Expression of the proto-oncogene, trk, receptors in the developing rat retina

The neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and NT-4/5 are important in a variety of developmental processes in the peripheral and central nervous systems. These molecules bind to a low-affinity receptor and to distinct high-affinity receptors. The high-affinity receptor for NGF is the proto-oncogene product, p140 trkA ( trk A ). Isoforms of p140 trkA , p145 trkB ( trk B ), and p140 trkC ( trk C ), are the primary high-affinity receptors for BDNF and NT-3, respectively. We evaluated the developmental regulation of the high-affinity neurotrophin receptors in the rat retina using polyclonal antibodies directed to a highly conserved region of the C-terminus of the p140 trk A isoforms (pan trk ) and antibodies directed to unique amino-acid sequences of p140 trkA , p145 trkB , and p140 trkC . Immunoreactivities for trk A and trk B , as well as pan trk , were detected in the developing retina and showed similar distributions. At similar antibody concentrations, trk C immunoreactivity was not detected. In the embryo, immunoreactivities were present in cells located throughout the neuroblastic retina, especially in the inner retinal layers, and in fibers in the nerve fiber layer and optic nerve. In the newborn retina, immunoreactivities for these two receptor isoforms were localized to numerous somata in the inner nuclear layer (INL), as well as to cells in the ganglion cell layer (GCL) and axons in the nerve fiber layer and optic nerve. A similar pattern of immunostaining persisted throughout the first postnatal week. By postnatal day-10, immunostaining was confined to large-diameter cells in the GCL, both heavily stained and lightly stained cells in the INL and a plexus of processes in the inner plexiform layer (IPL). In the adult retina, specific immunoreactivity was present in sparsely distributed, lightly and moderately stained, large cells in the GCL, numerous lightly and moderately stained cells throughout the INL and in plexuses of processes in the IPL and outer plexiform layer. Specific immunostaining of photoreceptor cells was not observed. These observations indicate that high-affinity receptors for the neurotrophins are expressed in cells of the inner retina, including ganglion cells, during the period of retinal development. This is congruent with roles for neurotrophins in such processes as survival, differentiation and synapse formation of cells in the developing visual system.