Increased Ethanol Self-Administration in delta-Opioid Receptor Knockout Mice

Opioidergic tuning of social attachment: reciprocal relationship between social deprivation and opioid abuse

Individuals misusing opioids often report heightened feelings of loneliness and decreased ability to maintain social connections. This disruption in social functioning further promotes addiction, creating a cycle in which increasing isolation drives drug use. Social factors also appear to impact susceptibility and progression of opioid dependence. In particular, increasing evidence suggests that poor early social bond formation and social environments may increase the risk of opioid abuse later in life. The brain opioid theory of social attachment suggests that endogenous opioids are key to forming and sustaining social bonds. Growing literature describes the opioid system as a powerful modulator of social separation distress and attachment formation in rodents and primates. In this framework, disruptions in opioidergic signaling due to opioid abuse may mediate social reward processing and behavior. While changes in endogenous opioid peptides and receptors have been reported in these early-life adversity models, the underlying mechanisms remain poorly understood. This review addresses the apparent bidirectional causal relationship between social deprivation and opioid addiction susceptibility, investigating the role of opioid transmission in attachment bond formation and prosocial behavior. We propose that early social deprivation disrupts the neurobiological substrates associated with opioid transmission, leading to deficits in social attachment and reinforcing addictive behaviors. By examining the literature, we discuss potential overlapping neural pathways between social isolation and opioid addiction, focusing on major reward-aversion substrates known to respond to opioids.

Multiobjective Molecular Optimization for Opioid Use Disorder Treatment Using Generative Network Complex

Opioid use disorder (OUD) has emerged as a significant global public health issue, necessitating the discovery of new medications. In this study, we propose a deep generative model that combines a stochastic differential equation (SDE)-based diffusion model with a pretrained autoencoder. The molecular generator enables efficient generation of molecules that target multiple opioid receptors, including mu, kappa, and delta. Additionally, we assess the ADMET (absorption, distribution, metabolism, excretion, and toxicity) properties of the generated molecules to identify druglike compounds. We develop a molecular optimization approach to enhance the pharmacokinetic properties of some lead compounds. Advanced binding affinity predictors were built using molecular fingerprints, including autoencoder embeddings, transformer embeddings, and topological Laplacians. Our process yields druglike molecules that can be used in highly focused experimental studies to further evaluate their pharmacological effects. Our machine learning platform serves as a valuable tool for designing effective molecules to address OUD.

Pairing Binge Drinking and a High-Fat Diet in Adolescence Modulates the Inflammatory Effects of Subsequent Alcohol Consumption in Mice

Alcohol binge drinking (BD) and poor nutritional habits are two frequent behaviors among many adolescents that alter gut microbiota in a pro-inflammatory direction. Dysbiotic changes in the gut microbiome are observed after alcohol and high-fat diet (HFD) consumption, even before obesity onset. In this study, we investigate the neuroinflammatory response of adolescent BD when combined with a continuous or intermittent HFD and its effects on adult ethanol consumption by using a self-administration (SA) paradigm in mice. The inflammatory biomarkers IL-6 and CX3CL1 were measured in the striatum 24 h after BD, 3 weeks later and after the ethanol (EtOH) SA. Adolescent BD increased alcohol consumption in the oral SA and caused a greater motivation to seek the substance. Likewise, mice with intermittent access to HFD exhibited higher EtOH consumption, while the opposite effect was found in mice with continuous HFD access. Biochemical analyses showed that after BD and three weeks later, striatal levels of IL-6 and CX3CL1 were increased. In addition, in saline-treated mice, CX3CL1 was increased after continuous access to HFD. After oral SA procedure, striatal IL-6 was increased only in animals exposed to BD and HFD. In addition, striatal CX3CL1 levels were increased in all BD- and HFD-exposed groups. Overall, our findings show that adolescent BD and intermittent HFD increase adult alcohol intake and point to neuroinflammation as an important mechanism modulating this interaction.

Episodic Ethanol Exposure in Adolescent Rats Causes Residual Alterations in Endogenous Opioid Peptides

Adolescent binge drinking is associated with an increased risk of substance use disorder, but how ethanol affects the central levels of endogenous opioid peptides is still not thoroughly investigated. The aim of this study was to examine the effect of repeated episodic ethanol exposure during adolescence on the tissue levels of three different endogenous opioid peptides in rats. Outbred Wistar rats received orogastric (i.e., gavage) ethanol for three consecutive days per week between 4 and 9 weeks of age. At 2 h and 3 weeks, respectively, after the last exposure, beta-endorphin, dynorphin B and Met-enkephalin-Arg⁶Phe⁷ (MEAP) were analyzed with radioimmunoassay. Beta-endorphin levels were low in the nucleus accumbens during ethanol intoxication. Remaining effects of adolescent ethanol exposure were found especially for MEAP, with low levels in the amygdala, and high in the substantia nigra and ventral tegmental area three weeks after the last exposure. In the hypothalamus and pituitary, the effects of ethanol on beta-endorphin were dependent on time from the last exposure. An interaction effect was also found in the accumbal levels of MEAP and nigral dynorphin B. These results demonstrate that repeated episodic exposure to ethanol during adolescence affected opioid peptide levels in regions involved in reward and reinforcement as well as stress response. These alterations in opioid networks after adolescent ethanol exposure could explain, in part, the increased risk for high ethanol consumption later in life.

Critical Role for Gi/o-Protein Activity in the Dorsal Striatum in the Reduction of Voluntary Alcohol Intake in C57Bl/6 Mice

The transition from non-dependent alcohol use to alcohol dependence involves increased activity of the dorsal striatum. Interestingly, the dorsal striatum expresses a large number of inhibitory G-protein-coupled receptors (GPCRs), which when activated may inhibit alcohol-induced increased activity and can decrease alcohol consumption. Here, we explore the hypothesis that dorsal striatal G_i/o-protein activation is sufficient to reduce voluntary alcohol intake. Using a voluntary, limited-access, two-bottle choice, drink-in-the-dark model of alcohol (10%) consumption, we validated the importance of G_i/o signaling in this region by locally expressing neuron-specific, adeno-associated-virus encoded G_i/o-coupled muscarinic M₄ designer receptors exclusively activated by designer drugs (DREADD) in the dorsal striatum and observed a decrease in alcohol intake upon DREADD activation. We validated our findings by activating G_i/o-coupled delta-opioid receptors (DORs), which are natively expressed in the dorsal striatum, using either a G-protein biased agonist or a β-arrestin-biased agonist. Local infusion of TAN-67, an in vitro-determined G_i/o-protein biased DOR agonist, decreased voluntary alcohol intake in wild-type and β-arrestin-2 knockout (KO) mice. SNC80, a β-arrestin-2 biased DOR agonist, increased alcohol intake in wild-type mice; however, SNC80 decreased alcohol intake in β-arrestin-2 KO mice, thus resulting in a behavioral outcome generally observed for G_i/o-biased agonists and suggesting that β-arrestin recruitment is required for SNC80-increased alcohol intake. Overall, these results suggest that activation G_i/o-coupled GPCRs expressed in the dorsal striatum, such as the DOR, by G-protein biased agonists may be a potential strategy to decrease voluntary alcohol consumption and β-arrestin recruitment is to be avoided.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Cannabinoid, melanocortin and opioid receptor expression on DRD1 and DRD2 subpopulations in rat striatum

The striatum harbors two neuronal populations that enable action selection. One population represents the striatonigral pathway, expresses the dopamine receptor D1 (DRD1) and promotes the execution of motor programs, while the other population represents the striatopallidal pathway, expresses the dopamine receptor D2 (DRD2) and suppresses voluntary activity. The two populations integrate distinct sensorimotor, cognitive, and emotional information streams and their combined activity enables the selection of adaptive behaviors. Characterization of these populations is critical to the understanding of their role in action selection, because it aids the identification of the molecular mechanisms that separate them. To that end, we used fluorescent in situ hybridization to quantify the percentage of striatal cells that (co)express dopaminergic receptors and receptors of the cannabinoid, melanocortin or opioid neurotransmitters systems. Our main findings are that the cannabinoid 1 receptor is equally expressed on both populations with a gradient from dorsal to ventral striatum, that the opioid receptors have a preference for expression with either the DRD1 or DRD2 and that the melanocortin 4 receptor (MC4R) is predominantly expressed in ventral parts of the striatum. In addition, we find that the level of MC4R expression determines its localization to either the DRD1 or the DRD2 population. Thereby, we provide insight into the sensitivity of the two dopaminoceptive populations to these neurotransmitters and progress the understanding of the mechanisms that enable action selection.

κ-Opioid receptors in the central amygdala regulate ethanol actions at presynaptic GABAergic sites

Human and animal studies indicate that κ-opioid receptors (KORs) are involved in ethanol drinking and dependence (Xuei et al., 2006; Walker and Koob, 2008; Walker et al., 2011). Using in vitro single-cell recording techniques in mouse brain slices, we examined the physiologic effects of KOR activation in the central amygdala (CeA) on GABAergic neurotransmission and its interaction with acute ethanol. A selective KOR agonist (U69593, 1 μM) diminished evoked GABAergic inhibitory postsynaptic currents (IPSCs) by 18% (n = 10), whereas blockade of KORs with a selective antagonist (nor-binaltorphimine, 1 μM) augmented the baseline evoked GABAergic IPSCs by 14% (P < 0.01; n = 34), suggesting that the KOR system contributes to tonic inhibition of GABAergic neurotransmission in the CeA. In addition, the enhancement by acute ethanol of GABAergic IPSC amplitudes was further augmented by pharmacologic blockade of KORs, from 14% (n = 36) to 27% (n = 26; P < 0.01), or by genetic deletion of KORs, from 14% in wild-type mice (n = 19) to 34% in KOR knockout mice (n = 13; P < 0.01). Subsequent experiments using tetrodotoxin to block activity-dependent neurotransmission suggest that KORs regulate GABA release at presynaptic sites. Our data support the idea that KORs modulate GABAergic synaptic responses and ethanol effects as one of multiple opioid system-dependent actions of ethanol in the CeA, possibly in a circuit-specific manner.

Dual efficacy of delta opioid receptor-selective ligands for ethanol drinking and anxiety

Alcoholism and anxiety disorders have a huge impact on society and afflict 17.6 million and 40 million people in the United States, respectively. A strong comorbidity exists between alcoholism and anxiety disorders. Indeed, alcohol withdrawal-induced anxiety is a primary contributing factor for relapse, and anxiolytics are a common adjuvant therapy prescribed for treatment-seeking alcoholics. It is thought that the use of alcohol to self-medicate and relieve anxiety contributes to the development of addiction. Treatment for anxiety disorders and alcoholism exist but are not universally effective. The delta opioid receptor (DOR) plays a role in both alcohol consumption and anxiety, making it a very interesting clinical target. Two pharmacologically distinct DORs have been described: DOR1 and DOR2. We find here that the relative specificity of DOR agonists for DOR1 or DOR2 can greatly affect the effects they exert on ethanol consumption and anxiety. The DOR1 agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydro-quinolino[2,3,30g]isoquinoline (TAN-67), although not effective in decreasing anxiety-like behavior in naive mice, has anxiolytic-like properties in ethanol-withdrawn mice. In contrast, a less subtype-selective agonist, (+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), while also reducing anxiety-like behavior, increases ethanol consumption. In addition, we found that the conical anxiolytic diazepam [DZ; 7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2(1H)-one] is a less effective anxiolytic in ethanol-withdrawn mice than in naive mice. Together, our findings suggest that selective DOR agonists can decrease anxiety-like behavior and are more effective than diazepam at reducing ethanol consumption. We believe the dual efficacy of DOR1 agonists makes these receptors an interesting therapeutic target for treatment-seeking alcoholics.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Mu-opioid receptors selectively regulate basal inhibitory transmission in the central amygdala: lack of ethanol interactions

Endogenous opioid systems are implicated in the actions of ethanol. For example, mu-opioid receptor (MOR) knockout (KO) mice self-administer less alcohol than the genetically intact counterpart wild-type (WT) mice (Roberts et al., 2000). MOR KO mice also exhibit less anxiety-like behavior than WT mice (Filliol et al., 2000). To investigate the neurobiological mechanisms underlying these behaviors, we examined the effect of ethanol in brain slices from MOR KO and WT mice using sharp-electrode and whole-cell patch recording techniques. We focused our study in the central nucleus of the amygdala (CeA) because it is implicated in alcohol drinking behavior and stress behavior. We found that the amplitudes of evoked inhibitory postsynaptic currents (IPSCs) or inhibitory postsynaptic potentials (IPSPs) were significantly greater in MOR KO mice than WT mice. In addition, the baseline frequencies of spontaneous and miniature GABA(A) receptor-mediated inhibitory postsynaptic currents were significantly greater in CeA neurons from MOR KO than WT mice. However, ethanol enhancements of evoked IPSP and IPSC amplitudes and the frequency of miniature IPSCs were comparable between WT and MOR KO mice. Baseline spontaneous and miniature excitatory postsynaptic currents (EPSCs) and ethanol effects on EPSCs were not significantly different between MOR KO and WT mice. Based on knowledge of CeA circuitry and projections, we hypothesize that the role of MOR- and GABA receptor-mediated mechanisms in CeA underlying reinforcing effects of ethanol operate independently, possibly through pathway-specific responses within CeA.

Restraint stress and ethanol consumption in two mouse strains

BACKGROUND

This study examined the interaction between restraint stress and ethanol drinking in mice that consume low and high amounts of ethanol.

METHODS

Two strains of mice (129SVEV and C57BL/6J) underwent 1 hour of restraint stress twice per day for 4 days in the presence of a CRF-1 receptor antagonist, a glucocorticoid receptor antagonist or vehicle. Ethanol preference and consumption were assessed using a two bottle choice design. In another study, mice were implanted with pellets containing corticosterone; ethanol preference and consumption were assessed using a two bottle choice design.

RESULTS

Restraint stress significantly increased ethanol preference and consumption in 129SVEV mice but not in C57BL/6J mice. Then 129SVEV mice underwent the identical stress procedure; however, mice received either the CRF-1 receptor antagonist, R121919 (15 or 20 mg/kg, ip) or vehicle 30 minutes prior to stress. R121919 did not block the stress-induced change in ethanol preference despite causing a significant blunting in the HPA axis. Negative results were also obtained using the CRF-1 receptor antagonist, Antalarmin (20 mg/kg, ip). In another study, 129SVEV mice were administered either the glucocorticoid receptor antagonist Mifepristone (25, 50 or 100 mug/kg, ip) or vehicle under the same procedure. Mifepristone did not alter ethanol preference. Moreover, the three receptor antagonist did not alter nonstress ethanol consumption either. In the last study, both mouse strains underwent active or sham adrenalectomy, then pellets containing corticosterone or placebo were implanted and preference for ethanol versus water was tested. Corticosterone administration decreased ethanol consumption in a strain-dependent manner.

CONCLUSION

These data show the restraint model for stress can modestly increase ethanol consumption in 129SVEV mice but not in C57BL/6J mice. Pharmacologic manipulation of CRF and corticosterone did not blunt baseline or stress-induced change in ethanol preference nor did administration of corticosterone mimic the effects of restraint stress on ethanol consumption. These findings suggest the mechanism responsible for increasing ethanol consumption in this model is independent of the HPA axis and extra-hypothalamic CRF.

Mouse inbred strain differences in ethanol drinking to intoxication

Recently, we described a simple procedure, Drinking in the Dark (DID), in which C57BL/6J mice self-administer ethanol to a blood ethanol concentration (BEC) above 1 mg/ml. The test consists of replacing the water with 20% ethanol in the home cage for 4 h early during the dark phase of the light/dark cycle. Three experiments were conducted to explore this high ethanol drinking model further. In experiment 1, a microanalysis of C57BL/6J behavior showed that the pattern of ethanol drinking was different from routine water intake. In experiment 2, drinking impaired performance of C57BL/6J on the accelerating rotarod and balance beam. In experiment 3, 12 inbred strains were screened to estimate genetic influences on DID and correlations with other traits. Large, reliable differences in intake and BEC were detected among the strains, with C57BL/6J showing the highest values. Strain means were positively correlated with intake and BEC in the standard (24 h) and a limited (4 h) two-bottle ethanol vs. water test, but BECs reached higher levels for DID. Strain mean correlations with other traits in the Mouse Phenome Project database supported previously reported genetic relationships of high ethanol drinking with low chronic ethanol withdrawal severity and low ethanol-conditioned taste aversion. We extend these findings by showing that the correlation estimates remain relatively unchanged even after correcting for phylogenetic relatedness among the strains, thus relaxing the assumption that the strain means are statistically independent. We discuss applications of the model for finding genes that predispose pharmacologically significant drinking in mice.

Presynaptic delta opioid receptors regulate ethanol actions in central amygdala

Endogenous opioid systems are implicated in the reinforcing effects of ethanol consumption. For example, delta opioid receptor (DOR) knockout (KO) mice show greater ethanol consumption than wild-type (WT) mice (Roberts et al., 2001). To explore the neurobiological correlates underlying these behaviors, we examined effects of acute ethanol application in brain slices from DOR KO mice using whole-cell patch recording techniques. We examined the central nucleus of amygdala (CeA) because the CeA is implicated in alcohol reinforcement (Koob et al., 1998). We found that the acute ethanol effects on GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were greater in DOR KO mice than in WT mice. Ethanol increased the frequency of miniature IPSCs (mIPSCs) significantly more in DOR KO mice than in WT mice. In CeA of WT mice, application of ICI 174864 [[allyl]2-Tyr-alpha-amino-isobutyric acid (Aib)-Aib-Phe-Leu-OH], a DOR inverse agonist, augmented ethanol actions on mIPSC frequency comparable with ethanol effects seen in DOR KO mice. Superfusion of the selective DOR agonist D-Pen(2),D-Pen(5)-enkephalin decreased the mean frequency of mIPSCs; this effect was reversed by the DOR antagonist naltrindole. These findings suggest that endogenous opioids may reduce ethanol actions on IPSCs of CeA neurons in WT mice through DOR-mediated inhibition of GABA release and that the increased ethanol effect on IPSCs in CeA of DOR KO mice could be, at least in part, due to absence of DOR-mediated inhibition of GABA release. This result supports the hypothesis that endogenous opioid peptides modulate the ethanol-induced augmentation of GABA(A) receptor-dependent circuitry in CeA (Roberto et al., 2003).

Increased consumption but not operant self-administration of ethanol in mice lacking the RIIbeta subunit of protein kinase A

BACKGROUND

Accumulating evidence indicates that adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) is involved in the neurobiological responses to ethanol. Previous reports indicate that mice lacking the RIIbeta subunit of PKA (RIIbeta(-/-)) voluntarily consume more ethanol than wild-type controls (RIIbeta(+/+)) using 2-bottle testing procedures. Although such procedures primarily measure consummatory behavior, operant self-administration procedures allow analysis of consummatory as well as appetitive or "ethanol-seeking" behavior (i.e., lever pressing is required to gain access to the ethanol solution). Therefore, we determined whether the high ethanol consumption characteristic of RIIbeta(-/-) mice would be complemented by increased appetitive ethanol-seeking behavior in an operant paradigm.

METHODS

RIIbeta(-/-) (n=8) and RIIbeta(+/+) (n=8) mice were initially sucrose-faded until they were lever responding for nonsweetened ethanol (10, 14, and 18%). Following the self-administration testing, RIIbeta(+/+) and RIIbeta(-/-) mice were given access to 2 bottles, one containing water and the other ethanol to replicate the voluntary ethanol drinking data previously from our laboratory. Finally, immediately after voluntary consumption all mice were again tested for self-administration of 10% ethanol. Alterations in the reinforcement schedule were also explored as RIIbeta(+/+) and RIIbeta(-/-) mice were tested for self-administration of 10% ethanol at FR-3 and FR-5 schedules.

RESULTS

The RIIbeta(-/-) mice displayed lower operant responding for ethanol and food reinforcement compared with RIIbeta(+/+) controls. However, this effect was driven by a significant increase in lever responses made by female RIIbeta(+/+) mice. When the excessive lever responses of the female RIIbeta(+/+) mice are accounted for, the RIIbeta(-/-) mice show ethanol lever responses comparable to controls. Following operant self-administration testing, RIIbeta(-/-) mice of both sexes consumed more ethanol solution compared with RIIbeta(+/+) mice during 2-bottle testing.

CONCLUSIONS

Increased ingestion of ethanol by RIIbeta(-/-) mice is likely the result of altered PKA activity within neuronal pathways that control ethanol-consummatory behaviors. Conversely, the RIIbeta subunit of PKA appears not to play a critical role in neuronal pathways that regulate appetitive behaviors directed at obtaining ethanol. Finally, increased operant self-administration of food and ethanol by female wild-type mice was absent in female RIIbeta(-/-) mice, suggesting that normal PKA signaling may be part of a general, and sex-dependent, mechanism involved with reinforcement-seeking behavior.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.

Schedule-induced alcohol drinking: non-selective effects of acamprosate and naltrexone

Acamprosate and naltrexone are therapeutically effective drugs that promote abstinence and prevent drinking relapse among alcohol-dependent patients, and dose-dependently decrease alcohol self-administration in animals. The purpose of this experiment was to investigate the behavioral specificity of acamprosate and naltrexone treatment in mice on alcohol drinking elicited in a schedule-induced polydipsia (SIP) task. Food-deprived male C57BL/6J (B6) mice were divided into three groups assigned to a 5% alcohol SIP, water SIP, or a 1-hour limited access regulatory water drinking task. Injections (intraperitoneal) of acute (0, 50, 100, 200, 400 mg/kg) and chronic (2 x 100 mg/kg, 10 days) acamprosate, or naltrexone (0, 1.0, 2.5, 5.0 mg/kg) were administered. Behavioral drug specificity was determined by comparing alterations in alcohol or water consumption in SIP with alterations in limited access drinking. Additionally, drug effects on drinking-specific measures (g/kg consumption and lick efficiency) were compared with those of non-drinking measures (head entries for food and locomotor activity) during SIP. In comparison with saline injections, acute acamprosate (400 mg/kg) reduced both alcohol and water drinking in both SIP and the regulatory drinking conditions, but had no significant effects on non-drinking measures. Chronic administration of acamprosate reduced both alcohol and water drinking during SIP, but did not significantly affect regulatory drinking or non-drinking measures. Naltrexone (1.0, 2.5, 5.0 mg/kg) reduced alcohol and water drinking in both paradigms, and at the highest dose, significantly reduced head entries for food. These results indicate that acamprosate (acute and chronic) and naltrexone are relatively non-selective in their effects on alcohol self-administration in this task.

Decreased oral self-administration of alcohol in kappa-opioid receptor knock-out mice

BACKGROUND

Although a large body of evidence suggests a role for the opioid system in alcoholism, the precise role of mu-, delta-, kappa-, and ORL1-opioid receptors and the physiological significance of their natural genetic variation have not been identified. The method of targeted gene disruption by homologous recombination has been used to knock out (KO) genes coding for opioid receptors, and study their effects on alcohol self-administration. Here we examined the effects of targeted disruption of kappa-opioid receptor (KOR) on oral alcohol self-administration and other behaviors.

METHODS

Oral alcohol, saccharin and quinine self-administration was assessed in a two-bottle choice paradigm using escalating concentrations of alcohol, or tastant solutions. In preference tests 12% alcohol, 0.033% and 0.066% saccharin, and 0.03 mM and 0.1 mM quinine solutions were used. Open-field activity was determined in an arena equipped with a computer-controlled activity-detection system. Subjects were tested for three consecutive days. Locomotor activity was assessed on days 1 and 2 (after saline injection, i.p.) and on day 3 (after alcohol injection, i.p.). Alcohol-induced locomotor activity was determined as the difference in activity between day 3 and day 2.

RESULTS

Male KOR KO mice in preference tests with 12% alcohol consumed about half as much alcohol as wild-type (WT) or heterozygous (HET) mice, showed lower preference for saccharin (0.033% and 0.066%) and higher preference to quinine (0.1 mM) than WT mice. Female KOR KO mice showed similar reduction in alcohol consumption in comparison to WT and HET mice. Partial deletion of KOR in HET mice did not change alcohol consumption in comparison to WT mice. In all genotype-groups females drank significantly more alcohol than males. MANOVA of locomotor activity among KO, WT, and HET mice indicated that strain and sex effects were not significant for alcohol-induced activation (p > 0.05), while strain x sex interaction effects on alcohol-induced activation could be detected (F(1,55) = 6.07, p < 0.05).

CONCLUSION

Our results indicating decreased alcohol consumption, lower saccharin preference, and higher quinine preference in KOR KO mice are in line with previous observations of opioid involvement in maintenance of food intake and raise the possibility that the deficient dynorphin/KOR system affects orosensory reward through central mechanisms which reduce alcohol intake and disrupt tastant responses, either as direct effects of absence of kappa-opioid receptors, or as effects of indirect developmental compensatory changes.

Stress enhancement of craving during sobriety: a risk for relapse

This report of the proceedings of a symposium presented at the 2004 Research Society on Alcoholism Meeting provides evidence linking stress during sobriety to craving that increases the risk for relapse. The initial presentation by Rajita Sinha summarized clinical evidence for the hypothesis that there is an increased sensitivity to stress-induced craving in alcoholics. During early abstinence, alcoholics who were confronted with stressful circumstances showed increased susceptibility for relapse. George Breese presented data demonstrating that stress could substitute for repeated withdrawals from chronic ethanol to induce anxiety-like behavior. This persistent adaptive change induced by multiple withdrawals allowed stress to induce an anxiety-like response that was absent in animals that were not previously exposed to chronic ethanol. Subsequently, Amanda Roberts reviewed evidence that increased drinking induced by stress was dependent on corticotropin-releasing factor (CRF). In addition, rats that were stressed during protracted abstinence exhibited anxiety-like behavior that was also dependent on CRF. Christopher Dayas indicated that stress increases the reinstatement of an alcohol-related cue. Moreover, this effect was enhanced by previous alcohol dependence. These interactive effects between stress and alcohol-related environmental stimuli depended on concurrent activation of endogenous opioid and CRF systems. A.D. Lê covered information that indicated that stress facilitated reinstatement to alcohol responding and summarized the influence of multiple deprivations on this interaction. David Overstreet provided evidence that restraint stress during repeated alcohol deprivations increases voluntary drinking in alcohol-preferring (P) rats that results in withdrawal-induced anxiety that is not observed in the absence of stress. Testing of drugs on the stress-induced voluntary drinking implicated serotonin and CRF involvement in the sensitized response. Collectively, the presentations provided convincing support for an involvement of stress in the cause of relapse and continuing alcohol abuse and suggested novel pharmacological approaches for treating relapse induced by stress.

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.

The Search for Genes Contributing to the Low Level of Response to Alcohol: Patterns of Findings Across Studies

BACKGROUND

Alcoholism is a complex genetically influenced disorder in which multiple phenotypes [e.g., disinhibition, alcohol-metabolizing patterns, and the low level of response (LR) to alcohol] contribute to the risk. A low LR to alcohol is one of the more thoroughly studied risk phenotypes; data indicate that LR relates to the risk status, predicts future alcoholism, and has a heritability as high as 60%. This article reviews data from animal and human studies regarding the LR to alcohol, searching for a convergence of results that might lead to the identification of relevant genes.

METHODS

A literature search was performed regarding animal and human genetic studies focusing on genes that might affect the LR to alcohol as a risk factor for alcoholism. The goal was to synthesize these results and highlight potential patterns.

RESULTS

Focusing on both genetic linkage and association studies, a number of chromosomal regions and genes potentially relevant to findings across two or more sources were identified. The genes of potential interest fell into several categories, including second-messenger systems (e.g., G proteins, adenylyl cyclase, and protein kinases); neurotransmitters or drug-related receptors (e.g., gamma-aminobutyric acid-A, glutamate, serotonin, and cannabinoid and opioid receptors); genes that affect alcohol metabolism; and genes that might relate to an overlap in the risk for alcoholism and some psychiatric conditions (e.g., catechol-O-methyltransferase regarding schizophrenia and bipolar disorder).

CONCLUSIONS

The review identifies several genes that may contribute to a low LR to alcohol and, thus, to an increased risk for alcohol use disorders. The chromosomal regions and genes highlighted here may form the basis for more focused genetic studies of alcohol use disorders, with the goals of developing more specific and effective prevention and treatment approaches.

Neuropeptides: implications for alcoholism

The role of neuromodulatory peptides in the aetiology of alcoholism has been relatively under-explored; however, the development of selective ligands for neuropeptide receptors, the characterization and cloning of receptors, and the development of transgenic mouse models have greatly facilitated this analysis. The present review considers the most recent preclinical evidence obtained from animal models for the role of two of the opioid peptides, namely b-endorphin and enkephalin; corticotropin-releasing factor (CRF), urocortin I and neuropeptide Y (NPY) in deleterious and excessive alcohol consumption, focussing on specific brain regions, in particular the central nucleus of the amygdala, that appear to be implicated in the pathophysiology of alcoholism. The review also outlines potential directions for further research to clarify neuropeptide involvement in neuromodulation within discrete brain nuclei pertinent to behavioural patterns.

Effect of chronic alcohol consumption on the activity of the hypothalamic-pituitary-adrenal axis and pituitary beta-endorphin as a function of alcohol intake, age, and gender

BACKGROUND

Experimental evidence indicates that components of the hypothalamic-pituitary-adrenal (HPA) axis and of the endogenous opioid system, such as beta-endorphin (beta-END), influence alcohol consumption, whereas chronic alcohol abuse alters the activity of both systems. Furthermore, gender and age differences have been reported in the activity of the HPA axis under basal conditions, in response to stress and acute alcohol challenge. The objective of the present studies was to investigate the hypothesis that chronic alcohol consumption alters the activity of the HPA axis and pituitary beta-END as a function of severity of alcohol abuse, gender, and age.

METHODS

Three age groups of each gender (18-29, 30-44, and 45-60 years old) were recruited. Each age and gender group included four subgroups: (a) nondrinkers, (b) light drinkers, (c) heavy drinkers, and (d) alcoholics in treatment. Demographic characteristics, alcohol consumption, and presence of alcohol dependence were recorded by using a structured interview. Blood samples were taken on the day of the interview. The levels of plasma adrenal corticotropic hormone (ACTH), cortisol, and beta-END were estimated as an index of the activity of the HPA-axis and pituitary beta-END.

RESULTS

Plasma ACTH and beta-END levels were significantly lower in females than males of all age and drinking category groups. Plasma cortisol levels were higher in 18- to 29-year-old female subjects compared with the 18- to 29-year-old male subjects. The plasma ACTH and beta-END levels were lower whereas plasma cortisol levels were higher in heavy drinkers than nondrinkers. This decrease in plasma ACTH and beta-END levels with heavy drinking was more pronounced in female than male subjects of the 30-44 and 45-60 age groups.

CONCLUSIONS

Chronic drinking, gender, and age influence the activity of the HPA-axis and pituitary beta-END, which in turn may influence drinking behavior.