Source Neuroscience Research Unit, Saint-Luc Hospital (CHUM), University of Montreal, Montreal, Quebec, Canada.
Acute liver failure (ALF) is characterized neuropathologically by cytotoxic brain edema and biochemically by increased brain ammonia and its detoxification product, glutamine. The osmotic actions of increased glutamine synthesis in astrocytes are considered to be causally related to brain edema and its complications (intracranial hypertension, brain herniation) in ALF. However studies using multinuclear (1)H- and (13)C-NMR spectroscopy demonstrate that neither brain glutamine concentrations per se nor brain glutamine synthesis rates correlate with encephalopathy grade or the presence of brain edema in ALF.
An alternative mechanism is now proposed whereby the newly synthesized glutamine is trapped within the astrocyte as a consequence of down-regulation of its high affinity glutamine transporter SNAT5 in ALF. Restricted transfer out of the cell rather than increased synthesis within the cell could potentially explain the cell swelling/brain edema in ALF. Moreover, the restricted transfer of glutamine from the astrocyte to the adjacent glutamatergic nerve terminal (where glutamine serves as immediate precursor for the releasable/transmitter pool of glutamate) could result in decreased excitatory transmission and excessive neuroinhibition that is characteristic of encephalopathy in ALF.
Paradoxically, in spite of renewed interest in arterial ammonia as a predictor of raised intracranial pressure and brain herniation in ALF, ammonia-lowering agents aimed at reduction of ammonia production in the gut have so far been shown to be of limited value in the prevention of these cerebral consequences. Mild hypothermia, shown to prevent brain edema and intracranial hypertension in both experimental and human ALF, does so independent of effects on brain glutamine synthesis; whether or not hypothermia restores expression levels of SNAT5 in ALF awaits further studies. While inhibitors of brain glutamine synthesis such as methionine sulfoximine, have been proposed for the prevention of brain edema in ALF, potential adverse effects have so far limited their applicability.