Gabapentin and it’s recently discovered GABA A activity

[DrunkenSkunk]

I stumbled across this fairly recent 2019 study here, though I need the help of some of you more pharmacology savvy folk in helping me interpret what exactly this means.

The study says that although gabapentin affects the GABA A receptor, it is not a PAM like benzodiazepines, rather it “increases cell-surface expression of δGABA A receptors”.

Does this mean that it doesn’t cause downregulation of the receptor like benzos, and therefor shares no cross-addiction potential? Different sub-target site (subtype?) altogether?

Also, can anyone explain why each of these studies contradict themselves on GABA B activity? Is it or is it not a GABA B agonist?

As it relates to me, I guess basically I’m simply wondering if there is any cross-tolerance/cross-addiction potential with these drugs because I have been physically dependent on benzos before and due to me being ignorant to new data on gabapentin, have been reckless at times with gabapentin usage and don’t want to retread that path.

Spoiler: Studies

Gabapentin increases expression of δ subunit-containing GABAA receptors

Gabapentin is a structural analog of the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Its anticonvulsant, analgesic and anxiolytic properties suggest that it increases GABAergic inhibition; however, the molecular basis for these effects ...

www.ncbi.nlm.nih.gov

Gabapentin robustly increases cell-surface expression of δGABAA receptors and increases a tonic inhibitory conductance in neurons. This enhanced δGABAA receptor function contributes to the ataxic and anxiolytic but not antinociceptive properties of gabapentin. Gabapentin does not increase levels of GABAA receptor agonists or several neurosteroids in the brain.

Gabapentin is not a GABA B agonist

Gabapentin is not a GABAB receptor agonist

Recent experiments have demonstrated that formation of functional type B gamma-aminobutyric acid (GABAB) receptors requires co-expression of two recep…

www.sciencedirect.com

Here we report that gabapentin was completely inactive at recombinant GABABheterodimers expressing either GABAB1a or GABAB1b receptor subunits in combination with GABAB2 receptor subunits. In addition, in both CA1 and CA3 pyramidal neurones from rodent hippocampal slices we were unable to demonstrate any agonist-like effects of gabapentin at either pre- or post-synaptic GABABreceptors. In contrast, gabapentin activated a GABAA receptor mediated chloride conductance.

Gabapentin is a GABA B agonist

The anticonvulsant, antihyperalgesic agent gabapentin is an agonist at brain gamma-aminobutyric acid type B receptors negatively coupled to voltage-dependent calcium channels - PubMed

Gabapentin (Neurontin, Pfizer Global R & D) is a novel anticonvulsant, antihyperalgesic, and antinociceptive agent with a poorly understood mechanism of action. In this study, we show that gabapentin (EC50 2 microM) inhibited up to 70 to 80% of the total K+-evoked Ca2+ influx via voltage-depende …

pubmed.ncbi.nlm.nih.gov

 

[DrunkenSkunk]

So I also just found this study right here, which is actually the the subject being quoted in the study about Gabapentin NOT being GABA B agonist.

So are these two studies basically debating whether or not it’s possible for a GABA B agonist to produce effects if not all GABA B specific target sites are activated?

Even then, why did the study claiming it’s not a GABA B agonist fail to replicate any agonist activity at the GABA B receptor?

I’m very confused, will someone please shed some light on what can be ascertained from all this about Gabapentin's MOA? Definitely a lot more than just voltage-gated calcium channels...

Spoiler: Study

y􏰂-Aminobutyric Acid Type B Receptors with Specific Heterodimer Composition and Postsynaptic Actions in Hippocampal Neurons Are Targets of Anticonvulsant Gabapentin Action

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.930.5612&rep=rep1&type=pdf

Gabapentin is the first GABAB receptor subtype-selective agonist identified providing proof of pharma- cologically and physiologically distinct receptor subtypes. This selective agonism of postsynaptic GABAB receptor subtypes by gabapentin in hippocampal neurons may be its key thera- peutic advantage as an anticonvulsant.

 

[Cotcha Yankinov]

Receptor affinity assays from different labs can occasionally produce conflicting results it seems.

I think the general consensus is that gabapentin is primarily an antagonist at a voltage dependent calcium channel (alpha2delta) and has no appreciable direct affinity for GABA-A or GABA-B receptors. There may be some effects on GABA/Glutamate metabolism..

I do recall that study finding some affinity for a very specific GABA receptor, but its difficult to say whether or not that's going to be appreciable in humans. If its likely to be an appreciable effect in humans, it should be easier for other labs to replicate those results, assuming they pull receptors from the same species and cells. My recollection is fuzzy but I'm not sure if they tested the exact same cells. You could check to see if they did a good job replicating that portion of the other lab's assay.

 

[DrunkenSkunk]

Thanks for your reply! I believe you’re referring to it’s speculated activity at GABA B1a

Excerpt taken from 2nd study

Spoiler

“However, Ng et al. [Mol. Pharmacol., 59 (2000) 144] have recently suggested a novel and important pharmacological difference between GABABreceptor heterodimers expressing the GABAB1aand GABAB1b receptor subunits. This study suggested that the antiepileptic GABA analogue gabapentin (Neurontin) is an agonist at GABABreceptors expressing the GABAB1a but not the GABAB1b receptor subunit.”

I’m aware of what gabapentin’s MOA has been thought to be (and still is, at least to some extent), however the reason for this post is that it’s currently being challenged in the 2019 study that hints at some sort of GABA A activity.

I guess I’m just looking into clarification into what that supposed activity actually means In comparison to benzos PAM activity on the same receptor

To put it bluntly what does this mean?

Spoiler

Gabapentin robustly increases cell-surface expression of δGABAA receptors and increases a tonic inhibitory conductance in neurons. This enhanced δGABAA receptor function contributes to the ataxic and anxiolytic but not antinociceptive properties of gabapentin.

These findings first identify δGABAA receptors as targets of gabapentin and challenge the widely-held notion that inhibition of α2δ subunits of voltage-dependent calcium channels is gabapentin's primary mechanism of action. Also, since dysregulation of δGABAA receptors contributes to several cognitive and psychiatric disorders, the therapeutic applications of gabapentin may be broader than currently recognized.

Also, aside from my general curiosity of what this new proposed MOA means, I assume this quote actually answers the benzo cross tolerance question?

Spoiler

Gabapentin did not modify cell-surface expression or total expression levels of α1 subunits, suggesting that it selectively modifies expression of δGABA A receptors

If the drug actually has no direct affinity for GABA target sites, why does gabapentin then seem to produce withdrawal symptoms that resemble GABA-ergic compounds? Are calcium channels really capable of producing such cessation symptoms?

 

[AlphaMethylPhenyl]

Can say that gabapentin can't cover the withdrawal of serious alcoholism. Usually ativan/librium are used for that.

 

[Cotcha Yankinov]

The answers to some of your questions lie beyond thinking of a drug's effects in terms of individual cells.

Let's pretend that we have a 3 cell assembly in 1 --> 2 --> 3 format

A glutamate cell (#1) projecting an axon that releases glutamate onto the dendrite of a GABA cell (#2) which expresses glutamate receptors, and then that GABA cell projects its GABA releasing axons onto yet another glutamate cell (#3) which expresses GABA receptors. Almost a neural circle jerk, if you will.

Imagine that the first glutamate cell releases its glutamate onto an AMPA receptor expressed on the GABA interneuron. The resulting AMPA receptor activation is excitatory, causing an influx of excitatory ions which increases the GABA cell's likelihood to fire and release GABA down onto cell #3, the final cell in our little train. Now imagine if you block the AMPA receptor and decrease the activation of the middle GABA interneuron, therefore starving the downstream glutamate cell (#3) of GABA. The #3 glutamate cell would react to this decreased GABA release by upregulating and sensitizing its GABA receptors.

Cross tolerance doesn't necessarily just form at the receptor level, it can also be part of a larger coordinated response to maintain homeostasis by multiple cells working in tandem. Much of tolerance to depressant drugs has to do with a compensatory upregulation of glutamate. This is in part why medications that help suppress glutamate (eg sodium channel antagonists) can be helpful with alcohol/benzo withdrawal. Delirium Tremens for example has partly to do with upregulation of a glutamate receptor called the NMDA receptor.

Gabapentin is a weird one because there is a wide range of individual response to it, both in terms of effects, tolerance and withdrawal. Some people receive very little withdrawals, other people receive benzo-like withdrawals. Gabapentin normally decreases the amount of glutamate released, and part of this isn't even mediated directly by blocking alpha2delta on glutamate neurons, some of it is by virtue of decreasing Substance P release (more related to its actions on neuropathic pain). So the effects of these drugs can be very roundabout.

With regards to your papers and gabapentinoid's rather tentative direct actions at GABA receptors, it would be important for those authors to provide evidence that gabapentin is exerting effects by binding directly to GABA receptors in order to draw the conclusion that it is a direct effect, rather than a roundabout effect (admittedly haven't read the paper to see what conclusions that may be trying to draw).

One way to do that would be a radioligand binding assay, with which you could prove a competitive interaction at a specific site of the GABA receptor. This can be difficult for some GABA ligands (even alcohol) because there are various interfaces between the GABA-A channel's subunits and many different subunit combinations. You might have to specially design a molecule in order to get it to bind to a specific GABA-A subunit interface with some selectivity, then add tritium to it, then run a radioligand binding study checking for a competitive interaction with the actual ligand that you want to test (in the below paper's case, alcohol)

From the Cover: Ethanol potently and competitively inhibits binding of the alcohol antagonist Ro15-4513 to α4/6β3δ GABAA receptors

Although GABA[A] receptors have long been implicated in mediating ethanol (EtOH) actions, receptors containing the “nonsynaptic” δ subunit only recently have been shown to be uniquely sensitive to EtOH. Here, we show that δ ...

www.ncbi.nlm.nih.gov

 

[Cotcha Yankinov]

Can say that gabapentin can't cover the withdrawal of serious alcoholism. Usually ativan/librium are used for that.

I definitely believe that, but I wonder if the potency of an alpha2delta antagonist could be pushed high enough to be helpful. I was never able to be nodded out from gabapentin, at the most it had the effects of a low dose benzo or a couple shots. People are always talking about the PK issues with gabapentin, it definitely felt ridiculous to me that I was taking 900mg 4 times a day but I never got to try Pregablin

 

[Snafu in the Void]

Can say that gabapentin can't cover the withdrawal of serious alcoholism. Usually ativan/librium are used for that.

I don't think anything really does. I've certainly tried a handful on benzos for it, as well as librium and valium scripts to quit...... I find it much more difficult to do compared to heroin -> bupe/done..... ethanol is it's own bitch that can't really be tamed with other drugs i.e. benzos.....


sorry if I sound pessimistic but I kinda am right now in terms of my alcoholism...

 

[DrunkenSkunk]

Once again, thank you. That was extremely informative.

Although the study proposing the GABA A activity did not, from what I can remember at least, use a delta GABA A antagonist on the target site to compete for binding, the GABA B antagonist “CGP55845” dose-dependently blocked the effects of gabapentin. Which would actually contradict the notion that the delta GABA A receptor is the primary cause of the drugs MOA.

Perhaps their methodology is flawed, though if I’m being honest that’s beyond my knowledge to call. To a layman like myself, it seems fairly shut and close but that’s only until you see the other papers failing to replicate these results

Spoiler

Moreover, activity of the agonist gabapentin was dose dependently and completely blocked with the GABA(B) antagonist CGP55845 and was nearly identical to the prototypic GABA(B) agonist baclofen in both extent and potency. Antisense knockdown of gb1a also completely blocked gabapentin activity, while gb1b antisense and control oligonucleotides had no effect, indicating that gabapentin inhibition of membrane Ca2+ mobilization in mIL cells was dependent on a functional GABA(B) (gb1a-gb2) heterodimer receptor.

The anticonvulsant, antihyperalgesic agent gabapentin is an agonist at brain gamma-aminobutyric acid type B receptors negatively coupled to voltage-dependent calcium channels - PubMed

Gabapentin (Neurontin, Pfizer Global R & D) is a novel anticonvulsant, antihyperalgesic, and antinociceptive agent with a poorly understood mechanism of action. In this study, we show that gabapentin (EC50 2 microM) inhibited up to 70 to 80% of the total K+-evoked Ca2+ influx via voltage-depende …

pubmed.ncbi.nlm.nih.gov

This paper right here is very intriguing, and posits a mostly solid explanation for the conflicting reports of GABA B activity.

Spoiler

Amid fanciful notions of alternative binding sites, it would appear that a somewhat more obvious explanation might have been overlooked. GBP binds with high affinity to the α2δ subunit of the voltage-gated calcium channel, but these channels are functionally associated with presynaptic GABABreceptors and are intimately involved in neurotransmitter release. As a result, selective inhibition of specific GABAB-linked presynaptic calcium channels containing the α2δ subunit could, in theory, explain the findings of Parker and colleagues and equally support all evidence to the contrary. The only argument against this simplistic proposition would be the ability of GABAB antagonists to reverse the effects of GBP, a phenomenon that could be accounted for by indirect physiologic antagonism, particularly in brain-slice preparations in which endogenous GABA may act as a residual agonist.

Not Another Gabapentin Mechanism!

www.ncbi.nlm.nih.gov

I do apologize if it seems like I’m just throwing a ton of papers at you and saying “do the homework for me”, that’s definitely not my intentions and I’m doing my best to quote what I feel is relevant info from these papers I’m sorting through.

Should the discussion go no further, I’m already walking out of this thread a bit more educated on the topic.

 

[AlphaMethylPhenyl]

I don't think anything really does. I've certainly tried a handful on benzos for it, as well as librium and valium scripts to quit...... I find it much more difficult to do compared to heroin -> bupe/done..... ethanol is it's own bitch that can't really be tamed with other drugs i.e. benzos.....


sorry if I sound pessimistic but I kinda am right now in terms of my alcoholism...

No you're right. Alcohol has a lot of other effects that add to the experience. It's unique in having all of these diverse mechanisms. Benzos just cover your mortality.

 

[Cotcha Yankinov]

So for that first paper there, some of those findings are either in single cell recordings or multi-cell recordings. They are essentially sticking leads into the cells to measure the ion flow as they add different molecules (GABA-B ligands etc) to probe the effects that they have. While that in vitro data is very useful, it's difficult to draw a conclusion about what is causing a specific effect with so many different things going on inside of the cells.

Part of what the authors are trying to do by using various techniques and various ligands is to control for all the variables to be able to say that it is gabapentin binding directly to a GABA-B receptor that is causing the effects seen, but I'm not sure if they've covered all the variables. You could try emailing some of the authors of those papers, many years later they might have very different views on it (that first paper is 20 years old). If they are Philip Seeman they can be blatantly wrong and will likely never change. Some authors will freely give out the complete paper too, we just have the abstract. Although they may not take kindly to our attempts to poke a hole in their paper

We also want to be able to say that the putative GABA-B agonism is producing an appreciable effect in vivo, on whatever metric you're interested in. Behavioral effects or neuropathic pain related effects could be very different. See this paper from Pfizer and other reputable groups concerning Dorsal Root Ganglion cells, a cell type that is probably more relevant than.. mouse pituitary cells. If this GABA-B receptor is so unique that that is where the Montreal group has to get it, maybe it's not relevant in humans.

But as an example, one of the issues with the first paper's conclusions is that they are using a GABA-B antagonist (CGP55845) to diminish some of gabapentin's effects and then drawing a conclusion (albeit not based solely off that data) that the GABA-B antagonist is competing directly at a site that gabapentin binds to on the GABA-B receptor (imagine two keys trying to enter the same lock). It would be very important to verify that the GABA-B antagonist is very selective for GABA-B, and is a silent antagonist.

An alternative explanation is that many receptor antagonists are not 100% "silent antagonists", and rather that many of them do have some activity at the receptor, compared to sitting silently on-top of it and blocking ligands from binding to it without causing a conformational change that affects the intracellular portion of the receptor. An inverse agonist for example binds to a receptor and activates it selectively in a way that causes downstream signaling but not the type of signaling associated with typical agonist receptor activation. Many ligands referred to as antagonists are actually ever so slightly partial agonists or inverse agonists.

They would have to have checked the eg GTPgammaS incorporation assays to determine that particular GABA-B antatgonist's intrinsic efficacy across different GABA-B receptors. A GABA-B ligand could be affecting the cellular compartments/microdomains further into the cell if it is not completely silent. Calcium ions have targets on the inside of the cells, and G-protein coupled receptors such as GABA-B receptors can certainly affect these targets. This could affect gabapentin's effects without gabapentin necessarily needing to be attempting to bind to the GABA-B receptor through that antagonist's blockade. Many G-protein coupled receptors also affect the expression of ion channels on the cell.

Receptor heterodimers are also very poorly understood, maybe there is some sort of dimerization-like phenomena occurring between the GABA-B receptors and VDCCs, but they at least have downstream signaling partners in common.

Another weird example is MDMA and the 5-HT2B receptor. Blockade of the 5-HT2B receptor blocks MDMA's serotonin releasing effects even though the serotonin releasing effects are from MDMA entering the cell as a substrate and causing the cell to essentially leak serotonin from the inside out. One explanation there would be that the activity of 5-HT2B is important for the intracellular functions that permit MDMA to release serotonin.

Welcome to Bluelight by the way! Always glad to have another hobbyist (most of us are)

CY

 

[JoEhJoEh]

Hey all,

I ordered pure GABA powder. Has anybody experiences with that? I konw it is just a mild anxiolytic, but perhaps you can boost other substances with it?

Would appreciate any answer, thanks,

JJ

 

[izo]

afaik it is inactive. the other time i heard it produces sort of a pressure feeling on the chest, reported to be not confortable, please report back.

 

[JoEhJoEh]

afaik it is inactive. the other time i heard it produces sort of a pressure feeling on the chest, reported to be not confortable, please report back.

There is an r- and an s-part of the Gaba - you need the r-Gaba, s-Gaba is inactive as far as i know or even inhibts some effects of the r-Gaba - correct me if I'm wrong................... -should be Amino-Acids... but I'm too high now to search i'm sorry : :D :D

J

 

[izo]

nope, gaba is achiral, it it gamma amino butyric acid which does not have an chiral center. not like the other alpha amino acids of which the proteins are composed.

 

[JoEhJoEh]

Why feel so many people better with it? Millions of placebo I can not imagine. I try it out.

 

[JoEhJoEh]

I try it and will feel it, let's see.

 

[sekio]

Why feel so many people better with it? Millions of placebo I can not imagine.

"Think of how stupid the average American is. Now realize, that means that 50% of them are dumber than that!" - George Carlin

GABA is a very poor drug, chemically speaking, it is very hydrophilic (logP of -3) and I don't think it makes it to the brain when consumed orally.

If GABA was centrally active and produced an effect like benzos/barbiturates/GHB it would have been documented by now, people would be using it widely, and would probably have been banned too.

 

[JoEhJoEh]

"Think of how stupid the average American is. Now realize, that means that 50% of them are dumber than that!" - George Carlin

GABA is a very poor drug, chemically speaking, it is very hydrophilic (logP of -3) and I don't think it makes it to the brain when consumed orally.

If GABA was centrally active and produced an effect like benzos/barbiturates/GHB it would have been documented by now, people would be using it widely, and would probably have been banned too.

but may it help against w/d of RC-Benzos? I mean now im dosing down but would it be helpful in the last days or even after such a w/d?

JJ

 

[JoEhJoEh]

Oh, and thank God im no American :D

So the Phenibut is much better when redosing and hopefully quitting then?