Güntürkün Distribution of BDNF , NT - 3 , trkB and trkC in the developing retino - tectal system of the pigeon ( Columba

The distribution of the neurotrophins BDNF and NT-3 as well as their corresponding high-affinity receptors trkB and trkC was characterized by immunohis-tochemistry in the developing retino-tectal system of the pigeon. These neurotrophins are known to be important for survival and development of neuronal tissues, but also for activity-dependent neuronal plasticity. In pigeons visual asymmetry is established at the morphological and behavioral level due to a natural asymmetrical light input before hatch, which is followed by a post-hatch period of consolidation with unbiased light stimulation. Since the retino-tectal system is the crucial entity of these events, we studied the retinal and the tectal distribution of these neurotrophins and their receptors during retino-tectal formation, to analyze the developmental sequences to which these neurotrophins are tuned. Here we demonstrate that in altricial pigeons no retinal immu-nolabeling of BDNF, NT-3 or their receptors could be detected before hatch, although a prominent tectal labeling pattern throughout most layers was evident. After hatch, both neurotrophins and their receptors showed a dramatic increase of retinal and tectal distribution. While the tectal and retinal protein synthesis of NT-3 vanished after 2 weeks, that of BDNF could still be revealed in adults. Therefore, the establishment of the retino-tectal system does not seem to depend on these neurotrophins before hatch, although they are probably utilized to shape the intratectal wiring pattern. In contrast, BDNF and NT-3 could play a prominent role in posthatch re-tino-tectal plasticity, as the consolidation of tectal asymmetries requires posthatch modifications of tectal circuits and proceeds within the first two posthatching weeks. These data are comparable with the distribution of neurotrophins in the retino-tectal system of chicks, although the onset of neurotrophin synthesis seems to be earlier in precocial chicks. Abbreviations BDNF Brain-derived neurotrophic factor · d deep tectal layers (13–15) · DAB 3,3'-diaminobenzidine · E embryonic day · GCL ganglion cell layer · im intermediate layers (8–12) · INL inner nuclear layer · IPL inner plexiform layer · NT-3 neurotrophin-3 · ONL outer nuclear layer · OPL outer plexiform layer · OT optic tectum · PBS phosphate buffered saline · Ph posthatching day · RGC retinal ganglion cells · s superficial layers (1–7) · trkB tyrosine-kinase receptor B · trkC tyrosine-kinase receptor C