Kisspeptin has been thought to play pivotal roles in the control of both pulse and surge modes of
gonadotropin-releasing hormone (
GnRH) secretion. To clarify loci of
kisspeptin action on
GnRH neurons, the present study examined the morphology of the
kisspeptin system and the associations between
kisspeptin and
GnRH systems in gonadally intact and castrated male goats.
Kisspeptin-immunoreactive (ir) and Kiss1-positive neurons were found in the medial preoptic area of intact but not castrated goats.
Kisspeptin-ir cell bodies and fibers in the arcuate nucleus (
ARC) and median eminence (ME) were fewer in intact male goats compared with castrated animals. Apposition of
kisspeptin-ir fibers on
GnRH-ir cell bodies was very rare in both intact and castrated goats, whereas the intimate association of
kisspeptin-ir fibers with
GnRH-ir nerve terminals was observed in the ME of castrated animals.
Neurokinin B immunoreactivity colocalized not only in
kisspeptin-ir cell bodies in the
ARC but also in
kisspeptin-ir fibers in the ME, suggesting that a majority of
kisspeptin-ir fibers projecting to the ME originates from the
ARC. A dual immunoelectron microscopic examination revealed that nerve terminals containing
kisspeptin-ir vesicles made direct contact with
GnRH-ir nerve terminals at the ME of castrated goats. There was no evidence for the existence of the typical synaptic structure between
kisspeptin- and
GnRH-ir fibers. The present results suggest that the
ARC kisspeptin neurons act on
GnRH neurons at the ME to control (possibly the pulse mode of)
GnRH secretion in males.