title: Modulation by adenosine of both muscarinic M1-facilitation and M2-inhibition of [3H]-acetylcholine release from the rat motor nerve terminals.
abstract: The crosstalk between adenosine and muscarinic autoreceptors regulating evoked [3H]-acetylcholine ([3H]-ACh) release was investigated on rat phrenic nerve-hemidiaphragm preparations. Motor nerve terminals possess facilitatory M1 and inhibitory M2 autoreceptors that can be activated by McN-A-343 (1-30 microm) and oxotremorine (0.3-100 microm), respectively. The muscarinic receptor antagonist, dicyclomine (3 nm-10 microm), caused a biphasic (inhibitory/facilitatory) effect, indicating that M1-facilitation prevails during 5 Hz stimulation trains. Concomitant activation of AF-DX 116-sensitive M2 receptors was partially attenuated, as pretreatment with M1 antagonists, muscarinic toxin 7 (MT-7, 0.1 nm) and pirenzepine (1 nm), significantly enhanced inhibition by oxotremorine. Activation of A2A-adenosine receptors with CGS 21680C (2 nm) (i) potentiated oxotremorine inhibition, and (ii) shifted McN-A-343-induced facilitation into a small inhibitory effect. Conversely, the A1-receptor agonist, R-N6-phenylisopropyl adenosine (R-PIA, 100 nm), attenuated the inhibitory effect of oxotremorine, without changing facilitation by McN-A-343. Synergism between A2A and M2 receptors is regulated by a reciprocal interaction with facilitatory M1 receptors, which may be prevented by pirenzepine (1 nm). During 50 Hz-bursts, facilitation (M1) of [3H]-ACh release by McN-A-343 disappeared, while the inhibitory (M2) effect of oxotremorine became predominant. This muscarinic shift results from the interplay with A2A receptors, as it was precluded by the selective A2A receptor antagonist, ZM 241385 (10 nm). In conclusion, when the muscarinic M1 positive feedback loop is fully operative, negative regulation of ACh release is mediated by adenosine A1 receptors. During high frequency bursts, tonic activation of A2A receptors promotes M2 autoinhibition by braking the M1 receptor operated counteraction.
authors.affiliation: Laboratório de Farmacologia/UMIB, Instituto de Ciências Biomédicas de Abel Salazar (ICBAS) - Universidade do Porto, Portugal.
authors.forename: Laura
authors.initials: L
authors.lastname: Oliveira
authors.forename: M Alexandrina
authors.initials: MA
authors.lastname: Timóteo
authors.forename: Paulo
authors.initials: P
authors.lastname: Correia-de-Sá
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000241:Adenosine / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017660:Autoreceptors / Q000187:drug effects / Q000378:metabolism*; D004558:Electric Stimulation; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D005260:Female; D008297:Male; D009046:Motor Neurons / Q000187:drug effects / Q000378:metabolism*; D018721:Muscarinic Agonists / Q000494:pharmacology; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism*; D010791:Phrenic Nerve / Q000187:drug effects / Q000378:metabolism; D058906:Purinergic P1 Receptor Agonists; D058915:Purinergic P1 Receptor Antagonists; D051381:Rats; D017208:Rats, Wistar; D043705:Receptor, Adenosine A2A; D043643:Receptor, Muscarinic M1; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D018047:Receptors, Purinergic P1 / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
chemical_list: D017660:Autoreceptors; D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D058906:Purinergic P1 Receptor Agonists; D058915:Purinergic P1 Receptor Antagonists; D043705:Receptor, Adenosine A2A; D043643:Receptor, Muscarinic M1; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic; D018047:Receptors, Purinergic P1; D000255:Adenosine Triphosphate; D000241:Adenosine; D000109:Acetylcholine