Abstract
Na+:K+:2Cl- cotransporter 1 (NKCC1) has been implicated in mediating ischemia-, trauma- or ammonia-induced astrocyte swelling/brain edema in mammals. This study aimed to determine the effects of ammonia or terrestrial exposure on ammonia concentrations in the plasma and brain, and the mRNA expression and protein abundance of nkcc/Nkcc in the brain, of the swamp eel Monopterus albus . Ammonia exposure led to a greater increase in the ammonia concentration in the brain of M. albus than terrestrial exposure. The brain ammonia concentration of M. albus reached 4.5 µmol g-1 and 2.7 µmol g-1 after 6 days of exposure to 50 mmol l-1 NH4Cl and terrestrial conditions, respectively. The full cDNA coding sequence of nkcc1b from M. albus brain comprised 3276 bp and coded for 1092 amino acids with an estimated molecular mass of 119.6 kDa. A molecular characterization indicated that it could be activated through phosphorylation and/or glycosylation by osmotic and/or oxidative stresses. Ammonia exposure for 1 day or 6 days led to significant decreases in the nkcc1b mRNA expression and Nkcc1b protein abundance in the brain of M. albus. In comparison, a significant decrease in nkcc1b mRNA expression was observed in the brain of M. albus only after 6 days of terrestrial exposure, but both 1 day and 6 days of terrestrial exposure resulted in significant decreases in the protein abundance of Nkcc1b. These results are novel because it has been established in mammals that ammonia up-regulates NKCC1 expression in astrocytes and NKCC1 plays an important role in ammonia-induced astrocyte swelling and brain edema. By contrast, our results indicate for the first time that M. albus is able to down-regulate the mRNA and protein expression of nkcc1b/Nkcc1b in the brain when confronted with ammonia toxicity, which could be one of the contributing factors to its extraordinarily high brain ammonia tolerance.
Highlights
The mitochondrial permeability transition could result from the permeation of cytosolic glutamine through the inner mitochondrial membrane into mitochondria, and the subsequent release of ammonia from glutamine catalyzed by glutaminase in the mitochondrial matrix of astrocytes [11]
Exposure to terrestrial conditions for 1 or 6 days led to significant increases, but at a slower rate, in the ammonia concentrations in the brain and plasma of M. albus (Figure 1B)
After 6 days of exposure to terrestrial conditions, the brain ammonia concentration was only 2.7 μmol g-1, which was comparable to the ammonia concentration in the brain of fish exposed to NH4Cl for 1 day but lower (60%) than that of fish exposed to NH4Cl for a 6-day period
Summary
Several theories (glutamatergic dysfunction, glutamine accumulation leading to astrocyte swelling, activation of N-methyl-ᴅ-aspartate-type glutamate receptors) have been proposed for the mechanisms of acute ammonia toxicity in mammalian brains [4,5]. The mitochondrial permeability transition could result from the permeation of cytosolic glutamine through the inner mitochondrial membrane into mitochondria, and the subsequent release of ammonia from glutamine catalyzed by glutaminase in the mitochondrial matrix of astrocytes [11]. All these theories of mechanisms of ammonia toxicity have yet to be confirmed in fish brain [12,13,14]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
