Benzodiazepine antagonist Ro 15-1788: binding characteristics and interaction with drug-induced changes in dopamine turnover and cerebellar cGMP levels

The recently discovered benzodiazepine antagonist Ro 15-1788 was characterized in binding studies, and its potency and selectivity were determined in vivo by interaction with drug-induced changes in dopamine turnover and cerebellar cGMP level. Ro 15-1788 reduced [3H]flunitrazepam binding in the brain in vivo with a potency similar to that of diazepam and effectively inhibited [3H]diazepam binding in vitro (IC50 = 2.3 +/- 0.6 nmol/liter). [3H]Ro 15-1788 bound to tissue fractions of rat cerebral cortex with an apparent dissociation (KD) of 1.0 +/- 0.1 nmol/liter. The in vitro potency of various benzodiazepines in displacing [3H]Ro 15-1788 from its binding site was of the same rank order as found previously in [3H]diazepam binding. Autoradiograms of [3H]Ro 15-1788 binding in sections of rat cerebellum showed the same distribution of radioactivity as with [3H]flunitrazepam. The attenuating effect of diazepam on the chlorpromazine- or stress-induced elevation of homovanillic acid in rat brain was antagonized by Ro 15-1788. Among a series of compounds which either decreased or increased the rat cerebellar cGMP level, only the effect of benzodiazepine receptor ligands (diazepam, zopiclone, CL 218 872) was antagonized by Ro 15-1788. Thus, Ro 15-1788 is a selective benzodiazepine antagonist acting at the level of the benzodiazepine receptor in the central nervous system. Peripheral benzodiazepine binding sites in kidney and schistosomes were not affected by Ro 15-1788.

Stimulation of benzodiazepine binding to rat brain membranes and solubilized receptor complex by avermectin B1a and gamma-aminobutyric acid

The binding of [3H]flunitrazepam to benzodiazepine receptors in synaptic membranes and a digitonin-solubilized receptor fraction of rat brain is increased by avermectin B1a and gamma-aminobutyric acid (GABA). The effects of avermectin B1a and GABA are both sensitive to inhibition by (+)-bicuculline. Avermectin B1a and GABA both decrease the Kd and increase the Bmax of [3H]flunitrazepam binding to membranes. Kinetic analysis of the binding of [3H]flunitrazepam to rat brain membranes indicates that avermectin B1a and GABA reduce the rate constants of both association and dissociation between the ligand and the receptor. These results suggest a similar mechanism of modulation of benzodiazepine binding by avermectin B1a and GABA. This modulation may involve in interaction among the receptors for benzodiazepine, GABA and avermectin B1a.

[3H]Propyl beta-carboline-3-carboxylate as a selective radioligand for the BZ1 benzodiazepine receptor subclass

Ethyl beta-carboline-3-carboxylate (beta-CCE) is a mixed-type inhibitor of [3H]flunitrazepam ([3H]FNM) binding to benzodiazepine receptors in noncerebellar regions of rat brain. These findings may represent the presence of either receptor multiplicity or negative cooperativity among benzodiazepine receptors. [3H]Propyl beta-carboline-3-carboxylate ([3H]PrCC) has previously been shown to bind specifically to benzodiazepine receptors of rat cerebellum. In the present study we found no indication of the presence of true negative cooperativity among benzodiazepine receptors when [3H]PrCC was used as radioligand. However, we observed that [3H]PrCC labelled only 57% of [3H]FNM binding sites in rat hippocampus (Bmax values) and 71% in rat cerebral cortex, whereas the number of receptors labelled by both ligands was equal in the cerebellum. Hofstee analyses of the shallow inhibition curves seen in hippocampus and cerebral cortex when [3H]FNM binding was inhibited by beta-CCE indicate that beta-CCE and some other beta-carboline-3-carboxylate derivatives interact preferentially with a subclass of receptors, and that the percentage of this subclass is equivalent to the number of receptors labelled by [3H]PrCC. We conclude that [3H]PrCC at low concentration (0.3-0.4 X 10(-9) M) labels a subclass of benzodiazepine receptors, BZ1, while another class, BZ2 receptors, are not labelled by [3H]PrCC when filtration assays are used. By parallel determinations of the proportion between [3H]FNM and [3H]PrCC binding we calculated the percentage of BZ1 receptors in several regions of rat, guinea pig and calf brain and in mouse forebrain. The values ranged from approximately 50% in hippocampus to 90% in the guinea pig pons.

Facilitation of benzodiazepine binding by levonantradol

Levonantradol enhanced binding of 3H-diazepam to rat cortical membranes. Scatchard analysis of this effect showed apparent KD and Bmax changes at 100 microM levonantradol and a KD decrease at 50 microM. Dextronantradol caused a similar enhancement, suggesting a lack of stereospecificity in vitro. Subsequently, levonantradol at pharmacologic doses (0.15 mg/kg subcutaneously) was found to enhance the binding of intravenous 3H-flunitrazepam to mouse brain. In contrast to the results in vitro, dextronantradol showed no enhancement of 3H-flunitrazepam binding at doses up to 15 mg/kg subcutaneously. This stereospecific interaction with benzodiazepine receptors in vivo suggests that levonantradol may facilitate the pharmacologic actions of benzodiazepines. Levonantradol, at doses of 0.32 and 3.2 mg/kg subcutaneously, which did not block the convulsant effect of pentylenetetrazol, enhanced both the potency and efficacy of diazepam in elevating the absolute threshold of pentylenetetrazol for eliciting clonic seizures. Consistent with its lack of facilitation of benzodiazepine binding, dextronantradol at 3.2 mg/kg, a dose without effect on pentylenetetrazol-induced convulsions, showed little or no enhancement of diazepam's anticonvulsant activity against the latter.

Binding of beta-carboline-3-carboxylic acid ethyl ester to mouse brain benzodiazepine receptors in vivo

beta-Carboline-3-carboxylic acid ethyl ester (beta-CEE) displaced [3H] flunitrazepam from the mouse brain benzodiazepine receptor in vivo with an i.v. ED50 of 2.1 mg/kg, an i.p. ED50 and 53.4 mg/kg, and an i.g. ED50 of 450 mg/kg. At 2.1 mg/kg i.v. beta-CEE displaced 37 +/- 9% of the label from hippocampal membranes and 76 +/- 7% from cerebellar membranes. The results suggest that the in vitro binding specificity of beta-CEE is also expressed in vivo and that the drug may act in vivo by binding to the benzodiazepine receptor.

Benzodiazepine binding and interactions with the GABA receptor complex in living cultures of rat cerebral cortex

Benzodiazepines bind to living cultures of dissociated rat cerebral cortex. This binding is saturable, and kinetic analyses indicate that the binding is to a single class on sites with kinetic constants very close to those obtained using neuronal membrane preparations. The efficacy of a number of benzodiazepines, xanthine derivatives and other drugs in competition experiments is similar to that seen in neuronal membrane preparations, and suggests that the benzodiazepine binding site studied in these investigations is the same as that found in neuronal membrane preparations and believed to be the pharmacologically active benzodiazepine binding site. GABA agonists increase the binding of benzodiazepines, and this increase has the same order of efficacy as their ability to hyperpolarize the neurons when applied at known concentrations with muscimol greater than GABA greater than THIP. At high concentrations THIP potentials benzodiazepine binding to the same level as GABA. Diazepam increases the ability of both GABA and THIP to hyperpolarize the neurons as well as the amplitude of spontaneous IPSPS which, in this system, are GABA-mediated. The competitive GABA antagonist bicuculline methiodide slightly decreased benzodiazepine binding and also antagonized the increase due to GABA. The non-competitive GABA antagonist picrotoxinin had no effect on benzodiazepine binding but did antagonize the GABA-induced increase in benzodiazepine binding. Replacement of Cl- in the incubation medium by acetate, which does not permeate the GABA-mediated Cl-- ionophore, increases benzodiazepine binding, and GABA no longer increases the binding. Picrotoxinin decreases the increase in benzodiazepine binding is Cl--free media, and this decrease is blocked by GABA. These results are discussed in terms of interactions at the GABA receptor complex consisting of a GABA recognition site, a benzodiazepine recognition site, a picrotoxinin recognition site, and a Cl- ionophore.

Regional specificity of benzodiazepine receptor down-regulation during chronic treatment of rats with flurazepam

[3H]Flunitrazepam binding was studied in synaptosomal membranes from rat brain following 4 weeks of chronic treatment with 100-150 mg/kg/day flurazepam. At 12 h after the end of treatment, the brain was removed and dissected into 8 areas: cerebral cortex, hippocampus, cerebellum, corpus striatum, hypothalamus, midbrain-thalamus, medulla-pons, and olfactory bulbs. Membranes from each area were extensively 'washed', and saturation binding studies performed. Chronic flurazepam treatment caused a reduction in the apparent number of binding sites (Bmax) that was confined to the cerebral cortex, hippocampus, and medulla-pons, with a possible smaller loss in the olfactory bulbs. The binding constant (KD) was unchanged in all areas studied.

A comparative study on the clinical effects of flunitrazepam and oxazepam as oral premedication

The clinical effects of flunitrazepam and oxazepam as oral premedicants were tested in a double-blind study of 69 otorhinolaryngologic patients. Flunitrazepam had a somewhat higher sedative effect (p less than 0.10) and moderated the increase in systolic blood pressure significantly (p less than 0.005) more than did oxazepam, but as regards the other parameters tested no significant differences were found (sleep, apprehension, excitement, dizziness, emetic effect, headache, increase in heart rate, venepuncture). In some patients a profuse salivary secretion was observed despite intravenous injection of atropine just before the induction of anesthesia. Our results support earlier claims of flunitrazepam's relatively strong sedative and anxiolytic properties, but on the whole the difference in clinical effects of these benzodiazepine derivatives was not marked.

Benzodiazepine receptor binding in young, mature and senescent rat brain and kidney

Clinical reports have described age-altered pharmacological effects of anxiolytic drugs especially an increased susceptibility to their sedative actions. In order to test whether such changes may be due to age-related alterations in central benzodiazepine receptors, 3H-flunitrazepam binding was assayed in the frontal cortex and cerebellum of young, mature and senescent rats. The numbers of 3H-flunitrazepam binding sites and their affinity was determined by Scatchard analysis of saturation isotherms and the relative abundance of type I and type II benzodiazepine receptors was assessed by drug-inhibition studies using diazepam and the triazolopyridazine, CL 218,872. In addition, age related changes in the kidney and hippocampus of the Ro5-4864-sensitive benzodiazepine receptor were studied using 3H-Ro5-4864. No age-related alterations were noted in the binding characteristics of 3H-flunitrazepam. Furthermore, drug-inhibition of 3H-flunitrazepam binding by diazepam and CL 218,872 was nearly identical in young, mature and senescent rats, indicating that also the ratio of type I and type II receptors does not change with age. Binding of 3H-Ro5-4864 to membranes from rat hippocampus was not age-related. However, a significant decrease in 3H-Ro5-4864 binding to kidney membranes was demonstrated. Hence, central benzodiazepine receptors appear unaltered in the senescent rat model of aging. The clinical findings of an increased susceptibility to the sedative effects of benzodiazepines in the elderly may therefore be attributed to pharmacokinetic variables, or to events occurring secondarily to receptor activation.

A simple, inexpensive filtration device suitable for neurotransmitter receptor binding studies in brain tissue

We describe the construction and use of an inexpensive filtration manifold suitable for radioligand binding assays. Experiments with [3H]flunitrazepam indicate that the binding data for benzodiazepine receptors in rat brain homogenate are almost indistinguishable from those obtained with a commercially available manifold.

Increase of benzodiazepine binding to the membranes isolated in the presence of diazepam

The preparation of brain cells in the presence of diazepam leads to a 60-70% increase in benzodiazepine receptor density. Diazepam may promote the dissociation of endogenous inhibitors (modulators) and/or retard the dissociation of benzodiazepin receptors from the membrane to the medium. Routinely used procedures for tissue preparation reveal only part of the benzodiazepine receptors originally present in the tissue.

Lateral olfactory tract lesions reveal neuronal localization of benzodiazepine recognition sites

The effects of neuronal degeneration on benzodiazepine binding parameters were assessed following bilateral electrolytic lesions of rat brain lateral olfactory tracts. [3H]Flunitrazepam binding was measured in olfactory bulb homogenates 1, 7 and 14 days post-lesion. Specific [3H]flunitrazepam binding was significantly decreased two weeks after lesions. Diminution in binding was associated with a decreased population of sites (Bmax) rather than affinity alterations (KD). The time course for the loss of receptors is compatible with retrograde degeneration and suggests that specific benzodiazepine binding sites are in part located on mitral cell-bodies.

Action of pyrazolopyridines as modulators of [3H]flunitrazepam binding to the gaba/benzodiazepine receptor complex of the cerebellum

The pyrazolopyridines etazolate (SQ 20009) and cartazolate (SQ 65396) have strong modulatory effects on the GABA/benzodiazepine receptor complex of rate cerebellum. Thus, etazolate and cartazolate directly stimulate [3H]flunitrazepam binding (with EC50 values of 1.2 microM and 0.3 microM respectively) by increasing the apparent affinity of [3H]flunitrazepam for its binding sites. Stimulation of [3H]flunitrazepam binding by pyrazolopyridines is dependent on the presence of certain anions like chloride, bromide, iodide, nitrite, nitrate but not fluoride, acetate, formate or sulfate. If is inhibited by bicuculline-methiodide, and by the "chloride channel drugs' picrotoxinin and IPTBO. isoTHAZ, a GABA analogue with GABA antagonist properties in vivo, fails to inhibit binding stimulated by etazolate but antagonizes [3H]flunitrazepam binding stimulated by GABA. The pyrazolopyridines have also indirect effects on benzodiazepine receptor binding since they enhance the apparent sensitivity of those GABA recognition sites which are coupled to benzodiazepine binding sites. Thus, in the presence of 10 microM etazolate, GABA and muscimol enhance [3H]flunitrazepam binding, with EC50 values of 109 nM and 12 nM respectively. This sensitization effect is partially dependent on the presence of chloride ions. The pyrazolopyridines facilitate also the stimulation of benzodiazepine receptor binding by beta-alanine and taurine and by the rigid and flattened GABA analogues THIP and piperidine-4-sulfonic acid. Taken together, these results suggest that the pyrazolopyridines modulate [3H]flunitrazepam binding by acting at a site closely related to GABA receptor-regulated chloride ion channels.

Autoradiographic localization of benzodiazepine receptors in immunocytochemically identified gamma-aminobutyrergic synapses

Benzodiazepine receptors can be visualized in regions of synaptic contact by electron microscopic autoradiography using [3H]flunitrazepam as a photoaffinity label in fresh brain tissue. Perfusion fixation of the tissue prior to photoaffinity labeling left the ligand binding characteristics and the light and electron microscopic distribution of benzodiazepine receptors unaltered. Therefore, the immunocytochemical localization of a neuronal marker in fixed tissue could be combined with photoaffinity labeling in order to identify the types of synapses containing benzodiazepine receptors. By using antiserum to glutamate decarboxylase, a marker of gamma-aminobutyrergic neurons, one-third of the photolabeled benzodiazepine receptors were found to be associated with immunocytochemically stained nerve endings. Thus, these synapses are the site of at lest some benzodiazepine receptors. The enhancement of gamma-aminobutyrergic synaptic transmission by benzodiazepines, shown electrophysiologically, appears to be a primary mechanism of action of this group of drugs.

In vitro modulation by avermectin B1a of the GABA/benzodiazepine receptor complex of rat cerebellum

Avermectin B1a, a macrocyclic lactone anthelmintic agent, causes a concentration-dependent increase of [3H]flunitrazepam binding to membranes from rat cerebellum by increasing the affinity and the number of binding sites. This effect appears to be independent of the concentration of chloride ions. The effects of avermectin B1a occur with high affinity (EC50 = 70 nM), and they persist after washing of the membranes with drug-free buffer. Pretreatment of the membranes with Triton X-100 completely abolishes the action of avermectin B1a. GABA and the GABA-mimetic compounds piperidine-4-sulfonic acid and THIP diminish the effects of avermectin B1a on benzodiazepine receptor binding in a bicuculline-methiodide-sensitive mode. In addition, the stimulation of [3H]flunitrazepam binding by avermectin B1a is decreased by the pyrazolopyridines etazolate and cartazolate. These observations suggest that avermectin B1a stimulates benzodiazepine receptor binding by acting on a modulatory site which is independent of the GABA recognition site and of the drug receptor for the pyrazolopyridines, but which is in functional interaction with these sites.

gamma-Aminobutyric acid and benzodiazepine receptors: copurification and characterization

gamma-Aminobutyric acid (GABA) and benzodiazepine receptors have been solubilized and purified by procedures such as gel filtration, ion-exchange, lectin, and affinity chromatographies. All of these procedures enhance the specific activity of each receptor to a similar extent. The drug specificities of [3H]muscimol and [3H]flunitrazepam binding sites are the same after extensive purification by affinity chromatography compared to the membrane bound and initially solubilized receptors. GABA and chloride stimulation of benzodiazepine binding is retained in pure receptors. Two bands are covalently labeled with [3H]flunitrazepam after ultraviolet irradiation of the purified receptor. The persistent association of GABA, benzodiazepine, and chloride recognition sites after extensive purification suggests that they may be part of a single macromolecular complex.

Benzodiazepine receptors in chick retina: development and cellular localization

Benzodiazepine receptors are present in high concentration in the chick retina. Their pharmacological properties are similar to those of the benzodiazepine receptors present in the brain. The retina receptors appear prior to, as well as during, the period of synaptogenesis. In the newborn chick retina the receptors are localized in the inner synaptic layer, probably on or close to synaptic connections.

Multiple benzodiazepine receptor localization by light microscopic radiohistochemistry

Benzodiazepine receptor binding studies have been reported suggesting the existence of at least two different benzodiazepine receptors. One distinguishing feature of these two sites is that one has a high affinity for triazolopyridazines, whereas the other has a low affinity. In this study, the regional localization of the two different receptors was examined by light microscopic radiohistochemical methods. [3H]Flunitrazepam was used to label all types of benzodiazepine receptors in slide-mounted tissue sections. CL218,872, a typical triazolopyridazine, was used to preferentially displace [3H]flunitrazepam from the subclass of receptors having a high affinity for CL218,872. Autoradiographs clearly showed that receptor binding in some regions was substantially affected by CL218,872 while that in other regions was affected to a lesser degree. Areas with receptors with high affinity for the triazolopyridazine (Type 1 receptors) included the cerebellum, globus pallidus and parts of the cerebral cortex. Areas with receptors having low affinity for the drugs (Type 2 receptors) included the superficial layer of the superior colliculus, the caudate-putamen and parts of the dentate gyrus. The results of this study may help explain the physiological differences between the benzodiazepine and triazolopyridazine drugs and should point out target sites in the brain for additional studies of the apparently two different receptors.