Co‐Localisation of Kisspeptin with Galanin or Neurokinin B in Afferents to Mouse GnRH Neurones

Abstract

The gonadotrophin-releasing hormone (GnRH) secreting neurones, which form the final common pathway for the central regulation of reproduction, are directly targeted by kisspeptin (KP) via the G protein-coupled receptor, GPR54. In these multiple labelling studies, we used ovariectomised mice treated with 17β-oestradiol (OVX + E(2)) or vehicle (OVX + oil) to determine: (i) the ultrastructural characteristics of KP-immunoreactive (IR) afferents to GnRH neurones; (ii) their galanin or neurokinin B (NKB) content; and (iii) the co-expression of galanin or NKB with KP in the two major subpopulations of KP neurones located in the rostral periventricular area of the third ventricle (RP3V) and the arcuate nucleus (Arc). Electron microscopic investigation of the neuronal juxtapositions revealed axosomatic and axodendritic synapses; these showed symmetrical or asymmetrical characteristics, suggesting a phenotypic diversity of KP afferents. Heterogeneity of afferents was also demonstrated by differential co-expression of neuropeptides; in OVX + E(2) mice, KP afferents to GnRH neurones showed galanin-immunoreactivity with an incidence of 22.50 ± 2.41% and NKB-immunoreactivity with an incidence of 5.61 ± 2.57%. In OVX + oil animals, galanin-immunoreactivity in the KP afferents showed a major reduction, appearing in only 5.78 ± 1.57%. Analysis for co-localisation of galanin or NKB with KP was extended to the perikaryal level in animal models, which showed the highest KP incidence; these were OVX + E(2) females for the RP3V and OVX + oil females for the ARC. In the RP3V of colchicine-treated OVX + E(2) animals, 87.84 ± 2.65% of KP-IR neurones were galanin positive. In the Arc of the colchicine-treated OVX + oil animals, galanin immunoreactivity was detected in only 12.50 ± 1.92% of the KP expressing neurones. By contrast, the incidence of co-localisation with NKB in the Arc of those animals was 98.09 ± 1.30%. In situ hybridisation histochemistry of sections from OVX + E(2) animals identified galanin message in more than a third of the KP neurones in the RP3V (38.67 ± 11.57%) and in the Arc (42.50 ± 12.52%). These data suggest that GnRH neurones are innervated by chemically heterogeneous KP cell populations, with a small proportion deriving from the Arc group. The presence of galanin within KP axons innervating GnRH neurones and the oestrogen-dependent regulation of that presence add a new dimension to the roles played by galanin in the central regulation of reproduction.

Selective optogenetic activation of arcuate kisspeptin neurons generates pulsatile luteinizing hormone secretion

“…The ARN kisspeptin, or KNDy cells, synthesize numerous neuropeptidergic and classic transmitters (20,23,35) and innervate only the distal processes of GnRH neurons in mice, either at the level of the dendron or their nerve terminals in the median eminence (36,37). We found that 2-Hz stimulation of ARN KNDy neurons was completely ineffective in modulating LH secretion.…”

Physiology, Signaling, and Pharmacology of Galanin Peptides and Receptors: Three Decades of Emerging Diversity

“…Alignment of peptide sequences reveals some sequence similarity among the three peptide groups, with SPX more closely related to galanin, and ligand-receptor interaction studies revealed that SPXs activate human GAL 2/3 but not GAL 1 , suggesting they may be natural ligands for GAL 2/3 . Furthermore, SPXs exhibited higher potency at GAL 3 than galanin (Kim et al, 2014), suggesting a possible role in endogenous regulation of GAL 3 signaling that should prompt further experimentation, particularly in relation to reproduction (e.g., Porteous et al, 2011;Kalló et al, 2012).…”

“…In addition to the neuronal actions of galanin already discussed, there are many other actions that, because of space restrictions, cannot be covered in detail. These include roles in arousal and sleep regulation (see Gaus et al, 2002;McGinty and Szymusiak, 2003;Saper, 2006), reproduction and associated behavior, neuroendocrine mechanisms, and hormone release, which are reviewed elsewhere, along with similar GALP actions (see, e.g., Gundlach, 2002;Gottsch et al, 2004;Crown et al, 2007;Kalló et al, 2012; see section VI).…”

Primary cilia enhance kisspeptin receptor signaling on gonadotropin-releasing hormone neurons

“…One possibility is that Kiss1r normally signals on the ciliary membrane and ciliary localization concentrates the receptors to optimize ligand binding and signal propagation. There is evidence for both synaptic (41,42) and volume (43) transmission of kisspeptin to GnRH neurons. Ciliary response to ligand should be involved in volume transmission as cilia project from the cell body and synaptic contacts have never been observed on neuronal cilia.…”