Sugars, Sweet Taste Receptors, and Brain Responses - PubMed (original) (raw)

Figure 1

The four major classes of taste cells. This classification incorporates ultrastructural features, patterns of gene expression, and the functions of each Types I, II (receptor), III (presynaptic), and IV (progenitor, not depicted) taste cells. Type I cells (blue) degrade or absorb neurotransmitters. They also may clear extracellular K+ that accumulates after action potentials (shown as bursts) in receptor (yellow) and presynaptic (green) cells. K+ may be extruded through an apical K channel such as renal outer medullary potassium channel (ROMK). Salty taste may be transduced by some Type I cells, but this remains uncertain. Sweet, bitter, and umami taste compounds activate receptor cells, inducing them to release ATP through pannexin 1 (Panx1) hemichannels. The extracellular ATP excites ATP receptors (P2X, P2Y) on sensory nerve fibers and on taste cells. Presynaptic cells, in turn, release serotonin (5-HT), which inhibits receptor cells. Sour stimuli (and carbonation, not depicted) directly activate presynaptic cells. Only presynaptic cells form ultrastructurally identifiably synapses with nerves. Tables below the cells list some of the proteins that are expressed in a cell type-selective manner. AADC, aromatic L-amino acid decarboxylase; Ca, voltage-gated calcium channel; Gα-gus, alpha-gustducin; Gγ13, Gγ13 subunit; GAD, glutamate decarboxylase; GLAST, glutamate aspartate transporter; 5-HT, 5-hydroxytryptamine; mGluRs, metabotropic glutamate receptor; Na, voltage-gated sodium channel; NCAM, neural cell adhesion molecule; NET, norepinephrine transporter; NTPDase; nucleoside triphosphate diphosphohydrolase; OXTR, oxytoxin receptor; Panx1, pannexin 1; PKD, polycystic kidney disease-like channel; PLCβ2, phospholipase C β2; ROMK, renal outer medullary potassium channel; SNAP, synaptosomal-associated protein; T1R, taste 1 receptor family; T2R, taste 2 receptor family; TRPM5, transient receptor potential cation channel M5; Adapted with permission from [6].