Beaulieu C, Campistron G, Crevier C
Département de Pathologie, Université de Montréal, Canada.
J Comp Neurol. 1994 Jan 22;339(4):559-72. doi: 10.1002/cne.903390407.
The number and size of synaptic contacts made by GABA-immunoreactive axonal boutons were estimated in each layer of the primary visual cortex (area Oc1M) of adult rats by using the dissector method. Immunoreactivity for GABA was detected with the postembedding immunogold technique on ultrathin sections. Targets of GABA synaptic contacts were also identified to predict the sites of GABA influence in the rat visual cortex. For the total cortical depth, 82 million out of an overall population of 666 million synaptic contacts per mm3 of tissue (or 1 in 8 contacts, 12%) were GABA. Layer IV averaged 62% more GABA contacts per unit volume than did any other cortical layer. Consequently, these represented a larger proportion (1 in 6, 17%) of the overall population of layer IV synaptic contacts. This higher number of GABA contacts was not due to a greater density of GABA boutons, but to an increased number of contacts made by each layer IV GABA bouton (mean of 1.4 contacts per bouton compared to 1.1 in other cortical layers). The total area occupied by the contacts on an average GABA bouton was similar in all layers; the higher number of contacts per GABA bouton in layer IV being compensated for by their smaller size. This observed constancy in the area of synaptic contacts suggests the presence of one or more regulatory mechanisms maintaining optimal numbers of the different macromolecules forming the synaptic contacts. The increased density of GABA contacts in layer IV compared to other cortical layers was due to their greater number targeting distal regions of the dendritic tree. Since layer IV receives the vast majority of thalamocortical terminals and since these axons preferentially target dendritic spines, the specific arrangement of GABA synaptic contacts in this layer could be designed to exert a precise inhibition near the site of the thalamic input and thus serve as the structural basis for the strong GABA-related hyperpolarization that followed the excitatory response after physiological stimulations of the thalamocortical pathway.
采用分割法，对成年大鼠初级视皮层（17区）各层中γ-氨基丁酸（GABA）免疫反应性轴突终扣所形成的突触接触的数量和大小进行了估计。利用包埋后免疫金技术在超薄切片上检测GABA的免疫反应性。还确定了GABA突触接触的靶点，以预测GABA在大鼠视皮层中的影响位点。在整个皮层深度，每立方毫米组织中6.66亿个突触接触的总体中，有8200万个（即每8个接触中有1个，占12%）是GABA突触。IV层每单位体积的GABA接触平均比其他任何皮层层多62%。因此，这些在IV层突触接触的总体中占更大比例（每6个中有1个，占17%）。GABA接触数量较多并非由于GABA终扣密度更大，而是由于IV层每个GABA终扣形成的接触数量增加（每个终扣平均1.4个接触，而其他皮层层为1.1个）。所有层中，平均一个GABA终扣上接触所占的总面积相似；IV层每个GABA终扣接触数量较多，但接触较小，相互抵消。观察到的突触接触面积的这种稳定性表明存在一种或多种调节机制，维持形成突触接触的不同大分子的最佳数量。与其他皮层层相比，IV层GABA接触密度增加是由于它们更多地靶向树突树的远端区域。由于IV层接受绝大多数丘脑皮质终末，且这些轴突优先靶向树突棘，该层中GABA突触接触的特定排列可能是为了在丘脑输入部位附近施加精确抑制，从而作为丘脑皮质通路生理刺激后兴奋性反应之后强烈的GABA相关超极化的结构基础。
