Trigeminal projections to hypoglossal and facial motor nuclei in the rat

This study was undertaken to identify the trigeminal nuclear regions connec ted to the hypoglossal (XII) and facial (VII) motor nuclei in rats. Anterog radely transported tracers (biotinylated dextran amine, biocytin) were inje cted into the various subdivisions of the sensory trigeminal complex, and l abeled fibers and terminals were searched for in the XII and VII. In a seco nd series of experiments, injections of retrogradely transported tracers (b iotinylated dextran amine, gold-horseradish peroxidase complex, fluoro-red, fluoro-green) were made into the XII and the VII, and labeled cells were s earched for in the principal sensory trigeminal nucleus, and in the pars or alis, interpolaris, and caudalis of the spinal trigeminal nucleus. Trigemin ohypoglossal projections were distributed throughout the ventral and dorsal region of the XII. Neurons projecting to the XII were found in all subdivi sions of the sensory trigeminal complex with the greatest concentration in the dorsal part of each spinal subnucleus and exclusively in the dorsal par t of the principal nucleus. Trigeminofacial projections reached all subdivi sions of the VII, with a gradual decreasing density from lateral to medial cell groups. They mainly originated from the ventral part of the principal nucleus. In the spinal nucleus, most of the neurons projecting to the VII w ere in the dorsal part of the nucleus, but some were also found in its cent ral and ventral parts. By using retrograde double labeling after injections of different tracers in the XII and VII on the same side, we examined whet her neurons in the trigeminal complex project to both motor nuclei. These e xperiments demonstrate that in the spinal trigeminal nucleus, neurons locat ed in the pars caudalis and pars interpolaris project by axon collaterals t o XII and VII. (C) 1999 Wiley-Liss, Inc.