The Effects of Carbachol on the Proximal and Distal Parts of Frog Motor Nerve Endings

Cholinomimetics not only activate postsynaptic cholinoreceptors in neuromuscular synapses, but also alter the process of acetylcholine secretion from nerve endings. However, the mechanism of action of cholinomimetics on the secretory process remains unidentified. We approached the question of the mechanism of the presynaptic action of cholinomimetics in the present study by investigating the effects of the n,m-cholinomimetic carbachol on nerve ending currents and postsynaptic membrane currents. Carbachol induced decreases in the postsynaptic response, without affecting the duration and amplitude of the nerve ending current in both the central and distal part of the nerve ending. However, carbachol increased the time between the arrival of the presynaptic action potential and the start of transmitter secretion. This effect on synaptic delay was more marked in the distal parts of the ending. The action of another potential modulator, extracellular potassium, was accompanied by decreases in presynaptic currents and also by increases in synaptic delay. These data provide evidence for the suppressive effect of carbachol on acetylcholine secretion acting via presynaptic metabotropic cholinoreceptors which control the level and time course of secretion of neurotransmitter quanta.