Dvoryanchikov Gennady, Tomchik Seth M, Chaudhari Nirupa
Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, Florida 33136, USA.
J Comp Neurol. 2007 Nov 20;505(3):302-13. doi: 10.1002/cne.21494.
Although adenosine triphosphate (ATP) is known to be an afferent transmitter in the peripheral taste system, serotonin (5-HT) and norepinephrine (NE) have also been proposed as candidate neurotransmitters and have been detected immunocytochemically in mammalian taste cells. To understand the significance of biogenic amines in taste, we evaluated the ability of taste cells to synthesize, transport, and package 5-HT and NE. We show by reverse transcriptase-polymerase chain reaction and immunofluorescence microscopy that the enzymes for 5-HT synthesis, tryptophan hydroxylase (TPH) and aromatic amino acid decarboxylase (AADC) are expressed in taste cells. In contrast, enzymes necessary for NE synthesis, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) are absent. Both TH and DBH are expressed in nerve fibers that penetrate taste buds. Taste buds also robustly express plasma membrane transporters for 5-HT and NE. Within the taste bud NET, a specific NE transporter, is expressed in some presynaptic (type III) and some glial-like (type I) cells but not in receptor (type II) cells. By using enzyme immunoassay, we show uptake of NE, probably through NET in taste epithelium. Proteins involved in inactivating and packaging NE, including catechol-O-methyltransferase (COMT), monoamine oxidase-A (MAO-A), vesicular monoamine transporter (VMAT1,2) and chromogranin A (ChrgA), are also expressed in taste buds. Within the taste bud, ChrgA is found only in presynaptic cells and may account for dense-cored vesicles previously seen in some taste cells. In summary, we postulate that aminergic presynaptic taste cells synthesize only 5-HT, whereas NE (perhaps secreted by sympathetic fibers) may be concentrated and repackaged for secretion.
虽然已知三磷酸腺苷（ATP）是外周味觉系统中的传入递质，但血清素（5-羟色胺，5-HT）和去甲肾上腺素（NE）也被认为是候选神经递质，并且已通过免疫细胞化学方法在哺乳动物味觉细胞中检测到。为了了解生物胺在味觉中的意义，我们评估了味觉细胞合成、转运和包装5-HT和NE的能力。我们通过逆转录聚合酶链反应和免疫荧光显微镜观察发现，5-HT合成所需的酶，色氨酸羟化酶（TPH）和芳香族氨基酸脱羧酶（AADC）在味觉细胞中表达。相比之下，NE合成所需的酶，酪氨酸羟化酶（TH）和多巴胺β-羟化酶（DBH）则不存在。TH和DBH均在穿透味蕾的神经纤维中表达。味蕾还强烈表达5-HT和NE的质膜转运体。在味蕾中，特定的NE转运体NET在一些突触前（III型）和一些胶质样（I型）细胞中表达，但在受体（II型）细胞中不表达。通过酶免疫测定，我们发现NE可能通过味觉上皮中的NET被摄取。参与NE失活和包装的蛋白质，包括儿茶酚-O-甲基转移酶（COMT）、单胺氧化酶-A（MAO-A）、囊泡单胺转运体（VMAT1、2）和嗜铬粒蛋白A（ChrgA），也在味蕾中表达。在味蕾内，ChrgA仅在突触前细胞中发现，可能与先前在一些味觉细胞中看到的致密核心囊泡有关。总之，我们推测胺能突触前味觉细胞仅合成5-HT，而NE（可能由交感神经纤维分泌）可能被浓缩并重新包装以进行分泌。
