EXPRESSION OF NEUROTROPHINS AND TRK RECEPTORS IN THE AVIAN RETINA

Using the RNase protection assay, we have found that nerve growth fact or (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) are expressed in the avian retina during development. The expr ession peaks around embryonic days 12-15, with decreasing levels at la ter stages of development. Abundant levels of NGF and BDNF but low lev els of NT-3 mRNA were found in the adult retina. We also found that li ght/darkness regulated the levels of NGF and BDNF mRNAs but not the le vels of NT-3 mRNA in the 5-day-old chicken retina. It was demonstrated that NGF and BDNF mRNA levels were up-regulated by light exposure. Th e cellular localization of mRNA expression for the neurotrophins and n eurotrophin receptors TrkA, TrkB, and TrkC in the retina was studied u sing in situ hybridization. The patterns of NGF and trkA mRNA expressi on were very similar and were localized to the external part of the in ner nuclear layer on the border with the outer plexiform layer and cor responded to the localization of horizontal cells. NT-3 labeling was a lso found over the external part of the inner nuclear layer, whereas t rkC mRNA was found over all layers in the retina. BDNF labeling was fo und over all layers in the retina, whereas TrkB labeling was intense o ver cells in the ganglion cell. layer, which is in agreement with the response of ganglion cells to BDNF stimulation. Functional neurotrophi n receptors were suggested by the response of retinal explants to neur otrophin stimulation. These data indicate that the neurotrophins play local roles in the retina that involve interactions between specific n euronal populations, which were identified by the localization of the Trk receptor expression. The data also suggest that NGF and BDNF expre ssion is regulated by normal neuron usage in the retina. (C) 1996 Wile y-Liss, Inc.