Joshi Ankur, Middleton Jason W, Anderson Charles T, Borges Katharine, Suter Benjamin A, Shepherd Gordon M G, Tzounopoulos Thanos
Departments of Otolaryngology and Neurobiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261.
Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, and.
J Neurosci. 2015 Feb 18;35(7):3112-23. doi: 10.1523/JNEUROSCI.0836-14.2015.
Auditory cortex (AC) layer 5B (L5B) contains both corticocollicular neurons, a type of pyramidal-tract neuron projecting to the inferior colliculus, and corticocallosal neurons, a type of intratelencephalic neuron projecting to contralateral AC. Although it is known that these neuronal types have distinct roles in auditory processing and different response properties to sound, the synaptic and intrinsic mechanisms shaping their input-output functions remain less understood. Here, we recorded in brain slices of mouse AC from retrogradely labeled corticocollicular and neighboring corticocallosal neurons in L5B. Corticocollicular neurons had, on average, lower input resistance, greater hyperpolarization-activated current (Ih), depolarized resting membrane potential, faster action potentials, initial spike doublets, and less spike-frequency adaptation. In paired recordings between single L2/3 and labeled L5B neurons, the probabilities of connection, amplitude, latency, rise time, and decay time constant of the unitary EPSC were not different for L2/3→corticocollicular and L2/3→corticocallosal connections. However, short trains of unitary EPSCs showed no synaptic depression in L2/3→corticocollicular connections, but substantial depression in L2/3→corticocallosal connections. Synaptic potentials in L2/3→corticocollicular connections decayed faster and showed less temporal summation, consistent with increased Ih in corticocollicular neurons, whereas synaptic potentials in L2/3→corticocallosal connections showed more temporal summation. Extracellular L2/3 stimulation at two different rates resulted in spiking in L5B neurons; for corticocallosal neurons the spike rate was frequency dependent, but for corticocollicular neurons it was not. Together, these findings identify cell-specific intrinsic and synaptic mechanisms that divide intracortical synaptic excitation from L2/3 to L5B into two functionally distinct pathways with different input-output functions.
听觉皮层（AC）的第5B层（L5B）包含皮质-丘系神经元（一种投射到下丘的锥体束神经元）和皮质-胼胝体神经元（一种投射到对侧AC的脑内神经元）。尽管已知这些神经元类型在听觉处理中具有不同作用且对声音有不同的反应特性，但塑造其输入-输出功能的突触和内在机制仍不太清楚。在这里，我们在小鼠AC脑片中记录了L5B中经逆行标记的皮质-丘系神经元和相邻的皮质-胼胝体神经元。皮质-丘系神经元平均具有较低的输入电阻、较大的超极化激活电流（Ih）、去极化的静息膜电位、更快的动作电位、初始峰电位双峰以及较少的峰频率适应。在单个L2/3神经元与标记的L5B神经元的配对记录中，对于L2/3→皮质-丘系连接和L2/3→皮质-胼胝体连接，单突触兴奋性突触后电流（EPSC）的连接概率、幅度、潜伏期、上升时间和衰减时间常数并无差异。然而，短串单突触EPSC在L2/3→皮质-丘系连接中未显示突触抑制，但在L2/3→皮质-胼胝体连接中则有明显抑制。L2/3→皮质-丘系连接中的突触电位衰减更快且显示出较少的时间总和，这与皮质-丘系神经元中Ih增加一致，而L2/3→皮质-胼胝体连接中的突触电位显示出更多的时间总和。以两种不同频率对L2/3进行细胞外刺激会导致L5B神经元产生动作电位；对于皮质-胼胝体神经元，动作电位发放频率与刺激频率相关，但对于皮质-丘系神经元则不然。总之，这些发现确定了细胞特异性的内在和突触机制，这些机制将从L2/3到L5B的皮质内突触兴奋分为两条功能不同、具有不同输入-输出功能的通路。
