Cross references:    GABA       GABA Metabotropic Receptor    
GABAA Receptor Evolution     Transmembrane Transport in General       
Ion Channels     Ligand-gated Ion Channel  

This page is about the GABA-gated ionotropic membrane channel.  For information on GABA as a ligand for GPCRs, please see:   GABA Metabotropic Receptor . 

GABAA receptor (Wiki) 
    "The GABAA receptor (GABAAR) is an ionotropic receptor and ligand-gated ion channel. Its endogenous ligand is γ-aminobutyric acid (GABA), the major inhibitory  neurotransmitter in the central nervous system. Upon activation, the GABAA receptor selectively conducts Cl- through its pore, resulting in hyperpolarization of the neuron. This causes an inhibitory effect on neurotransmission by diminishing the chance of a successful action potential occurring."

    "Schematic structure of the GABAA receptor. Left: GABAA monomeric subunit imbedded in a lipid bilayer (yellow lines connected to blue spheres). The four transmembrane α-helices (1–4) are depicted as cylinders. The disulfide bond in the N-terminal extracellular domain which is characteristic of the family of cys-loop receptors (which includes the GABAA receptor) is depicted as a yellow line. Right: Five subunits symmetrically arranged about the central chloride anion conduction pore. The extracellular loops are not depicted for the sake of clarity."  

    "Mild Inhibition of neuronal firing by drugs acting at the GABAA receptor causes a reduction of anxiety in the patient (an anxiolytic effect) while more pronounced inhibition induces sleep (sedation) and in extreme cases of overdose, may result in death." 

    "The receptor is a pentameric transmembrane receptor that consists of five subunits arranged around a central pore. Each subunit comprises four transmembrane domains with both the N- and C-terminus located extracellularly. The receptor sits in the membrane of its neuron, usually localized at a synapse, postsynaptically. However, some isoforms may be found extrasynaptically.[13] The ligand GABA is the endogenous compound that causes this receptor to open; once bound to GABA, the protein receptor changes conformation within the membrane, opening the pore in order to allow chloride anions (Cl) to pass down an electrochemical gradient. Because the reversal potential for chloride in most neurons is close to or more negative than the resting membrane potential, activation of GABAA receptors tends to stabilize or hyperpolarise the resting potential, and can make it more difficult for excitatory neurotransmitters to depolarize the neuron and generate an action potential. The net effect is typically inhibitory, reducing the activity of the neuron. The GABAA channel opens quickly and thus contributes to the early part of the inhibitory post-synaptic potential (IPSP).[14][15] "

    "Schematic diagram of a GABAA receptor protein ((α1)2(β2)2(γ2)) which illustrates the five combined subunits that form the protein, the chloride (Cl-) ion channel pore, the two GABA active binding sites at the α1 and β2 interfaces [1]"  

    "Subunits GABAA receptors are members of the large "Cys-loop" super-family of evolutionarily related and structurally similar ligand-gated ion channels that also includes nicotinic acetylcholine receptors, glycine receptors, and the 5HT3 receptor. There are numerous subunit isoforms for the GABAA receptor, which determine the receptor's agonist affinity, chance of opening, conductance, and other properties.[16] In humans, the units are as follows:[17]
There are three ρ units (GABRR1, GABRR2, GABRR3), however these do not coassemble with the classical GABAA units listed above,[18] but rather homooligomerize to form GABAA-ρ receptors (formerly classified as GABAC receptors but now this nomenclature has been deprecated[19] ). Five subunits can combine in different ways to form GABAA channels. The minimal requirement to produce a GABA-gated ion channel is the inclusion of both α and β subunits,[20] but the most common type in the brain is a pentamer comprising two α's, two β's, and a γ (α2β2γ).[17] The receptor binds two GABA molecules,[21] at the interface between an α and a β subunit.[17]"  
    "As GABAA receptors are responsible for most of the physiological activities of GABA in the central nervous system, subunits are expressed in many parts of the brain. Subunit composition can vary widely between regions and subtypes may be associated with specific functions. Interestingly, GABAA receptors can also be found in other tissues, including leydig cells, placenta, immune cells, liver, bone growth plates and several other endocrine tissues. Subunit expression varies between 'normal' tissue and malignancies and GABAA receptors can influence cell proliferation.[32]"  


gamma-Aminobutyric acid - Wikipedia, the free encyclopedia
    "GABAA receptors are chloride channels; that is, when activated by GABA, they allow the flow of chloride ions across the membrane of the cell. Whether this chloride flow is: 
     - excitatory/depolarizing (makes the voltage across the cell's membrane less negative),
     - shunting (has no effect on the cell's membrane) or
     - inhibitory/ hyperpolarizing (makes the cell's membrane more negative)
depends on the direction of the flow of chloride.

When net chloride flows out of the cell, GABA is excitatory or depolarizing; when the net chloride flows into the cell, GABA is inhibitory or hyperpolarizing. When the net flow of chloride is close to zero, the action of GABA is shunting. Shunting inhibition has no direct effect on the membrane potential of the cell, however it minimises the effect of any coincident synaptic input  ... That is to say, GABA's role changes from excitatory to inhibitory as the brain develops into adulthood.[3]

Cellular localization and differential distribution of GABAA receptor subunit proteins and messenger RNAs within hypothalamic magnocellular neurons.
Fenelon VS, Sieghart W, Herbison AE.
Neuroscience. 1995 Feb;64(4):1129-43.
PMID: 7753380   Related citations  

from the abstract
    "The inhibitory neurotransmitter GABA plays an important role in regulating the activity of magnocellular oxytocin and vasopressin neurons located in the supraoptic and paraventricular nuclei through occupancy of GABAA receptors. However, the GABAA receptor is a hetero-oligomeric protein comprised of different subunits and the subunit types expressed in a given receptor complex appear critical for its sensitivity to GABA, benzodiazepines and/or steroids. Thus, in order to understand fully the GABAergic control of oxytocin and vasopressin secretion, definition of the GABAA receptors synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei is required.  
    In the supraoptic nucleus, antibodies directed against the alpha 1, alpha 2 and beta 2/3 subunits of the GABAA receptor revealed similar strong antigen distribution on all magnocellular neurons. Using sequential double-immunoperoxidase staining, immunoreactivity for all three subunits was observed on both oxytocin and vasopressin neurons of the supraoptic nucleus.  
    In contrast, only alpha 2 subunit immunoreactivity was detected on the cell bodies of oxytocin and vasopressin neurons in the paraventricular nucleus.  
    No sex differences were detected.  
    In situ hybridization experiments using 35S-labelled oligonucleotides showed that all supraoptic neurons expressed alpha 1, alpha 2 and beta 2 subunit messenger RNA transcripts while magnocellular neurons in the paraventricular nucleus were only enriched in alpha 2 subunit messenger RNA. Quantitative analysis showed that the expression of alpha 1 and beta 2 subunit messenger RNAs in the paraventricular nucleus was half that observed in the supraoptic nucleus while expression of beta 3 subunit messenger RNA was very low in both nuclei.  
    These results show that all oxytocin and vasopressin neurons located in the supraoptic nucleus synthesize and express alpha 1, alpha 2 and beta 2 subunits of the GABAA receptor while those in the paraventricular nucleus are only immunoreactive for the alpha 2 subunit.  
    These observations suggest, therefore, that at least two pharmacologically distinct GABAA receptor isoforms exist on supraoptic neurons and that these are different to those expressed by paraventricular magnocellular cells. Thus, in addition to providing a definition of the subunits likely to form specific GABAA receptor isoforms on magnocellular neurons, this study gives direct evidence for GABAA receptor heterogeneity between supraoptic and paraventricular neurons, but not between oxytocin and vasopressin cells."     


Alpha 4 Beta Delta Receptors:   

Searching Google for "extrasynaptic α4βδ-­‐GABAA receptors" yielded 3,340 references.

Searching PubMed for "extrasynaptic α4βδ-­‐GABAA receptors" yielded 7 references.

Short-term steroid treatment increases delta GABAA receptor subunit expression in rat CA1 hippocampus: pharmacological and behavioral effects.
Shen H, Gong QH, Yuan M, Smith SS.
Neuropharmacology. 2005 Oct;49(5):573-86.
PMID:  15950994   Free PMC Article     Related citations

from the abstract
    "In this study, 48 h administration of 3alpha-OH-5beta-pregnan-20-one (3alpha,5beta-THP) or 17beta-estradiol (E2)+progesterone (P) to female rats increased expression of the delta subunit of the GABA(A) receptor (GABAR) in CA1 hippocampus. Coexpression of alpha4 and delta subunits was suggested by an increased response of isolated pyramidal cells to the GABA agonist 4,5,6,7- tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), following 48 h steroid treatment, and nearly complete blockade by 300 microM lanthanum (La3+).  
    Because alpha4betadelta GABAR are extrasynaptic, we also recorded pharmacologically isolated GABAergic holding current from CA1 hippocampal pyramidal cells in the slice. The La3+-sensitive THIP current, representative of current gated by alpha4betadelta GABAR, was measurable only following 48 h steroid treatment.  
    In contrast, the bicuculline-sensitive current was not altered by steroid treatment, assessed with or without 200 nM gabazine to block synaptic current. However, 48 h steroid treatment resulted in a tonic current insensitive to the benzodiazepine agonists lorazepam (10 microM) and zolpidem (100 nM).  
    These results suggest that 48 h steroid treatment increases expression of alpha4betadelta GABAR which replace the ambient receptor population. Increased anxiolytic effects of THIP were also observed following 48 h steroid treatment. The findings from the present study may be relevant for alterations in mood and benzodiazepine sensitivity reported across the menstrual cycle."  

My comment
    I'm amazed.  Although this is the oldest reference that PubMed found which mentions alpha4betadelta GABAR, the abstract speaks of it as though it expects the reader to be familiar with it.  This is not usually the case.  Usually the earlier references on a topic begin with a description of its first discovery, then there's a discussion of its characteristics, etc.  I'm probably going to need to look at the   Free PMC Article  and the   Related citations , but first I'm going to look at the abstracts of the other, later references that PubMed found.       

Extrasynaptic alphabeta subunit GABAA receptors on rat hippocampal pyramidal neurons.
Mortensen M, Smart TG.
J Physiol. 2006 Dec 15;577(Pt 3):841-56. Epub 2006 Oct 5.
PMID:  17023503   Free PMC Article     Related citations

from the abstract:   

    "Extrasynaptic GABA(A) receptors that are tonically activated by ambient GABA are important for controlling neuronal excitability. In hippocampal pyramidal neurons, the subunit composition of these extrasynaptic receptors may include alpha5betagamma and/or alpha4betadelta subunits. 
    Our present studies reveal that a component of the tonic current in the hippocampus is highly sensitive to inhibition by Zn(2+). This component is probably not mediated by either alpha5betagamma or alpha4betadelta receptors, but might be explained by the presence of alphabeta isoforms. Using patch-clamp recording from pyramidal neurons, a small tonic current measured in the absence of exogenous GABA exhibited both high and low sensitivity to Zn(2+) inhibition (IC(50) values, 1.89 and 223 microm, respectively). Using low nanomolar and micromolar GABA concentrations to replicate tonic currents, we identified two components that are mediated by benzodiazepine-sensitive and -insensitive receptors. The latter indicated that extrasynaptic GABA(A) receptors exist that are devoid of gamma2 subunits. To distinguish whether the benzodiazepine-insensitive receptors were alphabeta or alphabetadelta isoforms, we used single-channel recording. Expressing recombinant alpha1beta3gamma2, alpha5beta3gamma2, alpha4beta3delta and alpha1beta3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (approximately 25-28 pS) for gamma2 or delta subunit-containing receptors whereas alphabeta receptors were characterized by a lower main conductance state (approximately 11 pS). Recording from pyramidal cell somata revealed a similar range of channel conductances, indicative of a mixture of GABA(A) receptors in the extrasynaptic membrane. The lowest conductance state (approximately 11 pS) was the most sensitive to Zn(2+) inhibition in accord with the presence of alphabeta receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABA(A) receptors on hippocampal pyramidal neurons." 
 My comment:   
    Not only does this reference speak very casually of alpha4betadelta receptors.  It also speaks very casually of alpha5betagamma, alpha1beta3gamma2, alpha5beta3gamma2, alpha4beta3delta and alpha1beta3 receptors.  There's clearly a whole family of GABAA receptors.     

Presynaptic GABAA receptors facilitate GABAergic transmission to dopaminergic neurons in the ventral tegmental area of young rats.
Xiao C, Zhou C, Li K, Ye JH.
J Physiol. 2007 May 1;580(Pt.3):731-43. Epub 2007 Feb 15.
PMID  :17303643   Free PMC Article     Related citations  

from the abstract:   
    "Gamma-aminobutyric acid A receptor (GABA(A)R)-mediated postsynaptic currents (IPSCs) were recorded from dopaminergic neurons of the ventral tegmental area of young rats in acute brain slices and from mechanically dissociated neurons. Low concentrations (0.1-0.3 microm) of muscimol, a selective GABA(A)R agonist, increased the amplitude, and reduced the paired pulse ratio of evoked IPSCs. Moreover, muscimol increased the frequency but not the amplitude of spontaneous IPSCs (sIPSCs). These data point to a presynaptic locus of muscimol action. It is interesting that 1 microm muscimol caused an inhibition of sIPSCs, which was reversed to potentiation by the GABA(B) receptor antagonist CGP52432. Isoguvacine, a selective GABA(A)R agonist that belongs to a different class, mimicked the effects of muscimol on sIPSCs: it increased them at low (<or= 0.5 microm), and decreased them at a higher concentration (1 microm). Hence, the activation of presynaptic GABA(A)Rs facilitates GABA release, which is limited by presynaptic GABA(B)Rs. Furthermore, facilitation of sIPSCs by muscimol was eliminated in a medium containing tetrodotoxin or cadmium.  
    It is noteworthy that sIPSC frequency was greatly increased by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol(gaboxadol, or THIP), an agonist with preferential effects on extrasynaptic GABA(A)Rs containing alpha4betadelta subunits, or by guvacine, a GABA transport blocker, which increases ambient GABA levels. In addition, sIPSC frequency was attenuated by furosemide, a selective antagonist of alpha6 subunits. Thus, the presynaptic GABA(A)Rs may be situated at extrasynaptic sites and may contain alpha4/6betadelta subunits. Given the marked sensitivity of extrasynaptic GABA(A)Rs to ambient GABA, alcohols and anaesthetics, these receptors may present a critical site for regulating synaptic function in the developing brain in both physiological and pathological situations."   

My comment:   
    This reference mentioned alpha4betadelta subunits and alpha4/6betadelta subunits only very briefly.  I don't think it will be useful.      


Mechanisms of reversible GABAA receptor plasticity after ethanol intoxication.
Liang J, Suryanarayanan A, Abriam A, Snyder B, Olsen RW, Spigelman I.
J Neurosci. 2007 Nov 7;27(45):12367-77.
PMID:  17989301   Free Article     Related citations  

from the abstract:   
    "The time-dependent effects of ethanol (EtOH) intoxication on GABA(A) receptor (GABA(A)R) composition and function were studied in rats. A cross-linking assay and Western blot analysis of microdissected CA1 area of hippocampal slices obtained 1 h after EtOH intoxication (5 g/kg, gavage), revealed decreases in the cell-surface fraction of alpha4 and delta, but not alpha1, alpha5, or gamma2 GABA(A)R subunits, without changes in their total content. This was accompanied (in CA1 neuron recordings) by decreased magnitude of the picrotoxin-sensitive tonic current (I(tonic)), but not miniature IPSCs (mIPSCs), and by reduced enhancement of I(tonic) by EtOH, but not by diazepam. By 48 h after EtOH dosing, cell-surface alpha4 (80%) and gamma2 (82%) subunit content increased, and cell-surface alpha1 (-50%) and delta (-79%) and overall content were decreased. This was paralleled by faster decay of mIPSCs, decreased diazepam enhancement of both mIPSCs and I(tonic), and paradoxically increased mIPSC responsiveness to EtOH (10-100 mm). Sensitivity to isoflurane- or diazepam-induced loss of righting reflex was decreased at 12 and 24 h after EtOH intoxication, respectively, suggesting functional GABA(A)R tolerance. The plastic GABA(A)R changes were gradually and fully reversible by 2 weeks after single EtOH dosing, but unexplainably persisted long after withdrawal from chronic intermittent ethanol treatment, which leads to signs of alcohol dependence.  
    Our data suggest that early tolerance to EtOH may result from excessive activation and subsequent internalization of alpha4betadelta extrasynaptic GABA(A)Rs. This leads to transcriptionally regulated increases in alpha4 and gamma2 and decreases in alpha1 subunits, with preferential insertion of the newly formed alpha4betagamma2 GABA(A)Rs at synapses." 

My comment:   
    This reference mentions several GABA receptors.  I probably need to look at the subject of ionotropic GABA receptors in general.      

Modulation of spontaneous and GABA-evoked tonic alpha4beta3delta and alpha4beta3gamma2L GABAA receptor currents by protein kinase A.
Tang X, Hernandez CC, Macdonald RL.
J Neurophysiol. 2010 Feb;103(2):1007-19. doi: 10.1152/jn.00801.2009. Epub 2009 Nov 25.
PMID:  19939957   Free PMC Article     Related citations  

from the abstract
    "Protein kinase A (PKA) has been reported to regulate synaptic alphabetagamma gamma-aminobutyric acid type A (GABA(A)) receptor currents, but whether PKA regulates GABA(A) receptor peri- and extrasynaptic tonic currents is unknown.  
    GABA(A) receptors containing alpha4 subunits are important in mediating tonic inhibition and exist as both alpha4betadelta and alpha4betagamma receptors in the brain. To mimic GABA-independent and GABA-dependent tonic currents, we transfected HEK 293T cells with alpha4beta3delta or alpha4beta3gamma2L subunits and recorded spontaneous currents in the absence of applied GABA and steady-state currents in the presence of 1 muM GABA. Both alpha4beta3delta and alpha4beta3gamma2L receptors displayed spontaneous currents, but PKA activation increased spontaneous alpha4beta3delta currents substantially more than spontaneous alpha4beta3gamma2L currents. The increase in spontaneous alpha4beta3delta currents was due to an increase in single-channel open frequency. In contrast, PKA activation did not alter steady-state tonic currents recorded in the presence of 1 muM GABA. We concluded that PKA had a GABA concentration-dependent effect on alpha4beta3delta and alpha4beta3gamma2L currents. In the absence of GABA, spontaneous alpha4beta3delta and, to a lesser extent, alpha4beta3gamma2L currents could provide a basal, tonic current that could be regulated by PKA. With increasing concentrations of extracellular GABA, however, tonic alpha4beta3delta and alpha4beta3gamma2L currents would become more GABA dependent and less PKA sensitive. Thus in brain regions with fluctuating extracellular GABA levels, the dynamic range of GABA-activated tonic currents would be set by PKA and the increase in tonic current produced by increasing GABA would be reduced by PKA-mediated phosphorylation. When ambient GABA reaches micromolar concentrations, PKA would have no effect on steady-state tonic currents.   

My comment:   
    It's taken a long time, but the expression "alpha4 subunits", above, finally made me realize that the expression "alpha4betagamma" might not mean four alphas, one beta and one gamma. It may mean a receptor which includes an alpha of type 4, what the Wikipedia article, above, would call a GABRA4 plus one of the three types of beta and one of the three types of gamma.  However, if that's the case, then the designation "alpha4betagamma" leaves the number of each subunit open.  The diagram in the Wikipedia article, above, shows two alpha, two beta and one gamma, for a total of five.  How standard is this?          

Ethanol promotes clathrin adaptor-mediated endocytosis via the intracellular domain of δ-containing GABAA receptors.
Gonzalez C, Moss SJ, Olsen RW.
J Neurosci. 2012 Dec 5;32(49):17874-81. doi: 10.1523/JNEUROSCI.2535-12.2012.
PMID:  23223306   Free PMC Article     Related citations  

from the abstract:       
    "Pharmacological and genetic evidence reveals that GABA(A) receptor (GABA(A)-R) expression and localization are modulated in response to acute and chronic ethanol (EtOH) exposure.  To determine molecular mechanisms of GABA(A)-R plasticity in response to in vivo acute EtOH, we measured early time changes in GABA(A)-R subunit localization.  
    Single doses of EtOH (3 g/kg via i.p. injection in rats) produced decreases in surface levels of GABA(A)-R α4 and δ subunits at 5-15 min post-EtOH in hippocampus CA1 and dentate gyrus, verifying our earlier report (Liang et al., 2007).  
    Here we also examined the β3 subunit and its phosphorylation state during internalization. β3 also was internalized during 5-15 min after EtOH exposure, while phosphorylation of β3 was increased, then decreased at later times, ruling out β3 dephosphorylation-dependent endocytosis.  
    As early as 5 min post-EtOH, there is an initial increase in association between the δ subunits with clathrin adaptor proteins AP2-μ2 revealed by coimmunoprecipitation, followed by a decrease in association 15 min post-EtOH.  
    In vitro studies using glutathione S-transferase fused to the δ subunit intracellular domain (ICD) show that two regions, one containing a classical YxxΦ motif and the other an atypical R/K-rich motif, directly and differentially bind to AP2-μ2, with the former YRSV exhibiting higher affinity. Mutating both regions in the δ-ICD abolishes μ2 binding, providing a possible mechanism that can explain the rapid downregulation of extrasynaptic α4βδ-GABA(A)-R following in vivo EtOH administration, in which the δ-ICD increases in affinity for clathrin AP2-μ2 leading to endocytosis."   

My comment:   
    Although the GABA(A)-R plasticity reported here was in the hippocampus CA1 and dentate gyrus, the same phenomenon may occur in the nucleus accumbens.      

Tonic inhibition of accumbal spiny neurons by extrasynaptic α4βδ GABAA receptors modulates the actions of psychostimulants.
Maguire EP, Macpherson T, Swinny JD, Dixon CI, Herd MB, Belelli D, Stephens DN, King SL, Lambert JJ.
J Neurosci. 2014 Jan 15;34(3):823-38. doi: 10.1523/JNEUROSCI.3232-13.2014.
PMID:  24431441    Related citations  

My overall comments
1.  I found this abstract quite difficult.  It makes many specific claims which are easily confounded.  In order to make it more understandable, I've separated the various specific claims and offered hopefully explanatory comments on them. 
2.  I know almost nothing about genetics, so I may be mistaken when I interpret the expression "α4(-/-) mice" as describing a mouse which has inherited no α4 genes from either parent.  I tried searching both Google and PubMed but didn't find anything useful.  I'll continue to remain open to alternative meanings for this expression.   
from the abstract:     

    "Within the nucleus accumbens (NAc), synaptic GABAA receptors (GABAARs) mediate phasic inhibition of medium spiny neurons (MSNs) and influence behavioral responses to cocaine."  
My first two specific comments:   
1. Since the MSNs are themselves GABAergic, their inhibition will have a stimulatory effect on the recipients of their output. 
2.  GABA is not the only neurotransmitter which mediates phasic inhibition of MSNs.  Dopamine, norepinepherine, and perhaps other neurotransmitters play a similar role. 
    "We demonstrate that both dopamine D1- and D2-receptor-expressing MSNs (D-MSNs) additionally harbor extrasynaptic GABAARs incorporating α4, β, and δ subunits that mediate tonic inhibition, thereby influencing neuronal excitability.  
My third specific comment
3.  The important distinction here is between "phasic inhibition", which is time-limited by the destruction of GABA by monoamine oxidase (MAO) within the synaptic space, and "tonic inhibition" which is much more long lasting due to the absence of MAO in the extrasynaptic space.     

    Both the selective δ-GABAAR agonist THIP and DS2, a selective positive allosteric modulator, greatly increased the tonic current of all MSNs from wild-type (WT), but not from δ(-/-) or α4(-/-) mice.  
My fourth specific comment
4.  It's understandable that a delta-GABAAR agonist would not increase the tonic current of a delta(-/-) mouse, but why would it affect a alpha4(-/-) mouse? 

    Coupling dopamine and tonic inhibition, the acute activation of D1 receptors (by a selective agonist or indirectly by amphetamine) greatly enhanced tonic inhibition in D1-MSNs but not D2-MSNs.  
My fifth specific comment
5.  I think that this statement is at least misleading and probably just plain wrong.  The Wikipedia article referenced in Dopamine Receptors says that the D1 receptor "activates adenylyl cyclase, increasing the intracellular concentration of the second messenger cyclic adenosine monophosphate (cAMP)".  Although it's not stated specifically, I'm pretty sure that this means that "acute activation of D1 receptors" excites, rather than inhibits, the cell bearing the D1 receptor.  As noted in Comment 1, above, the MSNs are GABAergic and will, therefore, inhibit the neurons to which they project, but "acute activation of D1 receptors" will not "greatly enhanced tonic inhibition in D1-MSNs".  It will greatly enhance tonic excitation in D1-MSNs which will then exert a tonic inhibitory influence on the neurons to which they project.        
     In contrast, prolonged D2 receptor activation modestly reduced the tonic conductance of D2-MSNs.  
My sixth specific comment
6.  This is totally consistent with the fact that the D2 receptor is inhibitory.     

    Behaviorally, WT and constitutive α4(-/-) mice did not differ in their expression of cocaine-conditioned place preference (CPP).  
My seventh specific comment:  
7.  This implies that alpha4 subunits do not play a role in CPP.  However, they make up 2/3 of the alpha4 beta delta receptor.  Does this imply that there are receptors composed of only beta and delta subunits?     

    Importantly, however, mice with the α4 deletion specific to D1-expressing neurons (α4(D1-/-)) showed increased CPP.  
My eighth specific comment
8.  The only way this doesn't contradict my Comment 7, above, is if alpha4D1 and alpha4D2 balance one another and knocking out D1 unmasks the influence of D2. 
    Furthermore, THIP administered systemically or directly into the NAc of WT, but not α4(-/-) or α4(D1-/-) mice, blocked cocaine enhancement of CPP.  
My ninth specific comment
9.  I don't understand how this relates to the rest of the article.  THIP is a delta-GABAAR agonist.  What is it's relation to alpha4 (-/-) and alpha4(D1-/-) receptors?       

    In comparison, α4(D2-/-) mice exhibited normal CPP, but no cocaine enhancement.  
My tenth specific comment
10.  How can you have CPP without cocaine enhancement?  What, besides cocaine, would be causing the CPP?     

    In conclusion, dopamine modulation of GABAergic tonic inhibition of D1- and D2-MSNs provides an intrinsic mechanism to differentially affect their excitability in response to psychostimulants and thereby influence their ability to potentiate conditioned reward. Therefore, α4βδ GABAARs may represent a viable target for the development of novel therapeutics to better understand and influence addictive behaviors.  
My eleventh specific comment
11.  This article didn't really say much about "dopamine modulation of GABAergic tonic inhibition of D1- and D2-MSNs".   The "modulation of GABAergic tonic inhibition of D1- and D2-MSNs" was by extrasynaptic alpha4 beta delta GABA receptors, not dopamine.      

Segregation of different GABAA receptors to synaptic and extrasynaptic membranes of cerebellar granule cells.
    "Two types of GABAA receptor-mediated inhibition (phasic and tonic) have been described in cerebellar granule cells, although these cells receive GABAergic input only from a single cell type, the Golgi cell. In adult rats, granule cells express six GABAA receptor subunits abundantly (alpha1, alpha6, beta2, beta3, gamma2, and delta), which are coassembled into at least four to six distinct GABAA receptor subtypes. We tested whether a differential distribution of GABAA receptors on the surface of granule cells could play a role in the different forms of inhibition, assuming that phasic inhibition originates from the activation of synaptic receptors, whereas tonic inhibition is provided mainly by extrasynaptic receptors. The alpha1, alpha6, beta2/3, and gamma2 subunits have been found by immunogold localizations to be concentrated in GABAergic Golgi synapses and also are present in the extrasynaptic membrane at a lower concentration. In contrast, immunoparticles for the delta subunit could not be detected in synaptic junctions, although they were abundantly present in the extrasynaptic dendritic and somatic membranes. Gold particles for the alpha6, gamma2, and beta2/3, but not the alpha1 and delta, subunits also were concentrated in some glutamatergic mossy fiber synapses, where their colocalization with AMPA-type glutamate receptors was demonstrated. The exclusive extrasynaptic presence of the delta subunit-containing receptors, together with their kinetic properties, suggests that tonic inhibition could be mediated mainly by extrasynaptic alpha6beta2/3delta receptors, whereas phasic inhibition is attributable to the activation of synaptic alpha1beta2/3gamma2, alpha6beta2/3gamma2, and alpha1alpha6beta2/3gamma2 receptors."
    142 Related citations:     193 Cited by's:    
    Free full text

Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors.   
    "The proper functioning of the adult mammalian brain relies on the orchestrated regulation of neural activity by a diverse population of GABA (gamma-aminobutyric acid)-releasing neurons. Until recently, our appreciation of GABA-mediated inhibition focused predominantly on the GABA(A) (GABA type A) receptors located at synaptic contacts, which are activated in a transient or 'phasic' manner by GABA that is released from synaptic vesicles. However, there is growing evidence that low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a 'tonic' conductance. In this review, we consider the distinct roles of synaptic and extrasynaptic GABA receptor subtypes in the control of neuronal excitability."   
    206 Related citations:  
    454 Cited by's:      

Regulation of excitability by extrasynaptic GABA(A) receptors.   
    "Not only are GABA(A) receptors activated transiently by GABA released at synapses, but high affinity, extrasynaptic GABA(A) receptors are also activated by ambient, extracellular GABA as a more persistent form of signalling (often termed tonic inhibition). Over the last decade tonic GABA(A) receptor-mediated inhibition and the properties of GABA(A) receptors mediating this signalling have received increasing attention. Tonic inhibition is present throughout the central nervous system, but is expressed in a cell-type specific manner (e.g. in interneurons more so than in pyramidal cells in the hippocampus, and in thalamocortical neurons more so than in reticular thalamic neurons in the thalamus). As a consequence, tonic inhibition can have a complex effect on network activity. Tonic inhibition is not fixed but can be modulated by endogenous and exogenous modulators, such as neurosteroids, and by developmental, physiological and pathological regulation of GABA uptake and GABA(A) receptor expression. There is also growing evidence that tonic currents play an important role in epilepsy, sleep (also actions of anaesthetics and sedatives), memory and cognition. Therefore, drugs specifically aimed at targeting the extrasynaptic receptors involved in tonic inhibition could be a novel approach to regulating both physiological and pathological processes."  
    130 Related citations:   
    27 Cited by's:  
Extrasynaptic GABAA receptors: form, pharmacology, and function.  
    "GABA is the principal inhibitory neurotransmitter in the CNS and acts via GABA(A) and GABA(B) receptors. Recently, a novel form of GABA(A) receptor-mediated inhibition, termed "tonic" inhibition, has been described. Whereas synaptic GABA(A) receptors underlie classical "phasic" GABA(A) receptor-mediated inhibition (inhibitory postsynaptic currents), tonic GABA(A) receptor-mediated inhibition results from the activation of extrasynaptic receptors by low concentrations of ambient GABA. Extrasynaptic GABA(A) receptors are composed of receptor subunits that convey biophysical properties ideally suited to the generation of persistent inhibition and are pharmacologically and functionally distinct from their synaptic counterparts. This mini-symposium review highlights ongoing work examining the properties of recombinant and native extrasynaptic GABA(A) receptors and their preferential targeting by endogenous and clinically relevant agents. In addition, it emphasizes the important role of extrasynaptic GABA(A) receptors in GABAergic inhibition throughout the CNS and identifies them as a major player in both physiological and pathophysiological processes."     
    118 Related citations:  
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2011    117<519       
Subregion-Specific Modulation of Excitatory Input and Dopaminergic Output in the Striatum by Tonically Activated Glycine and GABA(A) Receptors.  
    "The flow of cortical information through the basal ganglia is a complex spatiotemporal pattern of increased and decreased firing. The striatum is the biggest input nucleus to the basal ganglia and the aim of this study was to assess the role of inhibitory GABA(A) and glycine receptors in regulating synaptic activity in the dorsolateral striatum (DLS) and ventral striatum (nucleus accumbens, nAc). Local field potential recordings from coronal brain slices of juvenile and adult Wistar rats showed that GABA(A) receptors and strychnine-sensitive glycine receptors are tonically activated and inhibit excitatory input to the DLS and to the nAc. Strychnine-induced disinhibition of glutamatergic transmission was insensitive to the muscarinic receptor inhibitor scopolamine (10 μM), inhibited by the nicotinic acetylcholine receptor antagonist mecamylamine (10 μM) and blocked by GABA(A) receptor inhibitors, suggesting that tonically activated glycine receptors depress excitatory input to the striatum through modulation of cholinergic and GABAergic neurotransmission. As an end-product example of striatal GABAergic output in vivo we measured dopamine release in the DLS and nAc by microdialysis in the awake and freely moving rat. Reversed dialysis of bicuculline (50 μM in perfusate) only increased extrasynaptic dopamine levels in the nAc, while strychnine administered locally (200 μM in perfusate) decreased dopamine output by 60% in both the DLS and nAc. Our data suggest that GABA(A) and glycine receptors are tonically activated and modulate striatal transmission in a partially subregion-specific manner."  
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Extrasynaptic GABA(A) receptors: their function in the CNS and implications for disease.   
    "Over the past two decades, research has identified extrasynaptic GABA(A) receptor populations that enable neurons to sense the low ambient GABA concentrations present in the extracellular space in order to generate a form of tonic inhibition not previously considered in studies of neuronal excitability. The importance of this tonic inhibition in regulating states of consciousness is highlighted by the fact that extrasynaptic GABA(A) receptors (GABA(A)Rs) are believed to be key targets for anesthetics, sleep-promoting drugs, neurosteroids, and alcohol. The neurosteroid sensitivity of these extrasynaptic GABA(A)Rs may explain their importance in stress-, ovarian cycle-, and pregnancy-related mood disorders. Moreover, disruptions in network dynamics associated with schizophrenia, epilepsy, and Parkinson's disease may well involve alterations in the tonic GABA(A)R-mediated conductance. Extrasynaptic GABA(A)Rs may therefore present a therapeutic target for treatment of these diseases, with the potential to enhance cognition and aid poststroke functional recovery."  
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Extrasynaptic GABA(A) receptors in the brainstem and spinal cord: structure and function.
    "γ-aminobutyric acid (GABA) plays many of its key roles in embryonic development and functioning of the central nervous system (CNS) by acting on ligand gated chloride-permeable channels known as GABAA receptors (GABAAR). Classically, GABAARmediated synaptic communication is tailored to allow rapid and precise transmission of information to synchronize the activity of large populations of cells to generate and maintain neuronal networks oscillations. An alternative type of inhibition mediated by GABAA receptors, initially described about 25 years ago, is characterized by a tonic activation of receptors that react to ambient extracellular GABA. The receptors that mediate this action are wide-spread throughout the nerve cells but are located distant from the sites of GABA release, and therefore they have been called extrasynaptic GABAA receptors. The molecular nature of the extrasynaptic GABAA receptors and the tonic inhibitory current they generate have been characterized in many brain structures, and due to its relevance in controlling neuron excitability they have become attractive pharmacological targets for a variety of neurological disorders such as schizophrenia, epilepsy and Parkinson disease. In the spinal cord, early studies have implicated these receptors in anesthesia, chronic pain, motor control, and locomotion. This review highlights past and present developments in the field of extrasynaptic GABAA receptors and emphasizes their subunit containing distribution and physiological role in the spinal cord."     
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GABAB Receptors Regulate Extrasynaptic GABAA Receptors.  
    "These results demonstrate for the first time a postsynaptic crosstalk between GABA(B) and GABA(A) receptors."      See:   GABA Metabotropic Receptor    for full Abstract, Related citations, Cited by's
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Homeostatic competition between phasic and tonic inhibition.  
    "The GABAA receptors are the major inhibitory receptors in the brain and are localized at both synaptic and extrasynaptic membranes. Synaptic GABAA receptors mediate phasic inhibition, whereas extrasynaptic GABAA receptors mediate tonic inhibition. Both phasic and tonic inhibitions regulate neuronal activity, but whether they regulate each other is not very clear. Here, we investigated the functional interaction between synaptic and extrasynaptic GABAA receptors through various molecular manipulations. Overexpression of extrasynaptic α6β3δ-GABAA receptors in mouse hippocampal pyramidal neurons significantly increased tonic currents. Surprisingly, the increase of tonic inhibition was accompanied by a dramatic reduction of the phasic inhibition, suggesting a possible homeostatic regulation of the total inhibition. Overexpressing the α6 subunit alone induced an up-regulation of δ subunit expression and suppressed phasic inhibition similar to overexpressing the α6β3δ subunits. Interestingly, blocking all GABAA receptors after overexpressing α6β3δ receptors could not restore the synaptic GABAergic transmission, suggesting that receptor activation is not required for the homeostatic interplay. Furthermore, insertion of a gephyrin-binding-site (GBS) into the α6 and δ subunits recruited α6(GBS)β3δ(GBS) receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission. Thus, it is not the positional effect of extrasynaptic α6β3δ receptors that causes the down-regulation of phasic inhibition. Overexpressing α5β3γ2 subunits similarly reduced synaptic GABAergic transmission. We propose a working model that both synaptic and extrasynaptic GABAA receptors may compete for limited receptor slots on the plasma membrane to maintain a homeostatic range of the total inhibition."   
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2013    117<1207 
Distinct roles of synaptic and extrasynaptic GABAAreceptors in striatal inhibition dynamics.  
    "Striatonigral and striatopallidal projecting medium spiny neurons (MSNs) express dopamine D1 (D1+) and D2 receptors (D2+), respectively. Both classes receive extensive GABAergic input via expression of synaptic, perisynaptic, and extrasynaptic GABAA receptors. The activation patterns of different presynaptic GABAergic neurons produce transient and sustained GABAA receptor-mediated conductance that fulfill distinct physiological roles. We performed single and dual whole cell recordings from striatal neurons in mice expressing fluorescent proteins in interneurons and MSNs. We report specific inhibitory dynamics produced by distinct activation patterns of presynaptic GABAergic neurons as source of synaptic, perisynaptic, and extrasynaptic inhibition. Synaptic GABAA receptors in MSNs contain the α2, γ2, and a β subunit. In addition, there is evidence for the developmental increase of the α1 subunit that contributes to faster inhibitory post-synaptic current (IPSC). Tonic GABAergic currents in MSNs from adult mice are carried by extrasynaptic receptors containing the α4 and δ subunit, while in younger mice this current is mediated by receptors that contain the α5 subunit. Both forms of tonic currents are differentially expressed in D1+ and D2+ MSNs. This study extends these findings by relating presynaptic activation with pharmacological analysis of inhibitory conductance in mice where the β3 subunit is conditionally removed in fluorescently labeled D2+ MSNs and in mice with global deletion of the δ subunit. Our results show that responses to low doses of gaboxadol (2 μM), a GABAA receptor agonist with preference to δ subunit, are abolished in the δ but not the β3 subunit knock out mice. This suggests that the β3 subunit is not a component of the adult extrasynaptic receptor pool, in contrast to what has been shown for tonic current in young mice. Deletion of the β3 subunit from D2+ MSNs however, removed slow spontaneous IPSCs, implicating its role in mediating synaptic input from striatal neurogliaform interneurons. "         
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