Melatonin-induced changes in the expression of thyroid hormone-converting enzymes in hypothalamus depend on the timing of melatonin injections and genetic background in mice.

Recent studies have identified TSHB, Dio2, and Dio3 as key genes for the photoperiodic regulation of gonads. In mammals, the expression of these genes is controlled by melatonin. Surprisingly, this effect of melatonin was shown to be conserved in several reproductively non-photoperiodic laboratory mouse strains that have thus become a valuable model to decipher the mechanisms through which melatonin controls the expression of TSHB, Dio2, and Dio3. In this study, we assessed the effects of intraperitoneal melatonin injections and of their timing on the expression of TSHB, TSHR, Dio2, and Dio3 in the hypothalamo-hypophysial systems of melatonin-proficient CBA/N and melatonin-deficient C57BL/6J mice kept under long-day conditions. In CBA/N mice, Dio3 expression was induced by a daily melatonin injection at ZT14 only, whereas in C57BL/6J mice, a daily melatonin injection induced Dio3 expression at all time points investigated (ZT8, 14, and 20) without changes in TSHB expression in both strains. Dio2 expression was suppressed by a daily melatonin injection only in C57BL/6J mice and only at ZT8. Effect of a daily melatonin injection on TSHR expression was strain- and region- specific. Melatonin levels elevated in plasma and hypothalamus after intraperitoneal injections of melatonin at ZT8 for 7days in C57BL/6J returned to basal levels within 1h after the final injection, while in CBA/N mice melatonin levels in hypothalamus remained high for at least 1h. These data suggest that Dio2 and Dio3 expression in the hypothalamus is differentially regulated by the timing of melatonin injections through strain-specific mechanisms.