Extracellular Gamma-aminobutyric Acid Levels Research Articles

Long-term intermittent theta burst stimulation enhanced hippocampus-dependent memory by regulating hippocampal theta oscillation and neurotransmitter levels in healthy rats

Intermittent theta burst stimulation (iTBS), an updated pattern of high-frequency repetitive transcranial magnetic stimulation, is a potential candidate for improving memory. The hippocampus has been shown to be involved in the memory-enhancing effect induced by iTBS. However, it remains largely unknown whether this effect is achieved by regulating hippocampal theta oscillation and neurotransmitters gamma-aminobutyric acid (GABA) and glutamate, which are strongly related to memory. Thus, we investigated the effect of 14 days of iTBS on hippocampus-dependent memory and further explored the roles of hippocampal theta oscillation and neurotransmitters GABA and glutamate in this effect. We found that compared to sham iTBS, real iTBS enhanced hippocampus-dependent memory measured by hole-board test and object place recognition test. Further, real iTBS increased the density of c-Fos positive neurons and normalized power of theta oscillation in the dorsal hippocampus (dHip) compared to sham iTBS. Interestingly, we observed a decrease in the level of extracellular GABA and an increase in the level of extracellular glutamate in the dHip after real iTBS. Our results suggest that long-term iTBS improved hippocampus-dependent memory, which may be attributed to the enhancement of theta oscillation and altered levels of extracellular GABA and glutamate in the dHip.

Adolescent Exposure to WIN 55212-2 Render the Nigrostriatal Dopaminergic Pathway Activated During Adulthood.

BackgroundDuring adolescence, neuronal circuits exhibit plasticity in response to physiological changes and to adapt to environmental events. Nigrostriatal dopaminergic pathways are in constant flux during development. Evidence suggests a relationship between early use of cannabinoids and psychiatric disorders characterized by altered dopaminergic systems, such as schizophrenia and addiction. However, the impact of adolescent exposure to cannabinoids on nigrostriatal dopaminergic pathways in adulthood remains unclear. The aim of this research was to determine the effects of repeated activation of cannabinoid receptors during adolescence on dopaminergic activity of nigrostriatal pathways and the mechanisms underlying this impact during adulthood.MethodsMale Sprague-Dawley rats were treated with 1.2 mg/kg WIN 55212-2 daily from postnatal day 40 to 65. Then no-net flux microdialysis of dopamine in the dorsolateral striatum, electrophysiological recording of dopaminergic neuronal activity, and microdialysis measures of gamma-aminobutyric acid (GABA) and glutamate in substantia nigra par compacta were carried out during adulthood (postnatal days 72–78).ResultsRepeated activation of cannabinoid receptors during adolescence increased the release of dopamine in dorsolateral striatum accompanied by increased population activity of dopamine neurons and decreased extracellular GABA levels in substantia nigra par compacta in adulthood. Furthermore, perfusion of bicuculline, a GABAa antagonist, into the ventral pallidum reversed the increased dopamine neuron population activity in substantia nigra par compacta induced by adolescent cannabinoid exposure.ConclusionsThese results suggest that adolescent exposure to cannabinoid agonists produces disinhibition of nigrostriatal dopamine transmission during adulthood mediated by decreased GABAergic input from the ventral pallidum.

Dynamic Cerebral Microdialysis during Pallidotomy and Thalamotomy in Parkinson’s Disease: A Preliminary Neurochemical Study

In Parkinson’s disease (PD), dopaminergic neurons reduce the regulation of glutamatergic (glutamate-Glu) input from the cortex to neostriatum (caudate and putamen nuclei) consequently leading to a hyperactivity of globus pallidus internae (GPi) neurons that release gamma-amino-butyric acid (GABA) into the thalamic ventrolateral (VL) nucleus. The objective of the present experiment was to measure changes in GABA and Glu in the caudate and the thalamus of 2 patients during the application of electrical stimuli following either a pallidotomy or a thalamotomy. Proper insertion of the electrode was tested by applying high frequency electrical pulses (HFEP). During these procedures, we obtained neurochemical information placing cerebral (CMD) microdialysis probes in caudate nucleus and VL nucleus of ipsi- and contra-lateral thalamus. In VL thalamus, extracellular GABA decreased during HFEP, tending to reach previous levels once HFEP was finalized. Following the pallido- or thalamotomy GABA decreased again. Similarly, in the contralateral VL thalamus, extracellular GABA levels showed a similar but less pronounced profile but did not show any decrement after the lesion. Caudate Glu decreases when HFEP is applied to the GPi and recovers to previous levels after HFEP, but did not decrease again after lesion (GPi-tomy), instead it continued to rise. These results suggest that HFEP exerts a similar but reversible biochemical effect as thermopallido- or thermothalamotomy on GABA extracellular concentration in the ipsilateral VL thalamus. We also observe a distant effect of HFEP, but not of thermolesion, on contralateral thalamic GABA and ipsilateral caudate Glu.

Investigation of the Role of AT2 Receptors in the Nucleus Tractus Solitarii of Normotensive Rats in Blood Pressure Control.

AimThe nucleus tractus solitarii (NTS) densely expresses angiotensin II type 2 receptors (AT2R), which are mainly located on inhibitory gamma-aminobutyric acid (GABA) neurons. Central AT2R stimulation reduces blood pressure, and AT2R stimulation in the rostral ventrolateral medulla (RVLM), mediates a hypotensive response through a GABAergic mechanism. We aimed to test the hypothesis that an AT2R mediated inhibition of the GABA release within the NTS might be involved in this hypotensive response, by assessing possible alterations in blood pressure and heart rate, as well as in GABA levels in normotensive Wistar rats.MethodsIn vivo microdialysis was used for measurement of extracellular GABA levels and for perfusion of the selective AT2R agonist, Compound 21, within the NTS. Our set-up allowed to determine simultaneously the excitatory glutamate dialysate levels. The mean arterial pressure and heart rate responses were monitored with a pressure transducer.ResultsLocal perfusion of Compound 21 into the NTS did not modify blood pressure and heart rate, nor glutamate and GABA levels compared to baseline concentrations. A putative effect was also not unmasked by concomitant angiotensin II type 1 receptor blockade with candesartan. Positive control experiments confirmed that the experimental set up had enough sensitivity to detect a reduction in GABA dialysate levels and blood pressure.ConclusionThe results did not provide evidence for a role of the AT2R within the NTS in the control of blood pressure, nor for an interaction with local GABAergic signaling in normotensive rats.

AT1 Receptor Mediated Hypertensive Response to Ang II in the Nucleus Tractus Solitarii of Normotensive Rats Involves NO Dependent Local GABA Release.

AimIt is well-established that angiotensin II exerts a dampening effect on the baroreflex within the nucleus tractus solitarii (NTS), the principal brainstem site for termination of baroreceptor afferents and which is densely populated with gamma-aminobutyric acid (GABA)ergic neurons and nerve terminals. The present study was designed to investigate whether local release of GABA is involved in the effects mediated by local angiotensin II within the NTS.MethodsIn vivo microdialysis was used for measurement of extracellular glutamate and GABA levels and for infusion of angiotensin II within the NTS of conscious normotensive Wistar rats. The mean arterial pressure (MAP) and heart rate response to local infusion of angiotensin II were subsequently monitored with a pressure transducer under anesthesia. The angiotensin II type 1 receptor (AT1R) antagonist, candesartan, was used to assess whether responses were AT1R dependent and the nitric oxide (NO) synthase inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME), was used to assess the involvement of NO in the evoked responses by infusion of angiotensin II. The MAP and heart rate responses were monitored with a pressure transducer.ResultsLocal infusion into the NTS of angiotensin II induced a significant to ninefold significantly increase in extracellular GABA levels; as well as MAP was increased by 15 mmHg. These responses were both abolished by co-infusion of either, the angiotensin II type 1 receptor antagonist, candesartan, or the NO synthase inhibitor, L-NAME, demonstrating that the effect is not only AT1R dependent but also NO dependent. The pressor response to angiotensin II was reversed by co-infusion with the GABAA receptor antagonist, bicuculline. Local blockade of NO synthase decreased both, GABA and glutamate concentrations.ConclusionOur results suggest that the AT1R mediated hypertensive response to angiotensin II within the NTS in normotensive rats is GABA and NO dependent. Nitric oxide produced within the NTS tonically potentiates local GABA and glutamate release.

Hypotensive Response to Angiotensin II Type 2 Receptor Stimulation in the Rostral Ventrolateral Medulla Requires Functional GABA-A Receptors.

Objectives: Angiotensin II, glutamate and gamma-aminobutyric acid (GABA) interact within the rostral ventrolateral medulla (RVLM) and the paraventricular nucleus (PVN) modulating the central regulation of blood pressure and sympathetic tone. Our aim was to assess the effects of local angiotensin II type 2 receptor stimulation within the RVLM and the PVN on neurotransmitter concentrations and mean arterial pressure (MAP).Methods: In vivo microdialysis was used for measurement of extracellular glutamate and GABA levels and for local infusion of the angiotensin II type 2 receptor agonist Compound 21 in the RVLM and the PVN of conscious normotensive Wistar rats. The MAP response to local Compound 21 was monitored with a pressure transducer under anaesthesia. Angiotensin II type 2 receptor selectivity was assessed using the angiotensin II type 2 receptor antagonist PD123319; the GABA-A receptor antagonist bicuculline was used to assess the involvement of GABA-A receptors.Results: Infusion of Compound 21 (0.05 μg/μl/h) in the RVLM significantly increased GABA levels and lowered blood pressure. These effects were abolished by co-infusion with PD123319. No changes in neurotransmitter levels or effects on blood pressure were seen with PD123319 infusion alone. Co-infusion of bicuculline abolished the Compound 21 evoked decrease in MAP. Infusion of Compound 21 within the PVN did not change extracellular neurotransmitter levels nor MAP.Conclusion: Selective stimulation of angiotensin II type 2 receptor within the RVLM by local Compound 21 infusion reduces blood pressure and increases local GABA levels in normotensive rats. This hypotensive response requires functional GABA-A receptors, suggesting that GABAergic neurons are involved in the sympatho-inhibitory action underlying this hypotensive response.

HYPOTENSIVE RESPONSE TO ANGIOTENSIN II TYPE 2 RECEPTOR STIMULATION IN THE ROSTRAL VENTROLATERAL MEDULLA: INTERACTION WITH GLUTAMATE AND GAMMA-AMINOBUTYRIC ACID

Event Abstract Back to Event HYPOTENSIVE RESPONSE TO ANGIOTENSIN II TYPE 2 RECEPTOR STIMULATION IN THE ROSTRAL VENTROLATERAL MEDULLA: INTERACTION WITH GLUTAMATE AND GAMMA-AMINOBUTYRIC ACID Laura LEGAT1*, Sofie Brouwers2, Ilse Smolders1 and Alain Dupont1, 2, 3 1 Vrije Universiteit Brussel, Department of Pharmaceutical Chemistry, Drug Analysis and Drug Information (FASC), Belgium 2 UZ Brussel, Department of Pharmacology and Cardiovascular Center, Belgium 3 UZ Brussel, Department of Clinical Pharmacology and Clinical Pharmacy (KFAR), Belgium Objectives Angiotensin II, glutamate and gamma-aminobutyric acid (GABA) interact within the rostral ventrolateral medulla and the paraventricular nucleus modulating the central regulation of blood pressure and sympathetic tone. Our aim was to assess the effects of local angiotensin II type 2 receptor stimulation within the rostral ventrolateral medulla and the paraventricular nucleus on neurotransmitter concentrations and mean arterial pressure. Methods In vivo microdialysis was used for measurement of extracellular glutamate and GABA levels and for local infusion of the angiotensin II type 2 receptor agonist Compound 21 in the rostral ventrolateral medulla and the paraventricular nucleus of conscious normotensive Wistar rats. The mean arterial pressure response to local Compound 21 was monitored with a pressure transducer under anaesthesia. Angiotensin II type 2 receptor selectivity was assessed using the angiotensin II type 2 receptor antagonist PD123319; the GABA(A) receptor antagonist bicuculline was used to assess the involvement of GABA(A) receptors. Results Infusion of C21 (0.05µg/µl/h) in the rostral ventrolateral medulla significantly increased GABA levels and lowered blood pressure. These effects were abolished by co-infusion with PD123319. No changes in neurotransmitter levels or effects on blood pressure were seen with PD123319 infusion alone. Co-infusion of bicuculline abolished the Compound 21 evoked decrease in mean arterial pressure. Infusion of Compound 21 within the paraventricular nucleus did not change extracellular neurotransmitter levels nor mean arterial pressure. Conclusion Selective stimulation of angiotensin II type 2 receptor within the rostral ventrolateral medulla by local Compound 21 infusion reduces blood pressure and increases local GABA levels in normotensive rats. This hypotensive response requires functional GABA(A) receptors, suggesting that GABAergic neurons are involved in the sympatho-inhibitory action underlying this hypotensive response. Keywords: Renin-angiotensin-system, angiotensin II type 2 receptors, compound 21, mean arterial pressure, rostral ventrolateral medualla, gamma-Aminobutyric Acid Conference: 12th National Congress of the Belgian Society for Neuroscience, Gent, Belgium, 22 May - 22 May, 2017. Presentation Type: Oral Presentation Topic: Sensory and Motor Systems Citation: LEGAT L, Brouwers S, Smolders I and Dupont A (2019). HYPOTENSIVE RESPONSE TO ANGIOTENSIN II TYPE 2 RECEPTOR STIMULATION IN THE ROSTRAL VENTROLATERAL MEDULLA: INTERACTION WITH GLUTAMATE AND GAMMA-AMINOBUTYRIC ACID. Front. Neurosci. Conference Abstract: 12th National Congress of the Belgian Society for Neuroscience. doi: 10.3389/conf.fnins.2017.94.00063 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 24 Apr 2017; Published Online: 25 Jan 2019. * Correspondence: Mrs. Laura LEGAT, Vrije Universiteit Brussel, Department of Pharmaceutical Chemistry, Drug Analysis and Drug Information (FASC), Jette, 1090, Belgium, laura_legat@hotmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Laura LEGAT Sofie Brouwers Ilse Smolders Alain Dupont Google Laura LEGAT Sofie Brouwers Ilse Smolders Alain Dupont Google Scholar Laura LEGAT Sofie Brouwers Ilse Smolders Alain Dupont PubMed Laura LEGAT Sofie Brouwers Ilse Smolders Alain Dupont Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

PP.35.02] EFFECT OF ANGIOTENSIN RECEPTOR STIMULATION IN THE ROSTRAL VENTROLATERAL MEDULLA ON BLOOD PRESSURE IN NORMOTENSIVE RATS

Objective: There is increasing evidence that glutamate and gamma-aminobutyric acid (GABA) interact within the rostral ventrolateral medulla (RVLM) modulating the central regulation of blood pressure and sympathetic tone. Angiotensin II (Ang II) is the most important effector in the renin-angiotensin-aldosterone system (RAAS) mediating its actions through the angiotensin II type 1 receptor (AT1R) and angiotensin II type 2 receptor (AT2R). The AT2R is part of the protective arm of the RAAS and mediates opposing effects counterbalancing the AT1R mediated actions. The selective and specific AT2R agonist Compound 21 (C21) is a useful research tool to investigate the AT2R mediated effects. Design and method: In the present study we assess through local stimulation and blockade of AT1R and AT2R the possible role of the brain RAAS at the level of the RVLM on the regulation of blood pressure. In vivo microdialysis, for measurement of extracellular glutamate and GABA levels, was carried out in the RVLM of conscious normotensive Wistar rats while locally infusing C21, Ang II, and PD123319. Glutamate and GABA levels were quantified by HPLC. Subsequently, the effect of central administration of C21, Ang II, and PD123319 on mean arterial pressure was assessed with a pressure transducer for continuous monitoring. Results: Infusion of Ang II into the RVLM significantly increased glutamate levels (P < 0.05) and significantly decreased GABA levels (P < 0.01). Infusion of the AT2R agonist C21 (0.05 μg/μl/h) tended to decrease glutamate and increase GABA levels, while absolutely no change was seen when co-infusing C21 together with the AT2 receptor antagonist PD123319. C21 infusion (0.05 μg/μl/h) significantly lowered blood pressure (P < 0.01). These responses were abolished by co-administration of C21 and PD123319. No changes in neurotransmitter levels or effects on blood pressure were seen with PD123319 infusion alone. Conclusions: These data demonstrate that acute specific stimulation of AT2R in the RVLM by local C21 infusion reduces blood pressure in normotensive rats, and suggests that decreases in local glutamate and increases in GABA levels may be involved in the sympatho-inhibitory action underlying this hypotensive response.

Pseudoginsenoside-F11 inhibits methamphetamine-induced behaviors by regulating dopaminergic and GABAergic neurons in the nucleus accumbens.

Although dependence to methamphetamine (METH) is associated with serious psychiatric symptoms and is a global health and social problem, no effective therapeutic approaches have been identified. Pseudoginsenoside-F11 (PF11) is an ocotillol-type saponin that is isolated from Panax quinquefolius (American ginseng) and was shown to have neuroprotective effects to promote learning and memory and to antagonize the pharmacological effects of morphine. Furthermore, PF11 also shows protective effects against METH-induced neurotoxicity in mice. However, the effects of PF11 on METH-induced preference and dopamine (DA) release have not been defined. We investigated the effects of PF11 administration on METH-induced hyperlocomotion and conditioned place preference (CPP) in mice. Subsequently, extracellular DA and gamma-aminobutyric acid (GABA) levels were determined in the nucleus accumbens (NAc) of mice after co-administration of PF11 and METH using in vivo microdialysis analyses. Moreover, the effects of PF11 administration on the μ-opioid neuronal responses, DAMGO (μ-opioid receptor agonist; [D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin)-induced hyperlocomotion and accumbal extracellular DA increase were investigated to elucidate how PF11 inhibits METH-induced dependence by dopaminergic neuronal hyperfunction. Co-administration of PF11 and METH for 6days attenuated METH-induced locomotor sensitization compared with treatment with METH alone. In the CPP test, PF11 administration also inhibited METH-induced place preference. In vivo microdialysis analyses indicated that co-administration of PF11 and METH for 7days prevented METH-induced extracellular DA increase in the NAc and repeated PF11 administration with or without METH for 7days increased extracellular GABA levels in the NAc, whereas single administration of PF11 did not. Furthermore, DAMGO-induced hyperlocomotion and accumbal extracellular DA increase were significantly inhibited by acute PF11 administration. The present data suggest that PF11 inhibits METH-induced hyperlocomotion, preference, and accumbal extracellular DA increase by regulating GABAergic neurons and μ-opioid receptors.

Alteration of glutamate/GABA balance during acute alcohol intoxication in rats: Effect of Xingnaojing injection

Ethnopharmacological relevanceXingnaojing Injection (XNJI) is a modern Chinese formula came from famous Chinese medicine An Gong Niu Huang Pill. XNJI has been used for treatment of cerebral diseases and stroke in China, and is approved by the State Food and Drug Administration of China for the treatment of acute alcohol intoxication (AAI). XNJI belongs to the ethnopharmacological family of medicines. In this study, we investigated the mechanisms of the XNJI effect on AAI. Aim of the studyTo investigate the effects of XNJI on glutamate, gamma-aminobutyric acid (GABA) and related receptor in lateral hypothalamic area (LHA) of AAI rat. Material and methodsAdult male Sprague-Dawley rats were implanted with microdialysis probes in LHA. Rats were randomly divided into control, model, 1.36mg/kg XNJI, 0.68mg/kg XNJI and 0.34mg/kg XNJI groups. During microdialysis, baseline samples were collected from 1h to 2.5h; thereafter, the rats were given an intraperitoneal injection of 52% ethanol, 5.2g/kg, or saline for control group. Twenty minutes later, three doses of XNJI was given by unilateral injection respectively, while saline for control and model groups, and samples were collected for the next 4h. The extracellular glutamate and GABA levels were measured in the LHA by a high performance liquid chromatography coupled with fluorescence detector (HPLC-FLU). The expression levels of related receptors N-methyl-d-aspartate receptor (NR) subunit NR2A, NR2B and GABAA were analyzed by reverse transcription polymerase chain reaction (RT-PCR). ResultsEthanol (5.2g/kg) significantly decreased the extracellular levels of glutamate and increased extracellular GABA in LHA. On the other hand ethanol significantly decreased NR2A and NR2B mRNAs expression, and increase GABAA mRNA expression. XNJI could increase the extracellular level of glutamate and decrease that of GABA; moreover, induced an increase in NR2A and NR2B mRNA expression, and a decrease in GABAA mRNA expression in LHA. ConclusionsThe current changes in glutamate, GABA and mRNA expressions of related receptors in LHA after injection of XNJI suggest that changes in these neurotransmitters and receptors as a potential mechanism of action for AAI.

Glutamate and GABA in the balance: Convergent pathways sustain seizures during status epilepticus

Seizures rapidly become self-sustaining and pharmacoresistant to benzodiazepines during status epilepticus (SE). A decrease in the number of postsynaptic gamma-aminobutyric acid (GABA)(A) receptors with SE causes a loss of synaptic inhibition, whereas increases in postsynaptic glutamatergic receptors further upset the balance between excitation and inhibition. Although extracellular GABA levels may increase during SE and contribute to postsynaptic GABA(A) receptor desensitization, other pathways involving glutamatergic activation ultimately may be responsible for the persistent down-regulation of postsynaptic GABA(A) receptors and erosion of synaptic inhibition.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

Gamma-vinyl GABA increases nonvesicular release of GABA and glutamate in the nucleus accumbens in rats via action on anion channels and GABA transporters.

gamma-Amino butyric acid (GABA) is a well-characterized inhibitory neurotransmitter in the central nervous system, which may also stimulate nonvesicular release of other neurotransmitters under certain conditions. We have recently reported that gamma-vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, elevates extracellular GABA but fails to alter dopamine release in the nucleus accumbens (NAc). Here, we investigated the mechanism(s) by which GVG elevates extracellular GABA levels and whether GVG also alters glutamate release in the NAc. In vivo microdialysis was used to simultaneously measure extracellular NAc GABA and glutamate before and after GVG administration in freely moving rats. Systemic administration of GVG or intra-NAc local perfusion of GVG significantly increased extracellular NAc GABA and glutamate. GVG-enhanced GABA was completely blocked by intra-NAc local perfusion of 5-nitro-2, 3-(phenylpropylamino)-benzoic acid (NPPB), a selective anion channel blocker and partially blocked by SKF89976A, a type 1 GABA transporter inhibitor. GVG-enhanced glutamate was completely blocked by NPPB or SKF89976A. Tetrodotoxin, a voltage-dependent Na(+)-channel blocker, failed to alter GVG-enhanced GABA and glutamate. These data suggest that GVG-enhanced extracellular GABA and glutamate are mediated predominantly by the opening of anion channels and partially by the reversal of GABA transporters. Enhanced extracellular glutamate may functionally attenuate the pharmacological action of GABA and prevent enhanced GABA-induced excess inhibition.

The effects of thioperamide on extracellular levels of glutamate and GABA in the rat prefrontal cortex

Histamine H3 receptors (H3R) are presynaptic heteroreceptors that negatively modulate the release of histamine and other neurotransmitters such as acetylcholine. Blocking H3 receptors with antagonists/inverse agonists has been shown to be procognitive and this effect has often been associated with increases in acetylcholine transmission. H3 receptors are abundantly expressed in the prefrontal cortex, an area associated with cognitive performance. While the procognitive effects of H3 receptor antagonists/inverse agonists may depend on alterations to acetylcholine or histamine release, other transmitters involved in cognitive processing such as glutamate and gamma-aminobutyric acid (GABA) may also be involved. The purpose of the present study was to examine the effects of thioperamide, an H3 receptor antagonist, on extracellular levels of glutamate and GABA in the prefrontal cortex. By means of in vivo microdialysis on freely moving Sprague Dawley rats, samples were collected and assayed via high-performance liquid chromatography coupled to electrochemical detection. Replacement of calcium with magnesium revealed that the release of GABA, but not glutamate, was calcium-dependent. Thioperamide (10-20 mg/kg) did not affect basal glutamate or GABA release. Perfusion with a high concentration of potassium (100 mM) increased GABA, but not glutamate, release and thioperamide (20 mg/kg) attenuated the effects of high potassium on GABA release. These data indicate that H3 receptors in the prefrontal cortex can enhance stimulated GABA release, but do not regulate basal levels of glutamate or GABA.

Alterations in Extracellular Levels of Gamma-Aminobutyric Acid in the Rat Basolateral Amygdala and Periaqueductal Gray During Conditioned Fear, Persistent Pain and Fear-Conditioned Analgesia

Evidence suggests an important role for supraspinal gamma-aminobutyric acid (GABA) in conditioned fear and pain. Using dual probe microdialysis coupled to HPLC, we investigated alterations in extracellular levels of GABA simultaneously in the rat basolateral amygdala and dorsal periaqueductal gray during expression of conditioned fear, formalin-evoked nociception, and fear-conditioned analgesia. Re-exposure to a context previously paired with footshock significantly increased the duration of freezing and 22-kilohertz ultrasonic vocalization, and reduced formalin-evoked nociceptive behavior. Upon re-exposure to the context, GABA levels in the basolateral amygdala were significantly lower in fear-conditioned animals compared with non-fear-conditioned controls, irrespective of intraplantar formalin/saline injection. GABA levels in the dorsal periaqueductal gray were lower in rats receiving intraplantar injection of formalin, compared with saline-treated controls. GABA levels sampled were sensitive to nipecotic acid and calcium infusion. No specific fear-conditioned analgesia-related alterations in GABA efflux were observed in these regions despite the ability of rats undergoing dual probe microdialysis to express this important survival response. In conclusion, expression of contextually induced fear- and pain-related behavior are accompanied by suppression of GABA release in the basolateral amygdala and dorsal periaqueductal gray, respectively, compared with non-fear, non-pain controls. This study investigates alterations in levels of the neurotransmitter GABA simultaneously in the rat amygdala and periaqueductal grey during expression of pain- and fear-related behavior and fear-induced analgesia. The results enhance our understanding of the role of this neurotransmitter in pain, memory of pain and control of pain during fear.

Possible relationship between the stress‐induced synaptic response and metaplasticity in the hippocampal CA1 field of freely moving rats

Hippocampal long-term potentiation (LTP) is suppressed not only by stress paradigms but also by low frequency stimulation (LFS) prior to LTP-inducing high frequency stimulation (HFS; tetanus), termed metaplasticity. These synaptic responses are dependent on N-methyl-D-aspartate receptors, leading to speculations about the possible relationship between metaplasticity and stress-induced LTP impairment. However, the functional significance of metaplasticity has been unclear. The present study elucidated the electrophysiological and neurochemical profiles of metaplasticity in the hippocampal CA1 field, with a focus on the synaptic response induced by the emotional stress, contextual fear conditioning (CFC). The population spike amplitude in the CA1 field was decreased during exposure to CFC, and LTP induction was suppressed after CFC in conscious rats. The synaptic response induced by CFC was mimicked by LFS, i.e., LFS impaired the synaptic transmission and subsequent LTP. Plasma corticosterone levels were increased by both CFC and LFS. Extracellular levels of gamma-aminobutyric acid (GABA), but not glutamate, in the hippocampus increased during exposure to CFC or LFS. Furthermore, electrical stimulation of the medial prefrontal cortex (mPFC), which caused decreases in freezing behavior during exposure to CFC, counteracted the LTP impairment induced by LFS. These findings suggest that metaplasticity in the rat hippocampal CA1 field is related to the neural basis of stress experience-dependent fear memory, and that hippocampal synaptic response associated stress-related processes is under mPFC regulation.

Neurotransmitter release during delay eyeblink classical conditioning: Role of norepinephrine in consolidation and effect of age

Delay classical eyeblink conditioning (EBC) is an important model of associative, cerebellar-dependent learning. Norepinephrine (NE) plays a significant modulatory role in the acquisition of learning; however, other neurotransmitters are also involved. The goal was to determine whether NE, gamma-aminobutyric acid (GABA) and glutamate (GLU) release are observed in cerebellar cortex during EBC, and whether such release was selectively associated with training. Further studies examined the role of the β-noradrenergic receptor in consolidation of the learned response by local infusion of propranolol at 5–120 min following training into the cerebellar cortex. In vivo microdialysis coupled to EBC was performed to examine neurotransmitter release. An increase in the extracellular level of NE was observed during EBC and was maximal on day 1 and diminished in amplitude with subsequent days of training. No changes in baseline NE release were observed in pseudoconditioning indicating that NE release is directly related to the associative learning process. The extracellular levels of GABA were also increased selectively during paired training however, the magnitude of GABA release increased over days of training. GLU release was observed to increase during both paired and unpaired training, suggesting that learning does not occur prior to the information arriving in the cerebellum. When propranolol was administered at either 5-, 60-, or 120-min post-training, there was an inhibition of conditioned responses, these data support the hypothesis that NE is important for consolidation of learning. In another set of experiments we demonstrate that the timing of release of NE, GABA and glutamate are significantly delayed in onset and lengthened in duration in the 22-month-old F344 rats. Over days of training the timing of release becomes closer to the timing of training and this is associated with increased learning of conditioned responses in the aged rats.

Restoration of the Luteinizing Hormone Surge in Middle-Aged Female Rats by Altering the Balance of GABA and Glutamate Transmission in the Medial Preoptic Area1

Hypothalamic glutamate and gamma-aminobutyric acid (GABA) neurotransmission are involved in the ovarian hormone-induced GnRH-LH surge in rodents. We previously reported that middle-aged rats have significantly less glutamate release in the medial preoptic area than young rats on the day of the LH surge. The present study tested the hypothesis that the delayed and attenuated LH surge in ovariohysterectomized middle-aged rats primed with ovarian steroids results from reduced hypothalamic glutamate and increased GABA(A) neurotransmission. Microdialysis results show that middle-aged rats with attenuated LH surges had reduced extracellular glutamate and increased extracellular GABA levels in the medial preoptic area compared with young rats. Blocking GABA(A) receptors with bicuculline or inhibiting synaptic glutamate reuptake with L-trans-pyrrolidine-2,4-dicarboxylic acid increased extracellular Glu in the medial preoptic area and partially restored LH surge amplitude in middle-aged rats without altering LH surge onset. Complete recovery of LH surge amplitude was observed in middle-aged rats treated with the combination of bicuculline and L-trans-pyrrolidine-2,4-dicarboxylic acid. This treatment also restored the extracellular glutamate:GABA ratio in the medial preoptic area of middle-aged rats to the level of young rats. Immunoblot analysis revealed that estradiol and progesterone treatment reduced SLC32A1(formerly known as vesicular GABA transporter) levels and increased SLC17A6 (formerly known as vesicular glutamate transporter 2) levels in the anterior hypothalamus of ovariohysterectomized young but not middle-aged rats. These data suggest that both reduced availability of glutamate and increased activation of GABA(A) receptors under estrogen-positive feedback conditions contribute to the age-related delay in onset and attenuated amplitude of the LH surge.

GAT1 and GAT3 expression are differently localized in the human epileptogenic hippocampus

The gamma amino butyric acid (GABA) transporters GAT-1 and GAT-3 were localized by immunohistochemistry in hippocampi removed for the control of medically intractable temporal lobe epilepsy (TLE). The study aimed to determine the relationship of GABA transporter expression to known patterns of hippocampal hyperexcitability and extracellular GABA levels. GAT-1 was localized in axon terminals and small neuronal cell bodies, and in non-sclerotic hippocampi was strongly expressed throughout all regions of the hippocampal formation. In the epileptogenic hippocampus exhibiting Ammon's horn sclerosis, immunoreactivity was reduced in the sclerotic regions CA3 and CA1, and around the cell bodies of dentate granule cells, but was increased along granule cell dendrites. GAT-3 was weakly expressed, if at all, in non-sclerotic hippocampi, but more prominently expressed in sclerotic hippocampi. GAT-3 expression was confined to cells resembling protoplasmic astrocytes, which were located in regions of relative neuronal sparing such as the dentate gyrus and hilus of the sclerotic hippocampus. The reduction in GAT-1 around granule cells in the sclerotic hippocampus could explain the prolonged GABA responses in this region. The loss of GAT-1 (a marker of GABAergic terminals) would also suggest a reduced GABAergic input to the granule cells, thus facilitating hyperexcitability. The increased GAT-3 expression in astrocytes in regions of relative neuronal sparing in the sclerotic hippocampus may be related to the overall low levels of extracellular GABA observed in the sclerotic hippocampus and their increased excitability.

Depression of extra-cellular GABA and increase of NMDA-induced nitric oxide following acute intra-nuclear administration of alcohol in the cerebellar nuclei of the rat.

Gamma-aminobutyric acid (GABA) and nitric oxide are two key-transmitters in cerebellar nuclei, the major output of cerebellar circuitry. The aims of this study were to investigate the effects of acute intra-cerebellar administration of ethanol (20 mM) on extra-cellular levels of GABA and on the NMDA-induced nitric oxide (NO) production using microdialysis in the rat. We also studied: (i) the effects of a pre-administration of DNQX, a specific antagonist of AMPA receptors, on NO production, (ii) the effects of a pre-administration of 7-NI (7-nitroindazole, an inhibitor of neuronal nitric oxide synthase NOS) and APV (D-2-amino-5-phosphonovaleric acid, a specific blocker of the NMDA type glutamate receptors) on the actions of alcohol/NMDA on glutamate receptors, and (iii) the in vivo interaction between DNQX, ethanol and NMDA receptor activation. We found that ethanol decreased the amount of extra-cellular GABA, and that this effect was counterbalanced by administration of tiagabine 1 mg/kg, a potent inhibitor of GAT-1 GABA transporter, given by the i.p. route. In loco administration of NMDA increased the levels of NO, as previously reported. A pre-administration of DNQX (500 microM) increased significantly the production of NO up to toxic levels, as well as ethanol administration. A pre-administration of 7-NI or APV reduced significantly the amounts of NO when NMDA and alcohol were infused simultaneously. The combination of ethanol with DNQX was associated with a marked enhancement of the concentrations of NO. The activity of GAT-1 in cerebellar nuclei and around this target, including in glial cells expressing GAT-1 activated by ambient GABA, seems to be spared by ethanol. Tiagabine could be considered as a candidate for future investigational treatments of acute ethanol-induced dysfunction of cerebellar nuclei. We found a potentiation of the production of NO when AMPA antagonists are given simultaneously to ethanol. The hypothesis of AMPA neurotoxicity, which has convincing arguments during chronic exposure, is challenged in this model of acute cerebellar nuclear toxicity of alcohol.