Plasma Melatonin Levels in the Scincid Lizard Trachydosaurus Rugosus: The Effects of Parietal Eye and Lateral Eye Impairment

Abstract

1. At a constant temperature of 24 °C there was a diel fluctuation in plasma melatonin concentration; highest levels occurring in the scotophase of a reversed daily light-cycle. Parietalectomy did not appear to affect melatonin titres under these conditions. 2. When lizards were subjected to a photoperiod together with a thermoperiod (31 °C in the photophase, 24 °C in the scotophase), nocturnal plasma melatonin levels were almost twice as high as those in animals subjected to a photoperiod at constant temperature. 3. Capping the lateral eyes of T. rugosus under these conditions did not alter the phase or amplitude of the rhythm in plasma melatonin content. However, removal of the parietal eye abolished the rhythm, owing mainly to reduced levels during the mid-scotophase and elevated levels during the mid-photophase. 4. It is concluded that plasma melatonin levels are regulated extraretinally, and that the parietal eye may help to mediate environmental input to centres secreting melatonin. It is suggested that the parietal eye may mediate thermal as well as photic information.