Estrogen-mediated regulation of cholinergic expression in basal forebrain neurons requires...

Beyond the role estrogen plays in neuroendocrine feedback regulation involving hypothalamic neurons, other roles for estrogen in maintaining the function of CNS neurons remains poorly understood. Primary cultures of embryonic rat neurons together with radiometric assays were used to demonstrate how estrogen alters the cholinergic phenotype in basal forebrain by differentially regulating sodium-coupled high-affinity choline uptake and choline acetyltransferase activity. High-affinity choline uptake was significantly increased 37% in basal forebrain cholinergic neurons grown in the presence of a physiological dose of estrogen (5 nM) from 4 to 10 days in vitro whereas choline acetyltransferase activity was not significantly changed in the presence of 5 or 50 nM estrogen from 4 to 10 or 10 to 16 days in vitro. Newly-synthesized acetylcholine was significantly increased 35% following 6 days of estrogen treatment (10 days in vitro). These effects are in direct contrast to those found for nerve growth factor; that is, nerve growth factor can enhance the cholinergic phenotype through changes in choline acetyltransferase activity alone. This is most surprising given that mitogen-activated protein kinase and extracellular-signal-regulated kinase1/2, kinases also activated in the signaling pathway of nerve growth factor, were found to participate in the estrogen-mediated changes in the cholinergic phenotype. Likewise, general improvement in the viability of the cultures treated with estrogen does not account for the effects of estrogen as determined by lactate dehydrogenase release and nerve growth factor-responsiveness. These findings provide evidence that estrogen enhances the differentiated phenotype in basal forebrain cholinergic neurons through second messenger signaling in a manner distinct from nerve growth factor and independent of improved survival.