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

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)

Effects of two convulsant beta-carboline derivatives, DMCM and beta-CCM, on regional neurotransmitter amino acid levels and on in vitro D-[3H]aspartate release in rodents

Clonic seizures were induced in Swiss or DBA/2 mice by methyl-6-7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), 0.048 mmol/kg i.p., or by methyl-beta-carboline-3-carboxylate (beta-CCM), 0.044 mmol/kg i.p. Measurement of regional brain (cortex, hippocampus, striatum, and cerebellum) amino acid levels after 15 min of seizure activity showed increases in gamma-aminobutyric acid (GABA) (in all regions after beta-CCM, and in cortex and hippocampus after DMCM), and an increase in glycine in the striatum after beta-CCM. Aspartate levels fell (in cortex and hippocampus) after DMCM, but were unchanged in all regions after beta-CCM. Glutamate levels fell in cortex after beta-CCM and in striatum after DMCM. Pretreatment with the excitatory amino acid receptor antagonist, 2-amino-7-phosphonoheptanoic acid, 0.5 mmol/kg i.p., 45 min prior to the beta-carboline, significantly increased the ED50 for DMCM-induced clonic seizures (4.68 mumol/kg vs. 9.39 mumol/kg). Similar pretreatment did not significantly alter the ED50 for beta-CCM (4.22 mumol/kg vs. 6.6 mumol/kg). Pretreatment with 2-amino-7-phosphonoheptanoic acid, 1.0 mmol/kg, blocked the increase in GABA content produced by DMCM but not the fall in cortical aspartate content. Potassium-induced release of preloaded D-[3H]aspartate from rat cortical or hippocampal minislices was enhanced in the presence of DMCM (100 microM). In contrast, stimulated release of D-[3H]aspartate (from cortex or hippocampus) was not altered in the presence of beta-CCM (100 microM). Although DMCM and beta-CCM are both considered to induce convulsion by acting at the GABA--benzodiazepine receptor complex, the convulsions differ in several pharmacological and biochemical respects. It is suggested that enhanced release of excitatory amino acid neurotransmitters plays a more important role in seizures induced by DMCM.

Regional changes in transmitter amino acids during focal and generalized seizures in rats

Measurement of glucose utilization after 20 minutes of seizure activity induced by the systemic administration of convulsants in paralyzed, ventilated rats demonstrates that the initial metabolic activation during kainic acid seizures is confined to the hippocampus. L-allylglycine-induced generalized seizures result in a global metabolic activation, whereas bicuculline seizures in fasted rats have little effect on brain glucose utilization after 20 minutes of seizure activity. Other metabolic changes associated with kainic acid seizures also predominate in the hippocampus, were decreases in the levels of aspartate, glutamate, taurine and glutamine, and an increase in the level of GABA are observed after 20 minutes of seizure activity. L-allylglycine seizures are associated with generalized decreases in regional GABA levels, and increases in regional glutamine levels. Bicuculline-induced changes include increases in hippocampal GABA and taurine levels, and increases in cerebellar glutamine and taurine levels. These changes can be tentatively explained in terms of known biochemical and neurophysiological mechanisms of these convulsants.

Excitatory and inhibitory effects of dopamine on synaptic transmission in the rat olfactory cortex slice

A study has been undertaken of the effects of dopamine on excitatory transmission at the lateral olfactory tract (LOT)-superficial pyramidal cell synapse of the rat olfactory cortex slice by measuring the effects of bath-applied dopamine on the amplitudes and latencies of the surface field potentials evoked on submaximal LOT stimulation in a total of 32 preparations. In 7 (22%) slices, dopamine had no detectable effects on transmission. In the remaining preparations, dopamine (1-250 microM) depressed transmission in a concentration-dependent manner. This action was unaffected by nadolol (10 microM), phentolamine (10 microM) and picrotoxin (25 microM) but was antagonized by chlorpromazine (10 microM) and trifluoperazine (0.2 and 0.5 microM) and mimicked by bromocriptine (0.01-5 microM) and apomorphine (0.25-25 microM). Investigation of the effects of dopamine on stimulus input-evoked potential output relationships indicated that the inhibitory effect of dopamine on transmission was mediated by a reduction in pyramidal cell excitability. In 6 slices (24% of those sensitive to dopamine) low dopamine concentrations (0.1-1 microM) facilitated transmission at the LOT-superficial pyramidal cell synapse. This excitatory effect was antagonized by nadolol and phentolamine (10 microM) and also by 100 microM 2-amino-5-phosphonovalerate (an antagonist of excitatory amino acid receptors of the N-methyl-D-aspartate type) but was unaffected by chlorpromazine (10 microM) and trifluoperazine (0.2 and 0.5 microM). By a comparison with the effects of noradrenaline on transmission, it is concluded that the excitatory effects of dopamine are mediated either indirectly by the release of noradrenaline or by a direct interaction of dopamine with adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory and inhibitory effects of noradrenaline on synaptic transmission in the rat olfactory cortex slice

An investigation has been made of the effects of noradrenaline on excitatory transmission at the lateral olfactory tract (LOT)-superficial pyramidal cell synapse of the rat olfactory cortex slice by measuring the effects of bath-applied noradrenaline on the amplitudes and latencies of the field potentials evoked on LOT stimulation. Low concentrations of noradrenaline (0.1-5 microM) facilitate transmission whereas higher doses (20-250 microM) depress transmission. Both these effects were completely blocked by non-selective alpha- and beta-adrenoceptor antagonists, by 2-amino-5-phosphonovaleric acid (an antagonist of excitatory amino acid receptors of the N-methyl-D-aspartate type) and by the methylxanthine theophylline. The depressant effects of noradrenaline were mimicked by bath application of GABA or adenosine and specifically antagonized by bicuculline and picrotoxin. In parallel experiments, noradrenaline (100 microM) significantly increased the potassium-evoked release of endogenous aspartate, glutamate and GABA, proposed transmitters of the olfactory cortex, although the effect on GABA release was specifically antagonized by 2-amino-5-phosphonovaleric acid. Noradrenaline (100 microM) also significantly increased the potassium-evoked release of D-[3H]aspartate, an effect antagonized by a number of alpha- and beta-adrenoceptor antagonists. It is concluded that at low concentrations, noradrenaline facilitates transmission at the LOT-superficial pyramidal cell synapse by increasing excitatory amino acid neurotransmitter release. This effect is mediated by both alpha- and beta-adrenoceptors although the primary site of release is unknown. At higher concentrations of noradrenaline, the increased levels of excitatory transmitters release sufficient endogenous GABA (and possibly adenosine) to cause an overall depression of transmission. These conclusions are supported by the results of a series of experiments in which the effects of noradrenaline on stimulus input-evoked field potential output relationships were assessed. It is not possible to exclude additional direct effects of noradrenaline on membrane excitability.

Inhibition by Diazepam and γ-Aminobutyric Acid of Depolarization-Induced Release of [¹⁴C]Cysteine Sulfinate and [³H]Glutamate in Rat Hippocampal Slices

Effects of diazepam and γ-aminobutyric acid-related compounds on the release of [¹⁴C]cysteine sulfinate and [³H]glutamate from preloaded hippocampal slices of rat brain were examined by a superfusion method. Diazepam markedly inhibited the release of cysteine sulfinate and glutamate evoked either by high K⁺ or veratridine without affecting that of other neurotransmitter candidates, e.g., γ-aminobutyric acid, acetylcholine, noradrenaline, and dopamine; IC₅₀ values for the release of cysteine sulfinate and glutamate were about 20 and 7 μM, respectively. γ-Aminobutyric acid (1 to 10 μM) and muscimol (100 μM) significantly reduced high K⁺-stimulated release of glutamate. Bicuculline, which had no effect on the release at a concentration of 50 μM by itself, antagonized the inhibitory effects of diazepam and γ-aminobutyric acid on glutamate release. Similar results were obtained with the release of cysteine sulfinate except that a high concentration (100 μM) of γ-aminobutyric acid was required for the inhibition. These results indicate the modulation by γ-aminobutyric acid innervation of the release of excitatory amino acids in rat hippocampal formation, and also suggest that some of the pharmacological effects of diazepam may be a consequence of inhibition of excitatory amino acid transmission.

Acute effects of sodium valproate and gamma-vinyl GABA on regional amino acid metabolism in the rat brain: incorporation of 2-[14C]glucose into amino acids

Amino acid concentrations have been determined in rat brain regions (cortex, striatum, cerebellum, and hippocampus) by HPLC after administration of acute anticonvulsant doses of sodium valproate (400 mg/kg, i.p.) and gamma-vinyl-GABA (1 g/kg, i.p.). After valproate administration the GABA level increases only in the cortex; aspartic acid concentration decreases in the cortex and hippocampus, and glutamic acid decreases in the hippocampus and striatum and increases in the cortex and cerebellum. There are no changes in the concentrations of glutamine, taurine, glycine, serine, and alanine following valproate administration. Only the GABA level increases in all the regions after gamma-vinyl-GABA administration. Cortical analyses 2, 4 and 10 minutes after pulse labeling with 2-[14C]glucose, i.v., show no change in the rate of cortical glucose utilization in the valproate treated group. The rate of labeling of glutamic acid is also unchanged, but the rate of labeling of GABA is reduced following valproate administration. After gamma-vinyl-GABA administration there is no change in the rate of labeling of GABA. These biochemical findings can be interpreted in terms of a primary anticonvulsant action of valproate on membrane receptors with secondary effects on the metabolism of amino acid neurotransmitters. This contrasts with the primary action of gamma-vinyl-GABA on GABA-transaminase activity.

Some effects of excitatory amino acid receptor antagonists on synaptic transmission in the rat olfactory cortex slice

A study has been made of the effects of a series of excitatory amino acid receptor antagonists on the field potentials evoked on electrical stimulation of the lateral olfactory tracts of olfactory cortex slices perfused in vitro. The antagonists studied included (+/-)-2-amino-5-phosphonovaleric acid, a potent, specific antagonist of N-methyl-D-aspartate (NMDA) receptors, gamma-D-glutamylglycine, an antagonist of NMDA and kainate receptors and (+/-)-cis-2,3-piperidine dicarboxylic acid and 2-amino-4-phosphonobutyric acid, drugs which in addition to antagonizing NMDA and kainate receptors also block responses to quisqualic acid. From the patterns of effects of the drugs it is proposed that quisqualate and NMDA but not kainate receptors are involved in mediating excitatory transmission in the olfactory cortex; quisqualate receptors are located at the lateral olfactory tract - superficial pyramidal cell synapse whereas NMDA receptors are present at the synapses of the superficial pyramidal cell collaterals with the deep pyramidal cell dendrites and/or at the synapses of the pyramidal cell collaterals and inhibitory interneurones. The results are discussed in terms of possible presynaptic and/or postsynaptic sites of antagonist action.

Baclofen: effects on evoked field potentials and amino acid neurotransmitter release in the rat olfactory cortex slice

A study has been made of the in vitro effects of (+/-)- and (-)-baclofen on the evoked field potentials and release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the excitatory input to the olfactory cortex slice, the lateral olfactory tract. Baclofen appears to reduce the excitatory input to the GABA-utilizing inhibitory interneurones; this action was manifest as a drug-induced abolition of the field potential known as the P-wave (IC50 for (-)-baclofen, 1.7 +/- 0.4 microM) together with a simultaneous reduction in the synaptically evoked release of aspartase and glutamate from the cut surface of slices. Both these actions of baclofen exhibited concentration dependence and stereospecificity and were not antagonized by picrotoxin (25 microM) thereby suggesting that they are directly related. The consequence of this action of baclofen was the abolition of GABA-mediated presynaptic and postsynaptic inhibition together with their respective field potential correlates, the late N- and I-waves. (+/-)-Baclofen (5 and 25 microM) also inhibited the potassium-evoked release of aspartate and glutamate from small cubes of tissue but, except at a high concentration (1 mM), had no effect on GABA release. Baclofen (up to 1 mM) did not affect transmission either at the lateral olfactory tract-superficial pyramidal cell synapse, a site where aspartate is the likely neurotransmitter, or at the superficial pyramidal cell collateral-deep pyramidal cell excitatory synapse. It is proposed that: (i) the actions of baclofen on the olfactory cortex are the result of inhibition of aspartate and glutamate release, probably from deep pyramidal cell collaterals; and (ii) not all neurones utilizing excitatory amino acids as their neurotransmitters are subject to the inhibitory action of baclofen.