Loss of entrainment and anatomical plasticity after lesions of the hamster retinohypothalamic tract.

The suprachiasmatic nuclei receive photic input information directly through a retinohypothalamic tract (RHT) and indirectly through a projection from the intergeniculate leaflet of the lateral geniculate complex, the geniculohypothalamic tract (GHT). Prior work has established that the RHT is sufficient for entrainment, but has not shown whether it is necessary because it has not been possible to transect that pathway. The present study addresses this problem by employing knife cuts to sever the RHT in male hamsters. Three knife cut procedures were used and one of these succeeded in separating the SCN from the optic chiasm in 8 animals with limited damage to the chiasm and the SCN. The effectiveness of the RHT lesion was confirmed by cholera toxin-HRP histochemistry which demonstrated that the knife cuts eliminate the normal retinal innervation of the SCN while sparing projections to thalamic and tectal visual centers. In a light-dark cycle, the lesioned animals exhibit free-running rhythms indicating that the RHT is necessary for entrainment. A surprising observation is the presence of extensive axonal sprouting of retinal fibers in brains of animals with RHT lesions. The newly-formed axons grow extensively into the SCN, anterior hypothalamus and basal forebrain, but form anomalous axonal plexuses which have no evident function.