Dynamics Of Excitatory Synaptic Transmission In The Hippocampus

大脑传递感觉刺激，处理信息，并在通过突触连接连接在一起的大型神经元网络中存储记忆。我们的实验室正在努力了解突触的特定特征如何影响它们的强度、可靠性和独立性，以及这些属性如何影响它们在网络功能中的作用。化学突触连接是通过释放可扩散的神经递质分子来建立的，这些分子与受体神经元上的受体结合；最近的证据表明，神经递质可能会逃离释放它的突触，扩散到邻近的突触中。神经递质在突触之间的这种“溢出”可能会对神经网络的信息能力以及在发育过程中管理其构建的规则产生深远的影响。我们致力于确定兴奋性神经递质谷氨酸在海马体突触之间溢出的程度，海马体是大脑中学习和记忆存储的主要部位。 在小鼠海马片上使用电生理技术，我们发现谷氨酸从释放的突触中逃逸，并扩散到邻近的突触中。这种扩散受到谷氨酸转运蛋白的严格调控，谷氨酸转运蛋白主要位于神经胶质膜上，与谷氨酸结合并将其从细胞外液中移除。正在继续研究这些机制的调制及其对海马神经和视网膜神经网络信息处理的影响。神经元谷氨酸转运体的数量比神经胶质细胞上的转运体少得多，但似乎限制了癫痫的发生，因此我们对神经元谷氨酸转运体如何有助于清除神经递质和突触连接的特异性特别感兴趣。 我们最近提交的工作首次表明，海马区缓冲区中的神经元转运蛋白在其释放部位附近突触地释放谷氨酸，延缓了谷氨酸从突触周围区域扩散出去，并限制了其对突触外NMDAR的激活。有趣的是，这些转运蛋白似乎通过调节NMDAR的一个特定亚型的激活来影响突触的可塑性，NR2B亚单位包含NMDAR。我们发现，神经元转运蛋白的遗传缺失会导致长时程增强(LTP)的减少，这种作用可以被一种专门阻断NR2B受体的药物逆转(挽救)。鉴于这些转运蛋白是电压依赖的，我们推测它们可能是突触可塑性活动依赖的调制的基础。 控制新生海马区受体激活和活性依赖性可塑性的规则知之甚少，但这是突触形成和网络组装的关键时期。有趣的是，谷氨酸在新生儿海马区的摄取容量要小得多，这表明谷氨酸可能会进一步扩散，以激活更远距离的受体，这意味着突触可塑性的一套不同的规则。我们的结果表明，较低的转运体表达被更大的细胞外体积所抵消，因此新生突触的突触特异性可能更多地通过稀释转运体而不是摄取来维持。一份描述这项工作的论文正在准备中。 我们还开始与华盛顿大学的大卫·库克合作，定量测量阿尔茨海默病小鼠模型中谷氨酸摄取的时间进程。

The brain transduces sensory stimuli, processes information and stores memory within large networks of neurons linked together by synaptic connections. Our laboratory is working to understand what particular features of synapses affect their strength, reliability and independence, and how these attributes contribute to their role in the function of the network. Chemical synaptic connections are made through the release of diffusible neurotransmitter molecules that bind to receptors on the recipient neuron; recent evidence suggests that the neurotransmitter may escape the synapse in which it is released and diffuse into neighboring synapses. This "spillover" of neurotransmitter between synapses could have a profound impact on the information capacity of neural networks and the rules governing their construction during development. We have worked to determine the extent to which the excitatory neurotransmitter glutamate spills over between synapses in the hippocampus, a major site of learning and memory storage in the brain. Using electrophysiological techniques in acutely prepared slices of mouse hippocampus, we have found that glutamate escapes the synapse from which it is released and diffuses into neighboring synapses. This diffusion is tightly regulated by glutamate transporters, pump proteins located primarily on glial membranes that bind glutamate and remove it from the extracellular fluid. Work is continuing to investigate the modulation of these mechanisms and their impact on information processing in hippocampal and retinal neural networks. We have become particularly interested in how neuronal glutamate transporters, which are much less numerous than those on glia but nonetheless appear to limit epileptogenesis, contribute to the clearance of neurotransmitter and the specificity of synaptic connections. Our most recent work, which has been submitted, indicates for the first time that neuronal transporters in the hippocampus buffer synaptically released glutamate close to its site of release, delaying its diffusion out of the perisynaptic region and limiting its activation of extrasynaptic NMDARs. Interestingly, these transporters appear to impact synaptic plasticity by regulating the activation of a particular subtype of NMDAR, the NR2B-subunit- containing NMDAR. We find that genetic deletion of neuronal transporters leads to a decrease in long-term potentiation (LTP) that can be reversed (rescued) by a drug that specifically blocks NR2B-containing receptors. Given that these transporters are voltage-dependent, we speculate that they may underlie an activity-dependent modulation of synaptic plasticity. The rules governing receptor activation and activity-dependent plasticity in the neonatal hippocampus is poorly understood, but this is a critical period of synapse formation and network assembly. Interestingly, glutamate uptake is much less capacious in neonatal hippocampus, suggesting that glutamate may diffuse further to activate receptors at a greater distance and implicating a different set of rules for synaptic plasticity. Our results suggest that the lower transporter expression is offset by a much greater extracellular volume, such that synaptic specificity of nascent synapses may be maintained more by dilution of transmitter rather than uptake. A paper describing this work is in preparation. We also have begin a collaboration with David Cook at the University of Washington to measure quantitatively the time course of glutamate uptake in a mouse model of Alzheimer's Disease.