A common property of transmitter amino acids such as GABA and glycine and transmitter amines such as dopamine, serotonin, and noradrenaline is a sodium-dependent high-affinity uptake system (for review see Fonnum et al., 1980). A similar uptake system also exists for glutamate. The uptake of glutamate shows an absolute requirement for sodium (Bennet et al., 1973), and studies with 22Na+show that two sodium ions are required for the uptake of one glutamate molecule (Stallcup et al., 1979). The order of association of the ions with the transport carrier has by kinetic studies been determined to be sodium-sodium-glutamate (Wheeler and Hollingsworth, 1978)
Glycine can be taken up into cells by specific transporter systems (GLYT1 and GLYT2) which comprise sodium/chloride-dependent transporters
2.4 Neurotransmitter transporters Neurotransmitter transporters are necessary for synaptic transmission just as receptors, ion channels, G proteins and effectors. Generally, neurotransmitters are removed from synaptic cleft by enzymatic degradation or by active transport to presynaptic button or to surrounding glia cells. There are three main classes of membrane transporters:
1. Transporters dependent on sodium and chlorine transport serotonin, norepinephrine or dopamine back into presynaptic part; this enables that neurotransmitters may be stored in vesicles and repeatedly released in response to action potential;
2. Vesicular transporters carry neurotransmitters into synaptic vesicles;
3. Sodium dependent transporters are localized in the membrane of glia cells and transport neurotransmitters such as GABA, glutamate or aspartate into glia cells.
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