Substance P-containing trigeminal sensory neurons project to the nucleus of the solitary tract.

Intense substance P-like immunoreactivity (SPLI) was identified in fiber bundles coursing between the spinal nucleus of the trigeminal nerve and the ventrolateral nucleus of the solitary tract at the level of the area postrema. These bundles were apparent only when tissue was stained for substance P immunoreactivity and were not visible in preparations treated with antisera to somatostatin or neurotensin. Following unilateral section of the trigeminal nerve, the SPLI-containing fiber bundles were absent ipsilateral to the nerve section. The fibers were absent bilaterally in rats which were previously injected with capsaicin. Unilateral removal of the nodose ganglion did not diminish the intensity or apparent number of SPLI fibers. These data indicate the presence of a trigeminosolitary projection which is composed of primary trigeminal sensory neurons containing substance P. The results provide an anatomical route by which substance P of trigeminal origin may modulate vagal or glossopharyngeal sensory information.