Dissociation between M1-facilitation of acetylcholine release and crosstalk with A2A- and M2-receptors on rat motoneurons

Acetylcholine (ACh) facilitates its own release acting at muscarinic M1 receptors on rat motoneurons. While the M1 positive feedback mechanism is operative there is a concomitant suppression of the ability of muscarinic M2-inhibitory and adenosine A2A-facilitatory receptors to control [3H]ACh release. We aimed at investigating whether M1-occlusion of M2 and A2A receptors function could result from interplay at second messengers level. Drugs blocking the IP3 pathway, like LiCl and 2-aminoethoxydiphenylborane (2-APB), but not the selective PKC inhibitor, chelerythrine, attenuated M1 facilitation by McN-A-343. PKC activation with phorbol 12-myristate 13-acetate mimicked the ability of McN-A-343 to suppress M2-inhibition and A2A-facilitation of [3H]ACh release caused by oxotremorine and CGS 21680C, respectively. Co-application of chelerythrine together with McN-A-343 restored oxotremorine-inhibition and CGS 21680C-facilitation, but this was not observed when the M1 agonist was applied together with LiCl and 2-APB. McN-A-343 also masked facilitation of [3H]ACh release caused by stimulators of the cyclic AMP pathway, forskolin and rolipram. Data suggest that M1-facilitation of ACh release results mainly from activation of the IP3 pathway. This mechanism can be dissociated from the way M1 receptor operates suppression of neuromodulation through M2-inhibitory and A2A-facilitatory receptors, which might involve secondary PKC activation.