Ona-Jodar Tiffany, Gerkau Niklas J, Sara Aghvami S, Rose Christine R, Egger Veronica
Neurophysiology, Institute of Zoology, Universität Regensburg Regensburg, Germany.
Institute of Neurobiology, Heinrich-Heine-Universität Düsseldorf Düsseldorf, Germany.
Front Cell Neurosci. 2017 Feb 28;11:50. doi: 10.3389/fncel.2017.00050. eCollection 2017.
Dendrodendritic synaptic interactions are a hallmark of neuronal processing in the vertebrate olfactory bulb. Many classes of olfactory bulb neurons including the principal mitral cells (MCs) and the axonless granule cells (GCs) dispose of highly efficient propagation of action potentials (AP) within their dendrites, from where they can release transmitter onto each other. So far, backpropagation in GC dendrites has been investigated indirectly via Ca imaging. Here, we used two-photon Na imaging to directly report opening of voltage-gated sodium channels due to AP propagation in both cell types. To this end, neurons in acute slices from juvenile rat bulbs were filled with 1 mM SBFI via whole-cell patch-clamp. Calibration of SBFI signals revealed that a change in fluorescence Δ/ by 10% corresponded to a Δ[Na] of ∼22 mM. We then imaged proximal axon segments of MCs during somatically evoked APs (sAP). While single sAPs were detectable in ∼50% of axons, trains of 20 sAPs at 50 Hz always resulted in substantial Δ/ of ∼15% (∼33 mM Δ[Na]). Δ/ was significantly larger for 80 Hz vs. 50 Hz trains, and decayed with half-durations τ ∼0.6 s for both frequencies. In MC lateral dendrites, AP trains yielded small Δ/ of ∼3% (∼7 mM Δ[Na]). In GC apical dendrites and adjacent spines, single sAPs were not detectable. Trains resulted in an average dendritic Δ/ of 7% (16 mM Δ[Na]) with τ ∼1 s, similar for 50 and 80 Hz. Na transients were indistinguishable between large GC spines and their adjacent dendrites. Cell-wise analysis revealed two classes of GCs with the first showing a decrease in Δ/ along the dendrite with distance from the soma and the second an increase. These classes clustered with morphological parameters. Simulations of Δ[Na] replicated these behaviors via negative and positive gradients in Na current density, assuming faithful AP backpropagation. Such specializations of dendritic excitability might confer specific temporal processing capabilities to bulbar principal cell-GC subnetworks. In conclusion, we show that Na imaging provides a valuable tool for characterizing AP invasion of MC axons and GC dendrites and spines.
树突 - 树突突触相互作用是脊椎动物嗅球神经元处理过程的一个标志。许多种类的嗅球神经元，包括主要的二尖瓣细胞（MCs）和无轴突颗粒细胞（GCs），在其树突内具有高效的动作电位（AP）传播能力，从那里它们可以相互释放递质。到目前为止，GC树突中的动作电位反向传播是通过钙成像间接研究的。在这里，我们使用双光子钠成像直接报告由于AP在两种细胞类型中传播导致的电压门控钠通道的开放。为此，通过全细胞膜片钳将来自幼年大鼠嗅球急性切片中的神经元用1 mM SBFI填充。SBFI信号的校准显示荧光变化Δ/ 10%对应于约22 mM的Δ[Na]。然后，我们在体细胞诱发的动作电位（sAP）期间对MCs的近端轴突段进行成像。虽然在约50%的轴突中可检测到单个sAP，但以50 Hz频率的20个sAP序列总是导致约15%（约33 mM Δ[Na]）的显著Δ/。80 Hz序列的Δ/比50 Hz序列显著更大，并且两种频率下的半衰期τ约为0.6 s时衰减。在MC侧向树突中，动作电位序列产生约3%（约7 mM Δ[Na]）的小Δ/。在GC顶端树突和相邻棘中，单个sAP不可检测。序列导致平均树突Δ/为7%（16 mM Δ[Na]），τ约为1 s，50 Hz和80 Hz时相似。大GC棘与其相邻树突之间的钠瞬变无法区分。细胞水平分析揭示了两类GCs，第一类显示随着距离胞体的距离树突上的Δ/降低，第二类则增加。这些类别与形态学参数聚集在一起。假设动作电位忠实反向传播，通过钠电流密度的负梯度和正梯度对Δ[Na]的模拟复制了这些行为。树突兴奋性的这种特殊性可能赋予嗅球主细胞-GC子网络特定的时间处理能力。总之，我们表明钠成像为表征AP侵入MC轴突以及GC树突和棘提供了一种有价值的工具。
