Bright light exposure reduces TH-positive dopamine neurons: implications of light pollution in Parkinson's disease epidemiology

Abstract

This study explores the effect of continuous exposure to bright light on neuromelanin formation and dopamine neuron survival in the substantia nigra. Twenty-one days after birth, Sprague-Dawley albino rats were divided into groups and raised under different conditions of light exposure. At the end of the irradiation period, rats were sacrificed and assayed for neuromelanin formation and number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra. The rats exposed to bright light for 20 days or 90 days showed a relatively greater number of neuromelanin-positive neurons. Surprisingly, TH-positive neurons decreased progressively in the substantia nigra reaching a significant 29% reduction after 90 days of continuous bright light exposure. This decrease was paralleled by a diminution of dopamine and its metabolite in the striatum. Remarkably, in preliminary analysis that accounted for population density, the age and race adjusted Parkinson's disease prevalence significantly correlated with average satellite-observed sky light pollution.

Figure 1. Light spectrum of the fluorescent lamp used to illuminate the animals.

(a) Light spectrum measured in the cages where the animals were housed and (b) light spectrum of the same lamp measured from the inside of the rat scalp and skull. The inset in panel (b) is a magnification of the spectrum between 400 and 500 nm to show the pick at 436.6 nm.

Figure 2. Fontana-Masson staining of neuromelanin granules in the substantia nigra and stereological counting of neuromelanin-positive neurons.

(a) Representative sections from rats raised in dim light – dark cycle (left panel) and rats raised in dim light – dark cycle for 70 days and than for 20 days under bright light (right panel). The asterisk on the right panel shows a cell with several tiny granules, while the arrow shows a large granule probably deriving from coalescence of tiny granules. Scale bar: 25 μm. (b) Stereological counting of neuromelanin-positive neurons in the substantia nigra of rats raised in dim light – dark cycle, and rats raised for 20 days or 3 months under bright light. *Significantly different from rats raised in dim light – dark cycle (F_7,7,7 = 51.42870, p = 0.00001, using one way ANOVA plus Scheffe's F test). The three subscript digits adjacent to the F value represent the numbers of rats used for the analysis, sequentially: rats raised in dim light – dark cycle, rats raised for 20 days under bright light and rats raised for 3 months under bright light.

Figure 3. TH immunohistochemistry and stereological counting of TH-positive neurons in ventral mesencephalon.

(a) Representative sections from rats raised in dim light – dark cycle (left panel) and rats raised in bright light for 3 months (right panel). Scale bar: 250 μm. (b) Stereological counting of TH-positive neurons in the substantia nigra of rats raised in dim light – dark cycle, and rats raised for 20 days or 3 months under bright light. *Significantly different from animals raised in dim light – dark cycle (F_10,8,8 = 5.01318, p = 0.01559, using one way ANOVA plus Scheffe's F test).

Figure 4. Levels of dopamine, DOPAC, 5-hydroxytryptamine and 5-HIAA in rats raised in dim light–dark cycle and rats raised for 20 days or 3 months under bright light.

The scale indicating dopamine and DOPAC levels is on the left y-axis while 5-hydroxytryptamine and 5-HIAA levels refer to the right y-axis. *Significantly different from animals raised in dim light – dark cycle (dopamine: F_10,8,8 = 9.01461, p = 0.00129; DOPAC: F_10,8,8 = 6.42800, p = 0.00606, using one way ANOVA plus Scheffe's F test).

Figure 5. TH immunohistochemistry and stereological counting of TH-positive neurons in ventral mesencephalon of the bilateral optic nerve transected rats.

(a) Surgical transection of the optic nerve. In panel 1 and 2 the curvilinear dashed lines indicate the incision of the scalp and the incision along the orbital rim to expose the retro-orbital tissue, respectively. Excess of retro-orbital fat (indicated by the arrow in panel 3) was removed and the orbit pulled forward to expose the optic nerve (indicated by the arrow in panel 4). The optic nerve was hooked with a surgical instrument (panel 5) and then cut. The arrow in panel 6 indicates the stump of the transected optic nerve. (b) Stereological counting of TH-positive neurons in the substantia nigra of rats raised in dim light – dark cycle, and rats raised under bright light for 4 months. *Significantly different from animals raised in dim light – dark cycle (t_8,8 = 5.87463, p = 0.00004, using two-samples t-Test). (c) Fontana-Masson staining of neuromelanin granules in the substantia nigra. Representative sections from rats raised in dim light – dark cycle (left panel) and rats raised under bright light (right panel). The asterisk on the right panel shows a cell with numerous tiny neuromelanin granules.

Figure 6. Similarity between Parkinson's disease prevalence and sky light pollution in U.S.A.

The prevalence of Parkinson's disease in U.S.A. (a) was reproduced from Willis et al. (with permission from S. Karger AG, Medical and Scientific Publishers, Allschwilerstrasse 10, 4009 Basel, Switzerland).The sky light pollution map of the U.S.A. (b) was obtained with permission from the Earth Observation Group (National Oceanic and Atmospheric Administration (NOAA) and the Department of Commerce, web site: http://www.ngdc.noaa.gov/dmsp/dmsp.html).