Pathological and protective roles of glia in chronic pain

Abstract

Glia have emerged as key contributors to pathological and chronic pain mechanisms. On activation, both astrocytes and microglia respond to and release a number of signalling molecules, which have protective and/or pathological functions. Here we review the current understanding of the contribution of glia to pathological pain and neuroprotection, and how the protective, anti-inflammatory actions of glia are being harnessed to develop new drug targets for neuropathic pain control. Given the prevalence of chronic pain and the partial efficacy of current drugs, which exclusively target neuronal mechanisms, new strategies to manipulate neuron-glia interactions in pain processing hold considerable promise.

Figure 1. Nociception

Normal pain signalling in the body is transmitted to the spinal cord dorsal horn through nociceptors. Nociceptive pain, such as a pin prick to the foot, leads to the release of pain transmitters from primary afferent terminals that project onto pain-projection neurons primarily to laminae I, IV and V in the spinal cord dorsal horns. It is notable that Aβ, Aδ and C fibres additionally project to laminae II–VI to a much lesser extent, but can alter pain-projection neuron activity. However, when tissue injury and skin inflammation ensue, chronic inflammatory signals are released from surrounding cells at the peripheral nerve terminal and lead to the sensitization of the nociceptors. Such factors include, but are not limited to, adenosine and its related mono- or polyphosphorylated compounds (AMP, ADP and ATP), bradykinin, glutamate, histamine, interleukin 1, interleukin 6, nerve growth factor, platelet-activating factor, prostaglandin E₂, protons, serotonin, substance P and tumour-necrosis factor-α. Nociceptive signalling from the dorsal root ganglia (DRG) is then relayed to the dorsal spinal cord, brain stem and brain, where the experience of pain occurs. The inset shows a cross section of spinal cord including the spinal cord dorsal horn and the DRG. The DRG contain pseudounipolar sensory neurons — so called because they give rise to a single axon that bifurcates, with one part projecting to the periphery and the other projecting to the dorsal horn of the spinal cord. The cell bodies of nociceptive neurons in the DRG are broadly classified into large and small types. Immunohistochemical staining studies have shown that slowly conducting C and Aδ fibres have small cell bodies, whereas faster-conducting Aβ fibres tend to have larger cell bodies.

Figure 2. Molecules involved in pain processing

a | Under healthy circumstances, low-frequency activation of Aδ and C fibre nociceptors by mild noxious stimuli leads to glutamate (Glu) release from the central presynaptic afferent nerve terminals in the spinal cord dorsal horn. Short-term activation of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid) and kainate subtypes of ionotropic glutamate receptors ensues. Although also present, the NMDA (N-methy-l-d-aspartate) ionotropic glutamate receptor subtype (NMDAR) remains silent because it is plugged by Mg²⁺. This signalling to dorsal horn pain-projection neurons provides information about the time of onset, duration and intensity of noxious stimuli from the periphery. Both astrocytes and microglia remain unchanged by these synaptic events. b | After repetitive synaptic communication, which can occur after a short barrage of nociceptive afferent input, there is an increase in the responsiveness of dorsal horn pain-projection neurons to subsequent stimuli (known as central sensitization) (see BOX 1). A co-release of glutamate and neurotransmitters such as substance P (sub P) and calcitonin gene-related peptide (CGRP) mediates NMDAR activation, leading to voltage-gated Ca²⁺ currents (VGCCs). In addition, inositol-1,4,5-trisphosphate (Ins(1,4,5)P₃) signalling and mitogen-activated protein kinases, such as extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK), are activated. In neurons, ERK can further sensitize excited AMPA receptors (AMPARs) and NMDARs. Activation of purinoreceptors (P2X₃) by ATP, activation of sub P receptors (the neurokinin 1 receptor (NK₁R)), activation of metabotropic glutamate receptors (mGluR), and release of brain-derived neurotrophic factor (BDNF) all contribute to enhanced nociceptive transmission. Astrocytes and microglia express various neurotransmitter receptors and are activated by glutamate, ATP and sub P. At synapses the glutamate transporters, glutamate transporter 1 (GLT1) and glutamate–aspartate transporter (GLAST), which are crucial for clearing synaptic glutamate, become dysregulated after prolonged exposure to high levels of synaptic glutamate. Ongoing excitation can induce ERK, p38 and JNK activation in microglia and astrocytes. Each of these kinases can activate the transcription factor nuclear factor-κB (NF-κB), which induces the synthesis of inflammatory factors. Upregulation of the V1 transient receptor potential channel (TRPV1) after inflammation further contributes to the sensitization to noxious signals. During this time, normally non-nociceptive Aβ fibres can also activate pain-projection neurons.

Figure 3. Glia activation: from pro-inflammatory to anti-inflammatory states

a |Pro-inflammatory roles for glia. If a noxious input persists, such as during chronic inflammation or nerve damage, sustained central sensitization leads to transcriptional changes in dorsal horn neurons that alter these neurons’ function for prolonged periods. Astrocytes respond to this ongoing synaptic activity by mobilizing internal Ca²⁺, leading to the release of glutamate (Glu), ATP that binds to P2X₄, tumour-necrosis factor-α (TNFα), interleukin 1β (IL-1 β), IL-6, nitric oxide (NO) and prostaglandin E₂ (PGE₂). Activated microglia are also a source of all of these pro-inflammatory factors. Matrix metalloproteinase 9 (MMP9) induces pro-IL-1β cleavage and microglial activation, whereas MMP2 induces pro-IL-1β cleavage and maintains astrocyte activation. The activation of p38 mitogen-activated protein kinase (p38 MAPK) is induced in both microglia and astrocytes on IL-1β signalling. Astrocytes and microglia express the chemokine receptors CX3CR1 (not shown) and CCR2 and become activated when the respective chemokines bind. After nerve damage, heat shock proteins (HSPs) are released and can bind to Toll-like receptors (TLRs) expressed on both astrocytes and microglia, leading to the further activation of these cell types. b | Activated glia can also be neuroprotective, as they release anti-inflammatory cytokines such as IL-10 and IL-4 and express cannabinoid receptors (CB₁ and CB₂) that have been shown to exert anti-inflammatory functions and to inhibit microglial toxicity by suppressing chemotactic responses and MAPK signal transduction, with consequent pro-inflammatory cytokine inhibition,. The glutamate transporters, glutamate transporter 1 (GLT1) and glutamate– aspartate transporter (GLAST), can resume normal glutamate clearance. Activation of microglial P2RX₇ purinoreceptors by ATP leads to TNFα release that protects neurons from glutamate-induced toxicity. Activated astrocytes reduce the spread of tissue degeneration after direct injury through the controlled removal of dying neurons and tissue debris, another neuroprotective effect. In quiescent cells (not shown), nuclear factor-κB (NF-κκB) is sequestered in the cytosol by inhibitor of κb (IκB), which binds to specific regions on NF-κB and thereby prevents exposure of the nuclear-localization signal. On stimulation with pro-inflammatory cytokines, IκB proteins are phosphorylated, leading to their ubiquitin-dependent degradation. As a consequence, NF-κB translocates to the nucleus and binds to elements in the promoters of target genes, leading to activation of pro-inflammatory cytokine genes