A monoamine releasing agent ( MRA), or simply monoamine releaser, is a drug that induces the synapse of one or more monoamine neurotransmitters from the synapse into the synapse, leading to an increase in the extracellular of the and hence enhanced cell signaling by those neurotransmitters. The monoamine neurotransmitters include serotonin, norepinephrine, and dopamine; MRAs can induce the release of one or more of these neurotransmitters.
MRAs work by reversing the direction of the monoamine transporters (MATs), including the serotonin transporter (SERT), norepinephrine transporter (NET), and/or dopamine transporter (DAT), causing them to promote efflux of synaptic vesicle monoamine neurotransmitter rather than reuptake of synapse monoamine neurotransmitter. Many, but not all MRAs, also reverse the direction of the vesicular monoamine transporter 2 (VMAT2), thereby additionally resulting in efflux of synaptic vesicle monoamine neurotransmitter into the cytoplasm.
A variety of different classes of drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters. These include and appetite suppressants acting as dopamine and norepinephrine releasers like amphetamine, methamphetamine, and phentermine; sympathomimetic agents acting as norepinephrine releasers like ephedrine and pseudoephedrine; non-stimulant appetite suppressants acting as serotonin releasers like fenfluramine and chlorphentermine; and acting as releasers of serotonin and/or other monoamines like MDMA. like phenethylamine and tryptamine, as well as the monoamine neurotransmitters themselves, are endogenous MRAs. It is thought that monoamine release by endogenous mediators may play some physiological regulatory role.
MRAs must be distinguished from monoamine reuptake inhibitors (MRIs) and monoaminergic activity enhancers (MAEs), which similarly increase synaptic monoamine neurotransmitter levels and enhance monoaminergic signaling but work via distinct mechanisms.
The differences in selectivity of MRAs is the result of different affinities as substrates for the monoamine transporters, and thus differing ability to gain access into monoaminergic neurons and induce monoamine neurotransmitter release.
As of present, no selective DRAs are known. This is because it has proven extremely difficult to separate DAT affinity from NET affinity and retain releasing efficacy at the same time. Several selective SDRAs, including tryptamine, (+)-α-ethyltryptamine (αET), 5-chloro-αMT, and 5-fluoro-αET, are known. However, besides their serotonin release, many of these compounds additionally act as non-selective serotonin receptor agonists, including of the serotonin 5-HT2A receptor (with accompanying hallucinogenic effects), and some of them are known to act as monoamine oxidase inhibitors.
Selective SRAs such as chlorphentermine have been described as dysphoria and lethargy. Less selective SRAs that also stimulate the release of dopamine, such as methylenedioxymethamphetamine (MDMA), are described as more pleasure, more reliably elevating mood and increasing mental energy and sociability. SRAs have been used as appetite suppressants and as . They have also been proposed for use as more effective and than selective serotonin reuptake inhibitors (SSRIs) because they can produce much larger increases in serotonin levels in comparison.
DRAs, usually non-selective for both norepinephrine and dopamine, have psychostimulant effects, causing an increase in energy, motivation, elevated mood, and euphoria. Other variables can significantly affect the subjective effects, such as infusion rate (increasing positive effects of DRAs) and psychological expectancy effects. They are used in the treatment of attention deficit hyperactivity disorder (ADHD), as appetite suppressants, wakefulness-promoting agents, to improve motivation, and are drugs of recreational use and drug misuse.
Selective NRAs are minimally psychoactive, but as demonstrated by ephedrine, may be distinguished from placebo, and may trends towards drug liking. They may also be performance-enhancing, in contrast to reboxetine which is solely a norepinephrine reuptake inhibitor. In addition to their central effects, NRAs produce peripheral sympathomimetic effects like increased heart rate, vasopressor, and force of heart contractions. They are used as nasal decongestants and , but have also seen use as wakefulness-promoting agents, appetite suppressants, and antihypotensive agents. They have additionally seen use as performance-enhancing drugs, for instance in sports.
The mechanisms by which MRAs induce MAT reverse transport and efflux are complex and incompletely understood. The process appears to depend on a number of intracellular changes, including sodium ion (Na+) and calcium ion (Ca2+) elevation, protein kinase C (PKC) activation, and Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIα) activation, among others.
The trace amine-associated receptor 1 (TAAR1) is a receptor for like β-phenethylamine and tryptamine, as well as for monoamine neurotransmitters like dopamine and serotonin, and is a known target of many MRAs, such as amphetamine and methamphetamine. The TAAR1 is a largely intracellular receptor expressed both in presynaptic and postsynaptic monoaminergic neurons and appears to be extensively co-localized with MATs in the brain. Some in-vitro studies have found that TAAR1 agonism by MAT substrates like MRAs can produce PKC activation and thereby induce MAT reverse transport and monoamine efflux. As such, TAAR1 agonism, coupled with MAT substrate activity, could mediate or contribute to the monoamine release of MRAs. However, findings in this area are conflicting, with other studies unable to replicate the results. In addition, MRAs can still induce monoamine efflux in the absence of TAAR1 in vitro, well-known MRAs like amphetamine and methamphetamine exhibit only low-potency human TAAR1 agonism that is of uncertain general significance in humans, many other MRAs are inactive as TAAR1 agonists in humans, the monoamine release and behavioral effects of amphetamines are not only preserved but substantially augmented in TAAR1 knockout mice, and the monoamine release and behavioral effects of amphetamines are strongly reduced or abolished in mice with TAAR1 overexpression. Besides induction of monoamine release, TAAR1 agonism, as well as other mechanisms, may mediate MAT internalization. MAT internalization may limit the capacity of MRAs to induce MAT reverse transport and monoamine efflux. TAAR1 signaling also activates G protein-coupled inwardly rectifying potassium channels (GIRKs) and thereby robustly inhibits the action potential of brain monoaminergic neurons and suppresses exocytotic monoamine release. Due to the preceding mechanisms, potent TAAR1 agonism by MRAs that possess this action may actually auto-inhibit and constrain their monoaminergic effects.
Although induction of MAT reverse transport and consequent monoamine efflux is the leading theory of how MRAs act, an alternative and more recent theory has proposed that amphetamine, at therapeutic doses, may not actually act by inducing DAT reverse transport and dopamine efflux, but instead by augmenting exocytotic dopamine release and hence by enhancing phasic rather than tonic dopaminergic signaling. According to this model, DAT reverse transport may only be relevant at supratherapeutic doses and may be more associated with toxicity, for instance induction of psychosis. It is unclear how amphetamine might act to enhance exocytotic dopamine release, and more research is needed to evaluate this theory.
Aside from the mechanisms mediating the monoamine release of MRAs, other targets of some MRAs, such as the intracellular sigma receptor σ1 receptor, have been found to inhibit MRA-induced monoamine efflux via interactions with the MATs. Conversely, activation of the sigma σ2 receptor has been found to potentiate amphetamine-induced dopamine efflux. The mechanism mediating this effect is unknown, but it has been postulated that it may be due to elevation of intracellular calcium and consequent downstream effects.
The enhancement of monoaminergic signaling by MRAs also differs from that with MRIs. Because MRIs block monoamine neurotransmitter reuptake and consequent inactivation following action potentials and exocytotic release, they preferentially augment phasic monoaminergic signaling rather than tonic signaling. In addition, inhibitory presynaptic and somatodendritic monoamine , including serotonin 5-HT1A and 5-HT1B autoreceptors, dopamine D2 and D3 autoreceptors, and α2-adrenergic autoreceptors, respond to elevated synaptic monoamine neurotransmitter levels by inhibiting presynaptic monoaminergic neuron firing rates, and this substantially limits the effects of MRIs. In contrast, MRAs do not depend on action potentials to induce monoamine release, and thus are able to largely bypass the negative feedback mediated by autoreceptors. Relatedly, MRAs can induce far greater maximal increases in monoamine neurotransmitter levels than MRIs. For instance, MRIs can achieve maximal elevations in brain monoamine levels of about 5- to 10-fold in animals, whereas MRAs can produce elevations of as much as 10- to 50-fold, with no clear ceiling limit. Since MRAs depend on uptake by the MATs to induce monoamine release, their mediation of monoamine release and consequent effects can be blocked by MRIs.
Release of monoamine neurotransmitters by themselves, for instance in the cases of serotonin, norepinephrine, and dopamine, has been referred to as "self-release". The physiological significance of the findings that monoamine neurotransmitters can act as releasing agents of themselves is unclear. However, it could imply that efflux is a common neurotransmitter regulatory mechanism that can be induced by any transporter substrate.
It is possible that monoamine neurotransmitter self-release could be a protective mechanism. It is notable in this regard that intracellular synaptic vesicle or dopamine is neurotoxicity to and that the vesicular monoamine transporter 2 (VMAT2) is neuroprotective by packaging this dopamine into . Along similar lines, MRAs induce the efflux of non-vesicular monoamine neurotransmitter and thereby move cytoplasmic neurotransmitter into the extracellular space. On the other hand, many MRAs but not all also act as VMAT2 inhibitors and reversers, and hence concomitantly induce the release of vesicular monoamine neurotransmitters like dopamine into the cytoplasm. Induction of VMAT2 efflux by MRAs appears to be related to their monoaminergic neurotoxicity.
Analogues of MDMA with retained MRA activity but reduced or no serotonergic neurotoxicity, like 5,6-methylenedioxy-2-aminoindane (MDAI) and 5-iodo-2-aminoindane (5-IAI), have been developed. Certain drugs have been found to block the neurotoxicity of MRAs in animals. For instance, the selective MAO-B inhibitor selegiline has been found to prevent the serotonergic neurotoxicity of MDMA in rodents.
Another method of measuring monoamine release involves the use of human HEK293 cells transfection with and gene expression monoamine transporters. However, MRAs show differing and much lower potencies in this system compared to rat brain synaptosomes, and it is much less frequently employed. The reasons for these differences are not entirely clear, but may be related to species differences, differences in release assay methods, and/or absence of important neuronal membrane proteins in non-neuronal HEK293 cells.
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Notes: (1) The smaller the value, the more potently the substance releases the neurotransmitter. (2) These values were from assays conducted using rat brain . Values from other methods of quantifying monoamine release, such as HEK293 cells transfection with monoamine transporters, are not fully analogous to neuronal cells and result in much different and lower potencies. As a result, they are not included in this table. |
In addition to the potencies of MRAs in terms of their MRA activity, data on the affinities (Ki) of various MRAs for the monoamine transporters (MATs) and their potencies () in acting as monoamine reuptake inhibitors (MRIs) have been published. Activities of MRAs at the vesicular monoamine transporter 2 (VMAT2) have been published as well.
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