A role for neurokinin B in pulsatile GnRH secretion in the ewe

Abstract

The recent description of infertility in humans with loss-of-function mutations in genes for neurokinin B (NKB) or its receptor (NK3R) has focused attention on the importance of this tachykinin in the control of GnRH secretion. In a number of species, NKB neurons in the arcuate nucleus also produce two other neuropeptides implicated in the control of GnRH secretion: (1) kisspeptin, which is also essential for fertility in humans, and (2) dynorphin, an inhibitory endogenous opioid peptide. A number of characteristics of this neuronal population led to the hypothesis that they may be responsible for driving synchronous release of GnRH during episodic secretion of this hormone, and there is now considerable evidence to support this hypothesis in sheep and goats. In this article, we briefly review the history of work on the NKB system in sheep and then review the anatomy of NKB signaling in the ewe. We next describe evidence from a number of species that led to development of a model for the role of these neurons in episodic GnRH secretion. Finally, we discuss recent experiments in sheep and goats that tested this hypothesis and led to a modified version of the model, and then broaden our focus to briefly consider the possible roles of NKB in other species and systems.

Fig 1

Confocal images (1.5 μm thickness) of the same section through the ovine ARC stained for dynorphin (green) and NKB (red). Panel C is a computer overlay of these two images so that cells containing both peptides appear as yellow. Magnification bar: 50 μm. Reprinted from Foradori et al. [9].

Fig 2

Reciprocal connections among dynorphin (red)/NKB (green) cells in the ovine arcuate nucleus. Confocal optical section (1.5 μm thickness) through the ARC showing individual channels (Panels A and B) and overlay (Panel C) illustrating close contacts (e.g., arrow) that contain both peptides onto a KNDy cell body. Magnification bar: 10 μm. Reprinted from Foradori et al. [9].

Fig. 3

Panel A: NKB-containing close contact immunostained using nickel-enhanced DAB (blue-black) onto a GnRH neuron immunostained using DAB (brown) in the POA. Magnification bar: 10 μm. Reprinted from Goubillon et al. [4]. Panel B: MBH GnRH neuron (green) with two close contacts (arrows) containing both dynorphin (red) and NKB (blue); note nearby KNDy neuron (magenta) also contains both NKB and dynorphin. Magnification bar: 10 μm. Reprinted from Lehman et al. [39].

Fig. 4

Sites of NK3R in the ovine diencephalon depicted on a schematic of a parasagittal section of the ovine hypothalamus. Insets are examples of ICC staining for NK3R next to coronal schematics containing camera lucida drawings of NK3R-ir cells (solid circles). Abbreviations: ac: anterior commissure; AHA: anterior hypothalamic area; ARC: arcuate nucleus; cp: cerebral peduncle; DMH: dorsomedial hypothalamus; fx: fornix; mARC: middle arcuate nucleus; MB: mammillary body; ME: median eminence; OC: optic chiasm; OVLT: organum vasculosum of lamina terminalis; POA: preoptic area; pt: pars tuberalis; PVN: paraventricular nucleus; RCh: retrochiasmatic area; VMH: ventromedial hypothalamus; vmPOA: ventromedial POA; 3v: third ventricle. Redrawn from Amstalden et al. [49] .

Fig. 5

The same section in the POA stained for NK3R (red) and GnRH (green). Note in overlay (Panel C) that the GnRH neuron contains no NK3R-ir. Magnification bar: 10 μm Reprinted from Amstalden et al. [49].

Fig. 6

Two sections through the ovine ARC stained for NK3R (red: panels A,D) and NKB (green, panels B & E). Panel C is overlay of Panels A and B, illustrating extensive NK3R in this NKB neuron. Panel F is overlay of Panels D and E, illustrating a non-NKB containing cell with NK3R. Note NKB-ir terminals in close contact with this neuron (e.g., arrow). Magnification bar: 10 μm. Reprinted from Amstalden et al. [49].

Fig. 7

Model for actions of KNDy peptides in initiation and termination of each GnRH pulse. Each GnRH pulse is initiated by NKB (purple) acting within the KNDy network (within dashed oval), which stimulates kisspeptin (green) release to drive GnRH (blue) secretion. GnRH release is then terminated by dynorphin (red) release from KNDy neurons acting directly on KNDy neurons. Note that the color in each terminal indicates the biologically active transmitter (possibly due to selective expression of post-synaptic receptors), not selective transport of that peptide to the terminal. Abbreviations: R_dyn: kappa opioid receptor; R_Ks: Kiss1r; R_NKB: NK3R. Redrawn from Lehman et al. [27].

Fig. 8

Effects of empty (open bars, top panels), or NKB-containing (shaded bars, bottom panels), microimplants placed in the ARC on LH pulse patterns in two individual OVX ewes. Solid circles depict peaks of identified LH pulses. Data from Goodman et al. [46].

Fig. 9

Effects of empty microimplants (open bars, top panels), or microimplants containing the NK3R antagonist, SB222200 (shaded bars, bottom panels) placed in the ARC on LH pulse patterns in two individual OVX ewes. Solid circles depict peaks of identified LH pulses. Individual animals selected to illustrate variation in duration of effects of the antagonist. Reprinted from Goodman et al. [46].

Fig. 10

Effects of ARC microimplants containing the NK3R antagonist, SB222200 (shaded bar), on GnRH secretion rates into hypophysial portal blood collected every 2 min (top panel), and LH concentrations in jugular blood collected every 10 min (bottom panel).

Fig. 11

Modified model for control of KNDy neural activity proposed to drive episodic GnRH secretion in sheep and goats. Each GnRH pulse is initiated by NKB (purple) acting within the KNDy network (within dashed oval), which stimulates kisspeptin (green) release to drive GnRH (blue) secretion and activates unidentified Kiss1r-containing ARC neurons (grey) that reinforce the stimulatory actions of NKB on KNDy neurons. GnRH release is then terminated by dynorphin (red) release from KNDy neurons acting either directly on KNDy neurons and/or the unidentified Kiss1r-containing neurons. Note that the color in each terminal indicates the biologically active transmitter (possibly due to selective expression of post-synaptic receptors), not selective transport of that peptide to the terminal.