Central vs. peripheral mediation of opioid effects on alcohol consumption in free-feeding rats

A critical review of front-loading: A maladaptive drinking pattern driven by alcohol's rewarding effects

Front-loading is a drinking pattern in which alcohol intake is skewed toward the onset of reward access. This phenomenon has been reported across several different alcohol self-administration protocols in a wide variety of species, including humans. The hypothesis of the current review is that front-loading emerges in response to the rewarding effects of alcohol and can be used to measure the motivation to consume alcohol. Alternative or additional hypotheses that we consider and contrast with the main hypothesis are that: (1) front-loading is directed at overcoming behavioral and/or metabolic tolerance and (2) front-loading is driven by negative reinforcement. Evidence for each of these explanations is reviewed. We also consider how front-loading has been evaluated statistically in previous research and make recommendations for defining this intake pattern in future studies. Because front-loading may predict long-term maladaptive alcohol drinking patterns leading to the development of alcohol use disorder (AUD), several future directions are proposed to elucidate the relationship between front-loading and AUD.

Pharmacologic dissociation between impulsivity and alcohol drinking in high alcohol preferring mice

BACKGROUND

Impulsivity is genetically correlated with, and precedes, addictive behaviors and alcoholism. If impulsivity or attention is causally related to addiction, certain pharmacological manipulations of impulsivity and/or attention may affect alcohol drinking, and vice versa. The current studies were designed to explore the relationship among impulsivity, drinking, and vigilance in selectively bred High Alcohol Preferring (HAP) mice, a line that has previously demonstrated both high impulsivity and high alcohol consumption. Amphetamine, naltrexone, and memantine were tested in a delay discounting (DD) task for their effects on impulsivity and vigilance. The same drugs and doses were also assessed for effects on alcohol drinking in a 2-bottle choice test.

METHODS

HAP mice were subjected to a modified version of adjusting amount DD using 0.5-second and 10-second delays to detect decreases and increases, respectively, in impulsive responding. In 2 experiments, mice were given amphetamine (0.4, 0.8, or 1.2 mg/kg), naltrexone (3 and 10 mg/kg), and memantine (1 and 5 mg/kg) before DD testing. Another pair of studies used scheduled access, 2-bottle choice drinking to assess effects of amphetamine (0.4, 1.2, or 3.0 mg/kg), naltrexone (3 and 10 mg/kg), and memantine (1 and 5 mg/kg) on alcohol consumption.

RESULTS

Amphetamine dose-dependently reduced impulsivity and vigilance decrement in DD, but similar doses left alcohol drinking unaffected. Naltrexone and memantine decreased alcohol intake at doses that did not affect water drinking but had no effects on impulsivity or vigilance decrement in the DD task.

CONCLUSIONS

Contrary to our hypothesis, none of the drugs tested here, while effective on either alcohol drinking or impulsivity, decreased both behaviors. These findings suggest that the genetic association between drinking and impulsivity observed in this population is mediated by mechanisms other than those targeted by the drugs tested in these studies.

The neurobiology of cocaine-induced reinforcement

Cocaine has potent pharmacological actions on a number of monoaminergic systems in the brain, including those that use noradrenaline, dopamine and serotonin as neurotransmitters. There is growing evidence that cocaine's effects on dopaminergic neurons, particularly those that make up the mesolimbic system, are closely associated with its rewarding properties. For example, low doses of dopamine receptor antagonists reliably influence cocaine self-administration, whereas noradrenaline and serotonin receptor antagonists are without consistent effects. Similarly, selective lesions of dopaminergic terminals in the nucleus accumbens, a major target of the mesolimbic dopamine projection, disrupt cocaine self-administration in a manner that is consistent with loss of cocaine-induced reward. The introduction of in vivo brain microdialysis as a tool with which to investigate the neurochemical correlates of motivated behaviour has provided new opportunities for investigating the role of dopamine in the nucleus accumbens in the acquisition and maintenance of cocaine self-administration. Although the body of literature that has been generated by this approach appears to contain some important inconsistencies, these probably reflect the use of inappropriate microdialysis conditions by some investigators. A critical review of the literature suggests that microdialysis results are generally consistent with a role for mesolimbic dopamine in cocaine-induced reward, although it does not seem to be the case that animals will work to maintain consistent increases in extracellular concentrations of dopamine in the nucleus accumbens in all experimental conditions. Elucidation of the complete neural circuitry of cocaine-induced reward remains an important priority for future research.

An investigation of the role of 5-HT(2C) receptors in modifying ethanol self-administration behaviour

We have previously reported that the 5-HT uptake blocker and releaser, dexfenfluramine, attenuates ethanol intake, and that this may be mediated via a 5-HT(2C) receptor mechanism. Our goals were to further determine the contribution made by this receptor subtype in mediating the reduction in ethanol self-administration induced by dexfenfluramine using the selective 5-HT(2C) antagonist, SB242,084. Additionally, we wanted to compare dexfenfluramine's effects on ethanol motivated responding with those elicited by the 5-HT(2C) receptor agonist Ro60-0175. In male Wistar rats trained to self-administer a 12% w/v ethanol solution on an FR-4 schedule, both dexfenfluramine (0.05--2.5 mg/kg ip) and Ro60-0175 (0.1--1 mg/kg sc) produced a significant dose-dependent reduction in ethanol self-administration, which was reversed by SB242,084 (0.5 mg/kg ip). Interestingly, SB242,084 alone (0.1--1 mg/kg ip) significantly increased ethanol motivated responding in both high and low ethanol drinking animals. While dexfenfluramine had no effect on ethanol's kinetic profile, the selective 5-HT(2C) agents used had opposing effects, with the agonist Ro60-0175 decreasing and the antagonist SB242,084 increasing blood ethanol levels. Since there were incongruent drug effects on ethanol self-administration and blood ethanol levels, these data support a role for 5-HT(2C) receptors in modifying ethanol intake independent of their effects on blood ethanol kinetics. Furthermore, 5-HT(2C) receptors may exert a tonic control over ethanol self-administration behaviour, since agonist and antagonist administration had opposing effects on this behaviour.

Genetic selection for high and low alcohol consumption in a limited-access paradigm

BACKGROUND

Several rat lines have been bred for their differences in alcohol consumption based on a continuous-access paradigm in which alcohol solution is available 24 hr/day. The limited-access paradigm (LAP), in which access to alcohol solution is restricted to a short period per day, however, has been used extensively to investigate the neurochemical mechanisms underlying alcohol consumption. There is evidence of possible differences in genetic determination of alcohol drinking in a continuous- versus limited-access condition. For these reasons, selective breeding for high- and low-alcohol consumption (HARF and LARF, respectively) based on a LAP was conducted.

METHODS

N/Nih rats were used as the breeding stock. A within-family breeding procedure was used to develop HARF and LARF lines with 10 families per line. Access to alcohol solution was restricted to 20 min/day. Alcohol was provided as 3%, 6% and 12% w/v solutions. Average intake of alcohol during the 12% phase was used as the selection criterion. Inbreeding began in the seventh generation.

RESULTS

After the sixth generation of selection, rats from the HARF line consumed an average of 1.2 g/kg, whereas rats from the LARF line consumed an average of 0.6 g/kg of alcohol during the 20-min access period. Alcohol consumption remained stable over the next eight generations of inbreeding. In the continuous-access-drinking paradigm, the HARF and LARF rats consumed an average of 5.5 to 7.0 g/kg and 1.0 to 2.0 g/kg of alcohol per day respectively. An estimated heritability of 0.25 was obtained.

CONCLUSIONS

These findings indicate that alcohol drinking in the LAP is influenced by genetic factors. Differences in alcohol drinking in the LAP also generalize to continuous access drinking. These rat lines will be very useful for investigations into the genetic and neurochemical mechanisms underlying alcohol drinking.

Caffeine promotes ethanol drinking in rats. Examination using a limited-access free choice paradigm

There is growing evidence that caffeine may alter the pattern of intake of a variety of drugs. The present study was designed to assess the effect of caffeine pretreatment on voluntary ethanol consumption. The first experiment examined the effect of caffeine on the acquisition of ethanol intake in a limited-access-choice procedure in which water and ethanol were presented concurrently. Male Wistar rats, exposed to food and water ad lib, were presented with a daily 1-h choice session between water and progressively increasing concentrations of ethanol (2-10%). Each ethanol concentration was made available for 4-6 days for a total of 20 days of access to ethanol. Intraperitoneal injections of caffeine (5 or 10 mg/kg) or saline were administered to the rats 30 min prior to each choice session. Caffeine produced a dose-related facilitation in ethanol drinking whereby the lower caffeine dose produced enhancement in ethanol drinking. The second experiment examined the effect of caffeine on the maintenance of established ethanol consumption. Male Wistar rats, initially acclimatized to increasing concentrations of ethanol (2%-10), were presented with an additional 18 ethanol (10%) presentations, comprised of a 6-day baseline period followed by 6 days of treatment where animals were given one of three doses of caffeine (2.5, 5 or 10 mg/kg) or saline prior to ethanol presentation. A final 6-day post-treatment period followed treatment. These results revealed an inverted-U effect of caffeine dose on ethanol ingestion where the low and high caffeine doses produced no effect but the moderate dose of 5 mg/kg enhanced ethanol drinking that persisted throughout the post-treatment period. A third experiment revealed that caffeine did not alter levels of blood ethanol within the time period used for the ethanol drinking session.

Peripheral opioid receptors may mediate a portion of the aversive and depressant effect of EtOH: CPP and locomotor activity

Previous investigators have reported that peripheral opioid receptors (located in the gut) may produce aversive effects when activated. Opioid receptors can be activated by endogenous opioids or by-products of ethanol (EtOH) metabolism [e.g., tetrahydroisoquinolines (TIQs)]; both are stimulated following EtOH consumption. Therefore, we tested the hypothesis that a portion of the aversive or depressant effects of EtOH may be mediated through, or modulated by, peripheral opioid receptors. Conditioned place preference (CPP) and locomotor activity were the dependent variables. Prior to EtOH gavage, we antagonized the peripheral opioid receptors with methylnaltrexone (MNTX), an opioid antagonist that does not easily pass through the blood-brain barrier. The effects of EtOH were found to be dose dependent: 1.5 g/kg was hedonically neutral but depressed locomotor activity; 2.25 g/kg EtOH was aversive and also depressed locomotor activity. MNTX (32 mg/kg) treatment was rewarding and stimulated motor activity (especially during the first conditioning session). When combined, 1.5 g/kg EtOH tended to enhance the rewarding effects of MNTX whereas MNTX blocked the aversive effects of 2.25 g/kg EtOH. During the first conditioning session EtOH attenuated the motor stimulant effects of MNTX whereas MNTX antagonized the motor depressant effects of EtOH; there was little effect of MNTX on EtOH-induced motor depression during subsequent conditioning sessions. Pretreatment with various doses of MNTX (0.1, 1.0, 10.0, 32.0 mg/kg) of rats receiving 1.5 g/kg EtOH indicated the effects of MNTX were dose dependent. Drug-induced locomotor activity and time spent in the conditioned compartment were positively correlated, suggesting that both behaviors were homologous. The data suggest that peripheral opioid receptors participate in mediating or modulating a portion of the behavioral effects of EtOH.

Acute ethanol exposure decreases the analgesic potency of morphine in mice

Chronic (7 days), forced ethanol drinking can decrease the analgesic potency of opioid agonists in mice. In the present study, the effect of short-term ethanol treatment was examined using forced ethanol access and ethanol injection protocols. Mice were given forced access to 1, 3 or 7% (v/v) ethanol for 24 hr and then tested for s.c. morphine analgesia using the tailflick assay. Controls had access to water. Another group of mice was injected i.p. with 2.5 g/kg ethanol or water 4 times over a 21 hr period and tested 3 hr after the final injection for morphine analgesia. Other mice were injected once i.p. with 1, 2 or 3 g/kg ethanol or water and tested 24 hr later using the tailflick. In the forced access study, ethanol dose-dependently decreased morphine's analgesic potency with the highest dose (7%) producing a 1.6-fold shift in the ED50. This decrease in morphine potency was similar to that found in a related study using 7% ethanol for 7 days (1.8-fold shift). Repeated ethanol injections significantly reduced the analgesic potency of morphine (1.9-fold shift), whereas, a single injection of 1, 2 or 3 g/kg ethanol did not alter the potency of morphine. Control studies indicated that neither 24 hr water nor food deprivation affected morphine potency. Overall, these data show that sustained exposure to ethanol over a 24 hr period will dose-dependently decrease morphine's analgesic potency.

Behavioral responses to ethanol in light and moderate social drinkers following naltrexone pretreatment

The opioid antagonist naltrexone has been shown to be effective in the treatment of alcoholism, possibly by dampening the subjective effects of ethanol. However, naltrexone does not consistently attenuate the effects of ethanol in social drinkers in laboratory-based challenge studies. In the present study, 25 healthy volunteers, who were either light drinkers (mean = 3 drinks per week) or moderate drinkers (mean = 16 drinks per week), participated in six evening sessions. At each session, subjects ingested a capsule containing naltrexone (25 or 50 mg) or placebo, and 1 hr later they consumed a beverage containing ethanol (0.25 g/kg, equivalent to about two standard alcoholic drinks) or placebo. Subjects received all combinations of pretreatments and beverages. They completed self-report mood questionnaires and psychomotor tests at regular intervals. This low dose of ethanol produced modest but significant effects on self-report measures such as ratings of feeling a drug effect and of liking the drug effect. However, naltrexone (25 or 50 mg) pretreatment had no dampening effect on subjects' responses to ethanol. These results indicate that acute doses of naltrexone that are effective when administered chronically to alcoholics do not attenuate the acute effects of a low dose of ethanol in non-problem drinkers.

The effect of ethanol drinking on opioid analgesia and receptors in mice

Recent studies suggest substantial interactions between opioids and ethanol (EtOH). Both in vivo and in vitro experiments indicate that EtOH can regulate opioid systems and that opioids can modify EtOH consumption. In the present studies, we examined if EtOH consumption altered opioid receptors and the potency of opioid analgesics. Mice were given unlimited access to 6-7% EtOH alone for 7 days or were allowed to drink increasing concentrations (3-6%) of EtOH over 13-14 days. Controls had access to water. The EtOH groups drank significantly less volume than controls, although there were no significant differences in body weight or baseline nociception. The analgesic (tail flick) potency of SC morphine was decreased by approximately 1.6-2.0-fold in EtOH-treated mice. A single acute dose of EtOH (1 g/kg) that produced blood alcohol levels in excess of that for 7 day exposure to EtOH, did not change morphine's analgesic ED50, suggesting that chronic exposure to EtOH was necessary for the reduction in potency. The change in morphine potency was not due to pharmacokinetic differences because EtOH consumption did not modify the concentration of morphine in brain and spinal cord. The analgesic potency of a delta-opioid receptor agonist (ICV DSLET) was also decreased by approximately 2-fold. Saturation binding studies indicated no changes in the density or affinity of brain and spinal cord delta-opioid ([3H]DPDPE, [3H]DSLET, [3H]DeltorphinII) and mu-opioid ([3H]DAMGO) receptors. Similarly, there was no significant effect of EtOH on delta-opioid receptor mRNA in either brain or spinal cord preparations. Taken together, these data suggest that EtOH consumption decreases the analgesic potency of opioids in mice through a mechanism that is unrelated to pharmacokinetics or opioid receptor changes in brain and cord.

Morphine enhances selection of both sucrose and ethanol in a two-bottle test

The influence of a low dose of morphine on the self-selection of alcohol and sucrose solutions is investigated. When given a choice between sucrose sweetened ethanol and plain water, rats show a significant preference for the sweetened ethanol. However, when given a choice between sweetened ethanol and sweetened water, rats increase consumption of sweetened water. These results suggest that the low-dose morphine enhancement of sweetened alcohol solutions is mediated by the reinforcing properties of sucrose not ethanol. However, when rats receive small doses of morphine and a choice between unsweetened ethanol and water, the rats increase consumption of ethanol. Therefore, a low dose of morphine enhances the self-selection of both sucrose and ethanol solutions. This provides additional confirmation that opioids may enhance the rewarding properties of a variety of appetite reinforcers.

Characterization of the decline in alcohol consumption by aminopeptidase inhibition

Bestatin, an aminopeptidase inhibitor, prolongs the action of angiotensin and enkephalin. Previous studies have shown that bestatin can reduce alcohol intake in heterogeneous as well as genetically selected alcohol preferring 'P' rats, but more detailed studies of the characteristics of this effect have not yet been done. In experiment 1, daily injections of 10, 20 or 40 mg/kg bestatin for 12 days produced a uniform reduction in alcohol intake (approximately 40%) which did not recover following suspension of bestatin, but did recover following a 7 day period of morphine-stimulated alcohol drinking. In experiment 2, the lower end of the dose range was explored (2.5, 5, 10 mg/kg) and a dose-dependent effect was observed with a threshold dose of 5 mg/kg. Again, alcohol drinking did not recover following suspension of the 10 mg/kg dose of bestatin. Using a limited access procedure, bestatin reduced the intake of a preferred sodium chloride solution but not an avidly-consumed glucose solution indicating that while bestatin's effect was selective, it was not specific to alcohol. In the final experiment, the role of angiotensin and enkephalin was explored by pretreating animals with losartan (5, 10, 20 mg/kg), an angiotensin receptor blocker and naltrexone, (0.25, 0.5, 1 mg/kg) an opiate receptor blocker. Both agents produced significant but rather small attenuations in bestatin's effect suggesting that other factors are involved in its action. These findings further characterize the inhibitory effect of bestatin on alcohol intake in rodents. In addition, the novel finding of a carry-over inhibition of alcohol intake in the bestatin-free period adds further impetus to the recommendation that this drug, which has been safely used in humans, might be examined as a possible therapeutic agent for alcohol abuse.

Genetic differences in naloxone enhancement of ethanol-induced conditioned taste aversion

The influence of the opioid system on acquisition of an ethanol-induced conditioned taste aversion was examined in alcohol-preferring and avoiding inbred strains of mice (C57BL/6J and DBA/2J). Fluid-deprived mice from each strain received either ethanol alone, naloxone alone, or both ethanol and naloxone immediately after access to a novel tasting fluid. Naloxone alone (1 or 3 mg/kg) did not induce a conditioned taste aversion in either strain of mice. Administration of ethanol (1.5 g/kg) to DBA/2J mice produced a moderate taste aversion that was not affected by co-administration of naloxone. Although ethanol administered alone (3 g/kg) did not cause a taste aversion in C57BL/6J mice, the combination of ethanol and the higher dose of naloxone produced a significant taste aversion that increased across trials. A second experiment addressed the possibility that naloxone failed to enhance the ethanol-induced condition taste aversion in DBA/2J mice due to a "floor" effect on consumption. A lower ethanol dose (1 g/kg) was given alone or in combination with naloxone (1 or 3 mg/kg). Again, ethanol produced a moderate conditioned taste aversion that was not potentiated by naloxone. Subsequent conditioning with a high ethanol dose produced further suppression of intake, confirming that naloxone's failure to enhance aversion on earlier trials was not due to a "floor" effect. These data demonstrate a strain specific interaction between the aversive effect of ethanol and naloxone. More specifically, the results indicate that blockade of opioid receptors enhances the aversive effect of ethanol in C57BL/6J but not DBA/2J mice, suggesting that genetically determined differences in the endogenous opioid system of alcohol-preferring mice may mitigate ethanol's aversive effect.

Morphine enhancement of sucrose palatability: analysis by the taste reactivity test

The ability of morphine to modify sucrose palatability was assessed by the taste reactivity test. In Experiment 1, rats were injected with morphine (0.0, 0.5, 2.0, and 10.0 mg/kg, subcutaneously), 30 min before receiving a 10-min intraoral infusion of 2% or 20% sucrose solution. A dose of 2.0 mg/kg morphine enhanced ingestive reactions elicited by both concentrations of sucrose solution. In Experiment 2, the interval between morphine pretreatment and the taste reactivity test was manipulated. Rats given 2.0 mg/kg morphine 30 or 120 min before testing displayed enhanced ingestive reactions elicited by 20% sucrose solution during the first 5 min of a 10-min test. The results support the hypothesis that morphine enhances the hedonic assessment of sucrose solution.

Differences between alcohol-preferring (AA) and alcohol-avoiding (ANA) rats in the prodynorphin and proenkephalin systems

The motivation to drink alcohol and the eventual risk of becoming addicted are in part genetically determined. Because opioid peptides are considered central to motivated behaviors, we have analyzed opioid peptides in relevant areas of the brain of two outbred lines of rats: the alcohol-preferring [Alko Alcohol (AA)] line who voluntarily drink alcohol and the alcohol-avoiding [Alko Non-Alcohol (ANA)] line with negligible intake. (Met)enkephalinArg6Phe7 (MEAP) was measured as a marker of proenkephalin, and dynorphin A, dynorphin B, and (Leu)enkephalinArg6 as markers of the prodynorphin system. The major line differences and effects of alcohol intake were observed in mesolimbic brain areas. The mesolimbic dopamine pathway, which projects from the ventral tegmental area (VTA) to the nucleus accumbens, is central in the reward system. Basal levels of MEAP and dynorphin peptides were low in the nucleus accumbens of AA rats, whereas (Leu)enkephalinArg6 levels were lower in the VTA of these rats. Alcohol drinking caused MEAP levels in the accumbens to rise, but had no effect on prodynorphin peptides. Opioids also influence the nigrostriatal dopamine pathway. However, this study showed no significant differences for any peptide between rat lines, or effect of alcohol intake, in either substantia nigra or striatum, except for a decrease of nigral and striatal (Leu)enkephalinArg6 levels in alcohol-drinking AA rats. Large line differences were observed in the pituitary gland. AA rats had high basal levels of MEAP, which became even higher after voluntary alcohol consumption for 4 weeks, and low levels of dynorphin peptides, not affected by alcohol drinking.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced activity of the brain beta-endorphin system by free-choice ethanol drinking in C57BL/6 but not DBA/2 mice

The objective of the present studies was to investigate the effect of voluntary ethanol consumption for 21 days on the brain beta-endorphin system of C57BL/6 (alcohol-preferring) and DBA/2 (alcohol-avoiding) strains of mice. As expected, C57BL/6 mice consumed a significantly higher quantity of the 10% ethanol solution than the DBA/2 mice. Under basal conditions the content of beta-endorphin like peptides differed only in the nucleus accumbens, higher levels being found in the DBA/2 mice. Voluntary ethanol consumption induced an increase in the hypothalamic content of mRNA coding for proopiomelanocortin, associated with a significant increase in the tissue content of beta-endorphin-like peptides in the arcuate nucleus and septum of the C57BL/6 mice, but did not alter the activity of the brain beta-endorphin system of the DBA/2 mice. Since voluntary ethanol consumption was not associated with nutritional deficits and stress, the ethanol-induced enhanced activity of the brain beta-endorphin system of the C57BL/6 mice must be a direct effect of ethanol and may be important in controlling the voluntary ethanol consumption by this strain of mice.

The effects of selective blockade of delta and mu opiate receptors on ethanol consumption by C57BL/6 mice in a restricted access paradigm

The effects of naltrexone, naltrindole (a selective delta opiate receptor antagonist) and beta-funaltrexamine (beta-FNA; a selective mu opiate receptor antagonist) on alcohol intake by C57BL/6 mice in a restricted access paradigm were examined. During the pretreatment baseline phase, mice consumed an average of 1.3 g/kg during 1 h access sessions to a 12% alcohol solution. Treatment with naltrexone reduced alcohol consumption to about 50% of that of the saline controls. Treatment with beta-FNA had no effect on alcohol consumption whereas naltrindole reduced consumption to the same extent as that observed with naltrexone. The pattern of findings indicate that naltrexone's ability to reduce alcohol consumption can be attributed to blockade of delta opiate receptors. Implications for treatment in human clinical trials are indicated.

A small dose of morphine leads rats to drink more alcohol and achieve higher blood alcohol concentrations

Male Sprague-Dawley rats were maintained on a daily regimen of 22 hr of fluid deprivation followed by a 2-hr opportunity to take a sweetened alcoholic beverage and water for over 6 months. During the week before the formal procedures of the experiment described herein, access to the alcoholic beverage was limited to 1.5 hr, but access to water was still for 2 hr. Intakes of ethanol, in terms of g/kg, were tabulated at 30 min for half of the rats and at 90 min for the rest. On the day of formal procedures, half of the rats of the 30- and 90-min measures were given 1 mg/kg of morphine sulfate just before the drinking session, whereas the rest received physiological saline. Morphine increased mean g/kg intakes of ethanol, as compared with controls, at 30 and 90 min. Blood alcohol levels were also increased. These data suggest that the well-documented ability of small doses of morphine to increase rats' intake of ethanol is probably not related to its ability to produce gastrointestinal effects, but rather due to its ability to modulate central motivational mechanisms associated with ingestion.

Involvement of mu-opioid receptors in alcohol drinking by alcohol-preferring AA rats

The present study examined the role of mu- and delta-opioid receptors in alcohol drinking using antagonists selective for these receptor types. Food- and water-sated male and female AA (Alko, alcohol) rats consistently drank 10% alcohol during daily 30-min access periods in their home cages in the middle of the 12-h light phase. On 3 consecutive days, the animals received the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP, 1 micrograms ICv), the delta-receptor antagonist N,N-diallyl-Try-Aib-Aib-Phe-Leu-OH (ICI 174,864, 3 micrograms ICV), or saline 15 min before the alcohol access period. Relative to saline, the mu-antagonist CTOP decreased alcohol drinking both by males and females progressively over the 3 treatment days, with a continued suppression on the first days after the termination of the administration. Treatment with the delta-antagonist ICI 174,864 had no effect on alcohol drinking in males, and produced transient hind limb dysfunction and barrel rolling in over half of the females. These results suggest that selective blockage of mu-opioid receptors is sufficient to suppress alcohol drinking in AA rats.

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.

Saccharin intake predicts ethanol intake in genetically heterogeneous rats as well as different rat strains

Saccharin and ethanol intakes were measured in seven strains of rats known to differ in their preferences for ethanol: The Fawn-Hooded (FH), alcohol-preferring (P) and Maudsley Reactive rats have been reported to drink ethanol voluntarily, whereas the alcohol-nonpreferring, Maudsley Nonreactive and Flinders Line (FSL and FRL) rats do not. Saccharin and ethanol intakes were highly correlated (r = +0.61) over all strains, with the FH rats drinking the most of both solutions. Correlation coefficients between pairs of drinking versus nondrinking rat strains were even higher. In a second experiment, genetically heterogeneous F2 progeny from cross-breeding the ethanol-preferring FH rats with the ethanol-nonpreferring Flinders Resistant Line (FRL) rats were studied. The results indicated a high positive correlation between saccharin and ethanol intakes (+0.65). These findings suggest that the association between saccharin and ethanol intakes previously reported in rat strains with different preferences for ethanol may have a similar genetic basis.

Reducing the desire to drink. Pharmacology and neurobiology

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

Effects of morphine on acquisition and maintenance of ethanol and water intake patterns in rats

Two experiments were conducted to assess the effects of chronic subcutaneous injections of morphine (1.0 mg/kg) or saline on the pattern and amount of sweetened ethanol and water intake in fluid restricted Long-Evans rats. Following daily injections, 2-h two-bottle choice tests were conducted with water and an ethanol solution (15% ethanol v/v in 5% sucrose w/v). During a 20-day acquisition phase (Experiment 1), ethanol intake patterns and amounts did not differ between saline (n = 6) and morphine (n = 6) groups. Both groups exhibited ethanol intake patterns that decreased exponentially throughout the session suggesting control by fluid restriction procedures. Morphine decreased water intake during initial periods of each session and increased intake during later periods. In Experiment 2, morphine and saline injections were reversed across three phases with the same rats. Morphine increased total ethanol consumption during the first few days of each 15-day phase, but the groups did not differ thereafter, and the initial increases produced no statistically significant group differences. Additionally, morphine augmented ethanol intake in early portions of sessions, while water intake was decreased and increased during early and later portions of each session, respectively. Analysis of the data from the last 5 days of each phase indicated that, when injected with morphine, the group which received saline during acquisition consumed significantly more ethanol solution than the group injected with morphine during acquisition. The effect on patterns of water and ethanol intake were observed, regardless of the drug injected during acquisition.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of morphine and naloxone on ethanol- and sucrose-reinforced responding in nondeprived rats

In the following series of experiments, effects of morphine (0.1, 0.3, 1.0, 3.0, and 10.0 mg/kg) and naloxone (0.1, 0.3, and 1.0 mg/kg) were assessed in nondeprived rats trained to leverpress with 10% ethanol, sweetened ethanol, or 5% sucrose and water as the reinforcers. Morphine, at doses of 0.1, 0.3, and 1.0 mg/kg had little effect on responding with ethanol or sweetened ethanol available on a fixed ratio 4 (FR4) schedule of reinforcement, but at the 3.0 mg/kg dose, morphine suppressed responding to near zero. Similar results were obtained when 10% ethanol and water were available on a concurrent FR4 FR4 schedule of reinforcement. When 5% sucrose and water were available concurrently, morphine suppressed responding at 3.0 and 10 mg/kg. Naloxone (0.1, 0.3, and 1.0 mg/kg) decreased responding for ethanol, sweetened ethanol, and sucrose solutions in a dose-dependent manner. Naloxone decreased total number of responses/session by shortening the duration of responding without affecting momentary rate. Overall, the data suggest that the endogenous opioid system plays a role in the ability of ethanol to reinforce operant behavior. However, this role does not appear to be specific to ethanol because similar results were observed with sucrose reinforcement. Failure to find enhanced ethanol intakes following morphine injections in the operant situation suggests that the method used to measure ethanol self-administration makes a difference in assessing the effects of drugs on ethanol intake.

Taste preferences in rat lines selected for low and high alcohol consumption

Alcohol-avoiding (ANA), alcohol-preferring (AA), and control Wistar rats were tested sequentially for their initial preferences for single concentration solutions of quinine, saccharin, salt, and citric acid, and then for an ascending series of saccharin concentrations. A similar study was subsequently conducted with the alcohol-nonpreferring (NP) and alcohol-preferring (P) rat lines. Both lines developed for low alcohol consumption drank much less saccharin than their respective lines developed for high alcohol intake when tested with the single concentration and with the ascending series. The ANAs also generally drank less of the bitter, salty, and sour solutions than the AAs or Wistars but little difference was found between the NPs and Ps with the other tastes. The curve relating saccharin consumption to concentration reached a maximum at about the same concentrations for AAs, Wistars, NPs, and Ps but for the ANAs, was shifted to the left. The results support a close relationship between the genetic factors influencing alcohol and saccharin intake in both line pairs. This relationship is probably not caused by saccharin tasting like alcohol to a rat, because other results indicate that the NPs do not have more negative reactions initially to the taste of alcohol, but it might be related to similar mechanisms mediating the reinforcement from sweet tastes and from systemic alcohol.

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.

Tests of opioid deficiency hypotheses of alcoholism

Eighty-three male rats were maintained on a daily regimen involving 22 h of deprivation of fluids followed by 2 h of access to water and a sweetened alcoholic beverage (12% ethanol, 5% sucrose). After about 3 weeks of such a regimen, rats eventually take considerable amounts of ethanol daily. In the present study, a series of injections of opioids was given subsequent to establishing stable daily intakes of ethanol. Specifically, before a day's opportunity to take fluids, some rats were given a small dose of morphine (2.0 mg/kg), while others were given a dose of naloxone (4.0 mg/kg). When morphine was given 0.5 h before the opportunity to drink, intake of ethanol was increased. However, when morphine was given 4.0 h before the opportunity, intake of ethanol was decreased. Nearly opposite effects were observed when naloxone was given. Other experiments tested the effects of giving morphine 4.0 h before the opportunity to drink plus the effects of a small dose of naloxone or plus the effects of a small dose of morphine. Morphine given 4.0 h before potentiated the effects of a small dose of naloxone and attenuated the effects of another dose of morphine. The effects of morphine were also shown to be similar among rats taking a solution of ethanol and water rather than a sweetened solution. These data provide support for the idea that surfeits, not deficits, in opioidergic activity increase propensity to take alcoholic beverages.

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.

Opiate antagonists reduce cocaine but not nicotine self-administration

Rats were trained to self-administer cocaine in 1-h sessions on a fixed ratio 5 (FR5) schedule of reinforcement. Acquisition was carried out at a unit dose of 0.3 mg/kg and responding was then stabilized at cocaine doses of 0.1, 0.3, and 1.0 mg/kg/infusion. Pretreatments with naltrexone (0.1-10 mg/kg, SC) 20 min prior to the start of self-administration sessions resulted in decreases in cocaine self-administration at doses of 0.1 and 0.3 mg/kg/infusion, but not at 1.0 mg/kg/infusion. Decreases depended on the dose of naltrexone used, with greater decreases in self-administration occurring at higher antagonist doses. In addition, treatment with the opiate antagonist naloxone also reduced cocaine self-administration at a unit dose of 0.3 mg/kg. A group of rats trained to self-administer nicotine at a dose of 0.03 mg/kg/infusion on the same schedule of reinforcement was unaffected by naltrexone treatment. These results may indicate that an endogenous opiate system plays a role in cocaine reinforcement.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 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 functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.

Intracerebroventricular morphine enhances alcohol consumption by rats

Previous experiments have shown that systemically administered low doses of opioid agonists increase subsequent alcohol consumption by rats. In this experiment, 10 micrograms of morphine were infused intracerebroventricularly (ICV) in free-feeding rats, daily for 6 days, 30 min prior to one-hour access to a 12% alcohol solution. Alcohol consumption was significantly increased in the morphine-treated group compared to that of a saline-treated control group, confirming that the locus of the effect is within the central nervous system.

Modulation of ethanol-intake by morphine: evidence for a central site of action

Previous studies have shown that subcutaneous administration of low doses of morphine increase, while subcutaneous naloxone decreases, ethanol-intake in rats. However, the site of action of morphine modulation of ethanol-intake remains unclear. In an attempt to elucidate this issue, seven graded doses of morphine were given intracerebroventricularly to rats 15 min prior to an opportunity to consume water and sweetened alcoholic beverage for 2 hr. Two lower doses of intracerebroventricular morphine (1, 3 micrograms) reliably increased ethanol-intake, while higher doses (3-30 micrograms) decreased intake of water. Preference ratios (ethanol-intake divided by total fluid-intake) were reliably increased by morphine doses of 1 microgram and higher. The present data provide support for a central site of morphine modulation of ethanol-intake.

Drugs to decrease alcohol drinking

A wide variety of drugs have been tested in experimental animals and several have been found that reduce voluntary alcohol drinking. The available evidence suggested that the same drugs also reduce alcohol drinking in alcoholics. Various factors limited or prevented the clinical use of the these drugs. Our working hypothesis has been that alcohol drinking is a learned response, reinforced primarily from alcohol in the brain, and that an alcoholic is a person in which this response and the related craving have become so strong that they dominate the behaviour and interfere with normal functioning. Learned responses are extinguished if they are made repeatedly while the reinforcement is blocked, and opiate antagonists appear to block the reinforcement from alcohol. A series of experiments support the hypothesis that drinking alcohol while an antagonist is present extinguishes the alcohol-drinking response in rats. The antagonists are non-addictive and at least naloxone appears to be safe. Clinical trials are now needed, but the present results suggest that this extinction procedure might be a useful adjunct to the treatment of alcoholism.

Effects of dopaminergic agents on alcohol consumption by rats in a limited access paradigm

The effects of several dopaminergic drugs on alcohol consumption were studied in free-feeding rats using a limited access paradigm. Ascending doses of amphetamine (0.1, 0.3 and 1.0 mg/kg), haloperidol (0.1, 0.3 and 1.0 mg/kg), SKF 38393 (a D1 receptor agonist - 0.3, 1.0 and 3.0 mg/kg), quinpirole (LY 171555, a D2 receptor agonist - 0.03, 0.1 and 0.3 mg/kg), SCH 23,390 (a D1 receptor blocker, 0.003, 0.01, 0.03 and 0.1 mg/kg) and spiperone (a D2 receptor blocker, 0.003, 0.01, 0.03 and 0.1 mg/kg) were administered IP to rats approximately 30 min prior to their 1-h per day access to alcohol. Each dose was administered for 5 successive days, and the effects of the drugs were compared to those of respective saline or 0.4% lactic acid solution controls. Although there was an overall significant dose effect of amphetamine on alcohol consumption, no single dose altered alcohol consumption significantly from baseline. SKF 38,393 specifically decreased alcohol consumption at the highest dose of 3 mg/kg. Quinpirole significantly increased water consumption at the highest dose but had no effect on alcohol consumption. The antagonist haloperidol decreased alcohol consumption but only at doses that also reduced water consumption. The specific antagonists SCH 23,390 and spiperone decreased water consumption at the highest doses tested without modifying alcohol consumption. Taken together, these data suggest that dopamine does not play as critical a role in mediating the reinforcing effects of alcohol (insofar as they are reflected by alcohol consumption) as it does in relation to other psychoactive drugs, particularly the psychomotor stimulants.