Age-related decrease in centrally-mediated pressor response to clonidine in conscious rats.

An age-related change in pressor response to intracerebroventricular (i.c.v.) injection of clonidine in conscious rats was investigated. In 15-week-old rats, clonidine (2 to 20 micrograms) caused a dose-dependent pressor response concomitant with decrease in heart rate. The depressor response to i.c.v. clonidine at lower doses (2 and 5 micrograms) but not higher doses (10 and 20 micrograms) was observed. The pressor response to clonidine in 20- and 30-week-old rats was markedly diminished, and the depressor response appeared at any of the doses injected. However, no age-related change in heart rate responses was observed. These results suggest that the centrally mediated pressor response to clonidine is age-related and masks the depressor response.