Objective   To investigate the effect of electromagnetic irradiation on cyclin dependent kinase 5 (CDK5) and its activators in primary cultured rat cortical neurons.    Methods   Rat cortical neurons were isolated and then primary cultured from neoborn rats. Then the cells were exposed to the electromagnetic wave (peak power 90 W/cm

2

) for 10 min. The mRNA level of CDK5 was determined by RT-PCR and the protein levels of CDK5 and its activator P35 and P25 were determined by Western blotting

in 0

3

6

12 and 24 h after exposure. The activity of CDK5 was quantified by liquid scintillation counting.    Results   There was no significant change in either mRNA or protein level of CDK5 in primary cultured cortical neurons after electromagnetic exposure at any time points (P0.05). The activity of CDK5 in primary cultured cortical neurons was increased significantly by 97% at 3 h and by 51% at 6 h after exposure (P0.05). Meanwhile

after the exposure

the protein level of P35 was not significantly changed (P0.05)

but that of P25 was significantly increased by 25% at 0 h

by 46% at 3 h and by 24% at 6 h (P0.05).    Conclusion   The electromagnetic irradiation induces significant increase of P25 protein level and CDK5 activity in primary cultured cortical neurons

indicating that P25/CDK5 may be involved in the process of the electromagnetic irradiation-induced neuron damage.