Further substantiation of a significant role for the sympathetic nervous system in inflammation

Mechanisms of referred visceral pain: uterine inflammation in the adult virgin rat results in neurogenic plasma extravasation in the skin

The purpose of this study was to investigate the mechanisms of referred pain observed in female patients with pain from the reproductive organs. We developed a model of inflammatory uterine pain in the rat. Inflammation of the uterus in rats pretreated with Evans Blue Dye resulted in dye extravasation in the skin over the abdomen, groin, lower back, thighs, perineal area and proximal tail, thus providing for the first time evidence for the trophic changes observed in the area of referred visceral pain in an animal model of uterine pain. The neuronal pathways mediating the observed dye extravasation in the skin after uterine inflammation may include dichotomizing afferent fibers, afferent-afferent interactions via a spinal cord pathway or a sympathetic reflex. This model will allow to gain further insight into the mechanisms of referred pain and the trophic changes observed in the area of referred pain in visceral disease.

Neural connection between the ventral portion of the lumbar intervertebral disc and the groin skin

This study was designed to investigate neural mechanisms of referred pain in lumbar intervertebral disc lesions. Patients with a degenerative disc in lower lumbar segments occasionally complain of groin pain, which cannot be explained anatomically as having a radicular origin. In rats pretreated with intravenous application of Evans blue dye, the dye extravasation appeared in the groin skin after application of capsaicin to the ventral portion of the L5-6 intervertebral disc. This response occurred even in rats with a sectioned L-5 spinal nerve and sympathetic trunks, but did not occur in rats with a sectioned genitofemoral nerve. Capsaicin topically applied to the sciatic nerve did not cause dye extravasation in the hindpaw. Therefore, groin dye extravasation was not due to a direct effect of capsaicin but, rather, presumably was caused by an "antidromic axon reflex" of dichotomizing C fibers or to a segmental sympathetic reflex causing vascular permeability. The present results indicate that the ventral portion of the lumbar discs is neurally connected to the groin skin via the upper (L-2) lumbar spinal nerves in rats. Groin pain coincident with low-back pain may be explained as referred pain, indicating that a lesion is present in the ventral portion of the lumbar intervertebral disc space.

Contribution of peripheral alpha 1A-adrenoceptors to pain induced by formalin or by alpha-methyl-5-hydroxytryptamine plus noradrenaline

We examined the peripheral adrenergic mechanisms involved in pain induced by alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT) plus (+/-)-noradrenaline or prostaglandin E2 and by intraplantar formalin. Agents were injected s.c. into the plantar surface of rats' paws, and the paw lifting and licking response scored. Pain produced by alpha-methyl-5-HT (10 micrograms) plus noradrenaline (10 micrograms) was blocked by pretreatment with the alpha-adrenoceptor antagonists, phentolamine (10 micrograms) and prazocin HCl (alpha 1; 40 micrograms), but not by timolol (beta; 10 micrograms) or idazoxan (alpha 2; 40 micrograms). Phenylepherine, but not clonidine, substituted for noradrenaline to induce pain when combined with alpha-methyl-5-HT. The alpha 1A-adrenoceptor antagonist, WB-4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane HCl), but not the alpha 1B- adrenoceptor antagonist, chloroethylclonidine, also blocked the pain response produced by alpha-methyl-5-HT plus noradrenaline. Neither of these agents altered pain produced by alpha-methyl-5-HT plus prostaglandin E2 (0.1 microgram). Formalin-induced pain (1%, 50 microliters) was biphasic, and timolol increased the first phase response. The second phase was attenuated by 40% by phentolamine (10 micrograms) injected 10 min before formalin or at the beginning of the second phase; 30 micrograms did not produce a larger effect. Prazosin and WP-4101, but not idazoxan or chloroethylclonidine, also attenuated the second phase. Thus, activation of alpha 1A-adrenoceptors can contribute to pain, but pain induced by alpha-methyl-5-HT plus prostaglandin E2 is independent of adrenergic function, indicating that adrenergic function is not necessary for induction of pain by inflammatory mediators. alpha 1A-Adrenoceptor blockade attenuates pain when administered after development of pain, implying that peripheral adrenergic mechanisms contribute to ongoing maintenance of pain.

Investigation of 6-hydroxydopamine-induced plasma extravasation in rat skin

Perfusion of 6-hydroxydopamine into the rat knee and trachea induces plasma extravasation, possibly by tissue-specific mechanisms involving sympathetic and sensory nerves respectively, and we aimed to identify the mediators which contribute to this response in skin. 6-Hydroxydopamine (both hydrobromide and hydrochloride salts), dose dependently increased plasma extravasation into rat dorsal skin, however, when compared to bradykinin or the tachykinin NK1 receptor agonist GR73632, high concentrations of 6-hydroxydopamine (1-10 mumol/site) were required. The response to 6-hydroxydopamine was not inhibited in chemically sympathectomised rats (6-hydroxydopamine, 300 mg/kg i.p. over 7 days) but was significantly reduced by co-administration with the histamine (H1) and the 5-HT receptor antagonists mepyramine and methysergide and in skin sites pre-injected with compound 48/80 (4 micrograms, -18 h) to degranulate dermal mast cells. The response was not inhibited by co-injection of the tachykinin NK1 receptor antagonist SRI40333 or by the cyclo-oxygenase inhibitor indomethacin (5 mg kg-1 i.p., -30 min) except at the lowest dose of 6-hydroxydopamine (1 mumol/site). We conclude that 6-hydroxydopamine is not a potent or selective mediator of increased vascular permeability in rat skin but, at high concentrations, may induce oedema formation via release of vasoactive amines from mast cells, augmented by generation of prostaglandins.

Phenotypic modification of primary sensory neurons: the role of nerve growth factor in the production of persistent pain

Inflammation results in an early and maintained elevation in nerve growth factor (NGF) levels in inflamed tissues. Neutralizing the action of the increased NGF with specific anti-NGF antibodies substantially diminishes inflammatory hypersensitivity, indicating that this neurotrophin is a key mediator in the production of inflammatory pain. The hyperalgesic actions of NGF may in part be the consequence of an increase in sensitivity of the peripheral terminals of high threshold nociceptors either as a result of a direct action of NGF on trkA expressing sensory fibres or indirectly via the release of sensitizing mediators from trkA expressing inflammatory cells and postganglionic sympathetic neurons. NGF is also, however, retrogradely transported in sensory neurons to the dorsal root ganglion where it alters transcription of a number of proteins and peptides. This chapter reviews evidence suggesting that an NGF-mediated modification of gene expression in the dorsal root ganglion during inflammation is central to the pathophysiology of persistent pain. The phenotype changes produced by NGF during inflammation include elevation of neuropeptides which may amplify sensory input signals in the spinal cord and augment neurogenic inflammation in the periphery and the upregulation of growth related molecules which may lead to a hyperinnervation of injured tissue by promoting terminal sprouting.

Pain due to nerve damage: are inflammatory mediators involved?

Damage to peripheral nerves often results in pain and hyperalgesia. We suggest that nerve damage causes an inflammatory response in which cells associated with the nerve release inflammatory mediators such as eicosanoids; these mediators may contribute to the hyperalgesia which results from nerve injury. The cell types most likely to be responsible include macrophages and postganglionic sympathetic neurones. A better understanding of the mechanisms involved should lead to improved therapies for neuropathic pain.

Mechanism of bradykinin-induced plasma extravasation in the rat knee joint

1. We have investigated the mechanism of bradykinin (BK)-induced plasma extravasation into the knee joint of the anaesthetized rat. Accumulation of [125I]-human serum albumin within the synovial cavity was used as a marker of increased vascular permeability. 2. Perfusion with BK (1 microM) produced significant plasma extravasation into the knee which was inhibited by co-perfusion of the selective bradykinin B2 receptor antagonist D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-bradykinin (Hoe 140, 200 nM). 3. The bradykinin B1 receptor agonist, [des-Arg9]-BK (up to 100 mM), did not induce plasma extravasation into the knee joint, over this time period. 4. Chemical sympathectomy by chronically administered 6-hydroxydopamine (6-OHDA) did not inhibit bradykinin-induced plasma extravasation. Acute intra-articular perfusion with 6-OHDA (to stimulate transmitter release from sympathetic nerve terminals) at concentrations up to 50 mM did not induce significant plasma extravasation. Intra-articular perfusion of 100 mM 6-OHDA induced significant plasma extravasation but produced severe systemic toxicity. 5. The selective neurokinin1 (NK1) receptor antagonist, RP67580 (230 nmol kg-1), or receptor antagonists for the mast cell products histamine and 5-hydroxytryptamine did not significantly inhibit BK-induced plasma extravasation. 6. Co-perfusion of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) (1 mM) did not significantly inhibit the response to BK. 133Xe clearance from L-NAME (1 mM)-injected joints was significantly (P < 0.05) reduced compared to D-NAME injected joints, suggesting a reduction in blood flow as a result of decreased basal NO production. Systemic administration of L-NAME at doses sufficient to produce significant and sustained elevation of blood pressure (5 or 30 mg kg-1, i.v. 15 min prior to BK perfusion) also failed to significantly inhibit the BK-induced response.7 We conclude that, in normal joints, BK induces plasma extravasation by acting on bradykinin B2 receptors and that this response is not dependent on secondary release of mediators from sympathetic nerve terminals, sensory nerves, mast cells or on generation of NO.

The involvement of sympathetic nerves in plasma extravasation induced by prostaglandin E2 and substance P

The effects of intravenous injection of prostaglandin E2 (PGE2), substance P (SP) and a metabolically stable SP analogue, [pGlu5,Me-Phe8,Sar9]-SP (5-11) on plasma extravasation of albumin in the rat after blockade of prostaglandin synthesis with indomethacin or chemical sympathectomy with guanethidine were studied. Blood pressure was decreased by all agonists, but only the hypotensive effects of SP were enhanced by pretreatment with indomethacin and guanethidine. The increase in plasma extravasation induced by PGE2 in the tongue, skin and lungs was blocked by both guanethidine and indomethacin. Pretreatment of the rats with guanethidine or indomethacin increased extravasation induced by SP in the tongue-tip, dorsal skin and foot, but decreased the enhanced permeability in the pinna, and did not alter the actions of the peptide in other tissues. In contrast, both guanethidine and indomethacin pretreatment increased vascular permeability responses to [pGlu5,Me-Phe8,Sar9]-SP (5-11) administration in 9 and 14 of 16 tissues examined, respectively. Thus, intact sympathetic nerves and functional cycloxygenase activity exert inhibitory constraints on the vascular permeability effects of intravenously administered SP or its analogue. On the other hand the integrity of the sympathetic nerves and prostaglandin synthesis are required for PGE2-induced increases in vascular leak.

Trypsin enhances sympathetic neuron-dependent plasma extravasation in the rat knee joint

Perfusion of 6-hydroxydopamine through the rat knee joint causes an increase in plasma extravasation by activation of sympathetic neuron terminals. Similarly, the increase in plasma extravasation in the rat knee joint produced by the inflammatory mediator bradykinin is dependent on the sympathetic post-ganglion neuron. There is evidence that both 6-hydroxydopamine and bradykinin release a number of mediators, some of which appear to enhance plasma extravasation and some which inhibit it. We attempted to determine the nature of inhibitory factor(s) by co-infusing trypsin (which rapidly cleaves peptides) with 6-hydroxydopamine. We observed a marked enhancement of 6-hydroxydopamine-induced plasma extravasation by trypsin. This effect appeared to be specific to neurogenic plasma extravasation since trypsin alone had little effect on plasma extravasation and trypsin did not affect non-neurogenic plasma extravasation (that produced by platelet activating factor). Taken together, the data suggests that 6-hydroxydopamine not only releases mediators from the sympathetic neuron that produce plasma extravasation, but also an inhibitor(s) of plasma extravasation that is peptide in nature.