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By Rebecca S. Hornung, Namrata GR Raut, Daisy J. Cantu, Lauren M Lockhart, and Dayna L. Averitt

Excerpt from the article published in Molecular Pain. January 2022. DOI: 10.1177/17448069211069255

Editor’s Highlights

• Pain signaled by the trigeminal nociceptors is common, often debilitating, and includes musculoskeletal, neurovascular, and neuropathic pain
• Trigeminal sensory neurons are an anatomical substrate for the reported antinociceptive activity of progesterone via Sig-1R and/or conversion to allopregnanolone.
• The trigeminal ganglia are thus an anatomical substrate for the antinociceptive actions of progesterone and its metabolite allopregnanolone on orofacial or craniofacial pain. 
• Sig-1Rs are available to progesterone in the trigeminal ganglia of female rats in similar concentrations across the estrous cycle and that progesterone could be metabolized locally in trigeminal sensory neurons as the required enzymes are present.

Abstract

Orofacial pain disorders are predominately experienced by women. Progesterone, a major ovarian hormone, is neuroprotective and antinociceptive. We recently reported that progesterone attenuates estrogen-exacerbated orofacial pain behaviors, yet it remains unclear what anatomical substrate underlies progesterone’s activity in the trigeminal system. Progesterone has been reported to exert protective effects through actions at intracellular progesterone receptors (iPR), membrane-progesterone receptors (mPR), or sigma 1 receptors (Sig-1R). Of these, the iPR and Sig-1R have been reported to have a role in pain. Progesterone can also have antinociceptive effects through its metabolite, allopregnanolone. Two enzymes, 5α-reductase and 3α-hydroxysteroid dehydrogenase (3α-HSD), are required for the metabolism of progesterone to allopregnanolone. Both progesterone and allopregnanolone rapidly attenuate pain sensitivity, implicating action of either progesterone at Sig-1R and/or conversion to allopregnanolone which targets GABA_A receptors. In the present study, we investigated whether Sig-1 Rs are expressed in nociceptors within the trigeminal ganglia of cycling female rats and whether the two enzymes required for progesterone metabolism to allopregnanolone, 5α-reductase and 3α-hydroxysteroid dehydrogenase, are also present. Adult female rats from each stage of the estrous cycle were rapidly decapitated and the trigeminal ganglia collected. Trigeminal ganglia were processed by either fluorescent immunochemistry or western blotting to for visualization and quantification of Sig-1R, 5α-reductase, and 3α-hydroxysteroid dehydrogenase. Here we report that Sig-1Rs and both enzymes involved in progesterone metabolism are highly expressed in a variety of nociceptive sensory neuron populations in the female rat trigeminal ganglia at similar levels across the four stages of the estrous cycle. These data indicate that trigeminal sensory neurons are an anatomical substrate for the reported antinociceptive activity of progesterone via Sig-1R and/or conversion to allopregnanolone.

Introduction

Orofacial pain is a general term used for pain conditions that affect structures of the head, face, neck, and oral cavity. These cranial and orofacial structures are innervated by trigeminal nociceptors that have cell bodies in the trigeminal ganglia and send nociceptive sensory input to the central nervous system. Pain signaled by the trigeminal nociceptors is common, often debilitating,¹ and includes musculoskeletal, neurovascular, and neuropathic pain.^2,3 In fact, orofacial pain affects 26% of the general population with the majority of patients being women.^4,5 Some orofacial pain disorders, such as temporomandibular joint disorders (TMD),^6–8 cervicogenic headaches,⁹ tension headaches,^10,11 and migraine,¹⁰ are more common prior to menopause, while others, such as trigeminal neuralgia ,¹² and burning mouth syndrome,¹³ are more common after menopause. Women also report greater pain sensitivity,¹⁴ less tolerance, and a lower pain threshold than men.¹⁵

Importantly, women report variance in their orofacial pain across their menstrual cycle with pain sensitivity increasing in the late luteal phase and peaking during menses .^16, 17 Women that are pregnant or postmenopausal report a decrease in orofacial pain sensitivity, particularly TMD, with some postmenopausal women reporting their TMD pain reemerges following hormone replacement therapy (HRT).^18–20 Preclinical studies also report a reduction in nociceptive responses in pregnant female rats following formalin injection into the temporomandibular joint (TMJ).²¹ Formalin or glutamate injection into the TMJ of female rats also results in increased pain sensitivity during diestrus when 17β-estradiol levels begin to rise.²² Despite the clear link between fluctuating gonadal hormones and TMD pain, the mechanisms underlying the effects of gonadal hormones on TMD pain remains elusive.

Estrogen and progesterone, the major female gonadal hormones, are primarily synthesized in the ovaries of females. These gonadal hormones are also neurosteroids as they can be locally synthesized in neurons and glial cells,^23,24 which allows these hormones to directly modulate both the central and peripheral nervous system. Although estrogen has been reported to be both pronociceptive and antinociceptive in various pain models,^22,25–29 the research literature exclusively reports that progesterone is anti-inflammatory and antinociceptive .^30–36 We have recently reported that estrogen increases pain behaviors in a rat model of inflammatory TMD pain which is reversed with progesterone treatment.³⁷Progesterone is also metabolized by 5α-reductase to 5α-dihydroprogesterone, which is then converted to the metabolite allopregnanolone by 3α-hydroxysteroid dehydrogenase (3α-HSD), thus progesterone’s protective effects can occur through its metabolite allopregnanolone. Allopregnanolone is a positive allosteric modulator of the γ-aminobutyric acid type A (GABA_A) receptor, which underlies its antinociceptive properties.³⁸ Indeed, we recently reported that injection of allopregnanolone can attenuate pain behaviors in a rat model of inflammatory TMD pain.³⁷ In support, mechanical and thermal pain thresholds in spinal cord injury animals were reduced following either an intrathecal injection of Provera, a pharmacological inhibitor of 3α-HSD activity,³⁹ or siRNA knockdown 3α-HSD.⁴⁰

Despite the evidence that progesterone and allopregnanolone are antinociceptive and attenuate orofacial pain, it is currently unknown whether progesterone or allopregnanolone can act directly at trigeminal sensory neurons. Progesterone can exert protective effects through the intracellular progesterone receptor (iPR), membrane-progesterone receptors (mPR), sigma-1 receptors (Sig-1R), and its neuroactive metabolite allopregnanolone.^41–43Both iPRs and mPRs are widely expressed within the brain⁴¹ and within the trigeminal ganglia.^44, 45 Although the role of mPRs in nociception is unknown, iPRs alone do not completely attenuate inflammatory TMJ allodynia⁴⁵ indicating other mechanisms, such as allopregnanolone or Sig-1R, at play. Progesterone may be metabolized locally in sensory neurons to allopregnanolone to inhibit pain via GABA_A receptors expressed in trigeminal sensory neurons. Alternatively, progesterone may act at the Sig-1R in trigeminal sensory neurons attenuating nociception. Sig-1R is a non-opioid receptor located within the plasma membrane of the endoplasmic reticulum. Agonists for the Sig-1R elicit nociceptive responses,^46–49 which are reversed by antagonists.^48–55 Progesterone acts as an antagonist of the Sig-1R.⁴²

The current study was designed to determine whether Sig-1Rs and/or the enzymes involved in the conversion of progesterone to allopregnanolone are present in the nociceptive population of sensory neurons of the female rat trigeminal ganglia. As female rats may have variations in expression levels of membrane proteins when ovarian hormones fluctuate across the estrous cycle,^56,57 we also examined whether the expression of the enzymes and Sig-1Rs display plasticity in expression levels across diestrus, proestrus, and estrus. Here we utilized fluorescent immunohistochemistry, confocal microscopy, and western blotting techniques to uncover two available mechanisms underlying the effects of progesterone on orofacial pain.

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Results

Sigma-1 receptors are present in nociceptive neural populations of the female rat trigeminal ganglia

Protein levels of Sig-1R expression were analyzed by western blot (Figure 1(a)) across the female rat estrous cycle. Levels of Sig-1R did not significantly vary across the rat estrous cycle [F (3, 12) = 1.036; p > 0.05] (Figure 1(b)). Sig-1R immunoreactivity (Figure 2(a) and (d)) and PGP9.5 immunoreactivity (Figure 2(b) and (e)) were observed within the same trigeminal ganglia sensory neuron populations (Figure 2(c) and (f)). Sig-1R immunoreactivity (Figure 2(g) and (j)) and NF200 immunoreactivity (Figure 2(h) and (k)) was also observed within the same trigeminal ganglia sensory neuron populations (Figure 2(i) and (l)). All PGP9.5+ cells and NF200+ cells also expressed Sig-1R (open bars; Figure 12(a) and (b)). Sig-1R immunoreactivity was observed in liver (positive control), but not following blocking peptide (negative control) (data not shown). These data reveal that trigeminal sensory neurons, including myelinated neurons, express Sig-1R.

Sig-1R immunoreactivity (Figure 3(a) and (d)) and Nav1.8 immunoreactivity (Figure 3(b) and (e)) were also observed within the same trigeminal ganglia neurons (Figure 3(c) and (f)). Approximately 79% of the Nav1.8 population co-expressed Sig-1R (open bar; Figure 12(c)). Trigeminal ganglia neurons that were immunoreactive for Sig-1R (Figure 3(h) and (k)) and isolectin IB4 (Figure 3(g) and (j)) were also observed in the female rat trigeminal ganglia in the same neuron populations (Figure 3(i) and (l)). Approximately 69% of the non-peptidergic population expressed Sig-1R (open bars; Figure 12(d)). These data reveal that nociceptive and non-peptidergic trigeminal sensory neurons largely express Sig-1R.

TRPV1 immunoreactivity was observed in a subpopulation of trigeminal ganglia neurons defined as nociceptive sensory neurons (Figure 4(a) and (e)). TRPV1-positive cells also expressed Sig-1R (Figure 4(b) and (f)), of which about 91% of TRPV1-positive were also positive for Sig-1R (Figure 12(e)). TRPV1 immunoreactivity overlapped with CGRP immunoreactivity, as expected (Figure 4(c) and (g)). The peptidergic TRPV1 population of sensory neurons highly expressed Sig-1R (Figure 4(d) and (h)). In line with the high expression of Sig-1R in the TRPV1-expressing neuron subpopulation, about 92% of the CGRPergic population also expressed Sig-1R positive (open bars; Figure 12(f)). These data reveal that the nociceptive, peptidergic population of trigeminal sensory neurons highly express Sig-1R.

Progesterone metabolizing enzymes are present in nociceptive neuron populations of the female rat trigeminal ganglia

The progesterone metabolizing enzyme 5α-reductase was detected by western blot in the female rat trigeminal ganglia at each stage of the estrous cycle (Figure 5(a)). The level of 5α-reductase did not significantly vary across the rat estrous cycle [F (3,12) = 2.171; p > 0.05] (Figure 5(b)). Further, the 3α-HSD enzyme was also detected by western blot in the female rat trigeminal ganglia at each stage of the estrous cycle (Figure 5(c)). The level of 3α-HSD also did not significantly vary across the rat estrous cycle [F (3,12) = 0.3112; p > 0.05] (Figure 5(d)). Double immunohistochemical staining of trigeminal ganglia of intact female rats revealed that PGP9. 5-positive neurons (Figure 6(a) and (d)) and 5α-reductase-positive cells (Figure 6(b) and (e)) overlapped in immunoreactivity (Figure 6(c) and (f)) with 100% of PGP9. 5 immunoreactive cells also being immunoreactive for 5α-reductase (gray bar; Figure 12(a)). Neurons immunoreactive for NF200 (Figure 6(g) and (j)) and cells immunoreactive for 5α-reductase (Figure 6(h) and (k)) also overlapped in labeling (Figure 6(i) and (l)). Again, all NF200-positive cells contained 5α-reductase (gray bar; Figure 12(b)). 5α-reductase immunoreactivity was observed in prostate tissue (positive control) but absent in testes (negative control) (data not shown). These data reveal that trigeminal sensory neurons, including myelinated neurons, contain an enzyme necessary for progesterone metabolism.

Further, expression of the sodium ion channel Nav1.8 identified a subpopulation of trigeminal nociceptive sensory neurons (Figure 7(a) and (d)) and localization of the progesterone metabolizing enzyme 5α-reductase (Figure 7(b) and (e)) in the Nav1.8-positive population (Figure 7(c) and (f)). About 97% of the Nav1.8 population was 5α-reductase-positive (gray bar; Figure 12(c)), which provides further evidence that progesterone metabolism occurs in nociceptive trigeminal sensory neurons in the female rat. Non-peptidergic cells labeled with Isolectin IB4 (Figure 7(g) and (j)) and cells that contained 5α-reductase enzyme (Figure 7(h) and (k)) were observed to be co-localized in a subpopulation of trigeminal sensory neurons (Figure 7(i) and (l)). Approximately 87% of Isolectin IB4-positive cells were also immunoreactive for 5α-reductase (gray bar; Figure 12(d)). Immunoreactivity for a subpopulation of nociceptive sensory neurons, the TRPV1 population, was observed in the trigeminal ganglia of female rats (Figure 8(a) and (e)). This population was highly peptidergic, as expected and observed as CGRP immunoreactivity (Figure 8(b) and (f)). Of the TRPV1-positive cells, 94% were also positive for 5α-reductase (gray bar; Figure 12(e) and 90% of the CGRP-positive cells contained 5α-reductase (gray bar; Figure 12(f)). Thus, the 5α-reductase enzyme (Figure 8(c) and (g)) was expressed in a subpopulation of peptidgergic, TRPV1-positive female rat trigeminal sensory neurons (Figure 8(d) and (h)).

Similar findings were observed for another progesterone metabolizing enzyme 3α-hydroxysteroid dehydrogenase (3α-HSD). Double immunohistochemical staining of the trigeminal ganglia of intact female rats revealed that PGP9.5-positive neurons (Figure 9(a) and (d)) and 3α-HSD-positive cells (Figure 9(b) and (e)) overlapped in immunoreactivity (Figure 9(c) and (f)), such that 96% of the PGP9.5 population were also immunoreactive for 3α-HSD (closed bars; Figure 12(a)). Neurons immunoreactive for NF200 (Figure 9(g) and (j)) and cells immunoreactive for 3α-HSD (Figure 9(h) and (k)) also overlapped in labeling (Figure 9(i) and (l)). All of NF200-positive neurons were also immunoreactive for 3α-HSD (closed bars; Figure 12(b)). 3α-HSD immunoreactivity was observed in prostate tissue (positive control) but absent in testes (negative control) (data not shown). Together these data reveal that trigeminal sensory neurons, including myelinated neurons, contain two enzymes necessary for progesterone metabolism.

Further, expression of the sodium ion channel Nav1.8 identified a subpopulation of trigeminal nociceptive sensory neurons (Figure 10(b) and (e)) and localization of progesterone metabolizing enzyme 3α-HSD (Figure 10(a) and (d)) in the Nav1.8-positive population (Figure 10(c) and (f)) provides further evidence that progesterone metabolism occurs in female rat trigeminal nociceptors. Approximately 99% of the Nav1.8 population was also immunoreactive for 3α-HSD (closed bars; Figure 12(c)). Non-peptidergic cells labeled with Isolectin IB4 (Figure 10 (g) and (j)) and cells that contained 3α-HSD enzyme (Figure 10(h) and (k)) were observed to be highly co-localized (Figure 10(i) and (l)). Of this non-peptidergic trigeminal neuron population, approximately 94% were 3α-HSD-positive (closed bar; Figure 12(d)). Immunoreactivity for TRPV1 was observed in the trigeminal ganglia of female rats (Figure 11(a) and (e)). This population was again highly peptidergic, observed as CGRP immunoreactivity (Figure 11(c) and (g)). The 3α-HSD enzyme (Figure 11(b) and (f)) was expressed in a subpopulation of peptidgergic, TRPV1-positive trigeminal sensory neurons (Figure 11(d) and (h)). All TRPV1-positive cells were also positive for 3α-HSD (closed bar; Figure 12(e) and 98% of the CGRP-positive cells contained 3α-HSD (closed bar; Figure 12(f)).

Discussion

Previously, we reported the rapid attenuation of high estradiol-evoked mechanical allodynia by two different doses of progesterone, as well as the acute, rapid attenuation by the progesterone metabolite, allopregnanolone, in female rats with persistent temporomandibular joint inflammation.³⁷ The orofacial region is innervated by the trigeminal nerve, which has nociceptors that are sensitive to noxious chemical, mechanical, and thermal stimuli. Nociceptors innervating the cranio-orofacial region have cell bodies located in the trigeminal ganglia and are excited by noxious chemical, mechanical, and thermal stimuli. Excitation of the nociceptors is relayed to the trigeminal nucleus subcaudalis in the medullary spinal cord, which transmits the signal on to the thalamus and somatosensory cortex.⁵⁸ Progesterone’s protective mechanisms can potentially occur within any of the peripheral and central nervous system anatomical locations mentioned above. Within the trigeminal system, estrogen upregulates inflammatory mediators,^25,59,60 ion channels,^26,29,61,62 and increases excitability of sensory neurons.^27,63,64 Although most studies focus on estrogen’s effects on nociception, several have investigated progesterone’s effects on the trigeminal system. Data from these studies indicates an antinociceptive role for progesterone within the trigeminal system.^22,37,65

Of the ovarian hormones, estrogen and progesterone, there is extensive evidence supporting a protective role for progesterone in nervous system diseases, disorders, and injuries. Although progesterone is known to be antinociceptive, the anatomical substrate for progesterone’s attenuation of inflammatory orofacial pain has not been clearly identified. Two major mechanisms known to have a role in progesterone’s antinociceptive effects are the Sig-1R and allopregnanolone. Progesterone is an antagonist at Sig-1R, but through metabolism to allopregnanolone, progesterone can indirectly potentiate GABA_A receptors. Conversion of progesterone to allopregnanolone requires two enzymes, 5α-reductase and 3α-HSD. Progesterone is first metabolized to 5α-dihydroprogesterone by 5α-reductase. 5α-dihydroprogesterone is then converted to allopregnanolone by the aldo-keto reductase, 3α-hydroxysteroid dehydrogenase. None of the neuroanatomical targets mentioned here have been reported in the female trigeminal ganglia. In the current study, we hypothesized that Sig-1R and the progesterone metabolizing enzymes are present in nociceptive sensory neurons of the female rat trigeminal ganglia as a potential anatomical substrate for the antinociceptive actions of progesterone. Here, we are the first to report that the Sig-1R, 5α-reductase, and 3α-HSD are found in nociceptive sensory neuron populations of the trigeminal ganglia of intact, naturally cycling female rats and do not vary across the estrous cycle.

Sig-1R is a non-opioid endoplasmic reticulum chaperone protein located within the mitochondrial-associated endoplasmic reticulum membrane⁶⁶ and highly expressed in several pain-related areas, including the dorsal root ganglia (DRG), periaqueductal gray, thalamus, and basolateral amygdala.^67–69 Our data contributes the finding that Sig-1R are highly expressed in the trigeminal ganglia of female rats and expression levels do not appear to be altered by fluctuating gonadal hormones. Sig-1Rs are found in both the TRPV1 and Nav1.8 subpopulations of sensory neurons that can be identified as nociceptors.⁷⁰ Further, we found that Sig-1R are found in myelinated (NF200+) neurons. Progesterone is known to have promyelinating effects within the nervous system,⁷¹ which may attenuate pain associated with demyelinating diseases, such as Charcot-Marie tooth disease or multiple sclerosis where patients experience “burning” and “stabbing” pain.^72,73

Nociceptors can also be peptidergic (CGRP-positive) and non-peptidergic (IB4-positive). Since CGRP is expressed in some non-nociceptive cells, we particularly highlight that Sig-1Rs are found in the CGRPergic TRPV1 population. Progesterone consistently downregulates inflammatory mediators,^74–76 including the proinflammatory and pronociceptive mediator CGRP.⁷⁷ In support, progesterone deficiency augments CGRP activity.⁷⁸ Thus, progesterone could potentially reduce CGRP activity within the trigeminal ganglia to contribute to the reduction in orofacial pain observed in our previous study.³⁷CGRP is released upon TRPV1 activation⁷⁹ and CGRPergic TRPV1 neurons are highly expressed within the trigeminal ganglia.⁸⁰ Our findings contribute evidence that the CGRPergic TRPV1 population in female rats contains targets for progesterone. In support, previous studies report Sig-1R agonists facilitate capsaicin-induced mechanical allodynia,⁴⁹which is reversed by Sig-1R antagonists.^81–83 Additionally, our data support previous research reporting that Sig-1R colocalizes with TRPV1 in the DRGs of male and female mice and progesterone reduces TRPV1 expression.⁶⁹ Non-peptidergic nociceptors have also been shown to express Sig-1R in the DRGs of male rats⁶⁸ and here we add that non-peptidergic nociceptors in the trigeminal ganglia of female rats also express Sig-1R.

Our previous study also reported that conversion to allopregnanolone by the enzymes 5α-reductase and 3α-HSD contributes to the antinociceptive properties of progesterone on orofacial pain,³⁷ as allopregnanolone is a positive allosteric modulator of the GABA_Areceptor. In support, progesterone metabolites and allopregnanolone can attenuate nociceptive behaviors.⁸⁴ 5α-reductase and 3α-HSD are expressed within the central nervous system,^85,86 the peripheral nervous system within the sciatic nerve,⁸⁷ and in the DRG.⁴⁰Here, we contribute to the known locations of 5α-reductase and 3α-HSD to include the nociceptive sensory neuron populations of the female rat trigeminal ganglia.

5α-reductase and 3α-HSD enzymes were found within peptidergic, non-peptidergic, and myelinated trigeminal ganglia neurons. Both enzymes were found in the Nav1.8 and TRPV1 subpopulations of nociceptive trigeminal sensory neurons. Expression in myelinated neurons was not surprising since both enzymes are found in male and female rat oligodendrocytes^85,88 and both alter myelin protein expression.⁸⁹ Since allopregnanolone has antinociceptive effects,⁸⁴ expression of 5α-reductase and 3α-HSD in nociceptive sensory neurons provides evidence that pain reduction can occur at the trigeminal ganglia.

Although the present study did not compare the levels or localization of Sig-1R or the enzymes between males and females, future studies are warranted given known sex differences in pain and sex differences in the prevalence of a variety of pain disorders. This comparison would help to determine any sex-specific mechanisms that may contribute to sex differences in pain. Further the present study focused on CGRPergic neurons, while another neuropeptide, substance P, may be involved in the antinociceptive effects of progesterone. Substance p has been shown to inhibit progesterone metabolism within the spinal cord, thus resulting in a decrease in circulating allopregnanolone.⁹⁰ If substance P can decrease progesterone metabolism in the trigeminal ganglia, then progesterone treatment may counter the drop in antinociceptive levels of allopregnanolone. Nevertheless, allopregnanolone is a positive allosteric modulator of the GABA_A receptor, which is expressed in the trigeminal ganglia,^{59–77,79–91} therefore, allopregnanolone may enhance GABA-mediated antinociception. This is an interesting avenue for exploration as GABA_A receptors are a potential progesterone target in female prevalent chronic pain conditions and expressed in male and female rodent and human sensory ganglia^91–95 with sex differences in nociceptive function reported^96,97 potentially due to alteration of subunit expression by chronic pain^92,93Future studies to investigate manipulation of these enzymes and observe their effects on orofacial pain behaviors in both male and female subjects are warranted, and our findings provide further evidence that GABA receptors in sensory ganglia may provide a novel therapeutic target for pain.

The trigeminal ganglia are not the only potential anatomical substrate for progesterone’s antinociceptive actions on orofacial pain. The trigeminal nucleus caudalis in the medullary spinal cord is also a likely target. CGRP⁹⁸ and TRPV1⁹⁹ are both expressed in the trigeminal nucleus caudalis and progesterone reduces both CGRP and TRPV1 levels at this brainstem site¹⁰⁰ to contribute to a reduction in pain. Chronic activation of the Sig-1R results in evoked nociception by activating the trigeminal nucleus caudalis,⁴⁷ thus progesterone-induced reduction in CGRP or TRPV1 at the brainstem may reduce pain. Additionally, GABA_Areceptors are expressed in the trigeminal subnucleus caudalis¹⁰¹ where allopregnanolone could enhance GABA-mediated antinociception. Future studies should also focus on mechanisms for progesterone’s activity within the trigeminal nucleus caudalis.

Overall, we provide evidence that Sig-1Rs are available to progesterone in the trigeminal ganglia of female rats in similar concentrations across the estrous cycle and that progesterone could be metabolized locally in trigeminal sensory neurons as the required enzymes are present. The trigeminal ganglia are thus an anatomical substrate for the antinociceptive actions of progesterone and its metabolite allopregnanolone on orofacial or craniofacial pain. Future studies directly targeting progesterone’s antinociceptive mechanisms within the trigeminal ganglia and observing trigeminal pain behaviors are warranted by our neuroanatomical findings. Improving knowledge on progesterone’s modes of action, especially for pain disorders that are more prevalent in women, may lead to sex-specific therapeutics for women in pain.