Do metals that translocate to the brain exacerbate traumatic brain injury?

Metal translocation to the brain is strictly controlled and often prevented by the blood-brain barrier. For the most part, only those metals required to maintain normal function are transported into the brain where they are under tight metabolic control. From the literature, there are reports that traumatic brain injury disrupts the blood-brain barrier. This could allow the influx of metals that would normally have been excluded from the brain. We also have preliminary data showing that metal pellets, surgically-implanted into the leg muscle of a rat to simulate a shrapnel wound, solubilize and the metals comprising the pellet can enter the brain. Surprisingly, rats implanted with a military-grade tungsten alloy composed of tungsten, nickel, and cobalt also showed significantly elevated uranium levels in their brains as early as 1 month after pellet implantation. The only source of uranium was low levels that are naturally found in food and water. Conversely, rats implanted with depleted uranium pellets demonstrated elevated uranium levels in brain resulting from degradation of the implanted pellets. However, when cobalt levels were measured, there were no significant increases in the brain until the rats had reached old age. The only source of cobalt for these rats was the low levels found in their food and water. These data suggest that some metals or metal mixtures (i.e., tungsten alloy), when embedded into muscle, can enhance the translocation of other, endogenous metals (e.g., uranium) across the blood-brain barrier. For other embedded metals (i.e., depleted uranium), this effect is not observed until the animal is of advanced age. This raises the possibility that metal body-burdens can affect blood-brain barrier permeability in a metal-specific and age-dependent manner. This possibility is disconcerting when traumatic brain injury is considered. Traumatic brain injury has been called the "signature" wound of the conflicts in Iraq and Afghanistan, often, an embedded metal fragment wound occurs simultaneously. Since the blood-brain barrier can be disrupted by traumatic brain injury, this raises the possibility of free access to the brain for any metals found in the body. Therefore, we hypothesize that this influx of metals overwhelms normal brain homeostasis, depletes the brain's antioxidant defense systems, and activates microglial cells resulting in the release of inflammatory mediators that can potentially exacerbate the adverse effects of traumatic brain injury.