August 13, 2009

The Versatile GABAa Chloride Channel Receptor Complex

In today’s industrialized society we are constantly exposed to work related stresses. Consequently, anxiety and insomnia (sleeplessness) have become quite common. No wonder, we are using anxiolytics and sedatives more often; to get relief from the anxiety and insomnia respectively.

Benzodiazepines such as diazepam (Valium), chlordiazepoxide (Librium) can effectively treat anxiety and insomnia. They do so by binding with a receptor (called Benzodiazepine-GABAa-chloride ion channel complex [henceforth to be referred to simply as GABAa receptor]) in nerve cell membranes. It is known that most drugs (medicines) exert their actions by combining with receptors: macromolecular complexes present in the cell membrane or within the cytosol or the nucleus.

The GABAa receptor is a very versatile receptor complex (a hypothetical model is shown at the bottom). Its main action is to inhibit transmission along neurons in which they are present. The GABAa benzodiazepine chloride channel receptorNormally, proper functioning of the brain is ensured by a balance between the action of excitatory and inhibitory neurotransmitters [henceforth to be referred to simply as NTs]. Simply put, excitatory NTs (for example, glutamate) give a green or go signal; while inhibitory NTs (such as GABA) tell the nerve not to fire (red or stop signal). In this connection, it must be said that GABA (gamma amino butyric acid) is the most important inhibitory NT. When GABA binds with the GABA receptor ionophore complex, the receptor changes shape (conformation); and then a centrally located chloride channel, that is a part of the receptor itself, opens. Since the concentration of the chloride ions (Cl-) is much more on the outside of the cell than on the inside, Cl- now rushes in due to the increase in chloride conductance. The cell voltage goes further down and the interior of the cell becomes more negative (hyperpolarized) with respect to the outside. The cell becomes less excitable and is thus inhibited.

Apart from maintaining the much needed critical balance already mentioned, they also ensure that the brain works in a relatively noise free environment. Billions of neuronal units are always firing in the background creating a constant ‘noise’. A constant release of GABA by the brain drowns out this noise thus improving the ‘signal to noise’ ratio, making the brain’s task of finding the proverbial ‘needle in a haystack’ a lot easier.

The GABAa receptor not only binds with GABA, but it is a binding site for various other ligands. the four transmembrane GABAa receptor pentamer showing sites of action of benzodiazepines, barbiturates, GABA and othersBut before we discuss them, let us briefly analyze its structure first. The receptor has a pentameric structure which means that it consists of five subunits, and each subunit has four membrane-spanning (transmembrane) domains (see picture). And there are many of the polypeptide subunits to choose from a vast array consisting of alpha, beta, gamma, delta, pi, rho and so on. (In addition, there are six different forms of alpha, 4 beta and 3 gamma subunits). Thus, it’s no wonder that a great variety of GABAa receptors will be found, given the possible permutations!

This receptor heterogeneity explains actions of various pharmaceuticals on the receptor. One major form of GABAa receptor (found throughout the brain) consists of two alpha1, two beta2 and one gamma2 subunits. In this isoform, GABA ‘somewhere’ between alpha1 and beta2 subunits, and benzodiazepines bind with the BZ1 (also called omega1) pockets located between alpha1 and gamma2 subunits. Benzodiazepines act only when the receptor isoform has one of the following alpha subunits: 1, 2, 3 or 5 and the subunit should have a conserved histidine residue in the N-terminal domain. In ‘knock-in’ mice where histidine has been replaced by arginine in the alpha1 subunit (alpha1H101R; H for histidine and R for arginine in the 101st residue of alpha1 subunit) there was no sedation or amnesia (as evidenced by their unchanged ‘energy’ and memory to electric shocks). It may be mentioned at this moment that the so called ‘date rape’ pills exploit the amnestic properties of benzodiazepines. The drug plays tricks with the victims’ memories. However, the anxiolytic and muscle relaxant properties were retained in these mice.

These mice also do not respond to the hypnotic effects of zolpidem and zaleplon, non-benzodiazepines that act at GABAa receptors containing alpha1 subunits. But in mice with selective histidine arginine mutation in the alpha2 subunit of GABAa receptors, resistance to the antianxiety action of benzodiazepines has been seen. Based on these observations, it is thought that alpha1 subunit mediates sedative and amnestic effects, while alpha2 takes care of the anxiolytic and muscle relaxant ones. It also seems that we are poised to make better benzodiazepines in future (like one that works in anxiety but doesn’t wreak the patients’ memory).

Lastly, the versatility. The GABAa receptor also binds barbiturates (urea derivatives used as anesthetics, anticonvulsants, Marilyn Monroe supposedly died of its overdose) in addition to the benzos. Alcohol, alphaxolone (a steroid anesthetic), etomidate (a short acting anesthetic), propofol (diprivan, Michael Jackson supposedly used it), volatile anesthetics like halothane, anticonvulsants like gabapentin and vigabatrin, anthelminthics like ivermectin, and neurosteroids (metabolites of androgen and progesterone) exert part or all of their actions by acting through this receptor, thereby hyperpolarizing the neuron. Conversely, convulsants picrotoxin blocks the chloride channel directly, while bicuculline blocks the receptor’s GABAa binding site causing depolarization and convulsion. There's a lot more than this mere exegesis, and I hope to discuss about it furher later.

ResearchBlogging.orgLast modified: never
Reference: Bertram G. Katzung, Basic and Clinical Pharmacology, ninth edition
Pharmacology: Rang, Dale, Ritter, Moore

Wisden, W., & Stephens, D. (1999). Pharmacology: Towards better benzodiazepines Nature, 401 (6755), 751-752 DOI: 10.1038/44482

3 comments:

AK said...

"Since the concentration of the chloride ions (Cl-) is much more on the outside of the cell than on the inside, Cl- now rushes in due to the increase in chloride conductance."

Evidently this isn't always true. See:

Excitatory Effect of GABAergic Axo-Axonic Cells in Cortical Microcircuits
Szabadics et al.
Science 13 January 2006: 233-235
DOI: 10.1126/science.1121325
(Free with registration)

Cation–chloride co-transporters in neuronal communication, development and trauma
Trends in Neurosciences, Volume 26, Issue 4, April 2003, Pages 199-206
John A. Payne, Claudio Rivera, Juha Voipio and Kai Kaila
(has a paywall)

In addition, The synaptic organization of the brain Edited by Gordon M. Shepherd chapter 5 (p199) references on this subject:

Rhoades BK, Freeman WJ (1990) Excitatory actions of GABA in the rat olfactory bulb.
Society for Neuroscience 20th Annual Meeting. St. Louis, MO. #170.12.
(Not available O/L, AFAIK)

Chloride is preferentially accumulated in a subpopulation of dendrites and periglomerular cells of the main olfactory bulb in adult rats
L. Siklós, M. Rickmann, F. Joó, W.J. Freeman and J.R. Wolff
Neuroscience 64: 165-172
doi:10.1016/0306-4522(94)00382-F

I haven't yet read the latter references (and probably never will read the one N/A O/L). In addition, there's a mention of more references in chapter 9, which I haven't had time to follow up.

amiya said...

Thanks AK. Although I haven't gone through the references yet, things like this may happen. Firstly, the flow depends both on electrical and concentration gradient (the electrical gradient will resist chloride ion flow to the inside, as the inside is more negative and Cl- is negatively charged). Added to this is the inherent stochastic nature of quantum molecular dynamics, reminiscent of Heisenberg Uncertainty principle, which might be expected at such nanoscale interactions.
I'll check your links when I have time. Thanks for your feedback.

AK said...

You may also be interested in:

GABAergic Depolarization of the Axon Initial Segment in Cortical Principal Neurons Is Caused by the Na–K–2Cl Cotransporter NKCC1
by Stanislav Khirug, Junko Yamada, Ramil Afzalov, Juha Voipio, Leonard Khiroug, and Kai Kaila
The Journal of Neuroscience, April 30, 2008, 28(18):4635-4639; doi:10.1523/JNEUROSCI.0908-08.2008

and

Complex Events Initiated by Individual Spikes in the Human Cerebral Cortex
by Gábor Molnár, Szabolcs Oláh, Gergely Komlósi, Miklós Füle, János Szabadics, Csaba Varga, Pál Barzó, and Gábor Tamás
PLoS Biol 6(9): e222. doi:10.1371/journal.pbio.0060222

In case you hadn't guessed, I'm researching a post on excitatory effects in GABA and glycine synapses, especially as regards the axon initial segment. Should be up sometime next week.