Effect of naltrexone on oral consumption of concurrently available ethanol and cocaine in the rat

Evaluation of the "Pipeline" for Development of Medications for Cocaine Use Disorder: A Review of Translational Preclinical, Human Laboratory, and Clinical Trial Research

Cocaine use disorder is a persistent public health problem for which no widely effective medications exist. Self-administration procedures, which have shown good predictive validity in estimating the abuse potential of drugs, have been used in rodent, nonhuman primate, and human laboratory studies to screen putative medications. This review assessed the effectiveness of the medications development process regarding pharmacotherapies for cocaine use disorder. The primary objective was to determine whether data from animal and human laboratory self-administration studies predicted the results of clinical trials. In addition, the concordance between laboratory studies in animals and humans was assessed. More than 100 blinded, randomized, fully placebo-controlled studies of putative medications for cocaine use disorder were identified. Of the 64 drugs tested in these trials, only 10 had been examined in both human and well-controlled animal laboratory studies. Within all three stages, few studies had been conducted for each drug and when multiple studies had been conducted conclusions were sometimes contradictory. Overall, however, there was good concordance between animal and human laboratory results when the former assessed chronic drug treatment. Although only seven of the ten reviewed drugs showed fully concordant results across all three types of studies reviewed, the analysis revealed several subject-related, procedural, and environmental factors that differ between the laboratory and clinical trial settings that help explain the disagreement for other drugs. The review closes with several recommendations to enhance translation and communication across stages of the medications development process that will ultimately speed the progress toward effective pharmacotherapeutic strategies for cocaine use disorder.

Evaluating the Impact of Naltrexone on the Rat Gambling Task to Test Its Predictive Validity for Gambling Disorder

Gambling Disorder has serious consequences and no medications are currently approved for the treatment of this disorder. One factor that may make medication development difficult is the lack of animal models of gambling that would allow for the pre-clinical screening of efficacy. Despite this, there is evidence from clinical trials that opiate antagonists, in particular naltrexone, may be useful in treating gambling disorder. To-date, the effects of naltrexone on pre-clinical models of gambling have not been evaluated. The purpose of the present study was to evaluate the effects of naltrexone in an animal model of gambling, the rat gambling task (rGT), to determine whether this model has some predictive validity. The rGT is a model in which rats are given a choice of making either a response that produces a large reward or a small reward. The larger the reward, the greater the punishment, and thus this task requires that the animal inhibit the ‘tempting’ choice, as the smaller reward option produces overall the most number of rewards per session. People with gambling disorder chose the tempting option more, thus the rGT may provide a model of problem gambling. It was found that naltrexone improved performance on this task in a subset of animals that chose the ‘tempting’, disadvantageous choice, more at baseline. Thus, the results of this study suggest that the rGT should be further investigated as a pre-clinical model of gambling disorder and that further investigation into whether opioid antagonists are effective in treating Gambling Disorder may be warranted.

Cocaine influences alcohol-seeking behavior and relapse drinking in alcohol-preferring (P) rats

BACKGROUND

The results of several studies suggest that there may be common neurocircuits regulating drug-seeking behaviors. Common biological pathways regulating drug-seeking would explain the phenomenon that seeking for 1 drug can be enhanced by exposure to another drug of abuse. The objective of this study was to assess the time course effects of acute cocaine administration on ethanol (EtOH) seeking and relapse.

METHODS

Alcohol-preferring (P) rats were allowed to self-administer 15% EtOH and water. EtOH-seeking was assessed through the use of the Pavlovian spontaneous recovery (PSR) model, while EtOH-relapse drinking was assessed through the use of the alcohol-deprivation effect.

RESULTS

Cocaine (0, 1, or 10 mg/kg), injected immediately, 30 minutes, or 4 hours prior to the first PSR testing session, dose-dependently increased responding on the EtOH lever compared to extinction responses and responding by saline controls. Under relapse conditions, cocaine given immediately prior to the relapse session had no effect (1 mg/kg) or reduced responding (10 mg/kg). In contrast, cocaine given 4 hours prior to the relapse session markedly enhanced EtOH responding compared to saline.

CONCLUSIONS

The enhanced expression of EtOH-seeking and EtOH-relapse behaviors may be a result of a priming effect of cocaine on neuronal circuits mediating these behaviors. The effect of cocaine on EtOH-relapse drinking is indicative of the complex interactions that can occur between drugs of abuse; production of conflicting behaviors (immediate), and priming of relapse/seeking (4-hour delay).

The vital role of constitutive GPCR activity in the mesolimbic dopamine system

The midbrain dopamine system has an important role in processing rewards and the stimuli associated with them, and is implicated in various psychiatric disorders. This system is tightly regulated by various G protein-coupled receptors (GPCRs). It is becoming increasingly clear that these receptors are not only activated by (endogenous) agonists but that they also exhibit agonist-independent intrinsic constitutive activity. In this review we highlight the evidence for the physiological role of such constitutive GPCR activity (in particular for cannabinoid 1, serotonin 2C and mu-opioid receptors) in the ventral tegmental area and in its output regions like the nucleus accumbens. We also address the behavioral relevance of constitutive GPCR signaling and discuss the repercussions of its abolition in dopamine-related psychiatric diseases.

Pharmacological enhancement of naltrexone treatment for opioid dependence: a review

PURPOSE: Opioid dependence (OD) is a serious and growing clinical condition with increasing social costs that requires expanding treatment beyond opioid agonist substitution. The opioid antagonist naltrexone has displayed a remarkable association of theoretical effectiveness and poor clinical utility in treating OD due to noncompliant behavior and low acceptability among patients, only partly modified by psychosocial interventions. We reviewed pharmacological studies, including naltrexone depot formulations and combination treatments. METHOD: We searched PubMed for clinical studies on the use of naltrexone implants and slow-release injections in OD, and investigations using adjunct medications to improve naltrexone maintenance therapy of OD. We discussed the results in view of their application to the clinical practice. RESULTS: Significant reduction in opioid use and improved retention in treatment have been found in several studies using depot naltrexone formulations, some of which are controlled clinical trials. Pilot investigations have gathered initial positive results on the use of naltrexone in combination with serotonin reuptake inhibitors, α-2 adrenergic, opioid, and γ-aminobutyric acid agonist medications. CONCLUSION: Current evidence suggests that more research on effectiveness and safety is needed in support of depot naltrexone treatment for OD. Further research comparing slow-release with oral naltrexone and opioid agonist medications will help characterize the role of opioid antagonist-mediated treatment of OD. Preliminary investigations on naltrexone combination treatments suggest the opportunity to continue study of new mixed receptor activities for the treatment of OD and other drug addictions.

CB1 receptor blockade decreases ethanol intake and associated neurochemical changes in fawn-hooded rats

BACKGROUND

This study was undertaken to identify the neurochemical changes underlying the attenuation of voluntary ethanol intake induced by the cannabinoid CB1 receptor antagonist AM251 in fawn-hooded rats.

METHODS

Rats were exposed to the 2-bottle-choice paradigm (ethanol 10% v/v or water) for 15 days. After this period, rats received AM251 (3 to 6 mg/kg, i.p.) or vehicle.

RESULTS

Voluntary ethanol intake decreased (30%) with the administration of incremental dosages of AM251 (3 mg/kg, 5 days and 6 mg/kg, 5 days) in rats with acquired high preferring ethanol consumption (>3.5 g of ethanol/kg/d). Ethanol intake significantly decreased proopiomelanocortin expression in the arcuate nucleus (38.31%) and micro-opioid-DAMGO-stimulated [(35)S]-GTPgamma binding in the caudate-putamen (40%), nucleus accumbens core (AccC) (32.87%), and shell (AccS) (34.21%). Moreover, ethanol intake increased tyrosine hydroxylase (TH) gene expression in the substantia nigra (24%) and ventral tegmental area (23%) and corticotrophin-releasing gene expression in the paraventricular hypothalamic nucleus (41.6%). The reduction of ethanol intake induced by AM251 was associated with blockade or significant reduction of the changes produced by ethanol in the expression of these genes in key regions related to drug dependence. Interestingly, treatment with AM251 reduced (20%) TH gene expression in rats drinking only water. In this respect, the action of AM251 in reducing TH gene expression may not be specific.

CONCLUSION

Taken together, these results revealed that blockade of cannabinoid CB1 receptors (CB1r) decreased voluntary ethanol intake in ethanol-habituated rats by normalizing the neurochemical alterations induced by ethanol.

Acamprosate and naltrexone treatment effects on ethanol and sucrose seeking and intake in ethanol-dependent and nondependent rats

RATIONALE

Two pharmacotherapies are approved for treating alcohol craving (acamprosate and naltrexone), but both have shown mixed findings in animals and humans.

OBJECTIVES

The present experiments utilized a "reinforcer blocking" approach (i.e., rats were able to consume ethanol during treatment) to better understand the efficacy of these treatments for ethanol seeking and drinking using ethanol-dependent and nondependent rats.

MATERIALS AND METHODS

In "nondependent" experiments, drugs (acamprosate 50, 100, and 200 mg/kg; naltrexone 0.1, 0.3, and 1.0 mg/kg) were administered over 3-week periods prior to operant sessions with a low response requirement to gain access to reinforcers for 20 min. For "dependent" experiments, rats were made dependent in vapor/inhalation chambers.

RESULTS

Acamprosate and naltrexone had similar effects on intake in nondependent and dependent rats; neither drug was selective for ethanol over sucrose drinking. In nondependent animals, naltrexone was more efficacious at more doses than acamprosate, and acamprosate's effects were limited to a dose that also had adverse effects on body weight. Both pharmacotherapies showed more selectivity when examining reinforcer seeking. In nondependent rats, acamprosate and naltrexone had response-attenuating effects in ethanol, but not sucrose, groups. In dependent animals, acamprosate had selective effects limited to a decrease in sucrose seeking. Naltrexone, however, selectively decreased ethanol-seeking in nondependent rats.

CONCLUSIONS

The naltrexone-induced decreases in seeking suggested a change in incentive motivation which was selective for ethanol in nondependent rats. The "nondependent" paradigm may model early stages of "problem drinking" in humans, and the findings suggest that naltrexone could be a good intervention for this level of alcohol abuse and relapse prevention.

Ontogenetic difference in ethanol reinforcing properties: the role of the opioid system

Previous data indicate that ethanol intoxication (3 g/kg, intragastric) on postnatal day (PD) 7 and 8 increases ethanol acceptance, but on PD 10 and 11 generates an aversion in infant rats. We investigated the participation of the opioid system in these effects. Subcutaneous administration of naloxone (1 or 10 mg/kg) followed by ethanol intoxication on PD 7 and 8 prevented the increased ethanol intake effect observed in the younger pups, but when ethanol intoxication occurred on PD 10 and 11, naloxone treatment did not affect the aversion observed at this age. An aversion to ethanol was evidenced in the younger pups administered ethanol and naloxone, but only when exposed to ethanol odor during ethanol intoxication. Results indicate that the increased ethanol acceptance induced by ethanol intoxication in the younger pups is mediated by the opioid system, and that ethanol may also induce conditioned aversions at this early age.

Gene transcription alterations associated with decrease of ethanol intake induced by naltrexone in the brain of Wistar rats

Preclinical and clinical studies suggest that the administration of the opioid antagonist naltrexone decreases the intake of ethanol. However, the neuroplastic adaptations in the brain associated to reduction of ethanol consumption remains to be elucidated. The aim of the study was to identify gene transcription alterations underlying the attenuation of voluntary ethanol intake by administration of naltrexone in rats. Increasing doses of naltrexone (0.7 mg/kg, 4 days and 1.4 mg/kg/day, 4 days) to rats with acquired high preferring ethanol consumption (>3.5 g of ethanol/kg/day) decreased voluntary ethanol intake (50%). Voluntary ethanol consumption altered mu-opioid receptor function in the cingulate cortex, caudate-putamen (CPu), nucleus accumbens core (Acb C) and shell (Acb S), the expression of tyrosine hydroxylase (TH) in the ventral tegmental area and substantia nigra, proenkephalin (PENK) in the piriform cortex, olfactory tubercle, CPu, Acb C and Acb S, ventromedial nucleus (VMN) and paraventricular nucleus (PVN) of the hypothalamus, corticotropin releasing factor (CRF) in PVN, cannabinoid CB(1) receptor (CB1-R) in the CPu, hippocampus and VMN, and serotonin transporter (5-HTT) in the dorsal and median raphe nuclei. The reduction of ethanol intake induced by naltrexone was associated with a blockade or significant reduction of the changes produced by ethanol in the expression of these genes in key regions related to drug dependence. These results point to a role for the mu-opioid receptor, TH, PENK, CRF, CB1-R, and 5-HTT genes in specific brain regions in the modulation of neuroadaptative mechanisms associated to the decrease of ethanol intake induced by naltrexone.

Delta receptor antagonism, ethanol taste reactivity, and ethanol consumption in outbred male rats

Naltrexone, a nonspecific opioid antagonist, produces significant changes in ethanol responsivity in rats by rendering the taste of ethanol aversive as well as producing a decrease in voluntary ethanol consumption. The present study investigated the effect of naltrindole, a specific antagonist of delta opioid receptors, on ethanol taste reactivity and ethanol consumption in outbred rats. In the first experiment, rats received acute treatment of naltrexone, naltrindole, or saline followed by the measurement of ethanol consumption in a short-term access period. The second experiment involved the same treatments and investigated ethanol palatability (using the taste-reactivity test) as well as ethanol consumption. Results indicated that treatment with 3 mg/kg naltrexone significantly affected palatability (rendered ethanol more aversive, Experiment 2) and decreased voluntary ethanol consumption (Experiments 1 and 2). The effects of naltrindole were inconsistent. In Experiment 1, 8 mg/kg naltrindole significantly decreased voluntary ethanol consumption but this was not replicated in Experiment 2. The 8 mg/kg dose produced a significant increase in aversive responding (Experiment 2) but did not affect ingestive responding. Lower doses of naltrindole (2 and 4 mg/kg) were ineffective in altering rats' taste-reactivity response to and consumption of ethanol. While these data suggest that delta receptors are involved in rats' taste-reactivity response to ethanol and rats' ethanol consumption, it is likely that multiple opioid receptors mediate both behavioral responses.

Effect of the melanocortin receptor stimulation or inhibition on ethanol intake in alcohol-preferring rats

It has been recently reported that acute intracerebroventricular injection of 1 nmol/rat of the non-selective melanocortin 3 and 4 receptor (MC3/4) agonist MTII reduces ethanol intake in female AA alcohol-preferring rats and alters opioid peptide levels in the ventral tegmental area of rats. To better understand the role of the MC system in the control of ethanol intake, we tested the acute and chronic effects of lateral ventricular (LV) injections of 0.01-1 nmol MTII, of 0.1-1 nmol of the MC3/4R receptor antagonist agouti related peptide (AgRP), and 0.1-0.5 nmol of the MC3/4R receptor antagonist SHU9119 on food, water, and 10% ethanol intake in Marchigian-Sardinian alcohol-preferring (msP) rats, which spontaneously ingest pharmacologically relevant quantities of ethanol both under short and long term access conditions. The data showed that with 2h/day ethanol access, LV MTII injections reduced intake of food and ethanol intakes. When food, water, and ethanol were available ad libitum and 0.01 nmol MTII was given by daily LV injection, however, ethanol intake was reduced for only the first 2 days, whereas food intake was reduced for all 5 days of treatment. Finally, acute LV injection of neither AgRP nor SHU9119 affected ethanol intake under ad libitum conditions, although both antagonists significantly increased food and water intake. In conclusion, these data fail to support a role for endogenous MC3/4R in the control of spontaneous ethanol intake in the msP rat. MC3/4R agonism, however, reduced ethanol intake in association with reduced food intake, suggesting that MTII might reduce nutrient-related controls of ethanol intake rather than, or in addition to, reward-related controls of ethanol intake.

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

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

Increased palatability of ethanol after prenatal ethanol exposure is mediated by the opioid system

Previous studies have shown that prenatal exposure to a moderate dose of ethanol (2 g/kg) during the last days of gestation of the rat (17-20) not only increases postnatal intake of the drug but also enhances the palatability of ethanol's taste when measured with a taste reactivity test. Prenatal administration of the opioid antagonist naloxone, together with ethanol, reduces ethanol intake. The aim of the present study was to analyze whether this decreased intake of ethanol after the administration of naloxone is accompanied by a reduction in ethanol's palatability. Results show that preweanling rats exposed prenatally to ethanol alone displayed more ethanol intake and more ingestive responses in reaction to its taste than non-exposed pups. Simultaneous prenatal administration of naloxone with ethanol prevented both the increased intake of ethanol and the higher amount of appetitive responses to its taste. These results indicate that the opioid system plays an important role in the effect of enhanced palatability of ethanol's taste after its prenatal exposure. Results also support the hypothesis of a conditioned response established in utero as a consequence of the association between ethanol's chemosensory and reinforcing aspects, the latter mediated by the opioid system.

The effect of cocaine on the expression of motor activity and conditioned place preference in high and low alcohol-preferring Wistar rats

Outbred rats show significant variability in their propensity to consume alcohol. These experiments were designed to examine the effect of cocaine on the expression of motor activation or place preference in outbred Wistar rats that consumed either high or low quantities of alcohol. These rats were exposed to a 2-bottle limited access procedure and dichotomized into 2 groups, high (mean 0.91 g/kg/h) and low alcohol consumption (0.36 g/kg/h), and then exposed to repeated daily cocaine, 10 or 20 mg/kg, or saline injections. The low alcohol-consuming rats showed a significant increase in motor behavior to a cocaine challenge across both doses of cocaine, which did not differ from each other. The high alcohol-consuming rats showed a significant increase in motor behavior only at the high dose of cocaine. In Experiment 2 both high and low alcohol-consuming rats were exposed to a conditioned place preference procedure using cocaine 10 mg/kg. High alcohol-consuming rats showed a significant place preference to the cocaine-paired side while low consuming rats did not. The differential effects of cocaine on motor activating behavior and reward obtained in these experiments suggest that those factors determining whether an outbred Wistar rat will consume high or low amounts of alcohol are related, in part, to differential sensitivity of those neural systems underlying the effects of those drugs.

The opioidergic-alcohol link : implications for treatment

Preclinical and clinical data implicate the endogenous opioid system in alcohol dependence. In vitro studies show that rodent pituitary and hypothalamic tissue responds to acute exposure to alcohol by releasing beta-endorphins. In vivo studies suggest differential activity of endogenous opioid receptors in rodents with high and low alcohol preference. Similarly, humans with a family history of alcohol dependence also show a heightened endorphin response to an acute challenge of alcohol compared with those with no family history of alcohol dependence.The effects of opioid agonists and antagonists on rodent and human alcohol consumption further support the opioid-alcohol link. In rodents and humans, small doses of opioid agonists increase alcohol consumption, while pretreatment with large doses decreases consumption. The opioid antagonist naltrexone decreases rodent alcohol consumption, particularly in low doses under acute and intermittent schedules. Most clinical trials in patients with alcohol dependence support modest therapeutic effects of naltrexone in decreasing alcohol consumption. Efforts to identify subgroups of alcohol-dependent patients responsive to naltrexone, as well as psychosocial and pharmacological augmentation strategies, may further improve the clinical usefulness of the drug.

Deficit in beta-endorphin peptide and tendency to alcohol abuse

Human and animal studies suggest that there is a correlation between endogenous opioid peptides, especially beta-endorphin, and alcohol abuse. It has been proven that the consumption of alcohol activates the endogenous opioid system. Consumption of alcohol results in an increase in beta-endorphin level in those regions of the human brain, which are associated with a reward system. However, it has also been observed that habitual alcohol consumption leads to a beta-endorphin deficiency. It is a well-documented phenomenon that people with a genetic deficit of beta-endorphin peptide are particularly susceptible to alcoholism. The plasma level of beta-endorphin in subjects genetically at high risk of excessive alcohol consumption shows lower basal activity of this peptide. Its release increases significantly after alcohol consumption. Clinical and laboratory studies confirm that certain genetically determined factors might increase the individual's vulnerability to alcohol abuse.

The effect of baclofen alone and in combination with naltrexone on ethanol consumption in the rat

Naltrexone has been evaluated in preclinical animal models of ethanol consumption and found to be effective in most reports. In clinical use, naltrexone has not proved to be as efficacious in preventing relapse. While naltrexone targets opioid receptors, many other neurotransmitter systems are targeted by ethanol and, to a greater or lesser extent, contribute to modulating ethanol's reinforcing effects. There has been indication that drugs active at the gamma amino butyric acid B (GABAB) receptors can affect the self-administration of many drugs with abuse potential. The experiments reported here evaluated the effect of three doses of baclofen (2.5, 5.0, or 7.5 mg/kg), a GABAB agonist, administered alone or in combination with a single dose of naltrexone (1.0 mg/kg). In Experiment 1, both naltrexone and baclofen, at the two higher doses tested, significantly reduced ethanol consumption in Wistar rats using a limited access procedure on Drug Days 1 and 2. When combined on Drug Days 3 and 4, baclofen/naltrexone was significantly more effective in reducing ethanol consumption than did either drug alone. Neither drug, alone or in combination, had an effect on water consumption. In Experiment 2, both baclofen and naltrexone again significantly reduced ethanol consumption, with no evidence that chronic administration across Drug Days 3 and 4 further reduced consumption compared with Drug Days 1 and 2. The clinical use of multiple pharmacotherapeutic agents in combination may allow for the use of lower doses of individual components, thereby reducing the negative side effects that contribute to lower compliance and higher relapse.