Adaptive changes in central and peripheral noradrenergic neurons in rats following chronic exercise

Abstract

The turnover rate of endogenous 14C-dopamine and 14C-noradrenaline after administration of 14C-tyrosine was studied in brain, heart and spleen of chronically exercised (trained) rats, while the animals were at rest. In the brains of trained animals the half lives of 14C-dopamine and 14C-noradrenaline were unchanged. However, the noradrenaline concentration of the brain was 26% greater in trained animals and so the calculated total amount of noradrenaline turning over per gram of brain tissue per hour was about one-third greater than in brains of untrained control rats. In peripheral organs there was a striking difference between trained and control rats in the half life of 14C-noradrenaline stores. In the hearts of trained rats, the apparent half life of 14C-noradrenaline was 79 h (statistically not significantly different from infinity), compared with 8.1 h in the heart of control rats. In the spleens, the corresponding apparent half lives of 14C-noradrenaline were infinity and 5.5 h, respectively. The results indicate firstly, that some noradrenergic neurons in the brain react to chronic increases in their impulse flow by increasing their stores of transmitter, just as do noradrenergic neurons in the heart and chromaffin cells in the adrenal medulla; secondly that chronically exercised animals display marked Adaptive changes in the functioning of peripheral sympathetic neurons, there being little if any sympathetic activity in heart and spleen during rest; and thirdly, that there is an inverse relationship between the changes in activity in central compared with peripheral noradrenergic neurons as a result of Adaptation to chronic exercise.