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Tyrosine kinase A (TrkA)is an essential component of the high affinity nerve growth factor (NGF)receptor necessary to the mediate the biological effects of the neurotrophins, NGF. This study examined the distribution of TrkA-immunoreactivity (IR)cells in the postnatal rat cerebral cortex and the changes that occur in postnatal development as a result of the expression of this protein. TrkA-IR was detected at postnatal day (PD)3, PD6, PD9 and PD15. Base upon their somatodendritic morphology, the most commonly labeled cell type was the pyramidal neurons. At PD3 and PD6, layer I, II, III and V was immunopositive for TrkA, at PD9, not only at layer I, II, III, and V but also at layer VI. At PD15, the TrkA-positive cells were distributed in all layers. These TrkA-positive cells were not detected at PD0. In contrast, there was significant increase in the percentage of cells exhibiting TrkA-IR with development and the highest level was detected at PD15. These results suggest that the cerebral cortex expresses TrkA strongly during the postnatal period. Moreover, the postnatal development-related increase in the expression of TrkA-cells shows that NGF may have a trophic effect on these cerebral cortex neurons from the postnatal period.
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