Direct and indirect thalamic afferents arising from the vestibular nuclear complex of rats: medial and spinal vestibular nuclei.

We examined the afferent pathways and terminations in the thalamus from the medial vestibular nucleus (MVe) and the spinal vestibular nucleus (SpVe) with anterograde transport of Phaseolus vulgaris leucoagglutinin (PHAL) and retrograde transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Following PHAL injection into the MVe, many anterogradely labeled fibers and terminal-like boutons were recognized in the central lateral nucleus of the thalamus. Sparse projections were found in the zona incerta, the ventral lateral geniculate nucleus and the compact region of the laterodorsal thalamic nucleus. When PHAL was injected into the SpVe, labeled ascending fibers ran along the lateral lemniscus forming a loose bundle, and terminated in the medial part of the rostrocaudal extent of the medial geniculate body, and the posterior and ventral thalamic nuclei. Furthermore, a few fibers projected to the central lateral nucleus. In order to confirm the indirect thalamic afferents via the spinal trigeminal nucleus, we injected WGA-HRP into the submedius nucleus of the thalamus after PHAL injection into the SpVe. Many anterogradely labeled terminal-like boutons from SpVe were recognized apposing retrogradely labeled spinal trigeminal neurons. Thus, the present results indicate the existence of dual vestibulo-thalamic pathways; a direct projection to the thalamus, and an indirect projection to the submedius nucleus via the spinal trigeminal nucleus.