Link extracellular glutamate signaling to the neuro-glio-vascular dynamic interaction with multi-modal fMRI

Here, we expressed genetically encoded fluorescent reporter iGluSnFR for extracellular glutamate (Glu) sensing and calcium indicator GCaMP6f for calcium sensing. We first acquired evoked neuronal calcium and Glu signals with simultaneous fMRI from the FP-S1 of two hemispheres, respectively. Fig. 1A shows the fMRI maps and the time course of BOLD signal, which is simultaneous acquired with the neuronal calcium and Glu spikes. Evoked neuronal spikes showed iGluSnFR with a more rapid temporal feature than neuronal GCaMP (Fig. 1 B). Also, the amplitude of the evoked Glu spike increased proportionally to the BOLD signals as a function of the stimulation intensity (Fig. 1 C). Besides the neuronal calcium, the evoked astrocytic calcium and Glu spikes were acquired with fMRI simultaneously (Fig. 2 A). Interestingly, we also observed the baseline drift of the Glu during the stimulation. This Glu baseline drift signal is increased proportionally upon the stimulation duration (Fig. 2B) and frequency (Fig. 2C). The linkage between the Glu with neuronal/astrocytic calcium and BOLD may indicate the clearance of Glu following synaptic Glu release or potential hemodynamic responses. Future study will further clarify the source for the Glu baseline drop to decipher the neurovascular coupling events. This platform offers us a more thorough interpretation of source signal contribution to fMRI; thus, would expand our understanding of the neurovascular coupling through the neuro-glio-vascula