Role of I1-imidazoline receptors within the caudal ventrolateral medulla in cardiovascular responses to clonidine in rats.
Although it is recognized that imidazoline receptors play an important role in the central regulation of cardiovascular activities, little is known about their role in the caudal ventrolateral medulla. In male Sprague-Dawley rats anesthetized with urethane, we used antagonists of I1-imidazoline receptor or alpha2-adrenoceptor to assess the function of these receptors in the caudal ventrolateral medulla in controlling the cardiovascular effects of clonidine. Unilateral microinjection of clonidine (6 nmol/50 nl) into the caudal ventrolateral medulla significantly (P<0.01) increased blood pressure and the discharge of the rostral ventrolateral medulla presympathetic neurons, while heart rate remained unchanged. Microinjection of yohimbine (a selective alpha2-adrenoceptor antagonist, 500 pmol/50 nl) into the caudal ventrolateral medulla did not modify blood pressure, heart rate, or the discharge of the rostral ventrolateral medulla presympathetic neurons, and failed to attenuate the local caudal ventrolateral medulla clonidine-induced blood pressure elevation. However, unilateral microinjection of idazoxan (a mixed antagonist of imidazoline receptor and alpha2-adrenoceptor, 2 nmol/50 nl) into the caudal ventrolateral medulla significantly (P<0.01) decreased mean arterial pressure, heart rate, and the discharge of the rostral ventrolateral medulla presympathetic neurons, and completely abolished the pressor effect of clonidine. In addition, bilateral microinjection of idazoxan (4 nmol in 100 nl for each side) into the caudal ventrolateral medulla effectively (P<0.01) blocked the depressor effects of clonidine administered intravenously (5 and 50 microg/kg). These results confirm that I1-imidazoline receptors within the caudal ventrolateral medulla are involved in maintaining the tonic cardiovascular activities and in the pressor effect of clonidine in the caudal ventrolateral medulla. In addition, it seems that the caudal ventrolateral medulla plays an important role in the antihypertensive effects of systemically administered clonidine in rats.
Wang
Wei-Zhong
WZ
Department of Physiology, Basic Medical College, Second Military Medical University, Shanghai, China. wwz1968@hotmail.com
Yuan
Wen-Jun
WJ
Ren
An-Jing
AJ
Pan
Yan-Xia
YX
Tang
Chao-Shu
CS
Su
Ding-Feng
DF
D000200:Action Potentials / Q000187:drug effects; D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D006339:Heart Rate / Q000187:drug effects; D019329:Idazoxan / Q000494:pharmacology; D054755:Imidazoline Receptors; D007275:Injections, Intravenous; D008297:Male; D008526:Medulla Oblongata / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008839:Microelectrodes; D008845:Microinjections; D009474:Neurons / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011955:Receptors, Drug / Q000037:antagonists & inhibitors / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D054755:Imidazoline Receptors; D011955:Receptors, Drug; C410887:imidazoline I1 receptors; D015016:Yohimbine; D003000:Clonidine; D019329:Idazoxan