Delayed-and-abbreviated environmental enrichment after traumatic brain injury confers neurobehavioral benefits similar to immediate-and-continuous exposure

Abstract

Environmental enrichment (EE) consists of increased living space, complex stimuli, and social interaction that collectively confer neurobehavioral benefits in preclinical models of traumatic brain injury (TBI). The typical EE approach entails implementation immediately after surgery and continual exposure, which is not clinically applicable, as TBI patients often only receive rehabilitation after critical care, and then only for a few hours per day. We are focused on developing a clinically relevant model of neurorehabilitation by refining the timing of initiation and duration of EE exposure after TBI. The goal of this experiment is to compare the typical EE approach to paradigms where EE is delayed by 3 or 7 days after TBI and then provided for only 6 h per day, which better mimics the clinic. The hypothesis is that the delayed-and-abbreviated EE paradigms will promote neurobehavioral benefits like the typical approach of immediate-and-continuous exposure. To test the hypothesis, anesthetized adult male rats underwent a controlled cortical impact of moderate severity (2.8 mm deformation at 4 m/s) or sham injury and then were randomly assigned to post-operative EE or standard (STD) housing. Motor ability, spatial learning, and memory retention were assessed. The hypothesis was confirmed as all EE-treated groups performed better than the STD group in all behavioral assessments (p < 0.05) and did not differ from one another (p > 0.05). The ability of EE to provide significant behavioral benefits even when delayed and delivered in moderation affords further support for EE as a preclinical model of neurorehabilitation and offers greater insight into the length of the therapeutic window.

Keywords: Behavior; Controlled cortical impact; Environmental enrichment; Learning and memory; Morris water maze; Recovery; Traumatic brain injury.

Fig. 1.

Enriched environment (EE) cage, which consists of three interconnected levels and a wide array of sensory stimuli (e.g., balls, ramps, tubes, and nesting materials). Ten rats, which included both TBI and sham controls, were housed for 6-h per day and were introduced to the environment according to the experimental design (i.e., 0-day, 3-days, or 7-days after TBI or sham injury). The combination of large space, sensory stimuli, and group housing (rats not depicted) constitutes multimodal rehabilitation.

Fig. 2.

Illustration of the experimental design. TBI or sham injury was performed at timepoint 0. Open circles indicate baseline motor assessment, closed circles indicate days of motor testing, and triangles indicate days of water maze training. Black corresponds to TBI + 24-h STD (0-day delay), green = TBI + 24-h EE (0-day delay), red = TBI + 6-h EE (3-day delay), and blue = TBI + 6-h EE (7-day delay).

Fig. 3.

Mean (± S.E.M.) time (s) to maintain balance on the beam was evaluated prior to, and after, TBI or SHAM injury on post-operative days 1–5 (0-day delay groups – TBI + 24 h STD, TBI + 24 h EE, and SHAM), 4–8 (3-day delay group), and 8–12 (7 day-delay group). The data were analyzed by repeated measures ANOVA followed by the Newman-Keuls multiple comparisons post-hoc test. *p < 0.05 vs. TBI + 24-h STD (0-day delay). ^#p < 0.05 vs. TBI + 24-h STD (0-day delay) and TBI + 24-h EE (0-day delay) groups. No difference was revealed among the TBI + 24-h EE (0-day delay), TBI + 6-h EE (3-day delay) and TBI + 6-h EE (7-day delay) groups or between the SHAM, TBI + 6-h EE (3-day delay) and TBI + 6-h EE (7-day delay) groups (p > 0.05).

Fig. 4.

Mean (± S.E.M.) time (s) to traverse an elevated narrow beam prior to, and after, TBI or SHAM injury on post-operative days 1–5 (0-day delay groups – TBI + 24 h STD, TBI + 24 h EE, and SHAM), 4–8 (3-day delay group), and 8–12 (7 day-delay group). The data were analyzed by repeated measures ANOVA followed by the Newman-Keuls multiple comparisons post-hoc test. *p < 0.05 vs. TBI + 24-h STD (0-day delay).^p < 0.05 vs. TBI + 24-h EE (0-day delay) and TBI + 6-h EE (3-day delay) groups. ^#p < 0.05 vs. all TBI groups. ^$No difference was observed overall between the TBI + 24-h STD (0-day delay) and TBI + 6-h EE (3-day delay) group, but single day analyses revealed a marked improvement in performance for the TBI + 6-h EE (3-day delay) group on the last two days of testing (p < 0.05).

Fig. 5.

Mean (± S.E.M.) beam-walk score derived from the number of foot slips while traversing a narrow beam prior to, and after, TBI or SHAM injury on post-operative days 1–5 (0-day delay groups – TBI + 24 h STD, TBI + 24 h EE, and SHAM), 4–8 (3-day delay group), and 8–12 (7 day-delay group). The data were analyzed by repeated measures ANOVA followed by the Newman-Keuls multiple comparisons post-hoc test. *p < 0.05 vs. TBI + 24-h STD (0-day delay).^p < 0.05 vs. TBI + 24-h EE (0-day delay) and TBI + 6-h EE (3-day delay). ^#p < 0.05 overall vs. all TBI groups, but during the last two days of testing, ^$no difference was observed between SHAM and TBI + 24-h EE (0-day delay), TBI + 6-h EE (3-day delay), and TBI + 6-h EE (7-day delay) groups. Lastly, no difference was revealed between the TBI + 24-h EE (0-day delay) and TBI + 6-h EE (3-day delay) groups (p > 0.05).

Fig. 6.

Mean (± S.E.M.) time (s) to locate a hidden and visible platform in the water maze after TBI or SHAM injury on post-operative days 14–19 (0-day delay groups – TBI + 24 h STD, TBI + 24 h EE, and SHAM), 17–22 (3-day delay group), and 21–26 (7 day-delay group). The data derived from the hidden platform training trials were analyzed by repeated measures ANOVA followed by the Newman-Keuls multiple comparisons post-hoc test. *p < 0.05 vs. TBI + 24-h STD (0-day delay). ^#p < 0.05 vs. all TBI groups. No differences were revealed among the TBI + 24-h EE (0-day delay), TBI + 6-h EE (3-day delay), and TBI + 6-h EE (7-day delay) groups (p > 0.05). A one-way ANOVA of the visible platform data revealed that all EE-treated groups located the platform quicker than the STD-housed control group (p < 0.05) and the SHAM group located it faster than the STD and TBI + 6-h EE (7-day delay) groups (p < 0.05).

Fig. 7.

Mean (± S.E.M.) percent time spent in the target quadrant after TBI or SHAM injury on post-operative days 19 (0-day delay groups – TBI + 24 h STD, TBI + 24 h EE, and SHAM), 22 (3-day delay group), and 26 (7 day-delay group). The data were analyzed by a one-way ANOVA followed by the Newman-Keuls multiple comparisons post-hoc test. The bar graph shows the % time that each group spent in the target quadrant (i.e., where the platform was located during acquisition training) and the horizontal dashed line depicts chance (25 %) level of exploration. *p < 0.05 vs. TBI + 24-h STD (0-day delay). No difference was revealed among the TBI + 24-h EE (0-day delay), TBI + 6-h EE (3-day delay), TBI + 6-h EE (7-day delay), and SHAM groups (p > 0.05).