Biomedical subjects

Koichi Noguchi

Publications and source records attributed to Koichi Noguchi.

At least 37 records · Page 2Linked to original sources

PubMed2004

Proteinase-activated receptor 2-mediated potentiation of transient receptor potential vanilloid subfamily 1 activity reveals a mechanism for proteinase-induced inflammatory pain.

Proteinase-activated receptor (PAR) 2 is expressed on a subset of primary afferent neurons and involved in inflammatory nociception. Transient receptor potential vanilloid subfamily 1 (TRPV1) is a sensory neuron-specific cation channel that responds to capsaicin, protons, or heat stimulus. Here, we show that TRPV1 is coexpressed with PAR2 but not with PAR1 or PAR3, and that TRPV1 can functionally interact with PAR2. In human embryonic kidney 293 cells expressing TRPV1 and PAR2, PAR2 agonists increased capsaicin- or proton-evoked TRPV1 currents through a PKC-dependent pathway. After application of PAR2 agonists, temperature threshold for TRPV1 activation was reduced from 42 degrees C to well below the body temperature. PAR2-mediated Fos expression in spinal cord was decreased in TRPV1-deficient mice. The functional interaction was also observed in mouse DRG neurons and proved at a behavioral level. These represent a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by PAR2 activation.

Animals↗

PubMed2004

MAPK activation in nociceptive neurons and pain hypersensitivity.

Extracellular signal-regulated protein kinase (ERK) is a mitogen-activated protein kinase (MAPK) that mediates intracellular signal transduction in response to a variety of stimuli. ERK is involved in cell proliferation and differentiation and in neuronal plasticity, including long-term potentiation, learning, and memory. Here, we present recently accumulating data about the roles of MAPK pathways in mediating the neuronal plasticity that contributes to pain hypersensitivity. The phosphorylation of ERK in the dorsal root ganglion (DRG) and dorsal horn neurons occurs in response to noxious stimulation of the peripheral tissue or electrical stimulation to the peripheral nerve, i.e., activity-dependent activation of ERK in nociceptive neurons. In addition, the activation of ERK occurs in these nociceptive neurons after peripheral inflammation and axotomy and contributes to persistent inflammatory and neuropathic pain, via transcriptional regulation of key gene products. On the other hand, peripheral inflammation and axotomy also induces p38 MAPK activation in DRG neurons. Taken together, these findings indicate that activation of MAPK in nociceptive neurons may participate in generating pain hypersensitivity through transcription-dependent and -independent means. Thus, inhibition of MAPK signaling in the primary afferents, as well as in the spinal cord, may provide a fruitful strategy for the development of novel analgesics.

Enzyme Activation↗

PubMed2004

Contribution of degeneration of motor and sensory fibers to pain behavior and the changes in neurotrophic factors in rat dorsal root ganglion.

To elucidate the role of the degeneration of motor and sensory fibers in neuropathic pain, we examined the pain-related behaviors and the changes of brain-derived neurotrophic factor (BDNF) in the L4/5 dorsal root ganglion (DRG) and the spinal cord after L5 ventral rhizotomy. L5 ventral rhizotomy, producing a selective lesion of motor fibers, produced thermal hyperalgesia and increased BDNF expression in tyrosine kinase A-containing small- and medium-sized neurons in the L5 DRG and their central terminations within the spinal cord, but not in the L4 DRG. Furthermore, L5 ventral rhizotomy up-regulated nerve growth factor (NGF) protein in small to medium diameter neurons in the L5 DRG and also in ED-1-positive cells in the L5 spinal nerve, suggesting that NGF synthesized in the degenerative fibers is transported to the L5 DRG and increases BDNF synthesis. On the other hand, L5 ganglionectomy, producing a selective lesion of sensory fibers, produced heat hypersensitivity and an increase in BDNF and NGF in the L4 DRG. These data indicate that degeneration of L5 sensory fibers distal to the DRG, but not motor fibers, might influence the neighboring L4 nerve fibers and induce neurotrophin changes in the L4 DRG. We suggest that these changes of neurotrophins in the intact primary afferents of neighboring nerves may be one of many complex mechanisms, which can explain the abnormal pain behaviors after nerve injury. The ventral rhizotomy and ganglionectomy models may be useful to investigate the pathophysiological mechanisms of neuropathic pain after Wallerian degeneration in motor or sensory or mixed nerve.

Animals↗

PubMed2004

Contribution of sensitized P2X receptors in inflamed tissue to the mechanical hypersensitivity revealed by phosphorylated ERK in DRG neurons.

The mechanism of mechanical hyperalgesia in inflammation might involve a 'mechanochemical' process whereby stretch evokes the release of adenosine 5'-triphosphate (ATP) from the damaged tissue that then excites nearby primary sensory nerve terminals. In the present study, phosphorylated extracellular signal-regulated protein kinase (pERK) immunoreactivity was used as a marker indicating functional activation of primary afferent neurons to examine the P2X receptor-mediated noxious response in DRG neurons in a rat model of peripheral inflammation. We found that very few pERK-labeled DRG neurons were detected in normal rats after alpha, beta methylene-ATP (alphabetame-ATP) intraplantar injection. However, a number of DRG neurons were labeled for pERK after alphabetame-ATP injection to the complete Freund's adjuvant (CFA) induced inflamed paw. Seventy-three percent of pERK-labeled DRG neurons co-expressed the P2X3 receptor. After mechanical noxious stimulation to the hind paw of CFA-inflamed rats, we found many more pERK-labeled neurons compared to those in the normal rats. Administration of the P2X3 receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid or 2'- (or 3')-O-(trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), significantly decreased the mechanical stimulation-evoked pERK labeling in CFA-inflamed rats, but not in normal rats. We also found the recruitment of neurons with myelinated A fibers labeled for pERK in CFA-inflamed rats, which was reversed by P2X3 receptor antagonists. Moreover, TNP-ATP dose dependently reduced the mechanical hypersensitivity of CFA rats. These data suggest that the P2X receptors in primary afferent neurons increase their activity with enhanced sensitivity of the intracellular ERK signaling pathway during inflammation and then contribute to the hypersensitivity to mechanical noxious stimulation in the inflammatory state.

Adenosine Triphosphate↗

PubMed2004

Activation of extracellular signal-regulated protein kinase in dorsal horn neurons in the rat neuropathic intermittent claudication model.

Extracellular signal-regulated protein kinase (ERK) is a mitogen-activated protein kinase (MAPK) that mediates several cellular responses to mitogenic and differentiation signals, and activation of ERK in dorsal horn neurons by noxious stimulation is known to contribute to pain hypersensitivity. In order to elucidate the pathophysiological mechanisms of the cauda equina syndrome, secondary to spinal canal stenosis, we evaluated walking dysfunction triggered by forced exercise and activation of ERK in the dorsal horn using a rat model of neuropathic intermittent claudication. Rats in the lumbar canal stenosis (LCS) group showed a shorter running distance from 1 to 14 days after surgery. Two minutes after running on the treadmill apparatus, phosphorylation of ERK was induced in neurons in the superficial laminae in the LCS group but not in the sham group, whereas there was no change in the deeper laminae. Intrathecal administration of the MAPK kinase inhibitor, U0126, 30 min before running, clearly increased the running distance, whereas there was no significant change in the vehicle control group 3 days after surgery. In addition, a prostaglandin E1 analog, OP-1206 alpha-CD, administered orally, improved the walking dysfunction, and further, inhibited activation of ERK following running 7 days after surgery. These findings suggest that intermittent claudication triggered by forced walking might affect the phosphorylation of ERK in the superficial laminae, possibly via transient (partial) ischemia of the spinal cord. ERK activation in the dorsal horn neurons may be involved in the transient pain in the neuropathic intermittent claudication model.

Alprostadil↗

PubMed2004

Tissue plasminogen activator in primary afferents induces dorsal horn excitability and pain response after peripheral nerve injury.

The extracellular protease cascade of plasminogen activators and plasminogen are known to regulate neuronal plasticity and extracellular matrix modification, and to be important factors involved in producing long-term potentiation in the CNS. The purpose of this study is to examine the expression of plasminogen activators in primary afferents and its role in nociceptive pathways after peripheral nerve injury. We found the induction of mRNAs for tissue type plasminogen activator (tPA) and urokinase plasminogen activator (uPA) in the rat dorsal root ganglia following sciatic nerve transection. Immunoreactivity for tPA was increased in laminae I and II of the dorsal horn and, importantly, the increase in proteolytic activity mediated by tPA was observed in the same area. As neither immunoreactivity for uPA nor uPA-mediated proteolysis was observed, we further examined the effects of tPA on dorsal horn excitability and neuropathic pain behaviour. Intrathecal injection of a specific inhibitor of tPA decreased electrical stimulation-induced Fos expression in dorsal horn neurons following axotomy, and also prevented the development of thermal hyperalgesia following partial sciatic nerve ligation. These findings suggest that the increased tPA in the dorsal horn due to mRNA expression in the dorsal root ganglia increases the dorsal horn excitability and has an important role in pain behaviour after peripheral nerve injury. The tPA-mediated hypersensitivity in dorsal horn neurons may be a novel molecular mechanism of neuropathic pain.

Afferent Pathways↗

PubMed2004

Enhancement of stimulation-induced ERK activation in the spinal dorsal horn and gracile nucleus neurons in rats with peripheral nerve injury.

It has been suggested that low-threshold sensory pathways have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. In the present study, we examined the involvement of these pathways in the development of hyperexcitability after sciatic nerve injury (SNI) by detecting the intracellular signal molecule. The rats that received a transection of the sciatic nerve 7 days before were electrically stimulated at 0.1 mA and 3 mA in the proximal region of the nerve injury site. We found a small number of phosphorylated extracellular signal-regulated kinase (pERK)-labelled neurons in laminae I-II and III-IV of the spinal dorsal horn in the control rats after 0.1 mA stimulation. By contrast, there was a marked increased of pERK-labelled neurons both in the superficial laminae and laminae III-IV after the same stimulation in the SNI rats. Enhancement of ERK activation induced by 3 mA stimulation was also observed. Immunoreactivity of pERK in gracile nucleus neurons was also dramatically increased after 0.1 mA stimulation to the injured nerve. These data suggest that the rats with peripheral nerve injury had an increased responsiveness to the low- or high-threshold peripheral stimuli in I-II, III-IV and gracile nucleus neurons. Furthermore, SNI rats that received neonatal capsaicin treatment showed a decreased number of pERK neurons after 0.1 mA stimulation in the dorsal horn and gracile nucleus neurons compared to the control rats. Thus, C-fibres may contribute to the enhanced excitability of the low-threshold sensory neurons after peripheral nerve injury.

Animals↗

PubMed2004

Differential activation of MAPK in injured and uninjured DRG neurons following chronic constriction injury of the sciatic nerve in rats.

To investigate the intracellular signal transduction pathways involved in the pathophysiological mechanisms of neuropathic pain after partial nerve injury, we examined the activation of extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) in the chronic constriction injury (CCI) model. The CCI induced an increase in the phosphorylation of ERK in predominantly injured medium-sized and large-sized DRG neurons and in satellite glial cells. Treatment with the MAPK kinase 1/2 inhibitor, U0126, suppressed CCI-induced mechanical allodynia and partially reversed the increase in neuropeptide Y (NPY) expression in damaged DRG neurons. In contrast, the CCI induced the activation of p38, mainly in uninjured small-to-medium-diameter DRG neurons and in satellite glial cells. The p38 inhibitor, SB203580, reversed the CCI-induced heat hyperalgesia and also the increase in brain-derived neurotrophic factor (BDNF) expression in intact DRG neurons. On the other hand, the nerve growth factor (NGF)-induced increase in BDNF expression in small-to-medium-diameter neurons was reversed by SB203580, whereas the anti-NGF-induced increase in NPY in medium-sized and large-sized neurons was partially blocked by U0126. Taken together, our results demonstrate that the activation of ERK and p38 and also the changes in NPY and BDNF expression may occur in different populations of DRG neurons after CCI, partially through alterations in the target-derived NGF. These changes in injured and intact primary afferents are likely to have a substantial role in pathological states, and MAPK pathways in nociceptors may be potential targets for the development of novel analgesics.

Activating Transcription Factor 3↗

PubMed2004

Changes in DRG neurons and spinal excitability in neuropathy.

An intracellular signalling pathway in the dorsal root ganglion (DRG) and spinal neurons is a popular target in pain research that is relevant to the neuroplastic changes that occur during chronic pain conditions. First, we examined the phosphorylation of ERK in DRG neurons after peripheral inflammation and sciatic nerve transection without any stimulation to the receptive field. We found an activation of ERK in different populations of DRG neurons after peripheral inflammation and axotomy, which developed from alterations in target-derived nerve growth factor (NGF). We observed that the ERK signalling regulates the brain-derived neurotrophic factor (BDNF) expression in DRG neurons in both conditions. We also demonstrated that very rapid phosphorylation of ERK occurred in DRG neurons that were involved in the transmission of various noxious signals under normal conditions. Further, we examined the pERK labelling after the mechanical stimuli into the inflamed tissue and found that the pERK labelling occurred through the P2X3 receptors in the terminals. This activity-dependent activation of the ERK signal pathway may be useful for identifying which DRG neurons are involved in transmission of noxious stimuli under normal and pathological conditions.

Animals↗

PubMed2003

Increase of preprotachykinin mRNA in the uninjured mandibular neurons after rat infraorbital nerve transection.

An increasing number of studies are suggesting that the adjacent uninjured primary afferents contribute to the mechanisms of neuropathic pain following peripheral nerve injury. In this study, we report that transection of the infraorbital nerve, a major branch of the maxillary nerve, causes exaggerated face grooming to normally innocuous mechanical stimuli in the skin territory of the uninjured mandibular nerve, and increases the expression of preprotachykinin mRNA in the primary afferent neurons in the mandibular zone in the trigeminal ganglia. Considering the various functions of substance P in the sensory transmission process, the increase in preprotachykinin mRNA in the uninjured primary afferent neurons may be one of the mechanisms of pain-related behavior in this neuropathic pain model.

Animals↗

PubMed2003

Possible involvement of P2Y2 metabotropic receptors in ATP-induced transient receptor potential vanilloid receptor 1-mediated thermal hypersensitivity.

The capsaicin receptor transient receptor potential V1 (TRPV1; also known as vanilloid receptor 1) is a sensory neuron-specific ion channel that serves as a polymodal detector of pain-producing chemical and physical stimuli. It has been reported that extracellular ATP potentiates the TRPV1 currents evoked by capsaicin or protons and reduces the temperature threshold for its activation through metabotropic P2Y receptors in a PKC-dependent pathway, suggesting that TRPV1 activation could trigger the sensation of pain at normal body temperature in the presence of ATP. Here, we show that ATP-induced thermal hyperalgesia was abolished in mice lacking TRPV1, suggesting the functional interaction between ATP and TRPV1 at a behavioral level. However, thermal hyperalgesia was preserved in P2Y1 receptor-deficient mice. Patch-clamp analyses using mouse dorsal root ganglion neurons indicated the involvement of P2Y2 rather than P2Y1 receptors. Coexpression of TRPV1 mRNA with P2Y2 mRNA, but not P2Y1 mRNA, was determined in the rat lumbar DRG using in situ hybridization histochemistry. These data indicate the importance of metabotropic P2Y2 receptors in nociception through TRPV1.

Adenosine Triphosphate↗

PubMed2003

The effects of dexmedetomidine and halothane on Fos expression in the spinal dorsal horn using a rat postoperative pain model.

We investigated the effect of an intrathecal injection of a selective alpha2 adrenergic receptor agonist, dexmedetomidine (Dex), and halothane anesthesia on Fos expression in the lumbar spinal dorsal horn after skin incision of the plantar surface of the hind paw, a postoperative pain model using rats. Fos immunoreactivity was induced particularly in the superficial layers of the spinal cord 2 h after surgery. Halothane anesthesia (0.5-1.5%) partially reversed Fos induction, but not in a dose-dependent manner. Preoperative spinal Dex (0.1-10 microg) dose-dependently reduced Fos immunoreactivity, while a relatively high dose of Dex (10 microg) was necessary to produce a profound effect. When used with halothane anesthesia, relatively low doses of Dex (1-3 microg) completely suppressed Fos induction in the superficial spinal layers. These findings indicate that preoperative Dex treatment may provide anesthesia that does not induce stress on spinal neurons.

Animals↗

PubMed2003

Differential activation of extracellular signal-regulated protein kinase in primary afferent neurons regulates brain-derived neurotrophic factor expression after peripheral inflammation and nerve injury.

To investigate the intracellular signal transduction pathways involved in regulating the gene expression of brain-derived neurotrophic factor (BDNF) in primary afferent neurons, we examined the activation of extracellular signal-regulated protein kinase (ERK) in dorsal root ganglion (DRG) neurons after peripheral inflammation and sciatic nerve transection. Peripheral inflammation induced an increase in the phosphorylation of ERK, mainly in tyrosine kinase A-containing small-to-medium-diameter DRG neurons. The treatment of the mitogen-activated protein kinase (MAPK) kinase 1/2 inhibitor U0126 reversed the pain hypersensitivity and the increase in phosphorylated-ERK (p-ERK) and BDNF in DRG neurons induced by complete Freund's adjuvant. On the other hand, axotomy induced the activation of ERK mainly in medium-and large-sized DRG neurons and in satellite glial cells. U0126 suppressed the axotomy-induced autotomy behavior and reversed the increase in p-ERK and BDNF. The intrathecal application of nerve growth factor (NGF) induced an increase in the number of p-ERK-and BDNF-labeled cells, mainly small neurons, and the application of anti-NGF induced an increase in p-ERK and BDNF in some medium-to-large-diameter DRG neurons. The activation of MAPK in the primary afferents may occur in different populations of DRG neurons after peripheral inflammation and axotomy, respectively, through alterations in the target-derived NGF. These changes, including the changes in BDNF expression, might be involved in the pathophysiological changes in primary afferent neurons.

Activating Transcription Factor 3↗

PubMed2003

Increased expression of 3 beta-hydroxysteroid dehydrogenase mRNA in dorsal root ganglion neurons of adult rats following peripheral nerve injury.

3beta-hydroxysteroid dehydrogenase (3beta-HSD) is an enzyme that converts pregnenolone to progesterone. It has been believed that 3beta-HSD is simply a converting enzyme of female steroid hormone. Recently, 3beta-HSD expressing cells were identified in the spinal cord. Steroid synthesis in the nervous system may indicate that steroid plays a role in the nervous system. We report here the increased expression of 3beta-HSD mRNA in the dorsal root ganglion (DRG) after peripheral nerve injury using reverse transcription-polymerase chain reaction and in situ hybridization histochemistry techniques. We detected only a few 3beta-HSD signals in the naïve DRG, and found that 3beta-HSD mRNA expression increased 3 days after injury and this increase was still observed at 14 days. Our results suggest that progesterone may have a role in the process against neuronal injury or in regeneration in the peripheral nervous system.

Animals↗

PubMed2003

Changes in phosphorylation of ERK and Fos expression in dorsal horn neurons following noxious stimulation in a rat model of neuritis of the nerve root.

Mechanical compression and chemical inflammation of the spinal nerve root are considered major sensory pathologies secondary to a lumbar disc herniation. In order to elucidate the dorsal horn responsiveness to noxious stimulation to the peripheral tissue in the neuritis model of the nerve root, we examined extracellular signal-regulated kinase (ERK) phosphorylation and Fos expression in spinal cord dorsal horn neurons. Male Sprague-Dawley rats received hemilaminectomies and the implantation of disc tissue that was obtained from coccygeal intervertebral discs. Three or 7 days after surgery, rats were perfused after receiving noxious mechanical stimulation of the plantar surface of the hind paw using a hemoclip, and the L4/5 spinal cord was processed for immunohistochemistry with antibodies for phospho-ERK and Fos. The number of Fos-immunoreactive (Fos-LI) neurons and phospho-ERK-immunoreactive (phospho-ERK-LI) neurons in the neuritis group after the noxious stimulation significantly increased compared to the sham-treated group at 3 and 7 days after surgery. The change in number of phospho-ERK-LI and Fos-LI neurons occurred mainly in the superficial dorsal horn. The number of Fos-LI neurons observed when the MEK inhibitor, U0126, was administered was significantly suppressed compared to the DMSO- (vehicle control) administered group. The increase in ERK phosphorylation and Fos expression in the spinal cord dorsal horn neurons indicates that responses/activation by the noxious stimulation applied to the periphery were elevated in spinal cord neurons in this neuritis model of the lumbar nerve root. Moreover, the increase in the Fos expression in the spinal cord dorsal horn may have been the result of the activation of the MAP kinase cascade.

Animals↗

PubMed2003

Induction of neuronal nitric oxide synthase by sympathetic denervation is mediated via alpha 2-adrenoceptors in the jejunal myenteric plexus.

Nitric oxide (NO) is an important nonadrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the gastrointestinal tract. In previous studies, neuronal nitric oxide synthase (nNOS) in the jejunal myenteric plexus, a key enzyme responsible for the release of NO, has been demonstrated to increase after splanchinic ganglionectomy (sympathetic nerve transection). The alpha2-adrenoceptor is known to be one of the most important receptors which controls intestinal motility. In the present study, we examined the effect of application of the alpha2-adrenoceptor agonist, clonidine hydrochloride, on nNOS expression in the rat jejunal myenteric plexus after splanchinic ganglionectomy. Clonidine (0.1-1 mg/kg, i.p.) or saline was administered for 5 days after the splanchinic ganglionectomy. The nNOS expression and nNOS mRNA were detected by immunohistochemistry and in situ hybridization for nNOS mRNA, respectively. In the rats treated with vehicle after the splanchinic ganglionectomy, nNOS expression in the myenteric plexus significantly increased compared with sham-operated rats. The increases in nNOS protein and mRNA after splanchinic ganglionectomy were significantly reversed by clonidine treatment. Clonidine-treated naive rats showed no difference in nNOS expression compared with sham-operated rats. These data suggest that nNOS expression in the jejunal myenteric plexus after splanchinic ganglionectomy is regulated by the alpha2-adrenoceptor and that the alpha2-adrenoceptor may play an important role in abnormal intestinal motility following splanchinic ganglionectomy in rat jejunum.

Animals↗

PubMed2003

Contribution of injured and uninjured dorsal root ganglion neurons to pain behavior and the changes in gene expression following chronic constriction injury of the sciatic nerve in rats.

Neuropathic pain models, such as the chronic constriction injury (CCI) model, are partial nerve injury models where there exist both intact and injured peripheral axons. Recent studies suggested that dorsal root ganglion (DRG) neurons with intact axons also show the alteration of excitability and gene expression and might have some role in the pathophysiological mechanisms of neuropathic pain. The incidence of pain-related behavior after the CCI is unstable and variable. In the present study, we used activating transcription factor 3 (ATF3) expression as a neuronal injury marker, and analyzed a relationship between the number of axotomized neurons and the incidence of pain-related behavior. We divided all rats into three groups according to the percentage of ATF3-immunoreactive (IR) neurons, group 1 (<12.5%), group 2 (12.5-25%), and group 3 (>25%). We found that rats in groups 2 and 3 showed thermal hyperalgesia, whereas only the rats in group 2 developed tactile allodynia from the third day to the fourteenth day after surgery. Rats in group 1 did not show thermal hyperalgesia or tactile allodynia. The DRG neurons in group 2 contained ATF3-IR neurons mainly in medium- and large-sized neurons. In order to investigate brain-derived neurotrophic factor (BDNF) and gamma-aminobutyric acid(A)-receptor (GABA(A)-R) regulation in both intact and injured primary afferent neurons after the CCI, we used a double-labeling method with immunohistochemistry and in situ hybridization, as well as double immunofluorescent staining. The CCI induced an increased number of BDNF-labeled neurons in the ipsilateral DRG and the increase in BDNF expression was observed mainly in small- and medium-sized neurons that were mainly ATF3-negative. On the other hand, the number of GABA(A)-Rgamma2 subunit mRNA-positive neurons decreased in the ipsilateral DRG and GABA(A)-R- and ATF3-labeled neurons rarely overlapped. These changes in molecular phenotype in intact and injured primary afferents may be involved in the pathophysiological mechanisms of neuropathic pain produced by partial nerve injury.

Activating Transcription Factor 3↗

PubMed2002

Phosphorylation of extracellular signal-regulated kinase in primary afferent neurons by noxious stimuli and its involvement in peripheral sensitization.

Alteration in the intracellular signal transduction pathway in primary afferent neurons may contribute to pain hypersensitivity. We demonstrated that very rapid phosphorylation of extracellular signal-regulated protein kinases (pERK) occurred in DRG neurons that were taking part in the transmission of various noxious signals. The electrical stimulation of Adelta fibers induced pERK primarily in neurons with myelinated fibers. c-Fiber activation by capsaicin injection induced pERK in small neurons with unmyelinated fibers containing vanilloid receptor-1 (VR-1), suggesting that pERK labeling in DRG neurons is modality specific. Electrical stimulation at the c-fiber level with different intensities and frequencies revealed that phosphorylation of ERK is dependent on the frequency. We examined the pERK in the DRG after application of natural noxious stimuli and found a stimulus intensity-dependent increase in labeled cell size and in the number of activated neurons in the c- and Adelta-fiber population. Immunohistochemical double labeling with phosphorylated ERK/VR-1 and pharmacological study demonstrated that noxious heat stimulation induced pERK in primary afferents in a VR-1-dependent manner. Capsaicin injection into the skin also increased pERK labeling significantly in peripheral fibers and terminals in the skin, which was prevented by a mitogen-activated protein kinase/ERK kinase inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(2-aminopheylthio)butadiene (U0126). Behavioral experiments showed that U0126 dose-dependently attenuated thermal hyperalgesia after capsaicin injection and suggested that the activation of ERK pathways in primary afferent neurons is involved in the sensitization of primary afferent neurons. Thus, pERK in primary afferents by noxious stimulation in vivo showed distinct characteristics of expression and may be correlated with the functional activity of primary afferent neurons.

Action Potentials↗