Intracerebroventricular morphine enhances alcohol consumption by rats

Sigma receptor-induced heavy drinking in rats: Modulation by the opioid receptor system

Alcohol use disorder (AUD) is a major cause of morbidity and mortality worldwide, for which new efficacious treatments are necessary. The opioid receptor system is a mediator of the rewarding effects of alcohol; in particular, while activation of μ opioid receptors enhances ethanol intake in rodents, opioid-receptor antagonists, such as naloxone and naltrexone, reduce its pleasurable and reinforcing effects, thereby decreasing alcohol. Sigma receptors (Sig-Rs) have been proposed as modulators of the effects of alcohol and, therefore, as a potential new pharmacological target for AUD. Somewhat analogously to μ opioid ligands, SigR agonists increase, while SigR antagonists decrease alcohol intake in animal models of excessive alcohol drinking. However, a potential cross-talk between these two receptor systems in relation to alcohol consumption has so far not been investigated. Here, we addressed this question pharmacologically, by testing the effects of either activating or inhibiting opioid receptors on the heavy alcohol drinking induced by chronic stimulation of SigR in alcohol-preferring rats. We found that the opioid receptor agonist morphine, which per se increases ethanol intake, at a sub-threshold dose reduces the binge-like drinking induced by the repeated treatment with the SigR agonist 1,3-di-o-tolylguanidine (DTG); conversely, the opioid receptor antagonist naltrexone, which per se reduces ethanol intake, at a sub-threshold dose potentiates the DTG-induced binge-like drinking. Our data show a cross-talk between the opioid and SigR systems relevant to the modulation of alcohol drinking, which provides important insights into the neurobiology of AUD and may lead to the development of novel therapies, either standalone or in combination.

Increased ethanol intake in prodynorphin knockout mice is associated to changes in opioid receptor function and dopamine transmission

The purpose of this study was to examine the role of the prodynorphin gene in alcohol sensitivity, preference and vulnerability to alcohol consumption. Handling-induced convulsion (HIC) associated to alcohol, alcohol-induced loss of righting reflex (LORR), hypothermic effects in response to acute ethanol challenge, blood ethanol levels (BELs), conditioned place preference, voluntary ethanol consumption and preference, tyrosine hydroxylase (TH), dopamine transporter (DAT) and proenkephalin (PENK) gene expression, and µ-, δ- and κ-opioid agonist-stimulated [S(35) ]- guanosine 5'-triphosphate-binding autoradiography were studied in prodynorphin knockout (PDYN KO) and wild-type (WT) mice. There were no differences in HIC, LORR or the decrease in body temperature in response to acute ethanol challenge between PDYN KO and WT mice. PDYN KO mice presented higher BEL, higher ethanol-conditioned place preference and more ethanol consumption and preference in a two-bottle choice paradigm than WT mice. These findings were associated with lower TH and higher DAT gene expression in the ventral tegmental area and substantia nigra, and with lower PENK gene expression in the caudate-putamen (CPu), accumbens core (AcbC) and accumbens shell (AcbSh) in PDYN KO. The functional activity of the µ-opioid receptor was lower in the CPu, AcbC, AcbSh and cingulate cortex (Cg) of PDYN KO mice. In contrast, δ- and κ-opioid receptor-binding autoradiographies were increased in the CPu and Cg (δ), and in the CPu, AcbC and Cg (κ) of PDYN KO. These results suggest that deletion of the PDYN gene increased vulnerability for ethanol consumption by altering, at least in part, PENK, TH and DAT gene expression, and µ-, δ- and κ-opioid receptor functional activity in brain areas closely related to ethanol reinforcement.

Addiction Research Centres and the Nurturing of Creativity. Substance abuse research in a modern health care centre: the case of the Centre for Addiction and Mental Health

The Centre for Addiction and Mental Health is one of the premier centres for research related to substance use and addiction. This research began more than 50 years ago with the Addiction Research Foundation (ARF), an organization that contributed significantly to knowledge about the aetiology, treatment and prevention of substance use, addiction and related harm. After the merger of the ARF with three other institutions in 1998, research on substance use continued, with an additional focus on comorbid substance use and other mental health disorders. In the present paper, we describe the structure of funding and organization and selected current foci of research. We argue for the continuation of this successful model of integrating basic, epidemiological, clinical, health service and prevention research under the roof of a health centre.

Differences in delta- and mu-opioid receptor blockade measured by positron emission tomography in naltrexone-treated recently abstinent alcohol-dependent subjects

Blockade of brain mu-opioid receptor (mu-OR) and delta-opioid receptor (delta-OR) was investigated in recently abstinent alcohol-dependent subjects (N=21) maintained on naltrexone. Subjects completed a 19-day inpatient protocol, which included alcohol abstinence followed by naltrexone treatment (50 mg) on days 15-19. Blood samples were collected after the first administration of naltrexone to evaluate serum levels of naltrexone and 6-beta-naltrexol. Regional brain mu-OR binding potential (BP) and delta-OR Ki was measured using [11C]carfentanil (CAR) positron emission tomography (PET) and [11C]methyl naltrindole ([11C]MeNTI) PET, respectively, before (day 5) and during naltrexone treatment (day 18). Naltrexone inhibition of [11C]CAR BP was near maximal across all brain regions of interest with little variability across subjects (mean+SD% inhibition=94.9+4.9%). Naltrexone only partially inhibited the [11C]MeNTI Ki and there was more variability across subjects (mean+SD% inhibition=21.1+14.49%). Peak serum levels of naltrexone were positively correlated with % inhibition of delta-OR Ki in neocortex and basal ganglia. Peak serum levels of naltrexone were not correlated with % inhibition of mu-OR BP. Peak levels of 6-beta-naltrexol were not significantly correlated with % inhibition of mu-OR BP or delta-OR Ki. Thus, the FDA recommended therapeutic dose of naltrexone was sufficient to produce near complete inhibition of the mu-OR in recently abstinent alcohol dependent subjects. The lower percent inhibition of delta-OR and greater variability in delta-OR blockade by naltrexone across subjects may contribute to individual differences in treatment outcomes to naltrexone. Further investigations on the relationship between individual differences in delta-OR blockade by naltrexone and clinical outcomes should be explored.

14-Methoxymetopon, a highly potent mu opioid agonist, biphasically affects ethanol intake in Sardinian alcohol-preferring rats

RATIONALE

Increased opioidergic activity is thought to increase the propensity to consume ethanol. However, the dose monotonicity and receptor subtype for this effect remain uncertain. 14-methoxymetopon is a centrally acting, selective micro opioid receptor agonist with greater systemic antinociceptive potency than morphine and a putatively improved therapeutic index.

OBJECTIVE

To determine whether 14-methoxymetopon influenced voluntary ethanol intake in Sardinian alcohol-preferring (sP) rats.

METHODS

Male sP rats with continuous 2-bottle choice access to ethanol (10% v/v) or water were subjects. The effects of systemic 14-methoxymetopon administration (2, 5, 12.25, 30 micro/kg, s.c.) on 4-h ethanol intake were determined. The ability of naltrexone (50 micro/kg, s.c.), an opioid antagonist, to block actions of 14-methoxymetopon (12.25, 30 micro/kg, s.c.) was examined as were the effects of 14-methoxymetopon (12.25 micro/kg, s.c.) on self-administered blood alcohol levels (BALs) and clearance of a passive ethanol bolus (1 g/kg). Finally, the effects of central 14-methoxymetopon administration (0.0003-100 ng, i.c.v.) on 4-h ethanol intake were evaluated.

RESULTS

Systemic 14-methoxymetopon very potently and dose-dependently suppressed ethanol and food intake for 30 min, followed by a greater, longer-lasting, and behaviorally specific increase in ethanol intake. The increased ethanol intake led to threefold higher BALs, was naltrexone-reversible, and not due to altered ethanol clearance. Intracerebroventricular 14-methoxymetopon administration rapidly altered ethanol intake per an inverted U-shaped dose-response function, increasing it at a 10 pg dose, while suppressing it at a 10,000-fold higher dose.

CONCLUSIONS

The novel mu analgesic increases ethanol intake, a potential therapeutic liability, and results suggest a non-monotonic influence of brain mu opioid receptor stimulation on ethanol intake.

Enhanced Morphine-Induced Ethanol Drinking in Alcohol-Preferring Alko Alcohol Rats Sensitized to Morphine

BACKGROUND

Alcohol-preferring alko alcohol (AA) rats are more susceptible to morphine-induced behavioral and neurochemical sensitization than alcohol nonpreferring alko nonalcohol (ANA) rats. Alko alcohol rats sensitized to morphine, however, do not show enhanced acquisition of ethanol drinking. The purpose of the present study was to clarify further interactions between morphine-induced behavioral sensitization and voluntary ethanol drinking in the AA rats.

METHODS

Alko alcohol rats drinking ethanol in a limited 6-hour access paradigm were sensitized to morphine with repeated injections of morphine (5-15 mg/kg). Injection days alternated with days of ethanol access. Controls had access only to water and/or were given injections of saline. After a 5-day washout period from ethanol and morphine, the rats were challenged with morphine or saline and subsequent ethanol drinking or locomotor activity was recorded.

RESULTS

Ethanol intake was suppressed during the repeated treatment with morphine, and the morphine-treated rats did not differ in ethanol intake from the controls when given access to ethanol after the washout. Intake of ethanol was, however, increased when the rats were challenged with morphine [1 or 10 mg/kg, subcutaneously (s.c.)], while in the controls an increase in ethanol intake was seen only after 1 mg/kg morphine. Sensitization to the locomotor stimulating effects of morphine was revealed in the morphine-treated rats after a challenge with morphine (3 or 10 mg/kg, s.c.). The controls that had been drinking ethanol also showed a sensitized response after morphine (3 mg/kg).

CONCLUSIONS

Ethanol did not interfere with the development of sensitization to morphine. Furthermore, the neuroadaptations induced by repeated exposure to ethanol were sufficient to cause behavioral cross-sensitization to morphine. Sensitization to the behavioral effects of morphine alone, however, neither enhances the reinforcing properties of voluntarily consumed ethanol nor contributes to increase in its intake. The increase in ethanol intake found after an acute dose of morphine was augmented in rats withdrawn from repeated treatment with morphine. The data suggest that the neuronal mechanisms underlying behavioral sensitization to morphine probably are distinct from those mediating reinforcement from ethanol and that the morphine-induced neuroadaptations contribute to the enhancement of increase in ethanol intake by morphine.

Galanin Microinjection in the Third Ventricle Increases Voluntary Ethanol Intake

BACKGROUND

The neuropeptide galanin increases food intake. Chronic ethanol (EtOH) increases the expression of galanin in the hypothalamus. The research presented here examines the effects of microinjection of galanin in the third ventricle on voluntary alcohol intake.

METHODS

Male Sprague Dawley rats with a cannula in the third ventricle were given access to increasing concentrations of EtOH for 12 hr/day until all acquired a preference for 7% EtOH over water in a two-bottle choice. Rats then received a microinjection of galanin (0, 1, and 3 nmol) alone or in combination with the galanin antagonist M40 (1 nmol) and with M40 alone to determine the effects on EtOH and water intake. Tests were conducted during both the light and dark periods of a 12:12-hr light-dark cycle with food available ad libitum. As a control for galanin-induced calorie intake, both EtOH and food were measured in a subset of rats during the dark.

RESULTS

Microinjections of galanin (1.0 and 3.0 nmol) increased EtOH consumption during both periods of the light-dark cycle. Galanin's effect on ethanol intake during the light was large relative to the very low intake of food and water during this period. Rats increased their intake of EtOH but not food. Receptor specificity for galanin (3 nmol) was shown by the galanin antagonist M40, which blocked the increase in EtOH intake. M40 alone decreased EtOH intake slightly.

CONCLUSIONS

These data show that galanin injected in the third ventricle increases EtOH consumption and that the effect can occur during both the light and the dark periods of the diurnal cycle in the presence of food and water. This suggests that galanin may play a role in augmenting voluntary alcohol intake and perhaps the development of alcohol dependence.

INTERACTIONS BETWEEN CANNABINOID AND OPIOID RECEPTOR SYSTEMS IN THE MEDIATION OF ETHANOL EFFECTS

Over the past few years, advances in the investigation of the neurochemical circuits involved in the development and treatment of alcohol dependence have identified peptides and receptors as potential key targets in the treatment of problems related to alcohol consumption. The endogenous opioid system is modified by alcohol intake in areas of the brain related to reward systems, and differential basal levels of opioid gene expression are found in rodents with a high preference for ethanol. This suggests a greater vulnerability to alcohol consumption in relation to differences in genetic background. Further evidence of the involvement of opioid peptides in alcohol dependence is the ability of the opioid antagonist naltrexone to reduce alcohol intake in animal models of dependence and in alcohol-dependent patients. Abundant evidence indicates that the activation of cannabinoid receptors stimulates the release of opioid peptides, therefore the cannabinoid receptor antagonists may presumably alter opioid peptide release, thus facilitating the reduction of ethanol consumption. However, little is known about the effects of ethanol on the endogenous cannabinoid system, the vulnerability of cannabinoid receptors to alcohol intake or their neurochemical implications in reducing consumption of alcohol. In this paper, we review the role of opioid and cannabinoid receptor systems, their vulnerability to alcohol intake and the development of dependence, and the targeting of these systems in the treatment of alcoholism.

Suppression by baclofen of the stimulation of alcohol intake induced by morphine and WIN 55,212-2 in alcohol-preferring rats

Administration of morphine and cannabinoids stimulates alcohol intake in rats. The present study investigated whether the promoting effect of morphine and of the cannabinoid receptor agonist, WIN 55,212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone], on alcohol intake was prevented by the gamma-aminobutyric (GABA)(B) receptor agonist, baclofen. Sardinian alcohol-preferring (sP) rats were given alcohol (10%, v/v) and water under the standard homecage two-bottle-free choice regimen with unlimited access for 24 h/day. Baclofen (0, 0.5 and 1 mg/kg; i.p.) was administered acutely 30 min before lights off. Morphine (0 and 1 mg/kg, s.c.) or WIN 55,212-2 (0 and 2 mg/kg, i.p.) was administered acutely 10 min after baclofen. Alcohol intake was recorded 60 min after lights off. As predicted, both morphine and WIN 55,212-2 produced a specific and marked increase in alcohol intake. Pretreatment with baclofen, which failed to alter alcohol intake when given alone, dose-dependently suppressed morphine- and WIN 55,212-2-induced promotion of alcohol drinking. These results suggest the involvement of the GABA(B) receptor in the neural circuitry mediating the stimulating effect of morphine and cannabinoids on alcohol consumption in sP rats.

Mu-opioid receptor binding measured by [11C]carfentanil positron emission tomography is related to craving and mood in alcohol dependence

BACKGROUND

The endogenous opioid system has been linked to alcohol dependence through animal and human studies. We investigated the relationship between alcohol craving and brain mu opioid receptors (mu-OR) in alcohol-dependent subjects.

METHODS

Regional brain mu-OR binding potential (BP) was measured using [(11)C]carfentanil positron emission tomography in eight male alcohol-dependent subjects undergoing alcohol withdrawal and eight matched control subjects. Self-reported alcohol craving, withdrawal, and mood were measured.

RESULTS

Lower mu-OR BP was associated with higher craving in the right dorsal lateral prefrontal cortex, the right anterior frontal cortex, and right parietal cortex. In these regions, alcoholics showed lower mean mu-OR BP compared with control subjects. Mu-OR BP in four other brain regions also correlated with craving, but there were no group differences in receptor binding potential. Mu-OR BP also correlated with depressive symptoms in five brain regions, three of which were identified in the craving analyses.

CONCLUSIONS

Results show a strong functional relationship between alcohol craving, mood, and mu-OR binding in specific brain regions of recently abstinent, alcohol-dependent men.

Ethanol Palatability and Consumption by High Ethanol-Drinking Rats: Manipulation of the Opioid System With Naltrexone

Three experiments examined the effect of acute naltrexone treatment on both taste reactivity and consumption of ethanol in high ethanol-preferring rat lines: Alko Alcohol-Accepting (AA) rats (Experiments 1 and 2) and Alcohol-Preferring (P) rats (Experiment 3). A 3.0 mg/kg naltrexone dose was ineffective at altering ethanol palatability for either line, whereas 7.5 mg/kg was effective at reducing palatability of 10% ethanol for AA, but not P, rats, as reflected by both a decrease in ingestive responding and an increase in aversive responding. The effects of naltrexone on ethanol consumption were quite consistent: At both dosages, acute naltrexone treatment significantly decreased consumption of 10% ethanol. Termination of naltrexone resulted in an immediate increase in ethanol consumption to control levels. Results show that ethanol palatability and consumption can be dissociated in the rat and that the organization of opioidergic mechanisms that mediate ethanol responses may vary between rat lines.

Boosting effect of morphine on alcohol drinking is suppressed not only by naloxone but also by the cannabinoid CB1 receptor antagonist, SR 141716

The present study investigated the effect of the cannabinoid CB(1) receptor antagonist, SR 141716 (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide), on the ability of low and high doses of morphine to, respectively, augment and suppress voluntary alcohol intake in selectively bred Sardinian alcohol-preferring rats. Acute administration of a low dose of morphine (1 mg/kg, s.c.) produced a specific and marked increase in alcohol intake, which correlated with an increase in blood alcohol levels and was prevented by either SR 141716 (0.3 mg/kg, i.p.) or naloxone (0.1 mg/kg, i.p.). A higher dose (10 mg/kg, s.c.) of morphine reduced both alcohol and food intakes and produced sedation and hypomotility. The suppressant effect of morphine on alcohol intake was blocked by naloxone (0.1 mg/kg, i.p.) but not by SR 141716 (0.3 mg/kg, i.p.). These results are in agreement with those showing the ability of SR 141716 to antagonize the appetitive and positive reinforcing properties of morphine and add further support to the hypothesis of the existence of a functional link between the action of opioids and of cannabinoids.

Low doses of naltrexone reduce palatability and consumption of ethanol in outbred rats

The opioid antagonist naltrexone, at doses of 1 and 3 mg/kg, has been shown to decrease the palatability and consumption of 10% ethanol in rats. However, a dose of 0.5 mg/kg of naltrexone has produced equivocal results. The purpose of the present study was to clarify the effects of low doses of naltrexone (0.0 [control], 0.25, 0.50, 0.75, and 1.0 mg/kg) on palatability and consumption of 10% ethanol. Sixty-four, male, Long-Evans hooded rats were divided into five groups matched for ethanol consumption. Each rat was injected over four consecutive days with one of five doses of naltrexone exactly 30 min before taste reactivity testing with 10% ethanol. When reactivity testing was completed, rats were acclimated to drink during a period of restricted access to fluid under conditions of mild fluid deprivation. Then, on four consecutive test days, rats were injected with naltrexone 10 min before 10% ethanol was made available for 30 min. Although each dose of naltrexone decreased ingestive responding to 10% ethanol over four days, this effect was not statistically reliable. However, all doses of naltrexone produced significant increases in aversive responding to the ethanol solution. Naltrexone, at the three highest doses, produced significant decreases in consumption of 10% ethanol. These results were consistent with the interpretation that naltrexone, even at low doses, reliably reduces palatability and consumption of 10% ethanol.

Quantitative autoradiography of mu-opioid receptors in the CNS of alcohol-naive alcohol-preferring P and -nonpreferring NP rats

The densities of mu-opioid binding sites in the CNS of alcohol-naive adult male P and NP rats (N = 9 each line) were examined using quantitative autoradiography. Coronal sections (20 microm) were prepared from frozen brains and incubated in 5 nM [3H]DAMGO to label mu-opioid receptor sites. Nonspecific binding was determined in the presence of 1 microM DAMGO. The amount of [3H]DAMGO binding was (a) 20-25% higher in the olfactory tubercle, nucleus accumbens shell and core, and basolateral and lateral amygdaloid nuclei; (b) 15% higher in the lateral septal intermediate nucleus and caudate-putamen patches; and (c) 10-30% lower in the pyramidal and radiatum layers in the CA1 region of the anterior dorsal hippocampus, ventral dentate gyrus and CA1 pyramidal layer of the posterior hippocampus, and posterior medial cortical amygdaloid nucleus of the P compared to the NP rat. No line differences were found in any of the other regions examined (e.g., the cerebral cortical subregions and layers, thalamic nuclei, ventral tegmental area, ventral pallidum, lateral hypothalamus, other regions of the hippocampus, and several subcortical structures). The innate differences in the amount of binding to mu-opioid recognition sites in certain limbic structures, such as the nucleus accumbens, amygdala, and olfactory tubercle, of the P and NP lines may be factors contributing to their disparate alcohol drinking characteristics.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Effects of opioids on the absorption of alcohol

Administration of opiate agonists and antagonists has been shown to increase and decrease alcohol consumption, respectively. Because opioids can affect gastric emptying and decrease intestinal motility, the present experiments were done to determine whether changes in alcohol consumption following opioid administration might be due to opioid-induced changes in the pharmacokinetics of alcohol. In experiment 1, morphine in doses ranging from 1.5 to 4.5 mg/kg dose dependently decreased the absorption of alcohol induced by oral intubation (1 g/kg) and reduced peak blood alcohol levels (BALs). Naltrexone in doses ranging from 1.5 to 4.5 mg/kg produced a small, but significant, reduction in the absorption of alcohol, but the effects were not dose related. Similar effects of morphine and naltrexone on alcohol absorption were observed in rats infused with alcohol (1 g/kg) through an implanted intragastric cannula. The effects of morphine on alcohol absorption were observed whether alcohol levels were determined from tail vein or arterial blood samples or from brain samples. The effects of morphine on alcohol absorption were not blocked by pretreatment with methyl-naltrexone. However, the peripherally acting opioid agonist loperamide reduced BALs in a manner similar to morphine. These studies indicate that although opiate agonists and antagonists modify alcohol absorption to different extents, their effects on BALs are not a sufficient condition to induce changes in alcohol consumption.

Endogenous opioids and alcohol dependence: opioid alkaloids and the propensity to drink alcoholic beverages

Rats consume alcoholic beverages in a wide variety of circumstances. Opioid antagonists, naloxone and naltrexone, decrease intake of many ingesta, including alcoholic beverages. Small doses of morphine increase intake of alcoholic beverages. Further, the effects of small doses of morphine are persistent and there is no sign that tolerance to morphine's ability to increase alcohol intake develops as seen with morphine's ability to produce analgesia. Morphine's effects can combine with other variables that enhance intake of alcoholic beverages to produce very large daily intakes of ethanol. These generalizations, from a large number of separate experiments, support the conclusion that alcoholism is a special case of an ingestive disorder involving opioidergic systems.

Zacopride, a 5-HT3 receptor antagonist, reduces voluntary ethanol consumption in rats

The effect of the selective 5-HT3 receptor antagonist, zacopride, was assessed in male Sprague-Dawley rats in free choice (6% ethanol and water) experiments. In Experiment 1, single zacopride (0.01-10 mg/kg, IP) injections failed to alter ethanol (ET) consumption during 1-h restricted ET access. In Experiment 2, zacopride (5.0 and 10 mg/kg, IP) injected twice daily for 5 days significantly reduced ET intake and ET preference during 24-h free access to 6% ET and water without altering the total volume of fluid consumed. Thus, the schedule of ET access (i.e., free vs. restricted) and/or the duration of drug treatment may determine the efficacy of pharmacological agents in altering ET preference. 5-HT3 receptor blockade may reduce serotonin/dopamine-mediated maintenance of ET preference; a process that may proceed via extinction mechanisms.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

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.