Amps releasing DA at high doses?

[MichelJacques]

Hi. I've read in numerous places that amphetamines are dopamine reuptake inhibitors at low dosages, but cause dopamine release at higher doses. However, I have only come across this on forums and not in the primary lit. Could anybody point me in the direction of some studies demonstrating this dose dependent activity? Thanks.

 

[sekio]

Compare and contrast Ki at DAT, NET, SERT versus EC50 at the same and then you have your answer.

(for d-amphetamine, units in nM, smaller numbers = more effective)
NE Release 7.07 ± 0.95
NE Reuptake 38.9 ± 1.8
5HT Release 1,765 ± 94
5HT Reuptake 3,830 ± 170
DA Release 24.8 ± 3.5
DA Reuptake 34 ± 6

http://www.ncbi.nlm.nih.gov/pubmed/11071707

 

[endotropic]

Compare and contrast Ki at DAT, NET, SERT versus EC50 at the same and then you have your answer.

(for d-amphetamine, units in nM, smaller numbers = more effective)
NE Release 7.07 ± 0.95
NE Reuptake 38.9 ± 1.8
5HT Release 1,765 ± 94
5HT Reuptake 3,830 ± 170
DA Release 24.8 ± 3.5
DA Reuptake 34 ± 6

http://www.ncbi.nlm.nih.gov/pubmed/11071707

Those numbers suggest the opposite pattern the OP suggested. It's not really clear what the "reuptake" numbers mean either, if the drug is already causing release, then the transporter is already working in reverse.

What would blocking reuptake even accomplish at that point? Unless the transporter can release transmitter and take it up in the same time period?

 

[sekio]

I think the figures are EC50 in nanomoles for release and Ki in nanomoles, respectively. Here's a PDF link.

[...] if the drug is already causing release, then the transporter is already working in reverse. What would blocking reuptake even accomplish at that point? Unless the transporter can release transmitter and take it up in the same time period?

Release and reuptake are separate assays: A transport reverser will "act like" a reuptake inhibitor in an assay for one, because it displaces dopamine or whatever from its active site on the transporter.

In contrast to uptake inhibitors, which were essentially inactive in the release assays, compounds known to be substrates were active in both uptake and release assays, although they were generally 5–10-fold more
potent in the release assays.

The data certainly indicate that amphetamine is a releaser more than a reuptake inhibitor, even at low doses, at least in vitro.

 

[endotropic]

Release and reuptake are separate assays: A transport reverser will "act like" a reuptake inhibitor in an assay for one, because it displaces dopamine or whatever from its active site on the transporter.


The data certainly indicate that amphetamine is a releaser more than a reuptake inhibitor, even at low doses, at least in vitro.

I understand how they got those results, I'm just trying to figure out what that all means in practice. In my mind the fact that a releasing agent has activity on a reuptake inhibition assay is meaningless, but I feel like I'm missing something.

 

[Lightning-Nl]

Actually it's the opposite. In lower dosages, amphetamines are really only monoamine releasers. Why is this? Because they're affinity for diffusing into cells is much higher than their affinities for the DAT, NET and SERT.

 

[ebola?]

This is not the opposite of the hypothetical condition posited, and it is also not true. Under conditions of low concentration, binding at monoaminergic transporters will matter more than diffusion through the cell membrane, as it is only high concentrations of drug that can drive such diffusion. Regardless, given the above discussion, it doesn't seem that there is any dose of amphetamine where we should expect the significance of reuptake inhibition to eclipse that of transporter reversal (unless there's some condition we can think of where binding occurs but transporter reversal does not).

ebola

 

[Lightning-Nl]

This is not the opposite of the hypothetical condition posited, and it is also not true. Under conditions of low concentration, binding at monoaminergic transporters will matter more than diffusion through the cell membrane, as it is only high concentrations of drug that can drive such diffusion. Regardless, given the above discussion, it doesn't seem that there is any dose of amphetamine where we should expect the significance of reuptake inhibition to eclipse that of transporter reversal (unless there's some condition we can think of where binding occurs but transporter reversal does not).

ebola

My mistake. I should have more correctly stated that amphetamine will just get uptaken by DAT, NET, and SERT - which it can then agonize TAAR and reverse the transporters.

In fact, it's effects are actually on all the trace-amines. Saying that it's selective for DAT specifically is a little understating. Also, it's affinity for Monoamine Oxidase probably contributes... But not at low dosages, now that I think about it...

 

[MichelJacques]

Thank you for posting that paper, Seiko.

This is not the opposite of the hypothetical condition posited, and it is also not true. Under conditions of low concentration, binding at monoaminergic transporters will matter more than diffusion through the cell membrane, as it is only high concentrations of drug that can drive such diffusion. Regardless, given the above discussion, it doesn't seem that there is any dose of amphetamine where we should expect the significance of reuptake inhibition to eclipse that of transporter reversal (unless there's some condition we can think of where binding occurs but transporter reversal does not).

My mistake. I should have more correctly stated that amphetamine will just get uptaken by DAT, NET, and SERT - which it can then agonize TAAR and reverse the transporters.

In fact, it's effects are actually on all the trace-amines. Saying that it's selective for DAT specifically is a little understating. Also, it's affinity for Monoamine Oxidase probably contributes... But not at low dosages, now that I think about it...

Hmm, this is a bit confusing to me. I think I have a misunderstanding of amphetamine's mode of action. I thought the drug was binding to the monoamine transporters on the exterior of the cell, causing the presynaptic cell to inhibit reuptake of the given monoamine. You're both making it sound more like amphetamine is being uptaken by the monoamine tranporter and then agonizing the transporter intracellularly, with the primary effect being release. Correct?

Either way, I'm with Endotropic. I don't understand how amphetamine could inhibit reuptake if it is already causing release. Doesn't release preclude reuptake?

Also, never heard of the trace amine-associated receptor. Would somebody be willing to give me a quick synopsis on how this is associated with the transporter? EDIT: nevermind, just found your thread on it, SwampFox (http://www.bluelight.org/vb/threads...ons-of-Trace-Amines-Phenethylamine-Derivative)

 

[sekio]

In fact, it's effects are actually on all the trace-amines. Saying that it's selective for DAT specifically is a little understating. Also, it's affinity for Monoamine Oxidase probably contributes... But not at low dosages, now that I think about it...

At "therapeutic" doses most of the effects of amphetamine are mediated by its ability to release norepinephrine, activity at downstream targets (DA receptors, 5ht receptors, adrenergic, TAAR, MAO) seems to be less important. (The effects of amphetamine on SERT are not significant at human dose levels.) This may also explain why drugs like e.g. ethcathinone - strong NE releasers - are still considered to be worthwhile stims.

Excerpt:

Our results show that the most potent action of the stimulants which produce amphetamine-like subjective effects is to release NE. Importantly, (+)-amphetamine, (-)-ephedrine, phentermine, and MDMA are 3.5-fold, 19-fold, 6.6-fold, and 4.8-fold more potent at NE release than DA release. These data raise the possibility that the release of NE contributes to the positive subjective effects produced by these substrate-type stimulants. We term this the “noradrenergic hypothesis.” A direct prediction of the release data and the noradrenergic hypothesis is that oral doses of these medications will produce sympathomimetic effects, which are mediated via release of NE, and amphetamine-like subjective effects, at lower doses than effects which are mediated by DA release. In humans, the noradrenergic effects of these compounds can be assessed via measurement of physiological parameters such as systolic blood pressure. The dopaminergic effects of these compounds can be assessed by their effect on plasma prolactin levels, which are decreased by dopaminergic agonists (Ascoli and Segaloff, 1996), and apparently not affected by agents which increase NE, such as the selective NE uptake inhibitor maprotiline (Steiger et al., 1993) or the selective NE releaser and adrenergic agonist ephedrine (Angrist et al., 1977). The noradrenergic hypothesis receives strong support from studies conducted in humans. Oral doses of D-amphetamine in the range of 30 – 40 mg produce sympathomimetic effects and subjective effects with the same time course (Heishman and Henningfield, 1991; Martin et al., 1971). Importantly, D-amphetamine at this dose range does not decrease plasma prolactin (Grady et al., 1996; Mas et al., 1999), which should occur if
D-amphetamine were releasing DA. Similar findings are reported for METH (Martin et al., 1971; Gouzoulis-Mayfrank et al., 1999).

The EC50 for DA versus NE suggest that if 30-40mg amphetamine is mainly a NE releaser, doses 2-4x as big would be expected to be DA releasers, or 60-120mg.

 

[MichelJacques]

The EC50 for DA versus NE suggest that if 30-40mg amphetamine is mainly a NE releaser, doses 2-4x as big would be expected to be DA releasers, or 60-120mg.

You're stating these as hypothetical as opposed to actual dose ranges? And either way, even at the lower dose range, DA is likely to be released, just not maximally as NE would be at the same low dose?

Sorry, still hung up on how reuptake occurs at higher doses than release. If somebody could explain this I would greatly appreciate it.

 

[sekio]

You're stating these as hypothetical as opposed to actual dose ranges?

"Oral doses of D-amphetamine [in humans] in the range of 30 – 40 mg produce sympathomimetic effects and subjective effects with the same time course. Importantly, D-amphetamine at this dose range does not decrease plasma prolactin, [indicative of low DA release]." Interpret that how you will.

Thought experiment: why are drugs like ephedrine and ethcathinone (primarily NE ligands/releasers) active as stimulants at all?

Sorry, still hung up on how reuptake occurs at higher doses than release. If somebody could explain this I would greatly appreciate it.

Amphetamine is more effective at reversing the DAT/NET transporter protien through phosphorylation than it is as a ligand for the dopamine site?

 

[Lightning-Nl]

Amphetamine is more effective at reversing the DAT/NET transporter protien through phosphorylation than it is as a ligand for the dopamine site?

There are very few studies determining what Amphetamines affinity for receptors is. However, I remember reading something a while ago that claimed that amphetamines half-maximal values were something like...

D1 - 200 nM
D2-like - 2000 (something) nM
D5 - 400 (something) nM
A1 - 90 nM
A2 - 10000+ nM
TAAR - 6.5 nM
5HT - 10000+ nM (no appreciable affinity.)

 

[sekio]

There are very few studies determining what Amphetamines affinity for receptors is.

There's more than a few. The problem is most databases have the results listed under many names... amphetamine, alpha-methylphenethylamine, 1-phenyl-2-propanamine, etc.

Also: EC50 values (functional efficacy) or Ki (binding affinity)?

 

[Lightning-Nl]

There's more than a few. The problem is most databases have the results listed under many names... amphetamine, alpha-methylphenethylamine, 1-phenyl-2-propanamine, etc.

Also: EC50 values (functional efficacy) or Ki (binding affinity)?

Binding affinity.