Sex differences in the behavioural response to persistent pain in rats

The role of female gonadal hormones in behavioral sex differences in persistent and chronic pain: Clinical versus preclinical studies

Clinical and preclinical studies have found sex-specific differences in the discrimination and perception of nociceptive stimuli. This article reviews the current literature concerning the biological basis of sex differences in the behavioral response to persistent inflammatory and chronic pain stimuli. The emerging picture from both clinical and preclinical studies suggests that the basis of these differences in nociceptive responses to such stimuli resides in the regulatory activity of gonadal hormones in the central nervous system. Published reports suggest that pain management targeted at female patients should consider hormonal factors during the female reproductive cycle.

Estradiol administration mediates the inflammatory response to formalin in female rats

Female rats demonstrate higher pain sensitivity than do males in various nociceptive assays of inflammation. In the present study, we found that estradiol (20%) replacement in ovariectomized rats attenuated the chronic phase of the formalin response but only at high formalin concentrations thought to rely on peripheral inflammation. An inactive isomer of estradiol, alpha-estradiol, failed to result in the same attenuation (P > 0.05). Our results suggest that estradiol's actions in inflammatory responses are mediated through genomic estrogen receptor-mediated mechanisms.

Nociceptive sensitivity and opioid antinociception and antihyperalgesia in Freund's adjuvant-induced arthritic male and female rats

The present study was designed to examine sex differences in complete Freund's adjuvant (CFA)-induced mechanical hyperalgesia and sex differences in opioid antinociception and anti-hyperalgesia. Female rats developed inflammation and hyperalgesia faster and exhibited greater peak hyperalgesia than male rats. In arthritic (CFA-treated) rats, lower thresholds were observed during estrus and proestrus, and in nonarthritic (vehicle-treated) rats, lower thresholds were observed during proestrus. Morphine and oxycodone were more potent in male than female arthritic rats, and butorphanol was more potent and effective in male than female arthritic rats. The potency of morphine was increased in arthritic rats, although to a greater magnitude in males. The potency of oxycodone was increased in male but not female arthritic rats. The potency of butorphanol was increased in arthritic male rats and the maximal antinociceptive effect of butorphanol was increased in arthritic female rats, but it did not result in greater than 20% antinociception. Morphine, oxycodone, and butorphanol all produced antihyperalgesic effects (returning thresholds of arthritic rats to the thresholds of nonarthritic rats) with greater potency in males than females. The peripherally acting opioid agonist loperamide produced intermediate levels of antinociception in male and female arthritic rats and no antinociception in nonarthritic rats. Loperamide was more potent in male than female arthritic rats at producing antihyperalgesia. These data demonstrate sex differences in arthritis-induced hyperalgesia and responsiveness to opioid analgesics. In arthritic rats, the antinociceptive effects of opioid agonists are most probably mediated by both central and peripheral opioid receptors, whereas their antihyperalgesic effects are mediated primarily by actions at peripheral opioid receptors.

Estrogen and mu-opioid receptor antagonists counteract the 17 beta-estradiol-induced licking increase and interferon-gamma reduction occurring during the formalin test in male rats

Women have a higher incidence of chronic pain syndromes than men and are generally more sensitive to experimental pain. Numerous studies have shown that the female gonadal hormones, estrogens, can profoundly affect the nervous and immune systems, including mechanisms involved in pain and nociception. In the present study, we used antagonists of estrogen receptors (ER) or mu-opioid receptors (mu OR) to evaluate the effects of estrogens on formalin-induced behavioural and immune responses in male rats. After two days of priming with 17 beta-estradiol or saline (i.c.v.), animals were subjected to the formalin test; 15 min prior to formalin (50 microl, 5%) or sham injection in the hind paw, animals were treated with an ER antagonist (ICI 182,780, ICI) or a mu OR antagonist (beta-funaltrexamine, FNA) or saline. The spontaneous behaviours, pain-related behaviours and interferon-gamma (IFN-gamma) production by peripheral blood mononuclear cells were studied in all groups. We found that central administration of estradiol increased the amount of licking of the formalin-injected paw in the second phase of the formalin test. Whereas ICI and FNA had no effect on pain behaviour in saline-pre-treated animals, both antagonists reversed the estradiol-induced increase in licking. The immune system was differently affected by formalin and estradiol treatment. Indeed, formalin injection per se decreased IFN-gamma production; estradiol had no effect on sham-injected animals but strongly reduce the decrease of IFN-gamma production in formalin-injected animals. The results demonstrate that centrally acting estrogens affect ER- and mu OR-mediated pain processing and influence immune function.

Sex differences in thermal nociception and prostaglandin-induced thermal hypersensitivity in rhesus monkeys

The present study examined thermal nociception in 4 male and 4 female rhesus monkeys. In the first experiment, monkeys were tested 5 days/week for 4 consecutive weeks in a warm-water, tail-withdrawal assay of thermal nociception. Thermal nociception did not vary by sex or menstrual cycle phase. However, male monkeys tended to be slightly more sensitive to thermal stimuli than female monkeys in the follicular phase of the menstrual cycle. Thermal nociception did not correlate with estradiol or progesterone levels in female monkeys. In the second experiment, thermal hypersensitivity was induced by administering prostaglandin E(2) (0.0032 to 0.1 mg subcutaneously) into the tail. Prostaglandin E(2) produced slightly greater thermal hypersensitivity in male than in follicular phase female monkeys, but male and luteal phase female monkeys did not differ, and there was not a significant difference between follicular and luteal phase female monkeys. Exposure to the behavioral procedures produced similar increases in blood levels of the stress-related hormones adrenocorticotropic hormone and cortisol in male and female monkeys, which suggests that measures of thermal nociception or thermal hypersensitivity were not confounded by sex differences in stress responses. These results suggest that sex and gonadal hormone levels have only a minor influence on thermal nociception or thermal hypersensitivity in rhesus monkeys.

PERSPECTIVE

These modest effects of sex and gonadal hormone levels on measures of pain in non-human primates could be interpreted to support the hypothesis that sex differences in pain perception in humans are due more to sociocultural factors than to a biological imperative.

Genetic evidence for the correlation of deep dorsal horn Fos protein immunoreactivity with tonic formalin pain behavior

The formalin test is commonly used as a model of persistent pain. Besides producing pain behavior, hind paw formalin injection induces the expression of the immediate-early gene, c-fos. A current controversy is whether noxious stimulus-induced Fos protein immunoreactivity can be considered a proxy (biomarker) of nociception in the spinal cord. We investigated this issue by exploiting our recent demonstration of genotype-dependent behavioral differences in response to formalin injection among inbred mouse strains. Accordingly, 6 inbred and 2 outbred strains were administered formalin (5% in 25 microL) into the ventral hind paw, monitored for licking behavior, and then sacrificed at 90 minutes after injection for Fos protein immunocytochemistry. Significant strain differences were observed in both licking behavior and Fos counts in superficial and deep laminae. We observed a significant correlation among strains between licking behavior in the late phase (10 to 60 minutes) of the formalin test and Fos expression in laminae V-VI (but not laminae I-II) of the dorsal horn (r = 0.94). These findings reinforce the use of the Fos technique to study the neuronal processing underlying pain but suggest that Fos labeling reliably reflects tonic pain behavior only in neurons located in the neck of the dorsal horn in mice.

Hyperalgesic priming in the rat demonstrates marked sexual dimorphism

In male rats, carrageenan (CAR)-induced inflammation or exposure to a selective protein kinase C epsilon (PKC epsilon ) agonist (psi epsilon RACK) produces prolongation of the hyperalgesia induced by a subsequent exposure to an inflammatory mediator, a phenomenon referred to as hyperalgesic priming. Since many chronic inflammatory conditions are sexually dimorphic, we tested the hypothesis that hyperalgesic priming is sexually dimorphic. Prior injection of CAR or psi epsilon RACK produced a prolongation of the hyperalgesia induced by a subsequent injection of prostaglandin E(2), from less than 3 h to greater than 24 h, but only in male rats. In ovariectomized female rats priming with CAR and psi epsilon RACK produced hyperalgesic priming effects similar to that observed in the male rat, and this effect was reversed by estrogen replacement. While gonadectomy in males had no effect on CAR and psi epsilon RACK induced hyperalgesic priming, female phenotype was observed following implantation of estrogen in males. Thus, mechanisms mediating the development of hyperalgesic priming produced by inflammation are suppressed by estrogen. This regulation of priming by estrogen appears to occur at or downstream of the activation of PKC epsilon.

Capsaicin-induced hyperalgesia and mu-opioid-induced antihyperalgesia in male and female Fischer 344 rats

The influence of sex in determining responses to opioid analgesics has been well established in rodents and monkeys in assays of short-lasting, phasic pain. The purpose of this investigation was to use a capsaicin model of tonic pain to evaluate sex differences in hyperalgesia and mu-opioid-induced antihyperalgesia in Fischer 344 (F344) rats. Capsaicin injected into the tail produced a dose-dependent thermal hyperalgesia in males and females, with the dose required to produce a comparable level of hyperalgesia being 3.0-fold higher in males than in females. These sex differences were modulated by gonadal hormones, inasmuch as gonadectomy increased the potency of capsaicin in males and decreased its potency in females. Morphine, buprenorphine, and dezocine administered by various routes [systemic (s.c.), local (in the tail), and central (i.c.v.)] generally produced marked antihyperalgesic effects in males and females. Although in most instances these opioids were equally potent and effective in males and females, selected doses of local and i.c.v. administered buprenorphine produced greater effects in females. When administered locally, the antihyperalgesic effects of morphine were mediated by peripheral opioid receptors in both males and females, since this effect was not reversed by i.c.v. naloxone methiodide. These data contrast with the finding that mu-opioids are more potent in male rodents in assays of phasic pain, thus suggesting that distinct mechanisms underlie male and female sensitivity to opioid antinociception in phasic and tonic pain models. These findings emphasize the need to test male and female rodents in tonic pain assays that may have greater relevance for human pain conditions.

Gonadectomy induces site-specific differences in nociception in rats

High prevalence of chronic orofacial pain in women and its relationship with ovarian states suggest that ovarian hormones may be involved in the control of orofacial nociception. Since the interaction between ovarian hormones and nociception seems more evident in the orofacial area than in many other parts of the body, a possible site specificity of an ovarian hormone effect on nociception was tested in rats. Two nociceptive tests were applied to three groups of male rats (n=46) and three groups of female rats (n=46), that were gonadectomised (n=17), sham-operated (n=15) or intact (n=14). Each rat in each group received a local subcutaneous injection of formalin in the upper lip and in the hindpaw. Upper lip injection resulted in an increased occurrence of upper lip rubbing for more than 45 min and hindpaw injection resulted in an increased occurrence of hindpaw licking for about 1h. The duration of the nociceptive behaviours was measured at 3 months after surgery. No significant difference was found between intact and sham-operated animals. A significant increase (54%) in the upper lip rubbing but not the hindpaw licking was observed in gonadectomised females. No difference was observed in castrated males for upper lip rubbing, but a tendency towards an increased duration (102%) of hindpaw licking was noted. The depletion in gonadal hormones was confirmed 3 months after gonadectomy and after the sacrifice of the animals, by the observed decline in the bone mineral density measured on the femur of 40 rats belonging to the six groups. A role of ovarian hormones was also suggested after immunostaining of oestrogen receptors in the lamina II of Caudalis subnucleus of the trigeminal sensory complex and cervical (C1-C2) spinal dorsal horn. The number of cells expressing oestrogen receptors displayed a small (13.6%) but significant (P=0.037) increase in ovariectomised compared with sham-operated rats. These results suggest that the lack of ovarian hormones induces a site-specific increase in the sensitivity to orofacial nociceptive stimulation, and that an up-regulation of oestrogen receptors in the Caudalis subnucleus and C1-C2 dorsal horn may be one of the factors involved in this effect.

Sex differences in opioid analgesia: "from mouse to man"

BACKGROUND

Numerous experimental studies, conducted primarily over the past 10 years, show that there are sex differences in opioid analgesia. This review summarizes the published literature on sex differences in analgesia produced by acute administration of drugs acting at mu-, kappa-, and delta-opioid receptors, in animals and humans. Additionally, methodological issues in research into opioid sex differences are discussed.

CONCLUSIONS

Procedural variables that may influence the outcome of studies examining sex differences in opioid analgesia include modality and intensity of the noxious stimulus used in the pain test, opioid type (efficacy and selectivity), and experimental design and data analytic techniques. Subject variables that may be important to consider include subject genotype and gonadal steroid hormone state of the subject at the time of analgesia testing. Evidence is provided for multiple mechanisms underlying sex differences in opioid analgesia, including both pharmacokinetic and pharmacodynamic factors. Future research directions are suggested, such as examining sex differences in opioid tolerance development, sex differences in opioid analgesia using models of acute inflammatory pain and chronic pain, and sex differences in effects of opioids other than analgesia, which may limit their therapeutic use.

Stress-induced visceral hypersensitivity in female rats is estrogen-dependent and involves tachykinin NK1 receptors

Hormonal cycling may be related to a higher incidence of pain syndrome in female. As tachykinins are pivotal in stress-induced colonic dysfunction, we investigated whether ovarian steroids influence stress-induced visceral hypersensitivity to rectal distension (RD) in female rats and further, whether this influence involves NK1 receptors. Female Wistar rats, either intact or ovariectomized (OVX), were equipped for abdominal muscle electromyography and submitted to 2-h partial restraint stress (PRS) or sham-PRS. First, the effect of PRS was evaluated in intact rats. Second, abdominal response to RD was recorded in OVX rats treated with either, progesterone, 17beta-estradiol, 17beta-estradiol-plus-progesterone, or vehicle, in both basal and PRS conditions. Third, the NK1 receptor-antagonist, SR140333, was tested in PRS-intact and PRS-OVX rats under 17beta-estradiol or 17beta-estradiol-plus-progesterone treatment. PRS induced visceral hypersensitivity to RD and this effect was prevented by ovariectomy. OVX rats treated with 17beta-estradiol or 17beta-estradiol-plus-progesterone, but not progesterone alone, exhibited visceral hypersensitivity after PRS similar to that of intact rats. Both stress-induced visceral hypersensitivity in intact rats and the hormonally-restored visceral hyper-responsiveness of OVX rats were antagonized by SR140333. It is concluded, therefore, that stress-induced visceral hypersensitivity in female rats is estrogens-dependent and mediated through NK1 receptor activation.

Maternal stress differently alters nociceptive behaviors in the formalin test in adult female and male rats

The long-term effects of restraint stress in Wistar rats during the last week of gestation were investigated on the acute and tonic phases of the specific biphasic nociceptive behavioral response in the formalin test in offspring, females and males, at 90 days. Prenatal stress produced significant changes in formalin-induced pain, which was more pronounced in females as compared to males. The distorted response in females was more at the supraspinal level with an increased intensity of the licking response in both phases as well as with an increased their duration. Results concerning changes of the interphase length indicate the impairments of inhibitory mechanisms in the central nervous system. Furthermore, profound difference in the effects of prenatal stress on the first phase but similarity in these effects on the second phase in females and males are indirect strong support of the view that the second phase in the formalin test can not be mediated by central sensitization alone but greatly depends on signals ongoing from nociceptive primary afferents. Finally, the results obtained in males are important argument in favor of assumption about different mechanisms of acute and tonic pain. Taken together, these studies show that prenatal stress alters nociceptive behaviors in the formalin test in rats at 90 days in a sex-specific manner.

Tonic pain response in mice: effects of sex, season and time of day

Seasonal and diurnal variations in tonic pain reactions were examined in female and male CBA/J mice maintained in a 12/12 dark/light cycle, at controlled temperature and humidity conditions. Animals were injected into the dorsum of one hindpaw with a dilute (20 microl, 1%) formalin solution. Pain-related behaviors were quantified as the time spent licking the injected paw and the number of flinching episodes. The experiments were performed during the first part of the light phase (Light: from 7 to 10 a.m.) or during the first part of the dark phase of the diurnal cycle (Dark: from 7 to 10 p.m.), in two different periods of the year: Spring (March-June) and Winter (November-January). Considering all data, females showed a slightly enhanced licking response, as well as an increase in the time spent in self-grooming, in comparison with males. In Spring, the licking and flinching responses were higher during the Dark phase than during the Light phase. This held for both sexes and for both phases of the behavioral response to formalin injection. By contrast, no significant diurnal variation in pain reactions was found in Winter. These seasonal and diurnal differences were not due to nonspecific changes in motor behavior, inasmuch as locomotor activity and self-grooming showed a different pattern: during the second phase after formalin, self-grooming was higher in the Light period in the experiments performed in Spring, whereas locomotor activity showed no significant seasonal changes. These results show that the behavioral reactions to prolonged noxious input, integrated both at spinal and supraspinal sites, undergo similar seasonal and diurnal variations in both sexes, strengthening the importance of chronobiological factors in the modulation of nociception.

The role of sex hormones on formalin-induced nociceptive responses

Many chronic pain conditions are more frequent in women than in men. This observation suggests that there is a potential role of sex hormones on pain perception. In the present study, we measured nociceptive responses to the formalin test in normal and gonadectomized male and female rats. The nociceptive responses to formalin injection were divided in four phases: acute (phase I), interphase and late phases (phases II and III). Four groups of rats were tested: (a) males (n = 15), (b) females (n = 16), (c) ovariectomized females (OVX) (n = 15) and (d) castrated males (CAST) (n = 15). Females presented significantly more nociceptive responses than males during phase I, interphase and phase II (P < 0.01). They also presented significantly more nociceptive responses than OVX females during the interphase (P < 0.05). CAST males presented significantly more nociceptive responses during the phases I (P < 0.01), II (P < 0.01) and III (P < 0.05) than the male rats. Finally, the responses of CAST males and OVX females were virtually identical, suggesting that the differences recorded between males and females in the formalin test were related to an activational effect of the sex hormones rather than an organizational effect. In conclusion, these results permit the support of the role of sex hormones on the modulation of pain perception. Interestingly, male and female sex hormones seem to act specifically on the different phases of the formalin test, suggesting some specific roles for sex hormones in different pain conditions.

Effects of the essential oil from citrus lemon in male and female rats exposed to a persistent painful stimulation

The ability of olfaction to modulate behavior in mammalian species has repeatedly been demonstrated. Here we tested the properties of the volatile components of lemon essential oil. Male and female rats were allowed to inhale the aroma while experiencing a persistent nociceptive input (50 microl formalin, 5%); in the same animals the c-Fos immunohistochemistry was used to test the degree of neuronal activation of areas belonging to the limbic system. In formalin-treated animals, lemon essential oil decreased licking the injected paw, in both sexes; flinching and flexing were decreased in males and increased in females in the interphase (5-20 min) of the formalin test. Essential oil increased the c-Fos expression in the arcuate n. of the hypothalamus. Essential oil and formalin increased c-Fos in the paraventricular n. of the hypothalamus and in the dentate gyrus of the hippocampus. In the paraventricular n. of the thalamus formalin induced higher c-Fos than control in both sexes; when formalin treatment was carried out in presence of essential oil, c-Fos further increased in males, but remained at control levels in females. The present results clearly indicate the ability of lemon essential oil to modulate the behavioral and neuronal responses related to nociception and pain.

Sex-related differences in mechanical nociception and antinociception produced by mu- and kappa-opioid receptor agonists in rats

Previous studies indicate that in antinociceptive procedures employing thermal, chemical and electrical stimuli, opioids are generally more potent in male than female rodents. The purpose of the present study was to examine nociception and opioid antinociception in male and female rats using a mechanical nociceptive stimulus. Results indicated that males had a higher threshold for nociception, and in tests in which a constant pressure was applied to the hindpaw, the paw withdrawal latencies were consistently longer in males. Opioids with activity at the mu receptor, including levorphanol, morphine, dezocine, buprenorphine, butorphanol and nalbuphine, were generally more potent and/or effective in males. In contrast, sex differences were not consistently observed with the kappa-opioid receptor agonists spiradoline, (5,7,8b)-N-methyl-N[2-1(1-pyrrolidinyl),1-oxaspiro[4,5]dec-8-yl benzeneacetamide (U69593), trans-(+/-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50488), enadoline, ethylketocyclazocine, and nalorphine. These findings suggest that males and females differ in their responsiveness to mechanical nociception and that sex differences in sensitivity to kappa-, but not mu-, opioid receptor agonists are specific to certain nociceptive stimulus modalities.

mu-opioid receptor-mediated antinociceptive responses differ in men and women

Sex differences in the experience of clinical and experimental pain have been reported. However, the neurobiological sources underlying the variability in pain responses between sexes have not been adequately explored, especially in humans. The endogenous opioid neurotransmitters and mu-opioid receptors are centrally implicated in responses to stress, in the suppression of pain, and in the action of opiate analgesic drugs. Here we examined sex differences in the activation of the mu-opioid system in response to an intensity-controlled sustained deep-tissue pain challenge with positron emission tomography and a mu-opioid receptor-selective radiotracer. Twenty-eight young healthy volunteers (14 men and 14 women) were studied during saline control and pain conditions using a double-blind, randomized, and counterbalanced design. Women were scanned during the early follicular phase of their menstrual cycles after ovulatory cycles. Significant sex differences in the regional activation of the mu-opioid system in response to sustained pain were detected compared with saline controls. Men demonstrated larger magnitudes of mu-opioid system activation than women in the anterior thalamus, ventral basal ganglia, and amygdala. Conversely, women demonstrated reductions in the basal state of activation of the mu-opioid system during pain in the nucleus accumbens, an area previously associated with hyperalgesic responses to the blockade of opioid receptors in experimental animals. These data demonstrate that at matched levels of pain intensity, men and women during their follicular phase differ in the magnitude and direction of response of the mu-opioid system in distinct brain nuclei.

Effects of gonadectomy and pain on interferon-gamma production in splenocytes of male and female rats

The role of gonadal hormones and persistent pain (formalin test) in the regulation of interferon-gamma (IFN-gamma) production in splenocytes was investigated in male and female rats. Animals were either sham-operated (Intact) or gonadectomized (GDX) and, 3 weeks later, were subcutaneously injected with formalin (50 microl, 10%) or only pricked with a syringe needle in the dorsal hind paw. Sixty minutes after treatment the animals were deeply anesthetized and the spleens were dissected under aseptic conditions. Blood was collected from the abdominal aorta for measurement of plasma steroids. IFN-gamma production was determined in vitro in the splenocytes after Con A stimulation. Splenocytes of Intact females showed higher IFN-gamma production than those of Intact males. This sex difference disappeared in GDX animals because of the lower levels in GDX females. Formalin decreased IFN-gamma in both Intact and GDX groups. In females, there was a positive correlation between IFN-gamma production in splenocytes and plasma estradiol levels. The present data demonstrate a sex difference in IFN-gamma production (due to the immunostimulating effect of estradiol in females) and an immunodepressive role of pain in both sexes.

Somatic symptom reporting in women and men

Women report more intense, more numerous, and more frequent bodily symptoms than men. This difference appears in samples of medical patients and in community samples, whether or not gynecologic and reproductive symptoms are excluded, and whether all bodily symptoms or only those which are medically unexplained are examined. More limited, but suggestive, literature on experimental pain, symptom reporting in childhood, and pain thresholds in animals are compatible with these findings in adults. A number of contributory factors have been implicated, supported by varying degrees of evidence. These include innate differences in somatic and visceral perception; differences in symptom labeling, description, and reporting; the socialization process, which leads to differences in the readiness to acknowledge and disclose discomfort; a sex differential in the incidence of abuse and violence; sex differences in the prevalence of anxiety and depressive disorders; and gender bias in research and in clinical practice. General internists need to keep these factors in mind in obtaining the clinical history, understanding the meaning and significance that symptoms hold for each patient, and providing symptom relief.

Estrous cycle modulation of nociceptive behaviors elicited by electrical stimulation and formalin

The impact of circulating ovarian hormones on nociceptive behaviors elicited by phasic and tonic stimuli was evaluated in rats using two behavioral tests: an operant escape task and the formalin test. The operant escape task was structured to separately evaluate hindlimb flexion reflexes, the latency of escape, and the amplitude of peak vocalization to a series of phasic electrocutaneous stimuli (0.05-0.8 mA), whereas the formalin test evaluated nociceptive behaviors elicited by tonic stimulation following a subcutaneous injection of dilute formalin (1%). Hindlimb reflex amplitude, escape latency, and peak vocalization varied across the estrous cycle, such that rats were most sensitive to electrical stimuli during proestrus (reflex and escape latency) and diestrus (vocalization). Furthermore, morphine-induced (3 mg/kg sc) attenuation of hindlimb reflex amplitude was sensitive to estrous cycling. During proestrus, morphine produced less attenuation of hindlimb reflex amplitude than during nonproestrus phases. However, estrous cycling did not alter nociceptive behaviors elicited by 1% formalin. These data support the notion that circulating ovarian hormones may differentially modulate behaviors associated with phasic and tonic pain.

Sex hormones regulate the contribution of PKCepsilon and PKA signalling in inflammatory pain in the rat

We have evaluated the contribution of differences in second messenger signalling to sex differences in inflammatory pain and its control by sex hormones. In normal male but not female rats, epinephrine-induced mechanical hyperalgesia was antagonized by inhibitors of protein kinase Cepsilon (PKCepsilon), protein kinase A (PKA) and nitric oxide synthetase (NOS). Similarly, in PKCepsilon knockout mice, a contribution of PKCepsilon to epinephrine-dependent mechanical hyperalgesia occurred in males only. In contrast, hyperalgesia induced by prostaglandin E2, in both females and males, was dependent on PKA and NO. In both sexes, inhibitors of mitogen-activated protein kinase/extracellular-signal related kinase kinase (MEK) inhibited epinephrine hyperalgesia. In gonadectomized females, the second messenger contributions to epinephrine hyperalgesia demonstrated the pattern seen in males. Administration of oestrogen to gonadectomized females fully reconstituted the phenotype of the normal female. These data demonstrate gender differences in PKCepsilon, PKA and NO signalling in epinephrine-induced hyperalgesia which are oestrogen dependent and appear to be exerted at the level of the beta-adrenergic receptor or the G-protein to which it is coupled.

Gender and gonadal hormone effects on vagal modulation of tonic nociception

We studied the influence of gender and gonadal hormones on modulation of tonic nociception exerted by vagal activity. In male rats, subdiaphragmatic vagotomy resulted in significantly reduced nociceptive behavior during phase 2 of the formalin test. Whereas gonadectomy alone had no effect, it completely eliminated the suppressive effect of subdiaphragmatic vagotomy; however, sex hormone replacement with either testosterone or dihydrotestosterone did not restore the ability of subdiaphragmatic vagotomy to suppress nociceptive behavior. These results suggest that, in males, a gonad-dependent but androgenic gonadal hormone-independent mechanism contributes to pronociceptive effects of vagal afferent activity. Although neither gonadectomy nor subdiaphragmatic vagotomy alone affected the response to formalin in females, gonadectomy plus vagotomy resulted in significantly reduced nociceptive behavior during phase 2. Reconstitution with 17 beta-estradiol implants in gonadectomized females not only prevented suppression of nociceptive behavior seen with gonadectomy plus vagotomy, but also led to increased nociceptive behavior in the interphase between phases 1 and 2. However, placement of 17 beta-estradiol implants in gonad-intact females had no effect on formalin-induced nociceptive behavior. The finding that estrogen produced an increase in nociceptive behavior in gonadectomized female rats after vagotomy but not in normal female rats (with intact gonads and subdiaphragmatic vagus) suggests that the interaction between estrogen and nociceptive afferent activity is suppressed by vagal function. In conclusion, a nonandrogenic action of testicular function in male rats and estrogen in females seems to influence the effect of vagal activity on formalin-induced nociceptive behavior.

Somatic symptom reporting in women and men

Women report more intense, more numerous, and more frequent bodily symptoms than men. This difference appears in samples of medical patients and in community samples, whether or not gynecologic and reproductive symptoms are excluded, and whether all bodily symptoms or only those which are medically unexplained are examined. More limited, but suggestive, literature on experimental pain, symptom reporting in childhood, and pain thresholds in animals are compatible with these findings in adults. A number of contributory factors have been implicated, supported by varying degrees of evidence. These include innate differences in somatic and visceral perception; differences in symptom labeling, description, and reporting; the socialization process, which leads to differences in the readiness to acknowledge and disclose discomfort; a sex differential in the incidence of abuse and violence; sex differences in the prevalence of anxiety and depressive disorders; and gender bias in research and in clinical practice. General internists need to keep these factors in mind in obtaining the clinical history, understanding the meaning and significance that symptoms hold for each patient, and providing symptom relief.

The role of gender in fibromyalgia syndrome

Fibromyalgia syndrome (FMS), characterized by widespread pain and tenderness on palpation (tender points), is much more common in women than in men in a proportion of 9:1. Two recent studies have shown important gender differences in various clinical characteristics of FMS. In a community and a clinic sample, women experienced significantly more common fatigue, morning fatigue, hurt all over, total number of symptoms, and irritable bowel syndrome. Women had significantly more tender points. Pain severity, global severity and physical functioning were not significantly different between the sexes, nor were psychologic factors, eg, anxiety, stress, and depression. Gender differences have also been observed in other related syndromes, eg, chronic fatigue syndrome, irritable bowel syndrome, and headaches. The mechanisms of gender differences in these illnesses are not fully understood, but are likely to involve an interaction between biology, psychology, and sociocultural factors.

Progesterone attenuates persistent inflammatory hyperalgesia in female rats: involvement of spinal NMDA receptor mechanisms

The relationship between endogenous gonadal steroid levels and persistent or chronic pain is poorly understood. These studies used an inflammation model to examine the role of the gonadal steroid, progesterone, in the development of persistent pain and hyperalgesia in lactating ovary-intact and ovariectomized rats. The results indicate that constant high plasma levels of progesterone attenuate inflammatory hyperalgesia by a mechanism involving inhibition of N-methyl-D-aspartate receptor activation at the spinal cord level. Since the pattern of high progesterone in lactating rats mimics the progesterone component of the luteal phase of the human menstrual cycle, these findings have significance in persistent or chronic pain conditions that are most prevalent in females.

Sex differences in thermal nociception and morphine antinociception in rodents depend on genotype

It has been appreciated for some time that the sexes can differ in their sensitivity to pain and its inhibition. Both the human and rodent literatures remain quite contentious, with many investigators failing to observe sex differences that others document clearly. Recent data from our laboratory have pointed to an interaction between sex and genotype in rodents, such that sex differences are observed in some strains but not others. However, these studies employed inbred mouse strains and are thus not directly relevant to existing data. We presently examined whether the observation of statistically significant sex differences in nociception and morphine antinociception might depend on the particular outbred rodent population chosen for study. Rats of both sexes and three common outbred strains were obtained from three suppliers (Long Evans, Simonsen; Sprague Dawley, Harlan; Wistar Kyoto, Taconic) and tested for nociceptive sensitivity on the 49 degrees C tail-withdrawal assay, and antinociception following morphine (1-10mg/kg, i.p.). In further studies, three outbred populations of mice (CD-1, Harlan; Swiss Webster, Harlan; Swiss Webster, Simonsen) were bred in our vivarium for several generations and tested for tail-withdrawal sensitivity and morphine antinociception (1-20male, and no significant difference. In a separate study in which the estrous cycle was tracked in female mice, we found evidence for an interaction between genotype and estrous phase relevant to morphine antinociception. However, estrous cyclicity did not explain the observed sex differences. These data are discussed with respect to the existing sex difference and pain literature, and also as they pertain to future investigations of these phenomena.

Gonadectomy and persistent pain differently affect hippocampal c-Fos expression in male and female rats

Hippocampal c-Fos expression was studied in male and female rats after gonadectomy and persistent pain. Three weeks after surgery, animals were sham- or formalin-injected (50 microl, 10%) and placed in a familiar testing apparatus. The formalin-evoked licking, flexing and jerking of the injected paw were recorded for 60 min, c-Fos was determined in the dorsal and ventral hippocampus: dentate gyrus (DG), CA1 and CA3. Gonadectomy induced higher c-Fos in the dorsal DG of both sexes, in all ventral subfields of males and in the ventral CA3 of females. In normal males and females, formalin increased c-Fos in the dorsal DG and in the male ventral subfields. In gonadectomized ones formalin decreased or did not change c-Fos. Gonadectomy induced longer flexing in males and females. These data indicate an important and sex-dependent interaction between gonadal hormones, nociceptive input and neuronal activity in the hippocampus.

Effects of formalin pain on hippocampal c-Fos expression in male and female rats

Immediate early genes are crucial intermediates in a cascade linking membrane stimulation to long-term alterations of neuronal activity. In the present experiment, we performed immunohistochemistry for c-Fos to determine the effects of persistent pain on cells of the hippocampus of male and female rats. Animals were subcutaneously injected with formalin (50 microl, 10%) and perfused: 2 h later, time 2; 24 h later, time 24; 24 h later after 20 min of the open-field test, time 24/OF. Controls were left undisturbed. In control, c-Fos was higher in females than in males in all hippocampal fields. In males at time 2, formalin increased c-Fos in the dentate gyrus (DG) and CA3 fields; at time 24, c-Fos returned to the control level; at time 24/OF, c-Fos was higher than in control in the DG, but not in the other fields. In the formalin-treated females at time 2 and at time 24, c-Fos levels were lower, or tended to be lower, than in control in all hippocampal fields; at time 24/OF, c-Fos levels in the DG were higher than in control and in males. In conclusion, persistent pain had different effects on c-Fos in the hippocampal subfields, depending on the time after treatment and the sex of the subject.

Sex differences in late behavioral response to subcutaneous formalin injection in mice

We investigated sex- or gender-dependent differences in nociception using the formalin test in mice. In addition to typical biphasic responses, a new, late phase (Phase 3) was observed. A local anesthetic QX-314 injected at the end of Phase 2 blocked Phase 3. Phase 3 in female mice was significantly greater than that in male mice regardless the stage of the estrus cycle.

Effects of novelty and pain on behavior and hippocampal extracellular ACh levels in male and female rats

In vivo microdialysis was used to assess the effects of novelty and pain on hippocampal ACh release in male and female rats. Experiments were carried out during the dark phase and consisted of 2 days of tests: on Day 1, after Baseline 1, animals were exposed to a new cage (Novelty) to which, 30 min later, a plastic cylinder (Object) was introduced. On Day 2, after Baseline 2, the Formalin test (50 microl of formalin 10%, s.c. injected in the dorsal hindpaw) was carried out in the animal's home cage. All behaviors were recorded. The extracellular levels of ACh in the dorsal hippocampus were estimated, in 10-min samples, by assay of ACh in the dialysates by HPLC. On Day 1 the raw values of ACh were higher in females than in males, but no sex difference was present when the percentage of change was considered. In both sexes the Novelty and Object tests induced an increase in ACh levels with respect to Baseline. Higher levels of exploration were present in females than males during the first 10 min of Novelty. On Day 2, ACh release increased in both sexes during the Formalin test. No sex difference in either ACh raw values or the percentages of change were found. Females showed higher levels of licking and lower levels of activity than males. The present study shows that novelty and pain induce similar hippocampal cholinergic activation in male and female rats but different behaviors. The results are discussed in light of the several anatomical and functional sex differences present in the hippocampus.

Behavioural and hormonal effects of restraint stress and formalin test in male and female rats

The formalin test was used to measure the analgesia induced by restraint in male and female rats. Animals were restrained for 30 min or left undisturbed in their cage and then (1) killed immediately to collect blood for hormonal determinations; or (2) subcutaneously injected with formalin in the hind paw (or sham-injected), introduced to an open field for recording of behaviour, and killed at the end of this procedure. In both experiments, corticosterone was found to be higher in females. In Experiment 1, the ability of restraint to be stressful was confirmed by the increase in corticosterone in both sexes and by the decrease of testosterone in males. In Experiment 2, restraint-treatment induced a reduction in licking and flexing that was limited to the second phase. The reduction occurred in different periods and to a different degree in the two sexes; it was greater in females. Spontaneous behaviours showed sex differences in restraint-treated but not in formalin-treated animals. The results show that the hormonal effects observed after restraint are not present after the formalin test and that the marked analgesia observed with phasic painful stimuli does not occur with a longer-lasting one such as that induced by formalin, after which only partial and short-lasting effects were observed.

Sex differences in pain-induced effects on the septo-hippocampal system

In addition to its role in the modulation of functions such as arousal and attention, learning and memory, the limbic system has repeatedly been described to be involved in the regulation of several behavioral aspects concerning the adaptation to aversive situations, including pain. A key role in these processes seems to be played by the septo-hippocampal system. This paper, far from being a comprehensive review of all the data available about the limbic system, describes some of the circuits participating in the septo-hippocampal system, with the aim of contributing to an understanding of the sex differences in the behavioral, hormonal and neuronal responses to aversive stimuli. It will appear that the complex anatomical and functional interactions between the different neurotransmitters acting at this level prevent one from indicating a certain substance as more important than others in determining a difference between the two sexes. This leads to the conclusion that the septo-hippocampal formation in toto plays a key role in determining the sex differences in the 'pain experience'.

Identification of a sex-specific quantitative trait locus mediating nonopioid stress-induced analgesia in female mice

It is increasingly appreciated that the sexes differ in their perception of noxious stimuli and in their responsivity to exogenous and endogenous analgesic manipulations. We previously reported the existence of qualitative sex differences in the neurochemical mediation of nonopioid (i.e., naloxone-insensitive) stress-induced analgesia (SIA) produced by forced swims and suggested that female mice possess a sex-specific SIA mechanism. This female-specific system is now known to be estrogen-dependent, to be ontogenetically organized, and to vary with reproductive status; however, its neurochemical identity remains obscure. In an attempt to identify candidate genes underlying SIA in both sexes, we performed a two-phase quantitative trait locus (QTL) mapping experiment using the BXD/Ty recombinant inbred (RI) set derived from DBA/2J (D2) and C57BL/6J (B6) inbred mouse strains and (B6xD2)F2 hybrid mice derived from these same progenitors. All mice were subjected to 3 min forced swims in 15 degrees C water; nociceptive sensitivity on the 54 degrees C hot-plate assay was assessed immediately before and 2 min after cessation of the swim. We report the localization of a QTL statistically associated with SIA magnitude [p = 0.00000012; logarithm of the odds (LOD) = 6.1] in female mice only. This female-specific QTL, which we name Fsia1, is located on chromosome 8 at 52-84 cM from the centromere and accounts for 17-26% of the overall trait variance in this sex. The present data provide further evidence of the existence of a female-specific SIA mechanism and highlight the important role of both genetic background and gender in the inhibition of pain.

Kappa-opioids produce significantly greater analgesia in women than in men

Sex differences in human responses to nociceptive stimuli and painful pathological conditions have generally indicated that women report higher pain levels or exhibit less tolerance than men for given stimulus intensities (reviewed in ref. 1 and 2). However, studies have not evaluated sex differences in analgesic responses. We recently reported that the opioid agonist-antagonist pentazocine, which acts predominantly at kappa-receptors, produced significantly better postoperative analgesia in females than in males in patients who underwent surgery for the removal of their third molars (wisdom teeth). In the current study, we evaluated the hypothesis that this sex difference is a characteristic of kappa-opioid agonism. In order to determine whether there are sex differences associated with kappa-opioid agonism, the analgesic efficacy of two other predominantly kappa-opioid analgesics, nalbuphine and butorphanol; was compared in males and females who underwent surgery for the removal of third molar teeth. We found that both nalbuphine and butorphanol produced significantly greater analgesia in females as compared with males. Considering our earlier findings, we conclude that kappa-opioid analgesia is greater in females than in males, probably reflecting a difference in kappa-opioid-activated endogenous pain modulating circuits.

Formalin-induced changes in adrenocorticotropic hormone and corticosterone plasma levels and hippocampal choline acetyltransferase activity in male and female rats

Formalin (10%) induces higher levels of licking and flexing in female than in male rats, as shown in the present study. In order to ascertain the neural and hormonal modifications that accompany these behavioural differences, we determined the activity of choline acetyltransferase in the hippocampus and the levels of adrenocorticotropic hormone and corticosterone in the plasma. Two concentrations of formalin were used (50 microliters; 0.1% or 10%). Formalin was injected subcutaneously in the dorsal part of the hindpaw, and the animal's behaviour was then recorded for 60 min in a familiar open-field apparatus. Hippocampal choline acetyltransferase activity did not differ between the two genders in controls, while a significant gender difference was present in both formalin-injected groups, with higher levels in females than in males. This was the result of a decrease in males but not in females. In contrast, adrenocorticotropic hormone was increased by both formalin concentrations in females; corticosterone was not affected by treatment in either gender. Results are discussed in the light of the morphological and functional differences between the two genders in the hippocampus and in the hypothalamo-pituitary-adrenal axis.

Gender difference in analgesic response to the kappa-opioid pentazocine

Gender difference in analgesia produced by the kappa-opiate pentazocine was investigated in a model of post-operative dental pain. In a recent study [Gordon et al., Neuroscience, 69 (1995) 345-349.] evaluating interaction between the GABAB agonist baclofen and opiates with respect to postoperative analgesia we found that females receiving pentazocine for the treatment of postoperative pain showed better analgesia than did males receiving similar treatment. To follow-up this result, we evaluated for the effect of gender on analgesia produced by pentazocine administered to participants not receiving another experimental medication. The analgesic response to pentazocine in ten females was compared to that in eight males. All participants were administered pentazocine after undergoing surgery for the removal of impacted third molars. We confirm our previous finding that pentazocine produces significantly greater analgesia in females than in males; no significant difference was observed in analgesia among females in different phases of the menstrual cycle.

Sex-related effects on behaviour and beta-endorphin of different intensities of formalin pain in rats

The effects of two intensities of formalin pain on behaviour and beta-Endorphin (beta-EP) concentration in the brain and pituitary were studied in male and female rats. The animals were familiarized with the Hole-Board apparatus for 3 days, and then, after a subcutaneous injection of formalin (50 microliter, 0.1 or 10%) or Sham-injection (Control) in the hindpaw, they were tested in the Hole-Board for 60 min. Licking, Flexing and Paw-Jerk of the injected limb were recorded. beta-EP concentration was determined in the hypothalamus (HYP), the periaqueductal gray matter (PAG), the anterior pituitary (AP) and the neurointermediate lobe (NIL). Licking and Flexing durations were greater in females than males only with formalin 10%. Sex differences in beta-EP concentration between the Control groups were found in all tissues except the HYP; beta-EP levels were higher in females in the PAG and NIL, but greater in the AP in males. beta-EP concentration increased in males in the HYP and NIL with formalin 10%; in females, a decrease was found in the HYP with formalin 0.1%. The present results suggest that: (a) there are differences between males and females in the responses to formalin pain, and the nature (pattern and duration) of the sex differences varies according to the pain intensity; (b) there are differences in beta-EP concentration between the two sexes in control animals, and male and female rats also exhibit differences in the modifications of beta-EP in response to formalin-induced pain.