GABAB receptor-mediated inhibition of histamine H1-receptor-induced inositol phosphate formation in slices of rat cerebral cortex

Functional and Mechanistic Neurotoxicity Profiling Using Human iPSC-Derived Neural 3D Cultures

Neurological disorders affect millions of people worldwide and appear to be on the rise. Whereas the reason for this increase remains unknown, environmental factors are a suspected contributor. Hence, there is an urgent need to develop more complex, biologically relevant, and predictive in vitro assays to screen larger sets of compounds with the potential for neurotoxicity. Here, we employed a human induced pluripotent stem cell (iPSC)-based 3D neural platform composed of mature cortical neurons and astrocytes as a model for this purpose. The iPSC-derived human 3D cortical neuron/astrocyte co-cultures (3D neural cultures) present spontaneous synchronized, readily detectable calcium oscillations. This advanced neural platform was optimized for high-throughput screening in 384-well plates and displays highly consistent, functional performance across different wells and plates. Characterization of oscillation profiles in 3D neural cultures was performed through multi-parametric analysis that included the calcium oscillation rate and peak width, amplitude, and waveform irregularities. Cellular and mitochondrial toxicity were assessed by high-content imaging. For assay characterization, we used a set of neuromodulators with known mechanisms of action. We then explored the neurotoxic profile of a library of 87 compounds that included pharmaceutical drugs, pesticides, flame retardants, and other chemicals. Our results demonstrated that 57% of the tested compounds exhibited effects in the assay. The compounds were then ranked according to their effective concentrations based on in vitro activity. Our results show that a human iPSC-derived 3D neural culture assay platform is a promising biologically relevant tool to assess the neurotoxic potential of drugs and environmental toxicants.

Activation of phosphoinositide turnover and protein kinase C by neurotransmitters that modulate calcium channels in embryonic chick sensory neurons

Gamma aminobutyric acid (GABA) and norepinephrine modulate the excitability of primary chick sensory neurons by decreasing the voltage dependent Ca current. Although previous electrophysiological studies indicate that neurotransmitter modulation of the Ca current in these neurons involves protein kinase C, the biochemical aspects of this mechanism have not been examined directly. We find that both norepinephrine (via a unique alpha receptor subtype) and GABA (via GABAb receptors) linked to pertussis toxin sensitive pathways, stimulate the metabolism of membrane phosphatidylinositol phospholipids in primary chick sensory neurons. In addition, norepinephrine causes the rapid translocation of C kinase activity from cytosolic to membrane associated distribution, consistent with its rapid activation in response to applied neurotransmitter. The pharmacology, pertussis toxin sensitivity and time course of the biochemical changes due to neurotransmitter treatment parallel the effects of these transmitters on calcium current modulation. These biochemical studies confirm the hypothesis that activation of protein kinase C is critically involved in calcium channel modulation in embryonic chick sensory neurons.

Chronic methamphetamine administration reduces histamine-stimulated phosphoinositide hydrolysis in mouse frontal cortex

In the present study, it was hypothesized that in vivo pretreatment with repeated methamphetamine would alter the agonist-stimulated phosphoinositide hydrolysis in mouse frontal cortical slices. Male ICR mice that received the methamphetamine injection (1.0mg/kg, intraperitoneally) once a day for five consecutive days showed behavioral sensitization to the same dose of methamphetamine 5 days after the last injection of the initial chronic treatment regimen (test day 10). On test day 10, the reduction of histamine (0.1-1.0mM)-stimulated phosphoinositide hydrolysis in the mouse frontal cortex was observed. The reduction was specific to histamine, but not to norepinephrine (10 microM-0.1mM) or L-glutamate (0.1-0.5mM). The reduction occurred without any change in the expression level of histamine H(1) receptor mRNA. The reduction recovered 25 days after the last injection of the initial chronic treatment regimen (test day 30). The direct application to the slices of a pharmacologically effective concentration of methamphetamine in vitro (10 microM) did not alter the histamine signal transduction. The present results suggest that the reduction is probably one of neuroadaptations in the frontal cortex contributing to behavioral sensitization.

GABAB receptors, monoamine receptors, and postsynaptic inositol trisphosphate-induced Ca2+ release are involved in the induction of long-term potentiation at visual cortical inhibitory synapses

gamma-Aminobutyric acid (GABA)A receptor-mediated inhibitory synaptic transmission in visual cortex undergoes long-term potentiation (LTP), which is input-specific and associative. The present study, conducted under a blockade of ionotropic glutamate receptors, demonstrates an induction mechanism of LTP considerably different from those of associative LTP at excitatory synapses. Inhibitory responses of layer V cells evoked by layer IV stimulation were studied in developing rat visual cortex slices by using intracellular and whole-cell recording methods. LTP induction was prevented by the application of an antagonist for GABAB receptors but not for GABAA or metabotropic glutamate receptors. Inhibition of postsynaptic G-proteins, phospholipase C, inositol trisphosphate (IP3) receptors, or Ca2+ increase prevented the generation of LTP, as did the blockade of GABAB receptors. In rat cerebral cortex, GABAB receptor activation is not known to affect the IP3 level by itself. However, it facilitates IP3 formation induced by the activation of alpha 1 adrenoceptors, which are believed to be located postsynaptically. Accordingly, I examined the involvement of these and other amine receptors, including histamine H1, muscarinic acetylcholine, and serotonin 5-HT2 receptors, all of which are coupled to IP3 formation. Only the blockade of alpha 1 adrenoceptors or serotonin 5-HT2 receptors prevented LTP induction in most, but not all, of the cells. These results suggest that LTP induction requires the activation of postsynaptic GABAB receptors and that its effect is mediated at least partly by facilitation of the monoamine-induced IP3 formation, which then causes Ca2+ release from the internal stores in postsynaptic cells.

Evidence for pharmacologically distinct GABAB receptors associated with cAMP production in rat brain

Gamma-Aminobutyric acid-B (GABAB) receptors mediate a variety of cellular functions, suggesting the possibility of pharmacologically and molecularly distinct receptors. To explore this possibility a number of GABAB receptor agonists and antagonists were examined for their ability to influence cAMP production in rat brain cerebral cortical slices. While the agonists did not differentiate between receptors associated with the augmentation of isoproterenol-induced cAMP production and those mediating inhibition of forskolin-stimulated second messenger accumulation, significant differences were noted between the potencies of some antagonists to inhibit these GABAB receptor-mediated responses. The results suggest at least two pharmacologically distinct subclasses of GABAB receptors regulate cAMP production in brain.

Role of peripheral GABAB receptors in the regulation of pepsinogen secretion in anaesthetized rats

The purpose of the present study was to investigate the role played by GABAB receptors in the regulation of gastric basal pepsinogen secretion in anaesthetized rats. Following parenteral administration, the GABAB receptor agonists (-)-baclofen and 3-aminopropylphosphinic acid (3-APPA) caused a dose-dependent increase in basal pepsinogen secretion which was associated with a parallel increment in acid output. The gastric stimulant effects induced by both agonists were not affected by intracerebroventricular injection of the GABAB receptor antagonists 2-hydroxy-saclofen, 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348) or phaclofen, whereas the excitatory actions were antagonized by intravenously administered 2-hydroxy-saclofen or CGP 35348, but not phaclofen. In addition, the (-)-baclofen-induced increases in both pepsinogen and acid output, were fully prevented by omeprazole or cimetidine, partly reduced by atropine and unaffected by pretreatment with capsaicin. When tested on rats undergoing bilateral cervical vagotomy, both (-)-baclofen and 3-APPA were still able to stimulate the basal pepsinogen and acid secretions, although at a lesser extent than in animals with intact vagus nerves. The stimulant actions elicited by (-)-baclofen in vagotomized rats were antagonized by 2-hydroxy-saclofen or CGP 35348, but not phaclofen. Moreover, these gastric excitatory effects were prevented by cimetidine or compound 48/80, while being unaffected by atropine. The present results show that peripheral GABAB receptors mediate an excitatory effect on gastric pepsinogen secretion which totally depends on an increase in acid output. It is also suggested that both vagal cholinergic and extravagal pathways, probably histaminergic in nature, take part in these GABAB receptor-mediated gastric stimulant actions.

Desensitization by cyclic AMP-dependent protein kinase of GABAB receptor expressed in Xenopus oocytes

Involvement of cyclic AMP-dependent protein kinase in the desensitization of GABAB receptor expressed in Xenopus oocytes was studied. GABA produced an outward current in the presence of bicuculline, under voltage-clamp conditions. Baclofen mimicked the effect of GABA, thereby indicating that the response to GABA in the presence of bicuculline is mediated by stimulation of GABAB receptor. The GABAB receptor-mediated response was suppressed by dibutyric cyclic AMP and forskolin, but not by 1,9-dideoxy-forskolin, and the effect of dibutyric cyclic AMP was inhibited by H-8. Application of GABA for 1 min at 20-min intervals induced a reliable response, while that at 10-min intervals produced a desensitization. The desensitization was also partially prevented by H-8. Dibutyric cyclic GMP, 8-bromo-cyclic GMP and Na-nitroprusside did not affect the GABAB receptor-mediated response. Thus, cyclic AMP-dependent protein kinase, but not cyclic GMP-dependent protein kinase participates in desensitization of the GABAB receptor expressed in Xenopus oocytes.

Activation of 5-HT1A receptors inhibits carbachol-stimulated inositol 1,4,5-trisphosphate mass accumulation in the rodent hippocampus

We have previously demonstrated that 5-HT1A receptor agonists partially prevent the stimulation by carbachol of [3H]-phosphoinositide hydrolysis in immature rat hippocampal slices. This negative modulation has been investigated further by measuring, using a radioreceptor assay, the mass accumulation of IP3. In hippocampal slices from developing rats and in hippocampal neurons, carbachol enhanced the accumulation of IP3 and this response was partially inhibited by 8-OH-DPAT with a potency compatible with the affinity of this agonist for 5-HT1A receptors. The inhibition of the carbachol response by 8-OH-DPAT was non-competitive in nature and 8-OH-DPAT did not affect the inhibitory potency of pirenzepine. The inhibitory effect of 8-OH-DPAT was maintained after washing the slices preincubated with this compound but was not observed on the carbachol-stimulated PIP2 hydrolysis in hippocampal membranes, suggesting that this compound induces long lasting changes of muscarinic receptors and/or their effector mechanism by an indirect action.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Potentiation by 1,4-dithiothreitol of histamine-induced inositol phosphate formation in rat cerebral cortex and human HeLa cells

Dithiothreitol (1 mM) caused a 340-fold shift of the concentration-response curve for histamine-induced [3H]inositol monophosphate ([3H]IP1) accumulation in slices of rat cerebral cortex prelabelled with [3H]inositol. Dithiothreitol had no significant effect on carbachol-induced [3H]IP1 accumulation. The effect of dithiothreitol appeared to be at the level of the H1-receptor, since curves of histamine inhibition of [3H]mepyramine binding to a membrane fraction from rat cerebral cortex were shifted to lower concentrations by dithiothreitol, with a similar concentration-dependence for dithiothreitol as observed for potentiation of [3H]IP1 accumulation. The small shift of the curve of histamine inhibition of [3H]mepyramine binding by a stable GTP analogue, 5'-guanylylimidodiphosphate, was not increased in the presence of dithiothreitol. Histamine-induced [3H]IP1 accumulation in human HeLa cells was also potentiated by dithiothreitol, but the decrease in the EC50 was only 3-fold. It is suggested that differences in the magnitude of the effect of dithiothreitol on histamine H1-receptor mediated responses between tissues may reflect in part the position of the concentration-response curve in the absence of dithiothreitol.

Interaction between histamine and adenosine in human cerebromicrovascular endothelial cells: modulation of second messengers

This study demonstrates the presence of histamine H1 and H2 receptors and purinoreceptors A1 and A2 on endothelial cells derived from human brain microvessels (HBEC). Histamine induced formation of both inositol triphosphate (IP3) (EC50 = 10.2 +/- 0.9 microM) and cyclic adenosine monophosphate (cAMP) (EC50 = 5.2 +/- 0.9 microM) in HBEC in a concentration-dependent fashion. IP3 formation was inhibited by H1 receptor antagonists mepyramine maleate and chlorphenyramine, but not by H2 receptor antagonist cimetidine. Production of cAMP was efficiently inhibited by cimetidine. Selective A1 receptor agonists decreased, whereas A2 receptor agonists increased cAMP production in HBEC. When added together with histamine to HBEC cultures, both A1 and A2 receptor agonists diminished histamine-induced IP3 stimulation. This effect was reversed in the presence of specific A1 and A2 receptor antagonists, respectively. Marked augmentation of histamine-induced cAMP production by HBEC was observed in the presence of A2 agonist. This response was dependent on H1 receptors, since it was reduced in the presence of H1-receptor antagonist. It is suggested that interaction between histamine and adenosine modulating induction of second messengers in HBEC may influence endothelium-dependent responses of brain microvascular compartments.

Association and agonistic action of DN-2327, a novel isoindoline derivative, GABAB receptor in brain

The association and action of DN-2327, 2-(7-chloro-1,8-naphthyridin-2-yl)-3-[(1,4-dioxa-8- azaspiro[4,5]dec-8-yl)carbonylmethyl]isoindolin-1-one, on the gamma-aminobutyric acid (GABA)B receptor in rat brain have been examined. DN-2327 inhibited the binding of [3H]GABA to GABAB receptor in crude synaptic membrane obtained from rat brain. The Scatchard analysis of [3H]GABA binding to GABAB receptor indicated that DN-2327 induced the decrease in affinity of both high and low affinity binding sites without changing the Bmax values. The forskolin-stimulated adenylate cyclase activity in slices from rat cerebral cortex was significantly suppressed by the addition of DN-2327. Furthermore, this inhibition by DN-2327 was eliminated by the simultaneous additions of 2-hydroxy saclofen or CGP 55845A, GABAB receptor antagonists. These results suggest that DN-2327 may have not only a high association with GABAB receptor but also an agonistic action on the receptor.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of Triton X-100 on ligand binding to GABAB receptors in mouse cerebral cortex, cerebellum and whole brain

1. Pretreatment of whole brain membranes with Triton X-100 (0.01-0.05 v/v%) increased the specific binding of [3H]baclofen without change of non-specific binding. Although the treatment increased the affinity of all the GABAB agonists tested, that of antagonists either tend to be decreased or unchanged. Furthermore, the treatment potentiated Gpp(NH)p-induced inhibition of [3H]baclofen binding. 2. Affinity for [3H]baclofen in the cerebral cortex was increased following Triton X-100 (0.02 v/v%) treatment, whereas binding characteristics in the cerebellum were not significantly changed. 3. These results suggest that the properties of cerebellar and cerebral cortical GABAB receptors differ from each other, and that pretreatment with Triton X-100 not only affects agonist and antagonist binding to GABAB receptors differentially, but also changes the coupling between GABAB receptors and guanine nucleotide regulatory proteins.

Structure and function of cerebral GABAA and GABAB receptors

The receptor for GABA (gamma-aminobutyric acid), an inhibitory neurotransmitter in the brain, has been classified into GABAA and GABAB types. The GABAA receptor was purified by means of affinity column chromatography using benzodiazepine as an immobilized ligand. The results indicated that the GABAA receptor consists of several subunits and forms a GABA-gated Cl- channel, which is coupled with the benzodiazepine receptor. The molecular weight of the GABAA receptor complex was estimated to be approximately 300 kDa. Furthermore, cDNA cloning of GABAA receptor subunits was performed and the primary structure of these subunits was deduced. The results suggested that these subunits possess four transmembrane domains in their structure which are important for the formation of the Cl- channel. On the other hand, activation of GABAB receptors induced the inhibition of adenylyl cyclase activity and phosphatidylinositol turnover via inhibitory GTP-binding proteins such as G(i) and/or G(o). The GABAB receptor was purified using baclofen affinity and immunoaffinity column chromatographies. It was confirmed that the purified GABAB receptor protein is about 80 kDa in its molecular weight. This protein is capable of inducing the inhibition of adenylyl cyclase when it is reconstituted with G(i)/G(o) protein in the phospholipid vesicle system. Currently available data indicate that GABAA and GABAB receptors in the central nervous system are distinct not only in terms of their molecules but also their signal transduction systems. However, the primary structure and synaptic localization of GABAB receptor molecules in the brain remain to be clarified.

Pharmacology, distribution, cellular localization, and development of GABAB binding in rodent cerebellum

Quantitative receptor autoradiography using [3H]GABA under selective conditions was used to characterize the pharmacology, distribution, cellular localization, and development of GABAB binding sites in rodent cerebellum. Pharmacologic analysis of [3H]GABA binding showed that drugs active at GABAB receptors displaced [3H]GABA with the following order of potency: 3-aminopropylphosphonous acid > CGP 35348 = 2-hydroxysaclofen > phaclofen. GTP-gamma-S and GDP-beta-S also diminished potently [3H]GABA binding in a dose-dependent manner. The pattern of [3H]GABA binding to GABAB binding sites was systematically mapped throughout the rat cerebellum. GABAB binding was greatest in the molecular layer and a pattern of parasagittal zonation was observed in the molecular layer of lobules VII-X in adult rats. The cellular localization of GABAB binding was investigated using lesion techniques. Neither methyl azoxymethanol lesions of cerebellar granule cells nor 3-acetylpyridine lesions of climbing fibers resulted in a decrease in [3H]GABA binding. Homozygote stumbler mutant mice, deficient in Purkinje cell dendrites, had a significant decrease in [3H]GABA binding in the molecular layer. These results suggest that the majority of cerebellar molecular layer GABAB binding sites detected by [3H]GABA autoradiography are located on Purkinje cell dendrites. Examination of [3H]GABA binding to GABAB binding sites during development revealed that binding in the molecular layer peaks between postnatal day 14 and postnatal day 28 and then decreases to adult levels. Transient expression of high levels of GABAB binding was observed in the deep cerebellar nuclei, peaking at postnatal day 3 and decreasing to adult levels by postnatal day 21. Our investigation of GABAB pharmacology yielded data in agreement with previously reported results. We have described a parasagittal pattern of GABAB binding in the cerebellar molecular layer and assigned the majority of cerebellar GABAB binding sites to Purkinje cell dendrites. Finally, development studies reveal transient peaks in GABAB binding in the cerebellar molecular layer and deep cerebellar nuclei.

Regional effects of pertussis toxin in vivo and in vitro on GABAB receptor binding in rat brain

Agonist binding to GABAB receptors modulates the activity of the guanine nucleotide binding proteins, Go and Gi. These G proteins are ADP-ribosylated by pertussis toxin and this prevents them from coupling to the GABAB receptor resulting in a reduction in high-affinity GABAB binding. GTP, which binds to a different site on the G protein alpha subunit, also reduces the affinity of the receptor for the G protein, and this can be used as a "marker" for G protein-GABAB receptor linkage. We have examined GABAB binding site distribution in rat brain after unilateral intrahippocampal pertussis toxin injection in vivo, and after incubating brain slices in pertussis toxin in vitro, using the technique of receptor autoradiography. The effect of pertussis toxin was compared with that of GTP gamma S on GABAB binding. Intrahippocampal pertussis toxin administration reduced GABAB but not GABAA receptor binding and the effects appeared to be limited by pertussis toxin diffusion. More widespread reductions in GABAB binding were seen after incubation of brain slices in vitro but the extent varied in different brain regions. No reduction was detected in the corpus striatum. GABAB binding was also reduced in membranes prepared from cerebral cortex, hippocampus and cerebellum but there was no significant reduction in the corpus striatum after pertussis toxin treatment. GTP gamma S reduced GABAB binding to a similar extent in all areas studied irrespective of their sensitivity to pertussis toxin suggesting that while GABAB binding sites are linked to G proteins throughout the rat brain, those in the corpus striatum may be predominantly pertussis toxin insensitive.

Monoclonal antibody to GABA binding protein, a possible GABAB receptor

A monoclonal antibody has been raised against a partially purified preparation for the GABAB receptor. The antibody recognized a protein of about 80 kDa in bovine brain synaptic membrane. Immunoabsorbent agarose beads conjugated with the antibody were able to remove, without visible changes in electrophoresed profiles of total proteins, over 90% of the baclofen suppressive GABA binding activity (designated herein, GABAB receptor binding activity) in the solubilized synaptic membrane fraction. Moreover, the addition of GB-1 antibody directly inhibited the GABA binding activity in the crude synaptic membrane fraction. These results indicate that the monoclonal antibody obtained here recognizes the GABA binding protein, or more specifically a GABAB receptor.

Regulation of GABAB receptors by histamine and neuronal activity in the isolated spinal cord of neonatal opossum in culture

The aim of these experiments has been to analyse the properties of receptors for the transmitter gamma-aminobutyric acid (GABA) in developing mammalian nervous system. Changes in responses of GABAB receptors have been measured after alterations of the chemical environment and the level of electrical activity. We have previously shown that when the central nervous system (CNS) of the new-born opossum, Monodelphis domestica, is cultured for three to five days in the presence of histidine, inhibition by baclofen, a GABAB agonist, disappears (Stewart et al. 1991). We have now investigated whether histidine acts indirectly by way of conversion to histamine. As with histidine, culture with 150 microM histamine for five days virtually abolished the inhibition by baclofen. The effects of histidine, as well as histamine, were blocked by mepyramine, a histamine H1-receptor antagonist, and by ranitidine, an H2-antagonist. Tetrodotoxin (TTX), which blocks all electrical activity, protected preparations from the action of histidine but not histamine. Our results suggest that histidine is converted to histamine, which reduces the efficacy of GABAB agonists. We conclude that, in the developing mammalian CNS, transmitter levels and electrical activity can selectively influence the properties of receptors.

Pharmacological and biochemical characteristics of partially purified GABAB receptor

Pharmacological and biochemical characteristics of the partially purified gamma-aminobutyric acid (GABA)B receptor using baclofen affinity column chromatography have been examined. The Scatchard analysis of [3H]GABA binding to the purified GABAB receptor showed a linear relationship and the KD and Bmax values were 60 nM and 118 pmol/mg of protein, respectively. Although GTP and Mg2+ did not affect on the GABAB receptor binding, Ca2+ significantly increased [3H]GABA binding to the purified GABAB receptor in a dose-dependent manner and showed its maximum effect at 2 mM. The enhancement of the binding by Ca2+ was found to be due to the increase of Bmax by the Scatchard analysis. The treatments with pronase and trypsin significantly decreased the binding of [3H]GABA, but phospholipase A2 had no significant effect on the binding. In addition, treatment with glycosidases such as glycopeptidase A and beta-galactosidase significantly decreased the binding of [3H]GABA to the purified GABAB receptor. These results suggest that purification of the solubilized GABAB receptor by the affinity column chromatography may result in the functional uncoupling of GABAB receptor with GTP-binding protein. Furthermore, the present results suggest that cerebral GABAB receptor may be a glycoprotein and membrane phospholipids susceptible to phospholipase A2 treatment may not be involved in the exhibition of the binding activity.

Characteristics of histamine-induced inositol phospholipid hydrolysis in HeLa cells

HeLa cells, a human epithelial carcinoma, contain Histamine H1-receptors, as defined by [3H] mepyramine binding studies. Histamine, via H1-receptors, stimulates the production of inositol phosphates (IP) with an EC50 of 1.2 microM. Preincubation of HeLa cells with histamine, followed by a further histamine challenge results in a slower time course of [3H] IP production, suggesting the occurrence of H1-receptor desensitisation.

Solubilization and partial purification of GABAB receptor from bovine brain

gamma-Aminobutyric acid (GABA)B receptor has been solubilized and partially purified by an affinity column chromatography. GABAB receptor was solubilized by 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) in the presence of asolectin. The solubilized GABAB receptor was adsorbed on baclofen-coupled epoxy-activated Sepharose 6B. The affinity matrix adsorbed 80% of the solubilized [3H]GABA binding activity to GABAB receptor, and approximately 75% of the adsorbed activity could be eluted with 1 M KC1. GABAB receptor binding in the fraction eluted from affinity column was displaced by GABA, baclofen and 2-hydroxy saclofen in a dose-dependent manner. Furthermore, the purified GABAB receptor showed approximately 2800-fold purification as compared with the original solubilized fraction and possessed the specific binding activity of 17.68 p mol/mg of protein. This binding consisted of a single binding site with a dissociation constant of 64.4 nM. The present results indicate that affinity column chromatographic procedures using baclofen-coupled epoxy-activated Sepharose 6B are suitable for the partial purification of GABAB receptor from cerebral tissues.

GABAB receptors expressed in Xenopus oocytes by guinea pig cerebral mRNA are functionally coupled with Ca2(+)-dependent Cl- channels and with K+ channels, through GTP-binding proteins

Transmembrane currents induced by (-)-baclofen (BAC), a specific agonist of the gamma-aminobutyric acid-B (GABAB) receptor, in Xenopus oocytes injected with guinea pig cerebral mRNA were electrophysiologically and pharmacologically characterized under a voltage-clamp condition. The oocytes injected with mRNA acquired responsiveness to BAC and showed two types of currents at a holding potential of -50 mV. One was the slow and smooth inward current which had a short latency and associated with a decrease in membrane conductance, and its amplitude was decreased by hyperpolarization and increased by depolarization. The other was the large fast oscillatory inward current with a long-latency, which was decreased in amplitude by depolarization and reversed at -26 mV. Both currents were not blocked by bicuculline but were depressed by 2-hydroxysaclofen (2-OH-SAC), though the smooth current was less sensitive to 2-OH-SAC; about 40% blockade at the 2-OH-SAC concentration capable of abolishing the oscillatory current. The smooth current was depressed by Ba2+. The oscillatory current was time-dependently attenuated and almost abolished by intracellularly injected pertussis toxin (PTX), while the smooth current was not depressed by this toxin even when the oscillatory current was nearly abolished. The intracellular injection of GTP-gamma-S into oocytes attenuated both oscillatory and smooth currents. These results suggest the possibility that GABAB receptors expressed in Xenopus oocytes by cerebral mRNA are functionally coupled with two signal transduction systems, one is the opening of Ca2(+)-dependent Cl- channels mediated by PTX-sensitive GTP-binding protein(s) and the other is the closure of K+ channels through PTX-insensitive GTP-binding protein(s).

Histamine H1-receptor in heart: unique electrophoretic mobility and autoradiographic localization

Histamine H1-receptors, visualized in the guinea pig heart by autoradiography using [125I]iodobolpyramine as a specific probe, are abundant in the nodal tissue and cardiac vessels but also occur heterogeneously in the myocardium. Following photoaffinity labeling with [125I]iodoazidophenpyramine and electrophoresis, the ligand binding domain of the heart H1-receptor was shown to be present on a major 68-kDa and a less abundant 54- to 58-kDa protein. The 68-kDa protein displayed a molecular size higher in heart than in all other tissues (56 kDa). This indicates the existence of at least two isoforms of the H1-receptor; the cardiac isoform, however, was pharmacologically indistinguishable from the common isoform studied in cerebellar membranes using available ligands. Its distinct electrophoretic properties suggest that the cardiac isoform may have a unique function.

gamma-Aminobutyric acid inhibition of histamine-induced inositol phosphate formation in guinea-pig cerebellum: comparison with guinea-pig and rat cerebral cortex

1. gamma-Aminobutyric acid (GABA), 2 mM, inhibited basal accumulation of [3H]-inositol monophosphate ([3H]-IP1) in lithium-treated slices of guinea-pig cerebellum preincubated with [3H]-inositol. In contrast, 2 mM GABA stimulated the accumulation of [3H]-IP1 in rat cerebral cortical slices over a 60 min incubation period, but had no significant effect in slices of guinea-pig cerebral cortex. The estimated IC50 for the inhibitory action of GABA in guinea-pig cerebellar slices was 0.52 +/- 0.12 mM. 2. GABA inhibited histamine-induced [3H]-IP1 accumulation in guinea-pig cerebellar slices in a non-competitive manner. The best-fit value for the maximum level of inhibition was 74 +/- 6%. The estimated IC50 for GABA was 0.77 +/- 0.15 mM and was not significantly different from the IC50 for inhibition of the basal accumulation of [3H]-IP1. The response to histamine in guinea-pig and rat cerebral cortical slices was also inhibited by 2 mM GABA. 3. In guinea-pig cerebellar slices 2 mM GABA potentiated histamine-induced [3H]-inositol bisphosphate ([3H]-IP2) accumulation, whereas in both guinea-pig and rat cerebral cortex the effect was inhibition. 4. Isoguvacine and muscimol, GABAA-selective agonists, and (-)-baclofen, GABA(B)-selective, had no significant effect on basal or histamine-stimulated accumulation of [3H]-IPs in guinea-pig cerebellar slices. (-)-Baclofen had only a weak inhibitory effect on [3H]-IP1 accumulation in guinea-pig-cerebral cortex (16 +/- 6% inhibition with 10 microM (-)-baclofen), whereas in rat cerebral cortex (-)-baclofen mimicked the inhibitory effect of GABA. 5. Nipecotic acid (1 mM) had qualitatively similar effects to those of 2mm GABA in guinea-pig cerebellar slices. 6. The competitive GABA uptake inhibitors SK&F 89976-A, SK&F 100330-A and SK&F 100561-A were potent histamine H,-receptor antagonists, as indicated by the inhibition of [3H]-mepyramine binding to homogenates of guinea-pig cerebellum and cerebral cortex. 7. GABA (2 mM) caused a small inhibition (12 + 3%) of [3H]-inositol incorporation into total inositol phospholipids in guinea-pig cerebellar slices, as in rat cerebral cortical slices, whereas 0.2mm histamine caused a small stimulation (15 + 4%). In the presence of both GABA and histamine, [3H]-inositol incorporation was unchanged from basal (101 + 5%). 8. GABA also inhibited [3H]-IP1 formation induced by endothelin-1 in guinea-pig cerebellar slices and increased, but not significantly, the amount of [3H]-IP2 accumulated. This, taken with the inhibitory effect on basal and histamine-stimulated accumulation, suggests that the action of GABA in guinea-pig cerebellar slices may be non-selective and may not be exerted through a specific GABA receptor.

Modulation of phosphoinositide metabolism in rat brain slices by excitatory amino acids, arachidonic acid, and GABA

In rat brain slices the synthesis of [3H]phosphoinositides and the production of [3H]inositol monophosphate (IP1) induced by norepinephrine (NE) were inhibited by glutamate. Calcium concentrations were varied to test if these inhibitory effects of glutamate were mediated by a calcium-dependent process. Although reducing calcium or addition of the calcium antagonist verpamil reduced the inhibitory effects of glutamate, these results were equivocal because reduced calcium directly decreased agonist-induced [3H]phosphoinositide synthesis. The inhibitory effects of glutamate were mimicked by quisqualate in a dose-dependent manner, but none of a variety of excitatory amino acid receptor antagonists modified the inhibition caused by quisqualate. It is suggested that glutamate activates a quisqualate-sensitive receptor (for which an antagonist is not available) and causes inhibition of phosphoinositide hydrolysis mediated in part by a direct or indirect inhibitory effect of calcium on phosphoinositide synthesis. Modulatory effects of arachidonic acid were examined because glutamate and calcium can activate phospholipase A2. Arachidonic acid caused a rapid and dose-dependent inhibition of [3H]phosphoinositide synthesis and of NE-stimulated [3H]IP1 production. A similar inhibition of the response to carbachol also occurred. The inhibition caused by arachidonic acid was unchanged by addition of inhibitors of cyclooxygenase or lipoxygenase. Activation of phospholipase A2 with melittin caused inhibitory effects similar to those of arachidonic acid. Inhibitors of phospholipase A2 were found to impair phosphoinositide metabolism, likely due to their lack of specificity for phospholipase A2. Further studies were carried out in slices that were prelabelled with [3H]inositol in an attempt to separate modulatory effects on [3H]phosphoinositide synthesis and agonist-stimulated [3H]IP1 production. Several excitatory amino acid agonists inhibited NE-stimulated [3H]IP1 production. This inhibitory interaction could be due to impaired synthesis of [3H]phosphoinositides because, even though the slices were prelabeled, addition of unlabelled inositol reduced NE-stimulated [3H]IP1 production, indicating that continuous regeneration of [3H]phosphoinositides is required. In contrast to the inhibitory effects of the excitatory amino acids, gamma-aminobutyric acid (GABA) enhanced the response to NE in cortical and hippocampal slices. GABA also enhanced the response to carbachol in hippocampal and striatal slices and to ibotenic acid in hippocampal slices. Baclofen potentiated the response to NE similarly to the effect of GABA and baclofen partially blocked the inhibitory effect of arachidonic acid but did not alter that of quisqualate.

Ca2(+)-dependence provides evidence for differing mechanisms of GABA-induced inositol phosphate formation and GABA potentiation of inositol phosphate formation induced by noradrenaline in rat cerebral cortex

[3H]Inositol phosphate formation ([3H]IP) induced by gamma-aminobutyric acid (GABA) in slices of rat cerebral cortex prelabelled with [3H]inositol was abolished in Krebs-Henseleit medium without added Ca2+, but could be restored by addition of Ca2+. In contrast, GABA potentiation of noradrenaline-induced [3H]IP accumulation in the same tissue was still observed in the absence of added Ca2+. This provides evidence (a) that the effect of GABA alone on [3H]IP formation is indirect and (b) that the mechanism of this effect is unlikely to be the same as for the potentiation by GABA of [3H]IP accumulation induced by noradrenaline.

Potentiation by gamma-aminobutyric acid of alpha 1-agonist-induced accumulation of inositol phosphates in slices of rat cerebral cortex

Noradrenaline-induced accumulation of 3H-labeled inositol mono-, bis-, and trisphosphate (IP1, IP2, and IP3, respectively) in lithium-treated slices of rat cerebral cortex preincubated with [3H]inositol was potentiated by gamma-aminobutyric acid (GABA). However, the effect on [3H]IP2 accumulation was much greater than that on [3H]IP1 or [3H]IP3 accumulation. The principal effect of GABA on noradrenaline concentration-response curves for both [3H]IP1 and [3H]IP2 was to cause an increase in the maximal response attainable. However, whereas the EC50 for GABA potentiation of [3H]IP1 formation was 0.5 mM, the curve for the potentiation of [3H]IP2 formation showed a marked upturn at GABA concentrations of greater than 1 mM. Prazosin (1 microM) blocked the noradrenaline-induced formation of all three inositol phosphates (IPs), in both the presence and the absence of 2 mM GABA. 3H-IP formation induced by phenylephrine and methoxamine was also potentiated by GABA, and again the greatest effect was on [3H]IP2 accumulation. The ratio of [3H]IP2/[3H]IP1 formed in response to 100 microM noradrenaline was increased by 2 mM GABA at all times from 10 to 60 min, whereas the ratio of [3H]IP3/[3H]IP1 was little altered. The effect of GABA was not mimicked by the GABAA agonists isoguvacine and 3-aminopropanesulphonic acid and was not blocked by bicuculline methiodide. (-)-Baclofen, a GABAB agonist, did produce some stimulation of the response to noradrenaline, but to a much lesser extent than GABA. Of the agents tested, nipecotic acid came nearest to reproducing the effect of GABA, in that the major effect was on [3H]IP2 accumulation. The effects of 2 mM GABA and 2 mM nipecotic acid were not additive. GABA potentiation of noradrenaline-induced 3H-IP formation was still apparent in the absence of Li+, but the increase of [3H]IP2 content was less than that of [3H]IP1 content.