TY - JOUR T1 - The Role of the Intergeniculate Leaflet in Entrainment of Circadian Rhythms to a Skeleton Photoperiod JF - The Journal of Neuroscience JO - J. Neurosci. SP - 372 LP - 380 DO - 10.1523/JNEUROSCI.19-01-00372.1999 VL - 19 IS - 1 AU - Kim Edelstein AU - Shimon Amir Y1 - 1999/01/01 UR - http://www.jneurosci.org/content/19/1/372.abstract N2 - Mammalian circadian rhythms are synchronized to environmental light/dark (LD) cycles via daily phase resetting of the circadian clock in the suprachiasmatic nucleus (SCN). Photic information is transmitted to the SCN directly from the retina via the retinohypothalamic tract (RHT) and indirectly from the retinorecipient intergeniculate leaflet (IGL) via the geniculohypothalamic tract (GHT). The RHT is thought to be both necessary and sufficient for photic entrainment to standard laboratory light/dark cycles. An obligatory role for the IGL–GHT in photic entrainment has not been demonstrated. Here we show that the IGL is necessary for entrainment of circadian rhythms to a skeleton photoperiod (SPP), an ecologically relevant lighting schedule congruous with light sampling behavior in nocturnal rodents. Rats with bilateral electrolytic IGL lesions entrained normally to lighting cycles consisting of 12 hr of light followed by 12 hr of darkness, but exhibited free-running rhythms when housed under an SPP consisting of two 1 hr light pulses given at times corresponding to dusk and dawn. Despite IGL lesions and other damage to the visual system, the SCN displayed normal sensitivity to the entraining light, as assessed by light-induced Fos immunoreactivity. In addition, all IGL-lesioned, free-running rats showed masking of the body temperature rhythm during the SPP light pulses. These results show that the integrity of the IGL is necessary for entrainment of circadian rhythms to a lighting schedule like that experienced by nocturnal rodents in the natural environment. ER -