Cell biology of taste buds and analysis of taste transduction mechanisms

胞外核苷酸如ATP是结合膜受体（P2X配体门控离子通道和g蛋白偶联的P2Y家族）的信号分子。在味觉系统中，P2X受体（P2X_2和P2X_3）仅在支配味蕾的神经纤维中表达。此外，在生理研究的基础上，发现P2Y受体在味蕾细胞的味觉信号转导中起一定的作用。然而，P2Y受体亚型的表达模式尚未被揭示。本研究采用RT - PCR、免疫组织化学和原位杂交技术检测了P2Y_1受体亚型在大鼠味觉乳头中的表达规律。这些分析表明P2Y_1受体在味蕾细胞中有表达。此外，我们发现p2y_1表达的细胞通过双免疫标记共同表达IP3R_3和SNAP-25。这些结果表明，ATP可能激活P2Y_1受体，导致Ca^<2+>通过IP3R3从内部储存释放。由于所有的SNAP-25免疫反应性味蕾细胞都共表达P2Y_1免疫反应性，提示表达P2Y_1的细胞可能与传入神经纤维形成突触。除了P2Y_1受体外，P2Y_4受体在味蕾细胞中也有表达。我们得出结论，P2Y和P2X受体似乎都是味蕾功能的重要调节因子。

Extracellular nucleotides such as ATP are the signaling molecules which bind to membrane receptors (P2X ligand-gated ion channels and G-protein coupled P2Y families). In the gustatory system, P2X receptors (P2X_2 and P2X_3) are expressed exclusively in nerve fibers innervating taste buds. Also, P2Y receptors are appeared to play some roles in taste signal transductions in taste bud cells on the basis of the physiological studies. However, the expression patterns of P2Y receptor subtypes have not been revealed. In this study, the expression patterns of P2Y_1 receptor subtypes were examined by using RT PCR, immunohistochemistry and in situ hybridization in rat gustatory papillae. These analyses showed that P2Y_1 receptor was expressed in taste bud cells. Furthermore, we found that P2Y_1-expressed cells coexpressed both IP3R_3 and SNAP-25 by double immunolabeling. These results suggest that ATP may activate P2Y_1 receptors resulting in Ca^<2+> release from internal stores via IP3R3. Since all SNAP-25 immunoreactive taste bud cells coexpressed P2Y_1 immunoreactive, it is suggested that P2Y_1 expressing cells may make synapses with afferent nerve fibers. In addition to P2Y_1 receptors, P2Y_4 receptors were expressed a subset of taste bud cells. We concluded that both P2Y and P2X receptors appear to be important regulators of taste bud functions.