Thermocyclic entrainment of lizard blood plasma melatonin rhythms in constant and cyclic photic environments

We assessed how chronic exposure to 6-h cryophase temperatures of 15 degree s C in an otherwise 33 degrees C environment entrains the rhythm of blood p lasma melatonin rhythms in Lizards (Tiliqua rugosa) subjected to constant d ark (DD), constant light (LL), and to 12:12-h Light-dark cycles (12L:12D). The peak of the melatonin rhythm was entrained by the cryophase temperature of the thermocycle in DD and LL, irrespective of the time at which the cry ophase temperature was applied. Comparable thermocycles of 6 h at 15 degree s C imposed on a 12L:12D photocycle, however, affected the amplitude and ph ase of the melatonin rhythm, depending on the phase relationship between li ght and temperature. Cold pulses in the early light period and at midday re sulted, respectively, either in low amplitude or nonexistent melatonin rhyt hms, whereas those centered in or around the dark phase elicited rhythms of high amplitude. Supplementary experiments in 12L:12D using two intermitten t 6-h 15 degrees C cryophases, one delivered in the midscotophase and anoth er in the midphotophase, elicited melatonin rhythms comparable to those in lizards subjected to constant 33 degrees C and 12L:12D. In contrast, lizard s subjected to 12L:12D and a 33 degrees C:15 degrees C thermocycle, whose t hermophase was aligned with the photophase, produced a threefold increase i n the amplitude of the melatonin rhythm. Taken together, these results supp ort the notion that there is an interaction between the external light and temperature cycle and a circadian clock in determining melatonin rhythms in Tiliqua rugosa.