Effects of intergeniculate leaflet lesions on circadian rhythms in Octodon degus

Abstract

The intergeniculate leaflet (IGL) modulates photic and nonphotic entrainment of circadian rhythms in nocturnal species, but nothing is known about its role in diurnal species. We investigated the significance of the IGL for circadian rhythm function in the diurnal rodent, Octodon degus, by determining the effects of bilateral electrolytic IGL lesions (IGL X) on: (i) photic entrainment; (ii) reentrainment rates to photic cues following a 6-h phase advance of the light–dark (LD) cycle; (iii) reentrainment rates to nonphotic social and photic cues following a 6-h phase advance of the LD cycle; and (iv) the circadian period ( τ) of the activity rhythm in constant darkness (DD). IGL X significantly lengthened the duration ( α) of the entrained activity rhythm and produced a significantly earlier phase of activity onset under entrained (LD 12:12) conditions, but did not change phase of activity offset, rhythm amplitude or mean daily activity levels. IGL X failed to modify τ of free-running activity rhythms in DD or alter reentrainment rates of circadian rhythms to nonphotic social and photic cues or photic cues alone. Thus, the IGL modulates two parameters of photic entrainment, but is not necessary for reentrainment to either nonphotic social or photic cues. Our results contribute to the growing comparative database on the neural mechanisms controlling circadian rhythms and indicate that the role of the IGL varies across species with no apparent relationship between diurnality–nocturnality and circadian function.