Opioid hyperalgesia and tolerance versus 5-HT1A receptor-mediated inverse tolerance

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Non-Peptide Opioids Differ in Effects on Mu-Opioid (MOP) and Serotonin 1A (5-HT1A) Receptors Heterodimerization and Cellular Effectors (Ca2+, ERK1/2 and p38) Activation

The importance of the dynamic interplay between the opioid and the serotonin neuromodulatory systems in chronic pain is well recognized. In this study, we investigated whether these two signalling pathways can be integrated at the single-cell level via direct interactions between the mu-opioid (MOP) and the serotonin 1A (5-HT1A) receptors. Using fluorescence cross-correlation spectroscopy (FCCS), a quantitative method with single-molecule sensitivity, we characterized in live cells MOP and 5-HT1A interactions and the effects of prolonged (18 h) exposure to selected non-peptide opioids: morphine, codeine, oxycodone and fentanyl, on the extent of these interactions. The results indicate that in the plasma membrane, MOP and 5-HT1A receptors form heterodimers that are characterized with an apparent dissociation constant Kdapp = (440 ± 70) nM). Prolonged exposure to all non-peptide opioids tested facilitated MOP and 5-HT1A heterodimerization and stabilized the heterodimer complexes, albeit to a different extent: Kd, Fentanylapp = (80 ± 70) nM), Kd,Morphineapp = (200 ± 70) nM, Kd, Codeineapp = (100 ± 70) nM and Kd, Oxycodoneapp = (200 ± 70) nM. The non-peptide opioids differed also in the extent to which they affected the mitogen-activated protein kinases (MAPKs) p38 and the extracellular signal-regulated kinase (Erk1/2), with morphine, codeine and fentanyl activating both pathways, whereas oxycodone activated p38 but not ERK1/2. Acute stimulation with different non-peptide opioids differently affected the intracellular Ca2+ levels and signalling dynamics. Hypothetically, targeting MOP−5-HT1A heterodimer formation could become a new strategy to counteract opioid induced hyperalgesia and help to preserve the analgesic effects of opioids in chronic pain.

Serotonin Plays a Key Role in the Development of Opioid-Induced Hyperalgesia in Mice

Opioid usage for pain therapy is limited by its undesirable clinical effects, including paradoxical hyperalgesia, also known as opioid-induced hyperalgesia (OIH). However, the mechanisms associated with the development and maintenance of OIH remain unclear. Here, we investigated the effect of serotonin inhibition by the 5-HT3 receptor antagonist, ondansetron (OND), as well as serotonin deprivation via its synthesis inhibitor para-chlorophenylalanine, on mouse OIH models, with particular focus on astrocyte activation. Co-administering of OND and morphine, in combination with serotonin depletion, inhibited mechanical hyperalgesia and astrocyte activation in the spinal dorsal horn of mouse OIH models. Although previous studies have suggested that activation of astrocytes in the spinal dorsal horn is essential for the development and maintenance of OIH, herein, treatment with carbenoxolone (CBX), a gap junction inhibitor that suppresses astrocyte activation, did not ameliorate mechanical hyperalgesia in mouse OIH models. These results indicate that serotonin in the spinal dorsal horn, and activation of the 5-HT3 receptor play essential roles in OIH induced by chronic morphine, while astrocyte activation in the spinal dorsal horn serves as a secondary effect of OIH. Our findings further suggest that serotonergic regulation in the spinal dorsal horn may be a therapeutic target of OIH. PERSPECTIVE: The current study revealed that the descending serotonergic pain-facilitatory system in the spinal dorsal horn is crucial in OIH, and that activation of astrocytes is a secondary phenotype of OIH. Our study offers new therapeutic targets for OIH and may help reduce inappropriate opioid use.

Persistent Rheb-induced mTORC1 activation in spinal cord neurons induces hypersensitivity in neuropathic pain

The small GTPase Ras homolog enriched in the brain (Rheb) can activate mammalian target of rapamycin (mTOR) and regulate the growth and cell cycle progression. We investigated the role of Rheb-mediated mTORC1 signaling in neuropathic pain. A chronic constriction injury (CCI) model was dopted. CCI induced obvious spinal Rheb expression and phosphorylation of mTOR, S6, and 4-E-BP1. Blocking mTORC1 signal with rapamycin alleviated the neuropathic pain and restored morphine efficacy in CCI model. Immunofluoresence showed a neuronal co-localization of CCI-induced Rheb and pS6. Rheb knockin mouse showed a similar behavioral phenotype as CCI. In spinal slice recording, CCI increased the firing frequency of neurons expressing HCN channels; inhibition of mTORC1 with rapamycin could reverse the increased spinal neuronal activity in neuropathic pain. Spinal Rheb is induced in neuropathic pain, which in turn active the mTORC1 signaling in CCI. Spinal Rheb-mTOR signal plays an important role in regulation of spinal sensitization in neuropathic pain, and targeting mTOR may give a new strategy for pain management.

Nocistatin inhibits pregnant rat uterine contractions in vitro: roles of calcitonin gene-related peptide and calcium-dependent potassium channel

The endogenous neuropeptide nociceptin/orphanin FQ, translated from the prepronociceptin gene, exerts a contraction-inhibitory effect on the rat uterus. As nocistatin has been reported to cause functional antagonism of the pro-nociceptive effects of nociceptin, we set out to investigate its effects on the pregnant rat uterus and to elucidate its signalling pathway. The expression of prepronociceptin mRNA in the uterus and nocistatin levels in the uterus and the plasma were confirmed by RT-PCR and radioimmunoassay. The uterine levels of prepronociceptin mRNA and nocistatin were significantly increased by the last day of pregnancy, while the plasma nocistatin levels remained unchanged. In the isolated organ bath studies nocistatin inhibited the prostaglandin- and the KCl-evoked contractions in the uterus dose-dependently. This latter effect was decreased by preincubation with capsaicin. Incubation with calcitonin gene-related peptide after capsaicin treatment caused an elevation in the contraction-inhibitory effect of nocistatin. The effect of nocistatin was also decreased by the Ca(2+)-dependent K(+) channel inhibitor paxilline, against spontaneous uterine contractions. Nociceptin potentiated the action of nocistatin. Naloxone decreased the effect of nocistatin administered either alone or in combination with nociceptin. In Ca(2+)-poor environment, this effect of naloxone was suspended. Enzyme immunoassay for the uterine intracellular cAMP levels partially confirmed the results of in vitro contractility studies. We conclude that nocistatin, generated locally in the uterus, exerts an inhibitory effect, the mechanism being mediated in part by Ca(2+)-dependent K(+) channels, the elevation of cAMP levels and sensory neuropeptides.

In vivo electrophysiological and neurochemical effects of the selective 5-HT1A receptor agonist, F13640, at pre- and postsynaptic 5-HT1A receptors in the rat

RATIONALE

F13640 (befiradol) is a novel 5-HT(1A) receptor agonist with exceptional selectivity vs. other receptors and binding sites. It shows analgesic activity in animal models and is currently developed for human use.

OBJECTIVES

Given the potential dual role of the serotonergic system in pain, through the modulation of ascending signals in spinal cord and their emotional processing by corticolimbic areas, we examined the in vivo activity of F13640 at somatodendritic autoreceptors and postsynaptic 5-HT(1A) heteroreceptors in medial prefrontal cortex (mPFC).

METHODS

In vivo single unit recordings and intracerebral microdialysis in the rat.

RESULTS

F13640 reduced the activity of dorsal raphe serotonergic neurons at 0.2-18.2 μg kg(-1), i.v. (cumulative doses; ED(50) = 0.69 μg kg(-1), i.v.) and increased the discharge rate of 80% of mPFC pyramidal neurons in the same dose range (ED(50) = 0.62 μg kg(-1), i.v.). Both effects were reversed by the subsequent administration of the 5-HT(1A) receptor antagonist (±)WAY100635. In microdialysis studies, F13640 (0.04-0.63 mg kg(-1), i.p.) dose-dependently decreased extracellular 5-HT in the hippocampus and mPFC. Likewise, F13640 (0.01-2.5 mg kg(-1), i.p.) dose-dependently increased extracellular DA in mPFC, an effect dependent on the activation of postsynaptic 5-HT(1A) receptors in mPFC. Local perfusion of F13640 in mPFC (1-1,000 μM) also increased extracellular DA in a concentration-dependent manner. Both the systemic and local effects of F13640 were prevented by prior (±)WAY100635 administration.

CONCLUSIONS

These results indicate that, upon systemic administration, F13640 activates both 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) receptors in prefrontal cortex with a similar potency. Both activities are likely involved in the analgesic properties of the compound.

Novel pyridylmethylamines as highly selective 5-HT(1A) superagonists

To further improve the maximal serotonergic efficacy and better understand the configurational requirements for 5-HT(1A) binding and activation, we generated and biologically investigated structural variants of the lead structure befiradol. For a bioisosteric replacement of the 3-chloro-4-fluoro moiety, a focused library of 63 compounds by solution phase parallel synthesis was developed. Target binding of our compound collection was investigated, and their affinities for 5-HT(2), α(1), and α(2)-adrenergic as well as D(1)-D(4) dopamine receptors were compared. For particularly interesting test compounds, intrinsic activities at 5-HT(1A) were examined in vitro employing a GTPγS assay. The investigation guided us to highly selective 5HT(1A) superagonists. The benzothiophene-3-carboxamide 8bt revealed almost exclusive 5HT(1A) recognition with a K(i) value of 2.7 nM and a maximal efficacy of 124%. To get insights into the bioactive conformation of our compound collection, we synthesized conformationally constrained bicyclic scaffolds when SAR data indicated a chair-type geometry and an equatorially dispositioned aminomethyl substituent for the 4,4-disubstituted piperidine moiety.

Progesterone rapidly recruits female-typical opioid-induced hyperalgesic mechanisms

Continuous morphine treatment can paradoxically increase nociception (i.e. hyperalgesia) in male and female mice, but sex differences have been reported. Here, we studied progesterone modulation of these differences by assessing nociception on the tail-withdrawal test in male and female mice rendered hyperalgesic during continuous infusion of two different morphine doses (1.6 and 40.0mg/kg/24h). Although the lower morphine infusion dose increased nociception in both sexes by infusion Day 4, this hyperalgesia dissipated by Day 6 in males and ovariectomized females, but not gonadally intact females. A single subcutaneous progesterone (0.0016mg/kg) injection to males and ovariectomized females on Day 6 caused hyperalgesia to recur within 30min and to persist for a minimum of 120min. The larger morphine infusion dose also increased nociception in both sexes on Days 4 and 6. However, the NMDA receptor antagonist MK-801 (0.05mg/kg) reversed hyperalgesia in males and ovariectomized females but not gonadally intact females on infusion Day 6. Subcutaneous progesterone (0.0016mg/kg) injection inhibited this reversal in male and ovariectomized female mice but had no effect on nociception in saline-infused mice of either sex. These data confirm our previous findings that male and female mice utilize distinct hyperalgesic mechanisms, and show for the first time that a single progesterone bolus dose can recruit female-typical hyperalgesia in ovariectomized females and males.

Sex-specific mediation of opioid-induced hyperalgesia by the melanocortin-1 receptor

BACKGROUND

N-Methyl-D-aspartate receptor antagonists reverse hyperalgesia during morphine infusion in male mice only. Because the melanocortin-1 receptor can act as a female-specific counterpart to N-methyl-D-aspartate receptors in kappa-opioid analgesic mechanisms, the authors assessed the contribution of melanocortin-1 receptors to the sex-specific mechanisms underlying morphine hyperalgesia.

METHODS

The tail-withdrawal test was used to compare the nociceptive responses of male and female C57BL/6J (B6) mice with those of C57BL/6J-Mc(1r(e/e)) mice, spontaneous mutants of the B6 background lacking functional melanocortin-1 receptors, during continuous morphine infusion (1.6 and 40.0 mgkg(-1) . 24 h(-1)). Separate groups of hyperalgesic B6 and outbred CD-1 mice were injected with MK-801 or MSG606, selective N-methyl-D-aspartate and melanocortin-1 receptor antagonists, respectively.

RESULTS

Morphine infusion (40.0 mg . kg(-1) . 24 h(-1)) reduced baseline withdrawal latencies by 45-55% in B6 mice of both sexes, indicating hyperalgesia; this increased nociception was manifest in male e/e mice only. Although MK-801 reversed hyperalgesia in male mice only, increasing latencies by 72%, MSG606 increased latencies by approximately 60% exclusively in females. A lower morphine infusion dose (1.6 mg . kg(-1) . 24 h(-1)) reduced baseline withdrawal latencies by 45-52% in B6 and e/e mice of both sexes, which was reversed by MK-801, but not MSG606, in both male and female B6 mice.

CONCLUSIONS

The data indicate the sex-specific mediation of high-dose morphine-induced hyperalgesia by N-methyl-d-aspartate and melanocortin-1 receptors in male and female mice, respectively, suggesting a broader relevance of this known sexual dimorphism. The data further indicate that the neural substrates contributing to hyperalgesia are morphine dose-dependent.

Continuous opioid treatment for chronic noncancer pain: a time for moderation in prescribing

Physicians have embraced the concept of long-term opioid treatment for chronic noncancer pain (CNCP), as evidenced by increased prescribing. Many patients have benefited from more liberal opioid prescribing, but many have not, and prescription opioid abuse has risen significantly coincident with increased prescribing. Because of the potentially serious adverse effects of opioids, physicians must balance potential benefits against risks, especially in individuals at risk for opioid misuse, abuse, or dependence. This article reviews long-term, continuous opioid treatment of CNCP, current treatment guidelines, addiction risk stratification, opioid-induced hyperalgesia, and endocrine dysfunction.

Efficacy of opioids for chronic pain: a review of the evidence

Opioid therapy for chronic pain has been popularized over the past few decades, and a concern has arisen that the analgesic efficacy of opioids is not always maintained over prolonged courses of treatment despite dose escalation and stable pain. Considering the potentially serious adverse effects of opioids, the idea that pain relief could diminish over time may have a significant impact on the decision to embark on this therapy, especially in vulnerable individuals. Possible loss of analgesic efficacy is especially concerning, considering that dependence may make it hard to withdraw opioid therapy even in the face of poor analgesia. This article first reviews the evidence on opioid efficacy when used for the treatment of chronic pain, and concludes that existing evidence suggests that analgesic efficacy, although initially good, is not always sustained during continuous and long-term opioid therapy (months to years). The theoretical basis for loss of analgesic efficacy over time is then examined. Mechanisms for loss of analgesic efficacy proposed are pharmacologic tolerance, opioid-induced hyperalgesia, subtle and intermittent withdrawal, and a number of psychologic factors including loss of the placebo component.

Naloxone rapidly evokes endogenous kappa opioid receptor-mediated hyperalgesia in naïve mice pretreated briefly with GM1 ganglioside or in chronic morphine-dependent mice

Low-dose naloxone-precipitated withdrawal hyperalgesia is a reliable indicator of physical dependence after chronic morphine treatment. A remarkably similar long-lasting (>3-4 h) hyperalgesia is evoked by injection of a low dose of naloxone (10 microg/kg, s.c.) in naïve mice after acute pretreatment with the glycolipid, GM1 ganglioside (1 mg/kg) (measured by warm-water-immersion tail-flick assays). GM1 treatment markedly increases the efficacy of excitatory Gs-coupled opioid receptor signaling in nociceptive neurons. Co-treatment with an ultra-low-dose (0.1 ng/kg, s.c.) of the broad-spectrum opioid receptor antagonist, naltrexone or the selective kappa opioid receptor antagonist, nor-binaltorphimine, blocks naloxone-evoked hyperalgesia in GM1-pretreated naïve mice and unmasks prominent, long-lasting (>4 h) inhibitory opioid receptor-mediated analgesia. This unmasked analgesia can be rapidly blocked by injection after 1-2 h of a high dose of naltrexone (10 mg/kg) or nor-binaltorphimine (0.1 mg/kg). Because no exogenous opioid is administered to GM1-treated mice, we suggest that naloxone may evoke hyperalgesia by inducing release of endogenous bimodally acting opioid agonists from neurons in nociceptive networks by antagonizing putative presynaptic inhibitory opioid autoreceptors that "gate" the release of endogenous opioids. In the absence of exogenous opioids, the specific pharmacological manipulations utilized in our tail-flick assays on GM1-treated mice provide a novel bioassay to detect the release of endogenous bimodally acting (excitatory/inhibitory) opioid agonists. Because mu excitatory opioid receptor signaling is blocked by ultra-low doses of naloxone, the higher doses of naloxone that evoke hyperalgesia in GM1-treated mice cannot be mediated by activation of mu opioid receptors. Co-treatment with ultra-low-dose naltrexone or nor-binaltorphimine may selectively block signaling by endogenous GM1-sensitized excitatory kappa opioid receptors, unmasking inhibitory kappa opioid receptor signaling, and converting endogenous opioid receptor-mediated hyperalgesia to analgesia. Co-treatment with kelatorphan stabilizes putative endogenous opioid peptide agonists released by naloxone in GM1-treated mice, so that analgesia is evoked rather than hyperalgesia. Acute treatment of chronic morphine-dependent mice with ultra-low-dose naltrexone (0.1 ng/kg) results in remarkably similar rapid blocking of naloxone (10 microg/kg)-precipitated withdrawal hyperalgesia and unmasking of prominent opioid analgesia. These studies may clarify complex mechanisms underlying opioid physical dependence and opioid addiction.

Nociception increases during opioid infusion in opioid receptor triple knock-out mice

Opioids are extensively used analgesics yet can paradoxically increase pain sensitivity in humans and rodents. This hyperalgesia is extensively conceptualized to be a consequence of opioid receptor activity, perhaps providing an adaptive response to analgesia, and to utilize N-methyl-D-aspartate (NMDA) receptors. These assumptions were tested here in opioid receptor triple knock-out (KO) mice lacking all three genes encoding opioid receptors (mu, delta, and kappa) by comparing their thermal nociceptive responses to the opioids morphine and oxymorphone with those of B6129F(1) controls. Injecting acute opioid bolus doses in controls caused maximal analgesia that was completely abolished in KO mice, confirming the functional consequence of the KO mouse opioid receptor deficiency. Continuous opioid infusion by osmotic pump in control mice also initially caused several consecutive days of analgesia that was shortly thereafter followed by several consecutive days of hyperalgesia. In contrast, continuously infusing KO mice with opioids caused no detectable analgesic response, but only immediate and steady declines in nociceptive thresholds culminating in several days of unremitting hyperalgesia. Finally, injecting the non-competitive NMDA receptor antagonist MK-801 during opioid infusion markedly reversed hyperalgesia in control but not KO mice. These data demonstrate that sustained morphine and oxymorphone delivery causes hyperalgesia independently of prior or concurrent opioid or NMDA receptor activity or opioid analgesia, indicating the contribution of mechanisms outside of current conceptions, and are inconsistent with proposals of hyperalgesia as a causative factor of opioid analgesic tolerance.

Opioid dependence and addiction during opioid treatment of chronic pain

Throughout the long history of opioid drug use by humans, it has been known that opioids are powerful analgesics, but they can cause addiction. It has also been observed, and is now substantiated by multiple reports and studies, that during opioid treatment of severe and short-term pain, addiction arises only rarely. However, when opioids are extended to patients with chronic pain, and therapeutic opioid use is not confined to patients with severe and short-lived pain, compulsive opioid seeking and addiction arising directly from opioid treatment of pain become more visible. Although the epidemiological evidence base currently available is rudimentary, it appears that problematic opioid use arises in some fraction of opioid-treated chronic pain patients, and that problematic behaviors and addiction are problems that need to be addressed. Since the potentially devastating effects of addiction can substantially offset the benefits of opioid pain relief, it seems timely to reexamine addiction mechanisms and their relevance to the practice of long-term opioid treatment for pain. This article reviews the neurobiological and genetic basis of addiction, its terminology and diagnosis, the evidence on addiction rates during opioid treatment of chronic pain and the implications of biological mechanisms in formulating rational opioid treatment regimes.

Immunology and homeopathy. 5. The rationale of the 'Simile'

The foundation of homeopathic medicine is the 'Similia Principle', also known as the 'Principle of Similarity' or also as the 'Simile', which reflects the inversion of pharmacological effects in healthy subjects as compared with sick ones. This article describes the inversion of effects, a widespread medical phenomenon, through three possible mechanisms: non-linearity of dose-response relationship, different initial pathophysiological states of the organism, and pharmacodynamics of body response to the medicine. Based on the systemic networks which play an important role in response to stress, a unitary and general model is designed: homeopathic medicines could interact with sensitive (primed) regulation systems through complex information, which simulate the disorders of natural disease. Reorganization of regulation systems, through a coherent response to the medicine, could pave the way to the healing of the cellular, tissue and neuro-immuno-endocrine homeodynamics. Preliminary evidence is suggesting that even ultra-low doses and high-dilutions of drugs may incorporate structural or frequency information and interact with chaotic dynamics and physical-electromagnetic levels of regulation. From the clinical standpoint, the 'simile' can be regarded as a heuristic principle, according to which the detailed knowledge of pathogenic effects of drugs, associated with careful analysis of signs and symptoms of the ill subject, could assist in identifying homeopathic remedies with high grade of specificity for the individual case.

Opioids and abnormal pain perception: New evidence from a study of chronic opioid addicts and healthy subjects

Recent evidence reported on increased pain sensitivity in animals following parenteral opioid administration and in humans subsequent to intravenously of short-acting opioids and possibly in drug addicts. The aims of the present study were to explore the possibilities that (1) pain perception is altered in chronic opioid addicts (OAs); (2) if indeed so, the cessation of opioid consumption resets their altered pain perception. Sixty heroin or methadone OAs who attended a 4-week inpatient detoxification program were exposed to the cold pressor test (CPT) upon entrance to the program, at 7 and 28 days subsequent to the cessation of opioid consumption (verified by repeated urine toxicology tests). Latency of pain onset (s), pain intensity (0-100 VAS), and tolerance (time for hand withdrawal) in response to the CPT were measured. In comparison with 70 healthy controls, the OAs demonstrated prolonged latency (6.6+/-3.5s versus 10.9+/-7.7s; p < 0.0001); decreased VAS (74+/-16 versus 55+/-20; p < 0.0001); shorter tolerance (56.4+/-51.3s versus 31.7+/-40.7s; p = 0.001). No differences between the three time points in any of the three measures were detected in the OAs. The results provide further evidence of opioid-induced hyperalgesia in the OA population, as manifested by their quicker hand withdrawal. In addition, it appears that detoxification from opioids does not reset pain perception for at least 1 month.

Oral opioid administration and hyperalgesia in patients with cancer or chronic nonmalignant pain

AIMS

Previous research has reported on reduced paw withdrawal latencies to heat and mechanical stimuli after parenteral administration of opioids in animals and on increased pain sensitivity in humans subsequent to postoperative infusions of short-acting opioids or in drug addicts. The aim of the present study was to explore the possibility that oral opioid treated patients with cancer-related or chronic nonmalignant pain differ in their pain sensitivity from patients treated with non-opioid analgesics.

METHODS

The study population consisted of 224 patients, including 142 in the opioid-treated group and 82 in the non-opioid-treated group. Pain thresholds for punctuate measured by von Frey filaments (g), mechanical pressure measured by pressure algometer (mmHg), heat stimuli measured by quantitative sensory testing (degrees C), as well as suprathreshold tonic heat pain intensity (46.5 degrees C for 1 min) measured by 0-10 numerical pain scale (NPS) were obtained at a nonpainful site (thenar eminence) in all patients.

RESULTS

No differences between the groups were found for gender, age, duration of pain, or duration of treatment (independent variables). No significant differences between the groups were found in punctuate (difference = 17.0 g (95% CI -8.8, 42.8), P = 0.19), pressure (2.2 mmHg (-28.7, 33.2), P = 0.89) and heat (-0.3 degrees C (-1.5, 0.9), P = 0.70) pain thresholds, or in suprathreshold heat pain intensity (difference between maximal pain intensities -0.4 NPS units (95% CI -1.2, 0.4), P = 0.31). Pearson correlations within the opioid-treated group failed to show significant relationships between any of the independent variables and the outcome measures. A further comparison of the outcomes between the 'weak' opioid-treated subgroup and the 'strong' opioid-treated subgroup again revealed insignificant results.

CONCLUSIONS

These results suggest that the administration of 'commonly used' dosages of oral opioids does not result in abnormal pain sensitivity beyond that of patients receiving non-opioid analgesia.

Morphine hyperalgesia in mice is unrelated to opioid activity, analgesia, or tolerance: Evidence for multiple diverse hyperalgesic systems

Hyperalgesia following chronic morphine treatment is thought to be a response to opioid receptor activation and analgesia and contribute to the development of analgesic tolerance. Here, the relationship between these variables was studied in mice tested for nociceptive sensitivity on the tail-withdrawal test during chronic infusion of various morphine doses. Hyperalgesic onset was preceded by dose-dependent analgesia except for the lowest morphine dose, which caused hyperalgesia 6 h after the start of infusion. Morphine ED50 values obtained at various infusion intervals demonstrated both analgesic tolerance in the absence of hyperalgesia and hyperalgesia in the absence of tolerance. Continuous opioid receptor antagonism using naltrexone pellets abolished analgesia during continuous morphine administration, transiently potentiated hyperalgesia, and revealed differences in hyperalgesic onset between morphine infusion doses. Acute injection of the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 attenuated hyperalgesia in naltrexone-treated mice, demonstrating a role for this receptor in morphine hyperalgesia unrelated to its effects upon morphine analgesia. In mice where hyperalgesia subsided after continuous infusion of the highest morphine dose (i.e., hyperalgesic adaptation), hyperalgesia was restored after infusing the lower but not higher morphine dose. In addition, acute injection of morphine-3beta-glucoronide (M3G) caused hyperalgesia that was cross-adaptive with the lower morphine dose only. The data demonstrate that morphine hyperalgesia is independent of prior or concurrent opioid receptor activity or analgesia and is unrelated to analgesic tolerance. Furthermore, the lack of hyperalgesic cross-adaptation between high and low morphine doses, and their differential cross-adaptation with M3G hyperalgesia, also suggests distinct morphine dose-dependent hyperalgesic systems.

Chronic pancreatitis patients show hyperalgesia of central origin: a pilot study

BACKGROUND

The pain of chronic pancreatitis remains challenging to manage, with treatment all too often being unsuccessful. A main reason for this is lacking understanding of underlying mechanisms of chronic pain in these patients.

AIM

To document, using somatic quantitative sensory testing, changes in central nervous system processing (neuroplasticity) associated with chronic pancreatitis pain and thus gain insight into underlying pain mechanisms.

PATIENTS AND METHODS

We studied 10 chronic pancreatitis patients on stable opioid analgesic medication. Ten matched surgical patients without pain served as controls. Pain verbal numeric rating scores (NRS) and thresholds to electric skin stimulation and pressure pain were measured in dermatomes T10 (pancreatic area), C5, T4, L1 and L4.

RESULTS

The pancreatitis patients had a median NRS pain score of 5 (range 3-8). Electric sensation and pain thresholds were significantly increased in the pancreatic region, tending to be more so in female pancreatitis patients. Pressure pain thresholds were significantly lower in pancreatitis patients than in controls, with men tending towards greater generalised relative hyperalgesia than women.

CONCLUSIONS

Chronic pancreatitis patients show pronounced generalised deep hyperalgesia that is present despite opioid therapy. These signs, consistent with central sensitisation, appear relatively more prominent in men than women. There is also evidence suggesting that women may have a better segmental inhibitory response than men, possibly explaining their relatively less prominent generalised deep tissue hyperalgesia compared to men.

Ultra-low dose naltrexone potentiates the anticonvulsant effect of low dose morphine on clonic seizures

Significant potentiation of analgesic effects of opioids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well understood. The bimodal modulation of pentylenetetrazole-induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms involved in opioid anti- (morphine at 0.5-3 mg/kg) versus pro-convulsant (20-100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg-10 ng/kg) significantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antagonism blocked this effect. Moreover, inhibition of opioid-induced excitatory signaling by naltrexone (1 ng/kg) unmasked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrexone could not increase the maximal anticonvulsant effect of morphine (1-3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while being completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids.

The novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 inhibits nociceptive responses, wind-up, and after-discharges in spinal neurons and withdrawal reflexes

Evidence shows that serotonin (5-HT) is involved in the transmission of nociception in the central nervous system. Using a new electrophysiological method of simultaneous recordings in rats we examined the actions of the novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 as well as those of the opioid receptor agonist fentanyl on simultaneously evoked responses of spinal dorsal horn (DH) wide-dynamic range (WDR) neurons and spinal withdrawal reflexes. Spinal withdrawal reflexes were studied by assessing the activity of single motor units (SMUs) electromyographically (EMG). Like that of 0.02 mg/kg fentanyl, intraperitoneal injection of 0.31 mg/kg of F 13640 markedly inhibited nociceptive pinch-evoked responses as well as C-fiber-mediated late responses including wind-up of both DH WDR neurons and SMUs to suprathreshold (1.5 x T) repeated (3 Hz) electrical stimulation. Specifically, in contrast to no significant depressive effects by fentanyl on 20 Hz electrically evoked after-discharge of DH WDR neurons, the after-discharges of DH WDR neurons and SMUs were significantly inhibited by F 13640 (P < 0.05 and P < 0.001, respectively). The inhibitory effects of F 13640 and fentanyl on responses of DH WDR neurons and SMUs were reversed by the specific antagonists WAY 100635 and naloxone, respectively, further indicating that this 5-HT1A receptor-modulated anti-nociception is mu-opioid receptor independent. For the first time, 5-HT1A receptors are clearly proved to be involved in the progressive wind-up to 3-Hz frequency of electrical stimulation as well as after-discharges of sensory input of DH WDR neurons, and simultaneously recorded motor output of spinal reflexes to 20-Hz frequency of electrical stimulation; this suggests that serotonin, through 5-HT1A receptors, exerts an inhibitory role in the control of obstinate pathological pain.

Dual, hyperalgesic, and analgesic effects of the high-efficacy 5-hydroxytryptamine 1A (5-HT1A) agonist F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt]: relationship with 5-HT1A receptor occupancy and kinetic parameters

The aim of the present study was to establish the relationship between the plasma and brain concentration-time profiles of F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt] after acute administration and both its hyper- and hypoanalgesic effects in rats. The maximal plasma concentration (C(max)) of F 13640 after i.p. administration of 0.63 mg/kg was obtained at 15 min and decreased to half its maximal value after about 1 h. The amount of F 13640 collected by means of in vivo microdialysis in hippocampal dialysates could be measured reliably after 0.63 and 2.5 mg/kg, reached its maximum at about 1 h, and fell to half of its maximal value at about 3 h. 5-Hydroxytryptamine 1A (5-HT(1A)) receptor occupancy was estimated by ex vivo binding in rat brain sections. F 13640 inhibited [(3)H]8-hydroxy-2-[di-n-propylamino] tetralin binding ex vivo in rat hippocampus, entorhinal cortex, and frontal cortex (ED(50), 0.34 mg/kg i.p.). Maximal inhibition was reached at approximately 30 min after 0.63 mg/kg F 13640 and fell to half of its value after about 4 to 8 h. After injection (15 min) in the paw pressure test, F 13640 (0.63 mg/kg i.p.) induced an initial hyperalgesia that was followed 4 h later by a paradoxical analgesia that lasted until 8 h. In contrast, in the formalin test, F 13640 inhibited pain behaviors until 4 h after drug administration. F 13640 also produced elements of the 5-HT syndrome that lasted up to 4 h after administration. These results demonstrate that F 13640 induces hyperalgesia and/or analgesia with a time course that parallels the occupancy of 5-HT(1A) receptors and the presence of the compound in blood and brain.