Routes of acetylcholine leakage from cytosolic and vesicular compartments of rat motor nerve terminals.

Acetylcholine efflux at the rat neuromuscular junction was assayed following blockage of ACh transport into synaptic vesicles by 2-(4-phenylpiperidino) cyclohexanol (AH5183). [2H4]Choline was used as a labeled precursor. AH5183 completely blocked ACh efflux from the cytosolic compartment but had comparatively less effect on release from the unlabeled vesicular pool. Tissue [2H4]ACh levels increased after AH5183 addition due to cytosolic ACh retention. Thus, ACh in the non-vesicular pool (calculated to be 34% of the total ACh) may efflux solely via the AH5183-sensitive ACh transporter inserted into the terminal membrane. ACh released from the vesicular fraction was about 100-fold more than could be accounted for by miniature end-plate potentials; possible causes of this overestimate are discussed.