Nicotinic receptors modulating ACh release in rat cortical synaptosomes: role of Ca2+ ions in their function and desensitization

Cholinergic nerve terminals in the central nervous system are endowed with both muscarinic and nicotinic autoreceptors, mediating inhibition, and enha ncement of acetylcholine release, respectively. Exogenous acetylcholine inh ibited the K+(15 mM)evoked overflow of [H-3]acetylcholine from superfused r at neocortical synaptosomes; however, in the presence of atropine, this mus carinic inhibition was reversed into a nicotinic potentiation when acetylch oline was added concomitantly with high-K+, but not before depolarization. Increasing concentrations of acetylcholine (plus atropine), nicotine and ()-anatoxin-a produced elevations of the K+-evoked [H-3]acetylcholine overfl ow resulting in bell-shaped concentration-response curves. Synaptosomes pre treated with different concentrations (10 mu M to 0.001 mu M) of acetylchol ine or nicotine responded to a subsequent nicotinic stimulus (10 mu M acety lcholine plus 0.1 mu M atropine, in 15 mM K+) in a manner reflecting varyin g degrees of desensitization. This desensitization could be reversed by was hings with standard medium and desensitization was attenuated when external Ca2+ ([Ca2+](e)) was decreased. Lowering of [Ca2+](e) or chelation of inte rnal Ca2+ with I,2-bis(2-aminophenoxy) ethone-N,N,N',N'-tetracetic acid ace toxymethylester (BAPTA-AM) permitted the nicotinic response to acetylcholin e alone (no atropine added) to prevail over the muscarinic response. Pretre atment with BAPTA-AM could however not prevent desensitization by acetylcho line (10 or 0.001 mu M). The data indicate that Ca2+ ions are involved in d etermining the balance between muscarinic and nicotinic autoreceptor functi on and in the desensitization of nicotinic autoreceptors. (C) 1999 Elsevier Science Ltd. All rights reserved.