Involvement of kappa type opioids on ethanol drinking

Δ 9-Tetrahydrocannabinol During Adolescence Reprograms the Nucleus Accumbens Transcriptome, Affecting Reward Processing, Impulsivity, and Specific Aspects of Cocaine Addiction-Like Behavior in a Sex-Dependent Manner

BACKGROUND

Cannabis exposure during adolescence is associated with emotional and motivational alterations that may entail an enhanced risk of developing psychiatric disorders. In rodent models, exposure to cannabinoids during adolescence leads to increased self-administration of opiates and cocaine, however, the psychological and neural mechanisms and the sex-specificity of this phenomenon are largely unknown.

METHODS

We exposed male and female adolescent rats to Δ9-tetrahydrocannabinol (THC) and studied at adulthood the effects of such treatment on psychological processes related to reward, such as Pavlovian conditioned approach, Pavlovian to instrumental transfer, habit formation and waiting impulsivity. In the light of these data and given the involvement of the nucleus accumbens in the processes examined, we performed an RNASeq transcriptomic study and assessed cocaine addiction-like behavior.

RESULTS

THC exposure increased goal-tracking (in males and females) and enhanced Pavlovian to instrumental transfer (especially in males) but did not affect habit formation. THC-exposed rats exhibited subtle, state-dependent changes in premature responding in the 2-CSRTT task. RNASeq data showed gene expression alterations in a marked sex-specific manner. While no effects were found on the acquisition of cocaine self-administration or punished drug-seeking, rats exposed to THC self-administered more cocaine under a progressive ratio schedule (males), had a higher rebound upon returning to continuous access to the drug (females) and showed reduced drug-seeking after 30 days of withdrawal (females).

CONCLUSIONS

Adolescent THC affects specific aspects of reward- (and cocaine-) guided behavior and the function of a key brain region mediating these effects, in a remarkable sex-specific manner.

Combination of Clinically Utilized Kappa-Opioid Receptor Agonist Nalfurafine With Low-Dose Naltrexone Reduces Excessive Alcohol Drinking in Male and Female Mice

BACKGROUND

Nalfurafine is the first clinically approved kappa-opioid receptor (KOP-r) agonist as an antipruritus drug with few side effects in humans (e.g., sedation, depression, and dysphoria). No study, however, has been done using nalfurafine on alcohol drinking in rodents or humans.

METHODS

We investigated whether nalfurafine alone or in combination with mu-opioid receptor (MOP-r) antagonist naltrexone changed excessive alcohol drinking in male and female C57BL/6J (B6) mice subjected to a chronic intermittent-access drinking paradigm (2-bottle choice, 24-hour access every other day) for 3 weeks. Neuronal proopiomelanocortin enhancer (nPE) knockout mice with brain-specific deficiency of beta-endorphin (endogenous ligand of MOP-r) were used as a genetic control for the naltrexone effects.

RESULTS

Single administration of nalfurafine decreased alcohol intake and preference in both male and female B6 mice in a dose-dependent manner. Pretreatment with nor-BNI (a selective KOP-r antagonist) blocked the nalfurafine effect on alcohol drinking, indicating a KOP-r-mediated mechanism. Pharmacological effects of a 5-dosing nalfurafine regimen were further evaluated: The repeated nalfurafine administrations decreased alcohol consumption without showing any blunted effects, suggesting nalfurafine did not develop a tolerance after the multidosing regimen tested. Nalfurafine did not produce any sedation (spontaneous locomotor activity), anhedonia-like (sucrose preference test), anxiety-like (elevated plus maze test), or dysphoria-like (conditioned place aversion test) behaviors, suggesting that nalfurafine had few side effects. Investigating synergistic effects between low-dose naltrexone and nalfurafine, we found that single combinations of nalfurafine and naltrexone, at doses lower than individual effective dose, profoundly decreased excessive alcohol intake in both sexes. The effect of nalfurafine on decreasing alcohol consumption was confirmed in nPE-/- mice, suggesting independent mechanisms by which nalfurafine and naltrexone reduced alcohol drinking.

CONCLUSION

The clinically utilized KOP-r agonist nalfurafine in combination with low-dose naltrexone has potential in alcoholism treatment.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

Role of the Dynorphin/Kappa Opioid Receptor System in the Motivational Effects of Ethanol

Evidence has demonstrated that dynorphin (DYN) and the kappa opioid receptor (KOR) system contribute to various psychiatric disorders, including anxiety, depression, and addiction. More recently, this endogenous opioid system has received increased attention as a potential therapeutic target for treating alcohol use disorders. In this review, we provide an overview and synthesis of preclinical studies examining the influence of alcohol (ethanol [EtOH]) exposure on DYN/KOR expression and function, as well as studies examining the effects of DYN/KOR manipulation on EtOH's rewarding and aversive properties. We then describe work that has characterized effects of KOR activation and blockade on EtOH self-administration and EtOH dependence/withdrawal-related behaviors. Finally, we address how the DYN/KOR system may contribute to stress-EtOH interactions. Despite an apparent role for the DYN/KOR system in motivational effects of EtOH, support comes from relatively few studies. Nevertheless, review of this literature reveals several common themes: (i) rodent strains genetically predisposed to consume more EtOH generally appear to have reduced DYN/KOR tone in brain reward circuitry; (ii) acute and chronic EtOH exposure typically up-regulate the DYN/KOR system; (iii) KOR antagonists reduce behavioral indices of negative affect associated with stress and chronic EtOH exposure/withdrawal; and (iv) KOR antagonists are effective in reducing EtOH consumption, but are often more efficacious under conditions that engender high levels of consumption, such as dependence or stress exposure. These results support the contention that the DYN/KOR system plays a significant role in contributing to dependence- and stress-induced elevation in EtOH consumption. Overall, more comprehensive analyses (on both behavioral and mechanistic levels) are needed to provide additional insight into how the DYN/KOR system is engaged and adapts to influence the motivation effects of EtOH. This information will be critical for the development of new pharmacological agents targeting KORs as promising novel therapeutics for alcohol use disorders and comorbid affective disorders.

Synergistic blockade of alcohol escalation drinking in mice by a combination of novel kappa opioid receptor agonist Mesyl Salvinorin B and naltrexone

Mesyl Salvinorin B (MSB) is a potent selective kappa opioid receptor (KOP-r) agonist that has potential for development as an anti-psychostimulant agent with fewer side-effects (e.g., sedation, depression and dysphoria) than classic KOP-r agonists. However, no such study has been done on alcohol. We investigated whether MSB alone or in combination with naltrexone (mu-opioid receptor antagonist) altered voluntary alcohol drinking in both male and female mice. Mice, subjected to 3weeks of chronic escalation drinking (CED) in a two-bottle choice paradigm with 24-h access every other day, developed rapid escalation of alcohol intake and high preference. We found that single, acute administration of MSB dose-dependently reduced alcohol intake and preference in mice after 3-week CED. The effect was specific to alcohol, as shown by the lack of any effect of MSB on sucrose or saccharin intake. We also used the drinking-in-the-dark (DID) model with limited access (4h/day) to evaluate the pharmacological effect of MSB after 3weeks of DID. However, MSB had no effect on alcohol drinking after 3-week DID. Upon investigation of potential synergistic effects between naltrexone and MSB, we found that acute administration of a combination of MSB and naltrexone reduced alcohol intake profoundly after 3-week CED at doses lower than those individual effective doses. Repeated administrations of this combination showed less tolerance development than repeated MSB alone. Our study suggests that the novel KOP-r agonist MSB both alone and in combination with naltrexone shows potential in alcoholism treatment models.

Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression

BACKGROUND

Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression.

METHODS

Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy.

RESULTS

Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH.

CONCLUSIONS

Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Exposure to ethanol on prenatal days 19-20 increases ethanol intake and palatability in the infant rat: involvement of kappa and mu opioid receptors

Prenatal exposure to ethanol on gestation Days 19-20, but not 17-18, increases ethanol acceptance in infant rats. This effect seems to be a conditioned response acquired prenatally, mediated by the opioid system, which could be stimulated by ethanol's pharmacological properties (mu-opioid receptors) or by a component of the amniotic fluid from gestation-day 20 (kappa-inducing factor). The latter option was evaluated administering non-ethanol chemosensory stimuli on gestation Days 19-20 and testing postnatal intake and palatability. However, prenatal exposure to anise or vanilla increased neither intake nor palatability of these tastants on postnatal Day 14. In experiment 2, the role of ethanol's pharmacological effect was tested by administering ethanol and selective antagonists of mu and kappa opioid receptors prenatally. Blocking the mu-opioid receptor system completely reversed the effects on intake and palatability, while antagonizing kappa receptors only partially reduced the effects on palatability. This suggests that the pharmacological effect of ethanol on the fetal mu opioid system is the appetitive reinforcer, which induces the prenatally conditioned preference detected in the preweanling period.

Involvement of kappa-opioid and endocannabinoid system on Salvinorin A-induced reward

BACKGROUND

The recreational drug, Salvinorin A, derived from the plant of Salvia divinorum, is a potent and selective kappa-opioid receptor agonist. The abuse of selective k-agonists is a novel phenomenon, the mechanism of which is not fully understood.

METHODS

We investigated salvinorin A given SC on the conditioned place preference (.05-160 microg/kg) and intracerebroventricular (ICV) self-administration (.01-1 microg/infusion) paradigms, in Wistar rats.

RESULTS

The present results demonstrate the rewarding effects of Salvinorin A in a range of doses between .1 and 40 microg/kg SC for conditioned place preference test and .1-.5 microg/infusion for ICV self-administration. Highest doses (160 microg/kg for conditioned place preference test and 1 microg/infusion for ICV self-administration) were aversive. The rewarding effect was antagonized by intraperitoneal (IP) pretreatment with the cannabinoid CB(1) receptor antagonist, rimonabant [N-piperidino-5-(4-chlorophenyl)1-(2,4-dichloro phenyl)-4 methyl pyrazole 3-carboxamide] (1 mg/kg), and the kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI) (10 mg/kg). In the shell of nucleus accumbens, dopamine extracellular levels were increased after administration of salvinorin A (40 microg/kg SC), reaching a maximum value of about 150%.

CONCLUSIONS

These data provide the demonstration of the rewarding effects of Salvinorin A through an interaction between kappa-opioid and (endo)cannabinoid system in rats.

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

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

A Microdialysis Profile of Dynorphin A1-8 Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence endorphinergic and enkephalinergic activity, while very few studies have examined its effect on dynorphinergic systems. The aim of the present study was to investigate the effect of alcohol administration or a mechanical stressor on extracellular levels of dynorphin A(1-8) in the rat nucleus accumbens-a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.5 microL/min in awake and freely moving animals and the dialysate was collected at 30-minute intervals. In one experiment, following a baseline period, rats were injected intraperitoneally with either physiological saline or 1 of 3 doses of alcohol, 0.8, 1.6, or 3.2 g ethanol/kg body weight. In a second experiment, following a baseline period, rats were applied a clothespin to the base of their tail for 20 minutes. The levels of dynorphin A(1-8) in the dialysate were analyzed with solid-phase radioimmunoassay.

RESULTS

Relative to saline-treated controls, an alcohol dose of 1.6 and 3.2 g/kg caused a transient increase in the extracellular levels of dynorphin A(1-8) in the first 30 minutes of alcohol administration. However, the effect resulting from the high 3.2 g/kg dose was far more pronounced and more significant than with the moderate dose. There was no effect of tail pinch on dynorphin A(1-8) levels in the nucleus accumbens.

CONCLUSIONS

In this experiment, a very high dose of alcohol was especially capable of stimulating dynorphin A(1-8) release in the nucleus accumbens. Dynorphin release in the accumbens has been previously associated with aversive stimuli and may thus reflect a system underlying the aversive properties of high-dose alcohol administration. However, the lack of effect of tail-pinch stress in the present study suggests that dynorphin A(1-8) is not released in response to all forms of stressful/aversive stimuli.

A single injection of the kappa opioid antagonist norbinaltorphimine increases ethanol consumption in rats

RATIONALE

Kappa opioid receptor (KOR) agonists interfere with the reinforcing effects of drugs of abuse. KOR agonists decrease heroin, cocaine, and ethanol self-administration, and block heroin and cocaine conditioned place preference (CPP) in rats. However, KOR agonists also produce emesis and dysphoria, making it difficult to determine if their effects on self-administration are due to an action on reward mechanisms or are secondary to the drug's direct aversive effects. Assuming that endogenous KOR ligands modulate circuits involved in drug and alcohol reward, selective KOR antagonists can be used to clarify these issues. If KOR antagonists increase drug self-administration then it is likely that endogenous KOR agonists directly modulate drug intake.

OBJECTIVES

To determine the effects of nor-BNI, the highly selective KOR antagonist, on ethanol consumption and CPP.

METHODS

Thirty-eight male Lewis rats were given free access to ethanol until stable self-administration was achieved. Animals were then administered a single injection of nor-BNI (10 mg kg(-1)) while ethanol intake was monitored.

RESULTS

A single injection of nor-BNI induces a long-lasting increase in ethanol consumption, but does not induce a CPP. A high/low split revealed that this effect was primarily due to an increase in drinking in nor-BNI-treated high drinkers, which drank significantly more than saline-treated high drinkers and also drank significantly more when compared to their own pretreatment baseline.

CONCLUSIONS

Blocking the KOR system increases ethanol self-administration, suggesting that the decrease in self-administration seen with KOR agonists is due to a direct modulation of reward circuitry.

Predictors of high ethanol consumption in RIIbeta knock-out mice: assessment of anxiety and ethanol-induced sedation

BACKGROUND

Genetic and pharmacological evidence suggests that the cyclic adenosine monophosphate-dependent protein kinase A pathway modulates neurobiological responses to ethanol. Mutant mice lacking the RIIbeta subunit of protein kinase A (RIIbeta(-/-)) are resistant to ethanol-induced sedation and drink significantly more ethanol than littermate wild-type mice (RIIbeta(+/+)). We determined whether high ethanol intake by the RIIbeta(-/-) mice on alternate genetic backgrounds is reliably predicted by high basal levels of anxiety or resistance to the sedative effects of ethanol.

METHODS

Two-bottle choice procedures and a battery of behavioral tests (elevated plus maze, open-field activity, and zero maze) were used to assess voluntary ethanol consumption and basal levels of anxiety in RIIbeta(-/-) and RIIbeta(+/+) mice on either a C57BL/6J or a 129/SvEv x C57BL/6J genetic background. Additionally, ethanol-induced sedation and blood ethanol levels were determined in RIIbeta(-/-) and RIIbeta(+/+) mice after intraperitoneal injection of ethanol (3.8 g/kg).

RESULTS

RIIbeta(-/-) mice on both genetic backgrounds consumed more ethanol and had a greater preference for ethanol relative to RIIbeta(+/+) mice. However, RIIbeta(-/-) mice showed reduced basal levels of anxiety when maintained on the C57BL/6J background but showed increased anxiety when maintained on the 129/SvEv x C57BL/6J background. Consistent with prior research, RIIbeta(-/-) mice were resistant to the sedative effects of ethanol, regardless of the genetic background. Finally, RIIbeta(-/-) and RIIbeta(+/+) mice showed similar blood ethanol levels.

CONCLUSIONS

These results indicate that high ethanol consumption is associated with resistance to the sedative effects of ethanol but that basal levels of anxiety, as well as ethanol metabolism, do not reliably predict high ethanol drinking by RIIbeta(-/-) mice.

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.

Anabolic androgenic steroids affects alcohol intake, defensive behaviors and brain opioid peptides in the rat

The present study investigated whether a relationship exists between nandrolone decanoate and voluntary ethanol intake in laboratory rats. Animals were subjected to daily subcutaneous injections with nandrolone decanoate (15 mg/kg) during 2 weeks. One group of animals was tested for voluntary alcohol intake 1 week after the end of the 2-week treatment period and another group received alcohol 3 weeks after the treatment. In addition, assessment of defensive behaviors and immunoreactivity (ir) levels of the brain opioid peptides dynorphin B and Met-enkephalin-Arg-Phe (MEAP) were performed. The nandrolone decanoate-treated animals were significantly more aggressive and showed lower fleeing and freeezing reaction than the oil-treated controls. Treatment with nandrolone decanoate enhanced voluntary alcohol intake, regardless if it was presented 1 or 3 weeks after end of the treatment period. These animals had a decreased activity of dynorphin B-ir in the nucleus accumbens, decreased levels of MEAP-ir in the periaqueductal gray (PAG) and higher levels of MEAP-ir in the hypothalamus compared to controls. In line with previous studies, this suggests that the altered dynorphin B-ir activity may promote the rewarding effects of ethanol and thereby increasing alcohol intake, whereas MEAP-ir may be associated with the ability to control the aggressive reaction. Abuse of nandrolone decanoate may thus constitute a risk factor for increased alcohol consumption and defensive aggression. In human, this constellation of behavioral symptoms is closely related to acts of crimes and violence and is often observed among those abusing anabolic androgenic steroids.

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.

Comparison of the proopiomelanocortin and proenkephalin opioid peptide systems in brain regions of the alcohol-preferring C57BL/6 and alcohol-avoiding DBA/2 mice

Differences in the activity of distinct components of the endogenous opioid system between ethanol-preferring and ethanol-avoiding animals may be important in controlling their voluntary alcohol consumption. The objective of the present studies was to compare the activity of two opioid peptide systems (enkephalin and beta-endorphin) in distinct regions of the brain, between the C57BL/6 and DBA/2 mice, using sensitive radioimmunoassays, in situ hybridization, and immunohistochemical techniques. The immunohistochemical studies indicated that there was no significant difference in the number of either beta-endorphin or enkephalin immunopositive cells between the C57BL/6 and DBA/2 mice. The in situ hybridization studies demonstrated a 27% higher content of proopiomelanocortin mRNA in the arcuate nucleus of the C57BL/6 than DBA/2 mice, p < 0.02. The content of proenkephalin mRNA was 25% higher in the nucleus accumbens, p < 0.005 and 23% higher in the caudate putamen, p < 0.01, of the C57BL/6 than DBA/2 mice. There was no significant difference in the content of beta-endorphin peptides in the distinct brain regions investigated. The content of met-enkephalin-arg6-phe7 in the nucleus accumbens and caudate was similar between the two strains of mice, while it was significantly lower in the amygdala, hippocampus, ventral tegmental area, and periaqueductal grey of the C57BL/6 than DBA/2 mice. Thus, there are significant differences in the activity of these two endogenous opioid peptide systems in distinct regions of the brain, between ethanol naive C57BL/6 and DBA/2 mice, which may play a role in controlling their alcohol consumption.

Nalmefene causes greater hypothalamic-pituitary-adrenal axis activation than naloxone in normal volunteers: implications for the treatment of alcoholism

Among other actions, opioid antagonists modulate the control endogenous opioids exert on the hypothalamic-pituitary-adrenal (HPA) axis. Naloxone, nalmefene, and naltrexone are the opioid antagonists approved for use in man and are primarily mu-opioid selective. Naltrexone and nalmefene have been demonstrated to be useful in the treatment of alcoholism. Compared with naloxone, nalmefene has a longer half-life, is more potent at the mu-receptor, and has a higher affinity for kappa- and delta-opioid receptors. We conducted an inpatient study comparing the effects of 10 and 30 mg doses of intravenous naloxone and nalmefene in normal, nonsubstance nor alcohol-abusing, volunteers. Significant increases in ACTH and cortisol were observed after both antagonists, without an apparent dose-response relationship; however, both doses of nalmefene resulted in greater HPA axis activation than either dose of naloxone (ACTH: p <0.005). These results indicate that kappa- and delta-opioids may play important roles in the regulation of the HPA axis; nalmefene may be useful as both a probe to explore the HPA axis physiology and as a pharmacotherapeutic agent.

Differences in the kappa opioid receptor mRNA content in distinct brain regions of two inbred mice strains

The inbred C57BL/6 mouse strain is known to prefer an ethanol solution in a two-bottle choice procedure, whereas the inbred DBA/2 mouse strain avoids drinking ethanol in this paradigm. The genetic basis of this behavior is still unclear but the endogenous opioid system is one of the factors thought to be involved. Therefore, we were interested to see if there are basal differences between the two lines of mice in the kappa opioid receptor (kappa OR) mRNA content in different brain regions. Because of the low expression level of this gene and the limited amount of tissue we developed a sensitive competitive reverse transcription-polymerase chain reaction (RT-PCR) assay for evaluation of levels of kappa OR mRNA in brain tissue. In septum and hypothalamus the DBA/2 mice showed a significantly higher basal level of kappa OR mRNA than did C57BL/6 mice. It is suggested that a difference in basal amount of kappa OR mRNA among these strains could lead to differences in kappa OR activity and subsequently to variations between the strains in distinct behaviors such as in ethanol preference.

Opioid involvement in alcohol drinking

A large body of evidence indicates that the endogenous opioid system plays an important role in maintaining alcohol drinking behavior. Evidence is reviewed that indicates that the reinforcing properties of alcohol that lead to continued and repeated bouts of drinking may be due, in part, to alcohol-induced activation of the endogenous opioid system. Much of this evidence is pharmacologic in nature. Blocking the action of endogenous opioid peptides via administration of opioid antagonists significantly attenuates alcohol consumption in animals under a variety of experimental conditions. In clinical trials, opioid receptor antagonists decrease alcohol consumption, relapse rates, subjective high, and alcohol craving in outpatient alcoholics. The potential clinical utility of opioid receptor antagonists in the treatment of alcoholism and alcohol dependence is discussed.

Genetics of alcoholism: role of the endogenous opioid system

At the present time alcoholism is recognized as a metabolic disease exhibiting the clinical features of craving for alcohol, loss of control over drinking, tolerance and physical dependence on alcohol, while both epidemiological and experimental studies have demonstrated that genetic factors may be important in determining whether an individual has a high or low vulnerability to develop alcoholism. Evidence also indicates that alcoholism is not characterized by a single gene single allele inheritance. Instead it seems that multiple genes and environmental factors interact to increase or decrease an individual's vulnerability to become an alcoholic. Current research is aimed at investigating whether certain behavioral, physiological and biochemical markers are highly associated with the incidence of alcoholism. Among the biochemical markers currently under investigation is the endogenous opioid system and its implication in mediating the reinforcing effects of ethanol. It is the objective of this manuscript to review current research on: (a) the interactions of ethanol with the endogenous opioid system at the molecular level; (b) the existence of genetically determined differences in the response of the endogenous opioid system to ethanol between subjects at high and low risk for excessive ethanol consumption, as well as between lines of animals showing preference or aversion for ethanol solutions; (c) the decrease of alcohol consumption following pretreatment with opioid antagonists; and (d) the possible use of specific opioid receptor antagonists together with behavioral therapy to modify drinking behavior, to control craving and to prevent relapse.

Reducing the desire to drink. Pharmacology and neurobiology

The past decade has witnessed major advances in understanding of neural functioning and neurobiological bases of alcohol consumption. Concurrent with this, a range of exciting investigations have been conducted on pharmacologic agents that may curb drinking behavior. Research is reviewed on several promising medications influencing neurotransmitter and endocrine systems with particular attention to the serotonergic and opioid systems. Following this overview, recommendations are offered regarding research methodology to support future pharmacotherapy trials.

The effect of single and repeated ethanol administration of hypothalamic opioid systems activity --an in vitro release study

The effect of ethanol administration on the in vitro release of alpha-neoendorphin (ANEO) and Met-enkephalin-Arg6-Gly7-Leu8 (MEAGL), peptides derived from prodynorphin and proenkephalin, respectively, was studied in the rat hypothalamus. Single ethanol administration had no effect on the tissue level and potassium-stimulated release of these peptides. Repeated ethanol administration increased the release of ANEO and reduced the release of MEAGL from hypothalamic slices. Two days after cessation of ethanol administration the release of MEAGL returned to control values, whereas the release of ANEO was significantly inhibited. On the other hand, the tissue level of those peptides remained unchanged after repeated ethanol or 2 days after its last administration. The present study shows that repeated treatment with ethanol may lead to an increase in the prodynorphin neurons' sensitivity to the depolarizing effect of potassium. In contrast, the sensitivity of the proenkephalin system to ethanol seems to be inhibited. Thus, ethanol appears to have an opposite effect on the sensitivity of hypothalamic proenkephalin and prodynorphin neurons.

D-Ala2-Met5-enkephalinamide impairs the acquisition of ethanol preference without influencing sucrose preference

The effects of subcutaneous (SC) administration of D-Ala2-Met5-enkephalinamide (DAME) (1, 10 and 100 micrograms/kg), synthetic analog of Met-enkephalin, on the acquisition of ethanol preference were studied in male Wistar rats. Under our procedural conditions, rats develop ethanol preference by a forced ethanol drinking session (conditioning session). Preconditioning administration of DAME (100 micrograms/kg) induced a reduction in ethanol consumption on the day of treatment and on subsequent testing days, but did not reliably modify later ethanol preference. Postconditioning administration of DAME (1, 10 and 100 micrograms/kg) markedly impaired the acquisition of ethanol preference. However, under the same schedule of treatment, DAME failed to affect subsequent rats' sucrose preference. These results suggest that, when administered after rats' first exposure to ethanol, DAME could interfere either with the reinforcement mechanisms of ethanol consumption induced by its intake, or with the storage of the information related to the ethanol incentive value.

Effects of the kappa opioid receptor antagonist MR-2266-BS on the acquisition of ethanol preference

Using a paradigm by which rats forced to drink a weak ethanol solution (2.5% w/v) (conditioning session) develop ethanol preference in consecutive retention testing days, the effects of the administration of the kappa opioid antagonist MR-2266-BS, prior to or after the forced ethanol session, were studied. Pre-conditioning subcutaneous (s.c.) administration of 1 mg/kg of MR-2266-BS induced a decrease in subsequent ethanol consumption without significantly modifying the acquisition of ethanol preference. Post-conditioning administration of MR-2266-BS (0.1, 1, 5 or 10 mg/kg) induced both a dose-dependent reduction in ethanol consumption and in preference throughout the three following days. The results of the present study provide further support of the involvement of kappa-type opioids on drinking behavior, and suggest that kappa receptors may be involved in the consumption and development of preference to ethanol.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Effects of ethanol in an open field apparatus: modification by U50488H and WIN 44441-3

The effects of U50488H, a kappa agonist, and WIN 44441-3, a kappa antagonist, and their modification of the effects of ethanol, on the behavior of rats in a modified open field apparatus, was examined. Crossover activity was increased by U50488H. Headpoke activity was decreased by WIN 44441-3 and increased by U50488H. Rearing activity was increased by WIN 44441-3 but was not affected by U50488H. The effect of both drugs was dose related, with the largest doses having no effect. Ethanol (0.5 g/kg) stimulated crossover activity while it depressed rearing, headpoke and corner activities; except for crossover activity the 2.0 g/kg dose of ethanol depressed these activities. Pretreatment with WIN 44441-3 (0.5 mg/kg) potentiated the stimulant effect of ethanol on crossover activity and partially reversed the depressant effect of ethanol on rearing and headpoke activities. U50488H potentiated the ethanol-induced depression of headpoke and reversed the depression of corner activity. Pretreatment with U50488H had no effect on ethanol's action on crossover and rearing behaviors. Our results indicate that kappa opiate receptors may mediate some behaviors exhibited by rats in a modified open field apparatus. Activation of these receptors increases locomotor and headpoke activity but had no effect on rearing activity. Furthermore, the 0.5 g/kg dose of ethanol has differential effects on different measures of open field behavior, while the 2.0 g/kg dose was largely depressant. Our data suggest that some of these effects of ethanol may be mediated via kappa opioid receptors.