The role of the intergeniculate leaflet in entrainment of circadian rhythms to a skeleton photoperiod

Mammalian circadian rhythms are synchronized to environmental light/dark (L D) cycles via daily phase resetting of the circadian clock in the suprachia smatic nucleus (SGN). Photic information is transmitted to the SGN directly from the retina via the retinohypothalamic tract (RHT) and indirectly from the retinorecipient intergeniculate leaflet (IGL) via the geniculohypothal amic tract (GHT). The RHT is thought to be both necessary and sufficient fo r photic entrainment to standard laboratory light/dark cycles. An obligator y role for the IGL-GHT in photic entrainment has not been demonstrated. Her e we show that the IGL is necessary for entrainment of circadian rhythms to a skeleton photoperiod (SPP), an ecologically relevant righting schedule c ongruous with light sampling behavior in nocturnal rodents. Rats with bilat eral electrolytic IGL lesions entrained normally to lighting cycles consist ing of 12 hr of light followed by 12 hr of darkness, but exhibited free-run ning rhythms when housed under an SPP consisting of two 1 hr light pulses g iven at times corresponding to dusk and dawn. Despite IGL lesions and other damage to the visual system, the SCN displayed normal sensitivity to the e ntraining light, as assessed by light-induced Fos immunoreactivity. In addi tion, all IGL-lesioned, free-running rats showed masking of the body temper ature rhythm during the SPP light pulses. These results show that the integ rity of the IGL is necessary for entrainment of circadian rhythms to a ligh ting schedule like that experienced by nocturnal rodents in the natural env ironment.