Muscarinic M 1 receptors stimulated by intracerebroventricular administration of McN-A-343 reduces the nerve injury-induced mechanical hypersensitivity via GABA B receptors rather than GABA A receptors in mice.

Cholinergic neurons play an important role in the higher functions of the brain, such as the memory, cognition, and nociception. However, the exact mechanism behind how the stimulation of all the muscarinic M receptors in the entire brain results in the alleviation of partial sciatic nerve ligation (PSNL)-induced mechanical hypersensitivity has not been investigated. Thus, we examined which subtype of GABA receptor was involved in the alleviation of PSNL-induce mechanical hypersensitivity produced by an intracerebroventricular administration of a muscarinic M receptor agonist, McN-A-343. Administering a GABA receptor antagonist, bicuculline, resulted in no changes to the McN-A-343-induced anti-hypersensitivity in PSNL mice whereas a GABA receptor antagonist, CGP35348, dose-dependently inhibited the anti-hypersensitivity. Furthermore, CGP35348 increased mechanical hypersensitivity in naïve mice, and the hypersensitivity was blocked by NMDA receptor antagonists, MK-801 and D-AP5. Additionally, muscarinic M receptors colocalized with GABA receptors and an NMDA receptor subunit, GluN2A, in a large region of the brain. Consequently, these results suggest that the activation of muscarinic M receptors in the entire brain reduces nerve injury-induced mechanical hypersensitivity via the GABA receptors, and the activation of the GABA receptors regulates glutamatergic transmission via NMDA receptors.