Vaginal Hyperalgesia Research Articles

The voltage-gated sodium channel NaV1.7 underlies endometriosis-associated chronic pelvic pain.

Chronic pelvic pain (CPP) is the primary symptom of endometriosis patients, but adequate treatments are lacking. Modulation of ion channels expressed by sensory nerves innervating the viscera has shown promise for the treatment of irritable bowel syndrome and overactive bladder. However, similar approaches for endometriosis-associated CPP remain underdeveloped. Here, we examined the role of the voltage-gated sodium (NaV) channel NaV1.7 in (i) the sensitivity of vagina-innervating sensory afferents and investigated whether (ii) NaV1.7 inhibition reduces nociceptive signals from the vagina and (iii) ameliorates endometriosis-associated CPP. The mechanical responsiveness of vagina-innervating sensory afferents was assessed with ex vivo single-unit recording preparations. Pain evoked by vaginal distension (VD) was quantified by the visceromotor response (VMR) in vivo. In control mice, pharmacological activation of NaV1.7 with OD1 sensitised vagina-innervating pelvic afferents to mechanical stimuli. Using a syngeneic mouse model of endometriosis, we established that endometriosis sensitised vagina-innervating pelvic afferents to mechanical stimuli. The highly selective NaV1.7 inhibitor Tsp1a revealed that this afferent hypersensitivity occurred in a NaV1.7-dependent manner. Moreover, in vivo intra-vaginal treatment with Tsp1a reduced the exaggerated VMRs to VD which is characteristic of mice with endometriosis. Conversely, Tsp1a did not alter ex vivo afferent mechanosensitivity nor in vivo VMRs to VD in Sham control mice. Collectively, these findings suggest that NaV1.7 plays a crucial role in endometriosis-induced vaginal hyperalgesia. Importantly, NaV1.7 inhibition selectively alleviated endometriosis-associated CPP without the loss of normal sensation, suggesting that selective targeting of NaV1.7 could improve the quality of life of women with endometriosis.

Eliapixant is a selective P2X3 receptor antagonist for the treatment of\xa0disorders\xa0associated with\xa0hypersensitive\xa0nerve fibers

ATP-dependent P2X3 receptors play a crucial role in the sensitization of nerve fibers and pathological pain pathways. They are also involved in pathways triggering cough and may contribute to the pathophysiology of endometriosis and overactive bladder. However, despite the strong therapeutic rationale for targeting P2X3 receptors, preliminary antagonists have been hampered by off-target effects, including severe taste disturbances associated with blocking the P2X2/3 receptor heterotrimer. Here we present a P2X3 receptor antagonist, eliapixant (BAY 1817080), which is both highly potent and selective for P2X3 over other P2X subtypes in vitro, including P2X2/3. We show that eliapixant reduces inflammatory pain in relevant animal models. We also provide the first in vivo experimental evidence that P2X3 antagonism reduces neurogenic inflammation, a phenomenon hypothesised to contribute to several diseases, including endometriosis. To test whether eliapixant could help treat endometriosis, we confirmed P2X3 expression on nerve fibers innervating human endometriotic lesions. We then demonstrate that eliapixant reduces vaginal hyperalgesia in an animal model of endometriosis-associated dyspareunia, even beyond treatment cessation. Our findings indicate that P2X3 antagonism could alleviate pain, including non-menstrual pelvic pain, and modify the underlying disease pathophysiology in women with endometriosis. Eliapixant is currently under clinical development for the treatment of disorders associated with hypersensitive nerve fibers.

Pharmacological modulation of voltage-gated sodium (NaV) channels alters nociception arising from the female reproductive tract.

Dyspareunia, also known as vaginal hyperalgesia, is a prevalent and debilitating symptom of gynaecological disorders such as endometriosis and vulvodynia. Despite this, the sensory pathways transmitting nociceptive information from female reproductive organs remain poorly characterised. As such, the development of specific treatments for pain associated with dyspareunia is currently lacking. Here, we examined, for the first time, (1) the mechanosensory properties of pelvic afferent nerves innervating the mouse vagina; (2) the expression profile of voltage-gated sodium (NaV) channels within these afferents; and (3) how pharmacological modulation of these channels alters vaginal nociceptive signalling ex vivo, in vitro, and in vivo. We developed a novel afferent recording preparation and characterised responses of pelvic afferents innervating the mouse vagina to different mechanical stimuli. Single-cell reverse transcription-polymerase chain reaction determined mRNA expression of NaV channels within vagina-innervating dorsal root ganglia neurons. Vagina-innervating dorsal root ganglia neuroexcitability was measured using whole-cell patch-clamp electrophysiology. Nociception evoked by vaginal distension was assessed by dorsal horn neuron activation within the spinal cord and quantification of visceromotor responses. We found that pelvic afferents innervating the vagina are tuned to detect various mechanical stimuli, with NaV channels abundantly expressed within these neurons. Pharmacological modulation of NaV channels (with veratridine or tetrodotoxin) correspondingly alters the excitability and mechanosensitivity of vagina-innervating afferents, as well as dorsal horn neuron activation and visceromotor responses evoked by vaginal distension. This study identifies potential molecular targets that can be used to modulate vaginal nociceptive signalling and aid in the development of approaches to manage endometriosis and vulvodynia-related dyspareunia.

Linaclotide treatment reduces endometriosis-associated vaginal hyperalgesia and mechanical allodynia through viscerovisceral cross-talk.

Endometriosis, an estrogen-dependent chronic inflammatory disease, is the most common cause of chronic pelvic pain. Here, we investigated the effects of linaclotide, a Food and Drug Administration-approved treatment for IBS-C, in a rat model of endometriosis. Eight weeks after endometrium transplantation into the intestinal mesentery, rats developed endometrial lesions as well as vaginal hyperalgesia to distension and decreased mechanical hind paw withdrawal thresholds. Daily oral administration of linaclotide, a peripherally restricted guanylate cyclase-C (GC-C) agonist peptide acting locally within the gastrointestinal tract, increased pain thresholds to vaginal distension and mechanical hind paw withdrawal thresholds relative to vehicle treatment. Furthermore, using a cross-over design, administering linaclotide to rats previously administered vehicle resulted in increased hind paw withdrawal thresholds, whereas replacing linaclotide with vehicle treatment decreased hind paw withdrawal thresholds. Retrograde tracing of sensory afferent nerves from the ileum, colon, and vagina revealed that central terminals of these afferents lie in close apposition to one another within the dorsal horn of the spinal cord. We also identified dichotomizing dual-labelled ileal/colon innervating afferents as well as colon/vaginal dual-labelled neurons and a rare population of triple traced ileal/colon/vaginal neurons within thoracolumbar DRG. These observations provide potential sources of cross-organ interaction at the level of the DRG and spinal cord. GC-C expression is absent in the vagina and endometrial cysts suggesting that the actions of linaclotide are shared through nerve pathways between these organs. In summary, linaclotide may offer a novel therapeutic option not only for treatment of chronic endometriosis-associated pain, but also for concurrent treatment of comorbid chronic pelvic pain syndromes.

Endometriosis‐associated vaginal hyperalgesia is mediated by the balance of reactive aldehyde production and metabolism.

BackgroundEndometriosis affects ~176 million women worldwide and is characterized by the growth of endometrial tissue outside the uterus (known as endometrial ectopic cysts). How endometriosis becomes associated with painful symptoms is not clear. Here we hypothesize that endometriosis‐associated vaginal hyperalgesia is mediated by an imbalance of reactive aldehyde production and metabolism.MethodsTo test this hypothesis, we used female Sprague‐Dawley rats (n=5/group) and subjected them to a cutting‐edge model of behavioral psychophysics to assess for rat vaginal nociception. Prior to subjecting the rodents to this model, we treated rodents with the reactive aldehyde, acetaldehyde (9 mg/kg intraperitoneal) to mimic an increased production of reactive aldehydes in the intraperitoneal space. To modulate ALDH2 activity, we administered to rodents either the ALDH2 activator, Alda‐1 (1 mg/kg, diluted in DMSO, given intraperitoneal) or the ALDH2 inhibitor disulfiram (15 mg/kg, diluted in DMSO, given intraperitoneal). The ALDH2 activator were delivered 10 minutes before or after acetaldehyde (Alda‐1 + aldehyde or aldehyde +Alda‐1). Disulfiram was given 10 minutes before acetaldehyde (Disulfiram + aldehyde). As controls Alda‐1, disulfiram, saline, and DMSO were delivered alone. Changes in vaginal nociception were expressed as area‐under‐the‐curve (AUC) units and calculated using standard trapezoid methods. For statistical analysis, a one‐way ANOVA with Bonferroni's multiple comparison test was used. Data presented as mean ± SEM.ResultsAcetaldehyde‐induced vaginal hyperalgesia (44±4* vs. saline 28±2, *P<0.05). Alda‐1 markedly attenuated hyperalgesia when given either before (26±3, *P<0.05 vs. acetaldehyde) or after (25±3, *P<0.05 vs. acetaldehyde) acetaldehyde treatment. Conversely, pretreatment with disulfiram before acetaldehyde increased vaginal hyperalgesia (61±5, *P<0.05 vs. acetaldehyde). Alda‐1, DMSO, and disulfiram alone produced no significant changes in vaginal nociception.ConclusionsThese data suggest that the mechanism of endometriosis‐induced hyperalgesia is mediated by changes in reactive aldehyde production and metabolism. Increasing ALDH2 activity in rats, both prevented and alleviated reactive aldehyde‐induced vaginal hyperalgesia. These data suggest a potential novel therapeutic for treating endometriosis by targeting reactive aldehyde metabolism.Support or Funding InformationThis work was supported by the National Institutes of Child Health and Human Development K99HD093858 (SLM), a Stanford Women & Sex Differences in Medicine (WSDM) Seed Grant, and an Endometriosis Foundation of America Research Award.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

(268) - The influence of chronic WIN 55, 212-2 treatment on vaginal hyperalgesia, VEGF, and NGF in a rat model of endometriosis

Endometriosis affects ~10% of women of reproductive age and is characterized by the growth of endometrial tissue outside the uterus. Approximately 50% of women with endometriosis suffer from debilitating chronic pelvic pain including dyspareunia (pain during sexual intercourse; vaginal hyperalgesia) and severe dysmenorrhea (menstrual pain). How the endometrial ectopic growths (cysts) contribute to painful symptoms is poorly understood. However, mounting evidence suggests the involvement of the endocannabinoid system in endometriosis and its associated pain. Cannabinoids and their receptors are known to be involved in uterine function and dysfunction, abnormal vascularization, neurite outgrowth, and neuronal guidance. Further, exogenous cannabinoids have been used for centuries to alleviate the endometriosis-associated painful symptom of dysmenorrhea. In a rat model of endometriosis (ENDO), uterus pieces are auto-transplanted onto abdominal arteries and form vascularized and innervated cysts similar to women. ENDO also produces painful symptoms similar to that of women with endometriosis, including vaginal hyperalgesia. Here, vaginal nociception was assessed behaviorally at baseline, for eight weeks after ENDO was induced (when vaginal hyperalgesia/cyst innervation stabilize), and then for eight weeks during chronic treatment with the CB1/CB2 agonist WIN 55, 212-2 (or in controls: DMSO or untreated). Cyst and uterus levels of vascular endothelial growth factor (VEGF) and nerve growth factor (NGF) were also assessed using protein array analysis. All data were analyzed in the proestrus stage. Our data demonstrate that treatment with WIN 55, 212-2 alleviates ENDO-induced vaginal hyperalgesia, significantly decreases VEGF expression in the cysts but not uterus, and does not significantly influence NGF expression in the cysts or uterus. Overall, these data suggest that cannabinoid receptor stimulation-mediated modulation of VEGF, potentially through VEGF's influence on vascularization and/or innervation, influences endometriosis-associated vaginal hyperalgesia. Further, these results also provide additional support for targeting the endocannabinoid system for endometriosis-associated pain.

Mo1541 - Oral Administration of the Gut-Restricted Guanylate Cyclase-C Agonist, Linaclotide, Reduces Endometriosis-Induced Vaginal Hyperalgesia

Mo1541 - Oral Administration of the Gut-Restricted Guanylate Cyclase-C Agonist, Linaclotide, Reduces Endometriosis-Induced Vaginal Hyperalgesia

Prostaglandin levels, vaginal innervation, and cyst innervation as peripheral contributors to endometriosis-associated vaginal hyperalgesia in rodents

Endometriosis is a painful condition characterized by growth of endometrial cysts outside the uterus. Here, we tested the hypothesis that peripheral innervation and prostaglandin levels contribute to endometriosis-associated pain. Female Sprague-Dawley rats (n = 16) were surgically instrumented by transplanting uterine tissue onto mesenteric arteries within the peritoneal cavity to create a model of endometriosis which forms extra-uterine endometrial cysts and vaginal hyperalgesia. Our results describe a significant positive correlation between endometriosis-induced vaginal hyperalgesia and cyst innervation density (sensory, r = 0.70, p = 0.003; sympathetic, r = 0.55, p = 0.03), vaginal canal sympathetic innervation density (r = 0.80, p = 0.003), and peritoneal fluid levels of the prostaglandins PGE2 (r = 0.65, p = 0.01) and PGF2α (r = 0.63, p = 0.02). These results support the involvement of cyst innervation and prostaglandins in endometriosis-associated pain. We also describe how sympathetic innervation density of the vaginal canal is an important predictor of vaginal hyperalgesia.

Resolvins RvD1 and 17(R)-RvD1 alleviate signs of inflammation in a rat model of endometriosis

To study the effects of two resolvins of D series, RvD1 and 17(R)-RvD1, on inflammatory signs associated with endometriosis (ENDO). Invivo research study. Research laboratory. Female Sprague-Dawley rats. Intravenous or intraperitoneal injections of RvD1 (300ng/kg) or 17(R)-RvD1 (300 and 900ng/kg) in rats with surgically induced ENDO. Vascular permeability of ectopic endometrial growths was assessed by Evans Blue extravasation; vaginal hyperalgesia was assessed with telemetered visceromotor response. Both resolvins, but not vehicle, significantly decreased vascular permeability in ectopic endometrial tissue. 17(R)-RvD1 also significantly alleviated severity of vaginal hyperalgesia. Our results suggest that RvD1 and 17(R)-RvD1 can be considered for further investigation of their therapeutic potential for treating ENDO.

Sprouted Innervation into Uterine Transplants Contributes to the Development of Hyperalgesia in a Rat Model of Endometriosis

Endometriosis is an enigmatic painful disorder whose pain symptoms remain difficult to alleviate in large part because the disorder is defined by extrauteral endometrial growths whose contribution to pain is poorly understood. A rat model (ENDO) involves autotransplanting on abdominal arteries uterine segments that grow into vascularized cysts that become innervated with sensory and sympathetic fibers. ENDO rats exhibit vaginal hyperalgesia. We used behavioral, physiological, and immunohistochemical methods to test the hypothesis that cyst innervation contributes to the development of this hyperalgesia after transplant. Rudimentary sensory and sympathetic innervation appeared in the cysts at two weeks, sprouted further and more densely into the cyst wall by four weeks, and matured by six weeks post-transplant. Sensory fibers became abnormally functionally active between two and three weeks post-transplant, remaining active thereafter. Vaginal hyperalgesia became significant between four and five weeks post-transplant, and stabilized after six to eight weeks. Removing cysts before they acquired functional innervation prevented vaginal hyperalgesia from developing, whereas sham cyst removal did not. Thus, abnormally-active innervation of ectopic growths occurs before hyperalgesia develops, supporting the hypothesis. These findings suggest that painful endometriosis can be classified as a mixed inflammatory/neuropathic pain condition, which opens new avenues for pain relief. The findings also have implications beyond endometriosis by suggesting that functionality of any transplanted tissue can be influenced by the innervation it acquires.

Telemetric Assessment of Referred Vaginal Hyperalgesia and the Effect of Indomethacin in a Rat Model of Endometriosis

Symptoms of endometriosis (ENDO), among others, include pelvic/abdominal and muscle pain. Non-steroidal anti-inflammatory agents are first-line treatment for this pain. Similar to women, rats with surgically induced ENDO, but not its surgical control, exhibit vaginal hyperalgesia, which in rats is evidenced by a decreased threshold for the visceromotor response (VMR) induced by vaginal distention. Here we assess the VMR in rats with implanted probes that telemetrically transmit EMG activity from the abdominal muscle. The feasibility and sensitivity of this technique for monitoring the VMR threshold across the estrous cycle and the influence of Indomethacin on ENDO-induced vaginal hyperalgesia were evaluated. VMR thresholds in response to vaginal distention with an infusion pump were measured in different estrous stages. Indomethacin (5 or 10 mg/kg i.p. or s.c.) was injected in proestrus rats and 40–60 min later the VMR threshold was measured. The VMR threshold varied across the estrous cycle only in ENDO rats, being lowest in proestrus. Indomethacin increased this threshold in proestrus ENDO rats. These results show that telemetric assessment of the VMR is a sensitive tool, suitable for long-term studies in conscious rats. The results with this technique also suggest that ENDO-associated vaginal hyperalgesia involves COX activity, the feature that also underlies inflammatory pains.

Endometriosis-induced vaginal hyperalgesia in the rat: Role of the ectopic growths and their innervation

Endometriosis is a painful disorder defined by extrauteral endometrial growths whose contribution to pain symptoms is poorly understood. Endometriosis is created in rats by autotransplanting on abdominal arteries pieces of either uterus (ENDO), which form cysts, or fat (shamENDO), which do not form cysts. ENDO, but not shamENDO induces vaginal hyperalgesia. We tested the hypothesis that the cysts are necessary to maintain vaginal hyperalgesia by assessing the effect of surgically removing them. Complete-cyst-removal eliminated ENDO-induced vaginal hyperalgesia up to 4months post-operatively. Sham-cyst-removal in ENDO rats, in which cysts were not removed, or partial cyst-removal increased the ENDO-induced hyperalgesia. The decreases and increases both took 3–6weeks to develop. Changes in ENDO-induced hyperalgesia did not occur in a control group of ENDO rats who had no surgery after ENDO. In a double-surgery control group, neither shamENDO surgery nor a subsequent sham surgery that mimicked “removal” of non-existent cysts influenced vaginal nociception. In a no-surgery control group, vaginal nociception remained stable for >6months. The increases in ENDO-induced hyperalgesia produced by the sham-cyst-removal surgery were smaller in proestrus than in other estrous stages. During the other stages (but not during proestrus), sympathetic innervation of the cysts increased. These results suggest that maintenance of ENDO-induced vaginal hyperalgesia requires continued presence of at least some ectopic endometrial tissue, and that surgical treatment that fails to remove ectopic endometrial tissue can exacerbate the hyperalgesia, possibly due in part to an increase in the cysts’ sympathetic innervation.

Endometriosis-induced vaginal hyperalgesia in the rat: Effect of estropause, ovariectomy, and estradiol replacement

Endometriosis (ENDO) is a painful disorder defined by extrauteral endometrial growths. It is created in rats by autotransplanting pieces of uterus (which form cysts), or, for shamENDO, fat (no cysts). ENDO induces vaginal hyperalgesia, likely via central sensitization. The severity of this hyperalgesia correlates with estradiol levels during the estrous cycle, suggesting the hyperalgesia is estradiol-modulated. If so, then hyperalgesic severity should track estradiol changes during reproductive senescence (estropause) when estradiol levels initially decrease, then increase. Using psychophysical methods to assess vaginal nociception, we found that the severity of ENDO-induced hyperalgesia paralleled estradiol changes during estropause: hyperalgesia first decreased, then returned. Furthermore, the return occurred regardless of the presence of the cysts (excised in some rats). This finding provides further support for ENDO’s likely centrally-mediated effects. Additionally, the results suggest that elimination of estradiol via ovariectomy (OVX) should alleviate ENDO-induced hyperalgesia and estradiol replacement should restore it. However, in healthy and shamENDO rats, OVX produces a vaginal hyperalgesia that is alleviated by estradiol, likely via estradiol’s peripheral influences on the vagina. Hence, we tested the hypothesis that OVX in ENDO rats would trigger a different type of vaginal hyperalgesia dependent on the loss of estradiol. We predicted that the opposing influences of estradiol on ENDO- and OVX-induced hyperalgesia would cancel each other. As predicted, OVX had no effect on ENDO-induced hyperalgesia and estradiol replacement alleviated it. These results suggest that, in intact rats, ENDO-induced vaginal hyperalgesia is exacerbated by estradiol, and that different mechanisms underlie ENDO-induced versus OVX-induced vaginal hyperalgesia.

A life of pelvic pain

Pelvic pain associated with menstruation, i.e., dysmenorrhea, is a chronic pelvic pain that not only interferes with a woman's wellbeing for a large part of her life but also often co-occurs with other chronic painful conditions such as interstitial cystitis and irritable bowel syndrome and others. Little has been known about mechanisms underlying these chronic pelvic pains. This paper reviews 37 years of research in my laboratory at Florida State University on such mechanisms. Our research, mostly on rats, has contributed to the following findings: (1) Female reproductive organs are innervated in a topographic fashion by afferents in the pelvic (vagina/cervix) and hypogastric (cervix/uterine horn) nerves. (2) The input contributes to uterine and vaginal perceptions (nociception) that are modified by reproductive status. (3) Throughout the CNS, neurons responsive to stimulation of the reproductive tract also respond to stimulation of skin and other internal organs, in a manner modifiable by reproductive status and peripheral pathophysiology. (4) This dynamic physiological convergence may reflect extensive anatomical divergence of and interconnections between pathways entering the CNS via gateways through the spinal cord, dorsal column nuclei, and solitary nucleus. (5) The convergence also indicates the existence of extensive cross-system, viscero–visceral interactions within the CNS, that, while organized for coherent bodily functioning, serves as a substrate by which pathophysiology in one organ can influence physiology and responses to pathophysiology in other organs. (6) Some cross-system effects observed so far include: (a) Bladder inflammation reduces the rate of uterine contractions and the effects of drugs on the uterus. (b) Colon inflammation produces signs of inflammation in the otherwise healthy bladder and uterus. (c) A surgical model of endometriosis produces vaginal hyperalgesia, exacerbates pain behaviors induced by a ureteral stone, and reduces volume voiding thresholds if the bladder. These cross-system effects, which likely involve CNS mechanisms, likely also underlie co-occurrence of painful clinical conditions. Research continues on details of these mechanisms and their relevance for clinical diagnosis and therapy. None of this work could have been done without collegial support of colleagues and technical staff at Florida State University.

Estrous changes in vaginal nociception in a rat model of endometriosis

A rat model of endometriosis, in which pieces of uterine horn (versus fat in controls) are autotransplanted into the abdomen where they form cysts, reduces fecundity and produces vaginal hyperalgesia. The cysts gradually enlarge over a 2-month period postsurgically and then plateau. Cysts regress with low estrogen levels and reappear when they rise. Based on the hypothesis that the vaginal hyperalgesia depends upon the cysts, this study tested two predictions: that (1) the hyperalgesia would develop postsurgically in parallel with the cysts, and (2) the hyperalgesia would vary with estrous, being greatest when estrogen levels are high (proestrus) and least when low (estrus). In rats trained to escape vaginal distention, percentage escape responses to different distention volumes were measured across the rat's 4-day estrous cycle for 2.5 months before and up to 4 months after autotransplantation of uterus ( n = 9) or fat ( n = 6) in abdominal sites. Vaginal pressures were also measured. In rats with uterine but not fat autotransplants, escape percentages increased postsurgically over a 2-month period and then plateaued. The increase was greatest in proestrus and failed to occur in estrus. Vaginal pressures were unchanged in all groups. These results strongly support the hypothesis that the vaginal hyperalgesia depends upon the cysts. Because the cysts were located in sites remote from the vagina, the hyperalgesia involves viscero-visceral interactions and is likely centrally mediated, whereas the estrous modulation could involve hormonal actions either on the cysts or, more likely, on vaginal afferent fibers, and/or on central neurons.

Estrogen replacement reverses ovariectomy-induced vaginal hyperalgesia in the rat

Objectives: The loss of ovarian function in women through aging or oophorectomy is often associated with the development of vaginal hyperalgesia that can be alleviated with estrogen replacement. This study examined if ovariectomy in rats would similarly give rise to vaginal hyperalgesia, and, if so, whether estrogen replacement would alleviate it. Methods: Female rats were trained to perform an operant response to escape vaginal distention delivered by inflating a balloon located in mid-vaginal canal. Percent escape responses to eight different volumes of distention measured in normally cycling rats were compared with measures made in the same rats following ovariectomy (OVX) or sham ovariectomy (shamOVX), and then, in the OVX group, estrogen replacement (OVX+E2). Pressures exerted by the eight volumes on the vaginal wall were also measured, thereby permitting assessment of vaginal tone. Results: Whereas overall escape response percentages after OVX, but not shamOVX, were significantly higher to the largest six distention volumes compared with responses during cycling, there were individual differences in the amount of hyperalgesia. Following OVX+E2, escape response percentages decreased in all but one rat. Vaginal tone after OVX, shamOVX or OVX+E2 did not differ from overall vaginal tone in cycling rats. Conclusions: Ovariectomy in rats evokes a variable amount of vaginal hyperalgesia that can be alleviated by estrogen replacement in most cases. Thus, the ovariectomized rat appears to provide a useful model for the study of mechanisms underlying the dyspareunia that is associated with loss of ovarian function in women.

Influence of endometriosis on pain behaviors and muscle hyperalgesia induced by a ureteral calculosis in female rats

Endometriosis and urinary calculosis can co-occur. Clinical studies have shown that both painful and non-painful endometriosis in women are associated with enhanced pain and referred muscle hyperalgesia from urinary calculosis, but the mechanisms underlying this phenomenon are still poorly understood. The aim of this study was to develop an animal model adequate to explore this viscero-visceral interaction in standardized conditions. Using a model of endometriosis previously developed to study reduced fertility and vaginal hyperalgesia, endometriosis ( endo) or sham-endometriosis ( sham-endo) was induced in rats by autotransplantation of small pieces of uterus (or, for sham-endo, fat) on cascade mesenteric arteries, ovary, and abdominal wall. After the endometrial, but not the fat autografts had produced fluid-filled cysts (3 weeks), urinary calculosis was induced by implanting an artificial stone into one ureter. Pain behaviors were monitored by continuous 24-h videotape recordings before and after stone implantation. Referred muscle hyperalgesia was assessed by measuring vocalization thresholds to electrical stimulation of the oblique musculature (L1 dermatome). The data were compared with previously reported data from rats that had received only the stone. Neither endo nor sham-endo alone induced pain behaviors. Following stone implantation, in endo rats compared to sham-endo and stone-only rats, pain behaviors specifically associated with urinary calculosis were significantly increased and new pain behaviors specifically associated with uterine pathology became evident. Muscle hyperalgesia was also significantly increased. To explore the relationship between the amount of endometriosis and that of ureteral pain behavior, two separate groups of endo rats were treated with either a standard non-steroidal anti-inflammatory drugs (ketoprofen) or placebo from the 12th to the 18th day after endometriosis induction. The stone was implanted on the 21st day. Ketoprofen treatment compared to placebo significantly reduced the size of the cysts and both ureteral and uterine pain behaviors post-stone implantation. The size of the cysts showed a significant linear correlation with the post-stone ureteral pain behaviors. In conclusion, endo increased pain crises and muscle hyperalgesia typically induced by a ureteral calculosis, and the ureteral calculosis revealed additional pain behaviors typically induced by uterine pathophysiology; and this enhancement was a function of the degree of endometriosis. This result closely reproduces the condition observed in humans and could be due to a phenomenon of ‘viscero-visceral’ hyperalgesia, in which increased input from the cyst implantation sites to common spinal cord segments (T10-L1) facilitates the central effect of input from the urinary tract.

Vaginal hyperalgesia in a rat model of endometriosis

This study examined whether a rat model of surgically-induced endometriosis that reduces fertility also evokes vaginal hyperalgesia along with changes in vaginal compliance. In nine rats trained to escape vaginal distention, percent escape responses to different volumes of vaginal distention were measured for 2.5 months before and after endometriosis or sham surgery. Vaginal pressures were also measured simultaneously to provide an estimate of vaginal compliance. Endometriosis (or sham) was induced by autotransplantation of small pieces of uterus (or fat) on mesenteric cascade arteries, abdomen, and ovary. Escape responses were significantly increased only in rats whose autotransplants had formed cysts. Vaginal pressures, however, remained unchanged. This vaginal hyperalgesia may involve a process of viscero-visceral referred hyperalgesia.