PLASMA MELATONIN IN THE SCINCID LIZARD, TRACHYDOSAURUS-RUGOSUS - DIEL RHYTHM, SEASONALITY, AND THE EFFECT OF CONSTANT LIGHT AND CONSTANT DARKNESS

Melatonin was assayed in the plasma of the scincid lizard, Trachydosaurus rugosus, using a specific radioimmunoassay. The levels of this indole exhibited a daily fluctuation similar to that observed in many other vertebrates. Plasma melatonin titers were low during the light phase and elevated during the dark phase when exposed to a lighting regimen of 13 hr light and 11 hr dark. In lizards captured in spring and transferred to this regimen, the amplitude of the plasma melatonin oscillation appeared damped in comparison to those sampled in the autumn. This damping was attributed to higher light phase and lower dark phase concentrations in spring. In one experiment, a correlation between size and plasma melatonin concentration was evident, higher levels being present in smaller animals. In contrast to birds and mammals, where a rhythm in blood melatonin content persists in constant dark, the plasma melatonin rhythm of T. rugosus was abolished by both constant light and constant darkness under constant temperature conditions. © 1979.