Capsaicin-induced sensitization of primate spinothalamic tract cells is prevented by a protein kinase C inhibitor

Quantitative Thermal Testing Profiles as a Predictor of Treatment Response to Topical Capsaicin in Patients with Localized Neuropathic Pain

There are no reliable predictors of response to treatment with capsaicin. Given that capsaicin application causes heat sensation, differences in quantitative thermal testing (QTT) profiles may predict treatment response. The aim of this study was to determine whether different QTT profiles could predict treatment outcomes in patients with localized peripheral neuropathic pain (PeLNP). We obtained from medical records QTT results and treatment outcomes of 55 patients treated between 2010 and 2013. Warm sensation threshold (WST) and heat pain threshold (HPT) values were assessed at baseline at the treatment site and in the asymptomatic, contralateral area. Responders were defined as those who achieved a > 30% decrease in pain lasting > 30 days. Two distinct groups were identified based on differences in QTT profiles. Most patients (27/31; 87.1%) with a homogenous profile were nonresponders. By contrast, more than half of the patients (13/24, 54.2%) with a nonhomogenous profile were responders (p = 0.0028). A nonhomogenous QTT profile appears to be predictive of response to capsaicin. We hypothesize patients with a partial loss of cutaneous nerve fibers or receptors are more likely to respond. By contrast, when severe nerve damage or normal cutaneous sensations are present, the pain is likely due to central sensitization and thus not responsive to capsaicin. Prospective studies with larger patient samples are needed to confirm this hypothesis.

Protein kinase C modulation of thermo-sensitive transient receptor potential channels: Implications for pain signaling

A variety of molecules are reported to be involved in chronic pain. This review outlines the specifics of protein kinase C (PKC), its isoforms and their role in modulating thermo-sensitive transient receptor potential (TRP) channels TRPV1-4, TRPM8, and TRPA1. Anatomically, PKC and thermo-sensitive TRPs are co-expressed in cell bodies of nociceptive dorsal root ganglion (DRG) neurons, which are used as physiological correlates of peripheral and central projections involved in pain transmission. In the past decade, modulation of painful heat-sensitive TRPV1 by PKC has received the most attention. Recently, PKC modulation of other newly discovered thermo-sensitive pain-mediating TRPs has come into focus. Such modulation may occur under conditions of chronic pain resulting from nerve damage or inflammation. Since thermo-TRPs are primary detectors of acute pain stimuli, their modulation by PKC can severely alter their function, resulting in chronic pain. Comprehensive knowledge of pain signaling involving interaction of specific isoforms of PKC with specific thermo-sensitive TRP channels is incomplete. Such information is necessary to dissect out modality specific mechanisms to better manage the complex polymodal nature of chronic pain. This review is an attempt to update the readers on current knowledge of PKC modulation of thermo-sensitive TRPs and highlight implications of such modulation for pain signaling

Complex regulation of capsaicin on intracellular second messengers by calcium dependent and independent mechanisms in primary sensory neurons

Intracellular second messengers play an important role in capsaicin- and analogous-induced sensitization and desensitization in pain. Fluorescence Ca²⁺ imaging, enzyme immunoassay and PKC assay kit were used to determine a novel mechanism of different Ca²⁺ dependency in the signal transduction of capsaicin-induced desensitization. On the average, capsaicin increased cAMP, cGMP concentration and SP release in bell-shaped concentration-dependent manner, with the maximal responses at concentrations around 1 μM, suggesting acute desensitization of TRPV1 receptor activation. Capsaicin-induced intracellular Ca²⁺ concentration ([Ca²⁺](i)) increase depended on extracellular Ca²⁺ influx as an initial trigger. The Ca²⁺ influx by capsaicin increased PKC activation and SP release. These increases were completely abolished in Ca²⁺-free solution, suggesting that the modulation of capsaicin on PKC and SP are Ca²⁺-dependent. Interestingly, the maximal cAMP increase by TRPV1 activation was not blocked Ca²⁺ removal, suggesting at least in part a Ca²⁺-independent pathway is involved. Further study showed that cAMP increase was totally abolished by G-protein and adenylate cyclase (AC) antagonist, suggesting a G-protein-dependent pathway in cAMP increase. However, SP release was blocked by inhibiting PKC, but not G-protein or AC, suggesting a G-protein independent pathway in SP release. These results suggest that both Ca²⁺-dependent and independent mechanisms are involved in the regulation of capsaicin on second messengers systems, which could be a novel mechanism underlying distinct desensitization of capsaicin and might provide additional opportunities in the development of effective analgesics in pain treatment.

PKMζ is essential for spinal plasticity underlying the maintenance of persistent pain

BACKGROUND

Chronic pain occurs when normally protective acute pain becomes pathologically persistent. We examined here whether an isoform of protein kinase C (PKC), PKMζ, that underlies long-term memory storage in various brain regions, also sustains nociceptive plasticity in spinal cord dorsal horn (SCDH) mediating persistent pain.

RESULTS

Cutaneous injury or spinal stimulation produced persistent increases of PKMζ, but not other atypical PKCs in SCDH. Inhibiting spinal PKMζ, but not full-length PKCs, reversed plasticity-dependent persistent painful responses to hind paw formalin and secondary mechanical hypersensitivity and SCDH neuron sensitization after hind paw capsaicin, without affecting peripheral sensitization-dependent primary heat hypersensitivity after hind paw capsaicin. Inhibiting spinal PKMζ, but not full-length PKCs, also reversed mechanical hypersensitivity in the rat hind paw induced by spinal stimulation with intrathecal dihydroxyphenylglycine. Spinal PKMζ inhibition also alleviated allodynia 3 weeks after ischemic injury in rats with chronic post-ischemia pain (CPIP), at a point when allodynia depends on spinal changes. In contrast, spinal PKMζ inhibition did not affect allodynia in rats with chronic contriction injury (CCI) of the sciatic nerve, or CPIP rats early after ischemic injury, when allodynia depends on ongoing peripheral inputs.

CONCLUSIONS

These results suggest spinal PKMζ is essential for the maintenance of persistent pain by sustaining spinal nociceptive plasticity.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Central sensitization in the trigeminal nucleus caudalis produced by a conjugate of substance P and the A subunit of cholera toxin

Individuals with chronic craniofacial pain experience symptoms that are consistent with central sensitization. In fact, central sensitization may constitute the major disease process in these conditions, particularly if the original injury has healed or the condition is idiopathic. To understand central sensitization we have developed a conjugate of substance P and cholera toxin (SP-CTA). SP-CTA is selectively taken up by cells that express neurokinin receptors. Twenty-four hours following intracisternal administration of SP-CTA, wild-type rats and mice demonstrated signs of persistent background nociception, but when tested for facial cold sensitivity, they did not differ from controls. However, treating the SP-CTA-injected animals with naloxone exposed cold hypersensitivity in the face. Mu-opioid receptor knockout mice treated with SP-CTA demonstrated hypersensitivity without naloxone treatment. These findings suggest that central sensitization leads to activation of an endogenous opioid system. The data also demonstrate that the intracisternal administration of SP-CTA in rodents is a useful model for studying central sensitization as a disease process without having to induce a peripheral injury.

PERSPECTIVE

Central sensitization is a concern in many craniofacial pain conditions. In this project, we utilize a conjugate of substance P and the catalytic subunit of cholera toxin to induce central sensitization in the nucleus caudalis of rodents. The data indicate that the injected animals become hypersensitive in the face.

Implication of endogenous beta-endorphin in the inhibition of the morphine-induced rewarding effect by the direct activation of spinal protein kinase C in mice

It has often been proposed that opioid addiction does not arise as a consequence of opioid treatment for pain. Recently, we demonstrated that activated protein kinase C (PKC) in the spinal cord associated with chronic pain-like hyperalgesia suppressed the morphine-induced rewarding effect in mice. In the present study, we investigated whether a gene deletion for an endogenous mu-opioid peptide beta-endorphin could affect pain-like behavior and the suppression of the morphine-induced rewarding effect by the direct activation of PKC in the spinal cord. We found that activation of spinal PKC by intrathecal (i.t.) treatment with phorbol 12,13-dibutyrate (PDBu), a specific PKC activator, caused thermal hyperalgesia, pain-like behaviors and suppression of the morphine-induced rewarding effect. This suppression of morphine reward was eliminated in mice that lacked beta-endorphin. In contrast, thermal hyperalgesia and pain-like behaviors were not affected in beta-endorphin knockout mice. These results suggest that the activation of PKC in the spinal cord may play an essential role in the suppression of the morphine-induced rewarding effect in mice with neuropathic pain through the constant release of beta-endorphin.

Changes in synaptic effectiveness of myelinated joint afferents during capsaicin-induced inflammation of the footpad in the anesthetized cat

The present series of experiments was designed to examine, in the anesthetized cat, the extent to which the synaptic efficacy of knee joint afferents is modified during the state of central sensitization produced by the injection of capsaicin into the hindlimb plantar cushion. We found that the intradermic injection of capsaicin increased the N2 and N3 components of the focal potentials produced by stimulation of intermediate and high threshold myelinated fibers in the posterior articular nerve (PAN), respectively. This facilitation lasted several hours, had about the same time course as the paw inflammation and was more evident for the N2 and N3 potentials recorded within the intermediate zone in the L6 than in the L7 spinal segments. The capsaicin-induced facilitation of the N2 focal potentials, which are assumed to be generated by activation of fibers signaling joint position, suggests that nociception may affect the processing of proprioceptive and somato-sensory information and, probably also, movement. In addition, the increased effectiveness of these afferents could activate, besides neurons in the intermediate region, neurons located in the more superficial layers of the dorsal horn. As a consequence, normal joint movements could produce pain representing a secondary hyperalgesia. The capsaicin-induced increased efficacy of the PAN afferents producing the N3 focal potentials, together with the reduced post-activation depression that follows high frequency autogenetic stimulation of these afferents, could further contribute to the pain sensation from non-inflamed joints during skin inflammation in humans. The persistence, after capsaicin, of the inhibitory effects produced by stimulation of cutaneous nerves innervating non-inflamed skin regions may account for the reported reduction of the articular pain sensations produced by trans-cutaneous stimulation.

PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids

The activity of GABAergic inhibitory interneurones located in lamina II of the spinal cord is of fundamental importance for the processing of peripheral nociceptive messages. We have recently shown that 3alpha-hydroxy ring A-reduced pregnane neurosteroids [3alpha5alpha-neurosteroids (3alpha5alphaNS)], potent allosteric modulators of GABA(A) receptors (GABA(A)Rs), are synthesized in the spinal cord and limit thermal hyperalgesia during inflammatory pain. Because changes in the expression of calcium-dependent protein kinases [protein kinase C (PKC)] are observed during pathological pain in the spinal cord, we examined the possible interactions between PKC and 3alpha5alphaNS at synaptic GABA(A)Rs. Using patch-clamp recordings of lamina II interneurones in the spinal cord of 15-20-day-old rats, we showed that synaptic inhibition mediated by GABA(A)Rs and its modulation by 3alpha5alphaNS in lamina II of the spinal cord largely depend on activation of PKC. Our experimental results suggested that activation of PKC locks synaptic GABA(A)Rs in a functional state precluding further positive allosteric modulation by endogenous and exogenous 3alpha5alphaNS. This effect was fully prevented by coadministration of chelerythrin, an inhibitor of PKC. Furthermore, application of chelerythrin alone rendered synaptic GABA(A)Rs hypersensitive to endogenously produced or exogenously applied 3alpha5alphaNS. These findings confirmed that there was a significant production of endogenous 3alpha5alphaNS in lamina II of the spinal cord but also indicated that PKC-dependent phosphorylation processes were tonically activated to control GABA(A)R-mediated inhibition under resting conditions. We therefore can conclude that PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids.

A critical role of the cAMP sensor Epac in switching protein kinase signalling in prostaglandin E2-induced potentiation of P2X3 receptor currents in inflamed rats

Sensitization of purinergic P2X receptors is one of the mechanisms responsible for exaggerated pain responses to inflammatory injuries. Prostaglandin E2 (PGE2), produced by inflamed tissues, is known to contribute to abnormal pain states. In a previous study, we showed that PGE2 increases fast inactivating ATP currents that are mediated by homomeric P2X3 receptors in dorsal root ganglion (DRG) neurons isolated from normal rats. Protein kinase A (PKA) is the signalling pathway used by PGE2. Little is known about the action of PGE2 on ATP currents after inflammation, although the information is crucial for understanding the mechanisms underlying inflammation-induced sensitization of P2X receptors. We therefore studied the effects of PGE2 on P2X3 receptor-mediated ATP currents in DRG neurons dissociated from complete Freund's adjuvant (CFA)-induced inflamed rats. We found that PGE2 produces a large increase in ATP currents. PKCepsilon, in addition to PKA, becomes involved in the modulatory action of PGE2. Thus, PGE2 signalling switches from a solely PKA-dependent pathway under normal conditions to both PKA- and PKC-dependent pathways after inflammation. Studying the mechanisms underlying the switch, we demonstrated that cAMP-responsive guanine nucleotide exchange factor 1 (Epac1) is up-regulated after inflammation. The Epac agonist CPT-OMe mimics the potentiating effect of PGE2 and occludes the PKC-mediated PGE2 action on ATP currents. These results suggest that Epac plays a critical role in P2X3 sensitization by activation of de novo PKC-dependent signalling of PGE2 after inflammation and would be a useful therapeutic target for pain therapies.

The impact of morphine after a spinal cord injury

Nociceptive stimulation, at an intensity that elicits pain-related behavior, attenuates recovery of locomotor and bladder functions, and increases tissue loss after a contusion injury. These data imply that nociceptive input (e.g., from tissue damage) can enhance the loss of function after injury, and that potential clinical treatments, such as pretreatment with an analgesic, may protect the damaged system from further secondary injury. The current study examined this hypothesis and showed that a potential treatment (morphine) did not have a protective effect. In fact, morphine appeared to exacerbate the effects of nociceptive stimulation. Experiment 1 showed that after spinal cord injury 20mg/kg of systemic morphine was necessary to induce strong antinociception and block behavioral reactivity to shock treatment, a dose that was much higher than that needed for sham controls. In Experiment 2, contused rats were given one of three doses of morphine (Vehicle, 10, 20mg/kg) prior to exposure to uncontrollable electrical stimulation or restraint alone. Despite decreasing nociceptive reactivity, morphine did not attenuate the long-term consequences of shock. Rats treated with morphine and shock had higher mortality rates, and displayed allodynic responses to innocuous sensory stimuli three weeks later. Independent of shock, morphine per se undermined recovery of sensory function. Rats treated with morphine alone also had significantly larger lesions than those treated with saline. These results suggest that nociceptive stimulation affects recovery despite a blockade of pain-elicited behavior. The results are clinically important because they suggest that opiate treatment may adversely affect the recovery of function after injury.

Protein kinase C in pain: involvement of multiple isoforms

Pain is the primary reason that people seek medical care. At present, chronic unremitting pain is the third greatest health problem after heart disease and cancer. Chronic pain is an economic burden in lost wages, lost productivity, medical expenses, legal fees and compensation. Chronic pain is defined as a pain of greater than 2 months duration. It can be of inflammatory or neuropathic origin that can arise following nerve injury or in the absence of any apparent injury. Chronic pain is characterized by an altered pain perception that includes allodynia (a response to a normally non-noxious stimuli) and hyperalgesia (an exaggerated response to a normally noxious stimuli). This type of pain is often insensitive to the traditional analgesics or surgical intervention. The study of the cellular and molecular mechanisms that contribute to chronic pain are of the up-most importance for the development of a new generation of analgesic agents. Protein kinase C isozymes are under investigation as potential therapeutics for the treatment of chronic pain conditions. The anatomical localization of protein kinase C isozymes in both peripheral and central nervous system sites that process pain have made them the topic of basic science research for close to two decades. This review will outline the research to date on the involvement of protein kinase C in pain and analgesia. In addition, this review will try to synthesize these works to begin to develop a comprehensive mechanistic understanding of how protein kinase C may function as a master regulator of the peripheral and central sensitization that underlies many chronic pain conditions.

Protein phosphatase 2A regulates central sensitization in the spinal cord of rats following intradermal injection of capsaicin

BACKGROUND

Intradermal injection of capsaicin into the hind paw of rats induces spinal cord central sensititzation, a process in which the responsiveness of central nociceptive neurons is amplified. In central sensitization, many signal transduction pathways composed of several cascades of intracellular enzymes are involved. As the phosphorylation state of neuronal proteins is strictly controlled and balanced by the opposing activities of protein kinases and phosphatases, the involvement of phosphatases in these events needs to be investigated. This study is designed to determine the influence of serine/threonine protein phosphatase type 2A (PP2A) on the central nociceptive amplification process, which is induced by intradermal injection of capsaicin in rats.

RESULTS

In experiment 1, the expression of PP2A protein in rat spinal cord at different time points following capsaicin or vehicle injection was examined using the Western blot method. In experiment 2, an inhibitor of PP2A (okadaic acid, 20 nM or fostriecin, 30 nM) was injected into the subarachnoid space of the spinal cord, and the spontaneous exploratory activity of the rats before and after capsaicin injection was recorded with an automated photobeam activity system. The results showed that PP2A protein expression in the spinal cord was significantly upregulated following intradermal injection of capsaicin in rats. Capsaicin injection caused a significant decrease in exploratory activity of the rats. Thirty minutes after the injection, this decrease in activity had partly recovered. Infusion of a phosphatase inhibitor into the spinal cord intrathecal space enhanced the central sensitization induced by capsaicin by making the decrease in movement last longer.

CONCLUSION

These findings indicate that PP2A plays an important role in the cellular mechanisms of spinal cord central sensitization induced by intradermal injection of capsaicin in rats, which may have implications in clinical pain therapy.

Activation of protein kinase B/Akt signaling pathway contributes to mechanical hypersensitivity induced by capsaicin

We investigated the involvement of the protein kinase B/Akt (PKB/Akt) signaling pathway in the mechanical hypersensitivity induced in rats by capsaicin. Intradermal injection of capsaicin results in activation of PKB/Akt in the lumbar spinal cord, most prominently in the dorsal horn, starting by 5 min after capsaicin injection and lasting at least 1h. The activated PKB/Akt in the spinal cord is in neurons, since phospho-PKB/Akt (p-PKB/Akt) colocalizes with the neuronal marker, neuronal-specific nuclear protein (NeuN). The mechanical hypersensitivity is shown by the enhanced paw withdrawal frequency to applications of von Frey filaments with different bending forces (30, 100, 200 mN) on the rat paw. Pre-treatment with several different PKB/Akt inhibitors, including SH-6, Akt inhibitor IV, and Akt inhibitor V, blocked the mechanical hypersensitivity induced by intradermal injection of capsaicin, a measure of spinal cord central sensitization. Two structurally unrelated phosphoinositide 3-Kinase (PI3K, upstream of PKB/Akt) inhibitors, Wortmannin and LY294002, also prevented the mechanical hypersensitivity induced by intradermal injection of capsaicin. Furthermore, post-treatment with the PI3K inhibitor, Wortmannin, or PKB/Akt inhibitors, such as NL-71-101, SH-6, Akt inhibitor IV, and inhibitor V significantly reduced the established mechanical hypersensitivity induced by capsaicin. The PKB/Akt signaling pathway in the spinal cord is therefore involved in pain hypersensitivity.

Participation of the L-arginine-nitric oxide-cyclic GMP-ATP-sensitive K+ channel cascade in the antinociceptive effect of rofecoxib

The antinociceptive effect of rofecoxib, a preferential inhibitor of cyclooxygenase-2, was assessed in the pain-induced functional impairment model in the rat. Systemic administration of rofecoxib generated a dose-dependent antinociceptive effect in rats injected with uric acid into the knee joint of the right hindlimb in order to produce nociception. Ipsilateral intra-articular pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, an inhibitor of nitric oxide (NO) synthesis), 1H-(1,2,4)-oxadiazolo (4,2-a)quinoxalin-1-one (ODQ, an inhibitor soluble guanylyl cyclase), and the ATP-sensitive potassium channel blocker glibenclamide reversed the antinociceptive effect of rofecoxib p.o. However, ipsilateral intra-articular pretreatment with L-arginine (a NO substrate), or 3-morpholino-sydnonimine-HCl (SIN-1, a non-enzymatic donor of NO), potentiated the antinociceptive effect induced by rofecoxib. The present results suggest that, in addition to cyclooxygenase-2 inhibition, the antinociceptive effect of rofecoxib could also involve activation of the L-arginine-NO-cyclic GMP (cGMP) pathway, followed by opening of ATP-sensitive K+ channels at the peripheral level.

Long-term potentiation in spinothalamic neurons

Sensitization of nociceptive dorsal horn neurons, including spinothalamic tract (STT) cells, is thought to underlie the development of secondary hyperalgesia and allodynia following tissue injury. In central sensitization, responses to stimulation of sensory receptors are enhanced without any change in the excitability of the primary afferent neurons. We hypothesize that central sensitization of STT neurons is a variety of long-term potentiation (LTP). Evidence that LTP occurs in the spinal cord is reviewed. Neurotransmitters that trigger central sensitization include excitatory amino acids and peptides. Evidence for this is that co-activation of N-methyl-D-aspartate and NK1 receptors can produce long-lasting increases in the responses of STT cells, and antagonists of these receptors prevent central sensitization. Responses to excitatory amino acids increase and those to inhibitory amino acids decrease during central sensitization, presumably accounting for the changed excitability of STT cells. We believe these changes result from the activation of signal transduction pathways, including the protein kinase C, NO/protein kinase G and protein kinase A cascades. Recent evidence shows that calcium/calmodulin dependent kinase II (CaMKII) is also upregulated early in the process of central sensitization and that several types of ionotropic glutamate receptors become phosphorylated. It is proposed that the phosphorylation of neurotransmitter receptors leads to alterations in the sensitivity of these receptors and to central sensitization. Comparable events occur during LTP in brain structures.

Human brain plasticity: an emerging view of the multiple substrates and mechanisms that cause cortical changes and related sensory dysfunctions after injuries of sensory inputs from the body

Injuries of peripheral inputs from the body cause sensory dysfunctions that are thought to be attributable to functional changes in cerebral cortical maps of the body. Prevalent theories propose that these cortical changes are explained by mechanisms that preeminently operate within cortex. This paper reviews findings from humans and other primates that point to a very different explanation, i.e. that injury triggers an immediately initiated, and subsequently continuing, progression of mechanisms that alter substrates at multiple subcortical as well as cortical locations. As part of this progression, peripheral injuries cause surprisingly rapid neurochemical/molecular, functional, and structural changes in peripheral, spinal, and brainstem substrates. Moreover, recent comparisons of extents of subcortical and cortical map changes indicate that initial subcortical changes can be more extensive than cortical changes, and that over time cortical and subcortical extents of change reach new balances. Mechanisms for these changes are ubiquitous in subcortical and cortical substrates and include neurochemical/molecular changes that cause functional alterations of normal excitation and inhibition, atrophy and degeneration of normal substrates, and sprouting of new connections. The result is that injuries that begin in the body become rapidly further embodied in reorganizational make-overs of the entire core of the somatosensory brain, from peripheral sensory neurons to cortex. We suggest that sensory dysfunctions after nerve, root, dorsal column (spinal), and amputation injuries can be viewed as diseases of reorganization in this core.

Group I metabotropic glutamate receptors in spinal cord injury: roles in neuroprotection and the development of chronic central pain

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.

Role of group II and group III metabotropic glutamate receptors in spinal cord injury

Spinal cord injury (SCI) produces an increase in extracellular excitatory amino acid (EAA) concentrations that results in glutamate receptor-mediated excitotoxic events. An important class of these receptors is the metabotropic glutamate receptors (mGluRs). mGluRs can activate a number of intracellular pathways that increase neuronal excitability and modulate neurotransmission. Group I mGluRs are known to modulate EAA release and the development of chronic central pain (CCP) following SCI; however, the role of group II and III mGluRs remains unclear. To begin evaluating group II and III mGluRs in SCI, we administered the specific agonists for group II, APDC, or group III, L-AP4, by interspinal injection immediately following SCI. Contusion injury was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod 2 mm in diameter) in 30 adult male Sprague-Dawley rats (175-200 g). Evoked and spontaneous behavioral measures of CCP, locomotor recovery, changes in mGluR expression, and amount of spared tissue were examined. Neither APDC nor L-AP4 affected locomotor recovery or the development of thermal hyperalgesia; however, L-AP4 and APDC attenuated changes in mechanical thresholds and changes in exploratory behavior indicative of CCP. APDC- and L-AP4-treated groups had higher expression levels of mGluR2/3 at the epicenter of injury on post contusion day 28; however, there was no difference in the amount of spared tissue between treatment groups. These results demonstrate that treatment with agonists to group II and III mGluRs following SCI affects mechanical responses, exploratory behavior, and mGluR2/3 expression without affecting the amount of tissue spared, suggesting that the level of mGluR expression after SCI may modulate nociceptive responses.

Extracellular glutamate, aspartate and arginine increase in the ventral posterolateral thalamic nucleus during nociceptive stimulation

Although there is evidence that the thalamus plays a remarkable role in pain processing few in vivo studies on the thalamic neurochemical correlates of pain have been done. In the present experiments a combination of capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) and microdialysis in freely moving rats was used to measure extracellular arginine, glutamate and aspartate in the thalamus during the formalin test. Microdialysis probes were implanted in the left ventral posterolateral (VPL) nucleus of the thalamus in rats. Samples were collected every 30 s, derivatized with fluorescein isothyocyanate and injected into a CZE-LIF instrument. After nine baseline samples, a subcutaneous formalin (5%, 50 microl) injection in the right hind paw caused an increase of arginine, glutamate and aspartate that lasted for about 3 min. These increases were calcium and nerve impulse dependent. These results indicate that the release of arginine, glutamate and aspartate may mediate rapid pain neural transmission in the VPL nucleus of the thalamus.

Effects of capsaicin on Ca(2+) release from the intracellular Ca(2+) stores in the dorsal root ganglion cells of adult rats

We have investigated the effect of capsaicin on Ca(2+) release from the intracellular calcium stores. Intracellular calcium concentration ([Ca(2+)](i)) was measured in rat dorsal root ganglion (DRG) neurons using microfluorimetry with fura-2 indicator. Brief application of capsaicin (1 microM) elevated [Ca(2+)](i) in Ca(2+)-free solution. Capsaicin-induced [Ca(2+)](i) transient in Ca(2+)-free solution was evoked in a dose-dependent manner. Resiniferatoxin, an analogue of capsaicin, also raised [Ca(2+)](i) in Ca(2+)-free solution. Capsazepine, an antagonist of capsaicin receptor, completely blocked the capsaicin-induced [Ca(2+)](i) transient. Caffeine completely abolished capsaicin-induced [Ca(2+)](i) transient. Dantrolene sodium and ruthenium red, antagonists of the ryanodine receptor, blocked the effect of capsaicin on [Ca(2+)](i). However, capsaicin-induced [Ca(2+)](i) transient was not affected by 2-APB, a membrane-permeable IP(3) receptor antagonist. Furthermore, depletion of IP(3)-sensitive Ca(2+) stores by bradykinin and phospholipase C inhibitors, neomycin, and U-73122, did not block capsaicin-induced [Ca(2+)](i) transient. In conclusion, capsaicin increases [Ca(2+)](i) through Ca(2+) release from ryanodine-sensitive Ca(2+) stores, but not from IP(3)-sensitive Ca(2+) stores in addition to Ca(2+) entry through capsaicin-activated nonselective cation channel in rat DRG neurons.

Strain and model differences in behavioral outcomes after spinal cord injury in rat

Spinal cord injury (SCI) results in loss of function below the level of injury and the development of chronic central pain (CCP) syndromes. Since different strains may develop and express chronic pain behaviors differently, we evaluated behavioral outcomes (locomotor recovery and the development of mechanical and thermal allodynia) in three commonly used strains of rats (Long-Evans, Wistar, and Sprague-Dawley) using two models of SCI. The two models examined were contusion at T10 (NYU impactor, 12.5 mm height) and the T13 hemisection. Mechanical stimulation (von Frey filaments) revealed significantly lower baseline responses for Long-Evans rats and significantly higher baseline paw withdrawal latencies to thermal stimulation for Wistar rats compared to the other strains. Following contusion SCI, Long-Evans rats had the highest percentage of animals that developed mechanical allodynia (73%), while Sprague-Dawley rats had the highest percentages (75%) following hemisection SCI. Interestingly, the Sprague-Dawley rats had the highest percentage (87%) to develop thermal allodynia following contusion SCI, while 100% of both Long-Evans and Sprague Dawley rats developed thermal allodynia in the hemisection model. Locomotor recovery after SCI was similar for each model in that Long-Evans rats recovered slower and to a lesser extent than the other strains. In each model, Sprague-Dawley rats recovered faster and achieved greater function. Overall, the hemisection model produced a larger percentage of animals that developed CCP and had greater responses to mechanical stimulation. Thus, it appears that strain selection has a greater impact on locomotor recovery and model selection has a greater impact on the development of CCP following SCI. Furthermore, these results suggest that genetic factors may play a role in recovery following SCI.

Cerebellar cortical stimulation increases spinal visceral nociceptive responses

The role of the cerebellum in modulating nociceptive phenomena is unclear. In this study, we focus on the effects of cerebellar cortical stimulation on the responses of midline neurons of the lumbosacral spinal cord to graded nonnoxious and noxious visceral (colorectal distension) as well as somatic (brush, pressure, pinch) stimuli. Extracellular recording was used for the isolation and recording of spinal nociceptive neurons, while electrical current pulses and chemical injection of D, L-homocysteic acid were used to stimulate the cortex of the posterior cerebellar vermis. Cerebellar cortical stimulation increased the responses of all isolated cells to colorectal distension, whereas the effect on the responses to somatic stimuli was variable. These findings indicate that the posterior cerebellar vermis may exert a pro-nociceptive effect on spinal visceroceptive neurons.

Characterisation using FLIPR of human vanilloid VR1 receptor pharmacology

A full pharmacological characterisation of the recently cloned human vanilloid VR1 receptor was undertaken. In whole-cell patch clamp studies, capsaicin (10 microM) elicited a slowly activating/deactivating inward current in human embryonic kidney (HEK293) cells stably expressing human vanilloid VR1 receptor, which exhibited pronounced outward rectification (reversal potential -2.1+/-0.2 mV) and was abolished by capsazepine (10 microM). In FLIPR-based Ca(2+) imaging studies the rank order of potency was resiniferatoxin>olvanil>capsaicin>anandamide, and all were full agonists. Isovelleral and scutigeral were inactive (1 nM-30 microM). The potencies of capsaicin, olvanil and resiniferatoxin, but not anandamide, were enhanced 2- to 7-fold at pH 6.4. Capsazepine, isovelleral and ruthenium red inhibited the capsaicin (100 nM)-induced Ca(2+) response (pK(B)=6.58+/-0.02, 5.33+/-0.03 and 7.64+/-0.03, respectively). In conclusion, the recombinant human vanilloid VR1 receptor stably expressed in HEK293 cells acted as a ligand-gated, Ca(2+)-permeable channel with similar agonist and antagonist pharmacology to rat vanilloid VR1 receptor, although there were some subtle differences.

Loose ligation of the rat sciatic nerve is accompanied by changes in the subcellular content of protein kinase C beta II and gamma in the spinal dorsal horn

This study examined whether loose ligation of the sciatic nerve was accompanied by specific changes in protein kinase C (PKC) betaII and gamma isozymes in the spinal dorsal horn. The isozyme staining pattern was visualized with immunocytochemistry. Their content in subcellular fractions was estimated from Western immunoblots. In control animals, PKC betaII immunoreactivity extended from lamina I into lamina III, while PKC gamma immunoreactivity was concentrated within laminae II and III. In ligated animals exhibiting thermal hyperalgesia, the content of both PKC betaII and gamma in the synaptosomal membrane fraction, but not crude cytosolic fraction, was significantly greater by an average of 40% from their respective controls. These data support suggestions that peripheral nerve injury engenders plastic changes in the dorsal horn to contribute to the development of persistent pain.

Characterization using FLIPR of rat vanilloid receptor (rVR1) pharmacology

The vanilloid receptor (VR1) is a ligand-gated ion channel, which plays an important role in nociceptive processing. Therefore, a pharmacological characterization of the recently cloned rat VR1 (rVR1) was undertaken. HEK293 cells stable expressing rVR1 (rVR1-HEK293) were loaded with Fluo-3AM and then incubated at 25 degrees C for 30 min with or without various antagonists or signal transduction modifying agents. Then intracellular calcium concentrations ([Ca(2+)](i)) were monitored using FLIPR, before and after the addition of various agonists. The rank order of potency of agonists (resiniferatoxin (RTX)>capsaicin>olvanil>PPAHV) was as expected, and all were full agonists. The potencies of capsaicin and olvanil, but not RTX or PPAHV, were enhanced at pH 6.4 (pEC(50) values of 7.47+/-0.06, 7.16+/-0.06, 8.19+/-0.06 and 6.02+/-0.03 respectively at pH 7.4 vs 7.71+/-0.05, 7.58+/-0.14, 8.10+/-0.05 and 6.04+/-0.08 at pH 6.4). Capsazepine, isovelleral and ruthenium red all inhibited the capsaicin (100 nM)-induced Ca(2+) response in rVR1-HEK293 cells, with pK(B) values of 7.52+/-0.08, 6.92+/-0.11 and 8.09+/-0.12 respectively (n=6 each). The response to RTX and olvanil were also inhibited by these compounds. None displayed any agonist-like activity. The removal of extracellular Ca(2+) abolished, whilst inhibition of protein kinase C with chelerythrine chloride (10 microM) partially (approximately 20%) inhibited, the capsaicin (10 microM)-induced Ca(2+) response. However, tetrodotoxin (3 microM), nimodipine (10 microM), omega-GVIA conotoxin (1 microM), thapsigargin (1 microM), U73122 (3 microM) or H-89 (3 microM) had no effect on the capsaicin (100 nM)-induced response. In conclusion, the recombinant rVR1 stably expressed in HEK293 cells acts as a ligand-gated Ca(2+) channel with the appropriate agonist and antagonist pharmacology, and therefore is a suitable model for studying the effects of drugs at this receptor.

The role of nitric oxide and prostaglandin signaling pathways in spinal nociceptive processing in chronic inflammation

Both nitric oxide (NO) and prostaglandins (PG) and their associated enzymes nitric oxide synthases (NOS) and cyclooxygenases (COX) (specifically COX-2) have been implicated in the development of hyperalgesia. This study examined the effects of naturally occurring chronic inflammation, chronic mastitis, on spinal nociceptive processing in sheep and focused on potential alterations in spinal PG and NO signaling pathways. Mechanical withdrawal thresholds were significantly lower in animals suffering from chronic inflammation (n=6) compared to control animals (n=6). Hyperalgesia was restricted to the side contralateral to the inflammation (decrease from ipsilateral side: hindlimb 33.2+/-5%, forelimb 19.4+/-5%). Neuronal NOS-immunoreactivity was significantly reduced bilaterally in lumbar and cervical spinal cord throughout laminae I-III (decrease 18.4+/-5% and 16.9+/-4%, respectively) and in lamina X (decrease 29.1+/-6% and 17.1+/-4%, respectively) in mastitic animals relative to control animals. No difference was detected in eNOS or iNOS-immunoreactivity or in NADPH-diaphorase staining, a marker of dynamically active NOS. RT-PCR failed to detect any change in levels of nNOS, eNOS, iNOS, COX-1 or COX-2 mRNAs. However, a marked increase in the PGE receptor, EP(3) (but not EP(2)) mRNA was detected in ipsilateral spinal cord tissue from animals with chronic inflammation. This increase in EP(3) receptor expression indicates that spinal PGs are important in the spinal response to chronic peripheral inflammation. Contralateral mechanical hyperalgesia may not be directly linked to changes in spinal EP(3) receptor mRNA expression, however, the bilateral changes in nNOS suggest that this pathway may contribute to the adaptive behavioural response observed.

Evidence for the participation of the nitric oxide-cyclic GMP pathway in the antinociceptive action of meloxicam in the formalin test

The involvement of the nitric oxide-cyclic GMP pathway in the antinociceptive action of the cyclooxygenase-2 preferential inhibitor meloxicam was assessed in the rat formalin test. Rats received local pretreatment with saline or meloxicam and then 50 microl of dilute formalin (1%). Local administration of meloxicam produced a dose-dependent antinociception in the second phase of the formalin test. The antinociception produced by meloxicam was due to a local action as its administration in the contralateral paw was ineffective. Local pretreatment of the paws with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not affect the antinociception produced by meloxicam. However, N(G)-L-nitro-arginine methyl ester (L-NAME, a NO synthesis inhibitor) or 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) blocked in a dose-dependent way the effect of meloxicam. It is concluded that the peripheral antinociceptive effect of meloxicam involves a local NO-cyclic GMP pathway.

Evidence for the participation of the nitric oxide-cyclic GMP pathway in the antinociceptive effect of nimesulide

The involvement of the nitric oxide-cyclic GMP pathway in the peripheral antinociception induced by the COX-2 preferential inhibitor nimesulide was assessed by using the formalin test in the rat. Intraplantar administration of nimesulide in the formalin-injured paw produced a significant antinociceptive effect that was due to a local action, because nimesulide administration in the contralateral paw was ineffective. Local pretreatment of the paws with saline or N(G)-D-nitro-arginine methyl ester (D-NAME, the inactive isomer of L-NAME) did not affect the antinociception produced by nimesulide. However, local administration of L-NAME (a nitric oxide synthesis inhibitor) or 1H-(1,2,4)-oxadiazolo(4, 2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) blocked the effect of nimesulide. Moreover, the antinociceptive effect of local nimesulide was potentiated by the coadministration of 3-morpholino-sydnonimine-HCl (SIN-1, a nitric oxide donor). It is concluded that nimesulide produces antinociception by a peripheral mechanism of action requiring activation of the nitric oxide-cyclic GMP pathway at the local level.

Role of metabotropic glutamate receptor subtype mGluR1 in brief nociception and central sensitization of primate STT cells

G-protein coupled metabotropic glutamate receptors (mGluRs) are important modulators of synaptic transmission in the mammalian CNS and have been implicated in various forms of neuroplasticity and nervous system disorders. Increasing evidence also suggests an involvement of mGluRs in nociception and pain behavior although the contribution of individual mGluR subtypes is not yet clear. Subtypes mGluR1 and mGluR5 are classified as group I mGluRs and share the ability to stimulate phosphoinositide hydrolysis and activate protein kinase C. The present study examined the role of group I mGluRs in nociceptive processing and capsaicin-induced central sensitization of primate spinothalamic tract (STT) cells in vivo. In 10 anesthetized male monkeys (Macaca fascicularis) extracellular recordings were made from 20 STT cells in the lumbar dorsal horn. Responses to brief (15 s) cutaneous stimuli of innocuous (BRUSH) and barely and substantially noxious (PRESS and PINCH, respectively) intensity were recorded before, during, and after the infusion of group I mGluR agonists and antagonists into the dorsal horn by microdialysis. Cumulative concentration-response relationships were obtained by applying different concentrations for at least 20 min each (at 5 microl/min). The actual concentrations reached in the tissue are 2-3 orders of magnitude lower than those in the microdialysis fibers (values in this paper refer to the latter). The group I antagonists were also applied at 10-25 min after capsaicin injection. S-DHPG, a group I agonist at both mGluR1 and mGluR5, potentiated the responses to innocuous and noxious stimuli (BRUSH > PRESS > PINCH) at low concentrations (10-100 microM; n = 5) but had inhibitory effects at higher concentrations (1-10 mM; n = 5). The mGluR5 agonist CHPG (1 microM-100 mM; n = 5) did not potentiate but inhibited all responses (10-100 mM; n = 5). AIDA (1 microM-100 mM), a mGluR1-selective antagonist, dose-dependently depressed the responses to PINCH and PRESS but not to BRUSH (n = 6). The group I (mGluR1 > mGluR5) antagonist CPCCOEt (1 microM-100 mM) had similar effects (n = 6). Intradermal injections of capsaicin sensitized the STT cells to cutaneous mechanical stimuli. The enhancement of the responses by capsaicin resembled the potentiation by the group I mGluR agonist S-DHPG (BRUSH > PRESS > PINCH). CPCCOEt (1 mM) reversed the capsaicin-induced sensitization when given as posttreatment (n = 5). After washout of CPCCOEt, the sensitization resumed. Similarly, AIDA (1 mM; n = 7) reversed the capsaicin-induced sensitization and also blocked the potentiation by S-DHPG (n = 5). These data suggest that the mGluR1 subtype is activated endogenously during brief high-intensity cutaneous stimuli (PRESS, PINCH) and is critically involved in capsaicin-induced central sensitization.

Pharmacological modulation of secondary mediator systems--cyclic AMP and cyclic GMP--on inflammatory hyperalgesia

1. The objective of the present paper was to evaluate the relevance of neuronal balance of cyclic AMP and cyclic GMP concentration for functional regulation of nociceptor sensitivity during inflammation. 2. Injection of PGE2 (10-100 ng paw-1) evoked a dose-dependent hyperalgesic effect which was mediated via a cyclic AMP-activated protein kinase (PKA) inasmuch as hyperalgesia was blocked by the PKA inhibitor H89. 3. The PDE4 inhibitor rolipram and RP73401, but not PDE3 and PDE5 inhibitors potentiated the hyperalgesic effects of PGE2. The hyperalgesic effect of dopamine was also enhanced by rolipram. Moreover, rolipram significantly potentiated hyperalgesia induced by carrageenan, bradykinin, TNF alpha, IL-1 beta, IL-6 and IL-8. This suggests that neuronal cyclic AMP mediates the prostanoid and sympathetic components of mechanical hyperalgesia. Moreover, in the neuron cyclic AMP is mainly metabolized by PDE4. 4. To examine the role of the NO/cyclic GMP pathway in modulating mechanical hyperalgesia, we tested the effects of the soluble guanylate cyclase inhibitor, ODQ. This substance counteracts the inhibitory effects of the NO donor, SNAP, on the hyperalgesia induced by PGE2. 5. The ODQ potentiated hyperalgesia induced by carrageenan, bradykinin, TNF alpha, IL-1 beta, IL-6 and IL-8. In contrast, ODQ had no significant effect on the hyperalgesia induced by PGE2 and dopamine. This indicates that the hyperalgesic cytokines may activate soluble guanylate cyclase, which down-regulate the ability of these substances to cause hyperalgesia. This event appears not to be mediated by prostaglandin or dopamine. 6. In conclusion, the results presented in this paper confirm an association between (i) hyperalgesia and elevated levels of cyclic AMP as well as (ii) antinociception and elevated levels of cyclic GMP. The intracellular levels of cyclic AMP that enhance hyperalgesia are controlled by the PDE4 isoform and appear to result in activation of protein kinase A whereas the intracellular levels of cyclic GMP results from activation of a soluble guanylate cyclase.

Activation of protein kinase C enhances peptide release from rat spinal cord slices

One possible mechanism to account for the pain enhancing effects of protein kinase C (PKC) activation may be a facilitation of neurotransmitter release from terminals of nociceptive sensory neurons in the spinal cord. To examine this notion, we studied whether treatment with a phorbol ester enhanced the resting and capsaicin-evoked release of immunoreactive substance P (iSP) and immunoreactive calcitonin gene-related peptide (iCGRP) using an in vitro spinal cord slice preparation. Exposing the spinal cord tissue to 100 nM phorbol 12,13 dibutyrate (PDBu), an activator of PKC, results in a two-fold increase in the basal and the capsaicin-evoked release of iSP and iCGRF compared to evoked peptide release without PDBu. When the tissue was perfused with 1 microM 4-alpha PDBu, an analog of PDBu that does not activate PKC, the peptide release was not significantly different from control. Pre-exposing slices to 1 microM bisindolylmaleimide I, an inhibitor of PKC activity, prevents the facilitation of peptide release induced by PDBu. These results suggest that activation of PKC can augment the release of peptides in the spinal cord, which could increase nociceptive sensory transmission and contribute to hyperalgesia.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.