Organisation of the cortical projection to the zona incerta of the thalamus.

In this study, we examined the organisation of the cortical projection to a small nucleus of the ventral thalamus called the zona incerta. Injections of biotinylated dextran were made into various cytoarchitectonically defined cortical areas of Sprague-Dawley rats, including the frontal (Fr1), cingulate (Cg1), parietal (Par1), forelimb (FL), and occipital (Oc1), and the patterns of anterograde labelling were examined in the zona incerta. In addition, injections of cholera toxin subunit B (CTB) were made into the zona incerta itself, and the laminar locations of retrogradely labelled cells were examined. Brains were then fixed in aldehyde and processed by using standard methods. After injections of dextran into the different cortical areas, the distribution and number of labelled terminals in the zona incerta were quite different. In terms of number, Cg1 had the heaviest projection to the zona incerta, whereas Oc1 had the weakest. In terms of distribution, three distinct patterns were evident. Firstly, labelled terminals were seen in distinct clusters in the dorsal and ventral sectors, usually one overlying the other (Fr1, FL, Par1). Secondly, labelled terminals were seen occupying much broader zones in the dorsal (and rostral) sector (Cg1). Thirdly, very few labelled terminals were seen in any incertal sector (Oc1). Thus, these results suggest that not all cortical areas project to the zona incerta equally, with some areas having stronger projections to this nucleus than others. From our CTB injections into the zona incerta itself, we show that all retrogradely labelled cells in the neocortex were limited to layer V. We suggest a possible thalamic circuit involving the zona incerta, the thalamic reticular nucleus (the other major nucleus of the ventral thalamus), and the two types of corticothalamic afferents (from layer VI and V). Copyright 1999 Wiley-Liss, Inc.