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Afferent pathways to the rostral reticular thalamic nucleus (Rt) in the rat were studied using anterograde and retrograde lectin tracing techniques, with sensitive immunocytochemical methods. The analysis was carried out to further investigate previously described subregions of the reticular thalamic nucleus, which are related to subdivisions of the dorsal thalamus, in the paraventricular and midline nuclei and three segments of the mediodorsal thalamic nucleus. Cortical inputs to the rostral reticular nucleus were found from lamina VI of cingulate, orbital and infralimbic cortex. These projected with a clear topography to lateral, intermediate and medial reticular nucleus respectively. Thalamic inputs were found from lateral and central segments of the mediodorsal nucleus to the lateral and intermediate rostral reticular nucleus respectively and heavy paraventricular thalamic inputs were found to the medial reticular nucleus. In the basal forebrain, afferents were found from the vertical and horizontal limbs of the diagonal band, substantia innominata, ventral pallidum and medial globus pallidus. Brainstem projections were identified from ventrolateral periaqueductal grey and adjacent sites in the mesencephalic reticular formation, laterodorsal tegmental nucleus, pedunculopontine nucleus, medial pretectum and ventral tegmental area. The results suggest a general similarity in the organisation of some brainstem Rt afferents in rat and cat, but also show previously unsuspected inputs. Furthermore, there appear to be at least two functional subdivisions of rostral Rt which is reflected by their connections with cortex and thalamus. The studies also extend recent findings that the ventral striatum, via inputs from the paraventricular thalamic nucleus, is included in the circuitry of the rostral Rt, providing further evidence that basal ganglia may function in concert with Rt. Evidence is also outlined with regard to the possibility that rostral Rt plays a significant role in visuomotor functions.

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