Binding of the ligand [3H]MK-801 to the MK-801 binding site of the N-methyl-D-aspartate receptor during experimental encephalopathy from acute liver failure and from acute hyperammonemia in the rabbit

The brain glutamate system in liver failure

Liver failure results in significant alterations of the brain glutamate system. Ammonia and the astrocyte play major roles in such alterations, which affect several components of the brain glutamate system, namely its synthesis, intercellular transport (uptake and release), and function. In addition to the neurological symptoms of hepatic encephalopathy, modified glutamatergic regulation may contribute to other cerebral complications of liver failure, such as brain edema, intracranial hypertension and changes in cerebral blood flow. A better understanding of the cause and precise nature of the alterations of the brain glutamate system in liver failure could lead to new therapeutic avenues for the cerebral complications of liver disease.

Increased extracellular brain glutamate in acute liver failure: decreased uptake or increased release?

Glutamatergic dysfunction has been suggested to play an important role in the pathogenesis of hepatic encephalopathy (HE) in acute liver failure (ALF). Increased extracellular brain glutamate concentrations have consistently been described in different experimental animal models of ALF and in patients with increased intracranial pressure due to ALF. High brain ammonia levels remain the leading candidate in the pathogenesis of HE in ALF and studies have demonstrated a correlation between ammonia and increased concentrations of extracellular brain glutamate both clinically and in experimental animal models of ALE Inhibition of glutamate uptake or increased glutamate release from neurons and/or astrocytes could cause an increase in extracellular glutamate. This review analyses the effect of ammonia on glutamate release from (and uptake into) both neurons and astrocytes and how these pathophysiological mechanisms may be involved in the pathogenesis of HE in ALF.

Effects of hyperammonemia and liver failure on glutamatergic neurotransmission

Glutamate is the main excitatory neurotransmitter in mammals. Glutamatergic neurotransmission involves several steps, beginning with release of glutamate from the presynaptic neuron. Glutamate in the extracellular space activates glutamate receptors present in the synaptic membranes, leading to activation of signal transduction pathways associated with these receptors. To avoid continuous activation of glutamate receptors, glutamate is removed from the synaptic cleft by specific glutamate transporters located mainly on astrocytes. All these steps are tightly modulated under physiological conditions, and alterations of any of the above steps may result in impairment of glutamatergic neurotransmission, leading to neurological alterations. There are studies in the literature reporting alterations in all these steps in hyperammonemia and/or hepatic failure. Glutamatergic neurotransmission modulates important cerebral processes. Some of these processes are altered in patients with liver disease and hepatic encephalopathy, who show altered sleep-wake patterns, neuromuscular coordination, and decreased intellectual capacity. The alterations in glutamatergic neurotransmission may be responsible for some of these neurological alterations found in hepatic encephalopathy. The effects of hyperammonemia and liver failure on different steps of glutamatergic neurotransmission including alterations of glutamate concentration in the extracellular fluid in brain, transport and transporters of glutamate, the content and function of different types of glutamate receptors and signal transduction pathways. Alterations induced by hyperammonemia and liver failure on the glutamate-nitric oxide-cGMP pathway in brain may result in changes in long-term potetiation and learning ability.

Effects of thioacetamide-induced hepatic failure on the N-methyl-D-aspartate receptor complex in the rat cerebral cortex, striatum, and hippocampus. Binding of different ligands and expression of receptor subunit mRNAs

Hepatic encephalopathy (HE) is characterized by symptoms pointing at disturbances in glutamatergic neurotransmission in the brain, particularly in the striatum. The binding parameters of ligands specific for different recognition sites in the N-methyl-D-aspartate (NMDA) receptor complex and the distribution of the receptor subunit mRNAs (NR1, NR2A-D) were assessed in rats with acute HE induced with a hepatotoxin, thioacetamide (TAA). The binding of: 1. L-[3H]glutamate (NMDA-displaceable); 2. [3H]dizocilpine and N-(1-[2-thienyl]-cyclohexyl) [3H]piperidine ([3H]TCP); and 3. The coactivator site agonist [3H]glycine was assayed in purified membranes of the cerebral cortex, hippocampus, and striatum. In HE rats, Bmax of NMDA-displaceable glutamate binding was increased in the cerebral cortex and hippocampus, but slightly decreased in the striatum. In this region, the binding affinity was also slightly increased. In HE, Bmax of [3H]dizocilpine binding was unchanged in the striatum and cerebral cortex, but substantially decreased in the hippocampus. Pretreatment with phorbol ester enhanced the binding of dizocilpine more in HE than in control rats. Bmax of [3H]TCP binding was decreased in the cerebral cortex and striatum, but increased in the hippocampus. The different responses of these two phencyclidine site antagonists to HE may be indicative of a conformational change within the ion channel and/or the presence of microdomains reacting differently to extrinsic factors. HE did not affect glycine binding, but potentiated the maximal stimulation of [3H]dizocilpine binding by glycine in the cerebral cortex. The results emphasize the brain region and domain specificity of the responses of the NMDA receptor complex to HE.

Release of [3H]dopamine from striatal and cerebral cortical slices from rats with thioacetamide-induced hepatic encephalopathy: different responses to stimulation by potassium ions and agonists of ionotropic glutamate receptors

The effects of depolarizing stimuli; high (50 mM) potassium ions and the glutamate receptor agonists N-methyl-D-aspartate, kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) on the release of newly-loaded [3H]dopamine were studied in frontal cortical and striatal slices from control rats and from rats with acute hepatic encephalopathy induced with a hepatotoxin, thioacetamide. Hepatic encephalopathy enhanced the stimulatory effect of potassium ions by 20% in striatal slices and by 34% in frontal cortical slices. In striatal slices the stimulatory effects of N-methyl-D-aspartate and kainate were depressed in hepatic encephalopathy by 46% and 21%, respectively, which may be taken to reflect impaired modulation of striatal dopamine release by glutamate acting at N-methyl-D-aspartate or kainate receptors. In frontal cortical slices, the stimulatory effect of kainate was enhanced by 35% in hepatic encephalopathy but N-methyl-D-aspartate-stimulated release was not affected. The release evoked by 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate was not affected in hepatic encephalopathy in either brain region. Stimulation of dopamine release in the frontal cortex by depolarization or glutamate acting at kainate receptors could inhibit the activity of descending corticostriatal glutamatergic pathways, further impairing regulation of dopamine release by glutamate in the striatum.

Neuroactive amino acids in hepatic encephalopathy

There is abundant evidence to suggest that alterations of excitatory and inhibitory amino acids play a significant role in the pathogenesis of hepatic encephalopathy (HE) in both acute and chronic liver diseases. Brain glutamate concentrations are reduced in patients who died in hepatic coma as well as in experimental HE, astrocytic reuptake of glutamate is compromised in liver failure and postsynaptic glutamate receptors (both NMDA and non-NMDA subclasses) are concomitantly reduced in density. Recent studies in experimental acute liver failure suggest reduced capacity of the astrocytic glutamate transporter in this condition. Together, this data suggests that neuron-astrocytic trafficking of glutamate is impared in HE. Other significant alterations of neuroactive amino acids in HE include a loss of taurine from brain cells to extracellular space, a phenomenon which could relate both to HE and to brain edema in acute liver failure. Increased concentrations of benzodiazepine-like compounds have been reported in human and experimental HE. Clinical trials with the benzodiazepine antagonist flumazenil reveal a beneficial effect in some patients with HE; the mechanism responsible for this effect, however, remains to be determined.

Ammonium acetate inhibits ionotropic receptors and differentially affects metabotropic receptors for glutamate

The effects of ammonium salts in concentration similar to those found in plasma in course of hepatic encephalopathy (2-4 mM) were studied in brain slices in order to clarify how glutamate synapses are affected by this pathological situation. Electrophysiological (mice cortical wedge preparations) and biochemical techniques (inositol phosphates and cyclic AMP measurements) were used so that the function of both the ionotropic and metabotropic glutamate receptors was evaluated. Ammonium acetate (2-4 mM), but not sodium acetate reduced the degree of depolarization of cortical wedges induced by different concentrations of N-methyl-D-aspartic acid (NMDA) or (S)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). This reduction was non-competitive in nature and did not reverse during the experimental period (90 min). In a similar manner, ammonium acetate reduced the formation of inositol phosphates induced by (1S,3R)-1-amynocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (100 microM), the prototype agonist of metabotropic glutamate receptors. When the metabotropic glutamate receptors negatively linked to the forskolin-stimulated cyclic AMP formation were evaluated, ammonium acetate significantly hampered forskolin effects and its actions were additive with those of the metabotropic glutamate receptor agonist 1S,3R-ACPD. In conclusion, our results suggest that toxic concentrations of ammonium impair the function of glutamate receptors of NMDA and AMPA type and of the metabotropic glutamate receptors linked to inositol phosphate formation while they functionally potentiate the action of glutamate agonists on the receptors negatively linked to adenylyl cyclase.