PATTERNS OF BRAIN-STEM PROJECTION TO THE THALAMIC RETICULAR NUCLEUS

To understand better how the brainstem may influence thalamocortical a ctivity, we have examined the projection patterns of different brainst em nuclei to the thalamic reticular nucleus. Iontophoretic injections of biotinylated dextran were made into various nuclei of the brainstem (superior colliculus, periaqueductal grey matter, parabrachial nucleu s, pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus, substantia nigra, ventral tegmental area, and locus coeruleus) of Spr ague-Dawley rats by using stereotaxic coordinates. Our results show th at afferents from each brainstem nucleus make distinct zones within th e reticular nucleus. For example, the superior colliculus projects lar gely to the dorsal parts of the reticular nucleus, whereas the peduncu lopontine nucleus projects to the ventral parts of the reticular nucle us. The substantia nigra, on the other hand, projects to the ventrolat eral edge of the reticular nucleus. We also examined the distribution of these brainstem afferents within the dorsal thalamus and compared t hese distributions with those seen in the reticular nucleus. We found three different patterns. First, a given brainstem nucleus projects to a particular dorsal thalamic nucleus as well as to the corresponding, functionally associated, reticular sector (e.g., from the substantia nigra). Second, a given brainstem nucleus projects to a particular dor sal thalamic nucleus but not to the corresponding reticular sector (e. g., from the superior colliculus). Finally, a given brainstem nucleus projects to a given reticular sector but not to the corresponding dors al thalamic nucleus (e.g., from the midbrain reticular nucleus). In ge neral, our results indicate that various brainstem nuclei project to p articular territories of the thalamic reticular nucleus. Through these reticular projections, brainstem nuclei may influence distinct thalam ocortical pathways in addition to those that are influenced by their d irect projection to the dorsal thalamus. (C) 1998 Wiley-Liss, Inc.