Department of Physiology and Biophysics, Miller School of Medicine, University of Miami, Miami, Florida 33136, USA.
J Neurosci. 2012 Jan 4;32(1):322-30. doi: 10.1523/JNEUROSCI.4070-11.2012.
Mammalian taste buds use ATP as a neurotransmitter. Taste Receptor (type II) cells secrete ATP via gap junction hemichannels into the narrow extracellular spaces within a taste bud. This ATP excites primary sensory afferent fibers and also stimulates neighboring taste bud cells. Here we show that extracellular ATP is enzymatically degraded to adenosine within mouse vallate taste buds and that this nucleoside acts as an autocrine neuromodulator to selectively enhance sweet taste. In Receptor cells in a lingual slice preparation, Ca(2+) mobilization evoked by focally applied artificial sweeteners was significantly enhanced by adenosine (50 μM). Adenosine had no effect on bitter or umami taste responses, and the nucleoside did not affect Presynaptic (type III) taste cells. We also used biosensor cells to measure transmitter release from isolated taste buds. Adenosine (5 μM) enhanced ATP release evoked by sweet but not bitter taste stimuli. Using single-cell reverse transcriptase (RT)-PCR on isolated vallate taste cells, we show that many Receptor cells express the adenosine receptor, Adora2b, while Presynaptic (type III) and Glial-like (type I) cells seldom do. Furthermore, Adora2b receptors are significantly associated with expression of the sweet taste receptor subunit, Tas1r2. Adenosine is generated during taste stimulation mainly by the action of the ecto-5'-nucleotidase, NT5E, and to a lesser extent, prostatic acid phosphatase. Both these ecto-nucleotidases are expressed by Presynaptic cells, as shown by single-cell RT-PCR, enzyme histochemistry, and immunofluorescence. Our findings suggest that ATP released during taste reception is degraded to adenosine to exert positive modulation particularly on sweet taste.
哺乳动物的味蕾使用 ATP 作为神经递质。味觉受体（II 型）细胞通过缝隙连接半通道将 ATP 分泌到味蕾内的狭窄细胞外空间中。这种 ATP 会激发初级感觉传入纤维，并刺激相邻的味蕾细胞。在这里，我们表明在小鼠味蕾中，细胞外 ATP 通过酶促降解为腺苷，并且这种核苷作为一种自分泌神经调质选择性地增强甜味。在舌切片制备中的受体细胞中，通过局部施加人工甜味剂引起的 Ca(2+)动员明显被腺苷（50 μM）增强。腺苷对苦味或鲜味反应没有影响，核苷也不影响 Presynaptic（III 型）味觉细胞。我们还使用生物传感器细胞测量从分离的味蕾释放的递质。腺苷（5 μM）增强了由甜味但不是苦味刺激引起的 ATP 释放。通过对分离的味蕾进行单细胞逆转录酶（RT）-PCR，我们表明许多受体细胞表达腺苷受体 Adora2b，而 Presynaptic（III 型）和 Glial-like（I 型）细胞很少表达。此外，Adora2b 受体与甜味受体亚基 Tas1r2 的表达显著相关。腺苷是在味觉刺激期间主要由外核苷酸酶，NT5E 的作用产生的，并且在较小程度上，由前列腺酸性磷酸酶产生。如单细胞 RT-PCR、酶组织化学和免疫荧光所示，这两种外核苷酸酶均由 Presynaptic 细胞表达。我们的发现表明，在味觉接收期间释放的 ATP 被降解为腺苷，以对甜味尤其进行正性调节。
