Cloning of a putative gamma-aminobutyric acid (GABA) receptor subunit rho 3 cDNA

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Differential modulation of human GABAC-ρ1 receptor by sulfur-containing compounds structurally related to taurine

BACKGROUND

The amino acid taurine (2-Aminoethanesulfonic acid) modulates inhibitory neurotransmitter receptors. This study aimed to determine if the dual action of taurine on GABA_C-ρ1R relates to its structure. To address this, we tested the ability of the structurally related compounds homotaurine, hypotaurine, and isethionic acid to modulate GABA_C-ρ1R.

RESULTS

In Xenopus laevis oocytes, hypotaurine and homotaurine partially activate heterologously expressed GABA_C-ρ1R, showing an increment in its deactivation time with no changes in channel permeability, whereas isethionic acid showed no effect. Competitive assays suggest that hypotaurine and homotaurine compete for the GABA-binding site. In addition, their effects were blocked by the ion-channel blockers picrotixin and Methyl(1,2,5,6-tetrahydropyridine-4-yl) phosphinic acid. In contrast to taurine, co-application of GABA with hypotaurine or homotaurine revealed that the dual effect is present separately for each compound: hypotaurine modulates positively the GABA current, while homotaurine shows a negative modulation, both in a dose-dependent manner. Interestingly, homotaurine diminished hypotaurine-induced currents. Thus, these results strongly suggest a competitive interaction between GABA and homotaurine or hypotaurine for the same binding site. "In silico" modeling confirms these observations, but it also shows a second binding site for homotaurine, which could explain the negative effect of this compound on the current generated by GABA or hypotaurine, during co-application protocols.

CONCLUSIONS

The sulfur-containing compounds structurally related to taurine are partial agonists of GABA_C-ρ1R that occupy the agonist binding site. The dual effect is unique to taurine, whereas in the case of hypotaurine and homotaurine it presents separately; hypotaurine increases and homotaurine decreases the GABA current.

Neurovascular interaction and the pathophysiology of diabetic retinopathy

Diabetic retinopathy (DR) is the most severe of the several ocular complications of diabetes, and in the United States it is the leading cause of blindness among adults 20 to 74 years of age. Despite recent advances in our understanding of the pathogenesis of DR, there is a pressing need to develop novel therapeutic treatments that are both safe and efficacious. In the present paper, we identify a key mechanism involved in the development of the disease, namely, the interaction between neuronal and vascular activities. Numerous pathological conditions in the CNS have been linked to abnormalities in the relationship between these systems. We suggest that a similar situation arises in the diabetic retina, and we propose a logical strategy aimed at therapeutic intervention.

Medicinal chemistry of ρ GABAC receptors

The inhibitory neurotransmitter, GABA, is a low-molecular-weight molecule that can achieve many low-energy conformations, which are recognized by GABA receptors and transporters. In this article, we assess the structure–activity relationship profiles of GABA analogs at the ionotropic ρ GABAC receptor. Such studies have significantly contributed to the design and development of potent and selective agonists and antagonists for this subclass of GABA receptors. With these tools in hand, the role of ρ GABAC receptors is slowly being realized. Of particular interest is the development of selective phosphinic acid analogs of GABA and their potential use in sleep disorders, inhibiting the development of myopia, and in improving learning and memory.

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.

GABAC-receptor stimulation activates cAMP-dependent protein kinase via A-kinase anchoring protein 220

In our previous study, anti-apoptotic effects of GABA(C)-receptor stimulation was suppressed by inhibitors of cAMP-dependent protein kinase (PKA), implying GABA(C) receptor-mediated PKA activation. The present study showed that GABA(C)-receptor stimulation with its agonist, cis-4-aminocrotonic acid (CACA), protected cultured hippocampal neurons from amyloid beta 25 - 35 (Abeta25 - 35) peptide-enhanced glutamate neurotoxicity. This protective effect of CACA was blocked by PKA inhibitors, KT 5720 and H-89, as well as a specific GABA(C)-receptor antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA). To test the possibility of GABA(C) receptor-mediated PKA activation, association of GABA(C) receptor with A-kinase anchoring proteins (AKAPs) and effect of an AKAP antisense oligonucleotide on the PKA activation were examined in primary cultured rat hippocampal neurons. Stimulation of the cells with CACA-activated PKA was assessed by the phosphorylated PKA substrate (135 kDa) level. Specific antibodies raised against GABA(C)-receptor rho subunits precipitated each rho subunit, AKAP220, and PKA regulatory and catalytic subunits from rat brain lysates, suggesting that rho is associated with the AKAP220/PKA complex. Furthermore, antisense oligonucleotide of AKAP220 suppressed such GABA(C) stimulation-induced PKA activation, suggesting that GABA(C)-receptor stimulation activates PKA via AKAP220.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

Streptozotocin-induced diabetes modulates GABA receptor activity of rat retinal neurons

Neural deficits suggestive of involvement of the GABA signaling pathway can often be detected early in the course of diabetic retinopathy, a leading cause of blindness in the United States. To examine in greater detail the nature of the neuronal changes associated with hyperglycemia, we investigated GABA receptor activity on retinal bipolar cells in streptozotocin-induced diabetic rats; cells from age-matched normal rats served as controls. Patch-clamp recordings from isolated rod-bipolar cells revealed that diabetes enhanced the whole cell currents elicited by GABA. Responses of the GABA(C) receptor, the predominant GABA receptor on rat rod bipolar cells, exhibited a greater sensitivity to GABA, larger maximum current responses, slower response kinetics, and a smaller single channel conductance among diabetic cells relative to those recorded from normal controls. Compared with the properties of homomeric rho1 and heteromeric rho1rho2 receptors formed in a heterologous expression system, these results suggested that there was a greater contribution from the rho1 subunit in the GABA(C) receptor-mediated response of diabetic cells. The levels of mRNA, measured with real-time RT-PCR, were consistent with this finding. There was a significant enhancement in the ratio of rho1/rho2 subunit expression in the retina of diabetic animals, although the levels of GABA rho1 subunit expression were comparable in diabetic and normal retinas. Taken together, the results suggest that diabetes modifies the subunit composition of the GABA(C) receptor on retinal neurons, most likely through its effect on the efficacy of gene transcription.

Autoradiographic analysis of GABAA receptors in mu-opioid receptor knockout mice

Anatomical evidence indicates that gamma-aminobutyric acid (GABA)-ergic and opioidergic systems are closely linked and act on the same neurons. However, the regulatory mechanisms between GABAergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are changes in GABA(A) receptors in mice lacking mu-opioid receptor gene. The GABA(A) receptor binding was carried out by autoradiography using [(3)H]-muscimol (GABA(A)), [(3)H]-flunitrazepam (FNZ, native type 1 benzodiazepine) and [(35)S]-t-butylbicyclophosphorothionate (TBPS, binding to GABA(A)-gated chloride channels) in brain slices of wild type and mu-opioid receptor knockout mice. The binding of [(3)H]-FNZ in mu-opioid receptor knockout mice was significantly higher than that of the wild type controls in most of the cortex and hippocampal CA1 and CA2 formations. mu-Opioid receptor knockout mice show significantly lower binding of [(35)S]-TBPS than that of the wild type mice in few of the cortical areas including ectorhinal cortex layers I, III, and V, but not in the hippocampus. There was no significant difference in binding of [(3)H]-muscimol between mu-opioid receptor knockout and wild type mice in the cortex and hippocampus. These data indicate that there are specific regional changes in GABA(A) receptor binding sites in mu-opioid receptor knockout mice. These data also suggest that there are compensatory up-regulation of benzodiazepine binding site of GABA(A) receptors in the cortex and hippocampus and down-regulation of GABA-gated chloride channel binding site of GABA(A) receptors in the cortex of the mu-opioid receptor knockout mice.

The GABA rho1 subunit interacts with a cellular retinoic acid binding protein in mammalian retina

Interactions between the intracellular domain of ligand-gated membrane receptors and cytoplasmic proteins play important roles in their assembly, clustering, and function. In addition, protein-protein interactions may provide an alternative mechanism by which neurotransmitters activate intracellular pathways. In this study, we report a novel interaction between the GABA rho1 subunit and cellular retinoic acid binding protein in mammalian retina that could serve as a link between the GABA signaling pathway and the control of gene expression in neurons. The interaction between the intracellular loop of the human GABA rho subunit and cellular retinoic acid binding protein was identified using a CytoTrap XR yeast two-hybrid system, and was further confirmed by co-precipitation of the human GABA rho subunit and cellular retinoic acid binding protein from baboon retinal samples. The cellular retinoic acid binding protein binding domain on the human rho1 subunit was located to the C-terminal region of human GABA rho subunit, and the interaction of the human GABA rho subunit with cellular retinoic acid binding protein could be antagonized by a peptide derived from within the binding domain of the rho1 subunit. Since cellular retinoic acid binding protein is a carrier protein for retinoic acid, we investigated the effect of GABA on retinoic acid activity in neuroblastoma cells containing endogenously expressed cellular retinoic acid binding protein. In the absence of the rho1 receptor, these cells showed enhanced neurite outgrowth when exposed to retinoic acid and GABA had no effect on their response to retinoic acid. In contrast, cells stably transfected with the human rho1 subunit showed a significantly reduced sensitivity to retinoic acid when exposed to GABA. These results suggest that the GABA receptor subunit effectively altered gene expression through its interaction with the cellular retinoic acid binding protein pathway.

Enantiomers of cis-constrained and flexible 2-substituted GABA analogues exert opposite effects at recombinant GABA(C) receptors

The effects of the enantiomers of a number of flexible and cis-constrained GABA analogues were tested on GABA(C) receptors expressed in Xenopus laevis oocytes using two-electrode voltage-clamp electrophysiology. (1S,2R)-cis-2-Aminomethylcyclopropane-1-carboxylic acid ((+)-CAMP), a potent and full agonist at the rho1 (EC(50) approximately 40 microM, I(max) approximately 100%) and rho 2 (EC(50) approximately 17 microM, I(max) approximately 100%) receptor subtypes, was found to be a potent partial agonist at rho3 (EC(50) approximately 28 microM, I(max) approximately 70%). (1R,2S)-cis-2-Aminomethylcyclopropane-1-carboxylic acid ((-)-CAMP), a weak antagonist at human rho1 (IC(50) approximately 890 microM) and rho2 (IC(50) approximately 400 microM) receptor subtypes, was also found to be a moderately potent antagonist at rat rho3 (IC(50) approximately 180 microM). Similarly, (1R,4S)-4-aminocyclopent-2-ene-1-carboxylic acid ((+)-ACPECA) was a full agonist at rho1 (EC(50) approximately 135 microM, I(max) approximately 100%) and rho2 (EC(50) approximately 60 microM, I(max) approximately 100%), but only a partial agonist at rho3 (EC(50) approximately 112 microM, I(max) approximately 37%), while (1S,4R)-4-aminocyclopent-2-ene-1-carboxylic acid ((-)-ACPECA) was a weak antagonist at all three receptor subtypes (IC(50)>>300 microM). 4-Amino-(S)-2-methylbutanoic acid ((S)-2MeGABA) and 4-amino-(R)-2-methylbutanoic acid ((R)-2MeGABA) followed the same trend, with (S)-2MeGABA acting as a full agonist at the rho1 (EC(50) approximately 65 microM, I(max) approximately 100%), and rho2 (EC(50) approximately 20 microM, I(max) approximately 100%) receptor subtypes, and a partial agonist at rho3 (EC(50) approximately 25 microM, I(max) approximately 90%). (R)-2MeGABA, however, was a moderately potent antagonist at all three receptor subtypes (IC(50) approximately 16 microM at rho1, 125 microM at rho2 and 35 microM at rho3). On the basis of these expanded biological activity data and the solution-phase molecular structures obtained at the MP2/6-31+G* level of ab initio theory, a rationale is proposed for the genesis of this stereoselectivity effect.

Interactions between ρ and γ2 subunits of the GABA receptor

The gamma-aminobutyric acid type C (GABA(C)) receptor is a ligand-gated chloride channel with distinct physiological and pharmacological properties. Although the exact subunit composition of native GABA(C) receptors has yet to be firmly established, there is general agreement that GABA rho subunits participate in their formation. Recent studies on white perch suggest that certain GABA rho subunits can co-assemble with the GABA(A) receptor gamma2 subunit to form a heteromeric receptor with electrophysiological properties that correspond more closely to the native GABA(C) receptor on retinal neurons than any of the homomeric rho receptors. In the present study we examined the interactions among various perch GABA rho and gamma2 subunits. When co-expressed in Xenopus oocytes, the gamma2 subunit co-immunoprecipitated with Flag-tagged perch rho1A, rho1B, and rho2B subunits, but not with the Flag-tagged perch rho2A subunit. Immunocytochemical studies indicated that the membrane surface expression of the gamma2 subunit was detected only when it was co-expressed with perch rho1A, rho1B, or rho2B subunit, but not with the perch rho2A subunit or when expressed alone. In addition, co-immunoprecipitation of perch rho1B and gamma2 subunits was also detected in protein samples of the teleost retina. Taken together, these findings suggest that a heteromeric rho(gamma2) receptor could represent one form of GABA(C) receptor on retinal neurons.

Functional characterization of rat ρ2 subunits expressed in HEK 293 cells

GABA(C) receptors are thought to be homo- or heteropentamers composed of rho1, rho2 and rho3 subunits. Previous work on rat rho2 subunits expressed in Xenopus oocytes has suggested that they do not form functional homo-oligomeric GABA(C) receptors, but do combine with rho1 or rho3 subunits to form hetero-oligomers. These findings are difficult to interpret because both human and mouse rho2 subunits do form functional homo-oligomeric receptors. Also, many regions of the rat brain express solely rho2 subunit transcripts which, according to presently available evidence, would not result in expression of functional GABA(C) receptors. We show here that homomeric rat rho2 receptors can be expressed in HEK 293 cells. Homo-oligomeric rat rho2 receptors expressed in mammalian cells matured slowly and displayed small but detectable GABA-induced currents with slow kinetics. Rat rho2 receptors also had a decreased sensitivity to picrotoxin and a marked sensitivity to the GABA(C) receptor agonist cis-aminocrotonic acid. Our results demonstrate for the first time the expression of functional homomeric rat rho2 receptors, and suggest that rho(2) subunits may contribute to brain function, including in areas not expressing other rho subunits.

Charged Residues at the 2′ Position of Human GABAC ρ1 Receptors Invert Ion Selectivity and Influence Open State Probability

The ability of members of the nicotinicoid superfamily of ligand-gated ion channels to selectively conduct anions or cations is critical to their function within the central nervous system. Recent work has demonstrated that residues at the intracellular end of the second transmembrane domain, between the -3' and 2' positions, form the ion selectivity filter of these receptors. In this study, the proline residue at the 2' position (Pro-2') at the intracellular end of the second transmembrane domain of the gamma-aminobutyric acid type C rho 1 subunit was mutated to glutamate (rho 1P2'E) and arginine (rho 1P2'R). Dilution potential experiments indicated that the charge selectivity of the rho 1P2'E receptor channels had been inverted, with the channels now becoming predominantly cation selective, indicating the ability of negatively charged residues at this 2' position to control charge selectivity. The mutation was also seen to have significantly decreased agonist potency and intrinsic efficacy. In contrast, the rho 1P2'R receptor channels were anion-selective but were now found to be constitutively open with high holding currents (inhibited by low gamma-aminobutyric acid doses and the competitive antagonist, 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid alone) and increased agonist activity. Hill coefficients of both mutants were decreased, but competitive antagonist studies indicated that their binding sites were not significantly affected.

Analysis of the Set of GABAA Receptor Genes in the Human Genome

The genes of the ionotropic gamma-aminobutyric acid receptor (GABR) subunits have shown an unusual chromosomal clustering, but only now can this be fully specified by analyses of the human genome. We have characterized the genes encoding the 18 known human GABR subunits, plus one now located here, for their precise locations, sizes, and exon/intron structures. Clusters of 17 of the 19, distributed between five chromosomes, are specified in detail, and their possible significance is considered. By applying search algorithms designed to recognize sequences of all known GABR-type subunits in species from man down to nematodes, we found no new GABR subunit is detectable in the human genome. However, the sequence of the human orthologue of the rat GABR rho3 receptor subunit was uncovered by these algorithms, and its gene could be analyzed. Consistent with those search results, orthologues of the beta4 and gamma4 subunits from the chicken, not cloned from mammals, were not detectable in the human genome by specific searches for them. The relationships are consistent with the mammalian subunit being derived from the beta line and epsilon from the gamma line, with mammalian loss of beta4 and gamma4. In their structures the human GABR genes show a basic pattern of nine coding exons, with six different genomic mechanisms for the alternative splicing found in various subunits. Additional noncoding exons occur for certain subunits, which can be regulatory. A dicysteine loop and its exon show remarkable constancy between all GABR subunits and species, of deduced functional significance.

Single cell RT-PCR demonstrates differential expression of GABAC receptor rho subunits in rat hippocampal pyramidal and granule cells

Although gamma-aminobutyric acid (GABA)C receptor rho1, rho2 and rho3 subunits are reportedly expressed in pyramidal and granule cells in the hippocampus at various developmental stages, it is not clear whether these three rho subunits are coexpressed in a single neuron. To attempt to answer this question, we performed single-cell RT-PCR for rho subunits from neurons of rat brain hippocampus. In hippocampal cultures, pyramidal cells were positive for rho1 mRNA expression in 89%, rho2 in 94% and rho3 in 94%, while granule cells were positive for rho1 mRNA in only 6%, rho2 in 36% and rho3 in 91%. Intensive amplification of the RT-PCR products by the second PCR revealed that all the three rho subunits were coexpressed in a single pyramidal and granule cells from both of the cultures and the slices. These results suggest that all the three GABAC receptor rho1, rho2 and rho3 subunits are present probably in different compositions in pyramidal and granule cells in the rat hippocampus.

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.

Expression profiling with arrays of randomly disrupted genes in mouse embryonic stem cells leads to in vivo functional analysis

DNA arrays are capable of profiling the expression patterns of many genes in a single experiment. After finding a gene of interest in a DNA array, however, labor-intensive gene-targeting experiments sometimes must be performed for the in vivo analysis of the gene function. With random gene trapping, on the other hand, it is relatively easy to disrupt and retrieve hundreds of genes/gene candidates in mouse embryonic stem (ES) cells, but one could overlook potentially important gene-disruption events if only the nucleotide sequences and not the expression patterns of the trapped DNA segments are analyzed. To combine the benefits of the above two experimental systems, we first created approximately 900 genetrapped mouse ES cell clones and then constructed arrays of cDNAs derived from the disrupted genes. By using these arrays, we identified a novel gene predominantly expressed in the mouse brain, and the corresponding ES cell clone was used to produced mice homozygous for the disrupted allele of the gene. Detailed analysis of the knockout mice revealed that the gene trap vector completely abolished gene expression downstream of its integration site. Therefore, identification of a gene or novel gene candidate with an interesting expression pattern by using this type of DNA array immediately allows the production of knockout mice from an ES cell clone with a disrupted allele of the sequence of interest.

Function of gamma-aminobutyric acid receptor/channel rho 1 subunits in spinal cord

gamma-Aminobutyric acid (GABA) receptor/channel rho 1 subunits are important components in inhibitory pathways in the central nervous system. However, the precise locations and roles of these receptors in the central nervous system are unknown. We studied the expression localization of GABA receptor/channel rho 1 subunit in mouse spinal cord and dorsal root ganglia (DRG). The immunohistochemistry results indicated that GABA receptor/channel rho 1 subunits were expressed in mouse spinal cord superficial dorsal horn (lamina I and lamina II) and in DRG. To understand the functions of the GABA receptor/channel rho 1 subunit in these crucial sites of sensory transmission in vivo, we generated GABA receptor/channel rho 1 subunit mutant mice (rho 1-/-). GABA receptor/channel rho 1 subunit expression in the rho 1-/- mice was eliminated completely, whereas the gross neuroanatomical structures of the rho 1-/- mice spinal cord and DRG were unchanged. Electrophysiological recording showed that GABA-mediated spinal cord response was altered in the rho 1-/- mice. A decreased threshold for mechanical pain in the rho 1-/- mice compared with control mice was observed with the von Frey filament test. These findings indicate that the GABA receptor/channel rho 1 subunit plays an important role in modulating spinal cord pain transmission functions in vivo.

GABAC receptor agonist suppressed ammonia-induced apoptosis in cultured rat hippocampal neurons by restoring phosphorylated BAD level

Ammonia-induced apoptosis and its prevention by GABAC receptor stimulation were examined using primary cultured rat hippocampal neurons. Ammonia (0.5-5 mm NH4Cl) dose-dependently induced apoptosis in pyramidal cell-like neurons as assayed by double staining with Hoechst 33258 and anti-neurofilament antibody. A GABAC receptor agonist, cis-4-aminocrotonic acid (CACA, 200 microm), but not GABAA and GABAB receptor agonists, muscimol (10 micro m) and baclofen (50 microm), respectively, inhibited the ammonia (2 mm)-induced apoptosis, and this inhibition was abolished by a GABAC receptor antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA, 15 microm). Expression of all three GABAC receptor subunits was demonstrated in the cultured neurons by RT-PCR. The ammonia-treatment also activated caspases-3 and -9 as observed in immunocytochemistry for PARP p85 and western blot. Such activation of the caspases was again inhibited by CACA in a TPMPA-sensitive manner. The anti-apoptotic effect of CACA was blocked by inhibitors for MAP kinase kinase and cAMP-dependent protein kinase, PD98059 (20 microm) and KT5720 (1 microm), suggesting possible involvement of an upstream pro-apoptotic protein, BAD. Levels of phospho-BAD (Ser112 and Ser155) were decreased by the ammonia-treatment and restored by coadministration of CACA. These findings suggest that GABAC receptor stimulation protects hippocampal pyramidal neurons from ammonia-induced apoptosis by restoring Ser112- and Ser155-phospho-BAD levels.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Co-assembly of GABA rho subunits with the GABA(A) receptor gamma(2) subunit cloned from white perch retina

Although it is well established that GABA(C) receptors are comprised in part of GABA rho subunits, the exact subunit composition of neuronal GABA(C) receptors is yet to be determined. A detailed comparison of GABA(C)-mediated neuronal responses elicited from retinal neurons with those obtained from receptors formed by GABA rho subunits revealed a number of significant differences both in the kinetics and the pharmacology of the responses. Our previous studies indicated that the human GABA(A) receptor gamma(2) subunit could co-assemble with one (rho(1B)) of the white perch GABA rho subunits to form a heterooligomeric receptor with properties that resembled those of the GABA(C) receptors on white perch bipolar cells. In this study, we cloned the white perch gamma(2) subunit, and investigated its co-assembly with four white perch GABA rho subunits. When expressed in Xenopus oocytes, perch gamma(2) and rho(1B) subunits form heterooligomeric receptors with distinct properties: the GABA-elicited responses have fast kinetics and are sensitive to pentobarbital modulation. The enhancement of GABA-elicited responses by pentobarbital on the heterooligomeric receptors could be eliminated by a single mutation in the third transmembrane domain of the gamma(2) subunit, indicating that pentobarbital sensitivity is mediated by the incorporated gamma(2) subunit. On the other hand, co-expression of the perch gamma(2) subunit with the other perch GABA rho subunits produced no detectable changes in the kinetics of GABA-elicited response or the sensitivity to pentobarbital modulation. These results suggest that the gamma(2) subunit can co-assemble only with one (rho(1B)), but not with other white perch GABA rho subunits.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

gamma-Aminobutyric acidA rho receptor subunits in the developing rat hippocampus

The RT-PCR approach was used to estimate the expression of gamma-aminobutyric acid (GABA)(A) rho receptor subunits in the hippocampus of neonatal and adult rats. All three rho subunits were detected at postnatal day (P) 2, the rho3 subunit being expressed at an extremely low level. The rho1 and rho2 products appeared to be developmentally regulated; they were found to be more pronounced in adulthood. In another set of experiments, to correlate gene expression with receptor function, GABA(A) rho subunit mRNAs were detected with single-cell RT-PCR in CA3 pyramidal cells (from P3-P4 hippocampal slices), previously characterized with electrophysiological experiments for their bicuculline-sensitive or -insensitive responses to GABA. In 6 of 19 cells (31%), pressure application of GABA evoked at -70 mV inward currents that persisted in the presence of 100 microM bicuculline (314 plus minus 129 pA). RT-PCR performed in two of these neurons revealed the presence of rho1 and rho2 subunits, the latter being present with the alpha2 subunit. A rho2 subunit was also found in 1 neuron (among 9) exhibiting a response to GABA, which was completely abolished by bicuculline. This might be due to the lack of putative accessory GABA(A) subunits that can coassemble with rho2 to make functional receptors. Similar experiments from 10 P15 CA3 pyramidal cells failed to reveal any rho1-3 transcripts. However, these neurons abundantly express alpha3 subunits. It is likely that in CA3 pyramidal cells of neonatal and adult hippocampus GABA(A) rho subunits are present but at very low levels of expression.

Gene expression and localization of GABA(C) receptors in neurons of the rat gastrointestinal tract

The effects of GABA in the CNS are mediated by three different GABA receptors: GABA(A), GABA(B) and GABA(C) receptors. GABA(A) and GABA(B) receptors, but not yet GABA(C) receptors, have been demonstrated in the enteric nervous system, where GABA has been proposed to be a transmitter. The purpose of this study was to determine whether GABA(C) receptors are present and thus may play a role in mediating the effects of GABA in the myenteric plexus of the rat gastrointestinal tract. We examined the expression of the three known GABA(C) receptor subunits, rho1, rho2 and rho3, in the rat duodenum, ileum and colon using the reverse transcriptase-polymerase chain reaction. We determined the localization of GABA(C) receptors in the myenteric plexus of these regions using two different antisera directed against GABA(C) receptor subunits. The polymerase chain reaction revealed that all three subunits were expressed in the gastrointestinal tract. When the layers of the intestine were separated and the layer containing myenteric neurons was assayed, the rho3 subunit was found in the ileum and colon, whereas rho1 was expressed in the duodenum and weakly in the colon and rho2 was expressed in the ileum. Immunocytochemistry revealed numerous labeled neurons in the myenteric plexus of each region. Colocalization showed that a large proportion of calbindin plus calretinin immunoreactive neurons (intrinsic primary afferent neurons) were immunoreactive for the GABA(C) receptor, and that 56% of nitric oxide synthase immunoreactive neurons (inhibitory motor neurons) exhibited the receptor. These results indicate that GABA(C) receptors of differing subunit compositions are expressed by neurons in the rat gastrointestinal tract. The effects of GABA on intrinsic sensory and on inhibitory motor neurons are likely to be mediated in part through GABA(C) receptors.

trans-4-Amino-2-methylbut-2-enoic acid (2-MeTACA) and (+/-)-trans-2-aminomethylcyclopropanecarboxylic acid ((+/-)-TAMP) can differentiate rat rho3 from human rho1 and rho2 recombinant GABA(C) receptors

1. This study investigated the effects of a number of GABA analogues on rat rho3 GABA(C) receptors expressed in Xenopus oocytes using 2-electrode voltage clamp methods. 2. The potency order of agonists was muscimol (EC(50)=1.9 +/- 0.1 microM) (+)-trans-3-aminocyclopentanecarboxylic acids ((+)-TACP; EC(50)=2.7 +/- 0.9 microM) trans-4-aminocrotonic acid (TACA; EC(50)=3.8 +/-0.3 microM) GABA (EC(50)=4.0 +/- 0.3 microM) > thiomuscimol (EC(50)=24.8 +/- 2.6 microM) > (+/-)-cis-2-aminomethylcyclopropane-carboxylic acid ((+/-)-CAMP; EC(50)=52.6 +/-8.7 microM) > cis-4-aminocrotonic acid (CACA; EC(50)=139.4 +/- 5.2 microM). 3. The potency order of antagonists was (+/-)-trans-2-aminomethylcyclopropanecarboxylic acid ((+/-)-TAMP; K(B)=4.8+/-1.8 microM) (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA; K(B)=4.8 +/-0.8 microM) > (piperidin-4-yl)methylphosphinic acid (P4MPA; K(B)=10.2+/-2.3 microM) 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; K(B)=10.2+/-0.3 microM) imidazole-4-acetic acid (I4AA; K(B)=12.6+/-2.7 microM) > 3-aminopropylphosphonic acid (3-APA; K(B)=35.8+/-13.5 microM). 4. trans-4-Amino-2-methylbut-2-enoic acid (2-MeTACA; 300 microM) had no effect as an agonist or an antagonist indicating that the C2 methyl substituent is sterically interacting with the ligand-binding site of rat rho3 GABA(C) receptors. 5. 2-MeTACA affects rho1 and rho2 but not rho3 GABA(C) receptors. In contrast, (plus minus)-TAMP is a partial agonist at rho1 and rho2 GABA(C) receptors, while at rat rho3 GABA(C) receptors it is an antagonist. Thus, 2-MeTACA and (+/-)-TAMP could be important pharmacological tools because they may functionally differentiate between rho1, rho2 and rho3 GABA(C) receptors in vitro.

The effects of cyclopentane and cyclopentene analogues of GABA at recombinant GABA(C) receptors

The pharmacological effects of the enantiomers of cis-3-aminocyclopentanecarboxylic acids ((+)- and (-)-CACP), the enantiomers of trans-3-aminocyclopentanecarboxylic acids ((+)- and (-)-TACP), and the enantiomers of 4-aminocyclopent-1-ene-1-carboxylic acids ((+)- and (-)-4-ACPCA) were studied on human homomeric rho(1) and rho(2) GABA(C) receptors expressed in Xenopus oocytes using two-electrode voltage clamp methods. These compounds are conformationally restricted analogues of gamma-aminobutyric acid (GABA) held in a five-membered ring. (+)-TACP (EC(50) (rho(1))=2.7+/-0.2 microM; EC(50) (rho(2))=1.45+/-0.22 microM), (+)-CACP (EC(50) (rho(1))=26.1+/-1.1 microM; EC(50) (rho(2))=20.1+/-2.1 microM) and (-)-CACP (EC(50) (rho(1))=78.5+/-3.5 microM; EC(50) (rho(2))=63.8+/-23.3 microM) were moderately potent partial agonists at rho(1) and rho(2) GABA(C) receptors, while (-)-TACP (100 microM inhibited 56% and 62% of the current produced by 1 microM GABA at rho(1) and rho(2) receptors, respectively) was a weak partial agonist with low intrinsic activity at these receptors. In contrast, (+)-4-ACPCA (K(i) (rho(1))=6.0+/-0.1 microM; K(i) (rho(2))=4.7+/-0.3 microM) did not activate GABA(C) rho(1) and rho(2) receptors but potently inhibited the action of GABA at these receptors, while (-)-4-ACPCA had little effect as either an agonist or an antagonist. The affinity order at both GABA(C) rho(1) and rho(2) receptors was (+)-TACP>(+)-4-ACPCA >> (+)-CACP>(-)-CACP >> (-)-TACP >> (-)-4-ACPCA. This study shows that the cyclopentane and cyclopentene analogues of GABA affect GABA(C) receptors in a unique manner, defining a preferred stereochemical orientation of the amine and carboxylic acid groups when binding to GABA(C) receptors. This is exemplified by the partial agonist, (+)-TACP, and the antagonist, (+)-4-ACPCA.

Developmentally regulated expression of GABA receptor rho1 and rho2 subunits, L7 and cone-rod homeobox (CRX) genes in mouse retina

In this study, we compared the temporal expression pattern of four retinal genes; rho1 and rho2 that encode subunits of GABA(c) receptors, L7 that encodes Purkinje cell protein and CRX that encodes the cone-rod homeobox transcription factor. A reverse-transcription-polymerase chain reaction (RT-PCR) strategy that generated a linear correlation between the amount of retinal RNA and the amount of amplified product was used to quantify transcripts from each gene. Results with this method showed that the rho1 and L7 have similar developmental patterns. Both exhibit basal level expression before P7. From P7 to P20, the RNA levels for both genes were increased about 12-fold. After P20, the RNA levels remained unchanged. Compared to rho1 and L7, expression of rho2 began later, since the rho2 RNA could not be detected until P10. At P10, the rho2 RNA level was about 10% of its level at P35. Expression of rho2 reached its peak at a later developmental stage compared to that of rho1 and L7. The different temporal patterns were confirmed by co-amplification of rho1, rho2, and L7 in a single PCR tube. CRX RNA was detected at embryonic day 15 (E15) and increased progressively, in agreement with a prior study using in situ hybridization. These data, combined with evidence that the tissue distribution of rho1 and L7 RNA in the CNS are similar, indicates that rho1 and L7 may share common transcriptional regulatory elements. Furthermore, the difference in the timing of rho subunit expression suggests that the subunit composition of GABA(c) receptors vary during retinal development.

GABA(C) receptors: a molecular view

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacological and electrophysiological properties. The predominant type, termed GABA(A), and a recently identified GABA(C) type, form ligand-gated chloride channels, whereas GABA(B) receptors activate separate cation channels via G proteins. Based on their homology to nicotinic acetylcholine receptors, GABA(C) receptors are believed to be oligomeric protein complexes composed of five subunits in a pentameric arrangement. To date up to five different GABA(C) receptors subunits have been identified in various species. Recent studies have shed new light on the biological characteristics of GABA(C) receptors, including the chromosomal localization of its subunit genes and resulting links to deseases, the cloning of new splice variants, the identification of GABA(C) receptor-associated proteins, the identification of domains involved in subunit assembly, and finally structure/function studies examining functional consequences of introduced mutations. This review summarizes recent data in view of the molecular structure of GABA(C) receptors and presents new insights into the biological function of this protein in the retina.

Molecular determinants of desensitization and assembly of the chimeric GABA(A) receptor subunits (alpha1/gamma2) and (gamma2/alpha1) in combinations with beta2 and gamma2

Two gamma-aminobutyric acid(A) (GABA(A)) receptor chimeras were designed in order to elucidate the structural requirements for GABA(A) receptor desensitization and assembly. The (alpha1/gamma2) and (gamma2/alpha1) chimeric subunits representing the extracellular N-terminal domain of alpha1 or gamma2 and the remainder of the gamma2 or alpha1 subunits, respectively, were expressed with beta2 and beta2gamma2 in Spodoptera frugiperda (Sf-9) cells using the baculovirus expression system. The (alpha1/gamma2)beta2 and (alpha1/gamma2)beta2gamma2 but not the (gamma2/alpha1)beta2 and (gamma2/alpha1)beta2gamma2 subunit combinations formed functional receptor complexes as shown by whole-cell patch-clamp recordings and [3H]muscimol and [3H]flunitrazepam binding. Moreover, the surface immunofluorescence staining of Sf-9 cells expressing the (alpha1/gamma2)-containing receptors was pronounced, as opposed to the staining of the (gamma2/alpha1)-containing receptors, which was only slightly higher than background. To explain this, the (alpha1/gamma2) and (gamma2/alpha1) chimeras may act like alpha1 and gamma2 subunits, respectively, indicating that the extracellular N-terminal segment is important for assembly. However, the (alpha1/gamma2) chimeric subunit had characteristics different from the alpha1 subunit, since the (alpha1/gamma2) chimera gave rise to no desensitization after GABA stimulation in whole-cell patch-clamp recordings, which was independent of whether the chimera was expressed in combination with beta2 or beta2gamma2. Surprisingly, the (alpha1/gamma2)(gamma2/alpha1)beta2 subunit combination did desensitize, indicating that the C-terminal segment of the alpha1 subunit may be important for desensitization. Moreover, desensitization was observed for the (alpha1/gamma2)beta2gamma2 receptor with respect to the direct activation by pentobarbital. This suggests differences in the mechanism of channel activation for pentobarbital and GABA.

Immunocytochemical and electrophysiological characterization of GABA receptors in the frog and turtle retina

The expression of GABA receptors (GABARs) was studied in frog and turtle retinae. Using immunocytochemical methods, GABA(A)Rs and GABA(C)Rs were preferentially localized to the inner plexiform layer (IPL). Label in the IPL was punctate indicating a synaptic clustering of GABARs. Distinct, but weaker label was also present in the outer plexiform layer. GABA(A)R and GABA(C)R mediated effects were studied by recording electroretinograms (ERGs) and by the application of specific antagonists. Bicuculline, the GABA(A)R antagonist, produced a significant increase of the ERG. Picrotoxin, when co-applied with saturating doses of bicuculline, caused a further increase of the ERG due to blocking of GABA(C)Rs. The putative GABA(C)R antagonist Imidazole-4-acidic acid (I4AA) failed to antagonize GABA(C)R mediated inhibition and, in contrast, appeared rather as an agonist of GABARs.

Structure and function of GABA(C) receptors: a comparison of native versus recombinant receptors

In less than a decade our knowledge of the GABA(C) receptor, a new type of Cl(-)-permeable ionotropic GABA receptor, has greatly increased based on studies of both native and recombinant receptors. Careful comparison of properties of native and recombinant receptors has provided compelling evidence that GABA receptor rho-subunits are the major molecular components of GABA(C) receptors. Three distinct rho-subunits from various species have been cloned and the pattern of their expression in the retina, as well as in various brain regions, has been established. The pharmacological profile of GABA(C) receptors has been refined and more specific drugs have been developed. Molecular determinants that underlie functional properties of the receptors have been assigned to specific amino acid residues in rho-subunits. This information has helped determine the subunit composition of native receptors, as well as the molecular basis underlying subtle variations among GABA(C) receptors in different species. Finally, GABA(C) receptors play a unique functional role in retinal signal processing via three mechanisms: (1) slow activation; (2) segregation from other inhibitory receptors; and (3) contribution to multi-neuronal pathways.

Expression of gamma-aminobutyric acid rho 1 and rho 1 Delta 450 as gene fusions with the green fluorescent protein

The functional characteristics and cellular localization of the gamma aminobutyric acid (GABA) rho 1 receptor and its nonfunctional isoform rho 1 Delta 450 were investigated by expressing them as gene fusions with the enhanced version of the green fluorescent protein (GFP). Oocytes injected with rho 1-GFP had receptors that gated chloride channels when activated by GABA. The functional characteristics of these receptors were the same as for those of wild-type rho 1 receptors. Fluorescence, because of the chimeric receptors expressed, was over the whole oocyte but was more intense near the cell surface and more abundant in the animal hemisphere. Similar to the wild type, rho 1 Delta 450-GFP did not lead to the expression of functional GABA receptors, and injected oocytes failed to generate currents even after exposure to high concentrations of GABA. Nonetheless, the fluorescence displayed by oocytes expressing rho 1 Delta 450-GFP was distributed similarly to that of rho 1-GFP. Mammalian cells transfected with the rho 1-GFP or rho 1 Delta 450-GFP constructs showed mostly intracellularly distributed fluorescence in confocal microscope images. A sparse localization of fluorescence was observed in the plasma membrane regardless of the cell line used. We conclude that rho 1 Delta 450 is expressed and transported close to, and perhaps incorporated into, the plasma membrane. Thus, rho 1- and rho 1 Delta 450-GFP fusions provide a powerful tool to visualize the traffic of GABA type C receptors.

Expression of gamma-aminobutyric acid rho 1 and rho 1 Delta 450 as gene fusions with the green fluorescent protein

The functional characteristics and cellular localization of the gamma aminobutyric acid (GABA) rho 1 receptor and its nonfunctional isoform rho 1 Delta 450 were investigated by expressing them as gene fusions with the enhanced version of the green fluorescent protein (GFP). Oocytes injected with rho 1-GFP had receptors that gated chloride channels when activated by GABA. The functional characteristics of these receptors were the same as for those of wild-type rho 1 receptors. Fluorescence, because of the chimeric receptors expressed, was over the whole oocyte but was more intense near the cell surface and more abundant in the animal hemisphere. Similar to the wild type, rho 1 Delta 450-GFP did not lead to the expression of functional GABA receptors, and injected oocytes failed to generate currents even after exposure to high concentrations of GABA. Nonetheless, the fluorescence displayed by oocytes expressing rho 1 Delta 450-GFP was distributed similarly to that of rho 1-GFP. Mammalian cells transfected with the rho 1-GFP or rho 1 Delta 450-GFP constructs showed mostly intracellularly distributed fluorescence in confocal microscope images. A sparse localization of fluorescence was observed in the plasma membrane regardless of the cell line used. We conclude that rho 1 Delta 450 is expressed and transported close to, and perhaps incorporated into, the plasma membrane. Thus, rho 1- and rho 1 Delta 450-GFP fusions provide a powerful tool to visualize the traffic of GABA type C receptors.

(+)- and (-)-cis-2-aminomethylcyclopropanecarboxylic acids show opposite pharmacology at recombinant rho(1) and rho(2) GABA(C) receptors

The effects of the enantiomers of (+/-)-CAMP and (+/-)-TAMP [(+/-)-cis- and (+/-)-trans-2-aminomethylcyclopropanecarboxylic acids, respectively], which are cyclopropane analogues of GABA, were tested on GABA(A) and GABA(C) receptors expressed in Xenopus laevis oocytes using two-electrode voltage clamp methods. (+)-CAMP was found to be a potent and full agonist at homooligomeric GABA(C) receptors (K:(D) approximately 40 microM: and I:(max) approximately 100% at rho(1); K:(D) approximately 17 microM: and I:(max) approximately 100% at rho(2)) but a very weak antagonist at alpha(1)beta(2)gamma(2L) GABA(A) receptors. In contrast, (-)-CAMP was a very weak antagonist at both alpha(1)beta(2)gamma(2L) GABA(A) receptors and homooligomeric GABA(C) receptors (IC(50) approximately 900 microM: at rho(1) and approximately 400 microM: at rho(2)). Furthermore, (+)-CAMP appears to be a superior agonist to the widely used GABA(C) receptor partial agonist cis-4-aminocrotonic acid (K:(D) approximately 74 microM: and I:(max) approximately 78% at rho(1); K:(D) approximately 70 microM: and I:(max) approximately 82% at rho(2)). (-)-TAMP was the most potent of the cyclopropane analogues on GABA(C) receptors (K:(D) approximately 9 microM: and I:(max) approximately 40% at rho(1); K:(D) approximately 3 microM: and I:(max) approximately 50-60% at rho(2)), but it was also a moderately potent GABA(A) receptor partial agonist (K:(D) approximately 50-60 microM: and I:(max) approximately 50% at alpha(1)beta(2)gamma(2L) GABA(A) receptors). (+)-TAMP was a less potent partial agonist at GABA(C) receptors (K:(D) approximately 60 microM: and I:(max) approximately 40% at rho(1); K:(D) approximately 30 microM: and I:(max) approximately 60% at rho(2)) and a weak partial agonist at alpha(1)beta(2)gamma(2L) GABA(A) receptors (K:(D) approximately 500 micro: and I:(max) approximately 50%). None of the isomers of (+/-)-CAMP and (+/-)-TAMP displayed any interaction with GABA transport at the concentrations tested. Molecular modeling based on the present results provided new insights into the chiral preferences for either agonism or antagonism at GABA(C) receptors.

Evidence for coassembly of mutant GABAC rho1 with GABAA gamma2S, glycine alpha1 and glycine alpha2 receptor subunits in vitro

Functional coassembly of gamma-aminobutyric acid (GABA)C rho1 subunits with GABAA (alpha1, beta2, and gamma2S) or glycine (alpha1, alpha2, and beta) subunits was examined using two-electrode voltage-clamp recordings in the Xenopus laevis oocyte expression system. To facilitate this study, we took advantage of the unique gating and pharmacological properties of two mutant rho1 subunits, rho1(T314A) and rho1(T314A/L317A). When the rho1(T314A) subunit was coexpressed with GABA gamma2S, glycine alpha1 or glycine alpha2 subunits, GABA response properties were different from those of homomeric rho1(T314A) receptors. Additionally, the sensitivity of heteromeric rho1(T314A) and gamma2S receptors to picrotoxinin (PTX) blockade of GABA-evoked responses was altered compared to that of homomeric rho1(T314A) receptors. Changes in GABA response properties and picrotoxinin sensitivity were also observed when rho1(T314A) subunits were coexpressed with wild-type rho1 subunits. When rho1(T314A/L317A) subunits were coexpressed with GABA gamma2S, glycine alpha1 or glycine alpha2 subunits, suppression by GABA of spontaneously active current was reduced compared to that of homomeric rho1(T314A/L317A) receptors. Recovery of the spontaneous current from inhibition by GABA for GABA rho1(T314A/L317A)/gamma2S heteromeric receptors displayed an additional component. Coinjection of wild-type rho1 with gamma2S cRNAs at a ratio of 1 : 1 resulted in a > 10-fold reduction in GABA-evoked current. Furthermore, coexpression of wild-type rho1 and gamma2S subunits was found to shift the GABA dose-response curve. Our results provide functional evidence that the GABAC rho1 subunit can coassemble with the GABAA gamma2S subunit, and, at least in its mutated form, rho1 can also form heteromeric receptors with glycine alpha1 or alpha2 subunits in vitro.

Stable GABA(A) receptor intermediates in SF-9 cells expressing alpha1, beta2 and gamma2 subunits

Spodoptera frugiperda insect cells (Sf-9 cells) were used to study GABA(A) receptor assembly. Time courses of the expression level of alpha1beta2 and alpha1beta2gamma2 receptor protein showed [(3)H]muscimol binding to appear 2 hr before [(3)H]flunitrazepam and [(35)S]TBPS binding. This indicates that muscimol may bind to pentamers with an immature conformation or to molecules smaller than the pentamer. Binding studies performed on fractions from sucrose gradients loaded with solubilized alpha1beta2 or alpha1beta2gamma2 containing membranes revealed no binding other than to the pentameric fractions. Western blotting on fractionated sucrose gradients, however, clearly revealed the existence of GABA(A) receptor intermediates. The alpha1 subunit was seen in fractions corresponding to molecules smaller than the pentamer only when co-expressed with gamma2, indicating that the gamma2 subunit is needed for the alpha1 to form relatively long lasting intermediates. Moreover, Western blots revealed multiple isoforms for each subunit. In general, it was primarily the lower molecular weight forms that were detected in the pentameric fractions. The exception being for the alpha1 and gamma2 forms in subunit combinations that did not contain both of these subunits (i.e., alpha1, gamma2, alpha1beta2, beta2gamma2), where higher molecular weight forms were strongly represented. These findings show that alpha1 and gamma2 prefer specific protein forms when expressed together.

Permeability and single channel conductance of human homomeric rho1 GABAC receptors

1. Homomeric human rho1 GABAC receptors were expressed in Xenopus oocytes and in human embryonic kidney cells (HEK293) in order to examine their conductance and permeability. 2. Reversal potentials of currents elicited by gamma-aminobutyric acid (GABA) were measured in extracellular solutions of various ionic composition to determine relative permeability of homomeric rho1 receptors. The rank order of anionic permeability was: SCN- > I- > NO3- > Br- > Cl- > formate (For-) > HCO3- > acetate (Ac-) approximately proprionate (Prop-) approximately isethionate (Ise-) approximately F- approximately PO4-. 3. In the oocyte expression system, relative permeabilities to SCN-, I-, NO3-, Br- and HCO3- were higher for rho1 GABAC receptors than alpha1beta2gamma2L GABAA receptors. 4. Expression of rho1 GABAC receptors in Xenopus oocytes and in HEK293 cells gave similar relative permeabilities for selected anions, suggesting that the expression system does not significantly alter permeation properties. 5. The pore diameter of the homomeric rho1 GABAC receptor expressed in oocytes was estimated to be 0.61 nm, which is somewhat larger than the 0.56 nm pore diameter estimated for alpha1beta2gamma2L GABAA receptors. 6. Homomeric rho1 GABA receptors expressed in oocytes had a single channel chord conductance of 0.65 +/- 0.04 pS (mean +/- s.e.m.) when the internal chloride concentration ([Cl-]i) was 20 mM. With a [Cl-]i of 100 mM, the single channel chord conductance was 1.59 +/- 0.24 pS. 7. The mean open time directly measured from 43 GABA-induced channel openings in six patches was 3. 2 +/- 0.8 s. The mean open time in the presence of 100 microM picrotoxin was 0.07 +/- 0.01 s (77 openings from 3 patches). 8. The differences observed in ionic permeabilities, pore size, single channel conductance and mean open time suggest that the rho1 homomeric receptor may not be the native retinal GABAC receptor reported previously.

Identification of 70 amino acids important for GABA(C) receptor rho1 subunit assembly

gamma-Aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in the mammalian central nervous system and gates at least three subclasses of receptors, termed GABA(A), GABA(B) and GABA(C). Accumulating evidence indicates that GABA(C) receptors are composed exclusively of rho subunits. The N-terminal half of the rho subunits has been shown to mediate formation of homo- and heterooligomeric GABA(C) receptors. In this study, we searched for specific sequences within the N-terminus of the rho1 subunit involved in the assembly process. Assembly sequences were localized to a 128-amino acid region by deletion of progressively larger regions of a chimeric rho1beta1 subunit previously shown to disrupt rho1 and rho2 assembly. To confirm this observation, a series of GABA(A) receptor beta subunit chimeras containing different regions of the rho1 N-terminus were tested for interference with rho1 and rho2 subunit assembly into functional GABA receptors. Transfer of 70 residues within the 128 amino acid region to the beta1 subunit created a chimera that disrupted rho1, but not rho2, assembly into functional receptors. These observations refine the location of signals involved in rho1 subunit assembly, and suggest that different signals exist for the formation of rho1 homooligomeric and rho1/rho2 heterooligomeric GABA(C) receptors.

Widespread expression of GABA(A) receptor subunits in peripheral tissues

The receptor subtypes involved in the physiological and pharmacological actions of gamma-amino butyric acid (GABA) in peripheral and endocrine tissues are not clear. Information about the molecular characteristics of GABA(A) receptors in peripheral endocrine tissues is only available for the pancreas and the adrenal medulla. Using reverse transcription (RT) polymerase chain reaction (PCR), the widespread expression of GABA(A) receptors subunits in rat peripheral tissues, including adrenal, ovary, testis, placenta, uterus, and small intestine is shown. It is shown that GABA(A) receptor subunits are expressed in multiple endocrine tissues in a tissue specific manner. These results give an insight into the likely pharmacological properties of these GABA(A) receptors in these tissues. The gonadal endocrine tissues such as the placenta, ovary and the testis express greater range of GABA(A) receptor subunits relative to the adrenal gland. The tissues with greater smooth muscle content, the small intestine and the uterus also express a smaller range of subunits subtypes.

Genetic linkage and radiation hybrid mapping of the three human GABA(C) receptor rho subunit genes: GABRR1, GABRR2 and GABRR3

GABA(C) receptors mediate rapid inhibitory neurotransmission in retina. We have mapped, in detail, the human genes which encode the three polypeptides that comprise this receptor: rho1 (GABRR1), rho2 (GABRR2) and rho3 (GABRR3). We show that GABRR1 and GABRR2 are located close together, in a region of chromosome 6q that contains loci for inherited disorders of the eye, but that GABRR3 maps to chromosome 3q11-q13.3. Our mapping data suggest that the rho polypeptide genes, which are thought to share a common ancestor with GABA(A) receptor subunit genes, diverged at an early stage in the evolution of this gene family.

GABA(A) and GABA(C) receptors have contrasting effects on excitability in superior colliculus

We have recently found that GABA(C) receptor subunit transcripts are expressed in the superficial layers of rat superior colliculus (SC). In the present study we used immunocytochemistry to demonstrate the presence of GABA(C) receptors in rat SC at protein level. We also investigated in acute rat brain slices the effect of GABA(A) and GABA(C) receptor agonists and antagonists on stimulus-evoked extracellular field potentials in SC. Electrical stimulation of the SC optic layer induced a biphasic, early and late, potential in the adjacent superficial layer. The late component was completely inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione or CoCl(2), indicating that it was generated by postsynaptic activation. Muscimol, a potent GABA(A) and GABA(C) receptor agonist, strongly attenuated this postsynaptic potential at concentrations >10 microM. In contrast, the GABA(C) receptor agonist cis-aminocrotonic acid, as well as muscimol at lower concentrations (0.1-1 microM) increased the postsynaptic potential. This increase was blocked by (1,2,5, 6-tetrahydropyridine-4-yl)methylphosphinic acid, a novel competitive antagonist of GABA(C) receptors. Our findings demonstrate the presence of functional GABA(C) receptors in SC and suggest a disinhibitory role of these receptors in SC neuronal circuitry.

A single amino acid in the second transmembrane domain of GABA rho subunits is a determinant of the response kinetics of GABAC receptors

The rho subunits that constitute the gamma-aminobutyric acid (GABA)C receptors of retinal neurons form a unique subclass of ligand-gated chloride channels that give rise to sustained GABA-evoked currents that exhibit slow offset (deactivation) kinetics. We exploited this property to examine the molecular mechanisms that govern the disparate response kinetics and pharmacology of perch GABA rho1B and rho2A subunits expressed in Xenopus oocytes. Using a combination of domain swapping and site-directed mutagenesis, we identified the residues at amino acid position 320 in the second transmembrane domain as an important determinant of the receptor kinetics of GABAC receptors. When the site contains a proline residue, as in wild-type rho1 subunits, the receptor deactivates slowly; when serine occupies the site, as in wild-type rho2 subunits, the time course of deactivation is more rapid. In addition, we found that the same site also altered the pharmacology of GABA rho receptors, e.g., when the serine residue of the rho2A receptor was changed to proline, the response of the mutant receptor to imidazole-4-acetic acid (I4AA) mimicked that of the rho1B receptor. However, despite gross changes in receptor pharmacology, the apparent binding affinity for the drug was not significantly altered. These findings provide further evidence that the second transmembrane domain is involved in the gating mechanism that governs the response properties of the various rho receptor subunits. It is noteworthy that the proline residue in native rho1 subunits and the serine residue of rho2 subunits are well conserved in all species, a good indication that the presence of multiple GABA rho subunits serves to generate GABAC receptors that display the wide range of response kinetics observed on various types of retinal neurons.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.