Eyes as gateways for environmental light to the substantia nigra: relevance in Parkinson's disease

Abstract

Recent data indicates that prolonged bright light exposure of rats induces production of neuromelanin and reduction of tyrosine hydroxylase positive neurons in the substantia nigra. This effect was the result of direct light reaching the substantia nigra and not due to alteration of circadian rhythms. Here, we measured the spectrum of light reaching the substantia nigra in 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 nigra in 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 fissure as possible gateways for environmental light to reach the mesencephalon.

Figure 1

Schematic representation of the transcerebellar trajectory of the optical probe implanted in the rat substantia nigra. (a) The final coordinates of the probe tip were AP +4.0, L 2.0, and DV 2.5 from the interaural line, according to the Rat Brain Atlas of Paxinos and Watson. The black circle represents the eye position with respect to the brain. (b) Coronal section (30 μm thick) stained with Cresyl Violet to evaluate the position (arrow) of the optical probe within the mesencephalon.

Figure 2

Analysis of the fluorescent light spectrum penetrating into the substantia nigra. (a) Spectrum of the fluorescent tube registered by the microoptic fibre directly facing the lamp. (b) Representative light spectrum measured by the optical probe inside the rat brain. The black line and the red line refer to readings taken with covered and uncovered eyes, respectively. The arrows indicate the peaks at ~710 nm.

Figure 3

Light spectrum of two different computer monitors. (a) Hitachi CM615 cathode-ray tube (CRT) monitor. (b) ACER AL1916 liquid-crystal display (LCD) monitor. The arrows indicate the peaks at ~710 nm.

Figure 4

CT and MRI analysis of a human head to trace a possible pathway for light from the outside to the substantia nigra. (a) Human head 3D CT scan showing the large superior orbital fissure that gives access to the internal cavity of the skull. (b) Sagittal CT scan of a human head. It is evident from this prospective that the mesencephalon (indicated by the white ellipse) is behind the orbit, not hidden by the posterior clinoid process. (c) Axial MRI scan of a human head. This image shows that CSF occupies a large volume between the superior orbital fissure and the mesencephalon. Furthermore, CSF is present around the optical nerve in the intraconal space. The white line indicates the estimated distance (79.78 mm) between the cornea and the mesencephalon.