The Neuron-Glia Unit in Neuropathology: Is it a Double-Edged Sword?

For decades, astrocytes were primarily recognized as structural components of the CNS. Only recently have we begun to understand their other functions within the context of CNS homeostasis. Astrocytes were previously thought of as passive cells, due primarily to the fact that they did not appear to respond to electrical stimulation or synaptic activity and showed only slight changes in membrane potential. It is now appreciated that astrocytes can, in fact, depolarize in response to neural activity [Haydon, 2001]. Additionally, astrocytes play an important role in distributing energy substrates to neurons [Dienel and Hertz 2001], Astrocyte-astrocyte, as well as astrocyte - neuron cross talk have been documented [Haydon, 2001). Other astrocytic functions include the production of trophic factors, the regulation of neurotransmitters and ion concentration, and the removal of debris from the extracellular environment [Aschner et al" 2002]. Thus, astrocytes play a number of essential roles in the CNS. When functioning optimally, they act the part of the charming astrocyte. However, astrocytes also play a role in generating reactive intermediates or impede brain remodeling and regeneration upon injury shifting them into the contrasting position of the "villain" astrocyte.