Kalloniatis M, Napper G A
Department of Optometry and Vision Sciences, University of Melbourne, Parkville, Victoria, Australia.
J Comp Neurol. 1996 Apr 15;367(4):518-36. doi: 10.1002/(SICI)1096-9861(19960415)367:4<518::AID-CNE4>3.0.CO;2-7.
Glutamate (E) is the putative amino acid neurotransmitter used by ganglion cells, photoreceptors, and bipolar cells. Aspartate (D) and glutamine (Q) are potential precursors of glutamate, and glutamate-utilizing neurons may use one or more of these amino acids to sustain production of glutamate. We used post-embedding immunocytochemistry for several amino acid neurotransmitters to characterize the amino acid signatures for displaced ganglion cells of the avian retina. We found two neurochemical signatures for displaced ganglion cells, EQ and EDQ, in mid-peripheral and far-peripheral retina, respectively. Differences in neurochemical signatures cannot be explained by the existence of two ganglion cell populations, and we propose that the two signature categories for the large-diameter displaced ganglion cells reflect variations in the aspartate precursor pool. The transamination reaction involved in glutamate production, aspartate/oxaloacetate and alpha-ketoglutarate/glutamate, requires an active TCA cycle, since the carbon skeleton of glutamate is derived from alpha-ketoglutarate, a TCA intermediary. We hypothesized that aspartate levels vary in the normal chicken retina because eccentricity-dependent differences in oxygen availability result in changes of alpha-ketoglutarate levels, and hence, alterations in the equilibrium of the transamination reaction. We tested this hypothesis by incubating isolated chicken retinas in anaerobic conditions and found elevated aspartate immunoreactivity in subpopulations of glutamate-utilizing neurons in the central retina. Under aerobic conditions, or in retinas placed directly into fixative, retinal samples from the central edge of the pecten did not show differential cellular staining for aspartate. We have, therefore, identified differences in neurochemical signatures for retinal neurons involving changes in active maintenance of precursor pools.
谷氨酸（E）被认为是神经节细胞、光感受器和双极细胞所使用的氨基酸神经递质。天冬氨酸（D）和谷氨酰胺（Q）是谷氨酸的潜在前体，利用谷氨酸的神经元可能使用这些氨基酸中的一种或多种来维持谷氨酸的产生。我们使用针对几种氨基酸神经递质的包埋后免疫细胞化学方法来表征鸟类视网膜中移位神经节细胞的氨基酸特征。我们分别在视网膜中周部和远周部发现了移位神经节细胞的两种神经化学特征，即EQ和EDQ。神经化学特征的差异不能用两种神经节细胞群体的存在来解释，我们提出大直径移位神经节细胞的这两种特征类别反映了天冬氨酸前体池的变化。谷氨酸产生过程中涉及的转氨反应，即天冬氨酸/草酰乙酸和α-酮戊二酸/谷氨酸反应，需要一个活跃的三羧酸循环，因为谷氨酸的碳骨架来自α-酮戊二酸，一种三羧酸循环中间产物。我们假设正常鸡视网膜中天冬氨酸水平存在差异是因为氧供应的偏心依赖性差异导致α-酮戊二酸水平变化，进而导致转氨反应平衡改变。我们通过在厌氧条件下孵育分离的鸡视网膜来验证这一假设，发现中央视网膜中利用谷氨酸的神经元亚群中天冬氨酸免疫反应性升高。在有氧条件下，或直接放入固定剂中的视网膜中，栉膜中央边缘的视网膜样本未显示出天冬氨酸的细胞染色差异。因此，我们确定了视网膜神经元神经化学特征的差异，这些差异涉及前体池主动维持的变化。
