Monosynaptic GABAergic projections from a subset of suprachiasmatic nucleus neuromedin S neurons suppress preoptic area kisspeptin neuron firing in female mice

Neural control of ovulation is critical for reproductive success. In spontaneous ovulators, this is achieved through hypothalamic neural circuits relaying ovarian follicle maturity cues to gonadotropin-releasing hormone (GnRH)-secreting neurons to drive the preovulatory luteinizing hormone (LH) surge. Within these circuits, kisspeptin-expressing neurons in the rostral periventricular region of the third ventricle (RP3V KISS1 neurons) are thought to stimulate GnRH neurons for the LH surge. In female rodents, the central circadian clock in the suprachiasmatic nucleus (SCN) regulates the timing of RP3V KISS1 and GnRH neuron activation, coordinating neuroendocrine control of ovulation and behavior. However, the mechanisms thereof are not fully understood. Here, we examined the potential regulation of RP3V KISS1 neuron activity by neuromedin S (NMS)-expressing SCN neurons – a neuronal population essential for circadian rhythms – using anatomical tract-tracing, optogenetics and electrophysiology in female mice. Our observations indicate that a subset of SCN NMS neurons, likely distinct from the canonical arginine vasopressin and vasoactive intestinal peptide SCN populations, projects to RP3V KISS1 neurons. Optogenetic activation of these projections evokes monosynaptic release of GABA onto ≈ 75% of RP3V KISS1 neurons. Whereas exogenous NMS only moderately affected RP3V KISS1 neuron activity, with no effect on firing and altered intracellular calcium concentration in subsets of cells, GABA release from SCN NMS fibers in the RP3V potently suppressed KISS1 neuron action potential firing, through activation of GABA A receptors. Together, these findings reveal additional complexity in the regulation of the GnRH neuronal network by the central circadian clock. Potential implications of these observations for the timing of the LH surge are discussed. Significance statement In females of multiple species, neuroendocrine circuits in the hypothalamus, which include preoptic area kisspeptin neurons and converge on the gonadotropin-releasing hormone (GnRH) neurons, integrate information about gonadal state with circadian cues to triggers ovulation. The mechanisms though which the central circadian clock in the suprachiasmatic nucleus (SCN) relays timing cues to the GnRH neuronal network are, however, incompletely understood. Using anatomical and functional approaches, we found that a previously unsuspected subpopulation of SCN neurons projects to preoptic area kisspeptin neurons and, unexpectedly, suppresses these cells’ activity through the release of the inhibitory neurotransmitter GABA. Our observations suggest that control of activity within the GnRH neuronal network for ovulation might extend beyond modulation by canonical SCN neuropeptides.