A set of questions

[aced126]

I was thinking yesterday about how a selective SRA like MDAI might have possible therapeutic values, specifically possibly as an antidepressant.

Now assuming that the drug shows efficacy during its half life, and daily dosing was started, how would it start to lose efficacy?

Another related question could be why daily dosing of MDMA wouldn't work. For one, TPH is inhibited (how?) so serotonin synthesis cannot take place. Could this be averted with 5HTP? Secondly we obviously have the problem of neurotoxicity, and taking MDMA every day will probably end up killing a lot of serotonin axons. With SRAs however, neurotoxicity is not observed (supporting the DA entering 5HT neurons and causing oxidative damage theory), but neurotoxicity is observed with SRA + DRA (5-IAI and amphetamine was the combination I think Nichols used to prove this).

And finally obviously there is the problem of receptor internalisation given excessive stimulation of those receptors. But that leads me on to another question which I've actually wondered for a long time. What are the actual receptors involved in mediating the effects of excessive serotonin release? I know that 8-OH-DPAT is a 1a agonist and induces empathic and serenic qualities in mice. 2b is also involved in mediating oxytocin release, which I think is relevant here as well.

Anyway coming back to receptor internalisation, it seems that most drugs tend to cap in the amount of tolerance they induce, and once that dose is reached then the patient can be steadily maintained on that dose. I mean, you don't keep on increasing the dose to infinity for patients who have been on any sort of drug for an extremely long time. There probably is a basal level of receptors which do not decrease in response to any amount of drug.

Basically I just don't really get why there hasn't been much work in trying to single out the proteins that mediate these drugs' effects and target them selectively to separate the therapeutic qualities from these types of drugs.

I understand depression is a lot more complicated than simply a result of "lack of monoamines". So while the pathophysiology of depression itself might be very intricate, it seems to be fixed (very temporarily of course) by increase in monoamines, or specifically for MDAI, increase in serotonin. Why can't this be exploited more?

 

[serotonin2A]

I don't think there is much evidence that depression can be "fixed" by acutely increasing monoamines. People certainly feel better after taking amphetamine or MDMA, but that is not specific to depression. It is generally accepted that SSRIs work due to effects downstream from monoamines, via pathways overlapping with the downstream effects of ketamine.

TPH is thought to be denatured due to oxidative stress, but I don't believe the exact mechanism has been worked out. MDMA depletes 5-HT because it causes the transmitter to leak out of vesicles.

The possibility of neurotoxicity and serotonin depletion with MDAI were factors that derailed possible development. I wouldn't say it is completely non-neurotoxic because it produced neurochemical changes at high doses. But MDAI certainly produced neurochemical changes in the presence of a DA releaser. However, it wasn't conclusively established that the direct neurochemical effects of DA are responsible for the neurotoxicity, as opposed to hyperthermia.

The evidence linking 5-HT1A to MDMA effects is spotty at best. Glennon found that 8-OH-DPAT substitutes for MDMA, but at that time it wasn't recognized that 8-OH-DPAT acts (weakly) as a serotonin releaser. The rats trained to discriminate MDMA are responding to an introceptive cue that probably does not correspond to what humans experience with MDMA, because generalization occurs with things like fenfluramine, PMA, and 3,4-DMA.

5-HT1B is believed to mediate some of the effects of MDMA. See:

https://www.ncbi.nlm.nih.gov/pubmed/1358088

https://www.ncbi.nlm.nih.gov/pubmed/8099482

https://www.ncbi.nlm.nih.gov/pubmed/11888548

http://link.springer.com/article/10.1007/s00213-016-4225-x

http://www.maps.org/images/pdf/1999_scearcelevie_1.pdf

http://jpet.aspetjournals.org/content/263/1/318.short

MDMA effects have also been linked to 5-HT2B receptors, but my opinion has always been that 5-HT2B plays more of a permissive role for interactions with SERT. So I don't think that MDMA is really acting through 5-HT2B.

 

[Cotcha Yankinov]

Basically I just don't really get why there hasn't been much work in trying to single out the proteins that mediate these drugs' effects and target them selectively to separate the therapeutic qualities from these types of drugs.

I understand depression is a lot more complicated than simply a result of "lack of monoamines". So while the pathophysiology of depression itself might be very intricate, it seems to be fixed (very temporarily of course) by increase in monoamines, or specifically for MDAI, increase in serotonin. Why can't this be exploited more?

It seems that the question here is if the brain compensates homeostatically for an increase in monoamines and such then how do you get around that? To me the answer is focusing on the therapeutic targets that mediate long term neuroplastic changes like TrkB/mTOR, which is to a large extent how SSRIs, aerobic exercise, ECT and I believe ketamine's metabolite HNK ultimately work.

http://www.nature.com/mp/journal/vaop/ncurrent/full/mp2016145a.html

You can see that chronic stress can cause a reduction in TrkB that is reversed by increasing 5-HT, so SSRIs might not be completely "missing the mark" in that respect. I believe amitriptyline is actually an agonist at TrkB, and it does have standout efficacy amongst other antidepressants.

I think the issue with using MDAI medicinally is that you may see vesicular depletion with VMAT2 inhibition (most substituted amphetamines are VMAT2 inhibitors and I assume MDAI is as well). The VMAT2 inhibition causes accumulation of 5-HT in the cytosol which then effluxes out through the reversed SERT. As vesicles are depleted, there wouldn't be many full vesicles to fuse with the membrane and mediate "natural" release of serotonin.

I think there have been theorized to be changes to vesicles/VMAT2 with chronic use of VMAT2 inhibitors as well. Normally those vesicles contain the harmful molecules and keep them from accumulating in the cytosol - this is protective when it comes to segregating harmful molecules like dopamine but I don't know if there would be consequences to not sequestering 5-HT.

The other thing to consider is that you might see addiction related neuroplastic changes with use of a releasing agent. A flood of 5-HT that is acting irrespective of the autoreceptors could help with immediate suicidality but it seems to me there would be abuse/overdose potential. Its been found with MDMA that you can negate the increase in dopamine seen in animals after MDMA administration with blockade of 5-HT2B, so I would assume that you're going to see similar abuseable effects with MDAI that you see with MDMA. You just can't abuse SSRIs like you can something like MDMA/MDAI.


On that note, whether its ibogaine for opioid use disorder or ketamine for treatment resistance depression, so many patients seem to ascribe (perhaps inaccurately) the efficacy of the drugs to the "mystical" experiences. As I understand it, there is a doctor at a ketamine clinic in Texas that likes to push the dose with ketamine because he thinks his patients are benefitting from the mystical experiences as well, whereas I think other clinics are purposely trying to keep the dosage low to avoid psychoactive effects.

 

[d1nach]

Lol I had a paper in my notebook where I was drawing a complex chart of how changes in mtorc1 and trkb could be the mechanism of both typical and experimental antidepressants and they laughed at me and said if we sent this to ant doctor any professional do you honestly think they would recognize anything your writting here in your drawings. These look nothing like what any professional would think.

 

[aced126]

It seems that the question here is if the brain compensates homeostatically for an increase in monoamines and such then how do you get around that? To me the answer is focusing on the therapeutic targets that mediate long term neuroplastic changes like TrkB/mTOR, which is to a large extent how SSRIs, aerobic exercise, ECT and I believe ketamine's metabolite HNK ultimately work.

http://www.nature.com/mp/journal/vaop/ncurrent/full/mp2016145a.html

You can see that chronic stress can cause a reduction in TrkB that is reversed by increasing 5-HT, so SSRIs might not be completely "missing the mark" in that respect. I believe amitriptyline is actually an agonist at TrkB, and it does have standout efficacy amongst other antidepressants.

I think the issue with using MDAI medicinally is that you may see vesicular depletion with VMAT2 inhibition (most substituted amphetamines are VMAT2 inhibitors and I assume MDAI is as well). The VMAT2 inhibition causes accumulation of 5-HT in the cytosol which then effluxes out through the reversed SERT. As vesicles are depleted, there wouldn't be many full vesicles to fuse with the membrane and mediate "natural" release of serotonin.

I think there have been theorized to be changes to vesicles/VMAT2 with chronic use of VMAT2 inhibitors as well. Normally those vesicles contain the harmful molecules and keep them from accumulating in the cytosol - this is protective when it comes to segregating harmful molecules like dopamine but I don't know if there would be consequences to not sequestering 5-HT.

The other thing to consider is that you might see addiction related neuroplastic changes with use of a releasing agent. A flood of 5-HT that is acting irrespective of the autoreceptors could help with immediate suicidality but it seems to me there would be abuse/overdose potential. Its been found with MDMA that you can negate the increase in dopamine seen in animals after MDMA administration with blockade of 5-HT2B, so I would assume that you're going to see similar abuseable effects with MDAI that you see with MDMA. You just can't abuse SSRIs like you can something like MDMA/MDAI.


On that note, whether its ibogaine for opioid use disorder or ketamine for treatment resistance depression, so many patients seem to ascribe (perhaps inaccurately) the efficacy of the drugs to the "mystical" experiences. As I understand it, there is a doctor at a ketamine clinic in Texas that likes to push the dose with ketamine because he thinks his patients are benefitting from the mystical experiences as well, whereas I think other clinics are purposely trying to keep the dosage low to avoid psychoactive effects.

Which brings me onto another question, which is why SSRIs do not induce the same effects as SRAs, like empathy mainly.

I know that SRAs are not subject to autoreceptor homeostasis, whereas SSRIs are, because autoreceptor activation only reduces rate of vesicle fusion.

But this does not explain why both DRAs and DRIs have similar effects.

Actually this might even suggest further that "DRIs" like cocaine and MPH don't actually simply block reuptake, but cause some release via inverse agonism.

https://www.ncbi.nlm.nih.gov/pubmed/24953830

However if this was the case, how would cocaine even have the opportunity to release DA? I can't imagine much DA floating around in the cytosol. It's likely to be picked up very quickly by VMAT2 into vesicles, or else it will cause oxidative stress.

 

[Cotcha Yankinov]

It could just be that the effect of synaptic concentrations of 5-HT on neural network activity isn't linear, Serotonin might be able to talk more about differential phasic vs. tonic firing and such induced by reuptake inhibitors vs. releasing agents (unfortunately I can't remember much).

I think Serotonin has mentioned its been theorized that vesicles may bind directly to the transporter when in reverse conformation, therefore not necessitating cytosolic DA when it comes to DAT inverse agonist related DA efflux, but I'm under the impression that synaptic 5-HT/NE also rise a fair bit with cocaine so maybe it is a substrate/VMAT2 inhibitor/TAAR1 agonist too? I'm honestly not at all familiar with cocaine

 

[aced126]

I don't think there is much evidence that depression can be "fixed" by acutely increasing monoamines. People certainly feel better after taking amphetamine or MDMA, but that is not specific to depression. It is generally accepted that SSRIs work due to effects downstream from monoamines, via pathways overlapping with the downstream effects of ketamine.

TPH is thought to be denatured due to oxidative stress, but I don't believe the exact mechanism has been worked out. MDMA depletes 5-HT because it causes the transmitter to leak out of vesicles.

The possibility of neurotoxicity and serotonin depletion with MDAI were factors that derailed possible development. I wouldn't say it is completely non-neurotoxic because it produced neurochemical changes at high doses. But MDAI certainly produced neurochemical changes in the presence of a DA releaser. However, it wasn't conclusively established that the direct neurochemical effects of DA are responsible for the neurotoxicity, as opposed to hyperthermia.

The evidence linking 5-HT1A to MDMA effects is spotty at best. Glennon found that 8-OH-DPAT substitutes for MDMA, but at that time it wasn't recognized that 8-OH-DPAT acts (weakly) as a serotonin releaser. The rats trained to discriminate MDMA are responding to an introceptive cue that probably does not correspond to what humans experience with MDMA, because generalization occurs with things like fenfluramine, PMA, and 3,4-DMA.

5-HT1B is believed to mediate some of the effects of MDMA. See:

https://www.ncbi.nlm.nih.gov/pubmed/1358088

https://www.ncbi.nlm.nih.gov/pubmed/8099482

https://www.ncbi.nlm.nih.gov/pubmed/11888548

http://link.springer.com/article/10.1007/s00213-016-4225-x

http://www.maps.org/images/pdf/1999_scearcelevie_1.pdf

http://jpet.aspetjournals.org/content/263/1/318.short

MDMA effects have also been linked to 5-HT2B receptors, but my opinion has always been that 5-HT2B plays more of a permissive role for interactions with SERT. So I don't think that MDMA is really acting through 5-HT2B.

In the studies you linked, they all measure locomotor activity but how well does this correlate to the desirable effects of MDMA experienced with humans, mainly empathy? Sure, RU-24,969 might induce locomotor activity by agonising 5HT1b, but I don't think this tells us much about empathy and trust.

It also mentions 5HT1b is involved in addiction, which is worrying. Would this be due to downstream DA release?

 

[serotonin2A]

In the studies you linked, they all measure locomotor activity but how well does this correlate to the desirable effects of MDMA experienced with humans, mainly empathy? Sure, RU-24,969 might induce locomotor activity by agonising 5HT1b, but I don't think this tells us much about empathy and trust.

It also mentions 5HT1b is involved in addiction, which is worrying. Would this be due to downstream DA release?

Well, first let me say first that the effects of MDMA on locomotor activity are mediated by the same circuitry that mediates the locomotor effects of amphetamine. Locomotor activation by drugs is thought to predict positive reinforcing effects:

Weiss A, Bozarth MA (1987) A psychomotor stimulant theory of addiction. Psychol Review 94, 469–492

But more importantly, the question you asked was "I just don't really get why there hasn't been much work in trying to single out the proteins that mediate these drugs' effects".

You didn't specifically ask about empathy in your post. If you only want to limit the discussion to studies that specifically examined empathy in humans then the answer is that other than the importance of serotonin, we don't know anything about the sites of action. There really isn't really any way to specifically study empathy in rodents, and in terms of human studies, there aren't antagonists available to look at individual serotonin receptors. So it is necessary to use animal models like drug discrimination and locomotor activity. Entactogens produce unique effects in both of those particular behavioral paradigms, so it is possible that the mechanistic data obtained in rodents is relevant to the unique effects of the entactogen class in humans.

 

[WSH]

there really isn't really any way to specifically study empathy in rodents

What's you opinion on adjacent lying, it certainly looks like humans on mdma having a cuddle party

 

[Cotcha Yankinov]

Hmmmmm is that distinct from mating/maternal/paternal and survival (warmth) behavior though? Although I suppose there may be some overlap with empathy and the aforementioned subjects.

With higher primates you would think you could study the brain of a monkey in vivo while it is looking at another monkey being injured/shocked, and maybe mirror neuron related tasks might be one arena to investigate. I don't know how much we want to differentiate between "theory of mind" (which rodents may not have a whole lot of) and "empathy" as a subjective feeling/emotion.

 

[WSH]

Hmmmmm is that distinct from mating/maternal/paternal and survival (warmth) behavior though?

Yes, they have excluded factors like "just warming each other up".

 

[serotonin2A]

Yes, they have excluded factors like "just warming each other up".

Social interaction isn't the same as empathy. One of the normal behaviors exhibited by rats is the instinct to congregate and huddle together. When I go to get rats from their cages, it is common to find them all together in a ball. On the other hand, humans don't normally behave like that. So you can't look at this behavior in rats and extrapolate that it has exactly the same meaning that it does in humans. MDMA may enhance this normal species-specific behavior in rats, but trying to equate this effect in rats with a specific MDMA effect in humans is really a case of anthropomorphizing behavior.

To give another example: When rats are hyperactive from MDMA, they exhibit thigmotaxis, meaning they tend to stay adjacent to the walls when they run around in a chamber. So basically the rats like to rub their whiskers along the walls when they are running, but no one would realistically argue that this is a rat analog of how humans who have taken MDMA like to touch objects.