MRI-guided robotic arm (MgRA) drives optogenetic activation of the rat corpus callosum
Yi Chen1,2, Pais Roldán Patricia1,2, Xuming Chen1, and Xin Yu1,3

1Research Group of Translational Neuroimaging and Neural Control, High-Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany, 2Graduate Training Centre of Neuroscience, University of Tuebingen, Tuebingen, Germany, 3The Werner Reichardt Centre for Integrative Neuroscience, University of Tuebingen, Tuebingen, Germany


An MRI-compatible robotic arm was developed to provide a precise fiber positioning for optogenetic fMRI of the rat brain. Corpus callosum connects two hemispheres through a thin sheet of spreading fiber bundle with only a few hundred micron thickness in the brain. This work shows that MgRA can guide fiber optic to precisely target the callosal fiber bundles. The optogenetically driven callosal axonal fiber-mediated neural activity leads to strong antidromic activation in the hemisphere with callosal neurons expressing ChR2 by fMRI.

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Scientists who are interested in small animal fMRI with cutting-edge techniques.


Functional changes across brain hemispheres have been reported after unilateral cortical or peripheral nerve injury1, 2, 3. Interhemispheric callosal connections usually underlie this cortico-cortical plasticity3, 4. By targeting the corpus callosal (CC) fiber expressing Channelrodopsin5, we could study the callosum mediated brain plasticity optogenetically. However, the thickness of the callosal fibers along the dorsal-ventral axis is only a few hundred micron in rats6. It is difficult to precisely target the callosal fibers by brain atlas-based conventional stereotaxic positioning. Here, we developed an MRI-guided robotic arm (MgRA) positioning system to target callosal fibers with high precision inside a 14.1T MR scanner and mapped the BOLD-fMRI functional patterns driven by optogenetic activation of the callosal fibers.


An MRI-compatible, automatic robotic control multi-degree robotic arm has been developed in 14.1T scanner, as shown in Fig.1a. Fig.1b illustrates the camera-based signals for experiment setup in the iso-center of the horizontal bore magnet. Alpha chloralose anesthetized rat is fixed by ear bars and tooth bar on the custom-designed MRI-compatible rat holder. A 24mm-diameter custom-designed transmit/receive surface coil was located on the rat brain while the fiber could be inserted into the brain to target CC through the hole of the coil driven by the robotic arm. MRI scans were performed using 2D rapid acquisition with relaxation enhancement (RARE) sequence: TR, 1200ms, TE, 7.5ms, 1.92cmX1.68cm FOV, 128X112 matrix, 0.15X0.15X0.6 mm3 spatial resolution. AAV5.CAG.ChR2.mcherry viral vectors were stereotaxically injected in the left S1FL (primary somatosensory cortex, forelimb region) in 4-week old rats. The detailed surgical procedures for optogenetic fMRI were described previously7. The optical fiber(~200um) delivered blue light pulses (473nm) at 5Hz, 20ms width with 15s duration for the fMRI block design. fMRI scans were performed using 3D Echo planar imaging sequence: TR, 1.5s, TE,11.5ms, 2.24X1.92X1.92cm3 FOV, 56X48X48 matrix, 400X400X400 um3 spatial resolution. MRI data analysis was performed using Analysis of Functional NeuroImages software(NIH, Bethesda).


Fig.1 illustrates that MgRA could drive fiber optic along the dorsal-ventral axis every 50 µm step, of which the accuracy is sufficient to guide the fiber optic to target CC. Channelorhodopsin-2 (ChR-2) was expressed in the callosal projection neurons in left hemisphere so that only the axonal fibers to the opposite direction was labeled(Fig. 2a). Fig.2b shows three locations of the fiber optic along the predefined trajectory—touching (step#1), nearly penetrating (step#2), and bypass (step#3) CC. Upon the optogenetic stimulation of the CC of step#1, there was significant increase of the fMRI signals of anti-dromic neuron activity from the axonal fibers backward to the soma of callosal projection neurons in the ipsilateral barrel cortex, as well as a small projection-induced BOLD signal in the contralateral hemisphere (Fig 2d). When the fiber optic was nearly penetrating or bypass the CC, less and no BOLD signals were detected, respectively. Fig. 2c illustrates the time courses from the anti-dromic activity-induced BOLD signals in the ipsilateral hemisphere.


The optogenetic activation of callosal axonal fibers led to strong antidromic activity in the hemisphere where the ChR2 expressing callosal neurons were located. The lack of fMRI signal in the contralateral hemisphere indicates that the net callosal inputs to the local excitatory-inhibitory circuits may not lead to sufficient BOLD signal detected by fMRI. MgRA provides sufficient targeting accuracy and flexibility for brain intervention for the multi-modal fMRI platform.


We thank Mr. Shanyi Yu for building up the first prototype of the robotic arm and Mr. Johannes Boldt for helping to improve the MgRA system. This work is supported by the Max-Planck-Society and the China Scholarship Council (PhD fellowship to Yi Chen).


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Fig. 1. Robotic arm setup and in vitro/vivo evaluation. a. View of robotic arm in MRI scanner. b, The snapshot of the fiber optic (tip with 589nm wavelength laser light, yellow arrow) positioned above the burr hole (green arrow) on the skull of an anesthetized rat with the fiber-driving piece. c. RARE anatomical image for fiber optic positioning and brain slice with CAG-ChR2-mCherry injected in barrel cortex, expressed in corpus callosum as well. d. Three continuous MRI anatomical images with step distance 50um.

Fig. 2. Optogenetic excitation of CAG-ChR2-mCherry expressed cc drives positive BOLD in FP in both hemispheres. a.Histological image demonstrates ChR2-mCherry expression. Red, AAV5.CAG.ChR2-mCherry; blue, 4′,6-Diamidin-2-phenylindol (DAPI). b. The time-lapsed images show fiber optic targeting different depth in the rat brain. c. The light-driven fMRI signal time courses from the activated ipsilateral voxeIs for three depth. d, The activation pattern upon optogenetic photo-activation of the CC in three locations.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)