PubMed Health⌕ Search

Biomedical subjects

J Bormann

Publications and source records attributed to J Bormann.

At least 37 records · Page 2Linked to original sources
PubMed1994

Decreased agonist affinity and chloride conductance of mutant glycine receptors associated with human hereditary hyperekplexia.

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor alpha 1 subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1R271L or alpha 1R271Q) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [3H]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1R271L, 64 pS; alpha 1R271Q, 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyperexcitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor.

Animals↗
PubMed1994

Cyanotriphenylborate: subtype-specific blocker of glycine receptor chloride channels.

The inhibitory glycine receptor is a ligand-gated ion-channel protein existing in different homo- and heterooligomeric isoforms. Here we show that the chloride channel of the recombinant alpha 1-subunit homooligomeric glycine receptor is efficiently blocked by cyanotriphenylborate (CTB) with a concentration effecting 50% inhibition (IC50) of 1.3 microM in the presence of 50 microM glycine. The antagonistic effect of CTB is noncompetitive, use dependent, and more pronounced at positive membrane potentials, suggesting open-channel block. In contrast to alpha 1-subunit receptors, alpha 2-subunit homooligomers are resistant to CTB (IC50 >> 20 microM). By exchanging the channel-lining transmembrane segment M2 of the alpha 1 polypeptide by that of the alpha 2 polypeptide, we could transfer this resistance to alpha 1 channels, indicating that a single glycine residue at position 254 of the alpha 1 subunit is critical for CTB sensitivity. The blocker did not affect the cation-selective channel of the nicotinic acetylcholine receptor. Thus, CTB may prove useful as a tool to probe the subunit structure of native glycine receptors in mammalian neurons.

Amino Acid Sequence↗
PubMed1994

Structure, diversity and synaptic localization of inhibitory glycine receptors.

The inhibitory glycine receptor (GlyR) mediates postsynaptic inhibition in spinal cord, brain stem and other regions of the vertebrate central nervous system. Biochemical and molecular approaches have identified different developmentally and regionally regulated GlyR isoforms that result from the differential expression of at least four genes coding for different variants of the ligand-binding alpha subunit. Molecular studies have allowed identification of GlyR subunit domains implicated in ligand binding, channel formation and receptor assembly. At the postsynaptic membrane, the GlyR colocalizes with a 93-kDa tubulin-binding peripheral membrane protein, gephyrin. Antisense inhibition of gephyrin expression prevents GlyR accumulation at postsynaptic membrane specialization. Thus, gephyrin is essential for postsynaptic receptor topology.

Animals↗
PubMed1993

Functional heterogeneity of hippocampal GABAA receptors.

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.

Animals↗
PubMed1993

Pharmacology of GABA receptor Cl- channels in rat retinal bipolar cells.

gamma-Aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABAA receptors and bicuculline-insensitive cation channels through GABAB receptors. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties. Here we report that GABA gates bicuculline-insensitive Cl- channels in rod bipolar cells of the rat retina, which were not modulated by flunitrazepam, pentobarbital and alphaxalone and were only slightly blocked by picrotoxinin. Moreover, the GABAB receptor agonist baclofen, and the antagonist 2-hydroxysaclofen had no effect. The underlying single-channel conductance was 7 pS and the open time 150 ms. These values are clearly different from those obtained for GABAA receptor channels recorded in other neurons of the same preparation, and in other parts of the brain. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA). Hence they may be similar to those receptors termed GABAC receptors.

Animals↗
PubMed1993

Residues within transmembrane segment M2 determine chloride conductance of glycine receptor homo- and hetero-oligomers.

We have expressed glycine receptor (GlyR) alpha and beta subunit cDNAs in HEK-293 cells to study the functional properties of homo- versus hetero-oligomeric GlyR channels. Dose-response curves of whole-cell currents in cells expressing alpha 1 subunits revealed an average Hill coefficient of h = 4.2. Co-expression with the beta subunit markedly increased glycine-gated whole-cell currents, which now exhibited a mean Hill coefficient of only h = 2.5. For alpha 1, alpha 2 and alpha 3 homo-oligomers, the main-state single-channel conductances were 86, 111 and 105 pS, respectively, recorded at symmetrical Cl- concentrations of 145 mM. The mutant alpha 1 G221A gave rise to a main-state of 107 pS. This indicates that the main-state of alpha homo-oligomers depends on residue 221 which is located within transmembrane segment M2. Importantly, the main-state conductances of alpha 1/beta, alpha 2/beta and alpha 3/beta hetero-oligomers were only 44, 54 and 48 pS, respectively. The latter values are similar to those found in spinal neurons, suggesting that native GlyRs are predominantly alpha/beta hetero-oligomers. Co-expression of alpha 1 with mutant beta subunits revealed that residues within and close to segment M2 of the beta subunit determine the conductance differences between homo- and hetero-oligomers.

Amino Acid Sequence↗
PubMed1993

Activation of Cl- channels by avermectin in rat cultured hippocampal neurons.

The actions of the insecticide avermectin (AVM) were studied in rat cultured hippocampal neurons with patch-clamp techniques. Application of micromolar concentrations of AVM to voltage-clamped cells gave rise to whole-cell currents, which showed a slow time-course of activation in the order of 10 s, and wash-out periods of typically 20 min. Dose-response curves revealed a half-maximally activating AVM concentration (EC50) of 2.0 +/- 0.6 microM and a Hill coefficient of 1.5 +/- 0.9. The current activated by AVM was carried predominantly by Cl- ions, as demonstrated by ion-substitution experiments. The Cl- channel blocker picrotoxinin (100 microM) substantially but transiently reduced the AVM response. Outside-out patch recording showed that AVM opened Cl- channels with a conductance of 40 +/- 12 pS. The open-time distribution was characterized by two time constants of 11 ms and 259 ms. It is suggested that AVM directly activates Cl- channels in mammalian central neurons, which resemble the channels activated by the physiological transmitters GABA and glycine.

Animals↗
PubMed1992

gp120 of HIV-1 induces apoptosis in rat cortical cell cultures: prevention by memantine.

After incubation of rat cortical cell cultures with the human immunodeficiency virus type 1 (HIV-1) coat protein gp120 for 12 h, cells showed fragmentation of DNA at internucleosomal linkers, the characteristic feature of apoptosis. In a quantitative approach, it was determined that the percentage of DNA fragmentation increased from 7%, in the absence of gp120, to 62% following incubation with 24 ng/ml of gp120. Simultaneously, the percentage of viable cells decreased from 94% to 33%. Memantine (1-amino-3,5-dimethyladamantane), a drug currently used in the therapy of spasticity and Parkinson's disease as well as the NMDA antagonist MK-801 both prevented the effects of gp120 at micromolar concentrations. In human cultured astrocytes, gp120 was ineffective with respect to DNA fragmentation and cell toxicity. From these data, we conclude that the gp120-induced apoptosis may contribute to the neurological complications frequently associated with the immunodeficiency syndrome. The cytoprotective effect of memantine in cortical cell cultures may qualify the drug for the treatment of AIDS-related dementia.

Animals↗
PubMed1992

The atypical M2 segment of the beta subunit confers picrotoxinin resistance to inhibitory glycine receptor channels.

Purified preparations of the inhibitory glycine receptor (GlyR) contain alpha and beta subunits, which share homologous primary structures and a common transmembrane topology with other members of the ligand-gated ion channel superfamily. Here, a beta subunit-specific antiserum was shown to precipitate the [3H]strychnine binding sites localized on alpha subunits from membrane extracts of both rat spinal cord and mammalian cells co-transfected with alpha and beta cDNAs. Further, inhibition of alpha homo-oligomeric GlyRs by picrotoxinin, a non-competitive blocker of ion flow, was reduced 50- to 200-fold for alpha/beta hetero-oligomeric receptors generated by cotransfection. Site-directed mutagenesis identified residues within the second predicted transmembrane segment (M2) of the beta subunit as major determinants of picrotoxinin resistance. These data implicate the M2 segment in blocker binding to and lining of the GlyR chloride channel.

Amino Acid Sequence↗
PubMed1991

Effects of the 1-amino-adamantanes at the MK-801-binding site of the NMDA-receptor-gated ion channel: a human postmortem brain study.

Recent studies from our laboratory have provided evidence that the 1-amino-adamantane derivative memantine (1-amino-3,5-dimethyl-adamantane) binds to the MK-801-binding site of the N-methyl-D-aspartate (NMDA)-receptor-gated ion channel. This action has been suggested to account for the antiparkinsonian and antispastic activity of the drug. In the present investigation we have extended our work by testing a series of 1-amino-adamantanes, including amantadine (1-amino-adamantane) and memantine, for their ability to compete with [3H]MK-801 binding in membrane homogenates of postmortem human frontal cortex. The most potent substance (1-amino-3,5-diethyl-adamantane) had a Ki-value of 0.19 +/- 0.06 microM while the weakest substance (1-N-methyl-amino-adamantane) had a Ki-value of 21.72 +/- 1.63 microM. The Ki-value of amantadine was 10.50 +/- 6.10 microM. In agreement with our earlier investigation, the Ki-value of memantine was 0.54 +/- 0.23 microM. The results indicate that 1-amino-adamantanes, in general, may produce their pharmacological effects through an interaction with the NMDA-receptor-gated ion channel. The displacement of [3H]MK-801 binding thus may provide the basis to predict the antiparkinsonian and antispastic activity of novel substituted 1-amino-adamantanes and possibly of other drugs.

Adamantane↗
PubMed1991

Electrophysiological characterization of diazepam binding inhibitor (DBI) on GABAA receptors.

The gamma-aminobutyric acid (GABAA) receptor complex is a hetero-oligomeric protein which contains an integral chloride channel and several modulatory domains. The ligands of benzodiazepine recognition sites can up- or down-regulate the activity of the GABAA receptor. The effects of DBI (diazepam binding inhibitor) on GABAA receptors have been studied in cultured mammalian central neurons. Experiments performed with patch-clamp techniques, as well as with conventional intracellular microelectrodes, have revealed a reversible reduction of GABA-induced responses by micromolar concentrations of DBI. This effect was prevented by Ro 15-1788 (flumazenil), a selective benzodiazepine receptor antagonist. From these data, DBI is capable of reducing the activity of the GABAA receptor complex by specifically interacting with the benzodiazepine recognition site. The idea of DBI being a negative allosteric modulator of GABAA receptor channels is in agreement with biochemical, as well as behavioral, pharmacology data.

Animals↗
PubMed1989

Effect of atrial natriuretic factor (ANF) on nicotinic acetylcholine receptor channels in bovine chromaffin cells.

Bovine chromaffin cells have binding sites for rat atrial natriuretic factor (ANF), as demonstrated autoradiographically by using the 125I-labelled peptide. Patch-clamp recording revealed that ANF reduces acetylcholine (ACh)-induced membrane currents in chromaffin cells at physiological membrane potentials. The effect was dose-dependent with the IC50 value being 5.2 microM ANF and the Hill coefficient close to 1. The channel block was absent at positive membrane potentials, indicating a non-competitive interaction of ANF with the open ACh receptor channel. Fragments of ANF had a much less pronounced action, which is possible due to their structure and molecular charge being different to ANF. The block of nicotinic ACh receptor channels may enable ANF to control the secretion of catecholamines from adrenal chromaffin cells.

Adrenal Medulla↗