Barbituate versus benzodiazapine potency

[Skorpio]

Benzos can be active at sub mg doses, while most barbiturates are dosed on the range of 100 mg to grams. Anybody know why this is?

 

[serotonin2A]

Barbituares and benzodiazepines both work through GABA-A receptors, but benzodiazepines tend to have higher affinity than barbiturates, meaning that benzodiazepines also tend to have much higher potency.

 

[WSH]

Barbituares and benzodiazepines both work through GABA-A receptors, but benzodiazepines tend to have higher affinity than barbiturates, meaning that benzodiazepines also tend to have much higher potency.

sero, can you tell me about the different binding sites for benzos and barbs (and ethanol) at the GABA ion channel?

 

[serotonin2A]

sero, can you tell me about the different binding sites for benzos and barbs (and ethanol) at the GABA ion channel?

BZ sites are located at the interface between alpha-gamma subunits

barbiturates can bind at the interface between alpha-beta subunits, as well as the interface between gamma-beta subunits

supposedly the ethanol site is immediately adjacent to the BZ site

 

[WSH]

BZ sites are located at the interface between alpha-gamma subunits

barbiturates can bind at the interface between alpha-beta subunits, as well as the interface between gamma-beta subunits

supposedly the ethanol site is immediately adjacent to the BZ site

are there any ligands that displace ethanol from its site?

 

[serotonin2A]

Yes, Ro 15-4513

 

[WSH]

Yes, Ro 15-4513

Does ethanol have an intrinsic activity / efficacy at its site?
and if so, then what is it?

 

[serotonin2A]

Does ethanol have an intrinsic activity / efficacy at its site?
and if so, then what is it?

Ethanol is a positive allosteric modulator. It does have efficacy, but the level is difficult to define because it is not clear whether the ethanol site has an endogenous ligand (intrinsic activity is usually defined based on the activity of an endogenous ligand).

 

[WSH]

Ethanol is a positive allosteric modulator. It does have efficacy, but the level is difficult to define because it is not clear whether the ethanol site has an endogenous ligand (intrinsic activity is usually defined based on the activity of an endogenous ligand).

ethanol is barb-like in the sense that it can DIRECTLY activate the GABA-receptor, right?
benzos only increase the affinity for endogenous GABA, right?

PS: is there anything that you do NOT know?

 

[Solipsis]

No, that would make ethanol an agonist but it is a positive allosteric modulator, making the endogenous ligand more effective. Does there have to be an endogenous ligand at the ethanol site though? It is allosteric, so doesn't it just modulate GABA's effects at the agonist sites? Is it that normal for there to be an endogenous ligand at allosteric sites? I thought the whole point was that they are a bit more 'rogue'. Is the activity of a PAM not defined by the modulation of activity of the agonistic ligand?

And yeah you are crazy knowledgeable ser2a, its good to have you on the forum

 

[Kdem]

Can't help myself ...when we are talking about these type of things, why is muscimol so different ? PAMs (positive allosteric modulators) versus agonists ...

 

[serotonin2A]

No, that would make ethanol an agonist but it is a positive allosteric modulator, making the endogenous ligand more effective. Does there have to be an endogenous ligand at the ethanol site though? It is allosteric, so doesn't it just modulate GABA's effects at the agonist sites? Is it that normal for there to be an endogenous ligand at allosteric sites? I thought the whole point was that they are a bit more 'rogue'. Is the activity of a PAM not defined by the modulation of activity of the agonistic ligand?

And yeah you are crazy knowledgeable ser2a, its good to have you on the forum

There certainly doesn't have to be an endogenous ligand. But often times allosteric sites evolved for a purpose. There are lots of biochemicals that are about the right size and structure to bind to the alcohol site, so it wouldn't be surprising if there is an endogenous ligand.

Thanks for the kind words

 

[belligerent drunk]

Can't help myself ...when we are talking about these type of things, why is muscimol so different ? POMs versus agonists ...

Perhaps because it's selective for GABAA alpha1 subunit? Z-drugs like zolpidem and zopiclone are notorious for their hallucinogenic effects as well as their hypnotic action. They're also selective alpha1 subunit ligands. Benzos on the other hand are rather nonselective for all 5 subunits, thus the hallucinogenic effect is of much lesser intensity (as compared to other effects such as sedation, muscle relaxation) and less noticeable. Seems to make sense to me.

 

[Solipsis]

@ muscimol and zolpidem differences: that topic is discussed here http://www.bluelight.org/vb/threads/814584-Why-is-zolpidem-so-different-cmared-to-other-benzo-s

@ RO-15-4513: It appears that this drug does not reduce currents from the ethanol-sensitive extrasynaptic GABA A subtypes at concentrations that block ethanol's activity. If there were an endogenous ligand for this site, wouldn't its activity be blocked similarly, yielding modulated activity compared to baseline activity? Or do you think that RO-15-4513 may simply not be competitive enough vs. that ligand?

Also I saw someone quoted saying here: https://www.scripps.edu/newsandviews/e_20020225/koob2.html that ethanol is an indirect GABA agonist. Not sure if that is what WSH meant... The guy quoted is just slightly misleadingly trying to explain it being a PAM?

 

[serotonin2A]

@ muscimol and zolpidem differences: that topic is discussed here http://www.bluelight.org/vb/threads/814584-Why-is-zolpidem-so-different-cmared-to-other-benzo-s

@ RO-15-4513: It appears that this drug does not reduce currents from the ethanol-sensitive extrasynaptic GABA A subtypes at concentrations that block ethanol's activity. If there were an endogenous ligand for this site, wouldn't its activity be blocked similarly, yielding modulated activity compared to baseline activity? Or do you think that RO-15-4513 may simply not be competitive enough vs. that ligand?

Also I saw someone quoted saying here: https://www.scripps.edu/newsandviews/e_20020225/koob2.html that ethanol is an indirect GABA agonist. Not sure if that is what WSH meant... The guy quoted is just slightly misleadingly trying to explain it being a PAM?

It is possible that the endogenous ligand is not tonically active. Alternatively, it could be a vestigial site; the ethanol site may have evolved to bind a particular endogenous ligand, but that doesn't mean that the system still functions that way.

I think the quote was just poorly phrased. If you read over the patch clamp literature, ethanol reportedly "potentiates" the response to GABA.

 

[WSH]

It is possible that the endogenous ligand is not tonically active. Alternatively, it could be a vestigial site; the ethanol site may have evolved to bind a particular endogenous ligand, but that doesn't mean that the system still functions that way.

I think the quote was just poorly phrased. If you read over the patch clamp literature, ethanol reportedly "potentiates" the response to GABA.

Here's another (kinda off-topic) question:

The affinity of sero for 5-ht1a is 1.6nm, which is also (roughly) the extracellular concentration of sero in the mPFC.

So let's assume 50% occupancy by endogenous sero.

Now let's add an partial agonist (e.g. gepirone, but the specifics are not important). Does the partial agonist first occupy the remaining 50% of the empty receptors and then compete with sero for the first 50%?
Or does it occupy the last 50% and simultaneously compete with sero for the first 50%?

 

[serotonin2A]

Here's another (kinda off-topic) question:

The affinity of sero for 5-ht1a is 1.6nm, which is also (roughly) the extracellular concentration of sero in the mPFC.

So let's assume 50% occupancy by endogenous sero.

Now let's add an partial agonist (e.g. gepirone, but the specifics are not important). Does the partial agonist first occupy the remaining 50% of the empty receptors and then compete with sero for the first 50%?
Or does it occupy the last 50% and simultaneously compete with sero for the first 50%?

In an idealized situation, where gepirone has Ki = 1.6 nM and is also present at 1.6 nM, it would initially occupy 25% of the remaining sites. Then gepirone and 5-HT would reach an equlibrium where 66% of the total sites are occupied by a mixture of 5-HT and gepirone. But in the real world how the equlibrium at both stages plays out depends on the kinetics of association and dissociation for both substances.

 

[WSH]

In an idealized situation, where gepirone has Ki = 1.6 nM and is also present at 1.6 nM, it would initially occupy 25% of the remaining sites. Then gepirone and 5-HT would reach an equlibrium where 66% of the total sites are occupied by a mixture of 5-HT and gepirone. But in the real world how the equlibrium at both stages plays out depends on the kinetics of association and dissociation for both substances.

Why 25% of the remaining sites?

If gepirone is applied (let's say with reverse microdialysis, but like i said the specifics are not important) at its Ki, then it would block 50% of the available receptors (which would be the 50% unoccupied ones) 50% x 50% = 25%, i understand that, but wouldn't it be 25% of the total sites intead of the remaining ones?

And how do you get to 66%?
50% sero + 25% gepi = 75%???
50% sero + (25% x 50%) gepi = 62.5%???

I know that you most likely used this formula: 2 / (2 + 1) = ~66%

But how much is the proportion of both? I think it should be 33% sero and 33% gepi, because both of them are at their Ki?
I think I answered my own question!

But the first question still exists: Why 25% of the remaining sites instead of the total sites?
Shouldn't gepirone occupy 50% of the available ones (50%) which would be 25% of the total sites?

 

[serotonin2A]

Why 25% of the remaining sites?

If gepirone is applied (let's say with reverse microdialysis, but like i said the specifics are not important) at its Ki, then it would block 50% of the available receptors (which would be the 50% unoccupied ones) 50% x 50% = 25%, i understand that, but wouldn't it be 25% of the total sites intead of the remaining ones?

And how do you get to 66%?
50% sero + 25% gepi = 75%???
50% sero + (25% x 50%) gepi = 62.5%???

I know that you most likely used this formula: 2 / (2 + 1) = ~66%

But how much is the proportion of both? I think it should be 33% sero and 33% gepi, because both of them are at their Ki?
I think I answered my own question!

But the first question still exists: Why 25% of the remaining sites instead of the total sites?
Shouldn't gepirone occupy 50% of the available ones (50%) which would be 25% of the total sites?

Use of the word displacement makes it sound like gepirone should be able to knock bound serotonin out of sites, but that is not what actually happens. What is displaced is the equilibrium of serotonin dissociating and re-associating with the receptor. Ligand dissociation happens at a constant rate whether or not there are other ligand molecules in the vicinity of the receptor. So once serotonin (or another ligand) binds to a site, the site is effectively non-existant for other ligands until serotonin dissociates, at which time some other ligand can displace it (ie, prevent serotonin from re-binding). So if 50% of the binding sites are occupied by serotonin, then none of those sites can be occupied by gepirone until some of the bound serotonin dissociates.

 

[WSH]

Use of the word displacement makes it sound like gepirone should be able to knock bound serotonin out of sites, but that is not what actually happens. What is displaced is the equilibrium of serotonin dissociating and re-associating with the receptor. Ligand dissociation happens at a constant rate whether or not there are other ligand molecules in the vicinity of the receptor. So once serotonin (or another ligand) binds to a site, the site is effectively non-existant for other ligands until serotonin dissociates, at which time some other ligand can displace it (ie, prevent serotonin from re-binding). So if 50% of the binding sites are occupied by serotonin, then none of those sites can be occupied by gepirone until some of the bound serotonin dissociates.

OK, but going back to the percentages: 50% are already occupied by sero and the addition of gepi at its Ki should occupy 50% of the remaining 50% that are not occupied by sero (like you said, gepi can't "drag" sero off the sites), so therefore the total occupancy should be 50% sero + 50% x 50% gepi = 75%, so why did you say it's 66% (I know that you are right and I'm wrong, because this specific receptor theory stuff isn'T my specialty, i just wanna know why 75% is wrong)