Loss of the diurnal variation of pyridostigrnine-induced growth hormone responses in depression

Abstract

Growth hormone (GH) plasma concentrations reflect a balance between stimulation via GH-releasing hormone and inhibition by somatostatin. Cholinergic agonists enhance GH release by inhibiting somatostatin secretion and in health, stimulated GH release undergoes diurnal variation. We investigated the influence of cortisol on pyridostigmine-induced GH responses by testing six patients with DSM-III-R major depression at 09.00 and 14.00 h. There were no differences in GH responses to pyridostigmine between 09.00 and 14.00 h despite a preservation of the circadian variation of cortisol levels. If cortisol plays an important role in regulating cholinergic activity one would expect the diurnal variation of pyridostigmine-induced GH release to be preserved. As it is not, a reasonable assumption to make is that the muscarinic supersensitivity observed in depression may be independent of the prevailing steroid milieu.