Elevated corticosterone levels. A possible cause of reduced axon sprouting in aged animals.

Measurements of serum corticosterone taken at three times in the diurnal cycle (08.00, 18.00, and 23.30 h) showed that aged male Sprague-Dawley rats have higher nonstressed circulating levels at two time points measured. To determine if such elevated levels of steroids were sufficient to interfere with lesion-induced sprouting, the corticosterone peak at 18.00 h of either aged or young adult animals was maintained in young adrenalectomized rats by use of subcutaneous corticosterone pellets. The normal young adult animal levels resulted in mild suppression of the adrenergic sprouting response seen in hippocampus following transection of the fimbria-fornix. Young animals maintained at the elevated corticosterone levels of normal senescent rats had marked suppression of sprouting. The levels of circulating glucocorticoids reached by aged rats are sufficient to retard sprouting, and may therefore interfere with synaptic turnover and the response of the senescent brain to damage.