Melatonin exerts direct inhibitory actions on ACTH responses in the human adrenal gland

In nonhuman primates and rodents, melatonin acting directly on the adrenal gland, inhibits glucocorticoid response to ACTH. In these species, an intrinsic adrenal circadian clock is involved in ACTH-stimulated glucocorticoid production. We investigated whether these findings apply to the human adrenal gland by determining i) expression of clock genes in vivo and ii) direct effects of melatonin in ACTH-stimulated adrenal explants over a) expression of the clock genes PER1 (Period 1) mRNA and BMAL1 [Brain-Muscle (ARNT)-like] protein, ACTH-induced steroidogenic acute regulatory protein (StAR), and 3β-hydroxysteroid dehydrogenase (3β-HSD) and b) over cortisol and progesterone production. Adrenal tissue was obtained from 6 renal cancer patients undergoing unilateral nephrectomy-adrenalectomy. Expression of the clock genes PER1, PER2, CRY2 (Cryptochrome 2), CLOCK (Circadian Locomotor Output Cycles Kaput) and BMAL1, was investigated by RT-PCR in a normal adrenal and in an adenoma. In independent experiments, explants from 4 normal adrenals were preincubated in culture medium (6 h) followed by 12 h in: medium alone; ACTH (100 nM); ACTH plus melatonin (100 nM); and melatonin alone. The explants' content of PER1 mRNA (real-time PCR) and StAR, 3β-HSD, BMAL1 (immuno slot-blot), and their cortisol and progesterone production (RIA) were measured. The human adrenal gland expresses the clock genes PER1, PER2, CRY2, CLOCK, and BMAL1. ACTH increased PER1 mRNA, BMAL1, StAR, and 3β-HSD protein levels, and cortisol and progesterone production. Melatonin inhibited these ACTH effects. Our study demonstrates, for the first time, direct inhibitory effects of melatonin upon several ACTH responses in the human adrenal gland.