SIGNALING MECHANISMS MEDIATING MUSCARINIC ENHANCEMENT OF GABAergic SYNAPTIC TRANSMISSION IN THE SPINAL CORD

Activation of muscarinic acetylcholine receptors (mAChRs) inhibits spinal nociceptive transmission by potentiation of GABAergic tone through M-2, M-3, and M-4 subtypes. To study the signaling mechanisms involved in this unique mAChR action, GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of lamina 11 neurons were recorded using whole-cell patch clamp techniques in rat spinal cord slices. The mAChR agonist oxotremorine-M caused a profound increase in the frequency of GABAergic sIPSCs, which was abolished in the Ca2+-free solution. Inhibition of voltage-gated Ca2+ channels with Cd2+ and Ni2+ largely reduced the effect of oxotremorine-M on sIPSCs. Blocking nonselective cation channels (NSCCs) with SKF96365 or 2-APB also largely attenuated the effect of oxotremorine-M. However, the KCNQ channel blocker XE991 and the adenylyl cyclase inhibitor MDL12330A had no significant effect on oxotremorine-M-induced increases in sIPSCs. Furthermore, the phosphoinositide-3-kinase (PI3K) Inhibitor wortmannin or LY294002 significantly reduced the potentiating effect of oxotremorine-M on sIPSCs. In the spinal cord in which the M3 subtype was specifically knocked down by intrathecal small interfering RNA (siRNA) treatment, SKF96365 and wortmannin still significantly attenuated the effect of oxotremorine-M. In contrast, SKF96365 and wortmannin both failed to alter the effect of oxotremorine-M on sIPSCs when the M-2/M-4 mAChRs were blocked. Therefore, our study provides now evidence that activation of mAChRs increases synaptic GABA release through Ca2+ Influx and voltage-gated Ca2+ channels. The PI3K-NSCC signaling cascade Is primarily involved in the excitation of GABAergic interneurons by the M-2/M-4 mAChRs in the spinal dorsal horn. (C) 2009 IBRO. Published by Elsevier Ltd. All rights reserved.