Contribution of TRPC3-mediated Ca2+ entry to taste transduction.

ABSTRACT

The current concept of taste transduction implicates the TASR/PLCß2/IP3R3/TRPM5 axis in mediating chemo-electrical coupling in taste cells of the type II. While generation of IP3 has been verified as an obligatory step, DAG appears to be a byproduct of PIP2 cleavage by PLCß2. Here, we provide evidence that DAG-signaling could play a significant and not yet recognized role in taste transduction. In particular, we found that DAG-gated channels are functional in type II cells but not in type I and type III cells. The DAG-gated current presumably constitutes a fraction of the generator current triggered by taste stimulation in type II cells. Bitter stimuli and DAG analogs produced Ca2+ transients in type II cells, which were greatly decreased at low bath Ca2+, indicating their dependence on Ca2+ influx. Among DAG-gated channels, transcripts solely for TRPC3 were detected in the taste tissue, thus implicating this channel in mediating DAG-regulated Ca2+ entry. Release of the afferent neurotransmitter ATP from CV papillae was monitored online by using the luciferin/luciferase method and Ussing-like chamber. It was shown that ATP secretion initiated by bitter stimuli and DAG analogs strongly depended on mucosal Ca2+. Based on the overall findings, we speculate that in taste transduction, IP3-driven Ca2+ release is transient and mainly responsible for rapid activation of Ca2+-gated TRPM5 channels, thus forming the initial phase of receptor potential. DAG-regulated Ca2+ entry through apically situated TRPC3 channels extends the primary Ca2+ signal and preserves TRPM5 activity, providing a needful prolongation of the receptor potential.