In young male monkeys without testes, constantly giving kisspeptin-10 made the kisspeptin receptor stop working and also blocked the hormone‑releasing effect of a related peptide (NKB). Stopping the kisspeptin infusion let the system recover. Doing the opposite—continuously giving the NKB‑mimic—did not stop kisspeptin from triggering hormone release. This shows NKB works before kisspeptin in the chain that tells the brain to release reproductive hormones.

Human genetics have revealed that kisspeptin signaling and neurokinin B (NKB) signaling are both required for robust pulsatile gonadotropin-releasing hormone (GnRH) release, and therefore for puberty and maintenance of adult gonadal function. How these two peptides interact to affect GnRH pulse generation remains a mystery. To address the hierarchy of the NKB and kisspeptin signaling pathways that are essential for GnRH release, two experiments were conducted using agonadal, juvenile male monkeys. Pituitary responsiveness to GnRH was first heightened by a pulsatile GnRH infusion to use the in situ pituitary as a bioassay for GnRH release. In the first experiment (n = 3), the kisspeptin receptor (KISS1R) was desensitized by a continuous 99-hour i.v. infusion of kisspeptin-10 (100 μg/h). During the last 4 h of continuous kisspeptin-10 infusion, desensitization of KISS1R was confirmed by failure of an i.v. bolus of kisspeptin-10 to elicit GnRH release. Desensitization of KISS1R was associated with a markedly blunted GnRH response to senktide. The response to senktide was progressively restored during the 72 h following termination of continuous kisspeptin-10. An analogous design was employed in the second experiment (n = 2) to desensitize the NKB receptor (neurokinin 3 receptor, NK3R) by administration of a continuous 48-hour i.v. infusion of senktide (200 μg/h). While a bolus of senktide during the last 3 h of continuous senktide administration failed to elicit GnRH release, thus confirming desensitization of NK3R, the ability of kisspeptin to stimulate GnRH was unimpaired. The foregoing findings support the view that NKB stimulation of GnRH release is upstream from KISS1R.