Extrapineal melatonin: analysis of its subcellular distribution and daily fluctuations.
Whereas there is little doubt that melatonin is an important hormone which mediates the effects of the pineal gland, there is debate concerning the nature of the melatonin message which the animal interprets. This brief resume considers the two main features of the melatonin rhythm which the organism could "read" to determine whether it is in a long or a short day. The first scheme is what is referred to as the duration hypothesis. This hypothesis depends on the fact that the changing photoperiod likewise alters the duration of the daily melatonin peak and this signals the organism as to daylength and the appropriate endocrine adjustments are made. The second possibility depends on the synchronization of elevated melatonin levels with the sensitivity of a particular organ system to the melatonin peak; when this occurs the organ responds accordingly. Both the external and internal coincidence models are considered. The duration and coincidence models are fundamentally quite different. In the case of the former, the altered duration of the melatonin peak per se determines the response that will occur. In the case of the coincidence models, the elevated melatonin has a more passive role with the "decision" to respond being a function of end organ sensitivity. In the final analysis, it may be that organisms use a combination of absolute duration of the melatonin pulse, direction of change of the melatonin rhythm, and synchrony of peak melatonin with the increased sensitivity of the end organ before a response is forthcoming.