Muscarinic agonists modulate calcium-dependent outward currents in an identified insect motoneurone

The soma membrane of the fast coxal depressor (Df) motoneurone of the cockroach, Periplaneta americana possesses a population of acetylcholine receptors which respond to both nicotinic and muscarinic ligands. Activation of these 'mixed' acetylcholine receptors by McN-A-343 or oxotremorine results in the generation of an inward current at potentials positive to - 40 mV. A large proportion of the outward current induced by membrane depolarization in Df somata is due to a calcium-activated potassium conductance (IKCa) which shows a characteristic N-shape voltage-dependence. This N-shape of the membrane current-voltage relationship is suppressed by McN-A-343 or oxotremorine indicating that the agonist-induced inward current is due to a reduction in IKCa.