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Functional ionotropic nucleotidic receptors responding to diadenosine pentaphospate and nicotinic receptors responding to epibatidine coexpress in 19% of the total rat midbrain cholinergic terminals, as determined by the combination of immunological and microfluorimetric techniques. Activation of each independent receptor induces the intrasynaptosomal [Ca 2+ ] i and acetylcholine (ACh) release in a dose-dependent way. The responses are inhibited by antagonists of the dinucleotide receptor and nicotinic receptors, thus confirming the involvement of specific receptors in both functions. Stimulation of single cholinergic terminal with both agonists altogether results in a significant decrease of the [Ca 2+ ] i signaling compared with responses of each independent agonist. Inhibitory interaction between both receptors is reverted when one of them is blocked by specific antagonists, both in [Ca 2+ ] i , and subsequent ACh release. The receptor′s inhibitory cross talk confirm the involvement of calcium/calmodulin-dependent protein kinase II, CaMKII, as the inhibitory effects are reverted in the presence of the specific inhibitors KN-62 (2-[ N -(4′-methoxybenzenesulfonyl)]-amino- N -(4′-chlorophenyl)-2-propenyl- N -methylbenzylamine phosphate) and KN-93 ( N -(2-[ N -[4-chlorocinnamyl]- N -methylaminomethyl]phenyl)- N -(2-hydroxyethyl)-4-methoxybenzenesulphonamide). These results demonstrate the existence of an efficient interaction between these two channel populations, opening a new understanding of the functioning of the cholinergic synaptic terminals or terminals containing other neurotransmitters but exhibiting these receptor types or ones that are similar.