An autoradiographic study of the efferent connections of the ventral lateral geniculate nucleus in the albino rat and the cat^†

The efferent connections of the ventral lateral geniculate nucleus (LGN_v) of the albino rat and the cat have been studied using the autoradiographic method for tracing axonal pathways. Following the injection of ³H-proline or ³H-leucine into the LGN_v of the rat, label transported in the rapid phase of axonal flow was found bilaterally in the olivary pretectal nuclei, the lateral terminal nuclei of the accessory optic system, and the ventral portion of the suprachiasmatic nuclei of the hypothalamus, and ipsilaterally in the rostrolateral portion of the superior colliculus. Since these regions are known to receive a direct projection from the retina, comparisons have been made of the distribution of silver grains in autoradiographs of each region following injections of ³H-proline into the eye and into the LGN_v; in every nuclear region except the superior colliculus the grain distributions were found to overlap precisely and, in the suprachiasmatic nuclei there also appears to be a similarity in the relative intensity of the input to the nuclei on the two sides. In the superior colliculus, the retinal fibers end mainly within the more superficial laminae, whereas those from the LGN_v are distributed mainly to the deeper layers where they overlap the projection from the striate and peristriate cortex. The LGN_v has also been found to project to the zona incerta on the same side and to the contralateral LGN_v, In the cat a similar set of projections to the lateral terminal nuclei of the accessory optic tract, the suprachiasmatic nuclei, and the pretectal areas of both sides has been found, together with a projection to the ipsilateral superior colliculus and the zona incerta of both sides. No evidence could be found in either species for a projection from the LGN_v to the visual cortex.