Monoamine inhibition

The 2-isopropyUiydrazide derivative of 4-CPA is iproclozide [3544-35-2] a pharmaceutical that inhibits monoamine oxidase. 

Monoamine—Oxidase Inhibitors. In the mid-1950s, tuberculosis patients with depression being treated with iproniazid (42) were occasionally reported to become euphoric. This observation led to the discovery of irreversible monoamine—oxidase (MAO) inhibiting properties. Hydrazine and nonhydrazine-related MAO inhibitors were subsequentiy shown to be antidepressants (122). Three other clinically effective irreversible MAO inhibitors have been approved for treatment of major depression phenelzine (43), isocarboxazid (44), and tranylcypromine (45). 

Future Outlook for Antidepressants. Third-generation antidepressants are expected to combine superior efficacy and improved safety, but are unlikely to reduce the onset of therapeutic action in depressed patients (179). Many dmgs in clinical development as antidepressive agents focus on estabhshed properties such as inhibition of serotonin, dopamine, and/or noradrenaline reuptake, agonistic or antagonistic action at various serotonin receptor subtypes, presynaptic tt2-adrenoceptor antagonism, or specific monoamine—oxidase type A inhibition. Examples include buspirone (3) (only... 

Two important pathways for catecholamine metaboHsm are 0-methylation by COMT, which is cytoplasmicaHy localized, and oxidative deamination by the mitochondrial localized enzyme MAO. There are large amounts of MAO in tissues such as the fiver and the heart which are responsible for the removal of most of the circulating monoamine, including some taken in from the diet. Tyramine is found in high concentrations in certain foods such as cheese, and in wine. Normally, this tyramine is deaminated in the fiver. However, if MAO is inhibited, the tyramine may then be converted into octopamine [104-14-37] which may indirecdy cause release of NE from nerve terminals to cause hypertensive crisis. Thus MAO, which is relatively nonspecific, plays an important role in the detoxification of pharmacologically active amines ingested from the diet. 

Hydrazides of isonicotinic acid have been used as antidepressant agents by virtue of their monoamine oxidase-inhibiting activity the pyridine ring has been shown to be replaceable by an... 

Antidepressants are used in the treatment of neuropathic pain and headache. They include the classic tricyclic compounds and are divided into nonselective nor-adrenaline/5-HT reuptake inhibitors (e.g., amitriptyline, imipramine, clomipramine, venlafaxine), preferential noradrenaline reuptake inhibitors (e.g., desipramine, nortriptyline) and selective 5-HT reuptake inhibitors (e.g., citalopram, paroxetine, fluoxetine). The reuptake block leads to a stimulation of endogenous monoaminer-gic pain inhibition in the spinal cord and brain. In addition, tricyclics have NMDA receptor antagonist, endogenous opioid enhancing, Na+ channel blocking, and K+ channel opening effects which can suppress peripheral and central sensitization. Block of cardiac ion channels by tricyclics can lead to life-threatening arrhythmias. The selective 5-HT transporter inhibitors have a different side effect profile and are safer in cases of overdose [3]. 

Acute treatment with nonselective MAO inhibitors (iproniazid, tranylcypromine, phenelzine), as a consequence of inhibiting both forms of the enzyme, increase, brain levels of all monoamines (phenylethylamine, tryptamine, methylhistamine aminergic neurotransmitters (dopamine, noradr enaline, adrenaline and serotonin). By contrast MAO-A inhibitors (clorgyline) increase serotonin and noradrenaline, while MAO-B inhibitors (selegiline, rasagiline) increase brain levels... 

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. 

The patty drug MDMA (3,4-methylene-dioxymetham-phetamine) as well as amphetamine causes efflux of all monoamine neurotransmitters. The effects of MDMA are described as psychostimulant and hallucinogenic and are judged differently from those of amphetamine. This difference is due to the stronger inhibition of SERT by MDMA as compared with amphetamine, which is a more potent dopamine releaser and moreaddictivethan MDMA. 

Non-selective monoamine reuptake inhibitors (NSMRI) are a group of antidepressants, which function by inhibiting the reuptake of noradrenaline... 

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. 

The pharmacology of amphetamine is considerably more complex. It does not only block monoamine reuptake, but also directly inhibits the vesicular monoamine transporter, causing an increase in cytosolic but not vesicular dopamine concentration. This may lead to reverse transport of the amines via the membrane-bound transporters. Further mechanisms of amphetamine action are direct MAO inhibition and indirect release of both dopamine and serotonin in the striatum. 

At low doses, both psychostimulants could theoretically stimulate tonic, extracellular levels of monoamines, and the small increase in steady state levels would produce feedback inhibition of further release by stimulating presynaptic autoreceptors. While this mechanism is clearly an important one for the normal regulation of monoamine neurotransmission, there is no direct evidence to support the notion that the doses used clinically to treat ADHD are low enough to have primarily presynaptic effects. However, alterations in phasic dopamine release could produce net reductions in dopamine release under putatively altered tonic dopaminergic conditions that might occur in ADHD and that might explain the beneficial effects of methylphenidate in ADHD. 

VMATs are irreversibly inhibited by the potent antihypertensive drug reserpine. The depressive effects of reserpine helped to formulate the original monoamine hypothesis of affective disorders. Reseipine also appears to interact with the transporters near the site of substrate recognition. Tetrabenazine, which is used in treatment of movement disorders, inhibits VMAT2 much more potently than VMAT1, consistent with the less hypotensive action of this agent. 

VMATs are not inhibited by drugs such as cocaine, tricyclic antidqnessants and selective serotonin reuptake inhibitors that affect plasma membrane monoamine transport. Amphetamines have relatively selective effects on monoaminergic cells due to selective uptake by plasma membrane monoamine transporters, but their effect appears to be mediated by their ability as weak bases to reduce ApH, the driving force for vesicular monoamine transport that leads to efflux of the vesicular contents into the cytoplasm. 

The TCAs, such as amitriptyline (Elavil) and dox-epin (Sinequan), inhibit reuptake of norepinephrine or serotonin at the presynaptic neuron. Drug classified as MAOIs inhibit the activity of monoamine oxidase a complex enzyme system that is responsible for breaking down amines. This results in an increase in endogenous epinephrine, norepinephrine and serotonin in the nervous system. An increase in these neurohormones results in stimulation of the CNS. The action of the SSRIs is linked to their inhibition of CNS neuronal uptake of serotonin (a CNS neurotransmitter). The increase in serotonin levels is thought to act as a stimulant to reverse depression. 

Monoamine oxidase (MAO) inhibitors. Drugs that produce inhibition of... 

Medvedev AE, Veselovsky AV, Shvedov VI, Tikhonova OV, Moskvitina TA, Fedotova OA, et al. Inhibition of monoamine oxidase by pirlindole analogues 3D-QSAR and CoMFA analysis. / Chem Inf Comput Sci 1998 38 1137-44. Miller JR, Edmondson DE. Structure-activity relationships in the oxidation of para-substituted benzylamine analogues by recombinant human liver monoamine oxidase A. Biochemistry 1999 38 13670-83. 

Binda C, Li M, Hubalek E, Restelli N, Edmondson DE, Mattevi A. Insights into the mode of inhibition of human mitochondrial monoamine oxidase B from high-resolution crystal structures. Proc Natl Acad Sci USA 2003 100 9750-5. 

This description would cover the classical NTs such as glutamate, GABA, ACh, DA, NA, and 5-HT as well as some peptides and ATP. That is irrespective of whether the effect produced by them is basic to the actual process of transmitting an impulse from one neuron to another, as with glutamate and ACh, rapidly inducing inhibition (GABA) or just making the neuron more or less responsive to other inputs (monoamines, peptides). 

One limitation of this method is that the specific activity of the radiolabel is progressively diluted as the radiolabelled transmitter is released from neurons and replaced by that derived from unlabelled substrate. This method also assumes that there is no compartmentalisation of the terminal stores, yet there is ample evidence that newly synthesised acetylcholine and monoamines are preferentially released. An alternative approach is to monitor the rate at which the store of neurotransmitter is depleted after inhibition of its synthesis (Fig. 4.1). However, the rate of release of some neurotransmitters (e.g. 5-HT) is partly governed by their rate of synthesis and blocking synthesis blunts release. 

Neff, NH and Costa, E (1966) The influence of monoamine oxidase inhibition on catecholamine synthesis. Life Sci. 5 951-959. 

In other respects the storage of 5-HT resembles that of noradrenaline with its uptake by vesicles resting on energy-dependent, vesicular monoamine transporters (VMATs) (see Chapter 8). Functional disruption of this transporter, either through competitive inhibition (e.g. by methylenedioxymethamphetamine (MDMA, Ecstasy )) or dissipation... 

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