Decrease of glutamate decarboxylase activity after in vivo cortical infusion of gamma-aminobutyric acid

GABA promotes gastrin-releasing peptide secretion in NE/NE-like cells: Contribution to prostate cancer progression

In prostate cancer (PCa), neuroendocrine cells (NE) have been associated with the progression of the disease due to the secretion of neuropeptides that are capable of diffusing and influence surrounding cells. The GABAergic system is enriched in NE-like cells, and contributes to PCa progression. Additionally, γ-aminobutyric acid (GABA) stimulates the secretion of gastrin-releasing peptide (GRP) in peripheral organs. For the first time, in this study we show the role of GABA and GABA_B receptor 1 (GABBR1) expression in GRP secretion in NE-like prostate cancer cells. We demonstrated an increase in GRP levels in NE-like cell medium treated with GABA_B receptor agonist. Moreover, the blocking of this receptor inhibited GABA-induced GRP secretion. The invasive potential of PC3 cells was enhanced by either GRP or conditioned medium of NE-like cells treated with GABA. Additionally, we confirmed a positive correlation between GABA and GRP levels in the serum of PCa patients with NE markers. Finally, using public available data sets, we found a negative correlation between GABBR1 and androgen receptor (AR) expression, as well as a strong positive correlation between GABBR1 and enolase 2. These results suggest that GABA via GABBR1 induces GRP secretion in NE like cells involved in PCa progression.

GABA withdrawal syndrome: GABAA receptor, synapse, neurobiological implications and analogies with other abstinences

The sudden interruption of the increase of the concentration of the gamma-aminobutyric acid (GABA), determines an increase in neuronal activity. GABA withdrawal (GW) is a heuristic analogy, with withdrawal symptoms developed by other GABA receptor-agonists such as alcohol, benzodiazepines, and neurosteroids. GW comprises a model of neuronal excitability validated by electroencephalogram (EEG) in which high-frequency and high-amplitude spike-wave complexes appear. In brain slices, GW was identified by increased firing synchronization of pyramidal neurons and by changes in the active properties of the neuronal membrane. GW induces pre- and postsynaptic changes: a decrease in GABA synthesis/release, and the decrease in the expression and composition of GABAA receptors associated with increased calcium entry into the cell. GW is an excellent bioassay for studying partial epilepsy, epilepsy refractory to drug treatment, and a model to reverse or prevent the generation of abstinences from different drugs.

Allopregnanolone potentiates the glutamate-mediated seizures induced by 4-aminopyridine in rat hippocampus in vivo

Excitatory and inhibitory neurotransmission in the central nervous system can be modulated by neurosteroids. We previously found that in rat hippocampal slices allopregnanolone (3α-hydroxy-5α-pregnan-20-one), a positive GABA(A) receptor modulator, suppresses the epileptic discharges induced by 4-aminopyridine (4-AP), a convulsant K(+) channel blocker that stimulates glutamate release. Here, we tested the action of allopregnanolone on the epileptogenic and excitotoxic effects of the intrahippocampal administration of 4-AP in vivo. Drugs were perfused by a microdialysis cannula-electrode in the dorsal hippocampus and the EEG was recorded. Extracellular levels of aspartate, glutamate and GABA were analyzed by HPLC in the microdialysis fractions, and 24 h after the experiment the hippocampus was studied histologically. 4-AP induced intense epileptic discharges, increased the extracellular levels of aspartate, glutamate, and GABA by 383, 420, and 245%, respectively, and produced a notable neurodegeneration in CA1 and CA3 areas. Allopregnanolone administration alone did not affect the electrical activity, amino acids levels or cellular morphology, but when co-infused with 4-AP incremented 55-77% the duration of the epileptic discharges, and potentiated 32-49% the release of glutamate in comparison with 4-AP alone. The 4-AP-induced neurodegeneration was not modified by allopregnanolone. The NMDA receptor antagonist MK-801 protected against the epilepsy and neurodegeneration produced by 4-AP, and allopregnanolone did not affect this protection. We conclude that, differently from the observations in vitro, allopregnanolone potentiated the stimulatory effect of 4-AP on glutamate release and that this may explain the potentiation of the epileptogenic effect of 4-AP in vivo.

Chronic elevation of extracellular glutamate due to transport blockade is innocuous for spinal motoneurons in vivo

Glutamate-mediated excitotoxicity has been considered to play an important role in the mechanism of spinal motoneuron death in amyotrophic lateral sclerosis (ALS), and some reports suggest that this excitotoxicity may be due to a decreased glutamate transport and the consequent elevation of its extracellular level. We have previously shown that short lasting increments in extracellular glutamate due to administration of the non-selective glutamate transport blocker l-2,4-trans-pyrrolidine-dicarboxylate (PDC) by microdialysis in the rat spinal cord do not induce motoneuron damage. In the present work we examined the potential involvement of chronic glutamate transport blockade as a causative factor of spinal motoneuron death and paralysis in vivo. Using osmotic minipumps, we infused directly in the spinal cord for up to 10 days PDC and another glutamate transport blocker, dl-threo-beta-benzyloxyaspartate (TBOA), and we measured by means of microdialysis and HPLC the extracellular concentration of glutamate and other amino acids. We found that after the infusion of both PDC and TBOA the concentration of endogenous extracellular glutamate was 3-4-fold higher than that of the controls. Nevertheless, in spite of this elevation no motoneuron degeneration or gliosis were observed, assessed by histological examination and choline acetyltransferase and glial fibrillary acidic protein immunocytochemistry. In accord with this lack of toxic effect, no motor deficits, assessed by three motor activity tests, were observed. Because we had previously shown that under identical experimental conditions the infusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) induced progressive motoneuron death and paralysis, we conclude that prolonged elevation of extracellular glutamate due to its transport blockade in vivo is innocuous for spinal motoneurons and therefore that these results do not support the hypothesis that glutamate transport deficiency plays a crucial role as a causal factor of spinal motoneuron degeneration in ALS.

Loss of phase synchrony in an animal model of partial status epilepticus

Interrupting a focal, chronic infusion of GABA to the rat motor cortex initiates the progressive emergence of a sustained spiking electroencephalographic (EEG) activity, associated with myoclonic jerks of the corresponding body territory. This activity is maintained over several hours, has an average frequency of 1.5 Hz, is localized to the infusion site and never generalizes. The GABA withdrawal syndrome (GWS) has therefore features of partial status epilepticus. Changes in EEG signals associated with the GWS were studied in freely moving rats by measuring the phase synchrony between bilateral epidural records from the neocortex. Our results showed (i) epileptic activity was associated with a striking decrease in phase synchrony between all pairs of electrodes including the focus, predominantly in the 1-6 Hz frequency range. There was a mean decrease of 75.34+/-5.26% in phase synchrony levels between the period before GABA interruption and the period after epileptic activity appeared. (ii) This reduction in synchrony contrasted with an increase of power spectral density in the corresponding EEG channels over the same 1-6 Hz frequency range, (iii) neither changes in synchrony nor in nonlinear dynamics were detected before the first EEG spikes, (iv) systemic injection of ketamine, an antagonist of N-methyl-d-aspartic acid (NMDA) receptors, modified transiently both epileptic activity and the synchrony profile. (v) Spiking activity and synchrony changes were suppressed by reperfusion of GABA. Our data suggest that, during a partial status epilepticus, interactions between the epileptic focus and connected neocortical neuronal populations are dramatically decreased in low frequencies.

Convulsions induced by methylmalonic acid are associated with glutamic acid decarboxylase inhibition in rats: A role for GABA in the seizures presented by methylmalonic acidemic patients?

Methylmalonic acid (MMA) is an endogenous convulsing compound that accumulates in methylmalonic acidemia, an inborn error of the metabolism characterized by severe neurological dysfunction, including seizures. The mechanisms by which MMA causes seizures involves the activation of the N-methyl-D-aspartate (NMDA) receptors, but whether GABAergic mechanisms are involved in the convulsions induced by MMA is not known. Therefore, in the current study we investigated the involvement of GABAergic mechanisms in the convulsions induced by MMA. Adult rats were injected (i.c.v.) with muscimol (46 pmol/1 microl), baclofen (0.03, 0.1 and 0.3 micromol/1 microl), MK-801 (6 nmol/1 microl), pyridoxine (2 micromol/4 microl) or physiological saline (0.15 micromol/1 microl). After 30 min, MMA (0.3, 0.1 and 3 micromol/1 microl) or NaCl (6 micromol/1 microl, i.c.v.) was injected. The animals were immediately transferred to an open field and observed for the appearance of convulsions. After behavioral evaluation, glutamic acid decarboxylase (GAD) activity was determined in cerebral cortex homogenates by measuring the 14CO2 released from l-[14C]-glutamic acid. Convulsions were confirmed by electroencephalographic recording in a subset of animals. MMA caused the appearance of clonic convulsions in a dose-dependent manner and decreased GAD activity in the cerebral cortex ex vivo. GAD activity negatively correlated with duration of MMA-induced convulsions (r=-0.873, P<0.01), in an individual basis. Muscimol, baclofen, MK-801 and pyridoxine prevented MMA-induced convulsions, but only MK-801 and pyridoxine prevented MMA-induced GAD inhibition. These data suggest GABAergic mechanisms are involved in the convulsive action of MMA, and that GAD inhibition by MMA depends on the activation of NMDA receptors. While in this study we present novel data about the role of the GABAergic system in MMA-induced convulsions, the central role of NMDA receptors in the neurochemical actions of MMA is further reinforced since they seem to trigger GABAergic failure.

Ca2+-permeable AMPA receptors and intracellular Ca2+ determine motoneuron vulnerability in rat spinal cord in vivo

Excitotoxicity mediated by overactivation of glutamate receptors, particularly the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) type, has been implicated in motoneuron degeneration. AMPA receptors lacking the GluR2 subunit are permeable to Ca(2+) and the entrance of this cation might be responsible for the selective vulnerability of spinal motoneurons in amyotrophic lateral sclerosis (ALS). To evaluate this hypothesis in vivo, we have used a model of motoneuron death in which AMPA, perfused by microdialysis in the rat lumbar spinal cord, produces ipsilateral paralysis and a remarkable loss of spinal motoneurons. Perfusion of 1-naphthyl acetyl spermine, a selective blocker of the Ca(2+)-permeable AMPA receptors, and of the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM), prevented the AMPA-induced paralysis and reduced by about 50% the loss of motoneurons. In addition, perfusion of pyruvate, an energy metabolic substrate, similarly prevented the paralysis and the motoneuron death. These results suggest that functional AMPA receptors lacking the GluR2 subunit are present in the rat spinal cord, and that motoneuron death is triggered by an increase of intracellular Ca(2+) via such Ca(2+)-permeable AMPA receptors. Our finding that pyruvate also protected against the excitotoxic effects of AMPA suggests that the increased intracellular Ca(2+) probably interferes with the mitochondrial energetic metabolism.

Cerebral neurons of transgenic ALS mice are vulnerable to glutamate release stimulation but not to increased extracellular glutamate due to transport blockade

Mechanisms of motor neuron loss in amyotrophic lateral sclerosis (ALS) are unknown, but it has been postulated that excitotoxicity due to excessive glutamatergic neurotransmission by decreased efficiency of glutamate transport may be involved in both familial (FALS) and sporadic ALS. Using microdialysis in vivo, we tested the effects of the glutamate transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (PDC) and of 4-aminopyridine (4-AP), which stimulates glutamate release from nerve endings, in the hippocampus and motor cortex of wild type (WT) and transgenic SOD1/G93A mice, an established model of FALS. Perfusion of 4-AP induced convulsions, expression of the inducible stress-marker heat-shock protein 70 (HSP70) and hippocampal neuronal loss. These effects were similar in both WT and G93A mice, and, in both groups, they were prevented by the previous systemic administration of the NMDA receptor antagonist MK-801. In contrast, perfusion of PDC resulted in a large and long-lasting (2 h) increase of extracellular glutamate, but no convulsions, neuronal damage or HSP70 expression were observed in either the WT or the G93A mice. Our results demonstrate that SOD1 G93A mutation does not enhance the vulnerability to endogenous glutamate-mediated excitotoxicity in brain, neither by blocking glutamate transport nor by stimulating its release. Therefore, these data do not support the possibility that glutamate transport deficiency may be an important factor of brain neuronal degeneration in familial ALS.

Effects of retigabine on the neurodegeneration and extracellular glutamate changes induced by 4-aminopyridine in rat hippocampus in vivo

We have previously shown that microdialysis perfusion of the K+ channel blocker 4-aminopyridine (4-AP) in rat hippocampus induces convulsions and neurodegeneration, due to the stimulation of glutamate release from synaptic terminals. Retigabine is an opener of the KCNQ2/Q3-type K+ channel that possesses antiepileptic action and may be neuroprotective, and we have therefore studied its effect on the hyperexcitation, the neuronal damage and the changes in extracellular glutamate induced by 4-AP. Retigabine and 4-AP were co-administered by microdialysis in the hippocampus of anesthetized rats, with simultaneous recording of the EEG, and the extracellular concentration of glutamate was measured in the microdialysis fractions. In 70-80% of the rats tested retigabine reduced the 4-AP-induced stimulation of glutamate release and prevented the neuronal damage observed at 24 h in the CA1 hippocampal region. However, retigabine did not block the EEG epileptic discharges and their duration was reduced in only 20-25% of the tested animals. We conclude that the neuroprotective action of retigabine is probably due to the blockade of the 4-AP-induced stimulation of glutamate release. This inhibition, however, was not sufficient to block the epileptic activity.

LateN-methyl-d-aspartate receptor blockade rescues hippocampal neurons from excitotoxic stress and death after 4-aminopyridine-induced epilepsy

The intrahippocampal perfusion of 4-aminopyridine (4-AP) in the rat produces immediate seizures and delayed neuronal death, due to the overactivation of N-methyl-D-aspartate (NMDA) receptors by endogenous glutamate released from nerve endings. With the same time course, 4-AP also induces the expression of the cell stress marker heat shock protein 70 (HSP70) in the contralateral non-damaged hippocampus. We have used this experimental model to study the mechanisms of the delayed neuronal stress and death. The NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801), administered intraperitoneally 30 or 60 but not 120 min after 4-AP perfusion, when animals show intense electroencephalography epileptiform activity, prevented the delayed neurodegeneration whereas the seizures continued for about 3 h as in the control animals. With an identical time window, MK-801 treatment also modified the pattern of HSP70 expression; the protein was expressed in the protected perfused hippocampus but no longer in the undamaged contralateral hippocampus. The possible role of Ca2+ in the delayed cell death and HSP70 expression was also studied by coperfusing the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester) with 4-AP. This treatment resulted in protective and HSP70 effects very similar to those of MK-801. These results suggest that the seizures are not linked to neurodegeneration and that NMDA receptors need to be continuously overactivated by endogenous glutamate for at least 60 min in order to induce delayed neuronal stress and death, which are dependent on Ca2+ entry through the NMDA receptor channel.

AMPA receptor activation, but not the accumulation of endogenous extracellular glutamate, induces paralysis and motor neuron death in rat spinal cord in vivo

The mechanisms of motor neuron (MN) degeneration in amyotrophic lateral sclerosis (ALS) are unknown, but glutamate-mediated excitotoxicity may be involved. To examine directly this idea in vivo, we have used microdialysis in the rat lumbar spinal cord and showed that four- to fivefold increases in the concentration of endogenous extracellular glutamate during at least 1 h, by perfusion with the glutamate transport inhibitor L-2,4-trans-pyrrolidine-dicarboxylate, elicited no motor alterations or MN damage. Stimulation of glutamate release with 4-aminopyridine induced transitory ipsilateral hindlimb muscular twitches but no MN damage. In contrast, perfusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) did not modify glutamate levels but produced intense muscular spasms, followed by ipsilateral permanent hindlimb paralysis and a remarkable loss of MNs. These effects of AMPA were prevented by co-perfusion with the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline. Perfusion with NMDA or kainate produced no motor effects or MN damage. Thus, the elevation of endogenous extracellular glutamate in vivo due to blockade of its transport is innocuous for spinal MNs. Because this resistance is observed under the same experimental conditions in which MNs are highly vulnerable to AMPA, these results indicate that excitotoxicity due to this mechanism might not be an important factor in the pathogenesis of ALS.

Continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat

OBJECTIVE

To explore the effect of continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat.

METHODS

An electrode and cannula complex was implanted in adult male Wistar rats, the electrode being targeted to the left basolateral amygdala. The animals were subjected to a standard kindling procedure. Osmotic minipumps filled with either GABA or mannitol were connected to cannulas and allowed to infuse during 7 days. Kindling experiments measuring after-discharge and seizure thresholds, seizure severity and duration, and behavioral toxicity were performed before, during and after the drug infusion period.

RESULTS

Both after-discharge and seizure thresholds were significantly increased both during and after GABA infusion with respect to pre-infusion controls, while the group receiving mannitol showed no significant differences. Seizure duration was not affected by GABA infusion. Seizures were either completely blocked or fully developed.

CONCLUSIONS

Direct continuous GABA infusion within the epileptogenic focus raises the seizure thresholds without affecting the generalized components in the amygdala kindling model of epilepsy.

Epilepsy, neurodegeneration, and extracellular glutamate in the hippocampus of awake and anesthetized rats treated with okadaic acid

We have previously shown that the intrahippocampal microinjection of okadaic acid (OKA), a potent inhibitor of serine/threonine protein phosphatases, induces epileptic seizures, neuronal death, and the hyperphosphorylation of the NR2B subunit of the N-methyl-D-aspartate (NMDA) receptor. We administered OKA by reverse microdialysis in the hippocampus of awake and halothane-anesthetized rats, with simultaneous collection of microdialysis fractions and recording of the EEG activity, and subsequent histological analysis. OKA produced intense behavioral and persistent EEG seizure activity in the awake rats but not in the anesthetized animals, and did not significantly alter the extracellular concentration of glutamate and aspartate detected in the microdialysis fractions. One day after the experiment a remarkable neurodegeneration of CA1 hippocampal region was observed in both the awake and the anesthetized rats. We conclude that the OKA-induced epilepsy cannot be ascribed to increased extracellular glutamate, but to an increased sensitivity of NMDA receptor. We propose that halothane protected against the epilepsy because it blocks NMDA receptor overactivation, and that the neurodegeneration of CA1 region is independent of this overactivation and due probably to alterations of cytoskeletal proteins consequent to the OKA-induced hyperphosphorylation.

Expression of heat shock protein 70 induced by 4-aminopyridine through glutamate-mediated excitotoxic stress in rat hippocampus in vivo

The intrahippocampal administration of 4-aminopyridine (4-AP) induces epileptic seizures and neurodegeneration, due probably to stimulation of glutamate release from synaptic terminals. We have studied the time course of the neurodegenerative changes produced by 4-AP, perfused through microdialysis cannulas in rat hippocampus, and correlated them with the expression of the inducible heat shock protein 70 (HSP70), detected immunocytochemically. Electroencephalographic seizure activity appeared immediately after the beginning of 4-AP perfusion. The first signs of histological neuronal damage were observed in CA1 and CA3 subfields of the perfused hippocampus 3 h after treatment and progressed until reaching a maximal neuronal loss at 24 h. In 4-AP-treated rats HSP70 was expressed mainly in neurons of the contralateral hippocampus, with a time course and cellular distribution very similar to the neurodegeneration observed in the perfused hippocampus, but no neuronal damage was observed. The N-methyl-D-aspartate (NMDA) receptor antagonists MK-801 and (3-phosphonopropyl)-piperazine-2-carboxylic acid prevented the seizures, the neurodegeneration and the expression of HSP70. These data demonstrate that the 4-AP-induced release of endogenous glutamate overactivates NMDA receptors in the perfused hippocampus and that the resulting neuronal hyperexcitability propagates to the contralateral hippocampus, generating a glutamate-mediated neuronal stress sufficient to induce the expression of HSP70 but not to produce neurodegeneration. These findings provide a useful model for investigating the relationships between neuronal hyperexcitation, neurodegeneration and the role of HSP expression.

Biochemical characteristics of the gamma-aminobutyric acid system in the insulinoma cell lines HIT-T15, RIN-m5F, betaTC3, and comparison with rat brain

BACKGROUND

gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the mammalian brain. Both GABA and its synthesizing enzyme, L-glutamate decarboxylase (GAD), are also present in the insulin-secreting pancreatic beta cells, in which its physiologic role is unclear. We have studied several aspects of the GABA system in the insulinoma cell lines HIT-T15, RIN-m5F, and betaTC3 in comparison with rat brain tissue.

METHODS

Insulinoma cell lines and embryonic rat brain cortex neurons were cultured. GAD activity was determined by a radioenzymatic method and the presence of GAD(67) protein was assessed by immunocytochemistry. Amino acid content and the effect of different conditions on the release of endogenous GABA were measured by HPLC and fluorometric detection after o-phthaldialdehyde derivatization. [3H]GABA was used for measuring the uptake of the amino acid in the insulinoma cultures and in rat forebrain synaptosomes.

RESULTS

The three insulinoma lines possess GABA and GAD activity at levels of approximately 20% compared with adult rat brain cortex. Dissimilar from the latter, in insulinoma cultures enzyme activity was not enhanced by addition of an excess of the coenzyme pyridoxal-5'-phosphate. Immunocytochemical visualization of GAD showed that the cells in both neuronal cultures and insulinoma lines were GAD(67)-positive, similar to Purkinje cell somata of adult rat cerebellar cortex. [3H]GABA uptake in the cell lines was approximately 10% of that in rat forebrain synaptosomes and showed less ionic and temperature dependence. In both cultured cerebral neurons and RINm5F cells, the addition of arginine induced the release of GABA, whereas neither high K(+) concentration nor glucose had any effect.

CONCLUSIONS

The insulinoma cell lines studied possess the same GAD(67) form of the enzyme present in brain. RIN line cells are capable of transporting glutamate. In these cells as well as in cultured cortical neurons, arginine stimulates the release of GABA and glutamate probably as the result of its electrogenic transport. Insulinoma cell lines may therefore be useful to study GABA metabolism and function in pancreatic beta cells.

Seizures and neurodegeneration induced by 4-aminopyridine in rat hippocampus in vivo: role of glutamate- and GABA-mediated neurotransmission and of ion channels

Infusion of the K(+) channel blocker 4-aminopyridine in the hippocampus induces the release of glutamate, as well as seizures and neurodegeneration. Since an imbalance between excitation and inhibition, as well as alterations of ion channels, may be involved in these effects of 4-aminopyridine, we have studied whether they are modified by drugs that block glutamatergic transmission or ion channels, or drugs that potentiate GABA-mediated transmission. The drugs were administered to anesthetized rats subjected to intrahippocampal infusion of 4-aminopyridine through microdialysis probes, with simultaneous collection of dialysis perfusates and recording of the electroencephalogram, and subsequent histological analysis. Ionotropic glutamate receptor antagonists clearly diminished the intensity of seizures and prevented the neuronal damage, but did not alter substantially the enhancement of extracellular glutamate induced by 4-aminopyridine. None of the drugs facilitating GABA-mediated transmission, including uptake blockers, GABA-transaminase inhibitors and agonists of the A-type receptor, was able to reduce the glutamate release, seizures or neuronal damage produced by 4-aminopyridine. In contrast, nipecotate, which notably increased extracellular levels of the amino acid, potentiated the intensity of seizures and the neurodegeneration. GABA(A) receptor antagonists partially reduced the extracellular accumulation of glutamate induced by 4-aminopyridine, but did not exert any protective action. Tetrodotoxin largely prevented the increase of extracellular glutamate, the electroencephalographic epileptic discharges and the neuronal death in the CA1 and CA3 hippocampal regions. Valproate and carbamazepine, also Na(+) channel blockers that possess general anticonvulsant action, failed to modify the three effects of 4-aminopyridine studied. The N-type Ca(2+) channel blocker omega-conotoxin, the K(+) channel opener diazoxide, and the non-specific ion channel blocker riluzole diminished the enhancement of extracellular glutamate and slightly protected against the neurodegeneration. However, the two former compounds did not antagonize the 4-aminopyridine-induced epileptiform discharges, and riluzole instead markedly increased the intensity and duration of the disharges. Moreover, at the highest dose tested (8mg/kg, i.p.), riluzole caused a 75% mortality of the rats. We conclude that 4-aminopyridine stimulates the release of glutamate from nerve endings and that the resultant augmented extracellular glutamate is directly related to the neurodegeneration and is involved in the generation of epileptiform discharges through the concomitant overactivation of glutamate receptors. Under these conditions, a facilitated GABA-mediated transmission may paradoxically boost neuronal hyperexcitation. Riluzole, a drug used to treat amyotrophic lateral sclerosis, seems to be toxic when combined with neuronal hyperexcitation.

In vivo potentiation of glutamate-mediated neuronal damage after chronic administration of the glycolysis inhibitor iodoacetate

Neuronal damage associated with cerebral ischemia and hypoglycemia might be the consequence of the extracellular accumulation of excitatory amino acids. In previous studies we showed that elevation of glutamate and aspartate extracellular levels by inhibition of its uptake in vivo is not sufficient to induce neuronal damage unless mitochondrial energy metabolism is compromised. In the present study we show that chronic systemic administration of the glycolysis inhibitor iodoacetate (25 mg/kg) induces no damage to the brain per se but enhances neuronal vulnerability to glutamate-mediated neurotoxicity in the hippocampus. Tissue injury is well protected either by antagonizing NMDA glutamate receptors with MK-801 or by administration of pyruvate, a substrate of the tricarboxylic acid cycle. In contrast to systemic treatment, local infusions through a dialysis probe of 5 mM iodoacetate into the hippocampus induced acute lesions not sensitive to MK-801. Iodoacetate intrahippocampal perfusion induced substantial increases in the extracellular levels of glutamate (3.5-fold), taurine (8.8-fold), and particularly aspartate (35-fold). Neuronal damage under this conditions occurs very rapidly as revealed by the histological analysis of animals transcardially perfused immediately after iodoacetate perfusion. Aspartate might contribute to neuronal damage since intrahippocampal administration of this amino acid (600 nmol/microl) induces extensive lesions. The present study might suggest that impairment of glucose oxidation through the glycolytic pathway in vivo facilitates glutamate neurotoxicity. Additionally, the results indicate that pyruvate might prevent as efficiently as glutamate receptor antagonists glutamate-mediated neuronal damage associated with ischemia/hypoglycemia.

Action of 4-aminopyridine on extracellular amino acids in hippocampus and entorhinal cortex: a dual microdialysis and electroencehalographic study in awake rats

In order to study the role of amino acids in the hippocampus and the entorhinal cortex during the convulsive process induced by 4-aminopyridine (4-AP), we have used a device allowing the simultaneous microdialysis and the recording of their electrical activity of both regions in freely moving rats. We found that infusion of 4-AP into the entorhinal cortex resulted in a large increase in extracellular glutamate and glutamine and small increases in glycine and taurine levels. Likewise, infusion of 4-AP into the hippocampus resulted in a major increase in glutamate, as well as slight increases in taurine and glycine. In both infused regions the peak concentration of extracellular glutamate was observed 15 min after 4-AP administration. No significant changes were found in the non-infused hippocampus or entorhinal cortex of the same rats. Simultaneous electroencephalographic recordings showed intense epileptiform activity starting during 4-AP infusion and lasting for the rest of the experiment (1 h) in both the entorhinal cortex and the hippocampus. The discharges were characterized by poly-spikes and spike-wave complexes that propagated almost immediately to the other region studied. These findings suggest that increased glutamatergic synaptic function in the circuit that connects both regions is involved in the epileptic seizures induced by 4-AP.

Neocortical hyperexcitability after GABA withdrawal in vitro

The sharp interruption of the intracortical instillation of exogenous gamma-aminobutyric acid (GABA), generates an epileptic focus in mammals. Seizures elicited by GABA withdrawal last several days or weeks. The present work reports that GABA withdrawal-induced hyperexcitability can be produced in vitro: a sudden withdrawal of GABA (5 mM; 120 min) or benzodiazepine (60 microM flunitrazepam) from the superfusion, induced a gradual increase in the amplitude of the evoked population spike (PS) recorded on neocortical slices. PS enhancement reached 150% above the control value 2.5 h after GABA withdrawal. GABA withdrawal-induced hyperexcitability was facilitated by progesterone. PS enhancement induced by GABA withdrawal was associated with an impairment of GABA transmission occurring before epileptiform discharges were fully established. Paired pulse inhibition and evoked [3H]-GABA release appear decreased; suggesting that cortical hyperexcitability as a result of GABA withdrawal involves pre-synaptic changes. Specific muscimol binding decreased during GABA superfusion but recovered after GABA withdrawal. However, the sensitivity of the post-synaptic response to 3alpha-OH-5alpha-pregnan-20-one or allopregnanolone (alloP) was enhanced after GABA withdrawal, suggesting a functional change in the GABA(A) receptors. The changes described may be the cellular correlates of the withdrawal syndromes appearing after interruption of the administration of GABA(A) receptor agonists.

Relationships among seizures, extracellular amino acid changes, and neurodegeneration induced by 4-aminopyridine in rat hippocampus: a microdialysis and electroencephalographic study

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.

Transient inhibition of glutamate uptake in vivo induces neurodegeneration when energy metabolism is impaired

Impairment of glutamate transport during ischemia might be related to the elevation of the extracellular concentration of glutamate and ischemic neuronal damage. Additionally, impairment of energy metabolism in vivo leads to neurodegeneration apparently mediated by a secondary excitotoxic mechanism. In vitro observations show that glucose deprivation and inhibition of energy metabolism exacerbate the toxic effects of glutamate. We have previously shown that glutamate uptake inhibition in vivo by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) leads to a substantial elevation in the extracellular concentration of excitatory amino acids that is not associated with cell death. These observations suggest that energy depletion during ischemia might be determinant of ischemic neuronal damage. To investigate whether impairment of energy metabolism in vivo increases neuronal susceptibility to glutamate uptake inhibition, we studied the effect of glutamate accumulation induced by the intrahippocampal or intrastriatal administration of PDC in energy-deficient rats chronically treated with 3-nitropropionic acid (3-NP), which irreversibly inhibits the tricarboxylic acid cycle and electron transport chain. Extracellular glutamate levels were monitored by HPLC from fractions collected from microdialysis probes, and neuronal damage was evaluated by histological analysis. Our results show that glutamate uptake inhibition leads to marked neuronal damage in energy-deficient rats but not in intact animals, which apparently is not related to an additional elevation of glutamate levels induced by 3-NP.

Inhibition of glutamate uptake induces progressive accumulation of extracellular glutamate and neuronal damage in rat cortical cultures

It is known that neurons exposed to high concentrations of glutamate degenerate and die. The clearance of this amino acid from the extracellular space depends on their active transport by Na(+)-dependent high-affinity carriers. In the present study we tested whether inhibition of glutamate transport in mixed glial/neuronal cortical cultures induces accumulation of extracellular glutamate and whether such increase results in cell damage. Three inhibitors of glutamate transport were used: L-trans-pyrrolidine-2,4-dicarboxylate (PDC), DL-threo-beta-hydroxyaspartate (THA), and dihydrokainate (DHK). Cell damage was assessed by light microscopy observations, reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, and leakage of lactate dehydrogenase. PDC induced a significant concentration- and time-dependent neuronal damage, whereas pure glial cultures were not affected. A good correlation was found between this damage and elevations of glutamate concentration in the medium. These effects of PDC were similar in glutamine-free medium and in medium supplemented with glutamine. THA induced identical cell damage and elevations of extracellular glutamate to those produced by PDC, while DHK did not affect at all any of these parameters. PDC- and THA-induced toxicity was protected by the N-methyl-D-aspartate receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo-(a,d)cyclohepten-5,10-imine maleate but not by the non-N-methyl-D-aspartate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline.

Preferential stimulation of glutamate release by 4-aminopyridine in rat striatum in vivo

The potassium channel blocker 4-aminopyridine (4-AP) is a potent convulsant drug which, in vitro, stimulates the release of neurotransmitter amino acids. We have studied the effect of 4-AP in vivo on the extracellular concentration of amino acids in rat striatum, by means of microdialysis and HPLC. Perfusion with 4-AP in the awake animal produced intense motor alterations, including barrel turning and running fits. Therefore, most microdialysis experiments were carried out in anesthetized rats. Perfusion with 20-75 mM 4-AP for 12.5 min resulted in a massive increase in extracellular glutamate (up to 20-fold), smaller increases in aspartate and taurine (up to 10-fold) and slight increments in glutamine, alanine, glycine and GABA. In contrast, perfusion with 100 mM K+ produced, mainly, an increment in taurine (7-fold) and modest increases in glutamate and aspartate (100-300%), as well as a notable decrease in glutamine. Tetraethylammonium (TEA, 120 mM) perfusion induced taurine and glutamate elevations similar to those after high K+, but glutamine was not affected. In unanesthetized rats, perfusion with 40 mM 4-AP induced changes in extracellular amino acids similar to those observed under anesthesia. In these animals neither high K+ nor TEA affected significantly the motor behavior. The results suggest that an enhancement of glutamatergic synaptic transmission, rather than a general depolarizing action, is an important factor in the neuronal hyperexcitability induced by 4-AP, which is consistent with the previously demonstrated inhibition of its convulsant effect by glutamate receptor antagonists.

Allopregnanolone potentiates a GABA-withdrawal syndrome in the rat cerebral cortex

We have studied the neuromodulatory effect of the neurosteroid 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone-3 alpha-5 alpha P-) in the GABA-withdrawal syndrome (GWS). This is a model of partial epilepsy consisting of an enduring paraoxysmal activity recorded at the site of GABA infusion that depends, for its induction, on GABA receptor activation. Rats were chronically implanted for frontal and occipital EEG recording with infusion cannulae fixed on the somatomotor cortical region. When the neurosteroid was infused after or concurrently with GABA, a potentiation of the GWS (i.e. shorter latency and prolonged duration) was observed. No modifications in EEG activity were detected when allopregnanolone was administered alone or prior to GABA administration. These results indicate a neuromodulatory effect of allopregnanolone, dependent on the presence of GABA at the receptor site.

beta-Amyloid peptide fragment 25-35 potentiates the calcium-dependent release of excitatory amino acids from depolarized hippocampal slices

beta-Amyloid protein (beta AP) has been frequently associated with the neuropathology of Alzheimer's disease (AD), although the mechanisms by which it can induce neurodegeneration are still unknown. Some studies in hippocampal cultured neurons suggest that beta AP, particularly its fragment 25-35, may induce neural growth or render neurons more vulnerable to excitotoxic insults by a mechanism involving intracellular Ca2+ dyshomeostasis. We have studied the effect of fragment 25-35 on the release of endogenous amino acids from hippocampal slices of young adult (3-3.5-month-old) and aged (23-25-month-old) rats, under basal, K(+)-depolarization, and post-depolarization conditions, in the presence and absence of Ca2+. In both young and aged tissue, the basal release of amino acids was not affected by the peptide. By contrast, 1-hr preincubation of slices from young animals with 10 microM 25-35 fragment resulted in a 140% increase of glutamate and aspartate release stimulated by K+ depolarization, compared with the control-stimulated release. These effects were strictly dependent on external Ca2+. Neither the K(+)-stimulated release of gamma-amino butyric acid (GABA) nor the release of glycine, glutamine, taurine, or alanine, which was not stimulated by high K+, were affected. Substance P and a scrambled sequence of the 25-35 fragment were without any effect per se, but substance P blocked the stimulatory effect of fragment 25-35 on glutamate and aspartate release. In slices from aged rats the basal release of glutamate was significantly higher (260%) than that in young tissue, and the K(+)-induced release of both aspartate and glutamate was also higher.(ABSTRACT TRUNCATED AT 250 WORDS)