Biphasic fMRI responses to optogenetic stimulation of parvalbumin interneurons

Parvalbumin (PV) neurons are the largest population of cortical interneurons, thus their role in neurovascular coupling is highly important in interpreting fMRI data, especially optogenetic fMRI. However, PV-based hemodynamic response is not well-understood. Here, we observed the biphasic BOLD response with initial vasoconstriction and follow-up ultraslow vasodilation at the stimulation site by PV photostimulation, while negative BOLD was observed at the downstream output. CBV-fMRI revealed that vasoconstriction localizes at middle to deeper layers matching with the distribution of glutamatergic neurons, while vasodilation propagated from superficial layers driven by neuropeptide substance P signaling.