BACKGROUND AND AIMS: Hypothalamic kisspeptin (KP) neurons use peptidergic signaling to regulate reproduction and puberty. METHODS: Here we have carried out a series of immunofluorescent experiments on formalin-fixed post mortem histological samples to identify the neuropeptide co-transmitters of human KP cells. RESULTS: In these colocalization studies using confocal microscopy, we found no evidence for neurotensin, cholecystokinin, proopiomelanocortin-derivatives, agouti-related protein, neuropeptide Y, somatostatin or tyrosine hydroxylase (dopamine) expression in KP neurons. In contrast, neurokinin B (NKB) was present in a large subset of these cells, as reported earlier in laboratory animals. Dynorphin, which has also been described in KP neurons of rodents and sheep, was observed rarely in human KP cells and their axons, and similarly, human KP neurons did not contain signal for galanin, a neuropeptide colocalized earlier with KP in mice. To the opposite, 30–50% of KP and NKB neurons in humans expressed immunoreactivity for substance P (SP) and cocaine- and amphetamine-regulated transcript (CART), unlike in laboratory species. In addition, we have provided evidence for a sexually dimorphic co-expression of proenkephalin (pENK) with KP and NKB. The pENK signal was detectable in 12.5±5.1% of NKB-IR and 1.9±1.0% of KP-IR neurons and in 5.7±2.5% of NKB-IR and 4.9±1.8% of KP-IR axon varicosities in human males. This colocalization was absent in postmenopausal women. CONCLUSIONS: The presence of substance P, CART and pENK in human KP neurons, together with the absence of dynorphin and galanin in most of these cells, indicate that humans use considerably different neurotransmitter mechanisms than rodents, to regulate fertility.
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