The Role of the Intergeniculate Leaflet in Entrainment of Circadian Rhythms to a Skeleton Photoperiod

Dec 31, 1998The Journal of neuroscience : the official journal of the Society for Neuroscience

How a Brain Region Helps Reset Body Clocks to Short Light Cycles

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Abstract

The integrity of the intergeniculate leaflet (IGL) is necessary for entrainment of circadian rhythms to a skeleton photoperiod (SPP).

Rats with IGL lesions showed normal entrainment to standard 12-hour light/dark cycles but free-ran under an SPP of two 1-hour light pulses.
The SCN maintained normal sensitivity to light despite IGL lesions, as indicated by light-induced Fos immunoreactivity.
All IGL-lesioned rats exhibited masking of body temperature rhythm during light pulses of the SPP.
These findings suggest that the IGL plays a critical role in synchronizing circadian rhythms to ecologically relevant light schedules for nocturnal rodents.

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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.

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The Role of the Intergeniculate Leaflet in Entrainment of Circadian Rhythms to a Skeleton Photoperiod

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