EFFECTS OF L-THYROXINE ON BRAIN MONOAMINES DURING PARR-SMOLT TRANSFORMATION OF ATLANTIC SALMON (SALMO-SALAR L)

During spring, seaward migrating juvenile Atlantic salmon (Salmo salar ) undergo parr-smolt transformation (PST) which involves changes in ph ysiology, including one or two peaks in plasma thyroxine (T-4). To inv estigate if changes in plasma T-4 influence neural function, we measur ed levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylace tic acid (DOPAC) and homovanillic acid (HVA), and also measured seroto nin (5-hydroxytryptamine, 5-HT) and its metabolite, 5-hydroxyindoleace tic acid (5-HIAA) in brain regions of two groups of Atlantic salmon pa rr on an 8:16 h light/dark photoperiod. One group was treated with amb ient T-4 to simulate the natural smelt peak in plasma T-4. T-4 treatme nt depressed DOPAC levels as well as DOPAC/DA and 5-HIAA/5-HT ratios i n the olfactory system but with no changes in the optic tectum. We con clude that during PST monoaminergic functions in specific brain region s of juvenile Atlantic salmon are affected by T-4 treatment. (C) 1997 Elsevier Science Ireland Ltd.