THE SUBSTANTIA NIGRA AND PARKINSON’S DISEASE
Parkinson’s disease (PD) is a relatively common progressive neurological disease manifested as a movement disorder, associated with tremors, stiffness and slowed movement. It is caused by a loss of neurons in the substantia nigra (“black substance”), a dark structure in the midbrain where dopamine is synthesized.
The dark color is due to substantial production of neuromelanin, a close relative to the skin-tanning agent, melanin. Depigmentation of the substantia nigra due to loss of neuromelanin is a hallmark feature of PD.31
Studies that have measured serum vitamin D levels have found significant differences in PD patients versus controls. One study that compared 186 PD patients with non-PD controls revealed that the PD patients had significantly lower bone density as well as significantly lower serum vitamin D levels compared to controls.32
Another study, based in China, compared 201 newly-diagnosed PD patients with 199 controls and likewise found that low serum vitamin D was linked to Parkinson’s.33
The Chinese study also used a questionnaire to determine whether the study participants took vitamin D supplements and how much sun exposure they obtained. The frequency of Parkinson’s disease in the group in the highest quartile of sun exposure was only half of the rate for those in the lowest quartile. Interestingly, serum vitamin D levels were highly correlated with degree of sun exposure but not with vitamin D supplementation.
One way in which sun exposure may be beneficial in Parkinson’s is through exposure to the eyes! Bright light therapy has been shown to benefit PD patients, both in improving sleep and mood, but also in improving motor function.34 A remarkable study on rats was able to measure the amount of light reaching the mesencephalon (midbrain, which houses the substantia nigra) when light was shone on the eyes.35
They demonstrated that a sharp peak was observed at around 710 nm, which is in the range of infrared light. It is likely that sunlight stimulates the synthesis of neuromelanin, just as it stimulates the synthesis of melanin in the skin. The neuromelanin then likely protects the dopaminergic neurons from oxidative damage by mopping up free radicals.
"Sun exposure is important not just to the skin but also to the eyes, and perhaps more crucially, to the structures in the brain stem behind the eyes that control circadian rhythms (pineal gland), brain function, movement, motor planning, reward-seeking, and learning (substantia nigra)."