Chronic hyperammonemia alters protein phosphorylation and glutamate receptor-associated signal transduction in brain

Hepatic encephalopathy: how to test and treat

PURPOSE OF REVIEW

Hepatic encephalopathy causes significant cognitive impairment and morbidity in patients with cirrhosis; however, hepatic encephalopathy is considered a reversible syndrome once recognized clinically. Although hepatic encephalopathy is not a single clinical entity, the pathophysiology resulting in brain dysfunction is not fully understood, although it is believed that ammonia production is an important contributing factor. The purpose of this review is to highlight studies used to test for hepatic encephalopathy and those utilizing specific new treatments.

RECENT FINDINGS

A 'STROOP' smartphone app has been developed to allow clinicians to test for covert hepatic encephalopathy (CHE). Lactulose therapy was effective for cirrhotic patients as primary prophylaxis to prevent overt hepatic encephalopathy (OHE) episodes. In patients without prior OHE, probiotics can be useful in preventing OHE. Lactulose, probiotics, L-ornithine-L-aspartate, and potassium-iron-phosphate-citrate have been studied in the treatment of CHE. Rifaximin was found to be safe and well tolerated in long-term maintenance of remission from OHE; however, compared to lactulose therapy in CHE, it is not cost-effective.

SUMMARY

Refinement in clinical management strategies for patients with cirrhosis and hepatic encephalopathy appears to continue to contribute to improved patient outcomes.

Effect of the class I metabotropic glutamate receptor antagonist AIDA on certain behaviours in rats with experimental chronic hyperammonemia

PURPOSE

This study examines possible interactions between behavioral effects and mGluR1 (class I metabotropic glutamate receptor) by injecting AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] in rats with experimental chronic hyperammonemia (chHA).

MATERIAL/METHODS

The effects of mGluR1 antagonist on some behaviors were tested in control groups of rats and in rats with chHA. Experimental chHA was induced by intraperitoneal injection of ammonium acetate (12 mmol/kg) for five consecutive days. We used the following behavioural tests: the open field test, the passive avoidance test and the elevated "plus" maze.

RESULTS

In control rats AIDA administered intracerebroventricularly (i.c.v.) at the dose 100 nmol decreased the number of crossings and bar approaches in the open field test and impaired acquisition and recall in the passive avoidance situation. ChHA significantly inhibited locomotor and exploratory activity and profoundly impaired acquisition and recall processes in the passive avoidance test and significantly increased acute stress responses. AIDA increased locomotor activity in chHA rats (especially number of crossed fields and rearings) and produced anxiety enhancement in rats with chHA. AIDA used in rats with chHA significantly improved acquisition and retrieval processes.

CONCLUSIONS

The obtained results suggest that AIDA, the antagonist of mGluR1, had beneficial effects on learning and memory in rats with experimental chronic hyperammonemia.

Hepatic encephalopathy: pathophysiology and emerging therapies

Hepatic encephalopathy is characterized by neuropsychiatric abnormalities in patients with liver failure. Severe hepatic encephalopathy is an indication for liver transplantation as it portends poor outcome. Treatment of hepatic encephalopathy involves correction of precipitating factors such as sepsis, gastrointestinal bleeding, medications, and electrolyte imbalance. Effective therapies include lactulose and antibiotics such as neomycin, metronidazole, and rifaximin.

Altered modulation of motor activity by group I metabotropic glutamate receptors in the nucleus accumbens in hyperammonemic rats

One of the neurological complications in hepatic encephalopathy is the impairment of motor coordination and function. Clinical signs of basal ganglia, cortico-spinal and cerebellar dysfunction have been commonly detected in these patients. We are studying the molecular bases of the alterations in motor coordination and function in hepatic encephalopathy. Hyperammonemia is considered the main factor responsible for the neurological alterations in patients with hepatic encephalopathy. Activation of metabotropic glutamate receptors (mGluRs) in the nucleus accumbens (NAcc) induces locomotion in rats. Asa first step in our studies on the alterations in motor co-ordination and function in hyperammonemia and hepatic encephalopathy we studied whether the control of motor function by mGluRs in the NAcc is altered in hyperammonemic rats. The locomotor activity induced by injection into the nucleus accumbens (NAcc) of DHPG, an agonist of group I mGluRs was significantly increased in hyperammonemic rats. Injection of DHPG increased extracellular dopamine but not glutamate in the NAcc of control rats. In hyperammonemic rats DHPG-induced increase in dopamine was significantly reduced, and extracellular glutamate increased 6-fold. The content of mGluR 1 but not mGluR 5, is increased in the NAcc of hyperammonemic rats. Blockade of mGluR 1 completely prevented motor and neurochemical effects induced by DHPG. These results show that modulation of both motor function and extracellular concentration of neurotransmitters by mGluRs in the NAcc is altered in hyperammonemia. This may contribute to the alterations in motor function in hepatic encephalopathy.

Hepatic encephalopathy: a dynamic or static condition

Hepatic encephalopathy (HE) is a neuropsychiatric disorder associated with portal hypertension. The mechanism of this disorder is still being characterized and the management has relied primarily on lowering the amount of ammonia present in the gastrointestinal tract or reversing liver disease by replacing the diseased liver. It is, however, not established that all the effects of hepatic encephalopathy are reversed by liver transplantation. In this review, we have outlined the mechanisms underlying HE and the pros and cons of reversibility of HE.

Recovery from parkinson syndrome and prolonged visually evoked potentials in hepatic encephalopathy

The combination of Parkinson syndrome (PS) and prolonged visually evoked potentials (VEPs) in a single patient with hepatic encephalopathy (HE) has not been reported. A 52-year-old male with chronic HE developed PS in early 2001. Treatment with L-DOPA was only of minimal effect. In August 2001 he was admitted because of worsening PS and HE. There was anemia, hyperlipidaemia, markedly elevated liver-function-parameters, hyperammonemia, elevated resting-lactate, steatosis hepatis and hepatomegalia. VEPs showed markedly prolonged P100-latencies. Under L-DOPA, pramipexol, L-ornithin-L-aspartate, paromomycin-sulfate, and lactulose liver-function-parameters normalized and PS markedly improved. In February 2003, VEPs were normal. L-DOPA was discontinued by the patient in April 2003 and pramipexol in December 2003. Since then PS did not recur. This case shows that HE may go along with reversible PS and prolonged VEPs. Under adequate therapy liver-function-parameters and VEPs normalize and PS disappears.

Ammonia toxicity to the brain and creatine

Symptoms of hyperammonemia are age-dependent and some are reversible. Multiple mechanisms are involved. Hyperammonemia increases the uptake of tryptophan into the brain by activation of the L-system carrier while brain glutamine plays a still undefined role. The uptake of tryptophan by the brain is enhanced when the plasma levels of branched-chain amino acids competing with the other large neutral amino acids are low. Hyperammonemia increases the utilization of branched-chain amino acids in muscle when ketoglutarate is low, and this is further enhanced by glutamine depletion (as a result of therapy with ammonia scavengers like phenylbutyrate). Anorexia, most likely a serotoninergic symptom, might further aggravate the deficiency of indispensable amino acids (e.g., branched-chain and arginine). The role of increased glutamine production in astrocytes and the excitotoxic and metabotropic effects of increased extracellular glutamate have been extensively investigated and found to differ between models of acute and chronic hyperammonemia. Using an in vitro model of cultured embryonic rat brain cell aggregates, we studied the role of creatine in ammonia toxicity. Cultures exposed to ammonia before maturation showed impaired cholinergic axonal growth accompanied by a decrease of creatine and phosphocreatine, a finding not observed in mature cultures. By using different antibodies, we have shown that the phosphorylated form of the intermediate neurofilament protein is affected. Adding creatine to the culture medium partially prevents impairment of axonal growth and the presence of glia in the culture is a precondition for this protective effect. Adequate arginine substitution is essential in the treatment of urea cycle defects as creatine is inefficiently transported into the brain.

Chronic hyperammonemia alters motor and neurochemical responses to activation of group I metabotropic glutamate receptors in the nucleus accumbens in rats in vivo

Hyperammonemia leads to altered cerebral function and neurological alterations in patients with hepatic encephalopathy. We studied the effects of hyperammonemia in rats on the modulation by group I metabotropic glutamate receptors (mGluR) of motor and neurochemical functions in vivo. Locomotion induced by injection of the mGluR agonist DHPG into nucleus accumbens was increased in hyperammonemic rats. In control rats DHPG increased extracellular dopamine (ca. 400%) but not glutamate. In contrast, in hyperammonemic rats DHPG increased extracellular glutamate (ca. 600%), while DHPG-induced dopamine increase was reduced. Blocking mGluR1 receptor with CPCCOEt prevented all DHPG effects, indicating that this receptor mediates its locomotor and neurochemical effects. Hyperammonemic rats showed increased (32%) mGluR1alpha, but not mGluR5 content in nucleus accumbens. These results show that modulation of locomotor and neurochemical functions by mGluRs in nucleus accumbens is strongly altered in hyperammonemia. These alterations may contribute to the neurological alterations in hyperammonemia and liver failure.

Kynurenic acid synthesis in cerebral cortical slices of rats with progressing symptoms of thioacetamide-induced hepatic encephalopathy

Increased ammonia is a major pathogenic factor in hepatic encephalopathy (HE), a neurologic syndrome associated with glutamatergic dysfunction. Previous studies have shown that in rat cerebral cortical slices or a glia-derived cell line, acute treatment with ammonia in vitro and in vivo inhibits the production of a broad-spectrum antagonist of excitatory amino acid receptors, kynurenic acid (KYNA). The present study analyzed KYNA synthesis in cerebral cortical slices obtained from rats with progressing HE symptoms accompanying acute liver failure induced by one, two, or three intraperitoneal administrations of thioacetamide (TAA) at 24-hr intervals. KYNA synthesis was found decreased to 83% of control 24 hr after one administration of TAA and unaffected after two TAA injections, when moderate hyperammonemia was associated by metabolic and bioelectric activation of the central nervous system, but was not accompanied by typical HE symptoms. KYNA synthesis was elevated to 155% of control after three TAA administrations, a period in which the rats showed advanced HE symptoms including stupor or coma. KYNA synthesis at the advanced HE stage was inhibited by glutamate in a degree comparable to that observed in control slices. The elevation of KYNA synthesis was associated with increased activity of a kynurenine aminotransferase (KAT) isomer, KAT-II. KYNA synthesis did not differ from control 21 days after the third TAA administration when HE symptoms receded. The results suggest that alterations of KYNA synthesis may contribute to the imbalance between neural excitation and inhibition at different stages of HE.

Characterization of N-methyl-D-aspartate receptor subunits involved in acute ammonia toxicity

Rapid administration of large doses of ammonia leads to death of animals, which is largely prevented by pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists. The present study focuses on a subunit(s) of NMDA receptor involved in ammonia-induced death by use of NMDA receptor GluRepsilon subunit-deficient (GluRepsilon(-/-)) mice and the selective GluRepsilon2 antagonist CP-101,606. Acute ammonia intoxication was induced in mice (eight per group) by a single intraperitoneal (i.p.) injection of ammonium chloride. Appearance of neurological deteriorations depended on the doses of ammonium chloride injected. While wild-type, GluRepsilon1(-/-), GluRepsilon4(-/-), and GluRepsilon1(-/-)/epsilon4(-/-) mice all died by ammonium chloride at 12 mmol/kg during the first tonic convulsions, two of eight GluRepsilon3(-/-) mice survived. Pretreatment of wild-type mice with CP-101,606 prevented two mice from ammonia-induced death. Pretreatment of GluRepsilon3(-/-) mice with CP-101,606 prevented the death of three mice and prolonged the time of death of non-survivors. Similarly, the neuronal form of nitric oxide synthase (NOS) inhibitor 7-nitroindazole (7-NI) as well as the nonselective NOS inhibitor L-NMMA, but not the inducible NOS inhibitor 1400W, partially prevented the death of mice and prolonged the period of death. Furthermore, ammonium chloride prolonged the increase in intracellular free Ca2+ concentration ([Ca2+]i) and subsequent NO production induced by NMDA in the cerebellum. These results suggest that activation of NMDA receptor containing GluRepsilon2 and GluRepsilon3 subunits and following activation of neuronal NOS are involved in acute ammonia intoxication which leads to death of animals.