Fig. 6. Effects of the cAMP pathway mutations on quantal ACh release from growth cones. A, Example traces of spontaneous currents collected from WT,dnc1, dnc2, and PKA-RI growth cones. Subsets of ACh currents detected from myoballs in contact with mutant growth cones had prolonged or irregular time courses. The ACh current amplitudes were smaller in mutants. B, Direct comparison of the spontaneous ACh currents from a WT and adnc2
growth cone detected by the same myoball. In control experiments, a myoball was manipulated into contact with a WT neuron (WT, first contact), lifted up, then returned to the same contact site (WT, second contact). The ACh currents remained the same between the first and the second contact (mean ± SEM = 59.5 ± 53.1 pA for the first contact and 63.1 ± 46.2 pA for the second contact). In a separate experiment, same myoballs were manipulated into contact with a WT anddnc2 neurons, which were situated side by side in the same recording dish. Currents detected with the same myoball from WT, and dnc neurons showed clear differences in amplitudes and kinetics (see traces and Results)C, Acute application of membrane-permeable db-cAMP (0.5 mm) to WT neurons prolonged the time course and reduced the amplitude of the ACh currents.