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E A Barnard

Publications and source records attributed to E A Barnard.

At least 19 recordsLinked to original sources
PubMed1992

Guanine nucleotide-binding protein-coupled and -uncoupled states of opioid receptors and their relevance to the determination of subtypes.

Opioid receptors are currently classified as mu, delta, and kappa types, but various subtypes have also been proposed. We have investigated whether subtypes exist by using [3H]bremazocine. [3H]Bremazocine binds to twice as many naloxone-sensitive sites as other nonselective opioid agonists, as shown in four membrane types that have very different ratios of mu, delta, and kappa receptor types. [3H]Bremazocine binding is completely inhibited by an excess (in unlabeled form) of other opioid ligands, with Hill coefficients of 0.8-0.95. These paradoxes can be explained if there are high- and low-affinity states of the mu, delta, and kappa receptors and bremazocine binds with similar affinities to both states. We propose that these states are the guanine nucleotide-binding protein (G-protein)-coupled form and the uncoupled form of each receptor. As evidence for this proposal, the [3H]bremazocine binding suffered little or no loss with G-protein-uncoupling treatments, whereas binding of other opioid agonists was fully sensitive. We conclude that [3H]bremazocine offers a tool for the measurement of the total pools of coupled and uncoupled opioid receptors and that much of the previous characterization of opioid receptor subtypes reflects, instead, a significant pool of G-protein-uncoupled opioid receptors.

Analgesics↗
PubMed1992

Solubilization and molecular size determination of the P2x purinoceptor from rat vas deferens.

Membranes of the rat vas deferens were shown to contain a high density of binding sites for [3H] alpha, beta-methylene ATP ([3H] alpha, beta-MeATP), a ligand selective for the P2X purinoceptor. Analysis demonstrated two classes, of high affinity (Kd = 1.8 nM, Bmax (maximum density) = 9.3 pmol/mg of protein) and of low affinity (Kd = 34 nM, Bmax = 29 pmol/mg of protein). The high affinity [3H] alpha, beta-MeATP binding sites were successfully solubilized with 2% digitonin: the Kd was then 1.6 nM. Both the association and dissociation of the receptor-ligand complex were rapid (half-time for association = 6.5 min). The rank order of potency of purinergic ligands in displacing [3H] alpha, beta-MeATP binding from the solubilized preparation was in accord with the pharmacological criteria for P2X purinoceptors. The receptor-detergent complex was separated by sucrose gradient ultracentrifugation from the ATPase enzymes also present in the preparation. The sedimentation coefficient of the receptor-detergent complex was 12.1 S. It was shown that [3H] alpha, beta-MeATP can function as a photoaffinity labeling reagent upon exposure to ultraviolet light; in the rat vas deferens membranes, it thus became cross-linked in a specific manner to a polypeptide of apparent molecular mass = 62,000 daltons, proposed to be the ligand-binding subunit of the functional P2X purinoceptor.

Adenosine Triphosphatases↗
PubMed1992

Unusual effects of benzodiazepines and cyclodiene insecticides on an expressed invertebrate GABAA receptor.

We have previously reported [(1991) EMBO J. 10, 3239-3245] the sequence of an invertebrate gamma-aminobutyric acid (GABA) type A (GABAA) receptor polypeptide which forms homo-oligomeric GABA-gated, bicuculline-sensitive, chloride-ion channels upon heterologous expression. We now demonstrate that the benzodiazepines Ro5-4864 (4'-chlorodiazepam) and diazepam, that are active at mammalian peripheral benzodiazepine sites, and not those benzodiazepines specific for central sites, directly active the homo-oligomeric receptor and evoke larger maximal responses than those elicited by GABA. In addition, members of the cyclodiene class of insecticides block the channel of the receptor in a manner indistinguishable from that of picrotoxin.

Animals↗
PubMed1992

Stable expression of mammalian type A gamma-aminobutyric acid receptors in mouse cells: demonstration of functional assembly of benzodiazepine-responsive sites.

The differential sensitivity of type A gamma-aminobutyric acid (GABAA) receptors to benzodiazepine ligands seen in the mammalian nervous system is thought to be generated by the existence of a number of different receptor subtypes, assembled from a range of closely related subunits (alpha 1-6, beta 1-3, gamma 1-3, and delta) encoded by discrete genes. The characteristics of a given subtype can be determined by the coexpression of cloned cDNAs encoding the subunits of interest. Two transient expression systems have so far been employed in the study of the ligand-binding characteristics and chloride channel properties of such GABAA receptors--Xenopus oocytes and transfected mammalian cells. Here we report on the use of a steroid-inducible promoter expression system for the production of a permanently transfected clonal cell line expressing the alpha 1 beta 1 gamma 2L GABAA receptor subtype. Using both immunoprecipitation by subunit-specific antisera and gel-exclusion chromatography, we have shown that the alpha 1, beta 1, and gamma 2L subunits coassemble to form receptor macromolecules that are of the same size as native GABAA receptors. Additionally, the recombinant receptors have the same benzodiazepine pharmacology as native alpha 1-containing GABAA receptors and function as GABA-gated chloride channels. Such cell lines expressing individual GABAA receptor subtypes will prove important tools in the study of the structure, function, and pharmacology of GABAA receptors and in the development of subtype-specific drugs.

Allosteric Regulation↗
PubMed1992

Purified unitary kainate/alpha-amino-3-hydroxy-5-methylisooxazole-propionate (AMPA) and kainate/AMPA/N-methyl-D-aspartate receptors with interchangeable subunits.

We have purified and characterized two vertebrate excitatory amino acid ionotropic receptors from the Xenopus central nervous system. Each is a unitary receptor (i.e., having more than one class of excitatory amino acid agonist specificity within one protein oligomer). The first is a unitary non-N-methyl-D-aspartate (non-NMDA) receptor and the second is a unitary NMDA/non-NMDA receptor. The specific agonist-activated channel activity and pharmacology of each type were recognized by patch-clamping lipid bilayers in which the isolated protein was reconstituted. In the second case, the NMDA and the non-NMDA sites could not be physically separated and exhibited functional interaction. Parallel evidence for this was obtained when poly(A) RNA from Xenopus brain was translated in oocytes: a noncompetitive inhibition of the response to L-kainate is produced by NMDA to a maximum depression of 30% at 1 mM NMDA. Each isolated oligomer contains 42-kDa subunits of the non-NMDA ligand binding type, but the second type has an additional NMDA-receptor-specific 100-kDa subunit. Thus, a subunit-exchange hypothesis can account for the known multiplicity of excitatory amino acid receptor types.

Animals↗
PubMed1992

Effects of subunit types of the recombinant GABAA receptor on the response to a neurosteroid.

When vertebrate brain poly(A)+ RNA is expressed in Xenopus oocytes the response of the GABA receptors formed is found to be inhibited allosterically by a neurosteroid, pregnenolone sulphate (PS). This negative modulation was reproduced after expressing RNAs encoding bovine GABAA receptor subunits in the combinations alpha i + beta 1, or alpha i + beta 1 + gamma 2 (where i = 1, 2 or 3). The characteristics of this inhibition vary significantly with the type of the alpha subunit (alpha 1, alpha 2, or alpha 3) used. When the bovine gamma 2L alternate form of the gamma 2 subunit was replaced by the human gamma 2S subunit, the behaviour was unchanged: the human gamma 2S subunit used is a newly-cloned form, which encodes a polypeptide with two amino acid differences from the human gamma 2 subunit previously described. The results of co-application of PS and 3 alpha-hydroxy-5 alpha-pregnan-ol-20-one, a neurosteroid which is a positive modulator of the GABAA receptor, indicate that these act at different sites on the receptor. PS also increases the desensitisation of the receptor by GABA. This effect, also, is alpha-subunit-type dependent and occurs by an acceleration of the fast phase of desensitisation.

Animals↗
PubMed1992

Protection by opioid ligands against modification of the opioid receptor by a carbodiimide.

Opioid receptors in membranes prepared from guinea-pig cerebellum were modified irreversibly by treatment with a water soluble carbodiimide, 1-ethyl,3-(3-dimethylaminoethyl)carbodiimide (EDAC). This decreased the number of [3H]bremazocine binding sites (Bmax reduced from 140 to 100 fmol/mg by 1 mM EDAC) without changing their affinity. When the EDAC concentration used was sufficient (500 mM) to inactivate almost all of the opioid receptors, the modification was partly prevented by inclusion of high concentrations (100 microM) of opioid agonists ([D-Ala2, MePhe4, Glyol5]-enkephalin, [D-Ala2, D-Leu5]-enkephalin,(+)-trans-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl]benzo(b)thiophene-4-acetamide hydrochloride), although they exhibited equal efficacy irrespective of their mu, delta or kappa type selectivity. However, almost all of the opioid binding sites were protected when a guanine nucleotide analogue (GppNHP, 100 microM) was also included with the agonists during carbodiimide treatment.

Animals↗
PubMed1992

Evidence for hybrid NMDA/kainate receptors from protein reconstitution studies and expression of vertebrate CNS RNAs in Xenopus oocytes.

1. A review is presented of recent advances in glutamate receptor research with particular emphasis on studies which show that some glutamate receptors in the central nervous systems (CNS) of Xenopus and rat contain a mixture of N-methyl-D-aspartate-sensitive and kainate-sensitive subunits. 2. Protein isolated from Xenopus CNS using a domoic acid affinity column exhibits complex pharmacological properties. It binds both [3H]kainate and [3H]glycine: the binding of the latter is strychnine-insensitive. 3. When reconstituted into lipid bilayers, channels gated by kainate and NMDA can be elicited and the properties of these channels are similar to those gated by kainate receptors and NMDA receptors, respectively, in studies of vertebrate central neurones in situ. 4. The protein can be fractionated into two components; one of which is sensitive only to kainate and AMPA, the other exhibiting sensitivity to both kainate and NMDA. 5. When RNA isolated from Xenopus and rat CNS is injected into Xenopus oocytes, responses to kainate and NMDA can be seen within 2-3 days. The responses to co-application of these agonists support the contention that some of the glutamate receptors expressed in oocytes contain both kainate-sensitive and NMDA-sensitive subunits.

Animals↗
PubMed1992

Receptor classes and the transmitter-gated ion channels.

Transmitter-gated channels, which can be selective for cations or for anions, form an important class among the membrane receptors responsible for signal transduction. Thirteen principal types of these channels can now be recognized and most of these are available for analysis in recombinant form. It is instructive to contrast their characteristic structural features with those of the two other primary classes of the signal-transducing receptors of membranes.

Animals↗
PubMed1992

Expression of four alternative dystrophin transcripts in brain regions regulated by different promoters.

Cognitive impairment occurs in one-third of patients with Duchenne muscular dystrophy, a lethal X-linked, recessive disease caused by mutations in the dystrophin gene which is expressed in both brain and muscle, the two transcripts having alternative first exons. Previous reports have indicated that the 'brain-type' dystrophin transcript predominates in brain. Using in situ hybridisation with antisense oligonucleotides, expression of four distinct mRNAs in specific brain areas is demonstrated here; the 14 kb muscle-type and brain-type transcripts were found to coexist in cortical and hippocampal neurons and two new transcripts have been identified in dentate gyrus and cerebellar Purkinje neurons, respectively. The latter has a novel first exon which was isolated and sequenced from mouse and human, and which would encode a protein with a different amino-terminus from the known muscle- and brain-type isoforms. Mapping in human located this exon in a large intron between the muscle-type promoter and second exon of the dystrophin gene. This finding of four alternative transcripts regulated by different promoters in brain reveals a new complexity to dystrophin expression that may have important insights for mental retardation mechanisms.

Animals↗
PubMed1992

Assembly of functional GABAA receptors in insect cells using baculovirus expression vectors.

We have constructed recombinant baculoviruses containing cDNAs encoding either the alpha 1- or the beta 1-subunit of the bovine GABAA receptor. In Spodoptera frugiperda (IPLB-Sf-21) cells infected with recombinant virus expressing either the alpha 1- or beta 1-subunit, or in cells co-infected with both viruses, functional GABAA receptors were detected by whole-cell electrophysiological recordings. The threshold for the responses mediated by the homo-oligomeric channels (alpha- or beta-) was 2-3 x 10(-6) M GABA, and for the co-infected cells was 8 x 10(-8) M GABA, suggesting that hetero-oligomeric channels formed in these cells. All GABA-induced currents were found to be inhibited by bicuculline and picrotoxin, potentiated by pentobarbital but were insensitive to benzodiazepines.

Animals↗
PubMed1992

Characterisation of an allosteric modulatory protein associated with alpha-[3H]amino-3-hydroxy-5-methylisoxazolepropionate binding sites in chick telencephalon: effects of high-energy radiation and detergent solubilisation.

alpha-[3H]Amino-3-hydroxy-5-methylisoxazolepropionate ([3H]AMPA) binds to 1-day-old chick telencephalon membranes with KD and Bmax values of 138 nM and 2.56 pmol/mg of protein, respectively. High-energy radiation bombardment of intact frozen telencephalon resulted in a biphasic inactivation curve for [3H]AMPA binding. At a 5.8-Mrad radiation dose, the affinity of [3H]AMPA binding was increased (54 nM), but there was no apparent alteration in the Bmax value (2.76 pmol/mg of protein). We attribute this phenomenon to the inactivation of a high molecular weight modulatory protein that down-regulates the affinity of [3H]AMPA binding. The estimated molecular masses of the AMPA binding site and of the modulatory component were 59 and 108 kDa, respectively. Solubilisation with n-octyl-beta-glucopyranoside resulted in an increase in the Bmax (4.7 pmol/mg of protein) with no pronounced alteration in the affinity (109 nM) of [3H]AMPA binding. However, the solubilisation-induced increase in Bmax did not occur in telencephalon irradiated before solubilisation. In contrast, the increase in affinity induced by radiation treatment was still detected in solubilised extracts. These results suggest that the number and affinity of [3H]AMPA sites in chick telencephalon are closely regulated and that the modulatory systems involved are affected by both irradiation and solubilisation.

Allosteric Regulation↗
PubMed1991

Cloning of a cDNA that encodes an invertebrate glutamate receptor subunit.

A full-length cDNA which encodes a putative glutamate receptor polypeptide was isolated from the pond snail Lymnaea stagnalis, using a short stretch of exonic sequence and two variants of the polymerase chain reaction. In this first comparison of invertebrate and vertebrate glutamate receptor sequences, the mature molluscan polypeptide, which comprises 898 amino acids and has a predicted Mr of 100,913, displays between 37% and 46% amino-acid identity to the rat ionotropic glutamate receptor subunits, GluR1 to GluR6.

Amino Acid Sequence↗
PubMed1991

Autoradiographic localisations of glutamatergic ligand binding sites in Xenopus brain.

In the Xenopus central nervous system the binding sites for [3H]kainate and [3H]alpha-amino-3-hydroxy-5-methylisoxazolepropionate [( 3H]AMPA) are highly localised and the distributions of both ligands are largely coincident. The telencephalon was the most strongly labelled area with a relatively uniform distribution of binding sites. In addition, the infundibulum and the cerebellum were also heavily labelled. Areas containing a lower density of binding sites included the septum, the thalamus and the optic lobes. [3H]Kainate binding was potently inhibited by 1 microM AMPA in the presence of 0.1 M KSCN and [3H]AMPA binding was blocked by 1 microM kainate. These data are consistent with the hypothesis that kainate and AMPA bind to the same site on a single protein entity and that this unitary AMPA/KA binding protein may constitute the predominant type of excitatory amino acid receptor in the Xenopus brain.

Animals↗
PubMed1991

AMPA and kainate-operated channels reconstituted in artificial bilayers.

Cationic channels which can be activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and kainate play a key role in the generation of excitatory postsynaptic potentials in the brain of all vertebrates. On a protein carrying binding sites for both these ligands, affinity-purified from the Xenopus nervous system, electrical measurements have been performed to investigate its functional properties after the pure complex had been incorporated into planar lipid bilayers. Domoate, AMPA or kainate added to the reconstituted protein activated cationic channels, which were blocked by typical antagonists for this neurotransmitter system. These data suggest that the reconstituted protein is an ionotropic receptor of the unitary non-NMDA subtype.

Animals↗