Projections of the rat trigeminal sensory nuclear complex demonstrated by multiple fluorescent dye retrograde transport

The fluorescent dyes True Blue, Fast Blue, Nuclear Yellow and SITS were used to examine the connections of the rat brainstem sensory trigeminal nuclear complex (nV). Particular attention was paid to the following questions: do individual trigeminal neurons project to multiple targets via axon collaterals; and do primary afferent inputs to the various regions of nV arise from individual cells? Pairs of injections, using contrasting dyes, were made into the following target area combinations: ventrobasal thalamus-ipsilateral superior colliculus, cerebellum-contralateral thalamus, nucleus principalis of nV-contralateral thalamus, and nucleus principalis of nV-subnucleus caudalis of spinal nV. In general, numerous neurons throughout all subdivisions of nV and within the trigeminal ganglion were labeled by a single dye following the injections. In addition, many cells in a similar distribution were found to be doubly-labeled following such injection combination. These data demonstrate the existence of significant subpopulations of first- and second-order neurons that project to multiple targets via divergent axonal ramifications throughout the rat sensory trigeminal system.