[Neuron-glia interactions]

The progress of research in the Central Nervous System (CNS) had led to the consideration of neurons and glia as indissociable functional complexes. Neuron-glia interactions are essential for the maturation of the CNS. Glial cells release trophic factors for neurons (NGF) and neurons release trophic factors for glia (GGF). Furthermore, the latter provide a substrate for the migration of neurons and guidance of axons by mean of adhesion molecules. In adults, the interactions between neurons and glial cells serve to maintain homeostasis. Thus, the glial cells perform the restoration of the metabolic equilibrium overthrown by the transmission of the nerve impulse and provide the glucose required for neuronal activity. The nerve impulse provokes increases in the cellular space of CO2, K+, NH3 and neurotransmitters which must be taken up to allow neuronal activity to continue (in normal conditions). Astrocytes perform the uptake of the extracellular K+ by means of passive ionic channels, ionic voltage-dependent channels and a sodium-potassium-ATPase-dependent pump. The oligodendrocytes are involved in the metabolism of CO2 by converting CO2 into carbonic acid by means of carbonic anhydrase. Oligodendrocytes and astrocytes play a role in terminating neural transmission by the uptake of the amino acid neurotransmitters, such as GABA, glutamate and aspartate. The catabolism of glutamate to glutamine by means of glutamine synthetase allows both the conversion of an excitatory amino acid into a neutral amino acid (which can diffuse in the extracellular space without causing neural transmission) and the reduction of cerebral NH3 content.(ABSTRACT TRUNCATED AT 250 WORDS)