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.     

GABA induced changes in acetylcholine release from slices of guinea-pig brain

Summary The effect of GABA on acetylcholine (ACh) release was investigated on superfused slices of guinea-pig cerebral cortex (CC), caudate nucleus (CN), tuberculum olfactorium and brain stem.GABA (1–6×10^–3 mol/l) increased the spontaneous and KCl-evoked ACh overflow in CC and CN, reduced the electrically-evoked release in all areas tested (most evidently in CC and CN) and lowered the threshold of electric stimulation-induced ACh release in CC. These effects were also caused by 3-amino-1-propane sulphonic acid (1×10^–3mol/l) and ethanolamine-O-sulphate (2×10^–4mol/l), were reduced by bicuculline (1×10^–4mol/l) and fully antagonized by picrotoxin (8×10^–5mol/l), but they were not influenced by phentolamine, methysergide, spiroperidol or strychnine.Tetrodotoxin (TTX) (5×10^–7mol/l) blocked the facilitation of spontaneous ACh release by GABA only when the slices were perfused with normal Krebs solution, but not when perfused with a KCl-enriched medium. These results suggest that GABA affects the cholinergic transmitter release through bicuculline- and picrotoxin-sensitive receptors, showing low affinity toward the agonist. Moreover GABA modulation of resting ACh release requires action potentials only in normal [K⁺]₀, but not in high [K⁺]₀, suggesting that GABA-receptive sites are located at cholinergic terminals.     

3H-ACh release from guinea pig gallbladder evoked by GABA through the bicuculline-sensitive GABA receptor

Summary We investigated the effect of GABA on the spontaneous efflux of ³H-acetylcholine (ACh) from the isolated guinea pig gallbladder loaded with ³H-choline. Application of GABA (10^–5 M) caused a significant increase in the fractional rate of tritium efflux. This GABA-evoked efflux of ACh was inhibited by the perfusion of tetrodotoxin (10^–6 M) and Ca-free medium. Nipecotic acid (10^–4 M) did not affect the GABA-evoked release of ACh, indicating that ACh was not released by the entry of GABA into cholinergic nerve terminals. Bicuculline (10^–6 M) and furosemide (10^–6 M), the chloride ion channel blocker, inhibited the GABA-evoked ACh release. The application of muscimol (10^–5 M), but not baclofen (10^–5 M) also produced an increase in the fractional rate of ACh release. Thus, the GABA receptors involved in the increase of ACh release are bicuculline-sensitive. The GABA-evoked release of ACh was not altered by the perfusion with hexamethonium (10^–5 M), thus indicating the presence of GABA receptors on the postganglionic cholinergic neurons.These findings suggest that bicuculline-sensitive GABA receptors probably coupled to a Cl^– ionophore are present on postganglionic cholinergic neurons and are involved in the increase of ACh release in guinea pig gallbladder.This paper is part of a dissertation submitted by N. Saito to Kobe University School of Medicine, for the requirement of Doctor of Philosophy     

Effects of neuroleptics on release of 3H-dopamine from slices of rat corpus striatum

Summary The characteristics of ³H-DA release from striatal slices by electrical stimulation were analyzed and the effects of a number of neuroleptics thereon were examined under different experimental conditions. The butyrophenones, haloperidol and spiroperidol, already at low concentrations (0.1–1 M) increased basal tritium efflux in a dose-dependent manner. The phenothiazines, chlorpromazine and fluphenazine, were much less effective in this respect.The butyrophenones strongly inhibited the electrically stimulated overflow of both ³H-DA and ¹⁴C-GABA, while the phenothiazines again had little effect. The action of 1 M haloperidol on ³H-DA release could be blocked by 10 M cocaine, but not with 1 M apomorphine. Apomorphine itself had no significant effect on ³H-DA release.Our data do not support the suggestion that presynaptic DA receptors on dopaminergic nerve terminals may modulate the release of newly taken-up ³H-DA. Some neuroleptics, particularly the butyrophenones may have presynaptic effects not related to interaction with DA receptors. It is suggested that different mechanisms may be involved in the local presynaptic receptor-mediated feedback regulation of transmitter release in noradrenergic and dopaminergic systems in the CNS.     

Lamina-specific differences in GABA(B) autoreceptor-mediated regulation of spontaneous GABA release in rat entorhinal cortex

Spontaneous synaptic inhibition plays an important role in regulating the excitability of cortical networks. Here we have investigated the role of GABA(B) autoreceptors in regulating spontaneous GABA release in the entorhinal cortex (EC), a region associated with temporal lobe epilepsies. We have previously shown that the level of spontaneous inhibition in superficial layers of the EC is much greater than that seen in deeper layers. In the present study, using intracellular and whole cell patch clamp recordings in rat EC slices, we have demonstrated that evoked GABA responses are controlled by feedback inhibition via GABA(B) autoreceptors. Furthermore, recordings of spontaneous, activity-independent inhibitory postsynaptic currents in layer II and layer V neurones showed that the GABA(B) receptor agonist, baclofen, reduced the frequency of GABA-mediated currents indicating the presence of presynaptic GABA(B) receptors in both layers. Application of the antagonist, CGP55845, blocked the effects of baclofen and also increased the frequency of GABA-mediated events above baseline, but the latter effect was restricted to layer V. This demonstrates that GABA(B) autoreceptors are tonically activated by synaptically released GABA in layer V, and this may partly explain the lower level of spontaneous GABA release in the deep layer.     

Presynaptic muscarinic receptor subtypes involved in the inhibition of acetylcholine and noradrenaline release in bovine cerebral arteries

Summary Experiments were performed in bovine cerebral arteries preincubated with [³H]-choline or [³H]-noradrenaline to analyze the presynaptic muscarinic receptors involved in inhibition of acetylcholine and noradrenaline release induced by electrical stimulation (4 Hz, 200 mA, 0.3 ms, 1 min). For this purpose, the actions of several muscarinic receptor antagonists on the ³H overflow and on the carbacol-induced inhibition of this overflow were assessed. The evoked [³H]-acetylcholine release and [³H]-noradrenaline release were markedly reduced by the presence of tetrodotoxin, Ca²⁺-free medium, and the inhibitor of both choline transport and choline acetyltransferase, AF64A. Chemical sympathetic denervation with 6-hydroxydopamine (6-OHDA) decreased the uptake of[³H]-noradrenaline, and AF64A reduced mainly the uptake of [³H]-choline, but also of [³H]-noradrenaline. Carbachol reduced the evoked [³H]-noradrenaline and [³H]-acetylcholine release; the IC₅₀ values were 0.37 and 0.43 mol/l, respectively.Atropine and 4-DAMP, but not AF DX 116, methoctramine or pirenzepine, increased the evoked [³H]-acetylcholine release. However, these muscarinic antagonists failed to modify the evoked [³H]-noradrenaline release. Carbachol inhibited the release of both acetylcholine and noradrenaline. The inhibition was blocked by the antagonists. The rank orders of potency (based on plC₅₀ values) were, in the case of [³H]-acetylcholine release, atropine > 4-DAMP >AF-DX 116 >- pirenzepine >- methoctramine, and, in the case of [³H]-noradrenaline release, atropine > 4-DAMP > AF-DX 116 >- methoctramine >-pirenzepine. These results suggest (1) that the prosynaptic receptors that modulate endogenous acetylcholine release are likely of the M₃ subtype, whilst those involved on the effect of the exogenous agonist Carbachol are of M₂ subtype, and (2) that those which inhibit noradrenaline release are probably a mixture of M₂ and M₃ subtypes as well. The autoinhibition of the acetylcholine release was funtionally active under our experimental conditions, while noradrenaline release does not appear to be modulated by muscarinic receptors in physiological conditions.Send offprint requests to G. Balfagón at the above address     

Inhibitory effects of GABA,L-glutamic acid and nicotine on the potassium-evoked release of subsaance P in substantia nigra slices of the rat

Rat substantia nigra slices were superfused with a physiological medium containing a diluted substance P (SP) antiserum, bacitracin and serum albumin to measure SP released in superfusates. As shown by measuring the degradation of a SP-labelled derivative incubated with cerebellar slices, this medium prevented the enzymatic inactivation of SP. Potassium (K⁺, 50 mM) and veratridine (5 × 10^?5M) stimulated SP release and these effects were respectively prevented in absence of calcium and in presence of tetrodotoxin (5 × 10^?7 M). GABA (5 × 10^?5 M) nicotine (10^?6 M) and L-glutamic acid (5 × 10^?5 M) reduced the K⁺ (50 mM)-evoked release of SP. In contrast, glycine (5 × 10^?5 M), oxotremorine (5 × 10^?5 M), D-glutamic acid (5 × 10^?5 M) and serotonin (5 × 10^?5 M) were without effect. Pempidine (10^? M) prevented the inhibitory effect of nicotine (10^? M) on the K⁺-evoked release of SP. Glutamic acid diethyl ester (10^?4 M) completely abolished the L-glutamic acid-induced inhibition of the K⁺-evoked release of SP. Picrotoxin (5 × 10^?5 M) did not influence the L-glutamic acid inhibitory effect excluding the intervention of GABAergic mechanisms.     

Effects of nefiracetam on spatial memory function and acetylcholine and GABA metabolism in microsphere-embolized rats

The present study aimed to determine whether nefiracetam, N-(2,6-dimethylphenyl)-2-(2-oxo-1-pyrrolidinyl) acetamide, a cognition enhancer, has an effect on learning and memory function in sustained cerebral ischemia, and whether the effect, if any, may accompany modification of the cholinergic or gamma-aminobutyric acid (GABA)ergic system, which are conceived to be involved in the learning and memory function, in the ischemic brain. Sustained cerebral ischemia was induced by the injection of 700 microspheres into the right hemisphere of the rat. The animals were treated once daily with 10 mg/kg nefiracetam p.o. from 15 h after the operation to either 10 days for the water maze study, or 3 or 5 days after the operation for neurochemical examination. Microsphere-embolized rats showed stroke-like symptoms 15 h after the operation and lengthened the escape latency in the water maze task on days 7-10, suggesting a spatial learning dysfunction. The delayed treatment did not reduce the stroke-like symptoms, but effectively shortened the escape latency. The animals at days 3 and 5 after the operation showed decreases in acetylcholine content and choline acetyltransferase activity, which were not prevented by nefiracetam. The microsphere-embolized rats showed decreases in GABA content and glutamic acid decarboxylase activity. The delayed treatment appreciably restored GABA content in the hippocampus on day 5 and reversed glutamic acid decarboxylase activity in both brain regions on day 5. These results suggest that the GABAergic activity rather than the cholinergic activity may be, at least in part, involved in the pharmacological effects of nefiracetam in the ischemic brain.     

The metabotropic glutamate receptor 1 is not involved in the facilitation of glutamate release in cerebrocortical nerve terminals

In this study we have addressed the identification of the metabotropic glutamate receptor (mGluR) involved in the facilitation of glutamate release in nerve terminals from the cerebral cortex. mGluR1 and 5 are coupled to phosphoinositide hydrolysis and the activation of these receptors with the specific agonist 3,5-dihydroxyphenylglycine (DHPG) enhances the release of glutamate. We have examined whether mGluR1 is responsible for this modulatory effect by preparing nerve terminals from mGluR1 deficient mice. The Ca²⁺-dependent glutamate release evoked by a submaximal depolarization is enhanced by the agonist DHPG in nerve terminals from both wild and mutant mice. This result is consistent with the finding that the mGluR agonist also induces a similar increase in the levels of diacylglycerol (DAG) in the nerve terminals from wild and mutant mice. Moreover, the activity-dependent switch from facilitation to inhibition of release, observed when a second stimulation of the receptor is applied shortly after (5 min) the first pulse, was also observed in the mutant mice. These results indicate therefore, that the facilitation of glutamate release is unlikely to be due to the activation of mGluR1 but related to another phosphoinositide coupled mGluR.     

Adenosine A2A receptor stimulation increases release of acetylcholine from rat hippocampus but not striatum,and does not affect catecholamine release

Rat striatal and hippocampal slices, preincubated with [³H] dopamine (DA) {or [³H] noradrenaline (NA)} and [¹⁴C] choline, were superfused continuously and stimulated electrically. 2-chloroadenosine (2-CADO 0.001–100 μM), a non-selective adenosine receptor agonist, produced a concentration-dependent inhibition of the electrically evoked DA and acetylcholine (ACh) release from the striatal slices and of the electrically evoked NA and ACh release from the hippocampal slices. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 3, 30 and 200 nM), a selective adenosine A₁ receptor antagonist, caused a concentration-dependent, parallel, rightward shift of the 2-CADO concentration-response curve, consistent with competitive antagonism. The pA₂ values ranged between 8.4 and 8.8. In the case of ACh release from the hippocampus, but in no other case, was there an increase in release of radioactivity at low concentrations of 2-CADO in the presence of DPCPX. The stimulation in the hippocampus could be blocked by a selective adenosine A_2A receptor antagonist KF 17837. By itself KF 17837 (0.1–100 μM) had no effect on electrically evoked NA release from hippocampal slices, but decreased electrically evoked ACh release. This inhibition was counteracted by DPCPX (1 μM). These results show that, under the conditions used, DA release in the striatum, and NA release in the hippocampus, as well as ACh release from the striatum are regulated by adenosine A₁ but not by adenosine A_2A receptors. By contrast, ACh release from the hippocampus is tonically regulated both by adenosine A₁ receptors, which inhibit release, and by adenosine A_2A receptors which stimulate release. Received: 25 April 1996 / Accepted: 30 August 1996     

Inhibition of benzodiazepine and GABA receptor binding by amino-gamma-carbolines and other amino acid pyrolysate mutagens

The effect of several pyrolysate mutagens on the benzodiazepine and GABA receptors was investigated. Of amino-gamma-carbolines, Trp-P-1 antagonized the suppressive effect of diazepam on the pentylenetetrazol-induced convulsions and death, whereas Trp-P-2 by itself precipitated seizures and death in male mice. Both Trp-P-1 and Trp-P-2 inhibited the specific binding of [3H]diazepam and [3H]muscimol in rat brain membranes mainly by increasing Kd, indicating that these gamma-carbolines bind on benzodiazepine and GABA receptors. IC50S of Trp-P-1 and Trp-P-2 on specific [3H]flunitrazepam binding were not changed by addition of GABA. The Hill coefficient of Trp-P-1 for displacing [3H]diazepam binding was about unity whereas that of Trp-P-2 was less than unity. These results suggest that Trp-P-1 and Trp-P-2 act as active antagonists or inverse agonists at benzodiazepine receptors. The convulsant effect of the gamma-carbolines may be mediated by an action on the central benzodiazepine receptors; however, the role of the effect on GABA receptors is not clear.     

Beware the builders: Construction noise changes [C]GABA release and uptake from amygdaloid and hippocampal slices in the rat

The effects of exposure to chronic noise and vibration, produced by construction work, on the release and uptake of [³H]5-hydroxytryptamine ([³H]5-HT) and [¹⁴C]γ-aminobutyric acid ([¹⁴C]GABA) from rat amygdaloid and hippocampal slices were investigated. Noise-exposure resulted in increased release, with no significant change in uptake, of [¹⁴C]GABA from amygdaloid slices. In hippocampal slices, [¹⁴C]GABA release was also increased, but the changes in release were dependent upon the marked decrease in uptake of [¹⁴C]GABA into these slices. There was an increase in peak K⁺-evoked release of [³H]5-HT from hippocampal slices, but no other changes in [³H]5-HT release or uptake in either region were observed following noise-exposure. These findings may have important practical implications for the research carried out in laboratories exposed to construction noise and vibrations.     

Inhibition by suramin and reactive blue 2 of GABA and glutamate receptor channels in rat hippocampal neurons

Effects of suramin and reactive blue 2 (RB2), compounds known as antagonists at P₂-purinoceptors, on ionic currents mediated through GABA and glutamate receptor channels were investigated in rat hippocampal neurons. Under whole-cell voltage-clamp, suramin (30 to 300 M) and RB2 (10 to 100 M) inhibited a current activated by 10 M GABA in a concentration-dependent manner. Suramin (100 and 300 M) and R132 (10 and 30 M) also inhibited an inward current activated by kainic acid (100 M), an agonist at non-NMDA type glutamate receptor channels, and an inward current activated by N-methyl-D-aspartate (NMDA; 100 M), an agonist at NMDA type glutamate receptor channels. The inhibition by suramin or RB2 did not exhibit voltage-dependence between –30 and –90 mV in the case of the GABA- or the kainate-evoked current. In contrast, the inhibition by these compounds of the NMDA-evoked current exhibited voltage-dependence and was enhanced by hyperpolarization. When the concentration of agonists was increased by 5- or 10-fold, the magnitude of the inhibition by suramin of the kainate-evoked current and the magnitude of the inhibition by R132 of the NMDA-evoked current were attenuated. ,-Methylene ATP (100 M) did not affect the GABA-, kainate- or NMDA-activated current. The results suggest that suramin and RB 2 inhibit GABA receptor channels and glutamate receptor channels. The blockade of these channels must be taken into account when these compounds are used as pharmacological tools to examine an involvement of P₂-purinoceptors, especially in preparations where GABAergic or glutamatergic neurotransmission is expected.     

Pharmacologically distinct GABAB receptors that mediate inhibition of GABA and glutamate release in human neocortex

1. The release of endogenous γ-aminobutyric acid (GABA) and glutamic acid in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply located tumours.
2. The basal outflows of GABA and glutamate from superfused synaptosomes were largely increased during depolarization with 15 mM KCl. The K⁺-evoked overflows of both amino acids were almost totally dependent on the presence of Ca²⁺ in the superfusion medium.
3. The GABA_B receptor agonist (−)-baclofen (1, 3 or 10 μM) inhibited the overflows of GABA and glutamate in a concentration-dependent manner. The inhibition caused by 10 μM of the agonist ranged from 45–50%.
4. The effect of three selective GABA_B receptor antagonists on the inhibition of the K⁺-evoked GABA and glutamate overflows elicited by 10 μM (−)-baclofen was investigated. Phaclofen antagonized (by about 50% at 100 μM; almost totally at 300 μM) the effect of (−)-baclofen on GABA overflow but did not modify the inhibition of glutamate release. The effect of (−)-baclofen on the K⁺-evoked GABA overflow was unaffected by 3-amino-propyl (diethoxymethyl)phosphinic acid (CGP 35348; 10 or 100 μM); however, CGP 35348 (10 or 100 μM) antagonized (−)-baclofen (complete blockade at 100 μM) at the heteroreceptors on glutamatergic terminals. Finally, [3-[[(3,4-dichlorophenyl) methyl]amino]propyl] (diethoxymethyl) phosphinic aid (CGP 52432), 1 μM, blocked the GABA_B autoreceptor, but was ineffective at the heteroreceptors. The selectivity of CGP 52432 was lost at 30 μM, as the compound, at this concentration, inhibited completely the (−)-baclofen effect both on GABA and glutamate release.
5. It is concluded that GABA and glutamate release evoked by depolarization of human neocortex nerve terminals can be affected differentially through pharmacologically distinct GABA_B receptors.

     

Noradrenaline inhibits central serotonin release through alpha2-adrenoceptors located on serotonergic nerve terminals

Summary The effect of noradrenaline on the depolarization-evoked release of ³H-5-hydroxytryptamine was investigated in superfused synaptosomes prepared from rat cortex and hippocampus and prelabelled with the radioactive indoleamine. Noradrenaline reduced in a concentration-dependent way the release of ³H-5-hydroxytryptamine elicited by 15 mM KCl. The inhibition was counteracted by the alpha-adrenoceptor antagonists phentolamine or yohimbine, but not by prazosin. The results indicate that, in rat brain, the inhibition of 5-hydroxytryptamine release by noradrenaline is mediated by adrenoceptors of the alpha₂-type localized on the terminal serotonergic fibres.     

A cumulative dose-response technique for the characterization of presynaptic receptors modulating [3H]noradrenaline release from rat brain slices

A cumulative dose-response technique was developed for the characterization of presynaptic receptors involved in the modulation of [³H]noradrenaline (NA) release from rat hippocampus slices, using continuous K⁺ (20 mM) depolarization. The results obtained with this technique were compared with those obtained using a repetitive K⁺ stimulation procedure. The release of [³H]NA induced by continuous K⁺ stimulation as well as that caused by repetitive K⁺ stimulation was strongly Ca²⁺-dependent and consisted for more than 90% of unmetabolized [³H]NA. Using continuous K⁺ stimulation it was demonstrated that the presynaptic inhibition of ³H-NA release by exogenous NA reached a maximum 10 min after addition of NA. The inhibitory effect of NA appeared to be independent of the time of addition, suggesting that the sensitivity of the presynaptic α-adrenoceptors remained unchanged during the experiment. Cumulative dose-response curves were recorded by the successive addition, at 10 min intervals, of increasing concentrations of NA. It was shown that continuous stimulation and repetitive K⁺ stimulation were basically similar with regard to the characteristics of the resulting [³H]NA release as well as its presynaptic α-adrenoceptor-mediated modulation by exogeneous NA. However, the cumulative dose-response technique, which can be carried out only using continuous K⁺ stimulation, makes it possible to determine more rapidly and also more accurately the apparent affinities and intrinsic activities of drugs towards receptors involved in the modulation of neurotransmitter release from brain slices.     

Characterization of the interaction between a novel convulsant agent,norbiphen, and GABA(A) and other ligand-gated ion channels

A hybrid molecule composed of the antimicrobial, norfloxacin, linked to the non-steroidal anti-inflammatory drug (NSAID), biphenylacetic acid, which we have termed norbiphen, is a lethal convulsant in vivo and an antagonist of rodent GABA(A) receptors in vitro. In the present study, the selectivity, molecular site(s) and mechanism of action of this novel convulsant were investigated using electrophysiological techniques. Sub-maximal GABA-evoked currents recorded from rodent hippocampal neurons were reversibly inhibited by norbiphen (1 microM) to 5+/-2% of control whereas glutamate, NMDA and glycine activated responses were little or unaffected. Sub-maximal GABA-evoked currents recorded from oocytes expressing recombinant human alpha1beta2gamma2s or alpha1beta2 GABA(A) receptors were also reversibly inhibited by norbiphen (1-1000 nM) with an IC(50) (+/-s.e.m.) of 5.7+/-1 and 8.8+/-1 nM, respectively. Similarly, GABA currents recorded from alpha1beta1gamma2s, alpha1beta1 and beta2gamma2s receptors were inhibited with IC(50)s of 16.1+/-1, 18.8+/-1 and 4.2+/-1 nM, respectively. In contrast, norbiphen (100 nM) had little or no effect at rho1 GABA(C) homomers. At alpha1beta2gamma2s receptors, norbiphen had no affect on the GABA reversal potential, and inhibition was not voltage-dependent, suggesting that this compound does not act at the ion channel. The GABA concentration response curve was shifted in a competitive-like fashion by norbiphen (10-300 nM) and a Schild analysis of these data yielded a slope of 0.94+/-0.1 and a pA(2) of 7.77. Our data reveal a novel, selective and highly potent antagonist of GABA(A) receptors. Norbiphen should be a valuable agent in future studies of this receptor complex.     

Comparison of anaesthetic and non-anaesthetic effects on depolarization-evoked glutamate and GABA release from mouse cerebrocortical slices

1. Investigation with substances that are similar in structure, but different in anaesthetic properties, may lead to further understanding of the mechanisms of general anaesthesia.
2. We have studied the effects of two cyclobutane derivatives, the anaesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F3), and the non-anaesthetic, 1,2-dichlorohexafluorocyclobutane (F6), on K⁺-evoked glutamate and γ-aminobutyric acid (GABA) release from isolated, superfused, cerebrocortical slices from mice, by use of h.p.l.c. with fluorescence detection for quantitative analysis.
3. At clinically relevant concentrations, the anaesthetic, F3, inhibited 40 mM K⁺-evoked glutamate and GABA release by 72% and 47%, respectively, whereas the structurally similar non-anaesthetic, F6, suppressed evoked glutamate release by 70% but had no significant effects on evoked GABA release. A second exposure to 40 mM KCl after a ∼30 min washout of F3 or F6 showed recovery of K⁺-evoked release, suggesting that F3 and F6 did not cause any non-specific or irreversible changes in the brain slices.
4. Our findings suggest that suppression of excitatory neurotransmitter release may not be directly relevant to the primary action of general anaesthetics. A mechanism involving inhibitory postsynaptic action is implicated, in which a moderate suppression of depolarization-evoked GABA release by the anaesthetic may be consistent with the enhancement of postsynaptic GABAergic activities.

     

GABA and baclofen potentiate the K+-evoked release of methionine-enkephalin from rat striatal slices

GABA potentiates the potassium-evoked release of methionine-enkephalin (ME) from slices of rat corpus striatum. This potentiation is observed only when a submaximal concentration (30 mM) of K+ is used to evoke release. The effect of GABA is dose-dependent between 100 and 100 micrometers. The basal release of ME is not altered by these concentrations of GABA. Before, but not muscimol, mimics the effect of GABA on the evoked release of ME. This effect is not stereoselective as both the (+)- and (-)-isomers of baclofen enhance ME release. Picrotoxin (100 micrometers) blocks the enhancement of ME release produced by both GABA and baclofen. Bicuculline methiodide (100 micrometers) does not block the effect of GABA. The effect of GABA on ME release may be mediated by an atypical GABA receptor which is activated by baclofen.     

A steroid derivative,R 5135, antagonizes the GABA/benzodiazepine receptor interaction

A novel steroid derivative, R 5135 (3α-hydroxy-16-imino-5β-17-aza-androstan-11-one) showed high affinity for both the GABA and glycine receptors in vitro. It also displaced [³H]diazepam from the benzodiazepine receptor in a rat cortex membrane preparation, but in this case a plateau occurred in the displacement curve at a concentration of R 5135 between 10^?7 and 10^?6 M, where binding was decreased by about 50 %. The “high affinity” component of R 5135 inhibition no longer appeared when the endogenous GABA concentration was reduced by extensive washing of the membrane preparation and it reappeared when GABA was re-introduced. Thus R 5135 behaves as a powerful antagonist of the GABA stimulation of [³H]diazepam binding, being 500 times more active than the GABA-antagonist bicuculline. The dual interaction between R 5135 and GABA and glycine receptors suggests that these may share some common structural feature or that they have overlapping specificity.