Neurokinin B Stimulates GnRH Release in the Male Monkey (Macaca mulatta) and Is Colocalized with Kisspeptin in the Arcuate Nucleus

Abstract

Human genetics indicate that kisspeptin and neurokinin B (NKB) signaling are necessary for generating pulsatile LH release and therefore for initiation of puberty and maintaining gonadal function. In the present study, male monkeys were employed to examine 1) whether activation of the NKB receptor (NK3R) is associated with GnRH release, and 2) hypothalamic localization of these peptides using immunofluorescence histochemistry. Agonadal juveniles, in which pituitary responsiveness to GnRH was heightened by GnRH priming, were employed to indirectly examine GnRH-releasing actions of NK3R and kisspeptin receptor agonists by tracking LH after their i.v. injection. Castrated adults were used for immunohistochemistry. Single i.v. injections of NKB or senktide (an NK3R agonist) elicited robust LH discharges that were abolished by GnRH receptor antagonism (acyline) confirming the ligands' hypothalamic action. Intermittent infusion of senktide (1-min pulse every hour for 4 h), in contrast to that of kisspeptin, failed to sustain pulsatile GnRH release. Repetitive senktide injections did not compromise the GnRH-releasing action of kisspeptin. NKB and kisspeptin were colocalized in perikarya of the arcuate nucleus and in axonal projections to the median eminence, confirming earlier findings in sheep. These results are consistent with the human genetics, and indicate that although brief activation of NK3R stimulates GnRH release, repetitive stimulation of this pathway, in contrast to that of kisspeptin receptor, fails to sustain pulsatile GnRH release. In addition, the data provide a platform for future elucidation of the interactions between NKB and kisspeptin that are required for generating pulsatile GnRH release in primates.

ENDOCRINOLOGY AND ADOLESCENCE: Deciphering puberty: novel partners, novel mechanisms

“…In keeping with the hypogonadotropic phenotype of patients with inactivating mutations of the NKB system, a number of studies in ewes, monkeys, mice, and rats have documented stimulatory actions of the NKB agonist, senktide, on LH secretion (28,33,34,35). Of note, the stimulatory effects of senktide on LH secretion were abrogated in Gpr54 null mice, thus suggesting that these require the integrity of kisspeptin signaling to manifest (35), a contention that is also supported by recent findings in the monkey (36).…”

“…Efforts on this front have allowed the identification of neurotransmitters that are coexpressed with kisspeptins at specific neuronal populations. As a paradigmatic example, expression of neurokinin B (NKB) has been demonstrated in a subset of Kiss1 neurons in numerous species, including the sheep (24), the goat, the mouse (25,26), the rat (27), and the monkey (28). The translational relevance of such coexpression, and of NKB actions in the reproductive brain, is reinforced by data from human studies that demonstrated that lossof-function mutations in the genes encoding either NKB (TAC3 in humans) or its receptor, NK3R (TAC3R in humans), are associated with impuberism and iHH (29,30,31), a phenotype similar to that of GPR54 or KISS1 null humans (10).…”

“…Indeed, evidence for inhibitory effects of senktide on LH secretion comes mostly from animal models with pre-elevated LH concentrations, namely gonadectomy (GNX) without adequate sex steroid replacement (37,38,39). In this context, proof of desensitization of NKB-induced LH responses has been presented in the monkey (28). This desensitization might account for the null or even inhibitory effect of senktide on LH secretion in GNX rodents and ewes (25,33,34,37), as they are expected to have pre-elevated, endogenous levels of NKB.…”

Sertoli cell differentiation in rhesus monkey (Macaca mulatta) is an early event in puberty and precedes attainment of the adult complement of undifferentiated spermatogonia

“…In addition, testes from three additional juvenile monkeys (13-15 months of age) that were castrated for an unrelated study (Ramaswamy et al 2010) were also employed. Testes from these animals, which were also labeled with BrdU, were fixed in Bouin's or 4% paraformaldehyde and comprised a mid juvenile group.…”

“…Following castration, testes were weighed and cut into several fragments for immersion fixation overnight in Bouin's solution or 10% neutral buffered formalin or 4% paraformaldehyde and processed for paraffin embedding. Postoperatively, each monkey received antibiotic and analgesic treatments as described previously (Ramaswamy et al 2010).…”