, 2003) in combination with optical fiber-based monitoring of population Ca2+ signaling activity (Adelsberger et al., 2005). The tip of the optical fiber (diameter 200 μm) was implanted above the stained cortical or thalamic area (Figure 1B). A column-like region with a diameter

of about 400–500 μm in mouse primary visual Paclitaxel cell line cortex was stained with OGB-1 (Figure 1C). In conditions of isoflurane anesthesia, slow oscillation-associated population Ca2+ transients occurred in the visual cortex at frequencies ranging from 8 to 30 events/min (Figure 1D, see Figure S4E available online), depending on the level of anesthesia (Kerr et al., 2005). It has been shown that Ca2+ transients are mediated by Ca2+ influx during the spiking activity selleck products in a local group of active cortical neurons (Kerr et al., 2005; Rochefort et al., 2009; Stosiek et al., 2003). In line with the previously used terminology (e.g., Rochefort et al., 2009), we refer to these population Ca2+ transients as Ca2+ waves. Figure 1I shows that spontaneous cortical Ca2+ waves are similar to those evoked by visual stimulation (Figure 1E) in terms of amplitude and duration. It is important to note that the comparison of Ca2+ wave amplitudes is meaningful only for a given site of optical recording, because the population of Ca2+ transients depends on

many local parameters, including the level of Ca2+ indicator inside cells and the intensity of the excitation light. Previous work has provided STK38 evidence that slow oscillations are initiated in the cortex (Sakata and Harris, 2009; Sanchez-Vives and McCormick, 2000; Timofeev and Steriade, 1996). To obtain deeper insights into the process of slow-wave initiation and propagation, we implemented an optogenetic approach. First, we used a transgenic Thy-1-ChR2 mouse line that expresses ChR2 in layer 5 neurons of the neocortex (Figure 1F)

(Arenkiel et al., 2007). When applying a single brief (50 ms) pulse of blue light through the optical fiber (Figure 1G) placed in the visual cortex, we obtained a reliable initiation of Ca2+ waves (Figure 1H). Light stimulation in C57/Bl6 mice failed to induce Ca2+ waves. Spontaneous, visually evoked, and optogenetically evoked Ca2+ waves recorded at a given cortical location had similar waveforms (Figure 1I) and virtually identical duration times and amplitudes (Figures S2A and S2B). The latencies of the onset of Ca2+ waves evoked by visual stimulation are quite similar to those evoked by brief (50 ms) optogenetic stimulation (Figure 1J). However, with shorter stimuli, optogenetically induced Ca2+ waves occur at longer latencies (Figure 1K). Not too surprisingly, Ca2+ waves can be evoked optogenetically not only in visual cortex (Figure S1A) but also in other cortical areas such as the frontal cortex (Figure S1B).