Inhibition of glutamate decarboxylase activity by 3-mercaptopropionic acid has different time course in the immature and adult rat brains

Neurochemical investigation of multiple locally induced seizures using microdialysis sampling: Epilepsy effects on glutamate release

The objective of this study was to develop an in vivo model for locally induced epilepsy. Epilepsy is a prominent neurological disorder that affects millions of people worldwide. Patients may experience either global seizures, affecting the entire brain, or focal seizures, affecting only one brain region. The majority of epileptic patients experience focal seizures but they go undiagnosed because such seizures can be difficult to detect. To better understand the effects of focal epilepsy on the neurochemistry of a brain region with high seizure diathesis, an animal model for locally induced seizures in the hippocampus was developed. In this model, two seizure events were chemically induced by administering the epileptogenic agent, 3-mercaptopropionic acid (3-MPA), to the hippocampus to disturb the balance between excitatory and inhibitory neurotransmitters in the brain. Microdialysis was used for local delivery of 3-MPA as well as for collection of dialysate for neurochemical analyses. Two periods of seizures separated by varying inter-seizure recovery times were employed, and changes in the release of the excitatory transmitter, glutamate, were measured. Significant differences in glutamate release were observed between the first and second seizure episodes. Diminished glutamate biosynthesis, enhanced glutamate re-uptake, and/or neuronal death were considered possible causes of the attenuated glutamate release during the second seizure episode. Biochemical measurements were indicative that a combination of these factors led to the attenuation in glutamate release.

Monoterpenoid terpinen-4-ol exhibits anticonvulsant activity in behavioural and electrophysiological studies

Terpinen-4-ol (4TRP) is a monoterpenoid alcoholic component of essential oils obtained from several aromatic plants. We investigated the psychopharmacological and electrophysiological activities of 4TRP in male Swiss mice and Wistar rats. 4TRP was administered intraperitoneally (i.p.) at doses of 25 to 200 mg/kg and intracerebroventricularly (i.c.v.) at concentrations of 10, 20, and 40 ng/2 μL. For in vitro experiments, 4TRP concentrations were 0.1 mM and 1.0 mM. 4TRP (i.p.) inhibited pentylenetetrazol- (PTZ-) induced seizures, indicating anticonvulsant effects. Electroencephalographic recordings showed that 4TRP (i.c.v.) protected against PTZ-induced seizures, corroborating the behavioural results. To determine whether 4TRP exerts anticonvulsant effects via regulation of GABAergic neurotransmission, we measured convulsions induced by 3-mercapto-propionic acid (3-MP). The obtained results showed involvement of the GABAergic system in the anticonvulsant action exerted by 4TRP, but flumazenil, a selective antagonist of the benzodiazepine site of the GABA_A receptor, did not reverse the anticonvulsant effect, demonstrating that 4TRP does not bind to the benzodiazepine-binding site. Furthermore, 4TRP decreased the sodium current through voltage-dependent sodium channels, and thus its anticonvulsant effect may be related to changes in neuronal excitability because of modulation of these channels.

Exogenous γ-aminobutyric acid (GABA) affects pollen tube growth via modulating putative Ca2+-permeable membrane channels and is coupled to negative regulation on glutamate decarboxylase

γ-Aminobutyric acid (GABA) is implicated in pollen tube growth, but the molecular and cellular mechanisms that it mediates are largely unknown. Here, it is shown that exogenous GABA modulates putative Ca(2+)-permeable channels on the plasma membranes of tobacco pollen grains and pollen tubes. Whole-cell voltage-clamp experiments and non-invasive micromeasurement technology (NMT) revealed that the influx of Ca(2+) increases in pollen tubes in response to exogenous GABA. It is also demonstrated that glutamate decarboxylase (GAD), the rate-limiting enzyme of GABA biosynthesis, is involved in feedback controls of Ca(2+)-permeable channels to fluctuate intracellular GABA levels and thus modulate pollen tube growth. The findings suggest that GAD activity linked with Ca(2+)-permeable channels relays an extracellular GABA signal and integrates multiple signal pathways to modulate tobacco pollen tube growth. Thus, the data explain how GABA mediates the communication between the style and the growing pollen tubes.

Long-term fish oil supplementation attenuates seizure activity in the amygdala induced by 3-mercaptopropionic acid in adult male rats

Several studies have provided evidence of significant effects of omega-3 fatty acids on brain functionality, including seizures and disorders such as epilepsy. Fish oil (FO) is a marine product rich in unsaturated omega-3 fatty acids. Considering that the amygdala is one of the brain structures most sensitive to seizure generation, we aimed to evaluate the effect of long-term chronic FO supplementation (from embryonic conception to adulthood) on the severity of seizures and amygdaloid electroencephalographic activity (EEG) in a 3-mercaptopropionic acid (3-MPA)-induced seizure model using adult rats. Female Wistar rats were fed a commercial diet supplemented daily with FO (300mg/kg) from puberty through mating, gestation, delivery, and weaning of the pups. Only the male pups were then fed daily with a commercial diet supplemented with the same treatment as the dam up to the age of 150days postpartum, when they were bilaterally implanted in the amygdala to record behavior and EEG activity before, during, and after seizures induced by administering 3-MPA. Results were compared with those obtained from rats supplemented with palm oil (PO) and rats treated with a vehicle (CTRL). The male rats treated with FO showed longer latency to seizure onset, fewer convulsive episodes, and attenuated severity compared those in the PO and CTRL groups according to the Racine scale. Moreover, long-term FO supplementation was associated with a reduction of the absolute power (AP) of the fast frequencies (12-25Hz) in the amygdala during the seizure periods. These findings support the idea that chronic supplementation with omega-3 of marine origin may have antiseizure properties as other studies have suggested.

Microperfusion of 3-MPA into the brain augments GABA

In vivo effects of microperfusion of a GABA synthesis inhibitor (3-MPA) into the striatum and hippocampus on amino acid concentrations and electrical neuronal activity were investigated. Paradoxical elevations in GABA in the striatum (5-fold in anesthetized and 50-fold in awake rats) and hippocampus (2-fold in anesthetized and 15-fold in awake rats) were documented under steady-state concentrations of 3-MPA along with expected increases in glutamate (a 15-fold increase and a 250-fold increase in the striatum of anesthetized and awake rats, respectively; a 7-fold increase and a 25-fold increase in the hippocampus of anesthetized and awake rats, respectively). There was no clear epileptiform or seizure activity. Explanations for the paradoxical increase in GABA are offered, and emphasis is placed on the dependency of disinhibition on the model in which its effects are studied as well as on the prevailing level of activation of the probed network.

Thirty years beyond discovery--clinical trials in succinic semialdehyde dehydrogenase deficiency, a disorder of GABA metabolism

This review summarizes a presentation made at the retirement Symposium of Prof. Dr. Cornelis Jakobs in November of 2011, highlighting the progress toward clinical trials in succinic semialdehyde dehydrogenase (SSADH) deficiency, a disorder first recognized in 1981. Active and potential clinical interventions, including vigabatrin, L-cycloserine, the GHB receptor antagonist NCS-382, and the ketogenic diet, are discussed. Several biomarkers to gauge clinical efficacy have been identified, including cerebrospinal fluid metabolites, neuropsychiatric testing, MRI, EEG, and measures of GABAergic function including (11 C)flumazenil positron emission tomography (PET) and transcranial magnetic stimulation (TMS). Thirty years after its discovery, encompassing extensive studies in both patients and the corresponding murine model, we are now running an open-label trial of taurine intervention, and are poised to undertake a phase II trial of the GABAB receptor antagonist SGS742.

Maintenance of thalamic epileptiform activity depends on the astrocytic glutamate-glutamine cycle

The generation of prolonged neuronal activity depends on the maintenance of synaptic neurotransmitter pools. The astrocytic glutamate-glutamine cycle is a major mechanism for recycling the neurotransmitters GABA and glutamate. Here we tested the effect of disrupting the glutamate-glutamine cycle on two types of neuronal activity patterns in the thalamus: sleep-related spindles and epileptiform oscillations. In recording conditions believed to induce glutamine scarcity, epileptiform oscillations showed a progressive reduction in duration that was partially reversible by the application of exogenous glutamine (300 muM). Blocking uptake of glutamine into neurons with alpha-(methylamino) isobutyric acid (5 mM) caused a similar reduction in oscillation duration, as did blocking neuronal GABA synthesis with 3-mercaptoproprionic acid (10 muM). However, comparable manipulations did not affect sleep spindles. Together, these results support a crucial role for the glutamate-glutamine cycle in providing the neurotransmitters necessary for the generation of epileptiform activity and suggest potential therapeutic approaches that selectively reduce seizure activity but maintain normal neuronal activity.

Blockade of GABA synthesis only affects neural excitability under activated conditions in rat hippocampal slices

The primary goal of this study was to establish whether inhibition of GABA synthesis was sufficient to induce network hyperexcitability in a rat hippocampal slice model comparable to that seen with GABA receptor blockade. We used field and intracellular recordings from the CA1 region of rat hippocampal slices to determine the physiological effects of blocking GABA synthesis with the convulsant, 3-mercaptoproprionic acid (MPA). We measured the rate of synthesis of GABA and glutamate in slices using 2-13C-glucose as a label source and liquid chromatography-tandem mass spectrometry. There was little effect of 3.5mM MPA on evoked events under control recording conditions. Tissue excitability was enhanced following a series of stimulus trains; this effect was enhanced when GABA transport was blocked. Evoked inhibitory potentials (IPSPs) failed following repetitive stimulation and MPA. Spontaneous epileptiform activity was seen reliably with elevated extracellular potassium (5mM). GABA synthesis decreased by 49% with MPA alone and 45% with the combination of MPA and excess potassium; GABA content was not substantially altered. Our data indicate: (1) GABAergic inhibition cannot be significantly compromised by MPA without network activation; (2) GABAergic synaptic inhibition is mediated by newly synthesized GABA; (3) there is a depletable pool of GABA that can sustain GABAergic inhibition when synthesis is impaired under basal, but not activated conditions; (4) overt hyperexcitability is only seen when newly synthesized GABA levels are low.

An investigation into the pharmacokinetics of 3-mercaptopropionic acid and development of a steady-state chemical seizure model using in vivo microdialysis and electrophysiological monitoring

OBJECTIVES

The goal of the present study was to develop a chemical seizure model using the convulsant, 3-mercaptopropionic acid (3-MPA). A pharmacodynamics approach was taken, combining in vivo microdialysis sampling with electrophysiological methods to simultaneously monitor, in real-time, the 3-MPA concentration in the brain and the corresponding electrocorticographic (ECoG) activity.

METHODS

The 3-MPA was administered in two doses (50 and 100 mg/kg) in order to study its pharmacokinetics. Microdialysis samples were collected from the striatum, hippocampus, and jugular vein every 5 min. The microdialysates were analyzed using high-performance liquid chromatography with electrochemical detection (HPLC-EC). The ECoG activity was monitored via screws placed onto the cortex. Noncompartmental pharmacokinetics analysis was performed to obtain the elimination constants (K(e)), the maximum concentration (C(max)), the time to achieve maximum concentration (T(max)), and the area under the concentration-time curves (AUC(inf)).

RESULTS

The average brain K(e) for the 50 and the 100mg/kg doses were 0.060 and 0.018 min(-1), respectively. The brain AUC(inf) for the 50 and 100mg/kg doses were 353 and 2168 mg min(-1)mL(-1), respectively. This led to a 67-fold increase in the observed number of seizures in the higher dose with the average seizure intensity double that of the smaller dose. These data led to the dosing scheme for the chemical seizure model of administering a 3-MPA loading dose of 60 mg/kg followed by a constant infusion of 50 mg/(kg min(-1)).

CONCLUSIONS

This study describes, to our knowledge, the first successful attempt to combine in vivo microdialysis with electrophysiology to monitor in real-time, the concentration and effects of 3-MPA in the brain. This led to the development of a steady-state chemical seizure model.

Reduced dendrite growth and altered glutamic acid decarboxylase (GAD) 65- and 67-kDa isoform protein expression from mouse cortical GABAergic neurons following excitotoxic injury in vitro

The vulnerability of brain cells to neurologic insults varies greatly, depending on their neuronal subpopulation. However, cells surviving pathological insults such as ischemia or brain trauma may undergo structural changes, e.g., altered process growth, that could compromise brain function. In this study, we examined the effect of glutamate excitotoxicity on dendrite growth from surviving cortical GABAergic neurons in vitro. Glutamate exposure did not affect GABAergic neuron viability, however, it significantly reduced dendrite growth from GABAergic neurons. This effect was blocked by the AMPA receptor antagonists NBQX and CFM-2, and mimicked by AMPA, but not NMDA. Glutamate excitotoxicity also caused an NMDA receptor-mediated decrease in the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67) immunoreactivity from GABAergic neurons, measured using immunocytochemical and Western blot techniques. GAD is necessary for GABA synthesis; however, reduction of GABA by 3-mercaptopropionic acid (3-MPA), which inhibits GABA synthesis, did not alter dendrite growth. These results suggest that GABAergic cortical neurons are relatively resistant to excitotoxic-induced cell death, but they can display morphological and biochemical alterations which may impair their function.

Influence of convulsants on rat brain activities of alanine aminotransferase and aspartate aminotransferase

There exist differences between 12-day-old and adult rats in the onset of seizures induced by some inhibitors of glutamate decarboxylase (GAD). The aim of study was to investigate if there are differences between both groups in activities of rat brain alanine aminotransferase (ALT) and aspartate aminotransferase (AST), the enzymes involved in glutamate metabolism, after the administration of 3-mercaptopropionic acid as specific GAD inhibitor or isoniazid as less specific general inhibitor of pyridoxal enzymes. Activities of both aminotransferases in a supernatant 20,000 g of the whole brain (containing predominantly cytosolic isoforms of enzymes) were increased at the beginning of 3-mercaptopropionic acid-induced generalized tonic-clonic seizures. At isoniazid-induced generalized tonic-clonic seizures, a significant increase in both enzyme activities was observed in adult rat brain. In the 12-day-old rat brain, ALT and AST activities reached about 40% and about 50-60% of adult control levels, respectively. In in vitro experiments, no influence of 3-mercaptopropionic acid on transaminase activities was found and an inhibitory effect of isoniazid on the enzymes was confirmed. Increased aminotransferase activities might participate in the enhanced synthesis of excitatory amino acid neurotransmitters in the nervous system, which may take a part in the initiation of epileptic seizures. Alternatively, the increased AST activity may be connected with an increased transport of NADH from the cytosol to mitochondria, while the increased ALT activity would represent the transformation of pyruvate to alanine as a consequence of increased glycolysis.

Glutamate dependence of GABA levels in neurons of hypoxic and hypoglycemic rat hippocampal slices

Hypoxia may increase GABA levels in neurons by ATP depletion-induced activation of glutamate decarboxylase and by inhibiting GABA transaminase. Hypoglycemia, which also depletes ATP, reduces neuronal levels of GABA and its precursor glutamate. We examined whether differences in glutamate levels may contribute to these altered GABA levels in hippocampal slices. GABA levels were highly correlated with endogenous glutamate levels during both hypoxia and hypoglycemia (R=0.93 for combined data). Hypoxia maximally increased GABA levels (146+/-6.3% of control, S.E.M.) when glutamate remained above 90% of control levels and ATP was at 30% of control levels. Hypoglycemia with similar ATP levels and glutamate levels at 40% of control decreased GABA levels to 55% of control. Effects of inhibitors of glutamate decarboxylase and GABA transaminase suggested that increased synthesis and decreased catabolism may both contribute to increased hypoxic GABA levels. Immunocytochemical studies suggested that hypoxia increased GABA concentrations primarily in neurons and their processes, but not in glial cells. Severe hypoxic ATP depletion increased the release of both GABA and glutamate. Hypoxia increased GABA levels in neurons, while hypoglycemia with a similar severity of ATP depletion decreased GABA levels. Much of the difference may be related to lower levels of precursor glutamate during hypoglycemia. The twofold higher levels of neuroprotective GABA available for release during hypoxia may contribute to differences in the pathophysiology of these metabolic insults.

Selective susceptibility to inhibitors of GABA synthesis and antagonists of GABA(A) receptor in rats with genetic absence epilepsy

Thalamocortical spike-and-wave discharges characterize the nonconvulsive absence seizures that occur spontaneously in genetic absence epilepsy rats from Strasbourg (GAERS), a selected strain of Wistar rats. GABA is crucial in the generation of absence seizures. The susceptibility to convulsions induced by threshold doses of various GABA receptor antagonists and inhibitors of GABA synthesis, kainic acid and strychnine, was compared in GAERS and in nonepileptic rats from a selected control strain (NE). The brain structures involved in the drug-elicited convulsive seizures were mapped by c-Fos immunohistochemistry. Injection of various antagonists of the GABA(A) receptor, bicuculline and picrotoxin, and inverse agonists of the benzodiazepine site (FG 7142 and DMCM) induced myoclonic spike-and-wave discharges followed by clonic or tonic-clonic seizures with high paroxysmal activity on the cortical EEG. The incidence of the convulsions was dose-dependent and was higher in GAERS than in NE rats. Mapping of c-Fos expression showed that the frontoparietal cortex was constantly involved in the convulsive seizures elicited by a threshold convulsant dose, whereas limbic participation was variable. In contrast, GAERS were less susceptible than NE rats to the tonic-clonic convulsions induced by the inhibitors of glutamate decarboxylase, isoniazide and 3-mercaptopropionic acid. The GABA(B) receptor antagonist CGP 56999 and kainic acid induced a similar incidence of seizures in GAERS and NE rats and predominantly activated the hippocampus. No difference in the tonic seizures elicited by strychnine could be evidenced between the strains. These results suggest that an abnormal cortical GABAergic activity may underlie absence seizures in GAERS.

Simultaneous determination of gamma-aminobutyric acid and glutamic acid in the brain of 3-mercaptopropionic acid-treated rats using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

The measurement of gamma-aminobutyric acid (GABA) and glutamic acid (Glu) in the whole brain and in various regions of the brain in 3-mercaptopropionic acid (3-MPA)-treated rats has been developed using liquid chromatography-mass spectrometry with an atmospheric pressure ionization interface system. The recoveries of these compounds were 94.90+/-4.18% for GABA, 95.60+/-2.86% for Glu after ion-exchange treatment. The detection limits for GABA and Glu were 2.5+/-0.3 microg/ml and 5.0+/-0.8 microg/ml, respectively, when 20 microl sample were injected. GABA concentration in the whole brain decreased gradually to 5 min and reached 63% of normal value after administration of 3-MPA, and the concentration increased gradually thereafter until 60 min. Conversely, the concentration of Glu in the whole brain increased gradually to 10 min and reached 154% of normal value, and after that decreased gradually and reached almost normal level at 60 min after administration of 3-MPA. GABA concentration in various regions of brain decreased to 5 min in all regions after administration of 3-MPA, and reached normal levels at 60 min as in the whole brain. This method was found to be useful for studies of metabolism of GABA and Glu in biological samples.

Estradiol increases dendritic spine density by reducing GABA neurotransmission in hippocampal neurons

We have previously shown that estradiol causes a twofold rise in dendritic spine density in cultured rat hippocampal neurons, as it does in vivo. More recently, estrogen receptors have been localized to aspiny inhibitory hippocampal interneurons, indicating that their effect on spiny pyramidal neurons may be indirect. We therefore examined the possibility that estradiol affects spine density by regulating inhibition in cultured hippocampal interneurons. Immunocytochemically, estrogen receptors were found to be co-localized with glutamate decarboxylase (GAD)-positive neurons (approximately 21% of total neurons in the culture). Exposure of cultures to estradiol for 1 d caused a marked decrease (up to 80%) in the GAD content of the interneurons, measured both by immunohistochemistry and Western blotting. Also, the number of GAD-positive neurons in the cultures decreased to 12% of the total cell population. Moreover, GABAergic miniature IPSCs were reduced in both size and frequency by estradiol, whereas miniature EPSCs increased in frequency. We then mimicked the proposed effects of estradiol by blocking GABA synthesis with mercaptopropionic acid (MA). Cultures treated with MA expressed a dose-dependent decrease in GABA immunostaining that mimicked that seen with estradiol. MA-treated cultures displayed a significant 50% increase in dendritic spine density over controls, similar to that produced by estradiol. These results indicate that estradiol decreases GABAergic inhibition in the hippocampus, which appears to effectively increase the excitatory drive on pyramidal cells, and thus may provide a mechanism for formation of new dendritic spines.