Thyroid hormone affects colour vision, according to new European research
European researchers have made new discoveries which uncover a link between thyroid hormone deficiency and colour vision. The study, published in the Journal of Neuroscience, challenges the previously held assumption that colour sensitivity of the retina's cone cells is fixed within the adult retina. The researchers, from Germany and Austria, have shown that thyroid hormone plays an important role in the development of the eye and in particular the cone visual cells. Cone cells are the visual cells responsible for colour vision. Thyroid hormone has a crucial role during the body's development as well as that of the nervous system. Children born with a thyroid hormone deficiency often have serious physiological and mental deficiencies. This is why newborn babies are routinely checked for thyroid hormone deficiency, and hormone substitution therapy is given when indicated. By carrying out studies on mice and rats, the team were able to show that in mature mice and rats, the production of their visual pigment is regulated by the hormone. Based on the assumption that this mechanism exists in all mammals, it can be concluded that there is a high chance the production of the visual pigment is also regulated by the thyroid hormone in humans. Most mammals have two spectral cone-types containing either one of two visual pigments (opsins), one sensitive to shortwave light (UV/blue opsin), the other to middle-to-long wave light (green opsin). Cones express a thyroid hormone receptor, and its activation by the hormone suppresses the synthesis of UV/blue opsin and activates the production of green opsin. Until now, the control of opsin production by thyroid hormone was considered a developmental phenomenon. Experts assumed that in mature cones the developmentally established 'opsin programme' is fixed and needs no further regulation. The researchers began by analysing thyroid hormone involvement in the early postnatal development of mouse cones. 'We wanted to know how long the time window for the hormone effect was, at what point the hormone's influence on opsin production stopped', says Anika Glaschke from the Max Planck Institute for Brain Research, Germany. 'To our surprise we did not find such an endpoint, even several weeks after birth there was a hormone effect.' The team then analysed the cones in adult mice and rats that had been rendered hypothyroid for several weeks. In these mice, all cones switched to the production of UV/blue opsin and reduced green opsin production. After ending the treatment, hormone levels returned to normal and the cones reverted to the production of their 'regular' opsin - one cone type to green opsin, the other to UV/blue opsin. The researchers conclude that the spectral cone types, which are defined by the opsin they express, are dynamically and reversibly controlled by thyroid hormone throughout life. 'In addition to their importance for basic retinal research, our findings may also have clinical relevance', says Martin Glösmann, one of the researchers who took part in the study from the University of Veterinary Medicine, Vienna. 'If this mechanism also acts in human cones, the adult-onset of thyroid hormone deficiency - e.g. as a consequence of dietary iodine deficiency or removal of the thyroid - would also affect the cone opsins and colour vision'.For more information, please visit:Max Planck Institute for Brain Research:http://www.mpg.de/en
Countries
Austria, Germany