Topographic and laminar organizations of the incertocortical pathway in rats.

The topographic and laminar organizations of the projection system from the zona incerta to the neocortex were studied by using both retrograde and anterograde methods in the rat. Injections of retrograde fluorescent tracers into different cortical areas revealed that the incertocortical projection neurons have a rough topographic organization with respect to their cortical targets. Furthermore, the incertocortical projecting neurons were found mainly in the dorsal and rostral subdivisions of the zona incerta, and none were found in the ventral subdivision. In cases which included three different fluorescent tracers injected into the frontal, the parietal and the occipital cortices, retrogradely single-labelled cells were found intermingled within the dorsal zona incerta. Very few double-labelled cells were noted, and triple-labelled cells were absent. Injections of anterograde tracers into the dorsal zona incerta demonstrate that labelled fibres traverse the striatum and terminate most densely in the outer half of layer I of the neocortex. The density of incertocortical terminals was greatest in the somatosensory cortex, while the innervation of visual cortical areas was sparse. Very fine and sparse bouton-like swellings of labelled incertocortical fibres were found running parallel along the pial surface. Since it has recently been shown that the incertocortical projections derive from GABAergic neurons, the present results suggest that the diffuse and roughly topographic projection from the zona incerta to the cerebral cortex may play an inhibitory role in widespread areas of cerebral cortex. This inhibitory action may preferentially target the distal dendrites of cortical neurons, since the majority of incertocortical terminals were found in the outer part of layer I of the neocortex.