Smith G T, Lu Y, Zakon H H
Section of Neurobiology, School of Biological Sciences, University of Texas, Austin 78712, USA.
J Comp Neurol. 2000 Jul 31;423(3):427-39.
Gymnotiform weakly electric fish produce electric organ discharges (EODs) that function in electrolocation and communication. The command signal for the EOD is produced by the medullary pacemaker nucleus, which contains two well-characterized neuron types: pacemaker cells and relay cells. In this study, we characterized a third neuron type in the pacemaker nucleus. These neurons, which we have named parvocells, were smaller (7-15 microm in diameter) than relay and pacemaker cells. The parvocells were labeled with an antibody against the neuronal calcium-binding protein, parvalbumin, and were not labeled with several glial-specific antibodies. Parvocells had one to three fine processes that often terminated at the periphery of relay and pacemaker cell bodies. The parvalbumin-positive terminals of the parvocells colocalized with immunoreactivity for SV-2, suggesting that the parvocells form chemical synapses on the relay and pacemaker cells. Parvalbumin-positive neurons are frequently gamma-aminobutyric acid (GABA)ergic or glycinergic, and the cytoplasm of the parvocell somata was immunoreactive with a glycine antibody. Antibodies against glycine receptors and gephyrin, however, did not label any cells in the pacemaker nucleus, suggesting that the pacemaker nucleus does not contain glycine or GABA((A)) receptors. Electron microscopy revealed gap junctions between the membranes of parvocells and adjacent terminal-like structures. Furthermore, neurobiotin injected into individual pacemaker or relay cells labeled parvocells as well as other pacemaker and relay cells, demonstrating that the parvocells are dye-coupled to the other neuron types in the pacemaker nucleus. These findings indicate that the parvocells are histochemically distinct from relay and pacemaker cells and that they receive electrotonic inputs from and make chemical synapses back onto pacemaker and relay cells. Further study is needed to investigate the function of these neurons in regulating the EOD.
裸背电鳗目弱电鱼产生用于电定位和通信的电器官放电（EODs）。EOD的指令信号由延髓起搏器核产生，该核包含两种特征明确的神经元类型：起搏器细胞和中继细胞。在本研究中，我们对起搏器核中的第三种神经元类型进行了特征描述。我们将这些神经元命名为小细胞，它们比中继细胞和起搏器细胞小（直径7 - 15微米）。小细胞用抗神经元钙结合蛋白小白蛋白的抗体标记，而未用几种胶质细胞特异性抗体标记。小细胞有一到三个细小突起，这些突起常常终止于中继细胞和起搏器细胞体的周边。小细胞的小白蛋白阳性终末与SV - 2的免疫反应性共定位，表明小细胞在中继细胞和起搏器细胞上形成化学突触。小白蛋白阳性神经元通常是γ-氨基丁酸（GABA）能或甘氨酸能的，小细胞胞体的细胞质与甘氨酸抗体发生免疫反应。然而，抗甘氨酸受体和gephyrin的抗体未标记起搏器核中的任何细胞，这表明起搏器核中不含有甘氨酸或GABA(A)受体。电子显微镜显示小细胞的膜与相邻的终末样结构之间存在缝隙连接。此外，注入单个起搏器或中继细胞的神经生物素标记了小细胞以及其他起搏器和中继细胞，表明小细胞与起搏器核中的其他神经元类型存在染料偶联。这些发现表明，小细胞在组织化学上与中继细胞和起搏器细胞不同，它们从起搏器和中继细胞接收电紧张性输入，并向其返回形成化学突触。需要进一步研究来探究这些神经元在调节EOD中的功能。
