Melatonin and glucocorticoid hormones.

The synthetic glucocorticoid dexamethasone (2.5, 5 or 10 micrograms/day), administered in the drinking water for 7 days, significantly reduced body weight gain, thymus weight and adrenal weight in female rats. Concomitant administration of melatonin (0.2 or 1.0 mg, i.p.) at either 8.00 or 16.00 h did not alter the glucocorticoid effect on any of these parameters. In competition binding studies melatonin did not compete with 3H-dexamethasone for binding to glucocorticoid receptors in thymus cytosol from adrenalectomized rats. By these indices, therefore, melatonin does not appear to be a glucocorticoid antagonist.