Naltrexone-induced opiate receptor supersensitivity

Alcohol use disorder in the perioperative period: a summary and recommendations for anesthesiologists and pain physicians

Excessive alcohol consumption and alcohol use disorder (AUD) increase the risk of perioperative morbidity and mortality. Aspiration, malnutrition, coagulopathies, seizures, and hemodynamic alterations are only a few of the major concerns related to acute alcohol intoxication and AUD. There are also numerous physiological effects, changes in medication metabolism and pharmacology, and adverse events related to chronic alcohol consumption. These are all important considerations for the anesthesiologist in the perioperative management of a patient with AUD. Pain perception and thresholds are altered in patients with acute and chronic alcohol use. Medications used to manage AUD symptoms, particularly naltrexone, can have significant perioperative implications. Patients on naltrexone who continue or stop this medication in the perioperative period are at an increased risk for undertreated pain or substance use relapse. This review highlights key considerations for the anesthesiologist and pain physician in the perioperative management of patients with active AUD (or those in recovery). It discusses the effects of acute and chronic alcohol use on pain perception and thresholds, provides guidance on the perioperative management of naltrexone and low-dose naltrexone, and reviews a multimodal approach to pain management.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Opioid Receptor-Mediated and Non-Opioid Receptor-Mediated Roles of Opioids in Tumour Growth and Metastasis

Opioids are administered to cancer patients in the period surrounding tumour excision, and in the management of cancer-associated pain. The effects of opioids on tumour growth and metastasis, and their consequences on disease outcome, continue to be the object of polarised, discrepant literature. It is becoming clear that opioids contribute a range of direct and indirect effects to the biology of solid tumours, to the anticancer immune response, inflammation, angiogenesis and importantly, to the tumour-promoting effects of pain. A common misconception in the literature is that the effect of opioid agonists equates the effect of the mu-opioid receptor, the major target of the analgesic effect of this class of drugs. We review the evidence on opioid receptor expression in cancer, opioid receptor polymorphisms and cancer outcome, the effect of opioid antagonists, especially the peripheral antagonist methylnaltrexone, and lastly, the evidence available of a role for opioids through non-opioid receptor mediated actions.

Low-Dose Naltrexone (LDN)-Review of Therapeutic Utilization

Naltrexone and naloxone are classical opioid antagonists. In substantially lower than standard doses, they exert different pharmacodynamics. Low-dose naltrexone (LDN), considered in a daily dose of 1 to 5 mg, has been shown to reduce glial inflammatory response by modulating Toll-like receptor 4 signaling in addition to systemically upregulating endogenous opioid signaling by transient opioid-receptor blockade. Clinical reports of LDN have demonstrated possible benefits in diseases such as fibromyalgia, Crohn’s disease, multiple sclerosis, complex-regional pain syndrome, Hailey-Hailey disease, and cancer. In a dosing range at less than 1 μg per day, oral naltrexone or intravenous naloxone potentiate opioid analgesia by acting on filamin A, a scaffolding protein involved in μ-opioid receptor signaling. This dose is termed ultra low-dose naltrexone/naloxone (ULDN). It has been of use in postoperative control of analgesia by reducing the need for the total amount of opioids following surgery, as well as ameliorating certain side-effects of opioid-related treatment. A dosing range between 1 μg and 1 mg comprises very low-dose naltrexone (VLDN), which has primarily been used as an experimental adjunct treatment for boosting tolerability of opioid-weaning methadone taper. In general, all of the low-dose features regarding naltrexone and naloxone have been only recently and still scarcely scientifically evaluated. This review aims to present an overview of the current knowledge on these topics and summarize the key findings published in peer-review sources. The existing potential of LDN, VLDN, and ULDN for various areas of biomedicine has still not been thoroughly and comprehensively addressed.

Naltrexone maintenance fails to alter amphetamine effects on intracranial self-stimulation in rats

Pharmacotherapy to treat stimulant use disorders continues to be an unmet medical need. Some evidence supports both the role of opioids in mediating abuse-related amphetamine effects and the potential utility of opioid antagonists as therapeutic candidates for treating amphetamine abuse. This study used intracranial self-stimulation (ICSS) to evaluate effects of exposure to and termination of naltrexone maintenance on rewarding amphetamine effects in an ICSS procedure in rats. Morphine and cocaine were included as positive and negative controls, respectively. Male Sprague-Dawley rats (N = 40) were trained to lever press for electrical brain stimulation to the medial forebrain bundle via an implanted electrode. Rats were then implanted with osmotic pumps delivering naltrexone (0.001 mg/kg/h, SC, 0.01 mg/kg/h, SC, or 0.1 mg/kg/h, SC) or saline for 14 days. Cumulative dose-effect curves were determined for amphetamine (0.032 mg/kg to 0.32 mg/kg), cocaine (1 mg/kg to 10 mg/kg), and morphine (1 mg/kg to 10 mg/kg) during the 2nd week of naltrexone maintenance. Additionally, dose-effect curves for morphine and amphetamine were determined again 24 hr after pump removal. Our results suggest that (a) exposure to and termination of naltrexone maintenance do not affect baseline ICSS responding, (b) naltrexone doses sufficient to antagonize morphine did not alter amphetamine or cocaine effects, and (c) termination of naltrexone treatment produced weak evidence for increased morphine sensitivity but no change in amphetamine effects. Our results do not support naltrexone as a pharmacotherapy for amphetamine and cocaine abuse and also suggest that termination from chronic naltrexone does not increase sensitivity to abuse-related morphine or amphetamine effects in ICSS. (PsycINFO Database Record

Continuous delivery of naltrexone and nalmefene leads to tolerance in reducing alcohol drinking and to supersensitivity of brain opioid receptors

Opioid antagonist treatments reduce alcohol drinking in rodent models and in alcohol-dependent patients, with variable efficacy across different studies. These treatments may suffer from the development of tolerance and opioid receptor supersensitivity, as suggested by preclinical models showing activation of these processes during and after subchronic high-dose administration of the short-acting opioid antagonist naloxone. In the present study, we compared equipotent low and moderate daily doses of naltrexone and nalmefene, two opioid antagonists in the clinical practice for treatment of alcoholism. The antagonists were given here subcutaneously for 7 days either as daily injections or continuous osmotic minipump-driven infusions to alcohol-preferring AA rats having trained to drink 10% alcohol in a limited access protocol. One day after stopping the antagonist treatment, [³⁵ S]GTPγS autoradiography on brain cryostat sections was carried out to examine the coupling of receptors to G protein activation. The results prove the efficacy of repeated injections over infused opioid antagonists in reducing alcohol drinking. Tolerance to the reducing effect on alcohol drinking and to the enhancement of G protein coupling to μ-opioid receptors in various brain regions were consistently detected only after infused antagonists. Supersensitivity of κ-opioid receptors was seen in the ventral and dorsal striatal regions especially by infused nalmefene. Nalmefene showed no clear agonistic activity in rat brain sections or at human recombinant κ-opioid receptors. The findings support the as-needed dosing practice, rather than the standard continual dosing, in the treatment of alcoholism with opioid receptor antagonists.

A Combination of Naltrexone + Varenicline Retards the Expression of a Genetic Predisposition Toward High Alcohol Drinking

BACKGROUND

This study examined whether naltrexone (NTX) or varenicline (VAR), alone or in combination, can retard the phenotypic expression of a genetic predisposition toward high alcohol drinking in rats selectively bred for high alcohol intake when drug treatment is initiated prior to, or concomitantly with, the onset of alcohol drinking.

METHODS

Alcohol-naïve P rats were treated daily with NTX (15.0 mg/kg BW), VAR (1.0 mg/kg BW), a combination of NTX (15.0 mg/kg BW) + VAR (1.0 mg/kg BW), or vehicle (VEH) for 2 weeks prior to, or concomitantly with, their first opportunity to drink alcohol and throughout 21 days of daily 2-hour alcohol access. Drug treatment was then discontinued for 3 weeks followed by reinstatement of drug treatment for an additional 3 weeks.

RESULTS

When P rats were pretreated with drug for 2 weeks prior to onset of alcohol access, only NTX + VAR in combination blocked the acquisition of alcohol drinking in alcohol-naïve P rats. When drug treatment was initiated concomitantly with the first opportunity to drink alcohol, NTX alone, VAR alone, and NTX + VAR blocked the acquisition of alcohol drinking. Following termination of drug treatment, NTX + VAR and VAR alone continued to reduce alcohol drinking but by the end of 3 weeks without drug treatment, alcohol intake in all groups was comparable to that seen in the vehicle-treated group as the expression of a genetic predisposition toward high alcohol drinking emerged in the drug-free P rats. After 3 weeks without drug treatment, reinstatement of NTX + VAR treatment again reduced alcohol intake.

CONCLUSIONS

A combination of NTX + VAR, when administered prior to, or concomitantly with, the first opportunity to drink alcohol, blocks the acquisition of alcohol drinking during both initial access to alcohol and during a later period of alcohol access in P rats with a genetic predisposition toward high alcohol intake. The results suggest that NTX + VAR may be effective in curtailing alcohol drinking in individuals at high genetic risk of developing alcoholism.

The World Federation of Societies of Biological Psychiatry (WFSBP) guidelines for the biological treatment of substance use and related disorders. Part 2: Opioid dependence

OBJECTIVES

To develop evidence-based practice guidelines for the pharmacological treatment of opioid abuse and dependence.

METHODS

An international task force of the World Federation of Societies of Biological Psychiatry (WFSBP) developed these practice guidelines after a systematic review of the available evidence pertaining to the treatment of opioid dependence. On the basis of the evidence, the Task Force reached a consensus on practice recommendations, which are intended to be clinically and scientifically meaningful for physicians who treat adults with opioid dependence. The data used to develop these guidelines were extracted primarily from national treatment guidelines for opioid use disorders, as well as from meta-analyses, reviews, and publications of randomized clinical trials on the efficacy of pharmacological and other biological treatments for these disorders. Publications were identified by searching the MEDLINE database and the Cochrane Library. The literature was evaluated with respect to the strength of evidence for efficacy, which was categorized into one of six levels (A-F).

RESULTS

There is an excellent evidence base supporting the efficacy of methadone and buprenorphine or the combination of buprenorphine and naloxone for the treatment of opioid withdrawal, with clonidine and lofexidine as secondary or adjunctive medications. Opioid maintenance with methadone and buprenorphine is the best-studied and most effective treatment for opioid dependence, with heroin and naltrexone as second-line medications.

CONCLUSIONS

There is enough high quality data to formulate evidence-based guidelines for the treatment of opioid abuse and dependence. This task force report provides evidence for the efficacy of a number of medications to treat opioid abuse and dependence, particularly the opioid agonists methadone or buprenorphine. These medications have great relevance for clinical practice.

Irving Page Lecture: 5-HT(2A) serotonin receptor biology: interacting proteins, kinases and paradoxical regulation

5-Hydroxytryptamine(2A) (5-HT(2A)) serotonin receptors are important pharmacological targets for a large number of central nervous system and peripheral serotonergic medications. In this review article I summarize work mainly from my lab regarding serotonin receptor anatomy, pharmacology, signaling and regulation. I highlight the role of serotonin receptor interacting proteins and the emerging paradigm of G-protein coupled receptor functional selectivity.

Endogenous Opioids Regulate Expression of Experimental Autoimmune Encephalomyelitis: A New Paradigm for the Treatment of Multiple Sclerosis

Preclinical investigations utilizing murine experimental auto-immune encephalomyelitis (EAE), as well as clinical observations in patients with multiple sclerosis (MS), may suggest alteration of endogenous opioid systems in MS. In this study we used the opioid antagonist naltrexone (NTX) to invoke a continuous (High Dose NTX, HDN) or intermittent (Low Dose NTX, LDN) opioid receptor blockade in order to elucidate the role of native opioid peptides in EAE. A mouse model of myelin oligodendrocyte glycoprotein (MOG)-induced EAE was employed in conjunction with daily treatment of LDN (0.1 mg/kg, NTX), HDN (10 mg/kg NTX), or vehicle (saline). No differences in neurological status (incidence, severity, disease index), or neuropathological assessment (activated astrocytes, demyelination, neuronal injury), were noted between MOG-induced mice receiving HDN or vehicle. Over 33% of the MOG-treated animals receiving LDN did not exhibit behavioral signs of disease, and the severity and disease index of the LDN-treated mice were markedly reduced from cohorts injected with vehicle. Although all LDN animals demonstrated neuropathological signs of EAE, LDN-treated mice without behavioral signs of disease had markedly lower levels of activated astrocytes and demyelination than LDN- or vehicle-treated animals with disease. These results imply that endogenous opioids, evoked by treatment with LDN and acting in the rebound period from drug exposure, are inhibitory to the onset and progression of EAE, and suggest that clinical studies of LDN are merited in MS and possibly in other autoimmune disorders.

Regulation of dynamin 2 and G protein-coupled receptor kinase 2 in rat nucleus accumbens during acute and repeated cocaine administration

Exposure to cocaine causes many neuroadaptations including alterations in several neurotransmitter receptors and transporters. This study investigated potential mechanisms of cocaine-induced receptor and transporter regulation by measuring levels of two proteins involved in receptor and transporter trafficking, dynamin 2 and G protein-coupled receptor kinase 2 (GRK2). Male Fischer rats received three daily injections of cocaine, 15 mg/kg, in a binge-pattern (at 1 h intervals) for 1, 3, or 14 days. Brain regions of interest were collected 30 min after the last injection and proteins measured by Western blot. Acute binge-pattern cocaine administration produced a significant increase in both dynamin 2- and GRK2-immunoreactivity (227% and 358% of control) in the nucleus accumbens and GKR2 (150% of control) in the caudate putamen. Tolerance to this effect occurred, as levels of both proteins returned to baseline after 3 days of cocaine. In contrast, dynamin 2 and GRK2 were significantly decreased in the nucleus accumbens after chronic cocaine. This pattern of regulation was unique to the nucleus accumbens and not seen in the frontal cortex or substantia nigra. Pretreatment with either the dopamine (DA) D1 receptor antagonist SCH 23390 or D2 receptor antagonist eticlopride prior to acute cocaine blocked the upregulation of dynamin 2 and GRK2 in the nucleus accumbens. However, only eticlopride was effective in attenuating the decrease in these proteins following chronic cocaine exposure. These results demonstrate that two proteins involved in receptor and transporter trafficking are selectively regulated in the nucleus accumbens following acute versus chronic cocaine exposure, and dopamine receptor activation is required for this regulation.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Food intake and reward mechanisms in patients with schizophrenia: implications for metabolic disturbances and treatment with second-generation antipsychotic agents

Obesity is highly prevalent among patients with schizophrenia and is associated with detrimental health consequences. Although excessive consumption of fast food and pharmacotherapy with such second-generation antipsychotic agents (SGAs) as clozapine and olanzapine has been implicated in the schizophrenia/obesity comorbidity, the pathophysiology of this link remains unclear. Here, we propose a mechanism based on brain reward function, a relevant etiologic factor in both schizophrenia and overeating. A comprehensive literature search on neurobiology of schizophrenia and of eating behavior was performed. The collected articles were critically reviewed and relevant data were extracted and summarized within four key areas: (1) energy homeostasis, (2) food reward and hedonics, (3) reward function in schizophrenia, and (4) metabolic effects of the SGAs. A mesolimbic hyperdopaminergic state may render motivational/incentive reward system insensitive to low salience/palatability food. This, together with poor cognitive control from hypofunctional prefrontal cortex and enhanced hedonic impact of food, owing to exaggerated opioidergic drive (clinically manifested as pain insensitivity), may underlie unhealthy eating habits in patients with schizophrenia. Treatment with SGAs purportedly improves dopamine-mediated reward aspects, but at the cost of increased appetite and worsened or at least not improved opiodergic capacity. These effects can further deteriorate eating patterns. Pathophysiological and therapeutic implications of these insights need further validation via prospective clinical trials and neuroimaging studies.

Sustained-release naltrexone formulations for the treatment of alcohol and opioid dependence

Naltrexone, an opioid receptor antagonist, is effective in the treatment of alcohol and opioid dependence. An important limitation of the use of oral naltrexone is the lack of efficacy among patients who do not consistently take the medication. The advent of a sustained-release naltrexone addresses the problem of noncompliance. Naltrexone can be compounded into microcapsules that can be injected or implanted, producing a clinical response lasting from 30 days to several months. Several randomized, placebo-controlled clinical trials demonstrate that sustained-release naltrexone is safe and effective for the treatment of both alcohol and opioid dependence. Sustained-release formulations of naltrexone may offer several advantages over its oral formulation with respect to medication compliance, pharmacokinetics, side effect profile and ease of use. Recently, one formulation of sustained-release naltrexone received approval from the US FDA and offers a new option for addiction treatment.

A synopsis of the pharmacological rationale, properties and therapeutic effects of depot preparations of naltrexone for treating alcohol dependence

Although oral naltrexone has been shown to diminish alcohol reinforcement, its limitations as a medication include its small treatment effect size, plasma level fluctuation and adverse events. The pharmacokinetic profile of naltrexone could be optimised by intramuscular administration, sustaining its release over several weeks. As a result, plasma levels would remain relatively constant; high enough to reduce drinking, low enough to minimise side effects. Two injectable naltrexone depot preparations, Vivitrex and Naltrel, have been tested as pharmacotherapy for alcohol dependence. Their adverse-event profiles seem to be mild compared with oral naltrexone. Vivitrex has shown efficacy at reducing heavy drinking significantly among alcohol-dependent men. Naltrel helped reduce relapse and promote abstinence in two samples of alcohol-dependent individuals. Additional efficacy studies are warranted.

Vivitrex, an injectable, extended-release formulation of naltrexone, provides pharmacokinetic and pharmacodynamic evidence of efficacy for 1 month in rats

While oral naltrexone is effective in treating alcohol and opiate dependencies, poor patient adherence and widely fluctuating plasma levels limit its efficacy. To overcome these problems, an extended-release formulation of naltrexone (Vivitrex) was developed by encapsulating naltrexone into injectable, biodegradable polymer microspheres. Pharmacokinetic studies in rats demonstrated that this formulation produced stable, pharmacologically relevant plasma levels of naltrexone for approximately 1 month following either subcutaneous or intramuscular injections. While rats receiving placebo microspheres demonstrated a pronounced analgesic response to morphine in the hot-plate test, morphine analgesia was completely blocked in rats treated with extended-release naltrexone. This antagonism began on day 1 following administration and lasted for 28 days. Rats reinjected with extended-release naltrexone 34 days after the initial dose and tested for another 35 days showed consistent suppression of morphine analgesia for an additional 28 days. mu-Opioid receptor density, as measured by [(3)H]DAMGO autoradiography, increased up to two-fold following a single injection of extended-release naltrexone. Saturation binding assays using [(3)H]DAMGO showed changes in the midbrain and striatum at 1 week after extended-release naltrexone administration, and after 1 month in the neocortex. These receptor increases persisted for 2-4 weeks after dissipation of the morphine antagonist actions of naltrexone. These data suggest that therapeutically relevant plasma levels of naltrexone can be maintained using monthly injections of an extended-release microsphere formulation, and that changes in mu-opioid receptor density do not impact its efficacy in suppressing morphine-induced analgesia in the rat. Clinical trials of extended release naltrexone for treating alcohol and opiate dependency are currently ongoing.

Cortical and striatal μ-opioid receptors are altered by gonadal hormone treatment but not by prenatal morphine exposure in adult male and female rats

The cerebral cortex (CX), cingulate CX (cgCX), and striatum (STR) play an important role in locomotion, cognition, emotion, and reward-motivated behaviors, and are altered by prenatal morphine exposure. We have demonstrated that delta-opioid receptors in the CX and STR of adult male and female rats are altered by prenatal morphine exposure and gonadal hormonal treatment. Because morphine binds with greater affinity to mu- than delta-opioid receptors, the present study examined the effect of prenatal morphine exposure on mu-opioid receptor density in the CX, cgCX, and STR of adult male and female rats using receptor autoradiography. In Experiment 1, three groups of adult male rats were analyzed: intact, gonadally intact; GNX, gonadectomized; and TP, GNX and testosterone propionate (TP)-treated. In Experiment 2, four groups of adult females were analyzed: OVX, ovariectomized; EB, OVX and estradiol benzoate (EB)-treated; P, OVX and progesterone (P)-treated; and EB+P, OVX and EB- and P-treated. In male rats, GNX and TP males had lower mu-opioid receptor densities in all three brain regions than gonadally intact males regardless of prenatal drug exposure. In female rats, OVX, EB+P-treated females had lower mu-opioid receptor density in the STR than OVX only females regardless of prenatal drug exposure. There were no drug or gonadal hormone effects in the CX or in the cgCX of female rats. Thus, the present study demonstrates that gonadal hormones, and not prenatal morphine exposure, alter the density of mu-opioid receptors in the CX, cgCX, and STR of adult male and female rats.

Quantitative autoradiography of adenosine receptors in brains of chronic naltrexone-treated mice

1. Manipulation of micro opioid receptor expression either by chronic morphine treatment or by deletion of the gene encoding micro opioid receptors leads to changes in adenosine receptor expression. Chronic administration of the opioid receptor antagonist naltrexone leads to upregulation of micro receptor binding in the brain. 2. To investigate if there are any compensatory alterations in adenosine systems in the brains of chronic naltrexone-treated mice, we carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors in the brains of mice treated for 1 week with naltrexone (8 mg(-1) kg(-1) day(-1)), administered subcutaneously via osmotic minipump. 3. Adjacent coronal brain sections were cut from chronic saline- and naltrexone-treated mice for the determination of binding of [(3)H] D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H] DAMGO), [(3)H]1,3-dipropyl-8-cyclopentylxanthine ([(3)H] DPCPX) or [(3)H] 2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine ([(3)H] CGS21680) to micro, A(1) and A(2A) receptors, respectively. 4. A significant increase in micro and A(1) receptor binding was detected in chronic naltrexone-treated brains. The changes in micro receptors were significant in several regions, but changes in A(1) were relatively smaller but showed significant upregulation collectively. No significant change in A(2A) receptor binding was detected in chronic naltrexone-treated brains. 5. The results show that blockade of opioid receptors causes upregulation of A(1) receptors, but not A(2A) receptors, by as yet undefined mechanisms.

Area-specific increased density of mu-opioid receptor immunoreactive neurons in the cerebral cortex of drug-related fatalities

In animal experiments and in cell culture, chronic morphine treatment has been followed by "up-regulation" as well as "down-regulation" of the mu-opioid receptor (OR) number. The present postmortem morphometric study of morphine-related fatalities of drug-addicts (n=13, 20-35 years old, with blood unconjugated morphine levels from 27.1 ng/ml to 458 ng/ml, m.v. 198.5 ng/ml) versus a non-addicted control group (n=13, 10-44 years old) was intended to examine, whether chronic opiate exposure affects the numerical density of mu-OR expressing neurons in the human neocortex (areas 11, 24 and 25 according to Brodmann). For the immunohistochemical procedure, vibratome sections (100 microm) were incubated with a monoclonal antibody against the mu-OR, diluted 1:100, and immunolabelled sites were visualized using an immunoperoxidase protocol. The numerical densities of OR immunoreactive neuronal profiles and Nissl-stained central profiles were assessed morphometrically (camera lucida-drawings). In both groups, the anti-mu-OR-immunoreactivity was mainly localized in pyramidal neurons of layers (L) II/III and V and in multiform neurons of L VI. In the areas 24 and 25, the density of the immunoreactive neuronal profiles did not display a significant difference between the two examined groups. In the area 11, however, the number of immunolabelled neuronal profiles amounted to 2777+/-206 mm(3) in the drug-related fatalities and to 2320+/-124 mm(3) in the control group and thus was significantly increased.

Receptor-selective changes in mu-, delta- and kappa-opioid receptors after chronic naltrexone treatment in mice

Chronic treatment with the opioid antagonist naltrexone induces functional supersensitivity to opioid agonists, which may be explained by receptor up-regulation induced by opioid receptor blockade. In the present study, the levels of opioid receptor subtypes through the brain of mice were determined after chronic naltrexone treatment using quantitative in vitro autoradiography. This is the first complete mapping study in mice for micro-, delta- and kappa-opioid receptors after chronic naltrexone exposure. Treatment with naltrexone clearly induced up-regulation of micro- (mean 80%) and, to a lesser extent, delta-opioid receptors (mean 39%). The up-regulation of micro- and delta-opioid receptors was evident throughout the brain, although there was variation in the percentage change across brain regions. In contrast, consistent up-regulation of kappa-opioid receptors was observed in cortical structures only and was not so marked as for micro- and delta-opioid receptors. In noncortical regions kappa-opioid receptor expression was unchanged. Taken together, the present findings suggest opioid receptor subtype-selective regulation by chronic naltrexone treatment in mice.

Preclinical and clinical studies on naltrexone: what have they taught each other?

Proceedings of a symposium at the 2002 RSA/ISBRA Meeting in San Francisco, California; organized and co-chaired by Janice C. Froehlich and Stephanie O'Malley. The presentations were (1) Introduction, by Janice C. Froehlich and Stephanie O'Malley; (2) Preclinical studies on naloxone: genetics and site of action, by Petri Hyytiä; (3) Clinical studies on naltrexone for treating hazardous drinkers, by Dena Davidson; (4) Clinical studies on naltrexone and sertraline in the treatment of alcohol dependence, by Conor Farren; and (5) Discussion by Janice D. Froehlich, Stephanie O'Malley, and Rainer Spanagel. Both preclinical and clinical studies are critical in the development of effective pharmacotherapeutic approaches for the treatment of alcoholism. Nowhere has this been more evident than in the development of naltrexone for the treatment of alcohol relapse. As research continues on the optimal use of naltrexone for modifying alcohol intake, a number of factors have emerged that are likely to determine the efficacy of naltrexone as a pharmacotherapeutic agent for the treatment of alcoholism. Some of these factors include dose, frequency and duration of treatment, pattern and severity of alcohol drinking prior to initiation of naltrexone treatment, genetic aspects of responsive subpopulations, degree of alcohol craving, and susceptibility to adverse effects of naltrexone. New, as well as established, animal models are being used to determine the parameters that optimize the ability of naltrexone to modify alcohol drinking in acute and chronic alcohol access paradigms, under conditions of intermittent versus continuous alcohol intake, and in populations that vary in genetic predisposition toward alcohol drinking. Current clinical studies are exploring the ability of naltrexone to alter alcohol drinking when delivered in combination with pharmacotherapeutic agents that act on nonopioid transmitter systems and the difference in efficacy of naltrexone when administered in populations that differ in drinking frequency and intensity, family history of alcoholism, and alcohol craving. This symposium presented new research findings from both preclinical and clinical studies with the aim of facilitating the development of treatment regimens that optimize the therapeutic potential of naltrexone in the treatment of alcoholism.

A possible molecular mechanism for the interaction of defensin with the sensory neuron membrane

A local membrane potential clamping method was used to study the effects of defensin NP-1 on the membranes of rat spinal ganglion neurons. NP-1 led to decreases in the effective charge for the activation gating system. This process depended on the NP-1 concentration. Use of the Hill equation showed that Kd was 2.10(-12) M and the Hill coefficient was 0.9. The structure of the defensin molecule was optimized using quantum chemical calculations based on a molecular mechanics method. The results obtained from these calculations suggested that a single hydroxyl group directed towards the outer part of thedefensin molecule and forming the carboxyl group of amino acid Glu14 could form a hydrogen bond with the active center of the membrane receptor. This explains the experimentally observed 1:1 stoichiometry of the ligand-receptor binding interaction between the defensin and the sensory neuron membrane.

Regulation of dihydropyridine-sensitive Ca2+ channels during naloxone-induced opioid supersensitivity in rats

The Ca(2+) channel blocker, nimodipine, increases the chronic naltrexone-induced supersensitivity to morphine analgesia in mice without affecting the density of up-regulated mu-opioid receptors. In the present study, the change in dihydropyridine-sensitive Ca(2+) channels associated with chronic naloxone-induced supersensitivity to morphine analgesia was studied in rat whole-brain membranes using a radiolabeled Ca(2+) channel blocker, [3H]PN200-110 [isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-methoxycarbonylpyridine-3-carboxylate] (0.02-1.0 nmol/l). Rats were chronically treated with naloxone (1 mg/kg, i.p.), nimodipine (1 mg/kg, i.p.) or their respective vehicles twice daily for 10 days. On the 11th day, 16 h after the last injection of either nimodipine or naloxone, morphine (2 mg/kg, i.p.)-induced tail-flick analgesia was determined or rats were killed for the binding study. Chronic naloxone significantly potentiated (+84%) the morphine-induced analgesia. Chronic nimodipine also potentiated (+76%) morphine analgesia. In rats treated with nimodipine and naloxone, there was an additive increase (206%) in morphine analgesia. In binding studies, chronic naloxone resulted in a significant decrease (-39%) in the density (B(max)) of [3H]PN200-110 binding with no change in K(d) value when compared to the effect of chronic vehicle. Chronic nimodipine resulted in a slight but significant (+14.5%) increase in the B(max) of [3H]PN200-110. However, when nimodipine was co-administered with naloxone, it not only reversed the down-regulation but actually up-regulated (+25%) [3H]PN200-110 binding with no change in K(d) value. Our results show significant down-regulation of [3H]PN200-110 binding in association with naloxone-induced analgesic supersensitivity to morphine. The supersensitivity was also observed following chronic blockade of up-regulated Ca(2+) channels by nimodipine. These results indicate an important role of L-type Ca(2+) channel regulation in naloxone-induced analgesic supersensitivity to morphine.

Antagonist-induced micro-opioid receptor up-regulation decreases G-protein receptor kinase-2 and dynamin-2 abundance in mouse spinal cord

Chronic treatment with opioid receptor antagonists has been shown to increase the density of micro-, delta- and kappa-opioid receptors in cell culture and in the intact animal. Although opioid receptor antagonist-induced up-regulation is a robust phenomenon, the mechanisms responsible for the increase in receptor density remain unclear. In the present study, changes in a kinase and a GTPase that have been implicated in G-protein-coupled receptor regulation were examined following opioid receptor antagonist treatment. Mice were implanted s.c. with a naltrexone pellet or placebo pellet. On the eighth day following implantation, spinal cord was removed and G-protein receptor kinase-2 (GRK-2) and dynamin-2 abundance were determined using a quantitative immunoblot approach. Changes in micro-opioid receptor density were also determined. Naltrexone treatment produced a significant (145%) increase in micro-opioid receptor density. Naltrexone treatment was associated with a significant 36% decrease in GRK-2 and 30% decrease in dynamin-2 abundance in spinal cord. These data raise the possibility that opioid receptor antagonist-induced micro-opioid receptor up-regulation in the intact animal may be due to a reduction in constitutive internalization of opioid receptors.

Numerical density of mu opioid receptor expressing neurons in the frontal cortex of drug related fatalities

In animal and cell culture experiments, chronic morphine treatment has been followed by 'up'- as well as 'down-regulation' of the mu opioid receptor (mu OR) number. The present postmortem morphometric study of morphine-related fatalities of drug addicts (n=12, and 22-35 years old, with blood unconjugated morphine levels from 27.1 to 458 ng/ml, m.v. 198.5 ng/ml) versus a non-addicted control group (n=13 and 10-44 years old) was intended to examine whether chronic opiate exposure affects the numerical density of mu OR expressing neurons in the human neocortex (area 10 according to Brodmann). For the immunohistochemical procedure, thick (100 microm) vibratome sections were incubated with a monoclonal antibody against the mu OR [Arvidsson et al., J. Neurosci. 15 (1995) 3328] and immunoreactive sites were visualized using an immunoperoxidase protocol. The numerical densities of mu OR-expressing and Nissl-stained neurons were assessed morphometrically (camera lucida-drawings). In both collectives, the anti-mu OR immunoreactivity was mainly found in pyramidal neurons of layers (L) II/III and V and in multiform neurons of L VI. In the drug-related fatalities and the control group, the density of neurons expressing mu OR protein was similar, amounting for 2698 +/- 153 and 2688 +/- 172/mm(3), respectively. These findings extend the binding studies of opioid ligands in postmortem brains of heroin addicts [Gabilondo et al., Psychopharmacology 115 (1994) 135] revealing similar receptor densities and affinities by showing no difference in the density of mu OR-positive neurons.

Numerical density of delta-opioid receptor expressing neurons in the frontal cortex of drug-related fatalities

In animal experiment and in cell culture, chronic morphine treatment has been followed by a reduction as well as an increase of the delta-opioid receptor (OR) number. The present postmortem morphometric study of morphine-related fatalities of drug addicts (n=12, 22-35 years old, with blood unconjugated morphine levels from 27.1 to 407 ng/ml, m.v. 176.9 ng/ml) versus a non-addicted control group (n=13, 10-44 years old) is intended to examine whether chronic opiate exposure also affects the numerical density of deltaOR expressing neurons in the human neocortex (area 10 according to Brodmann (Vergleichende Lokalisationslehre der Grosshirnrinde (1909) Johann Ambrosius Barth, Leipzig)). For the immunohistochemical procedure, vibratome sections (100 microm) were incubated with a monoclonal antibody against the deltaOR diluted 1:100, and immunoreactive sites were visualized using an immunoperoxidase protocol. The numerical densities of OR expressing and Nissl-stained neurons were assessed morphometrically (camera lucida drawings). In both collectives, the anti deltaOR immunoreactivity was predominantly localized in pyramidal neurons of layers (L) II/III and V as well as in round and ovoid neurons of L VI. In the drug-related fatalities, the density of neurons expressing deltaOR protein amounted for 2515+/-240/mm(3), in the control group for 2616+/-204/mm(3), thus displaying no statistically significant difference. These findings go along with the binding behavior of opioid ligands in postmortem brains of heroin addicts revealing similar receptor densities and affinities in the control subjects and addicts.

Morphine decreases the voltage sensitivity of slow sodium channels

Cell membrane recordings were made in conditions of voltage clamping with tight attachment of the microelectrode-patch clamping--to study the effects of morphine on tetrodotoxin-resistant (TTXr) sodium channels in rat spinal ganglion neurons in culture. The effects of a number of biologically active substances which regulate the receptor-mediated actions of morphine were studied. The effects of morphine were found to involve a chain of sequential reactions leading to decreases in the transfer of effective charge (Zeff) by the activatory gate system of TTXr sodium channels, depending on the concentration of agonist in the extracellular solution. A value of 8 nM was obtained for KD. with a Hill coefficient of X = 0.5. Non-specific antagonists of opioid receptors blocked the actions of morphine; these included ouabain at a concentration of 100 microM. An inhibitor, and activator, and a blocker of G-proteins had no effect on the effective charge. These data provide evidence that morphine decreases the voltage sensitivity of TTXr sodium channels.

Reduction of alcohol drinking and upregulation of opioid receptors by oral naltrexone in AA rats

Rats of the high-drinking AA line were given 1 mg/kg naltrexone (NTX) or vehicle orally with a stress-free procedure just before 1 h of access to 10% ethanol daily for 8 days and again, 8 h later on the first 7 days. Forebrain homogenate binding studies using 0.03-6.00 nM [3H] naloxone were conducted from 1 to 4 days following treatment. NTX significantly suppressed alcohol intake, with the effect becoming progressively greater over days and continuing during the post-treatment period. Saturation binding studies in brain homogenate revealed that NTX had increased the B(max) for opioid receptors by 93%, 74%, 49%, and 28%, respectively, from post-treatment days 1 to 4 without altering K(d). B(max) was negatively correlated (r=-0.510, p=0.008) with alcohol intake during the preceding hour, but in control rats, it was positively correlated with changes in alcohol intake over time (r=+0.790, p=0.020). These results are consistent with the hypothesis that opioid receptors mediate reinforcement from alcohol and that NTX reduces subsequent alcohol drinking by extinction. Opioid receptor upregulation can develop simultaneously with suppression of drinking and may partially counteract the clinical benefits from NTX in the treatment of alcoholism.

The effect of nimodipine on opioid antagonist-induced upregulation and supersensitivity

Regulation of calcium flux has been suggested to play a role in acute and chronic effects of opioids. Previous studies have shown calcium channel blockers can inhibit opioid agonist-induced downregulation of mu-opioid receptors and may reduce the magnitude of tolerance. In the present study, we determined if calcium channel blockade would affect increases in opioid receptor density and functional supersensitivity produced by chronic opioid antagonist treatment in the mouse. Mice were implanted subcutaneously with a 15-mg naltrexone (NTX) or placebo pellet. Mice also were implanted with an osmotic minipump that infused nimodipine (100 microg/kg/day) or a second placebo pellet. This protocol yielded four groups: nimodipine-NTX; nimodipine-placebo; placebo-NTX; placebo-placebo. On the seventh day, pumps and pellets were removed. Twenty-four hours later, a morphine dose-response study was conducted (tail flick); or mice were sacrificed and saturation binding studies ([3H]DAMGO) were performed in whole brain. NTX treatment significantly increased the analgesic potency of morphine by approximately 60%. Nimodipine increased the potency of morphine by approximately 50%. For mice treated with both nimodipine and NTX, there was an additive effect on morphine potency ( approximately 120% increase). In binding studies, NTX increased the density of mu-opioid receptors similarly ( approximately 60-70%) in the presence and absence of nimodipine treatment, with no change in affinity. No effect of chronic nimodipine alone on mu-opioid receptor binding was observed. These data indicate that NTX-induced upregulation and supersensitivity are independent of calcium channel blockade by nimodipine. These results contrast with those from tolerance and downregulation studies, and confirm suggestions that different substrates mediate chronic opioid agonist and antagonist-induced effects in vivo. Finally, in a separate study, morphine potency was unaffected by acute nimodiopine (100 microg/kg; SC), suggesting that prolonged exposure to this calcium channel blocker is required to increase morphine potency.

Association analysis of polymorphisms in the μ opioid gene and heroin abuse in Chinese subjects

We examined four polymorphisms in the μ opioid receptor gene in 282 Chinese heroin addicts from Sichuan Province, Southwest China and compared the allele and genotype frequencies to those in 258 normal controls from the same geographic region. Two of these polymorphisms (Ala6Val and Ser147Cys) were not polymorphic in the Chinese, with only Ala6 and Ser147 observed. The frequencies of the two other polymorphisms were significantly different from those observed in Caucasians, African Americans and Native Americans. The Asn40Asp and IVS2 + 691G/C polymorphisms did not differ significantly for allele (p= 0.16; p = 0.21), genotype (p= 0.32; p = 0.09) or haplotype frequencies (p= 0.24) between the Chinese heroin-addicted cases and normal controls. Similarly, we did not detect any association when the population was stratified by gender, route of administration (nasal inhalation and/or injection) and age-at-onset (above or below 25 years). This indicates that the μ opioid receptor is not likely to be a major genetic risk factor for heroin abuse in this population.

Regulation of opioid receptors by opioid antagonists: implications for rapid opioid detoxification

Opioid receptor antagonists have long been used in the diagnosis of opioid dependence and in the treatment of both opioid overdose and addiction. More recently they have been used in rapid opioid detoxification, a technique which has generated much ethical and scientific controversy. Because of this, the present review aims to integrate and summarize the current state of knowledge on adaptational changes to opioid systems as a result of antagonist administration. It is generally accepted that chronic treatment with an opioid antagonist results in opioid receptor upregulation. However, the mechanism(s) underlying this resultant opioid supersensitivity remain unresolved. In addition, there is not yet consensus regarding whether changes in opioid receptor number are directly responsible for the functional changes observed after chronic opioid antagonist treatment. Moreover, changes in opioid receptor number and sensitivity to opioid agonists and antagonists after chronic opioid antagonist treatment are dependent on dosing regimes as well as the kinetic properties of the antagonist itself. The role of these variables is appraised critically given the implication that an opioid antagonist can enhance functional responses. For example, such responses are an important consideration in the use of opioids because of possible adverse outcomes, such as overdose, after cessation of administration. Based on the literature discussed in this review it is concluded that caution is essential in the use of opioid antagonists for rapid opioid detoxification.

Modulation of enteric cholinergic neurons by hetero- and autoreceptors: cooperation among inhibitory inputs

In the guinea-pig colon, acetylcholine (ACh) release from intrinsic cholinergic motor neurons is inhibited by adrenoceptors, opioid and muscarinic receptors. Chronic sympathetic denervation resulted in supersensitivity to the inhibitory effect of DAMGO (mu-opioid agonist) on ACh release and on the peristaltic reflex. After chronic treatment with naltrexone (NTX) supersensitivity to DAMGO and subsensitivity to UK14,304 (alpha2-adrenoceptor agonist) developed for both functional parameters. The facilitatory effect of scopolamine on ACh release remained unchanged after chronic NTX treatment, whereas it was potentiated after chronic sympathetic denervation. These data suggest the existence of a functional interaction between different inhibitory pathways modulating cholinergic motor neurons in the guinea-pig colon. Namely, chronic manipulation of an inhibitory pathway may entail adaptive sensitivity changes in another inhibitory pathway so that homeostasis can be maintained.

Opioid receptor upregulation in mu-opioid receptor deficient CXBK and outbred Swiss Webster mice

Chronic in vivo treatment with opioid antagonists increases opioid receptor density and the potency of opioid agonists without altering receptor mRNA levels. To determine if basal receptor density affects opioid receptor upregulation, we examined the effect of chronic naltrexone treatment on mu-opioid receptor density and mRNA in two mice strains that differ in mu-opioid receptor density. CXBK mice (mu-opioid receptor deficient) and outbred Swiss Webster mice were implanted s.c. with a placebo or 15 mg naltrexone pellet for 8 days, the pellets removed and 24 hr later opioid receptor density (mu, delta) and receptor mRNA level (mu) determined in whole brain; or morphine dose-response studies conducted. In placebo-treated CXBK mice, mu-opioid receptor density was approximately 40% less than in Swiss Webster mice, although mu-opioid receptor mRNA abundance was similar in both strains. In placebo-treated CXBK mice, morphine potency was approximately 6-fold less than Swiss Webster mice. Naltrexone treatment increased morphine potency (1.7-fold) and mu- (approximately 90%) and delta- (approximately 20-40%) opioid receptor density in CXBK and Swiss Webster mouse brain similarly. Mu-opioid receptor mRNA was unchanged by naltrexone treatment in either strain. There was no difference in the basal or naltrexone-treated whole brain G(i alpha2) protein levels in CXBK or Swiss Webster mouse. These data indicate that a deficiency in mu-opioid receptors does not alter the regulation of opioid receptors by opioid antagonists in vivo, and suggest that adaptive responses to chronic opioid antagonist treatment are independent of opioid receptor density.

Nalmefene causes greater hypothalamic-pituitary-adrenal axis activation than naloxone in normal volunteers: implications for the treatment of alcoholism

Among other actions, opioid antagonists modulate the control endogenous opioids exert on the hypothalamic-pituitary-adrenal (HPA) axis. Naloxone, nalmefene, and naltrexone are the opioid antagonists approved for use in man and are primarily mu-opioid selective. Naltrexone and nalmefene have been demonstrated to be useful in the treatment of alcoholism. Compared with naloxone, nalmefene has a longer half-life, is more potent at the mu-receptor, and has a higher affinity for kappa- and delta-opioid receptors. We conducted an inpatient study comparing the effects of 10 and 30 mg doses of intravenous naloxone and nalmefene in normal, nonsubstance nor alcohol-abusing, volunteers. Significant increases in ACTH and cortisol were observed after both antagonists, without an apparent dose-response relationship; however, both doses of nalmefene resulted in greater HPA axis activation than either dose of naloxone (ACTH: p <0.005). These results indicate that kappa- and delta-opioids may play important roles in the regulation of the HPA axis; nalmefene may be useful as both a probe to explore the HPA axis physiology and as a pharmacotherapeutic agent.

Quantitative immunolocalization of mu opioid receptors: regulation by naltrexone

The present study utilized a newly developed quantitative immunohistochemical assay to measure changes in mu opioid receptor abundance following chronic administration of the opioid receptor antagonist naltrexone. These data were compared with those obtained from mu receptor radioligand binding on adjacent tissue sections, in order to determine whether the characteristic antagonist-induced increase in radioligand binding is due to an increase in the total number of mu receptors and/or to an increase in the proportion of receptors that are in an active binding conformation in the absence of a change in the total number of receptors. Adult male Sprague-Dawley rats were administered naltrexone, 7-8 mg/kg per day, or saline continuously for seven days by osmotic minipumps, after which time their brains were processed for immunohistochemistry and receptor autoradiography on adjacent fresh frozen tissue sections. Semiquantitative immunohistochemistry was performed using a radiolabelled secondary antibody for autoradiographic determination and a set of radioactive standards. Results demonstrate an overall concordance between the distribution of mu opioid receptors as measured by the two different methods with a few exceptions. Following naltrexone administration, mu receptor immunoreactivity was significantly higher in the amygdala, thalamus, hippocampus, and interpeduncular nucleus as compared with the saline-treated control animals. [3H]D-Ala2,N-Me-Phe4,Gly-ol5-enkephalin binding to mu opioid receptors was significantly higher in the globus pallidus, amygdala, thalamus, hypothalamus, hippocampus, substantia nigra, ventral tegmental area, central gray, and interpeduncular nucleus of the naltrexone-treated rats. These findings indicate that in some brain regions chronic naltrexone exposure increases the total number of mu opioid receptors, while in other regions there is an increase in the percent of active receptors without an observable change in the total number of receptors. Quantitative receptor immunodetection together with ligand autoradiography provides a new approach for investigating the regulation of mu opioid receptors on tissue sections.

Chronic naltrexone differentially affects supraspinal delta-opioid receptor-mediated antinociception

The effects of chronic treatment with naltrexone, an opioid receptor antagonist, on delta1- and delta2-opioid receptor agonist-induced antinociception and ligand binding were investigated in mice. Antinociception by intracerebroventricular (i.c.v.) [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, agonists selective for delta1- and delta2-opioid receptors, respectively, was blocked following subcutaneous (s.c.) implantation of a naltrexone pellet (7.5 mg) for 7 days. Removal of the naltrexone pellet was followed 24 h later by a decrease of 7.5-fold in the ED50 value of [D-Ala2]deltorphin II, but not that of DPDPE. In a whole brain homogenate the binding of [3H][D-Ala2]deltorphin II was increased twice as much as that of [3H]DPDPE. Chronic naltrexone treatment also produced an 8.6-fold decrease in the ED50 value of i.c.v. administered morphine. The increase in morphine potency was reversed to a control (placebo-treated mice) value by the selective delta2-opioid receptor antagonist, naltriben (25 pmol, i.c.v.). Thus, chronic naltrexone selectively increases delta2-opioid receptor-mediated antinociception, supporting the existence of delta opioid receptor subtypes with distinct adaptive characteristics. The data also indicate that delta2-opioid receptors are critically involved in the expression of morphine supersensitivity.

Open-label trial of oral nalmefene therapy for the pruritus of cholestasis

The aims of this study were to determine whether long-term oral administration of the opiate antagonist nalmefene is associated with any beneficial effects in patients with pruritus secondary to cholestatic liver disease and to assess the safety of long-term administration of this drug to these patients. Fourteen patients with unrelieved chronic pruritus of cholestasis were studied. Scratching activity, independent of limb movements, was recorded continuously for 24-hour periods before and during treatment with an initial ameliorating dose of nalmefene. Simultaneously, during these periods, visual analogue scores (VASs) of pruritus were recorded every 4 hours while patients were awake. The dose of nalmefene, which initially was 2 mg orally twice daily, was increased during the study, usually until a satisfactory clinical response was achieved. Five patients experienced a transient opioid withdrawal-like reaction that did not preclude continuing with nalmefene therapy. Serum biochemical indices of cholestasis did not change appreciably during treatment. Thirteen patients reported amelioration of the perception of pruritus on nalmefene. In 5 patients, exacerbations of pruritus occurred approximately 4 weeks after an initial ameliorating dose had been reached; these exacerbations were managed by increasing the dose. Baseline mean values for VAS and scratching activity were higher than corresponding means during nalmefene therapy in 13 (P = .002) and 12 (P = .013) patients, respectively. Possible tolerance to nalmefene occurred in 3 patients. Three patients experienced marked exacerbation of pruritus after nalmefene therapy was suddenly discontinued. Blood levels of nalmefene were consistent with normal pharmacokinetics of the drug. These results suggest that nalmefene may have a favorable risk-to-benefit ratio when it is administered orally long-term to patients with the pruritus of cholestasis.

In vivo homologous regulation of mu-opioid receptor gene expression in the mouse

Regulation of the mu-opioid receptor gene by opioid analgesic drugs has not been observed in rats and mice following in vivo treatments that produce tolerance. Although in vivo heterologous regulation of mu-opioid receptor mRNA by non-opioid compounds has been reported, the failure to observe changes in mu-opioid receptor mRNA levels in vivo after treatment with opioid agonists raised the possibility that in vivo homologous regulation by agonists may not occur. Therefore, in the present study, the effect of a high intrinsic efficacy opioid receptor agonist on opioid receptor density, gene expression and tolerance was determined. Mice were infused with etorphine for 7 days using an osmotic minipump, then the pump was removed and studies conducted 16-168 h later. Etorphine (50-250 microg/kg/day) infusion produced significant dose-dependent tolerance to the analgesic (tailflick) effects of etorphine, as well as dose-dependent mu-opioid receptor downregulation in brain at 16 h following the end of the infusion. Mu-opioid receptor density returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. Similarly, the magnitude of tolerance decreased over the same period. Evaluation of changes in brain mu-opioid receptor mRNA 16 h following etorphine infusion indicated that there was dose-dependent increase in steady-state levels, with no significant change in GAPDH mRNA. The increase in mu-opioid receptor mRNA was approximately 55-65% over control at the highest etorphine infusion dose. Mu-opioid receptor mRNA returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. These data suggest that the increase in mu-opioid receptor mRNA following the termination of etorphine treatment may drive the recovery of mu-opioid receptors. These data are the first demonstration of in vivo homologous regulation of mu-opioid receptor gene expression in the mouse by an opioid receptor agonist that produces tolerance and receptor downregulation.

The effect of mu-opioid receptor antisense on morphine potency and antagonist-induced supersensitivity and receptor upregulation

The present study examined the effect of in vivo antisense oligodeoxynucleotide treatment on naltrexone (NTX)-induced functional supersensitivity and mu-opioid receptor up-regulation in mice. On day 1 mice were implanted S.C. with a NTX or placebo pellet and injected I.T. and I.C.V. with dH2O or oligodeoxynucleotides. The oligodeoxynucleotides were designed so that they were either perfectly complementary to the first 18 bases of the coding region of mouse mu-opioid receptor mRNA, or had one (Mismatch-1) or four (Mismatch-4) mismatches. On days 3, 5, 7, and 9, mice were again injected I.T. and I.C.V. with dH2O or one of the oligodeoxynucleotides. After the final injections on day 9, placebo and NTX pellets were removed, and 24 h later mice were tested for morphine analgesia or sacrificed for saturation binding studies ([3H]DAMGO). Naltrexone increased the analgesic potency of morphine in dH2O treated mice by approximately 70%. In binding studies, NTX significantly increased density of brain (approximately 60%) and spinal cord (approximately 140%) mu-opioid receptors without affecting affinity. The mu-opioid antisense and the oligodeoxynucleotide with one mismatch (Mismatch-1) significantly reduced the potency of morphine by approximately twofold in placebo-treated mice. The oligodeoxynucleotide with four mismatches (Mismatch-4) did not significantly alter morphine potency. When placebo-treated mice were treated with either the antisense to the mouse mu-opioid receptor, Mismatch-4 or Mismatch-1 there were no significant changes in the density of mu-opioid receptors. Thus, mu-opioid antisense significantly reduced morphine potency without changing mu-opioid receptor density. When NTX and oligodeoxynucleotide treatments were combined, there was no change in NTX-induced supersensitivity and mu-opioid receptor upregulation. These data suggest that opioid antagonist-induced supersensitivity and upregulation of mu-opioid receptors does not involve changes in gene expression.

Downregulation of mu-opioid binding sites following chronic administration of neuropeptide FF (NPFF) and morphine

The effect of continuous ICV infusion of NPFF (10 micrograms/microliter) and morphine (40 micrograms/microliter) on mu-opioid binding sites was examined in rats using the in vitro radiolabeled techniques of whole-brain homogenate receptor binding and quantitative autoradiography. Mu receptors were labeled with [3H][D-Ala2-MePhe4,Glyol5] enkephalin in the homogenate binding experiments and with [125I][D-Ala2-MePhe4,Gly-ol5]enkephalin in autoradiographic studies. In homogenate binding studies, chronic administration of NPFF or morphine significantly downregulated mu receptors by 20% and 44%, respectively. Quantitative autoradiographic experiments demonstrated downregulation of mu opioid receptors in specific brain nuclei for both NPFF- and morphine-treated animals. Within the striatum and several nuclei of the thalamus, the mu receptors of the NPFF- and morphine-treated animals were decreased by 20-30% and 38-73%, respectively. These results suggest that NPFF may modulate opioid-mediated responses in part by altering the density of mu-opioid receptors.

Chronic opioid antagonist administration upregulates mu opioid receptor binding without altering mu opioid receptor mRNA levels

Chronic administration of opioid antagonists has been shown to increase radioligand binding to brain opioid receptors. The present study was conducted to determine whether chronic exposure to the opioid antagonist naltrexone would similarly increase mu opioid receptor gene expression as measured by mRNA levels. Male Sprague-Dawley rats were administered naltrexone, 7-8 mg/kg/day, or saline by osmotic minipumps for 7 days. As expected, the density of mu opioid receptor binding sites was significantly higher in the brains of animals treated chronically with naltrexone as compared with saline-treated control animals. However, mu opioid receptor mRNA content determined by a solution hybridization RNase protection assay was not significantly altered in any brain region investigated. These results indicate that the upregulation of mu opioid receptors as measured by radioligand binding is not accompanied by increased levels of mu receptor mRNA.

Gene-peptide relationships in the developing rat brain: the response of preproenkephalin mRNA and [Met5]-enkephalin to acute opioid antagonist (naltrexone) exposure

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor during brain development in addition to its role as a neurotransmitter. This study examined the relationship of gene and peptide expression in the developing (postnatal day 6) rat brain by disrupting peptide-receptor interaction with either a brief (4-6 h) or continuous opioid receptor blockade using a single injection of 1 or 50 mg/kg naltrexone (NTX), respectively; such perturbations result in growth inhibition or acceleration, respectively. In the caudate putamen, an area that has completed neurogenesis by postnatal day 6 and has an abundance of PPE mRNA and enkephalins in adulthood, NTX did not influence PPE mRNA in either NTX group, or the enkephalin levels in the 1 mg/kg NTX group. [Met5]-enkephalin values in the neostriatum, however, were 67-183% greater than controls in rats given 50 mg/kg NTX, beginning 5 min after drug injection. In the cerebellum, PPE mRNA expression was depressed from 5 min to 4 h in the 1 mg/kg NTX group, and was normal thereafter; mRNA levels in the 50 mg/kg NTX group were markedly subnormal for 24 h. Enkephalin levels were significantly depressed within 5 min of drug injection and remained so for 4 h in the 1 mg/kg NTX group, but were elevated to approximately 135% of control values at 8, 16, and 24 h. Enkephalin levels were not changed in the cerebellum of the 50 mg/kg NTX group, or in the plasma of either NTX group. These data suggest that a single exposure to NTX can affect transcriptional and translational mechanisms related to PPE mRNA and opioid peptide expression in a rapid and sustained manner, and that this treatment elicits a specific pattern of alterations dependent upon the brain region sampled, drug dosage, and/or the duration of opioid receptor blockade. Additionally, our results indicate that the decreased DNA synthesis in external germinal cells occurring after opioid receptor blockade as recorded earlier may be related to an increase in the potent opioid growth factor, [Met5]-enkephalin.

Assessment of delta opioid antinociception and receptor mRNA levels in mouse after chronic naltrexone treatment

The antinociceptive potency of the delta opioid receptor (DOR) agonist [D-Ala2]Deltorphin II and the levels of DOR mRNA were measured in mice chronically treated with naltrexone. ED50 determinations for [D-Ala2]Deltorphin II, using the tail-flick test with mice that had been treated with naltrexone for 7 days followed by a 24 h naltrexone free period (study day 8), revealed a 7.7-fold increase in antinociceptive potency, indicating functional supersensitivity. Utilization of a micro-dissection technique followed by quantitative solution hybridization measurements of RNA extracts from mouse CNS revealed levels of DOR mRNA ranging from 2.8 pg/microgram RNA in the caudate-putamen to 0.3 pg/microgram RNA in cerebellum. However, despite the functional increase in DOR sensitivity, the DOR mRNA levels in selected brain areas and spinal cord of naltrexone-treated and control mice did not differ. Assessment of DOR mRNA levels in whole brain and selected CNS regions after shorter treatment intervals (1, 6 and 12 h and 2 and 7 days) in naltrexone-treated and control mice revealed a similar pattern of results. Northern blot analysis of mouse whole brain RNA extracts after 7 days of naltrexone treatment did not show any alteration in the size of the DOR transcript. These data demonstrate that DOR mRNA levels are not altered during and after chronic naltrexone treatment and therefore are not associated with opioid-induced DOR up-regulation or DOR functional supersensitivity.

Cellular distribution of the mRNA for the kappa-opioid receptor in the human neocortex: a non-isotopic in situ hybridization study

Opioid receptors (OR) provide primary interaction sites of the human brain with opiates. Presently kappa-OR mRNA expression was studied in different cortical areas (A4, A10, A17) by in situ hybridization using digoxigenin-labeled oligonucleotides and an alkaline phosphatase-mediated color reaction. kappa-OR mRNA was expressed mainly in layers II/III and V pyramidal and layer VI multiform neurons. A4 giant pyramidal and A17 giant stellate neurons stood out labeled. These findings fit in with our data on kappa-OR protein distribution. Combined cellular assessment of protein and mRNA will enable the study kappa-OR expression under physiological and pathological conditions.