Central monoaminergic responses to salinity and temperature rises in common carp

Juvenile common carp, Cyprinus carpio, were exposed to increased levels of salinity (1 % NaCl) at 25 °C and 30 °C. Levels of the monoamine neurotransmitters dopamine (DA) and serotonin (5-HT) and their metabolites dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid were determined in different brain parts. Whereas the elevated temperature only resulted in higher levels of the metabolites, increased salinity caused increased levels of DA and 5-HT as well. Increased levels appeared after the first day of exposure and most effects were further enhanced after 1 week in 1 % NaCl. Increases in DA and 5-HT levels were most pronounced in the hypothalamus, which is the major integrative centre controlling the release of hormones. Thus, one possible role of these changes in neurotransmitter metabolism could be to control the release of prolactin and cortisol, two major hormones involved in the regulation of ion homeostasis in teleosts.