Verbny Yakov I, Erdélyi Ferenc, Szabó Gábor, Banks Matthew I
Department of Anesthesiology, University of Wisconsin, 1300 University Avenue, Room 4605, Madison, WI 53706, USA.
J Neurophysiol. 2006 Dec;96(6):3194-208. doi: 10.1152/jn.00484.2006. Epub 2006 Sep 13.
Feedforward inhibition triggered by thalamocortical (TC) afferents sharpens onset responses and shapes receptive fields of pyramidal cells in auditory cortex (ACx). Previous studies focused only on interneurons located in and around layer IV in primary ACx, target of the dense thalamic projections from ventral medial geniculate. We investigated a population of feedforward interneurons located throughout layers I-V and activated by both afferents from primary and nonprimary thalamus using recordings from auditory TC brain slices obtained from mice expressing green fluorescent protein under control of the glutamic acid decarboxylase (GAD65) promoter in a subpopulation of cortical GABAergic cells. We studied the responses of these interneurons and of pyramidal cells in ACx to thalamic stimulation and to hyper- and depolarizing current pulses. Most interneurons exhibited monosynaptic responses to thalamic stimulation, but this excitation was weak and subthreshold. Interneurons had multipolar dendritic morphology with widespread and dense axonal projections extending several hundred micrometers from the soma. In pyramidal cells from layers II-IV, thalamic excitatory postsynaptic potentials were significantly larger than in interneurons and were superthreshold in 40% of cells, but in these cells, there was no evidence of feedforward inhibition. By contrast, feedforward inhibition was observed in 12 of 18 layer V pyramidal cells. Thus feedforward inhibition in supragranular layers of ACx is weak, and these interneurons require coincident excitation to be activated by thalamic inputs.
丘脑皮质（TC）传入纤维触发的前馈抑制可增强听觉皮层（ACx）中锥体细胞的起始反应并塑造其感受野。以往的研究仅聚焦于位于初级ACx第IV层及其周围的中间神经元，这些区域是来自腹内侧膝状体密集丘脑投射的靶点。我们使用从在皮质GABA能细胞亚群中谷氨酸脱羧酶（GAD65）启动子控制下表达绿色荧光蛋白的小鼠获得的听觉TC脑片记录，研究了分布在I-V层且由初级和非初级丘脑传入纤维激活的一群前馈中间神经元。我们研究了这些中间神经元以及ACx中锥体细胞对丘脑刺激以及超极化和去极化电流脉冲的反应。大多数中间神经元对丘脑刺激表现出单突触反应，但这种兴奋较弱且低于阈值。中间神经元具有多极树突形态，其轴突投射广泛且密集，从胞体延伸数百微米。在II-IV层的锥体细胞中，丘脑兴奋性突触后电位明显大于中间神经元，并且在40%的细胞中超过阈值，但在这些细胞中，没有前馈抑制的证据。相比之下，在18个V层锥体细胞中有12个观察到前馈抑制。因此，ACx颗粒上层的前馈抑制较弱，这些中间神经元需要同时兴奋才能被丘脑输入激活。
