Optic enucleation eliminates circadian rhythm shifts induced by stimulating the intergeniculate leaflet in Syrian hamsters

The intergeniculate leaflet (IGL) is a region of the lateral geniculate complex that is part of the circadian system. It receives direct innervation by specialized retinal ganglion cells involved in circadian rhythm entrainment and is also reciprocally connected to the suprachiasmatic nucleus (SCN), which is the principal circadian pacemaker. Electrical stimulation in the IGL results in shifts of circadian rhythms with a pattern of phase dependence that resembles that elicited by periods of darkness. IGL stimulation also increases levels of c-Fos in the dorsolateral part of the caudal SCN. A previous study showed that optic enucleation prevents increases in c-Fos in the SCN, suggesting the hypothesis that this increase is related to antidromic activation of retinal ganglion cells which bifurcate and project to both SCN and IGL. We tested whether phase shifts induced by IGL stimulation are also dependent on intact retinal innervation. Electrical stimulation of the IGL for 60 min at circadian time (CT)9 (with CT12 defined as activity onset) induced phase advances in nine hamsters with electrodes in the IGL, while other placements did not evoke shifts. After optic enucleation, six of these hamsters received an identical second stimulation; none showed substantial phase shifts. These results are consistent with the hypothesis that phase shifts induced by IGL stimulation depend on antidromic activation of retinal ganglion cells.