AIM To investigate the roles of the
caudal ventrolateral medulla (CVLM) I 1 imidazoline receptors (I 1R) and α 2 adrenoceptors
(α 2 AR) in cental depressor mechanism of clonidine. MEHTODS Theeffects of locally administrated antagonists of I 1R and α 2 AR within CVLM on the resting
blood pressure (BP), heart rate (HR) and the depressor action of intravenously administrated clonidine were observed in urethane anesthetized SD rats. RESULTS Bilateral microinjection of either yohimbine (a selective antagonist of α 2 AR, 500 pmol·L -1 in 100 nl for each side, n =8) or idazoxan (a mixed antagonist of I 1R and α 2 AR, 2 nmol·L -1 in 100 nl for each side, n =10) not only produced significant ( P <0 01) decreases in both BP and HR, but also significantly ( P <0 01) attenuated clonidine induced depressor action. In addition, the attenuation of clonidine depressor action by idazoxan is more significant ( P <0 01) than by yohimbine. CONCLUSION Both I 1R and α 2 AR in the CVLM involved in maintaining the tonic cardiovascular activities and mediating the depressor action of clonidine.