Stimulatory effect of melatonin on the 5′-monodeiodinase activity in the liver, kidney, and brown adipose tissue during the early neonatal period of the rabbit

The response of type I 5'-monodeiodinase activity (5'-MD) to a sc injection or oral administration of melatonin was studied in 3-, 5-, and 7-day-old rabbits. Melatonin-treated animals showed higher activity of the type I 5'-MD in the liver and kidney and of type II 5'-MD in brown adipose tissue (BAT). This respond to melatonin treatment was age dependent. The stimulatory effect of melatonin on renal 5'-MD activity was observed only in 3- and 5-day-old rabbits and in the liver and BAT during the first week of life. Oral melatonin administration tended to exert a more marked effect on enzyme activity than sc injection of the hormone. Changes in 5'-MD activities were accompanied by an increase in serum iodothyronine (T-4, T-3, and rT(3)) concentrations. The T-3 and rT(3) increases may result from the deiodinating processes by the type I 5'-MD and 5-MD, respectively, whereas the rise in the serum T-4 was probably due to the stimulatory effect of melatonin on the secretory activity of the thyroid gland itself. These results are the first description of the effects evoked by melatonin treatment during the early neonatal period in newborns of the altricial type.