CHANGING PATTERNS OF EXPRESSION AND SUBCELLULAR-LOCALIZATION OF TRKB IN THE DEVELOPING VISUAL-SYSTEM

Neurotrophins play important roles in the survival, differentiation, a nd maintenance of CNS neurons. To begin to investigate specific roles for these factors in the mammalian visual system, we have examined the cellular localization of the neurotrophin receptor trkB within the de veloping cerebral cortex and thalamus of the ferret using extracellula r domain-specific antibodies. At prenatal ages (gestation is 41 d), tr kB-immunostained fibers were observed in the internal capsule and as t wo distinct fascicles within the intermediate zone of the cerebral cor tex. The staining of these fiber tracts declined with increasing age, whereas soma and dendrite staining of cortical neurons was first evide nt in early postnatal life and increased during subsequent development . Staining of subplate neurons [by prenatal day 5 (P5)] was followed b y staining of cortical layer 5 neurons (at P10). By P31, trkB immunore activity was particularly prominent in layers 3 and 5 but was absent f rom subplate neurons. Staining included cells, especially pyramidal ne urons, in all cortical layers by P45, and this pattern was maintained into adulthood. The optic tract and fibers within the lateral genicula te nucleus (LGN) were also strongly trkB immunoreactive at prenatal ag es. Cellular staining of a subset of LGN neurons, those within the C-l ayers and perigeniculate nucleus, was apparent by P10 and maintained u ntil P45, when the adult pattern of highly trkB-immunoreactive neurons in all layers of the LGN first appeared. The pattern of trkB immunore activity suggests that specific subsets of cortical and thalamic neuro ns may respond to neurotrophins such as brain-derived neurotrophic fac tor and/or NT-4/5 at discrete developmental times and locations. The a ppearance of trkB on axon fibers early in development and then on cell bodies and dendritic processes later is consistent with roles for bot h long-range and local, including autocrine and/or paracrine, delivery of neurotrophins in cell survival and maturation.