Regulation of neurite outgrowth by intermittent irradiation of visible light.

The effect of neurite outgrowth of PC12 cells on collagen-coated glass plates under intermittent light irradiation at 525 nm and 0.4 mW/cm2 of intensity was investigated. Neurite outgrowth of PC12 cells was significantly suppressed when PC12 cells were cultivated under intermittent light irradiation with a total irradiation time of more than 2 min/h. No temperature increase was observed in the culture medium under either continuous or intermittent light irradiation. Therefore, suppression of neurite outgrowth under light irradiation was not due to the increase of temperature in the culture medium, but rather the effect of light on the PC12 cells, especially the signal transmittance of light to PC12 cells. The light irradiation interval also affected the neurite outgrowth of PC12 cells when the total irradiation time was constant. A high extension ratio of neurite outgrowth was observed under a long time interval of nonirradiation between light irradiations (1 min of irradiation every hour) as compared with frequent light irradiation intervals (5 s of irradiation every 5 min) with the same total irradiation period per hour. The neurite outgrowth ratio was thought to be dependent on the light intensity, the total time of light irradiation in the intermittent light irradiation, and the interval of light irradiation in the intermittent light irradiation.