Modulation of a glial blood-brain barrier.

CNS interstitial fluid homeostasis by the glial perineurial blood-brain barrier in the crayfish and cockroach is dependent on glial uptake mechanisms, low paracellular permeability, and the cation-binding properties of the extracellular matrix. Potassium selective permeability of the crayfish perineurium is modulated by a Ca(2+)-dependent mechanism at the basolateral membranes of the glial barrier and is ion and voltage dependent. In addition, extracellular charged sites are significant in perineurial K+ and Ca2+ homeostasis and may be modified by changes in pH. In the cockroach, and probably the crayfish, perineurial K+ transport may also be modulated by receptor-mediated changes in glial membrane permeability. The factors acting at the crayfish and cockroach blood-brain barrier are summarized in FIGURE 8 and would be well suited for providing efficient K+ spatial buffering of the CNS. Analogous processes have been described in vertebrate glial cells and in the endothelial blood-brain barrier, which implies a common primary function. The CNS is protected from large fluctuations in the body fluids by the blood-brain barrier, whereas glial uptake mechanisms control the composition of the brain interstitial fluid, and modulation of both barrier permeability and glial transport by the altered chemical environment following neuronal activity allows precise adjustment of the brain extracellular fluids to the changing needs of the CNS. The insect and crustacean ventral nerve cord and perineurial blood-brain barrier provide an excellent preparation in which the interactions between these factors can be investigated in intact CNS tissue.