Dorsal thalamic connections of the ventral lateral geniculate nucleus of rats

In order to understand better the organisation of the ventral lateral geniculate nucleus of the ventral thalamus, this paper has examined the patterns of connections that this nucleus has with various nuclei of the dorsal thalamus in rats. Injections of biotinylated dextran or cholera toxin subunit B were made into the parafascicular, central lateral, posterior thalamic, medial dorsal, lateral dorsal, lateral posterior, dorsal lateral geniculate, anterior, ventral lateral, ventrobasal and medial geniculate nuclei of Sprague-Dawley rats and their brains were processed using standard tracer detection methods. Three general patterns of ventral lateral geniculate connectivity were seen. First, the parafascicular, central lateral, medial dorsal, posterior thalamic and lateral dorsal nuclei had heavy connections with the parvocellular (internal) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown previously to receive heavy inputs from many functionally diverse brainstem nuclei. Second, the visually related dorsal lateral geniculate and lateral posterior nuclei had heavy connections with the magnocellular (external) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown by previous studies to receive heavy inputs from the visual cortex and the retina. Finally, the anterior, ventral lateral, ventrobasal and medial geniculate nuclei had very sparse, if any, connections with the ventral lateral geniculate nucleus. Overall, our results strengthen the notion that one can package the ventral lateral geniculate nucleus into distinct visual (magnocellular) and non-visual (parvocellular) components.