Modulation of tyrosine hydroxylase gene expression in the rat adrenal gland by exercise: effects of age

Both aging and exercise are associated with alterations in circulating levels of catecholamines. To determine the interactions of age and exercise on tyrosine hydroxylase (TH) activity and TH mRNA, Fischer-344 female rats aged 5 months (young) and 25 months (old) were trained by treadmill running for 10 weeks. The elevation in maximum oxygen consumption in both groups was equivalent following exercise, indicating that training had occurred. In control rats, both TH activity and TH mRNA were greater in the older groups when compared with the younger animals. In young rats, exercise decreased TH activity by 25% and TH mRNA by 27%. In older rats, exercise was not associated with a decrease in TH activity and TH mRNA. Choline acetyltransferase activity (ChAT) was decreased and glutamic acid decarboxylase activity (GAD) was increased by exercise in young rats. The decrease in ChAT activity and increase in GAD activity suggest that transynaptic mechanisms play a role in the exercise-induced alteration of TH gene expression. Neither ChAT nor GAD was altered by exercise in older groups. Our data suggest that the previously reported diminution in catecholamines associated with exercise may be due to a decrease in TH mRNA and a resulting decrease in TH activity. There was no effect of exercise in the old rats, supporting previous observations that the plasticity of the sympathoadrenal system diminishes with age.