Entrainment of circadian rhythms: retinofugal pathways and unilateral suprachiasmatic nucleus lesions.

The role of retinohypothalamic input to the suprachiasmatic nuclei (SCN) as well as SCN afferents from the ventrolateral geniculate nuclei (LGNV) in the entrainment of circadian drinking rhythms was investigated in the rat. Bilateral lesions of the LGNV and the primary optic tracts had no affect on the entrainment of drinking rhythms to a light-dark cycle, the response to a 12 hr phase shift of the light-dark cycle, or on the period of the free-running circadian rhythm in constant light or constant darkness. Unilateral blinding in rats with or without LGNV lesions retarded the rate of phase shifting by 2 days and decreased the period of the free-running rhythm in constant light. For rats with unilateral SCN lesions, or such lesions combined with either ipsi- or contra-lateral blinding, the rate of re-entrainment was intermediate between intact and unilaterally blinded rats indicating that unilateral SCN lesions partially reversed the effects of unilateral blinding. Unilateral SCN lesions had no effect on the period of the free-running rhythm in constant light or darkness. These results are consistent with the interpretation that the asymmetrical innervation of the two SCN by the RHT in unilaterally blinded rats delays re-entrainment by changing the phase response curve of the circadian system. This change may be mediated by neural connections between the two SCN.