Soriano E, Nitsch R, Frotscher M
Institute of Anatomy, Johann Wolfgang Goethe University, Frankfurt am Main, Federal Republic of Germany.
J Comp Neurol. 1990 Mar 1;293(1):1-25. doi: 10.1002/cne.902930102.
Synaptic transmission can be blocked very efficiently by inhibitory synapses on axon initial segments. Inhibitory chandelier cells forming synapses on the axon initial segment of pyramidal neurons have been found in the neocortex and hippocampus proper. Here we describe an axo-axonic local circuit neuron in the rat fascia dentata that establishes synaptic contacts with axon initial segments of numerous dentate granule cells. Examination of a large number of Golgi-impregnated nongranule cells in the fascia dentata of rats revealed a group of neurons with characteristics of chandelier cells. Thus these cells exhibited an extensive axonal plexus within the granular layer that characteristically formed vertical aggregations of axonal varicosities. The cell bodies of these neurons were located in the inner molecular layer or in the outer part of the granular layer. Their dendrites invaded the molecular layer, suggesting an afferent innervation similar to that of the granule cells. Well impregnated putative axo-axonic cells were gold-toned for an electron microscopic analysis. The cell bodies and dendrites of these neurons exhibited characteristic ultrastructural features of nongranule cells, i.e., large amounts of perinuclear cytoplasm, infoldings of the nuclear membrane, and a large number of synaptic contacts on the perikaryon and on the smooth dendritic shafts. The axon originating from the cell body or from a proximal dendrite gave rise to numerous vesicle-filled varicosities that almost exclusively formed symmetric synaptic contacts with axon initial segments. A semiquantitative study of five axonal complexes demonstrated that 92.3% of identified postsynaptic elements were initial segments of granule cell axons. Immunostaining with antibodies against glutamate decarboxylase (GAD) and parvalbumin (PARV) revealed a subpopulation of neurons that very much resembled the Golgi-impregnated axo-axonic cells with regard to cell body location, dendritic arborization, and fine structural characteristics of perikarya and dendrites. GAD and PARV were found to be coexistent in these cells. Moreover, we found GAD- and PARV-immunoreactive terminals in symmetric synaptic contact with axon initial segments of granule cells. The present study has shown a hitherto unknown axo-axonic cell in the rat fascia dentata. On the basis of our immunocytochemical findings, we hypothesize that this cell exerts a strong inhibitory effect on dentate granule cells. This way, signal transmission from the fascia dentata to the hippocampus proper within the "trisynaptic pathway" can efficiently be controlled by a group of highly specialized neurons.
轴突起始段上的抑制性突触能够非常有效地阻断突触传递。在新皮层和海马本部已发现形成突触于锥体神经元轴突起始段的抑制性吊灯细胞。在此，我们描述了大鼠齿状回中的一种轴 - 轴突局部回路神经元，它与众多齿状颗粒细胞的轴突起始段建立突触联系。对大鼠齿状回中大量高尔基染色的非颗粒细胞进行检查，发现了一组具有吊灯细胞特征的神经元。因此，这些细胞在颗粒层内呈现出广泛的轴突丛，其特征是形成轴突膨体的垂直聚集。这些神经元的胞体位于内分子层或颗粒层的外部。它们的树突侵入分子层，表明其传入神经支配与颗粒细胞相似。对染色良好的假定轴 - 轴突细胞进行金染色以进行电子显微镜分析。这些神经元的胞体和树突呈现出非颗粒细胞特有的超微结构特征，即大量核周细胞质、核膜内褶以及在胞体和光滑树突干上有大量突触联系。源自胞体或近端树突的轴突产生大量充满囊泡的膨体，这些膨体几乎仅与轴突起始段形成对称突触联系。对五个轴突复合体的半定量研究表明，92.3%的已识别突触后成分是颗粒细胞轴突的起始段。用抗谷氨酸脱羧酶（GAD）和小白蛋白（PARV）抗体进行免疫染色显示，在细胞体位置、树突分支以及胞体和树突的精细结构特征方面，有一群神经元与高尔基染色的轴 - 轴突细胞非常相似。发现GAD和PARV在这些细胞中共存。此外，我们发现GAD和PARV免疫反应性终末与颗粒细胞轴突起始段形成对称突触联系。本研究显示了大鼠齿状回中一种迄今未知的轴 - 轴突细胞。基于我们的免疫细胞化学发现，我们推测这种细胞对齿状颗粒细胞发挥强大的抑制作用。通过这种方式，在“三突触通路”中从齿状回向海马本部的信号传递能够被一组高度特化的神经元有效控制。
