Morphine treatment induced calcitonin gene-related peptide and substance P increases in cultured dorsal root ganglion neurons

Gene expression in the dorsal root ganglion and the cerebrospinal fluid metabolome in polyneuropathy and opioid tolerance in rats

First-line pharmacotherapy for peripheral neuropathic pain (NP) of diverse pathophysiology consists of antidepressants and gabapentinoids, but only a minority achieve sufficient analgesia with these drugs. Opioids are considered third-line analgesics in NP due to potential severe and unpredictable adverse effects in long-term use. Also, opioid tolerance and NP may have shared mechanisms, raising further concerns about opioid use in NP. We set out to further elucidate possible shared and separate mechanisms after chronic morphine treatment and oxaliplatin-induced and diabetic polyneuropathies, and to identify potential diagnostic markers and therapeutic targets. We analysed thermal nociceptive behaviour, the transcriptome of dorsal root ganglia (DRG) and the metabolome of cerebrospinal fluid (CSF) in these three conditions, in rats. Several genes were differentially expressed, most following oxaliplatin and least after chronic morphine treatment, compared with saline-treated rats. A few genes were differentially expressed in the DRGs in all three models (e.g. Csf3r and Fkbp5). Some, e.g. Alox15 and Slc12a5, were differentially expressed in both diabetic and oxaliplatin models. Other differentially expressed genes were associated with nociception, inflammation, and glial cells. The CSF metabolome was most significantly affected in the diabetic rats. Interestingly, we saw changes in nicotinamide metabolism, which has been associated with opioid addiction and withdrawal, in the CSF of morphine-tolerant rats. Our results offer new hypotheses for the pathophysiology and treatment of NP and opioid tolerance. In particular, the role of nicotinamide metabolism in opioid addiction deserves further study.

Association between Body Mass Index and Medication-Overuse Headache among Individuals with Migraine: A Cross-Sectional Study

INTRODUCTION

Medication-overuse headache (MOH) is a secondary chronic headache disorder that occurs in individuals with a pre-existing primary headache disorder, particularly migraine disorder. Obesity is often combined with chronic daily headaches and is considered a risk factor for the transformation of episodic headaches into chronic headaches. However, the association between obesity and MOH among individuals with migraine has rarely been studied. The present study explored the association between body mass index (BMI) and MOH in people living with migraine.

METHODS

This cross-sectional study is a secondary analysis of data from the Survey of Fibromyalgia Comorbidity with Headache study. Migraine and MOH were diagnosed using the criteria of the International Classification of Headache Disorders, 3rd Edition. BMI (kg/m2) is calculated by dividing the weight (kg) by the square of the height (m). Multivariable logistic regression analysis was used to evaluate the association between BMI and MOH.

RESULTS

A total of 2,251 individuals with migraine were included, of whom 8.7% (195/2,251) had a concomitant MOH. Multivariable logistic regression analysis, adjusted for age, sex, education level, headache duration, pain intensity, headache family history, chronic migraine, depression, anxiety, insomnia, and fibromyalgia, demonstrated there was an association between BMI (odds ratio [OR], 1.05; 95% confidence interval [CI], 1.01-1.11; p = 0.031) and MOH. The results remained when the BMI was transformed into a category. Compared to individuals with Q2 (18.5 kg/m2 ≤ BMI ≤23.9 kg/m2), those with Q4 (BMI ≥28 kg/m2) had an adjusted OR for MOH of 1.81 (95% CI, 1.04-3.17; p = 0.037). In the subgroup analyses, BMI was associated with MOH among aged more than 50 years (OR, 1.13; 95%, 1.03-1.24), less than high school (OR, 1.08; 95%, 1.01-1.15), without depression (OR, 1.06; 95%, 1.01-1.12), and without anxiety (OR, 1.06; 95%, 1.01-1.12). An association between BMI and MOH was found in a sensitivity analysis that BMI was classified into four categories according to the World Health Organization guidelines.

CONCLUSION

In this cross-sectional study, BMI was associated with MOH in Chinese individuals with migraine.

Central sensitization mechanisms in chronic migraine with medication overuse headache: a study of thalamocortical activation and lateral cortical inhibition

BACKGROUND

It is unclear whether cortical hyperexcitability in chronic migraine with medication overuse headache (CM-MOH) is due to increased thalamocortical drive or aberrant cortical inhibitory mechanisms.

METHODS

Somatosensory evoked potentials (SSEP) were performed by electrical stimulation of the median nerve (M), ulnar nerve (U) and simultaneous stimulation of both nerves (MU) in 27 patients with CM-MOH and, for comparison, in 23 healthy volunteers (HVs) of a comparable age distribution. We calculated the degree of cortical lateral inhibition using the formula: 100 - [MU/(M + U) × 100] and the level of thalamocortical activation by analyzing the high frequency oscillations (HFOs) embedded in parietal N20 median SSEPs.

RESULTS

Compared to HV, CM-MOH patients showed higher lateral inhibition (CM-MOH 52.2% ± 15.4 vs. HV 40.4% ± 13.3; p = 0.005), which positively correlated with monthly headache days, and greater amplitude of pre-synaptic HFOs (p = 0.010) but normal post-synaptic HFOs (p = 0.122).

CONCLUSION

Our findings suggest that central neuronal circuits are highly sensitized in CM-MOH patients, at both thalamocortical and cortical levels. The observed changes could be due to the combination of dysfunctional central pain control mechanisms, hypersensitivity and hyperresponsiveness directly linked to the chronic intake of acute migraine drugs.

Medication overuse headache

Medication overuse headache (MOH) is a secondary headache disorder attributed to overuse of acute headache medications by a person with an underlying headache disorder, usually migraine or tension-type headache. MOH is common among individuals with 15 or more headache days per month. Although MOH is associated with substantial disability and reductions in quality of life, this condition is often under-recognized. As MOH is both preventable and treatable, it warrants greater attention and awareness. The diagnosis of MOH is based on the history and an unremarkable neurological examination, and is made according to the diagnostic criteria of the International Classification of Headache Disorders third edition (ICHD-3). Pathophysiological mechanisms of MOH include altered descending pain modulation, central sensitization and biobehavioural factors. Treatment of MOH includes the use of headache preventive therapies, but essential to success is eliminating the cause, by reducing the frequency of use of acute headache medication, and perhaps withdrawing the overused medication altogether. Appropriate treatment is usually highly effective, leading to reduced headache burden and acute medication consumption.

A Pharmacological Review of Calcitonin Gene-Related Peptide Biologics and Future Use for Chronic Pain

Calcitonin gene-related peptide (CGRP) antagonist medications have become the mainstay of acute and chronic migraine management in the outpatient setting and look to become more widely utilized by clinicians once the medications become available in generic form. However, their role in practice has remained limited to the treatment of migraines despite the ubiquitous presence of the molecule throughout the body. The literature surrounding expansion of the utility of these medications is limited; however, there have been several promising publications, and further studies are in the process to quantify their utility in the treatment of other pain-related disorders. This is a qualitative review of the current literature surrounding CGRP, particularly in relation to the treatment of non-migraine pain conditions, and looks to suggest potential utility in the field of chronic pain.

Neurobiology of migraine progression

Chronic migraine is one of the most devastating headache disorders. The estimated prevalence is 1.4-2.2% in the population. The factors which may predispose to the process of migraine progression include high frequency of migraine attacks, medication overuse, comorbid pain syndromes, and obesity. Several studies showed that chronic migraine results in the substantial anatomical and physiological changes in the brain. Despite no clear explanation regarding the pathophysiologic process leading to the progression, certain features such as increased sensory sensitivity, cutaneous allodynia, impaired habituation, identify the neuronal hyperexcitability as the plausible mechanism. In this review, we describe two main mechanisms which can lead to this hyperexcitability. The first is persistent sensitization caused by repetitive and prolonged trigeminal nociceptive activation. This process results in changes in several brain networks related to both pain and non-pain behaviours. The second mechanism is the decrease in endogenous brainstem inhibitory control, hence increasing the excitability of neurons in the trigeminal noceptive system and cerebral cortex. The combination of increased pain matrix connectivity, including hypothalamic hyperactivity and a weak serotonergic system, may contribute to migraine chronification.

Therapies targeting CGRP signaling for medication overuse headache

PURPOSE OF REVIEW

Medication overuse headache (MOH) affects more than 60 million individuals worldwide causing enormous personal and social burden. Only repurposed drugs are available for MOH that share limited evidence for efficacy. The preclinical data suggesting that activation of the calcitonin gene-related peptide (CGRP) pathway is involved in headache chronification along with clinical evidence that monoclonal antibodies targeting CGRP (anti-CGRP mAbs) have good efficacy in preventing chronic migraine, triggered this review that aims to summarize the current data on the effectiveness and safety of mAbs against CGRP in MOH.

RECENT FINDINGS

Post hoc analyses of phase-3 trials of erenumab, fremanezumab, galcanezumab, and eptinezumab for the prevention of chronic migraine revealed that patients with MOH benefit from the treatment over placebo. Several real-world studies confirm the efficacy of erenumab and galcanezumab in patients with MO. However, all published trials evaluated treatments in patients with chronic migraine with MO collectively, not in patients with MOH exclusively.

SUMMARY

The available data indicate that anti-CGRP mAbs represent a good mechanism-based and disease-specific therapeutical option with for MOH as long as detoxification and additional nonpharmaceutical interventions are operated. Future research should focus on long-term-controlled trials in MOH populations exclusively.

Repeated Morphine Administration Increases TRPV1 mRNA Expression and Autoradiographic Binding at Supraspinal Sites in the Pain Pathway

Repeated morphine administration induces tolerance to its analgesic effects. A previous study reported that repeated morphine treatment activates transient receptor potential vanilloid type 1 (TRPV1) expression in the sciatic nerve, dorsal root ganglion, and spinal cord, contributing to morphine tolerance. In the present study, we analyzed TRPV1 expression and binding sites in supraspinal pain pathways in morphine-tolerant mice. The TRPV1 mRNA levels and binding sites were remarkably increased in the cortex and thalamus of these animals. Our data provide additional insights into the effects of morphine on TRPV1 in the brain and suggest that changes in the expression of, and binding to TRPV1 in the brain are involved in morphine tolerance.

Pharmacological blockade of neurokinin1 receptor restricts morphine-induced tolerance and hyperalgesia in the rat

OBJECTIVES

The most notable adverse side effects of chronic morphine administration include tolerance and hyperalgesia. This study investigated the involvement of dorsal root ganglion (DRG) protein kinase Cɛ (PKCɛ) expression during chronic morphine administration and also considered the relationship between DRG PKCɛ expression and the substance P- neurokinin1 receptor (SP- NK1R) activity.

METHODS

Thirty-six animals were divided into six groups (n=6) in this study. In the morphine and sham groups, rats received 10 µg intrathecal (i.t.) morphine or saline for eight consecutive days, respectively. Behavioral tests were performed on days 1 and 8 before and after the first injections and then 48 h after the last injection (day 10). In the treatment groups, rats received NK1R antagonist (L-732,138, 25 µg) daily, either alone or 10 min before a morphine injection, Sham groups received DMSO alone or 10 min before a morphine injection. Animals were sacrificed on days 8 and 10, and DRG PKCɛ and SP expression were analyzed by western blot and immunohistochemistry techniques, respectively.

RESULTS

Behavioral tests indicated that tolerance developed following eight days of chronic morphine injection. Hyperalgesia was induced 48 h after the last morphine injection. Expression of SP and PKCɛ in DRG significantly increased in rats that developed morphine tolerance on day 8 and hyperalgesia on day 10, respectively. NK1R antagonist (L-732,138) not only blocked the development of hyperalgesia and the increase of PKCɛ expression but also alleviated morphine tolerance.

CONCLUSIONS

Our results provide evidence that DRG PKCɛ and SP-NK1R most likely participated in the generation of morphine tolerance and hyperalgesia. Pharmacological inhibition of SP-NK1R activity in the spinal cord suggests a role for NK1R and in restricting some side effects of chronic morphine. All experiments were performed by the National Institute of Health (NIH) Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 80-23, revised1996) and were approved by the Animal Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran (IR.SBMU.MSP.REC.1396.130).

The Evolution of Medication Overuse Headache: History, Pathophysiology and Clinical Update

Medication overuse headache (MOH), the development or worsening of chronic headache resulting from frequent and excessive intake of medications used for acute treatment of headache, is a common secondary headache disorder and is associated with significant personal and societal burdens. The plausible physiologic mechanism is that chronic exposure to acute care migraine treatment leads to suppression of endogenous antinociceptive systems, consequently facilitating the trigeminal nociceptive process via up-regulation of the calcitonin gene-related peptide (CGRP) system. Recognizing and preventing its development is an integral aspect of migraine management, as medication overuse is a modifiable risk factor in the progression from episodic to chronic migraine. Over the years, MOH has been difficult to treat and has generated much controversy. Ongoing debates exist over the diagnostic criteria and treatment strategies, particularly regarding the roles of formal detoxification and preventive treatment. The arrival of the anti-CGRP monoclonal antibodies has also challenged our views of MOH and its treatment. This review outlines the evolution of MOH diagnostic criteria, presents the current understanding of MOH pathogenesis and discusses the debates over its development and treatment. Data on the efficacy of anti-CGRP monoclonal antibodies in the setting of medication overuse is also presented. These results indicate that patients with medication overuse, who are treated with these new medications, may not need to be detoxified in order to treat MOH. In light of these developments, it is likely that in the future MOH will be more readily diagnosed and treatment will result in better outcomes.

The need for new acutely acting antimigraine drugs: moving safely outside acute medication overuse

Background

The treatment of migraine is impeded by several difficulties, among which insufficient headache relief, side effects, and risk for developing medication overuse headache (MOH). Thus, new acutely acting antimigraine drugs are currently being developed, among which the small molecule CGRP receptor antagonists, gepants, and the 5-HT_1F receptor agonist lasmiditan. Whether treatment with these drugs carries the same risk for developing MOH is currently unknown.

Main body

Pathophysiological studies on MOH in animal models have suggested that decreased 5-hydroxytryptamine (5-HT, serotonin) levels, increased calcitonin-gene related peptide (CGRP) expression and changes in 5-HT receptor expression (lower 5-HT_1B/D and higher 5-HT_2A expression) may be involved in MOH. The decreased 5-HT may increase cortical spreading depression frequency and induce central sensitization in the cerebral cortex and caudal nucleus of the trigeminal tract. Additionally, low concentrations of 5-HT, a feature often observed in MOH patients, could increase CGRP expression. This provides a possible link between the pathways of 5-HT and CGRP, targets of lasmiditan and gepants, respectively. Since lasmiditan is a 5-HT_1F receptor agonist and gepants are CGRP receptor antagonists, they could have different risks for developing MOH because of the different (over) compensation mechanisms following prolonged agonist versus antagonist treatment.

Conclusion

The acute treatment of migraine will certainly improve with the advent of two novel classes of drugs, i.e., the 5-HT_1F receptor agonists (lasmiditan) and the small molecule CGRP receptor antagonists (gepants). Data on the effects of 5-HT_1F receptor agonism in relation to MOH, as well as the effects of chronic CGRP receptor blockade, are awaited with interest.

4218T/C polymorphism associations with post-cesarean patient-controlled epidural fentanyl consumption and pain perception

BACKGROUND

The utilization of intrathecal opioids is an efficacious component of post-cesarean section pain management. Given that growing evidence indicates that calcitonin gene-related peptide (CGRP) plays a key role in the development of peripheral sensitization and is associated with enhanced pain, we hypothesized that CGRP 4218T/C polymorphism is associated with the variability in fentanyl consumption for post-cesarean analgesia.

METHODS

We recruited 548 patients who presented for elective cesarean delivery, and used polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method to analyze CGRP 4218T/C polymorphism. We examined the association of CGRP 4218T/C polymorphism and post-operative fentanyl consumption for analgesia as well as adverse reactions to fentanyl in those patients who received cesarean section surgeries.

RESULTS

We found that the CGRP 4218T/C polymorphism has a significant effect on pain perception, analgesic requirement, and nausea and vomiting for the first 24 h after cesarean delivery in patients who received PCEA fentanyl. Individuals with the C/C genotype had more pain, required more PCEA fentanyl, and experienced a lower incidence of nausea and vomiting.

CONCLUSION

These results indicated that patients with C/C genotype may have reduced sensitivity to fentanyl analgesia and/or increased pain perception, and were more willing to use PCEA fentanyl to manage their pain.

Antinociception of spirocyclopiperazinium salt compound LXM-10-M targeting α7 nicotinic receptor and M4 muscarinic receptor and inhibiting CaMKIIα/CREB/CGRP signaling pathway in mice

The present study was designed to investigate the antinociception of spirocyclopiperazinium salt compound LXM-10-M (2,4-dimethyl-9-β-m-hydroxyphenylethyl-3-oxo-6, 9-diazaspiro [5.5] undecane chloride) in thermal and chemical pain models, and further to explore the molecular target and potential signal pathway. We assessed the antinociception of LXM-10-M in hot-plate test, formalin test and acetic acid writhing test in mice. The possible changes of calcium/calmodulin-dependent protein kinase IIα (CaMKIIα)/cAMP response element-binding protein (CREB)/calcitonin gene related peptide (CGRP) signaling pathway were detected by Western Blot in mice. Administration of LXM-10-M produced significant antinociception in hot-plate test, formalin test and acetic acid writhing test in mice, with no obvious toxicity. The antinociceptive effects were blocked by pretreatment with methyllycaconitine citrate (MLA, α7 nicotinic receptor antagonist) or tropicamide (TRO, M4 muscarinic receptor antagonist). Western blot analysis showed that the upregulations of p-CaMKIIα, p-CREB and CGRP in the spinal cord were reduced by LXM-10-M in chemical pain model in mice, and the effects were blocked by MLA or TRO pretreatment. This is the first paper to report that LXM-10-M exerted significant antinociception, which may be attributed to the activation of α7 nicotinic receptor and M4 muscarinic receptor and thereby triggering the inhibition of CaMKIIα/CREB/CGRP signaling pathway in mice.

Discovery of tripeptide-derived multifunctional ligands possessing delta/mu opioid receptor agonist and neurokinin 1 receptor antagonist activities

Several bifunctional peptides were synthesized and characterized based on the pentapeptide-derived ligand NP30 (1: Tyr-DAla-Gly-Phe-Gly-Trp-O-[3',5'-Bzl(CF3)2]). Modification and truncation of amino acid residues were performed, and the tripeptide-derived ligand NP66 (11: Dmt-DAla-Trp-NH-[3',5'-(CF3)2-Bzl]) was obtained based on the overlapping pharmacophore concept. The Trp(3) residue of ligand 11 works as a message residue for both opioid and NK1 activities. The significance lies in the observation that the approach of appropriate truncation of peptide sequence could lead to a tripeptide-derived chimeric ligand with effective binding and functional activities for both mu and delta opioid and NK1 receptors with agonist activities at mu and delta opioid and antagonist activity at NK1 receptors, respectively.

Hydrogen sulfide inhibits opioid withdrawal-induced pain sensitization in rats by down-regulation of spinal calcitonin gene-related peptide expression in the spine

Hyperalgesia often occurs in opioid-induced withdrawal syndrome. In the present study, we found that three hourly injections of DAMGO (a μ-opioid receptor agonist) followed by naloxone administration at the fourth hour significantly decreased rat paw nociceptive threshold, indicating the induction of withdrawal hyperalgesia. Application of NaHS (a hydrogen sulfide donor) together with each injection of DAMGO attenuated naloxone-precipitated withdrawal hyperalgesia. RT-PCR and Western blot analysis showed that NaHS significantly reversed the gene and protein expression of up-regulated spinal calcitonin gene-related peptide (CGRP) in naloxone-treated animals. NaHS also inhibited naloxone-induced cAMP rebound and cAMP response element-binding protein (CREB) phosphorylation in rat spinal cord. In SH-SY5Y neuronal cells, NaHS inhibited forskolin-stimulated cAMP production and adenylate cyclase (AC) activity. Moreover, NaHS pre-treatment suppressed naloxone-stimulated activation of protein kinase C (PKC) α, Raf-1, and extracellular signal-regulated kinase (ERK) 1/2 in rat spinal cord. Our data suggest that H2S prevents the development of opioid withdrawal-induced hyperalgesia via suppression of synthesis of CGRP in spine through inhibition of AC/cAMP and PKC/Raf-1/ERK pathways.

Opioid receptor-triggered spinal mTORC1 activation contributes to morphine tolerance and hyperalgesia

The development of opioid-induced analgesic tolerance and hyperalgesia is a clinical challenge for managing chronic pain. Adaptive changes in protein translation in the nervous system are thought to promote opioid tolerance and hyperalgesia; however, how opioids drive such changes remains elusive. Here, we report that mammalian target of rapamycin (mTOR), which governs most protein translation, was activated in rat spinal dorsal horn neurons after repeated intrathecal morphine injections. Activation was triggered through μ opioid receptor and mediated by intracellular PI3K/Akt. Spinal mTOR inhibition blocked both induction and maintenance of morphine tolerance and hyperalgesia, without affecting basal pain perception or locomotor functions. These effects were attributed to the attenuation of morphine-induced increases in translation initiation activity, nascent protein synthesis, and expression of some known key tolerance-associated proteins, including neuronal NOS (nNOS), in dorsal horn. Moreover, elevating spinal mTOR activity by knocking down the mTOR-negative regulator TSC2 reduced morphine analgesia, produced pain hypersensitivity, and increased spinal nNOS expression. Our findings implicate the μ opioid receptor-triggered PI3K/Akt/mTOR pathway in promoting morphine-induced spinal protein translation changes and associated morphine tolerance and hyperalgesia. These data suggest that mTOR inhibitors could be explored for prevention and/or reduction of opioid tolerance in chronic pain management.

Pathophysiology of medication overuse headache--an update

The pathogenesis of medication overuse headache is unclear. Clinical and preclinical studies have consistently demonstrated increased excitability of neurons in the cerebral cortex and trigeminal system after medication overuse. Cortical hyperexcitability may facilitate the development of cortical spreading depression, while increased excitability of trigeminal neurons may facilitate the process of peripheral and central sensitization. These changes may be secondary to the derangement of central, probably serotonin (5-HT)-, and perhaps endocannabinoid-dependent or other, modulating systems. Increased expression of excitatory cortical 5-HT2A receptors may increase the susceptibility to developing cortical spreading depression, an analog of migraine aura. A reduction of diffuse noxious inhibitory controls may facilitate the process of central sensitization, activate the nociceptive facilitating system, or promote similar molecular mechanisms to those involved in kindling. Low 5-HT levels also increase the expression and release of calcitonin gene-related peptide from the trigeminal ganglion and sensitize trigeminal nociceptors. Thus, derangement of central modulation of the trigeminal system as a result of chronic medication use may increase sensitivity to pain perception and foster or reinforce medication overuse headache.

Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance

The age-dependency of opioid analgesia and tolerance has been noticed in both clinical observation and laboratory studies. Evidence shows that many molecular and cellular events that play essential roles in opioid analgesia and tolerance are actually age-dependent. For example, the expression and functions of endogenous opioid peptides, multiple types of opioid receptors, G protein subunits that couple to opioid receptors, and regulators of G protein signaling (RGS proteins) change with development and age. Other signaling systems that are critical to opioid tolerance development, such as N-methyl-D-aspartic acid (NMDA) receptors, also undergo age-related changes. It is plausible that the age-dependent expression and functions of molecules within and related to the opioid signaling pathways, as well as age-dependent cellular activity such as agonist-induced opioid receptor internalization and desensitization, eventually lead to significant age-dependent changes in opioid analgesia and tolerance development.

Pathophysiology of medication-overuse headache: implications from animal studies

Recent animal experiments have shown that chronic medication exposure profoundly affects the function of several areas in the nervous system related to headache pathogenesis. These changes include upregulation of calcitonin gene-related peptide, substance P, and nitric oxide synthase in trigeminal ganglia; expansion of receptive field and decreased nociceptive threshold of central trigeminal neurons; decrease in diffuse noxious inhibitory control; and increased susceptibility to develop cortical spreading depression (CSD). These changes indicate an increase in excitability of cortical and trigeminal neurons. The neuronal hyperexcitability may be the result of derangement of a central, possibly serotonin (5-HT)-dependent, modulating control system. Experiments with animals with low 5-HT showed that the processes of CSD and trigeminal nociception are enhanced in this condition. Derangement in the central 5-HT-dependent modulating system as a result of chronic medication use may underlie the chronification of headache as observed in patients with medication-overuse headache.

Acute morphine induces matrix metalloproteinase-9 up-regulation in primary sensory neurons to mask opioid-induced analgesia in mice

Background

Despite decades of intense research efforts, actions of acute opioids are not fully understood. Increasing evidence suggests that in addition to well-documented antinociceptive effects opioids also produce paradoxical hyperalgesic and excitatory effects on neurons. However, most studies focus on the pronociceptive actions of chronic opioid exposure. Matrix metalloproteinase 9 (MMP-9) plays an important role in neuroinflammation and neuropathic pain development. We examined MMP-9 expression and localization in dorsal root ganglia (DRGs) after acute morphine treatment and, furthermore, the role of MMP-9 in modulating acute morphine-induced analgesia and hyperalgesia in mice.

Results

Subcutaneous morphine induced a marked up-regulation of MMP-9 protein in DRGs but not spinal cords. Morphine also increased MMP-9 activity and mRNA expression in DRGs. MMP-9 up-regulation peaked at 2 h but returned to the baseline after 24 h. In DRG tissue sections, MMP-9 is expressed in small and medium-sized neurons that co-express mu opioid receptors (MOR). In DRG cultures, MOR agonists morphine, DAMGO, and remifentanil each increased MMP-9 expression in neurons, whereas the opioid receptor antagonist naloxone and the MOR-selective antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) suppressed morphine-induced MMP-9 expression. Notably, subcutaneous morphine-induced analgesia was enhanced and prolonged in Mmp9 knockout mice and also potentiated in wild-type mice receiving intrathecal injection of MMP-9 inhibitors. Consistently, intrathecal injection of specific siRNA targeting MMP-9 reduced MMP-9 expression in DRGs and enhanced and prolonged morphine analgesia. Subcutaneous morphine also produced heat hyperalgesia at 24 h, but this opioid-induced hyperalgesia was not enhanced after MMP-9 deletion or inhibition.

Conclusions

Transient MMP-9 up-regulation in DRG neurons can mask opioid analgesia, without modulating opioid-induced hyperalgesia. Distinct molecular mechanisms (MMP-9 dependent and independent) control acute opioid-induced pronociceptive actions (anti-analgesia in the first several hours and hyperalgesia after 24 h). Targeting MMP-9 may improve acute opioid analgesia.

Intrathecal PKA-selective siRNA treatment blocks sustained morphine-mediated pain sensitization and antinociceptive tolerance in rats

Sustained morphine treatment has been shown to produce paradoxical pain sensitization (opioid-induced hyperalgesia) and also causes increase in spinal pain neurotransmitter, such as calcitonin gene related peptide (CGRP), concentration in experimental animals. Studies have also shown that cyclic adenosine-monophosphate (cAMP)-dependent protein kinase (PKA) plays a major role in the regulation of presynaptic neurotransmitter (such as CGRP and substance P) synthesis and release. We have previously shown that in cultured primary sensory dorsal root ganglion (DRG) neurons sustained in vitro opioid agonist treatment upregulates cAMP levels (adenylyl cyclase (AC) superactivation) and augments basal and capsaicin evoked CGRP release in a PKA dependent manner. In the present study, we investigated the in vivo role of PKA in sustained morphine-mediated pain sensitization. Our data indicate that selective knock-down of spinal PKA activity by intrathecal (i.th.) pretreatment of rats with a PKA-selective small interference RNA (siRNA) mixture significantly attenuates sustained morphine-mediated augmentation of spinal CGRP immunoreactivity, thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance. The present findings indicate that sustained morphine-mediated activation of spinal cAMP/PKA-dependent signaling may play an important role in opioid induced hyperalgesia.

Sustained morphine-mediated pain sensitization and antinociceptive tolerance are blocked by intrathecal treatment with Raf-1-selective siRNA

BACKGROUND AND PURPOSE

Long-term morphine treatment enhances pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] levels in the spinal cord. It has been suggested previously that increased spinal CGRP may contribute to sustained morphine-mediated paradoxical pain sensitization and antinociceptive tolerance. Previous in vitro studies from our group indicated that Raf-1 kinase-mediated adenylyl cyclase superactivation played a crucial role in sustained morphine-mediated augmentation of basal and evoked CGRP release from cultured primary sensory neurons. The present study was aimed to evaluate the physiological significance of this molecular mechanism in vivo, in rats.

EXPERIMENTAL APPROACH

Rats were intrathecally (i.th) injected with a Raf-1-selective small interfering RNA (siRNA) mixture for 3 days and were subsequently infused with saline or morphine, s.c. for 7 days. Thermal and mechanical sensory thresholds of the animals were assessed by daily behavioural tests. After final behavioural testing (day 6), spinal cords were isolated from each animal group and spinal CGRP and Raf-1 protein levels were measured using elisa and immunohistochemistry.

KEY RESULTS

Selective knockdown of spinal Raf-1 protein levels by i.th Raf-1-selective siRNA pretreatment significantly attenuated sustained morphine-mediated up-regulation of CGRP immunoreactivity in the spinal cord of rats and prevented the development of thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance.

CONCLUSIONS AND IMPLICATIONS

Raf-1 played a significant role in sustained morphine-mediated paradoxical pain sensitization and antinociceptive tolerance in vivo. These findings suggest novel pharmacological approaches to improve the long-term utility of opioids in the treatment of chronic pain.

Biological and conformational evaluation of bifunctional compounds for opioid receptor agonists and neurokinin 1 receptor antagonists possessing two penicillamines

Neuropathic pain states and tolerance to opioids can result from system changes in the CNS, such as up-regulation of the NK1 receptor and substance P, lead to antiopioid effects in ascending or descending pain-signaling pathways. Bifunctional compounds, possessing both the NK1 antagonist pharmacophore and the opioid agonist pharmacophore with delta-selectivity, could counteract these system changes to have significant analgesic efficacy without undesirable side effects. As a result of the introduction of cyclic and topological constraints with penicillamines, 2 (Tyr-cyclo[d-Pen-Gly-Phe-Pen]-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)-Bzl]) was found as the best bifunctional compound with effective NK1 antagonist and potent opioid agonist activities, and 1400-fold delta-selectivity over the mu-receptor. The NMR structural analysis of 2 revealed that the relative positioning of the two connected pharmacophores as well as its cyclic and topological constraints might be responsible for its excellent bifunctional activities as well as its significant delta-opioid selectivity. Together with the observed high metabolic stability, 2 could be considered as a valuable research tool and possibly a promising candidate for a novel analgesic drug.

Effects of capsazepine, a transient receptor potential vanilloid type 1 antagonist, on morphine-induced antinociception, tolerance, and dependence in mice

BACKGROUND

Repeated morphine treatment has been shown to induce transient receptor potential vanilloid type 1 (TRPV1) expression in the spinal cord, dorsal root ganglion (DRG), and sciatic nerve of a rat model. Increased TRPV1 expression may therefore play a role in morphine tolerance. In this study, we evaluated the hypothesis that blockage of TRPV1 may be useful as an adjunctive pain management therapy. We investigated whether blockage of TRPV1 by capsazepine, a TRPV1 antagonist, affected antinociception, development of tolerance, and physical dependence on morphine in mice.

METHODS

Institute of Cancer Research mice were pretreated with capsazepine and post-treated with morphine acutely and repeatedly. Antinociception and its tolerance were assessed using the hot-plate test. Morphine dependence was examined through the manifestation of withdrawal symptoms induced by naloxone in morphine-dependent mice.

RESULTS

Acute capsazepine treatment (5 mg kg⁻¹, i.p.) potentiated the antinociceptive effects of morphine, as measured by the hot-plate test. Repeated co-treatment of capsazepine (2.5 mg kg⁻¹ i.p.) with morphine attenuated the development of tolerance to the antinociceptive effect of morphine. The development of morphine dependence was also reduced by capsazepine (1.25 or 2.5 mg kg⁻¹ i.p.).

CONCLUSIONS

Our results suggest that TRPV1 antagonists can be used adjunctively to morphine treatment because they strengthen morphine antinociception and prevent the development of tolerance, and also physical dependence, on morphine.

Improving metabolic stability by glycosylation: bifunctional peptide derivatives that are opioid receptor agonists and neurokinin 1 receptor antagonists

In order to obtain a metabolically more stable analgesic peptide derivative, O-beta-glycosylated serine (Ser(Glc)) was introduced into TY027 (Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3',5'-Bzl(CF(3))(2)) which was a previously reported bifunctional compound with delta/micro opioid agonist and neurokinin-1 receptor antagonist activities and with a half-life of 4.8 h in rat plasma. Incorporation of Ser(Glc) into various positions of TY027 gave analogues with variable bioactivities. Analogue 6 (Tyr-d-Ala-Gly-Phe-Nle-Pro-Leu-Ser(Glc)-Trp-NH-3',5'-Bzl(CF(3))(2)) was found to have effective bifunctional activities with a well-defined conformation with two beta-turns based on the NMR conformational analysis in the presence of DPC micelles. In addition, 6 showed significant improvement in its metabolic stability (70 + or - 9% of 6 was intact after 24 h incubation in rat plasma). This improved metabolic stability, along with its effective and delta selective bifunctional activities, suggests that 6 could be an interesting research tool and possibly a promising candidate as a novel analgesic drug.

The ERK 1 and 2 pathway in the nervous system: from basic aspects to possible clinical applications in pain and visceral dysfunction

The extracellular signal-regulated kinases 1 and 2 (ERK) cascade, member of the mitogen-activated protein kinases superfamily of signalling pathways, is one of the best characterized pathways as many protein interactions and phosphorylation events have been systematically studied. Traditionally, ERK are associated with the regulation of proliferation and differentiation as well as survival of various cell types. Their activity is controlled by phosphorylation on specific aminoacidic residues, which is induced by a variety of external cues, including growth-promoting factors.

In the nervous system, ERK phosphorylation is induced by binding of neurotrophins to their specific tyrosine kinase receptors or by neuronal activity leading to glutamate release and binding to its ionotropic and metabotropic receptors. Some studies have provided evidence of its importance in neuroplastic events. In particular, ERK phosphorylation in the spinal cord was shown to be nociceptive-specific and its upregulation, occurring in cases of chronic inflammatory and neuropathic pain, seems to be of the utmost importance to behavioural changes observed in those conditions. In fact, experiments using specific inhibitors of ERK phosphorylation have proved that ERK directly contributes to allodynia and hyperalgesia caused by spinal cord injury or chronic pain. Additionally, spinal ERK phosphorylation regulates the micturition reflex in experimental models of bladder inflammation and chronic spinal cord transection.

In this review we will address the main findings that suggest that ERK might be a future therapeutic target to treat pain and other complications arising from chronic pain or neuronal injury.

Involvement of protein kinase C in morphine tolerance at spinal levels of rats

The present study was performed to investigate the possible role of protein kinase C (PKC) in morphine tolerance at spinal levels of rats. Intrathecal injection of 10 μg of morphine induced increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. After intrathecal injections of 10 μg of morphine (twice a day) lasted for 5 days, the antinociceptive effects induced by intrathecal injections of morphine decreased significantly in rats. Interestingly, we found that there were significant increases in the content of PKC in the dorsal horn of the spinal cord and the dorsal root ganglion, but not in the ventral horn of the spinal cord, in rats with morphine tolerance determined by Western blot, suggesting that PKC is involved in morphine tolerance at spinal levels of rats. Furthermore, our results demonstrated that chronic intrathecal injection of the PKC inhibitor significantly inhibited the development of morphine tolerance. Moreover, we found that the maintenance of morphine tolerance was blocked by intrathecal administration of a PKC inhibitor in rats, and the inhibitory effects of the PKC inhibitor on morphine tolerance lasted for more than two days. Taken together, the present study clearly showed that PKC is involved in morphine tolerance at the spinal level of rats and that intrathecal administration of a PKC inhibitor can block the development and maintenance of morphine tolerance.

Sustained morphine-induced sensitization and loss of diffuse noxious inhibitory controls in dura-sensitive medullary dorsal horn neurons

Overuse of medications used to treat migraine headache can produce a chronic daily headache, termed medication overuse headache (MOH). Although "overuse" of opioids, triptans, and over-the-counter analgesics can all produce MOH, the neuronal mechanisms remain unknown. Headache pain is likely to be produced by stimulation of primary afferent neurons that innervate the intracranial vasculature and the resulting activation of medullary dorsal horn (MDH) neurons. The present study compared the receptive field properties of MDH dura-sensitive neurons in rats treated with morphine to those given vehicle. Animals were implanted with osmotic minipumps or pellets for sustained subcutaneous administration of morphine or vehicle 6-7 d before recording from dura-sensitive neurons. Electrical and mechanical activation thresholds from the dura were significantly lower in chronic morphine-treated animals when compared to vehicle controls. In addition, sustained morphine increased the cutaneous receptive field sizes. The presence of diffuse noxious inhibitory controls (DNICs) was examined by placing the tail in 55 degrees C water during concomitant noxious thermal stimulation of the cutaneous receptive field, usually located in the ophthalmic region. The DNIC stimulus produced significant inhibition of heat-evoked activity in vehicle- but not chronic morphine-treated animals. Inactivation of the rostral ventromedial medulla with 4% lidocaine reinstated DNICs in chronic morphine-treated animals. These results are consistent with studies demonstrating a loss of DNICs in patients that suffer from chronic daily headache and may partially explain why overuse of medication used to treat migraine can induce headaches.

The biological activity and metabolic stability of peptidic bifunctional compounds that are opioid receptor agonists and neurokinin-1 receptor antagonists with a cystine moiety

In order to improve metabolic stability, a ring structure with a cystine moiety was introduced into TY027 (Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)Bzl]), which is a lead compound of our developing bifunctional peptide possessing opioid agonist and NK1 antagonist activities. TY038 (Tyr-cyclo[D-Cys-Gly-Phe-Met-Pro-D-Cys]-Trp-NH-[3',5'-(CF(3))(2)Bzl]) was found as a highly selective delta opioid agonist over mu receptor in conventional tissue-based assays, together with an effective NK1 antagonist activity and good metabolic stability with more than 24h half life in rat plasma.

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin). In the present study, we demonstrate that sustained morphine-mediated augmentation of CGRP release from isolated primary sensory dorsal root ganglion neurons is dependent on protein kinase A and Raf-1 kinase. Our data indicate that, in addition to neural system adaptations, sustained opioid agonist treatment also produces intracellular compensatory adaptations in primary sensory neurons, leading to augmentation of evoked pain neurotransmitter release from these cells.

Opiate-induced persistent pronociceptive trigeminal neural adaptations: potential relevance to opiate-induced medication overuse headache

Medication overuse headache (MOH) is a challenging, debilitating disorder that develops from the frequent use of medications taken for the treatment of migraine headache pain. MOH affects an estimated 3-5% of the general population. The mechanisms underlying the development of MOH remain unknown. Opiates are one of the major classes of medications used for the treatment of migraine at least in some countries, including the USA. Although the effects of repeated opiate use for headache are unknown, it is possible that opiate use may contribute to increased frequency and occurrence of such headaches. Recent preclinical studies exploring the neuroadaptive changes following sustained exposure to morphine may give some insights into possible causes of MOH. Peripherally, these changes include increased expression of calcitonin gene-related peptide (CGRP) in trigeminal primary afferent neurons. Centrally, they include increased excitatory neurotransmission at the level of the dorsal horn and nucleus caudalis. Critically, these neuroadaptive changes persist for long periods of time and the evoked release of CGRP is enhanced following morphine pretreatment. Stimuli known to elicit migraine, such as nitric oxide donors or stress, produce hyperalgesia in morphine- but not in saline-pretreated rats even long after the discontinuation of the opiate. CGRP plays a prominent role in initiating vasodilation of the intracranial blood vessels and subsequent headache. Furthermore, studies have demonstrated increased excitability of the nociceptive pathway in migraine sufferers, and CGRP receptor antagonists have been shown to be efficacious in migraine pain. Thus, such persistent neuroadaptive changes may be relevant to the processes that promote MOH.

The importance of micelle-bound states for the bioactivities of bifunctional peptide derivatives for delta/mu opioid receptor agonists and neurokinin 1 receptor antagonists

To provide new insight into the determining factors of membrane-bound peptide conformation that might play an important role in peptide-receptor docking and further biological behaviors, the dodecylphosphocholine (DPC) micelle-bound conformations of bifunctional peptide derivatives of delta-preferring opioid agonists and NK1 antagonists (1: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF 3) 2; 2: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3,5-Bzl(CF 3) 2; 3: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-Bzl) were determined based on 2D NMR studies. Although the differences in the primary sequence were limited to the C-terminus, the obtained NMR conformations were unexpectedly different for each compound. Moreover, their biological activities showed different trends in direct relation to the compound-specific conformations in DPC micelles. The important result is that not only were the NK1 antagonist activities different (the pharmacophore located at the C-terminus)but the opioid agonist activities (this pharmacophore was at the structurally preserved N-terminus) also were shifted, suggesting that a general conformational change in the bioactive state was induced due to relatively small and limited structural modifications.

TRPV1 receptor in expression of opioid-induced hyperalgesia

Opiates are currently the mainstay for treatment of moderate to severe pain. However, prolonged administration of opiates has been reported to elicit hyperalgesia in animals, and examples of opiate-induced hyperalgesia have been reported in humans as well. Despite the potential clinical significance of such opiate-induced actions, the mechanisms of opiate-induced hypersensitivity remain unknown. The transient receptor potential vanilloid1 (TRPV1) receptor, a molecular sensor of noxious heat, acts as an integrator of multiple forms of noxious stimuli and plays an important role in the development of inflammation-induced hyperalgesia. Because animals treated with opiates show thermal hyperalgesia, we examined the possible role of TRPV1 receptors in the development of morphine-induced hyperalgesia using TRPV1 wild-type (WT) and knock-out (KO) mice and with administration of a TRPV1 antagonist in mice and rats. Administration of morphine by subcutaneous implantation of morphine pellets elicited both thermal and tactile hypersensitivity in TRPV1 WT mice but not in TRPV1 KO mice. Moreover, oral administration of a TRPV1 antagonist reversed both thermal and tactile hypersensitivity induced by sustained morphine administration in mice and rats. Immunohistochemical analyses indicate that sustained morphine administration modestly increases TRPV1 labeling in the dorsal root ganglia. In addition, sustained morphine increased flinching and plasma extravasation after peripheral stimulation with capsaicin, suggesting an increase in TRPV1 receptor function in the periphery in morphine-treated animals. Collectively, our data indicate that the TRPV1 receptor is an essential peripheral mechanism in expression of morphine-induced hyperalgesia.

PERSPECTIVE

Opioid-induced hyperalgesia possibly limits the usefulness of opioids, emphasizing the value of alternative methods of pain control. We demonstrate that TRPV1 channels play an important role in peripheral mechanisms of opioid-induced hyperalgesia. Such information may lead to the discovery of analgesics lacking such adaptations and improving treatment of chronic pain.

Activation of TRPV1 contributes to morphine tolerance: involvement of the mitogen-activated protein kinase signaling pathway

Tolerance to the analgesic effects of opioids occurs after their chronic administration, a pharmacological phenomenon that has been associated with the development of abnormal pain sensitivity such as hyperalgesia. In the present study, we investigated the role of TRPV1, which is crucial for the transduction of noxious chemical and thermal stimuli, in morphine tolerance and tolerance-associated thermal hyperalgesia. After chronic morphine treatment, a marked increase in TRPV1 immunoreactivity (IR) was detected in L4 dorsal root ganglion (DRG) neurons, spinal cord dorsal horn, and sciatic nerve. Real-time reverse transcription (RT)-PCR demonstrated that TRPV1 mRNA was upregulated in spinal cord and sciatic nerve but not in the DRG. Intrathecal pretreatment with SB366791 [N-(3-methoxyphenyl)-4-chlorocinnamide], a selective antagonist of TRPV1, attenuated both morphine tolerance and associated thermal hyperalgesia. Chronic morphine exposure induced increases in phosphorylation of mitogen-activated protein kinases (MAPKs), including p38 MAPK-IR, extracellular signal-regulated protein kinase (ERK)-IR, and c-Jun N-terminal kinase (JNK)-IR, in L4 DRG neurons. Intrathecal administration of the selective p38, ERK, or JNK inhibitors not only reduced morphine tolerance and associated thermal hyperalgesia but also suppressed the morphine-induced increase of TRPV1-IR in DRG neurons, spinal cord, and sciatic nerve and of mRNA levels in spinal cord and sciatic nerve. Together, we have identified a novel mechanism by which sustained morphine treatment results in tolerance and tolerance-associated thermal hyperalgesia, by regulating TRPV1 expression, in a MAPK-dependent manner. Thus, blocking TRPV1 might be a way to reduce morphine tolerance.

Effect of chronic administration of morphine on the expression of bovine adrenal medulla 22-like immunoreactivity in the spinal cord of rats

The aim of the present study was to investigate the effects of chronic administration of morphine on the expression of an endogenous opioid peptide in the spinal dorsal horn. Bovine adrenal medulla 22-like immunoreactivity (BAM22-IR) was found in the superficial layers of the spinal cord. Intrathecal (i.t.) administration of morphine (20 microg) for 6 days, but not 2 days, significantly reduced the expression of BAM22-IR whereas i.t. administration of saline for 2 and 6 days did not alter the expression of BAM22-IR. The present study suggests that reduction of BAM22-IR in the spinal cord is involved in the development of morphine tolerance.

Sustained morphine treatment augments basal CGRP release from cultured primary sensory neurons in a Raf-1 dependent manner

Recent studies suggest that sustained morphine-mediated paradoxical pain may play an important role in the development of analgesic tolerance. The intracellular signal transduction pathways involved in sustained opioid mediated augmentation of spinal pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release are not fully clarified. Cyclic AMP (cAMP)-dependent protein kinase (PKA) plays an important role in the modulation of presynaptic neurotransmitter release. Moreover, we have shown earlier that sustained opioid agonist treatment leads to a Raf-1-dependent sensitization of adenylyl cyclase(s) (AC superactivation), augmenting forskolin-stimulated cAMP formation upon opioid withdrawal (cAMP overshoot). Therefore, in the present study we examined the role of Raf-1 in sustained morphine-mediated regulation of cAMP formation and basal CGRP release in vitro, in cultured neonatal rat dorsal root ganglion (DRG) neurons. We found that sustained morphine treatment significantly augments intracellular cAMP production as well as basal CGRP release from cultured neonatal rat DRG neurons. The selective PKA inhibitor, H-89, attenuates the sustained morphine-mediated augmentation of basal CGRP release, indicating that the cAMP/PKA pathway plays an important role in regulation of CGRP release from sensory neurons. Since our present data also demonstrated that selective Raf-1 inhibitor, GW 5074, attenuated both the cAMP overshoot and the augmentation of CGRP release mediated by sustained morphine in neonatal rat DRG neurons, we suggest that Raf-1-mediated sensitization of the intracellular cAMP formation may play an important role in sustained morphine-mediated augmentation of spinal pain neurotransmitter release.

A structure-activity relationship study and combinatorial synthetic approach of C-terminal modified bifunctional peptides that are delta/mu opioid receptor agonists and neurokinin 1 receptor antagonists

A series of bifunctional peptides with opioid agonist and substance P antagonist bioactivities were designed with the concept of overlapping pharmacophores. In this concept, the bifunctional peptides were expected to interact with each receptor separately in the spinal dorsal horn where both the opioid receptors and the NK1 receptors were found to be expressed, to show an enhanced analgesic effect, no opioid-induced tolerance, and to provide better compliance than coadministration of two drugs. Compounds were synthesized using a two-step combinatorial method for C-terminal modification. In the method, the protected C-terminal-free carboxyl peptide, Boc-Tyr( tBu)- d-Ala-Gly Phe-Pro-Leu-Trp(Boc)-OH, was synthesized as a shared intermediate using Fmoc solid phase chemistry on a 2-chlorotrityl resin. This intermediate was esterified or amidated in solution phase. The structure-activity relationships (SAR) showed that the C-terminus acted as not only a critical pharmacophore for the substance P antagonist activities, but as an address region for the opioid agonist pharmacophore that is structurally distant from the C-terminal. Among the peptides, H-Tyr- d -Ala-Gly-Phe-Pro-Leu-Trp-NH-Bzl ( 3) demonstrated high binding affinities at both delta and mu receptors ( K i = 10 and 0.65 nM, respectively) with efficient agonist functional activity in the mouse isolated vas deferens (MVD) and guinea pig isolated ileum (GPI) assays (IC 50 = 50 and 13 nM, respectively). Compound 3 also showed a good antagonist activity in the GPI assay with substance P stimulation ( K e = 26 nM) and good affinity for the hNK1 receptor ( K i = 14 nM). Consequently, compound 3 is expected to be a promising and novel type of analgesic with bifunctional activities.

Morphine treatment accelerates sarcoma-induced bone pain, bone loss, and spontaneous fracture in a murine model of bone cancer

Metastatic bone cancer causes severe pain that is primarily treated with opioids. A model of bone cancer pain in which the progression of cancer pain and bone destruction is tightly controlled was used to evaluate the effects of sustained morphine treatment. In cancer-treated mice, morphine enhanced, rather than diminished, spontaneous, and evoked pain; these effects were dose-dependent and naloxone-sensitive. SP and CGRP positive DRG cells did not differ between sarcoma or control mice, but were increased following morphine in both groups. Morphine increased ATF-3 expression only in DRG cells of sarcoma mice. Morphine did not alter tumor growth in vitro or tumor burden in vivo but accelerated sarcoma-induced bone destruction and doubled the incidence of spontaneous fracture in a dose- and naloxone-sensitive manner. Morphine increased osteoclast activity and upregulated IL-1 beta within the femurs of sarcoma-treated mice suggesting enhancement of sarcoma-induced osteolysis. These results indicate that sustained morphine increases pain, osteolysis, bone loss, and spontaneous fracture, as well as markers of neuronal damage in DRG cells and expression of pro-inflammatory cytokines. Morphine treatment may result in "add-on" mechanisms of pain beyond those engaged by sarcoma alone. While it is not known whether the present findings in this model of osteolytic sarcoma will generalize to other cancers or opioids, the data suggest a need for increased understanding of neurobiological consequences of prolonged opioid exposure which may allow improvements in the use of opiates in the effective management of cancer pain.

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

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

PERSPECTIVE

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

Design, synthesis, and biological evaluation of novel bifunctional C-terminal-modified peptides for delta/mu opioid receptor agonists and neurokinin-1 receptor antagonists

A series of bifunctional peptides that act as agonists for delta and mu opioid receptors with delta selectivity and as antagonist for neurokinin-1 (NK1) receptors were designed and synthesized for potential application as analgesics in various pain states. The peptides were characterized using radioligand binding assays and functional assays using cell membrane and animal tissue. Optimization was performed on the fifth residue which serves as an address moiety for both receptor recognitions. It had critical effects on both activities at delta/mu opioid receptors and NK1 receptors. Among the synthesized peptides, H-Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF3) 2 (5) and H-Tyr-D-Ala-Gly-Phe-Nle-Pro-Leu-Trp-O-3,5-Bzl(CF3)2 (7) had excellent agonist activity for both delta opioid and mu opioid receptors and excellent antagonist activity for NK1 receptors. These results indicate that the rational design of multifunctional ligands with opioid agonist and neurokinin-1 antagonist activities can be accomplished and may provide a new tool for treatment of chronic and several pain states.

Involvement of cannabinoid (CB1)-receptors in the development and maintenance of opioid tolerance

Sustained exposure to opioid agonists such as morphine increases levels of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn, a response implicated in the development of opioid tolerance and physical dependence. Recent evidence suggests that both the opioid-induced increase in CGRP and the development of opioid physical dependence are suppressed by blockade of spinal cannabinoid (CB1)-receptors. The present study examined whether CB1-receptor activity also has a role in the development of opioid tolerance. In rats implanted with spinal catheters, repeated acute injections of morphine (15 microg) delivered over 4 h resulted in a rapid decline of thermal and mechanical antinociception and a significant loss of analgesic potency, reflecting development of acute opioid tolerance. In another set of experiments, chronic administration of spinal morphine (15 microg) once daily for 5 days produced a similar loss of analgesic effect and a marked increase in CGRP-immunoreactivity in the superficial laminae of the dorsal horn. Consistent with the in vivo findings, primary cultures of adult dorsal root ganglion (DRG) neurons exposed to morphine for 5 days showed a significant increase in the number of CGRP-immunoreactive neurons. Co-administration of acute or chronic morphine with a CB1-receptor antagonist/inverse agonist, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM-251), inhibited the development of both acute and chronic analgesic tolerance. In animals already exhibiting tolerance to morphine, intervention with AM-251 restored morphine analgesic potency. Co-administration with AM-251 attenuated the morphine-induced increase in CGRP-immunoreactivity in the spinal cord and in DRG cultured neurons. Collectively, the results of this study suggest that activity of endocannabinoids, mediated via CB1-receptors, contributes to both the development and maintenance of opioid tolerance by influencing the opioid-induced increase in spinal CGRP.

Spinal modulation of calcitonin gene-related peptide by endocannabinoids in the development of opioid physical dependence

Studies implicate endocannabinoids in the acute and chronic actions of opioid drugs, including the genesis of physical dependence. Previous evidence suggests that spinal release of calcitonin gene-related peptide (CGRP) and activation of its receptors contribute to opioid physical dependence. The release of CGRP at the spinal level is modulated by cannabinoid (CB1)-receptors. Thus, this study examined whether CB1-receptor activity mediates changes in CGRP underlying development of opioid physical dependence. Systemic morphine administration for 5-days elevated CGRP-immunoreactivity in the rat spinal dorsal horn. In situ hybridization of dorsal root ganglion (DRG) neurons revealed an increase in CGRP mRNA during initial (day 1-3) but not later phase (day 4-5) of morphine treatment. CGRP-immunoreactivity in DRG neurons, however, was increased in the later phase of morphine treatment. Naloxone challenge to morphine-treated animals precipitated an intense withdrawal syndrome that depleted CGRP-immunoreactivity and increased Fos expression in the dorsal horn. The Fos-response primarily occurred in neurons that expressed CGRP receptor component protein (RCP) suggesting CGRP activity contributes to neuronal activation during precipitated withdrawal. Spinal slices obtained from morphine-treated animals showed higher levels of CGRP release than from saline controls. Intrathecal co-administration of CB1-receptor antagonists, AM-251 or SR141716A, with daily morphine attenuated the behavioral manifestations of withdrawal. Treatment with AM-251 also reduced the depletion of CGRP, suppressed Fos-induction, and prevented the increase in capsaicin-evoked spinal CGRP release. Altogether, this study suggests that endocannabinoid activity, expressed via CB1-receptors, contributes to the induction of opioid physical dependence through spinal modulation of CGRP.

Characterization and effects on cAMP accumulation of adrenomedullin and calcitonin gene-related peptide (CGRP) receptors in dissociated rat spinal cord cell culture

Adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) have structural similarities, interact with each others receptors (calcitonin receptor-like receptor (CLR)/receptor-activity-modifying proteins (RAMPs)) and show overlapping biological activities. AM and CGRP receptors are chiefly coupled to cAMP production. In this study, a method of primary dissociated cell culture was used to investigate the presence of AM and CGRP receptors and their effects on cAMP production in embryonic spinal cord cells. Both neuronal and non-neuronal CLR immunopositive cells were present in our model. High affinity, specific [(125)I]-AM binding sites (K(d) 79 +/- 9 pM and B(max) 571 +/- 34 fmol mg(-1) protein) were more abundant than specific [(125)I]-CGRP binding sites (K(d) 12 +/- 0.7 pM and B(max) 32 +/- 2 fmol mg(-1) protein) in embryonic spinal cord cells. Specific [(125)I]-AM binding was competed by related molecules with a ligand selectivity profile of rAM > hAM(22-52) > rCGRPalpha > CGRP(8-37) >> [r-(r(*),s(*))]-N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-,1-piperidinecarboxamide (BIBN4096BS). Specific [(125)I]-CGRP binding was competed by rCGRPalpha > rAM > or = CGRP(8-37) > or = BIBN4096BS > hAM(22-52). Cellular levels of cAMP were increased by AM (pEC(50) 10.2 +/- 0.2) and less potently by rCGRPalpha (pEC(50) 8.9 +/- 0.4). rCGRPalpha-induced cAMP accumulation was effectively inhibited by CGRP(8-37) (pA(2) 7.63 +/- 0.44) and hAM(22-52) (pA(2) 6.18 +/- 0.21) while AM-stimulation of cAMP levels was inhibited by CGRP(8-37) (pA(2) 7.41+/- 0.15) and AM(22-52) (pA(2) 7.26 +/- 0.18). BIBN4096BS only antagonized the effects of CGRP (pA(2) 8.40 +/- 0.30) on cAMP accumulation. These pharmacological profiles suggest that effects of CGRP are mediated by the CGRP(1) (CLR/RAMP1) receptor in our model while those of AM are related to the activation of the AM(1) (CLR/RAMP2) receptor subtype.

Alternation of galanin in nociceptive modulation in the central nervous system of rats during morphine tolerance: A behavioral and immunohistochemical study

The present study was performed to investigate the alternation of galanin in nociceptive modulation and galanin-like immunoreactivity in the central nervous system of rats after morphine tolerance. The hindpaw withdrawal latencies to both thermal and mechanical stimulation increased significantly after intracerebroventricular injection of 3 nmol of galanin in opioid-naive rats. The antinociceptive effect induced by galanin was attenuated remarkably at the same dose in morphine-tolerant rats. Furthermore, an up-regulation of galanin-like immunoreactivity in the arcuate nucleus of hypothalamus of morphine-tolerant rat was observed by immunohistochemical methods, whereas no significant changes were detected in periaqueductal gray. The present study demonstrated that there are alternations in both galanin-induced antinociception and galanin-like immunoreactivity in the brain of rat after morphine tolerance. The results suggest an involvement of galanin in the central nervous system in morphine tolerance.

Inhibitory M2 muscarinic receptors are upregulated in both axotomized and intact small diameter dorsal root ganglion cells after peripheral nerve injury

Acetylcholine reduces nociceptive input in part by activating inhibitory M2 muscarinic receptors on primary sensory neurons, and acetylcholinesterase inhibitors and muscarinic agonists produce analgesia in humans and animals. M2 muscarinic receptors are upregulated in animals with diabetic neuropathy, but their level of expression and function after peripheral nerve injury has not been previously examined. This study tested, using intracellular Ca(2+) response to membrane depolarization, the effect of the M2 muscarinic receptor agonist bethanechol on individual dorsal root ganglion cells from normal and L5-6 spinal nerve-ligated rats, followed by M2 muscarinic receptor immunostaining. We also examined functional transient receptor potential for vanilloids-1 activity by determining intracellular Ca(2+) response evoked by capsaicin in M2 muscarinic receptor immunoreactive cells. In normal dorsal root ganglion cells, bethanechol inhibited the Ca(2+) response in a concentration-related fashion, and this inhibition was blocked by the M2 muscarinic receptor antagonist gallamine. Cells expressing M2 muscarinic receptors by immunostaining were significantly inhibited by bethanechol, whereas those lacking positive staining were not. The proportion of studied dorsal root ganglion neurons with positive M2 muscarinic receptor staining increased significantly in the injured ipsilateral L5-6 and the uninjured ipsilateral L4 ganglia, but not in the contralateral dorsal root ganglion neurons compared with normals. In contrast, the proportion of neurons responding to capsaicin significantly decreased in the injured ipsilateral L5-6 dorsal root ganglion cells. These results suggest that inhibitory M2 muscarinic receptors are upregulated in small- and medium-sized axotomized dorsal root ganglion neurons and their uninjured neighbors following nerve injury, and may represent an appropriate target for analgesia in this setting.

Opioid receptor-mediated hyperalgesia and antinociceptive tolerance induced by sustained opiate delivery

Opiates are commonly used to treat moderate to severe pain and can be used over prolonged periods in states of chronic pain such as those associated with cancer. In addition, to analgesic actions, studies show that opiate administration can paradoxically induce hyperalgesia. At the pre-clinical level, such hyperalgesia is associated with numerous pronociceptive neuroplastic changes within the primary afferent fibers and the spinal cord. In rodents, sustained opiate administration also induces antinociceptive tolerance. The mechanisms by which prolonged opiate exposure induces hyperalgesia and the relationship of this state to antinociceptive tolerance remain unclear. The present study was aimed at determining whether sustained opiate-induced hyperalgesia, associated neuroplasticity and antinociceptive tolerance are the result of specific opiate interaction at opiate receptors. Enantiomers of oxymorphone, a mu opioid receptor agonist, were administered to rats by spinal infusion across 7 days. Sustained spinal administration of (-)-oxymorphone, but not its inactive enantiomer (+)-oxymorphone or vehicle, upregulated spinal dynorphin content, produced thermal and tactile hypersensitivity, and produced antinociceptive tolerance. These results indicate that these pronociceptive actions of sustained opiate administration require specific interaction with opiate receptors and are unlikely to be the result of accumulation of potentially excitatory metabolic products. While the precise mechanisms, which may account for these pronociceptive changes remain to be unraveled, the present data point to plasticity initiated by opiate receptor interaction.

Enhanced prolactin levels in opium smokers

In Iran, opium is smoked for pleasure or as a medication by some people. It is a complex mixture of 40 different alkaloids, including morphine and codeine along with many impurities. Although it is well established that opioids or tobacco affect many physiological functions in humans, to our knowledge there has been no specific study looking at these effects in opium smokers. To assess that, we investigated the circulating levels of prolactin, TSH, LH, FSH and testosterone in male opium smokers who also smoke cigarettes (n=23, aged 28.4+/- 4.1 years), and comparing this with the corresponding values for nicotine abusers (n=12, 15-25 cigarettes/day) or a healthy control group (n=20) of the same age. Our results showed that 86.96% of the opium-dependent and 41.67 % of the nicotine-dependent group displayed high prolactin values (p<0.002). In addition, there was a positive correlation between the dose of opium and the plasma prolactin level of opium dependents (p=0.748, p<0.001). Low FSH was detected in 43.48% of the opium smokers and 50% of the cigarette smokers (p<0.001) with normal LH and testosterone levels. TSH of the opium smokers was also lower than that of the other two groups (p<0.002). In conclusion, the present data indicate that chronic opium and cigarette smoking may synergistically influence pituitary hormone production through the effects on neuropeptides produced either locally or systemic.