Pharmacology of flavor preference conditioning in sham-feeding rats: effects of naltrexone.

Relatively little is known about the neurochemical and pharmacological mechanisms involved in flavor preference learning. The present study examined the ability of the opioid antagonist, naltrexone to alter the acquisition and expression of flavor preferences conditioned by the sweet taste of sucrose. This was accomplished by adding a novel flavor (the CS+) to a sucrose solution, and a different flavor (the CS-) to a less-preferred saccharin solution. Rats were trained to drink these solutions with an open gastric fistula (sham-feeding), which minimized postingestive actions. Food-restricted (Experiments 1 and 2A) and ad lib-fed (Experiment 2B) rats were given either limited (Experiment 1) or unlimited (Experiment 2) access to the CS+ and CS- solutions during one-bottle training. Preferences were assessed in two-bottle tests (with the CS+ and CS- flavors presented in mixed sucrose-saccharin solutions) following vehicle or naltrexone (0.1-10 mg/kg, SC) treatment. The rats displayed significant CS+ preferences following vehicle, particularly after unlimited access training. In four of five experiments, naltrexone significantly reduced total intakes during the two-bottle, sham-feeding tests. Except for one instance, however, the drug failed to block the preference for the CS+ flavor over the CS- flavor. The effects of naltrexone (0.1 mg/kg) on the acquisition of flavor preferences were studied in sham-feeding rats under limited (Experiment 3A) and unlimited (Experiment 3B) training access conditions. Rats treated with naltrexone during training displayed similar CS+ preferences as did saline-treated rats, even though they consumed less CS+ during training. The naltrexone-trained rats also displayed smaller reductions in total or CS+ intakes than did saline-trained rats when all rats were treated with a 2.5 mg/kg dose of naltrexone during testing. As in previous studies, these results show that naltrexone significantly reduces the intake of sweet solutions, yet it has little or no effect on the acquisition or expression of flavor preferences conditioned by sucrose in sham-feeding rats.     

A potential role of saline trials in morphine-induced place-preference conditioning

The necessary conditions to alter rats' initial preferences for two sides of a shuttlebox were investigated, using procedures that are often used in the study of drug reinforcement. In Experiment 1, pairings of morphine sulfate (15 mg/kg, intraperitoneally) and either the nonpreferred side or a holding box was factorially combined with alternate-day pairings of saline and either the preferred side or a holding box. Pairings of saline and the preferred side were necessary and sufficient to increase preferences for the initially nonpreferred side. In Experiment 2, pairings of saline and the nonpreferred side, but not the holding box, strengthened the initial preference, regardless of whether morphine or saline injections preceded alternate-day holding-box placements. In Experiment 3, injection and placement in the preferred side in an unpaired manner, or placement only, decreased preferences for that side more than saline injections alone or no treatment. Paired saline injections and placement produced a greater change in preference than no treatment.     

Pharmacology of flavor preference conditioning in sham-feeding rats: effects of dopamine receptor antagonists

Opioid and dopamine systems are both implicated in the response to sweet solutions. Our laboratory previously reported that the opioid antagonist, naltrexone, reduced the intake of sweet solutions, yet had little or no effect on sucrose-conditioned flavor preferences in sham-feeding rats. The present study examined the role of dopamine D(1) and D(2) receptors in the expression of flavor preferences conditioned by the sweet taste of sucrose. All sessions were conducted under sham-feeding conditions to minimize postingestive influences. Training was accomplished by adding a novel flavor (CS+) to a 16% sucrose solution, a different flavor (CS-) to a less-preferred 0.2% saccharin solution in alternating, one-bottle sessions. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in mixed sucrose (8%)-saccharin (0.1%) solutions following systemic doses of 0, 50, 200, 400, or 800 nmol/kg of the D(2) antagonist, raclopride (Experiment 1) or the D(1) antagonist, SCH23390 (Experiment 2) under either food-restricted or unrestricted conditions. Rats significantly preferred the CS+ solutions in vehicle tests, and displayed equipotent and dose-dependent reductions in total intake and CS+ preference following either D(1) or D(2) receptor antagonism. Similar results were obtained with SCH23390 and raclopride in Experiment 3 conducted with water-restricted rats. These data indicate that dopaminergic D(1) and D(2) receptors play pivotal and functionally equivalent roles in the expression of flavor preferences conditioned by the sweet taste of sucrose.     

Ethanol-reinforced behaviour in the rat: effects of naltrexone.

It has been repeatedly reported that endogenous opioid pathways play an important role in ethanol drinking behaviour. In line with these findings, a non-selective opioid receptor antagonist, naltrexone, seems to reduce relapse rates in detoxified alcoholics. The aim of the present study was to evaluate the effects of naltrexone on (i) ethanol self-administration; (ii) extinction of responding for ethanol; (iii) reinstatement of ethanol-seeking induced by non-contingent presentations of ethanol-associated stimuli. Male Wistar rats were trained to lever-press for 8% ethanol in an operant procedure where ethanol was introduced in the presence of sucrose. The selectivity of naltrexone's actions was assessed by studying its effects on water-reinforced behaviour in separate control experiments. Acute injections of naltrexone (1 or 3 mg/kg) did not alter ethanol self-administration. Repeated treatment with naltrexone (3 mg/kg, before three consecutive self-administration sessions) progressively reduced ethanol intake. In the extinction procedure, acute administration of 3 mg/kg naltrexone suppressed responding previously reinforced with ethanol. Similarly, naltrexone (1-3 mg/kg) potently and dose-dependently inhibited reinstatement of ethanol-seeking produced by non-contingent deliveries of the liquid dipper filled with 8% ethanol. In the control experiments, lower doses of naltrexone (1-3 mg/kg) did not exert any effect on either reinforced or non-reinforced (extinction) lever-pressing for water. These results indicate that: (i) subchronic treatment with naltrexone leads to progressive reduction of ethanol self-administration; (ii) single doses of naltrexone may increase extinction and attenuate cue-induced reinstatement of ethanol-reinforced behaviour.     

Discriminative stimulus effects of cyclorphan: selective antagonism with naltrexone

The opioid antagonist, naltrexone, was used to identify some of the receptor mechanisms responsible for the discriminative stimulus effects of cyclorphan in the pigeon. Subjects were trained to discriminate 10 mg/kg IM injections of either morphine or dextrorphan from saline injections in a two key drug discrimination procedure in which responding was maintained by food presentation. The dextrorphan-trained birds generalized tol-cyclorphan at 10 mg/kg; naltrexone did not alter thel-cyclorphan dose-response curve for this effect. In the morphine-trained group,l-cyclorphan produced only partial generalization, and naltrexone greatly increased the dose ofl-cyclorphan necessary to produced only These results are consistent with the conclusion that in morphine-trained pigeons the partial generalization tol-cyclorphan is mediated by opioid receptors. Moreover, limited intrinsic efficacy at mu opioid receptors may be the characteristic ofl-cyclorphan that prevents full generalization in morphine-trained pigeons.d-Cyclorphan produced partial generalization in both groups, but the involvement of opioid receptor mechanisms could not be confirmed, as 1 mg/kg naltrexone did not antagonized-cyclorphan in either group.     

Depot naltrexone: long-lasting antagonism of the effects of heroin in humans

RATIONALE: Naltrexone, an opioid antagonist, is currently approved as a treatment for heroin dependence. However, naltrexone is generally not well accepted by patients, and medication non-compliance is a difficult obstacle to treatment. A sustained-release form of naltrexone may improve compliance. OBJECTIVE: The present study was designed to evaluate the time course, safety, and effectiveness of a depot formulation of naltrexone (Depotrex). METHODS: Twelve heroin-dependent individuals participated in an 8-week inpatient study. After a 1-week detoxification period, six participants received 192 mg naltrexone base and six participants received 384 mg naltrexone base. For safety, the low dose of depot naltrexone was tested before the high dose. The effects of heroin (0, 6.25, 12.5, 18.75, 25 mg, i.v.) were evaluated for the next 6 weeks. One dose of heroin was tested per day on Mondays through Fridays, and the entire dose range was tested each week. Active heroin doses were administered in ascending order during the week, while placebo could be administered on any day. Subjective, performance, and physiological effects were measured both before and after heroin administration. The hypotheses were that depot naltrexone would antagonize the effects of heroin, and that the high dose of depot naltrexone would produce a more effective and longer-lasting antagonism than the low dose. RESULTS: The low and high doses of depot naltrexone antagonized heroin-induced subjective ratings for 3 and 5 weeks, respectively. Plasma levels of naltrexone remained above 1 ng/ml for approximately 3 and 4 weeks after administration of 192 mg and 384 mg naltrexone. Other than the initial discomfort associated with the injection of depot naltrexone, there were no untoward side-effects. CONCLUSIONS: These results suggest that this depot formulation of naltrexone provides a safe, effective, long-lasting antagonism of the effects of heroin.     

Suppressant effects of naltrexone on water intake in rats

Parenteral administration of naltrexone to water-deprived female rats suppressed water intake when injected 4, but not 12 hours prior to the drinking session. Intraperitoneal injection 5 mg/kg naltrexone twice daily or oral self-administration of naltrexone in doses sufficient to block morphine-induced analgesia failed to reduce water intake or to alter body weight in non-deprived animals. These findings suggest that the suppressant effects of naltrexone on appetitive behavior are mediated by a different population of opiate receptors than those mediating morphine-induced analgesia.     

Adverse effects of oral naltrexone: analysis of data from two clinical trials

RATIONALE: Naltrexone treatment of alcohol dependence is associated with adverse events that may limit its effectiveness. Consequently, understanding the impact of adverse events on medication compliance and treatment retention may enhance naltrexone therapy of alcoholism. OBJECTIVES: To examine the relations among adverse events, drinking behavior, medication compliance and study retention in alcoholics receiving naltrexone for relapse prevention. METHODS: The current report is based on analysis of data from 92 subjects who participated in two previously published studies. Moderate or severe adverse effects were monitored weekly and categorized as either neuropsychiatric (NP) or gastrointestinal (GI). Medication compliance was determined by weekly urinary riboflavin testing. Study retention was determined by the proportion of study weeks completed by the subject. The causal relations among adverse events, medication compliance and study retention were analyzed separately for NP and GI adverse events using regression-based recursive path models. RESULTS: Both the NP and GI models fit the data well [NP model: chi 2(4) = 0.59, P = 0.96; GI model: chi 2(4) = 2.81, P = 0.59]. NP adverse events exerted little influence on medication compliance (beta = -0.17, P = 0.071), but directly decreased the length of study retention (beta = -0.35, P < 0.001). In contrast, there was a significant impact of GI adverse events on medication compliance (beta = -0.29, P = 0.002), but not directly on study retention (beta = -0.14, P = 0.081). CONCLUSION: Future studies aimed at enhancing the effectiveness of naltrexone should examine ways of reducing both NP and GI adverse events, in order to enhance both medication compliance and treatment retention.     

Chronic naltrexone treatment of rats: effects on gastrointestinal opioid peptide content.

The gastrointestinal tract contains immunoreactive enkephalins and beta-endorphin. The objective of the current study was to determine whether chronic treatment of rats with naltrexone altered the gastrointestinal tissue content of these opioid peptides. Opioid activity measured by radioreceptor assay was detectable throughout the gastrointestinal tract. There were regional differences in the [Met5]enkephalin: [Leu5]enkephalin-immunoreactivity (IR) ratios, possibly due to cell specific differential processing of precursor molecules or degradation of the peptides. Chronic naltrexone treatment increased opioid activity in the duodenum and jejunum, decreased [Met5]enkephalin-IR in the duodenum and [Leu5]enkephalin-IR in the gastric corpus, and increased beta-endorphin-IR in the duodenum. However, the changes were small, and it is unlikely that any functional changes resulting from naltrexone treatment can be reliably ascribed to such changes in tissue content.     

Effect of amphetamine on sucrose-reinforced lever pressing: interaction with food deprivation

Rats were trained to lever press on a Fixed Ratio Schedule 8 using sucrose reinforcement in one of two feeding conditions: ad lib food and water available in the home cage; reduced feeding in order to maintain the animals at 80% of their free feeding body weight. The effect of three doses of d-amphetamine (0.10, 0.25 and 0.50 mg/kg) on lever pressing was examined for each feeding condition. A systematic decrease in responding as dose increased was found in the ad lib feeding condition while only the highest dose had any effect on responding in the food restricted animals. Thus, it appeared that the effect of food deprivation was to shift the amphetamine dose-response curve to the right.     

Effects of test conditions on the outcome of place conditioning with morphine and naltrexone in mice

 Drug administration during test trials can increase the expression of place conditioning, offering an opportunity to determine the specificity of this enhanced response. Prior to training, Swiss-Webster mice spent similar durations in each of the distinctive compartments of a two-compartment box during three 900-s tests. During a 4-day conditioning period, daily injections of morphine (5–20 mg/kg, SC) or vehicle were differentially paired with one of two compartments of the box using an unbiased place conditioning procedure. Post-conditioning tests were conducted 2 and 3 days after the last conditioning day. Mice pre-treated during post-conditioning tests with vehicle did not show significant preference for the morphine-paired compartment when conditioned with morphine. Pretreatment with morphine (2.5–30 mg/kg, SC) led to a dose-dependent increase in time spent in the morphine-paired compartment. Post-conditioning tests in other groups of mice were conducted with heroin (0.1–3 mg/kg), fentanyl (0.01–0.3 mg/kg), cocaine (10–30 mg/kg) and pentobarbital (10–30 mg/kg), and results suggested that none of the tested drugs facilitated the expression of the morphine-conditioned place preference. In another experiment, naltrexone (0.1–10 mg/kg, SC) was administered as the conditioning drug. When tested with naltrexone (0.1–10 mg/kg), there was a dose-dependent avoidance of the naltrexone-paired compartment. Overall, the present data indicated that: (1) failure to exhibit place preference or place aversion when tested in a drug-free state does not imply the failure of conditioning procedure; and (2) effects of the morphine cue reinstatement during the post-conditioning tests appeared to be related to the unique pharmacological profile of the morphine stimulus. Received: 28 November 1997 / Final version: 23 July 1998     

Behavioural and endocrine effects of naltrexone in male talapoin monkeys

The effect of treating captive male talapoin monkeys with naltrexone hydrochloride (500 μg/kg intra-muscular injection twice daily) was studied both in socially living and singly caged animals. The behaviour of the group males and endocrine changes in all treated animals were monitored during the course of treatment and on drug withdrawal. Naltrexone significantly reduced sexual behaviour in previously active males, while increasing grooming interactions. Aggressive behaviour did not change. There was an overall significant elevation in testosterone, LH and cortisol during drug treatment and a significant decrease on withdrawal. Changes in prolactin in response to naltrexone depended upon the pre-treatment level of this hormone; in males in which levels were low, there was a significant elevation in prolactin, while in those with high pre-treatment prolactin, levels were unchanged by the drug. The behavioural changes reported for this primate are in direct contrast to changes reported in rodents, while the hormonal changes, except for prolactin, are comparable to others reported.     

Differential effects of CGS 8216 and naltrexone on ingestional behaviour

Effects of the pyrazoloquinoline CGS 8216 (a partial benzodiazepine receptor inverse agonist) and the opiate antagonist, naltrexone, were compared in several tests of ingestion in non-deprived and deprived male rats. Both naltrexone (0.1-10.0 mg/kg, SC) and CGS 8216 (1.25-10.0 mg/kg, IP) significantly reduced the consumption of a highly palatable saccharin-glucose solution by non-deprived rats. Both compounds were also effective in reducing, dose-dependently, the intake of palatable sweet or oily mash by non-deprived animals. Hence, naltrexone and CGS 8216 attenuated palatability-induced ingestional responses, and sweet taste was not necessary for this effect to occur. The two drugs also reduced the intake of the saccharin-glucose solution in food-deprived rats, but their effects diverged in water-deprived animals. CGS 8216 had relatively little effect in the thirsty animals, whereas the effect of naltrexone was enhanced. This difference was underscored in a final test of deprivation-induced consumption of water. Naltrexone reduced the drinking, but CGS 8216 had no effect. Taken together, these data indicate that CGS 8216 was more selective in its effects on ingestion.     

Chronic naltrexone supersensitizes the reinforcing and locomotor-activating effects of morphine

Rats were implanted for 10 days with a slow-release naltrexone pellet and then the pellet was removed. Sham-control animals were treated similarly, except no pellet was implanted. One day after pellet removal or sham treatment, animals were assessed for morphine-induced conditioned place preference (CPP) or locomotor activity. CPP was evident in sham animals following two conditioning trials using 5 mg/kg subcutaneous morphine (Experiment 1) and following one conditioning trial using 8 mg/kg intravenous morphine (Experiment 2). Animals conditioned while implanted with a naltrexone pellet showed no morphine-induced CPP. More important, one day after pellet removal, naltrexone-pretreated animals given one conditioning trial with 5 mg/kg intravenous morphine displayed a greater preference for morphine-associated cues relative to sham animals given morphine (Experiment 3 and 4). This single IV morphine dose was insufficient to produce CPP in sham animals, suggesting that naltrexone-induced supersensitization may only be evident at a morphine dose below the reinforcing threshold in control animals. Further, chronic naltrexone potentiated the locomotor-activating effect of 2 mg/kg subcutaneous morphine but not of either 1 or 5 mg/kg morphine (Experiment 5). Behavioral supersensitization assessed by morphine-induced locomotor activation was transient, as it was evident one day, but not either three or 10 days following pellet removal (Experiment 6). These results confirm the functional significance of opiate receptor up-regulation following chronic opioid blockade.     

Memantine enhances the inhibitory effects of naltrexone on ethanol consumption

Effects of the opioid receptor antagonist naltrexone (0.1; 0.3; 1.0 mg/kg i.p.) on operant ethanol self-administration alone and in combination with the non-competitive NMDA antagonist memantine (0.5 and 1 mg/kg, i.p.) were studied in rats. Acute administration of naltrexone (0.1; 0.3; 1.0 mg/kg i.p.) inhibited ethanol self-administration in a dose-dependent manner. Memantine (1.0 mg/kg) significantly enhanced the effects of naltrexone at 0.1 mg/kg, failing per se to inhibit ethanol consumption. Thus, low, sub-effective dose of memantine in combination with low doses of naltrexone blocked the reinforcing properties of ethanol in rats. It is suggested that the combination of sub-effective doses of memantine and naltrexone may have therapeutic value in the treatment of alcoholism particularly in a subgroup of alcoholic patients who have high sensitivity to the adverse side effects of naltrexone.     

Aversive effects of naltrexone in subjects not dependent on opiates

Naltrexone was given to ten opiate-free volunteer subjects following the same dosage schedule used for initiating treatment of opiate-dependent persons. During the three-week initiation period, three subjects dropped from the study owing to aversive effects of the drug. The remaining seven subjects reported similar unpleasant but tolerable effects. A separate group of ten volunteer subjects was given single doses of 50 or 100 mg of naltrexone or a naltrexone placebo on three separate occasions using blind controls. These subjects also reported aversive effects. The principal symptoms reported were loss of energy, gastrointestinal disturbances and mental depression. It is possible that these aversive reactions of naltrexone have limited acceptance of the drug as a treatment for opiate-dependent persons.     

Associative factors in drug pretreatment effects on gustatory conditioning: Cross-drug effects

The pretreatment effect (PE) in gustatory avoidance conditioning refers to the fact that pretreatment with a variety of pharmacological agents subsequently reduces the ability of the same agents to induce gustatory aversion. Explanations of this phenomenon emphasize either tolerance or associative interference. Any explanation of the phenomenon must also account for the present findings which demonstrate the PE when agents of pretreatment and conditioning were pharmacologically dissimilar. Rats were pretreated with d-amphetamine and tested for acquisition of an aversion to saccharin conditioned by amphetamine or morphine. The PE was obtained regardless of the drug used in conditioning. An associative manipulation involving nonreinforced presentation of the drug administration cues (i.e., injections followed by saline instead of drug), that attenuated the PE when pretreatment and conditioning were with amphetamine, was also effective when the pretreatment agent was amphetamine and the conditioning agent was morphine. The findings were interpreted within a framework of compensatory conditioning of a general physiological mechanism common to all gustatory avoidance.