Abstract

Significance Hypothalamic oscillators that generate pulsatile patterns of hormone secretion represent a fundamental physiological feature regulating homeostatic systems. How individual cells within these neural ensembles generate and coordinate episodic activity and resultant pulse secretion is unknown. Recently, arcuate KNDy (kisspeptin/neurokinin B/dynorphin) cells were identified as a critical component of the gonadotrophin-releasing hormone (GnRH) pulse generator required for reproduction. Using in vivo calcium imaging of KNDy neurons in freely moving mice, we reveal that, prior to each GnRH pulse, individual KNDy cells demonstrate synchronized activity with striking temporal order, with subsets of cells behaving as “leaders” or “followers.” Future work to distinguish these novel subpopulations and define mechanisms underlying the temporal ordering of cellular synchronization may provide avenues to regulate pulse secretion. A hypothalamic pulse generator located in the arcuate nucleus controls episodic release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and is essential for reproduction. Recent evidence suggests this generator is composed of arcuate “KNDy” cells, the abbreviation based on coexpression of kisspeptin, neurokinin B, and dynorphin. However, direct visual evidence of KNDy neuron activity at a single-cell level during a pulse is lacking. Here, we use in vivo calcium imaging in freely moving female mice to show that individual KNDy neurons are synchronously activated in an episodic manner, and these synchronized episodes always precede LH pulses. Furthermore, synchronization among KNDy cells occurs in a temporal order, with some subsets of KNDy cells serving as “leaders” and others as “followers” during each synchronized episode. These results reveal an unsuspected temporal organization of activation and synchronization within the GnRH pulse generator, suggesting that different subsets of KNDy neurons are activated at pulse onset than afterward during maintenance and eventual termination of each pulse. Further studies to distinguish KNDy “leader” from “follower” cells is likely to have important clinical significance, since regulation of pulsatile GnRH secretion is essential for normal reproduction and disrupted in pathological conditions such as polycystic ovary syndrome and hypothalamic amenorrhea.