Growth hormone response to clonidine as a probe of noradrenergic receptor responsiveness in affective disorder patients and controls

The growth hormone (GH) response to the α-adrenergic agonist clonidine was blunted in 19 depressed patients compared to 20 controls. This difference remained significant when age- and sex-matched pairs of patients and controls were compared from this sample, either including or excluding subjects with elevated GH baseline levels. Plasma levels of free 3-methoxy-4-hydroxyphenylglycol (MHPG) were assayed in blood samples drawn just before the clonidine infusion. A modest negative correlation was found between the plasma MHPG values and the magnitude of the GH responses to clonidine, although baseline plasma MHPG levels were not significantly different between patients and controls. The diminished GH response to clonidine observed suggests that many depressed patients may have decreased α-adrenoceptor responsiveness. Decreased responsiveness may in some cases be associated with relatively increased indices of presynaptic noradrenergic availability. Such a model might have implications for understanding the functional status of the noradrenergic neurotransmitter system in depressed patients and the possible subtyping of affective disorder patients.