Time-dependency for the effect of different stressors on rat pineal melatonin content.

This study describes the effects of two stressors, swimming for 15 min or i.p. beta-carboline (beta-CCE) injection, on rat pineal melatonin levels when applied either at the end of the light period of daily photoperiod (1800 h) or at the expected melatonin maximum (at 2200 h). Neither procedure affected melatonin levels at 1800 h. In contrast, both stressors were effective to augment pineal melatonin content when applied at 2200 h. When applied at light-dark transition, only beta-CCE injection increased pineal melatonin levels at the middle of the night. Neither procedure was effective to counteract the inhibition of pineal melatonin content caused by prolonging the light phase for 4 h. Deprivation for two days of water, but not of food, augmented pineal melatonin levels in rats killed at the end of the light period, up to levels that were undistinguishable from those found at 2200 h. No differences were found between the two stressors and controls when tested at 2200 h. The present results further support the concept that stressful stimuli can influence differentially pineal melatonin production.