Multiple benzodiazepine receptors and their regulation by gamma-aminobutyric acid

Potential antidepressant properties of SR 57746A, a novel compound with selectivity and high affinity for 5-HT1A receptors

SR 57746A, 4-(3-trifluoromethylphenyl)-N-[2-(naphth-2-yl)ethyl]-1,2,3,6- tetrahydropyridine HCl, was studied for its specific 5-HT1A receptor agonist action and antidepressant-like effects in the rat. The compound showed a high affinity for 5-HT1A specific binding sites in the rat hippocampus (IC50 3 nM), moderate affinity (10(-7)-10(-6) M) for dopamine D2 receptor, 5-HT uptake, 5-HT2 and alpha 1-adrenoceptor binding sites and practically no effect on binding sites of monoamine, GABAA, benzodiazepine and histamine receptors. It inhibited forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes at concentrations of 10(-6) and 10(-5) M. The effect of 10(-6) M SR 57746A on forskolin-stimulated adenylate cyclase activity was completely antagonized by 10(-6) M (-)-propranolol. Administered per os as a three-dose course to rats, SR 57746A significantly increased struggling in the forced swimming test at doses from 0.3 to 3 mg/kg. Single doses had no such effect. The effect of a three-dose course with 1 mg/kg SR 57746A on rats' struggling was antagonized by pretreatment with 5 mg/kg i.p. metergoline, a non-selective 5-HT receptor antagonist, and by 20 mg/kg i.p. (-)-propranolol, an antagonist at 5-HT1 receptors. Three oral doses of 100 mg/kg parachlorophenylalanine, an inhibitor of 5-HT synthesis, and 100 mg/kg i.p. (+/-)-sulpiride, an antagonist at dopamine D2 receptors, also antagonized the effect of SR 57746A in the forced swimming test. The results show that SR 57746A has selectivity and high affinity for 5-HT1A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of strychnocarpine and related beta-carbolines to brain receptors in vitro

The affinity of strychnocarpine and related beta-carbolines for serotonin, benzodiazepine, tryptamine, opiate and GABA receptors in rat brain was studied. Strychnocarpine showed a low to very low affinity for all the receptors tested. The weak binding to tryptamine receptors might explain part of the tremorigenic effects found earlier in the in vivo studies.

Differential ontogenesis of type I and II benzodiazepine receptors in mouse cerebellum

The ontogeny of type I and type II benzodiazepine binding sites was studied in mouse cerebellum by displacement of [3H]flunitrazepam binding by zolpidem, a ligand specific for the type I sites. Type I binding sites predominate throughout development and in the adult while type II sites account for 25% of total cerebellar benzodiazepine binding sites at birth and, during development, decrease to 10% or less in the adult. On a per cerebellum basis type II sites increase during the first postnatal week and then remain at a steady level while type I sites increase until adulthood. These results may indicate a specific localization of the type II sites (and of the corresponding alpha-protein subunits in the GABA/benzodiazepine receptor complex) in structures already present at birth and developing during a short early postnatal period. The affinity of zolpidem for its high affinity (type I) binding sites increases during cerebellar ontogeny, this increase possibly indicates an epigenetic (post-translational) 'maturation' process of the corresponding receptor molecule. Hill numbers indicate the existence of an additional binding site heterogeneity greater during development but still present in the adult; probably this is to be related to the simultaneous presence of different 'maturation' stages during development and with a certain variety of the final products.

The interaction of substituted benzamides with brain benzodiazepine binding sites in vitro

1. The interaction of substituted benzamides with brain benzodiazepine (BDZ) binding sites was examined by their ability to displace [3H]-flunitrazepam ([3H]-FNM) from specific binding sites in bovine cortical membranes in vitro. 2. Clebopride, Delagrange 2674, Delagrange 2335 and BRL 20627 displayed concentration-dependent displacement of [3H]-FNM with IC50 values of 73 nM, 132 nM, 7.7 microM and 5.9 microM, respectively. Other substituted benzamides including metoclopramide, sulpiride, tiapride, sultopride and cisapride were inactive at 10(-5) M. 3. Inhibition by clebopride and Delagrange 2674 of [3H]-FNM binding was apparently competitive and readily reversible. 4. In the presence of gamma-aminobutyric acid (GABA), the ability of diazepam and Delagrange 2674 to displace [3H]-Ro 15-1788 binding was increased 3.6 and 1.6 fold respectively, compared to the absence of GABA, while ethyl beta-carboline-3-carboxylate (beta CCE) and clebopride were less potent in the presence of GABA. 5. Diazepam was 30 fold less potent at displacing [3H]-Ro 15-1788 in membranes that had been photoaffinity labelled with FNM than in control membranes, whereas the potency of beta CCE did not differ. Clebopride and Delagrange 2674 showed a less than two fold loss of potency in photoaffinity labelled membranes. 6. The pattern of binding of clebopride and Delagrange 2674 in these in vitro tests is similar to that found previously with partial agonists or antagonists at BDZ binding sites. 7. Clebopride and Delagrange 2674 inhibited [3H]-FNM binding with similar potency in rat cerebellar and hippocampal membranes, suggesting they have no selectivity for BDZ1 and BDZ2 binding sites. 8. Clebopride and Delagrange 2674 are structurally dissimilar to other BDZ ligands and represent another chemical structure to probe brain BDZ binding sites.

Benzodiazepine receptor sites in the human brain: autoradiographic mapping

Receptor autoradiography was used to localize and quantify the distribution of benzodiazepine receptor sites in human post mortem materials using [3H]flunitrazepam. The distribution and density of these sites was analysed in the brains of 21 patients dying without reported neurological disease. The distribution of benzodiazepine receptors in the human brain was found to be comparable from case to case although differences in the density occurred among the brains examined. No influence of the post mortem delay, age, gender or pre mortem drug treatment on the distribution and densities was observed in our series. The highest densities of benzodiazepine receptors in human brain were localized in cortical and hippocampal areas, nucleus accumbens, amygdala and mammillary bodies. Intermediate densities were found in the basal ganglia and thalamic and hypothalamic nuclei. [3H]Flunitrazepam binding was low in the brainstem nuclei and very low in white matter. The triazolopyridazine Cl 218872, reported to differentiate between type I and type II benzodiazepine receptor sites, exhibited regional differences in affinity when used to block [3H]flunitrazepam binding. Benzodiazepine receptors in the cerebellar cortex were more sensitive to this compound than those in the dentate gyrus of the hippocampus and the tuberal nuclei of the hypothalamus. An enrichment in the concentration of type I benzodiazepine receptor Cl 218872-sensitive sites was observed in motor areas as compared to structures of the limbic system. The addition of GABA to the incubation medium resulted in an increase of [3H]flunitrazepam binding, suggesting the coupling of these sites to a GABAA receptor. The increase in binding was directly proportional to the density of benzodiazepine receptors but unrelated to the density of high-affinity GABAA sites. The distribution of benzodiazepine receptor sites in the human brain compares well with that previously described in the rat brain. The high densities of receptors localized in the limbic system and in the cortical areas suggest that the effects of benzodiazepines are mediated through an interaction with the sites we have visualized in these anatomical structures. Our results provide a detailed map of the distribution of benzodiazepine receptors and a basis for the understanding of pharmacological effects of these drugs in humans and for future studies of modifications of these receptors in neurological and neuropsychiatric conditions in humans.

Effect of PK 11195, an antagonist of benzodiazepine receptors, on platelet aggregation in a model of anxiety in the rat

1. Anxiety induced by forced swimming increases maximal intensity (h) of platelet aggregation (PAG) and time to reach it (t). 2. PK 11195 pretreatment (12.5 and 25 mg/kg) reverses anxiety-induced PAG changes. At 6.25 mg/kg it inhibits PAG. 3. Changes induced by anxiety on PAG may be mediated by peripheral-benzodiazepine receptors.

Further characterization of benzodiazepine receptor differences in long-sleep and short-sleep mice

Molecular and conformational characteristics of benzodiazepine (BZ) receptors in cortex and cerebellum from long-sleep and short-sleep mice were investigated using heat inactivation and beta-carboline competition techniques. To investigate differences in the allosteric coupling between GABA and BZ receptors, the protection of BZ receptors from heat inactivation, by GABA, was also evaluated. The two genotypes do not differ in the affinity or number of BZ receptors in the cortex or cerebellum. They do, however, appear to differ in the molecular structure and/or regulation of the conformational state of the receptor in the cortex, as indicated by a greater sensitivity of LS mice to both heat inactivation and beta-carboline competition of 3H-flunitrazepam (FNZ) binding in this region. Evidence for differences in the nature of coupling between GABA and BZ receptors is provided by the finding that in both regions, GABA protected BZ receptors from inactivation to a greater degree in LS mice. The relationship between these differences and the multiplicity of expression of BZ receptors is discussed.

Effects of benzodiazepines on laryngeal reflexes. Comparison of lormetazepam and Diazemuls

Of 20 volunteers, five were given intravenous Diazemuls 15 mg over 15 seconds, and three groups of five were given lormetazepam 2 mg intravenously over 10, 20 and 60 seconds, respectively. Laryngeal reactivity and psychomotor function were tested at intervals from prior to injection until 4 hours after injection. For equivalent degrees of depression of psychomotor function, lormetazepam depressed the laryngeal reflex less than Diazemuls (p = 0.004). Lormetazepam give over 60 seconds depressed the laryngeal reflex more than when given over 10 seconds (p = 0.008) or over 20 seconds (p = 0.048), although a significant difference was not demonstrated between the 10-second and 20-second groups. These results concur with experimental evidence that benzodiazepine receptor multiplicity exists, which allows various members of the benzodiazepine group of drugs to exhibit differing therapeutic ratios for their various effects.

The GABA/benzodiazepine receptor-chloride ionophore complex: nature and modulation

1. A high affinity, saturable, stereospecific binding site for Benzodiazepines has been found to be functionally and possibly structurally related to a GABA receptor-chloride ionophore complex. 2. There are both central (CNS) as well as "peripheral" binding sites, involving multiple organs. 3. Evidence strongly suggests that mutually exclusive Benzodiazepine agonists and antagonists bind to the same receptor, possibly in an agonist-antagonist-inverse agonist continuum. 4. The search for an endogenous ligand has been inconclusive and the question of such a substance remains open. 5. Although the relationship between this receptor and the Limbic System remains unclear, it seems certain that the Benzodiazepine receptor plays an important role in the modulation of Limbic System excitability.

Effect of long term amineptine treatment on pre- and postsynaptic mechanisms in rat brain

The effect of amineptine and its two metabolites on monoamine uptake, release and receptor binding was studied in vitro. Amineptine and its two metabolites did not displace labelled ligands for known neurotransmitters and drug receptor sites. Amineptine and its two metabolites did not influence [3H]-5-hydroxytryptamine ([3H]-5-HT) uptake or release by rat brain synaptosomes. Amineptine inhibited [3H]-dopamine and [3H]-noradrenaline ([3H]-NA) accumulation, with IC50 values of 1.4 and 10 microM, respectively. The effect was retained, though with lower efficacy, by the two metabolites. Amineptine released [3H]-dopamine from preloaded synaptosomes. Metabolite 1 had no effect on catecholamine release, and metabolite 2 was about half as active as the parent compound on [3H]-dopamine release. The releasing effect of amineptine on [3H]-dopamine was potentiated by reserpine pretreatment, suggesting that the drug acts on the cytoplasmic neurotransmitter pool. Chronic treatment with amineptine (20 mg kg-1, twice daily for 15 days followed by a 3 days drug withdrawal period) resulted in a decrease of [3H]-spiperone binding sites in striatum, and of [3H]-dihyroalprenolol and [3H]-clonidine in cortex. Chronic treatment with amineptine reduced basal [3H]-dopamine accumulation in striatal synaptosomes, without affecting [3H]-NA or [3H]-5-HT accumulation. The adaptive changes in the pre- and postsynaptic dopamine mechanisms observed after long term treatment with amineptine are consistent with the drug acting as an indirect dopamine agonist. The down regulation of beta- and alpha 2-noradrenoceptors observed after long term amineptine treatment may play a role in the antidepressant activity of the drug.

CGS 8216 and CGS 9896, novel pyrazoloquinoline benzodiazepine ligands with benzodiazepine agonist and antagonist properties

CGS 8216 and CGS 9896 are two recently described compounds which interact with benzodiazepine binding sites but have pharmacological, biochemical and behavioral characteristics which distinguish them from classical benzodiazepines. CGS 8216 shows properties of a weak inverse agonist, while CGS 9896 shows properties of a mixed agonist/antagonist. Experiments using quantitative autoradiography to determine benzodiazepine binding site interactions of these compounds in discrete anatomical areas are described. Results indicate that [3H]-CGS 8216 does not show any regional differentiation in binding characteristics in 7 brain areas studied. CGS 9896 preferentially inhibited [3H]-flunitrazepam from cerebellar sites compared to hippocampal dentate gyrus sites, but the magnitude of this effect was small. These data support the conclusion that CGS 9896 is acting preferentially at putative benzodiazepine type 1 sites and is consistent with the mixed agonist/antagonist profile of the compound.

Pre- versus postsynaptic localization of benzodiazepine and beta-carboline binding sites

[3H]Flunitrazepam (FNP) and [3H]methyl beta-carboline-3-carboxylate (MCC) binding was examined in soluble and particulate fractions from membranes solubilized with Triton X-100 or in subfractions of synaptosomal membranes obtained by a physical separation technique. Results using both methods demonstrate that benzodiazepine and beta-carboline sites reside on both pre- and postsynaptic membranes. Further, subfractionation experiments indicate that the binding sites for both ligands are unequally distributed within the synapse and among brain regions. For example, in cerebral cortical presynaptic membranes there are twice as many FNP as MCC sites whereas in postsynaptic membranes this ratio is reversed. The number of FNP and MCC sites are equal in the presynaptic fraction from cerebellum. The postsynaptic membranes derived from cerebellum have three times the number of FNP compared to MCC sites. In hippocampus this ratio varies between 1.5 and 2.8 in each subfraction. These results support the idea that benzodiazepine and beta-carboline binding sites represent different recognition sites.

Heterogeneity of brain benzodiazepine receptors: effects of physiological conditions

A number of investigators have shown compelling evidence for multiplicity of benzodiazepine (BDZ) receptors. The present study addresses the query of BDZ receptor heterogeneity, in vitro, with respect to temperature. In competition studies involving rat cerebellar tissue, CL 218,872 produced Hill slopes near unity at both 0 degree C and 37 degrees C. In contrast, similar experiments utilizing cortical tissue from rats and mice produced Hill slopes of 0.69 and 0.66 at 0 degree C and 37 degrees C respectively. 3H-Flunitrazepam-photoaffinity labeling of cortical and cerebellar membranes was conducted at 0 degree C and 37 degrees C. SDS-PAGE fluorographic analyses of photolysed 3H-flunitrazepam (3H-Flu) revealed one intensely labeled 51K band in the cerebellum at both temperatures, which was specifically chased by diazepam. Similar experiments conducted in cortical tissue revealed photoaffinity labeling of at least three distinct macromolecules, one intense 51K and two less intense 55K and 59K bands. Labeling of each of these bands was chased specifically by diazepam. These data, taken together, indicate the existence of regional BDZ receptor heterogeneity under physiological conditions.

Benzodiazepine receptor binding following amygdala-kindled convulsions: differing results in washed and unwashed brain membranes

The binding of [3H]flunitrazepam and [3H]RO5-4864 was measured in unwashed brain homogenates and in extensively washed brain membranes from amygdala-kindled and "yoked" control rats sacrificed 2 weeks following the sixth stage 5 convulsion. In unwashed homogenates, [3H]flunitrazepam binding was reduced in both the hypothalamus and ipsilateral right cortex of kindled rats (unchanged in other areas). In washed brain membranes, [3H]flunitrazepam binding was unaltered in these regions; it was bilaterally elevated, however, in both the amygdala and hippocampus (unchanged in other areas). In washed membranes, the in vitro addition of gamma-aminobutyric acid enhanced [3H]flunitrazepam binding to a similar extent in kindled and control membranes. These data indicate that the type of benzodiazepine binding abnormality observed after kindling depends on the type of tissue preparation employed in the assay procedure.

Benzodiazepine receptors on human blood platelets

Binding studies conducted on membrane preparation from human platelets using (3H) Ro5-4864 and (3H) diazepam showed specific and saturable binding. Scatchard analysis revealed a single class of binding sites with KD = 10.8 +/- 0.9 nM and Bmax = 775 +/- 105 fmol/mg protein for (3H) Ro5-4864 and KD = 10.5 +/- 1.1 nM and Bmax = 133 +/- 19 fmol/mg for (3H) diazepam. We were unable to detect any GABA binding site on crude membrane preparation, nor did GABA enhance the binding of (3H) Ro5-4864 or (3H) diazepam. This suggests that benzodiazepine receptors are uncoupled to GABA system on human platelets. Ro15-1788, a specific antagonist for "central type" benzodiazepine (BDZ) binding sites was inactive in displacing (3H) Ro5-4864 from membrane receptors, while PK 11195 (a specific ligand for the "peripheral type" receptor) was the most potent of the drugs tested in inhibiting (3H) Ro5-4864 binding. These results indicate that human blood platelets bear "peripheral-type" BDZ receptor. Moreover, we could not detect any (3H) propyl beta carboline specific binding on platelet membranes. Results on benzodiazepine receptors on human circulating lymphocytes are also reported and similarity in pharmacological properties with platelet benzodiazepine receptors is suggested.

CGS 8216, Ro 15-1788 and methyl-beta-carboline-3-carboxylate, but not EMD 41717 antagonize the muscle relaxant effect of diazepam in genetically spastic rats

Diazepam (0.4-4 mg/kg i.p.) reduced the spontaneous tonic activity in the electromyogram (EMG) recorded from the gastrocnemius-soleus muscle of spastic mutant Han-Wistar rats in a dose-dependent manner. The muscle relaxant effect of diazepam was antagonized by the benzodiazepine antagonists Ro 15-1788 (5 mg/kg i.p.), beta-CCM (2 mg/kg i.p.) and CGS 8216 (5 mg/kg i.p.), but not by EMD 41717 (50 mg/kg i.p.). These results add further support to the hypothesis that Ro 15-1788, CGS 8216 and beta-CCM do antagonize all pharmacological effects of benzodiazepines while EMD 41717 displays more selectivity in antagonizing the different actions of benzodiazepines.

In vitro interaction of premazepam with benzodiazepine receptors in rat brain regions

Premazepam (PRZ) in vitro competitively displaced 3H-diazepam (DIA), 3H-flunitrazepam (FLU) and 3H-RO 15-1788 from their binding sites on rat brain synaptosomes, with a potency intermediate to other benzodiazepines (BDZs), and Hill coefficients near 1 in different brain regions. Incubation at 37 degrees C reduced premazepam's affinity for BDZ receptors to a lower extent than other benzodiazepines and had no effect on the Hill coefficient. The IC50 of PRZ on 3H-RO 15-1788 and 3H-FLU binding was markedly reduced by GABA in rat cortex, like those of reference classical BDZs, but was GABA-independent in the cerebellum. The IC50 of the BDZ antagonist, RO 15-1788 was unaffected by GABA in both brain areas. The possibility that PRZ behaves as a partial agonist in the cortex and as an antagonist in the cerebellum is discussed.

Effects of various 2-amino-6-alkyldithiopurines on brain specific [3H]diazepam binding

Various derivatives of 2-amino-6- methylthiopurine with substituents at the 6-position of purine were tested for their abilities to displace [3H]diazepam binding to rat brain membranes. The potency was dependent on the carbon chain-length in the 6-position of purine. Among the derivatives tested, 2-amino-6-n- pentyldithiopurine had the highest potency, with a Ki value of 0.92 microM.

Effect of taurine on a benzodiazepine-GABA-chloride ionophore receptor complex in rat brain membranes

Taurine at 10 mM had no effect on basal binding of [3H]diazepam to the membranes, while it significantly inhibited a GABA-stimulated binding of [3H]diazepam in cerebral cortex, hippocampus, but not in cerebellum. The inhibition by taurine in the presence of GABA (1 microM to 1 mM) was not competitive. At low concentrations (0.04 to 0.2 nM) the binding of [3H]propyl-beta-carboline-3-carboxylate, a ligand exhibiting higher affinity for type I than type II benzodiazepine receptors, was not enhanced by GABA, while the binding of higher concentrations (0.5 nM) was. This GABA enhancement of [3H]propyl-beta-carboline-3-carboxylate binding was also selectively blocked by taurine. Pentobarbital increased the binding of [3H]diazepam in a medium containing chloride and this effect was potentiated by taurine at 1-10 mM. These findings may be relevant to the modulatory role of taurine in the central nervous system.

Inhibition of benzodiazepine and GABA receptor binding by amino-gamma-carbolines and other amino acid pyrolysate mutagens

The effect of several pyrolysate mutagens on the benzodiazepine and GABA receptors was investigated. Of amino-gamma-carbolines, Trp-P-1 antagonized the suppressive effect of diazepam on the pentylenetetrazol-induced convulsions and death, whereas Trp-P-2 by itself precipitated seizures and death in male mice. Both Trp-P-1 and Trp-P-2 inhibited the specific binding of [3H]diazepam and [3H]muscimol in rat brain membranes mainly by increasing Kd, indicating that these gamma-carbolines bind on benzodiazepine and GABA receptors. IC50S of Trp-P-1 and Trp-P-2 on specific [3H]flunitrazepam binding were not changed by addition of GABA. The Hill coefficient of Trp-P-1 for displacing [3H]diazepam binding was about unity whereas that of Trp-P-2 was less than unity. These results suggest that Trp-P-1 and Trp-P-2 act as active antagonists or inverse agonists at benzodiazepine receptors. The convulsant effect of the gamma-carbolines may be mediated by an action on the central benzodiazepine receptors; however, the role of the effect on GABA receptors is not clear.

Molecular aspects of the action of benzodiazepine and non-benzodiazepine anxiolytics: a hypothetical allosteric model of the benzodiazepine receptor complex

The availability of radiolabeled benzodiazepines has resulted in the identification of high affinity receptors in the central nervous system for this class of psychotherapeutic agent which are linked to recognition sites for the inhibitory neurotransmitter, GABA. Evaluation of new, synthetic compounds in the benzodiazepine radioligand binding assay has resulted in the identification of nine classes of non-benzodiazepine putative anxiolytic agents, some of which may be more anxioselective than the benzodiazepines. At least three and possibly five subclasses of benzodiazepine receptor have been identified in mammalian tissues using radioligand binding assays. The possibility exists that one of these receptor subclasses may mediate the anxiolytic effects of the benzodiazepines while the remainder may be involved in the mediation of the sedative, ataxic and anticonvulsant properties associated with benzodiazepine-like agents. Several endogenous ligands for the benzodiazepine receptor(s) have been postulated. These include various proteins and peptides, purines and the beta-carbolines. This latter group, which competitively antagonizes the pharmacological and biochemical effects of the benzodiazepines, has the highest affinity for the benzodiazepine receptor of all compounds thus far examined; however, none of these compounds has been conclusively identified as the endogenous ligand akin to the enkephalins and endorphins at the opiate receptor. The majority of available evidence would indicate that the endogenous ligand for the benzodiazepine receptor(s) is an antagonist of the benzodiazepines and other putative anxiolytic agents.

Investigation of the effects of muscimol on different components of [3H]propyl β-carboline-3-carboxylate binding to rat hippocampal and cerebellar membranes

The binding of [3H]propyl beta-carboline-3-carboxylate [( 3H]PCC) to rat hippocampal and cerebellar membranes was investigated together with the displacement by PCC of [3H]flunitrazepam [( 3H]FNM). Results were consistent with the presence of a single recognition site in cerebellum and two sites in hippocampys, but the affinity of neither of the hippocampal sites correlated with that in the cerebellum. Muscimol did not alter the affinity or capacity of any of these sites under the conditions used, indicating that PCC does not show the benzodiazepine-like property of GABAergic modulation, at any of the subpopulations of its binding sites.

Multiple sites of action for anxiogenic drugs: behavioural, electrophysiological and biochemical correlations

This review describes animal models of anxiety that are able to identify an anxiogenic drug effect. Evidence is reviewed for the anxiogenic action of several drugs that act at the GABA-benzodiazepine-chloride ionophore complex in the brain. The effects of their combinations with various other drugs thought to act at the same sites are discussed. The classification of these drugs on the basis of their behavioural profiles is compared with their classification based on biochemical and electrophysiological studies.

Comparison of the distribution of convulsant/barbiturate and benzodiazepine receptors using light microscopic autoradiography

Some convulsant drugs elicit CNS excitation by blocking neuronal activity at GABAergic synapses whereas depressant compounds may result in the enhancement of GABAergic transmission. These effects are thought to involve drug actions at a multireceptor complex involving a benzodiazepine receptor, GABA receptor, picrotoxin receptor and a chloride ionophore. A radiolabeled convulsant, [35S]t-butylbicyclophosphorothionate [( 35S]-TBT) has been developed and used to characterize the binding to the "picrotoxin" or convulsant/barbiturate site. The microscopic distribution of the convulsant/barbiturate sites are reported in this communication, as demonstrated by receptor autoradiography after labeling tissue sections with [35S]-TBT. Comparison of the distribution of these sites with those of the benzodiazepine receptors show a close regional correlation in many areas. The convulsant/barbiturate sites and the benzodiazepine receptors, however, are unevenly distributed in the rat cerebellum and exist in separate lamina.

Benzodiazepine receptor multiplicity

Receptors for gamma-aminobutyric acid (GABA) are phylogenetically old and extensive GABA receptor multiplicity had already evolved in invertebrate species. High affinity, "brain specific", benzodiazepine (BZ) receptors, present in the central nervous systems of virtually all vertebrate species, represent a heterogeneous sub-class of GABA receptors. Functional GABA-BZ-ion receptor complexes are aggregates consisting of different kinds of sub-units which are probably coded for by separate genes. These sub-units may be combined in different ways to yield different benzodiazepine receptor complexes. Different GABA-BZ-ion receptor complexes probably subserve different physiological functions and more selective drugs modifying these functions will probably be found.

Benzodiazepine receptor mediated discriminative cues: effects of GABA-ergic drugs and inverse agonists

Rats were exposed to a two-lever drug discrimination procedure using the benzodiazepine (BZ) receptor inverse agonists N'-methyl-beta-carboline-3-carboxamide (FG 7142) or methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). FG 7142 (30 mg/kg) failed to acquire discriminative stimulus control, although it did suppress responding. The same group of animals was trained successfully to discriminate diazepam (DZP, 2.5 mg/kg) from vehicle. The DZP cue was potentiated by the GABA agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol (THIP, 1,3 mg/kg); THIP alone produced vehicle-appropriate responding. In addition, clonazepam (0.2 mg/kg) and chlordiazepoxide (5 mg/kg) substituted for DZP (with potencies of 7.5 and 0.25 times that of DZP, respectively). In antagonism tests, FG 7142 (5-17.5 mg/kg), methyl-beta-carboline-3-carboxylate (beta-CCM, 2.5 mg/kg) did not effect, bicuculline (2 mg/kg) and DMCM (1 mg/kg) partially blocked, and the BZ receptor antagonist Ro 15-1788 (40 mg/kg) completely blocked the discriminative stimulus effects of DZP. In animals trained to discriminate DMCM (0.2 mg/kg) from vehicle, 95.2% substitution occurred with bicuculline (2 mg/kg); DZP (1-5 mg/kg) completely antagonized DMCM. These results indicate that the DZP cue is mediated by GABA-coupled BZ receptors and that GABA may modulate the efficacy of a BZ at its receptor site. However, since inverse BZ receptor agonists (FG 7142, DMCM and beta-CCM) were, at best, only marginally effective in antagonizing DZP, the DZP cue may be mediated by a distinct subclass of BZ receptors.

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.

Photoaffinity labeling of benzodiazepine receptors in rat brain with flunitrazepam alters the affinity of benzodiazepine receptor agonist but not antagonist binding

Recently, it was proposed that beta-carbolines interact with a subset of benzodiazepine (BZD) binding sites in mouse brain. This postulate was based upon evidence showing changes in binding properties of the BZD receptor following photoaffinity labeling of membranes with flunitrazepam (FLU). Under conditions in which 80% of specific [3H]diazepam binding was lost in photolabeled membranes, specific [3H]propyl beta-carboline-3-carboxylate [( 3H]PCC) binding was spared. In this study, the binding of the BZD antagonists [3H]PCC, [3H]Ro15 1788 and [3H]CGS 8216 was examined in rat brain membranes following photoaffinity labeling with FLU. No significant changes in the apparent KD and small reductions in the Bmax of 3H antagonist binding were observed. However, in the same membranes, up to 89% of specific [3H]FLU binding was lost. When [3H]PCC (0.05 nM) was used to label the receptors in control and photolabeled membranes, the ability of BZD receptor agonists to inhibit [3H]PCC binding was greatly diminished in the photolabeled membranes. In contrast, the potency of BZD antagonists remained the same in both control and treated membranes. Based upon PCC/[3H]Ro15 1788 competition experiments, the ability of PCC to discriminate between BZD receptor subtypes was unaffected by photoaffinity labeling of cortical membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentylenetetrazol may discriminate between different types of benzodiazepine receptors

The effect of various concentrations of pentylenetetrazol (PTZ) on [3H]flunitrazepam binding to benzodiazepine receptors was investigated by Hofstee and Hill plot analyses. These analyses indicate the presence of two PTZ binding sites in forebrain, whereas a single PTZ binding site is present in cerebellum. The relative proportions of the two PTZ binding sites in forebrain are close to those of benzodiazepine Type II and Type I receptors, respectively. These results suggest that PTZ may actually discriminate between different types of benzodiazepine receptors.

Autoradiographic differentiation of multiple benzodiazepine receptors by detergent solubilization and pharmacologic specificity

We have used light microscopic autoradiography to localize differentially Type 1 and 2 benzodiazepine receptors using pharmacologic and detergent treatment techniques. The triazolopyridazine CL218872 has preferential affinity for Type 1 receptors from which it displaces [3H]flunitrazepam selectivity. In biochemical experiments [3H]flunitrazepam binding sites with pharmacologic properties of Type 2 receptors are preferentially solubilized from brain membranes by detergents. We have now treated rat brain slices either with CL218872 or 2% sodium cholate and evaluated the autoradiographic distribution of [3H]flunitrazepam-labeled receptors. Whether calculated from the drug or detergent treated preparations, the relative densities of apparent Type 1 and 2 receptors are the same. Thus differential sensitivity to determine solubilization distinguishes the same two populations of receptors as Type 1 and 2 specific drugs.

[3H]diazepam binding sites on chick neurons in primary culture

High affinity [3H]diazepam binding sites were identified on neurons prepared from the hemispheres of 8-day-old chick embryos and grown in serum-containing or serum-free medium. Clonazepam (IC50 = 3 nM) was more potent than Ro 5-4864 (IC50 greater than 1000 nM) in displacing [3H]diazepam binding. GABA and pentobarbital, in the presence of chloride ions were able to stimulate [3H]diazepam binding synergistically. These interactions were found to be comparable to those observed in mammalian brain.

Binding of [3H]DMCM, a convulsive benzodiazepine ligand, to rat brain membranes: preliminary studies

DMCM (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) produces convulsions in mice and rats, probably by interacting with benzodiazepine (BZ) receptors. Investigation of specific binding of [3H]DMCM to rat hippocampus and cortex revealed polyphasic saturation curves, indicating a high-affinity site (KD = 0.5-0.8 nM) and a site with lower affinity (KD = 3-6 nM). BZ receptor ligands of various chemical classes, but not other agents, displace [3H]DMCM from specific binding sites--indicating that [3H]DMCM binds to BZ receptors in rat brain. The regional distribution of [3H]DMCM binding is complementary to that of the BZ1-selective radioligand [3H]PrCC. Specific binding of [3H]DMCM (0.1 nM) was reduced by gamma-aminobutyric acid (GABA) receptor agonist to approximately 20% of the control value at 37 degrees C in chloride-containing buffers; the reduction was bicuculline methiodide- and RU 5135-sensitive. The effective concentrations of 10 GABA analogues in reducing [3H]DMCM binding correlated closely to published values for their GABA receptor affinity. Specific binding of [3H]DMCM is regulated by unknown factors; e.g. enhanced binding was found by Ag+ treatment of membranes, in the presence of picrotoxinin, or by exposure to ultraviolet light in the presence of flunitrazepam. In conclusion, [3H]DMCM appears to bind to high-affinity brain BZ receptors, although the binding properties are different from those of [3H]flunitrazepam and [3H]PrCC. These differences might relate in part to subclass selectivity and in part to differences in efficacy of DMCM at BZ receptors.

Effect of GABA and benzodiazepine antagonists on [3H]flunitrazepam binding to cerebral cortical membrane

The effects of GABA on the binding of [3H]flunitrazepam to bovine cerebral cortical membranes was investigated in presence and in absence of benzodiazepine antagonists. The percentage stimulation of [3H]flunitrazepam binding by GABA is higher when approximately 50% of the binding is displaced by benzodiazepine antagonists. The observed increase in percent stimulation of [3H]flunitrazepam binding by GABA might reflect the preferential displacement of the ligand by benzodiazepine antagonists from a GABA-insensitive conformational state or site of the receptor.

Brain benzodiazepine receptors increase after chronic ethyl-beta-carboline-3-carboxylate

Rats were treated with repeated intraventricular injections of ethyl-beta-carboline-3-carboxylate (beta-CCE) (10 micrograms/rat, twice daily for 8 days), 36 h after the last injection, the total number of 3H-diazepam binding sites was increased in the cerebral cortex, cerebellum and hippocampus by 63, 51 and 38%, respectively. On the other hand, there were no significant differences in the dissociation constants (KD) between beta-CCE and solvent treated rats. In contrast, chronic beta-CCE administration failed to change the number of the apparent affinity of 3H-beta-CCE binding sites in all the brain areas examined. The results suggest that beta-CCE is an antagonist at the 3H-diazepam binding sites.

Suriclone: a new cyclopyrrolone derivative recognizing receptors labeled by benzodiazepines in rat hippocampus and cerebellum

Suriclone (RP 31,264), like zopiclone (RP 27,267), belongs to the family of cyclopyrrolones and is chemically entirely different from the benzodiazepines (BZDs). However, it possesses a pharmacological profile close to that of the BZDs and proved to be useful in therapeutics as an anxiolytic agent. In the present paper it is shown that suriclone possesses a high affinity for flunitrazepam binding sites and that tritiated suriclone binds specifically with high affinity in rat hippocampus (KD = 0.44 +/- 0.03 nM) and rat cerebellum (KD = 0.53 +/- 0.12 nM). Further, suriclone binding sites are recognized by BZDs or zopiclone, similarly in the two regions. The affinities of four BZD derivatives--nitrazepam, flunitrazepam, diazepam, and chlordiazepoxide--are similar for suriclone and flunitrazepam binding sites. Suriclone binding sites are, like flunitrazepam sites, protected from thermal inactivation by gamma-aminobutyric acid (GABA) (10 microM), but only flunitrazepam binding is enhanced by GABA. It could be postulated from this that suriclone interacts with a subpopulation of receptors that might be modulated differently from flunitrazepam binding sites. Our results indicate that suriclone could be a new probe for investigating the so-called BZD receptors.

Effect of chronic ethanol consumption on central and peripheral type benzodiazepine binding sites in the mouse brain

Following chronic exposure of C57/BL6 mice to ethanol the binding of [3H]Ro5-4864 and [3H]propyl-beta-carboline-3-carboxylate to benzodiazepine binding sites in the brain was studied. Peripheral-type benzodiazepine binding sites were measured using the probe [3H]Ro5-4864. Chronic ethanol treatment resulted in a significant increase in [3H]Ro5-4864 binding due to a 43% increase in receptor density. The affinity of [3H]Ro5-4864 for the receptor was not significantly affected. The binding of [3H]propyl-beta-carboline-3-carboxylate to central-type benzodiazepine receptors was not affected by chronic ethanol treatment.

Regional heterogeneity of benzodiazepine receptors at 37 degrees C: an in vitro study in various regions of the rat brain

The most compelling pharmacological evidence in support of benzo-diazepine (BZD) receptor heterogeneity is derived from the study of the complex interactions of CL 218872 and propyl beta-carboline-3-carboxylate (PCC) with brain BZD receptors. In the present study, we provide evidence to support the hypothesis that intraregional BZD receptor heterogeneity in rat brain is a result of the different conformational states of a single receptor. This hypothesis is based upon the observation that CL 218872 and PCC lose the ability to effectively discriminate BZD receptor subtypes in rat cerebral cortex, hippocampus and pons-medulla at physiological temperature (37 degrees C). Interestingly, both PCC and CL 218872 show higher affinity for BZD receptors in the cerebellum when compared to other brain regions at 37 degrees C. This observation suggests that interregional BZD receptor heterogeneity occurs under physiologically relevant temperatures. We propose that distinct cerebellar and non-cerebellar type BZD receptors exist in vivo while marked differences in the affinity of the type I and type II BZD receptor subtypes postulated by Klepner et al. 1979 may only occur in vitro at 0 degree--4 degree C.

The anxiogenic action of benzodiazepine antagonists

Two benzodiazepine antagonists were tested in an animal model of anxiety, the social interaction test. Ethyl beta-carboline-3-carboxylate (1 and 2 mg/kg) had a potent anxiogenic action; the imidazodiazepine RO 15-1788 (4-10 mg/kg) had a weak anxiogenic effect that with a larger dose (20 mg/kg) disappeared and RO 15-1788 (10 mg/kg) significantly counteracted the anxiogenic effect of the beta-carboline (1 mg/kg). The implications of these results for the understanding of the pharmacological basis of anxiety and for the existence and nature of an endogenous ligand for the benzodiazepine binding site are discussed.

Binding of beta-carbolines and caffeine on benzodiazepine receptors: correlations to convulsions and tremor

Compounds from both the beta-carboline (BC) and xanthine groups have been suggested to be the natural ligands for benzodiazepine (BZ) receptors. In this study we examined the effects of several BC's and caffeine, 1,3,7-trimethylxanthine, on the binding of 3H-flunitrazepam (3H-FZ) and beta-3H-carboline-3-carboxylic acid ethyl ester (3H-BCCE) to the BZ receptors of rat and mouse brain. In mice, convulsion-producing doses of caffeine (120 mg/kg intravenously) and harmane (30 mg/kg intravenously) lowered the specific binding of 3H-FZ in vivo by 12-31%. A tremorogenic dose of harmaline (30 mg/kg intravenously) increased binding by 31%. Caffeine and harmane also slightly decreased the in vivo binding of 3H-BCCE, a compound that binds preferentially to the cerebellar type of BZ receptors. Harmaline stimulated the binding of 3H-BCCE only in the forebrain. Both harmaline and harmane increased by 41-111% the amount of 3H-BCCE that was distributed to the brain. In vitro BC's and caffeine displaced 3H-FZ from receptors in the rat brain with various Ki values (4.7 to 206.9 microM). The antagonism for BZ binding was competitive and in Scatchard analysis produced linear plots. Exceptions were harmaline and caffeine in the forebrain: both exhibited curvilinear plots for 3H-FZ binding. Harmaline increased the binding, and caffeine decreased it by altering the affinity of a subgroup of BZ receptors. In the hindbrain both harmaline and caffeine inhibited binding and produced linear plots. BC-induced tremor and convulsions unveil a large number of spare receptors in the brain, and these seem to be of the cerebellar type of BZ receptors. In addition, our results show that tremorogenic and convulsive BC's act differently on BZ receptors: during harmaline-induced tremor the affinity of some BZ receptors is increased, while harmane-induced convulsions are connected to direct occupation of BZ receptors.