This study was supported by the following grants (to D.A.T.): NIA RO1 AG074999, NIA R21AG051103, VA I21RX002223, and VA I21 BX003023.

All the animal procedures were approved by the Institutional Animal Care and Use Committee at Duke University or the equivalent local authority regulating research involving animals. See the Table of Materials for details about all the materials, instruments, and equipment used in this protocol.
1. Instrument preparation
<ol>
	<li>Make sure that all the required items and surgical instruments are in place (Figure 2): scalp clean...

Mapping Cerebral Blood Flow and Electrical Fields: Electrode Placement in Mice

<ol>
	<li>Bestmann, S., Walsh, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcranial+electrical+stimulation.">Transcranial electrical stimulation.</a> Current Biology. 27 (23), R1258-R1262 (2017).</li><li>Bikson, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rigor+and+reproducibility+in+research+with+transcranial+electrical+stimulation:+An+NIMH-sponsored+workshop.">Rigor and reproducibility in research with transcranial electrical stimulation: An NIMH-sponsored workshop.</a> Brain Stimulation. 11 (3), 465-480 (2018).</li><li>Turner, D. A., Degan, S., Galeffi, F., Schmidt, S., Peterchev, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid,+dose-dependent+enhancement+of+cerebral+blood+flow+by+transcranial+AC+stimulation+in+mouse.">Rapid, dose-dependent enhancement of cerebral blood flow by transcranial AC stimulation in mouse.</a> Brain Stimulation. 14 (1), 80-87 (2020).</li><li>Shah, S., Chhatbar, P. Y., Feld, J. A., Feng, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integrating+tDCS+into+routine+inpatient+rehabilitation+practice+to+boost+post-stroke+recovery.">Integrating tDCS into routine inpatient rehabilitation practice to boost post-stroke recovery.</a> Brain Stimulation. 13 (4), 953-954 (2020).</li><li>Voroslakos, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+effects+of+transcranial+electric+stimulation+on+brain+circuits+in+rats+and+humans.">Direct effects of transcranial electric stimulation on brain circuits in rats and humans.</a> Nature Communications. 9 (1), 483 (2018).</li><li>Alekseichuk, I., Mantell, K., Shirinpour, S., Opitz, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+modeling+of+transcranial+magnetic+and+electric+stimulation+in+mouse,+monkey,+and+human.">Comparative modeling of transcranial magnetic and electric stimulation in mouse, monkey, and human.</a> Neuroimage. 194, 136-148 (2019).</li><li>Tavakoli, A. V., Yun, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcranial+alternating+current+stimulation+(tACS)+mechanisms+and+protocols.">Transcranial alternating current stimulation (tACS) mechanisms and protocols.</a> Frontiers in Cellular Neuroscience. 11, 214 (2017).</li><li>Yavari, F., Jamil, A., Mosayebi Samani, M., Vidor, L. P., Nitsche, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Basic+and+functional+effects+of+transcranial+electrical+stimulation+(tES)-An+introduction.">Basic and functional effects of transcranial electrical stimulation (tES)-An introduction.</a> Neuroscience and Biobehavioral Reviews. 85, 81-92 (2018).</li><li>Wachter, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcranial+direct+current+stimulation+induces+polarity-specific+changes+of+cortical+blood+perfusion+in+the+rat.">Transcranial direct current stimulation induces polarity-specific changes of cortical blood perfusion in the rat.</a> Experimental Neurology. 227 (2), 322-327 (2011).</li><li>Han, C. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hemodynamic+responses+in+rat+brain+during+transcranial+direct+current+stimulation:+A+functional+near-infrared+spectroscopy+study.">Hemodynamic responses in rat brain during transcranial direct current stimulation: A functional near-infrared spectroscopy study.</a> Biomedical Optics Express. 5 (6), 1812-1821 (2014).</li><li>Ayata, C., Lauritzen, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spreading+depression,+spreading+depolarizations,+and+the+cerebral+vasculature.">Spreading depression, spreading depolarizations, and the cerebral vasculature.</a> Physiological Reviews. 95 (3), 953-993 (2015).</li><li>Berenyi, A., Belluscio, M., Mao, D., Buzsaki, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Closed-loop+control+of+epilepsy+by+transcranial+electrical+stimulation.">Closed-loop control of epilepsy by transcranial electrical stimulation.</a> Science. 337 (6095), 735-737 (2012).</li><li>Hoang, K. B., Cassar, I. R., Grill, W. M., Turner, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomarkers+and+stimulation+algorithms+for+adaptive+brain+stimulation.">Biomarkers and stimulation algorithms for adaptive brain stimulation.</a> Frontiers in Neuroscience. 11, 564 (2017).</li><li>Turner, D., A, D. S., Hoffmann, U., Galleffi, F., Colton, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CVN-AD+Alzheimer's+mice+show+premature+reduction+in+neurovascular+coupling+in+response+to+spreading+depression+and+anoxia+compared+to+aged+controls.">CVN-AD Alzheimer's mice show premature reduction in neurovascular coupling in response to spreading depression and anoxia compared to aged controls.</a> Alzheimer's and Dementia. 17 (7), 1109-1120 (2021).</li><li>Colton, C. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=mNos2+deletion+and+human+NOS2+replacement+in+Alzheimer+disease+models.">mNos2 deletion and human NOS2 replacement in Alzheimer disease models.</a> Journal of Neuropathology and Experimental Neurology. 73 (8), 752-769 (2014).</li><li>Castano-Prat, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Altered+slow+(&lt;1+Hz)+and+fast+(beta+and+gamma)+neocortical+oscillations+in+the+3xTg-AD+mouse+model+of+Alzheimer's+disease+under+anesthesia.">Altered slow (&lt;1 Hz) and fast (beta and gamma) neocortical oscillations in the 3xTg-AD mouse model of Alzheimer's disease under anesthesia.</a> Neurobiology of Aging. 79, 142-151 (2019).</li><li>Etter, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optogenetic+gamma+stimulation+rescues+memory+impairments+in+an+Alzheimer's+disease+mouse+model.">Optogenetic gamma stimulation rescues memory impairments in an Alzheimer's disease mouse model.</a> Nature Communications. 10 (1), 5322 (2019).</li><li>Iaccarino, H. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gamma+frequency+entrainment+attenuates+amyloid+load+and+modifies+microglia.">Gamma frequency entrainment attenuates amyloid load and modifies microglia.</a> Nature. 540 (7632), 230-235 (2016).</li><li>Martorell, A. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multi-sensory+gamma+stimulation+ameliorates+Alzheimer's-associated+pathology+and+improves+cognition.">Multi-sensory gamma stimulation ameliorates Alzheimer's-associated pathology and improves cognition.</a> Cell. 177 (2), 256-271 (2019).</li><li>Dawson, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vagus+nerve+stimulation+paired+with+rehabilitation+for+upper+limb+motor+function+after+ischaemic+stroke+(VNS-REHAB):+A+randomised,+blinded,+pivotal,+device+trial.">Vagus nerve stimulation paired with rehabilitation for upper limb motor function after ischaemic stroke (VNS-REHAB): A randomised, blinded, pivotal, device trial.</a> Lancet. 397 (10284), 1545-1553 (2021).</li><li>Hacker, M. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deep+brain+stimulation+in+early-stage+Parkinson+disease:+Five-year+outcomes.">Deep brain stimulation in early-stage Parkinson disease: Five-year outcomes.</a> Neurology. 95 (4), e393-e401 (2020).</li><li>Duun-Henriksen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+era+in+electroencephalographic+monitoring?+Subscalp+devices+for+ultra-long-term+recordings.">A new era in electroencephalographic monitoring? Subscalp devices for ultra-long-term recordings.</a> Epilepsia. 61 (9), 1805-1817 (2020).</li><li>Haneef, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sub-scalp+electroencephalography:+A+next-generation+technique+to+study+human+neurophysiology.">Sub-scalp electroencephalography: A next-generation technique to study human neurophysiology.</a> Clinical Neurophysiology. 141, 77-87 (2022).</li></ol>

This protocol focuses on the in vivo, anesthetized measurement of the CBF response as a biomarker to estimate the brain response to tES14. Longer-term biomarkers of the tES response include histological treatment effects, such as the prevention of or changes in amyloid plaque formation (i.e., with gamma stimulation at 40 Hz in several AD models)16,17,18,19, but ...

Transcranial electrical stimulation (tES; with sine wave stimulation, tACS) is a common, external, non-invasive approach to brain neuromodulation1,2. Previously, we hypothesized that at certain doses, tES (and particularly tACS) may increase the cerebral blood flow (CBF) in the underlying brain regions3. Further, a dose-response relationship may exist between either the external current applied or the intracranial electrical field and the resulting CBF responses. However, most clinical stimulation protocols have focused on a maximal comfortable skin level of stimulation (i.e., ~ 2 mA) for scheduled periods of time (i.e., 30-45 min) as a treatment protocol4,5. In rodents, it is possible to use invasive, extracranial brain electrodes applied directly to the skull to investigate the electrical fields in the brain induced by tES6. Hence, the goal of this approach is to determine the effects of the intensity of tACS at relevant frequencies on CBF changes in terms of the dose-response relationship. This dose-response curve is based on a short-term physiological biomarker-direct measurements of the CBF-in relation to the electrical field imposed on the brain3. We have previously shown that, at larger amplitudes, typically beyond the range of electrical fields within the brain induced by tACS clinically, there is a near-linear correlation between the induced electrical field and the CBF in the cortex3. However, smaller-field stimulation (i.e., 1-5 mV/mm intensity) may be more relevant and feasible for use in humans; hence, we have modified our techniques to detect smaller CBF changes.
This paper describes a protocol to analyze the effects of lower-field strength tES alternating sine currents (tACS) on CBF (i.e., 0.5-2.0 mA current, 1-5 mV/mm electrical field), which can be tolerated by awake rodents5. This protocol involves the use of novel laser speckle imaging during tACS, as well as dual intracranial glass electrodes, to determine both the spread of active tACS within the brain (as monitored by the CBF) and the intracranial electrical field intensity, which is shown both as a diagram and an actual experimental photograph (Figure 1). There are many possible physiological effects of tES within the brain, including direct neuronal modulation, neural plasticity, and astrocyte activation7,8. Though CBF has been measured with tDCS9,10, these measurements were slow, indirect, and insufficient for assessing the dose-response function in the brain. Therefore, by using appropriate short-term biomarkers (i.e., CBF, electrical fields) and rapid on/off sequences of tACS, we can now estimate the dose-response function more accurately. Further, we can apply different techniques to measure the CBF, including both focal laser Doppler probes (LD) and laser speckle imaging (LSI) with defined regions of interest.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65195/65195fig01.jpg" /> 
Figure 1: Transcranial stimulation diagram and photographic example. (A) Diagram of the transcranial stimulation setup. The diagram shows a mouse skull with coronal and sagittal sutures. The transcranial electrodes are placed laterally and symmetrically on the skull and are mounted with surgical glue and conductive paste between the electrodes and the skull. These electrodes are connected to a human-compatible, constant-current stimulation device, which can specify the frequency, amplitude, and duration of stimulation. For the assessment of intracranial electrical fields, bilateral glass electrodes (~2 M&#8486;) are placed in the cerebral cortex (i.e., within 1 mm of the inner aspect of the skull through small burr holes), and these are sealed with mineral oil and have AgCl grounds in the neck muscle (shown as larger wires in the center buried into the subcutaneous neck tissue). These glass electrodes are connected to a DC amplifier, and their outputs are recorded through a digitizer with at least four channels. Bilateral laser Doppler probes are also placed on the skull for recordings. The entire skull is also imaged with either a laser speckle imaging device or a high-resolution (at least 1,024 x 1,024 pixels, 12-14 bit pixel depth) cooled camera for intrinsic optical signal detection. The hemoglobin isosbestic frequency is typically chosen (i.e., 562 nm) for illumination for blood flow imaging. (B) A close-up image of an actual experiment, showing the bilateral laser Doppler probes (to the left), the (bilateral) intracranial glass recording microelectrodes placed through the burr holes, and with the tACS stimulating electrodes laterally. Abbreviation: tACS = transcranial alternating current stimulation. <a href="https://www.jove.com/files/ftp_upload/65195/65195fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
As a way of assessing the mechanisms, we can also interrogate interactions with other physiological processes that also alter the CBF, such as K+-induced spreading depolarization11. Further, rather than scheduled sessions at regular times, it is also possible to develop a closed-loop system based on additional biomarkers for a variety of diseases, as has been proposed for epilepsy treatment12 (i.e., clinical Neuropace devices). For example, closed-loop brain stimulation for Parkinson&#39;s disease is commonly based on the intrinsic, abnormal local field potentials (LFPs) intrinsic to this disease in the absence of sufficient dopamine (typically &#946;-band LFPs)13.

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			<td>C57BI mice</td>
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			<td>Copper skull electrods</td>
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			<td>Major</td>
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			<td>In house preparation</td>
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			<td>Laser Doppler Probes</td>
			<td>Moor Instruments</td>
			<td>0.46 mm laser doppler probes</td>
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			<td>Mouse anesthesia fixation</td>
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			<td>3M Transpore</td>
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placement of extracranial stimulating electrodes and measurement of cerebral blood flow and intracranial electrical fields in anesthetized mice

The detection of cerebral blood flow (CBF) responses to various forms of neuronal activation is critical for understanding dynamic brain function and variations in the substrate supply to the brain. This paper describes a protocol for measuring CBF responses to transcranial alternating current stimulation (tACS). Dose-response curves are estimated both from the CBF change occurring with tACS (mA) and from the intracranial electric field (mV/mm). We estimate the intracranial electrical field based on the different amplitudes measured by glass microelectrodes within each side of the brain. In this paper, we describe the experimental setup, which involves using either bilateral laser Doppler (LD) probes or laser speckle imaging (LSI) to measure the CBF; as a result, this setup requires anesthesia for the electrode placement and stability. We present a correlation between the CBF response and the current as a function of age, showing a significantly larger response at higher currents (1.5 mA and 2.0 mA) in young control animals (12-14 weeks) compared to older animals (28-32 weeks) (p &lt; 0.005 difference). We also demonstrate a significant CBF response at electrical field strengths &lt;5 mV/mm, which is an important consideration for eventual human studies. These CBF responses are also strongly influenced by the use of anesthesia compared to awake animals, the respiration control (i.e., intubated vs. spontaneous breathing), systemic factors (i.e., CO2), and local conduction within the blood vessels, which is mediated by pericytes and endothelial cells. Likewise, more detailed imaging/recording techniques may limit the field size from the entire brain to only a small region. We describe the use of extracranial electrodes for applying tACS stimulation, including both homemade and commercial electrode designs for rodents, the concurrent measurement of the CBF and intracranial electrical field using bilateral glass DC recording electrodes, and the imaging approaches. We are currently applying these techniques to implement a closed-loop format for augmenting the CBF in animal models of Alzheimer's disease and stroke.

Author Spotlight: Stimulation-Based Approach to Improve Cerebral Blood Flow in Alzheimer's Model 

Placement of Extracranial Stimulating Electrodes and Measurement of Cerebral Blood Flow and Intracranial Electrical Fields in Anesthetized Mice

The cerebral blood flow can be measured by the degree of saturation of oxy deoxy hemoglobin. This increase in saturation is reduced with aging and in Alzheimer's model. We're estimating the strength of electrical stimulation using both applied currents and the intracranial electrical field density.The most recent developments include improving extracranial stimulation for application to animal models of Alzheimer's disease. Currently, multiple imaging technologies in animal models, including laser spectral imaging, dual photo microscopy, and the physiological recording are used. The challenge is understanding the mechanistic basis for changes in dynamic cerebral blood flow responses in models of Alzheimer's disease.We found the deficits in responses to metabolic challenges in Alzheimer's models mice, and applying extra cranial stimulation improves these deficits. The advantage of our protocol is that a stimulation-based approach improves cerebral blood flow.

Representative results are shown in Figure 4, Figure 5, and Figure 6. Figure 4 shows an example of the four channels with the two intracranial recording electrodes on the upper channels and the CBF responses on the lower channels. The tACS is symmetrical across the skull, but generally, the intracranial field response is slightly asymmetrical for applied AC currents, with one side showing a larger respo...

Watch this Scientific Journal Video about Stimulation-Based Approach to Improve Cerebral Blood Flow in Alzheimer's Model at JoVE.com

We describe a protocol for assessing dose-response curves for extracranial stimulation in terms of brain electrical field measurements and a relevant biomarker-cerebral blood flow. Since this protocol involves invasive electrode placement into the brain, general anesthesia is needed, with spontaneous breathing preferred rather than controlled respirations.

The detection of cerebral blood flow (CBF) responses to various forms of neuronal activation is critical for understanding dynamic brain function and ...