Tryptophan metabolism in developing chick brain.

The normal patterns of the occurrence of tryptophan, 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and monoamine oxidase (MAO) activity in chick brain during development were investigated. Tryptophan level in the brain on the 7th day of embryonic development was rather high and decreased gradually until the 17th day, but increased markedly from the 17th day to hatching, then increased slightly from hatching to the 14th day after hatching. 5-HT, 5-HIAA and MAO were already present on the 7th day of incubation, increased gradually with a temporary peak on the 14th day, and increased remarkably from the 17th day to hatching, after which they reached a plateau and remained unchanged from the 7th to the 14th day after hatching. Since the increase between the 17th day and hatching was observed in the patterns of tryptophan, 5-HT, 5-HIAA and MAO activity, the critical period of tryptophan metabolism in the brain during development seems to exist in this period. In order to examine the formation of 5-HT from tryptophan in vivo, newly hatched chicks and chicks 2 weeks after hatching were injected intraperitoneally with L-tryptophan and brain 5-HT was measured. 5-HT was increased markedly, even in newborn chick brain. This might indicate that the activity of tryptophan hydroxylase in newborn chick brain would be as high as that in adult chick brain.