Metabolism of the inhibitory neurotransmitter gamma-aminobutyric acid in a rabbit model of fulminant hepatic failure

Endozepine-4 levels are increased in hepatic coma

AIM

To evaluate the serum levels of endozepine-4, their relation with ammonia serum levels, the grading of coma and the severity of cirrhosis, in patients with hepatic coma.

METHODS

In this study we included 20 subjects with Hepatic coma, 20 subjects with minimal hepatic encephalopathy (MHE) and 20 subjects control. All subjects underwent blood analysis, Child Pugh and Model for End - stage liver disease (MELD) assessment, endozepine-4 analysis.

RESULTS

Subjects with hepatic coma showed significant difference in endozepine-4 (P < 0.001) and NH3 levels (P < 0.001) compared both to MHE and controls patients. Between NH3 and endozepine-4 we observed a significant correlation (P = 0.009; Pearson correlation 0.570). There was a significant correlation between endozepine-4 and MELD (P = 0.017; Pearson correlation = 0.529). In our study blood ammonia concentration was noted to be raised in patients with hepatic coma, with the highest ammonia levels being found in those who were comatose. We also found a high correlation between endozepine-4 and ammonia (P < 0.001). In patients with grade IV hepatic coma, endozepine levels were significantly higher compared to other groups.

CONCLUSION

This study suggests that an increased level of endozepine in subjects with higher levels of MELD was observed. In conclusion, data concerning involvement of the GABA-ergic system in HE coma could be explained by stage-specific alterations.

Update on management of patients with overt hepatic encephalopathy

Hepatic encephalopathy (HE) is a multifactorial neuropsychiatric disease that affects patients with cirrhosis. We review the clinical impact, pathogenesis, evaluation, management, and prevention of overt HE in patients with cirrhosis. Articles published between January 1960 and November 2012 were acquired through a MEDLINE search of different combinations of the terms hepatic encephalopathy, pathophysiology, treatment, prophylaxis, prevention, prognosis, and recurrence. The Healthcare Cost and Utilization Project database was used to obtain prevalence and cost information related to hospitalizations of patients with HE. The literature describes significant morbidity and mortality of HE in patients with cirrhosis. Overt HE develops in 30% to 45% of patients with cirrhosis and is associated with a substantial pharmacoeconomic burden, particularly HE-related hospitalizations. The development of HE in patients with cirrhosis portends a worsened prognosis and is incorporated into the Child-Pugh classification of the severity of liver disease. In the hospitalized patient, the development of HE is associated with precipitating events (eg, gastrointestinal bleeding, dehydration, infection), and in some patients, its course is characterized by frequent and severe relapses. In addition, hospitalized patients with overt HE have a 3.9-fold increased mortality risk. Patient management employs nonabsorbable disaccharides, the nonsystemic antibiotic rifaximin, or both, to treat acute HE episodes and prevent HE relapse. In open-label trials, use of the nonabsorbable disaccharide lactulose reduced the risk of overt HE recurrence in patients compared with no-lactulose control groups for ≤ a median of 14 months. In a randomized, placebo-controlled trial, rifaximin 550 mg twice daily was more effective in maintaining HE remission compared with placebo and was associated with a reduction in HE-related hospitalizations. Recent advances in treatment and preventative therapies may reduce the personal, societal, and economic impact of this disorder.

Increasing cell membrane potential and GABAergic activity inhibits malignant hepatocyte growth

Increasing hepatocyte membrane potentials by augmenting GABAergic activity inhibits nonmalignant hepatocyte proliferative activity. The objectives of this study were to document 1) potential differences (PDs) of four malignant hepatocyte cell lines, 2) GABAA receptor mRNA expression in the same cell lines, and 3) effects of restoring malignant hepatocyte PDs to levels approximating those of resting, nonmalignant hepatocytes. Hepatocyte PDs were documented in nonmalignant and malignant (Chang, HepG2, HuH-7, and PLC/PRF/5) hepatocytes with a fluorescent voltage-sensitive dye and GABAA receptor expression by RT-PCR and Western blot analyses. Compared with nonmalignant human hepatocytes, all four malignant cell lines were significantly depolarized (P < 0.0001, respectively). Only PLC/PRF/5 cells had detectable GABAA-beta3 receptor mRNA expression and all cell lines were negative for GABAA-beta3 receptor protein by Western blot analysis. Stable transfection of Chang cells with GABAA-beta3 receptor cDNA resulted in significant increases in PD and decreases in proliferative activity as manifest by decreased [3H]thymidine and bromodeoxyurieine incorporation rates, 4-[3-(4-lodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate activity, a lower mitotic index, prolongation of cell-doubling times, and attenuated growth patterns compared with cells transfected with vector alone. Colony formation in soft agar and the number of abnormal mitoses were also significantly decreased in GABAA-beta3 receptor transfected cells. The results of this study indicate 1) relative to healthy hepatocytes, malignant hepatocytes are significantly depolarized, 2) GABAA-beta3 receptor expression is absent in malignant hepatocyte cell lines, and 3) increasing the PD of malignant hepatocytes is associated with less proliferative activity and a loss of malignant features.

Sequence and chromosomal assignment of a human novel cDNA: similarity to gamma-aminobutyric acid transporter

Gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian brain. Although initially thought to be confined to the central nervous system, GABAergic activity has also been described in other tissues throughout the body. In the present study, we report the cloning and localization of human GABA transporter cDNA and document its expression in various human tissues. A human liver cDNA library was initially screened by a 32P-labeled murine brain GABA transporter 3 (GAT-3) cDNA probe, and full-length cDNA was cloned by employing Marathon-Ready(tm) human kidney cDNA. The human GABA transporter cDNA encoded a 569 amino acid hydrophobic protein with 12 transmembrane domains (TMs). Search of published sequences revealed high homology with rat GAT-2, murine GAT-3 cDNA, human solute carrier family 6 member 13 (SLC6A13), and a human peripheral betaine/GABA transporter. Northern blot analyses demonstrated that the human GABA transporter is expressed strongly in the kidney and to a lesser extent in the liver and brain. The sequence was well matched with human chromosome 12p13.3, suggesting the human GABA transporter contains 14 exons. The above findings confirm the existence of and further characterize a specific GABA transporter in human tissues.Key words: sequence, chromosome, GABA, GABA transporter.

Effect of ammonia on GABA uptake and release in cultured astrocytes

While the pathogenesis of hepatic encephalopathy (HE) is unclear, there is evidence of enhanced GABAergic neurotransmission in this condition. Ammonia is believed to play a major pathogenetic role in HE. To determine whether ammonia might contribute to abnormalities in GABAergic neurotransmission, its effects on GABA uptake and release were studied in cultured astrocytes, cells that appear to be targets of ammonia neurotoxicity. Acutely, ammonium chloride (5 mM) inhibited GABA uptake by 30%, and by 50-60% after 4-day treatment. GABA uptake inhibition was associated with a predominant decrease in Vmax; the Km was also decreased. Ammonia also enhanced GABA release after 4-day treatment, although such release was initially inhibited. These effects of ammonia (inhibition of GABA uptake and enhanced GABA release) may elevate extracellular levels of GABA and contribute to a dysfunction of GABAergic neurotransmission in HE and other hyperammonemic states.

The effects of ethanol and gamma aminobutyric acid alone and in combination on hepatic regenerative activity in the rat

BACKGROUND/AIMS

Both ethanol and gamma aminobutyric acid (GABA) have been reported to inhibit hepatic regenerative activity in the rat. Because alcoholic beverages contain appreciable amounts of GABA, we documented whether the inhibitory effects of alcohol on the liver are derived from ethanol alone or the combination of ethanol plus GABA.

METHODS

Adult male Sprague-Dawley rats (n=6/group) were treated with either ethanol (3 g/kg), GABA (500 mg/kg) or ethanol plus GABA (3 kg and 500 mg/kg, respectively), beginning 1 h prior to a 70% partial hepatectomy and continued every 4 h thereafter for a total of 24 h. Rats were then sacrificed and hepatic regenerative activity was documented by 3H-thymidine incorporation into hepatic DNA.

RESULTS

DNA synthesis was significantly inhibited by ethanol (-37%, p<0.005) and GABA (-19%, p<0.05). Maximum inhibition was achieved with the combination of ethanol plus GABA (-52%, p<0.001). To determine whether the additive effects of ethanol plus GABA were mediated by ethanol-induced enhancement of hepatic GABA(A) receptor activity, additional rats (n=6/group) receiving the combination of ethanol plus GABA were pre-treated with a single injection of either ciprofloxacin (50 mg/kg), a GABA(A) receptor antagonist, or an equal volume of saline. In these experiments, ciprofloxacin pre-treatment prevented the inhibitory effects of the ethanol plus GABA combination.

CONCLUSIONS

The results of this study indicate that the combination of ethanol plus GABA has a greater inhibitory effect on hepatic DNA synthesis following partial hepatectomy than ethanol alone. The clinical implication of this finding is that, when standardized for ethanol content, not all alcoholic beverages would be expected to have the same inhibitory effect on hepatic regeneration.

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

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is not solely located in the CNS, it and the enzymes responsible for its synthesis (glutamic acid decarboxylase, GAD, EC 4.1.1.15) and catabolism (GABA-transaminase, GABA-T, EC 2.6.1.19) are also present in non-neuronal organs. Following 2, 8 and 21 day oral administration of ethanolamine-O-sulphate (EOS) and gamma-vinyl GABA (GVG), two irreversible inhibitors of GABA-T, the GABA content and activities of GAD and GABA-T in rat brain, liver and kidney, and the GABA content of plasma were determined: GABA-T activity was significantly decreased (over 80%) in liver, brain and kidney, although there was 2-3 times the residual activity left in the brain compared with the peripheral organs. GABA content was subsequently significantly elevated in the liver (300-1500%), plasma (200-300%) and brain (200-300%), although, surprisingly, the kidney GABA content was reduced (by 60-70%) compared with control. GAD activity was decreased following 8 day treatment in liver and brain. Kidney GAD was reduced at all time points. These two compounds are anticonvulsant, GVG is used clinically for the treatment of epilepsy but it seems that these drugs have significant peripheral effects.

gamma-Aminobutyric acid (GABA) metabolism in mammalian neural and nonneural tissues

4-Aminobutyric acid (GABA), a major inhibitory neurotransmitter of mammalian central nervous system, is found in a wide range of organisms, from prokaryotes to vertebrates. GABA is widely distributed in nonneural tissue including peripheral nervous and endocrine systems. GABA acts on GABAA and GABAB receptors. GABAA receptors are ligand-gated chloride channels modulated by a variety of drugs. GABAB receptors are essentially presynaptic, usually coupled to potassium or calcium channels, and they function via a GTP binding protein. In neural and nonneural tissues, GABA is metabolized by three enzymes--glutamic acid decarboxylase (GAD), which produces GABA from glutamic acid, and the catabolic enzymes GABA-transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). Production of succinic acid by SSADH allows entry of the GABA carbon skeleton into the tricarboxylic acid cycle. Alternate sources of GABA include putrescine, spermine, spermidine and ornithine, which produce GABA via deamination and decarboxylation reactions, while L-glutamine is an additional source of glutamic acid via deamination. GAD from mammalian brain occurs in two molecular forms, GAD65 and GAD67 (referring to subunit relative molecular weight (Mr) in kilodaltons). These different forms of GAD are the product of different genes, differing in nucleotide sequence, immunoreactivity and subcellular localization. The presence and characteristics of GAD have been investigated in a wide variety of nonneural tissues including liver, kidney, pancreas, testis, ova, oviduct, adrenal, sympathetic ganglia, gastrointestinal tract and circulating erythrocytes. In some tissues, one form (GAD65 or GAD67) predominates. GABA-T has been located in most of the same tissues, primarily through histochemical and/or immunochemical methods; GABA-T is also present in a variety of circulating cells, including platelets and lymphocytes. SSADH, the final enzyme GABA catabolism, has been detected in some of the tissues in which GAD and GABA-T have been identified, although the presence of this enzyme has not been in mammalian pancreas, ova, oviduct, testis or sympathetic ganglia.

Amino acid release from cerebral cortex in experimental acute liver failure, studied by in vivo cerebral cortex microdialysis

Both increased gamma-aminobutyric acid (GABA)-ergic and decreased glutamatergic neurotransmission have been suggested relative to the pathophysiology of hepatic encephalopathy. This proposed disturbance in neurotransmitter balance, however, is based mainly on brain tissue analysis. Because the approach of whole tissue analysis is of limited value with regard to in vivo neurotransmission, we have studied the extracellular concentrations in the cerebral cortex of several neuroactive amino acids by application of the in vivo microdialysis technique. During acute hepatic encephalopathy induced in rats by complete liver ischemia, increased extracellular concentrations of the neuroactive amino acids glutamate, taurine, and glycine were observed, whereas extracellular concentrations of aspartate and GABA were unaltered and glutamine decreased. It is therefore suggested that hepatic encephalopathy is associated with glycine potentiated glutamate neurotoxicity rather than with a shortage of the neurotransmitter glutamate. In addition, increased extracellular concentration of taurine might contribute to the disturbed neurotransmitter balance. The observation of decreasing glutamine concentrations, after an initial increase, points to a possible astrocytic dysfunction involved in the pathophysiology of hepatic encephalopathy.

The effects of benzodiazepine-receptor antagonists and partial inverse agonists on acute hepatic encephalopathy in the rat

Two benzodiazepine-receptor partial inverse agonists (Ro 15-4513, Ro 15-3505) and one benzodiazepine-receptor antagonist (flumazenil) were administered to rats with hepatic encephalopathy due to acute liver ischemia. Significant improvement (P less than 0.002) of both the clinical grade of hepatic encephalopathy and the electroencephalographic abnormalities was observed after administration of the benzodiazepine-receptor partial inverse agonists: comatose rats with no spontaneous righting reflex regained consciousness immediately after injection of the drug. Only slight improvement in clinical hepatic encephalopathy grade was seen after administration of 25 mg/kg of flumazenil. The present data strongly support a role of increased gamma-aminobutyric acid-ergic tone in the pathogenesis of acute hepatic encephalopathy and provide a rationale for trials of benzodiazepine-receptor partial inverse agonists to restore consciousness in hepatic encephalopathy in humans in the near future.

Changes in brain metabolism during hyperammonemia and acute liver failure: results of a comparative 1H-NMR spectroscopy and biochemical investigation

The effects of hyperammonemia on brain function have been studied in three different experimental models in the rat: acute liver ischemia, urease-treated animals and methionine sulfoximine-treated animals. To quantify the development of encephalopathy, clinical grading and electroencephalographic spectral analysis were used as indicators. In all three experimental models brain ammonia concentrations increased remarkably associated with comparable increases in severity of encephalopathy. Furthermore, in vivo 1H-nuclear magnetic resonance spectroscopy of a localized cerebral cortex region showed a decrease in glutamate concentration in each of the aforementioned experimental models. This decreased cerebral cortex glutamate concentration was confirmed by biochemical analysis of cerebral cortex tissue post mortem. Furthermore, an increase in cerebral cortex glutamine and lactate concentration was observed in urease-treated rats and acute liver ischemia rats. As expected, no increase in cerebral cortex glutamine was observed in methionine sulfoximine-treated rats. These data support the hypothesis that ammonia is of key importance in the pathogenesis of acute hepatic encephalopathy. Decreased availability of cerebral cortex glutamate for neurotransmission might be a contributing factor to the pathogenesis of hyperammonemic encephalopathy. A surprising new finding revealed by 1H-nuclear magnetic resonance spectroscopy was a decrease of cerebral cortex phosphocholine compounds in all three experimental models. The significance of this finding, however, remains speculative.

Increased brain uptake of gamma-aminobutyric acid in a rabbit model of hepatic encephalopathy

Transfer of the inhibitory neurotransmitter gamma-aminobutyric acid across the normal blood-brain barrier is minimal. One prerequisite for gamma-aminobutyric acid in plasma contributing to the neural inhibition of hepatic encephalopathy would be that increased transfer of gamma-aminobutyric acid across the blood-brain barrier occurs in liver failure. The aim of the present study was to determine if brain gamma-aminobutyric acid uptake is increased in rabbits with stage II-III (precoma) hepatic encephalopathy due to galactosamine-induced fulminant hepatic failure. A modification of the Oldendorf intracarotid artery-injection technique was applied. [3H] gamma-aminobutyric acid, [14C] butanol, and 113mIn-labeled serum protein (transferrin) were injected simultaneously 4 s before decapitation. The ipsilateral brain uptake index of gamma-aminobutyric acid was determined from measurements of the 3 isotopes in 5 brain regions. Uncorrected or simple brain uptake indices of [3H] gamma-aminobutyric acid and [113mIn] transferrin were calculated using [14C] butanol as the highly extracted reference compound. The [113mIn] transferrin data were also used to "correct" the brain uptake index of [3H] gamma-aminobutyric acid for intravascular retention of [3H] gamma-aminobutyric acid. The methodology adopted minimized problems attributable to rapid [3H] gamma-aminobutyric acid metabolism, and slow brain washout and recirculation of the radiolabeled tracers. Both the uncorrected and corrected brain uptake indices of gamma-aminobutyric acid as well as the simple brain uptake index of transferrin were significantly increased in both stage II and III hepatic encephalopathy in all brain regions studied. Moreover, these brain uptake indices were significantly greater in stage III hepatic encephalopathy than in stage II hepatic encephalopathy. These findings indicate that transfer of gamma-aminobutyric acid from plasma to brain extracellular fluid is increased in the model of hepatic encephalopathy studied; hence, they provide support for the hypothesis that plasma-derived gamma-aminobutyric acid may contribute to the neural inhibition of hepatic encephalopathy due to fulminant hepatic failure.

Uptake and catabolism of gamma-aminobutyric acid by the isolated perfused rat liver

Serum concentrations of gamma-aminobutyric acid (GABA) are increased in liver failure, possibly because of decreased hepatic GABA catabolism. To study in detail the role of the liver in GABA metabolism, uptake and catabolism of GABA by isolated perfused liver from normal rats and rats with galactosamine- or carbon tetrachloride-induced liver failure were measured. Hepatic GABA uptake was almost complete at GABA concentrations of up to 10 microM and approached saturation at a concentration of 50 microM. The apparent affinity of hepatic GABA uptake was 38 microM and the apparent maximal velocity was 158 nmol/g.min. Hepatic GABA uptake was sodium-dependent. gamma-Aminobutyric acid taken up by the liver was rapidly catabolized as measured by 14CO2 formation from [U-14C]GABA. Aminooxyacetic acid, a GABA transaminase inhibitor, completely and irreversibly inhibited hepatic GABA catabolism and thereby also inhibited hepatic GABA uptake. Although uptake of GABA by livers of carbon tetrachloride- or galactosamine-treated rats was decreased (apparent maximal velocity, 103 and 98 nmol/g.min, respectively), at physiologic GABA concentrations in the perfusate GABA uptake and catabolism was not different from that of untreated controls. The observed impairment of hepatic GABA uptake or catabolism by the diseased liver would be expected to contribute to increased GABA levels in peripheral blood plasma in liver failure. However, the magnitude of the observed impairment would be insufficient to account for a 10-fold increase in such levels.

Regional brain GABA metabolism and release during hepatic coma produced in rats chronically treated with carbon tetrachloride

Hepatic coma was induced in rats chronically treated with CCl4, by means of a single injection of ammonium acetate. The activities of glutamate decarboxylase (GAD) and GABA transaminase (GABA-T), as well as the synaptosomal uptake and release of [3H]GABA, were measured in the following brain areas of the comatose rats: cortex, striatum, hypothalamus, hippocampus, midbrain and cerebellum. Hepatic coma was associated with a general decrease of GAD activity, whereas GABA-T activity was diminished only in the hypothalamus, striatum and midbrain. During hepatic coma, the K+-stimulated [3H]GABA release was notably diminished in the striatum and cerebellum, whereas a significant increase was observed in the hippocampus. [3H]GABA uptake increased in most regions after CCl4 treatment, independently of the presence of coma. The results indicate that GABAergic transmission seems to be decreased in most cerebral regions during hepatic coma.

Is the hypotension of cirrhosis a GABA-mediated process?

Systolic and diastolic blood pressures were recorded in 176 ambulant patients with chronic liver disease, including 36 patients with compensated cirrhosis (Group I), 119 patients with noncirrhotic chronic liver disease (Group II) and 21 patients with benign structural or functional liver disease (Group III). Group I patients had significantly lower systolic (113.0 +/- 2.2 mm Hg, mean +/- S.E.) and diastolic (65.3 +/- 1.7 mm Hg) pressures than Group II patients (125.8 +/- 3.5 and 76.6 +/- 1.5 mm Hg, respectively (p less than 0.0001) or Group III patients (125.1 +/- 3.4 and 77.5 +/- 2.4 mm Hg, respectively) (p less than 0.0001). Serum levels of GABA, a potent amino acid neurotransmitter with known vasodilatory effects in vitro, were higher in Group I patients (1.12 +/- 0.26 microM, mean +/- S.E.) than in Group II patients (0.41 +/- 0.05 microM) (p less than 0.005) or Group III patients (0.34 +/- 0.03 mM) (p less than 0.05). A constant infusion of GABA into the systemic circulation of six adult dogs, at rates required to achieve serum GABA levels within one order of magnitude of those observed in humans with cirrhosis, resulted in a 17.0 +/- 4.3 mm Hg decrease in systolic pressure (p less than 0.05) and a 10.8 +/- 3.7 mm Hg decrease in diastolic pressure (p less than 0.05). Control amino acids were not vasoactive. The results of this study suggest that, in addition to other vasoactive compounds, a GABA-mediated process might contribute to the hypotension observed in patients with compensated cirrhosis.

Overestimation of serum concentrations of gamma-aminobutyric acid in patients with hepatic encephalopathy by the gamma-aminobutyric acid-radioreceptor assay

Sera of patients with hepatic encephalopathy strongly inhibit the specific binding of gamma-aminobutyric acid to synaptic membranes. In a previous study, this inhibition of specific gamma-aminobutyric acid binding was attributed to gamma-aminobutyric acid itself, and it was assumed that serum gamma-aminobutyric acid is increased 5- to 30-fold in patients with hepatic encephalopathy. The findings of that study, however, were not confirmed by other analytical methods. Therefore, the validity of the gamma-aminobutyric acid-radioreceptor assay was tested. In view of the increased serum concentrations of several amino acids in hepatic encephalopathy, the effects of L-alpha-amino acids on the assay were studied. Five amino acids inhibited specific gamma-aminobutyric acid binding at a concentration of 0.5 mM or lower: glutamine; glutamate; taurine; proline, and OH-proline. Equimolar amounts of aminooxyacetate prevented the inhibition of specific gamma-aminobutyric acid binding by glutamine and glutamate but had no effect on that of gamma-aminobutyric acid, taurine, proline and OH-proline. Aminooxyacetate had no effect on specific gamma-aminobutyric acid binding itself. The inhibitory activity of a serum sample from a patient with hepatic encephalopathy was inhibited by 0.5 mM aminooxyacetate. The gamma-aminobutyric acid binding inhibitory activity of a serum sample of a patient with hepatic encephalopathy was purified by gel chromatography and contained several amino acids at concentrations of about 0.1 mM, 3.5 mM glutamine but no detectable gamma-aminobutyric acid. Accordingly, the gamma-aminobutyric acid binding inhibitory activity is not mediated by gamma-aminobutyric acid alone and is most likely due to glutamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic encephalopathy: lack of changes of gamma-aminobutyric acid content in plasma and cerebrospinal fluid

The aim of the study was to verify the role of gamma-aminobutyric acid in the pathogenesis of hepatic encephalopathy occurring in cirrhotic patients by attempting to correlate plasma and cerebrospinal fluid content of authentic gamma-aminobutyric acid with the neurological manifestations of hepatic encephalopathy. For this purpose, plasma and cerebrospinal fluid gamma-aminobutyric acid levels were measured by means of mass fragmentography in 17 cirrhotic patients with hepatic encephalopathy and in 6 cirrhotics without neurological symptoms. Moreover, in all patients, a second sample was obtained during the clinical course of hepatic encephalopathy. The mean plasma and cerebrospinal fluid gamma-aminobutyric acid levels were not different in patients with or without hepatic encephalopathy and did not change during the evolution of the neurological symptoms. The lack of changes in the gamma-aminobutyric acid content in plasma and cerebrospinal fluid during hepatic encephalopathy is in contrast with the hypothesized importance of increased entry into the brain of gamma-aminobutyric acid in the pathogenesis of hepatic encephalopathy.

gamma-Aminobutyric acid plasma levels and brain binding in Eck fistula dogs

It has been hypothesized that gamma-aminobutyric acid (GABA), the principle inhibitory neurotransmitter of the mammalian brain, contributes to the neural inhibition of hepatic encephalopathy. Eck fistulae were created in seven dogs and celiotomy alone performed in five dogs to determine plasma GABA levels, brain GABA binding, and synaptic membrane changes in dogs after creation of Eck fistulae. Eck fistula dogs lost 19 +/- 9% body weight, lost hair, ate poorly, and developed atrophic livers with classic hepatic histological changes. Control dogs maintained body weight, normal behavior, and normal liver histology. Plasma GABA levels were elevated significantly in the Eck fistula dogs (312 +/- 105.9 nM) both as compared to controls (154.4 +/- 69.8) and to preoperative levels (161.6 +/- 56.7, P less than 0.05). Brain GABA binding for animals sacrificed at 6-9 weeks was not statistically different from sham-operated dogs sacrificed at 6 weeks (1.87 +/- .54 pmole/mg protein vs 1.186 +/- 24, P less than 0.2). Synaptic membrane fluidity and cholesterol were likewise unchanged. Plasma GABA levels are increased significantly following complete portal diversion but do not correlate with the degree of encephalopathy. GABA binding to neural membranes are not significantly increased in Eck fistula dogs. These findings do not support a direct relationship between plasma GABA levels and neurologic impairment in Eck fistula dogs.

Enzymes of cerebral GABA metabolism and synaptosomal GABA uptake in acute liver failure in the rabbit: evidence for decreased cerebral GABA-transaminase activity

Measurements of the activities of the two key enzymes in cerebral GABA metabolism--glutamate decarboxylase (GAD) and GABA-transaminase (GABA-T)--were performed in normal rabbits and in rabbits with hepatic encephalopathy due to galactosamine-induced liver failure. Furthermore the uptake of GABA by synaptosomes was studied. Hepatic encephalopathy was associated with a marked decrease in the activity of GABA-T. This decrease in activity was already apparent in galactosamine-treated rabbits before the onset of hepatic encephalopathy. Sera and serum ultrafiltrates of rabbits with hepatic encephalopathy but not of normal rabbits or of rabbits with uremic encephalopathy were shown to inhibit GABA-T activity in vitro. Cerebral GAD activity and synaptosomal GABA uptake in rabbits with hepatic encephalopathy and in untreated animals were not different. These later findings indicate that hepatic encephalopathy is not associated with alterations of presynaptic GABA nerve terminals in the central nervous system. The demonstration of a decrease in cortical GABA-T activity provides indirect evidence for decreased GABA turnover in the brains of rabbits with hepatic encephalopathy and thus is compatible with augmented GABA-ergic inhibitory neurotransmission contributing to the neural inhibition of hepatic encephalopathy.

Identification of an acceptor system for gamma-aminobutyric acid on isolated rat hepatocytes

gamma-Aminobutyric acid (GABA) is a potent inhibitory neurotransmitter which is synthesized by the enteric bacterial flora and delivered into portal venous blood. To determine whether the liver is likely to play an important role in regulating serum GABA levels, the uptake and metabolism of [3H]GABA by three populations of cells isolated from rat liver were studied. GABA was specifically taken up by hepatocytes but not by endothelial or Kupffer cells. Uptake by hepatocytes was saturable, as well as time and sodium dependent. At 0.5 degrees C, a temperature at which binding of GABA to the cell surface is considered to be the predominant component of the uptake process, the apparent affinity constant (Km) was 0.82 microM and a minimum value for binding velocity (Vmax) was 0.13 microM per min per 5 X 10(5) cells. Uptake of [3H]GABA by hepatocytes was markedly inhibited by excess unlabeled GABA (95%), alpha-aminoisobutyric acid (66%) and bicuculline (58%), but was inhibited much less by alanine (16%) and leucine (29%). These findings suggest that GABA binds specifically to the high affinity acceptor of the A amino acid transport system of rat hepatocytes. Impaired function of this transport system in liver failure could contribute to increased circulating levels of GABA.