Eyes as Gateways for Environmental Light to the Substantia Nigra: Relevance in Parkinson’s Disease

Recent data indicates that prolonged bright light exposure of rats induces production of neuromelaninand reduction of tyrosine hydroxylasepositive neurons in the substantia nigra. This effect was the result of direct light reaching the substantia nigraand not due to alteration of circadian rhythms. Here, we measured the spectrum of light reaching the substantia nigrain rats and analysed the pathway that light may take to reach this deep brain structure in humans. Wavelength range and light intensity, emitted from a fluorescent tube, were measured, using a stereotaxically implanted optical fibre in the rat mesencephalon. The hypothetical path of environmental light from the eye to the substantia nigrain humans was investigated by computed tomography and magnetic resonance imaging. Light with wavelengths greater than 600 nm reached the rat substantia nigra, with a peak at 709 nm. Eyes appear to be the gateway for light to the mesencephalon since covering the eyes with aluminum foil reduced light intensity by half. Using computed tomography and magnetic resonance imaging of a human head, we identified the eye and the superior orbital fissureas possible gateways for environmental light to reach the mesencephalon.