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Introduction
Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) is widely used as a measure of neuronal activity. However, the neural basis of BOLD signal is still elusive[1]. In this work, we established a robust light-driven fMRI platform mediated by channelrhodopsin-2 (ChR2). Meanwhile, we performed the in vivo recording to characterize the light-evoked local field potential from the activated cortex. This platform will allow us to target specific cell types by optical stimulation, and directly study the coupled fMRI signal from the local cerebrovasculature[2] .
Methods
All images were acquired with a 14.1 T/26cm horizontal bore magnet (Magnex), interfaced to an AVANCE III console (Bruker) and equipped with a 12 cm gradient set, capable of providing 100 G/cm with a rise time of 150 us (Resonance Research). A transreceiver surface coil with 10 mm diameter was used to acquire fMRI images. ChR2 was expressed by AAV5 virus in the barrel cortex with CaMKII promoter for optical stimulation. Fiber optic (400um) was inserted into the deep layer cortex for optical stimulation. The optical pulse was modulated with frequency from 1 to 10Hz. The pulse duration was tested from 1ms to 50ms. And the light power was set from 0.05mw, 0.25mw, 1mw and 1.8mw, which will not lead to the heating-induced pseudo fMRI signal. The block design was set with light on for 5s, 15s and 30s. Light-driven data were acquired from 2 rats. Local field potential was recorded by the ERS module from Biopac. Animal surgical procedures were described previously [3]. The light pulse was delivered through the 470nm laser (2Hz, 6s duration, pulse duration=45ms, 15 epoch). AFNI software was used to perform the linear regression analysis to acquire functional maps.
Results
Fig 1 shows the fiber optic insertion into the deep layer cortex, where the ChR2 was expressed by AAV viral vectors. The fiber optic trace was also visible in the brain slice by immunostaining. The light-driven fMRI signal was detected in the barrel cortex close to the fiber tips, where the time course from the activated cortical ROI was shown with different duration of optical stimulation (Fig 1B). Fig 2. The time course of the light-driven fMRI response with different frequency, pulse duration and power level was represented. Fig 3 demonstrated the local field potential recorded in the FP-S1 by optical stimulation.
Conclusions
After optical stimulation through optical fiber, it shows robust fMRI and neural response in the cortex close to the optical fiber where expressing ChR2. This work provides us a reliable platform to study the neurovascular coupling mechanism of BOLD-fMRI signal.
