N-Methyl-D-aspartate (NMDA) at a subtoxic concentration (100 microM) promotes neuronal survival against glutamate-mediated excitotoxicity via a brain-derived neurotrophic factor (BDNF) autocrine loop in cultured cerebellar granule cells. The signal transduction mechanism(s) underlying NMDA neuroprotection, however, remains elusive. The mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3-K) pathways alter gene expression and are involved in synaptic plasticity and neuronal survival. This study tested whether neuroprotective activation of NMDA receptors, together with TrkB receptors, coactivated the MAPK or PI3-K pathways to protect rat cerebellar neurons. NMDA receptor activation caused a concentration- and time-dependent activation of MAPK lasting 24 hr. This activation was blocked by the NMDA receptor antagonist MK-801 but was attenuated only partially by the tyrosine kinase inhibitor k252a, suggesting that activation of both NMDA and TrkB receptors are required for maximal neuroprotection. The MAPK kinase (MEK) inhibitor U0126 (10 microM) partially blocked NMDA neuroprotection, whereas LY294002, a selective inhibitor of the PI3-K pathway, did not affect the neuroprotective activity of NMDA. Glutamate excitotoxicity decreased bcl-2, bcl-X(L), and bax mRNA levels,. NMDA increases Bcl-2 and Bcl-X(L) protein levels and decreases Bax protein levels. NMDA and TrkB receptor activation thus converge on the extracellular signal-regulated kinase (ERK) 1/2 signaling pathway to protect neurons against glutamate-mediated excitotoxicity. By increasing antiapoptotic proteins of the Bcl-2 family, NMDA receptor activation may also promote neuronal survival by preventing apoptosis.
(c) 2005 Wiley-Liss, Inc.