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First published online August 9, 2007
Journal of Experimental Biology 210, 2905-2911 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.003905

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

P. A. Wright^*, S. L. Steele, A. Huitema and N. J. Bernier

Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1 Canada

^* Author for correspondence (e-mail: patwrigh{at}uoguelph.ca)

Accepted 5 June 2007

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 µmol l^-1 NH₄Cl (75% of the 96 h-LC₅₀ value). The mRNA of all four GSase isoforms were detected in brain (not liver). After 9 h of NH₄Cl 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 NH₄Cl 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.

Key words: ammonia toxicity, liver, enzyme activity, gene expression, mRNA, nitrogen metabolism, glutamine, sublethal ammonia exposure

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