Reticular thalamic region in the rabbit: organisation of efferents to the superior colliculus.

Although it is well-established that the reticular thalamic nucleus provides a strong GABAergic input to the dorsal thalamus, the existence of reticular efferents to other subcortical centres is less certain. In this study, we investigate whether the reticular nucleus projects to a major brainstem centre, the superior colliculus. The neuronal tracer, biotinylated dextran, was injected into superficial and deep layers of the superior colliculus of rabbits and the resultant labelling in the reticular region was examined. After large injections, which encompassed both superficial and deep collicular layers, two discrete populations of retrogradely labelled cells are seen in the region of the reticular nucleus. One population of retrogradely labelled cells lies in the dorsocaudal regions of the reticular nucleus, the classically defined visual sector. This group of retrogradely labelled reticular cells is also seen after injections into the superficial layers of the superior colliculus, but not after injections limited to the deeper collicular layers. The other population lies close to the ventromedial edge of the main body of the reticular nucleus, within a region referred to as the inner small-celled region. This group of small cells has been commonly thought to be part of the reticular nucleus, but our immunohistochemical studies suggest that this is a clearly separate region, a region continuous ventrally with zona incerta. The retrogradely labelled cells in the inner small-celled region are seen also after injections limited to the deeper collicular layers, but not after injections limited to the superficial collicular layers. Our results suggest functional heterogeneity within the reticular nucleus: Specifically, it suggests that the nucleus is in a position to influence the processing of visual information at both the dorsal thalamic and midbrain levels.