Assessing Changes in Synaptic Plasticity Using an Awake Closed-Head Injury Model of Mild Traumatic Brain Injury

Summary

Here, it is demonstrated how an awake closed-head injury model can be used for examining the effects of repeated mild traumatic brain injury (r-mTBI) on synaptic plasticity in the hippocampus. The model replicates important features of r-mTBI in patients and is used in conjunction with in vitro electrophysiology.

Abstract

Mild traumatic brain injuries (mTBIs) are a prevalent health issue in North America. There is increasing pressure to utilize ecologically valid models of closed-head mTBI in the preclinical setting to increase translatability to the clinical population. The awake closed-headed injury (ACHI) model uses a modified controlled cortical impactor to deliver closed-headed injury, inducing clinically relevant behavioral deficits without the need for a craniotomy or the use of an anesthetic.

This technique does not normally induce fatalities, skull fractures, or brain bleeds, and is more consistent with being a mild injury. Indeed, the mild nature of the ACHI procedure makes it ideal for studies investigating repetitive mTBI (r-mTBI). Growing evidence indicates that r-mTBI can result in a cumulative injury that produces behavioral symptoms, neuropathological changes, and neurodegeneration. r-mTBI is common in youths playing sports, and these injuries occur during a period of robust synaptic reorganization and myelination, making the younger population particularly vulnerable to the long-term influences of r-mTBI.

Further, r-mTBI occurs in cases of intimate partner violence, a condition for which there are few objective screening measures. In these experiments, synaptic function was assessed in the hippocampus in juvenile rats that had experienced r-mTBI using the ACHI model. Following the injuries, a tissue slicer was utilized to make hippocampal slices to evaluate bidirectional synaptic plasticity in the hippocampus at either 1 or 7 days following the r-mTBI. Overall, the ACHI model provides researchers with an ecologically valid model to study changes in synaptic plasticity following mTBI and r-mTBI.

Introduction

Traumatic brain injury (TBI) is a significant health issue, with ~2 million cases in Canada and the United States every year^1,2. TBI affects all age groups and genders and has an incidence rate greater than any other disease, notably including breast cancer, AIDS, Parkinson's disease, and multiple sclerosis³. Despite the prevalence of TBI, its pathophysiology remains poorly understood, and treatment options are limited. In part, this is because 85% of all TBIs are classified as mild (mTBI), and mTBI has previously been thought to produce only limited and transient behavioral changes with no long-term neuropsychiatric consequences^4,5. It is now recognized that mTBI recovery can take weeks to years^5,6, precipitate more serious neurological conditions⁴, and that even repeated "sub-concussive" impacts affect the brain⁷. This is alarming as athletes in sports such as hockey/football have >10 head sub-concussive impacts per game/practice session^7,8,9,10.

Adolescents have the highest incidence of mTBI, and in Canada, roughly one in 10 teens will seek medical care for a sport-related concussion annually^11,12. In reality, any sub-concussive head impact or mTBI can cause diffuse damage to the brain, and this could also create a more vulnerable state for subsequent injuries and/or more serious neurological conditions^{13,14,15,16,17}. In Canada, it is recognized legally via Rowan's law that prior injury can increase the vulnerability of the brain to further injury¹⁸, but mechanistic understanding of r-mTBI remains woefully inadequate. It is clear, however, that single and r-mTBI can impact learning capacity during school years^19,20, have sex-specific outcomes^21,22,23,24, and impair cognitive capacity later in life^16,25,26. Indeed, cohort analyses strongly associate r-mTBI early in life with dementia later on^27,28. r-mTBI is also potentially associated with chronic traumatic encephalopathy (CTE), which is characterized by the accumulation of hyperphosphorylated tau protein and progressive cortical atrophy and precipitated by significant inflammation^27,29,30,31. Although the links between r-mTBI and CTE are currently controversial³², this model will allow them to be explored in greater detail in a preclinical setting.

An mTBI is often described as an "unseen injury," as it occurs within a closed skull and is difficult to detect even with modern imaging techniques^33,34. An accurate experimental model of mTBI should adhere to two tenets. First, it should recapitulate the biomechanical forces normally observed in the clinical population³⁵. Second, the model should induce heterogeneous behavioral outcomes, something that is also highly prevalent in clinical populations^36,37,38. Currently, the majority of preclinical models tend to be more severe, involving craniotomy, stereotaxic head restraint, anesthesia, and controlled cortical impacts (CCI) that produce significant structural damage and more extensive behavioral deficits than normally observed clinically³³. Another concern with many preclinical models of concussion that involve craniotomies is that this procedure itself creates inflammation in the brain, and this can exacerbate mTBI symptoms and neuropathology from any subsequent injury^39,40. Anesthesia also introduces several complex confounds, including reducing inflammation^41,42,43, modulating microglial function⁴⁴, glutamate release⁴⁵, Ca²⁺ entry through NMDA receptors⁴⁶, intracranial pressure, and cerebral metabolism⁴⁷. Anesthesia further introduces confounds by increasing blood-brain barrier (BBB) permeability, tau hyperphosphorylation, and corticosteroid levels, while reducing cognitive function^48,49,50,51. Additionally, diffuse, closed-headed injuries represent the vast majority of clinical mTBIs⁵². They also allow one to better study the multitude of factors that can influence behavioral outcomes, including sex²¹, age⁵³, inter-injury-interval¹⁵, severity⁵⁴, and the number of injuries²³.

The direction of the accelerative/decelerative forces (vertical or horizontal) is also an important consideration for behavioral and molecular outcomes. Research from Mychasiuk and colleagues have compared two models of diffuse closed-headed mTBI: weight-drop (vertical forces) and lateral impact (horizontal forces)⁵⁵. Both the behavioral and molecular analyses revealed heterogeneous model- and sex-dependent outcomes following mTBI. Thus, animal models that help avoid surgical procedures, while incorporating linear and rotational forces, are more representative of the physiological conditions under which these injuries normally occur^33,56. The ACHI model was created in response to this need, allowing for the rapid and reproducible induction of mTBI in rats while avoiding procedures (i.e., anesthesia) that are known to bias sex differences⁵⁷.

Protocol

Approval for all animal procedures was provided by the University of Victoria Animal Care Committee in compliance with Canadian Council on Animal Care (CCAC) standards. All male Long-Evans rats were bred in-house or purchased (see the Table of Materials).

1. Housing and breeding conditions

1. Allow the animals to acclimate to their housing environment for 1 week before weaning at postnatal day (PND) 21.
2. Maintain the rats in standard cage housing at 22.5 °C ± 2.5 °C, with ad libitum access to food and water, on a 12 h light/dark cycle.
3. Group and house the animals with two or three sex-matched littermates and randomly assign them to either sham or r-mTBI conditions.
4. Perform all procedures between 7:30 AM and 11:30 PM.

2. Setup of awake closed-head injury procedure

1. Position a 2.75 in. low-density foam pad (100 cm x 15 cm x 7 cm) underneath the impactor to allow for rotational head movement.
   NOTE: The foam pad had a spring constant of ~2,500 N/m but can vary between 3,100 and 5,600 N/m⁵⁸. The level of firmness (low, medium, and high) has not shown to be predictive of injury outcome⁵⁹. The foam pad is a non-consumable material. It is normally replaced yearly or if soiled or damaged.
2. Turn on the modified cortical impact device (Figure 1A), and set the velocity to 6 m/s.
   ​NOTE: These specifications are designed to elicit acute neurological impairment in juvenile and adolescent aged rats that are analogous to features of an mTBI, but such parameters may not be suitable for older animals or other species (e.g., mice or ferrets). For a review of common ACHI parameters, see⁶⁰.

3. Induction of mTBI

1. When the rats reach PND 24, move them into the procedure room where the procedures will be performed. Ensure this room is separate from their normal housing environment.
2. Gently place the rat in a restraint cone, ensuring that the snout and nostrils are close to the cone's small opening to allow for adequate ventilation. Use a plastic hair clip to hold the cone closed at the caudal end to prevent movement once the rat is placed in the restraint cone.
   1. Use restraint scores to record the animals' compliance or tolerance with the restraint cone and ACHI procedure.
      NOTE: The restraint score can be used as an assessment of stress in the animals. Thus, exclusion criteria can be developed using the restraint score to reduce variability between subjects that arises due to an excessive stress response.
      1. Give a score from 0 to 4 based on the animal's willingness to enter the cone, their movements, and vocalizations. Give a score of 0 if there is no resistance to the restraint, while a score of 1 corresponds with the animal turning 1-2x and little-to-no vocalization or squirming. Give a score of 2 if the animal has turned 2-3x and exhibits some vocalization or squirming. Give a restraint score of 3 if the animal has turned 5-10x and exhibits more vocalizations and squirming. Finally, give a score of 4 if the animal has turned more than 10x with frequent vocalizations and squirming.
         NOTE: This information is also on the scoring sheet itself (Supplementary Table S1 and Supplementary Table S2).
3. While the rat is restrained, manually position the helmet (Figure 1B) over the midline, with the targeting disk over the left parietal lobe (Figure 1C,D).
4. Place the rat on the foam pad and manually set the impactor to the Extend position. Manually lower the impactor tip so that it comes in contact with the targeting disk on the helmet. Manually set the impactor to the Retract position to make the impactor withdraw 10 mm above the helmet.
5. Use the dial on the stereotaxic arm to lower the impact tip by 10 mm so that it is again touching the targeting disk on the helmet. Flip the Impact switch so that the animal's head is rapidly accelerated for 10 mm at 6 m/s.
6. Once the device has been activated, immediately remove the animal from the restraint cone and proceed to perform an immediate neurological assessment protocol (NAP).
   ​NOTE: For the current experiments, this protocol was repeated eight times in total at 2 h intervals.

4. Induction of sham injury

1. Follow all experimental procedures as described above in section 3 but place the rat adjacent to the path of the impact piston, so no injury is delivered.

5. Neurological assessment protocol

NOTE: The NAP can be used to measure the level of consciousness, as well as cognitive and sensorimotor functioning.

1. At baseline and immediately following induction of the mTBI or sham injury, assess the rats using the NAP as described in^56,61. On a table, place the rats' home cage and a recovery cage spaced 100 cm apart. Evenly center the balance beam on top of both cages. Additionally, place a folded towel or additional cushioning underneath the balance beam.
2. If required, assess the level of consciousness. If animals are non-responsive after the mTBI, assess the apnea (cessation of breathing) and any delay in the righting reflex by using a stopwatch to record the time taken for the animal to resume breathing and/or right themselves from a supine into the prone position.
   NOTE: Loss of righting reflex and apnea are rare with the ACHI model but they can occasionally be observed in juvenile animals.
3. Assess the rat's cognitive and sensorimotor function using the following sequence of tests. Administer these tests rapidly in succession following the assessment of consciousness.
   NOTE: The summation of these four tests yields a total score out of 12, if there are no observed behavioral deficits. Deficits detract from this score.
   1. Startle response
      1. Place the rat in the empty recovery cage and clap loudly (50 cm) over the cage. Record the animal's response to the noise using the following scoring system:
         3 = Quick startle reaction to sound (e.g., ear movement/twitches, jump, whole body freezes).
         2 = Slow reaction or slight freezing reaction to sound.
         1 = Only ear movements observed.
         0 = No reaction to sound.
   2. Limb extension
      1. With the beam (100 cm long x 2 cm wide x 0.75 cm thick) placed horizontally across the rat's home and recovery cages, pick up the rat by the base of the tail and hold it near the beam. Ensure the rat is close enough to be able to easily grasp it. Assess the rat's ability to extend both limbs out to the beam with the following scoring system:
         3 = Full extension of both forelimbs and grasps the beam.
         2 = Only one limb is extended.
         1 = Intermittent extension or retraction of forelimbs.
         0 = Forelimbs are limp/no extension.
   3. Beam walk
      1. Place the animal in the center of the horizontal beam at the 50 cm mark facing its home cage. Ensure the beam is spaced equally between the rat's home cage and recovery cage (placed ~80 cm apart). Allow the rat to walk across the beam. Assess the rat's ability to balance and walk with the following scoring system:
         3 = Successfully walks across the beam with less than two foot slips within 10 s.
         2 = Successfully walks the beam, but more than two foot slips are observed.
         1 = Non-locomotive movement, 'swimming' motion.
         0 = Unable to walk along the beam or unable to move within 10 s.
   4. Rotating beam
      1. Reposition the rat at the center of the beam, ensuring the rat is balanced. Lift the beam 80 cm above a towel or padded surface and begin manually rotating the beam at a rate of one rotation per second for 4 s (a total of four rotations). Assess the rat's ability to remain on the beam as it rotates with the following scoring system:
         3 = Rat remains on the beam for all four rotations.
         2 = Rat falls on the fourth rotation.
         1 = Rat falls on the second or third rotation.
         0 = Rat falls during the first rotation.
4. Upon completion of NAP, return mTBI and sham rats to their home cages. Repeat as necessary for r-mTBI procedures. Monitor the well-being of the animals following injuries with the Cage-side Monitoring Checklist (Supplementary File 1). If there is any indication of abnormality (any score that is not N) during cage-side monitoring, a full pain score should be taken with the Pain Scale and Advanced Monitoring Checklist after Head Impact (Supplementary File 2).

6. Slice preparation

NOTE: In the current study, synaptic plasticity was assessed in animals following r-mTBI at either 1 or 7 days after mTBI. On these days, the animals were brought individually into the laboratory in covered cages prior to sacrifice.

1. Refrigerate (-20 °C) overnight all surgical tools (Figure 2A) required for making hippocampal slices: standard scissors, dissecting scissors, forceps, rongeurs, spatulas, and chilling block.
   NOTE: The tissue glue and incubation chamber should not be refrigerated.
2. Prepare artificial cerebrospinal fluid (aCSF) containing 125 mM NaCl, 2.5 mM KCl, 1.25 mM NaHPO₄, 25 mM NaHCO₃, 2 mM CaCl₂, 1.3 mM MgCl₂, and 10 mM dextrose (300 ± 10 mOsm; pH 7.2-7.4).
   NOTE: The main solution of aCSF must be continuously bubbled with carbogen (95% O₂/5% CO₂) for the duration of the protocol.
3. Before euthanizing the animal (step 6.8), prepare 12.5 mL of agarose. Dissolve 0.25 g of agarose in 12.5 mL of phosphate-buffered saline (1x PBS) by microwaving in a 50 mL conical tube in 10 s increments.
4. Keep the agarose warm (42 °C) and shake in a heating plate to prevent it from solidifying.
5. Set up a cutting station on ice, including a Petri dish and a small beaker (50 mL) filled with ice-cold aCSF (4 °C) and an overturned Petri dish with a piece of wetted filter paper on top (Figure 2A). Continuously bubble the aCSF in the small beaker with carbogen.
6. Warm the water bath to 32 °C. Fill the recovery chamber with aCSF and continuously bubble with carbogen (Figure 2B).
7. Transport the animal to the experimental room.
8. Anesthetize the animal using 5% isoflurane as an inhalant (until lack of a withdrawal reflex) and then rapidly decapitate it using a small guillotine.
9. Dissect the brain from the skull in the Petri dish filled with ice-cold (4 °C) aCSF, holding the skull submerged in the aCSF to help rapidly cool the tissue.
   NOTE: This procedure normally requires under 5 min, but the speed of brain removal is not a critical factor if the brain is submerged in chilled aCSF.
10. Place the brain into the small beaker of chilled and carbogenated aCSF to further clean and chill the sample.
11. Move the brain to the upside-down Petri dish and place it on the filter paper. Use a sharp scalpel to remove the cerebellum and prefrontal cortex to "block" the brain. Separate the two hemispheres by making a cut down the midline of the brain.
    NOTE: The following protocol is performed one hemisphere at a time. It is imperative that the hemisphere not currently being prepared remains submerged in the beaker of ice-cold (4 °C) carbogenated aCSF.
12. To create transverse hippocampal slices, place the hemisphere on the medial surface. Tilt the blade of a scalpel at ~30° inward and remove a thin slice from the dorsal surface of the brain to provide a flat surface for the brain to be mounted on the piston used by the slicer. Flip the brain onto the dorsal surface and gently dab the tissue on dry filter paper to remove any excess aCSF. Using cyanoacrylate glue, attach the dorsal surface of the brain to the piston, leaving the ventral surface upright.
    NOTE: Ensure that the glue does not run over the edge of the piston, as this will cause it to adhere to the metal tube used to contain the agarose and prevent movement of the piston.
13. Extend the outer tube of the piston over the brain and pour the liquid agarose into the tube until the brain is completely covered. Quickly solidify the agarose by clamping a chilling block over the piston tube (Figure 2A).
14. Position the piston into the chamber of the slicer and secure the chamber with a screw. Secure the blade and add ice-cold, oxygenated aCSF to the slicer chamber.
15. On the slicer (Figure 2B), set the cutting speed to 4, oscillation to 6, and toggle the continuous/single slicing switch to continuous. Push start to begin sectioning the brain at 400 µm.
16. As the slicer sections the brain, use a large diameter Pasteur pipette to transfer each slice to the recovery bath of oxygenated aCSF as it is sectioned (Figure 2C).
    NOTE: As each slice is cut, it can be placed sequentially in the different wells of the recovery bath. This protocol usually yields between six and eight slices, containing the hippocampus for each hemisphere. A rat atlas⁶² can be used to identify the dorsal-ventral position of individual slices in the rat brain.
17. Let the slices recover at 32 °C for 30 min and then leave to recover for an additional 30 min at room temperature (23 °C).
18. Repeat these steps to create slices from the second hemisphere.

7. Field electrophysiology

NOTE: To acquire extracellular field recordings from the dentate gyrus (DG), perform the following steps. Following the 60 min recovery, individual hippocampal slices are ready for extracellular field recordings.

1. Using a commercially available micropipette puller, pull recording electrodes (1-2 MΩ) from 10 cm borosilicate glass capillaries with an outer diameter of 1.5 mm and an inner diameter of 1.1 mm.
   NOTE: Recording electrode should have a resistance of ~1 MΩ and the tips should be ~1 mm in size. Consistency in electrode parameters is important for good recordings.
2. Turn on the computer and equipment to be used for recordings: the amplifier, digitizer, stimulator, micromanipulator, temperature regulator, microscope light, and vacuum pump.
3. Fill a beaker with aCSF and connect it to a gravity-controlled perfusion system. Open the aCSF valve on the perfusion system to begin a flow of aCSF through the perfusion chamber. Maintain a flow rate of approximately one or two drips/s or 2 mL/min. Continuously carbogenate aCSF for the duration of electrophysiological recordings.
   NOTE: It is imperative to maintain a constant drip rate of carbogenated aCSF during field recordings. It is also imperative that the reference electrode is completely submerged in aCSF.
4. Use a Pasteur pipette to transfer a hippocampal slice from the recovery bath to the perfusion chamber that is continuously perfused with carbogenated aCSF and maintained at 30 ± 0.5 °C. Orientate the brain slice so that the dentate gyrus and granule cell layer are visible in the field of view. Stabilize the slice with bent wire weights. Start the computer software for data acquisition.
   NOTE: It can be helpful to turn off the vacuum pump during this step to allow for free manipulation of the tissue. This should be done quickly as too much manipulation can damage the tissue. Additionally, the perfusion chamber can overflow with aCSF if this takes too much time. Once the tissue is orientated properly and stabilized, turn on the vacuum pump.
5. Use an upright microscope to visualize the DG with oblique optics. Position a concentric bipolar stimulating electrode to activate the medial perforant path (MPP) fibers in the middle third of the molecular layer. Then, position a glass micropipette, filled with aCSF in the MPP (Figure 3A,B). Begin with the electrodes further apart (i.e., the stimulating electrode near CA3 and the recording electrode just above the genu of the DG), as touching the tissue will cause damage to the fibers.
   NOTE: Optimally, all recordings should have the electrodes placed equidistant from the cell layer, approximately 200 μm apart.
6. Once the stimulating and recording electrodes are positioned, visualize the evoked field responses using an amplifier, a digitizer, and recording software.
7. To find a suitable field excitatory postsynaptic potential (fEPSP), stimulate the tissue with 0.12 ms current pulses at 0.2 Hz (every 5 s) when the user is proficient at finding responses, or at 0.067 Hz (every 15 s) for less proficient users to avoid overstimulation. Ensure that the fEPSP has a minimum amplitude of 0.7 mV with a clear fiber volley that is smaller than the fEPSP.
   NOTE: It is critical to position both electrodes equidistant from the cell layer to obtain maximal field responses and far enough apart (i.e., ~200 μm) to generate a small fiber volley. Small adjustments in electrode position may help enhance the amplitude of the response, although these should be kept to a minimum to avoid tissue damage.
8. Determine the maximum fEPSP amplitude by increasing the stimulation intensity and then set the simulating intensity so that the fEPSP is at 70% of the maximum amplitude.
   NOTE: The maximum amplitude is set to 70% for long-term depression (LTD) studies and to 50% for long-term potentiation (LTP) studies. The maximum amplitude is determined by adjusting the stimulation strength until the fEPSP no longer increases in amplitude. For a fEPSP with a 2 mV maximum amplitude, the response size would then be adjusted to 1.4 mV for LTD studies and 1.0 mV for LTP studies, to allow room for the fEPSP to depress or potentiate (respectively).
9. Establish a stable preconditioning baseline for 20 min with 0.12 ms pulses delivered at 0.067 Hz. For slices to be considered stable, look for <10% variability in the initial slope of the fEPSP and for the slope of the line of best fit through the plotted fEPSP slopes to be <0.5. Proceed with the next steps of the recording when EPSPs are verified to be stable for 20 min.
   NOTE: Various receptor antagonists can be added to the aCSF to block or enhance LTD and LTP. If they are required, ensure the slices are exposed to these pharmacological agents during this baseline period and that the requirements for stable recordings are met. For examples, see^63,64,65.
10. First, determine changes in basic synaptic properties by using paired-pulse stimuli and by constructing stimulus-response input-output curves. For the paired-pulse test, apply a series of paired pulses with an interpulse interval of 50 ms at 0.033 Hz. For the input-output curves, apply a series (10) of increasing stimulus intensities (0.0-0.24 ms) at 0.033 Hz to plot the fEPSP response size change.
11. To study LTD that is primarily dependent on the activation of CB1 receptors^64,66, employ a 10 Hz protocol (6,000 pulses at 10 Hz). This protocol takes 10 min to administer.
12. For postconditioning recordings, resume using single-pulse stimulation (0.12 ms at a frequency of 0.067 Hz) for an additional 60 min.
13. Following the postconditioning recording, again administer the paired-pulse stimuli, followed by an input-output curve. Compare these to baseline recordings to observe alterations in presynaptic release properties and help assess the health of the slice for long-term recordings.
14. During analysis, be conservative and adhere to the exclusion criteria when determining if the data from individual slices should be retained in the synaptic plasticity data set. Exclude slices that display a large slope in a line of best fit of fEPSP slopes during the preconditioning baseline (slope >0.5), instability in preconditioning baseline (>10% change), and or instability in the postconditioning period (slope >1.5 in 50-60 min of postconditioning).

Representative Results

The awake closed-head injury model is a viable method of inducing r-mTBI in juvenile rats. Rats exposed to r-mTBI with the ACHI model did not show overt behavioral deficits. Subjects in these experiments did not exhibit latency to right or apnea at any point during the r-mTBI procedure, indicating that this was indeed a mild TBI procedure. Subtle behavioral differences did emerge in the NAP; as described above, the rats were scored on four sensorimotor tasks (startle response, limb extension, beam walk, and rotating beam) on a scale from 0 to 3, with 3 representing no impairment with the task. Thus, the lower the NAP score, the more impaired the animal was. At baseline, there were no differences in the NAP scores between sham and r-mTBI rats. Following all ACHI sessions, the r-mTBI rats showed significant impairments within the NAP tasks when compared to shams (Figure 4). However, as reported previously for impacts delivered over multiple days (i.e., 2 or 4 days), the subsequent addition of injuries over the course of the day did not compound or produce additional behavioral deficits. Thus, the ACHI model of r-mTBI produces subtle, yet significant, behavioral deficits during these acute post injury time points.

Following the injury protocol, evoked field responses and synaptic plasticity were examined in the MPP input to the DG of the hippocampus on post injury day 1 (PID1), and PID7. Slice health was examined using fEPSPs in response to an ascending series of pulse widths in each slice. As is shown in Figure 3C, there was no difference in the input-output curves generated in slices obtained from sham and r-mTBI rats. To examine presynaptic transmitter release, a series of paired pulses (50 ms interpulse interval) were administered, and the ratio of the size of the second fEPSP was calculated relative to the first fEPSP. The paired-pulse ratios did not differ between sham and r-mTBI rats (Figure 3D). Thus, these data indicate that r-mTBI did not alter basic synaptic physiology in the MPP input to the DG. To examine LTD, a 10 Hz LTD protocol was administered to induce an LTD dependent on endocannabinoids⁶⁴. On PID1, there was a significant decrease in the capacity of the MPP input to the DG to sustain LTD (Figure 3E). This reduction in LTD was transient, however, and by PID7, slices from sham and r-mTBI animals displayed equivalent LTD (Figure 3F), although there was an indication of a slight trend for slices from r-mTBI animals to exhibit an increase in LTD.

Figure 1: The ACHI procedure setup used to model r-mTBI. (A) A modified controlled cortical impactor was used to rapidly displace the animal's head 10 mm at a velocity of 6.0 m/s. (B,C) Custom 3D printed helmet with a left parietal cortex target site. (D) Subjects were placed in a plastic restraint bag on a foam platform, with the helmet placed around the restraint cone and positioned so the target site is directly under the impactor tip. Abbreviations: ACHI = awake closed-head injury; r-mTBI = repeated mild traumatic brain injury. Please click here to view a larger version of this figure.

Figure 2: Materials and setup required for slice preparation. (A) Tools used for brain extraction, mounting, slicing, and incubation: (a) culture dish with filter paper; (b) various dissection tools, including standard scissors, dissecting scissors, forceps, a rongeur, and spatulas; (c) tissue adhesive; (d) Compresstome piston and specimen tube; (e) feather blade and blade holder; (f) chilling block; (g) slice incubation chamber. (B) Compresstome tissue slicer. (C) Slices incubating in a bath containing artificial cerebrospinal fluid that is continuously oxygenated with 95% O₂/5% CO₂. Please click here to view a larger version of this figure.

Figure 3: Acute impairments in synaptic plasticity in juvenile male rats due to r-mTBI using the ACHI model. (A) The major hippocampal pathways. The medial perforant pathway is comprised of the input from the entorhinal cortex into the dentate gyrus (blue). The medial perforant path inputs synapse onto granule cells in the dentate gyrus (purple). (B) Brightfield photomicrograph of a hippocampal brain slice (4x magnification), showing the actual placement of a bipolar stimulating electrode (left) and a glass recording electrode pipette (right) in the medial performant path of the dentate gyrus. (C) Input-output plot (fEPSP slope) for different simulation intensities (10-300 µs) on PID1 and PID7 for sham and r-mTBI rats. (D) Paired-pulse ratios for sham and r-mTBI rats (50 ms interpulse interval). (E) Time course of fEPSP changes prior to, and following, administration of an LTD induction paradigm in hippocampal slices obtained from sham and r-mTBI rats at PID1. (F) Time course of fEPSP changes prior to, and following, administration of an LTD induction paradigm in hippocampal slices obtained from sham and r-mTBI rats at PID7. Abbreviations: ACHI = awake closed-head injury; r-mTBI = repeated mild traumatic brain injury; PID = post injury day; fEPSP = field excitatory postsynaptic potential. Please click here to view a larger version of this figure.

Figure 4: Acute neurological impairment in juvenile male rats due to r-mTBI using the ACHI model. The rats underwent eight ACHI procedures at 2 h intervals over 1 day, with a neurological assessment protocol conducted at baseline and after each injury. The NAP consisted of four tasks: startle response, limb extension, beam walk, and rotating beam. Each task was scored out of 3, giving a total possible score of 12 for each session. Data presented as mean ± SEM. (*) indicates p < 0.05. Abbreviations: ACHI = awake closed-head injury; r-mTBI = repeated mild traumatic brain injury; NAP = neurological assessment protocol. Please click here to view a larger version of this figure.

Supplementary Table S1: ACHI procedure animal and impact information. Abbreviation: ACHI = awake closed-head injury. Please click here to download this File.

Supplementary Table S2: Restraint scoring for awake mTBI. Abbreviation: mTBI = mild traumatic brain injury. "Turn around in restraint" refers to the researcher placing the animal in the restraint, before closing the bag around the tail. After the bag is closed, the animal should not be able to turn around. Vocalization and squirming should be scored after the bag is closed. Please click here to download this File.

Supplementary File 1: Cage-side monitoring checklist. Please click here to download this File.

Supplementary File 2: Pain scale and advanced monitoring checklist. Please click here to download this File.

Discussion

Most preclinical research has utilized models of mTBI that do not recapitulate the biomechanical forces seen in the clinical population. Here, it is shown how the ACHI model can be used to induce r-mTBIs in juvenile rats. This closed-headed model of r-mTBI has significant advantages over more invasive procedures. First, the ACHI does not normally cause skull fractures, brain bleeds, or fatalities, all of which would be contraindications of a "mild" TBI in clinical populations⁶¹. Second, the ACHI does not require the use of craniotomies, which is significant because they are known to cause inflammatory responses that can exacerbate symptomologies and neuropathology⁶⁷. Third, the ACHI does not require the use of anesthesia. This is also significant, as anesthesia can have neuroprotective properties and can impair synaptic plasticity, in addition to learning and memory performance^{48,49,50,51,68}. Finally, the ACHI can produce subtle transient changes in neurological function that can be assessed immediately post injury.

As the ACHI does not normally induce loss of consciousness or apnea, this model mimics mTBI in a significant proportion of the clinical population^69,70,71. Despite this, the ACHI model produced a significant reduction in the NAP scores. This reduction persisted with repeated administrations of the ACHI procedure but did not exacerbate sensorimotor impairments within the r-mTBI group. This indicates that the ACHI model induces a mild injury analogous to that observed following concussive or sub-concussive head impacts in clinical populations^72,73. A primary advantage of the NAP is the detection of subtle behavioral deficits seen in the acute timeframe following r-mTBI. This quick examination may allow researchers to categorize rats based on their behavioral responses. However, the use of more robust behavioral tests at subacute and chronic time points may be necessary to detect motor, cognitive, and affective symptomologies^74,75,76. It is important to note that while there were no differences in NAP scores over the eight injuries, rodent behavior can be influenced by changes in environment and familiarity with the experimenter^77,78. Rats should be allowed to acclimate to the procedure room prior to administering r-mTBI or sham injuries. In addition, it is important for one individual to be responsible for administering the impacts to help ensure consistency.

Despite the previously mentioned benefits of the ACHI model, it is not without limitations. First, the paradigm was designed to mimic the cumulation of impacts in a single session and not repetitive injuries following a recovery period. Following injury, the brain resides in a window of cerebral vulnerability that extends from 1 to 5 days post injury in rodents^15,79,80. Receiving eight injuries on a singular day does not allow for acute and subacute injury cascades to develop. Therefore, depending on the research question of interest, the injury paradigm may need to be adjusted within the window of vulnerability. Second, while it is beneficial to limit the use of anesthetic, an unintended consequence of the ACHI model is subjecting the rats to restraint stress. It has been shown that exposure to acute and chronic stressors may initiate an inflammatory response, influence a variety of behaviors, and alter synaptic plasticity in the hippocampus^81,82,83.

The protocol described above provides a clear-cut method to produce high-quality transverse hippocampal slices from r-mTBI-administered animals with the ACHI model. Additionally, the protocol allows for stable electrophysiological recordings and shows that the hippocampus is still capable of exhibiting synaptic plasticity following r-mTBI, although there may be transient disruptions. With any electrophysiological recordings, slice health is paramount for the ability to record suitable fEPSPs. To preserve brain tissue, prior to slicing, it is imperative that the brain remains ice-cold in carbogenated aCSF. The brain's removal and slicing should be done quickly, but not if this comes at the expense of care. This protocol on juvenile animals utilizes aCSF as cutting solution, but depending on the age of the animal, protective cutting solutions (such as choline-, sucrose-, NMDG-, or glycerol-based solutions) may be required^84,85,86.

Field electrophysiological recordings allow researchers to gauge hippocampal synaptic plasticity. However, there are a number of limitations to the technique. The process of slicing the brain has shown to cause changes in spine numbers⁸⁷, which could affect synaptic plasticity. The use of in vivo recordings would preserve pathways and allow for measurement of synaptic plasticity in anesthetized or live animals⁸⁸. Additionally, the use of field recordings probes the properties of groups of neurons but does not inform about changes in individual neurons. The use of whole-cell patch-clamp recordings can give temporally detailed information about neuronal properties in response to pharmacological or optogenetic manipulations⁸⁹. Additionally, the combination of electrophysiological recordings with complementary techniques, such as calcium imaging, Western blotting, immunohistochemistry, or electron microscopy, would allow researchers to gain insight into the mechanisms of action.

Cognitive deficits are commonly reported following r-mTBI, and the current protocol can help investigate some of the underlying physiological processes associated with these deficits. In particular, the mild nature of the ACHI procedure opens up the possibility of examining changes in synaptic physiology across the lifespan of animals that have incurred r-mTBI. The ACHI model appears to be an ecologically valid model of mTBI than can be used to study r-mTBI. Preliminary studies utilizing the ACHI model have showed acute neurological impairment without overt structural damage, administering one, four, and eight repeated injury paradigms^61,90. Future studies will examine how r-mTBI can impact synaptic plasticity during developmental periods and in the aging brain. By better understanding the pathophysiology of mTBI and r-mTBI for synaptic function, the hope is to better direct potential therapeutic interventions to help reduce cognitive function.

Materials

Name	Company	Catalog Number	Comments
3D-printed helment			Designed and constructed by Christie laboratory (See Specifications in Christie et al. (2019), Current Protocols in Neuroscience)
Agarose	Fisher Scientific (BioReagents)	BP160500
Anesthesia chamber	Home Made	N/A	Plexiglass Container
Automatic Heater Controller	Warner Electric	TC-324B
Axon Digidata	Molecular Devices	1440A	Low-noise Data Acquisition System
Balance beam			Can be constructed or purchased (100 cm long x 2 cm wide x 0.75 cm thick)
Calcium Chloride	Bio Basic Canada Inc.	CD0050	For aCSF
Camera	Dage MTI	NC-70
Carbogen tank	Praxair	MM OXCD5C-K	Carbon Dioxide 5%, Oxygen 95%
Clampex Software	Molecular Devices	Clampex 10.5 Version
Compresstome Vibrating Microtome	Precisionary	VF 310-0Z
Concentric Bipolar Electrode	FHC Inc.	CBAPC75
Dextrose (D-Glucose)	Fisher Scientific (Chemical)	D16-3	aCSF
Digital Stimulus Isolation Amplifier	Getting Instruments, Inc.	Model 4D
Disodium Phosphate	Fisher Scientific (Chemical)	S373-500	PBS
Dissection Tools
Feather Double Edge Blade	Electron Microscopy Sciences	72002-10
Filter Paper	Whatman 1	1001-055
Flaming/Brown Micropipette Puller	Sutter Instrument	P-1000
Hair Claw Clip			Can be obtained from any department store
Home and Recovery Cages			Normal rat cages from animal care unit.
Hum Bug Noise Eliminator	Quest Scientific	726300
Isoflurane USP	Fresenius Kabi	CP0406V2
Isotemp 215 Digital Water Bath	Fisher Scientific	15-462-15
Leica Impact One CCI unit	Leica Biosystems		Tip is modified to hold 7mm rubber impact tip
Long-Evans rats, male	Charles River Laboratories (St. Constant, PQ)
Low-Density Foam Pad			3" polyurethane foam sheet
Magnesium Chloride	Fisher Scientific (Chemical)	M33-500	aCSF
Male Long Evans Rats	Charles River Laboratories		Animals ordered from Charles River Laboratories, or pups bred at the University of Victoria
MultiClamp 700B Amplifier	Molecular Devices	Model 700B
pH Test Strips	VWR Chemicals BDH	BDH83931.601
Potassium Chloride	Fisher Scientific (Chemical)	P217-500	aCSF, PBS
Potassium Phosphate	Sigma	P9791-500G	PBS
Push Button Controller	Siskiyou Corporation	MC1000e	Four-axis Closed Loop Controller Push-Button
Sample Discs	ELITechGroup	SS-033	For use with Vapor Pressure Osmometer
Small towel
Sodium Bicarbonate	Fisher Scientific (Chemical)	S233-500	aCSF
Sodium Chloride	Fisher Scientific (Chemical)	S271-3	For aCSF, PBS
Sodium Phosphate	Fisher Scientific (Chemical)	S369-500	aCSF
Soft Plastic Restraint Cones	Braintree Scientific	model DC-200
Stopwatch			Many lab members use their iPhone for this
Table or large cart with raised edges			For NAP and ACHI
Thin Wall Borosilicate Glass (with Filament)	Sutter Instrument	BF150-110-10	Outside diameter: 1.5 mm; Inside diameter: 1.10 mm; Length: 10 cm
Upright Microscope	Olympus	Olympus BX5OWI	5x MPlan 0.10 NA Objective lens
Vapor Pressure Osmometer	Vapro	Model 5600	aCSF should be 300-310 mOSM
Vetbond Tissue Adhesive	3M	1469SB
Vibraplane Vibration Isolation Table	Kinetic Systems	9101-01-45