Chronic clonidine treatment and its withdrawal: effects on blood pressure and catecholamine synthesizing enzymes in brain-stem nuclei.

We have studied the effects of withdrawal from chronic clonidine treatment in the adult male spontaneously hypertensive rat (SHR). SHR received clonidine, 0.1 mg X kg-1 X day-1 i.v. for 10 days. Clonidine was delivered via osmotic minipumps. After 7 days of treatment there was a 16.5 +/- 2.5 mm Hg fall in mean arterial pressure. This was accompanied by a decrease in the dopamine-beta-hydroxylase and phenylethanolamine-N-methyl transferase activities of the A1/C1 region. Withdrawal from clonidine was characterized by tachycardia and an increase in mean arterial pressure and heart rate lability. Phenylethanolamine-N-methyl transferase of the the dopamine-beta-hydroxylase activity remained diminished. The dopamine-beta-hydroxylase activity of the A2/C2 region was also diminished during withdrawal. We suggest that the blood pressure lowering effect of clonidine is accompanied by a decreased capacity to synthesize adrenaline in the A1/C1 region where adrenaline could mediate a pressor effect. Increased blood pressure lability during withdrawal is accompanied by a restoration of synthesis of adrenaline in the A1/C1 region. There is also a decrease in the capacity of synthesis of noradrenaline in the A2/C2 region where adrenaline may mediate a vasodepressor effect.