Modulation of estrogen receptors during development inhibits neurogenesis of precursors to GnRH-1 neurones: In vitro studies with explants of ovine olfactory placode

doi:10.1016/j.brainres.2008.05.026

Brain Research

Volume 1223, 5 August 2008, Pages 34-41

https://doi.org/10.1016/j.brainres.2008.05.026 Get rights and content

Abstract

The aim of the present study was to explore the putative effects of agonists and antagonists of the estradiol receptor on the early phase of GnRH-1 neuron development. To address this question we used an in vitro model of GnRH-1 neurons using cultured olfactory placode from sheep embryos on day 26 of gestation. Previous studies on this model have shown that in vitro the development of GnRH-1 neurons mimics in vivo development up to the start of pulsatile GnRH-1 secretion, To address the effects of modulating the estrogen receptor, cultures were treated with the endogenous and synthetic ligands of estradiol receptors: 17β-estradiol, 17α-estradiol and tamoxifen. Neurogenesis was measured by incorporation of [³H]-thymidine. Morphometric parameters were evaluated by image analysis. The main results are that antagonism of estradiol receptors induced an important decrease in neurogenesis but had little effect on morphometric parameters, suggesting that during this early phase of development, maternal estrogens are important to achieve correct development of the GnRH-1 neuronal network.

Introduction

Gonadotropin releasing hormone-1 (GnRH-1) is a decapeptide neurohormone that controls different aspects of vertebrate reproductive life (Ordog et al., 1997). At the end of gestation and/or during perinatal life, GnRH-1 is secreted in a pulsatile fashion, and stimulates the production of steroid hormones by the fetal and neonatal gonads contributing to the sexual differentiation of the external genitalia and the central nervous system. The secretion of GnRH-1 is strongly suppressed during infancy before being reactivated at the time of puberty to ensure normal gametogenesis and endocrine function of the gonads, the secretion of GnRH-1 by GnRH-1 neurons then continues during adult reproductive life and is subject to internal (steroid feedback, regulatory peptides, etc) and external regulation (photoperiod, nutritional status, social cues). Despite its functional importance, GnRH-1 is synthesized and secreted by a small number of neurons (800 in the mouse (Hoffman and Finch, 1986, Wray et al., 1989), 3000 in the sheep (Caldani et al., 1988) scattered between the preoptic area (POA) and the medial basal hypothalamus (MBH). Another characteristic of GnRH-1 neurons is their embryological origin: as far as we know, they are the only neurons in the central nervous system (CNS) originating outside the brain (Wray et al., 1989, Schwanzel-Fukuda and Pfaff, 1989). Their precursors appear in the medial part of the olfactory placode and migrate along the main and accessory olfactory nerves towards the cribiform plate. In contrast to olfactory projections, which project to the main and accessory olfactory bulbs, precursors of GnRH-1 neurons continue their progression along the terminal nerves between the telencephalic vesicles towards the POA and the anterior hypothalamus, their final destination. This sequence of events has now been well documented in various species. In chicken (Mulrenin et al., 1999) and sheep (Bruneau et al., 2003), the GnRH-1 neuron precursors are located in the medial olfactory region and their neurogenesis is prolonged compared to mice (Wray et al., 1989). Whereas numerous studies have explored the role of gonadal or synthetic steroids on the regulation of GnRH-1 neurons in prepubertal or adult animals (see for reviews (Foster et al., 2006, Tena-Sempere, 2006, Gamba and Pralong, 2006, Clarkson and Herbison, 2006), few studies have focused on the role of these steroids during the ontogenesis of the GnRH-1 neurons.

Thus, in the present study we examined the effects of the natural estrogen receptor ligands: 17-β-estradiol (E 17β), 17α-estradiol (E 17α) and the synthetic estrogen receptor ligand: tamoxifen (Tx) on early development of GnRH-1 neurons in a model of ovine olfactory placode explants maintained in primary culture.

Section snippets

Neuronal morphology

All treatments stimulated neurite length with a greater effect found in the E17β-treated group: 57.24 ± 6.97 μm (Mean ± sem, n = 6) versus 38.46 ± 1.59 μm (Mean ± sem, n = 6) for the control group which corresponds to a 34% increase. The mean neurite length was also increased in the E17α and Tx treated groups with mean neurite lengths of 56.42 ± 5.38 (Mean ± sem, n = 4) and 53.59 ± 5.54 μm (Mean ± sem, n = 8) respectively (Fig. 1). This difference was statistically significant (ANOVA, Newman–Keuls post hoc test p ≤

Discussion

The aim of this study was to evaluate the effect of ER-modulation of GnRH-1 neurons in early development. Using an in vitro model of primary culture of GnRH-1 neurons derived from the olfactory placode of ovine embryos we have shown that ligands of estradiol receptors such as tamoxifen and 17-α-estradiol decreased neurogenesis of the precursors of GnRH-1 neurons in a drastic manner whereas supplementation with 17-β-estradiol had no statistically significant effect on the neurogenesis of GnRH-1

Experimental procedures

A total of 53 explants were tested: 17 in the control group (C), 13 in the17β-Estradiol-treated group (E 17β), 10 in the17α-Estradiol-treated group (E 17α) and 13 in the tamoxifen-treated group (Tx).

Acknowledgments

Thanks to Dr Corinne Cotinot who provided us SRY-probes. Thanks to the shepherds at the Experimental Unit (UPEA) in Nouzilly who looked after the animals.

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Research ReportModulation of estrogen receptors during development inhibits neurogenesis of precursors to GnRH-1 neurones: In vitro studies with explants of ovine olfactory placode

Abstract

Introduction

Section snippets

Neuronal morphology

Discussion

Experimental procedures

Acknowledgments

Neuroscience

Mol. Cell. Endocrinol.

Mol. Cell. Neurosci.

Brain Res.

Mol. Cell. Endocrinol.

Neuroscience

Mol. Cell. Endocrinol.

Dev. Biol.

Mol. Cell. Endocrinol.

Gen. Comp. Endocrinol.

Brain Res. Rev.

Front. Neuroendocrinol.

Neurobiol. Aging

Front. Neuroendocrinol.

FEBS Lett.

J. Steroid Biochem. Mol. Biol.

Neuroscience

Dev. Brain Res.

Neuroscience

Exp. Neurol.

Neuroprotective effects of estradiol in the adult rat hippocampus: interaction with insulin-like growth factor-I signaling.

J. Neurosci. Res.

Serotonin mediates estrogen stimulation of cell proliferation in the adult denate gyrus.

Eur. J. Neurosci.

Neuroanatomical organization of gonadotropin-releasing hormone neurons during the oestrus cycle in the ewe.

BMC Neurosci.

Prolonged neurogenesis during early development of gonadotropin-releasing hormone neurones in sheep (Ovis aries): in vivo and in vitro studies

Neuroendocrinology

LHRH-immunoreactive structures in the sheep brain

Histochemistry

Evidence that the mediobasal hypothalamus is the primary site of action of estradiol in inducing the preovulatory gonadotropin releasing hormone surge in the ewe

Endocrinology

Disordered follicle development in ovaries of prenatally androgenized ewes

J. Endocrinol.

Sexual differentiation of the neuroendocrine control of gonadotrophin secretion: concepts derived from sheep models

Reproduction

Nonclassical estrogen receptor alpha signaling mediates negative feedback in the female mouse reproductive axis

Proc. Natl. Acad. Sci. U. S. A.

Importance of perinatal testosterone in sexual differentiation in the male rat

J. Endocrinol.

Sexual differentiation of the surge mode of gonadotropin secretion: prenatal androgens abolish the gonadotropin-releasing hormone surge in the sheep

J. Neuroendocrinol.

Stress in pregnancy activates neurosteroid production in the fetal brain

Neuroendocrinology

Research Report
Modulation of estrogen receptors during development inhibits neurogenesis of precursors to GnRH-1 neurones: In vitro studies with explants of ovine olfactory placode