Seasonality and role of SCN in entrainment of lizard circadian rhythms to daily melatonin injections

Role of suprachiasmatic nuclei in circadian and light-entrained behavioral rhythms of lizards

To establish whether the suprachiasmatic nuclei (SCN) of the Ruin lizard (Podarcis sicula) play a role in entrainment of circadian rhythms to light, we examined the effects of exposure to 24-h light-dark (LD) cycles on the locomotor behavior of lizards with SCN lesions. Lizards became arrhythmic in response to complete SCN lesion under constant temperature and constant darkness (DD), and they remained arrhythmic after exposure to LD cycles. Remnants of SCN tissue in other lesioned lizards were sufficient to warrant entrainment to LD cycles. Hence, the SCN of Ruin lizards are essential both to maintain locomotor rhythmicity and to mediate entrainment of these rhythms to light. We also asked whether light causes expression of Fos-like immunoreactivity (Fos-LI) in the SCN. Under LD cycles, the SCN express a daily rhythm in Fos-LI. Because Fos-LI is undetectable in DD, the rhythm seen in LD cycles is caused by light. We further showed that unilateral SCN lesions in DD induce dramatic period changes. Altogether, the present data support the existence of a strong functional similarity between the SCN of lizards and the SCN of mammals.