Involvement of estrogen in rapid pain modulation in the rat spinal cord

Local Synthesis of Estradiol in the Rostral Ventromedial Medulla Protects against Widespread Muscle Pain in Male Mice

Animal studies consistently demonstrate that testosterone is protective against pain in multiple models, including an animal model of activity-induced muscle pain. In this model, females develop widespread muscle hyperalgesia, and reducing testosterone levels in males results in widespread muscle hyperalgesia. Widespread pain is believed to be mediated by changes in the central nervous system, including the rostral ventromedial medulla (RVM). The enzyme that converts testosterone to estradiol, aromatase, is highly expressed in the RVM. Therefore, we hypothesized that testosterone is converted by aromatase to estradiol locally in the RVM to prevent development of widespread muscle hyperalgesia in male mice. This was tested through pharmacological inhibition of estrogen receptors (ERs), aromatase, or ER-α in the RVM which resulted in contralateral hyperalgesia in male mice (C57BL/6J). ER inhibition in the RVM had no effect on hyperalgesia in female mice. As prior studies show modulation of estradiol signaling alters GABA receptor and transporter expression, we examined if removal of testosterone in males would decrease mRNA expression of GABA receptor subunits and vesicular GABA transporter (VGAT). However, there were no differences in mRNA expression of GABA receptor subunits of VGAT between gonadectomized and sham control males. Lastly, we used RNAscope to determine expression of ER-α in the RVM and show expression in inhibitory (VGAT+), serotonergic (tryptophan hydroxylase 2+), and μ-opioid receptor expressing (MOR+) cells. In conclusion, testosterone protects males from development of widespread hyperalgesia through aromatization to estradiol and activation of ER-α which is widely expressed in multiple cell types in the RVM.

Estrogen receptor GPR30 in the anterior cingulate cortex mediates exacerbated neuropathic pain in ovariectomized mice

Menopausal women experience neuropathic pain 63% more frequently than men do, which may attribute to the estrogen withdrawal. However, the underlying mechanisms remain unclear. Here, the role of estrogen receptors (ERs) in ovariectomized (OVX) female mice following chronic constriction injury (CCI) was investigated. With 17β-estradiol (E2) supplemented, aggravated mechanical allodynia in OVX mice could be significantly alleviated, particularly after intra-anterior cingulate cortex (ACC) E2 delivery. Pharmacological interventions further demonstrated that the agonist of G-protein-coupled estrogen receptor 30 (GPR30), rather than ERα or ERβ in the ACC, exhibited the similar analgesic effect as E2, whereas antagonist of GPR30 exacerbated allodynia. Furthermore, OVX surgery reduced GPR30 expression in the ACC, which could be restored with estrogen supplementation. Selective downregulation of GPR30 in the ACC of naïve female mice induces mechanical allodynia, whereas GPR30 overexpression in the ACC remarkedly alleviated OVX-exacerbated allodynia. Collectively, estrogen withdrawal could downregulate the ACC GPR30 expression, resulting in exacerbated neuropathic pain. Our findings highlight the importance of GPR30 in the ACC in aggravated neuropathic pain during menopause, and offer a potential therapeutic candidate for neuropathic pain management in menopausal women.

GPER involvement in inflammatory pain

Chronic pain is a worldwide refractory health disease that causes major financial and emotional burdens and that is devastating for individuals and society. One primary source of pain is inflammation. Current treatments for inflammatory pain are weakly effective, although they usually replace analgesics, such as opioids and non-steroidal anti-inflammatory drugs, which display serious side effects. Emerging evidence indicates that the membrane G protein-coupled estrogen receptor (GPER) may play an important role in the regulation of inflammation and pain. Herein, we focus on the consequences of pharmacological and genetic GPER modulation in different animal models of inflammatory pain. We also provide a brief overview of the putative mechanisms including the direct action of GPER on pain transmission and inflammation.

A Guide to Preclinical Models of Zoster-Associated Pain and Postherpetic Neuralgia

Reactivation of latent varicella-zoster virus (VZV) causes herpes zoster (HZ), which is commonly accompanied by acute pain and pruritus over the time course of a zosteriform rash. Although the rash and associated pain are self-limiting, a considerable fraction of HZ cases will subsequently develop debilitating chronic pain states termed postherpetic neuralgia (PHN). How VZV causes acute pain and the mechanisms underlying the transition to PHN are far from clear. The human-specific nature of VZV has made in vivo modeling of pain following reactivation difficult to study because no single animal can reproduce reactivated VZV disease as observed in the clinic. Investigations of VZV pathogenesis following primary infection have benefited greatly from human tissues harbored in immune-deficient mice, but modeling of acute and chronic pain requires an intact nervous system with the capability of transmitting ascending and descending sensory signals. Several groups have found that subcutaneous VZV inoculation of the rat induces prolonged and measurable changes in nociceptive behavior, indicating sensitivity that partially mimics the development of mechanical allodynia and thermal hyperalgesia seen in HZ and PHN patients. Although it is not a model of reactivation, the rat is beginning to inform how VZV infection can evoke a pain response and induce long-lasting alterations to nociception. In this review, we will summarize the rat pain models from a practical perspective and discuss avenues that have opened for testing of novel treatments for both zoster-associated pain and chronic PHN conditions, which remain in critical need of effective therapies.

Role of sex hormones in modulating breast and ovarian cancer associated pain

According to the National Cancer Institute in 2020 there will be an estimated 21,750 new ovarian cancer cases and 276,480 new breast cancer cases. Both breast and ovarian cancer are hormone dependent cancers, meaning they cannot grow without the presence of hormones. The two most studied hormones in these two cancers are estrogen and progesterone, which are also involved in the modulation of pain. The incidence of pain in breast and ovarian cancer is very high. Research about mechanisms involved in modulation of pain by hormones are still being debated, as some studies find estrogen to be anti-nociceptive and others pro-nociceptive in pain studies. Moreover, analgesic treatments for breast and ovarian cancer-associated pain are limited and often ineffective. In this review, we will focus on estrogen and progesterone mechanisms of action in modulation of pain and cancer. We will also discuss new treatment options for these types of cancer and associated-pain.

G protein-coupled estrogen receptor in the rostral ventromedial medulla contributes to the chronification of postoperative pain

Aims

Chronification of postoperative pain is a common clinical phenomenon following surgical operation, and it perplexes a great number of patients. Estrogen and its membrane receptor (G protein‐coupled estrogen receptor, GPER) play a crucial role in pain regulation. Here, we explored the role of GPER in the rostral ventromedial medulla (RVM) during chronic postoperative pain and search for the possible mechanism.

Methods and Results

Postoperative pain was induced in mice or rats via a plantar incision surgery. Behavioral tests were conducted to detect both thermal and mechanical pain, showing a small part (16.2%) of mice developed into pain persisting state with consistent low pain threshold on 14 days after incision surgery compared with the pain recovery mice. Immunofluorescent staining assay revealed that the GPER‐positive neurons in the RVM were significantly activated in pain persisting rats. In addition, RT‐PCR and immunoblot analyses showed that the levels of GPER and phosphorylated μ‐type opioid receptor (p‐MOR) in the RVM of pain persisting mice were apparently increased on 14 days after incision surgery. Furthermore, chemogenetic activation of GPER‐positive neurons in the RVM of Gper‐Cre mice could reverse the pain threshold of pain recovery mice. Conversely, chemogenetic inhibition of GPER‐positive neurons in the RVM could prevent mice from being in the pain persistent state.

Conclusion

Our findings demonstrated that the GPER in the RVM was responsible for the chronification of postoperative pain and the downstream pathway might be involved in MOR phosphorylation.

Protein Kinases Mediate Anti-Inflammatory Effects of Cannabidiol and Estradiol Against High Glucose in Cardiac Sodium Channels

Background: Cardiovascular anomalies are predisposing factors for diabetes-induced morbidity and mortality. Recently, we showed that high glucose induces changes in the biophysical properties of the cardiac voltage-gated sodium channel (Nav1.5) that could be strongly correlated to diabetes-induced arrhythmia. However, the mechanisms underlying hyperglycemia-induced inflammation, and how inflammation provokes cardiac arrhythmia, are not well understood. We hypothesized that inflammation could mediate the high glucose-induced biophyscial changes on Nav1.5 through protein phosphorylation by protein kinases A and C. We also hypothesized that this signaling pathway is, at least partly, involved in the cardiprotective effects of cannabidiol (CBD) and 17β-estradiol (E₂).

Methods and Results: To test these ideas, we used Chinese hamster ovarian (CHO) cells transiently co-transfected with cDNA encoding human Nav1.5 α-subunit under control, a cocktail of inflammatory mediators or 100 mM glucose conditions (for 24 h). We used electrophysiological experiments and action potential modeling. Inflammatory mediators, similar to 100 mM glucose, right shifted the voltage dependence of conductance and steady-state fast inactivation and increased persistent current leading to computational prolongation of action potential (hyperexcitability) which could result in long QT3 arrhythmia. We also used human iCell cardiomyocytes derived from inducible pluripotent stem cells (iPSC-CMs) as a physiologically relevant system, and they replicated the effects produced by inflammatory mediators observed in CHO cells. In addition, activators of PK-A or PK-C replicated the inflammation-induced gating changes of Nav1.5. Inhibitors of PK-A or PK-C, CBD or E₂ mitigated all the potentially deleterious effects provoked by high glucose/inflammation.

Conclusion: These findings suggest that PK-A and PK-C may mediate the anti-inflammatory effects of CBD and E₂ against high glucose-induced arrhythmia. CBD, via Nav1.5, may be a cardioprotective therapeutic approach in diabetic postmenopausal population.

Ablation of spinal cord estrogen receptor α-expressing interneurons reduces chemically induced modalities of pain and itch

Estrogens are presumed to underlie, at least in part, the greater pain sensitivity and chronic pain prevalence that women experience compared to men. Although previous studies revealed populations of estrogen receptor-expressing neurons in primary afferents and in superficial dorsal horn neurons, there is little to no information as to the contribution of these neurons to the generation of acute and chronic pain. Here we molecularly characterized neurons in the mouse superficial spinal cord dorsal horn that express estrogen receptor α (ERα) and explored the behavioral consequences of their ablation. We found that spinal ERα-positive neurons are largely excitatory interneurons and many coexpress substance P, a marker for a discrete subset of nociceptive, excitatory interneurons. After viral, caspase-mediated ablation of spinal ERα-expressing cells, we observed a significant decrease in the first phase of the formalin test, but in male mice only. ERα-expressing neuron-ablation also reduced pruritogen-induced scratching in both male and female mice. There were no ablation-related changes in mechanical or heat withdrawal thresholds or in capsaicin-induced nocifensive behavior. In chronic pain models, we found no change in Complete Freund's adjuvant-induced thermal or mechanical hypersensitivity, or in partial sciatic nerve injury-induced mechanical allodynia. We conclude that ERα labels a subpopulation of excitatory interneurons that are specifically involved in chemically evoked persistent pain and pruritogen-induced itch.

Progesterone and Allopregnanolone Rapidly Attenuate Estrogen-Associated Mechanical Allodynia in Rats with Persistent Temporomandibular Joint Inflammation

Temporomandibular joint disorder (TMD) is associated with pain in the joint (temporomandibular joint, TMJ) and muscles involved in mastication. TMD pain dissipates following menopause but returns in some women undergoing estrogen replacement therapy. Progesterone has both anti-inflammatory and antinociceptive properties, while estrogen's effects on nociception are variable and highly dependent on both natural hormone fluctuations and estrogen dosage during pharmacological treatments, with high doses increasing pain. Allopregnanolone, a progesterone metabolite and positive allosteric modulator of the GABAA receptor, also has antinociceptive properties. While progesterone and allopregnanolone are antinociceptive, their effect on estrogen-exacerbated TMD pain has not been determined. We hypothesized that removing the source of endogenous ovarian hormones would reduce inflammatory allodynia in the TMJ of rats and both progesterone and allopregnanolone would attenuate the estrogen-provoked return of allodynia. Baseline mechanical sensitivity was measured in female Sprague-Dawley rats (150-175 g) using the von Frey filament method followed by a unilateral injection of complete Freund's adjuvant (CFA) into the TMJ. Mechanical allodynia was confirmed 24 h later; then rats were ovariectomized or received sham surgery. Two weeks later, allodynia was reassessed and rats received one of the following subcutaneous hormone treatments over 5 days: a daily pharmacological dose of estradiol benzoate (E2; 50 μg/kg), daily E2 and pharmacological to sub-physiological doses of progesterone (P4; 16 mg/kg, 16 μg/kg, or 16 ng/kg), E2 daily and interrupted P4 given every other day, daily P4, or daily vehicle control. A separate group of animals received allopregnanolone (0.16 mg/kg) instead of P4. Allodynia was reassessed 1 h following injections. Here, we report that CFA-evoked mechanical allodynia was attenuated following ovariectomy and daily high E2 treatment triggered the return of allodynia, which was rapidly attenuated when P4 was also administered either daily or every other day. Allopregnanolone treatment, whether daily or every other day, also attenuated estrogen-exacerbated allodynia within 1 h of treatment, but only on the first treatment day. These data indicate that when gonadal hormone levels have diminished, treatment with a lower dose of progesterone may be effective at rapidly reducing the estrogen-evoked recurrence of inflammatory mechanical allodynia in the TMJ.

Jingshu Keli and its Components Notoginsenoside R1 and Ginsenoside Rb1 Alleviate the Symptoms of Cervical Myelopathy through Kir3.1 Mediated Mechanisms

Background & Objective::

Cervical Spondylotic Myelopathy (CSM) is one of the most serious spinal cord disorders in adults. Pharmacological modulation of ion channels is a common strategy to interfere with CSM and prevent neuronal damage.

Methods:

Here, we investigated the effects of Jingshu Keli (JSKL), a traditional Chinese herbal formula, on CSM-related gait abnormality, mechanical allodynia and thermal hyperalgesia, and assessed the neuronal mechanisms of JSKL on cultured brainstem cells. Behavioral tests and patch clamp recordings were performed to make this assessment.

Results:

In our study, we found that JSKL significantly recovered the gait performance (P<0.001) and decreased the levels of mechanical pain in 18.9% (P<0.01) and thermal pain in 18.1% (P<0.05). Further investigation suggested that JSKL and its containing ginsenoside Rb1 (GRb1), notoginsenoside R1 (NGR1) reduced the action potential frequency in 38.5%, 27.2%, 25.9%, and hyperpolarized resting membrane potential in 15.0%, 13.8%, 12.1%, respectively. Kir channels, not KV channels and KCa channels, were the major intermediate factors achieving treatment effects. Finally, immunostaining results showed that the phosphorylation of Kir3.1 was promoted, whereas the total expression level did not change.

Conclusion:

Our study reveals a novel strategy of treating CSM by using Traditional Chinese Medicines (TCMs) containing active components.

Estradiol Acts in Lateral Thalamic Region to Attenuate Varicella Zoster Virus Associated Affective Pain

Varicella zoster virus (VZV) results in chicken pox and herpes zoster. Female rats show a higher level of herpes zoster associated pain than males, consistent with human studies. In this study, we addressed the novel hypothesis that sex difference in herpes zoster associated pain is due, in part, to estradiol modulating activity in the thalamus. To test this hypothesis a high and low physiological dose of estradiol was administered to castrated and ovariectomized rats and the affective pain response was measured after injection of VZV into the whisker pad. Thalamic infusion of the estrogen receptor antagonist ICI 182,780 concomitant with a high dose of estradiol addressed the role of estradiol binding to its receptor to effect pain. Phosphorylated extracellular signal-regulated protein kinase (pERK) positive cells were measured in excitatory (glutaminase positive) and inhibitory (glutamate decarboxylase 67 positive) cells of the lateral thalamic region. Our results show that a high dose of estradiol significantly reduced the pain response in both males and females. pERK significantly increased in excitatory cells after treatment with a low dose of estradiol and increased in inhibitory cells after treatment with a high dose of estradiol. Administration of ICI 182,780 significantly increased the pain response, reduced expression of GABA related genes in the thalamic region and significantly reduced the number of inhibitory cells expressing pERK. The results suggest that estradiol attenuates herpes zoster pain by increasing the activity of inhibitory neurons within the thalamus and that this reduction includes an estrogen receptor dependent mechanism.

Sex Differences in Estradiol Secretion by Trigeminal Brainstem Neurons

Estrogen status is a significant risk factor in the development of temporomandibular joint disorders (TMD). Classically, estrogen status is thought to derive mainly from ovarian sources; however, it is well known that estradiol (E2) also is synthesized by neurons in the brain. This study tested the hypothesis that E2 is produced by neurons in trigeminal subnucleus caudalis (Vc), the principal site of termination for sensory afferents that supply the temporomandibular joint (TMJ), to modify evoked responses in a model of TMJ nociception in male and female rats. Intra-TMJ injection of the small fiber excitant, allyl isothiocyanate (AIC), increased the levels of E2 collected from microdialysis probes sites at Vc of ovariectomized (OvX) female rats, ipsilateral to the stimulus, whereas males displayed no change. Dialysate levels of E2 collected from probe sites in the contralateral Vc or cerebellum in OvX rats were not affected by TMJ stimulation. Reverse dialysis of anastrozole, an aromatase (ARO) inhibitor, via the probe reduced perfusate levels of E2 in Vc. Systemic administration of letrozole, a non-steroid ARO inhibitor, for 4 days prevented TMJ-evoked increases in masseter muscle electromyography (MMemg) activity. ARO-positive neurons were distributed mainly in superficial laminae (I-III) at Vc and cell counts revealed no significant difference between OvX and male rats. Intra-TMJ injection of AIC revealed similar numbers of ARO/Fos dual-labeled neurons in OvX and male rats. By contrast, the percentage of ARO neurons co-labeled for glutamic acid decarboxylase (GAD), the biosynthetic enzyme for GABA, was greater in OvX (35%) than male rats (14%). Few ARO-positive neurons were co-labeled for estrogen receptor alpha. These data indicate that E2 is secreted continuously by Vc neurons and that acute stimulation of TMJ nociceptors evokes further secretion in a sex-dependent manner. Reduced TMJ-evoked MMemg activity after ARO inhibition suggests that locally produced E2 by Vc neurons acts via paracrine mechanisms to modify TMJ nociception in female rats.

Aromatase Derived Estradiol Within the Thalamus Modulates Pain Induced by Varicella Zoster Virus

Herpes zoster or shingles is the result of varicella zoster virus (VZV) infection and often results in chronic pain that lasts for months after visible symptoms subside. Testosterone often attenuates pain in males. Previous work demonstrates ovarian estrogen effects γ-aminobutyric acid (GABA) signaling in the thalamus, reducing pain but the role of testosterone within the thalamus is currently unknown. Because aromatase affects pain and is present in the thalamus we tested a hypothesis that testosterone converted to estrogen in the thalamus attenuates herpes zoster induced pain. To address this hypothesis, male Sprague-Dawley rats received whisker pad injection of either MeWo cells or MeWo cells containing VZV. To reduce aromatase derived estrogen in these animals we injected aromatase inhibitor letrozole systemically or infused it into the thalamus. To test if estrogen was working through the estrogen receptor (ER) agonist, 4, 4′, 4″-(4-Propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) was infused concomitant with letrozole. Motivational and affective pain was measured after letrozole and/or PPT treatment. Vesicular GABA transporter (VGAT) is important in pain signaling. Because estrogen effects VGAT expression we measured its transcript and protein levels after letrozole treatment. Virus injection and letrozole significantly increased the pain response but thalamic infusion of PPT reduced zoster pain. Letrozole increased the number of thalamic neurons staining for phosphorylated ERK (pERK) but decreased VGAT expression. The results suggest in male rats aromatase derived estradiol interacts with the ER to increase VGAT expression and increase neuronal inhibition in the thalamus to attenuate VZV induced pain.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

Physical Linkage of Estrogen Receptor α and Aromatase in Rat: Oligocrine and Endocrine Actions of CNS-Produced Estrogens

Rapid-signaling membrane estrogen receptors (mERs) and aromatase (Aro) are present throughout the central nervous system (CNS), enabling acute regulation of CNS estrogenic signaling. We previously reported that spinal membrane Aro (mAro) and mERα oligomerize (1). As their organizational relationship would likely influence functions of locally produced estrogens, we quantified the mAro and mERα that are physically associated and nonassociated in two functionally different regions of rat CNS: the spinal cord, which has predominantly neural functionalities, and the hypothalamus, which has both neural and endocrine capabilities. Quantitative immunoprecipitation (IP), coimmunoprecipitation, and Western blot analysis were used to quantify the associated and nonassociated subpopulations of mAro and mERα. Regardless of estrous-cycle stage, virtually all mAro was oligomerized with mERα in the spinal cord, whereas only ∼15% was oligomerized in the hypothalamus. The predominance of nonassociated mAro in the hypothalamus, in combination with findings that many hypothalamic Aro-immunoreactive neurons could be retrogradely labeled with peripherally injected Fluoro-Gold, suggests that a portion of hypothalamic estrogens is secreted, potentially regulating pituitary function. Moreover, circulating estrogens increased hypothalamic Aro activity (quantified by the tritiated water-release assay) in the absence of increased Aro protein, revealing nongenomic regulation of Aro activity in the mammalian CNS. The demonstrated presence of associated and nonassociated mAro and mERα subpopulations in the CNS suggests that their selective targeting could restore impaired estrogen-dependent CNS functionalities while minimizing unwanted effects. The full physiological ramifications of brain-secreted estrogens remain to be explored.

Toward an effective peripheral visceral analgesic: responding to the national opioid crisis

This minireiew summarizes recent new developments in visceral analgesics. This promising field is important, as a new approach to address abdominal pain with peripheral visceral analgesics is considered a key approach to addressing the current opioid crisis. Some of the novel compounds address peripheral pain mechanisms through modulation of opioid receptors via biased ligands, nociceptin/orphanin FQ opioid peptide (NOP) receptor, or dual action on NOP and μ-opioid receptor, buprenorphine and morphiceptin analogs. Other compounds target nonopioid mechanisms, including cannabinoid (CB2), N-methyl-d-aspartate, calcitonin gene-related peptide, estrogen, and adenosine A_2B receptors and transient receptor potential (TRP) channels (TRPV1, TRPV4, and TRPM8). Although current evidence is based predominantly on animal models of visceral pain, early human studies also support the evidence from the basic and animal research. This augurs well for the development of nonaddictive, visceral analgesics for treatment of chronic abdominal pain, an unmet clinical need.

Expression of aromatase in the rostral ventromedial medulla and its role in the regulation of visceral pain

AIMS

Estrogens are known to exert a wide spectrum of actions on brain functions including modulation of pain. Besides the circulating estrogens produced mainly by the ovaries, many brain regions are also capable of de novo synthesizing estrogens, which may exert important modulatory effects on neuronal functions. This study was aimed to test the hypothesis that aromatase, the enzyme that catalyzes the conversion of testosterone to estradiols, may be distributed in the rostral ventromedial medulla (RVM), where it may impact on visceral pain.

METHODS AND RESULTS

Adult female rats were treated with cyclophosphamide (CPM, 50 mg/kg, ip, once every 3 days) or saline. At approximately day 10 following the 3rd injection, CPM-treated rats exhibited colorectal hyperalgesia as they showed significantly greater abdominal withdrawal responses (AWR) to graded colorectal distension (CRD, 0-100 mm Hg) than the saline group. Immunofluorescent staining and Western blot assay revealed that CPM-induced colorectal hyperalgesia was associated with significantly increased expression of aromatase and phosphorylated μ-type opioid receptor (pMOR) and decreased expression of total MOR in the RVM. Intracisternal application of aromatase inhibitors, fadrozole, and letrozole reversed CPM-induced colorectal hyperalgesia and restored pMOR and MOR expression in the RVM.

CONCLUSIONS

Our observations confirmed the expression of aromatase in the RVM, a pivotal brain region in descending modulation of pain and opioid analgesia. The results support the hypothesis that locally produced estrogens in the RVM may be involved in the maintenance of chronic visceral hyperalgesia and the downstream signaling may involve phosphorylation of MOR.

Neuronal aromatase expression in pain processing regions of the medullary and spinal cord dorsal horn

In both acute and chronic pain conditions, women tend to be more sensitive than men. This sex difference may be regulated by estrogens, such as estradiol, that are synthesized in the spinal cord and brainstem and act locally to influence pain processing. To identify a potential cellular source of local estrogen, here we examined the expression of aromatase, the enzyme that catalyzes the conversion of testosterone to estradiol. Our studies focused on primary afferent neurons and on their central targets in the spinal cord and medulla as well as in the nucleus of the solitary tract, the target of nodose ganglion-derived visceral afferents. Immunohistochemical staining in an aromatase reporter mouse revealed that many neurons in laminae I and V of the spinal cord dorsal horn and caudal spinal trigeminal nucleus and in the nucleus of the solitary tract express aromatase. The great majority of these cells also express inhibitory interneuron markers. We did not find sex differences in aromatase expression and neither the pattern nor the number of neurons changed in a sciatic nerve transection model of neuropathic pain or in the Complete Freund's adjuvant model of inflammatory pain. A few aromatase neurons express Fos after cheek injection of capsaicin, formalin, or chloroquine. In total, given their location, these aromatase neurons are poised to engage nociceptive circuits, whether it is through local estrogen synthesis or inhibitory neurotransmitter release.

G protein-coupled estrogen receptor and estrogen receptor ligands regulate colonic motility and visceral pain

BACKGROUND

Diarrhea-predominant irritable bowel syndrome (IBS-D) is a functional gastrointestinal (GI) disorder, which occurs more frequently in women than men. The aim of our study was to determine the role of activation of classical estrogen receptors (ER) and novel membrane receptor, G protein-coupled estrogen receptor (GPER) in human and mouse tissue and to assess the possible cross talk between these receptors in the GI tract.

METHODS

Immunohistochemistry was used to determine the expression of GPER in human and mouse intestines. The effect of G-1, a GPER selective agonist, and estradiol, a non-selective ER agonist, on muscle contractility was characterized in isolated preparations of the human and mouse colon. To characterize the effect of G-1 and estradiol in vivo, colonic bead expulsion test was performed. G-1 and estradiol activity on the visceral pain signaling was assessed in the mustard oil-induced abdominal pain model.

KEY RESULTS

GPER is expressed in the human colon and in the mouse colon and ileum. G-1 and estradiol inhibited muscle contractility in vitro in human and mouse colon. G-1 or estradiol administered intravenously at the dose of 20 mg/kg significantly prolonged the time to bead expulsion in females. Moreover, G-1 prolonged the time to bead expulsion and inhibited GI hypermotility in both genders. The injection of G-1 or estradiol resulted in a significant reduction in the number of pain-induced behaviors in mice.

CONCLUSIONS AND INFERENCES

GPER and ER receptors are involved in the regulation of GI motility and visceral pain. Both may thus constitute an important pharmacological target in the IBS-D therapy.

Sex differences underlying orofacial varicella zoster associated pain in rats

BACKGROUND

Most people are initially infected with varicella zoster virus (VZV) at a young age and this infection results in chickenpox. VZV then becomes latent and reactivates later in life resulting in herpes zoster (HZ) or "shingles". Often VZV infects neurons of the trigeminal ganglia to cause ocular problems, orofacial disease and occasionally a chronic pain condition termed post-herpetic neuralgia (PHN). To date, no model has been developed to study orofacial pain related to varicella zoster. Importantly, the incidence of zoster associated pain and PHN is known to be higher in women, although reasons for this sex difference remain unclear. Prior to this work, no animal model was available to study these sex-differences. Our goal was to develop an orofacial animal model for zoster associated pain which could be utilized to study the mechanisms contributing to this sex difference.

METHODS

To develop this model VZV was injected into the whisker pad of rats resulting in IE62 protein expression in the trigeminal ganglia; IE62 is an immediate early gene in the VZV replication program.

RESULTS

Similar to PHN patients, rats showed retraction of neurites after VZV infection. Treatment of rats with gabapentin, an agent often used to combat PHN, ameliorated the pain response after whisker pad injection. Aversive behavior was significantly greater for up to 7 weeks in VZV injected rats over control inoculated rats. Sex differences were also seen such that ovariectomized and intact female rats given the lower dose of VZV showed a longer affective response than male rats. The phase of the estrous cycle also affected the aversive response suggesting a role for sex steroids in modulating VZV pain.

CONCLUSIONS

These results suggest that this rat model can be utilized to study the mechanisms of 1) orofacial zoster associated pain and 2) the sex differences underlying zoster associated pain.

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.

Association between estrogen levels and temporomandibular disorders: a systematic literature review

Introduction

To evaluate whether the hypothesis that estrogen levels are associated with temporomandibular disorders (TMD) in humans can be confirmed or contradicted by available literature.

Material and methods

A systematic review based on the content of PubMed, Scopus, and Cochrane Library databases was performed. Studies were identified using a combination of key words ‘temporomandibular disorder’ and ‘estrogen’. Nine studies were included into our review.

Results

The relationship between estrogen levels and TMD was found in seven out of nine reviewed papers. Results from two papers suggest that a high estrogen level is associated with an increased prevalence of TMD. Five additional papers found a relationship between a low estrogen level and an increase in TMD pain. In considering the value of evidence and inconsistencies of results in the reviewed publications, we state that there is weak evidence to support the hypothesis that estrogen levels are associated with TMD.

Conclusions

Results of reviewed studies were divergent and sometimes contradictory. One possible explanation is that estrogen influences TMD pain processing differently than temporomandibular joints (TMJ) structures, as shown in many animal studies. Estrogen may influence TMD pain processing differently than TMJ structures. We suggest consideration of the dual action of estrogen when planning future studies on its association with TMD.

Pioglitazone rapidly reduces neuropathic pain through astrocyte and nongenomic PPARγ mechanisms

Repeated administration of peroxisome proliferator-activated receptor gamma (PPARγ) agonists reduces neuropathic pain-like behavior and associated changes in glial activation in the spinal cord dorsal horn. As PPARγ is a nuclear receptor, sustained changes in gene expression are widely believed to be the mechanism of pain reduction. However, we recently reported that a single intrathecal (i.t.) injection of pioglitazone, a PPARγ agonist, reduced hyperalgesia within 30 minutes, a time frame that is typically less than that required for genomic mechanisms. To determine the very rapid antihyperalgesic actions of PPARγ activation, we administered pioglitazone to rats with spared nerve injury and evaluated hyperalgesia. Pioglitazone inhibited hyperalgesia within 5 minutes of injection, consistent with a nongenomic mechanism. Systemic or i.t. administration of GW9662, a PPARγ antagonist, inhibited the antihyperalgesic actions of intraperitoneal or i.t. pioglitazone, suggesting a spinal PPARγ-dependent mechanism. To further address the contribution of nongenomic mechanisms, we blocked new protein synthesis in the spinal cord with anisomycin. When coadministered intrathecally, anisomycin did not change pioglitazone antihyperalgesia at an early 7.5-minute time point, further supporting a rapid nongenomic mechanism. At later time points, anisomycin reduced pioglitazone antihyperalgesia, suggesting delayed recruitment of genomic mechanisms. Pioglitazone reduction of spared nerve injury-induced increases in GFAP expression occurred more rapidly than expected, within 60 minutes. We are the first to show that activation of spinal PPARγ rapidly reduces neuropathic pain independent of canonical genomic activity. We conclude that acute pioglitazone inhibits neuropathic pain in part by reducing astrocyte activation and through both genomic and nongenomic PPARγ mechanisms.

Colocalization of aromatase in spinal cord astrocytes: differences in expression and relationship to mechanical and thermal hyperalgesia in murine models of a painful and a non-painful bone tumor

While spinal cord astrocytes play a key role in the generation of cancer pain, there have been no studies that have examined the relationship of tumor-induced astrocyte activation and aromatase expression during the development of cancer pain. Here, we examined tumor-induced mechanical hyperalgesia and cold allodynia, and changes in Glial fibrillary acid protein (GFAP) and aromatase expression in murine models of painful and non-painful bone cancer. We demonstrate that implantation of fibrosarcoma cells, but not melanoma cells, produces robust mechanical hyperalgesia and cold allodynia in tumor-bearing mice compared to saline-injected controls. Secondly, this increase in mechanical hyperalgesia and cold allodynia is mirrored by significant increases in both spinal astrocyte activity and aromatase expression in the dorsal horn of fibrosarcoma-bearing mice. Importantly, we show that aromatase is only found within a subset of astrocytes and not in neurons in the lumbar spinal cord. Finally, administration of an aromatase inhibitor reduced tumor-induced hyperalgesia in fibrosarcoma-bearing animals. We conclude that a painful fibrosarcoma tumor induces a significant increase in spinal astrocyte activation and aromatase expression and that the up-regulation of aromatase plays a role in the development of bone tumor-induced hyperalgesia. Since spinal aromatase is also upregulated, but to a lesser extent, in non-painful melanoma bone tumors, it may also be neuroprotective and responsive to the changing tumor environment.

Migraine and estrogen

PURPOSE OF REVIEW

The aim is to systematically and critically review the relationship between migraine and estrogen, the predominant female sex hormone, with a focus on studies published in the last 18 months.

RECENT FINDINGS

Recent functional MRI (fMRI) studies of the brain support the existence of anatomical and functional differences between men and women, as well as between participants with migraine and healthy controls. In addition to the naturally occurring changes in endogenous sex hormones over the lifespan (e.g. puberty and menopause), exogenous sex hormones (e.g. hormonal contraception or hormone therapy) also may modulate migraine. Recent data support the historical view of an elevated risk of migraine with significant drops in estrogen levels. In addition, several lines of research support that reducing the magnitude of decline in estrogen concentrations prevents menstrually related migraine (MRM) and migraine aura frequency.

SUMMARY

Current literature has consistently demonstrated that headache, in particular migraine, is more prevalent in women as compared with men, specifically during reproductive years. Recent studies have found differences in headache characteristics, central nervous system anatomy, as well as functional activation by fMRI between the sexes in migraine patients. Although the cause underlying these differences is likely multifactorial, considerable evidence supports an important role for sex hormones. Recent studies continue to support that MRM is precipitated by drops in estrogen concentrations, and minimizing this decline may prevent these headaches. Limited data also suggest that specific regimens of combined hormone contraceptive use in MRM and migraine with aura may decrease both headache frequency and aura.

Acute estrogen surge enhances inflammatory nociception without altering spinal Fos expression

Chronic pain is a major neurological disorder that can manifest differently between genders or sexes. The complex actions of sex hormones may underlie these differences; previous studies have suggested that elevated estrogen levels can enhance pain perception. The purpose of this study was to investigate the hypothesis that acute, activational effects of estradiol (E2) increase persistent inflammatory nociception, and anatomically where this modulation occurs. Spinal expression of Fos is widely used as a marker of nociceptive activation. This study used formalin-evoked nociception in ovariectomized (OVX) adult female rats and measured late-phase hindlimb flinching and Fos expression in the spinal cord, and their modification by acute estrogen supplementation similar to a proestrus surge. Six days after ovariectomy, female rats were injected subcutaneously (s.c.) with 10μg/kg E2 or vehicle. Twenty-four hours later, 50μL of 1.25% or 100μL of 5% formalin was injected into the right hindpaw; hindlimb flinches were counted, and spinal cords removed 2h after formalin injection. The numbers of Fos-expressing neurons in sections of the lumbar spinal cord were analyzed using immunohistochemistry. Formalin-induced inflammation produced a dose-dependent increase in late-phase hindlimb flinching, and E2 pretreatment increased flinching following 5%, but not 1.25% formalin injection. Despite the modification of behavior by E2, the number of spinal Fos-positive neurons was not altered by E2 pretreatment. These findings demonstrate that an acute proestrus-like surge in serum estrogen can produce a stimulus-intensity-dependent increase in inflammation-evoked nociceptive behavior. However, the lack of effect on spinal Fos expression suggests that this enhancement of nociceptive signaling by estrogen is independent of changes in peripheral activation of, expression of the immediate early gene Fos by, or signal throughput of spinal nociceptive neurons.

Effects of estradiol on voltage-gated potassium channels in mouse dorsal root ganglion neurons

Voltage-gated potassium channels are regulators of membrane potentials, action potential shape, firing adaptation, and neuronal excitability in excitable tissues including in the primary sensory neurons of dorsal root ganglion (DRG). In this study, using the whole-cell patch-clamp technique, the effect of estradiol (E2) on voltage-gated total outward potassium currents, the component currents transient "A-type" current (I A) currents, and "delayed rectifier type" (I KDR) currents in isolated mouse DRG neurons was examined. We found that the extracellularly applied 17β-E2 inhibited voltage-gated total outward potassium currents; the effects were rapid, reversible, and concentration-dependent. Moreover, the membrane impermeable E2-BSA was as efficacious as 17β-E2, whereas 17α-E2 had no effect. 17β-E2-stimulated decrease in the potassium current was unaffected by treatment with ICI 182780 (classic estrogen receptor antagonist), actinomycin D (RNA synthesis inhibitor), or cycloheximide (protein synthesis inhibitor). We also found that I A and I KDR were decreased after 17β-E2 application. 17β-E2 significantly shifted the activation curve for I A and I KDR channels in the hyperpolarizing direction. In conclusion, our results demonstrate that E2 inhibited voltage-gated K(+) channels in mouse DRG neurons through a membrane ER-activated non-genomic pathway.

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.

Activation of estrogen receptor α enhances bradykinin signaling in peripheral sensory neurons of female rats

Numerous studies have demonstrated that females have a higher risk of experiencing several pain disorders with either greater frequency or severity than males. Although the mechanisms that underlie this sex disparity remain unclear, several studies have shown an important role for sex steroids, such as estrogen, in the modulation of nociception. Receptors for estrogen are present in primary afferent neurons in the trigeminal and dorsal root ganglia, and brief exposure to estrogen increases responses to the inflammatory mediator bradykinin (BK). However, the mechanism for estrogen-mediated enhancement of BK signaling is not fully understood. The aim of the present study was to evaluate the relative contributions of estrogen receptor α (ERα), ERβ, and G protein-coupled estrogen receptor 1 (GPER) to the enhanced signaling of the inflammatory mediator BK by 17β-estradiol (17β-E2) in primary sensory neurons from female rats in culture (ex vivo) and in behavioral assays of nociception in vivo. The effects of 17β-E2 on BK responses were mimicked by ERα-selective agonists and blocked by ERα-selective antagonists and by small interfering RNA knockdown of ERα. The data indicate that ERα is required for 17β-E2-mediated enhancement of BK signaling in peripheral sensory neurons in female rats.

Exposure to bisphenol A exacerbates migraine-like behaviors in a multibehavior model of rat migraine

Migraine is a common and debilitating neurological disorder suffered worldwide. Women experience this condition 3 times more frequently than men, with estrogen strongly implicated to play a role. Bisphenol A (BPA), a highly prevalent xenoestrogen, is known to have estrogenic activity and may have an effect in migraine onset, intensity, and duration through estrogen receptor signaling. It was hypothesized that BPA exposure exacerbates migraine symptoms through estrogen signaling and downstream activation of nociception related pathways. Utilizing a multibehavior model of migraine in ovariectomized female rats, changes in locomotion, light and sound sensitivity, grooming, and acoustic startle were examined. Furthermore, changes in the expression of genes related to estrogen (ERα, GPR30), and nociception (extracellular signal regulated kinase, ERK, sodium gated channel, Nav1.8, and fatty acid amide hydrolase, FAAH) were studied following behavioral experiments. The following results were obtained: BPA treatment significantly exacerbated migraine-like behaviors in rats. Rats exposed to BPA demonstrated decreased locomotion, exacerbated light and sound aversion, altered grooming habits, and enhanced startle reflexes. Furthermore, BPA exposure increased mRNA expression of estrogen receptors, total ERK mRNA and ERK activation, as well as Nav1.8, and FAAH mRNA, indicative of altered estrogen signaling and altered nociception. These results show that BPA, an environmentally pervasive xenoestrogen, exacerbates migraine-like behavior in a rat model and alters expression of estrogen and nociception-related genes.

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.

Effects of estradiol on voltage-gated sodium channels in mouse dorsal root ganglion neurons

Estrogen has multiple actions in the brain to modulate homeostasis, synaptic plasticity, neuroprotection and pain sensitivity. Previous studies have demonstrated that estradiol may affect the ion channel function. The role of voltage-gated sodium channels in the transmission of nociceptive and neuropathic pain messages is well-established. Herein, we report the effects of estradiol (E2) on TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na(+) currents, using a conventional whole-cell patch clamp technique from acutely isolated mouse dorsal root ganglion neurons. We found that the extracellularly 17β-E2 inhibited TTX-S Na(+) currents and TTX-R Na(+) currents; the effects were rapid, reversible and in a concentration-dependent manner. Moreover, 17β-E2 did not significantly affect the activation curve for Na(+) channel, and shifted the steady-state inactivation curve for TTX-S and TTX-R Na(+) channels in the hyperpolarizing direction. We also found that the membrane impermeable E2-BSA was as efficacious as 17β-E2, whereas 17α-E2 had no effect. Blockers of PKC (GÖ-6983) and PKA (H-89) abrogated these acute effects of 17β-E2. In conclusion, E2 inhibited voltage-gated Na(+) channels in mouse DRG neurons through a membrane ER-activated PKC-PKA signaling pathway. Through the modulation of voltage-gated sodium currents, estradiol could affect cell excitability, firing properties.