Epinephrine produces a beta-adrenergic receptor-mediated mechanical hyperalgesia and in vitro sensitization of rat nociceptors

Capsaicin-induced reactivation of latent herpes simplex virus type 1 in sensory neurons in culture

Herpes simplex virus type 1 (HSV-1) produces a life-long latent infection in neurons of the peripheral nervous system, primarily in the trigeminal and dorsal root ganglia. Neurons of these ganglia express high levels of the capsaicin receptor, also known as the vanilloid receptor-1 (VR-1). VR-1 is a non-selective ion channel, found on sensory neurons, that primarily fluxes Ca(2+) ions in response to various stimuli, including physiologically acidic conditions, heat greater than 45 degrees C and noxious compounds such as capsaicin. Using an in vitro neuronal model to study HSV-1 latency and reactivation, we found that agonists of the VR-1 channel - capsaicin and heat - resulted in reactivation of latent HSV-1. Capsaicin-induced reactivation of HSV-1 latently infected neurons was dose-dependent. Additionally, activation of VR-1 at its optimal temperature of 46 degrees C caused a significant increase in virus titres, which could be attenuated with the VR-1 antagonist, capsazepine. VR-1 activation that resulted in HSV-1 reactivation was calcium-dependent, since the calcium chelator BAPTA significantly reduced reactivation following treatment with caspsaicin and forskolin. Taken together, these results suggest that activation of the VR-1 channel, often associated with increases in intracellular calcium, results in HSV-1 reactivation in sensory neurons.

Tumor necrosis factor receptor type-1 in sensory neurons contributes to induction of chronic enhancement of inflammatory hyperalgesia in rat

Carrageenan-induced inflammatory pain lasting hours to days produces a protein kinase C epsilon (PKC epsilon )-dependent 'primed' state lasting several weeks, during which time injection of prostaglandin E2 induces hyperalgesia which is markedly enhanced and prolonged compared to PGE2-induced hyperalgesia in normal 'unprimed' rats. In the present study, we demonstrate that while inhibition of prostaglandin synthesis and antagonism of beta2-adrenergic receptors markedly attenuated the hyperalgesia induced by carrageenan, these interventions did not affect hyperalgesic priming. Tumor necrosis factor-alpha (rat recombinant; rrTNFalpha), another mediator of carrageenan-induced inflammation, alone produced hyperalgesia and priming, which were attenuated and prevented, respectively, by intrathecal administration of antisense to PKC epsilon. Inhibition of TNFalpha with thalidomide or a rat polyclonal anti-TNFalpha antibody attenuated carrageenan-induced hyperalgesia and prevented priming. Intrathecal administration of antisense to tumour necrosis factor receptor type-1 (TNFR1) reduced the level of TNFR1 transported toward the peripheral terminals of sensory neurons, and attenuated both carrageenan- and rrTNFalpha-induced priming. Acute hyperalgesia induced by carrageenan or rrTNFalpha remained intact in animals treated with TNFR1 antisense. Our results demonstrate that the generation of the primed state does not require production of hyperalgesia and that TNFalpha, which is generated during acute inflammation, can act on sensory neurons to induce hyperalgesic priming by activating neuronal PKC epsilon.

beta 2-Adrenoceptor regulation of CGRP release from capsaicin-sensitive neurons

Previous studies have suggested that neurotransmitter substances from the sympatho-adrenomedullary system regulate pulpal blood flow (PBF), in part, by the inhibition of vasoactive neuropeptide release from pulpal sensory neurons. However, no study has evaluated the role of beta-adrenoceptors. We evaluated the hypothesis that activation of beta-adrenoceptors inhibits immunoreactive calcitonin gene-related peptide (iCGRP) release from capsaicin-sensitive nociceptive neurons via in vitro superfusion of bovine dental pulp. Either norepinephrine or epinephrine inhibited capsaicin-evoked iCGRP. The norepinephrine effect was blocked by the selective beta(2)-adrenoceptor antagonist, ICI 118,551, but not by pre-treatment with the selective beta(1)-adrenoceptor antagonist, atenolol. In addition, application of albuterol, a selective beta(2)-adrenoceptor agonist, significantly blocked capsaicin-evoked release of iCGRP. Collectively, these studies demonstrate that activation of beta(2)-adrenoceptors in dental pulp significantly reduces exocytosis of neuropeptides from capsaicin-sensitive nociceptors. This effect may have physiologic significance in regulating PBF. Moreover, since capsaicin selectively activates nociceptors, beta(2)-adrenoceptor agonists may have clinical utility as peripherally acting therapeutics for dental pain and inflammation.

Vagal modulation of nociception is mediated by adrenomedullary epinephrine in the rat

Vagal afferent activity modulates mechanical nociceptive threshold and inflammatory mediator-induced hyperalgesia, effects that are mediated by the adrenal medulla. To evaluate the role of epinephrine, the major hormone released from the adrenal medulla, the beta2-adrenergic receptor antagonist ICI 118,551 was chronically administered to vagotomized rats and epinephrine to normal rats. In vagotomized rats, chronic administration of ICI 118,551 markedly attenuated vagotomy-induced enhancement of bradykinin hyperalgesia but had no effect on nociceptive threshold. In normal rats, chronic epinephrine had the opposite effect, enhancing bradykinin hyperalgesia. Like vagotomy-, epinephrine-induced enhancement of hyperalgesia developed slowly, taking 14 days to reach its peak. Vagotomy induced a chronic elevation in plasma concentrations of epinephrine. We suggest that ongoing activity in vagal afferents inhibits the release of epinephrine from the adrenal medulla. Chronically elevated levels of epinephrine, occurring after vagotomy, desensitize peripheral beta2-adrenergic receptors and lead to enhancement of bradykinin hyperalgesia. The ability of prolonged elevated plasma levels of epinephrine to sensitize bradykinin receptors could contribute to chronic generalized pain syndromes.

Protein kinase C stimulates the acid-sensing ion channel ASIC2a via the PDZ domain-containing protein PICK1

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular protons. They are expressed in central and sensory neurons where they are involved in neuromodulation and in pain perception. Recently, the PDZ domain-containing protein PICK1 (protein interacting with C-kinase) has been shown to interact with ASIC1a and ASIC2a, raising the possibility that protein kinase C (PKC) could regulate ASICs. We now show that the amplitude of the ASIC2a current, which was only modestly increased ( approximately +30%) by the PKC activator 1-oleyl-2-acetyl-sn-glycerol (OAG, 50 microm) in the absence of PICK1, was strongly potentiated ( approximately +300%) in the presence of PICK1. This PICK1-dependent regulatory effect was inhibited in the presence of a PKC inhibitory peptide and required the PDZ domain of PICK1 as well as the PDZ-binding domain of ASIC2a. We have also shown the direct PICK1-dependent phosphorylation of ASIC2a by [(32)P]phosphate labeling and immunoprecipitation and identified a major phosphorylation site, (39)TIR, on the N terminus part of ASIC2a. The OAG-induced increase in ASIC2a current amplitude did not involve any change in the unitary conductance of the ASIC2a channel, whether co-expressed with PICK1 or not. These data provide the first demonstration of a regulation of ASICs by protein kinase phosphorylation and its potentiation by the partner protein PICK1.

Sympathetic-independent bradykinin mechanical hyperalgesia induced by subdiaphragmatic vagotomy in the rat

Bradykinin-induced mechanical hyperalgesia is sympathetically dependent and B(2)-type bradykinin receptor-mediated in the rat; however, a sympathetically independent component of bradykinin hyperalgesia is shown after subdiaphragmatic vagotomy. We evaluated the mechanism of this bradykinin-induced sympathetic-independent mechanical hyperalgesia. The dose-response curve for bradykinin mechanical hyperalgesia in sympathectomized plus vagotomized rats was similar in magnitude to that for sympathetically dependent bradykinin hyperalgesia in normal rats. Although bradykinin mechanical hyperalgesia was mediated by the B(2)-type bradykinin receptors after sympathectomy plus vagotomy, it had a much more rapid latency to onset. This hyperalgesia was significantly attenuated by inhibition of protein kinase A but not protein kinase C, similar to the hyperalgesia produced by prostaglandin E(2), an agent that directly sensitizes primary afferent nociceptors. However, unlike prostaglandin E(2)-induced mechanical hyperalgesia in normal rats, after sympathectomy plus vagotomy, bradykinin-induced hyperalgesia was not attenuated by inhibition of nitric oxide synthesis. Peripheral administration of a mu opioid agonist, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin, significantly attenuated bradykinin mechanical hyperalgesia after sympathectomy plus vagotomy. These data suggest that after sympathectomy plus subdiaphragmatic vagotomy, bradykinin acts directly on primary afferents to produce mechanical hyperalgesia via a novel protein kinase A-dependent signaling mechanism.

Protein kinase C(alpha) is required for vanilloid receptor 1 activation. Evidence for multiple signaling pathways

Activation of vanilloid receptor (VR1) by protein kinase C (PKC) was investigated in cells ectopically expressing VR1 and primary cultures of dorsal root ganglion neurons. Submicromolar phorbol 12,13-dibutyrate (PDBu), which stimulates PKC, acutely activated Ca(2+) uptake in VR1-expressing cells at pH 5.5, but not at mildly acidic or neutral pH. PDBu was antagonized by bisindolylmaleimide, a PKC inhibitor, and ruthenium red, a VR1 ionophore blocker, but not capsazepine, a vanilloid antagonist indicating that catalytic activity of PKC is required for PDBu activation of VR1 ion conductance, and is independent of the vanilloid site. Chronic PDBu dramatically down-regulated PKC(alpha) in dorsal root ganglion neurons or the VR1 cell lines, whereas only partially influencing PKCbeta, -delta, -epsilon, and -zeta. Loss of PKC(alpha) correlated with loss of response to acute re-challenge with PDBu. Anandamide, a VR1 agonist in acidic conditions, acts additively with PDBu and remains effective after chronic PKC down-regulation. Thus, two independent VR1 activation pathways can be discriminated: (i) direct ligand binding (anandamide, vanilloids) or (ii) extracellular ligands coupled to PKC by intracellular signaling. Experiments in cell lines co-expressing VR1 with different sets of PKC isozymes showed that acute PDBu-induced activation requires PKC(alpha), but not PKC(epsilon). These studies suggest that PKC(alpha) in sensory neurons may elicit or enhance pain during inflammation or ischemia.

Alpha(1)-adrenoceptor-mediated depolarization and beta-mediated hyperpolarization in cultured rat dorsal root ganglion neurones

The mechanism of sympathetic - sensory coupling after nerve injury is still not well understood. We have studied the changes in resting potential and excitability of sensory neurones induced by adrenergic stimulation, using whole-cell and perforated-patch recordings in cultured dorsal root ganglion neurones from normal rats. Adrenaline (1-100 microM) depolarized 18 of 39 neurones (46%) and hyperpolarized seven neurones (18%); excitability was increased and decreased, respectively. Stimulating the neurones with 10 microM phenylephrine (alpha(1)-agonist) induced depolarization and increased excitability, while 10 microM isoprenaline (beta-agonist) induced hyperpolarization and reduced excitability. We conclude that alpha(1)- and beta-receptors have opposing effects on membrane potential and excitability in cultured dorsal root ganglion neurones, and the differing effects of adrenaline can be explained by different degrees of expression of each receptor type.

The role of the sympathetic efferents in endotoxin-induced localized inflammatory hyperalgesia and cytokine upregulation

The sympathetic system (SNS) is considered to be a major component of the neurogenic contribution to inflammation and hyperalgesia. We have investigated the role of the SNS in the local inflammatory pain induced by intraplantar (i.pl) injections of bacterial endotoxin (ET). Treatment of rats with an alpha-adrenoceptor antagonist (phentolamine, 0.25-1 mg/kg, i.p.), a beta-adrenoceptor antagonist (propranolol, 1-10 mg/kg, p.o.) or a sympathetic neuron-blocking agent (guanethedine, 30 mg/kg, s.c.) resulted in a dose-dependent reduction of the thermal hyperalgesia induced by ET. Mechanical hyperalgesia, however, was less sensitive to inhibition by propranolol and guanethedine but significantly inhibited by phentolamine. ET injection produced significant upregulation of tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), IL-6, and nerve growth factor (NGF). Treatment with any one of the three sympatholytics abolished the upregulation of NGF and IL-6, while phentolamine and guanethedine also reversed the upregulation of TNF-alpha. IL-1 beta was resistant to all of the sympatholytic treatments. We conclude that the SNS can contribute to the local inflammation and hyperalgesia following injection of ET. The resistance to sympatholytics shown by IL-1 beta, known to play a key role in the inflammatory cascade, suggests that ET can initiate inflammation and hyperalgesia independently of peripheral and central sympathetic mechanisms.

[Nociceptors and mediators in acute inflammatory pain]

OBJECTIVE

To bring together the most recent data concerning the physiology of nociceptors at a time when there has been significant progress in the understanding of these.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

Nociceptive impulses are generated at the periphery in unmyelinated fibres called nociceptors, the responses of which depend on the tissue environment. Numerous mediators can activate, sensitise or "wake up" nociceptor: kinins (bradykinin, kallidin and their metabolites), pro-inflammatory cytokines (TNF alpha, IL-6, IL-1 beta, IL-8), anti-inflammatory cytokines (IL-4, IL-6, IL-10, IL-12, IL-13), prostanoids (PGE2, PGI2), lipo-oxygenases (leucotrienes such LTB4 or 15-HETE), the "central mediators of the immune response" (NF-kappa B), growth factors such as neurotrophins (NGF, BDNF, NT-3 and NT-4/5), peptides (substance P, CGRP, Neurokinin A), nitric oxide, histamine, serotonin, proteases, excitatory amino acids, adrenergic amines and opioids. The release of neuromediators by primary afferent fibres in the spinal cord may be summarised by successively considering calcium channels, presynaptic receptors, excitatory amino acids and peptides.

CONCLUSION

Sensitisation phenomena are not exclusively peripheral but also central in origin and these are interlinked.

Nociceptor sensitization by extracellular signal-regulated kinases

Inflammatory pain, characterized by a decrease in mechanical nociceptive threshold (hyperalgesia), arises through actions of inflammatory mediators, many of which sensitize primary afferent nociceptors via G-protein-coupled receptors. Two signaling pathways, one involving protein kinase A (PKA) and one involving the epsilon isozyme of protein kinase C (PKCepsilon), have been implicated in primary afferent nociceptor sensitization. Here we describe a third, independent pathway that involves activation of extracellular signal-regulated kinases (ERKs) 1 and 2. Epinephrine, which induces hyperalgesia by direct action at beta(2)-adrenergic receptors on primary afferent nociceptors, stimulated phosphorylation of ERK1/2 in cultured rat dorsal root ganglion cells. This was inhibited by a beta(2)-adrenergic receptor blocker and by an inhibitor of mitogen and extracellular signal-regulated kinase kinase (MEK), which phosphorylates and activates ERK1/2. Inhibitors of G(i/o)-proteins, Ras farnesyltransferases, and MEK decreased epinephrine-induced hyper-algesia. In a similar manner, phosphorylation of ERK1/2 was also decreased by these inhibitors. Local injection of dominant active MEK produced hyperalgesia that was unaffected by PKA or PKCepsilon inhibitors. Conversely, hyperalgesia produced by agents that activate PKA or PKCepsilon was unaffected by MEK inhibitors. We conclude that a Ras-MEK-ERK1/2 cascade acts independent of PKA or PKCepsilon as a novel signaling pathway for the production of inflammatory pain. This pathway may present a target for a new class of analgesic agents.

Sex hormones regulate the contribution of PKCepsilon and PKA signalling in inflammatory pain in the rat

We have evaluated the contribution of differences in second messenger signalling to sex differences in inflammatory pain and its control by sex hormones. In normal male but not female rats, epinephrine-induced mechanical hyperalgesia was antagonized by inhibitors of protein kinase Cepsilon (PKCepsilon), protein kinase A (PKA) and nitric oxide synthetase (NOS). Similarly, in PKCepsilon knockout mice, a contribution of PKCepsilon to epinephrine-dependent mechanical hyperalgesia occurred in males only. In contrast, hyperalgesia induced by prostaglandin E2, in both females and males, was dependent on PKA and NO. In both sexes, inhibitors of mitogen-activated protein kinase/extracellular-signal related kinase kinase (MEK) inhibited epinephrine hyperalgesia. In gonadectomized females, the second messenger contributions to epinephrine hyperalgesia demonstrated the pattern seen in males. Administration of oestrogen to gonadectomized females fully reconstituted the phenotype of the normal female. These data demonstrate gender differences in PKCepsilon, PKA and NO signalling in epinephrine-induced hyperalgesia which are oestrogen dependent and appear to be exerted at the level of the beta-adrenergic receptor or the G-protein to which it is coupled.

Regulation of prostacyclin and prostaglandin E(2) receptor mediated responses in adult rat dorsal root ganglion cells, in vitro

1. Primary cultures of adult rat dorsal root ganglia (DRG) were prepared to examine the properties of prostacyclin (IP) receptors and prostaglandin E(2) (EP) receptors in sensory neurones. 2. IP receptor agonists, cicaprost and iloprost, stimulated adenylyl cyclase activity with EC(50) values of 22 and 28 nM, respectively. Prostaglandin E(1) (PGE(1)) and prostaglandin E(2) (PGE(2)) were 7 fold less potent than cicaprost and iloprost, with PGE(2) displaying a lower maximal response. 3. Adenylyl cyclase activation by iloprost, PGE(1) and PGE(2), but not by forskolin, was highly dependent on DRG cell density. Although the potency of iloprost and PGE(2) for stimulating adenylyl cyclase was unchanged, their maximal responses were significantly increased at low cell density. 4. Both IP and EP(2/4) receptors could be down-regulated by agonist pretreatment, however the presence of cyclo-oxygenase (COX) inhibitors did not prevent this apparent down-regulation of IP and EP(2/4) receptors at high DRG cell densities. 5. Stimulation of adenylyl cyclase by the neuropeptide calcitonin gene-related peptide was also decreased at high DRG cell density, whereas the responses to beta-adrenoceptor agonists were increased at high DRG cell density. 6. Addition of nerve growth factor (NGF), or the addition of anti-neurotrophin antibodies during the 5-day culture of DRG cells, had no effect on IP receptor-mediated responses. 7. These results indicate that G(s)-coupled receptors involved in nociception are regulated in a variable manner in adult rat sensory neurones, and that this cell density-dependent regulation may be agonist-independent for IP and EP(2/4) receptors.

Sodium channels and pain therapy

Researchers have characterized changes in the nervous system that occur in response to tissue injury in order to identify possible targets for novel therapeutic interventions for the treatment of pain. That blockers of voltage-gated sodium channels (VGSCs) are clinically effective for the treatment of pain associated with certain types of tissue injury suggests that these channels constitute such a target. Furthermore, there are changes in biophysical properties, expression, and/or distribution of VGSCs in subpopulations of primary afferent and central nervous system neurons in response to injury that are consistent with a role for VGSCs in the generation and maintenance of pain. Injury-induced changes in four unique VGSCs have been described. However, each of these channels appears to contribute to pain associated with different forms of injury in different ways.

The addition of dilute epinephrine produces equieffectiveness of bupivacaine enantiomers for cutaneous analgesia in the rat

We investigated the effectiveness for cutaneous analgesia of bupivacaine (Bup) stereoisomers in male rats. As a model of infiltration anesthesia, inhibition of a nocifensive reflex by subcutaneous injection of 0.6 mL of different concentrations of R-, S-, and racemic-Bup was evaluated quantitatively by the fraction of times a pinprick failed to evoke a nocifensive motor response. R-Bup was more potent in the extent of block; however, S-Bup had a longer-lasting action at smaller doses. This significant difference was apparent when R-Bup and S-Bup were administered in equipotent doses of 0.06% and 0.075%, respectively. Co-injection of epinephrine (Epi) with these equipotent doses enhanced and prolonged the blocking effects of both Bup stereoisomers, although at dilutions of 1:100,000 to 1:1,000,000 Epi itself induced partial, transient analgesia. At 1:2,000,000 dilution, Epi alone had no analgesic effect; however, when co-injected with the shorter-acting R-Bup (0. 06%), Epi prolonged its blocking effect to equal the duration of block evoked by equipotent S-Bup (0.075%). We conclude R-Bup is more potent for cutaneous analgesia and that the longer duration of block by S-Bup probably originates from vasoconstrictor activity.

IMPLICATIONS

Here we show that the more potent optical R-isomer of bupivacaine (Bup) can be used at a smaller dose (80%) than the S-isomer of Bup to give equal pain relief of a skin prick. Although the analgesia from R-Bup is briefer than that from equipotent S-Bup solutions, the durations become equal when a very dilute solution of the vasoconstrictor epinephrine is mixed with the R-isomer. The resulting vasoconstriction thus reduces vascular drug uptake and peak blood levels of systemic drug, reducing potential toxicity.

Chronic hypersensitivity for inflammatory nociceptor sensitization mediated by the epsilon isozyme of protein kinase C

We have identified a mechanism, mediated by the epsilon isozyme of protein kinase C (PKCepsilon) in peripheral neurons, which may have a role in chronic inflammatory pain. Acute inflammation, produced by carrageenan injection in the rat hindpaw, produced mechanical hyperalgesia that resolved by 72 hr. However, for up to 3 weeks after carrageenan, injection of the inflammatory mediators prostaglandin E(2) or 5-hydroxytryptamine or of an adenosine A(2) agonist into the same site induced a markedly prolonged hyperalgesia (>24 hr compared with 5 hr or less in control rats not pretreated with carrageenan). A nonselective inhibitor of several PKC isozymes and a selective PKCepsilon inhibitor antagonized this prolonged hyperalgesic response equally. Acute carrageenan hyperalgesia could be inhibited by PKA or PKG antagonists. However, these antagonists did not inhibit development of the hypersensitivity to inflammatory mediators. Our findings indicate that different second messenger pathways underlie acute and prolonged inflammatory pain.

Neuronal plasticity: increasing the gain in pain

We describe those sensations that are unpleasant, intense, or distressing as painful. Pain is not homogeneous, however, and comprises three categories: physiological, inflammatory, and neuropathic pain. Multiple mechanisms contribute, each of which is subject to or an expression of neural plasticity-the capacity of neurons to change their function, chemical profile, or structure. Here, we develop a conceptual framework for the contribution of plasticity in primary sensory and dorsal horn neurons to the pathogenesis of pain, identifying distinct forms of plasticity, which we term activation, modulation, and modification, that by increasing gain, elicit pain hypersensitivity.

A novel nociceptor signaling pathway revealed in protein kinase C epsilon mutant mice

There is great interest in discovering new targets for pain therapy since current methods of analgesia are often only partially successful. Although protein kinase C (PKC) enhances nociceptor function, it is not known which PKC isozymes contribute. Here, we show that epinephrine-induced mechanical and thermal hyperalgesia and acetic acid-associated hyperalgesia are markedly attenuated in PKCepsilon mutant mice, but baseline nociceptive thresholds are normal. Moreover, epinephrine-, carrageenan-, and nerve growth factor- (NGF-) induced hyperalgesia in normal rats, and epinephrine-induced enhancement of tetrodotoxin-resistant Na+ current (TTX-R I(Na)) in cultured rat dorsal root ganglion (DRG) neurons, are inhibited by a PKCepsilon-selective inhibitor peptide. Our findings indicate that PKCepsilon regulates nociceptor function and suggest that PKCepsilon inhibitors could prove useful in the treatment of pain.

The primary afferent nociceptor as pattern generator

One of the most important advances in our understanding of the pain experience was the introduction of the 'gate control' theory which stimulated analysis of activity pattern in nociceptive pathways and its modulation. Advances in cellular and molecular biology have recently begun to provide detailed information on the mechanisms of stimulus transduction within primary afferent nociceptors as well as mechanisms that modulate the transduction process. From these new insights into the sensory physiology of the nociceptive nerve ending emerges a concept of the primary afferent as the first site of pattern generation in the nociceptive pathway, in which dynamic tuning of gain in the mosaic of inputs to individual primary afferents occurs. The electrical properties of the nociceptor membrane that converts the generator potential to a pattern of action potentials is also actively adjusted. Our present understanding of the intracellular mechanisms that modulate the pattern of activity in nociceptive primary afferents is summarized, and implications for future efforts to unravel the meaning of patterning in nociceptor activity are discussed.