Abstract
Introduction: Retinal cone photoreceptor cells contain short (S) and medium (M) wavelength opsins, which are light-sensitive substances involved in color vision and visual acuity by sensing lights of different wavelengths. Thyroid hormones promote M-opsin expression and suppress S-opsin expression during the differentiation of cone photoreceptors. It was previously reported that M-opsin expression was delayed and S-opsin expression increased in TSH receptor-deficient mice and methimazole-induced hypothyroid mice. In addition, no M-opsin expression and increased S-opsin expression were observed in thyroid hormone receptor (TR) β2-deficient mice (Ng L et al, Nature Genetics. 2001; 27(1): 94-98.). This suggested that impaired thyroid function affects opsin development. We therefore examined retinal development in TRH-deficient mice, which are a model of central hypothyroidism established in our laboratory. Methods: We performed HE staining of the retina at postnatal 30 days and electroretinography at postnatal 10 weeks using TRH-/- and wild-type (WT) mice. We also examined expression levels of S/M opsin mRNA in WT, TRH-/- and TRH-/- pups born from TRH-/- dams at postnatal 12,17 and 30 days, and TRβΔ337T knock-in mice (TRβmut/mut) at postnatal 30 days. Furthermore, we performed immunohistochemistry to examine S/M opsin protein expression in these mice. Results: The retinal structures by HE staining and retinal functions by electroretinography in TRH-/- mice were unchanged compared with those in WT mice. Although M-opsin expression was not detected and S-opsin expression was higher in TRβmut/mut mice than in WT mice, the mRNA and protein expression levels of S/M-opsin did not significantly differ between TRH-/- pups born from TRH+/- dams and WT pups at all postnatal days. TRH-/- pups born from TRH-/- dams exposed to maternal hypothyroidism had similar serum total T4 levels to TRH-/- pups born from TRH+/- with normal maternal thyroid function. In contrast, the mRNA expression level of M-opsin was significantly lower (1.00±0.06 vs 0.64±0.05: mean ± SE, p<0.01) and the protein expression level was lower in TRH-/- pups born from TRH-/- dams than in WT pups at postnatal 12 days. However, these differences disappeared after postnatal 17 days, and there was no difference in M-opsin expression in TRH-/- pups born from TRH-/- dams compared with WT pups. Conclusions: Although no delay in opsin development was observed in TRH-/- pups born from TRH+/- dams, TRH-/- pups born from central hypothyroid dams exhibited delayed opsin development, suggesting that maternal hypothyroidism affects the development of retinal opsin in the neonatal period.
Hypothyroidism and Thyroid Function Alterations During the Neonatal Period