Gonchar Y, Burkhalter A
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Comp Neurol. 1999 Apr 12;406(3):346-60. doi: 10.1002/(sici)1096-9861(19990412)406:3<346::aid-cne4>3.0.co;2-e.
In rat visual cortex, forward and feedback interareal pathways innervate both pyramidal and gamma-aminobutyric acid (GABA)ergic (Johnson and Burkhalter [1996] J. Comp. Neurol. 368:383-398). GABAergic neurons consist of different cell types of which the largest group expresses parvalbumin (PV; Gonchar and Burkhalter [1997] Cereb. Cortex 4:347-358). Here, we report that PV neurons in layers 2/3 are synaptic targets of forward and feedback projections between area 17 and the lateromedial area (LM) of rat visual cortex. In both forward and feedback pathways, approximately 90% of axon terminals in layer 2/3 labeled by tracing with biotinylated dextran amine formed synapses with PV-negative profiles. In both pathways, most of these profiles resembled dendritic spines. Although there were no differences in the innervation of PV-negative targets, the two pathways differed in the innervation of PV-positive neurons. In each pathway, approximately 10% of terminals formed synapses with PV-positive profiles. However, in the forward pathway, the size of the contacted PV-positive profiles was larger than in the feedback pathway. Moreover, in the forward pathway, axon terminals on PV-positive profiles were larger, contained more mitochondria and docked synaptic vesicles than feedback synapses on PV neurons. Our results show that PV neurons provide a major target for area 17 <-> LM forward and feedback pathways terminating in upper layers. In each pathway, the proportion of axons contacting PV neurons is similar. However, both pathways differ in the subcellular localization and morphology of synapses on PV neurons. These asymmetries may contribute to the inequality in the strength of disynaptic inhibition evoked by forward and feedback inputs (Shao and Burkhalter [1996] J. Neurosci. 16:7353-7365).
在大鼠视觉皮层中，前向和反馈区域间通路支配锥体细胞和γ-氨基丁酸（GABA）能神经元（约翰逊和伯克哈尔特[1996]《比较神经学杂志》368:383 - 398）。GABA能神经元由不同细胞类型组成，其中最大的一组表达小白蛋白（PV；贡恰尔和伯克哈尔特[1997]《大脑皮层》4:347 - 358）。在此，我们报告称，第2/3层中的PV神经元是大鼠视觉皮层17区与外侧内侧区（LM）之间前向和反馈投射的突触靶点。在前向和反馈通路中，用生物素化葡聚糖胺追踪标记的第2/3层中约90%的轴突终末与PV阴性形态形成突触。在这两条通路中，这些形态大多类似于树突棘。虽然在对PV阴性靶点的支配上没有差异，但这两条通路在对PV阳性神经元的支配上有所不同。在每条通路中，约10%的终末与PV阳性形态形成突触。然而，在前向通路中，所接触的PV阳性形态的尺寸大于反馈通路中的。此外，在前向通路中，PV阳性形态上的轴突终末更大，含有更多线粒体，且与PV神经元上的反馈突触相比，停靠的突触小泡更多。我们的结果表明，PV神经元是终止于上层的17区<->LM前向和反馈通路的主要靶点。在每条通路中，与PV神经元接触的轴突比例相似。然而，这两条通路在PV神经元上突触的亚细胞定位和形态上存在差异。这些不对称性可能导致前向和反馈输入所诱发的双突触抑制强度不等（邵和伯克哈尔特[1996]《神经科学杂志》16:7353 - 7365）。
