**1. Introduction**

Even for students just beginning to study biochemistry and physiology, it is immediately apparent that amino acids (AAs) are among the most important molecules in nature. Their functions are broad and varied. Indeed, protein synthesis relies on the well-known polymerization of AAs to form a peptide bond. This property is the most famous aspect of AAs. However, many AAs have specific individual functions, such as neurotransmission [1], cellular energy metabolism [2], and detoxification [3, 4]. Accumulating evidence in recent years has demonstrated that AAs also regulate both the expression of genes and the protein phosphorylation cascade. Moreover, hormones and different low-molecular-weight biologically important chemical compounds can be synthesized from AAs [5]. AAs can be divided into essential and nonessential categories. If the body cannot synthesize the carbon skeleton of an amino acid, then it is considered nutritionally essential.

Indeed, the diet must contain such AAs. The dietary essentiality of other AAs (e.g., arginine, glycine, proline, and taurine) is determined by the developmental stage and species [6]. In contrast, if AAs can be synthesized de novo in a speciesdependent manner, they are considered nonessential. Accumulating evidence has led to the concept of functional AAs (FAAs), which are defined as AAs that regulate key metabolic pathways to improve the health, survival, growth, development, lactation, and reproduction of organisms [7]. Since the late 1970s, researchers have generally agreed that amino acids can also function as inhibitory or excitatory neurotransmitters [8]. It should be noted that in neurochemistry, the term "neurotransmitter" is usually used synonymously with "neuromediator," another term for a chemical participant in connections between neurons and neuroglia cells. Because these terms are exchangeable, they will both be used in the text. Based on their effects on vertebrate nerve cells, γ-aminobutyric acid (GABA), glycine, and taurine fall into the class of inhibitory amino acids, whereas glutamate and aspartate fall into the class of excitatory compounds [9]. Indeed, GABA is considered the main inhibitory neurotransmitter in the central nervous system (CNS) [10], but it is not truly a member of the AA family. Although taurine also plays a role in inhibitory neuromediation [11] and serves as an osmoeffector to regulate volume in astrocytes [12], this compound is considered a derivative of cysteine, and, similar to GABA, not a true amino acid. Thus, the remaining excitatory/inhibitory amino acid neurotransmitters are glutamate, aspartate, and glycine. The first and third are the most prominent members of the AA family. The processes that regulate glutamate and glycine in the CNS are (i) transportation, (ii) biochemical transformations in metabolic pathways, and (iii) interactions with membrane receptors. In the current chapter, the crosstalk between the processes mentioned above for both glutamate and glycine is presented because the final state of neurons seems to be a result of the balance between these excitatory and inhibitory influences.
