Differential inhibitory noradrenergic responses to clonidine in 25 depressed patients and 25 normal control subjects.

The authors measured plasma 3-methoxy-4-hydroxyphenylglycol (MHPG), plasma norepinephrine, blood pressure, and heart rate responses to the alpha 2-adrenergic agonist clonidine in 25 depressed patients and 25 normal control subjects. In the control subjects clonidine reduced plasma norepinephrine, blood pressure, and heart rate significantly more than placebo. In the depressed patients clonidine reduced blood pressure and the percent fall in plasma norepinephrine but not plasma MHPG or heart rate significantly more than placebo. The absolute and percent reductions in plasma MHPG and heart rate following clonidine were significantly less than in control subjects. These results raise the possibility that the sensitivity of the alpha 2-adrenergic receptors inhibitory to noradrenergic output may be reduced in depression.