Purpose: To investigate the anxiolytic and antidepressant effects of α- and β-amyrins in a mouse model of mild traumatic brain injury (mTBI), and the underlying mechanism(s) of action.   

Methods: Male Swiss mice (n = 165), weighing 25 - 40 g (mean weight = 32.5 ± 7.5 g), were used in this study. Existing mTBI model was modified and optimized for mild injury to brain capable of producing neurobehavioral changes. Forced swim test (FST) and tail suspension test (TST) were used to measure the antidepressant effects of α- and β-amyrins, while elevated plus maze (EPM) and light-dark model (LDM) were used for anxiety assessment. The probable mechanism of action of amyrin was also investigated through kinetics of serotonin uptake and activities of monoamine oxidases A (MAO A) and B (MAO B) in mouse hippocampus and cortex.   

Results: Induction of mTBI produced anxiety- and depression-like behaviors in mice. The 5th hitting righting time was 259 ± 25.11 s while duration of apnea was 27.33 ± 3.84 s. Apnea was significantly reduced on 5th and 7th hits, when compared with 4th and 6th hits (p < 0.05). The immobility time of mice was significantly reduced in FST and TST. A combination of the two forms of amyrin was more effective in reducing duration of immobility, relative to when each was used alone (p < 0.05). Amyrin significantly and dose-dependently increased entries, and time spent in open arms and light zone (p < 0.05). Mice in mTBI group exhibited a high degree of hopelessness, when compared with control group (p < 0.05). However, amyrin at a dose of 50 mg/kg significantly reduced the degree of hopelessness in the mice (p < 0.05). The specific activity of MAO A in hippocampal tissue (265.00 ± 12.07 µmol/min/g protein) was significantly higher than that of cortex (61.85 ± 5.14 µmol/min/g protein).  In both tissues, there were no significant differences in the activity of MAO B among the groups (p > 0.05). Amyrin significantly reversed the effects of mTBI on the levels of amino acids in mice hippocampus and cortex (p < 0.05). The results of synaptosomal uptake of serotonin show that fluoxetin exhibited competitive inhibition of serotonin uptake, while amyrin exhibited mixed type inhibition.   

Conclusion: The results obtained show that α- and β-amyrins exert potent anxiolytic and antidepressant effects via mechanisms involving MAO and GABA in the hippocampus.