What exactly is tolerance?

[bow-viper1]

People usually explain it by saying that they are chemicals naturally produced by the body, so when you put more of those chemicals into yourself over a period of time your body stops producing it since you are already getting enough of it.

But what are the mechanisms involved? Once it is removed what is causing the withdrawals, any why does it hurt so much? Chemically what is happening here? I'm interested in the way the body is handling these changes, so any explaination is appreciated.

If you need a specific example, let's use the opiate class.

 

[BilZ0r]

people usually explain it by saying that they are chemicals naturally produced by the body, so when you put more of those chemicals into yourself over a period of time your body stops producing it since you are already getting enough of it.

Yeah, no. You need to stop listening to those people. Tolerance comes mainly from the desensitization of the receptors directly effected by the drug in question. Opioid receptors in the case of opioids.

To quote from a textbook we're working on writing

G-protein coupled receptors (GPCR) [like opioid receptors] are also subject to desensitization. Activation of GPCRs increases the activity of G-protein receptor kinases, which phosphorylate the receptor and decrease their signaling efficacy (usually through a decrease in ligand or G-protein affinity). This phosphorylation is reversible, but also allows the binding of proteins called arrestins to the intracellular side of the GPCR. Arrestins not only completely cut the GPCR off from activating G-proteins but also allows the binding of other molecules, classical the clathrins, which pull the receptor into an intracellular vesicle (internalization). Once internalized, (where receptor is separated from interactions with ligands) it awaits one of two fates, reinsertion back into the membrane, or degradation by protiolytic enzymes. Hence long term treatment with agonists lead to a long-term depletion in receptor number (down-regulation), which only the synthesis of new receptors can resolve. It is worth noting that GPCRs can also interact with genes, altering expression and after chronic agonist application sufficient to induces internalization it is common to note a decrease in the production no that receptors mRNA, which will also reduce receptor numbers and further slow recovery times.

In conclusion. Taking agonist drugs like opioids, down-regulate the receptors they work on, by making them less sensitive to the drugs, and by phyically removing them from the tissue the were located on.

Not all withdrawal is that bad. There are essentially no withdrawal symptomes to amphetamines, even though there is tolerance. This is because the system that is downregulated is not as physiologically important as the opioid system.

Once you downregulate your opioid system, it takes a long time for it to reach a normal level of signalling again. Throughout this time, your body is chronically low in the signals sent by endogenous opioids, i.e. all the things they regulate go to shit, and this includes the withdrawal symptomes you know and love, your gastrointestinal system, the pain system, thermo-regulation, reward, stress etc etc etc...

 

[EN21]

What about benzos?
To my knowledge there is no real endogenous ligand.
Or is the whole ion channel desensitized?

 

[snowshovel]

Tolerance to a substance can also be metabolic, ie you produce more of whatever enzyme(s) your body uses to break the substance down.

I've no specific knowledge of what causes benzo tolerance though.

 

[blab]

in layman's terms: when it takes 80mgs of oxy to get you off now but when you first started it took 5mgs.

 

[BilZ0r]

What about benzos?
To my knowledge there is no real endogenous ligand.
Or is the whole ion channel desensitized?

Well benzos don't act on a G-protein coupled receptor, so their down-regulation mechanism is different. But it's a similar theme if my memory serves. By the receptor spending more time in a active state, it allows protein kinases to phosphorylate certain sites on the GABA-A receptor (or the receptors where the benzos have bound) which leads to internalization of receptors.

 

[ayjay]

^would love a reference on GABA receptor down regulation. Just had an in-service at work on benzo withdrawal - tolerance was explained in way bow-viper described in initial post above, which is not strictly true? Anyway - I'd like to be able to back it up when I challenge my co-workers' knowledge...

 

[BilZ0r]

More from the erowid.org textbook

The exact mechanism remains unclear, but evidence shows that it is likely that prolonged benzodiazepine treatment renders GABA-A receptors insensitive to benzodiazepine modulation. It seems likely that this is primarily due to the receptor being pulled into an intracellular vesicle (internalization), presumabley after the action of a kinase. The receptor is then modulated in some way, possibly by removing benzodiazepine sensitive subunits, so that it is insensitive to benzodiazepines and returned to the membrane surface. It seems that only very high doses or very long treatments with benzodiazepines lead to a total decrease in GABA-A receptor number, and this may be through reduction in GABA-A receptor subunit mRNA expression[1].

[1] Bateson AN. Basic pharmacologic mechanisms involved in benzodiazepine tolerance and withdrawal. Curr Pharm Des. 2002;8(1):5-21

The reference is available here

 

[lifeisforliving]

^^^ So with drugs such as zopiclone and zopiderm the "Resistance" to tolerance seen in many people may be from the relatively short half-life of it's ligand bound self. Since both eliminate from the body at such a fast rate, the body is essentially free of the drug for at least part of a 24 hour day. (if simply used to conquer insomnia)?

Also:Since from what I gather so far, there are a number greater than one (1) of binding sites per receptor - would drugs that bind to "all" or "most" of the receptor sites increase the activity of the receptor more than if just one site was occupied? Such that drugs that just "pound" the GABA system like benzodiazipines have a greater downregulation effect, than the more selective sub-type binding affinities like zopiclone/zopiderm?

Or is it like enzyme active sites where once ONE ligand has bound to the receptor, that the receptor contorts such that other drugs may not bind to it?
(specifically as related to benzos..).

 

[BilZ0r]

I don't really understand you questions. The new, non-benzodiazepine drugs, which are still essentially benzodiazepines, like zopiclone etc... they are claimed not to produce tolerance and dependence, yet they really still do, though maybe not as bad. They reason why they do this, if they do this, because they a) bind more selectively to a subset of GABA-A receptors (the so called BZ-1 receptors... a GABA-A receptor made up of a specific set of subunits). and b) what conformation they drugs stabalize the receptor into doesn't favour the molecular cascade that causes internalization.

The GABA-A receptor has two classical binding sites, for GABA. And I believe that the channel can open from either of them, though it is an easier transission to open from the doubley bound state.

The amount the receptor is downregulated by a drug would be a function of the amount of benzodiazepine bound to the GABA-A receptor, which would be a function of the affinity the drug has for the receptor and the concentration of the drug. And of course, certain drugs might stabalize conformations which favour internalization.

 

[lifeisforliving]

^ Well from my poorly constructed questions, you gave the answers I was looking for. I'm not that far in my studies for this stuff, which is why I ask here.

So from what you said:

1) Different ligands that bind to the same receptor site could have different rates of internalization (and therefore tolerance building)?

2) From your last paragraph:

The amount the receptor is downregulated by a drug would be a function of the amount of benzodiazepine bound to the GABA-A receptor, which would be a function of the affinity the drug has for the receptor and the concentration of the drug. And of course, certain drugs might stabalize conformations which favour internalization.

Is there any preference for long half-life vs. short half-life benzodiazipines then? Or essentially is the tolerance effect the same say if you gave 3 doses in one day vs. 3 doses spaced out by three days? - Just that in the more spaced out doses, there is the normal rebound of new receptors being synthesized?

3)

And of course, certain drugs might stabalize conformations which favour internalization.

So, for example, this would be a similar issue when we talk of "reversible MAO-B inhibitor" vs. an irreveserible MAO-B inhibitor? So some drugs may bind and have increase the chances greatly that a receptor would be internalized?

 

[BilZ0r]

Different ligands that bind to the same receptor site could have different rates of internalization (and therefore tolerance building)?

It's possible, that is certainly the case with G-protein coupled receptors, I don't know about ionotropic receptors, but I suspect its likely. It's also likely, that GABA-A receptors made up from different subunits are more suceptable to internalizatoin and by binding to different GABA-A receptor subunits with different affinities, different drugs will cause internalization at different rates...

Is there any preference for long half-life vs. short half-life benzodiazipines then? Or essentially is the tolerance effect the same say if you gave 3 doses in one day vs. 3 doses spaced out by three days? - Just that in the more spaced out doses, there is the normal rebound of new receptors being synthesized?

Well the longer the drug spends bound to the receptor, the more downregulation you're going to get. In general, I would imagine the dosing regiems that produce equal AUC of the plasma concentration-time graph, would produce equal amounts of downregulation... apart from the extremes: increadibley long, low concentration dosing regeimes would produce less tolerance than short, high concentration doses, because with the low-long dose, the rate of receptor reinsertion would be about the same as internalization...

...But on the most part the AUC would be the big factor.

So, for example, this would be a similar issue when we talk of "reversible MAO-B inhibitor" vs. an irreveserible MAO-B inhibitor? So some drugs may bind and have increase the chances greatly that a receptor would be internalized?

I don't quite follow. The difference between reverisable and irreversible inhibitors is that reversable inhibitors don't bind covalently to the receptor, while irreversable ones do.

Drugs that favour internalization would bind to the receptor, and stabalize a conformation that is prone to internalzation. In a GPCR paradigm, this would mean the ligand would bind to the receptor is such a what the GRK would have high affinity for the receptor. This is how both agonists and antagonists can cause internalization, by causing states that GRK can be active. I imagine something similar for benzos.

 

[lifeisforliving]

First, thanks for replying so thoroughly.. I'm so interested in this stuff!

It's possible, that is certainly the case with G-protein coupled receptors, I don't know about ionotropic receptors, but I suspect its likely. It's also likely, that GABA-A receptors made up from different subunits are more suceptable to internalizatoin and by binding to different GABA-A receptor subunits with different affinities, different drugs will cause internalization at different rates...

Do we know exactly by what biological mechanism receptors are internalized? Is it based on the receptor subunits and the conformations induced by ligands binding? Is it simply a chemical/phsyical property that the conformation allows for a greater likelihood that the receptor could "dive" into the membrane?

Or is there a whole separate internal system that senses active receptors (as was quoted above by phosphorylating them...)?

I don't quite follow. The difference between reverisable and irreversible inhibitors is that reversable inhibitors don't bind covalently to the receptor, while irreversable ones do.

I was trying to make the comparison between:

1) A ligand binding to a receptor, activating it, and detaching. Leaving the receptor ready for another cycle. (reversible, no lasting effect to the receptor)

vs.

2) A ligand (that favours internalization) binding to a receptor, activating it, and setting in motion the mechanism for internalization (irreversible, receptor is no longer able to participate on the PM).

Not a great comparison I guess, but all I could come up with in my thought process at the time.

Drugs that favour internalization would bind to the receptor, and stabalize a conformation that is prone to internalzation. In a GPCR paradigm, this would mean the ligand would bind to the receptor is such a what the GRK would have high affinity for the receptor. This is how both agonists and antagonists can cause internalization, by causing states that GRK can be active. I imagine something similar for benzos.

Yep. Thanks. Time for me to do some more studying to get all that and reply intelligently :D

Have a good night.

 

[BilZ0r]

Do we know exactly by what biological mechanism receptors are internalized? Is it based on the receptor subunits and the conformations induced by ligands binding? Is it simply a chemical/phsyical property that the conformation allows for a greater likelihood that the receptor could "dive" into the membrane?

Or is there a whole separate internal system that senses active receptors (as was quoted above by phosphorylating them...)?

That paper I linked above I think is our limit of understanding. I don't think we know half as much about how GABA-A receptors are internalized vs GPCR internalization.