Requirement of Ras for the activation of mitogen-activated protein kinase by calcium influx, cAMP, and neurotrophin in hippocampal neurons.

Mitogen-activated protein (MAP) kinase plays important roles in the establishment of long-term potentiation both in vitro and in living animals. MAP kinase is activated in response to a broad range of stimuli, including calcium influx through NMDA receptor and L-type calcium channel, cAMP, and neurotrophins. To investigate the role of Ras in the activation of MAP kinase and cAMP response element-binding protein (CREB) in hippocampal neurons, we inhibited Ras function by overexpressing a Ras GTPase-activating protein, Gap1(m), or dominant negative Ras by means of adenovirus vectors. Gap1(m) expression almost completely suppressed MAP kinase activation in response to NMDA, calcium ionophore, membrane depolarization, forskolin, and brain-derived neurotrophic factor (BDNF). Dominant negative Ras also showed similar effects. On the other hand, Rap1GAP did not significantly inhibit the forskolin-induced activation of MAP kinase. In contrast to MAP kinase activation, the inactivation of Ras activity did not inhibit significantly NMDA-induced CREB phosphorylation, whereas BDNF-induced CREB phosphorylation was inhibited almost completely. These results demonstrate that Ras transduces signals elicited by a broad range of stimuli to MAP kinase in hippocampal neurons and further suggest that CREB phosphorylation depends on multiple pathways.