CNS tryptamine metabolism in hepatic coma

"The quantitative determination of indolic microbial tryptophan metabolites in human and rodent samples: A systematic review"

Concentrations reported for indolic microbial metabolites of tryptophan in human and rodent brain, cerebrospinal fluid, plasma, saliva and feces were compiled and discussed. A systematic review of the literature was accomplished by key word searches of Pubmed, Google Scholar and the Human Metabolome Data Base (HMDB), and by searching bibliographies of identified publications including prior reviews. The review was prompted by the increasing appreciation of the physiological importance of the indolic compounds in human health and disease. The compounds included were indoleacetic acid (IAA), indole propionic acid (IPA), indoleacrylic acid (IACR), indolelactic acid (ILA) indolepyruvic acid (IPY), indoleacetaldehyde (IAALD), indolealdehyde (IALD), tryptamine (TAM), indole (IND) and skatole (SKT). The undertaking aimed to vet and compare existing reports, to resolve apparent discrepancies, to draw biological inferences from the consideration of multiple analytes across sample types, to survey the analytical methodologies used, and to point out areas in need of greater attention.

Hypoxia Routes Tryptophan Homeostasis Towards Increased Tryptamine Production

The liver is the central hub for processing and maintaining homeostatic levels of dietary nutrients especially essential amino acids such as tryptophan (Trp). Trp is required not only to sustain protein synthesis but also as a precursor for the production of NAD, neurotransmitters and immunosuppressive metabolites. In light of these roles of Trp and its metabolic products, maintaining homeostatic levels of Trp is essential for health and well-being. The liver regulates global Trp supply by the immunosuppressive enzyme tryptophan-2,3-dioxygenase (TDO2), which degrades Trp down the kynurenine pathway (KP). In the current study, we show that isolated primary hepatocytes when exposed to hypoxic environments, extensively rewire their Trp metabolism by reducing constitutive Tdo2 expression and differentially regulating other Trp pathway enzymes and transporters. Mathematical modelling of Trp metabolism in liver cells under hypoxia predicted decreased flux through the KP while metabolic flux through the tryptamine branch significantly increased. In line, the model also revealed an increased accumulation of tryptamines under hypoxia, at the expense of kynurenines. Metabolic measurements in hypoxic hepatocytes confirmed the predicted reduction in KP metabolites as well as accumulation of tryptamine. Tdo2 expression in cultured primary hepatocytes was reduced upon hypoxia inducible factor (HIF) stabilisation by dimethyloxalylglycine (DMOG), demonstrating that HIFs are involved in the hypoxic downregulation of hepatic Tdo2. DMOG abrogated hepatic luciferase signals in Tdo2 reporter mice, indicating that HIF stability also recapitulates hypoxic rewiring of Trp metabolism in vivo. Also in WT mice HIF stabilization drove homeostatic Trp metabolism away from the KP towards enhanced tryptamine production, leading to enhanced levels of tryptamine in liver, serum and brain. As tryptamines are the most potent hallucinogens known, the observed upregulation of tryptamine in response to hypoxic exposure of hepatocytes may be involved in the generation of hallucinations occurring at high altitude. KP metabolites are known to activate the aryl hydrocarbon receptor (AHR). The AHR-activating properties of tryptamines may explain why immunosuppressive AHR activity is maintained under hypoxia despite downregulation of the KP. In summary our results identify hypoxia as an important factor controlling Trp metabolism in the liver with possible implications for immunosuppressive AHR activation and mental disturbances.

Phytohormones: Multifunctional nutraceuticals against metabolic syndrome and comorbid diseases

Metabolic syndrome is characterized by the co-occurrence of diverse symptoms initiating the development of type 2 diabetes, cardiovascular diseases, and a variety of comorbid diseases. The complex constellation of numerous comorbidities makes it difficult to develop common therapeutic approaches that ameliorate these pathological features simultaneously. The plant hormones abscisic acid, salicylic acid, auxin, and cytokinins, have shown promising anti-inflammatory and pro-metabolic effects that could mitigate several disorders relevant to metabolic syndrome. Intriguingly, besides plants, human cells and gut microbes also endogenously produce these molecules, indicating a role in the complex interplay between inflammatory responses associated with metabolic syndrome, the gut microbiome, and nutrition. Here, we introduce how bioactive phytohormones can be generated endogenously and through the gut microbiome. These molecules subsequently influence immune responses and metabolism. We also elaborate on how phytohormones can beneficially modulate metabolic syndrome comorbidities, and propose them as nutraceuticals.

Tryptophan Metabolism: A Versatile Area Providing Multiple Targets for Pharmacological Intervention

The essential amino acid L-tryptophan (Trp) undergoes extensive metabolism along several pathways, resulting in production of many biologically active metabolites which exert profound effects on physiological processes. The disturbance in Trp metabolism and disposition in many disease states provides a basis for exploring multiple targets for pharmaco-therapeutic interventions. In particular, the kynurenine pathway of Trp degradation is currently at the forefront of immunological research and immunotherapy. In this review, I shall consider mammalian Trp metabolism in health and disease and outline the intervention targets. It is hoped that this account will provide a stimulus for pharmacologists and others to conduct further studies in this rich area of biomedical research and therapeutics.

Development of a three-factor neuropsychological approach for detecting minimal hepatic encephalopathy

BACKGROUND

Minimal hepatic encephalopathy (HE) has profoundly negative effects on daily functioning ad quality of life. However, standard psychometric procedures have not been widely incorporated into efforts to develop a neuropsychological battery for this condition.

AIMS

To establish the construct and diagnostic validity of a neuropsychological approach for the recognition of minimal HE in patients with cirrhosis.

METHODS

A comprehensive battery of neuropsychological tests was administered to cirrhotic patients with at most grade 1 HE, recruited from the liver transplant and advanced liver disease clinics. An inflammatory bowel disease comparison group was similarly evaluated, thus controlling for the secondary effects of chronic illness on cognition. Testing results for the cirrhosis group were subjected to principal component analysis to establish the relevant cognitive constructs and associated measures. Factor analysis was applied to the neuropsychological battery of 20 tests to determine the cognitive factors to be used. Age-adjusted standardized neuropsychological factor scores were then compared for the two groups.

RESULTS

Factor analysis revealed that our battery of 20 tests was measuring three cognitive factors. Based on the pattern of factor loadings, we labeled these important cognitive factors: global cognitive function; psychomotor speed; and learning and memory. Logistic regression revealed that only impaired psychomotor speed distinguished cirrhotics with no more than grade 1 HE from medically ill controls.

CONCLUSIONS

The cirrhosis group was characterized by a pattern of preserved global cognitive functioning, mild memory impairment, and moderate psychomotor speed impairment.

DISCUSSION

This distinctive pattern of focal psychomotor speed deficits is suggestive of subcortical pathway involvement in minimal HE.

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.

Increased brain serotonin turnover correlates with the degree of shunting and hyperammonemia in rats following variable portal vein stenosis

BACKGROUND/AIMS

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication of chronic liver disease. Brain monoamines have been implicated in the pathogenesis of HE. We examined the relationship between monoamine dysfunction and the degree of portal-systemic shunting (PSS) in rats with varying degrees of PSS.

METHODS

Concentrations of catecholamines, serotonin, histamine, precursors and metabolites in frontal cortex of rats with varying degrees of PSS (9-99.8%) were measured by HPLC.

RESULTS

The concentrations of the serotonin precursor, tryptophan, and its metabolite, 5-HIAA were increased up to 4-fold in brains of rats with various degrees of PSS and were significantly correlated with the degree of shunting and with arterial ammonia levels. Brain levels of histamine, its precursor, l-histidine, and metabolite, tele-methylhistamine were significantly increased only following total shunting. Concentrations of catecholamines and their metabolites were not significantly correlated with degree of PSS or hyperammonemia.

CONCLUSIONS

Given the established role of the serotonin system in the regulation of sleep, circadian rhythmicity and locomotion these findings suggest that selective alterations of this system could be implicated in the pathogenesis of HE. Therapeutic approaches aimed at the normalization of serotonin turnover could be beneficial in the prevention and treatment of early neuropsychiatric symptoms of HE.

Neurobiology of ammonia

Hyperammonemia resulting from inherited urea cycle enzyme deficiencies or liver failure results in severe central nervous system dysfunction including brain edema, convulsions and coma. Neuropathologic evaluation in these disorders reveals characteristic alterations of astrocyte morphology ranging from cell swelling (acute hyperammonemia) to Alzheimer Type II astrocytosis (chronic hyperammonemia). Having no effective urea cycle, brain relies on glutamine synthesis for the removal of excess ammonia and the enzyme responsible, glutamine synthetase, has a predominantly astrocytic localization. Accumulation of ammonia in brain results in a redistribution of cerebral blood flow and metabolism from cortical to sub-cortical structures. In addition to changes in astrocyte morphology, increased brain ammonia concentrations result in alterations in expression of key astrocyte proteins including glial fibrillary acidic protein, glutamate and glycine transporters and "peripheral-type" (mitochondrial) benzodiazepine receptors. Such changes result in alterations of astrocytic volume and increased extracellular concentrations of excitatory and inhibitory substances. In addition, the ammonium ion has direct effects on excitatory-inhibitory transmission via distinct mechanisms involving cellular chloride extrusion and postsynaptic receptor function. Acute ammonia exposure leads to activation of NMDA receptors and their signal transduction pathways. Chronic hyperammonemia also results in increased concentrations of neuroactive L-tryptophan metabolites including serotonin and quinolinic acid. Therapy in hyperammonemic syndromes continues to rely on ammonia-lowering strategies via peripheral mechanisms (reduction of ammonia production in the gastrointestinal tract, increased ammonia removal by muscle).

Brain tryptophan/serotonin perturbations in metabolic encephalopathy and the hazards involved in the use of psychoactive drugs

Several combined pathogenetic factors such as hyperammonemia, different brain tryptophan metabolic disturbances and serotonin physiological/pharmacological alterations not yet defined in all details, will often give rise to the clinical neuropsychiatric condition known as hepatic encephalopathy (HE). Indeed, to this the probable exposure to novel potent CNS-monoamine acting drugs today may put such patients at certain risk for other pharmacodynamic (PD) responses than usually are expected from these "safe" drugs. Moreover, with a compromised liver function in HE, also pharmacokinetic (PK) features for the drugs are likely changed in these patients. Thus, the ultimate clinical outcome by this probable but unknown PD/PK-deviation for such psychoactive drugs when given to HE-patients needs further clarification. Accordingly, delineation of both PD- and PK-effects in experimental HE should shed light on this issue of relevance for monoamine-active drug safety as well as on some further details in the complex tryptophan/monoamine-related pathophysiology that comes into play in HE.

Complications of cirrhosis III. Hepatic encephalopathy

Hepatic encephalopathy (HE) is a major neuropsychiatric complication of cirrhosis. HE develops slowly in cirrhotic patients, starting with altered sleep patterns and eventually progressing through asterixis to stupor and coma. Precipitating factors are common and include an oral protein load, gastrointestinal bleeding and the use of sedatives. HE is common following transjugular intrahepatic portosystemic stent shunts (TIPS). Neuropathologically, HE in cirrhotic patients is characterized by astrocytic (rather than neuronal) changes known as Alzheimer type II astrocytosis and in altered expression of key astrocytic proteins. Magnetic resonance imaging in cirrhotic patients reveals bilateral signal hyperintensities particularly in globus pallidus on T1-weighted imaging, a phenomenon which may result from manganese deposition. Proton (1H) magnetic resonance spectroscopy shows increases in the glutamine resonance in brain, a finding which confirms previous biochemical studies and results no doubt from increased brain ammonia removal (glutamine synthesis). Additional evidence for increased brain ammonia uptake and removal in cirrhotic patients is provided by studies using positron emission tomography and 13NH3. Recent molecular biological studies demonstrate increased expression of genes coding for neurotransmitter-related proteins in chronic liver failure. Such genes include monoamine oxidase (MAO-A isoform), the peripheral-type benzodiazepine receptor and nitric oxide synthase (nNOS isoform). Activation of these systems has the potential to lead to alterations of monoamine and amino acid neurotransmitter function as well as modified cerebral perfusion in chronic liver failure. Prevention and treatment of HE in cirrhotic patients continues to rely on ammonia-lowering strategies which include assessment of dietary protein intake and the use of lactulose, neomycin, sodium benzoate and L-ornithine-aspartate. The benzodiazepine receptor antagonist flumazenil may be effective in certain cases. A more widespread use of central nervous system-acting drugs awaits a more complete understanding of the precise neurotransmitter systems involved in the pathogenesis of HE in chronic liver failure.

Nutritional support in hepatic encephalopathy

Hepatic encephalopathy (HE) is a syndrome of global cerebral dysfunction resulting from underlying liver disease or portal-systemic shunting. HE can present as one of four syndromes, depending on the rapidity of onset of hepatic failure and the presence or absence of preexisting liver disease. The precise pathogenesis is unknown but likely involves impaired hepatic detoxification of ammonia as well as alterations in brain transport and metabolism of amino acids and amines. The etiology of malnutrition in hepatic failure is multifactorial. Nutritional deficits may be clinically manifest as marasmus or kwashiorkor, or both. Nutritional support in HE is directed toward reducing morbidity related to underlying malnutrition and concurrent disease. However, reaching nutritional goals is often complicated by protein and carbohydrate intolerance. The use of protein restriction in HE is controversial. Modified formulas that are supplemented in branched chain amino acids may be of value in patients who exhibit protein intolerance with standard feeding solutions or in patients who present with advanced degrees of encephalopathy.

Alterations of neurotransmitter-related gene expression in human and experimental portal-systemic encephalopathy

Portal-systemic encephalopathy (PSE) is a serious neuropsychiatric condition that results from chronic liver failure and portal-systemic shunting of venous blood. PSE is particularly prevalent following treatment of portal hypertension or ascites by the TIPS procedure. Recent studies both in autopsied brain tissue from PSE patients as well as in experimental animal models of PSE reveal that chronic liver failure results in altered expression of several genes coding for proteins having key roles in the control of neuronal excitability. Such alterations include increased expression of monoamine oxidase (MAO-A isoform), the "peripheral-type" benzodiazepine receptor (PTBR) as well as constitutive, neuronal nitric oxide synthase (nNOS). Such changes result in altered protein expression and in increased degradation of monoamine neurotransmitters, increased synthesis of neurosteroids with inhibitory properties and increased production of nitric oxide (respectively) in brain in chronic liver failure. In the case of PTBR and nNOS, increases in expression result from exposure to ammonia and/or manganese, two neurotoxic agents shown previously to be increased in brain in chronic liver failure. Further elucidation of the consequences of neurotransmitter-related gene expression could identify new pathophysiologic mechanisms and result in new approaches to the prevention of PSE in chronic liver disease in humans.

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.

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

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

Alterations in the metabolism of monoamine neurotransmitters have been proposed to be involved in the development of the hepatic encephalopathy (HE) associated with experimental and human liver failure. In order to evaluate this hypothesis, the monoamines and some of their metabolites were measured in homogenates of caudate nucleus (CAU), prefrontal (PFCo) and frontal cortex (FCo) dissected from brains obtained at autopsy from nine cirrhotic patients who had died in hepatic coma and an equal number of control subjects, free from neurological, psychiatric and hepatic disorders, matched for age and time interval from death to freezing of autopsied brain samples. Monoamine measurements were performed by high-performance liquid chromatography with ion-pairing and electrochemical detection after a simple extraction procedure. In all three regions investigated, concentrations of dopamine (DA) were unchanged in cirrhotic patients vs controls while its metabolites, 3-methoxytyramine (3-MT) and homovanillic acid (HVA) were selectively affected i.e. 3-MT was found to be increased in CAU, while HVA levels were increased in FCo and CAU. DOPAC was also found to be unchanged in CAU. Noradrenaline (NA) levels were greatly increased in PFCo and FCo of cirrhotic patients but remained unchanged in CAU. No significant differences in the concentrations of either serotonin (5-HT) or of its precursor 5-hydroxytryptophan (5-HTP) were found in any of the three regions studied. However, 5-hydroxyindoleacetic acid (5-HIAA), the major metabolite of 5-HT, was increased in PFCo and CAU of cirrhotic patients. These findings show that selective alterations of catecholamine and 5-HT systems are involved in human HE and therefore, they may play an important role in the pathogenesis of certain neurological symptoms associated with this encephalopathy.

Tryptophan metabolism in D-galactosamine-induced liver injury

We have reported that in rats with D-galactosamine-induced liver injury, the serum level of indoleacetic acid (IAA), a metabolite of tryptophan (TRP), increases before the increase in serum transaminase activity. To determine whether this IAA is derived from hepatocytes, isolated hepatocytes were treated with D-galactosamine and loaded with TRP, and the changes in TRP and IAA levels in the culture supernatant of the isolated hepatocytes were measured at various time intervals. As a result, IAA level in the culture supernatant of hepatocytes treated with D-galactosamine and loaded with TRP significantly increased in a time-dependent manner. This indicates that in D-galactosamine-induced liver injury, a metabolic pathway which produces IAA from TRP through tryptamine is present in hepatocytes.

Increase in the content of quinolinic acid in cerebrospinal fluid and frontal cortex of patients with hepatic failure

Quinolinic acid (QUIN), an excitotoxic tryptophan metabolite, has been identified and measured in human cerebrospinal fluid (CSF) using a mass-fragmentographic method. Furthermore, its content has been evaluated in frontal cortex obtained at autopsy from the cadavers of patients who died after hepatic coma. During the coma, the concentration of QUIN in the CSF was 152 +/- 38 pmol ml-1. In contrast, the concentration in control patients affected by different pathologies was 22 +/- 7 pmol ml-1. In the frontal cortex of patients who died after episodes of hepatic encephalopathy, the content of QUIN was three times higher than in controls (2.6 +/- 0.6 versus 0.80 +/- 0.08 nmol/g wet weight). As a result of these investigations we are now able to extend our previous observations on the increase of QUIN in the brains of rats used as experimental models of hepatic encephalopathy to man. QUIN should therefore be added to the list of compounds possibly involved in the pathogenesis and symptomatology of brain disorders associated with liver failure.

Hypothalamic-pituitary dopaminergic function in hepatic failure in man

The growth hormone (GH) response to apomorphine HCl (Apo) (0.75 mg sc), a dopamine (DA) receptor agonist, was assessed in healthy chronic alcoholics without cirrhosis (N = 20) and in patients with alcoholic cirrhosis both with (N = 5) and without (N = 14) hepatic encephalopathy (HE). A significant number of cirrhotic patients with (P less than 0.004) and without (P less than 0.002) HE had an impaired GH response (peak increment less than 5 ng/ml) compared with non-cirrhotic individuals. An impaired GH response was independent of the presence of HE. The magnitude of the GH response was unrelated to plasma oestrone, oestradiol, or progesterone concentrations but was significantly correlated with plasma testosterone levels (P less than 0.01). None of the patients with an abnormally low testosterone concentration showed a normal GH response. None of the subjects with HE showed an arousal response to Apo. These results suggest that DA receptor sensitivity is decreased in liver cirrhosis and that this decrease is related to inadequate circulating levels of testosterone. The occurrence of HE is independent of impaired DA function. The present study only evaluates DA function in the hypothalamic-pituitary axis and therefore may not reflect changes in other regions of brain.

Effect of tryptophan administration on tryptophan, 5-hydroxyindoleacetic acid and indoleacetic acid in human lumbar and cisternal cerebrospinal fluid

Tryptophan 5-hydroxyindoleacetic acid and indoleacetic acid were measured in cerebrospinal fluid taken during pneumoencephalography from patients, some of whom took a 3 g or 6 g tryptophan load at various times before. Measurements were made on both lumbar and cisternal cerebrospinal fluid and the results showed similarities between indoleamine metabolism in human brain and spinal cord. Our data suggested that (1) the blood-brain barrier active transport system for tryptophan is not far from saturation with tryptophan and the rate-limiting enzyme in 5-hydroxytryptamine (5HT) synthesis, tryptophan hydroxylase, is about half saturated. Therefore, both 3 g and 6 g tryptophan loads produced the same maximum rise in 5HT synthesis of just under 100%, (2) tryptamine differs from 5HT in two respects. It is more sensitive to changes in tryptophan availability than 5HT and the 6 g load increased brain tryptamine metabolism more than the 3 g load; also some of the tryptamine in brain is derived from peripheral sources and diffuses from blood to brain, (3) although the brain tryptamine content is much lower than that of 5HT, its rate of metabolism as indicated by CSF metabolite levels is not. In controls the rate of tryptamine metabolism is 15% of the rate of 5HT metabolism and this can increase to 40% after a 6 g tryptophan load.