Study of the spatial correlation between neuronal activity and BOLD fMRI responses evoked by sensory and channelrhodopsin-2 stimulation in the rat somatosensory cortex

Abstract

In this work, we combined optogenetic tools with high-resolution blood oxygenation level-dependent functional MRI (BOLD fMRI), electrophysiology, and optical imaging of cerebral blood flow (CBF), to study the spatial correlation between the hemodynamic responses and neuronal activity. We first investigated the spatial and temporal characteristics of BOLD fMRI and the underlying neuronal responses evoked by sensory stimulations at different frequencies. The results demonstrated that under dexmedetomidine anesthesia, BOLD fMRI and neuronal activity in the rat primary somatosensory cortex (S1) have different frequency-dependency and distinct laminar activation profiles. We then found that localized activation of channelrhodopsin-2 (ChR2) expressed in neurons throughout the cortex induced neuronal responses that were confined to the light stimulation S1 region (<500 μm) with distinct laminar activation profile. However, the spatial extent of the hemodynamic responses measured by CBF and BOLD fMRI induced by both ChR2 and sensory stimulation was greater than 3 mm. These results suggest that due to the complex neurovascular coupling, it is challenging to determine specific characteristics of the underlying neuronal activity exclusively from the BOLD fMRI signals.

Figure 1. BOLD fMRI and neuronal activity evoked by sensory stimulation have different frequency-dependency in S1

a. Examples of BOLD fMRI activation maps in response to contralateral forepaw stimulation at 1 Hz, 3 Hz, 6 Hz, 9 Hz and 20 Hz. b. Averaged time courses of BOLD fMRI percentage changes in response to forepaw stimulation at different frequencies (mean, n=6). The gray color bar depicts the length of stimulation. c. Averaged time courses and amplitudes of LFP in response to forepaw stimulation at different frequencies (mean, n=5). d. Normalized group results (mean ± SEM) showed that the maximal magnitude of LFP and the spiking rate of MUA (n=5) was observed when the stimulus was presented at 3 Hz, while the peak of the BOLD fMRI percentage change and its extent was observed at 9 Hz and 6 Hz, respectively (n=6).

Figure 2. BOLD fMRI and neuronal activity evoked by sensory stimulation at 9 Hz have different laminar profile

a. Time courses and amplitudes of BOLD fMRI responses to forepaw stimulation across the cortical depth (mean ± SEM; n=6) demonstrate that hemodynamic responses initiated and had the highest amplitude in laminae II+III. b. Example of evoked LFP responses to forepaw stimulation demonstrates that neuronal responses initiated and had the highest amplitude in lamina IV, and then spread throughout the cortical depth. c. Normalized group results (mean ± SEM) of averaged LFP amplitude (n=5), spiking rates of MUA (n=5), and magnitude of BOLD fMRI (n=6) responses across the cortical depth.

Figure 3. Hemodynamic responses and neuronal activity evoked by sensory stimulation have similar spatial extent across S1

a. Examples of CBF activation z-map overlaid on the original CBF contrast images show that both forepaw and ChR2 stimulations resulted in extensive hemodynamic responses in the right S1. The tip of the optic fiber used for ChR2 was masked off from the images. b. Examples of BOLD fMRI activation z-map overlaid on the anatomical images. c. An illustration of the position of the nine recording electrodes (red dots) and the optic fiber (blue star) overlaid on a LSCI image of the rat S1. d. Forepaw stimulation resulted in LFP responses that were observed in all nine electrodes that were positioned at lamia IV and covered 2 mm × 2 mm of S1. e. Forepaw stimulation resulted in MUA that were observed in all nine electrodes across S1.

Figure 4. Hemodynamic responses and neuronal activity evoked by ChR2 stimulation are partially co-localized in S1

a. Examples of neurons within S1 immunostained with antibodies targeted towards EYFP (green, indicative of ChR2 expression) and CaMKIIα (red, marker of excitatory pyramidal neurons). A large field of view fluorescence image shows that ChR2 expressing neurons are located throughout the cortex. b. Example of evoked LFP responses to ChR2 stimulation shows that responses were confined to laminae II+III and IV. c. ChR2 stimulation resulted in evoked LFP responses that were observed only in electrodes proximate to the optic fiber with a distance less than 500 μm. d. ChR2 stimulation resulted in increased MUA spiking activities only in electrodes proximate to the optic fiber. e. Example of CBF activation z-map shows that ChR2 stimulations resulted in extensive CBF increases in S1. f. Example of BOLD fMRI activation z-map shows that ChR2 stimulations resulted in extensive BOLD fMRI responses in S1. g. Group average of the CBF responses across S1 representation (in the medial-lateral axis) demonstrates similar spatial extent of the hemodynamic responses induced by forepaw and ChR2 stimulations (mean ± SEM, n=5). h. Group average of the BOLD fMRI responses across lamina IV S1 representation (in the medial-lateral axis) demonstrates similar extent and localization of the hemodynamic responses induced by forepaw and ChR2 stimulations (mean ± SEM, n=5). I. Time courses of BOLD fMRI responses across the cortical laminae evoked by ChR2 (mean ± SEM, n=5).