Advances in the molecular understanding of GABA(B) receptors

Comparative study of the effects of prenatal sevoflurane exposure at different cortical stages on forebrain development and maturation in offspring

Introduction

Brain development involves several critical stages, such as proliferation, neuronal migration, axonal pathfinding, and connection formation. Sevoflurane, a γ-aminobutyric acid (GABA) receptor agonist, is widely used as an inhaled general anesthetic. However, its impact on brain development has raised increasing concerns, particularly regarding prenatal exposure. This study aims to investigate the effects of prenatal sevoflurane exposure (PSE) at different cortical stages, focusing on its impact on the migration of glutamatergic and GABAergic neurons and neuronal behavior in offspring.

Methods

PSE was administered at two critical prenatal stages: embryonic day (E) 12.5 and E18.5. Double in situ hybridization was used to identify the coexpression of GABA receptors in Pax6- and Mash1-positive cells in the forebrain. The radial migration of glutamatergic neurons and the tangential migration of GABAergic neurons were analyzed. Behavioral tests, including the open-field test, elevated plus-maze test, forced swim test, tail suspension test, sucrose preference test, and Morris water maze, were performed on offspring to assess anxiety-like behaviors, depression, and learning and memory impairments.

Results

PSE inhibits the radial migration of glutamatergic neurons and promotes the tangential migration of GABAergic neurons. Specifically, early exposure (E12.5) inhibited the expression of the Pax6-Tbr2-Tbr1 cascade and the radial migration of Tbr1 in the ventral prefrontal cortex (PFC), whereas late exposure (E18.5) inhibited this process on the dorsal side. In addition, offspring mice with PSE exhibited increased anxiety-like behaviors, rather than depression, as demonstrated by reduced time spent in the center of the open-field test and in the open arms of the elevated plus-maze test. No significant differences were observed in the forced swim test, tail suspension test, or sucrose preference test. Furthermore, learning and memory impairments were observed in the Morris water maze.

Conclusion

Our results indicate that PSE at E12.5 and E18.5 leads to abnormalities in the migration of glutamatergic and GABAergic neurons, affecting long-term anxiety-like behaviors and causing learning and memory impairments in offspring mice.

Acute Cerebellar Manifestations without Limbic Involvement in GABAB Receptor Autoimmune Encephalitis: Case Report and Literature Review

Autoimmune encephalitis is a rapidly progressive inflammatory brain disease. Gamma-aminobutyric acid type B (GABAB) receptor autoimmune encephalitis is a rare subtype characterized by distinct clinical features. Diagnosis can be especially challenging when typical limbic symptoms and neuroimaging findings are absent. This case report underscores the importance of identifying this condition and starting immunosuppressive treatment promptly. A 59-year-old man presented with gait disturbances, dysarthria, and severe ataxia without cognitive impairment. Initial examinations, including a brain MRI, were unremarkable, except for an elevated cell count and protein in the cerebrospinal fluid. Despite receiving initial empirical antiviral treatment, his symptoms worsened, prompting the administration of intravenous methylprednisolone and immunoglobulin. After these immunosuppressive therapies, the cerebellar symptoms showed gradual improvement. Subsequent testing for antibodies to the GABAB receptor was positive in both the serum and cerebrospinal fluid. Follow-up MRI revealed cerebellar atrophy, consistent with a diagnosis of GABAB receptor-associated acute cerebellitis. This case illustrates that cerebellar symptoms can occur in the absence of more common limbic manifestations in GABAB receptor autoimmune encephalitis. The progression of cerebellar atrophy following an initially normal MRI is a significant finding that offers supporting evidence for the diagnosis of cerebellitis. A review of the literature identified similar cases of acute cerebellitis without limbic symptoms, although neuroimaging abnormalities in the cerebellum were not reported. Our case underscores the importance of increased clinical awareness and consideration of autoimmune causes, even when neuroimaging appears normal. Early and appropriate immunosuppressive therapy may help change the course of the disease and enhance patient outcomes.

Decoding sunitinib resistance in ccRCC: Metabolic-reprogramming-induced ABAT and GABAergic system shifts

Sunitinib, a primary treatment for clear cell renal cell carcinoma (ccRCC), frequently encounters the challenge of resistance development. Metabolic reprogramming, a characteristic change in ccRCC, is likely linked to this resistance. Our research revealed a notable decrease in the expression of the key metabolic gene ABAT in ccRCC, which contributed to diminished sensitivity to sunitinib. Downregulation of ABAT led to an increase in the intracellular level of gamma-aminobutyric acid (GABA), triggering abnormal activation of the G-protein-coupled receptor GABA-B. This activation resulted in increased transactivation of the tyrosine kinase receptors SYK and LYN, thereby reducing the antitumor and antiangiogenic properties of sunitinib. However, the application of SYK and LYN inhibitors successfully inhibited this effect. The transactivation of SYK and LYN caused resistance to the antiangiogenic effects of sunitinib through the upregulation of PGF protein levels. Furthermore, the combined application of an LYN inhibitor with sunitinib has been shown to enhance therapeutic efficacy.

Comprehensive transcriptome and metabolome profiling reveal metabolic mechanisms of Nitraria sibirica Pall. to salt stress

Nitraria sibirica Pall., a typical halophyte that can survive under extreme drought conditions and in saline-alkali environments, exhibits strong salt tolerance and environmental adaptability. Understanding the mechanism of molecular and physiological metabolic response to salt stress of plant will better promote the cultivation and use of halophytes. To explore the mechanism of molecular and physiological metabolic of N. sibirica response to salt stress, two-month-old seedlings were treated with 0, 100, and 400 mM NaCl. The results showed that the differentially expressed genes between 100 and 400 mmol L^-1 NaCl and unsalted treatment showed significant enrichment in GO terms such as binding, cell wall, extemal encapsulating structure, extracellular region and nucleotide binding. KEGG enrichment analysis found that NaCl treatment had a significant effect on the metabolic pathways in N. sibirica leaves, which mainly including plant-pathogen interaction, amino acid metabolism of the beta alanine, arginine, proline and glycine metabolism, carbon metabolism of glycolysis, gluconeogenesis, galactose, starch and sucrose metabolism, plant hormone signal transduction and spliceosome. Metabolomics analysis found that the differential metabolites between the unsalted treatment and the NaCl treatment are mainly amino acids (proline, aspartic acid, methionine, etc.), organic acids (oxaloacetic acid, fumaric acid, nicotinic acid, etc.) and polyhydric alcohols (inositol, ribitol, etc.), etc. KEGG annotation and enrichment analysis showed that 100 mmol L^-1 NaCl treatment had a greater effect on the sulfur metabolism, cysteine and methionine metabolism in N. sibirica leaves, while various amino acid metabolism, TCA cycle, photosynthetic carbon fixation and sulfur metabolism and other metabolic pathways have been significantly affected by 400 mmol L^-1 NaCl treatment. Correlation analysis of differential genes in transcriptome and differential metabolites in metabolome have found that the genes of AMY2, BAM1, GPAT3, ASP1, CML38 and RPL4 and the metabolites of L-cysteine, proline, 4-aminobutyric acid and oxaloacetate played an important role in N. sibirica salt tolerance control. This is a further improvement of the salt tolerance mechanism of N. sibirica, and it will provide a theoretical basis and technical support for treatment of saline-alkali soil and the cultivation of halophytes.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

Effects of dietary gamma-aminobutyric acid supplementation on the intestinal functions in weaning piglets

This study aims to investigate the effects of dietary gamma-aminobutyric acid (GABA) supplementation on the growth performance, intestinal immunity, intestinal GABAergic system, amino acid profiles and gut microflora of the weaned piglets. Totally sixteen healthy piglets were randomly assigned into two groups to be fed with the basal diet (Con group) or the basal diet with GABA (20 mg kg-1) supplementation. Body weights and feed intakes were monitored weekly. Piglets were sacrificed after 3 weeks of GABA supplementation to collect the blood, ileum, ileal mucosa and luminal content. Immune-associated factors, GABAergic system, amino acid profiles, and microbiota in the ileum and serum amino acid profiles were explored. The results showed that GABA supplementation improved the growth performance and modulated the intestinal immunity with inhibiting the gene expressions of IL-22, proinflammatory cytokines (IL-1 and IL-18), and Muc1, but promoted the expressions of anti-inflammatory cytokines (IFN-γ, IL-4, and IL-10), TLR6 and MyD88. GABA regulated a few components of the intestinal GABAergic system, increased the levels of most amino acids in the ileal mucosa but reduced the serum amino acid profiles. GABA regulated the population and diversity of intestinal microbiota, such as the abundances of the dominant microbial populations, the community richness, and diversity of the ileal microbiota. In conclusion, GABA supplementation modulated the intestinal functions, including intestinal immunity, intestinal amino acid profiles and gut microbiota, and the results can be helpful for understanding the functions of GABA in the intestine.

Combination of cheminformatics and bioinformatics to explore the chemical basis of the rhizomes and aerial parts of Dioscorea nipponica Makino

OBJECTIVES

This study was aimed to explore the chemical basis of the rhizomes and aerial parts of Dioscorea nipponica Makino (DN).

METHODS

The pharmacokinetic profiles of the compounds from DN were calculated via ACD/I-Lab and PreADMET program. Their potential therapeutic and toxicity targets were screened through the DrugBank's or T3DB's ChemQuery structure search.

KEY FINDINGS

Eleven of 48 compounds in the rhizomes and over half of the compounds in the aerial parts had moderate or good human oral bioavailability. Twenty-three of 48 compounds in the rhizomes and 40/43 compounds from the aerial parts had moderate or good permeability to intestinal cells. Forty-three of 48 compounds from the rhizomes and 18/43 compounds in the aerial parts bound weakly to the plasma proteins. Eleven of 48 compounds in the rhizomes and 36/43 compounds of the aerial parts might pass across the blood-brain barrier. Forty-three 48 compounds in the rhizomes and 18/43 compounds from the aerial parts showed low renal excretion ability. The compounds in the rhizomes possessed 391 potential therapeutic targets and 216 potential toxicity targets. Additionally, the compounds from the aerial parts possessed 101 potential therapeutic targets and 183 potential toxicity targets.

CONCLUSIONS

These findings indicated that combination of cheminformatics and bioinformatics may facilitate achieving the objectives of this study.

Gamma Amino Butyric Acid Attenuates Brain Oxidative Damage Associated with Insulin Alteration in Streptozotocin-Treated Rats

The aim of the current study was to evaluate the role of γ-amino butyric acid (GABA) in insulin disturbance and hyperglycemia associated with brain oxidative damage in streptozotocin-treated rats. Streptozotocin (STZ) was administered to male albino rats as a single intraperitoneal dose (60 mg/kg body weight). GABA (200 mg/Kg body weight/day) was administered daily via gavages during 3 weeks to STZ-treated-rats. Male albino rats Sprague-Dawley (10 ± 2 weeks old; 120 ± 10 g body weight) were divided into 4 groups of 6 rats and treated in parallel. (1) Control group: received distilled water, (2) GABA group: received GABA, (3) STZ group: STZ-treated rats received distilled water, (4) STZ + GABA group: STZ-treated rats received GABA. Rats were sacrificed after a fasting period of 12 h next last dose of GABA. The results obtained showed that STZ-treatment produced hyperglycemia and insulin deficiency (similar to type1 Diabetes). These changes were associated with oxidative damage in brain tissue and notified by significant decreases of superoxide dismutase and catalase activities in parallel to significant increases of malondialdehyde and advanced oxidation protein products levels. The histopathology reports also revealed that STZ-treatment produced degeneration of pancreatic cells. The administration of GABA to STZ-treated rats preserved pancreatic tissue with improved insulin secretion, improved glucose level and minimized oxidative stress in brain tissues. It could be concluded that GABA might protect the brain from oxidative stress and preserve pancreas tissues with adjusting glucose and insulin levels in Diabetic rats and might decrease the risk of neurodegenerative disease in diabetes.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Discovery of a Negative Allosteric Modulator of GABAB Receptors

Initialized from the scaffold of CGP7930, an allosteric agonist of GABAB receptors, a series of noncompetitive antagonists were discovered. Among these compounds, compounds 3, 6, and 14 decreased agonist GABA-induced maximal effect of IP3 production in HEK293 cells overexpressing GABAB receptors and Gqi9 proteins without changing the EC50. Compounds 3, 6, and 14 not only inhibited agonist baclofen-induced ERK1/2 phosphorylation but also blocked CGP7930-induced ERK1/2 phosphorylation in HEK293 cells overexpressing GABAB receptors. The results suggested that compounds 3, 6, and 14 are negative allosteric modulators of GABAB receptors. The representative compound 14 decreased GABA-induced IP3 production with IC50 of 37.9 μM and had no effect on other GPCR Class C members such as mGluR1, mGluR2, and mGluR5. Finally, we showed that compound 14 did not bind to the orthosteric binding sites of GABAB receptors, demonstrating that compound 14 negatively modulated GABAB receptors activity as a negative allosteric modulator.

Complex GABAB receptor complexes: how to generate multiple functionally distinct units from a single receptor

The main inhibitory neurotransmitter, GABA, acts on both ligand-gated and G protein-coupled receptors, the GABA_A/C and GABA_B receptors, respectively. The later play important roles in modulating many synapses, both at the pre- and post-synaptic levels, and are then still considered as interesting targets to treat a number of brain diseases, including addiction. For many years, several subtypes of GABA_B receptors were expected, but cloning revealed only two genes that work in concert to generate a single type of GABA_B receptor composed of two subunits. Here we will show that the signaling complexity of this unit receptor type can be largely increased through various ways, including receptor stoichiometry, subunit isoforms, cell-surface expression and localization, crosstalk with other receptors, or interacting proteins. These recent data revealed how complexity of a receptor unit can be increased, observation that certainly are not unique to the GABA_B receptor.

Ontogenetic modifications of neuronal excitability during brain maturation: developmental changes of neurotransmitter receptors

The development of the human brain depends on a precisely orchestrated cascade of events, including proliferation, migration and maturation of neural progenitor cells. Different mechanisms coordinate these stages to reach a normal structural organization, producing appropriate excitatory and inhibitory networks. Here, we will briefly review the developmental changes of glutamate (Glu) and γ-aminobutyric acid (GABA) receptors, with particular attention to the development of the human brain. We will also briefly discuss recent evidence on the involvement of the endocannabinoid signaling in the regulation of neuronal excitability during early brain development..

Sleep-deprivation induces changes in GABA(B) and mGlu receptor expression and has consequences for synaptic long-term depression

Long term depression (LTD) in the CA1 region of the hippocampus, induced with a 20-Hz, 30 s tetanus to Schaffer collaterals, is enhanced in sleep-deprived (SD) rats. In the present study, we investigated the role of metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid (GABA) B receptors (GABA(B)-Rs) and N-methyl-D-aspartic acid receptors (NMDARs) in the LTD of the population excitatory postsynaptic potential (pEPSP). The requirement of Ca(2+) from L- and T-type voltage-gated calcium channels (VGCCs) and intracellular stores was also studied. Results indicate that mGluRs, a release of Ca(2+) from intracellular stores and GABA(B)-Rs are required for LTD. Interestingly, while mGlu1Rs seem to be involved in both short-term depression and LTD, mGlu5Rs appear to participate mostly in LTD. CGP 55845, a GABA(B)-R antagonist, partially suppressed LTD in normally sleeping (NS) rats, while completely blocking LTD in SD rats. Moreover, GS-39783, a positive allosteric modulator for GABA(B)-R, suppressed the pEPSP in SD, but not NS rats. Since both mGluRs and GABA(B)-Rs seem to be involved in the LTD, especially in SD rats, we examined if the receptor expression pattern and/or dimerization changed, using immunohistochemical, co-localization and co-immunoprecipitation techniques. Sleep-deprivation induced an increase in the expression of GABA(B)-R1 and mGlu1αR in the CA1 region of the hippocampus. In addition, co-localization and heterodimerization between mGlu1αR/GABA(B)-R1 and mGlu1αR/GABA(B)-R2 is enhanced in SD rats. Taken together, our findings present a novel form of LTD sensitive to the activation of mGluRs and GABA(B)-Rs, and reveal, for the first time, that sleep-deprivation induces alterations in the expression and dimerization of these receptors.

Positive regulation by GABA(B)R1 subunit of leptin expression through gene transactivation in adipocytes

BACKGROUND

The view that γ-aminobutyric acid (GABA) plays a functional role in non-neuronal tissues, in addition to an inhibitory neurotransmitter role in the mammalian central nervous system, is prevailing, while little attention has been paid to GABAergic signaling machineries expressed by adipocytes to date. In this study, we attempted to demonstrate the possible functional expression of GABAergic signaling machineries by adipocytes.

METHODOLOGY/PRINCIPAL FINDINGS

GABA(B) receptor 1 (GABA(B)R1) subunit was constitutively expressed by mouse embryonic fibroblasts differentiated into adipocytes and adipocytic 3T3-L1 cells in culture, as well as mouse white adipose tissue, with no responsiveness to GABA(B)R ligands. However, no prominent expression was seen with mRNA for GABA(B)R2 subunit required for heteromeric orchestration of the functional GABA(B)R by any adipocytic cells and tissues. Leptin mRNA expression was significantly and selectively decreased in adipose tissue and embryonic fibroblasts, along with drastically reduced plasma leptin levels, in GABA(B)R1-null mice than in wild-type mice. Knockdown by siRNA of GABA(B)R1 subunit led to significant decreases in leptin promoter activity and leptin mRNA levels in 3T3-L1 cells.

CONCLUSIONS/SIGNIFICANCE

Our results indicate that GABA(B)R1 subunit is constitutively expressed by adipocytes to primarily regulate leptin expression at the transcriptional level through a mechanism not relevant to the function as a partner of heterodimeric assembly to the functional GABA(B)R.

Intron 4 containing novel GABAB1 isoforms impair GABAB receptor function

Background

Gamma-aminobutyric acid type B (GABAB) receptors decrease neural activity through G protein signaling. There are two subunits, GABAB1 and GABAB2. Alternative splicing provides GABAB1 with structural and functional diversity. cDNA microarrays showed strong signals from human brain RNA using GABAB1 intron 4 region probes. Therefore, we predicted the existence of novel splice variants.

Methodology/Principal Findings

Based on the probe sequence analysis, we proposed two possible splice variants, GABAB1j and GABAB1k. The existence of human GABAB1j was verified by quantitative real-time PCR, and mouse GABAB1j was found from a microarray probe set based on human GABAB1j sequence. GABAB1j open reading frames (ORF) and expression patterns are not conserved across species, and they do not have any important functional domains except sushi domains. Thus, we focused on another possible splice variant, GABAB1k. After obtaining PCR evidence for GABAB1k existence from human, mouse, and rat, it was cloned from human and mouse by PCR along with three additional isoforms, GABAB1l, GABAB1m, and GABAB1n. Their expression levels by quantitative real-time PCR are relatively low in brain although they may be expressed in specific cell types. GABAB1l and GABAB1m inhibit GABAB receptor-induced G protein-activated inwardly rectifying K⁺ channel (GIRK) currents at Xenopus oocyte two-electrode voltage clamp system.

Conclusions/Significance

This study supports previous suggestions that intron 4 of GABAB1 gene is a frequent splicing spot across species. Like GABAB1e, GABAB1l and GABAB1m do not have transmembrane domains but have a dimerization motif. So, they also could be secreted and bind GABAB2 dominantly instead of GABAB1a. However, only GABAB1l and GABAB1m are N- and C-terminal truncated splicing variants and impair receptor function. This suggests that the intron 4 containing N-terminal truncation is necessary for the inhibitory action of the new splice variants.

GABAB receptor activation protects neurons from apoptosis via IGF-1 receptor transactivation

The G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) play key roles in cell-cell communication. Several studies revealed important synergisms between these two types of receptors, with some of the actions of either receptor being mediated through transactivation of the other. Among the large GPCR family, GABA(B) receptor is activated by the neurotransmitter GABA, and is expressed in most neurons where it mediates slow and prolonged inhibition of synaptic transmission. Here we show that this receptor is involved in the regulation of life and death decisions of cerebellar granule neurons (CGNs). We show that specific activation of GABA(B) receptor can protect neurons from apoptosis through a mechanism that involves transactivation of the IGF-1 receptor (IGF-1R). Further work demonstrated that this cross talk was dependent on G(i/o)-protein, PLC, cytosolic Ca(2+), and FAK1 but independent of PKC, while IGF-1R-induced signaling involved Src kinase, PI3 kinase, and Akt activation. These results reveal a new function for this important GPCR and further highlight the importance of functional cross-talk networks between GPCRs and RTKs. Our results reveal GABA(B) receptor as a potential drug target for the treatment of neurodegenerative disorders.

GABAB receptor agonism as a novel therapeutic modality in the treatment of gastroesophageal reflux disease

Defined pharmacologically by its insensitivity to the GABA(A) antagonist bicuculline and sensitivity to the GABA analogue baclofen, the G protein-linked gamma-aminobutyric acid type B (GABA(B)) receptor couples to adenylyl cyclase, voltage-gated calcium channels, and inwardly-rectifying potassium channels. On the basis of a wealth of preclinical data in conjunction with early clinical observations that baclofen improves symptoms of gastroesophageal reflux disease (GERD), the GABA(B) receptor has been proposed as a therapeutic target for a number of diseases including GERD. Subsequently, there has been a significant effort to develop a peripherally-restricted GABA(B) agonist that is devoid of the central nervous system side effects that are observed with baclofen. In this article we review the in vitro and in vivo pharmacology of the peripherally-restricted GABA(B) receptor agonists and the preclinical and clinical development of lesogaberan (AZD3355, (R)-(3-amino-2-fluoropropyl) phosphinic acid), a potent and predominately peripherally-restricted GABA(B) receptor agonist with a preclinical therapeutic window superior to baclofen.

GABAB receptors as drug targets to treat gastroesophageal reflux disease

For many years, acid-suppressive therapy has been at the forefront of treating gastroesophageal reflux disease (GERD), yet despite the advent of the proton pump inhibitors (PPIs) some patients continue to experience persistent GERD symptoms. Therapeutic (non-surgical) options for such patients are currently limited. To tackle this clinical issue, research efforts have begun to focus on 'reflux inhibition' as a potential therapeutic target - i.e. inhibition of transient lower esophageal relaxations (TLESRs), the predominant mechanism of gastroesophageal reflux. Preclinical research has identified a number of drug targets through which TLESRs can be modulated, and the gamma-aminobutyric acid (GABA) type B (GABA(B)) receptor has emerged as one of the most promising. Studies with baclofen, a well-known agonist of this receptor, have demonstrated that reflux inhibition is a valid concept in the clinical setting in that reducing the incidence of TLESRs improves GERD symptoms. But baclofen is associated with significant central nervous system (CNS) side effects, rendering it undesirable for use as a treatment for GERD. Further development work has yielded a number of novel GABA(B) receptor agonists with reduced CNS side effect profiles, and clinical trials are currently being performed with several agents. Compounds that target TLESRs may therefore present a new add-on treatment for patients with persistent GERD symptoms despite PPI therapy.

Developmental profile and mechanisms of GABA-induced calcium signaling in hippocampal astrocytes

GABA (gamma-aminobutyric acid) is a transmitter with dual action. Whereas it excites neurons during the first week of postnatal development, it represents the major inhibitory transmitter in the mature brain. GABA also activates astrocytes by binding to ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. This results in glial calcium transients which can induce the release of gliotransmitters, rendering GABA an important mediator of neuron-glia interaction. Using whole-cell patch-clamp and ratiometric calcium imaging in hippocampal slices from rats at postnatal days 3-34, we have analyzed the developmental profile as well as the cellular mechanisms of calcium signals induced by GABA(A) and GABA(B) receptor activation in astrocytes. We found that GABA-evoked glial calcium transients are mediated by both GABA(A) and GABA(B) receptors. Throughout development, GABA(A)-receptor activation resulted in immediate calcium transients in the vast majority of astrocytes, most likely by influx of calcium through voltage-gated calcium channels. GABA(B) receptor activation, in contrast, resulted in delayed calcium transients, which were blocked following depletion of intracellular calcium stores and during persistent activation of heterotrimeric G-proteins. GABA(B) receptor-mediated calcium signals exhibited a clear developmental profile with less than 10% of astrocytes responding at P3 or P32-34, and about 60% of cells between P11 and P15. Our data thus indicate that GABA(B) receptor-mediated calcium transients are due to calcium release from intracellular stores following G-protein activation. Moreover, GABA(B) receptor-mediated calcium signaling in astrocytes preferentially occurs at a period during postnatal development when hippocampal networks are established.

In vitro modulation of gamma amino butyric acid (GABA) receptor expression by bone marrow stromal cells

Bone marrow stromal cells (BMSC) have the capability to undergo a change of morphology, reminiscent of neuronal cells, after exposure to an inductive medium. These induced BMSC-derived neuron-like (BDNL) cells express several neuronal markers, including Microtubule-Associated Protein Tau, Neurofilament M, and Nestin as revealed by immunocytochemistry analysis. To assess whether the induction process has possible functional relevance, we have focused our attention on the expression of neurotransmitter receptors. In particular, we show that the expression of GABA(A) subunits alpha1, beta2/3, epsilon and GABA(B1) mRNAs is greatly enhanced in BMSC by the induction treatment. Similar results were obtained from rat skin fibroblasts subjected to the same induction protocol, with the exception for the GABA(B2) transcript that was expressed only by BMSC and BDNL. The presence of both GABA(B1) and GABA(B2) subunits in BDNL cells suggests that functional GABA(B) receptors might be assembled: we indeed found that a functional GABA(B) receptor, negatively linked to cyclic AMP production, is expressed in BDNL. Therefore, we suggest that BMSC can be converted into cells equipped with appropriate receptors coupled to transduction mechanisms, potentially responding to a specific neurotransmitter.

Baclofen, a GABAB receptor agonist, inhibits human hepatocellular carcinoma cell growth in vitro and in vivo

Gamma aminobutyric acid (GABA) has been reported to affect cancer development, but the activation of its type B receptor (GABABR) has shown contradictory effects on the progress of human carcinoma. In this study, we investigated the antitumor effect of the GABABR agonist baclofen (Bac) on growth of human hepatocellular carcinoma (HCC) in vitro and in vivo. We found Bac induced G(0)/G(1) phase arrest which was associated with down-regulation of intracellular cAMP level, and up-regulation of p21(WAF1) protein expression as well as its phosphorylation level. These in vitro effects could be abrogated by pretreatment with the specific GABABR antagonist phaclofen (Pha). Moreover, systemic administration of Bac significantly suppressed Bel-7402 xenograft tumor growth. Our data support the inhibitory effect of GABABR activation on HCC development, which would raise the possibility to develop Bac as a therapeutic drug for the treatment of HCC.

GABA(B) receptors: structure and function

In the basal ganglia the effects of gamma-aminobutyrate (GABA) are mediated by both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. Although the existence and widespread distribution in the CNS of the GABA(B) receptor had been established in the 1980s the field of GABA(B) research was revolutionized with the discovery that two related G-protein-coupled receptors (GPCRs) needed to dimerize to form the functional GABA(B) receptor at the cell surface. This finding lead to a number of studies of oligomerization in GPCRs and detailed pharmacological studies of the cloned receptors and their splice variants. Particular interest has focused on the proteins interacting with the receptor which may be important in mediating the longer term signalling effects of the receptor and modifying its cellular localization or physiology. The cloning of the GABA(B) receptors also lead to the identification of the first compounds interacting in an allosteric fashion with the receptor some of which may have therapeutic value. Most recently "knockouts" of both the GABA(B) subunits have been produced where in general as expected there is a loss of the majority of the inhibitory effects of the GABA(B) receptor.

Evolutionary conservation of alternative splicing in chicken

Alternative splicing represents a source of great diversity for regulating protein expression and function. It has been estimated that one-third to two-thirds of mammalian genes are alternatively spliced. With the sequencing of the chicken genome and analysis of transcripts expressed in chicken tissues, we are now in a position to address evolutionary conservation of alternative splicing events in chicken and mammals. Here, we compare chicken and mammalian transcript sequences of 41 alternatively-spliced genes and 50 frequently accessed genes. Our results support a high frequency of splicing events in chicken, similar to that observed in mammals.

Treatment of detrusor sphincter dyssynergia with baclofen and doxazosin

Detrusor sphincter dyssynergia (DSD) is an involuntary contraction of the external urethral sphincter during detrusor contraction. A high proportion of patients needing repeat surgery and long term failure have both been described in the literature. In the present study, we evaluated clinical characteristics, underlying disorders and outcomes of conservative medical treatment in 21 female patients. Two patients were newly diagnosed multiple sclerosis. Urodynamic studies were performed in all symptomatic patients, and consisted of measurement of post-micturition residuals, urethral pressure profilometry and EMG cystometry according to the criteria of the International Continence Society. All patients were treated with baclofen 15 mg/day and doxazosin 4 mg/day. Seven patients received tolterodine 4 mg/day in addition to baclofen and doxazocin because they had detrusor hyperreflexia (DH). In conclusion, treatment with either combined baclofen and doxazosin or anticholinergic agent tolterodine appeared to be effective. In addition, it should be kept in mind that DSD could be the first sign to any neurologic diseases.

GABAB receptors and synaptic modulation

GABA(B) receptors modulate transmitter release and postsynaptic membrane potential at various types of central synapses. They function as heterodimers of two related seven-transmembrane domain receptor subunits. Trafficking, activation and signalling of GABA(B) receptors are regulated both by allosteric interactions between the subunits and by the binding of additional proteins. Recent studies have shed light on the roles of GABA(B) receptors in plasticity processes at excitatory synapses. This review summarizes our knowledge of the localization, structure and function of GABA(B) receptors in the central nervous system and their use as drug targets for neurological and psychiatric disorders.

Immunohistochemical labeling of the mu opioid receptor carboxy terminal splice variant mMOR-1B4 in the mouse central nervous system

The mu opioid receptor gene Oprm is alternatively spliced into many variants, providing for the multiplicity of mu opioid receptor subtypes. One of the mouse variants, mMOR-1B4, is unique in that it displays high affinity towards a wide range of mu opioid receptor antagonists, but poor affinity towards most classical mu opioid agonists. The present study examined the immunohistochemical distribution of the mMOR-1B4 variant in mouse brain and spinal cord. mMOR-1B4-like immunoreactivity (mMOR-1B4-LI) was enriched in many regions of the brain, spinal cord and in the dorsal root ganglia. Some of the structures showing prominent mMOR-1B4-LI include the olfactory bulb, cerebral cortex, bed nucleus of stria terminalis, hippocampus, habenular nucleus, amygdala, thalamus, hypothalamus, medium eminence, substantia nigra, ventral tegmental area, oculomotor nucleus, red nucleus, raphe nuclei, periaqueductal gray, locus coeruleus, trigeminal nucleus, reticular formation, area postrema and Purkinje cell layer and deep nuclei of cerebellum. mMOR-1B4-LI was present in afferent neurons of the dorsal root ganglia and their projecting fibers into the superficial laminae of the spinal dorsal horn. Some motor neurons in the anterior horn of the spinal cord also were immunopositive. The overall distribution of mMOR-1B4-LI in the central nervous system is distinguishable from previously characterized variants such as MOR-1-LI, MOR-1C-LI and exon-11-LI. These studies provide evidence for the region- and neuron-specific processing of the Oprm gene and support the possibility of functional differences among the variants.

Regulation of Arabidopsis thaliana 14-3-3 gene expression by gamma-aminobutyric acid

The function in plants of the non-protein amino acid, gamma-aminobutyric acid (GABA) is poorly understood. In this study, we show that GABA down-regulates the expression of a large subset of 14-3-3 gene family members in Arabidopsis thaliana seedlings in a calcium, ethylene and abscisic acid (ABA)-dependent manner. Gene expression is not affected when seedlings are supplied with glutamate (GLU), a precursor of GABA. The repression of 14-3-3 gene expression by GABA is dependent on functional ethylene and ABA signalling pathways, because the response is lost in the etr1-1, abi1-1 and abi2-1 mutants. Calcium measurements show that in contrast to GLU, GABA does not elicit a cytoplasmic calcium elevation, suggesting that the GABA response is unlikely to be mediated by GLU receptors (GLRs), as has been suggested previously. We suggest that in addition to its role as a stress-related metabolite, GABA may regulate gene expression in A. thaliana, including members of the 14-3-3 gene family.

Successive alterations of hippocampal gamma-aminobutyric acid B receptor subunits in a rat model of febrile seizure

Febrile seizure (FS) is a frequently encountered seizure type in childhood. Changes of brain function following FS have clinical importance. The recently identified gamma-aminobutyric acid B receptor (GABA(B)R) is a metabotropic receptor of GABA. In this study, we used a rat model of recurrent FS to investigate the changes of GABA(B)R1a and GABA(B)R2 subunits in hippocampus after recurrent FS by using Western blot, quantitative RT-PCR, double immunofluorescence, in situ hybridization and immunoprecipitation/Western blot. After treatment of hyperthermia and the presence of induced seizures once every 2 days for 10 times, GABA(B)R1a and GABA(B)R2 subunits in hippocampus were decreased after 24 h of the last treatment. The decrease of GABA(B)R1a lasted for 15 days but that of GABA(B)R2 persisted for more than 30 days. The binding of GABA(B)R1a to GABA(B)R2 in hippocampus was also decreased significantly after 24 h of the last treatment and lasted for more than 30 days. In situ hybridization showed that GABA(B)R1a mRNA was significantly decreased in dentate gyrus, and GABA(B)R2 mRNA was considerably reduced in CA3 region. In H10 and FS1 groups in which hyperthermia treatment was the same but no (H10 group) or only one seizure (FS(1) group) was induced, the decrease of GABA(B)R1a and GABA(B)R2 subunits and the reduced binding capability between GABA(B)R1a and GABA(B)R2 subunits were also detected but with less severity, and the time recovering from these abnormalities was shorter. We conclude that GABA(B)R1a and GABA(B)R2 subunits and the binding of the 2 subunits decrease in hippocampus for a relatively long period of time after recurrent FS in immature rats. These changes may result in long-lasting imbalance of excitation/inhibition function in hippocampus, and are derived from the consequences of recurrent febrile seizures.

GABA(B) receptors in the centromedian/parafascicular thalamic nuclear complex: an ultrastructural analysis of GABA(B)R1 and GABA(B)R2 in the monkey thalamus

Strong gamma-aminobutyric acid type B (GABA(B)) receptor binding has been shown throughout the thalamus, but the distribution of the two GABA(B) receptor subunits, GABA(B) receptor subunit 1 (GABA(B)R1) and GABA(B) receptor subunit 2 (GABA(B)R2), remains poorly characterized. In primates, the caudal intralaminar nuclei, centromedian and parafascicular (CM/PF), are an integral part of basal ganglia circuits and a main source of inputs to the striatum. In this study, we analyzed the subcellular and subsynaptic distribution of GABA(B) receptor subunits by using light and electron microscopic immunocytochemical techniques. Quantitative immunoperoxidase and immunogold analysis showed that both subunits display a similar pattern of distribution in CM/PF, being expressed largely at extrasynaptic and perisynaptic sites in neuronal cell bodies, dendrites, and axon-like processes and less abundantly in axon terminals. Postsynaptic GABA(B)R1 labeling was found mostly on the plasma membrane (70-80%), whereas GABA(B)R2 was more evenly distributed between the plasma membrane and intracellular compartments of CM/PF neurons. A few axon terminals forming symmetric and asymmetric synapses were also labeled for GABA(B)R1 and GABA(B)R2, but the bulk of presynaptic labeling was expressed in small axon-like processes. About 20% of presynaptic vesicle-containing dendrites of local circuit neurons displayed GABA(B)R1/R2 immunoreactivity. Vesicular glutamate transporters (vGluT1)-containing terminals forming asymmetric synapses expressed GABA(B)R1 and/or displayed postsynaptic GABA(B)R1 at the edges of their asymmetric specialization. Overall, these findings provide evidence for multiple sites where GABA(B) receptors could modulate GABAergic and glutamatergic transmission in the primate CM/PF complex.

Molecular and functional properties of neurons in the human lateral amygdala

Neuronal properties were investigated through patch-clamp recording in situ in surgical specimens of the human lateral amygdala (LA) obtained from patients with intractable temporal lobe epilepsy. Projection neurons displayed spiny dendrites, action potentials with varying degree of frequency adaptation, and an inwardly rectifying K+ (Kir) conductance coupled to GABA(B) receptors. In interneurons, dendrites were spineless or sparsely spiny, action potentials were shorter than those in projection neurons and often occurred spontaneously, and GABA(B) receptor-mediated responses were lacking. Single-cell RT-PCR demonstrated expression of Kir channel subunits Kir3.1 and Kir3.2 and of vesicular glutamate transporters VGLUT1 and VGLUT2 in projection neurons. It is concluded that projection neurons and interneurons of the human LA can be distinguished based upon morphological, electrophysiological, and molecular biological criteria. The most striking difference relates to the expression of postsynaptic GABA(B) receptors coupled to Kir3 channels in projection neurons and the lack of functional GABA(B) receptors in interneurons.

Functional expression of metabotropic GABAB receptors in primary cultures of astrocytes from rat cerebral cortex

GABA(B) receptor subunits are widely expressed on neurons throughout the central nervous system (CNS), at both pre- and postsynaptic sites, where they mediate the late and slow component of the inhibitory response to the major inhibitory neurotransmitter GABA. Recently, GABA(B) receptors have been reported to be expressed in astrocytes and microglia in the rat CNS by immunocytochemistry. However, there are few reports available for the functional characterization of GABA(B) receptors on astrocytes. In the present study, we therefore investigated the functional expression and characteristics of GABA(B) receptors in primary cultures of astrocytes from rat cerebral cortex. In the presence of 10 microM GTP, forskolin concentration-dependently increased adenylylcyclase (AC) activity in membranes prepared from rat astrocytes. The selective GABA(B) agonist (R)-baclofen concentration-dependently reduced forskolin-stimulated AC activity in the presence of 10 microM GTP. This effect was reversed by the selective GABA(B) antagonists, CGP-55845 and CGP-54626, and was completely abolished by treatment of astrocytic membranes with pertussis toxin. In addition, RT-PCR, Western blotting, and immunocytochemistry clearly showed that metabotropic GABA(B) receptor isoforms (GABA(B)R1 and GABA(B)R2) are expressed in rat cerebrocortical astrocytes. Taken collectively, these results demonstrate that functionally active metabotropic GABA(B) receptors are expressed in rat cerebrocortical astrocytes.

Glutamate and GABA receptor signalling in the developing brain

Our understanding of the role played by neurotransmitter receptors in the developing brain has advanced in recent years. The major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, activate both ionotropic (ligand-gated ion channels) and metabotropic (G protein-coupled) receptors, and are generally associated with neuronal communication in the mature brain. However, before the emergence of their role in neurotransmission in adulthood, they also act to influence earlier developmental events, some of which occur prior to synapse formation: such as proliferation, migration, differentiation or survival processes during neural development. To fulfill these actions in the constructing of the nervous system, different types of glutamate and GABA receptors need to be expressed both at the right time and at the right place. The identification by molecular cloning of 16 ionotropic glutamate receptor subunits, eight metabotropic glutamate receptor subtypes, 21 ionotropic and two metabotropic GABA receptor subunits, some of which exist in alternatively splice variants, has enriched our appreciation of how molecular diversity leads to functional diversity in the brain. It now appears that many different types of glutamate and GABA receptor subunits have prominent expression in the embryonic and/or postnatal brain, whereas others are mainly present in the adult brain. Although the significance of this differential expression of subunits is not fully understood, it appears that the change in subunit composition is essential for normal development in particular brain regions. This review focuses on emerging information relating to the expression and role of glutamatergic and GABAergic neurotransmitter receptors during prenatal and postnatal development.

Pharmacological and biochemical aspects of GABAergic neurotransmission: pathological and neuropsychobiological relationships

1. The GABAergic neurotransmission has been implicated in the modulation of many neural networks in forebrain, midbrain and hindbrain, as well as, in several neurological disorders. 2. The complete comprehension of GABA system neurochemical properties and the search for approaches in identifying new targets for the treatment of neural diseases related to GABAergic pathway are of the extreme relevance. 3. The present review will be focused on the pharmacology and biochemistry of the GABA metabolism, GABA receptors and transporters. In addition, the pathological and psychobiological implications related to GABAergic neurotransmission will be considered.

cDNA microarray analysis of lactoferrin expression in non-neoplastic human hepatocyte PH5CH8 cells

Lactoferrin (LF), a milk protein belonging to the iron transporter transferrin family, is known as a primary defense protein against pathogenic microorganisms. Previously, we found that bovine and human LFs prevented hepatitis C virus infection in cultured human hepatocytes by a direct interaction with the virus. Since LF is proposed to have transcriptional regulatory activity in addition to its antimicrobial function, we sought to identify the target genes that these two types of LF have in common. To this end, we were the first to perform microarray analysis (9970 genes) using human hepatocytes that expressed bovine or human LF by retrovirus-mediated gene transfer. In the results, LF could give a variety of expression profiles in the human hepatocytes, and showed that 9 and 19 genes were commonly up-regulated (more than 2.0-fold) and down-regulated (less than 0.50-fold), respectively, in both bovine and human LF-expressing cells compared with control cells. Among these genes, we found that gamma-aminobutyric acid (GABA)-B receptor 2 was transcriptionally down-regulated by bovine and human LFs, but not by human transferrin. Furthermore, we obtained the suggestive result that LF may modulate the level of intracellular cAMP. This modulation is one of the cellular responses that the GABA-B receptor modifies. This is the first report of microarray analysis applied to search inclusively for the target genes of LF.

Immunocytochemical localization of GABABR1 receptor subunits in the basolateral amygdala

Gamma-aminobutyric acid B (GABAB) receptors (GBRs) are G-protein-coupled receptors that mediate a slow, prolonged form of inhibition in the basolateral amygdala (ABL) and other brain areas. Recent studies indicate that this receptor is a heterodimer consisting of GABABR1 (GBR1) and GABABR2 subunits. In the present investigation, antibodies to the GABABR1 subunit were used to study the neuronal localization of GBRs in the rat ABL. GBR immunoreactivity was mainly found in spine-sparse interneurons and astrocytes at the light microscopic level. Very few pyramidal neurons exhibited perikaryal staining. Dual-labeling immunofluorescence analysis indicated that each of the four main subpopulations of interneurons exhibited GBR immunoreactivity. Virtually 100% of large CCK+ neurons in the basolateral and lateral nuclei were GBR+. In the basolateral nucleus 72% of somatostatin (SOM), 73% of parvalbumin (PV) and 25% of VIP positive interneurons were GBR+. In the lateral nucleus 50% of somatostatin, 30% of parvalbumin and 27% of VIP positive interneurons were GBR+. Electron microscopic (EM) analysis revealed that most of the light neuropil staining seen at the light microscopic level was due to the staining of dendritic shafts and spines, most of which probably belonged to spiny pyramidal cells. Very few axon terminals (Ats) were GBR+. In summary, this investigation demonstrates that the distal dendrites of pyramidal cells, and varying percentages of each of the four main subpopulations of interneurons in the ABL, express GBRs. Because previous studies suggest that GBR-mediated inhibition modulates NMDA-dependent EPSPs in the ABL, these receptors may play an important role in neuronal plasticity related to emotional learning.

Independent maturation of the GABA(B) receptor subunits GABA(B1) and GABA(B2) during postnatal development in rodent brain

GABA(B) receptors mediate slow inhibitory GABAergic neurotransmission. They are encoded by two distinct subunits, GABA(B1) (GBR1) and GABA(B2) (GBR2), with two major isoforms of GBR1, GBR1a and GBR1b, arising from differential promoter usage. Heterodimerization of GBR1 and GBR2 is essential for GABA(B) receptor function, as shown in recombinant expression systems and in GBR1(-/-) mice. GABA(B) receptors are highly expressed during ontogeny, prior to synaptogenesis, but their developmental function remains elusive. Here we investigated the postnatal development of GABA(B) receptors in rodent brain, focusing on potential differences in the spatial and temporal expression pattern of GBR1 and GBR2. Immunohistochemistry with subunit-specific antibodies revealed a widespread staining for GBR1a and GBR2 in neonatal rodent brain. During the first 2 weeks, these two subunits exhibited largely overlapping regional distribution, but with profound distinctions in cellular and subcellular localization. The adult-like pattern was established during the third week, with a prominent up-regulation of GBR1b, extensively codistributed with GBR2. Several unexpected features were noted at early stages, notably, a selective GBR2 staining of axonal tracts, such as the corticothalamic projection, and a prominent GBR1 expression in astrocytes. The specificity of the antibody labeling was verified in GBR1- and GBR2-knockout mice. In addition, the analysis of these mutants revealed a partial preservation of GBR2 staining in GBR1(-/-) mice and vice versa. Altogether, the results suggest a functional role for GBR1 and GBR2 proteins in immature brain in addition to their contribution to dimeric GABA(B) receptor complexes.

Agonist-induced desensitization and endocytosis of heterodimeric GABAB receptors in CHO-K1 cells

gamma-Aminobutyric acid B (GABA(B)) receptor is the first discovered G protein-coupled receptor that requires two subunits, GB1 and GB2, to form a functional receptor. Whereas the molecular and functional characteristics of GABA(B) receptors have been recently extensively studied, the mechanisms underlying receptor desensitization and endocytosis are still poorly understood. We have investigated the effect of continuous agonist exposure on the human GABA(B) receptor functional response and redistribution when expressed in Chinese hamster ovary (CHO-K1) cells. The wild-type GABA(B) receptor-mediated inhibition of the adenylate cyclase activity appeared desensitized after 2 h in the presence of GABA (100 microM). Fusion proteins were generated by attachment of cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) to GB1 and GB2, respectively, and confocal microscopy experiments in intact living cells semi-stably expressing the constructs were performed. Incubation of co-expressing CFP-GB1 and YFP-GB2 cells in the presence of GABA (100 microM) for 2 h induced a profound receptor internalization, and CFP-GB1 and YFP-GB2 appeared co-localized in the endosome (labelled with Cy3-transferrin). The internalization was blocked by a selective GABA(B) receptor antagonist. These results represent the first clear visualization of agonist-induced internalization of the unique heterodimeric GABA(B) receptor.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

The heptahelical domain of GABA(B2) is activated directly by CGP7930, a positive allosteric modulator of the GABA(B) receptor

The gamma-aminobutyric acid, type B (GABA(B)) receptor is well recognized as being composed of two subunits, GABA(B1) and GABA(B2). Both subunits share structural homology with other class-III G-protein-coupled receptors. They are composed of two main domains: a heptahelical domain (HD) typical of all G-protein-coupled receptors and a large extracellular domain (ECD). Although GABA(B1) binds GABA, GABA(B2) is required for GABA(B1) to reach the cell surface. However, it is still not demonstrated whether the association of these two subunits is always required for function in the brain. Indeed, GABA(B2) plays a major role in the coupling of the heteromer to G-proteins, such that it is possible that GABA(B2) can transmit a signal in the absence of GABA(B1). Today only ligands interacting with GABA(B1) ECD have been identified. Thus, the compounds acting exclusively on the GABA(B2) subunit will be helpful in analyzing the specific role of this subunit in the brain. Here, we explored the mechanism of action of CGP7930, a compound described as a positive allosteric regulator of the GABA(B) receptor. We showed that it activates the wild type GABA(B) receptor but with a low efficacy. The GABA(B2) HD is necessary for this effect, although one cannot exclude that CGP7930 could also bind to GABA(B1). Of interest, CGP7930 could activate GABA(B2) expressed alone and is the first described agonist of GABA(B2). Finally, we show that CGP7930 retains its agonist activity on a GABA(B2) subunit deleted of its ECD. This demonstrates that the HD of GABA(B2) behaves similar to a rhodopsin-like receptor, because it can reach the cell surface alone, can couple to G-protein, and be activated by agonists. These data open new strategies for studying the mechanism of activation of GABA(B) receptor and examine any possible role of homomeric GABA(B2) receptors.

The subcellular localization of GABA(B) receptor subunits in the rat substantia nigra

The inhibitory effects of GABA within the substantia nigra (SN) are mediated in part by metabotropic GABA(B) receptors. To better understand the mechanisms underlying these effects, we have examined the subcellular localization of the GABA(B) receptor subunits, GABA(B1) and GABA(B2), in SN neurons and afferents using pre-embedding immunocytochemistry combined with anterograde or retrograde labelling. In both the SN pars compacta (SNc) and pars reticulata (SNr), GABA(B1) and GABA(B2) showed overlapping, but distinct, patterns of immunolabelling. GABA(B1) was more strongly expressed by putative dopaminergic neurons in the SNc than by SNr projection neurons, whereas GABA(B2) was mainly expressed in the neuropil of both regions. Immunogold labelling for GABA(B1) and GABA(B2) was localized in presynaptic and postsynaptic elements throughout the SN. The majority of labelling was intracellular or was associated with extrasynaptic sites on the plasma membrane. In addition, labelling for both subunits was found on the presynaptic and postsynaptic membranes at symmetric, putative GABAergic synapses, including those formed by anterogradely labelled striatonigral and pallidonigral terminals. Labelling was also observed on the presynaptic membrane and at the edge of the postsynaptic density at asymmetric, putative excitatory synapses. Double immunolabelling, using the vesicular glutamate transporter 2, revealed the glutamatergic nature of many of the immunogold-labelled asymmetric synapses. The widespread distribution of GABA(B) subunits in the SNc and SNr suggests that GABA(B)-mediated effects in these regions are likely to be more complex than previously described, involving presynaptic autoreceptors and heteroreceptors, and postsynaptic receptors on different populations of SN neurons.

GABA modulates presynaptic signalling mediated by dinucleotides on rat synaptic terminals

Diadenosine pentaphosphate (Ap(5)A) elicits Ca(2+) transients in isolated rat midbrain synaptic terminals acting through specific ionotropic dinucleotide receptors. The activation of GABA(B) receptors by baclofen changes the sigmoidal concentration-response curve for Ap(5)A (EC(50) = 44 microM) into biphasic curves. Thus, when GABA(B) receptors are activated, the curve shows a high-affinity component in the picomolar range (EC(50) = 77 pM) and a low-affinity component in the micromolar range (EC(50) = 17 microM). In addition, in the presence of GABA or baclofen, Ap(5)A calcium responses are increased up to 50% over the control values. Saclofen, a specific antagonist of GABA(B) receptors, blocks the potentiatory effect of baclofen. As occurs with Ap(5)A, GABA(B) receptors are also capable to modulate diguanosine pentaphosphate (Gp(5)G)-induced calcium responses. The combination of immunocytochemical and microfluorimetric techniques carried out on single synaptic terminals have shown that in the presence of baclofen, 64% of the terminals responding to 100 microM Ap(5)A are also able to respond to 100 nM Ap(5)A. This value is close to the percentage of synaptic terminals responding to Ap(5)A and labeled with the anti-GABA(B) receptor antibody (69%). The activity of cyclic AMP-dependent protein kinase (PKA) seems to be involved in the potentiatory effect of GABA(B) receptors on Ap(5)A calcium responses, because PKA activation by forskolin or dibutiryl cyclic AMP blocks the potentiatory effect of baclofen, whereas PKA inhibition facilitates calcium signaling mediated by Ap(5)A. These results demonstrate that the activation of presynaptic GABA(B) receptors is able to modulate dinucleotide responses in synaptic terminals.

Selective neuroinhibitory effects of taurine in slices of rat main olfactory bulb

Taurine is abundant in the main olfactory bulb, exceeding glutamate and GABA in concentration. In whole-cell patch-clamp recordings in rat olfactory bulb slices, taurine inhibited principal neurons, mitral and tufted cells. In these cells, taurine decreased the input resistance and caused a shift of the membrane potential toward the chloride equilibrium potential. The taurine actions were sustained under the blockade of transmitter release and were reversible and dose-dependent. At a concentration of 5 mM, typically used in this study, taurine showed 90% of its maximal effect. GABA(A) antagonists, bicuculline and picrotoxin, blocked the taurine actions, whereas the glycine receptor antagonist strychnine and GABA(B) antagonists, CGP 55845A and CGP 35348, were ineffective. These findings are consistent with taurine directly activating GABA(A) receptors and inducing chloride conductance. Taurine had no effect on periglomerular and granule interneurons. The subunit composition of GABA(A) receptors in these cells, differing from those in mitral and tufted cells, may account for taurine insensitivity of the interneurons. Taurine suppressed olfactory nerve-evoked monosynaptic responses of mitral and tufted cells while chloride conductance was blocked. This action was mimicked by the GABA(B) agonist baclofen and abolished by CGP 55845A; CGP 35348, which primarily blocks postsynaptic GABA(B) receptors, was ineffective. The taurine effect most likely was due to GABA(B) receptor-mediated inhibition of presynaptic glutamate release. Neither taurine nor baclofen affected responses of periglomerular cells. The lack of a baclofen effect implies that functional GABA(B) receptors are absent from olfactory nerve terminals that contact periglomerular cells. These results indicate that taurine decreases the excitability of mitral and tufted cells and their responses to olfactory nerve stimulation without influencing periglomerular and granule cells. Selective effects of taurine in the olfactory bulb may represent a physiologic mechanism that is involved in the inhibitory shaping of the activation pattern of principal neurons.

Differential expression of ?-aminobutyric acid type B receptor subunit mRNAs in the developing nervous system and receptor coupling to adenylyl cyclase in embryonic neurons

gamma-Aminobutyric acid type B receptors (GABA(B)Rs) mediate both slow inhibitory synaptic activity in the adult nervous system and motility signals for migrating embryonic cortical cells. Previous papers have described the expression of GABA(B)Rs in the adult brain, but the expression and functional significance of these gene products in the embryo are largely unknown. Here we examine GABA(B)R expression from rat embryonic day 10 (E10) to E18 compared with adult and ask whether embryonic cortical neurons contain functional GABA(B)R. GABA(B)R1 transcript levels greatly exceed GABA(B)R2 levels in the developing neural tube at E11, and olfactory bulb and striatum at E17 but equalize in most regions of adult nervous tissue, except for the glomerular and granule cell layers of the main olfactory bulb and the striatum. Consistent with expression differences, the binding affinity of GABA for GABA(B)Rs is significantly lower in adult striatum compared with cerebellum. Multiple lines of evidence from in situ hybridization, RNase protection, and real-time PCR demonstrate that GABA(B)R1a, GABA(B)R1b, GABA(B)R1h (a subunit subtype, lacking a sushi domain, that we have identified in embryonic rat brain), GABA(B)R2, and GABA(B)L transcript levels are not coordinately regulated. Despite the functional requirement for a heterodimer of GABA(B)R subunits, the expression of each subunit mRNA is under independent control during embryonic development, and, by E18, GABA(B)Rs are negatively coupled to adenylyl cyclase in neocortical neurons. The presence of embryonic GABA(B)R transcripts and protein and functional receptor coupling indicates potentially important roles for GABA(B)Rs in modulation of synaptic transmission in the developing embryonic nervous system.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Gabapentin actions on Kir3 currents and N-type Ca2+ channels via GABAB receptors in hippocampal pyramidal cells

Gabapentin is a clinically effective anticonvulsant with an unclear mechanism of action. It was described as a GABA(B(1a,2)) receptor subtype-selective agonist, activating postsynaptic K(+) currents and inhibiting postsynaptic Ca(2+) channels in CA1 pyramidal cells, but without presynaptic actions. These activities appeared controversial and we therefore sought to further clarify gabapentin actions in rat hippocampal slices by characterizing K(+) currents and Ca(2+) channels targeted by gabapentin using whole-cell recording and multiphoton Ca(2+) imaging. 1) We found that gabapentin and baclofen induced inwardly rectifying K(+) currents (K(Gbp) and K(Bac), respectively), sensitive to Ba(2+) and Cs(+). 2) A constitutively active K(IR) current, independent of GABA(B) receptor activation and sensitive to Ba(2+) and Cs(+) was also present. 3) K(Gbp), K(Bac), and K(IR) currents showed some differences in sensitivity to Ba(2+) and Cs(+), indicating the possible activation of distinct Kir3 currents, independent of K(IR), by gabapentin and baclofen. 4) Gabapentin inhibition of Ca(2+) channels was abolished by omega-conotoxin GVIA, but not by omega-agatoxin IVA and nimodipine, indicating a predominant action of gabapentin on N-type Ca(2+) channels. 5) Gabapentin actions were linked to activation of pertussis toxin-sensitive G-proteins since N-ethylmaleimide (NEM) blocked K(Gbp) activation and Ca(2+) channel inhibition by gabapentin. 6) Finally, gabapentin reduced epileptiform discharges in slices via GABA(B) receptor activation. The anticonvulsant actions of gabapentin in hippocampal cells may thus involve GABA(B) receptor coupling to G-proteins and modulation of Kir3 and N-type Ca(2+) channels. Moreover, gabapentin and baclofen activation of GABA(B) receptors may couple to distinct cellular targets.

Distribution and function of GABAB receptors in spider peripheral mechanosensilla

The mechanosensilla in spider exoskeleton are innervated by bipolar neurons with their cell bodies close to the cuticle and dendrites attached to it. Numerous efferent fibers synapse with peripheral parts of the mechanosensory neurons, with glial cells surrounding the neurons, and with each other. Most of these efferent fibers are immunoreactive to gamma-aminobutyric acid (GABA), and the sensory neurons respond to agonists of ionotropic GABA receptors with a rapid and complete inhibition. In contrast, little is known about metabotropic GABAB receptors that may mediate long-term effects. We investigated the distribution of GABAB receptors on spider leg mechanosensilla using specific antibodies against 2 proteins needed to form functional receptors and an antibody that labels the synaptic vesicles on presynaptic sites. Both anti-GABAB receptor antibodies labeled the distal parts of the sensory cell bodies and dendrites but anti-GABABR1 immunoreactivity was also found in the axons and proximal parts of the cell bodies and some glial cells. The fine efferent fibers that branch on top of the sensory neurons did not show GABAB receptor immunoreactivity but were densely labeled with anti-synapsin and indicated synaptic vesicles on presynaptic locations to the GABAB receptors. Intracellular recordings from sensory neurons innervating the slit sensilla of the spider legs revealed that application of GABAB receptor agonists attenuated voltage-activated Ca2+ current and enhanced voltage-activated outward K+ current, providing 2 possible mechanisms for controlling the neurons' excitability. These findings support the hypothesis that GABAB receptors are present in the spider mechanosensilla where their activation may modulate information transmission.

Reducing GABA receptors

A number of important drugs act on GABA(A) receptors, pentameric GABA-gated chloride channels assembled from among 19 known subunits. In trying to discover the roles in the brain of the subunits and their combinations, with the goal of developing more selective drugs, one tool has been to reduce expression of the subunits and examine the functional consequences. After briefly examining the properties of GABA(A) receptors, this review surveys the means available for receptor subunit reduction, and some of the observations to which their application has led. The methods discussed include radiation-induced deletion, gene knockout, knock-in mutations, antisense, ribozymes, RNA interference, dominant negative constructs, and transcriptional regulation, e.g., via decoy oligonucleotides.

Phosphorylation-independent desensitization of GABA(B) receptor by GRK4

Agonist-promoted desensitization of the heterodimeric metabotropic GABA(B) receptor was investigated. Whereas no desensitization was observed in HEK293 cells heterologously expressing the receptor, GABA and the synthetic agonist baclofen induced a robust desensitization in cerebellar granule cells endogenously expressing the receptor. Taking advantage of this cell-specific desensitization phenotype, we identified GRK4 as the kinase involved in the neuronal desensitization. Transfection of small interference RNA directed against GRK4 significantly reduced GRK4 levels in cerebellar granule cells and strongly inhibited the agonist-promoted desensitization. Reciprocally, transfection of GRK4 in HEK293 cells restored agonist-promoted desensitization, confirming that this kinase is sufficient to support desensitization. Surprisingly, this desensitization occurred in the absence of ligand-induced receptor phosphorylation and could be promoted by GRK4 mutants deleted of their kinase domain. Taken together, these results suggest that GRK4 plays a central role in the agonist-promoted desensitization of GABA(B) receptor and that it does so through an atypical mechanism that challenges the generally accepted model linking the kinase activity of GRKs to their role in receptor desensitization.