Endogenous opioid peptides in uterine fluid

The present study demonstrates the presence of the endogenous opioid peptides β-endorphin (β-EP) and methionine-enkephalin (MET-ENK), in the uterine fluid of fertile women and normally cycling and superovulated cows. The two peptides are undetectable in the uterine fluid of untreated postmenopausal women, whereas they are present following estrogen-progesterone treatment. Immunoreactive (IR) MET-ENK concentrations were higher in the secretory than in the proliferative phase of the menstrual cycle. IR β-EP and IR MET-ENK are present also in the follicular, oviductal, and uterine fluid of cows, and in the uterine fluid, concentrations of IR MET-ENK are higher in the superovulated than in the control animals. Because opioids play important roles on endocrine and immune functions, the present data support the potential physiologic role of endometrial secretions.