Morphological Evidence for Enhanced Kisspeptin and Neurokinin B Signaling in the Infundibular Nucleus of the Aging Man

Abstract

Peptidergic neurons synthesizing kisspeptin (KP) and neurokinin B (NKB) in the hypothalamic infundibular nucleus have been implicated in negative sex steroid feedback to GnRH neurons. In laboratory rodents, testosterone decreases KP and NKB expression in this region. In the present study, we addressed the hypothesis that the weakening of this inhibitory testosterone feedback in elderly men coincides with enhanced KP and NKB signaling in the infundibular nucleus. This central hypothesis was tested in a series of immunohistochemical studies on hypothalamic sections of male human individuals that were divided into arbitrary "young" (21-49 yr, n = 11) and "aged" (50-67 yr, n = 9) groups. 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, exceeded several times those of the KP-IR elements. Robust aging-dependent enhancements were identified in the regional densities of KP-IR perikarya and fibers and the incidence of afferent contacts they established onto GnRH neurons. The abundance of NKB-IR perikarya, fibers, and axonal appositions to GnRH neurons also increased with age, albeit to lower extents. In dual-immunofluorescent studies, the incidence of KP-IR NKB perikarya increased from 36% in young to 68% in aged men. Collectively, these immunohistochemical data suggest an aging-related robust enhancement in central KP signaling and a moderate enhancement in central NKB signaling. These changes are compatible with a reduced testosterone negative feedback to KP and NKB neurons. The heavier KP and NKB inputs to GnRH neurons in aged, compared with young, men may play a role in the enhanced central stimulation of the reproductive axis. It requires clarification to what extent the enhanced KP and NKB signaling upstream from GnRH neurons is an adaptive response to hypogonadism or, alternatively, a consequence of a decline in the androgen sensitivity of KP and NKB neurons.

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

“…This observation raises the possibility that the axonal transport, processing and use for neurotransmission of the co-synthesized neuropeptides might be regulated depending on the functional status of these neurons. Earlier studies of the human KP and NKB systems in our laboratory already provided evidence that the extent of colocalization between KP and NKB is sex dependent [8,26] and also age dependent, at least in men [8,27]. In addition to this difference in the labeling of perikarya, we noticed that the neuronal contacts that these cells establish with GnRH neurons exhibit sex-specific patterns of KP/NKB co-labeling, with a significantly higher incidence of double-labeled fibers in postmenopausal women compared with age-matched men [26].…”

“…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]. …”

“…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].…”

Neuroanatomy of the Human Hypothalamic Kisspeptin System

“…In humans, the largest KP cell population has been detected in the Inf (analogous to the ARC) both with in situ hybridization [34] and with immunohistochemistry [10,61] (fig. 1d, e, 2a).…”