Contributions of the GABAA receptor alpha6 subunit to phasic and tonic inhibition revealed by a naturally occurring polymorphism in the alpha6 gene

Modulation of glutamate release from parallel fibers by mGlu4 and pre-synaptic GABA(A) receptors

The regulation of pre-synaptic glutamate release is important in the maintenance and fidelity of excitatory transmission in the nervous system. In this study, we report a novel interaction between a ligand-gated ion channel and a G-protein coupled receptor which regulates glutamate release from parallel fiber axon terminals. Immunocytochemical analysis revealed that GABA(A) receptors and the high affinity group III metabotropic glutamate receptor subtype 4 (mGlu4) are co-localized on glutamatergic parallel fiber axon terminals in the cerebellum. GABA(A) and mGlu4 receptors were also found to co-immunoprecipitate from cerebellar membranes. Independently, these two receptors have opposing roles on glutamate release: pre-synaptic GABA(A) receptors promote, while mGlu4 receptors inhibit, glutamate release. However, coincident activation of GABA(A) receptors with muscimol and mGlu4 with the agonist (2S)-S-2-amino-4-phosphonobutanoic acid , increased glutamate release from [(3) H]glutamate-loaded cerebellar synaptosomes above that observed with muscimol alone. Further support for an interaction between GABA(A) and mGlu4 receptors was obtained in the mGlu4 knockout mouse which displayed reduced binding of the GABA(A) ligand [(35) S]tert-butylbicyclophosphorothionate, and decreased expression of the α1, α6, β2 GABA(A) receptor subunits in the cerebellum. Taken together, our data suggest a new role for mGlu4 whereby simultaneous activation with GABA(A) receptors acts to amplify glutamate release at parallel fiber-Purkinje cell synapses.

GABAA receptors increase excitability and conduction velocity of cerebellar parallel fiber axons

In the adult mammalian brain, GABA(A) receptors (GABA(A)Rs) are responsible for the predominant forms of synaptic inhibition, but these receptors can excite neurons when the chloride equilibrium potential (E(Cl)) is depolarized. In many mature neurons, GABA(A)Rs are found on presynaptic terminals where they exert depolarizing effects. To understand whether excitatory GABA action affects axonal function, we used transverse cerebellar slices to measure the effects of photolysis of caged GABA on the initiation and propagation of compound parallel fiber (PF) action potentials (APs). Photolysis of caged GABA increased the amplitude and conduction velocity of PF APs; GABA reuptake blockers and a positive modulator of GABA(A)Rs enhanced these effects. In contrast, a modulator selective for δ-subunit-containing GABA(A)Rs did not enhance these effects and responsiveness remained in δ(-/-) mice, arguing that δ-subunit-containing GABA(A)Rs are not required. Synaptically released GABA also increased PF excitability, indicating that the mechanism is engaged by physiological signals. A Hodgkin-Huxley-style compartmental model of the PF axon and granule cell body was constructed, and this model recapitulated the GABA-dependent decrease in AP threshold and the increase in conduction velocity, features that were sensitive to E(Cl) and to the voltage dependence of sodium channel inactivation. The model also predicts that axonal GABA(A)Rs could affect orthodromic spike initiation. We conclude that GABA acting on cerebellar PFs facilitates both spike generation and propagation, allowing axons of granule cells to passively integrate signals from inhibitory interneurons and influence information flow in the input layer to the cerebellar cortex.

GABA Potency at GABA(A) Receptors Found in Synaptic and Extrasynaptic Zones

The potency of GABA is vitally important for its primary role in activating GABA_A receptors and acting as an inhibitory neurotransmitter. Although numerous laboratories have presented information, directly or indirectly, on GABA potency, it is often difficult to compare across such studies given the inevitable variations in the methods used, the cell types studied, whether native or recombinant receptors are examined, and their relevance to native synaptic and extrasynaptic GABA_A receptors. In this review, we list the most relevant isoforms of synaptic and extrasynaptic GABA_A receptors that are thought to assemble in surface membranes of neurons in the central nervous system. Using consistent methodology in one cell type, the potencies of the endogenous neurotransmitter GABA are compared across a spectrum of GABA_A receptors. The highest potency for GABA is measured when activating extrasynaptic-type α6 subunit-containing receptors, whereas synaptic-type α2β3γ2 and α3β3γ2 receptors exhibited the lowest potency, and other GABA_A receptor subtypes that are found both in synaptic and extrasynaptic compartments, showed intermediate sensitivities to GABA. The relatively simple potency relationship between GABA and its target receptors is important as it serves as one of the major determinants of GABA_A receptor activation, with consequences for the development of inhibition, either by tonic or phasic mechanisms.

Molecular basis for the high THIP/gaboxadol sensitivity of extrasynaptic GABA(A) receptors

Extrasynaptic GABA(A) receptors (eGABARs) allow ambient GABA to tonically regulate neuronal excitability and are implicated as targets for ethanol and anesthetics. These receptors are thought to be heteropentameric proteins made up of two α subunits-either α4 or α6-two β2 or β3 subunits, and one δ subunit. The GABA analog 4,5,6,7-tetrahydroisoxazolo (5,4-c)pyridin-3(-ol) (THIP) has been proposed as a selective ligand for eGABARs. Behavioral and in vitro studies suggest that eGABARs have nanomolar affinity for THIP; however, all published studies on recombinant versions of eGABARs report micromolar affinities. Here, we examine THIP sensitivity of native eGABARs on cerebellar neurons and on reconstituted GABARs in heterologous systems. Concentration-response data for THIP, obtained from cerebellar granule cells and molecular layer interneurons in wild-type and δ subunit knockout slices, confirm that submicromolar THIP sensitivity requires δ subunits. In recombinant experiments, we find that δ subunit coexpression leads to receptors activated by nanomolar THIP concentrations (EC(50) of 30-50 nM for α4β3δ and α6β3δ), a sensitivity almost 1,000-fold higher than receptors formed by α4/6 and β3 subunits. In contrast, γ2 subunit expression significantly reduces THIP sensitivity. Even when δ subunit cDNA or cRNA was supplied in excess, high- and low-sensitivity THIP responses were often apparent, indicative of variable mixtures of low-affinity αβ and high-affinity αβδ receptors. We conclude that δ subunit incorporation into GABARs leads to a dramatic increase in THIP sensitivity, a defining feature that accounts for the unique behavioral and neurophysiological properties of THIP.

Profound desensitization by ambient GABA limits activation of δ-containing GABAA receptors during spillover

High-affinity extrasynaptic GABA(A) receptors (GABA(A)Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA(A)Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA(A)Rs. We found that δ subunit-containing GABA(A)Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" δ subunit-containing GABA(A)Rs exhibited profound desensitization, rendering them insensitive to brief synaptic- or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

PKCγ is required for ethanol-induced increases in GABA(A) receptor α4 subunit expression in cultured cerebral cortical neurons

Ethanol exposure produces alterations in GABA(A) receptor function and expression associated with CNS hyperexcitability, but the mechanisms of these effects are unknown. Ethanol is known to increase both GABA(A) receptor α4 subunits and protein kinase C (PKC) isozymes in vivo and in vitro. Here, we investigated ethanol regulation of GABA(A) receptor α4 subunit expression in cultured cortical neurons to delineate the role of PKC. Cultured neurons were prepared from rat pups on postnatal day 0-1 and tested after 18 days. GABA(A) receptor α4 subunit surface expression was assessed using P2 fractionation and surface biotinylation following ethanol exposure for 4 h. Miniature inhibitory post-synaptic currents were measured using whole cell patch clamp recordings. Ethanol increased GABA(A) receptor α4 subunit expression in both the P2 and biotinylated fractions, while reducing the decay time constant in miniature inhibitory post-synaptic currents, with no effect on γ2 or δ subunits. PKC activation mimicked ethanol effects, while the PKC inhibitor calphostin C prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression. PKCγ siRNA knockdown prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression, but inhibition of the PKCβ isoform with PKCβ pseudosubstrate had no effect. We conclude that PKCγ regulates ethanol-induced alterations in α4-containing GABA(A) receptors.

Insufficient augmentation of ambient GABA responsible for age-related cognitive deficit

Age-related degeneration of intracortical inhibition could underlie declines in cognitive function during senescence. Based on a hypothesis that a decrease in basal concentration of ambient (extrasynaptic) GABA with aging leads to depressing intracortical inhibition, we investigated how the basal concentration affects stimulus-evoked activity (as signal), ongoing-spontaneous activity (as noise) of neurons and their (signal-to-noise) ratio S/N. We simulated a neural network model equipped with a GABA transport system that regulates ambient GABA concentration in a neuronal activity-dependent manner. An increase in basal concentration augmented ambient GABA, increased GABA-mediated inhibitory current, and depressed ongoing-spontaneous activity while still keeping stimulus-evoked activity. This led to S/N improvement, for which it was necessary for the reversal potential of GABA transporter to be close to the resting potential of neurons. Above the resting potential, ongoing-spontaneous activity was predominantly enhanced due to excessive GABA-uptake from the extracellular space by transporters. Below the resting potential, stimulus-evoked activity was predominantly depressed, caused by excessive GABA-release. We suggest that the insufficient augmentation of ambient GABA due to a decrease in its basal concentration may be one of the possible causes of cognitive deficit with aging, increasing ongoing-spontaneous neuronal activity as noise. GABA transporter may contribute to improving S/N, provided that its reversal potential is close to the resting potential.

Allosteric Modulation of αβδ GABAA Receptors

GABA_A receptors mediate the majority of the fast inhibition in the mature brain and play an important role in the pathogenesis of many neurological and psychiatric disorders. The αβδ GABA_A receptor localizes extra- or perisynaptically and mediates GABAergic tonic inhibition. Compared with synaptically localized αβγ receptors, αβδ receptors are more sensitive to GABA, display relatively slower desensitization and exhibit lower efficacy to GABA agonism. Interestingly, αβδ receptors can be positively modulated by a variety of structurally different compounds, even at saturating GABA concentrations. This review focuses on allosteric modulation of recombinant αβδ receptor currents and αβδ receptor-mediated tonic currents by anesthetics and ethanol. The possible mechanisms for the positive modulation of αβδ receptors by these compounds will also be discussed.

The role of GABA(A) receptors in the acute and chronic effects of ethanol: a decade of progress

The past decade has brought many advances in our understanding of GABA(A) receptor-mediated ethanol action in the central nervous system. We now know that specific GABA(A) receptor subtypes are sensitive to ethanol at doses attained during social drinking while other subtypes respond to ethanol at doses attained by severe intoxication. Furthermore, ethanol increases GABAergic neurotransmission through indirect effects, including the elevation of endogenous GABAergic neuroactive steroids, presynaptic release of GABA, and dephosphorylation of GABA(A) receptors promoting increases in GABA sensitivity. Ethanol's effects on intracellular signaling also influence GABAergic transmission in multiple ways that vary across brain regions and cell types. The effects of chronic ethanol administration are influenced by adaptations in GABA(A) receptor function, expression, trafficking, and subcellular localization that contribute to ethanol tolerance, dependence, and withdrawal hyperexcitability. Adolescents exhibit altered sensitivity to ethanol actions, the tendency for higher drinking and longer lasting GABAergic adaptations to chronic ethanol administration. The elucidation of the mechanisms that underlie adaptations to ethanol exposure are leading to a better understanding of the regulation of inhibitory transmission and new targets for therapies to support recovery from ethanol withdrawal and alcoholism.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

Hyperammonemia increases GABAergic tone in the cerebellum but decreases it in the rat cortex

BACKGROUND & AIMS

GABAA receptors modulate the function of the glutamate-nitric oxide-guanosine 3',5'-cyclic monophosphate (cGMP) pathway, which is reduced in cerebellum in hyperammonemic rats. It has been proposed that hyperammonemia-induced increases in gamma-aminobutyric acid "(GABAergic) tone" contribute to the pathogenesis of hepatic encephalopathy (HE), although this has not been assessed in vivo in animal models. We studied whether chronic hyperammonemia in rats increases GABAergic tone in the cerebellum and/or cerebral cortex and whether this increase contributes to cognitive impairment.

METHODS

We blocked GABAA receptors of rats with bicuculline and analyzed the function of this pathway in cerebellum and effects on learning ability.

RESULTS

Hyperammonemia increased GABAergic tone in cerebellum but decreased it in the cerebral cortex of rats. Increased GABAergic tone in the cerebellum of rats with hyperammonemia could have been caused by increases in extracellular GABA; tetrahydrodeoxy-corticosterone (a neurosteroid that enhances GABAA receptor activation); or amounts of the alpha1, alpha6, and gamma2 subunits of GABAA receptors. The decrease in GABAergic tone observed in the cortex could have resulted from the reduced amount of GABAA receptors delta and gamma2 subunits or increased levels of pregnanolone (5-fold), which selectively reduces activation of GABAA receptors that contain alpha4 subunits (widely expressed in cortex but not in cerebellum). Treatment with bicuculline normalized GABAergic tone and restored the increase in cGMP that was induced by activation of N-methyl-D-aspartate receptors and learning ability in hyperammonemic rats.

CONCLUSIONS

Increased GABAergic tone in the cerebellum contributes to cognitive impairment in hyperammonemic rats.

Expression of gamma-aminobutyric acid receptor (subtype A) in prostate cancer

BACKGROUND

In prostate cancer, gamma-aminobutyric acid (GABA) has been previously reported to increase cellular proliferation via the ionotropic GABAa receptor (GABAar) and to promote cellular invasiveness via the metabotropic GABAb receptor.

METHODS

In this study, we have investigated, by immunohistochemistry, GABAar levels in 12 normal human prostate, 13 benign prostatic hyperplasia (BPH) and 148 human prostate cancer specimens. We have also examined the effect of several GABA agonists and antagonists on the in vitro proliferation of four human prostate cancer cell lines: LNCaP, MDA-PCA-2b, DU145 and PC3.

RESULTS

GABAar immunoreactivity was present in the stroma of ~75% of the normal and BPH specimens, and in 95% of the prostate cancer specimens. Also, low to moderate GABAar staining was observed in the acinar epithelium of 50 (33%) prostate cancer specimens. No correlation was observed between GABAar staining and patient age, Gleason Sum or TNM stage. A GABAa agonist isoguvacine, at doses between 5-50 microg/ml (31-310 microM), stimulated the proliferation of all four human prostate cancer cell lines, tested. Baclofen, a GABAb agonist (up to 50 microg/ml, 234 microM) had no effect on growth. Also, at concentrations up to 100 microg/ml, GABA antagonists, bicuculline (223 microM), picrotoxin (166 microM) and saclofen (400 microM), did not have significant growth-inhibitory effects. However, dihydroergotoxine, which binds the GABAar chloride ion-channel, inhibited cellular proliferation (IC(50) 18-38 microM).

CONCLUSIONS

These data indicate frequent expression of GABAar in prostate cancer and support a role for GABAar in the proliferation of prostate cancer cells.

Etomidate, propofol and the neurosteroid THDOC increase the GABA efficacy of recombinant alpha4beta3delta and alpha4beta3 GABA A receptors expressed in HEK cells

General anesthetics, once thought to exert their effects through non-specific membrane effects, have highly specific ion channel targets that can silence neuronal populations in the nervous system, thereby causing unconsciousness and immobility, characteristic of general anesthesia. Inhibitory GABA(A) receptors (GABA(A)Rs), particularly highly GABA-sensitive extrasynaptic receptor subtypes that give rise to sustained inhibitory currents, are uniquely sensitive to GABA(A)R-active anesthetics. A prominent population of extrasynaptic GABA(A)Rs is made up of alpha4, beta2 or beta3, and delta subunits. Considering the demonstrated importance of GABA receptor beta3 subunits for in vivo anesthetic effects of etomidate and propofol, we decided to investigate the effects of GABA anesthetics on "extrasynaptic" alpha4beta3delta and also binary alpha4beta3 receptors expressed in human embryonic kidney (HEK) cells. Consistent with previous work on similar receptor subtypes we show that maximal GABA currents through "extrasynaptic" alpha4beta3delta receptors, receptors defined by sensitivity to EtOH (30mM) and the beta-carboline beta-CCE (1microM), are enhanced by the GABA(A)R-active anesthetics etomidate, propofol, and the neurosteroid anesthetic THDOC. Furthermore, we show that receptors formed by alpha4beta3 subunits alone also show high GABA sensitivity and that saturating GABA responses of alpha4beta3 receptors are increased to the same extent by etomidate, propofol, and THDOC as are alpha4beta3delta receptors. Therefore, both alpha4beta3 and alpha4beta3delta receptors show low GABA efficacy, and GABA is also a partial agonist on certain binary alphabeta receptor subtypes. Increasing GABA efficacy on alpha4/6beta3delta and alpha4beta3 receptors is likely to make an important contribution to the anesthetic effects of etomidate, propofol and the neurosteroid THDOC.

Ethanol modulates synaptic and extrasynaptic GABAA receptors in the thalamus

Drinking alcohol is associated with the disturbance of normal sleep rhythms, and insomnia is a major factor in alcoholic relapse. The thalamus is a brain structure that plays a pivotal role in sleep regulation and rhythmicity. A number of studies have implicated GABA(A) receptors (GABA(A)-Rs) in the anxiolytic, amnestic, sedative, and anesthetic effects of ethanol. In the present study, we examined the effects of ethanol on both synaptic and extrasynaptic GABA(A)-Rs of relay neurons in the thalamus. We found that ethanol (> or =50 mM) elicits a sustained current in thalamocortical relay neurons from the mouse ventrobasal thalamus, and this current is associated with a decrease in neuronal excitability and firing rate in response to depolarization. The steady current induced by ethanol was totally abolished by gabazine and was absent in relay neurons from GABA(A)-R alpha(4) subunit knockout mice, indicating that the effect of ethanol is to enhance tonic GABA-mediated inhibition. Ethanol (50 mM) enhanced the amplitude of tonic inhibition by nearly 50%. On the other hand, ethanol had no effect on spontaneous or evoked inhibitory postsynaptic currents (IPSCs) at 50 mM but did prolong IPSCs at 100 mM. Ethanol had no effect on the paired-pulse depression ratio, suggesting that the release of GABA from presynaptic terminals is insensitive to ethanol. We conclude that ethanol, at moderate (50 mM) but not low (10 mM) concentrations, can inhibit thalamocortical relay neurons and that this occurs mainly via the actions of ethanol at extrasynaptic GABA(A)-Rs containing GABA(A)-R alpha(4) subunits.

Synaptic inputs to granule cells of the dorsal cochlear nucleus

The mammalian dorsal cochlear nucleus (DCN) integrates auditory nerve input with nonauditory signals via a cerebellar-like granule cell circuit. Although granule cells carry nonauditory information to the DCN, almost nothing is known about their physiology. Here we describe electrophysiological features of synaptic inputs to granule cells in the DCN by in vitro patch-clamp recordings from P12 to P22 rats. Granule cells ranged from 6 to 8 microm in cell body diameter and had high-input resistance. Excitatory postsynaptic currents consisted of both AMPA receptor-mediated and N-methyl-D-aspartate receptor-mediated currents. Synaptically evoked excitatory postsynaptic currents ranged from -25 to -140 pA with fast decay time constants. Synaptic stimulation evoked both short- and long-latency synaptic responses that summated to spike threshold, indicating the presence of a polysynaptic excitatory pathway in the granule cell circuit. Synaptically evoked inhibitory postsynaptic currents in Cl(-)-loaded cells ranged from -30 to -1,021 pA and were mediated by glycine and, to a lesser extent, GABA(A) receptors. Unlike cerebellar granule cells, DCN granule cells lacked tonic inhibition by GABA. The glycinergic synaptic conductance was mediated by heteromeric glycine receptors and was far stronger than the glutamatergic conductance, suggesting that glycinergic neurons may act to gate nonauditory signals in the DCN.

Ethanol sensitivity of GABAergic currents in cerebellar granule neurons is not increased by a single amino acid change (R100Q) in the alpha6 GABAA receptor subunit

Cerebellar granule neurons (CGNs) extrasynaptically express GABA(A) receptors containing alpha(6)beta(x)delta subunits, which mediate tonic inhibitory currents. Although it has been shown that the function of these receptors is potently and directly enhanced by ethanol, this finding has not been reproducible across different laboratories. In outbred Sprague-Dawley rats, a naturally occurring arginine (R) to glutamine (Q) mutation in position 100 of the alpha(6) subunit was reported to increase the ethanol sensitivity of these receptors. However, we did not detect an action of this mutation in selectively bred rats (alcohol-tolerant and alcohol-nontolerant). Consequently, we reexamined the effect of the mutation on ethanol sensitivity in Sprague-Dawley rats. Using patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that 25 mM ethanol increases the tonic current amplitude, tonic current noise, and spontaneous inhibitory postsynaptic current (sIPSC) frequency to a similar extent in alpha(6)-100R/100R and alpha(6)-100Q/100Q CGNs. Exposure to 80 mM ethanol increased the tonic current amplitude to a significantly greater extent in alpha(6)-100R/100R than in alpha(6)-100Q/100Q CGNs; however, the effects of 80 mM ethanol on the tonic current noise and sIPSC frequency were not significantly different between these groups. In the presence of tetrodo-toxin, a non-N-methyl-d-aspartate receptor antagonist, exogenous GABA, and a GABA transporter inhibitor, neither 8 nor 40 mM ethanol consistently affected tonic current amplitude or noise in alpha(6)-100R/100R or alpha(6)-100Q/100Q CGNs. Thus, the alpha(6)-R100Q GABA(A) receptor subunit polymorphism does not in-crease the acute ethanol sensitivity of extrasynaptic receptors, lending further support to the hypothesis that ethanol modulates these currents indirectly via a presynaptic mechanism.

Presynaptic GABAA receptors facilitate GABAergic transmission to dopaminergic neurons in the ventral tegmental area of young rats

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 ( Collapse

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MESH Headings

• Animals
• Cadmium/pharmacology
• Dopamine/metabolism
• GABA Agonists/pharmacology
• In Vitro Techniques
• Inhibitory Postsynaptic Potentials/drug effects
• Muscimol/pharmacology
• Neurons/metabolism
• Neurons/physiology
• Presynaptic Terminals/metabolism
• Protein Isoforms/physiology
• Rats
• Rats, Sprague-Dawley
• Receptors, GABA-A/physiology
• Synaptic Transmission/physiology
• Tetrodotoxin/pharmacology
• Ventral Tegmental Area/cytology
• Ventral Tegmental Area/metabolism
• Ventral Tegmental Area/physiology
• gamma-Aminobutyric Acid/physiology

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Grants

• R01 AA011989 NIAAA NIH HHS
• AT 001182 NCCIH NIH HHS
• R21 AT001182 NCCIH NIH HHS
• AA015925 NIAAA NIH HHS
• AA11989 NIAAA NIH HHS
• R21 AA015925 NIAAA NIH HHS

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Affiliation(s)

Cheng Xiao Department of Anesthesiology, Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, 185 South Orange Avenue, Newark, NJ 07103, USA
Chunyi Zhou
Keyong Li
Jiang-Hong Ye

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Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

Ethanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition

Based on the similarity of ethanol intoxication to the behavioral effects of drugs known to target gamma-aminobutyric acid type A (GABAA) receptors (GABARs), it has been suspected for decades that ethanol facilitates the activity of GABA. Even so, it has been surprisingly difficult to identify molecular targets of ethanol. Research conducted over the past several years suggests that a subclass of GABARs (those containing delta subunits) responds in a relevant concentration range to ethanol. Although delta subunit-containing GABARs are not ubiquitously expressed at inhibitory synapses like their gamma subunit-containing, synaptic counterparts, they are found in many neurons in extrasynaptic locations. Here, they give rise to a tonic form of inhibition that can potently suppress neuronal excitability. Studies have shown that both recombinant and native delta subunit-containing GABARs (1) are modulated by behaviorally relevant (i.e., low millimolar) concentrations of ethanol, (2) directly bind ethanol over the same concentration range, (3) show altered function upon single amino substitutions linked to changes in behavioral responsiveness to ethanol, and (4) are a site of action of Ro15-4513, a competitive antagonist of ethanol binding and a drug which prevents many of the behavioral aspects of ethanol intoxication. Despite such comprehensive evidence, however, the field is not free from controversy. This review evaluates published data for and against a central role of delta subunit-containing GABARs in ethanol actions and suggests future directions that might help settle points of controversy.

GABAA receptors in the thalamus: alpha4 subunit expression and alcohol sensitivity

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) has long been implicated in the anxiolytic, amnesic, and sedative behavioral effects of alcohol. A large number of studies have investigated the interactions of alcohol with GABA receptors. Many investigators have reported effects of "high concentrations" (50-100 mM) of alcohol on GABA-mediated synaptic inhibition, but effects of the "low concentrations" (1-30 mM) of alcohol normally associated with mild intoxication have been elusive until recently. A novel form of "tonic inhibition" has been described in the central nervous system (CNS) that is generated by the persistent activation of extrasynaptic gamma-aminobutyric acid type A receptors (GABAA-Rs). These receptors are specific GABAA-R subtypes and distinct from the synaptic subtypes. Tonic inhibition regulates the excitability of individual neurons and the activity and rhythmicity of neural networks. Interestingly, several reports show that tonic inhibition is sensitive to low concentrations of alcohol. The thalamus is a structure that is critically important in the control of sleep and wakefulness. GABAergic inhibition in the thalamus plays a crucial role in the generation of sleep waves. Among the various GABAA-R subunits, the alpha1, alpha4, beta2, and delta subunits are heavily expressed in thalamic relay nuclei. Tonic inhibition has been demonstrated in thalamocortical relay neurons, where it is mediated by alpha4beta2delta GABAA-Rs. These extrasynaptic receptors are highly sensitive to gaboxadol, a novel hypnotic, but insensitive to benzodiazepines. Tonic inhibition is absent in thalamic relay neurons from alpha4 knockout mice, as are the sedative and analgesic effects of gaboxadol. The sedative effects of alcohol can promote sleep. However, alcohol also disrupts the normal sleep pattern and reduces sleep quality. As a result, sleep disturbance caused by alcohol can play a role in the progression of alcoholism. As an important regulator of sleep cycles, inhibition in the thalamus may therefore be involved in both the sedative effects of alcohol and the development of alcoholism. Investigating the effects of alcohol on both synaptic and extrasynaptic GABAA-Rs in the thalamus should help us to understand the mechanisms underlying the interaction between alcohol and sleep.

Does ethanol act preferentially via selected brain GABAA receptor subtypes? the current evidence is ambiguous

In rodent models, gamma-aminobutyric acid A (GABAA) receptors with the alpha6 and delta subunits, expressed in the cerebellar and cochlear nucleus granule cells, have been linked to ethanol sensitivity and voluntary ethanol drinking. Here, we review the findings. When considering both in vivo contributions and data on cloned receptors, the evidence for direct participation of the alpha6-containing receptors to increased ethanol sensitivity is poor. The alpha6 subunit-knockout mouse lines do not have any changed sensitivity to ethanol, although these mice do display increased benzodiazepine sensitivity. However, in general the compensations occurring in knockout mice (regardless of which particular gene is knocked out) tend to fog interpretations of drug actions at the systems level. For example, the alpha6 knockout mice have increased TASK-1 channel expression in their cerebellar granule cells, which could influence sensitivity to ethanol in the opposite direction to that obtained with the alpha6 knockouts. Indeed, TASK-1 knockout mice are more impaired than wild types in motor skills when given ethanol; this might explain why GABAA receptor alpha6 knockout mice have unchanged ethanol sensitivities. As an alternative to studying knockout mice, we examined the claimed delta subunit-dependent/gamma2 subunit-independent ethanol/[3H]Ro 15-4513 binding sites on GABAA receptors. We looked at [3H]Ro 15-4513 binding in HEK 293 cell membrane homogenates containing rat recombinant alpha6/4beta3delta receptors and in mouse brain sections. Specific high-affinity [3H]Ro 15-4513 binding could not be detected under any conditions to the recombinant receptors or to the cerebellar sections of gamma2(F77I) knockin mice, nor was this binding to brain sections of wild-type C57BL/6 inhibited by 1-100 mM ethanol. Since ethanol may act on many receptor and channel protein targets in neuronal membranes, we consider the alpha6 (and alpha4) subunit-containing GABAA receptors unlikely to be directly responsible for any major part of ethanol's actions. Therefore, we finish the review by discussing more generally alcohol and GABAA receptors and by suggesting potential future directions for this research.

GABAA receptor subtypes: the "one glass of wine" receptors

This review discusses evidence for and apparent controversy about, gamma-aminobutyric acid type A (GABAA) receptor (GABAAR) subtypes that mediate alcohol effects experienced during social drinking. GABAARs that contain the beta3 and delta subunits were shown to be enhanced by alcohol concentrations that mirror the concentration dependence of alcohol responses in humans. A mutation (alpha6R100Q) previously found in alcohol nontolerant rats in the cerebellar GABAAR alpha6 subunit is sufficient for increased alcohol-induced ataxia in rats homozygous for this mutation (alpha6-100QQ) and further increases alcohol sensitivity of tonic GABA currents (mediated by alpha6betadelta receptors) in cerebellar granule cells of alpha6-100QQ rats and in recombinant alpha6R100Qbeta3delta receptors. This provided the first direct evidence that these types of receptors mediate behavioral effects of ethanol. Furthermore, the behavioral alcohol antagonist Ro15-4513 specifically reverses ethanol enhancement on alpha4/6beta3delta receptors. Unexpectedly, native and recombinant alpha4/6beta3delta receptors bind the behavioral alcohol antagonist Ro15-4513 with high affinity and this binding is competitive with EtOH, suggesting a specific and mutually exclusive (competitive) ethanol/Ro15-4513 site, which explains the puzzling activity of Ro15-4513 as a behavioral alcohol antagonist. Our conclusion from these findings is that alcohol/Ro15-4513-sensitive GABAAR subtypes are important alcohol targets and that alcohol at relevant concentrations is more specific than previously thought. In this review, we discuss technical difficulties in expressing recombinant delta subunit-containing receptors in oocytes and mammalian cells that may have contributed to negative results and confusion. Not only because we have reproduced detailed positive results numerous times, and we and many others have built extensively on basic findings, but also because we explain and combine many previously puzzling results into a coherent and highly plausible paradigm on how alcohol exerts an important part of its action in the brain, we are confident about our findings and conclusions. However, many important open questions remain to be answered.

A new meaning for "Gin & Tonic": tonic inhibition as the target for ethanol action in the brain

Gamma-aminobutyric acid (GABA) is the main chemical inhibitory neurotransmitter in the brain. In the central nervous system, it acts on two distinct types of receptor: an ion channel, that is, an "ionotropic" receptor permeable to Cl- and HCO3- (GABAA receptors [GABAARs]) and a G-protein coupled "metabotropic" receptor that is linked to various effector mechanisms (GABAB receptors). This review will summarize novel developments in the physiology and pharmacology of GABAARs, specifically those found outside synapses. The focus will be on a particular combination of GABAAR subunits responsible for mediating tonic inhibition and sensitive to concentrations of ethanol legally considered to be sobriety impairing. Since the same receptors are also a preferred target for the metabolites of steroid hormones synthesized in the brain (neurosteroids), the ethanol-sensitive tonic inhibition may be a common pathway for interactions between the effects of alcohol and those of ovarian and stress-related neurosteroids.

Electrophysiological properties and subunit composition of GABAA receptors in patients with gelastic seizures and hypothalamic hamartoma

Abnormalities in GABA(A) receptor structure and/or function have been associated with various forms of epilepsy in both humans and animals. Whether this is true for patients with gelastic seizures and hypothalamic hamartoma (HH) is unknown. In this study, we characterized the pharmacological properties and native subunit composition of GABA(A) receptors on acutely dissociated single neurons from surgically resected HH tissues using patch-clamp, immunocytochemical, and RT-PCR techniques. We found that 1) GABA induced an inward current (I(GABA)) at a holding potential of -60 mV; 2) I(GABA) was mimicked by the GABA(A) receptor agonist muscimol and blocked by the GABA(A) receptor antagonist bicuculline, suggesting that I(GABA) was mediated principally through the GABA(A) receptor; 3) the EC(50) and Hill coefficient derived from the I(GABA) concentration-response curve were 6.8 muM and 1.9, respectively; 4) the current-voltage curve was linear at a reversal potential close to zero; and 5) I(GABA) exhibited low sensitivity to zinc and diazepam but higher sensitivity to pentobarbital and pregnanolone. Additionally, using Xenopus oocytes microtransplanted with normal human hypothalamic tissue, we confirmed that the functional properties of GABA(A) receptors were similar to those seen in small isolated HH neurons. Finally, the expression profile of GABA(A) receptor subunits obtained from normal control human hypothalamic tissue was identical to that from surgically resected human HH tissue. Taken together, our data indicate that GABA(A) receptors on small HH neurons exhibit normal pharmacosensitivity and subunit composition. These findings bear relevance to a broader understanding of inhibitory neurotransmission in human HH tissue.

Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

GABAA receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABAA receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABAA receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABAA receptor subunit genes revealed the role of alpha subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABAA receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltage-independent K+ conductances, and upregulation of GABAA receptors in interneurons that are not affected directly by the mutation. Deletion of the alpha1 subunit gene leads to complete loss of GABAA receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of alpha1(0/0) mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABAA receptors are required for long-term maintenance of GABAergic synapses in Purkinje cells.

Modulation of extrasynaptic THIP conductances by GABAA-receptor modulators in mouse neocortex

THIP is a hypnotic drug, which displays a unique pharmacological profile, because it activates a subset of extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors containing delta-subunits. It is important to study the physiology and pharmacology of these extrasynaptic receptors and to determine how THIP interacts with other hypnotics and anesthetics. Here, we study the modulation of the extrasynaptic response to THIP using three classes of GABA(A)-receptor ligands. In whole cell recordings from mouse neocortical layer 2/3 pyramidal cells, THIP induced an extrasynaptic tonic current of 44 +/- 5 pA. The benzodiazepine site agonist and hypnotic zolpidem (500 nM), which displays selectivity for alpha(1/2/3)- and gamma(2)-containing receptors, did not alter the tonic current induced by THIP. The anesthetic etomidate (1 microM), which shows selectivity for beta(2)- and beta(3)-containing GABA(A) receptors, potentiated the THIP current by 126%. Etomidate also induced a small tonic GABA(A) current per se. The anesthetic propofol (1 microM), which displays broad-spectrum modulatory effects on several GABA(A)-receptor subtypes, enhanced the tonic THIP current by 117%. Finally, all three compounds modulated the function of intrasynaptic receptors activated by synaptically released GABA. Our study shows that the extrasynaptic GABA(A) receptors responsible for the tonic THIP conductance likely do not contain alpha(1)-, alpha(2)-, alpha(3)-, and gamma(2)-subunits. Thus the tonic GABAergic conductance in the neocortex is presumably mediated by alpha(4)beta(2/3)delta receptors, which are likely to play a major role for neocortical excitability. Furthermore, our study has deepened the knowledge about the cellular actions of THIP as well as THIP's interactions with other hypnotics and anesthetics.

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.

Multiple types of GABAA receptors mediate inhibition in brain stem parasympathetic cardiac neurons in the nucleus ambiguus

Recent work suggests neurons can have different types of gamma-aminobutyric acid type A (GABA(A)) receptors that mediate phasic inhibitory postsynaptic currents (IPSCs) and tonic currents. This study examines the diversity of GABAergic synaptic currents in parasympathetic cardioinhibitory neurons that receive rhythmic bursts of GABAergic neurotransmission. Focal application of gabazine (25 microM) to cardiac vagal neurons in vitro did not change the frequency of firing in spontaneously active neurons or the resting membrane potential; however, picrotoxin (100 microM) significantly depolarized cardiac vagal neurons and increased their firing. Similarly, gabazine (25 microM) selectively blocked GABAergic IPSCs but did not change holding current in cardiac vagal neurons, whereas picrotoxin (100 microM) not only blocked GABAergic IPSCs but also rapidly decreased the tonic current. Because the tonic current could be attributable to activation of GABA receptors by ambient GABA or, alternatively, spontaneous opening of constitutively active GABA channels, an antagonist for the GAT-1 GABA transporter NO-711 (10 microM) was applied to distinguish between these possibilities. NO-711 did not significantly alter the holding current in these neurons. The benzodiazepine flunitrazepam (1 microM) significantly increased the tonic current and GABAergic IPSC decay time; surprisingly, however, in the presence of gabazine flunitrazepam failed to elicit any change. These results suggest cardiac vagal neurons possess gabazine-sensitive GABA(A) receptors that mediate phasic synaptic currents, a gabazine-insensitive but picrotoxin-sensitive extrasynaptic tonic current that when blocked depolarizes and increases the firing rate of cardiac vagal neurons, and benzodiazepines recruit a third type of GABA(A) receptor that is sensitive to gabazine and augments the extrasynaptic tonic current.