Retinoic acid signalling within the hypothalamus is essential to the photoperiodic neuroendocrine response

All animals keep tight control of their body weight through a fine balance of food consumption and energy expenditure; you do not see fat animals in the wild. The region of the brain determining this balance is called the hypothalamus, the command centre that controls food intake and the metabolism that burns this off. Ideal animal models to study this system are those that undergo seasonal change, gaining weight when day length shortens (heading into winter) and losing weight when day length increases (heading into summer); by altering the number of hours of daylight exposure the hypothalamus can be switched to command either weight gain or weight loss and the mechanisms that control this switch can be studied and understood. Recent and emerging work which has begun to shed light on these mechanisms is generating many surprises. Thyroid hormone is normally produced by the thyroid gland and it is known for its effects on metabolic rate. However within the brain enzymes are able to convert thyroid hormone into its bioactive form and back again. Day length (the length of the light-dark cycle) controls the local production of thyroid hormone within the hypothalamus and this in turn is important to seasonal control of body weight and reproduction. We have now discovered an entirely new system of control in the hypothalamus that derives from an essential part of our diet, vitamin A. This vitamin is converted to a potent control switch of gene expression called retinoic acid that acts in a similar way to thyroid hormone. Also like thyroid hormone, retinoic acid can be produced locally within the hypothalamus of the brain, via the appropriate enzyme, and it can then be transported and detected by specialised receptors that activate target genes. Once again day length determines the ability of retinoic acid to control these genes in the brain by changing the production and state of sensitivity to retinoic acid. In animals on summer day length, the animals appear to make retinoic acid and receptors are sensitive to it, while those exposed to a winter day length appear to make less retinoic acid and receptors are less sensitive to it. This project will study the sophisticated mechanisms that underlie this retinoic acid controlled switch and determine how it may be manipulated to control body weight. We aim to identify where, when and how the brain changes in its responsiveness to retinoic acid and we will do this in animals (rats) that respond to changes in day length. We will also test whether the brain produces more retinoic acid in some conditions relative to others (ie summer versus winter day length). An important aim of this project is to understand the sequence of functional steps by which day length can alter retinoic acid signals within the brain and in turn we want to identify how retinoic acid then alters physiological status in terms of food intake and growth at a CNS level. Given the recognition that vitamin A deficiency is a major nutritional problem in many parts of the world, the unexpected discovery of an entirely new signalling mechanism in the hypothalamus, which controls hypothalamic function is clearly of global relevance and importance. The potential of manipulating the retinoic acid signalling pathway to influence food consumption and obesity may offer some novel therapeutic opportunities.

Photoperiod exerts profound effects on the physiology (body weight, growth and reproduction) in seasonal mammals through changes in the neuroendocrine axis. At the same time photoperiod stimulates marked changes in genes (CRBP1, RAR and RXRgamma) involved in retinoid signalling within the hypothalamus. This strongly suggests that retinoid signalling plays an important role in photoperiodic control of the neuroendocrine hypothalamus. Using photoperiodic F344 rats as a model, we will examine our hypothesis that thyroid stimulating hormone (TSH), synthesised and secreted by the pars tuberalis of the pituitary, mediates sustained expression of genes and proteins involved in retinoid transport and signalling within the hypothalamus seen in long photoperiod. This maintains RA at an appropriate level to act either independently or in concert with T3 to maintain the long photoperiod phenotype. This project aims to test this hypothesis by examining the sequential steps involved. In the first part of the study TSH activity will be blocked by knock down of the TSH receptor in the ependymal layer of the hypothalamus using viral mediated shRNA or conversely activity through the TSH receptor will be increased by injection of TSH into the 3rd ventricle. Changes in retinoid gene expression (Stra6, CRBP1, RAR and RXRgamma) together with food intake, growth and testes size will be measured. In the second part of the study, RA synthesis will be blocked by either disulfiram implants into the hypothalamus, or shRNA against RALDH, to test whether this inhibits food intake and growth. Conversely we will test whether RA administration can restore food intake and growth. The final part of the study will examine downstream RA-regulated gene expression by Affymetrix microarray. This study will also differentiate between retinoid signalling genes directly regulated by RA and those regulated by photoperiod.