Pharmacological manipulation of cyclic GMP levels in brain restores learning ability in animal models of hepatic encephalopathy: therapeutic implications

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome present in patients with liver disease that includes impaired intellectual function. To develop therapeutic treatments to restore cognitive function, it is important to understand the molecular mechanisms that impair cognitive function in HE. This review summarizes data showing that: (a) cognitive function and learning are impaired in patients with liver disease and in animal models of chronic liver failure or hyperammonemia; (b) the glutamate–NO–cGMP pathway modulates some forms of learning; and (c) the function of this pathway is impaired in brain in vivo in rats with chronic hyperammonemia or liver failure and from patients who died from HE. Learning ability of hyperammonemic rats was restored by increasing cGMP by: (1) continuous intracerebral administration of zaprinast, an inhibitor of the cGMP-degrading phosphodiesterase; (2) chronic oral administration of sildenafil, an inhibitor of the phosphodiesterase that crosses the blood–brain barrier; and (3) continuous intracerebral administration of cGMP. The data summarized indicate that impairment of learning ability in rats with chronic liver failure or hyperammonemia is due to impairment of the glutamate–NO–cGMP pathway. Moreover, increasing extracellular cGMP by pharmacological means may be a new therapeutic approach to improve cognitive function in patients with HE.