Organizational and activational effects of testosterone on carrageenan-induced inflammatory pain and morphine analgesia

Migraine in men

BACKGROUND

Migraine is a common primary headache disorder, less frequently affecting men than women, and often regarded as predominantly a "women's disease." Despite this, migraine in men presents with unique characteristics in terms of symptoms, treatment responses, comorbidities, and pain perception. Historically, research has focused more on migraine in women, overlooking critical male-specific aspects.

RESULTS

This review delves into the epidemiology, clinical presentation, and particular challenges of diagnosing and managing migraine in men. It addresses sex-specific triggers, hormonal influences, and comorbid conditions affecting migraine prevalence and severity in men. Additionally, the review evaluates current therapeutic strategies, underscoring the necessity for individualized approaches. Men with migraine often exhibit atypical symptoms compared to the ICHD-3 criteria and are less likely to report common associated symptoms. They also tend to have fewer psychological comorbidities, respond more favorably to pharmacological treatments, yet are less likely to seek medical support. The reasons for these sex disparities are complex, involving biological, psychosocial, and cultural factors, such as brain structural differences, differences in functional responses to painful stimuli, hormonal effects, and behavioral influences like adherence to masculine norms and stigma.

CONCLUSION

Men are underrepresented in clinical migraine research. In contrast, preclinical studies often focus solely in male animals as a result of various misconceptions. This disparity necessitates greater focus on sex-specific aspects of migraine to enhance diagnosis, treatment, and research. Addressing stigma, increasing healthcare access, and ensuring balanced sex and gender representation in future studies is crucial for a comprehensive understanding and effective management of migraine for all patients.

Female specific interactions of serotonin and testosterone in the rostral ventromedial medulla after activity-induced muscle pain

Classical preclinical studies show that serotonin (5-HT) injected into the rostral ventromedial medulla (RVM) produces analgesia that is blocked by 5-HT2 receptor antagonists. One key modulator of 5-HT activity is the serotonin transporter (SERT) which reduces serotonergic signaling through reuptake into the presynaptic terminal. In the activity-induced muscle pain model, females show widespread pain and increased SERT expression in the RVM whereas males show localized pain and no changes in SERT expression. Since prior studies show testosterone protects from the development of widespread pain, and females have widespread pain in the activity-induced pain model, we hypothesized that testosterone modulates serotonin signaling to enhance analgesia in female mice with widespread pain. We showed that testosterone reduced the enhanced SERT protein expression and increased 5-HT2A receptor mRNA expression in the RVM normally observed in the activity-induced pain model in females, but not males. Inhibition of SERT in the RVM was analgesic in both female and male mice; this analgesia was blocked by co-administration of 5-HT2A antagonist. Next, using in situ hybridization, we demonstrated co-expression of SERT, 5-HT2A receptor, and androgen receptor mRNA in cells within the RVM in female mice. Lastly, activation of androgen receptors using dihydrotestosterone reduced hyperalgesia in female mice. These data therefore show for the first time expression of androgen receptors in the RVM in female mice, that activation of androgen receptors reduces nociceptive behaviors, and endogenous testosterone modulates SERT and 5-HT2 receptor expression. Thus, we show a sex-specific role for how testosterone modulates analgesia in mice. PERSPECTIVE: This article presents novel mechanisms testosterone's protection against muscle pain in female mice showing modulation of the serotonin system in the rostral ventromedial medulla. Understanding the relationship between testosterone and serotonin could lead to better treatment of individuals with muscle pain.

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.

Impact of continuous testosterone exposure on reproductive physiology, activity, and pain-related behavior in young adult female rats

Testosterone may reduce pain in cisgender women and transgender men. Rodents can provide a useful model for investigating physiological effects of hormone therapy. To this end, continuous-release testosterone or blank (placebo) capsules were implanted s.c. into young adult female rats, and three weeks later rats were either ovariectomized or sham-ovariectomized. Testosterone treatment that mimicked previously reported endogenous levels in males eliminated estrous cycling and decreased uterine weight. Testosterone also significantly increased body weight and suppressed the increases in daily wheel running observed in placebo controls over time. Subsequent ovariectomy or sham-ovariectomy decreased wheel running in all groups, but testosterone-treated rats recovered significantly more quickly than did placebo-treated rats. Neither testosterone nor ovariectomy significantly altered hindpaw mechanical threshold. Two weeks after sham/ovariectomy surgery, injection of Complete Freund Adjuvant (CFA) into one hindpaw reduced wheel running and mechanical threshold in all groups; running significantly decreased from the first to second day after CFA in testosterone- but not in placebo-treated rats. Morphine 1.0 but not 3.2 mg/kg increased CFA-suppressed wheel running similarly in all groups, whereas both doses of morphine increased CFA-suppressed mechanical threshold. These data suggest that weeks-long testosterone treatment with or without ovariectomy may provide a useful physiological model of testosterone therapy as used in human gender transition. Although testosterone administered at levels similar to those in gonadally intact males tended to hasten female rats' recovery from surgery, it did not decrease maximal pain-related behaviors after surgery or hindpaw inflammatory insult, nor did it alter opioid antinociception.

Age and Gender, but Not Pain are Associated with Pressure Pain Thresholds in Patients with Temporomandibular Disorders: A Cross-Sectional Study

PURPOSE

This study aims to explore the association of pressure pain thresholds (PPTs) with age, gender, and pain in patients with temporomandibular disorders (TMD).

PATIENTS AND METHODS

A total of 301 TMD patients (248 female and 53 male) were recruited and classified into the high and low age groups according to their median age of 26 years. Patients' demographics, pain-related variables, TMD-related variables, and PPTs of both left and right temporomandibular joints (TMJs), masseter, and temporalis were collected.

RESULTS

Pain duration and visual analog scale of pain (VAS) showed no significant correlations with PPTs (P>0.05). Multiple linear regression analysis revealed a significant positive association of PPTs of all six sites with males (β=0.41-0.72 kg·cm-2, 95% CI (0.19-0.38, 0.74-0.99), P<0.001), as well as with the high age group [β=0.28-0.36 kg·cm-2, 95% CI (0.07-0.20, 0.47-0.53), P<0.020]. Furthermore, PPTs of the left TMJ showed a significant negative association with left pain-related TMD (PT) [β=-0.21 kg·cm-2, 95% CI (-0.38, -0.04), P=0.026], but PPTs of the remaining sites did not show a significant association with PT (P>0.05). Stratified analysis showed that PPTs in females were associated with the high age group [β=0.25-0.37 kg·cm-2, 95% CI (0.04-0.20, 0.45-0.56), P<0.020] and that PPT of the left TMJ was associated with left PT [β=-0.21 kg·cm-2, 95% CI (-0.39, -0.03), P=0.043]. The remaining PPTs did not show a significant association with PT (P>0.05). In males, PPTs did not show significant correlations with age, PT and VAS (P>0.05).

CONCLUSION

PPTs in the orofacial region are associated with gender and age in TMD patients. Pain duration and intensity show no significant correlations with PPTs in TMD patients. Researchers and dentists should take age and gender into account when using PPTs as auxiliary diagnostic indicators for PT.

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.

Sex differences in neuroimmune and glial mechanisms of pain

ABSTRACT

Pain is the primary motivation for seeking medical care. Although pain may subside as inflammation resolves or an injury heals, it is increasingly evident that persistency of the pain state can occur with significant regularity. Chronic pain requires aggressive management to minimize its physiological consequences and diminish its impact on quality of life. Although opioids commonly are prescribed for intractable pain, concerns regarding reduced efficacy, as well as risks of tolerance and dependence, misuse, diversion, and overdose mortality rates limit their utility. Advances in development of nonopioid interventions hinge on our appreciation of underlying mechanisms of pain hypersensitivity. For instance, the contributory role of immunity and the associated presence of autoimmune syndromes has become of particular interest. Males and females exhibit fundamental differences in innate and adaptive immune responses, some of which are present throughout life, whereas others manifest with reproductive maturation. In general, the incidence of chronic pain conditions, particularly those with likely autoimmune covariates, is significantly higher in women. Accordingly, evidence is now accruing in support of neuroimmune interactions driving sex differences in the development and maintenance of pain hypersensitivity and chronicity. This review highlights known sexual dimorphisms of neuroimmune signaling in pain states modeled in rodents, which may yield potential high-value sex-specific targets to inform future analgesic drug discovery efforts.

Association of temporomandibular disorders with pain sensitivity: A cohort study

BACKGROUND

Pain related to temporomandibular disorders (TMD) can be linked with multiple site pain (MSP), and may associate with increased pain sensitivity, more frequently among women than men. The aim of the study was to examine the associations of pressure pain threshold (PPT) and tolerance (PPTo) with TMD and associated MSP in the Northern Finland Birth Cohort 1966 (NFBC1966) study.

METHODS

Altogether 1961 NFBC1966 subjects attended clinical medical and dental examination at the Institute of Dentistry, University of Oulu in 2012-2013. Clinical examinations were carried out using a modified Diagnostic Criteria for TMD protocol (DC/TMD). MSP was defined based on questions regarding body pain sites. Additionally, PPT and PPTo were assessed using algometer measurements. Mann-Whitney U-test and Tobit regression models were used to analyse associations between TMD sub-diagnoses, MSP, PPT and PPTo, stratified by sex. Further models were adjusted with anxiety and depressive symptoms, which were assessed using Hopkins Symptom Checklist-25 (HSCL-25) and two-way interaction terms.

RESULTS

Among females, lower PPT and PPTo were associated with myalgia and arthralgia. Among males, lower PPT and PPTo were associated with MSP-linked TMD. Tobit regression analysis showed significantly lower PPT and PPTo values in the myalgia and arthralgia subgroups among female TMD subjects. Among females, disc displacement with reduction had an inverse association with PPT and PPTo. Among males, lower PPTo was associated with degenerative joint disease and MSP-linked TMD.

CONCLUSIONS

The pain regulatory mechanisms behind TMD act differently between the genders as local TMD among females and MSP-linked TMD among males were associated with pain sensitivity.

SIGNIFICANCE

The study shows that there are differences in the associations of painful TMD with pressure pain tolerance, pressure pain sensitivity and MSP between male and female subjects.

Behavioral sex differences in cocaine and opioid use disorders: The role of gonadal hormones

Females are more vulnerable than males to many aspects of cocaine use disorder. This vulnerability also translates to opioid use disorder, with females exhibiting stronger behavioral responses than males to drugs such as heroin and morphine. While there is evidence for many overlapping neural mechanisms underlying cocaine and opioid abuse, there is also a breadth of evidence indicating divergent effects of the drugs on synaptic plasticity. This makes it unclear whether the behavioral sex differences seen in substance use disorder across different drugs of abuse rely on the same mechanisms. Ovarian hormones have consistently been implicated as drivers of the behavioral sex differences in cocaine taking and seeking. While there are far fewer studies on the role of ovarian hormones in opioid use disorder, the existing data suggest that ovarian hormones may not drive these behavioral effects in the same manner as in cocaine use disorder. This review highlights evidence that behavioral sex differences in substance use disorder might be driven by different mechanisms depending on drug class.

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.

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.

Influence of androgenic blockade with flutamide on pain behaviour and expression of the genes that encode the NaV1.7 and NaV1.8 voltage-dependent sodium channels in a rat model of postoperative pain

Background

Experimental studies suggest that testosterone reduces the nociceptive response after inflammatory and neuropathic stimuli, however the underlying mechanisms have not been fully elucidated. The aims of this study were to evaluate the effect of peripheral blockade of testosterone on pain behaviour and on expression levels of the genes that encode the NaV1.7 and NaV1.8 channels, in dorsal root ganglia in an acute postoperative pain model, as well as the influence of androgen blockade on the expression of these genes.

Methods

Postoperative pain was induced by a plantar incision and the study group received flutamide to block testosterone receptor. The animals were submitted to behavioural evaluation preoperatively, 2 h after incision, and on the 1st, 2nd, 3rd and 7th postoperative days. Von Frey test was used to evaluate paw withdrawal threshold after mechanical stimuli and the guarding pain test to assess spontaneous pain. The expression of the genes encoding the sodium channels at the dorsal root ganglia was determined by real time quantitative polymerase chain reaction.

Results

Animals treated with flutamide presented lower paw withdrawal threshold at the 1st, 2nd, 3rd, and 7th postoperative days. The guarding pain test showed significant decrease in the flutamide group at 2 h and on the 3rd and 7th postoperative days. No difference was detected between the study and control groups for the gene expression.

Conclusions

Our data suggest an antinociceptive effect of androgens following plantar incision. The expression of genes that encode voltage-gated sodium channels was not influenced by androgen blockade.

Mouse MRI shows brain areas relatively larger in males emerge before those larger in females

Sex differences exist in behaviors, disease and neuropsychiatric disorders. Sexual dimorphisms however, have yet to be studied across the whole brain and across a comprehensive time course of postnatal development. Here, we use manganese-enhanced MRI (MEMRI) to longitudinally image male and female C57BL/6J mice across 9 time points, beginning at postnatal day 3. We recapitulate findings on canonically dimorphic areas, demonstrating MEMRI’s ability to study neuroanatomical sex differences. We discover, upon whole-brain volume correction, that neuroanatomical regions larger in males develop earlier than those larger in females. Groups of areas with shared sexually dimorphic developmental trajectories reflect behavioral and functional networks, and expression of genes involved with sex processes. Also, post-pubertal neuroanatomy is highly individualized, and individualization occurs earlier in males. Our results demonstrate the ability of MEMRI to reveal comprehensive developmental differences between male and female brains, which will improve our understanding of sex-specific predispositions to various neuropsychiatric disorders.

Sex differences occur in various aspects of neurodevelopment. Here the authors use manganese-enhanced MRI at nine different postnatal stages to detail the development of structural sex differences in the mouse brain.

Neonatal Handling Produces Sex Hormone-Dependent Resilience to Stress-Induced Muscle Hyperalgesia in Rats

Neonatal handling (NH) of male rat pups strongly attenuates stress response and stress-induced persistent muscle hyperalgesia in adults. Because female sex is a well established risk factor for stress-induced chronic muscle pain, we explored whether NH provides resilience to stress-induced hyperalgesia in adult female rats. Rat pups underwent NH, or standard (control) care. Muscle mechanical nociceptive threshold was assessed before and after water avoidance (WA) stress, when they were adults. In contrast to male rats, NH produced only a modest protection against WA stress-induced muscle hyperalgesia in female rats. Gonadectomy completely abolished NH-induced resilience in male rats but produced only a small increase in this protective effect in female rats. The administration of the antiestrogen drug fulvestrant, in addition to gonadectomy, did not enhance the protective effect of NH in female rats. Finally, knockdown of the androgen receptor by intrathecal antisense treatment attenuated the protective effect of NH in intact male rats. Together, these data indicate that androgens play a key role in NH-induced resilience to WA stress-induced muscle hyperalgesia.

PERSPECTIVE

NH induces androgen-dependent resilience to stress-induced muscle pain. Therefore, androgens may contribute to sex differences observed in chronic musculoskeletal pain and its enhancement by stress.

Sex differences in experimental measures of pain sensitivity and endogenous pain inhibition

It has been suggested that increased pain sensitivity and disruption of endogenous pain inhibitory processes may account, at least in part, for the greater prevalence and severity of chronic pain in women compared to men. However, previous studies addressing this topic have produced mixed findings. This study examined sex differences in pain sensitivity and inhibition using quantitative sensory testing (QST), while also considering the influence of other important factors such as depressive symptoms and sleep quality. Healthy men (n=24) and women (n=24) each completed a QST battery. This battery included an ischemic pain task (IPT) that used a submaximal effort tourniquet procedure as well as a conditioned pain modulation (CPM) procedure for the assessment of endogenous pain inhibition. Prior to QST, participants completed the Center for Epidemiologic Studies Depression Scale and the Pittsburgh Sleep Quality Index. Analyses revealed significant sex differences for the ischemic pain task and the conditioned pain modulation procedure, such that women tolerated the ischemic pain for a shorter amount of time and demonstrated less pain inhibition compared with men. This remained true even when accounting for sex differences in depressive symptoms and sleep quality. The results of this study suggest that women may be more pain sensitive and possess less-efficient endogenous pain inhibitory capacity compared with men. Whether interventions that decrease pain sensitivity and enhance pain inhibition in women ultimately improve their clinical pain outcomes is an area of research that deserves additional attention in the future.

Sex differences in opioid analgesia and addiction: interactions among opioid receptors and estrogen receptors

Opioids are widely used as the pain reliever and also notorious for being addictive drugs. Sex differences in the opioid analgesia and addiction have been reported and investigated in human subjects and animal models. Yet, the molecular mechanism underlying the differences between males and females is still unclear. Here, we reviewed the literature describing the sex differences in analgesic responses and addiction liabilities to clinically relevant opioids. The reported interactions among opioids, estrogens, opioid receptors, and estrogen receptors are also evaluated. We postulate that the sex differences partly originated from the crosstalk among the estrogen and opioid receptors when stimulated by the exogenous opioids, possibly through common secondary messengers and the downstream gene transcriptional regulators.

Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon

A clear majority of patients with chronic pain are women; however, it has been surprisingly difficult to determine whether this sex bias corresponds to actual sex differences in pain sensitivity. A survey of the currently available epidemiological and laboratory data indicates that the evidence for clinical and experimental sex differences in pain is overwhelming. Various explanations for this phenomenon have been given, ranging from experiential and sociocultural differences in pain experience between men and women to hormonally and genetically driven sex differences in brain neurochemistry.

Implication of allopregnanolone in the antinociceptive effect of N-palmitoylethanolamide in acute or persistent pain

We investigated the involvement of de novo neurosteroid synthesis in the mechanisms underlying the analgesic and antihyperalgesic effects of N-palmitoylethanolamine (PEA) in two models of acute and persistent pain, the formalin test and carrageenan-induced paw edema. The pivotal role of peroxisome proliferator-activated receptor (PPAR)-α in the antinocifensive effect of PEA was confirmed by the lack of this effect in PPAR-α-null mice. PEA antinociceptive activity was partially reduced when the animals were treated with aminoglutethimide or finasteride, implying that de novo neurosteroid synthesis is involved in the effect of PEA. Accordingly, in the spinal cord, the allopregnanolone (ALLO) levels were increased by PEA treatment both in formalin- and carrageenan-exposed mice, as revealed by gas chromatography-mass spectrometry. In agreement with those data, in both pain models, PEA administration in challenged mice specifically restored the expression of two proteins involved in neurosteroidogenensis, the steroidogenic acute regulatory protein (StAR) and cytochrome P450 side-chain cleavage (P450scc) in the ipsilateral horns of spinal cord, without affecting their expression in the contralateral side. These results provide new information about the involvement of de novo neurosteroid synthesis in the modulation of pain behavior by PEA.

Estradiol-induced antinociceptive responses on formalin-induced nociception are independent of COX and HPA activation

Estrogen modulates pain perception but how it does so is not fully understood. The aim of this study was to determine if estradiol reduces nociceptive responses in part via hypothalamic-pituitary-adrenal (HPA) axis regulation of cyclooxygenase (COX)-1/COX-2 activity. The first study examined the effects of estradiol (20%) or vehicle with concurrent injection nonsteroidal antiinflammatory drugs (NSAIDs) on formalin-induced nociceptive responding (flinching) in ovariectomized (OVX) rats. The drugs were ibuprofen (COX-1 and COX-2 inhibitor), SC560 (COX-1 inhibitor), or NS398 (COX-2 inhibitor). In a second study, estradiol's effects on formalin-induced nociception were tested in adrenalectomized (ADX), OVX, and ADX+OVX rats. Serum levels of prostaglandins (PG) PGE(2) and corticosterone were measured. Estradiol significantly decreased nociceptive responses in OVX rats with effects during both the first and the second phase of the formalin test. The nonsteroidal antiinflammatory drugs (NSAIDs) did not alter nociception at the doses used here. Adrenalectomy neither altered flinching responses in female rats nor reversed estradiol-induced antinociceptive responses. Estradiol alone had no effect on corticosterone (CORT) or prostaglandin levels after the formalin test, dissociating the effects of estradiol on behavior and these serum markers. Ibuprofen and NS398 significantly reduced PGE2 levels. CORT was not decreased by OVX surgery or by estradiol below that of ADX. Only IBU significantly increased corticosterone levels. Taken together, our results suggest that estradiol-induced antinociception in female rats is independent of COX activity and HPA axis activation.

Anabolic-androgenic steroid effects on nociception and morphine antinociception in male rats

The purpose of this study was to investigate the effects of acute and chronic administration of anabolic-androgenic steroids (AAS) on nociception and morphine antinociception in acute pain models, as well as on chronic inflammatory nociception. In Experiment 1, adult, gonadally intact male rats were injected s.c. for 28 days with either 5 mg/kg testosterone (T), dihydrotestosterone (DHT), stanozolol (STAN), or safflower oil vehicle (N=12-25/group). On day 28, rats in each group were tested on acute thermal and mechanical nociceptive assays, before and after morphine treatment. In Experiment 2, rats in each group (N=8-10/group) were injected with mineral oil or complete Freund's adjuvant (CFA) into one hindpaw after 28 days of AAS treatment, and then tested for thermal hyperalgesia, mechanical allodynia, inflammation and locomotor suppression intermittently for 28 days. Experiment 3 replicated nociceptive measurements in Experiments 1 and 2, but with a single AAS or vehicle injection occurring 3h prior to testing (N=10-12/group). While chronic AAS administration tended to decrease body weight gain and alter reproductive organ weights in the expected manner, it did not significantly alter acute nociception nor attenuate the development of various chronic pain indices after CFA administration. Morphine antinociceptive potency was significantly decreased by chronic DHT on the hot plate test only. Acute AAS administration also did not significantly alter acute or chronic nociception, or morphine antinociceptive potency. Comparisons between acute and chronic AAS administration suggest that steroid tolerance did not occur in rats treated with AAS chronically. Taken together, these data do not support the hypothesis that AAS exposure alters nociception or morphine antinociception in gonadally intact males.

Evaluation of menstrual cycle effects on morphine and pentazocine analgesia

Studies have demonstrated menstrual cycle influences on basal pain perception, but direct evidence of menstrual cycle influences on analgesic responses has not been reported in humans. Our aim was to determine whether the magnitude of morphine and pentazocine analgesia varied across the menstrual cycle. Sixty-five healthy women, 35 taking oral contraceptives (OC) and 30 normally cycling (NOC), underwent experimental pain assessment both before and after intravenous administration morphine (0.08mg/kg) or pentazocine (0.5mg/kg) compared to saline placebo. Both active drug and placebo were administered once during the follicular phase and once during the luteal phase. Measures of heat, ischemic, and pressure pain sensitivity were obtained before and after drug administration. Change scores in pain responses were computed to determine morphine and pentazocine analgesic responses, and medication side effects were recorded. The data were analyzed using mixed-model analyses of variance. NOC women showed slightly greater heat pain sensitivity in the follicular vs luteal phase, while the reverse pattern emerged for OC women (P=0.046). Also, OC women showed lower pressure pain thresholds compared to NOC women (P<0.05). Regarding analgesic responses, NOC women showed greater morphine analgesia for ischemic pain during the follicular vs the luteal phase (P=0.004). Likewise, side effects for morphine were significantly higher in NOC women in the follicular phase than in the luteal phase (P=0.02). These findings suggest that sex hormones may influence opioid responses; however, the effects vary across medications and pain modalities and are likely to be modest in magnitude. Limited menstrual cycle effects on baseline pain responses were observed; however, morphine analgesia and side effects were greater during the follicular phase.

Sexual differentiation of rat reproductive versus opioid antinociceptive systems

BACKGROUND

It has been suggested that sexual differentiation of opioid analgesic sensitivity may parallel sexual differentiation in reproductive systems.

OBJECTIVE

The present study compared organizational and activational roles of testosterone in sexual differentiation in reproductive versus opioid antinociceptive systems in the rat, to assess whether both systems were similarly testosterone dependent.

METHODS

Male rat pups (Sprague-Dawley and Fisher 344 [F344]) were either handled or castrated on postnatal day (PND) 1, and female pups were injected with testosterone propionate (100 or 1000 microg) on PND 2. In adulthood, all rats were gonadectomized (or simply anesthetized) and implanted with either testosterone filled or blank capsules (one 10-mm capsule/100 g of body weight).

RESULTS

Two hundred one Sprague-Dawley rats and 178 F344 rats were used. In gonadally intact adults of both rat strains, the antinociceptive potency of subcutaneously injected morphine was significantly greater in males than in females (P < or = 0.05). These sex differences were eliminated by neonatal castration in males or by neonatal androgenization in females. However, adult testosterone treatment reversed the effects of neonatal castration in males. Masculinization and defeminization of sexual behavior, ovary weight, and body weight generally met conventional expectations. Compared with male controls, neonatally castrated males gained less body weight, and displayed more lordosis behavior and compromised male sexual behaviors. Compared with female controls, neonatally androgenized females gained more body weight, developed smaller ovaries, and presented less lordosis behavior and more male sexual behaviors. Overall, neonatal testosterone manipulations sufficient to masculinize or defeminize rats in terms of reproductive behavior and physiology also masculinized or defeminized morphine antinociceptive sensitivity. The effects of neonatal castration were reversed by adult testosterone treatment, indicating that sexual differentiation of opioid antinociceptive systems begins before PND 1.

CONCLUSIONS

Sensitivity to opioid antinociception begins to diverge between males and females early in life. The relationship between gonadal hormone-mediated sexual differentiation of the reproductive and the opioid antinociceptive systems suggests that the 2 systems may be functionally linked. This finding has implications for the treatment of pain and analgesia in women and men.

Sex differences in opioid analgesia, hyperalgesia, tolerance and withdrawal: central mechanisms of action and roles of gonadal hormones

This article reviews sex differences in opiate analgesic and related processes as part of a Special Issue in Hormones and Behavior. The research findings on sex differences are organized in the following manner: (a) systemic opioid analgesia across mu, delta and kappa opioid receptor subtypes and drug efficacy at their respective receptors, (b) effects of the activational and organizational roles of gonadal steroid hormones and estrus phase on systemic analgesic responses, (c) sex differences in spinal opioid analgesia, (d) sex differences in supraspinal opioid analgesia and gonadal hormone effects, (e) the contribution of genetic variance to analgesic sex differences, (f) sex differences in opioid-induced hyperalgesia, (g) sex differences in tolerance and withdrawal-dependence effects, and (h) implications for clinical therapies.

Sex and gender differences in pain and analgesia

It is a clinical reality that women make up the large majority of chronic pain patients, and there is now consensus from laboratory experiments that when differences are seen, women are more sensitive to pain than men. Research in this field has now begun to concentrate on finding explanations for this sex difference. Although sex differences in sociocultural, psychological, and experiential factors likely play important roles, evidence largely from animal studies has revealed surprisingly robust and often qualitative sex differences at low levels of the neuraxis. Although not yet able to affect clinical practice, the continued study of sex differences in pain may have important implications for the development of new analgesic strategies.

Testosterone is essential for alpha(2)-adrenoceptor-induced antinociception in the trigeminal region of the male rat

Activation of the alpha(2)-adrenoceptor has been shown to produce antinociception. We have previously shown that the antinociceptive effect of clonidine, an alpha(2)-adrenoceptor agonist, is sex-specific and is abolished by exogenous estrogen in ovariectomized rats or high level of endogenous estrogen in proestrous females. Here, we investigated whether testosterone mediates the antinociceptive effect of clonidine in the trigeminal region of the male rat. Clonidine (7 microg/5 microl) was injected intracisternally through a PE-10 cannula implanted dorsal to the trigeminal region in orchidectomized (GDX) male Sprague-Dawley rats. In separate groups, testosterone propionate (250 microg/100 microl; GDX+T) or beta-estradiol benzoate (100 microg/100 microl; GDX+E) were injected subcutaneously 24 and 48 h respectively prior to the N-methyl-D-aspartic acid (NMDA)--or heat-evoked nociceptive test. NMDA-induced number of scratches or duration of scratching behavior did not change significantly in control groups with or without hormonal replacement. Clonidine significantly reduced both measures only in the GDX+T group but not in GDX or GDX+E group. Clonidine also significantly increased head withdrawal latency (HWL) in the GDX+T group, but not in GDX or GDX+E group. The antinociceptive effect of clonidine was reversed by yohimbine, an alpha(2)-adrenoceptor antagonist, in GDX+T group. We conclude that testosterone is required for the expression of antinociception produced by selective activation of the alpha(2)-adrenoceptor in the trigeminal region of the male rat. These findings further our understanding of sex-related differences in the modulation of nociception and may provide insight into development and administration of analgesic agents in young vs. aging men.

The organizational hypothesis and final common pathways: Sexual differentiation of the spinal cord and peripheral nervous system

In honor of the 50th anniversary of the "organizational hypothesis," this paper reviews work on sexual differentiation of the spinal cord and peripheral nervous system. Topics considered include the spinal nucleus of the bulbocavernosus, the ejaculation center, the cremaster nucleus, sensory and autonomic neurons, and pain. These relatively simple neural systems offer ample confirmation that early exposure to testicular hormones masculinizes the nervous system, including final common pathways. However, I also discuss findings that challenge, or at least stretch, the organizational hypothesis, with important implications for understanding sex differences throughout the nervous system.

Sex-specific responses to opiates: animal and human studies

It is widely reported that analgesic drugs acting at mu, kappa, and delta opioid-receptors display quantitative and qualitative differences in effect in males and females. These sex-related differences are not restricted to the analgesic/antinociceptive properties of opioids, but are also present in opioid-induced side effects, such as changes in respiration, locomotor activity, learning/memory, addiction, and changes in the cardiovascular system. An increasing number of well-controlled animal and human studies directly examining the issue of sex in the potency of opioids show that, although sex may affect opioid analgesia, the direction and magnitude of sex differences depend on many interacting variables. These include those specific to the drug itself, such as dose, pharmacology, and route and time of administration, and those particular to the subject, such as species, type of pain, genetics, age, and gonadal/hormonal status. In the current review, we systematically present these animal and human studies and discuss the data in relation to the depending variables. Although the observed sex differences in opioid effect may be clinically relevant, lack of knowledge on other factors involved in the large variability in patient opioid analgesic sensitivity should compel practitioners to customize their dosing regimens based on individual requirements.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Modulation of pain by estrogens

It has become increasingly apparent that women suffer a disproportionate amount of pain during their lifetime compared to men. Over the past 15 years, a growing number of studies have suggested a variety of causes for this sex difference, from cellular to psychosocial levels of analysis. From a biological perspective, sexual differentiation of pain appears to occur similarly to sexual differentiation of other phenomena: it results in large part from organizational and activational effects of gonadal steroid hormones. The focus of this review is the activational effects of a single group of ovarian hormones, the estrogens, on pain in humans and animals. The effects of estrogens (estradiol being the most commonly examined) on experimentally induced acute pain vs. clinical pain are summarized. For clinical pain, the review is limited to a few syndromes for which there is considerable evidence for estrogenic involvement: migraine, temporomandibular disorder (TMD) and arthritis. Because estrogens can modulate the function of the nervous, immune, skeletal, and cardiovascular systems, estrogenic modulation of pain is an exceedingly complex, multi-faceted phenomenon, with estrogens producing both pro- and antinociceptive effects that depend on the extent to which each of these systems of the body is involved in a particular type of pain. Forging a more complete understanding of the myriad ways that estrogens can ameliorate vs. facilitate pain will enable us to better prevent and treat pain in both women and men.