Effects of the glutamic acid decarboxylase inhibitor 3-mercaptopropionic acid on the synthesis of brain GABA in vivo and postmortally

Protective effect and mechanism of injection of glutamate into cerebellum fastigial nucleus on chronic visceral hypersensitivity in rats

AIMS

We investigated the effects of chemical stimulation of cerebellum fastigial nucleus (FN) on the chronic visceral hypersensitivity (CVH) and its possible mechanism in rats.

MAIN METHODS

We stimulated the FN by microinjecting glutamate into the FN, in order to explore whether the cerebellum fastigial nucleus played a role on CVH in rat. The model of CVH was established by colorectal distension (CRD) in neonatal rats. Abdominal withdrawal reflex (AWR) scores, pain threshold, and amplitude of electromyography (EMG) were used to assess the hyperalgesia.

KEY FINDINGS

We showed that microinjection of l-glutamate (Glu) into the FN markedly attenuated hyperalgesia. The protective effect of FN was prevented by pretreatment with the glutamate decarboxylase inhibitor, 3-mercaptopropionic acid (3-MPA) into the FN or GABA_A receptor antagonist, bicuculline (Bic) into the LHA (lateral hypothalamic area). The expressions of protein Bax, caspase-3 were decreased, but the expression of protein Bcl-2 was increased after chemical stimulation of FN. These results indicated that the FN participated in regulation of CVH, and was a specific area in the CNS for exerting protective effects on the CVH. In addition, LHA and GABA receptor may be involved in this process.

SIGNIFICANCE

Our findings might provide a new and improved understanding of the FN function, and might show an effective treatment strategy for the chronic visceral hypersensitivity.

Protective effect of histamine microinjected into cerebellar fastigial nucleus on stress gastric mucosal damage in rats

In the study, we investigated the effect of histamine microinjected into cerebellar fastigial nucleus (FN) on stress gastric mucosal damage (SGMD), and its mechanisms in rats. The model of SGMD was established by restraining and water (21±1°C)-immersion for 3h. The gastric mucosal damage index (GMDI) indicated the severity of gastric mucosal damage. Histamine or receptor antagonist was microinjected into the FN. The decussation of superior cerebellar peduncle (DSCP) and the lateral hypothalamic area (LHA) were destroyed, respectively. The pathological changes of gastric mucosa were evaluated using biological signal acquisition system, Laser-Doppler flowmeter, and western blotting. We found that the microinjection of histamine (0.05, 0.5, and 5μg) into FN significantly attenuated the SGMD, in a dose-dependent manner, whereas, the microinjection of histamine H2 receptor antagonist, ranitidine, and glutamic acid decarboxylase antagonist, 3-mercaptopropionic acid (3-MPA) exacerbated the SGMD. The protective effect of histamine on SGMD was abolished by electrical lesion of DSCP or chemical ablation of LHA. The microinjection of histamine decreased the discharge frequency of the greater splanchnic nerve, and the gastric mucosal blood flow was increased. In addition, the cellular proliferation was enhanced, but the cellular apoptosis was reduced in the gastric mucosa. Also the pro-apoptosis protein, Bax, and caspase-3 were down-regulated, and the anti-apoptosis protein, Bcl-2 was up-regulated following microinjection of histamine. In conclusion, the FN participated in the regulation of SGMD after histamine microinjected into FN, and cerebellar-hypothalamic circuits (include: DSCP, LHA) contribute to the process, which may provide a new therapeutic strategy for SGMD.

Protective effect of chemical stimulation of cerebellar fastigial nucleus on stress gastric mucosal injury in rats

AIMS

We investigated the protective effects of chemical stimulation of cerebellar fastigial nucleus (FN) on stress gastric mucosal injury (SGMI) and its possible neuro-regulatory mechanisms in rats.

MAIN METHODS

Chemical stimulation, electrical stimulation, chemical ablation, electrolytic lesion, and microinjection were used to investigate the effects of FN simulation on SGMI. The model of SGMI was established by restraint and water (21±1°C)-immersion (RWI) for 3h in rats. The gastric mucosal injury index indicated the severity of gastric mucosal injuries.

KEY FINDINGS

We showed that microinjection of L-glutamic acid into the FN or electrical stimulation of the FN markedly attenuated SGMI. Either chemical lesion of the FN or electrical ablation of the decussation of superior cerebellar peduncle (DSCP) obviously aggravated SGMI. The protective effect of FN stimulation on SGMI was reversed after chemical ablation of the lateral hypothalamic area (LHA). The protective effect of FN was prevented by pretreatment with the glutamic acid decarboxylase antagonist, 3-MPA into the FN or GABA(A) receptor antagonist, bicuculline into the LHA. The protective effect of FN was abolished by pretreatment with sympathectomy. The discharge frequency of greater splanchnic nerve (GSN) was decreased and gastric mucosal blood flow (GMBF) was increased after chemical stimulation of FN. These results indicate that the FN participates in regulation of SGMI, and is a specific area in the CNS for exerting protective effects on the SGMI. The DSCP, LHA and peripheral sympathetic nerve may be involved in this process.

SIGNIFICANCE

Our findings might provide a new and improved understanding of the cerebellar function and an effective treatment strategy for stress gastric mucosal injury.

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.

GABA content and synthesis in the aging rat brain

The content and synthesis of GABA were measured in the cortex and striatum of young adult (4 months), mature (14 months), and old (24 months) male Wistar rats. GABA synthesis was determined from the GABA accumulation induced by aminooxyacetic acid (AOAA). Aging did not affect GABA content in the cortex (1.03 +/- 0.04 mumoles/g in young and 1.06 +/- 0.04 mumoles/g in old rats), or in the striatum (1.63 +/- 0.04 mumoles/g in young and 1.56 +/- 0.05 mumoles/g in old rats). Aging did not significantly change the AOAA-induced GABA accumulation in the striatum (+34% in young, +16% in mature, and +28% in old rats), but significantly reduced it in the cortex where this process takes place to a greater extent than in the striatum: +164% in young, +116% in mature, and +120% in old animals. It can be concluded that while in the striatum aging did not affect AOAA-induced GABA accumulation, in the cortex this was less in mature than young rats, with no further change in the old ones. GABA content was not affected by aging in either region.

Biochemical evidence for the GABA regulation of vasopressin levels in microdissected brain structures after servo-controlled hypotension

Discrete brain structures were analysed for gamma-aminobutyric acid (GABA) and vasopressin content in normo- and hypotensive rats treated with the glutamic acid decarboxylase inhibitor, 3-mercaptopropionic acid (MPA) and the GABAA agonist muscimol. In the normotensive group treated with MPA only, the concentration of vasopressin increased in the supraoptic nucleus, indicating an inhibitory role for GABA. In the hypotensive group a rise in the vasopressin level in the nucleus of the solitary tract was detected and the GABA level decreased in the supraoptic nucleus. Muscimol decreased the concentration of vasopressin in the nucleus of the solitary tract. The changes in the concentration of vasopressin may be a result of increased or decreased activation of the GABAergic system. The results show that the GABA- and vasopressinergic systems somehow interact although the more precise way of action remains to be clarified.

Adaptation of the GABAA-receptor complex in rat brain during chronic elevation of GABA by ethanolamine O-sulphate

Slice preparations of rat cuneate nucleus were used for studies on the gamma-aminobutyric acid GABAA-receptor complex following chronic and acute pretreatment with GABA-alpha-ketoglutarate aminotransferase (GABA-T) inhibitors. The whole brain GABA concentration was significantly increased 2.9 fold and 2.6 fold following treatment with ethanolamine O-sulphate (EOS, orally) for 15-30 days and 56-64 days, respectively. One hour after a single injection of gamma-acetylenic GABA (GAG) i.p., there was a significant 2.1 fold increase in whole brain GABA. Superfusion of a slice with muscimol or the GABA uptake inhibitor nipecotic acid depolarized the afferent nerve fibres. These effects were potentiated by flurazepam (1 microM) and pentobarbitone (10 microM) and antagonized by picrotoxin (3 microM, 30 microM). Following 15-30 days of EOS-treatment, the depolarization response to muscimol was decreased and that to nipecotic acid increased. These changes were no longer significant by 56-64 days of pretreatment. The acute dose of GAG did not affect the depolarization response to muscimol but increased that to nipecotic acid. The potentiations of muscimol by flurazepam (1 microM) and pentobarbitone (10 microM) were enhanced following chronic EOS treatment (15-64 days). The enhancement of flurazepam was less after 56-64 days than after 15-30 days pretreatment whereas the enhancement of pentobarbitone was similar at both times. Acute GAG treatment had no effect. The potency of picrotoxin as an antagonist of muscimol was reduced following chronic EOS treatment; the enhancement was less after 56-64 days than after 15-30 days pretreatment. Acute GAG treatment caused only a very small reduction in picrotoxin potency. Possible adaptations in the GABAA-receptor complex and its modulation during chronic elevation of brain GABA are discussed.

Regional differences in the changes in rat brain GABA concentrations post mortem and following inhibition of the synthesis and metabolism

Experimentally induced changes in the concentration of GABA were studied in regions where GABA is present, predominantly in nerve terminals (substantia nigra, pituitary gland) or outside nerve terminals (corpus callosum). Some other regions (cerebellum, cuneate nucleus, corpus striatum, hippocampus) were included where the cellular localization of GABA is less certain. The post mortem increase in GABA was about equal percentage-wise in all regions indicating that it occurs in all locations where GABA is present. The increase in GABA following the GABA alpha-ketoglutaric acid aminotransferase inhibitor gamma-acetylenic GABA was much greater in the corpus callosum than in the other regions indicating that this enzyme is more important for the GABA present outside than inside the GABA nerve terminals. The glutamic acid decarboxylase inhibitor 4-deoxypyridoxine decreased the endogenous concentration of GABA and it inhibited the increase in GABA induced by gamma-acetylenic GABA in the substantia nigra and in the pituitary gland, but not in the corpus callosum, indicating that this compound inhibits the decarboxylase mainly in the GABA nerve terminals.

Lack of effects of apomorphine, haloperidol and clozapine on the synthesis and utilization of brain GABA

The synthesis of GABA was studied as the accumulation of GABA following inhibition of the GABA-alpha-ketoglutaric acid aminotransferase by gamma-acetylenic GABA (GAG) or gamma-vinyl GABA. The disappearance of GABA was studied by means of inhibition of glutamate decarboxylase by 4-deoxypyridoxine. Systemic administration of the dopamine receptor agonist apomorphine did not change the accumulation or the disappearance of GABA in the substantia nigra and the corpus striatum of the rat. However, a very high dose of apomorphine somewhat increased the GAG-induced GABA accumulation in the corpus striatum. The dopamine receptor antagonists haloperidol and clozapine did not modify the accumulation and disappearance of GABA in the two structures, except for a slight decrease in the GAG-induced GABA accumulation in the substantia nigra. Following an acute hemisection, the neural connections between the substantia nigra and the corpus striatum are interrupted on one side of the brain. Apomorphine did not influence the accumulation or the disappearance of GABA in the substantia nigra and in the corpus striatum on the sectioned side of the brain. These results indicate that the synthesis and the utilization of GABA in the two structures studied are not influenced to any greater extent by changes in dopamine receptor activity.

Effects of GABAergic drugs on the GABA turnover in the substantia nigra and the corpus striatum of the rat

Changes in the endogenous GABA concentration and in GABA turnover following GABA receptor stimulation or blockade were studied in the substantia nigra and the corpus striatum of the rat. The GABA agonists, muscimol, baclofen and THIP decreased the accumulation of GABA following inhibition of GABA-alpha-ketoglutaric acid aminotransferase by gamma-acetylenic GABA (GAG) in both structures investigated. Only the effect of muscimol in the substantia nigra was inhibited by the GABA antagonist, bicuculline. Muscimol, baclofen and progabide reduced the disappearance rate of GABA in the substantia nigra following inhibition of the glutamate decarboxylase by 4-deoxypyridoxine. The endogenous GABA concentration was decreased in the corpus striatum following muscimol, THIP or baclofen, probably due to a decreased synthesis of GABA. Smaller effects were seen on the endogenous GABA concentration in the substantia nigra, since both the synthesis and the utilization of GABA were decreased by muscimol and baclofen. Thus, the turnover of brain GABA might be regulated by changes in receptor activity.

Effects of KCl-induced depolarization on the GABA concentration in the corpus striatum and in the substantia nigra

The effects of potassium ion depolarization on the concentration, the synthesis and the utilization of GABA as well as the effects on the concentrations of glutamine, dopamine (DA) and DOPAC in the substantia nigra and in the corpus striatum of the rat were investigated. An intranigral KCl injection did not influence the GABA concentration or the synthesis and utilization of GABA in the substantia nigra. Following an intrastriatal KCl injection the GABA concentration increased and the glutamine concentration decreased in the corpus striatum. A marked increase in the DNA concentration in the corpus striatum was seen following an intranigral KCl injection, whereas the DA concentration decreased and the DOPAC concentration increased following an intrastriatal KCl injection. It is concluded that the GABA in the substantia nigra and in the corpus striatum is affected differently by potassium ions, perhaps due to the different cellular localization of GABA in the two structures. The increase of GABA in the corpus striatum was accompanied by a decreased glutamine concentration indicating that glutamine might be a precursor of GABA.

Hypophysial GABA after ether stress, dexamethasone or inhibition of GABA catabolism

Ether stress (2 X 2 min within 15 min) and dexamethasone treatment (1 mg/kg IP; 1, 3 and 12 hours before sacrifice), the procedures supposed to increase the activity of glutamate decarboxylase (GAD) in the hypothalamus, fail to affect the concentration of GABA in the rat hypophysis. Five and/or ten minutes post-mortem an increased GABA level in the hypothalamus and cingulate cortex, and a decreased GABA concentration in the hypophysis was found. Three and four hours after the IP administration of 1-cycloserine (50 mg/kg) and 1-glutamic acid-gamma-hydrazide (160 mg/kg) respectively (both drugs are inhibitors of GABA catabolism) the concentration of GABA raised in all the regions examined. On the basis of studies in the whole gland it might be concluded that the concentration of GABA in the hypophysis depends more on GABA release from extrahypophysial tissue and GABA degradation in the hypophysis than on the extrahypophysial GABA synthesis. Also on the basis of post-mortem studies in the whole gland no indication for the appearance of GABA synthesis in hypophysis could be found.

On the use of enzyme inhibitors to study the synthesis and utilization of brain GABA

Changes in the GABA concentration in different parts of the rat brain were studied following inhibition of the glutamic acid decarboxylase or the GABA-alpha-ketoglutaric acid aminotransferase. The GABA concentration was reduced by the decarboxylase inhibitors 4-deoxypyridoxine and isoniazid, but not by 3-mercaptopropionic acid and DL-allylglycine. The aminotransferase inhibitor gamma-acetylenic GABA increased the concentration of GABA and this effect was markedly inhibited by 3-mercaptopropionic acid and partly by 4-deoxypyridoxine and isoniazid. The 4-deoxypyridoxine-induced decrease in GABA concentration was approximately maximal after 400 mg/kg intraperitoneally and 90 min. The brain DOPA decarboxylase activity in vivo was not inhibited by 4-deoxypyridoxine. The GABA concentration in the substantia nigra was reduced by 75 per cent 4 days after section of the striato-nigral GABA neurones. In the denervated substantia nigra, 4-deoxypyridoxine did not change the concentration of GABA whereas the effect of gamma-acetylenic GABA was reduced by 70 per cent.