Role of the SCN in the entrainment of locomotor circadian rhythms in the lacertid lizard Podarcis sicula

Electrolytic lesion to the suprachiasmatic nuclei of the hypothalamus (SCN). were shown to abolish circadian rhythms of locomotor activity in the ruin lizard. Podarcis sicula maintained in constant darkness (DD) and constant temperature. This supports the view that the SCN contain the primary circadian pacemaker dri- ving locomotor rhythms in this lizard. The present experiments were carried out to test whether the SCN mediate entrainment of circadian locomotor rhythms to 24h light-dark cycles (LD). For this purpose, lizards subjected to electrolitic lesions of the SCN were allowed to freerun in constant temperature and DD and were then exposed to 24h LD cycles. Lizards whose SCN were completely lesioned became behaviorally arrhythmic in constant conditions and remained arrhythmic even after exposure to LD cycles. Lizards in which lesions covered 70-80% of the SCN became behaviorally arrhythmic in constant conditions, but resumed rhythmicity after exposure to 24h LD cycles and entrainment was clearly attained after a series of transients. Lizards in which lesions covered either the optic chiasm or the periventricular nuclei, but not the SCN remained rhythmic in constant conditions and entrained to the LD cycles. All this allows to conclude that: 1. The SCN play a central role in entrainment of circadian locomotor rhythms to LD cycles; 2. The SCN contain a population of circadian oscillators driving behavioral rhythms in P. sicula.