P2X2- and P2X3-positive fibers in fungiform papillae originate from the chorda tympani but not the trigeminal nerve in rats and mice

The subtype 2 and subtype 3 ionotropic purinergic receptors (P2X receptors) are crucial for gustation, but the distribution of these receptors in the geniculate ganglion (GG) and their colocalization in tongue papillae remain unknown. Here we investigated the expression and colocalization of P2X(2) and P2X(3) receptors in the GG and fungiform papillae in rats and mice by using in situ hybridization and immunohistochemistry. In both species, P2X(2) transcripts and immunoreactivity were detected in approximately 50-60% of GG neuronal somata, whereas those of P2X(3) were observed in almost all neurons. In each fungiform papilla, immunoreactivity for both receptors was mostly colocalized and was seen in nerve fibers and their bundles concentrated in the taste buds. Because it is well known that the P2X receptors are involved in not only taste but also nociception, we determined whether the expression originated from the chorda tympani nerve (CT, gustatory) or trigeminal nerve (somatosensory) by cutting the CT in both animals. Most P2X(2) and P2X(3) immunoreactivity in the fungiform papillae was abolished after transection, although the nerve fiber immunoreactivity of transient receptor potential V1 (a marker of somatosensory nerve fibers) remained unchanged, indicating that most fungiform papillae nerve fibers with P2X(2) and P2X(3) receptors were derived from CT. Taken together, these findings suggest that most P2X(2) and P2X(3) receptors in fungiform papillae are used for gustation rather than somatosensation.