A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of one or more monoamine neurotransmitters from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitters and hence enhanced signaling by those neurotransmitters. [2] [3] [4] [1] [5] The monoamine neurotransmitters include serotonin, norepinephrine, and dopamine; monoamine releasing agents can induce the release of one or more of these neurotransmitters. [2] [3] [4] [1] [5]
Monoamine releasing agents 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 non-vesicular cytoplasmic monoamine neurotransmitter rather than reuptake of synaptic monoamine neurotransmitter. [5] [6] [1] [7] Many, but not all monoamine releasing agents, also reverse the direction of the vesicular monoamine transporter 2 (VMAT2), thereby additionally resulting in efflux of vesicular monoamine neurotransmitter into the cytoplasm. [5]
A variety of different classes of drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters. [2] [3] These include psychostimulants 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 entactogens acting as releasers of serotonin and/or other monoamines like MDMA. [2] [3] Trace amines like phenethylamine and tryptamine, as well as the monoamine neurotransmitters themselves, are endogenous monoamine releasing agents. [2] [3] [4] It is thought that monoamine release by endogenous mediators may play some physiological regulatory role. [4]
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. [5] [1] [8] [9]
MRAs can be classified by the monoamines they mainly release, although these drugs lie on a spectrum: [2] [3] [4] [5]
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. [10] Several selective SDRAs, including tryptamine, (+)-α-ethyltryptamine (αET), 5-chloro-αMT, and 5-fluoro-αET, are known. [11] [12] However, besides their serotonin release, 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. [11] [12]
MRAs can produce varying effects depending on their selectivity for inducing the release of different monoamine neurotransmitters. [3]
Selective SRAs such as chlorphentermine have been described as dysphoric and lethargic. [13] [14] Less selective SRAs that also stimulate the release of dopamine, such as methylenedioxymethamphetamine (MDMA), are described as more pleasant, more reliably elevating mood and increasing energy and sociability. [15] SRAs have been used as appetite suppressants and as entactogens. They have also been proposed for use as more effective antidepressants and anxiolytics than selective serotonin reuptake inhibitors (SSRIs) because they can produce much larger increases in serotonin levels in comparison. [16]
DRAs, usually non-selective for both norepinephrine and dopamine, have psychostimulant effects, causing an increase in energy, motivation, elevated mood, and euphoria. [17] Other variables can significantly affect the subjective effects, such as infusion rate (increasing positive effects of DRAs) and psychological expectancy effects. [18] 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 misuse.
Selective NRAs are minimally psychoactive, but as demonstrated by ephedrine, may be distinguished from placebo, and may trends towards liking. [19] They may also be performance-enhancing, [20] in contrast to reboxetine which is solely a norepinephrine reuptake inhibitor. [21] [22] In addition to their central effects, NRAs produce peripheral sympathomimetic effects like increased heart rate, blood pressure, and force of heart contractions. They are used as nasal decongestants and bronchodilators, 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.
MRAs cause the release of monoamine neurotransmitters by various complex mechanisms of action. They may enter the presynaptic neuron primarily via plasma membrane transporters, such as the dopamine transporter (DAT), norepinephrine transporter (NET), and serotonin transporter (SERT). Some, such as exogenous phenethylamine, amphetamine, and methamphetamine, can also diffuse directly across the cell membrane to varying degrees. Once inside the presynaptic neuron, they may inhibit the reuptake of monoamine neurotransmitters through vesicular monoamine transporter 2 (VMAT2) and release the neurotransmitters stores of synaptic vesicles into the cytoplasm by inducing reverse transport at VMAT2. MRAs can also bind to the intracellular receptor TAAR1 as agonists, which triggers a phosphorylation cascade via protein kinases that results in the phosphorylation of monoamine transporters located at the plasma membrane (i.e., the dopamine transporter, norepinephrine transporter, and serotonin transporter); upon phosphorylation, these transporters transport monoamines in reverse (i.e., they move monoamines from the neuronal cytoplasm into the synaptic cleft). [23] The combined effects of MRAs at VMAT2 and TAAR1 result in the release of neurotransmitters out of synaptic vesicles and the cell cytoplasm into the synaptic cleft where they bind to their associated presynaptic autoreceptors and postsynaptic receptors. Certain MRAs interact with other presynaptic intracellular receptors which promote monoamine neurotransmission as well (e.g., methamphetamine is also an agonist at σ1 receptor).
In spite of findings that TAAR1 activation by amphetamines can reverse the monoamine transporters and mediate monoamine release however, [23] [24] [25] [26] major literature reviews on monoamine releasing agents by experts like Richard B. Rothman and David J. Heal state that the nature of monoamine transport reversal is not well understood and/or do not mention TAAR1 activation. [5] [6] [1] [7] Moreover, amphetamines continue to produce psychostimulant-like effects and induction of dopamine and norepinephrine release in TAAR1 knockout mice. [23] [27] [28] [29] [30] In fact, TAAR1 knockout mice are supersensitive to the effects of amphetamines and TAAR1 activation appears to inhibit the striatal dopaminergic effects of psychostimulants. [23] [28] [27] [29] [30] Additionally, many substrate-type MRAs that do not bind to and/or activate the (human) TAAR1 are known, including most cathinones, ephedrine, 4-methylamphetamine, and 4-methylaminorex derivatives, among others. [31] [32] [33] [34]
There is a constrained and relatively small molecular size requirement for compounds to act as monoamine releasing agents. [5] This is because they must be small enough to serve as substrates of the monoamine transporters and thereby be transported inside of monoaminergic neurons by these proteins, in turn allowing them to induce monoamine neurotransmitter release. [5] Compounds with chemical features extending beyond the size constraints for releasers will instead act as partial releasers, reuptake inhibitors, or be inactive. [5] Partial releasers show reduced maximal efficacy in releasing monoamine neurotransmitters compared to conventional full releasers. [5]
Dopamine reuptake inhibitors (DRIs) have been grouped into two types, typical or conventional DRIs like cocaine, WIN-35428 (β-CFT), and methylphenidate that produce potent psychostimulant, euphoric, and reinforcing effects, and atypical DRIs like vanoxerine (GBR-12909), modafinil, benztropine, and bupropion, which do not produce such effects or have greatly reduced such effects. [7] [6] [5] [35] It has been proposed that typical DRIs may not actually be acting primarily as DRIs but rather as dopamine releasing agents (DRAs) via mechanisms distinct from conventional substrate-type DRAs like amphetamines. [7] A variety of different pieces of evidence support this hypothesis and help to explain otherwise confusing findings. [7] Under this model, typical cocaine-like DRIs have been referred to with the new label of dopamine transporter (DAT) "inverse agonists" to distinguish them from conventional substrate-type DRAs. [7] An alternative theory is that typical DRIs and atypical DRIs stabilize the DAT in different conformations, with typical DRIs resulting in an outward-facing open conformation that produces differing pharmacological effects from those of atypical DRIs. [6] [5] [35] [36]
Some MRAs, like the amphetamines amphetamine and methamphetamine, as well as trace amines like phenethylamine, tryptamine, and tyramine, are additionally monoaminergic activity enhancers (MAEs). [8] [9] [37] That is, they induce the action potential-mediated release of monoamine neurotransmitters (in contrast to MRAs, which induced uncontrolled monoamine release independent of neuronal firing). [8] [9] [37] They are usually active as MAEs at much lower concentrations than those at which they induce monoamine release. [8] [9] [37] The MAE actions of MAEs may be mediated by TAAR1 agonism, which has likewise been implicated in monoamine-releasing actions. [38] [39] MAEs without concomitant potent monoamine-releasing actions, like selegiline (L-deprenyl), phenylpropylaminopentane (PPAP), and benzofuranylpropylaminopentane (BPAP), have been developed. [8] [9]
A number of endogenous compounds are known to act as MRAs. [4] [40] [41] [11] [5] These include the monoamine neurotransmitters dopamine (an NDRA), [40] norepinephrine (an NDRA), [40] and serotonin (an SRA) themselves, [40] as well as the trace amines phenethylamine (an NDRA), [5] [37] [42] [43] tryptamine (an SDRA or imbalanced SNDRA), [41] [11] and tyramine (an NDRA). [40] [4] Synthetic MRAs are substantially based on structural modification of these endogenous compounds, most prominently including the substituted phenethylamines and substituted tryptamines. [40] [2] [3] [41] [44] [45] [46]
Release of monoamine neurotransmitters by themselves, for instance in the cases of serotonin, norepinephrine, and dopamine, has been referred to as "self-release". [4] The physiological significance of the findings that monoamine neurotransmitters can act as releasing agents of themselves is unclear. [4] However, it could imply that efflux is a common neurotransmitter regulatory mechanism that can be induced by any transporter substrate. [4]
It is possible monoamine neurotransmitter self-release could be a protective mechanism. [4] It is notable in this regard that intracellular non-vesicular or cytoplasmic dopamine is toxic to neurons and that the vesicular monoamine transporter 2 (VMAT2) is neuroprotective by packaging this dopamine into synaptic vesicles. [47] [48] [49] Along similar lines, monoamine releasing agents induce the efflux of non-vesicular monoamine neurotransmitter and thereby move cytoplasmic neurotransmitter into the extracellular space. [5] However, many, though not all, monoamine releasing agents also act as VMAT2 inhibitors and reversers and hence concomitantly induce the release of vesicular monoamine neurotransmitter into the cytoplasm. [5]
Some MRAs have been found to act as monoaminergic neurotoxins and hence to produce long-lasting damage to monoaminergic neurons. [50] [51] Examples include dopaminergic neurotoxicity with amphetamine and methamphetamine and serotonergic neurotoxicity with methylenedioxymethamphetamine (MDMA). [50] [51] Amphetamine may produce significant dopaminergic neurotoxicity even at therapeutic doses. [52] [53] [54] [55] [56] [57] However, clinical doses of amphetamine producing neurotoxicity is controversial and disputed. [58] [52] [54] In contrast to amphetamines, monoamine reuptake inhibitors like methylphenidate lack apparent neurotoxic effects. [52]
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. [59] [60] Certain drugs have been found to block the neurotoxicity of MRAs in animals. [51] For instance, the selective MAO-B inhibitor selegiline has been found to prevent the serotonergic neurotoxicity of MDMA in rodents. [51]
Compound | 5-HT | NE | DA | Type | Class | Ref |
---|---|---|---|---|---|---|
2-Aminoindane | >10000 | 86 | 439 | NDRA | Aminoindane | [61] |
2-APBT | 8.9 | 21.6 | 38.6 | SNDRA | Aminopropylbenzothiophene | [62] |
2C-E | >100000 | >100000 | >100000 | IA | Phenethylamine | [63] |
2C-I | >100000 | >100000 | >100000 | IA | Phenethylamine | [63] |
3-APBT | 21.9 | 13.4 | 21.7 | SNDRA | Aminopropylbenzothiophene | [62] |
3-Chloroamphetamine | ND | 9.4 | 11.8 | ND | Amphetamine | [64] [4] |
3-Chloromethcathinone | 211 | 19 | 26 | SNDRA | Cathinone | [5] |
3-Fluoroamphetamine | 1937 | 16.1 | 24.2 | NDRA | Amphetamine | [65] [4] |
3-Methoxyamphetamine | ND | 58.0 | 103 | ND | Amphetamine | [4] |
3-Methoxy-4-hydroxymethamphetamine (HMMA) | 589 | 625 | 607–2884 | SNDRA | Amphetamine | [4] [66] |
3-Methoxymethcathinone (3-MeOMC) | 306 | ND (68% at 10 μM) | 129 | SDRA | Cathinone | [67] |
3-Methylamphetamine | 218 | 18.3 | 33.3 | NDRA | Amphetamine | [65] [64] [4] |
3-Methylmethcathinone | 292 | 27 | 70 | SNDRA | Cathinone | [5] |
3,4-Dihydroxyamphetamine (HHA) | ND | 33 | 3485 | ND | Amphetamine | [4] |
3,4-Dihydroxymethamphetamine (HHMA) | ND | 77 | 1729 | ND | Amphetamine | [4] |
4-APBT | 21.2 | 46.2 | 66.6 | SNDRA | Aminopropylbenzothiophene | [62] |
4-Chloroamphetamine | ND | 23.5 | 68.5 | SNDRA | Amphetamine | [64] [4] |
4-Fluoroamphetamine | 730–939 | 28.0–37 | 51.5–200 | NDRA | Amphetamine | [65] [63] [64] [4] |
4-Hydroxy-3-methoxyamphetamine (HMA) | 897 | 694 | 1450–3423 | ND | Amphetamine | [4] [66] |
4-Methoxyamphetamine | ND | 166 | 867 | ND | Amphetamine | [4] |
cis-4-Methylaminorex | 53.2 | 4.8 | 1.7 | NDRA | Aminorex | [68] |
4-Methylamphetamine | 53.4 | 22.2 | 44.1 | SNDRA | Amphetamine | [65] [64] [4] |
4-Methylphenethylamine | ND | ND | 271 | ND | Phenethylamine | [5] |
4-Methylthiomethamphetamine | 21 | ND | ND | ND | Amphetamine | [69] |
4,4'-Dimethylaminorex | ND | ND | ND | SNDRA | Aminorex | ND |
cis-4,4'-Dimethylaminorex | 17.7–18.5 | 11.8–26.9 | 8.6–10.9 | SNDRA | Aminorex | [68] [70] |
trans-4,4'-Dimethylaminorex | 59.9 | 31.6 | 24.4 | SNDRA | Aminorex | [70] |
5-APB | 19 | 21 | 31 | SNDRA | Amphetamine | [71] |
5-APBT | 10.3 | 38.4 | 92.8 | SNDRA | Aminopropylbenzothiophene | [62] |
5-(2-Aminopropyl)indole (5-IT) | 28–104.8 | 13.3–79 | 12.9–173 | SNDRA | Amphetamine | [12] [72] |
(R)-5-(2-Aminopropyl)indole | 177 | 81 | 1062 | SNRA | Amphetamine | [12] |
(S)-5-(2-Aminopropyl)indole | ND | ND | ND | SNDRA | Amphetamine | ND |
5-Chloro-αMT | 16 | 3434 | 54 | SDRA | Tryptamine | [11] [12] |
5-Fluoro-αET | 36.6 | 5334 | 150 | SDRA | Tryptamine | [11] |
5-Fluoro-αMT | 19 | 126 | 32 | SNDRA | Tryptamine | [12] |
5-MABB | ND | ND | ND | ND | Amphetamine | [73] [74] |
(R)-5-MABB | 49 | 850 | IA | SRA | Amphetamine | [73] [74] |
(S)-5-MABB | 31 | 158 | 210 | SNDRA | Amphetamine | [73] [74] |
5-MAPB (5-MBPB) | 64 | 24 | 41 | SNDRA | Amphetamine | [71] |
5-MeO-αMT | 460 | 8900 | 1500 | SNDRA | Tryptamine | [63] |
5-MeO-AI | 134 | 861 | 2646 | SNRA | Aminoindane | [61] |
5-MeO-DMT | >100000 | >100000 | >100000 | IA | Tryptamine | [63] |
6-APB | 36 | 14 | 10 | SNDRA | Amphetamine | [71] |
6-APBT | 10.7 | 13.6 | 7.2 | SNDRA | Aminopropylbenzothiophene | [62] |
6-(2-Aminopropyl)indole (6-IT) | 19.9 | 25.6 | 164.0 | SNDRA | Amphetamine | [72] |
6-Chloroamphetamine | ND | 19.1 | 62.4 | ND | Amphetamine | [4] |
6-Fluoroamphetamine | ND | 24.1 | 38.1 | ND | Amphetamine | [4] |
6-MABB (6-MBPB) | ND | ND | ND | ND | Amphetamine | [73] [74] |
(R)-6-MABB | 172 | 227 | IA | SNRA | Amphetamine | [73] [74] |
(S)-6-MABB | 54 | 77 | 41 | SNDRA | Amphetamine | [73] [74] |
6-MAPB | 33 | 14 | 20 | SNDRA | Amphetamine | [71] |
6-Methoxyamphetamine | ND | 473 | 1478 | ND | Amphetamine | [4] |
6-Methylamphetamine | ND | 37 | 127 | ND | Amphetamine | [4] |
7-APBT | 36.9 | 28.5 | 16.8 | SNDRA | Aminopropylbenzothiophene | [62] |
α-Ethyltryptamine | 23.2 | 640 | 232 | SDRA | Tryptamine | [11] |
α-Methyltryptamine | 21.7–68 | 79–112 | 78.6–180 | SNDRA | Tryptamine | [63] [11] |
Amfepramone (diethylpropion) | >10000 | >10000 | >10000 | PD | Cathinone | [75] |
Aminorex | 193–414 | 15.1–26.4 | 9.1–49.4 | SNDRA | Aminorex | [40] [68] [4] |
Amphetamine | ND | ND | ND | NDRA | Amphetamine | ND |
D-Amphetamine | 698–1765 | 6.6–10.2 | 5.8–24.8 | NDRA | Amphetamine | [40] [76] [4] |
L-Amphetamine | ND | 9.5 | 27.7 | NDRA | Amphetamine | [64] [4] |
BDB | 180 | 540 | 2,300 | NDRA | Amphetamine | [63] |
Benzylpiperazine | ≥6050 | 62–68 | 175–600 | NDRA | Arylpiperazine | [63] [77] [3] [4] |
BK-NM-AMT | 41.3 | ND (55% at 10 μM) | 92.8 | SDRA | Tryptamine | [67] [78] |
BK-5F-NM-AMT | 190 | ND | 620 | ND | Tryptamine | [79] |
BK-5Cl-NM-AMT | 200 | ND | 865 | ND | Tryptamine | [79] |
BK-5Br-NM-AMT | 295 | ND | 2100 | ND | Tryptamine | [79] |
Bufotenin | 30.5 | >10000 | >10000 | SRA | Tryptamine | [41] |
Butylamphetamine | ND | ND | IA | ND | Amphetamine | [5] |
Cathinone | 6100 | 23.6 | 83.1 | NDRA | Cathinone | [4] [67] |
D-Cathinone | ND | ND | ND | NRA | Cathinone | ND |
L-Cathinone | 2366 | 12.4 | 18.5 | NDRA | Cathinone | [80] |
Chlorphentermine | 30.9 | >10000 | 2650 | SRA | Amphetamine | [40] |
DMPP | 26 | 56 | 1207 | SNRA | Arylpiperazine | [69] |
DMT | 114 | 4166 | >10000 | SRA | Tryptamine | [41] |
Dopamine | >10000 (RI) | 66.2 | 86.9 | NDRA | Phenethylamine | [40] [4] |
DPT | >100000 | >100000 | >100000 | IA | Tryptamine | [63] [41] |
Ephedrine (racephedrine) | ND | ND | ND | NDRA | Cathinol | ND |
D-Ephedrine (ephedrine) | >10000 | 43.1–72.4 | 236–1350 | NDRA | Cathinol | [40] [4] |
L-Ephedrine | >10000 | 218 | 2104 | NRA | Cathinol | [40] [80] |
Epinephrine | ND | ND | ND | NDRA | Phenethylamine | ND |
Ethcathinone | 2118 | 99.3 | >1000 (RI) | NRA | Cathinone | [75] [4] |
Ethylamphetamine | ND | ND | 88.5 | ND | Amphetamine | [5] |
Fenfluramine | 79.3–108 | 739 | >10000 (RI) | SRA | Amphetamine | [40] [81] [82] [4] |
D-Fenfluramine | 51.7 | 302 | >10000 | SNRA | Amphetamine | [40] [81] |
L-Fenfluramine | 147 | >10000 | >10000 | SRA | Amphetamine | [81] [83] |
MBDB | 540 | 3300 | >100000 | SNRA | Amphetamine | [63] |
mCPP | 28–38.1 | ≥1400 | 63000 | SRA | Arylpiperazine | [63] [83] [84] |
MDA | 160–162 | 47–108 | 106–190 | SNDRA | Amphetamine | [82] [4] [71] |
(R)-MDA | 310 | 290 | 900 | SNDRA | Amphetamine | [82] [4] |
(S)-MDA | 100 | 50.0 | 98.5 | SNDRA | Amphetamine | [82] [4] |
MDAI | 114 | 117 | 1334 | SNRA | Aminoindane | [61] |
MDEA | 47 | 2608 | 622 | SNDRA | Amphetamine | [69] |
(R)-MDEA | 52 | 651 | 507 | SNDRA | Amphetamine | [69] |
(S)-MDEA | 465 | RI | RI | SRA | Amphetamine | [69] |
MDMA | 50–85 | 54–110 | 51–278 | SNDRA | Amphetamine | [40] [85] [72] [82] [4] [71] |
(R)-MDMA | 340 | 560 | 3700 | SNDRA | Amphetamine | [82] [4] |
(S)-MDMA | 74 | 136 | 142 | SNDRA | Amphetamine | [82] [4] |
MDMAR | ND | ND | ND | SNDRA | Aminorex | ND |
cis-MDMAR | 43.9 | 14.8 | 10.2 | SNDRA | Aminorex | [70] |
trans-MDMAR | 73.4 | 38.9 | 36.2 | SNDRA | Aminorex | [70] |
Mephedrone | 118.3–122 | 58–62.7 | 49.1–51 | SNDRA | Cathinone | [85] [76] |
Methamnetamine | 13 | 34 | 10 | SNDRA | Amphetamine | [69] |
Methamphetamine | ND | ND | ND | NDRA | Amphetamine | ND |
D-Methamphetamine | 736–1291.7 | 12.3–13.8 | 8.5–24.5 | NDRA | Amphetamine | [40] [85] [4] |
L-Methamphetamine | 4640 | 28.5 | 416 | NRA | Amphetamine | [40] [4] |
Methcathinone | ND | 22.4 | 49.9 | NDRA | Cathinone | [4] |
D-Methcathinone | ND | ND | ND | NRA | Cathinone | ND |
L-Methcathinone | 1772 | 13.1 | 14.8 | NDRA | Cathinone | [80] |
Methylone | 234–242.1 | 140–152.3 | 117–133.0 | SNDRA | Cathinone | [85] [76] |
MMAI | 31 | 3101 | >10000 | SRA | Aminoindane | [61] |
Naphthylisopropylamine | 3.4 | 11.1 | 12.6 | SNDRA | Amphetamine | [86] [4] |
Norephedrine (phenylpropanolamine) | ND | ND | ND | NDRA | Cathinol | ND |
D-Norephedrine | >10000 | 42.1 | 302 | NDRA | Cathinol | [80] |
L-Norephedrine | >10000 | 137 | 1371 | NRA | Cathinol | [80] |
Norepinephrine | >10000 | 164 | 869 | NDRA | Phenethylamine | [40] [4] |
Norfenfluramine | 104 | 168–170 | 1900–1925 | SNRA | Amphetamine | [81] [82] |
(+)-Norfenfluramine | 59.3 | 72.7 | 924 | SNRA | Amphetamine | [81] |
(–)-Norfenfluramine | 287 | 474 | >10000 | SNRA | Amphetamine | [81] |
Norpropylhexedrine | ND | ND | ND | NDRA | Cyclohexethylamine | ND |
Norpseudoephedrine | ND | ND | ND | NDRA | Cathinol | ND |
D-Norpseudoephedrine (cathine) | >10000 | 15.0 | 68.3 | NDRA | Cathinol | [80] |
L-Norpseudoephedrine | >10000 | 30.1 | 294 | NDRA | Cathinol | [80] |
oMPP | 175 | 39.1 | 296–542 | SNDRA | Arylpiperazine | [87] [5] |
PAL-738 | 23 | 65 | 58 | SNDRA | Phenylmorpholine | [69] |
PAL-874 | >10000 | 305 | 688 | NDRA | Phenylbutynamine | [69] |
Phenacylamine (β-ketophenethylamine) | >10000 | ND | 208 | ND | Phenethylamine | [5] [67] |
Phendimetrazine | >100000 | >10000 | >10000 | PD | Phenylmorpholine | [88] [4] |
Phenethylamine | >10000 | 10.9 | 39.5 | NDRA | Phenethylamine | [5] [64] [4] |
Phenmetrazine | 7765 | 50.4 | 131 | NDRA | Phenylmorpholine | [88] [4] |
Phentermine | 3511 | 39.4 | 262 | NDRA | Amphetamine | [40] [4] |
Phenylalaninol | ND | ND | ND | ND | Amphetamine | ND |
D-Phenylalaninol | >10000 | 106 | 1355 | NRA | Amphetamine | [87] |
L-Phenylalaninol | ND | ND | ND | ND | Amphetamine | ND |
Phenylisobutylamine | ND | ND | 225 | ND | Amphetamine | [5] |
Phenylpropylamine | ND | 222 | 1491 | NDRA | Phenylpropylamine | [64] [4] |
pMPP | 3200 | 1500 | 11000 | SNRA | Arylpiperazine | [63] |
pNPP | 43 | >10000 | >10000 | SRA | Arylpiperazine | [69] |
Propylamphetamine | ND | ND | RI (1013) | ND | Amphetamine | [5] |
Propylhexedrine | ND | ND | ND | NDRA | Cyclohexethylamine | ND |
Pseudoephedrine (racemic pseudoephedrine) | ND | ND | ND | NDRA | Cathinol | ND |
D-Pseudoephedrine | >10000 | 4092 | 9125 | NDRA | Cathinol | [80] |
L-Pseudoephedrine (pseudoephedrine) | >10000 | 224 | 1988 | NRA | Cathinol | [80] |
Pseudophenmetrazine | >10000 | 514 | RI | NRA | Phenylmorpholine | [88] |
Psilocin | 561 | >10000 | >10000 | SRA | Tryptamine | [69] [41] |
Serotonin | 44.4 | >10000 (RI) | ≥1960 | SRA | Tryptamine | [40] [4] |
TFMCPP | 33 | >10000 | >10000 | SRA | Arylpiperazine | [69] |
TFMPP | 121 | >10000 | >10000 | SRA | Arylpiperazine | [77] [4] |
Trimethoxyamphetamine | 16000 | >100000 | >100000 | IA | Amphetamine | [63] |
Tryptamine | 32.6 | 716 | 164 | SDRA | Tryptamine | [41] [11] |
Tyramine | 2775 | 40.6 | 119 | NDRA | Phenethylamine | [40] [4] |
Notes: The smaller the value, the more strongly the substance releases the neurotransmitter. | ||||||
Phenethylamine (PEA) is an organic compound, natural monoamine alkaloid, and trace amine, which acts as a central nervous system stimulant in humans. In the brain, phenethylamine regulates monoamine neurotransmission by binding to trace amine-associated receptor 1 (TAAR1) and inhibiting vesicular monoamine transporter 2 (VMAT2) in monoamine neurons. To a lesser extent, it also acts as a neurotransmitter in the human central nervous system. In mammals, phenethylamine is produced from the amino acid L-phenylalanine by the enzyme aromatic L-amino acid decarboxylase via enzymatic decarboxylation. In addition to its presence in mammals, phenethylamine is found in many other organisms and foods, such as chocolate, especially after microbial fermentation.
Tryptamine is an indolamine metabolite of the essential amino acid, tryptophan. The chemical structure is defined by an indole—a fused benzene and pyrrole ring, and a 2-aminoethyl group at the second carbon. The structure of tryptamine is a shared feature of certain aminergic neuromodulators including melatonin, serotonin, bufotenin and psychedelic derivatives such as dimethyltryptamine (DMT), psilocybin, psilocin and others.
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.
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.
Chlorphentermine, sold under the brand names Apsedon, Desopimon, and Lucofen, is a serotonergic appetite suppressant of the amphetamine family. Developed in 1962, it is the para-chloro derivative of the better-known appetite suppressant phentermine, which is still in current use.
Naphthylaminopropane (PAL-287) is an experimental drug under investigation as of 2007 for the treatment of alcohol and stimulant addiction.
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.
MDAI, also known as 5,6-methylenedioxy-2-aminoindane, is an entactogen drug of the 2-aminoindane group which is related to MDMA and produces similar subjective effects.
A serotonin releasing agent (SRA) is a type of drug that induces the release of serotonin into the neuronal synaptic cleft. A selective serotonin releasing agent (SSRA) is an SRA with less significant or no efficacy in producing neurotransmitter efflux at other types of monoamine neurons, including dopamine and norepinephrine neurons.
A dopamine releasing agent (DRA) is a type of drug which induces the release of dopamine in the body and/or brain.
A serotonin–dopamine releasing agent (SDRA) is a type of drug which induces the release of serotonin and dopamine in the body and/or brain.
3-Fluoroamphetamine is a stimulant drug from the amphetamine family which acts as a monoamine releaser with similar potency to methamphetamine but more selectivity for dopamine and norepinephrine release over serotonin. It is self-administered by mice to a similar extent to related drugs such as 4-fluoroamphetamine and 3-methylamphetamine.
Substituted cathinones, or simply cathinones, which include some stimulants and entactogens, are derivatives of cathinone. They feature a phenethylamine core with an alkyl group attached to the alpha carbon, and a ketone group attached to the beta carbon, along with additional substitutions. Cathinone occurs naturally in the plant khat whose leaves are chewed as a recreational drug.
A monoamine reuptake inhibitor (MRI) 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.
Pseudophenmetrazine is a psychostimulant compound of the morpholine class. It is the N-demethylated and cis-configured analogue of phendimetrazine as well as the cis-configured stereoisomer of phenmetrazine. In addition, along with phenmetrazine, it is believed to be one of the active metabolites of phendimetrazine, which itself is inactive and behaves merely as a prodrug. Relative to phenmetrazine, pseudophenmetrazine is of fairly low potency, acting as a modest releasing agent of norepinephrine (EC50 = 514 nM), while its (+)-enantiomer is a weak releaser of dopamine (EC50 = 1,457 nM) whereas its (−)-enantiomer is a weak reuptake inhibitor of dopamine (Ki = 2,691 nM); together as a racemic mixture with the two enantiomers combined, pseudophenmetrazine behaves overall more as a dopamine reuptake inhibitor (Ki = 2,630 nM), possibly due to the (+)-enantiomer blocking the uptake of the (−)-enantiomer into dopaminergic neurons and thus preventing it from inducing dopamine release. Neither enantiomer has any significant effect on serotonin reuptake or release (both Ki = >10,000 nM and EC50 = >10,000 nM, respectively).
4-Hydroxy-3-methoxymethamphetamine (HMMA) is an active metabolite of 3,4-methylenedioxymethamphetamine (MDMA). It is a slightly more potent stimulant than MDMA in rodents. The drug is substantially less potent than MDMA as a monoamine releasing agent in vitro. Nonetheless, HMMA has been found to induce the release of serotonin, norepinephrine, and dopamine with EC50Tooltip half-maximal effective concentration values of 589 nM, 625 nM, and 607–2884 nM, respectively, and hence acts as a lower-potency serotonin–norepinephrine–dopamine releasing agent (SNDRA). The predicted log P of HMMA is 1.2.
ortho-Methylphenylpiperazine (also known as oMPP, oMePP, 1-(2-methylphenyl)piperazine, 2-MPP, and 2-MePP) is a psychoactive designer drug of the phenylpiperazine group. It acts as a serotonin–norepinephrine–dopamine releasing agent (SNDRA), with EC50 values for induction of monoamine release of 175 nM for serotonin, 39.1 nM for norepinephrine, and 296–542 nM for dopamine. As such, it has about 4.5-fold preference for induction of norepinephrine release over serotonin, and about 7.6- to 13.9-fold preference for induction of norepinephrine release over dopamine.
Monoaminergic activity enhancers (MAE), also known as catecholaminergic/serotonergic activity enhancers (CAE/SAE), are a class of drugs that enhance the action potential-evoked release of monoamine neurotransmitters in the nervous system. MAEs are distinct from monoamine releasing agents (MRAs) like amphetamine and fenfluramine in that they do not induce the release of monoamines from synaptic vesicles but rather potentiate only nerve impulse propagation-mediated monoamine release. That is, MAEs increase the amounts of monoamine neurotransmitters released by neurons per electrical impulse.
Substituted β-hydroxyamphetamines, or simply β-hydroxyamphetamines, also known as substituted phenylisopropanolamines, substituted phenylpropanolamines, substituted norephedrines, or substituted cathinols, are derivatives of β-hydroxyamphetamine with one or more chemical substituents. They are substituted phenethylamines, phenylethanolamines (β-hydroxyphenethylamines), and amphetamines (α-methylphenethylamines), and are closely related to but distinct from the substituted cathinones (β-ketoamphetamines). Examples of β-hydroxyamphetamines include the β-hydroxyamphetamine stereoisomers phenylpropanolamine and cathine and the stereospecific N-methylated β-hydroxyamphetamine derivatives ephedrine and pseudoephedrine, among many others.
BK-NM-AMT, or βk-NM-αMT, also known as β-keto-N-methyl-αMT or as α,N-dimethyl-β-ketotryptamine, is a serotonin–dopamine releasing agent (SDRA) and putative entactogen of the tryptamine and α-alkyltryptamine families. Along with certain other tryptamines, such as α-ethyltryptamine (αET), 5-chloro-αMT and 5-fluoro-αET, it is one of the few SDRAs known.
TABLE 11-2 Comparison of the DAT- and NET-Releasing Activity of a Series of Amphetamines [...]
Converging lines of evidence have solidified the notion that DA releasers are substrates of the transporter and once translocated, they reverse the normal direction of transporter flux to evoke release of endogenous neurotransmitters. The nature of this reversal is not well understood, but the entire process is primarily transporter-dependent and requires elevated intracellular sodium concentrations, phosphorylation of DAT, and possible involvement of transporter oligomers (Khoshbouei et al., 2003, 2004; Sitte and Freissmuth, 2010).
It is reported that methamphetamine (METH) interacts with TAAR1 and subsequently inhibits DA uptake, enhance DA efflux and induces DAT internalization, and these effects are dependent on TAAR1 (Xie & Miller, 2009). For example, METH-induced inhibition of DA uptake was observed in TAAR1 and DAT cotransfected cells and WT mouse and monkey striatal synaptosomes but not in DAT-only transfected cells or in striatal synaptosomes of TAAR1-KO mice (Xie & Miller, 2009). TAAR1 activation was enhanced by co-expression of monoamine transporters and this effect could be blocked by monoamine transporter antagonists (Xie & Miller, 2007; Xie et al., 2007). Furthermore, DA activation of TAAR1 induced C-FOS-luciferase expression only in the presence of DAT (Xie et al., 2007).
While our data suggest a role for TAAR1 in eliciting amphetamine-like stimulant effects, it must be borne in mind that the observed in vivo effects are likely to result from interaction with both TAAR1 and monoamine transporters. Thus it has been shown that the selective TAAR1 agonist RO5166017 fully prevented psychostimulant-induced and persistent hyperdopaminergia-related hyperactivity in mice.42 This effect was found to be DAT-independent, since suppression of hyperactivity was observed in DAT-KO mice.42 The collected information leads us to conclude that TAAR1 is a stereoselective binding site for amphetamine and that TAAR1 activation by amphetamine and its congeners may contribute to the stimulant properties of this class of compounds.
Another feature that distinguishes [substituted cathinones (SCs)] from amphetamines is their negligible interaction with the trace amine associated receptor 1 (TAAR1). Activation of this receptor reduces the activity of dopaminergic neurones, thereby reducing psychostimulatory effects and addictive potential (Miller, 2011; Simmler et al., 2016). Amphetamines are potent agonists of this receptor, making them likely to self‐inhibit their stimulating effects. In contrast, SCs show negligible activity towards TAAR1 (Kolaczynska et al., 2021; Rickli et al., 2015; Simmler et al., 2014, 2016). [...] The lack of self‐regulation by TAAR1 may partly explain the higher addictive potential of SCs compared to amphetamines (Miller, 2011; Simmler et al., 2013).
The activation of human TAAR1 might diminish the effects of psychostimulation and intoxication arising from 7-APB effects on monoamine transporters (see 4.1.3. for more details). Affinity to mouse and rat TAAR1 has been shown for many psychostimulants, but species differences are common (Simmler et al. 2016). For example, [5-(2-aminopropyl)indole (5-IT)] and [4-methylamphetamine (4-MA)] bind and activate TAAR1 in the nanomolar range, but do not activate human TAAR1.
It has been suggested that the association between PD and ADHD may be explained, in part, by toxic effects of these drugs on DA neurons.241 [...] An important question is whether amphetamines, as they are used clinically to treat ADHD, are toxic to DA neurons. In most of the animal and human studies cited above, stimulant exposure levels are high relative to clinical doses, and dosing regimens (as stimulants) rarely mimic the manner in which these drugs are used clinically. The study by Ricaurte and colleagues248 is an exception. In that study, baboons orally self-administered a racemic (3:1 d/l) amphetamine mixture twice daily in increasing doses ranging from 2.5 to 20 mg/day for four weeks. Plasma amphetamine concentrations, measured at one-week intervals, were comparable to those observed in children taking amphetamine for ADHD. Two to four weeks after cessation of amphetamine treatment, multiple markers of striatal DA function were decreased, including DA and DAT. In another group of animals (squirrel monkeys), d/l amphetamine blood concentration was titrated to clinically comparable levels for four weeks by administering varying doses of amphetamine by orogastric gavage. These animals also had decreased markers of striatal DA function assessed two weeks after cessation of amphetamine.
Amphetamine treatment similar to that used for ADHD has been demonstrated to produce brain dopaminergic neurotoxicity in primates, causing the damage of dopaminergic nerve endings in the striatum that may also occur in other disorders with long-term amphetamine treatment (57).
Though the paradigm used by Ricaurte et al. 53 arguably still incorporates amphetamine exposure at a level above much clinical use,14,55 it raises important unanswered questions. Is there a threshold of amphetamine exposure above which persistent changes in the dopamine system are induced? [...]
Repeated exposure to moderate to high levels of methamphetamine has been related to neurotoxic effects on the dopaminergic and serotonergic systems, leading to potentially irreversible loss of nerve terminals and/or neuron cell bodies (Cho and Melega, 2002). Preclinical evidence suggests that d-amphetamine, even when administered at commonly prescribed therapeutic doses, also results in toxicity to brain dopaminergic axon terminals (Ricaurte et al., 2005).