Abstract

Previous work in our laboratory has shown that injection of reserpine into the yolk sac of fertilized, unincubated chicken eggs resulted in decreased whole brain catecholamines at 14 days of embryonic development while at 30 days after hatching catecholamines were significantly elevated. The possible involvement of tyrosine hydroxylase (TH) in these observations was studied. At 10 days of development, brain TH activity in reserpinetreated embryos was not significantly different from vehicle-injected or uninjected controls, although catecholamines were significantly reduced. By 15 days of embryonic development, TH activity increased 4-fold and reserpine treatment caused a significant increase in enzyme activity over controls. By 20 days of development TH activity increased an additional 8.5-fold, to about 33 times the activity at 10 days, but between this time and 29 days of age after hatching TH activity increased only an additional 40%. Most important, however, is the continued significantly elevated TH activity at 29 days of age in the reserpine-treated group, suggesting a longer lasting effect upon enzyme activity than has been shown previously. The data support the contention that nervous activity is essential for compensatory induction or activation and that a critical period for a set-point of enzymatic activity may exist during development.