Benzodiazepines and central inhibitory mechanisms

The aqueous crude extract of Montanoa frutescens produces anxiolytic-like effects similarly to diazepam in Wistar rats: involvement of GABAA receptor

ETHNOPHARMACOLOGICAL RELEVANCE

Cihuapatli is the Nahuatl name assigned to some medicinal plants grouped in the genus Montanoa, where Montanoa frutescens (Family: Asteraceae, Tribe: Heliantheae) is included. The crude extract from these plants has been used for centuries in the Mexican traditional medicine as a remedy for reproductive impairments and mood disorders. Experimental studies have systematically corroborated the traditional use of cihuapatli on reproductive impairments and sexual motivation, however, the effect on mood and "nervous" disorders, remains to be explored.

MATERIALS AND METHODS

The anxiolytic-like effect of aqueous crude extract of M. frutescens (25, 50 and 75 mg/kg) was investigated in male Wistar rats evaluated in the elevated plus-maze and compared with several doses of diazepam (1, 2 and 4 mg/kg) as a reference anxiolytic drug. Picrotoxin (1 mg/kg), a noncompetitive antagonist of the GABA(A) receptor, was used in experimental procedures to evaluate if this receptor could be involved in the anxiolytic-like effects produced by M. frutescens. To discard hypoactivity, hyperactivity, or no changes associated with treatments, which could interfere with the behavioral activity in the elevated plus-maze, rats were subjected to the open field test.

RESULTS

M. frutescens at 50 mg/kg showed anxiolytic-like activity similarly to 2 mg/kg of diazepam, without disrupts in general motor activity. The anxiolytic-like effect of M. frutescens detected in the elevated plus-maze was blocked by picrotoxin, indicating that GABA(A) receptors are involved in the modulation of this effect.

CONCLUSIONS

The results corroborate the use of M. frutescens in folk Mexican ethnomedicine as a potential anxiolytic agent and suggest that this effect is mediated by the GABA(A) receptors. Additionally, some sedative effects with high doses of M. frutescens were detected in the present study.

GABA receptor-mediated inhibition of reflex deglutition in the cat

In anesthetized cats, swallowing elicited by electrical stimulation of the superior laryngeal nerves (SLNs) was inhibited by the GABA-mimetic muscimol and by diazepam, an action that was reversed by picrotoxin and bicuculline. This inhibition supports the involvement of GABA receptors, specifically those of the GABAA subtype which both antagonists have been shown to block in various areas of the central nervous system. The inhibition of reflex swallowing and its reversal were unaltered by a transection of the brainstem at a midcollicular level. Stimulation of the SLNs also caused a bradycardia that was inhibited by both muscimol and diazepam and was restored by both GABA antagonists. Data from these experiments provide suggestive evidence for a role of GABA-ergic transmission in the central control of the deglutitory reflex.

Anxiolytic effect of neuropeptide FMRFamide in rats

Central administration of FMRFamide in rats dose dependently increased the duration of time spent in the open arm of an elevated plus maze and enhanced the number of drink contacts in the thirsty rat conflict test. Similarly in the social interaction test, animals pretreated with FMRFamide spent sufficient time in active social interaction as compared to controls. Neuropeptide FMRFamide antagonized the anxiogenic effect of yohimbine and enhanced the antianxiety effect of diazepam in rats. The results indicate anxiolytic action of FMRFamide and the mechanism of such an action may involve serotonergic transmission.

Effects of GABA and anxiolytics on the single unit discharge of suprachiasmatic neurons in rat hypothalamic slices

The effects of gamma-aminobutyric acid (GABA), muscimol, baclofen and the anxiolytics; diazepam (DZP), flurazepam (FZP) and zopiclone on single-unit neural activities in the suprachiasmatic nucleus (SCN) were investigated using the rat hypothalamic slice preparation. Exposure of the slice to GABA 10(-4) M produced inhibitory responses in 65% of the 49 SCN neurons examined. The threshold concentration of GABA ranged from 10(-6) to 10(-4) M. Neurons responsive to GABA were not found to be restricted to a subdivision of the SCN, but were diffusely distributed throughout the nucleus. DZP, FZP and zopiclone produced responses similar to those of GABA. The inhibitory effects of GABA (10(-5) M) were potentiated by coadministration of DZP (10(-5) M). Muscimol and baclofen (10(-7) M to 10(-4) M) also inhibited SCN neuronal activity in a dose-dependent manner. Bicuculline (10(-5) M-10(-4) M) scarcely affected the baclofen-induced inhibition (1/6) but strongly antagonized the effects of muscimol (6/6), GABA (6/8) and DZP (4/5). These results suggest that the receptors mediating the inhibitory effects of GABA and anxiolytics within the SCN may be GABAA and/or GABAB or GABA-BDZ receptor complex, respectively.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Diazepam potentiates postsynaptic inhibition in bulbar respiratory neurons of cats

The purpose of this study was to assess the effect of benzodiazepine on inhibitory postsynaptic potentials (IPSPs) of medullary respiratory neurons in decerebrate, paralyzed cats. Diazepam (0.05 and 0.1 mg/kg i.v.) reversibly increased the IPSP waves occurring during the inactive phase of the respiratory cycle in all inspiratory and postinspiratory neurons examined. Input resistances of these neurons were reduced at that phase. The reversal potential for the IPSP wave was unaltered. Intracellular injection of chloride ions reversed the IPSP to depolarization, and diazepam produced a purely depolarizing effect. The drug effects observed during the active phase of each neuron include a decrease in the firing rate and a shortening of the burst activity. The firing threshold and shape of these spikes, however, remained unaltered. These results suggest that diazepam depresses the bulbar respiratory neuronal activities specifically by potentiating the periodic postsynaptic inhibition.

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.

A paradigm for determination of direct drug-induced modulatory alterations in Purkinje cell activity during treadmill locomotion

This paper describes a paradigm employing chronic single unit recording techniques and videotape analysis of treadmill locomotion in order to determine drug-induced modulation of sensorimotor neuronal activity. Animals implanted with a chronic headstage microdrive unit and an indwelling jugular cannula are trained to walk on a treadmill (10 s on, 10 s off). Characteristically, cerebellar Purkinje cells recorded 1-1.5 mm from midline, exhibit increased rates of discharge in phase with movement of specific limbs during a particular stage of the step-swing cycle, as assessed by video analysis of locomotor patterns. Drug-induced alterations in this movement-correlated discharge relative to changes in the spontaneous firing rate can then be determined to assess drug-induced neuromodulatory effects beyond general non-specific excitatory or inhibitory actions.

Sex steroid effects on extrahypothalamic CNS. II. Progesterone, alone and in combination with estrogen, modulates cerebellar responses to amino acid neurotransmitters

In a preliminary report we have shown that both intravenous and local application of progesterone (P) are capable of increasing cerebellar Purkinje cell responsiveness to microiontophoretically applied gamma-aminobutyric acid (GABA) and decreasing responsiveness to glutamate (GLUT) in the urethane-anesthetized, ovariectomized adult rat. In the present study we have examined the time course of effects of several doses of P and different combinations of both E2 and P on responses of individual Purkinje cells to GABA and GLUT. Extracellular activity of single Purkinje neurons was recorded using multibarrel glass micropipets. Spontaneous firing rate and responses of neurons to microiontophoretic pulses (10 s pulses every 40 s) of GABA (10-50 nA) and GLUT (3-40 nA) were examined before and after jugular i.v. administration of P or E2/P combinations to ovariectomized rats. In some cases animals received s.c. injections of E2 (2 micrograms) at 24 and 48 h before the day of recording. This injection schedule results in maximal reproductive effects of P. Within 5-15 min after P administration (5,50 or 500 micrograms) to ovariectomized rats, Purkinje cell responses to GLUT were decreased by 87%, and inhibitory responses to GABA were increased by 50%, with no associated change in spontaneous firing rate. In addition, the magnitude of the change in amino acid response was directly proportional to the dose of P. In most cases, complete recovery was observed 20-45 min after P administration. E2 pretreatment did not alter these P-induced effects. Combinations of E2 (300 ng/kg) and P (50 or 500 micrograms) injected simultaneously resulted in effects on GLUT responsiveness which were similar to those seen with P alone, while effects similar to E2 alone were observed with administration of E2 plus P at 5 micrograms. The administration of a protein synthesis inhibitor, anisomycin (30 mg/kg, i.v.), 20 min before the recording session did not prevent any of the above steroid effects. These results indicate that sex steroids can act to alter neuronal responsiveness to putative neurotransmitters in a CNS region not known to contain steroid receptors and that the particular combination of steroids will determine the neuronal response. These findings further suggest that the observed steroid-induced alterations in Purkinje cell responsiveness do not appear to require genomic mechanisms.

Locally applied progesterone metabolites alter neuronal responsiveness in the cerebellum

Ongoing studies in this laboratory have demonstrated that both systemically and locally administered sex steroids 17 beta estradiol (E2) and progesterone (P) alter cerebellar Purkinje cell responses to microiontophoretically applied amino acid neurotransmitters GABA and glutamate (GLUT) in the urethane-anesthetized, ovariectomized adult rat. In the present study, we have examined the effects of several locally pressure ejected P metabolites on Purkinje cell responsiveness to GABA and GLUT: 5 alpha-pregnane-20-one (5 alpha DHP), 5 alpha-pregnane-3 alpha-ol-20-one (3 alpha OH-DHP) and 5 alpha-pregnane-3 beta-ol-20-one (3 beta OH-DHP). GABA-induced inhibition was markedly enhanced immediately after onset of local application of 3 alpha OH-DHP or 5 alpha DHP, unaccompanied by alterations in background discharge. Both metabolites also attenuated excitatory responses to GLUT by 0-3 min after initiation of steroid application. In both cases, recovery to control levels of response was observed 6-9 min after termination of pressure application. These results are similar to those seen after local or systemic injection of P. In contrast, 3 beta OH-DHP did not produce any alteration in Purkinje cell responses to either amino acid. As 5 alpha DHP and 3 alpha OH-DHP can be localized in cerebellar tissue after P administration, the results presented here suggest that the neuronal effects of systemic P may be mediated by local membrane actions of P or its metabolites.

Progesterone alters GABA and glutamate responsiveness: a possible mechanism for its anxiolytic action

In this study, the neuromodulatory effects of progesterone were tested in an intact neuronal circuit of a model extrahypothalamic CNS area. Spontaneous discharge and responses of single cerebellar Purkinje neurons to microiontophoretically applied gamma-aminobutyric acid (GABA) and glutamate were monitored before, during and after either systemic injection, at physiologic doses, or local application of the steroid. By both means of administration, progesterone significantly enhanced inhibitory responses of Purkinje cells to GABA and suppressed glutamate excitation within 3-10 min post-steroid. These results are consistent with the anxiolytic actions of the steroid.

Involvement of neuronal chloride channels in ethanol intoxication, tolerance, and dependence

Studies of the role of neuronal chloride channels in ethanol action have focused on chloride channels coupled to gamma-aminobutyric acid (GABA) receptors. Ethanol intoxication is hypothesized to result from enhancement of GABA action, leading to increased chloride conductance and decreased neuronal excitability. Chronic ethanol treatment is suggested to produce a subsensitivity to GABA, leading to decreased action of ethanol and hyperexcitability on withdrawal of ethanol. Behavioral, electrophysiological, and biochemical studies of acute and chronic ethanol treatments on the GABA-regulated chloride channels of brain are reviewed.

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.

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.

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.

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.

Drug-induced rhythmic burst activity of cerebellar neurons

The discharge of cerebellar neurons was investigated in the rabbit and the rat under the influence of pentobarbital, diazepam or medazepam. In the rabbit, these drugs are known to induce a rhythm ranging between 4 and 25 Hz in the red nucleus (RN) and the cerebellum (Cb). Purkinje cells (P cells) in the intermediate zone of the cerebellar cortex as well as neurons of the interposed nucleus (IPN) were found to discharge with burst patterns fully synchronized with the drug-induced RN rhythm. In contrast, P cells in the medial cerebellar zone responded to these drugs only with changes in their discharge rate. Since P cells of the intermediate longitudinal zone project to the RN mainly via the IPN, the present findings complement our previous results, indicating that the rhythmic electrical activity in the RN is initiated by the cerebellum. The three drugs had similar effects on the activity of cerebellar units in the rabbit and the rat. The investigation also shows that, in spite of the uniform morphological structure of the cerebellar cortex, P cells do not respond uniformly to a given drug: the diversity of findings published on the P cell response to barbiturates or benzodiazepine derivatives may be explained by differences in the recording sites.

Electrophysiological studies on benzodiazepine antagonists

The actions of the benzodiazepine (BDZ) antagonists 3-hydroxymethyl-beta-carboline (3-HMC), Ro 14-7437 and Ro 15-1788 were tested on single cell activity of rat hypothalamic neurons in tissue cultures and on membrane properties of CA1 hippocampal pyramidal neurons in transverse slices. In addition, we examined the interactions of some of these agents with inhibitions elicited by gamma-aminobutyric acid (GABA) as well as the ability of Ro 14-7437 to reverse the GABA-enhancing action of the BDZ agonist flurazepam. BDZ antagonists did not alter patterns of spontaneous activity of hypothalamic neurons and did not affect resting membrane potential or membrane conductance in CA1 pyramidal cells. Ro 14-7437 either partially or totally reversed the potentiation by flurazepam of GABA-elicited depression of hypothalamic neuronal activity. Small and inconsistent actions on GABA-mediated inhibitions of hypothalamic neurons were noted. Electrically-elicited inhibitions of hypothalamic neurons were either not altered or slightly reduced. In the hippocampal slice, the frequency of spontaneous IPSPs, the amplitude of stratum-radiatum evoked IPSPs and the conductance increase caused by stratum-radiatum stimulation were either not altered or slightly reduced. These findings demonstrate that non-convulsant BDZ antagonists block the action of BDZ agonists in facilitating GABA and further that the presence of a BDZ agonist is not required for these GABA-mediated events to occur. However, these experiments do not exclude a modulatory role for an endogenous BDZ agonist on GABA-mediated events.

The action of gamma-aminobutyric acid (GABA) and ethylenediamine (EDA) on Limulus and Helix central neurones and rat cerebellar and sympathetic ganglion neurones

Intracellular recordings were made from central Limulus and Helix neurones and extracellular recordings from rat cerebellar Purkinje cells and sympathetic ganglia. The actions of gamma-aminobutyric acid (GABA) and ethylenediamine (EDA) and related analogues on these preparations were investigated. On Limulus neurones inhibited by GABA, EDA and piperazine were 81 and 186 times respectively less potent than GABA. Both the GABA and EDA events were chloride mediated, having similar reversal potentials and were reversibly antagonised by picrotoxinin. The EDA response persisted in high magnesium Ringer. On Helix neurones inhibited by GABA, EDA was 92 times less potent while on neurones excited by GABA, EDA was 9.25 times less potent. The other analogues tested had little or no GABA-like effect on either preparation. On rat cerebellar Purkinje cells, EDA was equipotent with GABA and both compounds were antagonised by bicuculline. Flurazepam only potentiated the action of EDA on 3 out of 23 cells tested while the GABA response of all 23 cells was potentiated by the benzodiazepine. Diaminopropionic acid was a weak inhibitor of cerebellar Purkinje cell firing but flurazepam potentiated this response in 6 out of 10 cells tested. On rat cervical ganglion neurones, EDA was half the potency of GABA and likewise the other analogues were less potent than GABA as depolarising agents. Incubation with glutamic acid decarboxylase inhibitors had no effect on the EDA response. Cross desensitisation between GABA and EDA was demonstrated using the ganglion preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

The perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns

Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Mal type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative.

Diazepam potentiates GABA-, but not adenosine-mediated, inhibition of neurons of the nigral pars reticulata

Experiments were conducted to assess the relative roles of gamma-aminobutyric acid (GABA) and adenosine in mediating the inhibition of neuronal activity by diazepam injected intravenously. Recent studies have shown that benzodiazepines inhibit, in a dose-dependent manner, the firing of neurons in the substantia nigra pars reticulata. In support of a predominantly GABAergic mechanism for this inhibitory action, a small dose of diazepam (50 micrograms/kg, i.v.), which itself had little effect on cell firing, significantly potentiated the inhibitory responses of neurons of the pars reticulata to muscimol, a potent GABA agonist given intravenously, and significantly and selectively potentiated the inhibition of reticulata neurons by GABA applied iontophoretically. In contrast to their extreme sensitivity to GABAergic inhibition, neurons of the pars reticulata were comparatively insensitive to systemically and iontophoretically administered adenosine-related drugs. However, in those instances when inhibitions could be achieved with iontophoretically applied adenosine-5'-monophosphate, the inhibitory responses were not significantly modified by a 50 micrograms/kg (i.v.) dose of diazepam. These findings, considered in light of differences in GABA and adenosine receptor densities within the substantia nigra, suggest that the benzodiazepine-induced inhibition of neurons of the nigral pars reticulata most likely involves potentiation of GABA but not adenosine-mediated influences.

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.

Interactions between amino acid neurotransmitters and flurazepam in the neocortex of unanesthetized rats

The effects induced by the benzodiazepine flurazepam (FLU) upon neuronal responses to glutamic acid (GLUT), gamma-aminobutyric acid (GABA), and glycine (GLY) were studied in the cortex of unanesthetized rats using single-unit extracellular recordings in conjunction with iontophoretic techniques. The application of FLU (5-20 nA) did not affect excitatory responses to GLUT, but the spontaneous firing rate was depressed by equivalent doses of this benzodiazepine. A selective increase of GABA, but not of GLY-induced responses was seen when either low currents (5-10 nA) of FLU or GLUT driving currents were used to study the neuronal responses of the inhibitory amino acids upon steady neuronal firing. Our data demonstrate that in unanesthetized animals FLU does not affect GLUT-induced effects while it selectively increases GABA-mediated inhibition.

Chlordiazepoxide normalizes behaviour and adrenal response abnormalities in septal rats in a dose and time dependent fashion

Rats lesioned in the septal nuclei display a dramatic syndrome characterized by hypoactivity in a novel environment, but exaggerated behavioural and corticosterone responses to environmental stimuli. These abnormal responses are normalized 2 hours following 15 mg/kg i.p. of chlordiazepoxide (Seggie, CCNP, 1980). The present study was undertaken to map the dose and time response characteristics of this effect and see if behaviour and corticosterone respond in parallel fashion. Chlordiazepoxide in doses of 7.5-30 mg/kg affected the hyperreactivity of septal rats is a 'U' shaped function without affecting the behaviour of non-lesioned rats. Corticosterone levels in non-lesioned rats were unaffected by chlordiazepoxide, while drug treated septal rats had corticosterone levels undistinguishable from non-lesioned control rats. The drug effects were transitory, maximal at 2 hours after injection, but absent as 48 hours after injection. General abnormalities of behaviour and corticosterone responsiveness were altered in parallel fashion by the drug, but some evidence of differences in sensitivity to the effect of the drug were noticed. Locomotion, was unaffected by the drug in lesioned and non-lesioned rats. The possibility of a GABA mechanism underlying the abnormalities of responsiveness, but not activity in septal rats is 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.

Multiple actions of picomolar concentrations of flurazepam on the excitability of cultured mouse spinal neurons

Intracellular recordings from mouse spinal neurons grown dissociated in tissue culture were used to study the effects of the water soluble benzodiazepine, flurazepam, upon neuronal excitability. Low concentrations of this drug (1 pM to 10 nM) depressed excitability in three distinctly different ways: (1) by directly increasing Cl- conductance, (2) by potentiating responses to GABA, and (3) by elevating spike threshold and/or depressing repetitive spike firing. Bathing neurons with picrotoxin induced 'convulsive-like' activity which was attenuated by flurazepam. The direct effects of flurazepam on the passive and active properties of membrane excitability were insensitive to picrotoxin. However, when the dose of flurazepam was increased to 10 nM or greater this drug lost its effectiveness. These results show that flurazepam is a potent drug with multiple sites of action all of which are likely to contribute to its pharmacological actions in vivo.

Enhancement of cocaine-induced hyperactivity in mice by benzodiazepines: evidence for an interaction of GABAergic processes with catecholaminergic neurons?

The effects of nine benzodiazepines on the locomotor stimulation induced in mice by cocaine (4 mg . kg-1 i.p.) were studied. These benzodiazepines markedly enhanced cocaine-induced hyperactivity. This effect was observed at low doses, e.g. doses at least 8 times lower than those required to depress the stimulation caused by cocaine. Nitrazepam-induced enhancement of the hyperactivity elicited by cocaine was reduced or suppressed by blocking dopaminergic receptors with pimozide (0.015--0.03 mg . kg-1), by interrupting GABAergic transmission with picrotoxin (0.25--0.5 mg . kg-1) or blocking alpha- or beta-adrenergic receptors with prazosin (0.25 mg . kg-1) or dl-propranolol (4 mg . kg-1) respectively. At these doses, neither pimozide, picrotoxin, prazosin nor propranolol were able to modify the spontaneous locomotor activity or the stimulation elicited by cocaine alone. Strychnine (0.25--0.50 mg . kg-1) or methysergide (2 mg . kg-1) failed to alter the enhancement by nitrazepam of cocaine-induced hyperactivity. These results suggest that an interaction of benzodiazepines with some catecholaminergic processes, either directly or through the involvement of a GABAergic link, may account for their facilitatory activity on cocaine-induced locomotor stimulation.

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.

Effect of sodium valproate on hypothalamic neurons in vivo and in vitro

The effects of sodium valproate on extracellularly recorded spontaneous neuronal activity and its interaction with GABA-induced inhibition were assessed in the rat medial basal hypothalamus both in situ and in explant culture. In situ, valproate enhanced the spontaneous rate in 14 of 20 neurons sampled, 6 neurons were unaffected. In vitro, valproate had an inconsistent effect on firing rate; the spontaneous rate was decreased in 6 cells, elevated in 1, while 17 either were unchanged or demonstrated no consistent pattern of response. Both in vivo and in vitro, valproate produced a variable effect on GABA induced inhibition. In vivo, the modal response of valproate on GABA inhibition was antagonism whereas the response in vitro was less consistent, favoring no change in GABA inhibition. The results of this study suggest that valproate exerts an inconsistent action on hypothalamic neurons. These findings are in contrast to those from the cerebral cortex, suggesting that the action of valproate in the brain may be regionally specific.

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.

A loss of GABAergic hippocampus innervation in rats with cobalt-induced epilepsy demonstrated by Wolman's fluorescence method

The topohistochemistry of gamma-aminobutyric acid (GABA) was studied in the hippocampus of rats with epileptiform activity provoked by cortical implantation of cobalt-agar pellets and in control rats with implanted agar-pellets. Using the GABA specific fluorochroming procedure of Wolman (1971) a distinct decrease of GABA fluorescence was found especially in the pyramidal and granular layer, but only in the animals with a typical epileptiform activity of the EEG. The results support the hypothesis that there is a lack of GABAergic inhibition in epileptic individuals, but the anticonvulsive effect of GABA depends on structural and functional relations in the specific type of seizure.

Evidence of a role for GABA in benzodiazepine effects on food preference in rats

It has previously been shown that chronic treatment with the GABA-transaminase inhibitor ethanolamine-O-sulphate (EOS), which elevates brain GABA levels by around 200%, selectivity enhances novel food consumption in rats treated with chlordiazepoxide (CDP) and given a food preference test. To replicate and extend these findings, the effects of two doses of CDP with and without EOS pretreatment were compared with those of EOS or saline alone. EOS alone had no significant effects except to decrease eating rate but, in combination with 2.5 mg/kg CDP, it antagonised the increase in weight of familiar food eaten found with CDP alone and marginally increased weight eaten and duration of novel foot eating episodes. EOS magnified the effects of 5.0 mg/kg CDP to increase markedly the weight eaten and duration of episodes for novel chocolate drops. As no additive effects of EOS and CDP on rate of eating were found, the results are consistent with a facilitation of novel food consumption by an anxiolytic action of the two drugs, rather than by a rate-retarding action which might bias animals toward novel food. Finally, that EOS alone did not mimic the effects of CDP suggests that the role of GABA in the anxiolytic action of CDP may be indirect.

Benzodiazepines modify the agonist responses at a presynaptic GABA receptor

The stimulus-induced release of [3H]glutamic acid from striatal tissue was actively modulated by a presynaptic GABA receptor. The agonist responses of this GABA receptor were modified stereoselectively by benzodiazepines at low concentrations. The results correlate well with effects seen in [3H]GABA binding experiments and as they demonstrate benzodiazepine-induced changes in GABA receptor properties in a tissue prism preparation, these results suggest that the mechanisms described for extensively disrupted preparations may also apply to whole tissue.

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.