Underlying mechanisms of pronociceptive consequences of prolonged morphine exposure

Descending control of spinal nociception from the periaqueductal grey distinguishes between neurons with and without C-fibre inputs

Information about noxious events in the periphery is conveyed to the spinal cord in A- and C-fibre nociceptive afferents, which have largely distinct electrical and chemical properties and which convey different qualities of the pain signal. Descending control that originates in the different functional columns of the midbrain periaqueductal grey (PAG) has important roles in the modulation of spinal nociception in different behavioural and emotional states and, it is now believed, in animal models of chronic pain. However, few studies of descending control have considered differential modulation of A- versus C-nociceptor-evoked responses. Here, we report that descending inhibitory control from the rostrocaudal extent of the dorsolateral/lateral and ventrolateral columns of the PAG preferentially targets Class 2 deep dorsal horn neurons with C-fibre inputs. Pinch-evoked responses of these neurons were depressed significantly by -37+/-4.2% (P<0.0001). In contrast, the pinch-evoked responses of Class 2 neurons without C-fibre inputs (presumably A-fibre mediated) were enhanced significantly by +34+/-11.8% (P<0.01). Further experiments indicated these facilitatory effects were at least partly due to a reduction in C-fibre-mediated segmental inhibition. We suggest this differential control of spinal nociception would be appropriate in many of the varied situations in which the PAG is believed to become active, whether short term (e.g. fight or flight) or long term (e.g. chronic pain). Additionally, the pro-nociceptive effects observed in a subset of spinal neurons may be related to the descending facilitation that has been reported in animal models of chronic pain.

Laminar organization of spinal dorsal horn neurones activated by C- vs. A-heat nociceptors and their descending control from the periaqueductal grey in the rat

The periaqueductal grey can differentially control A- vs. C-nociceptor-evoked spinal reflexes and deep spinal dorsal horn neuronal responses. However, little is known about the control of A- vs. C-fibre inputs to lamina I and the lateral spinal nucleus, and how this correlates with the control of deeper laminae. To address this, the laminar distributions of neurones expressing Fos-like immunoreactivity were determined following preferential activation of A- or C-heat nociceptors, using fast or slow rates of skin heating, respectively, in the absence or presence of descending control evoked from the periaqueductal grey. In lamina I, numbers of Fos-positive neurones following both fast and slow rates of skin heating were reduced significantly following activation in the ventrolateral and dorsolateral/lateral periaqueductal grey. In contrast, in the deep dorsal horn (laminae III–VI), activation in both the ventrolateral and dorsolateral/lateral periaqueductal grey significantly reduced the numbers of Fos-positive neurones evoked by C- but not A-nociceptor stimulation. C- but not A-heat nociceptor activation evoked Fos bilaterally in the lateral spinal nucleus. Stimulation in the ventrolateral but not the dorsolateral/lateral periaqueductal grey significantly increased the numbers of Fos-positive neurones evoked by A- and C-nociceptor stimulation bilaterally in the lateral spinal nucleus. These data have demonstrated differences in the descending control of the superficial vs. the deep dorsal horn and lateral spinal nucleus with respect to the processing of A- and C-fibre-evoked events. The data are discussed in relation to the roles of A- and C-nociceptors in acute and chronic pain.

Decreased substance P and NK1 receptor immunoreactivity and function in the spinal cord dorsal horn of morphine-treated neonatal rats

Opiate analgesic tolerance is defined as a need for higher doses of opiates to maintain pain relief after prolonged opiate exposure. Though changes in the opioid receptor undoubtedly occur during conditions of opiate tolerance, there is increasing evidence that opiate analgesic tolerance is also caused by pronociceptive adaptations in the spinal cord. We have previously observed increased glutamate release in the spinal cord dorsal horn of neonatal rats made tolerant to the opiate morphine. In this study, we investigate whether spinal substance P (SP) and its receptor, the neurokinin 1 (NK1) receptor, are also modulated by prolonged morphine exposure. Immunocytochemical studies show decreased SP- and NK1-immunoreactivity in the dorsal horn of morphine-treated rats, whereas SP mRNA in the dorsal root ganglia is not changed. Electrophysiological studies show that SP fails to activate the NK1 receptor in the morphine-treated rat. Taken together, the data indicate that chronic morphine treatment in the neonatal rat is characterized by a loss of SP effects on the NK1 receptor in lamina I of the neonatal spinal cord dorsal horn. The results are discussed in terms of compensatory spinal cord processes that may contribute to opiate analgesic tolerance.

PERSPECTIVE

This article describes anatomical and physiological changes that occur in the spinal cord dorsal horn of neonatal rats after chronic morphine treatment. These changes may represent an additional compensatory process of morphine tolerance and may represent an additional therapeutic target for the retention and restoration of pain relief with prolonged morphine treatment.

Cyclooxygenase-1-derived prostaglandins in the periaqueductal gray differentially control C- versus A-fiber-evoked spinal nociception

Nonsteroidal anti-inflammatory drugs (NSAIDs) exert analgesic effects by inhibiting peripheral cyclooxygenases (COXs). It is now clear that these drugs also have central actions that include the modulation of descending control of spinal nociception from the midbrain periaqueductal gray (PAG). Descending control is a powerful determinant of the pain experience and is thus a potential target for analgesic drugs, including COX inhibitors. Noxious information from the periphery is conveyed to the spinal cord in A- and C-fiber nociceptors, which convey different qualities of the pain signal and have different roles in chronic pain. This in vivo study used different rates of skin heating to preferentially activate A- or C-heat nociceptors to further investigate the actions of COX inhibitors and prostaglandins in the PAG on spinal nociceptive processing. The results significantly advance our understanding of the central mechanisms underlying the actions of NSAIDs and prostaglandins by demonstrating that (1) in the PAG, it is COX-1 and not COX-2 that is responsible for acute antinociceptive effects of NSAIDs in vivo; (2) these effects are only evoked from the opioid-sensitive ventrolateral PAG; and (3) prostaglandins in the PAG exert tonic facilitatory control that targets C- rather than A-fiber-mediated spinal nociception. This selectivity of control is of particular significance given the distinct roles of A- and C-nociceptors in acute and chronic pain. Thus, effects of centrally acting prostaglandins are pivotal, we suggest, to both the understanding of nociceptive processing and the development of new analgesic drugs.

Development of novel enkephalin analogues that have enhanced opioid activities at both mu and delta opioid receptors

Enkephalin analogues with a 4-anilidopiperidine scaffold have been designed and synthesized to achieve therapeutic benefit for the treatment of pain due to mixed mu and delta opioid agonist activities. Ligand 16, in which a Dmt-substituted enkephalin-like structure was linked to the N-phenyl-N-piperidin-4-yl propionamide moiety, showed very high binding affinities (0.4 nM) at mu and delta receptors with an increased hydrophobicity (aLogP = 2.96). This novel lead compound was found to have very potent agonist activities in MVD (1.8 nM) and GPI (8.5 nM) assays.

The narcotic bowel syndrome: clinical features, pathophysiology, and management

Narcotic bowel syndrome (NBS) is a subset of opioid bowel dysfunction that is characterized by chronic or frequently recurring abdominal pain that worsens with continued or escalating dosages of narcotics. This syndrome is underrecognized and may be becoming more prevalent. In the United States this may be the result of increases in using narcotics for chronic nonmalignant painful disorders, and the development of maladaptive therapeutic interactions around its use. NBS can occur in patients with no prior gastrointestinal disorder who receive high dosages of narcotics after surgery or acute painful problems, and among patients with functional gastrointestinal disorders or other chronic gastrointestinal diseases who are managed by physicians who are unaware of the hyperalgesic effects of chronic opioids. The evidence for the enhanced pain perception is based on the following: (1) activation of excitatory antianalgesic pathways within a bimodal opioid regulation system, (2) descending facilitation of pain at the rostral ventral medulla and pain facilitation via dynorphin and cholecystokinin activation, and (3) glial cell activation that produces morphine tolerance and enhances opioid-induced pain. Treatment involves early recognition of the syndrome, an effective physician-patient relationship, graded withdrawal of the narcotic according to a specified withdrawal program, and the institution of medications to reduce withdrawal effects.

Mechanisms of Opioid-Induced Tolerance and Hyperalgesia

Opioid tolerance and opioid-induced hyperalgesia are conditions that negatively affect pain management. Tolerance is defined as a state of adaptation in which exposure to a drug induces changes that result in a decrease of the drug's effects over time. Opioid-induced hyperalgesia occurs when prolonged administration of opioids results in a paradoxic increase in atypical pain that appears to be unrelated to the original nociceptive stimulus. Complex intracellular neural mechanisms, including opioid receptor desensitization and down-regulation, are believed to be major mechanisms underlying opioid tolerance. Pain facilitatory mechanisms in the central nervous system are known to contribute to opioid-induced hyperalgesia. Recent research indicates that there may be overlap in the two conditions. This article reviews known and hypothesized pathophysiologic mechanisms surrounding these phenomena and the clinical implications for pain management nurses.

Control of chronic pain by the ubiquitin proteasome system in the spinal cord

Chronic pain is maintained in part by long-lasting neuroplastic changes in synapses and several proteins critical for synaptic plasticity are degraded by the ubiquitin-proteasome system (UPS). Here, we show that proteasome inhibitors administered intrathecally or subcutaneously prevented the development and reversed nerve injury-induced pain behavior. They also blocked pathological pain induced by sustained administration of morphine or spinal injection of dynorphin A, an endogenous mediator of chronic pain. Proteasome inhibitors blocked mechanical allodynia and thermal hyperalgesia in all three pain models although they did not modify responses to mechanical stimuli, but partially inhibited responses to thermal stimuli in control rats. In the spinal cord, these compounds abolished the enhanced capsaicin-evoked calcitonin gene-related peptide (CGRP) release and dynorphin A upregulation, both elicited by nerve injury. Model experiments demonstrated that the inhibitors may act directly on dynorphin-producing cells, blocking dynorphin secretion. Thus, the effects of proteasome inhibitors on chronic pain were apparently mediated through several cellular mechanisms indispensable for chronic pain, including those of dynorphin A release and postsynaptic actions, and of CGRP secretion. Levels of several UPS proteins were reduced in animals with neuropathic pain, suggesting that UPS downregulation, like effects of proteasome inhibitors, counteracts the development of chronic pain. The inhibitors did not produce marked or disabling motor disturbances at doses that were used to modify chronic pain. These results suggest that the UPS is a critical intracellular regulator of pathological pain, and that UPS-mediated protein degradation is required for maintenance of chronic pain and nociceptive, but not non-nociceptive responses in normal animals.

The duration of spinal anesthesia with 5% lidocaine in chronic opium abusers compared with nonabusers

BACKGROUND

It has been demonstrated that chronic opium abusers have lower thresholds for pain. In this study we sought to determine whether chronic opium abuse has any effect on the duration of spinal block by local anesthetics.

METHODS

In a case-controlled study, 50 opium abusers and 50 nonabusers undergoing lower abdomen operations were selected from among the patients admitted to a university hospital for elective surgery. All patients received 100 mg hyperbaric preservative-free 5% lidocaine in dextrose, intrathecally.

RESULTS

The duration of anesthesia was much shorter in the opium abusers (60 +/- 7 min) than in the nonabusers (83 +/- 10 min) (P < 0.0001).

CONCLUSION

The study documents a shortened duration of spinal block in opium abusers.

Restless legs syndrome: diagnostic assessment and the advantages and risks of dopaminergic treatment

In the past few years, major advances have been made in the field of restless legs syndrome (RLS). New tools have been developed to assess the presence and severity of RLS and its complications. Furthermore new concepts of the phenotype are emerging.With a high likelihood a slight dopaminergic hypofunction contributes essentially to the pathophysiology of most phenotypes of RLS. Dopaminergic substitution either with L-DOPA or with dopamine agonists ameliorates symptoms in the large majority of patients. Too high of doses of either type of drug may be involved in the development of augmentation caused by treatment-induced alterations in dopaminergic neurotransmission. Dopaminergic agents are currently the agents of first choice to treat RLS, and large multicenter trials support the evidence of efficacy. Very careful tailoring of the dose is required to avoid the development of treatment complications, specifically augmentation.

The beta2 adrenergic receptor regulates morphine tolerance and physical dependence

Adaptations to the chronic administration of opioids reduce the utility of these drugs in treating pain and support addiction. Recent genetics-based approaches have implicated the beta2 adrenergic receptor (beta2-AR) in controlling some of these responses. We do not know, however, whether this receptor can modulate tolerance, dependence or changes in gene expression caused by chronic opioid administration. For our studies we used C57BL/6 mice and beta2-AR knockout mice in the FVB background. Morphine dose-response relationships were established both prior to and after chronic morphine treatment. In some cases, the selective beta2-AR antagonist butoxamine was administered along with or after morphine. Physical dependence was assessed using naloxone-precipitated withdrawal. The expression of calcitonin gene related peptide (CGRP) and substance P (SP) were measured in spinal cord and dorsal root ganglion (DRG) tissues using both real-time PCR and enzyme-linked immunoassay (ELISA). Both the co-administration of butoxamine with morphine and the administration of butoxamine after chronic morphine reversed morphine tolerance. Morphine failed to cause tolerance in beta2-AR knockout mice. Physical dependence was reduced under the same circumstances. The chronic administration of butoxamine with morphine reduced or eliminated the normally observed up-regulation of CGRP and SP in spinal cord and DRG tissues. Our results suggest that the beta2-AR modulates both opioid tolerance and physical dependence. Activation of beta2-ARs appears to be required for some of the key neurochemical changes which characterize chronic opioid administration. Therefore, beta2-AR antagonists show some promise as agents to enhance chronic opioid analgesic therapy.

Systems-level adaptations explain chronic tolerance development to nitrous oxide hypothermia in young and mature rats

RATIONALE

Nitrous oxide (N(2)O) can initially lower core temperature (T (core)), but hypothermic tolerance develops with chronic administration. Therefore, one or both of T (core)'s controlling determinants, heat production (HP) and heat loss (HL), must adapt across repeated N(2)O administrations. Simultaneous measurements of HP, HL, and T (core) during chronic N(2)O administrations will elucidate this adaptive process and constitute a rigorous model for studying the systems-level dynamics of tolerance in both mature and young animals. This approach is justified by the need to better understand the increased vulnerability to addiction associated with adolescent drug use.

OBJECTIVES

The objective of the study was to measure HL and HP across repeated steady-state administrations of 60% N(2)O in young and mature rats.

MATERIALS AND METHODS

Synchronous measurements of HP (indirect calorimetry), HL (direct calorimetry), and T (core) (telemetry) were obtained during 60% N(2)O administrations in adolescent (28-45 days, n = 11) and mature rats (>90 days, n = 8). Rats received five 90-min drug exposures (every other day).

RESULTS

Compared to mature rats, adolescents initially exhibited greater hypothermia, but acquired tolerance more rapidly and actually developed hyperthermia during the fifth administration. In both groups, N(2)O consistently increased HL, but progressive increases of intrasessional HP over repeated administrations prevented hypothermia and subsequently promoted hyperthermia in adolescent rats.

CONCLUSIONS

Adolescent rats hyper-adapt to N(2)O hypothermia. Increases of intrasessional HP across N(2)O administrations explained both tolerance to N(2)O hypothermia and the unexpected hyperthermia observed in adolescents. These findings raise the possibility that the increased vulnerability to addiction associated with adolescent drug use involves a hyper-adaptive tolerance mechanism.

Morphine priming in rats with chronic inflammation reveals a dichotomy between antihyperalgesic and antinociceptive properties of deltorphin

We previously showed that prolonged morphine treatment and chronic inflammation both enhanced delta opioid receptor (deltaOR) cell surface density in lumbar spinal cord neurons. Here, we sought to determine whether administration of morphine to rats with chronic inflammation would further increase the bio-availability of deltaOR, and thereby the analgesic properties of the deltaOR agonist deltorphin, over that produced by inflammation alone. We found that chronic inflammation produced by injection of complete Freund's adjuvant (CFA) into the hind paw resulted in a bilateral increase in the binding and internalization of fluorescent deltorphin in neurons of the lumbar spinal cord as did prolonged morphine treatment [Morinville A, Cahill CM, Aibak H, Rymar VV, Pradhan A, Hoffert C, Mennicken F, Stroh T, Sadikot AF, O'Donnell D, Clarke PB, Collier B, Henry JL, Vincent JP, Beaudet A (2004a) Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord. J Neurosci 24:5549-5559]. This effect was accompanied by an increase in the antinociceptive efficacy of intrathecal deltorphin as measured using the tail-flick test. Treatment of CFA-injected rats with morphine decreased the cell surface availability of deltaOR in neurons of the dorsal horn of the lumbar spinal cord as compared with treatment with CFA alone. Behaviorally, it significantly enhanced the antihyperalgesic effects of deltorphin (plantar test; % maximum possible antihyperalgesic effect (MPAHE)=113.5%+/-32.4% versus 26.1%+/-11.6% in rats injected with CFA alone) but strongly reduced the antinociceptive efficacy of the drug (tail-flick test; % maximum possible antinociceptive effect (MPE)=29.6%+/-3.6% versus 66.6%+/-6.3% in rats injected with CFA alone) suggesting that the latter, but not the former, is linked to the deltaOR trafficking events observed neuroanatomically. These results demonstrate that in chronic inflammation, the antihyperalgesic effects of deltaOR agonists may be enhanced by morphine pre-treatment. They also reveal a dichotomy between mechanisms underlying antihyperalgesic and antinociceptive effects of deltaOR agonists.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Spinal NK-1 receptor expressing neurons mediate opioid-induced hyperalgesia and antinociceptive tolerance via activation of descending pathways

Opioids can induce hyperalgesia in humans and in animals. Mechanisms of opiate-induced hyperalgesia and possibly of spinal antinociceptive tolerance may be linked to pronociceptive adaptations occurring at multiple levels of the nervous system including activation of descending facilitatory influences from the brainstem, spinal neuroplasticity, and changes in primary afferent fibers. Here, the role of NK-1 receptor expressing cells in the spinal dorsal horn in morphine-induced hyperalgesia and spinal antinociceptive tolerance was assessed by ablating these cells with intrathecal injection of SP-saporin (SP-SAP). Ablation of NK-1 receptor expressing cells prevented (a) morphine-induced thermal and mechanical hypersensitivity, (b) increased touch-evoked spinal FOS expression, (c) upregulation of spinal dynorphin content and (d) the rightward displacement of the spinal morphine antinociceptive dose-response curve (i.e., tolerance). Morphine-induced hyperalgesia and antinociceptive tolerance were also blocked by spinal administration of ondansetron, a serotonergic receptor antagonist. Thus, NK-1 receptor expressing neurons play a critical role in sustained morphine-induced neuroplastic changes which underlie spinal excitability reflected as thermal and tactile hypersensitivity to peripheral stimuli, and to reduced antinociceptive actions of spinal morphine (i.e., antinociceptive tolerance). Ablation of these cells likely eliminates the ascending limb of a spinal-bulbospinal loop that engages descending facilitation and elicits subsequent spinal neuroplasticity. The data may provide a basis for understanding mechanisms of prolonged pain which can occur in the absence of tissue injury.

Affective Modulation of Pain in Substance-Dependent Veterans

OBJECTIVE

Prior work suggests that positive affect inhibits pain while negative affect facilitates it. The current study sought to determine whether: 1) affective modulation of pain extends to a patient population; 2) cocaine and alcohol dependence influences the pattern of modulation; and 3) affective modulation of pain is mediated by changes in arm temperature.

DESIGN

Thirty-seven participants with and without substance dependence (14 alcohol, 13 cocaine, 10 none) attended three experimental sessions intended to induce emotions (negative, neutral, positive) by picture-viewing. Following emotion-induction, participants were asked to submerge their arm in 33 degrees F water and keep it there until they reached tolerance. During submersion, pain ratings were made on a mechanical visual analog scale (M-VAS).

OUTCOME MEASURES

Latency from submersion to first movement of the M-VAS (pain threshold) and latency to arm removal (pain tolerance) were measured. Arm temperature and manipulation checks for emotion-induction (corrugator electromyogram, heart rate, skin conductance, self-report) were also recorded.

RESULTS

Manipulation checks confirmed that targeted affective states were achieved. Pain threshold and tolerance were higher after viewing pleasant pictures than after unpleasant ones. Although arm temperature did vary based on the affect induced, analyses suggested that temperature did not influence pain outcomes.

CONCLUSIONS

Affect modulates pain perception in patients and does not appear to be mediated by changes in arm temperature. Additionally, pain modulation was not significantly influenced by cocaine or alcohol dependence. These data are encouraging, because they suggest that nonpharmacological methods of pain modulation may be effective in substance-dependent individuals.

Less is more: pathophysiology of dopaminergic-therapy-related augmentation in restless legs syndrome

Therapy-related augmentation of the symptoms of restless legs syndrome (RLS) is an important clinical problem reported in up to 60% of patients treated with levodopa and, to a lesser extent, with dopamine agonists. The efficacy of low-dose dopaminergic drugs for RLS has been established, but the mode of action is unknown. Here, we review the existing data and conclude that augmentation is a syndrome characterised by a severely increased dopamine concentration in the CNS; overstimulation of the dopamine D1 receptors compared with D2 receptors in the spinal cord may lead to D1-related pain and generate periodic limb movements; iron deficiency may be a main predisposing factor of augmentation, probably caused by a reduced function of the dopamine transporter; therapy with levodopa or dopamine agonists should remain at low doses and; iron supplementation and opiates are the therapy of choice to counter augmentation.

Evidence for spinal dorsal horn hyperexcitability in rats following sustained morphine exposure

Repeated or sustained exposure to opioids can not only induce analgesia but also long lasting enhancements in pain sensitivity, a phenomenon reported clinically and in animals. In rats, opioid-induced abnormal pain can be readily measured following continued delivery of morphine and the recruitment of descending facilitatory influences appears essential for the genesis of this state. Here, we provide evidence that an increased excitability develops in neurons of the deep dorsal horn (DH), following 7-10 days of sustained delivery of morphine. Electrophysiological recordings were made in halothane-anesthetised animals implanted with osmotic minipumps containing either morphine (45 microg/0.5 microl/h and 90 microg/0.5 microl/h) or saline. A separate group of naïve animals was also used as controls. Sustained morphine exposure resulted in dose-related and modality-specific enhancements of DH neurons (C-fibre and Adelta-fibre evoked responses, non-potentiated responses) and expanded neuronal receptive fields, as mapped with low intensity mechanical punctate stimuli. Heat and mechanical stimulation of the hindpaw (brush and von Frey filaments) produced similar enhancements in morphine-treated rats compared to control rats, reflecting spinal hyperexcitability. Interestingly, wind-up itself was unaltered. These neuroadaptive changes could form the neuronal basis for the documented clinical and experimental reports of abnormal paradoxical pain after opioids. Since brainstem excitatory controls actively operate during prolonged opioid exposure, the sustained triggering of such influences may act alongside spinal mechanisms such as wind-up to enhance central sensitisation and alter CNS excitability.

Differential effect of glutamate transporter inhibition on EPSCs in the morphine naïve and morphine tolerant neonatal spinal cord slice

Opioid analgesic tolerance is a phenomenon defined as a need for increasingly higher doses of opiates to maintain suitable pain relief following repeated drug exposure. Research suggests that analgesic tolerance may result from heightened NMDA receptor (NMDAR) activity, but little is known regarding the mechanisms by which this elevated NMDAR activity develops. Recent evidence suggests that glutamate transporter down-regulation follows repeated opiate exposure and contributes to heightened pain sensitivity. Though glutamate transporter inhibition has been shown to increase activity of spinal cord neurons, it is unknown whether this increase contributes to the heightened NMDAR activity that underlies opiate tolerance. We directly tested this hypothesis by comparing the effects of glutamate transporter inhibition on excitatory post-synaptic currents (EPSCs) in the spinal cord dorsal horn of opiate naïve and opiate tolerant rats. We show that non-selective glutamate transporter inhibition increases the rate of spontaneous excitatory post-synaptic currents (sEPSCs) in the opiate naïve, but not opiate tolerant slice. This potentiation occurs in the presence of the sodium channel blocker tetrodotoxin (TTX) and is blocked by the NMDAR antagonist D-2-amino-5-phosphonovalerate (APV). The sEPSC rate is elevated at baseline in the opiate tolerant spinal cord slice compared to the opiate naïve slice, and glutamate transporter inhibition eliminates this difference. Taken together, we conclude that glutamate transporter inhibition directly contributes to heightened NMDAR activity. Furthermore, we propose that the increased neural activity observed in the opiate tolerant slice is due to a state of glutamate transporter down-regulation and resultant heightened NMDAR activity.

Effects of Glycogen Synthase Kinase 3β and Cyclin-Dependent Kinase 5 Inhibitors on Morphine-Induced Analgesia and Tolerance in Rats

Repeated administration of morphine is associated with the development of tolerance, yet the mechanism underlying this phenomenon is still poorly understood. Recent evidence implicating glycogen synthase kinase 3 (GSK3) in opioid receptor signaling pathways has prompted us to investigate its role in morphine tolerance. Administration of 10 mg/kg morphine i.p. to Wistar rats twice daily for 8 days resulted in complete tolerance to its analgesic effects as measured by the tail-flick test. When injections of morphine were preceded by intrathecal (i.t.) administration of either an inhibitor of GSK3 [(3-(2,4-dichlorophenyl)-4-(1-methyl-1H-indol-3-yl)-1H-pyrrole-2,5-dione (SB216763) or 6-bromoindirubin-3'oxime] or an inhibitor of cyclin-dependent kinase (Cdk), roscovitine, development of tolerance to morphine analgesia was completely abolished. In addition, a single i.t. injection of either kinase inhibitor was able to restore in a dose-dependent manner the analgesic effect of morphine in morphine-tolerant rats. None of the inhibitors in doses used in the present study had analgesic effects of their own nor an effect on the analgesic potency of morphine. Repeated i.t. administration of either inhibitor had caused an increase in abundance of GSK-3beta phosphorylated at Ser(9) in the dorsal lumbar part of the spinal cord of rats that were chronically treated with morphine. Furthermore, reversal of morphine tolerance by a single injection of either inhibitor was always associated with increased abundance of phospho-GSK3beta. In conclusion, our data indicate that chronic morphine treatment activates a highly efficient pathway by means of which Cdk5 regulates GSK3beta activity.

Structure-activity relationships of bifunctional peptides based on overlapping pharmacophores at opioid and cholecystokinin receptors

Cholecystokinin (CCK) has been identified as a pronociceptive endogenous peptide which also possesses antiopioid actions. CCK may be upregulated in conditions of chronic pain or during sustained morphine administration resulting in attenuation of opioid-mediated pain relief. These complex interactions between opioids and endogenous CCK receptor systems have suggested the need for a new paradigm in drug design for some states of chronic pain. In these circumstances the rational design of potential drugs for the treatment of these conditions must be based on one ligand for multiple targets. We have designed a single peptide which can interact with delta and mu opioid receptors as agonists and with CCK receptors as antagonists. The ligands were designed based on a model of overlapping pharmacophores of opioid and CCK peptide ligands, which incorporates opioid pharmacophores at the N-terminal and CCK tetrapeptide pharmacophores at the C-terminal of the designed ligands. We measured binding and activities of our bifunctional peptides at opioid and CCK receptors. Compound 11 (Tyr-d-Ala-Gly-d-Trp-NMeNle-Asp-Phe-NH(2)) demonstrated opioid agonist properties at delta and mu receptors (IC(50) = 63 +/- 27 nM and 150 +/- 65 nM, respectively in MVD and GPI tissue assays) and high binding affinity at CCK-1 and CCK-2 receptors (K(i) = 320 and 1.5 nM, respectively). Compound 9 (Tyr-d-Nle-Gly-Trp-Nle-Asp-Phe-NH(2)) displayed potent agonist activity at delta and mu receptors (IC(50) = 23 +/-10 nM and 210 +/- 52 nM, respectively in MVD and GPI tissue assays), with a balanced binding affinity for CCK-1 and CCK-2 receptors (K(i) = 9.6 and 15 nM, respectively). These results provide evidence supporting the concept that opioid and CCK receptors have overlapping pharmacophores required for binding affinity and biological activity and that designing overlapping pharmacophores of two peptides into a single peptide is a valid drug design approach.

Descending facilitation from the rostral ventromedial medulla maintains visceral pain in rats with experimental pancreatitis

BACKGROUND & AIMS

Pain is a main complaint of patients with pancreatitis. We hypothesized that such pain is mediated through ascending pathways via the nucleus gracilis (NG) and is dependent on descending facilitatory influences from the rostral ventromedial medulla (RVM).

METHODS

A rat model of persistent experimental pancreatitis was used. After establishment of pancreatitis, rats received microinjection of lidocaine in the NG or in the RVM to determine the importance of neural activity at these supraspinal sites in the expression of abdominal hypersensitivity evoked by von Frey filaments (ie, pancreatic pain). Rats also were pretreated for 28 days before induction of pancreatitis with a single RVM microinjection of dermorphin-saporin to eliminate cells that drive descending facilitation. Dynorphin content was measured in the spinal cord of pancreatitic rats and the effects of spinal antidynorphin antiserum in pancreatic pain were assessed.

RESULTS

Microinjection of lidocaine into either the NG or the RVM produced a time-related reversal of pancreatitis-induced pain. Pancreatitis significantly increased thoracic spinal dynorphin content and spinal antidynorphin antiserum elicited a time-related reversal of abdominal hypersensitivity. RVM dermorphin-saporin injection prevented the maintenance, but not the expression, of pancreatitis abdominal hypersensitivity and also prevented the increase of spinal dynorphin content in animals with pancreatitis.

CONCLUSIONS

Our findings suggest that descending facilitation from the RVM plays a critical role in the maintenance, but not the expression, of pancreatic pain. These results provide a novel insight into the role of descending pathways and spinal plasticity in the maintenance of visceral pain from pancreatitis.

Assessment of Oxidative Damage Induced by Acute Doses of Morphine Sulfate in Postnatal and Adult Rat Brain

The aim of the present study is to evaluate the oxidative damage in rats of different ages. Weaned rats of 25 g and adults of 300 g were used in groups of 6, a single i.p. dose of morphine sulfate of 3, 6 or 12 mg/kg was administered. All animals were sacrificed to measure GSH and 5-HT levels in brain by liquid chromatography, as well as Na(+), K(+)-ATPase and total ATPase enzymatic activity. 5-HT levels decreased significantly (p < 0.05) in adult animals that received 3 and 6 mg morphine. Na(+), K(+)-ATPase activity increased significantly (p < 0.05) in all groups of weaned animals. In adult animals, Na(+), K(+)-ATPase and total ATPase partially diminished. GSH levels diminished significantly (p < 0.05) both in weaned and in adult groups. The results indicate age-induced changes in cellular regulation and biochemical responses to oxidative stress induced by morphine.

Role of delivery and trafficking of delta-opioid peptide receptors in opioid analgesia and tolerance

Changes in the number of receptors on the cell surface lead to modulations of physiological functions and pharmacological responses of neurons. Recent studies show that delta-opioid peptide (DOP) and mu-opioid peptide (MOP) receptors have distinct subcellular localizations in neurons. In nociceptive small neurons in the dorsal root ganglia, DOP receptors are sorted into neuropeptide-containing secretory vesicles, enabling the stimulus-induced cell surface expression of these receptors. MOP receptors are constitutively expressed on the cell surface. The physical interaction between DOP receptors and MOP receptors seems to be an important mechanism for the modulation of receptor functions. Experiments in animals show that MOP-receptor-mediated spinal analgesia is enhanced and morphine tolerance does not develop when DOP receptor functions are pharmacologically or genetically attenuated. Thus, the delivery and trafficking of DOP receptors are crucial processes that modulate opioid analgesia and tolerance.

Midbrain control of spinal nociception discriminates between responses evoked by myelinated and unmyelinated heat nociceptors in the rat

Descending control of spinal nociception is a major determinant of normal and chronic pain. Myelinated (A-fibre) and unmyelinated (C-fibre) nociceptors convey different qualities of the pain signal (first and second pain, respectively), and they play different roles in the development and maintenance of chronic pain states. It is of considerable importance, therefore, to determine whether descending control has differential effects on the central processing of A- vs. C-nociceptive input. In anaesthetised rats, biceps femoris EMG was recorded to monitor the thresholds and encoding properties of responses evoked by fast (7.5 degrees Cs(-1)) or slow (2.5 degrees Cs(-1)) rates of skin heating of the dorsal surface of a hindpaw to preferentially activate myelinated or unmyelinated heat nociceptors, respectively. Activation of neurones in the periaqueductal grey (PAG) by microinjection of dl-homocysteic acid (DLH) or bicuculline (BIC) significantly increased response thresholds to slow rates of heating (P<0.001), but not those to fast rates of heating (P>0.05). The ability of the EMG to encode the stimulus intensity of fast rates of skin heating remained intact and unaltered (r2=0.99, P<0.001) following BIC but not DLH injection. In contrast, encoding of the stimulus intensity of slow rates of skin heating was abolished following BIC and DLH injection. The functional significance of differential descending control of the central processing of C- and A-nociceptive inputs is discussed with respect to role of the PAG in mediating antinociception as part of active coping strategies in emergency situations and the role of C- and A-nociceptive inputs in animal models of chronic pain.

Antinociceptive pharmacology of N-[[4-(4,5-dihydro-1H-imidazol-2-yl)phenyl]methyl]-2-[2-[[(4-methoxy-2,6-dimethylphenyl) sulfonyl]methylamino]ethoxy]-N-methylacetamide, fumarate (LF22-0542), a novel nonpeptidic bradykinin B1 receptor antagonist

The antinociceptive pharmacology of N-[[4-(4,5-dihydro-1H-imidazol-2-yl)phenyl]methyl]-2-[2-[[(4-methoxy-2,6-dimethylphenyl) sulfonyl]methylamino]ethoxy]-N-methylacetamide fumarate (LF22-0542), a novel nonpeptidic B1 antagonist, was characterized. LF22-0542 showed high affinity for human and mouse B1 receptors with virtually no affinity for the human B2 receptor; a selectivity index of at least 4000 times was obtained when LF22-0542 was profiled throughout binding or cell biology assays on 64 other G-protein-coupled receptor, 10 ion channels, and seven enzymes. LF22-0542 was a competitive B1 receptor antagonist and elicited significant antinociceptive actions in the mouse acetic acid-induced writhing assay, as well as in the second phases of formalin-induced nociception in mice and in both the first and second phases of the formalin response in rats. LF22-0542 was active after s.c. but not p.o. administration. In B1 receptor knockout (KO) mice, acetic acid and formalin responses were significantly reduced and LF22-0542 had no additional effects in these animals. LF22-0542 alleviated thermal hypersensitivity in both acute (carrageenan) and persistent inflammatory (complete Freund's adjuvant) pain models in rats. LF22-0542 produced a full reversal of experimental neuropathic thermal hypersensitivity but was inactive in reversing nerve injury-induced tactile hypersensitivity in rats. In agreement with this observation, B1 KO mice subjected to peripheral nerve injury did not show thermal hypersensitivity but developed nerve injury-induced tactile hypersensitivity normally. The data demonstrate the antihyperalgesic actions of a selective systemically administered B1 receptor antagonist and suggest the utility of this class of agents for the treatment of inflammatory pain states and for some aspects of neuropathic pain.

Chronic pain and genetic background interact and influence opioid analgesia, tolerance, and physical dependence

Opioids are commonly used in the treatment of moderate to severe pain. However, their chronic use is limited by analgesic tolerance and physical dependence. Few studies have examined how chronic pain affects the development of tolerance or dependence, and essentially no studies have looked at the role of both genetics and pain together. For these studies we used 12 strains of inbred mice. Groups of mice from each strain were tested at baseline for morphine analgesic sensitivity, mechanical nociceptive threshold, and thermal nociceptive threshold. Mice were then given morphine in a 4-day escalating morphine administration paradigm followed by reassessment of the morphine dose-response relationship. Finally, physical dependence was measured by administering naloxone. Parallel groups of mice underwent hind paw injection of complete Freund's adjuvant (CFA) to induce chronic hind paw inflammation 7 days prior to the beginning of testing. The data showed that CFA treatment tended to lower baseline ED(50) values for morphine and enhanced the degree of analgesic tolerance observed after 4 days of morphine treatment. In addition, the degree of jumping behavior indicative of physical dependence was often altered if mice had been treated with CFA. The influence of background strain was substantial for all traits measured. In silico haplotypic mapping of the tolerance and physical dependence data demonstrated that CFA pretreatment altered the pattern of the predicted associations and greatly reduced their statistical significance. We conclude that chronic inflammatory pain and genetics interact to modulate the analgesic potency of morphine, tolerance, and physical dependence.

Recent advances in elucidating pain mechanisms

PURPOSE OF REVIEW

The purpose of this review is to highlight the important recent advances in this fast developing field of pain mechanisms. It is now recognized that acute tissue and neural injuries can result in nociceptor sensitization (primary hyperalgesia) and spinal cord hyperexcitability or central sensitization that results in secondary hyperalgesia and allodynia.

RECENT FINDINGS

New findings regarding peripheral and central sensitization are presented in this review. Newer ideas on pain modulation, pain states and pain syndromes, mechanisms of chronic pain, newer opioid and non opioid medications including newer N-methyl-D-asparate antagonists, cyclooxygenase-2 inhibitors and membrane stabilizing analgesics as well as pain control at the genetic level are discussed.

SUMMARY

The research of the last decade has focused on the biochemical and structural plasticity of the nervous system following tissue and nerve injury. The mechanisms involved in the transition from acute to chronic pain are complex with the involvement of interacting receptor systems and intracellular ion flux, second messenger systems, new synaptic connections and apoptosis.