Fiber-optic implant for simultaneous fluorescence-based calcium recordings and BOLD fMRI in mice

Abstract

Despite the growing popularity of blood oxygen level-dependent (BOLD) functional MRI (fMRI), understanding of its underlying principles is still limited. This protocol describes a technique for simultaneous measurement of neural activity using fluorescent calcium indicators together with the corresponding hemodynamic BOLD fMRI response in the mouse brain. Our early work using small-molecule fluorophores in rats gave encouraging results but was limited to acute measurements using synthetic dyes. Our latest procedure combines fMRI with optical detection of cell-type-specific virally delivered GCaMP6, a genetically encoded calcium indicator (GECI). GCaMP6 fluorescence, which increases upon calcium binding, is collected by a chronically implanted optical fiber, allowing longitudinal studies in mice. The chronic implant, placed horizontally on the skull, has an angulated tip that reflects light into the brain and is connected via fiber optics to a remote optical setup. The technique allows access to the neocortex and does not require adaptations of commercial MRI hardware. The hybrid approach permits fiber-optic calcium recordings with simultaneous artifact-free BOLD fMRI with full brain coverage and 1-s temporal resolution using standard gradient-echo echo-planar imaging (GE-EPI) sequences. The method provides robust, cell-type-specific readouts to link neural activity to BOLD signals, as emonstrated for task-free ('resting-state') conditions and in response to hind-paw stimulation. These results highlight the power of fiber photometry combined with fMRI, which we aim to further advance in this protocol. The approach can be easily adapted to study other molecular processes using suitable fluorescent indicators.

The Hemodynamic Mass Action of a Central Pattern Generator

“…There is compelling evidence that the changes in the BOLD fMRI signal are related to changes in deoxy-Hb, total-Hb, and regional cerebral blood volume during a variety of sensory, motor, cognitive tasks and resting states (Sakatani et al, 2007;Cui et al, 2011;Duan et al, 2012;Quaresima et al, 2012;Sasai et al, 2012;Tong et al, 2012;Sato et al, 2013;Yuan and Ye, 2013;Fabiani et al, 2014;Hocke et al, 2015;Noah et al, 2015;Anwar et al, 2016;Vannasing et al, 2016). Our findings are also supported by studies in mice with the simultaneous recording of fluorescentbased calcium recordings and BOLD fMRI signals (Schlegel et al, 2018). In this context, our findings suggest that a variant of the fNIRS in DC mode could also be developed to explore functional optical signals related to specific changes in deoxy-Hb, oxy-Hb, total-Hb, and/or regional cerebral blood volume contributing to the hemodynamic response of the spinal cord and the brainstem.…”

High-density multi-fiber photometry for studying large-scale brain circuit dynamics

“…Many further opportunities exist for hybrid approaches, combining multi-fiber arrays with other existing technologies. For example, high-density fiber arrays could expand the use of fiber optics in fMRI experiments [40][41][42] and thereby contribute to a better understanding of the relationship between neuronal, glial, and vascular dynamics. A combination with extracellular silicon probes 6 or emerging optoelectronic neural probes 43,44 may provide new possibilities for relating individual neuron activity to larger-scale population activity 45 , even up to the mesoscale 46 .…”

“…Multifiber photometry should be equally well applicable in rats, non-human primates 48 , and other mammalian species. Similar to single-fiber approaches, the multi-fiber approach will gain its full strength through the clever combination with modern genetic tools that permit cell-type or pathway-specific labeling with optical indicators, not only for neuronal calcium signals but also for membrane potential 21 , dopamine release 49 , glial dynamics 41,42 , metabolic substrates 50 , etc.. Likewise, multi-fiber implants could be useful in photo-activation experiments employing calcium integrators for marking of, or inducing targeted gene expression in, neuronal populations that are activated during specific time windows 51,52 .…”

Simultaneous mesoscopic Ca2+ imaging and fMRI: Neuroimaging spanning spatiotemporal scales

“…Fluorescence imaging using single fibers in combination with fMRI, or wide-field fluorescence and intrinsic hemodynamic signal imaging, have been described previously. [23][24][25][26][27][28][47][48][49] Both these dual imaging approaches measure Ca 2+ and BOLD (or a proxy for BOLD), but lack spatial coverage. Single-fiber time-series data, while quite useful for studying evoked responses, cannot reveal large scale network interactions compared to the whole-cortex Ca 2+ imaging presented here.…”

“…The optical imaging device presented here is quite different from previous implementations combining MRI with simultaneous fluorescent Ca 2+ data collection, where typically only single fibers were used to measure the fluorescence as a function of time from one or more ROIs. [23][24][25][26][27][28] Here, we obtained dynamic high-resolution movies (FOV: 14.5x14.5mm 2 , resolution: 25x25μm 2 ) of Ca 2+ fluorescence activity spanning a large fraction of the mouse cortex. The combined dualimaging set-up was assembled within a plastic sled that was inserted and secured within an 11.7T preclinical Bruker magnet (Bruker, Billerica, MA).…”