Molecular Dynamics and Electron Density Topology Reveal Ligand-Specific Interaction Patterns at the Dopamine D(2) Receptor.

Hepatic encephalopathy (HE) is a complex of pathological processes in the central nervous system leading to progressive neurological and mental illness in patients with severe liver disease or portosystemic shunt []. Due to the presence of multiple causes of brain pathology, the etiology of HE is not completely understood. According to numerous evidences, the mechanism of the brain injury is multifactorial and ammonia is a key postulated neurotoxin triggering a cascade of pathological reactions responsible for toxic-metabolic brain damage and clinical manifestations of HE []. However, it is well established that both forms of ammonia presented in the blood (NH/NH) readily cross the membranes of both neuronal and non-neuronal cells []. Consequently, when blood ammonia levels are abnormally elevated (hyperammonemia, HA), this toxin causes systemic toxicity leading to multiple organ failure, rather than solely direct brain damage [,]. Furthermore, this systemic disruption impairs interorgan metabolic communication, restricting the production and delivery of vital energy substrates to the brain []. This limitation is particularly critical because the brain, unlike other organs, exhibits an exceptionally high rate of oxidative metabolism to support its myriad of functions and, having extremely low energy reserves, strictly requires a constant supply of highly oxygenated and glucose-rich blood [,]. In the case of an inadequate supply of blood glucose to the brain, the only biochemical pathways that maintain the normal endogenous glucose levels in the cerebral tissues are glycogenolysis and gluconeogenesis (GNG). At the same time, glycogen stores are limited and the rate of de novo glucose synthesis in the brain is extremely low []. When the brain’s demand for energy substrates is not met by endogenous resources, normoglycemia is achieved through the release of glucose from the liver as a result of glycogenolysis and the GNG pathway, which produces glucose “de novo” using non-carbohydrate substrates. Like glucose, ketone bodies produced by the liver are also released into the bloodstream, and serve as additional energy sources for the brain []. Thus, the obligatory dependence of the vital activity of the brain on the metabolic function of the liver is axiomatic. Unfortunately, despite extensive research into the mechanisms of HE, the literature on ammonia-induced disturbances of liver metabolic pathways (except for the urea cycle) remains fragmented. This scarcity of data persists because it is generally assumed that the liver, being evolutionarily adapted for ammonia detoxification, cannot be damaged by the toxin itself [,]. However, this traditional view fails to account for the systemic nature of HE, where ammonia acts as a multiorgan toxin that directly impairs the hepatic metabolic capacity itself [,]. Therefore, to identify the true mechanisms of this pathology, a comprehensive analysis of recent advances is crucial. Understanding how ammonia derails highly integrated metabolic pathways in the liver, thereby disrupting blood glucose homeostasis, will help clarify how it leads to an energy crisis in the brain and, ultimately, to encephalopathy.