Ontogenesis of iodothyronine deiodinase activities in brain and liver of the chick embryo

The ontogeny of 5'-monodeiodinase activity (5'-MA) was analyzed in chick embryo brain and liver tissue. The enzymatic type predominant in this path and the activity of the deactivating pathway (5 MA-III) were determined during certain periods of neural development in both organs. Results show that T3 is predominantly formed in both organs during the first third of embryogenesis (from day 5) until neuroblast proliferation (day 13). Within this lapse, the slow type II enzyme (insensible to propylthiouracil) is present in the brain, whereas in the liver the predominating enzyme is type I (the rapid enzyme). Further along the synaptogenesis period (day 14-17), 5' deiodination virtually disappears in the brain, the hepatic type I enzyme switches to the slow autoconsume enzyme (type II), and 5 MA-III levels increase significantly in both organs. Finally, on days 18-20 (perinatal period) the 5' pathway reaches the highest levels observed throughout the study in both tissues. Associated to this increase, liver enzymatic activity returns to type I. During this period, 5 MA-III is reduced by 40% in the brain and disappears from the liver. Together, these data strengthen the notion of a protective mechanism against brain overexposure to T3 during synaptogenesis and suggest that the protective mechanism also involves the regulation of extraneural deiodinases.