Functional activity of the M2 and M4 receptor subtypes in the spinal cord studied with muscarinic acetylcholine receptor knockout mice

Stimulation of spinal muscarinic acetylcholine receptors (mAChRs) produces potent analgesia. Both M-2 and M-4 mAChRs are coupled to similar G proteins (G(i/o) family) and play a critical role in the analgesic action of mAChR agonists. To determine the relative contribution of M 2 and M 4 subtypes to activation of G(i/o) proteins in the spinal cord, we examined the receptor-mediated guanosine 5'-O-(3-[S-35]thio)triphosphate ([S-35]GTP gamma S) binding in M-2 and M-4 subtype knockout (KO) mice. Basal [S-35]GTP gamma S binding in the spinal cord was similar in the wild-type controls, M-2 and M-4 single-KO, and M-2/M-4 double-KO mice. The spinal [S-35]GTP gamma S binding stimulated by either muscarine or oxotremorine-M was not significantly different among three groups of wild-type mouse strains. In M 2 single-KO and M-2/M-4 double-KO mice, the agonist-stimulated [S-35]GTP gamma S binding was completely abolished in the spinal cord. Furthermore, the agonist-stimulated [S-35]GTP gamma S binding in the spinal cord of M 4 single-KO mice was significantly reduced (similar to 15%), compared with that in wild-type controls. On the other hand, the spinal [S-35]GTP gamma S binding stimulated by a mu-opioid agonist was not significantly different between wildtype and M-2 and M-4 KO mice. This study provides complementary new evidence that M-2 is the most predominant mAChR subtype coupled to the G(i/o) proteins in the spinal cord. Furthermore, these data suggest that a small but functionally significant population of M-4 receptors exists in the mouse spinal cord. The functional activity of these M-4 receptors seems to require the presence of M-2 receptors.