Female gonadal hormone effects on microglial activation and functional outcomes in a mouse model of moderate traumatic brain injury

Abstract

Aim: To address the hypothesis that young, gonad-intact female mice have improved long-term recovery associated with decreased neuroinflammation compared to male mice.

Methods: Eight to ten week-old male, female, and ovariectomized (OVX) mice underwent closed cranial impact. Gonad-intact female mice were injured only in estrus state. After injury, between group differences were assessed using complementary immunohistochemical staining for microglial cells at 1 h, mRNA polymerase chain reaction for inflammatory markers at 1 h after injury, Rotarod over days 1-7, and water maze on days 28-31 after injury.

Results: Male mice had a greater area of injury (P = 0.0063), F4/80-positive cells (P = 0.032), and up regulation of inflammatory genes compared to female mice. Male and OVX mice had higher mortality after injury when compared to female mice (P = 0.043). No group differences were demonstrated in Rotarod latencies (P = 0.62). OVX mice demonstrated decreased water maze latencies compared to other groups (P = 0.049).

Conclusion: Differences in mortality, long-term neurological recovery, and markers of neuroinflammation exist between female and male mice after moderate traumatic brain injury (MTBI). Unexpectedly, OVX mice have decreased long term neurological function after MTBI when compared to gonad intact male and female mice. As such, it can be concluded that the presence of female gonadal hormones may influence behavioural outcomes after MTBI, though mechanisms involved are unclear.

Keywords: Functional recovery; Inflammation; Microglia; Sex; Traumatic brain injury.

Figure 1

Ratio of inflammatory gene expression in experimental male and female mice. Since the ratio is a male/female value, genes with a ratio of less than one are genes that are less expressed in male mice and genes with ratios of greater than two are more expressed in male mice. Genes with ratios close to one are evenly expressed between both sexes.

Figure 2

Short- and long-term neurobehavioral measurements after mild traumatic injury in mice. Rotarod latencies (A) were not different over Days 1-7 after moderate traumatic brain injury (MTBI) (P = 0.62; ANOVA). Significant water maze (WM) latencies (B) differences were demonstrated between groups over Days 28-31 after MTBI. WM latencies did not differ between male and female mice after MTBI, but ovariectomized mice demonstrated longer latencies over the testing period (P = 0.04; ANOVA).

Figure 3

Water maze latencies for sham male, female and ovariectomized mice. No sex interaction was seen between male, female, and ovariectomized (OVX) mice over Days 28-31 after moderate traumatic brain injury, thus no significant sex differences are observed for shams.