Tyrosine Kinases

16 May

Receptor Tyrosine Kinases  The receptors for growth factors are tyrosine kinases that  are autophosphorylated upon  ligand binding.  In particular,  NGF and BDNF mediate their effects largely via TrkA and TrkB, respectively. Although NGF is required  for the survival of sensory neurons  during  fetal develop- ment, it is not necessary for the survival of mature sensory neurons. Rather it maintains  the phenotype  of sensory neurons  in the adult. NGF plays an essential role in peripheral sensitization; local and systemic injection of NGF produces  hyperalgesia. Although BDNF is synthesized  in primary  sensory neurons, it can be released from central terminals in the spinal cord following intense noxious stimulation and plays an important role in central sensitiza- tion. Intrathecal  infusion of a scavenger for TrkA and TrkB (BDNF receptor) or a general Trk inhibitor k252a reduces both inflammatory  and neuropathic pain (McMahon et al. 1995; Mannion et al. 1999; Ji and Strichartz 2004). A re- cent study has also shown that receptor tyrosine kinase EphB is required for central  sensitization  and  the development  of inflammatory  pain  (Battaglia et al. 2003).

Non-receptor Tyrosine Kinases  Among this large group of signaling molecules, the Src family is the best known, which includes Src, Fyn, Lck, Lyn, and Yes. Src is activated by receptor tyrosine kinases as well as by G protein-coupled receptor  (GPCR) and PKC. Activation of Src is implicated in p44/42 MAPK activation (Kawasaki et al. 2004). Salter and his collaborators have shown an important role of Src in regulating the activity of NMDA receptors, an essential mediator of central sensitization (Salter and Kalia 2004). Thus, Src inhibitor was shown to suppress central sensitization (Guo et al. 2002). It is not clear whether other nonreceptor  tyrosine kinases also contribute to pain sensitization.

Mitogen-Activated Proteins Kinases (MAPKs)

The MAPKs are a family of evolutionally conserved molecules that play a critical role in cell signaling. This family includes three major members—extracellular signal-regulated kinase (ERK or p44/42 MAPK), p38, and c-Jun N-terminal ki- nase (JNK)—that represent three different signaling cascades. MAPKs trans- duce a broad range of extracellular stimuli into diverse intracellular responses by both transcriptional and nontranscriptional regulation  (Widmann  et al.

1999). Early studies  indicated  a critical role of ERK in regulating  mitosis, proliferation,  differentiation,  and survival of mammalian  cells during devel- opment.  Recent evidence shows that  ERK also plays an important role in neuronal plasticity and inflammatory responses in the adult. p38 and JNK are typically activated by cellular stress (ultraviolet  irradiation,  osmotic shock, heat shock), lipopolysaccharide (LPS), and certain proinflammatory cytokines

such as TNF-α and IL-1β (Widmann  et al. 1999; Ji and Woolf 2001). There- fore, these two kinases, especially JNK, are also called stress-activated  pro-

tein kinase (SAPK), contributing  importantly  to inflammatory  responses and neuronal degeneration. All the family members are activated by different up- stream MAPK kinases (MKKs). The corresponding  MKKs for ERK, p38, and

JNK are MKK1/2 (also called MEK1/2), MKK3/6, MKK4/7. The ERK5 is the least-known member of MAPK family and is activated by MKK5. MKKs are activated by MAPK kinase kinase. Studies on MAPKs greatly benefit from

specific inhibitors available to explore the function of each pathway and from phosphorylation-specific antibodies available to investigate the activation of each pathway. Like other kinase inhibitors, MAPK inhibitors do not affect basal

pain perception.

ERK/MAPK (p44/42 MAPK) Pathway

ERK is the first and the most studied  member  of the MAPK family. It was originally identified as a primary effector of growth factor receptor signaling, a cascade that involves sequential activation of Ras, Raf (MAPK kinase kinase), MEK (MAPK kinase), and ERK (MAPK). However, the activation of the ERK cascade is not restricted to growth factor signaling. ERK is activated by per- sistent neural activity and pathological stimuli. A growing body of evidence demonstrates an involvement of ERK in neuronal plasticity, such as learning and memory, as well as pain hypersensitivity  (Widmann  et al. 1999; Ji and Woolf 2001; Ji et al. 2003).

Activation of ERK in Primary Sensory Neurons  There is an activity-dependent ERK activation in DRG neurons.  Depolarization  of adult DRG neuronal  cul- tures induces a very rapid and transient  ERK phosphorylation (pERK). This

transient  pERK induction  is also observed in vivo following peripheral nox- ious stimuli, reaching  peak at 2 min and almost returning  to the baseline after 10 min (Dai et al. 2002). In addition to neuronal activity, NGF, capsaicin, and epinephrine can all activate ERK in a subset of DRG neurons. ERK is not only induced  in the soma of DRG neurons  but also in peripheral  axons of DRG neurons  (Dai et al. 2002; Zhuang et al. 2004). Intraplantar injection of capsaicin increases pERK-labeled nerve fibers in the epidermis within 2 min. Inhibition  of the ERK pathway by MEK inhibitors  attenuates  heat hyperalge- sia by capsaicin and NGF as well as mechanical hyperalgesia by epinephrine (Aley et al. 2001; Dai et al. 2002; Zhuang et al. 2004). ERK is likely to mediate heat hyperalgesia by inducing TRPV1 sensitization  (Dai et al. 2002; Zhuang et al. 2004).

Activation of ERK in Dorsal Horn Neurons  We have shown that ERK activation in spinal dorsal horn neurons  is nociceptive activity-dependent  (Ji et al. 1999). Injection of the C fiber nociceptor activator capsaicin into a hindpaw of rats induces a remarkable ERK phosphorylation. pERK is induced in dorsal horn neurons as early as 1 min after C fiber activation. This activation exactly fol- lows the rule of spinal topographic  organization:  pERK-labeled neurons  are found in the medial superficial dorsal horn of the spinal cord on the stimu- lated side where primary nociceptive afferents from the hindpaw terminate. The pERK can only be induced by thermal noxious (heat and cold) and me- chanical noxious (prick) stimulus, but not by innocuous stimulus (light touch) (Ji et al. 1999). It appears  that  the duration  of noxious  stimulation  is also important;  a very brief noxious stimulation  (<10 s) may not induce pERK. Noxious stimulation also induces ERK activation in the trigeminal spinal nu- cleus (Huang et al. 2000).

Moreover, pERK can be induced in a spinal cord slice preparation, where different  types of afferent  fibers  in  the  attached  dorsal  root  can  be elec-

trically stimulated  (Ji et al. 1999; Lever et al. 2003). A bath  application  of capsaicin to spinal slices, which will activate TRPV1 receptors  in presynap- tic C fiber terminals  to stimulate  the release of neurotransmitter acting on

postsynaptic receptor, strongly activates ERK in superficial dorsal horn neu- rons in spinal slices (Fig. 1a, b). Using this simple and reliable in vitro (or ex vivo) model, we have investigated the molecular mechanisms involved in

the regulation of C fiber-induced  ERK activation (Kawasaki et al. 2004). We found that multiple neurotransmitter receptors, including NMDA, AMPA, and metabotropic  glutamate receptors, substance P NK-1 receptor, and TrkB re-

ceptor all contribute  to C fiber-evoked ERK activation (Kawasaki et al. 2004; Fig. 1c).

To investigate the functional significance of ERK activation, a MEK (ERK

kinase) inhibitor  (PD98059) was tested in the formalin model, where injec- tion of diluted formalin into a hindpaw of rats elicits a pain behavior lasting for an hour. Intrathecal injection of PD98059 blocks the central sensitization-

Fig. 1a–c  ERK activation  in the superficial  dorsal  horn  of spinal  slices as indicated  by phospho  ERK (pERK) immunostaining. a There  are  very few pERK-labeled neurons in the control  slices. b A bath application  of capsaicin (3 μM, 5 min) induces pERK in many neurons  in the stimulated  slices. Scale, 50 μm. c This panel shows the number  of pERK-positive neurons in the laminae I–II of spinal slices after capsaicin stimulation in the presence of different receptor antagonists. Capsaicin-induced pERK is reduced by blocking NMDA (MK-801, 100 μM), AMPA (CNQX, 20 μM), mGluR (CPCCOEt, 1 μM), NK-1 (GR205171A, 100 μM), and TrkB receptors (k252a 100 nM). *, p<0.05; **, p<0.01, compared to capsaicin group, ANOVA. Mean±SEM (n=5). (See Kawasaki et al. 2004 for more details)

mediated second phase of the painful response to formalin injection (Ji et al.

1999). After this initial  study, multiple  studies  in different  animal  models from different labs have confirmed that the ERK pathway contributes  impor-

tantly to the development  of central sensitization  (Karim et al. 2001; Galan et al. 2003; Kawasaki et al. 2004; Yu and Chen 2005; Yu and Yezierski 2005). Injection of CFA into a hindpaw produces  persistent  (>2 weeks) inflamma- tory pain. CFA also induces sustained  ERK activation  in dorsal horn  neu- rons, whereas capsaicin and formalin only induce transient ERK activation (Ji et al. 1999; Ji et al. 2002a). Intrathecal infusion of the MEK inhibitor U0126 re- duces the late phase of inflammatory pain. This delayed action of ERK is likely to be caused by transcriptional regulation (Ji et al. 2002a). MEK inhibitors are also shown to alleviate neuropathic  pain (Obata and Noguchi 2004; Zhuang et al. 2005).

Although overall pERK induction  is persistent  after CFA inflammation, the peak induction  only lasts a few hours.  The duration  of ERK activation is controlled  by phosphatases.  pERK could be inactivated  by MKP-1 (MAP kinase phosphatase-1)  and PP2A (protein  phosphatase  2A). Neuronal activ- ity not only induces ERK activation but also rapidly increases the expression of immediate early gene MKP-1. Intrathecal  injection of okadaic acid, a gen- eral PP2A inhibitor,  enhances central sensitization  by prolonging capsaicin- induced mechanical hyperalgesia and allodynia. Phosphatase inhibitor might prolong capsaicin-induced  ERK activation, resulting in prolonged pain facili- tation (Zhang et al. 2003; Ji and Strichartz 2004).

pERK Expression as a Marker for Nociceptive Activity and as an Assay for Screening Analgesic Compounds  The expression  of the immediate  early-gene c-fos has been extensively used as a marker  for demonstrating the activity of spinal nociceptive neurons (Hunt et al. 1987; Presley et al. 1990). Like the expression of c-Fos protein,  pERK expression can also serve as a marker  for neuronal activity following nociceptive input. Compared to c-Fos expression, pERK ex- pression is more rapid and transient, following neuronal activity more closely. Importantly,  pERK expression is functional, contributing  critically to dorsal horn neuron  sensitization. The function of ERK activation can be easily as- sessed by blocking this activation with specific MEK inhibitors. Further, pERK expression can be reliably studied in spinal slice preparation. Since many slices (>10) can be obtained from spinal lumbar enlargement of one rat and C fiber can be easily stimulated by bath capsaicin, pERK expression by capsaicin in spinal slices can be used for the screening of potential analgesic drugs (Ji 2004). Interestingly, pERK can be induced by tactile stimulation (touch) after nerve injury, which may underlie tactile allodynia after neuropathic  pain.

p38  MAPK Pathway

p38 is typically activated by cellular stress and inflammatory  mediators. p38 activation can also be activity-dependent.  Systematic p38 inhibitors  produce antiinflammatory effects in animal models (Ji and Woolf 2001). Interestingly,

phospholipase A2 (PLA2) is downstream to p38. The activation of PLA2 leads to the generation of arachidonic acid for prostaglandin  production,  eliciting pain (Svensson et al. 2005). Activated p38 is also translocated  to the nucleus phosphorylating the transcriptional factors and induces gene expression. The

biosynthesis of TNF-α and IL-1β, as well as many other inflammatory media- tors, is positively regulated by p38 (Ji and Woolf 2001).

p38 Activation in the DRG and Spinal Cord  Phospho-p38 (p-p38), the active form of p38, is normally expressed in 10%–15% of DRG neurons that are primary C fiber nociceptors (Ji et al. 2002b; Obata et al. 2004). p38 is activated in DRG neurons following peripheral inflammation and nerve injuries (Kim et al. 2002; Ji et al. 2002b; Jin et al. 2003; Schafers et al. 2003; Obata and Noguchi 2004). However, total p38 (nonphosphorylated and phosphorylated) levels do not increase after injury, indicating that the increase in p-p38 is caused by increased phosphorylation, rather than elevated substrate (Ji et al. 2002b). After nerve injury, p38 is activated not only in DRG neurons with axonal injury, but also

in adjacent neurons  without axonal injury (Obata et al. 2004). While TNF-α

contributes to an early activation of p38 after nerve injury, NGF, via retrograde

transport,  is important  for persistent  p38 activation after inflammation  and nerve injury (Ji et al. 2002b; Schafers et al. 2003; Obata et al. 2004). Unlike ERK, p38 is not activated in spinal cord neurons  in either control or pathological pain conditions (Jin et al. 2003; see further discussion in Sect. 3.4).

p38 and Pathological Pain The  pyridinyl  imidazole  compounds   SB203580, SB202190, and PD169316 are regarded as specific inhibitors for p38. SB203580 is the most frequently used p38 inhibitor. It does not inhibit the phosphoryla- tion of p38 MAPK, but rather binds to the ATP pocket in the enzyme, inhibiting activity of the enzyme. CNI-1493 is a potent antiinflammatory agent and was initially used as a monocyte synthesis inhibitor  to block glial activation and later recognized as a p38 inhibitor (Milligan et al. 2003). FR167653 is another p38 inhibitor. Unlike SB203580, CNI-1493 and FR167653 can block the phos- phorylation  of p38 (Koistinaho  and Koistinaho 2002; Obata et al. 2004). To examine whether p38 activation in the DRG is involved in the generation of inflammatory  pain, SB203580 was administered  into the intrathecal  space to target p38 activity in the DRG. This inhibitor reduced inflammation-induced heat hyperalgesia and suppressed CFA-induced TRPV1 upregulation, but had no effect on CFA-induced inflammation  (Ji et al. 2002b). Intrathecal injection of p38 inhibitors SD-282 and SB203580 also reduced substance P- and NMDA- induced pain hypersensitivity and suppressed COX-2 upregulation  and PGE2 release in the spinal cord (Svensson et al. 2003a; Svensson et al. 2003b). p38 in- hibitor can also alleviate neuropathic  pain. Intrathecal SB203580 prevents the development mechanical allodynia after spinal nerve ligation (Jin et al. 2003; Schafers et al. 2003; Tsuda et al. 2004). SB203580, CNI-1493, FR167653, and SD-282 further  reverses neuropathic  pain after spinal nerve ligation, sciatic

inflammatory neuropathy (SIN), and diabetic neuropathy (Jin et al. 2003; Mil- ligan et al. 2003; Obata et al. 2004; Sweitzer et al. 2004). The mechanisms of p38 regulation of pain sensitization are discussed in Sects. 3.1 and 3.4.

JNK/MAPK Pathway

JNK is the least studied member of MAPK family regarding its role in pain regulation. JNK can be activated by cell stress such as heat shock, direct DNA damage, and generation of reactive oxygen species and plays an important role in the induction of apoptosis. JNK has three isoforms: JNK1, JNK2, and JNK3, and JNK3 is closely correlated with neuronal  function. Activated JNK phos- phorylates the transcription factors c-Jun and ATF-2 (Widmann  et al. 1999). Nerve injury induces JNK activation in DRG neurons (Obata et al. 2004; Zhuang et al. 2006). JNK is also induced in glial cells in the spinal cord by nerve lesion. Intrathecal  injection of the JNK inhibitor SP600125 and D-JNKI-1 could pre- vent and reverse neuropathic  pain (Ma and Quirion 2002; Obata et al. 2004; Zhuang et al. 2006; also see Sect. 3.4).

Mechanisms of Pain Sensitization by Protein Kinases

Peripheral Mechanisms

Induction of Peripheral Sensitization: Posttranslational Regulation

As discussed in Sect. 1.2, TRPV1 and TTXR sodium channels are expressed in nociceptive primary sensory neurons and play a pivotal role in the induction of peripheral sensitization (Fig. 2). TRPV1 is essential for the generation of heat hyperalgesia. Inflammatory  heat hypersensitivity following bradykinin, NGF, CFA, and carrageenan is significantly reduced in TRPV1 knockout mice (Cate- rina et al. 2000; Chuang et al. 2001). TTXR sodium channels are crucial for the generation of hyperexcitability of sensory neurons. Knockdown of Nav1.8 with antisense oligodeoxynucleotides results in decreased inflammatory  pain and neuropathic  pain (Porreca et al. 1999). Hypersensitivity of TRPV1 and TTXR following stimulation  of inflammatory  mediators  underlies the induction  of peripheral sensitization (Julius and Basbaum 2001).

Several protein  kinases such as PKA, PKC, CaMK-II, PI3K and ERK are implicated in TRPV1 sensitization. PKA and PKC are also involved in regulating the sensitivity of TTXR. A membrane  translocation  of PKCε appears  to be important  for its activation of TTXR. The PI3K pathway could contribute  to peripheral sensitization by inducing the trafficking and membrane  insertion

Protein Kinases as Potential Targets for the Treatment of Pathological Pain

Fig. 2 Induction  of peripheral  sensitization  by protein  kinases in nociceptors.  Following tissue injury, inflammatory  mediators PGE2 , bradykinin, and NGF activate corresponding G protein-coupled receptors EP1–4 and B1/B2, and tyrosine kinase receptor TrkA on no- ciceptor terminals, axons, and soma, leading to the activation of PKA, PKC, CaMK-II, and ERK. These protein kinases increase the sensitivity of TRPV1 and TTX-resistant sodium channels Nav 1.8/1.9 by posttranslational regulation, causing peripheral sensitization

of critical ion channels (Gold et al. 1998; Julius and Basbaum 2001; Ji and Strichartz 2004; Zhuang et al. 2004). Since the action of these kinases could happen within minutes and occur in nociceptor terminals, it should involve posttranslational regulation of protein kinases.

Maintenance of Peripheral Sensitization: Transcriptional and Translational Regulation

Tissue and nerve injuries induce gene transcription and protein synthesis in primary sensory neurons in the DRG, contributing  to persistent pain hyper- sensitivity. For example, peripheral inflammation  increases the transcription of substance P, calcitonin gene-related protein (CGRP), and BDNF. After nerve injury, these genes are also induced in noninjured DRG neurons that are adja- cent to injured DRG neurons. NGF is believed to be critical for the expression of these genes (reviewed in Ji and Strichartz 2004). Since p38 and ERK can be activated by NGF; these two MAPKs appear to mediate NGF-induced expres- sion of substance P, CGRP, and BDNF via the transcription factor CREB. The cAMP-response element (CRE) sites are found in many genes expressed in the DRG including substance P, CGRP, and BDNF (Ji and Strichartz 2004).

In addition to transcriptional regulation, MAPK also mediates gene expres- sion via translational  regulation.  Peripheral  inflammation  and NGF induce

increase in TRPV1 protein levels but not in TRPV1 messenger RNA (mRNA)

levels in the DRG. CFA increases p-p38 and TRPV1 expression in C fiber noci-

ceptors, and p-p38 is heavily colocalized with NGF receptor TrkA and TRPV1. Further, intrathecal inhibition of p38 blocked inflammation-induced upregula- tion of TRPV1. p38 is likely to regulate the translation of TRPV1 via translation initiation factor eIF-4E in a NGF-dependent way (Ji et al. 2002b). NGF levels increase in the inflamed paw after CFA injection. NGF is taken up by nerve terminals and retrogradely transported to neuronal soma in the DRG, induc- ing p38 activation  and TRPV1 translation.  Finally, TRPV1 is anterogradely transported from DRG cell body to peripheral nerve terminals, contributing to persistent inflammatory  heat hyperalgesia (Ji et al. 2002b). The NGF–p38– TRPV1 cascade in also important for heat hyperalgesia after nerve injury, in which NGF is produced from injured axons after Wallerian degeneration and taken up by adjacent intact axons (Obata et al. 2004).

3.2

Spinal Mechanisms

3.2.1

Induction of Central Sensitization: Posttranslational Regulation

Glutamate is a predominant excitatory neurotransmitter in all nociceptors. Mild noxious  stimulation  generates  fast excitatory  postsynaptic  potentials, lasting milliseconds. This fast synaptic transmission is mediated by ionotropic glutamate AMPA and kainate receptors. Additional activation of intracellular signaling cascades is required  for the development  of central  sensitization (Woolf and Salter 2000; Ji and Woolf 2001).

Activation of second messenger  systems, especially protein  kinases, can phosphorylate  AMPA and NMDA receptors via posttranslational regulation, enhancing synaptic transmission and producing central sensitization (Fig. 3). For example, a PKC-mediated phosphorylation of NMDA receptor in dorsal

horn neurons removes its voltage-dependent Mg2+ block, which in turn enables glutamate to generate a greater inward current through the NMDA ion channel at resting membrane  potentials (Chen and Huang 1992). Tyrosine kinase Src

enhances NMDA current  via phosphorylation of NR2A/B subunit  of NMDA receptor. The tyrosine phosphorylation appears to increase channel open time and kinetics. Noxious simulation and inflammation  induce the phosphoryla-

tion of NMDA receptor subunits in dorsal horn neurons (Zou et al. 2000; Guo et al. 2002; Salter and Kalia 2004). CaMK and PKA have been implicated in the phosphorylation of AMPA receptors, leading to an increase of AMPA current

(Ji et al. 2003).

Recently it was shown that painful stimulation induces trafficking and in- sertion of AMPA receptor  subunits  to the plasma membrane  of spinal cord neurons (Galan et al. 2004). CaMK and ERK appear to mediate membrane in- sertion of AMPA receptors during synaptic plasticity. ERK could also regulate the activity of Kv4.2 potassium  channels, increasing the excitability of dor-

Protein Kinases as Potential Targets for the Treatment of Pathological Pain

Fig. 3 Induction of central sensitization by protein kinases in dorsal horn neurons. Injury- evoked spontaneous  activity induces the release of the neurotransmitters glutamate and the neuromodulators substance P and BDNF from primary  afferents in the dorsal horn, activating corresponding  ionotropic  NMDA and AMPA receptors,  metabotropic  mGluR receptors,  and tyrosine kinase TrkB receptors,  leading to subsequent  activation of PKA, PKC, CaMK-II, ERK, and Src in postsynaptic dorsal horn neurons. These protein kinases increase the sensitivity of AMPA and NMDA receptors and suppress the activity of Kv 4.2 potassium channels by posttranslational regulation, causing central sensitization

sal horn neurons (Hu et al. 2003). In addition to posttranslational regulation, nociceptive activity also induces a rapid increase of CaMK-II protein (within

10 min), which may involve translational regulation (Fang et al. 2002).

Maintenance of Central Sensitization: Transcriptional Regulation

Intense noxious stimulation,  inflammation,  and nerve injury produce an in- crease in the expression of immediate  early genes (e.g., c-fos, Zif268, Cox-2) and later response  genes (e.g., prodynorphin, NK-1, and TrkB) in the dor- sal horn of spinal cord. A continuous  production  of the protein products  of these genes could maintain  central sensitization  (reviewed in Ji et al. 2003). Upon activation,  pERK is translocated  to the nucleus of dorsal  horn  neu- rons. ERK activation is likely to maintain pain hypersensitivity via regulating gene expression. Inhibition  of ERK activation blocks inflammation-induced upregulation  of c-fos, prodynorphin, NK-1, as well as CREB phosphorylation (Ji et al. 2002a; Kawasaki et al. 2004). pERK is shown to activate CREB via a CREB kinase RSK2 (Fig. 4). CREB-binding site CRE has been identified in the promoter regions of numerous genes expressed in the dorsal horn, includ- ing those mentioned  above. It is of particular  interest that ERK activation is downstream  to many other  kinases, such as PKA, PKC, PI3K, Trk, and Src (Kawasaki et al. 2004; Fig. 4). Convergence of multiple signal pathways on ERK

activation indicates a pivotal role of the ERK pathway in intracellular  signal transduction.

Recently the concept for the “memory of pain” has been proposed  to ex- plain the persistence of pain. Studies on neural plasticity in the spinal cord and in the hippocampus  reveal similar mechanisms for central sensitization

Tyrosine Kinases

Fig. 4 Maintenance of central sensitization by protein kinases in dorsal horn neurons. ERK is classically activated by the Ras–Raf–MEK pathway following stimulation of growth factor receptors (TrkB). Several kinases such as PKA, PKC, Src, and PI3K can converge on ERK activation. Meanwhile, ERK is inhibited  by the phosphatases  MKP-1 and PP2A. MEK is inhibited  by the inhibitors  PD98059 and U0126. Upon phosphorylation, pERK activates the transcription factor CREB via CREB kinase Rsk2, leading to the transcription of CRE- mediated genes including immediate early genes Zif268, Cox-2, c-fos and late response genes NK-1, TrkB, and prodynorphin in dorsal horn neurons. Central sensitization is maintained by the protein products of these genes

and long-term potentiation (LTP), which are believed to underlie generation of pain hypersensitivity and memory, respectively (Sandkuhler 2000; Willis 2002; Ji et al. 2003). LTP is also induced in dorsal horn neurons following intense nox- ious stimulation (Sandkuhler 2000). Whereas long-term memory requires gene transcription, short-term  memory only requires posttranslational modifica- tions. This same dichotomy appears to apply to central sensitization-mediated pain hypersensitivity: persistent  pain (chronic  pain) but not acute pain re- quires gene transcription (Ji et al. 2003). In particular, the transcription factor CREB is believed to play an essential role in long-term neuronal plasticity in both hippocampal  and dorsal horn neurons.  CREB can maintain  long-term neural plasticity not only by inducing gene transcription but also by forming new synapses (Lonze and Ginty 2002).

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