Spinal PKCα inhibition and gene-silencing for pain relief: AMPAR trafficking at the synapses between primary afferents and sensory interneurons

Abstract

Upregulation of Ca2+-permeable AMPA receptors (CP-AMPARs) in dorsal horn (DH) neurons has been causally linked to persistent inflammatory pain. This upregulation, demonstrated for both synaptic and extrasynaptic AMPARs, depends on the protein kinase C alpha (PKCα) activation; hence, spinal PKC inhibition has alleviated peripheral nociceptive hypersensitivity. However, whether targeting the spinal PKCα would alleviate both pain development and maintenance has not been explored yet (essential to pharmacological translation). Similarly, if it could balance the upregulated postsynaptic CP-AMPARs also remains unknown. Here, we utilized pharmacological and genetic inhibition of spinal PKCα in various schemes of pain treatment in an animal model of long-lasting peripheral inflammation. Pharmacological inhibition (pre- or post-treatment) reduced the peripheral nociceptive hypersensitivity and accompanying locomotive deficit and anxiety in rats with induced inflammation. These effects were dose-dependent and observed for both pain development and maintenance. Gene-therapy (knockdown of PKCα) was also found to relieve inflammatory pain when applied as pre- or post-treatment. Moreover, the revealed therapeutic effects were accompanied with the declined upregulation of CP-AMPARs at the DH synapses between primary afferents and sensory interneurons. Our results provide a new focus on the mechanism-based pain treatment through interference with molecular mechanisms of AMPAR trafficking in central pain pathways.

Cross-Talk of Toll-Like Receptor 5 and Mu-Opioid Receptor Attenuates Chronic Constriction Injury-Induced Mechanical Hyperalgesia through a Protein Kinase C Alpha-Dependent Signaling

“…To examine the influence of nerve decompression in relieving pain hypersensitivity after CCI, we conducted behavioral assessments and compared the difference between CCI and Decompression groups (Figure 1). In CCI group, the decreases of withdrawal threshold were revealed on ipsilateral sides, indicating mechanical hyperalgesia, from post-operative week (POW) 2 to POW 8 (F (3,46) = 259.9; p < 0.0001) ( Figure 1A). More importantly, in Decompression group, withdrawal thresholds showed on ipsilateral sides have no considerable difference with that on contralateral sides at POW 8 (p = 0.6066) ( Figure 1A).…”

Spinal AMPA receptors: Amenable players in central sensitization for chronic pain therapy?

“…Meanwhile, the postsynaptic AMPAR at nociceptive synapses – between primary nociceptive afferents and the DH neurons – consist of a mixed population of GluA2-containing (Ca 2+ -impermeable) receptors, with a relatively large proportion of GluA2-lacking, CP-AMPAR. This appears due to the compelling blocking effects of i) intracellular polyamines (i.e., the inwardly rectified I–V curve) and ii) selective antagonists of CP-AMPAR on the excitatory postsynaptic AMPAR-mediated currents induced by primary nociceptive afferent stimulation (EPSC) [ 22 , 34 , 77 ]. Aside from functional studies, quantitative estimates of the GluA1-labeled particles at the synapses of the superficial DH were found to exceed those of GluA2 [ 63 ].…”

“…This was effectively blocked by the pharmacological inhibition of PKC [ 100 ]. Using genetic approaches (gene-silencing) combined with the spinal delivery of genetic materials, the PKCα-dependent upregulation of GluA1-containing CP-AMPAR has been validated in persistent inflammatory pain; moreover, the transient knockdown of spinal PKCα recovered both upregulated AMPAR-mediated currents and Ca 2+ influx in the lamina II DH neurons, returning them back to normal levels [ 76 ] and alleviating persistent inflammatory pain [ 22 ].…”

“…The pharmacological inhibition of spinal PKCα (using a selective PKCα inhibitor peptide, C2-4) or genetic inhibition of PKCα (with antisense oligonucleotides) has both effectively reduced nociceptive hypersensitivity in animals, and also provided relief in pain-induced locomotive deficit and anxiety in persistent peripheral inflammation [ 22 , 76 ]. At the cellular level, therapeutic effects were manifested in the form of restored AMPAR-mediated currents in the lamina II DH neurons and at nociceptive synapses [ 22 , 76 ]. Targeting scaffold proteins regulating the postsynaptic trapping of AMPAR at PSD can be another alternative ( Figure 2 ).…”

“…The implications of the role of spinal AMPAR in cellular mechanisms mediating the development and maintenance of chronic pain have emerged from several lines of experimental evidence. Studies from in vivo models of pain have clearly demonstrated that the pharmacological inhibition of spinal AMPAR significantly alleviates nociceptive hypersensitivity in animals experiencing pain of different origin: neuropathic [18], inflammatory [22][23][24], injury-induced [25,26] or postoperative pain [27][28][29]. Spinal AMPAR mediates sensory signaling, both at nociceptive and nonnociceptive circuits, and are heavily involved in the development [23,30,31] and maintenance of chronic pain [14,23,[31][32][33].…”