| Anatomy | Cell Biology | Physiology and Behavior | Clinical |
The monoamines will be considered as a group in discussing the cell biology of their 1) synthesis, 2) storage and 3) release. Monoamine receptors and termination of action of each monoamine will be considered separately.
Cell Biology - Biosynthesis of Monoamines
All monoamine (MA) neurotransmitters are synthesized from amino acids through a series of enzyme catalyzed reactions in which hydroxylation, decarboxylation and/or methylation convert the precursor amino acid into the active monoamine neurotransmitter.
Biosynthesis of Dopamine (DA), Norepinephrine (NE), and 5-hydroxytryptamine (5-HT)
Biosynthesis of all monoamines occurs primarily in the nerve terminal. As shown in Figure 12.7, the first step in the synthesis of catecholamines (DA and NE, as well as E, not shown) is the hydroxylation of the tyrosine to form DOPA. An analogous reaction, the hydroxylation of tryptophan to 5 hydroxytryptophane (5-HTP) is the first step in the biosynthesis of 5-HT. Both tyrosine hydroxylase and tryptophan hydroxylase are the rate-limiting steps in the biosynthetic pathway of the respective monoamines. Both enzymes are mixed function mono-oxygenases requiring molecular oxygen, iron and the cofactor, tetrahydrobiopterin (BH4) for activity. BH4 is converted to BH2 during the hydroxylation and must be regenerated to BH4 in order for monoamine biosynthesis to continue. As shown in Figure 12.7, the enzyme pteridine reductase regenerates the active cofactor. Pteridine reductase is therefore also an essential enzyme in the synthesis of catecholamines. The next step in the biosynthesis of monoamines is the decarboxylation by aromatic amino acid decarboxylase (AADC) to form the corresponding monoamine (Dopamine and 5 hydroxytryptamine 5-HT, respectively). NE is then formed from dopamine through an additional reaction, the hydroxylation of the 2nd carbon of the DA side chain. This last hydroxylation step occurs within the monoamine storage vesicle (see Figure 12.9a) and is catalyzed by dopamine β hydroxylase.
Figure 12.7a |
Figure 12.7b |
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Biosynthesis of the DA and NE precursor L-DOPA and the 5-HT precursor 5-HTP through hydroxylation using tyrosine hydroxylase (TH) and tryptophan hydroxylase (TryH.) These intermediates are then decarboxylated by a nonspecific decarboxylase, aromatic amino acid decarboxylase (AADC) to form the respective monoamines. Pteridine reductase regenerates the cofactor BH4.Biosynthesis of the DA and NE precursor L-DOPA and the 5-HT precursor 5-HTP through hydroxylation using tyrosine hydroxylase (TH) and tryptophan hydroxylase (TryH.) These intermediates are then decarboxylated by a nonspecific decarboxylase, aromatic amino acid decarboxylase (AADC) to form the respective monoamines. Pteridine reductase regenerates the cofactor BH4. |
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Two additional cofactors are required for the synthesis of monoamines; vitamin B6 is necessary as a cofactor for AADC catalyzed decarboxylation. Vitamin C is required as a cofactor for DA conversion to NE in the storage vesicle (see Figure 12.9a).
Biosynthesis of Epinephrine (E)
Epinephrine is synthesized in adrenal medulla and CNS by methylation of NE on the amino-terminus (not shown). The enzyme that catalyzes this reaction is phenyl ethanolamine N methyl transferase (PNMT). This enzyme uses S-adenysyl methionine as the methyl donor to methylate norepinephrine to form epinephrine (the nor refers to the lack of the methyl group). PNMT's localization outside the storage vesicle requires that norepinephrine shuttle out of the vesicle to be converted to epinephrine and then back into the storage vesicle for storage and release.
Biosynthesis of Histamine (HA)
Figure 12.8 |
As shown in Figure 12.8, in contrast to the catecholamines and 5-HT, the biosynthesis of histamine does not require hydroxylation. Histamine is the product of the decarboxylation of the amino acid, histidine, to form the monoamine, histamine, in a single step that is analogous to the decarboxylation of DOPA and 5-HTP. A different enzyme is used to decarboxylate histidine, histidine decarboxylase, as shown in Figure 12.8. This enzyme, like AADC, requires vitamin B6.
Regulation of Catecholamine Biosynthesis
The concentration of catecholamines in nerve terminals remains relatively constant despite frequent fluctuations in neuronal activity. This homeostasis is achieved through the regulation of TH activity. TH is phosphorylated and activated by both calcium and cAMP dependent protein kinases. A longer-term regulation of CA synthesis also occurs. This regulation is mediated through altered transcription of TH mRNA and altered TH mRNA stability. Both mechanisms lead to increased levels of TH protein.
Regulation of Serotonin Biosynthesis
The level of serotonin is regulated principally by the amount of tryptophan available to serotonergic neurons. This has two important implications for the level of serotonin in the brain. First, because tryptophan is not synthesized in mammals, the level of tryptophan available for serotonin biosynthesis is dependent on diet. Thus, diets high in tryptophan can markedly elevate serotonin levels. Second, because tryptophan is transported across the blood brain barrier by a transport system which also transports certain other amino acids, diets high in these amino acids can reduce the level of serotonin in the brain by competing with tryptophan for transport into the CNS. As will be discussed later, altered serotonin level in the CNS can have marked consequences on behavior.
Regulation of Histamine Biosynthesis
Thus far the mechanism for the regulation of histamine biosynthesis is unknown.
Storage of Monoamines
Monoamine neurotransmitters are stored in vesicles that appear dark at the EM level and are thus referred to as dense core vesicles. MA neurotransmitters are stored at a high concentration and are complexed with ATP and several proteins called chromagranins. One of these chromagranins is the enzyme, dopamine β hydroxylase (DβH), that converts DA to NE. As shown in Figure 12.9, MA neurotransmitters are taken into the vesicles by an exchange of H+ for the MA. In NE cells DA is taken up and converted to NE by DβH. As described above in the synthesis section, DβH hydroxylates the amino side chain. The uptake of MA neurotransmitters into storage vesicles is inhibited by the drug reserpine.
Figure 12.9a |
Figure 12.9b |
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An antiporter that exchanges protons for monoamines (MA) mediates storage of monoamines in dense core vesicles. Left: In NE cells, DA is taken up then converted to NE within the vesicle by the enzyme DBH. Right: All other monoamine cells merely store the MA neurotransmitters. |
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Release of Monoamines
Figure 12.10 |
Neuronal activation elicits the release of MA neurotransmitters by a calcium-dependent exocytosis, as described in Lecture 10, under Secretory Mechanism. The vesicular contents are released from the nerve terminal into the extracellular space during secretion. Because there is no classic postsynaptic specialization associated with the majority of MA nerve endings, the released MA neurotransmitters diffuse to postsynaptic cells in the vicinity where they stimulate MA receptors (volume neurotransmission).
MA neurotransmitters also act on the presynaptic cell, as shown in Figure 12.10 to influence their cell biology in a feed back manner. The interaction with the presynaptic receptors (termed autoreceptors) can both stimulate MA biosynthesis and inhibit the further release of neurotransmitter. Both the pre- and postsynaptic MA receptors are G protein linked, seven trans-membrane receptors. Their structure is similar to the muscarinic receptors discussed in the Lecture 11, Cholinergic Neurotransmission.
