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1.
The present work was undertaken to characterize kinetics, including activation, desensitization and deactivation, of responses mediated by GABAA and GABAC receptors on carp retinal bipolar cells, using the whole-cell patch-clamp technique. It was revealed that the GABAC response was generally slower in kinetics than the GABAA response. Activation kinetics of both the receptors could be well fit by monoexponential functions with time constants t, being 44.57 ms (GABAC) and 10.86 ms (GABAA) respectively. Desensitization of the GABAAresponse was characterized by a fast and a slow exponential component with time constants of τfast = 2.16 s and τslow = 19.78 s respectively, whereas desensitization of the GABAC response was fit by a monoexponential function of the time constant τ = 6.98 s. Deactivation at both the receptors was adequately described by biexponential functions with time constants being much higher for the GABAC response (τfast= 674.8 ms; τslow = 2 090 ms) than those for the GABAA response (τfast = 42.07 ms; τslow = 275.1 ms). These differences in kinetics suggest that GABAC and GABAA receptors may be involved in processing signals in different frequency domains.  相似文献   

2.
ABSTRACT

In the search for yet unknown subtypes of GABAB receptors, the subunit architecture of GABAB receptors in the retina was analyzed using selective antisera. Immunopurification of the splice variants GABAB1a and GABAB1b demonstrated that both were associated with GABAB2. Quantitative immunoprecipitation experiments indicated that practical the entire GABAB receptor population in the retina consists of the receptor subtypes GABAB1a/GABAB2 and GABAB1b/GABAB2, although low levels of GABAB1c/GABAB2 cannot be excluded. The data rule out the existence of GABAB receptors containing the splice variants GABAB1d and GABAB1e. Moreover, no evidence for homomeric GABAB1 receptors was found. Among the splice variants, GABAB1a is by far the predominant one in neonatal and adult retina, whereas GABAB1b is expressed only late in postnatal development and in the adult retina. Since GABAB1a is expressed at high levels before functional synapses are formed, this specific receptor subtype might be involved in the maturation of the retina. Finally, subcellular fractionation demonstrated that GABAB1a, but not GABAB1b, is present in postsynaptic densities, suggesting a differential pre- and postsynaptic localisation of both splice variants.  相似文献   

3.
Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (not reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.  相似文献   

4.
Gamma-amino butyric acid (GABA), in addition to being a metabolic intermediate and the main inhibitory neurotransmitter in the synaptic cleft, is postulated as a neurohormone, a paracrine signaling molecule, and a trophic factor. It acts through pre- and post-synaptic receptors, named GABAA and GABAC (ionotropic receptors) and GABAB (metabotropic receptor). Here we reviewed the participation of GABAB receptors in the regulation of the hypothalamic-pituitary-gonadal axis, using physiological, biochemical, and pharmacological approaches in rats, as well as in GABAB1 knock-out mice, that lack functional GABAB receptors. Our general conclusion indicates that GABAB receptors participate in the regulation of pituitary hormone secretion acting both in the central nervous system and directly on the gland. PRL and gonadotropin axes are affected by GABAB receptor activation, as demonstrated in the rat and also in the GABAB1 knock-out mouse. In addition, hypothalamic and pituitary GABAB receptor expression is modulated by steroid hormones. GABA participation in the brain control of pituitary secretion through GABAB receptors depends on physiological conditions, being age and sex critical factors. These results indicate that patients receiving GABAB agonists/antagonists should be monitored for possible endocrine side effects.  相似文献   

5.
GABAA receptors are the major inhibitory transmitter receptors in the central nervous system. They are chloride ion channels that can be opened by γ-aminobutyric acid (GABA) and are the targets of action of a variety of pharmacologically and clinically important drugs. GABAA receptors are composed of five subunits that can belong to different subunit classes. The existence of 19 different subunits gives rise to the formation of a large variety of distinct GABAA receptor subtypes in the brain. The majority of GABAA receptors seems to be composed of two α, two β and one γ subunit and the occurrence of a defined subunit stoichiometry and arrangement in αβγ receptors strongly indicates that assembly of GABAA receptors proceeds via defined pathways. Based on the differential ability of subunits to interact with each other, a variety of studies have been performed to identify amino acid sequences or residues important for assembly. Such residues might be involved in direct protein-protein interactions, or in stabilizing direct contact sites in other regions of the subunit. Several homo-oligomeric or hetero-oligomeric assembly intermediates could be the starting point of GABAA receptor assembly but so far no unequivocal assembly mechanism has been identified. Possible mechanisms of assembly of GABAA receptors are discussed in the light of recent publications.  相似文献   

6.
γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system and exerts its actions via ionotropic (GABAA and GABAC) and metabotropic (GABAB) receptors. The GABAB receptor is a dimer composed of R1 and R2 components. In addition to their location on neurons, GABA and functional GABAB receptors also have been detected in some peripheral tissues. In the present study, we combined immunohistochemistry, immunoblot and tension recording to determine if the human fallopian tube express glutamic acid decarboxylase (GAD65/67), two isoforms for synthesis of GABA and functional GABAB receptors. Immunoblots showed that the human fallopian tube tissue contained GABABR1 protein which was localized in the epithelial cells and smooth muscle cells by immunohistochemistry. In addition, epithelial cells also expressed GAD65/67. Tension recording found that both GABA and baclofen, a GABAB receptor agonist increased the spontaneous activity of human fallopian tube. The expressions of GABABR and GAD65/67 were significantly upregulated in the ectopic pregnancy group than in the intrauterine pregnancy group. We conclude that the human fallopian tube is capable of synthesizing GABA and expresses functionally active GABAB receptors. An upregulation of GABA synthesis and corresponding GABAB receptors may involve in ectopic pregnancy.  相似文献   

7.
In the central nervous system (CNS), the inhibitory transmitter GABA interacts with three subtypes of GABA receptors, type A, type B, and type C. Historically, GABA receptors have been classified as either the inotropic GABAA receptors or the metabotropic GABAB receptors. Over the past 10 yr, studies have shown that a third class, called the GABAC receptor, also exists. GABAC receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Although GABAA and GABAC receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The ρ subunit of the GABAC receptor, which has about 35% amino acid homology to GABAA receptor subunits, was cloned from the retina and, when expressed inXenopus oocytes, has properties similar to retinal GABAC receptors. There are probably distinct roles for GABAC receptors in the retina, because they are found on only a subset of neurons, whereas GABAA receptors are ubiquitous. This article reviews recent electrophysiological and molecular studies that have characterized the unique properties of GABAC receptors and describes the roles that these receptors may play in visual information processing in the retina.  相似文献   

8.
Sixteen known 5-HT3 receptor blockers, including clozapine, fully or partially reverse the inhibitory effect of 1 M GABA on [35S]TBPS binding, indicating that they are also GABAA antagonists, some of them selective for subsets of GABAA receptors. The 5-HT3 receptor blocker, ondansetron, has been reported to produce some antipsychotic and anxiolytic effects. However, no antipsychotic effects have been reported for a large number of highly potent 5-HT3 receptor blockers. Like clozapine, ondansetron partially reverses the inhibitory effect of GABA on [35S]TBPS binding. Additivity experiments suggest that ten 5-HT3 receptor blockers tested at low concentrations preferentially block subtypes of GABAA receptors that are among those blocked by clozapine. Wiley and Porter (29) reported that MDL-72222, the most potent GABAA antagonist decribed here, partially generalizes (71%) with clozapine in rats trained to discriminate an interoceptive clozapine stimulus, but only at a dose that severly decreases responding. Tropisetron (ICS-205,930) exhibits both GABA-positive and GABA-negative effects. R-(+)-zacopride is 6-fold more potent than S-(–)-zacopride as a GABAA antagonist. We conclude that the observed antipsychotic and, possibly, anxiolytic effects of some 5-HT3 receptor blockers are due to selective antagonism of certain GABAA receptors, and not to blockade of 5-HT3 receptors. We speculate that the anxiolytic and sedative effects of clozapine and several other antipsychotic drugs may be due to selective blockade of 122 GABAA receptors which are preferentially located on certain types of GABAergic interneurons (probably parvalbumin positive). Blockade of these receptors will increase the inhibitory output of these interneurons. So far, no highly potent GABAA antagonists with clozapine-like selectivity have been identified. Such compounds may exhibit improved clozapine-like antipsychotic activity.  相似文献   

9.
Hippocampal pyramidal neurons potentially express multiple subtypes of GABAA receptors at extrasynaptic locations that could therefore respond to different drugs. We activated extrasynaptic GABAA receptors in cultured rat hippocampal pyramidal neurons and measured single-channel currents in order to compare the actions of two drugs that potentially target different GABAA receptor subtypes. Despite the possible difference in receptor targets of etomidate and diazepam, the two drugs were similar in their actions on native extrasynaptic GABAA receptors. Each drug produced three distinct responses that differed significantly in current magnitude, implying heterogeneous GABAA receptor populations. In the majority of patches, drug application increased both the single-channel conductance (>40 pS) and the open probability of the channels. By contrast, in the minority of patches, drug application caused an increase in open probability only. In the third group high-conductance channels were observed upon GABA activation and drug application increased their open probability only. The currents potentiated by etomidate or diazepam were substantially larger in patches displaying high-conductance GABA channels compared to those displaying only low-conductance channels. Factors contributing to the large magnitude of these currents were the long mean open time of high-conductance channels and the presence of multiple channels in these patches. In conclusion, we suggest that the local density of extrasynaptic GABAA receptors may influence their single-channel properties and may be an additional regulating factor for tonic inhibition and, importantly, differential drug modulation. This work is dedicated to the memory of Professor P. W. Gage.  相似文献   

10.
Major pelvic ganglia (MPG) are relay centers for autonomic reflexes such as micturition and penile erection. MPG innervate the urogenital system, including bladder. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system, and may also play an important role in some peripheral autonomic ganglia, including MPG. However, the electrophysiological properties and function of GABAA receptor in MPG neurons innervating bladder remain unknown. This study examined the electrophysiological properties and functional roles of GABAA receptors in bladder-innervating neurons identified by retrograde Dil tracing. Neurons innervating bladder showed previously established parasympathetic properties, including small membrane capacitance, lack of T-type Ca2+ channel expression, and tyrosine-hydroxylase immunoreactivity. GABAA receptors were functionally expressed in bladder innervating neurons, but GABAC receptors were not. GABA elicited strong depolarization followed by increase of intracellular Ca2+ in neurons innervating bladder, supporting the hypothesis GABA may play an important role in bladder function. These results provide useful information about the autonomic function of bladder in physiological and pathological conditions.  相似文献   

11.
GABAC receptors are ligand-gated chloride channels and have important roles in some neurological functions like vision. Recent investigations demonstrated that these receptors are also expressed in the somatosensory cortex. In this study, we investigated the effect of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) (GABAC receptor antagonist) on the properties of the neuronal response to natural stimuli (whisker deflection) in deep layers of rat barrel cortex. Twenty-eight male Wistar rats, weighing 230–260?g, were used in this study. TPMPA (100?μmol/rat) was administered intracerebroventricularly (ICV). Neuronal responses to deflection of principal (PW) and adjacent (AW) whiskers were recorded in barrel cortex using tungsten microelectrodes. A computer-controlled mechanical displacement was used to deflect whiskers individually or in combination at 30?ms inter-stimulus intervals. ON and OFF responses for PW and AW deflections were measured. A condition-test ratio (CTR) was computed to quantify neuronal responses to whisker interactions. Our data suggest that ICV administration of TPMPA increased neuronal spontaneous activity, ON and OFF responses to PW, and/or AW deflections. However, CTR for neither ON nor OFF responses changed following TPMPA administration. The results of this study demonstrated that inhibition of GABAC receptors by TPMPA can modulate neuronal response properties in rat barrel cortex.  相似文献   

12.
We found that Tyr-Leu (YL) dose-dependently exhibits potent anxiolytic-like activity (0.1-1 mg/kg, i.p.) comparable to diazepam in the elevated plus-maze test in mice. YL was orally active (0.3-3 mg/kg). A retro-sequence peptide or a mixture of Tyr and Leu was inactive. The anxiolytic-like activity of YL was inhibited by antagonists for serotonin 5-HT1A, dopamine D1 and GABAA receptors; however, YL had no affinity for them. We also determined the order of their activation is 5-HT1A, D1 and GABAA receptors using selective agonists and antagonists. Taken together, YL may exhibit anxiolytic-like activity via activation of 5-HT1A, D1 and GABAA receptors.  相似文献   

13.
The expression of GABAA receptors in rat cerebellar granules in culture has been studied by β2/3 subunit immunocytochemistry and fluorescence confocal microscopy. These cells show labeling all over the cell bodies' plasma membrane and dendrites. Treatment with the protein tyrosine kinase (PTK) inhibitor genistein results in a decrease of the labeling associated with the β2/3 subunit in both cell bodies and dendrites. No effect was found with an inactive genistein analogue, daidzein. A similar effect was found with a protein kinase C (PKC) activator, phorbol myristate acetate (PMA). The effects of genistein and PMA are additive.The interpretation of the results is that PTK inhibition blocks exocytotic deposit of newly synthesized GABAA receptors onto the neuronal plasma membrane. On the other hand, PKC activation speeds up endocytotic removal of GABAA receptors.  相似文献   

14.
A radioiodinated probe, [125I]-CGP 71872, containing an azido group that can be photoactivated, was synthesized and used to characterize GABAB receptors. Photoaffinity labeling experiments using crude membranes prepared from rat brain revealed two predominant ligand binding species at 130 and 100 kDa believed to represent the long (GABABR1a) and short (GABABR1b) forms of the receptor. Indeed, these ligand binding proteins were immunoprecipitated using a GABAB receptor-specific antibody confirming the receptor specificity of the photoaffinity probe. Most convincingly, [125I]-CGP 71872 binding was competitively inhibited in a dose-dependent manner by cold CGP 71872, GABA, saclofen, (−)-baclofen, (+)-baclofen and ( )-glutamic acid with a rank order and stereospecificity characteristic of the GABAB receptor. Photoaffinity labeling experiments revealed that the recombinant GABABR2 receptor does not bind [125I]-CGP 71872, providing surprising and direct evidence that CGP 71872 is a GABABR1 selective antagonist. Photoaffinity labeling experiments using rat tissues showed that both GABABR1a and GABABR1b are co-expressed in the brain, spinal cord, stomach and testis, but only the short GABABR1b receptor form was detected in kidney and liver whereas the long GABABR1a form was selectively expressed in the adrenal gland, pituitary, spleen and prostate. We report herein the synthesis and biochemical characterization of the nanomolar affinity [125I]-CGP 71872 and CGP 71872 GABABR1 ligands, and differential tissue expression of the long GABABR1a and short GABABR1b receptor forms in rat and dog.  相似文献   

15.
Previously we have reported the presence of endogenous ligands that are involved in the regulation of the binding of muscimol to the GABA binding site of the GABAA receptors. Here, we report the presence of multiple forms of endogenous ligands in the brain which modulate the binding of flunitrazepam (FNZP) to the benzodiazepine (BZ) binding site of the GABAA receptor. Furthermore, one of the endogenous ligands for the BZ receptors, referred to as EBZ, has been identified as inosine based on the following observations: (1) standard inosine and the EBZ have identical NMR and UV spectra; (2) the elution profile of inosine and the EBZ from a HPLC column are indistinguishable, and (3) inosine and the EBZ show identical activity in inhibiting [3H]FNZP binding.  相似文献   

16.

Background

Magnolia bark preparations from Magnolia officinalis of Asian medicinal systems are known for their muscle relaxant effect and anticonvulsant activity. These CNS related effects are ascribed to the presence of the biphenyl-type neolignans honokiol and magnolol that exert a potentiating effect on GABAA receptors. 4-O-methylhonokiol isolated from seeds of the North-American M. grandiflora was compared to honokiol for its activity to potentiate GABAA receptors and its GABAA receptor subtype-specificity was established.

Methods

Different recombinant GABAA receptors were functionally expressed in Xenopus oocytes and electrophysiological techniques were used determine to their modulation by 4-O-methylhonokiol.

Results

3 μM 4-O-methylhonokiol is shown here to potentiate responses of the α1β2γ2 GABAA receptor about 20-fold stronger than the same concentration of honokiol. In the present study potentiation by 4-O-methylhonokiol is also detailed for 12 GABAA receptor subtypes to assess GABAA receptor subunits that are responsible for the potentiating effect.

Conclusion

The much higher potentiation of GABAA receptors at identical concentrations of 4-O-methylhonokiol as compared to honokiol parallels previous observations made in other systems of potentiated pharmacological activity of 4-O-methylhonokiol over honokiol.

General significance

The results point to the use of 4-O-methylhonokiol as a lead for GABAA receptor potentiation and corroborate the use of M. grandiflora seeds against convulsions in Mexican folk medicine.  相似文献   

17.
Subunit Composition and Function of GABAA Receptors of Rat Spermatozoa   总被引:1,自引:0,他引:1  
GABA triggers mammalian sperm acrosome reaction (AR). Here, evidence is presented, showing that rat spermatozoa contain GABAA receptors, composed of 5, 1 and 3 subunits. The effects of GABAA receptor agonist and antagonist on the induction of AR in rat spermatozoa were assessed using the chlortetracycline assay. Muscimol, a GABAA receptor agonist, triggered AR; whereas bicuculline, a GABAA receptor antagonist and picrotoxin, a GABAA receptor/Cl channel blocker, inhibited the ability of GABA or progesterone to induce AR. In conclusion, GABAA receptors appear to mediate the action of progesterone in inducing AR in rat spermatozoa.  相似文献   

18.
The age-related development of GABABreceptors and their coupling to adenylate cyclase were studied in the brains of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, the specific [3H]GABA binding to GABABreceptors showed a significant decrease not only in the posterior hypothalamus, midbrain, hippocampus and striatum of eleven-week-old SHR, which maintain a hypertensive state, but also in the posterior hypothalamus of four-week-old normotensive SHR. Similarly, the GABABreceptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity showed a decrease in the posterior hypothalamus of four-week-old SHR as well as in the posterior hypothalamus and striatum of eleven-week-old SHR. These results suggest that the functions of the GABABreceptor in the brain of SHR may be decreased independently from the occurrence of blood pressure elevation and that such changes may even be involved in the pathogenesis of SHR.  相似文献   

19.
Subunit-specific antibodies to all the γ subunit isoforms described in mammalian brain (γ1, γ2S, γL, and γ3) have been made. The proportion of GABAA receptors containing each γ subunit isoform in various brain regions has been determined by quantitative immunoprecipitation. In all tested regions of the rat brain, the γ1, and γ3 subunits are present in considerable smaller proportion of GABAA receptor than the γ2 subunit. Immunocytochemistry shows that γ1 immunoreactivity concentrates in the stratum oriens and stratum radiatum of the CA1 region of the hippocampus. In the dentate gyrus, γ1 immunoreactivity concentrates on the outer 2/3 of the molecular layer coinciding with the localization of the axospinous synapses of the perforant pathway. In contrast, γ3 immunoreactivity concentrates on the basket cells and other GABAergic local circuit neurons of the hilus. These cells are also rich in γ2S. In the cerebellu, γ1 immunolabeling was localized on the Bergmann glia. The γ2S and γ2L subunits are differentially expressed in various brain regions. Thus the γ2S is highly expressed in the olfactory bulb and hippocampus whereas the γ2L is very abundant in inferior colliculus and cerebellum, particularly in Purkinje cells, as immunocytochemistry, in situ hybridization and immunoprecipitation techniques have revealed. The γ2S and γ2L coexist in some brain areas and cell types. Moreover, the γ2S and γ2L subunits can coexist in the same GABAA receptor pentamer. We have shown that this is the case in some GABAA receptors expressed in cerebellar granule cells. These GABAA receptors also have α and β subunits forming the pentamer. Immunoblots have shown that the rat γ1, γ2S, γ2L and γ3 subunits are peptides of 47, 45, 47 and 44 kDa respectively. Results also indicate that there are aging-related changes in the expression of the γ2S and γ2L subunits in various brain regions which suggest the existence of aging-related changes in the subunit composition of the GABAA receptors which in turn might lead to changes in receptor pharmacology. The results obtained with the various γ subunit isoforms are discussed in terms of the high molecular and binding heterogeneity of the native GABAA receptors in brain. Special issue dedicated to Dr. Kinya Kuriyama  相似文献   

20.
Gabapentin, a novel anticonvulsant and analgesic, is a -aminobutyric acid (GABA) analogue but was shown initially to have little affinity at GABAA or GABAB receptors. It was recently reported to be a selective agonist at GABAB receptors containing GABAB1a-GABAB2 heterodimers, although several subsequent studies disproved that conclusion. In the present study, we examined whether gabapentin is an agonist at native GABAB receptors using a rat model of postoperative pain in vivo and periaqueductal gray (PAG) slices in vitro; PAG contains GABAB receptors, and their activation results in antinociception. An intrathecal injection of gabapentin or baclofen, a GABAB receptor agonist, induced antiallodynia in this postoperative pain model. Intrathecal injection of GABAB receptor antagonists CGP 35348 and CGP 55845 antagonized baclofen- but not gabapentin-induced antiallodynia. In ventrolateral PAG neurons, baclofen activated G-protein-coupled inwardly rectifying K+ (GIRK) channels in a manner blocked by CGP 35348 or CGP 55845. However, gabapentin displayed no effect on the membrane current. In neurons unaffected by gabapentin, baclofen activated GIRK channels through GABAB receptors. It is concluded that gabapentin is not an agonist at GABAB receptors that are functional in baclofeninduced antiallodynia in the postoperative pain model in vivo and in GIRK channel activation in ventrolateral PAG neurons in vitro.  相似文献   

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