Age-related alterations in cardiovascular responsiveness to clonidine infused into the nucleus tractus solitarii in rats.

The cardiovascular response to the unilateral injection of clonidine into the nucleus tractus solitarii in old compared to young rats was evaluated. In 3-month old rats clonidine (0.25, 0.5 and 1 microgram) injected into the nucleus tractus solitarii in anaesthetized rats produced a significant fall in blood pressure (BP) and a significant decrease in heart rate (HR). 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 solitarii did not produce any significant change in the cardiovascular parameters studied. In conclusion, the present experiments provide evidence that during ageing there is a progressive decrease in the cardiovascular response to alpha 2-adrenoceptor stimulation in the nucleus tractus solitarii. In addition, it is conceivable that such a decrease and subsequently the lack in response may be related to a progressive decrease in the number and/or affinity of the specific alpha 2-adrenoceptor binding sites at this level.