Ontogeny of Rapid Estrogen-Mediated Extracellular Signal-Regulated Kinase Signaling in the Rat Cerebellar Cortex: Potent Nongenomic Agonist and Endocrine Disrupting Activity of the Xenoestrogen Bisphenol A

In addition to regulating estrogen receptor-dependent gene expression, 17β-estradiol (E₂) can directly influence intracellular signaling. In primary cultured cerebellar neurons, E₂ was previously shown to regulate growth and oncotic cell death via rapid stimulation of ERK1/2 signaling. Here we show that ERK1/2 signaling in the cerebellum of neonatal and mature rats was rapidly responsive to E₂ and during development to the environmental estrogen bisphenol A (BPA). In vivo dose-response analysis for each estrogenic compound was performed by brief (6-min) intracerebellar injection, followed by rapid fixation and phosphorylation-state-specific immunohistochemistry to quantitatively characterize changes in activated ERK1/2 (pERK) immunopositive cell numbers. Beginning on postnatal d 8, E₂ significantly influenced the number of pERK-positive cells in a cell-specific manner that was dependent on concentration and age but not sex. In cerebellar granule cells on postnatal d 10, E₂ or BPA increased pERK-positive cell numbers at low doses (10⁻¹² to 10⁻¹⁰m) and at higher (10⁻⁷ to 10⁻⁶m) concentrations. Intermediate concentrations of either estrogenic compound did not modify basal ERK signaling. Rapid E₂-induced increases in pERK immunoreactivity were specific to the ERK1/2 pathway as demonstrated by coinjection of the mitogen-activated, ERK-activating kinase (MEK)1/2 inhibitor U0126. Coadministration of BPA (10⁻¹² to 10⁻¹⁰m) with 10⁻¹⁰m E₂ dose-dependently inhibited rapid E₂-induced ERK1/2 activation in developing cerebellar neurons. The ability of BPA to act as a highly potent E₂ mimetic and to also disrupt the rapid actions of E₂ at very low concentrations during cerebellar development highlights the potential low-dose impact of xenoestrogens on the developing brain.