Circadian rhythms of plasma melatonin in the ruin lizard Podarcis sicula: effects of pinealectomy.

Plasma melatonin was measured in lizards (Podarcis sicula) at six different times of day under conditions of constant temperature and darkness. Intact animals showed a circadian rhythm of melatonin with a peak in the subjective night of 207 pg/ml (median) and a trough during the subjective day that was below the minimum detection level of the assay (50 pg/ml). Pinealectomy abolished the circadian rhythm of plasma melatonin; median levels were near or below the minimum detection level at all times sampled. The data suggest that the pineal is the only source of rhythmic blood-borne melatonin in Podarcis sicula, and are consistent with the hypothesis that changes in the free-running period of the locomotor rhythm induced by pinealectomy in this species are due to withdrawal of rhythmic melatonin from the blood.