Abstract:
Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with kisspeptin, Dynorphin A (Dyn),
and their receptors [G-protein-coupled receptor-54 (GPR54)] and -opioid receptor (KOR), respectively]
within kisspeptin/NKB/Dyn (KNDy) neurons in the hypothalamic arcuate nucleus (ARC), the
proposed site of the GnRH pulse generator. Much previous research has employed intracerebroventricular
(icv) administration of KNDy agonists and antagonists to address the functions of KNDy
neurons.Weperformed a series of in vivo neuropharmacological experiments aiming to determine
the role of NKB/NK3R signaling in modulating the GnRH pulse generator and elucidate the interaction
between KNDy neuropeptide signaling systems, targeting our interventions to ARC KNDy
neurons. First,weinvestigated the effect of intra-ARC administration of the selectiveNK3Ragonist,
senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain
blood samples from freely moving ovariectomized 17 -estradiol-replaced rats. Our results show
that senktide suppresses LH pulses in a dose-dependent manner. Intra-ARC administration of
U50488, a selective KOR agonist, also caused a dose-dependent, albeit more modest, decrease in
LH pulse frequency. Thus we tested the hypothesis that Dyn/KOR signaling localized to the ARC
mediates the senktide-induced suppression of the LH pulse by profiling pulsatile LH secretion in
response to senktide in rats pretreated with nor-binaltorphimine, a selective KOR antagonist. We
show that nor-binaltorphimine blocks the senktide-induced suppression of pulsatile LH secretion
but does not affect LH pulse frequency per se. In order to address the effects of acute activation
of ARC NK3R, we quantified (using quantitative RT-PCR) changes in mRNA levels of KNDy-associated
genes in hypothalamic micropunches following intra-ARC administration of senktide. Senktide
down-regulated expression of genes encoding GnRH and GPR54 (GNRH1 and Kiss1r, respectively),
but did not affect the expression of Kiss1 (which encodes kisspeptin).Weconclude that NKB
suppresses the GnRH pulse generator in a KOR-dependent fashion and regulates gene expression
in GnRH neurons.
