Nitsch R, Leranth C
Institute of Anatomy, Humboldt University Clinic, Charité, Berlin, Federal Republic of Germany.
J Comp Neurol. 1996 Jan 15;364(3):425-38. doi: 10.1002/(SICI)1096-9861(19960115)364:3<425::AID-CNE4>3.0.CO;2-6.
Fibers of supramammillary origin establish putatively excitatory asymmetric synaptic connections with dentate granule cells. The present study was designed to determine whether hippocampal gamma-aminobutyric acid (GABA)-ergic nonprincipal cells are also targets of these calretinin (CR)-containing subcortical afferents. Light and electron microscopic double immunostaining for CR and parvalbumin (PA) or calbindin (CB) were performed in the rat dentate gyrus ipsilateral and contralateral to a unilateral fimbria-fornix transection. GABA-postembedding immunostaining was performed on ultrathin sections of this double-labeled material. Contralateral to the transection, CR-immunoreactive fibers formed multiple large boutons in the inner molecular layer. These fibers also impinged on PA-containing basket cells located adjacent to the granular layer and on CB-immunoreactive hilar neurons. Ipsilateral to the transection, CR-containing fibers in the inner molecular layer and boutons impinging on PA-containing or CB-immunoreactive neurons were absent. Parent cell bodies of extrinsic CR-containing afferents were traced using wheat germ agglutinin-conjugated horseradish peroxidase. Additional CR immunostaining of the subcortical region unveiled retrogradely labeled neurons that were also immunostained for CR only in the supramammillary area and the nucleus reuniens. The latter projection, however, terminates in CA1 and not in the dentate gyrus. Subcortical afferents impinging on dentate nonprincipal cells formed exclusively asymmetric synapses. Postembedding immunostaining demonstrated that CB-containing cells contain GABA, whereas CR-positive axon terminals forming asymmetric synapses are devoid of this labeling. These data indicate that dentate inhibitory neurons receive a putative excitatory input originating from the supramammillary nucleus. Thus, the supramamillo-hippocampal pathway may exert a powerful feed-forward inhibitory control of the signal flow in the rat dentate gyrus.
乳头体上核起源的纤维与齿状颗粒细胞建立了推测为兴奋性的不对称突触连接。本研究旨在确定海马γ-氨基丁酸（GABA）能非主细胞是否也是这些含钙视网膜蛋白（CR）的皮质下传入纤维的靶点。对大鼠单侧穹窿-海马伞横断同侧和对侧齿状回进行了CR与小白蛋白（PA）或钙结合蛋白（CB）的光镜和电镜双重免疫染色。对这种双重标记材料的超薄切片进行GABA包埋后免疫染色。在横断的对侧，CR免疫反应性纤维在内分子层形成多个大的终扣。这些纤维还作用于颗粒层附近含PA的篮状细胞和CB免疫反应性的门区神经元。在横断的同侧，内分子层中含CR的纤维以及作用于含PA或CB免疫反应性神经元的终扣缺失。使用小麦胚凝集素结合辣根过氧化物酶追踪含CR的外在传入纤维的母细胞体。对皮质下区域进行额外的CR免疫染色发现了逆行标记的神经元，这些神经元仅在乳头体上核和 reuniens 核中也被CR免疫染色。然而，后一种投射终止于CA1区而非齿状回。作用于齿状非主细胞的皮质下传入纤维仅形成不对称突触。包埋后免疫染色表明，含CB的细胞含有GABA，而形成不对称突触的CR阳性轴突终扣则没有这种标记。这些数据表明，齿状抑制性神经元接受来自乳头体上核的推测性兴奋性输入。因此，乳头体上核-海马通路可能对大鼠齿状回中的信号流施加强大的前馈抑制控制。
