Contributions of synaptic glutamate versus neuronal spiking activity to cerebral vascular responses in awake mice

Neurovascular coupling is the temporal relationship between neuronal activity and regional blood flow changes presumably to meet the high metabolic demands of the brain. Prior fMRI studies have reported excitatory synaptic transmission as more metabolically demanding than neuronal spiking, thus correlating better with cerebral hemodynamics. To investigate this finding with newer optical imaging techniques, we used fluorescent markers for extracellular synaptic glutamate and intracellular neuronal calcium to directly measure relationships between synaptic and spiking activities on local vascular changes in awake mice under evoked and spontaneous conditions. To determine which signal better predicts hemodynamic responses, we used a linear convolution model. Using wide field optical imaging (WFOI), we observed peak fluorescence values of 0.38% and 5.60% in synaptic glutamate and neuronal calcium, respectively, to whisker stimulation, and values of 3.13% and 35.77%, respectively, using two-photon microscopy (2PM). Following whisker stimulation, mean R² values were 0.64 and 0.79 for synaptic glutamate and neuronal calcium, respectively, with WFOI and 0.67 and 0.56, respectively, with 2PM. From WFOI resting-state, mean R² values were 0.73 and 0.68 for synaptic glutamate and neuronal calcium, respectively. Altogether, both signals perform similarly in predicting hemodynamic responses, with no significant differences in their prediction efficacy.