Thus TSH acts locally within the hypothalamus, through stimulation of the TSH-receptor expressing cells, a response also seen in the quail . This leads to upregulation of expression of type II deiodinase , a key enzyme controlling thyroid hormone bioactivity. Dio2 converts thyroxine into the bioactive tri-iodothyronine in TSH-receptor expressing cells located in the Tofacitinib tanycytes of the ependymal layer of the hypothalamus. Increased expression of Dio2 should increase T3 availability within this region. In the quail, decreased expression of the T3 catabolizing enzyme, type 3 deiodinase has been reported in long days which should reduce the clearance of active T3 in long days. Following a switch in photoperiod from long day to short day , Dio2 decreases whereas the expression of Dio3 increases, which should produce a net decrease in hypothalamic T3 levels in SD. Consistent with this, the hypothalamic levels of T3 were shown to be ten fold lower in SD than LD whilst plasma levels were similar in both photoperiods in the quail . Similar changes in Dio2 gene expression occur in mammalian species such as the Soay sheep, Syrian hamster, the photoperiodically sensitive Fischer 344 rat, and mice . The Siberian hamster differs in that only Dio3 changes with photoperiod whilst no Dio3 was observed in the ependymal cells in the Syrian hamster . Despite this, it can be anticipated that the net effect of the changes in species including the Siberian hamster, would be high levels of hypothalamic T3 in LD and low levels during SD. Consistent with this, studies in the Siberian hamster have shown that central thyroid hormone metabolism plays a critical role in the seasonal control of body weight and reproduction . The F344 rat also shows profound reductions in food intake and body weight in response to SD . Previous investigations of hypothalamic genes involved in the food intake and body weight response to altered photoperiod in F344 rats demonstrated marked, but Epoxomicin customer reviews opposite changes in neuropeptide Y and agouti-related peptide expression in the arcuate nucleus . It was postulated that upregulation of AgRP in LD was associated with the higher levels of food intake, whereas upregulation of NPY in the SD was associated with a reduced drive for growth. Study of the thyroid hormone signalling system in response to photoperiod in F344 rats has been limited to consideration of Dio2 expression, which showed lower levels in SD than LD .