Hepatic encephalopathy: Lessons from preclinical studies

Table 1 Advantages and disadvantages of animal models of hepatic encephalopathy

	Model features	Advantages	Disadvantages
Type A Model. Encephalopathy associated with acute liver failure	Type A models have been developed by exclusion (anhepatic models), partial removal of the liver or from the administration of a hepatotoxins	Anhepatic model: The course of HE is relatively rapid. Applied to studies of brain metabolism, neurotransmission abnormalities, gene expression and brain inflammation in ALF. The model responds to hypothermia, ammonia-lowering agents and anti-inflammatory drugs; Hepatotoxic model: Different hepatotoxins could be used to create Type A models. Generally, these models produce hypothermia, hypoglycemia and other systemic complications; Thioacetamide: Model with good repeatability, easy operation, and high similarity to human HE; Acetaminophen: Easy preparation, low price and dose-dependence; D-galactosamine: Good repeatability. Simulates the pathophysiological changes of acute liver failure. Shows manifestations of liver injury that are similar to viral hepatic failure	Anhepatic model: Neither procedure could lead to a potential recovery. The surgical procedure causes great trauma to the animal. Absence of injured or necrotic hepatic cells. The toxic substances and inflammatory mediators present in the injured liver are not perfused into the blood circulation; Partial hepatectomy: The surgery is difficult to control. Severe hypoglycemia leads to death; Hepatotoxic model: Each of these toxins could produce hepatitis with variable pathological nature. Animal-to-animal variations lead to a lack of reproducibility. Some hepatotoxins show extrahepatic toxicity; Acetaminophen: Poor reproducibility. Shows side effects in kidneys and other organs; D-galactosamine: High cost, short survival time and poor stability
Type B Model. Encephalopathy associated with portosystemic bypass without liver disease	Type B models have been developed by portosystemic shunting (portacaval anastomosis, congenital portacaval shunts, graded portal vein stenosis, and biliary duct ligation). Different HE aspects could be assessed using different shunt methods and species (pig, dog, rabbit, rat, and mouse)	Portacaval anastomosis: Can better simulate the clinical mild HE phenotypes in different animal models. (Dog) EEG changes and neurological status correlate with ammonia in the plasma; Congenital portacaval shunts: (Dogs) Naturally develop psychomotor dysfunction, reduced hepatic function, and hyperammonemia and are susceptible to high-protein diets; Graded portal vein stenosis: Easy to perform. The surgery may be reversed. (Rat) Provides a Minimal Hepatic Encephalopathy model. (Rat) Develop loss of activity, altered circadian rhythm, hyperammonemia, and altered ammonia/glutamine in the brain	Portacaval anastomosis: Cause severe coma due to hypersensitivity to ammonia. (Rabbit) Portacaval anastomosis may lead to death of animal in most cases. (Rat) Needs high surgical skills to perform; Congenital portacaval shunts: Access to animals with this congenital alteration
Type C Model. Encephalopathy associated with liver disease	HE associated with cirrhosis and portal hypertension (Type C) is the most common form of HE in patients. At present, there is no appropriate model to study HE that occurs in liver cirrhosis; nevertheless, some models have been developed	Biliary duct ligation: Animals develop liver failure, jaundice, portal hypertension, immune system dysfunction, and bacterial translocation. (Rat) Reproducible model of biliary cirrhosis with the development of hyperammonemia, low-grade encephalopathy, and decreased locomotor activity. (Rat) Bile duct-ligated animals fed with ammonium salts provide a model that reproduces human Alzheimer Type II astrocytosis	No satisfactory Type C animal model induced by alcoholic liver disease or viral hepatitis exists at the present time; Biliary duct ligation: Immune system dysfunction and other diseases may affect the final HE phenotypes. (Rat) Weight loss due to hunger suppression; CCl4: Animal-to-animal variations lead to a lack of reproducibility. Limited neurobehavioral assessment due to the presence of ascites. Possible health damage when a researcher uses this chemical
Models of acute/chronic hyperammonemia	These models are designed to study the effects of hyperammonemia on brain function without liver dysfunction	Inexpensive and simple to perform. The model shows alterations of multiple neurotransmitter systems in the brain. The model shows impairments in learning and memory	Limited to rats and mice. Time-consuming; not suitable for long-term studies. Lacks liver failure

Table 2 Brief description of type A animal models of hepatic encephalopathy

	Species used	Main findings
Surgical models
Hepatic devascularization	Rat, rabbit, pig	Increased AST, hypoglycemia, lethargy and coma
Hepatectomy	Rat, pig	Increased AST, TNF, PT, NH3, lactate, hypoglycemia, hepatic necrosis
Pharmacological models
Galactosamine (IP, IV, SC)	Rat, rabbit, guinea pig	Increased AST, PT, NH3, hepatic necrosis
Acetaminophen (IP, IV, SC, oral)	Rat, dog, pig	Increased AST, NH3, bilirubin, hypoglycemia, metabolic acidosis, centrilobular coagulative necrosis
Thioacetamide (IP, oral)	Rat	Increased AST, PT, NH3, metabolic acidosis, centrilobular necrosis
Azoxymethane (IP, SC)	Mouse	Increased AST, NH3 and bilirubin, hepatic necrosis

AST: Aspartate transaminase; TNF: Tumor necrosis factor; PT: Prothrombin time; NH₃: Ammonia; IP: Intraperitoneal; IV: Intravenous; SC: Subcutaneous.

Table 3 Brief description of findings obtained with types B and C experimental models of hepatic encephalopathy

Experimental model	Animal species	Biological findings	Clinical Signs
Portacaval anastomosis	Rats, dog, rabbit, pig	Increased brain ammonia/glutamine	Altered circadian cycle, hypokinesia, reduced memory and learning ability
Congenital portacaval shunts	Dogs, cats	Hyperammonemia	Hepatic dysfunction, psychomotor dysfunction, motor signs
Graded portal vein stenosis	Rats	Increased brain ammonia/glutamine	Minimal hepatic encephalopathy, loss of activity, altered circadian cycle
Carbon tetrachloride (CCl4)	Rats, mice	Generation of free radicals, lipoperoxidation, tissue fibrosis, increased hepatic membrane permeability	Hepatic failure, motor activity dysfunction
Bile duct ligation	Rats	Bacterial translocation, immune system dysfunction, hyperammonemia	Liver failure, portal hypertension, decreased locomotor activities due to low-grade encephalopathy