Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with
kisspeptin,
Dynorphin A (Dyn), and their
receptors [G-protein-coupled receptor-54 (GPR54)] and κ-
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. We performed 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, we investigated the effect of intra-
ARC administration of the selective NK3R agonist,
senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain blood samples from freely moving ovariectomized 17β-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). We conclude that NKB suppresses the
GnRH pulse generator in a KOR-dependent fashion and regulates gene expression in
GnRH neurons.