Loss of Acid sensing ion channel-1a and bicarbonate administration attenuate the severity of traumatic brain injury

Abstract

Traumatic brain injury (TBI) is a common cause of morbidity and mortality in people of all ages. Following the acute mechanical insult, TBI evolves over the ensuing minutes and days. Understanding the secondary factors that contribute to TBI might suggest therapeutic strategies to reduce the long-term consequences of brain trauma. To assess secondary factors that contribute to TBI, we studied a lateral fluid percussion injury (FPI) model in mice. Following FPI, the brain cortex became acidic, consistent with data from humans following brain trauma. Administering HCO3 (-) after FPI prevented the acidosis and reduced the extent of neurodegeneration. Because acidosis can activate acid sensing ion channels (ASICs), we also studied ASIC1a(-/-) mice and found reduced neurodegeneration after FPI. Both HCO3 (-) administration and loss of ASIC1a also reduced functional deficits caused by FPI. These results suggest that FPI induces cerebral acidosis that activates ASIC channels and contributes to secondary injury in TBI. They also suggest a therapeutic strategy to attenuate the adverse consequences of TBI.

Figure 1. HCO₃ ⁻ administration prevents the FPI-induced decrease in brain pH.

Data are cortical brain pH one hr after FPI. NaHCO₃ was administered as indicated immediately after FPI. Animals were anesthetized and mechanically ventilated. N = 9 control, 6 FPI, and 7 FPI plus NaHCO₃ mice. Each data point indicates an individual animal. * indicates P<0.05; ANOVA with Bonferroni post-hoc test.

Figure 2. Loss of ASIC1a and NaHCO₃ administration reduce FPI-induced neuronal degeneration.

A. Brain was removed one day after FPI. Data are fluoro-jade positive neurons per microscopic field in the cortex (0.9 μm²). N = 5 wild-type and 5 ASIC1a^−/− mice. Scale bar = 0.1 mm. Each data point indicates an individual animal. * indicates P<0.05, unpaired t-test. B. Brain was removed four days after FPI. N = 9 sham-treated mice, 11 FPI-treated wild-type mice 3 of which received NaCl after FPI, 9 FPI-treated mice with NaHCO₃ administered immediately after FPI, 7 FPI-treated mice with NaHCO₃ administered 1 hr after FPI, and 5 FPI-treated ASIC1a^−/− mice. * indicates P<0.05, ANOVA with Bonferroni post-hoc test.

Figure 3. Disruption of the ASIC1a gene and NaHCO₃ administration reduce the FPI-induced decrement in spatial memory.

A. Data from 4 days of training on the Barnes maze. Data are latency to find the escape hole. N = 11 wild-type, sham-treated mice, 17 FPI-treated wild-type mice, 14 ASIC1a^−/− sham-treated mice, 19 ASIC1a^−/− FPI-treated mice, and 10 FPI-treated wild-type mice that received NaHCO₃ immediately after FPI. None of the groups showed a statistically significant difference. B. Data are time during 90 sec. that mice spent in the quadrant that had contained the escape hole. * indicates P<0.05 compared to sham treated wild-type mice; ANOVA with Bonferroni post-hoc test.

Figure 4. NaHCO₃ administration improves performance in fear conditioning assay done after FPI.

A. Percentage of time freezing during 8 min on the training day. All the mice were wild-type. N = 25 sham-treated, 10 FPI-treated controls, 14 FPI-treated with NaHCO₃ administered immediately after FPI, and 6 FPI-treated with NaCl administered immediately after FPI. There were no statistically significant differences between groups. B. Percentage of time freezing during 6 min on the testing day. * indicates P<0.05; ANOVA with Bonferroni post-hoc test. ** indicates P<0.05 compared to FPI control and FPI treated with NaCl; ANOVA with Bonferroni post-hoc test.