Effects of melatonin administration on the circadian activity rhythm of the lizard Anolis carolinensis

The green anole, Anolis carolinensis, is the most thoroughly studied North American lizard species, yet little is known about the circadian system of this species. Studies in several lizard species, including Anolis, support the hypothesis that the pineal organ is the site of a major circadian pacemaker that controls the phasing and frequency of circadian clocks located elsewhere. We examined the effects of exogenous administration of the pineal hormone, melatonin, to determine if the pineal secretion of melatonin could be the mechanism whereby the pineal communicates with the rest of the circadian system in Anolis. Continuous melatonin administration via subcutaneous silastic implants to pineal-intact anoles exposed to light-dark (LD) cycles 6:18 and LD 6:20 and to pinealectomized anoles entrained to LD 6:18 completely suppressed activity that preceded lights-on suggesting a direct inhibitory (masking) effect on activity. Continuous exogenous administration of lower dosages of melatonin to anoles expressing their endogenous circadian activity rhythm (free-running) in dim continuous light caused either arrhythmicity, a shortening of the free-running period, bouts of arrhythmicity and rhythmicity, or a suppression of activity. It was concluded that melatonin can affect both the expression of the activity rhythm as well as the circadian oscillator driving the activity rhythm. The results support the hypothesis that the circadian pacemaker in the pineal organ controls other (subordinate) circadian clocks via the rhythmic synthesis and secretion of melatonin.