Friauf E, McConnell S K, Shatz C J
Department of Neurobiology, Stanford University School of Medicine, California 94305.
J Neurosci. 1990 Aug;10(8):2601-13. doi: 10.1523/JNEUROSCI.10-08-02601.1990.
Among the first postmitotic cells of the cerebral cortex is a special population located below the cortical plate: the subplate neurons. These neurons reach a high degree of morphological maturity during fetal life, well before the neurons of the cortical layers have matured, yet nearly all of these cells die after birth in the cat. Subplate neurons are also known to receive synaptic contacts. Here we have investigated whether these contacts are functional by making intracellular recordings from subplate neurons in cortical slices maintained in vitro. Subplate neurons were identified based on their location and morphology by injecting them with biocytin following the intracellular recordings. At all ages studied between embryonic day 50 and postnatal day 9, electrical stimulation of the optic radiations elicited EPSPs and synaptic and antidromic spikes in subplate neurons, indicating that some of the synapses seen at the ultrastructural level are indeed capable of synaptic transmission. The spiking patterns of 39 morphologically identified subplate neurons were examined by injecting depolarizing current, which revealed that a large majority gave only a single spike or a brief train of spikes in response to maintained depolarization, in contrast to the regular spiking pattern found in many neurons of adult cortex. Biocytin injections into subplate neurons revealed that they are a morphologically heterogeneous population with respect to their dendritic branching patterns; roughly half were inverted pyramids, the classic subplate neuron morphology. The axonal processes of subplate neurons were remarkable in that many not only arborized within the subplate, but also entered the cortical plate and terminated in the marginal zone. At early postnatal ages, these axons also gave off collaterals within cortical layer 4. The results of this study indicate that subplate neurons participate in synaptic microcircuits during development. While the presynaptic identity of the input to subplate neurons is not known conclusively, it is likely that geniculocortical axons, which wait in close proximity to subplate neurons, contribute significantly. The pattern of axonal branching of subplate neurons also implies that information conferred to subplate neurons may be relayed, in turn, to the neurons of cortical layer 4. Finally with the death of subplate neurons, the geniculocortical axons leave the subplate and invade the cortical plate to innervate directly the neurons of layer 4. Thus, subplate neurons may function as a crucial, but transient synaptic link between waiting geniculocortical axons and their ultimate target cells in the cortex.
大脑皮层最早的有丝分裂后细胞中，有一类特殊细胞群位于皮层板下方，即亚板神经元。这些神经元在胎儿期就达到了高度的形态成熟，远早于皮层各层神经元成熟，但在猫出生后，几乎所有这些细胞都会死亡。已知亚板神经元会接受突触联系。在此，我们通过对体外培养的皮层切片中的亚板神经元进行细胞内记录，来研究这些联系是否具有功能。细胞内记录后，通过向亚板神经元注射生物胞素，根据其位置和形态来识别它们。在胚胎第50天至出生后第9天的所有研究年龄段，对视辐射进行电刺激都会在亚板神经元中引发兴奋性突触后电位（EPSP）以及突触和逆向动作电位，这表明在超微结构水平上看到的一些突触确实能够进行突触传递。通过注入去极化电流，对39个形态学上已识别的亚板神经元的放电模式进行了检查，结果显示，与成年皮层中许多神经元的规则放电模式不同，绝大多数亚板神经元在持续去极化时仅产生单个动作电位或一串短暂的动作电位。向亚板神经元注射生物胞素后发现，就其树突分支模式而言，它们是形态各异的细胞群；大约一半是倒金字塔形，这是经典的亚板神经元形态。亚板神经元的轴突很显著，许多轴突不仅在亚板内分支，还进入皮层板并在边缘区终止。在出生后早期，这些轴突还会在皮层第4层发出侧支。本研究结果表明，亚板神经元在发育过程中参与突触微电路。虽然亚板神经元输入的突触前身份尚未最终确定，但很可能紧邻亚板神经元的膝状体皮质轴突起到了重要作用。亚板神经元的轴突分支模式还意味着传递给亚板神经元的信息可能会依次传递给皮层第4层的神经元。最后，随着亚板神经元的死亡，膝状体皮质轴突离开亚板并侵入皮层板，直接支配第4层的神经元。因此，亚板神经元可能是等待中的膝状体皮质轴突与其在皮层中的最终靶细胞之间关键但短暂的突触联系。
