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was observed in G-CEPIA1er responses. Responses become smaller and slower at the peripheral regions. This strongly suggests lateral diffusion of Ca2+ from the peripheral region to the central Ca2+-depleted region through the lumen of the ER. The importance of diffusion-dependent refilling was confirmed by the negligible impact of the SERCA inhibitor on recovery dynamics of ER Ca2+ (Figure 4C). This diffusion-dependent refilling can recover Ca2+ concentration within an approximately 10-second time scale after mGluR-dependent Ca2+ depletion. Therefore, these findings suggest that the ER functions as a Ca2+ store that rapidly and efficiently redistributes Ca2+ throughout dendrites and spines (Figure 2).Concluding remarksFluorescence imaging of mGluR signaling compo-nents revealed a clustered synaptic input as the unit of mGluR signaling, cooperative IP3 production as a booster of mGluR signaling and the ER as Ca2+ pipeline (Figure 2). By introducing my work on the fluorescence imaging of mGluR signaling, I intended to highlight the following points. First, spatio-temporal dynamics at various levels is essential for brain functions. Second, to investigate dynamics in the brain, fluorescence imaging is a powerful and indispensable tool. Third, the devel-opment of fluorescent probes and specialized optics have greatly advanced neuroscience. However, it should be emphasized that what we are capable of observing now is just the tip of the iceberg. Novel optical techniques currently in development will lead to the further discovery of novel dynamic processes and help decipher the complex and intri-cate mechanisms of brain functions.AcknowledgementsWork introduced in this review was performed at the Department of Pharmacology, Graduate School of Medicine, The University of Tokyo, and the Department of Cellular and Molecular Pharma-cology, Juntendo University Graduate School of Medicine. I would like to express my gratitude to Prof. Masamitsu Iino, Prof. Kenzo Hirose and Prof. Takashi Sakurai for their kind guidance. Work introduced in this review was supported by the Japan Society for the Promotion of Science KAKENHI (18790176, 20790202, 23689015, 16K08543 and 19K06936), and by grants from the Pharmaco-logical Research Foundation, the Tokyo Society of Medical Sciences, and Brain Science Foundation.YO performed the experiments for work introduced in this review. YO wrote the manuscript. YO approved the final manuscript.The author declares that there are no conflicts of interest. 1) Chen T-W, Wardill TJ, Sun Y, et al. Ultrasensitive fluo-rescent proteins for imaging neuronal activity. Nature. 2013; 499: 295-300. doi:10.1038/nature12354 2) Denk W, Strickler JH, Webb WW: Two-Photon Laser Scanning Fluorescence Microscopy. Science (80-). 1990; 248: 73-76. doi:10.1126/SCIENCE.2321027 3) Svoboda K, Yasuda R: Principles of Two-Photon Exci-tation Microscopy and Its Applications to Neuroscience. Neuron. 2006; 50: 823-839. doi:10.1016/J.NEURON.2006. 05.019 4) Lee SJR, Escobedo-Lozoya Y, Szatmari EM, Yasuda R: Activation of CaMKII in single dendritic spines during long-term potentiation. Nat 2009 4587236. 2009; 458: 299-304. doi:10.1038/nature07842 5) Matsuzaki M, Honkura N, Ellis-Davies GCR, Kasai H: Structural basis of long-term potentiation in single dendritic spines. Nat 2004 4296993. 2004; 429: 761-766. doi:10.1038/nature02617 6) Ohki K, Chung S, Kara P, Hübener M, Bonhoeffer T, Reid RC: Highly ordered arrangement of single neurons in orientation pinwheels. Nat 2006 4427105. 2006; 442: 925-928. doi:10.1038/nature05019 7) Xu T, Yu X, Perlik AJ, et al: Rapid formation and selec-tive stabilization of synapses for enduring motor memories. Nat 2009 4627275. 2009; 462: 915-919. doi: 10.1038/nature08389 8) Sun F, Zeng J, Jing M, et al: A Genetically Encoded Fluorescent Sensor Enables Rapid and Specific Detec-tion of Dopamine in Flies, Fish, and Mice. Cell. 2018; 174: 481-496.e19. doi:10.1016/J.CELL.2018.06.042/ATTACHMENT/3F0576E2-A395-41AF-9CE9-4E57991EA8A2/MMC1.XLSX 9) Okubo Y, Sekiya H, Namiki S, et al. Imaging extrasyn-aptic glutamate dynamics in the brain. Proc Natl Acad Sci U S A. 2010; 107: 6526-6531.10) Okubo Y: Visualization of metabotropic glutamate-re-ceptor signaling. Folia Pharmacol Jpn. 2014; 144: 76-80. doi:10.1254/fpj.144.7611) Okubo Y, Iino M: Visualization of glutamate as a volume transmitter. J Physiol. 2011; 589: 481-488.12) Okubo Y, Kakizawa S, Hirose K, Iino M: Visualization of IP3 dynamics reveals a novel AMPA receptor-trig-gered IP3 production pathway mediated by volt-age-dependent Ca2+ influx in Purkinje cells. Neuron. 161FundingAuthor contributionsConflicts of Interest statementReferences