Which type of receptor mediates the sweet taste response?

• A. Ligand-gated ion channels
• B. G-protein coupled receptors
• C. Receptor tyrosine kinases
• D. Nuclear receptors

Answer: (B)

Sweet taste detection uses a G-protein-coupled receptor mechanism. The receptors on sweet-detecting taste cells form a heterodimer of T1R2 and T1R3, which are GPCRs. When a sweet molecule binds, the receptor activates a G protein (gustducin), triggering a phospholipase Cβ2–IP3–Ca2+ signaling cascade. The rise in intracellular Ca2+ opens TRPM5 channels, depolarizing the cell and leading to the release of ATP as a neurotransmitter to gustatory neurons. This GPCR-based pathway, not direct ligand gating of an ion channel, is characteristic of sweet (as well as bitter and umami) taste transduction. Ligand-gated ion channels, receptor tyrosine kinases, and nuclear receptors do not mediate the sweet taste response in this pathway.