Brief exposure to NMDA produces long-term protection of cerebellar granule cells from apoptosis

Cerebellar granule cells (CGCs) require excitatory inputs to survive during their postnatal migration from the external to the internal granule cell layers. The lack of innervation of mossy fibres induces CGC death by apoptosis. In vitro, CGCs die by apoptosis in the presence of physiological concentrations of KCI (5 mM or K5) but they survive in the presence of depolarizing concentrations of KCI (25 mM or K25) or N-methyl-D-aspartate (NMDA) by a mechanism dependent on calcium influx. The addition of NMDA or K25, for only 24 h, to immature CGCs cultures [2 days in vitro (DIV)] was able to produce a remarkable and long-term protection until 8 DIV. Moreover, our data show that NMDA and K25-mediated long-lasting protection was related to an inhibition of caspase-3 activity. By using different protein kinase inhibitors, we have shown that the inhibition of caspase-3 activation by NMDA was dependent on the activation of tyrosine kinases and phosphatidylinositol 3-kinase (PI3-kinase). Moreover, an impairment in NMDA-mediated neuroprotection and caspase-3 inhibition was observed when the action of brain- derived neurotrophic factor (BDNF) was blocked. By contrast, K25-mediated neuroprotection was BDNF-independent and was mediated by a mitogen-activated protein kinase- and PI3-kinase-dependent inhibition of caspase-3.