We aim to study the mechanisms underlying the neurotrophic effect of daidzein (Dz) in hippocampal neurons: Dz-enhanced axonal outgrowths manifested growth cone formation and increased immunostaining intensity of growth-associated protein 43 (GAP-43) in growth cones. Consistent with this, Dz increased GAP-43 phosphorylation and its membrane translocation without affecting total GAP-43 levels. In the presence of Dz, significant increase in the immunoreactivity for estrogen receptor (ER) beta, but not ER alpha, was observed on the membrane of cell bodies and growing axons. Dz also induced the activation of protein kinase C alpha, (PKC alpha), which was inhibited by the ICI182,780 pretreatment. Similarly, Dz-promoted axonal elongation was blocked by ICI182,780 and Go6976. Moreover, Dz-stimulated activation of GAP-43 was specifically abolished by Go6976, suggesting PKC alpha being the upstream effector of GAP-43. Taken together, our data suggest that Dz triggers an ER beta/PKC alpha/GAP-43 signaling cascade to promote axonal outgrowths in cultured hippocampal neurons. (C) 2007 Elsevier Inc. All rights reserved.