PATTERNS OF CONNECTIONS BETWEEN ZONA INCERTA AND BRAIN-STEM IN RATS

To understand better the organisation of zona incerta of the thalamus, this study has examined the patterns of connections that this nucleus has with various nuclei of the brainstem. injections of biotinylated dextran or cholera toxin subunit B were made into the dorsal raphe, mi dbrain reticular nucleus, pedunculopontine tegmental nucleus, periaque ductal grey matter, pontine reticular nucleus, substantia nigra, super ior colliculus, and ventral tegmental area of Sprague-Dawley rats, and their brains were processed by using standard tracer-detection method s. In general, our results show that zona incerta forms the major zone in the thalamus where these ascending brainstem axons terminate and f rom which descending axons that travel back to these same brainstem ce ntres originate. These incertal inputs and outputs are limited largely to a distinct sector of zona incerta, the dorsal sector. An exception to this pattern is evident in the incertal projection to the deep lay ers of the superior colliculus; this projection, unlike all of the oth ers, arises from cells in the ventral sector of zona incerta. Our resu lts also show little evidence for a well-defined topography of project ion between the brainstem and the zona incerta. For instance, small in jections into each brainstem nucleus result in labelled terminals and in cells spread throughout much of the dorsal sector of zona incerta, with no local zone of concentration within the sector. Again, an excep tion to this pattern is seen in the incertal projection to the superio r colliculus. This projection, unlike the others, shows a clear topogr aphical organisation: A medial-lateral shift in the injection site in the colliculus results in a lateral-medial shift in the position of la belled cells in zona incerta. Curiously, even though the incertal proj ection to the colliculus appeals to be mapped, the collicular projecti on back to zona incerta is not mapped. In conclusion, then, a number o f brainstem nuclei (except for the deep collicular layers) have strong and overlapping connections within the same sector of zona incerta. T his convergence of many functionally diverse brainstem afferents withi n zona incerta places this nucleus in a strategic position to sample t he general activity of the brainstem and, perhaps, acts as a relay of this information to higher centres, such as the dorsal thalamic relay nuclei and the cerebral hemispheres. (C) 1998 Wiley-Liss, Inc.