Differential roles of neurokinin 1 and neurokinin 2 receptors in the development and maintenance of heat hyperalgesia induced by acute inflammation

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Liquid chromatography-electrospray linear ion trap mass spectrometry analysis of targeted neuropeptides in Tac1(-/-) mouse spinal cords reveals significant lower concentration of opioid peptides

Tachykinin and opioid peptides play a central role in pain transmission, modulation and inhibition. The treatment of pain is very important in medicine and many studies using NK1 receptor antagonists failed to show significant analgesic effects in humans. Recent investigations suggest that both pronociceptive tachykinins and the analgesic opioid systems are important for normal pain sensation. The analysis of opioid peptides in Tac1(-/-) spinal cord tissues offers a great opportunity to verify the influence of the tachykinin system on specific opioid peptides. The objectives of this study were to develop an HPLC-MS/MRM assay to quantify targeted peptides in spinal cord tissues. Secondly, we wanted to verify if the Tac1(-/-) mouse endogenous opioid system is hampered and therefore affects significantly the pain modulatory pathways. Targeted neuropeptides were analyzed by high performance liquid chromatography linear ion trap mass spectrometry. Our results reveal that EM-2, Leu-Enk and Dyn A were down-regulated in Tac1(-/-) spinal cord tissues. Interestingly, Dyn A was almost 3 fold down-regulated (p<0.0001). No significant concentration differences were observed in mouse Tac1(-/-) spinal cords for Met-Enk and CGRP. The analysis of Tac1(-/-) mouse spinal cords revealed noteworthy decreases of EM-2, Leu-Enk and Dyn A concentrations which strongly suggest a significant impact on the endogenous pain-relieving mechanisms. These observations may have insightful impact on future analgesic drug developments and therapeutic strategies.

Peripheral scaffolding and signaling pathways in inflammatory pain

Peripheral injury precipitates the release and accumulation of extracellular molecules at the site of injury. Although these molecules exist in various forms, they activate specific receptor classes expressed on primary afferent neurons to mediate cellular and behavioral responses to both nonpainful and painful stimuli. These inflammatory mediators and subsequent receptor-mediated effects exist to warn an organism of future injury, thereby resulting in protection and rehabilitation of the wounded tissue. In this chapter, inflammatory mediators, their target receptor classes, and downstream signaling pathways are identified and discussed within the context of inflammatory hyperalgesia. Furthermore, scaffolding mechanisms that exist to support inflammatory signaling in peripheral afferent neuronal tissues specifically are identified and discussed. Together, the mediators, pathways, and scaffolding mechanisms involved in inflammatory hyperalgesia provide a unique knowledge point from which new therapeutic targets can be understood.

Sensory-motor behavioral characterization of an animal model of Maroteaux-Lamy syndrome (or Mucopolysaccharidosis VI)

Maroteaux-Lamy disease, also known as mucopolysaccharidosis (MPS) VI, is an MPS disorder caused by mutations in the ARSB gene encoding for the lysosomal enzyme arysulfatase B (ARSB). Deficient ARSB activity leads to lysosomal accumulation of dermatan sulfate in a wide range of tissues and organs. There are various animal models of MPS VI that have been well characterized from a biochemical and morphological point of view. In this study, we report the sensory-motor characterization of MPS VI rats carrying homozygous null ARSB mutations. We show that adult MPS VI rats are specifically impaired in vertical activity and motor endurance. All together, these data are consistent with biochemical findings that show a major impairment in connective tissues, such as joints and bones. The behavioral abnormalities of MPS VI rats represent fundamental endpoints for studies aimed at testing the pre-clinical safety and efficacy of novel therapeutic approaches for MPS VI.

Dolichos falcata Klein attenuated the inflammation induced by monosodium urate crystals in vivo and in vitro

ETHNOPHARMACOLOGICAL RELEVANCE

Dolichos falcata Klein (DF), a Chinese Dai ethnic medicine popularly known as "Tuoyeteng" in Yunnan province of China, has been widely used as a traditional herbal medicine for the treatment of fracture and beriberoid disease for a long time in China. The present study was carried out to investigate the anti-inflammatory activity and the bioactive chemical constituents of DF, and further to assess its possible mechanism on gouty arthritis in an animal model of the MSU crystals-induced gouty inflammation.

MATERIALS AND METHODS

The ethanol extract (EE) of DF at the doses of 10, 20 and 40 mg/kg was administered to the rats treated with MSU crystals to evaluate the anti-gouty arthritis effect. Subsequently, the components of EE were isolated and identified using classical methods. Phyto-chemical analysis of EE was further carried out by HPLC-DAD. Finally, the anti-inflammatory effect of EE and two isolated components were assessed using the MSU crystals-treated monocyte/macrophage cell line RAW 264.7 in vitro.

RESULTS

EE (10, 20 and 40 mg/kg) significantly attenuated the pain threshold value, the joint swelling degree, the inflammatory cell infiltration of articular tissue and the increased levels of pro-inflammatory cytokines in MSU crystals-treated rats. Moreover, doliroside A (DA) and medicagenic acid-3-O-β-D-glucopyranoside (MG) were isolated and identified from EE. The major components of EE, including DA, MG and other triterpenoids, were well confirmed by HPLC. A further study revealed that EE, DA and MG (10, 20, 40μg/mL) exhibited stronger inhibitory effects on the production of pro-inflammatory cytokines (including interleukin-1β, interleukin-6 and tumor necrosis factor-α) in MSU crystals-treated RAW 264.7 cells.

CONCLUSION

These findings indicate that the major triterpenoids present in DF have a remarkable effect on improving symptoms of acute gouty arthritis induced by MSU crystals through inhibiting the production of pro-inflammatory cytokines.

Targeted liquid chromatography quadrupole ion trap mass spectrometry analysis of tachykinin related peptides reveals significant expression differences in a rat model of neuropathic pain

Animal models are widely used to perform basic scientific research in pain. The rodent chronic constriction injury (CCI) model is widely used to study neuropathic pain. Animals were tested prior and after CCI surgery using behavioral tests (von Frey filaments and Hargreaves test) to evaluate pain. The brain and the lumbar enlargement of the spinal cord were collected from neuropathic and normal animals. Tachykinin related peptides were analyzed by high performance liquid chromatography quadrupole ion trap mass spectrometry. Our results reveal that the β-tachykinin₅₈₋₇₁, SP and SP₃₋₁₁ up-regulation are closely related to pain behavior. The spinal β-tachykinin₅₈₋₇₁, SP and SP₃₋₁₁ concentrations were significantly up-regulated in neuropathic animals compared with normal animals (p<0.001; p<0.001 and p<0.05, respectively). In contrast, the spinal SP5₅₋₁₁ concentration in neuropathic animals revealed a significant down-regulation compared with normal animals (p<0.05). The brain β-tachykinin₅₈₋₇₁ and SP concentrations were significantly up-regulated (p<0.05 and p<0.001, respectively). Interestingly, no significant concentration differences were observed in the spinal cord and brain for NKA, β-tachykinin₅₈₋₇₁, SP₁₋₇ and SP₆₋₁₁ (p>0.05). The β-tachykinin₅₈₋₇₁, SP and C-terminal SP metabolites could potentially serve as biomarkers in early drug discovery.

Decision-making using fMRI in clinical drug development: revisiting NK-1 receptor antagonists for pain

Substance P (SP) and neurokinin-1 receptors (NK-1R) are localized within central and peripheral sensory pain pathways. The roles of SP and NK-1R in pain processing, the anatomical distribution of NK-1R and efficacy observed in preclinical pain studies involving pain and sensory sensitization models, suggested that NK-1R antagonists (NK-1RAs) would relieve pain in patient populations. Despite positive data available in preclinical tests for a role of NK-1RAs in pain, clinical studies across several pain conditions have been negative. In this review, we discuss how functional imaging-derived information on activity in pain-processing brain regions could have predicted that NK-1RAs would have a low probability of success in this therapeutic domain.

Intra-articular administration of tachykinin NK₁ receptor antagonists reduces hyperalgesia and cartilage destruction in the inflammatory joint in rats with adjuvant-induced arthritis

Persistent pain associated with inflammatory arthritis is an aggravating factor that decreases patients' quality of life. Current therapies for joint pain have limited effectiveness and produce unwanted negative side effects. Although the involvement of substance P and its cognate tachykinin receptor, NK(1), in joint inflammation has been extensively documented through animal experiments, the development of oral tachykinin NK(1) receptor antagonists against arthritis-induced pain has been unsuccessful in humans to date. To explore the possibility of using tachykinin NK(1) receptor antagonists as local therapeutic agents for inflammatory arthritis, we examined the effects of tachykinin NK(1) receptor antagonists administered into the rat ankle joint on hyperalgesia in complete Freund's adjuvant (CFA)-induced inflammatory monoarthritis. Administration of the tachykinin NK(1) receptor antagonist WIN 51708 or GR 82334 into the affected ankle joint at day 3 following intra-articular CFA injection reduced the mechanical hyperalgesia 12 h after the tachykinin NK(1) receptor antagonist injection and their analgesic effects persisted for at least 2 days. Histological examinations revealed that intra-articular WIN 51708 reduced the CFA-induced destructive changes in the cartilage. These findings suggest that intra-articular injection of tachykinin NK(1) receptor antagonists is a promising strategy for relieving the hyperalgesia that occurs in inflammatory arthritis.

Characterization of protein tyrosine phosphatase H1 knockout mice in animal models of local and systemic inflammation

Background

PTPH1 is a protein tyrosine phosphatase expressed in T cells but its effect on immune response is still controversial. PTPH1 dephosphorylates TCRzeta in vitro, inhibiting the downstream inflammatory signaling pathway, however no immunological phenotype has been detected in primary T cells derived from PTPH1-KO mice. The aim of the present study is to characterize PTPH1 phenotype in two in vivo inflammatory models and to give insights in possible PTPH1 functions in cytokine release.

Methods

We challenged PTPH1-KO mice with two potent immunomodulatory molecules, carrageenan and LPS, in order to determine PTPH1 possible role in inflammatory response in vivo. Cytokine release, inflammatory pain and gene expression were investigated in challenged PTPH1-WT and KO mice.

Results

The present study shows that carrageenan induces a trend of slightly increased spontaneous pain sensitivity in PTPH1-KO mice compared to WT (wild-type) littermates, but no differences in cytokine release, induced pain perception and cellular infiltration have been detected between the two genotypes in this mouse model. On the other hand, LPS-induced TNFα, MCP-1 and IL10 release was significantly reduced in PTPH1-KO plasma compared to WTs 30 and 60 minutes post challenge. No cytokine release modulation was detectable 180 minutes post LPS challenge.

Conclusion

In conclusion, the present study points out a slight potential role for PTPH1 in spontaneous pain sensitivity and it indicates that this phosphatase might play a role in the positive regulation of the LPS-induced cytokines release in vivo, in contrast to previous reports indicating PTPH1 as potential negative regulator of immune response.

Persistent pain is dependent on spinal mitochondrial antioxidant levels

Reactive oxygen species (ROS) scavengers have been shown to relieve persistent pain; however, the mechanism is not clearly understood. Superoxide produced from mitochondrial oxidative phosphorylation is considered the major source of ROS in neurons during excitation where mitochondrial superoxide levels are normally controlled by superoxide dismutase (SOD-2). The present study hypothesizes that capsaicin-induced secondary hyperalgesia is a consequence of superoxide build-up in spinal dorsal horn neurons and SOD-2 is a major determinant. To test this hypothesis, the spinal levels of SOD-2 activity, inactivated SOD-2 proteins, and mitochondrial superoxide were measured and correlated to the levels of capsaicin-induced secondary hyperalgesia in mice with and without SOD-2 manipulations. The data suggest that superoxide accumulation is a culprit in the abnormal sensory processing in the spinal cord in capsaicin-induced secondary hyperalgesia. Our studies also support the notion that SOD-2 nitration is a critical mechanism that maintains elevated superoxide levels in the spinal cord after capsaicin treatment. Finally, our findings suggest a therapeutic potential for the manipulation of spinal SOD-2 activity in pain conditions.

Sensory-nerve-derived neuropeptides: possible therapeutic targets

This review examines our developing understanding of the families and activities of some of the best known sensory-nerve-derived inflammatory neuropeptides, namely substance P, calcitonin gene-related peptide and galanin. Evidence to date shows involvement of these transmitters in a wide range of systems that includes roles as inflammatory modulators. There is an increasing understanding of the mechanisms involved in the release of the peptides from sensory nerves and these are key in understanding the potential of neuropeptides in modulating inflammatory responses and may also provide novel targets for anti-inflammatory therapy. The neuropeptides released act via specific G protein coupled receptors, most of which have now been cloned. There is knowledge of selective agonists and antagonists for many subtypes within these families. The study of neuropeptides in animal models has additionally revealed pathophysiological roles that in turn have led to the development of new drugs, based on selective receptor antagonism.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Techniques for assessing knee joint pain in arthritis

The assessment of pain is of critical importance for mechanistic studies as well as for the validation of drug targets. This review will focus on knee joint pain associated with arthritis. Different animal models have been developed for the study of knee joint arthritis. Behavioral tests in animal models of knee joint arthritis typically measure knee joint pain rather indirectly. In recent years, however, progress has been made in the development of tests that actually evaluate the sensitivity of the knee joint in arthritis models. They include measurements of the knee extension angle struggle threshold, hind limb withdrawal reflex threshold of knee compression force, and vocalizations in response to stimulation of the knee. A discussion of pain assessment in humans with arthritis pain conditions concludes this review.

Low-speed subcellular fractionation method for determining noxious stimulus-evoked spinal neurokinin-1 receptor internalization

Substance P release from nociceptive primary afferents activates post-synaptic neurokinin-1 (NK-1) receptors causing subsequent NK-1 receptor internalization. Fluorescent immunohistochemistry is typically used to quantify NK-1 receptor internalization, an indirect measure of substance P (SP) release. However, this technique entails several limitations that restrict its application. Using simple subcellular fractionation and immunoblotting methods, we demonstrate that intrathecal SP invokes a rapid and dose-dependent increase in dorsal horn cytoplasmic NK-1 receptors. We also show that hind paw compression and noxious thermal stimulation increase cytoplasmic NK-1 receptor, when compared to sham stimulations. Fluorescent immunohistochemistry confirmed that increases in cytoplasmic NK-1 corresponded with increased NK-1 receptor internalization. Herein, we report that low-speed centrifugation and Western immunoblotting provide NK-1 internalization results consistent with those obtained by more traditional methods. These data support previous findings demonstrating a role for spinal NK-1 receptors in nociceptive processing.

The role of ERK signaling and the P2X receptor on mechanical pain evoked by movement of inflamed knee joint

Pain during inflammatory joint diseases is enhanced by the generation of hypersensitivity in nociceptive neurons in the peripheral nervous system. To explore the signaling mechanisms of mechanical hypersensitivity during joint inflammation, experimental arthritis was induced by injection of complete Freund's adjuvant (CFA) into the synovial cavity of rat knee joints. As a pain index, the struggle threshold of the knee extension angle was measured. In rats with arthritis, the phosphorylation of extracellular signal-regulated kinase (ERK), induced by passive joint movement, increased significantly in dorsal root ganglion (DRG) neurons innervating the knee joint compared to the naïve rats that received the same movement. The intrathecal injection of a MEK inhibitor, U0126, reduced the phosphorylation of ERK in DRG neurons and alleviated the struggle behavior elicited by the passive movement of the joint. In addition, the injection of U0126 into the joint also reduced the struggle behavior. These findings indicate that the ERK signaling is activated in both cell bodies in DRG neurons and peripheral nerve fibers and may be involved in the mechanical sensitivity of the inflamed joint. Furthermore, the phosphorylated ERK-positive neurons co-expressed the P2X3 receptor, and the injection of TNP-ATP, which antagonizes P2X receptors, into the inflamed joint reduced the phosphorylated ERK and the struggle behavior. Thus, it is suggested that the activation of the P2X3 receptor is involved in the phosphorylation of ERK in DRG neurons and the mechanical hypersensitivity of the inflamed knee joint.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Calcitonin gene-related peptide receptor activation produces PKA- and PKC-dependent mechanical hyperalgesia and central sensitization

Calcitonin gene-related peptide (CGRP), acting through CGRP receptors, produces behavioral signs of mechanical hyperalgesia in rats and sensitization of wide dynamic range (WDR) neurons in the spinal cord dorsal horn. Although involvement of CGRP receptors in central sensitization has been confirmed, the second-messenger systems activated by CGRP receptor stimulation and involved in pain transmission are not clear. This study tested whether the hyperalgesia and sensitizing effects of CGRP receptor activation on WDR neurons are mediated by protein kinase A or C (PKA or PKC) signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was also attenuated significantly by a PKA inhibitor (H89) or a PKC inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP-induced sensitization of spinal dorsal horn neurons. The CGRP effect could be blocked by CGRP8-37. Either a PKA or PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP1 receptors in the spinal cord dorsal horn and suggest that the effects of CGRP are mediated by both PKA and PKC second-messenger pathways.

Attenuation of nociception in a model of acute pancreatitis by an NK-1 antagonist

Substance P (SP) acting at the NK-1 neurokinin receptor has a well-documented role in the transmission and maintenance of nociceptive information. SP is found in the majority of fibers innervating the pancreas, and it is up-regulated after pancreatic inflammation. The aim of this study was to investigate the role of the NK-1 receptors in the maintenance of pancreatic nociception. Using a newly developed rat model of acute pancreatic nociception that persists for 1 week, the NK-1 receptor expression in the spinal cord and pancreas was examined using immunohistochemistry and Western blotting procedures. The effects of a specific NK-1 antagonist, CP99,994, on the behavioral manifestations of pancreatic nociception were determined. The antagonist was administered intraperitoneally and intrathecally to differentiate peripheral and central effects. Injection of CP-100,263, the inactive enantiomer of CP-99,994 was used as a control for nonspecific effects of the antagonist. Immunohistochemistry and Western blotting analysis revealed an up-regulation of the NK-1 receptor occurs in the pancreas but not at the spinal cord level. The NK-1 antagonist was able to attenuate the nociceptive behaviors in rats with pancreatitis when applied intraperitoneally with a short duration of effectiveness. Intrathecal application of the antagonist was ineffective. These results suggest the involvement of pancreatic NK-1 receptors in the maintenance of nociception during pancreatic inflammation.

Involvement of spinal neurokinins, excitatory amino acids, proinflammatory cytokines, nitric oxide and prostanoids in pain facilitation induced by Phoneutria nigriventer spider venom

The major local symptom of Phoneutria nigriventer envenomation is an intense pain, which can be controlled by infiltration with local anesthetics or by systemic treatment with opioid analgesics. Previous work showed that intraplantar (i.pl) injection of Phoneutria nigriventer venom in rats induces hyperalgesia, mediated peripherally by tachykinin and glutamate receptors. The present study examined the spinal mechanisms involved in pain-enhancing effect of this venom. Intraplantar injection of venom into rat hind paw induced hyperalgesia. This phenomenon was inhibited by intrathecal (i.t.) injection of tachykinin NK1 (GR 82334) or NK2 (GR 94800) receptor antagonists, a calcitonin gene-related peptide (CGRP) receptor antagonist (CGRP8-37) and N-methyl-D-aspartate (NMDA; MK 801 and AP-5), non-NMDA ionotropic (CNQX), or metabotropic (AIDA and MPEP) glutamate receptor antagonists, suggesting the involvement of spinal neurokinins and excitatory amino acids. The role of proinflammatory cytokines, nitric oxide (NO), and prostanoids in spinally mediated pain facilitation was also investigated. Pharmacological blockade of tumour necrosis factor-alpha (TNFalpha) or interleukin-1beta (IL-1beta) reduced the hyperalgesic response to venom. Intrathecal injection of L-N6-(1-iminoethyl)lysine (L-NIL), but not of 7-nitroindazole (7-NI), inhibited hyperalgesia induced by the venom, indicating that NO, generated by the activity of the inducible form of nitric oxide synthase, also mediates this phenomenon. Furthermore, indomethacin, an inhibitor of cyclooxigenases (COX), or celecoxib, a selective inhibitor of COX-2, abolished venom-induced hyperalgesia, suggesting the involvement of spinal prostanoids in this effect. These data indicate that the spinal mechanisms of pain facilitation induced by Phoneutria nigriventer venom involves a plethora of mediators that may cooperate in the genesis of venom-induced central sensitization.

A role for substance P in arthritis?

Substance P is a neuropeptide that is released from sensory nerves and which has a number of pro-inflammatory effects. In this article, we review the evidence for a role of substance P in arthritis, both in experimental animal models and rheumatoid arthritis patients. Substance P expression is altered in the joint and dorsal horn of arthritic animals, exogenous substance P and neurokinin 1 (NK(1)) receptor antagonists modulate responses in the joint, and there is some evidence for a role of substance P in human joint disease. However, the therapeutic potential of NK(1) receptor antagonists in the treatment of rheumatoid arthritis remains controversial.

Capsular neuronal elements and their relation to pain reduction and functional improvement following total hip replacement

We studied changes of pain intensity and functional impairment in 22 patients with osteoarthrosis undergoing total hip replacement. Using a visual analogue scale, the mean scores for pain and disability before surgery were 71.7 and 70.9 respectively. Both scores showed gradual improvement during a 1-year follow-up period, with more than 90% of the total improvement occurring within the first 3 months. After 1 year, the scores for pain and disability were 11.9 and 4.1 respectively. The hip joint capsule was studied using immunohistochemistry to detect neurofilaments. Neurofilament immunoreactivity was observed in 16/22 cases and was correlated with pain and disability scores. However, there were no correlations between pre- and postoperative pain scores, the score changes, and the quantity of capsular neurofilaments. Thus, other factors than capsular neurofilaments influence the scores of pain and disability in osteoarthritis.

Characterization of wide dynamic range neurons in the deep dorsal horn of the spinal cord in preprotachykinin-a null mice in vivo

We previously reported that mice with a deletion of the preprotachykinin-A (pptA) gene, from which substance P (SP) and neurokinin A (NKA) are derived, exhibit reduced behavioral responses to intense stimuli, but that behavioral hypersensitivity after injury is unaltered. To understand the contribution of SP and NKA to nociceptive transmission in the spinal cord, we recorded single-unit activity from wide dynamic range neurons in the lamina V region of the lumbar dorsal horn of urethane-anesthetized wild-type and ppt-A null mutant (-/-) mice. We found that intensity coding to thermal stimuli was largely preserved in the ppt-A -/- mice. Neither the peak stimulus-evoked firing nor the neuronal activity during the initial phase (0-4 s) of the 41-49 degrees C thermal stimuli differed between the genotypes. However, electrophysiological responses during the late phase of the stimulus (5-10 s) and poststimulus (11-25 s) were significantly reduced in ppt-A -/- mice. To activate C-fibers and to sensitize the dorsal horn neurons we applied mustard oil (MO) topically to the hindpaw. We found that neither total MO-evoked activity nor sensitization to subsequent stimuli differed between the wild-type and ppt-A -/- mice. However, the time course of the sensitization and the magnitude of the poststimulus discharges were reduced in ppt-A -/- mice. We conclude that SP and/or NKA are not required for intensity coding or sensitization of nociresponsive neurons in the spinal cord, but that these peptides prolong thermal stimulus-evoked responses. Thus whereas behavioral hypersensitivity after injury is preserved in ppt-A -/- mice, our results suggest that the magnitude and duration of these behavioral responses would be reduced in the absence of SP and/or NKA.

The coding of noxious mechanical and thermal stimuli of deep dorsal horn neurones is attenuated in NK1 knockout mice

Previous behavioural evidence has shown that NK1 receptor gene knockout (NK1 -/-) mice display altered nociceptive responses following tissue or peripheral nerve injury. A single electrophysiological study reported an attenuation of wind-up and responses to mustard oil application. Although the behavioural results implicate SP and its receptor (NK1-R) in the transmission of noxious high intensity pain, little is known regarding the spinal neuronal substrates and the modalities involved. We have addressed this using in vivo electrophysiology and recordings of deep dorsal horn neurones in urethane-anaesthetised C57B6 x 129/sv mice to reveal a marked deficit in mechanical and thermal coding, selectively encompassing the suprathreshold range of noxious stimuli. The frequency-dependent increase in neuronal activity following repetitive C-fibre stimulation (wind-up) was also abolished in spinal neurones of NK1-R knockout mice. Quantification of the receptive field size of spinal neurones, mapped with low- and high-intensity mechanical punctate stimuli, revealed no differences between NK1 -/- and wildtype mice. We conclude that NK1-Rs are important in the high intensity noxious signalling of acute peripheral (mechanical/thermal) stimuli and this may result from the lack of wind-up and/or the disruption of spinal-bulbo-spinal loops.

Involvement of spinal neurokinin-1 receptors in the maintenance but not induction of carrageenan-induced thermal hyperalgesia in the rat

The study was undertaken to assess the antihyperalgesic effect of L-732,138, (N-acetyl-L-tryptophan-3,5-bistrifluoromethyl benzyl ester), a non-peptide neurokinin-1 (NK1) receptor antagonist in rats when given intrathecally. The peripheral inflammation associated with behavioral hyperalgesia to a thermal stimulus was induced by intraplantar (i.pl.) injection of carrageenan. The thermal hyperalgesia was measured by paw withdrawal latency. Intrathecal (i.t.) injection of L-732,138 (100 nmol) at 3h after carrageenan markedly attenuated the paw withdrawal latency of the inflamed paw, but not that of the non-inflamed paw. L-732,138 (100 nmol, i.t.) given 10 min prior to carrageenan injection had no effect on the carrageenan-induced decrease in paw withdrawal latency to noxious thermal stimulus. The results demonstrate that NK1 receptor is involved in the maintenance but not the induction and development of thermal hyperalgesia evoked by carrageenan.

Microdialysis in pain research

In vivo microdialysis has been used in preclinical pain research for more than a decade. This valuable tool allows correlations between nociceptive behavior and neurotransmitter release in pain-related CNS sites. However, several methodological issues must be considered to adequately interpret microdialysis data. Thus, the aim of this review is to describe key considerations, potential pitfalls, and important control experiments. We focus on animal experiments which evaluate the effects of noxious stimulation on CNS neurotransmitter release, particularly those that address clinically relevant problems in patients with long-lasting painful conditions.

Effects of protein kinase a activation on the responses of primate spinothalamic tract neurons to mechanical stimuli

Behavioral and anatomical studies by our group have suggested that the protein kinase A (PKA) signal transduction cascade contributes to long-term changes in nociceptive processing at the spinal cord level. In this study, we have examined the effects of activation of the PKA cascade on the responses of spinothalamic tract (STT) neurons to peripheral mechanical stimuli in anesthetized and paralyzed monkeys. PKA in the spinal cord was activated by intra-spinal infusion of forskolin, an activator of adenylate cyclase, by microdialysis. There was a consistent increase in responses to mechanical pressure and pinch stimuli in all STT cells tested when forskolin was administered. Enhanced responses remained at relatively high levels when forskolin had been washed out for 30 min. However, in most STT cells tested (65%), the responses to brushing stimuli were not obviously changed when forskolin was given. Background activity was slightly increased when forskolin was administered. An inactive isomer of forskolin, D-forskolin, did not produce significant effects on cellular activity. The sensitization of STT cells to noxious mechanical stimuli produced by forskolin could be blocked by pretreatment of the spinal cord with the PKA inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamine (H89). The same dose of H89 did not affect the enhanced responses to mechanical stimuli produced by activation of protein kinase G by intra-spinal infusion of 8-bromo-cGMP, indicating that the effect of forskolin was selective. The present data suggest that activation of PKA can preferentially enhance the responses of STT cells to noxious mechanical stimuli without producing an increase in responses to innocuous brushing stimuli. We speculate that the PKA signal transduction cascade may contribute more to secondary mechanical hyperalgesia than to secondary mechanical allodynia.

The neurokinin-2 receptor is not involved in the sensitization of primary afferents of the rat knee joint

Using electrophysiological methods, we aimed in the present study to determine whether the NK(2) receptor is involved in the sensitization of articular afferents of the rat. Impulse activity from 27 single fine nerve fibres innervating knee joints was recorded during non-noxious and noxious joint rotations. Close intraarterial application of the NK(2) receptor agonist [beta-Ala(8)]NKA(4-10) at doses of 0.2-200 nmol did not sensitize the afferents from normal knee joints to mechanical stimuli whereas the application of substance P (20 nmol) increased their mechanosensitivity. These data further support the hypothesis that the NK(2) receptor is not involved in the sensitization of primary afferents in normal knee joints to mechanical stimuli.

A novel control mechanism based on GDNF modulation of somatostatin release from sensory neurones

Small-diameter sensory neurones found in the rat dorsal root ganglia (DRG) include cells sensitive to glial cell line-derived neurotrophic factor (GDNF), which express the inhibitory peptide somatostatin (SOM). Here we addressed the functional relationship between GDNF and sensory neurone-derived SOM. Topical application of GDNF through the rat isolated dorsal horn of the spinal cord promoted activity-induced release of SOM from central terminals of sensory neurones. Once released by sensory neurones, SOM is known to act, at least in part, by opposing the action of Substance P (SP) in neurogenic inflammation. Therefore, we evaluated GDNF ability to modulate two well-documented effects of peripherally and centrally administered SP. Local application of GDNF in the mouse air pouch reduced SP-induced leukocyte migration. This effect of GDNF was mimicked by the SOM analog octreotide (OCT) and required intact SOM neuronal pools. Intrathecal injection of GDNF activated rat lumbar dorsal horn neurones and inhibited intrathecal SP-induced thermal hypersensitivity. This effect of GDNF was reversed by the SOM antagonist c-SOM and mimicked by OCT. In conclusion we propose GDNF regulation of neuronal SOM release as a novel mechanism that, if explored, may lead to new therapeutic strategies based on local release of somatostatin.

Two variables that can be used as pain indices in experimental animal models of arthritis

Since pain is an important symptom in arthritis, useful behavioral indices for pain in experimental arthritis animal models are important tools for investigative work on arthritis. The purpose of this study was to develop simple and quantifiable behavioral tests, which would represent the level of arthritic pain that develops after induction of inflammation in the knee. Two models of knee joint arthritis were produced: (1) KC model-injection of kaolin and carrageenan into the synovial cavity of the knee, and (2) CFA model-injection of complete Freund's adjuvant into the synovial cavity of the knee. The following three variables were measured before and at various times after the induction of arthritis. As an estimate of the degree of edema, the circumference of the knee was measured. As pain indices, (1) the vocalization threshold of compression force applied to the knee joint was measured to represent tenderness of the joint, and (2) the struggle threshold of the knee extension angle was measured to represent a reduction in range of motion in the arthritic joint. A time course study showed that behavioral changes last for at least 1 week for the KC model and at least 2 weeks for the CFA model. Correlation studies showed that all three variables significantly correlated with each other in both the KC and CFA arthritic models. Systemically injected morphine produced a partial reversal of these indices with the expected time course and dose response of a morphine-induced analgesic. It is concluded that two variables, the struggle threshold for knee extension and the vocalization threshold for knee compression, could be used as simple and useful pain indices in experimental models of arthritis.

Local neurokinin-1 receptor in the knee joint contributes to the induction, but not maintenance, of arthritic pain in the rat

Substance P is known to exert various pro-inflammatory effects that are mediated by neurokinin-1 (NK-1) receptor in peripheral tissues. This study examined the effect of the NK-1 receptor antagonist cis-2-[diphenylmethyl]-N-[(2-iodophenyl)-1-azabicyclo[2.2.2]octan-3-amine] (L-703,606) on nociceptive response following carrageenan injection (2%, 50 microl) into the knee joint cavity of the right hind leg. L-703,606 injection (0.1 or 1 mM, 50 microl) into the same joint cavity immediately before the carrageenan injection significantly reduced the nociceptive response. However, antagonist treatment at 5 h after carrageenan injection was ineffective in alleviating nociception. Neither intraperitoneal injection of the antagonist (1 mM, 50 microl) immediately before the carrageenan injection was effective. These results suggest that local NK-1 receptor contributes to the induction, but not maintenance, of arthritic pain.

The neurokinin-1 receptor antagonist RP 67580 reduces the sensitization of primary afferents by substance P in the rat

The inflammatory mediator substance P (SP) produces a variety of biological effects in several tissues by binding to the tachykinin receptor neurokinin 1 (NK1) and, to a lesser extent, by binding to the neurokinin 2 receptor (NK2). To assess the sensitizing effect of SP on articular afferent fibres the NK1receptor antagonist RP 67580 was applied in normal and acutely inflamed rat knee joints. Altogether 38 fine afferent nerve fibres from the rat knee with conduction velocities of 0.71-13.5 m/s were recorded as single units, during non-noxious and noxious joint rotations. SP, injected i.a. as a bolus close to the knee joint, was able to sensitize 45.5% (10 of 22) of the units recorded from normal joints and 33.3% (five of 15) of afferents from inflamed joints. The following i.a. application of RP 67580 in a range of 20-200 nmol antagonized in a dose-dependent manner the sensitizing effect of SP in a large proportion of slowly conducting articular afferents from normal (66.7%) and inflamed (46.2%) knee joints. Subsequent SP application enhanced the afferent sensitivity further. The electrophysiological results presented here further support the suggestion that the sensitization of afferents by SP in the rat knee joint is mediated mainly by the NK1 receptor, which is probably located on the primary afferents.

Adjuvant-induced joint inflammation causes very rapid transcription of beta-preprotachykinin and alpha-CGRP genes in innervating sensory ganglia

Neuropeptides synthesized in dorsal root ganglia (DRG) have been implicated in neurogenic inflammation and nociception in experimental and clinical inflammatory arthritis. We examined the very early changes in response to adjuvant injection in a rat model of unilateral tibio-tarsal joint inflammation and subsequent monoarthritis. Within 30 min of adjuvant injection ipsilateral swelling and hyperalgesia were apparent, and marked increases in beta-preprotachykinin-A (beta-PPT-A) and alpha-calcitonin gene-related peptide (CGRP)-encoding mRNAs were observed in small-diameter L5 DRG neurones innervating the affected joint. This response was augmented by recruitment of additional small-diameter DRG neurones expressing beta-PPT-A and CGRP transcripts. The increased mRNA was paralleled by initial increases in L5 DRG content of the protein products, substance P and calcitonin gene-related peptide. Within 15 min of adjuvant injection there were increases in electrical activity in sensory nerves innervating a joint. Blockade of this activity prevented the rapid induction in beta-PPT-A and CGRP mRNA expression in DRG neurones. Increased expression of heteronuclear (intron E) beta-PPT-A RNA suggests that increases in beta-PPT-A mRNA levels were, at least in part, due to transcription. Pre-treatment with the protein synthesis inhibitor cycloheximide had no effect upon the early rise in neuropeptide mRNAS: This and the rapid time course of these changes suggest that increased sensory neural discharge and activation of a latent modulator of transcription are involved.

NK-1 receptor gene expression is related to pain in chronic pancreatitis

Recent theories of pathogenesis of pain in chronic pancreatitis (CP) are neuroimmune interactions of intrapancreatic nerves and inflammatory cells and increase in levels of pain neurotransmitters such as substance P (SP). This study analyzed the expression and localization of neurokinin 1 receptor (NK-1R), which binds SP, and its association with pain and inflammation in CP. Pancreatic tissues from 31 patients (22 males, nine females; mean age 45.9+/-9.4 years) with CP were evaluated. Nine normal pancreases (five males, four females; mean age 42.9+/-9.5 years) served as controls. Quantitative PCR was used to determine the NK-1R mRNA expression levels and in situ hybridization and immunohistochemistry were used to localize expression sites of NK-1R mRNA and protein, respectively. We also analyzed whether an association exists between NK-1R mRNA expression and pain and inflammation. In CP samples, in situ hybridization and immunohistochemistry localized NK-1R mRNA expression and protein mainly in the nerves, ganglia, blood vessels, inflammatory cells and occasionally in fibroblasts. In patients with mild to moderate and strong intensity of pain, NK-1R mRNA levels were increased 14- and 30-fold over controls, respectively. There was a significant relationship between NK-1R mRNA levels and intensity of pain (r=0.46, P=0.03), NK-1R mRNA and the frequency of pain (r=0.51, P=0.04), and NK-1 mRNA and duration of pain (r=0.46, P=0.01) in CP patients, but not with the degree of tissue inflammation. NK-1R signaling may be involved in the pain syndrome of CP. The expression of NK-1R in inflammatory cells and blood vessels also points to an interaction of immunoreactive substance P nerves, inflammatory cells and blood vessels, and further supports the existence of a neuroimmune interaction that probably influences the pain syndrome and chronic inflammatory changes so characteristic of CP.

Deficits in visceral pain and hyperalgesia of mice with a disruption of the tachykinin NK1 receptor gene

Studies in mice lacking genes encoding for substance P or its receptor (NK1), or with NK1 antagonists, have shown that this system contributes to nociception, but the data are complex. Here, we have further examined the role of NK1 receptors in pain and hyperalgesia by comparing nociceptive responses to mechanical and chemical stimulation of viscera and the resulting hyperalgesia and inflammation in NK1 knockout (-/-) and wild-type (+/+) mice. We concentrated on visceral nociception because substance P is expressed by a much greater proportion of visceral than cutaneous afferents. NK1 -/- mice showed normal responses to visceral mechanical stimuli, measured as behavioural responses to intraperitoneal acetylcholine or hypertonic saline or reflex responses to colon distension in anaesthetized mice, although -/- mice failed to encode the intensity of noxious colon distensions. In contrast, NK1 -/- mice showed profound deficits in spontaneous behavioural reactions to an acute visceral chemical stimulus (intracolonic capsaicin) and failed to develop referred hyperalgesia or tissue oedema. However, in an identical procedure, intracolonic mustard oil evoked normal spontaneous behaviour, referred hyperalgesia and oedema in -/- mice. The inflammatory effects of capsaicin were abolished by denervation of the extrinsic innervation of the colon in rats, whereas those of mustard oil were unchanged, showing that intracolonic capsaicin evokes neurogenic inflammation, but mustard oil does not. Tests of other neurogenic inflammatory stimuli in NK1 -/- mice revealed impaired behavioural responses to cyclophosphamide cystitis and no acute reflex responses or primary hyperalgesia to intracolonic acetic acid. We conclude that NK1 receptors have an essential role mediating central nociceptive and peripheral inflammatory responses to noxious stimuli that evoke neurogenic inflammation, and modulating responses to noxious mechanical stimuli. We propose that two separate hyperalgesia pathways exist, one of which is NK1 receptor dependent, whereas the other does not require intact substance P/NK1 signalling.

Nerve injury-induced mechanical but not thermal hyperalgesia is attenuated in neurokinin-1 receptor knockout mice

Mice lacking the gene encoding for substance P and neurokinin A, or the NK-1 receptor, exhibit alterations in behavior to various acute nociceptive stimuli. However, behavioral responses of NK-1 mutant animals have not been well characterized in models of chronic pain. We studied the behavioral responses of NK-1 knockout and wild-type control mice to thermal and mechanical stimuli before and after inducing chronic neuropathic pain by unilateral ligation of the L5 spinal nerve. Mechanical hyperalgesia was evaluated by determining the frequency of withdrawal to von Frey monofilaments applied to the hind paws. Nerve injury-induced hyperalgesia to thermal stimuli was examined by determining responses to radiant heat and cooling stimuli. The contribution of the sympathetic nervous system to mechanical hyperalgesia was evaluated by administering 3 mg/kg phentolamine, an alpha-adrenergic antagonist, subcutaneously. Following spinal nerve injury, withdrawal frequencies to mechanical stimulation increased in wild-type mice within 1 day and persisted during the 9-week observation period, whereas in the knockout mice, withdrawal frequencies did not increase significantly. In contrast, withdrawal latencies to radiant heat decreased up to 2 weeks after nerve injury in both the NK-1 and the wild-type mice. Similarly, the increase in withdrawal frequency to the cooling stimuli following the nerve injury was not different in the NK-1 knockout and wild-type mice. Mechanical hyperalgesia in the wild-type mice was not reversed by systemic administration of phentolamine, suggesting that the pain is not sympathetically maintained. The results indicate that NK-1 receptors contribute to the development of mechanical, but not thermal, hyperalgesia in neuropathic pain.

Role of tachykinin NK2 receptors in normal and altered rectal sensitivity in rats

Irritable bowel syndrome is characterized by visceral hyperalgesia commonly associated with stress and inflammatory processes. We investigated the role of tachykinin NK2 receptors in the ability of trinitrobenzenesulphonic acid (TNBS) and stress to enhance the sensitivity of the rat rectum to distension using a selective tachykinin NK2 receptor antagonist (MEN 11420). Rats were fitted with electrodes implanted in the striated muscles of the abdomen. Rectal distension (RD) was performed with a balloon inflated by steps of 0.4 ml from 0 to 1.6 ml. Five groups were submitted to RD performed 3 days before and after intrarectal instillation of TNBS. Fifteen minutes before RD, rats were treated with saline or MEN 11420 (5 - 100 microg kg(-1) i.v.). Two other groups, submitted to 2 h restraint or sham stress sessions were randomly treated i.v. with saline or MEN 11420 (10 - 200 microg kg(-1)) prior to RD applied 20 min later. The basal response to RD was characterized by a significant increase in the number of abdominal contractions. This response occurred with a threshold volume of 0.8 ml and was dose-dependently reduced by MEN 11420 (5 - 100 microg kg(-1) i.v.). Rectal inflammation lowered the volume of distension producing abdominal contractions to 0.4 ml (allodynia). This effect was either reduced or suppressed by MEN 11420. A similar allodynia was observed after a stress session and this effect was reduced (49%) or suppressed by MEN 11420 at 200 and 100 microg kg(-1), respectively. Tachykinin NK2 receptors are involved in rectal hypersensitivity associated with inflammation and stress. British Journal of Pharmacology (2000) 129, 193 - 199

Tachykinin receptor modulators: novel therapeutics for rheumatoid arthritis

The activation of a cellular immune response in a genetically susceptible individual is widely recognised as a main step in triggering rheumatoid arthritis (RA). The tachykinins, substance P (SP) and neurokinin A (NKA), can play a major role in different immune diseases. In patients with inflammatory joint disease, elevated levels of SP have been demonstrated in the synovial fluid of affected joints. It is well known that SP and, to a lesser extent, NKA are deeply involved in the processing of nociceptive signals and exert many pro-inflammatory actions, which may be elicited by an increased neuronal neurokinin release in arthritis; the mechanism behind this increase remains to be fully elucidated. Different observations suggest that one approach to the treatment of RA might be to inhibit the local effects of neurokinins in the affected joints. This review will summarise the more relevant aspects of this topic.

Evidence for tonic activation of NK-1 receptors during the second phase of the formalin test in the Rat

Behavioral, electrophysiological, and autoradiographic experiments were done to study the second nociceptive phase in the formalin test. In initial experiments, this second phase was attenuated by 1-10 mg of the NK-1 receptor antagonist CP-99,994, given subcutaneously 10, 30, or 60 min before formalin (n = 8-10) and by 20 microgram given intrathecally 20 min after formalin (n = 13); the inactive isomer CP-100,263 was ineffective. In electrophysiological experiments on single dorsal horn neurons in vivo, the excitatory responses to subcutaneous formalin injection (50 microliter, 2.5%) were attenuated by subsequent intravenously administration of the NK-1 receptor antagonist CP-96,345 (0.5 mg/kg; n = 8), given 35-40 min after formalin, but not by the inactive enantiomer CP-96,344 (0.5 mg/kg; n = 9). Finally, autoradiographic binding of exogenous [(125)I]BH-substance P in the lumbar cord was reduced at 5 and 25 min after formalin (50 microliter, 1 or 5%), with an intermediate level of reduction at 12 min. These data are interpreted as evidence that the second phase of nociceptive scores in the formalin test is attributable at least partially to tonic activation of NK-1 receptors at the spinal level, whether because of a temporally limited release of substance P, for example only during the first phase, but a slow removal or breakdown of substance P, or, more likely, because of tonic release from primary afferents throughout the second phase. Irrespective of the mechanism, it can be concluded that at least some of the persistent nociceptive effects associated with peripheral inflammation, or at least those provoked by subcutaneous injection of formalin, are mediated via continuous activation of NK-1 receptors at the level of the spinal dorsal horn; this may relate directly to mechanisms underlying prolonged nociceptive pains in humans.

Evidence for tonic activation of NK-1 receptors during the second phase of the formalin test in the Rat

Behavioral, electrophysiological, and autoradiographic experiments were done to study the second nociceptive phase in the formalin test. In initial experiments, this second phase was attenuated by 1-10 mg of the NK-1 receptor antagonist CP-99,994, given subcutaneously 10, 30, or 60 min before formalin (n = 8-10) and by 20 microgram given intrathecally 20 min after formalin (n = 13); the inactive isomer CP-100,263 was ineffective. In electrophysiological experiments on single dorsal horn neurons in vivo, the excitatory responses to subcutaneous formalin injection (50 microliter, 2.5%) were attenuated by subsequent intravenously administration of the NK-1 receptor antagonist CP-96,345 (0.5 mg/kg; n = 8), given 35-40 min after formalin, but not by the inactive enantiomer CP-96,344 (0.5 mg/kg; n = 9). Finally, autoradiographic binding of exogenous [(125)I]BH-substance P in the lumbar cord was reduced at 5 and 25 min after formalin (50 microliter, 1 or 5%), with an intermediate level of reduction at 12 min. These data are interpreted as evidence that the second phase of nociceptive scores in the formalin test is attributable at least partially to tonic activation of NK-1 receptors at the spinal level, whether because of a temporally limited release of substance P, for example only during the first phase, but a slow removal or breakdown of substance P, or, more likely, because of tonic release from primary afferents throughout the second phase. Irrespective of the mechanism, it can be concluded that at least some of the persistent nociceptive effects associated with peripheral inflammation, or at least those provoked by subcutaneous injection of formalin, are mediated via continuous activation of NK-1 receptors at the level of the spinal dorsal horn; this may relate directly to mechanisms underlying prolonged nociceptive pains in humans.

Time-course of spinal sensitization following carrageenan-induced inflammation in the young rat: a comparative electrophysiological and behavioural study in vitro and in vivo

Inflammation of peripheral tissues evokes spontaneous pain and an increased responsiveness to external stimuli known as hyperalgesia, produced by both peripheral and central changes. The central component is initiated by a sustained afferent barrage produced by sensitized peripheral nociceptors, but it is unclear to which extent ongoing nociceptive input is required to maintain these central changes. Here, we have used an isolated preparation of the spinal cord in vitro obtained from eight- to 12-day-old rats to examine spinal plasticity in the absence of naturally occurring afferent inputs. Spinal reflex responses in preparations obtained from naive rats were compared with those from animals with carrageenan-induced inflammation of one hindpaw of 3 h, 6 h and 20 h duration prior to the extraction of the cord. Measurements of thermal (heat) and mechanical hyperalgesia in awake animals were also made at the same time-points. At 6 h post-carrageenan, there was a significant increase in the wind-up evoked by trains of high-intensity (C-fibre) stimuli, and at 20 h increased responses to both trains and single high-intensity stimuli, and a novel wind-up to low-intensity (Abeta-fibre) trains were observed. In contrast, maximal behavioural hyperalgesia was observed by 3 h post-carrageenan, and thermal hyperalgesia had resolved by 20 h, although mechanical hyperalgesia remained. These results show that the induction of spinal plasticity independent of peripheral input is a progressive process with a slow time-course, since significant hyperreflexia in the isolated spinal preparation appears 6 h after inflammation and develops further within 20 h. We conclude that during the first 3 h following inflammation, hyperalgesia is the result of peripheral sensitization and of central mechanisms that depend on an ongoing peripheral input and thus changes were not observed in the isolated spinal cord.

Distinct neurochemical features of acute and persistent pain

To address the neurochemistry of the mechanisms that underlie the development of acute and persistent pain, our laboratory has been studying mice with deletions of gene products that have been implicated in nociceptive processing. We have recently raised mice with a deletion of the preprotachykinin-A gene, which encodes the peptides substance P (SP) and neurokinin A (NKA). These studies have identified a specific behavioral phenotype in which the animals do not detect a window of "pain" intensities; this window cuts across thermal, mechanical, and chemical modalities. The lowered thermal and mechanical withdrawal thresholds that are produced by tissue or nerve injury, however, were still present in the mutant mice. Thus, the behavioral manifestations of threshold changes in nociceptive processing in the setting of injury do not appear to require SP or NKA. To identify relevant neurochemical factors downstream of the primary afferent, we are also studying the dorsal horn second messenger systems that underlie the development of tissue and nerve injury-induced persistent pain states. We have recently implicated the gamma isoform of protein kinase C (PKCgamma) in the development of nerve injury-induced neuropathic pain. Acute pain processing, by contrast, is intact in the PKCgamma-null mice. Taken together, these studies emphasize that there is a distinct neurochemistry of acute and persistent pain. Persistent pain should be considered a disease state of the nervous system, not merely a prolonged acute pain symptom of some other disease conditions.