P.1.b.011 Flexibility and spatial learning deficits after experimental traumatic brain injury in mice: a consequence of white matter lesion?

Abstract

Purpose of the study: Traumatic brain injury (TBI) results in white matter injury (WMI), evidenced by demyelination. WMI may originate from neuroinflammation and is associated with late emotional and cognitive disorders in patients. Experimental research focuses on brain lesions and neurological deficits occurring days after injury, occulting long-term consequences of TBI. The present study aims to evaluate the time-course of neuroinflammation, WMI and behavioral disorders related to anxiety, compulsiveness, depression, behavioral flexibility and spatial memory after TBI. Methods used: Three groups of male Swiss mice were studied: non-operated, sham-operated and traumatic brain injured mice. TBI was induced by a controlled cortical impact. Neuroinflammation was determined by microglia activation using CD11b immunostaining. The expression of myelin basic protein (MBP), a marker of myelination, was measured in the corpus callosum using Western blot (for all biomarkers evaluations n = 5−6/group). At 15 days, 1, 2 and 3 months, sensorimotor performances were determined with pole test and spontaneous locomotion with actimeter. Spatial learning and memory, and behavioral flexibility were assessed using Barnes maze: two weeks after TBI, mice were trained to find a target hole 3 trials/day during 4 days, followed by a probe test (i.e. in absence of target box) on the last day. At 1, 2 and 3 months, mice were trained to find a new target hole location as previously described. Compulsiveand anxiety-like behaviors using the marble-burying, the open field and the elevated plus maze tests and immobility/escape behavior in the forced swim test, more relevant to depression, were assessed 3 months postTBI (for all behavioral measurements: n = 9−12/group). Data were analyzed with one-way ANOVA. Results: Number of CD11b+ cells was increased from 24 h (P< 0.001) up to 7 days post-TBI (P< 0.001), showing neuroinflammation. MBP expression was decreased at 7 days and 3 months (P< 0.01) in ipsiand contralateral corpus callosum demonstrating acute and long-term bilateral WMI. Spontaneous locomotor activity, motor coordination performances, anxiety-like and despair-like behavioral measurements were undisturbed by TBI. Although, spatial learning performances were not disturbed at 2 weeks post-TBI, deficit was apparent when moving the target hole location 1, 2 and 3 months after TBI. While all mice discriminated the target zone correctly during the probe tests, latencies to find the new target hole, distances travelled and number of errors, were all significantly higher in injured mice during the first training day (P< 0.05), suggesting that TBI induces an early deficit in learning processes, behavioral adaptability and flexibility disorder. Conclusions: These data clearly demonstrate that TBI induces acute microglial activation that precedes acute and long term demyelination, suggesting the involvement of neuroinflammation in WMI. Moreover, the latter is associated with a reduction of flexibility and a spatial learning deficit, in the absence of sensorimotor and emotional states impairments, suggesting that changes in the escape behavior to the target hole, were related to behavioral flexibility rather than defense behavior. Behavioral evaluation is still ongoing until 12 months after TBI. This model will provide an overview of various late behavioral disorders and demyelination that could help to validate pharmacological strategies.