Free amino acids in fish brain: normal levels and changes upon exposure to high ammonia concentrations in vivo, and upon incubation of brain slices

Nutrition and metabolism of glutamate and glutamine in fish

Glutamate (Glu) and glutamine (Gln) comprise a large proportion of total amino acids (AAs) in fish in the free and protein-bound forms. Both Glu and Gln are synthesized de novo from other α-amino acids and ammonia. Although these two AAs had long been considered as nutritionally non-essential AAs for an aquatic animal, they must be included adequately in its diet to support optimal health (particularly intestinal health) and maximal growth. In research on fish nutrition, Glu has been used frequently as an isonitrogenous control on the basis of the assumption that this AA has no nutritional or physiological function. In addition, purified diets used for feeding fish generally lack glutamine. As functional AAs, Glu and Gln are major metabolic fuels for tissues of fish (including the intestine, liver, kidneys, and skeletal muscle), and play important roles not only in protein synthesis but also in glutathione synthesis and anti-oxidative reactions. The universality of Glu and Gln as abundant intracellular AAs depends on their enormous versatility in metabolism. Dietary supplementation with Glu and Gln to farmed fish can improve their growth performance, intestinal development, innate and adaptive immune responses, skeletal muscle development and fillet quality, ammonia removal, and the endocrine status. Glu (mainly as monosodium glutamate), glutamine, or AminoGut (a mixture of Glu and Gln) is a promising feed additive to reduce the use of fishmeal, while gaining the profitability of global aquaculture production. Thus, the concept of dietary requirements of fish for Glu and Gln is a paradigm shift in the nutrition of aquatic animals (including fish).

Air-breathing and excretory nitrogen metabolism in fishes

During water-land transition, ancient fishes acquired the ability to breathe air, but air-breathing engendered problems in nitrogenous waste excretion. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, and the degradation of these nitrogen-containing compounds releases ammonia. Ammonia is toxic and must be removed. Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills. Hence, ancient fishes that participated in water-land transition must have developed means to deal with ammonia toxicity. Extant air-breathing fishes, particularly amphibious ones, can serve as models to examine adaptations which might have facilitated the emergence of ancient fishes from water. Some of these fishes can actively emerge from water and display complex behaviors on land, while a few can burrow into mud and survive for years during drought. Many of them are equipped with mechanisms to ameliorate ammonia toxicity during emersion. In this review, the mechanisms adopted by air-breathing fishes to deal with ammonia toxicity during emersion were organized into seven disparate strategies. In addition, eight extant air-breathing fishes with distinctive terrestrial behaviors and peculiar natural habitats were selected to describe in detail how these seven strategies could be adopted in disparate combinations to ameliorate ammonia toxicity during emersion.

Reversible brain swelling in crucian carp (Carassius carassius) and goldfish (Carassius auratus) in response to high external ammonia and anoxia

Increased internal ammonia (hyperammonemia) and ischemic/anoxic insults are known to result in a cascade of deleterious events that can culminate in potentially fatal brain swelling in mammals. It is less clear, however, if the brains of fishes respond to ammonia in a similar manner. The present study demonstrated that the crucian carp (Carassius carassius) was not only able to endure high environmental ammonia exposure (HEA; 2 to 22 mmol L(-1)) but that they experienced 30% increases in brain water content at the highest ammonia concentrations. This swelling was accompanied by 4-fold increases in plasma total ammonia (TAmm) concentration, but both plasma TAmm and brain water content were restored to pre-exposure levels following depuration in ammonia-free water. The closely related, ammonia-tolerant goldfish (Carassius auratus) responded similarly to HEA (up to 3.6 mmol L(-1)), which was accompanied by 4-fold increases in brain glutamine. Subsequent administration of the glutamine synthetase inhibitor, methionine sulfoximine (MSO), reduced brain glutamine accumulation by 80% during HEA. However, MSO failed to prevent ammonia-induced increases in brain water content suggesting that glutamine may not be directly involved in initiating ammonia-induced brain swelling in fishes. Although the mechanisms of brain swelling are likely different, exposure to anoxia for 96 h caused similar, but lesser (10%) increases in brain water content in crucian carp. We conclude that brain swelling in some fishes may be a common response to increased internal ammonia or lower oxygen but further research is needed to deduce the underlying mechanisms behind such responses.

Adaptations of a deep sea scavenger: high ammonia tolerance and active NH₄⁺ excretion by the Pacific hagfish (Eptatretus stoutii)

The Pacific hagfish (Eptatretus stoutii) has an exceptional ability to both withstand and recover from exposure to high external ammonia (HEA). This tolerance is likely due to the feeding behavior of this scavenger, which feeds on intermittent food falls of carrion (e.g. fish, large marine mammals) during which time it may be exposed to high concentrations of total ammonia (T(Amm)=NH3+NH4(+)) while burrowed inside the decomposing carcass. Here we exposed hagfish to 20 mmol L(-1) T(Amm) for periods of up to 48 h and then let animals recover in ammonia-free seawater. During the 48 h HEA exposure period, plasma T(Amm) increased 100-fold to over 5000 μmol L(-1) while ammonia excretion (J(amm)) was transiently inhibited. This increase in plasma T(Amm) resulted from NH3 influx down massive inwardly directed ΔP(NH3) gradients, which also led to a short-lived metabolic alkalosis. Plasma [T(Amm)] stabilized after 24-48 h, possibly through a reduction in NH3 permeability across the body surface, which lowered NH3 influx. Ammonia balance was subsequently maintained through the re-establishment of J(amm) against an inwardly directed ΔP(NH3). Calculations of the Nernst potential for ammonia strongly indicated that J(amm) was also taking place against a large inwardly directed NH4(+) electrochemical gradient. Recovery from HEA in ammonia-free water was characterized by a large ammonia washout, and the restoration of plasma TAmm concentrations to near control concentrations. Ammonia clearance was also accompanied by a residual metabolic acidosis, which likely offset the ammonia-induced metabolic alkalosis seen in the early stages of HEA exposure. We conclude that restoration of J(amm) by the Pacific hagfish during ammonia exposure likely involves secondary active transport of NH4(+), possibly mediated by Na(+)/NH4(+) (H(+)) exchange.

Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.

Regulation of amino acid metabolism as a defensive strategy in the brain of three freshwater teleosts in response to high environmental ammonia exposure

Many teleosts have evolved mechanisms to cope with ammonia toxicity in the brain when confronted with high environmental ammonia (HEA). In the present study, the possible role of conversion of accumulated ammonia to glutamine and other free amino acids in the brain of three freshwater teleosts differing in their sensitivities to ammonia was investigated. The detoxification mode of ammonia in brain is suggested to be through amination of glutamate to glutamine by the coupled activities of glutamate dehydrogenase (GDH), transaminase (aspartate aminotransaminase 'AST' and alanine aminotransaminase 'ALT') and glutamine synthetase (GSase). We investigated the metabolic response of amino acids in the brain of highly sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less sensitive cyprinid Cyprinus carpio (common carp) and the highly resistant cyprinid Carassius auratus (goldfish) when exposed to 1mM ammonia (as NH4HCO3; pH 7.9) for 0 h (control), 3 h, 12 h, 24 h, 48 h, 84 h and 180 h. Results show that HEA exposure increased ammonia accumulation significantly in the brain of all the three species from 12h onwards. Unlike in trout, ammonia accumulation in carp and goldfish was restored to control levels (48-84h); which was accompanied with a significant increase in glutamine content as well as GSase activity. In trout, glutamine levels also increased (84-180 h) but GSase was not activated. The elevated glutamine level in trout was accompanied by a significant depletion of the glutamate pool in contrast to the stable glutamate levels seen in carp and goldfish. This suggests a simultaneous increase in the rate of glutamate formation to match with the demand of glutamine formation in cyprinids. The activity of GDH was elevated significantly in carp and goldfish but remained unaltered in trout. Also, the transaminase enzymes (AST and ALT) were elevated significantly in exposed carp and goldfish while only ALT was up-regulated in trout. Consequently, in carp and goldfish both aspartate and alanine were utilized under HEA, whereas only alanine was consumed in trout. With ammonia treatment, significant changes in concentrations of other amino acids also occurred. None of the species could detoxify brain ammonia into urea. This study suggests that protective strategies to combat ammonia toxicity in brain are more pronounced in carp and goldfish than in trout.

The relationship between NMDA receptor function and the high ammonia tolerance of anoxia-tolerant goldfish

Acute ammonia toxicity in vertebrates is thought to be characterized by a cascade of deleterious events resembling those associated with anoxic/ischemic injury in the central nervous system. A key event is the over-stimulation of neuronal N-methyl-D-aspartate (NMDA) receptors, which leads to excitotoxic cell death. The similarity between the responses to acute ammonia toxicity and anoxia suggests that anoxia-tolerant animals such as the goldfish (Carassius auratus Linnaeus) may also be ammonia tolerant. To test this hypothesis, the responses of goldfish were compared with those of the anoxia-sensitive rainbow trout (Oncorhynchus mykiss Walbaum) during exposure to high external ammonia (HEA). Acute toxicity tests revealed that goldfish are ammonia tolerant, with 96 h median lethal concentration (LC(50)) values of 199 μmol l(-1) and 4132 μmol l(-1) for NH(3) and total ammonia ([T(Amm)]=[NH(3)]+[NH(4)(+)]), respectively. These values were ~5-6 times greater than corresponding NH(3) and T(Amm) LC(50) values measured in rainbow trout. Further, the goldfish readily coped with chronic exposure to NH(4)Cl (3-5 mmol l(-1)) for 5 days, despite 6-fold increases in plasma [T] to ~1300 μmol l(-1) and 3-fold increases in brain [T(Amm)] to 6700 μmol l(-1). Muscle [T(Amm)] increased by almost 8-fold from ~900 μmol kg(-1) wet mass (WM) to greater than 7000 μmol kg(-1) WM by 48 h, and stabilized. Although urea excretion rates (J(Urea)) increased by 2-3-fold during HEA, the increases were insufficient to offset the inhibition of ammonia excretion that occurred, and increases in urea were not observed in the brain or muscle. There was a marked increase in brain glutamine concentration at HEA, from ~3000 μmol kg(-1) WM to 15,000 μmol kg(-1) WM after 48 h, which is consistent with the hypothesis that glutamine production is associated with ammonia detoxification. Injection of the NMDA receptor antagonists MK801 (0.5-8 mg kg(-1)) or ethanol (1-8 mg kg(-1)) increased trout survival time by 1.5-2.0-fold during exposure to 2 mmol l(-1) ammonia, suggesting that excitotoxic cell death contributes to ammonia toxicity in this species. In contrast, similar doses of MK801 or ethanol had no effect on ammonia-challenged (8-9.5 mmol l(-1) T(Amm)) goldfish survival times, suggesting that greater resistance to excitotoxic cell death contributes to the high ammonia-tolerance of the goldfish. Whole-cell recordings measured in isolated brain slices of goldfish telencephalon during in vitro exposure to 5 mmol l(-1) or 10 mmol l(-1) T(Amm) reversibly potentiated NMDA receptor currents. This observation suggested that goldfish neurons may not be completely resistant to ammonia-induced excitotoxicity. Subsequent western blot and densitometric analyses revealed that NMDA receptor NR1 subunit abundance was 40-60% lower in goldfish exposed to 3-5 mmol l(-1) T(Amm) for 5 days, which was followed by a restoration of NR1 subunit abundance after 3 days recovery in ammonia-free water. We conclude that the goldfish brain may be protected from excitotoxicity by downregulating the abundance of functional NMDA receptors during periods when it experiences increased internal ammonia.

Inhibition of glutamine synthetase during ammonia exposure in rainbow trout indicates a high reserve capacity to prevent brain ammonia toxicity

Glutamine synthetase (GSase), the enzyme that catalyses the conversion of glutamate and ammonia to glutamine, is present at high levels in vertebrate brain tissue and is thought to protect the brain from elevated ammonia concentrations. We tested the hypothesis that high brain GSase activity is critical in preventing accumulation of brain ammonia and glutamate during ammonia loading in the ammonia-intolerant rainbow trout. Trout pre-injected with saline or the GSase inhibitor methionine sulfoximine (MSOX, 6 mg kg(-1)), were exposed to 0, 670 or 1000 micromol l(-1) NH(4)Cl in the water for 24 and 96 h. Brain ammonia levels were 3- to 6-fold higher in ammonia-exposed fish relative to control fish and MSOX treatment did not alter this. Brain GSase activity was unaffected by ammonia exposure, while MSOX inhibited GSase activity by approximately 75%. Brain glutamate levels were lower and glutamine levels were higher in fish exposed to ammonia relative to controls. While MSOX treatment had little impact on brain glutamate, glutamine levels were significantly reduced by 96 h. With ammonia treatment, significant changes in the concentration of multiple other brain amino acids occurred and these changes were mostly reversed or eliminated with MSOX. Overall the changes in amino acid levels suggest that multiple enzymatic pathways can supply glutamate for the production of glutamine via GSase during ammonia exposure and that alternative transaminase pathways can be recruited for ammonia detoxification. Plasma cortisol levels increased 7- to 15-fold at 24 h in response to ammonia and MSOX did not exacerbate this stress response. These findings indicate that rainbow trout possess a relatively large reserve capacity for ammonia detoxification and for preventing glutamate accumulation during hyperammonaemic conditions.

Induction of four glutamine synthetase genes in brain of rainbow trout in response to elevated environmental ammonia

The key strategy for coping with elevated brain ammonia levels in vertebrates is the synthesis of glutamine from ammonia and glutamate, catalyzed by glutamine synthetase (GSase). We hypothesized that all four GSase isoforms (Onmy-GS01-GS04) are expressed in the brain of the ammonia-intolerant rainbow trout Oncorhynchus mykiss and that cerebral GSase is induced during ammonia stress. We measured GSase activity and the mRNA expression of Onmy-GS01-GS04 in fore-, mid- and hindbrain and liver, as well as ammonia concentrations in plasma, liver and brain of fish exposed to 9 or 48 h of 0 (control) or 670 micromol l(-1) NH(4)Cl (75% of the 96 h-LC(50) value). The mRNA of all four GSase isoforms were detected in brain (not liver). After 9 h of NH(4)Cl exposure, brain, liver and plasma ammonia content were elevated by two- to fourfold over control values. Midbrain, hindbrain and liver GSase activities were 1.3- to 1.5-fold higher in ammonia-exposed fish relative to control fish. Onmy-GS01-GS04 mRNA levels in brain (not liver) of ammonia-exposed fish (9 h) were significantly elevated by two- to fourfold over control values. After 48 h of the NH(4)Cl treatment, ammonia content and GSase activity, but not mRNA levels, in all tissues examined remained elevated compared to control fish. Taken together, these findings indicate that all four GSase isoforms are constitutively expressed in trout brain and are inducible under high external ammonia conditions. Moreover, elevation of GSase activities in fore-, mid- and hindbrain in response to environmental ammonia underlines the importance of brain GSase in the ammonia-stress response.

Defences against ammonia toxicity in tropical air-breathing fishes exposed to high concentrations of environmental ammonia: a review

In the tropics, air-breathing fishes can be exposed to environmental ammonia when stranded in puddles of water during the dry season, during a stay inside a burrow, or after agricultural fertilization. At low concentrations of environmental ammonia, NH(3) excretion is impeded, as in aerial exposure, leading to the accumulation of endogenous ammonia. At high concentrations of environmental ammonia, which results in a reversed NH(3) partial pressure gradient (DeltaP(NH3)), there is retention of endogenous ammonia and uptake of exogenous ammonia. In this review, several tropical air-breathing fishes (giant mudskipper, African catfish, oriental weatherloach, swamp eel, four-eyed sleeper, abehaze and slender African lungfish), which can tolerate high environmental ammonia exposure, are used as examples to demonstrate how eight different adaptations can be involved in defence against ammonia toxicity. Four of these adaptations deal with ammonia toxicity at branchial and/or epithelial surfaces: (1) active excretion of NH(4)(+); (2) lowering of environmental pH; (3) low NH(3) permeability of epithelial surfaces; and (4) volatilization of NH(3), while another four adaptations ameliorate ammonia toxicity at the cellular and subcellular levels: (5) high tolerance of ammonia at the cellular and subcellular levels; (6) reduction in ammonia production; (7) glutamine synthesis; and (8) urea synthesis. The responses of tropical air-breathing fishes to high environmental ammonia are determined apparently by behavioural adaptations and the nature of their natural environments.

Dogmas and controversies in the handling of nitrogenous wastes: ammonia tolerance in the oriental weatherloach Misgurnus anguillicaudatus

The oriental weatherloach Misgurnus anguillicaudatus is an extremely ammonia-tolerant fish. Many ammonia-protection mechanisms have been reported in this fish. Six strategies used by this fish to deal with the problem of excess ammonia are described. The fish can (1) reduce ammonia production through reduction in protein and/or amino acid catabolism; (2) reduce ammonia production and obtain energy through partial amino acid catabolism leading to alanine formation; (3) detoxify ammonia to glutamine; (4) tolerate very high ammonia levels in its tissues; (5) get rid of ammonia as NH(3) gas and, probably, (6) possesses background K(+) channels that are impermeable to NH(4)(+). The effects of extracellular ammonia on the contraction performance of the heart from this fish were found to be the same as in rainbow trout, an ammonia-sensitive fish. It suggests that the hearts of most, if not all, fish species are protected against ammonia. MK-801, an NMDA receptor blocker, was found to have a protective effect against ammonia intoxication in the oriental weatherloach, which suggests that the NMDA receptor, as in mammals, is involved in ammonia toxicity.

Role of amino acid metabolism in an air-breathing catfish, Clarias batrachus in response to exposure to a high concentration of exogenous ammonia

The air-breathing ureogenic walking catfish (Clarias batrachus) faces various environmental constraints throughout the year leading to the problem of accumulation of toxic ammonia. In the present study, the possible role of conversion of accumulated ammonia to various non-essential free amino acids (FAAs) was tested in this fish under hyper-ammonia stress caused by exposing the fish at 25 mM NH(4)Cl for 7 days. Significant accumulation of ammonia of approximately two- to threefold was observed in different tissues (except in the brain), which was accompanied with the significant accumulation of non-essential FAAs in the NH(4)Cl-exposed fish. There was approximately two- to threefold increase of non-essential FAAs in different tissues and in the plasma of the NH(4)Cl-exposed fish compared to the control fish after 7 days of exposure, which was mainly attributable to the increase of Asp, Ala, Gly, Glu, Gln and taurine (Tau) concentrations in general, with certain tissue-specific variations. This was also accompanied with significant increase of activity of certain amino acid metabolism-related enzymes such as the glutamine synthetase (approx. two- to threefold), glutamate dehydrogenase (ammonia utilizing direction) (approx. twofold), aspartate and alanine aminotransaminases (approx. twofold) mainly in the liver, kidney and muscle of the NH(4)Cl-exposed fish. Thus, it appears that the walking catfish has the capacity of active conversion of accumulated ammonia to non-essential FAAs under condition of high concentrations of external ammonia. However, the increase of urea excretion rate due to active conversion of ammonia to urea via the induced urea cycle appears to be quantitatively much more important pathway than the increase of tissue levels of FAAs in dealing with a severe ammonia load.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Glutamine synthetase expression in liver, muscle, stomach and intestine ofBostrichthys sinensisin response to exposure to a high exogenous ammonia concentration

A previous study provided evidence that the adaptive strategy used by the teleost fish Bostrichthys sinensis (sleeper) for detoxifying ammonia during extended periods of air exposure was to synthesize and store glutamine,primarily in the muscle, accompanied by an increase in glutamine synthetase(GSase) activity in liver. The aim of the present study was to assess the effect on GSase expression in various tissues of exposure of B. sinensis to exogenous ammonia. Exogenous ammonia increases internal ammonia concentrations in fish, mimicking environmental situations such as air exposure that preclude loss of ammonia across the gills, and thus triggering alternative mechanisms for ammonia detoxification. The results reveal relatively high levels of GSase activity, not only in liver but also,unexpectedly, in muscle, and even higher levels in intestine and, in particular, stomach. Exposure to ammonia results in significant increases in GSase activity, GSase protein and GSase mRNA levels in all of these tissues except stomach. The amino acid sequences of GSases from liver and stomach deduced from the cDNA sequences are essentially identical and are >97 %identical to the amino acid sequences of GSases from Gulf toadfish(Opsanus beta) and marble goby (Oxyeleotris marmoratus).

The effect of sub-lethal ammonia exposure on fed and unfed rainbow trout: the role of glutamine in regulation of ammonia

Many species of fishes have evolved mechanisms for coping with ammonia caused by either high ammonia environments or an inability to excrete nitrogenous wastes. Rainbow trout (Oncorhynchus mykiss), have not been known to have such a mechanism. The present study investigated whether rainbow trout can use amino acid synthesis and storage to cope with ammonia. Experiments were performed on fed and unfed rainbow trout under both control and elevated ammonia conditions (0 and 10 mgN/l (total ammonia nitrogen), pH 7.2). The results indicate that both feeding and ammonia exposure increased plasma ammonia significantly 6 h postprandial and post ammonia exposure. After 48 h the fed/ammonia exposed fish had plasma ammonia levels that were not significantly different than the fed/control fish. Plasma ammonia was reduced by more than 50%, attributable to ammonia being converted to glutamine in brain, liver and muscle tissue. Feeding alone also increased glutamine levels in brain tissue. Activity of glutamine synthetase in brain and liver was increased corresponding to an increase in glutamine concentrations when fish were exposed to ammonia. This is the first report showing that rainbow trout can detoxify endogenous and exogenous ammonia.

Ammonia toxicity in fish

Ammonia is present in the aquatic environment due to agricultural run-off and decomposition of biological waste. Ammonia is toxic to all vertebrates causing convulsions, coma and death, probably because elevated NH4+ displaces K+ and depolarizes neurons, causing activation of NMDA type glutamate receptor, which leads to an influx of excessive Ca2+ and subsequent cell death in the central nervous system. Present ammonia criteria for aquatic systems are based on toxicity tests carried out on, starved, resting, non-stressed fish. This is doubly inappropriate. During exhaustive exercise and stress, fish increase ammonia production and are more sensitive to external ammonia. Present criteria do not protect swimming fish. Fish have strategies to protect them from the ammonia pulse following feeding, and this also protects them from increases in external ammonia, as a result starved fish are more sensitive to external ammonia than fed fish. There are a number of fish species that can tolerate high environmental ammonia. Glutamine formation is an important ammonia detoxification strategy in the brain of fish, especially after feeding. Detoxification of ammonia to urea has also been observed in elasmobranches and some teleosts. Reduction in the rate of proteolysis and the rate of amino acid catabolism, which results in a decrease in ammonia production, may be another strategy to reduce ammonia toxicity. The weather loach volatilizes NH3, and the mudskipper, P. schlosseri, utilizes yet another unique strategy, it actively pumps NH4+ out of the body.

Ammonia detoxification and localization of urea cycle enzyme activity in embryos of the rainbow trout (Oncorhynchus mykiss) in relation to early tolerance to high environmental ammonia levels

The present study investigated the role of ammonia as a trigger for hatching, mechanisms of ammonia detoxification and the localization of urea cycle enzymes in the early life stages of freshwater rainbow trout (Oncorhynchus mykiss). The key urea cycle enzyme carbamoyl phosphate synthetase III was found exclusively in the embryonic body (non-hepatic tissues); related enzymes were distributed between the liver and embryonic body. ‘Eyed-up’ trout embryos were exposed either acutely (2h) to 10mmoll−1 NH4Cl or chronically (4 days) to 0.2mmoll−1 NH4Cl. Time to hatching was not affected by either acute or chronic NH4Cl exposure. Urea levels, but not ammonia levels in the embryonic tissues, were significantly higher than in controls after both acute and chronic NH4Cl exposure, whereas there were no significant changes in urea cycle enzyme activities. Total amino acid levels in the embryonic tissues were unaltered by chronic ammonia exposure, but levels of most individual amino acids and total amino acid levels in the yolk were significantly lower (by 34–58%) than in non-exposed controls. The data indicate that trout embryos have an efficient system to prevent ammonia accumulation in embryonic tissue, by conversion of ammonia to urea in embryonic tissues and through elevation of ammonia levels in the yolk.

The loach Misgurnus anguillicaudatus reduces amino acid catabolism and accumulates alanine and glutamine during aerial exposure

The loach Misgurnus anguillicaudatus inhabits rice fields in Southern China. It encounters drought during summer and ammonia loading during agricultural fertilization. In the laboratory, aerial exposure led to decreases in its ammonia and urea excretion. Ammonia accumulated to very high levels in the muscle and liver. Urea synthesis through the ornithine-urea cycle was not involved in ammonia detoxification in M. anguillicaudatus. However, M. anguillicaudatus was capable of partial amino acid catabolism leading to the accumulation of alanine in the first 24 h of aerial exposure. This was apparently coupled to a possible decrease in protein/amino acid catabolism. These are not detoxification mechanisms but mechanisms that avoid internal fouling by ammonia. Misgurnus anguillicaudatus was also capable of detoxifying internally produced ammonia in part to glutamine, which appears to be an important adaptation after 24 h of aerial exposure. However, unlike the case of the marble goby (Oxyeleotris marmoratus), there was no alteration to the kinetic properties of the hepatic glutamine synthetase. During dry seasons, M. anguillicaudatus moves actively on land until it encounters soft mud in which it can bury itself through several strong wriggling actions of the body. Hence, it is possible that M. anguillicaudatus uses partial amino acid catabolism to fuel its short period of activities on land and switches to the formation of glutamine to detoxify internally produced ammonia when it remains relatively inactive in the mud.

Changes in free amino acid synthesis in the perfused liver of an air-breathing walking catfish, Clarias batrachus infused with ammonium chloride: a strategy to adapt under hyperammonia stress

The changes in the free amino acid (FAA) levels, the rate of efflux of FAAs from the perfused liver, and the activity of some enzymes related to amino acid metabolism such as glutamate dehydrogenase (GDH, both reductive amination and oxidative deamination), glutamine synthetase (GS), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) were studied in the liver of a freshwater air-breathing teleost, the walking catfish, Clarias batrachus, perfused with 5 and 10 mM NH(4)Cl. The level of the various non-essential FAAs increased significantly, with a total increase of about 150%, which was accompanied by a significant increase of both ammonia and urea-N in the perfused liver both with 5 and 10 mM NH(4)Cl. The rate of efflux of these non-essential FAAs from the perfused liver also increased significantly with a total increase of about 115% and 160% at 5 and 10 mM NH(4)Cl, respectively. The activity of the mentioned amino acid metabolism-related enzymes in the perfused liver also got stimulated, except for GDH in the ammonia forming direction and ALT, under a higher ammonia load. The activity (both tissue and specific) of GDH in the glutamate forming direction increased maximally, followed by AST and GS in a decreasing order. Owing to these physiological adaptive strategies related to amino acid metabolism along with the presence of a functional and regulatory urea cycle (reported earlier), it is believed that this catfish is able to survive in very high ambient ammonia or in the air or in the mud during habitat drying.

The marble goby oxyeleotris marmoratus activates hepatic glutamine synthetase and detoxifies ammonia to glutamine during air exposure

Ammonia levels in various tissues of the marble goby Oxyeleotris marmoratus remained constant throughout a 72 h period of air exposure. The rate of ammonia excretion in these experimental fish decreased to approximately one-fifth of that of the submerged control. Ammonia was not converted to urea during air exposure because there were no significant increases in urea content in the tissues. Also, urea excretion rate was lowered to one-fiftieth that of the submerged fish. After 24 h of air exposure, there was a significant increase in muscle glutamine content, which peaked at 48 h. The increase in glutamine content could account for the decreases in the amounts of ammonia and urea excretion during air exposure. The specific activities of hepatic glutamate dehydrogenase (amination) and glutamine synthetase in these experimental fish increased threefold and thirtyfold, respectively, in comparison with the submerged controls. Thus, O. marmoratus appears to be the first known teleost that responds to air exposure by activating hepatic glutamine synthetase to detoxify internally produced ammonia.

Urea production and transport in teleost fishes

Teleosts appear to have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureotelic (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It was surprising, therefore, to find the expression of four urea cycle enzymes during early life stages of rainbow trout. The urea cycle may play a role in ammonia detoxification during a critical time of development. Exposure to alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) increased urea excretion by several-fold in trout embryos, free embryos and alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney.

N-acetylhistidine, glutamate, and beta-alanine are concentrated in a receptor cell layer of the trout inner ear

An epithelial sheet isolated from the trout saccular macula, highly enriched in acousticolateralis receptor cells (hair cells), has been analyzed for primary amine-containing compounds. The hair cell preparation, compared to the saccular nerve, was found to contain elevated levels of the presumptive receptoneural transmitter, glutamate, as well as beta-alanine, and components eluting in the positions of the standards phosphoserine and phosphoethanolamine on cation-exchange HPLC. Saccular nerve contained a different spectrum of primary amines and was elevated specifically in carnosine/homocarnosine. Acid hydrolysis of perchlorate extracts of both hair cell and nerve fractions yielded large amounts of histidine. For the saccular nerve fraction, production of histidine by acid hydrolysis was matched by production of beta-alanine and gamma-aminobutyric acid (GABA) and disappearance of carnosine/homocarnosine. The dipeptides carnosine and homocarnosine have been chromatographically resolved by expanded HPLC and found to be present in saccular nerve in a ratio of approximately 10:1, respectively. Production of histidine in the hair cell extract was not coupled with production of beta-alanine and GABA. The hair cell histidine-containing unknown, present in millimolar concentration, has been identified as N-acetylhistidine by the hydrolysis and rechromatography of fractions from cation-exchange HPLC. The large and specific presence of N-acetylhistidine in the hair cell preparation, together with electrophysiological evidence for its facilitatory action on afferent fibers in the frog semicircular canal, is suggestive of a role for this molecule as well as glutamate in acousticolateralis receptoneural transmission.

Ammonia distribution and excretion in fish

This paper reviews the literature concerning ammonia production, storage and excretion in fish. Ammonia is the end product of protein catabolism and is stored in the body of fish in high concentrations relative to basal excretion rates. Ammonia, if allowed to accumulate, is toxic and is converted to less toxic compounds or excreted. Like other weak acids and bases, ammonia is distributed between tissue compartments in relation to transmembrane pH gradients. NH3 is generally equilibrated between compartments but NH4 (+) is distributed according to pH. Ammonia is eliminated from the blood upon passage through the gills. The mechanisms of branchial ammonia excretion vary between different species of fish and different environments, and primarily involves NH3 passive diffusion and NH4 (+)/Na(+) exchange. Water chemistry near the gill surface may also be important to ammonia excretion, but a more accurate measurement of the NH3 gradient across the gill epithelium is required before a more detailed analysis of NH3 and NH4 (+) excretion can be made.

Sublethal effect of ammonia on certain biochemical and haematological indicators in common carp (Cyprinus carpio L.)

Carp (Cyprinus carpio L.) fry were exposed for 3 weeks to a sublethal concentration of ammonia (0.102 +/- 0.059 mg NH3/dm3). Ammonia induced considerable changes in the level of free amino acids and unfavourable changes in the blood, i.e. leucopaenia and eosinophy. The observed increase of the concentration of most amino acids in fish intoxicated with NH3 suggests that the process reflects detoxication of ammonia which takes place both in the brain and muscles.

Postprandial level of free amino acids in fish brain

1. Free amino acids were examined in common carp (Cyprinus carpio L.) brain 3, 6, 24 and 72 hr after a meal. 2. In common carp brain glutamic acid (409.6 +/- 37.6 mg/100 g dry matter), threonine (232.8 +/- 24.6) and histidine (191.3 +/- 70.1) were the most abundant free amino acids. 3. The ratios between the brain and the whole blood concentrations (B/W) expressed as the amounts of moles per tissue water were examined. 4. Glutamic acid, histidine, threonine, alanine, methionine, serine and glycine were found in consistently higher concentration in fish brain than in blood.

Free amino acids in tissues of the skate Raja erinacea and the stingray Dasyatis sabina: effects of environmental dilution

Concentrations of individual free amino acids were determined in various tissues of the skate (Raja erinacea) and the stingray (Dasyatis sabina), and the relationship of cellular free amino acid concentrations to intracellular osmoregulation was investigated by adapting these elasmobranchs to half-strength seawater. Each tissue characteristically had high levels of certain specific amino acids. Skate sing muscle contained high concentrations of sarcosine and beta-alanine, skate heart had high concentrations of taurine, and skate erythrocytes had high levels of taurine and beta-alanine. Amino acid levels in skate plasma were very low. High concentrations of aturine and glutamate were found in stingray brain. Adaptation of skates and stingrays to half-strength seawater was accompanied by significant decreases in concentrations of the major free amino acids in skate wing muscle and erythrocytes and in stingray brain, but not in skate heart. The data suggest that in these elasmobranchs certain specific free amino acids are selectively involved in intracellular osmoregulatory mechanisms.