LOCALIZATION OF DOPAMINE IN THE FRESH-WATER HYDROZOAN HYDRA-ATTENUATA

High performance liquid chromatography (HPLC), with electrochemical detection, is an analytical method sensitive enough to permit quantification of dopamine, dihydroxyphenylalanine (DOPA) and 5-S-cysteinylDOPA in single or hemisected specimens of Hydra attenuata. Dopamine and 5-S-cysteinylDOPA appear to be the quantitatively predominant catechol compounds in H. attenuata, whereas DOPA is present in minor amounts. The presence of DOPA and 5-S-cysteinylDOPA, and the quantitative correlation between dopamine and these compounds in many specimens, suggests that dopamine in H. attenuata, as in higher animals, is formed through decarboxylation of DOPA. Contrary to the dopaminergic nerves in higher animals, DOPA in Hydra seems to be oxidized and 5-S-cysteinyl DOPA is formed as a by-product. The oxidation of DOPA indicates that the hydroxylation of tyrosine into DOPA in the tissues of H. attenuata is mediated by a tyrosinase rather than a tyrosine hydroxylase. Immunocytochemical methods demonstrate a highly variable distribution of dopamine in the tissues of different specimens of H. attenuata. Dopamine immunoreactivity is confined to ectodermal tissue and can be found in several different cell types including nerve cells, battery cells, nematocytes, epithelial cells and interstitial undifferentiated cells. The large amounts of dopamine found in some specimens of H. attenuata indicate some biological function, although its sporadic occurrence in neurites makes it less plausible as a generally utilized neurotransmitter in this animal.