Cyclic mRNA expression of thyrotropin subunits and deiodinases in red drum, Sciaenops ocellatus

• T4 circulates with a robust daily rhythm in red drum.
• Pituitary TSH subunit gene expression cycles inversely to this T4 cycle.
• Pituitary Dio3, but not Dio2, expression mirrors the TSH subunit cycle.
• Dio3 may play a pivotal role in feedback regulation of TSH subunit expression.

The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T4), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T4 cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T4 feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T4 cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6–12 h out of phase with, the T4 cycle, consistent with the hypothesis that TSH secretion drives the T4 cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T4 providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.