David J. Tracey

Depletion of macrophages reduces axonal degeneration and hyperalgesia following nerve injury

&NA; Inflammatory mechanisms are believed to play an important role in hyperalgesia resulting from nerve injury. Hyperalgesia following nerve injury is temporally linked with Wallerian degeneration and macrophage recruitment, and is reduced in WLD mice, in which Wallerian degeneration is delayed. We sought more direct evidence that macrophages contribute to hyperalgesia and Wallerian degeneration by depleting macrophages with liposomes loaded with dichloromethylene diphosphonate (clodronate, Cl2MDP). Rats were subjected to partial ligation of the sciatic nerve. Intravenous injection of liposome‐encapsulated clodronate reduced the number of macrophages in the injured nerve, alleviated thermal hyperalgesia and protected both myelinated and unmyelinated fibres against degeneration. The results confirm the role of circulating monocytes/macrophages in the development of neuropathic hyperalgesia and Wallerian degeneration due to partial nerve injury. Macrophage depletion immediately after nerve injury could have some clinical potential in prevention of neuropathic pain.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

&NA; Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non‐neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

Hyperalgesia due to nerve injury: role of neutrophils.

The hypothesis that the early inflammatory cell, the neutrophil, contributes to the hyperalgesia resulting from peripheral nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. The extent and time-course of neutrophilic infiltration of the sciatic nerve and innervated paw skin after partial nerve damage was characterized using immunocytochemistry. The number of endoneurial neutrophils was significantly elevated in sections of operated nerve compared to sections of sham-operated nerve for the entire period studied, i.e. up to seven days post-surgery. This considerable elevation in endoneurial neutrophil numbers was only observed at the site of nerve injury. Depletion of circulating neutrophils at the time of nerve injury significantly attenuated the induction of hyperalgesia. However, depletion of circulating neutrophils at day 8 post-injury did not alleviate hyperalgesia after its normal induction. It is concluded that endoneurial accumulation of neutrophils at the site of peripheral nerve injury is important in the early genesis of the resultant hyperalgesia. The findings support the notion that a neuroimmune interaction occurs as a result of peripheral nerve injury and is important in the subsequent development of neuropathic pain.

Hyperalgesia due to nerve damage: role of nerve growth factor.

The hypothesis that nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) contribute to hyperalgesia resulting from nerve damage was tested in rats in which the sciatic nerve was partially transected on one side. Administration of antisera raised against NGF and BDNF relieved mechanical and thermal hyperalgesia in these animals. It has been suggested that NGF may elicit hyperalgesia by inducing mast cells to release algesic agents such as serotonin (5-HT). We found that degranulation of mast cells with compound 48/80 relieved mechanical and thermal hyperalgesia produced by nerve damage. We also found that local injection of the 5-HT2A and 5-HT3 receptor antagonists ketanserin and ICS 205-930 into the affected hind paw relieved mechanical hyperalgesia in a dose-dependent fashion. These findings support the idea that in this rat model of hyperalgesia due to peripheral nerve damage, NGF acts on mast cells to induce release of 5-HT, which sensitizes nociceptors. Hyperalgesia due to nerve injury and hyperalgesia due to inflammation may share some common features.

Hyperalgesia due to nerve injury: role of prostaglandins.

The hypothesis that prostaglandins contribute to hyperalgesia resulting from nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. Subcutaneous injection of indomethacin (a classic inhibitor of cyclo-oxygenase) into the affected hindpaw relieved mechanical hyperalgesia for up to 10 days after injection. Subcutaneous injection of meloxicam or SC-58125 (selective inhibitors of cyclo-oxygenase-2) into the affected hindpaw also relieved mechanical hyperalgesia, but with a shorter time-course. Subcutaneous injection of SC-19220 (an EP1 prostaglandin receptor blocker) into the affected hindpaw produced significant relief of mechanical and thermal hyperalgesia. Comparable injections into the contralateral paw or abdomen had no effect on mechanical or thermal hyperalgesia, suggesting that the effects we observed were local rather than systemic. We conclude that prostaglandins, probably prostaglandin E1 or E2, contribute to the peripheral mechanisms underlying hyperalgesia following nerve injury. These data provide further evidence that inflammatory mediators contribute to neuropathic pain, and may warrant further study of peripherally administered non-steroidal anti-inflammatory drugs as a possible treatment for such pain in patients.

Peripheral hyperalgesia in experimental neuropathy: mediation by α2-adrenoreceptors on post-ganglionic sympathetic terminals

&NA; Rats in which the sciatic nerve is partially transected develop hyperalgesia which is relieved by sympathectomy. We carried out experiments using this model of experimental peripheral neuropathy to examine the peripheral mechanisms underlying sympathetically maintained pain. Subcutaneous injection of noradrenaline (NA) into the affected paw exacerbated the hyperalgesia but had no effect in control animals. Injection of the non‐specific &agr;‐adrenergic blocker phentolamine and the &agr;2‐adrenergic blocker yohimbine significantly relieved the hyperalgesia, while injection of the &agr;1‐adrenergic blocker prazosin had no effect. Peripheral injection of the &agr;2‐adrenergic agonist clonidine had no significant effect, while injection of the &agr;1‐adrenergic agonist phenylephrine produced slight exacerbation of mechanical hyperalgesia. Hyperalgesia was eliminated by peripheral injection of indomethacin into the affected paw. Following a chemical sympathectomy, hyperalgesia was eliminated and injection of NA into the hyperalgesic paw had no effect on pain thresholds. We concluded that NA exacerbates hyperalgesia in this experimental model by acting on &agr;2‐adrenoreceptors which are located on post‐ganglionic sympathetic terminals. Our results are consistent with the proposal (Levine et al. 1986) that activation of these adrenoreceptors brings about an increased release of prostaglandins which sensitises nociceptors.

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.

Peripheral hyperalgesia in experimental neuropathy: exacerbation by neuropeptide Y

Injury of peripheral nerves often results in hyperalgesia (an increased sensitivity to painful stimuli). This hyperalgesia is mediated in part by sympathetic neurotransmitters. We examined the effect of neuropeptide Y (NPY), specific Y1 and Y2 agonists, and an NPY antagonist on peripheral hyperalgesia in rats whose sciatic nerves had been partially transected. NPY and the Y2 agonist, N-acetyl [Leu28,Leu31] NPY 24-36 exacerbated both mechanical and thermal hyperalgesia, while the Y1 agonist, [Leu31, Pro34]NPY relieved thermal hyperalgesia. Mechanical and thermal hyperalgesia were both relieved by alpha-trinositol (PP56), a non-competitive antagonist of the actions of neuropeptide Y. Hyperalgesia was also relieved by surgical sympathectomy, which eliminated the effects of NPY and its agonists. These results suggest that neuropeptide Y contributes to peripheral hyperalgesia by actions at Y2 receptors, which may be located on postganglionic sympathetic terminals.

A preconditioning nerve lesion inhibits mechanical pain hypersensitivity following subsequent neuropathic injury

BackgroundA preconditioning stimulus can trigger a neuroprotective phenotype in the nervous system - a preconditioning nerve lesion causes a significant increase in axonal regeneration, and cerebral preconditioning protects against subsequent ischemia. We hypothesized that a preconditioning nerve lesion induces gene/protein modifications, neuronal changes, and immune activation that may affect pain sensation following subsequent nerve injury. We examined whether a preconditioning lesion affects neuropathic pain and neuroinflammation after peripheral nerve injury.ResultsWe found that a preconditioning crush injury to a terminal branch of the sciatic nerve seven days before partial ligation of the sciatic nerve (PSNL; a model of neuropathic pain) induced a significant attenuation of pain hypersensitivity, particularly mechanical allodynia. A preconditioning lesion of the tibial nerve induced a long-term significant increase in paw-withdrawal threshold to mechanical stimuli and paw-withdrawal latency to thermal stimuli, after PSNL. A preconditioning lesion of the common peroneal induced a smaller but significant short-term increase in paw-withdrawal threshold to mechanical stimuli, after PSNL. There was no difference between preconditioned and unconditioned animals in neuronal damage and macrophage and T-cell infiltration into the dorsal root ganglia (DRGs) or in astrocyte and microglia activation in the spinal dorsal and ventral horns.ConclusionsThese results suggest that prior exposure to a mild nerve lesion protects against adverse effects of subsequent neuropathic injury, and that this conditioning-induced inhibition of pain hypersensitivity is not dependent on neuroinflammation in DRGs and spinal cord. Identifying the underlying mechanisms may have important implications for the understanding of neuropathic pain due to nerve injury.

Pain and endometriosis

Endometriosis is the commonest cause of chronic pelvic pain in women (Fauconnier and Chapron, 2005). It is characterized by the presence of uterine endometrial tissue outside of the uterus, most commonly in the pelvic cavity. The disorder mainly affects women of reproductive age. Symptoms of endometriosis include recurrent painful periods, painful intercourse, painful defecation during menstruation, chronic lower abdominal pain and hypersensitivity, chronic lower back pain and infertility (Farquhar, 2007). For many women, endometriosis has a negative impact on the ability to work, on family relationships and self-esteem (Huntington and Gilmour, 2005). Many women with endometriosis describe a progression of symptoms over their menstrual life, which may include a mix of different pains and abnormal visceral sensations, indicative of viscero-visceral hyperalgesia and suggestive of neuropathic pain (Horowitz, 2007). Current medical treatments for endometriosis include oral contraceptives, progestogens, androgenic agents, gonadotrophin releasing hormone analogues, as well as laparoscopic surgical excision of the endometriotic lesions. However, management of pain in women with endometriosis is currently insufficient for many women. Here we review the involvement of the nervous system, immune cells and inflammatory response, and hormones in endometriosis as well as current practice in pain management. We suggest that