Name: Surgery for Stimulating Electrode Placement and Burr Hole Creation in Mouse
Uploaded: 2023-06-02T13:40:00
Description: Watch this Scientific Journal Video about Surgery for Stimulating Electrode Placement and Burr Hole Creation in Mouse at JoVE.com

surgery for stimulating electrode placement and burr hole creation in mouse

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

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.

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.

Watch this Scientific Journal Video about Surgery for Stimulating Electrode Placement and Burr Hole Creation in Mouse at JoVE.com

Surgery for Stimulating Electrode Placement and Burr Hole Creation in Mouse  

Begin by transferring the anesthetized animal to the stereotaxic frame, and then secure the head into the nose cone and ear bars. Place the probes for measuring the pulse, pulse oxygen saturation, blood pressure, and temperature of the animal. Apply eye ointment.Remove the hair with depilatory cream and aseptically clean the scalp with three passages of iodine and alcohol. For a terminal study, remove the scalp using surgical scissors and expose the skull at approximately three millimeters from the lamboid suture caudally and three millimeters frontal to the bregma. Excise the scalp parietally to expose the inner part of the temporal muscle on both sides.Then, remove any residual subcutaneous connective tissue so that the skull is clean and dry to apply the stimulating electrodes. Next, apply conductive gel to the side of the electrodes that will be in contact with the skull. Place the electrode on this temporal muscle and secure it with surgical super glue around the edge at intermittent spots.Then, apply lidocaine paste to the temporal muscle and scalp on both sides without disturbing the electrodes. Test the impedance of the extracranial stimulating electrodes. Then, place the mouse under a laser speckle imaging device without intracranial recording electrodes.To apply stimulation between the two skull tACS electrodes, use a commercial human compatible stimulating device that delivers a constant current. Before stimulation, obtain a clear baseline. Apply brief periods of on/off stimulation at various amplitudes on either side of the skull.Ensure a clear baseline after stimulation. After the extracranial stimulating electrodes are placed four millimeters laterally to each side of the skull, mark the position of the burr holes at two millimeters on each side of the midline, four millimeters apart from each other, and orthogonal to sagittal suture. Then, drill two burr holes for the glass electrodes.Fill these burr holes with sterile mineral oil to prevent current ingress into the skull from the extracranial electrodes. Then, fill pooled glass micro electrodes with 0.2 molar sodium chloride, and place them using a micro manipulator into the two burr holes placed laterally to the sagittal suture. Place the glass micro electrodes into the burr holes.Next, test the impedance of the extracranial stimulating electrodes after the burr hole placement to verify that these do not interfere with the current flow into the brain. To measure the cerebral blood flow resulting from extracranial stimulation, place the mouse under a laser speckle imaging device with intracranial recording electrodes. Once inserted into the brain, perform depth profiles at various symmetrical depths to ensure these glass micro electrodes are approximately one millimeter within the cerebral cortex.Use a digitizing system to record continuous data from the Doppler probes and micro electrodes over a sufficiently stable baseline duration. Then, apply on/off stimulation and test the extra cranial stimulation. For experiments to induce spreading depression, add a third burr hole on the right side of the skull, 1.5 millimeters rostral to coronal suture, and one millimeter lateral to posterior frontal suture.Fill this burr hole with KCL for later application. In response to low intensity tACS, data was obtained for the intracranial direct DC electrical recordings and the bilateral laser Doppler recordings of the cerebral blood flow. A small response was observed in response to a 0.75 milliampere stimulus, while a larger response was seen in response to a one milliamp stimulus.For both electrical and cerebral blood flow traces, the responses were asymmetrical between the right and left side of the skull. Laser speckle skull imaging of the cerebral blood flow during attacks showed increased blood flow in response to the stimulation, which was diffused evenly throughout the cortex. After the stimulation, the blood flow returned to baseline.

Research

JoVE Journal

Neuroscience

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