No involvement of nicotinic receptors in the facilitation of acetylcholine outflow in mouse cortex in the presence of neostigmine and atropine

Abstract

1 The role of nicotinic and muscarinic receptors in the modulation of acetylcholine release was studied using ®eld stimulated mouse cortex slices incubated with [ 3 H]-choline. 2 Both acetylcholine (100 mM) and the cholinesterase inhibitor neostigmine (100 mM) inhibited the stimulation-induced (S-I) out¯ow of radioactivity but in the presence of atropine (0.3 mM) an enhancement was seen, which may be indicative of facilitatory nicotinic receptors. 3 Mecamylamine (100 mM) was unable to antagonize the enhancement seen in the presence of acetylcholine and atropine. The nicotinic agonist dimethylphenylpiperazinium (30 mM) did not facilitate S-I out¯ow of radioactivity. 4 A range of nicotinic blockers had no eect on the enhancement seen in the presence of neostigmine and atropine, nor did indomethacin, the 5HT 3 antagonist MDL 7222 nor the NMDA antagonist MK-801. 5 The inability to block this eect suggests that nicotinic receptors are not involved. We postulate, at least for neostigmine, that the facilitation is an artefact because of the use of [3 H]-choline as a radiotracer whereby the eux of radioactivity is enhanced because the radiolabelled acetylcholine is not metabolized to choline and therefore¯ows out of the tissue more readily.