Regulation of High Frequency GnRH Release in Early Prepubertal Mice

Presentation Number: OR30-3
Date of Presentation: June 23rd, 2014

Katarzyna M M Glanowska*1, Laura L Burger2 and Suzanne M Moenter2
1University of Virginia, Charlottesville, VA, 2University of Michigan, Ann Arbor, MI

Abstract

The central nervous system exerts control over reproduction through pulsatile release of GnRH. In this context, puberty is defined as a sequence of developmental changes leading to an organized pattern of GnRH release that is capable of inducing pituitary synthesis and secretion of gonadotropins. Our understanding of the central events of the early prepubertal period is poor because of an inability to measure GnRH in very young animals. We used fast-scan cyclic voltammetry (FSCV) to monitor changes in GnRH concentration in brain slices; GnRH release frequency in the median eminence (ME) of slices from adults matches LH pulse frequency in vivo. Using FSCV, we showed that GnRH release in the ME is observed well before the onset of outward pubertal signs, and also that the frequency in 1wk old male mice is much greater than in adults. Here we studied the mechanisms of high frequency GnRH secretion at this age. First, we hypothesized that frequent GnRH release was due to stimulatory kisspeptin drive to the GnRH system. The frequency of GnRH release in kisspeptin knock out mice, however, was the same as in controls (5.0±0.4, n=5, vs 5.0±0.6, n=6 events/hr), suggesting high frequency GnRH release at 1wk is kisspeptin-independent. We next asked if high frequency GnRH secretion could be inhibited by steroid feedback or inhibitory neuromodulators. To test steroid feedback, 1wk old intact male mice were injected sc with 50µg/g T or vehicle 4h before brain slice preparation and spontaneous GnRH release monitored. T reduced release frequency compared to vehicle (0.6±0.2, n=7, vs 4.3±0.7, n=3, events/hr, p<0.05). Bath application of the inhibitory neuromodulator GnIH to the slices from control mice also reduced GnRH release frequency (2.2±0.5 vs 5.3±0.8, events/hr, n=6, p<0.05). GnIH neurons have been reported to be steroid sensitive so we tested if the GnIH receptor (GPR147) inhibitor RF9 could reverse the effect of testosterone. RF9 restored GnRH frequency in slices from T-injected mice (4.6±0.7 events/hr, n=5). This suggests testosterone acts at least in part via activation of GPR147. Finally, we examined LH release in vivo in response to GnRH (150 ng/kg). GnRH failed to increase serum LH at 15 min in 1wk old mice, but did in mice aged ≥2wk, which is after cessation of the high frequency spontaneous release observed at 1wk of age. These data indicate high frequency GnRH release in early prepubertal male mice is kisspeptin-independent and appears to result from lack of endogenous inhibition of the GnRH network, since the hypothalamic circuitry is mature enough to respond to testosterone feedback when it arrives. We postulate the high endogenous GnRH frequency at 1wk results from GnRH neuronal activity needed for proper synaptogenesis, and that the shut down of the pituitary by this high frequency protects the downstream reproductive system from premature development while this critical neuronal maturation proceeds.

 

Nothing to Disclose: KMMG, LLB, SMM