Prevention of ammonia and glutamate neurotoxicity by carnitine: molecular mechanisms

L-Carnitine in the Treatment of Psychiatric and Neurological Manifestations: A Systematic Review

OBJECTIVE

L-carnitine (LC), a vital nutritional supplement, plays a crucial role in myocardial health and exhibits significant cardioprotective effects. LC, being the principal constituent of clinical-grade supplements, finds extensive application in the recovery and treatment of diverse cardiovascular and cerebrovascular disorders. However, controversies persist regarding the utilization of LC in nervous system diseases, with varying effects observed across numerous mental and neurological disorders. This article primarily aims to gather and analyze database information to comprehensively summarize the therapeutic potential of LC in patients suffering from nervous system diseases while providing valuable references for further research.

METHODS

A comprehensive search was conducted in PubMed, Web Of Science, Embase, Ovid Medline, Cochrane Library and Clinicaltrials.gov databases. The literature pertaining to the impact of LC supplementation on neurological or psychiatric disorders in patients was reviewed up until November 2023. No language or temporal restrictions were imposed on the search.

RESULTS

A total of 1479 articles were retrieved, and after the removal of duplicates through both automated and manual exclusion processes, 962 articles remained. Subsequently, a meticulous re-screening led to the identification of 60 relevant articles. Among these, there were 12 publications focusing on hepatic encephalopathy (HE), while neurodegenerative diseases (NDs) and peripheral nervous system diseases (PNSDs) were represented by 9 and 6 articles, respectively. Additionally, stroke was addressed in five publications, whereas Raynaud's syndrome (RS) and cognitive disorder (CD) each had three dedicated studies. Furthermore, migraine, depression, and amyotrophic lateral sclerosis (ALS) each accounted for two publications. Lastly, one article was found for other symptoms under investigation.

CONCLUSION

In summary, LC has demonstrated favorable therapeutic effects in the management of HE, Alzheimer's disease (AD), carpal tunnel syndrome (CTS), CD, migraine, neurofibromatosis (NF), PNSDs, RS, and stroke. However, its efficacy appears to be relatively limited in conditions such as ALS, ataxia, attention deficit hyperactivity disorder (ADHD), depression, chronic fatigue syndrome (CFS), Down syndrome (DS), and sciatica.

Clinical Evidence of Acetyl-L-Carnitine Efficacy in the Treatment of Acute Ischemic Stroke: A Pilot Clinical Trial

Background. This study was undertaken to evaluate the influence of oral Acetyl-L-carnitine (ALC) in patients with acute ischemic stroke. Methods. Sixty-nine cases with acute ischemic stroke with the onset of symptoms less than 24 hours not candidates for reperfusion therapy were randomly assigned to either the ALC group (1000 mg three times per day for three consecutive days) or the matching placebo group. The study outcomes based on intention-to-treat criteria included the change in the modified Rankin Scale (mRS) and National Institutes of Health Stroke Scale (NIHSS) score from baseline to day 90, as well as the change in serum levels of the inflammatory and oxidative stress biomarkers over the 3-day treatment protocol. Results. The NIHSS score and mRS score on day 90 were improved by 5.82 and 0.94 scores, respectively, in the ALC-treated group compared to 2.83 and 0.11 scores, respectively, in the placebo-treated group, which demonstrated the superiority of ALC relative to placebo. By using the multivariable analysis after adjusting for other variables in the model, compared to the group treated with placebo, patients in the ALC group had lower NIHSS score ( $\beta$ : -2.40, 95% CI: -0.69, -4.10 ( $p=0.007$ )) and mRS score ( $\beta$ : -1.18, 95% CI: -0.52, -1.84 ( $p=0.001$ )) 90 days after the intervention. The percentage of patients with a favourable functional outcome at day 90, defined as mRS scores of 0 or 1, was significantly higher in the ALC group in comparison to the placebo group (52.9% versus 28.6%). Further, over the 3-day treatment protocol, in the patients receiving ALC, the serum levels of proinflammatory biomarkers, including soluble intercellular adhesion molecule-1 (sICAM-1), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-α), and neuron-specific enolase (NSE), showed a significant decrease, while the serum levels of antioxidant biomarkers, including glutathione peroxidase (GPx), superoxide dismutase (SOD), and total antioxidant capacity (TAC), as well as the total L-carnitine’s level showed a significant increase compared to those in patients receiving placebo indicating significant alteration. Conclusions. Although preliminary, these results suggested that ALC administration during the acute phase of ischemic stroke might be helpful in improving functional and neurological outcomes that are probably linked to its anti-inflammatory and antioxidant properties. Trial Registration. This trial is registered with IRCT20150629022965N17 at Iranian Registry of Clinical Trials (registration date: 25/07/2018).

The Cerebral Effect of Ammonia in Brain Aging: Blood-Brain Barrier Breakdown, Mitochondrial Dysfunction, and Neuroinflammation

Aging occurs along with multiple pathological problems in various organs. The aged brain, especially, shows a reduction in brain mass, neuronal cell death, energy dysregulation, and memory loss. Brain aging is influenced by altered metabolites both in the systemic blood circulation and the central nervous system (CNS). High levels of ammonia, a natural by-product produced in the body, have been reported as contributing to inflammatory responses, energy metabolism, and synaptic function, leading to memory function in CNS. Ammonia levels in the brain also increase as a consequence of the aging process, ultimately leading to neuropathological problems in the CNS. Although many researchers have demonstrated that the level of ammonia in the body alters with age and results in diverse pathological alterations, the definitive relationship between ammonia and the aged brain is not yet clear. Thus, we review the current body of evidence related to the roles of ammonia in the aged brain. On the basis of this, we hypothesize that the modulation of ammonia level in the CNS may be a critical clinical point to attenuate neuropathological alterations associated with aging.

The Alteration of L-Carnitine Transport and Pretreatment Effect under Glutamate Cytotoxicity on Motor Neuron-Like NSC-34 Lines

L-Carnitine (LC) is essential for transporting fatty acids to the mitochondria for β-oxidation. This study was performed to examine the alteration of the LC transport system in wild type (WT, NSC-34/hSOD1^WT) and mutant type (MT, NSC-34/hSOD1^G93A) amyotrophic lateral sclerosis (ALS) models. The uptake of [³H]L-carnitine was dependent on time, temperature, concentration, sodium, pH, and energy in both cell lines. The Michaelis–Menten constant (K_m) value as well as maximum transport velocity (V_max) indicated that the MT cell lines showed the higher affinity and lower capacity transport system, compared to that of the WT cell lines. Additionally, LC uptake was inhibited by organic cationic compounds but unaffected by organic anions. OCTN1/slc22a4 and OCTN2/slc22a5 siRNA transfection study revealed both transporters are involved in LC transport in NSC-34 cell lines. Additionally, slc22a4 and slc22a5 was significantly decreased in mouse MT models compared with that in ALS WT littermate models in the immune-reactivity study. [³H]L-Carnitine uptake and mRNA expression pattern showed the pretreatment of LC and acetyl L-carnitine (ALC) attenuated glutamate induced neurotoxicity in NSC-34 cell lines. These findings indicate that LC and ALC supplementation can prevent the neurotoxicity and neuro-inflammation induced by glutamate in motor neurons.

Changes in the neurotransmitter profile in the central nervous system of marine medaka (Oryzias melastigma) after exposure to brevetoxin PbTx-1 - A multivariate approach to establish exposure biomarkers

A strategy to construct multivariate biomarkers for exposure to algal neurotoxins via correlating changes to the profiles of a series of neurotransmitters and their metabolites in the central nervous system (CNS) of exposed test organism is reported. 3-Month-old marine medaka (Oryzais melastigma) were exposed to waterborne brevetoxin PbTx-1 at two sub-lethal dose levels (0.5 and 2.5 μg-PbTx-1 L^-1) for a duration of 12 h before quantification of 43 selected neurotransmitters and metabolites in their CNS were measured via dansyl chloride derivatization and LC-MS/MS determination. The profiling data were analyzed by multivariate statistical analyses, including principle component analysis (PCA), projection on latent structure-discriminate analysis (PLS-DA) and orthogonal projection on latent structure-discriminate analysis (OPLS-DA). Neurotransmitters and metabolites related to activation of voltage-gated sodium channels (VGSCs), N-methyl-D-aspartic acid receptors (NMDARs) and cholinergic neurotransmission were found to contribute significantly to class separation in the corresponding OPLS-DA models. Those models obtained from different exposure dosages were correlated by the Shared and Unique Structures Plot (SUS-plot) to identify appropriate variables for the construction of exposure biomarkers in the form of multivariate predictive scores. The established biomarkers for male and female medaka fish were able to predict acute sub-lethal exposure to PbTx-1 with good sensitivity and specificity (male fish: sensitivity 94.7%, specificity 80.0%; female fish: sensitivity 91.4%, specificity 83.3%). Neurotransmitter profiles in the CNS of medaka fish that should have recovered from exposure to PbTx-1 have also been determined to reveal long-term impacts to the CNS of the affected organism even after the exposure has been interrupted.

Neurotoxicity of Ammonia

Abnormal liver function has dramatic effects on brain functions. Hyperammonemia interferes profoundly with brain metabolism, astrocyte volume regulation, and in particular mitochondrial functions. Gene expression in the brain and excitatory and inhibitory neurotransmission circuits are also affected. Experiments with a number of pertinent animal models have revealed several potential mechanisms which could underlie the pathological phenomena occurring in hepatic encephalopathy.

Hepatic encephalopathy in dogs and cats

OBJECTIVE

To comparatively review the pathogenesis, clinical presentation, diagnosis, and management of hepatic encephalopathy (HE) in dogs and cats.

DATA SOURCES

The Medline database was searched for articles related to HE in people, dogs, and cats. Articles published within the last 5 years were given special importance.

HUMAN DATA SYNTHESIS

The pathogenesis of HE is complex and incompletely understood, but ammonia appears to play a central role. Hyperammonemia leads to accumulation of glutamine in astrocytes, with subsequent astrocyte swelling and neurological dysfunction. The development of HE in patients with hepatic cirrhosis is a poor prognostic indicator. The fermentable disaccharide lactulose and the antimicrobial rifaximin are US Food and Drug Administration approved treatments for human HE. Severe protein restriction is no longer recommended for patients with this condition.

VETERINARY DATA SYNTHESIS

HE is often associated with portosystemic shunting in dogs and cats. Ammonia plays a central role in the pathogenesis of HE in dogs and cats, but other factors such as manganese and endogenous benzodiazepines may also contribute. Recently, a soy protein-based diet was found to be beneficial in treating canine HE. Severe dietary protein restriction is likely to be detrimental in affected animals. There have been no clinical trials of drugs routinely used in the management HE in veterinary medicine, but lactulose and antimicrobials such as metronidazole are well-established treatments.

CONCLUSIONS

HE is a potentially life-threatening condition that is probably underdiagnosed in companion animals. Although various treatment recommendations have been proposed, there is a lack of evidence in the veterinary literature regarding optimal strategies for the management of this condition. As our understanding of the pathogenesis of HE in dogs and cats evolves, novel diagnostic tests and therapeutic agents may become available.

Acetyl-L-carnitine in hepatic encephalopathy

Hepatic encephalopathy is a common complication of hepatic cirrhosis. The clinical diagnosis is based on two concurrent types of symptoms: impaired mental status and impaired neuromotor function. Impaired mental status is characterized by deterioration in mental status with psychomotor dysfunction, impaired memory, and increased reaction time, sensory abnormalities, poor concentration, disorientation and coma. Impaired neuromotor function include hyperreflexia, rigidity, myoclonus and asterixis. The pathogenesis of hepatic encephalopathy has not been clearly defined. The general consensus is that elevated levels of ammonia and an inflammatory response work in synergy to cause astrocyte to swell and fluid to accumulate in the brain which is thought to explain the symptoms of hepatic encephalopathy. Acetyl-L-carnitine, the short-chain ester of carnitine is endogenously produced within mitochondria and peroxisomes and is involved in the transport of acetyl-moieties across the membranes of these organelles. Acetyl-L-carnitine administration has shown the recovery of neuropsychological activities related to attention/concentration, visual scanning and tracking, psychomotor speed and mental flexibility, language short-term memory, attention, and computing ability. In fact, Acetyl-L-carnitine induces ureagenesis leading to decreased blood and brain ammonia levels. Acetyl-L-carnitine treatment decreases the severity of mental and physical fatigue, depression cognitive impairment and improves health-related quality of life. The aim of this review was to provide an explanation on the possible toxic effects of ammonia in HE and evaluate the potential clinical benefits of ALC.

Acetyl-L-carnitine reduces depression and improves quality of life in patients with minimal hepatic encephalopathy

BACKGROUND

Minimal hepatic encephalopathy (MHE) represents a common complication present in well-compensated cirrhotic patients that impairs patients' daily functioning and health-related quality of life (HRQL). Acetyl-L-carnitine (ALC) has been shown to be useful in improving blood ammonia and cognitive functions in cirrhotic patients with MHE.

OBJECTIVE

This study evaluated the effects of ALC treatment on HRQL and depression in patients with MHE.

STUDY DESIGN

This was a randomized, double-blind, placebo-controlled study. Sixty-seven patients with MHE were recruited to the study. They were randomly assigned to two groups and received either 2 g acetyl-L-carnitine twice a day (n = 33) or placebo (n = 34) for 90 days. The primary efficacy measures were changes in aspartate aminotransferase, alanine aminotransferase, γ-glutamyl-transpeptidase, albumin, alkaline phosphatase, prothrombin time, and ammonia. Clinical and laboratory assessments, psychometric tests and automated electroencephalogram (EEG) analysis were performed for all patients.

RESULTS

At the end of the study period, between the two groups, we observed a significant difference in physical function (p < 0.001), role physical (p < 0.001), general health (p < 0.001), social function (p < 0.05), role emotional (p < 0.05), mental health (p < 0.05), Beck Depression Inventory (p < 0.001), TMT-B s (p < 0.001), State Trait Inventory (p < 0.001), urea (p < 0.05), NH(4)(+) (p < 0.001), and bilirubin (p < 0.001).

CONCLUSIONS

This study shows that ALC treatment is associated with significant improvement in patient energy levels, general functioning and well-being. The improvement of quality of life is associated with reduction of anxiety and depression.

Branched chain amino acids supplemented with L-acetylcarnitine versus BCAA treatment in hepatic coma: a randomized and controlled double blind study

OBJECTIVE

Our earlier study has demonstrated that the administration of L-acetylcarnitine (LAC) improves neurological symptoms and serum parameters in hepatic coma. The aim of this work has been to evaluate the efficacy of the LAC and branched chain amino acids (BCAA) versus BCAA, administered in intravenous infusion, in patients with cirrhotic hepatic coma.

METHODS

Forty-eight highly selected patients were enrolled in the study and, after randomization, received blindly LAC+BCAA (n=24) versus BCAA (n=24). The two groups were similar in age, sex, pathogenesis of cirrhosis, and severity of liver disease. The comparison between values before and after LAC planned treatment showed statistical significant differences in neurological findings, evaluated by the Glasgow Scale, ammonia serum levels, blood urea nitrogen, and EEG.

RESULTS

After 60 min of the study period, the LAC+BCAA treated patients compared with BCCA treated showed a significant decrease of ammonia serum levels: 41.20 versus 10.40 mumol P<0.05. After 1 day of the study period, the LAC+BCAA treated patients compared with BCCA treated patients showed a significant increase of Glasgow's score: 3.60 versus 1.50 score P<0.05; a significant decrease of ammonia serum levels: 63.30 versus 27.00 mumol P<0.01; a significant improvement of EEG cps/s: 2.70 versus 0.6 P<0.001. No side-effects were observed in our study series.

CONCLUSION

Our study demonstrated that the administration of BCAA supplemented with LAC might improve neurological symptoms and serum ammonium levels in selected cirrhotic patients with hepatic coma.

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA(A) and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.

Valproate-induced hyperammonemic encephalopathy

Valproate-induced hyperammonemic encephalopathy (VHE) is an unusual complication characterized by a decreasing level of consciousness, focal neurological deficits, cognitive slowing, vomiting, drowsiness, and lethargy. We have thoroughly reviewed the predisposing factors and their screening, the biochemical and physiopathological mechanisms involved, the different treatments described, and those that are being investigated. Etiopathogenesis is not completely understood, although hyperammonemia has been postulated as the main cause of the clinical syndrome. The increase in serum ammonium level is due to several mechanisms, the most important one appearing to be the inhibition of carbamoylphosphate synthetase-I, the enzyme that begins the urea cycle. Polytherapy with several drugs, such as phenobarbital and topiramate, seems to contribute to hyperammonemia. Hyperammonemia leads to an increase in the glutamine level in the brain, which produces astrocyte swelling and cerebral edema. There are several studies that suggest that treatment with supplements of carnitine can lead to an early favorable clinical response due to the probable carnitine deficiency induced by a valproate (VPA) treatment. Development of the progressive confusional syndrome, associated with an increase in seizure frequency after VPA treatment onset, obliges us to rule out VHE by screening for blood ammonium levels and the existence of urea cycle enzyme deficiency, such as ornithine carbamoyltransferase deficiency. Electroencephalography (EEG) is characterized by signs of severe encephalopathy with continuous generalized slowing, a predominance of theta and delta activity, occasional bursts of frontal intermittent rhythmic delta activity, and triphasic waves. These EEG findings, as well as clinical manifestations and hyperammonemia, tend to normalize after VPA withdrawal.

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Evaluation of cardiac lesions and risk factors associated with myocarditis and dilated cardiomyopathy in southern sea otters (Enhydra lutris nereis)

OBJECTIVE

To describe cardiac lesions and identify risk factors associated with myocarditis and dilated cardiomyopathy (DCM) in beach-cast southern sea otters.

ANIMALS

Free-ranging southern sea otters.

PROCEDURE

Sea otters were necropsied at the Marine Wildlife Veterinary Care and Research Center from 1998 through 2001. Microscopic and gross necropsy findings were used to classify sea otters as myocarditis or DCM case otters or control otters. Univariate, multivariate, and spatial analytical techniques were used to evaluate associations among myocarditis; DCM; common sea otter pathogens; and potential infectious, toxic, and nutritional causes.

RESULTS

Clusters of sea otters with myocarditis and DCM were identified in the southern aspect of the sea otter range from May to November 2000. Risk factors for myocarditis included age, good body condition, and exposure to domoic acid and Sarcocystis neurona. Myocarditis associated with domoic acid occurred predominantly in the southern part of the range, whereas myocarditis associated with S. neurona occurred in the northern part of the range. Age and suspected previous exposure to domoic acid were identified as major risk factors for DCM. A sample of otters with DCM had significantly lower concentrations of myocardial L-carnitine than control and myocarditis case otters.

CONCLUSIONS AND CLINICAL RELEVANCE

Cardiac disease is an important cause of death in southern sea otters. Domoic acid toxicosis and infection with S. neurona are likely to be 2 important causes of myocarditis in sea otters. Domoic acid-induced myocarditis appears to progress to DCM, and depletion of myocardial L-carnitine may play a key role in this pathogenesis.

Valproic acid-induced hyperammonemia: a case report

The authors present a case of a patient treated with valproic acid for seizure disorder who presented with acute mental status changes consistent with encephalopathy. Notably, her serum ammonia level was 3 times the upper limit of normal, despite an only mildly elevated aspartate aminotransferase and normal bilirubin. Her serum valproic acid level was in the therapeutic range. Her symptoms resolved with discontinuation of valproic acid and supportive care. The authors review the possible mechanisms of valproic acid-associated hyperammonemia with encephalopathy and propose clinical practice modifications to minimize the incidence of this adverse reaction to this generally well-tolerated and clinically important psychotropic medication.