LIGHT DARK-INDUCED EFFECTS ON BEHAVIORAL RHYTHMS IN SUPRACHIASMATIC NUCLEUS-LESIONED RATS IRRESPECTIVE OF THE PRESENCE OF FUNCTIONAL SUPRACHIASMATIC NUCLEUS BRAIN IMPLANTS

Suprachiasmatic nucleus (SCN)-lesioned rats which had received a fetal SCN graft were kept in constant red light for three months. After this period it was examined whether those rats that showed a recovered free-running circadian rhythm could be entrained to light/dark cycles. To this end, they were subjected to a 12 h light/12 h dark schedule, followed by a 12 h light shift and again to dark conditions. In addition, the same regime was imposed on SCN-grafted rats without recovered circadian rhythms and on sham-grafted animals with a lesion, which were studied as controls. The presence of an SCN graft was identified immunocytochemically by the presence of vasopressin, vasoactive intestinal polypeptide and somatostatin cells. Drinking, eating and wheel-running rhythms were found to synchronize to the light/dark cycles in all rats, not with standing the presence of an SCN graft was. A 12 h light shift was immediately followed by a shift in the three rhythms. Under final dark conditions, free-running patterns reappeared in rhythm-recovered animals, without any convincing evidence for entrainment of the rhythms in the pattern of transition. Behavioral rhythms in SCN-lesioned rats are apparently masked by 12 h light/dark schedules via other visual pathways than the direct projection from the retina to the SCN.