Brain serotonin metabolism and behavior in rats with carbon tetrachloride-induced liver cirrhosis

Extracellular vesicles from mesenchymal stem cells alter gut microbiota and improve neuroinflammation and motor impairment in rats with mild liver damage

Gut microbiota perturbation and motor dysfunction have been reported in steatosis patients. Rats with mild liver damage (MLD) show motor dysfunction mediated by neuroinflammation and altered GABAergic neurotransmission in the cerebellum. The extracellular vesicles (EV) from mesenchymal stem cells (MSC) have emerged as a promising therapeutic proxy whose molecular basis relies partly upon TGFβ action. This study aimed to assess if MSC-EVs improve motor dysfunction in rats with mild liver damage and analyze underlying mechanisms, including the role of TGFβ, cerebellar neuroinflammation and gut microbiota. MLD in rats was induced by carbon tetrachloride administration and EVs from normal (C-EVs) or TGFβ-siRNA treated MSCs (T-EV) were injected. Motor coordination, locomotor gait, neuroinflammation and TNF-α-activated pathways modulating GABAergic neurotransmission in the cerebellum, microbiota composition in feces and microbial-derived metabolites in plasma were analyzed. C-EVs reduced glial and TNFα-P2X4-BDNF-TrkB pathway activation restoring GABAergic neurotransmission in the cerebellum and improving motor coordination and all the altered gait parameters. T-EVs also improved motor coordination and some gait parameters, but the mechanisms involved differed from those of C-EVs. MLD rats showed increased content of some Bacteroides species in feces, correlating with decreased kynurenine aside from motor alterations. These alterations were all normalized by C-EVs, whereas T-EVs only restored kynurenine levels. Our results support the value of MSC-EVs on improving motor dysfunction in MLD and unveil a possible mechanism by which altered microbiota may contribute to neuroinflammation and motor impairment. Some of the underlying mechanisms are TGFβ-dependent.

Preclinical models of hepatic encephalopathy

Hepatic encephalopathy is a multifactorial neuropsychiatric syndrome accompanying acute or chronic liver failure. Techniques for developing animal models of hepatic encephalopathy associated with acute or chronic liver failure, or vascular shunting are illustrated. In addition, the behavioral and biochemical characteristics of these models are described.

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.

Effect of carbon tetrachloride on inositol 1,4,5-trisphosphate dependent and independent regulation of rat brain microsomal Ca2+ flux

Carbon tetrachloride (CCl4) is a highly toxic industrial solvent with pronounced effects on the liver and brain. CCl4 is enzymatically cleaved to produce free radicals which attack membrane components, including proteins. Earlier reports indicated that CCl4 affects Ca(2+)-regulated events in the brain. Hence, the present study was initiated to determine whether CCl4 affects inositol 1,4,5-trisphosphate (IP3) receptor binding, free-Ca2+ movements across the microsomal membrane and protein kinase C (PKC) activity in rat brain, since IP3, Ca2+ and PKC are known to be involved in signal transduction. [3H]IP3 binding, free-Ca2+ movements and 45Ca2+ uptake were determined using rat brain microsomes and PKC activity was determined in the cytosolic fraction. CCl4 in vitro decreased [3H]IP3 binding to microsomes. IP3 mediated Ca2+ release from microsomes was inhibited and also the reuptake of IP3-released Ca2+ into microsomes was decreased in the presence of CCl4. CCl4 at concentrations < 2 microM independently released Ca2+ from microsomes. Uptake of total Ca2+ into microsomes was inhibited by CCl4 as observed with 45Ca(2+)-uptake studies. CCl4 at 1 microM inhibited PKC activity by 50%. Thus, perturbations in the binding of IP3 to its receptor sites, changes in the Ca2+ flux across the microsomal membrane and modulation of PKC activity by CCl4 in vitro suggested that CCl4 may exert neurotoxicity by altering signal transduction pathways.

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.

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.

Intrasplenic hepatocellular transplantation corrects hepatic encephalopathy in portacaval-shunted rats

The aim of this work was to evaluate the effect of intrasplenic hepatocellular transplantation on hepatic encephalopathy in an experimental model of chronic liver failure induced by end-to-side portacaval shunt in the rat. Inbred male Wistar Furth rats were divided into three groups: rats subjected to portacaval shunt (n = 10), rats subjected to portacaval shunt and intrasplenic hepatocellular transplantation of 10(7) hepatocytes isolated from livers of syngeneic rats (n = 10) and sham-operated rats (n = 10). Behavior tests were performed in a blind fashion at 3 wk, at 2 mo and at 3 mo after surgery. Spontaneous activity and nose-poke exploration by individual rats were studied in automated open field boxes equipped with infrared cells. Each cell beam interruption was automatically recorded on a microcomputer and transformed into a score index (counts/hour). Plasma levels of amino acids, ammonia and total biliary acids were measured. Portacaval shunt rats showed reduced spontaneous activity and nose-poke exploration scores. Intrasplenic hepatocellular transplantation significantly increased spontaneous activity after 2 mo and improved nose-poke exploration after 3 wk. At 3 mo, spontaneous activity and nose-poke exploration in portacaval shunt/intrasplenic hepatocellular transplantation rats were not significantly different from those of sham rats. Increases in plasma ammonia levels after portacaval shunt were not corrected. Amino acid imbalance and bile acid concentration in plasma were partially corrected by intrasplenic hepatocellular transplantation. These data show that intrasplenic hepatocellular transplantation can correct the neurological symptoms of hepatic encephalopathy in an experimental model of chronic liver failure and suggest that intrasplenic hepatocellular transplantation might be of therapeutic interest in chronic liver failure.

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.