Monoamine oxidase inhibitors mechanism

Two rather broad structural classes account for the large majority of drugs that have proven useful in the clinic for treating depression. Each of these has associated with it some clearly recognized side effects the monoamine oxidase inhibitors, most commonly derivatives of hydrazine, tend to have undesirable effects on blood pressure the tricyclic compounds on the other hand may cause undesirable changes in the heart. Considerable effort has thus been expended toward the development of antidepressants that fall outside those structural classes. An unstated assumption in this work is the belief that very different structures will be associated with a novel mechanism of action and a different set of ancillary activities. One such compound, trazodone... 

Listing of antidepressants grouped by principal mechanism of action in the synapse. Abbreviations MAOI—irreversible = irreversible monoamine oxidase inhibitor MAOI—reversible = reversible monoamine oxidase inhibitor NDRl = norepinephrine/ dopamine reuptake inhibitor NRI = norepinephrine reuptake inhibitor NSRl = norepinephrine/serotonin reuptake inhibitor NSSA = norepinephrine/specific serotonin agonist SRI = serotonin reuptake inhibitor SRl/serotonin-2 blocker = serotonin reuptake inhibitor and serotonin-2 receptor antagonist. 

The process of oxidative deamination is the most important mechanism whereby all monoamines are inactivated (i.e. the catecholamines, 5-HT and the numerous trace amines such as phenylethylamine and tryptamine). Monoamine oxidase occurs in virtually all tissues, where it appears to be bound to the outer mitochondrial membrane. Whereas there are several specific and therapeutically useful monoamine oxidase inhibitors, inhibitors of catechol-O-methyltransferase have found little application. This is mainly due to the fact that at most only 10% of the monoamines released from the nerve terminal are catabolized by this enzyme. The main pathways involved in the catabolism of the catecholamines are shown in Figure 2.16. 

Serendipity has played a major role in the discovery of most classes of psychotropic drugs. For example, the observation that the first antidepressants, the tricyclic antidepressants and the monoamine oxidase inhibitors, impeded the reuptake of biogenic amines into brain slices, or inhibited their metabolism, following their acute administration to rats, provided the experimenter with a mechanism that could be easily investigated in vitro. Such methods led to the development of numerous antidepressants that differed in their potency, and to some extent in their side effects (for example, the selective serotonin reuptake inhibitors) but did little to further the development of novel antidepressants showing greater therapeutic efficacy. The accidental discovery of atypical antidepressants such as mianserin led to the broadening of the basis of the animal models... 

The precise mechanism by which the first-generation tricyclic antidepressants, monoamine oxidase inhibitors, and the newer-generation antidepressants exert their effects is uncertain. However, it is clear that antidepressants exert their effects at both pre- and postsynaptic receptor sites (Figure 43.3 and Figure 43.4). 

Antidepressants were first introduced into the market in the 1950s with the serendipitous discovery of the antidepressant effect of two drugs initially evaluated for other medical uses Iproniazide, a monoamine oxidase inhibitor (MAOI), and Imipramine, a tricyclic antidepressant (TCA). Since then, a whole new generation of chemically and pharmacologically unrelated compounds have been introduced, which appear to be safer and better tolerated due to a more specific mechanism of action. These include selective serotonin reuptake inhibitors (SSRIs), serotonin and... 

Polasek TM, Elliot DJ, Somogyi AA, et al. An evaluation of potential mechanism-based inactivation of human drug metabolizing cytochromes P450 by monoamine oxidase inhibitors, including isoniazid. Br J Clin Pharmacol 2006 61(5) 570-584. 

A mechanism of irreversible toxicity of/wnz-chloroamphetamine (PC A, 120a) to serotonergic neurons has recently been proposed. According to this proposal, PCA induces 5HT release from serotonergic neurons. The ability of PCA also to function as a monoamine oxidase inhibitor leads to an accumulation of extraneuronal 5HT. Nonenzymatic oxidation produces the neurotoxin, 6-hydroxy-5HT, which is taken up into the neuron where cross-linking to macromolecular structures leads to neuronal destruction221,222. 

MAO inhibitors can cause bradycardia. A report of two cases of interactions of monoamine oxidase inhibitors with beta-blockers (nadolol and metoprolol) is of interest, and several possible mechanisms were discussed (14). 

Some antidepressants—specifically, tricyclics like imipramine (trade name Tofranil) and amitryptiline (trade name Elavil)—are thought to exert their antidepressant effect through inhibition of a reuptake mechanism that sucks back the neurotransmitters from the synapse into the neuron for storage and future use, a process mentioned in Chapter 1. The resulting net effect is an increase of these molecules at the synapse and thus a more robust neurotransmission. A different category of antidepressants—monoamine oxidase inhibitors (MAOIs)—display a different mechanism of action but with the same net effect of increasing norepinephrine and serotonin neurotransmission they inhibit the metabolism (breakdown) of the molecules stored in the neurons, thus creating more abundant supplies for neurotransmission. 

Knoll J (1969) The theory of active reflexes. An analysis of some fundamental mechanisms of higher nervous activity. Hungarian Academy of Sciences, Budapest Hafner Publishing Company, New York Knoll J (1976) Analysis of the pharmacological effects of selective monoamine oxidase inhibitors. In Wolstenholme GES, Knight J (eds) Monoamine oxidase and its inhibition. Ciba foundation Symposium 39 (new series), Elsevier, Amsterdam, pp 131-161... 

Previous reports that some TCA and non-TCA, like Iprindol and mianserin, were inhibitors of histamine-sensitive adenylate cyclase from mammalian brain Iri vitro were confirmed for a wide range of antidepressants.22 xhe effect appeared to be mediated by H2-receptors but neither monoamine oxidase inhibitors (MAOI) nor selective serotonin (5-HT) uptake inhibitors were very effective. However, TCA also blocked histamine-stimulated cyclic GMP formation in cultured nerve cells by a mechanism which involved Hx-receptors.23 The relevance of these findings, and indeed of the role of histamine in the central nervous sytem, has still to be defined. 

Drug interactions with sulfonylureas and biguanides are common non-steroidal antiinflammatory drugs, warfarin, alcohol, monoamine oxidase inhibitors, some uricosurics, some antibacterials and some antifungals can interact with them. All increase the risk of hypoglycaemia. The mechanism is probably competition for metabolizing enzymes or displacement from plasma protein binding sites. 

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