Quantification of enkephalinergic immunoreactivity in the periaqueductal gray of an animal model of endometriosis

Endometriosis is a debilitating disease characterized by pelvic pain, dysmenorrhea and dyspareunia. These symptoms produce stress on the individual and impact daily life activities. It is still poorly understood how stress affects endometriosis associated pain. We aimed to measure levels of the endogenous opioid peptide enkephalin (ENK) in the periaqueductal gray (PAG) of an animal model of endometriosis since PAG plays a critical role in descending modulation of pain via opioidergic mechanisms. Four groups of female rats received either: sham surgery, or surgical endometriosis then no‐stress, controllable or uncontrollable swim stress during 10 consecutive days. Sixty days after surgery, rats were killed and perfused. The brains were removed and sections containing the PAG processed for quantitative light immunohistochemistry for ENK. Results showed that uncontrollable stress exacerbated the vesicle grade in animals with endometriosis as compared to no‐stress or controllable stress. Within the PAG, ENK was mostly observed in the dorsal and lateral regions thru the rostral‐caudal span. Optical densitometry quantification revealed no differences between groups in ENK levels suggesting that neither stress nor endometriosis is affecting ENK levels in the PAG. Future experiments will elucidate whether other opioid peptides or their receptors alter in response to stress and endometriosis. R15AT006373