• Neuroscience & Pharmacology Discussion Welcome Guest
    Posting Rules Bluelight Rules
    Recent Journal Articles Chemistry Mega-Thread
    FREE Chemistry Databases! Self-Education Guide

  • N&PD Moderators: Skorpio

MDMA does not induce reverse serotonin transport

5-HT2

5-HT2

Bluelight Crew
Joined
Oct 7, 2001
Messages
7,158
Location
Europe
I thought I'd return to Bluelight with a juicy little tidbit of as-of-yet unpublished research. Michael Quick, whose lab at USC studies neurotransmitter transporters, recently came to my school to give a talk. He presented some of his work on the serotonin transporter (SERT), including how it is modulated by MDMA as well as the protein syntaxin 1A, a component of the synaptic vesicle release machinery (most of it has been published in this article: Quick MW, 2003. Regulating the conducting states of a mammalian serotonin transporter. Neuron 40(3):537-49.). After the talk, I asked him what the implications of his findings were for the two major postulated mechanisms of MDMA action: namely, reverse transport of serotonin and stimulation of vesicular release. Excited by all the work on amphetamine-induced reverse transport of dopamine that has been published recently, his lab has investigated whether similar things were going on with the SERT and MDMA-induced serotonin efflux. Surprisingly, they found nothing. The implication of this is that MDMA must induce serotonin efflux by somehow stimulating vesicular release of serotonin. In its native environment, the SERT seems to be closely associated with or bound to syntaxin 1A. This raises the possibility that MDMA directly stimulates the vesicular release machinery by inducing a cascade of protein conformational changes from SERT to syntaxin to SNAREs.

Perhaps a FRET-based assay comparing syntaxin conformational change induced by serotonin and MDMA, and looking for a FRET effect that is selectively induced by MDMA but not sertonin, could elucidate whether this is the mechanism.
 
I am not as knowledgable as some people on this board, but the theory of reverse transport MDMA-induced serotonin release has never sat well with me. Maybe it's because it just logically did not make sense. But anyways, I will be watching this thread, cause this is a really interesting subject.
 
^^^Hey, it happens with the most closely related transporter (dopamine), so it was a plausible hypothesis at one point.
 
^^^I guess for that exact reason it did not sit well with me. If dopamine does that then serotonin must do too? I have never heard more proof or reasoning behind it. For example, and again you are talking to an amateur here, why cant serotonin release happen indirectly as a result of DA/NE release? I am not saying this theory is wrong, but it's just too cute of an explanation and not much theory behind it, that is why you got reaction out of me with your post.

Plus, if these transporters can become a two-way street, you would need a lot of them going in reverse at the same time for the flood of serotonin to happen. It is just much easier (and therefore more likely) for it to happen going directly out of vesicle...

Anyways, big applause for your post, truly fascinating!!!
 
Last edited:
I definitely believe neurotransmitter transporters are bidirectional. Twin studies performed by Lin et al (1996) and Galli et al (1996) on the 5-HT transporter (SERT) and the noradrenalin transporter (NAT), respectively, demonstrated that neurotransmitter transporters have ionic currents similar to ion channels (indeed, they are ion channels too) and that the transporters have reversal potentials. In this conductance state, direction of monoamine flow is dependent on membrane potential. Strangely, however, the authors also demonstrated that transporters have another conductance state: classical fixed-stoichiometry transmitter/ion uptake and exchange. So, while the picture is a little muddy, reverse transport is certainly possible.

The work of Quick et al has really been a major source for reviews that I have written concerning transporters. The Quick (2003) paper was especially fascinating--he definitely demonstrated that syntaxin 1A not only interacts with SERT, but that syntaxin 1A is indeed regulating the conduction state of SERT via protein-protein interactions. I didn't know that Quick was studying MDMA, but if his (or your) hypothesis is that MDMA causes serotonin efflux by the following cascade: SERT --> sytaxin 1A --> SNARE complex binding --> Vesicular Release, then there should be a simple way of testing this hypothesis. Administration of botulism toxin C (BoToxC) should completely cleave syntaxin 1A, stopping the cascade, rendering MDMA ineffective. If MDMA causes reverse transport, BoToxC should do nothing to prevent its effects.
 
^^^There are some MDMA experiments in the 2003 Neuron paper, in the last figure.

In my opinion, that paper provided pretty strong evidence that SERT is bound to syntaxin 1A in its native environment (i.e. plasma membrane of neurons). Because syntaxin 1A locks the transporter to an electroneutral rather than electrogenic mode of transport, it seems unlikely that reverse transport can take place in neurons unless there is some vesicle-release-induced change in syntaxin 1A conformation that makes electrogenic transport possible. So perhaps Quick's lab has just not looked for reverse transport under the appropriate conditions. Still, I think it is unlikely. One of us should e-mail Quick to ask if he's done the MDMA/BoToxC experiment.
 
How were the experiments conducted? I used to have a copy of that article at home, but I've lost it now, and science direct isn't bringing up the full article.

I've got to say 'huh?', I mean, this goes against such a huge weight of research... One thing that definatly sticks out is, Quick is patching these cells isn't he? (Judging by the Na current stuff), it's quite likely then that hes removed PKC from the intracellular environment, which has been shown to be central to amphetamine mediated release, through the NET and the DAT, and is also stimulated by MDMA.
 
Well he said he did "essentially the same experiments" as the people who are looking at amphetamine-induced reverse transport of dopamine, whatever that means. I should have quizzed him about it more thoroughly but its been a few months since I've read that literature.
 
So like synaptosomes, and then HPLC... People have shown MDMA release serotonin from cortical synaptosomes heaps of times. Maybe he just screwed up producing the synaptosomes... did he validate with K+ evoked release.... I geuss you don't know... he should get the results out!
 
I just found this paper, pretty odd stuff. I don't buy it, but it's still interesting.
Mol Pharmacol. 2005 Jan;67(1):140-51.

Amphetamines take two to tango: an oligomer-based counter-transport model of neurotransmitter transport explores the amphetamine action.

Seidel S, Singer EA, Just H, Farhan H, Scholze P, Kudlacek O, Holy M, Koppatz K, Krivanek P, Freissmuth M, Sitte HH.

Institute of Pharmacology, Medical University Vienna, Wahringer Str. 13a, A-1090 Vienna, Austria. [email protected]

Amphetamine congeners [e.g., 3,4-methylenedioxymetamphetamine (MDMA), or "ecstasy"] are substrates for monoamine transporters (i.e., the transporters for serotonin, norepinephrine, and dopamine); however, their in vivo-action relies on their ability to promote monoamine efflux. The mechanistic basis for this counter transport remains enigmatic. We tested the hypothesis that outward transport is contingent on the oligomeric nature of neurotransmitter transporters by creating a concatemer of the serotonin transporter and the amphetamine-resistant GABA transporter. In cells expressing the concatemer, amphetamine analogs promoted GABA efflux and blunted GABA influx. In contrast, the natural substrates serotonin and GABA only cause mutual inhibition of influx via the other transporter moiety in the concatemer. GABA efflux through the concatemer that was promoted by amphetamine analogs was blocked by the protein kinase C inhibitors GF109203X (bisindoylmaleimide I) and Go6983 (2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide). Thus, based on our observations, we propose that, in the presence of amphetamine analogs, monoamine transporters operate as counter-transporters; influx and efflux occur through separate but coupled moieties. Influx and efflux are coupled via changes in the ionic gradients, but these do not suffice to account for the action of amphetamines; the activity of a protein kinase C isoform provides a second stimulus that primes the inward facing conformation for outward transport.
 
BilZ0r said:
So like synaptosomes, and then HPLC... People have shown MDMA release serotonin from cortical synaptosomes heaps of times. Maybe he just screwed up producing the synaptosomes... did he validate with K+ evoked release.... I geuss you don't know... he should get the results out!
Click to expand...

Don't synaptosomes also contain the vesicular release machinery and vesicles?
 
Yeah, definatly. So... I thought this guy would have shown that synaptosomes didn't realease 5-HT under any circumstances.
 
Ummm…no.

A direct protein-protein conformational chain interaction starting with the SERT could stimulate vesicular release from synaptosomes even in the absence of reverse transport.
 
I don't get you
Excited by all the work on amphetamine-induced reverse transport of dopamine that has been published recently, his lab has investigated whether similar things were going on with the SERT and MDMA-induced serotonin efflux. Surprisingly, they found nothing
Click to expand...

I'm assuming from this, they made synaptosomes, added some MDMA, and recorded no 5-HT release. If this is not what they did, can you say what they did. If this is what they did, then you're theory doesn't make sense as there would still be vesicular machinary.
 
I don't know what he did, because he didn't describe the experiments in detail and I foolishly didn't ask. I'm guessing he inactivated some component of the release machinery (probably SNAREs) with one of the botulinum toxins and then failed to observe any serotonin efflux.
 
You must log in or register to reply here.
Top