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 corticothalamic neurons from the rat primary visual cortex. Microiontophoretic injections of biocytin were made in cortical laminae V and VI to label small pools of corticothalamic cells and their intrathalamic axonal projections. After a survival period of 48 h, the animals were perfused and the tissue was processed for biocytin histochemistry. On the basis of the intrathalamic distribution of axonal fields and the types of terminations found in the thalamus, three types of corticothalamic projections were identified. (1) Cells of the upper part of lamina VI projected to the dorsal lateral geniculate nucleus where they arborized in rostrocaudally oriented bands or "rods" parallel to the lines of projection of retinal afferents. (2) Cells of the lower part of lamina VI projected to the lateral part of the lateral posterior nucleus and they also sent collaterals to the dorsal lateral geniculate nucleus where they participated in the formation of rods. (3) The corticothalamic projection of lamina V cells originated from collaterals of corticofugal cells whose main axons reached the tectum and/or the pontine nuclei. These collaterals never terminated within the dorsal lateral geniculate nucleus; they arborized in the lateral posterior, lateral dorsal and ventral lateral geniculate nuclei. All corticothalamic cells from lamina VI displayed the same type of axonal network made of long branches decorated by terminal boutons emitted "en passant" at the tip of fine stalks. Corticothalamic fibers arising from lamina V, however, generated varicose endings in restricted regions of their target nuclei. All corticothalamic axons derived from lamina VI cells, but not those derived from lamina V cells, gave off collaterals as they traversed the thalamic reticular complex. These results demonstrate that corticothalamic fibers arising from the rat primary visual cortex display a lamina-dependent projection pattern. In the light of previous studies on the topographical organization of corticothalamic projections, it is proposed that a similar organizational plan characterizes corticothalamic relationships in other sensory systems in the rat and in other species.