Mmu and D2 receptor antisense oligonucleotides injected in nucleus accumbens suppress high alcohol intake in genetic drinking HEP rats

Accumbal μ-Opioid Receptors Modulate Ethanol Intake in Alcohol-Preferring Alko Alcohol Rats

BACKGROUND

The nucleus accumbens shell is a key brain area mediating the reinforcing effects of ethanol (EtOH). Previously, it has been shown that the density of μ-opioid receptors in the nucleus accumbens shell is higher in alcohol-preferring Alko Alcohol (AA) rats than in alcohol-avoiding Alko Non-Alcohol rats. In addition, EtOH releases opioid peptides in the nucleus accumbens and opioid receptor antagonists are able to modify EtOH intake, all suggesting an opioidergic mechanism in the control of EtOH consumption. As the exact mechanisms of opioidergic involvement remains to be elucidated, the aim of this study was to clarify the role of accumbal μ- and κ-opioid receptors in controlling EtOH intake in alcohol-preferring AA rats.

METHODS

Microinfusions of the μ-opioid receptor antagonist CTOP (0.3 and 1 μg/site), μ-opioid receptor agonist DAMGO (0.03 and 0.1 μg/site), nonselective opioid receptor agonist morphine (30 μg/site), and κ-opioid receptor agonist U50488H (0.3 and 1 μg/site) were administered via bilateral guide cannulas into the nucleus accumbens shell of AA rats that voluntarily consumed 10% EtOH solution in an intermittent, time-restricted (90-minute) 2-bottle choice access paradigm.

RESULTS

CTOP (1 μg/site) significantly increased EtOH intake. Conversely, DAMGO resulted in a decreasing trend in EtOH intake. Neither morphine nor U50488H had any effect on EtOH intake in the used paradigm.

CONCLUSIONS

The results provide further evidence for the role of accumbens shell μ-opioid receptors but not κ-opioid receptors in mediating reinforcing effects of EtOH and in regulating EtOH consumption. The results also provide support for views suggesting that the nucleus accumbens shell has a major role in mediating EtOH reward.

Alcohol addiction and the mu-opioid receptor

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a mu-opioid receptor antagonist which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part due to noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a mis-sense single nucleotide polymorphism (rs179919 or A118G) in the mu-opioid receptor gene predicts a favorable response to naltrexone if an individual carries a 'G' allele. This chapter will review the evidence for this hypothesis. The data are promising that the 'G' allele predisposes to a beneficial naltrexone response among alcohol addicted persons, but additional research is needed to prove this hypothesis in prospective clinical trials.

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

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

Opioid neuroscience for addiction medicine: From animal models to FDA approval for alcohol addiction

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol-addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a mu opioid receptor antagonist which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part due to noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a mis-sense single-nucleotide polymorphism (rs179919 or A118G) in the mu opioid receptor gene predicts a favorable response to naltrexone if an individual carries a "G" allele. This chapter will review the evidence for this hypothesis. The data suggest that the "G" allele has a complex role in alcohol addiction, increasing the rewarding valence of alcohol. Whether the G allele increases risk for alcoholism and whether it predisposes to a beneficial naltrexone response among alcohol-addicted persons must await additional research with large sample sizes of multiple ethnicities in prospective clinical trials.

The Molecular Neurobiology of Twelve Steps Program & Fellowship: Connecting the Dots for Recovery

There are some who suggest that alcoholism and drug abuse are not diseases at all and that they are not consequences of a brain disorder as espoused recently by the American Society of Addiction Medicine (ASAM). Some would argue that addicts can quit on their own and moderate their alcohol and drug intake. When they present to a treatment program or enter the 12 Step Program & Fellowship, many addicts finally achieve complete abstinence. However, when controlled drinking fails, there may be successful alternatives that fit particular groups of individuals. In this expert opinion, we attempt to identify personal differences in recovery, by clarifying the molecular neurobiological basis of each step of the 12 Step Program. We explore the impact that the molecular neurobiological basis of the 12 steps can have on Reward Deficiency Syndrome (RDS) despite addiction risk gene polymorphisms. This exploration has already been accomplished in part by Blum and others in a 2013 Springer Neuroscience Brief. The purpose of this expert opinion is to briefly, outline the molecular neurobiological and genetic links, especially as they relate to the role of epigenetic changes that are possible in individuals who regularly attend AA meetings. It begs the question as to whether "12 steps programs and fellowship" does induce neuroplasticity and continued dopamine D2 receptor proliferation despite carrying hypodopaminergic type polymorphisms such as DRD2 A1 allele. "Like-minded" doctors of ASAM are cognizant that patients in treatment without the "psycho-social-spiritual trio," may not be obtaining the important benefits afforded by adopting 12-step doctrines. Are we better off with coupling medical assisted treatment (MAT) that favors combining dopamine agonist modalities (DAM) as possible histone-deacetylase activators with the 12 steps followed by a program that embraces either one or the other? While there are many unanswered questions, at least we have reached a time when "science meets recovery," and in doing so, can further redeem joy in recovery.

Hypothesizing dopaminergic genetic antecedents in schizophrenia and substance seeking behavior

The dopamine system has been implicated in both substance use disorder (SUD) and schizophrenia. A recent meta-analysis suggests that A1 allele of the DRD2 gene imposes genetic risk for SUD, especially alcoholism and has been implicated in Reward Deficiency Syndrome (RDS). We hypothesize that dopamine D2 receptor (DRD2) gene Taq1 A2 allele is associated with a subtype of non-SUD schizophrenics and as such may act as a putative protective agent against the development of addiction to alcohol or other drugs of abuse. Schizophrenics with SUD may be carriers of the DRD2 Taq1 A1 allele, and/or other RDS reward polymorphisms and have hypodopaminergic reward function. One plausible mechanism for alcohol seeking in schizophrenics with SUD, based on previous research, may be a deficiency of gamma type endorphins that has been linked to schizophrenic type psychosis. We also propose that alcohol seeking behavior in schizophrenics, may serve as a physiological self-healing process linked to the increased function of the gamma endorphins, thereby reducing abnormal dopaminergic activity at the nucleus accumbens (NAc). These hypotheses warrant further investigation and cautious interpretation. We, therefore, encourage research involving neuroimaging, genome wide association studies (GWAS), and epigenetic investigation into the relationship between neurogenetics and systems biology to unravel the role of dopamine in psychiatric illness and SUD.

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.

Opioid pharmacogenetics of alcohol addiction

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol-addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a μ-opioid receptor antagonist, which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part because of noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a missense single nucleotide polymorphism (rs179919 or A118G) in the μ-opioid receptor gene predicts a favorable response to naltrexone if an individual carries a "G" allele. This work will review the evidence for this hypothesis. The data are promising that the "G" allele predisposes to a beneficial naltrexone response among alcohol-addicted persons, but additional research is needed to prove this hypothesis in prospective clinical trials.

DRD2/ANKK1 TaqIA and SLC6A3 VNTR polymorphisms in alcohol dependence: association and gene-gene interaction study in a population of Central Italy

Dopamine is a neurotransmitter whose functions are mediated by five receptors expressed in several organs and tissues. Dopaminergic system dysfunctions are involved in the etiology or treatment of several pathological conditions, including drug addiction. Alcohol dependence (AD) is a widespread psychiatric disorder, affecting 5.4% of the general population lifetime. Family and twins studies support the role of a genetic component in AD. Since dopamine neurotransmission has been shown to be involved in drug reward, related genes are plausible candidates for susceptibility to AD. Here, we evaluated both the DRD2/ANKK1 TaqIA (rs1800497) and SLC6A3 40 bp-VNTR SNP and gene-gene interaction analysis in AD patients from a population of Central Italy. The study design was a case-control. In total, 280 alcoholic subjects (213 men and 67 woman) and 280 age- and sex-matched control subjects were recruited for this study. Case subjects met the DSM-IV criteria for AD and they are free from any psychiatric co-morbidities. Controls were subjects who had non-alcohol problem either never drank; those who have smoked at least one pack of cigarettes per day for at least 1 year were excluded. Genotyping was performed by allele-specific PCR and RFLP-PCR. SLC6A3 40 bp 3'UTR-VNTR displays no association with AD. DRD2/ANKK1 TaqIA genotype distribution is significantly associated to AD (O.R.=1.551, p=0.023), with A1* allele displaying an O.R.=1.403 (p=0.029). Gene-gene interaction analysis using three-way contingency table analysis by a log-linear model yielded no significant result. Our study in a population of Central Italy extends and confirms previous results and, for the first time, tested the gene-gene interaction between SLC6A3 and DRD2 in AD.

Generational association studies of dopaminergic genes in reward deficiency syndrome (RDS) subjects: selecting appropriate phenotypes for reward dependence behaviors

Abnormal behaviors involving dopaminergic gene polymorphisms often reflect an insufficiency of usual feelings of satisfaction, or Reward Deficiency Syndrome (RDS). RDS results from a dysfunction in the "brain reward cascade," a complex interaction among neurotransmitters (primarily dopaminergic and opioidergic). Individuals with a family history of alcoholism or other addictions may be born with a deficiency in the ability to produce or use these neurotransmitters. Exposure to prolonged periods of stress and alcohol or other substances also can lead to a corruption of the brain reward cascade function. We evaluated the potential association of four variants of dopaminergic candidate genes in RDS (dopamine D1 receptor gene [DRD1]; dopamine D2 receptor gene [DRD2]; dopamine transporter gene [DAT1]; dopamine beta-hydroxylase gene [DBH]).

METHODOLOGY

We genotyped an experimental group of 55 subjects derived from up to five generations of two independent multiple-affected families compared to rigorously screened control subjects (e.g., N = 30 super controls for DRD2 gene polymorphisms). Data related to RDS behaviors were collected on these subjects plus 13 deceased family members.

RESULTS

Among the genotyped family members, the DRD2 Taq1 and the DAT1 10/10 alleles were significantly (at least p < 0.015) more often found in the RDS families vs. controls. The TaqA1 allele occurred in 100% of Family A individuals (N = 32) and 47.8% of Family B subjects (11 of 23). No significant differences were found between the experimental and control positive rates for the other variants.

CONCLUSIONS

Although our sample size was limited, and linkage analysis is necessary, the results support the putative role of dopaminergic polymorphisms in RDS behaviors. This study shows the importance of a nonspecific RDS phenotype and informs an understanding of how evaluating single subset behaviors of RDS may lead to spurious results. Utilization of a nonspecific "reward" phenotype may be a paradigm shift in future association and linkage studies involving dopaminergic polymorphisms and other neurotransmitter gene candidates.

Ethanol action on dopaminergic neurons in the ventral tegmental area: interaction with intrinsic ion channels and neurotransmitter inputs

The dopaminergic system originating in the midbrain ventral tegmental area (VTA) has been extensively studied over the past decades as a critical neural substrate involved in the development of alcoholism and addiction to other drugs of abuse. Accumulating evidence indicates that ethanol modulates the functional output of this system by directly affecting the firing activity of VTA dopamine neurons, whereas withdrawal from chronic ethanol exposure leads to a reduction in the functional output of these neurons. This chapter will provide an update on the mechanistic investigations of the acute ethanol action on dopamine neuron activity and the neuroadaptations/plasticities in the VTA produced by previous ethanol experience.

Opioids in the nucleus accumbens stimulate ethanol intake

The nucleus accumbens (NAc) participates in the control of both motivation and addiction. To test the possibility that opioids in the NAc can cause rats to select ethanol in preference to food, Sprague-Dawley rats with ethanol, food, and water available, were injected with two doses each of morphine, the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), the delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), the k-receptor agonist (+/-)-trans-U-50488 methanesulfonate (U-50,488H), or the opioid antagonist naloxone methiodide (m-naloxone). As an anatomical control for drug reflux, injections were also made 2mm above the NAc. The main result was that morphine in the NAc significantly increased ethanol and food intake, whereas m-naloxone reduced ethanol intake without affecting food or water intake. Of the selective receptor agonists, DALA in the NAc increased ethanol intake in preference to food. This is in contrast to DAMGO, which stimulated food but not ethanol intake, and the k-agonist U-50,488H, which had no effect on intake. When injected in the anatomical control site 2mm dorsal to the NAc, the opioids had no effects on ethanol intake. These results demonstrate that ethanol intake produced by morphine in the NAc is driven in large part by the delta-receptor. In light of other studies showing ethanol intake to increase enkephalin expression in the NAc, the present finding of enkephalin-induced ethanol intake suggests the existence of a positive feedback loop that fosters alcohol abuse. Naltrexone therapy for alcohol abuse may then act, in part, in the NAc by blocking this opioid-triggered cycle of alcohol intake.

Effects of aripiprazole on subjective and physiological responses to alcohol

BACKGROUND

Aripiprazole is an atypical antipsychotic with partial agonist activity at D(2) receptors, which could reduce the reinforcing effects of alcohol. The present study examined whether aripiprazole modifies the behavioral and physiological effects of a moderate dose of alcohol in a group of social drinkers.

METHODS

Eighteen healthy subjects (9 men; mean age = 27.6 years) completed a double-blind, within-subject study with 3 experimental sessions in a randomized sequence, during which they received no medication, aripiprazole 2.5 mg, or aripiprazole 10 mg on the day prior to the laboratory session. During the session, subjects consumed alcohol that was served as three standardized drinks (i.e., a total of 0.8 g/kg for men and 0.7 g/kg for women). Breath alcohol concentration (BrAC), heart rate, blood pressure, static ataxia, and subjective effects were measured regularly throughout the laboratory sessions.

RESULTS

Alcohol consumption produced physiological and subjective responses that were consistent with the literature on its effects. Pre-treatment with aripiprazole was generally well tolerated, with tiredness being the most commonly reported adverse event. The medication was associated with modest physiological effects. It also significantly and dose-dependently increased the sedative effects of alcohol and, to a lesser degree, decreased the euphoric effects of alcohol.

CONCLUSIONS

These findings require replication in a larger subject sample that includes heavy drinkers and in a study that employs a placebo session. Based on its capacity to increase the sedative effects and decrease the euphoric effects of alcohol, aripiprazole could be of value in the treatment of heavy drinking.

Manipulation of catechol-O-methyl-transferase (COMT) activity to influence the attenuation of substance seeking behavior, a subtype of Reward Deficiency Syndrome (RDS), is dependent upon gene polymorphisms: A hypothesis

There are common genetic mechanisms responsible for both drug effects and subsequent seeking behavior. In 1996, we coined the term Reward Deficiency Syndrome (RDS). Past and current treatment of substance seeking behavior, a subtype of Reward Deficiency Syndrome (RDS), is considered by most to be inadequate. Recently, we evaluated a complex named Synaptamine [Haveos (SG8839R)]. The main difference with an older studied variant and the latest variant is the inclusion of a proprietary form of Rhodiola rosea, a known catechol-O-methyl-transferase inhibitor (COMT) to potentially enhance the activity of presynaptic released dopamine. In this regard, based on the current literature we hypothesize that manipulation of catechol-O-methyl-transferase (COMT) activity to influence the attenuation of substance seeking behavior, is dependent upon gene polymorphisms. In this regard we hypothesize that carrying the LL genotype with low COMT activity should as theorized, increase the reward induced by substance-induced dopamine release and may indeed increase the propensity to type 1 alcoholism and possibly other drugs that activate the dopaminergic system. Thus when alcohol is present in low COMT LL genotype, increasing COMT activity, not inhibiting it should assist in the reduction of social consumption or abuse. Alternatively, under physiological conditions (no psychoactive substances present (e.g. alcohol) carrying the DRD2 A1 allele with associated low D2 receptors should, as theorized, increase craving behavior because of a low or hypodopaminergic state causing the individual to seek out substances that increase the release of dopamine for subsequent activation of unbound D2 sites in the nucleus accumbens. Thus, in the absence of alcohol or other psychoactive drugs (dopamine releasers), especially during recovery or rehabilitation, decreasing, not increasing COMT activity, should result in enhanced synaptic dopamine as physiologically released, thereby proliferating D2 receptors while reducing stress, increasing well-being, reducing craving behavior and preventing relapse. Based on this hypothesis, we believe that adding the COMT inhibitor R. rosea (as Rhodimin) to our amino-acid and chromium combination in DUI offenders and other illegal drug-related crimes, increases the potential for more targeted neurochemical rebalancing and enhanced relapse prevention. Finally, we hypothesize that these data coupled together provide evidence that the combination of enkephalinase inhibition, neurotransmitter precursor loading, brain tryptophan enhancing and COMT inhibition as well as DNA analysis of the individual's genome, may be useful as an adjunct to therapy when used in outpatient recovery, specifically to assist in reducing craving behavior and preventing relapse.

Reduced alcohol consumption in mice lacking preprodynorphin

Many studies suggest a role for endogenous opioid peptides and their receptors in regulation of ethanol intake. It is commonly accepted that the kappa-opioid receptors and their endogenous ligands, dynorphins, produce a dysphoric state and therefore may be responsible for avoidance of alcohol. We used mutant mice lacking preprodynorphin in a variety of behavioral tests of alcohol actions. Null mutant female, but not male, mice showed significantly lower preference for alcohol and consumed lower amounts of alcohol in a two-bottle choice test as compared with wild-type littermates. In the same test, knockout mice of both sexes showed a strong reduction of preference for saccharin compared to control mice. In contrast, under conditions of limited (4 h) access (light phase of the light/dark cycle), null mutant mice did not show any differences in consumption of saccharin, but they showed significantly reduced intake of sucrose. To determine the possible cause for reduction of ethanol preference and intake, we studied other ethanol-related behaviors in mice lacking the preprodynorphin gene. There were no differences between null mutant and wild-type mice in ethanol-induced loss of righting reflex, acute ethanol withdrawal, ethanol-induced conditioned place preference, or conditioned taste aversion to ethanol. These results indicate that deletion of preprodynorphin leads to substantial reduction of alcohol intake in female mice, and suggest that this is caused by decreased orosensory reward of alcohol (sweet taste and/or palatability).

Hyperpolarization-activated cation current (Ih) is an ethanol target in midbrain dopamine neurons of mice

Ethanol stimulates the firing activity of midbrain dopamine (DA) neurons, leading to enhanced dopaminergic transmission in the mesolimbic system. This effect is thought to underlie the behavioral reinforcement of alcohol intake. Ethanol has been shown to directly enhance the intrinsic pacemaker activity of DA neurons, yet the cellular mechanism mediating this excitation remains poorly understood. The hyperpolarization-activated cation current, Ih, is known to contribute to the pacemaker firing of DA neurons. To determine the role of Ih in ethanol excitation of DA neurons, we performed patch-clamp recordings in acutely prepared mouse midbrain slices. Superfusion of ethanol increased the spontaneous firing frequency of DA neurons in a reversible fashion. Treatment with ZD7288, a blocker of Ih, irreversibly depressed basal firing frequency and significantly attenuated the stimulatory effect of ethanol on firing. Furthermore, ethanol reversibly augmented Ih amplitude and accelerated its activation kinetics. This effect of ethanol was accompanied by a shift in the voltage dependence of Ih activation to more depolarized potentials and an increase in the maximum Ih conductance. Cyclic AMP mediated the depolarizing shift in Ih activation but not the increase in the maximum conductance. Finally, repeated ethanol treatment in vivo induced downregulation of Ih density in DA neurons and an accompanying reduction in the magnitude of ethanol stimulation of firing. These results suggest an important role of Ih in the reinforcing actions of ethanol and in the neuroadaptations underlying escalation of alcohol consumption associated with alcoholism.

The mesolimbic dopamine system: the final common pathway for the reinforcing effect of drugs of abuse?

In this review we will critically assess the hypothesis that the reinforcing effect of virtually all drugs of abuse is primarily dependent on activation of the mesolimbic dopamine system. The focus is on five classes of abused drugs: psychostimulants, opiates, ethanol, cannabinoids and nicotine. For each of these drug classes, the pharmacological and physiological mechanisms underlying the direct or indirect influence on mesolimbic dopamine transmission will be reviewed. Next, we evaluate behavioral pharmacological experiments that specifically assess the influence of activation of the mesolimbic dopamine system on drug reinforcement, with particular emphasis on animal experiments using drug self-administration paradigms. There is overwhelming evidence that all five classes of abused drugs increase dopamine transmission in limbic regions of the brain through interactions with a variety of transporters, ionotropic receptors and metabotropic receptors. Behavioral pharmacological experiments indicate that increased dopamine transmission is clearly both necessary and sufficient to promote psychostimulant reinforcement. For the other four classes of abused substances, self-administration experiments suggest that although increasing mesolimbic dopamine transmission plays an important role in the reinforcing effects of opiates, ethanol, cannabinoids and nicotine, there are also dopamine-independent processes that contribute significantly to the reinforcing effects of these compounds.

Spatial learning in rats is impaired by microinfusions of protein kinase C-gamma antisense oligodeoxynucleotide within the nucleus accumbens

The nucleus accumbens (NAcc) has been shown to play a role in motor and spatial learning. Protein kinase C (PKC) has been implicated in the mechanisms of initiation and maintenance of long-term potentiation that is thought to be involved in the storage of long-term memory. In the present study, the importance of de novo synthesis of PKC-gamma within the NAcc in the acquisition and retention of spatial discrimination learning was assessed using an antisense knockdown approach. Separate groups of Long-Evans rats were exposed to acute microinfusions (6microg/microl) of PKC-gamma antisense oligodeoxynucleotide (AS-ODN), control oligodeoxynucleotide (C-ODN) or vehicle into the NAcc at 24 and 3h before each training session. Behavioral findings showed that the blockade of NAcc-PKC-gamma translation caused impairments in the early phase of learning and retention of spatial information. Biochemical experiments showed that PKC-gamma expression was reduced and Ca(2+)/phospholipid-dependent protein kinase C (PKC) activity was blocked significantly in the AS-ODN-treated rats in comparison with control rats. The present findings suggest that NAcc-PKC-gamma plays a role during the early acquisition of spatial learning. Also, retention test results suggest that NAcc-PKC-gamma may be working as an intermediate factor involved in the onset of molecular mechanisms necessary for spatial memory consolidation within the NAcc.

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.

Effects of single and dual administration of cocaine and ethanol on opioid and ORL1 receptor expression in rat CNS: an autoradiographic study

The co-abuse of cocaine and ethanol is common among human addicts and has been reported to produce a stronger increase of euphoria as compared to either drug given alone. Both cocaine and ethanol increase the extracellular dopamine concentration in the nucleus accumbens, a terminal region in the mesolimbic dopamine pathway. In addition, both cocaine and ethanol affect the endogenous opioid system, which in turn alters the activity of the mesolimbic dopamine pathway. We have carried out quantitative autoradiography mapping of the opioid receptors as well as the opioid receptor-like 1 receptor in the brains of rats treated with both single and dual cocaine and ethanol. Rats received acute cocaine, ethanol or both drugs in combination. Ethanol alone or in combination with cocaine modulated the receptor densities in rat central nervous system. The kappa receptor densities were generally decreased, while both the mu and the opioid receptor-like 1 receptors were up-regulated. The mu opioid receptor levels were mainly increased in non-cortical regions, whereas the opioid receptor-like 1 receptors were increased in cortical structures. No changes in delta opioid receptors were observed. Cocaine alone did not influence the receptor levels in any of the treatment groups.

Ten years of antisense inhibition of brain G-protein-coupled receptor function

Antisense oligonucleotides (AOs) are widely used as tools for inhibiting gene expression in the mammalian central nervous system. Successful gene suppression has been reported for different targets such as neurotransmitter receptors, neuropeptides, ion channels, trophic factors, cytokines, transporters, and others. This illustrates their potential for studying the expression and function of a wide range of proteins. AOs may even find therapeutic applications and provide an attractive strategy for intervention in diseases of the central nervous system (CNS). However, a lack of effectiveness and/or specificity could be a major drawback for research or clinical applications. Here we provide a critical overview of the literature from the past decade on AOs for the study of G-protein-coupled receptors (GPCRs). The following aspects will be considered: mechanisms by which AOs exert their effects, types of animal model system used, detection of antisense action, effects of AO design and delivery characteristics, non-antisense effects and toxicological properties, controls used in antisense studies to assess specificity, and our results (failures and successes). Although the start codon of the mRNA is the most popular region (46%) to target by AOs, targeting the coding region of GPCRs is almost as common (41%). Moreover, AOs directed to the coding region of the GPCR mRNA induce the highest reductions in receptor levels. To resist degradation by nucleases, the modified phosphorothioate AO (S-AO) is the most widely used and effective oligonucleotide. However, the end-capped phosphorothioate AOs (ECS-AOs) are increasingly used due to possible toxic and non-specific effects of the S-AO. Other parameters affecting the activity of a GPCR-targeting AO are the length (mostly an 18-, 20- or 21-mer) and the GC-content (mostly varying from 30 to 80%). Interestingly, one-third of the AOs successfully targeting GPCRs possess a GC/AT ratio of 61-70%. AO-induced reductions in GPCR expression levels and function range typically from 21 to 40% and 41 to 50%, respectively. In contrast to many antisense reviews, we therefore conclude that the functional activity of a GPCR after AO treatment correlates mostly with the density of the target receptors (maximum factor 2). However, AOs are no simple tools for experimental use in vivo. Despite successful results in GPCR research, no general guidelines exist for designing a GPCR-targeting AO or, in general, for setting up a GPCR antisense experiment. It seems that the correct choice of a GPCR targeting AO can only be ascertained empirically. This disadvantage of antisense approaches results mostly from incomplete knowledge about the internalisation and mechanism of action of AOs. Together with non-specific effects of AOs and the difficulties of assessing target specificity, this makes the use of AOs a complex approach from which conclusions must be drawn with caution. Further antisense research has to be carried out to ensure the adequate use of AOs for studying GPCR function and to develop antisense as a valuable therapeutic modality.

Effects of alcohol detoxification on dopamine D2 receptors in alcoholics: a preliminary study

Imaging studies in patients with Type II alcohol dependence have revealed significant reductions in dopamine (DA) D2 receptor availability. Here we assessed the effects of alcohol detoxification in DA D2 receptors in alcoholic subjects. We evaluated 14 patients with Type II alcohol dependence tested within 6 weeks of detoxification and then re-tested 1-4 months later while alcohol free. The comparison group comprised 11 healthy controls. PET was used with [11C]raclopride to measure DA D2 receptors. Eight alcoholics and all control subjects were tested with a CTI 931 PET scanner and six alcoholics with a Siemens HR+ PET scanner. Data were analyzed separately for the studies done in the different scanners. Comparisons between early and late alcohol detoxification showed no significant changes in DA D2 receptor availability (B(max)/K(d)) for the studies done with the CTI and the HR+ scanners. Comparison with controls showed lower DA D2 receptor levels in caudate and putamen in alcoholics tested during early detoxification and in caudate during late detoxification. These studies replicate previous findings of lower striatal DA D2 receptors in alcoholics than in controls and absence of significant recovery during alcohol detoxification. These findings suggest that low DA D2 receptor availability in alcoholics is not due to alcohol withdrawal and may reflect a predisposing factor.

Behavioral neurobiology of alcohol addiction: recent advances and challenges

Addictive behavior associated with alcoholism is characterized by compulsive preoccupation with obtaining alcohol, loss of control over consumption, and development of tolerance and dependence, as well as impaired social and occupational functioning. Like other addictive disorders, alcoholism is characterized by chronic vulnerability to relapse after cessation of drinking. To understand the factors that compel some individuals to drink excessively, alcohol research has focused on the identification of brain mechanisms that support the reinforcing actions of alcohol and the progression of changes in neural function induced by chronic ethanol consumption that lead to the development of dependence. More recently, increasing attention has been directed toward the understanding of neurobiological and environmental factors in susceptibility to relapse.

Role of endogenous cyclo(His-Pro) in voluntary alcohol consumption by alcohol-preferring C57BL mice

Cyclo (His-Pro) or CHP is a cyclic dipeptide endogenous to the brain of a variety of animal species including man. Administration of exogenous peptide to rodents has been shown to exhibit a variety of biological activities some of which appear to be mediated via a dopaminergic mechanism. Since a hypodopaminergic state has been associated with excessive drinking in animal models as well as man, we have explored the potential role of CHP in alcohol-preferring C57BL mice. The results of this study show that the level of CHP, a peptide that mimics dopamine in many of its pharmacologic actions, is lower in brains of alcohol-preferring C57BL mice compared to alcohol non-preferring DBA2 mice. Furthermore, administration of exogenous CHP to C57BL mice caused a pronounced decrease in their voluntary alcohol consumption. In conclusion, endogenous CHP may play a role in risk for developing excessive alcohol use by modulating central dopaminergic tone.

Genetic polymorphisms of the promoter region of dopamine D2 receptor and dopamine transporter genes and alcoholism among four aboriginal groups and Han Chinese in Taiwan

This study aims to examine the relationship between the functional polymorphism at the promoter region of the dopamine D2 receptor (DRD2) gene (i.e. -141C Ins/Del) and variable number of tandem repeat polymorphism at the 3' untranslated region of the dopamine transporter (DAT) gene (SLC6A3) with alcoholism in a case-control study. The cases (n = 203) were alcohol dependents with withdrawal symptoms, and the controls (n = 213) were sex- and ethnicity-matched individuals who were screened to exclude those with alcohol problems among four aboriginal groups (Atayal, Ami, Bunun, and Paiwan) and Han Chinese in Taiwan. To control for potential confounding factors, we excluded tobacco abusers from control subjects in part of the analysis and compared the distribution of the genetic polymorphisms in alcoholics with severe medical complications versus those with less severe medical complications. There were no differences in allele and genotype frequencies of these two distinct genetic markers between alcoholics and control subjects in these five different ethnic groups. There was no significant linkage disequilibrium between the -141C polymorphism and two other DRD2 polymorphisms (TaqI A and NcoI). The results remained unchanged when cases were limited to alcoholics with more severe medical complications or when tobacco abusers were excluded from control subjects. The results suggest that both the DRD2 promoter region and the DAT gene do not play a significant role in conferring vulnerability to alcoholism.

Influence of the dopamine D2 receptor (DRD2) exon 8 genotype on efficacy of tiapride and clinical outcome of alcohol withdrawal

We tested the hypothesis that allelic variants of the human dopamine D2 receptor E8 genotype are associated with (i) dopamine D2 antagonist tiapride dose in treatment of alcohol withdrawal (n = 50) and (ii) with anxiety and depression in patients during alcoholism detoxification therapy (admission n = 87; discharge n = 50). DRD2 E8 A/A genotype was associated with increased dose of tiapride during a 9-day detoxification therapy and with increased anxiety and depression scores on admission and 2 weeks later. The findings suggest a pharmacogenetic influence of DRD2 E8 genotype on tiapride efficacy in alcohol withdrawal. In an earlier report, DRD2 E8 A/A genotype was associated with reduced responsiveness to the dopamine D2 agonist apomorphine; however, it is not clear whether both findings share the same biological basis. Earlier findings concerning association of DRD2 E8 A/A with increased anxiety and depression are replicated for the first time.

Overexpression of dopamine D2 receptors reduces alcohol self-administration

The mechanism(s) underlying predisposition to alcohol abuse are poorly understood but may involve brain dopamine system(s). Here we used an adenoviral vector to deliver the dopamine D2 receptor (DRD2) gene into the nucleus accumbens of rats, previously trained to self-administer alcohol, and to assess if DRD2 levels regulated alcohol preference and intake. We show that increases in DRD2 (52%) were associated with marked reductions in alcohol preference (43%), and alcohol intake (64%) of ethanol preferring rats, which recovered as the DRD2, returned to baseline levels. In addition, this DRD2 overexpression similarly produced significant reductions in ethanol non-preferring rats, in both alcohol preference (16%) and alcohol intake (75%). This is the first evidence that overexpression of DRD2 reduces alcohol intake and suggests that high levels of DRD2 may be protective against alcohol abuse.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.