Endocrine Abstracts

Puberty is a transition period of reproductive development from juvenile stages to adulthood and depends upon activity of gonadotropin-releasing hormone (GnRH) neurons. GnRH neurons are initially activated in utero, but remain quiescent throughout the juvenile period. Premature reactivation of GnRH neurons results in precocious puberty in mice and humans, but the mechanisms underlying developmental control of GnRH neuron activity remain unknown. The neuropeptide kisspeptin, a potent activator of GnRH neurons that is implicated as a critical permissive signal triggering puberty and a major regulator of the adult female hypothalamus-pituitary-gonadal (hpg) axis, is paradoxically produced by neurons in the developing brain well before puberty onset. We have delineated the underlying neural circuitry using conditional genetic transsynaptic tracing in female mouse embryos. We find that kisspeptin-producing neurons in the arcuate nucleus (ARC) already communicate with a specific subset of GnRH neurons in utero. We show that ARC kisspeptin neurons are upstream of GnRH neurons and that GnRH neuron connectivity to ARC kisspeptin neurons does not depend on their spatial position in the brain. Furthermore, we demonstrate that the neural circuits between ARC kisspeptin and GnRH neurons are fully established and operative before birth. Most GnRH neurons express the kisspeptin receptor GPR54 upon circuit formation, suggesting that the signaling system implicated in gatekeeping puberty becomes operative in the embryo. Distinct roles have been proposed for AVPV and ARC kisspeptin neurons during reproductive maturation and in mediating estrogen feedback on the hpg axis in adult females. However, little is known about kisspeptin neuron connectivity in adult female mice. We analyzed the connectivity of individual kisspeptin neurons with the GnRH neuron population in adult female mice with a single cell resolution. Only subsets of AVPV and ARC kisspeptin neurons are synaptically connected with GnRH neurons demonstrating functional specialization within the two kisspeptin neuron populations.