Traumatic brain injury (TBI) is commonly associated with memory impairment. Here, we present a protocol to assess spatial working memory after TBI via a metric task. A metric test is a useful tool to study spatial working memory impairment after TBI.
Impairments to sensory, short-term, and long-term memory are common side effects after traumatic brain injury (TBI). Due to the ethical limitations of human studies, animal models provide suitable alternatives to test treatment methods, and to study the mechanisms and related complications of the condition. Experimental rodent models have historically been the most widely used due to their accessibility, low cost, reproducibility, and validated approaches. A metric test, which tests the ability to recall the placement of two objects at various distances and angles from one another, is a technique to study impairment in spatial working memory (SWM) after TBI. The significant advantages of metric tasks include the possibility of dynamic observation, low cost, reproducibility, relative ease of implementation, and low stress environment. Here, we present a metric test protocol to measure impairment of SWM in adult rats after TBI. This test provides a feasible way to evaluate physiology and pathophysiology of brain function more effectively.
The prevalence of neurological deficits such as attention, executive function, and certain memory deficits after moderate traumatic brain injury (TBI) is more than 50 percent1,2,3,4,5,6,7,8. TBI can lead to severe impairments in spatial short-term, long-term, and working memory9. These memory impairments have been observed in rodent models of TBI. Rodent models have enabled the development of techniques to test memory, allowing for deeper examinations into the effect of TBI on memory processing in neural memory systems.
Two tests, related to topological and metric spatial information processing respectively, assist with measuring spatial working memory (SWM). The topological test depends on changing the size of environmental space or related spaces of connection or enclosure around an object, while the metric test assesses changes in angles or distance between objects10,11. Goodrich-Hunsaker et al. first adapted the human topological test for rats10 and applied the metric task to dissociate the roles of the parietal cortex (PC) and dorsal hippocampus in spatial information processing11. Similarly, Gurkoff and colleagues evaluated metric, topological, and temporal ordering memory tasks after lateral fluid percussion injury9. There is a correlation between damage to certain regions of the brain and impairment of metric or topological memory. It has been suggested that metric memory impairment is related to lesions in bilateral dorsal dentate gyrus and cornu ammonis (CA) sub-region CA3 of the hippocampus, and that topological memory impairment is related to bilateral parietal cortex lesions10,12.
The purpose of this protocol is to assess spatial memory deficit in a rat population via a metric task. This method is a suitable alternative to investigate mechanisms of SWM after brain injury, and its advantages include the relative ease of implementation, high sensitivity, low cost of reproducibility, the possibility of dynamic observation, and a low stress environment. Compared to other behavioral tasks such as the Barnes maze13,14, Morris water navigation task15,16,17, or spatial maze tasks18,19, this metric test is less complicated. Due to its ease of implementation, the metric test requires a shorter and less stressful training period and takes place over only 2 days9: 1 day for habituation and 1 day for the task. Moreover, our proposed test is easier to perform than other low stress tests, such as the novel object recognition (NOR) task, and does not require the extra day of habituation20.
This paper provides a straightforward model for evaluating SWM after brain injury. This assessment of post-TBI SWM may assist in a more comprehensive investigation of its pathophysiology.
The experiments were performed following the recommendations of the Declarations of Helsinki and Tokyo and the Guidelines for the Use of Experimental Animals of the European Community. The experiments were approved by the Animal Care Committee of Ben-Gurion University of the Negev. A protocol timeline is illustrated in Figure 1.
1. Surgical procedures and fluid percussion TBI
2. Evaluation of Neurological Severity Score (NSS)
NOTE: Assessment of neurological deficit was performed and graded using an NSS, as previously described23,24. The maximum score of alteration in motor function and behavior is 24 points. A score of 0 indicates an intact neurological status and 24 indicates severe neurological dysfunction, as previously described24.
3. Preparing for the metric task
4. Performing the metric task
NOTE: Performing the metric task consists of two periods: 1) habituation (15 min) and 2) test (5 min) period.
5. Data analysis
NOTE: Data analysis is performed by video tracking software specifically designed for animal behavior studies that automatically records animal activity and movement (see Table of Materials). This software automates a range of behavioral variables, including mobility, activity, and explorative behavior.
The significance of comparisons between groups was determined using the Mann-Whitney test. Statistical significance of results was considered at P < 0.05, while statistically high relevance was measured at P < 0.01.
The results showed no differences in NSS between all groups before intervention and 28 days after TBI. Each group consisted of 12 female or 12 male rats. The NSS scores obtained 48 h after TBI are presented in Table 1. Rats from the TBI group that showed significant neurological deficit on day 28 after injury were excluded from the experiment. The data is measured as counts and presented as median ± range.
The sham-operated control group did not show any neurological deficit at 48 h after the first day of the study (NSS-0). Neurological deficit at 48 h after TBI was significantly greater for the male TBI rats than for the male sham-operated rats (5.5(4-7) vs. 0(0-0), U = 0, p < 0.01, r = -0.89), and for the female TBI rats than for the female sham-operated rats (4.5(3.25-6) vs. 0(0-0), U = 0, p < 0.01, r = -0.91), according to the Mann-Whitney test (Table 1).
A Mann-Whitney test indicated that object exploration time during the metric task was significantly shorter for the male TBI rats vs. male sham-operated rats (130% ± 44.3% vs. 1978% ± 59.2%), U = 0, p < 0.01, r = -0.85 (see Figure 3a,b). The data is measured as seconds expressed in % of baseline point and presented as mean ± SEM. Baseline is measured as the time of exploration during the first 5 min of the habituation period. The remaining three time points (5-10 min, 10-15 min, and 20-25 min) were calculated as a percentage of the baseline.
A Mann-Whitney test indicated that object exploration time during the metric task was significantly shorter for the female TBI rats vs. female sham-operated rats (89% ± 43.5% vs. 2160% ± 43.6%), U = 0, p < 0.01, r = -0.85 (see Figure 4a,b). The data is measured as seconds expressed in % of baseline point and presented as mean ± SEM. Baseline is measured as the time of exploration during the habituation period.
There was no significant difference found between male and female groups.
Figure 1: Protocol schematic with timelines. This figure shows protocol timeline. Groups of rats at different times included a sham-operated control group and TBI group and were assessed by NSS score at -1 h, 48 h, and 28 days after injury. Please click here to view a larger version of this figure.
Figure 2: Representative data analysis. Screen captures of the video tracking software for (A) Trial control settings (B) Trial list and (C) Acquisition, and example data exported into Excel (D). See text and video for details. Please click here to view a larger version of this figure.
Figure 3: Metric task for male rats. The object exploration time during the metric task was significantly shorter for the male TBI rats vs. the male sham-operated rats (see Figure 3a,b, which illustrates the data on different y-axis scales). Please click here to view a larger version of this figure.
Figure 4: Metric task for female rats. The object exploration time during the metric task was significantly shorter for female TBI rats vs. female sham-operated rats (see Figure 4a,b, which illustrates the data on different y-axis scales). Please click here to view a larger version of this figure.
NSS values of the study groups at 48 h after TBI | Median (range) | |||||
Animal Group | N | Baseline | 48h | 1w | 2w | 4w |
Sham-operated female/male rats | 12 | 0(0-0) | 0(0-0) | 0(0-0) | 0(0-0) | 0(0-0) |
TBI male rats | 12 | 0(0-0) | 5.5(4-7)* | 2(1-6)* | 1.5(0-2)* | 0(0-2) |
TBI female rats | 12 | 0(0-0) | 4.5(3.25-6)* | 1.5(0.25-2.8)* | 1(0-2)* | 0(0-0.8) |
Table 1: Determination of neurological performance. Neurological deficit at 48 h after TBI was significantly greater for the male TBI rats than for the male sham-operated rats and female TBI rats than for the female sham-operated rats.
By specifically targeting the metric spatial information process, this metric test provides a necessary tool toward understanding memory deficiency after TBI. The protocol presented in this paper is a modification of previously described behavioral tasks11. One previously described metric task used two different paradigms, each consisting of three habituation sessions and one testing session. The first paradigm consisted of moving the familiar objects closer together after habituation and the second paradigm moved the objects farther away11.
Compared to the Barnes maze, which is performed over five13 or fourteen14 days, the metric task presented here is performed within 2 days, the first day for habituation and the second day for the task9. The task in this protocol is less stressful than comparable behavioral tasks such as the Morris water maze, due to the stress induced by swimming in the maze and the longer duration of the task15,16,17. Maze tests for spatial memory require a significant learning period; even a simple T maze requires at least 5 days of training18. For more complex radial mazes, 15-20 days of daily testing is recommended19.
This protocol contains several critical steps. One crucial component is the need to treat the arena with an alcohol solution as well as the objects on it. It is also necessary that the surface of the arena is dry and clean, since the smell of alcohol and scents left over from previous animals can change the behavior of the animal under study. In addition, constantly adequate ventilation of the behavior room is vital. Since noise is one of the stress factors that can change the behavior of animals, we recommend proper soundproofing. Additionally, the platform height of 80 cm and the relative distance of the platform from other objects is necessary in order for the rat not to jump or climb onto another object. Further, maintaining consistent settings in processing recorded video files during set-up will help avoid incorrect interpretation of the data.
The neurological deficit that develops as a result of TBI must be considered in the assessment of memory. Neurological deficits after head trauma are a contributing factor that is part of this disease. Assessment of neurological deficits is very important in the rodent model of brain injury and is a highly-sensitive and frequently-used outcome26. However, severe neurological deficits can have an effect on behavioral tests, especially on tests that measure memory assessment27. The comparable Morris water maze task also assesses memory impairment28. A low score on the Morris test in TBI or stroked rats is highly correlated with neurological deficits and, in fact, reflects not memory or cognitive impairment, but rather neurological performance and the ability to withstand stress.
To minimize the effect of TBI-related neurological deficits on memory scores, we used the following approaches: 1) we used models of TBI of mild to moderate severity, which spontaneously recover neurological performance after 1 month. 2) Rats that showed neurological deficit 28 days after TBI were excluded from behavioral experiments, based on our observations that all rats with mild injury recover. In groups of 10-20 rats affected with severe TBI, one rat on average has a significant neurological deficit which may affect mobility. 3) To assess memory after trauma, we did not use tests related to movement, the results of which may be influenced by neurological deficiency (as in the Morris water maze). While the Barnes test and related tests are useful to assess memory in models of TBI and stroke, the metric test is better suited to assess SWM. Thus, the metric test is the test of choice for assessing the SWM of rats after TBI.
A limitation of this protocol is the use of a metric test alone rather than a topological test. We envision future studies that also incorporate topological tests to measure other aspects of SWM. Surprisingly, according to our results, no statistically significant difference was found between male and female rats. A large number of studies show sex differences after TBI29, many based on the difference in concentrations of reproductive hormones. Estrogen and progesterone play a neuroprotection role after TBI, which are shown to decrease intracranial pressure and improve neurological function score respectively30. According to a meta-analysis study, men more frequently suffer from TBI, but women have worse prognoses31. Cognitive impairments, the most common complication after TBI, trend toward gender differences, with women showing greater improvement on spatial positioning tasks and men performing better on verbal tasks32,33,34. Our results, however, indicate the possibility for uncertainty about gender-related spatial memory differences.
Among the various types of TBI models, the model of fluid percussion induced TBI is well documented and described, is easily reproducible, and has lower variability than other models35,36. However, it is important to note that the metric test has broad utility and may be used effectively with other TBI models. The metric test described in this protocol also allows for further research into memory impairment in comparable models of neurological damage, such as models of diffuse axonal brain injury24,37 and stroke38. This protocol may also be useful for studying the efficacy of various treatment modalities in restoring SWM after TBI.
The authors have nothing to disclose.
We thank Professor Olena Severynovska; Maryna Kuscheriava M.Sc; Maksym Kryvonosov M.Sc; Daryna Yakumenko M.Sc; Evgenia Goncharyk M.Sc; and Olha Shapoval, PhD candidate at the Department of Physiology, Faculty of Biology, Ecology, and Medicine, Oles Honchar Dnipro University, Dnipro, Ukraine for their supportive and useful contributions. The data was obtained as part of Dmitry Frank's PhD dissertation.
2% chlorhexidine in 70% alcohol solution | SIGMA – ALDRICH | 500 cc | For general antisepsis of the skin in the operatory field |
Bupivacaine 0.1 % | |||
4 boards of different thicknesses (1.5cm, 2.5cm, 5cm and 8.5cm) | This is to evaluate neurological defect | ||
4-0 Nylon suture | 4-00 | ||
Bottles | Techniplast | ACBT0262SU | 150 ml bottles filled with 100 ml of water and 100 ml 1%(w/v) sucrose solution |
Bottlses (four) for topological an metric tasks | For objects used two little bottles, first round (height 13.5 cm) and second faceted (height 20 cm) shape and two big faceted bottles, first 9×6 cm (height 21 cm) and second 7×7 cm (height 21 cm). | ||
Diamond Hole Saw Drill 3mm diameter | Glass Hole Saw Kit | Optional. | |
Digital Weighing Scale | SIGMA – ALDRICH | Rs 4,000 | |
Dissecting scissors | SIGMA – ALDRICH | Z265969 | |
Ethanol 99.9 % | Pharmacy | 5%-10% solution used to clean equipment and remove odors | |
EthoVision XT (Video software) | Noldus, Wageningen, Netherlands | Optional | |
Fluid-percussion device | custom-made at the university workshop | No specific brand is recommended. | |
Gauze Sponges | Fisher | 22-362-178 | |
Gloves (thin laboratory gloves) | Optional. | ||
Heater with thermometer | Heatingpad-1 | Model: HEATINGPAD-1/2 | No specific brand is recommended. |
Horizon-XL | Mennen Medical Ltd | ||
Isofluran, USP 100% | Piramamal Critical Care, Inc | NDC 66794-017 | Anesthetic liquid for inhalation |
Office 365 ProPlus | Microsoft | – | Microsoft Office Excel |
Olympus BX 40 microscope | Olympus | ||
Operating forceps | SIGMA – ALDRICH | ||
Operating Scissors | SIGMA – ALDRICH | ||
PC Computer for USV recording and data analyses | Intel | Intel® core i5-6500 CPU @ 3.2GHz, 16 GB RAM, 64-bit operating system | |
Plexiglass boxes linked by a narrow passage | Two transparent 30 cm × 20 cm × 20 cm plexiglass boxes linked by a narrow 15 cm × 15 cm × 60 cm passage | ||
Purina Chow | Purina | 5001 | Rodent laboratory chow given to rats, mice and hamster is a life-cycle nutrition that has been used in biomedical researc for over 5 |
Rat cages (rat home cage or another enclosure) | Techniplast | 2000P | No specific brand is recommended |
Scalpel blades 11 | SIGMA – ALDRICH | S2771 | |
SPSS | SPSS Inc., Chicago, IL, USA | 20 package | |
Stereotaxic Instrument | custom-made at the university workshop | No specific brand is recommended | |
Timing device | Interval Timer:Timing for recording USV's | Optional. Any timer will do, although it is convenient to use an interval timer if you are tickling multiple rats | |
Topological and metric tasks device | Self made in Ben Gurion University of Negev | White circular platform 200 cm in diameter and 1 cm thick on table | |
Video camera | Logitech | C920 HD PRO WEBCAM | Digital video camera for high definition recording of rat behavior under plus maze test |
Windows 10 | Microsoft |