Daily melatonin infusions entrain the locomotor activity of pinealectomized lizards

Previously, it was shown that the locomotor activity rhythms of pineal-intact lizards (Sceloporus occidentalis) could be entrained to a periodicity of 24 h by 10-micrograms melatonin injections administered every other day at the same time. The present study examined the response of the circadian activity rhythm of pinealectomized S. occidentalis to daily 12-h infusions of smaller quantities of melatonin (0.1 or 5 micrograms melatonin/day). The results show that entrainment is achieved by infusion of 0.1 microgram of melatonin/day in pinealectomized lizards, as well as by 5 micrograms of melatonin/day in pinealectomized and pineal-intact lizards. Serum melatonin levels in pinealectomized lizards receiving 0.1 microgram melatonin/day (measured in the middle of the infusion period) were comparable to mid-dark levels in intact lizards. These results provide further support for the hypothesis that the pineal, via its daily rhythm of melatonin secretion, plays an important role in the circadian organization of lizards.