Evidence for estrogenic regulation of gonadotropin‐releasing hormone neurons by glutamatergic neurons in the ewe brain: An immunohistochemical study using an antibody against vesicular glutamate transporter‐2

Abstract

Gonadotropin‐releasing hormone (GnRH) secretion is controlled by various factors, including the excitatory neurotransmitter glutamate. Estrogen (E) regulates GnRH secretion by means of E‐responsive cells in the brain that relay the feedback effects to the preoptic area (POA). We used an antibody to vesicular glutamate transporter 2 (VGluT2) to label glutamatergic neurons in the areas of the ewe brain that control GnRH secretion. VGluT2‐immunoreactive cells were observed in the arcuate nucleus (ARC)/ventromedial hypothalamic nucleus (VMH) complex, POA, bed nucleus of stria terminalis (BnST), and A1 and A2 cell groups in the brainstem. In three ewes, E receptor‐α was detected in 52–61% of glutamatergic neurons in ARC/VMH, 37–52% of neurons in the POA, and 37–58% of neurons in the BnST. E injection (i.m. or i.v.) increased the percentage of glutamatergic cells that expressed Fos protein in the ARC (P< 0.01 andP< 0.001, respectively). In six ewes, injection of the retrograde tracer Fluoro‐Gold into the POA labeled cells in the ARC and 6–29% of these were also VGluT2‐immunoreactive. Double‐labeling of varicosities in the POA showed colocalization of VGluT2 in 12.5 ± 3% of dopamine β‐hydroxylase–immunoreactive terminals, indicating that a subset of glutamatergic inputs could arise from brainstem noradrenergic neurons cells. In the POA, 60% of GnRH neurons had close appositions that were VGluT2‐immunoreactive. We conclude that E‐responsive glutamatergic neurons arising from the brainstem, the BnST, and ARC/VMH provide input to the POA and may be involved in the regulation of GnRH secretion. J. Comp. Neurol. 465:136–144, 2003.