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 peri odicity of 24 h by 10-mu g melatonin injections administered every oth er day at the same time. The present study examined the response of th e circadian activity rhythm of pinealectomized S. occidentalis to dail y 12-h infusions of smaller quantities of melatonin (0.1 or 5 mu g mel atonin/day). The results show that entrainment is achieved by infusion of 0.1 mu g of melatonin/day in pinealectomized lizards, as well as b y 5 mu g of melatonin/day in pinealectomized and pineal-intact lizards . Serum melatonin levels in pinealectomized lizards receiving 0.1 mu g melatonin/day (measured in the middle of the infusion period) were co mparable to mid-dark levels in intact lizards. These results provide f urther support for the hypothesis that the pineal, via its daily rhyth m of melatonin secretion, plays an important role in the circadian org anization of lizards.