Hamster circadian rhythms are phase-shifted by electrical stimulation of the geniculo-hypothalamic tract

The suprachiasmatic nuclei (SCN) contain the major pacemaker for mammalian circadian rhythms. The SCN receive photic input both directly, via the retinohypothalamic tract (RHT), and indirectly, via the geniculohypothalamic tract (GHT), which originates in cells in the intergeniculate leaflet (IGL) and anterior portions of the ventral lateral geniculate nucleus (vLGN). We tested whether electrical stimulation of the GHT would induce phase shifts in wheel-running activity rhythms of Syrian hamsters housed in continuous darkness or continuous illumination. In both lighting conditions, electrical stimulation of the GHT induced mainly phase advances when given during the late subjective day and small phase delays when given during the late subjective night and early subjective day. Stimulation in the thalamus outside the GHT failed to produce similar phase shifts. Repeated daily stimulation had only a weak entraining effect on the activity rhythm. Activation of GHT neurons appears to influence the pacemaker for activity rhythms in a phase-dependent manner.