Low Degree of Overlap Between Kisspeptin, Neurokinin B, and Dynorphin Immunoreactivities in the Infundibular Nucleus of Young Male Human Subjects Challenges the KNDy Neuron Concept

Abstract

Previous immunohistochemical and in situ hybridization studies of sheep, goats, and rodents indicated that kisspeptin (KP), neurokinin B (NKB), and dynorphin A (DYN) are extensively colocalized in the hypothalamic arcuate nucleus, thus providing a basis for the KP/NKB/DYN (KNDy) neuron concept; in both sexes, KNDy neuropeptides have been implicated in the generation of GnRH neurosecretory pulses and in the negative feedback effects of sexual steroids to the reproductive axis. To test the validity and limitations of the KNDy neuron concept in the human, we carried out the comparative immunohistochemical analysis of the three neuropeptides in the infundibular nucleus (Inf; also known as arcuate nucleus) and stalk of young male human individuals (<37 yr). Results of quantitative immunohistochemical experiments established that the regional densities of NKB immunoreactive (IR) perikarya and fibers, and the incidence of afferent contacts they formed onto GnRH neurons, were about 5 times as high as those of the KP-IR elements. Dual-immunofluorescent studies confirmed that considerable subsets of the NKB-IR and KP-IR cell bodies and fibers are separate, and only about 33% of NKB-IR perikarya and 75% of KP-IR perikarya were dual labeled. Furthermore, very few DYN-IR cell bodies could be visualized in the Inf. DYN-IR fibers were also rare and, with few exceptions, distinct from the KP-IR fibers. The abundance and colocalization patterns of the three immunoreactivities showed similar trends in the infundibular stalk around portal blood vessels. Together these results indicate that most NKB neurons in the Inf do not synthesize detectable amounts of KP and DYN in young male human individuals. These data call for a critical use of the KNDy neuron terminology when referring to the putative pulse generator system of the mediobasal hypothalamus. We conclude that the functional importance of these three neuropeptides in reproductive regulation considerably varies among species, between sexes, and at different ages.

Hypophysiotropic Gonadotropin-Releasing Hormone Projections Are Exposed to Dense Plexuses of Kisspeptin, Neurokinin B and Substance P Immunoreactive Fibers in the Human: A Study on Tissues from Postmenopausal Women

“…The GQ2 antibodies were used successfully in previous immunohistochemical experiments on hypothalamic sections from the rhesus monkey [19,49] and the human [7,9,26,27]. NKB synthesizing neuronal fibers were visualized with a previously characterized [7,9,26,27,49] rabbit polyclonal antiserum (IS-682; P. Ciofi; 1:100,000) directed against the C-terminal 28 amino acids of human pro-NKB or alternatively, with a mouse monoclonal antibody against human NKB (Biosensis Pty. Ltd., Thebarton, S.A., Australia; M-871-100; 1:50,000).…”

“…1018; 1:50,000) [26]. GnRH-IR elements were visualized with the biotinylated secondary antibody-ABC technique and brown diaminobenzidine chromogen, as in previous dual-label immunoperoxidase experiments [7,9,26,27]. …”

“…While homologous KP and NKB neurons are also present in the human infundibular nucleus [6,7], species differences are also likely to exist in the neurotransmitter complement that these neurons use for communication [8]. Notably, morphological studies have found only poor evidence for dynorphin expression in KP-immunoreactive (IR) neurons in the infundibular nucleus of young men [9]. On the other hand, substance P (SP) has been revealed in considerable subsets of KP and NKB neurons in the human [10], but not in the rodent or sheep, KNDy neurons.…”

“…Accordingly, GnRH neurons receive KP-IR afferent contacts [7,17,18,19,20] and show depolarization [12,21,22] and cFos expression [11,23] in response to KP. Similarly to KP fibers, NKB-IR axons also establish axo-somatic and axo-dendritic contacts with GnRH neurons in rats [24], mice [25] and humans [9,26,27] and 16% of the GnRH-IR perikarya in the preoptic area of the rat exhibit NK3 immunoreactivity [14]. Somewhat conflictingly, mouse GnRH neurons do not give electrophysiological responses to the NK3 receptor agonist senktide in slice preparations [28], and senktide does not induce GnRH release from the preoptic area which contains the GnRH cell bodies [29].…”

Kisspeptin across the human lifespan:evidence from animal studies and beyond

“…Although evidence from human studies has at times appeared conflicting with Hrabovszky and coworkers reporting a low prevalence of Dyn fibres in the infundibular nucleus of human males (Hrabovszky et al 2012), studies have consistently found Dyn neurons to be present in females in both animal and human studies Rance 2008, Goodman et al 2014). Furthermore, naloxone, an opioid receptor antagonist, has been shown to increase LH secretion in women in the prefollicular and luteal phases with naltrexone, an opioid agonist, increasing LH release in hypothalamic amenorrhoea (HA), a condition characterised by low-circulating gonadotrophins and sex steroids (Genazzani et al 1995).…”