The saphenous nerve in foot and ankle surgery: its variable anatomy and relevance.

BACKGROUND: Several studies have raised doubt regarding the role of the saphenous nerve (SN) in the foot, and some authors omit the SN from ankle blocks. Our aim was to assess the SN anatomy with reference to foot and ankle surgery. METHODS: In 29 cadaveric feet the SN was traced to its termination. At the ankle, the distances from the SN to the tibialis anterior tendon (TAT) and the long saphenous vein (LSV) were recorded. RESULTS: In 24 specimens, a SN was present at the ankle, and in 19 specimens extended to the foot. The mean distances from the nerve to the TAT and LSV were 15 mm and 4mm respectively. The nerve reached the first metatarsal (MT) in 28% of specimens. CONCLUSION: Although the SN anatomy is less extensive than previously described, it often reaches the first MT and therefore should routinely be included in ankle blocks for forefoot surgery.

Neuropathic pain: emerging treatments.

Neuropathic pain remains one of the most challenging of all neurological diseases and presents a large unmet need for improved therapies. Many mechanistic details are still lacking, but greater knowledge of overlapping mechanisms and disease comorbidities has highlighted key areas for intervention. These include peripheral and central hyperexcitability. Among the molecular drivers are ion channels (Nav1.7, Nav1.8, Nav1.3, Cav2.2, and alpha2-delta subunits) whose expression is changed during neuropathic pain and their block shows therapeutic utility. Block of a number of ligand-gated channels [transient receptor potential (TRP)V1, TRPM8, and neuronal nicotinic receptors (NNRs)], important in neural sensitization, may also prove beneficial. Other approaches, such as the modulation of peripheral excitability via CB1 receptors, reduction of spinal excitability through block of glutamate receptors (metabotropic glutamate receptor 5 and alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate), block of activated spinal neuroglial (CCR2 and P2X7), or increasing spinal inhibition by enhancing monoaminergic activity, all offer exciting opportunities currently being validated in the clinic. Finally of note is the emergence of biological approaches, for example, antibodies, siRNA, gene therapy, offering powerful therapeutic additions with which to redress the neurological disease imbalances causing neuropathic pain.

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.

Modulation of spinal excitability: co-operation between neurokinin and excitatory amino acid neurotransmitters.

Activation of C fibres with strong 'potentially tissue damaging' chemical, mechanical or thermal stimuli produces painful sensations that are significantly enhanced during pathological conditions, such as neuropathy and inflammation. The pronounced painful symptoms of hyperalgesia and allodynia are induced, in part, by the development of spinal hyperexcitability. This involves plastic changes in synaptic transmission between primary afferents and dorsal horn neurones induced by sustained activity of peripheral nociceptors. L. Urban, S. W. N. Thompson and A. Dray describe some of the central mechanisms that account for central hyperexcitability occurring in hyperalgesia and allodynia based on evidence from experiments both in vivo and in vitro with neurokinin and N-methyl-D-aspartate receptor antagonists.

Pharmacology of chronic pain.

Chronic pain, which is associated with prolonged tissue damage or injuries to the peripheral or central nervous system, results from a number of complex changes in nociceptive pathways. These include alterations of cell phenotype and changes in the expression of proteins such as receptors, transmitters and ion channels, as well as modifications of neural structure, for example, cell loss, nerve regeneration and synaptic reorganizations. The resultant increase in neural excitability can be reduced with receptor-selective drugs that block peripheral or central chemical mediators or that control ectopic activity or cellular phenotype changes. In this article, Andy Dray, Laszlo Urban and Anthony Dickenson focus on some current mechanistic aspects of chronic pain imposed by inflammation and peripheral neuropathy, and review in particular the molecular changes involving the pharmacology of nociceptive pathways since these have important implications for the management of pain.

Ions in the fire: recent ion-channel research and approaches to pain therapy.

Ion channels form a diverse and sophisticated collection of membrane-bound proteins. They are influenced by many endogenous compounds and physiological stimuli and modulate neuronal activity. It is thus not surprising that they provide attractive targets for the design of novel therapeutics. In this article, recent ion channel research and its relevance to modulation of sensory transmission is assessed. In pain research, specific blockade or activation of ion channels has long been considered a desired route for identification of analgesics. Historically, this has proven difficult to attain due to the incidence of side-effects associated with most ion-channel modulators. The recent discovery of several novel classes of ion channels, each of which has a specific distribution and role in sensory processing and nociception, has provided a plethora of targets for pharmaceutical intervention with the promise of an improved therapeutic index.

Immunochemical detection of photoaffinity-labelled capsaicin-binding proteins from sensory neurons.

Capsaicin is a plant neurotoxin which depolarises a subset of mammalian sensory neurons. A photoaffinity probe (4-azidophenylpropionamide) with capsaicin-like agonist activity (EC50 5 microM) has been used to covalently label rat and chick sensory neurons in culture, as well as membrane preparations from both neurons and other tissues. Dorsal root ganglion cell specific capsaicin-binding proteins, including a major band of apparent molecular mass 58,000, have been identified by means of Western blotting, using a specific anti-capsaicin antiserum characterised by radioimmunoassay with a large range of capsaicin congeners. Using the same radioimmunoassay, no endogenous capsaicin-like immunoreactive material in normal or inflamed tissue has, however, been detected.

Cobalt accumulation in neurons expressing ionotropic excitatory amino acid receptors in young rat spinal cord: morphology and distribution.

Excitatory amino acids (EAA) acting on N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate receptors play an important role in synaptic transmission in the spinal cord. Quantitative autoradiography and physiological experiments suggest that NMDA receptors are localized mainly in lamina II while kainate and AMPA receptors are found on both dorsal and ventral horn neurons. However the cell types expressing EAA receptors and their laminar distribution is not known. We have used a cobalt uptake method to study the morphology and distribution of spinal cord neurons expressing AMPA, kainate, or NMDA excitatory amino acid receptors in the lumbar enlargement of the rat spinal cord. The technique involved superfusion of hemisected spinal cords of 14 day-old rat pups in vitro with excitatory amino acid receptor ligands in the presence of CoCl2. Cobalt has been shown to enter cells through ligand-gated ion channels in place of Ca2+. Cells which accumulated cobalt ions following activation by ionotropic excitatory amino acid receptors were visualized histochemically. The cobalt uptake generated receptor-specific labeling of cells, as the NMDA receptor antagonist D-(-)-2-amino-(5)-phosphonovaleric acid (D-AP-5) (20 microM) blocked the NMDA, but not kainate-induced cobalt uptake. The kainate-induced cobalt labeling was reduced by the non-selective excitatory amino acid receptor antagonist kynurenic acid (4 mM). Passive opening of the voltage-gated Ca(2+)-channels by KCl (50 mM) did not result in cobalt uptake, indicating that cobalt enters the cells through ligand-gated Ca(2+)-channels. AMPA (500 microM), kainate (500 microM), or NMDA (500 microM) each induced cobalt uptake with characteristic patterns and distributions of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal perikarya while NMDA-induced uptake was the lowest. AMPA and kainate, but not NMDA superfusion, resulted in cobalt labeling of glial cells. Our results show that the cobalt uptake technique is a useful way to study the morphology and distribution of cells expressing receptors with ligand-gated Ca2+ channels.

Systemic capsaicin and olvanil reduce the acute algogenic and the late inflammatory phase following formalin injection into rodent paw.

Systemic capsaicin and an analogue, olvanil (NE-19550, 4-hydroxy-3-methoxyphenyl methyl-9Z-octadecenamide), were tested for antinociceptive activity in a model of persistent pain produced by the subcutaneous injection of formalin into the rodent hind paw. Formalin induced a biphasic nociceptive response in mice and rats which was measured (a) by the time spent licking the injected paw in mice and (b) by making electrophysiological recordings of single nociceptive neurone discharges in L1-L3 of the spinal dorsal horn of halothane-anaesthetised rats. In mice the initial phase of the response was reduced by systemic administration of morphine, capsaicin and olvanil but not by indomethacin. The second, more prolonged, inflammatory phase of the response was reduced by each agent. In rats, similar concentrations of capsaicin and olvanil reduced both the first and second components of the formalin response. These data show that capsaicin and a non-pungent analogue, olvanil, are efficacious antinociceptive agents in a model of prolonged chemical nociception induced by formalin. Their activity compares favourably with that of morphine and appears superior to that of indomethacin.

Hyperalgesia in rats following intracerebroventricular administration of endotoxin: effect of bradykinin B1 and B2 receptor antagonist treatment.

The present study investigated the development of thermal and mechanical hyperalgesia following intracerebroventricular (i.c.v.) injections of E. coli lipopolysaccharide (LPS). Hind paw withdrawal to von Frey filament stimulation and thermal withdrawal latencies were measured before and up to 24 or 48 h following an i.c.v. injection of LPS (dose range: 0.02--200 micrograms). Thermal and mechanical hyperalgesia were evident by 6 h after LPS injection. LPS-induced hyperalgesia was reversed by the B2 receptor antagonist, HOE 140 (10--30 pmol), when administered i.c.v. but not systemically (0.01--1 mmol/kg, i.v.). Central co-administration of the B1 receptor antagonists, des-Arg9-Leu8 Bk (0.1--1 nmol) or des-Arg10 HOE 140 (0.1--1 nmol) had no effect on thermal or mechanical hyperalgesia. LPS-induced hyperalgesia was also inhibited by indomethacin administered either i.c.v. (10 nmol) or i.v. (1 mumol/kg). These results indicate that administration of endotoxin to the CNS induces the development of hyperalgesia and that this response involves the activity of kinins, via the stimulation of centrally located B2 receptors, and the formation of prostanoids.

A spinal mechanism of action is involved in the antinociception produced by the capsaicin analogue NE 19550 (olvanil).

We have studied the effect of NE 19550 (olvanil, N-(4-hydroxy-3-methoxyphenyl) methyl-9Z-octadecenamide), a capsaicin analogue with approximately equipotent antinociceptive activity in vivo compared with capsaicin, on nociceptive responses recorded from spinal dorsal horn neurones in vivo and from a spinal ventral root in vitro. In adult rats anaesthetized with halothane, antinociceptive doses of olvanil (20-40 mumol/kg, s.c.) reduced C-fibre responses evoked in wide dynamic range, lumbar dorsal horn neurones, by peripheral transcutaneous electrical stimulation. Intradermal injection of olvanil, localized to a discrete region of the peripheral receptive field, did not activate C-fibres nor change C-fibre evoked activation of dorsal horn neurones. Spinal intrathecal administration of olvanil attenuated C-fibre evoked responses and, at the highest concentration, significantly reduced A beta-fibre evoked activity. In the neonatal rat spinal cord/tail preparation maintained in vitro, superfusion of the cord with olvanil (500 nM-5 microM) did not evoke a depolarization but responses to peripheral noxious stimulation were attenuated. In a similar in vitro preparation of the neonatal rat spinal cord, the release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) was measured in spinal cord superfusates. Capsaicin (2-10 microM) evoked a large release of CGRP-LI but olvanil (2-10 microM) produced only a small or undetectable release. Following the administration of each substance, however, the release of CGRP-LI evoked by a depolarizing potassium stimulus was significantly attenuated. These data indicate that C-fibre input to the dorsal horn was attenuated by acute systemic doses of olvanil that were antinociceptive in behavioural tests. This effect was unlikely to be due to impairment of C-fibre function by a peripheral locus of action but was more consistent with an action in the spinal cord in which the reduced release of a neurotransmitter substance from afferent nerve terminals may play a prominent role.

Nerve growth factor induces mechanical allodynia associated with novel A fibre-evoked spinal reflex activity and enhanced neurokinin-1 receptor activation in the rat.

A single dose of nerve growth factor (NGF, 1 microgram/g, i.p.) administered to rats aged between postnatal days (PND) 12 and 14 resulted in a behavioural hypersensitivity of the hindlimb flexion withdrawal reflex to mechanical stimuli which developed 2 h after NGF and remained significant for 24 h. Heat hyperalgesia occurred some 4 h following NGF injection and lasted for 24 h. Isolated spinal cords were prepared from animals treated with NGF and were maintained in vitro for physiological and pharmacological analysis of lumbar spinal reflex activity. Repetitive, low-frequency group I/II A beta-fibre stimulation evoked a novel wind-up response after NGF injection similar to that produced by C-fiber group III/IV stimulation in normal animals. The neurokinin-1 (NK1) receptor antagonist RP67580 reduced the C fiber-evoked responses following NGF treatment but not in naive preparations. The novel A beta fiber-evoked wind-up response was also reduced by RP67580. The NGF-induced changes in NK1 receptor responses occurred in the absence of any detectable changes in either spinal cord NK1 receptor dose-response relationships or NK1 receptor mRNA levels. These findings are likely to be related to the behavioural allodynia observed in the present study and to central excitability changes observed after chronic inflammation where NGF levels are increased.

Mu and delta opioid ligands inhibit reflex contractions of the urinary bladder in the rat by different central mechanisms.

The supraspinal and spinal mechanisms of opioid-induced inhibition of reflex contractions of the urinary bladder were studied in female rats, anesthetized with urethane. A variety of central manipulations was made to distinguish the effects produced by [D-Ala2-Me-Phe4-Gly(ol)5]-enkephalin (DAGO), a selective mu-opioid receptor ligand, from those of the delta ligand [2-D-penicillamine, 5-L-penicillamine]-enkephalin (DPLPE), administered by either intracerebroventricular (i.c.v.) or by spinal intrathecal (i.t.) injection. The effect of intraventricular but not of intrathecal administration of DPLPE was abolished 4-5 hr after the systemic administration of reserpine (5 mg/kg, i.p.). Reserpine did not modify the actions of DAGO, given by either route. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, 200 micrograms, i.c.v.) attenuated the effect of DPLPE given intraventricularly but not when given intrathecally, measured 7 days later. The effect produced by DPLPE given by either route was unchanged by pretreatment with 6-hydroxydopamine (6-OHDA, 150 micrograms i.c.v.). Neither 5,7-DHT nor 6-OHDA altered the effect of administrations of DAGO. The effect of DPLPE given intraventricularly was attenuated or abolished, in a dose-related and reversible manner, following the administration of naloxone or methysergide intrathecally but not by phentolamine, propranolol or atropine. The effect of DAGO given intraventricularly was antagonised by naloxone but not by any of the other antagonists. These observations suggested that the supraspinally- and spinally-mediated inhibition of reflex contractions of the urinary bladder produced by mu or delta receptor ligands can be dissociated. The supraspinal effect of DPLPE involved a descending serotoninergic, but not adrenergic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Meptazinol: unusual in vivo opioid receptor activity at supraspinal and spinal sites.

Systemic (1-10 mg/kg, s.c.), intracerebroventricular (i.c.v. 20-80 micrograms) and spinal intrathecal (i.t., 5-20 micrograms) administration of meptazinol hydrochloride produced dose-related inhibition of reflex contractions of the urinary bladder, recorded isometrically in urethane-anesthetized rats. The effects of meptazinol were reversed by naloxone administered by the same route. Indeed, this was achieved with intracerebroventricular or intrathecal administration of naloxone (2 micrograms), which also selectively antagonized the mu-receptor ligand [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO). However ICI 174,864 (3 micrograms, i.c.v. or i.t.), a delta-opioid receptor antagonist, did not affect the actions of meptazinol given intracerebroventricularly or intrathecally though it consistently abolished the equieffective actions of a selective delta-receptor ligand (2-D-penicillamine, 5-L-penicillamine) enkephalin (DPLPE). Naloxonazine (5 micrograms, i.c.v. or i.t.), an irreversible mu 1-opioid receptor antagonist, produced prolonged antagonism of the effects of DPLPE and meptazinol. The effects of DPLPE partially or completely recovered by 24 hr, indicating that naloxonazine produced prolonged antagonism of delta-opioid receptors. The effects of maptazinol however only recovered after 72 hr, suggesting that antagonism by naloxonazine of this ligand was irreversible and was mediated through a unique opioid receptor interaction. Subthreshold doses of meptazinol (10 micrograms, i.c.v.; 3 micrograms, i.t.) consistently antagonized the effects of morphine given intracerebroventricularly or intrathecally but not the equieffective doses of DPLPE or DAGO. These observations suggest that meptazinol inhibited reflex contractions of the bladder by supraspinal and spinal mu-opioid receptor activation. Furthermore, its agonistic effect and its antagonistic actions were compatible with interactions at a subpopulation of opioid receptors, possibly mu 1-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxonazine and opioid-induced inhibition of reflex urinary bladder contractions.

Spontaneous volume-induced contractions of the urinary bladder were recorded isometrically in urethane-anesthetized rats. Contractions were inhibited by alternate submaximal but equieffective doses of the selective mu and delta-opioid ligands [D-Ala2-Me-Phe4,Gly(ol)5] enkephaline (DAGO) and [2-D-penicillamine, 5-D-penicillamine] enkephalin (DPDPE), respectively, administered by the intracerebroventricular (i.c.v.) or spinal intrathecal (i.t.) route. Naloxonazine, postulated to be an irreversible mu 1-opioid receptor antagonist, administered by the same route, antagonized the effects of both DAGO and DPDPE. The antagonism of the effect of DAGO was reversed 3-4 hr later but that of DPDPE was more prolonged. Recovery of the effect of DPDPE was observed some 24 hr later. A similar pattern of activity against DAGO and DPDPE given intraventricularly or intrathecally was observed following intravenous injection of naloxonazine (10 mg/kg). Also naloxonazine (i.c.v., i.t. or i.v.) antagonized the effect of morphine given intraventricularly or intrathecally, but antagonism was not observed when morphine was retested 3-4 hr and 24 hr later. Naloxonazine increased the frequency of contraction of the bladder after each route of administration. This effect lasted 1-3 hr and was not seen 24 hr later. Systemic administration of naloxone (10 mg/kg, i.v) also increased the frequency of bladder contraction and attenuated or abolished the effect of DAGO given intraventricularly or intrathecally and the delta-receptor agonist [2-D-penicillamine, 5-L-penicillamine] enkephaline (DPLPE).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of bradykinin sensitivity in peripheral sensory fibres of the neonatal rat by nitric oxide and cyclic GMP.

Bradykinin-induced activation of peripheral sensory fibres was studied using an in vitro preparation of the neonatal rat spinal cord with attached tail. Noxious heat stimulation, as well as the applications of bradykinin and capsaicin, to the tail evoked reproducible responses recorded as a depolarization of a lumbar ventral root. Prolonged administration of a supramaximal concentration of bradykinin invariably induced a complete but selective desensitization to a subsequent bradykinin challenge. Bradykinin-induced desensitization was significantly attenuated by concanavalin-A and the effect of concanavalin-A was prevented by alpha-methyl mannoside. Both cyclic GMP and sodium nitroprusside induced a long lasting reduction of bradykinin responsiveness in peripheral fibres. The effect of nitroprusside was prevented by concanavalin-A, and by methylene blue, an inhibitor of guanylyl cyclase. Methylene blue also reduced bradykinin-induced desensitization. L-arginine, but not D-arginine, induced a desensitization to bradykinin. On the other hand, 7-nitroindazole (7-NI, 200-500 nM), an inhibitor of NOS, reduced the desensitization of bradykinin responses but higher concentrations of 7-NI (IC50 = 6.7 +/- 0.9 microM) selectively attenuated responses to bradykinin. The effects of 7-NI were attenuated by L-arginine pretreatment. These data suggest that bradykinin-induced desensitization of peripheral sensory fibres is mediated in part via NO and cyclic GMP dependent mechanisms: possibly NO production is required for guanylate cyclase activation.

A comparison of the release of substance P and some synthetic analogues from micropipettes by microiontophoresis or pressure.

The release of substance P (SP) and two analogues by iontophoresis or pressure from microelectrodes was compared. Substance P was released linearly by iontophoresis from electrodes while no release of the analogues was detected. [N-methylphenylalanine8, N-methylglycine9-] SP5-11 (DiMeC7) and [methyl-2-aminoethyl]11 SP (SP-DAE) were released from electrodes by pressure ejection with linear relationships in all cases between pressure and the amounts released. Under the tested experimental conditions, release of substance P by iontophoresis was between 2 and 3 orders of magnitude less than that by pressure over a given time. The release of substance P and the uncharged analogue DiMeC7 by pressure was very similar while release of SP-DAE was one order of magnitude less.

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 2. The amide bond "B-region".

A series of compounds incorporating replacements for the amide bond "B-region" moiety of capsaicin have been synthesized, including vanillylamides and esters, homovanillic acid amides and esters, ureas, and thioureas. These have been tested in an in vitro assay for agonism (45Ca2+ influx into dorsal root ganglia neurones), which is predictive of analgesic activity, to investigate the requirements in this region of capsaicin for activity. N-(4-Hydroxy-3-methoxybenzyl)-N'-octylthiourea (14a) emerged as the most potent analogue (EC50 = 0.06 microM). An operational model based on multiple hydrogen-bonding interactions is proposed to explain the structure-activity profile observed. In combination with studies on the other regions of the capsaicin molecule these results describe a picture of the molecular interactions of capsaicin with its putative receptor.

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 1. The aromatic "A-region".

A series of analogues of capsaicin, the pungent principle of chilli peppers, was synthesized and tested in assays for capsaicin-like agonism in vitro. The results of these assays were compared with activities in an acute nociceptive model and a correlation was observed which established that the results of these in vitro assays were predictive of analgesia. Using a modular approach the structure-activity profile of specific regions of capsaicin congeners was established using an in vitro assay measuring 45Ca2+ uptake into neonatal rat dorsal root ganglia neurones. Substituted benzylnonanamides 2a-z and N-octyl-substituted phenylacetamides 4a-v were made to test the requirements for activity in the aromatic "A-region" of the molecule. Compounds with the natural substitution pattern (2b and 4c) and the corresponding catechols (2i and 4g) were the most potent, although the catechols were less potent in vivo. Other substitution patterns have reduced activity. These results have established stringent structural requirements for capsaicin-like activity in this part of the molecule.

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 3. The hydrophobic side-chain "C-region".

Structural variants of the hydrophobic side chain ("C region") of the capsaicin molecule have been incorporated into a series of vanillylamides and vanillylthioureas. These compounds have been tested in an in vitro assay for agonism (45Ca2+ influx into dorsal root ganglia neurones), previously shown to be predictive of analgesic activity. The results of this study have established the requirement for a hydrophobic substituent of limited size (molar refractivity, MR, < 55) in order to obtain high potency. Combination of the information gained here about the "C-region" of the capsaicin molecule with the studies described in the preceding two papers provides a rational basis for the design of compounds of increased potency.