Dynamics Of Excitatory Synaptic Transmission In The CNS

中枢神经系统兴奋性突触传递的动力学

• 批准号: 7143910
• 负责人: JEFFREY S DIAMOND
• 金额: --
• 依托单位: NATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKE
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/7143910
• 关键词: GABA receptor; NMDA receptors; amacrine cells; calcium flux; confocal scanning microscopy; electrophysiology; glutamate receptor; glutamate transporter; glutamates; hippocampus; laboratory rat; neural information processing; neural transmission; neurotransmitter transport; pyramidal cells; receptor binding; retina; retinal ganglion; synapses; tissue /cell culture; visual stimulus

The brain stores information in patterns of synaptic connections within large networks of neurons. New information is incorporated into a neural network through the modification of connections via mechanisms that are incompletely understood. One fundamental question is whether individual connections behave independently, or whether they are influenced by the activity of neighboring synapses. 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 synaptic connections would have a profound impact on the information capacity of neural networks and the mechanisms by which they are constructed during development. Work in this laboratory is directed towards determining 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, and in the retina, where visual stimuli is encoded for transmission along the optic nerve. Using electrophysiological techniques in acutely prepared slices of rat retina and 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 cerebrospinal fluid. Moreover, it appears that the electrical state of the recipient neuron influence whether the receptors are responsive to low levels of glutamate released from a distant synapse. Work is continuing to investigate the modulation of these mechanisms and their impact on information processing in networks of neurons. In addition, we have recorded transporter-mediated synaptic responses in hippocampal astrocytes to estimate quantitatively how fast synaptically released glutamate is cleared from the extracellular space. In the adult rat hippocampus, glutamate is taken up within 1 millisecond following release. This rate is so fast that it suggests that uptake is actually limited by the efficiency of transporters, i.e., the probability that they will transport glutamate when they bind it rather than unbind it. Transporters release about 50% of the glutamate they bind back into the extracellular space, at a rate that approximates our measured rate of uptake. This suggests that transporters buffer the diffusion of glutamate and that glutamate actually diffuses less far than expected from the its measured extracellular lifetime. Work continues to determine what kind of extrasynaptic receptors are activated despite this rapid glutmaate uptake and a possible role for neuronal transporters in providing substrate for the synthesis of inhbitory neurotransmitter and, consequently, limiting epileptogenesis. Our work in the retina indicates that certain typed of receptors may be localized specifically to limit their activation under certain conditions. On ganglion cells, NMDA-type glutamate receptors appear to be located perisynaptically, such that their activation is prevented by glutamate transporters unless many vesicles of glutamate are released simultaneously. More recent work in the lab indicates that these perisynaptic receptors extend the range over which ganglion cells respond to light stimulation. In addition, NMDA receptors containing different subunit compositions may mediate defferent functional inputs to the same ganglion cell. A great deal of work in the lab is now directed toward understanding the molecular mechanisms underlying this unique targeting of NMDA receptors. Other experiments in which we record simultaneously from synaptically coupled retinal neurons indicate that ribbon synapses are capable of very fast transmitter release, even though their physiological release is slow. In addition, our experiments indicate that ribbon synapses coordinate the simultaneous release of multiple vesicles during evoked responses. In addition, we have discovered that synaptic depression at this synapse is due almost entirely to the depletion of neurotransmitter vesicles. While this is thought to be a common mechanism for depression at high-probability synapses, our experiments have provided relatively quantitative evidence for this idea. These results may provide new insights into the function of the synaptic ribbon. We also study inhbitory, GABAergic feedback from A17 amacrine cells onto rod bipolar cells. This feedback appears to be mediated by a complex combination of GABA-A and GABA-C receptor-mediated components. We continue to test the idea that these two components of feedback may be activated under different release conditions. Recent work indicates that GABA release from the A17 amacrine cell is elicited by calcium entering through calcium-permeagble AMPA-type glutamate receptors, not voltage-gated calcium channels. We have found that we can ablate A17 amacrine cells specifically with a toxic serotonin analog, allowing us to compare the characteristics of reciprocal feedback from A17s and feedback coming from other amacrine cell types.

大脑在庞大的神经元网络中以突触连接的方式存储信息。新的信息通过不完全了解的机制通过连接的修改被纳入神经网络。一个基本问题是，单个连接是独立运作，还是受到邻近突触活动的影响。突触连接是通过释放可扩散的神经递质分子来实现的，这些递质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。神经递质在突触连接之间的这种“溢出”将对神经网络的信息容量及其在发育过程中构建的机制产生深远的影响。该实验室的工作旨在确定兴奋性神经递质谷氨酸在海马体（大脑中学习和记忆存储的主要部位）和视网膜（视觉刺激被编码并沿视神经传递的地方）突触之间溢出的程度。我们利用电生理技术在大鼠视网膜和海马的急性制备切片中发现，谷氨酸从其释放的突触中逃脱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结合谷氨酸并将其从脑脊液中移除的胶质膜上。此外，受体神经元的电状态似乎影响受体是否对远端突触释放的低水平谷氨酸作出反应。研究这些机制的调节及其对神经元网络信息处理的影响的工作仍在继续。此外，我们记录了海马星形胶质细胞中转运体介导的突触反应，以定量估计突触释放的谷氨酸从细胞外空间清除的速度。在成年大鼠海马中，谷氨酸在释放后1毫秒内被吸收。这个速率如此之快，表明摄取实际上受到转运体效率的限制，也就是说，当它们结合而不是解除结合谷氨酸时，它们运输谷氨酸的可能性。转运体释放大约50%的谷氨酸它们结合回细胞外空间，其速率接近我们测量的摄取速率。这表明转运蛋白缓冲了谷氨酸的扩散，谷氨酸的扩散实际上比其测量的细胞外寿命所预期的要少。尽管谷氨酸的快速摄取和神经元转运体在为合成抑制性神经递质提供底物并因此限制癫痫发生方面的可能作用，但仍有工作继续确定哪种突触外受体被激活。