János Bolyai Research Scholarship of the Hungarian Academy of Sciences
In diabetic subjects, early visual functional alterations such as color vision deficiencies
(CVDs) are known to precede clinically apparent diabetic retinopathy. Prominent photoreceptor
outer segment degeneration and an increase in the number of retinal dual cones (co-expressing
S- and M-opsins simultaneously) have been described in diabetic rat models, suggesting
a connection with the development of CVDs. As cone opsin expression is controlled
by thyroid hormones, we investigated the diabetic retina in association with thyroid
hormone alterations.In rat models of type 1 and 2 diabetes, dual cones were labeled
by immunohistochemistry, and their numbers were analyzed in relation to free triiodothyronine
(fT3) and free thyroxine (fT4) levels. Quantification of dual cones was also performed
in human postmortem retinas. Additionally, a cross-sectional case-control study was
performed where thyroid hormone levels were measured and color vision was assessed
with Lanthony desaturated D15 discs.A higher number of dual cones was detectable in
diabetic rats, correlating with fT4 levels. Dual cones were also present in postmortem
human retinas, with higher numbers in the three diabetic retinas. As expected, age
was strongly associated with CVDs in human patients, and the presence of diabetes
also increased the risk. However, the current study failed to detect any effect of
thyroid hormones on the development of CVDs.Our results point toward the involvement
of thyroid homeostasis in the opsin expression changes in diabetic rats and human
samples. The evaluation of the possible clinical consequences warrants further research.
