Background: Excessive accumulation of ammonia in the brain is a causative factor of an array of neurological manifestations of hyperammonemic encephalopathies (“hyperammonemias”, HA) among which hepatic encephalopathy (HE) is a major epidemiologic and therapeutic challenge. While ammonia neurotoxicity is symptomatically and mechanistically very complex, there is a consensus with regard to the leading role in its pathogenesis of: i) astrocytes being the primary cellular target of ammonia toxicity; ii) alterations of glutamate (Glu)-dependent neurotransmission (over-excitation followed by inhibition of glutamatergic tone) being the cornerstone of its neurophysiological manifestations; and iii) brain edema, an often lethal consequence of astrocytic swelling, being among other factors caused by the retention of glutamine (Gln) in these cells. Objective: This article critically evaluates the present literature attempting to relate manifestations of HA to changes in astrocytic Glu and Gln transport as observed in different in vivo and in vitro HA and/or HE models. Emphasis is put on two disproportions in the state of the art: i) the paucity of available data regarding ammonia-dependent changes in Glu transport activity vs the relative abundance of information on the expression of astrocytic Glu transporters (GLT-1/EAAT2 and GLAST/EAAT1); ii) the just emerging still not very conclusive knowledge on the response of astrocytic Gln transporters SN1 and SN2. Conclusion: The review on the above issues is complemented by own recent data which fill some of the many gaps in the knowledge. A brief account is included on the roles of heteromeric cell membrane Glu/arginine (Arg) exchanger y+LAT2 and on the mitochondrial Gln transport.

Ammonia is considered the main pathogenic toxin in hepatic encephalopathy (HE). However, the molecular mechanisms involved have been disputed. As altered glutamatergic and GABAergic neurotransmission has been reported in HE, we investigated whether four members of the solute carrier 38 (Slc38) family of amino acid transporters—involved in the replenishment of glutamate and GABA—contribute to ammonia neurotoxicity in HE. We show that ammonium ion exerts multiple actions on the Slc38 transporters: It competes with glutamine for the binding to the system N transporters Slc38a3 and Slc38a5, consequently inhibiting bidirectional astroglial glutamine transport. It also competes with H ⁺ , Na ⁺ , and K ⁺ for uncoupled permeation through the same transporters, which may perturb astroglial intracellular pH, membrane potential, and K ⁺ ‐buffering. Knockdown of Slc38a3 in mice results in cerebral cortical edema and disrupted neurotransmitter synthesis mimicking events contributing to HE development. Finally, in a mouse model of acute liver failure (ALF), we demonstrate the downregulation of Slc38a3 protein, impeded astroglial glutamine release, and cytotoxic edema. Altogether, we demonstrate contribution of Slc38 transporters to the ammonia‐induced impairment of glutamine recycling between astrocytes and neurons, a phenomenon underlying acute ammonia neurotoxicity in the setting of ALF.

Excessive glutamine (Gln) synthesis in ammonia-overloaded astrocytes contributes to astrocytic swelling and brain edema, the major complication of hepatic encephalopathy (HE). Much of the newly formed Gln is believed to enter mitochondria, where it is recycled to ammonia, which causes mitochondrial dysfunction (a “Trojan horse” mode of action). A portion of Gln may increase osmotic pressure in astrocytes and the interstitial space, directly and independently contributing to brain tissue swelling. Here we discuss the possibility that altered functioning of Gln transport proteins located in the cellular or mitochondrial membranes, modulates the effects of increased Gln synthesis. Accumulation of excess Gln in mitochondria involves a carrier-mediated transport which is activated by ammonia. Studies on the expression of the cell membrane N-system transporters SN1 (SNAT3) and SN2 (SNAT5), which mediate Gln efflux from astrocytes rendered HE model-dependent effects. HE lowered the expression of SN1 at the RNA and protein level in the cerebral cortex (cc) in the thioacetamide (TAA) model of HE and the effect paralleled induction of cerebral cortical edema. Neither SN1 nor SN2 expression was affected by simple hyperammonemia, which produces no cc edema. TAA-induced HE is also associated with decreased expression of mRNA coding for the system A carriers SAT1 and SAT2, which stimulate Gln influx to neurons. Taken together, changes in the expression of Gln transporters during HE appear to favor retention of Gln in astrocytes and/or the interstitial space of the brain. HE may also affect arginine (Arg)/Gln exchange across the astrocytic cell membrane due to changes in the expression of the hybrid Arg/Gln transporter y⁺LAT2. Gln export from brain across the blood–brain barrier may be stimulated by HE via its increased exchange with peripheral tryptophan.

Neurological symptoms of acute liver failure (ALF) reflect decreased excitatory transmission, but the status of ALF-affected excitatory synapse has not been characterized in detail. We studied the effects of ALF in mouse on synaptic transmission and plasticity ex vivo and its relation to distribution of (i) synaptic vesicles (sv) and (ii) functional synaptic proteins within the synapse. ALF-competent neurological and biochemical changes were induced in mice with azoxymethane (AOM). Electrophysiological characteristics (long-term potentiation, whole-cell recording) as well as synapse ultrastructure were evaluated in the cerebral cortex. Also, sv were quantified in the presynaptic zone by electron microscopy. Finally, presynaptic proteins in the membrane-enriched (P2) and cytosolic (S2) fractions of cortical homogenates were quantitated by Western blot. Slices derived from symptomatic AOM mice presented a set of electrophysiological correlates of impaired transmitter release including decreased field potentials (FPs), increased paired-pulse facilitation (PPF), and decreased frequency of spontaneous and miniature excitatory postsynaptic currents (sEPSCs/mEPSCs) accompanied by reduction of the spontaneous transmitter release-driving protein, vti1A. Additionally, an increased number of sv per synapse and a decrease of P2 content and/or P2/S2 ratio for sv-associated proteins, i.e. synaptophysin, synaptotagmin, and Munc18–1, were found, in spite of decreased content of the sv-docking protein, syntaxin-1. Slices from AOM-treated asymptomatic mice showed impaired long-term potentiation (LTP) and increased PPF but no changes in transmitter release or presynaptic protein composition. Our findings demonstrate that a decrease of synaptic transmission in symptomatic ALF is associated with inefficient recruitment of sv proteins and/or impaired sv trafficking to transmitter release sites.