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 bisphenolA(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 10-10 M) and at higher (10-7 to 10-6 M) 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 10-10 M) with 10-10 M 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.
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