Substance P inhibits the action potentials in bullfrog sympathetic ganglion cells.

Substance P (0.5-5 microM) depressed the spike peak and after-hyperpolarization of action potentials of bullfrog sympathetic ganglion cells. It also depressed the after-hyperpolarization and prolonged the falling phase in Ca2+ spikes. The voltage-dependent K+ currents, both the delayed rectifier K+ current (Ik1) and the M current (Ik2), were suppressed by substance P, suggesting that the depression of the after-hyperpolarization may be due to suppression of these K+ currents.