Zona incerta: Substrate for contralateral interconnectivity in the thalamus of rats

We have shown previously that the zona incerta (ZI), a small nucleus derivi ng from the ventral thalamus, has extensive ipsilateral connections with th e higher order and intralaminar nuclei of the dorsal thalamus and that ther e are many ipsilateral interconnections between the different cytoarchitect onic sectors of the ZI. In this study, we explore the contralateral connect ions that the ZI has with its opposing nucleus as well as with the other nu clei of the thalamus. Injections of biotinylated dextran or cholera toxin s ubunit B were made into each of the different ZI sectors (rostral, dorsal, ventral, and caudal) and into intralaminar and higher order dorsal thalamic nuclei of Sprague-Dawley rats by using stereotaxic coordinates. Brains wer e fixed in aldehyde and processed using standard methods. Our results show that, after injections limited to a given ZI sector, labelled terminal-like elements and cells were seen across the other sectors of the ZI of the con tralateral side. Furthermore, after each of these ZI injections, labelling was seen in the intralaminar (e.g., parafascicular, central lateral, and ce ntral medial) and higher order (e.g., posterior thalamic, lateral posterior , and lateral dorsal) nuclei of the contralateral side. These patterns of l abelling were cofirmed after tracer injections into intralaminar and higher order nuclei; after such injections, labelling was seen in the contralater al ZI. In all cases, there was labelling on the ipsilateral side as well, a nd this was generally heavier than on the contralateral side. Overall, our results indicate that there is a network of interconnections between the ZI of both sides of the thalamus and that the ZI has contralateral connection s with the intralaminar and higher order nuclei. Hence, the ZI furnishes a substrate that spreads activity to both sides of the brain. J. Comp. Neurol . 436:52-63, 2001. (C) 2001 Wiley-Liss, Inc.