Growth hormone-releasing hormone influences neuronal expression in the developing chick brain. I. Catecholaminergic...

We have examined catecholaminergic expression during development in the chick embryonic brain using tyrosine hydroxylase (TH) activity as a biochemical marker for catecholaminergic neurons. TH activity was detectable as early as after 4 days of incubation in whole brain homogenates and increased throughout embryonic development. The greatest increase in enzyme activity was observed between embryonic days 8 and 15, a period of active neuronal maturation and synaptogenesis. Growth hormone-releasing hormone (GHRH) was tested for its influence on TH activity during embryonic development. Eight-day-old embryos that received GHRH (50 ng/50 microliters) in ovo on days 1, 3, 5 and 7 exhibited a significant (P less than 0.001) increase in TH activity. Similar results were obtained when GHRH was administered in a single 50 ng/50 microliter dose on day 1 or day 3 of development. However, embryos receiving the same dose of GHRH on day 5 exhibited no significant difference in TH activity as compared to controls. When growth hormone (GH, 100 ng/50 microliters) was administered during the same critical period (day 3) no difference was observed in TH activity as compared to controls. Thus, the effects of GHRH on TH activity do not appear to be mediated through GH. We interpret these data to mean that GHRH can enhance catecholaminergic phenotypic expression in the chick embryonic brain when administered during a discrete critical period of development from days 1 to 3 of embryonic age.