Dynamics Of Excitatory Synaptic Transmission In The Hippocampus

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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.