Increased GABA Levels Research Articles

Studies on the involvement of GABA in the aggression directed by groups of intact or gonadectomized male and female mice towards lactating intruders

The aims of the present studies were (a)_to determine the effects of pharmacological elevation of GABA by treatment with DPA (competitive inhibitor of GABA transaminase) on a form of aggression displayed by grouped female mice towards lactating and non-lactating intruders; (b) to estimate GABA levels in six different brain areas of intact and gonadectomized male and female mice. The results revealed that DPA treatment considerably reduced this form of aggression. Increased GABA levels, modulated by the hormonal state of the animals, were observed in most brain areas studied.

Protection against hyperbaric oxygen toxicity by pargyline, succinic acid and ascorbic acid: Role of brain GABA and brain ammonia

Exposure to hyperbaric oxygen (OHP) (60 psig) produced convulsions, and elevation in weight, water content and hemoglobin content of the lung and post-exposure delayed lethality. Pretreatment with pargyline, succinic acid or ascorbic acid provided protection against all of the above effects of OHP toxicity. In OHP exposed mice there was a decrease in brain γ-aminobutyric acid (GABA) levels concomitant with an inhibition of its synthesizing enzyme, glutamic acid decarboxylase, without any effect on the activity of its catabolizing enzyme GABA transaminase (GABA-T). Both pargyline and succinic acid produced an increase in GABA levels in the mice exposed to room air at normal pressure, and prevented the OHP-induced decrease in GABA. Pargyline treatment inhibited GABA-T activity without altering the activity of GAD. Conversely, succinic acid increased the activity of both GAD and GABA-T and prevented the OHP-induced decrease in GAD activity. Ascorbic acid was without effect on any of the components of the GABA system and it did not prevent any of the OHP-induced alterations in GABA or GAD. Brain ammonia levels are increased in the OHP exposed mice with a concomitant decrease in the glutamine levels. All three agents protected the OHP-exposed mice against these OHP-induced alterations. There data are consonant with the hypothesis that decreased brain GABA and increased brain ammonia levels are involved in the etiology of OHP toxicity.

Inhibition of kindling-induced generalized seizures by aminooxyacetic acid.

The anticonvulsant effect of aminooxyacetic acid (AOAA) was examined on a model of experimental epilepsy (kindling) induced by daily appropriate amygdaloid stimulation in the rat. Doses from 5 to 30 mg/kg were intraperitoneally administered in fully kindled animals 3--4 h before triggering a seizure. At low doses (less than 15 mg/kg) AOAA had no effect whereas at higher doses (greater than 15 mg/kg) it reduced the severity of the generalized kindled seizures in over half the cases, and even sometimes completely blocked them. The inhibition of epileptic activity by AOAA is in accordance with the hypothesis that an increase in GABA level is associated with a reduction of epileptic sensitivity. An unexpected lengthening of the afterdischarge duration was also observed in about 20% of the cases, independently of the amount administered. This fact is discussed in regard to the complex action of AOAA on gamma-aminobutyric acid related enzymes. Finally, since the afterdischarge threshold was shown to be unaffected by the drug, it is suggested that it may act on the afterdischarge propagation rather than at the focal amygdaloid level.

Evaluation of increases in nerve terminal-dependent vs nerve terminal-independent compartments of GABA in vivo

Evaluation of increases in nerve terminal-dependent and nerve terminal-independent compartments of GABA in vivo. Changes in GABA occurring in the absence and in the presence of GABAergic nerve terminals were estimated using rats in which the GABAergic projection to the substantia nigra (SN) was destroyed on one side of the brain. The net increase in GABA content of the GABA-denervated SN was compared with that of the intact SN after injection of amino-oxyacetic acid (AOAA), n-dipropylacetate (DPA) and γ-vinyl GABA (GVG). One week post-operative, GABA concentration in the SN on the transected hemisphere was 10–20% of control. AOAA (30 mg/kg) produced a two-fold increase in GABA content in the denervated SN, whereas DPA (300 mg/kg) was without effect. Since DPA caused an increase in GABA in the intact SN which was similar to that caused by AOAA (25–30%), it appears that the DPA induced elevation of GABA depends upon the presence of GABAergic nerve terminals whereas AOAA primarily elevates GABA in non-nerve-terminal compartments. GVG, an irreversible GABA-T inhibitor, increased GABA levels in both compartments. The predominant effect of GVG was on nerve terminal-independent GABA (4-fold increase after 900 mg/kg); the GABA increase associated with nerve terminals was smaller (30%) and did not appear until 60 hr after a single injection of GVG. The ability of DPA, AOAA and GVG to protect against chemically- and electrically-induced seizures was directly correlated with increases in nerve-terminal GABA, and not related to increases in other GABA pools.

Benzodiazepine binding after in vivo elevation of GABA

[ 3H]Diazepam binding was measured in rat cortical membranes. Acute and chronic amino-oxyacetic acid (AOAA) pretreatment greatly increased GABA levels, but did not alter diazepam binding. The GABA normally present was sufficient/to maximally stimulate diazepam binding. In in vivo binding studies, acute and chronic AOAA treatment increased the amount of [ 3H]diazepam in the brain at the time of sacrifice, thus increasing both specific and nonspecific binding and leaving unchanged the fraction of drug specifically bound. Fluctuation of brain GABA may not affect benzodiazepine binding unless the normal concentration is greatly depressed.

Dissociation between drug-induced increases in nerve terminal and non-nerve terminal pools of GABA in vivo

To examine whether increased GABA levels produced by n-dipropylacetate (DPA) and amino-oxyacetic acid (AOAA) are associated with nerve terminals, we compared the effect of these drugs on the GABA content of substantia nigra (SN) in rats in which the GABAergic afferent projections to SN had been unilaterally destroyed. In the SN largely devoid of GABAergic nerve terminals, AOAA (30 mg/kg) produced a 2-fold increase in GABA, whereas DPA (300 mg/kg) was without effect. Since DPA and AOAA both increased GABA to a similar extent in the intact SN, it appears that the GABA increase produced by DPA is associated with GABAergic nerve terminals, while AOAA primarily elevates GABA in non-nerve terminal components (neural perikarya and glial cells) which are not destroyed by our lesions.

Dyskinetic effects of intrastriatally injected GABA-transaminase inhibitors

Injection of GABA antagonists into the striatum of rats induces abnormal involuntary movements that are blocked by increasing GABA levels in this area. Attempts to increase GABA by intrastriatal (i.s.) injection of GABA-transaminase (GABA-T) inhibitors surprisingly induced identical dyskinesias. This property was shared by all GABA-T inhibitors tested except ethanolamine-O-sulphate. This dyskinesia is easily blocked by i.s. injection of GABA and muscimol, as well as by intraperitoneal pretreatment with the GABA-T inhibitors themselves. These observations suggest that some GABA-T inhibitors may behave as GABA antagonists when locally applied in the brain at high concentrations.

Identification of diazepam binding in intact animals

An in vivo method for labeling specific benzodiazepine (BDZ) binding sites in brain was developed using intravenously injected [ 3H]diazepam. Labeling of these sites is blocked by pretreatment of animals with high doses of pharmacologically active BDZs (but not by an inactive BDZ). Using this in vivo binding technique, specific BDZ binding is enhanced by pretreatment of rats with the GAB̵A agonist muscimol or with amino-oxyacetic acid, which increases GABA levels in brain.

Inhibition of isolation-induced aggressive behavior with GABA transaminase inhibitors.

Administration of aminooxyacetic acid (AOAA) and gamma-acetylenic GABA (GAG) resulted in a dose-dependent suppression of aggressive behavior and a concomitant increase in brain GABA. The increase in GABA levels persisted beyond the time when AOAA and GAG exerted an anti-aggressive effect. Brain GABA levels remained significantly elevated over controls for 120 h after a single injection of AOAA and 46 h after a single dose of GAG.

Effect of amino-oxyacetic acid (AOAA) on focal penicillin seizures

The biochemical and behavioral effects of the anticonvulsant amino-oxyacetic acid (AOAA) have been studied in a model of focal penicillin seizures in rats. At 20 mg/Kg AOAA treatment results in a progressive 11 fold increase in GABA levels in cortex over three hours. There is a decrease in aspartate, ketoglutarate, alanine and glutamine, and an initial decrease followed by an increase in pyruvate and glutamate. These results reflect a functional inhibition of several B-6 dependent aminotransferase enzymes. When rats are pretreated 30 min before the onset of focal penicillin seizures there is a 60% reduction in the number of discharges and a 34% reduction in seizure duration. Pretreatment beyond 75 min results in progressively less anticonvulsant effect, such that seizures eventually become more severe than control. There is an increase in the number and duration of discharges, seizure spikes become complex, and tonic-clonic events develop. Penicillin seizures do not cause a change in levels of GABA, but result in a decrease in glutamate within the focus. AOAA pretreatment initially prevents this decrease in glutamate but later accentuates it. The biochemical effects of AOAA are complex, but biphasic anticonvulsant properties coincide in time with a change in the balance of excitatory and inhibitory amino acids in the seizure focus.

Effect of gabaergic drugs on benzodiazepine binding site sensitivity in rat cerebral cortex

Pretreatment of rats in vivo with γ-aminobutyric acid (GABA) agonists and drugs which increase GABA levels in brain leads to an increased affinity of [ 3H]diazepam binding sites in rat cerebral cortex. These drugs include amino-oxyacetic acid, muscimol, γ-butyrolactone and β-( p-chlorophenyl)-GABA; pentobarbital and thiosemicarbazide are without major effect. Bicuculline and picrotoxin (at high concentrations) reverse this increase in [ 3H]diazepam binding site affinity.

Effect of diazepam and pentobarbital on aminooxyacetic acid-induced accumulation on GABA.

Abstract— The effect of diazepam and pentobarbital on γ‐aminobutyric acid (GABA) levels, the aminooxyacetic acid (AOAA)‐induced accumulation of GABA, and the in vitro activity of l‐glutamate 1‐carboxyl‐lyase (EC 4.1.1.15) [GAD] were studied in various regions of rat brain. Diazepam increased GABA levels in the substantia nigra, diminished the AOAA‐induced accumulation of GABA in the caudate nucleus, cingulate, parietal and entorhinal cortex and had no effect on GABA accumulation in the pyriform and cerebellar cortex. After pentobarbital, GABA levels were elevated in the caudate nucleus but decreased in the parietal and pyriform cortex; the AOAA‐induced accumulation of GABA also diminished in all cortical regions studied. No correlation was found between the apparent changes in GABA synthesis, as estimated by accumulation after inhibition of 4‐aminobutyrate‐2‐oxoglu‐tarate (EC 2.6.1.19) [GABA‐T] with AOAA, and the changes in GABA levels induced by these drugs. The reduction in AOAA‐induced GABA accumulation after diazepam and pentobarbital treatment was most pronounced in regions which showed the greatest accumulation of GABA after AOAA administration. Neither diazepam nor pentobarbital administration affected the activity of GAD in homogenates of cingulate cortex. Chlorpromazine, at a dose which decreased spontaneous activity, enhanced the AOAA‐induced GABA accumulation in the cingulate cortex, suggesting that drug‐induced sedation is not necessarily associated with decreased GABA synthesis. While regional differences were observed in the effects of diazepam and pentobarbital on GABA synthesis, both agents appear to inhibit GABA synthesis in vivo and both do so, in at least some brain areas, at subsedative doses.

Brain ?-Aminobutyric acid and acoustic priming in C57BL/6Bg mice

An initial auditory stimulus (IAS) to 19-day-old C57BL/6Bg mice is known to induce susceptibility to audiogenic seizures as tested at 28 days of age. Immediately following exposure to the IAS, whole brain levels of γ-aminobutyric acid (GABA) were transiently reduced, whereas the levels of aspartate, glutamate, norepinephrine, and 5-hydroxytryptamine remained unchanged. Pretreatment prior to the IAS with aminooxyacetic acid (AOAA) and hydrazine, two inhibitors of GABA transaminase activity that increase the levels of GABA in the brain, attenuated the acoustic priming of audiogenic seizure susceptibility. Hydroxylamine, a similar pyridoxal antagonist that does not increase GABA levels, had no effect on the acoustic priming by the IAS. Several substances acting on brain GABA system, including β-alanine, diaminobutyric acid, glutamate, and GABA itself, were also found to attenuate the acoustic priming effect of the IAS. These results indicate that GABA may be a neurochemical factor in the sequence of events following the IAS that lead to an increase in audiogenic seizure risk. Since bicuculline had no effect on acoustic priming and did not reverse the effect of AOAA, it is possible that GABA may be involved in a metabolic or regulatory role rather than in its function in postsynaptic inhibition.

Centrally-acting drugs and the formation of brain γ-aminobutyric acid

1. 1. The effects of a number of centrally-acting compounds were tested on the levels of GABA and glutamate and on the formation of GABA from glutamate in rat brain. 2. 2. Pargyline and desipramine increased the brain concentration of GABA but were without effect on GABA production. 3. 3. Imidazoleacetic acid increased GABA levels, decreased glutamate levels but did not alter GABA production, whereas γ-hydroxybutyric acid had an effect only on GABA formation. 4. 4. Isonicotinic acid hydrazine caused a marked decrease in the formation of GABA from glutamate. 5. 5. Chlorpromazine, diphenylhydantoin, bicucculine, and l-DOPA had no effect on any of the biochemical parameters measured. 6. 6. The central effects of these drugs are discussed in relation to the above findings.

GABA metabolism and cerebral protein synthesis

Glutamate decar☐ylase (GAD) activity and [ 14C]leucine incorporation into brain protein in vivo were measured in mice injected with l-glutamic acid -γ-hydrazide and 60 min later with pyridoxal-5′-phosphate; this combined treatment produces a decrease in GAD activity and an increase in GABA levels. Under such conditions, protein synthesis was inhibited to the same extent as GAD activity. A parallelism between the decrease in GAD activity and that in protein synthesis was also observed in brain cortex slices treated with GAD inhibitors. This treatment did not affect leucine incorporation into protein in liver slices. The results support the previously suggested hypothesis that a certain pool of GABA may have a role in the regulation of protein synthesis in brain, and further suggest that the synthesis of GABA may be more important in this respect than its total concentration.

The level of GABA in the brain and locomotor behavior

GABA content was measured in the brains of animals injected with AOAA, DPA or Saline. Significant increases in GABA were found in the motor cortex and cerebellum after treatment with both drugs as compared to saline injected controls. Increased GABA levels were associated with interference with the smooth execution of learned locomotor acts, especially where balancing and coordination of the hind limbs were necessary.

Sustained drug-induced elevation of brain GABA in the rat.

Abstract— The contents of GABA, homocarnosine, and β‐alanine can be raised in rat brain for long periods of time by the continued administration of phenelzine, aminooxyacetic acid (AOAA), or isonicotinic acid hydrazide (INH). These 3 compounds apparently act by preferential inhibition of the enzyme GABA aminotransferase (GABA‐T). Oral administration of phenelzine (20 mg/kg per day) caused a 25–50 per cent increase in GABA levels in rat brain, but produced appreciable toxic side effects. A similar increase in GABA levels in brain resulted from oral administration to rats of INH in a dosage of 60 mg/kg per day, without production of any obvious toxic effects. Simultaneous administration of large doses of pyridoxine did not abolish the GABA‐elevating effect of INH. Brain GABA levels in the rat were increased by approx. 50 per cent by daily injections of AOAA (2.5 mg/kg per day). At this low dosage, AOAA injections in rats could be continued for at least 6 weeks without producing evident toxic effects. Oral administration of large amounts of GABA, on the other hand, failed to increase the content of GABA in the brains of rats not treated with GABA‐T inhibitors, and failed to produce any further increase of brain GABA levels in rats treated with AOAA.

Analysis of the major amino acids of rat brain after in vivo inhibition of GABA transaminase by ethanolamine O-sulphate.

AbstractThe effect of in vivo inhibition of GABA transaminase by ethanolamine O‐sulphate on the content of the free amino acids in rat brain has been studied. Intracisternal injection of 2.0 mg/kg resulted in a progressive increase in GABA levels with time, to reach after 8 h a 100 per cent increase over saline‐injected control animals. The effect of injection of 0.5, 1.0 and 2.0 mg/kg was studied 24 h after injection and the results showed that the increased GABA levels were dependent on the dose of inhibitor employed. Apart from the substantial increase in the GABA concentration of the brain there were no significant changes in the content of the other amino acids except for a small but significant decrease in aspartic acid in one experiment. When the extent of inhibition of the transaminase was correlated with the rise in GABA concentration it was shown that no elevation occurred until more than half of the enzymic activity had been inhibited.

The role of GABA in learning in the cockroach?

1. 1. Injection of NH 2OH (hydroxylamine) and AOAA (amino-oxyacetate) into headless cockroaches decreased learning time. 2. 2. INH (isonicotinic acid hydrazide) increased learning time, while semicarbide had no effect. 3. 3. It is inferred that learning is accompanied by an increase in GABA levels. 4. 4. A model, based on this assumption, is proposed to account for a possible role of GABA in the learning process. It postulates simultaneous facilitation of the specified (learning) pathway by ACh and inhibition of alternative pathways by GABA.

Free amino acids in brain of mice treated with l-glutamic acid-γ-hydrazide

Glutamate decar☐ylase (GAD) activity and [14C]leucine incorporation into brain protein in vivo were measured in mice injected withl-glutamic acid -γ-hydrazide and 60 min later with pyridoxal-5′-phosphate; this combined treatment produces a decrease in GAD activity and an increase in GABA levels. Under such conditions, protein synthesis was inhibited to the same extent as GAD activity.A parallelism between the decrease in GAD activity and that in protein synthesis was also observed in brain cortex slices treated with GAD inhibitors. This treatment did not affect leucine incorporation into protein in liver slices. The results support the previously suggested hypothesis that a certain pool of GABA may have a role in the regulation of protein synthesis in brain, and further suggest that the synthesis of GABA may be more important in this respect than its total concentration.