Comparative pharmacological profiles of morphine and oxycodone under a neuropathic pain-like state in mice: evidence for less sensitivity to morphine

Comparison of oxycodone hydrochloride and flurbiprofen axetil on analgesia in mechanically ventilated patients with respiratory failure in a multicenter study

The design of this study is to compare the effectiveness of two analgesic drugs in the intervention of pain events for patients on mechanical ventilation. 414 patients from three hospitals with respiratory failure requiring mechanical ventilation were randomly assigned to oxycodone hydrochloride or flurbiprofen axetil. The primary endpoints is the difference in the proportion of patients with a Behavioral Pain Scale (BPS) score > 5 within 48 h. The secondary endpoints is to compare the dosage of sedative drugs (midazolam, propofol, dexmedetomidine) and to assess the clinical outcomes such as duration of mechanical ventilation. There was no significant difference in BPS scores between the two groups at enrollment, and BPS scores in oxycodone group were significantly lower than those in flurbiprofen axetil group at 24 and 48 h of enrollment. The proportion of patients with BPS less than 5 points in the Oxycodone hydrochloride group was also significantly lower than that in the flurbiprofen axetil group. For patients with Acute Physiology and Chronic Health Evaluation II (APACHE II) score greater than 10, subgroup analysis showed that the mechanical ventilation time of oxycodone hydrochloride group was significantly lower than that of flurbiprofen axetil group with statistical significance, and the dosage of midazolam was significantly lower than that of flurbiprofen axetil group. The length of ICU stay was significantly lower than that of flurbiprofen axetil group. Oxycodone hydrochloride was more potent than flurbiprofen axetil for analgesia for patients with respiratory failure requiring mechanical ventilation.

The immunological and pharmacokinetic evaluation of Lipid-PLGA hybrid nanoparticle-based oxycodone vaccines

The current opioid epidemic is one of the most profound public health crises facing the United States. Despite that it has been under the spotlight for years, available treatments for opioid use disorder (OUD) and overdose are limited to opioid receptor ligands such as the agonist methadone and the overdose reversing drugs such as naloxone. Vaccines are emerging as an alternative strategy to combat OUD and prevent relapse and overdose. Most vaccine candidates consist of a conjugate structure containing the target opioid attached to an immunogenic carrier protein. However, conjugate vaccines have demonstrated some intrinsic shortfalls, such as fast degradation and poor recognition by immune cells. To overcome these challenges, we proposed a lipid-PLGA hybrid nanoparticle (hNP)-based vaccine against oxycodone (OXY), which is one of the most frequently misused opioid analgesics. The hNP-based OXY vaccine exhibited superior immunogenicity and pharmacokinetic efficacy in comparison to its conjugate vaccine counterpart. Specifically, the hNP-based OXY vaccine formulated with subunit keyhole limpet hemocyanin (sKLH) as the carrier protein and aluminum hydroxide (Alum) as the adjuvant (OXY-sKLH-hNP(Alum)) elicited the most potent OXY-specific antibody response in mice. The induced antibodies efficiently bound with OXY molecules in blood and suppressed their entry into the brain. In a following dose-response study, OXY-sKLH-hNP(Alum) equivalent to 60 μg of sKLH was determined to be the most promising OXY vaccine candidate moving forward. This study provides evidence that hybrid nanoparticle-based vaccines may be superior vaccine candidates than conjugate vaccines and will be beneficial in treating those suffering from OUD.

Interactions of pain and opioids on conditioned place preference in rodents

RATIONALE

Opioid analgesics are the most effective medications used for the treatment of pain, however there are significant risks associated with repeated opioid use including opioid misuse and opioid use disorder development. Chronic pain affects millions of adults in the United States, and opioid misuse is often comorbid with pain conditions in individuals who are repeatedly treated with opioids. In addition to providing pain relief, opioids produce rewarding effects, but in chronic pain states, reward processing can become dysregulated. The conditioned place preference task is commonly used to measure the rewarding properties of opioids in rodents. During this task, opioid administration is paired with a distinct environment through repeated conditioning and the change in an animal's preference for the paired environment indicates whether the opioid is rewarding or not.

OBJECTIVES

Rodent pain models can be combined with conditioned place preference to examine the effects of pain on opioid reward. The existing preclinical literature on pain effects on conditioned place preference is conflicting, where pain conditions have been reported to enhance, suppress, or have no effect on opioid reward. This review will discuss several factors that may contribute to these discordant findings including conditioning session duration and number, rodent strain differences in opioid sensitivity, analgesic properties of opioids at tested doses, locomotor effects at tested doses, and diurnal variation in pain sensitivity. Future studies should consider how these factors contribute to opioid conditioned place preference in both pain and pain-free animals to have a better understanding of the interactions between pain and opioid reward.

Brain region-specific neural activation by low-dose opioid promotes social behavior

The opioid system plays crucial roles in modulating social behaviors in both humans and animals. However, the pharmacological profiles of opioids regarding social behavior and their therapeutic potential remain unclear. Multiple pharmacological, behavioral, and immunohistological c-Fos mapping approaches were used to characterize the effects of μ-opioid receptor agonists on social behavior and investigate the mechanisms in naive mice and autism spectrum disorder-like (ASD-like) mouse models, such as prenatally valproic acid-treated mice and Fmr1-KO mice. Here, we report that low-dose morphine, a μ-opioid receptor agonist, promoted social behavior by selectively activating neurons in prosocial brain regions, including the nucleus accumbens, but not those in the dorsomedial periaqueductal gray (dmPAG), which are only activated by analgesic high-dose morphine. Critically, intra-dmPAG morphine injection counteracted the prosocial effect of low-dose morphine, suggesting that dmPAG neural activation suppresses social behavior. Moreover, buprenorphine, a μ-opioid receptor partial agonist with less abuse liability and a well-established safety profile, ameliorated social behavior deficits in two mouse models recapitulating ASD symptoms by selectively activating prosocial brain regions without dmPAG neural activation. Our findings highlight the therapeutic potential of brain region-specific neural activation induced by low-dose opioids for social behavior deficits in ASD.

Chronic inflammatory pain reduces fentanyl intake during early acquisition of fentanyl self-administration, but does not change motivation to take fentanyl in male and female rats

The co-occurrence of chronic pain and opioid misuse has led to numerous preclinical investigations of pain-opioid interactions to examine how pain manipulations alter the reinforcing properties of opioids. However, preclinical investigations of chronic pain effects on opioid drug self-administration have produced inconsistent results. Our previous work demonstrated that established fentanyl self-administration is resistant to change by induction of chronic inflammatory pain (Complete Freund's Adjuvant; CFA) in male and female rats, while other laboratories have shown that CFA increased fentanyl self-administration in male but not female rats when pain induction precedes self-administration, which may be a critical factor in determining the effects of chronic pain on self-administration. The present study was designed similarly to Higginbotham et al. (2022) to test the effects of CFA on fentanyl self-administration in rats that underwent pain prior to acquisition of fentanyl self-administration. Male and female rats treated with hindpaw CFA or saline were trained to intravenously self-administer (IVSA) fentanyl for 3 weeks under limited access to fentanyl (2 h per day) conditions. After 3 weeks of fentanyl IVSA acquisition, we tested motivation to take fentanyl using progressive ratio testing and dose-response testing. CFA male and female rats self-administered less fentanyl than saline-treated controls during week 1 of acquisition, but not during weeks 2-3 of acquisition. Intra-session analysis of week 1 data demonstrated that chronic inflammatory pain suppressed fentanyl intake towards the end of week 1 and at the end of each operant session. We also report no effects of chronic inflammatory pain on motivation to take fentanyl. We discuss potential methodological explanations for differences between these results and prior reports. Our findings demonstrate that CFA temporarily suppresses fentanyl IVSA in animals without changing motivation to take fentanyl or promoting escalation of opioid use, suggesting that chronic inflammatory pain is unlikely to promote long-term risk of opioid misuse.

N-methyl-D-aspartate Receptors and Depression: Linking Psychopharmacology, Pathology and Physiology in a Unifying Hypothesis for the Epigenetic Code of Neural Plasticity

Uncompetitive NMDAR (N-methyl-D-aspartate receptor) antagonists restore impaired neural plasticity, reverse depressive-like behavior in animal models, and relieve major depressive disorder (MDD) in humans. This review integrates recent findings from in silico, in vitro, in vivo, and human studies of uncompetitive NMDAR antagonists into the extensive body of knowledge on NMDARs and neural plasticity. Uncompetitive NMDAR antagonists are activity-dependent channel blockers that preferentially target hyperactive GluN2D subtypes because these subtypes are most sensitive to activation by low concentrations of extracellular glutamate and are more likely activated by certain pathological agonists and allosteric modulators. Hyperactivity of GluN2D subtypes in specific neural circuits may underlie the pathophysiology of MDD. We hypothesize that neural plasticity is epigenetically regulated by precise Ca2+ quanta entering cells via NMDARs. Stimuli reach receptor cells (specialized cells that detect specific types of stimuli and convert them into electrical signals) and change their membrane potential, regulating glutamate release in the synaptic cleft. Free glutamate binds ionotropic glutamatergic receptors regulating NMDAR-mediated Ca2+ influx. Quanta of Ca2+ via NMDARs activate enzymatic pathways, epigenetically regulating synaptic protein homeostasis and synaptic receptor expression; thereby, Ca2+ quanta via NMDARs control the balance between long-term potentiation and long-term depression. This NMDAR Ca2+ quantal hypothesis for the epigenetic code of neural plasticity integrates recent psychopharmacology findings into established physiological and pathological mechanisms of brain function.

Cannabidiol Protects against the Reinstatement of Oxycodone-Induced Conditioned Place Preference in Adolescent Male but Not Female Rats: The Role of MOR and CB1R

Cannabidiol (CBD), a phytocannabinoid, appeared to satisfy several criteria for a safe approach to preventing drug-taking behavior, including opioids. However, most successful preclinical and clinical results come from studies in adult males. We examined whether systemic injections of CBD (10 mg/kg, i.p.) during extinction of oxycodone (OXY, 3 mg/kg, i.p.) induced conditioned place preference (CPP) could attenuate the reinstatement of CPP brought about by OXY (1.5 mg/kg, i.p.) priming in adolescent rats of both sexes, and whether this effect is sex dependent. Accordingly, a priming dose of OXY produced reinstatement of the previously extinguished CPP in males and females. In both sexes, this effect was linked to locomotor sensitization that was blunted by CBD pretreatments. However, CBD was able to prevent the reinstatement of OXY-induced CPP only in adolescent males and this outcome was associated with an increased cannabinoid 1 receptor (CB1R) and a decreased mu opioid receptor (MOR) expression in the prefrontal cortex (PFC). The reinstatement of CCP in females was associated with a decreased MOR expression, but no changes were detected in CB1R in the hippocampus (HIP). Moreover, CBD administration during extinction significantly potentialized the reduced MOR expression in the PFC of males and showed a tendency to potentiate the reduced MOR in the HIP of females. Additionally, CBD reversed OXY-induced deficits of recognition memory only in males. These results suggest that CBD could reduce reinstatement to OXY seeking after a period of abstinence in adolescent male but not female rats. However, more investigation is required.

Pregabalin versus Gabapentin Efficacy in the Management of Neuropathic Pain Associated with Failed Back Surgery Syndrome

OBJECTIVE

Failed back surgery syndrome (FBSS) is a common long-term complication following spine surgeries characterized by chronic persistent pain; different strategies of management were employed to deal with it. This clinical trial aims to compare the efficacy of Pregabalin and Gabapentin in the management of this condition.

METHODS

A double-blind, randomized, comparative study (clinical trial registry NCT05324761 on 11th April 2022) with two parallel arms with Pregabalin and Gabapentin were used in arms one and two, respectively. Visual analog scale was used for basal and endpoint assessment of pain. T-test and analysis of covariance were used to deal with different variables. A pairwise test was used to compare pairs of means.

RESULTS

Of 66 patients referred to the trial, 64 were eligible, with 60 patients completing the 30 days trial. Both pregabalin and gabapentin effectively reduce pain, with significant p-values of 0.001 for each group. However, the pregabalin group was superior to gabapentin in pain reduction (p=0.001). Gender was an insignificant factor (p=0.574 and p=0.445 for the pregabalin and gabapentin groups, respectively, with a non-significant reduction (p=0.393) for both groups in total. Location of stenosis before surgery and type of surgery performed show non-significant effect on pain reduction for both groups.

CONCLUSION

Both pregabalin and gabapentin effectively and safely relieve neuropathic pain associated with FBSS; pregabalin was significantly more effective irrespective of the patients' gender.

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.

An evaluation method for developing abuse-deterrent opioid formulations with agonist and antagonist combinations using conditioned place preference

Although opioids are useful narcotic analgesics in clinical settings, their misuse and addiction in the United States of America and other countries are rapidly increasing. Therefore, the development of abuse-deterrent formulations is an urgent issue. We herein investigated how to select the ratio of an opioid and the opioid receptor antagonist, naloxone in abuse-deterrent formulations for mice. The conditioned place preference (CPP) test was used to evaluate the rewarding effects of abused drugs. The opioids morphine (30 μmol/kg), oxycodone (3 μmol/kg), fentanyl (0.4 μmol/kg), and buprenorphine (0.5 μmol/kg) significantly induced place preference in mice. We also examined the optimal ratio of naloxone and opioids to inhibit the rewarding effects of the latter. Naloxone (3-5 μmol/kg) effectively inhibited place preference induced by the opioids tested. We calculated theoretical drug doses that exerted the same pharmacodynamic effects based on two parameters: μ-opioid receptor binding affinity and blood-brain barrier (BBB) permeability. Theoretical doses were very close to the drug doses at which mice showed place preference. Therefore, the CPP test is useful as a behavioral method for evaluating abuse-deterrent formulations of opioids mixed with an antagonist. The ratio of naloxone with opioids, at which mice did not show place preference, may be an effective index for developing abuse-deterrent formulations. Ratios may be calculated for other opioids based on μ-opioid receptor binding affinity and BBB permeability.

Opioids alter paw placement during walking, confounding assessment of analgesic efficacy in a postsurgical pain model in mice

Introduction:

Hind paw–directed assays are commonly used to study the analgesic effects of opioids in mice. However, opioid-induced hyperlocomotion can obscure results of such assays.

Objectives:

We aimed to overcome this potential confound by using gait analysis to observe hind paw usage during walking in mice.

Methods:

We measured changes in the paw print area after induction of postsurgical pain (using the paw incision model) and treatment with oxycodone.

Results:

Paw incision surgery reduced the paw print area of the injured hind paw as mice avoided placing the incised section of the paw on the floor. Surprisingly, oxycodone caused a tiptoe-like gait in mice, reducing the paw print area of both hind paws. Further investigation of this opioid-induced phenotype revealed that analgesic doses of oxycodone or morphine dose-dependently reduced the hind paw print area in uninjured mice. The gait changes were not dependent on opioid-induced increases in the locomotor activity; speed and paw print area had no correlation in opioid-treated mice, and other analgesic compounds that alter locomotor activity did not affect the paw print area.

Conclusion:

Unfortunately, the opioid-induced “tiptoe” gait phenotype prevented gait analysis from being a viable metric for demonstrating opioid analgesia in injured mice. However, this work reveals an important, previously uncharacterized effect of treatment with analgesic doses of opioids on paw placement. Our characterization of how opioids affect gait has important implications for the use of mice to study opioid pharmacology and suggests that scientists should use caution when using hind paw–directed nociceptive assays to test opioid analgesia in mice.

Aerobic exercise as a promising nonpharmacological therapy for the treatment of substance use disorders

Despite the prevalence and public health impact of substance use disorders (SUDs), effective long-term treatments remain elusive. Aerobic exercise is a promising, nonpharmacological treatment currently under investigation as a strategy for preventing drug relapse. Aerobic exercise could be incorporated into the comprehensive treatment regimens for people with substance abuse disorders. Preclinical studies of SUD with animal models have shown that aerobic exercise diminishes drug-seeking behavior, which leads to relapse, in both male and female rats. Nevertheless, little is known regarding the effects of substance abuse-induced cellular and physiological adaptations believed to be responsible for drug-seeking behavior. Accordingly, the overall goal of this review is to provide a summary and an assessment of findings to date, highlighting evidence of the molecular and neurological effects of exercise on adaptations associated with SUD.

Epidural Oxycodone for Acute Pain

Epidural analgesia is commonly used in labour analgesia and in postoperative pain after major surgery. It is highly effective in severe acute pain, has minimal effects on foetus and newborn, may reduce postoperative complications, and enhance patient satisfaction. In epidural analgesia, low concentrations of local anaesthetics are combined with opioids. Two opioids, morphine and sufentanil, have been approved for epidural use, but there is an interest in evaluating other opioids as well. Oxycodone is one of the most commonly used opioids in acute pain management. However, data on its use in epidural analgesia are sparse. In this narrative review, we describe the preclinical and clinical data on epidural oxycodone. Early data from the 1990s suggested that the epidural administration of oxycodone may not offer any meaningful benefits over intravenous administration, but more recent clinical data show that oxycodone has advantageous pharmacokinetics after epidural administration and that epidural administration is more efficacious than intravenous administration. Further studies are needed on the safety and efficacy of continuous epidural oxycodone administration and its use in epidural admixture.

A limited access oral oxycodone paradigm produces physical dependence and mesocorticolimbic region-dependent increases in DeltaFosB expression without preference

The abuse of oral formulations of prescription opioids has precipitated the current opioid epidemic. We developed an oral oxycodone consumption model consisting of a limited access (4 h) two-bottle choice drinking in the dark (TBC-DID) paradigm and quantified dependence with naloxone challenge using mice of both sexes. We also assessed neurobiological correlates of withdrawal and dependence elicited via oral oxycodone consumption using immunohistochemistry for DeltaFosB (ΔFosB), a transcription factor described as a molecular marker for drug addiction. Neither sex developed a preference for the oxycodone bottle, irrespective of oxycodone concentration, bottle position or prior water restriction. Mice that volitionally consumed oxycodone exhibited hyperlocomotion in an open field test and supraspinal but not spinally-mediated antinociception. Both sexes also developed robust, dose-dependent levels of opioid withdrawal that was precipitated by the opioid antagonist naloxone. Oral oxycodone consumption followed by naloxone challenge led to mesocorticolimbic region-dependent increases in the number of ΔFosB expressing cells. Naloxone-precipitated withdrawal jumps, but not the oxycodone bottle % preference, was positively correlated with the number of ΔFosB expressing cells specifically in the nucleus accumbens shell. Thus, limited access oral consumption of oxycodone produced physical dependence and increased ΔFosB expression despite the absence of opioid preference. Our TBC-DID paradigm allows for the study of oral opioid consumption in a simple, high-throughput manner and elucidates the underlying neurobiological substrates that accompany opioid-induced physical dependence.

Oxycodone in the Opioid Epidemic: High 'Liking', 'Wanting', and Abuse Liability

It is estimated that nearly a third of people who abuse drugs started with prescription opioid medicines. Approximately, 11.5 million Americans used prescription drugs recreationally in 2016, and in 2018, 46,802 Americans died as the result of an opioid overdose, including prescription opioids, heroin, and illicitly manufactured fentanyl (National Institutes on Drug Abuse (2020) Opioid Overdose Crisis. https://www.drugabuse.gov/drugs-abuse/opioids/opioid-overdose-crisis . Accessed 06 June 2020). Yet physicians will continue to prescribe oral opioids for moderate-to-severe pain in the absence of alternative therapeutics, underscoring the importance in understanding how drug choice can influence detrimental outcomes. One of the opioid prescription medications that led to this crisis is oxycodone, where misuse of this drug has been rampant. Being one of the most highly prescribed opioid medications for treating moderate-to-severe pain as reflected in the skyrocketed increase in retail sales of 866% between 1997 and 2007, oxycodone was initially suggested to be less addictive than morphine. The false-claimed non-addictive formulation of oxycodone, OxyContin, further contributed to the opioid crisis. Abuse was often carried out by crushing the pills for immediate burst release, typically by nasal insufflation, or by liquefying the pills for intravenous injection. Here, we review oxycodone pharmacology and abuse liability as well as present the hypothesis that oxycodone may exhibit a unique pharmacology that contributes to its high likability and abuse susceptibility. We will discuss various mechanisms that likely contribute to the high abuse rate of oxycodone including clinical drug likability, pharmacokinetics, pharmacodynamics, differences in its actions within mesolimbic reward circuity compared to other opioids, and the possibility of differential molecular and cellular receptor interactions that contribute to its selective effects. We will also discuss marketing strategies and drug difference that likely contributes to the oxycodone opioid use disorders and addiction.

Effect of intravenous oxycodone on the physiologic responses to extubation following general anesthesia

BACKGROUND

Endotracheal intubation and extubation may cause undesirable hemodynamic changes. Intravenous oxycodone has recently been introduced and used for relieving hemodynamic alterations in response to intubation, but there is insufficient information regarding its application in stabilizing hemodynamics during extubation in the patients emerging from general anesthesia.

METHODS

One hundred patients, who had undergone assorted laparoscopic surgeries under general anesthesia, were randomly assigned to Control group (saline injection, 50 cases) and Study group (intravenous injection of 0.08 mg/kg oxycodone immediately after completion of the surgical procedure, 50 cases). Blood pressure, heart rate, blood oxygen saturation (SpO₂) as well as blood concentrations of epinephrine, norepinephrine, and cortisol were recorded or measured immediately before extubation (T₀), during extubation (T₁), as well as one minute (T₂), 5 min (T₃), and 10 min after extubation (T₄). In addition, coughing and restlessness, time of eye-opening, and duration from completing surgery to extubation as well as Ramsay Sedation Scale were analyzed.

RESULTS

Blood pressure and heart rate as well as blood concentrations of epinephrine, norepinephrine, and cortisol were significantly higher in the Control group compared with the Study group at the time of extubation as well as 1, 5, and 10 min after extubation (P < 0.05). When the patients emerged from general anesthesia, 70 % of the Control group had cough, which was significantly higher than that of Study group (40 %, P < 0.05). Significantly higher number of patients manifested restlessness in the Control group before (40 %) and after extubation (20 %) compared with that in the Study group (20 and 2 %, respectively, P < 0.05). In addition, patients of Control group had lower Ramsay score at extubation (1.7 ± 0.7) as well as 30 min after extubation (2.4 ± 0.9) compared to that of the patients of Study group (2.2 ± 0.9, and 3.0 ± 0.8, respectively, P = 0.003 and 0.001).

CONCLUSIONS

Intravenous oxycodone attenuated alterations of hemodynamics and blood hormones associated with extubation during emergence from general anesthesia.

TRIAL REGISTRATION

Chinese Clinical Trial Registry: ChiCTR2000040370 (registration date: 11-28-2020) "'retrospectively registered".

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.

Morphine Antinociception Restored by Use of Methadone in the Morphine-Resistant Inflammatory Pain State

The antinociceptive effect of methadone in the morphine-resistant inflammatory pain state was described in the paw-withdrawal test using the complete Freund’s adjuvant (CFA)-induced mouse inflammatory pain model. After intraplantar (i.pl.) injection of CFA, thermal hyperalgesia was observed in the ipsilateral paw. The antinociceptive effects of subcutaneous (s.c.) injection of morphine, fentanyl, and oxycodone against thermal hyperalgesia in the inflammatory pain state were reduced in the ipsilateral paw 7 days after CFA pretreatment. On the contrary, the antinociceptive effect of s.c. injection of methadone was maintained in the ipsilateral paw 7 days after CFA pretreatment. The suppressed morphine antinociception in the CFA model mice was bilaterally restored following s.c. treatment with methadone 20 min prior to or 3 days after CFA pretreatment. The suppressed morphine antinociception was also bilaterally restored by intraperitoneal treatment with MK-801 30 min prior to CFA pretreatment; however, the s.c. injection of morphine 30 min prior to CFA pretreatment failed to restore the suppressed morphine antinociception in the CFA model mice. The expression level of mRNA for µ-opioid receptors 7 days after i.pl. pretreatment was not significantly changed by i.pl. pretreatment with CFA or s.c. pretreatment with methadone. In conclusion, methadone is extremely effective against thermal hyperalgesia in the morphine-resistant inflammatory pain state, and restores suppressed morphine antinociception in the inflammatory pain state without altering the expression level of mRNA for µ-opioid receptors.

Effects of Oxycodone Combined With Flurbiprofen Axetil on Postoperative Analgesia and Immune Function in Patients Undergoing Radical Resection of Colorectal Cancer

The influence of surgery and anesthesia on immune function during the perioperative period should not be neglected. In this study, we evaluated the effects of oxycodone combined with flurbiprofen axetil on postoperative analgesia and immune function in patients undergoing radical resection of colorectal cancer (CRC). One hundred and thirty-three were randomized into the oxycodone combined with flurbiprofen axetil (OF) group or the sufentanil combined with flurbiprofen axetil (SF) group. Patients in the OF group were prescribed oxycodone hydrochloride 0.1 mg/kg combined with flurbiprofen axetil 3 mg/kg for postoperative analgesia, whereas the SF group received sufentanil 0.1 μg/kg combined with flurbiprofen axetil 3 mg/kg. The primary outcome was visual analog scale (VAS) score. Secondary outcomes included the quantities of CD4⁺ , CD8⁺ , and natural killer (NK) T cells, tumor necrosis factor (TNF)-α level, and interleukin (IL)-6 in peripheral blood, the consumption of analgesics, and the incidence of adverse reactions, and so forth. The VAS scores at rest were similar in both group. However, the VAS scores at cough in the OF group at 8, 12, and 24 hours postsurgery were lower than those in the SF group. Compared with the SF group, the count of CD4⁺ T cells and ratio of CD4⁺ /CD8⁺ were higher in the OF group at 12, 24, 48, and 72 hours postsurgery, although the count of CD8⁺ and NK T cells was higher than that in the SF group at 48 and 72 hours postsurgery. In addition, the serum level of TNF-α and IL-6 at 12, 24, 48, and 72 hours postsurgery in the OF group was lower than that in the SF group. In addition, the incidence of postoperative nausea, postoperative vomiting, and pruritus was lower, the time to first flatus and bowel movement was earlier in the OF group. Oxycodone combined with flurbiprofen axetil applied for patient-controlled intravenous analgesia could effectively reduce pain intensity, particularly for visceral pain, and help to reverse the status of immunosuppression during radical resection of CRC.

The antagonistic activity profile of naloxone in μ-opioid receptor agonist-induced psychological dependence

Naloxone is a μ-opioid receptor antagonist that has been used to prevent overdose-related respiratory depression and deaths by the illicit use of opioids. Naloxone can also deter the abuse potential of opioids, but little has been reported regarding its antagonistic activity profile against opioid-induced psychological dependence. This study aimed to confirm the antagonistic activity profile of naloxone against several μ-opioid receptor agonists and investigate whether naloxone could affect the psychological dependence induced by widely used μ-opioid receptor agonist, oxycodone. In the Guanosine-5'-o-(3-thio) triphosphate (GTPγS) binding assay, naloxone (30-30,000 nM) inhibited the GTPγS binding induced by oxycodone, hydrocodone, morphine, and fentanyl. It elicited parallel rightward shifts in the concentration-response curves, indicating that naloxone possessed a competitive antagonistic activity profile against these μ-opioid receptor agonists. In the conditioned place preference test, oxycodone (0.01-1 mg/kg, i.v.) produced dose-dependent increases in place preference. The increased place preference induced by oxycodone (1 mg/kg) was significantly attenuated by co-administration of naloxone at a dose of 0.5 mg/kg but not 0.01 mg/kg. Naloxone (0.5 mg/kg, i.v.) also blocked oxycodone (1 mg/kg)-induced dopamine release in nucleus accumbens; however, at a lower dose (0.01 mg/kg), it did not affect the intrinsic dopamine release by oxycodone. These results indicate that the psychological dependence of oxycodone could be antagonized by naloxone, depending on the dose. This characterization might lead to a better understanding of the competitive antagonistic activity profile of naloxone for μ-opioid receptor in the brain.

Oxycodone self-administration and withdrawal behaviors in male and female Wistar rats

RATIONALE

Over the last decade, oxycodone has become one of the most widely abused drugs in the USA. Oxycodone use disorder (OUD) is a serious health problem that has prompted a need to develop animal models of OUD that have both face and predictive validity. Oxycodone use in humans is more prevalent in women and leads to pronounced hyperalgesia and irritability during withdrawal. However, unclear is whether current animal models of oxycodone self-administration recapitulate these characteristics in humans.

OBJECTIVES

We assessed the face validity of a model of extended-access oxycodone self-administration in rats by examining the escalation of oxycodone intake and behavioral symptoms of withdrawal, including irritability-like behavior and mechanical nociception, in male and female Wistar rats.

RESULTS

Both male and female rats escalated their oxycodone intake over fourteen 12-h self-administration sessions. After escalation, female rats administered more drug than male rats. No differences in plasma oxycodone levels were identified, but males had a significantly higher level of oxycodone in the brain at 30 min. Extended access to oxycodone significantly decreased aggressive-like behavior and increased defensive-like behaviors when tested immediately after a 12-h self-administration session, followed by a rebound increase in aggressive-like behavior 12 h into withdrawal. Tests of mechanical nociception thresholds during withdrawal indicated pronounced hyperalgesia. No sex differences in irritability-like behavior or pain sensitivity were observed.

CONCLUSIONS

The present study demonstrated the face validity of the extended access model of oxycodone self-administration by identifying sex differences in the escalation of oxycodone intake and pronounced changes in pain and affective states.

Effects of oxycodone applied for patient-controlled analgesia on postoperative cognitive function in elderly patients undergoing total hip arthroplasty: a randomized controlled clinical trial

BACKGROUND

Postoperative cognitive dysfunction (POCD) is a common complication after orthopedic surgery, which is not conducive to the prognosis of the elderly.

AIMS

We performed this study to investigate the effects of oxycodone applied for patient-controlled intravenous analgesia (PCIA) on postoperative cognitive function in elderly patients after total hip arthroplasty (THA).

METHODS

Ninety-nine participants were enrolled and allocated into two groups: oxycodone group (group O) and sufentanil group (group S). The primary outcome was the incidence of POCD, diagnosed according to the changes in the Mini-mental status examination (MMSE) and Montreal Cognitive Assessment (MoCA) scores. The secondary outcomes included the plasma levels of S-100B protein and neuron-specific enolase (NSE), the amount of postoperative analgesic consumption and the incidence of adverse reactions.

RESULTS

The incidence of POCD was significantly lower in patients receiving oxycodone up to the 3rd postoperative day (POD, 1st POD 27.3% vs. 51.1%, P = 0.021; 3rd POD 20.5% vs. 40.0%, P = 0.045), as compared to patients receiving sufentanil. The MMSE and MoCA scores of both groups decreased to varying degrees. However, compared with group S, the MMSE scores at 1st POD, 3rd POD, 5th POD and 7st POD in group O were higher than that in group S, while MoCA scores at 1st POD, 3rd POD and 5th POD in group O were higher. Compared with group S, the plasma levels of S-100B protein in group O at 4 h, 8 h, 12 h post-surgery were lower. While the plasma levels of NSE in group O at 4 h, 8 h, 12 h, 24 h post-surgery were lower. Number of PCIA boluses and consumption of analgesic drug during the first two POD were similar between two groups. However, postoperative incidence of nausea, vomiting and pruritus was significantly lower in patients receiving oxycodone.

CONCLUSION

Oxycodone applied for PCIA in elderly patients after THA could reduce the incidence of POCD, improve postoperative cognitive function and decrease the adverse reactions.

In vitro profiling of opioid ligands using the cAMP formation inhibition assay and the β-arrestin2 recruitment assay: No two ligands have the same profile

Previously, we showed that no two of seven opioids administered by the intracerebroventricular route had the same potency rank order for evoking antinociception, constipation and respiratory depression in rats. To gain insight at the cellular level, this study was designed to systematically investigate the activity profiles of six commonly used opioid ligands using the forskolin-stimulated cAMP assay and a β-arrestin2 recruitment assay in cultured HEK-293 cells transfected with MOP(μ), DOP(δ) or KOP(κ) receptors(-r). Morphine was a potent agonist at the MOP-r in the cAMP assay whereas it was a weak agonist at the KOP-r and DOP-r. Oxycodone had moderate efficacy and low potency at the MOP-r. Buprenorphine was a potent MOP-r and DOP-r agonist; its efficacy rank order was DOP > MOP > KOP. Fentanyl was a potent agonist at the MOP-r; its efficacy rank order was MOP > DOP > KOP. For DPDPE, its agonist efficacy was confined to the DOP-r, whereas for U69593, its efficacy rank order was KOP>> MOP. For the β-arrestin2 assay, fentanyl had full efficacy at the MOP-r whereas morphine and oxycodone were weak with insignificant efficacy at DOP and KOP receptors. Buprenorphine did not recruit β-arrestin2 at all three opioid-receptors. DPDPE and U69593 had full efficacy for β-arrestin2 recruitment to the DOP-r and KOP-r respectively. Despite the low efficacy and potency of morphine, oxycodone and buprenorphine in recruiting β-arrestin2 to the MOP-r herein, these opioids all evoked respiratory depression and constipation in rats. Together, our findings discount a key role for β-arrestin2 recruitment at the MOP-r in evoking opioid-related side-effects.

Drug-specific differences in the ability of opioids to manage burn pain

Burn injury pain is a significant public health problem. Burn injury treatment has improved tremendously in recent decades. However, an unintended consequence is that a larger number of patients now survive more severe injuries, and face intense pain that is very hard to treat. Although many efforts have been made to find alternative treatments, opioids remain the most effective medication available. Burn patients are frequently prescribed opioids in doses and durations that are significantly higher and longer than standard analgesic dosing guidelines. Despite this, many continue to experience unrelieved pain. They are also placed at a higher risk for developing dependence and opioid use disorder. Burn injury profoundly alters the functional state of the immune system. It also alters the expression levels of receptor, effector, and signaling molecules within the spinal cord's dorsal horn. These alterations could explain the reduced potency of opioids. However, recent studies demonstrate that different opioids signal preferentially via differential signaling pathways. This ligand-specific signaling by different opioids implies that burn injury may reduce the antinociceptive potency of opioids to different degrees, in a drug-specific manner. Indeed, recent findings hint at drug-specific differences in the ability of opioids to manage burn pain early after injury, as well as differences in their ability to prevent or treat the development of chronic and neuropathic pain. Here we review the current state of opioid treatment, as well as new findings that could potentially lead to opioid-based pain management strategies that may be significantly more effective than the current solutions.

Preemptive Anti-Stress Response Effects of Oxycodone Versus Sufentanil for Patients Undergoing Cardiac Valve Replacement-A Randomized Controlled Trial

Patients undergoing cardiac valve replacement may experience cardiovascular adverse events during the preoperative period before anesthesia. The study was to compare the preemptive anti-stress response effects of oxycodone versus sufentanil for patients undergoing cardiac valve replacement. Ninety-four patients were enrolled and assigned to group Oxy, group Suf and group NS. Patients in group Oxy were administrated with oxycodone 0.1 mg/kg, group Suf received sufentanil 0.1 μg/kg and group NS were given equivalent volume of normal saline. The primary outcomes included serum levels of cortisol, norepinephrine, and adrenaline. The secondary outcomes involved bispectral index value and the observer's assessment of awareness/sedation grade, levels of mean arterial pressure, heart rate, and the adverse reactions. Compared to group NS, the serum levels of cortisol at T₁ to T₅ (P < .05), and levels of norepinephrine and adrenaline at T₃ to T₅ (P < .05) in group Oxy and Suf were lower. The bispectral index value and observer's assessment of awareness/sedation grade T₁ to T₂ (P < .05) in group Suf were lower than those in group Oxy and NS. Compared with group NS, the levels of mean arterial pressure and heart rate in group Oxy and Suf at T₃ to T₅ (P < .05) were lower. The incidence of coughing was significantly higher in group Suf (23.3%), but not in group NS (6.7%), than that in group Oxy (3.3%). The preemptive analgesia of oxycodone may be used to inhibit the stress response, without leading to excessive sedation and respiratory depression, which may also help to stabilize hemodynamics during preoperative period.

Preparation of bivalent agonists for targeting the mu opioid and cannabinoid receptors

In order to obtain novel pharmacological tools and to investigate a multitargeting analgesic strategy, the CB₁ and CB₂ cannabinoid receptor agonist JWH-018 was conjugated with the opiate analgesic oxycodone or with an enkephalin related tetrapeptide. The opioid and cannabinoid pharmacophores were coupled via spacers of different length and chemical structure. In vitro radioligand binding experiments confirmed that the resulting bivalent compounds bound both to the opioid and to the cannabinoid receptors with moderate to high affinity. The highest affinity bivalent derivatives 11 and 19 exhibited agonist properties in [³⁵S]GTPγS binding assays. These compounds activated MOR and CB (11 mainly CB₂, whereas 19 mainly CB₁) receptor-mediated signaling, as it was revealed by experiments using receptor specific antagonists. In rats both 11 and 19 exhibited antiallodynic effect similar to the parent drugs in 20 μg dose at spinal level. These results support the strategy of multitargeting G-protein coupled receptors to develop lead compounds with antinociceptive properties.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Comparison of the antinociceptive profiles of morphine and oxycodone in two models of inflammatory and osteoarthritic pain in rat

Oxycodone and morphine are two opioid drugs commonly used for the treatment of moderate to severe pain. However, their use in the management of noncancer pain remains a controversial issue and, in this respect, the evidence on their effectiveness and safety, particularly in osteoarthritis, is being questioned. In order to analyse their analgesic profile, two different pain models in rats were used: the formalin-induced inflammatory pain and the monosodium iodoacetate (MIA)-induced knee osteoarthritic pain. Drugs were administered systemically (i.p.) and their antinociceptive effect and potency were assessed. In the formalin test, both morphine and oxycodone produced a dose-dependent antinociceptive effect, but oxycodone outdid morphine in terms of effectiveness and potency (nearly two times) in the early (acute nociceptive) as in the late phase (inflammatory). In the osteoarthritis model, both drugs reduced movement-evoked pain (knee-bend test), mechanical allodynia (von Frey test) and heat hyperalgesia (Plantar test). Pretreatment with naloxone and naloxone methiodide reduced morphine and oxycodone effects. Peripheral mu-opioid receptors play a crucial role in the antinociceptive effect of both drugs on movement-evoked pain and heat hyperalgesia, but not on tactile allodynia. The main finding of our study is that oxycodone has a better antinociceptive profile in the inflammatory and osteoarthritic pain, being more effective than morphine at 14 days post-MIA injection (phase with neuropathic pain); it overcame the morphine effect by improving the movement-induced pain, tactile allodynia and heat hyperalgesia. Therefore, oxycodone could be an interesting option to treat patients suffering from knee osteoarthritis when opioids are required.

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.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

GpTx-1 and [Ala5 , Phe6 , Leu26 , Arg28 ]GpTx-1, two peptide NaV 1.7 inhibitors: analgesic and tolerance properties at the spinal level

BACKGROUND AND PURPOSE

The voltage-gated sodium channel Na_V 1.7 is considered a therapeutic target for pain treatment based on human genetic evidence. GpTx-1 and its potent analogue [Ala⁵ , Phe⁶ , Leu²⁶ , Arg²⁸ ]GpTx-1 (GpTx-1-71) were recently characterized as Na_V 1.7 inhibitors in vitro. Furthermore, the present work was conducted to investigate the analgesic properties of these two peptides in different pain models after spinal administration.

EXPERIMENTAL APPROACH

The antinociceptive activities of both GpTx-1 and GpTx-1-71 were investigated in mouse models of acute, visceral, inflammatory and neuropathic pain. Furthermore, the side effects of GpTx-1 and GpTx-1-71 were evaluated in rotarod, antinociceptive tolerance, acute hyperlocomotion and gastrointestinal transit tests.

KEY RESULTS

The i.t. administration of both GpTx-1 and GpTx-1-71 dose-dependently produced powerful antinociception in the different pain models. This effect was attenuated by the opioid receptor antagonist naloxone, suggesting the involvement of the opioid system. In this study, repeated administration of these two_peptides produced spinal analgesia without a loss of potency over 8 days in mouse models of acute, inflammatory and neuropathic pain. Moreover, spinal administration of GpTx-1 and GpTx-1-71 did not induce significant effects on motor coordination, evoke acute hyperlocomotion or increase gastrointestinal transit time.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that the Na_V 1.7 peptide inhibitors GpTx-1 and GpTx-1-71 produce powerful, nontolerance-forming analgesia in preclinical pain models, which might be dependent on the endogenous opioid system. In addition, at the spinal level, the limited side effects imply that these Na_V 1.7 peptide inhibitors could be potentially developed as GpTx-1-based drugs for pain relief.

Effect of oxycodone hydrochloride combined with flurbiprofen axetil for intravenous patient-controlled analgesia in lower abdominal patients: A randomized trial

BACKGROUND

Problems like postoperative pain are still common phenomena after general anesthesia. Oxycodone hydrochloride is a semisynthetic opioid with a safe and excellent therapeutic effect on visceral pain. Flurbiprofen axetil has the efficacy of targeted analgesia. We hypothesize that different doses of oxycodone hydrochloride combined with flurbiprofen axetil would generate great results on postoperative intravenous analgesia in lower abdominal patients.

METHODS

In the clinical trial, 90 American Society of Anesthesiologists I or II patients scheduled for elective general anesthesia were randomly divided into 3 groups, 30 cases in each group. Group I: oxycodone hydrochloride 0.5 mg/kg + flurbiprofen axetil 150 mg, group II: oxycodone hydrochloride 0.75 mg/kg + flurbiprofen axetil 150 mg, group III: oxycodone hydrochloride 1.0 mg/kg + flurbiprofen axetil 150 mg. Dilute them with 0.9% saline to 150 mL, respectively, with the background dose of 2 mL/h, patient-controlled analgesia 2 mL per time, with an interval of 10 min, and the loading dose of 0.1 mL/kg. Record the preoperative situation, 24 h (T0) before surgery, postoperative situation, 1 h (T1), 4 h (T2), 8 h (T3), 12 h (T4), 24 h (T5), 48 h (T6), 72 h (T7) after the surgery, including the mean arterial pressure, heart rate, saturation of pulse oximetry, static and dynamic pain rating (NRS) and Ramsay sedation score, effective pressing and total pressing ratio (referred to as the pressing ratio), patient satisfaction, and occurrence of adverse reactions.

RESULTS

There was no significant statistic difference in mean arterial blood pressure, heart rate, arterial oxygen saturation, and adverse reactions among the 2 groups at each time point (P > .05). Compared with group I, the static NRS rating in group II and group III were significantly lower than that in group I (P < .05) from T1 to T5. The dynamic NRS rating of group II from T1 to T4 and that of group III from T1 to T5 were significantly lower (P < .05). The effective pressing and total pressing ratio was significantly higher (P < .05). There was no significant statistic difference between group II and group III in NRS rating and the effective pressing and total pressing ratio (P > .05). Compared with group III, the Ramsay sedation scores of group I and group II were significantly lower from T1 to T4 (P < .05).

CONCLUSION

The dose of 0.75 mg/kg oxycodone hydrochloride combined with flurbiprofen axetil can provide safe and effective postoperative analgesia for lower abdominal patients, with fewer adverse reactions.

Relationship between receptor occupancy and the antinociceptive effect of mu opioid receptor agonists in male rats

The analgesic mechanisms of mu opioid receptor (MOR) agonists, including receptor occupancy at the site of action, are not completely understood. The aims of the present study were to evaluate: (i) receptor occupancy in the rat brain after administration of MOR agonists; (ii) the relationship between occupancy and the antinociceptive effect. Morphine (2 or 4 mg/kg) or oxycodone (1 or 3 mg/kg) was subcutaneously administered to rats. The antinociceptive effect of these drugs was measured by the hot-plate test. MOR occupancy in the thalamus was assessed by conducting an ex vivo receptor binding assay using [³H] [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin, followed by autoradiographic analysis. Both drugs produced antinociception in a dose-dependent manner, and these effects disappeared after the time point at which the maximal effect was elicited. Thalamic MOR occupancy was observed in a dose-dependent manner at the time point at which maximal antinociception was elicited, and relatively low occupancy was observed when the antinociceptive effect was decreasing. Good correlation between thalamic MOR occupancy and the antinociceptive effect was observed. These findings provide direct evidence for the receptor occupancy of MOR agonists at the site of action and its relationship with the analgesic effect.

Effect of oxycodone patient-controlled intravenous analgesia after cesarean section: a randomized controlled study

Background

Oxycodone is a semisynthetic μ-opioid receptor agonist with a potentially good analgesic efficacy in visceral pain. This study aims to compare the efficacy of oxycodone with sufentanil patient-controlled intravenous analgesia (PCIA).

Methods

One hundred and twenty primiparas undergoing elective cesarean section were randomized into four groups by different drugs of PCIA: group S (sufentanil 100 μg), group OS1 (sufentanil 70 μg, oxycodone 30 mg), group OS2 (sufentanil 50 μg, oxycodone 50 mg), and group O (oxycodone 100 mg). Ramosetron 0.3 mg was added to each group. In all groups, drugs were diluted to 100 mL and managed with a continuous infusion of 1 mL·h⁻¹, a bolus dose of 2 mL, and a lockout interval of 15 min. The maximum dose of PCIA per hour was 10 mL. After surgery, pain scores, PCIA doses, and side effects were compared among groups.

Results

At all time points (6, 12, and 24 h after surgery), Numerical Rating Scale (NRS) of uterine cramping pain (NRS-U) scores in group O were lower than those in groups OS1 and S (P<0.008) and NRS-U scores in groups OS2 and OS1 were lower than that in group S (P<0.008). NRS of moving into the sitting position (NRS-S) scores in group O were lower than those in the other groups (P<0.008). NRS-S scores in group OS2 were lower than those in groups OS1 and S (P<0.008). At 12 and 24 h after surgery, NRS of incision pain at rest (NRS-R) scores in group O were lower than those in the other groups (P<0.008). At all time points, NRS-R scores in group OS2 were lower than those in groups OS1 and S (P<0.008). The number of PCIA boluses and amount of opioid consumption in group O were lower than those in groups OS1 and S at all time points (P<0.008).

Conclusion

Oxycodone PCIA may be more effective than sufentanil PCIA for pain relief after cesarean section but the incidence of side effects needs further investigation.

Blockade of P2X4 Receptors Inhibits Neuropathic Pain-Related Behavior by Preventing MMP-9 Activation and, Consequently, Pronociceptive Interleukin Release in a Rat Model

Neuropathic pain is still an extremely important problem in today's medicine because opioids, which are commonly used to reduce pain, have limited efficacy in this type of pathology. Therefore, complementary therapy is needed. Our experiments were performed in rats to evaluate the contribution of the purinergic system, especially P2X4 receptor (P2X4R), in the modulation of glia activation and, consequently, the levels of nociceptive interleukins after chronic constriction injury (CCI) of the right sciatic nerve, a rat model of neuropathic pain. Moreover, we studied how intrathecal (ith.) injection of a P2X4R antagonist Tricarbonyldichlororuthenium (II) dimer (CORM-2) modulates nociceptive transmission and opioid effectiveness in the CCI model. Our results demonstrate that repeated ith. administration of CORM-2 once daily (20 μg/5 μl, 16 and 1 h before CCI and then daily) for eight consecutive days significantly reduced pain-related behavior and activation of both spinal microglia and/or astroglia induced by CCI. Moreover, even a single administration of CORM-2 on day 7 after CCI attenuated mechanical and thermal hypersensitivity as efficiently as morphine and buprenorphine. In addition, using Western blot, we have shown that repeated ith. administration of CORM-2 lowers the CCI-elevated level of MMP-9 and pronociceptive interleukins (IL-1β, IL-18, IL-6) in the dorsal L4-L6 spinal cord and/or DRG. Furthermore, in parallel, CORM-2 upregulates spinal IL-1Ra; however, it does not influence other antinociceptive factors, IL-10 and IL-18BP. Additionally, based on our biochemical results, we hypothesize that p38MAPK, ERK1/2 and PI3K/Akt but not the NLRP3/Caspase-1 pathway are partly involved in the CORM-2 analgesic effects in rat neuropathic pain. Our data provide new evidence that P2X4R may indeed play a significant role in neuropathic pain development by modulating neuroimmune interactions in the spinal cord and DRG, suggesting that its blockade may have potential therapeutic utility.

Preventive Analgesia with Pregabalin in Neuropathic Pain from “Failed Back Surgery Syndrome”: Assessment of Sleep Quality and Disability

OBJECTIVE

Pregabalin group (PGB) is an antiepileptic used to treat neuropathic pain. We evaluated analgesic efficacy and safety for postoperative/chronic pain, disability, and sleep quality in patients who underwent spine surgery administered with PGB, or not, during the presurgical and postsurgical periods.

DESIGN

Retrospective cohort study of 60 patients (two groups with 30 patients) with full information on 50 (29 with PGB and 21 without PGB). Ten patients were dismissed as information was lacking. The PGB group (P) (29 patients) received 75 mg/12 hours before surgery, 150 mg 10 hours after surgery, and 150 mg/12 hours 3 days after surgery. The control group (C; 21 patients) took no PGB.

METHODS

Neuropathic pain was assessed before surgery, and 2 and 6 months later using visual analog scales (VAS), DN4, disability (Oswestry), and sleep quality. No serious adverse events occurred with PGB.

RESULTS

The median VAS pain score at rest was lower in the PGB group at 2 months postsurgery (1 vs 2, P = 0.032), as was the median DN4 score (0 vs 3, P = 0.032) and the median Oswestry disability index (ODI: 12 vs 18, P = 0.001). At 6 months postsurgery, pain scores were also lower in the PGB group for VAS (0 vs 4, P = 0.001), DN4 score (0 vs 4, P = 0.001) and the ODI (10 vs 24, P = 0.001). Improvement in the functionality and sleep quality of the PGB group was noteworthy (P = 0.018).

CONCLUSIONS

PGB has analgesic/antihyperalgesic effects on postoperative neuropathic pain after surgery for lumbar disc hernia. Our findings show that this benefit increases with time.

Suppression of the acute upregulation of phosphorylated-extracellular regulated kinase in ventral tegmental area by a μ-opioid receptor agonist is related to resistance to rewarding effects in a mouse model of bone cancer

We investigated the mechanisms underlying the suppression of the rewarding effects of opioids using the femur bone cancer (FBC) mouse model. The rewarding and antinociceptive effects of subcutaneously administered morphine and oxycodone in the FBC model mice were assessed using the conditioned place preference test and the von-Frey test. In FBC mice, antinociceptive doses of morphine (30 mg/kg) and oxycodone (5 mg/kg) did not produce the rewarding effects but excessive doses of morphine (300 mg/kg) and oxycodone (100 mg/kg) did. Western blot analyses revealed a transient and significant increase in phosphorylated-extracellular regulated kinase (p-ERK) levels in ventral tegmental area (VTA) 5 min after the administration of morphine in sham-group. Interestingly, in FBC group, a regular dose of morphine did not increase p-ERK levels but a high dose of morphine caused an increase in p-ERK level 5 min after administration. The rewarding effects of a regular dose of and a high dose of morphine in the sham-operation and FBC model, respectively, were significantly inhibited by the MEK inhibitor. The suppression of p-ERK might result in resistance to these rewarding effects under the conditions of bone cancer.

Involvement of mesolimbic dopaminergic network in neuropathic pain relief by treadmill exercise: A study for specific neural control with Gi-DREADD in mice

Background

Exercise alleviates pain and it is a central component of treatment strategy for chronic pain in clinical setting. However, little is known about mechanism of this exercise-induced hypoalgesia. The mesolimbic dopaminergic network plays a role in positive emotions to rewards including motivation and pleasure. Pain negatively modulates these emotions, but appropriate exercise is considered to activate the dopaminergic network. We investigated possible involvement of this network as a mechanism of exercise-induced hypoalgesia.

Methods

In the present study, we developed a protocol of treadmill exercise, which was able to recover pain threshold under partial sciatic nerve ligation in mice, and investigated involvement of the dopaminergic reward network in exercise-induced hypoalgesia. To temporally suppress a neural activation during exercise, a genetically modified inhibitory G-protein-coupled receptor, hM4Di, was specifically expressed on dopaminergic pathway from the ventral tegmental area to the nucleus accumbens.

Results

The chemogenetic-specific neural suppression by Gi-DREADD system dramatically offset the effect of exercise-induced hypoalgesia in transgenic mice with hM4Di expressed on the ventral tegmental area dopamine neurons. Additionally, anti-exercise-induced hypoalgesia effect was significantly observed under the suppression of neurons projecting out of the ventral tegmental area to the nucleus accumbens as well.

Conclusion

Our findings suggest that the dopaminergic pathway from the ventral tegmental area to the nucleus accumbens is involved in the anti-nociception under low-intensity exercise under a neuropathic pain-like state.

Chronic intrathecal infusion of T-type calcium channel blockers attenuates CaV3.2 upregulation in nerve-ligated rats

OBJECTIVE

T-type channel (TCC) Ca_V3.2 plays a pivotal role in pain transmission. In this study, we examined the effects of intrathecal TCC blockers on Ca_V3.2 expression in a L5/6 spinal nerve ligation (SNL) pain model. The neurotoxicity of TCC blockers were also evaluated.

METHODS

Male Sprague-Dawley rats (200-250 g) were used for right L5/6 SNL to induce neuropathic pain. Intrathecal infusion of saline or TCC blockers [mibefradil (0.7 μg/h) or ethosuximide (60 μg/h)] was started after surgery for 7 days. Fluorescent immunohistochemistry and Western blotting were used to determine the expression pattern and protein level of Ca_V3.2. Hematoxylin-eosin and toluidine blue staining were used to evaluate the neurotoxicity of tested agents.

RESULTS

Seven days after SNL, Ca_V3.2 protein levels were upregulated in ipsi-lateral L5/6 spinal cord and dorsal root ganglia (DRG) in immunofluorescence and Western blotting studies. Compared with the saline-treated group, rats receiving mibefradil or ethosuximide showed significant lower Ca_V3.2 expression in the spinal cord and DRG. No obvious histopathologic change in hematoxylin-eosin and toluidine blue staining were observed in all tested groups.

CONCLUSION

In this study, we demonstrate that SNL-induced Ca_V3.2 upregulation in the spinal cord and DRG was attenuated by intrathecal infusion of mibefradil or ethosuximide. No obvious neurotoxicity effects were observed in all the tested groups. Our data suggest that continuous intrathecal infusion of TCC blockers may be considered as a promising alternative for the treatment of nerve injury-induced pain.

Sex differences in responsiveness to the prescription opioid oxycodone in mice

Over-prescription and increased nonmedical use of oxycodone has become a major concern. Despite its increased use, preclinical data concerning oxycodone's effects are still limited, especially in rodent models. To address this, we examined oxycodone's effects on place preference, locomotor activation, corticosterone levels, and thermal analgesia across a range of doses (between 0.3 and 10mg/kg) in gonadally intact, adult male and female C57BL/6J mice. Males and females showed oxycodone-induced conditioned place preference and did not show significant between-sex differences in their place preference behavior. During both CPP conditioning sessions and open field assay, locomotor activity was increased by 1, 3, and 10mg/kg oxycodone in females and by 3 and 10mg/kg oxycodone in males. Plasma corticosterone levels were higher in females (compared to males) at baseline as well as following acute oxycodone injection and open field testing. The time course of oxycodone-induced analgesia was similar in males and females, however the total antinociceptive effect (AUC0-120min) was larger in males compared to females at the highest dose tested (10mg/kg). Taken together, these data suggest that male and female mice are modestly different in their responses to oxycodone.

Effects of dextromethorphan and oxycodone on treatment of neuropathic pain in mice

Background

Neuropathic pain is a very troublesome and difficult pain to treat. Although opioids are the best analgesics for cancer and surgical pain in clinic, only oxycodone among opioids shows better efficacy to alleviate neuropathic pain. However, many side effects associated with the use of oxycodone render the continued use of it in neuropathic pain treatment undesirable. Hence, we explored whether dextromethorphan (DM, a known N-methyl-D-aspartate receptor antagonist with neuroprotective properties) could potentiate the anti-allodynic effect of oxycodone and underlying mechanisms regarding to glial cells (astrocytes and microglia) activation and proinflammatory cytokines release in a spinal nerve injury (SNL) mice model.

Results

Oxycodone produced a dose-dependent anti-allodynic effect. Co-administration of DM at a dose of 10 mg/kg (i.p.) (DM10) which had no anti-allodynic effect by itself enhanced the acute oxycodone (1 mg/kg, s.c.) effect. When the chronic anti-allodynic effects were examined, co-administration of DM10 also significantly enhanced the oxycodone effect at 3 mg/kg. Furthermore, oxycodone decreased SNL-induced activation of glial cells (astrocytes and microglia) and plasma levels of proinflammatory cytokines (IL-6, IL-1β and TNF-α). Co-administration of DM10 potentiated these effects of oxycodone.

Conclusion

The combined use of DM with oxycodone may have therapeutic potential for decreasing the effective dose of oxycodone on the treatment of neuropathic pain. Attenuation of the glial activation and proinflammatory cytokines in the spinal cord may be important mechanisms for these effects of DM.

Involvement of μ- and δ-opioid receptor function in the rewarding effect of (±)-pentazocine

Most opioid receptor agonists have abuse potential, and the rewarding effects of opioids can be reduced in the presence of pain. While each of the enantiomers of pentazocine has a differential pharmacologic profile, (±)-pentazocine has been used clinically for the treatment of pain. However, little information is available regarding which components of pentazocine are associated with its rewarding effects, and whether the (±)-pentazocine-induced rewarding effects can be suppressed under pain. Therefore, the present study was performed to investigate the effects of pain on the acquisition of the rewarding effects of (±)-pentazocine, and to examine the mechanism of the rewarding effects of (±)-pentazocine using the conditioned place preference paradigm. (±)-Pentazocine and (-)-pentazocine, but not (+)-pentazocine, produced significant rewarding effects. Even though the rewarding effects induced by (±)-pentazocine were significantly suppressed under pain induced by formalin, accompanied by increase of preprodynorphin mRNA levels in the nucleus accumbens, a high dose of (±)-pentazocine produced significant rewarding effects under pain. In the normal condition, (±)-pentazocine-induced rewarding effects were blocked by a low dose of naloxone, whereas the rewarding effects induced by high doses of pentazocine under pain were suppressed by naltrindole (a δ-opioid receptor antagonist). Interestingly, (±)-pentazocine did not significantly affect dopamine levels in the nucleus accumbens. These findings suggest that the rewarding effects of (-)-pentazocine may contribute to the abuse potential of (±)-pentazocine through μ- as well as δ-opioid receptors, without robust activation of the mesolimbic dopaminergic system. We also found that neural adaptations can reduce the abuse potential of (±)-pentazocine under pain.