Organ-specific changes in norepinephrine turnover against various stress conditions in thermoneutral mice

The effects of three stressors of different categories, namely cold exposure, immobilization, and lipopolysaccharide (LPS) treatment, on sympathetic nerve activity were examined by assessing its biochemical index norepinephrine (NE) turnover in peripheral organs of C57BL/6 mice. NE turnover was assessed by measuring the decrease in the organ NE concentration 3 h after inhibition of catecholamine biosynthesis with alpha-methyl-p-tyrosine. NE turnover in brown adipose tissue (BAT) in the room temperature (23 degrees C) control group was as high as that in the cold exposure (4 degrees C) group. Similarly, the mRNA level of the thermogenic marker uncoupling protein 1 (UCP1) in the room temperature control group was as high as that in the cold exposure group. As sympathetic stimulation upregulates the UCP1 mRNA level, we thought that sympathetic nerve tonus in BAT was already accelerated at room temperature. To exclude factors affecting basal sympathetic nerve activity, mice housed at thermoneutral temperature (30 degrees C) were used as controls for the subsequent experiments. In this condition, cold exposure accelerated NE turnover in the BAT, as well as heart and pancreas. The corticosterone level showed a higher trend in the cold exposure group in comparison to the control group. Immobilization accelerated NE turnover in the spleen, pancreas, and white adipose tissue and elevated the corticosterone level. LPS (3 mg/kg, i.p.) did not affect NE turnover in all peripheral organs but elevated the corticosterone level. In summary, the sympathetic nervous and adrenocortical responses to three stressors differed greatly. In particular, sympathetic responses showed clear organ-specific acceleration patterns. This important feature may improve our understanding of the multiplicity of biological responses.