Effects of chronic oral treatment with GABA-transaminase inhibitors on the GABA system in brain, liver, kidney, and plasma of the rat

A critical role of hepatic GABA in the metabolic dysfunction and hyperphagia of obesity

Hepatic lipid accumulation is a hallmark of type II diabetes (T2D) associated with hyperinsulinemia, insulin resistance, and hyperphagia. Hepatic synthesis of GABA, catalyzed by GABA-transaminase (GABA-T), is upregulated in obese mice. To assess the role of hepatic GABA production in obesity-induced metabolic and energy dysregulation, we treated mice with two pharmacologic GABA-T inhibitors and knocked down hepatic GABA-T expression using an antisense oligonucleotide. Hepatic GABA-T inhibition and knockdown decreased basal hyperinsulinemia and hyperglycemia and improved glucose intolerance. GABA-T knockdown improved insulin sensitivity assessed by hyperinsulinemic-euglycemic clamps in obese mice. Hepatic GABA-T knockdown also decreased food intake and induced weight loss without altering energy expenditure in obese mice. Data from people with obesity support the notion that hepatic GABA production and transport are associated with serum insulin, homeostatic model assessment for insulin resistance (HOMA-IR), T2D, and BMI. These results support a key role for hepatocyte GABA production in the dysfunctional glucoregulation and feeding behavior associated with obesity.

2-Pyrrolidinone and Succinimide as Clinical Screening Biomarkers for GABA-Transaminase Deficiency: Anti-seizure Medications Impact Accurate Diagnosis

Broad-scale untargeted biochemical phenotyping is a technology that supplements widely accepted assays, such as organic acid, amino acid, and acylcarnitine analyses typically utilized for the diagnosis of inborn errors of metabolism. In this study, we investigate the analyte changes associated with 4-aminobutyrate aminotransferase (ABAT, GABA transaminase) deficiency and treatments that affect GABA metabolism. GABA-transaminase deficiency is a rare neurodevelopmental and neurometabolic disorder caused by mutations in ABAT and resulting in accumulation of GABA in the cerebrospinal fluid (CSF). For that reason, measurement of GABA in CSF is currently the primary approach to diagnosis. GABA-transaminase deficiency results in severe developmental delay with intellectual disability, seizures, and movement disorder, and is often associated with death in childhood. Using an untargeted metabolomics platform, we analyzed EDTA plasma, urine, and CSF specimens from four individuals with GABA-transaminase deficiency to identify biomarkers by comparing the biochemical profile of individual patient samples to a pediatric-centric population cohort. Metabolomic analyses of over 1,000 clinical plasma samples revealed a rich source of biochemical information. Three out of four patients showed significantly elevated levels of the molecule 2-pyrrolidinone (Z-score ≥2) in plasma, and whole exome sequencing revealed variants of uncertain significance in ABAT. Additionally, these same patients also had elevated levels of succinimide in plasma, urine, and CSF and/or homocarnosine in urine and CSF. In the analysis of clinical EDTA plasma samples, the levels of succinimide and 2-pyrrolidinone showed a high level of correlation (R = 0.73), indicating impairment in GABA metabolism and further supporting the association with GABA-transaminase deficiency and the pathogenicity of the ABAT variants. Further analysis of metabolomic data across our patient population revealed the association of elevated levels of 2-pyrrolidinone with administration of vigabatrin, a commonly used anti-seizure medication and a known inhibitor of GABA-transaminase. These data indicate that anti-seizure medications may alter the biochemical and metabolomic data, potentially impacting the interpretation and diagnosis for the patient. Further, these data demonstrate the power of combining broad scale genotyping and phenotyping technologies to diagnose inherited neurometabolic disorders and support the use of metabolic phenotyping of plasma to screen for GABA-transaminase deficiency.

Alleviation by gamma amino butyric acid supplementation of chronic heat stress-induced degenerative changes in jejunum in commercial broiler chickens

High ambient temperature adversely influences poultry production. In the present study, gamma amino butyric acid (GABA) supplementation was used to alleviate the adverse changes due to heat stress (HS) in a broiler chicken strain (Ross 308). At 21 days of age, the birds were divided into four groups of 13. Two groups were housed under normal room temperature, one group was given orally 0.2 ml 0.9% physiological saline (CN) daily, the other group received 0.2 ml of 0.5% GABA solution orally (GN). A third group was exposed to environmental HS (33 ± 1 °C lasting for 2 weeks) + physiological saline (CH) and a fourth group was exposed to HS + GABA supplementation (GH). GABA supplementation during HS significantly reduced the birds' increased body temperature (p <.0001) and increased their body weight gain (p <.0001). This effect was associated with increases in the heat stress-induced reductions in jejunal villus length, crypt depth and mucous membrane thickness, and decreases in the vascular changes occurred due to HS. Additionally, GABA supplementation significantly modulated HS-induced changes in glucose facilitated transporter 2 (GLUT2), peptide transporter 1 (PEPT1) and heat shock protein 70 (HSP70) mRNA expression in the jejunal mucosa (p < .0001). GABA supplementation also significantly elevated the triiodothyronine (T₃) hormone level and hemoglobin levels and decreased the heterophil-lymphocyte ratio (H/L ratio) (p <.0001). Furthermore, it induced higher hepatic glutathione peroxidase enzyme (GSH-Px) activities and decreased the malondialdehyde dehydrogenase (MDA) content. These results indicate that GABA supplementation during HS may be used to alleviate HS-related changes in broiler chickens.

Rat submandibular gland perfusion method for clarifying inhibitory regulation of GABAA receptor

GABA is an inhibitory transmitter found in rat salivary gland. However, the inhibitory potential of GABA on salivary secretion is unclear. Using an in vivo cannulation method, intraperitoneal administration of GABA was ineffective in the absence of gabaculine, a GABA transaminase inhibitor, on pilocarpine-induced salivary secretion, suggesting that GABA was rendered metabolically inactive before reaching the salivary gland. We hypothesized that the action of a drug on the salivary glands could be measured directly using a submandibular gland perfusion system. The submandibular gland artery, veins, and duct were cannulated in situ so that physiological functions such as innervation would not be compromised. Hank's balanced salt solution (pH 7.4) was perfused at a rate of 0.5 ml/min together with 1 μM carbachol (CCh) over a 5-min period every 30 min. Amount of secreted saliva showed no change to the recurrent addition of CCh to the perfusate. GABA or muscimol dose-dependently inhibited CCh-induced salivary secretion. This effect was blocked by bicuculline, a GABA(A)-receptor (GABA(A)-R) antagonist, and enhanced by clonazepam, a central-type benzodiazepine-receptor agonist. These results suggest that salivary secretion is suppressed by GABA(A)-R in rat salivary gland and that the perfusion method used was effective in clarifying inhibitory regulation of GABA(A)-R.

Deletion of the γ-aminobutyric acid transporter 2 (GAT2 and SLC6A13) gene in mice leads to changes in liver and brain taurine contents

The GABA transporters (GAT1, GAT2, GAT3, and BGT1) have mostly been discussed in relation to their potential roles in controlling the action of transmitter GABA in the nervous system. We have generated the first mice lacking the GAT2 (slc6a13) gene. Deletion of GAT2 (both mRNA and protein) neither affected growth, fertility, nor life span under nonchallenging rearing conditions. Immunocytochemistry showed that the GAT2 protein was predominantly expressed in the plasma membranes of periportal hepatocytes and in the basolateral membranes of proximal tubules in the renal cortex. This was validated by processing tissue from wild-type and knockout mice in parallel. Deletion of GAT2 reduced liver taurine levels by 50%, without affecting the expression of the taurine transporter TAUT. These results suggest an important role for GAT2 in taurine uptake from portal blood into liver. In support of this notion, GAT2-transfected HEK293 cells transported [(3)H]taurine. Furthermore, most of the uptake of [(3)H]GABA by cultured rat hepatocytes was due to GAT2, and this uptake was inhibited by taurine. GAT2 was not detected in brain parenchyma proper, excluding a role in GABA inactivation. It was, however, expressed in the leptomeninges and in a subpopulation of brain blood vessels. Deletion of GAT2 increased brain taurine levels by 20%, suggesting a taurine-exporting role for GAT2 in the brain.

Proteomic changes associated with diabetes in the BB-DP rat

These studies were structured with the aim of utilizing emerging technologies in two-dimensional (2D) gel electrophoresis and mass spectrometry to evaluate protein expression changes associated with type 1 diabetes. We reasoned that a broad examination of diabetic tissues at the protein level might open up novel avenues of investigation of the metabolic and signaling pathways that are adversely affected in type 1 diabetes. This study compared the protein expression of the liver, heart, and skeletal muscle of diabetes-prone rats and matched control rats by semiquantitative liquid chromatography-mass spectrometry and differential in-gel 2D gel electrophoresis. Differential expression of 341 proteins in liver, 43 in heart, and 9 (2D gel only) in skeletal muscle was detected. These data were assembled into the relevant metabolic pathways affected primarily in liver. Multiple covalent modifications were also apparent in 2D gel analysis. Several new hypotheses were generated by these data, including mechanisms of net cytosolic protein oxidation, formaldehyde generation by the methionine cycle, and inhibition of carbon substrate oxidation via reduction in citrate synthase and short-chain acyl-CoA dehydrogenase.

Inhibition of GABA shunt enzymes’ activity by 4-hydroxybenzaldehyde derivatives

4-Hydroxybenzaldehyde (HBA) derivatives were examined as inhibitors for GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). Investigation of structure-activity relation revealed that a carbonyl group or an amino group as well as a hydroxy group at the para position of the benzene ring are important for both enzymes' inhibition. HBA was shown to give competitive inhibition of GABA-T with respect to alpha-ketoglutarate and competitive inhibition of SSADH. 4-Hydroxybenzylamine (HBM) also showed the competitive inhibition on GABA-T with respect to GABA. In conclusion, the inhibitory effects of HBA and HBM on both enzymes could result from the similarity between both molecules and the two enzymes' substrates in structure, as well as the conjugative effect of the benzene ring. This suggested that the presence of the benzene ring may be accepted by the active site of both enzymes, HBA and HBM may be considered as lead compounds to design novel GABA-T inhibitors.

Differential effects of vigabatrin and zonisamide on the neuropeptide Y system in the hippocampus of seizure prone gerbil

Changed neuropeptide Y (NPY) system in the hippocampus has been reported in various experimental epileptic models. However, there have been little data concerning the alteration in the NPY system in the epileptic hippocampus following treatment of anti-epileptic drugs (AEDs). In the present study, therefore, we performed analyses of effects of vigabatrin (VGB) and zonisamide (ZNS) treatment on the NPY system in the hippocampus of the seizure sensitive (SS) gerbils. In SS gerbil, NPY immunoreactivity in the hippocampus was lower than that in seizure resistant gerbil. Following VGB treatment, the number of NPY immunoreactive neurons and NPY mRNA expression were increased in the hilus and the hippocampus proper. In contrast, ZNS treatment markedly elevated only the density of NPY immunoreactive fibers in the dentate gyrus, not in the hippocampus proper, as compared with saline-treated animals. These patterns were observed in the dose-dependent manners. These findings suggest that AEDs treatments may distinctly affect the NPY system in the SS gerbil hippocampus.

Focal neurometabolic alterations in mice deficient for succinate semialdehyde dehydrogenase

Metabolite profiling in succinate semialdehyde dehydrogenase (SSADH; Aldh5a1-/-) deficient mice previously revealed elevated gamma-hydroxybutyrate (GHB) and total GABA in urine and total brain and liver extracts. In this study, we extend our metabolic characterization of these mutant mice by documenting elevated GHB and total GABA in homogenates of mutant kidney, pancreas and heart. We quantified beta-alanine (a GABA homolog and putative neurotransmitter) to address its potential role in pathophysiology. We found normal levels of beta-alanine in urine and total homogenates of mutant brain, heart and pancreas, but elevated concentrations in mutant kidney and liver extracts. Amino acid analysis in mutant total brain homogenates revealed no abnormalities except for significantly decreased glutamine, which was normal in mutant liver and kidney extracts. Regional amino acid analysis (frontal cortex, parietal cortex, hippocampus and cerebellum) in mutant mice confirmed glutamine results. Glutamine synthetase protein and mRNA levels in homogenates of mutant mouse brain were normal. We profiled organic acid patterns in mutant brain homogenates to assess brain oxidative metabolism and found normal concentrations of Kreb's cycle intermediates but increased 4,5-dihydroxyhexanoic acid (a postulated derivative of succinic semialdehyde) levels. We conclude that SSADH-deficient mice represent a valid metabolic model of human SSADH deficiency, manifesting focal neurometabolic abnormalities which could provide key insights into pathophysiologic mechanisms.

Vigabatrin protects against hippocampal damage but is not antiepileptogenic in the lithium-pilocarpine model of temporal lobe epilepsy

In temporal lobe epilepsy (TLE), the nature of the structures involved in the development of the epileptogenic circuit is still not clearly identified. In the lithium-pilocarpine model, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as well as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). In order to determine whether protection of some brain areas could prevent the epileptogenesis induced by status epilepticus (SE) and to identify the cerebral structures involved in the genesis of TLE, we studied the effects of the chronic exposure to Vigabatrin (gamma-vinyl-GABA, GVG) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE. The animals were subjected to SE and GVG treatment (250 mg/kg) was initiated at 10 min after pilocarpine injection and maintained daily for 45 days. These pilo-GVG rats were compared with rats subjected to SE followed by a daily saline treatment (pilo-saline) and to control rats not subjected to SE (saline-saline). GVG treatment induced a marked, almost total neuroprotection in CA3, an efficient protection in CA1 and a moderate one in the hilus of the dentate gyrus while damage in the entorhinal cortex was slightly worsened by the treatment. All pilo-GVG and pilo-saline rats became epileptic after the same latency. Glutamic acid decarboxylase (GAD67) immunoreactivity was restored in pilo-GVG rats compared with pilo-saline rats in all areas of the hippocampus, while it was increased over control levels in the optical layer of the superior colliculus and the substantia nigra pars reticulata. Thus, the present data indicate that neuroprotection of principal cells in the Ammon's horn of the hippocampus is not sufficient to prevent epileptogenesis, suggesting that the hilus and extra-hippocampal structures, that were not protected in this study, may play a role in the genesis of spontaneous recurrent seizures in this model. Furthermore, the study performed in non-epileptic rats indicates that chronic treatment with a GABAmimetic drug upregulates the expression of the protein GAD67 in specific areas of the brain, independently from the seizures.

Reversal of the activity-dependent suppression of GABA-mediated inhibition in hippocampal slices from gamma-vinyl GABA (vigabatrin)-pretreated rats

The antiepileptic drug, gamma-vinyl GABA (GVG, vigabatrin), is an irreversible inhibitor of GABA-transaminase, the enzyme responsible for the breakdown of GABA. In hippocampal slices prepared from rats pretreated with either an anticonvulsant dose of GVG (1500 mg/kg) or saline, electrophysiological recordings were performed in order to examine the effects of GVG pretreatment on GABAergic neurotransmission. Although GVG had no effect on the effectiveness of GABA-mediated inhibition when elicited by a single stimulus, it reversed the activity-dependent depression of inhibition which is typically observed when inhibitory pathways are activated repetitively by a train of stimuli delivered at low frequency. Similarly, GVG pretreatment prevented the progressive decline in the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) during low-frequency stimulation of inhibitory interneurons. Thus, in slices from GVG pretreated rats, the amplitudes of both the fast and slow components of the last of a series of IPSPs evoked by a 5 Hz, 4 s train were maintained at 91.5 +/- 6.6% and 87.7 +/- 6.5%, respectively, compared to 61.1 +/- 3.9% and 57.1 +/- 5.0% in control slices. Finally, in slices from GVG pretreated rats, we observed a reduction in the ability of the GABA(B) receptor agonist, baclofen, to decrease the amplitude of monosynaptic inhibitory postsynaptic currents. These results suggest that GVG may produce its frequency-dependent actions by reducing the function of release regulating presynaptic GABA(B) autoreceptors. The frequency-dependent reinforcement of inhibition by GVG may importantly contribute to the anticonvulsant effectiveness of this compound.

Effect of chronic treatment with the GABA transaminase inhibitors gamma-vinyl GABA and ethanolamine O-sulphate on the in vitro GABA release from rat hippocampus

1. The effects of 2, 8 and 21 day oral treatment with the specific gamma-aminobutyric acid transaminase (GABA-T) inhibitors gamma-vinyl GABA (GVG) and ethanolamine O-sulphate (EOS) on brain GABA levels, GABA-T activity, and basal and stimulated GABA release from rat cross-chopped brain hippocampal slices was investigated. 2. Treatment with GABA-T inhibitors lead to a reduction in brain GABA-T activity by 65-80% compared with control values, with a concomitant increase in brain GABA content of 40-100%. 3. Basal hippocampal GABA release was increased to 250-450% of control levels following inhibition of GABA-T activity. No Ca2+ dependence was observed in either control or treated tissues. 4. GVG and EOS administration led to a significant elevation in the potassium stimulated release of GABA from cross-chopped hippocampal slices compared with that of controls. Although stimulated GABA release from control tissues was decreased in the presence of a low Ca2+ medium, GVG and EOS treatment abolished this Ca2+ dependency. 5. GABA compartmentalization, Na+ and Cl- coupled GABA uptake carriers and glial release may provide explanations for the loss of the Ca2+ dependency of stimulated GABA release observed following GVG and EOS treatment. 6. Administration of GABA-T inhibitors led to increases in both basal and stimulated hippocampal GABA release. However, it is not clear which is the most important factor in the anticonvulsant activity of these drugs, the increased GABA content 'leaking' out of neurones and glia leading to widespread inhibition, or the increase in stimulated GABA release which may occur following depolarization caused by an epileptic discharge.