Seasonal fluctuations in the hypothalamic irisin and its correlation with the hypothalamic-pituitary-gonadal -axis activity in male rhesus monkeys (Macaca mulatta).

Regulatory mechanisms underlying seasonal fluctuations in the activity of the hypothalamic-pituitary-gonadal (HPG)-axis are poorly understood. Since 2016, following the publication of a medical hypothesis highlighting irisin's involvement in reproductive function, extensive research on this aspect has been conducted; however, studies on seasonal variation remain limited. The purpose of this study was to examine seasonal variations in hypothalamic irisin/FNDC5 expression relative to the seasonally fluctuating HPG-axis activity in male rhesus monkeys. Hypothalamic tissues for immunocytochemistry (ICC), RT-qPCR, and testicular tissues for H&E staining were used. The experimental model was validated by assessing mean testicular volume, seminiferous epithelial height, seminiferous tubular diameter, and plasma testosterone, which were significantly higher in reproductively active than in reproductively inactive male monkeys. The present study revealed that reproductively active monkeys exhibited significantly higher mean numbers of hypothalamic irisin-like immunoreactive cell bodies and elevated FNDC5 mRNA expression levels compared with reproductively inactive monkeys. This showed a positive correlation with testicular volume, seminiferous tubular diameter/epithelial height, and plasma testosterone levels. The current study suggests the possibility that irisin may be a novel positive regulator of HPG-axis activity during the breeding season in male rhesus monkeys.