Immunohistochemistry for tyrosine hydroxylase (TH) was performed on the dorsal root ganglia (DRG) in wild-type, heterozygous and Brn-3a knockout mice at embryonic day 18.5. TH-immunoreactive (-IR) neurons were detected in the DRG of wild-type and heterozygous mice, but their proportion was greatly increased by the loss of Brn-3a function (wild-type and heterozygot, 8.4%; knockout, 20.9%). IR neurons were of various sizes in wild-type (mean ± S.D. = 118.1 ± 55.4 μm2, range = 26.6-306.3 μm2) and heterozygous mice. In the knockout mice, however, TH-IR neurons were mostly small (mean ± S.D. = 68.2 ± 34.3 μm2, range = 11.8-166.8 μm2). The present study suggests that Brn-3a may normally suppress TH expression in many small DRG neurons but activate TH expression in large DRG neurons.
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