Abstract
Inconsistent gender differences in the outcome of TBI have been reported. The mechanism is unknown. In a recent male animal study, repeated stress followed by TBI had synergistic effects on brain gene expression and caused greater behavioral deficits. Because females are more likely to develop anxiety after stress and because anxiety is mediated by cannabinoid receptors (CBRs) (CB1 and CB2), there is a need to compare CB1 and CB2 expression in stressed males and females. CB1 and CB2 mRNA expression was determined in the amygdala, hippocampus, prefrontal cortex (PFC), and hypothalamus of adolescent male and female rats after 3 days of repeated tail-shock stress using qPCR. PFC CB1 and CB2 protein levels were determined using Western blot techniques. Both gender and stress had significant effects on brain CB1 mRNA expression levels. Overall, females showed significantly higher CB1 and CB2 mRNA levels in all brain regions than males (p < 0.01). Repeated stress reduced CB1 mRNA levels in the amygdala, hippocampus, and PFC (p < 0.01, each). A gender × stress interaction was found in CB1 mRNA level in the hippocampus (p < 0.05), hypothalamus (p < 0.01), and PFC (p < 0.01). Within-sex one-way ANOVA analysis showed decreased CB1 mRNA in the hippocampus, hypothalamus, and PFC of stressed females (p < 0.01, each) but increased CB1 mRNA levels in the hypothalamus of stressed males (p < 01). There was a gender and stress interaction in prefrontal CB1 receptor protein levels (p < 0.05), which were decreased in stressed females only (p < 0.05). Prefrontal CB2 protein levels were decreased in both male and female animals after repeated stress (p < 0.05, each). High basal levels of CBR expression in young naïve females could protect against TBI damage whereas stress-induced CBR deficits could predict a poor outcome of TBI in repeatedly stressed females. Further animal studies could help evaluate this possibility.
Highlights
There is a growing body of literature supporting a gender effect on the acute response and long-term outcomes of TBI, yet the findings are inconsistent [1,2,3,4,5]
Stressed animals exhibited reduced CB1 mRNA levels in the amygdala, hippocampus, and the prefrontal cortex (PFC) when compared to those brain regions of the control animals (p < 0.01, each) (Figure 1)
In the hypothalamus there was no significant difference between the CB1 mRNA levels in the stress and control groups (p > 0.05)
Summary
There is a growing body of literature supporting a gender effect on the acute response and long-term outcomes of TBI, yet the findings are inconsistent [1,2,3,4,5]. Several studies suggest that gender differences in TBI outcome may be age-dependent. In a recent retrospective mortality study, involving 10,135 prepubescent (0–12 years), and 10,145 pubescent (12–18 years) hospitalized patients who sustained isolated moderate-to-severe TBI (defined as a head Abbreviated Injury Scale (AIS) score of 3 or greater). Females in the pubescent but not in the prepubescent age group showed a significantly greater decrease in mortality than males. Groswasser et al [7] reported a significantly better predicted-outcome for young females than for males under the age of 18 with comparable levels of TBI severity. Other studies demonstrated that older women took significantly longer time than men to recover from TBI, after controlling for age, injury severity, mechanism of injury, and comorbidities [13,14,15]. The mechanism for the inconsistent gender effect across different age groups is unknown
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