Monoamines and metabolites in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

Tyrosine uptake and regional brain monoamine metabolites in a rat model resembling congenital hyperammonemia

Hyperammonemia found in congenital disorders has a toxic effect on the central nervous system. Disturbances of brain neurotransmitter metabolism have been proposed, such as an increased transport of tryptophan into the brain and an increased flux through the serotonin pathway. Results concerning the catecholamine pathway are, however, contradictory. We therefore studied whether hyperammonermia increases brain uptake of the neurotransmitter precursor amino acid tyrosine and whether these changes affect the concentration of neurotransmitters and their metabolites in different brain areas (frontal cortex, caudatus-putamen, thalamus, hypothalamus, hippocampus/substantia nigra, brainstem) of rats made hyperammonemic with urease. The brain uptake of tyrosine was measured in the forebrain, brainstem, and cerebellum. The brain areas were analyzed for dopamine, 3,4-hydroxyphenylacetic acid; homovanillic acid, norepinephrine, and vanillylmandelic acid. The brain uptake index of tyrosine was increased in the forebrain and brainstem of the hyperammonemic rats with concomitantly elevated concentrations in the forebrain of tyrosine, phenylalanine, and tryptophan. The homovanillic acid content was significantly increased in the hypothalamus, hippocampus/substantia nigra and brainstem. The concentrations of norepinephrine, dopamine, and 3, 4-hydroxyphenylacetic acid were not significantly changed. Vanillylmandellic acid was decreased in the caudatus-putamen, thalamus, and hypothalamus. The data indicate an undisturbed neurotransmitter synthesis and, taken with the augmented tyrosine uptake at the blood-brain barrier, an increased flux through the dopamine pathway. These changes observed in the hyperammonemic animal model could contribute to the understanding of the pathogenic mechanisms and offer an explanation for the neuropsychiatric disturbances observed in children with congenital hyperammonemia.

Manganese toxicity, dopaminergic dysfunction and hepatic encephalopathy

Patients with chronic liver disease manifest a high incidence (> 75%) of pallidal signal hyperintensity on T1-weighted Magnetic Resonance Imaging (MRI), the intensity of which correlates with blood manganese levels and the presence of extrapyramidal symptoms. A major cause of pallidal hyperintensity on T1-weighted MRI is manganese deposition; chronic manganese intoxication in the absence of liver disease results in pallidal MR signal hyperintensity, in extrapyramidal symptoms and in selective effects on the dopaminergic neurotransmitter system in basal ganglia. Direct measurements in globus pallidus obtained at autopsy from patients with chronic liver disease who died in hepatic coma reveal 2 to 7-fold increases of pallidal manganese and a concomitant loss of dopamine D2 binding sites. Liver transplantation results in normalization of pallidal MR signals and of blood manganese levels. These findings suggest that (1) pallidal MR signal hyperintensity in patients with chronic liver disease is the result of manganese deposition and (2) alterations of dopaminergic function due to the toxic effects of manganese may contribute to the extrapyramidal symptoms in these patients.

Effect of probenecid on 5-hydroxyindoleacetic acid in cisternal cerebrospinal fluid of rats with portacaval anastomosis

Portal-systemic encephalopathy (PSE) is characterized by a neuropsychiatric disorder progressing through personality changes, to stupor and coma. Previous studies have revealed alterations of serotonin and of its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in brain tissue and CSF in experimental (rat) and human PSE. Increased brain 5-HIAA concentrations could result from its decreased removal rather than to increased serotonin metabolism. In order to evaluate this possibility, CSF 5-HIAA concentrations were measured using an indwelling cisterna magna catheter technique at various times following end-to-side portacaval anastomosis in rats (the most widely used animal model of PSE) treated with probenecid, a competitive inhibitor that blocks the active transport of acid metabolites out of the brain and CSF. Following portacaval anastomosis and probenecid treatment, CSF concentrations of 5-HIAA were increased to a greater extent than in sham-operated controls. When data were expressed as per-cent baseline values, the relative increase of CSF 5-HIAA in portacaval shunted rats following probenecid treatment was not significantly different from sham-operated controls. These findings confirm that increased 5-HIAA in the CNS in experimental PSE results from increased 5HT metabolism or turnover and that the probenecidsensitive acid metabolite carrier is intact in PSE.

The relationship between plasma and brain quinolinic acid levels and the severity of hepatic encephalopathy

BACKGROUND/AIMS

Quinolinic acid is an endogenous neuroexcitant derived from tryptophan. Brain quinolinic acid concentrations are reportedly elevated in chronic liver failure. The aim of this study was to determine if brain quinolinic acid levels correlate with the severity of hepatic encephalopathy.

METHODS

Postmortem samples of selected brain regions and plasma samples taken at several stages of encephalopathy were obtained from patients with acute and chronic liver failure. Quinolinic acid levels were measured by mass spectroscopy using [18O]quinolinic acid.

RESULTS

Plasma quinolinic acid levels were significantly increased by stage I encephalopathy in patients with acute liver failure and by stages II and III in patients with chronic liver failure. Brain quinolinic acid levels were elevated only in patients with acute liver failure and were uniformly distributed at concentrations below those observed in plasma.

CONCLUSIONS

The uniform distribution of quinolinic acid at subplasma concentrations in the brains of patients with acute liver failure suggests that it is synthesized peripherally and enters the brain across a permeabilized blood-brain barrier. Whereas the elevation of brain quinolinic acid levels in patients who died of acute but not chronic liver failure suggests that the involvement of quinolinic acid in the pathogenesis of hepatic encephalopathy is minimal, it could predispose these patients to seizures.

Trace amines in hepatic encephalopathy

In 1971 Fischer and Baldessarini proposed the hypothesis that hepatic encephalopathy (HE), a neuropsychiatric syndrome associated with hepatic dysfunction, could result from the direct decarboxylation of amino acids leading to trace amines such as tyramine and octopamine which could then act as false neurotransmitters. This was supported by the observation that the clinical symptoms of HE appeared to improve following treatment with L-Dopa, which cannot be metabolized to either of these trace amines. In addition to serum and urine levels of octopamine correlating roughly with the grade of clinical HE, levels of octopamine were also significantly increased in rat brain following coma induced by hepatic devascularization and in portacaval-shunted rats fed high aromatic amino acid content diets. This hypothesis was questioned, however, given the lack of observable adverse behavioural effects following treatments with octopamine. Finally, the equivocal results of a limited number of clinical trials (using L-Dopa) argued against a direct intervention by catecholamine-like trace amines in HE. An alternative hypothesis was advanced by Sourkes in 1978 implicating increased tryptophan metabolism as a factor in the etiology of HE. Hepatic dysfunction in humans alters CNS concentrations of tryptophan which correlate well with levels of the tryptamine metabolite indoleacetic acid (IAA). Furthermore, regional densities of [3H]tryptamine receptors in HE patient brain tissue are significantly decreased. These data support a pathophysiologic role for tryptophan and its neuroactive trace amine metabolite tryptamine in HE.

Regional alterations of dopamine and its metabolites in rat brain following portacaval anastomosis

Hyperammonemia and changes in brain monoamine metabolism have been proposed to contribute to the pathogenesis of the neuropsychiatric symptoms characteristic of human portal-systemic encephalopathy (PSE) resulting from chronic liver disease. Portacaval anastomosis (PCA) in the rat leads to sustained hyperammonemia and mild encephalopathy. In order to evaluate the role of dopamine (DA) metabolism in PSE, levels of DA and its metabolites were measured by HPLC with electrochemical detection in brain regions of rats with PCA at various stages of encephalopathy precipitated by ammonium acetate administration. Following ammonium acetate administration, rats with PCA rapidly develop severe neurological signs of encephalopathy progressing through loss of righting reflex to coma; sham-operated control animals administered ammonium acetate showed no such neurological deterioration. Concentrations of the DA metabolites DOPAC and HVA as well as [DA metabolites]/[DA] ratios, an indirect measure of DA turnover in brain, were increased in caudate-putamen, in cingulate and pyriform entorhinal cortices as well as in raphe nucleus and locus coeruleus. Increased DA metabolites, however, did not worsen at coma stages of PSE. Increased DA turnover thus appears to relate to early neuropsychiatric and extrapyramidal symptoms of PSE.

Choline acetyltransferase and acetylcholinesterase activities are unchanged in brain in human and experimental portal-systemic encephalopathy

Activities of choline acetyltransferase, acetylcholinesterase and butyrylcholinesterase were studied in the frontal cortex, temporal cortex, cerebellum and caudate nucleus obtained at autopsy from eight alcoholic cirrhotic patients who died in hepatic coma and from an equal number of age-matched subjects free from hepatic, neurological or psychiatric disorders. Activities of these enzymes were unaltered in the brains of cirrhotics compared to controls. Choline acetyltransferase and cholinesterase activities were also studied in the cerebral cortex, cerebellum, brain stem and striatum of rats four weeks following portacaval anastomosis and their sham-operated controls. Portacaval-shunting did not cause any statistically significant differences in the activities of choline acetyltransferase, acetyl or butyrylcholinesterases. These results argue against a presynaptic cholinergic lesion in human and experimental portal-systemic encephalopathy.

Alterations of [3H]8-OH-DPAT and [3H]ketanserin binding sites in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

Affinities and densities of binding sites for the 5HT1A receptor ligand [3H]8-hydroxy(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and the 5HT2 receptor ligand [3H]ketanserin were measured using a rapid filtration assay in crude membrane preparations from frontal cortex and hippocampus of nine cirrhotic patients who died in hepatic encephalopathy and from an equal number of age-matched subjects free from hepatic, neurological or psychiatric disorders. Binding site densities (Bmax) obtained by Scatchard analysis of saturation binding isotherms for [3H]8-OH-DPAT were decreased in frontal cortex (by 56%, P < 0.05) and hippocampus (by 30%, P < 0.05). [3H]ketanserin binding sites were concomitantly increased (by 55%, P < 0.05) in hippocampus of cirrhotic patients. Ligand binding affinities were within normal ranges in all cases. Previous reports have described the accumulation of the 5HT metabolite 5-hydroxyindoleacetic acid and increased activities of the 5HT-metabolizing enzyme MAOA in this same material from patients with hepatic encephalopathy. Taken together, these findings suggest that alterations of serotoninergic function in brain could be responsible for some of the neuropsychiatric symptoms of hepatic encephalopathy observed in humans with chronic liver disease.

Activities of monoamine oxidase-A and -B are altered in the brains of congenitally hyperammonemic sparse-fur (spf) mice

Activities of monoamine oxidases, MAOA and MAOB, were measured using radiometric assays in different brain regions of the sparse-fur (spf/Y) mouse, a model of congenital hyperammonemia resulting from an X-chromosomal defect of ornithine transcarbamylase. MAOA activities were decreased in cerebellum (by 23%, P < 0.05) and brainstem (by 16%, P < 0.05) of spf mice; activities of MAOB were concomitantly increased in cerebellum (by 22%, P < 0.05), brainstem (by 20%, P < 0.05) and cerebral cortex (by 22%, P < 0.05). These findings offer a rational explanation for previous findings of increased acidic metabolites of monoamines in the brain of spf mice. Altered monoaminergic function could be a key factor in the pathogenesis of neurological dysfunction in congenital hyperammonemias.

Selective loss of pallidal dopamine D2 receptor density in hepatic encephalopathy

The binding parameters of [3H]SCH 23390 and [3H]spiperone (radioligands for dopamine D1 and D2 receptors, respectively) were investigated in autopsied frontal cortex, caudate nucleus and globus pallidus/putamen of cirrhotic patients who died in hepatic coma as well as in age- and sex-matched controls. Specific [3H]SCH 23390 binding site densities were unchanged in all regions; in contrast, specific [3H]spiperone binding site density was decreased (by 44%, P < 0.001) in the globus pallidus/putamen of patients with HE. Decreased densities of pallidal D2 binding sites could relate to the motor dysfunctions commonly encountered in human HE.

Increased activities of MAOA and MAOB in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

Using radioenzymatic assays, activities of MAOA and MAOB were measured in autopsied brain tissue from cirrhotic patients who died in hepatic coma and in material from an equal number of age-matched subjects who were free from hepatic, neurological or psychiatric disorders. Activities of both MAOA and MAOB were significantly increased in frontal cortex and caudate nucleus, two brain regions shown previously to be the site of functional and morphological alterations of astrocytes and increased concentrations of the acid metabolites of dopamine and serotonin. These findings suggest that increased monoamine metabolism and subsequent modifications of monoaminergic synaptic function could contribute to the pathogenesis of hepatic encephalopathy.

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.

Psychiatric aspects of liver disease

This paper reviews the literature on the effects of liver disease on mental health, a topic which has been relatively neglected in the recent psychiatric literature. It discusses both the encephalopathy which may be associated with liver disease of almost any type and the psychological consequences of specific liver disorders. Also considered are the effects of liver disease on sexual function; the relationship between alcohol and hepatic disorder in causing mental disturbance; the effects of childhood liver disease; psychiatric aspects of liver transplantation; and the use of psychotropic drugs in patients with hepatic dysfunction.

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.

Effect of ammonia on brain serotonin metabolism in relation to function in the portacaval shunted rat

Four weeks following portacaval anastomosis (PCA) in the rat, severe liver atrophy, sustained hyperammonemia, and increased plasma and brain tryptophan are observed. Administration of ammonium acetate (NH4Ac) to rats with PCA precipitates severe signs of hepatic encephalopathy (HE) (loss of righting reflex progressing to loss of consciousness and ultimately deep coma). To evaluate the relationship between the deterioration of neurological status in HE and serotonin (5-HT) metabolism, the levels of 5-HT, its precursor 5-hydroxytryptophan, and its major metabolite 5-hydroxy-indole-3-acetic acid (5-HIAA) were measured by HPLC with ion-pairing and electrochemical detection in three well-defined areas of the cerebral cortex: anterior cingulate, piriform and entorhinal, and frontoparietal; as well as in the caudate-putamen, the raphe nuclei, and the locus ceruleus in rats with PCA at different stages of HE, before and after injection of NH4Ac, as well as in sham-operated controls. The results demonstrate increased 5-HIAA/5-HT ratios after PCA and NH4Ac loading, suggesting increased 5-HT turnover in the brains of these animals. However, these changes do not appear to be related to the precipitation of coma as no significant difference in 5-HT turnover was observed between precoma and coma stages of HE. Increased 5-HT turnover in brain of shunted rats may be related to early symptoms of HE such as altered sleep patterns and disorders of motor coordination.