The monoamines

The enzymes involved in the formation of the catecholamines are of low specificity. DOPA decarboxylase, or an enzyme closely akin to it, is concerned in the formation of 5-hydroxytryptamine dopamine-/9-oxidase has been shown to be capable of hydroxylating the jd-carbon atom of a number of tyramine derivatives - and phenylethanolamine A-methyltransferase is equally unselective in its A-methylation of noradrenaline derivatives . This lack of specificity suggests the possibility that alternative pathways for the formation of noradrenaline and adrenaline might exist in vivo. Some of the putative intermediaries in these other pathways have been shown to occur naturally and one of them, octopamine (/) is found in the brain . 

Adrenaline is the major component of the hormonal secretion of the adrenal medulla and in the adrenal gland noradrenaline can be regarded simply as a precursor of the active substance. In sympathetic nerves, however, catecholamine synthesis proceeds no further than noradrenaline, which exists there as a transmitter substance in its own right, with properties different from those of adrenaline. Noradrenaline and small amounts of adrenaline are present in the brain also, but in addition, certain areas of the brain contain amounts of dopamine quite out of prop ortion to their noradrenaline content and there is suggestive evidence that dopamine has an independent neurohumoral function. The unique situation thus arises that one, or probably two of the precursors of adrenaline have specialized functions of their own. 

Axelrod has shown that, in man, circulating adrenaline and noradrenaline are metabolized predominantly by catechol 0-methyltransferase, approximately 70 per cent of an administered dose being inactivated by 0-methyla-tion and 20 per cent by deamination. Generally similar results were obtained in studies on other mammals. The principal urinary metabolite of the catechol amines is 3-methoxy-4-hydroxymandelic acid. 

The greater importance of 0-methylation in the inactivation of circulating catechol amines is due to the fact that, in the liver, where most of the breakdown takes place, the activity of catechol 0-methyltransferase is very much higher than that of monoamine oxidase. This relationship does not, however, apply to all other organs in the heart, for instance, monoamine oxidase is about five times as active as the 0-methyltransferase, and monoamine oxidase also plays a part in catechol amine metabolism in the brain. 

Catechol-O-methyltransferase is found in cell cytopl2ism and monoamine oxidase occurs in the mitochondria. In the mitochondria in the brain, noradrenaline also occurs in a bound form, probably in two compartments , 

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Normally, dietary tyramine is broken down in the gastrointestinal tract by MAO and is not absorbed. In the presence of MAOI, however, all of its potent sympathomimetic actions are seen. Other side effects of MAOI include excessive CNS stimulation, orthostatic hypotension, weight gain, and in rare cases hepatotoxicity. Because the monoamine oxidase inhibitors exhibit greater toxicity, yet no greater therapeutic response than other, newer agents, clinical use has been markedly curtailed. The primary use for MAOIs is in the treatment of atypical depressions, eg, those associated with increased appetite, phobic anxiety, hypersomnolence, and fatigues, but not melancholia (2). 

Pharmacologically useful isoxazoles (B-82MI41600) include antibacterial sulfonamides (614), (615) and (616), semisynthetic penicillins (617), (618), (619) and (620), semisynthetic cephalosporin (621), anabolic steroid (622), the monoamine oxidase inhibitor (623) (used in psychotherapy), antiinflammatory agent (624) and antitumor agent (625). 

Neurotransmitter transport can be electrogenic if it results in the net translocation of electrical charge (e.g. if more cations than anions are transferred into the cell interior). Moreover, some transporters may direction-ally conduct ions in a manner akin to ligand-gated ion channels this ion flux is not coupled to substrate transport and requires a separate permeation pathway associated with the transporter molecule. In the case of the monoamine transporters (DAT, NET, SERT) the sodium current triggered by amphetamine, a monoamine and psychostimulant (see Fig. 4) is considered responsible for a high internal sodium concentration... 

Cocaine and desipramine inhibit the reuptake of monoamine neurotransmitters whereas amphetamine, which is a phenylalkylamine - similar in structure to the catecholamines, see Fig. 4 - competes for uptake and more importantly, evokes efflux of the monoamine neurotransmitters. All of them exert antidepressant effects. Cocaine and amphetamine are addictive whereas tricyclic antidepressants and their modern successors are not. The corollaty of the addictive properties is interference with DAT activity. Blockade of DAT by cocaine or efflux elicited by amphetamine produces a psychostimulant effect despite the different mechanisms even the experienced individual can hardly discern their actions. Because of the risk associated with inhibiting DAT mediated dopamine clearance the antidepressant effects of psychostimulants has not been exploited. 

Methylphenidate like cocaine largely acts by blocking reuptake of monoamines into the presynaptic terminal. Methylphenidate administration produces an increase in the steady-state (tonic) levels of monoamines within the synaptic cleft. Thus, DAT inhibitors, such as methylphenidate, increase extracellular levels of monoamines. In contrast, they decrease the concentrations of the monoamine metabolites that depend upon monoamine oxidase (MAO), that is, HVA, but not catecholamine-o-methyltransferase (COMT), because reuptake by the transporter is required for the formation of these metabolites. By stimulating presynaptic autoreceptors, methylphenidate induced increase in dopamine transmission can also reduce monoamine synthesis, inhibit monoamine neuron firing and reduce subsequent phasic dopamine release. 

There are numerous transmitter substances. They include the amino acids glutamate, GABA and glycine acetylcholine the monoamines dopamine, noradrenaline and serotonin the neuropeptides ATP and NO. Many neurones use not a single transmitter but two or even more, a phenomenon called cotransmission. Chemical synaptic transmission hence is diversified. The basic steps, however, are similar across all neurones, irrespective of their transmitter, with the exception of NO transmitter production and vesicular storage transmitter release postsynaptic receptor activation and transmitter inactivation. Figure 1 shows an overview. Nitrergic transmission, i.e. transmission by NO, differs from transmission by other transmitters and is not covered in this essay. 

The term pasaon flower is used to denote many of the approximately 400 species of the herb. F saon flower has been used in medicine to treat pain, anxiety, and insomnia. Some herbalists use the herb to treat symptoms of parkinsonism. F saon flower is often used in combination with other herbs , such a valerian, chamomile, and hops, for promoting relaxation, rest and sleep. Although no adverse reactions have been reported, large doses may cause CNS depression. The use of passion flower is contraindicated in pregnancy and in patientstaking the monoamine oxidase inhibitors (MAOIs). Fission flower contains coumarin, and the risk of bleeding may be increased when used in patientstaking warfarin and pasaon flower. 

Levodopa interacts with many different drugs. When levodopa is used with phenytoin, reserpine, and papaverine, there is a decrease in response to levodopa The risk of a hypertensive crisis increases when levodopa is used with the monoamine oxidase inhibitors (see Chap. 31). Foods high in pyridoxine (vitamin B6) or vitamin B6 preparations reverse the effect of levodopa However, when carbidopa is used with levodopa, pyridoxine has no effect on the action of levodopa hi fact, when levodopa and carbidopa are given together, pyridoxine may be prescribed to decrease the adverse effects associated with levodopa... 

The COMT inhibitors should not be administered with the monoamine oxidase (MAO) inhibitors (see Chap. 31) because there is an increased risk of toxicity. If the COMT inhibitors are administered with norepinephrine, dopamine, dobutamine, methyldopa, or epinephrine, there is a risk of increased heart rate, arrhythmias, and excessive blood pressure changes. 

When dextromethorphan is administered with the monoamine oxidase inhibitors (see Chap. 31), patients may experience hypotension, fever, nausea, jerking motions to the leg, and coma... 

Upon mutagenesis of the monoamine oxidase from Aspergillus niger (MAO-N) within several rounds of directed evolution [65], variant biocatalysts were identified with largely expanded substrate acceptance, enabling also the deracemization of tertiary amines incorporating straight-chain and cyclic structural motifs [66]. 

Reaction of 2,4-(lH,3H)-quinazolinedione 250 for 8h at 130°C with two equivalents of N-2-hydroxyethylpiperazine 251 (the alcohol group of which is in-situ protected by silylation), HMDS 2, and catalytic amounts of TSOH H2O, and subsequent transsilylation with boiling methanol, affords 68% of the monoaminated product 252 and methoxytrimethylsilane 13 a. Reaction of 250 with fivefold excess of N-2-hydroxyethylpiperazine 251 in the presence of octamethylcyclotetrasilazane... 

Figure 1.1 The chemical structures of the main neurotransmitters. The relatively simple structure of acetylcholine, the monoamines and the amino acids contrasts with that of the peptides, the simplest of which are the enkephalins which consists of five amino acids substance P has eleven...

Monoamines ean also be found in terminals at both symmetrie and asymmetrie synapses, but this may be partly beeause they eo-exist with the elassieal transmitters glutamate and GABA. The faet that vesieular and neuronal uptake transporters for the monoamines ean be deteeted outside a synapse along with appropriate postsynaptie reeeptors does suggest, however, that some monoamine effeets ean oeeur distant from the synaptie junetion (see Piekel, Nirenberg and Milner 1996, and Chapter 6). 

In between the above two extremes are the monoamines (1-lOnmol/g) which are preformed and stored in terminals but at much lower concentrations than the amino acids and when released are removed primarily by reuptake for re-use, or intraneuronal metabolism to inactive metabolites. Thus the appropriate synaptic organisation, biochemistry and receptor pharmacology of the NTs also varies in keeping with their function. It is often assumed, incorrectly, that the NTs found in the highest concentration are the most potent. In fact the opposite is true. Those like the amino acids while having high affinity for their receptors have low potency while the peptides found at much lower concentration have high potency but low affinity. 

It is generally felt that a substance is more likely to be a NT if it is unevenly distributed in the CNS although if it is widely used it will be widely distributed. Certainly the high concentration (5-10 pmol/g) of dopamine, compared with that of any other monoamine in the striatum or with dopamine in other brain areas, was indicative of its subsequently established role as a NT in that part of the CNS. This does not mean it cannot have an important function in other areas such as the mesolimbic system and parts of the cerebral cortex where it is present in much lower concentrations. In fact the concentration of the monoamines outside the striatum is very much lower than that of the amino acids but since the amino acids may have important biochemical functions that necessitate their widespread distribution, the NT component of any given level of amino acid is difficult to establish. 

If a substance is to be a NT it should be possible to demonstrate appropriate enzymes for its synthesis from a precursor at its site of action, although peptides are transported to their sites of location and action after synthesis in the axon or distal neuronal cell body. The specificity of any enzyme system must also be established, especially if they are to be modified to manipulate the levels of a particular NT, or used as markers for it. Thus choline acetyltransferase (ChAT) may be taken as indicative of ACh and glutamic acid decarboxylase (GAD) of GABA but some of the synthesising enzymes for the monoamines lack such specificity. 

In the monoamine receptors the ligand-binding domain is located within the transmembrane helices. A pocket is formed between TM3, TM5 and TM6 where the agonist binds. A conserved aspartate residue in TM3 (Asp-113 in the /(-adrenoceptor) and a... 

By far the most studied family of the G-protein-coupled receptors are the rhodopsin-like receptors. These are also the largest group of receptors in number as they include receptors not only for the monoamines, nucleotides, neuropeptides and peptide hormones, but they also include the odorant receptors which number several hundreds of related receptors. These receptors have short N-termini, a conserved disulphide bridge between the TM2-TM3 and TM4—TM5 extracellular domains, and variable-length C-termini. In some cases the C-terminus is myristolyated which by tying the C-terminus to the cell membrane generates a fourth intracellular loop. 

While the agonist binding domain is thought to be within the transmembrane domains for the monoamine and nucleotide receptors, neuropeptides are thought to bind close to the membrane surface on the extracellular domains of the receptor. It is still not clear whether non-peptide antagonists bind at the same or a different site on the receptor. 

It is already evident that the turnover rate of a transmitter is only a crude measure of its release rate. Further limitations are that there is appreciable intraneuronal metabolism of some neurotransmitters notably, the monoamines. In such cases, turnover will overestimate release rate. Another problem, again affecting monoamines, is that some of the released neurotransmitter is taken back into the nerve terminals and recycled. This leads to an underestimate of release rate. Despite these drawbacks, studies of turnover rates uncovered some important features of transmitter release. In particular, they provided the first evidence for distinct functional pools of monoamines, acetylcholine and possibly other neurotransmitters a release pool, which could be rapidly mobilised for release, and a storage or reserve pool which had a slower turnover rate. 

Until the recent development of appropriate HPLC techniques capable of detecting pmol amounts (see Flentge et al. 1997) ACh could only be measured chemically by relatively lengthy and expensive procedures (e.g. gas chromotography), which were not always very sensitive, or by bioassays. Although the latter, using muscle preparations that responded to ACh, such as the dorsal muscle of the leech, the rectus abdominus of the frog or certain clam hearts, were reasonably sensitive they were tiresome and not easily mastered. Thus studies on the release and turnover of ACh have not been as easy as for the monoamines. 

Figure 12.1 A comparison of the production, release and fate of classical transmitters such as glutamate and the monoamines and a peptide. (Most neurons use both classical and peptide transmitters)...

There have been few attempts to manipulate the monoamines in AzD and those using selegiline, the MOAb inhibitor, have shown little effect although the 5-HT3 antagonist, ondansetron, may give a slight improvement. 

All these animal models express behavioural deficits that are paralleled by some abnormality in noradrenaline and/or 5-HT function but it is unlikely that the monoamines are the only neurotransmitters to influence these complex behaviours. Nevertheless, the behavioural deficits all respond, with varying degrees of specificity, to established antidepressants and central monoamines appear to have a crucial role in the therapeutic effects of these drugs. For a more detailed review of this subject see Stanford (1995). 

The monoamine hypothesis predicts that drugs which increase the concentration of noradrenaline and/or 5-HT in the synapse should relieve depression. This could be achieved in two ways, as illustrated in Figure 20.1 ... 

The main problems with early, irreversible MAOIs were adverse interactions with other drugs (notably sympathomimetics, such as ephedrine, phenylpropanolamine and tricyclic antidepressants) and the infamous "cheese reaction". The cheese reaction is a consequence of accumulation of the dietary and trace amine, tyramine, in noradrenergic neurons when MAO is inhibited. Tyramine, which is found in cheese and certain other foods (particularly fermented food products and dried meats), is normally metabolised by MAO in the gut wall and liver and so little ever reaches the systemic circulation. MAOIs, by inactivating this enzymic shield, enable tyramine to reach the bloodstream and eventually to be taken up by the monoamine transporters on serotonergic and noradrenergic neurons. Fike amphetamine, tyramine reduces the pH gradient across the vesicle membrane which, in turn, causes the vesicular transporter to fail. Transmitter that leaks out of the vesicles into the neuronal cytosol cannot be metabolised because... 

A logical conclusion from this work was that depression is caused by hyperresponsive )S-adrenoceptors. At first, this might seem to undermine Schildkraut s suggestion that depression is caused by a deficit in noradrenergic transmission. However, proliferation of receptors is the normal response to a deficit in transmitter release and so the opposite change, dowmegulation of jS-adrenoceptors by antidepressants, would follow an increase in the concentration of synaptic noradrenaline. This would be consistent with both their proposed mechanism of action and the monoamine theory for depression. 

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