Impact of surgical excision of lesions on pain in a rat model of endometriosis

Recent advances in the treatment of pain in endometriosis: A bibliometric analysis of experimental models

Background and Aim

Treatment of endometriosis involves pain relief which is achieved through the administration of analgesics and non-steroidal anti-inflammatory drugs, with or without the addition of hormone therapy. At present, studies investigating endometriosis pain management using experimental rat models and the use of medications are scarce. Therefore, this study aimed to systematically evaluate research trends and critical points in the field of endometriosis pain management using experimental models.

Materials and Methods

A total of 30 publications related to this topic that were published from 2012 to 2022 were retrieved from various databases, including Web of Science, Scopus, PubMed, Embase, and CINAHL, using appropriate English keywords. The quality of the publications was evaluated using impact metrics, productivity, term density mapping, and author network.

Results

The average publication rate was three articles per year, reaching its peak in 2021 at five articles per year. The United States and China were found to be the most productive countries, with 12 and 10 publications per year, respectively. The field of medicine (37.0%) was the most abundant, although the H-index was relatively low (13:13). Term density mapping involved the search of 542 keywords, of which 35 were selected, with only 8 exhibiting significant density.

Conclusion

In the past decade, there has been a shortage of publications that have addressed pain in endometriosis in experimental models and, within this context the majority of the production and publication in this field has been performed by the United States and China. After performing this bibliometric review, it can be inferred that more research is required in this field, to develop new approaches and treatments for endometriotic pain.

The Unmet Needs for Studying Chronic Pelvic/Visceral Pain Using Animal Models

The different definitions of chronic pelvic/visceral pain used by international societies have changed over the years. These differences have a great impact on the way researchers study chronic pelvic/visceral pain. Recently, the role of systemic changes, including the role of the central nervous system, in the perpetuation and chronification of pelvic/visceral pain has gained weight. Consequently, researchers are using animal models that resemble those systemic changes rather than using models that are organ- or tissue-specific. In this review, we discuss the advantages and disadvantages of using bladder-centric and systemic models, enumerating some of the central nervous system changes and pain-related behaviors occurring in each model. We also present some drawbacks when using animal models and pain-related behavior tests and raise questions about possible, yet to be demonstrated, investigator-related bias. We also suggest new approaches to study chronic pelvic/visceral pain by refining existing animal models or using new ones.

Endometriosis-Associated Pain - Do Preclinical Rodent Models Provide a Good Platform for Translation?

Pelvic pain is a common symptom of endometriosis. Our understanding of its etiology remains incomplete and medical management is limited by poor translation from preclinical models to clinical trials. In this review, we briefly consider the evidence, or lack thereof, that different subtypes of lesion, extra-uterine bleeding, and neuropathic pathways add to the complex and heterogeneous pain experience of women with the condition. We summarize the studies in rodent models of endometriosis that have used behavioral endpoints (evoked and non-evoked) to explore mechanisms of endometriosis-associated pain. Lesion innervation, activation of nerves by pronociceptive molecules released by immune cells, and a role for estrogen in modulating hyperalgesia are key endometriosis-associated pain mechanisms replicated in preclinical rodent models. The presence of ectopic (full thickness uterus or endometrial) tissue may be associated with changes in the spinal cord and brain, which appear to model changes reported in patients. While preclinical models using rats and mice have yielded insights that appear relevant to mechanisms responsible for the development of endometriosis-associated pain, they are limited in scope. Specifically, most studies are based on models that only resulted in the formation of superficial lesions and use induced (evoked) behavioral 'pain' tests. We suggest that translation for patient benefit will be improved by new approaches including models of ovarian and deep infiltrating disease and measurement of spontaneous pain behaviors. Future studies must also capitalize on new advances in the wider field of pain medicine to identify more effective treatments for endometriosis-associated pain.

Effects of inhibiting the PI3K/Akt/mTOR signaling pathway on the pain of sciatic endometriosis in a rat model

This study investigated the relationship between the pain of sciatic endometriosis and the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) signaling pathway. Adult female Sprague–Dawley rats successfully received sciatic endometriosis induction. Mechanical paw withdrawal threshold and paw withdrawal latency were recorded to assess the mechanical hypersensitivity and thermal hyperalgesia. Quantitative real-time PCR, Western blotting, and enzyme-linked immunosorbent assays were used to detect PI3K, Akt, and mTOR expressions and their phosphorylation as well as the expressions of substance P, calcitonin gene-related peptide (CGRP), and nerve growth factor (NGF). Mechanical paw withdrawal threshold and paw withdrawal latency significantly decreased after sciatic endometriosis induction in rats; this decrease was ameliorated by inhibiting the PI3K/Akt/mTOR signaling pathway using LY294002. Compared with controls, rats with sciatic endometriosis showed increased PI3K, Akt, and mTOR expressions and elevated p-PI3K, p-Akt, and p-mTOR protein expressions. Higher NGF, substance P, and CGRP expressions were also found in the superficial dorsal horn of the spinal cord in rats with sciatic endometriosis than in control rats 21 days after surgery. Following the injection of LY294002 into rats with sciatic endometriosis, there was a significant decrease in the expressions of NGF, substance P, and CGRP. In conclusion, the inhibition of the PI3K/Akt/mTOR signaling pathway may alleviate endometriosis-associated sciatic nerve pain in a rat model of sciatic endometriosis.

Research Priorities for Endometriosis

The 3rd International Consensus Workshop on Research Priorities in Endometriosis was held in São Paulo on May 4, 2014, following the 12th World Congress on Endometriosis. The workshop was attended by 60 participants from 19 countries and was divided into 5 main sessions covering pathogenesis/pathophysiology, symptoms, diagnosis/classification/prognosis, disease/symptom management, and research policy. This research priorities consensus statement builds on earlier efforts to develop research directions for endometriosis. Of the 56 research recommendations from the 2011 meeting in Montpellier, a total of 41 remained unchanged, 13 were updated, and 2 were deemed to be completed. Fifty-three new research recommendations were made at the 2014 meeting in Sao Paulo, which in addition to the 13 updated recommendations resulted in a total of 66 new recommendations for research. The research recommendations published herein, as well as those from the 2 previous papers from international consensus workshops, are an attempt to promote high-quality research in endometriosis by identifying and agreeing on key issues that require investigation. New areas included in the 2014 recommendations include infertility, patient stratification, and research in emerging nations, in addition to an increased focus on translational research. A revised and updated set of research priorities that builds on this document will be developed at the 13th World Congress on Endometriosis to be held on May 17-20, 2017, in Vancouver, British Columbia, Canada.

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.

Sciatic endometriosis induces mechanical hypersensitivity, segmental nerve damage, and robust local inflammation in rats

BACKGROUND

Endometriosis is a common cause of pain including radicular pain. Ectopic endometrial tissue may directly affect peripheral nerves including the sciatic, which has not been modelled in animals.

METHODS

We developed a rat model for sciatic endometriosis by grafting a piece of autologous uterine tissue around the sciatic nerve. Control animals underwent a similar surgery but received a graft of pelvic fat tissue.

RESULTS

The uterine grafts survived and developed fluid-filled cysts; the adjacent nerve showed signs of swelling and damage. 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. Histological signs of inflammation in the uterine graft and in the adjacent nerve were observed at 3 weeks but were resolving by 7 weeks. 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-1α, 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

This model shared many features with other rat models of endometriosis, but also had some unique features more closely related to neuropathic pain models.

WHAT DOES THIS STUDY/REVIEW ADD

Some especially painful forms of endometriosis are essentially neuropathic, because peripheral nerves are directly affected by nearby ectopic endometrial tissue. We modelled endometriosis by implanting autologous uterine tissue around rat sciatic nerve. We observed mechanical and cold pain behaviours along with signs of inflammation and nerve damage and increased pro-inflammatory cytokines at the implant site. Pain behaviours correlated with signs of nerve inflammation and damage rather than with cyst survival.