Fig. 3. Serotonergic activity in the hypothalamus (A), telencephalon (B), and posterior brain (C) regions of rainbow trout exposed to 0, 500, 750 or 1000 µmol l^-1 NH₄Cl for either 24 or 96 h periods. Serotonergic activity is expressed as a ratio of the metabolite 5-hydroxyindole acetic acid (5-HIAA) to serotonin (5-HT). Values are means + 1 S.E.M. Treatments that do not share a common letter for a given time and brain region are significantly different from each other. ^*Significant differences between exposure periods for a given treatment (P<0.05; N=8-11). (D) Relationship between plasma [ammonia] and hypothalamic serotonergic activity after 24 h NH₄Cl exposure (r²=0.39, P<0.001).