Defective Stimulation of Thyroxine 5′-Deiodinase Activity by Cold Exposure and Norepinephrine in Brown Adipose Tissue of Monosodium Glutamate-Obese Mice

In order to examine the possible change in the thyroid hormone metabolism in the monosodium glutamate (MSG)-obese mice, we determined the iodothyronine deiodinase activity of brown adipose tissue (BAT), liver and kidney of male and female mice. There was no significant difference in the type II thyroxine 5'-deiodinase (T4 5'DII) activity in BAT between MSG-obese and control mice when they were kept at the ambient temperature of 22 degrees C. T4 5'DII activity in BAT of control mice increased markedly after exposure to cold (4 degrees C) for 4 h; however, the extent of cold-induced increase in T4 5'DII activity in BAT of MSG-obese mice was greatly reduced. Injection of norepinephrine (NE) (0.8 mg/kg, s.c.) 4 h previously increased T4 5'DII activity in BAT of control mice, but NE-induced increase in T4 5'DII activity was also markedly reduced in BAT of MSG-obese mice. Both cold- and NE-induced increase in T4 5'DII activity was greater in female, although similar tendency was obtained in male mice. Type I 3,3',5'-triiodothyronine deiodinase (rT3 5'DI) activity of liver and kidney, and serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels in MSG-obese mice were essentially the same as those of the control male and female mice irrespective of cold exposure. These results suggest that defective stimulation of T4 5'DII activity of BAT by cold in the MSG-obese mice is due to deficient sympathetic input to BAT and/or to diminished response of BAT to NE, and may contribute to a possible cause of inability of MSG-obese mice to maintain body temperature under cold exposure.