Substance P induced enhancement of inhibitory synaptic transmission in deep dorsal horn

Substance P (SP) is the most popular neuropeptide currently considered in detail, and has been widely studied as an excitatory factor of nociception. However, analgesic effects were not clearly shown by inactivation of SP receptors by the use of SP receptor antagonists or SP knock-out mice in some recent studies. In order to investigate the mechanism, we studied the excitatory and inhibitory effects of SP and SP receptor agonists on deep dorsal horn (DH) neurons of the spinal slice preparation by using patch clamp method. We used neurokinin 1 (NK1) receptor agonist ([Sar9, Met(O2)11]SP: 1.0 µM), because SP has highest affinity for NK1 receptor in three SP receptors (NK1, NK2, and NK3). Bath application of SP or NK1 receptor agonist for 1 minute increased the frequency of spontaneous excitatory postsynaptic currents (sEPSC) mediated by glutamate in deep DH. SP and NK1 receptor agonist induced enhancement of glutamate release from interneurons and/or primary afferent fibers onto deep DH neurons. On the other hand, the application of SP or NK1 receptor agonist largely increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) and miniature inhibitory post synaptic currents (mIPSC) mediated by GABA and/or glycine in deep DH. These findings suggested that SP and NK1 receptors are expressed in GABAergic and/or glycinergic interneurons and that their activations facilitated GABA and/or glycine release onto deep DH neurons. In summary, it has been thought SP is generally participating in the enhancement of excitatory nociceptive pathway, but these results made clear that SP induced enhancement of inhibitory nociceptive transmission in deep DH. Clinical treatment of SP receptor antagonists against pain has been proposed for a long time, but is not established yet. Our observation in this study revealed it is necessary to consider the inhibitory effects of SP in the spinal deep dorsal horn in clinical application of SP receptor antagonists for the treatment of various pain sensation.