Expressions of Kisspeptin System and Ki-67 in the Reproductive Tissues of Cyclic Bitches

Kisspeptin is a peptide that plays a pivotal role in the central regulation of gonadotropins. It is regarded as a key regulator of reproductive processes, including follicular and luteal development and endometrial remodelling. The objective of this study was to investigate the expression of KISS1, its receptor KISS1R and the proliferation marker Ki-67 in the uterus, oviduct and ovary of bitches at different stages of the oestrous cycle using reverse transcription quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry. Thirty-five healthy female dogs were divided into five groups based on reproductive stage as follows: anestrus, proestrus, estrus, diestrus and prepubertal. Each group consisted of an equal number of animals (n = 7). KISS1 mRNA expression was significantly elevated in estrus, while KISS1R mRNA expression remained consistently low across all tissues. Ki-67 mRNA expression increased significantly in the uterus during estrus, whereas the highest levels in the oviduct and ovary were observed in anestrus. Immunohistochemical analysis revealed a marked increase in KISS1 expression in the endometrium during estrus, accompanied by elevated Ki-67 positivity in the uterus, oviduct and ovary, particularly in glandular epithelial and luteal cells. Conversely, KISS1R protein showed strong localisation during estrus despite low transcript levels, suggesting possible post-transcriptional regulation. The synchronised upregulation of KISS1 and Ki-67 in the endometrium during estrus supports their coordinated roles in endometrial proliferation and remodelling. These findings emphasise the significance of local kisspeptin signalling in canine reproductive tissues and suggest its contribution to tissue adaptations during the estrous cycle. The results provide new insights into the peripheral functions of the kisspeptin system and highlight its potential as a target for improving reproductive management in bitches.