MUSCARINIC AGONISTS MODULATE CALCIUM-DEPENDENT OUTWARD CURRENTS IN AN IDENTIFIED INSECT MOTONEURON

The soma membrane of the fast coxal depressor (D-f) 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 depolarizaion in D-f 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.