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

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

κ-opioid receptors are implicated in the increased potency of intra-accumbens nalmefene in ethanol-dependent rats

Previously, it was shown that ethanol-dependent animals display increased sensitivity to the general opioid receptor antagonist nalmefene compared to naltrexone. It was hypothesized that the dissociable effects of the two antagonists were attributable to a κ-opioid receptor mechanism. Nucleus accumbens dynorphin is upregulated following chronic ethanol exposure and such neuroadaptations could contribute to nalmefene's increased potency in ethanol-dependent animals. To test this hypothesis, male Wistar rats were trained to self-administer ethanol using an operant conditioning procedure. Animals were then implanted with bilateral intra-accumbens shell guide cannulae and assigned to either a chronic intermittent ethanol vapor-exposure condition (to induce dependence) or an air-exposed control group. Following a one-month exposure period, nalmefene, nor-binaltorphimine (nor-BNI; selective for κ-opioid receptors) or a combination of the selective opioid receptor antagonists CTOP and naltrindole (selective for the μ- and δ-opioid receptors, respectively) were site-specifically infused into the nucleus accumbens shell prior to ethanol self-administration sessions during acute withdrawal. Nalmefene and CTOP/naltrindole dose-dependently reduced ethanol self-administration in nondependent and dependent animals, whereas nor-BNI selectively attenuated ethanol self-administration in ethanol-dependent animals without affecting the self-administration of nondependent animals. Further analysis indentified that intra-accumbens shell nalmefene was more potent in ethanol-dependent animals and that the increased potency was attributable to a κ-opioid receptor mechanism. These data support the concept that dysregulation of DYN/κ-opioid receptor systems contributes to the excessive self-administration observed in dependent animals and suggest that pharmacotherapeutics for ethanol dependence that target κ-opioid receptors, in addition to μ- and δ-opioid receptors, are preferable to those that target μ- and δ-opioid receptor mechanisms alone.     

Effects of oral loperamide on efficacy of naltrexone, baclofen and AM-251 in blocking ethanol self-administration in rats

Naltrexone is a μ-opioid receptor antagonist that has been extensively studied for its ability to block the rewarding effects of ethanol. Opioid receptors are widely distributed within the gastrointestinal tract (GIT). Typically, naltrexone is administered by parenteral routes in nonclinical studies. We initially tested if opioid receptors within the GIT would influence the ability of oral naltrexone to inhibit ethanol oral self-administration in rats using the co-administration of oral loperamide, a peripherally restricted opioid agonist. As expected, oral naltrexone only had modest effects on ethanol intake, and the response was not dose-dependent. However in rats, treatment with loperamide prior to the administration of naltrexone resulted in a suppression of ethanol intake which approached that observed with naltrexone given by the subcutaneous (SC) route. Importantly, administration of loperamide prior to administration of naltrexone did not alter blood concentrations of naltrexone. We then evaluated if oral loperamide would enhance effects of baclofen (a GABA_B receptor agonist) and AM-251 (a CB-1 receptor antagonist) and found that pre-treatment with loperamide did potentiate the action of both drugs to reduce ethanol self-administration. Finally, the specific opioid receptor type involved was investigated using selective μ- and κ-receptor antagonists to determine if these would affect the ability of the AM-251 and loperamide combination to block ethanol drinking behavior. The effect of loperamide was blocked by ALKS 37, a peripherally restricted μ-receptor antagonist. These data suggest an important role for opioid receptors within the GIT in modulating central reward pathways and may provide new insights into strategies for treating reward disorders, including drug dependency.     

Dissociation between opioid and CRF1 antagonist sensitive drinking in Sardinian alcohol-preferring rats

Rationale The role of positive vs negative ethanol reinforcement in ethanol intake of Sardinian alcohol-preferring (sP) rats is unclear.Objectives To test the hypothesis that spontaneous ethanol self-administration of sP rats was sensitive to the opioid receptor antagonist naltrexone, whereas withdrawal-induced, but not spontaneous, ethanol self-administration would be sensitive to corticotropin-releasing factor₁ (CRF₁) antagonists, implicating differential roles for positive and negative reinforcement, respectively.Methods Male sP rats operantly (FR1, 30 min/day) self-administered ethanol (10% v/v) until responding stabilized. One group (n=11) was made ethanol dependent through intermittent ethanol vapor exposure. Both nondependent (n = 10) and dependent rats received the CRF₁ antagonist LWH-63 (5, 10, and 20 mg/kg, s.c.). Separate nondependent sP rats (n = 10) received the opioid antagonist naltrexone (16, 50, 150, and 450 μg/kg, s.c.). Finally, CRF₁ antagonists (MJL-1-109-2, LWH-63, and R121919) were studied for their actions on home-cage ethanol drinking in nondependent sP rats (n = 6–8/group) under continuous, limited-access, or stressed conditions.Results Naltrexone potently reduced ethanol self-administration in nondependent sP rats. LWH-63 reduced heightened ethanol self-administration of vapor-sensitive, dependent sP rats. CRF₁ antagonists did not reduce ethanol intake in nondependent sP rats. R121919 (10 mg/kg, s.c.) retained antistress activity in sP rats, blunting novelty stress-induced suppression of ethanol intake.Conclusions Spontaneous ethanol self-administration of sP rats was opioid dependent with CRF₁ receptors implicated in withdrawal-induced drinking. Opioid and CRF₁ receptors play different roles in ethanol reinforcement and perhaps the ethanol addiction cycle. Such distinctions may apply to subtypes of alcoholic patients who differ in their motivation to drink and ultimately treatment response.V.S. and P.C. contributed equally to this work.Supported by NIAAA Alcohol Research Center Grant P60 AA0006420-21 (G.F.K., E.P.Z.), NIDDK 64871 (E.P.Z.), by the Intramural Research Program of the NIH, the National Institute of Diabetes and Digestive and Kidney Diseases, and the National Institute on Drug Abuse (M.J.L. and K.C.R.), by the Pearson Center for Alcoholism and Addiction Research (G.F.K., E.P.Z.), by a merit fellowship award from the University of Palermo (V.S.), and by a merit fellowship award from the University of Rome La Sapienza (P.C.).     

Inhibition of kappa opioid receptors attenuated increased cocaine intake in rats with extended access to cocaine

Objective  Previous studies demonstrated that the dynorphin/κ opioid system was up-regulated upon repeated cocaine self-administration. In the present study, we tested the hypothesis that increased cocaine self-administration with extended access was associated with increased activity of the κ opioid system in rats. Materials and methods  Rats self-administered 0.5 mg/kg per injection of cocaine on a fixed-ratio (FR) schedule in either 1-h (short access, ShA) or 6-h (long access, LgA) sessions. After cocaine intake in the LgA rats increased to a maximum, the effects of κ opioid receptor antagonists and a partial agonist were tested on cocaine intake in ShA and LgA rats. Results  Cocaine self-administration increased under FR and progressive-ratio (PR) schedules in LgA rats. Nor-BNI (15–30 mg/kg), a κ receptor antagonist, decreased cocaine intake in LgA rats under a PR schedule (ShA, +1.7%; LgA, −27.4% from baseline), whereas naltrexone (0.3–10 mg/kg) and SG-II-49 (0.025–0.1 mg/kg), a nonspecific opioid receptor antagonist and a partial agonist, respectively, decreased cocaine intake in both groups (PR data: SG-II-49, ShA −28.6%, LgA −19.8%; naltrexone, ShA −34.6%, LgA −11.8% compared with vehicle data). Conclusions  The present study demonstrated that the antagonism of κ opioid receptors attenuated only the increased cocaine intake in LgA rats under a PR schedule, whereas the antagonism of μ and κ receptors decreased cocaine intake in both ShA and LgA groups. The data suggest that increased motivation for cocaine in rats with extended access may be related to increased κ opioid activity and may contribute to compulsive use. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Opioids and rate of positively reinforced behavior: differential antagonism by naltrexone

Lever pressing by rats was maintained under a fixed-ratio 30 schedule of food presentation. The response rate-decreasing effects of several opioid compounds that vary in selectivity for, and activity at, μ, kappa, and non-opioid receptors, were examined alone and in combination with the opioid antagonist naltrexone. Naltrexone (0.01-1.0mg/kg) produced dose-dependent and generally parallel rightward shifts in the dose-effect curves for morphine, fentanyl, butorphanol and nalbuphine. Apparent pA(2) values for naltrexone against these agonists ranged from 7.05 to 7.29, and the slopes of the regression lines fitted to the Schild plots approximated theoretical unity (-1.0), suggesting a competitive interaction at μ-opioid receptors. In contrast, although at least one dose of naltrexone (0.01-10.0mg/kg) antagonized the response rate-decreasing effects of bremazocine, U50,488, (-)-pentazocine and nalorphine, suggesting some opioid activity, these effects differed from those of the μ agonists in that: (a) they were less sensitive to naltrexone antagonism; (b) maximal rightward shifts were smaller; (c) antagonism patterns were not directly related to naltrexone dose and, in some cases, were influenced by the response rate-decreasing effects of the larger naltrexone doses; and (d) there were considerable between-subjects differences in sensitivity to naltrexone antagonism. Naltrexone (0.1-10.0mg/kg) did not antagonize the effects of the non-opioid control compound pentobarbital. The present results suggest that patterns of naltrexone antagonism can provide a basis for making inferences about receptor activity related to the effects of some opioids on rate of schedule-controlled behavior. With some opioids, however, such inferences are limited by the direct response rate-decreasing effects of naltrexone itself and by differences in patterns of antagonism across subjects.     

Diminution of contractile response by κ-opioid receptor agonists in isolated rat ventricular cardiomyocytes is mediated via a pertussis toxin-sensitive G protein

Opioids directly decrease the contractile response of isolated ventricular cardiomyocytes to electrical stimulation. To investigate whether these effects are mediated via GTP-binding G_i/o proteins we examined the influence of pertussis toxin on the effects of the κ-opioid receptor agonist trans-(±)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide (U-50,488) methanesulphonate and on the as yet undescribed effects of the opioid peptide dynorphin A (1–8) on contraction. In isolated, electrically driven, rat ventricular cardiomyocytes both agents concentration dependently reduced cell shortening within 15 min, decreasing the contractile response by 79±4% (n=5) and 62±2% (n=6) of control values at maximal effective concentrations of 10 μM (U-50,488) and 1 μM [dynorphin A (1–8)], respectively. Pertussis toxin pre-treatment (200 ng/ml; 4.5–5 h) completely abolished the effects of U-50,488 and dynorphin A (1–8) on the contractile response, indicating that these effects are mediated via G_i/o proteins. In addition, the non-selective opioid receptor antagonist (–)-naloxone and the κ-opioid receptor antagonist nor-binaltorphimine antagonized the effects of U-50,488 and dynorphin A (1–8) on the contractile response. Furthermore, the μ- and δ-opioid receptor agonist (D-Ala², D-Leu⁵)-enkephalin (DADLE) had no effects on contraction. These results indicate that the decrease in cell shortening is due to stimulation of κ-opioid receptors. The direct effect of κ-opioid receptor agonists on the contractile response thus represents an additional mechanism for decreasing cardiac contractility, besides the M-cholinoceptor- or adenosine receptor-mediated pathway. It is conceivable that increased release of endogenous dynorphins from the heart during hypoxia may protect the heart in a similar manner to adenosine. Received: 16 September 1997 / Accepted: 15 June 1998     

Endogenous opioid systems and alcohol addiction

 Alcohol exerts numerous pharmacological effects through its interaction with various neurotransmitters and neuromodulators. Among the latter, the endogenous opioids play a key role in the rewarding (addictive) properties of ethanol. Three types of opioid receptors (μ, δ and κ) represent the respective targets of the major opioid peptides (β-endorphin, enkephalins and dynorphins, respectively). The rewarding (reinforcing) properties of μ- and δ-receptor ligands are brought about by activation of the mesolimbic dopamine system which ascends from the ventral tegmentum of the midbrain (VTA) to rostral structures; of these, the nucleus accumbens (NAC) is of particular importance in drug addiction. In contrast, dysphoria results from activation of κ-receptors. The neurochemical manifestations of these opposing effects are, respectively, increases and decreases in dopamine release in the NAC. Several lines of evidence indicate that alcohol interferes with endogenous opioid mechanisms which are closely linked with dopamine transmission in the mesolimbic pathway. The view that condensation products of dopamine and alcohol-derived aldehyde (tetrahydroisoquinolines) play a role remains controversial. There is, however, much information on the direct (acute and chronic) effects of alcohol on the binding properties of opioid receptors, as well as modulation of opioid peptide synthesis and secretion (e.g. a suggested increase in β-endorphin release). In view of the reinforcing properties of alcohol, it is relevant to consider behavioural studies involving alcohol self-administration in rodents and primates. Low doses of morphine have been found to increase, and higher doses of the opiate to decrease, alcohol consumption. Conversely, opioid antagonists such as naloxone and naltrexone (which bind to non-selectively opioid receptors) have been shown to decrease alcohol consumption under various experimental conditions. Similar results have been reported when selective μ- or δ-receptor antagonists are administered. Results obtained in genetic models of high preference for alcohol also support the view that alcohol intake depends on the activity of the endogenous opioid reward system and that alcohol consumption may serve to compensate for inherent deficits in this system. One hypothetical model proposes that reward results from activation of μ-opioid receptors in the VTA and/or δ-receptors in the NAC; both these nuclei are targets of endogenous β-endorphin. It is suggested that alcohol interferes with this reward pathway either directly or indirectly. The available experimental data accord well with those obtained from clinical studies in which opioid antagonists have been used to prevent relapse in alcoholics. Conceptual considerations concerning communalities between various forms of addictions are also discussed in this review. Received: 12 June 1996 / Final version: 30 August 1996     

Motor stimulant effects of ethanol and acetaldehyde injected into the posterior ventral tegmental area of rats: role of opioid receptors

Rationale  A recently published study has shown that microinjections of ethanol, or its metabolite, acetaldehyde into the substantia nigra pars reticulata, are able to produce behavioral activation in rats. Another brain site that could participate in such effects is the ventral tegmental area (VTA). Objectives  We have investigated the locomotor-activating effects of local microinjections of ethanol and acetaldehyde into the posterior VTA of rats and the role of opioid receptors in such effects. Materials  Cannulae were placed into the posterior VTA to perform microinjections of ethanol (75 or 150 nmol) or acetaldehyde (25 or 250 nmol) in animals not previously microinjected or microinjected with either the nonselective opioid antagonist naltrexone (13.2 nmol) or the irreversible antagonist of the μ-opioid receptors β-funaltrexamine (β-FNA; 2.5 nmol). After injections, spontaneous activity was monitored for 60 min. Results  Injections of ethanol or acetaldehyde into the VTA increased the locomotor activity of rats with maximal effects at doses of 150 nmol for ethanol and 250 nmol for acetaldehyde. These locomotor-activating effects were reduced by previously administering naltrexone (13.2 nmol) or β-FNA (2.5 nmol) into the VTA. Conclusions  The posterior VTA is another brain region involved in the locomotor activation after the intracerebroventricular administration of ethanol or acetaldehyde. Our data indicate that opioid receptors, particularly the μ-opioid receptors, could be the target of the actions of these compounds in the VTA. These results are consistent with the hypothesis that acetaldehyde could be a mediator of some ethanol effects.     

Effect of naltrexone on alcohol consumption during chronic alcohol drinking and after a period of imposed abstinence in free-choice drinking rhesus monkeys

Relapse into problematic alcohol drinking is a serious problem in the treatment of alcoholism. Free-choice drinking rhesus monkeys show relapse-like behaviour after imposed abstinence of alcohol, by immediately reinitiating ethanol intake at an increased level. The relapse-like behaviour of the monkeys seems not induced by physical withdrawal, but rather argues for a resistance to extinction of ethanol-reinforced behaviour. It has been suggested that endogenous opioids play a role in the positive reinforcing effect of ethanol. In this study, the effect of the opiate antagonist naltrexone was investigated in eight adult male rhesus monkeys(Macaca mulatta) who had about 1 year experience with alcohol drinking, under two conditions: 1) (expt 1) during continuous and concurrent supply of drinking water and two ethanol/water solutions (16% and 32% (v/v)), and 2) (expt 2) after 2 days of alcohol abstinence. In both experiments, each monkey received six doses of naltrexone (0.02, 0.06, 0.17, 0.5, 1.0, 1.5 mg · kg^–1); each dose was paired with a placebo injection (im) in a cross-over design. Consumption was measured from 16.00 hours in the afternoon (30 min after injection) to 9.00 hours the next morning. In experiment 1 naltrexone reduced total net ethanol intake in a graded dose-dependent manner. The effect of naltrexone was apparent shortly after injection, and lasted until the following day. Consumption of drinking water was reduced only shortly after injection. In expt 2, reduction of net ethanol intake was largely restricted to the first few hours of reinitiation of alcohol drinking, i.e. the period in which the abstinence-induced increase was manifest. Consumption of drinking water was not affected by naltrexone. Naltrexone hardly influenced consumption of the non-preferred ethanol solution of 32%. It is postulated that the opioid modulation specifically interacted with positively reinforced behaviour. In expt 2 naltrexone reduced ethanol intake at a lower dose (0.17 mg · kg^–1) compared to expt 1 (0.50 mg · kg^–1), but net ethanol intakes however remained higher. It might be that alcohol abstinence resulted in altered opioid activity, leading to increased ethanol-seeking behaviour. The renewed presentation of ethanol solutions (also) might have stimulated reinitiation of alcohol drinking, representing conditioned incentive stimuli. The reported monkey model of relapse in alcohol drinking could be a useful tool to evaluate new hypotheses and experimental treatments with respect to human alcoholism.     

Dopamine depletion augments endogenous opioid-induced locomotion in the nucleus accumbens using bothμ1 andδ opioid receptors

The aim of this study is to analyze further the opioid receptor subtypes involved in the augmentation of behavioral activity after dopamine depletion in the nucleus accumbens of rats. Initially, the opioid receptors involved in the augmentation of locomotion produced by endogenous opioids were evaluated by microinjection of kelatorphan, an inhibitor of proteolytic enzymes that inactivates enkephalin, with or without specific antagonists forμ ₁ orδ-opioid receptors, naloxonazine or naltrindole, respectively. Kelatorphan produced a dose-dependent increase in horizontal photocell counts and vertical movements. At all doses examined the behavioral response was augmented in rats sustaining accumbal dopamine lesions. The augmentation in dopamine-depleted rats was partially blocked by naloxonazine or naltrindole. Since the motor stimulant response to intra-accumbens microinjection of theδ-opioid agonist, [d-penicillamine^2,5]-enkephalin, was not augmented in a previous study, we tested the behavioral response to a new endogenousδ-opioid agonist, [d-Ala²] deltorphin I. The locomotor response to deltorphin was slightly augmented in dopamine-depleted rats. These data suggest that the augmentation in the motor response elicited by endogenous opioids after dopamine lesions in the nucleus accumbens involves bothμ ₁ andδ-opioid receptors.     

Preferential involvement of D3 versus D2 dopamine receptors in the effects of dopamine receptor ligands on oral ethanol self-administration in rats

There is evidence that dopamine transmission is involved in reinforcement processes and the present study investigated the relative involvement of D₃ versus D₂ dopamine receptors in the effects of dopamine ligands on the reinforcing action of ethanol. Rats were trained to self-administer ethanol (10% v/v) orally in a free-choice two-lever operant task using a saccharin-fading procedure. When preference in responding for ethanol over water had developed the rats were tested with several dopamine agonists and antagonists. Pretreatment with the non-selective dopamine agonist, apomorphine (0.01–0.1 mg/kg), the preferential D₂ agonist, bromocriptine (1–10 mg/kg) and the selective D₃ agonists, 7-OH-DPAT (0.003–0.1 mg/kg), PD 128907 (0.1–3 mg/kg), (+)3PPP (0.3–3 mg/kg), quinelorane (0.0001–0.003 mg/kg) and quinpirole (0.003–0.03 mg/kg), resulted in dose-dependent decreases in responding for ethanol. The relative potencies of the dopamine agonists to decrease ethanol self-administration were highly correlated with their published potencies to produce in vitro functional D₃ but not D₂ responses. Active doses could be considered as those selectively stimulating receptors involved in the control of dopamine release, suggesting that reduction of dopamine transmission was associated with a decrease in ethanol-reinforced responding. This conclusion was further supported by the finding that pretreatment with the D₂/D₃ dopamine antagonists, haloperidol (0.1–0.4 mg/kg) and tiapride (10–60 mg/ kg), decreased responding for ethanol at doses which have been shown previously to block dopamine transmission. Received: 25 January 1998/Final version: 24 April 1998     

The nociceptin/orphanin FQ receptor agonist Ro 64-6198 reduces alcohol self-administration and prevents relapse-like alcohol drinking.

Effects of the opioid receptor like-1 (ORL-1) receptor agonist Ro 64-6198 (0.1, 0.3, and 1.0 mg/kg intraperitoneally (i.p.)) on operant ethanol self-administration and activation of self-administration by ethanol deprivation were studied in male Wistar rats. Acute administration of Ro 64-6198 caused a dose-dependent reduction of ethanol self-administration. In comparison, the opioid antagonist naltrexone (0.1, 0.3, and 1.0 mg/kg i.p.) inhibited ethanol self-administration at all doses tested. Ethanol deprivation for 10 days significantly increased ethanol self-administration during the first 2 days after deprivation. Daily pretreatment with Ro 64-6198 (0.3 mg/kg) or naltrexone (0.3 mg/kg) during the last 3 days of ethanol deprivation abolished the deprivation-induced increase in ethanol intake. Thus, stimulation of the ORL-1 receptors by Ro 64-6198 reduced the acute reinforcing effects of ethanol and prevented relapse-like behavior in the ethanol-deprivation model in a similar manner as a blockade of opioid receptors by naltrexone. Ro 64-6198 at 0.1 and 0.3 mg/kg doses did not alter self-administration of 0.2% saccharin solution, indicating an apparent selectivity of this compound in modification of ethanol reward. These findings add further support to the idea that Ro 64-6198 and potentially other synthetic ORL-1 receptor agonists are as effective as naltrexone in blocking the actions of ethanol important for its addictive potential in animal experiments, and therefore may have therapeutic value in the treatment of alcoholism.     

Ethanol-reinforced behaviour in the rat: effects of naltrexone.

It has been repeatedly reported that endogenous opioid pathways play an important role in ethanol drinking behaviour. In line with these findings, a non-selective opioid receptor antagonist, naltrexone, seems to reduce relapse rates in detoxified alcoholics. The aim of the present study was to evaluate the effects of naltrexone on (i) ethanol self-administration; (ii) extinction of responding for ethanol; (iii) reinstatement of ethanol-seeking induced by non-contingent presentations of ethanol-associated stimuli. Male Wistar rats were trained to lever-press for 8% ethanol in an operant procedure where ethanol was introduced in the presence of sucrose. The selectivity of naltrexone's actions was assessed by studying its effects on water-reinforced behaviour in separate control experiments. Acute injections of naltrexone (1 or 3 mg/kg) did not alter ethanol self-administration. Repeated treatment with naltrexone (3 mg/kg, before three consecutive self-administration sessions) progressively reduced ethanol intake. In the extinction procedure, acute administration of 3 mg/kg naltrexone suppressed responding previously reinforced with ethanol. Similarly, naltrexone (1-3 mg/kg) potently and dose-dependently inhibited reinstatement of ethanol-seeking produced by non-contingent deliveries of the liquid dipper filled with 8% ethanol. In the control experiments, lower doses of naltrexone (1-3 mg/kg) did not exert any effect on either reinforced or non-reinforced (extinction) lever-pressing for water. These results indicate that: (i) subchronic treatment with naltrexone leads to progressive reduction of ethanol self-administration; (ii) single doses of naltrexone may increase extinction and attenuate cue-induced reinstatement of ethanol-reinforced behaviour.     

Involvement of the opioid system in the effects induced by nicotine on anxiety-like behaviour in mice

Rationale Recent studies have revealed the participation of the endogenous opioid system in several behavioural responses induced by nicotine including antinociception, rewarding properties, and physical drug dependence. Objectives The present study was designed to examine the possible involvement of the various opioid receptors in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice. Methods The acute administration of low (0.05) or high (0.8 mg/kg) doses of nicotine subcutaneously produced opposite effects in the elevated plus maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. Animals were only exposed once to nicotine. The effects of the pretreatment with the μ-opioid receptor antagonist, β-funaltrexamine (5 mg/kg), the δ-opioid antagonist, naltrindole (2.5 mg/kg) and the κ-opioid antagonist, nor-binaltorphimine (2.5 mg/kg) intraperitoneally were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine. Results β-funaltrexamine, but not nor-binaltorphimine or naltrindole, abolished nicotine-induced anxiolytic-like effects, suggesting an involvement of μ-opioid receptors in this behavioural response. On the other hand, naltrindole, but not nor-binaltorphimine or β-funaltrexamine, increased the anxiogenic-like responses of nicotine, suggesting an involvement of δ-receptors in this behavioural effect. Conclusions These results demonstrate that the endogenous opioid system is involved in the effects induced by nicotine on anxiety-like behaviour and provide new findings to further clarify the interaction between these two neurochemical systems.     

Benzodiazepines enhance the consumption and palatability of alcohol in the rat

This study examined the effect of the benzodiazepine, midazolam, on the consumption and palatability of 6% ethanol in male Wistar rats. In the first experiment, it was found that midazolam (5 mg/kg) increased home cage ethanol drinking 0–2 h after administration. Another intake experiment, in which ethanol was infused directly into the oral cavity through an indwelling catheter, also showed that midazolam (10 mg/kg) stimulated alcohol ingestion. The affective response to intraoral infusions of ethanol (1 ml during 1 min) was subsequently monitored in benzodiazepine-treated rats. Taste reactivity testing showed that midazolam (5–10 mg/kg) significantly increased the occurrence of hedonic orofacial responses and suppressed the number of passive drippings. A similar response pattern was observed following administration of diazepam (5 mg/kg) and chlordiazepoxide (10 mg/kg), but not after exposure to cis(Z)flupentixol (10 mg/kg). Midazolam also increased the incidence of hedonic responses in alcohol-naive rats with no previous access to ethanol in the home cages. Hedonic responsiveness did not appear to diminish with repeated benzodiazepine exposure: the behaviour of rats given five midazolam injections (10 mg/kg every second day) was similar to that shown by rats with no benzodiazepine pre-exposure. The increased hedonic response to ethanol induced by midazolam was blocked by pretreatment with the benzodiazepine receptor antagonist flumazenil (10 mg/kg), the latter drug exerting no effects on its own. The present results suggest that benzodiazepines, by acting on GABA_A receptors, may facilitate ethanol intake by increasing ethanol’s taste hedonic properties. Received: 22 April 1997/Final version: 14 October 1997     

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.     

Effect of selective antagonism of mu(1)-, mu(1/2)-, mu(3)-, and delta-opioid receptors on the locomotor-stimulating actions of ethanol

Previous studies have demonstrated that administration of nonspecific opioid antagonists such as naltrexone or naloxone reduces ethanol-induced locomotor activity in mice. However, because of their broad pharmacological profile, it remains unclear through which opioid receptor this antagonism is achieved. Therefore, the present study was aimed at further investigating the role of the different opioid receptors in ethanol-induced (2.5 g/kg) locomotion in mice. First, we compared the effect of naltrexone (0-2 mg/kg) on ethanol-induced locomotion with that of the selective delta-opioid receptor antagonist, naltrindole (0-10 mg/kg). Results of this first set of data revealed that naltrexone completely blocked this effect of ethanol at doses suggested to occupy only mu-opioid receptors, and naltrindole did not modify ethanol-induced locomotion. In a second set of experiments, we further investigated the involvement of mu-opioid receptors in ethanol-stimulated motor activity by assessing the implication of mu(1)-, mu(1/2)-, and mu(3)-opioid receptor subtypes. Results revealed that mu(1/2)-, and to a lesser extent mu(3)-, but not mu(1)-opioid receptor subtypes are involved in the psychomotor actions of ethanol. Data are discussed together with previous results which have emphasized the critical dependence of ethanol-induced motor behaviors on opioid receptors, as well as, of the integrity of beta-endorphin synthesizing neurons from the hypothalamic Arcuate Nucleus.     

Sex differences in the potency of κ opioids and mixed-action opioids administered systemically and at the site of inflammation against capsaicin-induced hyperalgesia in rats

Rationale Sex differences in the potency of the antinociceptive effects of κ opioids have been reported in various acute pain models with evidence suggesting that these sex differences are mediated by activity in the N-methyl-d-aspartate (NMDA) system. Objectives The purpose of the present study was to evaluate sex differences in the antihyperalgesic actions of selected κ and mixed-action opioids in a persistent pain model and determine if the NMDA system modulates these effects in a sexually dimorphic manner. Methods Using gonadally intact male and female F344 rats, hyperalgesia was induced by local administration of capsaicin in the tail, after which the tail was immersed in a mildly noxious thermal stimulus (45°C water), and tail-withdrawal latency measured. Opioids were then administered systemically (s.c.) and locally (in the tail) alone, and in selected combinations with the noncompetitive NMDA antagonist dextromethorphan. Results When administered systemically and locally, the κ opioids spiradoline, U69,593 and U50,488, and the mixed-action opioids butorphanol and nalbuphine, produced dose-dependent antihyperalgesic effects. Whereas the κ opioids were generally more potent in males, sex differences were not observed with the mixed-action opioids. Peripheral receptor activity was confirmed for local administration of κ opioids by the antagonism observed after local, but not intracerebroventricular (i.c.v.), administration of the κ antagonist nor-binaltorphamine (nor-BNI). Dextromethorphan was equally potent in attenuating the antihyperalgesia induced by κ opioids in both males and females. Conclusions These findings demonstrate sex differences in κ opioid activity in a persistent pain model. Although an NMDA antagonist blocked the effects of κ opioids in this model, these effects were not sexually dimorphic as reported in most acute pain models.     

Autoradiographic mapping of μ opioid receptor changes in rat brain after long-term haloperidol treatment: relationship to the development of vacuous chewing movements

Brain opioid systems modulating basal ganglia function may be involved in the development of neuroleptic-induced orofacial dyskinesias. This study examined changes in μ opioid receptors labeled with [³H]D-Ala², N-MePhe⁴, Gly-ol⁵-enkephalin ([³H]DAMGO) in 79 different brain regions of rats showing vacuous chewing movements after 21 weeks of treatment with haloperidol decanoate (HAL). Dopamine D₂ receptors labeled with [³H]raclopride were also examined in the adjacent sections of the same brains. For brain analyses HAL-treated rats were divided into a group showing high incidence of vacuous chewing movements (VCMs) and a group showing low incidence of VCMs. As expected, long-term HAL resulted in a pronounced elevation of D₂ receptors in caudate-putamen, n. accumbens, globus pallidus and olfactory bulbs (range: 27–70% increases) compared to controls. These changes were equal in magnitude in both HAL-treated groups, irrespective of the frequency of VCMs. In HAL-treated rats [³H]DAMGO was significantly decreased in several parts of the basal ganglia, including n. accumbens (−21%, P < 0.01), patchy area of the anterior caudate-putamen (−12%, P < 0.05), ventral pallidum (−27%, P < 0.01) and globus pallidus (−21%, P < 0.02). Statistically significant decreases were also seen in the subthalamic nucleus (−12%, P < 0.05) and ventrolateral thalamus (−21%, P < 0.05), both of which are targets of basal ganglia output. However, as in the case of [³H]raclopride binding, [³H]DAMGO changes were generally seen both in the High VCM and the Low VCM groups. These results confirm that long-term haloperidol leads to a decrease in μ-opioid binding in basal ganglia and related structures, similar to what is seen after 6-OHDA denervation. The observed μ-receptor binding changes may be a contribution factor, but do not appear sufficient to account for the differential development of neuroleptic-induced vacuous chewing movements. Received: 2 February 1996 / Final version: 10 June 1996