Luna Victor M, Schoppa Nathan E
Department of Physiology and Biophysics, University of Colorado at Denver, Anschutz Medical Campus, Aurora, Colorado 80045, USA.
J Neurosci. 2008 Aug 27;28(35):8851-9. doi: 10.1523/JNEUROSCI.2385-08.2008.
Odor coding in mammals is widely believed to involve synchronized gamma frequency (30-70 Hz) oscillations in the first processing structure, the olfactory bulb. How such inputs are read in downstream cortical structures however is not known. Here we used patch-clamp recordings in rat piriform cortex slices to examine cellular mechanisms that shape how the cortex integrates inputs from bulb mitral cells. Electrical stimulation of mitral cell axons in the lateral olfactory tract (LOT) resulted in excitation of pyramidal cells (PCs), which was followed approximately 10 ms later by inhibition that was highly reproducible between trials in its onset time. This inhibition was somatic in origin and appeared to be driven through a feedforward mechanism, wherein GABAergic interneurons were directly excited by mitral cell axons. The precise inhibition affected action potential firing in PCs in two distinct ways. First, by abruptly terminating PC excitation, it limited the PC response to each EPSP to exactly one, precisely timed action potential. In addition, inhibition limited the summation of EPSPs across time, such that PCs fired action potentials in strong preference for synchronized inputs arriving in a time window of <5 ms. Both mechanisms would help ensure that PCs respond faithfully and selectively to mitral cell inputs arriving as a synchronized gamma frequency pattern.
哺乳动物的气味编码普遍被认为涉及在首个处理结构即嗅球中同步的伽马频率（30 - 70赫兹）振荡。然而，下游皮质结构如何读取这些输入尚不清楚。在这里，我们使用大鼠梨状皮质切片的膜片钳记录来研究塑造皮质整合来自嗅球二尖瓣细胞输入方式的细胞机制。电刺激外侧嗅束（LOT）中的二尖瓣细胞轴突会导致锥体细胞（PCs）兴奋，大约10毫秒后会出现抑制，这种抑制在不同试验之间的起始时间高度可重复。这种抑制起源于体细胞，似乎是通过前馈机制驱动的，其中GABA能中间神经元被二尖瓣细胞轴突直接兴奋。精确的抑制以两种不同方式影响PCs的动作电位发放。首先，通过突然终止PCs的兴奋，它将PCs对每个兴奋性突触后电位（EPSP）的反应限制为恰好一个精确计时的动作电位。此外，抑制限制了EPSP随时间的总和，使得PCs强烈倾向于对在<5毫秒时间窗口内到达的同步输入发放动作电位。这两种机制都将有助于确保PCs对以同步伽马频率模式到达的二尖瓣细胞输入做出忠实且选择性的反应。
