Expression of TrkA mRNA in neonatal sympathetic neurons cultured in the presence of 10 ng/ml NGF for 7 days, followed by addition of 200 ng/ml NGF globally (G), in the center compartments (C), or on distal axons (D) for 48 or 72 hr (A) or for 7 days (B). The Northern blots and ethidium bromide-stained gels are as described in Fig. 1, and the RNA in each panel was derived from sister cultures.