Oxycodone-induced tolerance to respiratory depression: reversal by ethanol, pregabalin and protein kinase C inhibition

Characterization and validation of a spontaneous acute and protracted oxycodone withdrawal model in male and female mice

Opioid use disorder (OUD) is a serious health problem that may lead to physical dependence, in addition to affective disorders. Preclinical models are essential for studying the neurobiology of and developing pharmacotherapies to treat these problems. Historically, chronic morphine injections have most often been used to produce opioid-dependent animals, and withdrawal signs indicative of dependence were precipitated by administering an opioid antagonist. In the present studies, we have developed and validated a model of dependence on oxycodone (a widely prescribed opioid) during spontaneous withdrawal in male and female C57BL/6J mice. Dependence was induced by chronically administering oxycodone through osmotic minipumps at different doses for 7 days. Somatic withdrawal signs were measured after 3, 6, 24, and 48 h following minipump removal. Additionally, sensitivity to mechanical, thermal, and cold stimuli, along with anxiety-like behavior, were also measured. Our results indicated that spontaneous withdrawal following discontinuation of oxycodone produced an increase in total withdrawal signs after 60 and 120 mg/kg/day regimens of oxycodone administration. These signs were reversed by the administration of clinically approved medications for OUD. In general, both female and male mice showed similar profiles of somatic signs of spontaneous withdrawal. Spontaneous withdrawal also resulted in mechanical and cold hypersensitivity lasting for 24 and 14 days, respectively, and produced anxiety-like behaviors after 2 and 3 weeks following oxycodone removal. These results help validate a new model of oxycodone dependence, including the temporally distinct emergence of somatic, hyperalgesic, and anxiety-like behaviors, potentially useful for mechanistic and translational studies of opioid dependence.

Assessment of the potential of novel and classical opioids to induce respiratory depression in mice

BACKGROUND AND PURPOSE

Opioid-induced respiratory depression limits the use of μ-opioid receptor agonists in clinical settings and is the main cause of opioid overdose fatalities. The relative potential of different opioid agonists to induce respiratory depression at doses exceeding those producing analgesia is understudied despite its relevance to assessments of opioid safety. Here we evaluated the respiratory depressant and anti-nociceptive effects of three novel opioids and relate these measurements to their in vitro efficacy.

EXPERIMENTAL APPROACH

Respiration was measured in awake, freely moving male CD-1 mice using whole body plethysmography. Anti-nociception was measured using the hot plate test. Morphine, oliceridine and tianeptine were administered intraperitoneally, whereas methadone, oxycodone and SR-17018 were administered orally. Receptor activation and arrestin-3 recruitment were measured in HEK293 cells using BRET assays.

KEY RESULTS

Across the dose ranges examined, all opioids studied depressed respiration in a dose-dependent manner, with similar effects at the highest doses, and with tianeptine and oliceridine showing reduced duration of effect, when compared with morphine, oxycodone, methadone and SR-17018. When administered at doses that induced similar respiratory depression, all opioids induced similar anti-nociception, with tianeptine and oliceridine again showing reduced duration of effect. These data were consistent with the in vitro agonist activity of the tested compounds.

CONCLUSION AND IMPLICATIONS

In addition to providing effective anti-nociception, the novel opioids, oliceridine, tianeptine and SR-17018 depress respiration in male mice. However, the different potencies and kinetics of effect between these novel opioids may be relevant to their therapeutic application in different clinical settings.

An automated respiratory data pipeline for waveform characteristic analysis

Comprehensive and accurate analysis of respiratory and metabolic data is crucial to modelling congenital, pathogenic and degenerative diseases converging on autonomic control failure. A lack of tools for high-throughput analysis of respiratory datasets remains a major challenge. We present Breathe Easy, a novel open-source pipeline for processing raw recordings and associated metadata into operative outcomes, publication-worthy graphs and robust statistical analyses including QQ and residual plots for assumption queries and data transformations. This pipeline uses a facile graphical user interface for uploading data files, setting waveform feature thresholds and defining experimental variables. Breathe Easy was validated against manual selection by experts, which represents the current standard in the field. We demonstrate Breathe Easy's utility by examining a 2-year longitudinal study of an Alzheimer's disease mouse model to assess contributions of forebrain pathology in disordered breathing. Whole body plethysmography has become an important experimental outcome measure for a variety of diseases with primary and secondary respiratory indications. Respiratory dysfunction, while not an initial symptom in many of these disorders, often drives disability or death in patient outcomes. Breathe Easy provides an open-source respiratory analysis tool for all respiratory datasets and represents a necessary improvement upon current analytical methods in the field. KEY POINTS: Respiratory dysfunction is a common endpoint for disability and mortality in many disorders throughout life. Whole body plethysmography in rodents represents a high face-value method for measuring respiratory outcomes in rodent models of these diseases and disorders. Analysis of key respiratory variables remains hindered by manual annotation and analysis that leads to low throughput results that often exclude a majority of the recorded data. Here we present a software suite, Breathe Easy, that automates the process of data selection from raw recordings derived from plethysmography experiments and the analysis of these data into operative outcomes and publication-worthy graphs with statistics. We validate Breathe Easy with a terabyte-scale Alzheimer's dataset that examines the effects of forebrain pathology on respiratory function over 2 years of degeneration.

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.

Systematic assessment of non-medical use of prescription drugs using doctor-shopping indicators: A nation-wide, repeated cross-sectional study

AIMS

The aim of this study was to present the first nation-wide, systematic, repeated assessment of doctor-shopping (i.e. visiting multiple physicians to be prescribed the same drug) during 10 years for more than 200 psychoactive prescription drugs in the 67 million inhabitants in France.

DESIGN

This was a nation-wide, repeated cross-sectional study.

SETTING AND PARTICIPANTS

Data are from the French National Health Data System in 2010, 2015 and 2019 for 214 psychoactive prescription drugs (i.e. anaesthetics, analgesics, antiepileptics, anti-Parkinson drugs, psycholeptics, psychoanaleptics, other nervous system drugs and antihistamines for systemic use).

MEASUREMENTS

The detection and quantification of doctor-shopping relied upon an algorithm that detects overlapping prescriptions from repeated visits to different physicians. We used two doctor-shopping indicators aggregated at population level for each drug dispensed to more than 5000 patients: (i) the quantity doctor-shopped, expressed in defined daily doses (DDD), which measures the total quantity doctor-shopped by the study population for a given drug; and (ii) the proportion doctor-shopped, expressed as a percentage, which standardizes the quantity doctor-shopped according to the use level of the drug.

FINDINGS

The analyses included approximately 200 million dispensings to approximately 30 million patients each year. Opioids (e.g. buprenorphine, methadone, morphine, oxycodone and fentanyl), benzodiazepines and non-benzodiazepine hypnotics (Z-drugs) (e.g. diazepam, oxazepam, zolpidem and clonazepam) had the highest proportions doctor-shopped during the study period. In most cases, the proportion and the quantity doctor-shopped increased for opioids and decreased for benzodiazepines and Z-drugs. Pregabalin had the sharpest increase in the proportion doctor-shopped (from 0.28 to 1.40%), in parallel with a sharp increase in the quantity doctor-shopped (+843%, from 0.7 to 6.6 DDD/100 000 inhabitants/day). Oxycodone had the sharpest increase in the quantity doctor-shopped (+1000%, from 0.1 to 1.1 DDD/100 000 inhabitants/day), in parallel with a sharp increase in the proportion doctor-shopped (from 0.71 to 1.41%). Detailed results for all drugs during the study period can be explored interactively at: https://soeiro.gitlab.io/megadose/.

CONCLUSIONS

In France, doctor-shopping occurs for many drugs from many pharmacological classes, and mainly involves opioid maintenance drugs, some opioids analgesics, some benzodiazepines and Z-drugs and pregabalin.

Effects of Gabapentinoids on Heroin-Induced Ventilatory Depression and Reversal by Naloxone

Despite an increasing prevalence of gabapentinoids (gabapentin and pregabalin) in opioid overdose deaths, little research has evaluated potentially harmful interactions between gabapentinoids and opioids. This study sought to determine the effects of gabapentinoids on the ventilatory depressive effects of heroin and their reversal by naloxone. Rats were given gabapentin, pregabalin, or saline prior to receiving increasing doses of heroin while ventilation was monitored using whole-body plethysmography. In some sessions naloxone was administered following the largest dose of heroin. The primary outcomes of this study were minute volume and Pause. Heroin dose-dependently reduced minute volume and increased Pause. Administration of naloxone dose-dependently reversed the effects of heroin on ventilation. Gabapentinoids did not alter the ventilatory depressive effects of heroin alone but reduced the potency of naloxone to reverse heroin-induced ventilatory depression. These preliminary findings emphasize the need for further research evaluating interactions between gabapentinoids and opioids related to substance misuse and overdose.

A novel G protein-biased agonist at the μ opioid receptor induces substantial receptor desensitisation through G protein-coupled receptor kinase

BACKGROUND AND PURPOSE

G protein-biased μ opioid receptor agonists have the potential to induce less receptor desensitisation and tolerance than balanced opioids. Here, we investigated if the cyclic endomorphin analogue Tyr-c[D-Lys-Phe-Tyr-Gly] (Compound 1) is a G protein-biased μ agonist and characterised its ability to induce rapid receptor desensitisation in mammalian neurones.

EXPERIMENTAL APPROACH

The signalling and trafficking properties of opioids were characterised using bioluminescence resonance energy transfer assays, enzyme-linked immunosorbent assay and phosphosite-specific immunoblotting in human embryonic kidney 293 cells. Desensitisation of opioid-induced currents were studied in rat locus coeruleus neurones using whole-cell patch-clamp electrophysiology. The mechanism of Compound 1-induced μ receptor desensitisation was probed using kinase inhibitors.

KEY RESULTS

Compound 1 has similar intrinsic activity for G protein signalling as morphine. As predicted for a G protein-biased μ agonist, Compound 1 induced minimal agonist-induced internalisation and phosphorylation at intracellular μ receptor serine/threonine residues known to be involved in G protein-coupled receptor kinase (GRK)-mediated desensitisation. However, Compound 1 induced robust rapid μ receptor desensitisation in locus coeruleus neurons, to a greater degree than morphine. The extent of Compound 1-induced desensitisation was unaffected by activation or inhibition of protein kinase C (PKC) but was significantly reduced by inhibition of GRK.

CONCLUSION AND IMPLICATIONS

Compound 1 is a novel G protein-biased μ agonist that induces substantial rapid receptor desensitisation in mammalian neurons. Surprisingly, Compound 1-induced desensitisation was demonstrated to be GRK dependent despite its G protein bias. Our findings refute the assumption that G protein-biased agonists will evade receptor desensitisation and tolerance.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Metabolic interactions of benzodiazepines with oxycodone ex vivo and toxicity depending on usage patterns in an animal model

BACKGROUND AND PURPOSE

Opioids and benzodiazepines are frequently combined in medical as well as in non-medical contexts. At high doses, such combinations often result in serious health complications attributed to pharmacodynamics interactions. Here, we investigate the contribution of the metabolic interactions between oxycodone, diazepam and diclazepam (a designer benzodiazepine) in abuse/overdose conditions through ex vivo, in vivo and in silico approaches.

EXPERIMENTAL APPROACH

A preparation of pooled human liver microsomes was used to study oxycodone metabolism in the presence or absence of diazepam or diclazepam. In mice, diazepam or diclazepam was concomitantly administered with oxycodone to mimic acute intoxication. Diclazepam was introduced on Day 10 in mice continuously infused with oxycodone for 15 days to mimic chronic intoxication. In silico modelling was used to study the molecular interactions of the three drugs with CYP3A4 and 2D6.

KEY RESULTS

In mice, in acute conditions, both diazepam and diclazepam inhibited the metabolism of oxycodone. In chronic conditions and at pharmacologically equivalent doses, diclazepam drastically enhanced the production of oxymorphone. In silico, the affinity of benzodiazepines was higher than oxycodone for CYP3A4, inhibiting oxycodone metabolism through CYP3A4. Oxycodone metabolism is likely to be diverted towards CYP2D6.

CONCLUSION AND IMPLICATIONS

Acute doses of diazepam or diclazepam result in the accumulation of oxycodone, whereas chronic administration induces the accumulation of oxymorphone, the toxic metabolite. This suggests that overdoses of opioids in the presence of benzodiazepines are partly due to metabolic interactions, which in turn explain the patterns of toxicity dependent on usage.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Understanding and countering opioid-induced respiratory depression

Respiratory depression is the proximal cause of death in opioid overdose, yet the mechanisms underlying this potentially fatal outcome are not well understood. The goal of this review is to provide a comprehensive understanding of the pharmacological mechanisms of opioid-induced respiratory depression, which could lead to improved therapeutic options to counter opioid overdose, as well as other detrimental effects of opioids on breathing. The development of tolerance in the respiratory system is also discussed, as are differences in the degree of respiratory depression caused by various opioid agonists. Finally, potential future therapeutic agents aimed at reversing or avoiding opioid-induced respiratory depression through non-opioid receptor targets are in development and could provide certain advantages over naloxone. By providing an overview of mechanisms and effects of opioids in the respiratory network, this review will benefit future research on countering opioid-induced respiratory depression. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

The Role of Gabapentinoids in the Substance Use Pattern of Adult Germans Seeking Inpatient Detoxification Treatment - A Pilot Study

To shed more light on the addictive power of the gabapentinoids (GPTs) gabapentin and pregabalin, we performed a structured face-to-face interview with GPT-users about DSM-IV-dependence-criteria (sedatives), consume-motives and cessation-needs. Among 100 patients consecutively admitted to a detoxification-ward, fifteen (15%) reported lifetime GPT-use (18-50 years old, 2 females): seven (7%) used gabapentin, twelve (12%) pregabalin and four had lifetime experiences with both GPTs. Of the seven gabapentin-users, three patients were dependent including one person with a spontaneous remission. Of the 12 pregabalin-users, five were dependent, including two persons with a spontaneous remission. Fourteen of fifteen cases reported GPT-use side-by-side with an opioid-use, mostly for sparing opioids. Twelve GPT-users additionally co-used benzodiazepines. In no case, a GPT was the reason for detoxification treatment or reported to be involved in an emergency event. Altogether, every 7^th patient (n = 15) of our inpatient detoxification-seeking sample reported GPT-use including 50% (n = 8) who were dependent. Among them, 35% (3/8) had been already spontaneously remitted. As GPT-users reported no cession-need and the vast majority were primarily affected by co-occurring opioid- and benzodiazepine-addiction, we assume that GPTs more likely played a bystander-role than mediating the addictive behavior of this population with multiple recreational drug use experience.

Role of Acetaldehyde in Ethanol Reversal of Tolerance to Morphine-Induced Respiratory Depression in Mice

Background: Opioid users regularly consume other drugs such as alcohol (ethanol). Acute administration of ethanol rapidly reverses tolerance to morphine-induced respiratory depression. However, recent research has suggested that the primary metabolite of ethanol, acetaldehyde, may play a key role in mediating the CNS effects seen after ethanol consumption. This research investigated the role of acetaldehyde in ethanol reversal of tolerance to morphine-induced respiratory depression.

Methods: Tolerance was induced in mice by 6-days implantation of a 75 mg morphine pellet with control mice implanted with a placebo pellet. Tolerance was assessed by acute morphine administration on day 6 and respiration measured by plethysmography. Levels of acetaldehyde were inhibited or enhanced by pre-treatments with the acetaldehyde chelator D-penicillamine and the inhibitor of acetaldehyde dehydrogenase disulfiram respectively.

Results: Morphine pellet implanted mice displayed tolerance to an acute dose of morphine compared to placebo pellet implanted controls. Acute acetaldehyde administration dose-dependently reversed tolerance to morphine respiratory depression. As previously demonstrated, ethanol reversed morphine tolerance, and this was inhibited by D-penicillamine pre-treatment. An acute, low dose of ethanol that did not significantly reverse morphine tolerance was able to do so following disulfiram pre-treatment.

Conclusion: These data suggest that acetaldehyde, the primary metabolite of ethanol, is responsible for the reversal of morphine tolerance observed following ethanol administration.

Mice with high FGF21 serum levels had a reduced preference for morphine and an attenuated development of acute antinociceptive tolerance and physical dependence

Because of increased opioid misuse, there is a need to identify new targets for minimizing opioid tolerance, and physical and psychological dependence. Previous studies showed that fibroblast growth factor 21 (FGF21) decreased alcohol and sweet preference in mice. In this study, FGF21-transgenic (FGF21-Tg) mice, expressing high FGF21 serum levels, and wildtype (WT) C57BL/6J littermates were treated with morphine and saline to determine if differences exist in their physiological and behavioral responses to opioids. FGF21-Tg mice displayed reduced preference for morphine in the conditioned place preference assay compared to WT littermates. Similarly, FGF21-Tg mice had an attenuation of the magnitude and rate of acute morphine antinociceptive tolerance development, and acute and chronic morphine physical dependence, but exhibited no change in chronic morphine antinociceptive tolerance. The ED50 values for morphine-induced antinociception in the 55 °C hot plate and the 55 °C warm-water tail withdrawal assays were similar in both strains of mice. Likewise, FGF21-Tg and WT littermates had comparable responses to morphine-induced respiratory depression. Overall, FGF21-Tg mice had a decrease in the development of acute analgesic tolerance, and the development of physical dependence, and morphine preference. FGF21 and its receptor have therapeutic potential for reducing opioid withdrawal symptoms and craving, and augmenting opioid therapeutics for acute pain patients to minimize tolerance development.

Antinociceptive and Antipruritic Effects of HSK21542, a Peripherally-Restricted Kappa Opioid Receptor Agonist, in Animal Models of Pain and Itch

Kappa opioid receptor (KOR) agonists have been promising therapeutic candidates, owing to their potential for relieving pain and treating intractable pruritus. Although lacking morphine-like central nervous system (CNS) effects, KOR agonists do elicit sedation, dysphoria and diuresis which seriously impede their development. Peripherally-restricted KOR agonists have a poor ability to penetrate into the CNS system, so that CNS-related adverse effects can be ameliorated or even abolished. However, the only approved peripherally-restricted KOR agonist CR845 remains some frequent CNS adverse events. In the present study, we aim to address pharmacological profiles of HSK21542, with an expectation to provide a safe and effective alternative for patients who are suffering from pain and pruritus. The in vitro experimental results showed that HSK21542 was a selective and potent KOR agonist with higher potency than CR845, and had a brain/plasma concentration ratio of 0.001, indicating its peripheral selectivity. In animal models of pain, HSK21542 significantly inhibited acetic acid-, hindpaw incision- or chronic constriction injury-induced pain-related behaviors, and the efficacy was comparable to CR845 at 15 min post-dosing. HSK21542 had a long-lasting analgesic potency with a median effective dose of 1.48 mg/kg at 24 h post-drug in writhing test. Meanwhile, the antinociceptive activity of HSK21542 was effectively reversed by a KOR antagonist nor-binaltorphimine. In addition, HSK21542 had powerful antipruritic activities in compound 48/80-induced itch model. On the other hand, HSK21542 had a weak ability to produce central antinociceptive effects in a hot-plate test and fewer effects on the locomotor activity of mice. HSK21542 didn't affect the respiratory rate of mice. Therefore, HSK21542 might be a safe and effective KOR agonist and promising candidate for treating pain and pruritus.

Respiratory depressant effects of fentanyl analogs are opioid receptor-mediated

Opioid-related fatalities involving synthetic opioids have reached unprecedented levels. This study evaluated the respiratory depressant effects of seven fentanyl analogs that have either emerged in the illicit drug supply or been identified in toxicological analyses following fatal or non-fatal intoxications. Adult male Swiss Webster mice were administered fentanyl analogs (isobutyrylfentanyl, crotonylfentanyl, para-methoxyfentanyl, para-methoxybutyrylfentanyl, 3-furanylfentanyl, thiophenefentanyl, and benzodioxolefentanyl) and their effects on minute volume as compared to mu-opioid receptor (MOR) agonist standards (fentanyl, morphine, and buprenorphine) were measured using whole body plethysmography (WBP). All drugs elicited significant (p ≤ 0.05) hypoventilation relative to vehicle for at least one dose tested: morphine (1, 3.2, 10, 32 mg/kg), buprenorphine, (0.032, 0.1, 0.32, 1, 3.2 mg/kg), fentanyl (0.0032, 0.01, 0.032, 0.1, 1, 32 mg/kg), isobutyrylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), crotonylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxyfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxybutyrylfentanyl (0.32, 1, 3.2, 10 mg/kg), 3-furanylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), thiophenefentanyl (1, 3.2, 10, 32, 100 mg/kg), and benzodioxolefentanyl (3.2, 10, 32, 100 mg/kg). The ED₅₀ values for hypoventilation showed a rank order of potency as follows: fentanyl (ED₅₀ = 0.96 mg/kg) > 3-furanylfentanyl (ED₅₀ = 2.60 mg/kg) > crotonylfentanyl (ED₅₀ = 2.72 mg/kg) > para-methoxyfentanyl (ED₅₀ = 3.31 mg/kg) > buprenorphine (ED₅₀ = 10.8 mg/kg) > isobutyrylfentanyl (ED₅₀ = 13.5 mg/kg) > para-methoxybutyrylfentanyl (ED₅₀ = 16.1 mg/kg) > thiophenefentanyl (ED₅₀ = 18.0 mg/kg) > morphine (ED₅₀ = 55.3 mg/kg) > benzodioxolefentanyl (ED₅₀ = 10,168 mg/kg). A naloxone pretreatment (10 mg/kg) attenuated the hypoventilatory effects of all drugs. These results establish that the respiratory depressant effects of these fentanyl analogs are at least in part mediated by the MOR.

Different Doses of Oxycodone for Endoscopic Injection Sclerotherapy of Esophageal Varices

Background

The aim of the present study was to evaluate the effects of different doses of oxycodone during endoscopic injection sclerotherapy (EIS) for esophageal varices with painless sclerosing agents.

Material/Methods

A total of 119 patients were randomly divided into 3 groups: Group A, midazolam and 0.075 mg/kg oxycodone (n=40); Group B, midazolam and 0.1 mg/kg oxycodone (n=40); and Group C, midazolam and 0.125 mg/kg oxycodone (n=39). The main observation index was the incidence of body movement during the perioperative period. The secondary indices were additional propofol usage; postoperative analgesic usage; other adverse effects, such as hypoxia, myoclonus, and cough; and satisfaction scores for surgeons and patients.

Results

The incidence rates for body movement during the perioperative period in groups A, B, and C were 33%, 13%, and 0, respectively (P<0.001). The satisfaction scores for surgeons and patients were highest in Group C (0.125 mg/kg oxycodone). The incidence rates for hypoxia before EIS were 15%, 8%, and 33% (P=0.026) and during EIS were 23%, 3%, and 0% (P<0.001), respectively. There were no significant between-group differences with respect to other adverse effects.

Conclusions

The ideal dose of oxycodone for perioperative analgesia during EIS for esophageal varices is 0.125 mg/kg.

Sexually dimorphic neuroimmune response to chronic opioid treatment and withdrawal

Opioid use disorder is a leading cause of morbidity and mortality in the United States. Increasing pre-clinical and clinical evidence demonstrates sex differences in opioid use and dependence. However, the underlying molecular mechanisms contributing to these effects, including neuroinflammation, are still obscure. Therefore, in this study, we investigated the effect of oxycodone exposure and withdrawal on sex- and region-specific neuroimmune response. Real-time PCR and multiplex cytokine array analysis demonstrated elevated neuroinflammation with increased pro-inflammatory cytokine levels, and aberrant oligodendroglial response in reward neurocircuitry, following withdrawal from chronic oxycodone treatment. Chronic oxycodone and withdrawal treated male mice had lower mRNA expression of TMEM119 along with elevated protein levels of pro-inflammatory cytokines/chemokines and growth factors (IL-1β, IL-2, IL-7, IL-9, IL-12, IL-15, IL17, M-CSF, VEGF) in the prefrontal cortex (PFC) as compared to their female counterparts. In contrast, reduced levels of pro-inflammatory cytokines/chemokines (IL-1β, IL-6, IL-9, IL-12, CCL11) was observed in the nucleus accumbens (NAc) of oxycodone and withdrawal-treated males as compared to female mice. No treatment specific effects were observed on the mRNA expression of putative microglial activation markers (Iba1, CD68), but an overall sex specific decrease in the mRNA expression of Iba1 and CD68 was found in the PFC and NAc of male mice as compared to females. Moreover, a sex and region-specific increase in the mRNA levels of oligodendrocyte lineage markers (NG2, Sox10) was also observed in oxycodone and withdrawal treated animals. These findings may open a new avenue for the development of sex-specific precision therapeutics for opioid dependence by targeting region-specific neuroimmune signaling.

Prevalence of and Factors Associated with Gabapentinoid Use and Misuse Among Texas Medicaid Recipients

BACKGROUND AND OBJECTIVES

Gabapentin and pregabalin have been considered relatively safe opioid-sparing adjuncts for pain management. However, rising prescribing trends, presence of gabapentinoids in opioid-related overdoses, and the growing body of evidence regarding gabapentinoid misuse and abuse, have caused gabapentinoids to emerge as a drug class of public health concern. This study aimed to assess the prevalence of, and factors associated with gabapentinoid use and misuse.

METHODS

This retrospective study of Texas Medicaid data from 1/1/2012 to 30/8/2016 included patients aged 18-63 years at index date, with ≥ 1 gabapentinoid prescription, and continuously enrolled 6 months pre-index and 12 months post-index. Gabapentinoid misuse was defined as ≥ 3 claims exceeding daily doses of 3600 mg for gabapentin and 600 mg for pregabalin. Age, gender, concurrent opioid use, neuropathic pain diagnoses and gabapentinoid type were independent variables. Descriptive and inferential statistics were used.

RESULTS

Of included subjects (N = 39,000), 0.2% (N = 81) met study criteria for gabapentinoid misuse. Overall, the majority (76.4%) of gabapentinoid users were aged 41-63 years with a mean ± SD age of 48.2 ± 10.7 years. Those patients meeting the study criteria for gabapentinoid misuse were significantly younger (45.1 ± 11.0 vs 48.2 ± 10.7, p = 0.0084). Majority of the study sample was female (68.1%). However, a significantly higher proportion of males met the study criteria for gabapentinoid misuse compared to females (0.3% vs 0.2%, p = 0.0079). Approximately one-half (51.9%) of the study sample had neuropathic pain, and gabapentinoid misuse was significantly higher in neuropathic pain patients compared to those without neuropathic pain (0.3% vs 0.1%, p = 0.0078). Over three-quarters (77.4%) of patients were using gabapentin; however, gabapentinoid misuse was significantly higher among pregabalin users (0.4% vs 0.2%, p = 0.0003). Approximately 20% (17.3%) of gabapentinoid users had ≥ 90 days of concurrent opioid use. However, there was no significant difference in gabapentinoid misuse among patients with concurrent opioid use compared to patients without (0.3% vs 0.2%, p = 0.1440). Factors significantly associated with misuse included: male sex (odds ratio [OR] 0.486; 95% confidence interval [CI] 0.313-0.756; p = 0.0013); neuropathic pain (OR 2.065; 95% CI 1.289-3.308; p = 0.0026); and pregabalin versus gabapentin use (OR 2.337, 95% CI 1.492-3.661; p = 0.0002). Concurrent opioid use was not significantly associated with gabapentinoid misuse (OR 1.542, 95% CI 0.920-2.586; p = 0.1006).

CONCLUSION

Prevalence of gabapentinoid misuse was low (0.2%) among Texas Medicaid recipients. Younger age, male gender, neuropathic pain diagnosis and pregabalin use were significantly associated with higher levels of gabapentinoid misuse.

Gabapentinoid Abuse in France: Evidence on Health Consequences and New Points of Vigilance

Introduction: Gabapentinoid drugs (gabapentin and pregabalin) are widely used worldwide for epileptic and pain disorders. First signals of gabapentinoid abuse occurred in the last decade. This study aims to describe clinical characteristics of gabapentinoid use related disorders and health consequences in France. Materials and Methods: We designed a multisource investigation reviewing data reported to the French Addictovigilance Network (FAN) with pregabalin and gabapentin from 2010 to 2019. Information was obtained through the analysis of Spontaneous Reports (SRs) notified by health professionals and the pharmacoepidemiological surveys OSIAP (suspicious prescriptions forms indicators of potential abuse), OPPIDUM (observation of illicit drugs and misuse of psychotropic medications), DRAMES (death related to prescription drugs and other substances), and DTA (toxic deaths due to analgesics). Results: Over 2010-2019 period, were collected: (i) 265 SRs (258 pregabalin; 7 gabapentin); (ii) 816 forged prescription forms (805 pregabalin, 10 gabapentin, 1 involving both drugs); (iii) 145 cases of gabapentinoid use in people who use drugs (121 pregabalin; 24 gabapentin) and (iv) 31 cases of gabapentinoid-related deaths (25 pregabalin; 6 gabapentin). Risk factors of gabapentinoid abuse were opioid use disorders or psychiatric history, but cases of primary abuse in subjects without any substance abuse history were observed. Adverse outcomes concern almost exclusively pregabalin, with coma, dyspnea, convulsion, and conduction disorders. Treatment demands increased from 10.6% in 2018 to 23.1% in 2019, with pregabalin cited as the first substance leading to addictological care in the 2019 OPPIDUM survey. Gabapentinoid-related deaths increased over time. Pregabalin has become the first drug mentioned in forged prescriptions in 2019 (23.8% of OSIAP), while it ranked at the 15th position in 2017 (2.6%). Discussion: This study shows the importance of addictovigilance monitoring for gabapentinoids. Addictovigilance data helped to make visible the gabapentinoid-abuse related health harms (hospitalization for serious neurologic, psychiatric or cardiac effects, requests for addictological support and deaths) and to confirm the intrinsic abuse potential of pregabalin. These data highlight new points of vigilance considering observed primary abuse. At this point in France, the risk of abuse and related complications is very apparent with pregabalin. Still, it is identical to that observed elsewhere with gabapentin.

Association of Gabapentinoids With the Risk of Opioid-Related Adverse Events in Surgical Patients in the United States

IMPORTANCE

The use of gabapentinoids in multimodal postoperative analgesia is increasing; however, when coadministered with opioids, these drugs may potentiate central nervous system and respiratory depression.

OBJECTIVE

To evaluate the association between perioperative coadministration of gabapentinoids and opioids with inpatient opioid-related adverse events in surgical patients.

DESIGN, SETTING, AND PARTICIPANTS

This cohort study used propensity score trimming, stratification, and weighting of adults admitted for a major surgery between October 2007 and December 2017 who were treated with opioids on the day of surgery and included in the Premier Research database. Data analysis was conducted from February to April 2020.

EXPOSURE

Gabapentinoids (gabapentin or pregabalin) coadministered with opioids starting the day of surgery vs opioid therapy without gabapentinoids.

MAIN OUTCOMES AND MEASURES

Primary outcome was opioid overdose. Secondary outcomes included respiratory complications, unspecified adverse effects of opioid use, and a composite of these 3 outcomes. Patients were followed up for as long as 30 days from the day of surgery until deviation from the initial treatment regimen or discharge.

RESULTS

Gabapentinoids with opioids were administered to 892 484 of 5 547 667 eligible admissions (16.1%; mean [SD] age, 63.5 [11.8] years; 353 315 [39.6%] men). Among the 4 655 183 patients who received opioids only, the mean (SD) age was 63.7 (14.7) years, and 1 913 284 (41.1%) were men. Overall, 441 overdose events were identified, with absolute risks of 1.4 per 10 000 patients with gabapentinoid exposure and 0.7 per 10 000 patients receiving opioids only. Following propensity score trimming, the cohort included 737 383 patients exposed to gabapentinoids and 3 002 480 patients receiving opioids only. The primary analysis yielded the adjusted hazard ratio of 1.95 (95% CI, 1.49-2.55), and the number needed to treat for an additional overdose to occur was 16 914 patients (95% CI, 11 556-31 537 patients). Adjusted hazard ratios for secondary outcomes were 1.68 (95% CI, 1.59-1.78) for respiratory complications, 1.77 (95% CI, 1.61-1.93) for unspecified adverse effects of opioids, and 1.70 (95% CI, 1.62-1.79) for the composite outcome. The results were consistent across sensitivity analyses and subgroups identified by key clinical factors.

CONCLUSIONS AND RELEVANCE

In this real-world cohort study of patients who underwent major surgery, concomitant use of gabapentinoids with opioids was associated with increased risk of opioid overdose and other opioid-related adverse events; however, the absolute risk of adverse events was low.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Quantifying enhanced risk from alcohol and other factors in polysubstance-related deaths

BACKGROUND

To quantify how alcohol, polysubstance use and other factors influence opioid concentrations in drug-related deaths in West Virginia (WV), United States.

METHODS

Multiple linear regression models were employed to identify relationships among alcohol, other factors, and the concentrations of four commonly identified opioids (fentanyl, hydrocodone, oxycodone, methadone), accounting for demographic, toxicological and comorbid characteristics in WV drug-related deaths from 2005 to 2018.

RESULTS

Alcohol concentrations of 0.08% or above were associated with significant reductions in blood concentrations of fentanyl (27.5%), hydrocodone (30.5%) and methadone (32.4%). Significantly lower predicted concentrations of all opioids studied were associated with multiple opioid vs. single opioid presence, with predicted concentration reductions ranging from 13.7% for fentanyl to 65-66% for hydrocodone and oxycodone. Benzodiazepine presence was associated with small, non-statistically significant changes in opioid concentrations, while stimulant presence was associated with statistically significant reductions in hydrocodone and oxycodone concentrations.

CONCLUSIONS

Co-ingestion of alcohol, multiple opioids or stimulants were associated with significantly decreased predicted concentrations of commonly identified opioids in drug deaths. Further evidence is provided for enhanced risks from polysubstance use with opioids, which has important public health implications.

Fentanyl depression of respiration: Comparison with heroin and morphine

BACKGROUND AND PURPOSE

Fentanyl overdose deaths have reached "epidemic" levels in North America. Death in opioid overdose invariably results from respiratory depression. In the present work, we have characterized how fentanyl depresses respiration, and by comparing fentanyl with heroin and morphine, the active breakdown product of heroin, we have sought to determine the factors, in addition to high potency, that contribute to the lethality of fentanyl.

EXPERIMENTAL APPROACH

Respiration (rate and tidal volume) was measured in awake, freely moving mice by whole body plethysmography.

KEY RESULTS

Intravenously administered fentanyl produced more rapid depression of respiration than equipotent doses of heroin or morphine. Fentanyl depressed both respiratory rate and tidal volume. Fentanyl did not depress respiration in μ-opioid receptor knockout mice. Naloxone, the opioid antagonist widely used to treat opioid overdose, reversed the depression of respiration by morphine more readily than that by fentanyl, whereas diprenorphine, a more lipophilic antagonist, was equipotent in reversing fentanyl and morphine depression of respiration. Prolonged treatment with morphine induced tolerance to respiratory depression, but the degree of cross tolerance to fentanyl was less than the tolerance to morphine itself.

CONCLUSION AND IMPLICATIONS

We propose that several factors (potency, rate of onset, lowered sensitivity to naloxone, and lowered cross tolerance to heroin) combine to make fentanyl more likely to cause opioid overdose deaths than other commonly abused opioids. Lipophilic antagonists such as diprenorphine may be better antidotes than naloxone to treat fentanyl overdose.

Prolonged ethanol administration prevents the development of tolerance to morphine-induced respiratory depression

BACKGROUND

Opioid users regularly consume other drugs such as alcohol (ethanol). Acute administration of ethanol can rapidly reverse tolerance to morphine-induced respiratory depression. However, alcohol consumption by opioid users is likely to occur over prolonged time periods. We have therefore sought to determine the effect of prolonged alcohol consumption on the development of tolerance to opioid respiratory depression.

METHODS

Mice were fed control or ethanol (5%) liquid diet for 16 days. On days 9-16 morphine tolerance was induced by administration of 3 priming injections of morphine followed by subcutaneous implantation of a morphine-filled osmotic mini-pump. Control mice received saline. Respiration was measured by plethysmography and the effect of an acute morphine challenge dose was measured on day 16 to assess the development of morphine tolerance.

RESULTS

Prolonged ethanol consumption for 14 days did not alter the respiratory depressant effect of an acute dose of morphine. Control mice treated with prolonged morphine developed tolerance to acute morphine respiratory depression whereas ethanol diet fed mice treated with prolonged morphine showed significant respiratory depression during morphine-pump treatment and remained sensitive to the respiratory depressant effect of the acute challenge dose of morphine. The ethanol consumption did not alter blood or brain levels of morphine, whilst conversely prolonged morphine treatment did not alter blood levels of ethanol.

CONCLUSIONS

Prolonged ethanol consumption prevents the development and maintenance of tolerance to the respiratory depressant effect of morphine. These data suggest that ethanol inhibition of tolerance will greatly increase the risk of fatal heroin overdose in humans.

Global patterns of opioid use and dependence: harms to populations, interventions, and future action

We summarise the evidence for medicinal uses of opioids, harms related to the extramedical use of, and dependence on, these drugs, and a wide range of interventions used to address these harms. The Global Burden of Diseases, Injuries, and Risk Factors Study estimated that in 2017, 40·5 million people were dependent on opioids (95% uncertainty interval 34·3-47·9 million) and 109 500 people (105 800-113 600) died from opioid overdose. Opioid agonist treatment (OAT) can be highly effective in reducing illicit opioid use and improving multiple health and social outcomes-eg, by reducing overall mortality and key causes of death, including overdose, suicide, HIV, hepatitis C virus, and other injuries. Mathematical modelling suggests that scaling up the use of OAT and retaining people in treatment, including in prison, could avert a median of 7·7% of deaths in Kentucky, 10·7% in Kiev, and 25·9% in Tehran over 20 years (compared with no OAT), with the greater effects in Tehran and Kiev being due to reductions in HIV mortality, given the higher prevalence of HIV among people who inject drugs in those settings. Other interventions have varied evidence for effectiveness and patient acceptability, and typically affect a narrower set of outcomes than OAT does. Other effective interventions focus on preventing harm related to opioids. Despite strong evidence for the effectiveness of a range of interventions to improve the health and wellbeing of people who are dependent on opioids, coverage is low, even in high-income countries. Treatment quality might be less than desirable, and considerable harm might be caused to individuals, society, and the economy by the criminalisation of extramedical opioid use and dependence. Alternative policy frameworks are recommended that adopt an approach based on human rights and public health, do not make drug use a criminal behaviour, and seek to reduce drug-related harm at the population level.

Lack of correlation between the activity of the mesolimbic dopaminergic system and the rewarding properties of pregabalin in mouse

RATIONALE

Pregabalin is a psychoactive drug indicated in the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Pregabalin acts on different neurotransmission systems by inactivating the alpha2-delta subunit of voltage-gated calcium channels. In light of this pharmacological property, the hypothesis has been raised that pregabalin may regulate the mesolimbic dopamine pathway and thereby display a potential for misuse or abuse as recently observed in humans. Although some preclinical data support this possibility, the rewarding properties of gabapentinoid are still a matter for debate.

OBJECTIVE

The aim of this work was to evaluate the rewarding properties of pregabalin and to determine its putative mechanism of action in healthy mice.

RESULTS

Pregabalin alone (60 mg/kg; s.c.) produced a rewarding effect in the conditioned place preference (CPP) test albeit to a lower extent than cocaine (30 mg/kg; s.c.). Interestingly, when assessing locomotor activity in the CPP, the PGB60 group, similarly to the cocaine group, showed an increased locomotor activity. In vivo single unit extracellular recording showed that pregabalin had mixed effects on dopamine (DA) neuronal activity in the ventral tegmental area since it decreased the activity of 50% of neurons and increased 28.5% of them. In contrast, cocaine decreased 75% of VTA DA neuronal activity whereas none of the neurons were activated. Intracerebal microdialysis was then conducted in awake freely mice to determine to what extent such electrophysiological parameters influence the extracellular DA concentrations ([DA]ext) in the nucleus accumbens. Although pregabalin failed to modify this parameter, cocaine produced a robust increase (800%) in [DA]ext.

CONCLUSIONS

Collectively, these electrophysiological and neurochemical experiments suggest that the rewarding properties of pregabalin result from a different mode of action than that observed with cocaine. Further experiments are warranted to determine whether such undesirable effects can be potentiated under pathological conditions such as neuropathic pain, mood disorders, or addiction and to identify the key neurotransmitter system involved.

Oxycodone concentrations in the central nervous system and cerebrospinal fluid after epidural administration to the pregnant ewe

The main sites of the analgesic action of oxycodone are the brain and spinal cord. The present study describes the concentrations of oxycodone and its metabolites in the brain and spinal cord after epidural administration to the ewe. Twenty pregnant ewes undergoing laparotomy were randomized into two groups to receive epidural oxycodone: infusion group (n = 10, 0.1 mg·kg^-1 bolus followed by continuous infusion of 0.05 mg·kg^-1 ·h^-1 for five days) or repeated boluses group (n = 10, 0.2 + 2x0.1 mg·kg^-1 bolus followed by a 0.2 mg·kg^-1 bolus every 12 hours for five days). After five days of oxycodone administration, arterial blood samples were collected, the sheep were killed, and a CSF sample and tissue samples from the cortex, thalamus, cerebellum and spinal cord were obtained for the quantification of oxycodone and its main metabolites. The median plasma and CSF concentrations of oxycodone were 9.0 and 14.2 ng·mL^-1 after infusion and 0.4 and 1.1 ng·mL^-1 after repeated boluses. In the infusion group, the cortex, thalamus and cerebellum oxycodone concentrations were 4-8 times higher and in the spinal cord 1310 times higher than in plasma. In the repeated boluses group, brain tissue concentrations were similar in the three areas, and in the spinal cord were 720 times higher than in plasma. Oxymorphone was the main metabolite detected, which accumulated in the brain and spinal cord tissue. In conclusion, first, accumulation of oxycodone and oxymorphone in the CNS was observed, and second, high spinal cord concentrations suggest that epidural oxycodone may provide segmental analgesia.

Comprehensive molecular pharmacology screening reveals potential new receptor interactions for clinically relevant opioids

Most clinically used opioids are thought to induce analgesia through activation of the mu opioid receptor (MOR). However, disparities have been observed between the efficacy of opioids in activating the MOR in vitro and in inducing analgesia in vivo. In addition, some clinically used opioids do not produce cross-tolerance with each other, and desensitization produced in vitro does not match tolerance produced in vivo. These disparities suggest that some opioids could be acting through other targets in vivo, but this has not been comprehensively tested. We thus screened 9 clinically relevant opioids (buprenorphine, hydrocodone, hydromorphone, morphine, O-desmethyl-tramadol, oxycodone, oxymorphone, tapentadol, tramadol) against 9 pain-related receptor targets (MOR, delta opioid receptor [DOR], kappa opioid receptor [KOR], nociceptin receptor [NOP], cannabinoid receptor type 1 [CB1], sigma-1 receptor [σ1R], and the monoamine transporters [NET/SERT/DAT]) expressed in cells using radioligand binding and functional activity assays. We found several novel interactions, including monoamine transporter activation by buprenorphine and σ1R binding by hydrocodone and tapentadol. Tail flick anti-nociception experiments with CD-1 mice demonstrated that the monoamine transporter inhibitor duloxetine selectively promoted buprenorphine anti-nociception while producing no effects by itself or in combination with the most MOR-selective drug oxymorphone, providing evidence that these novel interactions could be relevant in vivo. Our findings provide a comprehensive picture of the receptor interaction profiles of clinically relevant opioids, which has not previously been performed. Our findings also suggest novel receptor interactions for future investigation that could explain some of the disparities observed between opioid performance in vitro and in vivo.

Proteomic profiling reveals Arl6ip-1 as a candidate target in cancer-induced bone pain rat model after oxycodone treatment

Treatment of cancer-induced bone pain (CIBP) is challenging in clinics. Oxycodone is used to treat CIBP. However, the lack of understanding of the mechanism of CIBP limits the application of oxycodone. In this study, proteomic profiling of oxycodone-treated spinal dorsal cord of rats with CIBP was performed. Briefly, a total of 3519 proteins were identified in the Sham group; 3505 proteins in the CIBP group; and 3530 proteins in the CIBP-OXY treatment group. The 2-fold cut-off value was used as the differential protein standard for abundance reduction or increase (p < 0.05). Significant differences were found in the abundance of 16 proteins between Sham and CIBP group; 11 proteins in the CIBP group had increased abundance while 5 proteins had reduced abundance. Furthermore, fifteen proteins with differential abundance were identified between the CIBP group and the OXY group. Compared with the CIBP group, there were six increased abundances and nine reduced abundances in the OXY group. In addition, a reduced expression of ADP-ribosylation factor-like 6 binding factor 1 (Arl6ip-1), an endoplasmic reticulum protein that has an important role in cell conduction and material transport, was found in the CIBP group compared with the Sham group. Its expression increased after the administration of OXY. Proteomics results were further verified by Western-blot. Fluorescent staining revealed that Arl6ip-1 co-localized with spinal dorsal horn neurons, but not with astrocytes or microglia. Based on the observed results, we believe that Arl6ip-1 may be a potential drug target for OXY treatment of CIBP rats.

Oxycodone-induced tolerance to respiratory depression: reversal by ethanol, pregabalin and protein kinase C inhibition

BACKGROUND AND PURPOSE

Oxycodone, a prescription opioid, is a major drug of abuse, especially in the USA, and contributes significantly to opioid overdose deaths each year. Overdose deaths result primarily from respiratory depression. We have studied respiratory depression by oxycodone and have characterized how tolerance develops on prolonged exposure to the drug. We have investigated the role of PKC in maintaining tolerance and have examined whether ethanol or pregabalin reverses oxycodone-induced tolerance.

EXPERIMENTAL APPROACH

Respiration was measured in male CD-1 mice by whole-body plethysmography. Mice were preinjected with oxycodone then implanted with mini-pumps (s.c.) delivering 20, 45 or 120 mg·kg-1 ·day-1 oxycodone for 6 days and subsequently challenged with oxycodone (3 mg·kg-1 , i.p.) or morphine (10 mg·kg-1 , i.p.) to assess the level of tolerance.

KEY RESULTS

Oxycodone-treated mice developed tolerance to oxycodone and cross tolerance to morphine-induced respiratory depression. Tolerance was less with 20 mg·kg-1 ·day-1 than with 45 or 120 mg·kg-1 ·day-1 oxycodone treatment. At doses that do not depress respiration, ethanol (0.3 g·kg-1 ), pregabalin (20 mg·kg-1 ) and calphostin C (45 μg·kg-1 ) all reversed oxycodone-induced tolerance resulting in significant respiratory depression. Reversal of tolerance was less in mice treated with oxycodone (120 mg·kg-1 ·day-1 ). In mice receiving ethanol and calphostin C or ethanol and pregabalin, there was no greater reversal of tolerance than seen with either drug alone.

CONCLUSION AND IMPLICATIONS

These data suggest that oxycodone-induced tolerance is mediated by PKC and that reversal of tolerance by ethanol or pregabalin may be a contributory factor in oxycodone overdose deaths.