A monoamine reuptake inhibitor (MRI) [1] is a drug that acts as a reuptake inhibitor of one or more of the three major monoamine neurotransmitters serotonin, norepinephrine, and dopamine by blocking the action of one or more of the respective monoamine transporters (MATs), which include the serotonin transporter (SERT), norepinephrine transporter (NET), and dopamine transporter (DAT). This in turn results in an increase in the synaptic concentrations of one or more of these neurotransmitters and therefore an increase in monoaminergic neurotransmission.
The majority of currently approved antidepressants act predominantly or exclusively as MRIs, including the selective serotonin reuptake inhibitors (SSRIs), serotonin–norepinephrine reuptake inhibitors (SNRIs), and almost all of the tricyclic antidepressants (TCAs). [2] Many psychostimulants used either in the treatment of ADHD or as appetite suppressants in the treatment of obesity also behave as MRIs, although notably amphetamine (and methamphetamine), which do act to some extent as monoamine reuptake inhibitors, exerts their effects primarily as releasing agents. [3] [4] Additionally, psychostimulants acting as MRIs that affect dopamine such as cocaine and methylphenidate are often abused as recreational drugs. [5] As a result, many of them have become controlled substances, which in turn has resulted in the clandestine synthesis of a vast array of designer drugs for the purpose of bypassing drug laws; a prime example of such is the mixed monoamine reuptake inhibitor and releasing agent mephedrone. [6]
There are a variety of different kinds of MRIs, of which include the following:
Compound | SERT | NET | DAT | Type | Class | |
---|---|---|---|---|---|---|
Amfonelic acid | ND | ND | 207 | DRI | Stimulant | |
Amineptine* [8] [9] | >100,000 (rat) | 10,000 (rat) | 1,000–1,400 (rat) | DRI | Stimulant | |
Amitriptyline | 4.30 | 35 | 3,250 | SNRI | TCA | |
Amoxapine | 58 | 16.0 | 4,310 | SNRI | TeCA | |
Amphetamine | >100,000 | ND | ND | NDRA | Stimulant | |
D-Amphetamine | >100,000 | 530 | 2,900 | NDRA | Stimulant | |
L-Amphetamine | >100,000 | ND | ND | NRA | Stimulant | |
Atomoxetine | 77 | 5 | 1,451 | NRI | Stimulant | |
Bupropion | 9,100 | 52,000 | 520 | NDRI | Stimulant | |
Butriptyline | 1,360 | 5,100 | 3,940 | N/A (IA) | TCA | |
Chlorphenamine | 15.2 | 1,440 | 1,060 | SRI | Antihistamine | |
Citalopram | 1.16 | 4,070 | 28,100 | SRI | SSRI | |
Escitalopram [10] | 1.1 | 7,841 | 27,410 | SRI | SSRI | |
Clomipramine | 0.28 | 38 | 2,190 | SNRI | TCA | |
Cocaethylene [11] | 3,878 | >10,000 | 555 | SDRI | Stimulant | |
Cocaine [11] | 304 | 779 | 478 | SNDRI | Stimulant | |
Cocaine [12] | 313±17 (IC50) | 292±34 (IC50) | 211±19 (IC50) | SNDRI | Stimulant | |
Desipramine | 17.6 | 0.83 | 3,190 | SNRI | TCA | |
Desmethylcitalopram | 3.6 | 1,820 | 18,300 | SRI | SSRI | |
Desmethylsertraline | 3.0 | 390 | 129 | SRI | SSRI | |
Desmethylsibutramine [13] | 15 | 20 | 49 | SNDRI | SNRI | |
(R)-Desmethylsibutramine | 44 | 4 | 12 | SNDRI | SNRI | |
(S)-Desmethylsibutramine | 9,200 | 870 | 180 | SNDRI | SNRI | |
Desoxypipradrol [14] | 53,700 | 550 | 50 | NDRI | Stimulant | |
Desvenlafaxine* [15] | 47 | 531 | ND | SNRI | SNRI | |
Didesmethylsibutramine [13] | 20 | 15 | 45 | SNDRI | SNRI | |
(R)-Didesmethylsibutramine | 140 | 13 | 8.9 | SNDRI | SNRI | |
(S)-Didesmethylsibutramine | 4,300 | 62 | 12 | SNDRI | SNRI | |
Diphenhydramine | 3,800 | 960 | 2,200 | N/A (IA) | Antihistamine | |
Dosulepin (dothiepin) | 8.6 | 46 | 5,310 | SNRI | TCA | |
Doxepin | 68 | 29.5 | 12,100 | SNRI | TCA | |
Duloxetine* [16] | 3.7 | 20 | 439 | SNRI | SNRI | |
Etoperidone | 890 | 20,000 | 52,000 | SRI | SARI | |
Femoxetine | 11.0 | 760 | 2,050 | SRI | SSRI | |
Fluoxetine | 0.81 | 240 | 3,600 | SRI | SSRI | |
Fluvoxamine | 2.2 | 1,300 | 9,200 | SRI | SSRI | |
GBR-12935 [11] | 289 | 277 | 4.90 | DRI | Stimulant | |
Hydroxybupropion [17] | ND | 1.7 (IC50) | >10 (IC50) | NDRI | Stimulant | |
Imipramine | 1.40 | 37 | 8,500 | SNRI | TCA | |
Indatraline [11] | 3.10 | 12.6 | 1.90 | SNDRI | Stimulant | |
Iprindole | 1,620 | 1,262 | 6,530 | N/A (IA) | TCA | |
Lofepramine | 70 | 5.4 | 18,000 | SNRI | TCA | |
Maprotiline | 5,800 | 11.1 | 1,000 | NRI | TeCA | |
Mazindol | 39 | 0.45 | 8.1 | NDRI | Stimulant | |
MDPV [18] | 3,349 | 26 | 4.1 | NDRI | Stimulant | |
Methamphetamine | >100,000 | ND | ND | NDRA | Stimulant | |
D-Methamphetamine | >100,000 | 660 | 2,800 | NDRA | Stimulant | |
L-Methamphetamine | >100,000 | ND | ND | NRA | Stimulant | |
Methylphenidate | >10,000 | 788 | 121 | NDRI | Stimulant | |
D-Methylphenidate | >10,000 | 206 | 161 | NDRI | Stimulant | |
L-Methylphenidate | >6,700 | >10,000 | 2,250 | NDRI | Stimulant | |
Mianserin | 4,000 | 71 | 9,400 | NRI | TeCA | |
Milnacipran* [16] | 151 | 68 | >100,000 | SNRI | SNRI | |
Levomilnacipran* [19] | 19.0 | 10.5 | >100,000 | SNRI | SNRI | |
Mirtazapine | >100,000 | 4,600 | >100,000 | N/A (IA) | TeCA | |
Modafinil* [20] | >50,000 | 136,000 | 4,043 | DRI | Stimulant | |
Nefazodone | 200 | 360 | 360 | SNDRI | SARI | |
Nefopam [21] | 29 | 33 | 531 | SNDRI | Analgesic | |
Nisoxetine [11] | 427 | 2.3 | 1,235 | NRI | Stimulant | |
Nomifensine | 1,010 | 15.6 | 56 | NDRI | Stimulant | |
Norfluoxetine | 1.47 | 1,426 | 420 | SRI | SSRI | |
Nortriptyline | 18 | 4.37 | 1,140 | SNRI | TCA | |
Oxaprotiline | 3,900 | 4.9 | 4,340 | NRI | TeCA | |
Paroxetine | 0.13 | 40 | 490 | SRI | SSRI | |
Protriptyline | 19.6 | 1.41 | 2,100 | SNRI | TCA | |
Reboxetine [22] | 129 | 1.1 | >10,000 | NRI | Stimulant | |
Sertraline | 0.29 | 420 | 25 | SRI | SSRI | |
Sibutramine [13] | 298–2,800 | 350–5,451 | 943–1,200 | SNDRI | SNRI | |
Trazodone | 160 | 8,500 | 7,400 | SRI | SARI | |
Trimipramine | 149 | 2,450 | 3,780 | SRI | TCA | |
Vanoxerine [11] | 73.2 | 79.2 | 4.3 | DRI | Stimulant | |
Venlafaxine* [16] | 145 | 1,420 | 3,070 | SNRI | SNRI | |
Vilazodone* [23] | 0.2 | ~60 | ND | SRI | SMS | |
Viloxazine | 17,300 | 155 | >100,000 | NRI | Stimulant | |
Vortioxetine* [24] | 5.4 | 890 (rat) | 140 (rat) | SRI | SMS | |
Zimelidine | 152 | 9,400 | 11,700 | SRI | SSRI | |
Values are Ki (nM) or, in some cases, when denoted by an asterisk (*), IC50 (nM). The smaller the value, the more strongly the drug binds to or inhibits the transporter. |
An anxiolytic is a medication or other intervention that reduces anxiety. This effect is in contrast to anxiogenic agents which increase anxiety. Anxiolytic medications are used for the treatment of anxiety disorders and their related psychological and physical symptoms.
Monoamine transporters (MATs) are proteins that function as integral plasma-membrane transporters to regulate concentrations of extracellular monoamine neurotransmitters. The three major classes are serotonin transporters (SERTs), dopamine transporters (DATs), and norepinephrine transporters (NETs) and are responsible for the reuptake of their associated amine neurotransmitters. MATs are located just outside the synaptic cleft (peri-synaptically), transporting monoamine transmitter overflow from the synaptic cleft back to the cytoplasm of the pre-synaptic neuron. MAT regulation generally occurs through protein phosphorylation and post-translational modification. Due to their significance in neuronal signaling, MATs are commonly associated with drugs used to treat mental disorders as well as recreational drugs. Compounds targeting MATs range from medications such as the wide variety of tricyclic antidepressants, selective serotonin reuptake inhibitors such as fluoxetine (Prozac) to stimulant medications such as methylphenidate (Ritalin) and amphetamine in its many forms and derivatives methamphetamine (Desoxyn) and lisdexamfetamine (Vyvanse). Furthermore, drugs such as MDMA and natural alkaloids such as cocaine exert their effects in part by their interaction with MATs, by blocking the transporters from mopping up dopamine, serotonin, and other neurotransmitters from the synapse.
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The dopamine transporter is a membrane-spanning protein coded for in humans by the SLC6A3 gene, that pumps the neurotransmitter dopamine out of the synaptic cleft back into cytosol. In the cytosol, other transporters sequester the dopamine into vesicles for storage and later release. Dopamine reuptake via DAT provides the primary mechanism through which dopamine is cleared from synapses, although there may be an exception in the prefrontal cortex, where evidence points to a possibly larger role of the norepinephrine transporter.
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A spontaneous orgasm, or spontaneous ejaculation when it occurs in males, is an orgasm which occurs spontaneously and involuntarily without sexual stimulation. Nocturnal emissions may be considered a normal/physiological form of spontaneous orgasm. Pathological spontaneous orgasms can be experienced as pleasurable, non-pleasurable, or unpleasant, and can be distressing. Causes of pathological spontaneous orgasms include spinal cord lesions, psychological causes, rabies, and medications. Some cases may have no identifiable cause. Spontaneous orgasms may have no trigger or may be triggered by various non-sexual circumstances. They may occur in both males and females. Treatment of spontaneous orgasms include psychotherapy, selective serotonin reuptake inhibitors (SSRIs), the alpha-1 blocker silodosin, and anxiolytics.
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Reuptake inhibitors (RIs) are a type of reuptake modulators. It is a drug that inhibits the plasmalemmal transporter-mediated reuptake of a neurotransmitter from the synapse into the pre-synaptic neuron. This leads to an increase in extracellular concentrations of the neurotransmitter and an increase in neurotransmission. Various drugs exert their psychological and physiological effects through reuptake inhibition, including many antidepressants and psychostimulants.
A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of a monoamine neurotransmitter from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitter. Many drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters, e.g., trace amines, many substituted amphetamines, and related compounds.
A dopamine releasing agent (DRA) is a type of drug which induces the release of dopamine in the body and/or brain. No selective and robust DRAs are currently known. On the other hand, many releasing agents of both dopamine and norepinephrine and of serotonin, norepinephrine, and dopamine are known. Serotonin–dopamine releasing agents (SDRAs), for instance 5-chloro-αMT, are much more rare and are not selective for dopamine release but have also been developed. Examples of major NDRAs include the psychostimulants amphetamine and methamphetamine, while an example of an SNDRA is the entactogen methylenedioxymethamphetamine (MDMA). These drugs are frequently used for recreational purposes and encountered as drugs of abuse. Selective DRAs, as well as NDRAs, have medical applications in the treatment of attention deficit hyperactivity disorder (ADHD).
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Serotonin antagonist and reuptake inhibitors (SARIs) are a class of drugs used mainly as antidepressants, but also as anxiolytics and hypnotics. They act by antagonizing serotonin receptors such as 5-HT2A and inhibiting the reuptake of serotonin, norepinephrine, and/or dopamine. Additionally, most also antagonize α1-adrenergic receptors. The majority of the currently marketed SARIs belong to the phenylpiperazine class of compounds.
RTI-83 is a phenyltropane derivative which represents a rare example of an SDRI or serotonin-dopamine reuptake inhibitor, a drug which inhibits the reuptake of the neurotransmitters serotonin and dopamine, while having little or no effect on the reuptake of the related neurotransmitter noradrenaline. With a binding affinity (Ki) of 55 nM at DAT and 28.4 nM at SERT but only 4030 nM at NET, RTI-83 has reasonable selectivity for DAT/SERT over NET
A serotonin–dopamine reuptake inhibitor (SDRI) is a type of drug which acts as a reuptake inhibitor of the monoamine neurotransmitters serotonin and dopamine by blocking the actions of the serotonin transporter (SERT) and dopamine transporter (DAT), respectively. This in turn leads to increased extracellular concentrations of serotonin and dopamine, and, therefore, an increase in serotonergic and dopaminergic neurotransmission.
The pharmacology of antidepressants is not entirely clear. The earliest and probably most widely accepted scientific theory of antidepressant action is the monoamine hypothesis, which states that depression is due to an imbalance of the monoamine neurotransmitters. It was originally proposed based on the observation that certain hydrazine anti-tuberculosis agents produce antidepressant effects, which was later linked to their inhibitory effects on monoamine oxidase, the enzyme that catalyses the breakdown of the monoamine neurotransmitters. All currently marketed antidepressants have the monoamine hypothesis as their theoretical basis, with the possible exception of agomelatine which acts on a dual melatonergic-serotonergic pathway. Despite the success of the monoamine hypothesis it has a number of limitations: for one, all monoaminergic antidepressants have a delayed onset of action of at least a week; and secondly, there are a sizeable portion (>40%) of depressed patients that do not adequately respond to monoaminergic antidepressants. Further evidence to the contrary of the monoamine hypothesis are the recent findings that a single intravenous infusion with ketamine, an antagonist of the NMDA receptor — a type of glutamate receptor — produces rapid, robust and sustained antidepressant effects. Monoamine precursor depletion also fails to alter mood. To overcome these flaws with the monoamine hypothesis a number of alternative hypotheses have been proposed, including the glutamate, neurogenic, epigenetic, cortisol hypersecretion and inflammatory hypotheses. Another hypothesis that has been proposed which would explain the delay is the hypothesis that monoamines don't directly influence mood, but influence emotional perception biases.