Estradiol suppresses glutamatergic transmission to gonadotropin-releasing hormone neurons in a model of negative feedback in mice

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.

FIG. 1.

GnRH neurons receive glutamate transmission mediated by both AMPA/KA R and NMDAR subtypes. A–C) Examples of EPSCs recorded at 40 mV (A) and −70 mV (B and C). The EPSCs recorded at 40 mV show a longer time course and are blocked by the NMDAR antagonist APV (20 μM), whereas EPSCs recorded at −70 mV show a fast time course and are blocked by the AMPA/KA R blocker CNQX (10 μM). C) The portion of the recording indicated by the black bar in B is expanded to show individual AMPA/KA R EPSCs; note change in scale.

FIG. 2.

Percentage of GnRH neurons from OVX+E, but not OVX, mice exhibiting EPSC changes in a diurnal manner. A and B) Rate of AMPA/KA R and/or NMDAR EPSC expression in GnRH neurons from OVX mice (top) or OVX+E mice (bottom) during negative feedback (13 cells for OVX and 16 cells for OVX+E) (A) and during positive feedback (10 cells for OVX and 25 cells for OVX+E) (B).

FIG. 3.

Estradiol suppresses AMPA/KA R-mediated glutamate transmission to GnRH neurons during negative feedback. A) Mean ± SEM AMPA/KA EPSC frequency in OVX+E and OVX cells during negative feedback (31 cells for OVX and 10 cells for OVX+E) and during positive feedback (15 cells for OVX and 33 cells for OVX+E). B) Cumulative probability distribution for interevent interval created using all cells in each treatment group. *P < 0.05 OVX vs. OVX+E during negative feedback; ^#P < 0.05 during negative feedback vs. positive feedback for OVX+E groups.

FIG. 4.

Effects of estradiol on AMPA/KA R-mediated glutamate transmission to GnRH neurons are not dependent on firing activity. A) Examples of EPSCs recorded before (con) and during (TTX) in vitro TTX treatment. B) The EPSC frequency in individual GnRH neurons before and during in vitro TTX treatment. Values from OVX cells recorded during the normal time of negative feedback are on the left, and values from OVX+E cells recorded during positive feedback are on the right. The filled circles indicate cells that showed >30% change in EPSC frequency in TTX. C) Percentage change in EPSC frequency (freq) from control after in vitro TTX treatment. White bar indicates OVX negative feedback; black bar, OVX+E positive feedback.

FIG. 5.

Estradiol decreases EPSC amplitude during negative feedback. A) Averaged AMPA/KA R EPSC traces from representative OVX+E cells during negative feedback and positive feedback. B) Cumulative probability distribution for AMPA/KA R EPSC amplitude comparing events during negative feedback and positive feedback in both OVX and OVX+E cells. *P < 0.01 vs. OVX negative feedback; ^#P < 0.01 vs. OVX+E positive feedback.