Also cOatp1c1 was found to be present in abundance in the ependymal cells lining the ventro-lateral walls of the third ventricle, yet there was no detectable change in chicken mRNA expression with photoperiod . Thus, the local rapid equilibration of thyroid hormone levels possible with these transporters, which differ with photoperiod, may account for the difference in the level of detectable thyroid hormone in the rat compared to the quail . These results highlight not only that there may be differences in the regulation of thyroid hormone transport and metabolism between species, but also show how important it is to gain a complete picture of all the players and components regulating thyroid bioavailability in the AB1010 hypothalamus so as to fully Abmole SB203580 understand their relevance to physiological function. It also remains a possibility that there could be a phase angle difference in daily rhythmicity in the thyroid hormone and TRH responses. While the lack of large changes in hypothalamic T3 and T4 in this study, in contrast to quail , may seem surprising in view of the profound changes in gene expression of thyroid hormone related genes, there are physiological reasons why sustained perturbed changes in T3/T4 may not be expected. Thyroid hormones in the hypothalamus are known to feedback and inhibit TRH to provide a tightly regulated set-point of TRH and the thyroid endocrine axis . The unchanging hypothalamic thyroid hormone levels observed in this study and the modest change in TRH mRNA in the paraventricular nucleus are consistent with this model of set-point control. Nonetheless, it should be noted that photoperiod was not without some effect on TRH gene expression, causing a slight decrease in SD, indicating some effect of photoperiod on TRH neurons. Overall, the players necessary for the transduction of photoperiodic information, from the induction of TSHb and CGA in the PT, through retrograde signalling in to the hypothalamus to adjust deiodinase activities and thereby modulate local thyroid hormone signalling, all appear to be in place in the F344 photoperiodic rat, as described in other species. The lack of detectable thyroid hormone change may simply be because it is a transient event that is rapidly equilibrated after the downstream output is activated, emphasising the need to investigate hormone flux rather than static levels. These thyroid hormone-dependent changes are thought to impact on seasonal reproduction and body weight .