We then examined the possibility of monitoring activity-dependent Ca2+-transients in entire populations of iN cells using the genetically expressed Ca2+-sensor gCamp6M, which is an advanced version of gCamp5 (Akerboom et al., 2012). We found that Ca2+-transients induced even by single isolated action potentials could be detected in our iN cells (Fig. 5D). The amplitude of the Ca2+-signal correlated well with the number of action potentials elicited. Conversely, when we co-cultured iN cells with mouse neurons, we observed typical network activity in iN cells that was induced by addition of the GABA-receptor blocker picrotoxin (Fig. 5E). These Ca2+-imaging examples demonstrate that it is possible to use iN cells for monitoring network activity of iN cells over larger populations of cells, for example during drug screening projects.
