Negative crosstalk between M1 and M2 muscarinic autoreceptors involves endogenous adenosine activating A1 receptors at the rat motor endplate

At the rat motor nerve terminals, activation of muscarinic M1 receptors negatively modulates the activity of inhibitory muscarinic M2 receptors. The present work was designed to investigate if the negative crosstalk between muscarinic M1 and M2 autoreceptors involved endogenous adenosine tonically activating A1 receptors on phrenic motor nerve terminals. The experiments were performed on rat phrenic nerve-hemidiaphragm preparations loaded with [3H]-choline (2.5 μCi/ml). Selective activation of muscarinic M1 and adenosine A1 receptors with 4-(N-[3-clorophenyl]-carbamoyloxy)-2-butyryltrimethylammonium (McN-A-343, 3 μM) and R-N6-phenylisopropyladenosine (R-PIA, 100 nM), respectively, significantly attenuated inhibition of evoked [3H]-ACh release induced by muscarinic M2 receptor activation with oxotremorine (10 μM). Attenuation of the inhibitory effect of oxotremorine (10 μM) by R-PIA (100 nM) was detected even in the presence of pirenzepine (1 nM) blocking M1 autoreceptors, suggesting that suppression of M2-inhibiton by A1 receptor activation is independent on muscarinic M1 receptor activity. Conversely, the negative crosstalk between M1 and M2 autoreceptors seems to involve endogenous adenosine tonically activating A1 receptors. This was suggested, since attenuation of the inhibitory effect of oxotremorine (10 μM) by McN-A-343 (3 μM) was suppressed by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (2.5 nM), and by reducing extracellular adenosine with adenosine deaminase (0.5 U/mL) or with the adenosine transport blocker, S-(p-nitrobenzyl)-6-thioinosine (NBTI, 10 μM). The results suggest that the negative crosstalk between muscarinic M1 and M2 autoreceptors involves endogenous adenosine outflow via NBTI-sensitive (es) nucleoside transport system channelling to the activation of presynaptic inhibitory A1 receptors at the rat motor endplate.