Melatonin and N-acetylserotonin stress responses: effects of type of stimulation and housing conditions.

The effects of housing condition and type of stimulation on serum melatonin and N-acetylserotonin (NAS) were investigated. Male rats were housed under a 12/12-hour light-dark cycle, with ad libitum food and water, either individually or in groups of four. At the start of the light phase, separate groups were sacrificed at rest or subjected for 3 minutes to the stimulation of cold water, noise, novel environment, or ether vapour and then decapitated at 0, 5, 15, 30 or 60 minutes after the end of stimulation. Melatonin was measured by a modified radioimmunoassay and NAS by a specific radioimmunoassay. Melatonin levels responded to stimulation with an increase, while NAS levels responded with a decrease. Housing condition had no effect on hormone response. However, the pattern of response for each of the two hormones differed greatly among the stimuli. For melatonin, cold water was the most potent stimulus, followed by noise, novel environment, and ether. NAS responded most to ether, fleetingly to cold, and in a bimodal manner to noise. The data are interpreted as suggesting that separate mechanisms regulate serum melatonin and serum NAS is response to environmental stimulation and that under appropriate control conditions melatonin from the pineal is very responsive to environmental stimuli, in a manner similar to that of pituitary hormones.