Hypothalamic regulation of appetite by thyroid hormones

Thyroid hormone is one of the most important mechanisms for controlling body function. The effects of a large excess or the absence of thyroid hormone is well known. However despite its importance of thyroid hormones their role in the day to day regulation of food intake is unknown. It is known that a large excess of T3 increases metabolic rate and this is thought to result in an indirect increase in appetite. However, recently we found that low doses of thyroid hormone increased food intake without affecting metabolic rate. This effect seemed to be mediated by part of the brain called the hypothalamus, specifically a region of the hypothalamus called the ventromedial nucleus (VMN). We now plan to establish the role of thyroid hormone in the VMN in the day to day regulation of food intake and energy expenditure. Recently mice which have been genetically altered to allow levels of thyroid hormone or thyroid hormone receptors to be reduced in specific regions of the brain have been produced. Using these mice we will establish the role of thyroid hormones in regulating food intake on a day to day basis and identify the receptor which mediates these effects. This work will allow us to establish the role and importance of thyroid hormone in regulating food intake.

Whilst a great deal of information is known about the importance of thyroid hormones in development and their pharmacological effects, their physiological role is still unclear. Recently it has become evident that tissue levels of thyroid hormone are regulated by the balance of local enzymatic activation and inactivation rather than the plasma levels of the hormones. We have recently shown that tlow doses of thyroid hormone increase food intake without affecting metabolic rate; an effect that is mediated via the ventromedial nucleus (VMN) of the hypothalamus. The work in this grant is to establish the physiological role and importance of thyroid hormones in the VMN in regulating food intake. To achieve this we will utilise three newly developed strains of transgenic mice. The strains of transgenic mice are pROSA26D3 which has a floxed stop transcription cassette between the ROSA26 promoter and deiodinase-3 (D3), the enzyme responsible for inactivating thyroid hormone. These mice will be stereotaxically injected with recombinant adeno-associated virus expressing a Cre-GFP fusion (AAVCre-GFP) into the VMN. This will result in decreased levels of T3 within this nucleus and thus its physiological role in the regulation of food intake can be established. The other mice are TRaflox/flox and TRbflox/flox in which Cre recombinase sites have been inserted into either the thyroid receptor a or b genes which allows specific inactivation of these receptors using Cre. Mice which are homozygous either for TRaflox/flox or TRbflox/flox or bothTRaflox/flox and TRbflox/flox will be injected with AAVCre-GFP into the VMN to specifically delete the receptor(s) within this hypothalamic nucleus. This will allow the identity of the receptor through which T3 exerts its effects on food intake to be identified. Together the experiments in this proposal will establish the physiological role and importance of T3 in the regulation of food intake and the receptor through which it acts