Cortical and striatal expression of tyrosine hydroxylase mRNA in neonatal and adult mice

1. Elucidating the mechanisms underlying regulation of the dopamine (DA) phenotype during development and in adult animals was a major focus of many of the students and postdoctoral fellows in the Laboratory of Dr Donald Reis. In one series of studies, expression of tyrosine hydroxylase (TH), the first enzyme in the DA biosynthetic pathway, was induced in primary cultures prepared from the cortical anlage of embryonic day 13 (E13)-E17 rat embryos. On the basis of these data, the current studies investigated whether under appropriate conditions TH expression might occur in forebrain regions that do not normally contain DA neurons. 2. A transgenic mouse strain harboring a 9-kb TH promoter/EGFP (enhanced green fluorescent protein) reporter construct was analyzed as adults for coexpression of the fluorescent reporter and the endogenous gene, the latter using a sensitive nonradioactive in situ hybridization procedure. The latter procedure was also used to determine the development of neonatal cortical endogenous TH expression. 3. Cortical and striatal cells containing TH mRNA were observed at postnatal day 5 (P5), but not P2, increased in number at P7 and were found in adults. Many cells in the cortex and striatum coexpressed TH mRNA and EGFP, but TH protein was not detected in these brain regions indicating independent transcriptional and translational regulation of TH expression. Overlapping expression of the two transcriptional indicators and TH protein in olfactory bulb occurred only in those DA neurons that receive afferent stimulation from receptor cells. 4. These findings suggest that partial DAergic differentiation may occur in some cortical and striatal cells, but that full expression of the phenotype requires synaptic activation or activity-dependent release of an as-yet unidentified factor(s).