Dose-dependent switch in response of gonadotropin-releasing hormone (GnRH) neurons to GnRH mediated through the type I GnRH receptor

Pulsatile release of GnRH provides central control of reproduction. GnRH neuron activity is likely synchronized to produce hormone pulses, but the mechanisms are largely unknown. One candidate for communication among these neurons is GnRH itself. Cultured embryonic and immortalized GnRH neurons express GnRH receptor type I ( GnRHR- 1), but expression has not been shown in adult GnRH neurons. Using mice that express green fluorescent protein ( GFP) in GnRH neurons, we tested whether adult GnRH neurons express GnRHR- 1. GFP- positive ( n = 42) and - negative neurons ( n = 22) were harvested from brain slices, and single- cell RTPCR was performed with cell contents. Fifty- two percent of the GnRH neurons tested expressed GnRHR- 1, but only 9% of non- GnRH hypothalamic neurons expressed GnRHR- 1; no false harvest controls ( n = 13) were positive. GnRHR- 1 expression within GnRH neurons suggested a physiological ultrashort loop feedback role for GnRH. Thus, we examined the effect of GnRH on the firing rate of GnRH neurons. Low- dose GnRH ( 20 nM) significantly decreased firing rate in 12 of 22 neurons ( by 42 +/- 4%, P< 0.05), whereas higher doses increased firing rate ( 200 nM, five of 10 neurons, 72 +/- 26%; 2000 nM, nine of 13 neurons, 53 +/- 8%). Interestingly, the fraction of GnRH neurons responding was similar to the fraction in which GnRHR- 1 was detected. Together, these data demonstrate that a subpopulation of GnRH neurons express GnRHR- 1 and respond to GnRH with altered firing. The dose dependence suggests that this autocrine control of GnRH neurons may be not only a mechanism for generating and modulating pulsatile release, but it may also be involved in the switch between pulse and surge modes of release.