Ca²⁺-Independent Feedback Inhibition of Acetylcholine Release in Frog Neuromuscular Junction

The effect of membrane potential on feedback inhibition of acetylcholine (ACh) release was studied using the frog neuromuscular junction. It was found that membrane potential affects the functional affinity (K_i) of the presynaptic M₂ muscarinic receptor. The K_i for muscarine shifts from ∼0.23 μM (at resting potential) to ∼8 μM (at a high depolarization). Measurements of Ca²⁺ currents in axon terminals showed that the depolarization-mediated shift in K_i does not stem from depolarization-dependent changes in Ca²⁺ influx. Pretreatments with pertussis toxin (PTX) abolished the depolarization-dependent shift in K_i; at all depolarizations K_i was the same and higher (∼32 μM) than before PTX treatment. The inhibitory effect of muscarine on ACh release is produced by two independent mechanisms: a slow, PTX-sensitive process, which prevails at low to medium depolarizations and operates already at low muscarine concentrations, and a fast, PTX-insensitive and voltage-independent process, which requires higher muscarine concentrations. Neither of the two processes involves a reduction in Ca²⁺ influx.