Evidence for a large projection from the zona incerta to the dorsal thalamus

In an effort to understand better how the zona incerta may influence neocortical activity, this study has examined the patterns of projection that this nucleus has to the dorsal thalamus, the "gateway" to the neocortex. To this end, Sprague-Dawley rats were anaesthetised with Ketamil (100 mg/kg) and Rompun (10 mg/kg), and injections of biotinylated dextran or cholera toxin subunit B (CTB) were made into various dorsal thalamic nuclei, including the primary relay (dorsal lateral geniculate, medial geniculate, ventral posterior), association (lateral dorsal, lateral posterior, posterior thalamic), and intralaminar (central lateral, parafascicular) nuclear groups, by using stereotaxic coordinates. Brains were aldehyde fixed and processed with standard methods. Our results show that there is a large projection from the zona incerta to the dorsal thalamus. This projection does not blanket all nuclei of the dorsal thalamus but, rather, shows a clear preference for some nuclei over others. After CTB or dextran injections into the primary relay nuclei, very few cells are labelled in the zona incerta. After similar injections are made into the association or intralaminar nuclei, however, many more labelled incertal cells are seen. There are some differences in the distribution of labelled cells within the zona incerta after injections into the association nuclei compared with injections into the intralaminar nuclei. The association nuclei relate strongly to the ventral sector, whereas the intralaminar nuclei relate strongly to the dorsal sector of the zona incerta. After each of these injections into the dorsal thalamus, labelled terminals are seen in the zona incerta also, and their distribution mirrors the distribution of the labelled incertal cells described above. Thus, in summary, our results indicate that the zona incerta has a large and preferential projection to the dorsal thalamus, in particular from the association and intralaminar nuclei. Through this dorsal thalamic projection, the zona incerta is in a position to influence large areas of the neocortex.