Developmental differences in the microglia response to nerve injury are resistant to dietary sodium deprivation.

Surgically cutting the chorda tympani nerve (CTX) results in profound degradation in both the brainstem and tongue early in life but minor, transient effects in adulthood. We have previously shown that microglia, immune cells of the brain, increase in number around the damaged chorda tympani fibers at all injury ages, with the largest increase occurring in adulthood. Similarly, macrophages increase in number on the tongue following adult CTX, which is prevented by dietary sodium deprivation. Microglia and macrophages are both monocytic cells, suggesting that sodium deprivation may also attenuate the microglia response to CTX. To test this, 32 male and female adult rats underwent either CTX, a sham surgery, or were left intact. Within each surgical condition, half were placed on a low sodium diet and received an i.p. injection of the diuretic furosemide. Four days later, rats were sacrificed via intracardial perfusion and the brains were extracted, sectioned at 40 µm on a vibratome, and stained for microglia. The density and morphology of microglia in the brainstem were quantified using the program Neurolucida, and statistics were performed using SPSS. Sodium deprivation did not significantly influence microglia density or morphology in any surgical condition. However, sodium deprivation did increase average microglia size for female rats, suggesting a sex-specific influence of sodium on the immune system of the brain.