Region-specific targets of p42/p44MAPK signaling in rat brain

In vitro studies indicate that p42/p44(MAPK) phosphorylate both nuclear and cytoplasmic proteins. However, the functional targets of p42/p44(MAPK) activation in vivo remain unclear. To address this question, we localized activated p42/p44(MAPK) in hippocampus and cortex and determined their signaling effects after electroconvulsive shock treatment (ECT) in rats. Phosphorylated p42/p44(MAPK) content increased in the cytoplasm of hippocampal neurons in response to ECT. Consistent with this cytoplasmic localization, inhibition of ECT-induced p42/p44(MAPK) activation by the extracellular signal-regulated kinase kinase inhibitor PD098059 blocked phosphorylation of the cytoplasmic protein microtubule-associated protein 2c (MAP2c), but failed to inhibit the induction of the nuclear protein c-Fos in response to ECT. In contrast to hippocampal neurons, cortical neurons exhibited an increase in amount of phosphorylated p42/p44(MAPK) in both the nucleus and cytoplasm after ECT. Accordingly, PD098059 blocked the induction of Fos-like immunoreactivity in the nuclei of cortical neurons as well as MAP2c phosphorylation in the cytoplasm. Our data indicate that both nuclear and cytoplasmic substrates can be activated by p42/p44(MAPK) in vivo. However, the functional targets of p42/p44(MAPK) signaling depend on the precise location of p42/p44(MAPK) within different subcellular compartments of brain regions, These results indicate unique functional pathways of p42/p44(MAPK)-mediated signal transduction within different brain regions in vivo.