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PubMed1992

O-acylated lysergol and dihydrolysergol-I derivatives as competitive antagonists of 5-HT at 5-HT2 receptors of rat tail artery. Allosteric modulation instead of pseudoirreversible inhibition.

Twelve ergolines (O-acylated lysergol and dihydrolysergol-I derivatives) were synthesized to study their antagonism of 5-HT responses in comparison with methylsergide and LY 53857 [6-methyl-1-(1-methylethyl)-8 beta-ergoline carboxylic acid 2-hydroxy-1-methylpropyl-ester hydrogen maleate] in cylindrical segments of the isolated rat tail artery. With regard to (9.10-didehydro-6-methyl-8 beta-ergoline)methyl R,S-2-methylbutyrate, the most potent new ergoline derivative, we examined the phenomenon of insurmountable antagonism to 5-HT by methylsergide. O-Acylated lysergol and dihydrolysergol-I derivatives competitively antagonized 5-HT-induced contractions with calculated pA2 values of 7.30 +/- 0.42 for the weakest and 8.42 +/- 0.35 for the most potent ergoline derivative in this series. N1-isopropyl substitution did not generally enhance 5-HT2 receptor affinities but lowered affinities for alpha 1 adrenoceptors in rat aorta. Methysergide and LY 53857 were insurmountable antagonists of 5-HT in rat tail artery. Preincubation with (9.10-didehydro-6-methyl-8 beta-ergoline)methyl R,S-2-methylbutyrate (1 mumol/l) partially prevented the depression of 5-HT-induced contractions caused by methysergide (1-10 nmol/l). Methysergide (100 nmol/l) abolished the protective effect of (9.10-didehydro-6-methyl-8 beta-ergoline)methyl R,S-2-methylbutyrate. (9.10-Didehydro-6-methyl-8 beta-ergoline)methyl R,S-2-methylbutyrate (1 mumol/l), concomitantly incubated with methysergide (30 nmol/l), partially restored the maximum response to 5-HT that had been depressed by methysergide (30 nmol/l). Partial restoration could not be mimicked by washout of methysergide.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Allosteric modulation of the NMDA receptor by dihydrolipoic and lipoic acid in rat cortical neurons in vitro.

The mitochondrial cofactor dihydrolipoic acid (DHLA) was observed to potentiate N-methyl-D-aspartate (NMDA), but not non-NMDA, receptor-mediated whole-cell responses in cultured neurons. This potentiation was readily reversed by the oxidizing agent 5,5'-dithio-bis-(2-nitro-benzoic acid) (DTNB). DHLA was unable to increase NMDA responses previously potentiated by dithiothreitol, nor did it have an effect on NMDA receptors alkylated with N-ethylmaleimide. Single-channel recordings revealed that DHLA produced an increase in NMDA channel open frequency, with no change in single-channel conductance or open time. In contrast, lipoic acid reversed the potentiation of NMDA-evoked responses produced by dithiothreitol and had no effect on NMDA receptors previously oxidized by DTNB. DHLA and lipoic acid are pervasively found substances that readily permeate cellular membranes and thus may influence NMDA receptor activity in vivo by modifying its redox site.

Animals↗
PubMed2002

Arylalkylamines are a novel class of positive allosteric modulators at GABA(B) receptors in rat neocortex.

Using grease-gap recording from rat neocortical slices, the gamma-aminobutyric acid(B) (GABA(B)) receptor agonists baclofen (3-100 microM) and SKF 97541 (3-aminopropyl-methylphosphinic acid) (1-30 microM) elicited reversible and concentration-dependent hyperpolarizing responses, with EC(50) values of 10 and 3 microM, respectively. The hyperpolarizations were antagonised by the GABA(B) receptor antagonist Sch 50911 ((+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid) (1, 5 and 10 microM). Fendiline (N-[3,3-diphenylpropyl)-alpha-methylbenzylamine) (5-50 microM) and its congeners, prenylamine (N-[3,3-diphenylpropyl)-alpha-methylphenylethylamine) (10-100 microM) and F551 (N-[3,3-diphenylpropyl)-alpha-methyl-3-methoxybenzylamine) (1-30 microM) reversibly enhanced hyperpolarizing responses to the agonists; such effects were reduced by Sch 50911. These arylalkylamines produced leftward shifts of the concentration-response curves, with a marked increase in the maximal hyperpolarization obtained, compared with the agonists alone, F551 being the most potent. These findings suggest that these arylalkylamines represent a new class of positive modulators of GABA(B) receptor-mediated function.

Animals↗
PubMed2001

The highly conserved COOH terminus of troponin I forms a Ca2+-modulated allosteric domain in the troponin complex.

The primary structure of the COOH-terminal region of troponin I (TnI) is highly conserved among the cardiac, slow, and fast skeletal muscle TnI isoforms and across species. Although no binding site for the other thin filament proteins is found at the COOH terminus of TnI, truncations of the last 19-23 amino acid residues reduce the activity of TnI in the inhibition of actomyosin ATPase and result in cardiac muscle malfunction. We have developed a specific monoclonal antibody (mAb), TnI-1, against the conserved COOH terminus of TnI. Using this mAb, isolation of the troponin complex by immunoaffinity chromatography from muscle homogenate and immunofluorescence microscopic staining of myofibrils indicate that the COOH terminus of TnI forms an exposed structure in the muscle thin filament. Binding of this mAb to the COOH terminus of cardiac TnI induced extensive conformational changes in the protein, suggesting an allosteric role of this region in the functional integrity of troponin. In the absence of Ca2+, the binding of troponin C and troponin T to TnI had very little effect on the conformation of the COOH terminus of TnI as indicated by the unaffected mAb affinity for the TnI-1 epitope. However, Ca2+ significantly increased the accessibility of the TnI-1 epitope on TnI in the presence of troponin C and troponin T. The results provide evidence that the COOH terminus is an essential structure in TnI and participates in the allosteric switch during Ca2+ activation of contraction.

Allosteric Site↗
PubMed1996

The mechanism of velocity modulated allosteric regulation in D-3-phosphoglycerate dehydrogenase. Cross-linking adjacent regulatory domains with engineered disulfides mimics effector binding.

D-3-Phosphoglycerate dehydrogenase (PGDH) (EC 1.1.1.95) from Escherichia coli is an allosterically regulated enzyme of the Vmax type. It is a tetramer of identical subunits and each subunit is made up of three identifiable domains, the cofactor binding domain, the substrate binding domain, and the regulatory domain. Each subunit contacts two other subunits through adjacent cofactor binding domains and through adjacent regulatory domains. L-Serine, the physiological effector, inhibits catalytic activity by apparently tethering regulatory domains from adjacent subunits together through the formation of hydrogen bonds to each subunit. This investigation demonstrates that cross-linking adjacent regulatory domains with engineered disulfides produces catalytic inhibition in the absence of inhibitor in a manner similar to that produced by the inhibitor. The inhibition due to cross-linking can be completely reversed in a concentration dependent manner by dithiothreitol. The active mutant enzyme, containing the engineered cysteines in the reduced state, retains its ability to be inhibited by L-serine, although at a 100-fold higher concentration. Hill plots of the serine inhibition of mutant and native enzyme indicate that the number of interacting sites remains at 2 in the mutant enzyme. The reversible inhibition of enzyme activity that results from tethering adjacent regulatory domains with engineered disulfides suggests that these domains move in some manner relative to one another during the active to inhibited state transition. These observations support the model which predicts that catalytic activity is regulated by the movement of rigid domains about flexible hinges and that effector binding prevents this by locking the regulatory domains in a state that produces an open active site cleft.

Allosteric Regulation↗
PubMed1996

The mechanism of velocity modulated allosteric regulation in D-3-phosphoglycerate dehydrogenase. Site-directed mutagenesis of effector binding site residues.

D-3-Phosphoglycerate dehydrogenase (EC 1.1.1.95) from Escherichia coli catalyzes the first committed step in serine biosynthesis and is allosterically regulated by L-serine, the end product of the pathway. Each subunit of the homotetramer is made up of three distinct domains with one of the intersubunit contacts being between adjacent regulatory domains. Each regulatory domain interface contains two symmetrical serine binding sites such that serine forms hydrogen bonds to both domains across the interface. Previous work (Al-Rabiee, R., Lee, E. J., and Grant, G. A. (1996) J. Biol. Chem. 271, 13013-13017) demonstrated that when adjacent regulatory domains are covalently linked to one another by engineered disulfide bonds, the enzyme was inactivated. Breaking the disulfide bonds by reduction restored enzymatic activity. This study demonstrates that the complementary situation is also true. Site-directed mutagenesis of three residues at the effector binding site, His344, Asn346, and Asn364', render the enzyme increasingly less susceptible to inhibition by the effector. When mutations result in a situation where it is no longer possible to establish a stable hydrogen bonding network across the regulatory domain interface, the inhibitory capacity of the effector is lost. Furthermore, mutations that produce as much as 5 orders of magnitude decrease in the ability of L-serine to inhibit the enzyme have no appreciable effect on the Km or kcat of the enzyme. These observations support the model that predicts that catalytic activity in D-3-phosphoglycerate dehydrogenase is regulated by the movement of adjacent regulatory domains about a flexible hinge and that effector binding tethers the regulatory domains together producing a state that results in a stable, open active site cleft that is no longer able to promote catalysis.

Allosteric Regulation↗
PubMed1998

Estrogen response elements function as allosteric modulators of estrogen receptor conformation.

The estrogen receptor (ER) is a ligand-dependent transcription factor that regulates the expression of estrogen-responsive genes. ER-mediated transcriptional changes are brought about by interaction of the ER with the estrogen response element (ERE). In this study, we examined the interaction of the Xenopus laevis ER DNA binding domain (DBD) and the intact ER with the X. laevis vitellogenin A2 ERE and the human pS2 ERE. Using gel mobility shift, DNase I footprinting, and methylation interference assays, we demonstrated that the DBD bound only as a dimer to the A2 ERE. However, the DBD bound as a monomer to the consensus pS2 ERE half site at lower DBD concentrations and then as a homodimer to the consensus and imperfect pS2 ERE half site at higher DBD concentrations. Antibody supershift experiments carried out with partially purified, yeast-expressed full-length ER demonstrated that three ER-specific antibodies interacted differentially with A2 and pS2 ERE-bound ER, indicating that receptor epitopes were differentially exposed. Furthermore, partial digestion of the A2 and pS2 ERE-bound ER with chymotrypsin or trypsin produced distinct protease cleavage patterns. Taken together, these data provide evidence that differential interaction of the DBD with the A2 and pS2 EREs brings about global changes in ER conformation. The conformational changes in ER induced by individual ERE sequences could lead to association of the receptor with different transcription factors and assist in the differential modulation of estrogen-responsive genes in target cells.

Animals↗
PubMed1999

Ganaxolone: a novel positive allosteric modulator of the GABA(A) receptor complex for the treatment of epilepsy.

Ganaxolone is a member of a novel class of neuroactive steroids which modulates the GABA(A) receptor complex (GRC) via an unique recognition site, distinct from those of benzodiazepines and barbiturates. Preclinical data from an array of chemically- and electrically-induced seizure models demonstrate that ganaxolone possesses broad spectrum anticonvulsant activity with potential clinical utility in both generalised and partial seizures, as well as cocaine-induced seizures. Clinical data to date support a favourable safety profile with somnolence, an extension of GABAergic activity, being the most frequently reported adverse event at higher doses. Target indications include infantile spasms and complex partial seizures. Open-label data in paediatric patients with intractable epilepsy suggest that ganaxolone may be effective in treating infantile spasms. A controlled trial utilising an in-patient, monotherapy design demonstrated that ganaxolone effectively decreases complex partial seizure activity compared to placebo. An important area of further evaluation is ganaxolone's potential role in the treatment of women with catamenial epilepsy.

Journal Article↗
PubMed1995

Comparison of the effects of full and partial allosteric modulators of GABA(A) receptors on complex behavioral processes in monkeys.

Two baselines involving a repeated acquisition task were used to assess the effects of bretazenil, imidazenil, and triazolam. The first baseline was a multiple schedule of repeated acquisition and performance of conditional discriminations. In the first component, the subject acquired a four-response chain by responding sequentially on three keys in the presence of different combinations of colors and geometric forms displayed on a center key. Acquisition of the discrimination was defined by a decrease in errors as the session progressed. In the performance component, the four-response chain was the same each session. Incorrect responses in either component produced a 5s time out during which responding had no programmed consequence. The second procedure, which has been used to evaluate the effects of drugs on memory, involved the acquisition of a discrimination, followed by a 1h delay and a retest of the same discrimination to assess retention. Triazolam (0.32 and 0.56mg/kg) administered alone, produced dose-related decreases in response rate in each component. In addition, triazolam also produced a dose-related increase in percentage errors in acquisition with no effect in performance. Triazolam (0.32mg/kg) eliminated retention (0 percent savings) in the memory task. Bretazenil (0.1-5.6mg/kg) or imidazenil (0.1-1.8mg/kg) administered alone had little or no effect on either rate of responding or accuracy in either component. Furthermore, bretazenil but not imidazenil disrupted retention at the higher doses tested. The combination of imidazenil or bretazenil with triazolam produced dose-related attenuation of the disruptive effects of triazolam on both behavioral baselines. These data suggest that the disruptive effects of benzodiazepines on learning and memory may be a function of the intrinsic efficacy of these compounds at different GABA(A) receptor subtypes.

Journal Article↗
PubMed2004

[Allosteric modulators of AMPA type glutamate receptors a novel gubclass of physiologically active substances].

Properties of AMPA type glutamate receptors widely presented within mammalian central nervous system are reviewed. AMPA receptors belong to ionotropic subclass of glutamate receptors and participate in fast excitatory glutamatergic transmission in the brain. Molecular structure of AMPA receptors consists of several subunits (GluRA1-GluRA4), which display various sensitivity to receptor ligands. These receptors are named according to their selective agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). AMPA receptor modulators represented by several chemical groups: pyrrolidinones (aniracetam), benzothiodiazinedioxides, benzylpiperidines and biarylpropylsulphonamides. These compounds facilitate AMPA receptor-mediated glutamatergic neurotransmission. AMPA receptor modulators enhance cognitive functions (learning and memory), reveal anxiolytic and antidepressant action, suppress the effects of psychostimulant agents, potentiate the effects of antipsychotic drugs, display neuroprotective properties. Positive modulation of AMPA receptor-associated ionic channels was shown to underline the mode of action of these substances that results in enhancement of long-term potentiation (LTP) and increasing neurotrophic factors expression. Preliminary clinical results have shown positive therapeutic effect of these substances in patients with cognitive impairment.

Animals↗
PubMed2006

Differential modulation by the GABAB receptor allosteric potentiator 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930) of synaptic transmission in the rat hippocampal CA1 area.

The recently discovered GABAB receptor-positive allosteric modulators enhanced the potency and efficacy of GABAB receptor agonists in in vitro experiments. These GABAB modulators also attenuated reward and anxiety in behavioral experiments without causing the untoward side effects associated with GABAB receptor activation by agonist administration and hence exhibited potential therapeutic utility. However, the underlying molecular mechanisms enabling the GABAB allosteric modulators to dissociate from the GABAB agonistic side effects remain elusive. To address this question, we have examined the effects of a typical GABAB modulator, 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930), on GABAB receptor-mediated modulations of both the excitatory and the delayed inhibitory components of hippocampal CA1 synaptic transmission. Using baclofen as an agonist and a multielectrode recording system, we recorded GABAB receptor-mediated modulations of both the field excitatory postsynaptic potentials and the population spikes simultaneously, as well as the paired-pulse inhibition of the population spike. We found that CGP7930 selectively enhanced the baclofen-induced modulation of synaptic inhibition without having any significant effects on the synaptic excitation. Our experiments have therefore revealed a pathway-selective differential modulation of synaptic transmission by CGP7930. This finding provides a synaptic mechanism to support the hypothesis that GABAB potentiators may be a better therapeutic alternative than GABAB agonists for central nervous system disorders.

Action Potentials↗
PubMed2002

Mapping of the benzodiazepine recognition site on GABA(A) receptors.

Ligands of the benzodiazepine binding site of the GABAA receptor come in three flavors: positive allosteric modulators, negative allosteric modulators and antagonists, all of which can bind with high affinity. The GABA(A) receptor is a pentameric protein which forms a chloride selective ion channel and ligands of the benzodiazepine binding site stabilize three different conformations of this receptor channel. Classical benzodiazepines exert a positive allosteric effect by increasing the affinity of channel opening by the agonist gamma-aminobutyric acid (GABA). We concentrate here on the major adult isoform, the alpha1beta2gamma2 GABA(A) receptor. The binding pocket for benzodiazepines is located in a subunit cleft between gamma2 and alpha1 subunits in a position homologous to the agonist binding site for GABA that is located between alpha1 and beta2 subunits. It is reviewed here how we arrived at this picture. In particular, point mutations were performed in combination with subsequent analysis of the expressed mutant proteins using either electrophysiological techniques or radioactive ligand binding assays. The predictive power of these methods is assessed by comparing the results with the predictions that can be made on the basis of the recently published crystal structure of the acetylcholine binding protein that shows homology to the N-terminal, extracellular domain of the GABA(A) receptor.

Allosteric Regulation↗
PubMed1989

Unsurmountable antagonism to 5-hydroxytryptamine in rat kidney results from pseudoirreversible inhibition rather than multiple receptors or allosteric receptor modulation.

Experiments were conducted in the isolated perfused rat kidney to determine the mechanism of unsurmountable antagonism by metergoline toward the vasoconstrictor response to 5-hydroxytryptamine (5-HT). Three possible mechanisms were investigated: 1) multiple receptors for 5-HT; 2) pseudoirreversible inhibition; and 3) allosteric modulation of 5-HT receptors. Metergoline and six other 5-HT antagonists acted as potent unsurmountable antagonists, only (-)-propranolol acted competitively (pA2 = 6.5). Multiple receptors for 5-HT were ruled out as the mechanism of unsurmountable antagonism because 5-HT and four 5-HT analogs gave similar pA2 values with (-)-propranolol. In addition, the KA value for 5-HT (185 nM) and the order and relativity of agonist potency suggest the presence of a single 5-HT2 receptor. Pseudoirreversible inhibition, over allosteric modulation of the 5-HT2 receptor, is suggested from experiments in which reversal of metergoline-induced inhibition by (-)-propranolol (studied in the presence of metergoline) was matched, in magnitude, by wash-out of metergoline. (-)-Propranolol is suggested to mimic wash-out by occupying and protecting receptors from which metergoline has dissociated. This is accomplished by virtue of its faster kinetics, on-off the 5-HT receptor, allowing restoration of 5-HT responses via a competitive interaction. Pseudoirreversible antagonism, over allosteric receptor modulation, offers an explanation that does not involve the postulation of allosteric effector sites or changes in the state of the 5-HT2 receptor.

Allosteric Regulation↗
PubMed2001

Pressure-sensitive and -insensitive coupling in gamma-aminobutyric acid(A) receptors.

RATIONALE: Previous behavioral and biochemical studies suggest that allosteric coupling processes initiated by benzodiazepines, barbiturates and neuroactive steroids can be sub-categorized on the basis of their sensitivities to antagonism by increased atmospheric pressure. However, biochemical evidence supporting this hypothesis was limited to single concentration studies in long sleep (LS) mice. OBJECTIVE: The present paper addresses these issues by extending biochemical investigation of pressure effects on allosteric modulators across a range of concentrations that allosterically enhance gamma-aminobutyric acid (GABA)A receptor function and alter behavior using two mouse genotypes. In addition, the effects of pressure on ligand binding were explored to further investigate the mechanism of pressure antagonism of allosteric modulation. METHODS: The effects of 12 times normal atmospheric pressure (ATA) of helium-oxygen gas (heliox) on allosteric modulation of GABA(A) receptor function and [3H]flunitrazepam binding was tested in LS and C57BL mouse brain membranes (microsacs) using chloride flux and high-affinity binding assays. RESULTS: In both genotypes, exposure to 12 ATA heliox antagonized the allosteric enhancement of GABA(A) receptor function by flunitrazepam (0.1-10 microM) and pentobarbital (0.1-50 microM) but did not affect allosteric modulation by 3alpha-hydroxy-5beta-pregnan-20-one (0.1-1 microM). Pressure did not affect benzodiazepine receptor affinity (Kd) or the number of benzodiazepine receptors (Bmax). CONCLUSIONS THE RESULTS: (1) confirm that there are differences in sensitivity to pressure antagonism of allosteric coupling among GABA(A) allosteric modulators; (2) demonstrate that these differences are not concentration or genotype dependent; (3) add evidence that pressure antagonizes allosteric modulation by uncoupling the receptor and (4) support the hypothesis that allosteric modulation of receptor function can be sub-categorized on the basis of sensitivity to pressure antagonism.

Animals↗