Kisspeptin acts as a potent neuropeptide regulator of reproduction through modulation of the hypothalamic-pituitary-gonadal axis. Previous studies revealed sex differences in brain expression patterns as well as regulation of expression by estrogen. Alternatively
sex differences and estrogen regulation of the kisspeptin receptor (encoded by Kiss1r) have not been examined at cellular resolution. In the current study
we examined whether Kiss1r mRNA expression also exhibits estrogen sensitivity and sex-dependent differences using in situ hybridization. We compared Kiss1r mRNA expression between ovariectomized (OVX) rats and estradiol (E2)-replenished OVX rats to examine estrogen sensitivity
and compared expression between gonadally intact male rats and female rats in diestrus or proestrus to examine sex differences. In OVX rats
E2 replenishment significantly reduced Kiss1r expression specifically in the hypothalamic arcuate nucleus (ARC). A difference in Kiss1r expression was also observed between diestrus and proestrus rats in the hypothalamic paraventricular nucleus (PVN)
but not in the ARC. Thus
estrogen appears to have region- and context-specific effects on Kiss1r expression. However
immunostaining revealed minimal colocalization of estrogen receptor alpha (ERα) in Kiss1r-expressing neuronal populations of ARC and PVN
indicating indirect or ERα-independent regulation of Kiss1r expression. Surprisingly
unlike the kisspeptin ligand
no sexual dimorphisms were observed in either the brain distribution of Kiss1r expression or in the number of Kiss1r-expressing neurons within enriched brain nuclei. The current study reveals marked differences in regulation between kisspeptin and kisspeptin receptor
and provides an essential foundation for further study of kisspeptin signaling and function in reproduction.
