Altered sensitivity of alpha 2-adrenoceptors in the brain during aging in rats.

The soporific and cardiovascular responses to the injection of clonidine into the locus coeruleus and nucleus tractus solitarius, respectively, were studied in rats of different ages. Clonidine (0.09, 0.19, 0.28, and 0.56 nmol) microinfused into the LC of young rats induced dose-dependent behavioral and ECoG SWS with a significant increase in total voltage in the lower frequency bands. In contrast to young rats, clonidine (0.19 and 0.28 nmol) given into the LC did not affect the behavior and ECoG spectrum power in old rats. However, after higher doses of clonidine (0.56 and 1.2 nmol) a small, short-lasting period of behavioral and ECoG SWS was still evident. In 3-month-old rats clonidine (0.25, 0.5 and 1 micrograms) injected into the nucleus tractus solitarius in anesthetized rats produced a significant fall in blood pressure and a significant decrease in heart rate. In contrast, in 12-month-old rats the maximum fall in blood pressure and heart rate was significantly less than in young animals. In addition, in older rats (24-month old) clonidine at the same or larger doses given into the nucleus tractus solitarius did not produce any significant changes in the cardiovascular parameters studied. In conclusion, the present experiments provide evidence that during aging there is a progressive decrease in the soporific and cardiovascular responses to alpha 2-adrenoceptor stimulation in the locus coeruleus and nucleus tractus solitarius, respectively.