Postmortem increase of GABA levels in peripheral rat tissues: prevention by 3-mercapto-propionic acid

Impaired Expression of GABA Signaling Components in the Alzheimer's Disease Middle Temporal Gyrus

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter, playing a central role in the regulation of cortical excitability and the maintenance of the excitatory/inhibitory (E/I) balance. Several lines of evidence point to a remodeling of the cerebral GABAergic system in Alzheimer’s disease (AD), with past studies demonstrating alterations in GABA receptor and transporter expression, GABA synthesizing enzyme activity and focal GABA concentrations in post-mortem tissue. AD is a chronic neurodegenerative disorder with a poorly understood etiology and the temporal cortex is one of the earliest regions in the brain to be affected by AD neurodegeneration. Utilizing NanoString nCounter analysis, we demonstrate here the transcriptional downregulation of several GABA signaling components in the post-mortem human middle temporal gyrus (MTG) in AD, including the GABA_A receptor α₁, α₂, α₃, α₅, β₁, β₂, β₃, δ, γ₂, γ₃, and θ subunits and the GABA_B receptor 2 (GABA_BR2) subunit. In addition to this, we note the transcriptional upregulation of the betaine-GABA transporter (BGT1) and GABA transporter 2 (GAT2), and the downregulation of the 67 kDa isoform of glutamate decarboxylase (GAD₆₇), the primary GABA synthesizing enzyme. The functional consequences of these changes require further investigation, but such alterations may underlie disruptions to the E/I balance that are believed to contribute to cognitive decline in AD.

Towards a Better Understanding of GABAergic Remodeling in Alzheimer's Disease

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the vertebrate brain. In the past, there has been a major research drive focused on the dysfunction of the glutamatergic and cholinergic neurotransmitter systems in Alzheimer’s disease (AD). However, there is now growing evidence in support of a GABAergic contribution to the pathogenesis of this neurodegenerative disease. Previous studies paint a complex, convoluted and often inconsistent picture of AD-associated GABAergic remodeling. Given the importance of the GABAergic system in neuronal function and homeostasis, in the maintenance of the excitatory/inhibitory balance, and in the processes of learning and memory, such changes in GABAergic function could be an important factor in both early and later stages of AD pathogenesis. Given the limited scope of currently available therapies in modifying the course of the disease, a better understanding of GABAergic remodeling in AD could open up innovative and novel therapeutic opportunities.

Non-invasive diagnostic biomarkers for estimating the onset time of permanent cerebral ischemia

The treatments for ischemic stroke can only be administered in a narrow time-window. However, the ischemia onset time is unknown in ~30% of stroke patients (wake-up strokes). The objective of this study was to determine whether MR spectra of ischemic brains might allow the precise estimation of cerebral ischemia onset time. We modeled ischemic stroke in male ICR-CD1 mice using a permanent middle cerebral artery filament occlusion model with laser Doppler control of the regional cerebral blood flow. Mice were then subjected to repeated MRS measurements of ipsilateral striatum at 14.1 T. A striking initial increase in γ-aminobutyric acid (GABA) and no increase in glutamine were observed. A steady decline was observed for taurine (Tau), N-acetyl-aspartate (NAA) and similarly for the sum of NAA+Tau+glutamate that mimicked an exponential function. The estimation of the time of onset of permanent ischemia within 6 hours in a blinded experiment with mice showed an accuracy of 33±10 minutes. A plot of GABA, Tau, and neuronal marker concentrations against the ratio of acetate/NAA allowed precise separation of mice whose ischemia onset lay within arbitrarily chosen time-windows. We conclude that (1)H-MRS has the potential to detect the clinically relevant time of onset of ischemic stroke.

Glutamate acid decarboxylase 1 promotes metastasis of human oral cancer by β-catenin translocation and MMP7 activation

Background

Glutamate decarboxylase 1 (GAD1), a rate-limiting enzyme in the production of γ-aminobutyric acid (GABA), is found in the GABAergic neurons of the central nervous system. Little is known about the relevance of GAD1 to oral squamous cell carcinoma (OSCC). We investigated the expression status of GAD1 and its functional mechanisms in OSCCs.

Methods

We evaluated GAD1 mRNA and protein expressions in OSCC-derived cells using real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) and immunoblotting analyses. To assess the critical functions of GAD1, i.e., cellular proliferation, invasiveness, and migration, OSCC-derived cells were treated with the shRNA and specific GAD1 inhibitor, 3-mercaptopropionic acid (3-MPA). GAD1 expression in 80 patients with primary OSCCs was analyzed and compared to the clinicopathological behaviors of OSCC.

Results

qRT-PCR and immunoblotting analyses detected frequent up-regulation of GAD1 in OSCC-derived cells compared to human normal oral keratinocytes. Suppression of nuclear localization of β-catenin and MMP7 secretion was observed in GAD1 knockdown and 3-MPA-treated cells. We also found low cellular invasiveness and migratory abilities in GAD1 knockdown and 3-MPA-treated cells. In the clinical samples, GAD1 expression in the primary OSCCs was significantly (P < 0.05) higher than in normal counterparts and was correlated significantly (P < 0.05) with regional lymph node metastasis.

Conclusions

Our data showed that up-regulation of GAD1 was a characteristic event in OSCCs and that GAD1 was correlated with cellular invasiveness and migration by regulating β-catenin translocation and MMP7 activation. GAD1 might play an important role in controlling tumoral invasiveness and metastasis in oral cancer.

Metabolic products of [2-(13) C]ethanol in the rat brain after chronic ethanol exposure

Most ingested ethanol is metabolized in the liver to acetaldehyde and then to acetate, which can be oxidized by the brain. This project assessed whether chronic exposure to alcohol can increase cerebral oxidation of acetate. Through metabolism, acetate may contribute to long-term adaptation to drinking. Two groups of adult male Sprague-Dawley rats were studied, one treated with ethanol vapor and the other given room air. After 3 weeks the rats received an intravenous infusion of [2-(13) C]ethanol via a lateral tail vein for 2 h. As the liver converts ethanol to [2-(13) C]acetate, some of the acetate enters the brain. Through oxidation the (13) C is incorporated into the metabolic intermediate α-ketoglutarate, which is converted to glutamate (Glu), glutamine (Gln), and GABA. These were observed by magnetic resonance spectroscopy and found to be (13) C-labeled primarily through the consumption of ethanol-derived acetate. Brain Gln, Glu, and, GABA (13) C enrichments, normalized to (13) C-acetate enrichments in the plasma, were higher in the chronically treated rats than in the ethanol-naïve rats, suggesting increased cerebral uptake and oxidation of circulating acetate. Chronic ethanol exposure increased incorporation of systemically derived acetate into brain Gln, Glu, and GABA, key neurochemicals linked to brain energy metabolism and neurotransmission. The liver converts ethanol to acetate, which may contribute to long-term adaptation to drinking. Astroglia oxidize acetate and generate neurochemicals, while neurons and glia may also oxidize ethanol. When (13) C-ethanol is administered intravenously, (13) C-glutamine, glutamate, and GABA, normalized to (13) C-acetate, were higher in chronic ethanol-exposed rats than in control rats, suggesting that ethanol exposure increases cerebral oxidation of circulating acetate.

Biochemical and immunopathological changes in experimental neurotoxocariasis

Toxocariasis is a widespread soil-transmitted parasitic disease. Toxocara canis larvae migrate through the tissues with a special predilection for the central nervous system. Recently, neurotoxocariasis is being diagnosed in humans with increasing frequency due to improved diagnostic tools. The present study aimed at exploring the biochemical and immunopathological alterations in the brain in experimental T. canis infection. For this purpose, 75 Toxocara-infected mice were sacrificed at 2, 5, and 16 weeks post-infection. The brains were removed and assayed for total larval count, pro-inflammatory cytokines (TNF-alpha, IL-6), and central neurotransmitters (gamma-aminobutyric acid, glutamate, dopamine, norepinephrine, and serotonin). Brain sections were also stained for histopathological study, and for assessment of the expression of inducible nitric oxide synthase (iNOS), and glial fibrillary acidic protein (GFAP) by immunohistochemical methods. We found that larval recovery showed progressive increase over the course of infection. Furthermore, the infected mice displayed increased expression of pro-inflammatory cytokines and iNOS, as well as significant disturbances in neurotransmitter profile. Astrocytic activation, evidenced by enhanced expression of GFAP, was also manifest in infected animals. These changes were maximal in the chronic stage of infection or intensified over time. In conclusion, experimental neurotoxocariasis is associated with significant biochemical, immunological, and pathological changes.

Regional variation in gamma-aminobutyric acid turnover: effect of castration on gamma-aminobutyric acid turnover in microdissected brain regions of the male rat

This study compared the turnover of GABA neurons in different brain areas of the male rat and examined the effect of castration on GABA turnover in regions of the brain associated with the control of gonadotropin secretion. To estimate GABA turnover, GABA was quantified by HPLC in microdissected brain regions 0, 30, 60, 90, and 120 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, i.p.). GABA accumulation was linear in all areas for 90 min (p < 0.01), and GABA turnover was estimated as the slope of the line formed by increased GABA concentration versus time, determined by linear regression. There was considerable regional variation both in the initial steady-state concentrations of GABA and in the rates of GABA turnover. Of 10 discrete brain structures, GABA turnover was highest in the medial preoptic nucleus and lowest in the caudate nucleus. Turnover times in the terminal fields of known GABAergic projection neurons ranged sevenfold, from 2.6 h in the substantia nigra to 0.4 h in the lateral vestibular nucleus. The effect of castration on GABA turnover in 13 microdissected brain regions was investigated by measuring regional GABA concentrations before and 30 min after injection of aminooxyacetic acid in intact rats or 2 or 6 days postcastration. Following castration, steady-state GABA concentrations were increased, and GABA turnover decreased in the diagonal band of Broca, the medial preoptic area, and the median eminence. GABA turnover increased in the medial septal nucleus and was unaffected in the cortex, striatum, and hindbrain. These results are consistent with the hypothesis that testosterone negative-feedback control of luteinizing hormone-releasing hormone involves steroid-sensitive GABAergic neurons in the rostral and medial basal hypothalamus.

GABA content and synthesis in the aging rat brain

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

GABA and its neural regulation in rat brown adipose tissue

By using a radioreceptor assay GABA was detectable in rat interscapular brown adipose tissue (IBAT), the levels being 1% those of CNS and 10-fold those of peripheral plasma. Injection of the glutamic acid decarboxylase (GAD) inhibitor 3-mercaptopropionic acid lowered IBAT GABA levels by about half while injection of the GABA transaminase inhibitor gamma-acetylenic GABA increased them by 230%. Rats kept at 4 degrees C for 14 days exhibited IBAT GABA levels that were about half those found at 22 degrees C. Accumulation of IBAT GABA after gamma-acetylenic GABA increased by 2-fold in cold-exposed rats. Sympathetic denervation of IBAT prevented the effect of the cold environment on GABA content and impaired that on GABA accumulation. GAD activity was detectable in IBAT homogenates and isolated brown adipocytes. Exposure of rats to cold increased Vmax of GAD without modifying its Km, regardless of intactness of innervation. In binding studies with 3H-GABA as a ligand, two types of sites were uncovered of KD = 14 and 146 nM, respectively. In the presence of 2.5 mM Ca2+ bicuculline and baclofen were 57 and 46% as effective as GABA to displace 3H-GABA from IBAT binding sites. The results indicate existence, possible synthesis and type A and B receptors of GABA in rat IBAT.

Dietary pyridoxine and the susceptibility to limbic motor seizures in rats

Dietary pyridoxine (PN) deficiency in adult female rats produced a 32% decrease in hippocampal gamma-aminobutyric acid content, measured by a radioreceptor assay. No spontaneous seizures were observed in pyridoxine-deficient animals, but the seizure latency after a systemic kainic acid challenge decreased by 35%. The results suggest that latency is a useful measure of limbic seizure susceptibility; this susceptibility can be manipulated by diet; and in adults pyridoxine-dependent mechanisms normally participate in preventing, rather than initiating, limbic seizures.

Identification of GABA receptor binding sites in rat and rabbit uterus

The present study provides evidence for the presence of gamma-aminobutyric acid (GABA) and L-glutamate decarboxylase in the uteri of the rat and the rabbit. Furthermore, it has been demonstrated for the first time that the uterine tissue contains high-affinity GABAA receptor binding sites in a high density. The present findings indicate that GABA may have a role in the uterine functions.