Stress-Induced Analgesia and Morphine Responses Are Changed in Catechol-O-methyltransferase-Deficient Male Mice

Assessing effects of tamoxifen on tolerance, dependence, and glutamate and glutamine levels in frontal cortex and hippocampus in chronic morphine treatment

Tamoxifen has been shown to reduce glutamate release from presynaptic glutamatergic nerves and reverse tolerance to morphine-induced respiratory depression. Changes in glutamatergic neurotransmission in the central nervous system contribute to morphine tolerance, dependence, and withdrawal. This study, therefore, evaluated effects of tamoxifen on development of analgesic tolerance and dependence, and brain glutamate and glutamine levels in chronic morphine administration. Mice implanted with placebo or morphine pellets were injected with tamoxifen (0.6-2 mg/kg) or vehicle twice daily for 3 days. Nociceptive response was evaluated in the hot plate and tail immersion tests, 4, 48 and 72 h post-implant, and following a challenge dose of morphine (10 mg/kg). Withdrawal signs were determined after naloxone (1 mg/kg) administration. Morphine increased nociceptive threshold which declined over time. At 72 h, acute morphine elicited tolerance to the analgesic effect in the hot plate test in vehicle or tamoxifen administered animals. In the tail immersion test, however, tolerance to morphine analgesia was observed in tamoxifen, but not vehicle, co-administration. Tamoxifen did not reduce withdrawal signs. In contrast to previous reports, glutamate and glutamine levels in the hippocampus and frontal cortex did not change in the morphine-vehicle group. Confirming previous findings, tamoxifen (2 mg/kg) decreased glutamate and glutamine concentrations in the hippocampus in animals with placebo pellets. Both doses of tamoxifen significantly changed glutamate and/or glutamine concentrations in both regions in morphine pellet implanted animals. These results suggest that tamoxifen has no effect on dependence but may facilitate tolerance development to the antinociception, possibly mediated at the spinal level, in chronic morphine administration.

Effect of oxytocin pretreatment on the development of morphine tolerance and dependence in rats

Increased opioid synthesis and release, and enhanced alpha-2 adrenoceptor signaling have been suggested to mediate repeated oxytocin-induced long-lasting effects including elevated pain threshold in rats. This study evaluated whether oxytocin pretreatment would influence development of dependence and tolerance to the nociceptive and body temperature responses to morphine and enhance effects of alpha-2 adrenergic agonist clonidine on nociceptive threshold, body temperature and morphine withdrawal signs. Rats injected subcutaneously with saline or 1 mg/kg oxytocin for 5 days were implanted with placebo or morphine pellets 24 h after the treatment period. Body temperature and nociception were assessed, with nociception determined via by hot plate and tail immersion tests, before and 4, 24 and 48 h after pellet implantation, and following a challenge dose of morphine. Withdrawal signs were determined after naloxone administration. Oxytocin produced analgesia, as evidenced by increased paw withdrawal latency in the hot plate test. Morphine increased body temperature and nociceptive threshold which declined over time. Morphine challenge could not demonstrate tolerance to the body temperature response. Analgesic tolerance was observed in the hot plate test in saline and in both tests in oxytocin pretreated rats. Naloxone-precipitated withdrawal appeared to be less severe in oxytocin pretreatment. Clonidine was ineffective on the withdrawal signs but decreased body temperature and increased tail flick latency in the tail immersion test in oxytocin pretreated animals. These results, while producing evidence for a hyperresponsiveness in alpha-2 adrenoceptors, provide contrasting effects on morphine tolerance and dependence, and their partial mediation by opioidergic and adrenergic activation in repeated oxytocin treatment.

The effect of morphine upon DNA methylation in ten regions of the rat brain

Morphine is one of the most effective analgesics in medicine. However, its use is associated with the development of tolerance and dependence. Recent studies demonstrating epigenetic changes in the brain after exposure to opiates have provided insight into mechanisms possibly underlying addiction. In this study, we sought to identify epigenetic changes in ten regions of the rat brain following acute and chronic morphine exposure. We analyzed DNA methylation of six nuclear-encoded genes implicated in brain function (Bdnf, Comt, Il1b, Il6, Nr3c1, and Tnf) and three mitochondrially-encoded genes (Mtco1, Mtco2, and Mtco3), and measured global 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5 hmC) levels. We observed differential methylation of Bdnf and Il6 in the pons, Nr3c1 in the cerebellum, and Il1b in the hippocampus in response to acute morphine exposure (all P value < 0.05). Chronic exposure was associated with differential methylation of Bdnf and Comt in the pons, Nr3c1 in the hippocampus and Il1b in the medulla oblongata (all P value < 0.05). Global 5mC levels significantly decreased in the superior colliculus following both acute and chronic morphine exposure, and increased in the hypothalamus following chronic exposure. Chronic exposure was also associated with significantly increased global 5hmC levels in the cerebral cortex, hippocampus, and hypothalamus, but significantly decreased in the midbrain. Our results demonstrate, for the first time, highly localized epigenetic changes in the rat brain following acute and chronic morphine exposure. Further work is required to elucidate the potential role of these changes in the formation of tolerance and dependence.

Advanced cancer pain: the search for genetic factors correlated with interindividual variability in opioid requirement

AIM

To assess association between genetic variants and opioid requirement in cancer patients.

MATERIALS & METHODS

A prospective observational trial of 243 advanced cancer patients with inadequate analgesia treated by the palliative care team was analyzed for ABCB1, ARRB2, COMT, GCH1, IL1RN, KCNJ6, OPRM1, RHBDF2, SCN9A and Stat6 polymorphisms.

RESULTS

For patients carrying OPRM1 118AG/GG and COMT 472GG (Val158Val) or these genotypes alone, a significant higher median percentage dose increase was observed (95.2% [32.8-345]) compared with OPRM1 118AA and COMT 472GA/AA (158Met allele carriers; 48.5% [0-98.8]; p = 0.0016). No associations were found with morphine equivalent dose after consultation palliative care team or ketamine use.

CONCLUSION

Patients with the combined OPRM1 118AG/GG and COMT 472GG genotype required 50% higher dose increase for sufficient analgesia.

Chronic stress impacts the cardiovascular system: animal models and clinical outcomes

Psychological stresses are associated with cardiovascular diseases to the extent that cardiovascular diseases are among the most important group of psychosomatic diseases. The longstanding association between stress and cardiovascular disease exists despite a large ambiguity about the underlying mechanisms. An array of possibilities have been proposed including overactivity of the autonomic nervous system and humoral changes, which then converge on endothelial dysfunction that initiates unwanted cardiovascular consequences. We review some of the features of the two most important stress-activated systems, i.e., the humoral and nervous systems, and focus on alterations in endothelial function that could ensue as a result of these changes. Cardiac and hematologic consequences of stress are also addressed briefly. It is likely that activation of the inflammatory cascade in association with oxidative imbalance represents key pathophysiological components of stress-induced cardiovascular changes. We also review some of the commonly used animal models of stress and discuss the cardiovascular outcomes reported in these models of stress. The unique ability of animals for adaptation under stressful conditions lessens the extrapolation of laboratory findings to conditions of human stress. An animal model of unpredictable chronic stress, which applies various stress modules in a random fashion, might be a useful solution to this predicament. The use of stress markers as indicators of stress intensity is also discussed in various models of animal stress and in clinical studies.

Catechol-O-methyltransferase gene polymorphism and chronic human pain: a systematic review and meta-analysis

In human studies, low COMT (catechol-O-methyltransferase) activity has been associated with increased sensitivity to acute clinical preoperative or postoperative pain. We explored the association between the COMT genotype and three chronic pain conditions: migrainous headache, fibromyalgia, or chronic widespread pain and chronic musculoskeletal pain. Furthermore, we evaluated whether COMT genotype affects the efficacy of opioids in chronic pain. After a systematic literature review, we carried out meta-analyses on the three chronic pain conditions. The efficacy of opioids was evaluated using a systematic review only. The meta-analyses showed that fibromyalgia or chronic widespread pain is the only type of chronic pain that could be associated with the COMT single nucleotide polymorphism rs4680 (Val158Met). Met158, which results in the low-activity variant of COMT, is the risk allele. In chronic clinical pain, the effect of the COMT polymorphism depends on the pain condition. Low COMT activity is not associated with migrainous headache or chronic musculoskeletal pain conditions, but it may increase the risk for fibromyalgia or chronic widespread pain. Low COMT activity increases opioid receptors and enhances opioid analgesia and adverse effects in some cancer pains. Findings from animal studies that have utilized COMT inhibitors elucidate the mechanism behind these findings. In rodent pain models, COMT inhibitors are pronociceptive, except for neuropathic pain models, where nitecapone was found to be antiallodynic. The complex interplay between enhanced adrenergic and dopaminergic activity in different parts of the nociceptive system probably explains the complicated actions of low COMT activity.

Inhibitors of catechol-O-methyltransferase sensitize mice to pain

BACKGROUND AND PURPOSE

Catechol-O-methyltransferase (COMT) inhibitors are used in Parkinson's disease in which pain is an important symptom. COMT polymorphisms modulate pain and opioid analgesia in humans. In rats, COMT inhibitors have been shown to be pro-nociceptive in acute pain models, but also to attenuate allodynia and hyperalgesia in a model of diabetic neuropathy. Here, we have assessed the effects of acute and repeated administrations of COMT inhibitors on mechanical, thermal and carrageenan-induced nociception in male mice.

EXPERIMENTAL APPROACH

We used single and repeated administration of a peripherally restricted, short-acting (nitecapone) and also a centrally acting (3,5-dinitrocatechol, OR-486) COMT inhibitor. We also tested CGP 28014, an indirect inhibitor of COMT enzyme. Effects of OR-486 on thermal nociception were also studied in COMT deficient mice. Effects on spinal pathways were assessed in rats given intrathecal nitecapone.

KEY RESULTS

After single administration, both nitecapone and OR-486 reduced mechanical nociceptive thresholds and thermal nociceptive latencies (hot plate test) at 2 and 3 h, regardless of their brain penetration. These effects were still present after chronic treatment with COMT inhibitors for 5 days. Intraplantar injection of carrageenan reduced nociceptive latencies and both COMT inhibitors potentiated this reduction without modifying inflammation. CGP 28014 shortened paw flick latencies. OR-486 did not modify hot plate times in Comt gene deficient mice. Intrathecal nitecapone modified neither thermal nor mechanical nociception.

CONCLUSIONS AND IMPLICATIONS

Pro-nociceptive effects of COMT inhibitors were confirmed. The pro-nociceptive effects were primarily mediated via mechanisms acting outside the brain and spinal cord. COMT protein was required for these actions.

Effect of genetic modifications in the synaptic dopamine clearance systems on addiction-like behaviour in mice

During the last 15 years, genetically modified mouse lines have proved to be a valuable research tool. This review summarizes research that studied addiction-like behaviour in mice that had a targeted mutation in the genes of the synaptic dopamine removal systems, i.e. in the dopamine transporter (DAT), a vesicular monoamine transporter 2 (VMAT2) or two dopamine-metabolizing enzymes (monoamine oxidase, MAO, mainly MAO-A isoenzyme, and catechol-O-methyltransferase, COMT). Majority of the mice are knockouts but also some knock-in and knock down mouse lines are included. Most studies have explored DAT, and it has been shown to be the critical target in addiction to psychostimulants. Its role in the development of addiction-like behaviour to nicotine, opioids or ethanol is less clear. VMAT2 also seems to be linked to psychostimulant addiction. MAO-A and COMT have a minor role in addiction-like behaviour that is further complicated by a sexual dimorphism.

Lack of adrenomedullin in the central nervous system results in apparently paradoxical alterations on pain sensitivity

Adrenomedullin (AM) is a regulatory peptide, coded by the adm gene, which is involved in numerous physiological processes, including pain sensitivity. Previous studies have shown that intrathecal injection of AM induced hyperalgesia in the rat. Here, we explore pain sensitivity in a mouse conditional knockout for adm in neurons of the central nervous system, including the spinal cord and dorsal root ganglia. Double immunofluorescence in wild-type (WT) animals shows that AM immunoreactivity is found in calcitonin gene-related peptide-positive neurons of the dorsal root ganglia but not in neurons that bind isolectin B4. Mice lacking adm had modified expression of canonical sensorial neuropeptides, having significantly more calcitonin gene-related peptide and less substance P and enkephalin than their WT littermates. Furthermore, the spinal cord of adm knockout mice expressed higher levels of the AM receptor components, suggesting a compensation attempt to deal with the lack of afferent AM signaling. Behavioral nociceptive tests also found differences between genotypes. In the tail-flick test, which measures mostly spinal reflexes, the adm-null animals had a longer latency than their WT counterparts. On the other hand, in the hotplate test, which requires encephalic processing, mice lacking adm had shorter latencies than normal littermates. These results suggest that AM acts as a nociceptive modulator in spinal reflexes, whereas it may have an analgesic function at higher cognitive levels. This study confirms the important role of AM in pain sensitivity processing but unveils a more complex scenario than previously surmised.

Effect of S-COMT deficiency on behavior and extracellular brain dopamine concentrations in mice

INTRODUCTION

Catechol-O-methyltransferase (COMT) has soluble (S-COMT) and membrane bound (MB-COMT) isoforms. Our aims were to assess the behavioral phenotype of S-COMT mutant mice and to clarify the role of MB-COMT in dopamine metabolism in different brain areas.

METHODS

Behavioral phenotype of the S-COMT mutant mice was assessed using a test battery designed to describe anxiety phenotype, spontaneous locomotor activity, sensorymotor gating, social behavior, and pain sensitivity. Microdialysis was used to explore the effect of S-COMT deficiency on extracellular dopamine under an L: -dopa load (carbidopa /L: -dopa 30/10 mg/kg i.p.).

RESULTS

In behavioral tests, mature adult S-COMT mutants that only possessed MB-COMT exhibited enhanced acoustic startle without alterations in sensorimotor gating. They also showed barbering of vibrissae and nonaggressive social dominance, suggesting a change in their social interactions. In addition, S-COMT deficiency slightly and sex-dependently affected spinal pain reflex and the effect of morphine on hot-plate latency. In microdialysis studies under L: -dopa load, S-COMT mutants of both sexes had higher accumbal dopamine levels, but male S-COMT mutant mice showed paradoxically lower prefrontal cortical dopamine concentrations than wild-type animals. S-COMT deficiency induced the accumulation of 3,4-dihydroxyphenylacetic acid in all brain areas, which was accentuated after L: -dopa loading. The lack of S-COMT decreased extracellular homovanillic acid levels. However, after L: -dopa loading, homovanillic acid concentrations in the prefrontal cortex of S-COMT mutants were similar to those of wild-type mice.

CONCLUSION

A lack of S-COMT has a notable, albeit small, brain-area and sex-dependent effect on the O-methylation of dopamine and 3,4-dihydroxyphenylacetic acid in the mouse brain. It also induces subtle changes in mouse social interaction behaviors and nociception.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).