Thyroid hormone receptor-β gene expression in the brain of the frog Pelophylax esculentus: Seasonal, hormonal and temperature regulation

Thyroid hormone receptor-β (trβ) cDNA was identified in the adult of Pelophylax esculentus (previously: Rana esculenta), a seasonally breeding species, in order to detect spatial brain trβ expression, its levels through the seasons and in response to 6-n-propyl-2-thiouracil, T4 and T3 administrations as well as to thermal manipulations. The deduced amino acid sequence of P. esculentus trβ showed a high similarity to the homologous of other vertebrates. By in situ hybridization we found trβ mRNA signal in the anterior preoptic nucleus, the habenulae, the hypothalamic-pituitary region and the ependyma. Brain trβ transcript levels varied through the seasons, and they were well correlated with brain T4 levels but only partially with T3 levels. Experimentally-induced hypothyroidism decreased brain trβ expression. The administration of exogenous thyroid hormones increased brain trβ expression, with T4 appearing more potent than T3. The experiments of thermal manipulations further strengthen the hypothesis that T4 is more effective than T3 in brain trβ regulation. This study also shows that, as in other vertebrates, deiodinase enzymes could modulate trβ expression via thyroid hormone regulation. © 2012.