Dhillon A, Jones R S
University Department of Pharmacology, Mansfield Road, OX1 3QT, Oxford, UK.
Neuroscience. 2000;99(3):413-22. doi: 10.1016/s0306-4522(00)00225-6.
Paired intracellular recordings were used to investigate recurrent excitatory transmission in layers II, III and V of the rat entorhinal cortex in vitro. There was a relatively high probability of finding a recurrent connection between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurrent synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generating synchronized epileptiform activity. In general, recurrent excitatory postsynaptic potentials in layers III and V of the entorhinal cortex had similar properties to those recorded in other cortical areas, although the probabilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent potentials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole receptors, although there appeared to be a slow component mediated by N-methyl-D-aspartate receptors. In layer III, recurrent transmission failed on about 30% of presynaptic action potentials evoked at 0.2Hz. This failure rate increased markedly with increasing (2, 3Hz) frequency of activation. In layer V the failure rate at low frequency was less (19%), and although it increased at higher frequencies this effect was less pronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neurons. We have previously suggested that layers IV/V of the entorhinal cortex readily generate synchronized epileptiform discharges, whereas layer II is relatively resistant to seizure generation. The present demonstration that recurrent excitatory connections are widespread in layer V but not layer II could support this proposal. The relatively high degree of recurrent connections and electrical coupling between layer III cells may be a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions.
采用配对细胞内记录技术，在体外研究大鼠内嗅皮层II、III和V层中的反复兴奋性传递。在V层（约12%）和III层（约9%）的锥体细胞对之间发现反复连接的概率相对较高。与之形成鲜明对比的是，我们未能在II层的主要神经元之间发现任何反复性突触连接，这可能是该层在产生同步癫痫样活动时相对具有抗性的一个重要因素。总体而言，内嗅皮层III和V层中的反复兴奋性突触后电位与在其他皮层区域记录到的电位具有相似的特性，尽管连接概率是所报道的最高概率之一。在V层记录到的反复兴奋性突触后电位比在III层记录到的电位更小，上升时间更快。在这两层中，反复电位主要由作用于α-氨基-3-羟基-5-甲基-4-异恶唑受体的谷氨酸介导，尽管似乎有一个由N-甲基-D-天冬氨酸受体介导的慢成分。在III层，以0.2Hz诱发的约30%的突触前动作电位会出现反复传递失败。随着激活频率增加（2、3Hz），这种失败率显著增加。在V层，低频时的失败率较低（19%），尽管在较高频率时失败率也增加，但这种效应不如在III层明显。最后，在III层，有证据表明锥体细胞之间存在相对较高的电耦合概率。我们之前曾提出，内嗅皮层的IV/V层容易产生同步癫痫样放电，而II层对癫痫发作相对具有抗性。目前关于反复兴奋性连接在V层广泛存在而在II层不存在的证明可能支持这一观点。III层细胞之间相对较高程度的反复连接和电耦合可能是其在慢性癫痫状态下易发生神经退行性变的一个因素。
