Abstract
The effects of clonidine on a high K+-induced release of noradrenaline (NA) were studied in 70-d- and 2-year-old rats. Treatment with 0.1 and 1.0 .mu.M clonidine significantly (p < 0.01) inhibited 20 mM KCl-evoked L-[3H]NA release from cerebral cortical slices in 70-d-old rats. The amounts of high K+-evoked L-[3H]NA release were markedly decreased at 2 years, compared to that at 70 d. In addition, the inhibitory effects of clonidine on high K+-evoked L-[3H]NA release were no longer observed in 2-year-old animals. These results suggested that presynaptic functions, including the modulation of NA release by .alpha.2-adrenoceptors, could become low in the brains of the 2-year-old rats.