Distinct roles of spinal muscarinic receptor subtypes in control of glycinergic input revealed by muscarinic receptor knockout mice

Abstract

Spinal muscarinic acetylcholine receptors (mAChRs) play an important role in regulation of nociception. The M2, M3 and M4 mAChR subtypes are present in the spinal dorsal horn. To determine the role of individual mAChR subtypes in the control of synaptic glycine release, spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded in lamina II neurons using whole-cell recordings in spinal cord slices of wild-type (WT) and mAChR subtype knockout (KO) mice. In 56.6 % neurons in WT mice, bath application of the mAChR agonist oxotremorine-M (3–10 μM) dose-dependently decreased the frequency of glycinergic sIPSCs. In M3-KO mice, oxotremorine-M significantly decreased the frequency of glycinergic sIPSCs in all cells tested. However, in M2/M4 double-KO mice, oxotremorine-M consistently increased the frequency of sIPSCs and mIPSCs in all neurons tested, and this effect was abolished by 4-DAMP (an M3 subtype-preferring muscarinic antagonist). Collectively, this study strongly suggests that stimulation of presynaptic M2 and M4 subtypes predominantly attenuates glycinergic inputs to spinal dorsal horn neurons. In contrast, activation of the M3 subtype increases synaptic glycinergic release in the spinal dorsal horn in mice. This study provides new information that the role of M2 and M4 subtypes in the control of glycinergic input to the spinal dorsal horn in mice is distinctly different from that in rats.