The blood-brain barrier and glutamate

Abstract

Glutamate concentrations in plasma are 50-100 micromol/L; in whole brain, they are 10,000-12,000 micromol/L but only 0.5-2 micromol/L in extracellular fluids (ECFs). The low ECF concentrations, which are essential for optimal brain function, are maintained by neurons, astrocytes, and the blood-brain barrier (BBB). Cerebral capillary endothelial cells form the BBB that surrounds the entire central nervous system. Tight junctions connect endothelial cells and separate the BBB into luminal and abluminal domains. Molecules entering or leaving the brain thus must pass 2 membranes, and each membrane has distinct properties. Facilitative carriers exist only in luminal membranes, and Na(+)-dependent glutamate cotransporters (excitatory amino acid transporters; EAATs) exist exclusively in abluminal membranes. The EAATs are secondary transporters that couple the Na(+) gradient between the ECF and the endothelial cell to move glutamate against the existing electrochemical gradient. Thus, the EAATs in the abluminal membrane shift glutamate from the ECF to the endothelial cell where glutamate is free to diffuse into blood on facilitative carriers. This organization does not allow net glutamate entry to the brain; rather, it promotes the removal of glutamate and the maintenance of low glutamate concentrations in the ECF. This explains studies that show that the BBB is impermeable to glutamate, even at high concentrations, except in a few small areas that have fenestrated capillaries (circumventricular organs). Recently, the question of whether the BBB becomes permeable in diabetes has arisen. This issue was tested in rats with diet-induced obesity and insulin resistance or with streptozotocin-induced diabetes. Neither condition produced any detectable effect on BBB glutamate transport.

FIGURE 1

The blood-brain barrier exists around the entire central nervous system. The photograph shows a mouse injected systemically with trypan blue (a dye that adheres to plasma proteins) and dissected from the dorsal side to reveal the central nervous system. Note that all structures throughout the body, except the central nervous system, take up the dye. Milton W Brightman and Thomas S Reese of the National Institute of Neurological Disorders and Stroke prepared the image, which was kindly provided by MW Brightman.

FIGURE 2

Diagram of the blood-brain barrier. A: The blood-brain barrier exists at the level of the endothelial cells of cerebral capillaries. The endothelial cells are joined together by an extensive network of tight junctions. A basement membrane, within which pericytes reside, surrounds the endothelial cells as does a layer composed of astrocyte processes (so-called end feet). The pericytes are numerous and most likely function as phagocytes. The astrocyte layer serves as a metabolic barrier. For example, astrocytes incorporate NH₄⁺ into glutamine and metabolize short-chain fatty acids. B: An electron micrograph of a cerebral capillary shows the basic elements (provided through the courtesy of Robert Page, Pennsylvania State University College of Medicine). Reproduced with permission from reference .

FIGURE 3

A: Horseradish peroxidase injected into the general circulation. Note that the enzyme is restricted to capillaries except in the median eminence (ME), which has fenestrated capillaries. No horseradish peroxidase reaches any of the ventricles, including the third ventricle (3V). B: Horseradish peroxidase injected into the 3V. Note that the peroxidase readily penetrates the brain parenchyma, but not the area of the ME. It may be concluded that there is a barrier at the level of the epithelial lining of the ventricle that prevents leakage of metabolites into the ventricles. Milton W Brightman and Thomas S Reese of the National Institute of Neurological Disorders and Stroke prepared the image, which was kindly provided by MW Brightman.

FIGURE 4

Extracellular fluid (ECF) concentrations of amino acid groups compared with plasma. Note that the excitatory amino acids glutamate and aspartate are almost undetectable. Glutamine, which is believed to be nontoxic, has similar concentrations in the plasma and the ECF. Reproduced with permission from reference .

FIGURE 5

Glutamate and glutamine transport between neurons, astrocytes, and endothelial cells of the blood-brain barrier. Glutamate is the most abundant excitatory neurotransmitter in the mammalian nervous system. At chemical synapses, glutamate is stored in vesicles. Nerve impulses trigger the release of glutamate from the presynaptic cell. Na⁺-dependent glutamate transporters (excitatory amino acid transporters; EAATs) are found in neuronal and glial membranes. These transporters play the important role of regulating concentrations of glutamate in the extracellular space, which maintains low concentrations. After glutamate is released as the result of an action potential, glutamate transporters quickly remove it from the extracellular space and thereby terminate the synaptic transmission. Without the activity of glutamate transporters, glutamate would build up and kill cells in a process called excitotoxicity, in which excessive amounts of glutamate act as a toxin to neurons. The activity of glutamate transporters also allows glutamate to be recycled. In cases of brain injury or oxygen insufficiency, the EAATs can work in reverse, and excess glutamate can accumulate outside cells, which rapidly halts neurotransmission. At least 3 EAATs are present in the abluminal membrane of the blood-brain barrier. These EAATs move glutamate into the endothelial cells from which egress is possible through the facilitative transporters in the luminal membrane. There are transporters capable of pumping glutamine from the extracellular fluid into endothelial cells, and glutaminase within endothelial cells may also hydrolyze glutamine to glutamate and NH₄⁺. No carrier is necessary for NH₄⁺, which may diffuse as NH₃. A, Na⁺-dependent system A; N, Na⁺-dependent system N; X_G^–, facilitative glutamate transporter; n, facilitative glutamine transporter. The lightning symbols indicate Na⁺ dependence, and P indicates stimulation by pyroglutamate. Reproduced with permission from reference .