Three‐dimensional properties of GnRH neuroterminals in the median eminence of young and old rats

Abstract

The decapeptide GnRH that regulates reproduction in all vertebrates is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 month) and old (22-24 month) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded, and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals, and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.

Endocannabinoids and prostaglandins both contribute to GnRH neuron-GABAergic afferent local feedback circuits

“…Furthermore, estradiol acts on endothelial cells to induce retraction of tanycyte processes from GnRH neuron terminals (de Seranno et al 2010); similar mechanisms may exist to regulate astrocyte apposition near the somata of these neurons. In this regard, several studies have shown that changes in steroid milieu, and in particular estradiol, alter glial apposition to GnRH neurons in a manner correlated with number of synapses (Baroncini et al 2007;King and Letourneau 1994;Kozlowski and Coates 1985;Meister et al 1988;Prevot et al 1998Prevot et al , 1999Witkin et al 1991Witkin et al , 1995Witkin et al , 1997Xiong et al 1997;Yin et al 2009aYin et al , 2009b. This apposition can change in a diurnal manner consistent with the estradiol-induced changes in GnRH neuron activity (Cashion et al 2003).…”

Initiation and Propagation of Action Potentials in Gonadotropin-Releasing Hormone Neuron Dendrites

“…Where then is the GnRH neuron axon? It has been shown that the terminal projection zone of GnRH neurons (the median eminence) contain GnRH fibers with secretary endings (Gross, 1976;Lehman et al, 1988;Yin et al, 2009). It is also known that GnRH neurons Figure 3.…”

Gonadotropin-Releasing Hormone Neuroterminals and Their Microenvironment in the Median Eminence: Effects of Aging and Estradiol Treatment

“…Previously, we reconstructed GnRH terminals from young (ϳ5 months) and old (ϳ24 months) ovariectomized rats in three dimensions, thereby revealing novel features of these cells and their microenvironment (12). Here, we extended that previous study by using both descriptive and quantitative measures in three age groups (young, middleaged, and old), and two hormone treatments (estradiol or vehicle) at the light and electron microscopic level.…”