Cis-4-amino-crotonic acid activates alpha 6 subunit-containing GABA(A) but not GABA(C) receptors in granule cells of adult rat cerebellar slices

Metabolomic Approaches to Defining the Role(s) of GABAρ Receptors in the Brain

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) acts through various types of receptors in the central nervous system. GABAρ receptors, defined by their characteristic pharmacology and presence of ρ subunits in the channel structure, are poorly understood and their role in the cortex is ill-defined. Here, we used a targeted pharmacological, NMR-based functional metabolomic approach in Guinea pig brain cortical tissue slices to identify a distinct role for these receptors. We compared metabolic fingerprints generated by a range of ligands active at GABAρ and included these in a principal components analysis with a library of other metabolic fingerprints obtained using ligands active at GABAA and GABAB, with inhibitors of GABA uptake and with compounds acting to inhibit enzymes active in the GABAergic system. This enabled us to generate a metabolic "footprint" of the GABAergic system which revealed classes of metabolic activity associated with GABAρ which are distinct from other GABA receptors. Antagonised GABAρ produce large metabolic effects at extrasynaptic sites suggesting they may be involved in tonic inhibition.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

Neurochemicals for the investigation of GABA(C) receptors

GABA(C) receptors are being investigated for their role in many aspects of nervous system function including memory, myopia, pain and sleep. There is evidence for functional GABA(C) receptors in many tissues such as retina, hippocampus, spinal cord, superior colliculus, pituitary and the gut. This review describes a variety of neurochemicals that have been shown to be useful in distinguishing GABA(C) receptors from other receptors for the major inhibitory neurotransmitter GABA. Some selective agonists (including (+)-CAMP and 5-methyl-IAA), competitive antagonists (such as TPMPA, (±)-cis-3-ACPBPA and aza-THIP), positive (allopregnanolone) and negative modulators (epipregnanolone, loreclezole) are described. Neurochemicals that may assist in distinguishing between homomeric ρ1 and ρ2 GABA(C) receptors (2-methyl-TACA and cyclothiazide) are also covered. Given their less widespread distribution, lower abundance and relative structural simplicity compared to GABA(A) and GABA(B) receptors, GABA(C) receptors are attractive drug targets.

γ-Hydroxybutyrate and the GABAergic footprint: a metabolomic approach to unpicking the actions of GHB

Gamma-hydroxybutyrate is found both naturally in the brain and self-administered as a drug of abuse. It has been reported to act at endogenous γ-hydroxybutyrate (GHB) receptors and GABA(B) receptors [GABA(B)R], and may also be metabolized to GABA. Here, the metabolic fingerprints of a range of concentrations of GHB were measured in brain cortical tissue slices and compared with those of ligands active at GHB and GABA-R using principal components analysis (PCA) to identify sites of GHB activity. Low concentrations of GHB (1.0 μM) produced fingerprints similar to those of ligands active at GHB receptors and α4-containing GABA(A)R. A total of 10 μM GHB clustered proximate to mainstream GABAergic synapse ligands, such as 1.0 μM baclofen, a GABA(B)R agonist. Higher concentrations of GHB (30 μM) clustered with GABA(C)R agonists and the metabolic responses induced by blockade of the GABA transporter-1 (GAT1). The metabolic responses induced by 60 and 100 μM GHB were mimicked by simultaneous blockade of GAT1 and GAT3, addition of low concentrations of GABA(C)R antagonists, or increasing cytoplasmic GABA concentrations by incubation with the GABA transaminase inhibitor vigabatrin. These data suggest that at concentrations > 30 μM, GHB may be active via metabolism to GABA, which is then acting upon an unidentified GABAergic master switch receptor (possibly a high-affinity extrasynaptic receptor), or GHB may itself be acting directly on an extrasynaptic GABA-R, capable of turning off large numbers of cells. These results offer an explanation for the steep dose-response curve of GHB seen in vivo, and suggest potential target receptors for further investigation.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Evidence that GABA rho subunits contribute to functional ionotropic GABA receptors in mouse cerebellar Purkinje cells

Ionotropic gamma-amino butyric acid (GABA) receptors composed of heterogeneous molecular subunits are major mediators of inhibitory responses in the adult CNS. Here, we describe a novel ionotropic GABA receptor in mouse cerebellar Purkinje cells (PCs) using agents reported to have increased affinity for rho subunit-containing GABA(C) over other GABA receptors. Exogenous application of the GABA(C)-preferring agonist cis-4-aminocrotonic acid (CACA) evoked whole-cell currents in PCs, whilst equimolar concentrations of GABA evoked larger currents. CACA-evoked currents had a greater sensitivity to the selective GABA(C) antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) than GABA-evoked currents. Focal application of agonists produced a differential response profile; CACA-evoked currents displayed a much more pronounced attenuation with increasing distance from the PC soma, displayed a slower time-to-peak and exhibited less desensitization than GABA-evoked currents. However, CACA-evoked currents were also completely blocked by bicuculline, a selective agent for GABA(A) receptors. Thus, we describe a population of ionotropic GABA receptors with a mixed GABA(A)/GABA(C) pharmacology. TPMPA reduced inhibitory synaptic transmission at interneurone-Purkinje cell (IN-PC) synapses, causing clear reductions in miniature inhibitory postsynaptic current (mIPSC) amplitude and frequency. Combined application of NO-711 (a selective GABA transporter subtype 1 (GAT-1) antagonist) and SNAP-5114 (a GAT-(2)/3/4 antagonist) induced a tonic GABA conductance in PCs; however, TPMPA had no effect on this current. Immunohistochemical studies suggest that rho subunits are expressed predominantly in PC soma and proximal dendritic compartments with a lower level of expression in more distal dendrites; this selective immunoreactivity contrasted with a more uniform distribution of GABA(A) alpha1 subunits in PCs. Finally, co-immunoprecipitation studies suggest that rho subunits can form complexes with GABA(A) receptor alpha1 subunits in the cerebellar cortex. Overall, these data suggest that rho subunits contribute to functional ionotropic receptors that mediate a component of phasic inhibitory GABAergic transmission at IN-PC synapses in the cerebellum.

Evidence for a functional role of GABAC receptors in the rat mature hippocampus

Both gamma-aminobutyric acid (GABA)(C) receptor subunit mRNA and protein are expressed in the stratum pyramidale in the CA1 area of the adult rat hippocampus, but so far no conclusive evidence about functional hippocampal GABA(C) receptors has been presented. Here, the contribution of GABA(C) receptors to stimulus-evoked postsynaptic potentials was studied in the hippocampal CA1 area with extracellular and intracellular recordings at the age range of 21-47 postnatal days. Activation of GABA(C) receptors with the specific agonist cis-4-aminocrotonic acid (CACA) suppressed postsynaptic excitability and increased the membrane conductance. The GABA(C) receptor antagonist 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic acid (TPMPA), but not the GABA(A) receptor antagonist bicuculline, inhibited the effects of CACA. GABA-mediated long-lasting depolarizing responses evoked by high-frequency stimulation of local inhibitory interneurons in the CA1 area in the presence of ionotropic glutamate receptor and GABA(B) receptor blockers were prolonged by TPMPA, indicating that GABA(C) receptors are activated under these conditions. For weaker stimulation, the effect of TPMPA was enhanced after GABA uptake was inhibited. Our data demonstrate that GABA(C) receptors can be activated by endogenous synaptic transmitter release following strong stimulation or under conditions of reduced GABA uptake. The lack of GABA(C) receptor activation by less intensive stimulation under control conditions suggests that these receptors are extrasynaptic and activated via spillover of synaptically released GABA.

Evidence for inhibition mediated by coassembly of GABAA and GABAC receptor subunits in native central neurons

Fast inhibition in the nervous system is commonly mediated by GABA(A) receptors comprised of 2alpha/2beta/1gamma subunits. In contrast, GABA(C) receptors containing only rho subunits (rho1-rho3) have been predominantly detected in the retina. However, here using reverse transcription-PCR and in situ hybridization we show that mRNA encoding the rho1 subunit is highly expressed in brainstem neurons. Immunohistochemistry localized the rho1 subunit to neurons at light and electron microscopic levels, where it was detected at synaptic junctions. Application of the GABA(C) receptor agonist cis-4-aminocrotonic acid (100-800 microM) requires the rho1 subunit to elicit responses, which surprisingly are blocked independently by antagonists to GABA(A) (bicuculline, 10 microM) and GABA(C) [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA); 40-160 microM] receptors. Responses to GABA(C) agonists were also enhanced by the GABA(A) receptor modulator pentobarbitone (300 microM). Spontaneous and evoked IPSPs were reduced in amplitude but never abolished by TPMPA, but were completely blocked by bicuculline. We therefore tested the hypothesis that GABA(A) and GABA(C) subunits formed a heteromeric receptor. Immunohistochemistry indicated that rho1 and alpha1 subunits were colocalized at light and electron microscopic levels. Electrophysiology revealed that responses to GABA(C) receptor agonists were enhanced by the GABA(A) receptor modulator zolpidem (500 nm), which acts on the alpha1 subunit when the gamma2 subunit is also present. Finally, coimmunoprecipitation indicated that the rho1 subunit formed complexes that also containedalpha1 and gamma2 subunits. Taken together these separate lines of evidence suggest that the effects of GABA in central neurons can be mediated by heteromeric complexes of GABA(A) and GABA(C) receptor subunits.

Ionotropic GABA receptors with mixed pharmacological properties of GABAA and GABAC receptors

Ionotropic gamma-aminobutyric acid (GABA) receptors form a large family of molecular isoforms with distinct properties. We have characterized a distinct new type of GABA receptors in CA1 pyramidal cells in rat hippocampal slices. Somatic application of GABA induced currents which were partially suppressed by (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific antagonist of GABA(C) receptors. This sensitivity was enhanced when we evoked the currents by the GABA(C) receptor agonist cis-4-aminocrotonic acid (CACA). However, both GABA- and CACA-evoked currents were sensitive towards bicuculline and thus lack the defining feature of GABA(C) receptors, which are insensitive towards this antagonist. Spontaneous miniature post-synaptic currents (mIPSCs) revealed a similar pharmacological behaviour. We conclude that juvenile CA1 pyramidal cells express a fraction of ionotropic GABA receptors with mixed pharmacological properties of both, GABA(A) and GABA(C) receptors.

Competitive GABA(A) receptor antagonists increase the proportion of functional high-affinity alpha6 subunit-containing receptors in granule cells of adult rat cerebellum

To investigate the properties of alpha6 subunit-containing GABA(A) receptors, whole-cell patch-clamp recordings were made from granule cells in adult rat cerebellar slices. In control, only currents evoked by low concentrations of GABA were significantly reduced in amplitude by furosemide, the alpha6 subunit-containing receptor antagonist. However, in the presence of competitive GABA(A) receptor antagonists, the furosemide block of currents evoked by higher GABA concentrations was markedly increased. Zinc, which preferentially blocks alpha6 subunit-containing receptors, also produced an increased block in the presence of bicuculline. To investigate whether similar effects occurred at synaptic receptors, inhibitory postsynaptic currents (IPSCs) were recorded. In most cells, furosemide produced little or no reduction in evoked IPSC amplitude. However in the presence of SR95531, a competitive antagonist, furosemide markedly reduced IPSC amplitude. One hypothesis, which could account for these observations, is that competitive antagonists prevent the continual activation of alpha6beta2/3gamma2 receptors by endogenous GABA and thus prevent their desensitisation. This hypothesis appears feasible as prolonged applications of low concentrations of GABA to recombinant alpha6beta2gamma2s receptors resulted in their desensitisation.

Gamma-aminobutyric acid-induced calcium signalling in rat superior collicular neurones

Ionotropic gamma-aminobutyric acid (GABA) receptors are known to mediate excitation in neonatal neurones as a crucial developmental factor. In the present study we employed calcium imaging techniques with the calcium indicator Fura-2-AM to study the pharmacology of GABA-induced calcium responses in cultures prepared from neonatal rat superficial superior colliculus (SC), after immunocytochemical labelling confirmed the presence of GABA(C) rho(1) subunits in 35% of neurones. Rises in neuronal intracellular calcium were obtained in response to GABA and also to the subtype-specific GABA(A) agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. However, the GABA(C) agonist cis-4-aminocrotonic acid induced calcium response only at unspecifically high concentrations (500 microM). Co-application of GABA antagonists revealed that both GABA(A&C) agonists' actions could be blocked by the GABA(A) antagonist bicuculline but not the GABA(C) antagonists 1,2,5,6-tetrahydro-(pyridin-4-yl) methylphosphinic acid. This suggests that activation of GABA(A) but not GABA(C) receptors contributes to excitatory GABA responses and related calcium signals in neonatal SC neurones.

Activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells

Amacrine cells in the vertebrate retina receive glutamatergic input from bipolar cells and make synapses onto bipolar cells, ganglion cells, and other amacrine cells. Recent studies indicate that amacrine cells express metabotropic glutamate receptors (mGluRs) and that signaling within the inner plexiform layer (IPL) of the retina might be modulated by mGluR activity. This study tests the hypothesis that activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells. Whole cell voltage-clamp recordings were combined with pharmacology to establish the identity of the ionotropic GABA receptors expressed in primary cultures of chick amacrine cells and to determine how mGluR5 activity affected the behavior of those receptors. Application of GABA (20 microM) produced currents that reversed at -58.2 +/- 0.9 mV, near the predicted Cl(-) reversal potential of -59 mV. The GABA(A) receptor antagonist, bicuculline (50 microM), completely blocked the GABA-gated currents. cis-4-Aminocrotonic acid (CACA; 100 microM), a GABA(C) receptor agonist, produced small currents that were not blocked by the GABA(C) antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA; 20 microM), but were completely blocked by bicuculline. These results indicate that cultured amacrine cells express GABA(A) receptors exclusively. Activating mGluR5 with (RS)-2-chloro-5-hydroxyphenylglycine (CHPG; 300 microM) enhanced GABA-gated currents by 10.0 +/- 1.5%. Buffering internal Ca(2+) with BAPTA (10 mM) blocked the CHPG-dependent enhancement. Activation of PKC with the cell-permeable PKC activators (-)-7-octylindolactam V, phorbol 12-myristate 13 acetate (PMA), or 1-oleoyl-2-acetyl-sn-glycerol (OAG) also enhanced GABA-gated currents in a dose-dependent manner. Preactivation of PKC occluded the mGluR5-dependent enhancement, and inhibition of Ca-dependent PKC isotypes with Gö6976 (35 nM) suppressed the effects of mGluR5 activation, suggesting that mGluR5 and PKC are part of the same pathway. To determine if mGluR5-dependent enhancement occurred at synaptic GABA(A) receptors, postsynaptic currents were recorded in the presence of CHPG. On average, the mean amplitudes of the quantal events were increased by about 18% when mGluR5 was activated. These results indicate that activation of mGluR5 enhances GABA-gated current in cultured amacrine cells in a manner that is both Ca(2+)- and PKC-dependent. These results support the possibility that glutamate released from bipolar cells can modulate the function of GABAergic amacrine cells and alter signaling in the inner plexiform layer.

Furosemide reveals heterogeneous GABA(A) receptor expression at adult rat Golgi cell to granule cell synapses

The contribution that alpha6 subunit-containing GABA(A) receptors make to inhibitory synaptic transmission to granule cells was investigated by making whole-cell patch clamp recordings from granule cells in adult rat cerebellar slices and applying furosemide, the specific alpha6 subunit-containing GABA(A) receptor antagonist. Endogenous, extracellular GABA continually activated GABA(A) receptors producing a tonic current. Since this current was markedly reduced by furosemide it was probably produced by alpha6 subunit-containing receptors. In contrast, furosemide had little effect on the amplitude or kinetics of fast spontaneous inhibitory postsynaptic currents (sIPSCs), although such sIPSCs were abolished by bicuculline and SR95331. However, the amplitude of evoked IPSCs with a very slow rise and decay were markedly reduced by furosemide. These IPSCs probably resulted from the spillover of GABA from neighbouring synapses activating high affinity alpha6 subunit-containing receptors. In the rest of the cells (40 out of 46), evoked IPSCs had rise and decay kinetics that lay in-between fast sIPSCs and slow 'spillover' IPSCs. Such IPSCs had variable kinetics and also exhibited considerable variation in the magnitude of furosemide block. Thus the GABA(A) receptors present at adult Golgi cell-granule cell synapses, at a developmental stage where receptor expression is complete, are highly heterogeneous.