Postnatal elevation of brain tyrosine hydroxylase activity, without concurrent increases in steady-state catecholamine levels, resulting from dl-alpha-methylparatyrosine administration during embryonic development.

Increased specific activity of tyrosine hydroxylase (TH) in the brains of embryonic and 29-day-old chickens results from exposure to reserpine during the early stages of embryogenesis. We have also reported significant increases in the steady-state concentrations of catecholamines (CAs) in the brains of these 29-day-old chickens, suggesting permanent alterations in the control mechanisms during a critical period of development. The specificity of embryonic CA depletion in the above findings was examined usind dl-alpha-methyl-para-tyrosine (AMPT) as a depleting agent. AMPT, injected in varying doses into the yolk sac of fertilized chicken eggs prior to incubation, caused embryonic CA depletion by day 10 of embryogenesis but repletion had occurred by 20 days of embryogenesis. TH activity in whole brain and brain parts was elevated in a dose-related fashion at 29 days postnatally by AMPT. However, no changes in steady-state CA levels in whole brain were observed at this time after AMPT. Differences in the responses to reserpine and AMPT are discussed with respect to their pharmacological actions. The data support the contention that early embryonic CA depletion can result in long-term increases in TH activity postnatally, but steady-state levels of product need not necessarily be altered.