Central actions of benzodiazepines

Autoradiographic analysis of GABAA receptors in mu-opioid receptor knockout mice

Anatomical evidence indicates that gamma-aminobutyric acid (GABA)-ergic and opioidergic systems are closely linked and act on the same neurons. However, the regulatory mechanisms between GABAergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are changes in GABA(A) receptors in mice lacking mu-opioid receptor gene. The GABA(A) receptor binding was carried out by autoradiography using [(3)H]-muscimol (GABA(A)), [(3)H]-flunitrazepam (FNZ, native type 1 benzodiazepine) and [(35)S]-t-butylbicyclophosphorothionate (TBPS, binding to GABA(A)-gated chloride channels) in brain slices of wild type and mu-opioid receptor knockout mice. The binding of [(3)H]-FNZ in mu-opioid receptor knockout mice was significantly higher than that of the wild type controls in most of the cortex and hippocampal CA1 and CA2 formations. mu-Opioid receptor knockout mice show significantly lower binding of [(35)S]-TBPS than that of the wild type mice in few of the cortical areas including ectorhinal cortex layers I, III, and V, but not in the hippocampus. There was no significant difference in binding of [(3)H]-muscimol between mu-opioid receptor knockout and wild type mice in the cortex and hippocampus. These data indicate that there are specific regional changes in GABA(A) receptor binding sites in mu-opioid receptor knockout mice. These data also suggest that there are compensatory up-regulation of benzodiazepine binding site of GABA(A) receptors in the cortex and hippocampus and down-regulation of GABA-gated chloride channel binding site of GABA(A) receptors in the cortex of the mu-opioid receptor knockout mice.

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: valproic acid, clonazepam, and alpha-methyl-alpha-phenylsuccinimide

Spontaneous thalamocortical epileptiform activity was elicited in rodent thalamocortical slices by a medium containing no added Mg2+. Multiple varieties of activity were generated in these slices, including simple thalamocortical burst complex (sTBC) activity that resembled the spike-wave discharges of generalized absence epilepsy, and complex thalamocortical burst complex (cTBC) activity that resembled generalized tonic-clonic seizure discharges. In a further pharmacological characterization of this activity, the effects of the broad-spectrum anticonvulsants valproic acid, alpha-methyl-alpha-phenylsuccinimide (the active metabolite of methsuximide) and clonazepam were studied. All three drugs were found to be effective in controlling both sTBC and cTBC activity when applied in clinically relevant concentration ranges. The effectiveness of valproic acid against spontaneous rhythms in vitro was not due to augmentation of GABAergic inhibition. No effect of valproic acid on GABA-activated chloride currents was evident in patch-clamp recordings of acutely isolated thalamic or cortical neurons. The equivalent general clinical and experimental spectrum of action of broadly effective anticonvulsants provided an additional correlation between the clinical efficacy of anticonvulsant drugs and their effects against epileptiform discharges in rodent thalamocortical slices. This further validates spontaneous generalized low-Mg2+ thalamocortical activity as a potentially valuable in vitro model of the primary generalized epilepsies, in which the cellular mechanisms underlying generation and control of these seizure discharges can be studied.

Identifiable Achatina giant neurones: their localizations in ganglia, axonal pathways and pharmacological features

1. An African giant snail (Achatina fulica Férussac), originally from East Africa, is now found abundantly in tropical and subtropical regions of Asia, including Okinawa in Japan. This is one of the largest land snail species in the world. The Achatina central nervous system is composed of the buccal, cerebral and suboesophageal ganglia. The 37 giant neurones were identified in these ganglia by the series of studies conducted over about 20 years. The identifications were made by the localization of these neurones in the ganglia, their axonal pathways and their pharmacological features. 2. In the left buccal ganglion, the four giant neurones, d-LBAN, d-LBMB, d-LBCN and d-LBPN, were identified. In the left and right cerebral ganglia, d-LCDN, d-RCDN, v-LCDN and v-RCDN were identified. The suboesophageal ganglia are further composed of the left and right parietal, the visceral, the left and right pleural, and the left and right pedal ganglia. In the right parietal ganglion, PON, TAN, TAN-2, TAN-3, RAPN, d-RPLN, BAPN, LPPN, LBPN, LAPN and v-RPLN were identified. In the visceral ganglion, VIN, FAN, INN, d-VLN, v-VLN, v-VAN, LVMN, RVMN and v-VNAN were identified. In the left parietal ganglion, v-LPSN was identified. In the left and right pedal ganglia, LPeNLN, RPeNLN, d-LPeLN, d-LPeCN, d-RPeAN, d-LPeDN, d-LPeMN and d-LPeEN were identified. 3. Of the small molecule compounds tested, dopamine, 5-hydroxytryptamine, GABA, L-glutamic acid, threo- or erythro-beta-hydroxy-L-glutamic acid were effective on the Achatina giant neurones. We suppose that these compounds act as the neurotransmitters for these neurones. 4. Of the neuroactive peptides, achatin-I(Gly-D-Phe-Ala-Asp). APGW-amide(Ala-Pro-Gly-Trp-NH2) and Achatina cardioexcitatory peptide (ACEP-1)(Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2) were proposed as neurotransmitters, because these were effective on the Achatina giant neurones and their presence was demonstrated in the Achatina ganglia. Further, myomodulin (Pro-Met-Ser-Met-Leu-Arg-Leu-NH2), buccalin (Gly-Met-Asp-Ser-Leu-Ala-Phe-Ser-Gly-Gly-Leu-NH2), FMRFamide (Phe-Met-Arg-Phe-NH2). [Ser2]-Mytilus inhibitory peptide ([Ser2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2), catch-relaxing peptide (CARP) (Ala-Met-Pro-Met-Leu-Arg-Leu-NH2), oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2) and small cardioactive peptideB (SCPB) (Met-Asn-Tyr-Leu-Ala-Phe-Pro-Arg-Met-NH2) could also be neurotransmitters because these peptides were also effective on the Achatina giant neurones, though their presence in the ganglia of this animal has not yet been demonstrated. 5. Calcium current (ICa) was recorded from Achatina giant neurones in the Na(+)-free solution containing K(+)-channel blockers under voltage clamp. The Ca2+ antagonistic effects of brovincamine, verapamil, eperisone, diltiazem, monatepil, etc., were compared using the ICa of the Achatina neurones. 6. Almost all of the mammalian small molecule neurotransmitters were effective on the Achatina giant neurones, suggesting that these compounds are acting on the neurones of a wide variety of animal species. However, the pharmacological features of the Achatina neurone receptors to these compounds were not fully comparable to those of the mammalian receptors. For example, we proposed that beta-hydroxy-L-glutamic acid (either threo- or erythro-) could be an inhibitory neurotransmitter for an Achatina neurone. 7. In contrast, the Achatina giant neurones appear to have no receptor for the mammalian neuroactive peptides, except for oxytocin and Arg-vasotocin. On the other hand, many neuroactive peptides were isolated from invertebrate nervous tissues, including achatin-I, a neuroexcitatory tetrapeptide having a D-phenylalanine residue.

Pharmacological characteristics of two different types of inhibitory GABA receptors on Achatina fulica neurones

GABA (gamma-aminobutyric acid) receptors of Achatina fulica neurones have been classified into two types associated with neuronal inhibition and one type with excitation. The pharmacological features of muscimol I and baclofen types associated with inhibition were investigated in this study. Activation of muscimol I type receptors on TAN (tonically autoactive neurone) by GABA, muscimol and trans-4-aminocrotonic acid (TACA) produced a transient outward current (Iout) with an increase in membrane conductance (g). Their relative potencies at GABA ED50 (approximately 10(-4) M) were: GABA: muscimol: TACA = 1:0.6:0.3. The relation between Iout and g increase (delta g) induced by various concentrations of these compounds was linear. The Hill coefficients for GABA were close to 1.0. The GABA effects were potentiated by pentobarbitone, antagonized competitively by pitrazepin and non-competitively by picrotoxin and diazepam, and unaffected by bicuculline. The reversal potentials of the effects of GABA, muscimol and TACA on TAN changed under various [Cl-]0 according to the Nernst equation for Ec1, but not under various [K+]0 and [Na+]0. Activation of baclofen type GABA receptors on RPeNLN (right pedal nerve large neurone) by GABA and (+/-)-baclofen produced a slow Iout with an increase in g. The two compounds were almost equipotent (ED50: approximately 3 x 10(-4) M). The relation between Iout and delta g produced by various concentrations was linear. The Hill coefficients for GABA were also close to 1.0. The reversal potentials of GABA and (+/-)-baclofen on RPeNLN changed under various [K+]0 according to the Nernst equation for EK, but not under various [Cl-]0 and [Na+]0. The two compounds hardly affected the voltage-gated and slowly inactivating calcium current. The Iout produced by GABA and (+/-)-baclofen was reduced by tetraethylammonium chloride, but was unaffected by 4-aminopyridine, bicuculline, pitrazepin and picrotoxin. In conclusion, the pharmacological features of muscimol I type GABA receptors are partly comparable to those of mammalian GABAA receptors, except for the influences of bicuculline and diazepam: the features of the baclofen type GABA receptor, which did not occur with muscimol I type receptors in the same neurone, were similar to those of GABAB.

GABAergic mechanisms in the pathogenesis and treatment of epilepsy

1. Evidence relating to the role of GABA in the pathogenesis of epilepsy is reviewed. 2. Impaired GABAergic function appears to contribute to seizure susceptibility in a variety of genetically-determined syndromes in animals, e.g. genetically epilepsy prone rats showing sound-induced seizures, gerbils with genetically determined epilepsy, and baboons, Papio papio, with photosensitive epilepsy. 3. In epilepsy secondary to a cerebral insult there is some morphological and biochemical evidence for impaired GABAergic function in experimental situations, but little definitive evidence in man. 4. Pharmacological approaches to enhancing GABAergic inhibition include the use of GABA agonists (or prodrugs), GABA-transaminase inhibition, GABA uptake inhibition, and action at the GABA/benzodiazepine allosteric site. 5. Experimental data suggest that the best prospect for potent anticonvulsant action with fewest side effects (myoclonus, sedation, ataxia) is at present offered by GABA-transaminase inhibitors or novel agents acting on the benzodiazepine receptor site.

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)

The role of adenosine in the central actions of the benzodiazepines

1. Evidence is presented which indicates that the central actions of the benzodiazepines cannot be fully accounted for by assuming an action only at the GABAA-Cl- channel supramolecular complex. 2. The hypothesis is presented, together with supporting evidence, that inhibition of adenosine uptake can account for many of the actions of the benzodiazepines. 3. New findings showing that Ro 15-1788 and Ro 5-4864 have both potentiative and antagonistic interactions with adenosine are discussed. 4. The proconvulsant beta-carbolines are shown to be adenosine antagonists. 5. The concept that benzodiazepine action may involve several mechanisms is presented.

Primary afferent terminal excitability in the normal and spastic mutant mouse spinal cord

A microcomputer-based system has been used to apply the technique of excitability testing to the study of the actions of a range of pharmacological agents on the excitability of single primary afferent terminals in the mouse spinal cord in vitro. GABAA analogues all evoked increases in excitability that were bicuculline sensitive. GABA itself also evoked biphasic changes in excitability, or occasionally only suppressed terminal excitability. This latter effect was often enhanced in the presence of bicuculline, and resembled the action of the GABAB agonist, baclofen. The GABAA action could be enhanced by concurrent application of either benzodiazepine, midazolam or flurazepam. Bicuculline alone frequently decreased excitability. This action could be abolished by blocking synaptic activity with a low Ca2+ high Mg2+ superfusate, and was therefore considered to be due to reduction of the tonic action of GABA released at synaptic connections. Comparison of the action of these agents on terminals in the spastic mutant mouse showed an increased sensitivity of the GABA response to the benzodiazepines in mutant animals.

Diazepam increases gamma-aminobutyric acid in human cerebrospinal fluid

In 11 neurological patients, levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) were determined in cerebrospinal fluid (CSF) before and 1, 3, 5, and 8 min after intravenous injection of diazepam (2 or 5 mg). GABA levels increased progressively after intravenous injection of 5 but not 2 mg of the benzodiazepine, the differences from preinjection values being significant at 3, 5, and 8 min. Furthermore, when relative CSF GABA alterations determined after injection of diazepam were compared to those determined in sequential CSF aliquots of 10 patients without diazepam injection, mean GABA increases after diazepam were significantly different from controls in all CSF fractions. The data suggest that, in addition to its well-known effects on postsynaptic GABA function, diazepam may exert effects on endogenous GABA concentrations and/or on GABA release in the human CNS as reflected by elevation of GABA levels in human CSF.

Diazepam action on gamma-aminobutyric acid-activated chloride currents in internally perfused frog sensory neurons

The Cl- current (ICl) in gamma-aminobutyric acid (GABA)-sensitive frog sensory neuron was separated from other Na+, Ca2+, and K+ currents using a suction pipette technique which allows internal perfusion under a single-electrode voltage clamp. Diazepam (DZP) itself evoked no response but facilitated the dose- and time-dependently GABA-induced ICl without changing the GABA equilibrium potential (EGABA) at concentrations ranging widely, from 3 X 10(-9) to 10(-4) M. In the presence of DZP, the GABA dose-response curve shifted to the left without changing the maximum current, indicating that DZP modifies the interaction between GABA and its receptor rather than affecting directly the channel activation step. The enhancement of the GABA-induced ICl by DZP depended neither on the membrane voltage nor on the inward or outward direction of the ICl. DZP also potentiated the ICl elicited by GABA agonists such as beta-alanine, taurine, homotaurine, 5-aminovaleric acid, l-GABOB, d-GABOB, glycine, and muscimol. The GABA response enhanced by pentobarbital (PB) was further enhanced by adding DZP, indicating that DZP and PB do not act in the same way. Ro5-3663, a diazepam analogue, enhanced the GABA-induced ICl only in a narrow range of the concentrations but inhibited the current at concentrations higher than 2 X 10(-6) M.

Comparison of the effects of the benzodiazepine midazolam and three serotonin antagonists on a consummatory conflict paradigm

A consummatory conflict procedure that involves an abrupt reduction in magnitude of an expected reward (negative contrast) has been shown to be particularly sensitive to the effects of anxiolytic agents. As previously reported with chlordiazepoxide, another benzodiazepine (BDZ), midazolam released suppressed consummatory performance in a dose-dependent manner. This effect was not due to a general appetitie stimulatory effect of the drug. The effects of three 5-HT antagonists on negative contrast were examined to evaluate the role serotonin may play in the anxiolytic action of BDZ. Methysergide was found to be ineffective, cinanserin tended to reduce contrast at two intermediate doses, and cyproheptadine eliminated the contrast effect in a similar fashion as midazolam. The effectiveness of cyproheptadine may not be attributed to its anticholinergic or antihistaminergic actions since scopolamine and pyrilamine did not produce similar effects. The results are discussed in terms of the role serotonin may play in the anti-conflict action of BDZ, as well as possible interactional effects of GABA.

Blockade of a putative GABA-mediated neurotransmission in the cerebellum by benzodiazepine receptor inverse agonists

An exponential relationship was observed between the firing rate of cerebellar Purkinje cells in urethane-anaesthetized rats and the duration of inhibition evoked in these cells by electrical stimulation of the nearby cortical surface. Benzodiazepines, administered i.v., decreased cell firing and increased the duration of the inhibitory response but did not alter the relationship between the two parameters. These effects of one benzodiazepine, RU 32007, were reversed by the benzodiazepine antagonist Ro15-1788 which had little effect alone. The benzodiazepine inverse agonists methyl- or ethyl-beta-carboline-3-carboxylate increased cell firing with the expected reductions in duration of inhibitory response in some cases. However, in 50% of recordings the inhibitory response disappeared, independent of the firing rate. All the effects of the beta-carboline esters were reversed by Ro15-1788 or the benzodiazepine, RU 32007. This action of the benzodiazepine receptor inverse agonists represents an in vivo blockade of an endogenous synaptic inhibition which is thought to be mediated by release of GABA.

Residual benzodiazepine (BZ) binding in the cortex of pcd mutant cerebella and qualitative BZ binding in the deep cerebellar nuclei of control and mutant mice: an autoradiographic study

In mutant mice 'Purkinje cell degeneration' (pcd), there is an almost complete degeneration of Purkinje cells followed subsequently by a partial degeneration of granule cells. Recent neurochemical studies have revealed a 50% decrease in benzodiazepine (BZ) receptors in 45-day-old pcd mutants after degeneration of the Purkinje cells. At 300 days there is an 80% decrease in BZ receptors concomitant with granule cell losses. To determine the histological localization of these receptor changes this autoradiographic analysis was conducted. An in vitro autoradiographic technique was used to explore [3H]flunitrazepam binding. BZ receptors were found to be more concentrated in the molecular than the granular layer of mutant and control cerebellar cortices. There was, nonetheless, no statistically significant difference in grain counts between control and mutant mice in any layer. Substantial atrophy of cerebellar structures, particularly of the molecular layer, occurred in the mutant mice. It began even before 45 days of age but was extreme at 300 days. When the appropriate mathematical correction factor was introduced for the layer atrophy there was a 60% decrease in grain count in 45-day-old mutants in the molecular layer and a 84% decrease in 300-day-old mutants compared to controls. The initial decrease in total BZ receptors in the 45-day-old mutant animals is associated with a selective loss of Purkinje cells. The amount of receptor binding which persists in the 300-day-old mutants in the molecular layer would appear to reflect binding in the remaining parallel fibers from granule cells which remain.(ABSTRACT TRUNCATED AT 250 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 mesencephalic locomotor region in cat: effects of local applications of diazepam and gamma-aminobutyric acid

In decorticate unanaesthetized cats, displaying sequences of stereotyped locomotor movements in response to electrical stimulation of their footpad, local injections were made into the mesencephalic locomotor region (MLR). Diazepam (at 10 micrograms) and GABA (at 500 micrograms) induced complete suppression for 5-10 min, followed by partial amplitude recovery. It is suggested that: (i) MLR contains both diazepam and GABA receptors, and (ii) diazepam (and GABA) blockade is only short lasting because of a substitution (vicariancy) process through other structures also involved in locomotion.

Antispasticity drugs: mechanisms of action

Several different drugs are now used, or are potentially useful, to treat patients with spasticity. Although these compounds vary in their actions on spinal neurons and reflex arcs, it is possible to formulate reasonable hypotheses regarding their modes of action. The benzodiazepines bind to specific benzodiazepine receptors linked to classic gamma-aminobutyric acid (GABA) receptors located on the terminals of primary afferent fibers. This binding results in an increased affinity of the GABA receptor for the amino acid, an augmented flux of chloride ions across the terminal membrane, and an increase in the amount of presynaptic inhibition. Baclofen activates GABAB receptors putatively located on the same terminals. Activation of these receptors retards the influx of calcium ions into the terminals, thereby reducing the evoked release of excitatory amino acids and possibly other transmitters. Progabide and its metabolites act on both classic and GABAB receptors. Glycine works on specific inhibitory receptors located on spinal interneurons and motoneurons. The phenothiazines act on the brainstem to alter the function of fusimotor fibers. Phenytoin and carbamazepine reduce the afferent output of muscle spindles. Dantrolene diminishes the activation of the contractile process in muscle fibers by reducing the release of calcium ions from the sarcoplasmic reticulum. This review summarizes the data supporting these concepts.

Benzodiazepines both enhance gamma-aminobutyrate responses and decrease calcium action potentials in guinea-pig myenteric neurones

The effect of two benzodiazepines, midazolam and diazepam, was studied in guinea-pig myenteric neurones, using intracellular recording techniques. Both these benzodiazepines (100-300 pM) potentiated the rapidly desensitizing, bicuculline-sensitive depolarization, induced by alpha-aminobutyrate ionophoresis. Concentrations of midazolam and diazepam higher than 100 nM depressed the gamma-aminobutyrate-induced depolarization. The potentiating effect of the benzodiazepines was reversibly abolished by Ro 15-1788 (1-100 nM) and by pentylenetetrazol (100 microM). A second effect of midazolam and diazepam (100-300 pM) was a reversible depression of the amplitude and duration of the directly evoked action potential in 29% of neurones, without affecting membrane potential or conductance. The effect was very marked when electrodes were filled with CsCl, and was also seen in the presence of tetrodotoxin. In some but not all of these neurones, the amplitude and duration of the action potentials was reduced also by gamma-aminobutyrate (1-10 microM). Ro 15-1788 and pentylenetetrazol reversibly abolished the effect of benzodiazepines on the action potential, but not that of gamma-aminobutyrate. Thus, benzodiazepines have two effects on myenteric neurones. The first is an enhancement of the gamma-aminobutyrate response (activation of Cl conductance); the second is a depression of the calcium action potential, which appears to be independent of gamma-aminobutyrate.

Gabaergic mechanisms and anxiety: an overview and a new neurophysiological model

GABA is one of the principal inhibitory neurotransmitters in the mammalian brain and an ever increasing wealth of information suggests that GABAergic mechanisms have a special role in the neurophysiology of anxiety. All of the most commonly used antianxiety drugs (the benzodiazepines, the barbiturates, ethanol) selectively enhance only GABA-mediated synaptic transmission. Furthermore, the relative affinities of pharmacologically active benzodiazepines for the benzodiazepine receptor correlate well with their ability to antagonize GABA-modulin (the endogenous inhibitor of GABA receptors) in vitro, as well as with their ability to potentiate GABA-mediated electrically evoked cortical inhibition in vivo. Finally, it is of interest for the neurophysiology of anxiety that repetitive stimulation of the recurrent inhibitory GABAergic pathway in the rat hippocampus leads to a remarkable reduction of the effectiveness of GABA; this elimination of GABAergic "inhibition" is counteracted by antianxiety drugs. On the basis of the above a neurophysiological model of anxiety is proposed.

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.

GABAergic agents as anticonvulsants in baboons with photosensitive epilepsy

GABAergic agents have been evaluated for acute anticonvulsant activity in baboons, Papio papio with photosensitive epilepsy. The potent GABAA agonists muscimol and THIP are proconvulsant. (-)Baclofen, 2 mg/kg suppresses myoclonic responses; higher doses facilitate EEG paroxysmal activity. (S) gamma-vinyl GABA, 100-200 mg/kg, suppresses myoclonic responses for more than 24 h. Some derivatives of esters of beta-carboline-3-carboxylate that bind to the benzodiazepine receptor, e.g. ZK 91296 and ZK 93423, suppress myoclonus with a potency at least as great as diazepam.

Effect of diazepam on vestibular compensation in squirrel monkeys

The effect of the chronic administration of diazepam on vestibular compensation (post-unilateral labyrinthectomy) was studied in squirrel monkeys. An intramuscular injection of diazepam (2 mg/kg) was given daily for a period of 4 weeks post-operatively. After daily injection, reduction of the slow-phase eye velocity ( SPEV ) of spontaneous nystagmus and enhancement of locomotor body dysequilibrium were found in the early post-operative stage. However, these pharmacological effects decreased gradually and almost no remarkable effect was found about 3 weeks after surgery. When compared to control animals, no marked differences was found in SPEV reduction of spontaneous nystagmus in injected animals during the vestibular compensation period. Platform runway tests showed that the average number of calendar days needed to regain the pre-operative level of locomotor balance function in the group injected with diazepam was even less than that of the control group. Thus, post-operative daily diazepam injections did not retard the vestibular compensation. The possible neural mechanism responsible for this phenomenon is discussed.

Structure-activity studies on the potentiation of benzodiazepine receptor binding by ethylenediamine analogues and derivatives

The effect of ethylenediamine analogues on in vitro binding of [3H]-diazepam to crude cerebral cortical synaptosomal membranes in the rat was studied. Ethylenediamine significantly increased [3H]-diazepam binding to a maximum potentiation of 154% control (EC50 = 1.8 X 10(-4) M) and was the most active compound studied in terms of both potency and the maximum potentiation observed. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is dependent on carbon-chain length, appears to require two terminal amino groups, and is not observed in the rigid analogues studied. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is mediated largely by a change in receptor number and not receptor affinity. Results are discussed in terms of the possible nature of the ethylenediamine binding site.

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.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of 1-methyl cyclohexane carboxylic acid on electrical activity of Purkinje cells in the rat: evidence for a potentiation of intracerebellar inhibition

The effect of an anticonvulsant compound (Simiand, Ferrandes, Lacolle and Eymard, 1979), 1-methyl cyclohexane carboxylic acid (CCA), upon the electrical activity of Purkinje cells (PCs) was studied in the cerebellar cortex of the rat in vivo. Cyclohexane carboxylic acid (200-400 mg/kg i.v.) decreased the spontaneous simple spike (SS) activity of the Purkinje cells tested without modifying the complex spike (CS) frequency. Two effects of CCA upon intracortical inhibition were observed: (1) the decrease in firing rate that followed surface stimulation of the parallel fibres (LOC stimulation) was enhanced after injection of CCA; (2) the depression of the antidromic field potential of Purkinje cells by a conditioning stimulation was also enhanced after injection of CCA. This latter effect was suppressed in a reversible manner by injection of bicuculline. These results strongly suggest that the effect of CCA upon electrical activity of Purkinje cells is related to an enhancement of the inhibition exerted on Purkinje cells by GABAergic, cerebellar interneurones. The possible mechanisms of action of CCA are discussed.

Ethylenediamine and GABA potentiation of [3H]diazepam binding to benzodiazepine receptors in rat cerebral cortex

Specific binding of [3H]diazepam at a free concentration of 2 nM was found to be maximally potentiated by 117% in Tris-HCl buffer and 160% in Tris-citrate buffer by ethylenediamine (EDA), but only at relatively high concentrations of EDA (ED50 = 5 X 10(-5) M), although this potentiation was susceptible to a low dose (6 microM) of bicuculline. Dose-response curves show that EDA differs from GABA with respect to both potency and efficacy. In additivity experiments no evidence was found that EDA could act as a partial agonist at GABA receptors, and it was concluded that EDA and GABA apparently do not potentiate [3H]diazepam binding by acting on the same receptor. Scatchard analysis lends support to this hypothesis, indicating that the potentiation of [3H]diazepam binding by 3.16 X 10(-3) M EDA is due to an increase in receptor number (from 930 to 1170 fmol/mg protein) and not receptor affinity (remaining constant about 20 nM). Subsequent studies showed the potentiation to be reversible. It is concluded that EDA can act on the GABA-benzodiazepine receptor ionophore complex but that this is probably not a direct action on the GABA receptor. It is suggested that EDA can be used to differentiate GABA receptors linked to benzodiazepine receptors from those not so linked.

Dissimilar influences of some injectable anaesthetics on the responses of reticulo-spinal neurones to inhibitory transmitters in the lamprey

1 Intracellular recordings were made from identified bulbar reticulo-spinal neurones in the medulla of lamprey ammocoetes. Responses to iontophoretically applied inhibitory transmitters were measured as changes in membrane potential and input resistance. 2 Dose-dependent alterations in the responses to gamma-aminobutyric acid (GABA) and glycine during bath application of injectable anaesthetic drugs were measured; the compounds used were pentobarbitone, ketamine, metomidate and the steroid mixture alphaxalone/alphadolone (Saffan). 3 GABA responses were potentiated by pentobarbitone (1-3 X 10(-4) M) and prolonged by ketamine (3.7 X 10(-5) M); but depressed by high concentrations (10(-3) M) of all drugs, as well as by anaesthetic concentrations of alphaxalone (1-3 X 10(-5) M). 4 Glycine responses were depressed by alphaxalone (1-3 X 10(-5) M) and by supra-anaesthetic concentrations of ketamine (3.7 X 10(-4) M) and metomidate (1.8 X 10(-3) M). No drug potentiated the glycine responses. 5 In the absence of an effect common to the 4 anaesthetics, it is concluded that neither potentiation nor inhibition of all GABA or glycine responses is an essential feature of anaesthesia. However, effects comparable to those described here may contribute to the overall clinical picture during anaesthesia of higher vertebrates. The findings do not support the notion that all anaesthetic agents act on biological membranes by a single mechanism.

Effects of benzodiazepines on single unit activity in the substantia nigra pars reticulata

Intravenous administration of two benzodiazepines, flurazepam and diazepam, had an inhibitory effect on the firing rates of neurons of the substantia nigra pars reticulata, a brain region with an identified GABAergic innervation. Diazepam was more potent than flurazepam. Bicuculline and picrotoxin, two drugs which block GABAergic transmission, and caffeine and theophylline, two methylxanthines which inhibit benzodiazepine binding, all reversed the inhibition produced by diazepam. The action of theophylline was less consistent than that of caffeine. Similarly, Ro 15-1788, an imidazodiazepine which putatively functions as a specific benzodiazepine antagonist, reversed the diazepam-induced inhibition. These findings are consistent with previous reports which suggest that the benzodiazepines may act through a GABAergic mechanism. In a separate group of experiments, caffeine or Ro 15-1788 was administered alone. While caffeine excited all reticulata, generally had little excitatory effect. These results suggest: 1) that cells of the substantia nigra pars reticulata may not receive a substantial, tonic inhibition mediated by an endogenous benzodiazepine-like substance; and 2) that the methylxanthines may increase reticulata cell firing, at least in part, through mechanisms unrelated to the blockade of benzodiazepine receptors.

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.

GABA autoreceptors are not coupled to benzodiazepine receptors in rat cerebral cortex

Depolarization-induced release of [3H]gamma-aminobutyric acid ([3H]-GABA) from preloaded slices of rat cerebral cortex was inhibited by muscimol and THIP in a dose-dependent fashion. This inhibition of release was prevented by the GABA antagonists bicuculline and picrotoxin. These results confirm previous reports postulating the existence of GABA autoreceptors on GABAergic terminals. Since benzodiazepines are known to facilitate post-synaptic GABA actions, the effect of flunitrazepam on the inhibition of GABA release mediated through the autoreceptors has been examined. At a concentration of 1 microM or 10 microM, flunitrazepam had no effect on the IC50 values for muscimol or THIP in inhibiting stimulated GABA release. It thus seems that GABA autoreceptors are not functionally coupled to benzodiazepine receptors in rat cerebral cortex.

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.

The effects of chlordiazepoxide on synaptic transmission and amino acid neurotransmitter release in slices of rat olfactory cortex

The rat olfactory cortex slice has been used to investigate the effects of chlordiazepoxide on evoked field potentials and the release of endogenous amino acid neurotransmitters (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the lateral olfactory tract. When single, low frequency stimuli were employed, chlordiazepoxide (2 microM-1 mM) depressed the amplitude of the field potential correlate of the depolarizing actions of the lateral olfactory tract excitatory transmitter (aspartate?) although aspartate release was unaffected. The field potential correlate of GABA-mediated presynaptic inhibition (late N-wave) was also depressed in amplitude but low drug concentrations (between approximately 2 and 50 microM) increased its peak duration . Effects of chlordiazepoxide on evoked inhibition were analyzed by giving paired stimuli such that the second stimulus occurred during the field potentials evoked by the first stimulus. Chlordiazepoxide (1-20 microM) increased the depression in amplitudes of the presynaptic massed action potential and late N-wave evoked by the second of a pair of stimuli compared with those evoked by the first stimulus suggesting that presynaptic inhibition was potentiated. These effects of chlordiazepoxide were accompanied by a significant reduction in aspartate release from the lateral olfactory tract terminals. Moreover, the drug effects on presynaptic inhibition and aspartate release were antagonized by picrotoxin (5 microM). On the other hand, chlordiazepoxide (1-50 microM) had no significant effect on postsynaptic inhibition. The results are discussed in terms of both the sites (presynaptic or postsynaptic) and mechanisms of action of chlordiazepoxide.

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.

GABA agonists. Development and interactions with the GABA receptor complex

This review describes the development of GABA receptor agonists with no detectable affinity for other recognition sites in GABA-mediated synapses. The key compounds are THIP, isoguvacine, and piperidine-4-sulphonic acid (P4S), developed via extensive structural modifications of the potent but not strictly specific GABA agonist muscimol. The structural parameters, which have to be considered in the design of GABA agonists are discussed on the basis of the structures and biological activities of these GABA agonists and a number of related compounds. A model, which summarizes our present knowledge of the structure of the postsynaptic GABA receptors complex, is presented, and the interaction of GABA agonists with various sites in this complex is discussed. Of particular interest are the effects of GABA agonists on the binding of diazepam to the benzodiazepine binding site, assumed to be a structural unit of the GABA receptor complex. While rigid molecules like THIP are capable of activating the GABA receptors, a certain degree of conformational mobility of GABA agonists apparently is a prerequisite for stimulation of diazepam binding in vitro at 0 degree C. The findings suggest that GABA receptor functions involve conformational changes of certain elements, including the attempts to develop GABA agonists with desirable pharmacokinetic and toxicological characteristics. While muscimol is a toxic compound, THIP is well tolerated by animals, and in contrast to isoguvacine, THIP penetrates into the brain after systemic administration to animals, a difference which can be explained on the basis of their protolytic properties. The attempts to develop pro-drugs of isoguvacine capable of penetrating the blood-brain barrier with subsequent decomposition in the brain tissue to isoguvacine are described.

An endogenous ligand to the benzodiazepine receptor: preliminary evaluation of its bioactivity

Chromatographic separation of aqueous brain extracts yields a peptide containing fraction which competitively inhibits 3H-diazepam binding to its receptor. An intracerebral-ventricular injection of this isolated fraction results in altered responses in pharmacological and behavioral tests which are similar to those observed when diazepam is administered in the same fashion. The most pronounced effect was obtained in the conflict test. Changes observed in other tests, such as blocking pentylenetetrazole convulsions, altering motility or reducing hyperthermia, were also consistent with the actions of diazepam. At the dose used, neither diazepam nor the brain extract altered muscular co-ordination in two ataxia evaluations. Thus, the animals' performance in the other paradigms would not be adversely influenced by immobilization side-effects. The results reported here support the notion that an endogenous factor does exist in brain which can act like the benzodiazepine drugs when tested for bioactivity in animal studies.

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.

Involvement of central GABA receptors in the regulation of the urinary bladder function of anaesthetized rats

Cystometric recordings were performed in pentobarbitone anaesthetized rats and the effects of gammaaminobutyric acid (GABA) mechanisms on urinary bladder function were evaluated as their influence on a bladder hyperactivity induced by 1-dihydroxyphenylalanine (L-DOPA) after peripheral decarboxylase inhibition. The bladder response was inhibited by intracerebroventricular (i.c.v., 4th ventricle) injections of GABA (250 microgram), muscimol (0.2 microgram) and glycine (1,000 microgram) as well as by systemically administered muscimol (4 mg/kg) and diazepam (2 mg/kg). Intravenous (i.v.) bicuculline, but not i.v. strychnine, antagonized the inhibitory actions of intraperitoneal (i.p.) and i.c.v. muscimol and i.v. diazepam while the opposite was true for the inhibitory action of i.c.v. glycine. In rats not pretreated with L-DOPA, i.p. administration of bicuculline (4 mg/kg) after 15 min caused prominent detrusor contractions that were prevented by an infracollicular brain transection. It is suggested that GABA synapses in the pontinemesencephalic brain region may be involved in the modulation of urinary bladder function.