Muscarinic modulation of acetylcholine release from slices of guinea pig nucleus basalis magnocellularis.

Spontaneous and electrically evoked endogenous acetylcholine release and [3H]-choline efflux from slices of guinea pig nucleus basalis magnocellularis (nbM) were studied. Tetrodotoxin reduced the spontaneous endogenous release by 55%, while the Ca(2+)-free medium reduced it by about 30%. Evoked [3H]-choline efflux was Na+ and Ca2+ dependent and frequency related. Physostigmine, 30 microM, nearly halved the stimulation-evoked efflux; atropine, 0.15 microM, not only antagonized, but even reversed this effect into facilitation. Pirenzepine, 1 microM, and AFDX 116, 1 microM, were less effective than atropine, and reversed the inhibitory effect of physostigmine only when applied together. 4-DAMP, 0.01 microM, was ineffective. These findings indicate that acetylcholine release in guinea pig nbM slices is inhibited by the cooperation of muscarinic autoreceptors, possibly belonging to the M1 and M2 subclasses.