Melatonin in non-mammalian vertebrates: Physiological role in reproduction?

Although the pineal organ has been clearly implicated in the photoperiodic control of seasonal reproduction in several non-mammalians, the possible physiological role for melatonin, the major pineal hormone, is far from clear. In many studies on birds, reptiles, amphibians, fishes and even some invertebrates, it has been reported that melatonin administration can influence (usually suppress) reproductive parameters. However, in the vast majority of these studies, melatonin has been administered in an unphysiological manner, usually by injection, and/or dose. It is notable that in most studies where melatonin has been administered either using continuous-release implants or by treatments attempting to mimick the natural diurnal melatonin rhythm, no or limited effects on reproduction were observed. In conclusion, the data available to date do not support the concept that melatonin plays an important physiological role in the photoperiodic control of reproduction in non mammalians. (C) 1997 Elsevier Science Inc.