Blockade of GABAB receptors accelerates amygdala kindling development.

The aim of this study was to investigate the putative role of GABAB receptors in the development of amygdala kindling in rats. The effects of the GABAB blocker CGP 35348 and the GABAB agonist baclofen on the progressive development of behavioural seizure symptoms (stages 1-5 classified by Racine) and duration of after-discharges (AD) were studied. CGP 35348 at a dose of 300 mg/kg i.p., which blocks central GABAB receptors, moderately but consistently accelerated the development of behavioural seizure symptoms. CGP 35348 had no marked effect on the duration of ADs corresponding to the different seizure stages. L-baclofen (6 mg/kg i.p.) had a dual effect on kindling development. It retarded the development of the behavioural symptoms, but increased the duration of AD. In conclusion, the results suggest that synaptically-released GABA activated GABAB receptors and thereby exerted a depressant effect on kindling development.     

Molecular and functional heterogeneity of GABAergic synapses

Knowledge of the functional organization of the GABAergic system, the main inhibitory neurotransmitter system, in the CNS has increased remarkably in recent years. In particular, substantial progress has been made in elucidating the molecular mechanisms underlying the formation and plasticity of GABAergic synapses. Evidence available ascribes a key role to the cytoplasmic protein gephyrin to form a postsynaptic scaffold anchoring GABA(A) receptors along with other transmembrane proteins and signaling molecules in the postsynaptic density. However, the mechanisms of gephyrin scaffolding remain elusive, notably because gephyrin can auto-aggregate spontaneously and lacks PDZ protein interaction domains found in a majority of scaffolding proteins. In addition, the structural diversity of GABA(A) receptors, which are pentameric channels encoded by a large family of subunits, has been largely overlooked in these studies. Finally, the role of the dystrophin-glycoprotein complex, present in a subset of GABAergic synapses in cortical structures, remains ill-defined. In this review, we discuss recent results derived mainly from the analysis of mutant mice lacking a specific GABA(A) receptor subtype or a core protein of the GABAergic postsynaptic density (neuroligin-2, collybistin), highlighting the molecular diversity of GABAergic synapses and its relevance for brain plasticity and function. In addition, we discuss the contribution of the dystrophin-glycoprotein complex to the molecular and functional heterogeneity of GABAergic synapses.     

The influence of GABA on discharges of cortical epileptogenic focus

Summary GABA, when applied locally, acted similarly on both primary and mirror cortical focus: the negative component of the spike discharge was suppressed or inverted in polarity, whereas the late slow negative wave was strongly potentiated. Recordings from deep cortical layers suggested a different origin of these 2 surface-negative components of focal discharges.     

Bicuculline and picrotoxin block gamma-aminobutyric acid-gated Cl- conductance by different mechanisms

Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for gamma-aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.     

Releasable GABA in tubular epithelium of rat kidney

The distribution of gamma-aminobutyric acid (GABA) in the rat kidney was examined by immunocytochemical techniques. GABA-like immunoreactivity (GABA-LI) was predominantly confined to the renal tubules, including the ascending parts of the distal tubules, and the loops of Henle, the collecting tubules and ducts, and the connective parts of the convoluted tubules. In GABA-positive cortical tubules, about half of the epithelial cells were labelled. The labelled cell type showed the ultrastructural features of principal cells. Depolarizing stimulation by ouabain and high K+ concentration evoked the efflux of endogenous GABA from kidney slices. The present findings, along with previous results, suggest that GABA released from renal tubular epithelium, and transported with the urine, might be involved in the modulation of contractility in the urinary tract.     

Releasable GABA in tubular epithelium of rat kidney

Summary The distribution of gamma-aminobutyric acid (GABA) in the rat kidney was examined by immunocytochemical techniques. GABA-like immunoreactivity (GABA-LI) was predominantly confined to the renal tubules, including the ascending parts of the distal tubules, and the loops of Henle, the collecting tubules and ducts, and the connective parts of the convoluted tubules. In GABA-positive cortical tubules, about half of the epithelial cells were labelled. The labelled cell type showed the ultrastructural features of principal cells. Depolarizing stimulation by ouabain and high K⁺ concentration evoked the efflux of endogenous GABA from kidney slices. The present findings, along with previous results, suggest that GABA released from renal tubular epithelium, and transported with the urine, might be involved in the modulation of contractility in the urinary tract.     

Bicuculline and picrotoxin block γ-aminobutryic acid-gated Cl− conductance by different mechanisms

Summary Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for -aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.To whom reprint requests should be addressed.Acknowledgments. We thank Drs S. Minakami and S. Yasui for helpful discussions and comments.     

Role of neurotransmitter release and cyclic AMP-dependent membrane phosphorylation in low voltage myocardial automaticity

Low voltage myocardial automaticity (LVA) was investigated by pharmacological modulations of the presynaptic and postsynaptic processes. The sensitivity of LVA both to inhibitor and stimulator of neurotransmitter release suggests its involvement in LVA genesis. Moreover, LVA is blocked by the inhibition of the cyclic AMP system, supporting the participation of the c-AMP-dependent membrane phosphorylation in calcium-mediated cardiac electrogenesis.     

The glycinergic inhibitory synapse

Glycine is one of the most important inhibitory neurotransmitters in the spinal cord and the brainstem, and glycinergic synapses have a well-established role in the regulation of locomotor behavior. Research over the last 15 years has yielded new insights on glycine neurotransmission. Glycinergic synapses are now known not to be restricted to the spinal cord and the brainstem. Presynaptic machinery for glycine release and uptake, the structure and function of postsynaptic receptors and the factors (both pre- and postsynaptic) which control the strength of glycinergic inhibition have been extensively studied. It is now established that glycinergic synapses can be excitatory in the immature brain and that some inhibitory synapses can corelease γ-aminobutyric acid (GABA) and glycine. Moreover, the presence of glycine transporters on glial cells and the capacity of these cells to release glycine suggest that glycine may also act as a neuromodulator. Extensive molecular studies have revealed the presence of distinct subtypes of postsynaptic glycine receptors with different functional properties. Mechanisms of glycine receptors aggregation at postsynaptic sites during development are better understood and functional implications of variation in receptor number between postsynaptic sites are partly elucidated. Mutations of glycine receptor subunits have been shown to underly some human locomotor disorders, including the startle disease. Clearly, recent work on glycine receptor channels and the synapses at which they mediate inhibitory signalling in both young and adult animals necessitates an update of our vision of glycinergic inhibitory transmission. Received 8 September 2000; received after revision 1 December 2000; accepted 21 December 2000     

Benzodiazepine receptors resolved

To date, attempts to map the distribution and density of benzodiazepine receptors in the CNS have been dominated by radiohistochemical techniques with conventional receptor binding. Their limited resolution, however, prompted us to try an immunohistochemical approach. Purified GABA/benzodiazepine receptors, prepared from bovine cerebral cortex, have been used to raise monoclonal antibodies for this purpose. Immunoreactive sites in rat brain, spinal cord and retina as well as in bovine and post-mortem human brain were found to be concentrated on neuronal cell bodies and processes in those regions known to be innervated by GABAergic neurons. Electron microscopic analysis revealed a selective staining of axosomatic and axodendritic pre- and postsynaptic contacts.     

Benzodiazepine receptors resolved

Summary To date, attempts to map the distribution and density of benzodiazepine receptors in the CNS have been dominated by radiohistochemical techniques with conventional receptor binding. Their limited resolution, however, prompted us to try an immunohistochemical approach. Purified GABA/benzodiazepine receptors, prepared from bovine cerebral cortex, have been used to raise monoclonal antibodies for this purpose. Immunoreactive sites in rat brain, spinal cord and retina as well as in bovine and post-mortem human brain were found to be concentrated on neuronal cell bodies and processes in those regions known to be innervated by GABAergic neurons. Electron microscopic analysis revealed a selective staining of axosomatic and axodendritic pre- and postsynaptic contacts.     

Role of neurotransmitter release and cyclic AMP-dependent membrane phosphorylation in low voltage myocardial automaticity

Summary Low voltage myocardial automaticity (LVA) was investigated by pharmacological modulations of the presynaptic and postsynaptic processes. The sensitivity of LVA both to inhibitor and stimulator of neutrotransmitter release suggests its involvement in LVA genesis. Moreover, LVA is blocked by the inhibition of the cyclic AMP system, supporting the participation of the c-AMP-dependent membrane phosphorylation in calcium-mediated cardiac electrogenesis.Acknowledgment. The generous gift of RM1 12330A from Prof. G. Guellen, Paris, and of tolbutamide from Prof. T. Hayakawa, Tokyo, are gratefully acknowledged.     

On the mechanism of the amphetamine induced vasodilatation at the rat's cerebral cortex

Summary Cerebral cortical blood flow was measured with the hydrogen clearance technique. It was found that the increase in CoBF induced by amphetamine is blocked by atropine or chlorpromazine.This work was supported by grants from Consejo Nacional de Investigaciones Cientificas y Tecnicas and Consejo de Investigaciones de la Provincia de Santa Fé, Argentina.     

Ethanol and the benzodiazepine-GABA receptor-ionophore complex

Ethanol has a pharmacological profile similar to that of classes of drugs like benzodiazepines and barbiturates, which enhance GABAergic transmission in the mammalian CNS. Several lines of behavioral, electrophysiological and biochemical studies suggest that ethanol may bring about most of its effects by enhancing GABAergic transmission. Recently, ethanol at relevant pharmacological concentrations has been shown to enhance GABA-induced 36Cl-fluxes in cultured spinal cord neurons, synaptoneurosomes and microsacs. These enhancing effects of ethanol were blocked by GABA antagonists. Ro15-4513, an azido analogue of classical BZ antagonist Ro15-1788, reversed most of the behavioral effects of ethanol and other effects involving 36Cl-flux studies. The studies summarized below indicate that most of the pharmacological effects of ethanol can be related to its effects on GABAergic transmission.     

Ethanol and the benzodiazepine-GABA receptor-ionophore complex

Summary Ethanol has a pharmacological profile similar to that of classes of drugs like benzodiazepines and barbiturates, which enhance GABAergic transmission in the mammalian CNS. Several lines of behavioral, electrophysiological and biochemical studies suggest that ethanol may bring about most of its effects by enhancing GABAergic transmission. Recently, ethanol at relevant pharmacological concentrations has been shown to enhance GABA-induced³⁶Cl-fluxes in cultured spinal cord neurons, synaptoneurosomes and microsacs. These enhancing effects of ethanol were blocked by GABA antagonists. Ro15-4513, an azido analogue of classical BZ antagonist Ro15-1788, reversed most of the behavioral effects of ethanol and other effects involving³⁶Cl-flux studies. The studies summarized below indicate that most of the pharmacological effects of ethanol can be related to its effects on GABAergic transmission.     

gamma-Aminobutyric acid uptake by rat kidney brush-border membrane vesicles

Brush-border membrane vesicles (BBMV) from rat kidney cortex possessed two uptake systems for gamma-aminobutyric acid (GABA), a high affinity system (Km = 10.9 microM) and a low affinity system (Km = 1203 microM). Both uptake systems were inhibited by p-hydroxymercuribenzoic acid and ouabain, and by the action of neuraminidase, whereas the GABA analogs nipecotic acid, beta-alanine, 2,4-diaminobutyric acid and 4,5,6,7-tetrahydroisoxazolo-[4,5 c]-pyridin-3-ol had no effect on the GABA uptake activity. The BBMV uptake systems were clearly different from the GABA transport systems present in brain tissue.     

γ-Aminobutyric acid (GABA) signalling in plants

The role of γ-aminobutyric acid (GABA) as a signal in animals has been documented for over 60 years. In contrast, evidence that GABA is a signal in plants has only emerged in the last 15 years, and it was not until last year that a mechanism by which this could occur was identified—a plant ‘GABA receptor’ that inhibits anion passage through the aluminium-activated malate transporter family of proteins (ALMTs). ALMTs are multigenic, expressed in different organs and present on different membranes. We propose GABA regulation of ALMT activity could function as a signal that modulates plant growth, development, and stress response. In this review, we compare and contrast the plant ‘GABA receptor’ with mammalian GABA_A receptors in terms of their molecular identity, predicted topology, mode of action, and signalling roles. We also explore the implications of the discovery that GABA modulates anion flux in plants, its role in signal transduction for the regulation of plant physiology, and predict the possibility that there are other GABA interaction sites in the N termini of ALMT proteins through in silico evolutionary coupling analysis; we also explore the potential interactions between GABA and other signalling molecules.     

Existence of different populations of unitary evoked postsynaptic potentials at the frog neuromuscular junction]

The postsynaptic response to monoquantic evoked transmitter release (mean quantal content about 0.3) has been studied at temperatures from 10 to 23 degrees C. The delay between nerve stimulation (1 to 10 Hz) and the unitary postsynaptic potentials fluctuates by steps. The existence of preferential delay sites can always be detected (mean number 13.5 +/- 3.1). Identical delay unitary postsynaptic potentials often shows identical amplitude and identical time to peak. These results suggest that few emitting sites are preferentially activated along the nerve terminal at low level release during long lasting stimulation. The "single process" assumption used in statistical studies of transmitter release is probably oversimplified.     

Action ofβ-(4-chlorophenyl-GABA) on uptake and metabolism of GABA in different subcellular fractions of rat brain

Summary -(4-chlorophenyl)-GABA, a GABA mimetic compound, acts as an inhibitor of GABA metabolism in both synaptosomal and extrasynaptosomal compartments. It has no significant action on GABA or Glu uptake by synaptosomes.     

Structural mechanisms of the agrin–LRP4–MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin–LRP4–MuSK signaling pathway.