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Making Sense of the Fact that MDMA has a poor affinity for SERT

nuke

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It's pretty clear from several different studies that MDMA and MDA have pretty much no affinity for human SERT.

Cloned human SERT
MDA: 107,000.000000nM, hot ligand 3H-5HT (Jones DC, et. al., 2004)

MDMA: 102,000.000000nM, hot ligand 3H-5HT (Jones DC, et. al., 2004)
> 10,000.000000nM, hot ligand 3H-CITALOPRAM (PDSP certified data)

The story is similar for NET and DAT.

In fact, amphetamine has much higher affinity for human SERT:

Cloned human SERT
d-AMP: 1,765.000000, Functional assay (Rothman RB, et al., 2003a)

So, what's the story? Does MDMA simply not displace 5HT, DA or NE somehow? Are functional assay the only clear way to get reasonable affinity values for amphetamine-type monoamine releasers? This latter question is curious too, since d-AMP displaces very well other substrates at the dopamine receptor. Something strange appears to be happening that's not in line with the theories produced so far as to pharmacological mechanism of action, which were all mostly based on data from lower order mammals.
 
Hahaha this is awesome. It always irked me how most people imagine that "moar serotonin = happpppyyyy!".

As for the question, perhaps it is pertinent to compare which parts (as in organs rather than receptors) of the brain does MDMA effect most and comparing to d-amp?
 
^agreed. I'd imagine a lot of mdma's empathic effects have quite a bit to do with prolactin, vasopressin, & oxytocin, though i'm unclear through what route it affects these hormomes.

Also, it seems serotonin release≠SERT affinity.

I could be wrong about these things, but from my limited knowledge base, they seem reasonable. Do tell if i'm incorrect

I'd really like to see some radiotagging of mdma & studies of live humans.
 
I thought it did something funny with SERT as in it is DISTINCT from you typical antidepressants. I.e Antidepressants generally make you feel rotten to start with and beneficial effects only building up later whilst MDMA hits all the NICE buttons straight away.
 
If I remember correctly the same is true for tianeptine though; it fails to displace (3H)5HT from SERT yet alters serotonin transport, apparently through SERT, when measured both by in vitro cell based assays and animal studies.

Most likely this would mean they are just binding to a seperate modulatory site distinct from where 5HT binds, the fact 5HT isn't displaced just means they are not competing for the same site on SERT, it doesn't mean they don't bind to SERT at all.
 
I don't buy it, as citalopram was tried as well. I mean, it's a protein of size 600 amino acids, there's only so many places it could go. There's a possibility it may form a homomeric dimer (which they're now beginning to believe that DAT is), but even then... It just seems really unlikely. There are quite a lot of spots for allosteric modulation in something huge like GABA receptors, but not for something that's believed to be as small as SERT.

And given the structural similar to DA and NE, why wouldn't it bind to the DAT and NET as well? Yet at the same time, d-AMP binds very well to the human SERT?
 
I thought it did something funny with SERT as in it is DISTINCT from you typical antidepressants. I.e Antidepressants generally make you feel rotten to start with and beneficial effects only building up later whilst MDMA hits all the NICE buttons straight away.

the (positive) effects of SSRIs are dubious at best, but the more popular theory right now is that when they work (i've only seen better-than-marginal statistical evidence for ssris efficacy in women with near debilitating or debilitating depression) it is due to an increase in hippocampal volume, however there are some studies that seem to show that even when hippocampal neurogenesis is blocked, there is still an effect, in rats in a forced swim test anyway... I suspect the changes in the hippocampus are consequent to some change in activity in the medial pFC, but that's just a guess.

Back to MDMA, what about other metabolites having SERT affinity? I haven't looked, but I'd imagine MDA's are similar to MDMA, but what about HMMA? And the affinity, albeit weak, for 5ht1 seems to be enough to cause some significant hormonal changes, so it may be a combination of individually weak, but synergistic effects.
 
It's pretty clear from several different studies that MDMA and MDA have pretty much no affinity for human SERT.

Are you absolutely sure that this is the general consensus? Though it makes me sound like an annoyingly skeptical prick, I have to ask, as I've read far too many papers that either directly take measurements at the transporter or mention the drug's supposedly high affinity for the mentioned site. In other words, this is the first I've heard of it. If anything, I can be sure that the substantial volume of related literature I've read on the subject accounts for a large enough portion of the research community that your claim of 100,000nM seems more than a bit outlandish.

On the other hand, if such an odd discrepancy were genuine, it doesn't make too much of a difference. MDMA's objective effects are clearly either mediated by significant 5-HT release [among a few other nuances], or some mysterious and unlikely mechanism that near-exactly mimics the former. But again, even if the releasing agent's affinity as an actively transported substrate is less than significant, the compound could still quite easily be admitted into the presynaptic terminal button via direct passive diffusion across the membrane, after which the transporter phosphorylation--->reversal can take full effect.

Back to MDMA, what about other metabolites having SERT affinity?

Good point.
 
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I haven't bothered looking at the literature on MDMA for a quite a long time,
if we go with the assumption that the primary effects of MDMA come from it being a monoamine releaser then
there are a few things that are interesting, It seems MDMA does not require SERT to be taken up by the neuron, it seems that it can be transported just as easily by DAT or NET, SERT can be reasonably selctively blocked yet MDMA still ends up inside.
MDMA like most of the releasing amphetamines seems primarily to interact with VMAT-2 and otherwise cause disruption of vesicular uptake of monoamines, it goes further and causes the vesicles to dump the monoamines to the cytosol./plasm I suspect that the high cytoplasmic concentrations of monoamines then drive the reuptake transporters backwards.
this kind of fits with a couple of observations, the use of a SERT inhibitor blocks some of the subjective effects of MDMA, the inhibitor is not transported instead it locks the transporter in the extracellular configuration, the transporter then cannot do anything, and it cannot dump serotonin from the high intracellular concentration to the lower synaptic concentration.
the other thing is that the usual way of determining transporter affinity using a hot ligand might be flawed in the case of MDMA and similar, because it is quite possible that MDMA can interact with the transporter but not displace the hot ligand. there is already evidence with DAT that there is a high affinity site for dopamine and a high affinity site for the phenyltropane hot ligands, so using hot dopamine would give erroneous data for Phenyl tropane affinity and vice versa. or maybe there are distinct DAT populations?
its all a mystery, the more we learn the less we know
 
the other thing is that the usual way of determining transporter affinity using a hot ligand might be flawed in the case of MDMA and similar, because it is quite possible that MDMA can interact with the transporter but not displace the hot ligand.

I've given this idea quite a bit of thought in the past, though I could never find any sound evidence in the literature for a mechanism of transporter uptake that wouldn't displace the commonly used reference ligands. There may be multiple distinct binding sites for uptake inhibitors/ligands, but for substrates? Wouldn't it have to be, like, a "double transporter" or something?

or maybe there are distinct DAT populations?

This one seems like much more of a stretch. I'm pretty sure we have the full sequence homology for the transporter, and I've never heard of any variations massive enough to accommodate multiple subtypes. But I suppose that still doesn't account for any potential in vivo modification by intracellular mechanisms [e.g. phosphorylation] that could create significant inter-region variation in transporter activity and/or binding.

I suspect that the high cytoplasmic concentrations of monoamines then drive the reuptake transporters backwards

Okay. But HOW? Why aren't they just quickly swept up by MAO, apropos reserpine (the amphetamines' MAO inhibition is nowhere near potent enough to address the issue)? I've been puzzling over the amine releasers' mechanism for a long time, and am repeatedly frustrated by the relative dearth of satisfying answers. None of the proposed mechanisms with which people seem to be satisfied make much sense in light of other conflicting or muddling results of the agonizing number of studies on the topic. [See the 'short questions' sticky for a more comprehensive rant]
 
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a few points, it is not necessary for there to be a different sequence of amino acids for multiple populations of the transporter to exist, post translational modification is more than sufficient. IMHO the biggest failure in molecular biology is the constant over simplification to a gene sequence. The proteome is far larger than the genome and biologists need to get their heads round it, it isn't neat and tidy and is much harder to work out than the DNA sequences but it is the answer to a lot of questions in a lot of fields.

I have a pile of papers on the subject of the binding of dopamine transporter inhibitors related to the DA uptake, a few are available online free http://www.exciteddelirium.org/StaleyJPET94.pdf
The most interesting work is a series of very solid experiments originally done for a PhD thesis in the states, here the Dopamine uptake inhibition potency varies with time and strongly suggests there are changes to the populations of DAT. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855629/ there are unfortunately no simplistic predigested reviews on this topic and it requires a lot of reading to get the overview.

With respect to amphetamine mechanism, there are a couple of fairly recent reviews, the mechanism is complex and it interacts with multiple cellular systems. I think if you review the data you will find you are mistaken in thinking that MAO rapidly chews up the monoamines dumped from the vesicles, measurements of the cytoplasmic concentration of monoamines shows that AMP and METH increase cytoplasmic concentrations massively.

there is still a lot to figure out and some of the data is conflicting, there is not a grand unified theory of AMP action, I suspect there never will be. The elements will almost certainly include MAT phosphorylation, MAT population changes probably a subtle conformal change, VMAT-2 inhibition or reversal, changes to the storage vesicles bringing them into the releasable pool, weak base effect on the vesicles, gene activation, and direct and indirect agonism at various receptors.

I am no expert in this field, it is some time since I last had good reason to look at it, I would love for someone to actually trawl through the literature and make a decent review, there is enough data, just a lack of skeptical and critical analysis of the data.





I've given this idea quite a bit of thought in the past, though I could never find any sound evidence in the literature for a mechanism of transporter uptake that wouldn't displace the commonly used reference ligands. There may be multiple distinct binding sites for uptake inhibitors/ligands, but for substrates? Wouldn't it have to be, like, a "double transporter" or something?



This one seems like much more of a stretch. I'm pretty sure we have the full sequence homology for the transporter, and I've never heard of any variations massive enough to accommodate multiple subtypes. But I suppose that still doesn't account for any potential in vivo modification by intracellular mechanisms [e.g. phosphorylation] that could create significant inter-region variation in transporter activity and/or binding.



Okay. But HOW? Why aren't they just quickly swept up by MAO, apropos reserpine (the amphetamines' MAO inhibition is nowhere near potent enough to address the issue)? I've been puzzling over the amine releasers' mechanism for a long time, and am repeatedly frustrated by the relative dearth of satisfying answers. None of the proposed mechanisms with which people seem to be satisfied make much sense in light of other conflicting or muddling results of the agonizing number of studies on the topic. [See the 'short questions' sticky for a more comprehensive rant]
 
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Bumping this from the dead.

Like a number of us, I've been watching the Coursera "drugs and the brain course" (much delayed) - https://www.coursera.org/course/drugsandbrain

In this Prof Lester simply states that MDMA diffuses passively through the cell membrane and the vesicle membrane, thus it doesn't require any particular affinity for SERT or VMAT. This is supported by vektors point that serotonin does not need SERT to get into the cytoplasm.

So affinity for otherwise for SERT actually has very little to do with MDMA's mechanism.

P A states there is a tonne of literature measuring MDMA's affinity for SERT. I am unfamiliar with this literature, is anyone able to provide some basic refs?
 
^agreed. I'd imagine a lot of mdma's empathic effects have quite a bit to do with prolactin, vasopressin, & oxytocin, though i'm unclear through what route it affects these hormomes.

Also, it seems serotonin release≠SERT affinity.

I could be wrong about these things, but from my limited knowledge base, they seem reasonable. Do tell if i'm incorrect

I'd really like to see some radiotagging of mdma & studies of live humans.
It appears that 5HT-1A receptors are implicated in oxytocin release, and that oxytocin is accountable for at least some of MDMA empathogenic effects.http://www.sciencedirect.com/science/article/pii/S0306452207001960
 
I find this really interesting, whilst I obviously dont have the same level of knowledge of many who post in ADD, I've got a few things to throw in. Obviously these arent questions that could be answered, but just add to the discussion of it all. Skip to the bold if you dont want to read my getting technical and explaining the reason to the bold statements.

1. Going under the idea that MDMA has little or no affinity for the SERT, why do SSRIs block MDMAs effect if it doesnt use the transporter to work.

Im asssuming that it means, whilst MDMA having no affinity for the SERT itself actually does make it inside the neuron and does something to cause serotonin to be availible for the SERT to pump it out on its own actions and obviously the SSRI is preventing it that way.

Alternatively, SSRI's and MDMA work on the same mechanism that isnt actually directly involving the SERT?

2. I find it odd how MDMAs effects come from the massive release of serotonin... If it simply did dump serotonin like that surely it would be activating all the serotonin receptors and more serotonin as said before does not mean SMILES AND EUPHORIA. MDMAs effects from serotonin agonism all appear to be from the 5HT1A (and all 5HT1 subsets, either way) and also the 5HT2A (and like 5HT1 also the 5HT2 subsets like 5HT2B and 5HT2C) but the typical effects that come from the agonism of other serotonin receptors, 5HT3 for example which causes vommitting (yes which is seen in some people whether comming up, or rolling WAY to hard never the less) its like this receptor is never activated in the way the 5HT1 and 5HT2 type recptors are. Yes it is known that MDMA is a direct agonist at the 5HT1A and B and slightly less at the 5HT2A, enough at the 5HT2B to show cardiac fibrosis like fenfluramine does, and also the 5HT2C which IIRC is resposible for its body warmth sensation and touch enhancment anyway its besides the point here.

This is obvious that whilst MDMA is said to cause a massive release of serotonin as methamphetamine causes a massive release of dopamine, however while amp does not release dopamine EVERYWHERE and in the specific parts of the brain most notably the striatum/reward centres, the release causes all types of dopamine receptors to be antagonised however if MDMA really did cause a release of serotonin like believed and subsqeuently binding to all those recptors, it would cause in theory much different effects due to all serotonin receptors beign agonised.

My point being here, contrarry to what we believe in MDMA being a serotonin realeaser like meth is a dopamine releaser, I feel it isnt. It seems like MDMA is somehow directly, and strongly for that matter, agonising 5HT1 receptors and slightly less the 5HT2 receptors - an offset note, MDA is more selective for its 5HT2 agonism than 5HT1 which makes sense why it is more psycedelic and less lovey-dovey than MDMA.

5HT1A agonism has been shown to be repsonsible for the realease of some of the hormones involved in creating MDMA what it is, ie oxytocin for bonding etc.

3. I've seen studies that post MDMA use, there is a reduction in SERT aswell as SERT binding. How so if MDMA does not actually rely that much on SERT.

Or is the answer simply so much serotonin being pumped around naturally it is the body trying to be in homeostasis downregulating sert like it does with serotonin receptors when over firing/used.

4. MDMA has also been shown to directly affect the IIRC amygdala to reduce ones defense mechanisms hence contributing them to "opening-up" and not recognise such angry faces (studies done on that people on MDMA can no recognise anger/fear facial expressions aswell when sober).

So...

Maybe MDMA doesnt release serotonin, and if it did, why wouldnt it just blunt emotion and create a flat effect like SSRIS? Hmmm... not sure about you but I know sometimes myself and many others find that amphetamine and meth also actually do release serotonin obviously meth much more... however the effects of amp/meth serotonin release do seem to fit in with simple realease and all round serotonin agonism... Essentially what I am saying is SSRIs cause equal serotonin recptor activation and its constant which is why it gives such a dull effect on mood and comes with side effects that happen when other serotoin receptors besides 1A/B and 2A/B/C are activated such as vommiting from 5HT3... meth and amp can give that dulled out serotonergic feel and are known serotonin releasers too a much extent than dopamine/noradrenaline and that all makes sense and that MDMAs effects come from the somehow activation of 5HT1A through its direct activation and release of all the associated hormones, and it also interacts directly IIRC with the amygdala.

But, what really does not make sense is this: if MDMA is really, allthough primarily a serotonin releaser but also a releaser of dopamine and norepinphrine... why do SSRIs seem to block ALL the effects of MDMA if they only interact with the SERT and prevent anythign getting into the neuron, MDMA's supposed mechanism of getting into the neuron, but more importantly, why are the no noradrenergic or dopaminergic effects occuring in people who are on SSRIS and take MDMA - bear in mind this includes people having no amp/MDMA tolerance or use and even people using hefty amounts of MDMA to feel something when on SSRIS surely the would feel the effects of amphetamine at the least...amphetamine can be felt and used and IS used for people on SSRIs.

That meaning, maybe MDMA is not a releaser of either of the 3 monoamines.

I know its hard to read and comprehend what I am saying, took about 30 minutes to get it all out, but there is sense in it and I feel like I am bringing some important points here if you can understand what I am saying.
 
Going way back to P A's comment: SERT is a double transporter as its a sodium-neurotransmitter symporter (both are transferred into the cell). Now granted, SERT is a small protein but I would not be surprised if it had some weird binding site ala modafinil at DAT.

Perhaps it binds intercellularly, or activates a PKC(beta?) dependent phosphorylation once inside and avoids SERT binding entirely. Or you know maybe it's retardidly high dose relative to other amphetamines is due to its membrane penetration rather than it's metabolism?
 
I

1. Going under the idea that MDMA has little or no affinity for the SERT, why do SSRIs block MDMAs effect if it doesnt use the transporter to work.

Im asssuming that it means, whilst MDMA having no affinity for the SERT itself actually does make it inside the neuron and does something to cause serotonin to be availible for the SERT to pump it out on its own actions and obviously the SSRI is preventing it that way.

Yes. SSRIS block the effect of MDMA not by stopping MDMA being taken up into the presynaptic terminal, but by blocking the efflux of 5HT that MDMA causes by shifting the concentration of 5HT in the cytosol that then drives SERT in reverse.

Alternatively, SSRI's and MDMA work on the same mechanism that isnt actually directly involving the SERT?
No, they seem to work via different mechanisms.

My point being here, contrarry to what we believe in MDMA being a serotonin realeaser like meth is a dopamine releaser, I feel it isnt. It seems like MDMA is somehow directly, and strongly for that matter, agonising 5HT1 receptors and slightly less the 5HT2 receptors - an offset note, MDA is more selective for its 5HT2 agonism than 5HT1 which makes sense why it is more psycedelic and less lovey-dovey than MDMA.

5HT1A agonism has been shown to be repsonsible for the realease of some of the hormones involved in creating MDMA what it is, ie oxytocin for bonding etc.
Except there's plenty of research showing massive increases in extracellular serotonin under MDMA. So it does appear to be a releaser.

3. I've seen studies that post MDMA use, there is a reduction in SERT aswell as SERT binding. How so if MDMA does not actually rely that much on SERT.

Or is the answer simply so much serotonin being pumped around naturally it is the body trying to be in homeostasis downregulating sert like it does with serotonin receptors when over firing/used.
If the brain was responding to there being too much 5HT in the synaptic gap, then if anything you would see an upregulation of SERT to remove the excess 5HT, but I'm not sure that happens. IIRC the feedback loop involves 5HT1A autoreceptors that reduce release of presynaptic 5HT.

Apparent reductiion in SERT binding via PET is, I think, supposed to show neurotoxicity, the reduction of SERT indicating pruning.



But, what really does not make sense is this: if MDMA is really, allthough primarily a serotonin releaser but also a releaser of dopamine and norepinphrine... why do SSRIs seem to block ALL the effects of MDMA if they only interact with the SERT and prevent anythign getting into the neuron, MDMA's supposed mechanism of getting into the neuron, but more importantly, why are the no noradrenergic or dopaminergic effects occuring in people who are on SSRIS and take MDMA - bear in mind this includes people having no amp/MDMA tolerance or use and even people using hefty amounts of MDMA to feel something when on SSRIS surely the would feel the effects of amphetamine at the least...amphetamine can be felt and used and IS used for people on SSRIs.

That meaning, maybe MDMA is not a releaser of either of the 3 monoamines.

I know its hard to read and comprehend what I am saying, took about 30 minutes to get it all out, but there is sense in it and I feel like I am bringing some important points here if you can understand what I am saying.

I can't remember off the top of my head what MDMA's NE and DA releasing/re-uptake blocking effects are, but my feeling is they are pretty minor (compared to d-amp or meth) so they are more just adding "colour" to the MDMA experience and on their own are not enough to be particularly noticeable. That's pure speculation, I'm sure there are reference that could back this up or disprove it.
 
There are dozens of studies that find MDMA to have a reasonably high affinity for SERT.

I don't buy the study this is taken from- there are far too many other studies which contradict the findings in this study. Unless there are a bunch of newer studies which find it to have no affinity, and someone comes up for a really good explanation for why all of the other studies come up with very different numbers, I don't think there's any reason to be wringing our hands looking for a new explanation for MDMA's psychoactivity.

The fact that this thread has generated such long, detailed posts, makes me a little sad because there are more important questions which deserve attention.

I'm getting tired, so I'm not going to put too much effort into this, but consider the numbers they get here- http://www.jneurosci.org/content/31/19/7190.full (that's even the full article)

R-MDMA exhibited moderate affinity for 5-HT2A receptors (Ki = 4.7 ± 1.1 μM) and lower affinity for SERT (Ki = 24.5 ± 0.8 μM). R-MDMA exhibited no appreciable affinity for either 5-HT1A receptors, NET, or DAT (Ki > 50 μM) (Table 2). In contrast, S-MDMA exhibited no affinity at 5-HT1A and 5-HT2A receptors (Ki > 50 μM), but high affinity for SERT (Ki = 222 ± 62 nM) and moderate affinity at both DAT and NET (Ki = 2.3 ± 0.4 μM and 7.8 ± 2.1 μM, respectively) (Table 2).

The Journal of Neuroscience, 11 May 2011, 31(19): 7190-7198; doi: 10.1523/​JNEUROSCI.1171-11.2011

Also fairly new.

Full article available for this one too- http://link.springer.com/article/10.1007/s00213-005-0174-5?LI=true. It's a good study, they're clear about the number of trials they're giving each assay (3 for all, but it looks like they did 7 for MDMA vs. SERT), and they used a hot ligand, not an antagonist.

It finds the following for MDMA: [3H](+)MDMA Ki= 447nM (+/-) 197

This study introduces a measure I hadn't seen before, though- the velocity of transport. I haven't read this fully yet, but it appears that [+]-MDMA is transported by SERT extremely fast- about 10x faster than it transports serotonin! that's super interesting, IMHO, and may have significance for explaining MDMA's psychoactivity.
 
My point being here, contrarry to what we believe in MDMA being a serotonin realeaser like meth is a dopamine releaser, I feel it isnt. It seems like MDMA is somehow directly, and strongly for that matter, agonising 5HT1 receptors and slightly less the 5HT2 receptors - an offset note, MDA is more selective for its 5HT2 agonism than 5HT1 which makes sense why it is more psycedelic and less lovey-dovey than MDMA.

In addition to what Hammilton said, the major problem with this theory is this:

With MDMA, the usual assignments of activity to optical isomers is reversed from all of the known psychedelic drugs. The more potent isomer is the "S" isomer, which is the more potent form of amphetamine and methamphetamine. This was one of the first clear distinctions that was apparent between MDMA and the structurally related psychedelics (where the "R" isomers are the more active). Tolerance studies also support differences in mechanisms of action. In one study, MDMA was consumed at 9:00 AM each day for almost a week (120 milligrams the first day and 160 milligrams each subsequent day) and by the fifth day there were no effects from the drug except for some mydriasis. And even this appeared to be lost on the sixth day. At this point of total tolerance, there was consumed (on day #7, at 9:00 AM) 120 milligrams of MDA and the response to it was substantially normal with proper chronology, teeth clench, and at most only a slight decrease in mental change. A complete holiday from any drug for another 6 days led to the reversal of this tolerance, in that 120 milligrams of MDMA had substantially the full expected effects. The fact that MDMA and MDA are not cross-tolerant strengthens the argument that they act in different ways, and at different sites in the brain.

http://www.erowid.org/library/books_online/pihkal/pihkal109.shtml
 
I find it extremely interesting that MDMA and MDA do not share cross tolerance... Amp and Methamp do?
 
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