GABA receptor mechanisms in the central nervous system

The effect of nucleus basalis magnocellularis deep brain stimulation on memory function in a rat model of dementia

Background

Deep brain stimulation has recently been considered a potential therapy in improving memory function. It has been shown that a change of neurotransmitters has an effect on memory function. However, much about the exact underlying neural mechanism is not yet completely understood.

We therefore examined changes in neurotransmitter systems and spatial memory caused by stimulation of nucleus basalis magnocellularis in a rat model of dementia.

Methods

We divided rats into four groups: Normal, Lesion, Implantation, and Stimulation. We used 192 IgG-saporin for degeneration of basal forebrain cholinergic neuron related with learning and memory and it was injected into all rats except for the normal group. An electrode was ipsilaterally inserted in the nucleus basalis magnocellularis of all rats of the implantation and stimulation group, and the stimulation group received the electrical stimulation. Features were verified by the Morris water maze, immunochemistry and western blotting.

Results

All groups showed similar performances during Morris water maze training. During the probe trial, performance of the lesion and implantation group decreased. However, the stimulation group showed an equivalent performance to the normal group. In the lesion and implantation group, expression of glutamate acid decarboxylase65&67 decreased in the medial prefrontal cortex and expression of glutamate transporters increased in the medial prefrontal cortex and hippocampus. However, expression of the stimulation group showed similar levels as the normal group.

Conclusion

The results suggest that nucleus basalis magnocellularis stimulation enhances consolidation and retrieval of visuospatial memory related to changes of glutamate acid decarboxylase65&67 and glutamate transporter.

Hydrogen peroxide modulates synaptic transmission in ventral horn neurons of the rat spinal cord

KEY POINTS

Excessive production of reactive oxygen species (ROS) is implicated in many central nervous system disorders; however, the physiological role of ROS in spinal ventral horn (VH) neurons remains poorly understood. We investigated how pathological levels of H2O2, an abundant ROS, regulate synaptic transmission in VH neurons of rats using a whole-cell patch clamp approach. H2O2 increased the release of glutamate and GABA from presynaptic terminals. The increase in glutamate release involved N-type voltage-gated calcium channels (VGCCs), ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP3 Rs); the increase in GABA release, which inhibited glutamatergic transmission, involved IP3 R. Inhibiting N-type VGCCs and RyRs attenuates excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may represent a novel strategy for treating H2O2-induced motor neuron disorders resulting from trauma or ischaemia-reperfusion injury. Excessive production of reactive oxygen species (ROS) is a critical component of the cellular and molecular pathophysiology of many central nervous system (CNS) disorders, including trauma, ischaemia-reperfusion injury, and neurodegenerative diseases. Hydrogen peroxide (H2O2), an abundant ROS, modulates synaptic transmission and contributes to neuronal damage in the CNS; however, the pathophysiological role of H2O2 in spinal cord ventral horn (VH) neurons remains poorly understood, despite reports that these neurons are highly vulnerable to oxidative stress and ischaemia. This was investigated in the present study using a whole-cell patch clamp approach in rats. We found that exogenous application of H2O2 increased the release of glutamate from excitatory presynaptic terminals and γ-aminobutyric acid (GABA) from inhibitory presynaptic terminals. The increase of glutamate release was induced in part by an increase in Ca(2+) influx through N-type voltage-gated calcium channels (VGCCs) as well as by ryanodine receptor (RyR)- and inositol trisphosphate receptor-mediated Ca(2+) release from the endoplasmic reticulum (ER). In inhibitory presynaptic neurons, increased IP3 R-mediated Ca(2+) release from the ER increased GABAergic transmission, which served to rescue VH neurons from excessive release of glutamate from presynaptic terminals. These findings indicate that inhibiting N-type VGCCs or RyRs may attenuate excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may therefore represent a novel and targeted strategy for preventing and treating H2O2-induced motor neuron disorders.

Activation of GABA-A receptors during postnatal brain development increases anxiety- and depression-related behaviors in a time- and dose-dependent manner in adult mice

Disturbances of the gamma-amino butyric acid-ergic (GABAergic) system during postnatal development can have long-lasting consequences for later life behavior, like the individual's response to stress. However, it is unclear which postnatal windows of sensitivity to GABA-ergic modulations are associated with what later-life behavioral outcomes. Therefore, we sought to determine whether neonatal activation of the GABA-A receptor during two postnatal periods, an early window (postnatal day 3-5) and a late window (postnatal day 14-16), can affect anxiety- and depression-related behaviors in male mice in later life. To this end, mice were treated with either saline or muscimol (50, 100, 200, 300 and 500μg/kg) during the early and late postnatal periods. An additional group of mice was treated with the GABA-A receptor antagonist bicuculline+muscimol. When grown to adulthood male mice were exposed to behavioral tests to measure anxiety- and depression-related behaviors. Baseline and stress-induced corticosterone (CORT) levels were also measured. The results indicate that early postnatal and to a lesser extent later postnatal exposure to the GABA-A receptor agonist muscimol increased anxiety-like behavior and stress-induced CORT levels in adults. Moreover, the early postnatal treatment with muscimol increased depression-like behavior with increasing baseline CORT levels. The anxiogenic and depression-like later-life consequences could be antagonized by bicuculline. Our findings suggest that GABA-A receptor signaling during early-life can influence anxiety- and depression-related behaviors in a time- and dose-dependent manner in later life. Our findings help to increase insight in the developmental mechanisms contributing to stress-related disorders.

Capillary electrophoresis determination of non-protein amino acids as quality markers in foods

Non-protein amino acids mainly exist in food as products formed during food processing, as metabolic intermediates or as additives to increase nutritional and functional properties of food. This fact makes their analysis and determination an attractive field in food science since they can give interesting information on the quality and safety of foods. This article presents a comprehensive review devoted to describe the latest advances in the development of (achiral and chiral) analytical methodologies by capillary electrophoresis and microchip capillary electrophoresis for the analysis of non-protein amino acids in a variety of food samples. Most relevant information related to sample treatment, experimental separation and detection conditions, preconcentration strategies and limits of detection will be provided.

Neurosteroids differentially modulate fast and slow interictal discharges in the hippocampal CA3 area

Two types of spontaneous interictal discharge, identified as fast and slow events, can be recorded from the hippocampal CA3 area in rat brain slices during application of 4-aminopyridine (4AP) (50 μm). Here, we addressed how neurosteroids modulate the occurrence of these interictal events and of the associated high-frequency oscillations (HFOs) (ripples, 80-200 Hz; fast ripples, 250-500 Hz). Under control conditions (i.e. during 4AP application), ripples and fast ripples were detected in 12.3 and 17.5% of fast events, respectively; in contrast, the majority of slow events (> 98%) did not co-occur with HFOs. Application of 0.1, 1 or 5 μm allotetrahydrodeoxycorticosterone (THDOC) to 4AP-treated slices caused a dose-dependent decrease in the duration of the fast events and an increase in the occurrence of ripples, but not fast ripples; in contrast, the duration of slow events increased. THDOC potentiated the slow events that were recorded during pharmacological blockade of glutamatergic transmission, but had no effect on interictal discharges occurring during GABAA receptor antagonism. These results demonstrate that potentiation of GABAA receptor-mediated signaling by THDOC differentially affects slow and fast interictal discharges; these differences may provide insights into how hyperexcitable activity is influenced by neurosteroids.

Structural analogues of the natural products magnolol and honokiol as potent allosteric potentiators of GABA(A) receptors

Biphenylic compounds related to the natural products magnolol and 4'-O-methylhonokiol were synthesized, evaluated and optimized as positive allosteric modulators (PAMs) of GABA(A) receptors. The most efficacious compounds were the magnolol analog 5-ethyl-5'-hexylbiphenyl-2,2'-diol (45) and the honokiol analogs 4'-methoxy-5-propylbiphenyl-2-ol (61), 5-butyl-4'-methoxybiphenyl-2-ol (62) and 5-hexyl-4'-methoxybiphenyl-2-ol (64), which showed a most powerful potentiation of GABA-induced currents (up to 20-fold at a GABA concentration of 3μM). They were found not to interfere with the allosteric sites occupied by known allosteric modulators, such as benzodiazepines and N-arachidonoylglycerol. These new PAMs will be useful as pharmacological tools and may have therapeutic potential for mono-therapy, or in combination, for example, with GABA(A) receptor agonists.

The integrative analysis of microRNA and mRNA expression in Apis mellifera following maze-based visual pattern learning

The honeybee (Apis mellifera) is a social insect with strong sensory capacity and diverse behavioral repertoire and is recognized as a good model organism for studying the neurobiological basis of learning and memory. In this study, we analyzed the changes in microRNA (miRNA) and messenger RNA (mRNA) following maze-based visual learning using next-generation small RNA sequencing and Solexa/lllumina Digital Gene Expression tag profiling (DGE). For small RNA sequencing, we obtained 13 367 770 and 13 132 655 clean tags from the maze and control groups, respectively. A total of 40 differentially expressed known miRNAs were detected between these two samples, and all of them were up-regulated in the maze group compared to the control group. For DGE, 5 681 320 and 5 939 855 clean tags were detected from the maze and control groups, respectively. There were a total of 388 differentially expressed genes between these two samples, with 45 genes up-regulated and 343 genes down-regulated in the maze group, compared to the control group. Additionally, the expression levels of 10 differentially expressed genes were confirmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and the expression trends of eight of them were consistent with the DGE result, although the degree of change was lower in amplitude. The integrative analysis of miRNA and mRNA expression showed that, among the 40 differentially expressed known miRNAs and 388 differentially expressed genes, 60 pairs of miRNA/mRNA were identified as co-expressed in our present study. These results suggest that both miRNA and mRNA may play a pivotal role in the process of learning and memory in honeybees. Our sequencing data provide comprehensive miRNA and gene expression information for maze-based visual learning, which will facilitate understanding of the molecular mechanisms of honeybee learning and memory.

Benzodiazepine modulation of homomeric GABAAρ1 receptors: differential effects of diazepam and 4'-chlorodiazepam

GABA(A) receptors (GABA(A)Rs) are ligand-gated ion channels that mediate inhibitory neurotransmission in the central nervous system (CNS). They are members of the Cys-loop receptor family and display marked structural and functional heterogeneity. Many GABA(A)Rs receptor subtypes are allosterically modulated by benzodiazepines (BDZs), which are drugs extensively used as anxiolytics, sedative-hypnotics and anticonvulsants. One high-affinity site and at least three additional low-affinity sites for BDZ recognition have been identified in several heteromeric and homomeric variants of the GABA(A)Rs (e.g.: α1β2γ2, α1β2/3, β3, etc.). However, the modulation of homomeric GABA(A)ρRs by BDZs was not previously revealed, and these receptors, for a long a time, were assumed to be fully insensitive to the actions of these drugs. In the present study, human homomeric GABA(A)ρ1 receptors were expressed in Xenopus oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of BDZs. GABA(A)ρ1 receptor-mediated responses were modulated by diazepam and 4'-chlorodiazepam in the micromolar range, in a concentration-dependent, voltage-independent and reversible manner. Diazepam produced potentiating effects on GABA-evoked Cl(-) currents and 4'-Cl diazepam induced biphasic effects depending on the GABA concentration, whereas Ro15-4513 and alprazolam were negative modulators. BDZ actions were insensitive to flumazenil. Other BDZs showed negligible activity at equivalent experimental conditions. Our results suggest that GABA(A)ρ1 receptor function can be selectively and differentially modulated by BDZs.

Neuron type- and input pathway-dependent expression of Slc4a10 in adult mouse brains

Slc4a10 was originally identified as a Na(+) -driven Cl(-) /HCO3 (-) exchanger NCBE that transports extracellular Na(+) and HCO3 (-) in exchange for intracellular Cl(-) , whereas other studies argue against a Cl(-) -dependence for Na(+) -HCO3 (-) transport, and thus named it the electroneutral Na(+) /HCO3 (-) cotransporter NBCn2. Here we investigated Slc4a10 expression in adult mouse brains by in situ hybridization and immunohistochemistry. Slc4a10 mRNA was widely expressed, with higher levels in pyramidal cells in the hippocampus and cerebral cortex, parvalbumin-positive interneurons in the hippocampus, and Purkinje cells (PCs) in the cerebellum. Immunohistochemistry revealed an uneven distribution of Slc4a10 within the somatodendritic compartment of cerebellar neurons. In the cerebellar molecular layer, stellate cells and their innervation targets (i.e. PC dendrites in the superficial molecular layer) showed significantly higher labeling than basket cells and their targets (PC dendrites in the basal molecular layer and PC somata). Moreover, the distal dendritic trees of PCs (i.e. parallel fiber-targeted dendrites) had significantly greater labeling than the proximal dendrites (climbing fiber-targeted dendrites). These observations suggest that Slc4a10 expression is regulated in neuron type- and input pathway-dependent manners. Because such an elaborate regulation is also found for K(+) -Cl(-) cotransporter KCC2, a major neuronal Cl(-) extruder, we compared their expression. Slc4a10 and KCC2 overlapped in most somatodendritic elements. However, relative abundance was largely complementary in the cerebellar cortex, with particular enrichments of Slc4a10 in PC dendrites and KCC2 in molecular layer interneurons, granule cells and PC somata. These properties might reflect functional redundancy and distinction of these transporters, and their differential requirements by individual neurons and respective input domains.

Localized GABAergic inhibition of dendritic Ca(2+) signalling

Neuronal circuits are defined by synaptic connections between their cellular constituents. In this article, I highlight several recent studies emphasizing the surprising level of precision exhibited by inhibitory GABAergic synapses within the neocortex and hippocampus. Specifically, GABAergic inputs to dendritic shafts and spines of pyramidal cells have a key role in the localized regulation of neuronal Ca(2+) signalling. These findings provide important new insights into the cellular mechanisms underlying the contributions of inhibitory transmission to both normal and abnormal brain activity.

The role of the cerebellum in the pathogenesis of cortical myoclonus

The putative involvement of the cerebellum in the pathogenesis of cortical myoclonic syndromes has been long hypothesized, as neuropathological changes in patients with cortical myoclonus have most commonly been found in the cerebellum rather than in the suspected culprit, the primary somatosensory cortex. A model of increased cortical excitability due to loss of cerebellar inhibitory control via cerebello-thalamo-cortical connections has been proposed, but evidence remains equivocal. Here, we explore this hypothesis by examining syndromes that present with cortical myoclonus and ataxia. We first describe common clinical characteristics and underlying neuropathology. We critically view information on cerebellar physiology with regard to motorcortical output and compare findings between hypothesized and reported neurophysiological changes in conditions with cortical myoclonus and ataxia. We synthesize knowledge and focus on neurochemical changes in these conditions. Finally, we propose that the combination of alterations in inhibitory neurotransmission and the presence of cerebellar pathology are important elements in the pathogenesis of cortical myoclonus.

In vivo pharmacological characterization of AC-3933, a benzodiazepine receptor partial inverse agonist for the treatment of Alzheimer's disease

GABAergic neurons are known to inhibit neural transduction and therefore negatively affect excitatory neural circuits in the brain. We have previously reported that 5-(3-methoxyphenyl)-3-(5-methyl-1,2,4-oxadiazol-3-yl)-1,6-naphthyridin-2(1H)-one (AC-3933), a partial inverse agonist for the benzodiazepine receptor (BzR), reverses GABAergic inhibitory effect on cholinergic neurons, and thus enhances acetylcholine release from these neurons in rat hippocampal slices. In this study, we evaluated AC-3933 potential for the treatment of Alzheimer's disease, a disorder characterized by progressive decline mainly in cholinergic function. Oral administration of AC-3933 (0.01-0.03mg/kg) resulted in the amelioration of scopolamine-induced amnesia, as well as a shift in electroencephalogram (EEG) relative power characteristic of pro-cognitive cholinergic activators, such as donepezil. In addition, treatment with AC-3933 even at the high dose of 100mg/kg p.o. produced no seizure or anxiety, two major adverse effects of BzR inverse agonists developed in the past. These findings indicate that AC-3933 with its low risk for side effects may be useful in the treatment of Alzheimer's disease.

Role of synaptic activity in the regulation of amyloid beta levels in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Accumulation of amyloid-beta (Aβ) peptides is regarded as the critical component associated with AD pathogenesis, which is derived from the amyloid precursor protein (APP) cleavage. Recent studies suggest that synaptic activity is one of the most important factors that regulate Aβ levels. It has been found that synaptic activity facilitates APP internalization and influences APP cleavage. Glutamatergic, cholinergic, serotonergic, leptin, adrenergic, orexin, and gamma-amino butyric acid receptors, as well as the activity-regulated cytoskeleton-associated protein (Arc) are all involved in these processes. The present review summarizes the evidence for synaptic activity-modulated Aβ levels and the mechanisms underlying this regulation. Interestingly, the immediate early gene product Arc may also be the downstream signaling molecule of several receptors in the synaptic activity-modulated Aβ levels. Elucidating how Aβ levels are regulated by synaptic activity may provide new insights in both the understanding of the pathogenesis of AD and in the development of therapies to slow down the progression of AD.

Comparison of the effects of the GABAB receptor positive modulator BHF177 and the GABAB receptor agonist baclofen on anxiety-like behavior, learning, and memory in mice

γ-Aminobutyric acid B (GABAB) receptor activation is a potential therapeutic approach for the treatment of drug addiction, pain, anxiety, and depression. However, full agonists of this receptor induce side-effects, such as sedation, muscle relaxation, tolerance, and cognitive disruption. Positive allosteric modulators (PAMs) of the GABAB receptor may have similar therapeutic effects as agonists with superior side-effect profiles. The present study behaviorally characterized N-([1R,2R,4S]-bicyclo[2.2.1]hept-2-yl)-2-methyl-5-(4-[trifluoromethyl]phenyl)-4-pyrimidinamine (BHF177), a GABAB receptor PAM, in mouse models of anxiety-like behavior, learning and memory. In addition, the effects of BHF177 were compared with the agonist baclofen. Unlike the anxiolytic chlordiazepoxide, baclofen (0.5, 1.5, and 2.5 mg/kg, intraperitoneally) and BHF177 (10, 20, and 40 mg/kg, orally) had no effect on anxiety-like behavior in the elevated plus maze, light/dark box, or Vogel conflict test. Baclofen increased punished drinking in the Vogel conflict test, but this effect may be attributable to the analgesic actions of baclofen. At the highest dose tested (2.5 mg/kg), baclofen-treated mice exhibited sedation-like effects (i.e., reduced locomotor activity) across many of the tests, whereas BHF177-treated mice exhibited no sedation-like effects. BHF177 exhibited pro-convulsion properties only in mice, but not in rats, indicating that this effect may be species-specific. At doses that were not sedative or pro-convulsant, baclofen and BHF177 had no selective effects on fear memory retrieval in contextual and cued fear conditioning or spatial learning and memory in the Barnes maze. These data suggest that BHF177 has little sedative activity, no anxiolytic-like profile, and minimal impairment of learning and memory in mice.

Baclofen into the lateral parabrachial nucleus induces hypertonic sodium chloride intake during cell dehydration

BACKGROUND

Activation of GABA(B) receptors with baclofen into the lateral parabrachial nucleus (LPBN) induces ingestion of water and 0.3 M NaCl in fluid replete rats. However, up to now, no study has investigated the effects of baclofen injected alone or combined with GABA(B) receptor antagonist into the LPBN on water and 0.3 M NaCl intake in rats with increased plasma osmolarity (rats treated with an intragastric load of 2 M NaCl). Male Wistar rats with stainless steel cannulas implanted bilaterally into the LPBN were used.

RESULTS

In fluid replete rats, baclofen (0.5 nmol/0.2 μl), bilaterally injected into the LPBN, induced ingestion of 0.3 M NaCl (14.3 ± 4.1 vs. saline: 0.2 ± 0.2 ml/210 min) and water (7.1 ± 2.9 vs. saline: 0.6 ± 0.5 ml/210 min). In cell-dehydrated rats, bilateral injections of baclofen (0.5 and 1.0 nmol/0.2 μl) into the LPBN induced an increase of 0.3 M NaCl intake (15.6 ± 5.7 and 21.5 ± 3.5 ml/210 min, respectively, vs. saline: 1.7 ± 0.8 ml/210 min) and an early inhibition of water intake (3.5 ± 1.4 and 6.7 ± 2.1 ml/150 min, respectively, vs. saline: 9.2 ± 1.4 ml/150 min). The pretreatment of the LPBN with 2-hydroxysaclofen (GABA(B) antagonist, 5 nmol/0.2 μl) potentiated the effect of baclofen on 0.3 M NaCl intake in the first 90 min of test and did not modify the inhibition of water intake induced by baclofen in cell-dehydrated rats. Baclofen injected into the LPBN did not affect blood pressure and heart rate.

CONCLUSIONS

Thus, injection of baclofen into the LPBN in cell-dehydrated rats induced ingestion of 0.3 M NaCl and inhibition of water intake, suggesting that even in a hyperosmotic situation, the blockade of LPBN inhibitory mechanisms with baclofen is enough to drive rats to drink hypertonic NaCl, an effect independent of changes in blood pressure.

Recent progress in GABAergic excitation from mature brain

The excitatory effect of γ-Aminobutyric acid (GABA) has been recognized in very young animals and in seizure generation, but not so much in animals after weaning age or in adults. The existence of this phenomenon in mature brain is still controversial. In the course of debate, creative studies have identified and characterized the phenomenon in suprachiasmatic nucleus, cortex, hippocampus and basolateral amygdala, albeit mostly in single neurons. In neural circuit activity, presumed GABAergic excitation was observed in basolateral amygdala during the study of a neuropeptide, cholecystokinin. Though the functional meaning of this phenomenon in vivo remains to be uncovered, it may be implicated in epilepsy or anxiety in the adult brain.

Cocaine-induced adaptations in metabotropic inhibitory signaling in the mesocorticolimbic system

The addictive properties of psychostimulants such as cocaine are rooted in their ability to activate the mesocorticolimbic dopamine (DA) system. This system consists primarily of dopaminergic projections arising from the ventral tegmental area (VTA) and projecting to the limbic and cortical brain regions, such as the nucleus accumbens (NAc) and prefrontal cortex (PFC). While the basic anatomy and functional relevance of the mesocorticolimbic DA system is relatively well-established, a key challenge remaining in addiction research is to understand where and how molecular adaptations and corresponding changes in function of this system facilitate a pathological desire to seek and take drugs. Several lines of evidence indicate that inhibitory signaling, particularly signaling mediated by the Gi/o class of heterotrimeric GTP-binding proteins (G proteins), plays a key role in the acute and persistent effects of drugs of abuse. Moreover, recent evidence argues that these signaling pathways are targets of drug-induced adaptations. In this review we discuss inhibitory signaling pathways involving DA and the inhibitory neurotransmitter GABA in two brain regions - the VTA and PFC - that are central to the effects of acute and repeated cocaine exposure and represent sites of adaptations linked to addiction-related behaviors including sensitization, craving, and relapse.

Gene expression analysis following olfactory learning in Apis mellifera

The honeybee has a strong learning and memory ability, and is recognized as the best model organism for studying the neurobiological basis of learning and memory. In this study, we analyzed the gene expression difference following proboscis extension response-based olfactory learning in the A. mellifera using a tag-based digital gene expression (DGE) method. We obtained about 5.71 and 5.65 million clean tags from the trained group and untrained group, respectively. A total of 259 differentially expressed genes were detected between these two samples, with 30 genes up-regulated and 229 genes down-regulated in trained group compared to the untrained group. These results suggest that bees tend to actively suppress some genes instead of activating previously silent genes after olfactory learning. Our DGE data provide comprehensive gene expression information for olfactory learning, which will facilitate our understanding of the molecular mechanism of honey bee learning and memory.

Intraspinal transplantation of GABAergic neural progenitors attenuates neuropathic pain in rats: a pharmacologic and neurophysiological evaluation

Dysfunctional γ-aminobutyric acid (GABA)-ergic inhibitory neurotransmission is hypothesized to underlie chronic neuropathic pain. Intraspinal transplantation of GABAergic neural progenitor cells (NPCs) may reduce neuropathic pain by restoring dorsal horn inhibition. Rat NPCs pre-differentiated to a GABAergic phenotype were transplanted into the dorsal horn of rats with unilateral chronic constriction injury (CCI) of the sciatic nerve. GABA signaling in antinociceptive effects of NPC grafts was tested with the GABA(A) receptor antagonist bicuculline (BIC), GABA(B) receptor antagonist CGP35348 (CGP) and GABA reuptake inhibitor SKF 89976A (SKF). NPC-treated animals showed decreased hyperalgesia and allodynia 1-3week post-transplantation; vehicle-injected CCI rats continued displaying pain behaviors. Intrathecal application of BIC or CGP attenuated the antinociceptive effects of the NPC transplants while SKF injection induced analgesia in control rats. Electrophysiological recordings in NPC treated rats showed reduced responses of wide dynamic range (WDR) neurons to peripheral stimulation compared to controls. A spinal application of BIC or CGP increased wind-up response and post-discharges of WDR neurons in NPC treated animals. Results suggest that transplantation of GABAergic NPCs attenuate pain behaviors and reduce exaggerated dorsal horn neuronal firing induced by CCI. The effects of GABA receptor inhibitors suggest participation of continuously released GABA in the grafted animals.

Development of the chloride homeostasis in the auditory brainstem

Inhibitory neurotransmission plays a substantial role in encoding of auditory cues relevant for sound localization in vertebrates. While the anatomical organization of the respective afferent auditory brainstem circuits shows remarkable similarities between mammals and birds, the properties of inhibitory neurotransmission in these neural circuits are strikingly different. In mammals, inhibition is predominantly glycinergic and endowed with fast kinetics. In birds, inhibition is mediated by gamma-Aminobutiric acid (GABA) and too slow to convey temporal information. A further prominent difference lies in the mechanism of inhibition in the respective systems. In auditory brainstem neurons of mammals, [Cl(-)](i) undergoes a developmental shift causing the actions of GABA and glycine to gradually change from depolarization to the 'classic' hyperpolarizing-inhibition before hearing onset. Contrary to this, in the mature avian auditory brainstem Cl(-) homeostasis mechanisms accurately adjust the Cl(-) gradient to enable depolarizing, but still very efficient, shunting inhibition. The present review considers the mechanisms underlying development of the Cl(-) homeostasis in the auditory system of mammals and birds and discusses some open issues that require closer attention in future studies.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Binding sites for bilobalide, diltiazem, ginkgolide, and picrotoxinin at the 5-HT3 receptor

Bilobalide (BB), ginkgolide B (GB), diltiazem (DTZ), and picrotoxinin (PXN) are 5-hydroxytryptamine type 3 (5-HT(3)) receptor antagonists in which the principal sites of action are in the channel. To probe their exact binding locations, 5-HT(3) receptors with substitutions in their pore lining residues were constructed (N-4'Q, E-1'D, S2'A, T6'S, L7'T, L9'V, S12'A, I16'V, D20'E), expressed in Xenopus laevis oocytes, and the effects of the compounds on 5-HT-induced currents were examined. EC(50) values at mutant receptors were less than 6-fold different from those of wild type, indicating that the mutations were well tolerated. BB, GB, DTZ, and PXN had pIC(50) values of 3.33, 3.14, 4.67, and 4.97, respectively. Inhibition by BB and GB was abolished in mutant receptors containing T6'S and S12'A substitutions, but their potencies were enhanced (42- and 125-fold, respectively) in S2'A mutant receptors. S2'A substitution also caused GB ligand trap. PXN potency was modestly enhanced (5-fold) in S2'A, abolished in T6'S, and reduced in L9'V (40-fold) and S12'A (7-fold) receptors. DTZ potency was reduced in L7'T and S12'A receptors (5-fold), and DTZ also displaced [(3)H]granisetron binding, indicating mixed competitive/noncompetitive inhibition. We conclude that regions close to the hydrophobic gate of M2 are important for the inhibitory effects of BB, GB, DTZ, and PXN at the 5-HT(3) receptor; for BB, GB, and PXN, the data show that the 6' channel lining residue is their major site of action, with minor roles for 2', 9', and 12' residues, whereas for DTZ, the 7' and 12' sites are important.

GABAB receptor modulation of voltage-sensitive calcium channels in spines and dendrites

Although primarily studied at the cell body, GABA(B) receptors (GABA(B)Rs) are abundant at spines and dendrites of cortical pyramidal neurons, where they are positioned to influence both synaptic and dendritic function. Here, we examine how GABA(B)Rs modulate calcium (Ca) signals evoked by action potentials (APs) in spines and dendrites of layer 2/3 pyramidal neurons in mouse prefrontal cortex. We first use two-photon microscopy to show that GABA(B)Rs inhibit AP Ca signals throughout the entire dendritic arbor of these neurons. We then use local pharmacology and GABA uncaging to show that dendritic GABA(B)Rs also decrease the input resistance, shorten the AP afterdepolarization, and generate inhibitory postsynaptic potentials. However, we find that these electrophysiological effects recorded at the cell body do not correlate with the inhibition of AP Ca signals measured in spines and dendrites. Instead, we use voltage-clamp recordings to show that GABA(B)Rs directly inhibit several subtypes of voltage-sensitive calcium channels (VSCCs) in both spines and dendrites. Given the importance of VSCC-mediated Ca signals for neuronal function, our results have implications for the functional role of dendritic GABA(B)Rs in the prefrontal cortex and throughout the brain.

Implications for treatment: GABAA receptors in aging, Down syndrome and Alzheimer's disease

In addition to progressive dementia, Alzheimer's disease (AD) is characterized by increased incidence of seizure activity. Although originally discounted as a secondary process occurring as a result of neurodegeneration, more recent data suggest that alterations in excitatory-inhibitory (E/I) balance occur in AD and may be a primary mechanism contributing AD cognitive decline. In this study, we discuss relevant research and reports on the GABA(A) receptor in developmental disorders, such as Down syndrome, in healthy aging, and highlight documented aberrations in the GABAergic system in AD. Stressing the importance of understanding the subunit composition of individual GABA(A) receptors, investigations demonstrate alterations of particular GABA(A) receptor subunits in AD, but overall sparing of the GABAergic system. In this study, we review experimental data on the GABAergic system in the pathobiology of AD and discuss relevant therapeutic implications. When developing AD therapeutics that modulate GABA it is important to consider how E/I balance impacts AD pathogenesis and the relationship between seizure activity and cognitive decline.

GABAB receptor modulation of synaptic function

Neuromodulators have complex effects on both the presynaptic release and postsynaptic detection of neurotransmitters. Here we describe recent advances in our understanding of synaptic modulation by metabotropic GABAB receptors. By inhibiting multivesicular release from the presynaptic terminal, these receptors decrease the synaptic glutamate signal. GABAB receptors also inhibit the Ca2+ permeability of NMDA receptors to decrease Ca2+ signals in postsynaptic spines. These new findings highlight the importance of GABAB receptors in regulating many aspects of synaptic transmission. They also point to novel questions about the spatiotemporal dynamics and sources of synaptic modulation in the brain.

Neuroimaging insights into the role of cortical GABA systems and the influence of nicotine on the recovery from alcohol dependence

This paper reviews evidence suggesting that nicotine and tobacco smoke profoundly modulate the effects of alcohol on γ-aminobutyric acid (GABA) neuronal function, specifically at the GABA(A)-benzodiazepine receptor (GABA(A)-BZR). The focus of this paper is on recent neuroimaging evidence in preclinical models as well as clinical experiments. First, we review findings implicating the role of alcohol at the GABA(A)-BZR and discuss the changes in GABA(A)-BZR availability during acute and prolonged alcohol withdrawal. Second, we discuss preclinical evidence that suggests nicotine affects GABA neuronal function indirectly by a primary action at neuronal nicotinic acetylcholine receptors. Third, we show how this evidence converges in studies that examine GABA levels and GABA(A)-BZRs in alcohol-dependent smokers and nonsmokers, suggesting that tobacco smoking attenuates the chemical changes that occur during alcohol withdrawal. Based on a comprehensive review of literature, we hypothesize that tobacco smoking minimizes the changes in GABA levels that typically occur during the acute cycles of drinking in alcohol-dependent individuals. Thus, during alcohol withdrawal, the continued tobacco smoking decreases the severity of the withdrawal-related changes in GABA chemistry. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Neurosteroids and GABA-A Receptor Function

Neurosteroids represent a class of endogenous steroids that are synthesized in the brain, the adrenals, and the gonads and have potent and selective effects on the GABAA-receptor. 3α-hydroxy A-ring reduced metabolites of progesterone, deoxycorticosterone, and testosterone are positive modulators of GABA(A)-receptor in a non-genomic manner. Allopregnanolone (3α-OH-5α-pregnan-20-one), 5α-androstane-3α, 17α-diol (Adiol), and 3α5α-tetrahydrodeoxycorticosterone (3α5α-THDOC) enhance the GABA-mediated Cl(-) currents acting on a site (or sites) distinct from the GABA, benzodiazepine, barbiturate, and picrotoxin binding sites. 3α5α-P and 3α5α-THDOC potentiate synaptic GABA(A)-receptor function and activate δ-subunit containing extrasynaptic receptors that mediate tonic currents. On the contrary, 3β-OH pregnane steroids and pregnenolone sulfate (PS) are GABA(A)-receptor antagonists and induce activation-dependent inhibition of the receptor. The activities of neurosteroid are dependent on brain regions and types of neurons. In addition to the slow genomic action of the parent steroids, the non-genomic, and rapid actions of neurosteroids play a significant role in the GABA(A)-receptor function and shift in mood and memory function. This review describes molecular mechanisms underlying neurosteroid action on the GABA(A)-receptor, mood changes, and cognitive functions.

GABAergic actions on cholinergic laterodorsal tegmental neurons: implications for control of behavioral state

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) play a critical role in regulation of behavioral state. Therefore, elucidation of mechanisms that control their activity is vital for understanding of how switching between wakefulness, sleep and anesthetic states is effectuated. In vivo studies suggest that GABAergic mechanisms within the pons play a critical role in behavioral state switching. However, the postsynaptic, electrophysiological actions of GABA on LDT neurons, as well as the identity of GABA receptors present in the LDT mediating these actions is virtually unexplored. Therefore, we studied the actions of GABA agonists and antagonists on cholinergic LDT cells by performing patch clamp recordings in mouse brain slices. Under conditions where detection of Cl(-) -mediated events was optimized, GABA induced gabazine (GZ)-sensitive inward currents in the majority of LDT neurons. Post-synaptic location of GABA(A) receptors was demonstrated by persistence of muscimol-induced inward currents in TTX and low Ca(2+) solutions. THIP, a selective GABA(A) receptor agonist with a preference for δ-subunit containing GABA(A) receptors, induced inward currents, suggesting the existence of extrasynaptic GABA(A) receptors. LDT cells also possess GABA(B) receptors as baclofen-activated a TTX- and low Ca(2+)-resistant outward current that was attenuated by the GABA(B) antagonists CGP 55845 and saclofen. The tertiapin sensitivity of baclofen-induced outward currents suggests that a G(IRK) mediated this effect. Further, outward currents were never additive with those induced by application of carbachol, suggesting that they were mediated by activation of GABA(B) receptors linked to the same G(IRK) activated in these cells by muscarinic receptor stimulation. Activation of GABA(B) receptors inhibited Ca(2+) increases induced by a depolarizing voltage step shown previously to activate VOCCs in cholinergic LDT neurons. Baclofen-mediated reductions in depolarization-induced Ca(2+) were unaltered by prior emptying of intracellular Ca(2+) stores, but were abolished by low extracellular Ca(2+) and pre-application of nifedipine, indicating that activation of GABA(B) receptors inhibits influx of Ca(2+) involving L-type Ca(2+) channels. Presence of GABA(C) receptors is suggested by the induction of inward current by (E)-4- amino-2-butenoic acid (TACA) and its inhibition by 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic (TPMPA), a relatively selective agonist and antagonist, respectively, of GABA(C) receptors. All of these GABA-mediated actions were found to occur in histochemically-identified cholinergic neurons. Taken together, these data indicate for the first time that cholinergic neurons of the LDT exhibit functional GABA(A, B and C) receptors, including extrasynaptically located GABA(A) receptors, which may be tonically activated by synaptic overflow of GABA. Accordingly, the activity of cholinergic LDT neurons is likely to be significantly affected by GABAergic tone within the nucleus, and so, demonstrated effects of GABA on behavioral state may be mediated, in part, via direct actions on cholinergic neurons in the LDT.

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

GABAB receptors modulate NMDA receptor calcium signals in dendritic spines

Metabotropic GABA(B) receptors play a fundamental role in modulating the excitability of neurons and circuits throughout the brain. These receptors influence synaptic transmission by inhibiting presynaptic release or activating postsynaptic potassium channels. However, their ability to directly influence different types of postsynaptic glutamate receptors remains unresolved. Here we examine GABA(B) receptor modulation in layer 2/3 pyramidal neurons from the mouse prefrontal cortex. We use two-photon laser-scanning microscopy to study synaptic modulation at individual dendritic spines. Using two-photon optical quantal analysis, we first demonstrate robust presynaptic modulation of multivesicular release at single synapses. Using two-photon glutamate uncaging, we then reveal that GABA(B) receptors strongly inhibit NMDA receptor calcium signals. This postsynaptic modulation occurs via the PKA pathway and does not affect synaptic currents mediated by AMPA or NMDA receptors. This form of GABA(B) receptor modulation has widespread implications for the control of calcium-dependent neuronal function.

In-capillary derivatization and stacking electrophoretic analysis of gamma-aminobutyric acid and alanine in tea samples to redeem the detection after dilution to decrease matrix interference

An in-capillary derivatization and stacking capillary electrophoresis (CE) technique has been applied to redeem the detection of dilute analytes in the analysis of gamma-aminobutyric acid (GABA) and alanine (Ala) in tea samples. Extracts from samples were diluted to eliminate matrix interference before introduction into the CE system. GABA and Ala in the diluted sample zone were derivatized with o-phthaldialdehyde/2-mercaptoethanol (OPA/2-ME) to form fluorescence-labeled products in the stacking process, and the labeled derivatives were then enriched by online stacking. Optimal conditions for the stacking, such as the concentration of the background buffer solution, the matrix of the sample zone (sample solution), and the volume of the sample injection, were investigated and then applied to real sample analysis. Under optimum conditions, the detections were linear in the range of 5.0 nM-2.5 microM with the square of correlation coefficients (R2) of 0.9995 and 0.9992 for GABA and Ala, respectively. Detection limits were found to be 0.7 and 0.8 nM for GABA and Ala, respectively. Tea samples were analyzed with recoveries between 92.33 and 97.87% and between 94.36 and 96.46% for GABA and Ala, respectively. This method is a rapid, convenient, and sensitive process for determining GABA and Ala in complicated matrix samples such as tea samples.

Genetic association studies of glutamate, GABA and related genes in schizophrenia and bipolar disorder: a decade of advance

Schizophrenia (SZ) and bipolar disorder (BD) are debilitating neurobehavioural disorders likely influenced by genetic and non-genetic factors and which can be seen as complex disorders of synaptic neurotransmission. The glutamatergic and GABAergic neurotransmission systems have been implicated in both diseases and we have reviewed extensive literature over a decade for evidence to support the association of glutamate and GABA genes in SZ and BD. Candidate-gene based population and family association studies have implicated some ionotrophic glutamate receptor genes (GRIN1, GRIN2A, GRIN2B and GRIK3), metabotropic glutamate receptor genes (such as GRM3), the G72/G30 locus and GABAergic genes (e.g. GAD1 and GABRB2) in both illnesses to varying degrees, but further replication studies are needed to validate these results. There is at present no consensus on specific single nucleotide polymorphisms or haplotypes associated with the particular candidate gene loci in these illnesses. The genetic architecture of glutamate systems in bipolar disorder need to be better studied in view of recent data suggesting an overlap in the genetic aetiology of SZ and BD. There is a pressing need to integrate research platforms in genomics, epistatic models, proteomics, metabolomics, neuroimaging technology and translational studies in order to allow a more integrated understanding of glutamate and GABAergic signalling processes and aberrations in SZ and BD as well as their relationships with clinical presentations and treatment progress over time.

Influence of 17beta-estradiol and progesterone on GABAergic gene expression in the arcuate nucleus, amygdala and hippocampus of the rhesus macaque

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain, and the responsiveness of neurons to GABA can be modulated by sex steroids. To better understand how ovarian steroids influence the GABAergic system in the primate brain, we evaluated the expression of genes encoding GABA receptor subunits, glutamic acid decarboxylase (GAD) and a GABA transporter in the brains of female rhesus macaques. Ovariectomized adults were subjected to a hormone replacement paradigm involving either 17beta-estradiol (E), or E plus progesterone (E+P). Untreated animals served as controls. Using GeneChip microarray analysis and real-time RT-PCR (qPCR), we examined gene expression differences within and between the amygdala (AMD), hippocampus (HPC) and arcuate nuclei of the medial basal hypothalamus (MBH). The results from PCR corresponded with results from representative GeneChip probesets, and showed similar effects of sex steroids on GABA receptor subunit gene expression in the AMD and HPC, and a more pronounced expression than in the MBH. Exposure to E+P attenuated GAD1, GAD2 and SLC32A1 gene expression in the AMD and HPC, but not in the MBH. GABA receptor subunit gene expression was generally higher in the AMD and HPC than in the MBH, with the exception of receptor subunits epsilon and gamma 2. Taken together, the data demonstrate differential regulation of GABA receptor subunits and GABAergic system components in the MBH compared to the AMD and HPC of rhesus macaques. Elevated epsilon and reduced delta subunit expression in the MBH supports the hypothesis that the hypothalamic GABAergic system is resistant to the modulatory effects of sex steroids.

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

Focal decreases of cortical GABAA receptor binding remote from the primary seizure focus: what do they indicate?

PURPOSE

To determine the electroclinical significance and histopathological correlates of cortical gamma-aminobutyric acid(A)(GABA(A)) receptor abnormalities detected in and remote from human neocortical epileptic foci.

METHODS

Cortical areas with decreased(11)C-flumazenil (FMZ) binding were objectively identified on positron emission tomography (PET) images and correlated to intracranial electroencephalography (EEG) findings, clinical seizure variables, histology findings, and surgical outcome in 20 patients (mean age, 9.9 years) with intractable partial epilepsy of neocortical origin and nonlocalizing magnetic resonance imaging (MRI).

RESULTS

Focal decrease of cortical FMZ binding was detected in the lobe of seizure onset in 17 (85%) patients. Eleven patients (55%) had 17 remote cortical areas with decreased FMZ binding outside the lobe of seizure onset. Thirteen of those 16 (81%) of the 17 remote cortical regions that were covered by subdural EEG were around cortex showing rapid seizure spread on intracranial EEG. Remote FMZ PET abnormalities were associated with high seizure frequency and, when resected, showed gliosis in all six cases where material was available. Higher number of unresected cortical regions with decreased FMZ binding was associated with poorer surgical outcome.

CONCLUSIONS

Focal decreases of cortical GABA(A) receptor binding on PET may include cortical regions remote from the primary focus, particularly in patients with high seizure frequency, and these regions are commonly involved in rapid seizure propagation. Although these regions may not always need to be resected to achieve seizure freedom, a careful evaluation of cortex with decreased GABA(A) receptor binding prior to resection using intracranial EEG may facilitate optimal surgical outcome in patients with intractable neocortical epilepsy.