This thread started in 2013 and it was updated in 2014. I'd like to bounce it up. Does anyone know whether research in this area continues?

To set the scene:

Romeo et al. wrote [1]:

"In the human subject analysed, this passage between the eye surface and the substantia nigra ... was 79.78 mm long ... . Within this length, 22.66 mm was comprised of the light transparent tissues of the eye (lens and vitreous humour) and 23.35 mm of liquid behind the superior orbital fissure inside the skull ... . ... [P]art of the remaining path could be inside the CSF around the optic nerve (9.73 mm). Taken together, the light transparent portion of this path could be as long as 55.74 mm, meaning that at some angle, only 24.04 mm of biological tissue stands between the external light environment and the substantia nigra in this patient."

and:

"[W]e have recently shown that continuous (three months) bright light exposure of rats induces formation of neuromelanin and reduces tyrosine hydroxylase positive neurons in the substantia nigra".

Wikipedia states [2]:

"Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA)."

PwP partially avoid this constraint because of the drugs that we take provide levodopa. But, many of us will be still producing endogenous levodopa and will need the TH for this.

I can (13 years post-diagnosis) still go overnight without drugs, and several hours in the morning before I need to take them. I've always thought of this being due to having reserves of levodopa, which build up during the night.

As a final point, if light does prove to reach the SN, it would suggest that wearing sunglasses, perhaps with lenses chosen to stop harmful frequencies of light, might be a partial therapy for PD.

References:

[1] "Eyes as Gateways for Environmental Light to the Substantia Nigra: Relevance in Parkinson's Disease"
Stefania Romeo, 1 Daniela Di Camillo, 2 Alessandra Splendiani, 1 Marta Capannolo, 1 Cristina Rocchi, 1 Gabriella Aloisi, 1 Irene Fasciani, 1 Giovanni U. Corsini, 3 Eugenio Scarnati, 1 Luca Lozzi, 2 and Roberto Maggio
ScientificWorldJournal 2014
Eyes as Gateways for Environmental Light to the Substantia Nigra: Relevance in Parkinson's Disease

[2] Wikipedia
Tyrosine hydroxylase - Wikipedia

John