**2. The membrane transport system of amino acids**

Glutamate and glycine are nonessential amino acids; their levels differ depending on the location. The extracellular glutamate concentration around quiescent neurons is less than 1 μM, while its concentration in the cytoplasm is much higher, at approximately 2 mM [13]. The brain sequesters glycine in concentrations of 600 μM [14], with a basal concentration in the cerebrospinal fluid (CSF) of ~6 μM [15], compared to a plasma concentration of ~250 μM [16]. Because no extracellular enzymes degrade glutamate and glycine, maintaining these low extracellular concentrations requires cellular uptake of both compounds. Thus, the activity of the carriers directly regulates receptor response to neuron activation. Indeed, glutamate and glycine serve as neuromediators in the extracellular fluid because the binding site of AA receptors is exposed to the outer surface of cells. Consequently, the release of AA into the extracellular fluid controls receptor activation and active states are controlled by the removal of AAs from the extracellular fluid [17]. This uptake is catalyzed by a family of transporter proteins located on the cell surface of both astrocytes and neurons [17]. A high-affinity glutamatergic uptake system was observed in the mammalian brain in the 1970s. Subsequently, excitatory amino acid transporters (EAATs) were experimentally identified. They transport glutamate and aspartate across the plasma membrane. Notably, EAATs are part of the well-known solute carrier 1 (SLC1) family of transmembrane amino acid transporters [18]. Thus, released glutamate molecules can be removed from the synaptic cleft by the brain transporters; this process will initiate the glutamate-glutamine cycle, eventually restoring the pool of the neuromediator in synaptic vesicles [19]. Five EAAT isoforms, human EAAT1-5, have been

### *Amino Acids as Neurotransmitters. The Balance between Excitation and Inhibition… DOI: http://dx.doi.org/10.5772/intechopen.103760*

identified; they correspond to GLAST1/GLT-1/EAAC1/EAAT4/EAAT5 in rodents, respectively [20]. In addition, the EAAT4 and EAAT5 subtypes were identified, with EAAT5 predominantly expressed in the retina. Notably, the transport cycle times of EAATs are relatively slow and their high affinity for glutamate makes it possible to sequester low glutamate concentrations from the extracellular space, preventing excitotoxicity. The slow transportation rate may in part be overcome by rapid surface diffusion and transporter tracking of EAATs upon glutamate stimulation [21]. The SLC1 family also contains two neutral amino acid transporters, alanine serine cysteine transporters 1 and 2 (ASCT1 and 2), which share high sequence homology with the EAATs [22]. EAAT1 and EAAT2 are glutamate transporters that are mostly expressed in astrocytes. These two glutamate transporters are responsible for most of the glutamate clearance in the brain. EAAT2 is widely expressed in the cerebral cortex and the hippocampus [13]. Moreover, GLT-1/EAAT2 accounts for approximately 90% of the total glutamate uptake in the brain, and thus, it is considered the most important glutamate transporter subtype in the CNS. This transporter is predominantly but not exclusively expressed in astrocytes [22]. Glutamate transporters couple glutamate uptake to the transport of inorganic ions. It is now generally accepted that 3 Na+ ions and 1 H+ ion are cotransported and 1 K+ ion is counter-transported with the uptake of each glutamate molecule. Based on this stoichiometry, glutamate transporters were calculated to concentrate glutamate up to 5 × 106 -fold inside cells under physiological conditions. This glutamate transport is electrogenic [23].

The extracellular levels of glycine in inhibitory and excitatory synapses are controlled by glycine transporters (GlyTs). Both subtypes, GlyT1 and GlyT2, belong

#### **Figure 1.**

*Membrane carriers are responsible for clearance of glutamate/glycine from interstitial fluid (ISF) in the CNS. The scheme indicates two types of neurons. Some are excitatory and glutamatergic (the upper part of the scheme). Other neurons are inhibitory and glycinergic (the lower part of the scheme). Both types of neurons are interconnected with astrocytes. Moreover, glycine and glutamate are accessible for both types of cells. AA transporters (EAAT, GlyT, etc.) are found in all cell membranes but have differing isoenzyme compositions.*

to the sodium-dependent solute carrier 6 (SLC6) family of transporters, but they have different regional and cellular expression patterns in the CNS, different stoichiometries (that is, different numbers of sodium ions that are co-transported with every glycine molecule) and varying abilities to reverse-transport glycine into the extracellular space. To date, five variants of GlyT1 (GlyT1a, GlyT1b, GlyT1c, GlyT1d, and GlyT1e) and three variants of GlyT2 (GlyT2a, GlyT2b, and GlyT2c) have been identified and occur as a result of alternative promoter usage and/or splicing, but the relative distributions of these within the CNS have not been fully characterized [21].

The essential function of membrane transporters is to accumulate neuromediators in vesicles. At presynaptic terminals, vesicular glutamate transporters (vGluTs; SLC17A7, -6, and -8) load glutamate into synaptic vesicles. The two subtypes of vGluTs, vGluT1, and vGluT2, are expressed in excitatory neurons in a complementary manner in the brain, composing two subsets of excitatory neurons [13]. Glycine also actively accumulates in synaptic vesicles through vesicular inhibitory amino acid transporter (VIAAT); currently, only one type of transporter (SLC32A1) is known to be responsible for this process [18]. The scheme of balanced neuromediator transport is represented in **Figure 1**.

Remarkably, both glutamate and glycine transporters have mechanisms that include sodium ion transport. This means that neuromediator uptake is accompanied by changes in membrane potential. Moreover, the intake of both glutamate and glycine initiates several metabolic reactions in neurons and astrocytes. However, these reactions are spatially distributed, and the fate of the neuromediators is functionally determined by different cells. Interestingly, the metabolic transformations of AAs are closely related to ATP production by mitochondria and the oxidation of glucose.
