The effects of opiate antagonists on the discriminative stimulus properties of ethanol

Opioid receptors and the discriminative stimulus effects of ethanol in squirrel monkeys: Mu and delta opioid receptor mechanisms

Mu and delta opioid receptors modulate the reinforcing effects of ethanol, however, their role in the subjective effects of ethanol is not well understood. This study evaluated the contribution of mu and delta opioid receptors to the subjective effects of ethanol using drug discrimination procedures. Monkeys were trained to discriminate ethanol from saline under a schedule of food delivery. In tests, ethanol engendered increases in drug-lever responding, reaching a maximum of >80%. The mu opioid receptor agonists fentanyl and buprenorphine and the delta opioid receptor agonists SNC 80 and SNC 162 did not substitute for the discriminative stimulus effects of ethanol. As pretreatments, the full agonists fentanyl and SNC 80 enhanced the effects of low doses of ethanol and fentanyl attenuated the effects of the ethanol training dose. Although the possibility of pharmacological antagonism of the effects of ethanol cannot be ruled out, a more likely alternative is that the diminished effects of ethanol were due to perceptual masking of the ethanol stimulus. In contrast, the partial agonists buprenorphine and SNC 162 did not alter ethanol's effects. Finally, the discriminative stimulus effects of ethanol were attenuated following administration of presumably mu-selective doses of the antagonist naltrexone, but not after administration of the delta opioid receptor antagonist naltrindole. The ability of naltrexone to block the discriminative stimulus effects of ethanol likely reflects its capacity to attenuate ethanol-induced increases in endogenous opioids, in particular beta-endorphin, because attenuation of the ethanol stimulus was not accompanied by significant suppression of response rate.

Can experimental paradigms and animal models be used to discover clinically effective medications for alcoholism?

Evaluating medications in animal laboratory paradigms can reveal whether the compound is effective in an established alcoholism model, at clinically relevant doses and exposure conditions, when administered orally (or transdermally) and without serious limiting side effects. Positive outcomes constitute a possible discovery for relevance to alcoholism and, under favorable marketing conditions, encourage further development. Medication testing using animal models of alcoholism might also guide clinical testing by discriminating clinically effective from clinically ineffective compounds. This ability rests on whether there are tests or, more reasonably, batteries of tests having this discriminative ability. The present paper examines this possibility. Effects of naltrexone and acamprosate in animal paradigms which model behavioral aspects of alcoholism are reviewed and compared with the effects of compounds which have limited effects in alcoholics. It is not clear at present whether any single paradigm or combination of paradigms differentiates clinically effective from clinically limited compounds. Steps are suggested to improve the use of preclinical laboratory tests to predict which compounds are likely to be effective medications for reducing drinking and sustaining abstinence in human alcoholics.

A multi-site dose ranging study of nalmefene in the treatment of alcohol dependence

The opiate antagonist nalmefene has been shown in 2 single-site studies to reduce alcohol consumption and relapse drinking in alcohol-dependent individuals. This safety and preliminary multisite efficacy study evaluated 3 doses of nalmefene (5, 20, or 40 mg) in a double-blind comparison to placebo over a 12-week treatment period in 270 recently abstinent outpatient alcohol-dependent individuals. Participants concomitantly received 4 sessions of a motivational enhancement therapy (with a medication compliance component) delivered from trained counselors. Although more subjects in the active medication groups terminated the study early secondary to adverse events, the rates did not differ significantly from that of placebo. The 20-mg/d group experienced more insomnia, dizziness, and confusion, while the 5-mg group also had more dizziness and the 40-mg group had more nausea than the placebo group. Most of these symptoms were mild and improved over time. Although all subjects had a reduction in heavy drinking days, craving, gamma-glutamyl transferase, and carbohydrate-deficient transferrin concentrations over the course of the study, there was no difference between the active medication and placebo groups on these measures. The time to first heavy drinking day was also not significantly different between the placebo and the active treatment groups. This relatively small multisite trial showed that nalmefene was reasonably well tolerated in recently abstinent alcoholics. However, possibly because of variation among the sites or the comparatively small sample size, there was no evidence of superior efficacy outcomes with nalmefene treatment.

Opiate delta-2-receptor antagonist naltriben does not alter discriminative stimulus effects of ethanol

The ability of a selective 2-opiate receptor antagonist, naltriben, to modulate ethanol discrimination was investigated in a rat model using a drug discrimination procedure. Rats were trained to discriminate ethanol (1.25 g/kg, IP) from saline on a fixed-ratio schedule, FR10. Once rats had acquired the ethanol-saline discrimination, ethanol dose-response tests were conducted with 15-min pretest injections. Following the characterization of the ethanol dose-response curve, the effect of naltriben on ethanol's discriminative stimulus was assessed by administering naltriben (0. 032-5.6 mg/kg, IP) 15 min before the ethanol administration. In the present study, naltriben did not have any modulatory effect on ethanol discrimination, suggesting that either Delta(2)-opiate receptors are not involved in the formation of ethanol's discriminative stimulus or the antagonism of Delta(2)-opiate receptors is not sufficient to alter ethanol's compound discriminative stimulus.

The influence of opioid antagonists on the discriminative stimulus effects of ethanol

The aim of the present study was to investigate the role of different endogenous opioid systems in the expression of ethanol's discriminative stimulus effects in a two-lever operant drug discrimination paradigm. Wistar rats trained to make differential responses following the administration of ethanol (1 g/kg, i.p.) or saline. The correct response (fixed-ratio schedule; FR10) resulted in the presentation of food. Once rats had acquired the discrimination an ethanol dose-response test was conducted. The effects of opioid antagonists on the discrimination were assessed by administering the mu-opioid receptor antagonists naloxone (0.5-20 mg/kg s.c.) and cyprodime (5-100 mg/kg s.c.) and the delta-opioid receptor antagonist naltrindole (0.1-25 mg/kg s.c.) 15-30 min before the discrimination test. Furthermore, the selective kappa-opioid antagonist nor-binaltorphimine (5 mg/kg s.c.) given 24 h before the test session was examined. Results of generalization testing demonstrate that ethanol discrimination was dose dependent. Pretreatment with naloxone produced only at the highest dose a partial, but significant, antagonism, whereas cyprodime failed to alter the ethanol cue. This suggested the involvement of other opioid receptor subtypes. However, neither naltrindole nor nor-binaltorphimine had any effect on the ethanol-saline discrimination. These results demonstrate that the expression of the ethanol cue is only partly dependent on the function of endogenous opioid systems.

Naloxone, but not Tyr-MIF-1, reduces volitional ethanol drinking in rats: correlation with degree of spontaneous preference

The possible relationship between the actions of ethanol and opiates led us to examine the effect of opiate antagonists on ethanol intake in rats with a free choice of water. Naloxone (NAL) significantly reduced intake of ethanol. This effect was much greater in "high-preferring" (ethanol/total fluid intake greater than 60%) than in "low-preferring" (ethanol/total fluid intake less than 30%) rats. Furthermore, a correlation was found between the degree of spontaneous preference (ethanol/total fluid intake ratio) and the reduction of ethanol drinking by NAL. Sensitivity to NAL increased with increased preference for ethanol. Neither Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) nor MIF-1 (Pro-Leu-Gly-NH2) caused a significant modification of ethanol intake. This study shows that NAL can reduce volitional ethanol intake in rats and provides further evidence that Tyr-MIF-1 does not always act like NAL.

The role of dopamine and serotonin receptors in the mediation of the ethanol interoceptive cue

The drug discrimination paradigm was used to evaluate the contribution of dopamine or serotonin receptors in the mediation of the stimulus properties of ethanol. Briefly, rats were trained to discriminate between ethanol (600 mg/kg, IP) and water vehicle. Dose-response relationships were observed for ethanol and rats were tested with various dopamine and serotonin receptor agonists and antagonists. The specific dopamine receptor agonists SKF 38393 (DA1) and LY 171555 (DA2) failed to produce appreciable ethanol-like stimulus effects. Furthermore, the dopamine receptor antagonists SCH 23390 (DA1) and haloperidol (DA2) did not affect ethanol-appropriate responding when administered in combination with the training dose of ethanol. A number of specific serotonergic receptor ligands were also tested. Quipazine, 5-MeODMT, buspirone, 8-OH-DPAT elicited intermediate ethanol-like stimulus properties in rats. The serotonin receptor blockers pizotifen, pirenperone and (-)propranolol were ineffective in blocking the interoceptive cue produced by 600 mg/kg ethanol. However, TFMPP produced strong ethanol-like discriminative properties and completely substituted for the training dose of ethanol. These results indicate that the stimulus properties of TFMPP are similar to those of a low dose of ethanol.

Enkephalinase inhibition: regulation of ethanol intake in genetically predisposed mice

This is the first report of alteration in alcohol intake in mice with a genetic predisposition to alcohol preference and known to have innate brain enkephalin deficiencies. We have been able to significantly attenuate both volitional and forced ethanol intake respectively by acute and chronic treatment with hydrocinnamic acid and D-phenylalanine, known carboxypeptidase (enkephalinase) inhibitors. Since these agents, through their enkephalinase inhibitory activity, raise brain enkephalin levels, we propose that excessive alcohol intake can be regulated by alteration of endogenous brain opioid peptides.

Alcohol and opioid peptides: neuropharmacological rationale for physical craving of alcohol

Until recently alcoholism was regarded as an incurable psychological problem. During the last decade a chain of research has led to important hypotheses about the etiology of the physical craving of alcohol. Recent discoveries indicate that the brain has receptor sites for naturally occurring opiatelike substances (endorphins, enkephalins, and dynorphins) which are produced by the nervous system. Opiates such as morphine or heroin and some of the metabolic products of alcohol (tetrahydroisoquinolines) can also attach themselves to these receptors. It has been further discovered that the physiological craving for alcohol may be the result of a deficiency of the naturally occurring opiatelike substances as well as other neurochemical deficits (i.e., dopaminergic, GABAergic, and serotonergic). These neurochemical deficits can occur genetically or as a result of long-term heavy drinking.

Evidence from behavioural and in vitro receptor binding studies that the enkephalinergic system does not mediate acute ethanol effects

The behavioural disturbances produced by acute exposure to ethanol have been related to changes in function of the opioid systems in the CNS. However, evidence in the literature is conflicting. The present report concerns the possible role of the enkephalinergic system in the mediation of acute ethanol effects. We used rats to study the ability of a selective opioid delta receptor antagonist (ICI 154129) to prevent the effect of ethanol on pain sensitivity, body temperature, sensorimotor performance and level of consciousness. Furthermore, in vitro receptor binding was measured to investigate whether or not ethanol, within a non-lethal concentration range, would change the binding parameters of the delta receptor ligand [3H][D-ala2, D-Leu5]enkephalin. ICI 154129 did not significantly influence the effects of ethanol in the behavioural experiments. Ethanol did not significantly change the binding parameters whether saturation or competition was measured in the receptor binding experiments. Thus, there was no evidence that the enkephalinergic system mediated the acute ethanol effects.

A drug discrimination analysis of ethanol-induced behavioral excitation and sedation: the role of endogenous opiate pathways

The drug discrimination technique was used to characterize the biphasic behavioral effects of ethanol (ETOH) and examine the role of opiate pathways in mediating ETOH's excitatory (EX) and sedative (SED) phase effects. Forty Sprague-Dawley rats were trained to perform ETOH vs. saline discriminations during either ETOH's EX (6 min post-dose) or SED (30 min post-dose) phase of action in a double-lever food reinforced operant paradigm. After animals had achieved the required performance criterion, dose-response and phase-generalization tests were conducted in each group. In addition, preliminary studies were conducted to examine the effect of naloxone (NLX) pretreatment on discrimination of ETOH's EX and SED phase stimulus effects. Results of generalization testing demonstrate that discrimination of ETOH was dose-dependent and there was no generalization between phases regardless of the initial training condition. NLX pretreatment significantly attenuated discrimination of ETOH's EX phase effects but was ineffective in antagonizing ETOH's SED effects. These data demonstrate that the stimulus properties of ETOH are phase-dependent and suggest that opiate pathways are critical for the expression of ETOH's EX phase behavioral effects.

Ethanol and tetrahydroisoquinoline alkaloids do not produce narcotic discriminative stimulus effects

Male Sprague-Dawley rats were trained in a two lever food-reinforced procedure to discriminate between the effects of saline and the synthetic narcotic analgesic fentanyl (0.04 mg/kg). After acquisition of this discrimination, generalization tests with morphine, ethanol and some tetrahydroisoquinoline alkaloids were conducted. The rats dose-dependently generalized the effect of morphine but did not generalize the effects of either ethanol, tetrahydropapaveroline, salsolinol or 3-carboxysalsolinol to the fentanyl discriminative stimulus. Thus, these date do not support a biochemical link between ethanol and opiates.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Naloxone alters alcohol drinking induced in the rat by tetrahydropapaveroline (THP) infused ICV

In male rats of the Sprague-Dawley or Long-Evans strain, intracerebroventricular (ICV) cannulae were implanted permanently using stereotaxic techniques. Tetrahydropapaveroline (THP) was infused ICV for up to 14 days either chronically around the clock or acutely once per day in doses previously found to induce an abnormally high intake of alcohol. During these periods, alcohol preference for individual rats was determined by a self-selection procedure in which the concentration of alcohol was increased from 3 to 30% on each day of a 12-day interval. Those rats which displayed a substantial increase in their intake of alcohol were selected for naloxone treatment and subsequently assigned a fixed concentration of alcohol at which maximum consumption occurred. Naloxone was administered subcutaneously two to six times per day for three consecutive days in total daily doses ranging from 1.5 to 3.0 mg/kg. Each rat served as its own control and was given 0.9% saline isovolumetrically according to the same temporal schedule. Naloxone generally suppressed alcohol intake in animals by 20% to 45%, but the reduction in drinking depended upon the injection regimen as well as the prior level of alcohol consumption. In "high drinking" rats, the mean alcohol intake of 6.2 g/kg/day was reduced to 3.7 g/kg/day by naloxone whereas in "low drinkers" the mean intake of 3.5 g/kg/day was suppressed to 2.8 g/kg/day by the opiate antagonist. These results further support the suggestion of a possible opiate receptor link in the pathogenesis and maintenance of aberrant drinking of alcohol, the mechanism of which may involve the endogenous action of an amine-aldehyde condensation product in the brain.