Sex differences in morphine analgesia: an experimental study in healthy volunteers

Probiotic supplementation mitigates sex-dependent nociceptive changes and gut dysbiosis induced by prenatal opioid exposure

The gut microbiome has emerged as a promising target for modulating adverse effects of opioid exposure due to its significant role in health and disease. Opioid use disorder (OUD) has become increasingly prevalent, specifically in women of reproductive age, contributing to an increased incidence of offspring exposed to opioids in utero. Recent studies have shown that prenatal opioid exposure (POE) is associated with notable changes to the maternal gut microbiome, with subsequent implications for the offspring's microbiome and other adverse outcomes. However, the role of the gut microbiome in mediating sex-based differences in pain sensitivity has not yet been investigated. In this study, both male and female C57BL/6 offspring were used to determine sex-based differences in nociception and gut microbial composition as a result of POE. Our data reveals significant sex-based differences in offspring prenatally exposed to opioids. The gut microbiome of opioid-exposed females showed an enrichment of commensal bacteria including Lactobacillus compared to opioid-exposed males. Additionally, POE females demonstrated decreased nociceptive sensitivity, while males demonstrated increased nociceptive sensitivity. RNA sequencing of the prefrontal cortex showed sex-based differences in several canonical pathways, including an increase in the opioid signaling pathway of opioid-exposed females, which was not observed in males. Microbiome modification via maternal probiotic supplementation attenuated sex-based differences throughout the early stages of life. Together, our study provides further insight on sex-based differences arising from POE and highlights the pivotal role of the gut microbiome as a modifiable target for mitigating its negative effects.

Morphine and hydromorphone pharmacokinetics in human volunteers: population-based modelling of interindividual and opioid-related variability

BACKGROUND

Morphine and hydromorphone have differing onsets, magnitudes, and durations of effects and side-effects. Differences between opioids in their interindividual variabilities in pharmacokinetics and pharmacodynamics might influence rational drug selection. Crossover drug studies can provide more informative interindividual variability data than parallel group studies. Using data from a crossover study of i.v. morphine and hydromorphone in healthy volunteers, we tested the hypothesis that morphine and hydromorphone differ in their interindividual pharmacokinetic variability.

METHODS

Arterial opioid and metabolite concentrations from a randomised crossover study in 51 volunteers receiving a 2-h infusion of hydromorphone (0.05 or 0.1 mg kg-1 i.v.) or morphine (total 0.1 or 0.2 mg kg-1 i.v.) 1-2 weeks apart were evaluated with a three-compartmental model for parent opioid and incorporating glucuronides using population modelling (NONMEM). The primary outcome was interindividual variability in pharmacokinetics, based on the coefficient of variation (%CV) of individual model parameters, calculated as √[exp(ω2)-1]×100 where ω2 is the interindividual variability.

RESULTS

Data were analysed per drug and in a combined morphine-hydromorphone model. Both analyses indicate that interindividual variabilities for hydromorphone and morphine were comparable with %CV ranging from 9% to 31% for structural model parameters (combined analysis). Similarly, additive and relative residual errors had comparable variabilities, 20-40% and 72-87%, respectively, for morphine and hydromorphone (combined analysis).

CONCLUSIONS

Morphine and hydromorphone did not differ in a statistically significant or clinically meaningful manner in their interindividual pharmacokinetic variability. Interindividual pharmacokinetic variability does not appear a meaningful consideration in the choice between these two opioids.

EC50 and EC95 of Remifentanil for Inhibiting Bronchoscopy Responses in Elderly Patients During Fiberoptic Bronchoscopy Under Ciprofol Sedation: An Up-and-Down Sequential Allocation Trial

Background

Opioids are used to suppress cough during fiberoptic bronchoscopy (FOB). However, evidence regarding the optimal dose of remifentanil during FOB under ciprofol sedation is limited. This study aimed to investigate the effective concentration (EC) of remifentanil required to suppress bronchoscopy responses during FOB under ciprofol sedation in elderly patients.

Materials and Methods

Elderly patients aged 60 to 90 years with American Society of Anesthesiologists (ASA) physical status I-III, scheduled for FOB, were enrolled. Patients were assigned to either the male or the female group. Remifentanil was administered intravenously prior to ciprofol administration. The endpoints included responses to FOB, such as vocal cords movement, coughing, and body movement. The EC50 and EC95 values of remifentanil required to alleviate the responses to FOB were calculated using Dixon's up-and-down method for both male and female groups. Probit analysis was used to generate a dose-response curve.

Results

Thirty-nine patients (19 males and 20 females) were enrolled. The EC50 values (plasma concentration) of remifentanil for blunting FOB responses under ciprofol sedation were 3.25 (2.75 to 3.26) ng/mL and 2.25 (1.75 to 2.25) ng/mL in males and females, respectively (p = 0.0023). Probit analysis indicated that the EC50 of remifentanil required to suppress responses to FOB under ciprofol sedation was 3.102 [95% confidence interval (CI):2.694 to 3.749] ng/mL and 2.052 [95% CI: 1.345 to 2.750] ng/mL in males and females, respectively. The EC95 of remifentanil required to suppress responses to FOB under ciprofol sedation was 3.741 [95% CI: 3.366 to 7.699] ng/mL and 2.943 [95% CI: 2.456 to 9.533] ng/mL in males and females, respectively.

Conclusion

The results indicate differences between males and females in the EC50 and EC95 of remifentanil needed to suppress responses to FOB under ciprofol sedation while preserving spontaneous breathing in elderly patients.

Trial Registration

The study was registered with chictr.org.cn (ChiCTR2300077720; 17 th November 2023).

Molecular Sex Differences and Clinical Gender Efficacy in Opioid Use Disorders: From Pain Management to Addiction

Opioids have been utilized for both medical and recreational purposes since their discovery. Primarily recognized for their analgesic properties, they are also associated with the development of tolerance and dependence, contributing to a significant public health concern worldwide. Sex differences in opioid use disorder reveal that while men historically exhibit higher rates of abuse, women may develop dependence more quickly and are more susceptible to the addictive nature of opioids. This narrative review explores sex differences in opioid response in both clinical and experimental models, focusing on opioid receptor mechanisms, pain modulation, and hormonal influences. Additionally, it discusses the complexities of opioid addiction and withdrawal, highlighting sex-specific responses and the role of opioid replacement therapies. Diverse experimental outcomes, together with observational data, underscore the need for further research into sex-specific opioid biological mechanisms in a wider context, including demographic, cultural, and health-related factors. A comprehensive understanding of these complexities holds the potential to enhance personalized opioid therapies.

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.

Associations of dexamethasone's effect on morphine consumption after total knee arthroplasty-Subgroup analyses

The DEXamethasone twice for pain treatment after Total Knee Arthroplasty (DEX-2-TKA) trial showed that adding one and two doses of 24 mg intravenous dexamethasone to paracetamol, ibuprofen and local infiltration analgesia, reduced morphine consumption (primary outcome) within 48 h after TKA. We aimed to explore the differences in the effect of dexamethasone on morphine consumption in different subgroups. Quantile regression adjusted for site was used to test for significant interaction between the predefined dichotomised subgroups and treatment group. The subgroups were defined based on baseline data: sex (male/female), age (≤65 years/>65 years), American Society of Anaesthesiologists (ASA)-score (ASA I + II/III), visual analogue score of preoperative pain at rest (≤30 mm/>30 mm), pain during mobilisation (≤30 mm/>30 mm), type of anaesthesia (spinal anaesthesia/general anaesthesia and spinal converted to general anaesthesia), and prior daily use of analgesics (either paracetamol and/or NSAID/neither). These analyses were supplemented with post hoc multivariate linear regression analyses. Test of interaction comparing sex in the pairwise comparison between DX2 (dexamethasone [24 mg] + dexamethasone [24 mg]) versus placebo (p = .02), showed a larger effect of dexamethasone on morphine consumption in male patients compared to females. Test of interaction comparing age in the pairwise comparison between DX1 (dexamethasone [24 mg] + placebo) versus placebo (p = .04), showed a larger effect of dexamethasone on morphine consumption in younger patients (≤65 years) compared to older. All remaining subgroup analyses showed no evidence of a difference. The supplemental multivariate analyses did not support any significant interaction for sex (p = .256) or age (p = .730) but supported a significant interaction with the type of anaesthesia (p < .001). Our results from the quantile regression analyses indicate that the male sex and younger age (≤65 years) may be associated with a larger analgesic effect of dexamethasone than the effects in other types of patients. However, this is not supported by post-hoc multivariate linear regression analyses. The two types of analyses both supported a possible interaction with the type of anaesthesia.

Addressing Ancestry and Sex Bias in Pharmacogenomics

The association of an individual's genetic makeup with their response to drugs is referred to as pharmacogenomics. By understanding the relationship between genetic variants and drug efficacy or toxicity, we are able to optimize pharmacological therapy according to an individual's genotype. Pharmacogenomics research has historically suffered from bias and underrepresentation of people from certain ancestry groups and of the female sex. These biases can arise from factors such as drugs and indications studied, selection of study participants, and methods used to collect and analyze data. To examine the representation of biogeographical populations in pharmacogenomic data sets, we describe individuals involved in gene-drug response studies from PharmGKB, a leading repository of drug-gene annotations, and showcaseCYP2D6, a gene that metabolizes approximately 25% of all prescribed drugs. We also show how the historical underrepresentation of females in clinical trials has led to significantly more adverse drug reactions in females than in males.

Morphine and Hydromorphone Effects, Side Effects, and Variability: A Crossover Study in Human Volunteers

BACKGROUND

Balancing between opioid analgesia and respiratory depression continues to challenge clinicians in perioperative, emergency department, and other acute care settings. Morphine and hydromorphone are postoperative analgesic standards. Nevertheless, their comparative effects and side effects, timing, and respective variabilities remain poorly understood. This study tested the hypothesis that IV morphine and hydromorphone differ in onset, magnitude, duration, and variability of analgesic and ventilatory effects.

METHODS

The authors conducted a randomized crossover study in healthy volunteers. Forty-two subjects received a 2-h IV infusion of hydromorphone (0.05 mg/kg) or morphine (0.2 mg/kg) 1 to 2 weeks apart. The authors measured arterial opioid concentrations, analgesia in response to heat pain (maximally tolerated temperature, and verbal analog pain scores at discrete preset temperatures to determine half-maximum temperature effect), dark-adapted pupil diameter and miosis, end-expired carbon dioxide, and respiratory rate for 12 h after dosing.

RESULTS

For morphine and hydromorphone, respectively, maximum miosis was less (3.9 [3.4 to 4.2] vs. 4.6 mm [4.0 to 5.0], P < 0.001; median and 25 to 75% quantiles) and occurred later (3.1 ± 0.9 vs. 2.3 ± 0.7 h after infusion start, P < 0.001; mean ± SD); maximum tolerated temperature was less (49 ± 2 vs. 50 ± 2°C, P < 0.001); verbal pain scores at end-infusion at the most informative stimulus (48.2°C) were 82 ± 4 and 59 ± 3 (P < 0.001); maximum end-expired CO2 was 47 (45 to 50) and 48 mmHg (46 to 51; P = 0.007) and occurred later (5.5 ± 2.8 vs. 3.0 ± 1.5 h after infusion start, P < 0.001); and respiratory nadir was 9 ± 1 and 11 ± 2 breaths/min (P < 0.001), and occurred at similar times. The area under the temperature tolerance-time curve was less for morphine (1.8 [0.0 to 4.4]) than hydromorphone (5.4°C-h [1.6 to 12.1] P < 0.001). Interindividual variability in clinical effects did not differ between opioids.

CONCLUSIONS

For morphine compared to hydromorphone, analgesia and analgesia relative to respiratory depression were less, onset of miosis and respiratory depression was later, and duration of respiratory depression was longer. For each opioid, timing of the various clinical effects was not coincident. Results may enable more rational opioid selection, and suggest hydromorphone may have a better clinical profile.

EDITOR’S PERSPECTIVE

Morphine pharmacokinetics and opioid transporter expression at the blood-retina barrier of male and female mice

Opioids are effective analgesics for treating moderate to severe pain, however, their use must be weighed against their dangerous side effects. Investigations into opioid pharmacokinetics provide crucial information regarding both on- and off-target drug effects. Our recent work showed that morphine deposits and accumulates in the mouse retina at higher concentrations than in the brain upon chronic systemic exposure. We also found reduced retinal expression of P-glycoprotein (P-gp), a major opioid extruder at the blood-brain barrier (BBB). Here, we systematically interrogated the expression of three putative opioid transporters at the blood-retina barrier (BRB): P-gp, breast cancer resistance protein (Bcrp) and multidrug resistance protein 2 (Mrp2). Using immunohistochemistry, we found robust expression of P-gp and Bcrp, but not Mrp2, at the inner BRB of the mouse retina. Previous studies have suggested that P-gp expression may be regulated by sex hormones. However, upon acute morphine treatment we found no sex differences in morphine deposition levels in the retina or brain, nor on transporter expression in the retinas of males and females with a high or low estrogen:progesterone ratio. Importantly, we found that P-gp, but not Bcrp, expression significantly correlated with morphine concentration in the retina, suggesting P-gp is the predominant opioid transporter at the BRB. In addition, fluorescence extravasation studies revealed that chronic morphine treatment did not alter the permeability of either the BBB or BRB. Together, these data suggest that reduced P-gp expression mediates retinal morphine accumulation upon systemic delivery, and in turn, potential effects on circadian photoentrainment.

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.

Comparing the Effects of Low-Dose Ketamine, Fentanyl, and Morphine on Hemorrhagic Tolerance and Analgesia in Humans

Hemorrhage is a leading cause of preventable battlefield and civilian trauma deaths. Ketamine, fentanyl, and morphine are recommended analgesics for use in the prehospital (i.e., field) setting to reduce pain. However, it is unknown whether any of these analgesics reduce hemorrhagic tolerance in humans. We tested the hypothesis that fentanyl (75 µg) and morphine (5 mg), but not ketamine (20 mg), would reduce tolerance to simulated hemorrhage in conscious humans. Each of the three analgesics was evaluated independently among different cohorts of healthy adults in a randomized, crossover (within drug/placebo comparison), placebo-controlled fashion using doses derived from the Tactical Combat Casualty Care Guidelines for Medical Personnel. One minute after an intravenous infusion of the analgesic or placebo (saline), we employed a pre-syncopal limited progressive lower-body negative pressure (LBNP) protocol to determine hemorrhagic tolerance. Hemorrhagic tolerance was quantified as a cumulative stress index (CSI), which is the sum of products of the LBNP and the duration (e.g., [40 mmHg x 3 min] + [50 mmHg x 3 min] …). Compared with ketamine (p = 0.002 post hoc result) and fentanyl (p = 0.02 post hoc result), morphine reduced the CSI (ketamine (n = 30): 99 [73-139], fentanyl (n = 28): 95 [68-130], morphine (n = 30): 62 [35-85]; values expressed as a % of the respective placebo trial's CSI; median [IQR]; Kruskal-Wallis test p = 0.002). Morphine-induced reductions in tolerance to central hypovolemia were not well explained by a prediction model including biological sex, body mass, and age (R2=0.05, p = 0.74). These experimental data demonstrate that morphine reduces tolerance to simulated hemorrhage while fentanyl and ketamine do not affect tolerance. Thus, these laboratory-based data, captured via simulated hemorrhage, suggest that morphine should not be used for a hemorrhaging individual in the prehospital setting.

S-Ketamine oral thin film—Part 2: Population pharmacodynamics of S-ketamine, S-norketamine and S-hydroxynorketamine

Ketamine is a versatile drug used for many indications and is administered via various routes. Here, we report on the pharmacodynamics of sublingual and buccal fast-dissolving oral-thin-films that contain 50 mg of S-ketamine in a population of healthy male and female volunteers. Twenty volunteers received one or two 50 mg S-ketamine oral thin films in a crossover design, placed for 10 min sublingually (n = 15) or buccally (n = 5). The following measurements were made for 6 h following the film placement: antinociception using three distinct pain assay; electrical, pressure, and heat pain, and drug high on an 11-point visual analog scale. Blood samples were obtained for the measurement of plasma S-ketamine, S-norketamine, and S-hydroxynorketamine concentrations. A population pharmacodynamic analysis was performed in NONMEM to construct a pharmacodynamic model of S-ketamine and its metabolites. P-values < 0.01 were considered significant. The sublingual and buccal 50 and 100 mg S-ketamine oral thin films were antinociceptive and produced drug high with effects lasting 2–6 h, although a clear dose-response relationship for antinociception could not be established. The effects were solely related to the parent compound with no contribution from S-norketamine or S-hydroxynorketamine. S-ketamine potency was lower for antinociception (C₅₀ ranging from 1.2 to 1.7 nmol/mL) than for drug high (C₅₀ 0.3 nmol/ml). The onset/offset of effect as defined by the blood-effect-site equilibration half-life did not differ among endpoints and ranged from 0 to 5 min. In conclusion, the 50-mg S-ketamine oral thin film was safe and produced long-term antinociception in all three nociceptive assays with side effects inherent to the use of ketamine. The study was registered at the trial register of the Dutch Cochrane Center (www.trialregister.nl) under identifier NL9267 and the European Union Drug Regulating Authorities Clinical Trials (EudraCT) database under number 2020-005185-33.

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 (14 females/15 males; 29 ± 6 yr; 26 ± 4 kg·m-2, means ± 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 2 min) before and ∼35 min 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 postdrug/placebo time points) using paired, two-tailed t tests. Before drug/placebo infusion, perceived pain (P = 0.92), ΔMSNA burst frequency (n = 14, P = 0.21), and Δmean 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 Δmean BP (morphine: 10 ± 7 vs. placebo: 13 ± 8 mmHg, P = 0.003), but not ΔMSNA burst frequency (morphine: 10 ± 11 vs. placebo: 13 ± 11 bursts·min-1, P = 0.12), during the CPT. Reductions in pain perception and Δmean BP were only weakly related (r = 0.34, P = 0.07; postmorphine CPT minus postplacebo CPT). These data provide valuable information regarding how low-dose morphine affects autonomic cardiovascular responses during an experimental painful stimulus.NEW & NOTEWORTHY In this randomized, crossover, placebo-controlled trial, we found that low-dose morphine administration reduced pain perception and blood pressure responses during the cold pressor test via attenuated increases in heart rate and cardiac output. We also determined that muscle sympathetic outflow responses during the cold pressor test seem to be unaffected by low-dose morphine administration. Finally, our exploratory analysis suggests that biological sex does not influence morphine-induced antinociception in healthy adults.

Low-dose morphine reduces tolerance to central hypovolemia in healthy adults without affecting muscle sympathetic outflow

Hemorrhage is a leading cause of preventable battlefield and civilian trauma deaths. Low-dose (i.e., an analgesic dose) morphine is recommended for use in the prehospital (i.e., field) setting. Morphine administration reduces hemorrhagic tolerance in rodents. However, it is unknown whether morphine impairs autonomic cardiovascular regulation and consequently reduces hemorrhagic tolerance in humans. Thus, the purpose of this study was to test the hypothesis that low-dose morphine reduces hemorrhagic tolerance in conscious humans. Thirty adults (15 women/15 men; 29 ± 6 yr; 26 ± 4 kg·m-2, means ± SD) completed this randomized, crossover, double-blinded, placebo-controlled trial. One minute after intravenous administration of morphine (5 mg) or placebo (saline), we used a presyncopal limited progressive lower-body negative pressure (LBNP) protocol to determine hemorrhagic tolerance. Hemorrhagic tolerance was quantified as a cumulative stress index (mmHg·min), which was compared between trials using a Wilcoxon matched-pairs signed-rank test. We also compared muscle sympathetic nerve activity (MSNA; microneurography) and beat-to-beat blood pressure (photoplethysmography) during the LBNP test using mixed-effects analyses [time (LBNP stage) × trial]. Median LBNP tolerance was lower during morphine trials (placebo: 692 [473-997] vs. morphine: 385 [251-728] mmHg·min, P < 0.001, CI: -394 to -128). Systolic blood pressure was 8 mmHg lower during moderate central hypovolemia during morphine trials (post hoc P = 0.02; time: P < 0.001, trial: P = 0.13, interaction: P = 0.006). MSNA burst frequency responses were not different between trials (time: P < 0.001, trial: P = 0.80, interaction: P = 0.51). These data demonstrate that low-dose morphine reduces hemorrhagic tolerance in conscious humans. Thus, morphine is not an ideal analgesic for a hemorrhaging individual in the prehospital setting.NEW & NOTEWORTHY In this randomized, crossover, placebo-controlled trial, we found that tolerance to simulated hemorrhage was lower after low-dose morphine administration. Such reductions in hemorrhagic tolerance were observed without differences in MSNA burst frequency responses between morphine and placebo trials. These data, the first to be obtained in conscious humans, demonstrate that low-dose morphine reduces hemorrhagic tolerance. Thus, morphine is not an ideal analgesic for a hemorrhaging individual in the prehospital setting.

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.

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5'-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G's behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

Ketamine Psychedelic and Antinociceptive Effects Are Connected

BACKGROUND

Ketamine produces potent analgesia combined with psychedelic effects. It has been suggested that these two effects are associated and possibly that analgesia is generated by ketamine-induced dissociation. The authors performed a post hoc analysis of previously published data to quantify the pharmacodynamic properties of ketamine-induced antinociception and psychedelic symptoms. The hypothesis was that ketamine pharmacodynamics (i.e., concentration-effect relationship as well as effect onset and offset times) are not different for these two endpoints.

METHODS

Seventeen healthy male volunteers received escalating doses of S- and racemic ketamine on separate occasions. Before, during, and after ketamine infusion, changes in external perception were measured together with pain pressure threshold. A population pharmacokinetic-pharmacodynamic analysis was performed that took S- and R-ketamine and S- and R-norketamine plasma concentrations into account.

RESULTS

The pharmacodynamics of S-ketamine did not differ for antinociception and external perception with potency parameter (median [95% CI]) C50, 0.51 (0.38 to 0.66) nmol/ml; blood-effect site equilibration half-life, 8.3 [5.1 to 13.0] min), irrespective of administration form (racemic ketamine or S-ketamine). R-ketamine did not contribute to either endpoint. For both endpoints, S-norketamine had a small antagonistic effect.

CONCLUSIONS

The authors conclude that their data support an association or connectivity between ketamine analgesia and dissociation. Given the intricacies of the study related to the pain model, measurement of dissociation, and complex modeling of the combination of ketamine and norketamine, it is the opinion of the authors that further studies are needed to detect functional connectivity between brain areas that produce the different ketamine effects.

EDITOR’S PERSPECTIVE

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.

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.

Interleukin-1 receptor-associated kinase 4 (IRAK4) in the nucleus accumbens regulates opioid-seeking behavior in male rats

Opioid addiction remains a severe health problem. While substantial insights underlying opioid addiction have been yielded from neuron-centric studies, the contribution of non-neuronal mechanisms to opioid-related behavioral adaptations has begun to be recognized. Toll-like receptor 4 (TLR4), a pattern recognition receptor, has been widely suggested in opioid-related behaviors. Interleukin-1 receptor-associated kinase 4 (IRAK4) is a kinase essential for TLR4 responses, However, the potential role of IRAK4 in opioid-related responses has not been examined. Here, we explored the role of IRAK4 in cue-induced opioid-seeking behavior in male rats. We found that morphine self-administration increased the phosphorylation level of IRAK4 in the nucleus accumbens (NAc) in rats; the IRAK4 signaling remained activated after morphine extinction and cue-induced reinstatement test. Both systemic and local inhibition of IRAK4 in the NAc core attenuated cue-induced morphine-seeking behavior without affecting the locomotor activity and cue-induced sucrose-seeking. In addition, inhibition of IRAK4 also reduced the cue-induced reinstatement of fentanyl-seeking. Our findings suggest an important role of IRAK4 in opioid relapse-like behaviors and provide novel evidence in the association between innate immunity and drug addiction.

Involvement of T-type calcium channels in the mechanism of low dose morphine-induced hyperalgesia in adult male rats

It has been shown that systemic and local administration of ultra-low dose morphine induced a hyperalgesic response via mu-opioid receptors. However, its exact mechanism(s) has not fully been clarified. It is documented that mu-opioid receptors functionally couple to T-type voltage dependent Ca⁺² channels. Here, we investigated the role of T-type calcium channels, amiloride and mibefradil, on the induction of low-dose morphine hyperalgesia in male Wistar rats. The data showed that morphine (0.01 μg i.t. and 1 μg/kg i.p.) could elicit hyperalgesia as assessed by the tail-flick test. Administration of amiloride (5 and 10 μg i.t.) and mibefradil (2.5 and 5 μg i.t.) completely blocked low-dose morphine-induced hyperalgesia in spinal dorsal horn. Amiloride at doses of 1 and 5 mg/kg (i.p.) and mibefradil (9 mg/kg ip) 10 min before morphine (1 μg/kg i.p.) inhibited morphine-induced hyperalgesia. Our results indicate a role for T-type calcium channels in low dose morphine-induced hyperalgesia in rats.

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.

Sex and Estrous Cycle Differences in Analgesia and Brain Oxycodone Levels

Sex differences in opioid analgesia occur in rodents and humans, and could be due to differences in drug and metabolite levels. Thus, we investigated the sex and cycle differences in analgesia (nociception) from oxycodone in rats and related these to sex and cycle differences in brain and plasma oxycodone and metabolite levels. Since numerous opioids are CYP2D enzyme substrates and variation in CYP2D alters opioid drug levels and response, we also initiated studies to see if the sex and cycle differences observed might be due to differences in brain CYP2D activity. Across oxycodone doses, females in diestrus had higher analgesia (using tail flick latency) compared to males and females in estrus; we also demonstrated a direct effect of estrous cycle on analgesia within females. Consistent with the analgesia, females in diestrus had highest brain oxycodone levels (assessed using microdialysis) compared to males and females in estrus. Analgesia correlated with brain oxycodone, but not brain oxymorphone or noroxycodone levels, or plasma drug or metabolite levels. Propranolol (a CYP2D mechanism-based inhibitor), versus vehicle pre-treatments, increased brain oxycodone, and decreased brain oxymorphone/oxycodone drug level ratios (an in vivo CYP2D activity phenotype in the brain) in males and females in estrus, but not in females in diestrus. Brain oxymorphone/oxycodone inversely correlated with analgesia. Together, both sex and estrous cycle impact oxycodone analgesia and brain oxycodone levels, likely through regulation of brain CYP2D oxycodone metabolism. As CYP2D6 is expressed in human brain, perhaps similar sex and cycle influences also occur in humans.

Effect and Safety of Remifentanil Patient-Controlled Analgesia Compared with Epidural Analgesia in Labor: An Updated Meta-Analysis of Randomized Controlled Trials

PURPOSE

The study was aimed to systematically assess the effect and safety of remifentanil patient-controlled analgesia (rPCA) versus epidural analgesia (EA) during labor.

METHODS

Eligible trials were retrieved from PubMed, EMBASE, ScienceDirect, and Cochrane Library before April 2020. The primary outcomes were patient satisfaction with pain relief and average visual analog scale (VAS) pain scores during labor; the secondary outcomes were rate of spontaneous delivery, oxygen desaturation, maternal hyperthermia, and neonatal Apgar scores <7 at 1 and 5 min.

RESULTS

Eleven studies involving 3,039 parturients were included. We found that parturients receiving rPCA were similarly satisfied with pain relief compared to those receiving EA (standardized mean difference: -0.19; 95% confidence interval [CI]: -0.57, 0.18), though had significantly higher VAS pain scores during labor (weighted mean difference: 1.41; 95% CI: 0.32, 2.50). The rate of spontaneous delivery was comparable. rPCA increased the risk of maternal oxygen desaturation (risk ratio [RR]:3.23, 95% CI: 1.98, 5.30). There was no statistical significance regarding hyperthermia (RR: 0.49, 95% CI: 0.24, 1.01). No significant difference was found for neonatal Apgar scores <7 at 1 and 5 min.

CONCLUSION

rPCA could be an optional alternative for pain relief to EA without worsening maternal satisfaction with pain relief, delivery modes, or neonatal morbidity. However, rPCA was associated with higher pain intensity during labor and higher incidence of maternal oxygen desaturation. The routine use of rPCA in labor must be armed with close respiratory monitoring. Continued well-designed studies are required to provide more robust evidence.

Postoperative respiratory depression after hysterectomy

To investigate if sex-specific physiologic characteristics could impact postoperative respiratory depression risks in women, we studied incidence and risk factors associated with postoperative respiratory depression in a gynecologic surgical cohort. Only hysterectomies performed under general anesthesia from 2012 to 2017 were included to minimize interprocedural variability. Respiratory depression was defined as episodes of apnea, hypopnea, hypoxemia, pain-sedation mismatch, unplanned positive airway pressure device application, or naloxone administration in the post-anesthesia care unit. Multivariable logistic regression was used to explore the association with clinical characteristics. From 1,974 hysterectomies, 253 had postoperative respiratory depression, yielding an incidence of 128 (95% confidence interval, 114-144) per 1,000 surgeries. Risk factors associated with respiratory depression were older age (odds ratio 1.22 [95% confidence interval 1.02-1.46] per decade increase, p = 0.03), lower body weight (0.77 [0.62-0.94] per 10 kg/m2, p = 0.01), and higher intraoperative opioid dose (1.05 [1.01-1.09] per 10 mg oral morphine equivalents, p = 0.01); while sugammadex use was associated with a reduced risk (0.48 [0.30-0.75], p = 0.002). Respiratory depression was not associated with increased hospital stay, postoperative complications, or mortality. Postoperative respiratory depression risk in women increased with age, lower weight, and higher intraoperative opioids and decreased with sugammadex use; however, it was not associated with postoperative pulmonary complications.

Gender-related differences in migraine

Migraine is considered mostly a woman’s complaint, even if it affects also men. Epidemiological data show a higher incidence of the disease in women, starting from puberty throughout life. The sex-related differences of migraine hold clinical relevance too. The frequency, duration, and disability of attacks tend to be higher in women. Because of this, probably, they also consult specialists more frequently and take more prescription drugs than men. Different mechanisms have been evaluated to explain these differences. Hormonal milieu and its modulation of neuronal and vascular reactivity is probably one of the most important aspects. Estrogens and progesterone regulate a host of biological functions through two mechanisms: nongenomic and genomic. They influence several neuromediators and neurotransmitters, and they may cause functional and structural differences in several brain regions, involved in migraine pathogenesis. In addition to their central action, sex hormones exert rapid modulation of vascular tone. The resulting specific sex phenotype should be considered during clinical management and experimental studies.

Periaqueductal gray/dorsal raphe dopamine neurons contribute to sex differences in pain-related behaviors

Sex differences in pain severity, response, and pathological susceptibility are widely reported, but the neural mechanisms that contribute to these outcomes remain poorly understood. Here we show that dopamine (DA) neurons in the ventrolateral periaqueductal gray/dorsal raphe (vlPAG/DR) differentially regulate pain-related behaviors in male and female mice through projections to the bed nucleus of the stria terminalis (BNST). We find that activation of vlPAG/DRDA+ neurons or vlPAG/DRDA+ terminals in the BNST reduces nociceptive sensitivity during naive and inflammatory pain states in male mice, whereas activation of this pathway in female mice leads to increased locomotion in the presence of salient stimuli. We additionally use slice physiology and genetic editing approaches to demonstrate that vlPAG/DRDA+ projections to the BNST drive sex-specific responses to pain through DA signaling, providing evidence of a novel ascending circuit for pain relief in males and contextual locomotor response in females.

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.

Current Understanding of the Involvement of the Insular Cortex in Neuropathic Pain: A Narrative Review

Neuropathic pain is difficult to cure and is often accompanied by emotional and psychological changes. Exploring the mechanisms underlying neuropathic pain will help to identify a better treatment for this condition. The insular cortex is an important information integration center. Numerous imaging studies have documented increased activity of the insular cortex in the presence of neuropathic pain; however, the specific role of this region remains controversial. Early studies suggested that the insular lobe is mainly involved in the processing of the emotional motivation dimension of pain. However, increasing evidence suggests that the role of the insular cortex is more complex and may even be related to the neural plasticity, cognitive evaluation, and psychosocial aspects of neuropathic pain. These effects contribute not only to the development of neuropathic pain, but also to its comorbidity with neuropsychiatric diseases. In this review, we summarize the changes that occur in the insular cortex in the presence of neuropathic pain and analgesia, as well as the molecular mechanisms that may underlie these conditions. We also discuss potential sex-based differences in these processes. Further exploration of the involvement of the insular lobe will contribute to the development of new pharmacotherapy and psychotherapy treatments for neuropathic pain.

Physiologically based pharmacokinetic/pharmacodynamic model for the prediction of morphine brain disposition and analgesia in adults and children

Morphine is a widely used opioid analgesic, which shows large differences in clinical response in children, even when aiming for equivalent plasma drug concentrations. Age-dependent brain disposition of morphine could contribute to this variability, as developmental increase in blood-brain barrier (BBB) P-glycoprotein (Pgp) expression has been reported. In addition, age-related pharmacodynamics might also explain the variability in effect. To assess the influence of these processes on morphine effectiveness, a multi-compartment brain physiologically based pharmacokinetic/pharmacodynamic (PB-PK/PD) model was developed in R (Version 3.6.2). Active Pgp-mediated morphine transport was measured in MDCKII-Pgp cells grown on transwell filters and translated by an in vitro-in vivo extrapolation approach, which included developmental Pgp expression. Passive BBB permeability of morphine and its active metabolite morphine-6-glucuronide (M6G) and their pharmacodynamic parameters were derived from experiments reported in literature. Model simulations after single dose morphine were compared with measured and published concentrations of morphine and M6G in plasma, brain extracellular fluid (ECF) and cerebrospinal fluid (CSF), as well as published drug responses in children (1 day– 16 years) and adults. Visual predictive checks indicated acceptable overlays between simulated and measured morphine and M6G concentration-time profiles and prediction errors were between 1 and -1. Incorporation of active Pgp-mediated BBB transport into the PB-PK/PD model resulted in a 1.3-fold reduced brain exposure in adults, indicating only a modest contribution on brain disposition. Analgesic effect-time profiles could be described reasonably well for older children and adults, but were largely underpredicted for neonates. In summary, an age-appropriate morphine PB-PK/PD model was developed for the prediction of brain pharmacokinetics and analgesic effects. In the neonatal population, pharmacodynamic characteristics, but not brain drug disposition, appear to be altered compared to adults and older children, which may explain the reported differences in analgesic effect.

Developmental processes in children can affect pharmacokinetics: “what the body does to the drug” as well as pharmacodynamics: “what the drug does to the body”. A typical example is morphine, of which the analgesic response is variable and particularly neonates suffer more often from respiratory depression, even when receiving doses corrected for differences in elimination. One way to mathematically incorporate developmental processes is by employing physiologically based pharmacokinetic/pharmacodynamic (PB-PK/PD) models, where physiological differences between individuals are incorporated. In this study, we developed a morphine PB-PK/PD model to predict brain drug disposition as well as analgesic response in adults and children, as both processes could potentially contribute to developmental variability in the effect of morphine. We found that age-related variation in BBB expression of the main morphine efflux transporter P-glycoprotein was not responsible for differences in brain exposure. In contrast, pharmacodynamic modelling suggested an increased sensitivity to morphine in neonates.

Pharmacokinetics of Morphine and Morphine-6-Glucuronide During Postoperative Pain Therapy in Cardiac Surgery Patients

BACKGROUND AND OBJECTIVE

Morphine is a standard analgesic drug for postoperative pain therapy. This study aimed to evaluate the pharmacokinetics of morphine and its active metabolite morphine-6-glucuronide (M6G) in cardiac surgery  patients during postoperative analgesia.

METHODS

Twenty-five adult patients undergoing cardiac surgery received postoperative pain therapy by patient-controlled analgesia with intravenous bolus doses of morphine. Plasma concentrations of morphine and M6G were determined from arterial samples. Population pharmacokinetic parameters were estimated using nonlinear mixed-effects modeling.

RESULTS

Data from twenty-one patients (aged 44-79 years) were analyzed. Pharmacokinetics were best described by a three-compartment model for morphine and a two-compartment model for M6G, linked by a transit compartment. Mean (±SD) population estimates for morphine were: clearance (CL) = 1.35±0.40 L/min, central volume of distribution (V₁) = 8.1±2.2 L, steady-state volume of distribution (V_ss) = 207±83 L, terminal elimination half-life (T_1/2γ) = 177±50 min. Clearance of morphine was proportional to cardiac output. Mean (±SD) population estimates for M6G were: CL = 0.098±0.037 L/min, V₁ = 5.5±0.8 L, V_ss = 15.8±0.8 L, T_1/2β = 227±74 min. The time to peak concentration of M6G after a bolus dose of morphine was 53±20 min. Clearance of M6G was proportional to estimated glomerular filtration rate.

CONCLUSIONS

The pharmacokinetics of morphine and M6G in pain therapy of cardiac surgery  patients could be well described by standard compartmental models. Cardiac output was identified as a significant covariate for morphine clearance, whereas renal function was identified as the most significant covariate for clearance of M6G. These effects should be particularly considered if morphine is administered as a continuous infusion. The developed pharmacokinetic model also enables patient-controlled target-controlled infusion for pain therapy with morphine.

TRIAL REGISTRATION

Clinical Trials NCT02483221 (June 26, 2015).

Sex differences in the human reward system: convergent behavioral, autonomic and neural evidence

Several studies have suggested that females and males differ in reward behaviors and their underlying neural circuitry. Whether human sex differences extend across neural and behavioral levels for both rewards and punishments remains unclear. We studied a community sample of 221 young women and men who performed a monetary incentive task known to engage the mesoaccumbal pathway and salience network. Both stimulus salience (behavioral relevance) and valence (win vs loss) varied during the task. In response to high- vs low-salience stimuli presented during the monetary incentive task, men showed greater subjective arousal ratings, behavioral accuracy and skin conductance responses (P < 0.006, Hedges' effect size g = 0.38 to 0.46). In a subsample studied with functional magnetic resonance imaging (n = 44), men exhibited greater responsiveness to stimulus salience in the nucleus accumbens, midbrain, anterior insula and dorsal anterior cingulate cortex (P < 0.02, g = 0.86 to 1.7). Behavioral, autonomic and neural sensitivity to the valence of stimuli did not differ by sex, indicating that responses to rewards vs punishments were similar in women and men. These results reveal novel and robust sex differences in reward- and punishment-related traits, behavior, autonomic activity and neural responses. These convergent results suggest a neurobehavioral basis for sexual dimorphism observed in the reward system, including reward-related disorders.

Methadone for postoperative analgesia: contribution of N-methyl-D-aspartate receptor antagonism: A randomised controlled trial

BACKGROUND

Over the past number of years, N-methyl-D-aspartate (NMDA) inhibitory drugs, like ketamine, have been introduced as adjuvant treatments for postoperative acute pain, within a multimodal approach. A further extension of this strategy could be the use of opioids with NMDA receptor (NMDAr) antagonism activity for control of postoperative pain. Methadone has a unique pharmacodynamic profile: it is both a μ-agonist and an NMDAr-blocker.

OBJECTIVE

We designed this study to investigate the precise contribution of NMDAr antagonism in methadone-induced analgesia.

DESIGN

Single-centre, prospective, randomised, double-blind study.

SETTING

National Cancer Center - Fondazione IRCCS Istituto Nazionale Tumori Milano; patients were recruited between March 2010 and June 2012.

PATIENTS

Ninety-six patients scheduled for an open laparotomy for anterior resection of the rectum.

INTERVENTIONS

We randomly assigned patients to four groups: 0-Mo (placebo and morphine), K-Mo [S(+)-ketamine and morphine], 0-Me (placebo and methadone), K-Me [S(+)-ketamine and methadone].

MAIN OUTCOME MEASURES

The primary end-point was the extent of mechanical static (punctuate) hyperalgesia to von Frey hair stimulation lateral to the surgical incision.

RESULTS

Peri-incisional hyperalgesia was 8.4 cm (95% confidence interval, 1.5 to 15.41) lower in the treatment group (K-Me) compared with the control group (0-Mo) at 24 h after surgery (P = 0.02). No significant differences were observed between the groups at 48 h after surgery (P = 0.88). Both groups treated with methadone had significantly lower pain during rest and movement, as measured with a Numerical Rating Scale at 24 h. At 48 h, only the movement Numerical Rating Scale was significantly lower. No difference occurred in opioid consumption.

CONCLUSION

Methadone provides effective control of acute postoperative pain, independently, by modulation of the hyperalgesia mechanism.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, no.: NCT01594047.

Gender differences in use of prolonged non-operative therapies prior to index ACDF surgery

Prior to anterior cervical discectomy and fusion (ACDF) surgery, patients suffering from cervical stenosis traditionally trial non-operative treatments for pain management. There is a paucity of data evaluating gender disparities in the prolonged utilization of conservative therapy prior to ACDF surgery. Therefore, the purpose of this study was to assess for gender-based differences in the utilization and cost of maximal non-operative therapy (MNT) for cervical stenosis prior to ACDF surgery. Medical records from patients with symptomatic cervical stenosis undergoing 1, 2, or 3-level index ACDF procedures between 2007 and 2016 were gathered from an insurance database consisting of 20.9 million covered lives. The utilization of MNTs within 5 years prior to index ACDF surgery was assessed. A total of 2254 patients (females: 53.1%) underwent an index ACDF surgery. There were a significantly greater percentage of female patients that utilized NSAIDs (p < 0.0001), opioids (p = 0.0019), muscle relaxants (p < 0.0001), cervical epidural steroid injections (p = 0.0428), and physical therapy/occupational therapy treatments (p < 0.0001). The total direct cost associated with all MNT prior to index ACDF was $4,833,384. On average, $2028.01 was spent per male patient while $2247.29 was spent per female patient. When normalized by number of pills billed per patient utilizing therapy, female patients utilized more NSAIDs (males: 591.8 pills, females: 669.3 pills), opioids (male: 1342.0 pills, female: 1650.1 pills), and muscle relaxants (males: 823.7 pills, females: 1211.1 pills). The results suggest that there may be gender differences in the utilization of non-operative therapies for symptomatic cervical stenosis prior to ACDF surgery.

A Mechanism-Based Forensic Investigation into the Postmortem Redistribution of Morphine

The interpretation of postmortem drug levels is complicated by the change in drug blood levels during the postmortem period, a phenomenon known as postmortem drug redistribution. We investigated the postmortem redistribution (PMR) of morphine, morphine-3-glucuronide and normorphine in the rat. Morphine (10 mg/kg) was intravenously injected into rats, followed by euthanasia 1 h post-injection. The carcasses were placed in a supine position at room temperature, and tissues including heart blood, femoral blood, liver, lung and brain were collected at different time points: 0, 8, 16 or 24 h postmortem. The samples were analyzed with a validated (following modified Scientific Working Group for Forensic Toxicology (SWGTOX) (20) guidelines) liquid chromatography-tandem mass spectrometry method. The use of a mechanism-based approach (involving the used set doses of drug with the study performed in controlled environment) to assess PMR using systematic and statistical analyses provides important information that has not previously been presented in PMR literature. While previous human studies focus on central to peripheral ratios as well as peripheral to tissue ratio, this work focused on the change in morphine and metabolite concentrations over the course of the postmortem interval in relation to each other in addition to the comparison to additional matrices at each postmortem interval. Postmortem redistribution was identified in several tissues across the postmortem interval; however, there was minimal statistical difference observed among each matrix at a given postmortem interval with the exception of normorphine and morphine-3-glucuronide. Combined, our study provides a valuable resource and reference information that can aide toxicologists, medical examiners or coroners when assessing postmortem drug concentrations of morphine and metabolites when they are making determinations of cause of death.

Analgesic Effect of Morphine Added to Bupivacaine in Serratus Anterior Plane Block Following Modified Radical Mastectomy. Only a Local Effect? Randomized Clinical Trial

Background

Serratus anterior plane (SAP) block, a novel regional anesthetic procedure, involves the anterolateral chest wall. Opioid receptors have been found on peripheral nerve terminals, so morphine may have a local action.

Objective

This work aimed at exploring the analgesic efficacy of morphine added to bupivacaine in SAPB in patients for whom modified radical mastectomy was conducted and whether it is a mere local effect.

Methods

Forty female patients were planned to have modified radical mastectomy participated in the study. Patients were randomly divided into two groups; Control group (C): received ultrasound-guided serratus anterior plane block with 20 mL of bupivacaine hydrochloride 0.25%; Morphine group (M): received the same in addition to 10 mg morphine sulfate. Intra- and post-operative blood samples were taken for the assessment of morphine serum levels. All patients were assessed for VAS scores during rest and movement (VAS-R and VAS-M). Time to the first request and the total amount of the rescue analgesia were recorded.

Results

In group M, Morphine was not detected in the plasma of all patients. Both VAS-R and VAS-M were significantly higher in group C than in group M (P<0.001) and (P≤0.003), respectively. Time to the first request of rescue analgesia was 8.5 h in group C compared to 20 h in group M (P=0.005) with a median dose of acetaminophen consumption of 2 g in group C compared to 1 g in group M (P=0.006).

Conclusion

Ten mg of morphine, when added to bupivacaine in SAPB, improved postoperative analgesia in patients to whom modified radical mastectomy was conducted. This effect seems to be attributed merely to local mechanisms.

Registration

The registration number of this study is NCT02962024 at www.clinicaltrial.gov.

Carfentanil toxicity in the African green monkey: Therapeutic efficacy of naloxone

Carfentanil is an ultra-potent opioid with an analgesic potency 10,000 times that of morphine but has received little scientific investigation. Three experiments were conducted to evaluate the toxicity of carfentanil and the efficacy of naloxone in adult male African green monkeys. The first experiment determined the ED₅₀ (found to be 0.71 μg/kg) of subcutaneous carfentanil for inducing bradypnea and/or loss of posture. Experiment 2 attempted to establish the ED₅₀ of naloxone for rapidly reversing the bradypnea/loss of posture induced by carfentanil (1.15 μg/kg). Experiment 3 evaluated the effects of carfentanil (0.575 μg/kg) alone, the safety of naloxone (71-2841 μg/kg), and the efficacy of naloxone (71-710 μg/kg) administration at two time points following carfentanil (1.15 μg/kg) on operant choice reaction time. Collectively, these experiments characterized the temporal progression of carfentanil-induced toxic signs, determined the range of naloxone doses that restored respiratory and gross behavioral function, and determined the time course and range of naloxone doses that partially or completely reversed the effects of carfentanil on operant choice reaction time performance in African green monkeys. These results have practical relevance for the selection of opioid antagonists, initial doses, and expected functional outcomes following treatment of synthetic opioid overdose in a variety of operational/emergency response contexts.

Effect of sex differences in remifentanil requirements for inhibiting the response to a CO2 pneumoperitoneum during propofol anesthesia: an up-and-down sequential allocation trial

Background

A CO₂ pneumoperitoneum during a laparoscopic procedure causes violent hemodynamic changes. However, the remifentanil required to inhibit the cardiovascular response to a CO₂ pneumoperitoneum combined with propofol remains unknown. Moreover, the sex of the patient may influence the response to opioids, which can affect this requirement. The main objective of this study was to compare the required median effective concentration (EC₅₀) of remifentanil for inhibiting the cardiovascular response to a CO₂ pneumoperitoneum between female and male patients during propofol anesthesia.

Methods

The current study is an up-and-down sequential allocation trial. Forty-six patients with American Society of Anesthesiologists physical status I or II, a body mass index 18 to 30 kg/m², aged 20 to 60 years, and scheduled for laparoscopic surgery were enrolled. Induction of anesthesia was performed by target-controlled infusion. The effective effect-site concentration (Ce) of propofol was 4 μg/ml. The Ce of remifentanil was initially 4 ng/ml and then adjusted to a predetermined level after I-gel laryngeal mask airway insertion. The Ce of remifentanil for each patient was determined by the response of the previous patient using the modified Dixon “up-and-down” method. The first patient received remifentanil at 5.0 ng/ml Ce, and the step size between patients was 0.5 ng/ml.

Results

Patients characteristics including age, body mass index, American Society of Anesthesiologists physical status, type of surgery and surgery duration, were comparable between male and female patients. The EC₅₀ of remifentanil required to inhibit the response to a CO₂ pneumoperitoneum based on the Dixon “up-and-down” method in women (4.17 ± 0.38 ng/ml) was significantly lower than that in men (5.00 ± 0.52 ng/ml) during propofol anesthesia (P = 0.01).

Conclusions

The EC₅₀ of remifentanil required to inhibit the response to a CO₂ pneumoperitoneum was lower in women than in men during propofol anesthesia.

Trial registration

The study was registered at http://www.chictr.org.cn (ChiCTR-IOR-17011906, 8th, July, 2017).

Gender differences in association of prescription opioid use and mortality: A propensity-matched analysis from the REasons for Geographic And Racial Differences in Stroke (REGARDS) prospective cohort

BACKGROUND

Prescription opioids (PO) have been widely used for chronic non-cancer pain, with commensurate concerns for overdose. The long-term effect of these medications on non-overdose mortality in the general population remains poorly understood. This study's objective was to examine the association of prescription opioid use and mortality in a large cohort, accounting for gender differences and concurrent benzodiazepine use, and using propensity score matching. Methods: 29,025 US community-dwellers were enrolled in the REasons for Geographic And Racial Differences in Stroke (REGARDS) cohort between 2003 and 2007, and followed through December 31, 2012. At baseline there were 1907 participants with PO; 1864 of them were matched to participants without PO, based on the model-derived propensity to receive opioid prescriptions. Causes of death were expert-adjudicated. Results: Over median follow-up of 6 years there were 4428 deaths (413 among persons with PO). The risk for all-cause mortality was 12% higher, in absolute terms, for persons with PO compared to those without PO in the overall sample, with gender differences (interaction p = .0008). The risk of death was increased for women with PO (hazard ratio [HR] 1.21 [95% Confidence Interval (CI) 1.04-1.40]), but not men (HR 0.92 [95% CI 0.77-1.10]). Women with PO were at higher risk of cardiovascular disease (CVD) death (HR 1.43 [95% CI 1.12-1.84]), sudden death (HR 2.02 [95% CI 1.29-3.15]) (a subset of CVD death), and accidents (HR 2.18 [95% CI 1.03-4.60]). These risks were not observed for men with PO. Conclusion: Over 6 years of follow-up, women but not men who had opioid prescriptions were at higher risk of all-cause mortality, CVD death, sudden death, and accidents. Special caution in prescribing opioids for women may be warranted until these findings are confirmed.

Analgesic and Respiratory Depressant Effects of R-dihydroetorphine

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What We Already Know about This Topic

What This Article Tells Us That Is New

Background

There is an ongoing need for potent opioids with less adverse effects than commonly used opioids. R-dihydroetorphine is a full opioid receptor agonist with relatively high affinity at the μ-, δ- and κ-opioid receptors and low affinity at the nociception/orphanin FQ receptor. The authors quantified its antinociceptive and respiratory effects in healthy volunteers. The authors hypothesized that given its receptor profile, R-dihydroetorphine will exhibit an apparent plateau in respiratory depression, but not in antinociception.

Methods

The authors performed a population pharmacokinetic–pharmacodynamic study (Eudract registration No. 2009-010880-17). Four intravenous R-dihydroetorphine doses were studied: 12.5, 75, 125, and 150 ng/kg (infused more than 10 min) in 4 of 4, 6 of 6, 6 of 6, and 4 of 4 male subjects in pain and respiratory studies, respectively. The authors measured isohypercapnic ventilation, pain threshold, and tolerance responses to electrical noxious stimulation and arterial blood samples for pharmacokinetic analysis.

Results

R-dihydroetorphine displayed a dose-dependent increase in peak plasma concentrations at the end of the infusion. Concentration-effect relationships differed significantly between endpoints. R-dihydroetorphine produced respiratory depression best described by a sigmoid EMAX-model. A 50% reduction in ventilation in between baseline and minimum ventilation was observed at an R-dihydroetorphine concentration of 17 ± 4 pg/ml (median ± standard error of the estimate). The maximum reduction in ventilation observed was at 33% of baseline. In contrast, over the dose range studied, R-dihydroetorphine produced dose-dependent analgesia best described by a linear model. A 50% increase in stimulus intensity was observed at 34 ± 11 pg/ml.

Conclusions

Over the dose range studied, R-dihydroetorphine exhibited a plateau in respiratory depression, but not in analgesia. Whether these experimental advantages extrapolate to the clinical setting and whether analgesia has no plateau at higher concentrations than investigated requires further studies.

Gender based differences, pharmacogenetics and adverse events in chronic pain management

Safety data in chronic non-cancer pain (CNCP) with long-term opioid therapy has been poorly studied and can be differently influenced by gender. Furthermore, pharmacogenetics (PGx) could possibly be used to tailor pain medication based on the individual's genetic background. The aim was to assess whether PGx applied to a pharmacovigilance system could help to improve a patient's security profile. A pharmacovigilance data recording system was conducted over 24 months, including genotyping of OPRM1 variants (opioid receptor, A118G) and COMT (enzyme that degrades catecholamines such as norepinephrine, G1947A). Pain intensity (visual analogue scale, VAS), morphine equivalent daily dose (MEDD), adverse events (AEs) and suspected adverse drug reactions (ADRs) were recorded and analysed by gender. The Ethics Committee approved the study and data were analysed with R 3.6.0 software. A total of 748 patients were recruited in the study (67% female, VAS 62 ± 29 mm, MEDD 119 ± 114 mg/day) reporting a median of 6 (3.5-9) AEs/patient. Women presented more nausea, headaches, insomnia, loss of appetite, weight change, depression and dizziness than men. Analysis by genotype demonstrated that PGx influenced the prevalence of vomiting and depression in men, dizziness in women and sexual dysfunction in both. Physicians notified 150 ADRs mostly in females (79%) related to nervous system disorders. PGx applied to a pharmacovigilance recording system provides important information to achieve a better knowledge about AEs in CNCP pharmacological therapy. OPRM1 and COMT polymorphisms were associated with AEs in CNCP patients that differed according to gender.

The Duration of Nerve Block from Local Anesthetic Formulations in Male and Female Rats

PURPOSE

It is unknown whether there are sex differences in response to free or encapsulated local anesthetics.

METHODS

We examined nerve block duration and toxicity following peripheral nerve blockade in male and female rats. We studied the local anesthetic bupivacaine (free or encapsulated) as well as tetrodotoxin, which acts on a different site of the same voltage-gated channel.

RESULTS

Sensory nerve blockade was 158.5 [139-190] minutes (median [interquartile range]) (males) compared to 173 [134-171] minutes (females) (p = 0.702) following bupivacaine injection, N = 8 male, 8 female. Motor nerve blockade was 157 [141-171] minutes (males) compared to 172 [146-320] minutes (females) (p = 0.2786). Micellar bupivacaine (N = 8 male, 8 female) resulted in sensory nerve blockade of 266 [227-320] minutes (males) compared to 285 [239-344] minutes (females) (p = 0.6427). Motor nerve blockade was 264 [251-264] minutes (males) compared to 287 [262-287] minutes (females) (p = 0.3823). Liposomal bupivacaine (N = 8 male, 8 female) resulted in sensory nerve blockade of 240 [207-277] minutes (males) compared to 289 [204-348] minutes (females) (p = 0.1654). Motor nerve blockade was 266 [237-372] minutes (males) compared to 317 [251-356] minutes (females) (p = 0.6671). Following tetrodotoxin injection (N = 12 male,12 female) sensory nerve blockade was 54.8 [5-117] minutes (males) compared to 54 [14-71] minutes (females) (p = 0.6422). Motor nerve blockade was 72 [40-112] minutes (males) compared to 64 [32-143] minutes (females) (p = 0.971).

CONCLUSIONS

We found no statistically significant sex differences associated with the formulations tested. In both sexes, durations of nerve block were similar between micellar and liposomal bupivacaine formulations, despite the micellar formulation containing less drug.

Epidural Analgesia With Sufentanil in Relation to OPRM1 and ABCB1 Polymorphisms

The aim of this study was to evaluate the association between OPRM1 and ABCB1 polymorphisms on pain relief with epidural sufentanil in 69 patients after rectosigma resection for cancer. The median number of injections (SD) 2.31 (1.36), IQR=1, required by 118AA subjects was significantly lower in comparison with 118AG group 5.25 (3.13), IQR=6.5, (χ2=9.75, p=0.001); correspondingly median drug consumption of 1.16 (0.79), IQR=1.083, defined daily doses (DDD) was significantly less in the 118AA group in comparison with 2.14 (1.17), IQR=2.23, DDD in 118AG subjects, (χ2=7.00, p=0.008). Opioid-induced adverse effects were observed in 15 % and 33 % of patients in 118AA and 118AG groups, respectively (χ2=8.16, p=0.004). The median number of injections (SD) required by women and men was 3.30 (2.16), IQR=2, and 2.80 (1.59), IQR=1, respectively (χ2=6.25, p=0.012). Opioid-induced adverse effects were observed in 26 % and 12 % of women and men, respectively (χ2=5.49, p=0.011). Heterozygotes of OPRM1 polymorphism and women were more difficult to treat subpopulations that required higher doses of rescue analgesic medication and suffered more adverse effects.

Sex differences in pain and opioid mediated antinociception: Modulatory role of gonadal hormones

Sex-related differences in pain and opioids has been the focus of many researches. It is demonstrated that women experience greater clinical pain, lower pain threshold and tolerance, more sensitivity and distress to experimentally induced pain compared to men. Sex differences in response to opioid treatment revealed inconsistent results. However, the etiology of these disparities is not fully elucidated. It is, therefore, conceivable now that this literature merits to be revisited comprehensively. Possible multifaceted factors seem to be associated. These include neuroanatomical, hormonal, neuroimmunological, psychological, social and cultural aspects and comorbidities. This review aims at providing an overview of the substantial literature documenting the sex differences in pain and analgesic response to opioids from animal and human studies within the context of the modulatory effects of the aforementioned factors. A detailed and critical discussion of the cellular and molecular signaling pathways underlying the modulatory actions of gonadal hormones in the sexual dimorphism in pain processing and opioid analgesia is extensively presented. It is indicated that sexual dimorphic activation of certain brain regions contributes to differential pain sensitivity between females and males. Plausible crosstalk between sex hormones and neuroimmunological signaling pertinent to toll-like and purinergic receptors is uncovered as causal cues underlying sexually dimorphic pain and opioid analgesia. Conceivably, a thorough understanding of these factors may aid in sex-related advancement in pain therapeutic management.

Sex effects on behavioral markers of emergence from propofol and isoflurane anesthesia in rats

Clinical studies have demonstrated sex-related differences in recovery from surgical anesthesia. This study aimed to characterize the emergence pattern following two anesthesia regimens in both sexes of rats. We considered six different markers of emergence from anesthesia: sigh, eye blinking, forelimb movement, mastication, neck extension, and recovery of the righting reflex (RORR). Spontaneous motor activity 24 h after the anesthesia induction was also examined. Our results showed that the rank order of the emergence latency after intraperitoneal propofol, PRO, exposure was forelimb movement < sigh < blink < mastication < neck extension < RORR, while after inhaled isoflurane, ISO, anesthesia the sequence was changed as sigh < blink < mastication < forelimb movement < neck extension < RORR in both male and female rats. Moreover, the latency to emergence after PRO in female rats was significantly higher than male rats, although following ISO there was no difference between the sexes (P < 0.001; P > 0.05, respectively). Open-field testing revealed no difference in PRO and ISO spontaneous locomotor activity due to drug administration (P > 0.05). These two anesthetics presented different emergence sequences. Although clinical data suggests that females arouse faster than males from anesthesia with propofol, our intraperitoneal technique in a rodent model had the opposite effect. Pharmacokinetic analysis demonstrated increased absorption of injected propofol for the female rats in our study, emphasizing the role of sexual dimorphism in drug distribution in rodents. Despite these pharmacokinetic differences, the pharmacodynamic effects of the drugs were remarkably consistent among both sexes through emergence.

GABAB receptors modulate morphine antinociception: Pharmacological and genetic approaches

Previous studies in our laboratory showed an interaction between the GABAergic and opioid systems involved in the analgesic effect of baclofen (BAC). Furthermore, it is known that sex differences exist regarding various pharmacological responses of morphine (MOR) and they are related to an increased sensitivity to MOR effects in males. The aims of the present study were to evaluate the possible involvement of the GABA_B receptors in the antinociceptive responses induced by MOR (1, 3 and 9 mg/kg, s.c.) administration using both pharmacological (BAC 2 mg/kg, i.p.; and 2-OH-saclofen, SAC 0.3 mg/kg, intra cisterna magna) and genetic approaches (GABA_B1 knockout mice; GABA_B1 KO) in mice of both sexes. In addition, we explored the alterations in c-Fos expression of different brain areas involved in the antinociceptive effect of MOR using both approaches. The pharmacological approach showed a higher dose-dependent antinociceptive effect of MOR in male mice compared to female mice. BAC and SAC pretreatment potentiated and attenuated the antinociceptive effect of MOR, respectively, in both sexes. The genetic approach revealed a dose-dependent antinociceptive effect of MOR in the wild type mice, but not in the GABA_B1 KO mice and no sex differences were observed. Additionally, BAC and SAC pretreatment and the lack of GABA_B1 subunit of the GABA_B receptor prevented the changes observed in c-Fos expression in the cingulate cortex and nucleus accumbens of male mice. Our results suggest that the GABA_B receptors are involved in the MOR antinociceptive effect of both male and female mice.

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.