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

A New Threat to Dopamine Neurons: The Downside of Artificial Light

Growing awareness of adverse impacts of artificial light on human health has led to recognize light pollution as a significant global environmental issue. Despite, a large number of studies in rodent and monkey models of Parkinson's disease have reported that near infrared light has neuroprotective effects on dopaminergic neurons, recent findings have shown that prolonged exposure of rodents and birds to fluorescent artificial light results in an increase of neuromelanin granules in substantia nigra and loss of dopaminergic neurons. The observed detrimental effect seems to be dependent on a direct effect of light on the substantia nigra rather than a secondary effect of the alterations of circadian rhythms. Moreover, inferences from animal models to human studies have shown a positive correlation between the prevalence of Parkinson's disease and light pollution. The present article discusses experimental evidence supporting a potentially deleterious impact of light on dopaminergic neurons and highlights the mechanisms whereby light might damage neuronal tissue. Moreover, it analyses epidemiological evidence that suggests light pollution to be an environmental risk factor for Parkinson's disease.

The effect of directed photic stimulation of the pineal on experimental Parkinson's disease

The role of the circadian system in Parkinson's disease (PD) is a topic of increasing scientific interest. This has emerged from recent studies demonstrating an altered response of PD patients to treatment in relation to the phase of the light/dark cycle and from other work defining the functional significance of melanocytes in PD: a cell type that the nigro-striatal dopamine (NSD) system and circadian system both contain. The present study was undertaken to determine the sensitivity of the pineal, as the final common pathway of the circadian system, to light delivered directly to the pineal via surgical implantation of LEDs. Direct photic stimulation of the pineal altered the course of experimental PD while anatomical controls receiving stimulation of the frontal cortex exhibited a negative impact on the course of recovery of these animals. These effects were closely linked to the phase of the light/dark cycle. The present results suggest that while pineal photoreceptors are regarded as vestigial, functional photo-reactivity of the pineal remains. It is inferred that melanocytes are the active cells responsible for the observed effect since they remain functionally intact in mammalian pineal even though pineal photoreceptors are functionally inert. Although the stimuli applied in the present study may be regarded as artificial this study demonstrates that brain parenchyma remains differentially reactive to direct light exposure and presents a novel mechanism in circadian structures that needs to be explored.

Fluorescent light induces neurodegeneration in the rodent nigrostriatal system but near infrared LED light does not

We investigated the effects of continuous artificial light exposure on the mouse substantia nigra (SN). A three month exposure of C57Bl/6J mice to white fluorescent light induced a 30% reduction in dopamine (DA) neurons in SN compared to controls, accompanied by a decrease of DA and its metabolites in the striatum. After six months of exposure, neurodegeneration progressed slightly, but the level of DA returned to the basal level, while the metabolites increased with respect to the control. Three month exposure to near infrared LED light (∼710nm) did not alter DA neurons in SN, nor did it decrease DA and its metabolites in the striatum. Furthermore mesencephalic cell viability, as tested by [³H]DA uptake, did not change. Finally, we observed that 710nm LED light, locally conveyed in the rat SN, could modulate the firing activity of extracellular-recorded DA neurons. These data suggest that light can be detrimental or beneficial to DA neurons in SN, depending on the source and wavelength.

640-Slice CT Measurement of Superior Orbital Fissure as Gateway for Light into the Brain: Statistical Evaluation of Area and Distance

OBJECTIVE

Our aim was to provide normative data concerning superior orbital fissure area (SOFA), ocular skin and the substantia nigra (D-SS) and orbital fissure and the substantia nigra (D-SOF-S) distances by CT scan in adult Caucasian population.

METHODS

The area of the superior orbital fissure (SOF), the distance between the ocular skin and the substantia nigra and the distance between the superior orbital fissure and the substantia nigra using CT and 3D-CT images.

RESULTS

Normative data stratified for age and gender were obtained. The data here reported show that some degree of variability in SOFA, D-SS and D-SOF-S measurements can be observed healthy Caucasian subjects. Gender stratified prediction intervals (mean +/- 2 Standard Deviations) for SOFA and D-SOF-S were 69.2 (+/-15.8) and 38.4 (+/-7.6) for male and 56.8 (+/-11.9) and 36.5 (+/-6.1) for female, respectively. Age and gender significantly impacted on D-SS values and normative data were constructed generating data stratified for these two variables. D-SS was 89.4 (+/-10.3) and 86.4 (+/-9.7) for male and female, respectively.

CONCLUSIONS

Here we provide adjunctive anatomical information on specific anatomical cerebral zones. Our data may have implications for surgeons actively committed to treat pathological conditions involving these cerebral areas. Additionally, the anatomical variability found with respect to SOF and the potential different exposure of the substanzia nigra to the bright light could play a role in Parkinson's disease as already speculated in literature.