Selective antagonists of benzodiazepines

Benzodiazepines produce most, if not all, of their numerous effects on the central nervous system (CNS) primarily by increasing the function of those chemical synapses that use gamma-amino butyric acid (GABA) as transmitter. This specific enhancing effect on GABAergic synaptic inhibition is initiated by the interaction of benzodiazepines with membrane proteins of certain central neurones, to which drugs of this chemical class bind with high affinity and specificity. The molecular processes triggered by the interaction of these drugs with central benzodiazepine receptors, and which result in facilitation of GABAergic transmission, are still incompletely understood. Theoretically, benzodiazepines could mimic the effect of hypothetical endogenous ligands for the benzodiazepine receptors, although there is no convincing evidence for their existence; in vitro studies indicate that benzodiazepines might compete with a modulatory peptide which is present in the supramolecular assembly formed by GABA receptor, chloride ionophore and benzodiazepine receptor and which reduces the affinity of the GABA receptor for its physiological ligand. The mechanisms of action of benzodiazepines at the molecular level are likely to be better understood following our recent discovery of benzodiazepine derivatives, whose unique pharmacological activity is to prevent or abolish in a highly selective manner at the receptor level all the characteristic centrally mediated effects of active benzodiazepines. Here, we describe the main properties of a representative of this novel class of specific benzodiazepine antagonists.     

Co-localization of GABA receptors and benzodiazepine receptors in the brain shown by monoclonal antibodies

The most abundant inhibitory neurotransmitter in the central nervous system, gamma-aminobutyric acid (GABA), exerts its main effects via a GABAA receptor that gates a chloride channel in the subsynaptic membrane. These receptors can contain a modulatory unit, the benzodiazepine receptor, through which ligands of different chemical classes can increase or decrease GABAA receptor function. We have now visualized a GABAA receptor in mammalian brain using monoclonal antibodies. The protein complex recognized by the antibodies contained high- and low-affinity binding sites for GABA as well as binding sites for benzodiazepines, indicative of a GABAA receptor functionally associated with benzodiazepine receptors. As the pattern of brain immunoreactivity corresponds to the autoradiographical distribution of benzodiazepine binding sites, most benzodiazepine receptors seem to be part of GABAA receptors. Two constituent proteins were identified immunologically. Because the monoclonal antibodies cross-react with human brain, they provide a means for elucidating those CNS disorders which may be linked to a dysfunction of a GABAA receptor.     

Molecular heterogeneity of benzodiazepine receptors

Benzodiazepines exhibit reversible, stereospecific high affinity binding to mammalian brain membranes, and the respective binding sites for 3H-flunitrazepam represent pharmacologically and clinically relevant receptors for benzodiazepines. Recently it has been demonstrated that reversibly bound 3H-flunitrazepam becomes irreversibly attached to a specific membrane protein with apparent molecular weight of 50,000 when incubations are performed in the presence of UV light. Irreversible binding of 3H-flunitrazepam to this protein had pharmacological properties similar to reversible benzodiazepine receptor binding, indicating that 3H-flunitrazepam is a photoaffinity label for the benzodiazepine receptor. Using irreversible binding of 3H-flunitrazepam and subsequent electrophoretic separation of the labelled proteins in SDS-gels followed by fluorography, we found that in hippocampus and several other brain regions at least two different types of benzodiazepine receptors exist. Each seems to be associated with a gamma-aminobutyric acid (GABA) receptor.     

Importance of a novel GABAA receptor subunit for benzodiazepine pharmacology

Neurotransmission effected by GABA (gamma-aminobutyric acid) is predominantly mediated by a gated chloride channel intrinsic to the GABAA receptor. This heterooligomeric receptor exists in most inhibitory synapses in the vertebrate central nervous system (CNS) and can be regulated by clinically important compounds such as benzodiazepines and barbiturates. The primary structures of GABAA receptor alpha- and beta-subunits have been deduced from cloned complementary DNAs. Co-expression of these subunits in heterologous systems generates receptors which display much of the pharmacology of their neural counterparts, including potentiation by barbiturates. Conspicuously, however, they lack binding sites for, and consistent electrophysiological responses to, benzodiazepines. We now report the isolation of a cloned cDNA encoding a new GABAA receptor subunit, termed gamma 2, which shares approximately 40% sequence identity with alpha- and beta-subunits and whose messenger RNA is prominently localized in neuronal subpopulations throughout the CNS. Importantly, coexpression of the gamma 2 subunit with alpha 1 and beta 1 subunits produces GABAA receptors displaying high-affinity binding for central benzodiazepine receptor ligands.     

Effect of age on benzodiazepine-induced behavioural convulsions in rats

The benzodiazepines are a class of drugs used to alleviate anxiety. As such they constitute one of the most commonly prescribed compounds, due in part to their efficacy and safety. The physiological effect of these drugs is probably through interactions with a low affinity benzodiazepine binding site and two (types 1 and 2) higher affinity sites. The ontogenesis of these latter two binding sites in the rat differs, with the type 2 binding site being predominant at birth and the type 1 binding site increasing in number after the second week after birth. The differential development of these two receptor types is important because the immature organism may have different physiological and behavioural responses from the adult. Here we demonstrate an important difference: that a prototypic benzodiazepine, chlordiazepoxide, and a water-soluble benzodiazepine, flurazepam, produce behavioural convulsions in the preweanling rat. The convulsions are antagonized by the benzodiazepine blocker Ro-15-1788. The triazolopyridizine CL-218872, specific to the type 1 receptor, does not share this action. We suggest that this paradoxical convulsant effect of chlordiazepoxide and flurazepam is due to activation of the type 2 receptor in the absence of the type 1 receptor in the immature rat.     

Cerebellar GABAA receptor selective for a behavioural alcohol antagonist

Benzodiazepines are widely prescribed anxiolytics and anticonvulsants which bind with high affinity to sites on the GABAA receptor/Cl- channel complex and potentiate the effect of the neurotransmitter GABA (gamma-aminobutyric acid). The heterogeneity of benzodiazepine recognition sites in the central nervous system was revealed by studies showing different classes of GABAA receptor subunits (classes alpha, beta and gamma) and variant subunits in these classes, particularly in the alpha-class. Expression of recombinant subunits produces functional receptors; when certain alpha-variants are coexpressed with beta- and gamma-subunits the resulting receptors have pharmacological properties characteristic of GABAA-benzodiazepine type I or type II receptors. The alpha-variants are differentially expressed in the central nervous system and can be photoaffinity-labelled with benzodiazepines. Here we report a novel alpha-subunit (alpha 6) of cerebellar granule cells. We show that recombinant receptors composed of alpha 6, beta 2 and gamma 2 subunits bind with high affinity to the GABA agonist [3H]muscimol and the benzodiazepine [3H]Ro15-4513 but not the other benzodiazepines or beta-carboniles. The same distinctive pharmacology is observed with GABAA receptors from rat cerebellum immunoprecipitated by an antiserum specific for the alpha 6 subunit. We conclude that this alpha-subunit is part of a cerebellar receptor subtype, selective for Ro15-4513, an antagonist of alcohol-induced motor incoordination and ataxia.     

Specific tricyclic antidepressant binding sites in rat brain.

The discovery of high-affinity binding sites for psychoactive drugs such as benzodiazepines, opiates and neuroleptics has opened up new approaches to the study of these drugs and their mechanisms of action. Although most tricyclic antidepressants inhibit neuronal uptake of noradrenaline and serotonin, their mechanism of action remains unclear. Changes in the sensitivity of the beta-receptor after chronic tricyclic antidepressant treatment suggest that they modulate noradrenergic neurotransmission. Tricyclic antidepressants also act directly on cholinergic, histaminergic, alpha-adrenergic and serotonergic receptors. It is not clear, however, which, if any, of these effects are related to the primary antidepressant effect or whether they are simply responsible for some of the side effects. We have thus investigated the possibility that specific binding sites for tricyclic antidepressants exist in the central nervous system. So far, binding studies using 3H-labelled tricyclic antidepressant drugs have only detected binding to histaminergic H2 and cholinergic muscarinic receptors and low-affinity binding. We demonstrate here a population of specific high-affinity binding sites for 3H-imipramine on brain membranes which may be responsible for the antidepressant effects of these drugs.     

5,7-二取代-[1,2,4]三唑[1,5-a]嘧啶衍生物的合成及其抗癫痫活性

通过合成一系列5,7-二取代-[1,2,4]三唑[1,5-a]嘧啶衍生物,对其体内抗癫痫活性进行研究.各种查尔酮和3-氨基-1,2,4-三氮唑在二甲基甲酰胺中加热反应分别得到5,7-二取代-4,7-二氢-[1,2,4]三唑[1,5-a]嘧啶(2a-2e)和它们的脱氢产物5,7-二取代-[1,2,4]三唑[1,5-a]嘧...     

beta-Carboline-3-carboxylic acid ethyl ester antagonizes diazepam activity

Analogous to the progression of events in the opiate receptor-enkaphalin area, the first reports that benzodiazepines have selective and specific high-affinity binding sites in brain have stimulated a search for the endogenous 'ligand' or substance that might normally act at these sites. Braestrup and co-workers have extracted from human urine a gamma-fraction (ref. 10) which they have recently identified as beta-carboline-3-carboxylic acid ethyl ester (beta CEE). They reported that this substance is extremely potent in displacing 3H-diazepam from brain binding sites and proposed that a beta-carboline-3-carboxylic acid derivative might, in part, be the endogenous ligand for the brain benzodiazepine receptor. We have examined several synthetically derived beta-carboline-3-carboxylic acid analogues and now present data obtained from testing only the beta CEE described by Braestrup et al. In addition to confirming these workers' observation that this compound is a potent displacer of 3H-diazepam from brain tissue, our pharmacological data indicate that beta CEE has activity that is opposite to, rather than similar to, that of diazepam.     

Synthesis and anticonvulsant activity of 1-substituted benzyl-N-substituted-1,2,3-triazole-4-formamides

Substituted benzyl azids were synthesized through the reaction of substituted benzyl chloride and sodium azid，which subsequently underwent cyclization with ethyl propiolate and amidation to give thirteen 1-substituted benzyl-N-substituted-1,2,3-triazole-4-formamide derivatives (3a-3m). The structure of the synthesized compounds was confirmed by IR, H-NMR, MS and elemental analysis. Their anticonvulsant activity against maximal electrolshock (MES) induced seizure was tested and the result showed that all these compounds possess anticonvulsant activity in different degrees. Among those, the compounds containing chloro atoms on the phenyl ring were less potent in anticonvulsant activity, while introducing one or two fluorin atoms on benzyl system increased its activity. Furthermore, their activity decreased when there was substituent on the nitrogen atom of carboxamide, and the larger the substituent, the lower the activity.     

1-(3-苯氧基丙基)-1H-1,2,4-三氮唑衍生物的合成及其抗惊厥活性

目的合成一系列1-(3-苯氧基丙基)-1H-1,2,4-三氮唑衍生物,对其体内抗惊厥活性进行研究。方法以取代酚为起始原料,先后与1-溴-3-氯丙烷及1,2,4-三氮唑进行烷基化反应,得到目标化合物2a-2m。化合物2a-2m通过核磁共振氢谱、碳谱和质谱进行结构确证;采用最大电休克发作模型和皮下戊四唑模型评价目标化合物的抗惊厥活性,旋转棒法评价目标化合物的神经毒性。结果合成了一系列单烷基取代的三唑类衍生物,这些化合物对MES模型和Sc-PTZ模型均有效,大部分在100 mg/kg剂量下表现出抗惊厥活性,使得该类化合物具有广谱的抗癫痫潜能。结论新合成的(3-苯氧基丙基)-1H-1,2,4-三氮唑衍生物具有较好的抗惊厥活性,使得该类化合物具有广谱的抗癫痫潜能。本研究进一步丰富了三唑类化合物的抗癫痫构效关系,同时为研究大发作和失神发作类癫痫的治疗药物提供了一定的基础。     

Differential localization of type I and type II benzodiazepine binding sites in substantia nigra

A number of studies have suggested the existence of multiple benzodiazepine binding sites in the brain. We have recently reported the physical separation of two apparent benzodiazepine binding site subtypes, the pharmacological properties, and distribution in tissue sections of which correspond to the putative type I and type II sites. Benzodiazepine and gamma-aminobutyric acid (GABA) receptors have been shown to interact, and lesions of the GABAergic striatonigral pathway, which lead to GABA supersensitivity, both increase the numbers of GABA binding sites and enhance GABA-stimulated benzodiazepine binding. We demonstrate here that degeneration of striatonigral fibres increases the density of putative type I benzodiazepine binding sites in the substantia nigra and decreases the density of the putative type II sites. This suggests that type I sites that increase after denervation are postsynaptic, whereas the type II sites reduced by the lesion may be localized to axons or terminals of the striatonigral pathways.     

A site for the potentiation of GABA-mediated responses by benzodiazepines

The benzodiazepines have been well characterised as minor tranquillizers and attempts to explain their unique spectrum of activity have included suggestions that they may interact with a variety of neurotransmitter systems. Recently, a possible interaction with the gamma-aminobutyric acid (GABA) system has received most attention. Benzodiazepines potentiate the actions of both synaptically released and exogenously administered GABA on mammalian neuronal preparations but the site of action within the GABA response mechanism has not been determined. Binding studies suggest that benzodiazepines combine with highly specific sites in the neuronal membrane and that these sites have some indirect association with GABA receptors. To investigate this association further in a functioning GABA system, quantitative studies have been made in vitro on neuronal depolarisations mediated by GABA receptor activation. Evidence has already been presented that bicuculline is most probably a competitive antagonist at the GABA receptor while picrotoxin acts as an antagonist at a separate site. Here flurazepam is shown to attenuate preferentially the action of picrotoxin rather than bicuculline and a model is suggested for the site of action of these drugs within the GABA response mechanism.     

Biologically active phorbol esters specifically alter affinity of epidermal growth factor membrane receptors

TPA (12-O-tetradecanoyl-phorbol-13-acetate) reversibly inhibits the binding of (125)I-labelled epidermal growth factor (EGF) to treated mouse and human cells, but does not affect the binding of various other ligands to their membrane receptors. It alters the affinity of the receptors for EGF without changing the total number of available receptors per cell. Those phorbol esters which stimulate cell growth in culture and have tumour-promoting activity in vivo alter the EGF-receptor affinity, while the biologically inactive derivatives fail to change the affinity of EGF for its receptors.     

Inhibitory receptors bind ANGPTLs and support blood stem cells and leukaemia development

How environmental cues regulate adult stem cell and cancer cell activity through surface receptors is poorly understood. Angiopoietin-like proteins (ANGPTLs), a family of seven secreted glycoproteins, are known to support the activity of haematopoietic stem cells (HSCs) in vitro and in vivo. ANGPTLs also have important roles in lipid metabolism, angiogenesis and inflammation, but were considered 'orphan ligands' because no receptors were identified. Here we show that the immune-inhibitory receptor human leukocyte immunoglobulin-like receptor B2 (LILRB2) and its mouse orthologue paired immunoglobulin-like receptor (PIRB) are receptors for several ANGPTLs. LILRB2 and PIRB are expressed on human and mouse HSCs, respectively, and the binding of ANGPTLs to these receptors supported ex vivo expansion of HSCs. In mouse transplantation acute myeloid leukaemia models, a deficiency in intracellular signalling of PIRB resulted in increased differentiation of leukaemia cells, revealing that PIRB supports leukaemia development. Our study indicates an unexpected functional significance of classical immune-inhibitory receptors in maintenance of stemness of normal adult stem cells and in support of cancer development.     

Synthesis and anticonvulsant activity of 7-benzylamino-4,5-dihydro-［1,2,4］triazolo［4,3-a］quinolines

A series of 7-substituted-benzylamino-4,5-dihydro-[1,2,4]triazolo[4,3-a]quinoline derivatives was synthesized and evaluated for their anticonvulsant activity.The subcutaneous pentylenetetrazole test(sc-PTZ)demonstrated that the most effective compound in controlling the sc-PTZ induced seizure was 7-(3-bromine-benzylamino)-4,5-dihydro-[1,2,4]triazolo[4,3-a]quinoline(4j)with an ED50 of 5.0 mg/kg and the PI of 20.7,which was also safer than the reference drugs.And the maximal electroshock test(MES)demonstrated that among these derivatives,7-(3-fluorobenzylamino)-4,5-dihydro-[1,2,4]trizolo[4,3-a]quinoline(4i),with an ED50 of 15.3 mg/kg and the PI of 7.2,was the safest in MES test.Furthermore,their neurotoxicities were measured by the rotarod neurotoxicity test,and the results showed that all derivatives possessed lower neurotoxicity.     

青霉素惊厥与脑中GABA_A和GABA_B受体亲和力的关系

脑室微量注射青霉素（１１．９ｍｇ·ｍｌ－１，１５μｌ）制作小白鼠惊厥模型；并以同位素示踪法研究大脑皮层、小脑、海马、下丘脑四个脑区ＧＡＢＡＡ和ＧＡＢＡＢ受体亲和力的变化。结果显示，青霉素惊厥时大脑皮层和小脑ＧＡＢＡＡ受体亲和力显著减弱，而海马、下丘脑ＧＡＢＡＡ受体亲和力无变化；青霉素惊厥使四个脑区中ＧＡＢＡＢ受体均显著下降。提示，除了海马和下丘脑的ＧＡＢＡＡ受体以外，四个脑区的ＧＡＢＡＡ和ＧＡＢＡＢ受体均参与了青霉素的致惊厥过程。青霉素可能通过竞争内源性ＧＡＢＡ与ＧＡＢＡＡ和ＧＡＢＡＢ受体的结合，阻断了ＧＡＢＡ介导的突触前和突触后抑制效应并增加了兴奋性递质的释放，显示了惊厥效应。     

Proton inhibition of N-methyl-D-aspartate receptors in cerebellar neurons.

Mammalian neurons contain at least three types of excitatory amino-acid receptors, selectively activated by N-methyl-D-aspartate (NMDA) or aspartate, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate ((S)-AMPA) and kainate. An important aspect of NMDA receptors is their regulation by a variety of factors such as glycine, Mg2+ and Zn2+ that are present in vivo. We show here that NMDA receptor responses are selectively inhibited by protons, with a 50% inhibitory concentration (IC50) that is close to physiological pH, implying that NMDA receptors are not fully active under normal conditions. (S)-AMPA and kainate responses remain unchanged at similar pH levels. Proton inhibition is voltage-insensitive and does not result either from fast channel block, a change in channel conductance, or an increase in the 50% excitatory concentration (EC50) of aspartate/NMDA or glycine. Instead, protons seem to decrease markedly the opening frequency of 30-50 pS NMDA channels, and reduce the relative proportion of longer bursts. This feature of NMDA receptors could be relevant to neurotoxic activation of NMDA receptors during ischaemia, as well as to seizure generation, as extracellular proton changes occur during both of these pathological situations. Furthermore, these results may have implications for normal NMDA receptor function as transient changes in extracellular protons occur during synaptic transmission.