Interactions between benzodiazepines and GABA in the cerebellar cortex

Diazepam-induced changes of optokinetic nystagmus fast phase

Since specific benzodiazepine (Bz) binding sites have been found in the vision and oculomotor control areas of the central nervous system (CNS), the fast phases of optokinetic nystagmus (OKN) should be affected by Bz administration. In this study, we examine the effects of Bzs on OKN fast phases under closed- and open-loop experimental conditions. Six normal subjects participated in the experiments. The eye movements were measured by the magnetic field, search coil technique, 90 min after diazepam or placebo administration. The study was performed in a randomized, double-blind fashion. After diazepam, the mean amplitude (MAmp) and mean peak velocity (MVel) of OKN fast phases decreased significantly under both experimental conditions. The percentage decreases in MAmp and MVel under the open-loop condition were significantly larger than those under the closed-loop condition. The results indicate that the fast phases of OKN could sensitively reflect the pharmacodynamic effects of Bzs on the CNS.

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)

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.

The benzodiazepine antagonist Ro 15-1788 reverses the effect of methyl-beta-carboline-3-carboxylate but not of harmaline on cerebellar cGMP and motor performance in mice

The cerebellar cGMP level in mice was decreased in a dose-dependent manner 30 min after diazepam (ED50 = 2 mg/kg p.o.). This effect was reversed by the specific benzodiazepine antagonist Ro 15-1788. Methyl-beta-carboline-3-carboxylate (beta-CCM) and harmaline increased cGMP. Ro 15-1788 dose dependently counteracted the beta-CCM- but not the harmaline-induced increase in cGMP. In the horizontal wire test Ro 15-1788 antagonized the impairment of motor performance induced by beta-CCM, but not that induced by harmaline. These findings further support the view that harmaline in contrast to beta-carboline-3-carboxylates does not act through benzodiazepine receptors, and that Ro 15-1788 antagonizes only those convulsants and stimulants that act through specific benzodiazepine receptors.

Autoradiography of benzodiazepine receptor binding in the central nervous system of the normal C57BL6J mouse

[3H]flunitrazepam has been used as a photoaffinity label for the specific, clonazepam-displaceable 1,4-benzodiazepine binding sites in sections of normal C57BL6J mouse brain and spinal cord. Binding was visualized by light microscope autoradiography and quantified by a simple microdensitometric procedure. Specific flunitrazepam binding was seen to be highest in the colliculi, cerebral cortex, hippocampal formation, interpeduncular nucleus, mamillary body, hypothalamus, olfactory tubercle, and in the molecular layer and deep nuclei of the cerebellum. The distribution of specific flunitrazepam binding sites in mouse brain and spinal cord is discussed in terms of the known actions of the benzodiazepines.

Loss of Purkinje cell-associated benzodiazepine receptors spares a high affinity subpopulation: a study with pcd mutant mice

In order to identify the relative number of benzodiazepine (BZ) receptors in Purkinje and granule cells, the Purkinje cell degeneration (pcd) mutant mouse was used at different ages. In these mice, Purkinje cells have degenerated almost completely by 45-50 days of age. Granule cell loss occurs only later, and is most severe between 180 and 300 days. [3H]Flunitrazepam (FNZ) and [3H]ethyl-carboline-3-carboxylate (beta-CC) were used as ligands. In the 45-50-day-old pcd mice, it was found that there is approximately a 50% decrease in the number of receptors as labeled by [3H]beta-CC or [3H]FNZ, when the binding is expressed as fmol/cerebellum. The binding decreased by approximately 80% in 300-day-old pcd mice (fmol/cerebellum). [3H]FNZ was not displaced by 1 microM RO5-4864, ruling out binding to glial cells. Nonlinear regression analysis of FNZ saturation data provided evidence for two populations of receptors (high and low affinity sites). Only the low-affinity sites were reduced in number at 45 days. [3H]beta-CC saturation data showed, however, only one population of receptors. The total number of receptors (Bmax) was significantly lower for beta-CC than for FNZ in the control mice. It appears that 50% of the total BZ receptors is associated with Purkinje cells. In addition, our data on 300-day-old pcd mutants strongly suggest the existence of granule cell-associated BZ receptors.

Chemical structure and biological activity of the diazepines

Since the introduction of chlordiazepoxide and diazepam many diazepines have been developed. Use of these drugs is increasing and considerable knowledge has accumulated about their mechanisms of action. The structural and pharmacological properties of these drugs are surveyed briefly.

Differential effects of diazepam upon vestibulo- and visual-oculomotor responses in the rabbit

The effects of intravenously administered diazepam (0.6 mg/kg) on vestibulo-ocular reflex (VOR), optokinetic nystagmus (OKN), optovestibular reflex (OVR) and their after-nystagmus were examined in rabbits. These reflexes were evoked by velocity step of 20 degrees/sec of chair or drum rotation. Slow phase eye velocity (SPEV) of OVR shows algebraic summation of those of VOR and OKN. Although SPEVs of VOR, OKN and OVR significantly decreased at 10 and 30 min after diazepam injection (p less than 0.05, t-test), OKN shows most distinctive reduction. SPEV reduction of OVR after diazepam administration was also equal to the algebraic summation of VOR and OKN reductions.

Diazepam enhances cerebellar inhibition on vestibular neurons

The spontaneous neuronal activity of the lateral (LVN) and the superior (SVN) vestibular nuclei was analysed before and after the intravenous (i.v.) injection of diazepam in encéphale isolé', decerebrate and cerebellectomized rabbits. The inhibition of vestibular neurons was dependent on the integrity of cerebellar connections with LVN, while these links were partially responsible for the diazepam inhibition on SVN. A role of spinal and telediemesencephalic structures was not recognized. Considering that diazepam does not increase the activity of Purkinje cells, the drug effect ought to be exerted at the level of the Purkinje cell junctions with the cerebellar nuclei and with the vestibular neurons. GABA being the neurotransmitter released by Purkinje cells evidence is provided for a diazepam potentiation of the GABAergic mechanism at the level of vestibular system.

Some pharmacological studies on the spastic mouse

1 Full-wave rectification and integration of the EMG signal recorded from the hamstring muscles of the spastic mouse was used to evaluate the actions of a variety of drugs on the muscle rigidity of these mutants, animals in which no histological lesion has yet been found. 2 Profound and long-lasting muscle relaxant responses were consistently observed upon the injection of diazepam (2 mg/kg, i.p.) and flunitrazepam (2 mg/kg, i.p.). Such responses were always greater than those obtained upon injection of 40% (v/v) propylene glycol (10 ml/kg) alone, the vehicle for the benzodiazepines. 3 The muscle relaxant action of a low dose (0.25 mg/kg i.p.) of the benzodiazepine Roll-6896 was not shared by the same dose of its enantiomer Roll-6893. 4 Profound and long-lasting muscle relaxation was caused by sodium valproate (696 mg/kg, i.p.). Consistent muscle relaxant responses were also observed upon the injection of pentobarbitone (30 mg/kg, i.p.), but not phenobarbitone (30 mg/kg, i.p.). 5 Other drugs that had little or no detectable effect on the muscle rigidity of the spastic mouse included diphenylhydantoin (30 mg/kg, i.p.) and bromocriptine (10 mg/kg, s.c.) while, in some animals, benztropine (2 mg/kg, i.p.) and baclofen (10 mg/kg, i.p.) increased muscle rigidity. 6 The development of full muscle relaxant responses to flunitrazepam (2 mg/kg, i.p.) and to sodium valproate (696 mg/kg, i.p.) was shown to depend upon mild warming of the animals with radiant heat, a procedure which can increase muscle spindle afferent input to the spinal cord. 7 The results suggest a hyperactivity of stretch reflexes in the spastic mouse, ameliorated selectively by those drugs that enhance the GABA-mediated presynaptic inhibition of such pathways.

6 -Hydroxydopamine decreases benzodiazepine but not GABA receptor binding in rat cerebellum

The noradrenergic innervation to the cerebellum was lesioned by intracerebroventricular 6-hydroxydopamine and the effects on GABA and benzodiazepine receptors followed by radiolabelling experiments. This lesion produced a 20% decrease in benzodiazepine receptor labelling in the cerebellum with no change in the labelling of GABA receptors. This provides further evidence for a lack of identity between the GABA and benzodiazepine receptors.

The GABA postsynaptic membrane receptor-ionophore complex. Site of action of convulsant and anticonvulsant drugs

The function of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), has been implicated in the mode of action of many drugs which excite or depress the central nervous system. Many convulsant agents appear to block GABA action whereas anticonvulsants enhance GABA action. Some of these drug effects involve altered GABA-mediated synaptic transmission at the level of GABA biosynthesis, release from nerve endings, uptake into cells, and metabolic degradation. A greater number of agents of diverse classes appear to affect GABA action at the postsynaptic membrane, as determined from both electrophysiological and biochemical studies. The recently developed in vitro radioactive receptor binding assays have led to a wealth of new information about GABA action and its alteration by drugs. GABA inhibitory transmission involves the regulation, by GABA binding to its receptor site, of chloride ion channels. In this GABA receptor-ionophore system, other drug receptor sites, one for benzodiazepines and one for barbiturates/picrotoxinin (and related agents) appear to form a multicomponent complex. In this complex, the drugs binding to any of the three receptor categories are visualized to have an effect on GABA-associated chloride channel regulation. Available evidence suggests that the complex mediates many of the actions of numerous excitatory and depressant drugs showing a variety of pharmacological effects.

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.

Diazepam binding of dissociated hippocampal cultures from fetal mice

Primary cultures of dissociated hippocampi from fetal mice examined for the presence of binding sites for [3H]diazepam. The binding assays were done with living cells still attached to the culture dish. The cells contain high affinity binding sites for [3H]diazepam, Kd = 5 nM, which are completely inhibited with 20 nM R05-4864 but only 26% with 20 nM lorazepam. The binding was inhibited by purinergic compounds and by quinidine. The living cell did not exhibit increased binding of [3H]diazepam in the presence of GABA and in fact a slight decrease in binding was found. This was also found when live, intact C6 glial cells were investigated. These observations suggest that the use of living cells to study the benzodiazepine receptor is valuable and maybe necessary to fully characterize this receptor.

The influence of diazepam on the activity of secondary vestibular neurons in the rabbit

We have studied the influence of intravenously administered diazepam on the activity of single secondary vestibular neurons in unanesthetized, paralyzed rabbits evoked by sinusoidal angular accelerations about the vertical and longitudinal axes. Intravenous injections of diazepam (20-100 micrograms/kg) caused a decreased sensitivity of all secondary vestibular neurons which were tested. The reduction in sensitivity was sometimes preceded by a transient increase in excitability which lasted 10-40 sec. The duration of the decreased sensitivity to vestibular stimulation following intravenous injections of diazepam was dose-dependent, lasting 15-60 min. These data suggest that the diazepam-induced reduction of vestibuloocular reflexes is caused, at least in part, by the depressant action of diazepam upon secondary vestibular neurons.

Receptors for the age of anxiety: pharmacology of the benzodiazepines

Investigation of the actions of the benzodiazepines has provided insights into the neurochemical mechanisms underlying anxiety, seizures, muscle relaxation, and sedation. Behavioral, electrophysical, pharmacological, and biochemical evidence indicates that the benzodiazepines exert their therapeutic effects by interacting with a high-affinity binding site (receptor) in the brain. The benzodiazepine receptor interacts with a receptor for gamma-aminobutyric acid, a major inhibitory neurotransmitter, and enhances its inhibitory effects. The benzodiazepine receptor may also interact with endogenous substances and several naturally occurring compounds, including the purines and nicotinamide, are candidates for this role. Both the purines and nicotinamide possess some benzodiazepine-like properties in vivo, although further work will be required to confirm their possible roles as endogenous benzodiazepines.