Octopamine modulates ionic currents and spiking in dorsal unpaired median (DUM) neurons

MODULATORY effects of octopamine on ionic currents and spiking in isolated cockroach dorsal unpaired median neurons were investigated by means of the chopped voltage-/current-clamp and the patch-clamp technique. Octopamine increased the spiking frequency at concentrations less than or equal to 10 mu M and reduced it at > 10 mu M. It enhanced a low voltage-activated Ca2+ current at 1 to 100 mu M. At concentrations up to 10 mu M two components of Ca2(+)-activated K+ current were potentiated. At > 10 mu M octopamine in addition reduced a high voltage-activated Ca2+ current and the Ca2+-activated K+ current. A membrane permeant cAMP-analogue imitated the effects obtained at high octopamine concentration. Octopamine and cAMP in addition reduced a depolarizing resting current at both low and high concentration. Possible mechanisms and physiological significance are discussed for the opposite effects of octopamine observed at low versus high concentrations.