Chronic Sciatica In A Novel Rat Model Of Endometriosis

Objective:Endometriosis (EMS) is a common gynecological disease, which is defined as endometrial tissues appear outside the uterus cavity, affecting approximately 10% women of reproductive age. The main symptoms of EMS are chronic pelvic pain and infertility. Recent researches showed that about 40% EMS patients are suffering from chronic sciatica, mostly related to menstrual cycle-cyclic sciatica -- without lesion in spine under CT or MRI. Lesions around sciatic nerve roots or directly affecting peripheral sciatic nerve can cause nerve irritation and then sciatica. Due to the feature that EMS is estrogen-dependent, the sciatica is usually menstrual-related and is considered as radiated pain or neuropathic pain, with the former one induced by lesions locate at posterior-lateral pelvic wall irritating sciatic nerve roots; while the latter one induced by lesions directly affect peripheral sciatic nerve causing inflammation and nerve damage. Meanwhile, surgery findings suggested that lesions around sciatic nerve is the main cause of EMS cyclic sciatica. However, at the early stage, EMS patients would usually go to department of orthopaedics or neurology, which can cause delay of precise diagnose. In this experiment, we successfully established a novel model of rat endometriosis, mimicking chronic sciatica of EMS patients, then showed some prelimilary data regarding mechanisms of chronic sciatica induced by EMS.Methods:We successfully developed a rat model for sciatic endometriosis by grafting a piece of autologous uterine tissue (using connective tissue around uterus in control group) around the sciatic nerve, which was verified by observing specific structures of endometrial tissues using H&E and immunohistochemistry methods. After the model was established, animal behavior tests were performed for up to 7 weeks to test pain threshold. In another set of experiment, animals were behavioral tested as well as defining estrous cycle, to examine whether pain-like behavior is related to estrous cycle. Then, some animals were performed lesion-removal surgeries and pain threshold was tested after the surgeries to analyze the difference of before and after. In H&E and immunohistochemistry essays, we determined whether there was inflammation in the lesions or structural damage in the sciatic nerve, following was cytokine essay testing the actual protein level in ectopic tissue. Based on the idea of neuropathic pain, we used GAP43 as nerve regeneration marker to determine newly grown fibers in the ectopic tissue; we also used in vivo single fiber recording to determine spontaneous activity in the lesion section of sciatic nerve.Results:(1) The uterine grafts survived and developed fluid-filled cysts, showing no signs of invading surrounding muscle tissues; the adjacent nerve showed signs of swelling and damage. Newly developed blood vessels were easy to observe. H&E showed typical endometrial gland structure at post-operative week 3, while immunohistochemistry staining showed no sign that ectopic tissue invading sciatic nerve.(2) Mechanical and cold hypersensitivity and allodynia of the ipsilateral hindpaw developed gradually over the first 2 weeks after the surgery, peaked at 2-5 weeks, and was almost resolved by 7 weeks. Control animals showed only minor changes in these pain behaviours.(3) Histological signs of inflammation in the uterine graft and in the adjacent nerve, which were massive infiltration of inflammatory cells and swelling of nerve fibers, were observed at 3 weeks but were resolving by 7 weeks.(4) In vivo fibre recording showed increased spontaneous activity, especially of C-fibres, in sciatic nerve proximal to the uterine graft. Several pro-inflammatory cytokines including interluekin-18, VEGF, fractalkine, and MIP-la, were elevated in the uterine graft plus sciatic nerve samples, compared to samples from normal nerve or nerve plus fat graft. Growth associated protein 43 (GAP43), a marker of regenerating nerve fibres, was observed in the adjacent sciatic nerve as well as in the uterine graft.Conclusions:(1) The mechanism of pain in sciatic endometriosis may include:the liberation of inflammatory factors and the regeneration or injury of nerve fibers.(2) We try to speculate pain of endometriosis may be one of the neuropathic pain in that production and liberation of inflammatory factors, regeneration of nerve fibers, spontaneous activity enhancement of A & C sensory nerve fiber may participate in the occurrence and development of pain mechanism in endometriosis.(3) Compared with other endometriosis rat model, the sciatic endometriosis rat model do well in mimicking neuropathic pain, and in the future it is beneficial to study the inter reaction between inflammatory and nerve fibers in endometriosis. However, this model has its limitations, for example, it has nothing to do with menstrual cycle.