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 are recording transporter-mediated synaptic responses in hippocampal astrocytes in an effort to estimate more 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. 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. 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.
