The Blood-Brain Barrier
Effects of Electromagnetic Fields From
upon the Blood-Brain Barrier
Leif G. Salford, MD, PhD
Senior Professor Department of Neurosurgery Lund University
Henrietta Nittby, MD, PhD Department of Neurosurgery Lund University Hospital
Bertil R. R. Persson PhD, MD h.c.
Prof. em. Department of Medical Radiation Physics Lund University
Prepared for the BioInitiative Working Group September 2012
Some organs of crucial importance for the function of our bodies are protected from exposure to potentially harmful compounds in the blood. Thus the brain, the eyes (which are protrusions of the brain), the testes and the follicles of the ovaries have special barriers between the capillaries and the tissue. In the normal brain, the passage of compounds over this barrier, the Blood-Brain Barrier (BBB), is highly restricted. The BBB is a hydrophobic barrier formed by the vascular endothelial cells of the capillaries in the brain with tight junctions between them leaving no openings between the vessel lumen and the surrounding brain. There is scientific evidence that the BBB exists not only in vertebrates, but also in insects (1), crustaceans and cephalopod molluscs (such as the cuttlefish) (2) and in elasmobranchs (cartilaginous fishes such as sharks) (3) and helices (landsnails) (4), maintaining ionic integrity of the neuronal bathing fluid. The BBB seems to be present very early in the foetal development. Also, at an early stage, there seems to be a cerebrospinal fluid barrier, which excludes cerebrospinal fluid (CST) protein from the brain extracellular space. The tight junctions of the BBB are composed of tight junction proteins (occludin, claudin and zonula occludens, where the zonula occludens is the intracellular peripheral membrane protein that anchors claudin and occludin to the actin cytoskeleton. An important part is the binding of claudin proteins on opposing membranes, where claudin-5 in particular is crucial in the BBB. Astrocytes are surrounding the outer surface of the endothelial cells with protrusions, called end feet, and are implicated in the maintenance, functional regulation and repair of the BBB. The astrocytes form a connection between the endothelium and the neurons and constitute a second barrier to hydrophilic molecules. Other periendothelial accessory structures of the BBB include pericytes and a bilayer basal membrane which surrounds the endothelial cells and pericytes. The basement membrane (basal lamina) supports the ablumenal surface of the endothelium and may act as a barrier to passage of macromolecules. The pericytes are a type of macrophages, expressing macrophage markers with capacity for phagocytosis but also for antigen presentation. In fact, the pericytes, which cover about 25% o the capillary surface, seem to be in a position to significantly contribute to central nervous system (CNS) immune mechanisms. The pericytes also have other functional roles: with their capability for contractility they seem to serve as a smooth muscle equivalent, and through regulation of endothelial cells they maintain the stability of blood vessels. Additionally, the pericytes seem to be highly involved in many diseases, both infectious and autoimmune, and also in other diseases such as Alzheimer’s by production of amyloid. Also, by regulating their vascular permeability, the pericytes are supposed to play an important role in inflammatory diseases. Physiologically, the microvasculature of the central nervous system (CNS) differs from that of peripheral organs. It is characterized not only by its tight junctions, which seal cell-to-cell contacts between adjacent endothelial cells, but also by the low number of pinocytotic vesicles for nutrient transport through the endothelial cytoplasm and its lack of fenestrations, and the five-fold higher number of mitochondria in BBB endothelial cells compared to muscular endothelia in rat. The tight junctions and low number of pinocytotic vesicles guarantee that proteins cannot pass freely into the brain parenchyma. Solutes which are not highly lipid soluble, or which do not bind to selective transporters with high affinity, are excluded from free exchange. BBB has a capacity to metabolize certain solutes, such as drugs and nutrients. Fourthly, active transporters maintain the levels of certain solutes at specific values within the brain interstitial fluid, made possible by active transport against the concentration gradients. These enzyme systems are differently distributed between the luminal and the ablumenal membranes of the endothelial cells, thus gaining the BBB polarity properties. For example, Na+-K+-ATPase is located on the antilumenal membrane: this mechanism, requires energy to function properly. The normal selective permeability of the BBB can be altered in several pathological conditions such as epileptic seizures or extreme hypertension and also transient openings of the BBB might lead to permanent tissue damage. It has also been shown that an increased permeability of the BBB is seen in cases of oxidative stress, where BBB dysfunction and neurodegeneration were shown to be mediated through an excitotoxicity mechanism by the serine protease tissue plasminogen activator, with NO and ONOO– as downstream mediators.
In early studies on the effects of low-intensity EMFs on the BBB, various compounds were injected intravenously, followed by EMF exposure and comparisons of the penetration into the brain tissue between sham and exposed animals. Frey et al. found increases in the BBB permeability of rats to fluorescein after 30 min of exposure to both pulsed and continuous waves (CWs) at 1.2GHz with average power densities of 0.2mW/cm2. Similar observations were made in a study with 180 animals by Oscar and Hawkins. Exposure of anaesthetized rats for 20 min to 1.3GHz of pulsed EMFs with average power densities of 0.3mW/cm2 resulted in leakage of 14C-mannitol, dextran, and inulin into the cerebellar brain tissue, as well as inulin and dextran leakage from capillaries into hypothalamic and medullar tissue. Also, BBB permeability to mannitol was investigated in un-anaesthetised rats, which were exposed to pulsed radiation or sham exposed for 20 min. The animals were sacrificed at different time intervals after the exposure. In an attempt to repeat the findings of Oscar and Hawkins (26), Preston et al. found no increase in the uptake of 14C-mannitol in anaesthetised rats after 2450MHz CW exposure for 30 min at power densities of 0.1 to 30mW/cm2. Preston et al. further concluded that the increased BBB permeability, which had been observed by Oscar and Hawkins in cerebellum and medulla, possibly had been misinterpreted and was not due to the EMF exposure. Rather, changes in blood flow and water influx or egress were supposed to be responsible for the BBB permeability in these caudal parts of the brain. No alteration of BBB permeation of 14C-sucrose and 3H-inulin was found by Ward et al. after exposure of anaesthetised rats to CW at 2450MHz for 30 min at power densities of 0, 10, 20, or 30 mW/cm2 after correction for thermal effects. Similarly, Ward and Ali (32) observed no permeation after 1.7GHz exposure at SAR of 0.1 W/kg, using the same exposure duration and injected tracers as Ward et al. (31). Absence of EMF induced BBB permeability was also reported by Gruenau et al. (33), after injection of 14C-sucrose in conscious rats and exposure 30 min pulsed energy (2.8GHz at 0, 1, 5, 10, or 15mW/cm2) or continuous wave (2.8 GHz, 0, 10, or 40 mW/cm2). In further studies, more attention was directed towards the effects of hyperthermia, resulting from exposure at high SAR-levels, on BBB permeability: there is a demonstrable increases of BBB permeability associated with intense, microwave-induced hyperthermia. With more research into the area of EMF induced BBB permeability, it became evident that with high-intensity EMF exposure resulting in tissue heating, the BBB permeability is temperature dependent