We are interested in examining mechanisms underlying estrogen actions during neuronal differentiation in the central nervous system (CNS). Our research has focused on one possible mechanism, the developmental interactions between estrogen and the neurotrophins (nerve growth factor [NGF], brain derived neurotrophic factor [BDNF] and neurotrophin-3 [NT-3]). Using combined isotopic and non-isotopic in situ hybridization, we found that neurons in developmental estrogen targets (e.g., the cerebral cortex), co-localized mRNAs for the neurotrophins (NGF or BDNF) with their cognate receptors (p75NGFR [the pan-neurotrophin receptor] and trkA or trkB [the tyrosine kinase receptors]), suggesting a localization of neurotrophin-autocrine loops to these estrogen-sensitive neurons. In contrast, the basal forebrain, which is estrogen-sensitive in the adult and during development, only expressed neurotrophin receptor mRNAs, suggesting that this region was not an autocrine neurotrophin target. We examined the potential for developmental estrogen-neurotrophin interactions, using a model neurotrophin-sensitive system, i.e., differentiating PC12 cells. NGF significantly increased estrogen receptor density in PC12 cells. Reciprocally, estrogen up-regulated trkA mRNA and transiently down-regulated p75NGFR mRNA, suggesting that estrogen may increase the efficiency of NGF binding in PC12 cells. Similar estrogen-dependent regulation of NGF receptor mRNAs were also observed in the adult dorsal root ganglia, suggesting that estrogen may regulate NGF sensitivity in adult neurotrophin targets as well. Such estrogen-neurotrophin interactions may have an important role during differentiation and in the adult, following injury.