Trigeminal projections to the peribrachial region in the muskrat

Abstract

The anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase was used to study the trigeminoperibrachial pathway in the muskrat after injections of tracer into either the medullary dorsal horn or the dorsolateral pons. After injections into the medullary dorsal horn, labeled fibers ascended into the ipsilateral dorsolateral pons via the spinal trigeminal tract, within the neuropil of the trigeminal 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 Kölliker-Fuse nucleus continued into the lateral parabrachial nucleus. At intermediate levels ipsilaterally, the Kölliker-Fuse nucleus again was labeled densely, as were areas analogous to the external lateral and external medial subnuclei of the parabrachial nucleus in the rat. A thin band of label along the ventral spinocerebellar tract outlined an unlabeled 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 sparser 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 lateral subnucleus was labeled densely via fibers ascending in the trigemino thalamic 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 peribrachial area. An exception, however, was that the caudal medial parabrachial nucleus was also labeled after the dorsal paratrigeminal injection. Also, only scant label was found in the rostral third of the dorsolateral pons on either side after these injections. Both trigeminothalamic and trigeminolemniscal pathways were labeled contralaterally after these injections. These trigeminal projections to the dorsolateral pons were compared to the projections from the nucleus tractus solitarii and the ventrolateral medulla. Numerous trigeminal neurons were labeled retrogradely after injections of wheat germ agglu tinin-horseradish peroxidase into the dorsolateral pons. In the medullary dorsal horn, they were found almost exclusively in laminae I and V. Labeled neurons 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 the interface of the subnucleus caudalis and subnucleus interpolaris, as well as with those found in the ventral and dorsal paratrigeminal nuclei. Fewer labeled neurons were found along the ventromedial border of subnucleus interpolaris caudally and in the dorsomedial portion of the subnucleus oralis. These data illustrate a robust trigmeminoperibrachial pathway in the muskrat. Since the parabrachial nucleus receives both somatosensory and visceral sensory information, it may play a role in somatovisceral reflex behaviors.