Ultra-low-dose naltrexone reduces the rewarding potency of oxycodone and relapse vulnerability in rats

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

μ-opioid receptor agonists and psychedelics: pharmacological opportunities and challenges

Opioid misuse and opioid-involved overdose deaths are a massive public health problem involving the intertwined misuse of prescription opioids for pain management with the emergence of extremely potent fentanyl derivatives, sold as standalone products or adulterants in counterfeit prescription opioids or heroin. The incidence of repeated opioid overdose events indicates a problematic use pattern consistent with the development of the medical condition of opioid use disorder (OUD). Prescription and illicit opioids reduce pain perception by activating µ-opioid receptors (MOR) localized to the central nervous system (CNS). Dysregulation of meso-corticolimbic circuitry that subserves reward and adaptive behaviors is fundamentally involved in the progressive behavioral changes that promote and are consequent to OUD. Although opioid-induced analgesia and the rewarding effects of abused opioids are primarily mediated through MOR activation, serotonin (5-HT) is an important contributor to the pharmacology of opioid abused drugs (including heroin and prescription opioids) and OUD. There is a recent resurgence of interest into psychedelic compounds that act primarily through the 5-HT2A receptor (5-HT 2A R) as a new frontier in combatting such diseases (e.g., depression, anxiety, and substance use disorders). Emerging data suggest that the MOR and 5-HT2AR crosstalk at the cellular level and within key nodes of OUD circuitry, highlighting a major opportunity for novel pharmacological intervention for OUD. There is an important gap in the preclinical profiling of psychedelic 5-HT2AR agonists in OUD models. Further, as these molecules carry risks, additional analyses of the profiles of non-hallucinogenic 5-HT2AR agonists and/or 5-HT2AR positive allosteric modulators may provide a new pathway for 5-HT2AR therapeutics. In this review, we discuss the opportunities and challenges associated with utilizing 5-HT2AR agonists as therapeutics for OUD.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

Introduction

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures.

Methods

In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-h sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests.

Results

In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase.

Discussion

These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures. In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-hr sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests. In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase. These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Ligand-Free Signaling of G-Protein-Coupled Receptors: Relevance to μ Opioid Receptors in Analgesia and Addiction

Numerous G-protein-coupled receptors (GPCRs) display ligand-free basal signaling with potential physiological functions, a target in drug development. As an example, the μ opioid receptor (MOR) signals in ligand-free form (MOR-μ*), influencing opioid responses. In addition, agonists bind to MOR but can dissociate upon MOR activation, with ligand-free MOR-μ* carrying out signaling. Opioid pain therapy is effective but incurs adverse effects (ADRs) and risk of opioid use disorder (OUD). Sustained opioid agonist exposure increases persistent basal MOR-μ* activity, which could be a driving force for OUD and ADRs. Antagonists competitively prevent resting MOR (MOR-μ) activation to MOR-μ*, while common antagonists, such as naloxone and naltrexone, also bind to and block ligand-free MOR-μ*, acting as potent inverse agonists. A neutral antagonist, 6β-naltrexol (6BN), binds to but does not block MOR-μ*, preventing MOR-μ activation only competitively with reduced potency. We hypothesize that 6BN gradually accelerates MOR-μ* reversal to resting-state MOR-μ. Thus, 6BN potently prevents opioid dependence in rodents, at doses well below those blocking antinociception or causing withdrawal. Acting as a ‘retrograde addiction modulator’, 6BN could represent a novel class of therapeutics for OUD. Further studies need to address regulation of MOR-μ* and, more broadly, the physiological and pharmacological significance of ligand-free signaling in GPCRs.

Pharmacological treatment of pain among persons with opioid addiction: A systematic review and meta-analysis with implications for drug development

The clinical features and neurobiology of pain and opioid use disorder (OUD) are inextricably linked. Despite emerging evidence supporting the negative impact of ongoing pain in the treatment of OUD, the pharmacological management of pain in the presence of OUD has received limited attention. We sought to systematically review the studies investigating pharmacotherapies for pain among persons with OUD. Eligible studies had participants with OUD and outcomes including evoked or spontaneous pain. We searched Scopus, Cochrane Database of Systematic Reviews, Medline, and Embase. Out of 1,097 studies that met the search criteria, 12 studies provided data relevant to the research question-five laboratory studies and seven clinical trials. Random effects pooled estimates suggested no significant difference between groups at baseline but a response favoring the active treatment group over placebo, with nonsignificant heterogeneity between studies. Findings from these studies provide preliminary evidence for analgesic and antihyperalgesic effects of gabapentin, GABA agonists, and NMDA antagonists among persons with OUD. To establish the tradeoffs between the analgesic effects and abuse liability of these compounds, further well-controlled clinical trials are required among persons with OUD. This review also underscores the need for methodological enhancement in drug development for pain in OUD. Future research should address the clinical and neurobiological overlap between pain- and addiction-related phenomena. Transdisciplinary approaches may identify biomarkers of these shared phenomena and their neural substrates. The development of novel therapeutics for pain in OUD may be accelerated by such integration of pain and addiction research.

High fructose corn syrup alters behavioural and neurobiological responses to oxycodone in rats

There are indications that sugars in the diet can play a role in vulnerability to opioid abuse. The current study examined a range of neuro-behavioural interactions between oxycodone (OXY) and high fructose corn syrup (HFCS). Male Sprague-Dawley rats had access to HFCS (0 or 50%) over 26 days in their home cages and were subsequently tested on place conditioning induced by 0, 0.16 and 2.5 mg/kg OXY (3 pairings of drug and saline, each 30 min), as well as on locomotor responses to 0, 0.16 and 2.5 mg/kg OXY, and in-vivo microdialysis was employed to measure dopamine (DA) levels in the nucleus accumbens (NAc) in response to 0 and 2.5 mg/kg OXY. A complex set of interactions between HFCS exposure and responses to OXY were observed: HFCS increased place preference induced by OXY, it enhanced the suppressant effect of OXY on locomotion, and it attenuated OXY-induced elevation in DA overflow in the NAc. Taken together, these findings suggest that nutrition has the potential to influence some responses to opioids which may be relevant to their abuse.

Relapse-like behavior in a mouse model of the OPRM1 (mu-opioid receptor) A118G polymorphism: Examination with intravenous oxycodone self-administration

The widely abused prescription opioid oxycodone is a mu-opioid receptor (MOP-r) agonist and addiction to such opioids is a relapsing disorder. The human MOP-r gene (OPRM1) has an important functional single nucleotide polymorphism (SNP), A118G, which affects risk of severe opioid use disorders. A112G (G/G) knock-in mice are models of human A118G carriers. We examined oxycodone self-administration (SA) in male and female G/G versus wild type (A/A) mice in SA sessions and in relapse-like behavior. Adult male and female G/G and A/A mice self-administered oxycodone (0.25 mg/kg/infusion, FR1) for 10 consecutive days. Following 10-day home cage drug free withdrawal, the mice were re-exposed to oxycodone SA for a further 10 days. MOP-r receptor mRNA in various brain regions were examined immediately after the last re-exposure session. We found that G/G mice had greater oxycodone SA than A/A mice in the initial and in re-exposure sessions. Mice of both genotypes had greater oxycodone intake during the re-exposure period than during the initial exposure. We also detected differences in MOP-r gene expression due to genotype, sex and oxycodone SA history in the dorsal striatum, hippocampus, and prefrontal cortex. These studies may improve our understanding of MOP-r-agonist self-exposure and relapse in human carriers of the A118G SNP.

Biased Opioid Antagonists as Modulators of Opioid Dependence: Opportunities to Improve Pain Therapy and Opioid Use Management

Opioid analgesics are effective pain therapeutics but they cause various adverse effects and addiction. For safer pain therapy, biased opioid agonists selectively target distinct μ opioid receptor (MOR) conformations, while the potential of biased opioid antagonists has been neglected. Agonists convert a dormant receptor form (MOR-μ) to a ligand-free active form (MOR-μ*), which mediates MOR signaling. Moreover, MOR-μ converts spontaneously to MOR-μ* (basal signaling). Persistent upregulation of MOR-μ* has been invoked as a hallmark of opioid dependence. Contrasting interactions with both MOR-μ and MOR-μ* can account for distinct pharmacological characteristics of inverse agonists (naltrexone), neutral antagonists (6β-naltrexol), and mixed opioid agonist-antagonists (buprenorphine). Upon binding to MOR-μ*, naltrexone but not 6β-naltrexol suppresses MOR-μ*signaling. Naltrexone blocks opioid analgesia non-competitively at MOR-μ*with high potency, whereas 6β-naltrexol must compete with agonists at MOR-μ, accounting for ~100-fold lower in vivo potency. Buprenorphine's bell-shaped dose-response curve may also result from opposing effects on MOR-μ and MOR-μ*. In contrast, we find that 6β-naltrexol potently prevents dependence, below doses affecting analgesia or causing withdrawal, possibly binding to MOR conformations relevant to opioid dependence. We propose that 6β-naltrexol is a biased opioid antagonist modulating opioid dependence at low doses, opening novel avenues for opioid pain therapy and use management.

The kappa-opioid receptor agonist, nalfurafine, blocks acquisition of oxycodone self-administration and oxycodone's conditioned rewarding effects in male rats

Mu-opioid receptor (MOR) agonists are highly efficacious for the treatment of pain but have significant abuse liability. Recently, we reported that nalfurafine, when combined with oxycodone at a certain ratio, reduced the reinforcing effects of oxycodone in rats while producing additive antinociceptive effects. Questions remain, however, including if the combination will function as a reinforcer in drug-naïve rats, and if the combination produces aversive effects that could explain nalfurafine's ability to reduce oxycodone self-administration? In the present study, we investigated nalfurafine's ability to reduce acquisition of oxycodone self-administration when the two were self-administered as a mixture in drug-naïve rats and nalfurafine's ability to attenuate a conditioned place preference (CPP) induced by oxycodone. In the self-administration study, male Sprague-Dawley rats self-administered intravenous injections of oxycodone (0.056 mg/kg/injection), an oxycodone/nalfurafine combination (0.056/0.0032 mg/kg/injection), or saline under fixed-ratio schedules of reinforcement for 20 days to compare rates of acquisition of drug taking. In the CPP assay, male Sprague-Dawley rats received subcutaneous injections of either saline, oxycodone (3.2 mg/kg), nalfurafine (0.18 mg/kg), or an oxycodone/nalfurafine combination at the same ratio used in the self-administration study (3.2 mg/kg/0.18 mg/kg). All subjects self-administering oxycodone alone met acquisition criteria. However, only 13% of subjects self-administering oxycodone/nalfurafine met criteria, and no subjects acquired self-administration of saline. Oxycodone, but not nalfurafine alone or the oxycodone/nalfurafine combination, produced rewarding effects in rats in the CPP test. These findings suggest that the combination of oxycodone and nalfurafine will be less habit forming in opioid-naïve patients than oxycodone alone.

Combining a Candidate Vaccine for Opioid Use Disorders with Extended-Release Naltrexone Increases Protection against Oxycodone-Induced Behavioral Effects and Toxicity

Opioid use disorders (OUDs) and opioid-related fatal overdoses are a significant public health concern in the United States and worldwide. To offer more effective medical interventions to treat or prevent OUD, antiopioid vaccines are in development that reduce the distribution of the targeted opioids to brain and subsequently reduce the associated behavioral and toxic effects. It is of critical importance that antiopioid vaccines do not interfere with medications that treat OUD. Hence, this study tested the preclinical proof of concept of combining a candidate oxycodone vaccine [oxycodone-keyhole limpet hemocyanin (OXY-KLH)] with an FDA-approved extended-release naltrexone (XR-NTX) depot formulation in rats. The effects of XR-NTX on oxycodone-induced motor activity and antinociception were first assessed in nonvaccinated naïve rats to establish a baseline for subsequent studies. Next, OXY-KLH and XR-NTX were coadministered to determine whether the combination would affect the efficacy of each individual treatment, and it was found that the combination of OXY-KLH and XR-NTX offered greater efficacy in reducing oxycodone-induced motor activity, thigmotaxis, antinociception, and respiratory depression over a range of repeated or escalating oxycodone doses in rats. These data support the feasibility of combining antibody-based therapies with opioid receptor antagonists to provide greater or prolonged protection against opioid-related toxicity or overdose. Combining antiopioid vaccines with XR-NTX may provide prophylactic measures to subjects at risk of relapse and accidental or deliberate exposure. Combination therapy may extend to other biologics (e.g., monoclonal antibodies) and medications against substance use disorders. SIGNIFICANCE STATEMENT: Opioid use disorders (OUDs) remain a major problem worldwide, and new therapies are needed. This study reports on the combination of an oxycodone vaccine [oxycodone-keyhole limpet hemocyanin (OXY-KLH)] with a currently approved OUD therapy, extended-release naltrexone (XR-NTX). Results demonstrated that XR-NTX did not interfere with OXY-KLH efficacy, and combination of low doses of XR-NTX with vaccine was more effective than each individual treatment alone to reduce behavioral and toxic effects of oxycodone, suggesting that combining OXY-KLH with XR-NTX may improve OUD outcomes.

Effect of the dopamine stabilizer (-)-OSU6162 on potentiated incubation of opioid craving after electric barrier-induced voluntary abstinence

In the classical incubation of drug craving rat model, drug seeking is assessed after homecage forced abstinence. However, human abstinence is often voluntary because negative consequences of drug seeking outweigh the desire for the drug. Here, we developed a rat model of incubation of opioid craving after electric barrier-induced voluntary abstinence and determined whether the dopamine stabilizer (-)-OSU6162 would decrease this new form of incubation. We trained male and female rats to self-administer oxycodone (0.1 mg/kg/infusion, 6 h/day) for 14 days. We then exposed them to either homecage forced abstinence or voluntary abstinence induced by an electric barrier of increasing intensity near the drug-paired lever. On abstinence days 1, 15, or 30, we tested the rats for oxycodone seeking without shock and drug. We also examined the effect of (-)-OSU6162 (7.5 and 15 mg/kg) on oxycodone seeking on abstinence day 1 or after 15 days of either voluntary or forced abstinence. Independent of sex, the time-dependent increase in oxycodone seeking after cessation of opioid self-administration (incubation of opioid craving) was stronger after voluntary abstinence than after forced abstinence. In males, (-)-OSU6162 decreased incubated (day 15) but not non-incubated (day 1) oxycodone seeking after either voluntary or forced abstinence. In females, (-)-OSU6162 modestly decreased incubated oxycodone seeking after voluntary but not forced abstinence. Results suggest that voluntary abstinence induced by negative consequences of drug seeking can paradoxically potentiate opioid craving and relapse. We propose the dopamine stabilizer (-)-OSU6162 may serve as an adjunct pharmacological treatment to prevent relapse in male opioid users.

Lasting effects of repeated ∆9 -tetrahydrocannabinol vapour inhalation during adolescence in male and female rats

BACKGROUND AND PURPOSE

Adolescents are regularly exposed to ∆⁹ -tetrahydrocannabinol (THC) via smoking and, more recently, vaping cannabis extracts. Growing legalization of cannabis for medical and recreational purposes, combined with decreasing perceptions of harm, makes it increasingly important to determine the consequences of frequent adolescent exposure for motivated behaviour and lasting tolerance in response to THC.

EXPERIMENTAL APPROACHES

Male and female rats inhaled THC vapour, or that from the propylene glycol (PG) vehicle, twice daily for 30 min from postnatal day (PND) 35-39 and PND 42-46 using an e-cigarette system. Thermoregulatory responses to vapour inhalation were assessed by radio-telemetry during adolescence and from PND 86-94. Chow intake was assessed in adulthood. Blood samples were obtained from additional adolescent groups following initial THC inhalation and after 4 days of twice daily exposure. Additional groups exposed repeatedly to THC or PG during adolescence were evaluated for intravenous self-administration of oxycodone as adults.

KEY RESULTS

Female, not male, adolescents developed tolerance to the hypothermic effects of THC inhalation in the first week of repeated exposure despite similar plasma THC levels. Each sex exhibited tolerance to THC hypothermia in adulthood after repeated adolescent THC. However, enhanced potency was found in females. Repeated THC male rats consumed more food than their PG-treated control group, without significant bodyweight differences. Adolescent THC did not alter oxycodone self-administration in either sex but increased fentanyl self-administration in females.

CONCLUSIONS AND IMPLICATIONS

Repeated THC vapour inhalation in adolescent rats has lasting consequences observable in adulthood.

Delta-opioid receptor antagonist naltrindole reduces oxycodone addiction and constipation in mice

Oxycodone, a widely prescribed and very potent oral opioid analgesic agent, is highly addictive and has many side effects, including troublesome constipation. Our studies in mice indicated that pretreatment of naltrindole did not significantly affect the analgesic efficacy of oxycodone but attenuated the tolerance and withdrawal induced by chronic oxycodone administration. Naltrindole also attenuated the oxycodone-induced rewarding and re-instatement behaviors, as shown by the conditioned place preference test. Further, oxycodone-induced decrease in intestinal transit (i.e., constipation) was reduced by naltrindole. However, naltrindole did not block the respiratory depression produced by oxycodone. Taken together, these data suggest that naltrindole can attenuate some major side effects while retaining the analgesic efficacy of oxycodone in mice. Naltrindole and oxycodone may have the potential to be a potent analgesic combination with much lower levels of oxycodone's side effects of addictive liability and constipation.

Prophylactic vaccination protects against the development of oxycodone self-administration

Abuse of prescription opioids is a growing public health crisis in the United States, with drug overdose deaths increasing dramatically over the past 15 years. Few preclinical studies exist on the reinforcing effects of oxycodone or on the development of therapies for oxycodone abuse. This study was conducted to determine if immunopharmacotherapy directed against oxycodone would be capable of altering oxycodone-induced antinociception and intravenous self-administration. Male Wistar rats were administered a small-molecule immunoconjugate vaccine (Oxy-TT) or the control carrier protein, tetanus toxoid (TT), and trained to intravenously self-administer oxycodone (0.06 or 0.15 mg/kg/infusion). Brain oxycodone concentrations were 50% lower in Oxy-TT rats compared to TT rats 30 min after injection (1 mg/kg, s.c.) whereas plasma oxycodone was 15-fold higher from drug sequestration by circulating antibodies. Oxy-TT rats were also less sensitive to 1-2 mg/kg, s.c. oxycodone on a hot water nociception assay. Half of the Oxy-TT rats failed to acquire intravenous self-administration under the 0.06 mg/kg/infusion training dose. Oxycodone self-administration of Oxy-TT rats trained on 0.15 mg/kg/infusion was higher than controls; however under progressive ratio (PR) conditions the Oxy-TT rats decreased their oxycodone intake, unlike TT controls. These data demonstrate that active vaccination provides protection against the reinforcing effects of oxycodone. Anti-oxycodone vaccines may entirely prevent repeated use in some individuals who otherwise would become addicted. Vaccination may also reduce dependence in those who become addicted and therefore facilitate the effects of other therapeutic interventions which either increase the difficulty of drug use or incentivize other behaviors.

The novel dopamine D3 receptor antagonists/partial agonists CAB2-015 and BAK4-54 inhibit oxycodone-taking and oxycodone-seeking behavior in rats

The use of prescription opioid analgesics, particularly oxycodone, has dramatically increased, and parallels escalated opioid abuse and drug-related deaths worldwide. Understanding the molecular mechanisms underlying the development of opioid dependence and expanding treatment options to counter prescription opioid abuse has become a critical public health matter. In the present study, we first evaluated the reinforcing effects of oxycodone in a rat model of self-administration and then explored the potential utility of two novel high affinity dopamine D3 receptor (D3R) antagonists/partial agonists, CAB2-015 and BAK4-54, for treatment of prescription opioid abuse and dependence. We found that rats acquired oxycodone self-administration rapidly within a range of unit doses that was similar to that for heroin, confirming that oxycodone has significant abuse potential. Strikingly, pretreatment with either CAB2-015 or BAK4-54 (0.4-10 mg/kg, i.p.) dose-dependently decreased oxycodone self-administration, and shifted the oxycodone dose-response curve downward. Repeated pretreatment with CAB2-015 or BAK4-54 (0.4-4 mg/kg) facilitated extinction and inhibited oxycodone-induced reinstatement of drug-seeking behavior. In addition, pretreatment with CAB2-015 or BAK4-54 (4-10 mg/kg) also dose-dependently decreased oxycodone-enhanced locomotor activity, but only CAB2-015 decreased oral sucrose self-administration. These data suggest that D3R antagonists may be suitable alternatives or adjunctive to opioid-based medications currently used clinically in treating opioid addiction and that the D3R-selective ligands (CAB2-015 or BAK4-54) provide new lead molecules for development.

Lorcaserin Suppresses Oxycodone Self-Administration and Relapse Vulnerability in Rats

Opioid use disorder (OUD) is a major public health problem. High relapse rates and poor treatment retention continue to pose major challenges in OUD treatment. Of the abused opioids, oxycodone is well described to maintain self-administration and evoke the durable conditioned responses ("cue reactivity") that result from pairing of opioid-related stimuli (e.g., paraphernalia) with repeated abuse. Serotonin (5-HT) neurotransmission, particularly through the 5-HT_2C receptor (5-HT_2CR), regulates psychostimulant reward and cue reactivity, and in the present experiments, we investigated the hypothesis that the selective 5-HT_2CR agonist lorcaserin, which is approved by the United States Food and Drug Administration (FDA) for the treatment of obesity, will suppress oxycodone self-administration and oxycodone-associated cue reactivity in rats. We found that lorcaserin inhibited oxycodone intake, an effect blocked by the selective 5-HT_2CR antagonist SB242084. Lorcaserin also decreased responding for the discrete cue complex ("cue reactivity") previously associated with delivery of oxycodone (i.e., stimulus lights, infusion pump sounds) in both abstinence and extinction-reinstatement models. The selected dose range of lorcaserin (0.25-1 mg/kg) does not overtly alter spontaneous behaviors nor operant responding on inactive levers in the present study. Taken together, the ability of lorcaserin to reduce the oxycodone self-administration and decrease cue reactivity associated with relapse highlights the therapeutic potential for lorcaserin in the treatment of OUD.

Effects of an oxycodone conjugate vaccine on oxycodone self-administration and oxycodone-induced brain gene expression in rats

Prescription opioid abuse is an increasing public health concern in the USA. A vaccine comprising a hapten (OXY) conjugated to the carrier protein keyhole limpet hemocyanin (OXY-KLH) has been shown to attenuate the antinociceptive effects of oxycodone. Here, the vaccine's ability to prevent acquisition of intravenous (i.v.) oxycodone self-administration was studied in rats. Effects of vaccination on oxycodone-induced changes in the expression of several genes within the mesolimbic system, which are regulated by chronic opiate use, were also examined. Vaccination with OXY-KLH reduced the proportion of rats acquiring i.v. self-administration of oxycodone under a fixed ratio (FR) 3 schedule of reinforcement compared to control rats immunized with the unconjugated KLH carrier protein. Vaccination significantly reduced the mean number of infusions at FR3, total number of infusions, and total oxycodone intake during the entire protocol. Compared to oxycodone self-administering control rats immunized with the carrier alone, rats vaccinated with the OXY-KLH immunogen showed increased levels of adenylate cyclase 5 (Adcy5) and decreased levels of early growth response protein 2 (Egr2) and the early immediate gene c-Fos in the striatum. These data suggest that vaccination with OXY-KLH can attenuate the reinforcing effects of oxycodone at a clinically-relevant exposure level. Analysis of mRNA expression identified some addiction-relevant markers that may be of interest in understanding oxycodone effects or the protection provided by vaccination.

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.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Oxycodone combined with opioid receptor antagonists: efficacy and safety

INTRODUCTION

A mu receptor antagonist combined with oxycodone (OXY) may improve pain control, reduce physical tolerance and withdrawal, minimizing opioid-related bowel dysfunction and act as an abuse deterrent.

AREAS COVERED

The authors cover the use of OXY plus ultra-low-dose naltrexone for analgesia and the use of sustained-release OXY plus sustained-release naloxone to reduce the opioid bowel syndrome. The authors briefly describe the use of sustained-release OXY and naltrexone pellets as a drug abuse deterrent formulation. Combinations of ultra-low-dose naltrexone plus OXY have been in separate trials involved in patients with chronic pain from osteoarthritis and idiopathic low back pain. High attrition and marginal differences between ultra-low-dose naltrexone plus OXY and OXY led to discontinuation of development. Prolonged-release (PR) naloxone combined with PR OXY demonstrates a consistent reduction in opioid-related bowel dysfunction in multiple randomized controlled trials. However, gastrointestinal side effects, including diarrhea, were increased in several trials with the combination compared with PR OXY alone. Analgesia appeared to be maintained although non-inferiority to PR OXY is not formally established. There were flaws to trial design and safety monitoring. Naltrexone has been combined with OXY in individual pellets encased in a capsule. This combination has been reported in a Phase II trial and is presently undergoing Phase III studies.

EXPERT OPINION

Due to the lack of efficacy the combination of altered low-dose naltrexone with oxycodone should cease in development. The combination of sustained release oxycodone plus naloxone reduces constipation with a consistent benefit. Safety has been suboptimally evaluated which is a concern. Although the drug is commercially available in several countries, ongoing safety monitoring particularly high doses would be important.

Effect of yohimbine stress on reacquisition of oxycodone seeking in rats

RATIONALE

Stress, a powerful precipitant of drug seeking during abstinence, may also accelerate the return to pathological patterns of intake after initial instances of drug reuse.

OBJECTIVE

To explore the effect of stress on a learning process underlying relapse, this study assessed the effect of yohimbine on reacquisition of oxycodone seeking.

METHODS

One hundred thirty-two male Sprague-Dawley rats underwent place conditioning with oxycodone (2 mg/kg, SC; ×6 days), extinction (vehicle × 6 days), and reconditioning with 0, 0.25, 2, or 5 mg/kg oxycodone (2 days). Yohimbine (0, 2.5, or 5 mg/kg, IP) was administered 30 min prior to reconditioning.

RESULTS

Pretreatment with 2.5 mg/kg yohimbine increased, while 5 mg/kg yohimbine decreased, reacquisition of oxycodone-induced place preference. A follow-up study (n = 30) further indicated that the effect of yohimbine was specific to reacquisition.

CONCLUSION

The observation that yohimbine can enhance reacquisition of oxycodone seeking supports the hypothesis that stress can facilitate learning processes involved in the unfolding of relapse.

Effects of combined opioids on pain and mood in mammals

The authors review the opioid literature for evidence of increased analgesia and reduced adverse side effects by combining mu-opioid-receptor (MOR) agonists, kappa-opioid-receptor (KOR) agonists, and nonselective low-dose-opioid antagonists (LD-Ant). We tested fentanyl (MOR agonist) and spiradoline (KOR agonist), singly and combined, against somatic and visceral pain models. Combined agonists induced additive analgesia in somatic pain and synergistic analgesia in visceral pain. Other investigators report similar effects and reduced tolerance and dependence with combined MOR agonist and KOR agonist. LD-Ant added to either a MOR agonist or KOR agonist markedly enhanced analgesia of either agonist. In accordance with other place-conditioning (PC) studies, our PC investigations showed fentanyl-induced place preference (CPP) and spiradoline-induced place aversion (CPA). We reduced fentanyl CPP with a low dose of spiradoline and reduced spiradoline CPA with a low dose of fentanyl. We propose combined MOR agonist, KOR agonist, and LD-Ant to produce superior analgesia with reduced adverse side effects, particularly for visceral pain.

Human abuse liability assessment of oxycodone combined with ultra-low-dose naltrexone

RATIONALE

Prescription opioid abuse has risen dramatically in the United States as clinicians have increased opioid prescribing for alleviation of both acute and chronic pain. Opioid analgesics with decreased risk for abuse are needed.

OBJECTIVE

Preclinical and clinical studies have shown that opioids combined with ultra-low-dose naltrexone (NTX) may have increased analgesic potency and have suggested reduced abuse or dependence liability. This study addressed whether addition of ultra-low-dose naltrexone might decrease the abuse liability of oxycodone (OXY) in humans.

MATERIALS AND METHODS

This double-blind, placebo-controlled study systematically examined the subjective and physiological effects of combining oral OXY and ultra-low NTX doses in 14 experienced opioid abusers. Seven acute drug conditions given at least 5 days apart were compared in a within-subject crossover design: placebo, OXY 20 mg, OXY 40 mg, plus each of the active OXY doses combined with 0.0001 and 0.001 mg NTX.

RESULTS

The methods were sensitive to detecting opioid effects on abuse liability indices, with significant differences between all OXY conditions and placebo as well as between 20 and 40 mg OXY doses on positive subjective ratings (e.g., "I feel a good drug effect" or "I like the drug"), on observer- and participant-rated opioid agonist effects, and on a drug-versus-money value rating. There were no significant differences or evident trends associated with the addition of either NTX dose on any abuse liability indices.

CONCLUSIONS

The addition of ultra-low-dose NTX to OXY did not decrease abuse liability of acutely administered OXY in experienced opioid abusers.

Naltrexone attenuation of conditioned but not primary reinforcement of nicotine in rats

RATIONALE

Opioid neurotransmission has been implicated in reinforcement-related processes for several drugs of abuse, including opiates, stimulants, and alcohol. However, less is known about its role in the motivational effects of nicotine and nicotine-associated environmental cues.

OBJECTIVE

This study investigated whether pretreatment with naltrexone, an opioid receptor antagonist, alters conditioned incentive salience of nicotine cues under two conditions: cue-induced reinstatement of nicotine-seeking after extinction and cue-maintained responding during extinction. The effect of naltrexone on nicotine self-administration during the maintenance phase was also examined.

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to self-administer nicotine (0.03 mg/kg/infusion, i.v.) on a fixed-ratio 5 schedule and associate a conditioned stimulus (CS) with each nicotine delivery. Once responding was extinguished by saline substitution for nicotine and omission of the CS, the reinstatement tests were conducted following subcutaneous administration of naltrexone (0, 0.25, 1, 2 mg/kg). In separate groups of rats, naltrexone (0, 2 mg/kg) was chronically given before each extinction sessions, where responses on the active lever resulted in presentations of the CS without nicotine infusion (saline substitution). Self-administration/naltrexone tests were conducted in different groups of rats receiving similar nicotine self-administration training.

RESULTS

Naltrexone significantly attenuated the CS-reinstated responding on the active, previously nicotine-reinforced lever in the reinstatement tests and the CS-maintained active lever responding during the extinction tests. In contrast, neither acute nor chronic naltrexone produced an effect on nicotine self-administration behavior.

CONCLUSIONS

These results indicate that activation of opioid receptors is implicated in mediation of the conditioned incentive properties of nicotine cues but not in the maintenance of nicotine self-administration. Therefore, these findings suggest that opioid receptor antagonists might have clinical potential for prevention of smoking relapse associated with exposure to environmental cues.

Naloxone's pentapeptide binding site on filamin A blocks Mu opioid receptor-Gs coupling and CREB activation of acute morphine

Chronic morphine causes the mu opioid receptor (MOR) to switch its coupling from Gi/o to Gs, resulting in excitatory signaling via both Gαs and its Gβγ dimer. Ultra-low-dose naloxone (NLX) prevents this switch and attenuates opioid tolerance and dependence. This protective effect is mediated via a high-affinity interaction of NLX to a pentapeptide region in c-terminal filamin A (FLNA), a scaffolding protein interacting with MOR. In organotypic striatal slice cultures, we now show that acute morphine induces a dose-dependent Go-to-Gs coupling switch at 5 and 15 min that resolves by 1 hr. The acute Gs coupling induced by 100 µM morphine was completely prevented by co-treatment with 100 pM NLX, (+)NLX, or naltrexone (NTX), or their pentapeptide binding site (FLNA_2561–2565), which we show can act as a decoy for MOR or bind to FLNA itself. All of these co-treatments presumably prevent the MOR–FLNA interaction. Since ultra-low-dose NTX also attenuates the addictive properties of opioids, we assessed striatal cAMP production and CREB phosphorylation at S¹³³. Correlating with the Gs coupling, acute morphine induced elevated cAMP levels and a several-fold increase in pS¹³³CREB that were also completely blocked by NLX, NTX or the FLNA pentapeptide. We propose that acute, robust stimulation of MOR causes an interaction with FLNA that allows an initially transient MOR–Gs coupling, which recovers with receptor recycling but persists when MOR stimulation is repeated or prolonged. The complete prevention of this acute, morphine-induced MOR–Gs coupling by 100 pM NLX/NTX or 10 µM pentapeptide segment of FLNA further elucidates both MOR signaling and the mechanism of action of ultra-low-dose NLX or NTX in attenuating opioid tolerance, dependence and addictive potential.

High-affinity naloxone binding to filamin a prevents mu opioid receptor-Gs coupling underlying opioid tolerance and dependence

Ultra-low-dose opioid antagonists enhance opioid analgesia and reduce analgesic tolerance and dependence by preventing a G protein coupling switch (Gi/o to Gs) by the mu opioid receptor (MOR), although the binding site of such ultra-low-dose opioid antagonists was previously unknown. Here we show that with approximately 200-fold higher affinity than for the mu opioid receptor, naloxone binds a pentapeptide segment of the scaffolding protein filamin A, known to interact with the mu opioid receptor, to disrupt its chronic opioid-induced Gs coupling. Naloxone binding to filamin A is demonstrated by the absence of [³H]-and FITC-naloxone binding in the melanoma M2 cell line that does not contain filamin or MOR, contrasting with strong [³H]naloxone binding to its filamin A-transfected subclone A7 or to immunopurified filamin A. Naloxone binding to A7 cells was displaced by naltrexone but not by morphine, indicating a target distinct from opioid receptors and perhaps unique to naloxone and its analogs. The intracellular location of this binding site was confirmed by FITC-NLX binding in intact A7 cells. Overlapping peptide fragments from c-terminal filamin A revealed filamin A_2561-2565 as the binding site, and an alanine scan of this pentapeptide revealed an essential mid-point lysine. Finally, in organotypic striatal slice cultures, peptide fragments containing filamin A_2561-2565 abolished the prevention by 10 pM naloxone of both the chronic morphine-induced mu opioid receptor–Gs coupling and the downstream cAMP excitatory signal. These results establish filamin A as the target for ultra-low-dose opioid antagonists previously shown to enhance opioid analgesia and to prevent opioid tolerance and dependence.

Naloxone attenuates incubated sucrose craving in rats

RATIONALE

Cue-induced craving precedes drug relapse and contributes to eating disorders. Opiate antagonists have been demonstrated to be effective at reducing cravings for drugs and food. Craving, as defined as responding for a stimulus previously associated with a reward, increases, or incubates, over forced abstinence in an animal model of relapse.

OBJECTIVES

This paper aims to determine anticraving effects of the opiate antagonist, naloxone, on the incubation of sucrose craving.

METHODS

106 male Long-Evans rats lever pressed for 10% sucrose solution 2 h/day for 10 days. On either day 1 or 30 of forced abstinence, rats responded in extinction for 6 h and then were injected (ip) with either saline or naloxone (0.001, 0.01, 0.1, 1, or 10 mg/kg). The rats then responded for 1 h for presentation of a tone + light cue previously presented with every sucrose delivery during self-administration training.

RESULTS

The rats responded more in extinction and following saline on day 30 vs day 1 (an incubation of craving). Except for a trend for a decrease in responding following 10 mg/kg on day 1, naloxone was primarily effective on day 30. On day 30, naloxone significantly reduced responding at all doses except for 0.1 mg/kg.

CONCLUSIONS

The time-dependent increase in sensitivity to an opiate antagonist is consistent with time-dependent changes in the opiate system following forced abstinence from sucrose. These changes may partly underlie the incubation of sucrose craving. In addition, these findings could be used to support the use of naloxone as an anticraving medication in protracted abstinence.

Oxytrex: an oxycodone and ultra-low-dose naltrexone formulation

Oxytrex (Pain Therapeutics, Inc.) is an oral opioid that combines a therapeutic amount of oxycodone with an ultra-low dose of the antagonist naltrexone. Animal data indicate that this combination minimizes the development of physical dependence and analgesic tolerance while prolonging analgesia. Oxytrex is in late-stage clinical development by Pain Therapeutics for the treatment of moderate-to-severe chronic pain. To evaluate the safety and efficacy of the oxycodone/naltrexone combination, three clinical studies have been conducted, one in healthy volunteers and the other two in patients with chronic pain. The putative mechanism of ultra-low-dose naltrexone is to prevent an alteration in G-protein coupling by opioid receptors that is associated with opioid tolerance and dependence. Opioid agonists are initially inhibitory but become excitatory through constant opioid receptor activity. The agonist/antagonist combination of Oxytrex may reduce the conversion from an inhibitory to an excitatory receptor, thereby decreasing the development of tolerance and physical dependence.

Gbetagamma that interacts with adenylyl cyclase in opioid tolerance originates from a Gs protein

We previously demonstrated that chronic morphine induces a change in G protein coupling by the mu opioid receptor (MOR) from Gi/o to Gs, concurrent with the instatement of an interaction between Gbetagamma and adenylyl cyclase types II and IV. These two signaling changes confer excitatory effects on the cell in place of the typical inhibition by opioids and are associated with morphine tolerance and dependence. Both signaling changes and these behavioral manifestations of chronic morphine are attenuated by cotreatment with ultra-low-dose naloxone. In the present work, using striatum from chronic morphine-treated rats, we isotyped the Gbeta within Gs and Go heterotrimers that coupled to MOR and compared these to the Gbeta isotype of the Gbetagamma that interacted with adenylyl cyclase II or IV after chronic morphine treatment. Isotyping results show that chronic morphine causes a Gs heterotrimer associated with MOR to release its Gbetagamma to interact with adenylyl cyclase. These data suggest that the switch to Gs coupling by MOR in response to chronic morphine, which is attenuated by ultra-low-dose opioid antagonist cotreatment, leads to a two-pronged stimulation of adenylyl cyclase utilizing both Galpha and Gbetagamma subunits of the Gs protein novel to this receptor.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Diffusion of naltrexone across reconstituted human oral epithelium and histomorphological features

In transbuccal absorption a major limitation could be the low permeability of the mucosa which implies low drug bioavailability. The ability of naltrexone hydrochloride (NLX) to penetrate a resembling histologically human buccal mucosa was assessed and the occurrence of any histomorphological changes observed. We used reconstituted human oral (RHO) non-keratinised epithelium as mucosal section and a Transwell diffusion cells system as bicompartmental model. Buccal permeation was expressed in terms of drug flux (J(s)) and permeability coefficients (K(p)). Data were collected using both artificial and natural human saliva. The main finding was that RHO does not restrain NLX permeation. Drug transport across the epithelium was observed also in presence of various concentrations of penetration enhancers, without any significant differences. On the contrary, the flux throughout the mucosa was extensively affected by iontophoresis. Histologically, no sign of flogosis was observed in any specimen under experiment without iontophoresis, whereas cytoarchitectural changes, up to nuclear pycnosis or cellular swelling, were determined as a consequence of the application of electric fields.

Ultra-low-dose naltrexone suppresses rewarding effects of opiates and aversive effects of opiate withdrawal in rats

RATIONALE

Ultra-low-dose opioid antagonists enhance opiate analgesia and attenuate tolerance and withdrawal.

OBJECTIVES

To determine whether ultra-low-dose naltrexone (NTX) coadministration alters the rewarding effects of opiates or the aversive effects of opiate withdrawal.

METHODS

We used the conditioned place preference (CPP) and conditioned place aversion (CPA) paradigms to assess whether ultra-low-dose NTX alters the acute rewarding effects of oxycodone or morphine, or the aversive aspect of withdrawal from either drug. To assess the dose response for ultra-low-dose NTX, a range of NTX doses (0.03-30 ng/kg) was tested in the oxycodone CPP experiment. In order to avoid tolerance or sensitization effects, we used single conditioning sessions and female rats, as females are more sensitive to the conditioning effects of these drugs.

RESULTS

Ultra-low-dose NTX (5 ng/kg) blocked the CPP to morphine (5 mg/kg) and the CPA to withdrawal from chronic morphine (5 mg/kg, for 7 days). Coadministration of ultra-low-dose NTX (30 pg/kg) also blocked the CPA to withdrawal from chronic oxycodone administration (3 mg/kg, for 7 days). The effects of NTX on the CPP to oxycodone (3 mg/kg) revealed a biphasic dose response. The two lowest doses (0.03 and 0.3 ng/kg) blocked the CPP, the middle dose (3 ng/kg) was ineffective, and oxycodone combined with the highest dose (30 ng/kg) produced a trend toward a CPP.

CONCLUSIONS

Ultra-low-dose NTX coadministration blocks the acute rewarding effects of analgesic doses of oxycodone or morphine as well as the anhedonia of withdrawal from chronic administration.