Specific high-affinity receptors for neurotrophin-3 on sympathetic neurons
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When used at concentrations allowing interactions only with its high-affinity receptors, neurotrophin-3 (NT-3) promotes the survival of sensory neurons isolated from embryonic day 8 (E8) chicks, but not the survival of E11 sympathetic neurons. These sympathetic neurons (which can be rescued by the addition of NGF) display high-affinity receptors for NT-3 (Kd of 1.6 x 10(-11) M) that cannot be distinguished from the high-affinity NT-3 receptors on sensory neurons using equilibrium binding or kinetic criteria. This represents the first example of embryonic neurons that cannot be rescued by the in vitro addition of a neurotrophin in spite of the presence of corresponding neurotrophin high-affinity receptors. At elevated concentrations, beyond the saturation of its high-affinity receptors, NT-3 supports the survival of some E11 sympathetic neurons, an effect that might be mediated by the high-affinity NGF receptor. Using E7 sympathetic neurons, about 40% of the cells initially plated can be rescued in vitro by the addition of low concentrations of NT-3 (but not of NGF) and produce profuse neurites. This indicates that NT-3 may play a role in the early development of some sympathetic neurons.
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