EFFECTS OF 3-IODO-L-TYROSINE, A TYROSINE-HYDROXYLASE INHIBITOR, ON EYE PIGMENTATION AND BIOGENIC-AMINES IN THE PLANARIAN, DUGESIA-DOROTOCEPHALA

Planarians (Dugesia dorotocephala) were evaluated as bioassay organism s to detect inhibition of tyrosine hydroxylase, the rate-limiting enzy me in the synthesis of catecholamines. Thirty planaria per dose were e xposed to 0 (control), 0.001, 0.01, 0.1, or 1 mM 3-iodo-L-tyrosine (mo noiodotyrosine or MIT) in standard test media beginning 24 hr before d ecapitation and continuing for 13 days. Complete regeneration of norma l heads occurred over the first 6 days in all dose groups, a response reported to be partially dependent on catecholamines. Beginning on Day 7, the black eye pigments began fading in the 0.1 and 1 mM dose group s and were completely absent macroscopically and histologically by Day 11. The 0, 0.001, and 0.01 mM dose groups did not lose visible eye pi gments. On Day 13, 3 planaria/dose were harvested for histopathology; 15 planaria/dose were decapitated a second time and remained in MIT so lutions; and 12 planaria/dose were left intact, placed in fresh contro l media, and evaluated for eye repigmentation. Normal head regeneratio n (including eyes) was detected grossly in all groups, even in those a nimals devoid of eye pigments at the time of decapitation. As before, eye pigments began fading 7 days after decapitation (Day 20 of experim ent) and were completely absent in 73 and 33% of the animals in the 0. 1 and 1 mM groups, respectively, on Day 25. All animals moved to contr ol media reformed eye pigments, beginning within 48 hr. Analysis of th e decapitated heads by HPLC-ECD on Day 13 revealed a significant decre ase in dopamine and 5-hydroxytryptamine (serotonin) concentrations in MIT-exposed animals. Tyrosine hydroxylase activity (and possibly tyros inase activity) was shown to be inhibited by the highest two concentra tions for whole planaria homogenates in vitro. (C) 1996 Society of Tox icology