Department of Cell Biology & Neuroscience, Rutgers University, Piscataway, NJ 08854, USA.
Hear Res. 2011 Aug;278(1-2):52-68. doi: 10.1016/j.heares.2011.01.016. Epub 2011 Jan 31.
As with other elements of the peripheral auditory system, spiral ganglion neurons display specializations that vary as a function of location along the tonotopic axis. Previous work has shown that voltage-gated K(+) channels and synaptic proteins show graded changes in their density that confers rapid responsiveness to neurons in the high frequency, basal region of the cochlea and slower, more maintained responsiveness to neurons in the low frequency, apical region of the cochlea. In order to understand how voltage-gated calcium channels (VGCCs) may contribute to these diverse phenotypes, we identified the VGCC α-subunits expressed in the ganglion, investigated aspects of Ca(2+)-dependent neuronal firing patterns, and mapped the intracellular and intercellular distributions of seven VGCC α-subunits in the spiral ganglion in vitro. Initial experiments with qRT-PCR showed that eight of the ten known VGCC α-subunits were expressed in the ganglion and electrophysiological analysis revealed firing patterns that were consistent with the presence of both LVA and HVA Ca(2+) channels. Moreover, we were able to study seven of the α-subunits with immunocytochemistry, and we found that all were present in spiral ganglion neurons, three of which were neuron-specific (Ca(V)1.3, Ca(V)2.2, and Ca(V)3.3). Further characterization of neuron-specific α-subunits showed that Ca(V)1.3 and Ca(V)3.3 were tonotopically-distributed, whereas Ca(V)2.2 was uniformly distributed in apical and basal neurons. Multiple VGCC α-subunits were also immunolocalized to Schwann cells, having distinct intracellular localizations, and, significantly, appearing to distinguish putative compact (Ca(V)2.3, Ca(V)3.1) from loose (Ca(V)1.2) myelin. Electrophysiological evaluation of spiral ganglion neurons in the presence of TEA revealed Ca(2+) plateau potentials with slopes that varied proportionately with the cochlear region from which neurons were isolated. Because afterhyperpolarizations were minimal or absent under these conditions, we hypothesize that differential density and/or kinetics of one or more of the VGCC α-subunits could account for observed tonotopic differences. These experiments have set the stage for defining the clear multiplicity of functional control in neurons and Schwann cells of the spiral ganglion.
与外周听觉系统的其他元素一样，螺旋神经节神经元表现出随着音调轴上位置的变化而产生的特化。以前的工作表明，电压门控 K(+)通道和突触蛋白的密度呈梯度变化，使耳蜗高频、基底区域的神经元具有快速反应性，而对耳蜗低频、顶端区域的神经元具有较慢、更持久的反应性。为了了解电压门控钙通道 (VGCC) 如何促成这些不同的表型，我们鉴定了在神经节中表达的 VGCC α-亚基，研究了 Ca(2+) 依赖性神经元放电模式的各个方面，并在体外对螺旋神经节中七个 VGCC α-亚基的细胞内和细胞间分布进行了映射。使用 qRT-PCR 的初步实验表明，十个已知的 VGCC α-亚基中有八个在神经节中表达，电生理分析显示的放电模式与 LVA 和 HVA Ca(2+) 通道的存在一致。此外，我们能够使用免疫细胞化学研究七个 α-亚基，我们发现所有这些亚基都存在于螺旋神经节神经元中，其中三个是神经元特异性的 (Ca(V)1.3、Ca(V)2.2 和 Ca(V)3.3)。对神经元特异性 α-亚基的进一步表征表明，Ca(V)1.3 和 Ca(V)3.3 呈音调分布，而 Ca(V)2.2 在顶端和基底神经元中均匀分布。多个 VGCC α-亚基也被免疫定位到施万细胞中，具有不同的细胞内定位，并且重要的是，它们似乎区分了致密 (Ca(V)2.3、Ca(V)3.1) 和疏松 (Ca(V)1.2) 髓鞘。在 TEA 存在的情况下对螺旋神经节神经元进行电生理评估，发现 Ca(2+) 平台电位的斜率与神经元分离的耳蜗区域成比例变化。因为在这些条件下后超极化很小或不存在，所以我们假设一个或多个 VGCC α-亚基的密度和/或动力学的差异可以解释观察到的音调差异。这些实验为确定螺旋神经节神经元和施万细胞中功能控制的明显多样性奠定了基础。
