Evaluation of potential sex differences in the subjective and analgesic effects of morphine in normal, healthy volunteers

Disruption of positive- and negative-feature morphine interoceptive occasion setters by dopamine receptor agonism and antagonism in male and female rats

RATIONALE

Internally perceived stimuli evoked by morphine administration can form Pavlovian associations such that they can function as occasion setters (OSs) for externally perceived reward cues in rats, coming to modulate reward-seeking behaviour. Though much research has investigated mechanisms underlying opioid-related reinforcement and analgesia, neurotransmitter systems involved in the functioning of opioids as Pavlovian interoceptive discriminative stimuli remain to be disentangled despite documented differences in the development of tolerance to analgesic versus discriminative stimulus effects.

OBJECTIVES

Dopamine has been implicated in many opioid-related behaviours, so we aimed to investigate the role of this neurotransmitter in expression of morphine occasion setting.

METHODS

Male and female rats were assigned to positive- (FP) or negative-feature (FN) groups and received an injection of morphine or saline before each training session. A 15-s white noise conditioned stimulus (CS) was presented 8 times during every training session; offset of this stimulus was followed by 4-s access to liquid sucrose on morphine, but not saline, sessions for FP rats. FN rats learned the reverse contingency. Following stable discrimination, rats began generalization testing for expression of morphine-guided sucrose seeking after systemic pretreatment with different doses of the non-selective dopamine receptor antagonist, flupenthixol, and the non-selective dopamine receptor agonist, apomorphine, combined with training doses of morphine or saline in a Latin-square design.

RESULTS

The morphine discrimination was acquired under both FP and FN contingencies by males and females. Neither flupenthixol nor apomorphine at any dose substituted for morphine, but both apomorphine and flupenthixol disrupted expression of the morphine OS. This inhibition was specific to sucrose seeking during CS presentations rather than during the period before CS onset and, in the case of apomorphine more so than flupenthixol, to trials on which access to sucrose was anticipated.

CONCLUSIONS

Our findings lend support to a mechanism of occasion setting involving gating of CS-induced dopamine release rather than by direct dopaminergic modulation by the morphine stimulus.

Effects of selective dopamine D3 receptor partial agonist/antagonists on oxycodone self-administration and antinociception in monkeys

Recent studies suggest that dopamine D3 receptors (D3R) may be a therapeutic target for opioid use disorders (OUD). This study examined the effects of the D3R partial agonist (±)VK4-40 and the D3R-selective antagonist (±)VK4-116, compared to the mu-opioid receptor antagonist naltrexone (NTX), in nonhuman primate models of OUD and antinociception. Adult male and female (N = 4/sex) cynomolgus monkeys were trained to self-administer oxycodone (0.003-0.1 mg/kg/injection) first under a fixed-ratio (FR) and then a progressive-ratio (PR) schedule of reinforcement during daily 1- and 4-hr sessions, respectively. Under the FR schedule, intravenous NTX (0.01-0.1 mg/kg), (±)VK4-116 (1.0-10 mg/kg), and (±)VK4-40 (1.0-10 mg/kg) were studied in combination with the peak oxycodone dose and a dose on the descending limb of the dose-effect curve; NTX and (±)VK4-40 were also studied at the peak of the PR dose-response curve (N = 4). Following saline extinction, each compound was examined on oxycodone-induced reinstatement. Finally, these compounds were assessed in adult male rhesus monkeys (N = 3) in a warm-water (38 °C, 50 °C, 54 °C) tail withdrawal assay. NTX decreased responding on the peak of the FR oxycodone dose-response curve, but increased responding on the descending limb. (±)VK4-40, but not (±)VK4-116, significantly decreased peak oxycodone self-administration; (±)VK4-40 did not increase responding on the descending limb. NTX and (±)VK4-40, but not (±)VK4-116, attenuated oxycodone-induced reinstatement. Under PR responding, NTX and (±)VK4-40 decreased breakpoints. Oxycodone-induced antinociception was attenuated by NTX, but not by (±)VK4-40 or (±)VK4-116. Together, these results suggest that further research evaluating the effects of (±)VK4-40 as a novel pharmacotherapy for OUD is warranted.

Opioid analgesic effects on subjective well-being in the operating theatre

Exposure to opioid analgesics due to surgery increases the risk of new persistent opioid use. A mechanistic hypothesis for opioids' abuse liability rests on the belief that, in addition to pain relief, acute opioid treatment improves well-being (e.g. via euphoria) and relieves anxiety. However, opioids do not consistently improve mood in laboratory studies of healthy non-opioid users. This observational study determined how two commonly used opioid analgesics affected patients' subjective well-being in standard clinical practice. Day surgery patients rated how good and how anxious they felt before and after an open-label infusion of remifentanil (n = 159) or oxycodone (n = 110) in the operating theatre before general anaesthesia. One minute after drug injection, patients reported feeling intoxicated (> 6/10 points). Anxiety was reduced after opioids, but this anxiolytic effect was modest (remifentanil Cohen's d = 0.21; oxycodone d = 0.31). There was moderate to strong evidence against a concurrent improvement in well-being (Bayes factors > 6). After remifentanil, ratings of 'feeling good' were significantly reduced from pre-drug ratings (d = 0.28). After oxycodone, one in three participants felt better than pre-drug. Exploratory ordered logistic regressions revealed a link between previous opioid exposure and opioid effects on well-being, as only 14 of the 80 opioid-naïve patients reported feeling better after opioid injection. The odds of improved well-being ratings after opioids were higher in patients with previous opioid exposure and highest in patients with > 2 weeks previous opioid use (adjusted OR = 4.4). These data suggest that opioid-induced improvement of well-being is infrequent in opioid-naïve patients. We speculate that peri-operative exposure could increase risk of persistent use by rendering subsequent positive opioid effects on well-being more likely.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

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.

Oxycodone self-administration and reinstatement in male and female squirrel monkeys: Effects of alternative reinforcer availability

The use of non-drug alternative reinforcers has long been utilized as a component of therapeutic interventions for the management of substance use disorder; however, the conditions under which alternative reinforcers are most effective are not well characterized. This study evaluated the impact of varying the magnitude of an alternative reinforcer on oxycodone self-administration and reinstatement in male and female squirrel monkeys. Subjects (n=4/sex) were trained under concurrent second-order schedules of reinforcement for intravenous oxycodone (0.001-0.1mg/kg/inj) on one lever, and sweetened condensed milk (5, 10, 20, 30% in water) on another. Oxycodone-primed reinstatement was evaluated by administering 0.32mg/kg oxycodone prior to sessions in which saline was available on the drug-paired lever. During oxycodone self-administration sessions, milk availability decreased oxycodone self-administration and preference in a concentration-dependent manner; low milk concentrations were more effective at decreasing oxycodone’s reinforcing potency in males. During reinstatement tests, milk significantly attenuated oxycodone-primed responding in both males and females; low milk concentrations were more effective at decreasing the priming effects of oxycodone in females. That alternative reinforcers differentially impacted self-administration and reinstatement in a sex-dependent manner suggests that treatment strategies that utilize alternative reinforcers may be more effective in males or females depending on when they are implemented.

Sublingual dexmedetomidine (BXCL501) reduces opioid withdrawal symptoms: findings from a multi-site, phase 1b/2, randomized, double-blind, placebo-controlled trial

Background: Like other alpha-2-adrenergic receptor agonists, dexmedetomidine may reduce the severity of opioid withdrawal but with fewer adverse cardiovascular effects.Objective: This study assessed the safety of sublingual dexmedetomidine (BXCL501) and its preliminary efficacy in treating opioid withdrawal (ClinicalTrials.gov: NCT04470050).Methods: Withdrawal was induced among individuals with physiological dependence on opioids via discontinuation of oral morphine (Days 1-5). Participants were randomized to receive placebo or active BXCL501: 30, 60, 90, 120, 180, and 240 μg twice daily (Days 6-12). Treatment-emergent adverse events (TEAEs) were the primary outcome measure. Secondary outcomes included the Clinical and Subjective Opiate Withdrawal Scales (COWS and SOWS-Gossop, respectively), and the Agitation and Calmness Evaluation Scale (ACES).Results: Of 225 participants enrolled, 90 discontinued during morphine stabilization. Post-BXCL501 randomization (Day 6) data were available from 135 participants (73% male), with 33% completing thru Day 12. In total, 36 subjects reported 1 or more TEAE. Higher doses of BXCL501 (i.e. 180 and 240 µg, twice daily) increased the frequency of: hypotension, orthostatic hypotension, and somnolence. TEAEs related to BXCL501 were mild or moderate in severity, except for one participant in the 120 µg condition whose orthostatic hypotension and bradycardia were classified as severe. Higher BXCL501 dose conditions (120, 180, and 240 µg) resulted in statistically significant reductions in COWS & SOWS scores. Mean ratings on the ACES were between 3 (mild), 4 (normal), and 5 (mild calmness), with few significant differences as a function of dose.Conclusions: These findings support the continued development of BXCL501 for the management of opioid withdrawal.

Opioid-blunted cortisol response to stress is associated with increased negative mood and wanting of social reward

Animal research suggests a central role of the μ-opioid receptor (MOR) system in regulating affiliative behaviors and in mediating the stress-buffering function of social contact. However, the neurochemistry of stress-related social contact seeking in humans is still poorly understood. In a randomized, double-blind, between-subjects design, healthy female volunteers (N = 80) received either 10 mg of the µ-opioid agonist morphine sulfate, or a placebo. Following a standardized psychosocial stress induction, participants engaged in a social reward task, in which the motivation to obtain skin-to-skin social touch and the hedonic reactions elicited by such touch were assessed. Morphine prevented the increase of salivary cortisol typically observed following acute stress exposure. Notably, this altered HPA axis responsivity was associated with increased negative affect in response to psychosocial stress, and with enhanced subjective wanting of highly rewarding social contact. These findings provide novel evidence on the effect of exogenous opioids administration on the reactions to psychosocial stress and point to a state-dependent regulation of social motivation.

Low-Dose Morphine Reduces Pain Perception and Blood Pressure, but Not Muscle Sympathetic Outflow, Responses During the Cold Pressor Test

Our knowledge about how low-dose (analgesic) morphine affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose morphine affects human autonomic cardiovascular responses during painful stimuli in conscious humans. Therefore, we tested the hypothesis that low-dose morphine reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-nine participants (14F/15M; 29±6 y; 26±4 kg•m^-2, mean ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ~0.4 °C ice bath for two minutes) before and ~35 minutes after drug/placebo administration (5 mg IV morphine or saline). We compared pain perception (100 mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography; 14 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and post-drug/placebo time points) using paired, two-tailed t-tests. Before drug/placebo infusion, perceived pain (p=0.92), Δ MSNA burst frequency (n=14, p=0.21), and Δ mean BP (p=0.39) during the CPT were not different between trials. After the drug/placebo infusion, morphine versus placebo attenuated perceived pain (morphine: 43±20 vs. placebo: 57±24 mm,p<0.001) and Δ mean BP (morphine: 10±7 vs. placebo: 13±8 mmHg,p=0.003), but not Δ MSNA burst frequency (morphine: 10±11 vs. placebo: 13±11 bursts/minute,p=0.12), during the CPT. Reductions in pain perception and Δ mean BP were only weakly related (r=0.34,p=0.07; post-morphine CPT minus post-placebo CPT). These data provide valuable information regarding how low-dose morphine affects autonomic cardiovascular responses during an experimental painful stimulus.

Sex-Related Differences in Pharmacological Response to CNS Drugs: A Narrative Review

In the last decades, both animal and human studies have neglected female subjects with the aim of evading a theorized intricacy of feminine hormonal status. However, clinical experience proves that pharmacological response may vary between the two sexes since pathophysiological dissimilarities between men and women significantly influence the pharmacokinetics and pharmacodynamics of drugs. Sex-related differences in central nervous system (CNS) medication are particularly challenging to assess due to the complexity of disease manifestation, drugs’ intricate mechanisms of action, and lack of trustworthy means of evaluating the clinical response to medication. Although many studies showed contrary results, it appears to be a general tendency towards a certain sex-related difference in each pharmacological class. Broadly, opioids seem to produce better analgesia in women especially when they are administered for a prolonged period of time. On the other hand, respiratory and gastrointestinal adverse drug reactions (ADRs) following morphine therapy are more prevalent among female patients. Regarding antidepressants, studies suggest that males might respond better to tricyclic antidepressants (TCAs), whereas females prefer selective serotonin reuptake inhibitors (SSRI), probably due to their tolerance to particular ADRs. In general, studies missed spotting any significant sex-related differences in the therapeutic effect of antiepileptic drugs (AED), but ADRs have sex variations in conjunction with sex hormones’ metabolism. On the subject of antipsychotic therapy, women appear to have a superior response to this pharmacological class, although there are also studies claiming the opposite. However, it seems that reported sex-related differences regarding ADRs are steadier: women are more at risk of developing various side effects, such as metabolic dysfunctions, cardiovascular disorders, and hyperprolactinemia. Taking all of the above into account, it seems that response to CNS drugs might be occasionally influenced by sex as a biological variable. Nonetheless, although for each pharmacological class, studies generally converge to a certain pattern, opposite outcomes are standing in the way of a clear consensus. Hence, the fact that so many studies are yielding conflicting results emphasizes once again the need to address sex-related differences in pharmacological response to drugs.

Subjective and Cardiovascular Responses to an Acute Laboratory Gambling Task in Men and Women

Men have previously been overrepresented in gambling for money but in recent years there has been an increase in recognition that women who gamble are "catching up" with their male counterparts. There have been few experimental studies investigating the subjective effects of gambling, and even less have explored the gender differences. As gender differences previously have been reported in the subjective effects of several drugs of abuse such as opioids, amphetamines and alcohol, we sought to investigate if the subjective effects of gambling also differed by gender. The present article analyzes if gender modulates the subjective and physiological effects of an acute laboratory gambling task in healthy men and women. Eighty-two men and women (n = 35 men, n = 47 women) were tested with an online slot machine gambling session and self-report questionnaires of mood and blood pressure were taken before and after gambling. Both men and women showed stimulatory effects of gambling i.e., feelings of high and euphoria and but no differences were found between genders. Findings suggest that both men and women equally experience a pattern of stimulatory effects of gambling from the gambling situation. Gambling therefore seems to have the same abuse potential in both men and women. Although the gap between men and women is narrowing, immediate subjective and physiologic responses do not explain gender differences in the epidemiology of pathological gambling. The contexts and factors that foster or hinder the evolution of gambling addiction in males and females should be further explored. This conclusion is interesting in light of that men are over three times more at risk to experience gambling related problems than women and this risk may depend on other factors involved in the development of addiction.

No antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol in male and female rats with persistent inflammatory pain

Studies have demonstrated antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol (THC) in animals, but whether such synergy occurs against all types of pain and in humans is unclear. Because a majority of chronic pain patients are women, and sex differences in morphine and THC potencies have been observed in rodents, the present study examined sex-specific effects of morphine and THC given alone and in combination, in rats with persistent inflammatory pain. On day 1, baseline mechanical and thermal response thresholds, hindpaw weight-bearing, locomotor activity, and hindpaw thickness were determined. Inflammation was then induced via hindpaw injection of complete Freund's adjuvant (CFA). Three days later, morphine (s.c.), THC (i.p) or a morphine-THC combination (1:1, 3:1 and 1:3 dose ratios) was administered, and behavioral testing was conducted at 30-240 min postinjection. Morphine alone was antiallodynic and antihyperalgesic, with no sex differences, but at some doses increased weight-bearing on the CFA-treated paw more in males than females. THC alone reduced mechanical allodynia with similar potency in both sexes, but reduced thermal hyperalgesia and locomotor activity with greater potency in females than males. All morphine-THC combinations reduced allodynia and hyperalgesia, but isobolographic analysis of mechanical allodynia data showed no significant morphine-THC synergy in either sex. Additionally, whereas morphine alone was antinociceptive at doses that did not suppress locomotion, morphine-THC combinations suppressed locomotion and did not increase weight-bearing on the inflamed paw. These results suggest that THC is unlikely to be a beneficial adjuvant when given in combination with morphine for reducing established inflammatory pain.

Sex differences in type and occurrence of adverse reactions to opioid analgesics: a retrospective cohort study

OBJECTIVES

Sex as a biological variable affects response to opioids. However, few reports describe the prevalence of specific adverse reactions to commonly prescribed opioids in men and women separately. A large cohort was used to investigate sex differences in type and occurrence of adverse reactions associated with use of codeine, tramadol, oxycodone and hydrocodone.

DESIGN

Retrospective cohort study.

SETTING

Participants in the Right Drug, Right Dose, Right Time (RIGHT) Study.

PARTICIPANTS

The medical records of 8457 participants in the RIGHT Study who received an opioid prescription between 1 January 2004 and 31 December 2017 were reviewed 61% women, 94% white, median age (Q1-Q3)=58 (47-66).

PRIMARY AND SECONDARY OUTCOME MEASURES

Adverse reactions including gastrointestinal, skin, psychiatric and nervous system issues were collected from the allergy section of each patient's medical record. Sex differences in the risk of adverse reactions due to prescribed opioids were modelled using logistic regression adjusted for age, body mass index, race and ethnicity.

RESULTS

From 8457 participants (of which 449 (5.3%) reported adverse reactions), more women (6.5%) than men (3.4%) reported adverse reactions to at least one opioid (OR (95% CI)=2.3 (1.8 to 2.8), p<0.001). Women were more likely to report adverse reactions to tramadol (OR (95% CI)=2.8 (1.8 to 4.4), p<0.001) and oxycodone (OR (95% CI)=2.2 (1.7 to 2.9), p<0.001). Women were more likely to report gastrointestinal (OR (95% CI)=3.1 (2.3 to 4.3), p<0.001), skin (OR (95% CI)=2.1 (1.4 to 3.3), p=0.001) and nervous system issues (OR (95% CI)=2.3 (1.3 to 4.2), p=0.004).

CONCLUSIONS

These findings support the importance of sex as a biological variable to be factored into pain management studies.

κ-Opioid receptor antagonism reverses heroin withdrawal-induced hyperalgesia in male and female rats

Although opioids are potent analgesics, a consequence of chronic opioid use is hyperalgesia during withdrawal, which may contribute to opioid misuse. Dynorphin, the endogenous ligand of κ-opioid receptors (KORs), is upregulated in opioid-dependent rats and in animal models of chronic pain. However, the role of KORs in opioid withdrawal-induced hyperalgesia remains to be determined. We hypothesized that KOR antagonism would reverse opioid withdrawal-induced hyperalgesia in opioid-dependent rats. Male and female Wistar rats received daily injections of heroin (2-6 mg/kg, SC) and were tested for mechanical sensitivity in the electronic von Frey test 4-6 h into withdrawal. Female rats required significantly more heroin than male rats to reach comparable levels of both heroin-induced analgesia and hyperalgesia (6 mg/kg vs. 2 mg/kg). Once hyperalgesia was established, we tested the effects of the KOR antagonists nor-binaltorphimine (norBNI; 30 mg/kg, SC) and 5'-guanidinonaltrindole (5'GNTI; 30 mg/kg, SC). When the animals continued to receive their daily heroin treatment (or saline treatment in the repeated saline group) five times per week throughout the experiment, both KOR antagonists reversed heroin withdrawal-induced hyperalgesia. The anti-hyperalgesia effect of norBNI was more prolonged in males than in females (14 days vs. 7 days), whereas 5'GNTI had more prolonged effects in females than in males (14 days vs. 4 days). The behavioral effects of 5'GNTI coincided with higher 5'GNTI levels in the brain than in plasma when measured at 24 h, whereas 5'GNTI did not reverse hyperalgesia at 30 min posttreatment when 5'GNTI levels were higher in plasma than in the brain. Finally, we tested the effects of 5'GNTI on naloxone-induced and spontaneous signs of opioid withdrawal and found no effect in either male or female rats. These findings indicate a functional role for KORs in heroin withdrawal-induced hyperalgesia that is observed in rats of both sexes.

Cues conditioned to withdrawal and negative reinforcement: Neglected but key motivational elements driving opioid addiction

Opioid use disorder (OUD) is a debilitating disorder that affects millions of people. Neutral cues can acquire motivational properties when paired with the positive emotional effects of drug intoxication to stimulate relapse. However, much less research has been devoted to cues that become conditioned to the aversive effects of opioid withdrawal. We argue that environmental stimuli promote motivation for opioids when cues are paired with withdrawal (conditioned withdrawal) and generate opioid consumption to terminate conditioned withdrawal (conditioned negative reinforcement). We review evidence that cues associated with pain drive opioid consumption, as patients with chronic pain may misuse opioids to escape physical and emotional pain. We highlight sex differences in withdrawal-induced stress reactivity and withdrawal cue processing and discuss neurocircuitry that may underlie withdrawal cue processing in dependent individuals. These studies highlight the importance of studying cues associated with withdrawal in dependent individuals and point to areas for exploration in OUD research.

Sex- and Gender-Based Pharmacological Response to Drugs

In humans, the combination of all sex-specific genetic, epigenetic, and hormonal influences of biologic sex produces different in vivo environments for male and female cells. We dissect how these influences of sex modify the pharmacokinetics and pharmacodynamics of multiple drugs and provide examples for common drugs acting on specific organ systems. We also discuss how gender of physicians and patients may influence the therapeutic response to drugs. We aim to highlight sex as a genetic modifier of the pharmacological response to drugs, which should be considered as a necessary step toward precision medicine that will benefit men and women. SIGNIFICANCE STATEMENT: This study discusses the influences of biologic sex on the pharmacokinetics and pharmacodynamics of drugs and provides examples for common drugs acting on specific organ systems. This study also discusses how gender of physicians and patients influence the therapeutic response to drugs.

Pain in Women: A Perspective Review on a Relevant Clinical Issue that Deserves Prioritization

Introduction

Gender equity and gender medicine are opportunities not to be missed, and this Expert Group Opinion Paper on pain in women aims to review the treatment of pain conditions mainly affecting women, as well as the fundamental aspects of the different clinical response to drug treatment between the genders, and what can be done for gender-specific rehabilitation.

Methods

Perspective review.

Results

Genotypic and phenotypic differences in pain between the sexes are conditioned by anatomical, physiological, neural, hormonal, psychological, social, and cultural factors, such as the response to pharmacological treatment to control pain. The examination of these factors shows that women are affected by pain diseases more frequently and severely than men and that they report pain more frequently and with a lower pain threshold than men. Some forms of pain are inherently related to gender differences, such as pain related to the genitourinary system. However, other forms of chronic pain are seen more frequently in women than men, such as migraine, rheumatological, and musculoskeletal pain, in particular fibromyalgia.

Discussion

Research is needed into the pathophysiological basis for gender differences in the generation of acute pain and maintenance of chronic pain, including the factors that put women at higher risk for developing chronic pain. In addition, different specialties need to collaborate to develop gender-related diagnostic and therapeutic guidelines, and healthcare professionals need to upskill themselves in the appropriate management of pain using existing diagnostic tools and therapeutic options.

The opioid crisis: a contextual, social-ecological framework

The prevalence of opioid use and misuse has provoked a staggering number of deaths over the past two and a half decades. Much attention has focused on individual risks according to various characteristics and experiences. However, broader social and contextual domains are also essential contributors to the opioid crisis such as interpersonal relationships and the conditions of the community and society that people live in. Despite efforts to tackle the issue, the rates of opioid misuse and non-fatal and fatal overdose remain high. Many call for a broad public health approach, but articulation of what such a strategy could entail has not been fully realised. In order to improve the awareness surrounding opioid misuse, we developed a social-ecological framework that helps conceptualise the multivariable risk factors of opioid misuse and facilitates reviewing them in individual, interpersonal, communal and societal levels. Our framework illustrates the multi-layer complexity of the opioid crisis that more completely captures the crisis as a multidimensional issue requiring a broader and integrated approach to prevention and treatment.

Genetics and Gender in Acute Pain and Perioperative Opioid Analgesia

Experimental and clinical acute pain research in relation to biological sex and genetics started in the 1980s. Research methods became more powerful and sensitive with the advancement in affordable gene sequencing methods and high-throughput genetic assays. Decades of research has identified several potential pharmaceutical targets, providing insights into future research direction, and understanding of acute pain and opioid analgesic effects in the clinical setting. However, there is insufficient evidence to make generalized recommendations for using genetic tests for clinical practice of acute pain management.

Sex differences in opioid reinforcement under a fentanyl vs. food choice procedure in rats

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure^® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure^® reinforcement in Sprague-Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure^® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure^® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

Intact responses to non-drug rewards in long-term opioid maintenance treatment

Disruption of non-drug reward processing in addiction could stem from long-term drug use, addiction-related psychosocial stress, or a combination of these. It remains unclear whether long-term opioid maintenance treatment (OMT) disrupts reward processing. Here, we measured subjective and objective reward responsiveness in 26 previously heroin-addicted mothers in >7 years stable OMT with minimal psychosocial stress and illicit drug use. The comparison group was 30 healthy age-matched mothers (COMP). Objective reward responsiveness was assessed in a two-alternative forced-choice task with skewed rewards. Results were also compared to performance from an additional 968 healthy volunteers (meta-analytic approach). We further compared subprocesses of reward-based decisions across groups using computational modelling with a Bayesian drift diffusion model of decision making. Self-reported responsiveness to non-drug rewards was high for both groups (means: OMT = 6.59, COMP = 6.67, p = 0.84, BF₁₀ = 0.29), yielding moderate evidence against subjective anhedonia in this OMT group. Importantly, the mothers in OMT also displayed robust reward responsiveness in the behavioral task (t₁₉ = 2.72, p = 0.013, BF₁₀ = 3.98; d = 0.61). Monetary reward changed their task behavior to the same extent as the local comparison group (reward bias OMT = 0.12, COMP = 0.12, p = 0.96, BF₁₀ = 0.18) and in line with data from 968 healthy controls previously tested. Computational modelling revealed that long-term OMT did not even change decision subprocesses underpinning reward behavior. We conclude that reduced sensitivity to rewards and anhedonia are not necessary consequences of prolonged opioid use.

Neuronal and glial factors contributing to sex differences in opioid modulation of pain

Morphine remains one of the most widely prescribed opioids for alleviation of persistent and/or severe pain; however, multiple preclinical and clinical studies report that morphine is less efficacious in females compared to males. Morphine primarily binds to the mu opioid receptor, a prototypical G-protein coupled receptor densely localized in the midbrain periaqueductal gray. Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray, and its descending projections to the rostral ventromedial medulla and spinal cord, as an essential descending inhibitory circuit mediating opioid-based analgesia. Remarkably, the majority of studies published over the following 30 years were conducted in males with the implicit assumption that the anatomical and physiological characteristics of this descending inhibitory circuit were comparable in females; not surprisingly, this is not the case. Several factors have since been identified as contributing to the dimorphic effects of opioids, including sex differences in the neuroanatomical and neurophysiological characteristics of the descending inhibitory circuit and its modulation by gonadal steroids. Recent data also implicate sex differences in opioid metabolism and neuroimmune signaling as additional contributing factors. Here we cohesively present these lines of evidence demonstrating a neural basis for sex differences in opioid modulation of pain, with a focus on the PAG as a sexually dimorphic core of descending opioid-induced inhibition and argue for the development of sex-specific pain therapeutics.

Impact of sex on pain and opioid analgesia: a review

Chronic pain is a debilitating condition that impacts tens of millions each year, resulting in lost wages for workers and exacting considerable costs in health care and rehabilitation. A thorough understanding of the neural mechanisms underlying pain and analgesia is critical to facilitate the development of therapeutic strategies and personalized medicine. Clinical and epidemiological studies report that women experience greater levels of pain than men and have higher rates of pain-related disorders. Studies in both rodents and humans report sex differences in the anatomical and physiologic properties of the descending antinociceptive circuit, mu opioid receptor (MOR) expression and binding, morphine metabolism, and immune system activation, all of which likely contribute to the observed sex differences in pain and opioid analgesia. Although more research is needed to elucidate the underlying mechanisms, these sex differences present potential therapeutic targets to optimize pain management strategies for both sexes.

The Pharmacokinetics of Morphine and Codeine in Human Plasma and Urine after Oral Administration of Qiangli Pipa Syrup

Papaveris pericarpium, a natural source of morphine and codeine, is the principal active component in many antitussive traditional Chinese medicines. We herein report the first PK study of papaveris pericarpium in human plasma and urine following oral administration of single (15, 30, 60 mL) and multiple dose (15 mL) of Qiangli Pipa Syrup (MOR 0.1 mg/mL, COD 0.028 mg/mL) by monitoring morphine and codeine using a HPLC-MS/MS method. Their T_max and t_1/2 values are independent of dosages, while the AUC_0-t linearly increased with higher dosages, indicating linear PK characteristics. AUC_0-t increased obviously after multiple doses, indicating possible risk of accumulative toxicity. Urine studies suggested risks of positive opiate drug tests with a cutoff of 300 ng/mL, which lasted 6-14 h at different doses. These results provide important information for clinical safety, efficacy and rational drug use of Qiangli Pipa Syrup and also guide the related judicial expertise of its administration.

Sex differences in innate immunity and its impact on opioid pharmacology

Morphine has been and continues to be one of the most potent and widely used drugs for the treatment of pain. Clinical and animal models investigating sex differences in pain and analgesia demonstrate that morphine is a more potent analgesic in males than in females. In addition to binding to the neuronal μ-opioid receptor, morphine binds to the innate immune receptor toll-like receptor 4 (TLR4), located on glial cells. Activation of glial TLR4 initiates a neuroinflammatory response that directly opposes morphine analgesia. Females of many species have a more active immune system than males; however, few studies have investigated glial cells as a potential mechanism driving sexually dimorphic responses to morphine. This Mini-Review illustrates the involvement of glial cells in key processes underlying observed sex differences in morphine analgesia and suggests that targeting glia may improve current treatment strategies for pain. © 2016 Wiley Periodicals, Inc.

Sex and gender differences in substance use disorders

The gender gap in substance use disorders (SUDs), characterized by greater prevalence in men, is narrowing, highlighting the importance of understanding sex and gender differences in SUD etiology and maintenance. In this critical review, we provide an overview of sex/gender differences in the biology, epidemiology and treatment of SUDs. Biological sex differences are evident across an array of systems, including brain structure and function, endocrine function, and metabolic function. Gender (i.e., environmentally and socioculturally defined roles for men and women) also contributes to the initiation and course of substance use and SUDs. Adverse medical, psychiatric, and functional consequences associated with SUDs are often more severe in women. However, men and women do not substantively differ with respect to SUD treatment outcomes. Although several trends are beginning to emerge in the literature, findings on sex and gender differences in SUDs are complicated by the interacting contributions of biological and environmental factors. Future research is needed to further elucidate sex and gender differences, especially focusing on hormonal factors in SUD course and treatment outcomes; research translating findings between animal and human models; and gender differences in understudied populations, such as those with co-occurring psychiatric disorders and gender-specific populations, such as pregnant women.

Sex differences among opioid-abusing patients with chronic pain in a clinical trial

BACKGROUND

The characteristics of patients with co-occurring chronic pain and prescription opioid abuse have not been well described, and even less is known about differences between men and women in this population.

OBJECTIVES

This study evaluated sex differences in the demographic, diagnostic, and behavioral attributes of patients with chronic pain and opioid abuse.

METHODS

Data were collected via self-report and semistructured clinical interviews from 162 patients (120 men and 42 women) who screened for a study investigating the abuse liability of prescription opioids.

RESULTS

There were no differences between men and women in age, race, education, marital status, or employment status. Participants had used prescription opioids for 5.4 ± 6.7 years. The majority of participants (60%) had low back pain in addition to opioid dependence as defined by the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition. More women reported more arthritic pain than men, but otherwise there were no differences in types of pain. Pain exerted a greater effect in women on mood, walking ability, and social relations. Men reported more of certain aberrant behaviors, including abuse of alcohol or illicit drugs, unauthorized dose increases, contact with street culture, and being arrested by police. Women were more depressed than men.

CONCLUSIONS

The demographic profile of opioid-abusing patients with chronic pain presenting for treatment in a clinical trial was similar between sexes; however, some important differences were observed. Women reported more psychiatric comorbidity and endorsed greater pain-related physical and social impairment. Men reported more aberrant behaviors. These differences suggest that men with chronic pain and opioid abuse/dependence may benefit by closer monitoring of aberrant behaviors whereas women may benefit from closer attention paid to physical and psychological effects of pain.

Lack of Sex Difference in Minimum Local Analgesic Concentration of Ropivacaine for Ultrasound-Guided Supraclavicular Brachial Plexus Block

Background

Sex differences, which may be an important variable for determining anesthetic requirements, have not been well investigated in the aspect of local anesthetic. This investigation aimed to compare the minimum local analgesic concentration (MLAC) of ropivacaine for ultrasound-guided supraclavicular brachial plexus block (US-SCB) between men and women.

Material/Method

Patients aged 18–45 years undergoing elective forearm, wrist, or hand surgeries under US-SCB were divided into 2 groups according to sex. The initial concentration was 0.375% ropivacaine 20 mL and the concentration for the next patient was determined by the up-down technique at 0.025% intervals. Success was defined as the absence of any pain in response to a pinprick in the region of all 4 terminal nerves and the skin incision within 45 min. The primary outcome was the MLAC of ropivacaine, which was estimated by the Dixon and Massey method. The analgesia duration, which was defined as the time from the end of the US-SCB injection to the time of feeling discomfort and need for additional analgesics, was observed for each patient.

Results

The MLAC of ropivacaine 20 mL for US-SCB was 0.2675% (95% confidence interval [CI], 0.2512–0.2838%) in men and 0.2675% (95% CI, 0.2524–0.2826%) in women. There was no significant difference in MLAC or the analgesia duration between the 2 groups (P>0.05).

Conclusions

We found no significant sex-related differences in MLAC or analgesia duration of ropivacaine for US-SCB.

Considerations when using pharmacokinetic/pharmacodynamic modeling to determine the effectiveness of simple analgesics in children

INTRODUCTION

Assessment of analgesic drugs includes comparative studies to other analgesics and local anesthesia blockade, number needed to treat estimates and opioid sparing descriptions. An additional methodology is to define the concentration-response relationship using pharmacokinetic/pharmacodynamic (PK/PD) modeling.

AREAS COVERED

A concentration-response relationship allows analgesic effect comparison between drugs for different acute pain types. Covariates such as size, age and organ function impact greatly on PK in children. The cumulative effect of confounding factors (e.g., pharmacogenetics, placebo and changes in baseline pain over time) complicates PD. Other factors (outcome measures, method of measurement, failure to account for study attrition) impact on outcome. Population PK/PD modeling approaches allow us to account for these various factors to some extent.

EXPERT OPINION

Nonlinear mixed effects models help interpret analgesic data and their use is increasing. The PK is relatively well understood. The next investigative step will involve investigation into covariate effects for PD. Mathematical functions for both placebo models and dropout models are well described and should be incorporated into analgesic effectiveness studies that investigate a range of doses. Improvements in pain assessment tools and a greater understanding of pharmacogenomics factors will help individualize analgesic therapy.

Confronting the challenges of effective pain management in children following tonsillectomy

Tonsillectomy is an extremely common surgical procedure associated with significant morbidity and mortality. The post-operative challenges include: respiratory complications, post-tonsillectomy hemorrhage, nausea, vomiting and significant pain. The present model of care demands that most of these children are managed in an ambulatory setting. The recent Federal Drug Agency (FDA) warning contraindicating the use of codeine after tonsillectomy in children represents a significant change of practice for many pediatric otolaryngological surgeons. This introduces a number of other safety concerns when deciding on a safe alternative to codeine, especially since most tonsillectomy patients are managed by lay primary caregiver's at home. This review outlines the safety issues and proposes, based on currently available evidence, a preventative multi-modal strategy to manage pain, nausea and vomiting without increasing the risk of post-tonsillectomy bleeding.

Inhibition of c-Kit signaling is associated with reduced heat and cold pain sensitivity in humans

The tyrosine kinase receptor c-Kit is critically involved in the modulation of nociceptive sensitivity in mice. Ablation of the c-Kit gene results in hyposensitivity to thermal pain, whereas activation of c-Kit produces hypersensitivity to noxious heat, without altering sensitivity to innocuous mechanical stimuli. In this study, we investigated the role of c-Kit signaling in human pain perception. We hypothesized that subjects treated with Imatinib or Nilotinib, potent inhibitors of tyrosine kinases including c-Kit but also Abl1, PDFGFRα, and PDFGFRβ, that are used to treat chronic myeloid leukemia (CML), would experience changes in thermal pain sensitivity. We examined 31 asymptomatic CML patients (14 male and 17 female) receiving Imatinib/Nilotinib treatment and compared them to 39 age- and sex-matched healthy controls (12 male and 27 female). We used cutaneous heat and cold stimulation to test normal and noxious thermal sensitivity, and a grating orientation task to assess tactile acuity. Thermal pain thresholds were significantly increased in the Imatinib/Nilotinib-treated group, whereas innocuous thermal and tactile thresholds were unchanged compared to those in the control group. In conclusion, our findings suggest that the biological effects of c-Kit inhibition are comparable in mice and humans in that c-Kit activity is required to regulate thermal pain sensitivity but does not affect innocuous thermal and mechanical sensation. The effect on experimental heat pain observed in our study is comparable to those of several common analgesics; thus modulation of the c-Kit pathway can be used to specifically modulate noxious heat and cold sensitivity in humans.

Sex and gender aspects in anesthetics and pain medication

The influence of sex and gender on anesthesia and analgesic therapy remains poorly understood, nevertheless the numerous physiological and pharmacological differences present between men and women. Although in anesthesiology sex-gender aspects have attracted little attention, it has been reported that women have a greater sensitivity to the non-depolarizing neuroblocking agents, whereas males are more sensitive than females to propofol. It has been suggested that men wake slower than women after general anesthesia and have less postoperative nausea and vomiting. Sexual hormones seem to be of importance in the onset of differences. Nevertheless, in the last years, sex-gender influences on pain and analgesia have become a hot topic and data regarding sex-gender differences in response to pharmacologic and non-pharmacologic pain treatments are still scanty, inconsistent, and non-univocal. In particular, females seem to be more sensitive than males to opioid receptor agonists. Women may experience respiratory depression and other adverse effects more easily if they are given the same doses as males. Evidently, there is an obvious need for more research, which should include psychological and social factors in experimental preclinical and clinical paradigms in view of their importance on pain mechanism, in order to individualize analgesia to optimize pain relief.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Time of admission, gender and age: challenging factors in emergency renal colic - a preliminary study

BACKGROUND

Nephrolithiasis is a relatively common problem and a frequent Emergency Department (ED) diagnosis in patients who present with acute flank/abdominal pain. The pain management in these patients is often challenging.

OBJECTIVES

To investigate the most effective dose of morphine with the least side effects in emergency renal colic patients.

MATERIALS AND METHODS

150 renal colic patients who experienced a pain level of 4 or greater, based on visual analog scale (VAS) at admission time were included. Pain was scored on a 100 mm VAS (0 = no pain, 100 = the worst pain imagined). When patients arrived at ED, a physician would examine the patients and assessed initial pain score, then filled a questionnaire according to the patient information. Patients were assigned to receive 2.5 mg morphine sulfate intravenously. We monitored patients' visual analog scale (VAS), and adverse events at different time points (every 15 minutes) for 90 minutes. Additional doses of intravenous morphine (2.5 mg) were administered if the patient still had pain. (Max dose: 10 mg). The cumulative dose of morphine, defined as the total amount of morphine prescribed to each patient during the 90 minutes of the study, was recorded. Patients were not permitted to use any nonsteroidal anti-inflammatory drugs as coadjuvant analgesics during the study period. Subjects with inadequate pain relief at 90 minutes received rescue morphine and were excluded from the study. The primary end point in this study was pain relief at 90 minutes, defined as either VAS<40 or decrease of 50% or more as compared to the initial VAS. The secondary objective was to detect the occurrence of adverse effects at any time points in ED.

RESULTS

The studied patients consisted of 104 men and 46 women with the mean age of 43 ±14 years (range, 18 to 75 years). There was no statistically significant difference between the mean age and gender differences in pain response. Rescue analgesia at 30 minutes were given in 54.5% receiving morphine. The average time to painless was 35 minutes. But there were no statistically significant differences between the mean age and gender differences in pain response (P > 0.05). Older patients responded sooner to morphine than the young. Most of the patients had a pain score of 90 -100 (77.3 %) at the beginning that was reduced to 29.4% during the 30 minutes follow up. During the first hour, we found that 94.7% of the patients had no pain or significant pain reduction and only 2.1% of the patients still had pain.

CONCLUSIONS

We conclude that there were no significant differences among the gender, time of admission and side - effects in renal colic patients in response to morphine.

Do initial responses to drugs predict future use or abuse?

Individuals vary in their initial reactions to drugs of abuse in ways that may contribute to the likelihood of subsequent drug use. In humans, most drugs of abuse produce positive subjective states such as euphoria and feelings of well-being, which may facilitate repeated use. In nonhumans, many drugs initially increase locomotor activity and produce discriminative stimulus effects, both of which have been considered to be models of human stimulant and subjective states. Both humans and nonhumans vary in their sensitivity to early acute drug effects in ways that may predict future use or self-administration, and some of these variations appear to be genetic in origin. However, it is not known exactly how the initial responses to drugs in either humans or nonhumans relate to subsequent use or abuse. In humans, positive effects of drugs facilitate continued use of a drug while negative effects discourage use, and in nonhumans, greater genetic risk for drug intake is predicted by reduced sensitivity to drug aversive effects; but whether these initial responses affect escalation of drug use, and the development of dependence is currently unknown. Although early use of a drug is a necessary step in the progression to abuse and dependence, other variables may be of greater importance in the transition from use to abuse. Alternatively, the same variables that predict initial acute drug effects and early use may significantly contribute to continued use, escalation and dependence. Here we review the existing evidence for relations between initial direct drug effects, early use, and continued use. Ultimately, these relations can only be determined from systematic longitudinal studies with comprehensive assessments from early drug responses to progression of problem drug use. In parallel, additional investigation of initial responses in animal models as predictors of drug use will shed light on the underlying mechanisms.

Striatal μ-opioid receptor availability predicts cold pressor pain threshold in healthy human subjects

Previous PET studies in healthy humans have shown that brain μ-opioid receptor activation during experimental pain is associated with reductions in the sensory and affective ratings of the individual pain experience. The aim of this study was to find out whether brain μ-opioid receptor binding at the resting state, in absence of painful stimulation, can be a long-term predictor of experimental pain sensitivity. We measured μ-opioid receptor binding potential (BP(ND)) with μ-opioid receptor selective radiotracer [(11)C]carfentanil and positron emission tomography (PET) in 12 healthy male subjects. Later, we recruited these subjects to participate in a separate psychophysical testing session to measure cold pressor pain threshold, cold pressor pain tolerance and tactile sensitivity with von Frey monofilaments. We used both voxel-by-voxel and region-of-interest image analyses to examine the potential associations between μ-opioid receptor BP(ND) and psychophysical measures. The results show that striatal μ-opioid receptor BP(ND) predicts cold pressor pain threshold, but not cold pressor pain tolerance or tactile sensitivity. This finding suggests that striatal μ-opioid receptor density is involved in setting individual pain threshold.

Effects of morphine on thermal sensitivity in adult and aged rats

There are contradictory data regarding older individuals' sensitivity to pain stimulation and opioid administration. Adult (12-16 months; n = 10) and aged (27-31 months; n = 7) male F344xBN rats were tested in a thermal sensitivity procedure where the animal chooses to remain in one of two compartments with floors maintained at various temperatures ranging from hot (45°C) through neutral (30°C) to cold (15°C). Effects of morphine were determined for three temperature comparisons (ie, hot/neutral, cold/neutral, and hot/cold). Aged rats were more sensitive to cold stimulation during baseline. Morphine produced antinociception during hot thermal stimulation, but had no effect on cold stimulation. The antinociceptive (and locomotor-altering) effects of morphine were attenuated in aged rats. These data demonstrate age-related differences in baseline thermal sensitivity and responsiveness to opioids. Based on behavioral and physiological requirements of this procedure, it is suggested that thermal sensitivity may provide a relevant animal model for the assessment of pain and antinociception.

Lack of sex differences to the subjective effects of nitrous oxide in healthy volunteers

BACKGROUND

Although numerous studies have assessed subjective effects of nitrous oxide, few studies have analyzed for sex differences. Since sex differences have been reported in subjective effects of several drugs such as opioids, nicotine and alcohol, we sought to determine if sex modulates the subjective effects of the inhalant, nitrous oxide, in healthy volunteers.

METHODS

Thirty-eight females and seventy-two males from nine studies that were conducted in our laboratory were included in this retrospective analysis. All experimental studies utilized randomized, placebo-controlled, repeated measures designs in which subjects inhaled 30% nitrous oxide in oxygen and 100% oxygen (placebo). Dependent measures in this analysis were subjective effects measured at baseline and 15 min into the inhalation period.

RESULTS

Nitrous oxide produced a number of subjective effects, including those that could be considered abuse liability-related ("elated," "having pleasant thoughts," drug liking), but sex did not modulate these effects.

CONCLUSIONS

Females and males showed similar subjective responses to 30% nitrous oxide. Future prospective studies might assess other concentrations, other measures (choice, analgesic response), and other inhaled general anesthetics to more comprehensively characterize the role of sex in response to inhalants.

Do sex differences exist in opioid analgesia? A systematic review and meta-analysis of human experimental and clinical studies

Although a contribution of sex in opioid efficacy has garnered much attention, the confirmation and direction of any such difference remain elusive. We performed a systematic review of the available literature on sex differences in μ and mixed μ/κ opioid effect on acute and experimental pain. Fifty unique studies (including three unpublished studies) were included in the analyses. Across the 25 clinical studies on μ-opioids there was no significant sex-analgesia association. Restricting the analysis to patient-controlled analgesia (PCA) studies (irrespective of the opioid) yielded greater analgesia in women (n=15, effect size 0.22, 95% c.i. 0.02-0.42, P=0.028). Further restricting the analysis to PCA morphine studies yielded an even greater effect in women (n=11, effect size=0.36, 95% c.i. 0.17-0.56, P=0.003). Meta-regression indicated that the longer the duration of PCA, the difference in effect between the sexes further increased. Across experimental pain studies on μ-opioids women had greater antinociception from opioids (n=11, effect size=0.35; 95% c.i. 0.01-0.69, P=0.047), which was predominantly due to 6 morphine studies. Female patients had greater μ/κ opioid analgesia (n=7, effect size 0.84; 95% c.i. 0.25-1.43, P=0.005), but no sex-analgesia association was present in experimental studies (n=7). Sex differences exist in morphine-induced analgesia in both experimental pain studies and clinical PCA studies, with greater morphine efficacy in women. The data on non-morphine μ and mixed μ/κ-opioids are less convincing and require further study.

Psychopharmacological effects of oxycodone in healthy volunteers: roles of alcohol-drinking status and sex

BACKGROUND

Studies have shown that alcohol-drinking status modulates psychopharmacological effects of several drugs. We sought to determine if drinking status modulates the effects of a prescription opioid, oxycodone, in healthy volunteers. We included sex of the volunteer in the statistical analyses since this is a factor that is known to alter several pharmacodynamic effects of opioids in nonhumans and humans.

METHODS

Fifteen light drinkers (eight males) and 14 moderate drinkers (eight males) participated in a crossover, randomized, double-blind study in which they received 0, 10, and 20mg of oxycodone (p.o.). Dependent measures were subjective, psychomotor/cognitive, reinforcing, and physiological effects.

RESULTS

Self-reported alcohol-drinking status did not modulate the effects of oxycodone. However, there were a number of Sex x Dose interactions with females reporting more and larger unpleasant effects than males (e.g., visual analog scale ratings of "nauseated" greater in females than in males).

CONCLUSIONS

Studies have established that moderate drinkers report a greater degree of abuse liability-related effects than do light drinkers with several different drugs, including diazepam, amphetamine, and nitrous oxide, but we were unable to establish this with the prescription opioid, oxycodone. However, we did observe sex differences in several subjective effects of oxycodone, a finding that is consistent with the extant literature showing sex differences in pharmacodynamic effects of opioids.