Suprachiasmatic nucleus transplants function as an endogenous oscillator only in constant darkness.

Rats which sustained lesions of the suprachiasmatic nucleus (SCN) were transplanted with fetal hypothalamic tissue which included the SCN. After the graft, animals were kept under a 12:12 light-dark (LD) cycle during 6 weeks at which time half the animals were placed in constant light (LL) and half in constant darkness (DD). The results showed that all grafted rats recovered the photoperiod under LD, but under constant conditions rats began to free-run only in DD. The results suggest that the grafted tissue functions as a self-sustained oscillator only in darkness.