Contribution of TRPC3-mediated Ca entry to taste transduction.

The current concept of taste transduction implicates the TASR/PLCβ2/IPR3/TRPM5 axis in mediating chemo-electrical coupling in taste cells of the type II. While generation of IP has been verified as an obligatory step, DAG appears to be a byproduct of PIP cleavage by PLCβ2. 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 Ca transients in type II cells, which were greatly decreased at low bath Ca, indicating their dependence on Ca influx. Among DAG-gated channels, transcripts solely for TRPC3 were detected in the taste tissue, thus implicating this channel in mediating DAG-regulated Ca 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 Ca. Based on the overall findings, we speculate that in taste transduction, IP-driven Ca release is transient and mainly responsible for rapid activation of Ca-gated TRPM5 channels, thus forming the initial phase of receptor potential. DAG-regulated Ca entry through apically situated TRPC3 channels extends the primary Ca signal and preserves TRPM5 activity, providing a needful prolongation of the receptor potential.