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Abstract:
Introduction: Emerging studies have combined fMRI with fibre optic-based optogenetics, fluorescence Ca2+ recording,and neuropharmacology(i.e.,multi-modal fMRI platform) to decode neural circuitry and decipher brain function1,2.The MOF3 could guide light by a conventional higher-index core modified by the presence of air holes for fluid delivery,providing the possibility to integrate multiple functions in one probe. Here,we developed a pure silica MOF-based probe for optogenetically and sensory-driven single-vessel fMRI4 with simultaneous Ca2+ signals and Mn2+ injection to optimize multi-modal fMRI platform.
Methods: CaMKII.ChR2.mCherry was injected into the right barrel cortex (BC)(Fig.1e) while the GCaMP6f was expressed in the left BC of rats (Fig.2b). The MOF-based probe was inserted in the BC(Fig.1b) to deliver blue light excitation pulses (488 nm) at 3 Hz, 4 s duration, 10 ms width, 15 mW for the optogenetic stimulation and continuously at 5 μW for calcium recording. 0.1mM MnCl2 solution was used as the MR contrast agent with a modified MPRAGE sequence (FOV:1.92×1.92 cm2, 0.7 mm thickness, TR, 4000 ms; Echo TR/TE = 15/1.7 ms;TI,1000 ms;number of segments:4). fMRI block design was 4 s stim on/17.5 s off, 8 epochs for the whole brain EPI:1.5s TR, 400 μm isotropic resolution (Fig.1d,f) and SSFP single vessel BOLD:TR,1.5 s;FOV,9.6×9.6 mm2; 500 μm thickness; 100 μm in-plane resolution (Fig.1g, Fig.2c).
Results/Discussion: First, as proof of concept, a MOF-based optofluidic probe was evaluated in a 14 T scanner for simultaneous optogenetic stimulation and Mn2+ injection (Fig.1b). As Fig.1c shows, the Mn2+ was delivered into the left BC effectively with negligible liquid leaking in the right BC with MOFs of ~230 μm outer diameter. Upon optogentic stimulation, the robust BOLD in the right BC demonstrates sufficient light propagation. In addition, high resolution single-vessel BOLD was acquired (Fig.1e,f,g). Next, the MOF-based probe was glued to a multi-mode fibre for MRI-compatible calcium recording and Mn2+ injection upon sensory stimulation. The use of a photomultiplier chip makes it MRI-compatible and simplifies the conventional lightpath (Fig.2a). The BOLD for the vessels surrounding the probe and the Ca2+ signals demonstrates its reliability (Fig.2c). Interestingly, the decreased Ca2+ baseline after each injection shows that the extracellular Mn2+ quenches the fluorescence Ca2+ signals5 (Fig.2d,e).
Conclusions: We present a MOF-based fully functional optofluidic probe and a simplified Ca2+ recording lightpath in a 14 T scanner to reduce tissue damage for optogenetic and fluid injection, and simplification of the conventional Ca2+ recording lightpath. The MOF-based probe integrates optogenetic-driven brain fMRI studies with simultaneous Ca2+ recording and drug delivery, which may contribute to uncovering the basis of neuropsychiatric diseases.
