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 enhancement of acetylcholine release, respectively. Exogenous acetylcholine inhibited the K+(15 mM)evoked overflow of [H-3]acetylcholine from superfused rat neocortical synaptosomes; however, in the presence of atropine, this muscarinic inhibition was reversed into a nicotinic potentiation when acetylcholine 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 overflow resulting in bell-shaped concentration-response curves. Synaptosomes pretreated with different concentrations (10 mu M to 0.001 mu M) of acetylcholine or nicotine responded to a subsequent nicotinic stimulus (10 mu M acetylcholine plus 0.1 mu M atropine, in 15 mM K+) in a manner reflecting varying degrees of desensitization. This desensitization could be reversed by washings with standard medium and desensitization was attenuated when external Ca2+ ([Ca2+](e)) was decreased. Lowering of [Ca2+](e) or chelation of internal Ca2+ with I,2-bis(2-aminophenoxy) ethone-N,N,N',N'-tetracetic acid acetoxymethylester (BAPTA-AM) permitted the nicotinic response to acetylcholine alone (no atropine added) to prevail over the muscarinic response. Pretreatment with BAPTA-AM could however not prevent desensitization by acetylcholine (10 or 0.001 mu M). The data indicate that Ca2+ ions are involved in determining the balance between muscarinic and nicotinic autoreceptor function and in the desensitization of nicotinic autoreceptors. (C) 1999 Elsevier Science Ltd. All rights reserved.