Veratridine-induced release in vivo and in vitro of amino acids in the rabbit olfactory bulb

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Neuroprotective Mechanism of Taurine due to Up-regulating Calpastatin and Down-regulating Calpain and Caspase-3 during Focal Cerebral Ischemia

AIMS

Taurine as an endogenous substance possesses a number of cytoprotective properties. In the study, we have evaluated the neuroprotective effect of taurine and investigated whether taurine exerted neuroprotection through affecting calpain/calpastatin or caspase-3 actions during focal cerebral ischemia, since calpain and caspase-3 play central roles in ischemic neuronal death.

METHODS

Male Sprague-Dawley rats were subjected to 2 h of middle cerebral artery occlusion (MCAo), and 22 h of reperfusion. Taurine was administrated intravenously 1 h after MCAo. The dose-responses of taurine to MCAo were determined. Next, the effects of taurine on the activities of calpain, calpastatin and caspase-3, the levels of calpastatin, microtubule-associated protein-2 (MAP-2) and alphaII-spectrin, and the apoptotic cell death in penumbra were evaluated.

RESULTS

Taurine reduced neurological deficits and decreased the infarct volume 24 h after MCAo in a dose-dependent manner. Treatment with 50 mg/kg of taurine significantly increased the calpastatin protein levels and activities, and markedly reduced the m-calpain and caspase-3 activities in penumbra 24 h after MCAo, however, it had no significant effect on mu-calpain activity. Moreover, taurine significantly increased the MAP-2 and alphaII-spectrin protein levels, and markedly reduced the ischemia-induced TUNEL staining positive score within penumbra 24 h after MCAo.

CONCLUSIONS

Our data demonstrate the dose-dependent neuroprotection of taurine against transient focal cerebral ischemia, and suggest that one of protective mechanisms of taurine against ischemia may be blocking the m-calpain and caspase-3-mediated apoptotic cell death pathways.

Selective neuroinhibitory effects of taurine in slices of rat main olfactory bulb

Taurine is abundant in the main olfactory bulb, exceeding glutamate and GABA in concentration. In whole-cell patch-clamp recordings in rat olfactory bulb slices, taurine inhibited principal neurons, mitral and tufted cells. In these cells, taurine decreased the input resistance and caused a shift of the membrane potential toward the chloride equilibrium potential. The taurine actions were sustained under the blockade of transmitter release and were reversible and dose-dependent. At a concentration of 5 mM, typically used in this study, taurine showed 90% of its maximal effect. GABA(A) antagonists, bicuculline and picrotoxin, blocked the taurine actions, whereas the glycine receptor antagonist strychnine and GABA(B) antagonists, CGP 55845A and CGP 35348, were ineffective. These findings are consistent with taurine directly activating GABA(A) receptors and inducing chloride conductance. Taurine had no effect on periglomerular and granule interneurons. The subunit composition of GABA(A) receptors in these cells, differing from those in mitral and tufted cells, may account for taurine insensitivity of the interneurons. Taurine suppressed olfactory nerve-evoked monosynaptic responses of mitral and tufted cells while chloride conductance was blocked. This action was mimicked by the GABA(B) agonist baclofen and abolished by CGP 55845A; CGP 35348, which primarily blocks postsynaptic GABA(B) receptors, was ineffective. The taurine effect most likely was due to GABA(B) receptor-mediated inhibition of presynaptic glutamate release. Neither taurine nor baclofen affected responses of periglomerular cells. The lack of a baclofen effect implies that functional GABA(B) receptors are absent from olfactory nerve terminals that contact periglomerular cells. These results indicate that taurine decreases the excitability of mitral and tufted cells and their responses to olfactory nerve stimulation without influencing periglomerular and granule cells. Selective effects of taurine in the olfactory bulb may represent a physiologic mechanism that is involved in the inhibitory shaping of the activation pattern of principal neurons.

In vitro ethanol exposure decreases potassium-stimulated, but not veratridine-stimulated, glutamate release in the guinea pig hippocampus

In this study we determined the effect of in vitro ethanol exposure on stimulated glutamate release in transverse hippocampal slices (400-microm thickness) of the young postnatal guinea pig (PD 12) by using two chemical stimuli with different mechanisms of action. Ethanol (50 mM) decreased K+ (45 mM)-, but not veratridine (10 microM)-, stimulated glutamate release. The study findings demonstrate that in vitro ethanol exposure produces differential inhibition of stimulated glutamate release in the hippocampus, dependent on the stimulating agent.

Recovery of high potassium-evoked dopamine release after depolarization challenge in the striatum of young and old male rats

The aim of this study was to assess the recovery of high potassium-evoked dopamine (DA) release after depolarization challenge in young (3-4 months) and old (21-25 months) male Wistar rats. Recovery of DA release was evaluated by comparison of the peak responses of DA release induced by two serial high potassium stimulations. Concentric microdialysis probes were stereotaxically implanted in the lateral striatum of rats, and microdialysis was commenced 24 h after surgery. Using a low flow rate of perfusion (1 microl/min), all rats received 2 x 20 min infusions of 100 mM potassium solution separated by either 60 or 140 min. No difference in the basal DA concentration or the potassium-evoked DA release or its recovery was seen between the two groups. Our results suggest that the vesicular DA store recovers rapidly after high potassium challenge in both young and old rats.

Brain microdialysis of GABA and glutamate: what does it signify?

Microdialysis has become a frequently used method to study extracellular levels of GABA and glutamate in the central nervous system. However, the fact that the major part of GABA and glutamate as measured by microdialysis does not fulfill the classical criteria for exocytotic release questions the vesicular origin of the amino acids in dialysates. Glial metabolism or reversal of the (re)uptake sites has been suggested to be responsible for the pool of nonexocytotically released amino-acid transmitters that seem to predominate over the neuronal exocytotic pool. The origin of extracellular GABA and glutamate levels and, as a consequence, the implications of changes in these levels upon manipulations are therefore obscure. This review critically analyzes what microdialysis data signify, i.e., whether amino-acid neurotransmitters sampled by microdialysis represent synaptic release, carrier-mediated release, or glial metabolism. The basal levels of GABA and glutamate are virtually tetrodotoxin- and calcium-independent. Given the fact that evidence for nonexocytotic release mediated by reversal of the uptake sites as a release mechanism relevant for normal neurotransmission is so far limited to conditions of "excessive stimulation," basal levels most likely reflect a nonneuronal pool of amino acids. Extracellular GABA and glutamate concentrations can be enhanced by a wide variety of pharmacological and physiological manipulations. However, it is presently impossible to ascertain that the stimulated GABA and glutamate in dialysates are of neuronal origin. On the other hand, under certain stimulatory conditions, increases in amino-acid transmitters can be obtained in the presence of tetrodotoxin, again suggesting that aspecific factors not directly related to neurotransmission underlie these changes in extracellular levels. It is concluded that synaptic transmission of GABA and glutamate is strictly compartmentalized and as a result, these amino acids can hardly leak out of the synaptic cleft and reach the extracellular space where the dialysis probe samples.

Long-term monitoring of extracellular dopamine concentration in the rat striatum by a repeated microdialysis procedure

This study examined a protocol for repeated measurement of the extracellular dopamine (DA) concentration in the rat striatum by microdialysis. Rats were implanted with a guide cannula in the striatum and the probe was inserted on each dialysing day, i.e. ten times over a 23 day period. During this period the animals were submitted to a control saline treatment. DA concentration was measured using the no-net-flux method. In these conditions, DA concentration remained remarkably constant over the 23 day period. The histological analysis using glial fibrillary acidic protein (GFAP), dopamine (DA) and tyrosine hydroxylase (TH) immunocytochemistry showed a moderate gliosis and a discrete increase of immunoreactivity of catecholaminergic fibres around the probe implantation site. This increase is probably related to a plasticity of the dopaminergic system in response to the lesion due to the probe implantations. This study shows that such a paradigm makes possible to measure the whole time course of the DA concentration in the rat striatum during chronic treatments with psychoactive drugs such cocaine or other compounds acting in the nigrostriatal system.

Extracellular levels of glutamate and aspartate in the entopeduncular nucleus of the rat determined by microdialysis: regulation by striatal dopamine D2 receptors via the indirect striatal output pathway?

This study used intracerebral microdialysis to monitor the outputs of excitatory amino acids in the entopeduncular nucleus (EPN) of conscious or halothane-anaesthetized rats, in an attempt to obtain direct biochemical evidence for the theory that neuronal inputs to the EPN by the indirect striatal output pathway are glutamatergic and regulated primarily by dopamine D2 receptors in the striatum. In dopamine-intact animals, both glutamate and asparate were readily detectable in EPN dialysates. Recoveries of both amino acids were increased bilaterally by local perfusion with veratridine (100 microM, given under halothane anaesthesia), pretreatment with reserpine (4 mg/kg, i.p., 24 h beforehand), unilateral pretreatment of the medial forebrain bundle with 6-OHDA (8 microg/4 microl), and by the systemic (1 mg/kg, i.p.) or bilateral intrastriatal (7 microg/0.5 microl under halothane anaesthesia) administration of the dopamine D2 receptor antagonist haloperidol, but not raclopride (2 mg/kg, i.p.). The dopamine D1 receptor antagonist SCH 23390 was ineffective both systemically (0.25 mg/kg, i.p.) and intrastriatally (0.125 microg/0.5 microl/side), as also were control intrastriatal injections of saline (0.5 microl/side). By contrast, the dopamine D2/3 receptor agonist quinpirole (4 mg/kg, i.p.) lowered the outputs of glutamate and aspartate in the EPN of reserpine-treated and normal individuals, whilst the dopamine D1 receptor agonist SKF 38393 (30 mg/kg, i.p.) was inactive; however, both drugs caused behavioural arousal. The dopamine D2/3 receptor agonist RU 24213 reversed reserpine-induced akinesia, yet paradoxically increased glutamate (not aspartate) output in the EPN still further. The combination of benserazide (30 mg/kg, i.p.) and L-DOPA (50 mg/kg, i.p.) evoked intense contraversive circling in unilaterally 6-OHDA-lesioned rats, together with a drop in EPN glutamate (but not aspartate) output in the intact but not lesioned hemisphere. These results offer biochemical support for the hypothesis that excitatory neurones innervating the EPN via the indirect striatal output pathway, may utilise glutamate and/or aspartate as their neurotransmitter. They further endorse the view that the EPN receives information from striatal D2 and not D1 receptors via excitatory synapses, which become hyperactive following dopamine depletion or inactivation, and which are subject to control by the contralateral as well as by the ipsilateral hemisphere. The results obtained with RU 24213 and L-DOPA, however, indicate that dopaminergic behaviours can also occur independently of glutamate or aspartate release in the EPN.

The determination of the extracellular concentration of brain glutamate using quantitative microdialysis

Quantitative microdialysis with two enzyme-based assays was used to determine the extracellular concentration of glutamate in the striatum of freely moving rats. From the difference between infused and dialysate glutamate a value of 3.0 +/- 0.6 microM for the extracellular glutamate concentration was computed by regression analysis. The in vivo recovery, derived from the slope of the regression line, was 50%.

Clinical microdialysis: the role of on-line measurement and quantitative microdialysis

The use of microdialysis in the clinic is examined in the light of lessons learnt from microdialysis in freely moving rats. Changes in concentrations of metabolites are an important index of the state of health of tissues. For effective therapeutic intervention rapid assays are essential Enzyme-based on-line assays for glucose and lactate are described. By combining two of these assays simultaneous measurements of glucose and lactate, sampled at 2 min intervals can be obtained. The relation between dialysate concentrations and the true extracellular concentration of an analyte is dependent on conditions in the tissue sampled and cannot be calculated from in vitro probe recoveries. Furthermore, with acute implantation of the probe and possibly rapidly changing tissue conditions, there will be changes in probe recovery in vivo. Quantitative microdialysis allows the measurement of the true extracellular concentration and the probe recovery in vivo. The clinical applicability of a number of quantitative microdialysis methods is discussed, and three approaches highlighted. By increasing membrane length and reducing flow rate recovery in vivo can be increased to 100%. In this case dialysate concentrations equal extracellular ones. By perfusing an inert exogenous compound an index of changes to extracellular volume and hence tissue oedema can be obtained. In the zero net flux method the infusion of a few concentrations of the analyte under study allows the direct determination of both the ECF concentration and the in vivo recovery. The latter can provide valuable information about changes in the physical as well as chemical state of the tissue. This can guide rapid effective therapeutic intervention.

The effect of anosmia on MK-801-induced behaviour in mice

Systemic administration of N-methyl-D-aspartate (NMDA) receptor antagonists induces a well defined behaviour in rodents characterized by, for example increased locomotion and ataxia. It is not clear in what brain region(s) NMDA antagonists induce this behaviour. We have studied the possible involvement of olfactory pathways by making adult mice anosmic via intranasal injection of zinc sulphate, a procedure that is known to destroy the olfactory epithelium. The NMDA antagonist MK-801 was given intraperitoneally (0.1-1.0 mg/kg) and the animals were scored for locomotion and ataxia 60-90 min later. Before MK-801 administration, olfactory-lesioned mice did not differ from non-lesioned controls with regard to locomotion or ataxia. MK-801 caused locomotor activation (> or = 0.2 mg/kg) and ataxia (> or = 0.5 mg/kg) in both groups. In general, olfactory-lesioned animals showed more locomotion and less ataxia after MK-801 administration than non-lesioned animals. Lesioned animals displayed 2.0- (P < 0.05) and 3.7-fold (P < 0.05) more extensive locomotor activation than non-lesioned animals after 0.5 and 1.0 mg/kg of MK-801, respectively. No difference in the degree of ataxia was seen between the two groups at 0.5 mg/kg, whereas non-lesioned animals showed a 2.1-fold higher degree of ataxia after 1.0 mg/kg of MK-801, indicating that the enhanced MK-801-induced locomotor activation in olfactory-lesioned mice was not simply due to less ataxia. These results suggest that olfactory input is involved in NMDA antagonist-induced behaviour.

Net taurine transport and its inhibition by a taurine antagonist

P2-fractions were isolated from rat brain, and used to study net taurine transport. The fractions were incubated in increasing concentrations of [3H]taurine and the intraterminal concentration measured by liquid scintillation and amino acid analysis. The membrane potential of the isolated fractions was estimated using 86Rb+ as a marker for intracellular K+. Taurine was synthesized in the P2-fraction when incubated in taurine free medium. At external taurine concentrations below 370 microM a significant amount of the endogenous taurine was released to the incubation medium. Net taurine uptake into the P2-fraction was achieved at external taurine concentrations exceeding 370 microM. The taurine antagonist 6-aminomethyl-3-methyl-4H, 1, 2, 4-benzothiadiazine-1, 1-dioxide (TAG) competitively inhibited taurine and [3H]taurine transport into the P2-fraction. As the external concentration of taurine was increased, the accumulation of 86Rb+ into the P2-fraction was facilitated. This indicated an increasing hyperpolarization of the neuronal membrane as taurine transport shifted from release towards uptake. TAG reduced the hyperpolarization that paralleled taurine accumulation, in a dose dependent manner. Our results indicate that relatively low transmembranal gradients of taurine may be maintained by an electrogenic taurine transporter having a large transport capacity. Such a transporter may well serve the needs of osmotic regulation, i.e. to transport large amounts of taurine in any direction across the neuronal membrane.

Amino acid neurotransmitters in nucleus tractus solitarius: an in vivo microdialysis study

Amino acid neurotransmitters in the nucleus tractus solitarius (NTS) are thought to play a key role in the mediation of visceral reflexes and glutamate has been proposed as the neurotransmitter of visceral afferent nerves projecting to this region. The present studies sought to characterize the use of in vivo microdialysis to examine extracellular fluid levels of amino acids in the NTS of anesthetized rats. Using a microdialysis probe that was 450 microns in length and a sensitive HPLC assay for amino acids, amino acids could be measured in dialysate samples collected from the NTS. Perfusion of the microdialysis probe with 60 mM K+, to elicit depolarization of nerve terminals in the vicinity of the probe, resulted in increased dialysate fluid levels of aspartate, glutamate, glycine, taurine, and GABA. In contrast, glutamine and tyrosine were decreased and other amino acids were not significantly affected. Prior removal of the ipsilateral nodose ganglion did not alter the K(+)-evoked changes in dialysate levels of any of these amino acids. Electrical stimulation of the vagus nerves, using a variety of stimulus parameters, did not significantly alter dialysate levels of glutamate or any of the other amino acids that were measured. Blockade of glutamate uptake with dihydrokainate increased dialysate levels of glutamate, aspartate, and GABA, but in the presence of dihydrokainate vagal stimulation did not alter dialysate levels of these amino acids. The results show that in vivo microdialysis can be used to examine amino acid efflux in the rat NTS and provide further evidence for amino acidergic neural transmission in the NTS. However, these studies fail to support the hypothesis that vagal afferents release glutamate or aspartate.

H(+)-ATPase and transport of DOPAC, HVA, and 5-HIAA in monoamine neurons

The effects of N-methylmaleimide (N-MtM), a vacuolar H(+)-ATPase inhibitor, were evaluated in the putamen of the cat to study the in vivo transport mechanisms of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindolacetic acid (5-HIAA), using the brain focal microdialysis technique combined with HPLC. The addition of N-MtM to the perfusate altered invariably the flux of the DOPAC, HVA, and 5-HIAA in a similar pattern, resulting in a decrease of the extracellular levels of such metabolites, its extent being N-MtM concentration dependent, thus indicating that the mechanism(s) of such a decrease is (are) related most likely to decreased transport from the intracellular to the extracellular space as the consequence of the inhibition of the vacuolar H(+)-ATPase of DA and 5-HT neurons by the N-MtM. Furthermore, N-MtM masked the release of DA and 5-HT produced by KCl 120 mmol/l. Indeed, N-MtM increased the extracellular levels of such transmitters to values exceeding 4 to 6 times of those produced by KCl 120 mmol/l alone, which suggests that vacuolar H(+)-ATPase is probably involved also in the retention and/or reuptake process of DA and 5-HT.

Release of endogenous adenosine and its metabolites by the activation of NMDA receptors in the rat hippocampus in vivo

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.

Quantitative microdialysis: analysis of transients and application to pharmacokinetics in brain

The behavior of a microdialysis probe in vivo is mathematically described. A diffusion-reaction model is developed that not only accounts for transport of substances through tissues and probe membranes but also accounts for transport across the microvasculature and metabolism. Time-dependent equations are presented both for the effluent microdialysate concentration and for concentration profiles about the probe. The analysis applies either to measuring the tissue pharmacokinetics of drugs administered systemically, or for sampling of endogenously produced substances from tissue. In addition, an expression is developed for the transient concentration about the probe when it is used as an infusion device. All mathematical expressions are found to be a sum of an algebraic and an integral term. Theoretical prediction of time-dependent probe behavior in brain has been compared with experimental data for acetaminophen administered at 15 mg/kg to rats by intravenous bolus. Plasma and whole striatal tissue samples were used to describe plasma kinetics and to estimate a capillary permeability-area product of 0.07 min-1. Theoretical prediction of transient effluent dialysate concentrations exhibited close agreement with experimental data over 60 min. Terminal decline of the dialysate effluent concentration was slightly overestimated but theoretical concentrations still lay within the 95% confidence interval of the experimental data at 112 min. Microvasculature transport and metabolism play major roles in determining microdialysate transient responses. Extraction fraction (recovery) has been shown to be a declining function in time for five probe operating conditions. High rates of metabolism and/or capillary transport affect the time required to approach steady-state extraction, shortening the time as the rates increase. Conversely, for substances characterized by low permeabilities and negligible metabolism, experimental situations exist that are predicted to have very slow approaches to microdialysis steady state.

Endogenous release of gamma-aminobutyric acid from the medial preoptic area measured by microdialysis in the anaesthetised rat

The characteristics of gamma-aminobutyric acid (GABA) release as monitored by microdialysis have been investigated in the chloral hydrate anaesthetised rat. The high outflow of GABA following insertion of the microdialysis probe (membrane 2 mm in length, 0.5 mm in diameter) into the medial preoptic area was found to decline to a stable baseline level after 2 h. After this time, perfusion with a medium containing 100 mM potassium ions evoked a 56-fold increase in GABA outflow. The addition of the calcium channel blocker verapamil (100 microM) to the perfusion medium induced significant 25 and 50% reductions in basal and potassium-stimulated GABA outflow, respectively. In the same animals, verapamil caused an 80% decrease in potassium-stimulated noradrenaline outflow. The glutamic acid decarboxylase inhibitors 3-mercaptopropionic acid and L-allylglycine added to the perfusion medium at a concentration of 10 mM reduced basal GABA release by approximately 50% with different time-courses of action. Ethanolamine-O-sulfate, a GABA-transaminase inhibitor, induced significant increases in basal GABA outflow 90 min after inclusion in the perfusion medium. These results demonstrate that microdialysis is a suitable technique with which to monitor extracellular levels of GABA and provide in vivo data on GABA release and degradation mechanisms.

Sites of [3H]taurine Uptake in the Rat Substantia Nigra in Relation to the Release of Taurine from the Striatonigral Pathway

The autoradiographic localization of radiolabelled taurine taken up in the rat substantia nigra in vivo together with conditions of release of the [3H]taurine taken up into brain slices were studied to determine whether they are consistent with the hypothesis that taurine may act as a neurotransmitter in the striatonigral pathway. At the light microscopic level the main cellular elements that became radiolabelled following the injection of [3H]taurine into the substantia nigra could be identified as glial cells. Electron microscope autoradiography confirmed that a subpopulation of glial cells including astrocytes, pericytes, and oligodendrocytes were radiolabelled and that neuronal perikarya were not radiolabelled. In addition, axonal elements including both terminal and preterminal boutons were found to have silver grains overlying them and were thus considered to be radiolabelled. This was supported by a quantitative analysis of the distribution of the silver grains; whereas glial elements had a significantly higher number of grains associated with them than with any other structure, axonal elements had a significantly greater number of grains than dendritic structures. Release of the preloaded [3H]taurine from superfused slices of substantia nigra occurred in response to veratridine, was calcium-dependent and was sensitive to inhibition by high magnesium concentrations or tetrodotoxin. Following the destruction of neurons in the striatum by ibotenic acid injections, although the weight of the ipsilateral substantia nigra was reduced, the uptake of [3H]taurine was not altered. In contrast to this, the veratridine-stimulated release was markedly attenuated, implying that the destruction of striatal neurons causes the loss of sites in the substantia nigra from which exogenous taurine is released. These results add further support to previous suggestions that taurine might act as a neurotransmitter or neuromodulator in the striatonigral pathway.

Intracerebral microdialysis: I. Experimental studies of diffusion kinetics

Intracerebral microdialysis is a brain perfusion technique in which a tubular, semipermeable membrane perfused with a physiological solution is implanted into a selected brain region. Molecules in the extracellular space diffuse into the perfusate and may be recovered and their concentration determined. Hence, the level of substances such as neurotransmitters may be monitored, and the response to different treatments may be studied. The technique also allows for administration of substances locally to the region of the brain surrounding the perfused tubular membrane. Basic principles of the microdialysis technique are described, and the results from methodological experiments are examined. It is concluded that there is a direct linear relation between the concentration of a molecule in the medium surrounding the dialysis membrane and the concentration measured in the collected perfusate. Relative changes of molecular concentration in brain extracellular space may be calculated even when the molecular diffusion rate is unknown. In addition, a method is presented for calculating the real concentration of a substance in the extracellular space from its concentration in the perfusate. Applied in striatum of rat brain using microdialysis in vivo, the average extracellular concentration of the following substances is estimated to be: substance P, 0.9 nM; dopamine, 1 microM; and dihydroxyphenylacetic acid, 0.05 mM.

Ischemic flow threshold for extracellular glutamate increase in cat cortex

Extracellular glutamate (Glu), cerebral blood flow (CBF), and auditory-evoked potentials (AEPs) were measured concurrently using microdialysis and hydrogen clearance in the auditory cortex of anesthetized cats during global ischemia of various severities. A threshold-type relationship was observed between extracellular Glu and CBF: Glu increased at CBF levels below about 20 ml/100 g/min. The Glu increase was related to the impairment of AEPs. The results suggest that Glu neurotoxicity is an important factor for ischemic neuronal injury even in penumbra.

Activation of the bilateral corticostriatal glutamatergic projection by infusion of GABA into thalamic motor nuclei in the cat: an in vivo release study

The unilateral application of GABA (10(-5) M; 30 min) into thalamic motor nuclei of the cat increases the release of dopamine in both caudate nuclei. This effect has been suggested to be related to an activation of the bilateral corticostriatal glutamatergic projection, glutamate exerting a presynaptic facilitatory influence on dopamine release. To explore this hypothesis further, halothane-anesthetized cats implanted with push-pull cannulae were used in order to examine the effects of such a GABA application on the release of glutamate in both caudate nuclei. Aspartate, alanine, glutamine, serine and tyrosine were also measured in the superfusates. The unilateral application of GABA (10(-5) M; 30 min) into thalamic motor nuclei increased the release of glutamate bilaterally. Although less pronounced, ipsi- or bilateral increases in the efflux of alanine, glutamine and tyrosine were also observed. Contralateral changes in the efflux of glutamate, alanine and tyrosine were prevented following acute section of the corpus callosum. In addition, when applied continuously into one caudate nucleus, 2-amino-5-phosphonovaleric acid, a blocker of N-methyl-D-aspartate receptors, prevented the GABA-induced increase in alanine or tyrosine efflux but did not affect the enhanced release of glutamate. These results confirm that the unilateral application of GABA in thalamic motor nuclei activates a thalamo-cortico-striatal neuronal loop leading to the stimulation of glutamate release in both caudate nuclei. Changes in the efflux of other amino acids could be linked to increased metabolic activity of striatal target cells resulting from the increased release of glutamate and from its effect on N-methyl-D-aspartate receptors.

Endogenous release of neuronal serotonin and 5-hydroxyindoleacetic acid in the caudate-putamen of the rat as revealed by intracerebral dialysis coupled to high-performance liquid chromatography with fluorimetric detection

Extracellular levels of endogenous serotonin (5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were measured in the caudate-putamen of anesthetized and awake rats using intracerebral microdialysis coupled to HPLC with fluorimetric detection. A dialysis probe (of the loop type) was perfused with Ringer solution at 2 microliters/min, and samples collected every 30 or 60 min. Basal indole levels were followed for up to 4 days in both intact and 5,7-dihydroxytryptamine (5,7-DHT) lesioned animals. Immediately after the probe implantation, the striatal 5-HT levels were about 10 times higher than the steady-state levels that were reached after 7-8 h of perfusion. The steady-state baseline levels, which amounted to 22.5 fmol/30 min sampling time, remained stable for 4 days. In 5,7-DHT-denervated animals, the steady-state levels of 5-HT, measured during the second day after probe implantation, were below the limit of detection (less than 10 fmol/60 min). However, during the first 6 h post-implantation, the 5-HT output was as high as in intact animals, which suggests that the high 5-HT levels recovered in association with probe implantation were blood-derived. As a consequence, all other experiments were started after a delay of at least 12 h after implantation of the dialysis probe. In awake, freely moving animals, the steady-state 5-HT levels were about 60% higher than in halothane-anesthetized animals, whereas 5-HIAA was unaffected by anesthesia. KCl (60 and 100 mM) added to the perfusion fluid produced a sharp increase in 5-HT output that was eight-fold at the 60 mM concentration and 21-fold at the 100 mM concentration. In contrast, 5-HIAA output dropped by 43 and 54%, respectively. In 5,7-DHT-lesioned animals, the KCl-evoked (100 mM) release represented less than 5% of the peak values obtained for the intact striata. Omission of Ca2+ from the perfusion fluid resulted in a 70% reduction in baseline 5-HT output, whereas the 5-HIAA levels remained unchanged. High concentrations of tetrodotoxin (TTX) added to the perfusion medium (5-50 microM) resulted in quite variable results. At a lower concentration (1 microM), however, TTX produced a 50% reduction in baseline 5-HT release, whereas the 5-HIAA output remained unchanged. The 5-HT reuptake blocker, indalpine, increased the extracellular levels of 5-HT sixfold when added to the perfusion medium (1 microM), and threefold when given intraperitoneally (5 mg/kg). By contrast, the 5-HIAA level remained unaffected during indalpine infusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Fast HPLC estimation of gamma-aminobutyric acid in microdialysis perfusates: effect of nipecotic and 3-mercaptopropionic acids

A method for rapid, automated (less than 5 min), and sensitive (detection limit 50 fmol/10 microliter) determination of gamma-aminobutyric acid (GABA) is described. The method is based on precolumn derivatization with o-phthaldialdehyde/t-butylthiol reagent and separation by reverse-phase HPLC with electrochemical detection under isocratic conditions. A 100 X 4 mm Nucleosil 3 C18 column was used; the mobile phase consisted of 0.15 M sodium acetate, 1 mM EDTA (pH 5.4), and 50% acetonitrile; the flow rate was 0.8 ml/min. The potential of the glassy carbon working electrode was +0.75 V. The method allows for the monitoring of GABA levels in the extracellular fluid sampled by microdialysis as documented in the present study when 0.5 mM nipecotic acid is infused via the probe, or 3-mercaptopropionic acid is injected at a dose of 100 mg/kg i.p. There was a 15-fold increase of extracellular GABA after nipecotic acid, whereas in the second case the inhibition of GABA synthesis was followed by a 74% decrease of GABA as compared to basal levels.

Secretion of newly synthesized proteins into the extracellular fluid of the rabbit hippocampus

Extracellular proteins were collected continuously from the hippocampus of the rabbit by slow perfusion of a protein-permeable thin tubing. After pulse labelling with radioactive amino acids via the tubing a rapid appearance of labelled proteins was seen in the extracellular fluid. The half-life of the proteins in the extracellular space varied between 15 min and 2 h. Protein fractions were separated with gel electrophoresis or HPLC. The extracellular fraction differed distinctly from intracellular proteins with respect to the labelling pattern.

Extracellular amino acid concentrations in the dorsal spinal cord of freely moving rats following veratridine and nociceptive stimulation

In vivo microdialysis was used to sample extracellular concentrations of amino acids in the dorsal lumbar spinal cord of freely moving rats. Changes in the extracellular concentrations of amino acids were measured in response to infusion of veratridine (180 microM), a sodium channel activator, as well as during acute noxious stimulation by an injection of 5% formalin into the metatarsal region of the hindleg. Veratridine produced a tetrodotoxin (TTX)-sensitive increase in the extracellular concentration of Glu. Concentrations of Asp, taurine, Ala, Asn, and Gly were not significantly elevated following veratridine stimulation. Intradermal injection of formalin produced a TTX-sensitive increase in Asp concentration and a non-TTX-sensitive increase in Glu concentration. These data support the hypothesis that Glu and Asp are dorsal horn neurotransmitters involved in nociception.

Stimulus-coupled release of amino acids from cerebellar granule cells in culture

Cerebellar cultures greatly enriched in excitatory granule neurons were depolarized by exposure to either elevated K+ or veratrine. Stimulation of the release of not only Glu, but also of certain amino acids, including Gly, Ala and Ser, was observed. The effect was specific, as depolarization did not induce the release of all the estimated amino acids or of lactate dehydrogenase. In comparison with the characteristics of the evoked release of Glu, those of the responsive neutral amino acids were similar in terms of Ca2+-dependence, but differences were also noted. Thus, upon stimulation, the relative rise was smaller than for Glu and the degree of depolarization causing maximal release was lower. The questions of whether stimulus-coupled release of the non-transmitter amino acids from granule cells may play a neuromodulatory role in the cerebellum is discussed.

Quinolinic acid effects on amino acid release from the rat cerebral cortex in vitro and in vivo

1. The effect of quinolinic acid, N-methyl-D,L-aspartate (NMDLA) and kainate on the release of endogenous and exogenous amino acids from the rat cerebral cortex in vitro and in vivo was studied. 2. Neither quinolinic acid nor NMDLA had any effect on the basal or potassium-evoked release of [3H]-D-aspartate from slices of rat cerebral cortex either in the presence or absence of magnesium. Kainic acid failed to modify the basal efflux of [3H]-D-aspartate but significantly inhibited (by 34.4% +/- 0.04%, P less than 0.05) the potassium-evoked release. 3. Neither quinolinate nor NMDLA had any effect on the basal efflux of endogenous amino acids from rat cortical slices either in the presence or absence of magnesium ions at concentrations between 10 microM and 5 mM. 4. Both NMDLA (1 mM) and quinolinate (5 mM) produced an efflux of endogenous aspartate (371.4% +/- 11.6%; 389.3% +/- 12.1%) and glutamate (405.4% +/- 13.6%; 430.1 +/- 8.7%) respectively from the rat cerebral cortex in vivo (P less than 0.01). The quinolinic acid-evoked efflux was abolished by the NMDLA antagonist, 2-amino-5-phosphonovaleric acid (200 microM). 5. Kainic acid also caused an efflux of endogenous amino acids from the rat cerebral cortex in vivo. However, the profile of this release was different from that produced by quinolinate and NMDLA. 6. The results add further support to the suggestion that quinolinic acid acts at the NMDLA-preferring receptor and may also explain the requirement for intact afferent projections for the neurotoxic effects of quinolinate to be manifested.

Analysis of amino acids: neurochemical application

For high sensitivity analysis of neuroactive amino acids, liquid chromatography employing precolumn derivatisation with o-phthalaldehyde (OPA) is suitable for several reasons. The OPA reagent is non-fluorescent per se, the reaction occurs rapidly in alkaline aqueous solutions and forms highly fluorescent derivatives with primary amines.

Alterations in hippocampal extracellular amino acids and purine catabolites during limbic seizures induced by folate injections into the rabbit amygdala

The effects on hippocampal extra- and intracellular amino acids of focal injection of folic acid into the amygdala in the rabbit were studied with brain dialysis. Folate seizures were accompanied by pronounced elevations of extracellular alanine and phosphoethanolamine. The increase of extracellular alanine was related to an enhanced level of this amino acid in total hippocampal tissue, whereas phosphoethanolamine was unaltered in tissue biopsies. Folate seizures did not significantly affect extracellular aspartate and extracellular glutamate was only slightly elevated (50-75% over baseline values). The tissue concentration of glutamate remained at control levels during the seizures and tissue aspartate was decreased by 28%. Extracellular glutamine decreased rapidly after folate injection with a concomitant increase of total hippocampal glutamine. Neither extra- nor intracellular taurine was affected by folate epilepsy. The experiments also encompassed measurements of the extracellular purine catabolites inosine, hypoxanthine and xanthine. Folic acid-induced epilepsy produced changes in these compounds indicative of moderately accelerated degradation of adenosine 5'-triphosphate. The findings support the view of glutamate as a mediator of epilepsy-related brain damage. It is, however, questionable if the small enhancement of extracellular glutamate is enough to provoke neuronal necrosis associated with folate epilepsy.

In vivo studies on the extracellular, and veratrine-releasable, pools of endogenous amino acids in the rat striatum: effects of corticostriatal deafferentation and kainic acid lesion

The effects of corticostriatal deafferentation (decortication) and destruction of intrinsic neurons (intrastriatal kainate injection) on the extracellular concentration, and veratrine-releasable pools, of endogenous amino acids in the rat striatum were examined using the in vivo brain dialysis technique. Intracellular amino acid content was also determined. Decortication reduced selectively intra- and extracellular levels of glutamate (Glu) and aspartate (Asp). Extracellular changes were more pronounced than those in tissue content. gamma-Aminobutyric acid (GABA), taurine (Tau), and phosphoethanolamine (PEA) levels were not affected, whereas nonneuroactive amino acids were increased at 1 week but not at 1 month post-lesion. The intracellular pool of Glu and Asp was also reduced in kainate-lesioned striata. However, extracellular levels of these compounds were not affected significantly by this treatment. The tissue content of all other amino acids was decreased, the most prominent change being in the concentration of GABA. Extracellular GABA concentration was also reduced dramatically, whereas the concentrations of noneuroactive amino acids were increased to varying degrees. These data suggest that transmitter pools of neuroactive amino acids are an important supply for their extracellular pools. Lesion-induced alterations in nonneuroactive amino acids are discussed with regard to the loss of metabolic pools, glial reactivity, and changes in blood-brain barrier transport. Veratrine induced a massive release of neuroactive amino acids such as Glu, Asp, GABA, and Tau into the extracellular fluid, and a delayed increase in PEA. Extracellular levels of neuroactive amino acids were raised slightly. Decortication reduced, selectively, the amounts of Glu and Asp released by veratrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Free amino acids in the pre-retinal vitreous space. Effect of high potassium and nipecotic acid

The pre-retinal vitreous space of the eye of albino rabbits was perfused in vivo via a dialysis probe in order to measure the concentration of endogenous amino acids. Studies of isotope concentrations in the vitreal effluent following intracarotid injections of 3HOH and [14C]mannitol demonstrated that entrance of 3HOH into the dialysis probe occurred within a few minutes whereas [14C]mannitol was almost completely excluded. Among the amino acids, analysed with high-pressure liquid chromatography (HPLC), glutamine had a concentration similar to that in plasma and cerebrospinal fluid (CSF). Taurine was four times higher than in the CSF. Perfusion with 125 mM KCl media increased particularly the taurine and phosphoethanolamine content. Nipecotic acid, a potent inhibitor of gamma-aminobutyric acid (GABA) uptake, increased GABA concentration over 60 times. In addition, it increased taurine levels by almost 10 times.

In vivo determination of extracellular concentration of amino acids in the rat hippocampus. A method based on brain dialysis and computerized analysis

Extracellular (EC) concentrations of amino acids were determined in the rat dentate gyrus by means of non-linear regression analysis of 'in vivo' brain dialysis data, considering a simple model of diffusion through a dialysis membrane. The apparent diffusion constants (K) of several amino acids were also calculated in the 'in vivo' situation. While putative amino acid neurotransmitters (glutamate, aspartate and gamma-aminobutyric acid (GABA) were present in the EC fluid at the low micromolar range (0.8-2.9 microM), glutamine was by far the most prominent (193.4 microM). The values of intra/extracellular concentration ratios formed 3 groups: high (greater than 2000) for putative neurotransmitters; low (less than 100) for serine, glutamine, arginine and alpha-alanine; and intermediate (about 400) for taurine. The 'in vivo' calculated K values proved useful for estimation of both basal and changing EC concentrations of amino acids in relatively brief perfusions. These data were evaluated in terms of the functional significance of absolute EC concentrations and tissue-EC fluid ratios. Present findings indicate the simultaneous existence of both an inhibitory and an excitatory tonus as well as the utility of high intra/extracellular concentration ratios in determination of the possible neurotransmitter role of specific amino acids.

Increases in cerebral interstitial fluid adenosine concentration during hypoxia, local potassium infusion, and ischemia

This study used the brain dialysis technique to test the hypothesis that the adenosine concentration of cerebral interstitial fluid increases during situations in which cerebral oxygen supply is inadequate for oxygen demand. Sealed 300-micron hollow dialysis fibers were implanted in the caudate nucleus of pentobarbital-anesthetized rats and perfused at 2 microliter/min with artificial cerebrospinal fluid. In vitro tests indicated the recovery of adenosine, inosine, and hypoxanthine from the external medium to be approximately 20% at 2 microliter/min and close to 100% at 0.1 microliter/min. Three in vivo interventions were tested: hypoxia/hypotension (PaO2 = 41.9 mm Hg; MABP = 42.8 mm Hg; n = 9), local potassium infusion (n = 4), and cerebral anoxia/ischemia (n = 10). These interventions produced 10-, 4-, and 30-fold increases in perfusate adenosine concentration, respectively, as well as increases in perfusate concentrations of inosine and hypoxanthine. A separate group of rats (n = 9) perfused at 0.1 microliter/min yielded estimates of cerebral interstitial fluid adenosine, inosine, and hypoxanthine concentrations of 1.26, 3.30, and 7.19 microM, respectively. These results are consistent with the adenosine hypothesis for the regulation of CBF.

An analysis of the effects of excitatory amino acid receptor antagonists on evoked field potentials in the olfactory bulb

The effects of excitatory amino acid antagonists on extracellular field potentials in the olfactory bulb produced by lateral olfactory tract stimulation were analysed in vivo. The compounds tested D-2-amino-5-phosphonovalerate, L-(+)2-amino-4-phosphonobutyrate, gamma-D-glutamylglycine, L-glutamic acid diethylester and cis-2,3-piperidine dicarboxylic acid, were administered by brain dialysis. Of the compounds tested, only cis-2,3 piperidine-dicarboxylic acid and gamma-D-glutamylglycine were able to suppress the synaptic excitation of granule cells. This pharmacological profile suggests the involvement of non-N-methyl-D-aspartate receptors. However, the suppression was accompanied by a reduction in the amplitude of the presynaptic volley. A second finding was that D-2-amino-5-phosphono-valerate and gamma-D-glutamyl glycine attenuated granule cell mediated inhibition of mitral cells, suggesting the involvement of N-methyl-D-aspartate-sensitive receptors. The possibility that mitral cells and that either centrifugal fibres, or an intrinsic olfactory bulb feedback loop might use an excitatory amino acid as its neurotransmitter is therefore discussed.

Microdialysis in the study of extracellular levels of amino acids in the rat brain

Microdialysis has been used for in vivo studies of extracellular amino acids in rat brain. We describe a method where a probe was designed to be implanted vertically. This probe is suitable for regional stereotaxic studies of the rat brain. The dialysis probe was perfused with Ringer's solution and the perfusates were analysed for their amino acid content with a high performance liquid chromatography (HPLC) technique. An orthophthaldialdehyde derivative of the amino acids was formed before the sample was injected onto the column. In vitro studies of the dialysis probe show that the relative recovery of substances outside the membrane is dependent on perfusion speed and length of dialysing membrane but not on the concentration outside. We were also able to show that the probe was within the blood-brain barrier (BBB) when implanted into the brain since after intravenous injection of Na99mTcO4, a substance that cannot pass through the intact BBB, it was not possible to recover any isotope from the perfusate. We conclude that microdialysis is a unique method of studying regional neurochemical events within the BBB, for example, changes in putative amino acid neurotransmitters and their metabolites.

Regional distribution and extracellular levels of amino acids in rat central nervous system

The extracellular levels of aspartate, glutamate, serine, glutamine, glycine, alanine and GABA were studied in vivo with the microdialysis technique in 15 different regions of the rat brain. The effect of high K+ on the overflow of these amino acids was also studied. These results were compared with those from a regional dissection of 17 brain regions in which the tissue content of the same amino acids was determined. The in vivo data showed an unevenly distributed KCl response of aspartate, glutamate, taurine and GABA, all of which are putative neurotransmitters. It was not possible to predict the response to high K+ from the magnitude of the unstimulated overflow. Glutamine overflow was inversely related to that of glutamate during the high K+ stimulus, which is consistent with glutamine being the main precursor of glutamate. Only for GABA and alanine was overflow proportional to the tissue level in the different regions studied.

Release of endogenous amino acids from striatal neurons in primary culture

Endogenous amino acid release was examined in highly purified striatal neurons obtained from fetal mouse brain, and differentiated in primary culture. This study aimed to determine which amino acids are released from striatal neurons after a brief depolarization period induced by elevated potassium concentration or veratrine. Amino acids released into the extracellular medium, subsequent to a 3-min exposure of striatal neurons, were subjected to HPLC analysis. At 14 days in vitro potassium (56 mM) depolarization elicited a 25-fold increase in gamma-aminobutyric acid release, 85% of which was calcium-dependent. This effect was small but apparent at 7 days in vitro (two-fold increase) and greatly increased between 11 and 14 days in vitro, subsequent to the appearance of synaptic vesicles in nerve terminals. gamma-Aminobutyric acid release was readily reversible within minutes of return to the resting state. Veratrine induced a quantitatively similar but calcium-independent increase in gamma-aminobutyric acid release. Similar results were observed on aspartate and glutamate release, but the increase was very small even after 14 days in vitro (62.2 and 123.3% increase over basal release, respectively). Taurine and hypotaurine release increased during and after depolarization induced by potassium. This effect remained constant between 11 and 18 days in vitro. BAY K 8644, a dihydropyridine-sensitive calcium channel agonist, augmented the effect of 15 mM potassium on gamma-aminobutyric acid release, but this effect remained very small as compared to the potassium (56 mM) or veratrine effects. In addition, nifedipine inhibited this BAY K 8644-induced release. These results demonstrate the high level of differentiation among striatal neurons containing gamma-aminobutyric acid in this in vitro system.

In vivo release of endogenous amino acids from the rat striatum: further evidence for a role of glutamate and aspartate in corticostriatal neurotransmission

By means of the push-pull cannula method, the outflow of endogenous amino acids was studied in the striatum of halothane-anesthetized rats. Addition of K+ ions (30 mM for 4 min) to the superfusion fluid increased the release of aspartate (+116%), glutamate (+217%), taurine (+109%), and gamma-aminobutyric acid (GABA) (+429%) whereas a prolonged decrease in the outflow of glutamine (-28%) and a delayed reduction in the efflux of tyrosine (-25%) were observed. In the absence of Ca2+, the K+-induced release of aspartate, glutamate, and GABA was blocked whereas the K+-induced release of taurine was still present. Under these conditions, the decrease in glutamine efflux was reduced and that of tyrosine was abolished. Local application of tetrodotoxin (5 microM) decreased only the outflow of glutamate (-25%). One week following lesion of the ipsilateral sensorimotor cortex the spontaneous outflow of glutamine and of tyrosine was enhanced. Despite the lack of change in their spontaneous outflow, the K+-evoked release of aspartate and glutamate was less pronounced in lesioned than in control animals, whereas the K+-evoked changes in GABA and glutamine efflux were not modified. Our data indicate that the push-pull cannula method is a reliable approach for the study of the in vivo release of endogenous amino acids. In addition, they provide further evidence for a role for glutamate and aspartate as neurotransmitters of corticostriatal neurons.

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.

Kainic acid-induced changes of extracellular amino acid levels, evoked potentials and EEG activity in the rabbit olfactory bulb

The effect of kainic acid (KA) on the extracellular content of amino acids in the rabbit olfactory bulb was investigated, both in vivo with the brain dialysis technique and in vitro with a superfused tissue slice preparation. Olfactory bulb EEG activity and lateral olfactory tract (LOT)-evoked field potentials were monitored simultaneously during dialysis experiments. KA induced a rapid (within 5 min) increase of extracellular aspartate, glutamate, GABA, phosphoethanolamine and taurine in vivo. LOT-evoked potentials were altered concomitantly in a concentration- and time-dependent manner. The antidromic invasion of mitral/tufted cells was depressed and the synaptic activation of granule cells was abolished in the presence of KA. Olfactory bulb EEG activity was also affected. Oscillatory bursts in olfactory bulb EEG were abolished by 10 mM in most experiments, whereas sustained oscillations were induced by 1 mM KA. The effects of KA may partly be due to a blockade of amino acid reuptake since dihydrokainate (DKA) perfusion was found to increase extracellular aspartate and glutamate. However, DKA had no significant effect on EEG or evoked potentials. In vitro, aspartate and glutamate were selectively increased during KA perfusion.

Mass transfer in brain dialysis devices--a new method for the estimation of extracellular amino acids concentration

A simple mathematical description of mass transfer in brain dialysis devices is presented. The validity of this model is supported by results from both in vitro and in vivo dialysis experiments. Based on this description, an alternative approach for estimating the extracellular concentration of amino acids is outlined.

Ischemia-induced shift of inhibitory and excitatory amino acids from intra- to extracellular compartments

Brain ischemia was induced for 10 or 30 min by clamping the common carotid arteries in rabbits whose vertebral arteries had previously been electrocauterized. EEG and tissue content of high energy phosphates were used to verify the ischemic state and to evaluate the degree of postischemic recovery. Extracellular levels and total contents of amino acids were followed in the hippocampus during ischemia and 4 h of recirculation. At the end of a 30-min ischemic period, GABA had increased 250 times, glutamate 160 times, and aspartate and taurine 30 times in the extracellular phase. The levels returned to normal within 30 min of reflow. A delayed increase of extracellular phosphoethanolamine and ethanolamine peaked after 1-2 h of reflow. Ten minutes of ischemia elicited considerably smaller but similar effects. With respect to total amino acids in the hippocampus, glutamate and aspartate decreased to 30-50% of control while GABA appeared unaffected after 4 h of reflow. Alanine, valine, phenylalanine, leucine, and isoleucine increased severalfold. The importance of toxic extracellular levels of excitatory amino acids, as well as of high extracellular levels of inhibitory amino acids, are considered in relation to the pathophysiology of neuronal cell loss during cerebral ischemia.

Effect of glutamine on glutamate release from hippocampal slices induced by high K+ or by electrical stimulation: interaction with different Ca2+ concentrations

To characterize the effect of glutamine on the release of glutamate, aspartate, and gamma-aminobutyric acid (GABA), rat hippocampal slices were superfused with different concentrations of glutamine or Ca2+. Amino acids released and retained were analyzed by HPLC. Glutamine (0.5 mmol/L) increased more than threefold the release of glutamate evoked by 50 mmol/L K+ in the presence of 2.6 mmol/L Ca2+ without a corresponding increase in glutamate content, while the release of aspartate was increased less and that of GABA not at all by glutamine. The evoked release of all three amino acids, including the enhanced release of glutamate in the presence of glutamine, was strongly dependent on Ca2+ concentrations between 0.1 and 2.6 mmol/L. The potentiation of glutamate release by glutamine reached a plateau at 0.25 mmol/L glutamine. Intermittent electrical field stimulation increased the release of only glutamate and this release was nearly doubled by glutamine. The increased release was Ca2+ dependent and tetrodotoxin (TTX) sensitive. Results suggest that extracellular glutamine promotes primarily the formation of releasable glutamate and this enhancement is dependent on extracellular Ca2+.