CAMP: A Role in Sweet Taste Adaptation

Abstract

Sweet taste transduction is initiated by sugars or synthetic sweeteners binding to the G protein coupled receptor, T1R2+T1R3, and activating G protein(s) and downstream signaling effectors. Recent genetic and functional studies implicated phospholipase C (PLCβ2) and Ca 2+ release from intracellular stores in sweet transduction. Considerable evidence suggests that cAMP also plays a role in the sweet response. Initial observations pointed to cAMP as the second messenger, because sweet stimuli modulate cAMP levels in taste tissue, and because membrane permeant cAMP strongly influences the physiological response of taste buds to sweet stimuli. Further, enzymes that regulate cAMP levels, adenylyl cyclases (ACs) and phosphodiesterases (PDEs) and gustducin, a G protein that can activate PDEs, are all expressed in many sweet-sensitive taste cells. And, loss of gustducin impairs sweet responses. Yet, the precise role of cAMP in sweet taste remains unclear. Here, we review evidence that cAMP is produced as a direct consequence of receptor activation, that cAMP directly depolarizes mammalian taste cells, and that cAMP-dependent Protein Kinase (PKA) likely underlies adaptation to the sweet response. We suggest a model in which the role of α-gustducin is to keep cAMP levels low to prevent chronic adaptation of sweet-sensitive taste cells.