ADHD meds and Catecholamine levels in the PFC

[Mracid]

I was wondering about the increased levels of Catecholamines in the Prefrontal Cortex(PFC) from ADHD medication, I know all ADHD meds increase both NE and DA in the PFC by alot, but I found clear information of how much only about Atomoxetine(ATX) (3 fold increase). I found nothing about that on Methylphenydate (MPH) or Amphetamine(AMPH). Altho I found that MPH increase DA levels by 6 fold in the synaptic clef but nothing about general Catecholamine levels in the PFC.

Mostly the positive effects of ADHD meds come from that rise of stimulant neurotransmission in the PFC, and that despite their different mode of actions ATX, MPH and AMPH all work towards that goal. ATX blocks the NET which is abundant in the PFC, and because the NET also transport DA in the PFC this increase both DA and NE, MPH blocks DAT and NET which explains evidently how it raises DA and NE and AMPH reverse all 3 monoamine transporters, competitively inhibit them and has small MAOI activity.

Altho, I did not find anything about the specific changes of the Catecholamine levels in the PFC that arise from any of thos drugs, except the 3fold increase from ATX mentionned earlier. So I guess I am asking if anyone has any informations on the time dependant and dose dependant variations of Catecholamine levels in the PFC that arise from the use of those 3 drugs.

What I am trying to determine is which of those 3 on a equidose level raise DA and NE in the PFC in a more pronounced and sustained manner. If we look at the mode of actions:

-ATX inhibit their reuptake from the most abundant transporter in a very selective way, altho it is known that when NET is blocked both NE and DA can be reuptake by DAT and even if DAT is less abundant, it still has an important impact.

-MPH inhibit the reuptake from both transporters in a less selective way, so when both transporters are blocked there is no more way out for DA and NE, which would imply a more radical raise of neurotransmission, but the drug is about 10 to 20 times less selective than ATX and has a wider range of action, acting on other dopaminergic circuit in the brain. So even if on a synaptic level it is theoretically more effective, on a cortical level its effectiveness is not evidently superior.

-AMPH evidently cause a more pronounced and effective raise in all monoamine levels in the synaptic clef, due to being a releasing agent and a competitive inhibitor (and the small MAOI action just adds to it), altho with repetitive use the depletion of DA and NE makes it theoretically less effective in the long term since there is gradually less and less Neurotransmitters(NT) to release.

So theoretically there is absolutely no evidence on which of those 3 has a more effective and sustainable action on catecholamine levels in the PFC, and I found no informations based on experimental datas that can clear this blurr.

Thank you for reading and possibly discussing.

 

[Seppi]

At lower doses, amphetamine and methylphenidate promote cognitive control (aka "executive function") by increasing DA/NE neurotransmission in the PFC. This effect primarily serves to increase one's ability to control their behavior in a manner that's consistent with completing their current goals/objectives. However, higher order executive functions like planning, decision making, and reasoning are also promoted as a result of taking low doses of these drugs.

The more noticeable effect of these drugs is to amplify motivational salience for specific tasks (i.e., how motivated you are to get shit done). That arises through increased DA neurotransmission in the nucleus accumbens, which doesn't receive any significant noradrenergic projections. Consequently, atomoxetine doesn't provide the same motivational effects that amphetamine and methylphenidate confer.

It's also worth noting that there are a number of other brain structures which mediate cognitive control and which are innervated by dopaminergic and noradrenergic projections. For example, the caudate nucleus and subthalamic nucleus both receive dopaminergic innervation from the substantia nigra pars compacta and both are involved in mediating executive functions (specifically, inhibitory control). Several subcompartments in the parietal lobe are also involved in mediating certain executive functions, like working memory, attention, and cognitive flexibility; however, I'm not sure what neurotransmitters are implicated in modulating cognitive control in those regions.

Consequently, focusing on the PFC alone when comparing the effects of drugs with differing effects on DA and NE release and/or reuptake isn't going to give you an entirely accurate comparison of the therapeutic and/or desirable neuropsychological effects of these drugs.



A minor point of correction: amphetamine is both a competitive and a non-competitive reuptake inhibitor at DAT and NET. The non-competitive inhibition of reuptake occurs as a result of the internalization of these transporters.

 

[serotonin2A]

A minor point of correction: amphetamine is both a competitive and a non-competitive reuptake inhibitor at DAT and NET. The non-competitive inhibition of reuptake occurs as a result of the internalization of these transporters.

I'm not sure how much the non-competitive action is really part of what would be described as the acute mechanism-of-action of amphetamine -- the internalization is an adaptation that acts in opposition to the releasing action of amphetamine. By the same logic, you could describe mescaline as a 5-HT2A antagonist because it induces receptor internalization.

I guess the answer lies in analyzing wheather monoamine levels are higher when amphetamine is causing release vs. when it reduces reuptake due to DAT internalization. My gut feeling is that the loss of release due to DAT internalization is not completely offset by the resulting reduction of uptake capacity.

 

[Mracid]

I am not trying to determine the effectiveness to increase cognitive control and improve exetcutive functions, I am trying to determine which of them has a greater and more sustained effect on the PFC alone to be able to take it into account when I analyze the whole drug including other aspects, the effect on PFC is one of the pieces of the puzzle that I am missing, but I am still researching on the other effects, I just did not want to write on every single effect of those 3 drugs. I am just seeking an information that I am missing and am not finding.

 

[serotonin2A]

I am not trying to determine the effectiveness to increase cognitive control and improve exetcutive functions, I am trying to determine which of them has a greater and more sustained effect on the PFC alone to be able to take it into account when I analyze the whole drug including other aspects, the effect on PFC is one of the pieces of the puzzle that I am missing, but I am still researching on the other effects, I just did not want to write on every single effect of those 3 drugs. I am just seeking an information that I am missing and am not finding.

You need to be looking at imaging studies in humans to answer your question. Microdialysis will show you extracellular monoamine levels in PFC in rodents but the results may not be entitely relevant to humans.

 

[Mracid]

Thank you.

 

[Seppi]

I'm not sure how much the non-competitive action is really part of what would be described as the acute mechanism-of-action of amphetamine -- the internalization is an adaptation that acts in opposition to the releasing action of amphetamine. By the same logic, you could describe mescaline as a 5-HT2A antagonist because it induces receptor internalization.

I guess the answer lies in analyzing wheather monoamine levels are higher when amphetamine is causing release vs. when it reduces reuptake due to DAT internalization. My gut feeling is that the loss of release due to DAT internalization is not completely offset by the resulting reduction of uptake capacity.

I suppose that's one way of looking at it, but I don't think it's entirely accurate to say that reuptake inhibition due to internalization acts in opposition to neurotransmitter efflux. It's more of a neutralizing action since it also opposes reuptake and doesn't reverse neurotransmitter efflux.

I'm not sure if typical therapeutic doses of amphetamine pharmaceuticals actually induce the phosphorylation of all dopamine transporters in a dopamine neuron though. If it does, then I suppose that the protein kinases that phosphorylate DAT in order to induce internalization compete with the protein kinases that phosphorylate DAT to induce neurotransmitter efflux. However, if it doesn't phosphorylate all of the transporters, then these two processes more than likely act in parallel in a non-competitive manner. In that case, transporter internalization serves to neutralize reuptake while reverse transport/neurotransmitter efflux acts in opposition to reuptake. The neutralization of reuptake in this case would actually facilitate efflux by preventing the reuptake of effluxed neurotransmitters.

In any event, the application of the term "non-competetive reuptake inhibition" in relation to DAT internalization comes from this review (covered in more detail in this figure).


FWIW, the action of mescaline that you described at 5-HT2A receptors appears to be consistent with the mode of action of a non-competitive antagonist (i.e., "Unlike competitive antagonists, which affect the amount of agonist necessary to achieve a maximal response but do not affect the magnitude of that maximal response, non-competitive antagonists reduce the magnitude of the maximum response that can be attained by any amount of agonist"). The mechanism of action is largely irrelevant if the acute net effect (i.e., antagonistic mode of action in this context) on agonist binding is the same. NB: I say "acute net effect" here because if a drug induces neurodegeneration by causing a permanent loss of receptor density, it's obviously not acting as an antagonist.

 

[serotonin2A]

I suppose that's one way of looking at it, but I don't think it's entirely accurate to say that reuptake inhibition due to internalization acts in opposition to neurotransmitter efflux. It's more of a neutralizing action since it also opposes reuptake and doesn't reverse neurotransmitter efflux.

I'm not sure if typical therapeutic doses of amphetamine pharmaceuticals actually induce the phosphorylation of all dopamine transporters in a dopamine neuron though. If it does, then I suppose that the protein kinases that phosphorylate DAT in order to induce internalization compete with the protein kinases that phosphorylate DAT to induce neurotransmitter efflux. However, if it doesn't phosphorylate all of the transporters, then these two processes more than likely act in parallel in a non-competitive manner. In that case, transporter internalization serves to neutralize reuptake while reverse transport/neurotransmitter efflux acts in opposition to reuptake. The neutralization of reuptake in this case would actually facilitate efflux by preventing the reuptake of effluxed neurotransmitters.

In any event, the application of the term "non-competetive reuptake inhibition" in relation to DAT internalization comes from this review (covered in more detail in this figure).


FWIW, the action of mescaline that you described at 5-HT2A receptors appears to be consistent with the mode of action of a non-competitive antagonist (i.e., "Unlike competitive antagonists, which affect the amount of agonist necessary to achieve a maximal response but do not affect the magnitude of that maximal response, non-competitive antagonists reduce the magnitude of the maximum response that can be attained by any amount of agonist"). The mechanism of action is largely irrelevant if the acute net effect (i.e., antagonistic mode of action in this context) on agonist binding is the same. NB: I say "acute net effect" here because if a drug induces neurodegeneration by causing a permanent loss of receptor density, it's obviously not acting as an antagonist.

You are missing an important part of my explanation. Amphetamine must enter terminals through DAT in order to release dopamine. So DAT internalization does act in opposition to release, because if amphetamine cannot enter terminals then it cannot induce release. To the extent that most of the effects of amphetamine are due to release, DAT internalization is going to represent a mechanism that promotes tolerance.

The action of mescaline is not like a non-competitive antagonist, because it activates the receptor and induces an agonist response. Noncompetitive antagonists do not induce an agonist-like response.

 

[Cotcha Yankinov]

Random question if anyone cares to answer: let's say persistent transporter downregulation is noted after releasing agent use in humans but isn't thought to be due to nerve terminal loss.

Is the transporter downregulation a consequence of increased postsynaptic response and activation of transporter internalization signaling (which is then a cycle because that same tranaporter internalization will lead to more synaptic transmitters and a greater postsynaptic response, once again triggering internalization)?

 

[Seppi]

You are missing an important part of my explanation. Amphetamine must enter terminals through DAT in order to release dopamine. So DAT internalization does act in opposition to release, because if amphetamine cannot enter terminals then it cannot induce release. To the extent that most of the effects of amphetamine are due to release, DAT internalization is going to represent a mechanism that promotes tolerance.

The action of mescaline is not like a non-competitive antagonist, because it activates the receptor and induces an agonist response. Noncompetitive antagonists do not induce an agonist-like response.

Amphetamine can diffuse across membranes; it doesn't have to enter via DAT. I realize that the vast majority of amphetamine entering a DA neuron enters via DAT.

Amphetamine can not enter through an effluxing transporter, so I don't understand the point you're making. You're saying DAT internalization acts in opposition to DA efflux because amphetamine can't enter the neuron. By extension, DA efflux acts in opposition to DA efflux because amphetamine can't enter the neuron in that case either.

I didn't realize mescaline was a receptor agonist. However, what you described was a non-competitive antagonistic mode of action.

 

[Seppi]

Random question if anyone cares to answer: let's say persistent transporter downregulation is noted after releasing agent use in humans but isn't thought to be due to nerve terminal loss.

Is the transporter downregulation a consequence of increased postsynaptic response and activation of transporter internalization signaling (which is then a cycle because that same tranaporter internalization will lead to more synaptic transmitters and a greater postsynaptic response, once again triggering internalization)?

DAT internalization occurs as a result of protein kinase-mediated phosphorylation on specific residues. This is entirely presynaptic. It's not a persistent effect since it's reversed by dephosphorylation.

 

[serotonin2A]

Amphetamine can diffuse across membranes; it doesn't have to enter via DAT. I realize that the vast majority of amphetamine entering a DA neuron enters via DAT.

Amphetamine can not enter through an effluxing transporter, so I don't understand the point you're making. You're saying DAT internalization acts in opposition to DA efflux because amphetamine can't enter the neuron. By extension, DA efflux acts in opposition to DA efflux because amphetamine can't enter the neuron in that case either.

I didn't realize mescaline was a receptor agonist. However, what you described was a non-competitive antagonistic mode of action.

Very little amphetamine diffuses across membranes. They know this because it is possible to measure amphetamine entry into cells without transporters.

Amphetamine CAN enter through an effluxing transporter unless the transporter is in a channel mode or is phosphorylated. There is always some subset of the DAT population that is not phosphorylated or in channel mode at any given time, and hence available to transport amphetamine inside the cell (some DAT is always cycling back to the plasma membrane from intracellular compartments).

The release process I am taking about is called facilitated exchange...it is the classical way that amphetamine was known to release dopamine through DAT:

http://m.jbc.org/content/278/14/12070

As dopamine levels rise in the cytoplasm and it is pumped out of the cell due by DAT through reverse transport, the transport site is then exposed extracellularly, meaning that amphetamine can then bind and be transported to the cytoplasm. That is the classical mode of amphetamine action on DAT. So basically amphetamine entry in brings more dopamine out.

 

[Cotcha Yankinov]

DAT internalization occurs as a result of protein kinase-mediated phosphorylation on specific residues. This is entirely presynaptic. It's not a persistent effect since it's reversed by dephosphorylation.

Sorry I worded my thoughts horribly (late at night) - I meant postsynaptic as in the cell that would be releasing dopamine, and presynaptic as in the cell (e.g. glutamate) releasing NTs onto the dopamine cell, in that warped sense from the point of view of the glutamate neuron, the entire DA cell is "postsynaptic".

But if this phosphorylation isn't persistent (or cyclical), then either

A. There should be other mechanisms by which transporters can be persistently downregulated

Or

B. That downregulation really represents loss of transporter expressing tissues.

 

[serotonin2A]

Sorry I worded my thoughts horribly (late at night) - I meant postsynaptic as in the cell that would be releasing dopamine, and presynaptic as in the cell (e.g. glutamate) releasing NTs onto the dopamine cell, in that warped sense from the point of view of the glutamate neuron, the entire DA cell is "postsynaptic".

But if this phosphorylation isn't persistent (or cyclical), then either

A. There should be other mechanisms by which transporters can be persistently downregulated

Or

B. That downregulation really represents loss of transporter expressing tissues.

The terminal is still considered to be presymaptic even if it is downstream from another cell.

Phosphorylation can potentially regulate DAT function in both an acute and chronic manner. Phosphorylation starts out as an acute event, but it is possible for repeated cycles of phosphorylation and dephosphorylation to occur.

DAT can also be downregulated due to changes in gene expression.

 

[Seppi]

Amphetamine CAN enter through an effluxing transporter unless the transporter is in a channel mode or is phosphorylated. There is always some subset of the DAT population that is not phosphorylated or in channel mode at any given time, and hence available to transport amphetamine inside the cell (some DAT is always cycling back to the plasma membrane from intracellular compartments).

The release process I am taking about is called facilitated exchange...it is the classical way that amphetamine was known to release dopamine through DAT:

http://m.jbc.org/content/278/14/12070

As dopamine levels rise in the cytoplasm and it is pumped out of the cell due by DAT through reverse transport, the transport site is then exposed extracellularly, meaning that amphetamine can then bind and be transported to the cytoplasm. That is the classical mode of amphetamine action on DAT. So basically amphetamine entry in brings more dopamine out.

You are quoting a hypothesis which hasn't been demonstrated experimentally. What I've stated has been. https://www.ncbi.nlm.nih.gov/pubmed/27591044

 

[serotonin2A]

You are quoting a hypothesis which hasn't been demonstrated experimentally. What I've stated has been. https://www.ncbi.nlm.nih.gov/pubmed/27591044

There is experimental data that support the hypothesis (see for example http://jpet.aspetjournals.org/content/208/2/203.long). But it isn't really the point...the reason I brought up that model is that you argued that amphetamine isn't transported into terminals via DAT, and there is substantial evidence to support that conclusion.

http://www.nature.com/articles/ncomms10652

"Our data lead to a model for how pharmacologically relevant concentrations of amphetamines increase extracellular dopamine: (1) DAT imports and concentrates amphetamines in the cytoplasm. (2) Cytoplasmic amphetamines (and endogenous dopamine) are accumulated into vesicles by VMAT in a H⁺-antiport process that diminishes vesicular H⁺ concentration. (3) Diminished vesicular ΔpH promotes dopamine redistribution from vesicles into the cytoplasm through a mechanism that is still unclear. Vesicle deacidification alters the protonation state of luminal dopamine, which might be sufficient to increase its diffusion across the membrane. However, this mechanism does not readily explain protonophore-induced efflux of MPP⁺ from vesicles, since this compound is a quaternary ammonium and thus pH cannot alter MPP⁺’s protonation state. Whether VMAT itself might be a route of dopamine release from vesicles requires further study. (4) The redistributed cytoplasmic dopamine subsequently effluxes out of the cell through DAT via amphetamine-stimulated reverse transport. Our previous work demonstrated that phosphorylation of DAT is essential for dopamine efflux and for locomotor behaviour induced by amphetamine but not for the actions of methylphenidate, a competitive non-substrate inhibitor of DAT<sup>."


https://www.ncbi.nlm.nih.gov/pubmed/1903446

"The accumulation of d-[3H]AMPH into striatal synaptosomes was saturable, of high affinity, ouabain-sensitive and temperature-dependent, suggesting an active transport phenomenon. Eadee-Hofstee analysis of striatal d-[3H]AMPH transport (AMT) saturation isotherms indicated an apparent Km of 97 nM and a Vmax of 3.0 fmol/mg tissue/min. Lesion of the striatal dopaminergic innervation led to equivalent decreases of [3H] dopamine (DA) transport and AMT, indicating that AMT occurs in DA terminals. Furthermore, AMT was not evident in cerebral cortex, a brain region with a paucity of DA terminals. In competition studies, AMT was stereospecific; d-AMPH (IC50 = 60 nM) was an 8-fold more potent inhibitor of the transport than its I-isomer (IC50 = 466 nM). DA(IC50 = 257 nM), DA uptake blockers and substrates were found to be potent inhibitors of AMT"</sup>


http://www.sciencedirect.com/science/article/pii/S0165614714002120

"amphetamines enter monoaminergic terminals via DAT, SERT, or NET and subsequently accumulate in the synaptic vesicles by the action of VMATs."


https://www.ncbi.nlm.nih.gov/pubmed/6514010

"PC-12 cells (a clonal line of rat phaeochromocytoma cells) take up noradrenaline by a transport system which is identical with the neuronal amine transport system ("uptake1"). The uptake of 3H-noradrenaline into reserpine-pretreated PC-12 cells (monoamine oxidase inhibited) was saturable (Km = 0.6 +/- 0.1 mumol/l), dependent on sodium and chloride, and competitively inhibited by (+)-amphetamine (Ki = 0.18 +/- 0.04 mumol/l), cocaine (Ki = 0.55 +/- 0.15 mumol/l) and desipramine (Ki = 4.3 +/- 0.6 nmol/l). The uptake and accumulation of 3H (+)-amphetamine showed characteristics comparable to those of 3H-noradrenaline, since the uptake of 3H (+)-amphetamine (0.1 mumol/l) was reduced by omission of sodium or chloride from the incubation medium. The sodium-sensitive component of uptake and accumulation of 3H (+)-amphetamine was fully inhibited by cocaine and desipramine. The IC50 of desipramine for inhibition of the sodium-sensitive component of the 1-min uptake of 3H (+)-amphetamine (20 nmol/l) was about 2 nmol/l, i.e., identical with the Ki for inhibition of uptake of 3H-noradrenaline."

 

[Seppi]

There is experimental data that support the hypothesis (see for example http://jpet.aspetjournals.org/content/208/2/203.long). But it isn't really the point...the reason I brought up that model is that you argued that amphetamine isn't transported into terminals via DAT, and there is substantial evidence to support that conclusion.

Quote me then.

This is a massive straw man argument on your part because all I've stated is that amphetamine can't enter a neuron through an effluxing transporter. You are arguing that I've stated that amphetamine can't enter a neuron through a transporter. Reuptake and reverse transport are not the same thing. If you need clarification, read:
https://en.wikipedia.org/wiki/Transporter_reversal
https://en.wikipedia.org/wiki/Reuptake

There is experimental data that support the hypothesis (see for example http://jpet.aspetjournals.org/content/208/2/203.long).

This paper does not show that amphetamine reuptake occurs through an effluxing transporter. All it did was provide "circumstantial evidence" (their words) that amphetamine is a substrate for DAT.

 

[serotonin2A]

Quote me then.

This is a massive straw man argument on your part because all I've stated is that amphetamine can't enter a neuron through an effluxing transporter. You are arguing that I've stated that amphetamine can't enter a neuron through a transporter. Reuptake and reverse transport are not the same thing. If you need clarification, read:
https://en.wikipedia.org/wiki/Transporter_reversal
https://en.wikipedia.org/wiki/Reuptake



This paper does not show that amphetamine reuptake occurs through an effluxing transporter. All it did was provide "circumstantial evidence" (their words) that amphetamine is a substrate for DAT.

Seppi, you really have completely misunderstood my point and you are talking across me. I never said AMPH enters through an effluxing transporter, and I have no clue why you think I said that. You seem to be making an assumption that the effect of phosphorylation on DAT is all or none -- that amphetamine phosphorylates every transporter. That isn't what happens. Once amphetamine starts working, a subset of transporters will be releasing dopamine, but others will be not be in that conformation, and will be able to take up amphetamine into terminals.

 

[Seppi]

Seppi, you really have completely misunderstood my point and you are talking across me. I never said AMPH enters through an effluxing transporter, and I have no clue why you think I said that.

See below.

Amphetamine CAN enter through an effluxing transporter unless the transporter is in a channel mode or is phosphorylated.

You seem to be making an assumption that the effect of phosphorylation on DAT is all or none -- that amphetamine phosphorylates every transporter. That isn't what happens. Once amphetamine starts working, a subset of transporters will be releasing dopamine, but others will be not be in that conformation, and will be able to take up amphetamine into terminals.

I'm not making this assumption. I've stated that I have no clue if it actually does or not. See below. FWIW, I think it's unlikely that all transporters are phosphorylated at therapeutic doses.

I'm not sure if typical therapeutic doses of amphetamine pharmaceuticals actually induce the phosphorylation of all dopamine transporters in a dopamine neuron though. If it does, then I suppose that the protein kinases that phosphorylate DAT in order to induce internalization compete with the protein kinases that phosphorylate DAT to induce neurotransmitter efflux. However, if it doesn't phosphorylate all of the transporters, then these two processes more than likely act in parallel in a non-competitive manner. In that case, transporter internalization serves to neutralize reuptake while reverse transport/neurotransmitter efflux acts in opposition to reuptake. The neutralization of reuptake in this case would actually facilitate efflux by preventing the reuptake of effluxed neurotransmitters.

 

[serotonin2A]

See above.

I meant effluxing as in facilitated exchange, not channel mode. Did you read my post above?