GONADOTROPIN-RELEASING HORMONE PULSE GENERATOR ACTIVITY IN CLOSE PROXIMITY TO THE ARCUATE KISSPEPTIN NEURONS IN THE GOAT

Ohkura, S.1,2; Takase, K.2; Uenoyama, Y.2; Tsukamura, H.2; Maeda, K.-I.2; Okamura, H.1

1National Institute of Agrobiological Sciences, Tsukuba 305-0901, Japan
2Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan

Pulsatile release of gonadotropin-releasing hormone (GnRH) is indispensable to maintain normal activity of the pituitary-gonadal axis in mammals.  The only evidence for the presence of the GnRH pulse-generating mechanism, namely the GnRH pulse generator, has been the periodic increase in the multiple-unit activity (MUA) which is synchronized with luteinizing hormone (LH) pulses, but recorded from unknown neuronal substrates in the arcuate nucleus (ARC)/median eminence (ME) region in monkeys, rats and goats.  Kisspeptin (firstly named metastin)-GPR54 signaling is a promising candidate for the main component of the GnRH pulse generator because of their location in the brain and profound effect on gonadotropin release.  The present study aims to determine whether the ARC kisspeptin neuron is a component of the GnRH pulse generator.  For this purpose, the MUA was recorded at the posterior portion of the ARC where the majority of kisspeptin neuronal cell bodies are distributed.  MUA volleys accompanied by LH pulses occurred with regular intervals in castrated male goats fitted with recording electrodes in close apposition to the kisspeptin neuronal cell bodies in the posterior portion of the ARC, where GnRH neuronal fibers or cell bodies are not located.  Intravenous administration of human kisspeptin-10 (0.38 µmol/injection) increased plasma LH concentrations without affecting the GnRH pulse generator activity.  This also supports the idea that the ARC kisspeptin neuron is a component of the GnRH pulse generator that maintains its intrinsic rhythm during massive GnRH release induced by the exogenous kisspeptin-10 stimulation.  Immunohistochemical studies in castrated male goats revealed that a group of kisspeptin-immunoreactive cell bodies located only in the ARC with a number of fibers reaching to the external layer of the ME to which GnRH-immunoreactive fibers project, suggesting that kisspeptin directly stimulates GnRH release at the ME.  Overall, the present findings suggest that the ARC kisspeptin neuron is a component of the GnRH pulse generator and regulates GnRH release at the nerve terminals in the ME.  This work was supported in part by the Program for Promotion of Basic Research Activities for Innovative Biosciences (PROBRAIN) of Japan.  
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