Hyperammonemia Enhances GABAergic Neurotransmission in Hippocampus: Underlying Mechanisms and Modulation by Extracellular cGMP

Rats with chronic hyperammonemia reproduce the cognitive and motor impairment present in patients with hepatic encephalopathy. It has been proposed that enhanced GABAergic neurotransmission in hippocampus may contribute to impaired learning and memory in hyperammonemic rats. However, there are no direct evidences of the effects of hyperammonemia on GABAergic neurotransmission in hippocampus or on the underlying mechanisms. The aims of this work were to assess if chronic hyperammonemia enhances the function of GABA(A) receptors in hippocampus and to identify the underlying mechanisms. Activation of GABA(A) receptors is enhanced in hippocampus of hyperammonemic rats, as analyzed in a multielectrode array system. Hyperammonemia reduces membrane expression of the GABA transporters GAT1 and GAT3, which is associated with increased extracellular GABA concentration. Hyperammonemia also increases gephyrin levels and phosphorylation of the beta 3 subunit of GABA(A) receptor, which are associated with increased membrane expression of the GABA(A) receptor subunits alpha 1, alpha 2, gamma 2, beta 3, and delta. Enhanced levels of extracellular GABA and increased membrane expression of GABA(A) receptors would be responsible for the enhanced GABAergic neurotransmission in hippocampus of hyperammonemic rats. Increasing extracellular cGMP reverses the increase in GABA(A) receptors activation by normalizing the membrane expression of GABA transporters and GABA(A) receptors. The increased GABAergic neurotransmission in hippocampus would contribute to cognitive impairment in hyperammonemic rats. The results reported suggest that reducing GABAergic tone in hippocampus by increasing extracellular cGMP or by other means may be useful to improve cognitive function in hyperammonemia and in cirrhotic patients with minimal or clinical hepatic encephalopathy.

Automated summary

This study found that chronic hyperammonemia enhances the function of GABA(A) receptors in the hippocampus and promotes the transmission of GABA neurotransmitters by reducing the membrane expression of GABA transporters and increasing the membrane expression of GABA(A) receptor subunits. Increasing extracellular cGMP can reverse this increase in function. The results of this study suggest that modulating GABA regulation may improve cognitive function in patients with hyperammonemia and hepatic encephalopathy.