Enhanced GABA release in cerebral cortical slices derived from rats with thioacetamide-induced hepatic encephalopathy

The release of newly loaded [³H]GABA was studied in slices of different brain regions derived from rats in which acute hepatic encephalopathy (HE) was induced with a hepatotoxin thioacetamide. HE increased both spontaneous and high (50 mM) ammonium chloride-evoked GABA release in cerebral cortical slices by 38% and 50%, respectively. No effects of HE were noted in cerebellar or striatal slices. An increased release of GABA in the cerebral cortex may contribute to the endogenous benzodiazepine-mediated enhancement of GABAergic tone, which is thought to be partly responsible for the pathophysiological mechanism of HE.     

Increased GABA Release in the Human Brain Cortex as a Potential Pathogenetic Basis of Hyperosmolar Diabetic Coma

Human cerebral cortical slices preincubated with [³H]GABA, [³H]noradrenaline, or 5-[³H]hydroxytryptamine and superfused with Krebs solution or Mg²⁺-free Krebs solution were used to investigate the influence of increased D-glucose concentrations on the release of these [³H]-neurotransmitters evoked by high K⁺ content or NMDA receptor activation, respectively. An increase in level of D-glucose (normal content, 11.1 mM) by 32, 60, and/or 100 mM (a range characteristic for hyperosmolar diabetic coma) increased the [³H]GABA release and inhibited the [³H]noradrenaline release evoked by both methods of stimulation. The K⁺-induced 5-[³H]hydroxytryptamine release was also inhibited by high D-glucose content. Blockade of GABA_B receptors by p-(3-aminopropyl)-p-diethoxymethylphosphinic acid (CGP 35348) attenuated the inhibitory effect of high D-glucose content on the K⁺-evoked release of [³H]noradrenaline and 5-[³H]hydroxy-tryptamine, suggesting that the effect on monoamine release is, at least to a major part, the result of the increased GABA release and, as a consequence, of an increased GABA concentration at inhibitory GABA_B receptors. The membrane-impermeable sorbitol mimicked the increasing effect of D-glucose on [³H]GABA release and its inhibitory effect on 5-[³H]hydroxytryptamine release. However, dimethyl sulfoxide, which is known to permeate rapidly through biological membranes, had no effect at concentrations equiosmolar to D-glucose. It is concluded that a reduction in brain cell volume caused by increased extracellular, compared with cytoplasmic, osmolarity is crucial for the changes in neuronal function observed at high D- glucose and sorbitol content, In view of the fact that GABA is the main inhibitory neurotransmitter in the brain, the increased GABA release may be assumed to contribute to the pathogenesis of hyperosmolar diabetic coma.     

Characteristics of GABA Release in Mouse Brain Stem Slices under Normal and Ischemic Conditions

GABA is known to be the inhibitory neurotransmitter in the majority of brain stem nuclei. The release of GABA has been extensively studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release in the brain stem have not been systemically characterized. The properties of preloaded [³H]GABA were now investigated in mouse brain stem slices, using a superfusion system. The basal release was enhanced by K⁺ stimulation (50 mM K⁺) and under various cell-damaging conditions (ischemia, hypoglycemia, the presence of free radicals and metabolic poisons). No K⁺-stimulated release was discernible in the absence of Ca²⁺, indicating that the release was at least partly Ca²⁺-dependent. Moreover, the release was increased when Na⁺ or Cl⁻ was omitted from the superfusion medium. GABA and β-alanine stimulated the release, confirming the involvement of the reversed function of GABA transporters. Incubation of the slices with the anion channel inhibitors diisothiocyanostilbene and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonate and with the Cl⁻ uptake inhibitor 9-anthracenecarboxylic acid also reduced GABA release, demonstrating that a part of it comprises leakage through anion channels. All these mechanisms were involved in the ischemia-induced GABA release, which was over 4-fold greater than the release in normoxia. Contrary to the other brain areas, GABA release in the brain stem was not affected by ionotropic glutamate receptors but may be modulated by metabotropic receptors. This ischemia-induced GABA release might constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions. Special issue dedicated to Dr. Simo S. Oja     

H3-Receptors Control Histamine Release in Human Brain

The regulation of histamine release was studied on slices prepared from pieces of human cerebral cortex removed during neurosurgery and labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration induced [3H]histamine release, although to a lesser extent than from rat brain slices. Exogenous histamine reduced by up to 60% the K+-evoked release, with an EC50 of 3.5 +/- 0.5 X 10(-8) M. The H3-receptor antagonists impromidine and thioperamide reversed the histamine effect in an apparently competitive manner and enhanced the K+-evoked release, indicating a participation of endogenous histamine in the release control process. The potencies of histamine and the H3-receptor antagonists were similar to those of these agents at presynaptic H3-autoreceptors controlling [3H]histamine release from rat brain slices. It is concluded that H3-receptors control histamine release in the human brain.     

Quantum Properties of GABA Release in Cultured Neurons from the Rat Cortex

We analyzed the properties of inhibitory synaptic transmission between neurons in low-density cultures of cortical cells. Miniature, spontaneous, and evoked inhibitory postsynaptic currents were studied using a whole-cell path-clamp technique at a holding potential of -80 mV. These postsynaptic currents were identified as GABA release-activated Cl^- currents mediated by GABA_A receptors. Fitting amplitude histograms of these currents with Gaussian curves and an autocorrelation technique revealed the presence of equidistant peaks corresponding to a mean quantum size of 10 pA.     

Modulatory Action of Taurine on the Release of GABA in Cerebellar Slices of the Guinea Pig

Abstract: For the purpose of demonstrating the action of taurine as a neuromodulator in addition to its suggested neurotransmitter function, the effects of taurine and muscimol on the depolarization-induced Ca-dependent release of [³H]γ-aminobutyric acid (pH]GABA) and l -[³H]glutamate in cerebellar slices from guinea pigs were investigated. The release of [³H]GABA was found to be greatly decreased by a GABA agonist, muscimol, and by taurine, but not by glycine. The release of l -[³H]glutamate was little affected by taurine. The release of [³H]GABA was enhanced by bicuculline and strychnine, but not by picrotoxin, and the suppressive action of muscimol on the GABA release was antagonized by bicuculline, picrotoxin, and strychnine, suggesting the possible existence of presynaptic autoreceptors for GABA in the cerebellum. The suppressive action of taurine on the release of [³H]GABA, on the other hand, was blocked only by bicuculline. These results suggest that taurine reduced the release of [³H]GABA from cerebellar slices by acting on the GABA autoreceptors or, more likely, on other types of receptors that are sensitive to bicuculline. As a possible mechanism for this modulatory action of taurine, the blockade by this amino acid of the influx of Ca²⁺ into cerebellar tissues was tentatively suggested.     

GAT-1 regulates both tonic and phasic GABA(A) receptor-mediated inhibition in the cerebral cortex

γ-Aminobutyric acid 1 (GAT-1) is the most copiously expressed GABA transporter; we studied its role in phasic and tonic inhibition in the neocortex using GAT-1 knockout (KO) mice. Immunoblotting and immunocytochemical studies showed that GAT-2 and GAT-3 levels in KOs were unchanged and that GAT-3 was not redistributed in KOs. Moreover, the expression of GAD65/67 was increased, whereas that of GABA or VGAT was unchanged. Microdialysis studies showed that in KOs spontaneous extracellular release of GABA and glutamate was comparable in WT and KO mice, whereas KCl-evoked output of GABA, but not of glutamate, was significantly increased in KOs. Recordings from layer II/III pyramids revealed a significant increase in GABA_AR-mediated tonic conductance in KO mice. The frequency, amplitude and kinetics of spontaneous inhibitory post-synaptic currents (IPSCs) were unchanged, whereas the decay time of evoked IPSCs was significantly prolonged in KO mice. In KO mice, high frequency stimulation of GABAergic terminals induced large GABA_AR-mediated inward currents associated with a reduction in amplitude and decay time of IPSCs evoked immediately after the train. The recovery process was slower in KO than in WT mice. These studies show that in the cerebral cortex of GAT-1 KO mice GAT-3 is not redistributed and GADs are adaptively changed and indicate that GAT-1 has a prominent role in both tonic and phasic GABA_AR-mediated inhibition, in particular during sustained neuronal activity.     

Developmental changes in high-affinity uptake of GABA by cultured neurons

High-affinity uptake of [³H]-aminobutyric acid (GABA) was studied in cultures of neonatal rat cortical neurons grown on pre-formed monolayers of non-neuronal (glial) cells. Both the maximum rate (V _max) and, to a smaller extent, theK _m of [³H]GABA uptake increased with time. In addition, in parallel with these changes, 2,4-diaminobutyric acid and cis-3-aminocyclohexane-1-carboxylic acid (ACHC), compounds which are considered typical substrate/inhibitors of GABA uptake in neurons, became progressively stronger inhibitors of [³H]GABA uptake. Consequently, the present results may mean that the studies using uptake, of [³H]GABA, [³H]ACHC, or [³H]DABA as a specific marker for GABAergic neurons differentiating during the ontogenetic development of the central nervous system may have to be interpreted with caution.     

Effect of α-Amino-4-Phosphonobutyrate on the Release of Endogenous Glutamate and Aspartate from Cortical Synaptosomes of Epileptic Rats

The effect of the glutamate antagonist alpha-amino-4-phosphonobutyrate (APBA) on the release of endogenous amino acids from sensorimotor cortical synaptosomes of rats with a cortical cobalt focus and from non-epileptic rats was studied: (1) The release of endogenous glutamate, aspartate, and gamma-aminobutyric acid (GABA) from synaptosomal preparations of cobalt-induced epileptogenic tissues was increased compared with the release from the contralateral (sensorimotor) region or the sensorimotor cortex of normal animals. The intrasynaptosomal content of these amino acids was reduced in proportion to the amount released. The levels of other amino acids were unaffected or showed much smaller changes. (2) APBA (0.5-1 mM) decreased significantly the spontaneous release of aspartate and glutamate from the epileptic foci without affecting GABA or any other amino acid. (3) APBA produced no effect whatsoever on the release of any amino acid from synaptosomal preparations of nonepileptic focus.     

Modulation of GABA Release by Second Messenger Substances and NO in Mouse Brain Stem Slices Under Normal and Ischemic Conditions

GABA is the inhibitory neurotransmitter in most brain stem nuclei. The properties of release of preloaded [³H]GABA were now investigated with slices from the mouse brain stem under normal and ischemic (oxygen and glucose deprivation) conditions, using a superfusion system. The ischemic GABA release increased about fourfold in comparison with normal conditions. The tyrosine kinase inhibitor genistein had no effect on GABA release, while the phospholipase inhibitor quinacrine reduced both the basal and K⁺-evoked release in normoxia and ischemia. The activator of protein kinase C (PKC) 4β-phorbol 12-myristate 13-acetate had no effects on the releases, whereas the PKC inhibitor chelerythrine reduced the basal release in ischemia. When the cyclic guanosine monophosphate (cGMP) levels were increased by superfusion with zaprinast and other phosphodiesterase inhibitors, GABA release was reduced under normal conditions. The NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA) enhanced the basal and K⁺-stimulated release by acting directly on presynaptic terminals. Under ischemic conditions GABA release was enhanced when cGMP levels were increased by zaprinast. This effect was confirmed by inhibition of the release by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The NO-producing agents SNAP, HA, and sodium nitroprusside potentiated GABA release in ischemia. These effects were reduced by the NO synthase inhibitor N^G-nitro-l-arginine, but not by ODQ. The results show that particularly NO and cGMP regulate both normal and ischemic GABA release in the brain stem. Their effects are however complex.     

GABA Release Modified by Adenosine Receptors in Mouse Hippocampal Slices under Normal and Ischemic Conditions

The excitatory glutamatergic neurons in the hippocampus are modulated by inhibitory GABA-releasing interneurons. The neuromodulator adenosine is known to inhibit the presynaptic release of neurotransmitters and to hyperpolarize postsynaptic neurons in the hippocampus, which would imply that it is an endogenous protective agent against cerebral ischemia and excitotoxic neuronal damage. Interactions of the GABAergic and adenosinergic systems in regulating neuronal excitability in the hippocampus is of crucial importance, particularly under cell-damaging conditions. We now characterized the effects of adenosine receptor agonists and antagonists on the release of preloaded [³H]GABA from hippocampal slices prepared from adult (3-month-old) mice, using a superfusion system. The effects were tested both under normal conditions and in ischemia induced by omitting glucose and oxygen from the superfusion medium. Basal and K⁺-evoked GABA release in the hippocampus were depressed by adenosinergic compounds. Under normal conditions activation of both adenosine A₁ and A_2A receptors by the agonists R(-)N⁶-(2-phenylisopropyl)adenosine and CGS 21680 inhibited the K⁺-evoked release, which effects were blocked by their specific antagonists, 8-cyclopentyl-1,3-dipropyl-xanthine and 3,7-dimethyl-1-propargylxanthine, respectively. Under ischemic conditions the release of both GABA and adenosine is markedly enhanced. The above receptor agonists then depressed both the basal and K⁺-evoked GABA release, only the action of A_2A receptors being however receptor-mediated. The demonstrated depression of GABA release by adenosine in the hippocampus could be deleterious to neurons and contribute to excitotoxicity.     

Ubiquitin-Mediated Stress Response in a Rat Model of Brain Transient Ischemia/Hypoxia

Ubiquitin (Ub) is a small 76-residue protein, involved in intracellular protein degradation through a specific ATP-dependent system, which uses Ub as a tag to label proteins committed to be hydrolyzed by a specific 26 S protease. PGP-9.5 is another important component of the Ub system, i.e. a neuron-specific carboxyl-terminal hydrolase, which recycles Ub from Ub-polypeptide complexes. We have investigated the expression of Ub and PGP-9.5 in rat hippocampal neurons in an early phase of reperfusion in a model of transient global brain ischemia/hypoxia (bilateral occlusion of common carotid arteries for 10 min accompanied by mild hypoxia—15% O₂—for 20 min), by means of immunohistochemical methods using light and electron microscopy. The intensity of Ub and PGP-9.5 immunoreactivity was evaluated by image analysis. We have detected a marked increase of Ub immunoreactivity (UIR) in neurons of CA1, CA2, CA3, CA4, and dentate gyrus subfields 1 hr after ischemia/hypoxia (but not after hypoxia only), statistically significant as confirmed by image analysis. Such increase in immunoreactivity in ischemic/hypoxic rats was localized essentially in the nuclei of hippocampal neurons. There were no changes in PGP-9.5 immunoreactivity. The data suggest that in the present model of rat brain ischemia/hypoxia Ub is involved in the neuronal stress response.     

Effect of β-Bungarotoxin on the Release of Endogenous Amino Acids from the Sensorimotor Cortex

Abstract: β-Bungarotoxin, a snake neurotoxin purified from the venom of Bungarus multicinctus , caused a significant increase in the in vivo release of glutamate from the superfused sensorimotor cortex of awake animals. A smaller effect on GABA release was observed, but no change was detected in the release of six other amino acids measured. The effects on glutamate and GABA release were entirely blocked by tetrodotoxin (1 μM) and were reversible when the cortical tissue was washed with saline.     

Regulation of Histamine Release and Synthesis in the Brain by Muscarinic Receptors

The cholinergic modulation of histamine release and synthesis was studied in rat brain slices or synaptosomes labeled with L-[3H]histidine. Carbachol in increasing concentrations progressively reduced the K+-induced [3H]histamine release from cortical slices. Pirenzepine, a preferential M1-receptor antagonist, reversed the carbachol effect in an apparently competitive manner and with Ki values of 1-6 X 10(-8) M. 11-[(2-[(Diethylamino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116), considered a preferential M2-receptor antagonist, reversed the carbachol effect with a mean Ki of approximately 2 X 10(-7) M. Oxotremorine behaved as a partial agonist in the modulation of histamine release. Neostigmine, an acetylcholinesterase inhibitor, inhibited the K+-induced release of [3H]histamine from cortical slices, and the effect was largely reversed by pirenzepine, an observation suggesting a modulation by endogenous acetylcholine. The effects of carbachol and pirenzepine were observed with slices of other brain regions known to contain histaminergic nerve terminals or perikarya, as well as with cortical synaptosomes. The two drugs also modified, in opposite directions, [3H]histamine formation in depolarized cortical slices. In vivo oxotremorine inhibited [3H]histamine formation in cerebral cortex, and this effect was reversed by scopolamine. When administered alone, scopolamine failed to enhance significantly the 3H- labeled amine formation, a finding suggesting that muscarinic receptors are not activated by endogenous acetylcholine released under basal conditions. It is concluded that muscarinic heteroreceptors, directly located on histaminergic nerve terminals, control release and synthesis of histamine in the brain. These receptors apparently belong to the broad M1-receptor category and may correspond to a receptor subclass displaying a rather high affinity for AF-DX 116.     

Effect of Nipecotic Acid, a γ-Aminobutyric Acid Transport Inhibitor, on the Turnover and Release of γ-Aminobutyric Acid in Rat Cortical Slices

Abstract: To see the effect of a γ-aminobutyric acid GABA uptake inhibitor on the efflux and content of endogenous and labeled GABA, rat cortical slices were first labeled with [³H]GABA and then superfused in the absence or presence of 1 mM nipecotic acid. Endogenous GABA released or remaining in the slices was measured with high performance liquid chromatography, which was also used to separate [³H]GABA from its metabolites. In the presence of 3 mM K⁺, nipecotic acid released both endogenous and [³H]GABA, with a specific activity four to five times as high as that present in the slices. The release of labeled metabolite(s) of [³H]GABA was also increased by nipecotic acid. The release of endogenous GABA evoked by 50 mM K⁺ was enhanced fourfold by nipecotic acid but that of [³H]GABA was only doubled when expressed as fractional release. In a medium containing no Ca²⁺ and 10 mM Mg²⁺, the release evoked by 50 mMK⁺ was nearly suppressed in either the absence or the presence of nipecotic acid. In the absence of nipecotic acid electrical stimulation (bursts of 64 Hz) was ineffective in evoking release of either endogenous or [³H]GABA, but in the presence of nipecotic acid it increased the efflux of endogenous GABA threefold, while having much less effect on that of [³H]GABA. Tetrodotoxin (TTX) abolished the effect of electrical stimulation. Both high K⁺ and electrical stimulation increased the amount of endogenous GABA remaining in the slices, and this increase was reduced by omission of Ca²⁺ or by TTX. The results suggest that uptake of GABA released through depolarization is of major importance in removing GABA from extracellular spaces, but the enhancement of spontaneous release by nipecotic acid may involve intracellular heteroexchange. Depolarization in the presence of Ca²⁺ leads to an increased synthesis of GABA, in excess of its release, but the role of this excess GABA remains to be established.     

当归对宫内缺氧新生大鼠大脑皮质神经元与VEGF mRNA表达的影响

目的 探讨宫内缺氧对新生大鼠大脑皮质神经元与VEGF mRNA表达的影响以及当归的调控作用.方法 孕14 d健康SD雌性大鼠15只,随机分为对照组、缺氧组和当归组各5只,于孕14 d开始将当归组与缺氧组孕鼠置于低张氧浓度三气培养箱中,制作胎鼠宫内缺氧模型,此前一小时按8 mL/kg分别给予当归和生理盐水尾静脉注射,对照组不缺氧,余同缺氧组.三组孕鼠分娩当日每窝随机选取新生鼠4只,取脑组织多聚甲醛固定,石蜡包埋切片、NSE mRNA、VEGF mRNA原位杂交,400倍拍照、IPP6.0软件图像分析.结果 缺氧组新生大鼠大脑皮质NSE mRNA阳性细胞数较对照组减少,积分光密度值(IOD)降低(P＜0.05),VEGF mRNA阳性细胞IOD值升高(P＜0.05);当归组新生大鼠大脑皮质NSE mRNA阳性细胞数较缺氧组增多、IOD值增高(P＜0.05),VEGF mRNA阳性细胞IOD值增高(P＜0.05).结论 宫内缺氧可致新生大鼠大脑皮质神经元受损,当归注射液对此损伤有一定保护作用,其机制可能是通过上调大脑皮质VEGF mRNA的表达而使缺氧环境改善.     

Characteristics of GABA Release Induced by Free Radicals in Mouse Hippocampal Slices

The release of the inhibitory neurotransmitter GABA is generally enhanced under potentially cell-damaging conditions. The properties and regulation of preloaded [³H]GABA release from mouse hippocampal slices were now studied in free radical-containing medium in a superfusion system. Free radical production was induced by 0.01% of H₂O₂ in the medium. H₂O₂ markedly potentiated GABA release, which was further enhanced about 1.5-fold by K⁺ stimulation (50 mM). In Ca²⁺-free media this stimulation was not altered, indicating that the release was mostly Ca²⁺-independent. Moreover, omission of Na⁺ increased the release, suggesting that it is mediated by Na⁺-dependent transporters operating outwards, a conception confirmed by the enhancement with GABA homoexchange. Inhibition of the release with the ion channel inhibitors diisothiocyanostilbene-2,2′-disulphonate and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonate indicates that Cl⁻ channels also participate in the process. This release was not modified by the adenosine receptor (A₁ and A_2a) agonists and ionotropic glutamate receptor agonists kainate, N-methy-d-aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, whereas the agonists of metabotropic glutamate receptors of group I [(S)-3,5-dihydroxyphenylglycine] and of group II [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] enhanced it by receptor-mediated mechanisms, the effects being abolished by their respective antagonists. The group III agonist l(+)-2-amino-4-phosphonobutyrate reduced the evoked GABA release, but this was not affected by the antagonist. Furthermore, the release was reduced by activation of protein kinase C by 4β-phorbol 12-myristate 13-acetate and by inhibition of tyrosine kinase by genistein and of phoshoplipase by quinacrine. On the other hand, increasing cGMP levels with the phosphodiesterase inhibitor zaprinast, selective for PDE5, 6 and 9, and NO production with the NO-generating compounds hydroxylamine, sodium nitroprusside and S-nitroso-N-penicillamine enhanced the release. The regulation of GABA release induced by free radical production proved thus to be rather complex. Under potentially cell-damaging conditions, the potentiation of GABA release may be a mechanism to counteract hyperactivity and reduce the effects of excitatory amino acid release. On the other hand, reduction of GABA release could be harmful and contribute to excitotoxic damage and neuronal degeneration.     

Release of Purines, Noradrenaline, and GABA from Rat Hippocampal Slices by Field Stimulation

Labelled adenine, noradrenaline (NA), and gamma-aminobutyric acid (GABA) were taken up by the transversely cut hippocampal slice. [3H]NA and [14C]GABA were retained as such, [3H]- (or [14C]-) adenine mainly as adenine nucleotides. There was a spontaneous overflow of all three types of compounds ranging from 0.1 (GABA) to 0.21 (NA) %/min. The rate of [3H]NA overflow increased rapidly during electrical field stimulation. The release rate was well maintained over a 15-min period. The rate of [14C]GABA release also increased rapidly but it was not maintained over a 15-min period even if uptake and/or metabolism was inhibited by nipecotic acid (1 mM) and aminooxyacetic acid (AOAA, 0.1 mM). The bulk of the purines was released after the stimulation period. For all compounds the amounts released were frequency- and calcium-dependent. At a frequency of 3 Hz a 10 V stimulation was sufficient to cause a maximal [3H]NA release and 20 V to cause maximal [14C]GABA release, but 14C-purine release was increased further by increasing the voltage to 40 V. The evoked purine release was inhibited by a nucleoside uptake inhibitor (dipyridamole). On stimulation of [3H]NA-labelled slices the released radioactivity was composed of greater than 95% unchanged NA. The specific activities of NA in the slice and in the superfusate were practically identical. In [3H]adenine-labelled slices the released radioactivity was composed of adenosine, inosine, and hypoxanthine, but the activity in the slice of ATP, ADP, and AMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Desmethylimipramine Enhances the Release of Endogenous GABA and Other Neuro trans mitter Amino Acids from the Rat Thalamus

The influence of desmethylimipramine (DMI) on the release of endogenous gamma-aminobutyric acid (GABA) and some other amino acids from the rat thalamus was studied with a push-pull perfusion technique. Following HPLC the amino acids were fluorimetrically estimated. Added to the perfusion medium at a concentration of 10 mumol L-1, DMI caused a 5- to 10-fold increase in the release of GABA. Similar effects were found with imipramine, trimeprimine, haloperidol, and propranolol. The elevation of GABA release induced by DMI was Ca dependent. The release of aspartate and glutamate was also increased by DMI, but in contrast to K ions, DMI did not reduce the thalamic output of glutamine.