Extracellular GABA Concentrations Research Articles

Effects of activation of NMDA and AMPA glutamate receptors on the extracellular concentrations of dopamine, acetylcholine, and GABA in striatum of the awake rat: a microdialysis study.

The effects of activation of the AMPA and NMDA ionotropic glutamate receptors on the extracellular concentration of dopamine, acetylcholine, (ACh) and GABA in striatum of the awake rat was investigated. Also the levels of DOPAC, HVA, and choline (Ch) were included in this study. Seven to eight days after stereotaxical implantation of a guide-cannulae assembly, microdialysis experiments were performed. The dopamine and ACh content of samples were measured by HPLC coupled to electrochemical detection. GABA was measured using fluorometric detection. Perfusion of AMPA (1, 20, 100 microM) produced a dose-related increase of dopamine and a dose-related decrease of DOPAC and HVA. AMPA 100 microM decreased extracellular concentrations of ACh and increased the extracellular concentration of Ch and GABA. Perfusion of NMDA 500 microM increased the concentration of dopamine and decreased DOPAC and HVA. Also, NMDA 100 microM decreased DOPAC. NMDA 500 microM decreased the extracellular concentrations of ACh and increased the concentrations of Ch and GABA. Perfusion of the AMPA/kainate-antagonist DNQX (100 microM) blocked the effects of AMPA (100 microM) on dopamine, DOPAC, HVA, ACh, and GABA concentrations. Perfusion of the NMDA-antagonist CPP (100 microM) blocked the effects of NMDA 500 microM on dopamine, DOPAC, HVA, ACh, Ch, and GABA concentrations. These results suggest an interaction between glutamate-dopamine-ACh-GABA in striatum of the awake rat.

Unaltered control of extracellular GABA-concentration through GAT-1 in the hippocampus of rats after pilocarpine-induced status epilepticus

The uptake of the inhibitory transmitter GABA (γ-aminobutyric acid) limits the efficacy of synaptic and tonic inhibition in brain tissue. It has been reported that GABA-uptake is down-regulated in temporal lobe epilepsy. This down-regulation may increase the inhibitory action of GABA but may also limit the anticonvulsant activity of GABA-uptake blockers. We have directly compared the function of GABA-uptake in hippocampal slices from normal and chronically epileptic rats. We raised the global extracellular concentration of GABA by bath-application of the agonist in the absence and presence of the GABA-uptake blocker tiagabine. GABA-induced currents were measured in dentate granule cells and CA1 pyramidal neurons in hippocampal slices. The potentiation of currents by tiagabine was taken as a measure for the efficacy of GABA-uptake in the hippocampal tissue. There was no difference between cells from control- or pilocarpine-treated animals in the response to GABA or in the conductance increase following application of tiagabine. Our data show that in the chronic phase of the pilocarpine-model GABA-uptake maintains its ability to control the extracellular background concentration of GABA.

NMDA and AMPA/kainate glutamatergic agonists increase the extracellular concentrations of GABA in the prefrontal cortex of the freely moving rat: modulation by endogenous dopamine

Using microdialysis in the prefrontal cortex, this study investigated first the effects of the ionotropic glutamatergic agonists NMDA and AMPA on extracellular concentrations of GABA, and second, the modulation of these effects by increasing endogenous dopamine. NMDA (20, 100, and 500 μM) and AMPA (1, 20, and 100 μM), perfused through the microdialysis probe for 60 min, produced a dose-related increase of extracellular concentrations of GABA in the prefrontal cortex of the awake rat. NMDA 100 and 500 μM produced a maximal increase of extracellular GABA of 150 ± 38% and 245 ± 75% of baseline, respectively. AMPA 20 and 100 μM produced a maximal increase of extracellular GABA of 140 ± 17% and 195 ± 41% of baseline, respectively. NMDA and AMPA also increased extracellular concentrations of glutamate. Increases of extracellular GABA, and also of glutamate, produced by NMDA (500 μM) and AMPA (100 μM) were significantly blocked by the NMDA antagonist CPP (100 μM) and the AMPA/kainate antagonist DNQX (100 μM), respectively. To investigate whether dopamine modulates the increases of GABA produced by NMDA and AMPA, endogenous dopamine was increased with the dopamine uptake inhibitor nomifensine. Nomifensine (1, 100, and 1000 μM) produced a dose-related increase of dialysate dopamine (from 0.1 to 1.0 nM) but did not modify basal extracellular concentrations of GABA in the prefrontal cortex. However, increases of endogenous dopamine at 0.5–0.7 nM did potentiate the increases of extracellular GABA produced by AMPA (20 μM) (from 140% to 240% of baseline), but not by NMDA (100 μM), in this area of the brain. These effects were attenuated by the perfusion of (−)sulpiride (D2 antagonist), but not by the perfusion of SCH-23390 (D1 antagonist). These results suggest that glutamate, through the activation of both NMDA and AMPA/kainate ionotropic receptors, facilitates GABAergic transmission in the prefrontal cortex, and that dopamine can modulate the effects of glutamate through AMPA/kainate receptors on GABA transmission in this area of the brain.

Convulsant and subconvulsant doses of norfloxacin in the presence and absence of biphenylacetic acid alter extracellular hippocampal glutamate but not gamma-aminobutyric acid levels in conscious rats.

Fluoroquinolones are antibiotics with central excitatory side effects. These adverse effects presumably result from inhibition of gamma-aminobutyric acid (GABA) binding to GABA(A) receptors. This GABA antagonistic effect is greatly potentiated by the active metabolite of fenbufen, biphenylacetic acid (BPAA). Nevertheless, it remains questionable whether GABA receptor antagonism alone can explain the convulsant activity potentials of these antimicrobial agents. The present study was undertaken to investigate the possible effects of norfloxacin, both in the absence and in the presence of BPAA, on the extracellular hippocampal levels of GABA and glutamate, the main central inhibitory and excitatory amino acid neurotransmitters, respectively. This in vivo microdialysis approach with conscious rats allows monitoring of behavioral alterations and concomitant transmitter modulation in the hippocampus. Peroral administration of 100 mg of BPAA per kg of body weight had no effect on behavior and did not significantly alter extracellular GABA or glutamate concentrations. Intravenous perfusion of 300 mg of norfloxacin per kg did not change the rat's behavior or the concomitant neurotransmitter levels in about half of the experiments, while the remaining animals exhibited severe seizures. These norfloxacin-induced convulsions did not affect extracellular hippocampal GABA levels but were accompanied by enhanced glutamate concentrations. Half of the rats receiving both 100 mg of BPAA per kg and 50 mg of norfloxacin per kg displayed lethal seizures, while the remaining animals showed no seizure-related behavior. In the latter subgroup, again no significant alterations in extracellular GABA levels were observed, but glutamate overflow remained significantly elevated for at least 3 h. In conclusion, norfloxacin exerts convulsant activity in rats, accompanied by elevations of extracellular hippocampal glutamate levels but not GABA levels, even in the presence of BPAA.

Involvement of metabotropic glutamate receptors in excitatory amino acid and GABA release following spinal cord injury in rat.

Spinal cord injury (SCI) leads to an increase in extracellular excitatory amino acid (EAA) concentrations resulting in glutamate receptor-mediated excitotoxic events. The glutamate receptors include ionotropic (iGluRs) and metabotropic (mGluR) receptors. Of the three groups of mGluRs, group-I activation can initiate intracellular pathways that lead to further transmitter release. Groups II and III mGluRs function mainly as autoreceptors to regulate neurotransmitter release. In an effort to examine the role of mGluRs in the increase in EAAs following SCI, we administered AIDA, a potent group-I mGluR antagonist immediately after injury. To determine subtype specific roles of the group-I mGluRs, we evaluated EAA release following LY 367385 (mGluR1 antagonist) and MPEP (mGluR5 antagonist) administration. To evaluate group-II and -III mGluRs we administered APDC (group-II agonist) and L-AP4 (group-III agonist) immediately following injury; additionally, we initiated treatment with CPPG (group-II/-III antagonist) and LY 341495 (group-II antagonist) 5 min prior to injury. Subjects were adult male Sprague-Dawley rats (225-250 g), impact injured at T10 with an NYU impactor (12.5 mm drop). Agents were injected into the epicenter of injury, amino acids where collected by microdialysis fibers inserted 0.5 mm caudal from the edge of the impact region and quantified by HPLC. Treatment with AIDA significantly decreased extracellular EAA and GABA concentrations. MPEP reduced EAA concentrations without affecting GABA. Combining LY 367385 and MPEP resulted in a decrease in EAA and GABA concentrations greater than either agent alone. L-AP4 decreased EAA levels, while treatment with LY 341495 increased EAA levels. These results suggest that mGluRs play an important role in EAA toxicity following SCI.

Role of GABA within the nucleus tractus solitarii in the hypoxic ventilatory decline of awake rats.

The purpose of this study was to examine our hypothesis that gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS) may be related to the hypoxic ventilatory decline (HVD) and that chemoreceptor stimulation was essential to activate this mechanism. We used unanesthetized, freely moving rats in this study. An in vivo microdialysis technique was used to measure the extracellular GABA concentration ([GABA]o), and an in vivo microinjection technique was used to examine the effects of the GABA agonists and antagonists on the ventilation during hypoxia. The GABA agonists injected into the NTS attenuated the ventilation during hypoxia. By hypoxic exposure, [GABA]o was increased during the HVD. However, by carotid body denervation (CBD), this GABA increase was abolished. Although GABA antagonists microinjected into the NTS during the HVD phase significantly increased the depressed ventilation, this effect on the ventilation was abolished by CBD. These results suggest that the GABA in the NTS has a pivotal role in the HVD and that this mechanism is not activated without chemoreceptor stimulation.

Noradrenergic and GABAergic systems in the medial hypothalamus are activated during hypoglycemia.

Noradrenergic and GABAergic systems in the medial hypothalamus influence plasma glucose and may be activated during glucoprivation. Microdialysis probes were placed into the ventromedial nucleus (VMH), lateral hypothalamus (LHA), and paraventricular nucleus (PVH) of male Sprague-Dawley rats to monitor extracellular concentrations of norepinephrine (NE) and GABA. During systemic hypoglycemia, induced by insulin (1.0 U/kg), NE concentrations increased in the VMH (P < 0.05) and PVH (P = 0.06) in a bimodal fashion during the first 10 min and 20-30 min after insulin administration. In the VMH, GABA concentrations increased (P < 0.05) in a similar manner as NE. Extracellular NE concentrations in the LHA were slightly lower (P = 0.13), and GABA levels remained at baseline. The increases in NE and GABA in the VMH were absent during euglycemic clamp; however, NE in the PVH still increased, reflecting a direct response to hyperinsulinemia. On the basis of these data, we propose that the activity of noradrenergic afferents to the medial hypothalamus is increased during hypoglycemia and influences the activity of local GABAergic systems to activate appropriate physiological compensatory mechanisms.

Involvement of NMDA and AMPA/kainate receptors in the effects of endogenous glutamate on extracellular concentrations of dopamine and GABA in the nucleus accumbens of the awake rat

We have investigated the effects of perfusion of the N-methyl-D-aspartate (NMDA) receptor antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and the α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) on the endogenous glutamate-evoked changes of extracellular dopamine and α-aminobutyric acid (GABA) in the nucleus accumbens of the awake rat. Local infusion of the glutamate uptake inhibitor L- trans-pyrrolidine-2,4-dicarboxilic acid in the nucleus accumbens produced an increase in extracellular concentrations of glutamate, dopamine, and GABA. At the dose of 4 mM, the increase of extracellular glutamate, dopamine, and GABA were 3.73 ± 0.46 μM ( n = 8; p < 0.001), 4.70 ± 0.92 nM ( n = 6; p < 0.001) and 0.36 ± 0.08 μM ( n = 8; p < 0.001), respectively. Perfusion of the NMDA-receptor antagonist CPP attenuated the increases of dopamine by 90% ( n = 5; p < 0.001), but enhanced the increases of GABA by 70% ( n = 7; p < 0.01). Perfusion of the AMPA-receptor antagonist DNQX did not attenuate the increases of GABA. These results suggest a differential mediation of ionotropic glutamatergic receptors in the actions of endogenous glutamate on extracellular concentration of dopamine and GABA.

Progesterone regulation of GABAA receptor plasticity in adult rat supraoptic nucleus.

Marked plasticity in GABAA receptor signalling occurs in adult oxytocin neurons of the supraoptic nucleus (SON) through the modulation of GABAA receptor alpha subunits during pregnancy. The present studies were undertaken to examine the potential mechanisms underlying this plasticity. In vivo microdialysis experiments in conscious rats revealed that no significant changes in extracellular GABA concentrations occurred within the SON over the last two days of pregnancy and the time of parturition itself. In situ hybridization studies examined the effects of gonadal steroid manipulation upon the GABAA receptor subunits expressed by SON neurons (alpha1, alpha2, beta2 and gamma2 subunits) and demonstrated that cellular levels of the alpha1 subunit were increased following 8 days oestrogen and progesterone treatment. Estrogen alone or allopregnanolone, the progesterone derivative, had no effect on alpha1 subunit mRNA expression in the SON. Immunocytochemical experiments demonstrated progesterone receptors in many neural populations but not within the SON of late pregnant rats. These studies indicate that alterations in endogenous GABA release within the SON are unlikely to be responsible for the GABAA receptor plasticity exhibited by oxytocin neurons in late pregnancy. Rather, data demonstrate that the fluctuating concentrations of progesterone during pregnancy act indirectly on SON neurons to modulate alpha1 subunit mRNA expression. Together, these experiments provide evidence for the ligand-independent induction of GABAA receptor plasticity in the adult brain by progesterone.

Effects of the crude venom of the social wasp Agelaia vicina on γ‐aminobutyric acid and glutamate uptake in synaptosomes from rat cerebral cortex

Glutamate (L-glu) is the most important excitatory neurotransmitter in the mammalian central nervous system. Its action is terminated by transporters located in the plasma membrane of neurons and glial cells, which have a critical role in preventing glutamate excitotoxicity under normal conditions. The neurotransmitter γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system. Venoms of solitary wasps and orb-spiders are composed of large proteins, medium-size peptides, polyamine amides (PAs), and other neuroactive components that are highly selective to nervous tissues. The abnormal operation of uptake systems is involved in several failures. Several studies indicate alterations in extracellular GABA and glutamate concentrations in epilepsy conditions that may relate to transporter functions. The effects of the crude and boiled venom of the social wasp Agelaia vicina, “cassununga,” on GABA and L-glu uptake in rat cerebral cortex synaptosomes are related. The venom uncompetitively inhibited high- and low-affinity GABA uptake by 91.2% and by 76%, respectively. This kind of inhibition was also found to affect high- (99.6%) and low-affinity (90%) uptake of L-glu. These results suggest that the effects observed in these experiments indicate the venom of A. vicina to be a useful tool to further characterize GABA- and L-glu-uptake systems. © 2000 John Wiley & Sons, Inc. J Biochem Toxicol 14: 88–94, 2000

Effects of Aging on the Interaction Between Glutamate, Dopamine, and GABA in Striatum and Nucleus Accumbens of the Awake Rat

The aim of the present study was to investigate, using microdialysis, the effects of aging on the glutamate/dopamine/GABA interaction in striatum and nucleus accumbens of the awake rat. For that, the effects of an increase of the endogenous concentration of glutamate on the extracellular concentration of dopamine and GABA in striatum and nucleus accumbens of young (2-4 months), middle-aged (12-14 months), aged (27-33 months), and very aged (37 months) male Wistar rats were studied. Endogenous extracellular glutamate was selectively increased by perfusing the glutamate uptake inhibitor L-trans-pyrrolidine-3,4-dicarboxylic acid (PDC) through the microdialysis probe. In young rats, PDC (1, 2, and 4 mM) produced a dose-related increase of dialysate concentrations of glutamate in both striatum and nucleus accumbens. PDC also increased dialysate dopamine and GABA in both structures. These increases were significantly correlated with the increases of glutamate but not with the PDC dose used, which strongly suggests that the increases of dopamine and GABA were produced by glutamate. In striatum, there were no significant differences in the dopamine/glutamate and GABA/glutamate correlations between young and aged rats. This means that the effects of glutamate on dopamine and GABA do not change during aging. On the contrary, in the nucleus accumbens of aged rats, the increases of dopamine, when correlated with the increases of glutamate, were significantly lower than in young rats. Moreover, the ratio of dopamine to glutamate increases at maximal increases of glutamate was negatively correlated with aging. On the contrary, the ratio of GABA to glutamate increases in nucleus accumbens was positively correlated with aging, which suggests that the effects of endogenous glutamate on GABA tend to be higher in the nucleus accumbens of aged rats. The findings of this study suggest that aging changes the interaction between endogenous glutamate, dopamine, and GABA in nucleus accumbens, but not in striatum, of the awake rat.

Endogenous GABA potentiates the potassium-induced release of dopamine in striatum of the freely moving rat: a microdialysis study

Using microdialysis, a study was made of the effects of an increase of endogenous GABA on basal and potassium-stimulated release of dopamine in striatum of the awake rat. The dopamine metabolites DOPAC and HVA were also measured. Extracellular concentrations of GABA were increased by inhibiting its uptake with nipecotic acid. TTX (10 μM) reduced basal extracellular concentrations of dopamine, and dopamine metabolites, but not GABA. Nipecotic acid (200, 500, and 1000 μM) produced a dose-related increase in basal extracellular concentrations of GABA, but did not change basal extracellular concentrations of dopamine and dopamine metabolites. However, nipecotic acid significantly enhanced the dopamine release produced by perfusion of potassium (50 mM) and also enhanced the extracellular increase of GABA produced by high potassium. These results suggest that an increase of endogenous GABA is facilitating the stimulated release, but not the basal release, of dopamine in the striatum of the awake rat.

Action of bicyclic isoxazole GABA analogues on GABA transporters and its relation to anticonvulsant activity

The inhibitory action of bicyclic isoxazole γ-aminobutyric acid (GABA) analogues and their 4,4-diphenyl-3-butenyl (DPB) substituted derivatives has been investigated in cortical neurones and astrocytes as well as in human embryonic kidney (HEK 293) cells transiently expressing either mouse GABA transporter-1 (GAT-1), GAT-2, -3 or -4. It was found that 4,5,6,7-tetrahydroisoxazolo(4,5- c)pyridin-3-ol (THPO) and 5,6,7,8-tetrahydro-4 H-isoxazolo[4,5- c]azepin-3-ol (THAO) displayed some inhibitory activity on GAT-1 and GAT-2, where the compounds exhibited a slightly lower potency on GAT-2 compared to GAT-1. DPB substituted THPO displayed higher inhibitory potency than the parent compound regarding the ability to inhibit GABA uptake via GAT-1 and GAT-2. Concerning the inhibitory mechanism, THPO, THAO and DPB–THPO were competitive inhibitors on GAT-1 transfected HEK 293 cells and the same mechanism was observed for THPO in GAT-3 transfected cells. Regarding GABA uptake into neurones and astroglia cells THAO and DPB–THAO both displayed competitive inhibitory action. The observations that THPO, THAO as well as their DPB derivatives act as competitive inhibitors together with earlier findings such as potent anticonvulsant activity, lack of proconvulsant activity and the ability of THPO to increase extracellular GABA concentration, indicate that these bicyclic isoxazole GABA analogues and their DPB derivatives may be useful lead structures in future search for new antiepileptic drugs.

Periaqueductal gray matter glutamate and GABA decrease following subcutaneous formalin injection in rat.

Glutamate and GABA are important nociception modulating transmitters in specific brain regions, i.e. the spinal cord, the thalamic nuclei and the periaqueductal gray (PAG). However, quantitative and topographical changes in glutamate and GABA release in these brain regions during peripheral inflammation episodes have not been characterized in awake animals. To address this issue, an in vivo microdialysis study was carried out in freely moving rats in order to analyze PAG extracellular glutamate and GABA concentrations following unilateral formalin injection into the dorsal skin of the right hind-paw. Both glutamate and GABA release decreased after the injection of formalin during phase I and phase II of hyperalgesia. Because naloxone prevented the decrease of GABA and glutamate release induced by formalin, this study shows that, in vivo, a nociceptive stimulation may activate opioidergic fibres into the PAG. The increased release of endogenous opioids may, in turn, inhibit the activity of the GABAergic neurons (i.e. opioid disinhibition). Formalin injection also decreased extracellular glutamate concentration. However, we found that intra-PAG perfusion with tetrodotoxin only decreased GABA, but not glutamate dialysate values. Although it should be reasonable to speculate that opioids also inhibit glutamate fibres, further investigation is needed to clarify whether or not the dialysate glutamate we measured reflects change in the metabolism or neurotransmitter pool of this amino acid.

Modulation of GABA release by dopamine in the substantia nigra.

The role of specific dopamine receptor subtypes in the regulation of GABA release in the substantia nigra was investigated using microdialysis in the awake rat. Both basal and potassium-stimulated changes in the extracellular concentrations of GABA were examined in response to the local perfusion of tetrodotoxin (TTX), the D1 agonist SKF 38393, or the D2 agonist LY 171555 through the microdialysis probe in the substantia nigra. Although TTX (1 microM) did not alter the basal extracellular concentrations of GABA in the substantia nigra, it attenuated the potassium-stimulated (80 mM K+) release of GABA. SKF 38393 had no effect on basal extracellular concentrations of GABA, but did potentiate K+ -stimulated release of GABA in a concentration-dependent manner. The potentiated response at the highest concentration of SKF 38393 (100 microM) was blocked by the D1 antagonist SCH 23390. In contrast to the effect of the D1 agonist, the D2 agonist LY 171555 attenuated the stimulated release of GABA. These data indicate that although basal extracellular concentrations of GABA in the substantia nigra may not be derived from neuronal pools, K+ -stimulated release of GABA is impulse-mediated and is modulated by the D1 and the D2 receptors. Local interactions between dopamine and GABA in the substantia nigra may have important implications for the direct regulation of basal ganglia efferent activity and motor behavior.

Endogenous dopamine increases extracellular concentrations of glutamate and GABA in striatum of the freely moving rat: involvement of D1 and D2 dopamine receptors.

Interactions between endogenous dopamine, glutamate, GABA, and taurine were investigated in striatum of the freely moving rat by using microdialysis. Intrastriatal infusions of the selective dopamine uptake inhibitor nomifensine (NMF) were used to increase the endogenous extracellular dopamine. NMF produced a dose-related increase in extracellular dopamine and also increased extracellular concentrations of glutamate, GABA, and taurine. Extracellular increases of dopamine were significantly correlated with extracellular increases of glutamate and GABA, but not taurine. To investigate whether the increased extracellular dopamine produced by NMF was responsible for the concomitant increase of glutamate and GABA, D1, and D2 receptor antagonists were used. Dopamine receptor antagonists D1 (SCH23390) and D2 (sulpiride) significantly attenuated the increases of glutamate and GABA produced by NMF. These data suggest that endogenous dopamine, through both D1 and D2 dopamine receptors, plays a role in releasing glutamate and GABA in striatum of the freely moving rat.

Effects of endogenous glutamate on extracellular concentrations of GABA, dopamine, and dopamine metabolites in the prefrontal cortex of the freely moving rat: involvement of NMDA and AMPA/KA receptors.

Using microdialysis, interactions between endogenous glutamate, dopamine, and GABA were investigated in the medial prefrontal cortex of the freely moving rat. Interactions between glutamate and other neurotransmitters in the prefrontal cortex had already been studied using pharmacological agonists or antagonists of glutamate receptors. This research investigated whether glutamate itself, through the increase of its endogenous extracellular concentration, is able to modulate the extracellular concentrations of GABA and dopamine in the prefrontal cortex. Intracortical infusions of the selective glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) were used to increase the endogenous extracellular glutamate. PDC (0.5, 2, 8, 16 and 32 mM) produced a dose-related increase in dialysate glutamate in a range of 1-36 microM. At the dose of 16 mM, PDC increased dialysate glutamate from 1.25 to 28 microM. PDC also increased extracellular GABA and taurine, but not dopamine; and decreased extracellular concentrations of the dopamine metabolites DOPAC and HVA. NMDA and AMPA/KA receptor antagonists were used to investigate whether the increases of extracellular glutamate were responsible for the changes in the release of GABA, and dopamine metabolites. The NMDA antagonist had no effect on the increase of extracellular GABA, but blocked the decreases of extracellular DOPAC and HVA, produced by PDC. In contrast, the AMPA/KA antagonist blocked the increases of extracellular GABA without affecting the decreases of extracellular DOPAC and HVA produced by PDC. These results suggest that endogenous glutamate acts preferentially through NMDA receptors to decrease dopamine metabolism, and through AMPA/KA receptors to increase GABAergic activity in the medial prefrontal cortex of the awake rat.

Changes in extracellular glutamate and GABA levels in the hippocampal CA 3 and CA 1 areas and the induction of glutamic acid decarboxylase-67 in dentate granule cells of rats treated with kainic acid

For the evaluation of glutamatergic and GABAergic transmission during seizures, rat hippocampal CA 1 and CA 3 areas were separately assessed by brain microdialysis, and extracelluar glutamate and GABA were measured through the course of the seizures after a systemic administration of kainic acid (KA). The generalized convulsion started at about 1.5 h and was suppressed by diazepam at 2 h after the KA treatment. In the CA 3 area, extracellular glutamate started to increase soon after the KA injection and returned to the control level at about 1.5 h. A decrease and then slight increase of the extracellular glutamate level in CA 3 followed the diazepam injection. In the CA 1 area, in contrast, a long-lasting decrease of extracellular glutamate was observed. The extracellular GABA concentration in the CA 3 area increased immediately after the systemic administration of KA and returned to the normal level at about 3.5 h. A second increase in the extracellular GABA in this area began at about 4.5 h after the KA treatment. In the CA 1 area, an increase of extracellular GABA began at about 3.5 h after KA administration (much later than that observed in the CA 3 area) and was maintained throughout the observation. In situ hybridization showed a transient expression of glutamic acid decarboxylase (GAD)-67 mRNA in the granule cell layer of the dentate gyrus at 4 and 6 h, whereas GAD65 mRNA was unaffected. GABA immunoreactivity in the same area and mossy fibers in the CA 3 were increased most significantly at 8 h after administration of KA. The possible relation of GABA induction in mossy fibers with the delayed increase in extracellular GABA in CA 3 was discussed.

GAMMA-VINYL-GABA POTENTIATES THE SEVERITY OF NALOXONE-PRECIPITATED ABSTINENCE SIGNS IN MORPHINE-DEPENDENT RATS

Effects of gamma-vinyl-GABA (GVG), an antiepileptic drug that inhibits GABA transaminase and increases extracellular GABA concentrations in the brain, were investigated on the morphine abstinence syndrome (AS) in male Wistar rats. Two morphine pellets (75 mg morphine base in each) were implanted subcutaneously on the back of the rats. Seventy-two hours after the morphine implantation, naloxone (NL, 2 mg kg−1) was injected intraperitoneally (i.p.) to induce precipitated morphine AS. GVG was administered at the doses of 250 mg kg−1(n=11) and 500 mg kg−1(n=11) i.p. 24 h prior to AS and at the dose of 500 mg kg−1(n=13) i.p. 6 h prior to AS. Immediately after NL injections, rats were observed for 5 min and AS signs (jumping, teeth chattering, wet dog shake, diarrhoea, ptosis and defecation) were assessed. The behavioural signs of GVG-treated rats were compared with the control groups (n=10) during the AS. Jumping, wet dog shake, teeth chattering were found to be significantly increased in all of the GVG-treated groups. Ptosis was found to have increased in only 500 mg kg−1GVG groups. GVG potentiated the severity of morphine AS signs. GVG does not seem to have any therapeutic potential for treatment of morphine abstinence unlike some other drugs that enhance GABAergic transmission.

Amphetamine releases GABA in striatum of the freely moving rat: involvement of calcium and high affinity transporter mechanisms

Using microdialysis the effect was investigated of amphetamine (AMPH) infusions into the striatum on the release of GABA in the freely moving rat. AMPH (5, 10 and 20 μg/μl), infused through a microdialysis probe at the rate of 2.5 μl/min, produced a dose-related increase in extracellular concentrations of GABA. At the highest dose (20 μg/μl), AMPH increased GABA from 0.08±0.01 to 0.67±0.14 μM. Increases in extracellular GABA produced by AMPH were both calcium-dependent and high affinity GABA transporter-mediated. A medium free of calcium reduced the increase of extracellular GABA produced by AMPH by 37%. Nipecotic acid (2, 4 and 8 mM), a specific GABA re-uptake blocker, significantly attenuated increases in extracellular GABA, but not GLU, produced by AMPH (20 μg/μl). This study is the first in vivo evidence showing the release of GABA produced by AMPH through a high affinity transporter mechanism.