Imaging of extracellular Ca<sup>2+</sup> in the hippocampus by a novel CMOS ion image sensor

Intracellular calcium ion (Ca2+i) is one of the most important cation that controls several cellular functions, and thus there were already huge number of literature about Ca2+i imaging in various cells. Although it is believed that Ca2+i increase is accompanied by extracellular Ca2+(Ca2+o) decrease, the kinetics of Ca2+o decrease remain unknown because of the limited imaging options. To overcome this limitation, we developed a Ca2+ image sensor (CIS) that is highly selective to Ca2+ but not to other cations within wide dynamic range (from 100 mM to 100 mM). We used CIS for the imaging of Ca2+o in acute hippocampal slices. Stimulation with glutamate (Glu) decreased Ca2+o to around 200 nM within 3 s, which returned to the baseline level (2 mM) with slow kinetics (5 min). Glu stimulates both neurons and glial cells to evoke Ca2+i elevation, thereby reducing Ca2+o. Thus, we tested NMDA to selectively stimulate NMDA receptor in neurons. NMDA mimicked Glu-evoked Ca2+o decrease. Interestingly, Ca2+o decrease was initiated at hippocampal CA1-2, before spreading along the pyramidal layers. Glutamate- and NMDA-evoked Ca2+o decreases were inhibited by a NMDA receptor antagonist D-APV, suggesting involvement of neuronal NMDA receptors in decrease in Ca2+o. So far, many scientists neglected Ca2+o, but the CIS would tell us its importance for understanding brain functions.