Serum Melatonin after a Single Aqueous Subcutaneous Injection in Syrian Hamsters

摘要

The reproductive system regresses and serum thyroxine concentration falls in male and female Syrian hamsters within 2-3 months during exposure to a short photoperiod (less than 12 h light/day) or after surgical blinding. These endocrine effects of reduced photoperiod are blocked by pinealectomy. They have been mimicked in intact animals kept in a long photoperiod when the pineal hormone melatonin was injected for 2-3 months. A single injection of 25 micrograms melatonin was given subcutaneously in 0.1 ml saline either at 3 h into the light phase (morning) in female Syrian hamsters or at 1 h before the beginning of the dark phase (evening) in males (L/D 14/10 h). In both cases, the maximal mean serum values seen at 20 min after injection (50 ng/ml) were more than 1,000 times the normal nocturnal melatonin concentration. By 1 h after injection, serum melatonin had fallen to 10% (morning) or 4% (evening) of the respective 20-min value. The average half-times for plasma concentration during the second hour after injection were 14 min in the morning and 13 min in the evening. After either morning or evening injection, serum melatonin reached normal daytime concentration between 2 and 4 h after injection.