Trigeminocerebellar, trigeminotectal, and trigeminothalamic projections: a double retrograde axonal tracing study in the mouse.

Double retrograde axonal tracing experiments were carried out in order to reveal potential patterns of divergence in axonal projections from the two major sensory nuclei of the mouse brainstem trigeminal complex: the principal sensory and spinal trigeminal nuclei (oralis, interpolaris, and caudalis divisions). The tracers wheat germ agglutinin, N-[acetyl-3H] and horseradish peroxidase were used in paired injection strategies within portions of the cerebellum, superior colliculus, and thalamic ventrobasal complex and/or posterior group of adult ICR white mice. Trigeminal neurons with projections to tactile areas of the cerebellar cortex or underlying deep cerebellar nuclei were found scattered throughout the principal sensory nucleus and interpolaris division, and mainly in dorsal regions of the oralis division of the spinal trigeminal nucleus. Injections of either tracer which involved lateral portions of the rostral half of the superior colliculus labeled trigeminotectal neurons mainly in the contralateral interpolaris division, ventral half of the oralis division, and a ventral region of the principal sensory nucleus near the oralis border. Fewer trigeminotectal neurons were found scattered throughout the principal sensory nucleus and the magnocellular layer of the caudalis divisions, although an occasional labeled neuron wa also found in the marginal layer. Contralaterally projecting trigeminothalamic neurons were observed throughout the principal sensory nucleus, interpolaris division, and within the marginal and magnocellular layers of caudalis. Double-labeled neurons were observed only after paired injections of the tracers in the thalamus and ipsilateral superior colliculus, and they were found within the caudoventral portion of the principal sensory nucleus near the oralis border, throughout the interpolaris division, within the magnocellular layer of caudalis, and only a few double-labeled neurons were also found within the marginal layer. After such injections, 50% of the labeled tectum-projecting neurons in the principal sensory nucleus, 64% in the interpolaris division, and 57% in the caudalis division are branched neurons which have collateralized projections to both the superior colliculus and thalamus. These projections, which have not been described before, appear to arise from more than one class of projection neuron which is differentially distributed within different regions of the trigeminus.