Long-term treatment with JNK inhibitor continues to reversibly prevent axon formation. a, Neurons transfected to express soluble tomato fluorescent protein and immunostained for tau-1. Top panels, Representative neuron chronically treated with SP600125 for 4 d (4 dy), showing continued inhibition of axon formation. Bottom panels, Representative neuron cultured for 4 d in the chronic presence of SP600125, followed by washout for 24 h. The neuron formed a single axon. b, Neurons at 7 d (7 dy) in culture immunostained for tau-1 and MAP2. Top panels, Representative neurons chronically treated with SP600125. The neurons did not form axons but had MAP2-immunopositive neurites that resembled dendrites. Bottom panels, Representative neurons cultured in the presence of SP600125 for 6 d, followed by washout for 24 h. Two of the four neurons formed axons (arrows). Scale bars: a, 25 μm (top panels) and 50 μm (bottom panels); b, 50 μm. SP, SP600125.