Developmental regulation of tryptophan hydroxylase messenger RNA expression and enzyme activity in the raphe and its target fields

Tryptophan hydroxylase is the rate-limiting enzyme in the synthesis of sero tonin and during development, brain serotonin levels and tryptophan hydroxy lase activities increase. Increased tryptophan hydroxylase activity could r esult from alterations in tryptophan hydroxylase messenger RNA levels, tran slation, and/or post-translational regulation. Tryptophan hydroxylase messe nger RNA levels in the dorsal raphe nucleus increased 35-fold between embry onic day 18 and postnatal day 22, measured by quantitative in situ hybridiz ation, then decreased by 40% between postnatal days 22 and 61. These change s correlated with tryptophan hydroxylase enzyme activities in the raphe nuc lei as expected, but not in cortical or hippocampal targets. Tryptophan hyd roxylase messenger RNA expression in the nucleus raphe obscuris increased 2 .5-fold between postnatal days 8 and 22 but did not correlate with enzyme a ctivity in the spinal cord. Using an in vitro model of serotonergic raphe n euron differentiation, serotonergic differentiation was associated with an increase in both tryptophan hydroxylase promoter activity and protein expre ssion. In vivo, tryptophan hydroxylase messenger RNA levels per single cell and per brain section were correlated during development up to postnatal d ay 22, but not beyond for both the dorsal raphe nucleus and nucleus raphe o bscuris. Between postnatal days 22 and 61 single cell levels of tryptophan hydroxylase messenger RNA in the dorsal raphe nucleus did not change yet th e levels per brain section significantly decreased by 40%. During the same period in the nucleus raphe obscuris, tryptophan hydroxylase messenger RNA levels per single cell signifcantly increased by 30% yet levels per brain s ection did not change. Comparison of tryptophan hydroxylase messenger RNA l evels per cell and per brain section indicated a serotonergic loss between postnatal days 22 and 61 in both the dorsal raphe nucleus and nucleus raphe obscuris and may reflect either a loss of neurotransmitter phenotype or ce ll death. This study is the first to characterize the expression of brain tryptophan hydroxylase messenger RNA during rat development. In addition, this study i s the first to report the activity of tryptophan hydroxylase in the spinal cord and hippocampus in the embryonic and neonatal rat. Together, the data provide a better understanding of the intricate relationship between patter ns of tryptophan hydroxylase messenger RNA expression and enzyme activity. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.