Dynamic regulation of glycinergic input to spinal dorsal horn neurones by muscarinic receptor subtypes in rats

Activation of spinal muscarinic acetylcholine receptors (mAChRs) inhibits nociception. However, the cellular mechanisms of this action are not fully known. In this study, we determined the role of mAChR subtypes in regulation of synaptic glycine release in the spinal cord. Whole-cell voltage-clamp recordings were performed on lamina II neurones in the rat spinal cord slices. The mAChR agonist oxotremorine-M significantly increased the frequency of glycinergic sIPSCs but not mIPSCs. Surprisingly, the effect of oxotremorine-M on sIPSCs was largely attenuated at a higher concentration. On the other hand, 1-10 μ oxotremorine-M also dose-dependently incresed the frequency of sIPSCs in the presence of himbacine (an M₂/M₄ mAChR antagonist) or AF-DX116 (an M₂ mAChR antagonist). The M₃ mAChr antagonist 4-DAMP abolished the stimulatory effect of oxotremorine-M on sIPSCs. Interestingly, the GABA_B receptor antagonist CGP55845 potentiated the stimulatory effect of oxotremorine-M on sIPSCs. In the presence of CGP55845, both himbacine and AF-DX116 similarly reduced the potentiating effect of oxotremorine-M on sIPCs. Collectively, these data suggest that the M₃ subtype is present on the somatodendritic site of glycinergic neurones and is mainly responsible for muscarinic potentiation of glycinergic input to spinal dorsal horn neurones. Concurrent stimulation of mACHRs on adjacent GABAergic interneurones attenuates synaptic glycine release through presynaptic GABA_B receptors on glycinergic interneurones. This study illustrates a complex dynamic interaction between GABAergic and glycinergic synapses in the spinal cord dorsal horn.