EXPRESSION OF HYPOCALCIN IN THE DEVELOPING RAT-BRAIN

Expression of hippocalcin in the developing rat brain was investigated by a combination of Northern blot, in situ hybridization, immunoblot and immunohistochemical methods. In the hippocampus, hippocalcin mRNA and immunoreactivity first appeared in the CA3 pyramidal cells on embr yonic day 19 (E19) and postnatal day 1 (Pi), respectively, and extende d throughout Ammon's horn. After P14, the hippocampal pyramidal cells, especially in the CA1 region, maintained the highest expression level among the brain regions. The dentate granule cells expressed a small amount of hippocalcin mRNA and immunoreactivity from P7 and maintained a low level throughout the developmental stages. In the cerebral cort ex, hippocalcin mRNA and immunoreactivity appeared in the pyramidal ce lls of the piriform cortex from P1 and P4, respectively. Their express ion extended throughout the cerebral cortex and reached the maximum le vel on P14, and then declined gradually with age to half of the maximu m level by adults. In the cerebellum, a few Purkinje cells expressed a small amount of hippocalcin mRNA and immunoreactivity on P7. Their ex pression became evident in most of the Purkinje cells on P14 and incre ased gradually by P28. Then, their expression declined with age; howev er, the immunoreactivity was concentrated in the cell bodies and proxi mal segments of the dendrites in adults. These results suggest that th e expression of hippocalcin mRNA and protein is strictly controlled by both the cell type and the developmental process and that hippocalcin plays a role in neuronal differentiation in the early stages of devel opment and may relate to other neuronal function in the adult brain.