Gábriel R, Straznicky C
Department of Zoology, Janus Pannonius University, Pécs, Hungary.
J Hirnforsch. 1995;36(3):329-40.
The central termination and the transmitter content of the optic fibers and the neurochemical nature of their synaptic targets was investigated in the optic tectum of the toad Bufo marinus. Retinal ganglion cells were retrogradely filled from the tectum with the fluorescent dye DiI and the retinal wholemounts were immunostained for glutamate. Most of the dye-filled cells could be double labeled. In addition, double-labeling for gamma-aminobutyric acid an glutamate were also made, when colocalization of these markers was not observed in the neurons of the retinal ganglion cell layer. In order to identify retinal terminals in the optic tectum, optic axons were retrogradely filled with horseradish peroxidase. Postembedding immunocytochemistry showed that 88% of the optic axon terminals were glutamate-like immunoreactive, 6% gamma-aminobutyric acid-immunoreactive and 6% were negative for both GABA and glutamate. Optic fibre terminals synapsed on gamma-aminobutyric acid- or glutamate-containing postsynaptic profiles (58% and 7%, respectively), while the rest on immunonegative elements. Optic fibres containing glutamate rarely synapsed with glutamate-like immunoreactive postsynaptic elements. In contrast, 67% of the gamma-aminobutyric acid-immunoreactive optic terminals synapsed onto gamma-aminobutyric acid-positive dendrites. It has been observed after combination of anterograde tracer transport and double-label immunocytochemistry, that 57% of the optic terminals synapsed on gamma-aminobutyric acid-immunoreactive elements and 38% on dendrites containing neither gamma-aminobutyric acid- nor glutamate-immunoreactive materials. These results suggest that (1) a large number of ganglion cells use glutamate and some gamma-aminobutyric acid as a transmitter, (2) a substantial proportion of the optic axons terminate on gamma-aminobutyric acid-containing inhibitory interneurons in the tectum, (3) some intrinsic neurons in the tectum are glutamate-like immunoreactive. We also propose, that (4) gamma-aminobutyric acid-immunoreactive optic axons may form an effective disinhibitory circuit in the tectum by synapsing preferentially with local inhibitory interneurons.
在海蟾蜍（Bufo marinus）的视顶盖中，研究了视神经纤维的中央终末、递质含量及其突触靶点的神经化学性质。用荧光染料DiI从视顶盖逆行标记视网膜神经节细胞，并对视网膜整装片进行谷氨酸免疫染色。大多数被染料标记的细胞可进行双重标记。此外，当在视网膜神经节细胞层的神经元中未观察到γ-氨基丁酸和谷氨酸的共定位时，也进行了γ-氨基丁酸和谷氨酸的双重标记。为了识别视顶盖中的视网膜终末，用辣根过氧化物酶逆行标记视神经轴突。包埋后免疫细胞化学显示，88%的视神经轴突终末呈谷氨酸样免疫反应阳性，6%呈γ-氨基丁酸免疫反应阳性，6%对γ-氨基丁酸和谷氨酸均呈阴性。视神经纤维终末与含γ-氨基丁酸或谷氨酸的突触后成分形成突触（分别为58%和7%），其余与免疫阴性成分形成突触。含谷氨酸的视神经纤维很少与谷氨酸样免疫反应阳性的突触后成分形成突触。相反，67%的γ-氨基丁酸免疫反应阳性的视神经终末与γ-氨基丁酸阳性的树突形成突触。在顺行示踪剂运输和双重标记免疫细胞化学相结合后观察到，57%的视神经终末与γ-氨基丁酸免疫反应阳性成分形成突触，38%与既不含γ-氨基丁酸也不含谷氨酸免疫反应物质的树突形成突触。这些结果表明：（1）大量神经节细胞使用谷氨酸和一些γ-氨基丁酸作为递质；（2）相当一部分视神经轴突终止于视顶盖中含γ-氨基丁酸的抑制性中间神经元；（3）视顶盖中的一些内在神经元呈谷氨酸样免疫反应阳性。我们还提出：（4）γ-氨基丁酸免疫反应阳性的视神经轴突可能通过优先与局部抑制性中间神经元形成突触，在视顶盖中形成有效的去抑制回路。
