Elsevier

Behavioural Brain Research

Volume 365, 3 June 2019, Pages 222-230
Behavioural Brain Research

Research report
Single mild traumatic brain injury results in transiently impaired spatial long-term memory and altered search strategies

https://doi.org/10.1016/j.bbr.2018.02.040Get rights and content

Highlights

  • A mouse model for weak, single mild traumatic brain injury (minimal TBI) is used.

  • Minimal TBI results in transient deficits in long-term memory reconsolidation.

  • Minimal TBI causes relapse to less effective search strategies in the water maze.

  • Minimal TBI leads to a transient increase in freezing during fear conditioning.

  • A transient rise in hippocampal GFAP after minimal TBI indicates astrogliosis.

Abstract

Mild traumatic brain injury (mTBI) can lead to diffuse neurophysical damage as well as cognitive and affective alterations. The nature and extent of behavioral changes after mTBI are still poorly understood and how strong an impact force has to be to cause long-term behavioral changes is not yet known. Here, we examined spatial learning acquisition, retention and reversal in a Morris water maze, and assessed search strategies during task performance after a single, mild, closed-skull traumatic impact referred to as "minimal" TBI. Additionally, we investigated changes in conditioned learning in a contextual fear-conditioning paradigm. Results show transient deficits in spatial memory retention, which, although limited, are indicative of deficits in long-term memory reconsolidation. Interestingly, minimal TBI causes animals to relapse to less effective search strategies, affecting performance after a retention pause. Apart from cognitive deficits, results yielded a sub-acute, transient increase in freezing response after fear conditioning, with no increase in baseline behavior, an indication of a stronger affective reaction to aversive stimuli after minimal TBI or greater susceptibility to stress. Furthermore, western blot analysis showed a short-term increase in hippocampal GFAP expression, most likely indicating astrogliosis, which is typically related to injuries of the central nervous system. Our findings provide evidence that even a very mild impact to the skull can have detectable consequences on the molecular, cognitive and affective-like level. However, these effects seemed to be very transient and reversible.

Introduction

Traumatic brain injury (TBI) typically occurs when the head hits or is struck by an external object, leading to structural or functional brain alterations from brain acceleration and deceleration or the penetration of an object into the brain [1]. Although 80–90% of all mTBIs resolve spontaneously within a couple of weeks, mTBI may initiate a complex condition called post-concussion syndrome [2]. Patients suffering from the aftermath of concussive brain injury may develop persistent symptoms, including cognitive and psychological complaints [3]. There is, however, some controversy about the prognosis of long-term symptoms and there is a lack of clear evidence of cognitive symptoms attributable to mTBI [4], which complicates the development of new lead compounds and therapeutic targets. Furthermore, the neuropathology of mTBI is characterized by its heterogeneous nature [5,6] with a lack of consensus on how much, or how little, it takes to initiate long-term behavioural consequences [7]. As a result, mTBI represents not a single neuropathological event but a complex disease process. Among all brain structures, the hippocampus has been reported to be particularly susceptible to TBI in general and mTBI specifically [[8], [9], [10]]. Animal models can help to elucidate the functional and structural alterations caused by mTBI and ultimately support the identification of potential therapeutic targets. However, while a number of experimental animal studies reported short- and long-term behavioural alterations reflecting cognitive and affective deficits after mTBI [8,[11], [12], [13], [14]], results across studies are diverse and inconsistent. As such, the modelling and replication of key features typically seen in humans after concussion remains a major but important challenge (for review see Dewitt et al. [15]).

Given the current lack of knowledge regarding the relation between the severity of a single mild head impact and the resulting transient or longer-term behavioural and biochemical consequences, we examined the effects of very mild single closed-skull mTBI (herein referred to as minimal TBI) on performance in established paradigms of hippocampus-dependent spatial and associative learning (Morris water maze and contextual fear conditioning, respectively) and on the expression of several key proteins of synaptic function.

Section snippets

Animals

63 male and female C57/Bl6J mice, aged 8–12 weeks at the time of surgery, were randomized into a sham control group (n = 31) and minimal TBI group (n = 32). All animals were group-housed in standard animal cages under conventional laboratory conditions (12 h/12 h light-dark cycle, ∼22 °C), with ad libitum access to food and water. Animal husbandry and experiments were conducted in accordance with the KU Leuven Ethical Committee (P097/2014) and the European Directive (2010/63/EU).

Surgery

Experimental

Results

Fig. 1 presents an overview and timeline of all experimental procedures. Closed-skull injury resulted in a brief and transient period of apnoea ranging from about 10 to 30 s in the minimal TBI group. Microscopic examination revealed no focal lesions in any of the animals subjected to injury (Fig. 2). There were no skull fractures, cerebral haemorrhages or contusions identified after closed-skull injury.

Statistical analyses of the water maze acquisition data for the two groups yielded no

Discussion

The nature and extent of behavioural changes seen after mTBI are still poorly understood. In the present study, we induced a very mild single, closed-skull mTBI (minimal TBI) using the weight-drop device described by Flierl et al. [16]. This non-repetitive protocol induces a very mild head impact. In agreement with the latter and the definition of mTBI, histological examination did not reveal any gross anatomical abnormalities. Immunoblotting experiments revealed a transiently increased

Conclusions

Our results demonstrate that exposure to a very mild single closed-skull mTBI (minimal TBI) causes behavioural abnormalities that manifest as temporary deficits in long-term memory during the early phase of spatial learning in the water maze, accompanied by a higher proportion of non-spatial search strategies and compensated by further training. These deficits were accompanied by an increase of GFAP in the hippocampus. In addition, minimal TBI results in an increased anxiety-like response to an

Acknowledgements

This work was funded by the Research Foundation Flanders (FWO) (G.0D76.14), the University of Copenhagen (scholarship to LM), the Agency for Innovation by Science and Technology in Flanders (IWT) (scholarship 141698 to AS) and an interdisciplinary research grant from KU Leuven (GOA) (12/008). The authors wish to thank R. D’Hooge for supporting the behavioural testing, F. Van Leuven for critical discussion and the donation of antibodies, the research group of L. Arckens (N. Lombaert, M. Hennes)

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    These authors contributed equally to this work.

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