Development of full-length Trk B-immunoreactive structures in the hippocampal formation of the macaque monkey

Distribution and morphological changes of cells containing the signal trans ducing neurotrophin receptor, full-length Trk B (fl-Trk B), were investigat ed in the hippocampal formation of the macaque monkey between embryonic day 140 and the adult stage. Western blot analysis showed that one main protei n band, which migrated at 141 kDa, was detected in both the embryonic and a dult hippocampal formation. In the pyramidal cells in CA1 and CA3 subfields , the subiculum, and the entorhinal cortex, fl-Trk B-immunoreactive dendrit es were observable in the embryonic stage. In contrast, in the granule cell s of the dentate gyrus, few dendrites were immunoreactive during embryonic and early developmental stages. This difference may be due to the later gro wth of the granule cells of the dentate gyrus. The existence of fl-Trk B im munoreactivity in the cell body and dendrites in the embryonic hippocampal neurons, suggests that BDNF and/or NT4/5 act on the hippocampal cells by au tocrine/paracrine mechanisms. In the entorhinal cortex, fl-Trk B immunoreac tivity became localized in the stellate cells in layer II and the pyramidal cells in layers III,V and VI in adulthood. This indicates that BDNF and/or NT4/5 an important for the maintenance of the projection neurons in the en torhinal cortex at the adult stage. The strongest fl-Trk B immunoreactivity in the hippocampal neurons occurred at postnatal month 4, corresponding to the period of greatest synapse production in the monkey hippocampus, sugge sting that BDNF and/or NT4/5 with fl-Trk B may play a role in synapse forma tion in the monkey hippocampus.