Apolipoprotein D gene expression in the rat brain and light and electron microscopic immunocytochemistry of apolipoprotein D expression in the cerebellum of neonatal, immature and adult rats

Apolipoprotein D gene and protein expression were investigated in the rat b rain and cerebellum, respectively, during development. Apolipoprotein D gen e expression was first observed in embryonic day 12 rat brain, with a moder ate increase in apolipoprotein D messenger RNA levels towards the later par t (embryonic days 15-17) of gestation. In the postnatal rat brain, a marked induction of apolipoprotein D messenger RNA occurred at postnatal day 10, with progressively higher levels of apolipoprotein D messenger RNA observed up to postnatal day 20. Somewhat lower, but none the less high, levels of apolipoprotein D messenger RNA continued to be present in brains of adult a nimals. In the immature cerebellum (day 3 up to one- to two-week-old rats), there were many densely labeled apolipoprotein D-immunoreactive cells that had features of oligodendrocyte precursors. Purkinje neurons showed apolip oprotein D immunoreactivity in one- to two-week-old animals, after which th ere appeared to be some decrease in staining. Oligodendrocytes in the cereb ella of two-week-old animals were strongly apolipoprotein D positive, with immunoreactivity declining in older animals. These results reveal a maturation-associated induction of apolipoprotein D gene expression in the rat brain, and expression of apolipoprotein D in gli al (immature oligodendrocyte) cells in the immature cerebellum, followed by specific expression of apolipoprotein D in Purkinje neurons. (C) 1999 IBRO . Published by Elsevier Science Ltd.