Simultaneous GCaMP6-based fiber photometry and fMRI in rats

Abstract

Background Understanding the relationship between neural and vascular signals is essential for interpretation of functional MRI (fMRI) results with respect to underlying neuronal activity. Simultaneously measuring neural activity using electrophysiology with fMRI has been highly valuable in elucidating the neural basis of the blood oxygenation-level dependent (BOLD) signal. However, this approach is also technically challenging due to the electromagnetic interference that is observed in electrophysiological recordings during MRI scanning. New Method Recording optical correlates of neural activity, such as calcium signals, avoids this issue, and has opened a new avenue to simultaneously acquire neural and BOLD signals. Results The present study is the first to demonstrate the feasibility of simultaneously and repeatedly acquiring calcium and BOLD signals in animals using a genetically encoded calcium indicator, GCaMP6. This approach was validated with a visual stimulation experiment, during which robust increases of both calcium and BOLD signals in the superior colliculus were observed. In addition, repeated measurement in the same animal demonstrated reproducible calcium and BOLD responses to the same stimuli. Comparison with Existing Method(s) Taken together, simultaneous GCaMP6-based fiber photometry and fMRI recording presents a novel, artifact-free approach to simultaneously measuring neural and fMRI signals. Furthermore, given the cell-type specificity of GCaMP6, this approach has the potential to mechanistically dissect the contributions of individual neuron populations with respect to BOLD signal, and ultimately reveal its underlying neural mechanisms. Conclusions The current study established the method for simultaneous GCaMP6-based fiber photometry and fMRI in rats.

Physiological and Pathological Brain Activation in the Anesthetized Rat Produces Hemodynamic-Dependent Cortical Temperature Increases That Can Confound the BOLD fMRI Signal

“…We were able to bypass these potential signal confounds using a FLASH based Gradient Echo sequence at a lower image temporal resolution, but same image/relaxation contrast, since the timeframe for the phantom to decrease in temperature was ∼3,000 s. This intrinsic inverse effect of temperature on BOLD signal intensity, as well as the increased metabolic rates and facilitated unloading of oxygen from hemoglobin as a result of activation-induced temperature increases, such as those presented here, should therefore be taken into consideration when interpreting BOLD fMRI signal changes in small animal models that possess negative brain-core temperature differentials, and which can be exacerbated by degree of brain exposure and anesthesia. This is particularly relevant to the growing number of studies examining the neurophysiological underpinnings of the BOLD signal using simultaneous BOLD fMRI and optical modalities, such as calcium imaging ( Schulz et al, 2012 ; Liang et al, 2017 ) and optogenetics ( Lee et al, 2010 ; Yu et al, 2016 ; Schmid et al, 2017 ), that are typically conducted using anesthetized and anesthetized rodent models (including the model described here), and require a craniotomy for imaging and/or illumination. It is also pertinent to studies using hypercapnia to calibrate fMRI signals to a baseline CMRO 2 and examine subsequent transients during functional activation ( Davis et al, 1998 ; Kida et al, 2007 ).…”

“…Remarkably, this confound has not been previously accounted for in BOLD neuroimaging studies using anesthetized rodent models, which possess profound negative brain-core temperature differentials and are thus predisposed to large increases in brain temperature during functional hyperemia ( Zhu et al, 2009 ). Importantly, this effect may also have implications for pre-clinical studies using craniotomies to elucidate the underpinnings of the BOLD signal through novel combination with other optical modalities such as optical imaging spectroscopy ( Kennerley et al, 2005 ), calcium imaging ( Schulz et al, 2012 ; Liang et al, 2017 ), optogenetics ( Lee et al, 2010 ; Schmid et al, 2017 ) and, potentially, two-photon microscopy ( Cui et al, 2017 ).…”

Genetic Reporters of Neuronal Activity: c-Fos and G-CaMP6

“…Moreover, the deeper the area of interest, the greater the required illumination, increasing the risk of photodamage to tissue, increasing the rate of photobleaching of the fluorophoer, and decreasing the signal-to-noise due to surface reflections of the illumination. Fiber photometry offers an opportunity to utilize GCaMP6 to image deeper structures, but the spatial resolution of the data suffers and most reported systems should be thought of as reporting an average calcium concentration for a local population rather than the sub-neuron resolution capable with 2-photon resonant scanning microscopes (Deisseroth, 2016; Liang et al, 2017). This technology may still have potential when combined with novel imaging approaches, however, for simultaneous imaging of spatially separated areas (Kim et al, 2016) and is the focus of ongoing technique development.…”

An open database of resting-state fMRI in awake rats

“…First, environmental and genetic background are relatively uniform, making it easier to separate their influences on brain networks and function ( Gorges et al, 2017 ). Second, fMRI can be combined with cutting-edge neuroscience techniques such as electrophysiology ( Majeed et al, 2011 ; Pan et al, 2010 ; Sloan et al, 2010 ), optogenetics ( Desai et al, 2011 ; Lee et al, 2010 ; Liang et al, 2015b ), calcium signal recording ( Liang et al, 2017 ; Schlegel et al, 2018 ) and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) ( Grayson et al, 2016 ), which can facilitate bridging wide-range information from the cellular to systems levels. Applying rsfMRI in transgenic rodent models can further link imaging discoveries to neural mechanisms at the genetic and molecular levels ( Asaad and Lee, 2018 ).…”