AbstractEffects of 4‐methycatechol (4MC), a potent stimulator of nerve growth factor and brain‐derived neurotrophic factor (BDNF) synthesis, on phosphorylation of cellular molecules in cultured rat cortical neurons were examined. 4MC stimulated tyrosine phosphorylation of various proteins of molecular weight from 10–300 kDa including Trks, which are high‐affinity neurotrophin receptors. Moreover, 4MC enhanced the phosphorylation of serine 133 of mitogen‐activated protein kinase (MAPK/ERK) in a dose‐dependent manner. Pretreatment of cultures with PD98059, a selective inhibitor of MAPK kinase (MEK‐1), inhibited 4MC‐induced phosphorylation of ERKs, demonstrating MEK‐1‐mediated activation. Therefore, it seems that 4MC triggered the phosphorylation of Trks, resulting in the activation of the subsequent MAPK/ERK signal cascade, or perhaps the involvement of BDNF action as 4MC can stimulate neuronal BDNF synthesis. The phosphorylation of MAPK/ERK was unaffected, however, in the presence of cycloheximide, a protein synthesis inhibitor, and K252a, a selective inhibitor of Trks, suggesting that the effect of newly synthesized BDNF was negligible on this event, and that primary sites of 4MC actions are not limited only to Trks. These results suggest that 4MC primarily activates multiple signal transduction molecules such as tyrosine kinases, including Trks. A significant increase in the survival rate of cortical neurons in the presence of 10 or 100 nM 4MC supported this idea, because the concentrations were much lower than those for stimulation of BDNF synthesis. Our results strongly suggest that the neurotrophic actions of 4MC found so far are mediated predominantly by direct activation of some intracellular signals including MAPK/ERK rather than by neurotrophin synthesis. © 2002 Wiley‐Liss, Inc.