Induction of Mild Traumatic Brain Injury in a Mouse Model

Secure an anesthetized, prepped mouse in a stereotaxic frame with its skull exposed. 

Identify the skull reference points. Then, position a head support apparatus under the animal’s head and inflate it to prevent ear damage. 

Align a stereotaxically guided electromagnetic impactor over the head. 

Reset the stereotaxic coordinates to precisely align the impactor probe over the target. Attach a contact sensor to the animal's ear.

Once the probe touches the skull, retract it. Set the desired impact depth and initiate the impact with the appropriate parameters. 

The probe delivers a controlled force to the skull, inducing axonal stretching and shearing in cortical neurons, which leads to mild traumatic brain injury.

Measure the animal’s righting time, the time taken to regain normal body position post-injury. 

Evaluate the mouse for visible skull fractures.

Then, remove the mouse from the frame, close the incision, and apply antibiotics.

Allow the mouse to recover.

Secure the mouse into the stereotaxic frame using lightweight, acetal resin, tapered point ear bars, a bite bar, and a mouse anesthesia mask. Then, carefully monitor the mouse's respiration to ensure the depth of anesthesia. Apply sterile eye lubricant to the eyes to prevent corneal drying. Sterilize the scalp with povidone iodine swabs.

After ensuring the depth of anesthesia, make an approximately 1 centimeter midline scalp incision between the eyes and neck using a scalpel, exposing the skull. Allow the skull to dry for 1 to 2 minutes, then, identify the bregma and lambda. Next, place the head support apparatus under the head and inflate the bulb with water until it is pressing against the bottom of the mouse's head but not lifting the head away from the bite bar.

Move the impactor into place over the mouse's head. Extend the impactor by placing the extend-retract toggle switch on extend. Then, line up the impactor until it is centered over the bregma. Reset the digital stereotaxic x and y-coordinates in the stereotaxic reader to 0. Then, align the probe over the impact location by moving the probe from the bregma over to the target coordinates as medial lateral to 0 millimeter and anterior posterior to minus 1.6 millimeters.

Click the contact sensor to the mouse ear. Slowly lower the probe tip with the extended probe until first contact with the surface is made. Stop at the beep. Then, reset the digital stereotaxic z-coordinates in the stereotaxic reader to 0.

Carefully inspect if the tip is flush with the skull. Then, retract the impactor by placing the toggle switch on the control box in the retract position. The tip withdraws and has no more contact with the head until the time of impact. Set the impact depth by adjusting the dorsal ventral depth to minus 1.2 millimeters. Carefully monitor the mouse's respiration to ensure the anesthesia depth and adjust the gas level as needed.

Induce the impact by pressing the right toggle switch to impact. The probe tip goes down at the displayed velocity, stays down for the set dwell time, and retracts. Start the timer immediately after the close head injury or CHI impact is delivered to record the righting times. The average righting reflex time is 5 to 15 minutes.

Evaluate the mouse for visible skull fractures, hemorrhages, and apnea. Remove the mouse from the stereotaxic frame. Close the scalp by stapling the skin together. Then apply triple antibiotic ointment with a sterile cotton-tipped applicator to the closed incision. Return the mouse to a clean holding cage for recovery.