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Department of Psychological and Brain Sciences, Johns Hopkins University, Baltimore, Maryland 21218
Address all correspondence and requests for reprints to: Gregory F. Ball, Department of Psychological and Brain Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218. E-mail: gball{at}jhu.edu.
One of the few robust generalizations that can be made about the endocrine regulation of photoperiodic responses in mammals and birds is that thyroid hormones are important (1). The initial observation was made over 30 yr ago based on thyroid ablation studies in European starlings (2). The general finding is that an intact thyroid gland is required for photoperiodic animals to make the transitions among reproductive states that normally occur as the breeding season progresses (3, 4). For example, in avian species there are marked transitions in seasonally breeding temperate zone species such that they go from being photosensitive or in a state of reproductive responsiveness to stimulating daylengths (characteristic of the fall and early winter) to being photostimulated, namely possessing a fully developed reproductive system (exhibited by birds in the spring) to the state of photorefractoriness, which is characterized as now being nonresponsive to previously stimulating daylengths (which occurs in many wild bird populations in the late summer). Although there are important species differences in the exact effects of thyroid hormones on seasonal changes in reproductive physiology, in general the smooth progression of these transitions is dependent on the bird having an intact thyroid gland (3). In recent years, the cellular and molecular events responsible for these effects of thyroid hormones on the photoperiodic regulation of gonadal function have begun to be elucidated. The group of Yoshimura, Ebihara, Yasuo, and colleagues in Japan have made some of the most significant contributions to this story in the past few years (5, 6). The article in this issue of Endocrinology by Nakao et al. (7) describes yet another advance from this group. In this paper, they focus on the thyroid hormone transporter system that seems to play a key role in getting thyroid hormones into target cells. For many years, there was some debate as to whether thyroid hormones could enter cells via passive diffusion, as is often the case for steroid hormones. Both steroids and thyroid hormones exert their effects at least in part via intracellular receptors that are part of the same superfamily of genes (8). In recent years, a family of proteins that facilitate transport of thyroid hormones across the cell membrane known as the organic anion transporting polypeptides (Oatp) has been identified (9). Previously, work on thyroid hormone transport in relation to thyroid function and photoperiodic responses had been limited to a discussion of transporters transthyretin or T4-binding globulin that may facilitate hormone transport in the blood or into the brain via the choroids plexus (10). The identification of the Oatp family raises an entire new class of genes/proteins that may influence thyroid hormone function in relation to seasonal reproduction. In this study, with the help of the chicken genome project, Nakao et al. (7) have identified 10 Oatps in birds and have analyzed their expression in Japanese quail maintained on short or long days. Although domestic chickens do exhibit many traits regulated by photoperiod (11), these breeds are all descended from a tropical species and do not readily exhibit traits such as photorefractoriness (nonresponsiveness to previously stimulating daylengths). Japanese quail in contrast are highly photoperiodic and exhibit a form of seasonal photorefractoriness, and there is a very large literature on the photoperiodic regulation of reproduction in these birds that provides a basis by which to investigate thyroid function in relation to the photoperiodic regulation of reproduction (12). Nakao et al. (7) identified the expression of four genes in the choroids plexus and of even more interest of two genes in the tuberal hypothalamus. The region of the tuberal hypothalamus where they observed expression is the same part of the brain where they identified photoperiod regulation of the expression of enzymes involved in the metabolism of T4 into T3 the thyroid hormone metabolite thought to be most effective in regulating photoperiodic responses. With the use of a Chinese hamster ovary cell expression system, they have convincingly demonstrated that the protein expressed by the gene cOatp1c1 that is expressed markedly in the basal hypothalamus is a highly specific transporter for T4. One of the main take-home messages of this study is that there is high expression of a specific transporter for T4 precisely in the brain area, which previous work has identified as a key site of hormone action for the regulation of the photoperiodic response in quail. Thus, another piece in the puzzle of the cellular actions of thyroid hormones in relation to the photoperiodic control of reproduction has been found.
Another interesting aspect of this study by Nakao et al. (7) is that they found no evidence for photoperiodic regulation of the expression of the Oatp genes. Birds held on long-day conditions vs. short-day conditions showed no difference in the expression of cOatp1c1 and cOatp1b1 in the basal part of the tuberal hypothalamus. This is in contrast to a study by Prendergast et al. (10), who found that the onset reproductive refractoriness to short days in Siberian hamsters is associated with a down-regulation of genes coding for thyroid transporters such as transthyretin or T4-binding globulin. Expression was restored when the animals were exposed to long days and the inhibitory response to short days was restored. These proteins are of course a completely different class of thyroid transporters than that represented by the Oatp family of genes. However, we will have to wait for future studies to see whether there are qualitative species differences in the degree to which different aspects of the thyroid hormone system are modulated in relation to the regulation of the photoperiodic response in seasonally breeding species.
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Received November 30, 2005.
Accepted for publication December 2, 2005.
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