Estradiol Suppresses Glutamatergic Transmission to Gonadotropin-Releasing Hormone Neurons in a Model of Negative Feedback in Mice1

Abstract

A surge of gonadotropin-releasing hormone (GnRH) release from the brain triggers the luteinizing hormone (LH) surge that causes ovulation. The GnRH surge is initiated by a switch in estradiol action from negative to positive feedback. Estradiol signals critical for the surge are likely transmitted to GnRH neurons at least in part via estradiol-sensitive afferents. Using an ovariectomized estradiol-treated (OVX+E) mouse model that exhibits daily LH surges, we examined changes in glutamate transmission to GnRH neurons during negative feedback and positive feedback. Spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptors (AMPA/ KA Rs) or N-methyl-D-aspartate receptors (NMDARs) were recorded in GnRH neurons from OVX+E and OVX mice. There were no diurnal changes in the percentage of GnRH neurons from OVX mice exhibiting EPSCs. In cells from OVX+E mice, the profile of AMPA/KA R-mediated and NMDAR-mediated EPSCs showed changes dependent on time of day. Comparison of AMPA/KA R-mediated EPSC frequency in OVX+E and OVX cells showed that estradiol suppressed transmission during negative feedback but had no effect during positive feedback. Tetrodotoxin treatment to block action potential firing did not affect AMPA/KA R-mediated EPSC frequency in OVX cells during negative feedback or in OVX+E cells during positive feedback, suggesting that estradiol-induced suppression of glutamate transmission may be primarily due to activity-independent changes. The diurnal removal of estradiol-induced suppression of AMPA/KA R-mediated glutamate transmission to GnRH neurons during positive feedback suggests that the primary role for estradiol-induced changes in glutamate transmission may be in mediating negative feedback.

Frequency-Dependent Recruitment of Fast Amino Acid and Slow Neuropeptide Neurotransmitter Release Controls Gonadotropin-Releasing Hormone Neuron Excitability

“…Intriguingly, no GnRH neurons showed AMPA and NMDA receptor coactivation following AVPV stimulation. This is reminiscent of the observation that only 19% of GnRH neurons have spontaneous postsynaptic currents mediated by both AMPA and NMDA receptors (Christian et al, 2009). The relative absence of NMDA receptor involvement is in agreement with other brain slice studies indicating that only 10 -30% of adult GnRH neurons express functional NMDA receptors (Spergel et al, 1999;Christian et al, 2009;Iremonger et al, 2010).…”

“…This result is surprising considering that glutamatergic neurons are plentiful in the AVPV (Ottem et al, 2004) and most GnRH neurons express functional glutamate receptors (Iremonger et al, 2010). Virtually all GnRH neurons exhibit spines (Campbell et al, 2005(Campbell et al, , 2009), indicative of glutamatergic inputs, and the majority exhibit electrophysiological evidence of functional glutamate receptors (Spergel et al, 1999;Christian et al, 2009). For GnRH neurons in which it was possible to differentiate between AMPA and NMDA receptor activation, ϳ70% of the AVPV glutamate current was mediated by AMPA receptors.…”

In VivoRecordings of GnRH Neuron Firing Reveal Heterogeneity and Dependence upon GABAAReceptor Signaling

“…The transpharyngeal exposure of GnRH neurons was adapted from a surgical approach for viewing the median eminence and pituitary gland (Bonnefont et al, 2005;Lafont et al, 2010) (see Fig. 1A).…”

“…Ever since the discovery of GnRH (Schally et al, 1971;Barry et al, 1973), understanding the properties of these cells has been an important goal in the field of reproductive biology. A variety of experimental preparations, including immortalized GnRH-secreting GT1 cells (Weiner et al, 1992), embryonic olfactory placode cultures (Terasawa et al, 1993;Fueshko and Wray, 1994), and transgenic rodent models, have been used to investigate GnRH neurons (Moenter, 2010;Constantin et al, 2012a).…”

“…This suggested that it might actually be possible to record from fluorescent GnRH neurons in vivo through a ventral surgical approach. Taking insight from methodologies enabling the visualization of the pituitary gland in vivo in the mouse (Bonnefont et al, 2005;Lafont et al, 2010), we report here the first recordings of GnRH neurons in vivo. We show that GnRH neurons exhibit even greater heterogeneity in their spontaneous firing rates than that found in the brain slice and that GABA A receptor activation by endogenous GABA is essential for GnRH neuron firing in vivo.…”