KCl and cAMP block the development of competence in NGF-deprived sympathetic neurons. Mouse sympathetic neurons were deprived of NGF in the presence of 1 μg/ml cycloheximide (−NGF+CHX), 35 mm KCl (−NGF+KCl), or 400 μm CPTcAMP (−NGF+CPTcAMP) for 36 hr. Neurons were then microinjected with cytochrome c (10 mg/ml), and the viability of the cells at 3, 6, and 12 hr after the injections was determined as described in the legend to Figure 1. Although the NGF-deprived, cycloheximide-saved cells developed competence (microinjection of cytochrome c induced rapid cell death), both KCl and CPTcAMP prevented the development of competence in these neurons (microinjection of cytochrome c did not induce cell death). Data shown are mean ± range of two experiments with ∼100 cells counted for each time point per experiment.