Photoperiod affects amplitude but not duration of in vitro melatonin production in the ruin lizard (Podarcis sicula)

The pineal gland and its major output signal melatonin have been demonstrated to play a central role in the seasonal organization of the ruin lizard Podarcis sicula. Seasonal variations in the amplitude of the nocturnal melatonin signal, with high values in spring as compared to low values in summer and autumn, have been found in vivo. The authors examined whether the pineal gland of the ruin lizard contains autonomous circadian oscillators controlling melatonin synthesis and whether previously described seasonal variations of in vivo melatonin production can also be found in isolated cultured pineal glands obtained from ruin lizards in summer and winter. In vitro melatonin release from isolated pineal glands of the ruin lizard persisted for 4 days in constant conditions. Cultured explanted pineal glands obtained from animals in winter and summer showed similar circadian rhythms of melatonin release, characterized by damping of the amplitude of the melatonin rhythm. Although different photoperiodic conditions were imposed on ruin lizards before explantation of pineal glands, the authors did not find any indication for corresponding differences in the duration of elevated melatonin in vitro. Differences were found in the amplitude of in vitro melatonin production in light/dark conditions and, to a lesser degree, in constant conditions. The presence of a circadian melatonin rhythm in vitro in winter, although such a rhythm is absent in vivo in winter, suggests that pineal melatonin production is influenced by an extrapineal oscillator in the intact animal that may either positively or negatively modulate melatonin production in summer and winter, respectively.