Seasonal expressions of oxytocin and oxytocin receptor in epididymis of the male muskrat (Ondatra zibethicus).

Oxytocin (OT) is presented in the male reproductive tract and has various physiological functions, such as stimulating contraction of the reproductive tract and aiding sperm transport. In this study we investigated seasonal expressions of OT and oxytocin receptor (OTR) in the epididymis of the muskrat during the breeding and non-breeding seasons. Morphologically, the weight and length of epididymis in the breeding season were significantly higher than those in the non-breeding season. In the breeding season, the lumen diameter and epithelial thickness of epididymis increased significantly, and there were a large number of sperms in the lumen. However, in the non-breeding season, the diameter of the lumen became significantly narrower, the epithelial height became thinner, and there was no sperm in the lumen. Immunohistochemical results showed that OT and OTR were presented in the cytoplasm of epithelial cells and smooth muscle cells within the epididymis, and the immunostainings of OT and OTR in the breeding season were significantly stronger compared with the non-breeding. In consistent with the immunohistochemical results, the mRNA levels of OT and OTR were higher in the whole epididymis during the breeding season than those in the non-breeding season. In addition, the concentrations of OT in the epididymis and sera were both significantly higher in the breeding season when compared to the non-breeding season. These results suggested that the epididymis of the muskrat was the direct target organ of OT, and OT might play a regulatory role in the epididymal function via endocrine or autocrine/paracrine manners.