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 inje ction 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 dep endent. The stimulatory effect of melatonin on renal 5'-MD activity wa s observed only in 3- and 5-day-old rabbits and in the liver and BAT d uring 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 a nd rT(3) increases may result from the deiodinating processes by the t ype 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 secreto ry activity of the thyroid gland itself. These results are the first d escription of the effects evoked by melatonin treatment during the ear ly neonatal period in newborns of the altricial type.