Abstract
The numbers of astrocytes in the human brain are higher than neurons. Although the existence of the cell within the brain tissue was found almost the same period as that of neurons, their roles in the brain have been thought only supporters for neurons. The functions as the blood-brain barrier, uptake of transmitters released from neurons and the energy supplies to neurons are important for maintaining brain function. Since astrocytes had been recognized as inert cells by electrophysiologists, they were treated as outsiders of the brain which worked as dynamic information-processing machinery using electricity. However, the end of 20th century, unexpected function of astrocyte was found by Ca²⁺ fluorometry. Astrocytes were found to express almost all kinds of neurotransmitter receptors, which were mostly G-protein coupled type, and responded to many kinds of neurotransmitters by dynamic Ca²⁺ oscillation. Furthermore, the astrocytes were found to be able to activate neurons by substances released by themselves such as glutamate, ATP and D-serine, which are called “gliotransmitters” . Astrocytes are no longer supporting players for the brain, but the cells would participate in the neuronal circuit forming tripartite-synapse, and play important roles in higher-order brain functions.
