Neurotransmitters

A wide range of amino acids and their derivatives act as neurotransmitters (Table 4.3).

Amino acids directly involved in this function are classified within the amino acidergic system, whereas those derived from the aromatic amino acids and tryptophan are categorized within the monoaminergic system (Bradford, 1986).

The synthesis of GABA from glutamate has already been shown [Reaction 4.9], The formation of dopamine and noradrenaline from tyrosine and of serotonin from tryptophan respectively are depicted in Figs 4.10 and 4.11.

Table 4.3. Amino acids and their derivatives as neurotransmitters: definitive and proposed compounds.

Group/precursor

Specific compound

Action

Amino acids

Arginine

Biogenic amines

•y-Aminobutyrate (GABA)

Glutamate

Aspartate

Glycine

Histamine

Taurine

Nitric oxide (NO)

Dopamine

Noradrenaline

Serotonin (5-hydroxytryptamine)

Inhibitory Excitatory Excitatory Inhibitory

Classical neurotransmitter; immunomodulator Modulator of neuronal activity Multifunctional Excitatory and inhibitory Excitatory and inhibitory Vasoconstrictor and stimulator of smooth muscle contraction

Tyrosine

Tryptophan

3,4-Dihydroxyphenylalanine (Dopa)

5-Hyd roxytryptophan

Dopamine

Noradrenaline

Adrenaline

Fig. 4.10. Synthesis of noradrenaline and adrenaline from tyrosine. Enzymes: 1, tyrosine hydroxylase; 2, aromatic decarboxyl 3S6, u i dopamine-p-hydroxyl 3S6 ; 4, transmethyl 3S6 •

5-Hydroxytryptamine (serotonin)

Fig. 4.11. Synthesis of 5-hydroxytryptamine from tryptophan. Enzymes: 1, tryptophan 5-monooxygenase; 2, 5-hydroxytryptophan decarboxyl 3S6>

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