of the cannabinoid CB2 receptor (CB2 r) on the vulnerability to ethanol consumption. The time-related and dose-response effects of ethanol on rectal temperature, handling-induced convulsions (HIC) and blood ethanol concentrations were evaluated in CB2 KO and wild-type (WT) mice. The reinforcing properties of ethanol were evaluated in conditioned place preference (CPP), preference and voluntary ethanol consumption and oral ethanol self-administration. Water-maintained behavior schedule was performed to evaluate the degree of motivation induced by a natural stimulus. Preference for non-alcohol tastants assay was performed to evaluate the differences in taste sensitivity. Tyrosine hydroxylase (TH) and μ-opioid receptor gene expressions were also measured in the ventral tegmental area and nucleus accumbens (NAcc), respectively. CB2 KO mice presented increased HIC score, ethanol-CPP, voluntary ethanol consumption and preference, acquisition of ethanol self-administration, and increased motivation to drink ethanol compared with WT mice. No differences were found between genotypes in the water-maintained behavior schedule or preference for non-alcohol tastants. Naïve CB2 KO mice presented increased μ-opioid receptor gene expression in NAcc. Acute ethanol administration (1-2 g/kg) increased TH and μ-opioid receptor gene expressions in CB2 KO mice, whereas the lower dose of ethanol decreased TH gene expression in WT mice. These results suggest that deletion of the CB2 r gene increased preference for and vulnerability to ethanol consumption, at least in part, by increased ethanol-induced sensitivity of the TH and μ-opioid receptor gene expressions in mesolimbic neurons. Future studies will determine the role of CB2 r as a target for the treatment of problems related with alcohol consumption.

Ghrelin increases GABAergic transmission and interacts with ethanol actions in the rat central nucleus of the amygdala

The neural circuitry that processes natural rewards converges with that engaged by addictive drugs. Because of this common neurocircuitry, drugs of abuse have been able to engage the hedonic mechanisms normally associated with the processing of natural rewards. Ghrelin is an orexigenic peptide that stimulates food intake by activating GHS-R1A receptors in the hypothalamus. However, ghrelin also activates GHS-R1A receptors on extrahypothalamic targets that mediate alcohol reward. The central nucleus of the amygdala (CeA) has a critical role in regulating ethanol consumption and the response to ethanol withdrawal. We previously demonstrated that rat CeA GABAergic transmission is enhanced by acute and chronic ethanol treatment. Here, we used quantitative RT-PCR (qRT-PCR) to detect Ghsr mRNA in the CeA and performed electrophysiological recordings to measure ghrelin effects on GABA transmission in this brain region. Furthermore, we examined whether acute or chronic ethanol treatment would alter these electrophysiological effects. Our qRT-PCR studies show the presence of Ghsr mRNA in the CeA. In naive animals, superfusion of ghrelin increased the amplitude of evoked inhibitory postsynaptic potentials (IPSPs) and the frequency of miniature inhibitory postsynaptic currents (mIPSCs). Coapplication of ethanol further increased the ghrelin-induced enhancement of IPSP amplitude, but to a lesser extent than ethanol alone. When applied alone, ethanol significantly increased IPSP amplitude, but this effect was attenuated by the application of ghrelin. In neurons from chronic ethanol-treated (CET) animals, the magnitude of ghrelin-induced increases in IPSP amplitude was not significantly different from that in naive animals, but the ethanol-induced increase in amplitude was abolished. Superfusion of the GHS-R1A antagonists D-Lys3-GHRP-6 and JMV 3002 decreased evoked IPSP and mIPSC frequency, revealing tonic ghrelin activity in the CeA. D-Lys3-GHRP-6 and JMV 3002 also blocked ghrelin-induced increases in GABAergic responses. Furthermore, D-Lys3-GHRP-6 did not affect ethanol-induced increases in IPSP amplitude. These studies implicate a potential role for the ghrelin system in regulating GABAergic transmission and a complex interaction with ethanol at CeA GABAergic synapses.

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.

Olfaction under metabolic influences

Recently published work and emerging research efforts have suggested that the olfactory system is intimately linked with the endocrine systems that regulate or modify energy balance. Although much attention has been focused on the parallels between taste transduction and neuroendocrine controls of digestion due to the novel discovery of taste receptors and molecular components shared by the tongue and gut, the equivalent body of knowledge that has accumulated for the olfactory system, has largely been overlooked. During regular cycles of food intake or disorders of endocrine function, olfaction is modulated in response to changing levels of various molecules, such as ghrelin, orexins, neuropeptide Y, insulin, leptin, and cholecystokinin. In view of the worldwide health concern regarding the rising incidence of diabetes, obesity, and related metabolic disorders, we present a comprehensive review that addresses the current knowledge of hormonal modulation of olfactory perception and how disruption of hormonal signaling in the olfactory system can affect energy homeostasis.

Influence of the OPRM1 A118G polymorphism on alcohol-induced euphoria, risk for alcoholism and the clinical efficacy of naltrexone

Alcohol-use disorders are thought to be heterogeneous in etiology, pathophysiology and response to treatment. One hypothesized contributor to this variability is the common A118G polymorphism of the µ-opioid receptor gene, OPRM1. This article critically evaluates the evidence that the A118G substitution affects subjective, behavioral and neurobiological responses to alcohol and the opioid receptor antagonist, naltrexone. Although screening of patients in a clinical setting remains premature, results suggest the A118G substitution may influence one etiological pathway to alcoholism, for which naltrexone pharmacotherapy is more effective.

Regulation of drug and palatable food overconsumption by similar peptide systems

This review is aimed at understanding some of the common neurochemical, behavioral and physiological determinants of drug and food overconsumption. Much current work has been devoted to determining the similarities between the brain circuits controlling excessive use of addictive drugs and the overconsumption of palatable foods. The brain systems involved likely include peptides of both mesolimbic and hypothalamic origin. Evidence gathered from expression and injection studies suggests that the consumption of drugs, such as ethanol and nicotine, and also of palatable foods rich in fat is stimulated by different orexigenic peptides, such as enkephalin, galanin, orexin, and melaninconcentrating hormone, acting within the hypothalamus or various limbic structures, while another peptide, neuropeptide Y, is closely related to carbohydrate consumption and shows an inverse relationship with ethanol and nicotine consumption. Moreover, studies in animal models suggest that a propensity to overconsume these reinforcing substances may result from preexisting disturbances in these same peptide systems. These neurochemical disturbances, in turn, may also be closely linked to specific behaviors associated with excessive consummatory behavior, such as hyperactivity or novelty-seeking, palatable food preference, and also fluctuations in circulating lipid levels. Clear understanding of the relationship between these various determinants of consummatory behavior will allow researchers to effectively predict and examine at early stages of exposure animals that are prone to drug and food overconsumption. This work may ultimately aid in the identification of inherent traits that increase the risk for drug abuse and palatable food overconsumption.

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.

Human and laboratory rodent low response to alcohol: is better consilience possible?

If people are brought into the laboratory and given alcohol, there are pronounced differences among individuals in many responses to the drug. Some participants in alcohol challenge protocols show a cluster of 'low level of responses to alcohol' determined by observing post-drinking-related changes in subjective, motor and physiological effects at a given dose level. Those individuals characterized as having low level of response (LR) to alcohol have been shown to be at increased risk for a lifetime diagnosis of alcohol dependence (AD), and this relationship between low LR and AD appears to be in part genetic. LR to alcohol is an area where achieving greater consilience between the human and the rodent phenotypes would seem to be highly likely. However, despite extensive data from both human and rodent studies, few attempts have been made to evaluate the human and animal data systematically in order to understand which aspects of LR appear to be most directly comparable across species and thus the most promising for further study. We review four general aspects of LR that could be compared between humans and laboratory animals: (1) behavioral measures of subjective intoxication; (2) body sway; (3) endocrine responses; and (4) stimulant, autonomic and electrophysiological responses. None of these aspects of LR provide completely face-valid direct comparisons across species. Nevertheless, one of the most replicated findings in humans is the low subjective response, but, as it may reflect either aversively valenced and/or positively valenced responses to alcohol as usually assessed, it is unclear which rodent responses are analogous. Stimulated heart rate appears to be consistent in animal and human studies, although at-risk subjects appear to be more rather than less sensitive to alcohol using this measure. The hormone and electrophysiological data offer strong possibilities of understanding the neurobiological mechanisms, but the rodent data in particular are rather sparse and unsystematic. Therefore, we suggest that more effort is still needed to collect data using refined measures designed to be more directly comparable in humans and animals. Additionally, the genetically mediated mechanisms underlying this endophenotype need to be characterized further across species.

Acute ethanol effects on local cerebral glucose utilization in select central nervous system regions of adolescent alcohol-preferring (P) and alcohol-nonpreferring (NP) rats

BACKGROUND

Alcohol abuse among adolescents is a major health and developmental problem. The 2-[(14)C]deoxyglucose (2-DG) technique allows for the in vivo quantification of local cerebral glucose utilization (LCGU) as a measure of functional neuronal activity.

METHODS

Local cerebral glucose utilization rates were examined after acute ethanol administration within selected brain regions of adolescent alcohol-preferring (P) and -nonpreferring (NP) rats. Postnatal day 45 male P and NP rats were injected with saline or 1.0 g/kg ethanol, i.p., 10 minutes prior to an intravenous bolus of [(14)C]-2-deoxyglucose (125 microCi/kg). Image densities were determined using quantitative autoradiography and LCGU values calculated.

RESULTS

Acute ethanol injection significantly decreased LCGU rates in select brain regions including the olfactory tubercles, the frontal cortex (Fr), and subregions of the posterior hippocampus (pCA1 and pCA3). Acute ethanol had no significant effects on LCGU rates in any region of the adolescent NP rats. Significant basal LCGU rate differences were apparent between the rat lines in a nearly global fashion with adolescent P rats having much higher basal LCGU rates compared with adolescent NP rats.

CONCLUSIONS

These findings suggest that the adolescent P and NP rats are less sensitive to the effects of acute ethanol than their adult counterparts. The adolescent P rat is relatively more sensitive to the initial effects of acute ethanol in select brain regions as compared with the adolescent NP rat. Additionally, the innate hyper-excited state of the adolescent P central nervous system is a likely factor in the development of their high alcohol drinking behaviors.

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.

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.

Naloxone attenuates incubated sucrose craving in rats

RATIONALE

Cue-induced craving precedes drug relapse and contributes to eating disorders. Opiate antagonists have been demonstrated to be effective at reducing cravings for drugs and food. Craving, as defined as responding for a stimulus previously associated with a reward, increases, or incubates, over forced abstinence in an animal model of relapse.

OBJECTIVES

This paper aims to determine anticraving effects of the opiate antagonist, naloxone, on the incubation of sucrose craving.

METHODS

106 male Long-Evans rats lever pressed for 10% sucrose solution 2 h/day for 10 days. On either day 1 or 30 of forced abstinence, rats responded in extinction for 6 h and then were injected (ip) with either saline or naloxone (0.001, 0.01, 0.1, 1, or 10 mg/kg). The rats then responded for 1 h for presentation of a tone + light cue previously presented with every sucrose delivery during self-administration training.

RESULTS

The rats responded more in extinction and following saline on day 30 vs day 1 (an incubation of craving). Except for a trend for a decrease in responding following 10 mg/kg on day 1, naloxone was primarily effective on day 30. On day 30, naloxone significantly reduced responding at all doses except for 0.1 mg/kg.

CONCLUSIONS

The time-dependent increase in sensitivity to an opiate antagonist is consistent with time-dependent changes in the opiate system following forced abstinence from sucrose. These changes may partly underlie the incubation of sucrose craving. In addition, these findings could be used to support the use of naloxone as an anticraving medication in protracted abstinence.

Functional gene expression differences between inbred alcohol-preferring and -non-preferring rats in five brain regions

The objective of this study was to determine if there are innate differences in gene expression in selected CNS regions between inbred alcohol-preferring (iP) and -non-preferring (iNP) rats. Gene expression was determined in the nucleus accumbens (ACB), amygdala (AMYG), frontal cortex (FC), caudate-putamen (CPU), and hippocampus (HIPP) of alcohol-naïve adult male iP and iNP rats, using Affymetrix Rat Genome U34A microarrays (n = 6/strain). Using Linear Modeling for Microarray Analysis with a false discovery rate threshold of 0.1, there were 16 genes with differential expression in the ACB, 54 in the AMYG, 8 in the FC, 24 in the CPU, and 21 in the HIPP. When examining the main effect of strain across regions, 296 genes were differentially expressed. Although the relatively small number of genes found significant within individual regions precluded a powerful analysis for over-represented Gene Ontology categories, the much larger list resulting from the main effect of strain analysis produced 17 over-represented categories (P < .05), including axon guidance, gliogenesis, negative regulation of programmed cell death, regulation of programmed cell death, regulation of synapse structure function, and transmission of nerve impulse. Co-citation analysis and graphing of significant genes revealed a network involved in the neuropeptide Y (NPY) transmitter system. Correlation of all significant genes with those located within previously established rat alcohol QTLs revealed that of the total of 313 significant genes, 71 are located within such QTLs. The many regional and overall gene expression differences between the iP and iNP rat lines may contribute to the divergent alcohol drinking phenotypes of these rats.

The alcohol-preferring P rat and animal models of excessive alcohol drinking

The alcohol-preferring, P, rat was developed by selective breeding to study ethanol drinking behavior and its consequences. Characterization of this line indicates the P rat meets all of the criteria put forth for a valid animal model of alcoholism, and displays, relative to their alcohol-non-preferring, NP, counterparts, a number of phenotypic traits associated with alcohol abuse and alcoholism. Behaviorally, compared with NP rats, P rats are less sensitive to the sedative and aversive effects of ethanol and more sensitive to the stimulatory effects of ethanol. Neurochemically, research with the P line indicates the endogenous dopaminergic, serotonergic, GABAergic, opiodergic, and peptidergic systems may be involved in a predisposition for alcohol abuse and alcoholism. Paralleling the clinical literature, genetically selected P rats display levels of ethanol intake during adolescence comparable to that seen during adulthood. Binge drinking has been associated with an increased risk for health and other problems associated with ethanol abuse. A model of binge-like drinking during the dark cycle indicates that P rats will consume 6 g/kg/day of ethanol in as little as three 1-hour access periods/day, which approximates the 24-hour intake of P rats with free-choice access to a single concentration of ethanol. The alcohol deprivation effect (ADE) is a transient increase in ethanol intake above baseline values upon re-exposure to ethanol access after an extended period of deprivation. The ADE has been proposed to be an animal model of relapse behavior, with the adult P rat displaying a robust ADE after prolonged abstinence. Overall, these findings indicate that the P rat can be effectively used in models assessing alcohol-preference, a genetic predisposition for alcohol abuse and/or alcoholism, and excessive drinking using protocols of binge-like or relapse-like drinking.

Perinatal ethanol exposure alters met-enkephalin levels of male and female rats

This study used a rat model of Fetal Alcohol Syndrome to investigate whether combined prenatal and postnatal ethanol exposure affects met-enkephalin levels in the brains of male and female Long-Evans adult rats. Intragastric ethanol was administered to a group of rats (ET) from gestational day (GD) 1 through 22 and from postnatal day (PD) 2 through 10. The control groups consisted of a nontreated control group (NTC) and an intubated control group (IC) that received the intragastric intubation procedure but no exposure to ethanol. We measured met-enkephalin levels in the prefrontal cortex, nucleus accumbens, hypothalamus, central and basolateral nucleus of amygdala and ventral tegmental area. Met-enkephalin levels in the hypothalamus of male and female ET animals were significantly higher than those in either the NTC or IC animals. Met-enkephalin levels in the central nucleus of the amygdala of male and female ET animals were significantly lower than the levels in the NTC animals. Met-enkephalin levels in the nucleus accumbens of ET females were significantly greater than those in the IC females. These results demonstrate that the combination of prenatal and postnatal ethanol exposure affects basal met-enkephalin levels in specific regions in a sex-specific manner. These changes in met-enkephalin levels may explain how early ethanol exposure affects opioid-regulated behaviors such as social play, sexual behavior, and other social behaviors.

Increased GABA release in the central amygdala of ethanol-dependent rats

The central nucleus of amygdala (CeA) is important in regulating alcohol consumption and plays a major role in the anxiogenic response to ethanol withdrawal. We showed previously that acute ethanol augments GABA(A) receptor-mediated IPSPs and IPSCs, possibly by a presynaptic mechanism. Here, we have examined the interaction of acute ethanol with the GABAergic system in chronic ethanol-treated (CET) rats using an in vitro CeA slice preparation and in vivo brain microdialysis. We found that in CeA slices from CET rats, the baseline evoked IPSP and IPSC amplitudes were increased, and paired-pulse facilitation ratios were lower than in naive rats, suggesting an increased GABAergic transmission after chronic ethanol treatment. Interestingly, acute ethanol (5-66 mm) significantly enhanced IPSPs and IPSCs equally in CET and naive rats, indicating a lack of tolerance for this effect of acute ethanol. Analysis of miniature IPSC frequency suggests that the increased GABAergic transmission by both acute and chronic ethanol arises from a presynaptic mechanism involving enhanced vesicular release of GABA. These data are supported by microdialysis studies showing that CET rats presented a fourfold increase in baseline GABA dialysate content compared with naive rats. In vivo administration of ethanol (0.1, 0.3, and 1.0 m) produced a dose-dependent increase in GABA release in the CeA dialysate in both CET and naive rats. These combined findings suggest that acute and chronic ethanol increases GABA release in CeA and support previous reports that the behavioral actions of ethanol are mediated, in part, by increased GABAergic transmission in the CeA.

Gene expression in the hippocampus of inbred alcohol-preferring and -nonpreferring rats

The hippocampus is sensitive to the effects of ethanol and appears to have a role in the development of alcohol tolerance. The objective of this study was to test the hypothesis that there are innate differences in gene expression in the hippocampus of inbred alcohol-preferring (iP) and -nonpreferring (iNP) rats that may contribute to differences in sensitivity to ethanol and/or in the development of tolerance. Affymetrix microarrays were used to measure gene expression in the hippocampus of alcohol-naive male iP and iNP rats in two experiments (n=4 and 6 per strain in the two experiments). Combining data from the two experiments, there were 137 probesets representing 129 genes that significantly differed (P < or = 0.01); 62 probesets differed at P < or = 0.001. Among the 36% of the genes that were expressed more in the iP than iNP rat at this level of significance, many were involved in cell growth and adhesion, cellular stress reduction and anti-oxidation, protein trafficking, regulation of gene expression, synaptic function and metabolism. Among the 64% of the genes that had lower expression in the hippocampus of iP than iNP rats were genes involved in metabolic pathways, cellular signaling systems, protein trafficking, cell death and neurotransmission. Overall, the data indicate that there are significant innate differences in gene expression in the hippocampus between iP and iNP rats, some of which might contribute to the differences observed in the development of alcohol tolerance between the selectively bred P and NP lines.

Chronic ethanol modulates delta and mu-opioid receptor expression in rat CNS: immunohistochemical analysis with quantitiative confocal microscopy

Ethanol consumption affects levels of endogenous opioids as well as opioid receptors in both animals and humans. We studied the expression of delta (delta) and mu (mu) opioid receptors (ORs) in brain sections of adult male Sprague-Dawley rats after 2 weeks of consuming ethanol in a liquid diet, with comparisons to sections from pair-fed control animals. Immunohistochemical staining for the ORs, using selective antibodies, and quantitation of confocal images, revealed increased expression of delta-ORs in hippocampal CA1 of the chronic ethanol-treated rats. In contrast, mu-ORs decreased in their expression after ethanol treatment in multiple brain areas, including cortex, hippocampus, midbrain colliculi, striatum and nucleus accumbens. The alterations in immunoreactive OR expression may be related to reduced functional coupling of the ORs to G-proteins, as found in prior studies in several brain regions, using the same chronic ethanol diet protocol. Changes in OR expression and functional coupling in the CNS may be factors in ongoing ethanol consumption and tolerance.

Alcohol Effects on Central Nervous System Gene Expression in Genetic Animal Models

This article summarizes the proceedings of a symposium presented at the 2004 annual meeting of the Research Society on Alcoholism in Vancouver, British Columbia, Canada. The organizers and chairs were William J. McBride and Michael F. Miles. The presentations were (1) Molecular Triangulation on Gene Expression Patterns in Behavioral Responses to Acute Ethanol, by Robnet T. Kerns; (2) Gene Expression in Limbic Regions After Ethanol Self-Infusion Into the Posterior Ventral Tegmental Area, by Zachary A. Rodd; (3) Microarray Analysis of CNS Limbic Regions of Inbred Alcohol-Preferring and -Nonpreferring rats and Effects of Alcohol Drinking, by Wendy N. Strother and Howard J. Edenberg; and (4) Microarray Analysis of Mouse Lines Selected for Chronic Ethanol Withdrawal Severity: The Convergence of Basal, Ethanol Regulated, and Proximity to Ethanol Quantitative Trait Loci to Identify Candidate Genes, by Joel G. Hashimoto and Kristine M. Wiren.

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.

Sugar-dependent rats show enhanced intake of unsweetened ethanol

Rats show signs of dependence on sugar when it is available intermittently, including bingeing, withdrawal, and cross-sensitization with amphetamine. In the current study, we sought to determine whether sugar-dependent rats would show increased intake of unsweetened ethanol and, conversely, whether intermittent access to ethanol would augment sugar consumption. In Experiment 1, with intermittent versus ad libitum access to ethanol, Sprague-Dawley rats were given escalating concentrations of ethanol (1%, 2%, 4%, 7%, and 9%) over the course of 20 days. Rats in the intermittent ethanol access group, with 12-h daily access, consumed more 4%, 7%, and 9% ethanol during the first hour of access, and more 9% ethanol daily, than did rats in the ad libitum ethanol access group. In Experiment 2, with ethanol as a gateway to sugar intake, the rats from Experiment 1 were switched to 10% sucrose with 12-h daily access for 1 week. Rats in the intermittent ethanol access group consumed significantly more sugar than was consumed by rats in a control group with no prior ethanol experience. In Experiment 3, with sugar as a gateway to ethanol to determine whether sugar dependence leads to increased ethanol intake, four groups were maintained for 21 days according to the following designations: intermittent access to sugar and chow, ad libitum access to sugar and chow, intermittent access to chow, or ad libitum access to chow. Four days later, all groups were switched to intermittent ethanol access, as described in Experiment 1. The group with intermittent access to sugar and chow consumed the most 9% ethanol, supporting the suggestion that sugar dependence alters a rat's proclivity to drink ethanol. These results may relate to the co-morbidity between binge-eating disorders and alcohol intake and the tendency of people abstaining from alcohol to consume excessive amounts of sugar. In conclusion, bingeing on either ethanol or sugar fosters intake of the other.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.

Central opioid receptors differentially regulate the nalmefene-induced suppression of ethanol- and saccharin-reinforced behaviors in alcohol-preferring (P) rats

The exact opioid-sensitive receptors participating in EtOH-seeking behaviors remains unclear. Previous studies have reported higher densities of micro-opioid receptor binding in the nucleus accumbens (NACC) of P relative to NP rats; however, no differences were seen in delta-receptor binding. In contrast to the NACC, substantially lower levels of micro-receptor binding have been observed in the ventral tegmental area (VTA) of both P and NP rats, albeit no line differences have been observed. In the present study, opioid receptors in the NACC, VTA, and hippocampus were evaluated for their capacity to regulate both EtOH- and saccharin-motivated behaviors in the genetically selected alcohol-preferring (P) rat. To accomplish this, nalmefene, an opiate antagonist with preferential binding affinity for the micro-opioid receptor was unilaterally or bilaterally infused during concurrent availability of 1 h daily EtOH (10% v/v) and saccharin (0.025 or 0.050% w/v) solutions. Rats performed under a two-lever fixed ratio (FR) schedule in which four responses on one lever produced the EtOH solution, and four on a second lever produced the saccharin solution. The results demonstrated that when responding maintained by both EtOH and saccharin are matched at basal levels, unilateral (1-60 microg) or bilateral (0.5-10 microg) microinjections of nalmefene into the NACC produced selective dose-dependent reductions on responding maintained by EtOH. Unilateral (40, 60 microg) and bilateral (10 microg) VTA infusions were also observed to selectively reduced EtOH responding; however, greater nalmefene doses were required and the magnitude of suppression on EtOH responding was markedly less compared with the NACC. The greater sensitivity of nalmefene to suppress EtOH responding in the NACC is likely due to the greater number of opioid receptors in the NACC relative to the VTA. Only bilateral infusion of the 40 microg dose in the NACC and VTA suppressed responding maintained by both EtOH and saccharin. In contrast, intrahippocampal infusions dose dependently suppressed EtOH- and saccharin-maintained responding over a range of doses (1-20 microg). The present study provides evidence that nalmefene suppresses EtOH-motivated behaviors via blockade of opioid receptors within the NACC and VTA, and under various dose conditions both reinforcer and neuroanatomical specificity can be observed.

Innate differences in protein expression in the nucleus accumbens and hippocampus of inbred alcohol-preferring and -nonpreferring rats

Two-dimensional gel electrophoresis (2-DE) was used to separate protein samples solubilized from the nucleus accumbens and hippocampus of alcohol-naïve, adult, male inbred alcohol-preferring (iP) and alcohol-nonpreferring (iNP) rats. Several protein spots were excised from the gel, destained, digested with trypsin, and analyzed by mass spectrometry. In the hippocampus, 1629 protein spots were matched to the reference pattern, and in the nucleus accumbens, 1390 protein spots were matched. Approximately 70 proteins were identified in both regions. In the hippocampus, only 8 of the 1629 matched protein spots differed in abundance between the iP and iNP rats. In the nucleus accumbens, 32 of the 1390 matched protein spots differed in abundance between the iP and iNP rats. In the hippocampus, the abundances of all 8 proteins were higher in the iNP than iP rat. In the nucleus accumbens, the abundances of 31 of 32 proteins were higher in the iNP than iP rat. In the hippocampus, only 2 of the 8 proteins that differed could be identified, whereas in the nucleus accumbens 21 of the 32 proteins that differed were identified. Higher abundances of cellular retinoic acid-binding protein 1 and a calmodulin-dependent protein kinase (both of which are involved in cellular signaling pathways) were found in both regions of the iNP than iP rat. In the nucleus accumbens, additional differences in the abundances of proteins involved in (i) metabolism (e.g., calpain, parkin, glucokinase, apolipoprotein E, sorbitol dehydrogenase), (ii) cyto-skeletal and intracellular protein transport (e.g., beta-actin), (iii) molecular chaperoning (e.g., grp 78, hsc70, hsc 60, grp75, prohibitin), (iv) cellular signaling pathways (e.g., protein kinase C-binding protein), (v) synaptic function (e.g., complexin I, gamma-enolase, syndapin IIbb), (vi) reduction of oxidative stress (thioredoxin peroxidase), and (vii) growth and differentiation (hippocampal cholinergic neurostimulating peptide) were found. The results of this study indicate that selective breeding for disparate alcohol drinking behaviors produced innate alterations in the expression of several proteins that could influence neuronal function within the nucleus accumbens and hippocampus.

Variants of kappa-opioid receptor gene and mRNA in alcohol-preferring and alcohol-avoiding mice

Results of recent studies have indicated an association between voluntary alcohol intake and activities of kappa-opioid receptor systems in animal models. We assessed the possibility that genetic differences observed in alcohol preference among mouse strains are related to possible polymorphisms of the kappa-opioid receptor gene (Oprk1). We compared DNA sequences of the coding region and the promoter/regulatory region of Oprk1 among C57BL/6ByJ (B6, alcohol-preferring), BALB/cJ (alcohol-avoiding), CXBI (alcohol-avoiding), and six B6.C and B6.I Recombinant QTL Introgression (RQI) strains, which carry approximately 3% of the donor BALB/cJ genome in the background B6 genome and showed various alcohol preferences. Although there were no sequence differences in the coding region, BALB/cJ had a single nucleotide polymorphism (SNP) in the promoter region, which was not detected in other strains. The results indicate that the difference in alcohol preference between B6 and BALB/cJ is not correlated with polymorphisms of Oprk1. However, results of further studies comparing Oprk1 mRNA expression between B6 and BALB/cJ showed that Oprk1 expression is regulated differently in these strains. Also, DBA/2J mice (alcohol-avoiding) showed expression of Oprk1 mRNA subtypes (alternatively spliced) different from B6 and BALB/cJ mice. Search of the Celera Genomics database indicated that DBA/2J had several SNP sites in the promoter/regulatory regions, which might explain the different expression of Oprk1 mRNA subtypes in this strain. The strain-dependent variation in the expression of alternatively spliced genes can be a significant source of phenotypic variation of complex traits such as alcohol preference.

Studying the neurobiology of stimulant and alcohol abuse and dependence in genetically manipulated mice

The ability to manipulate the genetic makeup of organisms by specific targeting of selected genes has provided a novel means of investigating the neurobiological mechanisms underlying drug abuse and dependence. However, as with other techniques, there are a number of potential pitfalls in the use of genetically manipulated animals (usually mice) in behavioural experiments. This review discusses the techniques involved in creating genetically manipulated mice, and points to opportunities and insights into addictive processes provided by the new science, while illustrating some of the potential problems encountered in interpretation of data obtained from such animals. The use of the mouse as an experimental animal also raises some specific problems which limit the usefulness of the technique at present. Examples taken from research into alcohol and psychostimulant abuse and dependence are used to illustrate the usefulness of genetically manipulated animals in addiction research, the problems of interpretation which sometimes arise, and how techniques are being developed to overcome present limitations to this exciting area of research.

The effect of chronic ethanol consumption and withdrawal on mu-opioid and dopamine D(1) and D(2) receptor density in Fawn-Hooded rat brain

Previous studies have implicated the dopamine and opioid systems in the induction and maintenance of ethanol consumption. This study investigated, in alcohol-preferring Fawn-Hooded (FH) rats, whether chronic free-choice ethanol consumption and subsequent withdrawal cause alterations in central mu-opioid, dopamine D(1), and D(2) receptor density using autoradiography. FH rats were given a free choice between a 5% ethanol solution and tap water (n = 25) and displayed a mean ethanol consumption of 5.6 g/kg/day. A parallel group of FH rats (n = 5) only had access to tap water. Rats were then withdrawn from ethanol for 0, 1, 2, 5, or 10 days and killed by cervical dislocation and decapitation. Increases in mu-opioid receptor density were observed in the nucleus accumbens and ventral tegmental area upon withdrawal compared with the ethanol naive group. In the lateral amygdala, binding in all withdrawal groups was significantly different from the ethanol naive FH rats, and also from the chronic ethanol rats. An increase in dopamine D(1) receptor density was observed in the substantia nigra, pars reticulata in the 5- and 10-day withdrawal groups compared with ethanol naive. Accumbal dopamine D(2) receptor density (+25-30%) increased in the 10-day withdrawal group compared with both naive and chronic ethanol groups. These findings demonstrate that the opioid and dopamine systems are susceptible to modulation by chronic ethanol consumption and withdrawal in the FH rat. Furthermore, although acute ethanol withdrawal results in modulation of mu-opioid receptors, effects on dopamine receptors are delayed and only become evident 5 to 10 days after withdrawal.

Regional central nervous system densities of delta-opioid receptors in alcohol-preferring P, alcohol-nonpreferring NP, and unselected Wistar rats

The densities of delta-opioid receptors in the central nervous system of alcohol-naive, adult, male, alcohol-preferring P, alcohol-nonpreferring NP, and Wistar rats were examined with the use of quantitative autoradiography. Slides with coronal 20-microm sections through the regions of interest were incubated in 5 nM [3H]-[D-Pen(2),D-Pen(5)]enkephalin (DPDPE) to label delta(1)-opioid receptor sites. Nonspecific binding was determined in the presence of 10 microM naloxone. Significant differences between the P and the NP rat lines were found in numerous cortical regions, the basolateral amygdala, and the posterior hippocampus, with 10%-20% lower [3H]-DPDPE binding found in the P line. In most regions examined, binding levels in the Wistar rats were intermediate between those of the P and the NP rats. Significantly lower [3H]-DPDPE binding levels in the P rat may indicate fewer delta(1)-opioid receptors or decreased binding affinity. The lower binding in certain limbic regions, such as the basolateral amygdala and posterior hippocampus, as well as cortical differences in the P rat may be associated with the divergent alcohol drinking behaviors found between the P and the NP lines.

The opioid receptor system and alcoholism: a genetic perspective

Over the past decade, mounting evidence has implicated the endogenous opioid receptor system as a central player in the etiology of alcohol drinking behavior in animals and alcoholism in humans. Much of this work is a product of a pharmacological approach, where differences in opioid receptor pharmacology have been found to predict drinking behavior in animal models of alcoholism, including rats and mice selectively bred for alcohol preference and avoidance. This review considers the opioid receptor system and alcoholism from a genetic standpoint, and discusses investigation into opioid receptor pharmacology in animal models of alcoholism as work that paved the way for the more recent molecular genetic studies implicating the delta-, and particularly, the mu opioid receptors as genetically linked to alcoholism-associated phenotypes in animal models of the disease. These genetic studies are set within the broader context of the candidate gene approach for alcoholism, where opioid receptor genes are taken to be partial, rather than complete, risk factors for alcoholism. Building upon these findings, the recent genetic association between alcoholism and the mu opioid receptor gene in humans is discussed. Finally, the translation of such genetic association studies between opioid receptor genes and alcoholism to a pharmacogenetic approach, allowing for the evaluation of putative relationships between genotype and pharmacological response profiles, is suggested to address the etiological question of what the molecular mechanism is underlying opioid receptor genetic risk for alcoholism phenotypes.

The effect of antagonists selective for mu- and delta-opioid receptor subtypes on alcohol consumption in C57BL/6 mice

Several studies have demonstrated that non-selective opioid receptor antagonists effectively reduce alcohol consumption in both animal models and at the clinical level. However, research examining the contribution of specific opioid receptor subtypes to this effect has yielded conflicting results. Some of these studies have shown that the effect is contingent upon the action of mu receptors while others have suggested that delta receptors are primarily responsible. The data reported here re-examine this question using the alcohol-preferring C57BL/6 mice. The results of this experiment demonstrate that D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), a mu-selective antagonist, and naltrindole, a delta-selective antagonist, are equally effective at reducing alcohol consumption in a limited access model compared to a saline control group. While there was no specific comparison of the effects of these drugs on alternative appetitive behavior, neither of these drugs had effects on measured off-session food or water consumption. The results of this experiment suggest that alcohol consumption is mediated by both mu- and delta-opioid receptor subtypes.

Opioid propeptide mRNA content and receptor density in the brains of AA and ANA rats

Recent evidence has indicated an association between the rewarding effects of ethanol intake and endogenous opioid activity. The present studies examine the presence of differences in opioid peptide mRNA content and mu and kappa opioid receptor densities, between ethanol naive AA and ANA rats bred selectively for their high and low alcohol consumption, respectively. In situ hybridization was used to compare the content of proopiomelanocortin, proenkephalin and prodynorphin mRNA in distinct brain regions known to be involved in the reinforcing properties of addictive drugs, between rats from each line. Results indicated that AA rats had a significantly greater content of proopiomelanocortin mRNA in the arcuate nucleus of the hypothalamus, of proenkephalin mRNA in the prefrontal cortex and of prodynorphin mRNA in the mediodorsal nucleus of the thalamus (p < or = .05). Receptor autoradiography was performed using 3H-labeled ligands specific for mu and kappa opioid receptors. AA rats were found to have a greater density of mu opioid receptors in the shell region of the nucleus accumbens and prefrontal cortex, but a lower density of kappa opioid receptors in the ventromedial hypothalamus, compared to ANA rats. The present data demonstrate the presence of inherited differences in the activity of distinct components of the endogenous opioid system in some brain regions associated with the processes of reward and reinforcement; and as such, may play a role in determining differences in ethanol drinking between AA and ANA rats.

Acoustic startle and fear-potentiated startle in alcohol-preferring (P) and -nonpreferring (NP) lines of rats

The objective of the present study was to determine whether alcohol-preferring P and -nonpreferring NP rats differ in their acoustic startle response and in fear-potentiated startle. In Experiment 1, male P and NP rats were tested on the startle response to acoustic stimuli ranging from 90-115 dB. Experiments 2 and 3 examined fear-potentiated startle and extinction of the response. In Experiment 2, rats received two light foot shock training sessions separated by 3-4 h. Testing consisted of ten acoustic startle (115 dB) and fear-potentiated startle (light preceding the acoustic startle) presentations administered every 24 h for 9 consecutive days. To test potentiated startle learning under reduced training conditions, a single training session was administered in Experiment 3, and a single within-session extinction test of 50 startle and 50 potentiated startle trials occurred the following day. Results of Experiment 1 indicated that P and NP rats did not differ in startle at any of the acoustic intensities tested. Following fear-potentiated startle conditioning in Experiment 2, however, both acoustic startle and potentiated startle responding were consistently greater in P than NP rats over most of the first 6 test days with P rats having approximately a 100% greater acoustic startle and 50-100% greater potentiated startle response. Moreover, following a single training session in Experiment 3, only P rats showed significant fear-conditioned startle. Additionally, P rats exhibited a 50-100% elevated acoustic startle response over that observed in NP rats. Taken together, the data indicate that, although experimentally naive male P and NP rats show similar acoustic startle responses, P rats become more responsive to both startle-alone and potentiated startle stimuli following fear conditioning. The change in general startle reactivity of the P rat following aversive conditioning, along with facilitated light foot shock learning, suggests that stress exposure may be an important variable in examining associations between anxiety and alcohol drinking behavior.