[The role of glial monoamine transporters in the central nervous system].

Monoamine transport systems play a very important role in determining the concentrations of monoamines in the synaptic cleft, and therefore the magnitude and duration of the effects of transmitters. Several transport systems for monoamines have been described. The first to be recognized were uptake, a Na(+)-dependent, high-affinity, cocaine-sensitive neuronal transporter, which includes dopamine transporter, norepinephrine transporter and serotonin transporter, and uptake1, a Na(+)-independent, low-affinity, high-capacity, steroid-sensitive extraneuronal transporter. Recently, molecular identification of the uptake2 transporter has been reported, and this has been called extraneuronal monoamine transporter in humans, and organic cation transporter3 in rats. Astrocytes contain these two transport systems that can remove monoamine neurotransmitters from the synaptic cleft by transporters present in the plasma membrane. Since monoamine oxidase and catechol-O-methyl-transferase are present in astroglial cells, their glial uptake systems are likely to play an important role in regulating extracellular monoamine concentrations. This uptake system may be characterized as a second line of defense that inactivates monoamines that have escaped neuronal re-uptake, and thus prevents uncontrolled spreading of the signal. In this review, the identification of monoamine transporters in astrocytes is described and the physiological role of glial monoamine transporters in monoaminergic neurotransmission is discussed.