Experimental Neuroimaging Laboratory
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Functional dissection of therapeutic deep brain stimulation circuitry using optogenetic fMRI: Deep brain stimulation (DBS) is a well-established neurosurgical therapy for multiple neurological and psychiatric disorders. In DBS, an electrode is stereotactically guided to a target cerebral nucleus and high frequency (~130 Hz) electrical stimulation is delivered through a pacemaker-like subcutaneous stimulating device. It is most commonly employed in the treatment of Parkinson’s disease (PD), generally in cases where other medical therapies have become inadequate or dyskinesias have become intolerable. When applied for the symptomatic treatment of PD, the subthalamic nucleus (STN) is frequently targeted, often resulting in a marked reduction in several hallmark PD symptoms, including resting tremor and rigidity. However, despite these benefits, many parkinsonian symptoms are frequently refractory to, or may worsen during STN-DBS. The STN is both anatomically heterogeneous and fiber-dense, and thus there is a high likelihood of recruitment of off-target circuits during STN-DBS, even with accurate electrode placements. A better understanding of how DBS exerts its therapeutic effects will allow optimization of this procedure to enhance therapeutic outcomes and reduce unwanted side-effects. Our project aims to address three critical, yet elusive questions of: 1) which neural circuits represent on- and off-target STN-DBS effects, 2) whether selective optogenetic stimulation of STN neurons ameliorate parkinsonian motor deficits, and 3) which neural circuits are necessary for therapeutic STN-DBS. To these ends, we will use state-of-the-art functional magnetic resonance imaging (fMRI), functional connectivity MRI (fcMRI), electrophysiology, optogenetics, and behavioral assessment to dissect therapeutic DBS circuitry in an animal model of PD, in which the amelioration of motor deficits are strongly DBS-dependent. Our group has extensive experience in high resolution DBS-fMRI studies in rodents (Lai et al., Neuroimage, 2013; Younce et al., J Vis Exp, 2013; Albaugh and Shih, Brain Connect, 2014; Lai et al., Magn Reson Med). Our collaborators (Drs. Garret Stuber, Warren Grill, and Wei Gao) are also leaders in understanding and continuing development of optogenetics, DBS, and brain network connectivity analysis approaches. It is our hope that this project will provide novel insights into DBS mechanisms, and lay a foundation to establish new DBS treatment targets and stimulus paradigms for a wide variety of neurological and psychiatric disorders.

    



Dissecting the source of fMRI signals using DREADDs:
Despite being widely used, the neurobiological basis of the fMRI signal remains unclear due to the lack of tool examining its cellular origin and complex, disproportionate susceptibility to cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen consumption (CMRO2) changes in vivo. This limitation seriously confounds fMRI data interpretation for both basic neuroscience research and clinical applications. We are currently asking a fundamental, yet controversial question: how neurons and astrocytes, the two major cell populations in the brain, orchestrate to generate BOLD fMRI signal? We will combine two genetic approaches, namely: 1) Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and 2) optogenetics to quantitatively determine the contribution of neurons or astrocytes to BOLD, CBF, CBV and CMRO2 in the same subject. While optogenetics involve the use of photosensitive ion-channels or opsins, DREADD is a variant of the G-protein coupled receptor (GPCR) that is inactive until triggered by an exogenous ligand – clozapine-N-oxide (CNO). Specifically, we will first determine the effect of optogenetic and pharmacogenetic (DREADD) stimulation and inhibition of neurons or astrocytes by using adeno-associated virus (AAV) under the calcium/calmoduin-dependent protein kinaseIIα (CaMKIIα) and glial fibriliary acidic protein (GFAP) promoters, respectively. After that, we will selectively stimulate either neurons or astrocytes while the other cell type is silenced to dissect the role of target cell population in manipulating fMRI responses. An array of cutting-edge MR tools will be used for data acquisition – BOLD and CBF will be simultaneously measured by a novel two-coil continuous arterial spin labeling technique, CBV will be assessed using monocrystalline iron-oxide nanocolloid contrast agent, and CMRO2 will be quantified using MR-oxygen extraction fraction method. These measurements will be performed noninvasively in the same subject before, during and after genetic manipulation of cellular activities. The outcomes of our studies should provide mechanistic insights into the roles of neurons and astrocytes in controlling CBF, CBV, CMRO2, and how these hemodynamic responses together generate BOLD fMRI signals.  

 


Peri-infarct spreading depolarization in stroke:
Peri-infarct spreading depolarization describes a series of propagating electrical potentials that silence brain activity, alter cerebral blood flow, induce cell swelling, and appear during the hyperacute phase of stroke.  PID has been shown to accelerate stroke progression, and suppressing PID is known to reduce infarct volume. Although PID is a potential therapeutic target for stroke, the spatiotemporal properties of PIDs remain to be characterized and how reperfusion affects PID signatures is largely unexplored.  Optical imaging and EEG/ECoG recording allow identification of PID, but both techniques are depth-limited and cannot provide volumetric tissue information with clinically relevant indices. To date, PID has not been systemically characterized by MRI due to technical constraints. Because PID appears immediately after stroke onset, we have developed a novel photothrombotic model that allows stroke induction inside the MRI system to capture the entire temporal evolution of PID features and stroke progression. Our goal is to quantitatively probe spatiotemporal PID signatures using perfusion and diffusion MRI in an experimental rat model with and without reperfusion. Our model can also induce an ischemic lesion in any predetermined brain area, which opens up new avenues for preclinical stroke research (Kao et al., Neurobiol Dis).
 
 


 
Interestingly, our recent data demonstrated for the first time that intravenous infusion of tissue plasminogen activator (tPA), the only thrombolytic agent approved by the FDA for acute stroke treatment, surprisingly increased the number of PIDs when vascular damage became significant. With these novel tools in hands, we are interested studying the
motor behavioral correlates of these imaging signatures, and further address two questions, namely: 1) whether tPA-induced PIDs exacerbate ischemic injury and 2) whether co-administration of PID-suppressing agents during tPA infusion improves outcomes. Our results should shed light on a novel strategy to reduce tPA-induced tissue toxicity. This project, if successful, could ultimately lead to a clinical trial improving the overall efficacy of intravenous tPA treatment in stroke.  
                   




Neurovascular coupling and uncoupling in the cortex and the striatum:
Our team developed multimodal MRI techniques to measure cerebral blood oxygenation, blood flow, blood volume, and oxygen metabolism changes in animal models (Shih et al., J Cereb Blood Flow and Metab, 2011). These techniques were used to investigate neurovascular function and fMRI signaling mechanisms in vivo. It is the general consensus that increased neural activity is accompanied by increases in hemodynamic responses. Our group challenged this concept by demonstrating sustained negative fMRI signals along with increases in neuronal activity in the striatum (Shih et al., J Neurosci, 2009; Shih et al., Exp Neurol, 2012; Chen et al., Neurobiol Dis, 2013; Shih et al., J Cereb Blood Flow and Metab 2014). These findings indicate that caution should be taken when interpreting perfusion-based functional imaging data, as the presence of vasoactive neurotransmitters may confound these imaging results.          
 
 
 


 
Technical development for simultaneous fMRI and electrophysiology:
Our lab is developing simultaneous fMRI/electrophysiology at high spatial resolution (ranged from 30 to 100 microns) to elucidate the neurophysiological origins of fMRI signals and the pathophysiological mechanisms of neurovascular diseases (Shih et al., Neuroimage, 2013). The MRI-compatible multichannel microelectrode array and micro-EEG cap are designed and fabricated using semiconductor production and microelectro-mechanical systems (MEMS) technology (Lai et al., J Neural Eng, 2012). Combing fMRI and electrophysiology provides additional neural information and has potential to complement current fMRI techniques that primarily measure hemodynamic responses.
 
 
                        


Ocular MRI: Our group have developed and applied multimodal MRI techniques to investigate eye function and ocular circulation. Ocular MRI in small animal models is challenging due to limited spatial resolution and insufficient signal-to-noise ratio. We implemented ultra-high resolution MRI to resolve different retinal layers and choroid as well as their blood flow supply in quantitative units (Shih et al., J Magn Reson Imaging, 2013). We also investigated light-evoked functional hyperemia in the retina and choroid with laminar resolution (Shih et al., Invest Ophthalmol Vis Sci, 2011, Shih et al., Curr Eye Res, 2013) and revealed how retinal and choroidal circulations respond to pharmacological (Shih et al., Magn Reson Med, 2012) and gas challenges (Shih et al., Radiology, 2012). These techniques will be used to study eye diseases such as retinal ischemia, retinal degeneration and diabetic retinopathy.