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

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.

Neuropeptides and alcohol addiction in monkeys

Neuropeptides have been implicated in experimental drug addiction. Desglycinamide (Arg8) vasopressin (DGAVP) attenuates heroin and cocaine intake during initiation of drug self-administration in rats. beta-Endorphin is self-administered in rats and a role of endogenous opioids in cocaine reward has been proposed. The present studies deal with voluntary alcohol consumption in monkeys under free choice conditions. Monkeys initiated alcohol drinking within a few days and after a stable drinking pattern was acquired increased their ethanol consumption during a short period following interruption of the alcohol supply (relapse). The alcohol drinking behavior seems under the control of reinforcement principles. DGAVP reduced the acquisition of alcohol drinking in the majority of treated monkeys. Initiation of alcohol drinking induced modifications in neuroendocrine homeostasis e.g. an increased plasma beta-endorphin. Both the opioid antagonist naltrexone and the opioid agonist morphine dose-dependently decreased alcohol intake during continuous supply and after imposed abstinence. The monkeys were more sensitive to both drugs after imposed abstinence. The effects are interpreted in the context of the endorphin compensation hypothesis of addictive behavior. It is suggested that endorphins may be particularly implicated in craving for addictive drugs and in relapse of addictive behavior.

Interaction between alcohol deprivation and morphine withdrawal in mice

Hybrid mice having had 12 weeks free access to flavored alcohol were deprived of it for 3 days. The increase in alcohol intake rate during first 1.5 h of renewal access as compared with its intake rate during the remaining 22.5 h (alcohol-deprivation effect, ADE) was used to select individuals having (HD) or lacking (LD) the ADE. The mice received morphine or placebo pellet implantation for 4 days followed by removal of alcohol and pellets. Cross-maze, acoustic startle response, and tail flick tests were conducted 1 day after morphine withdrawal and after 2 days of alcohol deprivation. Next day, during a second ADE test, placebo-treated HDs again showed ADE and LDs showed no ADE; the HDs during morphine withdrawal also still showed an ADE similar to those in placebo HDs, but LDs after morphine removal differed in showing an ADE. Significantly more hyperalgesia in HD than LD mice was observed during morphine withdrawal. Placebo HDs differed from placebo LDs in showing stereotyped behavior and kindling-like rise in acoustic startle response. Morphine withdrawal caused body weight loss, and converted the startle response curve in both LD and HD mice to a bell-shape curve. In HD mice the alcohol deprivation behavioral pattern was substituted by the corresponding morphine withdrawal pattern. The existence of distinct neural circuits responsible for the development of alcohol and morphine dependence is proposed.

L-glutamate abolishes differential responses to alcohol deprivation in mice

Hybrid mice produced by crossing CBA and C57BL lines were deprived of alcohol for three days after prolonged prior access. Subsequently, when access was first given to a flavored 30% alcohol solution, about half of the mice showed a greatly elevated rate of drinking during the first 1.5 h, characteristic of the alcohol-deprivation effect (ADE), but other mice showed no increase. Repeating the test three weeks later showed that having or lacking an ADE is a stable group characteristic. Behavioral differences were found on cross-maze and slip funnel tests between mice that had an ADE and those that lacked it. Topical application of L-glutamate to the frontal cortex prevented the subsequent elevation of alcohol drinking during the first 1.5 h after deprivation but did not alter drinking during the remaining 22.5 h. L-glutamate treatment also affected those cross-maze behaviors found to be related to the ADE. The results suggest that frontal cortex neurons sensitive to L-glutamate are necessary for the ADE and that comparisons between hybrid mice having and lacking an ADE might be used for determining the neuronal mechanisms responsible for the effect.

Responding for oral ethanol after naloxone treatment by alcohol-preferring AA rats

The present study examined the effect of a relatively nonselective opioid antagonist, naloxone, on lever pressing for oral ethanol by the alcohol-preferring AA rats. The AAs, housed continually in operant chambers with free access to food and water, learned to respond for 10% oral ethanol during daily 60-min alcohol access periods indicated by a stimulus light. The rats developed stable ethanol responding, resulting in mean ethanol intakes of 1.2 g/kg/60 min and measurable blood alcohol levels. In the first experiment, single systemic injections of naloxone (0.05-2.5 mg/kg) had no effect on the initial rate of responding; dose-dependent decreases were observed later during the alcohol access. The second experiment examined the effects of repeated injections of 0.5 and 2.5 mg/kg naloxone on 5 consecutive days. Naloxone suppressed responding dose-relatedly over the treatment days. In contrast to the effects of single injections, repeated injections with 2.5 mg/kg naloxone produced progressive decreases within the first minutes of access. The results suggest that naloxone may attenuate the reinforcing actions of ethanol.

The alcohol-deprivation effect in hybrid mice

Hybrid mice (F1 CBA x C57Bl/6) after prolonged access to saccharin-menthol flavored 30% alcohol (FA) were deprived of it on two occasions, followed by test choices between (1) FA and water, and (2) FA and similarly flavored water. Although many mice failed to show deprivation-induced increases in FA intake, the mean rate of FA drinking during the first 1.5 h of renewed access was significantly increased even in the second test. Comparisons between the two tests suggest that flavor factors are generally less important than ethanol-seeking behavior.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Differences in the brain and pituitary beta-endorphin system between the alcohol-preferring AA and alcohol-avoiding ANA rats

The content of pro-opiomelanocortin (POMC) mRNA was determined in the hypothalamus, as well as in the anterior and intermediate lobes of the pituitary gland of the alcohol-preferring AA and alcohol-avoiding ANA rats under basal conditions. In addition the content of beta-endorphin-like immunoreactivity (beta-EPLIR) was measured in nine brain regions, the anterior and intermediate lobes of the pituitary gland and the serum. The content of beta-EPLIR was significantly higher in the septum and significantly lower in the amygdala, and periaqueductal gray matter of the AA rats, while there was no significant difference between the AA and ANA rats in the arcuate nucleus plus median eminence, nucleus accumbens, caudate, hippocampus, and cortex. HPLC analysis indicated no significant differences in the relative proportions of non-acetyl and acetyl forms of beta-endorphin peptides in the hypothalamus, distinct brain regions, and anterior and neurointermediate lobes of the pituitary gland, between the AA and ANA rats. The content of POMC mRNA but not of beta-EPLIR was significantly higher in the hypothalamus and neurointermediate lobe of the AA rats, while the content of both beta-EPLIR and POMC mRNA were significantly higher in the anterior pituitary of the AA than of the ANA rats. Thus, there are genetically determined differences in the pituitary and brain beta-endorphin system between the AA and ANA rats, which may be important in controlling the differences in the voluntary ethanol consumption exhibited by these animals.

Low doses of morphine reduce voluntary alcohol consumption in rhesus monkeys

Experimental opioid modulation has been found to influence the consumption of alcohol in animals. Whereas it has generally been agreed upon that opiate antagonists reduce alcohol consumption, the results with opiate agonists are less consistent. The present study reports on the effect of low doses of morphine in 8 adult male rhesus monkeys that had a free choice in drinking water, a 16% and a 32% ethanol/water solution, (a) during continuous ad libitum access (Experiment I), and (b) after 2 days of alcohol abstinence (Experiment II). In both experiments each monkey received a single morphine injection (i.m.) in 5 different doses (0.03, 0.06, 0.17, 0.50, 1.50 mg.kg-1); each morphine injection (i.m.) was placebo-controlled in a cross-over design. Consumption was measured from 16.00 h in the afternoon (30 min after injection) to 08.30 h the next morning. In Experiment I after 0.50 and 1.50 mg.kg-1 of morphine ethanol intake and water consumption were both reduced during the first hours after injection; only ethanol intake remained reduced during the subsequent night. Effects lasted not longer than 24 h. In Experiment II, morphine administered 30 min before reintroduction of ethanol solutions reduced ethanol intake at doses of 0.17, 0.50 and 1.50 mg.kg-1; water consumption was unaffected. The reduction lasted for the subsequent night after the 2 highest doses. Records obtained of various spontaneous behavioural activities made it unlikely that the used dose range had induced some aspecific sedation; monkeys remained alert and active. The results are contradictory with studies in which low doses of morphine stimulated alcohol drinking in rats. The present results seem to support the hypothesis that at least in monkeys morphine can compensate for some effects of alcohol.