Immunocytochemical organization and sour taste activation in the rostral nucleus of the solitary tract of mice.

Sensory inputs from the oropharynx terminate in both the trigeminal brainstem complex and the rostral part of the nucleus of the solitary tract (nTS). Taste information is conveyed via the facial and glossopharyngeal nerves, while general mucosal innervation is carried by the trigeminal and glossopharyngeal nerves. In contrast, the caudal nTS receives general visceral information largely from the vagus nerve. Although the caudal nTS shows clear morphological and molecularly delimited subdivisions, the rostral part does not. Thus, linking taste-induced patterns of activity to morphological subdivisions in the nTS is challenging. To test whether molecularly defined features of the rostral nTS correlate with patterns of taste-induced activity, we combined immunohistochemistry for markers of various visceral afferent and efferent systems with c-Fos-based activity maps generated by stimulation with a sour tastant, 30 mM citric acid. We further dissociated taste-related activity from activity arising from acid-sensitive general mucosal innervation by comparing acid-evoked c-Fos in wild-type and "taste blind" P2X2 /P2X3 double knockout (P2X-dbl KO) mice. In wild-type mice, citric acid stimulation evoked significant c-Fos activation in the central part of the rostral nTS-activity that was largely absent in the P2X-dbl KO mice. P2X-dbl KO mice, like wild-type mice, did exhibit acid-induced c-Fos activity in the dorsomedial trigeminal brainstem nucleus situated laterally adjacent to the rostral nTS. This dorsomedial nucleus also showed substantial innervation by trigeminal nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP), a marker for polymodal nociceptors, suggesting that trigeminal general mucosal innervation carries information about acids in the oral cavity. J. Comp. Neurol. 525:271-290, 2017.