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

Subcellular localization of the GABAA receptor gamma 2 subunit in the rat spinal cord

The fine subcellular organization of the GABAA receptor complex in the adult rat spinal ventral horn was analysed by immunocytochemistry using a specific polyclonal antiserum raised against the gamma 2 subunit. This subunit confers benzodiazepine sensitivity on the chloride channel of the GABAA receptor. With both fluorescent and peroxidase staining, the immunoreactivity was mainly observed in the grey matter and more specifically in the dorsal and ventral horns on medium and large neurons. A high number of immunostained somata were clustered in regions corresponding to motor nuclei. On the neuronal surface, labelling appeared as fluorescent dots over the more diffuse staining that was present on the soma and proximal part of dendrites. At the ultrastructural level, peroxidase end product was in most cases associated with the internal side of postsynaptic differentiations facing terminal boutons enriched with pleiomorphic small clear vesicles. The positively stained synapses were encountered on proximal dendrites of neurons and throughout the neuropil of the ventral horn (layers VII-IX). An immunoreactivity on the postsynaptic membrane was occasionally found to decorate large pieces of membrane not directly apposed to presynaptic active zones. In addition, presynaptic labelling was observed at axoaxonic contacts and at extrasynaptic sites on membranes within boutons, sometimes themselves apposed to gamma 2 immunoreactivity. Finally, we also observed gamma 2 immunoreactivity at the cytosolic face of the plasma membrane of some glial elements. These results give morphological evidence for the involvement of GABAA receptors in both post- and presynaptic inhibition in the rat spinal ventral horn. The presence of gamma 2 subunit immunoreactivity at these different synaptic contacts suggests that the two types of inhibition can be modulated by benzodiazepine drugs. The findings also provide anatomical evidence for the possible regulation of GABA release through an autoreceptor, and for GABAergic communication between neuronal and glial components.

Modulation of GABA-stimulated Cl- flux by a benzodiazepine agonist and an 'inverse agonist' after chronic flurazepam treatment

Rats treated one week with flurazepam were killed while still on the drug or 48 h after termination of drug treatment. The brain 'microsac' preparation derived from the cerebral cortices was used for studying the GABA-stimulated chloride influx. There was no significant change in the basal or GABA-stimulated influx between control and treated groups. However, the effect of flunitrazepam to enhance 10 microM GABA-stimulated influx was significantly reduced, indicating tolerance. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3- carboxylate (DMCM), an 'inverse agonist' at benzodiazepine receptors, dose dependently inhibited 50 microM GABA-stimulated influx; chronic treatment did not alter the effect of DMCM. This study demonstrates that one week treatment with flurazepam produces tolerance to benzodiazepines without any change in the effect of GABA or DMCM. This indicates that GABA and benzodiazepine sites are differently modulated after chronic treatment with benzodiazepines. However, since both benzodiazepine and DMCM act on the same receptors it appears that the different 'domains' on the benzodiazepine receptor are differently altered during chronic treatment.

The search for the hypnogenic center

1. Electrophysiological and lesion studies have suggested that a number of specific sites in the brainstem and basal forebrain may be involved in the regulation of sleep and waking. In contrast, a study of glucose consumption as measured by the 2-deoxyglucose technique reported a generalized decrease in nonREM sleep compared to waking. The rate of protein synthesis was relatively unchanged in nonREM sleep. 2. Another approach to understanding sleep regulation is to study the mechanism by which hypnotic drugs affect the nervous system. This may be done at both a molecular and neuroanatomic level. Studies with B-carbolines, inverse agonists of benzodiazepines (BZs), indicate that sleep induction by BZs is mediated by binding at the BZ recognition site of the BZ receptor complex. Binding at this site by a long-acting B-carboline parallels the time course of its arousing effects. A study with an enantiomeric BZ indicates that the effects on sleep are stereospecific. It is conceivable that some inverse agonists or enantiomeric benzodiazepines might be developed for clinical use as analeptics. 3. Microinjection of a BZ into the dorsal raphe nucleus acutely increases wakefulness, while administration into the medial preoptic area of the hypothalamus enhances sleep maintenance.

GABA, GABA receptors and benzodiazepine receptors in the human spinal cord: an autoradiographic and immunohistochemical study at the light and electron microscopic levels

The regional, cellular and subcellular distribution of GABA, GABA receptors and benzodiazepine receptors was investigated by light and electron microscopy in the human lumbar spinal cord taken post-mortem from eight cases aged 20-76 years. Firstly, the regional distribution of GABA receptors and benzodiazepine receptors was studied using autoradiography following in vitro labelling of cryostat sections with tritiated ligands. This was followed by a detailed study of the cellular and subcellular distribution and localization of GABA and benzodiazepine/GABAA receptors by light and electron microscopy using immunohistochemical techniques with monoclonal antibodies to GABA and to the alpha and beta subunits of the benzodiazepine/GABAA receptor complex. The results showed a close correspondence in the regional distributions of GABA, GABA (GABAA and GABAB) receptors and benzodiazepine receptors. The highest density of GABA-like immunoreactivity, GABA receptors and benzodiazepine receptors was localized as a dense band within lamina II of the dorsal horn (especially inner lamina II) with moderately high densities in laminae I and III. The remaining laminae of the spinal gray matter showed much lower levels of labelling. A close correspondence was also seen in the distribution of GABA-like immunoreactivity and of benzodiazepine/GABAA receptor immunoreactivity at the cellular and subcellular levels. At the cellular level, the greatest number of GABA-immunoreactive cells was found in lamina II; they comprised small, round to oval cells and, on the basis of soma size, shape, orientation and dendromorphology, they corresponded to previously described islet and filamentous cells. Benzodiazepine/GABAA receptor immunoreactivity was also localized on the same cell types in lamina II. At the subcellular level in lamina II, GABA-immunoreactive axon terminals mainly established axodendritic synaptic contacts. Small numbers of GABA-immunoreactive axon terminals appear to form possible axo-axonic contacts in complex synaptic arrays. Benzodiazepine/GABAA receptors were localized within the same types of synaptic complexes in which GABA-immunoreactive axon terminals were found. In these synaptic complexes, benzodiazepine/GABAA receptor immunoreactivity was associated with presynaptic and postsynaptic membranes and on apparent non-synaptic membranes. These results show a high concentration of GABA, GABA receptors and benzodiazepine receptors in lamina II of the dorsal horn of the human spinal cord and suggest a possible role for GABA in spinal sensory functions.

Demonstration of a distinct class of high-affinity binding sites for [3H]norharman [( 3H]beta-carboline) in the rat brain

Specific binding sites were demonstrated for some beta-carbolines in the rat brain with [3H]norharman as a ligand. The ligand displayed a high affinity for synaptosomal membranes which had been fractionated by a sucrose gradient. The calculated apparent KD value was 1.55 nmol/l and the maximum number of binding sites 148 fmol/mg protein. Displacement studies showed an exclusive specificity for a small group of beta-carbolines but not for the previously described inverse agonists at the benzodiazepine receptor nor for tryptamine and other indoles, as well as pargyline, a monoamine oxidase inhibitor. Further analysis revealed other binding sites for [3H]norharman, with an apparent KD value of 36 nmol/l that are presumably located on mitochondrial membranes. Binding to these sites was also not displaced by pargyline. Pargyline displaced [3H]norharman from a third population of binding sites on mitochondrial membranes with the apparent KD value of 46 nmol/l. These findings could explain the pharmacological effects of norharman and other beta-carbolines in vivo.

Phylogenetic conservation of the benzodiazepine binding sites: pharmacological evidence

Phylogenetic research can help to elucidate the structure of the GABA/benzodiazepine receptor complex. In this study the evolution of the beta-carboline binding site was traced to see whether it paralleled that of the benzodiazepine binding site. The ratio of [3H]ethyl-beta-carboline-3-carboxylate (beta-CCE) to [3H]flunitrazepam (FNZ) binding sites was determined in several nonmammalian species. The results further substantiate the tight link between these two binding sites. Photoaffinity labelling of the benzodiazepine receptor (BZR) has revealed phylogenetic variation of the molecular weight of the benzodiazepine binding proteins. The IC50 values for inhibition of [3H]FNZ by various compounds which are active at the central benzodiazepine receptors were determined in three phylogenetically distant species that each showed distinct subunit patterns. In these species, the respective affinities of the compounds were remarkably similar, suggesting that the binding sites for benzodiazepines are conserved in higher bony fishes and tetrapods. The conserved binding sites, in addition to recent immunological results obtained in other research groups, provide further evidence for the existence of the GABA/BZR as an isoreceptor complex.

Behavioral measurement of benzodiazepine tolerance and GABAergic subsensitivity in the substantia nigra pars reticulata

Rotational behavior was elicited by unilateral microinjection of the benzodiazepine flurazepam, and the gamma-aminobutyric acid (GABA) agonist, muscimol, into the substantia nigra pars reticulata (SNpr). This response was used to quantitate benzodiazepine tolerance and GABAergic subsensitivity after chronic benzodiazepine treatment. Studies in naive rats established the dose requirements for inducing contralateral circling and demonstrated the reproducibility of the behavioral response as a measure of SNpr function. There was a large difference in potency between the two drugs for causing dose-related rotation. The response to microinjected flurazepam could be blocked by 16 mg/kg of the benzodiazepine antagonist, Ro15-1788. Tolerance to intranigral flurazepam (50 micrograms) was measured by a reduction in the turning response after a 1- or 4-week chronic flurazepam treatment. The time course for the reversal of tolerance after a 4-week benzodiazepine treatment correlates with the time course of the reversal of benzodiazepine receptor down-regulation in the SNpr. Subsensitivity of the GABAergic system was demonstrated by the decreased rotational response to muscimol (10 ng), confirming the idea that the GABAergic system is also functionally altered by chronic benzodiazepine treatment. The time course of the decreased sensitivity to muscimol does not coincide with the development and reversal of tolerance to the turning produced by flurazepam or with benzodiazepine receptor down-regulation. These data suggest differential regulation of SNpr sensitivity to benzodiazepine and GABA agonists following chronic benzodiazepine treatment and may provide a basis for differential tolerance; the development of tolerance to some but not other benzodiazepine actions.

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)

Sleep and benzodiazepine receptor sub-types

CL 218,872, which preferentially binds to benzodiazepine (BZ) type 1 receptors, and flurazepam, which is thought to bind to both types 1 and 2, were given alone and in combination to rats. CL 218,872 had little effect on sleep latency, but significantly increased total sleep time. As expected, flurazepam both shortened sleep latency and prolonged total sleep. Pretreatment with CL 218,872 had no effect on these alterations in sleep induced by flurazepam. The implications for possible significance of sub-typing of BZ receptors are discussed.

Bidirectional effects of beta-carbolines in reflex epilepsy

Derivatives of ethyl-beta-carboline-3-carboxylate, ZK 91296, ZK 93423 and ZK 95962 have potent anticonvulsant activity against sound-induced seizures in audiogenic DBA/2 mice and against photically-induced seizures in the baboon, Papio papio. The convulsant beta-carbolines, DMCM and beta-CCM, have proconvulsant and convulsant activity in the same animal models. DMCM and beta-CCM are similar in potency as convulsants in DBA/2 mice (ED50 value for DMCM: 1.3 mg/kg; ED50 value for beta-CCM; 0.8 mg/kg), but differ with respect to their profiles for protection by anticonvulsant drugs. The anticonvulsant potencies of diazepam and clobazam are similar against both types of beta-carboline-induced seizures, whereas quazepam protects better against beta-CCM seizures (4 fold elevation in ED50 value at 1 mg/kg quazepam IP) than against DMCM seizures (1.7 fold elevation in ED50 value), supporting a preferential action of beta-CCM on BZ1 receptors. Valproate (400 mg/kg) and gamma-vinyl-GABA (1.5 g/kg) protect better against beta-CCM seizures (9.5 and 5.9 fold elevations in ED50 values respectively) than against DMCM seizures (1.8 and 2.7 fold elevations in ED50 values respectively). The excitatory amino acid antagonist, 2-amino-7-phosphonoheptanoic acid, has significant anticonvulsant activity against DMCM seizures. The elevated regional GABA levels in brains of DBA/2 mice observed during beta-CCM seizures are eliminated by the pretreatment with Ro 15-1788, which also blocks the seizure activity.

Aging does not alter the sensitivity of benzodiazepine receptors to GABA modulation

The effects of GABA on benzodiazepine receptor binding in cerebral cortical, hippocampal, and cerebellar membranes from 2-3 months old and 28-32 months old rats were studied. GABA modulation of agonist, antagonist, and inverse agonist binding to the receptor was examined using the displacement of 3H-Ro15-1788 by diazepam, Ro15-1788, and beta-carboline-3-carboxylate methyl ester, respectively, in the absence and presence of 100 microM GABA and 150 mM sodium chloride. GABA modulation was alike in old and young rats, with respect to the particular ligand and brain region. The results support the hypothesis that, in the brain regions studied, the allosteric modulation of benzodiazepine receptor binding by GABA remains intact as a function of aging.

Benzodiazepine receptors and their relationship to the treatment of epilepsy

Benzodiazepines (BDZ) interact with components of neuronal membranes to modify excitability in three different ways. Action at a high affinity central receptor (dissociation constant, KD, of 3 nM) linked to the GABAA recognition site enhances the inhibitory action of GABA by increasing the number of openings of Cl- channels produced by a given concentration of GABA. This effect correlates with anticonvulsant activity as evaluated in the antipentylenetetrazol test in animals and with antimyoclonic activity in human beings. It also correlates with anxiolytic activity. Action at a lower affinity membrane site (KD 100 nM to 1 microM) limits repetitive firing as observed in isolated neurons (in a manner similar to the action of phenytoin or carbamazepine). This does not depend primarily on neurotransmitter mechanisms, but probably involves an increase in the population of sodium channels in the inactive state. Action at a lower affinity site (KD 45 microM) in presynaptic terminals decreases voltage sensitive Ca++ conductance and, by limiting Ca++ entry, decreases neurotransmitter release. The two lower affinity BDZ systems may be responsible for therapeutic action in status epilepticus and for sedative side-effects. The high affinity central benzodiazepine binding sites can be differentiated into BZ1 and BZ2 receptors by ligands (such as triazolopyridazines and Quazepam) that preferentially act on BZ1 sites. There are regional differences in the density of the two receptor subtypes, but these have not yet been correlated with specific actions of benzodiazepines. Differences between various 1,4- and 1,5-benzodiazepines in terms of therapeutic action in epilepsy and neurologic side-effects can probably be explained on the basis of variation in full or partial agonist action at the high affinity central receptor, or differing relative action at the high and low affinity receptors.