title: Nicotinic receptors modulating ACh release in rat cortical synaptosomes: role of Ca2+ ions in their function and desensitization.
abstract: 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 [3H]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 [3H]acetylcholine overflow resulting in bell-shaped concentration-response curves. Synaptosomes pretreated with different concentrations (10 microM to 0.001 microM) of acetylcholine or nicotine responded to a subsequent nicotinic stimulus (10 microM acetylcholine plus 0.1 microM 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 1,2-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 microM). 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.
authors.affiliation: Dipartimento di Medicina Sperimentale, Università di Genova, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Marchi
authors.forename: M
authors.initials: M
authors.lastname: Lupinacci
authors.forename: E
authors.initials: E
authors.lastname: Bernero
authors.forename: F
authors.initials: F
authors.lastname: Bergaglia
authors.forename: M
authors.initials: M
authors.lastname: Raiteri
mesh_terms: D000109:Acetylcholine / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D011978:Receptors, Nicotinic / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology
chemical_list: D015220:Calcium Channels; D011978:Receptors, Nicotinic; D001285:Atropine; D000109:Acetylcholine