The neurobiological mechanism underlying hypothalamic GnRH pulse generation: the role of kisspeptin neurons in the arcuate nucleus

Abstract

This review recounts the origins and development of the concept of the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator. It starts in the late 1960s when striking rhythmic episodes of luteinizing hormone secretion, as reflected by circulating concentrations of this gonadotropin, were first observed in monkeys and ends in the present day. It is currently an exciting time witnessing the application, primarily to the mouse, of contemporary neurobiological approaches to delineate the mechanisms whereby Kiss1/NKB/Dyn (KNDy) neurons in the arcuate nucleus of the hypothalamus generate and time the pulsatile output of kisspeptin from their terminals in the median eminence that in turn dictates intermittent GnRH release and entry of this decapeptide into the primary plexus of the hypophysial portal circulation. The review concludes with an examination of questions that remain to be addressed.

Keywords: GnRH; Kisspeptin; dynorphin; neurokinin B; pulse generation.

Figure 1.. Pulsatile luteinizing hormone secretion is tightly correlated with electrophysiological activity in the monkey mediobasal hypothalamus.

The relationship between luteinizing hormone (LH) “pulses”, observed in the peripheral circulation (open data points), and multiunit activity (MUA) (continuous trace) recorded in the mediobasal hypothalamus of two ovariectomized rhesus monkeys before (Panel A) and after (Panel B) anesthesia was induced with thiopental. Note the high-fidelity relationship between the two parameters regardless of frequency of pulsatile LH secretion. Reproduced from Wilson et al. with permission of Karger Publishers. Copyright © (1984) Karger Publishers, Basel, Switzerland.

Figure 2.. Intermittent kisspeptin administration drives a corresponding pattern of GnRH dependent luteinizing hormone discharges in monkey.

Intermittent intravenous administration of kisspeptin-10 (2 μg/min for 1 min/hour starting at 11 a.m. on day 1 and continuing for 48 hours, closed data points) induces a sustained train of luteinizing hormone (LH) “pulses” in a naturally GnRH-deficient primate model (juvenile male rhesus monkey) that matches that generated by an antecedent intermittent GnRH infusion, also administered at 1 pulse/hour before kisspeptin administration (9 to 11 a.m., day 1). The LH response to kisspeptin was abolished by prior treatment with a GnRH receptor antagonist (not shown). Results for vehicle are shown in the open data points. Although kisspeptin-10 or vehicle was administered every hour for 48 hours, LH responses were tracked for only two or three pulses per day. Black arrows indicate times of pulse infusions of kisspeptin-10 or vehicle that were selected for monitoring the LH response. White arrows indicate time of GnRH pulse infusions. Values are presented as mean ± standard error. GnRH, gonadotropin-releasing hormone. Reproduced from Plant et al. with permission of the Endocrine Society.

Figure 3.. Kisspeptin projections to the median eminence intermingle intimately with GnRH fibers in the monkey.

Interactions between kisspeptin (green fluorescence) and GnRH (red fluorescence) fibers in hemi-coronal sections at the anterior ( A), mid-tuberal ( B), and posterior ( C) aspects of the median eminence of a castrated adult male rhesus monkey. Note, in panel B, the heavy kisspeptin innervation of the internal zone of the median eminence and GnRH innervation of both the internal and external zones. Panel D shows high-power magnification of kisspeptin axonal beads in contact with GnRH fibers (white arrows) in the external zone of the median eminence shown in the inset. GnRH, gonadotropin-releasing hormone. Reproduced from Ramaswamy et al. with permission of the Endocrine Society.

Figure 4.. Kisspeptin elicits Ca ⁺⁺ increases in GnRH dendrons and terminals in the mouse median eminence.

One-minute discharges (“puffs”) of kisspeptin from a pipette locally applied to GnRH nerve terminals and dendrons in hypothalamic slices elicit increases in intracellular Ca ⁺⁺ (the trigger for GnRH release) monitored by the Ca ⁺⁺ indicator, GCaMP6 (green fluorescence), expressed in GnRH neurons of transgenic mice. Lower traces, Ca ⁺⁺ responses to a kisspeptin puff (horizontal gray bar) on a GnRH terminal ( Ai) or dendron ( Aii). The fluorescent signals observed at times 1, 2, and 3 on the Ca ⁺⁺ traces are shown above. ΔF/F, relative increase in fluorescence over baseline; GnRH, gonadotropin-releasing hormone. Reproduced from Iremonger et al. with permission of the Society for Neuroscience.

Figure 5.. Electrophysiological activity in KNDy neurons is tightly correlated with discharges of luteinizing hormone in mouse.

High-fidelity correlation between electrophysiological activity (reflected by intracellular Ca ⁺⁺) of kisspeptin (KNDy) neurons in the arcuate nucleus of gonadectomized male mice monitored in vivo using an optogenetic approach (black trace) and the pulsatile secretion of luteinizing hormone (LH) (red) as tracked by measuring plasma concentrations of the gonadotropin. Reproduced from Clarkson et al. with permission from the National Academy of Sciences.

Figure 6.. Simple schematic of the KNDy neuron model of the GnRH pulse generator.

Small green and red filled circles indicate neurokinin B and dynorphin auto-synaptic release, respectively, within the arcuate nucleus. The temporal dynamics of the release of these two neuropeptides at this location are unknown. GnRH, gonadotropin-releasing hormone; MMB, mammillary body; OC, optic chiasm.