The action of ammonia on astrocyte glycogen and glycogenolysis

Most of the brain glycogen, a major energy reserve that can be mobilized in response to increased neuronal activity, resides in the astrocyte, the site of the neuropathological abnormality found in hepatic encephalopathy (HE). Ammonia, a neurotoxin implicated in the pathogenesis of HE, has been reported to cause a depletion of glycogen in primary astrocyte cultures. To further investigate the action of ammonia on glycogen levels, cultured astrocytes were exposed to ammonium chloride (1-5 mM) for various times up to 7 d. Treatment with ammonia for 24 h did not alter deoxyglucose uptake, but significantly lowered peak glycogen values (found at 1.5 h following feeding with medium containing 5.5 mM glucose) in a concentration-dependent manner. This inhibitory effect was not observed after longer exposure times to ammonia. Three day treatment of cells did, however, significantly reduce norepinephrine-stimulated glycogenolysis, an effect not seen after 1 d of ammonia treatment. Part of the neurotoxic action of long term ammonia exposure in humans and experimental animals may be to inhibit the breakdown of glycogen. The effect of ammonia on astrocyte glycogen synthesis and/or breakdown may disrupt glial neuronal signaling and thus play a role in the pathogenesis of HE.