Effects Of Opioid Antagonists Research Articles

Stereotypy II: a review of neurobiological interpretations and suggestions for an integration with behavioral methods

Stereotypy is a relatively common behavioral disorder displayed by individuals with developmental disabilities, including autism. In this paper, we review selected studies on neurobiological interpretations of stereotypy and pharmacological interventions for stereotypy. Specifically, we review studies that evaluated the effects of serotonin uptake inhibitors (e.g., clomipramine) or opioid antagonists (e.g., naltrexone) on stereotypy displayed by humans. Throughout, suggestions are made for the incorporation of behavioral methods into this area of research.

The Opioidergic-Alcohol Link

Preclinical and clinical data implicate the endogenous opioid system in alcohol dependence. In vitro studies show that rodent pituitary and hypothalamic tissue responds to acute exposure to alcohol by releasing beta-endorphins. In vivo studies suggest differential activity of endogenous opioid receptors in rodents with high and low alcohol preference. Similarly, humans with a family history of alcohol dependence also show a heightened endorphin response to an acute challenge of alcohol compared with those with no family history of alcohol dependence.The effects of opioid agonists and antagonists on rodent and human alcohol consumption further support the opioid-alcohol link. In rodents and humans, small doses of opioid agonists increase alcohol consumption, while pretreatment with large doses decreases consumption. The opioid antagonist naltrexone decreases rodent alcohol consumption, particularly in low doses under acute and intermittent schedules. Most clinical trials in patients with alcohol dependence support modest therapeutic effects of naltrexone in decreasing alcohol consumption. Efforts to identify subgroups of alcohol-dependent patients responsive to naltrexone, as well as psychosocial and pharmacological augmentation strategies, may further improve the clinical usefulness of the drug.

P.6.022 Differential effects of tree selective opioid antagonists injected into the nucleus accumbens on tolerance to ethanol

P.6.022 Differential effects of tree selective opioid antagonists injected into the nucleus accumbens on tolerance to ethanol

The development of tolerance to locomotor effects of morphine and the effect of various opioid receptor antagonists in rats chronically treated with morphine

Behavioural measures are considered to be highly sensitive indices of opioid withdrawal. Opioids, depending on dose and time protocols may induce both reduction and enhancement of locomotor activity and chronic opioid treatment results in tolerance and sensitisation to these effects. In the present study the locomotor activity as experimental model was used to assess the development of tolerance to subcutaneous morphine challenge at different time points following morphine withdrawal in rats exposed to gradually increasing subcutaneous doses of morphine for 11 days. Tolerance developed to the inhibitory action of morphine (10 mg/kg) was observed even 8 weeks after morphine withdrawal, while tolerance to its locomotor activity enhancing effect (3 mg/kg) was detected 18 h after withdrawal, but not 3 weeks later. In the other series of experiments the locomotor activity of animals exposed to chronic morphine treatment was tested 18 h after spontaneous or subcutaneously administrated opioid antagonists precipitated withdrawal. Spontaneous withdrawal resulted in a moderate decrease of locomotion. Both the non-selective antagonist naloxone in low, μ opioid-receptor selective doses and the δ opioid-receptor selective naltrindole induced marked reduction of locomotor activity. The results provide further evidence that both μ and δ opioid-receptors might be affected during chronic morphine treatment.

Opioid receptor involvement in food deprivation-induced feeding: evaluation of selective antagonist and antisense oligodeoxynucleotide probe effects in mice and rats.

Central administration of general and selective opioid receptor subtype antagonists in the rat has revealed a substantial role for mu, a moderate role for kappa, and a minimal role for delta receptors in the mediation of deprivation-induced feeding. Antisense probes directed against the kappa opioid receptor (KOP), nociceptin opioid receptor (NOP), and delta opioid receptor (DOP) genes in rats result in reductions similar to kappa and delta antagonists, whereas antisense probes directed against the mu opioid receptor (MOP) gene produced modest reductions relative to mu antagonists, suggesting that isoforms of the MOP gene may mediate deprivation-induced feeding. Since these isoforms were initially identified in mice, the present study compared the effects of general and selective opioid receptor antagonists on deprivation-induced feeding in rats and mice and antisense probes directed against exons of the MOP, DOP, KOP, and NOP genes on deprivation-induced feeding in the mouse. Food-deprived (12 and 24 h) rats and mice displayed similar profiles of reductions in deprivation-induced feeding following general, mu, and kappa opioid antagonists. In contrast, mice, but not rats, displayed reductions in deprivation-induced intake following delta antagonism as well as DOP antisense probes, suggesting a species-specific role for the delta receptor. Antisense probes directed against the KOP and NOP genes also reduced deprivation-induced intake in mice in a manner similar to kappa antagonism. However, the significant reductions in deprivation-induced feeding following antisense probes directed against either exons 2, 4, 7, 8, or 13 of the MOP gene were modest compared with mu antagonism, suggesting a role for multiple mu-mediated mechanisms.

Classification and correlation of St. John's wort extracts by nuclear magnetic resonance spectroscopy, multivariate data analysis and pharmacological activity.

The use of proton NMR spectroscopy allows the analysis of complex multi-component mixtures such as plant extracts by simultaneous quantification of all proton-bearing compounds and consequently all relevant substance classes. Since the spectra obtained are too complicated to be analysed visually, the classification of spectra was carried out using multivariate statistical methods. The spectroscopic data of various extracts of St. John's wort (Hypericum perforatum) samples derived from 4 different accessions extracted with 6 distinct solvents were chemometrically evaluated and calibrated using the partial least square (PLS) algorithm. In a first approach, we found a consistent correlation for the spectroscopic pattern of the extracts and the corresponding IC (50) values derived from non-selective binding to opioid receptors. Consequently, the multivariate data analysis was used to predict the pharmacological efficacy of further St. John's wort extracts on the basis of their proton NMR spectra. In a second approach a PLS 2 model was used to predict the biological activity for eight St. John's wort extracts based on two pharmacological data sets: (i) non-selective binding to opioid receptors and (ii) antagonist effect at corticotrophin-releasing factor type 1 (CRF (1)) receptors. The PLS 2 model confirmed the useful application of the presented approach to assess the quality of medicinal herbs and extracts by spectroscopic analysis derived from bioactivity-related quality parameters.

Effect of novel non-peptidic delta opioid receptor antagonists on human T and B cell activation

The effects of the antagonist naltrindole (NTI) on cells of the immune system have been largely studied although the mechanisms of action are still unclear. The aim of this study is to evaluate, in vitro, the immunomodulatory activity of four new delta-selective opioid compounds structurally related to naltrindole. The effects at different concentrations of these opioid antagonists on proliferative response were studied on normal human peripheral blood mononuclear cells (PBMC) stimulated with different stimuli: mitogens, the antigen PPD, the anti-CD3 monoclonal antibodies (mAb), the superantigen Staphylococcus aureus Cowan strain 1 (SAC) and alloantigens in the mixed lymphocyte cultures (MLR). The immunomodulatory capacity of these compounds was evaluated by determining the interleukin-2 (IL-2) release in mitogen activated PBMC. The present study shows that all the new delta opioid antagonists at 10 −5 M concentration are immunosuppressive. The inhibitory action is also evident at lower concentrations when anti-CD3 mAb and SAC were used as stimulators. In addition, the production of IL-2 was inhibited by the opioid treatment, but this might not be the only mechanism of action.

Effects of Opioid Antagonism on Prolactin Secretion and c-Fos/TH Expression During Lactation in Rats

Many studies have established that dopamine (DA) secreted by tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus is the major inhibitory factor controlling prolactin (PRL) secretion from the anterior pituitary. Endogenous opioid peptides (EOPs), mainly the neuropeptide beta-endorphin, facilitate PRL secretion by decreasing TIDA neuronal inhibitory tone in a number of physiological conditions, including pregnancy and lactation. We have previously demonstrated that there are many more c-Fos-expressing neurons than TIDA neurons in the arcuate nucleus, and treatment with naloxone (NAL), an opioid antagonist, activated these neurons in pregnant rats. Our previous data also suggest that the rostral region of the arcuate nucleus is more important than the caudal region in regulating the nocturnal PRL surge in pregnant rats. The aim of this study was to investigate the effects of NAL in regulating TIDA neuronal activity and therefore facilitating PRL secretion during lactation in rats. NAL was continuously infused (0.2 mg/10 microL/min iv) for 1 h before the separated pups returned, and then for 2 or 5 h after the separated pups were returned. Radioimmunoassay (RIA) was used to measure plasma PRL levels, and the immunocytochemical (ICC) staining of c-Fos was performed to detect changes in transcriptional activity of neurons in the hypothalamus. ICC of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis, was performed to visualize TIDA neurons in the arcuate nucleus. The results showed that the peak of the PRL response to suckling was markedly delayed and dampened in NAL-treated rats (p<0.05). The percentage of c-Fos positive TH neurons in the arcute nucleus increased in rats treated with NAL for 5 h after return of pups, but not in rats treated with NAL for 2 h.

Morphine inhibits Fc-mediated phagocytosis through mu and delta opioid receptors

Previous studies have shown that acute exposure to morphine and other opioids inhibits both non-opsonic (Rojavin et al., 1993; Szabo et al., 1993) and Fc-mediated (Casellas et al., 1991; Tomei and Renaud, 1997) phagocytosis by murine peritoneal macrophages. Furthermore, subchronic exposure to morphine will result in a putative tolerant/dependent state, where phagocytosis is no longer inhibited by morphine and drug withdrawal is actually inhibitory (Lazaro et al., 2000). However, the nature of the opioid receptors mediating these effects is unknown, although mu, delta and kappa receptors have been identified in these cells, mostly by molecular techniques (Chao et al., 1996; Chuang et al., 1994, 1995; Gaveriaux et al., 1995; Lawrence et al., 1995; Roy and Loh, 1996). This is an important question, since opioid effects on immune cells have been postulated to be a co-factor in the spreading of infectious diseases among drug addicts, including HIV (Donahoe et al., 1991; Haverkos and Lange, 1990). Therefore, we undertook to identify these receptors by testing the effect of selective opioid receptor agonists and antagonists on Fc-mediated phagocytosis by murine macrophages, and determining how these observations are affected by the absence of the mu receptor, using mu-opioid receptor knockout (MORKO) mice (Loh et al., 1998).

Determinants of the behavioral effects of opioids and their antagonists: contributions of the Laboratory of Psychobiology.

The behavioral pharmacology of opioids has been influenced significantly by the research and writings of Drs. Peter B. Dews, Roger T. Kelleher, and William H. Morse, their colleagues, and their students. Their conceptual and methodological approach to the topic is reviewed briefly, and three areas of research are described to provide an empirical perspective. The objective of determining the general effects of opioids on behavior is described; the effects of opioids on schedule-controlled behavior and punished behavior are described and compared to non-opioids. The differential effects of opioid antagonists on responding reinforced by different stimuli are also presented. The conceptual and methodological approach taken by the group, as well as their discoveries in the behavioral pharmacology of opioids, will continue to exert a positive influence on the field.

Opiate antagonist therapy for the pruritus of cholestasis: the avoidance of opioid withdrawal-like reactions.

Increased opioidergic neurotransmission in the brain appears to contribute to the pruritus that complicates cholestasis and certain non-cholestatic chronic liver diseases. Opiate antagonists have been shown to decrease scratching activity in patients with the pruritus of cholestasis. Initiation of oral administration of an orally bioavailable opiate antagonist may precipitate a florid opioid-withdrawal-like reaction in patients with pruritus complicating cholestasis. Such reactions can be minimized, or avoided completely, by cautiously infusing naloxone before giving small oral doses of an orally bioavailable opiate antagonist. The infusion rate of naloxone should initially be very low; it should be increased gradually and stopped when a rate known to be associated with opioid antagonist effects has been attained. Oral therapy with an opiate antagonist can then be initiated.

The antinociceptive effect of mirtazapine in mice is mediated through serotonergic, noradrenergic and opioid mechanisms

The antinociceptive effects of the noradrenergic and specific serotonergic antidepressant (NaSSA) drug mirtazapine and its interaction with various opioid receptor subtypes were evaluated in mice with a hotplate analgesicmeter. Mirtazapine elicited an antinociceptive effect in a dose-dependent manner following doses from 1 to 7.5 mg/kg. As the mirtazapine dose increased beyond 10 mg/kg latencies returned to baseline, yielding a biphasic dose–response curve. The effect of opioid, adrenergic, and serotonergic receptor antagonists was examined as to their ability to block mirtazapine antinociception. Mirtazapine (at 10 mg/kg)-induced antinociception was significantly inhibited by naloxone, nor-BNI, and naltrindole, but neither by β-FNA nor by naloxonazine, implying the involvement of κ 1- and δ-opioid mechanisms. When adrenergic and serotonergic antagonists were used, both metergoline and yohimbine, decreased antinociception elicited by mirtazapine, implying a combined serotonergic and noradrenergic mechanism of antinociception. When mirtazapine was administered together with various agonists of the opioid receptor subtypes, it significantly potentiated antinociception mediated only by κ 3-opioid receptor subtypes. Summing up these results we conclude that the antinociceptive effect of mirtazapine is mainly influenced by the κ 3-opioid receptor subtype combined with both serotonergic and noradrenergic receptors. These results suggest a potential use of mirtazapine in the management of some pain syndromes, and raise questions regarding a possible indirect opioid-dependence induced by mirtazapine. However, further research is needed in order to establish both the exact clinical indications and the effective doses of mirtazapine when prescribed for pain.

Effects of opioid antagonists on unconditioned and conditioned hyperactivity to morphine

In a series of experiments, the ability of selective μ- (β-funaltrexamine, β-FNA), δ- (naltrindole, nalt) and κ- (nor-binaltorphimine, nor-BNI) opioid receptor antagonists to attenuate the unconditioned and conditioned hyperactive effects of morphine was examined. For comparison, the nonselective opioid receptor antagonist naloxone (nalx) was also examined. Locomotor activity served as the behavioral measure. Experiment 1 found that doses of 1 and 4, but not 16 mg/kg, of morphine effectively produced conditioned hyperactivity (CH). Experiments 2a–d found that β-FNA, nalt, nor-BNI and nalx, respectively, attenuated unconditioned morphine-induced hyperactivity. Experiments 3a–c, however, found that none of the selective antagonists, given individually, attenuated CH. In contrast, nalx did attenuate CH (Experiment 3d). Collectively results suggest that the unconditioned and conditioned hyperactive responses to morphine are mediated by different receptor systems and that activation of multiple opioid-receptor subtypes mediate expression of CH.

Effect of opioid receptor antagonists on hypothalamic–pituitary–adrenal activity in rhesus monkeys

Some opioid antagonists increase the release of adrenocorticotropic hormone (ACTH) and cortisol in humans and, therefore, may indicate that endogenous opioids modulate hypothalamic–pituitary–adrenal axis activity. The type of opioid receptor that may be related to these endocrine effects is unknown. The purpose of this experiment was to evaluate the ability of different opioid antagonists to increase ACTH and cortisol plasma levels in rhesus monkeys. Eight monkeys received intramuscular injections of various antagonists: 0.0032–1.0 mg/kg naltrexone, 0.1–3.2 mg/kg naltrindole (delta-selective), 0.032–0.32 mg/kg clocinnamox (mu-selective), and 1–3.2 mg/kg nor-binaltorphimine (kappa-selective). Naltrexone, 0.1–1.0 mg/kg, increased ACTH levels, whereas naltrindole and clocinnamox failed to increase ACTH levels. Nor-binaltorphimine, 1–3.2 mg/kg, increased ACTH concentrations on the day of injection, but not at a time when other assays continue to demonstrate κ-antagonism (24 h). Cortisol concentrations generally followed the same pattern as the ACTH concentrations, but the incremental differences in cortisol release between doses were less clear. Thus, opioid modulation of ACTH and cortisol plasma levels is not clearly associated with a particular opioid receptor. Although the κ-antagonist increased ACTH and cortisol release on the day of injection, some evidence suggests that this endocrine effect may be due to mechanisms other than those mediated by the κ-receptor. Alternatively, the naltrexone-induced increase of ACTH and cortisol plasma levels may be caused by activity at multiple opioid receptors or some uncharacterized receptor. Finally, the increased release of ACTH and cortisol may be a response to naltrexone’s aversive properties.

Opioid antagonists for alcohol dependence.

The results from animal studies suggest that opioid antagonists may prevent the reinforcing effects of alcohol consumption. Based on the results of those animal studies, some opioid antagonists, such as, naltrexone, nalmefene, have been studied for their benefits in treating alcohol dependence. To determine the effectiveness of opioid antagonists in attenuating or preventing the recommencement of alcohol consumption in patients with alcohol dependence in comparison to placebo, other medications and psychosocial treatments. In addition, discontinuation rate, death, patient satisfaction, functioning, health-related quality of life and economic outcomes were also evaluated. Electronic searches of Cochrane Controlled Trials Register (Cochrane Library 2001, issue 4), MEDLINE (1966 - October 2001), EMBASE (1980 - December 2001), and CINHAL (1982 - December 2001) were undertaken. Du Pont Pharmaceutical and Ivax Corporation were contacted for information regarding unpublished trials. The reference lists of the obtained papers were also examined. All relevant randomised controlled trials (RCTs) and controlled clinical trials (CCTs) were included. Participants were people with alcohol dependence. Naltrexone (NTX), nalmefene (NMF) and other opioid antagonists with/without other biological or psychosocial treatments were examined. Four primary outcomes of interest were number of patients who return to drinking, percentage or number of drinking days, number of standard drinks of alcohol and amount of alcohol consumed. A number of secondary outcomes were also considered. Two reviewers evaluated and extracted the data independently. The dichotomous data were extracted on an intention-to-treat basis. The Relative Risk with the 95% confidence interval was used to assess the dichotomous data. Weighted (or Standardised) Mean Difference with 95% confidence interval was used to assess the continuous data. The review included 19 RCTs or CCTs presented in 26 articles. In comparison to placebo, two of four short-term primary outcomes were significantly in favour of NTX. Those were number of patients who return to drinking (61% in NTX group vs 69% in placebo group) [RR (95% CI) = 0.88 (0.80 to 0.98), NNT = 14] and percentage or number of drinking days [WMD (95% CI) = -4.52 (-5.29 to -3.75)]. However, the short-term discontinuation rates were high and not different between NTX and placebo groups [RR (95% CI) = 0.96 (0.81 to 1.13)]. No medium-term outcomes of NTX and placebo groups showed any significant difference after the completion of NTX treatment for three to six months. However, those who were regularly treated with NTX treatment in both short and medium terms consumed smaller amounts of alcohol than placebo-treated patients. Because of the small sample sizes, there were few significant differences for other comparisons. NTX at the dose of 50 mg/day is effective for alcohol dependence in short-term treatment. The optimal duration of NTX treatment may be longer than 3 months. The evidence so far may be too little to support the superiority of NTX to acamprosate and the inferiority of NTX to disulfiram. NTX treatment should be concurrently given with a psychosocial intervention. Other patterns of NTX administration should not be used at present, e.g., a dose of three times a week, combined NTX with other biological treatments. NMF has no role for the treatment of alcohol dependence in clinical practice. Randomised, double-blind, placebo-controlled trials of NTX treatment in patients with alcohol dependence are still needed. Some issues should be concerned in further studies. Firstly, further trials should be conducted in larger sample sizes and over longer periods of time. Secondly, other than the outcomes relevant to alcohol use, some important outcomes should also be measured, e.g., functioning, health-related quality of life, economic cost. Thirdly, the comparisons between NTX and other treatments for alcohol dependence, both biological and psychosocial, should be investigated. Fourthly, combined treatments of NTX and other biological treatments for alcohol dependence may be in issue of interest. Lastly, high discontinuation rate in both treatment and control groups should be concerned.

Kappa opioid antagonist effects of the novel kappa antagonist 5'-guanidinonaltrindole (GNTI) in an assay of schedule-controlled behavior in rhesus monkeys.

Opioid receptors are divided into three types: kappa, mu, and delta receptors. Receptor-selective antagonists are useful experimental tools for evaluation of opioid receptor-mediated processes. 5'-Guanidinonaltrindole (GNTI) was recently developed as a novel kappa-selective antagonist. To evaluate the potency, time course, and selectivity of GNTI's opioid antagonist effects in rhesus monkeys in an assay of schedule-controlled responding. Five rhesus monkeys were trained to respond under a fixed ratio 30 schedule of food reinforcement. The rate-decreasing effects of the kappa agonists U50,488 and U69,593, the mu agonist morphine, and the delta agonist SNC80 were examined alone and after pretreatment with GNTI (0.1 and 1.0 mg/kg i.m.; 1 h to 14 days). U50,488, U69,593, morphine, and SNC80 dose-dependently decreased response rates in this procedure. GNTI produced a dose- and time-dependent antagonism of the rate-decreasing effects of U50,488. The kappa antagonist effects of GNTI had a slow onset and a long duration of action, and peak antagonist effects were observed after 24 h. A higher dose of 3.2 mg/kg GNTI eliminated responding in one monkey and was not studied further. The antagonist effects of GNTI were kappa selective, because 1.0 mg/kg GNTI also antagonized the effects of U69,593, but not those of morphine or SNC80. These results suggest that GNTI is a potent and selective kappa antagonist with a slow onset and long duration of action in rhesus monkeys. Relative to the prototype kappa antagonist nor-binaltorphimine, GNTI may have some advantages as a tool for the study of kappa receptor-mediated processes.

Endogenous Opioids in Dopaminergic Cell Body Regions Modulate Amphetamine-Induced Increases in Extracellular Dopamine Levels in the Terminal Regions

Opioid antagonists attenuate behavioral effects of amphetamine and amphetamine-induced increases in extracellular dopamine levels in nucleus accumbens and striatum of rats but do not alter those effects of cocaine. This study was performed to determine 1) if the effect of opioid antagonists on the dopamine response to amphetamine is mediated in either the terminal or cell body region of the nigrostriatal and mesolimbic pathways, and 2) if the enkephalinase inhibitor thiorphan, which slows degradation of endogenous opioid peptides, increases the dopamine response to amphetamine but not to cocaine. Microdialysis probes were placed either into a dopaminergic terminal region or into both a terminal and cell body region of rats. Naloxone methiodide (1.0 microM), a lipophobic opioid antagonist, was administered into either the terminal or cell body region by reverse dialysis, whereas extracellular dopamine was collected in the terminal region. Increases in extracellular dopamine in nucleus accumbens and striatum caused by amphetamine (0.1-6.4 mg/kg, s.c.) were reduced significantly (28-39%) by naloxone methiodide administered into either substantia nigra or ventral tegmentum but not into terminal regions. Thiorphan (10 microM) administered into substantia nigra increased significantly the dopamine response to amphetamine in the ipsilateral striatum by as much as 42% but did not affect the dopamine response to cocaine (3.0-56 mg/kg, i.p.). These results suggest that amphetamine promotes release of endogenous opioids, which, through actions in the ventral tegmentum and substantia nigra, contribute to amphetamine-induced increases in extracellular dopamine in the nucleus accumbens and striatum.

Blockade effect of mu and kappa opioid antagonists on the anti-nociception induced by intra-periaqueductal grey injection of oxytocin in rats

Intra-periaqueductal grey (PAG) injection of 1 nmol of oxytocin induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. The anti-nociceptive effect of oxytocin was attenuated significantly by subsequent intra-PAG injection of the mu opioid antagonist β-funaltrexamine (β-FNA) and the kappa opioid antagonist nor-binaltorphimine (nor-BNI), but not by the delta antagonist naltrindole. The results demonstrated that mu and kappa opioid receptors, not delta receptors, were involved in the oxytocin-induced anti-nociception in PAG of rats.

Naltrexone Suppresses Ethanol Intake in 6‐Hydroxydopamine–Treated Rats

The suppressive effect of opioid antagonists, such as naltrexone, on ethanol intake has been suggested to be based on the interference with ethanol-induced stimulation of dopamine release in the nucleus accumbens. The aim of this study was to determine whether reduction of dopamine innervation to the nucleus accumbens with the neurotoxin 6-hydroxydopamine (6-OHDA) alters naltrexone-induced suppression of ethanol consumption. Because the mesolimbic dopaminergic neurons have also been implicated in ethanol reinforcement, the effects of 6-OHDA on the maintenance and acquisition of ethanol intake were also studied. To damage accumbal terminals of the mesolimbic dopamine neurons, alcohol-preferring Alko Alcohol (AA) rats were given bilateral injections of 6-OHDA or vehicle into the nucleus accumbens after pretreatment with desipramine and pargyline. The effect of the lesion on the acquisition or maintenance of ethanol self-administration was studied in animals having continual access to ethanol solution (10% v/v) and water. Subsequently the effect of naltrexone on ethanol consumption was determined. Naltrexone (0.03-3.0 mg/kg subcutaneously) suppressed ethanol consumption in a dose-dependent manner both in 6-OHDA-treated and control animals given a daily 90-min access to ethanol solution. When the rats had continual access to ethanol, there was a clear day-to-day decline in ethanol intake during the first 5 days of the 7-day naltrexone treatment (10 mg/kg subcutaneously). 6-OHDA treatment had no effect on either the acquisition or maintenance of ethanol self-administration. Postmortem analysis of the brain dopamine content revealed approximately 92% depletion of dopamine in the nucleus accumbens of the 6-OHDA-treated rats. The suppressive effect of naltrexone does not depend on naltrexone's interaction with dopaminergic terminals in the nucleus accumbens. Furthermore, the role of the mesolimbic dopamine pathway is probably not central either in the acquisition or maintenance of ethanol self-administration in alcohol-preferring AA rats.

Interaction of morphine and naltrexone on oral ethanol self-administration in rhesus monkeys.

Opioid antagonists, such as naltrexone (NTX), reduce ethanol consumption and opioid agonists increase or decrease ethanol consumption in rats depending upon the dose. If the opioid antagonist and agonist effects on ethanol consumption are mediated by mu-opioid receptors, then NTX doses that reduce ethanol consumption should be similar to the doses necessary to antagonize the effects of opioid agonists on ethanol consumption. The purpose of these experiments was: (1) to determine whether morphine increases ethanol consumption in rhesus monkeys as it does in rodents; (2) to determine if the mu-receptor mediates the effects of morphine on ethanol consumption by conducting a pK(B) analysis using NTX; and (3) to determine if the mu-receptor also mediates the NTX-induced decreases in ethanol consumption by making comparisons between the NTX doses that affect ethanol consumption and the NTX doses that block the effects of morphine on ethanol consumption. Three male rhesus monkeys responded for 2% ethanol and water for 2 h/day on a fixed-ratio 4 schedule of reinforcement. Morphine doses as low as 0.0032mg/kg failed to increase ethanol fluid deliveries, whereas higher doses produced a dose-related decrease in ethanol fluid deliveries. Although 0.01 mg/kg NTX alone had no effect on ethanol fluid deliveries, it reduced the suppressant effects of morphine with a mu-receptor pK(B) of 8.21 (8.08-8.34). When given alone, 0.1 mg/kg NTX decreased ethanol fluid deliveries but failed to reverse the suppression caused by 1 mg/kg morphine. Therefore, monkeys may differ from rats in their response to morphine when ethanol consumption is the dependent variable. Furthermore, because the NTX dose that reduced the effects of morphine on responding for ethanol was smaller than the NTX doses that suppressed ethanol-reinforced responding when given alone, NTX may exert these two effects through different mechanisms.