The proportion of CGRP-immunoreactive and SP-mRNA containing dorsal root ganglion cells is increased by a unilateral inflammation of the ankle joint of the rat

Altered sensory innervation and pain hypersensitivity in a model of young painful arthritic joints: short- and long-term effects

BACKGROUND

Early life experience can cause long-term alterations in the nociceptive processes underlying chronic pain, but the consequences of early life arthritic joint inflammation upon the sensory innervation of the joint is not known. Here, we measure pain sensitivity and sensory innervation in a young, juvenile and adult rodent model of arthritic joints and test the consequences of joint inflammation in young animals upon adult arthritic pain and joint innervation.

METHODS

Unilateral ankle joint injections of complete Freund's adjuvant (CFA) (6-20 µl) were performed in young, postnatal day (P)8, adolescent (P21) and adult (P40) rats. A separate cohort of animals were injected at P8, and again at P40. Hindpaw mechanical sensitivity was assessed using von Frey monofilaments (vF) for 10 days. Nerve fibres were counted in sections through the ankle joint immunostained for calcitonin gene-related peptide (CGRP) and neurofilament 200 kDa (NF200).

RESULTS

Ankle joint CFA injection increased capsular width at all ages. Significant mechanical pain hypersensitivity and increased number of joint CGRP + ve sensory fibres occurred in adolescent and adult, but not young, rats. Despite the lack of acute reaction, joint inflammation at a young age resulted in significantly increased pain hypersensitivity and CGRP+ fibre counts when the rats were re-inflamed as adults.

CONCLUSIONS

Joint inflammation increases the sensory nociceptive innervation and induces acute pain hypersensitivity in juvenile and adult, but not in young rats. However, early life joint inflammation 'primes' the joint such that adult inflammatory pain behaviour and nociceptive nerve endings in the joint are significantly increased. Early life joint inflammation may be an important factor in the generation and maintenance of chronic arthritic pain.

In Situ Hybridisation Study of Neuronal Neuropeptides Expression in Models of Mandibular Denervation with or without Inflammation: Injury Dependant Neuropeptide Plasticity

Neuronal expression of neuropeptides is altered following peripheral tissue injury associated with inflammation or nerve injury. This results in neuropathic pain with or without neurogenic inflammation which is a major health problem regularly seen in trigeminal neuralgia. Activation of the trigeminal system results in the release of vasoactive neuropeptides substance P and Calcitonin Gene-related Peptide (CGRP) which contribute to nociception, pain and neurogenic inflammation in injured tissues.

Tumour-Derived Glutamate: Linking Aberrant Cancer Cell Metabolism to Peripheral Sensory Pain Pathways

Background

Chronic pain is a major symptom that develops in cancer patients, most commonly emerging during advanced stages of the disease. The nature of cancer-induced pain is complex, and the efficacy of current therapeutic interventions is restricted by the dose-limiting side-effects that accompany common centrally targeted analgesics.

Methods

This review focuses on how up-regulated glutamate production and export by the tumour converge at peripheral afferent nerve terminals to transmit nociceptive signals through the transient receptor cation channel, TRPV1, thereby initiating central sensitization in response to peripheral disease-mediated stimuli.

Results

Cancer cells undergo numerous metabolic changes that include increased glutamine catabolism and over-expression of enzymes involved in glutaminolysis, including glutaminase. This mitochondrial enzyme mediates glutaminolysis, producing large pools of intracellular glutamate. Up-regulation of the plasma membrane cystine/glutamate antiporter, system xc-, promotes aberrant glutamate release from cancer cells. Increased levels of extracellular glutamate have been associated with the progression of cancer-induced pain and we discuss how this can be mediated by activation of TRPV1.

Conclusion

With a growing population of patients receiving inadequate treatment for intractable pain, new targets need to be considered to better address this largely unmet clinical need for improving their quality of life. A better understanding of the mechanisms that underlie the unique qualities of cancer pain will help to identify novel targets that are able to limit the initiation of pain from a peripheral source–the tumour.

Rat model of cancer-induced bone pain: changes in nonnociceptive sensory neurons in vivo

Nonnociceptive sensory neurons relate to transient episodes of intense pain that characterize neuropathic pain. They are involved in the peripheral sensitization and tactile hypersensitivity.

Introduction:

Clinical data on cancer-induced bone pain (CIBP) suggest extensive changes in sensory function. In a previous investigation of an animal model of CIBP, we have observed that changes in intrinsic membrane properties and excitability of dorsal root ganglion (DRG) nociceptive neurons correspond to mechanical allodynia and hyperalgesia.

Objectives:

To investigate the mechanisms underlying changes in nonnociceptive sensory neurons in this model, we have compared the electrophysiological properties of primary nonnociceptive sensory neurons at <1 and >2 weeks after CIBP model induction with properties in sham control animals.

Methods:

Copenhagen rats were injected with 10⁶ MAT-LyLu rat prostate cancer cells into the distal femur epiphysis to generate a model of CIBP. After von Frey tactile measurement of mechanical withdrawal thresholds, the animals were prepared for acute electrophysiological recordings of mechanically sensitive neurons in the DRG in vivo.

Results:

The mechanical withdrawal threshold progressively decreased in CIBP model rats. At <1 week after model induction, there were no changes observed in nonnociceptive Aβ-fiber DRG neurons between CIBP model rats and sham rats. However, at >2 weeks, the Aβ-fiber low-threshold mechanoreceptors (LTMs) in CIBP model rats exhibited a slowing of the dynamics of action potential (AP) genesis, including wider AP duration and lower AP amplitude compared with sham rats. Furthermore, enhanced excitability of Aβ-fiber LTM neurons was observed as an excitatory discharge in response to intracellular injection of depolarizing current into the soma.

Conclusion:

After induction of the CIBP model, Aβ-fiber LTMs at >2 weeks but not <1 week had undergone changes in electrophysiological properties. Importantly, changes observed are consistent with observations in models of peripheral neuropathy. Thus, Aβ-fiber nonnociceptive primary sensory neurons might be involved in the peripheral sensitization and tumor-induced tactile hypersensitivity in CIBP.

Neuronal plasticity of trigeminal ganglia in mice following nerve injury

Background

Nerve injury may induce neuropathic pain. In studying the mechanisms of orofacial neuropathic pain, attention has been paid to the plastic changes that occur in the trigeminal ganglia (TGs) and nucleus in response to an injury of the trigeminal nerve branches. Previous studies have explored the impact of sciatic nerve injury on dorsal root ganglia (DRGs) and it has shown dramatic changes in the expression of multiple biomarkers. In large, the changes in biomarker expression in TGs after trigeminal nerve injury are similar to that in DRGs after sciatic nerve injury. However, important differences exist. Therefore, there is a need to study the plasticity of biomarkers in TGs after nerve injury in the context of the development of neuropathic pain-like behaviors.

Aim

The aim of this study was to investigate the plasticity of biomarkers associated with chronic persistent pain in TGs after trigeminal nerve injury.

Materials and methods

To mimic the chronic nature of the disorder, we used an intraoral procedure to access the infraorbital nerve (ION) and induced a nerve injury in mice. Immunohistochemistry and quantification were used for revealing the expression level of each biomarker in TGs after nerve injury.

Results

Two weeks after partial ION injury, immunohistochemistry results showed strongly upregulated expressions of activating transcription factor 3 and neuropeptide Y (NPY) in the ipsilateral TGs. Microglial cells were also activated after nerve injury. In regard to positive neuronal profile counting, however, no significant difference in expression was observed in galanin, substance P, calcitonin gene-related peptide, neuronal nitric oxide synthase, phosphorylated AKT, or P2X3 in ipsilateral TGs when compared to contralateral TGs.

Conclusion

In this study, the expression and regulation of biomarkers in TGs have been observed in response to trigeminal nerve injury. Our results suggest that NPY and Iba1 might play crucial roles in the pathogenesis of orofacial neuropathic pain following this type of injury. Further investigations on the relevance of these changes may help to target suitable treatment possibilities for trigeminal neuralgia.

Chemical Coding of Sensory Neurons Supplying the Hip Joint Capsule in the Sheep

Immunohistochemical properties of nerve fibres supplying the joint capsule were previously described in many mammalian species, but the localization of sensory neurons supplying this structure was studied only in laboratory animals, the rat and rabbit. However, there is no comprehensive data on the chemical coding of sensory neurons projecting to the hip joint capsule (HJC). The aim of this study was to establish immunohistochemical properties of sensory neurons supplying HJC in the sheep. The study was carried out on 10 sheep, weighing about 30-40 kg. The animals were injected with a retrograde neural tracer Fast Blue (FB) into HJC. Sections of the spinal ganglia (SpG) with FB-positive (FB+) neurons were stained using antibodies against calcitonin gene-related peptide (CGRP) substance P (SP), pituitary adenylate cyclase-activating peptide (PACAP), nitric oxide synthase (n-NOS), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), Leu-5-enkephalin (Leu-Enk), galanin (GAL) and vesicular acetylcholine transporter (VACHT). The vast majority of FB+ neurons supplying HJC was found in the ganglia from the 5th lumbar to the 2nd sacral. Immunohistochemistry revealed that most of these neurons were immunoreactive to CGRP or SP (80.7 ± 8.0% or 56.4 ± 4.8%, respectively) and many of them stained for PACAP or GAL (52.9 ± 2.9% or 50.6 ± 19.7%, respectively). Other populations of FB+ neurons were those immunoreactive to n-NOS (37.8 ± 9.7%), NPY (34.6 ± 6.7%), VIP (28.7 ± 4.8%), Leu-Enk (27.1 ± 14.6) and VACHT (16.7 ± 9.6).

Usefulness of intra-articular botulinum toxin injections. A systematic review

Botulinum toxin is a proven and widely used treatment for numerous conditions characterized by excessive muscular contractions. Recent studies have assessed the analgesic effect of botulinum toxin in joint pain and started to unravel its mechanisms.

LITERATURE-SEARCH-METHODOLOGY

We searched the international literature via the Medline database using the term "intraarticular botulinum toxin injection" combined with any of the following terms: "knee", "ankle", "shoulder", "osteoarthritis", "adhesive capsulitis of the shoulder".

RESULTS

Of 16 selected articles about intraarticular botulinum toxin injections, 7 were randomized controlled trials done in patients with osteoarthritis, adhesive capsulitis of the shoulder, or chronic pain after joint replacement surgery. Proof of anti-nociceptive effects was obtained in some of these indications and the safety and tolerance profile was satisfactory. The studies are heterogeneous. The comparator was usually a glucocorticoid or a placebo; a single study used hyaluronic acid. Pain intensity was the primary outcome measure.

DISCUSSION-CONCLUSION

The number of randomized trials and sample sizes are too small to provide a satisfactory level of scientific evidence or statistical power. Unanswered issues include the effective dosage and the optimal dilution and injection modalities of botulinum toxin.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Glutaminase immunoreactivity and enzyme activity is increased in the rat dorsal root ganglion following peripheral inflammation

Following inflammation, primary sensory neurons in the dorsal root ganglion (DRG) alter the production of several proteins. Most DRG neurons are glutamatergic, using glutaminase as the enzyme for glutamate production, but little is known about glutaminase following inflammation. In the present study, adjuvant-induced arthritis (AIA) was produced in rats with complete Freund's adjuvant into the hindpaw. At 7 days of AIA, DRG were examined with glutaminase immunohistochemistry, Western blot immunoreactivity, and enzyme activity. Image analysis revealed that glutaminase was elevated most in small-sized neurons (21%) (P < 0.05). Western blot analysis revealed a 19% increase (P < 0.05) in total glutaminase and 21% in mitochondrial glutaminase (P < 0.05). Glutaminase enzyme activity was elevated 29% (P < 0.001) from 2.20 to 2.83 moles/kg/hr. Elevated glutaminase in primary sensory neurons could lead to increased glutamate production in spinal primary afferent terminals contributing to central sensitization or in the peripheral process contributing to peripheral sensitization.

Interaction of histamine and calcitonin gene-related peptide in the formalin induced pain perception in rats

Histamine and calcitonin gene-related peptide (CGRP) contribute to the pain perception. The aim of the present study is to clarify the interaction of histamine and CGRP in the perception of inflammatory pain. The effects of a histamine H1 receptor antagonist (pyrilamine, i.p.), an H2 receptor antagonist (ranitidine, i.p.) and a CGRP antagonist (CGRP 8-37, i.t.) on the formalininduced pain was studied in rats. Pyrilamine and ranitidine produced a dose-dependent antinociceptive response in the first and the second phases of the formalin test. A single administration of pyrilamine (1 mg/kg, i.p.), ranitidine (10 mg/kg, i.p.) or CGRP 8-37 (10 µg/µL, i.t.) had no significant effects on the pain perception in the second phase. A combination of CGRP 8-37 and pyrilamine or ranitidine at these sub-effective doses, however, showed nociceptive response in the second phase. Moreover, a histamine (i.t.)-induced hyperalgesia was completely prevented by treatment with GGRP 8-37 at this dose. Our findings have raised the possibility that the CGRP system has interaction with histamine in the perception of inflammatory pain.

Changes in Abeta non-nociceptive primary sensory neurons in a rat model of osteoarthritis pain

Background

Pain is a major debilitating factor in osteoarthritis (OA), yet few mechanism-based therapies are available. To address the need to understand underlying mechanisms the aim of the present study was to determine changes in sensory neurons in an animal model of OA pain.

Results

The model displayed typical osteoarthritis pathology characterized by cartilage degeneration in the knee joint and also manifested knee pathophysiology (edema and increased vasculature permeability of the joint) and altered nociception of the affected limb (hind paw tenderness and knee articulation-evoked reduction in the tail flick latency). Neurons included in this report innervated regions throughout the entire hind limb. Aβ-fiber low threshold mechanoreceptors exhibited a slowing of the dynamics of action potential (AP) genesis, including wider AP duration and slower maximum rising rate, and muscle spindle neurons were the most affected subgroup. Only minor AP configuration changes were observed in either C- or Aδ-fiber nociceptors.

Conclusion

Thus, at one month after induction of the OA model Aβ-fiber low threshold mechanoreceptors but not C- or Aδ-fiber nociceptors had undergone changes in electrophysiological properties. If these changes reflect a change in functional role of these neurons in primary afferent sensory processing, then Aβ-fiber non-nociceptive primary sensory neurons may be involved in the pathogenesis of OA pain. Further, it is important to point out that the patterns of the changes we observed are consistent with observations in models of peripheral neuropathy but not models of peripheral inflammation.

Differential hypertrophy and atrophy among all types of cutaneous innervation in the glabrous skin of the monkey hand during aging and naturally occurring type 2 diabetes

Diabetic neuropathy (DN) is a common severe complication of type 2 diabetes. The symptoms of chronic pain, tingling, and numbness are generally attributed to small fiber dysfunction. However, little is known about the pathology among innervation to distal extremities, where symptoms start earliest and are most severe, and where the innervation density is the highest and includes a wide variety of large fiber sensory endings. Our study assessed the immunochemistry, morphology, and density of the nonvascular innervation in glabrous skin from the hands of aged nondiabetic rhesus monkeys and from age-matched monkeys that had different durations of spontaneously occurring type 2 diabetes. Age-related reductions occurred among all types of innervation, with epidermal C-fiber endings preferentially diminishing earlier than presumptive Adelta-fiber endings. In diabetic monkeys epidermal innervation density diminished faster, became more unevenly distributed, and lost immunodetectable expression of calcitonin gene-related peptide and capsaicin receptors, TrpV1. Pacinian corpuscles also deteriorated. However, during the first few years of hyperglycemia, a surprising hypertrophy occurred among terminal arbors of remaining epidermal endings. Hypertrophy also occurred among Meissner corpuscles and Merkel endings supplied by Abeta fibers. After longer-term hyperglycemia, Meissner corpuscle hypertrophy declined but the number of corpuscles remained higher than in age-matched nondiabetics. However, the diabetic Meissner corpuscles had an abnormal structure and immunochemistry. In contrast, the expanded Merkel innervation was reduced to age-matched nondiabetic levels. These results indicate that transient phases of substantial innervation remodeling occur during the progression of diabetes, with differential increases and decreases occurring among the varieties of innervation.

Muscle inflammation induces a rapid increase in calcitonin gene-related peptide (CGRP) mRNA that temporally relates to CGRP immunoreactivity and nociceptive behavior

Recent data support an important role for calcitonin gene-related peptide (CGRP) in deep tissue nociceptive processing. Using real-time reverse transcriptase polymerase chain reaction (RT-PCR), radioimmunoassay, immunohistochemistry and behavioral testing, we studied the early time course of CGRP mRNA and protein expression as well as nociceptive behavior following muscle inflammation. A rapid and significant increase in CGRP mRNA occurred in the mandibular division (V3) of the ipsilateral trigeminal ganglion at 30 minutes, 4 and 24 h after the injection of complete Freund's adjuvant as an inflammatory agent into rat masseter muscle. No change in mRNA occurred in the ipsilateral ophthalmic and maxillary divisions (V1/V2) or in the contralateral V3. The levels of immunoreactive calcitonin gene-related peptide (iCGRP) in the ipsilateral V3 significantly increased at 1, 4 and 24 h following muscle inflammation. In contrast, no change occurred in iCGRP levels in either the ipsilateral V1/V2 or contralateral V3. When saline was injected into the masseter muscle, the levels of mRNA or iCGRP did not change in the ipsilateral V3 suggesting that the biochemical changes are specific to CFA-induced muscle inflammation. The number of muscle afferent neurons immunoreactive for CGRP was significantly reduced compared with control at 1, 4 and 24 h in the ipsilateral but not in the contralateral trigeminal ganglion following inflammation. This decrease in the ipsilateral ganglion may indicate a loss of intrasomatic CGRP as a result of increased axonal transport away from the neuronal cell body and/or release of CGRP. Behavioral testing showed a reduction in head withdrawal thresholds bilaterally from 30 min through 24 h following muscle inflammation. Thus upregulation of CGRP mRNA and iCGRP levels are temporally related to the development of inflammation and lowered pain thresholds. The present data support the hypothesis that CGRP is upregulated during deep tissue inflammation and suggest that gene transcription is involved in this upregulation.

Localization of SP- and CGRP-immunopositive nerve fibers in the hip joint of patients with painful osteoarthritis and of patients with painless failed total hip arthroplasties

Using immunohistochemical methods we determined the presence of SP- and CGRP-immunopositive nerve fibers in the hip joint of patients with femoral neck fracture (controls, group 1), painful osteoarthritis (group 2), and painless failed total hip arthroplasties (group 3). Immunoreactive nerve fibers were found in the soft tissue of the fossa acetabuli as well as in the subintimal part of the synovial layer in the hip joint capsule of groups 1 and 2. In the capsule of controls the innervation density had a median of 5.7fibers/cm(2) for CGRP-ir and 3.2fibers/cm(2) for SP-ir afferents. In the osteoarthritic group, the density significantly increased to a median of 15.6fibers/cm(2) for CGRP-ir and 8.2fibers/cm(2) for SP-ir neurons (p=0.05). Patients with failed hip arthroplasties completely lacked these neuropeptide containing afferents. Innervation density in the fossa acetabuli of osteoarthritc patients showed a median of 14.1fibers/cm(2) for CGRP-ir and 5.9fibers/cm(2) for SP-ir afferents. From these data we assume that the hip joint capsule and the soft tissue of the fossa acetabuli are important triggers of nociception. This is supported by the fact, that patients with loosened total hip arthroplasties, where we failed to detect SP- and CGRP-immunoreactive fibers, did not feel pain. The upregulation of SP- and CGRP-positive neurons in response to arthritic stages suggests a mechanism involving neuropeptides in the maintenance of a painful degenerative joint disease and in mediating noxious stimuli from the periphery. Furthermore, these findings help to explain clinical observations, such as effectiveness of local therapy to control hip pain with intraarticular injection, synovectomy and denervation procedures.

Deep tissue inflammation upregulates neuropeptides and evokes nociceptive behaviors which are modulated by a neuropeptide antagonist

Promising recent developments in the therapeutic value of neuropeptide antagonists have generated renewed importance in understanding the functional role of neuropeptides in nociception and inflammation. To explore this relationship we examined behavioral changes and primary afferent neuronal plasticity following deep tissue inflammation. One hour following craniofacial muscle inflammation ipsilateral as well as contralateral head withdrawal thresholds and ipsi- and contralateral hindpaw withdrawal thresholds were lowered and remained reduced for 28 days. Elevated levels of calcitonin gene-related peptide (CGRP) within the trigeminal ganglion temporally correlated with this mechanical allodynia. Inflammation also induced an increase in the number of CGRP and substance P (SP)-immunopositive trigeminal ganglion neurons innervating inflamed muscle but did not evoke a shift in the size distribution of peptidergic muscle afferent neurons. Trigeminal proprioceptive muscle afferent neurons situated within the brainstem in the mesencephalic trigeminal nucleus did not express CGRP or SP prior to or following inflammation. Intravenous administration of CGRP receptor antagonist (8-37) two minutes prior to adjuvant injection blocked plasma extravasation and abolished both head and hindlimb mechanical allodynia. Local injection of CGRP antagonist directly into the masseter muscle prior to CFA produced similar, but less pronounced, effects. These findings indicate that unilateral craniofacial muscle inflammation produces mechanical allodynia at distant sites and upregulates CGRP and SP in primary afferent neurons innervating deep tissues. These data further implicate CGRP and SP in deep tissue nociceptive mechanisms and suggest that peptide antagonists may have therapeutic potential for musculoskeletal pain.

A model of organotypic rat spinal slice culture and biolistic transfection to elucidate factors that drive the preprotachykinin-A promoter

The tachykinin substance P (SP) is a neuropeptide that is expressed in some nociceptive primary sensory afferents and in discrete populations of spinal cord neurons. Expression of spinal SP and the preprotachykinin-A (PPT-A) gene that encodes SP exhibits plasticity in response to conditions such as peripheral inflammation but the mechanisms that regulate expression are poorly understood. We have developed a spinal cord organotypic culture system that is suitable for the analysis of PPT-A gene promoter activity following biolistic transfection of recombinant DNA constructs. Spinal cord organotypic slices showed good viability over a 7-day culture period. Immunostaining for phenotypic markers such as NeuN and beta-III tubulin demonstrated preservation of neurons and their structure, although there was evidence of axotomy-induced down-regulation of NeuN in certain neuronal populations. Neurokinin-1 receptor (NK-1R) immunostaining in laminae I and III was similar to that seen in acute slices. Biolistic transfection was used to introduce DNA constructs into neurons of these organotypic cultures. Following transfection with a construct in which expression of enhanced green fluorescent protein (EGFP) is controlled by the PPT-A promoter, we showed that induction of neuronal activity by administration of a forskolin analogue/high K(+) (10 microM/10 mM) for 24 h resulted in a fourfold increase in the number of EGFP-positive cells. Similarly, a twofold increase was obtained after treatment with the NK-1R-specific agonist [Sar(9),Met (O(2))(11)]-substance P (10 microM). These data demonstrate the usefulness of this model to study physiological and pharmacological factors relevant to nociceptive processing that can modulate PPT-A promoter activity.

Role of calcitonin gene-related peptide in the sensitization of dorsal horn neurons to mechanical stimulation after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the sensitization of dorsal horn neurons induced by intradermal injection of capsaicin in rats. Extracellular recordings were made from wide dynamic range (WDR) dorsal horn neurons with receptive fields on the hindpaw in the lumbar enlargement of anesthetized rats. The background activity and responses to brushing, pressing, and pinching the skin were assessed. A postsuperfusion or a presuperfusion of CGRP(8-37) paradigm was followed. When tested 30 min after capsaicin injection, there was an increase in background activity and responses to brush, press, and pinch applied to the receptive field. Superfusion of CGRP(8-37) into the spinal cord at 45 min after capsaicin injection significantly reversed the increased background activity and responses to brush, press, and pinch applied to the receptive field. On the other hand, spinal superfusion of CGRP(8-37) prior to capsaicin injection prevented the increased background activity and responses to brush, press, and pinch of WDR neurons that occurred following capsaicin injection in control experiments. A sensitization of spinal dorsal horn neurons could also be induced by superfusion of the spinal cord with CGRP. The effect could be blocked by CGRP(8-37) dose-dependently. Collectively, these results suggest that CGRP and its receptors are involved in the spinal cord central sensitization induced by intradermal injection of capsaicin.

The contribution of spinal cord neurokinin-1 receptor signaling to pain

Discovery of the occurrence of neurokinin-1 (NK-1) receptor internalization in response to agonist activation has provided researchers with a new tool for studying tachykinin actions. Using the readily observable end point of NK-1 receptor internalization as an activity marker, this observation has allowed for more detailed study of tachykinin systems in vivo and in vitro. What has this technique taught us about tachykinin function and activity in the spinal cord? Here we discuss recent findings, which shed light on the functional relevance of receptor internalization, the regulation of neuropeptide release from primary afferent nociceptors, and the signaling produced by tachykinins during nociception and injury. The potential consequences of these discoveries for the treatment of pain and understanding of the role of tachykinins in nociception are discussed.

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.

Neuropeptide- and tyrosine hydroxylase-immunoreactive nerve fibers in painful Morton's neuromas

We examined the expression of three neuropeptides that have been implicated in nociceptive transmission, and the sympathetic nerve fiber marker tyrosine hydroxylase, in 11 painful human Morton's neuromas, using immunohistochemistry. Antibodies against the neural markers RT97 and PGP 9.5 were used to map the general nerve fiber organization of the neuromas. Four specimens of normal human peripheral nerves were used as controls. Substance P, calcitonin gene-related peptide, and neuropeptide Y immunoreactivities were more pronounced in neuroma tissue than in control nerves. Neuropeptide immunofluorescence was seen both in larger nerve fiber trunks and in masses of disorganized axon profiles dispersed in loose connective tissue. Tyrosine hydroxylase immunoreactivity was present at varying levels of expression in neuroma nerve fiber trunks, in connective tissue nerve fiber bundles, and around some blood vessels. Our findings suggest that neuropeptides are involved in the response to injury in Morton's neuromas and that they could play a role in initiation or modulation of pain. In addition, pain from Morton's neuromas could be influenced by sympathetic nerve fibers.

Effects of the peripherally selective kappa-opioid asimadoline, on substance P and CGRP mRNA expression in chronic arthritis of the rat

We have previously shown that the kappa-opioid agonist, asimadoline, produces time-dependent changes in neuropeptide concentrations in the joints of rats with chronic arthritis. We hypothesized that asimadoline acts on peripheral terminals to modulate substance P (SP) release. To address this hypothesis, here we have examined neuropeptide expression in their source cells in dorsal root ganglia (DRG) that innervate the joint, as well as in non-neuronal tissue, after treatment with asimadoline. We found an increased production of SP and CGRP in untreated chronic arthritic animals which supports our previous finding of increased SP content in the joint. More importantly, the kappa-opioid asimadoline reduced the expression of both SP and calcitonin gene-related peptide-alpha (alpha-CGRP) in DRG cells but had no effect on the very low expression of neuropeptides in non-neuronal tissue. The fact that SP synthesis is attenuated by asimadoline accords with our hypothesis that the increased tissue levels of SP result from kappa-mediated pre-synaptic inhibition of release leading to augmented tissue stores. These in vivo data confirm literature findings that opioids inhibit SP release from peripheral endings of primary afferent fibres.

Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Although opioids can reduce stimulus-evoked efflux of Substance P (SP) from nociceptive primary afferents, the consequences of this reduction on spinal cord nociceptive processing has not been studied. Rather than assaying SP release, in the present study we examined the effect of opioids on two postsynaptic measures of SP release, Fos expression and neurokinin-1 (NK-1) receptor internalization, in the rat. The functional significance of the latter was first established in in vitro studies that showed that SP-induced Ca(2+) mobilization is highly correlated with the magnitude of SP-induced NK-1 receptor internalization in dorsal horn neurons. Using an in vivo analysis, we found that morphine had little effect on noxious stimulus-evoked internalization of the NK-1 receptor in lamina I neurons. However, internalization was reduced when we coadministered morphine with a dose of an NK-1 receptor antagonist that by itself was without effect. Thus, although opioids may modulate SP release, the residual release is sufficient to exert maximal effects on the target NK-1 receptors. Morphine significantly reduced noxious stimulus-induced Fos expression in lamina I, but the Fos inhibition was less pronounced in neurons that expressed the NK-1 receptor. Taken together, these results suggest that opioid analgesia predominantly involves postsynaptic inhibitory mechanisms and/or presynaptic control of non-SP-containing primary afferent nociceptors.

Calcitonin gene-related peptide gene expression in collagen-induced arthritis is differentially regulated in primary afferents and motoneurons: influence of glucocorticoids

Calcitonin gene-related peptide is involved in peripheral and spinal mechanisms of inflammatory pain. In this paper, we used collagen II-induced arthritis in the rat as a model to investigate the influence of chronic arthritic pain on calcitonin gene-related peptide gene expression in sensory and motor pathways. Additionally, we examined the effect of the glucocorticoid drug budesonide on arthritis-induced changes of calcitonin gene-related peptide expression and constitutive calcitonin gene-related peptide expression. Thirteen days after the immunization with native rat collagen type II rats developed a progressive and chronic polyarthritis which was scored with respect to the degree of swelling and/or redness of the paw and ankle joints. Budesonide significantly attenuated the extent of arthritis. Changes in calcitonin gene-related peptide expression were evaluated by semiquantitative in situ hybridization and immunocytochemistry on day 21 post-immunization. In sensory neurons of dorsal root ganglia of arthritic rats, a significant increase in calcitonin gene-related peptide messenger RNA and protein levels was seen. These increases were completely blocked by budesonide. Also in dorsal root ganglia of non-arthritic rats, budesonide had an effect, with reduced calcitonin gene-related peptide messenger RNA levels below constitutive concentrations. Image analysis of calcitonin gene-related peptide immunoreactivity revealed that changes in calcitonin gene-related peptide expression were due to alterations in calcitonin gene-related peptide expression levels rather than to de novo synthesis or changes in the numbers of calcitonin gene-related peptide expressing neurons. In spinal motoneurons of arthritic rats, marked decreases in calcitonin gene-related peptide messenger RNA and protein levels were measured. These reductions were attenuated by budesonide. The changes in calcitonin gene-related peptide expression in motoneurons correlated with the severity of arthritis in the ipsilateral hind paw. Budesonide had no effects on calcitonin gene-related peptide messenger RNA levels in motoneurons of non-arthritic rats. The opposite regulation of calcitonin gene-related peptide gene expression in primary sensory and spinal somatomotor pathways in collagen-induced arthritis suggests that calcitonin gene-related peptide plays a specific role in both chronic inflammatory pain and arthritis-induced motor dysfunction. The sensitivity of constitutive and inflammation-induced sensory calcitonin gene-related peptide expression to budesonide treatment may indicate that the beneficial effects of steroid treatment in inflammation is partly mediated by down-regulation of calcitonin gene-related peptide in sensory neurons involved in neurogenic inflammation.

Serotonin unmasks functional NK-2 receptors in vagal sensory neurones of the guinea-pig

1. The regulation of substance P (SP) responsiveness in acutely isolated nodose neurones from adult guinea-pigs was investigated using standard intracellular recording techniques. 2. In control neurones, SP produced no measurable electrophysiological effects. However, following incubation with serotonin (5-HT, 10 microM), 64% of neurones were depolarized by 10 +/- 0.6 mV (n = 84 of 132 neurones) by SP (100 nM). 5-HT-induced SP responses were inhibited by SR48968 (100 nM, n = 6), a neurokinin 2 (NK-2) receptor antagonist, but were unaffected by CP99,994 and SR142801, NK-1 and NK-3 receptor antagonists (n = 3 each), respectively. 3. 5-HT-induced unmasking of SP responses was maximal within 5 min. Increasing the 5-HT incubation time up to 120 min did not increase the mean response amplitude or the percentage of SP responsive neurones (P = 0.611 and 0.867, respectively). 4. 5-HT-induced unmasking of SP responses was dose dependent (EC50 = 14 nM). A 5-HT3 receptor agonist CPBG (1 microM), mimicked the unmasking effects of 5-HT (n = 10 of 19 neurones), while 5-CT (10 microM), a non-selective 5-HT agonist devoid of action at 5-HT3 receptors, did not (n = 18). ICS205-930 (1 microM), a 5-HT3 receptor antagonist, completely blocked the 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 5. In 68% of the neurones tested, bath-applied 5-HT (10 microM) evoked a 178 +/- 29.5 nM increase in [Ca2+]i (n = 16), which was blocked by nominally zero [Ca2+]o (n = 4) or by ICS205-930 (1 microM, n = 4). Nodose neurones incubated with 5-HT in the presence of nominally zero [Ca2+]o did not respond to SP (n = 12 of 13 neurones) in Locke solution containing normal [Ca2+]o, indicating that the 5-HT-mediated elevation of [Ca2+]i is required for unmasking of SP responses. Calmidazolium (100 nM), a calmodulin inhibitor, inhibited the unmasking effects of 5-HT (n = 5 of 5 neurones). 6. Incubating neurones with the nitric oxide (NO) donors papaNONOate (1 mM, 15-30 min) or SNAP (50 microM, 30-60 min) unmasked depolarizing SP responses in 71% and 45% of the neurones studied, respectively. L-NMMA (30 microM), a NO synthase inhibitor, blocked 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 7. In sum, these results suggest that stimulation of 5-HT3 receptors activates an intracellular signalling cascade that couples calcium-calmodulin and NO activation to NK-2 receptor unmasking in sensory neurones.

Examination of colocalization of calcitonin gene-related peptide- and substance P-like immunoreactivity in the knee joint of the dog

It is generally assumed that the majority of substance P (SP)-containing afferents are also immunoreactive for calcitonin gene-related peptide (CGRP). In order to determine whether this is also the case in articular afferents where the contents of these peptides are low, we carried out a double labeling study using Fast Blue (FB) as a retrograde tracer injected into the center of the knee joint cavity of the dog together with immunohistochemistry for SP and CGRP. After 7-36 days of survival, dorsal root ganglia (DRGs, L4-S1) were removed. Labeled cells were found mainly (94%) in L5 - 6 DRGs, and SP- and CGRP-like immunoreactivity was found in about 17 and 29% of FB-labeled cells, respectively. The coexistence of SP and CGRP was observed in 10.4% of articular afferents and only 62.7% of SP-positive articular neurons contained CGRP, a much lower ratio than in other afferents of the dog such as testicular afferents. Our data suggest that these peptides are not always released together and that they do not always work together in the joint under normal conditions.

CGRP upregulation in dorsal root ganglia and ileal mucosa during Clostridium difficile toxin A-induced enteritis

We have previously reported that pretreatment of rats with capsaicin (an agent that ablates sensory neurons) or CP-96345 (a substance P receptor antagonist) dramatically inhibits fluid secretion and intestinal inflammation in ileal loops exposed to Clostridium difficile toxin A. The aim of this study was to determine whether calcitonin gene-related peptide (CGRP), a neuropeptide also found in sensory afferent neurons, participates in the enterotoxic effects of toxin A. Administration of toxin A was also found to increase CGRP content in dorsal root ganglia and ileal mucosa 60 min after toxin exposure. Furthermore, immunohistochemical studies demonstrated increased neuronal staining for CGRP 2 h after toxin A treatment. Pretreatment of rats with CGRP-(8-37), a specific CGRP antagonist, before instillation of toxin A into ileal loops significantly inhibited toxin-mediated fluid secretion (by 48%), mannitol permeability (by 83%), and histological damage. We conclude that CGRP, like substance P, contributes to the secretory and inflammatory effects of toxin A via increased production of this peptide from intestinal nerves, including primary sensory afferent neurons.

Intrathecal CGRP(8-37) results in a bilateral increase in hindpaw withdrawal latency in rats with a unilateral thermal injury

The present study was performed to explore the effects of intrathecal administration of calcitonin gene-related peptide8-37 (CGRP(8-37)) on the hindpaw withdrawal latency (HWL) to pressure in rats with one thermally injured hindpaw. Furthermore, the interaction of CGRP(8-37)and naloxone was studied. Thermal injury was performed by dipping the left paw into 60 degrees C for 20 s. This induced a significant increase in the volume of the left hindpaw (P<0.001) and significant bilateral decreases of the latency of hindpaw withdrawal response to mechanical stimulation (Left: P<0.001; right: P<0.05). Intrathecal administration of 10, 20 and 40 nmol of CGRP(8-37), but not of 1 or 5 nmol, induced a significant bilateral increase in HWLs (P<0.001). The effect of CGRP(8-37) was partly reversed by intrathecal injection of naloxone at a dose of 32 and 64 microg respectively. Using radioimmunoassay, we found a significant bilateral increase in the concentration of CGRP-like immunoreactivity in the perfusate of both hindpaws 24 h after unilateral thermal injury (left: P< 0.001; right: P< 0.05). There was also an increase in the amount of CGRP-like immunoreactivity in the cerebrospinal fluid (P< 0.001), but not in plasma. The results indicate that CGRP plays a role in the transmission of nociceptive information in the spinal cord of thermally injured rats. Furthermore, our findings suggest that opioids can modulate CGRP-related effects in the spinal cord.

Sensory neuropeptides: their role in inflammation and wound healing

Vasoactive neuropeptides including substance P and calcitonin gene-related peptide (CGRP) are localised in sensory nerves which innervate blood vessels. These are the major vasoactive neuropeptides released from sensory nerve endings and both have been suggested to have roles in inflammatory and cardiovascular disease. The neuropeptides have potent effects on microvascular tone and permeability, which are seen soon after release from perivascular nerves. There is also evidence that neuropeptides can affect various activities of inflammatory cells and that sensory nerves play a role in the recovery of the healthy microcirculation during wound healing phases. This review concentrates on evidence that the neuropeptides substance P, acting via tachykinin NK1 and NK2 receptors, and CGRP, acting via CGRP1 receptors, play a pro-inflammatory role in disease and a beneficial role in wound healing. In addition, results from clinical trials of recently developed neuropeptide antagonists are discussed.

Peptides and neuromas: calcitonin gene-related peptide, substance P, and mast cells in a mechanosensitive human sural neuroma

We examined and compared a mechanosensitive human sural neuroma and a contralateral sural nerve collected simultaneously from a patient involved in a diabetic neuropathy research protocol. Using indirect immunofluorescence staining. we identified a striking difference in the content within axons of two neuropeptides, substance P (SP) and calcitonin, gene-related peptide (CGRP), between the contralateral nerve and the neuroma. Unlike the contralateral nerve, where immunofluorescence was sparse, a significant number of disorganized axon profiles that stained brightly positive for CGRP or SP were identified in the neuroma. In contrast, staining for tyrosine hydroxylase, a label of sympathetic axons, was largely absent except around one large arteriole. The neuroma specimen also contained large numbers of serotonin-containing mast cells, only noted occasionally in the contralateral nerve. The peptide staining and mast cell accumulation in the human neuroma closely resembled changes we have previously observed in an animal neuroma model. Local neuropeptides may play a role in the injury response of peripheral nerve, and may be related to mechanosensitivity.

Tachykinin NK1 receptor antagonist RP67580 attenuates progressive hypersensitivity of flexor reflex during experimental inflammation in rats

We have now examined whether the tachykinin NK1 receptor is involved in mediating progressive hypersensitivity of spinal flexor motoneurons induced by repeated peripheral stimulation of inflamed tissue in decerebrate-spinal rats. The mechanical threshold of spinal flexor motoneurons was significantly decreased, and the touch- and pinch-evoked responses significantly increased, 48 h after intra-plantar injection of 100 microliters complete Freund's adjuvant. The threshold was further progressively decreased and the touch- and pinch-evoked responses increased over the 80 min testing period. Subcutaneous injection of the tachykinin NK1 receptor antagonist RP67580 (2-[1-imino-2-(2-methoxy phenyl) ethyl]-7,7 diphenyl-4 perhydroisoindolone-(3aR,7aR)) (20 min prior to the beginning of the test) at 1 mg and 10 mg/kg significantly attenuated the progressive decrease of mechanical withdrawal threshold, and the progressive increase of the touch- and pinch-evoked responses. The inactive enantiomer RP68651 (2-[1-imino-2-(2-methoxy phenyl) ethyl]-7,7 diphenyl-4 perhydroisoindolone-(3aS,7aS)) at 1 mg and 10 mg/kg had no significant effect. The present results indicate that substance P and its preferred tachykinin NK1 receptor are involved in mediating progressive hypersensitivity during inflammation.

Progressive tactile hypersensitivity: an inflammation-induced incremental increase in the excitability of the spinal cord

Two established phenomena contribute to the generation of post-injury pain hypersensitivity: peripheral sensitization, an increase in transduction sensitivity of high threshold A delta and C-fibre nociceptors, and central sensitization, an increase in excitability of neurones in the spinal cord triggered exclusively by C-fibre inputs. We now describe a novel phenomenon: progressive tactile hypersensitivity, which contributes to a cumulative allodynia during inflammation. Behavioural measurements in conscious intact animals showed that repeated light touch stimuli delivered at 5-min intervals to an inflamed paw, established 48 h earlier by an intra-plantar injection of complete Freund's adjuvant (CFA), resulted in a progressive reduction in the mechanical withdrawal threshold by more than 75%, from its already hypersensitive basal level. This hypersensitive state persisted for several hours after discontinuing the touch stimuli and did not occur in non-inflamed animals. To monitor nociceptive processing and the afferent fibres responsible, we also measured activity in posterior biceps femoris/semitendinosus flexor motor neurones. In non-inflamed decerebrate-spinal rats, the cutaneous mechanical threshold and pinch-evoked activity of these neurones are stable when tested repeatedly at 5-min intervals and are characterised by absent or small responses to low intensity mechanical stimuli or electrical activation of A beta-fibres. In inflamed animals, the spontaneous activity, touch-, pinch- and A beta-afferent-evoked responses of hamstring flexor motor neurones are significantly increased. The flexor reflex becomes, moreover, progressively more sensitized by repetition every 5 min, of standard mechanical stimuli (touch and pinch), that do not modify excitability in control non-inflamed animals. A cumulative increase in A beta-afferent-evoked responses also occurs when the test stimulus only comprises stimulation of the sural nerve at A beta strength (10 Hz, 10 sec), showing that A beta-afferents have the capacity to produce progressive hypersensitivity. Progressive hypersensitivity, measured here as a progressive tactile allodynia after inflammation in either intact or decerebrate-spinal rats, with its gradual build-up and contribution from A beta fibres, is very different from the central sensitization induced by C-fibre stimulation which is characterised by a peak increase in excitability soon after the conditioning input followed by a steady decrement to baseline levels. Progressive hypersensitivity is likely to be the consequence of an alteration in the function and phenotype of afferents innervating inflamed tissue and the pattern of excitation they produce in spinal neurones. The phenomenon may have an important role in the development of inflammatory pain and hypersensitivity.

Intrathecal CGRP8-37-induced bilateral increase in hindpaw withdrawal latency in rats with unilateral inflammation

1. Recent work in our laboratory has demonstrated that intrathecal administration of a selective antagonist of calcitonin gene-related peptide (CGRP), CGRP8-37, increased the hindpaw withdrawal latency (HWL) to thermal stimulation and hindpaw withdrawal threshold (HWT) to pressure in normal rats, and that these effects were more pronounced than in rats with mononeuropathy. 2. The present study was performed to investigate the effects of intrathecal administration of CGRP8-37 on the HWL and HWT in rats with unilateral hindpaw inflammation induced by subcutaneous injection of carrageenin. The effect of naloxone was also studied. 3. Subcutaneous injection of 0.1 ml of carrageenin into the plantar region of the left hindpaw induced a significant increase in the volume of the ipsilateral hindpaw (P < 0.001), and significant bilateral decreases of the HWL to thermal stimulation (ipsilateral: P < 0.001; contralateral: P < 0.01) and HWT to pressure (ipsilateral: P < 0.001; contralateral: P < 0.01). 4. Intrathecal administration of 10 nmol of CGRP8-37, but not of 1 or 5 nmol, induced a significant bilateral increase in the HWL and HWT in rats with experimentally induced inflammation (thermal test: P < 0.001; mechanical test: P < 0.001). 5. The effect of intrathecal administration of 10 nmol CGRP8-37 on HWL and HWT was significantly more pronounced in intact rats than in rats with experimentally induced inflammation (ipsilateral: P < 0.001; contralateral: P < 0.001). 6. The effect of CGRP8-37 on withdrawal responses in the inflamed paw was partly reversed by intrathecal injection of naloxone at a dose of 88 nmol in the thermal (ipsilateral: P < 0.01; contralateral: P = 0.14) and mechanical tests (ipsilateral: P < 0.05; contralateral: P = 0.60). 7. A significant bilateral increase in the concentration of CGRP-like immunoreactivity in the perfusate of both hindpaws was demonstrated 24 h after unilateral injection of carrageenin (ipsilateral: P < 0.001; contralateral: P < 0.05). There was also an increase in the amount of CGRP-like immunoreactivity in the cerebrospinal fluid (P < 0.001), but not in plasma (P = 0.75). 8. The present study demonstrates that acute experimentally-induced unilateral hindpaw inflammation, induces bilateral increases in the amount of CGRP-like immunoreactivity in hindpaw perfusates. Intrathecal administration of CGRP8-37 increased the HWL to thermal stimulation and HWT to pressure bilaterally. 9. The results indicate that CGRP plays a role in the transmission of presumed nociceptive information in the spinal cord of rats with experimentally induced inflammation. Furthermore, our findings suggest that opioids can modulate CGRP-related effects in the spinal cord.

Antinociceptive profiles of non-peptidergic neurokinin1 and neurokinin2 receptor antagonists: a comparison to other classes of antinociceptive agent

This study compared the antinociceptive properties of systemic administration of selective, non-peptidergic antagonists at neurokinin (NK1 and NK2) receptors to those of other classes of antinociceptive agent. (All doses are in mg/kg.) In mice, the NK1 antagonist, CP 99,994, preferentially (inhibitory dose50 (ID50) = 4.4) inhibited the late phase (LP) as compared to the early phase (EP) (16.1) of formalin-induced licking (FIL). A high dose (17.6) elicited ataxia in the rotarod test. Acetic acid-induced writhing was reduced at intermediate doses (10.0) whereas the tail-flick (TF) response to thermal and mechanical stimuli was inhibited only at high doses (22.7 and 17.7, respectively). Modulation of stimulus intensity did not modify the influence of CP 99,994 upon the response to heat. A similar pattern of data was acquired with RP 67,580, although this NK1 antagonist more potently inhibited writhing (2.8). In contrast, RP 68,651, the inactive isomer of RP 67,580, neither reduced the LP of FIL nor modified writhing indicating that these actions of RP 67,580 were stereospecific. Three further NK1 antagonists, SR 140,333, WIN 51,708 and WIN 62,577, likewise inhibited the LP of FIL and failed to modify the TF response at non-ataxic doses. Further, SR 140,333 (0.5) and WIN 51,708 (1.4) were potent ligands in the writhing procedure. The NK2 antagonist, SR 48,966, mimicked NK1 antagonists in preferentially inhibiting the LP (7.7) as compared to the EP (26.9) of FIL. Further, only at doses higher than those evoking ataxia (20.9) did SR 48,968 modify the TF response (36.5 and 32.0 for heat and pressure, respectively). However, it differed to NK1 antagonists in being inactive in the writhing test (> 40.0). In comparison to these NK1 and NK2 antagonists, the mu-opioid agonists (morphine and fentanyl) and kappa-opioid agonists (enadoline and U 69,593) equipotently inhibited all nociceptive responses at doses not provoking ataxia. While the glycine B receptor partial agonist, (+)-HA 966, selectively blocked the LP of FIL and did not evoke ataxia, the NMDA receptor channel blocker, (+)-MK 801, elicited antinociception only at doses close to those provoking ataxia. Finally, the NSAIDs, indomethacin and ibuprofen, the BK2 antagonist, Hoe 140 and the nitric oxide synthase (NOS) inhibitors, L-NAME and 7 nitroindazole, inhibited the LP (but not the EP) of FIL and (except for L-NAME) also reduced writhing: in contrast, they did not evoke ataxia and were inactive in the TF procedures.(ABSTRACT TRUNCATED AT 400 WORDS)

NADPH-diaphorase reactivity in articular afferents of a normal and inflamed knee joint in the cat

The distribution of NADPH-diaphorase was studied in retrogradely labelled dorsal root ganglion cells innervating the knee joint of the cat. A strong staining reaction was found in 7.5 +/- 1.9% (mean +/- S.D. of 9 normal joints and 6393 labelled perikarya) of the articular afferents. An acute inflammation (32 h) significantly increased this proportion to 10.9 +/- 2.2% (mean +/- S.D. of 5 inflamed joints and 3933 labelled perikarya). The diameter distribution of the somata with a positive NADPH-diaphorase reaction ranged from 18 to 46 microns with a maximum at 24-28 microns. These data indicate that a small proportion of knee joint primary afferents may be able to release nitric oxide playing a role in synaptic transmission and in regulatory functions within the peripheral tissue under normal and pathophysiological conditions.