INFORMATION PROCESSING IN THE BRAIN NOT ONLY NEURONS' JOB
06 April 2007 - University of Bonn (Universität Bonn)

In human beings gliacytes (glia in Greek means 'glue') are far more numerous than nerve cells, i.e. the neurons: almost 90% of all brain cells are one of the three types of glia (astrocytes, oligodendrocytes, microgliacells). Nevertheless, for a long time researchers into the brain only saw the gliacyte as a kind of "wet nurse" which looked after and fed the real stars of the show, the neurons. However, for some years now the signs have been increasing that the neglected neural glue should be accorded a more important role.

The research team headed by Professor Christian Steinhäuser and his Swiss colleague Professor Andrea Volterra have now been able to convincingly substantiate these suppositions by investigating brain specimens using the electron microscope and the methods of molecular biology: "We have found specific transport proteins in astrocytes which were previously only known to exist in nerve cells," Professor Steinhäuser explains. In the neurons these "molecular conveyor belts" fill little internal cellular storage vesicles with the messenger substance glutamate. "We were able to demonstrate that there are glutamate vesicles like these in astrocytes, too, previously this assumption was vehemently opposed in some quarters. The transport proteins are located in the membrane of these vesicles." Using cell cultures the team proved that the vesicles really do work: "If the astrocytes are stimulated by activating a particular receptor, the vesicles merge with the membrane from within, thereby releasing their content, the glutamate, into the space between the astrocyte and its adjoining cells." The whole process takes place pretty quickly: within 0.2 seconds most of the storage vesicles have emptied, the transmission of chemical signals between two nerve cells in what is known as the "synaptic gap" takes place only a little faster. The messenger substance released also indirectly activated glutamate receptors in adjoining cells which the researchers had previously placed there as sensors to measure this activity.

The astrocytes studied were from the hippocampus, a cerebral structure resembling a seahorse, which plays an important part in learning and memory processes. Hippocampus astrocytes have at their disposal an enormous number of finely networked projections; a single astrocyte from this region of the brain can, by releasing glutamate, theoretically influence up to 140,000 synapses which do not necessarily have to be situated in the immediate proximity. "The astroglial glutamate signal might therefore serve to modulate a large number of neurons," Professor Steinhäuser explains. If this should be correct, the gliacytes would have unexpectedly graduated to being important players in the brain, with astrocytes as conductors which keep an orchestra of thousands of nerve cells playing in tune.

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