Seasonal changes in the expression of PACAP, VPAC1, VPAC2, PAC1 and testicular activity in the testis of the muskrat (Ondatra zibethicus)

Pituitary adenylate cyclase-activating polypeptide (PACAP) plays an important role in the steroidogenesis and spermatogenesis in the testis through its receptors PAC1, VPAC1 and VPAC2. In this study, we investigated the seasonal expressions of PACAP, PAC1, VPAC1, VPAC2, luteinizing hormone receptor (LHR), follicle stimulating hormone receptor (FSHR), steroidogenic acute regulatory protein (StAR), 3β-hydroxysteroid dehydrogenase (3β-HSD) and CYP17A1 in the testis of male muskrat during the breeding season and non-breeding season, respectively. Histologically, we observed the presence of Leydig cells, Sertoli cells and various types of germ cells in the testis during the breeding season, yet only Leydig cells, Sertoli cells, spermatogonia and primary spermatocyte during the non-breeding season. In addition, the immunohistochemical localizations of PACAP and VPAC1 were identified in the Leydig cells, spermatogonia and spermatozoa during the breeding season, while only in the Leydig cells and spermatogonia during the non-breeding season, and PAC1 and VPAC2 were localized in the Leydig cells in both seasons, among which LHR, StAR, 3β-HSD and CYP17A1 were also expressed. Meanwhile, the protein and mRNA expression levels of PACAP, PAC1, VPAC1, VPAC2, LHR, FSHR, StAR, 3β-HSD and CYP17A1 in the testis during the breeding season were significantly higher than those during the non-breeding season. These results suggested that PACAP is involved in the regulation of steroidogenesis and spermatogenesis via an endocrine, autocrine or paracrine manner in the testis of muskrat.