Central .ALPHA.2-Adrenoceptor-Mediated Pressor Response to Clonidine in Conscious, Spontaneously Hypertensive Rats.

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Pressor responses to intracerebroventricular (i.c.v.) injection of clonidine were investigated in conscious spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). Clonidine (1-10 μg, i.c.v.) caused a dose-dependent pressor response and decrease in heart rate in both SHR and WKY. In SHR, low doses (1, 2.5 μg) but not high doses (5, 10 μg) of i.c.v.-clonidine induced a depressor response following the pressor response. Both pressor and depressor responses to i.c.v.-clonidine were significantly greater in SHR than in WKY. In both SHR and WKY, pressor responses to i.c.v.-clonidine were abolished by pentobarbital anesthesia, pretreatment with i.v.-furosemide (5 mg/kg), 24-hr water deprivation and pretreatment with i.c.v.-yohimbine (100 μg), but not by pretreatment with i.v.-yohimbine (100 μg) and i.c.v.-prazosin (10 μg). On the 1st day after surgery for arterial catheter implantation, SHR reduced their water intake, and i.c.v.-clonidine (5 μg) caused a slight pressor response, whereas the same dose of clonidine on the 7th day after surgery resulted in a marked pressor response. These results suggest that clonidine caused a central α<SUB>2</SUB>-adrenoceptor-mediated pressor response, which is greater in SHR than in WKY and is sensitive to body fluid volume changes and anesthesia.
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