Administration of the pineal hormone melatonin (1 mg/day) to growing female rats provided significant protection against the injurious effects (decrease of body weight gain and atrophy of the thymus and adrenals) caused by a glucocorticoid, dexamethasone (0.4-0.01 mg/day). The protective effects were greater at lower doses of dexamethasone, and the adrenal/body weight ratio was well maintained. Histological examination of the thymus and adrenals confirmed both the injurious effect of dexamethasone and the protective effect of melatonin. Dexamethasone also increased glutamic pyruvic transaminase, free fatty acid, triglyceride and glucose in the serum, and melatonin significantly reduced the former two changes. It is proposed that the protection afforded by the pineal hormone melatonin against the injurious effects of dexamethasone is due to a direct anti-glucocorticoid action and dose not involve any other endocrine organ.
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