The role of GABA in mediating the anticonvulsant properties of benzodiazepines

Anxioselective anxiolytics: on a quest for the Holy Grail

The discovery of benzodiazepine receptors provided the impetus to discover and develop anxioselective anxiolytics ('Valium without the side effects'). The market potential for an anxioselective based on the γ-aminobutyric acid A (GABA(A)) receptor resulted in clinical trials of multiple compounds. In contrast to the anxioselective profile displayed in preclinical models, compounds such as bretazenil, TPA023, and MRK 409 produced benzodiazepine-like side effects (sedation, dizziness) in Phase I studies, whereas alpidem and ocinaplon exhibited many of the characteristics of an anxioselective in the clinic. Alpidem was briefly marketed for the treatment of anxiety, but was withdrawn because of liver toxicity. Reversible elevations in liver enzymes halted development of ocinaplon in Phase III. The clinical profiles of these two molecules demonstrate that it is possible to develop GABA(A) receptor-based anxioselectives. However, despite the formidable molecular toolbox at our disposal, we are no better informed about the GABA(A) receptors responsible for an anxioselective profile in the clinic. Here, I discuss the evolution of a quest, spanning four decades, for molecules that retain the rapid and robust anti-anxiety actions of benzodiazepines without the side effects that limit their usefulness.

Cardiovascular and somatic startle and defense: concordant and discordant actions of benzodiazepine receptor agonists and inverse agonists

Benzodiazepine receptor (BZR) agonists and inverse agonists yield generally opposing effects on GABAergic transmission, and the functional consequences of these ligands are often bidirectional. BZR agonists exert anxiolytic effects, whereas the BZR partial inverse agonist FG 7142 has been reported to have anxiogenic actions in a variety of paradigms. In keeping with this literature, we found that the cardioacceleratory defensive response is enhanced by FG 7142, and attenuated by the BZR agonist chlordiazepoxide. In contrast, both compounds attenuated basal and fear-potentiated somatic startle responses. This did not appear to reflect a global reduction of startle reactivity, however, as the cardiac startle response was not significantly altered. These findings support the view that multiple substrates underlie distinct aspects or features of fear and anxiety. The results are consistent with the suggestion that FG 7142 may selectively enhance those aspects of anxiety that depend on cortical-cognitive processing.

The autoradiographic perspective of central benzodiazepine receptors; a short review

1. We reviewed studies performed to characterize central benzodiazepine binding sites. 2. An overview of the different radioligands used to characterize BZ1 and BZ2 binding sites and a mapping of these central benzodiazepine sites are described. 3. Saturation studies carried out by autoradiogram quantification also are reviewed. 4. The specific use of the autoradiographic technique to carry out studies on ontogeny, development, and phylogeny is discussed, as well as studies performed using this technique on some diseases and experimental conditions, such as drug treatments or chemical and mechanical lesions.

GABAA receptor subtypes differentiated by their gamma-subunit variants: prevalence, pharmacology and subunit architecture

Native GABAA receptors containing different gamma-subunit variants were distinguished immunobiochemically with antisera selectively recognizing the gamma 1-, gamma 2- and gamma 3-subunits. While GABAA receptors containing the gamma 2-subunits were confirmed to be rather ubiquitous in the adult brain, receptors characterized by the gamma 1- or gamma 3-subunit were of low abundance, as shown by immunoprecipitation. The three receptor populations differed strikingly in their benzodiazepine (BZ) site ligand binding profiles. The gamma 3-receptor population displayed reduced affinity for the full agonists clonazepam flunitrazepam and virtually lacked sensitivity to zolpidem. The gamma 1-receptor population displayed low affinity for all benzodiazepine site ligands tested, except for flunitrazepam, and could be differentiated from the gamma 2- and gamma 3-receptors by its low affinity for the inverse agonist beta CCM and its lack of affinity for the partial inverse agonist Ro 15-4513 and the antagonist flumazenil. Since flumazenil antagonizes all major effects of BZ agonists, gamma 1-receptors are not involved in mediating these actions in vivo. In immunopurified receptors, the gamma-subunit variants were found to be assembled with different variants of alpha- and beta-subunits, indicating that not only the gamma 2-subunit gamma 1- and gamma 3-subunits are part of various receptor subtypes. In addition, the gamma 2- and gamma 3-subunits can be co-assembled in native receptors, consistent with the subunit stoichiometry of two alpha-, one beta- and two gamma-subunits proposed previously for recombinant receptors.

Fetal, neonatal and adult expression of benzodiazepine receptor subtypes in human visual cortex

We have investigated the relative distributions of type I and type II benzodiazepine receptors in fetal, neonatal and adult human visual cortex using [3H]flunitrazepam and the triazolopyridazine, CL218872. Microdensitometry of autoradiograms show that while both benzodiazepine subtypes are found in visual area 17 early in life, the type II benzodiazepine receptors predominate. In adult human visual cortex comparable mounts of type I and type II subtypes occur in a laminar specific distribution concentrated in layers 4c and 2/3. The results may suggest a role for age-dependent expression of benzodiazepine receptor subtypes in visual cortex development and neuroplasticity.

Involvement of GABAergic systems in benzodiazepine-induced impairment of passive avoidance learning in mice

The possible involvement of GABAergic neuronal systems in benzodiazepine (BZP)-induced impairment of a passive avoidance response was investigated. Chlordiazepoxide (CDP) impaired passive avoidance when administered prior to training. The CDP-induced impairment was antagonized by pretreatment with picrotoxin, but not by pretreatment with bicuculline or post-training administration of picrotoxin. On the contrary, when combined with muscimol, the dose at which CDP impaired the response was lower than the dose at which it did so alone. The synergy of muscimol and CDP was attenuated by pretreatment with flumazenil or bicuculline. From these results, we conclude that GABAergic systems play an important role in the BZP-induced impairment of passive avoidance.

Characterization of [3H]alprazolam binding to central benzodiazepine receptors

The binding of the triazolobenzodiazepine [3H]alprazolam was studied to characterize the in vitro interactions with benzodiazepine receptors in membrane preparations of rat brain. Studies using nonequilibrium and equilibrium binding conditions for [3H]alprazolam resulted in high specific to nonspecific (signal to noise) binding ratios. The binding of [3H]alprazolam was saturable and specific with a low nanomolar affinity for benzodiazepine receptors in the rat brain. The Kd was 4.6 nM and the Bmax was 2.6 pmol/mg protein. GABA enhanced [3H]alprazolam binding while several benzodiazepine receptor ligands were competitive inhibitors of this drug. Compounds that bind to other receptor sites had a very weak or negligible effect on [3H]alprazolam binding. Alprazolam, an agent used as an anxiolytic and in the treatment of depression, acts in vitro as a selective and specific ligand for benzodiazepine receptors in the rat brain. The biochemical binding profile does not appear to account for the unique therapeutic properties which distinguish this compound from the other benzodiazepines in its class.

In vivo labeling of central benzodiazepine receptors with the partial inverse agonist [3H]Ro 15-4513

Ro 15-4513 is an imidazobenzodiazepine and a partial inverse agonist at the central benzodiazepine receptors (BZDr). It has been shown to antagonize behavioral and biochemical effects of ethanol. In vivo binding of [3H]Ro 15-4513 was evaluated in mouse brain. After intravenous injection [3H]Ro 15-4513 was readily taken up by the brain and distributed to brain areas enriched in benzodiazepine receptors. Binding was specific for central BZDr, saturable and reversible. A high degree of specific binding, relative to non-specific binding, was achieved. Analysis of dissociation kinetics revealed that [3H]Ro 15-4513 was retained significantly longer in hippocampus compared to other brain regions. In view of the known distribution of benzodiazepine receptor subtypes, this suggests that, in vivo, [3H]Ro 15-4513 has a higher affinity for benzodiazepine receptors type II and may explain quantitative differences in the regional distribution of this ligand compared to the antagonist [3H]Ro 15-1788. We conclude from these studies that Ro 15-4513 is a suitable ligand for in vivo studies of benzodiazepine receptors. Labeled with a positron-emitting isotope, it could be used with positron emission tomography to study BZDr in man under a variety of conditions.

Type I and type II gamma-aminobutyric acid/benzodiazepine receptors: purification and analysis of novel receptor complex from neonatal cortex

The gamma-aminobutyric acid (GABA) type A receptor was purified several thousandfold by affinity chromatography from rat cerebellum, adult cortex, and neonatal cortex. Competition for the benzodiazepine binding site by CL 218872 indicated that cerebellar receptors were predominantly type I, adult cortical receptors were a mixture of subtypes, and neonatal cortex was enriched in type II receptor. The receptor purified from neonatal cortex contained predominantly a 54-kilodalton (kDa), beta-subunit-like protein, whereas receptors from cerebellum and adult cortex contained nearly equal amounts of a 50-kDa, alpha-subunit-like protein and a 54-kDa polypeptide. Peptide maps of trypsin-digested 54-kDa subunits from cerebellum, adult cortex, and neonatal cortex exhibited very similar profiles, a result indicating considerable homology between these proteins in the receptor subtypes. A 59-kDa subunit protein was detected in the receptor complex purified from neonatal cortex. Like the 50-kDa, alpha-subunit of the type I receptor, this protein was photolabeled with [3H]flunitrazepam. The photolabeled peptide fragments, produced by trypsin digestion of these alpha 50- and alpha 59-subunits, exhibited the same retention times on reverse-phase HPLC. A less highly purified GABAA receptor preparation from adult rat spinal cord possessed characteristics that were very similar to those of the receptors purified from neonatal cortex.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A quantitative cerebral and whole body autoradiographic study of a intravenously administered benzodiazepine antagonist 3H-Ro 15-1788 in mice

3H-Ro 15-1788, a benzodiazepine receptor antagonist, was injected IV into male and pregnant mice. Autoradiograms were prepared from sagittal sections of animals killed after 30 s to 48 h. In the adult animal there was a rapid and high initial accumulation of radioactivity in the brain as compared to other organs and tissues. The highest accumulation was found in cortical brain areas, such as the olfactory bulb and the frontal cortex. Cerebellar cortex, globus pallidus, amygdala, substantia nigra, colliculus, hippocampus and pons followed in rank order. The rate of decline of radioactivity was highest in the pons and lowest in the olfactory bulb. The initial disappearance of radioactivity from the cerebellum was higher than from the other brain regions. Ro 15-1788 was rapidly eliminated; 4 h after drug administration there was an almost complete clearance of radioactivity from all tissues. After 24 h no trace of activity remained in the animal. The distribution of radioactivity at later time points indicates that metabolites of Ro 15-1788 are eliminated by fecal, urinary and nasal secretion. In the fetus also there was an early accumulation of radioactivity in the central nervous system. The radioactivity in fetal organs was lower than in the mother at all time intervals.

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.

Mouse lines selected for genetic differences in diazepam sensitivity

Selective breeding techniques were used to alter allelic frequencies responsible for diazepam sensitivity and resistance. We used the rotarod test to determine the duration of diazepam-induced neurologic deficit in genetically heterogeneous mice. Males were more sensitive than females in the initial population. We then selectively bred for diazepam resistance and sensitivity. A significant difference between the lines was apparent in both sexes after two generations, and divergence has continued over seven generations. Brain benzodiazepine assays indicated that absorption and distribution of diazepam do not differ in the two lines. Differences in brain benzodiazepine concentrations at recovery from ataxia indicated that the two lines differ in central nervous system sensitivity. We found diazepam-induced rotarod impairment to be blocked in a dose-dependent manner by the specific benzodiazepine antagonist Ro 15-1788, indicating that this effect is mediated through BZ receptors. A dose-response curve obtained from generations 6 and 7 indicates a 9- to 14-fold difference in dose required to obtain similar effects in the two lines. These mice are expected to be useful experimental subjects in studies of benzodiazepine mechanisms.

High affinity [3H]zolpidem binding in the rat brain: an imidazopyridine with agonist properties at central benzodiazepine receptors

[3H]Zolpidem, a novel hypnotic drug possessing a chemical structure unrelated to that of benzodiazepine (BZD) was employed as a new ligand to determine its binding characteristics to membrane preparations of rat cerebral cortex and cerebellum. In both structures, the imidazopyridine [3H]zolpidem bound with high affinity to a single population of recognition sites. The cerebellum possessed a similar number of [3H]zolpidem and [3H]diazepam binding sites, while the cerebral cortex possessed a lower density of [3H]zolpidem than [3H]diazepam binding sites. In contrast to [3H]diazepam binding, [3H]zolpidem binding was not detectable in the spinal cord. In the cortex, BZDs had a similar potency to displace [3H]zolpidem and [3H]diazepam binding while non-BZDs were more potent to inhibit [3H] zolpidem binding than [3H]diazepam binding. The binding of [3H]zolpidem was enhanced by GABA to the same extent as [3H]diazepam binding. The increase in [3H] zolpidem binding caused by chloride ions was less pronounced than that in [3H]diazepam binding. It is concluded that [3H]zolpidem possesses selectivity for BZD receptors with the pharmacological characteristics and regional distribution of the BZD1 receptor subtype. [3H]Zolpidem as a radioligand offers a useful additional tool to study the mechanism of action of hypnotics acting through BZD receptor subtypes.

GABAergic drugs can enhance or attenuate chlordiazepoxide-induced sleep time in a heterogeneous strain of mice

Evidence supports the notion that differences between the Long-Sleep and Short-Sleep selectively-bred lines of mice are attributable to differences in brain excitability and that these differences are mediated by activity of the GABAergic system. The general applicability of this hypothesis to other populations of mice was tested by using an outbred strain of mice. Specifically, a heterogeneous strain of mice was administered several doses of the hypnotic chlordiazepoxide. Additionally, the indirect GABA agonist AOAA, and the GABA antagonists bicuculline, picrotoxin and pentylenetetrazol were administered to independent groups in conjunction with chlordiazepoxide. The results clearly demonstrate that chlordiazepoxide dose-dependently increased hypnosis, while AOAA enhanced, and the antagonists attenuated sleep time. These findings can be used to support the contention that GABA mediates the bidirectional response of Long-Sleep and Short-Sleep mice to CNS hypnotic-depressants; and, further, show that GABA mediation of sleep time in mice is a general phenomenon.

Postnatal development of proteins associated with different benzodiazepine receptors

The postnatal development of several proteins irreversibly labeled by [3H]flunitrazepam in membranes from rat cerebral cortex was investigated. It was demonstrated that in the early postnatal days proteins with apparent molecular weights 55,000 and 59,000 were predominantly labeled whereas irreversible labeling of a protein with apparent molecular weight 51,000 started to predominate only in the second postnatal week. Irreversible labeling of another protein with apparent molecular weight 62,000 was weak throughout development. All these proteins seem to be associated with central benzodiazepine receptors. Irreversible labeling at various time points after birth seems to parallel the postnatal development of these proteins, and the different time course of development and different binding properties of the individual proteins support the hypothesis that these proteins are associated with separate and distinct benzodiazepine receptor subtypes. The pharmacological properties of the individual receptor subtypes seem to be fully developed in the early postnatal days, and therefore newborn animals seem to be a good model system for the investigation of properties and function of these various benzodiazepine receptor subtypes.

Benzodiazepine receptors: multiple receptors or multiple conformations?

Several lines of evidence from reversible binding studies seem to indicate there are at least two "central" benzodiazepine receptor subtypes, the BZ1 and BZ2 receptors. Irreversible binding studies, using 3H-flunitrazepam as a photoaffinity label for benzodiazepine receptors, not only are in perfect agreement with the data from reversible binding studies but extend these studies by identifying P51, a protein with apparent molecular weight 51,000, as a protein associated with the BZ1 receptor and by suggesting that the BZ2 receptor might actually consist of several different benzodiazepine receptors associated with different and distinct proteins irreversibly labeled by 3H-flunitrazepam. Other reversible binding studies have accumulated indicating the existence of several different conformations of benzodiazepine receptors. Irreversible binding studies support this conclusion and in addition suggest the existence of four different benzodiazepine binding sites within the GABA-benzodiazepine receptor complex. It is therefore hypothesized that there are several different GABA-benzodiazepine receptor subtypes all of which have four distinct benzodiazepine binding sites which can exist in at least three different but freely interconvertible conformations. This hypothesis can account for all experimental observations obtained so far and might partially explain the distinct clinical effects of structurally similar benzodiazepines.

Triazolam, an anomalous benzodiazepine receptor ligand: in vitro characterization of alprazolam and triazolam binding

Both alprazolam and triazolam displaced clonazepam (but not Ro 5-4864) from rat brain membranes with high affinity, showing them to act at central but not peripheral benzodiazepine receptors. At 0 degrees C, 10 microM gamma-aminobutyric acid (GABA) increased the ability of alprazolam, but not of triazolam, to displace ethyl-beta-carboline-3-carboxylate (beta-CCE) and Ro 15-1788 from these receptors. At 37 degrees C, GABA increased the affinity of the receptors for both drugs, with a +GABA/-GABA ratio of 1.5 for each in promoting Ro 15-1788 binding displacement. As both triazolam and alprazolam act as anxiolytics in vivo, the results at 37 degrees C would be compatible with the hypothesis that GABA causes an increase in affinity of drugs that act in this way, but the results at 0 degrees C would not be compatible. At 37 degrees C, alprazolam had a higher IC50 for the benzodiazepine receptor than at 0 degrees C, whereas triazolam showed the reverse effect. The relative IC50 values in vitro at 37 degrees C correlated better with the potency in vivo than those obtained at 0 degrees C. At 0 degrees C, both drugs showed Hill plots with slopes of 0.9-1 with beta-CCE and Ro 15-1788. At 37 degrees C, the slopes with triazolam were much reduced, indicating that the drug may have a selective action on a subclass of central benzodiazepine receptors. In the studies reported here, alprazolam behaved like other benzodiazepines, whereas triazolam showed several anomalous properties. It would be of interest if these properties could be related either to the drug's use as a hypnotic or to the side effects it sometimes induces.

Diazepam antagonizes GABAmimetics in rats with spontaneous petit mal-like epilepsy

Wistar rats in our laboratory breeding colony spontaneously present petit mal-like, non-convulsive, epileptic seizures. In these rats, as in other animal petit mal models, GABAmimetics, agonists of GABA-A receptors such as 4, 5, 6, 7 tetrahydroisooxazolo (5,4-c) pyridin-3-ol (THIP), or inhibitors of GABA catabolism such as gamma-vinyl GABA (GVG) or L-cycloserine (CYC), aggravated the seizures. Diazepam not only abolished the spontaneous seizures but also completely blocked the effects of the GABAmimetics, totally suppressing seizures in rats given THIP, GVG or CYC. These findings show that the mode of action of benzodiazepines is not comparable to a non-specific potentiation of GABA transmission, and suggest that the anti-absence effects of the benzodiazepines could depend on interactions with neurotransmitter systems other than GABA.

Effect of picrotoxinin on benzodiazepine receptor binding

The effect of picrotoxinin on [3H]flunitrazepam binding to benzodiazepine receptors was investigated. In mouse forebrain membranes, picrotoxinin inhibited basal, GABA- and pentobarbital-stimulated [3H]flunitrazepam binding; this inhibitory activity was temperature- and chloride ion-dependent. Scatchard analysis of the data indicates that picrotoxinin decreases the number of binding sites without altering binding affinity. In cerebellar membranes, picrotoxinin did not alter [3H]flunitrazepam receptor binding.

On the location of gamma-aminobutyrate and benzodiazepine receptors in the cerebellum of the normal C3H and Lurcher mutant mouse

Binding of gamma-aminobutyrate and benzodiazepine receptor ligands has been studied in the cerebellum of adult normal (C3H) and Lurcher mutant mice. The adult mutant has lost all Purkinje cells and more than 90% of the granule cells in the cerebellar cortex. When compared with their normal littermates Lurcher mice displayed large decreases in the number of high-affinity binding sites for [3H]muscimol, a synaptic gamma-aminobutyrate receptor ligand, in washed cerebellar homogenates. This observation was consistent with the extensive loss of gamma-aminobutyrate receptive Purkinje and granule cells from the Lurcher cerebellum. However, specific binding of the benzodiazepine-receptor ligand [3H]flunitrazepam to Lurcher cerebellum remained unchanged. Indeed quantitative autoradiography, employing [3H]flunitrazepam as a photoaffinity label, showed no significant differences in the density of labelling between Lurcher and normal littermate mice in any region of the cerebellum. These benzodiazepine binding sites in washed homogenates or tissue sections displayed a gamma-aminobutyrate-induced enhancement of [3H]flunitrazepam binding which occurred to the same extent in both Lurcher and normal cerebellum, a facilitatory effect which could be blocked by the addition of bicuculline methobromide. Our results suggest that a large proportion of the high-affinity, specific benzodiazepine binding sites in mouse cerebellum are not coupled to the synaptic gamma-aminobutyrate receptors thought to be labelled by high affinity [3H]muscimol binding. Further, that benzodiazepine binding sites do not appear to be enriched on either the soma or dendrites of Purkinje cells, as has been suggested from previous studies. Investigations at the electron microscope level are now required to elucidate the cellular location of benzodiazepine binding sites in the cerebellar cortex and to examine whether or not they are likely to be exposed to gamma-aminobutyrate in vivo.

Pharmacological and neurochemical properties of 1,4-diazepines with two annelated heterocycles ('hetrazepines')

1,4-Diazepines with two annelated heterocycles ('hetrazepines') such as brotizolam (WE 941), WE 973 and WE 1008 bind with high affinities to benzodiazepine receptors in the central nervous system. Brotizolam has a pharmacologic spectrum of action similar to clinically useful benzodiazepines, while the closely related derivatives WE 973 and WE 1008 appear to lack hypnotic action. Unlike other benzodiazepine receptor ligands which share common pharmacologic properties with the benzodiazepines, the apparent affinities of WE 973 and WE 1008 are not increased significantly in the presence of GABA, even at an elevated incubation temperature. Furthermore, the apparent affinities of these compounds do not appear to be reduced as a result of increasing the incubation temperature. Brotizolam, like the benzodiazepines, facilitates GABAergic transmission in zona recitulata neurons of the substantia nigra. In contrast, at a dose which inhibits cell firing, WE 973 does not appear to significantly augment the inhibitory action of GABA in these cells. These observations suggest that the so-called 'GABA shift' may not be a valid means of distinguishing benzodiazepine-like compounds in vitro. Furthermore, these data suggest that facilitation of GABAergic transmission may be necessary for the hypnotic action of benzodiazepine receptor ligands, but not for the anticonflict or the anticonvulsant actions of such compounds.

The sedative effects of CL 218,872, like those of chlordiazepoxide, are reversed by benzodiazepine antagonists

The effects of CL 218,872, initially classified as a non-sedative anxiolytic, were investigated and compared with those of chlordiazepoxide in the holeboard. The ability of two drugs that antagonise the effects of benzodiazepines, CGS 8216 and Ro 15-1788, to reverse the effects of CL 218,872 and chlordiazepoxide were also investigated, to see whether their effects might be mediated via benzodiazepine receptors. CL 218,872 (10 mg/kg) was found to be significantly sedative in both mice and rats (i.e., both locomotor activity and head-dipping were significantly decreased). In mice, the effects of CL 218,872 and of chlordiazepoxide were very similar over a range of doses, except that the stimulatory effect seen with low doses of chlordiazepoxide on head-dipping just failed to reach significance with CL 218,872. This study is in agreement with recently published results from different tests showing that sedative effects can be obtained with doses of CL 218,872 that are low and not much higher than those leading to anxiolysis. The sedative effects of both CL 218,872 (10 mg/kg) and chlordiazepoxide (20 mg/kg) were significantly reversed by RO 15-1788 (10 and 20 mg/kg) and CGS 8216 (10 mg/kg), suggesting that their effects are mediated via benzodiazepine receptors. The increase in head-dipping seen with chlordiazepoxide (2.5 mg/kg) was also reversed by RO 15-1788 and CGS 8216.

Cerebrovascular and cerebral metabolic effects of flurazepam and a benzodiazepine antagonist, 3-hydroxymethyl-beta-carboline

There is a need in clinical practice for an antagonist which can reverse the sedative action of benzodiazepines. Recently, 3-hydroxymethyl-beta-carboline (3-HMC) has been reported to inhibit the sleep inducing effects of flurazepam. The effects of flurazepam (0.5, 5 and 50 mg/kg) on cerebral blood flow (CBF) and cerebral O2 consumption (CMRO2) were evaluated in rats and the ability of 3-HMC to reverse these changes was determined. Regional CBF was measured with radioactive microspheres and cortical CMRO2 was calculated from sagittal sinus-arterial O2 content differences and cortical CBF. Flurazepam produced dose dependent decreases in CBF and CMRO2 which were significant at 5 and 50 mg/kg. 3-HMC (5 mg/kg) inhibited flurazepam induced changes at the 5 mg/kg dose but had little effect on the CBF and CMRO2 depression produced by 50 mg/kg flurazepam. At a dose of 25 mg/kg, 3-HMC inhibited the effects of both 5 and 50 mg/kg flurazepam. Blood pressure and heart rate were also decreased by flurazepam but these variables were not reversed as effectively by 3-HMC treatment. The results indicate that 3-HMC is an active antagonist of the cerebrovascular and cerebral metabolic depression produced by flurazepam and can stimulate CBF and CMRO2 at high doses when given alone.

CL 218,872 binding to benzodiazepine receptors in rat spinal cord: modulation by gamma-aminobutyric acid and evidence for receptor heterogeneity

The binding of the triazolopyridazine CL 218,872 to central benzodiazepine receptors identified with [3H]Ro 15-1788 was studied in extensively washed homogenates of rat spinal cord and cerebral cortex. CL 218,872 displacement curves were shallow in both spinal cord (nH = 0.67) and cortex (nH = 0.54), suggesting the presence of type 1 and type 2 benzodiazepine receptors in both tissues. CL 218,872 had lower affinity in spinal cord (IC50 = 825 nM) than cortex (IC50 = 152 nM), possibly reflecting the presence of fewer type 1 sites in the cord. Activating gamma-aminobutyric acid (GABA) receptors with 10 microM muscimol resulted in a two- to threefold increase in CL 218,872 affinity in both tissues without changes in the displacement curve slope. This indicates that GABA enhances CL 218,872 affinity for both type 1 and type 2 sites in both spinal cord and cerebral cortex.

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.

Differential interactions of GABA agonists, depressant and convulsant drugs with [35S]-t-butylbicyclophosphorothionate binding sites in cortex and cerebellum

Effects of three GABA agonists, four GABA antagonists and convulsants (picrotoxinin, alpha-dihydropicrotoxinin [DHP], pentamethylenetetrazole [PTZ] and isopropylbicyclophosphate ester) and three depressant drugs (pentobarbital, (+)etomidate and etazolate) were investigated on [35S]t-butylbicyclophosphorothionate (TBPT) in cortex and cerebellum. All the convulsants tested were equipotent in inhibiting [35S]TBPT binding in cortex and cerebellum. Convulsants like picrotoxinin inhibited [35S]-TBPT binding competitively in both cortex and cerebellum. In contrast, gamma-aminobutyric acid (GABA) agonists (muscimol, GABA and 4,5,6,7-tetrahydroisoxazol[5,4-C]pyridine-3-ol [THIP]), and depressants like etazolate, (+)etomidate and pentobarbital were more potent inhibitors of [35S]TBPT binding in cerebellum than in cortex. GABA inhibition of [35S]TBPT binding appears to be mediated through a low-affinity site. GABA and pentobarbital inhibited [35S]TBPT binding in cortex and cerebellum noncompetitively. Depressants like pentobarbital appear to interact with the TBPT sites allosterically. These results suggest that depressant and convulsant drugs that modulate GABAergic transmission interact differently with the TBPT binding sites in cortex and cerebellum.

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.

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.

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.

Sustained improvement in tardive dyskinesia with diazepam: indirect evidence for corticolimbic involvement

A rater-bind, ABA's design study of 21 cases indicates that diazepam significantly improves tardive dyskinesia and that some of the improvement persists for an extended period after diazepam is withdrawn. Since benzodiazepine receptors and sites of action seem to be mainly in the neocortex (especially frontal), limbic cortex, and deep limbs nuclei, and these structures provide most of the input into the nigrostriatopallidal system that probably regulates its role in voluntary movement, it may be suggested that impaired corticolimbic control of basal ganglia may be a factor in the pathogenesis of tardive dyskinesia.

Irreversible binding of [3H]flunitrazepam to different proteins in various brain regions

Irreversible photolabeling by [3H]flunitrazepam of four proteins with apparent molecular weights 51,000 (P51), 53,000 (P53), 55,000 (P55), and 59,000 (P59) was investigated in various rat brain regions by SDS-polyacrylamide gel electrophoresis, fluorography, and quantitative determination of radioactivity bound to proteins. On maximal labeling of these proteins, only 15-25% of [3H]flunitrazepam reversibly bound to membranes becomes irreversibly attached to proteins. Results presented indicate that for every [3H]flunitrazepam molecule irreversibly bound to membranes, three molecules dissociate from reversible benzodiazepine binding sites. This seems to indicate that these proteins are either closely associated or identical with reversible benzodiazepine binding sites, and supports the hypothesis that four benzodiazepine binding sites are associated with one benzodiazepine receptor. When irreversible labeling profiles of proteins P51, P53, P55, and P59 were compared in different brain regions, it was found that labeling of individual proteins varied independently, supporting previous evidence that these proteins are associated with distinct benzodiazepine receptors.

Antagonism of the anxiolytic action of diazepam and chlordiazepoxide by the novel imidazopyridines, EMD 39593 and EMD 41717

The imidazopyridines EMD 35993 and EMD 41717 antagonized the anticonflict actions of diazepam and chlordiazepoxide in rodent models which are predictive for anxiolytic action in man. In contrast to other described benzodiazepine antagonists, these compounds did not antagonize either the anticonvulsant or muscle relaxant properties of either benzodiazepine. Both EMD 39593 and EMD 41717 competitively inhibit the binding of [3H]diazepam to brain membranes, but do not exhibit regional differences in potency. These observations suggest that both EMD 39593 and EMD 41717 display some selectivity in antagonizing the anxiolytic properties of benzodiazepines, and as such may be useful tools in identifying neuronal substrates of anxiety.

Diethylpyrocarbonate modification of benzodiazepine receptors from calf cerebral cortex

Diethylpyrocarbonate (DEP), an amino acid modifying reagent, causes complete inactivation of particulate and deoxycholate-solubilized benzodiazepine-receptors from calf cerebral cortex. No heterogeneity was observed in DEP-sensitivity of the receptors. Protection from DEP-induced inactivation was provided by the centrally active benzodiazepines, diazepam and nitrazepam, but not by the peripherally active Ro5-4864, suggesting that DEP modifies a residue which is essential for the central actions of benzodiazepines. GABA did not protect against inactivation or influence the protection afforded by diazepam, indicating that the DEP-modifiable residue is independent of GABA binding sites, or that GABA binding sites are also sensitive to DEP. DEP-induced inactivation of benzodiazepine-receptors proceeds much faster at pH 10.1 than at pH 8.1 or 6.0, indicating the modification of a high pKa side group, possibly the phenol of a tyrosyl residue. This postulation is in accord with our previous findings with the modifying reagents tetranitromethane and N-acetylimidazole.

Early postnatal chlordiazepoxide administration: permanent behavioural effects in the mature rat and possible involvement of the GABA-benzodiazepine system

The long term behavioural and biochemical effects of chronic chlordiazepoxide treatment during the period of neuronal maturation in the rat have been investigated. The administration to lactating mothers of chlordiazepoxide at very low doses (0.22 and 2.6 mg/kg) in their drinking water affects both behavioural and biochemical parameters in offspring at 60 days of age and undrugged since weaning. A deficit in the acquisition of the conditioned avoidance response in treated rats was observed, although no significant difference in spontaneous locomotor activity between control and treated rats was found. 3H-Flunitrazepam binding sites in cerebral cortex and hippocampus were decreased by the treatment, whereas no change was detected in cerebellum. Moreover, 3H-muscimol binding sites increased in hippocampus with no changes in cerebral cortex and cerebellum. According to the different regional distribution of benzodiazepine type 1 and type 2 receptors, we suggest that type 2 receptors are selectively affected by the treatment, and that the GABAergic receptor system is also permanently altered by administration of chlordiazepoxide during early postnatal life.

Postnatal development of proteins irreversibly labeled by [3H]flunitrazepam

The irreversible labeling by [3H]flunitrazepam of three proteins with apparent molecular weights of 51,000 (P51), 55,000 (P55) and 59,000 (P59) was investigated using hippocampal membranes isolated from rats at various timepoints after birth. The present results indicate that [3H]flunitrazepam predominantly labels P55 and P59 in the early days after birth whereas labeling of P51 starts to increase significantly in the second postnatal week. A possible association of P55 and P59 with type 2 and of P51 with type 1 benzodiazepine receptors is suggested.

Quantitative receptor autoradiography using [3H]ultrofilm: application to multiple benzodiazepine receptors

Radiohistochemical methods utilized to label drug and neurotransmitter receptors in vitro at the light microscopic level are quantitative. In this report, the response of tritium-sensitive sheet film ([3H]Ultrofilm) was characterized under conditions used for the light microscopic localization of neurotransmitter receptors by in vitro autoradiographic techniques. Radioactive standards containing varying concentrations of tritium were prepared from brain tissue and exposed to [3H]Ultrofilm for varying lengths of time, and the response of the film was measured by microdensitometry. A 1n optical density versus 1n radioactivity plot provided a useful linear relationship. Relationships established between radioactivity concentrations, exposure times and optical density were utilized to establish appropriate exposure conditions for several [3H]ligands in brain sections. The accuracy and pharmacological relevance of these methods were tested by studying the regional distribution of multiple benzodiazepine (BZ) receptors, and by analyzing the inhibitory potency of the triazolopyridazine, CL218,872, and methyl-beta-carbolinecarboxylate, two agents which discriminate between type 1 and 2 BZ receptors. The results obtained compared favorably with results previously obtained in tissue homogenates by biochemical methods. Overall, these results have practical implications. For example, quantitative autoradiography can be used to generate accurate kinetic and pharmacological displacement curves for small tissue areas. Also, since the film response curve is not linear with exposure (radioactivity X time), autoradiographs of the same sections can have relatively different density patterns depending on the time of exposure.

A novel pyrazoloquinoline that interacts with brain benzodiazepine receptors: characterization of some in vitro and in vivo properties of CGS 9896

The novel pyrazoloquinoline, CGS, 9896, was a potent inhibitor of specific [3H]-flunitrazepam binding in several brain regions with subnanomolar KI values. The inhibition of [3H] propyl beta-carboline-3-carboxylate ([3H]-PCC-) binding by CGS 9896 was enhanced by gamma-aminobutyric acid (GABA) but not by chloride ion. GABA enhancement of CGS 9896 inhibition of [3H]-PCC binding predicts this compound has benzodiazepine (BZD) agonist-type activity. Behavioral studies support this prediction. CGS 9896 was found to protect mice against bicuculline and metrazol induced seizure at doses that did not induce ataxia or sedation. CGS 9896 may represent a class of compounds with potential therapeutic value. The high affinity of this non-BZD compound suggests that CGS 9896 may also be of value as a high affinity ligand for the continued study of BZD receptors.

Characteristics of type 1 and type 2 benzodiazepine receptors in the ovine brain

Analysis of the displacement of 3H-diazepam binding to membranes prepared from the ovine frontal cortex by the triazolopyradiazine CL218,872 yielded a Hill coefficient significantly below unity. By analogy with similar studies of this drug in rat brain this suggested the existence of Types 1 and 2 benzodiazepine receptors. The degree of displacement of 3H-diazepam by CL218,872 (200 nM, Type 1; 800 nM, Type 2) in homogenates of brain regions differed, the rank order being cerebellum greater than parietal cortex greater than frontal cortex congruent to temporal cortex congruent to hippocampus greater than striatum. Displacement of 3H-diazepam by CL218,872 was enhanced by 10(-5) M GABA in the striatum (at 200 nM and 800 nM CL218,872) and cerebellum (at 200 nM CL218,872). Benzodiazepine receptors in the ovine frontal cortex were least sensitive to CL218,872 (200 nM) in young fetuses (54-68 days gestation) and achieved adult levels of sensitivity by late gestation. Finally, the potency of CL218,872 to displace 3H-diazepam was not effected by the 3H-ligand concentration (0.5 nM or 5.0 nM), suggesting that Types 1 and 2 benzodiazepine receptors are not identical to the high and low affinity 3H-diazepam binding sites we have previously identified in the ovine brain.

Peripheral metabolism of beta-carboline-carboxylic acid esters

Esters of beta-carboline-3-carboxylic acid have recently been identified as potent inhibitors of brain benzodiazepine receptors in vitro. Ethyl beta-carboline-3-carboxylate (beta-CCE), however, is a rather weak inhibitor in vivo of benzodiazepine receptors in mice. The ED50-value was 91 mg/kg intraperitoneally 35 min after administration (ED50 is that dose which inhibits by 50% the specific binding of 3H-flunitrazepam intravenously). ED50 for beta-CCE was 2-20 fold lower in mice pretreated with organophosphorus esterase inhibitors, concomitantly with the observation of strong inhibition of liver and kidney hydrolyzing activity, using 3H-propyl beta-carboline-3-carboxylate as substrate. The rat brain contains only approximately 0.1% of the hydrolyzing activity as compared to the liver. It is concluded that some esters of beta-carboline-3-carboxylate exhibit only weak effects on benzodiazepine receptors in living animals due to hydrolysis outside the brain.