A study involving rats reveals the mechanism responsible for learning impairment due to liver failure and shows that sildenafil restores cognitive function.
Liver disease sometimes causes hepatic encephalopathy, which involves brain damage, personality changes, and intellectual impairment due to hyperammonemia (high levels of ammonia in the blood). However, the mechanisms involved in both learning and how liver disease leads to learning impairment are unclear.
In a new study led by Vicente Felipo of the Laboratory of Neurobiology at the Fundacion Valenciana de Investigaciones Biomedicas in Valencia, Spain and published in the February 2005 issue of Hepatology, researchers hypothesized that impaired learning was due to a defect in the glutamate- nitric oxide-cGMP pathway in the brain and that administering sildenafil to increase cGMP would restore learning ability. Sildenafil, commonly known as Viagra, is known to prevent the destruction of cGMP and allow it to accumulate in the body.
Hepatology, the official journal of the American Association for the Study of Liver Diseases (AASLD), published by John Wiley & Sons, Inc. is available online via Wiley InterScience.
Researchers examined four groups of rats in their study:
- rats in which they constructed portacaval shunts (a treatment used to treat high blood pressure in the liver due to liver disease that is believed to be one of the causes of hepatic encephalopathy, and also a model of chronic liver failure in rats);
- rats with portacaval shunts that were given sildenafil;
- rats that were fed an ammonium-containing diet;
- and rats that were fed the diet and given sildenafil. They also used control groups consisting of rats fed a normal diet both with and without sildenafil. All animals were subjected to a maze learning test four weeks following surgery or from the date when drug treatment began. Levels of both cGMP and ammonia in brain were measured using a microdialysis probe.
Results showed that while rats with the portacaval shunt showed a reduced learning ability, treatment of shunted rats with sildenafil restored their ability to learn. Tests showed that the concentration of cGMP was reduced in the extracellular fluid in brains of shunted rats compared with controls and that treatment with sildenafil restored levels of cGMP in these animals. In addition, further tests showed a reduction of 74 percent in the function of the glutamate- nitric oxide-cGMP pathway in shunted rats, while treatment with sildenafil significantly enhanced the function of this pathway. These evaluations were also performed on rats with hyperammonemia. Results showed that chronic hyperammonemia significantly reduced the rats' ability to learn, but that treatment with sildenafil restored their learning ability. While sildenafil treatment restored levels of cGMP and enhanced the function of the glutamate-nitric oxide-cGMP pathway in hyperammonemic rats, it did not affect ammonia levels.
"The fact that rats with portacaval anastomosis [shunts] or with hyperammonemia without liver failure show the same alterations in the function of the [glutamate-nitric oxide- cGMP] pathway, extracellular cGMP and learning ability indicates that hyperammonemia, which is the only common alteration in both models, is responsible for the alteration of the function of the pathway and, subsequently, of the impairment of learning ability," the authors state. They note, however, that an excessive increase in cGMP may impair learning and that it must be kept high but below a certain threshold to reach maximum learning ability.
The authors conclude: "Although caution must be taken considering the possible deleterious increase in the existing vasodilatation in liver disease by sildenafil, pharmacological manipulation of cGMP in brain by safe procedures may be a useful treatment to restore cognitive and intellectual functions in patients with overt or minimal hepatic encephalopathy."