Expression of immunoreactivity to neurokinin-1 receptor by subsets of cranial parasympathetic neurons: Correlation with neuropeptides, nitric oxide synthase, and pathways

We examined the patterns of coexistence of immunoreactivity to the neurokinin-1 (NK₁) tachykinin receptor, nitric oxide synthase, and neuropeptides in the sphenopalatine and otic ganglia of guinea pigs using a combination of multiple-labeling immunohistochemistry and pathway tracing in vitro. Most neurons had immunoreactivity to vasoactive intestinal peptide (85-96%) and neuropeptide Y (60%). Subpopulations of vasoactive intestinal peptide-immunoreactive neurons also had immunoreactivity to nitric oxide synthase (37-48%) or enkephalin (25-35%), but these formed mutually exclusive populations. Almost all neurons expressing NK₁ receptor immunoreactivity contained immunoreactivity to enkephalin, vasoactive intestinal peptide, and neuropeptide Y, but not nitric oxide synthase. Using a combination of retrograde axonal tracing and axonal crushing, we found that most neurons with immunoreactivity to nitric oxide synthase projected along the nasopalatine and moidal nerves to the nasal mucosa. In contrast, most neurons with immunoreactivity to enkephalin followed the zygomatic nerve to the facial skin and lacrimal gland. Based on their peptide content, we conclude that the neurons with immunoreactivity to enkephalin and NK₁ receptor projected selectively to the skin. In both the sphenopalatine and the otic ganglia, about half of the neurons with NK₁ receptor immunoreactivity were surrounded by varicose nerve fibers with substance P immunoreactivity. Many of these fibers are likely to have originated in the trigeminal ganglion. Taken together, these observations establish a strong anatomical basis for a range of interactions between trigeminal and cranial parasympathetic pathways that may underlie pathophysiological conditions such as trigeminal neuralgia.