[Imaging non-synaptic transmission via glutamate].

Glutamate is the major excitatory neurotransmitter in the mammalian brain. The conventional view is that glutamate mediates synaptically confined point-to-point transmission at excitatory synapses. However, glutamate has also been suggested to escape from the synaptic cleft and mediate volume transmission, which is often referred to as glutamate spillover. This non-synaptic transmission via glutamate has been implicated in the regulation of a variety of important neural and glial functions. Despite the immense potential physiological and pathophysiological importance of glutamate spillover, the spatiotemporal dynamics of extrasynaptic glutamate concentrations have been only inferred indirectly, and their characteristics remain elusive because of a lack of appropriate technology. The recent development of fluorescent glutamate indicators has enabled the quantitative analysis of glutamate spillover. With a hybrid-type fluorescent indicator, we succeeded in imaging glutamate spillover in brain slices and in vivo. Our results showed that glutamate spillover was locally induced by the spatiotemporal cluster of synaptic activities due to the summation of glutamate originating from neighboring synapses. We also showed that glutamate spillover had magnitudes and durations that were sufficient for activating high-affinity glutamate receptors. Moreover, we successfully observed sensory input-induced glutamate spillover in the cerebral cortex in vivo. Thus, the present study clarified the features of non-synaptic transmission via glutamate and, furthermore, made it possible to directly visualize synaptic activity in live animals.