Characterization of IP₃ receptors in bitter taste transduction

Abstract

An important second messenger in taste transduction is inositol trisphosphate (IP3), which has been implicated in the transduction of bitter, sweet and umami compounds. Although the importance of IP3 is known its target has not been described previously. Here we provide evidence that the downstream target of IP3 is the type III IP3 receptor (IP3R3) in taste cells. This receptor is located on the endoplasmic reticulum and allows Ca2+ to pass from the ER into the cytosol. Rat taste buds contain several types of taste cells distinguishable by morphological characteristics, however, the physiological roles of these cell types is not clear. Knowing that IP3 is involved in bitter, sweet, and umami taste and that its target is IP3R3, we have utilized DAB immunoelectron microscopy to determine which cell types express IP3R3. Our results indicate that a large subset of Type II and small subset of Type III cells display IP3R3 immunoreactivity, suggesting these cell types are responsible for bitter, sweet, and umami transduction. Interestingly, many Immunoreactlve Type II cells lacked conventional synapses with nerve fibers, but many were found with subsurface cisternae (SSC) of smooth endoplasmic reticulum at close appositions with nerve fibers. Of the taste transduction pathways, the role of IP3 in bitter taste is best understood. Bitter compounds activate a heterotrlmeric G-proteIn consisting of Gαgustducin, Gβ3, and Gγ13. The Gβ3γ13 subunits raise cytosolic IP3 by activation of PLCβ2, while Gαgustducin decreases cyclic adenosine monophosphate (cAMP) via activation of phosphodiesterase. Gαgustducin null mice show a marked decrease in their ability to detect bitter substances, although the precise role of Gαgustducin in the transduction process is not understood. The finding that IP3R3 is the dominant and perhaps only IP3 receptor isoform in taste cells is intriguing because work in other systems has shown that IP3R3 is inhibited by cAMP dependent phosphorylation, suggesting a possible role for Gαgustducin in taste transduction. Further, phosphorylation has been shown to inhibit PLCβ2, another essential component in bitter transduction. In the last chapter we show preliminary data investigating a possible role of Gαgustducin in modulation of PLCβa and IP3R3.