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

The distribution of the neurotrophins BDNF and NT-3 as well as their corres ponding high-affinity receptors trkB and trkC was characterized by immunohi stochemistry in the developing retino-tectal system of the pigeon. These ne urotrophins are known to be important for survival and development of neuro nal tissues, but also for activity-dependent neuronal plasticity. In pigeon s visual asymmetry is established at the morphological and behavioral level due to a natural asymmetrical light input before hatch, which is followed by a posthatch period of consolidation with unbiased light stimulation. Sin ce the retino-tectal system is the crucial entity of these events, we studi ed the retinal and the tectal distribution of these neurotrophins and their receptors during retino-tectal formation, to analyze the developmental seq uences to which these neurotrophins are tuned. Here we demonstrate that in altricial pigeons no retinal immunolabeling of BDNF, NT-3 or their receptor s could be detected before hatch, although a prominent tectal labeling patt ern throughout most layers was evident. After hatch, both neurotrophins and their receptors showed a dramatic increase of retinal and tectal distribut ion. 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 est ablishment of the retino-tectal system does not seem to depend on these neu rotrophins before hatch, although they are probably utilized to shape the i ntratectal wiring pattern. In contrast, BDNF and NT-3 could play a prominen t role in posthatch retino-tectal plasticity, as the consolidation of tecta l asymmetries requires posthatch modifications of tectal circuits and proce eds within the first two posthatching weeks. These data are comparable with the distribution of neurotrophins in the retino-tectal system of chicks, a lthough the onset of neurotrophin synthesis seems to be earlier in precocia l chicks.