Unimpaired thermogenic response to noradrenaline in genetic (ob/ob) and hypothalamic (MSG) obese mice

The thermogenic response to noradrenaline administration was investigated at 25° in two models of obese mice (genetic ob/ob obesity of the ‘QEC’ strain and monosodium-glutamate-induced obesity) and in their respective lean littermates. Subcutaneous injections of a low dose of noradrenaline (I00 μg/kg body wt.) eJevated metabolic rate by about 3096 in both obese models but not in their respective lean counterparts. In contrast, the increase in metabolic rate after injections of a high dose of noradrenaline (600 μg/kg body wt.) was of a similar magnitude in both lean and obese animals: metabolic rate was increased by 70–80%. These results indicate that the overall whole body thermogenic capacity is unimpaired at room temperature in this ‘QEC’ strain of ob/ob mice and in the hypothalamic damaged obese mice. Obesity in these models is therefore not associated with a reduced ability to respond to noradrenaline but could rather be due to a failure to release noradrenaline.