Sex differences in micro-opioid receptor expression in the rat midbrain periaqueductal gray are essential for eliciting sex differences in morphine analgesia

The role of gonadal hormones in regulating opioid antinociception

Opioids are the most prescribed drugs for the alleviation of pain. Both clinical and preclinical studies have reported strong evidence for sex-related divergence regarding opioid analgesia. There is an increasing amount of evidence indicating that gonadal hormones regulate the analgesic efficacy of opioids. This review presents an overview of the importance of gonadal steroids in modulating opioid analgesic responsiveness and focuses on elaborating what is currently known regarding the underlyingmechanism. We sought to identify the link between gonadal hormones and the effect of oipiod antinociception.

The Influence of Subjective Sexual Arousal and Disgust on Pain

Current models propose that inhibited sexual arousal is a key component in maintaining sexual pain in women with Genito-Pelvic Pain/Penetration Disorder. It thus follows that enhancing sexual arousal may be an effective strategy to modulate pain, but this effect has not been successfully demonstrated with women, although it has been successful with men. This study built on previous works and examined if the pain-killing effect of sexual arousal might have been undermined by concurrently-elicited disgust. We tested whether women experience disgust as well as sexual arousal when viewing sex stimuli, and whether disgust has an exacerbating effect on pain. Female participants (N = 164) were randomly distributed to watch a porn, disgust, or neutral train-ride film. A cold pressor test (CPT) was utilized to induce pain at the same time that participants viewed their respective film. Pain was indexed by the duration that participants kept their hand in the cold water, and by self-reported pain intensity at the time they quit the CPT. The results showed no differences in pain across conditions. The sex stimulus elicited substantial disgust as well as sexual arousal, and there was a negative correlation between the two emotions. Disgust was not found to increase pain compared to both the neutral and sex conditions. Thus, the findings provide no support for a pain-modulatory effect of subjective sexual arousal on pain in women. This might, however, be due to the sex stimulus having elicited an ambivalent state between an appetitive and aversive emotion concurrently.

Pain mechanisms in the transgender individual: a review

Specific Aim

Provide an overview of the literature addressing major areas pertinent to pain in transgender persons and to identify areas of primary relevance for future research.

Methods

A team of scholars that have previously published on different areas of related research met periodically though zoom conferencing between April 2021 and February 2023 to discuss relevant literature with the goal of providing an overview on the incidence, phenotype, and mechanisms of pain in transgender patients. Review sections were written after gathering information from systematic literature searches of published or publicly available electronic literature to be compiled for publication as part of a topical series on gender and pain in the Frontiers in Pain Research.

Results

While transgender individuals represent a significant and increasingly visible component of the population, many researchers and clinicians are not well informed about the diversity in gender identity, physiology, hormonal status, and gender-affirming medical procedures utilized by transgender and other gender diverse patients. Transgender and cisgender people present with many of the same medical concerns, but research and treatment of these medical needs must reflect an appreciation of how differences in sex, gender, gender-affirming medical procedures, and minoritized status impact pain.

Conclusions

While significant advances have occurred in our appreciation of pain, the review indicates the need to support more targeted research on treatment and prevention of pain in transgender individuals. This is particularly relevant both for gender-affirming medical interventions and related medical care. Of particular importance is the need for large long-term follow-up studies to ascertain best practices for such procedures. A multi-disciplinary approach with personalized interventions is of particular importance to move forward.

Sex specific effects of buprenorphine on adult hippocampal neurogenesis and behavioral outcomes during the acute phase after pediatric traumatic brain injury in mice

Traumatic brain injury (TBI) in children often causes cognitive and mental dysfunctions, as well as acute and chronic pain. Adult hippocampal neurogenesis plays a key role in cognition, depression, and pain. Adult hippocampal neurogenesis can be modulated by genetic and environmental factors, such as TBI and opioids. Buprenorphine (BPN), a semisynthetic opioid, is commonly used for pain management in children, however, the effects of BPN on adult hippocampal neurogenesis after pediatric TBI are still unclear. This study investigated the sex-specific effects of BPN on adult hippocampal neurogenesis during acute phase after pediatric TBI. Male and female littermates were randomized on postnatal day 20-21(P20-21) into Sham, TBI+saline and TBI+BPN groups. BPN was administered intraperitoneally to the TBI+BPN mice at 30 min after injury, and then every 6-12 h (h) for 2 days (d). Bromodeoxyuridine (BrdU) was administered intraperitoneally to all groups at 2, 4, 6, and 8-h post-injury. All outcomes were evaluated at 3-d post-BrdU administration. We found that TBI induced significant cognitive impairment, depression, and reduced adult hippocampal neurogenesis in both male and female mice, with more prominent effects in females. BPN significantly improved adult hippocampal neurogenesis and depression in males, but not in females. We further demonstrated that differential expressions of opioid receptors, transcription factors and neuroinflammatory markers at the neurogenic niche might be responsible for the differential effects of BPN in males and females. In conclusion, this study elucidates the effects of BPN on adult hippocampal neurogenesis and behavioral outcomes at the acute phase after pediatric TBI.

Chronic Morphine Induces Adaptations in Opioid Receptor Signaling in a Thalamostriatal Circuit That Are Location Dependent, Sex Specific, and Regulated by μ-Opioid Receptor Phosphorylation

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) via activity at μ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in the DMS. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation) in male, but not female, mice. At MThal cell bodies, chronic morphine treatment decreased subsequent morphine activation of potassium conductance (morphine tolerance) in both male and female mice. In knock-in mice expressing phosphorylation-deficient MORs, chronic morphine treatment resulted in tolerance to, rather than facilitation of, subsequent morphine signaling at MThal-DMS terminals, suggesting phosphorylation deficiency unmasks adaptations that counter the facilitation observed at presynaptic terminals in wild-type mice. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry, and sex.

Mapping Astrocytic and Neuronal μ-opioid Receptor Expression in Various Brain Regions Using MOR-mCherry Reporter Mouse

The μ-opioid receptor (MOR) is a class of opioid receptors characterized by a high affinity for β-endorphin and morphine. MOR is a G protein-coupled receptor (GPCR) that plays a role in reward and analgesic effects. While expression of MOR has been well established in neurons and microglia, astrocytic MOR expression has been less clear. Recently, we have reported that MOR is expressed in hippocampal astrocytes, and its activation has a critical role in the establishment of conditioned place preference. Despite this critical role, the expression and function of astrocytic MOR from other brain regions are still unknown. Here, we report that MOR is significantly expressed in astrocytes and GABAergic neurons from various brain regions including the hippocampus, nucleus accumbens, periaqueductal gray, amygdala, and arcuate nucleus. Using the MOR-mCherry reporter mice and Imaris analysis, we demonstrate that astrocytic MOR expression exceeded 60% in all tested regions. Also, we observed similar MOR expression of GABAergic neurons as shown in the previous distribution studies and it is noteworthy that MOR expression is particularly in parvalbumin (PV)-positive neurons. Furthermore, consistent with the normal MOR function observed in the MOR-mCherry mouse, our study also demonstrates intact MOR functionality in astrocytes through iGluSnFr-mediated glutamate imaging. Finally, we show the sex-difference in the expression pattern of MOR in PV-positive neurons, but not in the GABAergic neurons and astrocytes. Taken together, our findings highlight a substantial astrocytic MOR presence across various brain regions.

Estradiol and Mu opioid-mediated reward: The role of estrogen receptors in opioid use

Opioid use and opioid use disorder are characterized by sex and gender differences, and some of these differences may be mediated by differences in the hormonal milieu within and across individuals. This review focuses on the role of ovarian hormones, and particularly estradiol, on the endogenous mu opioid receptor system. There is an abundance of data indicating that estradiol influences the activity of endogenous mu opioid peptides, the activation of mu opioid receptors, and the internalization and desensitization of mu opioid receptors. These effects have functional consequences on behaviors mediated by endogenous mu opioid receptor activity and on sensitivity to mu opioid agonists and antagonists. Recent behavioral data suggest these consequences extend to mu opioid reward, and preclinical studies report that estradiol decreases self-administration of mu opioid receptor agonists across a range of experimental conditions. Data collected in human laboratory studies suggest that estradiol may have functionally similar effects in clinical populations, and thus estrogen receptors may be a potential target in the development of novel therapeutics. This review summarizes data from cellular assays to clinical trials to explore how estradiol influences mu opioid receptor activity, as well as potential ways in which estrogen receptors may be targeted to address the problems of opioid use.

Morphine-context associative memory and locomotor sensitization in mice are modulated by sex and context in a dose-dependent manner

Sex differences in opioid use, development of opioid used disorder, and relapse behaviors indicate potential variations in opioid effects between men and women. The locomotor and interoceptive effects of opioids play essential roles in opioid addiction, and uncovering the neural mechanisms underlying these effects remain crucial for developing effective treatments. In this study, we examined the dose-dependent effects of morphine on locomotor sensitization and the strength and stability of morphine-context associations in the conditioned place preference (CPP) paradigm in male and female mice, as well as the relationships between these measures. We observed that while CPP is similar between sexes, the locomotor effects of repeated morphine administration and withdrawal differentially contributed to the strength and stability of morphine-context associations. Specifically, females exhibited higher morphine-induced hyperlocomotion than males regardless of the context in which morphine was experienced. Greater locomotor sensitization to morphine in females than males emerged in a dose-dependent manner only when there was sufficient context information for CPP to be established. Additionally, the relationships between the locomotor effects of morphine and the strength and stability of CPP were different in males and females. In females, positive acute and sensitizing locomotor effects of morphine were correlated with a higher CPP score, while the opposite direction of this relationship was found in males. These results suggest that different aspects of the subjective experience of morphine intoxication and withdrawal are important for morphine abuse-related behaviors and highlight the importance of sex-specific responses in the context of opioid addiction.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Brainstem Modulation of Nociception by Periaqueductal Gray Neurons Expressing the μ-Opioid Receptor in Mice

BACKGROUND

Pharmacologic manipulations directed at the periaqueductal gray have demonstrated the importance of the μ-opioid receptor in modulating reflexive responses to nociception. The authors hypothesized that a supraspinal pathway centered on neurons in the periaqueductal gray containing the μ-opioid receptor could modulate nociceptive and itch behaviors.

METHODS

The study used anatomical, optogenetic, and chemogenetic approaches in male and female mice to manipulate μ-opioid receptor neurons in the periaqueductal gray. Behavioral assays including von Frey, Hargreaves, cold plantar, chloroquine-induced itch, hotplate, formalin-induced injury, capsaicin-induced injury, and open field tests were used. In separate experiments, naloxone was administered in a postsurgical model of latent sensitization.

RESULTS

Activation of μ-opioid receptor neurons in the periaqueductal gray increased jumping (least-squares mean difference of -3.30 s; 95% CI, -6.17 to -0.44; P = 0.023; n = 7 or 8 mice per group), reduced itch responses (least-squares mean difference of 70 scratching bouts; 95% CI, 35 to 105; P < 0.001; n = 8 mice), and elicited modestly antinociceptive effects (least-squares mean difference of -0.7 g on mechanical and -10.24 s on thermal testing; 95% CI, -1.3 to -0.2 and 95% CI, -13.77 to -6.70, and P = 0.005 and P < 0.001, respectively; n = 8 mice). Last, the study uncovered the role of the periaqueductal gray in suppressing hyperalgesia after a postsurgical state of latent sensitization (least-squares mean difference comparing saline and naloxone of -12 jumps; 95% CI, -17 to -7; P < 0.001 for controls; and -2 jumps; 95% CI, -7 to 4; P = 0.706 after optogenetic stimulation; n = 7 to 9 mice per group).

CONCLUSIONS

μ-Opioid receptor neurons in the periaqueductal gray modulate distinct nocifensive behaviors: their activation reduced responses to mechanical and thermal testing, and attenuated scratching behaviors, but facilitated escape responses. The findings emphasize the role of the periaqueductal gray in the behavioral expression of nociception using reflexive and noxious paradigms.

EDITOR’S PERSPECTIVE

Changes in hypothalamic mu-opioid receptor expression following acute olanzapine treatment in female rats: Implications for feeding behavior

Advances have been made in recent years in using opioid receptor antagonists as an adjunct therapy to psychotropic medication to reduce debilitating weight gain and metabolic adverse effects associated with in particular second generation antipsychotics. However, it is unknown whether second generation antipsychotics produce a change in opioid receptor expression in the brain. The present study investigated early changes in opioid receptor expression in the female rat hypothalamus, a master controller of hunger and metabolic regulation, after acute treatment with olanzapine, a commonly used second generation antipsychotic. Using quantitative spatial in situ hybridization and receptor autoradiography, expression levels of the three opioid receptors; kappa, mu and delta, were determined at mRNA and protein level, respectively, in the five hypothalamic areas: paraventricular nucleus, arcuate nucleus, ventromedial nucleus, dorsomedial nucleus and lateral hypothalamus. After 48 h of olanzapine treatment at clinically relevant plasma concentration weight gain and food intake changes, and increased plasma glucose were observed in female rats. Olanzapine treatment also led to a significant increase in mu opioid receptor availability in the arcuate nucleus, which contains both satiety and hunger controlling neurons. No other areas showed any opioid receptor expressional changes with olanzapine treatment on neither at mRNA nor protein level. Technical difficulties made it impossible to analyze mRNA levels in the lateral hypothalamus and overall binding of delta opioid receptors. Thus, the present study provided insights in to how olanzapine at clinically relevant plasma levels already at an early stage modulated the opioid system in the hypothalamus.

Increased LPS-Induced Fever and Sickness Behavior in Adult Male and Female Rats Perinatally Exposed to Morphine

As a result of the current opioid crisis, the rate of children born exposed to opioids has skyrocketed. Later in life, these children have an increased risk for hospitalization and infection, raising concerns about potential immunocompromise, as is common with chronic opioid use. Opioids can act directly on immune cells or indirectly via the central nervous system to decrease immune system activity, leading to increased susceptibility, morbidity, and mortality to infection. However, it is currently unknown how perinatal opioid exposure (POE) alters immune function. Using a clinically relevant and translatable model of POE, we have investigated how baseline immune function and the reaction to an immune stimulator, lipopolysaccharide, is influenced by in utero opioid exposure in adult male and female rats. We report here that POE potentiates the febrile and neuroinflammatory response to lipopolysaccharide, likely as a consequence of suppressed immune function at baseline (including reduced antibody production). This suggests that POE increases susceptibility to infection by manipulating immune system development, consistent with the clinical literature. Investigation of the mechanisms whereby POE increases susceptibility to pathogens is critical for the development of potential interventions for immunosuppressed children exposed to opioids in utero.

Varied impacts of social relationships on neuroendocrine state

Social relationships, affiliative social attachments, are important for many species. The best studied types of relationships are monogamous pair bonds. However, it remains unclear how generalizable models of pair bonding are across types of social attachments. Zebra finches are a fascinating system to explore the neurobiology of social relationships because they form various adult bonds with both same- and opposite-sex partners. To test whether different bonds are supported by a single brain network, we quantified individuals' neuroendocrine state after either 24 h or 2 weeks of co-housing with a novel same- or opposite-sex partner. We defined neuroendocrine state by the expression of 22 genes related to 4 major signaling pathways (dopamine, steroid, nonapeptide, and opioid) in six brain regions associated with affiliation or communication [nucleus accumbens (NAc), nucleus taeniae of the amygdala (TnA), medial preoptic area (POM), and periaqueductal gray (PAG), ventral tegmental area, and auditory cortex]. Overall, we found dissociable effects of social contexts (same- or opposite-sex partnerships) and duration of co-housing. Social bonding impacted the neuroendocrine state of four regions in males (NAc, TnA, POM, and PAG) and three regions in females (NAc, TnA, and POM). Monogamous pair bonding specifically appeared to impact male NAc. However, the patterns of gene expression in zebra finches were different than has previously been reported in mammals. Together, our results support the view that there are numerous mechanisms regulating social relationships and highlight the need to further our understanding of how social interactions shape social bonds.

Translational research updates in female health anesthesiology: a narrative review

Background and Objective

Females represent 49.6% of the global population and constitute a significant proportion of surgical patients and hospital admissions. Little is known about the bi-directional effects of sex and anesthetics or the impact of anesthetic interventions on long-term female health outcomes. Sex differences in pain pathways can influence pain experience and treatment effectiveness. The impact of anesthetic management on the recurrence of breast cancer is poorly understood, as are the long-term consequences of cardiovascular disease and safe and effective treatments in pregnancy. This review aims to outline recent advances in translational science in female health anesthesiology research and highlight critical research opportunities in pain, cancer outcomes, and cardiovascular disorders.

Methods

We searched PubMed and summarized relevant articles published in English between December 2021 and June 2022.

Key Content and Findings

Studies reveal sex differences in pain pathways and highlight the importance of sex as a biological variable in experimental designs and translational medicine. Sex differences have also been observed in side effects attributed to opioid analgesics. We summarize some of the neural circuits that might underlie these differences. In the perioperative setting, specific anesthetics are implicated in metastatic seeding potential and acute and chronic pain outcomes, suggesting the importance of anesthetic selection in comprehensive care during oncologic surgery. In the peridelivery setting, preeclampsia, a cardiovascular disorder of pregnancy, affects maternal outcomes; however, biomarkers can risk-stratify females at risk for preeclampsia and hold promise for identifying the risk of adverse neurological and other health outcomes.

Conclusions

Research that builds diagnostic and predictive tools in pain and cardiovascular disease will help anesthesiologists minimize sex-related risks and side effects associated with anesthetics and peri-hospital treatments. Sex-specific anesthesia care will improve outcomes, as will the provision of practical information to patients and clinicians about the effectiveness of therapies and behavioral interventions. However, more research studies and specific analytic plans are needed to continue addressing sex-based outcomes in anesthesiology.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

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.

Persistent inflammation selectively activates opioid-sensitive phasic-firing neurons within the vlPAG

The ventrolateral periaqueductal gray (vlPAG) is a key brain area within the descending pain modulatory pathway and an important target for opioid-induced analgesia. The vlPAG contains heterogeneous neurons with respect to neurotransmitter content, receptor and channel expression, and in vivo response to noxious stimuli. This study characterizes intrinsic membrane properties of vlPAG neurons to identify neuron types that respond to inflammation and determine whether the pain-responsive neurons are inhibited by opioids. Surveying 382 neurons identified four neuron types with distinct intrinsic firing patterns: Phasic (48%), Tonic (33%), Onset (10%), and Random (9%). Mu-opioid receptor (MOR) expression was determined by the ability of a selective MOR agonist (DAMGO) to activate G protein-coupled inwardly rectifying potassium channel (GIRK) currents. Opioid-sensitive neurons were observed within each neuron type. Opioid sensitivity did not correlate with other intrinsic firing features, including low-threshold spiking that has been previously proposed to identify opioid-sensitive GABAergic neurons in the vlPAG of mice. Complete Freund's adjuvant (CFA)-induced acute inflammation (2 h) had no effect on vlPAG neuron firing patterns. However, persistent inflammation (5-7 days) selectively activated Phasic neurons through a significant reduction in their firing threshold. Opioid-sensitive neurons were strongly activated compared with the opioid-insensitive Phasic neurons. Overall, this study provides a framework to further identify neurons activated by persistent inflammation so that they may be targeted for future pain therapies.NEW & NOTEWORTHY Intrinsic firing properties define four distinct vlPAG neuron populations, and a subset of each population expresses MORs coupled to GIRK channels. Persistent, but not acute, inflammation selectively activates opioid-sensitive Phasic vlPAG neurons. Although the vlPAG is known to contribute to the descending inhibition of pain, the activation of a single physiologically defined neuron type in the presence of persistent inflammation represents a mechanism by which the vlPAG participates in descending facilitation of pain.

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.

Chronic morphine induces adaptations in opioid receptor signaling in a thalamo-cortico-striatal circuit that are projection-dependent, sex-specific and regulated by mu opioid receptor phosphorylation

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well-understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) and anterior cingulate cortex (ACC) via activity at μ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in both the DMS and ACC. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses, MThal-ACC synapses, and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation), but decreased subsequent morphine inhibition of transmission at MThal-ACC synapses (morphine tolerance) in a sex-specific manner; these adaptations were present in male but not female mice. Additionally, these adaptations were not observed in knockin mice expressing phosphorylation-deficient MORs, suggesting a role of MOR phosphorylation in mediating both facilitation and tolerance to morphine within this circuit. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry and sex.

Spatial quantification of single cell mRNA and ligand binding of the kappa opioid receptor in the rat hypothalamus

Detailed quantification of brain tissue provides a deeper understanding of changes in expression and function. We have created a pipeline to study the detailed expression patterns of the kappa opioid receptor in the rat hypothalamus using high resolution fluorescence microscopy and receptor autoradiography. The workflow involved structured serial sampling of rat hypothalamic nuclei, in situ detection of mRNA and receptor expression, and advanced image analysis. Our results demonstrate how maintaining spatial information can lead to increased understanding of RNA and protein expression. In addition, we show the detailed expression patterns of the kappa opioid receptor in the rat hypothalamus.

Activation of HCN channels caused by elevated cAMP levels in periaqueductal gray promotes bone cancer pain

The periaqueductal gray (PAG) is an important relay center for the descending pathways that regulate nociceptive information transduction. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play critical roles in the nerve injury-induced pain hypersensitivity. Previous studies have identified that HCN1 and HCN2 channel protein located in the ventral-lateral periaqueductal gray (vlPAG), a region important for pain regulation. However, it is not clear whether the HCN channel in vlPAG is involved in bone cancer pain (BCP). In this study, we assessed the role of HCN channels in BCP by measuring changes of HCN channel expression and activity in vlPAG neurons in bone cancer rats. In the present study, the BCP model was established by injecting SHZ-88 breast cancer cells into the right tibia bone marrow in rats. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were measured to evaluate pain behavior in rats. HCN1 and HCN2 channels expression in vlPAG were detected by using Western Blot and immunohistochemistry. In addition, the cAMP level in vlPAG neurons was detected by ELISA, and HCN channel current (I_h) of vlPAG neurons was recorded by whole cell patch-clamp to evaluate HCN channel activity. As a result, decreased MWT and TWL were observed in rats on 7d after SHZ-88 cell inoculation, and the allodynia was sustained until 21d after inoculation. At the same time, HCN1 and HCN2 channels expression and neuronal I_h in vlPAG were significantly increased in BCP rats. In addition, the level of cAMP in vlPAG also increased after SHZ-88 cell inoculation. Furthermore, intravlPAG injection of ZD7288 (HCN channels antagonist) could significantly reduce hyperalgesia and the elevation of cAMP in vlPAG in BCP rats. Our observations suggest that the elevation of cAMP may promote the activation of HCN channels in vlPAG in bone cancer rats, thereby promoting the development of bone cancer pain.

Evaluating the Antinociceptive Efficacy of Cannabidiol Alone or in Combination with Morphine Using the Formalin Test in Male and Female Mice

Introduction: Phytocannabinoids have emerged as a potential alternative treatment option for individuals experiencing persistent pain. However, evidence-based research regarding their clinical utility in both males and females remains incomplete. In addition, it is unknown whether combining readily available cannabinoids with opioids has a synergistic or subadditive effect on pain modulation. To begin to fill this knowledge gap, we investigated the antinociceptive effects of the phytocannabinoid, CBD, either alone or in combination with opioids in male and female C57BL/6J mice. Results: Using the formalin test, our results show that CBD (10 mg/kg, i.p.) treatment evoked antinociception in phase I, but not in phase II, of the formalin test in male mice. However, in female mice, CBD showed no significant antinociceptive effect. In addition, a direct sex comparison showed that CBD evoked a significant increase in nociceptive behaviors in female versus male mice during phase I of the formalin test. Furthermore, we show that CBD (10 mg/kg, i.p.) in combination with low-dose morphine (1 mg/kg, i.p.) was ineffective at eliciting a synergistic antinociceptive response in both male and female mice. Lastly, consistent with previous literature, we showed that females treated with a relatively higher dose of morphine (10 mg/kg, i.p.) displayed a significant increase in the variability of nociceptive behaviors compared to morphine-treated male mice. Conclusion: Overall, our results suggest that CBD treatment may have beneficial antinociceptive effects during the acute phase of persistent pain, but these effects are more beneficial to males than females. We provide further pre-clinical support that treatments geared toward reducing nociceptive behaviors differentially affect males and females.

Cannabigerol (CBG) attenuates mechanical hypersensitivity elicited by chemotherapy-induced peripheral neuropathy

BACKGROUND

Cannabigerol (CBG) is a non-psychoactive phytocannabinoid produced by the plant Cannabis sativa with affinity to various receptors involved in nociception. As a result, CBG is marketed as an over-the-counter treatment for many forms of pain. However, there is very little research-based evidence for the efficacy of CBG as an anti-nociceptive agent.

METHODS

To begin to fill this knowledge gap, we assessed the anti-nociceptive effects of CBG in C57BL/6 mice using three different models of pain; cisplatin-induced peripheral neuropathy, the formalin test, and the tail-flick assay.

RESULTS

Using the von Frey test, we found that CBG-attenuated mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy in both male and female mice. Additionally, we observed that this CBG-induced reduction in mechanical hypersensitivity was attenuated by the α₂ -adrenergic receptor antagonist atipamezole (3 mg/kg, i.p.) and the CB₁ R antagonist, AM4113 (3 mg/kg, i.p.), and blocked by the CB₂ R antagonist/inverse agonist, SR144528 (10 mg/kg, i.p.). We found that the TRPV1 antagonist, SB705498 (20 mg/kg, i.p.) was unable to prevent CBG actions. Furthermore, we show that CBG:CBD oil (10 mg/kg, i.p.) was more effective than pure CBG (10 mg/kg) at reducing mechanical hypersensitivity in neuropathic mice. Lastly, we show that pure CBG and CBG:CBD oil were ineffective at reducing nociception in other models of pain, including the formalin and tail flick assays.

CONCLUSIONS

Our findings support the role of CBG in alleviating mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy, but highlight that these effects may be limited to specific types of pain.

SIGNIFICANCE

There are few effective treatments for neuropathic pain and neuropathic pain is projected to increase with the aging population. We demonstrate that CBG (cannabigerol) and CBG:CBD oil attenuate neuropathy-induced mechanical hypersensitivity mice. Second, we identify receptor targets that mediate CBG-induced reduction in mechanical hypersensitivity in neuropathic mice. Third, we demonstrate that an acute injection of CBG is anti-nociceptive specifically for neuropathic pain rather than other forms of pain, including persistent pain and thermal pain.

Age-Induced Changes in µ-Opioid Receptor Signaling in the Midbrain Periaqueductal Gray of Male and Female Rats

Opioids have decreased analgesic potency (but not efficacy) in aged rodents compared with adults; however, the neural mechanisms underlying this attenuated response are not yet known. The present study investigated the impact of advanced age and biological sex on opioid signaling in the ventrolateral periaqueductal gray (vlPAG) in the presence of chronic inflammatory pain. Assays measuring µ-opioid receptor (MOR) radioligand binding, GTPγS binding, receptor phosphorylation, cAMP inhibition, and regulator of G-protein signaling (RGS) protein expression were performed on vlPAG tissue from adult (2-3 months) and aged (16-18 months) male and female rats. Persistent inflammatory pain was induced by intraplantar injection of complete Freund's adjuvant (CFA). Adult males exhibited the highest MOR binding potential (BP) and highest G-protein activation (activation efficiency ratio) in comparison to aged males and females (adult and aged). No impact of advanced age or sex on MOR phosphorylation state was observed. DAMGO-induced cAMP inhibition was highest in the vlPAG of adult males compared with aged males and females (adult and aged). vlPAG levels of RGS4 and RGS9-2, critical for terminating G-protein signaling, were assessed using RNAscope. Adult rats (both males and females) exhibited lower levels of vlPAG RGS4 and RGS9-2 mRNA expression compared with aged males and females. The observed age-related reductions in vlPAG MOR BP, G-protein activation efficiency, and cAMP inhibition, along with the observed age-related increases in RGS4 and RGS9-2 vlPAG expression, provide potential mechanisms whereby the potency of opioids is decreased in the aged population.SIGNIFICANCE STATEMENT Opioids have decreased analgesic potency (but not efficacy) in aged rodents compared with adults; however, the neural mechanisms underlying this attenuated response are not yet known. In the present study, we observed age-related reductions in ventrolateral periaqueductal gray (vlPAG) µ-opioid receptor (MOR) binding potential (BP), G-protein activation efficiency, and cAMP inhibition, along with the observed age-related increases in regulator of G-protein signaling (RGS)4 and RGS9-2 vlPAG expression, providing potential mechanisms whereby the potency of opioids is decreased in the aged population. These coordinated decreases in opioid receptor signaling may explain the previously reported reduced potency of opioids to produce pain relief in females and aged rats.

Estrogen modulation of the pronociceptive effects of serotonin on female rat trigeminal sensory neurons is timing dependent and dosage dependent and requires estrogen receptor alpha

ABSTRACT

The role of the major estrogen estradiol (E2) on orofacial pain conditions remains controversial with studies reporting both a pronociceptive and antinociceptive role of E2. E2 modulation of peripheral serotonergic activity may be one mechanism underlying the female prevalence of orofacial pain disorders. We recently reported that female rats in proestrus and estrus exhibit greater serotonin (5HT)-evoked orofacial nocifensive behaviors compared with diestrus and male rats. Further coexpression of 5HT 2A receptor mRNA in nociceptive trigeminal sensory neurons that express transient receptor potential vanilloid 1 ion channels contributes to pain sensitization. E2 may exacerbate orofacial pain through 5HT-sensitive trigeminal nociceptors, but whether low or high E2 contributes to orofacial pain and by what mechanism remains unclear. We hypothesized that steady-state exposure to a proestrus level of E2 exacerbates 5HT-evoked orofacial nocifensive behaviors in female rats, explored the transcriptome of E2-treated female rats, and determined which E2 receptor contributes to sensitization of female trigeminal sensory neurons. We report that a diestrus level of E2 is protective against 5HT-evoked orofacial pain behaviors, which increase with increasing E2 concentrations, and that E2 differentially alters several pain genes in the trigeminal ganglia. Furthermore, E2 receptors coexpressed with 5HT 2A and transient receptor potential vanilloid 1 and enhanced capsaicin-evoked signaling in the trigeminal ganglia through estrogen receptor α. Overall, our data indicate that low, but not high, physiological levels of E2 protect against orofacial pain, and we provide evidence that estrogen receptor α receptor activation, but not others, contributes to sensitization of nociceptive signaling in trigeminal sensory neurons.

Sex specific effects of buprenorphine on behavior, astrocytic opioid receptor expression and neuroinflammation after pediatric traumatic brain injury in mice

Children who suffered traumatic brain injury (TBI) often experience acute and chronic pain, which is linked to a poor quality of life. Buprenorphine (BPN) is commonly used to treat moderate to severe persistent pain in children, however, the efficacy and safety profile of BPN in the pediatric population is still inconclusive. This study investigated the sex-specific effects of BPN on body weight, motor coordination and strength, expression of opioid receptors in the white matter astrocytes, and neuroinflammation in a mouse impact acceleration model of pediatric TBI. Male and female littermates were randomized on postnatal day 20-21(P20-21) into Sham, TBI + saline and TBI + BPN groups. Mice in the TBI + saline and TBI + BPN groups underwent TBI, while the Sham group underwent anesthesia without injury. BPN (0.075 mg/kg) was administered to the TBI + BPN mice at 30 min after injury, and then every 6-12 h for 2 days. Mice in the TBI + saline group received the same amount of saline injections. The impact of BPN on body weight, motor function, opioid receptor expression, and neuroinflammation was evaluated at 1-day (d), 3-d and 7-d post-injury. We found that 1) TBI induced significant weight loss in both males and females. BPN treatment improved weight loss at 3-d post-injury in females. 2) TBI significantly impaired motor coordination and strength. BPN improved motor coordination and strength in both males and females at 1-d and 3-d post-injury. 3) TBI significantly decreased exploration activity at 1-d post-injury in males, and at 7-d post-injury in females, while BPN improved the exploration activity in females. 4) TBI significantly increased mRNA expression of mu-opioid receptors (MOR) at 7-d post-injury in males, but decreased mRNA expression of MOR at 1-d post-injury in females. BPN normalized MOR mRNA expression at 1-d post-injury in females. 5) MOR expression in astrocytes at corpus callosum significantly increased at 7-d post-injury in male TBI group, but significantly decreased at 1-d post-injury in female TBI group. BPN normalized MOR expression in both males and females. 6) TBI significantly increased the mRNA expression of TNF-α, IL-1β, IL-6 and iNOS. BPN decreased mRNA expression of iNOS, and increased mRNA expression of TGF-β1. In conclusion, this study elucidates the sex specific effects of BPN during the acute phase after pediatric TBI, which provides the rationale to assess potential effects of BPN on chronic pathological progressions after pediatric TBI in both males and females.

Darting across space and time: parametric modulators of sex-biased conditioned fear responses

Pavlovian fear conditioning is a widely used behavioral paradigm for studying associative learning in rodents. Despite early recognition that subjects may engage in a variety of both conditioned and unconditioned responses, the last several decades have seen the field narrow its focus to measure freezing as the sole indicator of conditioned fear. We previously reported that female rats were more likely than males to engage in darting, an escape-like conditioned response that is associated with heightened shock reactivity. To determine how experimental parameters contribute to the frequency of darting in both males and females, we manipulated factors such as chamber size, shock intensity, and number of trials. To better capture fear-related behavioral repertoires in our animals, we developed ScaredyRat, an open-source custom Python tool that analyzes Noldus Ethovision-generated raw data files to identify darters and quantify both conditioned and unconditioned responses. We found that, like freezing, conditioned darting occurrences scale with experimental alterations. While most darting occurs in females, we found that with an extended training protocol, darting can emerge in males as well. Collectively, our data suggest that darting reflects a behavioral switch in conditioned responding that is a product of an individual animal's sex, shock reactivity, and experimental parameters, underscoring the need for careful consideration of sex as a biological variable in classic learning paradigms.

Sex Differences in the Amygdaloid Projections to the Ventrolateral Periaqueductal Gray and Their Activation During Inflammatory Pain in the Rat

Preclinical and clinical studies have reported sex differences in pain and analgesia. These differences may be linked to anatomical structures of the central nervous system pain modulatory circuitry, and/or hormonal milieu. The midbrain periaqueductal gray is a critical brain region for descending inhibition of pain. The PAG projects to the rostral ventromedial medulla (RVM), which projects bilaterally to the spinal cord to inhibit pain. In addition to pain, this descending circuit (or pathway) can be engaged by endogenous opioids (i.e., endorphins) or exogenous opioids (i.e., morphine), and we have previously reported sex differences in the activation of this circuit during pain and analgesia. Forebrain structures, including the amygdala, project to and engage the PAG-RVM circuit during persistent inflammatory pain. However, there are limited studies in females detailing this amygdalar-PAG pathway and its involvement during persistent inflammatory pain. The objective of the present study was to delineate the neural projections from the amygdala to the PAG in male and female rats to determine if they are sexually distinct in their anatomical organization. We also examined the activation of this pathway by inflammatory pain and the co-localization of receptors for estrogen. Injection of the retrograde tracer fluorogold (FG) into the ventrolateral PAG (vlPAG) resulted in dense retrograde labeling in both the central amygdala (CeA) and medial amygdala (MeA). While the number of CeA-vlPAG neurons were comparable between the sexes, there were more MeA-vlPAG neurons in females. Inflammatory pain resulted in greater activation of the amygdala in males; however, females displayed higher Fos expression within CeA-vlPAG projection neurons. Females expressed higher ERα in the MeA and CeA and the same was true of the projection neurons. Together, these data indicate that although the MeA-vlPAG projections are denser in females, inflammatory pain does not significantly activate these projections. In contrast, inflammatory pain resulted in a greater activation of the CeA-vlPAG pathway in females. As females experience a greater number of chronic pain syndromes, the CeA-vlPAG pathway may play a facilitatory (and not inhibitory) role in pain modulation.

SEX-DEPENDENT IMPACT OF MICROBIOTA STATUS ON CEREBRAL μ -OPIOID RECEPTOR DENSITY IN FISCHER RATS

μ‐opioid receptors (MOPr) play a critical role in social play, reward and pain, in a sex‐ and age‐dependent manner. There is evidence to suggest that sex and age differences in brain MOPr density may be responsible for this variability; however, little is known about the factors driving these differences in cerebral MOPr density. Emerging evidence highlights gut microbiota's critical influence and its bidirectional interaction with the brain on neurodevelopment. Therefore, we aimed to determine the impact of gut microbiota on MOPr density in male and female brains at different developmental stages. Quantitative [³H]DAMGO autoradiographic binding was carried out in the forebrain of male and female conventional (CON) and germ‐free (GF) rats at postnatal days (PND) 8, 22 and 116–150. Significant ‘microbiota status X sex’, ‘age X brain region’ interactions and microbiota status‐ and age‐dependent effects on MOPr binding were uncovered. Microbiota status influenced MOPr levels in males but not females, with higher MOPr levels observed in GF versus CON rats overall regions and age groups. In contrast, no overall sex differences were observed in GF or CON rats. Interestingly, within‐age planned comparison analysis conducted in frontal cortical and brain regions associated with reward revealed that this microbiota effect was restricted only to PND22 rats. Thus, this pilot study uncovers the critical sex‐dependent role of gut microbiota in regulating cerebral MOPr density, which is restricted to the sensitive developmental period of weaning. This may have implications in understanding the importance of microbiota during early development on opioid signalling and associated behaviours.

GPR171 Agonist Reduces Chronic Neuropathic and Inflammatory Pain in Male, But Not Female Mice

Chronic pain is a growing public health crisis that requires exigent and efficacious therapeutics. GPR171 is a promising therapeutic target that is widely expressed through the brain, including within the descending pain modulatory regions. Here, we explore the therapeutic potential of the GPR171 agonist, MS15203, in its ability to alleviate chronic pain in male and female mice using a once-daily systemic dose (10 mg/kg, i.p.) of MS15203 over the course of 5 days. We found that in our models of Complete Freund's Adjuvant (CFA)-induced inflammatory pain and chemotherapy-induced peripheral neuropathy (CIPN), MS15203 did not alleviate thermal hypersensitivity and allodynia, respectively, in female mice. On the other hand, MS15203 treatment decreased the duration of thermal hypersensitivity in CFA-treated male mice following 3 days of once-daily administration. MS15203 treatment also produced an improvement in allodynia in male mice, but not female mice, in neuropathic pain after 5 days of treatment. Gene expression of GPR171 and that of its endogenous ligand BigLEN, encoded by the gene PCSK1N, were unaltered within the periaqueductal gray (PAG) in both male and female mice following inflammatory and neuropathic pain. However, following neuropathic pain in male mice, the protein levels of GPR171 were decreased in the PAG. Treatment with MS15203 then rescued the protein levels of GPR171 in the PAG of these mice. Taken together, our results identify GPR171 as a GPCR that displays sexual dimorphism in alleviation of chronic pain. Further, our results suggest that GPR171 and MS15203 have demonstrable therapeutic potential in the treatment of chronic pain.

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.

Maternal deprivation effect on morphine-induced CPP is related to changes in Opioid receptors in selected rat brain regions (hippocampus, prefrontal cortex, and nucleus accumbens)

Early-life environmental conditions affect offspring's development. Maternal deprivation (MD) can induce persistent changes that give rise to neuropsychiatric diseases including substance abuse disorders. However, long-lasting mechanisms that determine vulnerability to drug addiction remain unknown. We hypothesized that MD could induce changes in Opioid system, HPA (hypothalamic-pituitary-adrenal) axis, and BDNF (brain-derived neurotrophic factor), so may be involved in the drug abuse in later life. Male offspring of Wistar rats (n=8 per group) were subjected to 3h of daily MD during postnatal days 1-14. In adulthood, morphine-induced CPP (conditioned place preference) was investigated using two doses of morphine (3 and 5mg/kg). Serum corticosterone level was measured by ELISA method. The expression level of genes in selected brain regions (hippocampus, prefrontal cortex, and nucleus accumbens) was determined by qPCR (quantitative PCR). A greater morphine-induced CPP was observed in MD rats with 3 and 5mg/kg morphine compared to controls. MD group had a higher corticosterone level. A significant decrease was observed in the expression of BDNF gene (in all of the selected brain regions) and GR (glucocorticoid receptor) gene (in the hippocampus and nucleus accumbens) in MD rats. Also, a significant increase in the expression of μ Opioid receptor (in all of the selected brain regions) and κ Opioid receptor (in the prefrontal cortex and nucleus accumbens) was observed in MD rats. Our results suggest that MD induces alterations in the HPA axis function, BDNF level, and Opioid receptors system that enhance vulnerability to morphine at adulthood.

Sex-specific role for serotonin 5-HT2A receptor in modulation of opioid-induced antinociception and reward in mice

Opioids are among the most effective analgesics and the mainstay of pain management. However, concerns about safety and abuse liability have challenged their widespread use by the medical community. Opioid-sparing therapies include drugs that in combination with opioids have the ability to enhance analgesia while decreasing opioid requirement as well as their side effects. Sex differences in antinociceptive responses to opioids have received increasing attention in recent years. However, the molecular mechanisms underlying sex differences related to opioid-sparing adjuncts remain largely unexplored. Using warm water tail-withdrawal as a mouse model of acute thermal nociception, our data suggest that adjunctive administration of the serotonin 5-HT_2A receptor (5-HT_2AR) antagonist volinanserin dose-dependently enhanced potency of the opioid analgesic oxycodone in male, but not female, mice. This antinociceptive-like response induced by oxycodone was also augmented in 5-HT_2AR knockout (5-HT_2AR^-/-) male, but not female mice; an effect that was reversed by Cre-loxP-mediated selective expression of 5-HT_2AR in dorsal root ganglion (DRG) neurons of 5-HT_2AR^-/- littermates. Pharmacological inhibition with volinanserin or genetic deletion in 5-HT_2AR^-/- animals potentiated the ability of oxycodone to reduce DRG excitability in male mice. Adjunctive volinanserin did not affect oxycodone-induced conditioned place preference (CPP), whereas it reduced oxycodone-induced locomotor sensitization in male and female mice. Together, these results suggest that adjunctive volinanserin augments opioid-induced antinociception, but not abuse-related behavior, through a sex-specific signaling crosstalk mechanism that requires 5-HT_2AR expression in mouse DRG neurons. Ultimately, our results may pave the way for the clinical evaluation of volinanserin as a potential sex-specific opioid adjuvant.

The Lack of Contribution of 7-Hydroxymitragynine to the Antinociceptive Effects of Mitragynine in Mice: A Pharmacokinetic and Pharmacodynamic Study

Kratom (Mitragyna speciosa), a Southeast Asian tree, has been used for centuries in pain relief and mitigation of opium withdrawal symptoms. Mitragynine (MTG), the major kratom alkaloid, is being investigated for its potential to provide analgesia without the deleterious effects associated with typical opioids. Concerns have been raised regarding the active metabolite of MTG, 7-hydroxymitragynine (7HMG), which has higher affinity and efficacy at µ-opioid receptors than MTG. Here we investigated the hotplate antinociception, pharmacokinetics, and tissue distribution of MTG and 7HMG at equianalgesic oral doses in male and female C57BL/6 mice to determine the extent to which 7HMG metabolized from MTG accounts for the antinociceptive effects of MTG and investigate any sex differences. The mechanism of action was examined by performing studies with the opioid receptor antagonist naltrexone. A population pharmacokinetic/pharmacodynamic model was developed to predict the behavioral effects after administration of various doses of MTG and 7HMG. When administered alone, 7HMG was 2.8-fold more potent than MTG to produce antinociception. At equivalent effective doses of MTG and 7HMG, there was a marked difference in the maximum brain concentration of 7HMG achieved, i.e., 11-fold lower as a metabolite of MTG. The brain concentration of 7HMG observed 4 hours post administration, producing an analgesic effect <10%, was still 1.5-fold higher than the maximum concentration of 7HMG as a metabolite of MTG. These results provide strong evidence that 7HMG has a negligible role in the antinociceptive effects of MTG in mice. SIGNIFICANCE STATEMENT: Mitragynine (MTG) is being investigated for its potential to aid in pain relief, opioid withdrawal syndrome, and opioid use disorder. The active metabolite of MTG, 7-hydroxymitragynine (7HMG), has been shown to have abuse potential and has been implicated in the opioid-like analgesic effect after MTG administration. The results of this study suggest a lack of involvement of 7HMG in the antinociceptive effects of MTG in mice.

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Central amygdala inflammation drives pain hypersensitivity and attenuates morphine analgesia in experimental autoimmune encephalomyelitis

ABSTRACT

Chronic pain is a highly prevalent symptom associated with the autoimmune disorder multiple sclerosis (MS). The central nucleus of the amygdala plays a critical role in pain processing and modulation. Neuropathic pain alters nociceptive signaling in the central amygdala, contributing to pain chronicity and opioid tolerance. Here, we demonstrate that activated microglia within the central amygdala disrupt nociceptive sensory processing and contribute to pain hypersensitivity in experimental autoimmune encephalomyelitis (EAE), the most frequently used animal model of MS. Male and female mice with EAE exhibited differences in microglial morphology in the central amygdala, which was associated with heat hyperalgesia, impaired morphine reward, and reduced morphine antinociception in females. Animals with EAE displayed a lack of morphine-evoked activity in cells expressing somatostatin within the central amygdala, which drive antinociception. Induction of focal microglial activation in naïve mice via injection of lipopolysaccharide into the central amygdala produced a loss of morphine analgesia in females, similar to as observed in EAE animals. Our data indicate that activated microglia within the central amygdala may contribute to the sexually dimorphic effects of morphine and may drive neuronal adaptations that lead to pain hypersensitivity in EAE. Our results provide a possible mechanism underlying the decreased efficacy of opioid analgesics in the management of MS-related pain, identifying microglial activation as a potential therapeutic target for pain symptoms in this patient population.

Contextual control of conditioned pain tolerance and endogenous analgesic systems

The mechanisms underlying the transition from acute to chronic pain are unclear but may involve the persistence or strengthening of pain memories acquired in part through associative learning. Contextual cues, which comprise the environment in which events occur, were recently described as a critical regulator of pain memory; both male rodents and humans exhibit increased pain sensitivity in environments recently associated with a single painful experience. It is unknown, however, how repeated exposure to an acute painful unconditioned stimulus in a distinct context modifies pain sensitivity or the expectation of pain in that environment. To answer this question, we conditioned mice to associate distinct contexts with either repeated administration of a mild visceral pain stimulus (intraperitoneal injection of acetic acid) or vehicle injection over the course of 3 days. On the final day of experiments, animals received either an acid injection or vehicle injection prior to being placed into both contexts. In this way, contextual control of pain sensitivity and pain expectation could be tested respectively. When re-exposed to the noxious stimulus in a familiar environment, both male and female mice exhibited context-dependent conditioned analgesia, a phenomenon mediated by endogenous opioid signaling. However, when expecting the presentation of a painful stimulus in a given context, males exhibited conditioned hypersensitivity whereas females exhibited endogenous opioid-mediated conditioned analgesia. These results are evidence that pain perception and engagement of endogenous opioid systems can be modified through their psychological association with environmental cues. Successful determination of the brain circuits involved in this sexually dimorphic anticipatory response may allow for the manipulation of pain memories, which may contribute to the development of chronic pain states.

Involvement of the nociceptin opioid peptide receptor in morphine-induced antinociception, tolerance and physical dependence in female mice

Nociceptin opioid peptide (NOP) receptor modulates pain transmission and is considered a prospective target for pain management. Under acute pain conditions in rodents, however, no definitive conclusions about effects of systemically intervening NOP receptors on nociception, classical opioid-induced antinociception, tolerance and physical dependence have been drawn. Given that opioid analgesia has sex differences, and females experience greater pain and consume more opioids, clarifying these issues in females will help develop novel analgesics. To clarify the role of NOP receptors on the pharmacological profiles of µ-opioid receptor agonists, in this study, a selective agonist (SCH221510) and antagonist (SB612111) of the NOP receptor were subcutaneously administered in female mice in multiple animal models. In hot-plate test, neither SCH221510 (3 and 10 mg/kg, sc) nor SB612111 (10 mg/kg, sc) produced significant antinociception. SCH221510 (3 mg/kg, sc) attenuated but SB612111 (10 mg/kg, sc) enhanced morphine-induced antinociception, with rightward and leftward shift of morphine dose-response curves, respectively. SCH221510 (3 mg/kg, sc) combined with morphine (10 mg/kg, sc) accelerated the development of morphine antinociceptive tolerance. Conversely, SB612111 (10 mg/kg, sc) delayed morphine tolerance development. Neither SCH221510 (3 mg/kg, sc) nor SB612111 (10 mg/kg, sc) statistically significantly altered the development of morphine-induced physical dependence. Therefore, systemic activation of NOP receptors attenuated morphine antinociception to acute thermal stimuli, facilitated morphine-induced antinociceptive tolerance but did not robustly alter physical dependence in female mice. Systemic blockade of NOP receptors produced opposite actions. These findings demonstrate that N/OFQ-NOP receptor system plays diverse roles in modulating pharmacological profiles of µ-opioid receptor agonists.

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 Differences in Coping Strategies Based on Chronic Pain Intensity Among Older Adults

The aim of the current study was to investigate chronic pain coping strategies of older adults and the differences between sexes according to the intensity of pain. A cross-sectional study was performed in specialty clinics in midwestern Brazil. Participants comprised 276 older adults with chronic pain. The Brief Pain Inventory and Chronic Pain Coping Inventory-Brazilian version were used to assess self-reported pain intensity and pain coping strategies. Mann-Whitney U, Kruskal-Wallis, and Dunn tests were used for intergroup comparisons. The task persistence and coping self-statements subscales were the most used coping strategies. The strategies of exercise/stretching and task persistence were used differently between males and females. Males showed significant differences in asking for assistance, task persistence, and guarding subscales when pain intensity was taken into account, whereas females showed differences in the guarding and resting subscales. Adaptive coping was the most used coping strategy; however, some non-adaptive subscales were significant in both sexes when compared with levels of chronic pain intensity. [Journal of Gerontological Nursing, 47(10), 30-36.].

Gender differences in the prevalence of heroin and opioid analgesic misuse in the United States, 2015-2019

OBJECTIVES

Gender differences in the prevalence of opioid misuse continue to evolve and have not been well characterized in recent years. Our objective was to investigate gender differences in the prevalence of opioid misuse and use disorder in the US over the 5-year period from 2015 to 2019.

METHODS

We used annual survey data from the 2015-2019 National Survey on Drug Use and Health to estimate gender differences in the prevalence of opioid misuse. We examined past-year opioid analgesic misuse initiation, opioid analgesic misuse, heroin use, opioid analgesic use disorder and heroin use disorder. Logistic regression models were used to test gender differences, adjusting for sociodemographic variables.

RESULTS

In adjusted analyses, women had higher odds of having initiated opioid analgesic misuse in the past year compared to men. In contrast, men had higher odds of misuse of opioid analgesics, heroin use, and an opioid analgesic or heroin use disorder.

CONCLUSIONS

Although opioid misuse has historically been more prevalent in men, the gender difference in opioid analgesic misuse continues to narrow, with more women initiating misuse than men including higher rates of misuse in adolescent girls. Heroin use continues to be approximately twice as common in men as women.

Estrogen exacerbates the nociceptive effects of peripheral serotonin on rat trigeminal sensory neurons

Orofacial pain disorders involving trigeminal sensory neurons disproportionately affect women and can be modulated by hormones, especially estrogen (E2). Proinflammatory mediators, like serotonin (5HT), can act on sensory neurons expressing the transient receptor potential vanilloid 1 (TRPV1) ion channel, resulting in peripheral sensitization. We previously reported peripheral 5HT evokes greater pain behaviors in the hindpaw of female rats during proestrus and estrus, stages when E2 fluctuates. It is unknown if this interaction is comparable in the trigeminal system. We hypothesized that E2 exacerbates 5HT-evoked nocifensive pain behaviors and pain signaling in female trigeminal sensory neurons. We report 5HT-evoked nocifensive behaviors are significantly higher during estrus and proestrus, which is attenuated by blocking the 5HT2A receptor. The comparable dose of 5HT was not nociceptive in males unless capsaicin was also administered. When administered with capsaicin, a lower dose of 5HT evoked trigeminal pain behaviors in females during proestrus. Further, basal 5HT content in the vibrissal pad was higher in cycling females compared to males. Ex vivo, E2 enhanced 5HT-potentiated CGRP release from trigeminal neurons, which was not significantly reduced by blocking the 5HT2A receptor. Our data indicates that estrogen fluctuation influences the pronociceptive effects of 5HT on trigeminal sensory neurons.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Hyporesponsivity to mu-opioid receptor agonism in the Wistar-Kyoto rat model of altered nociceptive responding associated with negative affective state

ABSTRACT

Chronic pain is often comorbid with anxiety and depression, altering the level of perceived pain, which negatively affects therapeutic outcomes. The role of the endogenous mu-opioid receptor (MOP) system in pain-negative affect interactions and the influence of genetic background thereon are poorly understood. The inbred Wistar-Kyoto (WKY) rat, which mimics aspects of anxiety and depression, displays increased sensitivity (hyperalgesia) to noxious stimuli, compared with Sprague-Dawley (SD) rats. Here, we report that WKY rats are hyporesponsive to the antinociceptive effects of systemically administered MOP agonist morphine in the hot plate and formalin tests, compared with SD counterparts. Equivalent plasma morphine levels in the 2 rat strains suggested that these differences in morphine sensitivity were unlikely to be due to strain-related differences in morphine pharmacokinetics. Although MOP expression in the ventrolateral periaqueductal gray (vlPAG) did not differ between WKY and SD rats, the vlPAG was identified as a key locus for the hyporesponsivity to MOP agonism in WKY rats in the formalin test. Moreover, morphine-induced effects on c-Fos (a marker of neuronal activity) in regions downstream of the vlPAG, namely, the rostral ventromedial medulla and lumbar spinal dorsal horn, were blunted in the WKY rats. Together, these findings suggest that a deficit in the MOP-induced recruitment of the descending inhibitory pain pathway may underlie hyperalgesia to noxious inflammatory pain in the WKY rat strain genetically predisposed to negative affect.

Testosterone protects against the development of widespread muscle pain in mice

Chronic widespread pain conditions are more prevalent in women than men, suggesting a role for gonadal hormones in the observed differences. Previously, we showed that female mice, compared to male, develop widespread, more severe, and longer-duration hyperalgesia in a model of activity-induced muscle pain. We hypothesized testosterone protects males from developing the female pain phenotype. We tested whether orchiectomy of males before induction of an activity-induced pain model produced a female phenotype and whether testosterone administration produced a male phenotype in females. Orchiectomy produced longer-lasting, more widespread hyperalgesia, similar to females. Administration of testosterone to females or orchiectomized males produced unilateral, shorter-lasting hyperalgesia. Prior studies show that the serotonin transporter (SERT) is increased in the nucleus raphe magnus (NRM) in models of chronic pain, and that blockade of SERT in the NRM reduces hyperalgesia. We examined potential sex differences in the distribution of SERT across brain sites involved in nociceptive processing using immunohistochemistry. A sex difference in SERT was found in the NRM in the activity-induced pain model; females had greater SERT immunoreactivity than males. This suggests that testosterone protects against development of widespread, long-lasting muscle pain and that alterations in SERT may underlie the sex differences.

Chronic opioid exposure differentially modulates oxycodone self-administration in male and female rats

Withdrawal from opioid painkillers can produce short‐lived physical symptoms and protracted psychological symptoms including anxiety and depressive‐like states that often lead to opioid misuse and opioid use disorder (OUD). Studies testing the hypothesis that opioid withdrawal potentiates the reinforcing effects of opioid self‐administration (SA) are largely inconclusive and have focused on males. Although some clinical evidence indicates that women are more likely than men to misuse opioids to self‐medicate, preclinical studies in both sexes are lacking. Based on clinical reports, we hypothesized that withdrawal from escalating‐dose morphine injections that approximates a prescription painkiller regimen would lead to increased oxycodone SA to a greater extent in female compared to male rats. After escalating‐dose morphine (5–30 mg/kg or vehicle, twice/day for 12 days), rats underwent a 2‐week abstinence period during which withdrawal signs were measured. The impact of this treatment was assessed on oxycodone SA acquisition, maintenance, dose response, and progressive ratio responding, with additional analyses to compare sexes. We found that both sexes expressed somatic withdrawal, whereas only males demonstrated hyperalgesia in the warm water tail flick assay. During SA acquisition, males with prior morphine exposure took significantly more oxycodone than females. Finally, females with prior morphine exposure demonstrated the lowest motivation to SA oxycodone in the progressive ratio test. Contrary to our initial hypothesis, our findings suggest that prior opioid exposure increases vulnerability to initiate misuse more in males and decreases the reinforcing efficacy of oxycodone in females.

Positive allosteric modulation of the mu-opioid receptor produces analgesia with reduced side effects

Positive allosteric modulators (PAMs) of the mu-opioid receptor (MOR) have been hypothesized as potentially safer analgesics than traditional opioid drugs. This is based on the idea that PAMs will promote the action of endogenous opioid peptides while preserving their temporal and spatial release patterns and so have an improved therapeutic index. However, this hypothesis has never been tested. Here, we show that a mu-PAM, BMS-986122, enhances the ability of the endogenous opioid Methionine-enkephalin (Met-Enk) to stimulate G protein activity in mouse brain homogenates without activity on its own and to enhance G protein activation to a greater extent than β-arrestin recruitment in Chinese hamster ovary (CHO) cells expressing human mu-opioid receptors. Moreover, BMS-986122 increases the potency of Met-Enk to inhibit GABA release in the periaqueductal gray, an important site for antinociception. We describe in vivo experiments demonstrating that the mu-PAM produces antinociception in mouse models of acute noxious heat pain as well as inflammatory pain. These effects are blocked by MOR antagonists and are consistent with the hypothesis that in vivo mu-PAMs enhance the activity of endogenous opioid peptides. Because BMS-986122 does not bind to the orthosteric site and has no inherent agonist action at endogenously expressed levels of MOR, it produces a reduced level of morphine-like side effects of constipation, reward as measured by conditioned place preference, and respiratory depression. These data provide a rationale for the further exploration of the action and safety of mu-PAMs as an innovative approach to pain management.

Considering sex as a biological variable will require a global shift in science culture

For over half a century, male rodents have been the default model organism in preclinical neuroscience research, a convention that has likely contributed to higher rates of misdiagnosis and adverse side effects from drug treatment in women. Studying both sexes could help to rectify these public health problems, but incentive structures in publishing and career advancement deter many researchers from doing so. Moreover, funding agency directives to include male and female animals and human participants in grant proposals lack mechanisms to hold recipients accountable. In this Perspective, we highlight areas of behavioral, cellular and systems neuroscience in which fundamental sex differences have been identified, demonstrating that truly rigorous science must include males and females. We call for a cultural and structural change in how we conduct research and evaluate scientific progress, realigning our professional reward systems and experimental standards to produce a more equitable, representative and therefore translational body of knowledge.

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.