Andy Dray

Bradykinin and inflammatory pain

There is compelling evidence linking bradykinin (BK) with the pathophysiological processes that accompany tissue damage and inflammation, especially the production of pain and hyperalgesia. Several mechanisms have been proposed to account for hyperalgesia including the direct activation of nociceptors as well as sensitization of nociceptors through the production of prostanoids or the release of other mediators. In keeping with this, antagonists of the BK B2 receptor are efficacious analgesic and anti-inflammatory agents in acute inflammatory pain. More recently it has been suggested that when inflammation is prolonged, BK B1 receptors, which are not expressed in healthy tissues to a significant degree, also play an important role in the maintenance of hyperalgesia. This may be one of a number of adaptive mechanisms that occur peripherally and centrally following the prolonged activation of nociceptors during inflammation or injury.

Antinociceptive activity of the bradykinin B1 and B2 receptor antagonists, des-Arg9, [Leu8]-BK and HOE 140, in two models of persistent hyperalgesia in the rat

&NA; There has been recent evidence linking bradykinin (BK) receptors with inflammation. This study has investigated the involvement of BK receptors in two models of persistent inflammatory hyperalgesia in rats. In a Freunds adjuvant‐induced hyperalgesia model and an ultraviolet (UV)‐induced hyperalgesia model in rats the specific B2 antagonist, D‐Arg[Hyp3, Thi5, D‐Tic7, Oic8]‐BK (HOE 140), was either ineffective or weakly active in reversing hyperalgesia. The specific B1 antagonist, des‐Arg9, [Leu8]‐BK, was effective in reversing or preventing the development of hyperalgesia in both Freunds adjuvant‐induced hyperalgesia and UV‐induced hyperalgesia. The B1 agonist, des‐Arg9‐BK, produced a small exacerbation of hyperalgesia in both models. Data suggest that in persistent inflammatory conditions in the rat bradykinin B1 receptors are involved in the accompanying hyperalgesia.

Kinins and kinin receptors in the nervous system

Kinins, including bradykinin and kallidin, are peptides that are produced and act at the site of tissue injury or inflammation. They induce a variety of effects via the activation of specific B1 or B2 receptors that are coupled to a number of biochemical transduction mechanisms. In the periphery the actions of kinins include vasodilatation, increased vascular permeability and the stimulation of immune cells and peptide-containing sensory neurones to induce pain and a number of neuropeptide-induced reflexes. Mechanisms for kinin synthesis are also present in the CNS where kinins are likely to initiate a similar cascade of events, including an increase in blood flow and plasma leakage. Kinins are potent stimulators of neural and neuroglial tissues to induce the synthesis and release of other pro-inflammatory mediators such as prostanoids and cytotoxins (cytokines, free radicals, nitric oxide). These events lead to neural tissue damage as well as long lasting disturbances in blood-brain barrier function. Animal models for CNS trauma and ischaemia show that increases in kinin activity can be reversed either by kinin receptor antagonists or by the inhibition of kinin production. A number of other central actions have been attributed to kinins including an effect on pain signalling, both within the brain (which may be related to vascular headache) and within the spinal dorsal horn where primary afferent nociceptors can be stimulated. Kinins also appear to play a role in cardiovascular regulation especially during chronic spontaneous hypertension. Presently, however, direct evidence is lacking for the release of kinins in pathophysiological conditions of the CNS and it is not known whether spinal or central neurones, other than afferent nerve terminals, are sensitive to kinins. A more detailed examination of the effects of kinins and their central pharmacology is necessary. It is also important to determine whether the inhibition of kinin activity will alleviate CNS inflammation and whether kinin receptor antagonists are useful in pathological conditions of the CNS.

Bradykinin-induced activation of nociceptors: receptor and mechanistic studies on the neonatal rat spinal cord-tail preparation in vitro

1 The effects of bradykinin on nociceptors have been characterized on a preparation of the neonatal rat spinal cord with functionally connected tail maintained in vitro. Administration of bradykinin to the tail activated capsaicin‐sensitive peripheral fibres and evoked a concentration‐dependent (EC50 = 130 nm) depolarization recorded from a spinal ventral root (L3‐L5). 2 The response to bradykinin was unaffected by the peptidase inhibitors, bestatin (0.4 mm), thiorphan (1 μm), phosphoramidon (1 μm) and MERGETPA (10 μm) or by the presence of calcium blocking agents, cadmium (200 μm) and nifedipine (10 μm). 3 Inhibition of cyclo‐oxygenase with indomethacin (1–5 μm), aspirin (1–10 μm) and paracetamol (10–50 μm) consistently attenuated responses to bradykinin. 4 The effect of bradykinin was mimicked by the phorbol ester PDBu, an activator of protein kinase C. The response to bradykinin was attenuated following desensitization to PDBu but desensitization to bradykinin did not induce a cross‐desensitization to PDBu. The protein kinase C inhibitor staurosporine (10–500 nm) consistently attenuated the effects of PDBu and bradykinin. 5 Bradykinin responses were reversibly enhanced by dibutyryl cyclic AMP (100 μm). However dibutyryl cyclic GMP (0.5 mm) and nitroprusside (10 μm) produced prolonged block of responsiveness to bradykinin. Prolonged superfusion with pertussis toxin did not affect responses to bradykinin. 6 The B1‐receptor agonist des Arg9‐bradykinin (10–100 μm) was ineffective alone or after prolonged exposure of the tail to lipopolysaccharide (100 ng ml−1) or epidermal growth factor (100 ng ml−1) to induce B1 receptors. The B1‐receptor antagonist, des Arg9 Leu8‐bradykinin (10 μm) did not attenuate the response to bradykinin. A number of bradykinin B2 antagonists selectively and reversibly attenuated the response to bradykinin. The rank order potency was Hoe 140 > LysLys [Hyp3,Thi5,8,d‐Phe7]‐bradykinin > d‐Arg[Hyp3, Thi5,8, d‐Phe7]‐bradykinin = d‐Arg[Hyp2,Thi5,8, d‐Phe7]‐bradykinin. 7 These data show that bradykinin produces concentration‐dependent activation of peripheral nociceptors in the neonatal rat tail. The responses were unaffected by calcium channel block and were partially dependent on the production of prostanoids. Bradykinin‐evoked responses were consistent with the activation of protein kinase C‐dependent mechanisms. Cyclic GMP‐dependent mechanisms may be involved in bradykinin‐receptor desensitization whereas cyclic‐AMP dependent mechanisms increase fibre excitability and facilitate bradykinin‐induced responses. The effects of bradykinin were mediated by a B2 receptor.

Arthritis and pain. Future targets to control osteoarthritis pain

Clinical presentation of osteoarthritis (OA) is dominated by pain during joint use and at rest. OA pain is caused by aberrant functioning of a pathologically altered nervous system with key mechanistic drivers from peripheral nerves and central pain pathways. This review focuses on symptomatic pain therapy exemplified by molecular targets that alter sensitization and hyperexcitability of the nervous system, for example, opioids and cannabinoids. We highlight opportunities for targeting inflammatory mediators and their key receptors (for example, prostanoids, kinins, cytokines and chemokines), ion channels (for example, NaV1.8, NaV1.7 and CaV2.2) and neurotrophins (for example, nerve growth factor), noting evidence that relates to their participation in OA etiology and treatment. Future neurological treatments of pain appear optimistic but will require the systematic evaluation of emerging opportunities.

In vivo antinociceptive activity of anti-rat mGluR1 and mGluR5 antibodies in rats.

TO examine the specific roles of group I metabotropic glutamate receptors (mGluRs) in nociceptive processing, we examined the effects of intrathecal (i.t.) treatment with antibodies raised against the C-terminals of mGluR1 and mGluR5 in various rat pain models. The effects of anti-mGluR1 IgG and anti-mGluR5 IgG were assessed in a model of persistent pain induced by intrathecal administration of the mGluR1/5 agonist DHPG, as well as in models of heat pain (plantar test), chemical pain (formalin test) and neuropathic pain. DHPG-induced spontaneous nociceptive behaviours (SNB) were significantly attenuated by i.t. treatment with either anti-mGluR1 IgG (30 μg) or anti-mGluR5 IgG (10 and 3 0μg). Neither anti-mGluR1 IgG (30 μg) nor anti-mGluR5 IgG (30 μg) significantly increased response latencies to noxious heat in the plantar test, compared with anti-rat IgG (control IgG). Moreover, neither antibody (3 0μg) significantly reduced formalin pain scores as compared to control IgG. However, i.t. treatment with anti-mGluR1 IgG (30 μg) or anti-mGluR5 IgG (30 μg) significantly reduced cold hyper-sensitivity exhibited 8 days after constriction injury of the sciatic nerve, supporting the contention that group I mGluRs play a role in the development of neuropathic pain. Because these antibodies were effective against neuropathic pain, and not acute heat or chemical noxious stimuli, these results suggest that mGluRs are involved in nociceptive processing in chronic pain states rather than signaling acute noxious stimuli, and that DHPG-induced pain may be mediated by similar mechanisms as neuropathic pain.

Intrathecal nerve growth factor restores opioid effectiveness in an animal model of neuropathic pain

It is without dispute that the treatment of neuropathic pain is an area of largely unmet medical need. Available analgesics, such as morphine, either have minimal effects in neuropathic pain patients, or are not always well tolerated due to concurrent adverse effects. The chronicity of neuropathic pain is thought to be related to many neurochemical changes in the dorsal root ganglia (DRG) and spinal cord, including a reduction in the retrograde transport of nerve growth factor (NGF). In this study, we have determined the ability of chronic intrathecal (i.t.) infusion of NGF to reverse neuropathic pain symptoms and to restore morphines effectiveness in an animal model of neuropathic pain. Seven days after sciatic nerve constriction injury, NGF was administered to the spinal cord by continuous infusion (125 ng/microl/h) via osmotic pumps attached to chronically implanted i.t. catheters. Spinal infusion of NGF did not affect the expression of tactile allodynia or thermal (hot) hyperalgesia in neuropathic rats, although it significantly increased cold water responses frequency at day 14. Following infusion of vehicle, i.t. morphine (20 microg) was ineffective in altering somatosensory thresholds in neuropathic rats. In contrast, morphine substantially attenuated the neuropathy-induced warm and cold hyperalgesia, as well as tactile allodynia, in neuropathic rats chronically infused with i.t. NGF. In addition, we demonstrate that i.t. morphine-induced antinociception was augmented by a cholecystokinin (CCK) antagonist in animals chronically infused with i.t. antibodies directed against NGF. We hypothesize that NGF is critical in maintaining neurochemical homeostasis in the spinal cord of nociceptive neurons, and that supplementation may be beneficial in restoring and/or maintaining opioid analgesia in chronic pain conditions resulting from traumatic nerve injury.

Pro-nociceptive effects of neuromedin U in rat.

The neuropeptide neuromedin U (NMU) has been shown to have significant effects on cardiovascular, gastrointestinal and CNS functions. The peptide was first isolated from the porcine spinal cord and later shown to be present in spinal cords of other species. Little is known about the distribution of neuromedin U receptors (NMURs) in the spinal cord and the spinal action of the peptide. Here we report on the expression of NMURs and a potential role in nociception in the rat spinal cord using a combination of behavioral and electrophysiological studies. Receptor autoradiography showed that NMU-23 binding was restricted to the superficial layers of spinal cord, a region known to be involved in the control of nociception. In situ hybridization analysis indicated the mRNA of NMUR2 was located in the same region (laminae I and IIo) as NMU-23 binding, while the mRNA for NMU receptor 1 was observed in a subpopulation of small diameter neurons of dorsal root ganglia. Intrathecal (i.t.) administration of neuromedin U-23 (0.4-4.0 nmol/10 microl) dose-dependently decreased both the mechanical threshold to von Frey hair stimulation and the withdrawal latency to a noxious thermal stimulus. Mechanical allodynia was observed between 10 and 120 min, peaking at 30 min and heat hyperalgesia was observed 10-30 min after i.t. administration of NMU-23. A similar mechanical allodynia was also observed following i.t. administration of NMU-8 (0.4-4 nmol/10 microl). A significant enhancement of the excitability of flexor reflex was induced by intrathecal administration of NMU-23 (4 nmol/10 microl). Evoked responses to touch and pinch stimuli were increased by 439+/-94% and 188+/-36% (P<0.01, n=6) respectively. The behavioral and electrophysiological data demonstrate, for the first time, a pro-nociceptive action of NMU. The restricted distribution of NMU receptors to a region of the spinal cord involved in nociception suggests that this peptide receptor system may play a role in nociception.

NE-19550 and NE-21610, antinociceptive capsaicin analogues: studies on nociceptive fibres of the neonatal rat tail in vitro

When applied to peripheral fibres in a neonatal rat tail/spinal cord preparation in vitro, capsaicin (0.2-50 microM) induced an activation, selective desensitization and reduced responses to other noxious stimuli (heat, bradykinin). Similar concentrations of the antinociceptive analogues NE-19550 and NE-21610, did not affect peripheral fibre responsiveness but induced cross desensitization to capsaicin. At 500 microM both analogues produced similar effects to capsaicin. Capsaicin analogues may induce analgesia without initial activation of nociceptors.

Antisense oligonucleotide knockdown of mGluR1 alleviates hyperalgesia and allodynia associated with chronic inflammation.

Chronic inflammation induced by injection of complete Freunds adjuvant (CFA) into one hindpaw elicits thermal hyperalgesia and mechanical allodynia in the injected paw. Metabotropic glutamate receptors (mGluRs) have been implicated in dorsal horn neuronal nociceptive responses and pain associated with short-term inflammation. The goal of the present study was to assess the role of mGluR1 in the hyperalgesia and allodynia associated with the CFA model of chronic inflammation. Here we show that antisense (AS) oligonucleotide knockdown of spinal mGluR1 attenuates thermal hyperalgesia and mechanical allodynia in rats injected with CFA in one hindpaw. When intrathecal infusion of mGluR1 AS oligonucleotide (50 microg/day) began prior to CFA injection, mechanical allodynia was attenuated from Days 1 to 8 following CFA injection, whereas heat hyperalgesia was attenuated on Day 1 and then from Days 4 to 8. When intrathecal infusion of mGluR1 AS oligonucleotide was begun 2 days after CFA injection, both mechanical allodynia and heat hyperalgesia were attenuated at all time points following the oligonucleotide infusion. Thus, the present data suggest a role for mGluR1 in persistent inflammatory nociception.