SAT-412 Effects of the Ovary and Dihydrotestosterone on Firing Rate of GnRH Neurons in Prenatally Androgenized Mice

Abstract

GnRH pulse frequency drives downstream reproductive function by signaling for differential release of gonadotropins, which subsequently control ovarian function. In patients with polycystic-ovary syndrome (PCOS), neuroendocrine defects, including persistent high-frequency LH (and presumably GnRH) pulses, are coupled with dysregulated ovarian function. Prenatally androgenized (PNA) animal models exhibit features similar to PCOS, including increased androgens and high-frequency LH pulses. Adult PNA mice exhibit these phenotypes and also have increased GnRH neuron firing rate and increased excitatory GABAergic transmission to GnRH neurons. In contrast to adults, GnRH neuron firing rate at 3 weeks of age is reduced in PNA females. The causes of this age-dependent difference in firing rate are not known, however puberty occurs between these ages and androgens can increase GnRH neuron firing rate in adulthood. Consistent with this, ovariectomy in adult PNA mice decreased firing rate to that in control diestrous mice. Here we tested the hypothesis that a lack of ovarian androgens mediated this decrease. We also investigated if ovariectomy changed firing rate in 3-week old PNA mice. PNA mice were born from dams injected daily on days 16-18 of gestation with 225µg dihydrotestosterone (DHT). Ovariectomy (OVX) or sham surgery was performed and at this time some mice received constant-release DHT implants that produce sub-male levels of androgen (400 µg/implant) 5-7 days before recordings (surgery at 14-16 days of age for prepubertal mice). Hour-long extracellular recordings were made to ascertain spontaneous firing rate of GFP-identified GnRH neurons in acutely-prepared brain slices. In adult OVX PNA mice, DHT increased firing rate of GnRH neurons relative to OVX PNA, but firing rate was still less than in sham-operated PNA animals (two-way ANOVA/Fisher’s LSD, n= 7-11 cells/group). In adult OVX control mice, preliminary data indicate that DHT replacement alone does not alter firing rate, in contrast to previous results with DHT plus estradiol. At three weeks of age, there was no difference in firing rate between PNA sham and OVX mice (unpaired two-tailed Student’s t-test, n=7-8 cells/group). These studies suggest that increased GnRH firing rate in adult PNA females is driven at least in part by ovarian androgens, whereas changes in firing rate in 3-week-old PNA females are driven by a different mechanism, such as prenatal programming, rather than ovarian factors.