Definition of the hypothalamic GnRH pulse generator in mice.

The pulsatile release of luteinizing hormone (LH) is critical for mammalian fertility. However, despite several decades of investigation, the identity of the neuronal network generating pulsatile reproductive hormone secretion remains unproven. We use here a variety of optogenetic approaches in freely behaving mice to evaluate the role of the arcuate nucleus kisspeptin (ARNKISS) neurons in LH pulse generation. Using GCaMP6 fiber photometry, we find that the ARNKISS neuron population exhibits brief (∼1 min) synchronized episodes of calcium activity occurring as frequently as every 9 min in gonadectomized mice. These ARNKISS population events were found to be near-perfectly correlated with pulsatile LH secretion. The selective optogenetic activation of ARNKISS neurons for 1 min generated pulses of LH in freely behaving mice, whereas inhibition with archaerhodopsin for 30 min suppressed LH pulsatility. Experiments aimed at resetting the activity of the ARNKISS neuron population with halorhodopsin were found to reset ongoing LH pulsatility. These observations indicate the ARNKISS neurons as the long-elusive hypothalamic pulse generator driving fertility.

Anatomical location of mature GnRH neurons corresponds with their birthdate in the developing mouse.

The gonadotropin-releasing hormone (GnRH) neurons exhibit substantial functional, anatomical, and molecular heterogeneity, which has hampered their thorough examination. This study was undertaken in an effort to understand whether the anatomical distribution of GnRH neurons is related to their developmental history, because such an association may help explain differences within the population. Using bromodeoxyuridine pulse labeling of timed pregnant female mice we labeled dividing cells, including GnRH neuron progenitors in the olfactory placode, throughout the window of GnRH neuron differentiation. Our results indicate that cells that become postmitotic early tend to populate the rostral aspects of the adult GnRH neuron continuum, whereas later-generated cells tend to settle more caudally; an inside-out pattern reminiscent of neocortex. These observations suggest that the timing of differentiation influences the ability of postmitotic GnRH neurons to navigate to their adult location, and hence may be important in determining the ultimate wiring of the adult network.