Tyrosine hydroxylase activities in the brains of wild Norway rats and silver foxes selected for reduced aggressiveness towards humans

Tyrosine hydroxylase (TH) activity was measured in brains from Norway rats and silver foxes showing wild type aggressiveness and from their counterparts selected over 20-25 generations for reduced aggressiveness towards humans (tameness). TH activity in the brain stem and cortex was increased in tame animals of both species compared with aggressive counterparts. Selection increased hypothalamic TH activity in foxes, but decreased it in rats. There was no difference in TH activity in corpus striatum between the tame and aggressive animals. Fetal TH activity in the posterior part of the brain was higher in tame than aggressive rats at day 20 of embryogensis. Increased TH activity in the brain stem and cortex of adult aggressive rats was observed after treatment of their mothers with hydrocortisone on the days 16 and 18 of pregnancy. This elevation in TH activity was associated with attenuation of the defense behavior of aggressive rats. The data suggested that alterations in neural TH activity in tame rats and foxes may be part of the neurochemical basis of their behavioral phenotype which is developed by selection. © 1994 Wiley-Liss, Inc.