Decipher the neuronal and astrocytic calcium signal coupling with fMRI

Functional magnetic resonance imaging (fMRI) based on blood oxygen level dependent (BOLD) signals reects the changes of oxy/deoxy-hemoglobin ratio in vessels. Simultaneous fMRI studies with electrophysiological recordings have demonstrated that BOLDfMRI signal correlated with neuronal activity (Logothetis et al. 2001). In addition, Schulz et al acquired fMRI and calcium (Ca2+) signal simultaneously through Ca2+ sensitive dyes (Schulz et al. 2012). Using the similar strategy, we expressed the genetically encoded calcium indicators (GCaMP6f) specifically in either neurons or astrocytes in the rat cortex. The cell-type specific Ca2+ signal showed different coupling features to the BOLD fMRI signal. In contrast to the highly correlated neuronal Ca2+ signal to the fMRI signal, the astrocytic Ca2+ signal presented two different coupling features to the fMRI signal. The evoked astrocytic Ca2+ signal remained positively correlated to fMRI signal, but spontaneous astrocytic Ca2+ signal was negatively correlated to the fMRI signal. Moreover, the spontaneous astrocytic Ca2+ signal could be elicited by stimulation through the whole cortex, which is highly correlated with activation of the mediodorsal and centrolateral thalamic nucleus. This work demonstrated a novel neuron-glia-vascular coupling event mediated through the spontaneous astrocytic calcium signal, which may indicate specific brain state changes