Comparison of the effects of ammonia on brain mitochondrial function in rats and gulf toadfish

Abstract

We compared the effect of hyperammonemia on NADH levels in brain slices and on the rate of oxygen consumption from isolated nonsynaptic brain mitochondria in ammonia-sensitive Wistar rats with that in ammonia-tolerant gulf toadfish (Opsanus beta). The NADH content was significantly decreased (12% less than control after 45 min with 1 mM NH4Cl) in rat brain slices, but it was not affected in brain slices from toadfish (with both 1 and 6 mM NH 4Cl). The rates of oxygen consumption of different sets of enzymes of the electron transport chain (ETC; complexes I, II, III, and IV; II, III, and IV; and IV alone) were unaltered by hyperammonemic conditions in isolated nonsynaptic mitochondria from either rats or toadfish. These results lead us to conclude that the differing effects of ammonia on NADH levels in rat and toadfish brain slices must be due to aspects other than the direct effects of ammonia on enzymes of the ETC. Additionally, because these effects were seen in vitro, our studies enabled us to rule out the possibility that effects of ammonia on metabolism were via indirect systemic effects. These results are discussed in the context of current views on mechanisms of central nervous system damage in hyperammonemic states. hepatic encephalopathy; hyperammonemia; NADH; mitochondria; Opsanus beta; glutamine metabolism MAMMALS IN GENERAL are physiologically sensitive to increases in the concentration of ammonia in the body. In particular, excess brain ammonia 1 (i.e., as low as 500 to 1,000 mol/kg tissue wt) caused by liver failure (e.g., hepatic encephalopathy) or inborn errors in urea metabolism or by injection of ammonia in experimental animals leads to brain neuropathies. A general sequence of symptoms progresses from altered sleep patterns to muscular incoordination, stupor, coma, and death, at a rate that is dependent on the extent and rate of ammonia intoxication; a lethal dose of ammonia can cause symptoms within minutes [reviewed by Cooper and Plum (6) and more recently by Hazell and Butterworth (10)]. Much earlier literature focuses on the causes of these neurological symptoms being related to "cerebral energy failure." Indeed, in the early stages of hyperammonemia, brain metabolic rate decreases, with precipitous drops in whole brain creatine phosphate levels, whereas whole brain ATP content falls later in the onset of disease. Because the decline in whole brain ATP follows severe neurological impairment, many have argued that cerebral energy failure cannot be the cause of the impairment.Following this line of reasoning, many investigators have now focused attention on other changes associated with hyperammonemia and hepatic encephalopathy. One is the phenomenon of astrocyte swelling in hyperammonemia-induced brain pathologies. Astrocytes contain most of the brain's activity of the enzyme glutamine synthetase (GSase), which ostensibly detoxifies ammonia by combining it with glutamate to form glutamine (20). Thus, during hyperammonemia, glutamine accumulates in the brain and astrocytes then become swolle...

Piscine insights into comparisons of anoxia tolerance, ammonia toxicity, stroke and hepatic encephalopathy

“…However, exposure to sublethal concentrations of ammonia disrupted cerebral amino acid metabolism. A more recent toadfish study revealed that brain mitochondrial metabolism was also not severely disrupted by high concentrations of ammonia (up to 60% of the 96h LC 50 ) (Veauvy et al, 2002). It will be interesting to determine if, as in mammals, disruption of brain energy metabolism per se is not a key explanation for toxicity of ammonia in fish.…”

“…The potential for brain swelling in fish has only been examined in one study of a species that is highly tolerant to ammonia exposure (the gulf toadfish, Opsanus beta) where 96h LC 50 values are 10 mM, (Wang and Walsh, 2002). Veauvy et al (2005) recently demonstrated that despite substantial chronic exposure to sub-lethal concentrations of ammonia (1/3 of the 96h LC 50 ) or acute exposure to lethal concentrations of ammonia (1 to 3 times the 96h LC 50 ), toadfish showed no signs of brain swelling as assessed by water status using magnetic resonance imaging.…”

Effects of acute exposure to exogenous ammonia on cerebral monoaminergic neurotransmitters in juvenile Solea senegalensis

“…It is known that in mammals the brain is very sensitive to excess ammonia, and that neurotransmitter systems, mitochondrial function, brain energy metabolism and astrocytes are affected (Szerb and Butterworth 1992;Kosenko et al 1994;Butterworth 1998;Monfort et al 2002;Felipo and Butterworth 2002;Veauvy et al 2002). In fish, exposure to ammonia alters the predatorprey behaviour (Heath 1987) and inhibits feeding (Beamish and Tandler 1990;Ortega et al 2005); these types of behaviour are modulated by monoaminergic neurotransmission (5-HT, DA and NA) (De Pedro et al 1998a, b;Weis et al 2000;Ruibal et al 2002).…”

Interactions between cortisol and Rhesus glycoprotein expression in ureogenic toadfish, Opsanus beta

“…However, toadfish experience elevated plasma ammonia shortly following feeding [~300-500mmoll -1 (Rodela et al, 2012)] or during exposure to high ambient ammonia [~1000mmoll -1 Veauvy et al, 2002)]. In these circumstances, internal ammonia concentrations likely exceed the ammonia-trapping capacity of GS and, as a result, toadfish may rely more heavily on transport pathways to eliminate ammonia.…”