Reversal of the behavioral and electrophysiological abnormalities of an animal model of hepatic encephalopathy by benzodiazepine receptor ligands

Combination treatment of hepatic encephalopathy due to thioacetamide-induced fulminant hepatic failure in the rat with benzodiazepine and opioid receptor antagonists

BACKGROUND/AIMS

Treatment of hepatic encephalopathy with drugs acting on the target organ of this syndrome, the brain, is unsatisfactory. Combination treatment with different neurotransmitter receptor antagonists may be a rational option to optimize treatment.

METHODS

The effects of various doses of the benzodiazepine receptor antagonist Ro 15-3505 and the opioid receptor antagonist naloxone, alone or in combination, were tested on hepatic encephalopathy in rats with thioacetamide-induced hepatic failure in an open-field activity meter. Comparison of single and combination treatment was also done using a neurological test battery. In addition, we compared survival of treatment-responder rats with treatment non-responders.

RESULTS

Naloxone dose dependently increased ambulatory activity and improved neurological score. Ro 15-3505 also improved ambulatory activity and neurological score; however, the improvement was less evident at higher doses. Combination treatment was not superior to single treatment. Survival was increased in treatment-responder rats.

CONCLUSIONS

The failure of combination treatment with Ro 15-3505 and naloxone to further improve hepatic encephalopathy may suggest that the two neurotransmitter systems are interrelated or that hepatic encephalopathy may not be further improved by drugs acting on the brain.

The role of astrocytes in the development of hepatic encephalopathy

Thioacetamide (TAA), a hepatotoxin used to ascertain the role of astrocytes in hepatic encephalopathy, was administered to prepare four experimental groups of rats. (The TAA1D, TAA1.5D, TAA2D, and TAA2.5D group rats were perfusion fixated with formalin at 1, 1.5, 2, and 2.5 days, respectively, after initial administration of TAA. In addition, TAA was readministered to the TAA2D and TAA2.5D rats 24 h after the first dose.) Abnormalities of higher brain function and equilibrium that progressed with time were apparent in the rats receiving TAA. On the other hand, innate reflexes (e.g. pupillary reflex) were similar to those in the normal control group. Astrocyte cell areas in the hippocampus, neocortex, hypothalamus, cerebellum, and basal ganglia (striatum) from the TAA rats were significantly larger than in corresponding sites from the normal rats (maximum in TAA1D and TAA1.5D groups). However, there were no differences with respect to the midbrain. Any morphological difference was not observed in neurons between the hepatic encephalopathy and normal rats. Administration of TAA caused hepatic tissue injury that progressed over time. Surprisingly, encephalopathy was apparent even when hepatic injury was mild. These findings suggest that abnormalities in astrocytes, which precede any abnormal change in neurons, play a role in the development of hepatic encephalopathy.

Plasma levels of pipecolic acid, both L- and D-enantiomers, in patients with chronic liver diseases, especially hepatic encephalopathy

Pipecolic acid is regarded as a gamma-aminobutyric acid receptor agonist. Stereochemical studies of pipecolic acid were performed in patients with chronic liver diseases. Plasma D- and L-pipecolic acid were significantly elevated in 15 liver cirrhotic patients with no history of hepatic encephalopathy (1.05+/-0.24 micromol/l, 1.58+/-0.13 micromol/l, p < 0.01) and in 27 patients with chronic hepatic encephalopathy (1.58+/-0.50 micromol/l, 2.38+/-0.58 micromol/l, p<0.001) compared to 15 normal subjects. In seven patients with chronic hepatic encephalopathy orally treated with kanamycin, plasma pipecolic acid significantly decreased (D-acid: before 1.62+/-0.23 micromol/l, after 0.61+/-0.15 micromol/l; p<0.01, L-acid: before 2.43-0.52 micromol/l, after 2.23+/-0.11 micromol/l; p< 0.05). These results suggest that plasma pipecolic acid, particularly D-acid, is produced from D-lysine by intestinal bacteria in liver cirrhotic patients and that pipecolic acid could be involved in the pathogenesis of hepatic encephalopathy.

Does ammonia contribute to increased GABA-ergic neurotransmission in liver failure?

The ammonia and GABAergic neurotransmission hypotheses of the pathogenesis of hepatic encephalopathy (HE) have appeared to be unrelated and perhaps mutually exclusive. Observations in animal models of fulminant hepatic failure, that are consistent with increased GABAergic inhibitory neurotransmission contributing to the manifestations of HE, include: (i) abnormal visual evoked potential waveforms that resemble those induced by GABA(A)/benzodiazepine (BZ) receptor complex agonists; (ii) GABA(A)/BZ receptor complex antagonist-induced ameliorations of encephalopathy; (iii) increased resistance to drugs which decrease GABAergic tone; and (iv) hypersensitivity of CNS neurons to depression by GABA(A)/BZ receptor complex agonists. Mechanisms of increased GABAergic tone in HE may include the following: (i) increased brain concentrations of natural BZs; and (ii) increased GABA concentrations in synaptic clefts, possibly due to increased blood-brain-barrier permeability to GABA and a decrease in GABA(B) receptor density. Both neuroelectrophysiological and behavioral data indicate that ammonia concentrations in the range 0.75-2 mM induce increased excitatory neurotransmission. In contrast, recently, ammonia concentrations in the range 0.15-0.75 mM, i.e. concentrations that commonly occur in plasma in precoma HE, have been shown: (i) to increase GABA-induced chloride current in cultured neurons; and (ii) to enhance synergistically the binding of GABA(A)/BZ receptor agonists. In addition, increased ammonia concentrations enhance synthesis of neurosteroids in astrocytes, and some neurosteroids potently augment GABAergic neurotransmission. Thus, the modestly elevated concentrations of ammonia, that commonly occur in liver failure, may contribute to the manifestations of HE by enhancing GABAergic inhibitory neurotransmission. This concept appears to unify the ammonia and GABAergic neurotransmission hypotheses.

Improvement of chronic hepatic encephalopathy in dogs by the benzodiazepine-receptor partial inverse agonist sarmazenil, but not by the antagonist flumazenil

Therapeutic modulation of the increased GABAergic tone in chronic hepatic encephalopathy (HE) by the benzodiazepine receptor (BR) antagonist flumazenil (F) has led to conflicting results in humans and animal models for HE. The BR inverse agonist sarmazenil (S) has only been used in animal models of acute HE. Therefore we investigated the effects of intravenous injection of F and S in dogs with chronic HE 8 to 12 weeks after placement of a portocaval shunt and 40% hepatectomy (n=7), compared to sham-operated pair-fed controls (n=7). The HE dogs had hyperammonemia (298 +/- 48 microM v 33 +/- 3 before surgery (mean +/- SEM)) and signs of HE at the start of the experiments (0.9 +/- 0.1 (scale 0-4)). Three (S3) and 8 (S8) mg/kg of S resulted in a significant improvement of encephalopathy (grade 0.9 +/- 0.2 immediately before v 0.5 +/- 0.1 after injection (S3) and 0.7 +/- 0.1 v 0.3 +/- 0.1 (S8)) and increase in mean dominant frequency of the EEG (MDF; 9.1 +/- 0.7 Hz v 11.1 +/- 0.3 (S3) and 8.9 +/- 0.5 v 11.0 +/- 0.3 (S8)) in HE dogs, whereas 15 mg/kg of S, 3 and 8 mg/kg of F, and the vehicle had no significant effects. The efficacy of S in these dogs is consistent with an increased GABAergic tone in the pathogenesis of chronic HE. The lack of effects of F makes a role for endogenous benzodiazepines herein unlikely.

Endogenous benzodiazepine activity in the peripheral and portal blood of dogs with congenital portosystemic shunts

OBJECTIVE

The purpose of this study was to determine whether an endogenous benzodiazepine receptor ligand (EBZ) was present in the arterial and portal blood of dogs with congenital portosystemic shunts (CPSS).

STUDY DESIGN

The presence or absence of an EBZ was determined by the collection of systemic and portal blood from dogs with CPSS.

ANIMALS

Fifteen client-owned dogs with a confirmed CPSS. All dogs had historical signs compatible with hepatic encephalopathy. Eight healthy dogs were used as controls.

METHODS

In all dogs, systemic blood samples were collected after they were anesthetized. Portal blood samples were collected intraoperatively. EBZ was measured by radioreceptor assay.

RESULTS

In 10 of 15 dogs, the portal blood concentration of EBZ was significantly elevated compared with normal dogs (mean, 13.2 +/- 18.55 ng/mL). Five dogs had elevated systemic blood EBZ levels (mean, 8.2 +/- 16.08 ng/mL). Eleven of 15 dogs had a higher portal than systemic blood concentration of EBZ. In contrast, control dogs had extremely low EBZ concentrations detected in their portal blood (mean, 0.16 +/- 0.3 ng/mL) and systemic blood (0 ng/mL). The mean portal and systemic blood concentrations in dogs with CPSS were significantly greater than in control dogs (P < .05).

CONCLUSIONS

Elevated blood levels of EBZ were found in dogs with CPSS. The portosystemic gradient noted in 11 dogs suggests the gastrointestinal tract as a possible source for the endogenous ligand.

CLINICAL RELEVANCE

Generalized motor seizures have been reported in dogs after surgical correction of CPSS. If the presence of a CPSS results in stimulation of brain receptors for benzodiazepines, post-CPSS ligation seizures may result from a withdrawal of EBZ after ligation of the portosystemic shunt.

Behavioral and electroencephalographic studies of beagles with an Eck's fistula: suitability as a model of hepatic encephalopathy

Behavioral manifestations, electroencephalograms (EEGs) and visually evoked potentials (VEPs) were studied in beagles with Eck's fistula (portacaval shunt [PCS]), an established model of hyperammonemia, to determine whether they developed CNS disorders characteristic of hepatic encephalopathy. After PCS, behavioral changes occurred in the form of listlessness, sluggishness (altered gait, snapping and transient catatonia-like symptoms) and apparent blindness, which appeared in that order and progressed to coma and death in some animals. The EEGs from the frontal cortex showed a gradual decrease in voltage and frequency. Development of snapping and catatonia-like symptoms coincided with the occurrence of high voltage fast waves in the EEGs from the occipital cortex. In comatose Eck's fistula dogs. flattening of the EEGs was recorded from the frontal cortex and a lowered voltage was noted in the EEGs from the occipital cortex. After PCS, the latencies and amplitudes of the components of VEP were increased. The snapping and catatonia-like symptoms were markedly ameliorated by carbamazepine and the coma by flumazenil and thyrotropin-releasing hormone. These findings indicate that Eck's fistula dogs provide a useful model of hepatic encephalopathy.

Management of portal hypertension and its complications

In the setting of chronic liver disease, portal hypertension and its complications pose major challenges in management. Once it develops, portal hypertension is the source of potentially devastating sequelae, including life-threatening hemorrhage, infection, renal failure, and encephalopathy. Improved understanding of the pathophysiology of these conditions has led to advances in endoscopic, radiologic, medical, and surgical management. The possibility in selected cases of successful amelioration of portal hypertension and its dreaded complications by liver transplantation highlights the importance of timely and careful management of patients with decompensated cirrhosis.

GABAergic inhibition of the pituitary release of adrenocorticotropin and alpha-melanotropin is impaired in dogs with hepatic encephalopathy

gamma-Aminobutyric acid (GABA) is the principal depressant neurotransmitter system, but its possible role in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis has not yet been investigated in the dog. Moreover, GABA is one of the factors underlying the syndrome of hepatic encephalopathy (HE), and in dogs with HE, the regulation of the HPA axis is deranged. We have therefore investigated the role of the GABA system in the regulation of the HPC system in 10 healthy dogs and 10 dogs with HE due to congenital portosystemic shunts. The effect of an intravenous injection of the GABA antagonist bicuculline on the release of adrenocorticotropin (ACTH), alpha-melanotropin (MSH), and cortisol was measured in plasma. In healthy dogs, a dose of 1.0 mg/kg caused a marked release of ACTH, MSH, and cortisol, but doses of 0.001 to 0.5 mg/kg produced an inconsistent or no response. The high release of MSH after bicuculline administration indicated that the effect of GABA was predominantly in the neurointermediate lobe of the pituitary. In order to investigate whether the effect of GABA was exerted in the pituitary or at a suprapituitary level, the effect of incubation with GABA on basal and corticotropin-releasing hormone-induced ACTH release was measured in primary cultures of anterior and neurointermediate lobe cells from healthy dogs, and no response was observed. We conclude that in healthy dogs, GABA inhibits the release of ACTH and MSH from the neurointermediate lobe of the pituitary at a suprapituitary level. In dogs with HE, 1.0 mg/kg of bicuculline caused virtually no stimulation of the secretion of ACTH, MSH, or cortisol, indicating deranged GABAergic neurotransmission in HE. This may be explained by an increased GABA tone that prevents the effect of the antagonist. Such a high GABA tone associated with HE has been documented in several other species. Dogs with HE had significantly increased basal levels of ACTH, MSH, and cortisol in plasma, and their cortisol:creatinine ratios in 24-hr urine samples (63 +/- 14.10(-6)) were higher than those of healthy dogs (9 +/- 2.10(-6)). An increased basal HPA activity in dogs with HE is not in agreement with augmented GABAergic inhibition, but this contradiction may be explained by the predominance of effects of dopaminergic disinhibition that has been reported in such dogs.

The role of flumazenil in the critically ill

Flumazenil is a specific benzodiazepine antagonist. It competitively inhibits the action of benzodiazepines at gamma aminobutyric acid (GABA) receptors in the central nervous system and thus reverses the sedative effects of benzodiazepines. Usually, it is given intravenously as a bolus dose or as an infusion. It has a short duration of action. Flumazenil is extensively metabolized to compounds that have minimal benzodiazepine antagonistic properties. Reversal of sedation can be useful in many conditions that are often encountered in the critically ill. The adverse effects of its use are usually predictable and, with sufficient clinical monitoring, are usually avoidable. These properties make it a useful and safe drug when used appropriately.

Benzodiazepine receptor antagonism improves reaction time in latent hepatic encephalopathy

Endogenous benzodiazepine-like substances are thought to play a role in the development of hepatic encephalopathy (HE). Ten patients with sub-clinical or latent hepatic encephalopathy (LHE) and ten normal controls were cognitively assessed pre- and post-infusion of 0.2 mg of the benzodiazepine (BZ) antagonist flumazenil in a placebo-controlled, cross-over, double-blind design. Flumazenil infusion resulted in a significant improvement in simple reaction time in patients, but not in controls. Saline infusion had no effect on any of the cognitive measures in either group. Flumazenil appeared to have a particular enhancing effect on the cognitive, as opposed to the motor, component of the reaction time task. This finding supports the view that the benzodiazepine/GABA system is implicated in the bradyphrenia that is characteristic of chronic liver disease, even before hepatic encephalopathy is apparent. We conclude that benzodiazepine receptor antagonism may improve cognitive function, particularly speed of information processing, in patients with latent hepatic encephalopathy.

Gut bacteria provide precursors of benzodiazepine receptor ligands in a rat model of hepatic encephalopathy

Benzodiazepine receptor (BZR) ligands are elevated in animals and humans with fulminant hepatic failure (FHF) and contribute to the pathogenesis of the associated hepatic encephalopathy (HE). As gut factors are proposed to play a role in the pathogenesis of HE, we investigated gut flora as a source of BZR ligands. Rats received daily oral neomycin, vancomycin and metronidazole (AB +) or saline (AB -) before and concurrent with the induction of FHF with thioacetamide. BZR ligands were extracted from brain and plasma and quantified using radiometric techniques. Plasma BZR ligand concentrations in AB(+) and AB(-) rats with HE were higher than AB(+) and AB(-) control rats. Brain BZR ligand concentrations increased in AB(+) and AB(-) rats with HE. Stool cultures from antibiotic treated rats with HE indicated the presence of multidrug resistant Acinetobacter lwoffii. Although no significant concentrations of BZR ligands were detected in culture media inoculated with A. lwoffii, administering A. lwoffii to normal rats significantly elevated BZR ligand levels in brain, but not plasma. Thus, antibiotic treatment of rats is associated with the growth of a resistant strain of bacterium which produces an inactive BZR ligand precursor. BZR ligands may be synthesized from these precursors in the brain and efficiently cleared by a normal liver following brain-to-plasma transfer. Impairment of this clearance process in FHF facilitates their accumulation, enabling agonist BZR ligands to contribute to the pathogenesis of HE by further enhancing GABAergic neurotransmission.

Flumazenil as a diagnostic tool in the differential diagnosis of coma in a critically ill patient

The purpose of this report is to describe the use of flumazenil as a diagnostic aid in the differential diagnosis of coma in a patient with an inadvertent overdose of benzodiazepines. We report a patient with suspected septic encephalopathy whose level of consciousness markedly improved following flumazenil administration. Subsequent analysis revealed the presence of benzodiazepines and their metabolites in the blood and urine although the patient had not received benzodiazepines for over two weeks. The critically ill patient with multiorgan failure may have considerable derangement of benzodiazepine metabolism; therefore, if an obtunded patient's level of consciousness improves following flumazenil administration, benzodiazepine intoxication must be considered.

Animal models of hepatic encephalopathy and hyperammonemia

Animal models of chronic liver disease with hyperammonemia are currently available to investigators. Two in particular have been utilized extensively. Carbon tetrachloride induced (CCl4) cirrhosis in the rat and portacaval shunt in the same species and other animals particularly the dog. In regards to hepatic encephalopathy, however, the CCl4 cirrhosis rat model seems to display few behavioral changes unless very advanced decompensated cirrhosis is produced. Further work should be done on this model to verify the development of encephalopathy and to improve reproducibility. The portacaval shunt rat on the other hand clearly has a consistent albeit subtle set of behavioral changes. Recent improvements in detecting these changes and more importantly showing reversal or improvement by neomycin or a low protein diet are a major advance. Hopefully, more laboratories will be able to reproduce this reversible change in behavior. Experiences gained from 7 years of using the portacaval shunt rat and other models of liver disease are described.

Hepatic encephalopathy in rats with thioacetamide-induced acute liver failure is not mediated by endogenous benzodiazepines

BACKGROUND

To distinguish whether the improvement of hepatic encephalopathy by benzodiazepine receptor antagonists is mediated by their antagonistic or their inverse agonistic properties, the neurobehavioral effects of a variety of benzodiazepine receptor ligands in rats with thioacetamide-induced acute liver failure were tested.

METHODS

The neural inhibitory effect of the benzodiazepine agonist flunitrazepam and its reversibility by the "pure" antagonist Ro 14-7437 were examined in thioacetamide-treated rats and controls. The effects of Ro 14-7437, of the partial inverse agonist Ro 15-4513, and the inverse agonist DMCM in rats with hepatic encephalopathy grade II/III were tested. Encephalopathic rats were pretreated with Ro 14-7437 or vehicle and then injected with Ro 15-4513.

RESULTS

Thioacetamide-treated rats were more sensitive to flunitrazepam than controls. In both groups, its effect was completely antagonized with Ro 14-7437. Encephalopathy was significantly improved by Ro 15-4513, although Ro 14-7437 and vehicle had no effect. DMCM worsened the condition of encephalopathic rats but had no effect in controls. Pretreatment with Ro 14-7437 abolished the beneficial effects of Ro 15-4513.

CONCLUSIONS

In rats with thioacetamide-induced liver failure, endogenous benzodiazepines do not precipitate hepatic encephalopathy. The amelioration of hepatic encephalopathy is mediated by benzodiazepine receptor ligands with both antagonistic and inverse agonistic properties.

Temporary abolition of seizure activity by flumazenil in a case of valproate-induced non-convulsive status epilepticus

We report on a 33-year-old female suffering from frequent complex-partial seizures who developed a non-convulsive status epilepticus after one week of antiepileptic therapy with valproate (VPA) which had been added to a basic medication with barbexaclone (BBC) in rapidly increasing dosage. The electroencephalogram (EEG) showed continuous rhythmic generalized sharp and slow wave activity with a frontal maximum. Intravenous administration of 3.0 mg of the benzodiazepine (BZ) receptor antagonist flumazenil under monitoring with video-EEG led to an immediate and marked electroclinical improvement, whereas 6.0 mg of the BZ receptor agonist midazolam was followed by a deterioration both clinically and in the EEG. We discuss the concept of VPA-encephalopathy and the possible mechanisms of the action of flumazenil on VPA-induced as well as on other toxic and metabolic encephalopathies. Flumazenil might antagonize increased benzodiazepine receptor activity with agonistic and even convulsive properties in these encephalopathic syndromes. Further investigations are needed concerning the relation of drug-induced or metabolic encephalopathies and central benzodiazepine receptor activity. We recommend a therapeutic trial with flumazenil, if stupor or decreased seizure control develop in patients treated with valproate.

Effects of inhibition of ornithine aminotransferase on thioacetamide-induced hepatogenic encephalopathy

Repeated administration of thioacetamide (TAA) to CD1 mice produced hepatic failure and biochemical and behavioral effects characteristic of hepatogenic encephalopathy (HE). The symptoms in mice resembled those previously observed in rats after similar treatments. It is, however, obvious that both in rats and mice the severity of symptoms depends not only on dose and dosing schedule of TAA, but also on strain and body weight (age). Administration of 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase (OAT), significantly reduced mortality, and it ameliorated most of the TAA-induced pathologic symptoms, such as hypothermia, decreased locomotor and exploratory behavior, pathologic liver function and amino acid patterns. The most prominent biochemical consequence of 5FMOrn administration is the elevation of ornithine concentrations in tissues, including the brain, and in body fluids. Elevated ornithine concentrations are, therefore, the most likely basis for the therapeutic effects of 5FMOrn. In agreement with this notion is the enhancement of citrulline and urea formation. These findings and the observation that administration of ornithine in combination with a branched-chain 2-oxoacid ameliorated the pathologic symptoms of portal-systemic encephalopathy suggest inhibition of OAT in the treatment of this disease. The liver protective effect of 5FMOrn is not yet understood; the enhancement of regenerative processes is a likely explanation.

Do benzodiazepine ligands contribute to hepatic encephalopathy?

1. Levels of BZ receptor ligands are elevated in the brain of animal models of FHF and humans with FHF. Some of these ligands have agonist properties and some are known 1, 4-BZs which bind to the DS receptor. Much of the BZ receptor ligand activity in HE is unidentified and it is possible that some may bind to receptor subtypes other than the DS receptor. 2. Average levels of BZ receptor ligands in the brain in HE do not appear to be sufficient to augment GABAergic tone to a degree that would result in severe encephalopathy (i.e. coma). However, these ligands have a heterogeneous distribution in the brain and their neuroinhibitory effects may be potentiated by increased availability of GABA at GABAA receptors. Furthermore, that these ligands may contribute to HE is suggested by anecdotal reports of ameliorations of HE being induced in a majority of patients by the BZ receptor antagonist flumazenil. 3. The response of HE to flumazenil in humans is usually incomplete and in animal models may be modest. Potential explanations for these findings include pharmacokinetics, BZ receptor subtype specificity and higher levels of BZ receptor ligands in the brain in humans with HE than in animal models. 4. Certain BZ receptor ligands e.g. Ro 15-3505 and Ro 15-4513, that are structurally related to flumazenil, are more efficacious at ameliorating HE than flumazenil in animal models. These findings may be more dependent on differences in BZ receptor subtype specificity than differences in intrinsic activity. The properties of an ideal BZ receptor ligand for administration to a patient with HE would appear to be: (i) antagonist action at BZ receptors, (ii) no intrinsic activity apparent after a conventional pharmacologic dose, (iii) high specificity and affinity for BZ receptors, (iv) slow metabolism, and (v) absence of toxic effects. Promising ligands, such as Ro 15-3505, with weak partial inverse agonist actions and hence analeptic potential, require careful evaluation of their therapeutic index before clinical application. 5. BZ receptor ligands may be useful in the management of HE. Specifically, they may be given IV: (i) to reverse effects of exogenous BZs; (ii) to aid in the differential diagnosis of encephalopathy; (iii) to provide prognostic information; and (iv) to optimize brain function. They may also be given orally with the objective of reducing dietary protein intolerance in patients with chronic liver disease.

Recent developments in the pathophysiology and treatment of hepatic encephalopathy

The pathophysiology of HE has not yet been clarified. At present the main mechanisms under discussion are the combined effects of different toxins, such as ammonia, mercaptans, phenols and short- and medium-chain fatty acids, as well as a change particularly in GABAergic and glutamatergic neurotransmission. In this chapter the current views on the importance of these individual factors in the pathophysiology of HE are discussed; possible connections between changes in neurotransmission and the effect of different neurotoxins are presented. In addition, possible therapies resulting from recent knowledge of the pathophysiology of this disease are discussed, such as the use of Bz receptor antagonists.

Hepatic Encephalopathy

Hepatic encephalopathy occurs in a number of different species as a result of either congenital portacaval shunts or acquired liver disease. Despite intensive research, the neurochemical basis of the disorder has not been defined. Theories to explain the cerebral dysfunction that accompanies acute or chronic hepatic failure include 1) ammonia acting as the putative neurotoxin, 2) perturbed monoamine neurotransmission as a result of altered plasma amino acid metabolism, 3) an imbalance between excitatory amino acid neurotransmission, mediated by glutamate, and inhibitory amino acid neurotransmission, mediated by gamma-aminobutyric acid, and 4) increased cerebral concentrations of an endogenous benzodiazepine-like substance.

Modulation of thioacetamide-induced hepatocellular necrosis by prostaglandins is associated with novel histologic changes

Cytoprotective effects of the prostaglandins 16,16-dimethyl PGE2 (dmPGE2) and PGF2 alpha tromethamine (PGF2 alpha) were evaluated in the rat model of acute hepatocellular necrosis induced by thioacetamide (TAA). dmPGE2 (100 micrograms/kg SC 8 hourly) did not induce a significant increase in survival when started after the onset of TAA-induced fulminant hepatic failure. However, priming with dmPGE2 (100 micrograms/kg SC 30 min before TAA) reduced TAA-induced elevations in serum ALT (684 +/- 68 (SEM) vs 274 +/- 135 IU/1, p less than 0.01). This phenomenon did not occur if dmPGE2 was administered after TAA or by the IP route. Modulation of TAA-induced centrizonal hepatocellular necrosis by dmPGE2 was associated with a striking increase in centrizonal ballooning of hepatocytes (p less than 0.01), and, as assessed by stereology, less hepatocellular necrosis and degenerative changes. PGF2 alpha, which in contrast to dmPGE2 does not act via cAMP, had no effect on TAA-induced changes in serum ALT or hepatic histology. These findings suggest that dmPGE2 decreases hepatocellular necrosis by activating surface membrane adenylate cyclase and consequently stimulating cAMP. Ballooning of hepatocytes could occur secondary to these membrane events and appears to be a marker of dmPGE2-induced cytoprotection in this model.

Neurochemical studies of hepatic encephalopathy

Despite intensive research, the neurochemical basis of hepatic encephalopathy (HE) has not been defined. Theories that are currently favoured to explain the cerebral dysfunction that accompanies acute or chronic hepatic failure include: (1) ammonia acting as the putative neurotoxin; (2) perturbed monoamine neutrotransmission as a result of altered plasmo amino acid metabolism; (3) an imbalance between excitatory amino acid neurotransmission, mediated by glutamate, and inhibitory amino acid neurotransmission, mediated by gamma-aminobutyric acid (GABA); and (4) increased cerebral concentrations of an endogenous benzodiazepine-like substance. Studies of amino acid neurotransmitter receptors in HE have yielded conflicting results. The majority of studies in different animal models of acute and chronic HE and in patients have reported that brain GABA receptor density and affinity are unchanged. There have been fewer studies of excitatory amino acid receptors and these have also yielded conflicting results. However, the majority suggest that components of the glutamate receptor system are perturbed in HE. Further investigation is required to determine the significance of these findings to the pathogenesis of HE.

Beneficial effect of pharmacological modulation of the GABAA-benzodiazepine receptor on hepatic encephalopathy in the rat: comparison with uremic encephalopathy

Hepatic encephalopathy is ameliorated by drugs acting on the central GABAA-benzodiazepine receptor complex. To investigate whether these effects are specific for hepatic encephalopathy or just reflect a nonspecific arousal reaction, various benzodiazepine antagonists like flumazenil or with inverse agonistic properties (Ro 15-4513, Ro 15-3505) were studied in uremic encephalopathy in rats after bilateral ureteral ligation (n = 20) and compared with hepatic encephalopathy caused by thioacetamide-induced acute liver failure (n = 33). As soon as the animals developed clear signs of metabolic encephalopathy, their motor activity was recorded in an animal activity meter for 10 min. Furthermore, a composite score was calculated by grading various behavioral signs from 0 = absent to 3 = apparently normal. Rats were then injected with coded preparations of Ro 15-4513 (0.5, 2.5 and 5 mg/kg body wt intraperitoneally), flumazenil (2.5, 10, 25 and 40 mg/kg), Ro 15-3505 (10 mg/kg) or vehicle, and the measurements were repeated. The code was broken after the completion of the study. Pretreatment motor activity was decreased both in hepatic and uremic encephalopathy (20.7 +/- 6.4 [S.E.M.] and 41.3 +/- 37.1 movements/10 min). In hepatic encephalopathy motor activity and the composite score were improved both by 5 mg/kg Ro 15-4513 (by 293%, p less than 0.05) and by 10 mg/kg Ro 15-3505 (by 509%, p greater than 0.01), whereas vehicle and flumazenil had no effects. In uremic encephalopathy neither drug was effective in improving the neurobehavioral status.(ABSTRACT TRUNCATED AT 250 WORDS)

Flumazenil. A reappraisal of its pharmacological properties and therapeutic efficacy as a benzodiazepine antagonist

Flumazenil is a specific benzodiazepine antagonist which is indicated when the central effects of a benzodiazepine need to be attenuated or terminated. Following intravenous administration of up to 1 mg, flumazenil effectively reverses sedation and improves psychomotor performance following administration of short and longer acting benzodiazepines used for sedation, or general anaesthesia supplemented with benzodiazepines. The duration of action is short at generally 30 to 60 minutes and supplemental doses of flumazenil may be needed to maintain the desired level of consciousness in some patients. After poisoning with high dosages of benzodiazepines alone or combined with other drugs, the initial single dose of flumazenil will require supplementing with repeated low intravenous doses or an infusion to maintain wakefulness. In such patients, flumazenil also facilitates differential diagnosis and reduces the necessity for interventions. Flumazenil thus enhances recovery and allows more rapid discharge of patients sedated with benzodiazepines for diagnostic procedures and facilitates management of patients during the initial recovery period following general anaesthesia supplemented with benzodiazepines, but does not preclude normal monitoring during the recovery period. Flumazenil is clearly very useful in treating drug poisoning when benzodiazepines are a major component. By virtue of its specific benzodiazepine antagonist effects, flumazenil provides an innovative and well tolerated approach in clinical situations requiring rapid reversal of benzodiazepine-induced central nervous system depressant effects.

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.

Elevated brain concentrations of 1,4-benzodiazepines in fulminant hepatic failure

BACKGROUND

Increased gamma-aminobutyric acid (GABA) neurotransmission has been implicated in the pathogenesis of hepatic encephalopathy. The mechanism by which GABA-ergic activity is increased in hepatic failure is unclear, but recent studies in animals with encephalopathy due to fulminant hepatic failure suggest that GABA-ergic neurotransmission may be increased by the presence of elevated concentrations of benzodiazepine agonists such as diazepam and N-desmethyldiazepam.

METHODS AND RESULTS

Samples of frontal cortex were obtained at autopsy from 11 patients with hepatic encephalopathy who died of acetaminophen-induced fulminant hepatic failure and 8 patients who died of cardiovascular disease or trauma. None of the 19 patients had received benzodiazepines while hospitalized. Chromatographic analyses of extracts of these samples revealed 4 to 19 peaks representing substances that inhibited the binding of a radiolabeled imidazobenzodiazepine ([3H]flumazenil) to its receptors. Several of these peaks had retention times corresponding to those of known 1,4-benzodiazepines. Ultraviolet- and mass-spectroscopic analysis confirmed that two of these peaks represented diazepam and N-desmethyldiazepam. The patients who died of fulminant hepatic failure could be divided into two groups: six who had had significantly elevated brain concentrations (2-fold to 10-fold higher than normal) of substances inhibiting the binding of [3H]flumazenil and five who had normal concentrations.

CONCLUSIONS

Brain concentrations of substances inhibiting the binding of [3H]flumazenil to its receptors are increased in some patients with hepatic encephalopathy due to fulminant hepatic failure. The origin of these substances is unknown, but these findings provide a rational basis for trials of benzodiazepine-receptor antagonists in the management of this disorder.

The contribution of endogenous benzodiazepine receptor ligands to the pathogenesis of hepatic encephalopathy

The involvement of the gamma-aminobutyric acid A(GABAA) receptor complex in the pathogenesis of hepatic encephalopathy (HE) was examined in galactosamine-treated rabbits with HE caused by fulminant hepatic failure. Radioligand binding to the constituent components of the GABAA receptor complex was unchanged in rabbits with HE. However, partially purified extracts from encephalopathic rabbit brain were approximately three times more potent in inhibiting [3H]Ro 15-1788 binding to benzodiazepine (BZ) receptors than extracts from control rabbits. The inhibition of radioligand binding to the BZ receptor produced by these extracts was competitive and reversible and was significantly enhanced by GABA. Further purification of these extracts by high-performance liquid chromatography (HPLC) indicated that the inhibitory activity was localized in several peaks, some of which had retention times corresponding to 1,4-benzodiazepine standards. The presence of diazepam in these extracts was confirmed using mass spectroscopy. Both mass spectroscopic and radiometric techniques demonstrated that the concentration of diazepam in brain extracts from encephalopathic rabbits was approximately 4 times greater than control extracts. These findings link the presence of BZ receptor agonists to the development of a neuropathological condition, thereby providing a rational basis for the use of BZ receptor antagonists in the management of HE in man.

Gamma-aminobutyric acid (GABAA) receptor-function in a rat model of hepatic encephalopathy

The functional activity of the gamma-aminobutyric acid (GABAA) receptor-chloride ionophore complex was studied in rats with hepatic encephalopathy (HE) secondary to thioacetamide-induced fulminant hepatic failure (FHF). Muscimol stimulation and benzodiazepine potentiation of GABA receptor-mediated 36Cl- uptake into cerebral cortical synaptoneurosomes was compared in HE and control rats. [3H]Flumazenil binding assays were conducted to determine whether the levels of endogenous benzodiazepine-like ligands in extracts of cortex were increased with stages of encephalopathy in this animal model of HE. In both control and HE rats maximal uptake of 36Cl- via the GABAA receptor complex occurred at muscimol concentrations of 30 microM. Potentiation of muscimol-stimulated 36Cl- uptake into synaptoneurosomes by diazepam (5 microM) was equivalent in both groups. Aqueous extracts of proteolytically digested homogenates of cerebral cortices prepared from control and HE rats were effective in stimulating 36Cl- uptake into synaptoneurosomes. Alkaline organic extracts of proteolytically digested homogenates of cerebral cortices from HE rats were more effective than corresponding extracts from controls at inhibiting the binding of [3H]flumazenil. Inhibition of [3H] fumazenil binding by organic extracts derived from the cerebral cortices of HE rats did not increase with progression of encephalopathy. The results show that muscimol-stimulated 36Cl- uptake into synaptoneurosomes and, consequently, GABAA receptor-mediated chloride channel function are not significantly altered in the model of HE studied and are consistent with the hypothesis that HE results in an increased availability of one or more endogenous ligands which can augment GABA receptor-gated chloride conductance.

Endogenous benzodiazepine receptor ligands in human and animal hepatic encephalopathy

The role of endogenous benzodiazepine receptor ligands in the pathogenesis of hepatic encephalopathy was studied in humans and in rat models of hepatic encephalopathy. Endogenous benzodiazepine ligands were extracted from rat brain and human CSF by acid treatment and purification by HPLC. Detection and partial characterization of these endogenous benzodiazepine ligands were carried out using both radioreceptor binding assays and radioimmunoassays with anti-benzodiazepine antibodies. Four different benzodiazepine receptor ligands were identified in human and rat tissue, two of which may be diazepam and desmethyldiazepam, based on elution profiles and anti-benzo-diazepine antibody reactivity. Human CSF and serum from patients with hepatic encephalopathy contained approximately 10 times more endogenous benzodiazepine receptor ligand than CSF from controls or nonencephalopathic patients with liver disease. The levels of brain benzodiazepine receptor ligand compounds were also increased approximately 10-fold in rats suffering from fulminant hepatic failure, but not in rats with portacaval shunts, a model of chronic hepatic disease. The increased concentrations of these substances could be behaviorally significant and may contribute to the pathogenesis of hepatic encephalopathy.

Brain concentrations of benzodiazepines are elevated in an animal model of hepatic encephalopathy

Brain extracts from rats with hepatic encephalopathy due to thioacetamide-induced fulminant hepatic failure contained 4- to 6-fold higher concentrations of substances that inhibit radioligand binding to benzodiazepine receptors than corresponding control rat extracts. Both isocratic and gradient-elution HPLC indicated that this inhibitory activity was localized in 3-8 peaks with retention times corresponding to deschlorodiazepam, deschlorolorazepam, lorazepam, oxazepam, diazepam, and N-desmethyldiazepam. The presence of diazepam and N-desmethyldiazepam was confirmed by mass spectroscopy. Both mass spectroscopic and radiometric techniques indicated that the concentrations of N-desmethyldiazepam and diazepam in brain extracts from encephalopathic rats were 2-9 and 5-7 times higher, respectively, than in control brain extracts. While benzodiazepines have been identified previously in mammalian and plant tissues, this report demonstrates that concentrations of these substances are increased in a pathophysiological condition. These findings provide a rational basis for the use of benzodiazepine receptor antagonists in the management of hepatic encephalopathy in humans.

Hepatic encephalopathy, GABA-ergic neurotransmission and benzodiazepine receptor ligands

Evidence compatible with increased GABAergic tone contributing to the manifestations of hepatic encephalopathy (HE) in animal models of fulminant hepatic failure (FHF) includes: (i) increased resistance to drugs which induce seizures by reducing GABAergic tone; (ii) abnormalities of visual evoked responses (VERs) which resemble those induced by drugs which augment GABAergic tone; (iii) increased sensitivity of CNS neurons to a GABA agonist; and (iv) ameliorations of the encephalopathy induced by a GABA receptor antagonist. Evidence compatible with a benzodiazepine (BZ) receptor ligand with agonist properties contributing to increased GABAergic tone in animal models of FHF includes: (i) abnormalities of VERs which resemble those in BZ agonist-induced coma; (ii) increased sensitivity of CNS neurons to a BZ receptor agonist; (iii) excitation of CNS neurons induced by BZ receptor antagonists; (iv) reversal of the increased sensitivity of CNS neurons to a GABA agonist by a BZ receptor antagonist; (v) presence of a ligand(s) in brain which displaces a radiolabeled ligand from BZ receptors; and (vi) increased affinity of this ligand(s) for BZ receptors in the presence of GABA ("positive GABA shift").