Hormonal and other effects of naltrexone in normal men

Naltrexone alters alcohol self-administration behaviors and hypothalamic-pituitary-adrenal axis activity in a sex-dependent manner in rats

BACKGROUND

The mu-opioid antagonist, naltrexone (NTX), is a FDA-approved treatment for alcohol use disorder (AUD); however, the data on whether it differentially affects males vs. females are mixed. NTX increases hypothalamic-pituitary-adrenal (HPA) axis activity that associates with subjective responses to alcohol and craving in individuals with AUD. The present study tested for sex differences in the ability of NTX to decrease appetitive and consummatory behaviors in rats in operant alcohol self-administration. Because the opioid system and HPA axis are sexually dimorphic, we examined NTX's effect on adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels.

METHODS

Male and female Sprague-Dawley rats (n's = 6-8) were trained to lever press for alcohol (10% v/v) under a fixed-ratio 2 schedule of reinforcement. NTX doses (0, 0.1-10 mg/kg) were assessed in tests conducted under a progressive ratio schedule of reinforcement. Separate groups of alcohol and water drinking rats (n's = 8) were used to assess NTX's (10 mg/kg) effects on HPA axis hormones.

RESULTS

NTX decreased consummatory behaviors for alcohol in a dose-related manner, but not appetitive behaviors in males. In females, NTX decreased appetitive behaviors for alcohol in a dose-dependent manner, but only decreased consummatory behaviors at the highest (10 mg/kg) NTX dose. NTX increased ACTH levels in alcohol drinking females in diestrus, but not in other groups. However, NTX increased CORT levels for longer durations in alcohol drinking males relative to alcohol drinking females in diestrus.

CONCLUSIONS

Our findings suggest that NTX selectively reduces consummatory behaviors for alcohol in males and appetitive behaviors in females, while also showing differential sex effects on HPA hormones.

WIN 44,441: A Stereospecific and Long-Acting Narcotic Antagonist

The opiate antagonist WIN 44,441-3 is a potent, stereospecific antagonist of mu, delta, and kappa opiate receptors. This antagonist activity is of long duration (> 4 h) with no agonist activity being observed. It therefore appears that WIN 44,441-3 will be a useful long-acting opiate antagonist for in vivo studies.

Effect of opioid antagonists on sex hormone secretion

BACKGROUND

Endogenous opioids have roles in various functions in different parts of the body, including intestinal motility, suppression of pain, reinforcement of behavior, and regulation of the hypothalamic-pituitary-gonadal axis. The endogenous opioid system is also recognized to be involved in the negative-feedback regulation of the release of LH and testosterone.

AIM

The reviewed articles herein show the development of the current model of this regulation, the evidence supporting it, and also the observed effects of opioid antagonist (naloxone, naltrexone, and nalmefene) on the system.

MATERIALS AND METHODS

Review of the studies published during the years 1979-1996 (no significant studies made after that). Search from databases Pubmed, SciFinder, and Medline with search words opioid antagonists, hormones, LH, testosterone, and GnRH, in different combinations.

RESULTS/CONCLUSIONS

Opioid antagonists seem to increase the secretion of GnRH in the hypothalamus which then causes a pulsatile release of LH in the pituitary and secretion of testosterone. According to the experiments, the frequency of pulses and concentration of LH and testosterone in plasma seem to increase. These effects are seen in both men and women (at early follicular phase). More research is needed to investigate the consequences of these effects in general.

Naltrexone selectively elevates GABAergic neuroactive steroid levels in heavy drinkers with the Asp40 allele of the OPRM1 gene: a pilot investigation

BACKGROUND

Preclinical studies have implicated GABAergic neurosteroids in behavioral responses to alcohol. Naltrexone is thought to blunt the reinforcing effects of alcohol, and a few studies have found that the effects of naltrexone are moderated by the Asn40Asp polymorphisms of the OPRM1 gene. The present study seeks to integrate these lines of research by testing (i) the moderating role of the functional Asn40Asp polymorphism of the OPRM1 gene on naltrexone-induced alternations in GABAergic neurosteroid levels, namely (3alpha,5alpha)-3-hydroxypregnan-20-one (allopregnanolone, ALLO); and (ii) the combined effects of naltrexone or genotype with alcohol administration on neurosteroid levels in a sample of at-risk drinkers.

METHODS

Participants were 32 (9 females) nontreatment-seeking heavy drinkers who completed a placebo-controlled laboratory study of naltrexone (50 mg/d for 3 days) and provided complete sets of serum samples for ALLO assays before and after alcohol administration under both naltrexone and placebo conditions.

RESULTS

Naltrexone treatment raised ALLO levels among carriers of the Asp40 allele, but not homozygotes for the Asn40 allele. The Asn40Asp polymorphism did not moderate effects of naltrexone on cortisol levels. Ethanol infusion modestly reduced ALLO levels in all subjects, independent of genotype or naltrexone exposure.

CONCLUSIONS

Naltrexone increased ALLO levels among individuals with the Asn40Asp allele suggesting a potential neurosteroid contribution to the neuropharmacological effects of naltrexone among Asp40 carriers.

Antipruritic treatment with systemic μ-opioid receptor antagonists: a review

During the past two decades, systemic μ-opioid receptor antagonists (MORA) have been used in the treatment of various forms of chronic pruritus. In a number of case reports, case series, and controlled trials, treatment with MORA has demonstrated considerable antipruritic effects. In double-blind controlled studies, significant antipruritic relief has been achieved by MORA in cholestatic pruritus, chronic urticaria, and atopic dermatitis. In case reports and case series, antipruritic efficacy of MORA has been reported in prurigo nodularis, mycosis fungoides, postburn pruritus, aquagenic pruritus, hydroxyethyl starch-induced pruritus, and pruritus of unknown origin. However, most of the evidence remains anecdotal, the design of these trials varies, and comparison of results is difficult. In this review we aim to present an overview of these reports and to assess the evidence for the antipruritic action of the drugs naloxone, nalmefene, and naltrexone, which are currently in use for the treatment of chronic pruritus of different origins. We will also evaluate recommendations for the use of MORA in daily medical practice.

Effects of naltrexone on cortisol levels in heavy drinkers

The primary objectives of this study were to: (a) examine the neuroendocrine effects of naltrexone vs. placebo by comparing serum cortisol levels; and (b) test the biobehavioral correlates of naltrexone-induced changes in cortisol. Non-treatment seeking heavy drinkers (n=37) completed two intravenous alcohol administrations, one after naltrexone (50 mg) and one after placebo. Cortisol levels were measured at baseline and after alcohol intake (BrAC=0.06 g/dl) on both sessions, as were subjective responses to alcohol. Analyses revealed that naltrexone significantly raised overall cortisol levels compared to placebo. Cortisol levels decreased during alcohol administration and a stronger decrease was observed in the naltrexone condition. Cortisol levels were, in turn, inversely related to some of alcohol's the reinforcing effects (i.e., alcohol 'high,' vigor) and positively associated with some of its unpleasant effects (i.e., sedation and subjective intoxication). These results suggest that naltrexone alters cortisol levels in heavy drinkers and that its effects on subjective responses to alcohol may be related, in part, to naltrexone's ability to activate the HPA-axis.

Antagonist treatment of opioid withdrawal translational low dose approach

Although antagonist drugs are receiving increasing attention in the treatment of opioid withdrawal, the mechanisms of interaction of opiate agonists and antagonists remain largely to be investigated. We focused on the effects of very low quantities of opiate antagonists, following the clinical indication of their potential utility in detoxification. Upon reviewing the evidence on the administration of small doses of naloxone and naltrexone in the presence of agonist drugs, the effects of low-dose naltrexone during opiate administration and withdrawal are described. The application of a translational methodology allowed completing the clinical design with behavioral and cellular information obtained from a specifically developed animal model. The initial results indicate that low doses of naltrexone may help reducing the manifestation of opioid withdrawal, offer suggestions for further investigations and confirm the utility of a translational research approach to the clinical neurobiology of drug addiction.

Nociceptive flexion reflex and pain rating responses during endogenous opiate blockade with naltrexone in healthy young adults

The effect of opioid blockade on nociceptive flexion reflex (NFR) activity and subjective pain ratings was examined in 151 healthy young men and women. Using a within-subjects design, NFR threshold was assessed on 2 days after administration of either placebo or a 50mg dose of naltrexone. Electrocutaneous pain threshold and tolerance levels were measured after NFR threshold assessment on each day. Results indicated that administration of naltrexone was consistently associated with hypoalgesic responding. Specifically, participants exhibited lower levels of NFR activity and reported lower pain ratings for electrocutaneous stimulation delivered at pain threshold and tolerance levels following administration of naltrexone as compared to placebo. These findings indicate that opiate blockade using the current standard dose may elicit hypoalgesia. A potential moderating effect of dose of opiate-blockade medication and level of endogenous opioid activation should be carefully examined in future research.

Nalmefene induced elevation in serum prolactin in normal human volunteers: partial kappa opioid agonist activity?

In humans, mu- and kappa-opioid receptor agonists lower tuberoinfundibular dopamine, which tonically inhibits prolactin release. Serum prolactin is, therefore, a useful biomarker for tuberoinfundibular dopamine. The current study evaluated the unexpected finding that the relative mu- and kappa-opioid receptor selective antagonist nalmefene increases serum prolactin, indicating possible kappa-opioid receptor agonist activity. In all, 33 healthy human volunteers (14 female) with no history of psychiatric or substance use disorders received placebo, nalmefene 3 mg, and nalmefene 10 mg in a double-blind manner. Drugs were administered between 0900 and 1000 on separate days via 2-min intravenous infusion. Serial blood specimens were analyzed for serum levels of prolactin. Additional in vitro studies of nalmefene binding to cloned human kappa-opioid receptors transfected into Chinese hamster ovary cells were performed. Compared to placebo, both doses of nalmefene caused significant elevations in serum prolactin (p<0.002 for nalmefene 3 mg and p<0.0005 for nalmefene 10 mg). There was no difference in prolactin response between the 3 and 10 mg doses. Binding assays confirmed nalmefene's affinity at kappa-opioid receptors and antagonism of mu-opioid receptors. [(35)S]GTPgammaS binding studies demonstrated that nalmefene is a full antagonist at mu-opioid receptors and has partial agonist properties at kappa-opioid receptors. Elevations in serum prolactin following nalmefene are consistent with this partial agonist effect at kappa-opioid receptors. As kappa-opioid receptor activation can lower dopamine in brain regions important to the persistence of alcohol and cocaine dependence, the partial kappa agonist effect of nalmefene may enhance its therapeutic efficacy in selected addictive diseases.

Alcohol urge and plasma beta-endorphin change after alcohol challenge with naltrexone pretreatment in social drinkers

The authors have investigated the effect of naltrexone (NTX) on lowering the urge of alcohol drinking and the action mechanism of NTX. Fifteen healthy male social drinkers voluntarily participated. The experimental method was a double-blind, placebo-controlled cross-over design. To eliminate NTX effect, 1 week washout cross-over interval was taken. Subjects ingested NTX, 50 mg/day, or placebo for 1 week. Then, the alcohol (0.5 ml/kg) challenge test was done in the evening. Blood samples were taken immediately before drinking, at 20 min and at 60 min after alcohol drinking. Plasma beta-endorphin, plasma ACTH and serum cortisol levels were checked. Subjects completed self-report questionnaires such as the visual analog scales of drink urge and the alcohol sensation scales at regular intervals. In the case of NTX pretreatment, the subjects reported significantly (P=.013) less urge to drink alcohol on the self-reporting urge scales, especially at postdrinking 20 min and 60 min than placebo pretreatment. After alcohol challenge, the subjects reported significantly more dizziness (P=.015) in the case of NTX pretreatment, and reported less mood elevation trend, though not significant (P=.052). Basal plasma beta-endorphin levels were not different, but in the case of NTX pretreatment, the increasing degree of plasma beta-endorphin during 20 min after alcohol challenge was significantly (P=.039) higher than with placebo pretreatment. This results show that the NTX reduced the urge to drink alcohol with the mechanism of partially blocking the opioid positive reward system and partially mimicking the alcohol effect.

A risk-benefit assessment of naltrexone in the treatment of alcohol dependence

There is a great deal of interest in the use of naltrexone as a treatment for alcohol (ethanol) dependence since there is a rapidly expanding body of evidence to support its efficacy and tolerability in this indication. Naltrexone, a long-acting, nonselective opioid receptor antagonist has been shown to reduce alcohol intake when combined with behavioural treatment. Naltrexone may prevent the return to clinically significant drinking by blocking the pleasurable effects or "high' associated with alcohol drinking. Results from controlled studies showed that in alcohol dependent patients taking naltrexone 50 mg/day in combination with behavioural treatment, relapse rates were reduced by 50% compared with placebo treated patients. Historically, several factors have limited the use of effective pharmacological adjuncts in the treatment of alcohol dependence. These include safety considerations in this vulnerable population, and the fact that some treatment programmes discourage alcohol-dependent patients from taking medications. The most common adverse effects reported with the use of naltrexone at a dosage of 50 mg/day include nausea and vomiting. Naltrexone does not appear to be hepatotoxic in dosages recommended in the treatment of alcohol dependence, i.e. 50 mg/day. Thus, naltrexone appears to offer significant therapeutic benefits at a relatively low risk, when used judiciously and with behavioural treatment for alcohol dependent patients.

A bioequivalence study of oral controlled-release morphine using naltrexone blockade

Twenty-three normal volunteers who received morphine sulphate (MS Contin) with naltrexone completed this randomized, analytically blinded, two-way crossover comparison of the bioavailability of one 200-mg oral controlled-release morphine sulfate tablet with two 100-mg MSC tablets. Morphine effects were blocked by three 100-mg doses of naltrexone. The first dose of naltrexone was given 24 hours before MSC dosing, followed by a second dose at the time of MSC dosing and a third dose 24 hours after MSC administration. Compared with two 100-mg MSC tablets, the 200-mg tablet was 96% bioavailable (90% confidence interval, 88.14-105.74%). The 90% confidence intervals for mean Cmax and AUC0-24 for one 200-mg MSC tablet were within +/- 20% of the Cmax and AUC0-24 of two 100-mg tablets, indicating the two dosage forms are bioequivalent. Single 200-mg doses of MSC given with the naltrexone blockade were generally well tolerated, and adverse effects were similar to those reported for naltrexone alone and for lower doses of morphine without naltrexone. Naltrexone proved safe and effective in blocking the effects of controlled-release morphine, permitting bioequivalence studies of a high dose of morphine in normal volunteers.

Naltrexone. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic efficacy in the management of opioid dependence

Naltrexone is a long acting competitive antagonist at opioid receptors which blocks the subjective and objective responses produced by intravenous opioid challenge. It is suitable for oral administration, and has been studied as an adjunct for use in opioid addiction management programmes. In non-comparative clinical trials involving detoxified patients, oral naltrexone reduced heroin craving and between 23 and 62% of patients remained in treatment after 3 to 4 weeks. However, in two studies 32 to 58% of patients who continued in treatment were opioid-free between 6 and 12 months after stopping naltrexone. As might be expected studies involving highly motivated patients have shown this type of patient group to achieve greater treatment success rates during naltrexone therapy, and remain opioid-free longer than other groups of apparently less motivated patients. In addition, when naltrexone is combined with family support, psychotherapy and counselling, patients are more likely to remain opioid-free. Naltrexone produces a low incidence of side effects, with gastrointestinal effects being the most commonly reported symptoms. Thus, despite the overall high attrition rates from trials, in selected patient groups and in combination with appropriate support mechanisms and psychotherapy, naltrexone represents a useful adjunct for the maintenance of abstinence in the detoxified opioid addict.

d-Amphetamine raises cortisol levels in schizophrenic patients with and without chronic naltrexone pretreatment

Five drug-free schizophrenic patients received an infusion of 20 mg d-amphetamine and placebo on separate days in a pilot study. Plasma cortisol levels rose significantly at 1 and 2 hours after d-amphetamine compared to following placebo. A repeat infusion of d-amphetamine during naltrexone treatment led to a similar increase in cortisol concentrations.

Assessment of naltrexone in the treatment of schizophrenia

Naltrexone, a long acting opiate antagonist, and placebo were administered to eight schizophrenics in doses of 200 mg per day for 1 week in a double-blind, crossover design. No improvement was noted, and no side effects resembling the opiate withdrawal syndrome with naltrexone were found. Naltrexone does not appear to alter schizophrenic symptomatology.