Regional expression patterns of T1r family in the mouse tongue.

The T1r family is one of the receptor family belong to class C type of G protein coupled receptors, and comprised of three taste budspecific receptors, T1r1, T1r2 and T1r3 (Nelson et al., 2001). T1r1 and T1r2 are known to have distinctive patterns of regional expression, respectively (Hoon et al., 1999). T1r1 is expressed in taste buds in the fungiform papillae, but is rare in the taste buds of circumvallate papillae. In contrast, T1r2 is rarely expressed in fungiform papillae but is expressed in all taste buds of the circumvallate papillae. T1r3 is strongly expressed in both fungiform and circumvallate papillae (Kitagawa et al., 2001; Nelson et al., 2001) and forms an amino-acid (umami) receptor and a sweet receptor in combination with T1r1 and T1r2, respectively (Nelson et al., 2001; Li et al., 2002). These expression patterns suggest that taste cells in circumvallate papillae receive the sweet taste substances through the heterodimer of T1r2 and T1r3 (T1r2/T1r3), and those in fungiform papillae receive the umami substances through the heterodimer of T1r1 and T1r3 (T1r1/T1r3). However, physiological studies in mice showed that taste receptor cells in the anterior as well as in the posterior parts of the tongue were sensitive to both sweet and umami substances (Ninomiya et al., 1993). This contradiction may suggest an inadequacy of information about the expression of T1r family. We compared T1r receptors (T1rs) expression pattern between circumvallate and fungiform papillae in mice using double-colored in situ hybridization. Furthermore, we examined the expression patterns of T1rs and the taste cell specific G protein, gustducin, in order to compare taste signal transductions in circumvallate and fungiform papillae.