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°C in an otherwise 33°C environment entrains the rhythm of blood plasma melatonin rhythms in lizards (Tiliqua rugosa) subjected to constant dark (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 cryophase temperature was applied. Comparable thermocycles of 6 h at 15°C imposed on a 12L:12D photocycle, however, affected the amplitude and phase of the melatonin rhythm, depending on the phase relationship between light and temperature. Cold pulses in the early light period and at midday resulted, respectively, either in low amplitude or nonexistent melatonin rhythms, whereas those centered in or around the dark phase elicited rhythms of high amplitude. Supplementary experiments in 12L:12D using two intermittent 6-h 15°C cryophases, one delivered in the midscotophase and another in the midphotophase, elicited melatonin rhythms comparable to those in lizards subjected to constant 33°C and 12L:12D. In contrast, lizards subjected to 12L:12D and a 33°C: 15°C thermocycle, whose thermophase was aligned with the photophase, produced a threefold increase in the amplitude of the melatonin rhythm. Taken together, these results support 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.