Tuesday,
December 9, 2008 - 4:00 P.M., NSH 202
Biological sensory systems continue to surprise physical scientists with sensitivities and capabilities often surpassing those of technical devices. Much is understood about the biophysical basis of vision, hearing, touch, smell, and taste. However, despite more than 40 years of behavioral biological studies, the receptors and receptor mechanism(s) underlying the magnetic sense of migratory birds and other animals remains enigmatic. This picture is changing in recent years, in part due to a better conceptual and quantitative formulation of the possible biophysical mechanisms that allow detection of the very weak geomagnetic field. Here, we focus on the radical pair model, which has emerged as a frontrunner hypothesis for the basis of the physiological compass of birds. The radical pair model proposes that certain photochemical reactions can be influenced by magnetic fields by virtue of the Zeeman interaction with electron spins. Magnetic sensing thus becomes an indirect effect on light sensing. We will show that the radical pair model can explain puzzling behavioral observations such as the fact that the bird's magnetic compass is sensitive only to the axis, but not to the polarity of the geomagnetic field. We present a novel form of behavioral in vivo resonance spectroscopy as a diagnostic tool: theory predicts that weak oscillating magnetic fields in the MHz frequency range will interfere with radical pair reactions and thereby lead to disorientation in behavioral tests. By exposing birds and other animals to oscillating magnetic fields at various frequencies and intensities and monitoring the orientation responses in behavioral experiments, we could identify resonance patterns that elucidate the underlying mechanism. We continue to explore this remarkable link between molecular level calculations and observations of responses of living animals to obtain information about the chemical nature of the receptor molecules. With the photoreceptor cryptochrome emerging as a magnetoreceptor candidate, we will present current efforts to verify this hypothesis with molecular biological and genetic tools, suggesting that we may be only step away from solving one of the longest standing mysteries of sensory biology.