Receptor interaction and drug circulation

[Jamshyd]

This is one thing that, read as read can read, I couldn't find a discription of anywhere (thanks largely to scientists' poor writing skills).

So a drug is travelling in the blood-stream. Some of it is plasma-bound, while some is not. Assuming is crosses the BBB:

1. Just to make sure I have this right: only the non-plasma bound portion of the drug will actually reach the receptors, is that correct?

2. And this is my real question: Once the drug passes the BBB, it binds to a receptor on the appropriate nuron. Does this molecule simply "kiss" the receptor (being followed immediately by another "kiss" from another molecule), dissociate, go back into circulation only to cross the BBB again and kiss another receptor, and so on, until it reaches the liver and is metabolized?

This makes sense to me. The idea most writers seem to give (assuming they don't gloss over it, is that a molecule travels and finds a receptor, and lodges into the receptor untill it is elliminated, making it "one molecule per receptor" in a way. But this doesn't make sense on many levels.

Which is more correct?

 

[MattPsy]

The latter. Except that it will almost immediately dissociate and then bind again... but not necessarily or even very likely to the same receptor it bound to last time... or even on the same synapse!
After a while it will either pass back into the bloodstream (and then probably pass back into the brain after a circuit through the rest of the body, if it escapes destruction by the liver, or renal elimination), or some types of drugs will be eaten up by brain MAO.

 

[MurphyClox]

1. Just to make sure I have this right: only the non-plasma bound portion of the drug will actually reach the receptors, is that correct?

Yep. But there is a equilibrium between the bound and unbound fraction. So, the bound part can be released..and be bound again...and.... BUT, as long as it is bound to any plasma-protein it cannot pass the BBB.

Does this molecule simply "kiss" the receptor (being followed immediately by another "kiss" from another molecule), dissociate, go back into circulation only to cross the BBB again and kiss another receptor, and so on, until it reaches the liver and is metabolized?

Your idea of it is quite exact...It goes like: Cross the BBB, bind to receptor, do your job (i.e. agonist, antagonist,...), get released from receptor, keep on traveling...until you hit the liver.
Some molecules stay longer than others. This depends on their affinity to the target (e.g. the receptor) but staying too long (worst case: no release at all) makes the drug a heavy poison.

Peace! Murphy

 

[Jamshyd]

Alright!! Thank you so much. Now I can think of things much more confidently with this idea being confirmed - your comment about the drug being a poison if it stays attached for too long was another thing that got me thinking of the second scenario being wrong.

 

[The Monkey Mantra]

Jamshyd, the dissociation of an agonist from its receptor is very important to the effect. Nicotine, for example, stimulates cholinergic neurons. In the periphery, however, because it stimulates and then sticks to the receptor for a really long time, it ends up acting as an antagonist even though it's an agonist, blocking the receptor from its normal ligand, acetylcholine, and causing the paradoxical relaxing effect on the peripheral nervous system. Methadone *supposedly* works the same way when it "blocks" the heroin high. It binds, agonizes, and takes its sweet time to let go.

 

[Ham-milton]

Methadone *supposedly* works the same way when it "blocks" the heroin high. It binds, agonizes, and takes its sweet time to let go.

I don't think this is accurate. Methadones high affinity is what's blocking other drugs from getting to the receptor, not because it stays bound to the receptor for a longer period of time.

Some molecules stay longer than others. This depends on their affinity to the target (e.g. the receptor) but staying too long (worst case: no release at all) makes the drug a heavy poison.

Drugs that stay bound longer are not heavy poisons. They're called partial agonists. Buprenorphine binds and stays bound for a very long period of time. It also has a very high affinity- this is the double whammy that makes it so effective at blocking other drugs.

Pure agonists bind, agonize and release almost immediately. Partial agonists bind, agonize and stay bound for a long period. I have not seen any drug that binds and stays bound forever, though I did once hear that a fentanyl analogue did (staying bound until the receptor died and a new one formed), but I never found confirmation of that.

 

[mad_scientist]

Pure agonists bind, agonize and release almost immediately. Partial agonists bind, agonize and stay bound for a long period. I have not seen any drug that binds and stays bound forever, though I did once hear that a fentanyl analogue did (staying bound until the receptor died and a new one formed), but I never found confirmation of that.

Lofentanil and other high potency fentanyl agonists stay bound for a long time even though they are full agonists, which leads to rapid internalisation and deactivation of the opioid receptors. Might even have a reference for that somewhere if I go dig through my notes...

 

[Ham-milton]

Are you sure about that? I found this, but it doesn't say anything about a long dissociation hl from mu receptors.

J. M. Maloteaux1, J. N. Octave1, E. C. Laterre1 and P. M. Laduron2
(1) Laboratoire de Neurochimie, Université Catholique de Louvain, Avenue Hippocrate, B-1200 Brussels, Belgium
(2) Rhône-Poulenc Santé, Centre de Recherches, F-94403 Vitry-sur-Seine, France

Received: 7 January 1988 Accepted: 22 October 1988
Summary There was stereospecific binding of 3H-lofentanil (K D value = 1.53 nM) to membranes of neuroblastoma-glioma NG 108-15 cells which are known to bear high affinity binding sites for enkephalin derivatives (delta-opiate receptor subtype). There was no high affinity specific binding of the mgr-opiate specific ligand 3H-sufentanil. The specific binding of 3H-lofentanil to delta-opiate receptor subtype was down-regulated (decrease in B max value without change in the K D value) after prolonged incubation of the cells in the presence of leu- and met- enkephalin (0.1 mgrM). There was no down-regulation of the opiate receptors (3H-lofentanil and 3H-d-ala-d-leu-enkephalin specific binding) after incubation of NG 108-15 cells with drugs from the fentanyl series (alfentanil or sufentanil).
In cultured neurones from rat forebrain (15 day old embryos), the 3H-lofentanil binding was specific with high affinity (K D: 0.048 nM) and a slow dissociation rate similar to that in adult rat cortex. Drugs of the fentanyl series (4-anilino-piperidines) were potent displacers whereas agonists of the delta- (enkephalin derivatives), sgr (phencyclidine, haloperidol, 3-hydroxyphenyl-propylpiperidine) or K- (U 50488) opiate sites had a low affinity (K i > 0.5 mgrM) for 3H-lofentanil specific binding sites. Since there was also specific binding of 3H-sufentanil, the opiate receptors in cultured neurones seem to be mainly of the mgr-subtype and this is consistent with the ontogeny of opiate receptors subtypes. These receptors were down-regulated after incubation in the presence of etorphine, sufentanil and alfentanil but not enkephalin derivatives.
These results strongly suggest specific binding of 3H-sufentanil and 3H-lofentanil mainly to the so-called mgr-opiate receptors in cultured neurones and a specific binding of 3H-lofentanil to lower affinity delta-opiate receptors in neuroblastoma-glioma cells. The down-regulation of the mgr-opiate binding sites in cultured neurones and that of the delta-site in neuroblastoma × glioma hybrid cells were dose-and temperature-dependent, induced by the corresponding high affinity agonists and prevented by naloxone. Morphine did not induce down-regulation of mgr or delta receptor sites, possibly because of a partial antagonist effect on both receptor subtypes.

 

[Ham-milton]

I also found this, but it doesn't say what receptor they're talking about. I think it's safe to say mu, but I dunno.

Opiate receptors in brain labelled in vivo with [3H]lofentanil.
Laduron PM, Poncet J, Janssen PF.

The in vivo binding of [3H]lofentanil was studied in various regions of the brain in rat. After intravenous injection of [3H]lofentanil the disposition of the labelled drug in the brain paralleled exactly the regional distribution of opiate receptors measured in in vitro binding assays. The labelling was saturable and could be prevented by naloxone when given before [3H]lofentanil, in all the regions except in the cerebellum. The long-lasting occurrence of the specific labelling was entirely compatible with the extremely slow dissociation rate of lofentanil and its long duration of action. This explains why [3H]lofentanil is not displaceable by naloxone in vivo. Subcellular fractionation experiments revealed that all the labelling in the frontal cortex but not in the cerebellum was particulate-bound and entirely displaceable by naloxone. The advantages of [3H]lofentanil in vivo are its extremely low non-specific binding and its ability to reveal very low occupancy of opiate receptors in brain.

It was my understanding that any drug that remained bound for any substantial period of time is considered a partial agonist. Why then is lofentanil considered a full agonist?

When it stays bound, is it continually agonizing the receptor, whereas a partial agonist only antagonizes it for a brief period then stays bound occupying space?

If so, it'd certainly explain why it would cause monumental tolerance climbing.

 

[Black]

Drugs that stay bound longer are not heavy poisons. They're called partial agonists. Buprenorphine binds and stays bound for a very long period of time. It also has a very high affinity- this is the double whammy that makes it so effective at blocking other drugs.

Pure agonists bind, agonize and release almost immediately. Partial agonists bind, agonize and stay bound for a long period. I have not seen any drug that binds and stays bound forever, though I did once hear that a fentanyl analogue did (staying bound until the receptor died and a new one formed), but I never found confirmation of that.

if a drug has a higher affinity it also stays bound for a longer time.

also partial agonists don't stay for a longer time than full agonists. that depends on the affinity. the partial agonist only elecits less of a downstream response than a full agonist.

 

[Black]

When it stays bound, is it continually agonizing the receptor, whereas a partial agonist only antagonizes it for a brief period then stays bound occupying space?

the partial agonist avtivates the receptor but does not change the spacial conformation of the receptor quite in the right way, as a full agonist would. so the downstream events (dissociation of proteins/phosphorylation of proteins or whatever) don't take place to the same extent as with full receptor activation.

 

[Ham-milton]

if a drug has a higher affinity it also stays bound for a longer time.

That's not true at all. High affinity drugs like Fentanyl, Sufentanil and Alfentanil have relatively fast dissociation half-lives (F and A are about 10s, S roughly 1min). Carfentanil is another with high affinity and very short dissociation half life.

There are lots of drugs with lower potencies and longer dissociation half lives.

 

[Ham-milton]

the partial agonist avtivates the receptor but does not change the spacial conformation of the receptor quite in the right way, as a full agonist would. so the downstream events (dissociation of proteins/phosphorylation of proteins or whatever) don't take place to the same extent as with full receptor activation.

Okay, that makes sense. A few months ago someone (mobydick?) posted a primer on what exactly agonists/antagonists/partials did differently. Apparently the partial agonist portion was somewhat inaccurate or my memory is horrible

 

[negrogesic]

the partial agonist avtivates the receptor but does not change the spacial conformation of the receptor quite in the right way, as a full agonist would. so the downstream events (dissociation of proteins/phosphorylation of proteins or whatever) don't take place to the same extent as with full receptor activation.

Yes, that is similar to what I thought; they do occupy the receptor but only a portion of the receptors become active (therefore name, partial). But keep in mind there may be issues with this description, for example i remember reading that decamethonium does not meet this definition however is still considered a partial agonist because it does not contract skeletal muscle as well as ACh. So there may in fact be two definitions...

 

[MattPsy]

Okay, that makes sense. A few months ago someone (mobydick?) posted a primer on what exactly agonists/antagonists/partials did differently. Apparently the partial agonist portion was somewhat inaccurate or my memory is horrible

Hey, that's the one that I presented my understanding of partial agonists on (which was contray to what MobiusDick said) and you presented what he said as evidence against what I said!
So which is it!!?!?
I'm sooo confused now.

The understanding I have NOW is that partial agonists are ones that may bind to the receptor but not activate it sometimes - so sometimes they're an agonist, and other times an antagonist... leading to a kind of half-way activity when you have thousands of these drug molecules binding to their own receptors. Their efficacy (intrinsic activity) is thus the PROBABILLITY that receptor activition occurs on binding, compared to the endogenous agonist.
This same effect may also occur with agonists that have longer dissociation halflives than usual, I think, from what I can see, because they should amount to the same expression of activity... because if an agonist stays bound, the ion channels it is coupled to won't close... which means you won't get any additional action potentials generated as there should be no voltage gradient between the extracellular and intracellular space of the neuron if the channel is stuck open?

 

[Black]

@hammilton
you sure about the affinity? from what i know about equilibrium/dissociation constants and reaction kinetics the time bound should be proportional to the affinity, but my physical chemistry is a bit rusty and i haven't been able to find a satisfying definition of pharmacological affinity yet…

This same effect may also occur with agonists that have longer dissociation halflives than usual, I think, from what I can see, because they should amount to the same expression of activity... because if an agonist stays bound, the ion channels it is coupled to won't close... which means you won't get any additional action potentials generated as there should be no voltage gradient between the extracellular and intracellular space of the neuron if the channel is stuck open?

that's an interesting point. but i think it depends on the receptor. with ion channels you should have a constant action potential (not sure what that means for further signalling events, probably overstimulation), with receptor tyrisine kinases you should have constant activity if the ligand stays bound and with G-Protein coupled receptors things get interesting. the G protein dissociates from the receptor and inactivates itself after some time but cannot bind to the receptor again.
according to wikipedia an activated receptor may activate an inactive g protein again, but g proteins bound to the receptor change the affinities for ligands.

 

[MattPsy]

Why would you get a constant action potential?
For a voltage to be produce there has to be a electrostatic gradient. If the channel is stuck open, there won't be one, because the charge will be equalised.
For a postsynaptic potential to be created there has to be a sufficient voltage "spike" occur (generated from the synapse) to open voltage-gated ion channels at the axon hillock of the neuron. If no spike occurs... no action potential will be generated. Also, there is no such thing as a constant action potential - neuron firing always occurs in spikes.

 

[Black]

it's true that the action potential is a spike with neuronal transmittion, but a spike from polarisation to depolarisaiton. under normal (no signal transmission) conditions there's a electrostatic gradient between the inner and outer side of the the nerve cell and with the action potential there's a short spike of depolarisation. ion channels stuck open would lead to a constant state of depolarisation. (at least that's what would be logical in such a circumstance). i think that constant depolarisation would be quite toxic to the cell. i can't think of a drug that keeps ion channels open, only neurotoxins come to my mind…

 

[negrogesic]

^^^The only drugs I can think of are topical anesthetics, the rest are essentially neurotoxins, such as blowfish venom etc...

 

[Limpet_Chicken]

Barbs lock open the chloride channels on the GABAa receptor don't they, isn't that why they are compared to benzos, which bind allosterically and do not activate the receptor directly in the abscence of GABA, a lot more dangerous?

What about depolarising neuromuscular blocking agents like suxamethonium (or it might be succinylcholine, in my current state my capacity to look it up is diminished somewhat