Aged Mice Demonstrate Preserved Learning And Memory After Traumatic Brain Injury Compared

To Young MiceMecca B.A.R. Islam, MS1, Booker T Davis IV, PhD, MS 1 , Craig Weiss, PhD 2 and Steven J. Schwulst, MD 1

1Department of Surgery, Division Of Trauma and Critical Care 2 Behavioral Phenotyping Core, Department

CONCLUSION

RESULTSMETHODSOver 3 million Americans sustain a traumatic brain injury (TBI) annually. Age-relateddifferences in brain physiology and comorbidities may lead to divergent outcomes in agedversus young patients. However, few studies have addressed age as an independent variable.We hypothesized that aged mice would have greater deficits in learning, memory, and anxietybehavior after TBITBI or sham injury was induced in 14-week-old (n=21) and 80-week-old(n=24) male C57BL/6 mice via controlled cortical impact. 30-days post-TBI mice underwentopen field testing and cued fear conditioning to assess learning, memory and anxiety. Datawas analyzed using one-way ANOVA and Sidak’s multiple comparison test. TBI or shaminjury was induced in 14-week-old (n=21) and 80-week-old (n=24) male C57BL/6 mice viacontrolled cortical impact. 30-days post-TBI mice underwent zero-maze testing and cued fearconditioning to assess learning, memory and anxiety. Young TBI spent more time in the openregions of the zero-maze demonstrating marked disinhibition of normal anxiety-like behavior(Figure 4). Aged TBI mice demonstrate preserved associative learning and memory ascompared to young TBI mice after cued fear conditioning. Additionally there was a differencebetween Aged Sham and Young Sham (Figure 5).

ABSTRACT

RESEARCH OBJECTIVES

RESULTS

ACKNOWLEDGMENTS

INTRODUCTIONTrauma is the leading cause of death and disability in patients between the ages of 1-44 withTBIs contributing to a nearly a third of them. Presently, approximately 2% of the U.S.population are afflicted with disabilities and behavior deficits as a result of a TBI. Despitepromising preclinical data, clinical trials have failed to produce effective therapies for thishighly morbid injury process. If fact, most clinical trials of TBI have included patientsranging in age from adolescence (~15yrs) to more advanced age (>65yrs). This despiteepidemiological data which suggest that aged TBI patients have worse outcomes ascompared to younger TBI patients.

Our research objective was to assess age as an independent variable in neurocognitive testsof learning, memory, and anxiety after TBI. To further these Aims. We hypothesized thataged mice would have greater deficits in learning, memory, and anxiety behavior after TBI.

(Islam et al, JoVE 2019)

(A) Brain from a 12-week-old naïve mouse. (B) Brain from a 12-week-old mouse 24h after injury (C) Brain from a 12-week-old mouse 7 days after injury

Figure 1. Severe TBI viaMurine Model

Contrary to our hypothesis, aged mice had attenuated deficits in learning, memory,anxiety measures as compared to young mice after TBI:• These data suggest relative preserved connectivity between the hippocampus and

prefrontal cortex in aged mice after TBI.• Additionally, young TBI mice showed marked disinhibition of normal anxiety-like

behavior suggesting a greater loss of connectivity between the amygdala andhippocampus compared to aged TBI mice.

• These data demonstrate different pathophysiology in the aged vs. young brain afterTBI suggesting that different treatment and rehabilitation strategies are needed foraged and young TBI patients.

Figure 2. Murine Model Exhibit Severe TBI

(Islam et al, JoVE 2019)

(A) The grounding cable is clipped to the mouse’shind region and the impacting tip is lowered ontothe dura mater. This is the zero point. (B) Theimpacting tip is retracted, a 2 mm depth of injury isdialed into the stereotaxic frame, and the impact isapplied. (C) After the CCI is applied, the impactingtip is rotated out of the field and the mouse isrecovered from the stereotaxic frame

Figure 3. Histology and MRI Confirm Severe TBI

(Islam et al, JoVE 2019)

(A) Shows sham injury with craniectomy(B)CCI results in a severe TBI with massiveloss of cortex at the site of injury as well asloss and distortion of the underlyinghippocampal formation and thalamus . (C)MRI at 1-day post-TBI shows tissue traumaand edema over the left parietotemporalcortex. (D–E) Representative images frompost-injury days 7 and 14 showing increasedareas of hyperattenuation representingprogressive replacement of devitalized tissuewith cerebrospinal fluid.

(A) Aged mice show a relative attenuated loss of associated learning and memory as compared to young mice after TBI (47.68 ± 4.45% vs. 27.88 ±3.570% freezing, p=0.0114). Young TBI were the most disinhibited group showing a lack of preservation of these pathways (F3,25 =12.72, p <0.001).Young TBI were significantly different than young sham * (27.88 ± 3.57% vs. 44.16 ± 5.42% freezing, p = 0.0412) and aged sham (27.88 ± 3.57% vs.62.78 ± 3.62% freezing ,p<0.001). However, it was markedly less than that of aged sham ** (47.68 ± 4.45% vs. 62.78 ± 3.62% freezing, p = 0.0318) .This demonstrates preserved connectivity in the aged brain as compared to the young brain after TBI (B) Young animals experience a greater % change inlearning and memory loss in compare to aged animals

Figure 5. Aged TBI Shows Relative Preservation Of Connectivity between Amygdala, Hippocampus, and Prefrontal Cortex

(A) Zero Maze tracing of mice based on regions. The white boxes indicate open regions (B) Young TBI spent more time in the open regions indicating amarked disinhibition of anxiety-like behavior as compared to young sham (16.08 ± 2.3% vs. 5.70 ± 1.43% time, p =0.0007), aged sham (16.08 ± 2.3%vs. 9.2 ± 0.87% time, p <0.0400), and aged TBI (16.08 ± 2.3% vs. 5.85 ± 1.34% time, p =0.0008). Multiple comparison test shown no significantinteractions between the other groups. Furthermore there was an interaction between both age and injury (F1,30 =16.26 , p =0.0003)

Figure 4. Young TBI Show Disinhibition Of Anxiety-Like Behavior

This research was funded by NIH Grant 1R01GM130662