Tyrosine hydroxylase expression in rat adrenal medulla: influence of age and cold.

Chronic and cold exposure is associated with an increase in adrenal medullary tyrosine hydroxylase (TH) activity and expression that may be important for the regulatory response to cold. Senescent rats do not maintain their body temperature as well as young rats. We investigated the ability of the catecholaminergic system of older rats to respond to cold stimulus. TH activity, TH immunoreactivity, and TH mRNA were assessed in adrenal medullae of male F-344 rats of 3 and 24 months of age following 48 h of mild (8 degrees C) cold exposure. In control rats, basal levels of TH activity were increased by 2.9-fold, TH immunoreactivity by 1.3-fold, and TH mRNA by 2.3-fold with age. In the young rats there were increases after a 48-h cold exposure in TH activity, TH immunoreactivity, and TH mRNA per pair of adrenal medullae. In contrast, in senescent rats there were no significant changes in these parameters following cold exposure. These data suggest that the induction of TH activity is impaired in senescent rats following cold exposure and that there is a loss of plasticity with respect to the TH gene expression.