The genetic mediation of individual differences in sensitivity to pain and its inhibition

The maintenance of individual differences in the sensitivity of acute and neuropathic pain behaviors to electroacupuncture in rats

"Responder" Sprague-Dawley (SD) rats that were sensitive to electroacupuncture (EA) in an acute thermal pain test (i.e. tail flick latency [TFL] test) maintained sensitivity to EA in the warm allodynia test after peripheral nerve injury. Similarly, the "non-responder" SD rats that were insensitive to EA in the TFL test were also insensitive to EA in the allodynia test. The EA-induced analgesic effects in the TFL test were significantly higher in CCK-A receptor deficient, Otsuka Long-Evans Tokushima Fatty (OLETF) rats than in their littermates, Long-Evans Tokushima Otsuka (LETO) rats. Similarly, the anti-allodynic effects of EA were significantly greater in OLETF rats than in LETO rats. These results suggest that the individual differences in the sensitivity of acute pain behavior to EA were maintained in neuropathic pain behavior following peripheral nerve injury, and that CCK-A receptor expression plays an important role in mediating this phenomenon.

Morphine in postoperative patients: pharmacokinetics and pharmacodynamics of metabolites

BACKGROUND

There is great variability in the need for morphine in the postoperative period. We performed a pharmacokinetic-pharmacodynamic study considering the potential effect of the two main metabolites of morphine.

METHODS

Fifty patients with moderate to severe pain received morphine as an IV titration, followed by IM administration postoperatively. The plasma concentration of morphine, morphine-6-glucuronide (M-6-G), morphine-3-glucuronide (M-3-G), and pain intensity were measured at frequent intervals. Pharmacokinetic and pharmacodynamic fitting was performed with the software NONMEM.

RESULTS

The pharmacokinetics were largely predictable. M-6-G and M-3-G clearances were markedly decreased in patients with renal failure. The pharmacodynamics was less predictable, with an important interindividual variability. M-6-G was 7.8 times more potent than morphine, but the average time to peak concentration in the effect compartment after a bolus injection of morphine was 4.25 h for M-6-G, when compared to 0.33 h for morphine. M-3-G showed mild inhibition of the analgesic properties of morphine and of M-6-G. The time to M-3-G peak concentration in the effect compartment after a bolus injection of morphine was 10 h.

CONCLUSIONS

M-6-G is a potent opioid agonist and M-3-G a mild opioid antagonist. Both are poorly excreted in patients with renal failure. However, the metabolism of morphine was rapid when compared to the transfer of metabolites through the blood-brain barrier, which appears to be the limiting process. Because poor analgesia due to M-3-G's effect may occur in some patients after 1 or 2 days, a switch to other molecules should be considered.

Is there genetic polymorphism evidence for individual human sensitivity to opiates?

Opiate analgesics have been widely used for severe acute pain and chronic cancer-related pain. Individual differences in the effectiveness of opiates and their side effects limit the clinical benefits and increase risks of drug abuse. Genetic factors might affect variations of opiate sensitivity. The mu opioid peptide receptor (MOP) is the principal site of pharmacologic actions for most clinically important opiate drugs. Recent studies using various knockout mice and recombinant-inbred strain CXBK mice have indicated that the analgesic effect of morphine is dependent on the amount of the MOP. There are more than 100 polymorphisms identified in the human MOP (OPRM1) gene. These polymorphisms might be correlated with OPRM1 mRNA stability and opiate sensitivity, including opiate analgesia, tolerance, and dependence. More precise studies on the relationship between gene polymorphisms and opiate sensitivity will enable realization of personalized pain treatment by predicting opiate sensitivity and requirement for each patient.

The importance of genetic background on pain behaviours and pharmacological sensitivity in the rat spared serve injury model of peripheral neuropathic pain

Neuropathic pain conditions can encompass a diverse constellation of signs and symptoms consisting of sensory deficits, allodynia and hyperalgesia. Animal models of neuropathic pain have enabled the identification of key pathophysiological changes occurring within nociceptive pathways as a result of injury, and serve an invaluable role for preclinical screening of novel analgesic candidates. We have produced the first systematic description of the development and maintenance, and the pharmacological sensitivity of nociceptive behaviours in four rat strains with different genetic background (outbred Sprague-Dawley and inbred Brown Norway, Lewis and Fischer 344 rats), using the spared nerve injury model of peripheral neuropathic pain. Hindpaw mechanical hypersensitivity was evident from 7 to 30 days post-injury in all four strains, developing most quickly and severely in Fischer 344 rats; Lewis rats were least affected. Morphine (6 but not 3 mg/kg, s.c.) and gabapentin (100 but not 50 mg/kg, s.c.) had significant antiallodynic and antihyperalgesic actions in all four strains after spared nerve injury, although marked differences in potency across strains were observed. Two strains (Fischer 344 rats and Lewis) were insensitive to the antihyperalgesic properties of gaboxadol (15 mg/kg) whereas gaboxadol (6 mg/kg) was equipotent to morphine (6 mg/kg) in two other strains (Sprague-Dawley and Brown Norway). The observed pharmacogenetic variations have important implications for the preclinical testing of drugs, and provide a basis for development of pharmacogenomics in neuropathic pain.

Limitations of pharmacotherapy: behavioral approaches to chronic pain

Pharmacotherapy is most appropriate in acute pain, whereas in chronic pain states behavioral approaches or a combination of behavioral treatment and pharmacotherapy is more appropriate. In this chapter we first describe the role of learning and memory as well as other psychological factors in the development of chronic pain and emphasize that chronic pain must viewed as the result of a learning process with resulting central neuroplastic changes. We then describe operant behavioral and cognitive-behavioral treatments as well as biofeedback and relaxation techniques and present innovative treatment procedures aimed at altering central pain memories. We complete the section with a discussion of combined behavioral and pharmacological approaches and an interdisciplinary view.

Exploring joint effects of genes and the clinical efficacy of morphine for cancer pain: OPRM1 and COMT gene

Pain is a complex human trait. It is likely that the interaction of multiple genes, each with a small individual effect, along with the effect of environmental factors, influences the clinical efficacy of opioids rather than a single gene alone. Polymorphisms in genes coding for the mu-opioid receptor (A118G) and catechol-O-methyl transferase (Val158Met) may be important modulators of opioid efficacy. We assessed joint effects of the OPRM1 and COMT genes in predicting morphine dose for cancer pain relief. We used genotype and clinical data from a pharmacokinetic study of morphine in 207 inpatients treated with stable morphine dose for at least 3 days by Palliative Medicine Specialists. Results showed significant variation in morphine dose requirement by genotype groups: carriers of COMT Val/Val and Val/Met genotype required 63% and 23%, respectively, higher morphine dose compared to carriers of Met/Met genotype (p=0.02). Carriers of OPRM1 GG genotype required 93% higher morphine dose compared to carriers of AA genotypes (p=0.012). When we explored for joint effects, we found that carriers of the OPRM1 AA and COMT Met/Met genotype required the lowest morphine dose to achieve pain relief (87 mg/24 h; 95%CI=57,116) and those with neither Met/Met nor AA genotype needed the highest morphine dose (147 mg/24 h; 95%CI=100,180). The significant joint effects for the Met/Met and AA genotypes (p<0.012) persisted, even after controlling for demographic and clinical variables in the multivariable analyses. Future studies are needed to further characterize the joint effects of multiple genes, along with demographic and clinical variables, in predicting opioid dose.

Proposal for a standardized protocol for the systematic orofacial examination of patients with Hereditary Sensory Radicular Neuropathy

AIM

To apply a standardized protocol for the orofacial evaluation of two adult siblings (one male and one female) with Hereditary Sensory Radicular Neuropathy (HSRN) that presented with dental problems.

SUMMARY

The systematic evaluation consisted of (a) clinical questionnaire; (b) radiographs [orthopantomography and computarized tomography (CT)]; (c) orofacial psychophysical tests (pain, thermal, mechanical and electrical sensation); and (d) histology of gingiva and pulp (optical and transmission electronic microscopy). The female patient had complete insensitivity to orofacial pain and partial facial heat sensitivity, and received dental treatment without anaesthesia or pain. She had a severe and painless jaw infection due to pulp necrosis in tooth 37. The male patient had partial insensitivity to orofacial pain and required anaesthesia for dental treatment. Histological examination of gingivae and pulpal tissue revealed an altered proportion of unmyelinated and myelinated sensory nerve fibres.

KEY LEARNING POINTS

* Patients with HSRN may present with significant, silent dental disease. * A standard protocol is helpful when evaluating such patients. * If the opportunity arises, evaluation of pulp tissue may reveal an altered proportion of myelinated and unmyelinated nerve fibres. This may avoid the more estabilished sural nerve biopsy.

Safety and Efficacy of Hydromorphone as an Analgesic Alternative to Morphine in Acute Pain: A Randomized Clinical Trial

STUDY OBJECTIVE

We compare a standard weight-based dose of intravenous hydromorphone (Dilaudid) to a standard weight-based dose of intravenous morphine in adults presenting to the ED with acute severe pain.

METHODS

This was a prospective, randomized, double-blind, clinical trial conducted in an academic medical center. Of the 198 adult patients presenting to the ED with acute severe pain who were randomized to receive either intravenous hydromorphone at 0.015 mg/kg or intravenous morphine at 0.1 mg/kg, 191 patients had sufficient data for analysis. The main outcome measure was the difference between the 2 groups in pain reduction at 30 minutes as measured on a validated numeric rating scale. Adverse effects, pain reduction at 5 minutes and 2 hours postbaseline, and additional analgesics and antiemetics were tracked as secondary outcome measures.

RESULTS

The mean change of pain from baseline to 30 minutes postbaseline in patients allocated to intravenous hydromorphone was -5.5 numeric rating scale units versus -4.1 in patients allocated to intravenous morphine (difference -1.3; 95% confidence interval -2.2 to -0.5). Adverse effects were similar in both groups, with the exception of pruritus, which did not occur in patients receiving hydromorphone (0% versus 6% [difference -6%; 95% confidence interval -11% to -1%]). No patient required naloxone.

CONCLUSION

For the treatment of acute, severe pain in the emergency department, intravenous hydromorphone at 0.015 mg/kg represents a feasible alternative to intravenous morphine at 0.1 mg/kg.

Venlafaxine compromises the antinociceptive actions of gabapentin in rat models of neuropathic and persistent pain

RATIONALE

Neuropathic pain is associated with a number of disease states of diverse aetiology that can share common pathophysiological mechanisms. Antiepileptic drugs modulate ion channel function and antidepressants increase extracellular monoamine levels, and both drug classes variously attenuate signs and symptoms of neuropathic pain. Thus, coadministration of the antiepileptic gabapentin and the antidepressant venlafaxine may provide superior pain relief to administration of either drug alone.

OBJECTIVES

To systematically establish the pain relieving efficacies of venlafaxine and gabapentin alone and in combination.

MATERIALS AND METHODS

Gabapentin (50 and 100 mg/kg, s.c.) and venlafaxine (10, 25, 50 mg/kg, s.c.) were tested alone or in combination in the rat spared nerve injury (SNI) model of neuropathic pain and the rat formalin test of persistent pain. Diuresis was measured in a separate experiment after administration of venlafaxine.

RESULTS

Hindpaw mechanical allodynia was dose-dependently reversed by gabapentin (50 and 100 mg/kg, s.c.), whereas venlafaxine was ineffective (10 and 50 mg/kg, s.c.). Both gabapentin and venlafaxine also attenuated hindpaw mechanical hyperalgesia. Surprisingly, coadministration of venlafaxine (50 mg/kg) significantly lowered the antiallodynic effect of both doses of gabapentin by up to 60% in spared-nerve-injury rats and a negative antinociceptive interaction between gabapentin and venlafaxine was also observed in the rat formalin test. We demonstrated that venlafaxine administration was associated with a dose-dependent increase in urine output over the time course of the nociceptive experiments.

CONCLUSION

Venlafaxine compromises the antiallodynic effects of coadministered gabapentin most probably as consequence-increased diuresis.

Clinical Pharmacology and Pharmacotherapy of Opioid Switching in Cancer Patients

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.

Integrating molecular genetics analyses into clinical research

The integration of molecular genetics approaches into the study of complex health phenomena is an increasingly important and available strategy for researchers across the health science disciplines. Pain sensation and response to painful stimuli are examples of complex health phenomena that are particularly amenable to molecular genetics approaches. Both human and animal model research suggests that differences in these responses may be related, in part, to variation in the genes that modulate sensation and behavior. The authors are currently managing a large cross-disciplinary research effort to identify child characteristics, including genotypes, that predict the degree of distress displayed by children following a painful medical procedure (i.e., IV insertion). The purpose of this article is to describe the strategies used to integrate molecular genetics methods into this project. The authors discuss the steps needed to complete this process, including (a) establishing a collaboration with genetics researchers and laboratory facilities, (b) developing and implementing a plan to manage biologic samples, and (c) incorporating genetics into the informed consent process.

Can personality traits and gender predict the response to morphine? An experimental cold pain study

The aim of the present study was to examine the possible role of personality traits, in accordance with Cloninger's theory, and gender, in the variability of responsiveness to opioids. Specifically, it was intended to test whether or not the three personality dimensions - harm avoidance (HA), reward dependence (RD) and novelty seeking (NS) - as suggested by Cloninger, can predict inter-personal differences in responsiveness to morphine after exposure to experimental cold pain. Thirty-four healthy volunteers (15 females, 19 males) were given the cold pressor test (CPT). Pain threshold, tolerance, and magnitude (VAS) were measured before and after (six measures, 30 min apart) the administration of either 0.5 mg/kg oral morphine sulphate (n=21) or 0.33 mg/kg oral active placebo (diphenhydramine) (n=13) in a randomized, double blind design. Assessment of the three personality traits, according to Cloninger's Tridimensional Personality Questionnaire, was performed before the CPT. A high HA score (but not RD, NS, or baseline values of the three pain parameters) predicted a significantly larger pain relief following the administration of morphine sulphate (but not of the placebo). Women exhibited a larger response in response to both treatments, as indicated by a significantly increased threshold and tolerance following morphine sulphate as well as significantly increased tolerance and decreased magnitude following placebo administration. The present study confirms the existence of individual differences in response to analgesic treatment. It suggests that high HA personality trait is associated with better responsiveness to morphine treatment, and that females respond better than men to both morphine and placebo.

Parental substance abuse, reports of chronic pain and coping in adult patients with sickle cell disease

There is increasing interest from a social learning perspective in understanding the role of parental factors on adult health behaviors and health outcomes. Our review revealed no studies, to date, that have evaluated the effects of parental substance abuse on reports of chronic pain and coping in adult patients with sickle cell disease (SCD). We explored the effects of parental substance (alcohol or drug) abuse on reports of the sensory, affective and summary indices of pain in 67 adult patients, mean age 38.9 (13.5), with SCD. We also explored the effects of parental substance abuse on psychopathology associated with pain and active coping. Twenty-four percent of patients reported that their parent(s) abused substances. Patients whose parent(s) were characterized as substance abusers reported greater sensory (p=0.02), affective (p=0.01) and summary (VAS; p=0.02) indices of pain as compared to their counterparts, whose parent(s) were not characterized as substance abusers. Patients did not differ in average age, education or the propensity to respond in a socially acceptable manner. There was a significant trend towards patients who characterized their parents as abusers scoring higher than their counterparts on active coping. We propose a Social Learning Theory to explain the current findings and suggest a need for additional prospective research to simultaneously explore biological (genetic) and social factors that influence the interpretation, experience and reporting of chronic pain in adult patients with chronic disease.

Variation and homogeneity in affective responses to physical activity of varying intensities: an alternative perspective on dose-response based on evolutionary considerations

A model for systematic changes in patterns of inter-individual variation in affective responses to physical activity of varying intensities is presented, as a conceptual alternative to the search for a global dose-response curve. It is theorized that trends towards universality will emerge in response to activities that are either generally adaptive, such as moderate walking, or generally maladaptive, such as strenuous running that requires anaerobic metabolism and precludes the maintenance of a physiological steady state. At the former intensity the dominant response will be pleasure, whereas at the latter intensity the dominant response will be displeasure. In contrast, affective responses will be highly variable, involving pleasure or displeasure, when the intensity of physical activity approximates the transition from aerobic to anaerobic metabolism, since activity performed at this intensity entails a trade-off between benefits and risks. Preliminary evidence in support of this model is presented, based on a reanalysis of data from a series of studies.

Mitochondrial uncoupling proteins in the CNS: in support of function and survival

Mitochondrial uncoupling mediated by uncoupling protein 1 (UCP1) is classically associated with non-shivering thermogenesis by brown fat. Recent evidence indicates that UCP family proteins are also present in selected neurons. Unlike UCP1, these proteins (UCP2, UCP4 and BMCP1/UCP5) are not constitutive uncouplers and are not crucial for non-shivering thermogenesis. However, they can be activated by free radicals and free fatty acids, and their activity has a profound influence on neuronal function. By regulating mitochondrial biogenesis, calcium flux, free radical production and local temperature, neuronal UCPs can directly influence neurotransmission, synaptic plasticity and neurodegenerative processes. Insights into the regulation and function of these proteins offer unsuspected avenues for a better understanding of synaptic transmission and neurodegeneration.

Transcriptional profiling of spinal cord injury-induced central neuropathic pain

Central neuropathic pain (CNP) is an important problem following spinal cord injury (SCI), because it severely affects the quality of life of SCI patients. As in the patient population, the majority of rats develop significant allodynia (CNP rats) after moderate SCI. However, about 10% of SCI rats do not develop allodynia, or develop significantly less allodynia than CNP rats (non-CNP rats). To identify transcriptional changes underlying CNP development after SCI, we used Affymetrix DNA microarrays and RNAs extracted from the spinal cords of CNP and non-CNP rats. DNA microarry analysis showed significantly increased expression of a number of genes associated with inflammation and astrocytic activation in the spinal cords of rats that developed CNP. For example, mRNA levels of glial fibrilary acidic protein (GFAP) and Aquaporin 4 (AQP4) significantly increased in CNP rats. We also found that GFAP, S100beta and AQP4 protein elevation persisted for at least 9 months throughout contused spinal cords, consistent with the chronic nature of CNP. Thus, we hypothesize that CNP development results, in part, from dysfunctional, chronically "over-activated" astrocytes. Although, it has been shown that activated astrocytes are associated with peripheral neuropathic pain, this has not previously been demonstrated in CNP after SCI.

PK-PD modeling of buprenorphine in cats: intravenous and oral transmucosal administration1

The pharmacokinetics and thermal antinociceptive effects of buprenorphine after intravenous (i.v.) or oral transmucosal (OTM) administration were studied in six adult cats. Plasma buprenorphine concentrations were measured using radioimmunoassay in a crossover study after a dose of 20 microg/kg given by the i.v. or OTM route. Oral pH was measured. Blood for drug analyses was collected before, and at 1, 2, 4, 6, 10, 15, 30, and 60 min and at 2, 4, 6, 8, 12, and 24 h after treatment. Thermal thresholds were measured before treatment, then following treatment every 30 min to 6 h, every 1 hour to 12 h and at 24 hours postadministration. Plasma buprenorphine concentration effect relationships were analyzed using a log-linear effect model. Oral pH was 9 in each cat. Peak plasma buprenorphine concentration was lower and occurred later in the OTM group but median bioavailability was 116.3%. Thermal thresholds increased significantly between 30 and 360 min in both groups. Peak effect was at 90 min and there was no difference at any time between the two groups. There was distinct hysteresis between plasma drug concentration and effect in both groups. Overall, OTM administration of buprenorphine is as effective as i.v. treatment and offers a simple, noninvasive method of administration which produces thermal antinociception for up to 6 h in cats.

Sind μ-Opioidrezeptorpolymorphismen wichtig für die Opioidtherapie?

Polymorphisms in the mu-opioid receptor gene may potentially alter the clinical effects of opioid analgesics. A common mu-opioid receptor polymorphism occurring at an allelic frequency of 12% decreases the potency of opioid analgesics in humans. Interestingly, in carriers of this mutation, it appears to be possible to reach analgesia by increasing the opioid dose but side effects appear to occur less often despite the higher opioid dose. This suggests a broadened therapeutic range of the opioids. Other mutations of the mu-opioid receptor, for example three mutations within the third intracellular loop of the receptor, impair receptor signaling, but they are too rare to greatly affect pain therapy or have not yet been investigated in the context of pain therapy.

pain1: A neuropathic pain QTL on mouse chromosome 15 in a C3H×C58 backcross

We have produced a backcross (BC) population of 267 mice from the parental strains C3H/HeN and C58/J. The mice were phenotyped for neuropathic pain using the neuroma model. Subsequently all BC mice were genotyped in a region of chromosome 15 that has been previously suggested to contain a quantitative trait locus (QTL) for this trait. We have confirmed the linkage of the QTL, named pain1, to the central region of chromosome 15. Our finding provides the necessary robustness to justify efforts towards identification of the underlying gene.

The effect of antinociceptive drugs tested at different times after nerve injury in rats

Given the evolving nature of anatomical and functional changes in the nervous system that are involved in the development of neuropathic pain, it is possible that the differing time course after injury underlies the inconsistent efficacy of drugs in neuropathic pain patients. In the current study, we evaluated the behavioral effects of two standard drugs used clinically for neuropathic pain, the anticonvulsant gabapentin and antidepressant imipramine, in rats at different times after peripheral nerve injury. Rats that underwent the spared nerve injury procedure responded to an innocuous mechanical stimulus (von Frey filament) 2, 4, and 8 wk after injury. Gabapentin dose-dependently suppressed mechanical sensitivity at all time points tested but the potency of gabapentin was three-fold less 4 wk postinjury (135 mg/kg) compared with 2 and 8 wk postinjury (41 and 44 mg/kg, respectively). In contrast, imipramine lacked significant efficacy at 2 and 8 wk postinjury but slightly attenuated mechanical hypersensitivity at 4 wk postinjury. The results show that drug effects may change over time in the neuropathic state, which should be an important consideration in the evaluation of drugs in preclinical animal pain models and has implications for temporal approaches to therapy in the clinic.

The A118G single nucleotide polymorphism of the mu-opioid receptor gene (OPRM1) is associated with pressure pain sensitivity in humans

Responses to painful stimuli are characterized by tremendous interindividual variability, and genetic factors likely account for some proportion of this variability. However, few studies have identified genetic contributions to experimental pain perception in humans. This experiment investigated whether the A118G single nucleotide polymorphism of the mu-opioid receptor gene ( OPRM1 ) was associated with responses to three different experimental pain modalities in a sample of 167 healthy volunteers (96 female, 71 male). Responses to thermal, mechanical, and ischemic pain were assessed in all subjects, and genotyping of OPRM1 was performed, which revealed that the rare A118G allele occurred in 24 females (25%) and 12 males (17%). Statistical analyses indicated that subjects with a rare allele had significantly higher pressure pain thresholds than those homozygous for the common allele. Also, a sex by genotype interaction emerged for heat pain ratings at 49 degrees C, such that the rare allele was associated with lower pain ratings among men but higher pain ratings among women. These data indicate an association of a common single nucleotide polymorphism of OPRM1 with mechanical pain responses and that this genotype may be associated with heat pain perception in a sex-dependent manner. This study examines the association of the A118G SNP of OPRM1 to experimental pain sensitivity. The results indicate that the rare allele is associated with higher pressure pain thresholds. These results support previous contentions that OPRM1 may be a pain-relevant gene; however, replication of these findings is needed.

How individual sensitivity to opiates can be predicted by gene analyses

Opiate analgesics are widely used and abused drugs. Individual differences in opiate sensitivity can hamper effective pain treatments and increase risks of drug abuse. Although genetic factors might affect individual differences in opiate sensitivity, scientific evidence for specific genetic mechanisms that underlie these differences has been sparse. Recent studies using inbred and knockout mice have revealed that the mu opioid peptide (MOP) receptor encoded by the Oprm1 gene has a mandatory role in the analgesic and addictive properties of opiate drugs. Increasing evidence suggests that differences in Oprm1 gene sequences affect the amount of Oprm1 mRNA and sensitivity to opiates, and >100 polymorphisms have been identified in the human OPRM1 gene, some of which are related to vulnerability to drug dependence in some populations. Rapid advances in this research field are leading to improved understanding of the relationships between gene polymorphisms and opiate sensitivities that will enable more-accurate prediction of the opiate sensitivity and opiate requirements in individual patients.

Molecular biology of opioid analgesia

Opioids provide excellent pain relief in most patients. Yet the responses of patients to individual opioids can vary markedly, even among the mu opioids. Understanding this variability would greatly enhance our ability to treat patients appropriately. Classical pharmacological studies have long implied the existence of multiple subtypes of mu opioid receptors. More recently, a number of variants of the cloned mu opioid receptor have been described. These variants all show the same selectivity for mu opioids, confirming their classification as mu opioid receptors. Yet, they differ in their functional activation by opioids as well as in their localization within cells and regions in the brain. These multiple mu opioid receptors may help explain the range of responses seen clinically among patients for the various opioid drugs.

Pharmacokinetic-pharmacodynamic correlations and biomarkers in the development of COX-2 inhibitors

The mechanism by which COX inhibitors exert their analgesic effect is well established. However, data show no direct correlation between drug concentrations in plasma and the analgesic or adverse effects in chronic inflammatory conditions. This represents a major problem in the development of COX inhibitors, since it is difficult to predict the appropriate dosing regimen for the treatment of chronic inflammatory pain, based upon information from pre-clinical studies and eventually early clinical studies. The factors that determine response in inflammatory pain must be understood in order to make predictions about the time course of the analgesic effect. In this review the determinants of drug response and their variability will be discussed: physicochemical properties, pharmacokinetics (PK), pathophysiology and disease progression. From a mechanistic point of view, endogenous mediators of inflammation might be used as a biomarker for the analgesic effect and safety assessment. Such a biomarker can be an intermediate step between drug exposure and response. In addition, its concentration-effect relationship could be characterized by pharmacokinetic-pharmacodynamic (PK/PD) modelling. Indeed, recent investigations have shown that COX-2 inhibition, as determined by modelling of prostaglandin E2 (PGE2) levels in the whole blood assay in vitro can be used as a marker to predict drug effects (analgesia) in humans. A model-derived parameter, IC80, (total and unbound) was found to correlate directly with the analgesic plasma concentration of different COX inhibitors varying in enzyme selectivity. These findings indicate that PGE2 and thromboxane B2 inhibition can be used to predict and select efficacious doses in humans.

Are μ-opioid receptor polymorphisms important for clinical opioid therapy?

Mutations in the mu-opioid receptor--the primary site of action of opioid analgesics--are candidates for the variability of clinical opioid effects. This has been substantiated by recent advances in genetic research. A common mu-opioid receptor polymorphism was associated with higher demands for alfentanil or morphine for pain relief. It also decreased the potency of morphine for pupil constriction and experimental analgesia, but its molecular mechanisms are unclear. Another opioid receptor mutation greatly impaired receptor signalling in vitro, but is very rare. The accumulated evidence provides a solid basis for continuing research that should address the underlying molecular mechanisms and the role and benefits of OPRM1 genotyping for clinical pain therapy.

Comparison of the antinociceptive effect of morphine, methadone, buprenorphine and codeine in two substrains of Sprague-Dawley rats

Sprague-Dawley rats from two different vendors, Möllegård, Denmark and B&K Universal, Sweden, have been tested for the antinociceptive effect of morphine, methadone, buprenorphine and codeine on the hot plate. Morphine and methadone had significantly weaker effect in Möllegård rats compare to B&K rats. In contrast, the effect of buprenorphine was stronger in Möllegård rats than in B&K rats and the effect of codeine was similar in the two substrains. Plasma levels of morphine, morphine-6-glucuronide, morphine-3-glucuronide, buprenorphine and norbuprenorphine were determined at two time points after drug injection. Möllegård rats had significantly lower mean plasma level of morphine and significantly higher ratio of morphine-3-glucuronide/morphine at 30 min, compared to B&K rats. No difference was seen for the metabolism of buprenorphine in the two substrains. The results suggest that Möllegård rats metabolize morphine to morphine-3-glucuronide to a greater extent than B&K rats, and this may at least partly underlie the substrain difference in the effect of morphine. It is also suggested that the antinociceptive mechanisms of buprenorphne may be different from those of morphine and methadone.

Managing pain in feline patients

In the past 10 years, great strides have been made in the field of feline analgesia. A better understanding of the cat's unique metabolism has led researchers to realize that extrapolation across species boundaries is unwise,and this has resulted in feline-specific studies. The opioids are now used more commonly in cats, with good analgesic effect and few side effects. Excellent acute pain management is achievable in cats by using opioids, NSAIDs, alpha2-agonists, and local anesthetics. Although much of the research data has compared the use of single drugs, a multimodal approach using agents that work at different parts of the pain pathway is commonly used in clinical settings, with added benefit. Compared with dogs, few pain-scoring systems have been developed for cats, and this remains an important goal. Management of chronic pain in cats is a challenge because of the potential problems with long-term NSAID use; however, reports of low doses given at extended intervals are encouraging. As we gain experience with less traditional analgesics, such as amitriptyline, amantadine, and gabapentin, and critically evaluate complimentary therapies, our ability to provide comfort to this population of cats will improve.

Can coadministration of oxycodone and morphine produce analgesic synergy in humans? An experimental cold pain study

AIMS

The coadministration of subantinociceptive doses of oxycodone with morphine has recently been shown to result in a synergistic antinociceptive effect in rats. The present study was aimed to investigate the possibility that coadministration of morphine and oxycodone can produce a similar synergistic effect in humans exposed to an experimental model of cold pressor test (CPT).

METHODS

The enriched enrollment design was used to exclude 'stoic' and 'placebo responders' in a single-blind fashion. 'Nonstoic', placebo 'nonresponder' female volunteers (n = 30) were randomly assigned to receive 0.5 mg kg(-1) oral morphine sulphate, 0.5 mg kg(-1) oral oxycodone hydrochloride, and the combination of 0.25 mg kg(-1) morphine sulphate with 0.25 mg kg(-1) oxycodone hydrochloride, 1 week apart from each other, in a double-blind crossover design. Latency to pain onset (threshold), pain intensity (VAS), and pain tolerance (time until removal of the hand from the water) were measured six times over a 3-h period, subsequent to the administration of each medication, and were used to assess their antinociceptive effect.

RESULTS

The combination produced a significantly higher effect on latency to pain onset than that of morphine alone [difference in mean postbaseline value 2.2; 95% confidence interval (CI) 0.48, 3.9; P = 0.01] but the effect was nonsignificantly smaller that that of oxycodone alone. Similarly, the effect of the combination on pain tolerance was significantly larger than that of morphine alone (combination difference 8.4; 95% CI 2.5, 14.3; P = 0.007), whereas oxycodone alone caused a nonsignificantly larger effect than that of the combination treatment. Comparisons of pain magnitude failed to show any significant differences between the three treatments.

CONCLUSIONS

These results indicate that at the doses tested, morphine and oxycodone do not produce synergistic antinociceptive effects in healthy humans exposed to the CPT.

Sex differences in opioid analgesia: clinical and experimental findings

Sex differences in analgesic responses to opioids have received increasing attention in recent years. This article examines the literature on sex differences in opioid analgesia, including the results of studies from the authors' own laboratories. In general, nonhuman animal studies suggest more robust opioid analgesic responses in males relative to females; however, the human studies completed to date seem to indicate greater opioid analgesia among females. The most consistent evidence of sex differences in analgesia comes from studies of kappa-agonist-antagonists administered to patients following oral surgery. These data indicate more robust analgesia in females, and dose-response characteristics suggest that these agents possess both analgesic and antianalgesic properties, and the agonists may produce these effects in different proportions for women versus men. In contrast, the data from laboratory pain models in humans suggest greater analgesic effects in women in response mu-opioid agonists but not kappa-agonist-antagonists. Multiple mechanisms may explain sex differences in opioid analgesia, including gonadal hormonal effects, pharmacokinetics and pharmacodynamics, genetic influences, balance of analgesic/antianalgesic processes, and psychological factors. However, the disparity of results obtained from different pain models--animals versus humans and clinical pain versus experimental pain in humans--suggests that the models themselves are mechanistically different. Additional investigation is warranted in order to further explicate the nature of sex differences in opioid analgesia and to elucidate the underlying mechanisms.

Genetic basis for individual variations in pain perception and the development of a chronic pain condition

Pain sensitivity varies substantially among humans. A significant part of the human population develops chronic pain conditions that are characterized by heightened pain sensitivity. We identified three genetic variants (haplotypes) of the gene encoding catecholamine-O-methyltransferase (COMT) that we designated as low pain sensitivity (LPS), average pain sensitivity (APS) and high pain sensitivity (HPS). We show that these haplotypes encompass 96% of the human population, and five combinations of these haplotypes are strongly associated (P=0.0004) with variation in the sensitivity to experimental pain. The presence of even a single LPS haplotype diminishes, by as much as 2.3 times, the risk of developing myogenous temporomandibular joint disorder (TMD), a common musculoskeletal pain condition. The LPS haplotype produces much higher levels of COMT enzymatic activity when compared with the APS or HPS haplotypes. Inhibition of COMT in the rat results in a profound increase in pain sensitivity. Thus, COMT activity substantially influences pain sensitivity, and the three major haplotypes determine COMT activity in humans that inversely correlates with pain sensitivity and the risk of developing TMD.

Sex differences in temporal characteristics of descending inhibitory control: an evaluation using repeated bilateral experimental induction of muscle pain

Little is known about sex differences in the temporal pattern of descending inhibitory mechanisms, such as descending noxious inhibitory control (DNIC). Sex differences in temporal characteristics of DNIC were investigated by measuring pressure pain thresholds (PPTs) over time in the trapezius muscles (local pain areas) and the posterolateral neck muscles (referred pain areas) following repeated bilateral injection of hypertonic versus isotonic saline into both trapezius muscles. Ten females and 11 males received two consecutive bilateral injections, with 15 min interval, of either 5.8% hypertonic saline (0.5 ml in each side for each bilateral injection) or isotonic saline as a control in a randomized manner. Following hypertonic saline injection, the maximal pain intensities of the first and second bilateral injections were significantly higher in females than in males. The PPTs in the trapezius muscles were significantly lower in females than in males. Significantly higher PPTs (hypoalgesia) in men than in women were shown 15 min after the first bilateral injection, and 7.5 and 15 min after the second bilateral injection in the referred pain areas. Importantly, the second bilateral injection failed to further increase the PPTs for both sexes. These results showed that there were sex differences in temporal characteristics of descending inhibition with long-lasting hypoalgesia in men than in women. Repeated noxious muscular stimuli may inhibit further build-up of DNIC, which may reflect a mechanism of plasticity of the descending inhibitory systems following recurrent nociceptive barrage for both sexes.

Use of thermal threshold response to evaluate the antinociceptive effects of butorphanol in cats

OBJECTIVE

To characterize the antinociceptive actions of several doses of butorphanol by use of a thermal threshold testing device specifically designed for cats.

ANIMALS

6 domestic shorthair cats.

PROCEDURE

The study was a masked, randomized, crossover design. Thermal thresholds were measured by use of a thermal threshold-testing device specifically developed for cats. A small probe containing a heater element and temperature sensor was held with consistent contact against a shaved area of the cat's skin with an elasticized band. Skin temperature was recorded before each test, prior to activation of the heater. On detection of a response (eg, the cat flinched, turned, or jumped), the stimulus was terminated and the threshold temperature recorded. Three baseline measurements were recorded before IV injection of 0.1, 0.2, 0.4, or 0.8 mg of butorphanol/kg. Each cat received all doses in a randomized order at least 1 week apart. The investigator was unaware of the treatment received. Thermal thresholds were measured every 15 minutes for 6 hours.

RESULTS

Mean+/-SD pretreatment threshold temperature for all cats was 40.8+/-2.2 degrees C. There were no dose-related differences among treatments. There was a significant increase in threshold values for all treatments from 15 to 90 minutes after injection. Mydriasis was detected in all cats after treatment with butorphanol and dysphoric behavior was frequently exhibited.

CONCLUSIONS AND CLINICAL RELEVANCE

Results obtained by use of a thermal stimulus indicated that the duration of antinociceptive action of butorphanol was 90 minutes and there was no dose-response relationship in cats.

Rodent strain differences in the NPFF1 and NPFF2 receptor distribution and density in the central nervous system

The present study evaluates the putative differences between NPFF1 and NPFF2 receptor distribution and density throughout the central nervous system between rat and mouse strains by using in vitro quantitative autoradiography. The binding of [125I]YVP ([125I]YVPNLPQRF-NH2) and [125I]EYF ([125I]EYWSLAAPQRF-NH2), used to label NPFF1 and NPFF2 receptors, respectively, was compared between Sprague-Dawley and Wistar rats and between Swiss and C57BL/6-SV129 mice. In contrast to Wistar, Sprague-Dawley brains contained NPFF1 binding sites in the cortical and spinal cord areas, the accumbens nucleus, the anterodorsal thalamic nucleus, the parafascicular thalamic nucleus, the inferior colliculus and the nucleus of the solitary tract. The distribution of NPFF2 binding sites was also different between the two strains of rats. As compared to Swiss, C57BL/6-SV129 mice showed higher basal NPFF2 receptor levels in cortical areas, telencephalon and some other regions. In contrast, they showed lower amounts in thalamic structures, except the reuniens nucleus, and in mesencephalic and rhombencephalic regions. In the cervical spinal cord the levels of NPFF2 receptors were similar. The NPFF1 binding levels were nearly the same in telencephalic structures while distinct in the forebrain. Differences in amount of NPFF receptor subtypes among these strains of rats or mice could lead to differences in NPFF control of opioid nociception.

Spinal G-protein-gated K+ channels formed by GIRK1 and GIRK2 subunits modulate thermal nociception and contribute to morphine analgesia

G-protein-gated potassium (K+) channels are found throughout the CNS in which they contribute to the inhibitory effects of neurotransmitters and drugs of abuse. Recent studies have implicated G-protein-gated K+ channels in thermal nociception and the analgesic action of morphine and other agents. Because nociception is subject to complex spinal and supraspinal modulation, however, the relevant locations of G-protein-gated K+ channels are unknown. In this study, we sought to clarify the expression pattern and subunit composition of G-protein-gated K+ channels in the spinal cord and to assess directly their contribution to thermal nociception and morphine analgesia. We detected GIRK1 (G-protein-gated inwardly rectifying K+ channel subunit 1) and GIRK2 subunits, but not GIRK3, in the superficial layers of the dorsal horn. Lack of either GIRK1 or GIRK2 was correlated with significantly lower expression of the other, suggesting that a functional and physical interaction occurs between these two subunits. Consistent with these findings, GIRK1 knock-out and GIRK2 knock-out mice exhibited hyperalgesia in the tail-flick test of thermal nociception. Furthermore, GIRK1 knock-out and GIRK2 knock-out mice displayed decreased analgesic responses after the spinal administration of higher morphine doses, whereas responses to lower morphine doses were preserved. Qualitatively similar data were obtained with wild-type mice after administration of the G-protein-gated K+ channel blocker tertiapin. We conclude that spinal G-protein-gated K+ channels consisting primarily of GIRK1/GIRK2 complexes modulate thermal nociception and mediate a significant component of the analgesia evoked by intrathecal administration of high morphine doses

Structural, psychological, and genetic influences on low back and neck pain: a study of adult female twins

OBJECTIVE

To assess genetic and environmental influences on low back and neck pain in a classic twin design and to examine the extent to which these are explained by structural changes seen on magnetic resonance imaging (MRI) and psychological and lifestyle variables.

METHODS

The subjects comprised 1,064 unselected women (181 monozygotic [MZ] and 351 dizygotic [DZ] twin pairs) recruited from a national registry of twin volunteers. Outcome measures included lifetime history of low back and neck pain (using a range of increasingly stringent definitions), MRI scores of disc degeneration in the lumbar and cervical spine, psychological distress as assessed by the General Health Questionnaire (GHQ), and lifestyle variables assessed by questionnaire.

RESULTS

For all definitions of pain, there was a consistent excess concordance in MZ when compared with DZ twins, equating to a heritability for low back pain in the range of 52-68% and for neck pain in the range of 35-58%. The strongest associations were between low back pain and MRI change (odds ratio [OR] 3.6, 95% confidence interval [95% CI] 1.8-7.3]) and between neck pain and response on the GHQ (OR 3.3, 95% CI 2.1-5.0). These associations were mediated genetically.

CONCLUSIONS

Genetic factors have an important influence on back and neck pain reporting in women. These factors include the genetic determinants of structural disc degeneration and an individual's inherited tendency toward psychological distress. MRI changes are the strongest predictor of low back pain.

The tridimensional personality theory and pain: harm avoidance and reward dependence traits correlate with pain perception in healthy volunteers

The aim of the present study was to examine the possible role of personality traits in determining the variability of pain perception among individuals. More specifically, it was intended to test whether or not the three personality dimensions suggested by Cloninger in 1987 - mainly harm avoidance (HA), but also reward dependence (RD), and novelty seeking (NS), can predict interpersonal differences in responsiveness to experimental pain. Seventy healthy volunteers participated in the study. Their personality traits were evaluated through Cloninger's tridimensional personality questionnaire (TPQ). Pain threshold (latency to pain onset), pain magnitude (VAS), and pain tolerance (time to withdrawal) were measured by using the cold pressor test. Bonferroni-adjusted correlations were found between HA and the pain parameters as follows: a negative correlation between HA and threshold (rho=-0.297, P(adj)=0.039); no significant correlation between HA and tolerance (rho=-0.219, P(adj)=0.207); and a trend for a positive correlation between HA and VAS (rho=0.266, P(adj)=0.081). Possible correlations between pain perception and the various possible combinations of high and low scoring for each of the three traits were also investigated. Correlations were found only for the combinations of high/low HA and high/low RD. The low HA/low RD combination demonstrated the lowest responsiveness to pain (VAS 65.2+/-21.4; tolerance 107.6+/-71.8 s), whereas the high HA/low RD combination was correlated with the highest responsiveness (VAS 83.3+/-10.8; tolerance 30.8+/-28.4 s). The results indicate that HA personality trait correlates best with pain responsiveness. As such, a high HA are likely to predict a heightened pain response. RD may modify this pattern. The possible relevant behavioral and neuro-chemical mechanisms are discussed.

Inadequate analgesia in emergency medicine

Review of emergency department pain management practices demonstrates pain treatment inconsistency and inadequacy that extends across all demographic groups. This inconsistency and inadequacy appears to stem from a multitude of potentially remediable practical and attitudinal barriers that include (1) a lack of educational emphasis on pain management practices in nursing and medical school curricula and postgraduate training programs; (2) inadequate or nonexistent clinical quality management programs that evaluate pain management; (3) a paucity of rigorous studies of populations with special needs that improve pain management in the emergency department, particularly in geriatric and pediatric patients; (4) clinicians' attitudes toward opioid analgesics that result in inappropriate diagnosis of drug-seeking behavior and inappropriate concern about addiction, even in patients who have obvious acutely painful conditions and request pain relief; (5) inappropriate concerns about the safety of opioids compared with nonsteroidal anti-inflammatory drugs that result in their underuse (opiophobia); (6) unappreciated cultural and sex differences in pain reporting by patients and interpretation of pain reporting by providers; and (7) bias and disbelief of pain reporting according to racial and ethnic stereotyping. This article reviews the literature that describes the prevalence and roots of oligoanalgesia in emergency medicine. It also discusses the regulatory efforts to address the problem and their effect on attitudes within the legal community.

Genotype-dependence of gabapentin and pregabalin sensitivity: the pharmacogenetic mediation of analgesia is specific to the type of pain being inhibited

The antiepileptic drug, gabapentin, and another structurally related compound, pregabalin, are increasingly employed in the pharmacotherapy of chronic pain states, although their primary mechanism of action remains a topic of active study. A genomic approach to the study of these drugs may elucidate their potentially novel mechanisms. We examined the heritability of sensitivity to analgesia from gabapentin and pregabalin as a precursor to linkage mapping efforts. Accordingly, 11 inbred mouse strains were tested for inhibition of nociception by gabapentin or pregabalin (50-300 mg/kg, i.p.) in two different preclinical assays of inflammatory pain, the formalin test (5% formalin; 20 microl) and zymosan thermal hyperalgesia on the paw-withdrawal test (3 mg/ml zymosan; 20 microl). Significant strain-dependence of drug action was noted in each case, indicating that sensitivity to these analgesics is heritable. Furthermore, the pattern of strain sensitivities to gabapentin and pregabalin were mostly similar, supporting the notion that they act via similar genetic and physiological mechanisms. However, there was virtually no correlation between strain sensitivities to pregabalin inhibition of formalin nociception and zymosan thermal hyperalgesia. In light of previous data from our laboratory and others regarding morphine analgesia, we now establish and empirically demonstrate the general principle that pharmacogenetic mechanisms underlying analgesic sensitivity are specific to the type of pain being inhibited. This has considerable implications for ongoing pharmacogenetic investigations and, more generally, for the choices of preclinical models of pain used in drug development.

Characterizing the subjective, psychomotor, and physiological effects of oral propoxyphene in non-drug-abusing volunteers

BACKGROUND

The subjective, psychomotor, and physiological effects of a widely prescribed prescription opioid, propoxyphene, have not been studied in a population of non-drug-abusing people. The drug also has potential for abuse and it was of interest in the present study to determine if the drug had any abuse liability-related subjective effects in this population.

METHODS

Eighteen volunteers participated in a crossover, randomized, double-blind study in which they received, all p.o., placebo; 50 mg propoxyphene napsylate; 100 mg propoxyphene napsylate; 200 mg propoxyphene napsylate; 40 mg morphine sulfate; and 2 mg lorazepam. Measures were assessed before and for 300 min after drug administration.

RESULTS

Both morphine and lorazepam produced subjective effects. There were no statistically significant subjective effects obtained with any dose of propoxyphene in the group as a whole, but approximately 30-50% of the subjects did appear to experience subjective effects from the drug. Drug liking was not consistently observed in this subset. Propoxyphene, unlike lorazepam, did not impair psychomotor or cognitive performance. Both propoxyphene and morphine produced miosis.

CONCLUSIONS

There was a lack of statistically significant subjective effects of propoxyphene in the group as a whole, including a propoxyphene dose that was twice as high as the typical clinically-prescribed dose of 100 mg. However, there were some subjects who did report effects, consistent with the notion that patients differ in their sensitivity to opioid effects.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Mechanical sensation and pain thresholds in patients with chronic arthropathies

This study measured mechanical sensation and pain thresholds in the cutaneous field overlying the knee joints of rheumatoid arthritis (RA; N = 27) and osteoarthritis (OA; N = 28) patients, compared with age- and weight-matched normal control subjects (Norm; N = 27) by using graded von Frey monofilaments. A visual analog scale (VASpain), cutaneous joint temperature and circumference were measured for subjective ongoing pain and inflammation. Compared to Norm, RA and OA groups had (1) significantly higher VASpain scores, joint circumferences and (RA only) surface temperatures, (2) significantly increased average thresholds for innocuous mechanical sensation (0.014 +/- 0.003 vs 0.077 +/- 0.035 and 0.123 +/- 0.043 g, respectively) indicative of hypoesthesia and (3) significantly decreased pain thresholds, indicative of mechanical allodynia (446.683 +/- 0 vs 285.910 +/- 40.012 and 322.681 +/- 34.521 g for Norm vs RA and OA, respectively). Intrapatient joint temperature, circumference, and pain threshold were significantly correlated in RA. The highest scores in average mechanical sensation mapped to the same grid region as the lowest scores in average pain thresholds in RA and OA patients. The simultaneous hypoesthesia and allodynia, with paradoxical decrease in sensation and increased pain thresholds may reflect peripheral and central alterations in neuronal responsiveness to mechanical stimulation and suggests activation of a descending inhibitory system.

Strain differences in pain sensitivity and expression of preprodynorphin mRNA in rats following peripheral inflammation

The experience of pain is highly variable among individuals, which may be due in part to the effects of genetic factors on the central transmission and modulation of noxious inputs. This study examined behavioral responses and the expression of preprodynorphin (PPD) mRNA at the spinal level during complete Freund's adjuvant-induced inflammation of the unilateral hind paw in male Fischer 344 (F344), Sprague-Dawley (SD), and Lewis (LEW) rats. Experiments showed that F344 rats exhibited stronger hind paw hyperalgesia and greater spinal PPD mRNA induction than SD or LEW rats. These results indicate that genetic factors that determine the spinal PPD mRNA and dynorphin production underlie strain-dependent differences in pain perception.