Department of Otolaryngology, University of Pittsburgh Medical School, BST3 Building, 3501 Fifth Avenue #10016, Pittsburgh, PA, 15261, USA.
Department of Neurobiology, University of Pittsburgh, Pittsburgh, PA, USA.
Brain Struct Funct. 2017 Nov;222(8):3375-3393. doi: 10.1007/s00429-017-1408-0. Epub 2017 Apr 10.
The neurotransmitter receptor subtype, number, density, and distribution relative to the location of transmitter release sites are key determinants of signal transmission. AMPA-type ionotropic glutamate receptors (AMPARs) containing GluA3 and GluA4 subunits are prominently expressed in subsets of neurons capable of firing action potentials at high frequencies, such as auditory relay neurons. The auditory nerve (AN) forms glutamatergic synapses on two types of relay neurons, bushy cells (BCs) and fusiform cells (FCs) of the cochlear nucleus. AN-BC and AN-FC synapses have distinct kinetics; thus, we investigated whether the number, density, and localization of GluA3 and GluA4 subunits in these synapses are differentially organized using quantitative freeze-fracture replica immunogold labeling. We identify a positive correlation between the number of AMPARs and the size of AN-BC and AN-FC synapses. Both types of AN synapses have similar numbers of AMPARs; however, the AN-BC have a higher density of AMPARs than AN-FC synapses, because the AN-BC synapses are smaller. A higher number and density of GluA3 subunits are observed at AN-BC synapses, whereas a higher number and density of GluA4 subunits are observed at AN-FC synapses. The intrasynaptic distribution of immunogold labeling revealed that AMPAR subunits, particularly GluA3, are concentrated at the center of the AN-BC synapses. The central distribution of AMPARs is absent in GluA3-knockout mice, and gold particles are evenly distributed along the postsynaptic density. GluA4 gold labeling was homogenously distributed along both synapse types. Thus, GluA3 and GluA4 subunits are distributed at AN synapses in a target-cell-dependent manner.
神经递质受体亚型、数量、密度和分布与递质释放位点的位置有关，是信号转导的关键决定因素。包含 GluA3 和 GluA4 亚基的 AMPA 型离子型谷氨酸受体 (AMPARs) 在能够高频发放动作电位的神经元亚群中表达丰富，如听觉中继神经元。听神经 (AN) 在耳蜗核的两种中继神经元，即布什细胞 (BC) 和梭形细胞 (FC) 上形成谷氨酸能突触。AN-BC 和 AN-FC 突触具有不同的动力学特性；因此，我们使用定量冷冻断裂复制免疫金标记研究了这些突触中 GluA3 和 GluA4 亚基的数量、密度和定位是否存在差异。我们发现 AMPAR 的数量与 AN-BC 和 AN-FC 突触的大小之间存在正相关。两种类型的 AN 突触具有相似数量的 AMPAR；然而，AN-BC 的 AMPAR 密度高于 AN-FC 突触，因为 AN-BC 突触较小。在 AN-BC 突触中观察到更多和更高密度的 GluA3 亚基，而在 AN-FC 突触中观察到更多和更高密度的 GluA4 亚基。免疫金标记的内突触分布显示，AMPAR 亚基，特别是 GluA3，集中在 AN-BC 突触的中心。在 GluA3 敲除小鼠中，AMPAR 位于中心的分布不存在，金颗粒均匀分布在突触后密度上。GluA4 金标记均匀分布在两种突触类型上。因此，GluA3 和 GluA4 亚基在 AN 突触中的分布是靶细胞依赖性的。
