TRIGEMINAL PROJECTIONS TO THE PERIBRACHIAL REGION IN THE MUSKRAT

The anterograde and retrograde transport of wheat germ agglutinin-hors eradish peroxidase was used to study the trigeminoperibrachial pathway in the muskrat after injections of tracer into either the medullary d orsal horn or the dorsolateral pens. After injections into the medulla ry dorsal horn, labeled fibers ascended into the ipsilateral dorsolate ral pens via the spinal trigeminal tract, within the neuropil of the t rigeminal sensory complex and within the reticular formation adjacent to the spinal trigeminal nucleus. At caudal levels of the ipsilateral peribrachial area, dense terminal-like label distributed in the Kollik er-Fuse nucleus continued into the lateral parabrachial nucleus. At in termediate levels ipsilaterally, the Kolliker-Fuse nucleus again was l abeled densely, as were areas analogous to the external lateral and ex ternal medial subnuclei of the parabrachial nucleus in the rat. A thin band of label along the ventral spinocerebellar tract outlined an unl abeled area in the central portion of the lateral parabrachial nucleus . Rostrally near the pontomesencephalic junction, the area designated the superior lateral subnucleus in the hamster was labeled, while spar ser label was present more dorsally. Contralateral to the injections, caudal and intermediate levels of the peribrachial area contained only scant reaction product. However, the rostral area of the superior lat eral subnucleus was labeled densely via fibers ascending in the trigem inothalamic tract. Injections made just rostral to the obex and either centered in or including the dorsal or ventral paratrigeminal nuclei produced similar labeling at caudal and intermediate levels of the per ibrachial area. An exception, however, was that the caudal medial para brachial nucleus was also labeled after the dorsal paratrigeminal inje ction. Also, only scant label was found in the rostral third of the do rsolateral pens on either side after these injections. Both trigeminot halamic and trigeminolemniscal pathways were labeled contralaterally a fter these injections. These trigeminal projections to the dorsolatera l pens were compared to the projections from the nucleus tractus solit arii and the ventrolateral medulla. Numerous trigeminal neurons were l abeled retrogradely after injections of wheat germ agglutinin-horserad ish peroxidase into the dorsolateral pens. In the medullary dorsal hor n, they were found almost exclusively in laminae I and V. Labeled neur ons in lamina I were especially prominent in rostral ventral levels of the medullary dorsal horn. Labeled cells in lamina I were continuous with others found in the displaced band of substantia gelatinosa at th e interface of the subnucleus caudalis and subnucleus interpolaris, as well as with those found in the ventral and dorsal paratrigeminal nuc lei. Fewer labeled neurons were found along the ventromedial border of subnucleus interpolaris caudally and in the dorsomedial portion of th e subnucleus oralis. These data illustrate a robust trigmeminoperibrac hial pathway in the muskrat. Since the parabrachial nucleus receives b oth somatosensory and visceral sensory information, it may play a role in somatovisceral reflex behaviors.