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Neuroimmune interactions and pain during postnatal development 

Neuroimmune interactions and pain during postnatal development
Chapter:
Neuroimmune interactions and pain during postnatal development
Author(s):

David Vega-Avelaira

and Simon Beggs

DOI:
10.1093/med/9780199642656.003.0007
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Summary

The immune system is essential for identifying and mounting defensive responses to tissue damage and infection. In addition, it is increasingly recognized that interactions between immune cells and nociceptive pathways can modulate pain sensitivity. The role and function of immune cells in the central nervous system changes during postnatal development, and as a result, the impact of neuroimmune interactions on pain signalling varies with both age and the type of injury.

Introduction

Interactions between nociceptive pathways and the immune system are important for detecting and responding to potential harm from tissue damage and infection, and involve multiple different cell types in different regions (Figure 7.1). In the periphery, nociceptors release a range of mediators which not only contribute to peripheral sensitization, but also attract and activate immune cells (mast cells, dendritic cells, T lymphocytes) to facilitate removal of pathogens and wound healing (Chiu et al., 2012). Macrophages are T lymphocytes which are present in the dorsal root ganglion (DRG), and increased activity and recruitment following nerve injury can contribute to pain sensitivity (Hu et al., 2007). In the spinal cord, microglia are no longer thought of as having only immune surveillance and supportive roles. Following noxious afferent input, microglial reactivity is increased, and the released proinflammatory cytokines, chemokines, and extracellular proteases, interact with neurons and contribute to central sensitization (Beggs and Salter, 2010; Clark et al., 2011). Astrocytes, another subtype of glial cells, are also activated in the spinal cord following inflammation, plantar incision, and nerve injury, but the degree and time course differ from microglia, and varies with the type of injury (Chiang et al., 2012). Modulators of glial activity reduce pain sensitivity associated with injury, but have little effect on normal or baseline pain processing, suggesting that the main role of glial activation is in pathological pain states (Chiang et al., 2012).

Figure 7.1 Neuroimmune interactions in neuropathic pain. Peripheral nerve injury provokes activation of innate immune cells such as macrophages and mastocytes proximal to the damaged area (Hu and McLachlan, 2002, 2003; Watkins and Maier, 2002). As a consequence, both immune cells and Schwann cells secrete pro-inflammatory cytokines to prevent further injury and repair the damage. This promotes physiological, cellular, and molecular changes in the DRG neurons and macrophages which result in neuronal aberrant electrical discharges, macrophage activation and pro-inflammatory cytokine release (Gabay et al., 2011; Hu and McLachlan, 2002; Ozaktay et al., 2006; Suter et al., 2009). Cytokines, such as CCL2 are anterogradely transported from the ganglia to the primary afferents terminals in the spinal cord (Thacker et al., 2009). Acting through CCR2 and interleukin receptors they activate intracellular pathways (e.g. P38 mak kinase pathway), promote translocation of transcription factors like NF-kB to microglia nuclei, and become activated (Capiralla et al., 2012; Chang and Waxman, 2010; Ren and Dubner, 2007; Schäfers et al., 2003; Svensson et al., 2003). As part of the activation process, microglia produce and release cytokines with an autocrine and paracrine activity which create a feedback positive loop that will sensitize second-order neurons and initiate the neuropathy (X. Gao et al., 2005; Y.-J. Gao and Ji, 2009, 2010).

Figure 7.1
Neuroimmune interactions in neuropathic pain. Peripheral nerve injury provokes activation of innate immune cells such as macrophages and mastocytes proximal to the damaged area (Hu and McLachlan, 2002, 2003; Watkins and Maier, 2002). As a consequence, both immune cells and Schwann cells secrete pro-inflammatory cytokines to prevent further injury and repair the damage. This promotes physiological, cellular, and molecular changes in the DRG neurons and macrophages which result in neuronal aberrant electrical discharges, macrophage activation and pro-inflammatory cytokine release (Gabay et al., 2011; Hu and McLachlan, 2002; Ozaktay et al., 2006; Suter et al., 2009). Cytokines, such as CCL2 are anterogradely transported from the ganglia to the primary afferents terminals in the spinal cord (Thacker et al., 2009). Acting through CCR2 and interleukin receptors they activate intracellular pathways (e.g. P38 mak kinase pathway), promote translocation of transcription factors like NF-kB to microglia nuclei, and become activated (Capiralla et al., 2012; Chang and Waxman, 2010; Ren and Dubner, 2007; Schäfers et al., 2003; Svensson et al., 2003). As part of the activation process, microglia produce and release cytokines with an autocrine and paracrine activity which create a feedback positive loop that will sensitize second-order neurons and initiate the neuropathy (X. Gao et al., 2005; Y.-J. Gao and Ji, 2009, 2010).

Peripheral nerve injury

Neuropathic pain can be severe and difficult to treat (see also Walker, Chapter 21; Rastogi and Campbell, Chapter 49, this volume). Despite considerable effort the conditions that initiate and maintain neuropathic pain are as yet not fully understood. Peripheral nerve injury (PNI) activates multiple cellular and molecular pathways in both the peripheral and central nervous systems. Of considerable interest now is parsing these cellular and molecular pathways; PNI will evoke a nerve regeneration response and a stress response associated with interrupted metabolic pathways (Costigan et al., 2009). Clearly, explicit damage to the peripheral nervous system is countered by responses in multiple systems, and it is the interaction between immune system reactivity and injury-driven changes in nociceptive signalling that is now under scrutiny.

PNI in experimental animals under 3 weeks of age does not result in the development of pain hypersensitivity, although evidence suggests that the onset is delayed (Howard et al., 2005; Vega-Avelaira et al., 2012). This developmental anomaly is due, at least in part, to differences between the early postnatal and adult immune systems (Moss et al., 2007; Vega-Avelaira et al., 2007, 2009).

Microglia

Microglia are the resident macrophages of the central nervous system (CNS). Their complex repertoire of functions in the adult includes maintaining homeostasis, detecting and responding to infection, and promoting or suppressing neuroinflammation. These actions also link microglia inextricably to neurodegenerative diseases such as Alzheimer’s disease, Parkinsonism, and neuropathic pain (Beggs et al., 2012b; Kettenmann et al., 2011). Microglia respond to potentially damaging injury or disease to the CNS by adopting any of several reactive phenotypes. Morphological changes can be identified by changes in expression of the microglial marker, ionized calcium binding adaptor molecule1 (Iba-1; Beggs and Salter, 2007; Suter et al., 2009; Vega-Avelaira et al., 2007). Where once the concept of a monolithic microglial ‘activation’ was widely accepted, evidence now suggests there are different subpopulations with different reactive response profiles and capabilities (Scheffel et al., 2012).

Microglia are sensors for damage or disease in the CNS, capable of detecting pathogen-associated molecular patters (PAMPs) and damage-associated molecular patterns (DAMPs). One of the many classes of receptors expressed by microglia are the Toll-like receptors (TLRs). TLRs are part of the complex molecular sensor framework for detecting both PAMPs and DAMPs (Stewart et al., 2010; Zhang and Mosser, 2008) and members of this family of receptors have been implicated in the pathogenesis of neuropathic and inflammatory pain in adult animals (Sorge et al., 2011; Tanga et al., 2005).

Microglial function during development

Microglia are of haematopoietic origin, and first appear as primitive macrophages in the yolk sac in early embryogenesis from where they migrate into the CNS (Ginhoux et al., 2010). This process occurs throughout embryonic and into early postnatal life (Pont-Lezica et al., 2011). The actions of microglia during development are complex with a dual function as: (1) immune effectors in the removal of excess and exuberant axonal projections, and (2) a more classical glial role as cytotropic and cytotrophic factors (Schlegelmilch et al., 2011). Given the extensive remodelling and refining of connectivity throughout the CNS in the early postnatal period, it is clearly necessary for microglia to differentiate between normal physiology and pathology.

The induction of cytokines and chemokines resulting from TLR4 activation varies with postnatal age and the pattern is the same across the TLR family members, with a general hyporesponsivity that peaks at postnatal day (P) 21. The transient changes and hyporesponsivity of some TLRs may be a reflection of the substantial reorganization and refinement of connectivity within the dorsal horn (and CNS generally) that occurs postnatally. Normal developmental synaptic elimination and pruning of exuberant axonal projections would provide a temporary increase in DAMPs and provoke an unwanted immune response.

This time point marks the end of a critical period of maturation of both the nervous and immune systems, and also the point at which central neuroimmune interactions stimulated by PNI are capable of driving neuropathic behavioural pain behaviours (Howard et al., 2005; Moss et al., 2007). Intriguingly, this postnatal maturation is also marked by a switch in gene expression such that gene transcripts associated with the classical, pro-inflammatory M1 phenotype are downregulated while those of the alternative anti-inflammatory M2 phenotype start to predominate (Gordon and Taylor, 2005; Mosser and Edwards, 2008; Scheffel et al., 2012).

T lymphocytes

T cells, also known as T lymphocytes, play an important role in cell-mediated immunity, but there is also increasing evidence for a role of T cells in the responses to PNI that produce pain. Helper T cells can be divided into two broad subtypes; Th1 and Th2, classified as such by the array of cytokines they produce. Th1 is generally pro-inflammatory and released cytokines include interferon gamma (IFNγ), which is heavily implicated in neuropathic pain pathogenesis (Costigan et al., 2009; Masuda et al., 2012; Tsuda et al., 2009). Th2 is generally anti-inflammatory, and ideally a balance of Th1 and Th2 exists. In early life, the Th1 pro-inflammatory response is weak (Fadel and Sarzotti, 2000), and the balance is shifted to a Th2 predominance, with Th1 gradually strengthening with exposure to allergens.

The complex developmental regulation of the immune system is occurring at the same time as the nervous system is undergoing extensive postnatal refinement (Beggs et al., 2002; Fitzgerald, 2005; Fitzgerald et al., 1994), it is not surprising that interactions of the immune system with the CNS following PNI differ significantly in the early postnatal period from those seen in the adult. Taken together, these observations indicate that the immune system in the immediate postnatal period does not react to PNI in the same way as in the mature adult animal. The pattern of T-cell activation and infiltration following PNI differs significantly in young animals. The predominance of Th2 T cells in the neonate underlies this, imbuing the immature immune system with a tolerance for the new antigens exposed by normal developmental changes within the CNS (Costigan et al., 2009).

Neuroimmune interactions in dorsal root ganglia

Macrophages are phagocytic immune cells, capable of engulfing and digesting cellular debris and pathogens, and act to maintain the cellular environment and protect against potential infection in the DRG (Watkins and Maier, 2002). In keeping with resident microglia in the CNS, macrophages in the DRG adopt a ramified surveillance state and are homogenously distributed between neuronal cell bodies and projecting fibres (Beggs et al., 2012b; Hu and McLachlan, 2002, 2003; Vega-Avelaira et al., 2009). PNI provokes both a reactive response of the resident macrophages as well as infiltration of new macrophages from the circulation (Hu and McLachlan, 2002, 2003). Macrophages in the DRG are characterized by expression of the ‘macrophage scavenger receptor’ CD163, and can also be identified by expression of CD68, which is increased 3 days following experimental PNI in adult animals (Hu et al., 2007). CD168-positive cell number remains the same, suggesting a reactive phenotypic switch in the normal resident macrophage population. Macrophage reactivity peaks at 7 days post injury, as shown by the adoption of an amoeboid morphology and maximal proliferation, and persists for at least 3 months (Hu and McLachlan, 2002). Retrograde labelling studies have shown that these reactive macrophages cluster around damaged neurons and in particular the large cell bodies of myelinated afferents (Vega-Avelaira et al., 2009).

The injury-induced reactive phenotype of DRG macrophages is determined in part by their molecular phenotype. Increased expression of membrane receptors such as purinergic receptors (e.g. P2X4) and chemokine receptors (e.g. IL6R, TNFαR, CCR2) activate intracellular second-messenger pathways (e.g. MAP kinases pathway; Gao and Ji, 2010; Ji et al., 2003, 2009; Scholz and Woolf, 2007), leading to increased expression of molecules that drive a phagocytic phenotype. This is accompanied by the release of chemokines (Hu and McLachlan, 2003) which maintain this adaptive phenotype through both autocrine and paracrine positive feedback loops and by macrophage recruitment from the circulation. Chemokines also act directly to influence neuronal excitability (Scholz and Woolf, 2007), thereby modulating afferent signal transduction from the dorsal root ganglia to the spinal cord.

Signalling molecules

MCP-1/CCL2

Macrophage chemo-attractant protein 1 (MCP-1, now CCL2) and its receptor CCR2 have been associated with several models of nerve injury (Thacker et al., 2009; Vega-Avelaira et al., 2009). MCP-1 is expressed in small DRG neurons and is transported anterogradely to the spinal cord after PNI where it has chemotactic effects on both glia and neurons (Thacker et al., 2009) (Figure 7.1). Genetically modified CCR2 null or ‘knock-out’ mice do not exhibit enhanced pain behaviour after PNI (Abbadie et al., 2003; Zhang et al., 2007). Taken together, these studies directly implicate macrophage-neuronal interactions in pain behaviour.

Tumour necrosis factor α (TNFα)

TNFα is a pro-inflammatory cytokine and one of many factors released following PNI (Jeon et al., 2011; Kim et al., 2011). Macrophages are the main source of TNFα but de novo expression is also evident in small-diameter nociceptive cells after nerve injury. TNFα activates the MAP kinase p38 intracellular pathway which instigates a feed-forward mechanism to increase further production of TNFα. Pharmacological blockade of TNFα is sufficient to ameliorate pain behaviour in animals with partial axotomy (Schäfers et al., 2003).

Interleukin-6 (IL-6)

IL-6 is a pleiotropic cytokine (Simpson et al., 1997) with roles in neuronal survival, growth and differentiation (Grothe et al., 2000). It is synthesized and released by macrophages and Schwann cells. The IL-6 receptor (IL-6R or CD126) is expressed in the DRG (Brázda et al., 2009; Watkins and Maier, 2002; Zhang et al., 2007) and there is evidence for IL-6 involvement in both the hyperalgesia and allodynia associated with neuropathic pain (DeLeo et al., 1996). IL-6 production is increased in neurons and satellite glial cells in DRGs within 24 h of PNI and persists for at least 2 weeks (Brázda et al., 2009). Furthermore, IL-6 has roles in multiple independent pathways mediating processes including nerve regeneration (Qiu et al., 2005) and TNFα-induced apoptosis of DRG neurons (Murata et al., 2011). As contradictory evidence exists for both beneficial and detrimental effects of IL-6 after PNI, further investigation is warranted.

Interleukin-1β (IL-1β)

IL-1β is intimately associated with the initiation and maintenance of neuropathic pain (Gabay et al., 2011; Wolf et al., 2006). It is constitutively expressed in primary sensory neurons (Copray et al., 2001) and has pro-excitatory effects in small and medium non-peptidergic neurons (Stemkowski and Smith, 2012) via a G protein-coupled receptor mediated mechanism (von Banchet et al., 2011). PNI induces a rapid increase in IL-1β in DRG macrophages (Thacker et al., 2007), sensitizing DRG neurons and contributing to increased mechanosensitivity (Ozaktay et al., 2006).

Prostaglandins

Prostaglandins, such as cyclo-oxygenase (COX)-2 dependent PGE2, have an important role in the pathogenesis of neuropathic pain (Ma and Quirion, 2008; Ma et al., 2010). At the spinal cord level, COX-2 is upregulated within the first 3 days after injury (O’Rielly and Loomis, 2006), and in the periphery, COX-2 and PGE2 production is upregulated in Schwann cells and macrophages infiltrating damaged nerves. PGE2 receptors, such as EP1 receptors, are expressed on DRG neurons, and nerve injury causes the upregulation of EP1 in human and rat DRG neurons (Durrenberger et al., 2006; Ma et al., 2010). PGE2 sensitizes damaged axons to mechanical and heat stimuli (Ma and Quirion, 2008; Michaelis et al., 1998), and increases production of substance P and calcitonin gene-related peptide in DRG neurons (Segond von Banchet et al., 2003).

Nitric oxide (NO)

Macrophages are a main source of nitric oxide (NO) after nerve injury (Moalem and Tracey, 2006), induced by TNFα via inducible NO synthase (iNOS) (Lowenstein et al., 1994). NO upregulates prostaglandin production and contributes to neuronal sensitization (Mollace et al., 1997; Purwata, 2011).

Nerve injury and spinal microglial reactivity

Increased microglial reactivity has been demonstrated in several animal models of PNI associated with chronic pain (Coull et al., 2005; Tsuda et al., 2003; Vega-Avelaira et al., 2009; Zhuang et al., 2005). Initiation of the microglial response after peripheral nerve damage is triggered by increased neural activity of sensory afferent fibres (Suter et al., 2009). Nociceptive stimuli are predominantly transmitted by Aδ- and C-fibres (see Walker and Baccei, Chapter 6, this volume). However, following nerve injury, activity in Aβ-fibres which normally transduce innocuous tactile sensation can contribute to persistent pain (Matsumoto et al., 2008). Afferent blockade with the local anaesthetic bupivacaine, which blocks conduction in all sensory neurons (C, Aδ, and Aβ fibres) prevented nerve injury-induced spinal microglial activation; but resiniferatoxin, which selectively blocks peripheral input from C and Aδ nociceptive fibres (Mitchell et al., 2010) had no effect (Suter et al., 2009). This suggests that the large myelinated fibres have an important role in the switch of microglia to a reactive phenotype after nerve injury. It has also been suggested that a small proportion of microglia are activity-independent, responding to retrograde axonal transport and cytokine stimulation (Abe and Cavalli, 2008; O’Brien and Nathanson, 2007). Microglia also respond to chemotactic factors and migrate towards the area in the spinal cord that receives the projections of degenerating C-fibre primary afferents (Beggs and Salter, 2007; Davalos et al., 2005; Nimmerjahn et al., 2005; Shehab et al., 2008; Shields et al., 2003).

Microglia express multiple receptors that may be involved in altering reactivity (Beggs and Salter, 2010) that include: P2X and P2Y purinergic receptors (Trang et al., 2009; Tsuda et al., 2003), TLR-4 receptors (Ji and Strichartz, 2004; Lehnardt et al., 2003), and chemokine receptors such as the fractalkine receptor CX3CR1 (Clark et al., 2011; Hughes et al., 2002), the CCR2 receptor for MCP-1/CCL2 (Thacker et al., 2009), and the IFNγ receptor (Tsuda et al., 2009). Release of ATP, which acts via purinergic receptors, contributes to microglia changing to a reactive phenotype (Haynes et al., 2006). This is accompanied by morphological changes, increased proliferative capacity, and increased expression of membrane molecules such as complement receptor 3 (CD11b; Kreutzberg, 1996; Nakajima and Kohsaka, 1998) and Iba1 (Beggs and Salter, 2010).

Pro-inflammatory cytokines sensitize neurons by enhancing excitatory AMPA/NMDA currents and reducing inhibitory gamma-aminobutyric acid (GABA)/glycine currents (Kawasaki et al., 2008; Reeve et al., 2000; Sweitzer et al., 2001; Wang et al., 2005), but also contribute to neuropathic pain by stimulating microglia. Microglia are the main source of pro-inflammatory cytokines in the CNS (Hanisch, 2002) and increased functional microglial reactivity increases production and release of proinflammatory mediators (fractalkine IL-1β, IL-4, TNFα), prostaglandins, and ATP (Giaume et al., 2007; Kreutzberg, 1996; Segond von Banchet et al., 2003; Watkins et al., 2001). Following receptor activation, an increase in the intracellular concentration of Ca2+ activates signalling pathways, such as those mediated by the mitogen-activated protein kinase (MAPK) p38 (Ji and Suter, 2007) (Figure 7.1). p38 MAPK is a key factor in regulating the expression of proinflammatory cytokines, PGE2 and COX-2 (Jana et al., 2003; Ji and Suter, 2007; Wilms et al., 2003), and is involved in the recruitment of microglia to areas of injury (Tsuda et al., 2005). p38 activation is increased following nerve damage, whereas its inhibition suppresses mechanical allodynia (Ji and Suter, 2007; Jin et al., 2003; Svensson et al., 2003). Activation of the transcription factor NF-kB also promotes the production of proinflammatory cytokines, chemokines, COX-2 and other molecules related to immune activity (Y.-J. Gao and Ji, 2010).

IL-6 is upregulated in both the DRG and the spinal cord after nerve injury (Raghavendra et al., 2003), and also has a dual function by stimulating microglia (Galiano et al., 2001; Klein et al., 1997) and neuronal responses to injury (Latrémolière et al., 2008).

TLR4 receptor

The metabotropic receptor TLR-4 is important for pathogen recognition, and is activated by various substances, such as lipopolysaccharide from Gram-negative bacteria (H. Cao and Zhang, 2008; Capiralla et al., 2012). It is expressed on microglia and activation induces intracellular signalling cascades resulting in increased NF-κB signalling and subsequent expression of numerous genes regulating proinflammatory cytokines and other immunological molecules (Capiralla et al., 2012). Deficiency of this receptor or its blockade attenuates microglial reactivity, decreases levels of IFNγ, IL-1β, and TNF, and reduces pain behaviours associated with PNI (Tanga et al., 2005).

P2X4 receptor

P2X4 receptors are expressed by microglia following PNI (Tsuda et al., 2003). Blockade of this receptor prevents and reverses mechanical allodynia in animal models of PNI, and mice deficient in P2X4 do not develop mechanical hypersensitivity after nerve injury (Ulmann et al., 2008). P2X4R expression is increased through the activation of other microglial receptors such us IFNγ and fibronectin receptors (Masuda et al., 2012; Tsuda et al., 2008, 2009). P2X4R activation induces the synthesis and release of brain-derived neurotrophic factor (BDNF) by microglia (Coull et al., 2005; Trang et al., 2009) via a p38 MAPK-dependent pathway (Beggs and Salter, 2010; Jin et al., 2003). BDNF activates its neuronal receptor tyrosine kinase B (TrkB) on lamina I neurons in the spinal cord dorsal horn (Slack et al., 2005). This reduces the expression of the chloride transporter KCC2 leading to intracellular accumulation of Cl. The net effect of this is disinhibition; reducing the inhibitory action of GABA, and facilitating further neuronal sensitization (Coull et al., 2003, 2005). BDNF signalling in neurons also activates kinase pathways leading to phosphorylation of the NMDA receptor NR1 subunit, which also contributes to enhanced sensitivity (Ren and Dubner, 2007; Suen et al., 1997).

CX3CR1 receptor

Fractalkine (CX3CL1) is released by neurons and its receptor CX3CR1 is expressed on microglia, making it a prime candidate for neuronal-microglial signalling (Verge et al., 2004). The increased activity of primary afferents after nerve injury releases fractalkine into the extracellular space (Zhuang et al., 2007) and creates a chemotactic gradient for microglia (Chapman et al., 2000). Intrathecal injection of fractalkine upregulates expression of CX3CR1 and induces thermal hyperalgesia, which can be prevented by blocking fractalkine activity with CX3CR1 antibodies (Sun et al., 2007). Inhibiting microglial function with minocycline also prevents the appearance of mechanical allodynia and thermal hyperalgesia induced by injection of fractalkine (Milligan et al., 2005).

Lymphocyte infiltration and central sensitization

In addition to increased macrophage and microglial reactivity following PNI, other immunocompetent cells, such as T-lymphocytes also play a role. T lymphocytes can release excitatory cytokines, generating ectopic impulse activity in sensory neurons after nerve injury (L. Cao and DeLeo 2008; L. Cao et al., 2009; Costigan et al., 2009; Hu and McLachlan 2002). The dorsal root ganglia have a small population of lymphocytes which perform an immune surveillance function (Hu and McLachlan 2002). Peripheral nerve damage causes infiltration of CD8+ lymphocytes into the DRG and spinal cord (Costigan et al., 2009; Hu and McLachlan 2002) which suggests that PNI triggers an adaptive immune response. Moreover, this lymphocyte infiltration contributes to microglia reactivity via CD40 (a member of the TNF-receptor superfamily) (L. Cao et al., 2009) and mechanical hypersensitivity (Costigan et al., 2009; Hu and McLachlan 2002). The role of T cells is also demonstrated by the reduction in pain behaviours after nerve injury exhibited by the lymphocyte deficient Rag1-null mice (Costigan et al., 2009).

Postnatal differences in immune system activation after nerve injury

The role of neuroimmune interactions as mediators of neuropathic pain has focused on experimental models in adult animals. However, clinical studies suggest that traumatic nerve injury is less likely to produce neuropathic pain in early life; brachial plexus avulsion, which causes intense neuropathic pain in adults, is not painful when the injury is sustained at birth (Anand and Birch, 2002; see also Walker, Chapter 21, this volume). Laboratory studies conducted in animals throughout postnatal development are consistent with this: in nerve injury models of neuropathic pain, persistent mechanical allodynia does not develop if the injury is performed in the first 3 weeks of life, a developmental period which has parallels with human development from the neonate to adolescence (Howard et al., 2005; Ririe and Eisenach, 2006). Since infant rats and humans clearly respond to acute noxious stimuli and display both acute and chronic inflammatory pain behaviour from an early neonatal age, it appears that the mechanisms underlying neuropathic pain are differentially regulated over a prolonged postnatal period. Important differences include:

  • Microglia in the neonatal spinal cord are capable of being activated by exogenous stimuli (e.g. intraspinal NMDA), but nerve injury in young rats fails to induce either the spinal microgliosis or macrophage accumulation in the DRG, that are hallmarks of PNI in the adult (Moss et al., 2007; Vega-Avelaira et al., 2007).

  • Microarray analysis of gene expression in the DRG following nerve injury demonstrate changes associated with pro-inflammatory cytokines such us IL-6, CSF-1, and MCP-1 being upregulated in adult but not in young rats (Vega-Avelaira et al., 2009).

  • In the spinal cord, many genes are differentially regulated in the adult compared with the infant rat, and the majority of these are involved in immune function (Costigan et al., 2009).

  • Activation and infiltration of T-cells associated with the Th1 pro-inflammatory response in the dorsal horn is much greater in adult rats than in young animals after nerve injury (Costigan et al., 2009).

Neonatal incision and long-term priming of microglial response

The mechanisms and effectors previously described, that are in place to mute the postnatal immune response as part of the normal developmental process, would at first suggest that early postnatal peripheral injury does not result in persistent neuropathic pain. While the incidence of neuropathic pain following traumatic nerve injury is lower in infants and young children, the concept that an absence of pain to early life injury indicates a lack of a response, whether CNS or immune, is simplistic. A key function that unites all sensory systems is that adult connectivity is shaped by the balance of sensory input in early postnatal life. In much the same way that auditory and visual system connectivity in the brain is driven by sound and light, tactile sense is driven by touch (Bourne, 2010; Sanes and Bao, 2009). Neural activity during critical postnatal periods can influence normal development (Hensch, 2004) and this is also true for nociceptive pathways (Beggs et al., 2002; Ren et al., 2004). A considerable canon of evidence from both laboratory and clinical studies now shows that exposure to nociceptive stimuli in early life can produce long-term changes in sensory processing and the response to future injury (see Walker, Chapter 3; Grunau, Chapter 4, this volume). Importantly, priming of neuroimmune signalling by neonatal injury can contribute to persistent effects. ‘Priming’ or ‘neonatal programming’ of adult immune responses by early life immune stressors has been described (Boisse et al., 2004; Spencer et al., 2011), and an early immune challenge can alter pain sensitivity in adulthood (Boisse et al., 2005). As microglia are long-lived cells and can retain an innate immune memory (Perry, 2010) they are well suited to a role in persistent alterations. Plantar hindpaw incision, an established model of postoperative pain, performed during the first postnatal week, but not at older ages, enhances the response to subsequent injury 2 weeks later (Walker et al., 2009). The impact of neonatal injury persists until adulthood, with increases in both the degree and duration of incision-related hyperalgesia (Beggs et al., 2012a). These functional changes are mirrored by alterations in the time course and degree of microglial reactivity in the dorsal horn. The primed state arises from centrally-mediated changes in dorsal horn sensitivity or connectivity as:

  • Hyperalgesia is selectively reversed by intrathecal minocycline, which reduces microglial reactivity in the spinal cord.

  • The increased sensitivity and microglial reactivity following neonatal incision is also seen following direct electrical stimulation of the tibial nerve, which bypasses peripheral nociceptors (Beggs et al., 2012a).

Conclusion

Interactions between the immune system and nociceptive pathways play an important role in modulating pain sensitivity. Throughout postnatal development, neuroimmune interactions have a significant impact on age-related changes in the response to PNI, and also play a role in persistent changes in future pain response following neonatal surgical injury. Evaluating the relative risks and benefits of modulating microglial activity to prevent long-term changes in nociceptive pathways, or to manage enhanced pain sensitivity states, requires further research.

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