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 extrasynaptically in astrocytic membranes. 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 (Thomas, et al., 2011). In addition, we have begun to study how the relative location and number of calcium channels and vesicle release sites in the presynaptic terminals of CA1 synapses may influence short-term plasticity of synaptic transmission. We find that depletion shapes the relative size of response pairs even at low-probability synapses, due to uneven recruitment of release sites within single active zones. This work has been submitted. 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. A manuscript is in preparation.
