TYROSINE-HYDROXYLASE ACTIVITY AND DOPAMINE TURNOVER OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) BRAIN - THE SPECIAL STATUS OF THE HYPOTHALAMUS

The dynamics of catecholamine (CA)-synthesis enzymes have been poorly studied in fish. Tyrosine hydroxylase (TH), the rate-limiting enzyme o f CA synthesis has been only studied in in vitro conditions. In the pr esent report the in vivo CA synthesis and the CA metabolism were studi ed in different regions of the forebrain of the rainbow trout. Levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and the rate of accumulation of 3,4-dihydroxyphenylalanine (DO PA) were determined by HPLC following a treatment with hydroxybenzylhy drazine (NSD), a potential inhibitor of DOPA decarboxylase. Kinetics o f the accumulation of DOPA and of the decline of DOPAC were in agreeme nt with those found in rat, evidencing that the accumulation of DOPA f ollowing NSD can be used in trout to quantify the in vivo enzymatic ac tivity of tyrosine hydroxylase. Experiments using treatment with NSD o r with methyl-p-tyrosine reached a same conclusion: the DA neuronal ac tivity in trout is much higher than NE neuronal activity. However,the hypothalamus had high DA levels vs. low in vitro and in vivo TH activi ties and exhibited a low CA turnover.