Characterization of GnRH Pulse Generator Activity in Male Mice Using GCaMP Fiber Photometry

Kisspeptin neurons located in the hypothalamic arcuate nucleus are thought to represent the GnRH pulse generator responsible for driving pulsatile LH secretion. The recent development of GCaMP6 fiber photometry technology has made it possible to perform long-term recordings of the population activity of the arcuate nucleus kisspeptin (ARNKISS) neurons in conscious-behaving mice. Using this approach, we show that ARNKISS neurons in intact male mice exhibit episodes of synchronized activity that last ∼2 minutes and have a mean inter-episode interval of 166 minutes, with a very wide range (43 to 347 minutes). Gonadectomy resulted in dramatic changes in the dynamics of ARNKISS neuron behavior with temporally distinct alterations in synchronization episode (SE) amplitude (sevenfold increase), inter-SE frequency (range, 2 to 58 minutes), and duration (up to 28 minutes), including the frequent appearance of seemingly unstable clusters of doublet and triplet SEs. The combination of photometry with repeated blood sampling revealed a perfect correlation between ARNKISS neuron population SEs and LH pulses in intact and short-term gonadectomized (GDX) mice. No differences were detected in SE frequency across 24 hours in either intact or GDX mice. These observations further support a role for ARNKISS neurons as the GnRH pulse generator and show that it operates in a stochastic manner without diurnal variation in both intact and GDX male mice. The removal of gonadal steroids has multiple time-dependent effects upon ARNKISS neuron synchronizations, indicating their critical role in shaping pulse generator behavior.