Magnetoreception: activation of avian cryptochrome 1a in various light conditions.

The avian magnetic inclination compass is based on radical pair processes, with cryptochrome (Cry) assumed to form the crucial radical pairs; it requires short-wavelength light from UV to green. Under high-intensity narrow-band lights and when yellow light is added, the magnetic compass is disrupted: migratory birds no longer prefer their migratory direction, but show other orientation responses. The candidate receptor molecule Cry1a is located in the shortwavelength-sensitive SWS1 cone photoreceptors in the retina. The present analysis of avian retinae after the respective illuminations showed that no activated Cry1a was present under 565 nm green light of medium and high intensity, and hardly any under high intensity 502 nm turquoise, whereas we found activated Cry1a at all three tested intensities of 373 nm UV and 424 nm blue light. Activated Cry1a also was found when 590 nm yellow light was added to low-intensity light of the four colors; yet these light combinations result in impaired magnetic orientation. This indicates that the disruption of the magnetic compass does not occur at the receptor level in the retina, but at higher processing stages, where the unnatural, almost monochromatic or bichromatic illumination causes yet unknown responses that interfere with the inclination compass.