GPR30 estrogen receptor agonists induce mechanical hyperalgesia in the rat

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.

Gonadal hormone-dependent nociceptor sensitization maintains nociplastic pain state in female mice

ABSTRACT

Nociplastic pain conditions develop predominantly in women. We recently established a murine nociplastic pain model by applying postinjury thermal (40°C) stimulation to an injured (capsaicin-injected) area, triggering a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. The nociplastic pain state was centrally maintained by spinal microglia in males but peripherally by ongoing afferent activity at the previously injured area in females. Here, we investigated whether gonadal hormones are critical for the development of this peripherally maintained nociplastic pain state in females. Although the transition to a nociplastic pain state still occurred in ovariectomized females, the pain state was maintained neither by ongoing afferent activity at the previously injured area nor by spinal microglia. Estradiol reconstitution a week before the injury plus postinjury stimulation, but not after the transition had already occurred, restored the development of peripherally maintained nociplastic mechanical hypersensitivity in ovariectomized females. G protein-coupled estrogen receptor antagonism during the transition phase mimicked ovariectomy in gonad-intact females, whereas the receptor antagonism after the transition gradually alleviated the nociplastic mechanical hypersensitivity. At the previously injured area, afferents responsive to allyl isothiocyanate (AITC), a TRPA1 agonist, contributed to the maintenance of nociplastic mechanical hypersensitivity in gonad-intact females. In ex vivo skin-nerve preparations, only AITC-responsive afferents from the nociplastic pain model in gonad-intact females showed ongoing activities greater than control. These results suggest that gonadal hormones are critical for peripherally maintained nociplastic pain state in females by sensitizing AITC-responsive afferents to be persistently active.

The chronification mechanism of orofacial inflammatory pain: Facilitation by GPER1 and microglia in the rostral ventral medulla

Background

Chronic orofacial pain is a common and incompletely defined clinical condition. The role of G protein-coupled estrogen receptor 1 (GPER1) as a new estrogen receptor in trunk and visceral pain regulation is well known. Here, we researched the role of GPER1 in the rostral ventral medulla (RVM) during chronic orofacial pain.

Methods and Results

A pain model was established where rats were injected in the temporomandibular joint with complete Freund's adjuvant (CFA) to simulate chronic orofacial pain. Following this a behavioral test was performed to establish pain threshold and results showed that the rats injected with CFA had abnormal pain in the orofacial regions. Additional Immunostaining and blot analysis indicated that microglia were activated in the RVM and GPER1 and c-Fos were significantly upregulated in the rats. Conversely, when the rats were injected with G15 (a GPER1 inhibitor) the abnormal pain the CFA rats were experiencing was alleviated and microglia activation was prevented. In addition, we found that G15 downregulated the expression of phospholipase C (PLC) and protein kinase C (PKC), inhibited the expression of GluA1, restores aberrant synaptic plasticity and reduces the overexpression of the synapse-associated proteins PSD-95 and syb-2 in the RVM of CFA rats.

Conclusion

The findings indicate that GPER1 mediates chronic orofacial pain through modulation of the PLC-PKC signal pathway, sensitization of the RVM region and enhancement of neural plasticity. These results of this study therefore suggest that GPER1 may serve as a potential therapeutic target for chronic orofacial pain.

Estrogen-dependent regulation of transient receptor potential vanilloid 1 (TRPV1) and P2X purinoceptor 3 (P2X3): Implication in burning mouth syndrome

Sex differences in the nervous system have gained recent academic interest. While the prominent differences are observed in mood and anxiety disorders, growing number of evidences also suggest sex difference in pain perception. This review focuses on estrogen as the key molecule underlying such difference, because estrogen plays many functions in the nervous system, including modulation of transient receptor potential vanilloid 1 (TRPV1) and P2X purinoceptor 3 (P2X3), two important nociceptive receptors. Estrogen was shown in various studies to up-regulate TRPV1 expression through two distinct pathways, resulting in pro-nociceptive effect. However, estrogen alleviated pain in other studies, by down-regulating nerve growth factor (NGF)-activated pathways and TRPV1. Estrogen may also attenuate nociception by inhibiting P2X3 receptors and ATP-signaling. Understanding the mechanism underlying the pro- and anti-nociceptive effect of estrogen might be crucial to understand pathophysiology of the burning mouth syndrome (BMS), a common chronic orofacial pain disorder in menopausal women. The involvement of TRPV1 is strongly suspected because of burning sensation. Reduced estrogen level of the BMS patient might have caused increased activity of P2X3 receptors. Interestingly, the increased expression of TRPV1 and P2X3 in oral mucosa of BMS patients was reported. The combinational impact of differential modulation of TRPV1/P2X3 during menopause might be an important contributing factor of etiology of BMS. Understanding the estrogen-dependent regulation of nociceptive receptors may provide a valuable insight toward the peripheral mechanism of sex-difference in pain perception.

The Antitumor Peptide ERα17p Exerts Anti-Hyperalgesic and Anti-Inflammatory Actions Through GPER in Mice

Persistent inflammation and persistent pain are major medical, social and economic burdens. As such, related pharmacotherapy needs to be continuously improved. The peptide ERα17p, which originates from a part of the hinge region/AF2 domain of the human estrogen receptor α (ERα), exerts anti-proliferative effects in breast cancer cells through a mechanism involving the hepta-transmembrane G protein-coupled estrogen receptor (GPER). It is able to decrease the size of xenografted human breast tumors, in mice. As GPER has been reported to participate in pain and inflammation, we were interested in exploring the potential of ERα17p in this respect. We observed that the peptide promoted anti-hyperalgesic effects from 2.5 mg/kg in a chronic mice model of paw inflammation induced by the pro-inflammatory complete Freund's adjuvant (CFA). This action was abrogated by the specific GPER antagonist G-15, leading to the conclusion that a GPER-dependent mechanism was involved. A systemic administration of a Cy5-labeled version of the peptide allowed its detection in both, the spinal cord and brain. However, ERα17p-induced anti-hyperalgesia was detected at the supraspinal level, exclusively. In the second part of the study, we have assessed the anti-inflammatory action of ERα17p in mice using a carrageenan-evoked hind-paw inflammation model. A systemic administration of ERα17p at a dose of 2.5 mg/kg was responsible for reduced paw swelling. Overall, our work strongly suggests that GPER inverse agonists, including ERα17p, could be used to control hyperalgesia and inflammation.

GPR30 receptor promotes preoperative anxiety-induced postoperative hyperalgesia by up-regulating GABAA-α4β1δ subunits in periaqueductal gray in female rats

BACKGROUND

G-protein coupled estrogen receptor 30 (GPR30) was proved the specific estrogen receptor relating to mechanical hyperalgesia. Studies have shown that the GABA_A receptor subunits α4, β1, and δ in the periaqueductal gray (PAG) neurons promote the descending facilitation system. This study inquired into whether and how GPR30 and GABA_A-α4β1δ in the PAG promote preoperative anxiety-induced postoperative hyperalgesia in female rats.

METHODS

All the female rats were subjected to the single prolonged stress (SPS) to stimulate preoperative anxiety. Subsequently, mechanical allodynia was evaluated before and after the incision, based on the paw withdrawal mechanical threshold (PWMT). The selective GPR30 agonist G1 and antagonist G15 were locally microinjected into the PAG. The expression of GPR30, protein kinase A (PKA), and GABA_A receptor subunits α4, β1, and δ in the PAG neurons were detected using western blotting and immunofluorescence.

RESULTS

Behavioral testing revealed that Group S and Group I decreased the nociceptive threshold levels of PWMT in female rats. PWMT in Group S + I decreased more than that of Group S and Group I. Further, results of western blotting showed the expression of GPR30, PKA, and GABA_A α4, β1, and δ subunits significantly up-regulated in Group S + I, and immunofluorescence indicated that the neurons of PAG in Group S + I appeared simultaneously immunopositive for GPR30 and GABA_A α4, β1, and δ receptors. After microinjection of G1 into the PAG, female rats with plantar incision continued to exhibit significant hyperalgesia until postoperative 48 h. On the other hand, microinjection of G15 with SPS and plantar incision procedure relieved postoperative hyperalgesia in female rats. Western blotting demonstrated that intra-PAG injection of G15 markedly decreased the GPR30, PKA, and GABA_A α4, β1, and δ levels in Group G15 + I.

CONCLUSIONS

Our results indicate that the GPR30-PKA-GABA_Aα4β1δ pathway in the PAG promotes preoperative anxiety-induced postoperative hyperalgesia in female rats. This mechanism might be a potential novel therapeutic target for hyperalgesia in females.

High Estrogen Level Modifies Postoperative Hyperalgesia via GPR30 and MMP-9 in Dorsal Root Ganglia Neurons

The cycling of sex hormones is one of the factors affecting pain in females, and the mechanisms are not fully understood. G-protein coupled estrogen receptor 30 (GPR30) is the estrogen receptor known to be involved in mechanical hyperalgesia. Studies have demonstrated that matrix metalloproteinase-9 (MMP-9) is a critical component in peripheral/central nervous system hypersensitivity and neuroinflammation, both of which participate in hyperalgesia. Here, ovariectomized rats were treated with low or high dose estrogen replacement, and then plantar incisions were made. Subsequently, mechanical allodynia was evaluated by determining the paw withdrawal mechanical threshold before and after the incision. In rats with incisions, high estrogen levels induced postoperative hyperalgesia and upregulation of GPR30 and MMP-9 in dorsal root ganglia (DRGs). MMP-9 was expressed primarily in DRG neurons co-expressing GPR30, and led to the activation of IL-1β. After intrathecal injection of the GPR30 agonist G1, female rats with low estrogen and plantar incisions continued to exhibit significant hyperalgesia until 48 h post-incision. In high estrogen level rats with plantar incisions, intrathecal injection of GPR30 antagonist G15 significantly attenuated postoperative hyperalgesia. Intraperitoneal injection of N-acetyl-cysteine, a source of cysteine that prevents the oxidation of cysteine residues on MMP-9, significantly relieved high estrogen-induced postoperative hyperalgesia via suppression of MMP-9 and IL-1β activation in DRGs. These results demonstrate that high estrogen level in rats with incisions elicit GPR30 and MMP-9 upregulation in DRGs and subsequently activate IL-1β, leading to induced postoperative hyperalgesia.

Activation of the G Protein-Coupled Estrogen Receptor Elicits Store Calcium Release and Phosphorylation of the Mu-Opioid Receptors in the Human Neuroblastoma SH-SY5Y Cells

Estrogens exert extensive influences on the nervous system besides their well-known roles in regulation of reproduction and metabolism. Estrogens act via the nuclear receptor ERα and ERβ to regulate gene transcription (classical genomic effects). In addition, estrogens are also known to cause rapid non-genomic effects on neuronal functions including inducing fast changes in cytosolic calcium level and rapidly desensitizing the μ type opioid receptor (MOR). The receptors responsible for the rapid actions of estrogens remain uncertain, but recent evidence points to the G protein-coupled estrogen receptor (GPER), which has been shown to be expressed widely in the nervous system. In the current study, we test the hypothesis that activation of GPER may mediate rapid calcium signaling, which may promote phosphorylation of MOR through the calcium-dependent protein kinases in neuronal cells. By qPCR and immunocytochemistry, we found that the human neuroblastoma SH-SY5Y cells endogenously express GPER and MOR. Activation of GPER by 17β-estradiol (E2) and G-1 (GPER selective agonist) evoked a rapid calcium rise in a concentration-dependent manner, which was due to store release rather than calcium entry. The GPER antagonist G15, the PLC inhibitor U73122 and the IP3 receptor inhibitor 2-APB each virtually abolished the calcium responses to E2 or G-1. Activation of GPER stimulated translocation of PKC isoforms (α and ε) to the plasma membrane, which led to MOR phosphorylation. Additionally, E2 and G-1 stimulated c-Fos expression in SH-SY5Y cells in a PLC/IP3-dependent manner. In conclusion, the present study has revealed a novel GPER-mediated estrogenic signaling in neuroblastoma cells in which activation of GPER is followed by rapid calcium mobilization, PKC activation and MOR phosphorylation. GPER-mediated rapid calcium signal may also be transmitted to the nucleus to impact on gene transcription. Such signaling cascade may play important roles in the regulation of opioid signaling in the brain.

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.

G protein-coupled estrogen receptor activates cell type-specific signaling pathways in cortical cultures: relevance to the selective loss of astrocytes

Selective activation of the G protein-coupled estrogen receptor has been proposed to avoid some of the side effects elicited by the activation of classical estrogen receptors α and β. Although its contribution to neuroprotection triggered by estradiol in brain disorders has been explored, the results regarding ischemic stroke are contradictory, and currently, there is no consensus on the role that this receptor may play. The present study aimed to investigate the role of GPER in the ischemic insult. For that, primary cortical cultures exposed to oxygen and glucose deprivation (OGD) were used as a model. Our results demonstrate that neuronal survival was strongly affected by the ischemic insult and concurrent GPER activation with G1 had no further impact. In contrast, OGD had a smaller impact on astrocytes survival but G1, alone or combined with OGD, promoted their apoptosis. This effect was prevented by the GPER antagonist G15. The results also show that ischemia did not change the expression levels of GPER in neurons and astrocytes. In this study, we also demonstrate that selective activation of GPER induced astrocyte apoptosis via the phospholipase C pathway and subsequent intracellular calcium rise, whereas in neurons, this effect was not observed. Taken together, this evidence supports a direct impact of GPER activity on the viability of astrocytes, which seems to be associated with the regulation of different signaling pathways in astrocytes and neurons.

TRP Channels as Potential Targets for Sex-Related Differences in Migraine Pain

Chronic pain is one of the most debilitating human diseases and represents a social and economic burden for our society. Great efforts are being made to understand the molecular and cellular mechanisms underlying the pathophysiology of pain transduction. It is particularly noteworthy that some types of chronic pain, such as migraine, display a remarkable sex dimorphism, being up to three times more prevalent in women than in men. This gender prevalence in migraine appears to be related to sex differences arising from both gonadal and genetic factors. Indeed, the functionality of the somatosensory, immune, and endothelial systems seems modulated by sex hormones, as well as by X-linked genes differentially expressed during development. Here, we review the current data on the modulation of the somatosensory system functionality by gonadal hormones. Although this is still an area that requires intense investigation, there is evidence suggesting a direct regulation of nociceptor activity by sex hormones at the transcriptional, translational, and functional levels. Data are being accumulated on the effect of sex hormones on TRP channels such as TRPV1 that make pivotal contributions to nociceptor excitability and sensitization in migraine and other chronic pain syndromes. These data suggest that modulation of TRP channels' expression and/or activity by gonadal hormones provide novel pathways for drug intervention that may be useful for targeting the sex dimorphism observed in migraine.

The beta-adrenergic receptor agonist, terbutaline, reduces UVB-induced mechanical sensitization in humans

OBJECTIVES

Previously, we found in cultures of primary neurons and in animals that sensitized primary neurons can be desensitized by treatment with e.g. beta-adrenergic receptor agonists. We now tested whether also in human sensitization such as UVB-radiation induced sunburn-like hyperalgesia can be reduced by intradermal injection of the beta-adrenergic receptor agonist terbutaline.

METHODS

In our prospective randomized study, 17 participants received an individual UVB dose to cause a defined local sunburn-like erythema at four locations, two on each forearm. Twenty-four hours later, the sensitized four areas were injected intradermally with terbutaline pH 4.3, terbutaline pH 7.0, saline pH 4.3 or saline pH 7.0, respectively. Pain thresholds were examined before and after induction of UVB-sensitization, and 15, 30 and 60 min after injection of the respective solution. Mechanical pain thresholds of the skin and of deeper tissues were determined by pinprick and pressure algometer measurements, respectively.

RESULTS

UVB-irradiation decreased mechanical pain thresholds for pinprick and pressure algometer measurements demonstrating a successful sunburn-like sensitization. Intradermal injection of terbutaline pH 7.0 into the sensitized skin reduced the sensitization for all measured timepoints as determined by pinprick measurements. Pinprick measurements of sensitization were not reduced by injection of terbutaline pH 4.3, saline solution pH 7.0 or saline solution pH 4.3. Also, sensitization of deeper tissue nociceptors were not altered by any of the injections as measured with the pressure algometer.

CONCLUSIONS

Similar to our cellular observations, also in humans beta-adrenergic agonists such as terbutaline can reduce the sensitization of primary neurons in the skin.

SIGNIFICANCE

We previously showed in model systems that beta-adrenergic stimulation can not only sensitize but also desensitize nociceptors. Our study shows that also in humans beta-adrenergic agonists desensitize if injected into UVB-sensitized skin. This indicates an analgesic activity of adrenergic agonists in addition to their vasoconstrictory function.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

GPR30 disrupts the balance of GABAergic and glutamatergic transmission in the spinal cord driving to the development of bone cancer pain

Cancer induced bone pain is a very complicated clinical pain states that has proven difficult to be treated effectively due to poorly understand of underlying mechanism, but bone cancer pain (BCP) seems to be enhanced by a state of spinal sensitization. In the present study, we showed that carcinoma tibia implantation induced notable pain sensitization and up-regulation of G-protein-coupled estrogen receptor (GPR30) in the spinal cord of rats which was reversed by GPR30 knockdown. Further studies indicated that upregulation of GPR30 induced by cancer implantation resulted in a select loss of γ-aminobutyric acid-ergic (GABAergic) neurons and functionally diminished the inhibitory transmission due to reduce expression of the vesicular GABA transporter (VGAT). GPR30 contributed to spinal cord disinhibition by diminishing the inhibitory transmission via upregulation of α1 subunit and downregulation of γ2 subunits. GPR30 also facilitated excitatory transmission by promoting functional up-regulation of the calcium/calmodulin-dependent protein kinase II α (CaMKII α) in glutamatergic neurons and increasing the clustering of the glutamate receptor subunit 1 (GluR1) subunit to excitatory synapse.

Taken together, GPR30 contributed to the development of BCP by both facilitating excitatory transmission and inhibiting inhibitory transmission in the spinal cord. Our findings provide the new spinal disinhibition and sensitivity mechanisms underlying the development of bone cancer pain.

Estrogen signaling through both membrane and nuclear receptors in the liver of fathead minnow

Estradiol is a potent sex steroid hormone that controls reproduction and other cellular pathways in fish. It is known to regulate important proteins such as vitellogenin, the egg yolk precursor protein, and zona radiata proteins that form the eggshell for fish eggs. These proteins are made in the liver and transported out into the blood from where they are taken up into the ovary during oogenesis. Estradiol can exert its influence directly through soluble nuclear receptors (there are three in fish) or indirectly through membrane receptors and a phosphorylation cascade. Often there is coordination through both genomic and non-genomic pathways. We have used a toxicogenomics approach to determine the contribution of genomic and non-genomic regulation in the liver of fathead minnows exposed to 5ng ethinylestradiol per liter or to a mixture of 5ng ethinylestradiol and 100ng ZM189,154 (ZM) per liter. ZM has previously been shown to be a "perfect" antagonist for the fish nuclear estrogen receptors but has displayed agonistic activities for membrane receptors. We find that both nuclear and membrane receptors contribute to the biosynthesis of vitellogenin 1 and estrogen receptor one (Esr1), among others. In addition, lipid metabolism pathways appear to require both activities.

Estradiol Sensitizes the Transient Receptor Potential Vanilloid 1 Receptor in Pain Responses

Sex differences exist in chronic pain pathologies, and gonadal estradiol (E2) alters the pain sensation. The nocisensor transient receptor potential vanilloid 1 (TRPV1) receptor plays a critical role in triggering pain. Here we examined the impact of E2 on the function of TRPV1 receptor in mice sensory neurons in vitro and in vivo. Both mechano- and thermonociceptive thresholds of the plantar surface of the paw of female mice were significantly lower in proestrus compared with the estrus phase. These thresholds were higher in ovariectomized (OVX) mice and significantly lower in sham-operated mice in proestrus compared with the sham-operated mice in estrus phase. This difference was absent in TRPV1 receptor-deficient mice. Furthermore, E2 potentiated the TRPV1 receptor activation-induced mechanical hyperalgesia in OVX mice. Long pretreatment (14 hours) with E2 induced a significant increase in TRPV1 receptor messenger RNA expression and abolished the capsaicin-induced TRPV1 receptor desensitization in primary sensory neurons. The short E2 incubation (10 minutes) also prevented the desensitization, which reverted after coadministration of E2 and the tropomyosin-related kinase A (TrkA) receptor inhibitor. Our study provides in vivo and in vitro evidence for E2-induced TRPV1 receptor upregulation and sensitization mediated by TrkAR via E2-induced genomic and nongenomic mechanisms. The sensitization and upregulation of TRPV1 receptor by E2 in sensory neurons may explain the greater pain sensitivity in female mice.

Postpartum estrogen withdrawal impairs GABAergic inhibition and LTD induction in basolateral amygdala complex via down-regulation of GPR30

Postpartum estrogen (E2) withdrawal is known to be a particularly vulnerable time for depressive symptoms. In this study, ovariectomized (OVX) mice were treated with co-administration of estradiol benzoate and progesterone (E2/P4) followed by administration of E2 alone (E2) and a subsequent E2 withdrawal (EW) to mimic the hormonal changes during pregnancy and postpartum. The objective of this study was to investigate the influence of E2 withdrawal after hormone-simulated pregnancy on synaptic function and plasticity in basolateral amygdala complex (BLA). In comparison to control mice, EW mice spent less time in the central portion of open-field test and open arms of elevated plus-maze. Excitatory postsynaptic potentials (EPSPs) slopes at external capsule BLA synapse were reduced in E2/P4-mice, recovered in E2-mice, and increased in EW-mice. EW-mice showed a significant increase in duration of EPSPs and paired-pulse inhibition (PPI) with multi-spike responses of EPSPs and impairment of long-term depression (LTD) induction, which were corrected by GABA_AR agonist muscimol. Levels of estrogen receptor (ER) GPR30, ERα and ERβ expression in BLA of EW-mice were lower than those in control mice. The bath-application of GPR30 agonist G-1 in BLA of EW-mice recovered the GABA_AR-mediated inhibition and LTD indication, but ERβ agonist DPN or ERα agonist PPT could not. A single BLA-injection of G-1 rather than DPN or PPT in EW-mice could partially relieve the anxiety-like behaviors. The results indicate that postpartum E2 withdrawal causes dysfunction of GABA_AR-mediated inhibition in the BLA through reducing GPR30 expression, which impairs LTD induction and causes anxiety-like behaviors.

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.

Estrogen Receptor Beta Selective Agonists as Agents to Treat Chemotherapeutic-Induced Neuropathic Pain

Chemotherapy-induced neuropathic pain (CINP) remains a major unmet medical need. Estrogen receptor beta (ERβ)-selective agonists represent a novel strategy for treating CINP because they are neuroprotective and may also have anticancer activity. We confirmed that ERβ-selective agonists have antiallodynic effects in the spinal nerve ligation model of neuropathic pain. We then showed that structurally diverse ERβ-selective agonists also relieved allodynia in CINP caused by taxol, oxaliplatin, and vincristine. These effects were receptor subtype specific and mediated by ERβ receptors as ERα-selective and nonselective estrogen agonists were inactive, a mixture of an ERβ and ERα agonist was inactive, and ERβ-selective antagonists blocked the effects of the ERβ-selective agonists. The efficacy and potency of ERβ-agonists was greater in male rats than females. To address the possibility that AC-186 might stimulate proliferation of cancers, rendering it unsuitable for treating CINP, we evaluated proliferative effects of AC-186 on prostate cancer cells and found it inhibited growth (LNCaP cells) or had no effect (PC3 cells) on these cells. Thus, ERβ-selective agonists exhibit potential for treating CINP.

Regulation of cyclooxygenase-2 expression in rat oviductal epithelial cells: Evidence for involvement of GPR30/Src kinase-mediated EGFR signaling

The oviduct plays a crucial role in female reproduction by regulating gamete transport, providing a specific microenvironment for fertilization and early embryonic development. Cyclooxygenase (COX)-derived prostaglandins play essential role in carrying out these oviduct-specific functions. Estrogen upregulates COX-2 expression in rat oviduct; however, the mechanisms responsible for regulation of COX-2 expression in rat oviductal epithelial cells (OECs) remain unclear. In the present study, we proposed that estrogen induces COX-2 expression via G-protein coupled receptor i.e., GPR30 in OECs. To investigate this hypothesis, we examined the effects of E2-BSA, ICI 182,780, GPR30 agonist and GPR30 antagonist on COX-2 expression and explored potential signaling pathway leading to COX-2 expression. Co-localization experiments revealed GPR30 to be primarily located in the peri-nuclear space, which was also the site of E2-BSA-fluorescein isothiocyanate (E2-BSA-FITC) binding. The E2-BSA induced-COX-2 and prostaglandin release were subjected to regulation by both EGFR and PI3K signaling as inhibitors of c-Src kinase (PP2), EGFR (EGFR inhibitor) and PI-3 kinase (LY294002) attenuated E2-BSA mediated effect. These results suggest that EGFR transactivation leading to activation of PI-3K/Akt pathway participates in COX-2 expression in rat OECs. Interestingly, E2-BSA induced COX-2 expression and subsequent prostaglandin release were abolished by NF-κB inhibitor. In addition, E2-BSA induced the nuclear translocation of p65-NF-κB and up-regulated the NF-κB promoter activity in rat OECs. Taken together, results demonstrated that E2-BSA induced the COX-2 expression and consequent PGE2 and PGF2α release in rat OECs. These effects are mediated through GPR30-derived EGFR transactivation and PI-3K/Akt cascade leading to NF-κB activation.

What have we learned about GPER function in physiology and disease from knockout mice?

Estrogens, predominantly 17β-estradiol, exert diverse effects throughout the body in both normal and pathophysiology, during development and in reproductive, metabolic, endocrine, cardiovascular, nervous, musculoskeletal and immune systems. Estrogen and its receptors also play important roles in carcinogenesis and therapy, particularly for breast cancer. In addition to the classical nuclear estrogen receptors (ERα and ERβ) that traditionally mediate predominantly genomic signaling, the G protein-coupled estrogen receptor GPER has become recognized as a critical mediator of rapid signaling in response to estrogen. Mouse models, and in particular knockout (KO) mice, represent an important approach to understand the functions of receptors in normal physiology and disease. Whereas ERα KO mice display multiple significant defects in reproduction and mammary gland development, ERβ KO phenotypes are more limited, and GPER KO exhibit no reproductive deficits. However, the study of GPER KO mice over the last six years has revealed that GPER deficiency results in multiple physiological alterations including obesity, cardiovascular dysfunction, insulin resistance and glucose intolerance. In addition, the lack of estrogen-mediated effects in numerous tissues of GPER KO mice, studied in vivo or ex vivo, including those of the cardiovascular, endocrine, nervous and immune systems, reveals GPER as a genuine mediator of estrogen action. Importantly, GPER KO mice have also demonstrated roles for GPER in breast carcinogenesis and metastasis. In combination with the supporting effects of GPER-selective ligands and GPER knockdown approaches, GPER KO mice demonstrate the therapeutic potential of targeting GPER activity in diseases as diverse as obesity, diabetes, multiple sclerosis, hypertension, atherosclerosis, myocardial infarction, stroke and cancer.

International Union of Basic and Clinical Pharmacology. XCVII. G Protein-Coupled Estrogen Receptor and Its Pharmacologic Modulators

Estrogens are critical mediators of multiple and diverse physiologic effects throughout the body in both sexes, including the reproductive, cardiovascular, endocrine, nervous, and immune systems. As such, alterations in estrogen function play important roles in many diseases and pathophysiological conditions (including cancer), exemplified by the lower prevalence of many diseases in premenopausal women. Estrogens mediate their effects through multiple cellular receptors, including the nuclear receptor family (ERα and ERβ) and the G protein-coupled receptor (GPCR) family (GPR30/G protein-coupled estrogen receptor [GPER]). Although both receptor families can initiate rapid cell signaling and transcriptional regulation, the nuclear receptors are traditionally associated with regulating gene expression, whereas GPCRs are recognized as mediating rapid cellular signaling. Estrogen-activated pathways are not only the target of multiple therapeutic agents (e.g., tamoxifen, fulvestrant, raloxifene, and aromatase inhibitors) but are also affected by a plethora of phyto- and xeno-estrogens (e.g., genistein, coumestrol, bisphenol A, dichlorodiphenyltrichloroethane). Because of the existence of multiple estrogen receptors with overlapping ligand specificities, expression patterns, and signaling pathways, the roles of the individual receptors with respect to the diverse array of endogenous and exogenous ligands have been challenging to ascertain. The identification of GPER-selective ligands however has led to a much greater understanding of the roles of this receptor in normal physiology and disease as well as its interactions with the classic estrogen receptors ERα and ERβ and their signaling pathways. In this review, we describe the history and characterization of GPER over the past 15 years focusing on the pharmacology of steroidal and nonsteroidal compounds that have been employed to unravel the biology of this most recently recognized estrogen receptor.

The dual action of estrogen hypothesis

Estradiol (E2) can act in the brain in a relatively fast manner (i.e., seconds to minutes) usually through signaling initiated at the cell membrane. Brain-derived E2 has thus been considered as another type of neurotransmitter. Recent work found that behaviors indicative of male sexual motivation are activated by estrogenic metabolites of testosterone (T) in a fast manner, while sexual performance (copulatory behavior per se) is regulated by brain E2 in a slower manner via nucleus-initiated actions. This functional division between these two types of action appears to generalize to other behavioral systems regulated by E2. We propose the dual action of estrogen hypothesis to explain this functional distinction between these two different modes of action.

Estrogens Suppress Spinal Endomorphin 2 Release in Female Rats in Phase with the Estrous Cycle

BACKGROUND/AIMS

Male and female rats differ in their ability to utilize spinal endomorphin 2 (EM2; the predominant mu-opioid receptor ligand in spinal cord) and in the mechanisms that underlie spinal EM2 analgesic responsiveness. We investigated the relevance of spinal estrogen receptors (ERs) to the in vivo regulation of spinal EM2 release.

METHODS

ER antagonists were administered directly to the lumbosacral spinal cord of male and female rats, intrathecal perfusate was collected, and resulting changes in EM2 release were quantified using a plate-based radioimmunoassay.

RESULTS

Intrathecal application of an antagonist of either estrogen receptor-α (ERα) or the ER GPR30 failed to alter spinal EM2 release. Strikingly, however, the concomitant blockade of ERα and GPR30 enhanced spinal EM2 release. This effect was sexually dimorphic, being absent in males. Furthermore, the magnitude of the enhancement of spinal EM2 release in females was dependent upon estrous cycle stage, suggesting a relationship with circulating levels of 17β-estradiol. The rapid onset of enhanced EM2 release following intrathecal application of ERα/GPR30 antagonists (within 30-40 min) suggests mediation via ERs in the plasma membrane, not the nucleus. Notably, both ovarian and spinally synthesized estrogens are essential for membrane ER regulation of spinal EM2 release.

CONCLUSION

These findings underscore the importance of estrogens for the regulation of spinal EM2 activity and, by extension, endogenous spinal EM2 antinociception in females. Components of the spinal estrogenic mechanism(s) that suppress EM2 release could represent novel drug targets for improving utilization of endogenous spinal EM2, and thereby pain management in women.

Role of nociceptor estrogen receptor GPR30 in a rat model of endometriosis pain

Endometriosis, the most common cause of chronic pelvic pain, is an estrogen-dependent disease in which classic estrogen receptors (ERα, ERβ) play an important role. Although recent evidence suggests that the novel G protein-coupled estrogen receptor (GPR30) also plays a key role in the progression of endometriosis, whether it is also involved in endometriosis pain is still unknown. Here we tested the hypothesis that GPR30 expressed by nociceptors contributes to endometriosis pain. Intramuscular injection of the GPR30 agonists raloxifene or 17β-estradiol produced a fast-onset, persistent, mechanical hyperalgesia at the site of the injection. Intrathecal antisense (AS) oligodeoxynucleotides (ODN), but not mismatch (MM) ODN, targeting mRNA for GPR30 markedly inhibited its protein expression in nociceptors and attenuated the mechanical hyperalgesia induced by local raloxifene or 17β-estradiol. Pretreatment with the GPR30 antagonist G-36 also inhibited the hyperalgesia induced by raloxifene or 17β-estradiol in naive control rats. Surgical implant of autologous uterine tissue onto the gastrocnemius muscle, which induces endometriosis-like lesions, produced local mechanical hyperalgesia. Intrathecal AS, but not MM, ODN targeting GPR30 mRNA reversibly inhibited the mechanical hyperalgesia at the site of endometriotic lesions. Finally, intralesional injection of the GPR30 antagonist G-36 also inhibited the mechanical hyperalgesia at the site of ectopic uterine tissue. We conclude that local GPR30 agonists produce persistent mechanical hyperalgesia in naive female rats, whereas local GPR30 antagonists inhibit mechanical hyperalgesia in a model of endometriosis pain. Thus, GPR30 expressed by nociceptors innervating ectopic uterine lesions might play a major role in endometriosis pain.

Estrogen rapidly enhances incisional pain of ovariectomized rats primarily through the G protein-coupled estrogen receptor

It has become increasingly apparent that the pain threshold of females and males varies in an estrogen dependent manner. To investigate the modulation of pain by estrogen and the molecular mechanisms involved in this process. A total of 48 rats were ovariectomized (OVX). At 14 and 20 days after OVX, rats were divided into eight groups: groups 1–4 were administered drugs intravenously (IV); groups 5–8 were administered through intrathecal (IT) catheter. Hind paw incision was made in all animals to determine incisional pain. Paw withdraw threshold (PWT) was tested prior to and 24 h after incision. The test drugs were applied 24 h after the incision. Rats were either IV or IT administered with: 17-β-estradiol (E2), G protein-coupled estrogen receptor (GPER)-selective agonist (G1), GPER-selective antagonist (G15) and E2 (G15 + E2), or solvent. Before and 30 min after IV drug administration and 20 min during the IT catheter administration, PWT was tested and recorded. 24 h after incisional surgery, the PWT of all rats significantly decreased. Both in the IV group and IT group: administration of E2 and G1 significantly decreased PWT. Neither administration of G15 + E2 nor solvent significantly changed PWT. Estrogen causes rapid reduction in the mechanical pain threshold of OVX rats via GPER.

Role for monocyte chemoattractant protein-1 in the induction of chronic muscle pain in the rat

While raised levels of monocyte chemoattractant protein 1 (MCP-1) have been observed in patients with chronic muscle pain, direct evidence for its role as an algogen in skeletal muscle is still lacking. In the rat, MCP-1 induces a dose-dependent mechanical hyperalgesia lasting for up to 6weeks. Following recovery, rats exhibited a markedly prolonged hyperalgesia to an intramuscular injection of prostaglandin E2, hyperalgesic priming. Intrathecal pretreatment with isolectin B4 (IB4)-saporin, which selectively destroys IB4-positive (IB4+) nociceptors, markedly decreased MCP-1-induced hyperalgesia and prevented the subsequent development of priming. To evaluate the involvement of MCP-1 in stress-induced chronic pain we administered, intrathecally, antisense (AS) or mismatch oligodeoxynucleotides directed against CCR2 (the canonical receptor for MCP-1) mRNA, during the exposure to water-avoidance stress, a model of stress-induced persistent muscle pain. The AS treatment attenuated this hyperalgesia, whereas IB4-saporin abolished water-avoidance stress-induced muscle hyperalgesia and prevented stress-induced hyperalgesic priming. These results indicate that MCP-1 induces persistent muscle hyperalgesia and a state of latent chronic sensitization to other algogens, by action on its cognate receptor on IB4+ nociceptors. Because MCP-1 also contributes to stress-induced widespread chronic muscle pain, it should be considered as a player in chronic musculoskeletal pain syndromes.

Sex steroids and neuroprotection in spinal cord injury: a review of preclinical investigations

Spinal cord injury (SCI) is a debilitating condition that affects motor, sensory and autonomic functions. Subsequent to the first mechanical trauma, secondary events, which include inflammation and glial activation, exacerbate tissue damage and worsen functional deficits. Although these secondary injury mechanisms are amenable to therapeutic interventions, the efficacy of current approaches is inadequate. Further investigations are necessary to implement new therapies that can protect neural cells and attenuate some of the detrimental effects of inflammation while promoting regeneration. Studies on different animal models of SCI indicated that sex steroids, especially 17β-estradiol and progesterone, exert neuroprotective, anti-apoptotic and anti-inflammatory effects, ameliorate tissue sparing and improve functional deficits in SCI. As sex steroid receptors are expressed in a variety of cells including neurons, glia and immune system-related cells which infiltrate the injury epicenter, sex steroids could impact multiple processes simultaneously and in doing so, influence the outcomes of SCI. However, the translation of these pre-clinical findings into the clinical setting presents challenges such as the narrow therapeutic time window of sex steroid administration, the diversity of treatment regimens that have been employed in animal studies and the lack of sufficient information regarding the persistence of the effects in chronic SCI. The current review will summarize some of the major findings in this field and will discuss the challenges associated with the implementation of sex steroids as a promising treatment in human SCI.

G-protein oestrogen receptor 1: trials and tribulations of a membrane oestrogen receptor

Oestrogens are now recognised to be able to initiate rapid, fast responses, in addition to their classical, longer-term actions. There is a growing appreciation of the potential implications of this mode of action for oestrogenic signalling in both neuronal and non-neuronal systems. As such, much effort has been made to determine the mechanisms that are critical for transducing these rapid effects into cellular responses. Recently, an orphan G-protein-coupled receptor (GPCR), termed GPR30, was identified as an oestrogen-sensitive receptor in cancer cells. This receptor, now term G-protein oestrogen receptor 1 (GPER1) has been the subject of many investigations, and a role for this receptor in the nervous system is now emerging. In this review, we highlight some of the more recent advances in our understanding of the distribution and subcellular localisation of this receptor in the brain, as well as some of the evidence for the potential role that this receptor may play in the brain. We then discuss some of the controversies surrounding the pharmacology of this receptor, and attempt to reconcile these by suggesting that the 'agonist-specific coupling' model of GPCR function may provide a potential explanation for some of the divergent reports of GPER1 pharmacology.

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.

Sex differences in opioid analgesia and addiction: interactions among opioid receptors and estrogen receptors

Opioids are widely used as the pain reliever and also notorious for being addictive drugs. Sex differences in the opioid analgesia and addiction have been reported and investigated in human subjects and animal models. Yet, the molecular mechanism underlying the differences between males and females is still unclear. Here, we reviewed the literature describing the sex differences in analgesic responses and addiction liabilities to clinically relevant opioids. The reported interactions among opioids, estrogens, opioid receptors, and estrogen receptors are also evaluated. We postulate that the sex differences partly originated from the crosstalk among the estrogen and opioid receptors when stimulated by the exogenous opioids, possibly through common secondary messengers and the downstream gene transcriptional regulators.

Wound-healing growth factor, basic FGF, induces Erk1/2-dependent mechanical hyperalgesia

Growth factors such as nerve growth factor and glial cell line-derived neurotrophic factor are known to induce pain sensitization. However, a plethora of other growth factors is released during inflammation and tissue regeneration, and many of them are essential for wound healing. Which wound-healing factors also alter the sensitivity of nociceptive neurons is not well known. We studied the wound-healing factor, basic fibroblast growth factor (bFGF), for its role in pain sensitization. Reverse transcription polymerase chain reaction showed that the receptor of bFGF, FGFR1, is expressed in lumbar rat dorsal root ganglia (DRG). We demonstrated presence of FGFR1 protein in DRG neurons by a recently introduced quantitative automated immunofluorescent microscopic technique. FGFR1 was expressed in all lumbar DRG neurons as quantified by mixture modeling. Corroborating the mRNA and protein expression data, bFGF induced Erk1/2 phosphorylation in nociceptive neurons, which could be blocked by inhibition of FGF receptors. Furthermore, bFGF activated Erk1/2 in a dose- and time-dependent manner. Using single-cell electrophysiological recordings, we found that bFGF treatment of DRG neurons increased the current-density of NaV1.8 channels. Erk1/2 inhibitors abrogated this increase. Importantly, intradermal injection of bFGF in rats induced Erk1/2-dependent mechanical hyperalgesia.

PERSPECTIVE

Analyzing intracellular signaling dynamics in nociceptive neurons has proven to be a powerful approach to identify novel modulators of pain. In addition to describing a new sensitizing factor, our findings indicate the potential to investigate wound-healing factors for their role in nociception.

17β-estradiol rapidly attenuates P2X3 receptor-mediated peripheral pain signal transduction via ERα and GPR30

Estrogen has been reported to affect pain perception, although the underlying mechanisms remain unclear. In this investigation, pain behavior testing, patch clamp recording, and immunohistochemistry were used on rats and transgenic mice to determine which estrogen receptors (ERs) and the related signaling pathway are involved in the rapid modulation of estrogen on P2X3 receptor-mediated events. The results showed that 17β-estradiol (E2) rapidly inhibited pain induced by α,β-methylene ATP (α,β-me-ATP), a P2X1 and P2X3 receptor agonist in ovariectomized rats and normal rats in diestrus. The ERα agonist 4,49,499-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT) and G protein-coupled receptor 30 (GPR30) agonist G-1 mimicked the estrogen effect, whereas the ERβ agonist diarylpropionitrile (DPN) had no effect. In cultured rat dorsal root ganglion (DRG) neurons, PPT and G-1 but not DPN significantly attenuated α,β-me-ATP-mediated currents, with the dose-response curve of these currents shifted to the right. The inhibitory effect of E2 on P2X3 currents was blocked by G-15, a selective antagonist to the GPR30 estrogen receptor. E2 lacked this effect in DRG neurons from ERα-knockout mice but partly remained in those from ERβ-knockout mice. The P2X3 and GPR30 receptors were coexpressed in the rat DRG neurons. Furthermore, the ERK1/2 inhibitor U0126 reversed the inhibitory effect of E2 on α,β-me-ATP-induced pain and of PPT or G-1 on P2X3 receptor-mediated currents. The cAMP-protein kinase A (PKA) agonist forskolin, but not the PKC agonist phorbol-12-myristate-13-acetate (PMA), mimicked the estrogen-inhibitory effect on P2X3 receptor currents, which was blocked by another ERK1/2 inhibitor, PD98059. These results suggest that estrogen regulates P2X3-mediated peripheral pain by acting on ERα and GPR30 receptors expressed in primary afferent neurons, which probably involves the intracellular cAMP-PKA-ERK1/2 pathway.

The fundamental unit of pain is the cell

The molecular/genetic era has seen the discovery of a staggering number of molecules implicated in pain mechanisms [18,35,61,69,96,133,150,202,224]. This has stimulated pharmaceutical and biotechnology companies to invest billions of dollars to develop drugs that enhance or inhibit the function of many these molecules. Unfortunately this effort has provided a remarkably small return on this investment. Inevitably, transformative progress in this field will require a better understanding of the functional links among the ever-growing ranks of "pain molecules," as well as their links with an even larger number of molecules with which they interact. Importantly, all of these molecules exist side-by-side, within a functional unit, the cell, and its adjacent matrix of extracellular molecules. To paraphrase a recent editorial in Science magazine [223], although we live in the Golden age of Genetics, the fundamental unit of biology is still arguably the cell, and the cell is the critical structural and functional setting in which the function of pain-related molecules must be understood. This review summarizes our current understanding of the nociceptor as a cell-biological unit that responds to a variety of extracellular inputs with a complex and highly organized interaction of signaling molecules. We also discuss the insights that this approach is providing into peripheral mechanisms of chronic pain and sex dependence in pain.

Calcium supplementation increases circulating cholesterol by reducing its catabolism via GPER and TRPC1-dependent pathway in estrogen deficient women

BACKGROUND

Limited studies have addressed the effects of calcium supplementation (CaS) on serum total cholesterol (TC) in postmenopausal women and the results are inconclusive. Moreover, the potential mechanisms through which CaS regulates cholesterol metabolism in the absence of estrogen are still sealed for the limitation of human being study.

METHODS

Cross-sectional survey, animal and in vitro experiments were conducted to investigate the effect of CaS on endogenous cholesterol metabolism in estrogen deficiency and identify its potential mechanisms. Ovariectomized rats were used to mimic estrogen deficiency. In vitro, HepG2 cell line was exposed to estradiol and/or calcium treatment.

RESULTS

We demonstrated that CaS significantly increased serum TC and the risk of hypercholesterolemia and myocardial infarction in postmenopausal women. Increased serum TC in estrogen deficiency was caused mainly by decreased cholesterol catabolism rather than increased synthesis. This was mediated by reduced 7α-hydroxylase resulting from increased liver intracellular Ca(2+) concentrations, reduced intracellular basal cAMP and subsequent up-regulation of SREBP-1c and SHP expression. Estrogen had a protective role in preventing CaS-induced TC increase by activating the G-protein coupled estrogen receptor, which mediated the estrogen effect through the transient receptor potential canonical 1 cation channel.

CONCLUSIONS

CaS increases endogenous serum TC via decreasing hepatic cholesterol catabolism in estrogen deficiency. G-protein coupled estrogen receptor is shown to be a key target in mediating CaS-induced TC increase. CaS should be monitored for the prevention of serum TC increase during menopause.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Importance of sex to pain and its amelioration; relevance of spinal estrogens and its membrane receptors

Estrogens have a multitude of effects on opioid systems and are thought to play a key role in sexually dimorphic nociception and opioid antinociception. Heretofore, classical genomic actions of estrogens are largely thought to be responsible for the effects of these steroids on nociception and opioid antinociception. The recent discovery that estrogens can also activate estrogen receptors that are located in the plasma membrane, the effects of which are manifest in seconds to minutes instead of hours to days has revolutionized our thinking concerning the ways in which estrogens are likely to modulate pain responsiveness and the dynamic nature of that modulation. This review summarizes parameters of opioid functionality and nociception that are subject to modulation by estrogens, underscoring the added dimensions of such modulation that accrues from rapid membrane estrogen receptor signaling. Implications of this mode of signaling regarding putative sources of estrogens and its degradation are also discussed.

Ca++/CaMKII switches nociceptor-sensitizing stimuli into desensitizing stimuli

Many extracellular factors sensitize nociceptors. Often they act simultaneously and/or sequentially on nociceptive neurons. We investigated if stimulation of the protein kinase C epsilon (PKCε) signaling pathway influences the signaling of a subsequent sensitizing stimulus. Central in activation of PKCs is their transient translocation to cellular membranes. We found in cultured nociceptive neurons that only a first stimulation of the PKCε signaling pathway resulted in PKCε translocation. We identified a novel inhibitory cascade to branch off upstream of PKCε, but downstream of Epac via IP3-induced calcium release. This signaling branch actively inhibited subsequent translocation and even attenuated ongoing translocation. A second 'sensitizing' stimulus was rerouted from the sensitizing to the inhibitory branch of the signaling cascade. Central for the rerouting was cytoplasmic calcium increase and CaMKII activation. Accordingly, in behavioral experiments, activation of calcium stores switched sensitizing substances into desensitizing substances in a CaMKII-dependent manner. This mechanism was also observed by in vivo C-fiber electrophysiology corroborating the peripheral location of the switch. Thus, we conclude that the net effect of signaling in nociceptors is defined by the context of the individual cell's signaling history.

Functional adaptation in female rats: the role of estrogen signaling

Background

Sex steroids have direct effects on the skeleton. Estrogen acts on the skeleton via the classical genomic estrogen receptors alpha and beta (ERα and ERβ), a membrane ER, and the non-genomic G-protein coupled estrogen receptor (GPER). GPER is distributed throughout the nervous system, but little is known about its effects on bone. In male rats, adaptation to loading is neuronally regulated, but this has not been studied in females.

Methodology/Principal Findings

We used the rat ulna end-loading model to induce an adaptive modeling response in ovariectomized (OVX) female Sprague-Dawley rats. Rats were treated with a placebo, estrogen (17β-estradiol), or G-1, a GPER-specific agonist. Fourteen days after OVX, rats underwent unilateral cyclic loading of the right ulna; half of the rats in each group had brachial plexus anesthesia (BPA) of the loaded limb before loading. Ten days after loading, serum estrogen concentrations, dorsal root ganglion (DRG) gene expression of ERα, ERβ, GPER, CGRPα, TRPV1, TRPV4 and TRPA1, and load-induced skeletal responses were quantified. We hypothesized that estrogen and G-1 treatment would influence skeletal responses to cyclic loading through a neuronal mechanism. We found that estrogen suppresses periosteal bone formation in female rats. This physiological effect is not GPER-mediated. We also found that absolute mechanosensitivity in female rats was decreased, when compared with male rats. Blocking of adaptive bone formation by BPA in Placebo OVX females was reduced.

Conclusions

Estrogen acts to decrease periosteal bone formation in female rats in vivo. This effect is not GPER-mediated. Gender differences in absolute bone mechanosensitivity exist in young Sprague-Dawley rats with reduced mechanosensitivity in females, although underlying bone formation rate associated with growth likely influences this observation. In contrast to female and male rats, central neuronal signals had a diminished effect on adaptive bone formation in estrogen-deficient female rats.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

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

The pivotal role of estrogens in the pain sensitivity has been investigated in many ways. Traditionally, it is ascribed to the slow genomic changes mediated by classical nuclear estrogen receptors (ER), ERα and ERβ, depending on peripheral estrogens. Recently, it has become clear that estrogens can also signal through membrane ERs (mERs), such as G-protein-coupled ER1 (GPER1), mediating the non-genomic effects. However, the spinal specific role played by ERs and the underlying cellular mechanisms remain elusive. The present study investigated the rapid estrogenic regulation of nociception at the spinal level. Spinal administration of 17β-estradiol (E2), the most potent natural estrogen, acutely produced a remarkable mechanical allodynia and thermal hyperalgesia without significant differences among male, female and ovariectomized (Ovx) rats. E2-induced the pro-nociceptive effects were partially abrogated by ICI 182,780 (ERs antagonist), and mimicked by E2-BSA (a mER agonist). Inhibition of local E2 synthesis by 1,4,6-Androstatrien-3,17-dione (ATD, a potent irreversible aromatase inhibitor), or blockade of ERs by ICI 182,780 produced an inhibitory effect on the late phase of formalin nociceptive responses. Notably, lumbar puncture injection of G15 (a selective GPER1 antagonist) resulted in similar but more efficient inhibition of formalin nociceptive responses as compared with ICI 182,780. At the cellular level, the amplitude and decay time of spontaneous inhibitory postsynaptic currents were attenuated by short E2 or E2-BSA treatment in spinal slices. These results indicate that estrogen acutely facilitates nociceptive transmission in the spinal cord via activation of membrane-bound estrogen receptors.

Mechanisms of G protein-coupled estrogen receptor-mediated spinal nociception

Human and animal studies suggest that estrogens are involved in the processing of nociceptive sensory information and analgesic responses in the central nervous system. Rapid pronociceptive estrogenic effects have been reported, some of which likely involve G protein-coupled estrogen receptor (GPER) activation. Membrane depolarization and increases in cytosolic calcium and reactive oxygen species (ROS) levels are markers of neuronal activation, underlying pain sensitization in the spinal cord. Using behavioral, electrophysiological, and fluorescent imaging studies, we evaluated GPER involvement in spinal nociceptive processing. Intrathecal challenging of mice with the GPER agonist G-1 results in pain-related behaviors. GPER antagonism with G15 reduces the G-1-induced response. Electrophysiological recordings from superficial dorsal horn neurons indicate neuronal membrane depolarization with G-1 application, which is G15 sensitive. In cultured spinal sensory neurons, G-1 increases intracellular calcium concentration and induces mitochondrial and cytosolic ROS accumulation. In the presence of G15, G-1 does not elicit the calcium and ROS responses, confirming specific GPER involvement in this process. Cytosolic calcium concentration elevates faster and with higher amplitude following G-1 intracellular microinjections compared to extracellular exposure, suggesting subcellular GPER functionality. Thus, GPER activation results in spinal nociception, and the downstream mechanisms involve cytosolic calcium increase, ROS accumulation, and neuronal membrane depolarization.

PERSPECTIVE

Our results suggest that GPER modulates pain processing in spinal sensory neurons via cytosolic calcium increase and ROS accumulation. These findings extend the current knowledge on GPER involvement in physiology and disease, providing the first evidence of its pronociceptive effects at central levels and characterizing some of the underlying mechanisms.

17β-Estradiol regulates the gene expression of voltage-gated sodium channels: role of estrogen receptor α and estrogen receptor β

Estradiol (E2) plays a key role in pain modulation, and the biological effects of E2 are transduced by binding estrogen receptors (ERs). Voltage-gated sodium (Nav) channels are responsible for the generation and propagation of action potentials in the membranes of most neurons and excitable cells. Adult dorsal root ganglion (DRG) neurons can express the ERs (ERα and ERβ), and Nav channels (TTX-S: Nav1.1, Nav1.6, and Nav1.7; and TTX-R: Nav1.8, and Nav1.9). Although E2 modulates Nav channel currents, little is known about the molecular mechanisms involved. In this study, we investigate the mRNA expressions of Nav channel subtypes mediated differentially by the ERs in the DRGs of wild-type (WT) and estrogen receptor knockout (αERKO and βERKO) mice. By means of quantitative real-time PCR, we found that the expressions of Nav1.1, Nav1.7, Nav1.8, and Nav1.9 subtypes were elevated in αERKO and βERKO mice, whereas Nav1.6 mRNA decreased in αERKO, but not in βERKO mice. The mRNA expressions of Nav subtypes were increased in E2-treated WT ovariectomized animals. We also found that E2-regulation of Nav1.1 and Nav1.9 mRNA expressions is dependent on ERα, ERβ, and another ER, whereas E2-regulation of Nav1.8 appears to be in an ERβ-dependent manner.

IB4-saporin attenuates acute and eliminates chronic muscle pain in the rat

The function of populations of nociceptors in muscle pain syndromes remain poorly understood. We compared the contribution of two major classes, isolectin B4-positive (IB4(+)) and IB4-negative (IB4(-)) nociceptors, in acute and chronic inflammatory and ergonomic muscle pain. Baseline mechanical nociceptive threshold was assessed in the gastrocnemius muscle of rats treated with IB4-saporin, which selectively destroys IB4(+) nociceptors. Rats were then submitted to models of acute inflammatory (intramuscular carrageenan)- or ergonomic intervention (eccentric exercise or vibration)-induced muscle pain, and each of the three models also evaluated for the transition from acute to chronic pain, manifest as prolongation of prostaglandin E2 (PGE(2))-induced hyperalgesia, after recovery from the hyperalgesia induced by acute inflammation or ergonomic interventions. IB4-saporin treatment did not affect baseline mechanical nociceptive threshold. However, compared to controls, IB4-saporin treated rats exhibited shorter duration mechanical hyperalgesia in all three models and attenuated peak hyperalgesia in the ergonomic pain models. And, IB4-saporin treatment completely prevented prolongation of PGE(2)-induced mechanical hyperalgesia. Thus, IB4(+) and IB4(-) neurons contribute to acute muscle hyperalgesia induced by diverse insults. However, only IB4+ nociceptors participate in the long term consequence of acute hyperalgesia.

Identification of a GPER/GPR30 antagonist with improved estrogen receptor counterselectivity

GPER/GPR30 is a seven-transmembrane G protein-coupled estrogen receptor that regulates many aspects of mammalian biology and physiology. We have previously described both a GPER-selective agonist G-1 and antagonist G15 based on a tetrahydro-3H-cyclopenta[c]quinoline scaffold. The antagonist lacks an ethanone moiety that likely forms important hydrogen bonds involved in receptor activation. Computational docking studies suggested that the lack of the ethanone substituent in G15 could minimize key steric conflicts, present in G-1, that limit binding within the ERα ligand binding pocket. In this report, we identify low-affinity cross-reactivity of the GPER antagonist G15 to the classical estrogen receptor ERα. To generate an antagonist with enhanced selectivity, we therefore synthesized an isosteric G-1 derivative, G36, containing an isopropyl moiety in place of the ethanone moiety. We demonstrate that G36 shows decreased binding and activation of ERα, while maintaining its antagonist profile towards GPER. G36 selectively inhibits estrogen-mediated activation of PI3K by GPER but not ERα. It also inhibits estrogen- and G-1-mediated calcium mobilization as well as ERK1/2 activation, with no effect on EGF-mediated ERK1/2 activation. Similar to G15, G36 inhibits estrogen- and G-1-stimulated proliferation of uterine epithelial cells in vivo. The identification of G36 as a GPER antagonist with improved ER counterselectivity represents a significant step towards the development of new highly selective therapeutics for cancer and other diseases.

The G-protein-coupled estrogen receptor GPER in health and disease

Estrogens mediate profound effects throughout the body and regulate physiological and pathological processes in both women and men. The low prevalence of many diseases in premenopausal women is attributed to the presence of 17β-estradiol, the predominant and most potent endogenous estrogen. In addition to endogenous estrogens, several man-made and plant-derived molecules, such as bisphenol A and genistein, also exhibit estrogenic activity. Traditionally, the actions of 17β-estradiol are ascribed to two nuclear estrogen receptors (ERs), ERα and ERβ, which function as ligand-activated transcription factors. However, 17β-estradiol also mediates rapid signaling events via pathways that involve transmembrane ERs, such as G-protein-coupled ER 1 (GPER; formerly known as GPR30). In the past 10 years, GPER has been implicated in both rapid signaling and transcriptional regulation. With the discovery of GPER-selective ligands that can selectively modulate GPER function in vitro and in preclinical studies and with the use of Gper knockout mice, many more potential roles for GPER are being elucidated. This Review highlights the physiological roles of GPER in the reproductive, nervous, endocrine, immune and cardiovascular systems, as well as its pathological roles in a diverse array of disorders including cancer, for which GPER is emerging as a novel therapeutic target and prognostic indicator.

Estrogen destabilizes microtubules through an ion-conductivity-independent TRPV1 pathway

Recently, we described estrogen and agonists of the G-protein coupled estrogen receptor GPR30 to induce protein kinase C (PKC)ε-dependent pain sensitization. PKCε phosphorylates the ion channel transient receptor potential, vanilloid subclass I (TRPV1) close to a novel microtubule-TRPV1 binding site. We now modeled the binding of tubulin to the TRPV1 C-terminus. The model suggests PKCε phosphorylation of TRPV1-S800 to abolish the tubulin-TRPV1 interaction. Indeed, in vitro PKCε phosphorylation of TRPV1 hindered tubulin-binding to TRPV1. In vivo, treatment of sensory neurons and F-11 cells with estrogen and the GPR30 agonist, G-1, resulted in microtubule destabilization and retraction of microtubules from filopodial structures. We found estrogen and G-1 to regulate the stability of the microtubular network via PKC phosphorylation of the PKCε-phosphorylation site TRPV1-S800. Microtubule disassembly was not, however, dependent on TRPV1 ion conductivity. TRPV1 knock-down in rats inverted the effect of the microtubule-modulating drugs, Taxol and Nocodazole, on estrogen-induced and PKCε-dependent mechanical pain sensitization. Thus, we suggest the C-terminus of TRPV1 to be a signaling intermediate downstream of estrogen and PKCε, regulating microtubule-stability and microtubule-dependent pain sensitization.