TOPOGRAPHIC AND LAMINAR ORGANIZATIONS OF THE INCERTOCORTICAL PATHWAY IN RATS

The topographic and laminar organizations of the projection system fro m the zona incerta to the neocortex were studied by using both retrogr ade and anterograde methods in the rat. Injections of retrograde fluor escent tracers into different cortical areas revealed that the incerto cortical projection neurons have a rough topographic organization with respect to their cortical targets. Furthermore, the incertocortical p rojecting neurons were found mainly in the dorsal and rostral subdivis ions of the zona incerta, and none were found in the ventral subdivisi on. In cases which included three different fluorescent tracers inject ed into the frontal, the parietal and the occipital cortices, retrogra dely single-labelled cells were found intermingled within the dorsal z ona incerta. Very few double-labelled cells were noted, and triple-lab elled cells were absent. Injections of anterograde tracers into the do rsal zona incerta demonstrate that labelled fibres traverse the striat um and terminate most densely in the outer half of layer I of the neoc ortex. The density of incertocortical terminals was greatest in the so matosensory cortex, while the innervation of visual cortical areas was sparse. Very fine and sparse bouton-like swellings of labelled incert ocortical fibres were found running parallel along the pial surface. S ince it has recently been shown that the incertocortical projections d erive from GABAergic neurons, the present results suggest that the dif fuse and roughly topographic projection from the zona incerta to the c erebral cortex may play an inhibitory role in widespread areas of cere bral cortex. This inhibitory action may preferentially target the dist al dendrites of cortical neurons, since the majority of incertocortica l terminals were found in the outer part of layer I of the neocortex. (C) 1997 IBRO. Published by Elsevier Science Ltd.