Direct Action of Estradiol on Gonadotropin-Releasing Hormone-1 Neuronal Activity via a Transcription-Dependent Mechanism

Abstract

Pulsatile secretion of gonadotropin-releasing hormone-1 (GnRH-1) is essential for reproduction. GnRH-1 induces gonadotropin release and is regulated by 17␤-estradiol (E2). Although a subpopulation of GnRH-1 neurons expresses estrogen receptor (ER) ␤, it is unclear whether E2 acts directly on GnRH-1 neurons or indirectly through interneuronal connections. To test the hypothesis that E2 acts directly on GnRH-1 neurons to regulate neuronal activity, we used calcium imaging to monitor intracellular calcium oscillations in GnRH-1 neurons maintained in nasal explants. TTX was used to minimize synaptic input from other cells. Consistent with previous studies, TTX reduced the activity of individual GnRH-1 neurons to a basal level, while the population of cells maintained synchronized calcium oscillations. Exposure of GnRH-1 cells to TTX plus E2 increased the number of calcium peaks/cell, percentage of cells with Ն10 peaks, mean peak amplitude, and percentage of cells that contributed to each calcium pulse in explants maintained in vitro for 7 d (7 div) compared with TTX alone. These effects were induced within 30 min and were not mimicked by 17␣-estradiol, E2 conjugated to BSA (which does not cross the plasma membrane), or seen at 21 div, when the percentage of GnRH-1 cells expressing ER␤ transcripts declines. In addition, these effects were inhibited by the ER antagonist ICI 182,780 and prevented by inhibition of gene transcription. These data suggest that, via ER␤, E2 can rapidly act as a hormone-activated transcription complex and are the first to show that E2 directly increases GnRH-1 neuronal activity and synchronization.

Membrane-initiated estrogen signaling in hypothalamic neurons

“…In contrast, E2-induced calcium oscillations in the primate placode cultures are not blocked by ICI 182,780, and the antagonist ICI 182,780 alone has no effect suggesting that an ER may not be involved (Abe et al, 2007). The actions of E2-BSA in mouse GnRH neurons are abrogated by pertussis toxin treatment and not by inhibition of gene transcription, supporting a role for a G proteincoupled membrane receptor (Temple et al, 2004, Temple andWray, 2005). However, in vivo studies have provided evidence for ERβ in the E2-induced rapid (15 min) induction of pCREB in mouse GnRH neurons (Abraham et al, 2003).…”

“…The latter findings combined with recent technological advances, such as the development of ER mutants and transgenic animals expressing green fluorescent protein (GFP) in GnRH neurons, have greatly facilitated studies to understand the cellular mechanisms by which GnRH neurons are regulated by estrogen (Spergel et al, 1999, Suter et al, 2000, Kato et al, 2003, Han et al, 2005, Abraham et al, 2003, Smith et al, 2006, Wintermantel et al, 2006. A series of recent publications show that in mouse hypothalamic neuronal explants and in primate nasal placode cultures, E2 augments synchronous intracellular Ca 2+ oscillations in GnRH neurons (Temple et al, 2004, Temple and Wray, 2005, Abe et al, 2007. This relatively rapid effect is observed within 10-30 min of E2 application.…”

“…The calcium oscillations in the presence of TTX are most likely the result of estrogen action directly on GnRH neurons. Furthermore, E2 conjugated to BSA at the C-17 position (E2-17 BSA) or to a large nondegradable poly(amido)amine-dendrimer macromolecule (Harrington et al, 2006) mimics the effects of E2 on intracellular calcium oscillations (Temple et al, 2004, Temple and Wray, 2005, Abe et al, 2007. However in the mouse explant experiments (Temple et al, 2004, Temple andWray, 2005), the E2 -induced calcium oscillations are blocked by ICI 182,780 suggesting that an ER is involved.…”

“…Furthermore, E2 conjugated to BSA at the C-17 position (E2-17 BSA) or to a large nondegradable poly(amido)amine-dendrimer macromolecule (Harrington et al, 2006) mimics the effects of E2 on intracellular calcium oscillations (Temple et al, 2004, Temple and Wray, 2005, Abe et al, 2007. However in the mouse explant experiments (Temple et al, 2004, Temple andWray, 2005), the E2 -induced calcium oscillations are blocked by ICI 182,780 suggesting that an ER is involved. In contrast, E2-induced calcium oscillations in the primate placode cultures are not blocked by ICI 182,780, and the antagonist ICI 182,780 alone has no effect suggesting that an ER may not be involved (Abe et al, 2007).…”

Diurnal and estradiol-dependent changes in gonadotropin-releasing hormone neuron firing activity

“…The use of mice in studies investigating the GnRH system has increased greatly in recent years (19)(20)(21)(22)(23)(24)(25)(26). Previous studies in mice have used a regimen of ovariectomy followed by injections of estrogen and͞or progesterone at specific times to induce a LH surge (12,27).…”