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.     

Identification of presynaptic serotonin autoreceptors using a new ligand: 3H-PAT

Binding studies with appropriate labelled ligands have revealed the existence of two types of serotonin (5-HT) receptor, 5-HT1 and 5-HT2, in the central nervous system of mammals. The 5-HT1 type is characterized by a higher affinity for agonists than for antagonists, whereas the 5-HT2 type binds preferentially to antagonists. However, neither of these receptor types apparently corresponds to the presynaptic autoreceptor controlling 5-HT release. In an attempt to identify the presynaptic autoreceptor directly, we synthesized the tritiated derivative of 8-hydroxy-2-(di-n-propylamino) tetralin (PAT), a new tetralin derivative with potent 5-HT agonist properties and carried out binding studies with rat brain membranes. As we report here, in the hippocampus, the properties of 3H-PAT binding sites correspond closely to those of 5-HT1 sites. In contrast, in the striatum, 3H-PAT binding sites exhibit a subcellular distribution and pharmacological characteristics usually associated with presynaptic autoreceptors. Furthermore, a marked loss of 3H-PAT binding sites occurs in the striatum (but not in the hippocampus) after the selective degeneration of serotoninergic fibres in 5,7-hydroxytryptamine (5,7-HT)-treated rats. Conversely, the sprouting of additional 5-HT terminals in the brain stem of adult rats treated at birth with 5,7-HT is associated with an increased density of 3H-PAT binding sites in this region. 3H-PAT thus seems to be a useful ligand for studying the biochemical and pharmacological characteristics of presynaptic autoreceptors in selected regions of rat brain.     

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.     

Biological function of H1V-1 transmembrane protein gp41

Since 1992, the study of biological functions of HIV-1 gp41 has made great progress. Experimental evidence from several research groups demonstrated that gp41 has a putative cellular receptor. A recombinant soluble gp41 (aa539–684) and gp41 immunosuppressive peptide (aa583–599) could bind to human B lymphocytes and monocytes, but weakly bind to T lymphocytes. It was found that gp41 contains two cellular binding sites (aa583–599 and 641–675). GP41 could selectively inhibit cell proliferation of human T, B lymphocytes and monocytes, enhance human MHC class I, II and ICAM-1 molecule expression on cell surface. Gp41 binding proteins and a monoclonal antibody against the first binding site could inhibit this modulation effect. Amino acid sequence homology exists between gp41 and human type I interferons, and the homologous region is located in the first binding site on gp41 and in the receptor binding site on type I interferons. Studies in other groups indicate that both binding sites in gp41 may be associated with HIV infection of cells. Peptides containing two binding sites could respectively inhibit HIV infection of cells. A monoclonal antibody recognizing the second binding site could neutralize lab-strains and recently separated strains of HIV-1. Besides, antibodies against two regions (homologous with gp41 binding sites) of SIV transmembrane protein gp32 could protect macaques from SIV infection. These results suggest that the study of gp41 binding sites and cellular receptor could contribute to understanding the mechanism of HIV infection and to developing HIV vaccine and anti-HIV drugs.     

Biological function of HIV-1 transmembrane protein gp41──A study on a putative cellular receptor of gp41

Since 1992, the study of biological functions of HIV-1 gp41 has made great progress. Experimental evidence from several research groups demonstrated that gp41 has a putative cellular receptor. A recombinant soluble gp41 (aa539-684) and gp41 immunosuppressive peptide (aa583-599) could bind to human B lymphocytes and monocytes, but weakly bind to T lymphocytes. It was found that gp41 contains two cellular binding sites (aa583-599 and 641-675). GP41 could selectively inhibit cell proliferation of human T, B lymphocytes and monocytes, enhance human MHC class Ⅰ, Ⅱ and ICAM-1 molecule expression on cell surface. Gp41 binding proteins and a monoclonal antibody against the first binding site could inhibit this modulation effect. Amino acid sequence homology exists between gp41 and human type Ⅰ interferons, and the homologous region is located in the first binding site on gp41 and in the receptor binding site on type Ⅰ interferons. Studies in other groups indicate that both binding sites in gp41 may be associated with HIV infection of cells. Peptides containing two binding sites could respectively inhibit HIV infection of cells. A monoclonal antibody recognizing the second binding site could neutralize lab-strains and recently separated strains of HIV-1. Besides, antibodies against two regions (homologous with gp41 binding sites) of SIV transmembrane protein gp32 could protect macaques from SIV infection. These results suggest that the study of gp41 binding sites and cellular receptor could contribute to understanding the mechanism of HIV infection and to developing HIV vaccine and anti-HIV drugs.     

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.     

Identification and distribution of 5-HT3 receptors in rat brain using radioligand binding

Functional serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into three subtypes: 5-HT1-like, 5-HT2 and 5-HT3 (ref. 1). Brain binding sites have been identified for both the 5-HT1 and 5-HT2 subtypes. Receptors of the 5-HT3 type have been characterized on isolated peripheral tissue models such as the rat vagus nerve, guinea-pig ileum and isolated rabbit heart. Using these models, selective 5-HT3 receptor antagonists such as MDL 72222 (ref. 5), ICS 205-930 (ref. 6), GR38032F (ref. 7) and BRL 43694 (ref. 8) have been developed. Recently, GR38032F, MDL 72222 and ICS 205-930 have been shown to have behavioural effects in rodents and primates that undoubtedly reflect an action in the central nervous system (refs 9-11 and unpublished observations), suggesting the existence of 5-HT3 receptors in the brain. Here we report direct evidence for the existence of 5-HT3 receptors in rat brain tissue and their distribution, based on high affinity binding of the potent 5-HT3 receptor antagonist 3H-GR65630 to homogenates of rat entorhinal cortex. Selective 5-HT3 receptor antagonists and agonists inhibited binding of 3H-GR65630 with high affinities which correlated well with their actions on the rat isolated vagus nerve. Binding was differentially distributed throughout the brain with high concentrations in cortical and limbic areas.     

alpha 2-Adrenergic receptors appear in rat salivary glands after reserpine treatment

The regulation of central and peripheral adrenergic receptors by various chemical, physiological, pharmacological and pathological stimuli has been the subject of intense study. For example, drug treatments can produce relatively small changes in the density of existing receptor binding sites in a variety of tissues. The alpha-adrenergic receptors in rat salivary gland tissue have been studied using radioligand receptor binding techniques. We have recently identified and characterised alpha 1-adrenergic receptors in the rat submandibular gland, but surprisingly, alpha 2-adrenergic receptor binding was not detectable. We now report that a single treatment of reserpine results in the appearance of alpha 2-adrenergic binding sites within 12 h. Continued treatment with the drug produces further increases in the number of alpha 2-adrenergic receptors, such that after 7 days the levels of alpha 1- and alpha 2-adrenergic receptors are similar. This is the first example of a drug treatment resulting in the appearance of a receptor type which was not previously detectable.     

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.     

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.     

A functional correlate for the dihydropyridine binding site in rat brain

Calcium channels, controlling the influx of extracellular Ca2+ and hence neurotransmitter release, exist in the brain. However, drugs classed as calcium antagonists and which inhibit Ca2+ entry through voltage-activated Ca2+ channels in heart and smooth muscle, seem not to affect any aspect of neuronal function in the brain at pharmacologically relevant concentrations. Yet the dihydropyridine calcium antagonists (for example, nitrendipine) bind stereospecifically with high affinity to a recognition site on brain-cell membranes thought to represent the Ca2+ channel and consequently, the physiological relevance of these sites has been questioned. However, activation of voltage-dependent Ca2+ channels can increase cytoplasmic Ca2+ and neurotransmitter release in neuronal tissue. We show here that Bay K8644, a dihydropyridine Ca2+-channel activator, can augment K+-stimulated release of serotonin from rat frontal cortex slices and that these effects can be antagonized by low concentrations of calcium antagonists. As 3H-dihydropyridine binding to cortical membrane preparations resembles the binding in heart and smooth muscle where there are good functional correlates we conclude that the dihydropyridine binding sites in the brain represent functional Ca2+ channels that can be unmasked under certain circumstances.     

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.     

Cloning of a cDNA for a glutamate receptor subunit activated by kainate but not AMPA.

Fast excitatory transmission in the vertebrate central nervous system is mediated mainly by L-glutamate. On the basis of pharmacological, physiological and agonist binding properties, the ionotropic glutamate receptors are classified into NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate) and kainate subtypes. Sequence homology between complementary DNA clones encoding non-NMDA glutamate receptor subunits reveals at least two subunit classes: the GluR1 to GluR4 class and the GluR5 class. Here we report the cloning and expression of a functional rat glutamate receptor subunit cDNA, GluR6, which has a very different pharmacology from that of the GluR1-GluR4 class. Receptors generated from the GluR1-GluR4 class have a higher apparent affinity for AMPA than for kainate. When expressed in Xenopus oocytes the homomeric GluR6 receptor is activated by kainate, quisqualate and L-glutamate but not by AMPA, and the apparent affinity for kainate is higher than for receptors from the GluR1-GluR4 class. Desensitization of the receptor was observed with continuous application of agonist. The homomeric GluR6 glutamate receptor exhibits an outwardly rectifying current-voltage relationship. In situ hybridizations reveal a pattern of GluR6 gene expression reminiscent of the binding pattern obtained with [3H]kainate.     

Acceleration of activation and inactivation by the beta subunit of the skeletal muscle calcium channel.

The L-type voltage-dependent calcium channel is an important link in excitation-contraction coupling of muscle cells (reviewed in refs 2 and 3). The channel has two functional characteristics: calcium permeation and receptor sites for calcium antagonists. In skeletal muscle the channel is a complex of five subunits, alpha 1, alpha 2, beta, gamma and delta. Complementary DNAs to these subunits have been cloned and their amino-acid sequences deduced. The skeletal muscle alpha 1 subunit cDNA expressed in L cells manifests as specific calcium-ion permeation, as well as sensitivity to the three classes of organic calcium-channel blockers. We report here that coexpression of the alpha 1 subunit with other subunits results in significant changes in dihydropyridine binding and gating properties. The available number of drug receptor sites increases 10-fold with an alpha 1 beta combination, whereas the affinity of the dihydropyridine binding site remains unchanged. Also, the presence of the beta subunit accelerates activation and inactivation kinetics of the calcium-channel current.     

The mast cell binding site on human immunoglobulin E

Antibodies of the immunoglobulin E isotype sensitize mast cells and basophils for antigen-induced mediator release by binding through the Fc portion to a high-affinity receptor (Fc epsilon R1, Ka = 10(9)M-1) on the cell surface causing the clinical manifestations of type I hypersensitivity. As the amino acid sequence of the human epsilon chain is now known, attempts have been made to map the Fc epsilon R1 binding site on IgE to a fragment smaller than Fc epsilon using proteolytic cleavage products, none of which proved to be active. Cleavage between the C epsilon 2 and C epsilon 3 domains released two inactive fragments, suggesting that the junction between these segments could be important in receptor binding. This region is protected against protease digestion in the rat IgE complex with the receptor of rat basophilic leukaemia cells. Here we report the mapping of the mast cell receptor binding site on human IgE to a sequence of 76 amino acids at the C epsilon 2/C epsilon 3 junction. Recombinant peptides containing this sequence inhibit passive sensitization of skin mast cells in vivo and sensitize mast cells to degranulation by anti-IgE in vitro almost as efficiently as a myeloma IgE. Fragments containing the separate domains are inactive. Additional sequences are required for rapid assembly of fragments into disulphide-linked dimers, suggesting that a single chain can form the active site. In a three-dimensional model of the human Fc epsilon, the two identical segments are far apart. Each folds to generate a cleft between the C epsilon 2 and C epsilon 3 domains on the surface of the Fc epsilon. The docking of IgE on to mast cells could take place within this cleft.     

Dopaminergic D-3 binding sites are not presynaptic autoreceptors

Postsynaptic dopamine (DA) receptors have been classified biochemically and pharmacologically into two types: D-1 receptors mediate adenylate cyclase stimulation, demonstrating micromolar affinity for DA and butyrophenone antagonists; D-2 receptors mediate adenylate cyclase inhibition, demonstrating nanomolar affinity for DA and butyrophenone antagonists. D-1 receptors are labelled by 3H-thioxanthene antagonists, while D-2 receptors are labelled by both 3H-agonists and all 3H-antagonists. A third class of dopaminergic binding site, termed D-3, represents high-affinity 3H-agonist binding sites demonstrating low, micromolar, affinity for butyrophenones. In the rat striatum, D-3 sites were decreased 50% by 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal DA pathway, suggesting that such D-3 binding labels presynaptic DA autoreceptors on nigrostriatal terminals. However, nigrostriatal denervation produces a concomitant depletion of striatal DA. Here we demonstrate that a reserpine-induced depletion of DA produces a decrease in D-3 binding comparable to that seen with nigrostriatal denervation, independent of presynaptic terminal degeneration. This loss in binding, or that caused by 6-OHDA lesions, is recovered by preincubating the striatal membranes with DA or with the supernatant from control striatal membrane preparations. We therefore suggest that the loss of D-3 binding following 6-OHDA lesions results from the depletion of endogenous DA rather than the degeneration of terminals and their putatively associated autoreceptors.     

Anatomical distributions of four pharmacologically distinct 3H-L-glutamate binding sites

Glutamate is thought to serve as a major excitatory neurotransmitter throughout the central nervous system (CNS); electrophysiological studies indicate that its action is mediated by multiple receptors. Four receptors have been characterized by their selective sensitivity to N-methyl-D-aspartate (NMDA), kainic acid (KA), quisqualic acid (QA) and 2-amino-4-phosphonobutyric acid (APB). Electrophysiological evidence indicates that these receptors are all present in the rat hippocampus and that the anatomically discrete synaptic fields within the hippocampus exhibit differential sensitivity to the selective excitatory amino acid agents. Thus, we have used the hippocampus as a model system to investigate possible subpopulations of 3H-L-glutamate binding sites. By using quantitative autoradiography, the pharmacological specificity of 3H-L-glutamate binding in discrete terminal fields was determined. We report here that there are at least four distinct classes of 3H-L-glutamate binding sites which differ in their anatomical distribution, pharmacological profile and regulation by ions. Two of these sites seem to correspond to the KA and NMDA receptor classes, and a third site may represent the QA receptor. The fourth binding site does not conform to present receptor classifications. None of these binding sites corresponds to the major glutamate binding site observed in biochemical studies.     

Structure of the dimerized hormone-binding domain of a guanylyl-cyclase-coupled receptor

The atrial natriuretic peptide (ANP) hormone is secreted by the heart in response to an increase in blood pressure. ANP exhibits several potent anti-hypertensive actions in the kidney, adrenal gland and vascular system. These actions are induced by hormone binding extracellularly to the ANP receptor, thereby activating its intracellular guanylyl cyclase domain for the production of cyclic GMP. Here we present the crystal structure of the glycosylated dimerized hormone-binding domain of the ANP receptor at 2.0-A resolution. The monomer comprises two interconnected subdomains, each encompassing a central beta-sheet flanked by alpha-helices, and exhibits the type I periplasmic binding protein fold. Dimerization is mediated by the juxtaposition of four parallel helices, arranged two by two, which brings the two protruding carboxy termini into close relative proximity. From affinity labelling and mutagenesis studies, the ANP-binding site maps to the side of the dimer crevice and extends to near the dimer interface. A conserved chloride-binding site is located in the membrane distal domain, and we found that hormone binding is chloride dependent. These studies suggest mechanisms for hormone activation and the allostery of the ANP receptor.     

Carboxy terminus of vasopressin required for activity but not binding

Vasopressin antagonists are valuable pharmacological tools for investigating physiological and behavioural functions of the nonapeptide arginine-vasopressin (AVP). The removal of glycinamide from the carboxy terminus of AVP drastically reduces its characteristic vasopressor and antidiuretic activities. In contrast to this we show here that removal of the carboxy-terminal glycinamide or the glycine at position 9 from several vasopressin antagonists makes little difference to their ability to block vasopressor and antidiuretic responses to AVP. These data demonstrate the critical structural requirements of the carboxy-terminal position for receptor activation, in contrast to the lack of such requirements for receptor binding. They also provide an avenue to a wide variety of antagonists substituted at the carboxy terminus (for example radiolabelled derivatives and affinity ligands) and suggest clues for the development of more potent and/or selective antagonists.