Augmentation by chlordiazepoxide of the inhibitory effects of taurine, beta-alanine and gamma-aminobutyric acid on spike discharges in guinea-pig cerebellar slices

Modulation of GABA-mediated inhibition in rat cerebellar slices by benzodiazepine receptor ligands

1. Extracellular recordings were made from the Purkinje cell layer of rat cerebellar slices. Compounds were perfused over the slice and bipolar stimulating electrodes placed in the external layer of the slice close to the recording electrode. 2. Stimulus-evoked inhibition of Purkinje layer cell activity was sensitive to bicuculline methiodide and picrotoxin, suggesting it was gamma-aminobutyric acid (GABA) mediated. The benzodiazepine ligands RU 32007 and Ro 19-0528 reversibly increased the period of inhibition, as did pentobarbital. This benzodiazepine effect was antagonised by Ro 15-1788. 3. Five inverse agonists all reduced the period of stimulated inhibition and this effect was reversed by Ro 15-1788, suggesting the involvement of benzodiazepine receptors. 4. It is concluded that this system provides a convenient physiological and possibly quantitative model for studying the action of benzodiazepine receptor ligands.

Membrane channels activated by taurine in cultured mouse spinal cord neurons

Patch-clamp methods were used to compare biophysical properties of anion channels activated by taurine and gamma-aminobutyric acid in the membrane of cultured mouse spinal neurons. Outside-out patches were voltage clamped at -80 mV at a temperature of 21-23 degrees C. Bath application of GABA (1.5-2 microM) or taurine (5-40 microM) induced chloride-dependent single-channel currents in 14/20 patches tested. Amplitude distributions of these currents showed peaks corresponding to conductance levels of 8, 16, 27 and 46 pS. Only a few percent of GABA-induced events reached the 46 pS level, while 30% of taurine-induced currents were of this size. The average lifetime of taurine-activated channels in the open state was 1.0 +/- 0.07 ms, significantly shorter than the corresponding value for GABA (1.6 +/- 0.08 ms). Taurine-induced currents were abolished by 10 microM strychnine, but persisted in the presence of 50 microM bicuculline.

Identification of two taurine receptor subtypes on the primary afferent terminal of frog spinal cord

1. Effects of taurine on primary afferent terminals in the frog spinal cord were examined by a sucrose-gap method applied to a dorsal root (9th or 10th segment). 2. In a normal Ringer solution, taurine (1 mM, applied for 5s at a rate of 0.04 ml s-1, 0.2 mumol) caused a hyperpolarization, but a higher concentration (10 mM, applied at the same rate, 2.0 mumol) caused a biphasic response consisting of a hyperpolarization followed by a slow onset depolarization. A similar biphasic response could also be observed in tetrodotoxin-treated preparations. 3. When the concentration of extracellular Mg2+ was increased up to 9.0 mM, the depolarizing response to taurine was augmented. The rate of the augmentation was dependent upon the extracellular Mg2+ concentration. 4. The depolarizing effect was selectively antagonized by bicuculline in concentrations (10-30 microM) that had no significant antagonizing action on gamma-aminobutyric acid (GABA)-induced depolarization. On the other hand the hyperpolarizing effect of taurine was selectively reduced by strychnine (0.1 microM) which had no antagonizing effect on responses to glycine. 5. These results suggest that in the frog spinal cord there are at least two subtypes of taurine receptor whose pharmacological profiles resemble GABA and glycine receptors in the mammalian central nervous system, and whose sensitivity may be modulated by extracellular Mg2+.

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)

Reduction of inhibition by a benzodiazepine antagonist, Ro15-1788, in the rat hippocampal slice

The effects of extracellular applications of benzodiazepine agonists and the benzodiazepine antagonist, Ro15-1788, were investigated on pyramidal neurons in the CA1 region of rat hippocampal slices. The benzodiazepine agonists, chlordiazepoxide and diazepam, enhanced gamma-aminobutyrate synaptic inhibition, as tested by extracellular recordings during a paired-pulse inhibition paradigm. In contrast, Ro15-1788 (0.1-1 microM) depressed paired-pulse inhibition in a dose-dependent manner that suggested agonist activity at higher (10-100 microM) concentrations. Intracellular recordings from CA1 neurons showed that Ro15-1788 reduced both orthodromically and antidromically evoked inhibitory postsynaptic potentials. The reduction of the inhibitory postsynaptic potential probably resulted from a postsynaptic effect on the conductance mechanism of the inhibitory postsynaptic potential, since there were no changes in resting input resistance, the inhibitory postsynaptic reversal potential or the frequency of spontaneous inhibitory postsynaptic potentials. These data suggest that in the hippocampal slice preparation either (1) an endogenous benzodiazepine agonist exists that can be displaced by Ro15-1788 or (2) Ro15-1788 has inverse agonist activity.

Inhibitory effect on 3H-diazepam binding and potentiating action on GABA of ethyl loflazepate, a new minor tranquilizer

A new benzodiazepine compound, ethyl loflazepate (ethyl-7-chloro-2,3-dihydro-5-(2-fluorophenyl)-2-oxo-1H,1,4- benzodiazepine-3-carboxylate; CM6912) was studied using in vitro experimental systems for its displacement activity on 3H-diazepam binding to the synaptosomal membrane fraction of rat cerebrum and potentiating action on GABA. CM6912 inhibited the specific binding of 3H-diazepam by 25%, 75% and 90% at concentrations of 0.01 microM, 0.1 microM and 1 microM, respectively, while its metabolites CM6913 and CM7116, at 0.1 microM, completely inhibited the binding. Concentrations for 50% inhibition (IC50) were 25 nM for CM6912, 3.2 nM for CM6913 and 1.4 nM for CM7116. These results suggest that the metabolite CM7116 is stronger than its parent compound in displacing the 3H-diazepam binding, and they also suggest that the long-lasting anti-anxietic action of CM6912 might be due to the in vivo formation of CM7116. CM6912, CM7116 and diazepam potentiated the suppressive action of GABA on spontaneous spikes of Purkinje cells in guinea pig cerebellar slices in a dose-dependent manner. Concentrations for 50% suppression (IC50) were 96.0 microM for GABA alone, 75.0 microM for GABA plus diazepam (5 microM), 78.9 microM for GABA plus CM6912 (5 microM) and 60.8 microM for GABA plus CM7116 (5 microM). These findings suggest that CM6912 and CM7116 may potentiate the postsynaptic inhibitory action of GABA in a manner similar to and probably more strongly than diazepam.

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.

Contribution of cerebellar intracortical inhibition to Purkinje cell response during vestibulo-ocular reflex of alert rabbits

Ionophoretic application of bicuculline, an antagonist of gamma-aminobutyric acid (GABA), was used to examine the contribution of intracortical inhibition to vestibular responses of Purkinje cells in the cerebellar flocculus of alert rabbits. Purkinje cells were sampled extracellularly (with triple-barrelled micropipettes) from the floccular area where electrical stimulation through the micro-electrode evoked abduction of the ipsilateral eye, indicating its close functional relationship to the horizontal vestibulo-ocular reflex. These cells exhibited frequency modulation of simple spike discharges in-phase or out-phase with sinusoidal head rotation (0.5 cycles/s, 5 degrees peak-to-peak) in the horizontal plane. Bicuculline was ejected ionophoretically through one barrel with a 20-60 nA current. The pharmacological effectiveness of the ejected bicuculline was confirmed for each Purkinje cell by its blocking action upon the depressant action of GABA applied ionophoretically through another barrel. Bicuculline usually shifted the simple spike modulation in the in-phase direction: it reduced the amplitude of out-phase modulation in three cells, converted out-phase modulation to the in-phase type in four cells, and increased in-phase modulation in five cells. In three other cells, however, bicuculline shifted the modulation in the out-phase direction. Because bicuculline application usually increased the resting discharge level of a Purkinje cell, ionophoretic application of DL-homocysteate was used in ten Purkinje cells to control for the effect of a generalized increase in excitability. In contrast to bicuculline, DL-homocysteate generally induced a slight increase of the simple spike modulation regardless of the phase relationship. Since frequency modulation of the simple spike discharges of flocculus Purkinje cells is presumed to contribute to the control of vestibulo-ocular reflexes, these results point to an important functional role of intracortical post-synaptic inhibition in the cerebellar cortex.

Diazepam increases GABA mediated inhibition in the olfactory cortex slice

The effect of diazepam on inhibition has been examined using an in vitro preparation of the guinea-pig olfactory cortex. Diazepam (0.03-30 mumol/l) doubled the intensity and duration of the recurrent inhibitory conductance. Diazepam had no effect on single evoked excitatory post-synaptic potential (e.p.s.p.) nor any effects on the action potential or membrane electrical constants. Diazepam (0.003-100 mumol/l) also reduced the multisynaptic e.p.s.p. generated through a recurrent pathway directed at the soma when elicited during the time-course of the inhibitory conductance. Diazepam had a comparitively small effect on the monosynaptic e.p.s.p. generated on the distal dendrite. Pentobarbitone had a similar though more intense effect over a narrow concentration range (10-200 mumol/l). The inhibitory coductance is thought to be GABA-mediated. Diazepam doubled the potency of the GABA analogue, muscimol, when applied via the bathing solution, whereas a modest 50 mumol/l pentobarbitone increased muscimol potency by about four-fold. The mild but selective effect of diazepam contrasts with the more intense and general effects of pentobarbitone and supports the idea that these drugs act through different mechanisms at the GABA receptor/channel complex.

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

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

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

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

Distinction between the effects of barbiturates, benzodiazepines and phenytoin on responses to gamma-aminobutyric acid receptor activation and antagonism by bicuculline and picrotoxin

1 Interactions of depressant and anticonvulsant drugs with the neuronal gamma-aminobutyric acid (GABA) receptor + effector system have been examined on afferent fibres to the rat cuneate nucleus in vitro. Three types of interaction have been measured: (a) potentiation of depolarizing responses to the GABA analogue, muscimol: (b) reduction in the potency of bicuculline as an antagonist of muscimol at the GABA receptor: (c) reduction in the potency of picrotoxin as an antagonist of muscimol acting on the effector mechanism. 2 Phenobarbitone reduced the potency of picrotoxin in doses which did not affect the potency of bicuculline and which caused only a small potentiation of muscimol. Pentobarbitone did not show such selectivity, a reduction in potency of picrotoxin always being accompanied by a reduction in potency of bicuculline and a substantial potentiation of muscimol. 3 Flurazepam and lorazepam both reduced the potency of picrotoxin without affecting that of bicuculline and with very little potentiation of muscimol. Phenytoin had no effect on the potency of picrotoxin whilst potentiating muscimol to the same extent as phenobarbitone. 4 The spectrum of drug activity in reducing the potency of picrotoxin correlates well with the reported anticonvulsant effects of these drugs against kindled amygdaloid seizures. Potentiation of muscimol and reduction of bicuculline potency appear more closely related to hypnotic properties.

Inhibitory actions of taurocyamine, hypotaurine, homotaurine, taurine and GABA on spike discharges of Purkinje cells, and localization of sensitive sites, in guinea pig cerebellar slices

The effects of the possible metabolites of taurine--taurocyamine and hypotaurine--and of homotaurine on spike discharge frequency of guinea pig cerebellar Purkinje cells were investigated in comparison with taurine and GABA. Taurocyamine, hypotaurine and homotaurine all dose-dependently suppressed the spike frequency with potencies relative to taurine (ED50 = 1.0 mM) of 6, 0.5 and 500, respectively. Both picrotoxin and strychnine competitively blocked the inhibition by all the substances tested, except for the non-competitive antagonism by picrotoxin on taurocyamine. The sites sensitive to taurocyamine and hypotaurine were situated primarily in the dendritic region of the cerebellar molecular layer, while those to homotaurine were primarily near the Purkinje cell soma. The onset of the inhibitory action of taurine and related sulfur-containing substances was found to be slower than that of GABA.

Localization of sensitive sites to taurine, gamma-aminobutyric acid, glycine and beta-alanine in the molecular layer of guinea-pig cerebellar slices

1 The localization, in the molecular layer of guinea-pig cerebellar slices, of the sites most sensitive to iontophoretically applied taurine, gamma-aminobutyric acid (GABA), glycine and beta-alanine was investigated. 2 The most sensitive sites were located (expressed as the distance from the Purkinje cell body to the pial surface): 0 micrometers, 60 micrometers and 220 micrometers for taurine; 0 micrometers and 180 micrometers for GABA; 80 micrometers and 200 micrometers for glycine, and 80 micrometers, 180 micrometers and 300 micrometers for beta-alanine. 3 The sensitive site (at 0 micrometers) to GABA was considered to represent the basket synapses on the Purkinje cell soma, while the sites (60 to 80 micrometers and 200 to 300 micrometers) for taurine, glycine and beta-alanine were tentatively assigned to the synapses of the stellate neurones on the Purkinje cell dendrites. 4 Inhibitory actions of all four amino acids tested at the most sensitive sites were antagonized by both picrotoxin and strychnine. 5 The possibility that taurine might be the neurotransmitter of the stellate neurones in guinea-pig cerebellum is discussed.

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.