Localization of cholecystokinin mRNA to rat lingual epithelium using in situ hybridization
Taste buds are spindle-shaped collections of taste receptor cells located in the surface epithelium of the oral cavity. Taste receptor cells are specialized sensory epithelial cells that are responsible for the transmission of taste information to the taste nerves. Immunocytochemical evidence of the neuropeptide cholecystokinin in various taste cells of the rat has been presented by our lab (Herness 1991). These results have prompted our investigation of the messenger RNA encoding this peptide in rat taste cells. CCK may play an important role in taste signal transmission or modulation.Three areas of the oral cavity were investigated for the presence of CCK mRNA. Two of these areas contain collections of taste buds in well-defined structures called papillae. Circumvallate papillae, located on the posterior surface of the tongue, and foliate papillae, located on the lateral surfaces of the tongue, were sectioned and probed for CCK mRNA using non-radiographic in situ hybridization. The third area investigated was the nasoincisor duct, located on the roof of the oral cavity. This duct contains isolated taste buds within the surface epithelium near the opening to the oral cavity.The results of this study confirm the presence of CCK mRNA in all three areas studied. Taste buds located in the circumvallate papillae, foliate papillae and in the nasoincisor duct all contain taste cells that express CCK mRNA. These results are verified by the absence of labeled cells in negative control experiments. The negative controls consists of the omission of probe, anti-DIG antibody, and the application of a sense probe. The immunocytochemical results show only a subset of taste cells labeled for the CCK peptide while the in situ results depict all cells in the bud labeled for CCK mRNA. The in situ results very closely parallel the immunocytochemical results previously obtained by our lab, although with in situ hybridization epithelial staining is more prominent. The surface epithelium contains the messenger RNA encoding CCK likely because taste receptor cells are derived from the lingual epithelium.Several roles for CCK can be considered in taste physiology. Taste reception and taste signal transduction is not fully understood. Also the localization and pharmacology of CCK receptors in taste systems awaits investigation. These two areas must be studied further to understand the function of CCK in taste cells.