Long-term ovariectomy changes formalin-induced licking in female rats: the role of estrogens

Comparison of pain modulatory effect of the LPGi estragon receptor on inflammatory pain between pro-estrus and estrus phases and OVX rats

The present study has investigated whether circulating estrogen level variations in the pro-estrus and estrus phases of the intact rats and estrogen depletion in the ovariectomized animals (OVX) adjust the formalin-induced nociceptive behaviors. During the pro-estrus and estrus phases of rats' estrus cycle and in the OVX rats, 17β-estradiol and ICI 182,780 (estrogen receptor antagonist) were administered into the right paragigantocellularis lateralis (LPGi) nucleus. Then, the formalin-induced flexing and licking responses were recorded for 60 min. The findings of this study revealed that intra-LPGi administration of 17β-estradiol (0.8 μmol) reduced the formalin-induced flexing and licking duration in pro-estrus and estrus rats (P < 0.001), suggesting an analgesic effect. 17β-Estradiol injection into the LPGi nucleus of OVX rats increased the flexing duration (P < 0.05) while decreasing the licking duration (P < 0.05) of the formalin test. The pain modulatory effect of 17β-estradiol on the flexing response was reversed by ICI 182,780 (15 nmol) in the pro-estrus (P < 0.001) and estrus rats (P < 0.001) but not in the OVX rats. Also, pretreatment of LPGi nucleus with ICI 182,780 reversed the analgesic effect of 17β-estradiol on the licking response in the pro-estrus (P < 0.05), estrus (P < 0.001), and OVX rats (P < 0.001). These results suggest that the pain threshold in intact female rats is modulated independently of the estrus state. Still, the basal level of plasma estrogen and the activation of its receptors are necessary for pain modulation.

Ovariectomy induces hyperalgesia accompanied by upregulated estrogen receptor α and protein kinase B in the rat spinal cord

Hormone supplementation is one of the common therapies for menopause-related disorders. Among different tools, the ovariectomy (OVX) rodents are widely accepted as an appropriate menopausal pain model. Our previous study has showed that OVX produces a significant pain facilitation in both acute pain and tonic pain, however, the underlying mechanisms remain unclear. In this study, we examined the effects of OVX treatment and estradiol (E2) supplementation on formalin-induced nociceptive responses, and explored the associated spinal mechanisms. Female Sprague-Dawley rats underwent bilateral OVX, and E2 supplementation was given subcutaneously from the 5th week after surgery (30 μg/day for 7 days). Our results showed that formalin-induced nociceptive behaviors did not differ between diestrus and proestrus stages of the estrous in intact rats. However, OVX exacerbated formalin-evoked inflammatory pain, especially in the late phase at 4-5 weeks but not 2 weeks post-surgery. E2 supplementation significantly reversed the OVX-triggered hyperalgesia. Double immunofluorescence staining revealed that both ERα and ERβ in the spinal dorsal horn were co-labeled with the neuronal markers, but not with markers of astrocytes or microglia. The spinal ERα (but not ERβ) expression significantly increased in the OVX group, which was reversed by E2 supplementation. Moreover, the OVX individuals showed an increased protein kinase B (AKT) level in lumbar spinal cord, and E2 supplementation diminished the AKT expression in OVX rats. Finally, intrathecal injection Wortmannin, an inhibitor for AKT signaling, effectively reduced the nociceptive behaviors in the late phase and the number of c-fos positive cells. Together, our findings indicate that E2 supplementation alleviates the OVX-induced hyperalgesia, which might be involved in spinal ERα and AKT mechanisms.

Estrogenic impregnation alters pain expression: analysis through functional neuropeptidomics in a surgical rat model of osteoarthritis

PURPOSE

Several observational studies suggest that estrogens could bias pain perception. To evaluate the influence of estrogenic impregnation on pain expression, a prospective, randomized, controlled, blinded study was conducted in a Sprague-Dawley rat model of surgically induced osteoarthritis (OA).

METHODS

Female rats were ovariectomized and pre-emptive 17β-estradiol (0.025 mg, 90-day release time) or placebo pellets were installed subcutaneously during the OVX procedures. Thirty-five days after, OA was surgically induced on both 17β-estradiol (OA-E) and placebo (OA-P) groups. Mechanical hypersensitivity was assessed by static weight-bearing (SWB) and paw withdrawal threshold (PWT) tests. Mass spectrometry coupled with high-performance liquid chromatography (HPLC-MS) was performed to quantify the spinal pronociceptive neuropeptides substance P (SP), calcitonin gene-related peptide (CGRP), bradykinin (BK), somatostatin (SST), and dynorphin-A (Dyn-A).

RESULTS

Compared to control, ovariectomized rats presented higher SP (P = 0.009) and CGRP (P = 0.017) concentrations. OA induction increased the spinal level of SP (+ 33%, P < 0.020) and decreased the release of BK (- 20%, (P < 0.037)). The OA-E rats at functional assessment put more % body weight on the affected hind limb than OA-P rats at D7 (P = 0.027) and D56 (P = 0.033), and showed higher PWT at D56 (P = 0.009), suggesting an analgesic and anti-allodynic effect of 17β-estradiol. Interestingly, the 17β-estradiol treatment counteracted the increase of spinal concentration of Dyn-A (P < 0.016) and CGRP (P < 0.018).

CONCLUSION

These results clearly indicate that 17β-estradiol interfers with the development of central sensitization and confirm that gender dimorphism should be considered when looking at pain evaluation.

Estrogens and phytoestrogens in body functions

Estrogens are the hormones of reproduction in women as well as of many other important functions in the male and female body. They undergo significant changes in the different phases of life, e.g. during puberty, pregnancy or at menopause/andropause. Phytoestrogens are natural non-steroidal phenolic plant compounds that can mimic the activity of estrogens and their beneficial effects in women and in men. This narrative review summarizes the literature on the physiological role of estrogens and the several potential health benefits of phytoestrogens, with particular attention given to the possible role of phytoestrogens in aging.

Estrogen modulation of pain perception with a novel 17β-estradiol pretreatment regime in ovariectomized rats

Estrogen plays substantial roles in pain modulation; however, studies concerning sex hormones and nociception often yield confusing results. The discrepancy could be a result of lack of consensus to regard estrogen as a variable when working with animal models; thus, the influence of hormones’ fluctuations on nociception has continually been neglected. In the present study, we designed a novel hormone substitution model to aid us to evaluate the effects of estrogen’s long-term alterations on ovariectomy (OVX)-induced mechanical hyperalgesia and the expression of estrogen receptors(ERs). OVX rats were implanted with slow-release estrogen pellets at differently arranged time points and doses, such that a gradual elevation or decrease of serum estrogen levels following a relatively stable period of estrogen replacement was achieved in rats. Our results demonstrated that gradual estrogen depletion rather than elevation following the stable period of estrogen substitution in OVX rats alleviated OVX-induced mechanical hyperalgesia in a dose-independent manner, and the opposite estrogen increase or decrease paradigms differently regulate the expression of spinal ERs. Specifically, in rats rendered to continuously increased serum estrogen, the early phase estrogen-induced anti-nociception effect in OVX rats was eliminated, which was accompanied by an over-activation of ERα and a strong depression of ERβ, while in the OVX rats subject to gradual decrease of estrogen replacement, both ERα and ERβ increased modestly compared with the OVX group. Thus, the present study demonstrated that estrogen increase or decrease modulate nociception differently through change of spinal ERs.

The Differential Role of Cytokines on Stress Responses in a Menopause Rat Model

Menopause is a risk factor of anxiety and depression. Also, psychoneurological symptoms are shown in almost all women in the perimenopausal period. The present study investigated if repeated stress modulates behavioral changes or the balance of pro- and anti-inflammatory cytokines in ovariectomized (OVX) rats. Albino SD female rats were randomly divided into four groups: the naïve normal group (NOR), a surgically ovariectomized group (OVX), the only stressed group (ST), and the OVX and stressed groups (OVX + ST). We performed a battery of tests such as the forced swimming test (FST), the sucrose intake, and social exploration. In the same animals, corticosterone (CORT) was assessed in the serum, and also, two representative cytokines (IL-1β and IL-4) were examined in different brain regions after all the behavior sessions for all the experimental groups. The OVX + ST group showed more immobility time in FST than the OVX group or the ST group. Also, the OVX + ST group tended to have a decreased active social exploration and sucrose solution intake compared to the OVX group or ST group. The serum concentration of CORT of the OVX + ST group was higher than the OVX group or ST group and also the level of CORT in OVX + ST was markedly increased compared to the NOR group. In the brain, the number of IL-1β immunoreactive neurons of the OVX + ST group was increased compared to the NOR group. The OVX + ST group tended to have an increase in IL-1β-positive neurons compared to the OVX or ST group. However, the number of IL-4 immunoreactive neurons of the OVX + ST group was markedly decreased compared with the NOR group. Also, the IL-4-positive neurons in the OVX + ST group was significantly decreased when compared to the ST group. These results indicate that ovariectomy and stress combine to increase the depressive-like behaviors and neuroinflammatory responses. Together, these data show neuroinflammation as a potential contributor to depressive-like symptoms during menopausal transition.

Neurosteroids and neuropathic pain management: Basic evidence and therapeutic perspectives

Complex mechanisms involved in neuropathic pain that represents a major health concern make its management complicated. Because neurosteroids are bioactive steroids endogenously synthesized in the nervous system, including in pain pathways, they appear relevant to develop effective treatments against neuropathic pain. Neurosteroids act in paracrine or autocrine manner through genomic mechanisms and/or via membrane receptors of neurotransmitters that pivotally modulate pain sensation. Basic studies which uncovered a direct link between neuropathic pain symptoms and endogenous neurosteroid production/regulation, paved the way for the investigations of neurosteroid therapeutic potential against pathological pain. Concordantly, antinociceptive properties of synthetic neurosteroids were evidenced in humans and animals. Neurosteroids promote peripheral analgesia mediated by T-type calcium and gamma-aminobutyric acid type A channels, counteract chemotherapy-induced neuropathic pain and ameliorate neuropathic symptoms of injured spinal cord animals by stimulating anti-inflammatory, remyelinating and neuroprotective processes. Together, these data open interesting perspectives for neurosteroid-based strategies to manage/alleviate efficiently neuropathic pain.

Ovarian Hormone-dependent and Spinal ERK Activation-regulated Nociceptive Hypersensitivity in Female Rats with Acid Injection-induced Chronic Widespread Muscle Pain

Symptoms of chronic widespread muscle pain (CWP) meet most of the diagnostic criteria for fibromyalgia syndrome, which is prevalent in females. We used an acid injection-induced muscle pain (AIMP) model to mimic CWP. After female rats received an ovariectomy (OVX), acid saline solution was injected into the left gastrocnemius muscle. Time courses of changes in pain behaviours and p-ERK in the spinal cord were compared between groups. Intrathecal injections of oestradiol (E2) to the OVX group before two acid injections and E2 or progesterone (P4) injections in male rats were compared to evaluate hormone effects. We found that repeated acid injections produced mechanical hypersensitivity and enhanced p-ERK expression in the spinal dorsal horn. OVX rats exhibited significantly less tactile allodynia than did the rats in the other groups. The ERK inhibitor U0126 alleviated mechanical allodynia with lower p-ERK expression in the sham females but did not affect the OVX rats. Intrathecal E2 reversed the attenuated mechanical hypersensitivity in the OVX group, and E2 or P4 induced transient hyperalgesia in male rats. Accordingly, our results suggested that ovarian hormones contribute to AIMP through a spinal p-ERK-mediated pathway. These findings may partially explain the higher prevalence of fibromyalgia in females than males.

Neurosteroids in Pain Management: A New Perspective on an Old Player

Since the discovery of the nervous system’s ability to produce steroid hormones, numerous studies have demonstrated their importance in modulating neuronal excitability. These central effects are mostly mediated through different ligand-gated receptor systems such as GABA_A and NMDA, as well as voltage-dependent Ca²⁺ or K⁺ channels. Because these targets are also implicated in transmission of sensory information, it is not surprising that numerous studies have shown the analgesic properties of neurosteroids in various pain models. Physiological (nociceptive) pain has protective value for an organism by promoting survival in life-threatening conditions. However, more prolonged pain that results from dysfunction of nerves (neuropathic pain), and persists even after tissue injury has resolved, is one of the main reasons that patients seek medical attention. This review will focus mostly on the analgesic perspective of neurosteroids and their synthetic 5α and 5β analogs in nociceptive and neuropathic pain conditions.

Chronic Osteoporotic Pain in Mice: Cutaneous and Deep Musculoskeletal Pain Are Partially Independent of Bone Resorption and Differentially Sensitive to Pharmacological Interventions

Although the pathological changes in osteoporotic bones are well established, the characterization of the osteoporotic pain and its appropriate treatment are not fully elucidated. We investigated the behavioral signs of cutaneous and deep musculoskeletal pain and physical function; time-dependent changes in bone mineral density (BMD) and the emergence of the behavioral phenotype; and the effects of pharmacological interventions having different mechanisms of action (chronic intraperitoneal administration of pamidronate [0.25 mg/kg, 5x/week for 5 weeks] versus acute treatment with intraperitoneal morphine [10 mg/kg] and pregabalin [100 mg/kg]) in a mouse model of ovariectomized or sham-operated mice 6 months following surgery. We observed reduced BMD associated with weight gain, referred cutaneous hypersensitivity, and deep musculoskeletal pain that persisted for 6 months. Chronic bisphosphonate treatment, 6 months after ovariectomy, reversed bone loss and hypersensitivity to cold, but other behavioral indices of osteoporotic pain were unchanged. While the efficacy of acute morphine on cutaneous pain was weak, pregabalin was highly effective; deep musculoskeletal pain was intractable. In conclusion, the reversal of bone loss alone is insufficient to manage pain in chronic osteoporosis. Additional treatments, both pharmacological and nonpharmacological, should be implemented to improve quality of life for osteoporosis patients.

Nociceptive Alteration by High Sucrose Diet in Hypoestrogenic Wistar Rats

Preclinical Research Obesity is a risk factor associated with alterations in pain perception. The aim of this study was to analyse a time-course of nociceptive responses (plantar test) in hypoestrogenic rats after the induction of obesity. Animals (hypoestrogenic and naïve) received either a hypercaloric or regular diet for 24 weeks. Thermal nociception and body weight were measured during this period. At the 4th and 17th weeks after treatment, oral glucose tolerance, blood insulin levels, abdominal fat weight, and uric acid levels were measured. The hypoestrogenic rats on a high sucrose diet had higher body weight and abdominal fat weight than control rats. A biphasic response was observed in the ovariectomized group fed with sucrose with thermal latency being decreased in the fourth week. During weeks 12-18, thermal latency increased compared to that of the hypoestrogenic control. There were no differences in basal blood glucose levels at the 4th and 17th weeks; however, oral glucose tolerance, insulin, and uric acid levels were altered. This indicated that increased body weight and fat as well as alteration sin glucose tolerance, hyperinsulinemia and hyperuricemia, may be associated with the biphasic nociceptive response. Drug Dev Res 77 : 258-266, 2016. © 2016 Wiley Periodicals, Inc.

The relationship of bone-tumor-induced spinal cord astrocyte activation and aromatase expression to mechanical hyperalgesia and cold hypersensitivity in intact female and ovariectomized mice

Recently, our group established a relationship between tumor-induced spinal cord astrocyte activation and aromatase expression and the development of bone tumor nociception in male mice. As an extension of this work, we now report on the association of tumor-induced mechanical hyperalgesia and cold hypersensitivity to changes in spinal cord dorsal horn GFAP and aromatase expression in intact (INT) female mice and the effect of ovariectomy on these parameters. Implantation of fibrosarcoma cells produced robust mechanical hyperalgesia in INT animals, while ovariectomized (OVX) females had significantly less mechanical hyperalgesia. Cold hypersensitivity was apparent by post-implantation day 7 in INT and OVX females compared to their saline-injected controls and increased throughout the experiment. The decrease in mechanical hyperalgesia in OVX females was mirrored by significant decreases in spinal astrocyte activity in laminae I-II, III-IV, V-VI and X and aromatase expression in laminae V-VI and X in the dorsal horn of tumor-bearing animals. Administration of the aromatase inhibitor letrozole reduced tumor-induced hyperalgesia in INT females only suggesting that the tumor-induced increase in aromatase expression and its associated increase in spinal estrogen play a role in the development of bone tumor-induced hyperalgesia. Finally, intrathecal (i.t.) administration of 17β-estradiol caused a significant increase in tumor-induced hyperalgesia in INT tumor-bearing females. Since i.t. 17β-estradiol increases tumor pain and ovariectomy significantly decreases tumor pain, as well as spinal aromatase, estrogen may play a critical role in the spinal cord response to the changing tumor environment and the development of tumor-induced nociception.

Ovariectomy results in variable changes in nociception, mood and depression in adult female rats

Decline in the ovarian hormones with menopause may influence somatosensory, cognitive, and affective processing. The present study investigated whether hormonal depletion alters the nociceptive, depressive-like and learning behaviors in experimental rats after ovariectomy (OVX), a common method to deplete animals of their gonadal hormones. OVX rats developed thermal hyperalgesia in proximal and distal tail that was established 2 weeks after OVX and lasted the 7 weeks of the experiment. A robust mechanical allodynia was also occurred at 5 weeks after OVX. In the 5^th week after OVX, dilute formalin (5%)-induced nociceptive responses (such as elevating and licking or biting) during the second phase were significantly increased as compared to intact and sham-OVX females. However, chronic constriction injury (CCI) of the sciatic nerve-induced mechanical allodynia did not differ as hormonal status (e.g. OVX and ovarian intact). Using formalin-induced conditioned place avoidance (F-CPA), which is believed to reflect the pain-related negative emotion, we further found that OVX significantly attenuated F-CPA scores but did not alter electric foot-shock-induced CPA (S-CPA). In the open field and forced swimming test, there was an increase in depressive-like behaviors in OVX rats. There was no detectable impairment of spatial performance by Morris water maze task in OVX rats up to 5 weeks after surgery. Estrogen replacement retrieved OVX-induced nociceptive hypersensitivity and depressive-like behaviors. This is the first study to investigate the impacts of ovarian removal on nociceptive perception, negative emotion, depressive-like behaviors and spatial learning in adult female rats in a uniform and standard way.

Effects of gustatory nerve transection and/or ovariectomy on oral capsaicin avoidance in rats

The incidence of chronic oral pain such as burning mouth syndrome is greater in peri-menopausal females, and was postulated to be associated with gustatory nerve damage. We investigated whether bilateral transection of the chorda tympani, with or without accompanying ovariectomy, affected oral capsaicin avoidance in rats. Female rats had restricted access to 2 bottles, 1 bottle containing capsaicin (concentration range: 0.33-33 μM/L) and the other vehicle. Percent volume of capsaicin consumption and lick counts were measured. The concentration series was tested before and 0.5, 3, 6, 9, and 12 months after the following surgical procedures: (a) bilateral transection of the chorda tympani (CTx); (b) ovariectomy (OVx); (3) CTx plus OVx; or (4) sham CT surgery. Before surgery there was a concentration-dependent decrease in licks and volume of capsaicin consumed, with a threshold between 0.1 and 0.3 ppm. The majority of drink licks occurred during the first 9 minutes of access. Over the 12-month test period, the CTx group did not exhibit reduced capsaicin consumption, and consumed significantly more capsaicin at 6 and 9 months postsurgery. Rats in the OVx group consistently consumed significantly less capsaicin and exhibited significantly higher counts of capsaicin-evoked Fos-like immunoreactivity in the dorsomedial trigeminal subnucleus caudalis (Vc) compared to all other treatment groups. That CTx, with or without OVx, did not enhance capsaicin avoidance indicates that damage to the gustatory system does not disinhibit trigeminal nociceptive transmission.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Potential role of allopregnanolone for a safe and effective therapy of neuropathic pain

Because the treatment and management of neuropathic pain are extremely complicated, the characterization of novel analgesics and neuroprotectors with safe toxicological profiles is a crucial need to develop efficient therapies. Several investigations revealed that the natural neurosteroid allopregnanolone (AP) exerts analgesic, neuroprotective, antidepressant and anxiolytic effects. These effects result from AP ability to modulate GABA(A), glycine, L- and T-type calcium channels. It has been shown that AP treatment induced beneficial actions in humans and animal models with no toxic side effects. In particular, a multi-parametric analysis revealed that AP efficiently counteracted chemotherapy-evoked neuropathic pain in rats. It has also been demonstrated that the modulation of AP-producing enzyme, 3α-hydroxysteroid oxido-reductase (3α-HSOR), in the spinal cord regulates thermal and mechanical pain thresholds of peripheral nerve injured neuropathic rats. The painful symptoms were exacerbated by intrathecal injections of provera (pharmacological inhibitor of 3α-HSOR) which decreased AP production in the spinal cord. By contrast, the enhancement of AP concentration in the intrathecal space induced analgesia and suppression of neuropathic symptoms. Moreover, in vivo siRNA-knockdown of 3α-HSOR expression in healthy rat dorsal root ganglia increased thermal and mechanical pain perceptions while AP evoked a potent antinociceptive action. In humans, blood levels of AP were inversely associated with low back and chest pain. Furthermore, oral administration of AP analogs induced antinociception. Altogether, these data indicate that AP, which possesses a high therapeutic potential and a good toxicological profile, may be used to develop effective and safe strategies against chronic neuropathic pain.

Steroids, spinal cord and pain sensation

During the whole life, the nervous system is continuously submitted to the actions of different categories of hormones, including steroids. Therefore, the interactions between hormonal compounds and neural tissues are subjected to intense investigations. While a majority of studies focus on the brain, the spinal cord (SC) has received little attention, although this structure is also an important part of the central nervous system, controlling motor and sensory functions. To point out the importance of interactions between hormones and the SC in the regulation of neurobiological activities, we recapitulated and discussed herein various key data, revealing that the pivotal role played by the SC in nociception and pain modulation, directly depends on the SC ability to metabolize and synthesize steroidal molecules. The paper suggests that future investigations aiming to develop effective strategies against chronic pain, must integrate regulatory effects exerted by hormonal steroids on the SC activity, as well as the actions of endogenous neurosteroids locally synthesized in spinal neural networks.

Altered morphine-induced analgesia in neurotensin type 1 receptor null mice

Both neurotensin (NT) and opioid agonists have been shown to induce antinociception in rodents after central administration. Besides, previous studies have revealed the existence of functional interactions between NT and opioid systems in the regulation of pain processing. We recently demonstrated that NTS1 receptors play a key role in the mediation of the analgesic effects of NT in long-lasting pain. In the present study, we therefore investigated whether NTS1 gene deletion affected the antinociceptive action of mu opioid drugs. To this end, pain behavioral responses to formalin were determined following systemic administration of morphine in both male and female NTS1 knockout mice. Acute injection of morphine (2 or 5 mg/kg) produced strong antinociceptive effects in both male and female wild-type littermates, with no significant sex differences. On the other hand, morphine analgesia was considerably reduced in NTS1-deficient mice of both sexes compared to their respective controls, indicating that the NTS1 receptor actively participates in mu opioid alleviating pain. By examining specifically the flinching, licking and biting nociceptive behaviors, we also showed that the functional crosstalk between NTS1 and mu opioid receptors influences the supraspinally-mediated behaviors. Interestingly, sexual dimorphic action of morphine-induced pain inhibition was found in NTS1 null mice in the formalin test, suggesting that the endogenous NT system interacts differently with the opioid network in male and female mice. Altogether, these results demonstrated that NTS1 receptor activation operates downstream to the opioidergic transmission and that NTS1-selective agonists combined with morphine may act synergistically to reduce persistent pain.

Endogenous gonadal hormones regulate females' behavioral responses to formalin through prostaglandin E2 release

INTRODUCTION

This study aimed to determine if endogenous gonadal hormones affect the intracellular mechanisms in the spinal cord that control inflammatory pain responses.

METHODS

We analyzed behavioral responses to, and changes in, serum levels of prostaglandin E2, estradiol, progesterone, and corticosterone after administration of 5% formalin in intact and ovariectomized (OVX) female rats.

RESULTS

OVX females displayed significantly more flinching than did intact females during Phase I, and after formalin administration their corticosterone levels were significantly lower. No differences were seen across COX-1 and COX-2 protein expression in the spinal cord of either naive or formalin-treated rats. However, subsequent to formalin a main effect of gonadectomy was seen in prostaglandin E2 levels; OVX animals had significantly lower prostaglandin E2 levels than intact animals.

CONCLUSIONS

These results indicate that in female rats nociceptive responses to formalin are regulated through the levels of prostaglandin E2, an important mediator in inflammation, whereas protein levels of COX-1 and COX-2 play a more limited role.

Involvement of NTS2 receptors in stress-induced analgesia

Stress activates multiple neural systems that suppress pain sensation. This adaptive phenomenon referred as stress-induced analgesia (SIA) is mediated by the activation of endogenous pain inhibitory systems. Both opioid and non-opioid forms of SIA have been elicited in rodents according to stressor parameters and duration. There is accumulating evidence that the endogenous neurotensin (NT) system plays an important role in SIA. Especially, NT-deficient mice were shown to exhibit reduced SIA following water avoidance or restraint stress. Since central NT produces naloxone-insensitive analgesic effects by acting on spinal and supraspinal NTS2 receptors, we hypothesized that NT might mediate non-opioid SIA through NTS2 activation. Here, we evaluated the influence of an opioid-independent severe stress produced by a cold-water swim for 3 min at 15 degrees C on rodent offspring's pain perception. Our results demonstrated that mice lacking NTS2 exhibit significantly reduced SIA following cold-water swim stress. Indeed, NTS2 knockout mice submitted to both acute (plantar test) and tonic (formalin test) pain stimuli show a greater sensitivity to pain in comparison to wild-type littermates. Accordingly, pretreatment with the NT receptor antagonist SR142948A results in a hyperalgesic response to stress induced by cold-water swim. Endogenous NT regulates hypothalamic-pituitary-adrenal axis activity in stress condition by increasing corticosterone plasma levels. Accordingly, the plasma levels of corticosterone measured by radioimmunoassay are significantly reduced in non-stressed and stressed NTS2-deficient mice in comparison with wild-type mice. To further investigate the site of action of NT in mediating SIA, we microinjected NTS2 agonists in lumbar spinal cord and quantified post-stress sensitivity to pain in rats using the plantar test. Exogenously administered NTS2 analogs, JMV-431, beta-lactotensin and NT69L markedly enhance the magnitude and duration of stress antinociception in both 25- and 60-day-old rats. In sum, by using genetic and pharmacological approaches, we demonstrated here that NTS2 receptors mediate non-opioid SIA. Our results also revealed that the release of endogenous NT in response to stress requires the presence of NTS2 to stimulate corticotropin-releasing factor (CRF)-induced elevation of plasma corticosterone, and that NTS2 receptors localized at the lumbar spinal cord participate to the disinhibition of descending pain control pathways. Therefore, these data highlight the significance of NTS2 as a novel target for the treatment of pain and stress-related disorders.

Evidence for a key role of steroids in the modulation of pain

Neurotransmitters such as glutamate, substance P, serotonin and gamma-aminobutyric acid pivotally control pain mechanisms. It is also well known that inflammatory and/or neuropathic pain may depend on the action of diverse cytokines and other molecules including eicosanoids, endorphins, calcitonin-gene related peptide, free radicals and transcription factors. Because steroids control the development, activities and plasticity of the nervous system, these compounds are of particular interest in the modulation of pain. The paper discusses various data supporting the existence of key regulatory effects of steroids in the control of pain. In particular, we analyzed three categories of observations which historically contributed to demonstrate that endogenous and synthetic steroids play a crucial role in the regulation of neurobiological processes involved in pain sensation. The first series of data, which present the chemical characteristics enabling steroids to act on several tissues, also summarize pertinent results supporting the modulation of pain sensation by steroidal compounds. The second category of data evokes psychosocial, fundamental and clinical results suggesting the existence of sex steroid-based differences in pain perception. Finally, we discuss recent evidence showing the endogenous production of neurosteroids and their effects in the spinal cord which crucially controls pain transmission. Taken together, the data reviewed herein suggest that future investigations aiming to develop effective steroid-based strategies against chronic pain must integrate in a complementary manner anti-inflammatory properties of steroids, sex steroid-induced dimorphism in pain perception and regulatory effects exerted by endogenous neurosteroids in pain neural circuits.

Peripheral estradiol induces temporomandibular joint antinociception in rats by activating the nitric oxide/cyclic guanosine monophosphate signaling pathway

Recently, we have reported that high physiological estradiol level during the proestrus phase of the estrous cycle or systemic estradiol administration in ovariectomized rats decreases formalin-induced temporomandibular joint nociception. However, the mechanisms underlying the antinociceptive effect of estradiol are presently unknown. In this study, we used the temporomandibular joint formalin model in rats to investigate whether estradiol decreases nociception by a peripheral non-genomic mechanism, and if so, whether this mechanism is mediated by the activation of the nitric oxide-cyclic guanosine monophosphate signaling pathway and of opioid receptors. The administration of estradiol into the ipsilateral, but not into the contralateral temporomandibular joint significantly reduced formalin-induced temporomandibular joint nociception in ovariectomized and diestrus but not in proestrus females. However, the administration of the estrogen receptor antagonist ICI 182780 into the ipsilateral, but not into the contralateral temporomandibular joint blocked the antinociceptive effect of serum estradiol in proestrus females, suggesting that the physiological effect of estradiol in nociception is mediated, at least in part, by a peripheral mechanism. The administration of estradiol into the ipisilateral temporomandibular joint did not affect formalin-induced nociception in male rats. The antinociceptive effect of temporomandibular joint estradiol administration in ovariectomized and diestrus females was mimicked by estradiol conjugated with bovine serum albumin, which does not diffuse through the plasma membrane, and was blocked by the estrogen receptor antagonist ICI 182780. The administration of the nitric oxide synthase inhibitor (nitro-l-arginine) or of a guanylate cyclase inhibitor (1H-(1,2,4)-oxadiasolo (4,2-a) quinoxalin-1-one) into the ipsilateral, but not into the contralateral temporomandibular joint blocked the antinociceptive effect of estradiol and of estradiol conjugated with bovine serum albumin, while the opioid receptor antagonist naloxone had no effect. These findings suggest that estradiol decreases temporomandibular joint nociception in female rats through a peripheral non-genomic activation of the nitric oxide-cyclic guanosine monophosphate signaling pathway.

Estrogen-dependent changes in visceral afferent sensitivity

Many forms of chronic pain are more prevalent in women and this is interpreted as the consequence of a direct role of estrogens in the modulation of pain perception. Some functional pain states, i.e. those without a clear and demonstrable pathology, are also more prevalent in women and the pain in these conditions is also modulated by hormonal variations during the menstrual cycle. Increased pain sensitivity is commonly interpreted as the consequence of peripheral or central hyperexcitability of nociceptive pathways. Therefore a role has been suggested for estrogen in the modulation of the excitability of nociceptive afferents and central neurons. The literature on the sign of this modulation is not uniform, with reports pointing to estrogen as either pro- or anti-nociceptive. In our hands, a permanent reduction in the levels of estrogen, such as that induced by surgical ovariectomy (OVX) generates a hyperalgesic state of slow onset and long duration that can be prevented or reversed by exogenous administration of estrogen. The hyperalgesia is characterized by mechanical and thermal hyperalgesia in the abdominal and pelvic regions as well as by visceral hypersensitivity. The possible role of estrogen in the prevention of chronic painful states is discussed.

Evidence for a role of NTS2 receptors in the modulation of tonic pain sensitivity

BACKGROUND

Central neurotensin (NT) administration results in a naloxone-insensitive antinociceptive response in animal models of acute and persistent pain. Both NTS1 and NTS2 receptors were shown to be required for different aspects of NT-induced analgesia. We recently demonstrated that NTS2 receptors were extensively associated with ascending nociceptive pathways, both at the level of the dorsal root ganglia and of the spinal dorsal horn. Then, we found that spinally administered NTS2-selective agonists induced dose-dependent antinociceptive responses in the acute tail-flick test. In the present study, we therefore investigated whether activation of spinal NTS2 receptors suppressed the persistent inflammatory pain symptoms observed after intraplantar injection of formalin.

RESULTS

We first demonstrated that spinally administered NT and NT69L agonists, which bind to both NTS1 and NTS2 receptors, significantly reduced pain-evoked responses during the inflammatory phase of the formalin test. Accordingly, pretreatment with the NTS2-selective analogs JMV-431 and levocabastine was effective in inhibiting the aversive behaviors induced by formalin. With resolution at the single-cell level, we also found that activation of spinal NTS2 receptors reduced formalin-induced c-fos expression in dorsal horn neurons. However, our results also suggest that NTS2-selective agonists and NTS1/NTS2 mixed compounds differently modulated the early (21-39 min) and late (40-60 min) tonic phase 2 and recruited endogenous pain inhibitory mechanisms integrated at different levels of the central nervous system. Indeed, while non-selective drugs suppressed pain-related behaviors activity in both part of phase 2, intrathecal injection of NTS2-selective agonists was only efficient in reducing pain during the late phase 2. Furthermore, assessment of the stereotypic pain behaviors of lifting, shaking, licking and biting to formalin also revealed that unlike non-discriminative NTS1/NTS2 analogs reversing all nociceptive endpoint behaviors, pure NTS2 agonists specifically inhibited paw lifting, supporting a role of NTS2 in spinal modulation of persistent nociception.

CONCLUSION

The present study provides the first demonstration that activation of NTS2 receptors produces analgesia in the persistent inflammatory pain model of formalin. The dichotomy between these two classes of compounds also indicates that both NTS1 and NTS2 receptors are involved in tonic pain inhibition and implies that these two NT receptors modulate the pain-induced behavioral responses by acting on distinct spinal and/or supraspinal neural circuits. In conclusion, development of NT agonists targeting both NTS1 and NTS2 receptors could be useful for chronic pain management.

Effects of 17beta-estradiol on responses of viscerosomatic convergent thalamic neurons in the ovariectomized female rat

Ovarian hormones have been shown to exert multiple effects on CNS function and viscerosomatic convergent activity. Ovariectomized (OVX) female rats were used in the present study to examine the long-term effects of proestrus levels of 17beta-estradiol (EB) delivered by a 60-day time-released subcutaneous pellet on the response properties of viscerosomatic convergent thalamic neurons. In addition, avoidance thresholds to mechanical stimulation for one of the convergent somatic territories, the trunk, was assessed using an electro-von Frey anesthesiometer before and at the end of the 6-wk post-OVX/implant period prior to the terminal electrophysiological experiments, which were done under urethane anesthesia. Rats implanted with an EB-containing pellet, relative to placebo controls, demonstrated 1) altered thalamic response frequencies and thresholds for cervix and vaginal but not colon stimulation; 2) some response variations for just the lateral group of thalamic subnuclei; and 3) altered thalamic response frequencies and thresholds for trunk stimulation. Thalamic response thresholds for trunk pressure in EB versus placebo rats were consistent with the avoidance thresholds obtained from the same groups. In addition, EB replacement affected visceral and somatic thresholds in opposite ways (i.e., reproductive-related structures were less sensitive to pressure, whereas somatic regions showed increased sensitivity). These results have obvious reproductive advantages (i.e., decreased reproductive organ sensitivity for copulation and increased trunk sensitivity for lordosis posturing), as well as possible clinical implications in women suffering from chronic pelvic pain syndromes and/or neuropathic pain.

An immunohistochemical study of the antinociceptive effect of calcitonin in ovariectomized rats

BACKGROUND

Calcitonin is used as a treatment to reduce the blood calcium concentration in hypercalcemia and to improve bone mass in osteoporosis. An analgesic effect of calcitonin has been observed and reported in clinical situations. Ovariectomized (OVX) rats exhibit the same hormonal changes as observed in humans with osteoporosis and are an animal model of postmenopausal osteoporosis. The aim of this study to investigate antinociceptive effect of calcitonin in OVX rats using the immunohistochemical study.

METHODS

We assessed the antinociceptive effects of calcitonin in an ovariectomized (OVX) rat model, which exhibit osteoporosis and hyperalgesia, using the immunohistochemical method. Fifteen rats were ovariectomized bilaterally, and ten rats were received the same surgery expected for ovariectomy as a sham model. We used five groups: the OVX-CT (n = 5), the sham-CT (n = 5), and the OVX-CT-pcpa (n = 5) groups received calcitonin (CT: 4 U/kg/day), while OVX-vehi (n = 5) and the sham-vehi (n = 5) groups received vehicle subcutaneously 5 times a week for 4 weeks. The OVX-CT-pcpa-group was given traperitoneal injection of p-chlorophenylalanine (pcpa; an inhibitor of serotonin biosynthesis) (100 mg/kg/day) in the last 3 days of calcitonin injection. Two hours after 5% formalin (0.05 ml) subcutaneously into the hind paw, the L5 spinal cord were removed and the number of Fos-immunoreactive (ir) neurons were evaluated using the Mann-Whitney-U test.

RESULTS

The numbers of Fos-ir neurons in the OVX-CT and sham-CT groups were significantly less than in the OVX-vehi and sham-vehi groups, respectively (p = 0.0090, p = 0.0090). The number of Fos-ir neurons in the OVX-CT-pcpa-group was significantly more than that of the OVX-CT-group (p = 0.0283), which means pcpa inhibits calcitonin induced reduction of c-Fos production.

CONCLUSION

The results in this study demonstrated that 1) the increase of c-Fos might be related to hyperalgesia in OVX-rats. 2) Calcitonin has an antinociceptive effect in both OVX and sham rats. 3) The central serotonergic system is involved in the antinociceptive properties of calcitonin.

Estrogen modulation of ovariectomy-induced hyperalgesia in adult mice

Some chronic pain conditions are more prevalent in women. However, the evidence from both human and animal studies as to whether estrogen is pro- or anti-nociceptive is inconsistent. We have used a model of functional abdominal pain in mice to examine the role of estrogen in the modulation of a hyperalgesic state induced by ovariectomy. C57/BL6 female mice were either ovariectomized (OVX), received the same surgery without removing the ovaries or were tested without any surgical procedure. Mechanical hyperalgesia was assessed by von Frey filaments and thermal pain was measured using a hot-plate at 50 degrees C. OVX mice, but not sham-operated, developed mechanical hyperalgesia localized to the abdominal region, the hindlimbs and the proximal tail, 4-5 weeks after OVX as well as a reduction in response latency to the hot plate. OVX animals were implanted with 17beta-estradiol pellets or with similar pellets with no hormone five weeks after OVX, when the hyperalgesic state was fully developed, and the estrogen reversed both mechanical and thermal hyperalgesia. Vaginal smears were taken to record the phase of the cycle at the time of the test from all animals and no significant differences were detected in mechanical hyperalgesia or in thermal pain threshold between normal animals in different phases of their estrous cycle. These results show that OVX induces a hyperalgesic state of slow onset and long duration that can be reversed by estrogen. We have also observed no estrous cycle modulation of pain sensitivity in normal animals.

Estradiol replacement in ovariectomized rats is antihyperalgesic in the formalin test

A subcutaneous implant of 17beta-estradiol or progesterone provides steady-state serum hormone levels from 7 to 24 days after implantation and allows the evaluation of the effects of the replacement with these hormones on phase 1 and phase 2 formalin-induced behaviors in ovariectomized (OVX) rats. Graded doses of 17beta-estradiol (5% to 40%) reduce formalin-induced behavior by 35% to 49% during phase 2 but not during phase 1, as measured with an automated formalin apparatus. The maximal response is seen with 20% 17beta-estradiol. The antihyperalgesic effect of 20% 17beta-estradiol is significant at 8 days after implantation and persists at 21 days. In contrast, graded doses of progesterone have no effect on either phase of formalin. The estrogen receptor antagonist tamoxifen completely prevents the antihyperalgesic effect of the 20% 17beta-estradiol implant. Formalin-induced behaviors during phase 2 are significantly less in proestrus females and OVX rats given 20% 17beta-estradiol compared with OVX control rats. Also, the formalin-induced increase in serum corticosterone is attenuated in OVX control rats compared with proestrus females and OVX rats given 20% 17beta-estradiol. These results indicate that estrogen replacement in OVX rats restores the maximal corticosterone response to tonic pain and, by an estrogen receptor-mediated process, inhibits tonic pain.

PERSPECTIVE

Hormone replacement (HR) therapy remains a widely used modality. We used a pharmacokinetically based rat HR model that results in continuous physiological levels of 17beta-estradiol to demonstrate the analgesic (antihyperalgesic) effects of estrogen replacement in an inflammatory pain model (formalin). These results suggest a potentially important consequence of HR therapy.

Progesterone mediates gonadal hormone differences in tactile and thermal hypersensitivity following L5 nerve root ligation in female rats

Sex differences in the magnitude of response to thermal and tactile stimuli have been demonstrated in both clinical and animal studies. Female rats typically display lower thresholds to painful stimuli and display more robust responses following nerve injury as compared with males. There is a body of evidence implicating the sex hormones in mediating this sex difference. In the present study, we sought to determine which gonadal hormones were involved in mediating the observed female hypersensitivity in female rats both prior to and following experimental nerve root injury using a chronic hormone replacement paradigm. Female rats were ovariectomized and hormone pellets containing 17beta-estradiol, progesterone (P), 17beta-estradiol+progesterone or placebo were implanted s.c. Our results demonstrate that only the group of female rats that received progesterone alone maintained the hypersensitive phenotype following ovariectomy, compared with gonadally intact male rats. This result was observed both in response to thermal stimuli in non-injured female rats and to thermal and tactile stimuli following L5 nerve root ligation, a model of low back pain associated with lumbar radiculopathy. Postmortem analysis of serum gonadal hormone concentrations demonstrates that the hormonal manipulations were successful and the exogenous hormones were similar to physiological levels observed in the sham-ovariectomized controls. Taken together, these results demonstrate the critical role for progesterone in mediating enhanced female tactile and thermal hypersensitivity following L5 nerve root ligation.

Estrogen-dependent abdominal hyperalgesia induced by ovariectomy in adult mice: A model of functional abdominal pain

Some abdominal pain syndromes are characterized by persistent pain without demonstrable pathology. Many of them are prevalent in women and it is known that sex hormones are associated with differences in pain perception between males and females. To model a process of functional abdominal pain in females we studied the time course and estrogen dependency of a hyperalgesic state induced by ovariectomy in adult mice. Three groups of C57/BL6 mice were used: virgin mice, proven breeders (2 or 3 successful pregnancies) and retired breeders (more than three successful pregnancies). Within each group, a third of the mice were ovariectomized (OVX), a third received sham surgery and a third were controls. OVX mice, but not sham or controls, developed a robust mechanical hyperalgesia and allodynia in the abdomen, hindlimbs and proximal tail, but not in the forelimbs, that was established 4 weeks after OVX and lasted for the 7 weeks of the experiment. Increases in visceral sensitivity were also observed in OVX mice. Thermal pain thresholds (hot plate) remained unchanged. The reproductive history of the animals had no influence on the hyperalgesia. In another series of experiments a slow release pellet containing 17beta-estradiol was implanted in half of the OVX mice and a similar pellet without the hormone in the other half. Hormone replacement prevented the development of hyperalgesia in OVX animals but did not stop the involution of the internal reproductive organs. We conclude that OVX in mice provides a useful model for a hormonally dependent hyperalgesic state resembling functional pain.

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.

Lack of estrogen increases pain in the trigeminal formalin model: a behavioural and immunocytochemical study of transgenic ArKO mice

In order to examine the effect of estrogen on facial pain, we first compared the face-rubbing evoked by a formalin injection in the lip of aromatase-knockout (ArKO) mice, lacking endogenous estrogen production, 17 beta-estradiol-treated ArKO mice (ArKO-E2) and wild-type (WT) littermates. During the 'acute' phase of pain the time spent rubbing was similar in the three groups, whereas during the following 'interphase' and the second phase of pain, grooming was increased in ArKO mice. Estradiol-treatment restored a behaviour similar to WT group. To better understand estrogens modulation on pain processes, we examined changes in 5-HT and CGRP innervations of trigeminal nucleus caudalis (TNC) in ArKO, ArKO-E2 and WT groups sacrificed during the interphase. Whereas serotonin and CGRP immunoreactivities were comparable in WT and ArKO non-injected control groups, our data showed that 9 min after formalin injection, the density of serotoninergic terminals increased significantly in WT, but not in ArKO mice, while that of CGRP-immunoreactive fibers was lower in WT than in ArKO mice on the injected side. Estradiol-treatment only partially reversed these changes in ArKO-E2 mice. We conclude that estrogen deprivation in ArKO mice can be responsible for increased nociceptive response and that it is accompanied by transmitter changes favouring pro- over anti-nociceptive mechanisms in TNC during interphase of the formalin model. That estradiol-treatment completely reverses the behavioural abnormality suggests that estrogens absence produces chiefly functional activation-dependent changes. However, the fact that the immunohistochemical abnormalities were not totally normalized by estradiol-treatment suggested that some permanent developmental alterations may occur in ArKO mice.