Hypothesizing a More Complete Theory for Amphetamine's Mechanism of Action

[SpunkySkunk347]

It's been a while since I last dabbled in pharmacodynamics and pharmacokinets, but back in my day ( ) the prevailing theory was that amphetamine reversed reuptake of DA, NE, and 5-HT (as opposed to inhibiting reuptake) but it always became too hypothetical after that; I think it was generally believed that amphetamine interfered with a different protein from typical DRIs such as cocaine or methylphenidate, but at the same transport nonetheless.

I've thought of a few other factors however that perhaps maybe no one has yet considered:

1. Due to structural similarity to dopamine and other monoamine neurotransmitters, dopamine and amphetamine would be competing for metabolism by monoamine oxidase -- whether or not that could be significantly contributing to the perceived psychoactive effects of amphetmine, we have to really think and ask, just how many mols of dopamine are present in the brain, and what percentage of that is near the synapse or in presynaptic vesicles? And concerning the distribution of amphetamine in plasma, what percentage of that is close enough to be influencing synaptic activity, or imminently about to? If amphetamine is evenly distributed and saturated in plasma throughout the body (excluding the kidneys, bladder, and GI tract), certainly only a very small percentage would be in close proximity to synapses in the CNS. Then, at what rate do more amphetamine molecules circulate into regions nearby the synapses, and how quickly is monoamine oxidase ready to break down another monoamine?

2. With amphetamine's lack of any hydroxyl groups on its phenyl ring, would the oxygen added by monoamine oxidase when breaking a compound down be prone to attach to the phenyl ring rather than result in deamination? The addition of a methyl group in the way of the nitrogen atom in the a-methylphenethylamine molecule would maybe seem to also prevent deamination. In this case, the metabolite of amphetamine would be a molecule having much more of a resemblance to dopamine and norepinephrine, if indeed a hydroxyl group will attach to the phenyl ring instead of deamination occurring.
This proposed hypothesize likely has its answer found in the structural kinetics of monoamine oxidase itself, and I don't expect to get an answer anytime soon before someone with professional biochemistry software looks up whether the proposed reaction could occur within the structure of the MAO enzymes. My knowledge of reactions in organic chemistry is very rough and rudimentary, but would the double-bond between carbon in the phenyl ring give it properties similar to nitrogen grouped as an amine, due to the spatial similarity of both only sharing bonds with 3 other atoms?

According to this picture on wikipedia (https://commons.wikimedia.org/wiki/File:TAAR1_Dopamine.svg), amphetamine is being portrayed as filling up the presynaptic dopamine vesicle after leaving the dopamine transporter, not allowing enough room in the vesicle for dopamine to fit back in the vesicles and forcing them to continue back out to the synapse -- I don't think this is the traditional view held regarding reuptake "reversal"; I hypothesize that, due to the difference in what side of the structure differing functional groups are located on the dopamine and amphetamine molecules, the amphetamine molecule is able to fit into the same places as dopamine much easier when its a tight pass for the hydroxyl groups on dopamine's phenyl ring - and, the opposite is true concerning the amine side of the two molecules, where the extra methyl group likely gets jammed or doesn't fit through the same as dopamine.

3. Given the complexity of the entire synaptic system for a monoamine neurotransmitters, it may be that numerous phenomena similar to those mentioned are occurring as a result of the two compound's structural similarities, and happening spontaneously by chance, interference with the dopamine transport and other aspects of synaptic transmission occur giving rise to amphetamine's effects.

4. At the dopamine receptors of the postsynaptic neuron, although amphetamine may not be potent as a dopamine agonist, its structural similarity to dopamine might still lower the threshold of the receptors collectively across the postsynaptic neuron, especially if the hypothesis earlier was true that a metabolite of amphetamine has a hydroxyl group in a similar position as dopamine. This is another hypothesis that calls for someone with a technical knowledge of biochemistry, and either software that can accurately graph the behavior of enzymes and proteins or just an immense intellect capable of graphing it out in one's head.

 

[serotonin2A]

1. MAO is inside of neurons, in the cytoplasm. In general, amphetamine only weakly interacts with MAO. Drug action is dependent on concentration, so there is no need to think about the percentage of dose that is in proximity to its targets, but rather the concentration.

2. MAO isn't the enzyme that hydroxylates amphetamine -- p-hydroxylation is due to a cytochrome p450 enzyme (I think 2D6).
Amphetamine doesn't work by filling up vesicles, but rather causes monoamines to leak out of vesicles.

3. I would recommend reading a few recent articles about amphetamine's mechanism of action. Over the last few years, many new findings have been reported RE how amphetamine works in the brain.

4. The action you are describing (lowering the threshold for receptor activation) is called allosteric modulation. Amphetamine does not have such an action on dopamine receptors.

 

[d1nach]

I agree with 5ht2as point number 2

 

[Cotcha Yankinov]

RE Serotonin2A's 3. -

I think there have been different mechanisms described in regards to amphetamine's effect on reuptake transporters (not just TAAR1 mediated reversal of DAT), like RhoA and ROCK mediated internalization of DAT and RhoA mediated internalization of EAAT3 in dopamine neurons.

 

[sekio]

would the double-bond between carbon in the phenyl ring give it properties similar to nitrogen grouped as an amine, due to the spatial similarity of both only sharing bonds with 3 other atoms?

Not at all. The carbons in the phenyl ring are aromatic, that is to say, bonding and charge is delocalized around the ring. It's not technically correct to draw a benzene ring as a cyclohexane with three double bonds, as that implies the bond lengths are different between single and double. If you were to measure the benzene ring with an atomic force microscope then you'd see that the six carbon atoms are all equidistant from each other, that is to say, the bond lengths are all the same.

 

[serotonin2A]

I hypothesize that, due to the difference in what side of the structure differing functional groups are located on the dopamine and amphetamine molecules, the amphetamine molecule is able to fit into the same places as dopamine much easier when its a tight pass for the hydroxyl groups on dopamine's phenyl ring - and, the opposite is true concerning the amine side of the two molecules, where the extra methyl group likely gets jammed or doesn't fit through the same as dopamine.

I missed this point on my first read through -- it is difficult to read big blocks of text on my phone.

Binding occurs because molecules have complementary structural features that facilitate interaction, so the molecules "stick together" for a period of time after they bump into each other. The absence of hydroxy groups doesn't make amphetamine all that much smaller than dopamine, but it does remove two points of interaction that could contribute to binding. That is why amphetamine does not bind to dopamine receptors.

 

[d1nach]

Is the focus here on the action on reward pathways or cognition focus ect.?

 

[aced126]

I have a few points to add which may eludicate its mechanism.

My first point, or lemma if you like, is that substrates can be moved against their concentration gradient (active transport) in two ways - primary active transport and secondary active transport:

-Primary active transport occurs when transporter proteins harness energy derived from ATP hydrolysis to drive conformational changes within a protein to translocate the substrate.

-Secondary active transport occurs when transporters harness the energy stored in the concentration gradient of one substrate to drive transport of another substrate against its concentration gradient.

Often these 2 types of transport work in concert. Primary active transport generates the gradient, and secondary active transport harnesses it to drive transport of specific substrates against their concentration gradient. This is exactly what's going on at a dopamine neuron, both in DA uptake from the synaptic cleft, and DA storage in vesicles.

***

Now let's talk a bit about DAT. DAT is a symporter. What this means is that it cotransports 2 Na+ molecules along with every dopamine molecule. DAT does not use ATP, so how does it take up pretty much all the dopamine in the synaptic cleft after an action potential? It is the high concentration of Na+ in the synaptic cleft which drives dopamine back into the membrane, against its concentration gradient.

A quick though experiment at this point: imagine if there was no Na+ gradient. Would DAT then be able to take up dopamine? Well, not fully. It would be able to equilibriate the concentrations of dopamine in the cell and the synaptic cleft. What would happen if there was a really high concentration of dopamine in the neuron? It would just cause DAT to work in reverse until the concentrations are equilibriated once again. Thus, given a high enough concentration of dopamine inside the neuron (to overcome the high Na+ gradient on the outside), DAT will naturally work in reverse and pump dopamine out. DAT cannot disobey the laws of thermodynamics.

The dopamine is now in the neuron. It does not stay in the neuron - it gets packaged into vesicles. Once again, almost all the dopamine is packed into vesicles, thus transport of dopamine against its concentration gradient into vesicles is required.

How is this managed? Vesicles have a type of transporter called a V-type ATPase, similar in structure to ATP synthase. However this protein translocates H+ into the vesicle at the expense of ATP hydrolysis. The high [H+] inside the vesicle is now harnessed by an antiporter which transports dopamine into the vesicle against its concentration gradient. The energy input to achieve this active transport comes from the dissipation of the H+ gradient in the vesicle. The antiporter also must obey the laws of thermodynamics - if the cytosol is greatly acidified such that the cytosolic pH is lower than the vesicle, then the antiporter will work in reverse (transporting H+ into the vesicle and DA out of the vesicle).

***

Now I can lay out a simple mechanism for how amphetamine works -

1) It enters the neuron through DAT.

2) It then causes efflux of dopamine from the vesicles. I propose a few mechanisms for how this happens:

-Here's where I'm gonna get criticised. When in the synaptic cleft and entering the neuron, amphetamine is protonated. The cytosol of the neuron is a bit basic due to the V-type ATPases acidifying vesicles. Amphetamine makes the cytosol more acidic, increasing cytosolic [H+] past vesicular [H+]. This then causes H+ to enter the vesicle. It has to enter via the antiporter, thus dopamine must leave the vesicle as well, due to the contransportive nature of the antiporter.

-Now a lot might be thinking "the volume of a vesicle is tiny compared to the cytosol - how can amphetamine acidify the cytosol so greatly that it makes it more acidic than the vesicle?" Well, the only reason I can think of this at the moment is that amphetamine might bind transiently to the antiporter, or some other protein on the vesicle (seems feasible, since a mechanism like this might increase natural DA uptake rate into vesicles), and release its proton then when it binds. Thus a high local proton concentration is formed around the vesicle, leading the H+ influx and thus DA efflux.

-Amphetamine activates TAAR1. This seems to result in an increase in expression of trace amines. These trace amines, along with amphetamine itself, might be able to not only bind but act as a substrate of the antiporter. Thus, translocation of these amines into the vesicle occurs, and as equilibrium is approached, some of the DA in the vesicle will be effluxed and replaced by these amines. Here is another point: the vesicle antiporter, like the DAT symporter, is always catalysing both translocations - it might translocate a dopamine into the vesicle one moment, then the next moment it might translocate one out. On average, however, the concentration of dopamine in the vesicle at equilibrium will be related to the pH difference, as well as the concentration of these trace amines which act as a substrate as well.

3) Now that a lot of dopamine has been effluxed out of the vesicle, the concentration of dopamine in the cytosol is great enough to overcome the concentration of Na+ outside the neuron. Thus, efflux of dopamine into the synaptic cleft occurs - DAT can and does function in both ways at all times.

I'm a bit dubious about the whole phosphorylation of DAT thing, and how that comes into play. A lot of people say "phosphorylation of DAT makes it work in reverse". Well, the thing is that DAT is not coupled to ATP hydrolysis, and so it can only ever function according to the relative concentration gradients of its substrates.

I'll edit this post in time.

 

[serotonin2A]

2) It then causes efflux of dopamine from the vesicles. I propose a few mechanisms for how this happens:

-Here's where I'm gonna get criticised. When in the synaptic cleft and entering the neuron, amphetamine is protonated. The cytosol of the neuron is a bit basic due to the V-type ATPases acidifying vesicles. Amphetamine makes the cytosol more acidic, increasing cytosolic [H+] past vesicular [H+]. This then causes H+ to enter the vesicle. It has to enter via the antiporter, thus dopamine must leave the vesicle as well, due to the contransportive nature of the antiporter.

-Now a lot might be thinking "the volume of a vesicle is tiny compared to the cytosol - how can amphetamine acidify the cytosol so greatly that it makes it more acidic than the vesicle?" Well, the only reason I can think of this at the moment is that amphetamine might bind transiently to the antiporter, or some other protein on the vesicle (seems feasible, since a mechanism like this might increase natural DA uptake rate into vesicles), and release its proton then when it binds. Thus a high local proton concentration is formed around the vesicle, leading the H+ influx and thus DA efflux.

-Amphetamine activates TAAR1. This seems to result in an increase in expression of trace amines. These trace amines, along with amphetamine itself, might be able to not only bind but act as a substrate of the antiporter. Thus, translocation of these amines into the vesicle occurs, and as equilibrium is approached, some of the DA in the vesicle will be effluxed and replaced by these amines. Here is another point: the vesicle antiporter, like the DAT symporter, is always catalysing both translocations - it might translocate a dopamine into the vesicle one moment, then the next moment it might translocate one out. On average, however, the concentration of dopamine in the vesicle at equilibrium will be related to the pH difference, as well as the concentration of these trace amines which act as a substrate as well.

You did a great job summarizing the action of amphetamine...until you reached point two where rampant speculation took over. There has actually been a great deal of experimental data that has been collected about this topic.

http://sulzerlab.org/pdf_articles/Sulzer05AMPHreview.pdf
http://www.nature.com/articles/ncomms10652
https://www.ncbi.nlm.nih.gov/pubmed/18297690

The cytosol of neurons isn't basic because protons are pumped into vesicles, but rather because cytoplasmic pH is tightly regulated by Na/H exchange across the cell membrane and by the co-transport of Na and bicarbonate into neurons. It is necessary to tightly regulate cytoplasmic pH because channel protein function is often affected by pH.

 

[d1nach]

I thought amphetamine is a base because it can accept h+

Amphetamine + h2o
Amphetamine(h+) + oh

Or amphetamine + HCl
Amphetamine-hcl

 

[aced126]

You did a great job summarizing the action of amphetamine...until you reached point two where rampant speculation took over. There has actually been a great deal of experimental data that has been collected about this topic.

http://sulzerlab.org/pdf_articles/Sulzer05AMPHreview.pdf
http://www.nature.com/articles/ncomms10652
https://www.ncbi.nlm.nih.gov/pubmed/18297690

The cytosol of neurons isn't basic because protons are pumped into vesicles, but rather because cytoplasmic pH is tightly regulated by Na/H exchange across the cell membrane and by the co-transport of Na and bicarbonate into neurons. It is necessary to tightly regulate cytoplasmic pH because channel protein function is often affected by pH.

Okay, my pH argument might have been a bit silly. But I don't exactly think the pH argument given by nature is exactly sound.

The nature article is suggesting amphetamine enters the vesicle through VMAT and deacidifies it. But dopamine is also a primary amine - it would do the same thing. The only way this argument holds is if there is A LOT more amphetamine getting into the vesicles than dopamine. In that case, yes, the vesicle is deacidified. Protons enter through VMAT to compensate. What leaves in exchange? Not dopamine, but mainly amphetamine leaves (remember in this scenario there is a lot more amphetamine than dopamine in the vesicles). So I don't think this pH altering argument fully works.

I think the better argument here is that because amphetamine itself is a substrate of VMAT, it will start to equilibriate with dopamine concentrations in the vesicle. As I pointed out in my earlier post, dopamine is always going in and out of vesicles through VMAT. Thus adding another dopamine-like molecule will eventually result in some dopamine in the vesicle being exchanged for the other molecule.

I would have to do some calculations but my gut feeling is that at therapeutic concentrations, I don't think amphetamine itself isn't enough to cause massive displacement of dopamine. There just isn't enough of it. The experiments done in the nature article don't disprove the fact that other trace amines could be helping to displace dopamine out of vesicles, simply by shifting the equilibrium. These trace amines could be upregulated due to the presence of amphetamine.

I wanted to make another point to rationalise the upregulation of trace amines. There might be a "natural amphetamine" - phenethylamine (just as endorphins are to opioids), which is a good agonist of TAAR1.

 

[d1nach]

There is no stupid arguments only arguments with different degrees of evidence. This is complex stuff. I dont understand most of what you guys are saying I just know amines act as base in water .

 

[serotonin2A]

Okay, my pH argument might have been a bit silly. But I don't exactly think the pH argument given by nature is exactly sound.

The nature article is suggesting amphetamine enters the vesicle through VMAT and deacidifies it. But dopamine is also a primary amine - it would do the same thing. The only way this argument holds is if there is A LOT more amphetamine getting into the vesicles than dopamine. In that case, yes, the vesicle is deacidified. Protons enter through VMAT to compensate. What leaves in exchange? Not dopamine, but mainly amphetamine leaves (remember in this scenario there is a lot more amphetamine than dopamine in the vesicles). So I don't think this pH altering argument fully works.

I think the better argument here is that because amphetamine itself is a substrate of VMAT, it will start to equilibriate with dopamine concentrations in the vesicle. As I pointed out in my earlier post, dopamine is always going in and out of vesicles through VMAT. Thus adding another dopamine-like molecule will eventually result in some dopamine in the vesicle being exchanged for the other molecule.

I would have to do some calculations but my gut feeling is that at therapeutic concentrations, I don't think amphetamine itself isn't enough to cause massive displacement of dopamine. There just isn't enough of it. The experiments done in the nature article don't disprove the fact that other trace amines could be helping to displace dopamine out of vesicles, simply by shifting the equilibrium. These trace amines could be upregulated due to the presence of amphetamine.

I think you are underestimating the amount of amphetamine that is able to participate in this process -- compared to DA, there is basically and enless supply of amphetamine to disrupt vesicular function because AMPH is not just in the cytoplasm but also distributed in all the extracellular fluid.

The other point you are missing is that the vesicular packaging system was designed to function with a certain level of dopamine. It isn't necessarily a very efficient process because dopamine isn't synthesized at a very high rate. Just because the vesicular system functions normally doesn't mean that the function wouldn't be disrupted by the presence of another weak base that is transported by VMAT. There are enough protons in vesicles to buffer the relatively slow inward influx of dopamine, but a comparatively large bolus of amphetamine given on top of that may completely overwhelm the buffering capacity.

Amphetamine being a VMAT substrate is not sufficient to explain its action. You already brought up the reason -- dopamine in not just passively stored in vesicles, otherwise cytoplasmic dopamine would actually mimic the action of amphetamine. So amphetamine has to be doing something else, in addition to exchange, that allows dopamine to be transported out of vesicles. It has been experimentially verified that amphetamine deacidifies vesicles, and such an action would undoubtedly cause vesicles to leak dopamine.

You are correct that there would normally be some bi-directional exchange of DA through VMAT, but that would be limited because most of the DA in vesicles is protonated (which keeps it from bring transported out).

One thing you wrote that I didn't understand is:

"Protons enter through VMAT to compensate. What leaves in exchange? Not dopamine, but mainly amphetamine leaves (remember in this scenario there is a lot more amphetamine than dopamine in the vesicles)."

First, how would protons enter through VMAT? I believe that VMAT is unidirectional in terms of proton transport. It is true that dopamine can be transported in both directions, but that is because VMAT literally functions like a revolving door (although DA probably has lower affinity for VMAT when it is making the return trip). Even when dopamine is transported out of vesicles, protons would still be transported out.

Second, I don't believe that in any scenario there would be more amphetamine in vesicles than dopamine. The concentration of DA in vesicles is something like 100,000 times higher than the cytoplasmic concentration. It is hard to believe that the amphetamine concentration in vesicles would get that high with acute exposure.

RE trace amines --
Trace amines can cause reverse-transport of catacholamines. But how would amphetamine up-regulate trace amine production over the course of a few minutes (the time course over which amphetamine effects can peak after iv administration)? You would obviously have to increase the action of one or more enzymes. But that isn't magically going to increase trace amine levels -- depending on the synthesis rate, the availability of precursor, and buffering processes such as degredation by MAO, it might take hours to days to see a meaningful increase in cytoplasmic concentration.

 

[aced126]

I think you are completely underestimating the amount of amphetamine that is able to participate in this process -- compared to DA, there is basically and enless supply of amphetamine to disrupt vesicular function because it is not just in the cytoplasm but also distributed in all the extracellular fluid.

The other point you are missing is that the vesicular packaging system was designed to function with a certain level of dopamine. It isn't necessarily all that efficient because dopamine isn't necessarily packaged at a very high rate. Just because the vesicular system functions normally doesn't mean that the function wouldn't be disrupted by the presence of another weak base that is transported by VMAT. There are enough protons in vesicles to buffer the relatively slow inward influx of dopamine, but a comparatively large bolus of amphetamine given on top of that may completely overwhelm the buffering capacity.

You are correct that there would be some bi-directional exchange of DA through VMAT, but that would normally be limited because most of the DA in vesicles is protonated (which keeps it from bring transported).

Trace amines can cause reverse-transport of catacholamines. But how would amphetamine up-regulate trace amine production over the course of a few minutes (the time course over which amphetamine effects can peak after iv administration)? You would obviously have to increase the action of one or more enzymes. But that isn't magically going to increase trace amine levels -- depending on the synthesis rate, the availability of precursor, and buffering processes such as degredation by MAO, it might take hours to days to see a meaningful increase in cytoplasmic concentration.

I was thinking about the timescale for the last phenomenon to take effect. I'll need to research it more, but maybe if one or more of the trace amine synthesising enzymes is under phosphorylation control, synthesis will be fairly rapid.

If it is the case that amphetamine concentrations are much more than dopamine concentrations in the neuron, then I can believe that amphetamine itself will be able to displace vesicular dopamine. I would however like to see some evidence or a calculation showing that this is the case

 

[serotonin2A]

I was thinking about the timescale for the last phenomenon to take effect. I'll need to research it more, but maybe if one or more of the trace amine synthesising enzymes is under phosphorylation control, synthesis will be fairly rapid.

If it is the case that amphetamine concentrations are much more than dopamine concentrations in the neuron, then I can believe that amphetamine itself will be able to displace vesicular dopamine. I would however like to see some evidence or a calculation showing that this is the case

RE point one: Such mechanisms are great for signaling when the molecular players are confined to discrete regions of the cell and signaling is amplified by downstream steps. But here you are talking about bulk synthesis. If the trace amines are going to play a role, then they are going to have to be produced at concentrations similar to that of amphetamine (otherwise any effect they have would be overwhelmed by the effect of amphetamine, which acts in exactly the same manner as the trace amines).

RE point two: cytoplasmic DA levels are low due to the action of MAO, packaging in vesicles, and feedback regulation of synthesis. DA does not hang around in the cytoplasm for very long.

EDIT This paper indicates the cytoplasmic DA concentration is likely in the range of 2.2 uM:

http://www.sciencedirect.com/science/article/pii/016502709090036F

Its obviously not a perfect test, but DA levels in those cells would be under the same regulatory constraints as are found in mammalian DAergic neurons.

 

[Seppi]

According to this picture on wikipedia (https://commons.wikimedia.org/wiki/F...1_Dopamine.svg), amphetamine is being portrayed as filling up the presynaptic dopamine vesicle after leaving the dopamine transporter, not allowing enough room in the vesicle for dopamine to fit back in the vesicles and forcing them to continue back out to the synapse -- I don't think this is the traditional view held regarding reuptake "reversal"; I hypothesize that, due to the difference in what side of the structure differing functional groups are located on the dopamine and amphetamine molecules, the amphetamine molecule is able to fit into the same places as dopamine much easier when its a tight pass for the hydroxyl groups on dopamine's phenyl ring - and, the opposite is true concerning the amine side of the two molecules, where the extra methyl group likely gets jammed or doesn't fit through the same as dopamine.

That image simply depicts amphetamine entering vesicles through VMAT2, causing dopamine to be released by an unspecified mechanism. Neither the image nor the caption suggest that amphetamine forces dopamine out because there's "not enough room" for dopamine. When I captioned that image (which I also drew), I intentionally avoided attributing a mechanism for dopamine efflux/reuptake inhibition at VMAT2 because there are 2 phenomena that occur when amphetamine interacts with vesicles (i.e., collapse of the pH gradient and amphetamine binding to the TBZ binding site of VMAT2) and it's not entirely clear if one or both of these mechanisms are responsible for efflux/reuptake inhibition, or if other mechanisms are involved.

For a recent reference covering this topic, see http://www.sciencedirect.com/science/article/pii/S2210533615300411

Also, amphetamine doesn't bind to dopamine receptors.

 

[Seppi]

RE point one: Such mechanisms are great for signaling when the molecular players are confined to discrete regions of the cell and signaling is amplified by downstream steps. But here you are talking about bulk synthesis. If the trace amines are going to play a role, then they are going to have to be produced at concentrations similar to that of amphetamine (otherwise any effect they have would be overwhelmed by the effect of amphetamine, which acts in exactly the same manner as the trace amines).

RE point two: cytoplasmic DA levels are low due to the action of MAO, packaging in vesicles, and feedback regulation of synthesis. DA does not hang around in the cytoplasm for very long.

EDIT This paper indicates the cytoplasmic DA concentration is likely in the range of 2.2 uM:

http://www.sciencedirect.com/science/article/pii/016502709090036F

Its obviously not a perfect test, but DA levels in those cells would be under the same regulatory constraints as are found in mammalian DAergic neurons.

Phenethylamine has some minor pharmacodynamic differences with amphetamine, but I'm not entirely sure how notable these are in the context of DA neuronal signaling. E.g., PEA signals through hTAAR2 with greater potency relative to hTAAR1 (phenethylamine's EC50s are 300 nm for hTAAR1 [this is the same order of magnitude as amphetamine's EC50 at hTAAR1] and ~0.5 nm for hTAAR2), but I'm not aware of hTAAR2 being expressed on dopamine neurons (NB: hTAAR2 is expressed in the human brain per PMID 27424325). Amphetamine's binding profile at hTAAR2 doesn't appear to have been studied in any primary research as of right now.

FWIW, amphetamine does markedly increase PEA biosynthesis in individuals with ADHD. I'm not aware of how significant this effect is relative to its other pharmacodynamic mechanisms. Overall, it's likely a fairly minor point though because, as you stated, PEA and amphetamine have very similar pharmacodynamics in dopamine neurons.

 

[AlphaMethylPhenyl]

The way to push the dopamine out of the vesicles, is not to fill amphetamine with them (via VMAT)?

PEA + an MAOI, for the record (if it isn't already obvious), do not produce identical sympathomimetic effects as amphetamine.

For some reason I thought TAAR2 is only present in the PNS.

 

[Cotcha Yankinov]

I think disruption of VMAT2's function due to changing pH is how amphetamine causes the cytosol to fill with dopamine

 

[MrSpeedyG]

subbed