The oviduct of the Chinese brown frog (Rana dybowskii) expands during pre-brumation rather than the breeding period, exhibiting a special physiological feature. Vitamin A is essential for the proper growth and development of many organisms, including the reproductive system such as ovary and oviduct. Vitamin A is metabolized into retinoic acid, which is crucial for oviduct formation. This study examined the relationship between oviducal expansion and vitamin A metabolism. We observed a significant increase in the weight and diameter of the oviduct in Rana dybowskii during pre-brumation. Vitamin A and its active metabolite, retinoic acid, notably increased during pre-brumation. The mRNA levels of retinol binding protein 4 (rbp4) and its receptor stra6 gene, involved in vitamin A transport, were elevated during pre-brumation compared to the breeding period. In the vitamin A metabolic pathway, the mRNA expression level of retinoic acid synthase aldh1a2 decreased significantly during pre-brumation, while the mRNA levels of retinoic acid α receptor (rarα) and the retinoic acid catabolic enzyme cyp26a1 increased significantly during pre-brumation, but not during the breeding period. Immunohistochemical results showed that Rbp4, Stra6, Aldh1a2, Rarα, and Cyp26a1 were expressed in ampulla region of the oviduct. Western blot results indicated that Aldh1a2 expression was lower, while Rbp4, Stra6, RARα, and Cyp26a1 were higher during pre-brumation compared to the breeding period. Transcriptome analyses further identified differential genes in the oviduct and found enrichment of differential genes in the vitamin A metabolism pathway, providing evidences for our study. These results suggest that the vitamin A metabolic pathway is more active during pre-brumation compared to the breeding period, and retinoic acid may regulate pre-brumation oviductal expansion through Rarα-mediated autocrine/paracrine modulation.

The oviduct of Chinese brown frog (Rana dybowskii) displays seasonal morphological and functional changes, which expands specifically during pre-brumation. To uncover the molecular mechanism underlying this phenomenon, we firstly confirmed the increased weight and enlarged diameter of the oviduct in pre-brumation by morphological observation. Interestingly, the glycogen content in the oviduct increased significantly during pre-brumation, indicating Rana dybowskii stores energy in the oviduct before brumation. Transcriptome analysis further identified the differentially expressed genes in the synthesis and metabolism pathways of carbohydrates in the oviduct during pre-brumation. Based on that evidence, we focused on the mRNA and protein expression of glycogenic genes in the oviduct of Rana dybowskii. qPCR confirmed that the expression of glycolysis and glycogenesis-related genes were up-regulated while gluconeogenesis-related genes were down-regulated during pre-brumation. Western blot data showed that glucose transporter GLUT1 and glycogen synthesis-regulation proteins including GYS, and p-GSK-3β were highly expressed in the oviduct during pre-brumation. Moreover, immunohistochemical data showed that GLUT1, GYS, p-GYS, GSK-3β and p-GSK-3β were expressed regionally in the oviduct of Rana dybowskii. The data suggests that glycogen synthesis may be involved in the oviductal expansion of Rana dybowskii during the pre-brumation.