Resolving TRPV1- and TNF-α-Mediated Spinal Cord Synaptic Plasticity and Inflammatory Pain with Neuroprotectin D1

Abstract

Mechanisms of inflammatory pain are not fully understood. We investigated the role of TRPV1 and TNF-α, two critical mediators for inflammatory pain, in regulating spinal cord synaptic transmission. We found in mice lacking Trpv1 the frequency but not the amplitude of spontaneous EPSCs (sEPSCs) in lamina II neurons of spinal cord slices is reduced. Further, C-fiber-induced spinal long-term potentiation (LTP) in vivo is abolished in Trpv1 knockout mice. TNF-α also increases sEPSC frequency but not amplitude in spinal lamina IIo neurons, and this increase is abolished in Trpv1 knockout mice. Single-cell PCR analysis revealed that TNF-α-responding neurons in lamina IIo are exclusively excitatory (vGluT2+) neurons. Notably, neuroprotectin-1 (NPD1), an anti-inflammatory lipid mediator derived from omega-3 polyunsaturated fatty acid (docosahexaenoic acid) blocks TNF-α- and capsaicin-evoked sEPSC frequency increases but has no effect on basal synaptic transmission. Strikingly, NPD1 potently inhibits capsaicin-induced TRPV1 current (IC50=0.4 nM) in dissociated dorsal root ganglion neurons, and this IC50 is ≈ 500 times lower than that of AMG9810, a commonly used TRPV1 antagonist. NPD1 inhibition of TRPV1 is mediated by GPCRs, since the effects were blocked by pertussis toxin. In contrast, NPD1 had not effect on mustard oil-induced TRPA1 currents. Spinal injection of NPD1, at very low doses (0.1–10 ng), blocks spinal LTP and reduces TRPV1-dependent inflammatory pain, without affecting baseline pain. NPD1 also reduces TRPV1-independent but TNF-α-dependent pain hypersensitivity. Our findings demonstrate a novel role of NPD1 in regulating TRPV1/TNF-α-mediated spinal synaptic plasticity and identify NPD1 as a novel analgesic for treating inflammatory pain.

Endogenous Interleukin-1β in Neuropathic Rats Enhances Glutamate Release from the Primary Afferents in the Spinal Dorsal Horn through Coupling with Presynaptic N-Methyl-d-aspartic Acid Receptors*

“…Understanding the regulation of presynaptic receptors by proinflammatory cytokines would reveal synaptic mechanisms by which glial cells alter spinal nociceptive processing. In this regard, it was recently reported that TNF-␣ increases glutamate release from presynaptic terminals via TRPV1 receptors (73). Our findings that presynaptic NMDA receptors in the primary afferent terminals are the effector receptors used by endogenous IL-1␤ to increase glutamate release from the primary afferent terminals are the first to reveal the functional coupling between presynaptic NMDA receptors and IL-1␤.…”

“…Primary (73) increases glutamate release from the central terminals of primary afferents. Added to this complexity of presynaptic regulation mechanisms is the presynaptic NMDA receptor reported recently by us and others.…”

Posttraumatic stress disorder influences the nociceptive and intrathecal cytokine response to a painful stimulus in combat veterans

“…TNF␣ and IL-6 were significantly correlated with evoked pain measures in the PTSD group, whereas there was no correlation between proinflammatory cytokine concentrations and any pain measure in the CC group. CNS release of TNF␣ can lower pain thresholds by modulating the number of neuronal membrane ion channels and their functional activity (Choi et al, 2010), which can result in hyperexcitable pain circuits within dorsal horn (Reeve et al, 2000;Park et al, 2011;Kawasaki et al, 2008). TNF␣ binding to the microglial TNF␣ receptor (TNF␣-R) stimulates p38 mitogen-activated protein kinase (P38 MAPK) phosphorylation and activation.…”

“…Similar to TNF␣, both inflammatory and neuropathic pain models have shown that IL-1␤ is synthesized and secreted from astrocytes, neurons, and microglia. IL-1␤ enhances activity in the lamina II of the dorsal horn by multiple mechanisms (Reeve et al, 2000;Park et al, 2011;Kawasaki et al, 2008) and is known to amplify pain signaling. Increased IL-1␤ secretion in the PTSD-group may enhance pain nociception, which is a conclusion supported in our study by the following: 1) increases in spontaneously reported pain intensity and unpleasantness and 2) the increased evoked pain measure of temporal summation.…”

TLR3 deficiency impairs spinal cord synaptic transmission, central sensitization, and pruritus in mice

“…Whole-cell patchclamp recordings were performed at room temperature to measure inward currents (voltage clamp, at -60 mV holding potential) and action potentials (current clamp) in small-sized DRG neurons, using Axopatch-200B amplifier (49). Whole-cell patch-clamp recordings were also conducted in lamina II neurons in spinal cord slices (30) to measure sEPSC in a voltage clamp mode (-70 mV).…”