Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:hemicholinium-3, a depletor of endogenous acetylcholine (Parikh and Sarter, 2006); (ii) by enhancing cholinergic tone with AF-DX384, a selective M2/M4 autoreceptor antagonist, that blocks the self-inhibitory mechanism by which acetylcholine regulates its own release and (iii) by selectively blocking M1 muscarinic receptors, key modulators of both LTD and LTP (Calabresi et al., 2000; Wang et al., 2006; Bonsi et al., 2008).
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