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First published online May 5, 2005
Journal of Experimental Biology 208, 1855-1866 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01577

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Appetite-suppressing effects of ammonia exposure in rainbow trout associated with regional and temporal activation of brain monoaminergic and CRF systems

Van A. Ortega¹, Kenneth J. Renner² and Nicholas J. Bernier^1,*

¹ Department of Integrative Biology, University of Guelph, Guelph, ON, Canada, N1G 2W1
² Biology and Neuroscience, University of South Dakota, Vermillion, SD 57069, USA

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

Accepted 9 March 2005

To assess whether the brain's monoaminergic and/or corticotropin-releasing factor (CRF) systems may be involved in mediating the appetite-suppressing effects of high environmental ammonia levels, we exposed rainbow trout to one of four NH₄Cl treatments (0, 500, 750 1000 µmol l^-1) for 24 or 96 h and monitored changes in food intake, brain serotonin (5-HT) and dopamine (DA) activity, CRF and urotensin I (UI) mRNA levels, and plasma cortisol levels. Food intake decreased in a dose-dependent manner after 24 h of ammonia exposure and partially recovered in all groups after 96 h. Ammonia also elicited dose-dependent increases in serotonergic activity in the hypothalamus (HYP), telencephalon (TEL) and posterior brain (PB). Whereas the increase in serotonergic activity was timed with the 24 h food intake inhibition, TEL and PB serotonergic activity increased after 96 h. In the PB, exogenous ammonia also elicited dose-dependent increases in dopaminergic activity after both 24 and 96 h of exposure. Transient increases in TEL CRF and UI mRNA levels, HYP UI mRNA levels, and plasma cortisol concentrations were evidence that the hypothalamic-pituitary-interrenal (HPI) stress axis was primarily stimulated in the first 24 h of ammonia exposure when food intake was depressed. Overall, the transient nature of the appetite suppression during chronic ammonia exposure, and the time-dependent changes in brain monoaminergic and CRF systems, implicate 5-HT, DA, CRF and UI as potential mediators of the appetite-suppressing effects of ammonia. Among these anorexigenic signals, our results specifically identify hypothalamic 5-HT as a potentially key neurobiological substrate for the regulation of food intake during exposure to high external ammonia concentrations.

Key words: feeding behavior, anorexia, HPI axis, stress, corticotropin-releasing factor, urotensin I, cortisol, rainbow trout, Oncorhynchus mykiss

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