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

Menstrual migraine is caused by estrogen withdrawal: revisiting the evidence

OBJECTIVE

To explore and critically appraise the evidence supporting the role of estrogen withdrawal in menstrual migraine.

MAIN BODY

Menstrual migraine, impacting about 6% of reproductive-age women, manifests as migraine attacks closely related to the menstrual cycle. The estrogen withdrawal hypothesis posits that the premenstrual drop in estrogen levels serves as a trigger of migraine attacks. Despite its wide acceptance, the current body of evidence supporting this hypothesis remains limited, warranting further validation. Estrogen is believed to exert a modulatory effect on pain, particularly within the trigeminovascular system - the anatomic and physiologic substrate of migraine pathogenesis. Nevertheless, existing studies are limited by methodologic inconsistencies, small sample sizes, and variable case definitions, precluding definitive conclusions. To improve our understanding of menstrual migraine, future research should concentrate on untangling the intricate interplay between estrogen, the trigeminovascular system, and migraine itself. This necessitates the use of robust methods, larger sample sizes, and standardized case definitions to surmount the limitations encountered in previous investigations.

CONCLUSION

Further research is thus needed to ascertain the involvement of estrogen withdrawal in menstrual migraine and advance the development of effective management strategies to address unmet treatment needs.

Formalin-evoked pain triggers sex-specific behavior and spinal immune response

Mounting evidence shows sex-related differences in the experience of pain with women suffering more from chronic pain than men. Yet, our understanding of the biological basis underlying those differences remains incomplete. Using an adapted model of formalin-induced chemical/inflammatory pain, we report here that in contrast to male mice, females distinctly display two types of nocifensive responses to formalin, distinguishable by the duration of the interphase. Females in proestrus and in metestrus exhibited respectively a short-lasting and a long-lasting interphase, underscoring the influence of the estrus cycle on the duration of the interphase, rather than the transcriptional content of the dorsal horn of the spinal cord (DHSC). Additionally, deep RNA-sequencing of DHSC showed that formalin-evoked pain was accompanied by a male-preponderant enrichment in genes associated with the immune modulation of pain, revealing an unanticipated contribution of neutrophils. Taking advantage of the male-enriched transcript encoding the neutrophil associated protein Lipocalin 2 (Lcn2) and using flow cytometry, we confirmed that formalin triggered the recruitment of LCN2-expressing neutrophils in the pia mater of spinal meninges, preferentially in males. Our data consolidate the contribution of female estrus cycle to pain perception and provide evidence supporting a sex-specific immune regulation of formalin-evoked pain.

Evaluation of the Antinociceptive, Antiallodynic, Antihyperalgesic and Anti-Inflammatory Effect of Polyalthic Acid

Nonsteroidal anti-inflammatory drugs (NSAIDs) are very commonly used, but their adverse effects warrant investigating new therapeutic alternatives. Polyalthic acid, a labdane-type diterpenoid, is known to produce gastroprotection, tracheal smooth muscle relaxation, and antitumoral, antiparasitic and antibacterial activity. This study aimed to evaluate the antinociceptive, antiallodynic, antihyperalgesic and anti-inflammatory effect of polyalthic acid on rats. Moreover, the effectiveness of treating hyperalgesia with a combination of polyalthic acid and naproxen was analyzed, as well as the type of drug-drug interaction involved. Nociception was examined by injecting 1% formalin into the right hind paw and thermal hyperalgesia and inflammation by injecting a 1% carrageenan solution into the left hind paw of rats. Allodynia was assessed on an L5/L6 spinal nerve ligation model. Polyalthic acid generated significant antinociceptive (56-320 mg/kg), antiallodynic (100-562 mg/kg), and antihyperalgesic and anti-inflammatory (10-178 mg/kg) effects. Antinociception mechanisms were explored by pretreating the rats with naltrexone, ODQ and methiothepin, finding the effect blocked by the former two compounds, which indicates the participation of opioid receptors and guanylate cyclase. An isobolographic analysis suggests synergism between polyalthic acid and naproxen in the combined treatment of hyperalgesia.

Sex differences and the endocannabinoid system in pain

Cannabis use has been increasing in recent years, particularly among women, and one of the most common uses of cannabis for medical purposes is pain relief. Pain conditions and response to analgesics have been demonstrated to be influenced by sex, and evidence is emerging that this is also true with cannabinoid-mediated analgesia. In this review we evaluate the preclinical evidence supporting sex differences in cannabinoid pharmacology, as well as emerging evidence from human studies, both clinical and observational. Numerous animal studies have reported sex differences in the antinociceptive response to natural and synthetic cannabinoids that may correlate to sex differences in expression, and function, of endocannabinoid system components. Female rodents have generally been found to be more sensitive to the effects of Δ⁹-THC. This finding is likely a function of both pharmacokinetic and pharmacodynamics factors including differences in metabolism, differences in cannabinoid receptor expression, and influence of ovarian hormones including estradiol and progesterone. Preclinical evidence supporting direct interactions between sex hormones and the endocannabinoid system may translate to sex differences in response to cannabis and cannabinoid use in men and women. Further research into the role of sex in endocannabinoid system function is critical as we gain a deeper understanding of the impact of the endocannabinoid system in various disease states, including chronic pain.

Anti-oxidant, anti-inflammatory and antinociceptive properties of the acetone leaf extract of vernonia amygdalina in some laboratory animals

PURPOSE

Vernonia amygdalina is a medicinal plant of great importance that has its fresh leaves rich in vitamins and salt hence, it is valuable in human diet. The anti-oxidant, anti-inflammatory and analgesic activities of its acetone leaf extract were evaluated in this study to validate its folkloric use.

METHODS

The acetone extract is prepared by dissolving ground plant materials (200g) in 1 L of acetone for 48 h, filtered, and then dried using rotary evaporator before it is used for the pharmacological investigations. Standard phytochemical methods were used to test for the presence of phytoactive compounds in the plant. Acute toxicity was carried out in mice to determine safe doses for use. The anti-inflammatory activities were conducted using carrageenan and histamine to induce oedema in rats while analgesic activities were embarked upon using acetic acid- induced writhing test and formalin-induced paw lick test. The anti-oxidant activities were assessed in vitro using ABTS, DPPH, FRAP and total polyphenolics.

RESULTS

The results from this study showed that the 100 and 200 mg/kg doses of the acetone extract caused significant reduction in oedema induced by both carrageenan and histamine. Similar effect was observed in analgesic tests which were comparable to that of indomethacin, the reference drug used in the study.

CONCLUSION

The anti-oxidant effects were also good and the pharmacological activities may be due to the presence of polyphenols and other phytochemicals contained in the plant. The study may have thus validated the folkloric use of this plant as a medicinal and nutritional agent.

Participation of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain in rats

Metabotropic P2Y receptors subfamily consists of eight functional mammalian receptors. Specifically, P2Y1, P2Y6 and P2Y11 receptors have been described in the sensory nervous system, but their participation, at peripheral level, in behavioral pain models is scarcely understood. This study assessed the role of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain. Ipsilateral, but not contralateral peripheral pre-treatment with the endogenous P2Y1 (ADP, 100-1000nmol/paw), P2Y6 (UDP, 180-300nmol/paw) and P2Y11 (ATP, 100-1000nmol/paw), or selective P2Y1 (MRS2365, 0.1-10nmol/paw), P2Y6 (PSB0474, 0.1-0.10pmol/paw) and P2Y11 (NF546, 0.3-3nmol/paw) receptor agonists increased 0.5% formalin-induced flinching behavior. Concordantly, peripheral pre-treatment with the selective P2Y1 (MRS2500, 0.01-10pmol/paw), P2Y6 (MRS2578, 3-30nmol/paw) and P2Y11 (NF340, 1-10nmol/paw) receptor antagonists significantly decreased 1% formalin-induced flinching behavior. Furthermore, the pronociceptive effect of ADP (100nmol/paw) or MRS2365 (10nmol/paw), UDP (300nmol/paw) or PSB0474 (10pmol/paw) and ATP (1000nmol/paw) or NF546 (3nmol/paw) was blocked by the selective P2Y1 (MRS2500, 0.01nmol/paw), P2Y6 (MRS2578, 3nmol/paw), and P2Y11 (NF340, 1nmol/paw) receptor antagonists, respectively. Western blot analysis confirmed the presence of P2Y1 (66kDa), P2Y6 (36kDa) and P2Y11 (75kDa) receptors in dorsal root ganglia (DRG) and sciatic nerve. Results suggest that peripheral activation of P2Y1, P2Y6 and P2Y11 receptors plays a pronociceptive role in formalin-induced pain.

The transition from acute to chronic pain: understanding how different biological systems interact

PURPOSE

Although pain is an adaptive sensory experience necessary to prevent further bodily harm, the transition from acute to chronic pain is not adaptive and results in the development of a chronic clinical condition. How this transition occurs has been the focus of intense study for some time. The focus of the current review is on changes in neuronal plasticity as well as the role of immune cells and glia in the development of chronic pain from acute tissue injury and pain.

PRINCIPAL FINDINGS

Our understanding of the complex pathways that mediate the transition from acute to chronic pain continues to increase. Work in this area has already revealed the complex interactions between the nervous and immune system that result in both peripheral and central sensitization, essential components to the development of chronic pain. Taken together, a thorough characterization of the cellular mechanisms that generate chronic pain states is essential for the development of new therapies and treatments. Basic research leading to the development of new therapeutic targets is promising with the development of chloride extrusion enhancers. It is hoped that one day they will provide relief to patients with chronic pain.

CONCLUSIONS

A better understanding of how chronic pain develops at a mechanistic level can aid clinicians in treating their patients by showing how the underlying biology of chronic pain contributes to the clinical manifestations of pain. A thorough understanding of how chronic pain develops may also help identify new targets for future analgesic drugs.

The effect of social stress on chronic pain perception in female and male mice

The current investigations on social stress primarily point to the negative health consequences of being in a stressful social hierarchy. The repetitive nature of such stressors seems to affect behavioral response to pain both in rodents and humans. Moreover, a large discrepancy in the possibility of social stresses affecting pain perception in the two genders exists. The present study examined the effect of chronic social stress on nociceptive responses of both sexes by implementing of food deprivation, food intake inequality and unstable social status (cage-mate change every 3 days) for a period of 14 days in 96 Balb/c mice. In this regard we injected 20 µl formalin 2% into the plantar surface of hind paw at the end of stress period and scored pain behaviors of all subjects, then serum concentrations of proinflammatory cytokines were measured. Our results showed that there was significant difference in chronic phase of formalin test following implementation of food deprivation and inequality (P<0.05) as compared to control group, so that pain perception was decreased considerably and this decline in inequality exposed subjects was well above isolated ones (P<0.05); whereas unstable social situation did not affect pain perception. Moreover, IL-1 and IL-6 concentrations in serum of stressed mice of both genders were well above control group (p<0.05). Finally, despite chronic pain perception in control and unstable male subjects was larger than females; the decrease of chronic pain perception in male stressed animals (poverty and inequality experienced subjects) was much more than stressed females. These results revealed that although food deprivation and social inequality can induce hypoalgesia, some socioeconomic situations like social instability don't affect pain sensation, whereas there were similar increases of proinflammatory cytokines level in all socially stressed subjects. In addition, males display larger hypoalgesic responses to inequality as compared with females.

Influence of estrogen levels on thermal perception, pain thresholds, and pain tolerance: studies on women undergoing in vitro fertilization

We examined the relationship between estrogen and pain in women undergoing in vitro fertilization (IVF). Quantitative sensory tests (QST) were performed twice during the IVF-regimen: once during hormonal down-regulation and once during hormonal up-regulation. A group of healthy men and a group of women using monophasic contraceptives were also examined, to control for session-to-session effects. Among the women undergoing IVF, serum 17β-estradiol levels differed strongly between treatments as expected, and increased from 65.7 (SD = 26) pmol/L during the down-regulation phase, to 5,188 (SD = 2,524) pmol/L during the up-regulation phase. Significant outcomes in the QST were only seen for temperature perception thresholds (1.7 °C versus 2.2 °C; P = .003) and cold pain threshold (11.5 °C versus 14.5 °C; P = .04). A similar change in cold pain threshold was also seen in the 2 control groups, however, and statistical analysis suggested that this change was due to a session-to-session effect rather than being the result of hormonal modulation. Heat pain thresholds, heat tolerance, pressure pain thresholds, and the cold pressor test showed no significant differences between sessions. These data demonstrate that pain perception and pain thresholds in healthy women show little, if any, changes even with major variations in serum estradiol levels.

PERSPECTIVE

This study shows that pain perception and tolerance in women undergoing in vitro fertilization do not vary, despite the dramatic changes in 17β-estradiol levels induced by the treatment regimen. The result thus suggests that in humans, contrary to experimental animals, changes in estrogen levels have little influence on pain sensitivity.

Sex Differences in Pain Responses at Maturity Following Neonatal Repeated Minor Pain Exposure in Rats

There is mounting evidence of long-lasting changes in pain sensitivity in school-age children who were cared for in a neonatal intensive care unit. Such care involves multiple pain exposures, 70% of which are accounted for by heel lance to monitor physiological well-being. The authors sought to model the repeated brief pain resulting from heel lance by administering repeated paw needle stick to neonatal rat pups. Repeated needle stick during the first 8 days of life was sex-specific in altering responses to mechanical and inflammatory stimuli, but not to a thermal stimulus, at maturity. Specifically, neonatal paw needle stick males exhibited significantly greater mechanical sensitivity in response to von Frey hair testing, whereas neonatal paw needle stick females exhibited significantly greater pain behavior scores following hindpaw formalin injection. This is the first study to show such sex-dependent changes in pain responsiveness at maturity in animals having experienced repeated neonatal needle stick pain. These findings support existing evidence that there are long-term sensory sequelae following neonatal pain experiences in rats and further suggest that there are sex-linked differences in the nature of the consequences. If these relationships hold in humans, these findings suggest that even mild painful insults early in life are not without sensory consequences.

Vulnerability factors in anxiety determined through differences in active-avoidance behavior

The risk for developing anxiety disorders is greater in females and those individuals exhibiting a behaviorally inhibited temperament. Growth of behavioral avoidance in people is a significant predictor of symptom severity in anxiety disorders, including post-traumatic stress disorder. Using an animal model, our lab is examining how the process of learning avoidant behavior may lead certain individuals to develop anxiety. Here we examined whether the known vulnerabilities of female sex and behaviorally inhibited temperament have individual or additive effects upon the acquisition of an active-avoidance response. A discrete trial lever-press escape-avoidance protocol was used to examine the acquisition of behavioral avoidance in male and female Sprague-Dawley (SD) rats and behaviorally inhibited inbred Wistar-Kyoto (WKY) rats. Overall, WKY rats of both sexes were indistinguishable in their behavior during the acquisition of an active-avoidance response, exhibiting quicker acquisition of reinforced responses both between and within session compared to SD rats. Further WKY rats emitted more non-reinforced responses than SD rats. Sex differences were evident in SD rats in both the acquisition of the reinforced response and the emission of non-reinforced responses, with SD females acquiring the response quicker and emitting more non-reinforced responses following lever presses that led to an escape from shock. As vulnerability factors, behavioral inhibition and female sex were each associated with more prevalent reinforced and non-reinforced avoidant behavior, but an additive effect of these 2 factors was not observed. These data illustrate the importance of genetics (both strain and sex) in the assessment and modeling of anxiety vulnerability through the acquisition of active-avoidance responses and the persistence of emitting those responses in periods of non-reinforcement.

The effects of acute restraint stress on nociceptive responses evoked by the injection of formalin into the temporomandibular joint of female rats

The temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute restraint stress on the nociceptive behavioral responses of female rats during proestrus and estrus phases of the estrous cycle. Rats were subjected to one session of restraint stress (15, 30 min or 1 h). They were then either immediately killed to allow the collection of blood for hormonal radioimmunoassay determinations or subjected to TMJ formalin test to evaluate nociception. All stress protocols significantly raised the plasma concentrations of corticosterone. The performance of rats subjected to 15 and 30 min of restraint stress was similar to that of control rats, whereas rats that were stressed for 1 h showed a decrease in nociceptive responses, during both proestrus and estrus phases. The stress-induced analgesia (SIA) was greater in the proestrus phase. To evaluate the role of kappa-opioid receptors, the selective receptor kappa-opioid antagonist nor-binaltorphimine (nor-BNI; 200 microg or saline) was injected into the TMJ 24 h prior to the 1 h stress period and the TMJ formalin test. The local administration of nor-BNI partially reversed the SIA during the proestrus phase. These findings suggest that (1) acute stress for 1 h can produce analgesia both during proestrus and estrus phases; this effect is greater during the proestrus phase and (2) kappa-opioid receptor activation is involved in the SIA observed in the proestrus phase.

Menstrual cycle phase does not influence gender differences in experimental pain sensitivity

Influence of menstrual cycle phase on experimental pain sensitivity in women and on gender differences in pain sensitivity was examined in 48 men and 49 women in response to cold pressor, heat, and ischemic pain. Each woman was tested at three points in their menstrual cycle in randomized order, the early follicular, late follicular, and luteal phases, while men were also tested three times, controlling for number of days between test sessions. Cycle phase was confirmed via serum hormone levels. As expected, women were significantly more sensitive to cold pain (p<.01), to heat pain (p<.0001), and to ischemic pain (p<.01) than men. However, pain perception during each task was not influenced by the menstrual cycle in women, nor did the menstrual cycle influence the magnitude of the gender differences in pain sensitivity. These results indicate that although women are more sensitive to a variety of noxious stimuli than men, menstrual cycle phase does not appear to moderate those differences in healthy men and women.

The influence of sex and ovarian hormones on temporomandibular joint nociception in rats

The aim of this study was to investigate the influence of sex and ovarian hormones on formalin- and glutamate-induced temporomandibular joint (TMJ) nociception in rats. The influence of sex and ovarian hormones on the nociceptive behavior induced by formalin or glutamate was virtually the same. The nociceptive behavior of males was similar to that of females in the proestrus phase of the estrous cycle but was significantly lower than that in the diestrus phase. Since the serum level of estradiol but not of progesterone was significantly higher in the proestrus than in the diestrus phase, these data suggest that females with lower endogenous serum level of estradiol have an exacerbation of TMJ nociception. The nociceptive behavior of ovariectomized rats was similar to that of diestrus females and significantly greater than that of proestrus females. Although the administration of estradiol or progesterone in ovariectomized females significantly reduced TMJ nociception, the combination of both hormones did not increase the antinociceptive effect induced by each of them. These findings suggest that estradiol and progesterone decrease TMJ nociception in an independent way.

PERSPECTIVE

We report that ovarian hormones have an antinociceptive effect on the TMJ formalin and glutamate nociceptive behavior models. Therefore, the greater prevalence and severity of TMJ pain in women of reproductive age may be a consequence of hormonal fluctuation during the reproductive cycle, in that during low endogenous estradiol serum level TMJ pain sensitivity is increased, enhancing the risk of females experiencing TMJ pain.

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.

Inactivation of the periaqueductal gray attenuates antinociception elicited by stimulation of the rat medial preoptic area

The medial preoptic area (MPOA) is a sexually dimorphic structure that plays key roles in gonado-steroidal regulation and thermoregulation. The MPOA may be involved in sex-based differences in nociceptive processing and steroid hormones effect on pain thresholds. Consistent with this, there is evidence that MPOA can produce antinociception or hyperalgesia. MPOA stimulation inhibits spinal cord or trigeminal neuronal responses to noxious stimuli or produces analgesia, yet most of these studies utilized electrical stimulation which antidromically activates periaqueductal gray (PAG) and rostroventromedial medulla (RVM) neurons involved in descending modulation of nociception. Effects of selective activation of MPOA neurons on behavioral indices of antinociception and the site-specificity of such responses are unknown. To address these questions, we examined the influence of MPOA microinjections of d,l homocysteate (DLH) on hindlimb and tail nocifensive reflexes in lightly anesthetized rats. DLH, but not saline, microinjections into several MPOA subregions markedly increased withdrawal response latencies to noxious thermal stimuli. Antinociceptive effects of MPOA activation were abolished by microinjection of lidocaine into PAG. These results suggest that activation of MPOA neurons produces antinociception that is at least partly mediated by projections to PAG.

Maternal separation leads to persistent reductions in pain sensitivity in female rats

We determined responses to noxious thermal stimuli, before and after morphine, and mu-opioid receptor binding in brain regions involved in nociception in maternally separated (MS), neonatally handled (H) and nonhandled (NH) female rats. Long-Evans dams were randomly assigned to either 180-minute (MS) or 15-minute (H) minute daily separations from their litters or left undisturbed (NH). At 120 days of age, paw lick latency (50 degrees C hot plate) was determined in offspring during diestrous. Rats were then given 1, 2, 5, or 10 mg/kg morphine and paw lick latency was measured. Rats were killed during diestrous and mu-opioid receptor binding was determined in discrete brain regions, using [(3)H]DAMGO autoradiography. MS rats had significantly longer (P < .05) paw lick latencies compared with H rats. The percent maximal possible effect of morphine was significantly (P < .05) lower in MS compared with H rats for the 5 mg/kg dose. Mu-Opioid receptor binding capacity was significantly greater (P < .05) in MS rats compared with H rats in the medial preoptic nucleus. In conclusion, MS and H treatments led to antipodal differences in pain sensitivity in female rats and differential mu-opioid receptor binding in the medial preoptic nucleus.

PERSPECTIVE

This article describes the persistent impact of early life adversity on pain sensitivity and the analgesic potency of morphine. Clinically, early life history may play an important role in pain symptoms and responses to opioid analgesics.

Sex differences in opioid-mediated pain inhibitory mechanisms during the interphase in the formalin test

Many chronic pain conditions are more prevalent in women than men and both fundamental and clinical research supports the implication of endogenous pain inhibitory mechanisms. The goal of this study was to verify if sex differences on endogenous pain inhibitory mechanisms during the formalin test are opioidergic and modulated by sex hormones. Formalin tests were performed with naloxone hydrochloride, a non-selective opioid antagonist in intact and gonadectomized Sprague-Dawley rats of both sexes. Considering the sexual dimorphisms we found, where naloxone preferentially blocked the interphase in female rats, injections of all the possible combinations of mu- (naltrexone hydrochloride), delta- (naltrindole hydrochloride) and kappa-selective antagonists (norbinaltorphimine dihydrochloride) were given to evaluate the contribution of these opioid-receptor subtypes to the inhibitory mechanism during the interphase in intact females. Finally, the systemic administration of naloxone methiodide and intrathecal administration of naloxone hydrochloride in intact females allowed us to verify if the action of endogenous opioids that are liberated during the interphase takes place at the periphery or spinally, respectively. The results show that the interphase was almost completely inhibited by naloxone in females while it produced only a slight blockade in males. These results permitted us to conclude that opioids play a major role in the pain inhibitory mechanism of the interphase in females while a non-opioid mechanism seems to be responsible for this inhibitory pathway in males. Using gonadectomized animals of both sexes, we demonstrated the modulation of the opioidergic system of the interphase by sex hormones. The administration of different combinations of selective antagonists for mu-, kappa- and delta-opioid receptors in intact females permitted us to conclude that only the combination of kappa- and delta-selective antagonists significantly blocked the interphase. The same result was obtained with the combination of the three antagonists, confirming the results with systemic naloxone hydrochloride. Finally, intrathecal administration permitted us to support that the action of naloxone is primarily at the spinal level, even if a supraspinal action cannot be ruled out. These results are of particular interest in showing sexual dimorphisms in endogenous pain modulation mechanisms during the interphase of the formalin test. A clearer understanding of the difference between male and female endogenous pain inhibitory pathways should lead to a better understanding of the role of endogenous pain modulation deficits in certain chronic pain conditions.

Analgesia and hyperalgesia from CRF receptor modulation in the central nervous system of Fischer and Lewis rats

This study examines the contribution of central corticotropin-releasing factor (CRF) to pain behavior. CRF is the principal modulator of the hypothalamo-pituitary-adrenal (HPA) axis, in addition to acting on many other areas of the central nervous system. We compared nociceptive thresholds (heat and mechanical) and pain behavior in response to a sustained stimulus (formalin test) between Fischer and Lewis rats that have different HPA axis activity. Intracerebroventricular (i.c.v.) administration of CRF produced dose-dependent antinociception at a lower dose in Lewis (40 ng, paw pinch 71+/-0 g) compared to Fischer rats (200 ng, 112+/-3 g). The antinociceptive effect of CRF was mostly preserved in adrenalectomized Fischer rats. The i.c.v. administration of the CRF receptor antagonist, astressin, had a hyperalgesic effect, suggesting that CRF is tonically active. Lewis rats required higher doses of astressin (5 ng, paw pinch 51+/-1 g) to show nociceptive effects compared to Fischer rats (1 ng, 79+/-1 g). Only Lewis rats vocalized during mechanical stimulus, and this behavior was prevented by diazepam or morphine but was worsened by CRF, despite its antinociceptive property. In the formalin test, CRF and astressin had the largest effect on the interphase suggesting that they act on the endogenous pain inhibitory system. CRF also increased anxiety/fear-like behaviors in the forced swim and predator odor tests. Our results establish that central CRF is a key modulator of pain behavior and indicates that CRF effects on nociception are largely independent of its mood modulating effect as well as its control of the HPA axis.

Specificity of female and male sex hormones on excitatory and inhibitory phases of formalin-induced nociceptive responses

Several factors have been proposed to account for the differences observed between men and women in pain perception. One of these is female and male gonadal hormones. In order to verify this assumption, a hormone replacement (pellets inserted subcutaneously) of (1) 17beta-estradiol, (2) progesterone, (3) 17beta-estradiol + progesterone or (4) testosterone have been performed in gonadectomized female and male Sprague-Dawley rats. Twenty-one days after the hormonal replacement, a formalin test was performed. The nociceptive responses were divided in three distinct phases: acute (phase I), inhibitory (interphase) and tonic (phase II). After analysis, we observed that testosterone has a hypoalgesic effect on phases I and II of the formalin test. At the opposite, female hormones act only on the interphase: the combination of 17beta-estradiol and progesterone in gonadectomized rats reestablishes the weaker nociceptive pain reduction during the interphase as it is observed in the intact female. These effects were not gender specific since they had the same action in female and male. Our results permit to believe that testosterone plays a protective role in pain perception. Moreover, the female hormones act mainly on pain inhibition mechanisms (interphase), suggesting that the prevalence of certain chronic pain conditions in women could be related to a deficit of these pain inhibitory mechanisms rather than an increased nociceptive activity.

Influence of estrous cycle and gonadal hormone depletion on nociception and opioid antinociception in female rats of four strains

Evidence suggests that gonadal hormones can modulate sensitivity to nociceptive stimuli and opioid antinociception. However, cross-study comparisons addressing the nature of this modulation have been complicated by a number of methodologic factors, including the use of different rodent strains and opioids. The present study examined the influence of estrous cycle and gonadal hormone depletion (ovariectomy) on thermal nociception and opioid antinociception in female F344, Lewis, Long Evans, and Wistar rats. Estrous cycle-dependent differences in nociceptive sensitivity were not observed in any of the strains. Ovariectomy decreased nociceptive sensitivity relative to their intact female counterparts. In normal cycling females, morphine and buprenorphine were generally most potent in metestrus and proestrus and least potent in estrus. The magnitude of these differences was consistently larger with buprenorphine. Ovariectomy increased the antinociceptive potency of morphine and buprenorphine, with this effect also being larger with buprenorphine. These data suggest that in adult females of a number of rat strains, estrous cycle and gonadal hormone depletion modulate the antinociceptive potency of opioids, with the magnitude of this effect being dependent on the type of opioid. In contrast, depletion of gonadal hormones, but not estrous cycle, modulates thermal nociceptive sensitivity in adult female rats.

PERSPECTIVE

Gonadal hormones influence opioid antinociception, and this effect is apparent across different genetic backgrounds. These results suggest that the phase of the menstrual cycle might alter the effectiveness of certain opioids administered to relieve pain in women.

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.

Sexual dimorphism in the antinociception mediated by kappa opioid receptors in the rat temporomandibular joint

This study assessed the effect of the kappa opioid receptor agonist U50,488 administered into the rat temporomandibular joint (TMJ) on nociceptive behavioral responses evoked by formalin injected into the same site. Groups consisted of females, stratified into proestrus and diestrus phases of the estrous cycle, and males. Intra-TMJ formalin induced significantly different dose-dependent responses among the three groups, with diestrus females showing greater responses than males or proestrus females; therefore, equi-nociceptive formalin doses were chosen to test the effects of U50,488. U50,488 significantly reduced formalin-induced nociceptive behavior in all groups, but the reduction was significantly greater in females, especially those in diestrus. Pre-injection of the selective kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) into the same site significantly attenuated the effect of U50488; U50,488 injection into the contralateral TMJ failed to reduce nociceptive behavior. These findings support a role for kappa opioid receptors local to the site of inflammation to modulate inflammatory pain. Furthermore, since plasma levels of ovarian hormones are low during diestrus, these findings are consistent with the suggestion that sex hormones may play an antagonistic role in these peripheral kappa-mediated effects.

Estrous cycle stage-dependent expression of acute tolerance to morphine analgesia in rats

Both baseline pain sensitivity and the response to antinociceptive treatment are sensitive to an animal's sex and estrous cycle stage. Sex differences are also observed in the development of antinociceptive tolerance induced by repetitive exposure to opiate drugs such as morphine. Conventional tolerance study protocols do not assess the impact of the estrous cycle stage. The present study aimed to compare the development of acute tolerance to morphine-induced antinociception in male and female (cycling and ovariectomized) Wistar rats using the tail-flick test. Acute tolerance was induced by two consecutive subcutaneous injections of morphine (10 mg/kg) or saline separated by an interval of 6 h. It was found that rats pretreated with morphine were tolerant to the second morphine dose. Tolerance was most pronounced in proestrous female rats and, to a lesser degree, in male rats. It was absent in ovariectomized rats as well as during the estrus, metestrus and diestrus phases. Thus, the estrous cycle exerts dramatic effects on the induction of acute tolerance to morphine-induced antinociception. These results suggest that pain management strategies can be optimized through the use of sex- and estrous cycle-specific techniques.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (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 neurologic 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).

Two variables that can be used as pain indices in experimental animal models of arthritis

Since pain is an important symptom in arthritis, useful behavioral indices for pain in experimental arthritis animal models are important tools for investigative work on arthritis. The purpose of this study was to develop simple and quantifiable behavioral tests, which would represent the level of arthritic pain that develops after induction of inflammation in the knee. Two models of knee joint arthritis were produced: (1) KC model-injection of kaolin and carrageenan into the synovial cavity of the knee, and (2) CFA model-injection of complete Freund's adjuvant into the synovial cavity of the knee. The following three variables were measured before and at various times after the induction of arthritis. As an estimate of the degree of edema, the circumference of the knee was measured. As pain indices, (1) the vocalization threshold of compression force applied to the knee joint was measured to represent tenderness of the joint, and (2) the struggle threshold of the knee extension angle was measured to represent a reduction in range of motion in the arthritic joint. A time course study showed that behavioral changes last for at least 1 week for the KC model and at least 2 weeks for the CFA model. Correlation studies showed that all three variables significantly correlated with each other in both the KC and CFA arthritic models. Systemically injected morphine produced a partial reversal of these indices with the expected time course and dose response of a morphine-induced analgesic. It is concluded that two variables, the struggle threshold for knee extension and the vocalization threshold for knee compression, could be used as simple and useful pain indices in experimental models of arthritis.