雌激素对于疼痛的快速调节及其可能机制

安光惠（综述） 颜 涛 李士通（审校）

（上海交通大学附属上海市第一人民医院麻醉科 上海 200080）

IntroductionIt has been well described that pain behaviors，bot h in hu man and in ani mals，can be infl uenced by estrogen［1］．The basal mechanical pain t hreshol ds of male and female rats var y in an estrogen－dependent manner［2］．Also ad ministration of estrogen has been shown to result in sensitization of nociceptive neur ons［3－4］．

Estrogens′receptors，ERαand ERβ，locate in cell nucleus．When activated by t he ligands，t hey can modulate the gene expression．These effects happen in several hours or days．Recentl y，estrogen′s quick effect on nociceptive neurons has been reported．This non－geno mic action of estr ogen takes effect in minutes or within an hour and can not be mediated by t he nuclear ERs．The newl y defined G－protein－coupled estrogen receptor（GPER），l ocates bot h on t he cell membrane and in plas ma，and can act rapidly through the secondmessengers［1］．In t his review，we will su mmarize the role of GPER in mechanical pain modulation．And we will discuss t he possible signaling downstream pathway of GPER．

Estrogen′s quick ef f ects on mechanical hy per al gesia In rats，the paw withdraw threshold reduces significantly after intrathecal ad ministration of estrogen［5］．Blockade of the endogenous synthesis of estrogens in quail markedly reduced the behavioral responsiveness to painful ther mal stimulus in a minute［6］．And，estrogen had rapid sti mulatory effect on induction of mechanical hyperalgesia in rats［7］．G1（selective agonist of GPER）had the same results with estrogen．

Naming of GPER GPER was first separated from the B cell in 1996［8］，and named GPER in 1997 by Car meci［9］．The International Union of Phar macology officially named it GPER．

Str ucture of GPER As a member of t he G－protein－coupled receptors（GPCRs）family，GPER has a 7－trans membrane hydr ophobic do main，which comprises 20－26 amino acids interconnected by six alter nating extracell ular and intracell ular loops．GPER has an exceptionally conserved sequence among GPCRs，t he Asp－Ar g－Tyr triplet（DRY）．DRY locates in the second intracytoplasmic loop after the third transmembrane domain，and is believed to play a role in signal transduction［10］．

Ex pression of GPER In rat nervous system，GPER mRNA expressed in spinal cord，dorsal root ganglia，nodose ganglia，trigeminal ganglia，hippocampus，brain stem，and hypothalamus［11］．In spinal cor d，GPER is detected in the dorsal hor n，especially in the superficial layer［11－12］．GPER cells of lar ge and s mall neur ons are noted in t he l u mbar dorsal root ganglion［11］．

Interestingl y，ar o matase （estr ogen－synt hase，which catalyzes the conversion of C19 androgens into estr ogens）is also expressed exactly in the

Agonist——G1 GPER－specific co mpound 1（G1），a substituted tetrahydro－3 H－cyclopenta［c］－quinoline，does not bind or activate t he classical nuclear receptors，but binds to GPER with high affinity and selectivity［15］．

As the first GPER－specific agonist［15］，G1 has been used to investigate GPER－mediated biol ogical effects in a wide variety of different cell types and in vivo models［16］．

Agonist——ICI 182，780 ICI 182，780 was t he first pure antiestr ogen，which has been widel y used in the cure of hu man breast cancer．New studies f ound t hat it can bind to GPER and activate it［17－18］．The special physiological character of ICI 182，780 makes it a ver y i mport dr ug in t he study of GPER．

Antagonist——G15 G15，a G1 analog，binds to GPER with high affinity but has no activating effects．G15 does not bind to eit her ERαor ERβ．In vivo，G15 can block some of estrogen′s uterine and neur ol ogical responses．This reveales t hat GPER contributes to those effects initiated by estrogen［19］．superficial layer of t he dorsal hor n［13］．This colocation suggests that GPER may be involved in estrogens′s quick modulation on neuron．

GPER has also been f ound in dorsal root ganglion （DRG）neur ons bot h in male and female rats［11－12］．In DRG，GPER is observed in the cytoplas m of all size neur ons，while t he ERαwas mainly localized in the nuclear of s mall and mediu m neur ons．The expression of GPER in t he DRG was si milar in male and female rates．Interestingly，t here′s a significant higher expression of GPER than ERαin the lu mbar DRG in both sexes［12］．

Ver y little of t he mechanis ms f or regulation of GPER expression has been explored．GPER expression was down－regulated in DRG neur ons of ovariectomized female rats，the reduction in GPER can be recovered by estr ogen replace ment［12］．The GPER′s pro moters and other transcriptional activators remain unknown［14］．

Selective agonists and antagonists of GPER

Ot her membrane estr ogen receptors Rapid action of estrogen may mainly act through the membrane receptors like GPER．Recently，t wo new types of membrane were reported，ER－X and Gaq－ER［20］．Gaq－ER is also a member of t he GPCR family．But their roles in pain mechanisms have not been explored．

The possible signaling pathway of estrogen induced quick mechanical hyperalgesia

Estrogen－GPER－PKCε－TRVP1 pathway

PKCε PKCε，a member of the novel PKCs，has been identified to be an i mportant intracell ular mediator leading to the onset of mechanical hyperalgesia［21］．The st udy of Hucho，et al［7］study shows that estrogen controls PKCε－dependent mechanical hyperal gesia t hr ough direct action on nociceptive neurons．Kuhn，et al［1］got the same results．They f ound t hat G1 and ICI 182，780 lead to translocation of PKCεin DRG neurons to the plas ma membrane．Injection of G1 or ICI 182，780 into the hind paw of male rats rapidly induces PKCε－dependent mechanical hyperal gesia．And PKCε－specific inhibitory peptide εV1－2 could co mpletely abolish t he onset of estr ogen－induced mechanical hyperalgesia．These studies i mplicate t hat estr ogen induces quick mechanical hyperalgesia through a PKCε－dependent pathway．

GPER Studies show t hat estrogen can induce rapid mechanical hyperalgesia in rats in 30 minutes［1，7］．GPER′s selective agonist G1 has the same effects as estrogen．As the agonist of GPER and t he pure ERα，βantagonist，ICI 182，780 can also induce the rapid decrease in the nociceptive t hreshold［1］．Theref ore，t he receptor involved in estrogen induced quick mechanical hyperalgesia should be GPER but not classic ERs．

TRVP1 Transient receptor potential，vanilloid subclass 1 （TRPV1），is highly expressed in t he dorsal r oot ganglion．It has been pr oven as a molecular integrator of painf ul sti muli including noxious heat to endovanilloids in inflammation．TRPV1′s C－ter minus is one target of PKCε phosphor ylation，and it has been proposed to be the downstrea m of PKC in GPER－involved mechanical hyperalgesia．In their studies the authors identified that TRPV1 regulates PKCε－mediated and microtubule－dependent mechanical pain sensitization．GPER agonist G1 also can induce TRPV1－S800－dependent destabilization of microt ubules［22］．So TRPV1 may also be the downstream of estrogen－GPER－PKCεpat h way．

Estrogen－N MDA receptor path way N－met hyl－D－aspartate（NMDA）receptor（NR）plays a very i mportant role in the initiation and maintaining of nociception．ERɑ and the NR1 subunit of t he NMDA receptor are co－expressed in dorsal hor n neur ons，supporting a direct action of estradiol on NMDA receptors． Possible mechanis ms incl ude increasing the expression of NMDA receptors and changing the level of subunit phosphorylation［23］．

Estrogen－P2X receptor pathway P2X receptors are expressed in DRG neurons．Studies suggest that the P2X receptors in sensory neurons play a role in the generation and／or modulation of the pain signaling fro m t he peripher y to t he spinal cor d［24］．It has been reported t hat estr ogen inhibits the expression of P2X3 receptor t hr ough a geno mic mechanis m．And the up－regulation of P2X3 receptors in DRG neurons from the ovariectomized female rats may play a central r ole in mediating the abnor mal nociceptive responses［25］． But the mechanis ms underlying estrogen and P2X3 receptor－mediated nociceptive responses remain unclear．

ConclusionEstr ogen induced mechanical hyperalgesia has been well st udied．But t he r ole of GPER in the rapid action of estrogen and the downstream path way is still unknown．Further research will help to understand the mechanism of pain and find new therapies．

AcknowledgementsWe woul d like to t hank Wenhui Li from Shanghai First People′s Hospital，Shanghai Jiaotong University f or her outstanding advice．

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