Chronic very low dose naltrexone administration attenuates opioid withdrawal expression

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

Dose Escalation of Naltrexone to Reduce Stress Responses Associated With Opioid Antagonist Induction: A Double-blind Randomized Trial

CONTEXT

To describe the role of opioid antagonist induction in reducing stress response and withdrawal symptoms.

OBJECTIVE

Complexity of naltrexone induction is limiting broader applicability of opioid antagonist-assisted abstinence. The aim of this clinical trial was to assess the stress response to 2 low-dose naltrexone induction protocols under minimal oral sedation.

DESIGN

Double-blind randomized controlled trial.

SETTING

Open setting in-patient unit.

PARTICIPANTS

Adults with opioid use disorder, and at least a year-long history of opioid use.

INTERVENTION PROTOCOL

Patients received either a single 12.5 mg naltrexone oral dose (SI group) or escalating dosage regimen starting from 50 μg up to a cumulative dose of 12.5 mg (ED group).

MAIN OUTCOME MEASURE

Differences in cortisol and adrenocorticotropic hormone (ACTH) concentrations 1 hour after the start of naltrexone induction.

RESULTS

In all, 124 patients were enrolled and 68 remained in the trial at the point of randomization-33 in SI and 35 in ED group. Eight patients were excluded from final analysis. Plasma cortisol and ACTH concentrations were significantly higher in SI group; mean difference between groups 313 nmol/L (95% confidence interval [CI] 182-444, P < 0.001) and 36.9 pg/mL (95% CI 12.3-61.4, P = 0.004), respectively.

SECONDARY OUTCOMES

SI patients experienced significant increases in plasma cortisol and ACTH concentrations, and withdrawal scores. In ED group these measures remained at or below baseline throughout the 24-hour period from start of naltrexone induction.

CONCLUSIONS

Contrary to a single 12.5-mg dose, the escalating naltrexone dosing regimen produced no significant increase in stress response and withdrawal scores during antagonist induction.

Very low dose naltrexone in opioid detoxification: a double-blind, randomized clinical trial of efficacy and safety

Withdrawal syndrome is one of the initial focuses of opioid detoxification. Very low dose naltrexone (VLNTX) has been found to reduce opioid tolerance and dependence in animal and human clinical studies. The aim of this study was to determine the safety and efficacy of VLNTX during early stages of detoxification. In a multi-arm parallel, double-blind, randomized controlled trial, 63 opioid-dependent male participants referring to Imam Reza Rehabilitation Center were allocated to three equal groups using block randomization method. They received 0.125 mg, 0.250 mg of VLNTX or placebo daily for 10 days, together with the routine clonidine-based protocol. Self-reported and observer ratings of withdrawal severity and adverse events were measured on the 1st, 4th and 10th day of treatment. Runny eyes (p = 0.006), anxiety (p = 0.031) and dehydration (p = 0.014) were reduced during the whole 10 days in the 0.125 mg VLNTX-treated group compared to placebo. Only drowsiness (p = 0.043) and dysphoric mood (p < 0.001) were reduced in the 0.250 mg VLNTX-treated group. Results of 1st, 4th, and 10th-day assessment showed that most symptoms reductions were for the 0.125 mg VLNTX and the placebo group in the 1st and 4th days, respectively. On the 10th day, there was not any significant difference between 0.250 mg VLNTX-treated group and placebo group. No adverse effect was observed. In the starting days of detoxification, VLNTX can reduce the withdrawal symptoms, but the efficacy declined by passing time. Further studies are needed to test the utility of this new therapeutic approach.

Dissociation between morphine-induced spinal gliosis and analgesic tolerance by ultra-low-dose α2-adrenergic and cannabinoid CB1-receptor antagonists

Long-term use of opioid analgesics is limited by tolerance development and undesirable adverse effects. Paradoxically, spinal administration of ultra-low-dose (ULD) G-protein-coupled receptor antagonists attenuates analgesic tolerance. Here, we determined whether systemic ULD α2-adrenergic receptor (AR) antagonists attenuate the development of morphine tolerance, whether these effects extend to the cannabinoid (CB1) receptor system, and if behavioral effects are reflected in changes in opioid-induced spinal gliosis. Male rats were treated daily with morphine (5 mg/kg) alone or in combination with ULD α2-AR (atipamezole or efaroxan; 17 ng/kg) or CB1 (rimonabant; 5 ng/kg) antagonists; control groups received ULD injections only. Thermal tail flick latencies were assessed across 7 days, before and 30 min after the injection. On day 8, spinal cords were isolated, and changes in spinal gliosis were assessed through fluorescent immunohistochemistry. Both ULD α2-AR antagonists attenuated morphine tolerance, whereas the ULD CB1 antagonist did not. In contrast, both ULD atipamezole and ULD rimonabant attenuated morphine-induced microglial reactivity and astrogliosis in deep and superficial spinal dorsal horn. So, although paradoxical effects of ULD antagonists are common to several G-protein-coupled receptor systems, these may not involve similar mechanisms. Spinal glia alone may not be the main mechanism through which tolerance is modulated.

Long-Acting Injectable Naltrexone Induction: A Randomized Trial of Outpatient Opioid Detoxification With Naltrexone Versus Buprenorphine

OBJECTIVE

At present there is no established optimal approach for transitioning opioid-dependent adults to extended-release injection naltrexone (XR-naltrexone) while preventing relapse. The authors conducted a trial examining the efficacy of two methods of outpatient opioid detoxification for induction to XR-naltrexone.

METHOD

Participants were 150 opioid-dependent adults randomly assigned 2:1 to one of two outpatient detoxification regimens, naltrexone-assisted detoxification or buprenorphine-assisted detoxification, followed by an injection of XR-naltrexone. Naltrexone-assisted detoxification lasted 7 days and included a single day of buprenorphine followed by ascending doses of oral naltrexone along with clonidine and other adjunctive medications. Buprenorphine-assisted detoxification included a 7-day buprenorphine taper followed by a week-long delay before administration of XR-naltrexone, consistent with official prescribing information for XR-naltrexone. Participants from both groups received behavioral therapy focused on medication adherence and a second dose of XR-naltrexone.

RESULTS

Compared with participants in the buprenorphine-assisted detoxification condition, participants assigned to naltrexone-assisted detoxification were significantly more likely to be successfully inducted to XR-naltrexone (56.1% compared with 32.7%) and to receive the second injection at week 5 (50.0% compared with 26.9%). Both models adjusted for primary type of opioid use, route of opioid administration, and morphine equivalents at baseline.

CONCLUSIONS

These results demonstrate the safety, efficacy, and tolerability of low-dose naltrexone, in conjunction with single-day buprenorphine dosing and adjunctive nonopioid medications, for initiating adults with opioid dependence to XR-naltrexone. This strategy offers a promising alternative to the high rates of attrition and relapse currently observed with agonist tapers in both inpatient and outpatient settings.

Molecular Changes in Opioid Addiction: The Role of Adenylyl Cyclase and cAMP/PKA System

For centuries, opiate analgesics have had a considerable presence in the treatment of moderate to severe pain. While effective in providing analgesia, opiates are notorious in exerting many undesirable adverse reactions. The receptor targets and the intracellular effectors of opioids have largely been identified. Furthermore, much of the mechanisms underlying the development of tolerance, dependence, and withdrawal have been delineated. Thus, there is a focus on developing novel compounds or strategies in mitigating or avoiding the development of tolerance, dependence, and withdrawal. This review focuses on the adenylyl cyclase and cyclic adenosine 3,5-monophosphate (cAMP)/protein kinase A (AC/cAMP/PKA) system as the central player in mediating the acute and chronic effects of opioids. This chapter also reviews the neuronal adaptive changes in the locus coeruleus, amygdala, periaqueductal gray, and ventral tegmental area induced by acute and chronic actions of opioid because these neuronal adaptive changes in these regions may underlie the behavioral changes observed in opiate users and abusers.

Neuropeptide trefoil factor 3 attenuates naloxone-precipitated withdrawal in morphine-dependent mice

RATIONALE

The persistence of physical dependence and craving in addicts is considered to contribute to relapse. Increasing evidence indicates that neuropeptide systems are associated with several phases of drug addiction, but little is known about whether the neuropeptide trefoil factor affects withdrawal symptoms.

OBJECTIVES

This study aims to investigate the potential effects of the neuropeptide trefoil factor 3 (TFF3) on naloxone-precipitated withdrawal symptoms in morphine-dependent mice.

RESULTS

Mice received increasing doses of morphine over 3 days. On day 4, the mice were injected with TFF3 (1.0 mg/kg, i.p.) 30 min after the last dose of morphine. Thirty minutes after TFF3 treatment, naloxone (1 mg/kg, i.p.) was injected, and body weight, jumping behavior, wet-dog shakes, and locomotor activity were assessed 30 min later. Naloxone caused significant weight loss and increased jumping behavior and wet-dog shakes in morphine-dependent mice. TFF3 (1.0 mg/kg) reversed these behavioral symptoms caused by morphine withdrawal, suggesting that TFF3 might ameliorate physical dependence associated with opiate addiction. Furthermore, TFF3 pretreatment significantly reduced morphine withdrawal-induced increases in plasma corticosterone and adrenocorticotropic hormone levels. The glucocorticoid receptor agonist RU486 blocked the behavioral effects of TFF3 on morphine withdrawal symptoms. Finally, Fos expression in the medial prefrontal cortex which was decreased during morphine withdrawal was increased by TFF3 pretreatment.

CONCLUSION

These findings indicate that TFF3 might be a potential therapeutic candidate for opiate addiction by regulating glucocorticoid secretion and neuronal activation in the prefrontal cortex.

Injectable and implantable sustained release naltrexone in the treatment of opioid addiction

Sustained release technologies for administering the opioid antagonist naltrexone (SRX) have the potential to assist opioid-addicted patients in their efforts to maintain abstinence from heroin and other opioid agonists. Recently, reliable SRX formulations in intramuscular or implantable polymers that release naltrexone for 1-7 months have become available for clinical use and research. This qualitative review of the literature provides an overview of the technologies currently available for SRX and their effectiveness in reducing opioid use and other relevant outcomes. The majority of studies indicate that SRX is effective in reducing heroin use, and the most frequently studied SRX formulations have acceptable adverse events profiles. Registry data indicate a protective effect of SRX on mortality and morbidity. In some studies, SRX also seems to affect other outcomes, such as concomitant substance use, vocational training attendance, needle use, and risk behaviour for blood-borne diseases such as hepatitis or human immunodeficiency virus. There is a general need for more controlled studies, in particular to compare SRX with agonist maintenance treatment, to study combinations of SRX with behavioural interventions, and to study at-risk groups such as prison inmates or opioid-addicted pregnant patients. The literature suggests that sustained release naltrexone is a feasible, safe and effective option for assisting abstinence efforts in opioid addiction.

Drug safety evaluation of naltrexone/bupropion for the treatment of obesity

INTRODUCTION

Obesity is a known health risk for the development of several preventable diseases. Obesity-related metabolic alterations negatively impact different physiological mechanisms, which supports the rationale for the use of combined drug therapy. Naltrexone is an opioid antagonist for the treatment of opioid and alcohol dependency, whereas bupropion is a norepinephrine/dopamine reuptake inhibitor used to treat depression and smoking cessation. Although not effective as individual monotherapies for obesity, naltrexone and bupropion in combination produce weight loss and a metabolic profile beneficial for the potential treatment of obesity.

AREAS COVERED

This review examines the safety and antiobesity effects of naltrexone and bupropion alone and in combination. It reviews the results of four Phase III clinical trials of a novel fixed dose of sustained-released naltrexone/bupropion.

EXPERT OPINION

Naltrexone/bupropion has a greater weight loss efficacy than two FDA-approved medications, orlistat and lorcaserin. Although the weight loss produced by phentermine/topiramate is superior to naltrexone/bupropion, the safety profile of naltrexone/bupropion has less severe adverse effects. In addition, naltrexone/bupropion is well tolerated, with nausea being the most reported adverse event. Unlike other centrally acting medications, lorcaserin and phentermine/topiramate, naltrexone/bupropion has no abuse potential.

Smoking and opioid detoxification: behavioral changes and response to treatment

INTRODUCTION

The relevance of tobacco use in opioid addiction (OA) has generated a demand for available and more effective interventions. Thus, further analysis of less explored nicotine-opioid clinical interactions is warranted.

METHODS

A post-hoc analysis of OA participants in a double-blind, randomized very low dose naltrexone (VLNTX) inpatient detoxification trial evaluated measures of opioid withdrawal and tobacco use. Intreatment smokers were compared with nonsmokers, or smokers who were not allowed to smoke.

RESULTS

A total of 141 (81%) of 174 OA participants were smokers, all nicotine-dependent. Inpatient smoking was a predictor of opioid withdrawal discomfort. Intreatment smokers (n = 96) showed significantly higher opioid craving (F = 3.7, p < .001) and lower detoxification completion rate (χ(2) = 7.9, p < .02) compared with smokers who were not allowed to smoke (n = 45) or nonsmokers (n = 33). Smoking during treatment was associated with more elevated cigarette craving during detoxification (F = 4.1, p < .001) and a higher number of cigarettes smoked at follow-up (F = 3.6, p < .02). Among intreatment smokers, VLNTX addition to methadone taper was effective in easing opioid withdrawal and craving more than other treatments, whereas the combination VLNTX-clonidine was associated with significantly reduced cigarette craving and smoking during detoxification.

CONCLUSIONS

Failure to address tobacco use may negatively affect pharmacologically managed opioid discontinuation. Opioid detoxification may offer a window of opportunity to expand smoking cessation treatment, hence improving OA outcomes. The observed effects support testing of VLNTX-clonidine in smoking cessation trials among individuals with or without substance abuse.

Buprenorphine-mediated transition from opioid agonist to antagonist treatment: state of the art and new perspectives

Constant refinement of opioid dependence (OD) therapies is a condition to promote treatment access and delivery. Among other applications, the partial opioid agonist buprenorphine has been studied to improve evidence-based interventions for the transfer of patients from opioid agonist to antagonist medications. This paper summarizes PubMed-searched clinical investigations and conference papers on the transition from methadone maintenance to buprenorphine and from buprenorphine to naltrexone, discussing challenges and advances. The majority of the 26 studies we examined were uncontrolled investigations. Many small clinical trials have demonstrated the feasibility of in- or outpatient transfer to buprenorphine from low to moderate methadone doses (up to 60-70 mg). Results on the conversion from higher methadone doses, on the other hand, indicate significant withdrawal discomfort, and need for ancillary medications and inpatient treatment. Tapering high methadone doses before the transfer to buprenorphine is not without discomfort and the risk of relapse. The transition buprenorphine-naltrexone has been explored in several pilot studies, and a number of treatment methods to reduce withdrawal intensity warrant further investigation, including the co-administration of buprenorphine and naltrexone. Outpatient transfer protocols using buprenorphine, and direct comparisons with other modalities of transitioning from opioid agonist to antagonist medications are limited. Given its potential salience, the information gathered should be used in larger clinical trials on short and long-term outcomes of opioid agonist-antagonist transition treatments. Future studies should also test new pharmacological mechanisms to help reduce physical dependence, and identify individualized approaches, including the use of pharmacogenetics and long-acting opioid agonist and antagonist formulations.

The combination very low-dose naltrexone-clonidine in the management of opioid withdrawal

BACKGROUND

The management of withdrawal absorbs substantial clinical efforts in opioid dependence (OD). The real challenge lies in improving current pharmacotherapies. Although widely used, clonidine causes problematic adverse effects and does not alleviate important symptoms of opioid withdrawal, alone or in combination with the opioid antagonist naltrexone. Very low-dose naltrexone (VLNTX) has been shown to attenuate withdrawal intensity and noradrenaline release following opioid agonist taper, suggesting a combination with clonidine may result in improved safety and efficacy.

OBJECTIVES

We investigated the effects of a VLNTX-clonidine combination in a secondary analysis of data from a double-blind, randomized opioid detoxification trial.

METHODS

Withdrawal symptoms and treatment completion were compared following VLNTX (.125 or .25 mg/day) and clonidine (.1-.2 mg q6h) in 127 individuals with OD undergoing 6-day methadone inpatient taper at a community program.

RESULTS

VLNTX was more effective than placebo or clonidine in reducing symptoms and signs of withdrawal. The use of VLNTX in combination with clonidine was associated with attenuated subjective withdrawal compared with each medication alone, favoring detoxification completion in comparison with clonidine or naltrexone placebo. VLNTX/clonidine was effective in reducing symptoms that are both undertreated and well controlled with clonidine treatment and was not associated with significant adverse events compared with other treatments.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

Preliminary results elucidate neurobiological mechanisms of OD and support the utility of controlled studies on a novel VLNTX + low-dose clonidine combination for the management of opioid withdrawal.

Problem drinking and low-dose naltrexone-assisted opioid detoxification

OBJECTIVE

The influence of alcohol use on opioid dependence is a major problem that warrants a search for more effective treatment strategies. The addition of very-low-dose naltrexone (VLNTX) to methadone taper was recently associated with reduced withdrawal intensity during detoxification. In a secondary analysis of these data, we sought to determine whether problem drinking affects detoxification outcomes and whether symptoms are influenced by VLNTX use.

METHOD

Opioid-dependent patients (N = 174) received naltrexone (0.125 or 0.250 mg/day) or placebo in a double-blind, randomized design during methadone-based, 6-day inpatient detoxification. Alcohol consumption was assessed at admission using the Addiction Severity Index and selfreport. Outcome measures were opioid withdrawal intensity, craving, and retention in treatment.

RESULTS

Problem drinking-opioid dependent patients (n = 79) showed episodic heavy alcohol use and reported increased subjective opioid withdrawal intensity (p = .001), craving (p = .001), and significantly lower rate of retention in treatment (p = .02). Individuals with problem drinking and opioid dependence who were treated with VLNTX (n = 55) showed reduced withdrawal (p = .05) and a lower rate of treatment discontinuation (p = .03), resuming alcohol intake in smaller numbers the day following discharge (p = .03). Treatment effects were more pronounced on anxiety, perspiration, shakiness, nausea, stomach cramps, and craving. There were no group differences in use of adjuvant medications and no treatment-related adverse events.

CONCLUSIONS

Heavy drinking is associated with worse opioid detoxification outcomes. The addition of VLNTX is safe and is associated with reduced withdrawal symptoms and better completion rate in these patients. Further studies should explore the use of VLNTX in detoxification and long-term treatment of combined alcohol-opioid dependence and alcohol dependence alone.

Very low dose naltrexone addition in opioid detoxification: a randomized, controlled trial

Although current treatments for opioid detoxification are not always effective, medical detoxification remains a required step before long-term interventions. The use of opioid antagonist medications to improve detoxification has produced inconsistent results. Very low dose naltrexone (VLNTX) was recently found to reduce opioid tolerance and dependence in animal and clinical studies. We decided to evaluate safety and efficacy of VLNTX adjunct to methadone in reducing withdrawal during detoxification. In a multi-center, double-blind, randomized study at community treatment programs, where most detoxifications are performed, 174 opioid-dependent subjects received NTX 0.125 mg, 0.250 mg or placebo daily for 6 days, together with methadone in tapering doses. VLNTX-treated individuals reported attenuated withdrawal symptoms [F = 7.24 (2,170); P = 0.001] and reduced craving [F = 3.73 (2,107); P = 0.03]. Treatment effects were more pronounced at discharge and were not accompanied by a significantly higher retention rate. There were no group differences in use of adjuvant medications and no treatment-related adverse events. Further studies should explore the use of VLNTX, combined with full and partial opioid agonist medications, in detoxification and long-term treatment of opioid dependence.

Low dose naltrexone administration in morphine dependent rats attenuates withdrawal-induced norepinephrine efflux in forebrain

The administration of low dose opioid antagonists has been explored as a potential means of detoxification in opiate dependence. Previous results from our laboratory have shown that concurrent administration of low dose naltrexone in the drinking water of rats implanted with subcutaneous morphine pellets attenuates behavioral and biochemical signs of withdrawal in brainstem noradrenergic nuclei. Noradrenergic projections originating from the nucleus tractus solitarius (NTS) and the locus coeruleus (LC) have previously been shown to be important neural substrates involved in the somatic expression of opiate withdrawal. The hypothesis that low dose naltrexone treatment attenuates noradrenergic hyperactivity typically associated with opiate withdrawal was examined in the present study by assessing norepinephrine tissue content and norepinephrine efflux using in vivo microdialysis coupled to high performance liquid chromatography (HPLC) with electrochemical detection (ED). The frontal cortex (FC), amygdala, bed nucleus of the stria terminalis (BNST) and cerebellum were analyzed for tissue content of norepinephrine following withdrawal in morphine dependent rats. Naltrexone-precipitated withdrawal elicited a significant decrease in tissue content of norepinephrine in the BNST and amygdala. This decrease was significantly attenuated in the BNST of rats that received low dose naltrexone pre-treatment compared to controls. No significant difference was observed in the other brain regions examined. In a separate group of rats, norepinephrine efflux was assessed with in vivo microdialysis in the BNST or the FC of morphine dependent rats or placebo treated rats subjected to naltrexone-precipitated withdrawal that received either naltrexone in their drinking water (5 mg/L) or unadulterated water. Following baseline dialysate collection, withdrawal was precipitated by injection of naltrexone and sample collection continued for an additional 4 h. At the end of the experiment, animals were transcardially perfused and the brains were removed for verification of probe placement. Low dose naltrexone pre-treatment significantly attenuated withdrawal-induced increases of extracellular norepinephrine in the BNST, with a smaller effect in the FC. These findings suggest that alterations in norepinephrine release associated with withdrawal may be attenuated in forebrain targets of noradrenergic brainstem neurons that may underlie reduced behavioral signs of withdrawal following low dose naltrexone administration.

Effectiveness of low-dose naltrexone in the post-detoxification treatment of opioid dependence

BACKGROUND

The clinical use of naltrexone (NTX) in the treatment of opioid dependence has been limited because of poor compliance and inconsistent outcomes. In particular, the therapeutic benefit of extended treatment with NTX after opioid detoxification is unclear. The present study evaluated whether the augmentation with low-dose NTX during the post-detoxification treatment of opioid dependence would improve outcomes.

METHODS

In an open-label naturalistic design, 435 opioid-dependent patients who had completed inpatient detoxification were offered the choice of entering 1 of the 2 outpatient treatment arms: clonidine extended treatment (CET) (clonidine + psychosocial treatment), or enhanced extended treatment (EET) (oral NTX [1-10 mg/d] + CET) for 21 days. The primary outcome measure was retention in treatment. Secondary outcomes included abstinence from opioids, dropouts, and adherence to postdischarge care.

RESULTS

One hundred sixty-two patients (37.2%) accepted EET. Subjects receiving EET stayed longer in the program (F = 64.4; P = 0.000), were less likely to drop out, used less opioids, and followed through with referral to long-term outpatient treatment in a higher number, compared with patients in the CET arm (P = 0.000 in each case). The NTX + clonidine combination was safe and well tolerated.

CONCLUSIONS

This preliminary study indicates the potential benefit of augmentation with low-dose NTX to improve outcomes after opioid detoxification for a preferred group of patients. Randomized controlled trials are necessary to further evaluate the role of low-dose NTX in the outpatient treatment of opioid dependence.

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.

Elevated μ-opioid receptor expression in the nucleus of the solitary tract accompanies attenuated withdrawal signs after chronic low dose naltrexone in opiate-dependent rats

We previously described a decrease in withdrawal behaviors in opiate-dependent rats that were chronically treated with very low doses of naltrexone in their drinking water. Attenuated expression of withdrawal behaviors correlated with decreased c-Fos expression and intracellular signal transduction elements [protein kinase A regulatory subunit II (PKA) and phosphorylated cAMP response element binding protein (pCREB)] in brainstem noradrenergic nuclei. In this study, to determine whether similar cellular changes occurred in forebrain nuclei associated with drug reward, expressions of PKA and pCREB were analyzed in the ventral tegmental area, frontal cortex, striatum, and amygdala of opiate-treated rats that received low doses of naltrexone in their drinking water. No significant difference in PKA or pCREB was detected in these regions following drug treatment. To examine further the cellular mechanisms in noradrenergic nuclei that could underlie attenuated withdrawal behaviors following low dose naltrexone administration, the nucleus of the solitary tract (NTS) and locus coeruleus (LC) were examined for opioid receptor (OR) protein expression. Results showed a significant increase in muOR expression in the NTS of morphine-dependent rats that received low doses of naltrexone in their drinking water, and increases in muOR expression were also found to be dose dependent. Protein expression of muOR in the LC and deltaOR in either brain region remained unchanged. In conclusion, our previously reported decreases in c-Fos and PKA expression in the NTS following pretreatment with low doses of naltrexone may be partially explained by a greater inhibition of NTS neurons resulting from increased muOR expression in this region.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.