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Abstract
In response to taste stimulation, taste buds release ATP, which activates ionotropic ATP receptors (P2X2/P2X3) on taste nerves as well as metabotropic (P2Y) purinergic receptors on taste bud cells. The action of the extracellular ATP is terminated by ectonucleotidases, ultimately generating adenosine, which itself can activate one or more G-protein coupled adenosine receptors: A1, A2A, A2B, and A3. Here we investigated the expression of adenosine receptors in mouse taste buds at both the nucleotide and protein expression levels. Of the adenosine receptors, only A2B receptor (A2BR) is expressed specifically in taste epithelia. Further, A2BR is expressed abundantly only in a subset of taste bud cells of posterior (circumvallate, foliate), but not anterior (fungiform, palate) taste fields in mice. Analysis of double-labeled tissue indicates that A2BR occurs on Type II taste bud cells that also express Gα14, which is present only in sweet-sensitive taste cells of the foliate and circumvallate papillae. Glossopharyngeal nerve recordings from A2BR knockout mice show significantly reduced responses to both sucrose and synthetic sweeteners, but normal responses to tastants representing other qualities. Thus, our study identified a novel regulator of sweet taste, the A2BR, which functions to potentiate sweet responses in posterior lingual taste fields.
Highlights
In the peripheral gustatory system, ATP plays a crucial role in the transmission of information from taste buds to the gustatory nerve fibers [1], [2], [3], [4]
Type II ‘‘receptor’’ cells, originally termed ‘‘light cells’’, express the G protein-coupled receptors for umami (T1R1/T1R3), sweet (T1R2/T1R3) or bitter (T2Rs) [23,24] transduction. These taste receptors are expressed in largely non-overlapping subsets of Type II taste cells, but all couple to the same downstream signaling effectors including phospholipase C-b2 (PLCb2), inositol 1, 4, 5-trisphosphate receptor type 3 (IP3R3) and transient receptor potential channel M5 (TrpM5) [22]
Expression of adenosine receptors in taste epithelium To test for the presence of mRNAs for adenosine receptors, we used RT-PCR in mouse CV papillae along with whole brain and non-gustatory tongue epithelium
Summary
In the peripheral gustatory system, ATP plays a crucial role in the transmission of information from taste buds to the gustatory nerve fibers [1], [2], [3], [4]. Type II ‘‘receptor’’ cells, originally termed ‘‘light cells’’, express the G protein-coupled receptors for umami (T1R1/T1R3), sweet (T1R2/T1R3) or bitter (T2Rs) [23,24] transduction These taste receptors are expressed in largely non-overlapping subsets of Type II ( called ‘‘receptor’’) taste cells, but all couple to the same downstream signaling effectors including phospholipase C-b2 (PLCb2), inositol 1, 4, 5-trisphosphate receptor type 3 (IP3R3) and transient receptor potential channel M5 (TrpM5) [22]. These signaling proteins serve as well-characterized markers of Type II taste cells in all taste fields. Type III ( termed ‘‘presynaptic’’, [22]) cells were originally classified as ‘‘intermediate cells’’ because they have ultrastructural features intermediate
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