Fluorescent histochemical demonstration of catecholamines in brown adipose tissue from obese (ob/ob) and lean mice acclimated at different temperatures.

Fluorescent histochemistry was used to visualize catecholamines in brown adipose tissue (BAT) of lean and genetically obese mice after they had been acclimated at different temperatures. At all temperatures, strong catecholamine-dependent fluorescence, attributable to the sympathetic innervation, was seen around the blood vessels of BAT from both lean and obese animals. Additionally, catecholamine-dependent fluorescent varicosities, in direct contact with the adipocytes were seen in abundance in lean mice acclimated at 23 degrees, 13 degrees or 4 degrees C and in obese mice acclimated at 13 degrees C. This latter compartment was greatly reduced in lean mice acclimated at 33 degrees C and in obese mice acclimated at 23 degrees and 33 degrees C. Three acute treatments (pretreatment with a monoamine oxidase inhibitor; 24 h food deprivation; and short-term cold exposure followed by short-term warm exposure) all increased the varicose fluorescence associated with adipocytes in obese mice housed at 23 degrees C, which suggests that the low resting level in these animals is attributable, at least in part, to subthreshold concentrations of catecholamines in existing varicosities rather than the absence of sympathetic varicosities per se. These results are in accordance with the results from noradrenaline turnover studies which suggest that the difference in sympathetic nervous system (SNS) activity in BAT from lean and obese (ob/ob) mice is best demonstrated at normal environmental temperatures. The reduced SNS activity in BAT of obese mice (which our studies show to be at the 'cellular' level) is likely to be a major factor in their reduced non-shivering thermogenesis and resultant high efficiency of energy storage as previously suggested by other workers.