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

Sex-Differences in Post-Procedural Pain Experiences After Thermal Liver Ablations for Liver Tumors: A Retrospective Study

INTRODUCTION

Literature shows differences in pain experiences between sexes. The exact influence of thermal liver ablation on experienced pain is still not well-known. This study aims to investigate the maximum pain intensity at the recovery between men and women after percutaneous thermal liver ablation.

METHODS

Patients treated with percutaneous thermal liver ablation (radiofrequency or microwave ablation) in Maastricht University Medical Center + between 2018 and 2022 for primary or secondary liver tumors were included retrospectively. Outcomes included maximum numerical rating scale (NRS, scale:0-10) score at the recovery room, prevalence of post-procedural pain (defined as NRS score ≥ 4), duration of anesthesia, length of stay at recovery, and complications. Regression analyses were adjusted for age, ASA-score, BMI, tumor type, maximum diameter of lesion, chronic pain in patients' history, and history of psychological disorder.

RESULTS

183 patients were included of which 123 men (67%). Results showed higher average maximum NRS scores in women patients compared to men (mean:3.88 versus 2.73), but not after adjustments (aß:0.75, 95%CI:-0.13-1.64). Women suffered more from acute post-procedural pain (59% versus 35%; aOR:2.50, 95%CI:1.16-5.39), and needed analgesics more often at the recovery room (aOR:2.43, 95%CI:1.07-5.48) compared to men. NRS score at recovery arrival did not significantly differ (aß:0.37, 95%CI:-0.48-1.22). No differences were seen in the length of stay at the recovery, duration of anesthesia, procedure time, and complication rate. Location of the tumor (subcapsular or deep), total tumors per patient, and distinction between primary and secondary tumors had no influence on the NRS.

CONCLUSION

This retrospective single-center study shows higher post-procedural pain rates after thermal liver ablation in women, resulting in higher analgesics use at the recovery room. The results suggest considering higher dosage of analgesics during thermal liver ablation in women to reduce post-procedural pain. LEVEL OF EVIDENCE 3: Non-controlled retrospective cohort study.

Sex Differences in Pain Sensitivity in a Dutch Cohort: Cross-Sectional and Web-Based Multidimensional Study

BACKGROUND

Sex is an important factor influencing the development and treatment of chronic pain, but the extent of its influence is still unclear. Other demographic factors as well as nonpharmacological interventions might influence pain sensitivity differently in men and women.

OBJECTIVE

In this study, we aimed to investigate the influence of sex and other demographic, lifestyle, behavioral, clinical, and environmental factors on pain sensitivity in the Dutch population. Different films were used to investigate how they would impact pain sensitivity and what influence sex and other variables have on the effect of this simple intervention.

METHODS

We performed a study consisting of 2 parts: (1) a cross-sectional research to investigate pain sensitivity differences between men and women and the influence of other demographic variables on the pain sensitivity in a Dutch cohort and (2) an internet intervention study to determine whether a short film could skew pain sensitivity.

RESULTS

All respondents filled in a web-based demographic questionnaire and were randomized into 4 groups. The control group filled in the Pain Sensitivity Questionnaire without watching a preliminary film. A cross-sectional analysis was performed in the control group (n=1746). The other 3 groups watched short films: one group watched a film with scenes of nature (n=2650), another group watched a film on laughing people (n=2735), and the last group watched a film on physically painful events (n=2708). Immediately after the film viewing, participants were directed to the Pain Sensitivity Questionnaire to measure their pain sensitivity. The Pain Sensitivity Questionnaire score was stated as a mean per question on the numeric rating scale from 0-1. The cross-sectional study revealed no significant differences between men and women but showed male-female differences in the Pain Sensitivity Questionnaire when specific background factors were present. Watching a short film had a positive impact on the pain sensitivity of the respondents who had chronic pain, with a higher effect observed in female respondents.

CONCLUSIONS

Scientists performing pain research need to account for factors that can influence the outcome of their study and be aware that these factors can be sex-dependent, and pain sensitivity should be analyzed accordingly. Even relatively small interventions such as watching a film can impact pain sensitivity, especially in respondents with current chronic pain. This effect can vary as well when different background factors are present. Our findings warrant further explorations of the possibilities that simple interventions bring for patients in personalized medicine.

TRIAL REGISTRATION

Landelijk Trial Register NTR-new NL8182; https://onderzoekmetmensen.nl/en/trial/29537.

Sex Disparities in Opioid Prescription and Administration on a Hospital Medicine Service

INTRODUCTION

Decisions to prescribe opioids to patients depend on many factors, including illness severity, pain assessment, and patient age, race, ethnicity, and gender. Gender and sex disparities have been documented in many healthcare settings, but are understudied in inpatient general medicine hospital settings.

OBJECTIVE

We assessed for differences in opioid administration and prescription patterns by legal sex in adult patient hospitalizations from the general medicine service at a large urban academic center.

DESIGNS, SETTING, AND PARTICIPANTS

This study included all adult patient hospitalizations discharged from the acute care inpatient general medicine services at the University of California, San Francisco (UCSF) Helen Diller Medical Center at Parnassus Heights from 1/1/2013 to 9/30/2021.

MAIN OUTCOME AND MEASURES

The primary outcomes were (1) average daily inpatient opioids received and (2) days of opioids prescribed on discharge. For both outcomes, we first performed logistic regression to assess differences in whether or not any opioids were administered or prescribed. Then, we performed negative binomial regression to assess differences in the amount of opioids given. We also performed all analyses on a subgroup of hospitalizations with pain-related diagnoses.

RESULTS

Our study cohort included 48,745 hospitalizations involving 27,777 patients. Of these, 24,398 (50.1%) hospitalizations were female patients and 24,347 (49.9%) were male. Controlling for demographic, clinical, and hospitalization-level variables, female patients were less likely to receive inpatient opioids compared to male patents (adjusted OR 0.87; 95% CI 0.82, 0.92) and received 27.5 fewer morphine milligram equivalents per day on average (95% CI - 39.0, - 16.0). When considering discharge opioids, no significant differences were found between sexes. In the subgroup analysis of pain-related diagnoses, female patients received fewer inpatient opioids.

CONCLUSIONS

Female patients were less likely to receive inpatient opioids and received fewer opioids when prescribed. Future work to promote equity should identify strategies to ensure all patients receive adequate pain management.

Preoperative anxiety, postoperative pain tolerance and analgesia consumption: A prospective cohort study

BACKGROUND

Anxiety affects the patient's perception of postoperative pain and causes a significant increase in the consumption of analgesia postoperatively.

OBJECTIVE

This study assesses the relationship between preoperative anxiety, postoperative pain and postoperative pethidine consumption.

METHODS

A prospective cohort design was used. Data were collected from 100 patients who were undergoing a laparoscopic cholecystectomy at St Joseph Hospital, Jerusalem. Pain-controlled analgesia with pethidine was utilised to manage pain throughout the postoperative period. The visual analogue scale scores and pethidine consumption of all patients were recorded.

FINDINGS

Participants' mean level of pain was higher in the preoperative period (mean visual analogue scale = 1.3) compared with their mean level of pain in the postoperative period (mean visual analogue scale = 0.5). There is a statistically significant difference between the participants' level of anxiety and postoperative pain level (p < 0.001). Gender, weight, level of education and smoking were predictors of developing preoperative anxiety. Also, gender, smoking and medication were statistically significant predictors of developing postoperative pain. Furthermore, gender, medical history and medication were statistically significant predictors of pethidine consumption postoperatively.

CONCLUSIONS

The preoperative anxiety reduction intervention should be promoted and implemented as routine care for all surgical patients.

Incidence and predictors of chronic pain after inguinal hernia surgery: a systematic review and meta-analysis

PURPOSE

The mesh is currently the preferred treatment option for hernia repair surgery. Chronic postoperative inguinal pain (CPIP), lasting more than 3 months after surgery, is a complication that significantly impacts patients' quality of life. Currently, there is a lack of evidence-based information describing the incidence and independent predictive factors of chronic pain, posing a serious challenge in clinical practice for devising personalized prevention strategies. Hence, we conducted this systematic review and meta-analysis to investigate the incidence and predictive factors, aiming to provide a reference for developing plans to prevent chronic pain.

METHODS

We conducted a systematic search of PubMed, Cochrane, Embase, and Web of Science, with the retrieval cutoff date set at December 17, 2022. The included studies underwent assessment using the NOS scale, and subgroup analysis for the incidence was carried out based on different regions.

RESULTS

Ultimately, 18 studies were included, involving 29,466 patients. Meta-analysis showed that the pooled incidence of chronic pain was 17.01% (95%CI 12.78% ~ 21.71%). The incidence was 18.65% (95%CI 13.59% ~ 24.29%) in Europe, 14.70% (95%CI 7.87% ~ 23.17%) in Asia, and 6.04%(95%CI 4.62 ~ 7.64) in North America. Furthermore, We also found that the risk factors for CPIP are younger age [OR = 2.261 (95%CI 1.126 ~ 4.549)], presence of other postoperative complications [OR = 1.849 (95%CI 1.034 ~ 3.305)], hernial sac defect < 3 cm [OR = 1.370 (95%CI 1.012 ~ 1.853)], being female [OR = 1.885 (95%CI 1.024 ~ 3.472)], postoperative pain [OR = 1.553 (95%CI 1.276 ~ 1.889)], preoperative pain [OR = 2.321 (95%CI 1.354 ~ 3.979)], and having a history of ipsilateral inguinal hernia repair [OR = 2.706 (95% CI 1.445 ~ 5.069)].

CONCLUSIONS

The incidence of persistent pain following hernia repair surgery is high in current clinical practice, a concern that should not be overlooked. Stratified assessment tools need to be established for patients experiencing early chronic pain, and personalized follow-up strategies and preventive interventions should be developed for those with potentially high risks. These measures aim to enhance the quality of life for patients after hernia repair.

PBPK-PD model for predicting morphine pharmacokinetics, CNS effects and naloxone antagonism in humans

Morphine and morphine-6-glucuronide (M6G) produce central nervous system (CNS) effects by activating mu-opioid receptors, while naloxone is used mainly for the reversal of opioid overdose, specifically for the fatal complication of respiratory depression, but also for alleviating opioid-induced side effects. In this study we developed a physiologically-based pharmacokinetic-pharmacodynamic (PBPK-PD) model to simultaneously predict pharmacokinetics and CNS effects (miosis, respiratory depression and analgesia) of morphine as well as antagonistic effects of naloxone against morphine. The pharmacokinetic and pharmacodynamic parameters were obtained from in vitro data, in silico, or animals. Pharmacokinetic and pharmacodynamic simulations were conducted using 39 and 36 clinical reports, respectively. The pharmacokinetics of morphine and M6G following oral or intravenous administration were simulated, and the PBPK-PD model was validated using clinical observations. The Emax model correlated CNS effects with free concentrations of morphine and M6G in brain parenchyma. The predicted CNS effects were compared with observations. Most clinical observations fell within the 5th-95th percentiles of simulations based on 1000 virtual individuals. Most of the simulated area under the concentration-time curve or peak concentrations also fell within 0.5-2-fold of observations. The contribution of morphine to CNS effects following intravenous or oral administration was larger than that of M6G. Pharmacokinetics and antagonistic effects of naloxone on CNS effects were also successfully predicted using the developed PBPK-PD model. In conclusion, the pharmacokinetics and pharmacodynamics of morphine and M6G, antagonistic effects of naloxone against morphine-induced CNS effects may be successfully predicted using the developed PBPK-PD model based on the parameters derived from in vitro, in silico, or animal studies.

A pain in the pocket: Prevalence of pocket pain in patients with implantation of neuromodulation systems: A retrospective study

INTRODUCTION

Implantable pulse generator (IPG) is a neurostimulation therapy mediated by electrodes and surgically implanted in a subcutaneous "pocket" used for the control of numerous pathologies. This study examines both the prevalence of pain associated with IPG implantation ("pain pocket syndrome") and its associated characteristics.

MATERIALS AND METHODS

56 patients with an IPG were included in the study. A health questionnaire was conducted to determine the presence of pain associated with the pocket and its neuropathic characteristics, as well as associated aesthetic concerns, location, situations that accentuate or alleviate pain, medications used for baseline and pocket pain control and other factors associated.

RESULTS

Pain in the area of implantation of the IPG had a prevalence of 52.6% of patients (n = 27), in our sample, with a mean score on the visual analogic scale (VAS) of 4.9 points [3.9-5.8 points], with neuropathic characteristics in 53.3% (n = 16) of the patients with pain, with differences between the mean VAS score of the female (5.5 [4.3-5.8 points]) and males (3.5 points [2.1-4.9 points]) (p = 0.04).

CONCLUSION

Pocket pain is a condition with a higher prevalence than described in previous studies, being of a higher intensity in females, involving a moderate pain in the area of implantation of the neuromodulating therapy. This pain has neuropathic characteristics and could require a repositioning intervention. Hence, more studies in this field should be carried to detect and prevent this syndrome.

Sex differences in PD-L1-induced analgesia in paclitaxel-induced peripheral neuropathy mice depend on TRPV1-based inhibition of CGRP

AIMS

Paclitaxel (PTX) is extensively utilized in the management of diverse solid tumors, frequently resulting in paclitaxel-induced peripheral neuropathy (PIPN). The present study aimed to investigate sex differences in the behavioral manifestations and underlying pathogenesis of PIPN and search for clinically efficacious interventions.

METHODS

Male and female C57BL/6 mice (5-6 weeks and 12 months, weighing 18-30 g) were intraperitoneally (i.p.) administered paclitaxel diluted in saline (NaCl 0.9%) at a dose of 2 mg/kg every other day for a total of 4 injections. Von Frey and hot plate tests were performed before and after administration to confirm the successful establishment of the PIPN model and also to evaluate the pain of PIPN and the analgesic effect of PD-L1. On day 14 after PTX administration, PD-L1 protein (10 ng/pc) was injected into the PIPN via the intrathecal (i.t.) route. To knock down TRPV1 in the spinal cord, adeno-associated virus 9 (AAV9)-Trpv1-RNAi (5 μL, 1 × 1013 vg/mL) was slowly injected via the i.t. route. Four weeks after AAV9 delivery, the downregulation of TRPV1 expression was verified by immunofluorescence staining and Western blotting. The levels of PD-L1, TRPV1 and CGRP were measured via Western blotting, RT-PCR, and immunofluorescence staining. The levels of TNF-α and IL-1β were measured via RT-PCR.

RESULTS

TRPV1 and CGRP protein and mRNA levels were higher in the spinal cords of control female mice than in those of control male mice. PTX-induced nociceptive behaviors in female PIPN mice were greater than those in male PIPN mice, as indicated by increased expression of TRPV1 and CGRP. The analgesic effects of PD-L1 on mechanical hyperalgesia and thermal sensitivity were significantly greater in female mice than in male mice, with calculated relative therapeutic levels increasing by approximately 2.717-fold and 2.303-fold, respectively. PD-L1 and CGRP were partly co-localized with TRPV1 in the dorsal horn of the mouse spinal cord. The analgesic effect of PD-L1 in PIPN mice was observed to be mediated through the downregulation of TRPV1 and CGRP expression following AAV9-mediated spinal cord specific decreased TRPV1 expression.

CONCLUSIONS

PTX-induced nociceptive behaviors and the analgesic effect of PD-L1 in PIPN mice were sexually dimorphic, highlighting the significance of incorporating sex as a crucial biological factor in forthcoming mechanistic studies of PIPN and providing insights for potential sex-specific therapeutic approaches.

Influence of Age, Gender and Body Mass Index on Intravenous Pharmacokinetics of Buprenorphine in Children Undergoing Orthopedic Surgery

Background

Buprenorphine (BPN) is a widely used analgesic in the pediatric population, although there are few studies on the pharmacokinetics and pharmacodynamics of this drug.

Objective

The objective was to characterize the pharmacokinetics of BPN after intravenous administration and analyze the effect of age, gender, weight, height, body mass index (BMI), and drug-drug interactions as covariates.

Methods

Ninety-nine children (2-10 years), who underwent orthopedic surgery under regional, general, or combined anesthesia were included. Patients evaluated according to the American Society of Anesthesiologists Physical Status Classification, who received intravenous BPN 2 μg/kg were enrolled. Blood was collected from 1-240 min. Drug plasma concentrations were determined by LC-MS/MS. Population pharmacokinetic parameters were obtained with Monolix 2021R1 software. Pearson's correlation and/or ANOVA were used for statistical analysis.

Results

Age was associated with changes in clearance and central compartment volume and the female gender was associated with lower intercompartmental clearance, while BMI modified clearance, central and peripheral compartment volume. Concomitant administration of BPN with fentanyl and dexamethasone produced decreases in clearance.

Conclusions

The covariates of sex, age, and BMI are directly related to the increase or decrease in BPN pharmacokinetic parameters.

Effects of sex and hydration status on kappa opioid receptor-mediated diuresis in rats

Understanding the function of the kappa opioid receptor (KOP) is crucial for the development of novel therapeutic interventions that target KOP for the treatment of pain, stress-related disorders and other indications. Activation of KOP produces diuretic effects in rodents and man. Sex is a vital factor to consider when assessing drug response in pre-clinical and clinical studies. In this study, the diuretic effect of the KOP agonist, U50488 (1-10 mg/kg), was investigated in both adult female and male Wistar rats that were either normally hydrated or water-loaded. The KOP antagonist norbinaltorphimine (norBNI, 10 mg/kg) was administered 24 h prior to U50488 to confirm the involvement of KOP. U50488 elicited a significant diuretic response at doses ≥ 3 mg/kg in both female and male rats independent of hydration status. U50488 diuretic effects were inhibited by norBNI pre-administration. Water-loading reduced data variability for urine volume in males, but not in females, compared with normally hydrated rats. Sex differences were also evident in U50488 eliciting a significant increase in sodium and potassium ion excretion only in males. This may suggest different mechanisms of U50488 diuretic action in males where renal excretion mechanisms are directly affected more than in females.

Neuroimaging of opioid effects in humans across conditions of acute administration, chronic pain therapy, and opioid use disorder

Evidence of central nervous system (CNS) exogenous opioid effects in humans has been primarily gained through neuroimaging of three participant populations: individuals after acute opioid administration, those with opioid use disorder (OUD), and those with chronic pain receiving opioid therapy. In both the brain and spinal cord, opioids alter processes of pain, cognition, and reward. Opioid-related CNS effects may persist and accumulate with longer opioid use duration. Meanwhile, opioid-induced benefits versus risks to brain health remain unclear. This review article highlights recent accumulating evidence for how exogenous opioids impact the CNS in humans. While investigation of CNS opioid effects has remained largely disparate across contexts of opioid acute administration, OUD, and chronic pain opioid therapy, integration across these contexts may enable advancement toward effective interventions.

Biological sex influences sleep phenotype in mice experiencing spontaneous opioid withdrawal

Aversive symptoms, including insomnia experienced during opioid withdrawal, are a major drive to relapse; however, withdrawal-associated sleep symptomatology has been little explored in preclinical models. We describe here a model of opioid withdrawal in mice that resembles the sleep phenotype characteristic of withdrawal in humans. Male and female C57BL/6 mice were instrumented with telemeters to record electroencephalogram, electromyogram, activity and subcutaneous temperature. All mice received two treatments separated by a 16-day washout period: (1) saline (volume: 10 ml kg-1); or (2) ascending doses of morphine (5, 10, 20, 40 and 80 mg kg-1; volume: 10 ml kg-1) for 5 days at Zeitgeber time 1 and Zeitgeber time 13. Recordings for the first 71 hr after treatment discontinuation (withdrawal days 1-3) and for 24 hr on withdrawal days 5 and 7 were scored for sleep/wake state, and sleep architecture and electroencephalogram spectral data were analysed. Morphine was acutely wake- and activity-promoting, and non-rapid eye movement and rapid eye movement sleep were increased during the dark phase on withdrawal day 2 in both sexes. While non-rapid eye movement delta power (0.5-4.0 Hz), a measure of sleep intensity, was reduced during the light phase on withdrawal day 1 and the dark phase on withdrawal day 2 in both sexes, female mice also exhibited changes in the duration and the number of bouts of sleep/wake states. These observations of fragmented sleep on withdrawal days 1-3 suggest poorer sleep consolidation and a more pronounced withdrawal-associated sleep phenotype in female than in male mice. These data may indicate a greater sensitivity to morphine, a more distinct aversive sleep phenotype and/or a faster escalation to dependence in female mice.

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of 4 opioid receptor (Oprm1, Oprd1, Oprl1, Oprk1) and the cannabinoid CB1 receptor (Cnr1) genes in the DRG regulate nociception. In this study, we determined the role of REST in the control of their expression in the DRG induced by spared nerve injury (SNI) in both male and female mice. Transcriptomic analyses of male mouse DRGs followed by quantitative reverse transcription polymerase chain reaction analyses of both male and female mouse DRGs showed that SNI upregulated expression of Rest and downregulated mRNA levels of all 4 opioid receptor and Cnr1 genes, but Oprm1 was upregulated in female mice. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated differing levels of REST at these promoters in male and female mice. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in the DRG in both male and female mice. Our results suggest that nerve injury represses the transcription of Oprd1 and Cnr1 via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain.

Gender Differences in Pain Threshold, Unpleasantness, and Descending Pain Modulatory Activation Across the Adult Life Span: A Cross Sectional Study

The neurobiological underpinnings of gender differences in pain perception, and how these differences may be modified by age, are incompletely understood, placing patients at risk of suboptimal pain management. Using functional magnetic resonance imaging, we examined brain responses in the descending pain modulatory system (DPMS, specifically, dorsolateral prefrontal cortex, anterior cingulate cortex, insula, hypothalamus, amygdala, and periaqueductal gray, during an evoked pain task. We investigated the interaction of age and gender in our sample of healthy adults (27 females, 32 males, 30-86 years) on DPMS response. In a perceptually matched thermal pain paradigm, we investigated pain unpleasantness and neural responses for 3 heat pain percepts: just noticeable pain, weak pain, and moderate pain (MP). Females reported just noticeable pain at a lower temperature, but reported less unpleasantness at weak pain and MP percepts, compared to males. There was a significant age-by-gender interaction during moderate pain in the right anterior cingulate cortex and bilateral insula, such that, males had a stronger positive relationship between DPMS response and age compared to females in these regions. Our results indicate that differences in DPMS responses may explain some gender differences in pain perception and that this effect may change across the adult lifespan. PERSPECTIVE: Gender differences in pain have been well-documented but the brain mechanisms for these differences are still unclear. This article describes potential differences in brain functioning during different levels of pain that could explain differences in pain responses between men and women across the adult lifespan.

Does sex affect the efficacy of systemic pharmacological treatments of pain in knee osteoarthritis? A systematic review

Objective

To determine whether sex influences the analgesic efficacy of systemic pharmacological treatment in patients with knee osteoarthritis.

Design

A systematic review, guided by Cochrane methods, sourced studies from Medline, Cochrane Library, Embase, and CINAHL Plus with Full Text as of October 10, 2022. Eligible studies were double-blind RCTs evaluating systemic pharmacological treatments for knee osteoarthritis in adults, with minimum 30-day treatment duration, reporting sex-specific results or mentioning sex subgroup analysis for analgesic efficacy. The risk of bias was assessed using the Cochrane Risk of Bias tool version 2 (RoB 2).

Results

9 studies (5201 participants) met inclusion criteria, analyzing drugs including duloxetine, etoricoxib, tapentadol, naproxcinod, lutikizumab, and rofecoxib. Only one study reported sex-specific results. Review findings suggested no significant sex-based differences in treatment efficacy, however, data were limited due to a lack of sex-specific reporting or inclusion of sex in subgroup analyses.

Conclusions

Current evidence does not support the existence of sex differences in the analgesic efficacy of systemic knee osteoarthritis treatments. However, this conclusion is substantially limited by the paucity of sex-specific reporting of results or subgroup analyses in most primary studies, emphasizing the need for future research to report on sex-stratified data to allow for comprehensive, personalized treatment strategies.

Influence of a chronic beta-blocker therapy on perioperative opioid consumption - a post hoc secondary analysis

BACKGROUND

Beta-blocker (BB) therapy plays a central role in the treatment of cardiovascular diseases. An increasing number of patients with cardiovascular diseases undergoe noncardiac surgery, where opioids are an integral part of the anesthesiological management. There is evidence to suggest that short-term intravenous BB therapy may influence perioperative opioid requirements due to an assumed cross-talk between G-protein coupled beta-adrenergic and opioid receptors. Whether chronic BB therapy could also have an influence on perioperative opioid requirements is unclear.

METHODS

A post hoc analysis of prospectively collected data from a multicenter observational (BioCog) study was performed. Inclusion criteria consisted of elderly patients (≥ 65 years) undergoing elective noncardiac surgery as well as total intravenous general anesthesia without the use of regional anesthesia and duration of anesthesia ≥ 60 min. Two groups were defined: patients with and without BB in their regular preopreative medication. The administered opioids were converted to their respective morphine equivalent doses. Multiple regression analysis was performed using the morphine-index to identify independent predictors.

RESULTS

A total of 747 patients were included in the BioCog study in the study center Berlin. 106 patients fulfilled the inclusion criteria. Of these, 37 were on chronic BB. The latter were preoperatively significantly more likely to have arterial hypertension (94.6%), chronic renal failure (27%) and hyperlipoproteinemia (51.4%) compared to patients without BB. Both groups did not differ in terms of cumulative perioperative morphine equivalent dose (230.9 (BB group) vs. 214.8 mg (Non-BB group)). Predictive factors for increased morphine-index were older age, male sex, longer duration of anesthesia and surgery of the trunk. In a model with logarithmised morphine index, only gender (female) and duration of anesthesia remained predictive factors.

CONCLUSIONS

Chronic BB therapy was not associated with a reduced perioperative opioid consumption.

TRIAL REGISTRATION

This study was registered at ClinicalTrials.gov ( NCT02265263 ) on the 15.10.2014 with the principal investigator being Univ.-Prof. Dr. med. Claudia Spies.

Kappa Opioid Receptors Negatively Regulate Real Time Spontaneous Dopamine Signals by Reducing Release and Increasing Uptake

The role of the dynorphin/kappa opioid receptor (KOR) system in dopamine (DA) regulation has been extensively investigated. KOR activation reduces extracellular DA concentrations and increases DA transporter (DAT) activity and trafficking to the membrane. To explore KOR influences on real-time DA fluctuations, we used the photosensor dLight1.2 with fiber photometry in the nucleus accumbens (NAc) core of freely moving male and female C57BL/6 mice. First, we established that the rise and fall of spontaneous DA signals were due to DA release and reuptake, respectively. Then mice were systemically administered the KOR agonist U50,488H (U50), with or without pretreatment with the KOR antagonist aticaprant (ATIC). U50 reduced both the amplitude and width of spontaneous signals in males, but only reduced width in females. Further, the slope of the correlation between amplitude and width was increased in both sexes, suggesting that DA uptake rates were increased. U50 also reduced the frequency of signals in both males and females. All effects of KOR activation were stronger in males. Overall, KORs exerted significant inhibitory control over spontaneous DA signaling, acting through at least three mechanisms - inhibiting DA release, promoting DAT-mediated uptake, and reducing the frequency of signals.

Risk factors for discontinuing intravenous patient-controlled analgesia after thoracic surgery

PURPOSE

This study examined the risk factors of experiencing side effects from using intravenous patient-controlled analgesia (IV PCA) following lung and esophageal surgery.

METHODS

Our study included adult patients who underwent lung or esophageal surgery and received IV PCA for postoperative acute pain control between 2020 and 2022. We collected information on side effects from IV PCA use, the decision to discontinue PCA, and the PCA regimen from the daily reports of the acute pain management team and verified the accuracy using electronic records from ward nurses. The primary outcome was the risk factor associated with discontinuing IV PCA due to its side effects.

RESULTS

Out of the 1796 patients in our study, 1795 used PCA containing opioids; 196 patients stopped IV PCA due to unbearable side effects. Being female (adjusted odds ratio [aOR]: 2.65, 95% confidence interval [CI]: 1.70, 4.13) was linked to a higher chance of stopping PCA use. Having hypertension (aOR: 0.46, 95% CI: 0.26, 0.81) and being classified as the American Society of Anesthesiologists class 3 or higher (aOR: 0.48, 95% CI: 0.23, 0.86) were associated with a lower chance of discontinuing PCA use.

CONCLUSION

Our study determined the risk factors to stop using IV PCA due to side effects following lung or esophageal surgery. These results emphasize the need for personalized pain management plans that take into account the patient's characteristics and the type of surgery performed.

Evaluating opioid prescribing patterns following discharge from elective surgical procedures: a worrying trend during the 'opioid crisis' - an audit of elective surgical procedures

BACKGROUND

The 'opioid crisis' has reached epidemic proportions globally. Importantly, 30% of opioid dependency stem from opioids obtained on hospital discharge prescriptions.

AIM

The aim of this study is to evaluate opioid prescription patterns on discharge of post-operative patients in an Irish Hospital.

METHODS

A retrospective cohort study was undertaken in a single institution during the 5 year eligibility period (January 2017-October 2021). Comparisons in opioid prescription patterns following minor (inguinal hernia repair (IHR), intermediate (laparoscopic cholecystectomy (LC)) and major (colonic resection (CR)) were made. Descriptive statistics were performed using SPSS version 26.0 RESULTS: In total, 300 patients were included in this study with mean age 59.6 years (range: 20-92). Of these, 112 patients underwent IHR (37.3%), 116 patients underwent LC (38.7%), and 72 patients underwent CR (24.0%). The mean age at diagnosis was 61 years, 53 years and 58 years for IHR, LC and CR, respectively (P < 0.001). Patients undergoing CR were more likely to have greater comorbidity burden (3.1 vs. 1.2 (IHR) vs. 1.8 (LC) respectively (P = 0.030). On discharge, 27.8% of CR patients received opioids (20/72) compared to 24.1% of IHR (28/116) and 15.9% of LC (18/113) patients, respectively (P = 0.126).

CONCLUSION

We observed considerable variability in opioid prescribing patterns following minor, intermediate and major operations in our centre. Care is required when prescribing opioids in the post-operative setting, and opioid prescription guidelines are required to both tackle and prevent an escalation of this 'opioid crisis'.

Risk Factors for Severe Pain and Impairment of Daily Life Activities after Cesarean Section-A Prospective Multi-Center Study of 11,932 Patients

Cesarean section (CS) is the most widely performed and one of the most painful surgeries. This study investigated postoperative pain after CS using patient-related outcomes (PROs) to identify risk factors for severe pain. The secondary outcome was to evaluate the influence of surgery indication (primary CS (PCS) vs. urgent CS (UCS)). This multi-center, prospective cohort study included data submitted to the pain registry "quality improvement in postoperative pain treatment" (QUIPS) between 2010 and 2020. In total, 11,932 patients were evaluated. Median of maximal pain was 7.0 (numeric rating scale (NRS) 0 to 10); 53.9% suffered from severe pain (NRS ≥ 7), this being related to impairment of mood, ambulation, deep breathing and sleep, as well as more vertigo, nausea and tiredness (p < 0.001). Distraction, relaxation, mobilization, having conversations, patient-controlled analgesia (PCA) and pain monitoring were shown to be protective for severe pain (p < 0.001). Maximal pain in PCS and UCS was similar, but UCS obtained more analgesics (p < 0.001), and experienced more impairment of ambulation (p < 0.001) and deep breathing (p < 0.05). Severe pain has a major effect on daily-life activities and recovery after CS, and depends on modifiable factors. More effort is needed to improve the quality of care after CS.

Simultaneous application of lidocaine and ketamine during ambulatory infusion therapy: a retrospective analysis

Background: Infusions with lidocaine or ketamine have been separately established in the treatment of chronic pain. This study aims to retrospectively evaluate the effect of combined infusions of lidocaine and ketamine. Materials & methods: Patient records were screened for receipt of combined ambulatory infusions of lidocaine and ketamine from 2012 through 2021. A scoring system was designed to assess pain response retrospectively. Results: A total of 319 patients were included. Median pain reduction in days was 10.00 (interquartile range: 13.25). Side effects were limited to the acute phase of infusions. A total of 41.4% of patients who received concomitant pain medication reported a dose reduction. Conclusion: Our data support combined infusions as a safe therapy option, with good short-, medium- and long-term reductions in pain and great heterogeneity in treatment response. Clinical trial registration: ClinicalTrials.gov (NCT05103319).

Burrowing as an index of inflammatory pain in male vs. female rats

The study objective was to determine whether burrowing behavior is useful as a functional index of pain in both male and female rats, and whether a 'no-training' protocol can be used to increase testing efficiency. Adult Sprague-Dawley rats were injected in one or both hindpaws with oil vehicle or complete Freund's adjuvant (CFA); starting the next day, the amount of gravel each rat burrowed out of a tube in 1 h was measured daily for ≤7 days. Without preliminary training on the burrowing procedure, CFA reliably suppressed burrowing for 2-3 days compared to controls, in both sexes. However, whereas unilateral CFA completely suppressed burrowing 1-day post-CFA in nearly all males, bilateral CFA was required to do so in females. When administered 30 min before testing, once daily for 5 days post-CFA, the nonsteroidal anti-inflammatory drug ketoprofen (0.01-3.2 mg/kg) and the opioid morphine (0.1-3.2 mg/kg) significantly increased CFA-suppressed burrowing, whereas the purported cannabinoid analgesic Δ 9 -tetrahydrocannabinol (0.01-2.0 mg/kg) did not. The benzodiazepine chlordiazepoxide (1.25-10 mg/kg), included as a 'true negative' control, also did not restore CFA-suppressed burrowing in either sex. However, in CFA-treated males only, chlordiazepoxide decreased burrowing, suggesting that anxiety may contribute to burrowing in males but not females that are in pain. Overall these results suggest that burrowing is a valid, functional index of inflammatory pain in both sexes, and training on the burrowing procedure is not necessary. However, females are more avid burrowers than males, which should be considered when both sexes are used in inflammatory pain testing.

Consideration of sex and gender in Alzheimer's disease and related disorders from a global perspective

Sex or gender differences in the risk of Alzheimer's disease and related dementias (ADRD) differ by world region, suggesting that there are potentially modifiable risk factors for intervention. However, few epidemiological or clinical ADRD studies examine sex differences; even fewer evaluate gender in the context of ADRD risk. The goals of this perspective are to: (1) provide definitions of gender, biologic sex, and sexual orientation. and the limitations of examining these as binary variables; (2) provide an overview of what is known with regard to sex and gender differences in the risk, prevention, and diagnosis of ADRD; and (3) discuss these sex and gender differences from a global, worldwide perspective. Identifying drivers of sex and gender differences in ADRD throughout the world is a first step in developing interventions unique to each geographical and sociocultural area to reduce these inequities and to ultimately reduce global ADRD risk. HIGHLIGHTS: The burden of dementia is unevenly distributed geographically and by sex and gender. Scientific advances in genetics and biomarkers challenge beliefs that sex is binary. Discrimination against women and sex and gender minority (SGM) populations contributes to cognitive decline. Sociocultural factors lead to gender inequities in Alzheimer's disease and related dementias (ADRD) worldwide.

The power of integrating data: advancing pain research using meta-analysis

Publications related to pain research have increased significantly in recent years. The abundance of new evidence creates challenges staying up to date with the latest information. A comprehensive understanding of the literature is important for both clinicians and investigators involved in pain research. One commonly used method to combine and analyse data in health care research is meta-analysis. The primary aim of a meta-analysis is to quantitatively synthesise the results of multiple studies focused on the same research question. Meta-analysis is a powerful tool that can be used to advance pain research. However, there are inherent challenges when combining data from multiple sources. There are also numerous models and statistical considerations when undertaking a meta-analysis. This review aims to discuss the planning and preparation for completing a meta-analysis, review commonly used meta-analysis models, and evaluate the clinical implications of meta-analysis in pain research.

Sex differences in the impact of pain, morphine administration and morphine withdrawal on quality of life in rats

The disruptive effects of pain on quality of life are greater in men than in women, but the disruptive effects of opioid administration and withdrawal tend to be greater in women. These sex differences in pain, acute opioid effects, and opioid withdrawal tend to be opposite to sex differences reported in laboratory rats. We hypothesized that sex differences in humans and rats would more closely align if animal research measured quality of life as opposed to traditional evoked behaviors of pain (e.g., nociceptive reflexes) and opioid withdrawal (e.g., wet dog shakes). The present study assessed quality of life in adult female and male rats by measuring voluntary wheel running in the rat's home cage. Hindpaw inflammation induced by administration of Complete Freund's Adjuvant (CFA) into the right hindpaw caused a greater depression of wheel running in male compared to female rats. Twice daily injections of high morphine doses (5-20 mg/kg) and the subsequent morphine withdrawal caused a greater depression of wheel running in female compared to male rats. These sex differences are consistent with human data that shows the impact of pain on quality of life is greater in men than women, but the negative effects of opioid administration and withdrawal are greater in women. The present data indicate that the clinical significance of animal research would be enhanced by shifting the endpoint from pain and opioid evoked behaviors to measures of quality of life such as voluntary wheel running.

Using crowdsourcing to study the differential effects of cross-drug withdrawal for cigarettes and opioids in a behavioral economic demand framework

Smoking rates among those who use prescribed or recreational opioids are significantly higher than the general population. Hypothesized neuropharmacological interactions between opioids and nicotine may contribute to this pattern of polysubstance use, especially during withdrawal. However, little research has examined how the withdrawal of one substance may affect the consumption of the other (i.e., cross-drug withdrawal effects). Behavioral economic demand tasks (e.g., hypothetical purchase tasks) can be used to quickly assess the value of a drug. Crowdsourcing can be a convenient tool to gain preliminary insight into different processes in substance valuation that may otherwise be impossible or prohibitively difficult to study. The purpose of the present study was to provide a preliminary examination of the effects of hypothetical withdrawal of cigarettes and opioids on the consumption of those drugs among polysubstance users. Amazon Mechanical Turk workers who reported daily smoking and at least monthly opioid use completed a series of hypothetical purchase tasks for doses of opioids and cigarettes under various withdrawal conditions. Sensitivity to the price of both drugs decreased when under withdrawal for either, indicating a higher drug value of cigarettes and opioids due to effects of cross-drug withdrawal. Nicotine and opioid dependence severity, impulsive choice, and riskiness were also positively related to drug purchasing. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Sex and gender differences in pain

Chronic pain affects 20% of adults and is one of the leading causes of disability worldwide. Women and girls are disproportionally affected by chronic pain. About half of chronic pain conditions are more common in women, with only 20% having a higher prevalence in men. There are also sex and gender differences in acute pain sensitivity. Pain is a subjective experience made up of sensory, cognitive, and emotional components. Consequently, there are multiple dimensions through which sex and gender can influence the pain experience. Historically, most preclinical pain research was conducted exclusively in male animals. However, recent studies that included females have revealed significant sex differences in the physiological mechanisms underlying pain, including sex specific involvement of different genes and proteins as well as distinct interactions between hormones and the immune system that influence the transmission of pain signals. Human neuroimaging has revealed sex and gender differences in the neural circuitry associated with pain, including sex specific brain alterations in chronic pain conditions. Clinical pain research suggests that gender can affect how an individual contextualizes and copes with pain. Gender may also influence the susceptibility to develop chronic pain. Sex and gender biases can impact how pain is perceived and treated clinically. Furthermore, the efficacy and side effects associated with different pain treatments can vary according to sex and gender. Therefore, preclinical and clinical research must include sex and gender analyses to understand basic mechanisms of pain and its relief, and to develop personalized pain treatment.

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.

Opioid analgesia alters corticospinal coupling along the descending pain system in healthy participants

Opioids are potent analgesic drugs with widespread cortical, subcortical, and spinal targets. In particular, the central pain system comprising ascending and descending pain pathways has high opioid receptor densities and is thus crucial for opioid analgesia. Here, we investigated the effects of the opioid remifentanil in a large sample (n = 78) of healthy male participants using combined corticospinal functional MRI. This approach offers the possibility to measure BOLD responses simultaneously in the brain and spinal cord, allowing us to investigate the role of corticospinal coupling in opioid analgesia. Our data show that opioids altered activity in regions involved in pain processing such as somatosensory regions, including the spinal cord and pain modulation such as prefrontal regions. Moreover, coupling strength along the descending pain system, that is, between the anterior cingulate cortex, periaqueductal gray, and spinal cord, was stronger in participants who reported stronger analgesia during opioid treatment while participants that received saline showed reduced coupling when experiencing less pain. These results indicate that coupling along the descending pain pathway is a potential mechanism of opioid analgesia and can differentiate between opioid analgesia and unspecific reductions in pain such as habituation.

Focus on Sex and Gender: What We Need to Know in the Management of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease, affecting mostly women with a female/male ratio of 3:1. It is characterized by symmetrical polyarthritis, leading to progressive joint damage. Sex differences have been reported in terms of disease course and characteristics, influencing patients reported outcome measures (PROMs) and pain perception, ultimately leading to male–female disparities in treatment response. Notwithstanding, sex and gender discrepancies are still under-reported in clinical trials. Therefore, there is a consistent need for a precise reference of sex and gender issues in RA studies to improve treat-to-target achievement. This narrative review explores the above-mentioned aspects of RA disease, discussing the latest core principles of RA recommendations, from safety issues to early arthritis concept and management, treat-to-target and difficult-to-treat notions, up to the most recent debate on vaccination. Our final purpose is to evaluate how sex and gender can impact current management guidelines and how this issue can be integrated for effective disease control.

The PPARγ agonist pioglitazone produces a female-predominant inhibition of hyperalgesia associated with surgical incision, peripheral nerve injury, and painful diabetic neuropathy

Pioglitazone, an agonist at peroxisome proliferator-activated receptor gamma, is FDA-approved for the treatment of insulin resistance in type 2 diabetes. Numerous studies in male rodents suggest that pioglitazone inhibits inflammatory and neuropathic pain, but few included female subjects. To address this gap, we compared the effects of pioglitazone in both sexes in the intraplantar methylglyoxal model (MG) model of chemical pain and painful diabetic neuropathy (PDN), the plantar incision model (PIM) of postoperative pain, the spared nerve injury (SNI) model of traumatic nerve injury, and the ZDF rat and db/db mouse models of PDN. We administered pioglitazone by one-time intrathecal or intraperitoneal injection or by adding it to chow for 6 weeks, followed by measurement of hypersensitivity to non-noxious mechanical, noxious mechanical, heat, and/or cold stimuli. In all mouse models, injection of pioglitazone decreased pain-like behaviors with greater potency and/or efficacy in females as compared to males: heat and mechanical hypersensitivity in the MG model (0.1-10 mg/kg); mechanical hypersensitivity in the PIM model (10 μg); mechanical and cold hypersensitivity in the SNI model (100 mg/kg); and heat hypersensitivity in the db/db model (100 mg/kg). Furthermore, co-administration of low doses of morphine (1 mg/kg) and pioglitazone (10 mg/kg) decreased SNI-induced mechanical and cold hypersensitivity in female but not male mice. In the ZDF rat, pioglitazone (100 mg/kg) decreased heat and mechanical hypersensitivity with no sex difference. In the db/db model, pioglitazone had no effect when given into chow for 6 weeks at 0.3, 3 or 30 mg/kg doses. We conclude that females exhibit greater anti-hyperalgesic responses to pioglitazone in mouse models of chemical-induced nociception, postsurgical pain, neuropathic pain, and PDN. These findings set the stage for clinical trials to determine whether pioglitazone has analgesic properties across a broad spectrum of chronic pain conditions, particularly in women.

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

No significant influence of OCT1 genotypes on the pharmacokinetics of morphine in adult surgical patients

We investigated the impact of genetic variants in OCT1 (SLC22A1) on morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) pharmacokinetics in adult patients scheduled for major surgery. Blood samples were taken before and 5, 10, 15, 30, 45, 60 and 90 min after a bolus of morphine (0.15 mg/kg). Patients were genotyped for the genetic variants (rs12208357, rs34059508, rs72552763 and rs34130495) in OCT1. Eighty-six patients completed the trial. The mean difference (95% confidence interval) for dose adjusted morphine, M3G and M6G AUC was 0.9 (-0.7-2.4), -5.9 (-11.8 to -0.03) and -1.1 (-2.5-0.4) h/L*10-6 , respectively, in patients with two reduced function alleles compared to patients with no reduced function alleles in OCT1. Accordingly, the (AUCM3G/Dose )/(AUCmorphine/Dose ) and (AUCM6G/Dose )/(AUCmorphine/Dose ) ratio was reduced, -1.8 (-3.2 to -0.4) and -0.4 (-0.7 to -0.03), respectively, when comparing the same groups. OCT1 variants had no influence on the experience of pain, adverse events or the number of PCA doses used. In conclusion, genetic variants in OCT1 had a small and clinically unimportant impact on the exposure of morphine after intravenous administration. Our results do not support pre-emptive genotyping for OCT1 prior to morphine administration in patients scheduled for major surgery.

The Human SCN10AG1662S Point Mutation Established in Mice Impacts on Mechanical, Heat, and Cool Sensitivity

The voltage-gated sodium channel NAV1.8 is expressed in primary nociceptive neurons and is involved in pain transmission. Mutations in the SCN10A gene (encoding NAV1.8 channel) have been identified in patients with idiopathic painful small fiber neuropathy (SFN) including the SCN10A^G1662S gain-of-function mutation. However, the role of this mutation in pain sensation remains unknown. We have generated the first mouse model for the G1662S mutation by using homologous recombination in embryonic stem cells. The corresponding Scn10a^G1663S mouse line has been analyzed for Scn10a expression, intraepidermal nerve fiber density (IENFD), and nociception using behavioral tests for thermal and mechanical sensitivity. The Scn10a^G1663S mutants had a similar Scn10a expression level in dorsal root ganglia (DRG) to their wild-type littermates and showed normal IENFD in hindpaw skin. Mutant mice were more sensitive to touch than wild types in the von Frey test. In addition, sexual dimorphism was observed for several pain tests, pointing to the relevance of performing the phenotypical assessment in both sexes. Female homozygous mutants tended to be more sensitive to cooling stimuli in the acetone test. For heat sensitivity, male homozygous mutants showed shorter latencies to radiant heat in the Hargreaves test while homozygous females had longer latencies in the tail flick test. In addition, mutant males displayed a shorter reaction latency on the 54°C hot plate. Collectively, Scn10a^G1663S mutant mice show a moderate but consistent increased sensitivity in behavioral tests of nociception. This altered nociception found in Scn10a^G1663S mice demonstrates that the corresponding G1662 mutation of SCN10A found in SFN patients with pain contributes to their pain symptoms.

Influences of Gender on Intravenous Nalbuphine Actions After Major Abdominal Surgery: A Multicenter Study

INTRODUCTION

Nalbuphine, a synthetic kappa-opioid receptor (KOR) agonist and a partial µ-opioid receptor (MOR) antagonist, has been used for years as an effective analgesic. It has been shown to have a better safety profile than morphine. Considering the long history of use of this drug, it is interesting that only a limited amount of information exists on how gender differences influence nalbuphine responses. In this randomized double-blind comparative trial after major abdominal surgery, the analgesic effects of two doses of continuous intravenous infusion of nalbuphine were evaluated based on gender.

METHODS

Enrolled patients were divided into four groups (two females and two males with 32 patients in each group). Two of them (groups A1 and A2), one male and one female, received postoperative continuous intravenous infusions of nalbuphine at 2 mg/h via patient-controlled analgesia (PCA). Each patient had the potential of receiving a rescue bolus of 1 mg of nalbuphine with a lock out time of 15 min. The other two groups (groups B1 and B2) received half the infusion dose, 1 mg/h, and half the nalbuphine rescue dose with the PCA pump, 0.5 mg maximum every 15 min as needed. Patients' vital signs, numerical pain rating scores, rescue nalbuphine, and incidence of side effects were assessed immediately after the operation, and every 3 h during the first 12 h.

RESULTS

Nalbuphine 2 mg/h dosing led to significantly lower pain scores amongst females compared to males at 6, 9, and 12 h; while the 1 mg/h infusion pain scores were only lower at the 9-h time period. Females receiving the nalbuphine 2 mg dose at 6 h, and the 1 mg dose at 6, 9, and 12-h measurements needed significantly lower doses of rescue nalbuphine. Females on the 1 mg dose experienced significantly more nausea, vomiting, and sedation at the 6-, 9-, and 12-h measurement times. In the multivariate analysis, female gender was a negative predictor at all measurement times.

CONCLUSIONS

The current study supports the hypothesis that although nalbuphine was found to be an effective and well-tolerated analgesic after major abdominal surgery, females were statistically more responsive than males.

TRIAL REGISTRATION

The study was registered at the Pan African Clinical trials Registry PACTR201304000486309, and approved for the Ethical aspects.

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.

A scoping review of the diurnal variation in the intensity of neuropathic pain

BACKGROUND

Recent studies suggest that neuropathic pain exhibit a daily diurnal pattern with peak levels usually in the late afternoon to evening and trough in the morning hours, although literature on this topic has been sparse. This scoping review examines current evidence on the chronobiology of neuropathic pain in both animal models and in humans with neuropathic pain.

METHOD

Literature search was conducted on major medical databases for relevant articles on chronobiology of neuropathic pain in both animal models and in humans with neuropathic pain. Data extracted include details of specific animal models or specific neuropathic pain conditions in humans, methods and timing of assessing pain severity, and specific findings of diurnal variation in pain intensity or its surrogate markers.

RESULTS

Thirteen animal and eight human studies published between 1976 to 2020 were included in the analysis. Seven out of 13 animal studies reported specific diurnal variation in pain intensity, with five of the seven studies reporting a trend towards increased sensitivity to mechanical allodynia or thermal hyperalgesia in the late light to dark phase. All eight studies on human subjects reported a diurnal variation in the intensity of neuropathic pain where there was an increase in pain intensity through the day with peaks in late evening and early night hours.

CONCLUSIONS

Studies included in this review demonstrated a diurnal variation in the pattern of neuropathic pain that is distinct from the pattern for nociceptive pain. These findings have implications for potential therapeutic strategies for neuropathic pain.

Elevated customary alcohol consumption attenuates opioid effects

BACKGROUND

Regular alcohol consumption is on the rise among older adults and has the potential of altering the subjective experience of pain and response to pain medications. This study examined the cognitive, analgesic and side effect response to oxycodone in middle age and older adults with elevated levels of customary alcohol consumption in a human laboratory setting.

METHODS

After refraining from alcohol for one day, eligible participants underwent baseline assessment cognition and side effects by means of questionnaires that were repeated at three time points (90 min, 5 and 8 h) following administration of a 10 mg oral dose of oxycodone. Response to pain stimulus (Cold Pressor Test (CPT)), pupil size, and plasma oxycodone were also measured.

RESULTS

One hundred twenty-eight adults (age 35-85) completed the study day. Compared to those with lower customary alcohol consumption, participants with elevated alcohol consumption showed attenuated opioid-induced pupil constriction and cognitive decline on objective measures of working memory, sustained attention, inhibitory control, coordination on a simulated driving task, and subjective dysphoric effects with enhanced subjective euphoric effects. Oxycodone pharmacokinetics, pain tolerance to CPT, and Berg balance were impacted comparably between alcohol consumption groups. Women endorsed greater negative drug effects, whereas men endorsed positive drug effects.

CONCLUSION

Independent of subject's age, elevated customary alcohol consumption attenuates opioid central effects (i.e., pupil miosis, impaired cognition) and gender influences subjective drug effects. Clinicians should consider alcohol consumption and gender when prescribing opioid medications.

Physiological properties of pain-modulating neurons in rostral ventromedial medulla in female rats, and responses to opioid administration

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Physiological properties of RVM pain-modulating neurons were described in female rats.

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ON- and OFF-cells in females have fundamental properties comparable to those in males.

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As in males, RVM neuron output is altered in persistent inflammation and by morphine.

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This work provides a foundation for future studies of RVM in females.

Functional pain disorders disproportionately impact females, but most pain research in animals has been conducted in males. While there are anatomical and pharmacological sexual dimorphisms in brainstem pain-modulation circuits, the physiology of pain-modulating neurons that comprise a major functional output, the rostral ventromedial medulla (RVM), has not been explored in female animals. The goal of this study was to identify and characterize the activity of RVM cells in female, compared to male, rats. ON- and OFF-cells were identified within the RVM in females, with firing properties comparable to those described in males. In addition, both ON- and OFF-cells exhibited a sensitized response to somatic stimuli in females subjected to persistent inflammation, and both ON- and OFF-cells responded to systemically administered morphine at a dose sufficient to produce behavioral antinociception. These data demonstrate that the ON-/OFF-cell framework originally defined in males is also present in females, and that as in males, these neurons are recruited in females in persistent inflammation and by systemically administered morphine. Importantly, this work establishes a foundation for the use of female animals in studies of RVM and descending control.

Does aerobic exercise training alter responses to opioid analgesics in individuals with chronic low back pain? A randomized controlled trial

ABSTRACT

We tested whether aerobic exercise training altered morphine analgesic responses or reduced morphine dosages necessary for adequate analgesia. Patients with chronic back pain were randomized to an 18-session aerobic exercise intervention (n = 38) or usual activity control (n = 45). Before and after the intervention, participants underwent 3 laboratory sessions (double-blinded, crossover) to assess effects of saline placebo, i.v. morphine (0.09 mg/kg), and i.v. naloxone (12 mg) on low back pain and evoked heat pain responses. Differences in evoked and back pain measures between the placebo and morphine conditions indexed morphine analgesia, with pre-post intervention changes the primary outcome. Endogenous opioid analgesia was indexed by differences in evoked and low back pain measures between the naloxone and placebo conditions. A Sex X Intervention interaction on the analgesic effects of morphine on visual analogue scale back pain intensity was observed (P = 0.046), with a similar trend for evoked pain threshold (P = 0.093). Male exercisers showed reduced morphine analgesia pre-post intervention, whereas male controls showed increased analgesia (with no differences in females). Of clinical significance were findings that relative to the control group, aerobic exercise produced analgesia more similar to that observed after receiving ≈7 mg morphine preintervention (P < 0.045). Greater pre-post intervention increases in endogenous opioid function (from any source) were significantly associated with larger pre-post intervention decreases in morphine analgesia (P < 0.046). The overall pattern of findings suggests that regular aerobic exercise has limited direct effects on morphine responsiveness, reducing morphine analgesia in males only.

Men and Women Respond Equally Well to Spinal Cord and Dorsal Root Ganglion Stimulation

OBJECTIVES

The influence of gender on outcomes in individuals undergoing treatment for chronic pain is unclear. This retrospective, single-site study explored the impact of gender on pain, quality of life (QoL), revisions, and explants in patients with failed back surgery syndrome or visceral pain, who received a fully implanted 10 kHz spinal cord stimulation (SCS), burst SCS, or dorsal root ganglion (DRG) stimulation system.

MATERIALS AND METHODS

The following data were collected from paper and electronic records: gender, age, chronic pain diagnosis, system, baseline and follow-up scores (average pain [visual analog scale, VAS], worst pain [VAS], QoL [EQ-5D-3L]), revisions, and explants. Data were statistically analyzed by one-way ANCOVAs controlling for age, chi-square tests of independence and logistic regression.

RESULTS

The final sample comprised 387 patients (176 males and 211 females). Males were significantly older compared to females (mean difference: 2.33 years, p = 0.044). Controlling for age, baseline average pain was significantly lower in males than females (mean difference: -0.32, p = 0.049). Males and females responded equally well to 10 kHz SCS and burst SCS as well as DRG stimulation. A greater percentage of males (5%) than females (1%) had revisions due to lead fractures. Additionally, more females (13%) than males (6%) had an explant due to insufficient pain relief. Female gender and older age were associated with greater likelihood of having an explant compared to male gender and younger age.

CONCLUSION

Gender may play an influential role in pain severity at baseline but have little effect at follow-up. To help identify which patients may undergo a revision or explant, gender and age could be important factors and should be further scrutinized. Even though men and women responded equally well to SCS and DRG stimulation, more men had a revision due to lead fractures, and more women were explanted due to insufficient pain relief.

Sex-based differences in psychiatric symptoms and opioid abstinence during buprenorphine/naloxone treatment in adolescents with opioid use disorders

BACKGROUND

Recent studies indicate that sex-based differences exist in co-occurring psychiatric symptoms and disorders among individuals with opioid use disorders (OUD). Whether these associations are present in adolescent samples and change during OUD treatment is poorly understood.

OBJECTIVES

In the current study, we examined sex-based differences in psychiatric symptoms and relationships among sex, psychiatric symptoms, and opioid use outcomes in youth with OUD receiving buprenorphine/naloxone (Bup/Nal) and psychosocial treatment.

METHODS

The study randomly assigned one hundred and fifty-two youth (15-21 years old) diagnosed with OUD to either 12 weeks of treatment with Bup/Nal or up to 2 weeks of Bup/Nal detoxification with both treatment arms receiving weekly drug counseling as part of a multisite clinical trial (NIDA-CTN-0010). We compared psychiatric symptoms, assessed via the Youth Self Report (YSR) at baseline and week 12, across male and female OUD participants. The study used logistic regression models to identify sex and psychiatric symptom variables that were predictors of opioid positive urine (OPU) at week 12.

RESULTS

Compared to males, females with OUD had higher mean psychiatric symptom scores at baseline across broad-band and narrow-band symptom domains. The study observed significant reductions in psychiatric symptom scores in both males and females during treatment, and by week 12, females only differed from males on anxious-depressive symptom scores. Females, in general, and youth of both sexes presenting to treatment with higher anxious depression scores were less likely to have a week-12 OPU.

CONCLUSIONS

Clinically significant sex-based differences in psychiatric symptoms are present at baseline among youth with OUD receiving Bup/Nal-assisted treatment and mostly resolve during treatment.

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.

Exploring the Domains of Gender as Measured by a New Gender, Pain and Expectations Scale

Background: While sex- or gender-based differences in pain expression have been documented, exploration of traditionally genderized traits on pain has been hampered by the lack of strong measurement tools. This study evaluated the structural validity of a 16-item "Gender personality traits" subscale of a recently developed Gender, Pain and Expectations Scale (GPES). Methods: Data were drawn from an existing database of 248 participants (65.7% female). Maximum likelihood-based confirmatory factor analysis was carried out while considering the conceptual meaningfulness of subscales to evaluate the factor structure identified by these traits. Construct validity was explored using a priori hypotheses regarding anticipated mean differences in scores between biological male and female participants. Results: A meaningful factor structure could not be defined with all 16 items. Through conceptual and statistical triangulation a three-factor structure informed by 10 items was identified that satisfied acceptable fit criteria. The factors were termed "Emotive," "Relationship-Oriented," and "Goal-Oriented." Evidence of construct validity was supported through significant sex-based differences (p ≤ 0.02) in the expected directions for all three subscales. Conclusions: Review of the items in the three factors led the researchers to endorse a move away from naming these "masculine" and "feminine," rather focusing on the nature of the traits: "Relationship-oriented," "Emotive," and "Goal-oriented." Implications for researchers conducting sex/gender-based pain research are discussed. Clinical Trial Registration number: NCT02711085.

Comparison of Postoperative Opioid Utilization in an ACDF Cohort: Narcotic Naive Patients Versus Preoperative Opioid Users

STUDY DESIGN

A retrospective cohort study.

OBJECTIVE

To compare the postoperative opioid utilization rates and costs after anterior cervical discectomy and fusion (ACDF) procedures between groups of patients who were preoperative opioid users versus opioid naive.

SUMMARY OF BACKGROUND DATA

Opioid medications are frequently prescribed after ACDF procedures. Given the current opioid epidemic, there is increased emphasis on early identification of patients at risk for prolonged postoperative opioid use.

METHODS

Records from patients diagnosed with cervical stenosis who underwent a ≤3-level index ACDF surgery between 2007 and 2017 were collected from a large insurance database. International Classification of Diseases diagnosis/procedure codes, Current Procedural Terminology codes, and generic drug codes were used to search clinical records. Two cohorts were established: a group of patients who utilized opioids preoperatively and a group of patients who were opioid naive at the time of surgery. The 1-year utilization and costs of postoperative therapies were documented for each group.

RESULTS

The preoperative opioid use cohort contained 4485 patients (61.6%), whereas the opioid-naive cohort included 2799 patients (38.4%). Postoperatively, 86.6% of the preoperative opioid use group continued to use opioids, whereas 59.0% of the opioid-naive group began using opioids. Patients who utilized opioids preoperatively were 4.48 times more likely (95% confidence interval, 3.99-5.02, P<0.001) to use opioids postoperatively and 4.30 times more likely (95% confidence interval, 3.10-5.94, P<0.001) to become opioid dependent compared with opioid-naive patients. In addition, after normalization, patients in the preoperative opioid use group utilized 3.7 times more opioid units/patient and billed for 5.3 times more dollars/patient than opioid-naive patients.

CONCLUSIONS

In patients with cervical stenosis who undergo an ACDF procedure, the postoperative utilization and costs of opioids seem to be substantially higher in patients with preoperative opioid use compared with opioid-naive patients. Efforts should be made to avoid opioid use as a component of conservative management before surgery.

LEVEL OF EVIDENCE

Level III.