CORTICOTHALAMIC PROJECTIONS FROM THE PRIMARY VISUAL-CORTEX IN RATS - A SINGLE-FIBER STUDY USING BIOCYTIN AS AN ANTEROGRADE TRACER

This study investigates the pattern of axonal projections of single co rticothalamic neurons from the rat primary visual cortex. Microiontoph oretic injections of biocytin were made in cortical laminae V and VI t o label small pools of corticothalamic cells and their intrathalamic a xonal projections. After a survival period of 48 h, the animals were p erfused and the tissue was processed for biocytin histochemistry. On t he basis of the intrathalamic distribution of axonal fields and the ty pes of terminations found in the thalamus, three types of corticothala mic projections were identified. (1) Cells of the upper part of lamina VI projected to the dorsal lateral geniculate nucleus where they arbo rized in rostrocaudally oriented bands or ''rods'' parallel to the lin es of projection of retinal afferents. (2) Cells of the lower part of lamina VI projected to the lateral part of the lateral posterior nucle us and they also sent collaterals to the dorsal lateral geniculate nuc leus where they participated in the formation of rods. (3) The cortico thalamic projection of lamina V cells originated from collaterals of c orticofugal cells whose main axons reached the tectum and/or the ponti ne nuclei. These collaterals never terminated within the dorsal latera l geniculate nucleus; they arborized in the lateral posterior, lateral dorsal and ventral lateral geniculate nuclei. All corticothalamic cel ls from lamina VI displayed the same type of axonal network made of lo ng branches decorated by terminal boutons emitted ''en passant'' at th e tip of fine stalks. Corticothalamic fibers arising from lamina V, ho wever, generated varicose endings in restricted regions of their targe t nuclei. All corticothalamic axons derived from lamina VI cells, but not those derived from lamina V cells, gave off collaterals as they tr aversed the thalamic reticular complex. These results demonstrate that corticothalamic fibers arising from the rat primary visual cortex dis play a lamina-dependent projection pattern. In the light of previous s tudies on the topographical organization of corticothalamic projection s, it is proposed that a similar organizational plan characterizes cor ticothalamic relationships in other sensory systems in the rat and in other species.