Distribution of GABA receptor rho subunit transcripts in the rat brain

Action of the Photochrome Glyght on GABAergic Synaptic Transmission in Mouse Brain Slices

Glyght is a new photochromic compound described as an effective modulator of glycine receptors at heterologous expression, in brain slices and in zebrafish larvae. Glyght also caused weak inhibition of GABA_A-mediated currents in a cell line expressing α1/β2/γ2 GABA_A receptors. However, the effects of Glyght on GABAergic transmission in the brain have not been analysed, which does not allow a sufficiently comprehensive assessment of the effects of the compound on the nervous system. Therefore, in this study using whole-cell patch-clamp recording, we analysed the Glyght (100 µM) action on evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in mice hippocampal slices. Two populations of cells were found: the first responded by reducing the GABAergic eIPSCs’ amplitude, whereas the second showed no sensitivity to the compound. Glyght did not affect the ionic currents’ amplitude induced by GABA application, suggesting the absence of action on postsynaptic GABA receptors. Additionally, Glyght had no impact on the paired-pulse modulation of GABAergic eIPSCs, indicating that Glyght does not modulate the neurotransmitter release mechanisms. In the presence of strychnine, an antagonist of glycine receptors, the Glyght effect on GABAergic synaptic transmission was absent. Our results suggest that Glyght can modulate GABAergic synaptic transmission via action on extrasynaptic glycine receptors.

Crosstalk Between GABAergic Neurotransmission and Inflammatory Cascades in the Post-ischemic Brain: Relevance for Stroke Recovery

Adaptive plasticity processes are required involving neurons as well as non-neuronal cells to recover lost brain functions after an ischemic stroke. Recent studies show that gamma-Aminobutyric acid (GABA) has profound effects on glial and immune cell functions in addition to its inhibitory actions on neuronal circuits in the post-ischemic brain. Here, we provide an overview of how GABAergic neurotransmission changes during the first weeks after stroke and how GABA affects functions of astroglial and microglial cells as well as peripheral immune cell populations accumulating in the ischemic territory and brain regions remote to the lesion. Moreover, we will summarize recent studies providing data on the immunomodulatory actions of GABA of relevance for stroke recovery. Interestingly, the activation of GABA receptors on immune cells exerts a downregulation of detrimental anti-inflammatory cascades. Conversely, we will discuss studies addressing how specific inflammatory cascades affect GABAergic neurotransmission on the level of GABA receptor composition, GABA synthesis, and release. In particular, the chemokines CXCR4 and CX3CR1 pathways have been demonstrated to modulate receptor composition and synthesis. Together, the actual view on the interactions between GABAergic neurotransmission and inflammatory cascades points towards a specific crosstalk in the post-ischemic brain. Similar to what has been shown in experimental models, specific therapeutic modulation of GABAergic neurotransmission and inflammatory pathways may synergistically promote neuronal plasticity to enhance stroke recovery.

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].

Sex-specific alterations in GABA receptor-mediated responses in laterodorsal tegmentum are associated with prenatal exposure to nicotine

Smoking during pregnancy is associated with deleterious physiological and cognitive effects on the offspring, which are likely due to nicotine-induced alteration in the development of neurotransmitter systems. Prenatal nicotine exposure (PNE) in rodents is associated with changes in behaviors controlled in part by the pontine laterodorsal tegmentum (LDT), and LDT excitatory signaling is altered in a sex and age-dependent manner by PNE. As effects on GABAergic LDT signaling are unknown, we used calcium imaging to evaluate GABA_A receptor- (GABA_A R as well as GABA_A -_ρ R) and GABA_B receptor (GABA_B R)-mediated calcium responses in LDT brain slices from female and male PNE mice in two different age groups. Overall, in older PNE females, changes in calcium induced by stimulation of GABA_A R and GABA_B R, including GABA_A -_ρ R were shifted toward calcium rises. In both young and old males, PNE was associated with alterations in calcium mediated by all three receptors; however, the GABA_A R was the most affected. These results show for the first time that PNE is associated with alterations in GABAergic transmission in the LDT in a sex- and age-dependent manner, and these data are the first to show PNE-associated alterations in functionality of GABA receptors in any nucleus. PNE-associated alterations in LDT GABAergic transmission within the LDT would be expected to alter output to target regions and could play a role in LDT-implicated, negative behavioral outcomes following gestational exposure to smoking. Accordingly, our data provide further supportive evidence of the importance of eliminating the consumption of nicotine during pregnancy.

Analysis of Modulation of the ρ1 GABAA Receptor by Combinations of Inhibitory and Potentiating Neurosteroids Reveals Shared and Distinct Binding Sites

The ρ1 GABA_A receptor is prominently expressed in the retina and is present at lower levels in several brain regions and other tissues. Although the ρ1 receptor is insensitive to many anesthetic drugs that modulate the heteromeric GABA_A receptor, it maintains a rich and multifaceted steroid pharmacology. The receptor is negatively modulated by 5β-reduced steroids, sulfated or carboxylated steroids, and β-estradiol, whereas many 5α-reduced steroids potentiate the receptor. In this study, we analyzed modulation of the human ρ1 GABA_A receptor by several neurosteroids, individually and in combination, in the framework of the coagonist concerted transition model. Experiments involving coapplication of two or more steroids revealed that the receptor contains at least three classes of distinct, nonoverlapping sites for steroids, one each for the inhibitory steroids pregnanolone (3α5βP), 3α5βP sulfate, and β-estradiol. The site for 3α5βP can accommodate the potentiating steroid 5αTHDOC. The findings are discussed with respect to receptor modulation by combinations of endogenous neurosteroids. SIGNIFICANCE STATEMENT: The study describes modulation of the ρ1 GABA_A receptor by neurosteroids. The coagonist concerted transition model was used to determine overlap of binding sites for several inhibitory and potentiating steroids.

Human brain transcriptome analysis finds region- and subject-specific expression signatures of GABAAR subunits

Altered expression of GABA receptors (GABAARs) has been implicated in neurological and psychiatric disorders, but limited information about region-specific GABAAR subunit expression in healthy human brains, heteromeric assembly of major isoforms, and their collective organization across healthy individuals, are major roadblocks to understanding their role in non-physiological states. Here, by using microarray and RNA-Seq datasets-from single cell nuclei to global brain expression-from the Allen Institute, we find that transcriptional expression of GABAAR subunits is anatomically organized according to their neurodevelopmental origin. The data show a combination of complementary and mutually-exclusive expression patterns that delineate major isoforms, and which is highly stereotypical across brains from control donors. We summarize the region-specific signature of GABAR subunits per subject and its variability in a control population sample that can be used as a reference for remodeling changes during homeostatic rearrangements of GABAAR subunits after physiological, pharmacological or pathological challenges.

Enhanced GABAergic inhibition in the premammillary nucleus of photorefractory turkey hens via GABAA receptor upregulation

The premammillary nucleus (PMM) of the turkey mediobasal hypothalamus, where dopamine-melatonin (DA-Mel) neurons are localized, is a site for photoreception and photoperiodic time measurement, which is essential for the initiation of avian reproductive seasonality. In addition, this area could also be responsible for the onset and maintenance of photorefractoriness at the end of the breeding season due to the enhanced inhibitory effect of γ-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter in the central nervous system which interferes with the photosexual response in the turkey, a seasonally breeding bird. Here, we further characterized the GABAA receptor subunits in the PMM DA-Mel neurons related to reproductive seasonality and the onset of photorefractoriness. GABAA receptor subunits and GABA synthesis enzymes in the PMM of photosensitive and photorefractory turkey hens were identified using real-time qRT-PCR. The upregulation of GABAA receptor α1-3, β2-3, γ1-3, ρ1-3, δ, and θ mRNA expression were observed in the PMM of photorefractory birds when compared to those of photosensitive ones while there is no change observed in the GABA synthesis enzymes, glutamate decarboxylase 1 and 2. Those upregulated GABAA receptor subunits were further examined using immunohistochemical staining and they appeared to be co-localized within the PMM DA-Mel neurons. The upregulation of GABAA receptor subunits observed in the PMM of photorefractory birds coincides with a lack of responsiveness to a light stimulus provided during the photosensitive phase. This is supported by the absence of c-fos induction and TH upregulation in the PMM and a subsequence inhibition of c-fos and GnRH-I expression in the nucleus commissurae pallii. The augmented GABAA receptor subunits expression may mediate an enhancement of inhibitory GABAergic neurotransmission and the subsequent interference with the photosexual response. This could contribute to the state of photorefractoriness and the termination of breeding activities in the turkey, a temperate zone bird.

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.

Effects of electroacupuncture on the levels of retinal gamma-aminobutyric acid and its receptors in a guinea pig model of lens-induced myopia

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter of the retina and affects myopic development. Electroacupuncture (EA) is widely utilized to treat myopia in clinical settings. However, there are few reports on whether EA affects the level of retinal GABA during myopic development. To study this issue, in the present study, we explored the changes of retinal GABA content and the expression of its receptor subtypes, and the effects of EA stimulation on them in a guinea pig model with lens-induced myopia (LIM). Our results showed that the content of GABA and the expression of GABAA and GABAC receptors of retina were up-regulated during the development of myopia, and this up-regulation was inhibited by applying EA to Hegu (LI4) and Taiyang (EX-HN5) acupoints. Moreover, these effects of EA show a positional specificity. While applying EA at a sham acupoint, no apparent change of myopic retinal GABA and its receptor subtypes was observed. Taken together, our findings suggest that LIM is effective to up-regulate the level of retinal GABA, GABAA and GABAC receptors in guinea pigs and the effect may be inhibited by EA stimulation at LI4 and EX-HN5 acupoints.

An intracellular redox sensor for reactive oxygen species at the M3-M4 linker of GABAA ρ1 receptors

BACKGROUND AND PURPOSE

Reactive oxygen species (ROS) are normally involved in cell oxidative stress but also play a role as cellular messengers in redox signalling; for example, modulating the activity of neurotransmitter receptors and ion channels. However, the direct actions of ROS on GABAA receptors were not previously demonstrated. In the present work, we studied the effects of ROS on GABAA ρ1 receptor function.

EXPERIMENTAL APPROACH

GABAA ρ1 receptors were expressed in oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of ROS. Chemical protection of cysteines by selective sulfhydryl reagents and site-directed mutagenesis studies were used to identify protein residues involved in ROS actions.

KEY RESULTS

GABAA ρ1 receptor-mediated responses were significantly enhanced in a concentration-dependent and reversible manner by H₂O₂. Potentiating effects were attenuated by a free radical scavenger, lipoic acid or an inhibitor of the Fenton reaction, deferoxamine. Each ρ1 subunit contains only three cysteine residues, two extracellular at the Cys-loop (C¹⁷⁷ and C¹⁹¹) and one intracellular (C³⁶⁴) at the M3-M4 linker. Mutant GABAA ρ1 receptors in which C³⁶⁴ was exchanged by alanine were completely insensitive to modulation, implying that this site, rather than a cysteine in the Cys-loop, is essential for ROS modulation.

CONCLUSION AND IMPLICATIONS

Our results show that the function of GABAA ρ1 receptors is enhanced by ROS and that the intracellular C³⁶⁴ is the sensor for ROS actions.

Modulation of the human ρ1 GABAA receptor by inhibitory steroids

RATIONALE

Modulators of the ρ1 GABAA receptor may be useful in the treatment of visual, sleep, and cognitive disorders. Neuroactive steroids and analogues have been shown to modulate ρ1 receptor function, but the molecular mechanisms are poorly understood.

OBJECTIVES

We employed electrophysiology and voltage-clamp fluorometry to compare the actions of several neuroactive steroids and analogues on the human ρ1 GABAA receptor.

RESULTS

Results confirmed that P294S and T298F mutations affect modulation by steroids. The P294S mutation abolished inhibition by (3α,5β)-3-hydroxypregnan-20-one (3α5βP) while the T298F mutation eliminated inhibition by 17β-estradiol. Voltage-clamp fluorometry demonstrated that steroids differing in the presence of a charged group on C3 or nature of substituent on C17 uniquely modified fluorescence changes elicited by GABA in the extracellular domain. The I307Q mutation reversed the inhibitory effect of 3α5βP but was without effect on modulation by (3α,5β)-3-hydroxypregnan-20-one sulfate or 17β-estradiol. The effect of 3α5βP on the fluorescence change generated at Y241C was dependent on whether the steroid acted as an inhibitor or a potentiator. Further, the effect was limited to uncharged 5β-reduced steroids containing an acetyl group on C17.

CONCLUSIONS

The data demonstrate that steroids and analogues differ with respect to conformational changes elicited by these drugs as well as sensitivity to the effects of mutations. Steroids and analogues could be provisionally divided into three major groups based on their actions on the ρ1 GABAA receptor: 5β-reduced uncharged steroids, sulfated and carboxylated steroids, and 17β-estradiol. Further division among 5β-reduced uncharged steroids was based on substituent at position C17.

Nitric oxide potentiation of the homomeric ρ1 GABA(C) receptor function

BACKGROUND AND PURPOSE

NO is a highly diffusible and reactive gas produced in the nervous system, which acts as a neuronal signal mediating physiological or pathological mechanisms. NO can modulate the activity of neurotransmitter receptors and ion channels, including NMDA and GABA(A) receptors. In the present work, we examined whether GABA(C) receptor function can also be regulated by NO.

EXPERIMENTAL APPROACH

Homomeric ρ1 GABA(C) receptors were expressed in oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of the NO donor DEA. Chemical protection of cysteines by selective sulfhydryl reagents and site-directed mutagenesis were used to determine the protein residues involved in the actions of NO.

KEY RESULTS

GABAρ1 receptor responses were significantly enhanced in a dose-dependent, fast and reversible manner by DEA and the specific NO scavenger CPTIO prevented these potentiating effects. The ρ1 subunits contain only three cysteine residues, two extracellular at the Cys-loop (C177 and C191) and one intracellular (C364). Mutations of C177 and C191 render the ρ1 GABA receptors non-functional, but C364 can be safely exchanged by alanine (C364A). NEM, N-ethyl maleimide and (2-aminoethyl) methanethiosulfonate prevented the effects of DEA on GABAρ1 receptors. Meanwhile, the potentiating effects of DEA on mutant GABAρ1(C364A) receptors were similar to those observed on wild-type receptors.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the function of GABA(C) receptors can be enhanced by NO acting at the extracellular Cys-loop.

Identification of the minimal promoter for specific expression of the GABAρ1 receptor in retinal bipolar cells

γ-aminobutyric acid (GABA)ρ receptors regulate rapid synaptic ion currents in the axon end of retinal ON bipolar neurons, acting as a point of control along the visual pathway. In the GABAρ1 subunit knock out mouse, inhibition mediated by this receptor is totally eliminated, showing its role in neural transmission in retina. GABAρ1 mRNA is expressed in mouse retina after post-natal day 7, but little is known about its transcriptional regulation. To identify the GABAρ1 promoter, in silico analyses were performed and indicated that a 0.290-kb fragment, flanking the 5'-end of the GABAρ1 gene, includes putative transcription factor-binding sites, two Inr elements, and lacks a TATA-box. A rapid amplification of cDNA ends (RACE) assay showed three transcription start sites (TSS) clustered in the first exon. Luciferase reporter assays indicated that a 0.232-kb fragment upstream from the ATG is the minimal promoter in transfected cell lines and in vitro electroporated retinae. The second Inr and AP1 site are important to activate transcription in secretin tumor cells (STC-1) and retina. Finally, the 0.232-kb fragment drives green fluorescent protein (GFP) expression to the inner nuclear layer, where bipolar cells are present. This first work paves the way for further studies of molecular elements that control GABAρ1 transcription and regulate its expression during retinal development.

Receptor targets of amacrine cells

Amacrine cells are a morphologically and functionally diverse group of inhibitory interneurons. Morphologically, they have been divided into approximately 30 types. Although this diversity is probably important to the fine structure and function of the retinal circuit, the amacrine cells have been more generally divided into two subclasses. Glycinergic narrow-field amacrine cells have dendrites that ramify close to their somas, cross the sublaminae of the inner plexiform layer, and create cross talk between its parallel ON and OFF pathways. GABAergic wide-field amacrine cells have dendrites that stretch long distances from their soma but ramify narrowly within an inner plexiform layer sublamina. These wide-field cells are thought to mediate inhibition within a sublamina and thus within the ON or OFF pathway. The postsynaptic targets of all amacrine cell types include bipolar, ganglion, and other amacrine cells. Almost all amacrine cells use GABA or glycine as their primary neurotransmitter, and their postsynaptic receptor targets include the most common GABA(A), GABA(C), and glycine subunit receptor configurations. This review addresses the diversity of amacrine cells, the postsynaptic receptors on their target cells in the inner plexiform layer of the retina, and some of the inhibitory mechanisms that arise as a result. When possible, the effects of GABAergic and glycinergic inputs on the visually evoked responses of their postsynaptic targets are discussed.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Expression of GABAρ receptors in the neostriatum: localization in aspiny, medium spiny neurons and GFAP-positive cells

GABAergic transmission in the neostriatum plays a central role in motor coordination, in which a plethora of GABA-A receptor subunits combine to modulate neural inhibition. GABAρ receptors were originally described in the mammalian retina. These receptors possess special electrophysiological and pharmacological properties, forming a characteristic class of ionotropic receptors. In previous studies, we suggested that GABAρ receptors are expressed in the neostriatum, and in this report we show that they are indeed present in all the calretinin-positive interneurons of the neostriatum. In addition, they are located in calbindin-positive interneurons and projection neurons that express the dopamine D(2) receptor. GABAρ receptors were also located in 30% of the glial fibrillary acidic protein-positive cells, and may therefore also contribute to gliotransmission. Quantitative reverse transcription-PCR suggested that the mRNAs of this receptor do not express as much as in the retina, and that GABAρ2 is more abundant than GABAρ1. Electrophysiological recordings in brain slices provided evidence of neurons expressing a cis-4-aminocrotonic acid-activated, 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid-sensitive ionotropic GABA receptor, indicating the presence of functional GABAρ receptors in the neostriatum. Finally, electron-microscopy and immunogold located the receptors mainly in perisynaptic as well as in extrasynaptic sites. All these observations reinforce the importance of GABAρ receptors in the neostriatum and contribute to the diversity of inhibitory regulation in this area.

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.

The GABA(A)ρ receptors in hippocampal spontaneous activity and their distribution in hippocampus, amygdala and visual cortex

A bicuculline-resistant and TPMPA-sensitive GABAergic component was identified in hippocampal neurons in culture and in acute isolated brain slices. In both preparations, total GABAergic activity showed two inactivation kinetics: fast and slow. RT-PCR, in situ hybridization (ISH) and immunohistochemistry detected expression of GABAρ subunits. Immunogold and electron microscopy indicated that the receptors are mostly extrasynaptic. In addition, by RT-PCR and immunofluorescence we found GABAρ present in amygdala and visual cortex.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.

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.

Expression of GABAergic receptors in mouse taste receptor cells

Background

Multiple excitatory neurotransmitters have been identified in the mammalian taste transduction, with few studies focused on inhibitory neurotransmitters. Since the synthetic enzyme glutamate decarboxylase (GAD) for gamma-aminobutyric acid (GABA) is expressed in a subset of mouse taste cells, we hypothesized that other components of the GABA signaling pathway are likely expressed in this system. GABA signaling is initiated by the activation of either ionotropic receptors (GABA_A and GABA_C) or metabotropic receptors (GABA_B) while it is terminated by the re-uptake of GABA through transporters (GATs).

Methodology/Principal Findings

Using reverse transcriptase-PCR (RT-PCR) analysis, we investigated the expression of different GABA signaling molecules in the mouse taste system. Taste receptor cells (TRCs) in the circumvallate papillae express multiple subunits of the GABA_A and GABA_B receptors as well as multiple GATs. Immunocytochemical analyses examined the distribution of the GABA machinery in the circumvallate papillae. Both GABA_A-and GABA_B- immunoreactivity were detected in the peripheral taste receptor cells. We also used transgenic mice that express green fluorescent protein (GFP) in either the Type II taste cells, which can respond to bitter, sweet or umami taste stimuli, or in the Type III GAD67 expressing taste cells. Thus, we were able to identify that GABAergic receptors are expressed in some Type II and Type III taste cells. Mouse GAT4 labeling was concentrated in the cells surrounding the taste buds with a few positively labeled TRCs at the margins of the taste buds.

Conclusions/Significance

The presence of GABAergic receptors localized on Type II and Type III taste cells suggests that GABA is likely modulating evoked taste responses in the mouse taste bud.

GABAergic actions on cholinergic laterodorsal tegmental neurons: implications for control of behavioral state

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) play a critical role in regulation of behavioral state. Therefore, elucidation of mechanisms that control their activity is vital for understanding of how switching between wakefulness, sleep and anesthetic states is effectuated. In vivo studies suggest that GABAergic mechanisms within the pons play a critical role in behavioral state switching. However, the postsynaptic, electrophysiological actions of GABA on LDT neurons, as well as the identity of GABA receptors present in the LDT mediating these actions is virtually unexplored. Therefore, we studied the actions of GABA agonists and antagonists on cholinergic LDT cells by performing patch clamp recordings in mouse brain slices. Under conditions where detection of Cl(-) -mediated events was optimized, GABA induced gabazine (GZ)-sensitive inward currents in the majority of LDT neurons. Post-synaptic location of GABA(A) receptors was demonstrated by persistence of muscimol-induced inward currents in TTX and low Ca(2+) solutions. THIP, a selective GABA(A) receptor agonist with a preference for δ-subunit containing GABA(A) receptors, induced inward currents, suggesting the existence of extrasynaptic GABA(A) receptors. LDT cells also possess GABA(B) receptors as baclofen-activated a TTX- and low Ca(2+)-resistant outward current that was attenuated by the GABA(B) antagonists CGP 55845 and saclofen. The tertiapin sensitivity of baclofen-induced outward currents suggests that a G(IRK) mediated this effect. Further, outward currents were never additive with those induced by application of carbachol, suggesting that they were mediated by activation of GABA(B) receptors linked to the same G(IRK) activated in these cells by muscarinic receptor stimulation. Activation of GABA(B) receptors inhibited Ca(2+) increases induced by a depolarizing voltage step shown previously to activate VOCCs in cholinergic LDT neurons. Baclofen-mediated reductions in depolarization-induced Ca(2+) were unaltered by prior emptying of intracellular Ca(2+) stores, but were abolished by low extracellular Ca(2+) and pre-application of nifedipine, indicating that activation of GABA(B) receptors inhibits influx of Ca(2+) involving L-type Ca(2+) channels. Presence of GABA(C) receptors is suggested by the induction of inward current by (E)-4- amino-2-butenoic acid (TACA) and its inhibition by 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic (TPMPA), a relatively selective agonist and antagonist, respectively, of GABA(C) receptors. All of these GABA-mediated actions were found to occur in histochemically-identified cholinergic neurons. Taken together, these data indicate for the first time that cholinergic neurons of the LDT exhibit functional GABA(A, B and C) receptors, including extrasynaptically located GABA(A) receptors, which may be tonically activated by synaptic overflow of GABA. Accordingly, the activity of cholinergic LDT neurons is likely to be significantly affected by GABAergic tone within the nucleus, and so, demonstrated effects of GABA on behavioral state may be mediated, in part, via direct actions on cholinergic neurons in the LDT.

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

Changes in NMDA receptor contribution to synaptic transmission in the brain in a rat model of glaucoma

In the age-related, blinding disease glaucoma, retinal ganglion cells (RGCs) degenerate, possibly affecting glutamatergic retinofugal transmission to the brain. The superior colliculus (SC) is a major central target of retinofugal axons in the rodent, a much used disease model. We investigated the contribution of NMDA-type glutamate receptors to retinocollicular transmission in a rat glaucoma model, using a SC brain slice preparation to determine the sensitivity of synaptic responses to the NMDAR antagonist D-AP5. At 32 weeks after induction of experimental glaucoma, but not earlier, there was an increase in NMDAR contribution to SC synaptic responses in slices receiving input from glaucomatous eyes. This suggests that there are changes in NMDAR function after RGC degeneration in experimental glaucoma, which may represent functional SC compensation through plasticity via NMDARs. This has implications for studies carried out using rodent glaucoma models, especially those evaluating potential treatment strategies, as it suggests that functional changes in the central visual system need to be considered in addition to those in the eye. Furthermore, the data underline the need for early therapeutic intervention in order to pre-empt subsequent central functional changes.

The role of Loop F in the activation of the GABA receptor

Functional studies of the ligand gated ion channel family (nicotinic acetylcholine, serotonin Type 3, glycine and GABA receptors) along with the crystal structure of the acetylcholine binding protein (AChBP) and molecular dynamics simulations of the nAChR structure have resulted in a structural model in which the agonist-binding pocket comprises six loops (A-F) contributed by adjacent subunits. It is presumed that the binding of agonist results in a local structural rearrangement that is then transduced to the gate, causing the pore to open. Efforts are underway to better define the specific roles of the six binding loops. Several studies have suggested Loop F may play a direct role in linking the structural rearrangement within the binding pocket to the gate, although other investigations have indicated Loop F may be crucial for locking the agonist molecule into the binding site. This review will focus on the controversy surrounding the role of Loop F during GABA receptor activation.

Regulation of GABA(A) receptor subunit expression by pharmacological agents

The gamma-aminobutyric acid (GABA) type A receptor system, the main fast-acting inhibitory neurotransmitter system in the brain, is the pharmacological target for many drugs used clinically to treat, for example, anxiety disorders and epilepsy, and to induce and maintain sedation, sleep, and anesthesia. These drugs facilitate the function of pentameric GABA(A) receptors that exhibit widespread expression in all brain regions and large structural and pharmacological heterogeneity as a result of composition from a repertoire of 19 subunit variants. One of the main problems in clinical use of GABA(A) receptor agonists is the development of tolerance. Most drugs, in long-term use and during withdrawal, have been associated with important modulations of the receptor subunit expression in brain-region-specific manner, participating in the mechanisms of tolerance and dependence. In most cases, the molecular mechanisms of regulation of subunit expression are poorly known, partly as a result of neurobiological adaptation to altered neuronal function. More knowledge has been obtained on the mechanisms of GABA(A) receptor trafficking and cell surface expression and the processes that may contribute to tolerance, although their possible pharmacological regulation is not known. Drug development for neuropsychiatric disorders, including epilepsy, alcoholism, schizophrenia, and anxiety, has been ongoing for several years. One key step to extend drug development related to GABA(A) receptors is likely to require deeper understanding of the adaptational mechanisms of neurons, receptors themselves with interacting proteins, and finally receptor subunits during drug action and in neuropsychiatric disease processes.

Contribution of GABA(C) receptors to inhibition in the rodent accessory optic system

The medial terminal nucleus (MTN) of the mammalian accessory optic system controls vertical compensatory eye movements. It consists of two neuronal populations which respond best either to upward or to downward visual image shifts. The two cell classes are located spatially separate in the dorsal or in the ventral subdivision of the MTN, respectively. Pronounced GABAergic pathways have been described to exist between neurons in the two MTN subdivisions indicating that inhibitory interactions play a significant role for the generation of MTN cell response properties. Yet, the types of GABA receptors which mediate these inhibitory interactions are unknown. Functionally, it is of particular interest to know whether GABA(C) receptors, as in other subcortical visual centers, participate in inhibitory mechanisms in MTN neurons. We therefore performed whole-cell patch clamp recordings from MTN neurons in acute mouse midbrain slices. We monitored excitatory and inhibitory postsynaptic responses to afferent stimulation and applied specific GABA receptor agonists and antagonists to identify the GABA receptor types present in MTN neurons. We found that more than 80% of the neurons in both MTN subdivisions express functional GABA(C) receptors that can be activated by specific receptor agonists. A blockade of GABA(C) receptors, on the other hand, either reduced or enhanced postsynaptic inhibition, indicating that both postsynaptic and presynaptic functions are served by this receptor type. This, together with earlier results, suggests that GABA(C) receptors play a general role for the control of neuronal excitability in subcortical visual pathways.

GABAC receptor-mediated inhibition is altered but not eliminated in the superior colliculus of GABAC rho1 knockout mice

GABA(C) receptors (GABA(C)Rs) are widely expressed in the mammalian subcortical visual system, particularly in the retina and superior colliculus (SC). GABA(C)Rs are composed of specific rho1-3 subunits the expression of which varies among visual structures. Thus rho1 subunits are most abundant in retina, and their loss eliminates GABA(C)R expression and function. In the SC, rho2 subunit expression may be equal to or stronger than rho1 subunit expression; however, results across studies vary considerably. To more directly assess the expression of GABA(C)R subunits, we characterized inhibition in the SC of wild-type (WT) and GABA(C) rho1 Null mice that lack expression of GABA(C) rho1 subunits. We used whole cell patch-clamp recordings and evaluated GABA(C)R-mediated modulation of electrically evoked post synaptic currents using either agonists or antagonists in WT mice. In GABA(C) rho1 Null stratum griseum superficiale (SGS) cells, inhibitory postsynaptic currents were shorter in duration and their excitatory postsynaptic currents (EPSCs) were longer, indicating that a slow GABA(C)R-mediated inhibitory component was reduced in each case. In contrast to retina, GABA(C)R-mediated currents in the SC were altered but not eliminated in GABA(C) rho1 Null mice. In the majority of SC cells in GABA(C) rho1 Null mice, GABA(C)R activation could still be induced to alter EPSC peak amplitudes in putative interneurons and in many projection neurons. These results, compared with previously published data, indicate a fundamental difference between retina and SC in the control of GABA(C)R expression and subunit composition.

GABA A receptors: subtypes provide diversity of function and pharmacology

This mini-review attempts to update experimental evidence on the existence of GABA(A) receptor pharmacological subtypes and to produce a list of those native receptors that exist. GABA(A) receptors are chloride channels that mediate inhibitory neurotransmission. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. They are assembled from a family of 19 homologous subunit gene products and form numerous receptor subtypes with properties that depend upon subunit composition, mostly hetero-oligomeric. These vary in their regulation and developmental expression, and importantly, in brain regional, cellular, and subcellular localization, and thus their role in brain circuits and behaviors. We propose several criteria for including a receptor hetero-oligomeric subtype candidate on a list of native subtypes, and a working GABA(A) receptor list. These criteria can be applied to all the members of the LGIC superfamily. The list is divided into three categories of native receptor subtypes: "Identified", "Existence with High Probability", and "Tentative", and currently includes 26 members, but will undoubtedly grow, with future information. This list was first presented by Olsen & Sieghart (in press).

Inhibitory interaction between P2X4 and GABA(C) rho1 receptors

Reciprocal functional inhibition between P2X and GABA(A/C) receptors represents a novel mechanism fine-tuning neuronal excitability. However, the participating receptors and underlying mechanisms are not fully understood. P2X(4) receptor is widely found in neurons that express GABA(C) rho1 receptor. Thus, we co-expressed P2X(4) and rho1 receptors in HEK293 cells and, using patch-clamp recording, examined whether they have mutual functional inhibition. Currents evoked by simultaneous application of ATP and GABA (I(ATP+GABA)) were significantly smaller compared to the addition of I(ATP) and I(GABA). Furthermore, I(ATP) were strongly suppressed during rho1 receptor activation. Similarly, I(GABA) were greatly attenuated during P2X(4) receptor activation. Such mutual inhibition was absent in cells only expressing P2X(4) or rho1 receptor. Taken together, these functional data support negative cross-talk between P2X(4) and rho1 receptors.

Cyclothiazide: a subunit-specific inhibitor of GABAC receptors

We tested the effects of cyclothiazide (CTZ), an agent used to block desensitization of AMPA-type glutamate receptors, on heterologously expressed GABA(C) receptors formed by homomeric rho subunits. CTZ inhibition of GABA(C) receptors was subunit specific; it produced a dose-dependent reduction of the GABA-elicited current on homomeric rho2 receptors with an IC(50) of about 12 microm, but had no significant effect on homomeric rho1 receptors. This differential sensitivity was attributable to a single amino acid located on the second transmembrane domain of the rho subunits. Mutating the residue at this position from serine to proline on the rho2 subunit eliminated CTZ sensitivity, whereas switching proline to serine on the rho1 subunit made the receptor CTZ sensitive. The inhibitory properties of CTZ were consistent with its action as a channel blocker on the receptors formed by rho2 subunits. The effect showed a small degree of voltage dependence, and was due mainly to a non-competitive mechanism that reduced the maximum response elicited by GABA. In addition, the prominent membrane current rebound when co-application of GABA and CTZ was terminated suggests that the binding site for CTZ on the GABA(C) receptor is distinct from that for GABA, and that CTZ acts as a non-competitive antagonist on the GABA(C) receptor. CTZ inhibited the open channel of the GABA(C) receptor with a time constant of about 0.4 s, but the kinetics were approximately 10-fold slower when GABA is absent. The ability of CTZ to interact with various types of neurotransmitter receptors indicates that the drug has multiple actions in the CNS.

GABAC receptor subunit mRNA expression in the rat superior colliculus is regulated by calcium channels, neurotrophins, and GABAC receptor activity

The distribution of mRNA for the rho2 subunit of the GABA(C) receptor is much broader in organotypic SC cultures than in vivo, suggesting that GABA(C) receptor expression is regulated by environmental factors. Electrophysiological recordings indicate that neurons in SC cultures have functional GABA(C) receptors, although these receptors exhibited smaller conductance than in vivo, probably due to increased rho2 subunit expression. Adding cortical input, treatment with various neuromodulators, and blocking neuronal activity with TTX failed to affect the expression of rho2 subunits. Electrophysiological recordings revealed the presence of spontaneous Ca(2+) currents in SC cultures and preventing these, by treatment with blockers of L-type Ca(2+) channels, caused rho2 mRNA expression to decline to in vivo levels. In contrast, rho1 subunit mRNA levels remained unchanged, indicating that the two subunits are independently regulated. Surprisingly, both tonic activation and blockade of GABA(C) receptors upregulated rho1/rho2 mRNA expression. Further, NGF and BDNF promoted such expression during an early postnatal time window. In vivo, expression of the rho2 mRNA in the SC, and the rho2/rho3 mRNA in the retina increased with age. Expression of the rho2 mRNA in the visual cortex, and the rho1 mRNA in the retina and SC was constant. Subunit mRNA expression was similar in dark-reared animals, indicating that visual experience has no influence. These experiments suggest that GABA(C) receptor expression in the SC is regulated during postnatal development. While visual experience seems to have no influence on GABA(C) receptor subunits, spontaneous calcium currents selectively promote rho2 expression and both rho1 and rho2 are autoregulated both by GABA(C) receptor activity and by neurotrophic factors.

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.

A reciprocal connection between the ventral lateral geniculate nucleus and the pretectal nuclear complex and the superior colliculus: Anin vitrocharacterization in the rat

The ventral lateral geniculate nucleus (vLGN), the pretectal nuclear complex (PNC) and the superior colliculus (SC) are structures that all receive retinal input. All three structures are important relay stations of the subcortical visual system. They are strongly connected with each other and involved in circadian and/or visuomotor processes. However, the information transferred along these pathways is unknown and their possible functions are, therefore, not well understood. Here, we characterized multiple pathways between the vLGN, the PNC, and the SC electrophysiologically and anatomically in anin vitrostudy using acute rat brain slices. Using orthodromic and antidromic electrical stimulation, we first characterized vLGN neurons that receive pretectal input and those that project to the PNC. Morphological reconstructions of cells labeled after patch clamp recordings identified these neurons as geniculo-tectal neurons and as medium-sized multipolar neurons. We identified inhibitory connections in both pathways and we could show that inhibitory postsynaptic currents (IPSCs) evoked from the PNC in vLGN neurons are mediated only by GABAAreceptors, while IPSCs evoked in PNC neurons by vLGN stimulation are either mediated by both, GABAAand GABACreceptors or by a GABA receptor with mixed GABAAand GABACreceptor-like pharmacology. Finally, retrograde double labeling experiments with two different fluorescent dextran amines indicated that pretectal neurons which project to the ipsilateral vLGN also project to the ipsilateral SC.

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.

Establishing an ion pair interaction in the homomeric rho1 gamma-aminobutyric acid type A receptor that contributes to the gating pathway

gamma-Aminobutyric acid type A (GABA(A)) receptors are members of the Cys-loop superfamily of ligand-gated ion channels. Upon agonist binding, the receptor undergoes a structural transition from the closed to the open state, but the mechanism of gating is not well understood. Here we utilized a combination of conventional mutagenesis and the high precision methodology of unnatural amino acid incorporation to study the gating interface of the human homopentameric rho1 GABA(A) receptor. We have identified an ion pair interaction between two conserved charged residues, Glu(92) in loop 2 of the extracellular domain and Arg(258) in the pre-M1 region. We hypothesize that the salt bridge exists in the closed state by kinetic measurements and free energy analysis. Several other charged residues at the gating interface are not critical to receptor function, supporting previous conclusions that it is the global charge pattern of the gating interface that controls receptor function in the Cys-loop superfamily.

Colocalization of synaptic GABA(C)-receptors with GABA (A)-receptors and glycine-receptors in the rodent central nervous system

Fast inhibition in the nervous system is preferentially mediated by GABA- and glycine-receptors. Two types of ionotropic GABA-receptor, the GABA(A)-receptor and GABA(C)-receptor, have been identified; they have specific molecular compositions, different sensitivities to GABA, different kinetics, and distinct pharmacological profiles. We have studied, by immunocytochemistry, the synaptic localization of glycine-, GABA(A)-, and GABA(C)-receptors in rodent retina, spinal cord, midbrain, and brain-stem. Antibodies specific for the alpha1 subunit of the glycine-receptor, the gamma2 subunit of the GABA(A)-receptor, and the rho subunits of the GABA(C)-receptor have been applied. Using double-immunolabeling, we have determined whether these receptors are expressed at the same postsynaptic sites. In the retina, no such colocalization was observed. However, in the spinal cord, we found the colocalization of glycine-receptors with GABA(A)- or GABA(C)-receptors and the colocalization of GABA(A)- and GABA(C)-receptors in approximately 25% of the synapses. In the midbrain and brain-stem, GABA(A)- and GABA(C)-receptors were colocalized in 10%-15% of the postsynaptic sites. We discuss the possible expression of heteromeric (hybrid) receptors assembled from GABA(A)- and GABA(C)-receptor subunits. Our results suggest that GABA(A)- and GABA(C)-receptors are colocalized in a minority of synapses of the central nervous system.

A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.

Properties of cholinergic responses in neurons in the intermediate grey layer of rat superior colliculus

The intermediate grey layer (SGI) of superior colliculus (SC) receives cholinergic innervation from brainstem parabrachial region. To clarify the action of cholinergic inputs to local circuits in the SGI, we investigated the effect of cholinergic agonists and antagonists on a large number of randomly sampled neurons in Wistar rat SGI (n=246) using whole-cell patch clamp technique in slices of the rat SC. Responses of the recorded cells (n=98) to bath application of carbachol were classified into five patterns: (i) nicotinic inward only (n=14); (ii) nicotinic inward+muscarinic inward (n=26); (iii) nicotinic inward+muscarinic inward+muscarinic outward (n=39); (iv) nicotinic inward+muscarinic outward (n=13) and (v) muscarinic outward only (n=4). Among these, a majority of morphologically identified projection neurons exhibited either response pattern (ii) (9/28) or (iii) (15/28), which suggested that the primary action of cholinergic inputs on the SGI output is excitatory. Nicotinic receptor subtypes involved in the nicotinic current were examined by testing the effects of antagonists on the currents induced by bath application of 1,1-dimethyl-4-phenyl-piperazinium or transient pressure application of acetylcholine (ACh). Muscarinic receptor subtypes involved in the muscarinic inward and outward currents were investigated by examining the effects of antagonists on muscarine-induced currents. The results showed that nicotinic inward currents are mediated mainly by alpha4beta2 and partly by alpha7 nicotinic receptors and that muscarinic inward and outward currents are mediated by M3 (plus M1) and M2 muscarinic receptors, respectively.

High pH accelerates GABA deactivation on perch-rho1B receptors

The ionotropic GABA(C) receptor, formed by GABA rho subunits, is known to be modulated by a variety of endogenous compounds, as well as by changes in pH. In this study, we explore the proton sensitivity of the GABA rho subunits cloned from the perch retina, and report a novel action of high pH on the homomeric receptor formed by one of the GABA rho subunits, the perch-rho(1B) subunit. Raising extracellular pH to 9.5 significantly accelerated GABA deactivation responses elicited from oocytes expressing the perch-rho(1B) subunit, and reduced its sensitivity to GABA. The change in the kinetics of the GABA-offset response occurred without altering the maximum response amplitude, and the reduced GABA sensitivity was independent of membrane potential. Although acidification of the extracellular solution also accelerated GABA deactivation for all other GABA rho receptors examined in this study, the effects of high pH were unique to the homomeric receptor formed by the perch-rho(1B) subunit. In addition, we found that, unlike the effects on the response to the naturally occurring full agonist GABA, the responses elicited by partial agonists (imidazole-4-acetic acid (I4AA) and beta-alanine) in the presence of the high pH solution showed a significant reduction in the maximum response amplitude. When considered in terms of a model describing the activation of GABA(C) receptors, in which pH can potentially affect either the binding affinity or the rate of channel closure, the results were consistent with the view that external alkalization reduces the gating efficiency of the receptor. To identify the proton sensitive domain(s) of the perch-rho(1B) receptor, chimeras were constructed by domain swapping with other perch-rho subunits. Analysis of the pH sensitivities of the various chimeric receptors revealed that the alkaline-sensitive residues are located in the N-terminal region of the perch-rho(1B) subunit.

Random assembly of GABA rho1 and rho2 subunits in the formation of heteromeric GABA(C) receptors

1. Various combinations of the rho subunits (rho(1A), rho(1B), rho(2A), rho(2B)) of GABA(C) receptors cloned from white perch retina were expressed in Xenopus oocytes, and electrophysiological and pharmacological methods were used to test their ability to co-assemble into heteromeric receptors. Simultaneous injection of the two subunits, irrespective of their relative proportions, led invariably to the formation of a preponderance of heteromeric receptors. 2. The GABA deactivation responses elicited from these cells could be described by a single exponential decay, and their pharmacological responses deviated significantly from those expected of a simple mixture of two homomeric rho(1) and rho(2) receptors. In contrast, a double exponential function comprising fast and slow components was required to fit the GABA deactivation responses elicited from oocytes sequentially expressing rho(1) and rho(2) subunits, a condition that favors the formation of a mixture of homomeric rho(1) and rho(2) receptors. 3. Both the GABA-response kinetics and the sensitivity to picrotoxin of the heteromeric perch rho(1B)rho(2A) receptor varied with the proportion of the subunit RNA injected, indicating there is no fixed stoichiometry for their co-assembly into heteromeric rho(1)rho(2) receptors. 4. If native GABA(C) receptors in retinal neurons behave in a similar manner as in the oocyte expression system, these finding suggest that the properties of their GABA(C) receptors are likely to be influenced by the transcription/translation efficiency of GABA rho subunit genes.

Gamma-aminobutyric acid (GABA)-C receptor stimulation increases prolactin (PRL) secretion in cultured rat anterior pituitary cells

Gamma-aminobutyric acid (GABA) reportedly inhibits secretion of anterior pituitary hormones by directly acting on GABA-A and GABA-B receptors on anterior pituitary cells, but the roles of GABA-C receptors are little known. In this study, involvement of GABA-C receptors in the secretion of prolactin (PRL) was examined using cultured rat anterior pituitary cells. GABA-C receptor agonist, cis-4-aminocrotonic acid (CACA, 0.1-1 mM) increased PRL secretion dose-dependently, while GABA-A receptor agonist, 100 microM muscimol, but not GABA-B receptor agonist, 100 microM baclofen, decreased the secretion. GABA-C receptor antagonist, 15 microM (1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA), and GABA-A receptor antagonist, 100 microM bicuculline, not only reversed such an agonist-induced increase or decrease in PRL secretion, but also suppressed or enhanced spontaneous PRL secretion, raising a possibility of GABA-C or GABA-A receptor stimulation by intrinsic pituitary-derived GABA. GABA-C receptor subunits (rho1, rho2, rho3) and GABA synthesizing enzymes (GAD 65 and GAD 67) were shown to be expressed as assayed by RT-PCR, and GABA-C receptor stimulation by CACA obviously increased intracellular Ca2+ concentration in the anterior pituitary cells. Thus, PRL secretion from anterior pituitary cells appears to be enhanced via direct GABA-C receptor stimulation by GABA originating from the anterior pituitary cells besides well-known hypothalamic GABA.

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.

Cloning and functional expression of the bovine GABA(C) rho2 subunit. Molecular evidence of a widespread distribution in the CNS

GABA(C) receptors were first described as a non-desensitizing, bicuculline- and baclofen-insensitive component in Xenopus oocytes expressing bovine retina mRNA. However, the expression, tissue distribution and functional properties of GABA(C) receptors from other areas of the CNS still remain controversial. In previous experiments, the injection of rat cerebellum mRNA into Xenopus oocytes induced the expression of receptors that generated currents with both GABA(A) and GABA(C) characteristics; the latter component apparently being given by the rho2 subunit, suggesting the expression of GABA(C) receptors in the CNS and the formation of homooligomeric receptors. In this study, using RT-PCR, we found that the rho1 and rho2 subunits are widely expressed in the CNS including areas where they have not been previously described such as the bulb, pons and the caudate nucleus. To determine if the GABA(C) component of the GABA-currents elicited by oocytes expressing cerebellum mRNA was caused by activation of homomeric GABA rho2 receptors, we cloned the corresponding cDNA and expressed it in Xenopus oocytes. It was found that oocytes injected with rho2 cDNA, efficiently formed GABA-gated homooligomeric receptors. The GABA-dose-current response gave an EC50=1.19muM and the currents were resistant to bicuculline and reversibly antagonized by the specific GABA(C) receptor antagonist TPMPA. Altogether, our results indicate a widespread distribution of both rho1 and rho2 subunits in the bovine CNS and show further that the rho2 subunit cDNA isolated from cerebellum, forms fully functional receptors when expressed in Xenopus oocytes.