Annual variations in brain serotonin and related compounds of domesticated rainbow trout (Oncorhynchus mykiss)

The levels of tryptophan (Trp), 5-hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in two brain regions (hypothalamus and medulla oblongata) of rainbow trout (Oncorhynchus mykiss) were measured throughout the year using a sensitive high performance liquid chromatographic method with electrochemical detection. Trp was also quantified in serum, liver and brain regions. Trp concentrations were higher in sera than in tissues (brain and liver) throughout the year. Hypothalamic and medullary Trp offered similar annual patterns showing two peaks, the first on 10 May and the second on 15 June. 5-HT levels were always higher in the hypothalamus than in the medulla and much higher than the 5-HIAA levels in both regions. Moreover hypothalamic and medullar serotonin patterns were very different, in the latter region few variations were observed. In addition, the 5-HIAA/5-HT ratio was different in the brain regions, the hypothalamus showed two peaks (April 8 and June 15) whereas the equivalent was not observed in the medulla. All measured compounds showed significant variations during the year.