Rapid modulation of micro-opioid receptor signaling in primary sensory neurons

14-3-3γ mediates the long-term inhibition of peripheral kappa opioid receptor antinociceptive signaling by norbinaltorphimine

Long-term inhibition of kappa opioid receptor (KOR) signaling in peripheral pain-sensing neurons is a potential obstacle for development of peripherally-restricted KOR agonists that produce analgesia. Such a long-term inhibitory mechanism is invoked from activation of c-Jun N-terminal kinase (JNK) that follows a single injection of the KOR antagonist norbinaltorphimine (norBNI). This effect requires protein synthesis of an unknown mediator in peripheral pain-sensing neurons. Using 2D difference gel electrophoresis with tandem mass spectrometry, we have identified that the scaffolding protein 14-3-3γ is upregulated in peripheral sensory neurons following activation of JNK with norBNI. Knockdown of 14-3-3γ by siRNA eliminates the long-term reduction in KOR-mediated cAMP signaling by norBNI in peripheral sensory neurons in culture. Similarly, knockdown of 14-3-3γ in the rat hind paw abolished the norBNI-mediated long-term reduction in peripheral KOR-mediated antinociception. Further, overexpression of 14-3-3γ in KOR expressing CHO cells prevented KOR-mediated inhibition of cAMP signaling. These long-term effects are selective for KOR as heterologous regulation of other receptor systems was not observed. These data suggest that 14-3-3γ is both necessary and sufficient for the long-term inhibition of KOR by norBNI in peripheral sensory neurons.

Age-related changes in peripheral nociceptor function

Pain and pain management in the elderly population is a significant social and medical problem. Pain sensation is a complex phenomenon that typically involves activation of peripheral pain-sensing neurons (nociceptors) which send signals to the spinal cord and brain that are interpreted as pain, an unpleasant sensory experience. In this work, young (4-5 months) and aged (26-27 months) Fischer 344 x Brown Norway (F344xBN) rats were examined for nociceptor sensitivity to activation by thermal (cold and heat) and mechanical stimulation following treatment with inflammatory mediators and activators of transient receptor potential (TRP) channels. Unlike other senses that decrease in sensitivity with age, sensitivity of hindpaw nociceptors to thermal and mechanical stimulation was not different between young and aged F344xBN rats. Intraplantar injection of bradykinin (BK) produced greater thermal and mechanical allodynia in aged versus young rats, whereas only mechanical allodynia was greater in aged rats following injection of prostaglandin E₂ (PGE₂). Intraplantar injection of TRP channel activators, capsaicin (TRPV1), mustard oil (TRPA1) and menthol (TRPM8) each resulted in greater mechanical allodynia in aged versus young rats and capsaicin-induced heat allodynia was also greater in aged rats. A treatment-induced allodynia that was greater in young rats was never observed. The anti-allodynic effects of intraplantar injection of kappa and delta opioid receptor agonists, salvinorin-A and D-Pen²,D-Pen⁵]enkephalin (DPDPE), respectively, were greater in aged than young rats, whereas mu opioid receptor agonists, [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and morphine, were not effective in aged rats. Consistent with these observations, in primary cultures of peripheral sensory neurons, inhibition of cAMP signaling in response to delta and kappa receptor agonists was greater in cultures derived from aged rats. By contrast, mu receptor agonists did not inhibit cAMP signaling in aged rats. Thus, age-related changes in nociceptors generally favor increased pain signaling in aged versus young rats, suggesting that changes in nociceptor sensitivity may play a role in the increased incidence of pain in the elderly population. These results also suggest that development of peripherally-restricted kappa or delta opioid receptor agonists may provide safer and effective pain relief for the elderly.

Buprenorphine Exposure Alters the Development and Migration of Interneurons in the Cortex

The misuse of opioids has reached epidemic proportions over the last decade, with over 2.1 million people in the United States suffering from substance use disorders related to prescription opioid pain relievers. This increase in opioid misuse affects all demographics of society, including women of child-bearing age, which has led to a rise in opioid use during pregnancy. Opioid use during pregnancy has been associated with increased risk of obstetric complications and adverse neonatal outcomes, including neonatal abstinence syndrome. Currently, opioid use disorder in pregnant women is treated with long-acting opioid agonists, including buprenorphine. Although buprenorphine reduces illicit opioid use during pregnancy and improves infant outcomes at birth, few long-term studies of the neurodevelopmental consequences have been conducted. The goal of the current experiments was to examine the effects of buprenorphine on the development of the cortex using fetal brain tissue, 3D brain cultures, and rodent models. First, we demonstrated that we can grow cortical and subpallial spheroids, which model the cellular diversity, connectivity, and activity of the developing human brain. Next, we show that cells in the developing human cortex express the nociceptin opioid (NOP) receptor and that buprenorphine can signal through this receptor in cortical spheroids. Using subpallial spheroids to grow inhibitory interneurons, we show that buprenorphine can alter interneuron development and migration into the cortex. Finally, using a rodent model of prenatal buprenorphine exposure, we demonstrate that alterations in interneuron distribution can persist into adulthood. Together, these results suggest that more research is needed into the long-lasting consequences of buprenorphine exposure on the developing human brain.

Long-term antagonism and allosteric regulation of mu opioid receptors by the novel ligand, methocinnamox

Opioid overdose is a leading cause of death in the United States. The only treatment available currently is the competitive antagonist, naloxone (Narcan® ). Although naloxone is very effective and has saved many lives, as a competitive antagonist it has limitations. Due to the short half-life of naloxone, renarcotization can occur if the ingested opioid agonist remains in the body longer. Moreover, because antagonism by naloxone is surmountable, renarcotization can also occur in the presence of naloxone if a relatively larger dose of opioid agonist is taken. In such circumstances, a long-lasting, non-surmountable antagonist would offer an improvement in overdose treatment. Methocinnamox (MCAM) has been reported to have a long duration of antagonist action at mu opioid receptors in vivo. In HEK cells expressing the human mu opioid receptor, MCAM antagonism of mu agonist-inhibition of cAMP production was time-dependent, non-surmountable and non-reversible, consistent with (pseudo)-irreversible binding. In vivo, MCAM injected locally into the rat hindpaw antagonized mu agonist-mediated inhibition of thermal allodynia for up to 96 h. By contrast, antagonism by MCAM of delta or kappa agonists in HEK cells and in vivo was consistent with simple competitive antagonism. Surprisingly, MCAM also shifted the concentration-response curves of mu agonists in HEK cells in the absence of receptor reserve in a ligand-dependent manner. The shift in the [D-Ala2 ,N-MePhe4 ,Gly-ol5 ]-enkephalin (DAMGO) concentration-response curve by MCAM was insensitive to naloxone, suggesting that in addition to (pseudo)-irreversible orthosteric antagonism, MCAM acts allosterically to alter the affinity and/or intrinsic efficacy of mu agonists.

GRK2 Dictates a Functional Switch of the Peripheral Mu-Opioid Receptor

The peripheral mu-opioid receptor (MOR) has been recognized as a potential target to provide safer analgesia with reduced central side effects. Although analgesic incompetence of the peripheral MOR in the absence of inflammation was initially identified more than a decade ago, there has been very limited investigation into the underlying signaling mechanisms. Here we identify that G protein-coupled receptor kinase 2 (GRK2) constitutively interacts with the MOR in peripheral sensory neurons to suppress peripheral MOR activity. Brief exposure to bradykinin (BK) causes uncoupling of GRK2 from the MOR and subsequent restoration of MOR functionality in dorsal root ganglion (DRG) neurons. Interestingly, prolonged BK treatment induces constitutive activation of the MOR through a mechanism that involves protein kinase C (PKC) activation. After silencing Raf kinase inhibitory protein (RKIP) by RNA interference, BK-induced constitutive MOR activation is completely abrogated, which agrees with previous findings that BK activates PKC signaling to initiate GRK2 sequestration by RKIP. Furthermore, we demonstrate that constitutive, peripheral MOR activity requires GRK2 uncoupling and that the FDA-approved SSRI paroxetine promotes this state of uncoupling. Collectively, these results indicate that GRK2 tightly regulates MOR functional states and controls constitutive MOR activity in peripheral sensory neurons, supporting the potential for targeting the kinase to provide safer analgesia.

Modulation of μ-opioid receptor activation by acidic pH is dependent on ligand structure and an ionizable amino acid residue

Background and Purpose

Adverse side effects of conventional opioids can be avoided if ligands selectively activate peripheral opioid receptors in injured tissue. Injury and inflammation are typically accompanied by acidification. In this study, we examined influences of low pH and mutation of the ionizable amino acid residue H297^6.52 on μ‐opioid receptor binding and signalling induced by the μ‐opioid receptor ligands fentanyl, DAMGO, and naloxone.

Experimental Approach

HEK 293 cells stably transfected with μ‐opioid receptors were used to study opioid ligand binding, [³⁵S]‐GTPγS binding, and cAMP reduction at physiological and acidic pH. We used μ‐opioid receptors mutated at H297^6.52 to A (MOR‐H297^6.52A) to delineate ligand‐specific interactions with H297^6.52.

Key Results

Low pH and the mutant receptor MOR‐H297^6.52A impaired naloxone binding and antagonism of cAMP reduction. In addition, DAMGO binding and G‐protein activation were decreased under these conditions. Fentanyl‐induced signalling was not influenced by pH and largely independent of H297^6.52.

Conclusions and Implications

Our investigations indicate that low pH selectively impairs μ‐opioid receptor signalling modulated by ligands capable of forming hydrogen bonds with H297^6.52. We propose that protonation of H297^6.52 at acidic pH reduces binding and subsequent signalling of such ligands. Novel agonists targeting opioid receptors in injured tissue might benefit from lack of hydrogen bond formation with H297^6.52.

Dynamic Opioid Receptor Regulation in the Periphery

Opioids serve a vital role in the current analgesic array of treatment options. They are useful in acute instances involving severe pain associated with trauma, surgery, and terminal diseases such as cancer. In the past three decades, multiple receptor isoforms and conformations have been reported throughout literature. Most of these studies conducted systemic analyses of opioid receptor function, often generalizing findings from receptor systems in central nervous tissue or exogenously expressing immortalized cell lines as common mechanisms throughout physiology. However, a culmination of innovative experimental data indicates that opioid receptor systems are differentially modulated depending on their anatomic expression profile. Importantly, opioid receptors expressed in the peripheral nervous system undergo regulation uncommon to similar receptors expressed in central nervous system tissues. This distinctive characteristic begs one to question whether peripheral opioid receptors maintain anatomically unique roles, and whether they may serve an analgesic advantage in providing pain relief without promoting addiction.

Signaling characteristics and functional regulation of delta opioid-kappa opioid receptor (DOP-KOP) heteromers in peripheral sensory neurons

Receptor heteromers often display distinct pharmacological and functional properties compared to the individual receptor constituents. In this study, we compared the properties of the DOP-KOP heteromer agonist, 6'-guanidinonaltrindole (6'-GNTI), with agonists for DOP ([D-Pen2,5]-enkephalin [DPDPE]) and KOP (U50488) in peripheral sensory neurons in culture and in vivo. In primary cultures, all three agonists inhibited PGE₂-stimulated cAMP accumulation as well as activated extracellular signal-regulated kinase 1/2 (ERK) with similar efficacy. ERK activation by U50488 was Gi-protein mediated but that by DPDPE or 6'-GNTI was Gi-protein independent (i.e., pertussis toxin insensitive). Brief pretreatment with DPDPE or U50488 resulted in loss of cAMP signaling, however, no desensitization occurred with 6'-GNTI pretreatment. In vivo, following intraplantar injection, all three agonists reduced thermal nociception. The dose-response curves for DPDPE and 6'-GNTI were monotonic whereas the curve for U50488 was an inverted U-shape. Inhibition of ERK blocked the downward phase and shifted the curve for U50488 to the right. Following intraplantar injection of carrageenan, antinociceptive responses to either DPDPE or U50488 were transient but could be prolonged with inhibitors of 12/15-lipoxgenases (LOX). By contrast, responsiveness to 6'-GNTI remained for a prolonged time in the absence of LOX inhibitors. Further, pretreatment with the 12/15-LOX metabolites, 12- and 15- hydroxyeicosatetraenoic acid, abolished responses to U50488 and DPDPE but had no effect on 6'-GNTI-mediated responses either in cultures or in vivo. Overall, these results suggest that DOP-KOP heteromers exhibit unique signaling and functional regulation in peripheral sensory neurons and may be a promising therapeutic target for the treatment of pain.

Making Sense of Pharmacology: Inverse Agonism and Functional Selectivity

Constitutive receptor activity/inverse agonism and functional selectivity/biased agonism are 2 concepts in contemporary pharmacology that have major implications for the use of drugs in medicine and research as well as for the processes of new drug development. Traditional receptor theory postulated that receptors in a population are quiescent unless activated by a ligand. Within this framework ligands could act as agonists with various degrees of intrinsic efficacy, or as antagonists with zero intrinsic efficacy. We now know that receptors can be active without an activating ligand and thus display "constitutive" activity. As a result, a new class of ligand was discovered that can reduce the constitutive activity of a receptor. These ligands produce the opposite effect of an agonist and are called inverse agonists. The second topic discussed is functional selectivity, also commonly referred to as biased agonism. Traditional receptor theory also posited that intrinsic efficacy is a single drug property independent of the system in which the drug acts. However, we now know that a drug, acting at a single receptor subtype, can have multiple intrinsic efficacies that differ depending on which of the multiple responses coupled to a receptor is measured. Thus, a drug can be simultaneously an agonist, an antagonist, and an inverse agonist acting at the same receptor. This means that drugs have an additional level of selectivity (signaling selectivity or "functional selectivity") beyond the traditional receptor selectivity. Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools.

Allosterism within δ Opioid-κ Opioid Receptor Heteromers in Peripheral Sensory Neurons: Regulation of κ Opioid Agonist Efficacy

There is abundant evidence for formation of G protein-coupled receptor heteromers in heterologous expression systems, but little is known of the function of heteromers in native systems. Heteromers of δ and κ opioid receptors (DOR-KOR heteromers) have been identified in native systems. We previously reported that activation of DOR-KOR heteromers expressed by rat pain-sensing neurons (nociceptors) produces robust, peripherally mediated antinociception. Moreover, DOR agonist potency and efficacy is regulated by KOR antagonists via allosteric interactions within the DOR-KOR heteromer in a ligand-dependent manner. Here we assessed the reciprocal regulation of KOR agonist function by DOR antagonists in adult rat nociceptors in culture and in a behavioral assay of nociception. Naltrindole enhanced the potency of the KOR agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-pyrrolidin-1-ylethyl]acetamide (ICI-199441) 10- to 20-fold, but did not alter responses to 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50488). By contrast, the potency of U50488 was enhanced 20-fold by 7-benzylidenenaltrexone. The efficacy of 6'-guanidinonaltrindole (6'-GNTI) to inhibit nociceptors was blocked by small interfering RNA knockdown of DOR or KOR. Replacing 6'-GNTI occupancy of DOR with either naltrindole or 7-benzylidenenaltrexone abolished 6'-GNTI efficacy. Further, peptides derived from DOR transmembrane segment 1 fused to the cell membrane-penetrating HIV transactivator of transcription peptide also blocked 6'-GNTI-mediated responses ex vivo and in vivo, suggesting that 6'-GNTI efficacy in nociceptors is due to its positive allosteric regulation of KOR via occupancy of DOR in a DOR-KOR heteromer. Together, these results provide evidence for the existence of functional DOR-KOR heteromers in rat peripheral sensory neurons and that reciprocal, ligand-dependent allosteric interactions occur between the DOR and KOR protomers.

Orofacial antinociceptive effect of Mimosa tenuiflora (Willd.) Poiret

Mimosa tenuiflora (Willd.) Poiret, popularly known in Brazil as "jurema-preta" is widely used against bronchitis, fever, headache and inflammation. Its antioxidant, anti-inflammatory and antinociceptive potential has already been reported. To assess the orofacial antinociceptive effect of M. tenuiflora, ethanolic extracts of M. tenuiflora (leaves, twigs, barks and roots) were submitted to in vitro tests of antioxidant activity. The extract with the highest antioxidant potential was partitioned and subjected to preliminary chemical prospecting, GC-MS, measurement of phenolic content and cytotoxicity tests of the fraction with the highest antioxidant activity. The nontoxic fraction with the highest antioxidant activity (FATEM) was subjected to tests of acute and chronic orofacial nociception and locomotor activity. The possible mechanisms of neuromodulation were also assessed. The EtOAc fraction, obtained from the ethanolic extract of M. tenuiflora barks, was the one with the highest antioxidant potential and nontoxic (FATEM), and Benzyloxyamine was the major constituent (34.27%). FATEM did not alter the locomotor system of mice and reduced significantly the orofacial nociceptive behavior induced by formalin, glutamate, capsaicin, cinnamaldehyde or acidic saline compared to the control group. FATEM also inhibited formalin- or mustard oil-induced temporomandibular nociception. In addition, it also reduced mustard oil-induced orofacial muscle nociception. However, FATEM did not alter hypertonic saline-induced corneal nociception. Neuropathic nociception was reversed by treatment with FATEM. The antinociceptive effect of FATEM was inhibited by naloxone, L-NAME and glibenclamide. FATEM has pharmacological potential for the treatment of acute and neuropathic orofacial pain and this effect is modulated by the opioid system, nitric oxide and ATP-sensitive potassium channels. These results lead us to studies of isolation and characterization of bioactive principles.

B2-kinin receptors in the dorsal periaqueductal gray are implicated in the panicolytic-like effect of opiorphin

Reported results have shown that the pentapeptide opiorphin inhibits oligopeptidases that degrade brain neuropeptides, and has analgesic and antidepressant effects in experimental animals, without either tolerance or dependency after chronic administration. In a previous study we showed that opiorphin has a panicolytic-like effect in the dorsal periaqueductal gray (dPAG) electrical stimulation test (EST), mediated by the μ-opioid receptor (MOR). This study further analyzes the mechanism of opiorphin panicolytic action, using the EST and drug injection inside the dPAG. The obtained results showed that blockade of the 5-HT_1A receptors with WAY-100635 did not change the escape-impairing effect of opiorphin, and combined injection of sub-effective doses of opiorphin and the 5-HT_1A-agonist 8-OH-DPAT did not have a significant anti-escape effect. In contrast, the anti-escape effect of opiorphin was antagonized by pretreatment with the kinin B2 receptor blocker HOE-140, and association of sub-effective doses of opiorphin and bradykinin caused a significant anti-escape effect. The anti-escape effect of bradykinin was not affected by previous administration of WAY-100635. Therefore, the anti-escape effect of opiorphin in the dPAG seems to be mediated by endogenous bradykinin, acting on kinin B2 receptors, which previous results have shown to interact synergistically with MOR in the dPAG to restrain escape in two animal models of panic. Chemical compounds: Opiorphin (PubChem CID: 25195667); WAY100635 maleate salt (PubChem CID: 11957721); 8-OH-DPAT hydrobromide (PubChem CID: 6917794); Bradykinin (PubChem CID: 439201); HOE-140 (Icatibant) (PubChem CID: 6918173).

Regulation of δ Opioid Receptor-Mediated Signaling and Antinociception in Peripheral Sensory Neurons by Arachidonic Acid-Dependent 12/15-Lipoxygenase Metabolites

The function of δ opioid receptors (DOR) expressed by peripheral pain-sensing neurons (nociceptors) is regulated by both cyclooxygenase- and lipoxygenase (LOX)-dependent arachidonic acid (AA) metabolites. Whereas cyclooxygenase metabolites enhance responsiveness, LOX metabolites elicit a refractory, nonsignaling state of the DOR receptor system for antinociceptive signaling. In this study, using high-performance liquid chromatography-tandem mass spectrometry analyses, we have found that the 12-/15-LOX metabolites, 12-hydroxyeicosatetraenoic acid (HETE) and 15-HETE, were elevated after treatment of adult rat primary sensory neuron cultures with AA. Exogenously applied 12-HETE and 15-HETE, but not 5-HETE, completely prevented DOR and κ opioid receptor (KOR) agonist-mediated inhibition of prostaglandin E2 (PGE₂)-stimulated cAMP accumulation, but not inhibition, by the 5-HT₁ receptor agonist 5-carboxamidotryptamine in cultured peripheral sensory neurons and in Chinese hamster ovary (CHO) cells heterologously expressing DOR or KOR. Similarly, intraplantar injection of 12- or 15-HETE, either alone or in combination, prevented DOR agonist-mediated inhibition of PGE₂-evoked thermal allodynia. Further, both AA- and carrageenan-mediated induction of the nonresponsive state of the DOR system was blocked by an intraplantar coinjection of the 12-/15-LOX inhibitors baicalein and luteolin. In contrast to the regulation of cAMP signaling, pretreatment with 12- and 15-HETE had no effect on either DOR or KOR agonist- mediated activation of extracellular signal-regulated kinase in peripheral sensory neurons or CHO cells. These results suggest that the analgesic efficacy of peripherally restricted opioids for treatment of inflammatory pain may be enhanced by adjunct inhibition of LOX activity.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Effects of local and spinal administrations of mu-opioids on postoperative pain in aged versus adult mice

Introduction

Suboptimal management of postoperative pain leads to increased risk of chronic opioid therapy, especially in elderly patients.

Objectives

Although this age-dependent phenomenon has been observed clinically, basic mechanisms including baseline nociception, postoperative hypersensitivity, and mu-opioid efficiency in aged animals have never been evaluated.

Methods

We tested these criteria using incision model on adult (3–6 months) and aged (24 months) mice to assess translatability of postoperative animal studies to clinical observations.

Results

Thermal and mechanical testing revealed lower baseline nociception in aged vs adult mice, while behavioral assays after hind paw plantar incision showed similar hypersensitivity levels for both age groups. Efficiency of local and spinal mu-opioid injections on postoperative pain was assessed next. DAMGO, a pure mu-opioid, was effective in reducing postoperative hypersensitivity in aged and adult mice, although adult mice displayed increased sensitivity to higher doses (50 μg local; 1–15 μg spinal). Buprenorphine, a mixed mu-opioid agonist, produced dose-dependent antihypersensitivity with adult mice more sensitive to lower doses (0.1 μg local; 0.02 μg spinal), and aged mice more sensitive to higher doses (1, 10 μg local; 0.1, 1 μg spinal). Finally, exploratory locomotor activity was used to evaluate the suppression of incision-induced spontaneous pain by DAMGO. Spinal and systemic (intraperitoneal) DAMGO inhibited ongoing pain more in adults compared with aged mice.

Conclusion

As in humans, baseline nociception was lower in aged vs adult mice, while postoperative hypersensitivity magnitudes were comparable between groups. Unlike in humans, adult mice were more sensitive to mu-opioids, although higher doses of mixed mu-opioids were more effective for postoperative antihypersensitivity in aged mice.

Constitutive Desensitization of Opioid Receptors in Peripheral Sensory Neurons

Opioid receptors expressed by peripheral pain-sensing neurons are functionally inactive for antinociceptive signaling under most basal conditions; however, tissue damage or exposure to inflammatory mediators (e.g., bradykinin) converts these receptors from a nonresponsive state to a functionally competent state. Here we tested the hypothesis that the basal, nonresponsive state of the mu- and delta-opioid receptors (MOR and DOR, respectively) is the result of constitutive receptor activity that activates desensitization mechanisms, resulting in MOR and DOR receptor systems that are constitutively desensitized. Consistent with our previous findings, under basal conditions, neither the MOR agonist [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin nor the DOR agonist [d-Pen2,5]-enkephalin, inhibited prostaglandin E2 (PGE2)-stimulated cAMP accumulation in peripheral sensory neurons in culture (ex vivo) or inhibited PGE2-stimulated thermal allodynia in the rat hind paw in vivo. Prolonged treatment with naloxone induced MOR and DOR responsiveness both in vivo and ex vivo to a similar magnitude as that produced by bradykinin. Also similar to bradykinin, the effect of naloxone persisted for 60 minutes after washout of the ligand. By contrast, prolonged treatment with 6β-naltrexol, did not induce functional competence of MOR or DOR but blocked the effect of naloxone. Treatment with siRNA for β-arrestin-2, but not β-arrestin-1, also induced MOR and DOR functional competence in cultured peripheral sensory neurons. These data suggest that the lack of responsiveness of MOR and DOR to agonist for antinociceptive signaling in peripheral sensory neurons is due to constitutive desensitization that is likely mediated by β-arrestin-2.

Long-Term Reduction of Kappa Opioid Receptor Function by the Biased Ligand, Norbinaltorphimine, Requires c-Jun N-Terminal Kinase Activity and New Protein Synthesis in Peripheral Sensory Neurons

A single administration of the κ opioid receptor (KOR) antagonist, norbinaltorphimine (norBNI), produces long-term reduction in KOR function in heterologous expression systems and brain that is mediated by activation of c-Jun N-terminal kinase (JNK). In this study, we examined the long-term effects of norBNI on adult rat peripheral sensory neurons in vivo and ex vivo. Following a single intraplantar (i.pl.) injection of norBNI into the hind paw, peripheral KOR-mediated antinociception in the ipsilateral, but not the contralateral, hindpaw was abolished for at least 9 days. By contrast, the antinociceptive response to mu and delta opioid receptor agonists was unaltered. The long-term inhibitory effect on antinociception produced by pretreatment with norBNI required occupancy of peripheral KOR and was completely blocked by i.pl. injection of the JNK inhibitor, SP600125. In cultures of peripheral sensory neurons, norBNI activated JNK for at least 30 minutes. Furthermore, norBNI blocked KOR-mediated inhibition of adenylyl cyclase activity measured 24 hours later in a JNK-dependent manner, but did not block activation of extracellular signal-regulated kinase (ERK). The long-term inhibitory effect of norBNI on KOR function in vivo and ex vivo was blocked by inhibitors of mRNA translation, cycloheximide and rapamycin. These data suggest that in peripheral sensory neurons norBNI is a KOR-biased ligand for activation of JNK signaling, resulting in long-term blockade of some (antinociception, inhibition of adenylyl cyclase activity), but not all (ERK), KOR signaling. Importantly, norBNI elicits de novo protein synthesis in sensory neuron terminals that produces selective long-term regulation of KOR.

Cell-autonomous regulation of Mu-opioid receptor recycling by substance P

How neurons coordinate and reprogram multiple neurotransmitter signals is an area of broad interest. Here, we show that substance P (SP), a neuropeptide associated with inflammatory pain, reprograms opioid receptor recycling and signaling. SP, through activation of the neurokinin 1 (NK1R) receptor, increases the post-endocytic recycling of the mu-opioid receptor (MOR) in trigeminal ganglion (TG) neurons in an agonist-selective manner. SP-mediated protein kinase C (PKC) activation is both required and sufficient for increasing recycling of exogenous and endogenous MOR in TG neurons. The target of this cross-regulation is MOR itself, given that mutation of either of two PKC phosphorylation sites on MOR abolishes the SP-induced increase in recycling and resensitization. Furthermore, SP enhances the resensitization of fentanyl-induced, but not morphine-induced, antinociception in mice. Our results define a physiological pathway that cross-regulates opioid receptor recycling via direct modification of MOR and suggest a mode of homeostatic interaction between the pain and analgesic systems.

Opioid Receptors

Opioids are the oldest and most potent drugs for the treatment of severe pain. Their clinical application is undisputed in acute (e.g., postoperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny. This article reviews mechanisms underlying opioid analgesia and other opioid actions. It discusses the structure, function, and plasticity of opioid receptors; the central and peripheral sites of analgesic actions and side effects; endogenous and exogenous opioid receptor ligands; and conventional and novel opioid compounds. Challenging clinical situations, such as the tension between chronic pain and addiction, are also illustrated.

Functional selectivity of kappa opioid receptor agonists in peripheral sensory neurons

Activation of kappa opioid receptors (KORs) expressed by peripheral sensory neurons that respond to noxious stimuli (nociceptors) can reduce neurotransmission of pain stimuli from the periphery to the central nervous system. We have previously shown that the antinociception dose-response curve for peripherally restricted doses of the KOR agonist (-)-(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50488) has an inverted U shape. Here, we found that the downward phase of the U50488 dose-response curve was blocked by an inhibitor of extracellular signal-regulated kinase (ERK) activation U0126. Local administration of the selective KOR agonist salvinorin A (Sal-A), also resulted in an inverted U-shaped curve; however, the downward phase was insensitive to U0126. By contrast, inhibition of c-Jun N-terminal kinase (JNK) partially blocked the downward phase of the dose-response curve to Sal-A, suggesting a role for JNK. In cultures of peripheral sensory neurons, U50488 and Sal-A inhibited adenylyl cyclase activity with similar efficacies; however, their ability to activate ERK and JNK differed. Whereas U50488 activated ERK but not JNK, Sal-A activated JNK but not ERK. Moreover, although both U50488 and Sal-A produced homologous desensitization, desensitization to U50488 was blocked by inhibition of ERK activation, whereas desensitization to Sal-A was blocked by inhibition of JNK. Substitution of an ethoxymethyl ether for the C2 position acetyl group of Sal-A reduced stimulation of JNK, prevented desensitization by ethoxymethyl ether for the C2 position acetyl group of Sal-A, and resulted in a monotonic antinociception dose-response curve. Collectively, these data demonstrate the functional selectivity of KOR ligands for signaling in peripheral sensory neurons, which results in differential effects on behavioral responses in vivo.

Dual regulation of δ-opioid receptor function by arachidonic acid metabolites in rat peripheral sensory neurons

The regulation of opioid receptor system function in peripheral sensory neurons is not well understood. Opioid agonist efficacy to inhibit nociceptor function and to promote antinociception is generally weak under basal conditions and frequently no response occurs. However, in response to a cyclooxygenase-dependent metabolite of arachidonic acid (AA) after exposure to inflammatory mediators, such as bradykinin (BK) or exogenous AA, peripheral opioid receptor systems become much more responsive to opioid agonists. In this study, we examined the time course for the induction and maintenance of functional competence of the δ-opioid receptor (DOR) system in adult rat nociceptors in culture and in vivo. We found that the responsive state of DOR after pretreatment with BK or exogenous AA is transient (30-60 minutes) and persists for 15-30 minutes after a 15-minute exposure of nociceptors to BK or AA. Interestingly, whereas functional competence of the DOR system could be reinduced with a second application of BK 60 minutes after the first, responsiveness of the DOR system could not be reinduced after an initial exposure to AA. This nonresponsive state of DOR after exogenous AA was mediated by a lipoxygenase (LOX)-dependent metabolite of AA. Intraplantar carrageenan also produced transient DOR functional competence and responsiveness was also reinduced by inhibition of LOX. Thus, the DOR system expressed by peripheral sensory neurons is under dual regulation by cyclooxygenase- and LOX-dependent metabolites of AA.

Endogenous regulation of inflammatory pain by T-cell-derived opioids: when friend turns to foe

Painful sensation is a hallmark of microbe-induced inflammation. This inflammatory pain is downregulated a few days after infection by opioids locally released by effector T lymphocytes generated in response to microbe-derived antigens. This review focuses on the endogenous regulation of inflammatory pain associated with adaptive T-cell response and puts in perspective the clinical consequences of the opioid-mediated analgesic activity of colitogenic T lymphocytes in inflammatory bowel disease.

Activation of estrogen receptor α enhances bradykinin signaling in peripheral sensory neurons of female rats

Numerous studies have demonstrated that females have a higher risk of experiencing several pain disorders with either greater frequency or severity than males. Although the mechanisms that underlie this sex disparity remain unclear, several studies have shown an important role for sex steroids, such as estrogen, in the modulation of nociception. Receptors for estrogen are present in primary afferent neurons in the trigeminal and dorsal root ganglia, and brief exposure to estrogen increases responses to the inflammatory mediator bradykinin (BK). However, the mechanism for estrogen-mediated enhancement of BK signaling is not fully understood. The aim of the present study was to evaluate the relative contributions of estrogen receptor α (ERα), ERβ, and G protein-coupled estrogen receptor 1 (GPER) to the enhanced signaling of the inflammatory mediator BK by 17β-estradiol (17β-E2) in primary sensory neurons from female rats in culture (ex vivo) and in behavioral assays of nociception in vivo. The effects of 17β-E2 on BK responses were mimicked by ERα-selective agonists and blocked by ERα-selective antagonists and by small interfering RNA knockdown of ERα. The data indicate that ERα is required for 17β-E2-mediated enhancement of BK signaling in peripheral sensory neurons in female rats.

Activation of mu opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) via β-arrestin-2-mediated cross-talk

The transient receptor potential family V1 channel (TRPV1) is activated by multiple stimuli, including capsaicin, acid, endovanilloids, and heat (>42C). Post-translational modifications to TRPV1 result in dynamic changes to the sensitivity of receptor activation. We have previously demonstrated that β-arrestin2 actively participates in a scaffolding mechanism to inhibit TRPV1 phosphorylation, thereby reducing TRPV1 sensitivity. In this study, we evaluated the effect of β-arrestin2 sequestration by G-protein coupled receptors (GPCRs) on thermal and chemical activation of TRPV1. Here we report that activation of mu opioid receptor by either morphine or DAMGO results in β-arrestin2 recruitment to mu opioid receptor in sensory neurons, while activation by herkinorin does not. Furthermore, treatment of sensory neurons with morphine or DAMGO stimulates β-arrestin2 dissociation from TRPV1 and increased sensitivity of the receptor. Conversely, herkinorin treatment has no effect on TRPV1 sensitivity. Additional behavioral studies indicate that GPCR-driven β-arrestin2 sequestration plays an important peripheral role in the development of thermal sensitivity. Taken together, the reported data identify a novel cross-talk mechanism between GPCRs and TRPV1 that may contribute to multiple clinical conditions.

The Effects of Intraarticular Opioids in pain relief after Arthroscopic Menisectomy: A Randomized Clinical Trial Study

Objective: Knee arthroscopy is a painful procedure which if untreated will cause intensive and prolonged pain that may prevent rehabilitation of patients. This study was designed to compare the analgesic effects of different opioids in the early post operative period in comparison to control group.

Methodology: One hundred forty patients were prospectively assigned to four groups randomly. After arthroscopic menisectomy all patients received an intraarticular injection containing 9.5 ml bupivacaine 0.5% with 1:200000 epinephrine in a 10 ml syringe. The remainder of syringe was filled with one of the study solutions. Group I: 5mg methadone, group II: 5mg morphine, group III: 5 ml normal saline, group IV: 50 mg meperidine. At three stages in the ealy post operative period the need for analgesics was recorded. A statistical comparison was done afterwards.

Results: In morphine group (group II), the analgesic usage in hospitalized and outpatients compared with other groups was significantly low(P<0.05).

Conclusion: Morphine in comparison to meperidine or methadone is more beneficial in reducing pain or analgesic need when is added to bupivacain injection following arthroscopic menisectomy.

Signalling profile differences: paliperidone versus risperidone

BACKGROUND AND PURPOSE

Paliperidone is an active metabolite of the second-generation atypical antipsychotic, risperidone recently approved for the treatment of schizophrenia and schizoaffective disorder. Because paliperidone differs from risperidone by only a single hydroxyl group, questions have been raised as to whether there are significant differences in the effects elicited between these two drugs.

EXPERIMENTAL APPROACH

We compared the relative efficacies of paliperidone versus risperidone to regulate several cellular signalling pathways coupled to four selected GPCR targets that are important for either therapeutic or adverse effects: human dopamine D2 , human serotonin 2A receptor subtype (5-HT2A ), human serotonin 2C receptor subtype and human histamine H1 receptors.

KEY RESULTS

Whereas the relative efficacies of paliperidone and risperidone were the same for some responses, significant differences were found for several receptor-signalling systems, with paliperidone having greater or less relative efficacy than risperidone depending upon the receptor-response pair. Interestingly, for 5-HT2A -mediated recruitment of β-arrestin, 5-HT2A -mediated sensitization of ERK, and dopamine D2 -mediated sensitization of adenylyl cyclase signalling, both paliperidone and risperidone behaved as agonists.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the single hydroxyl group of paliperidone promotes receptor conformations that can differ from those of risperidone leading to differences in the spectrum of regulation of cellular signal transduction cascades. Such differences in signalling at the cellular level could lead to differences between paliperidone and risperidone in therapeutic efficacy or in the generation of adverse effects.

Functional role of peripheral opioid receptors in the regulation of cardiac spinal afferent nerve activity during myocardial ischemia

Thinly myelinated Aδ-fiber and unmyelinated C-fiber cardiac sympathetic (spinal) sensory nerve fibers are activated during myocardial ischemia to transmit the sensation of angina pectoris. Although recent observations showed that myocardial ischemia increases the concentrations of opioid peptides and that the stimulation of peripheral opioid receptors inhibits chemically induced visceral and somatic nociception, the role of opioids in cardiac spinal afferent signaling during myocardial ischemia has not been studied. The present study tested the hypothesis that peripheral opioid receptors modulate cardiac spinal afferent nerve activity during myocardial ischemia by suppressing the responses of cardiac afferent nerve to ischemic mediators like bradykinin and extracellular ATP. The nerve activity of single unit cardiac afferents was recorded from the left sympathetic chain (T₂-T₅) in anesthetized cats. Forty-three ischemically sensitive afferent nerves (conduction velocity: 0.32-3.90 m/s) with receptive fields in the left and right ventricles were identified. The responses of these afferent nerves to repeat ischemia or ischemic mediators were further studied in the following protocols. First, epicardial administration of naloxone (8 μmol), a nonselective opioid receptor antagonist, enhanced the responses of eight cardiac afferent nerves to recurrent myocardial ischemia by 62%, whereas epicardial application of vehicle (PBS) did not alter the responses of seven other cardiac afferent nerves to ischemia. Second, naloxone applied to the epicardial surface facilitated the responses of seven cardiac afferent nerves to epicardial ATP by 76%. Third, administration of naloxone enhanced the responses of seven other afferent nerves to bradykinin by 85%. In contrast, in the absence of naloxone, cardiac afferent nerves consistently responded to repeated application of ATP (n = 7) or bradykinin (n = 7). These data suggest that peripheral opioid peptides suppress the responses of cardiac sympathetic afferent nerves to myocardial ischemia and ischemic mediators like ATP and bradykinin.

Sensitivity of hippocampal 5-HT1A receptors to mild stress in BDNF-deficient mice

Serotonin 1A (5-HT(1A)) receptors in brain play an important role in cognitive and integrative functions, as well as emotional states. Decreased brain-derived neurotrophic factor (BDNF) expression and/or function, particularly in hippocampus, are implicated in the pathophysiology of stress-related disorders such as major depression. BDNF(+/-) mice are more vulnerable to stress than wild-type mice, exhibiting behavioural despair after mild handling stress. We examined the effect of mild handling stress on 5-HT(1A) receptor function, as measured by 8-OH-DPAT stimulated [(35)S]GTPγS binding, in BDNF(+/-) mice and mice with a forebrain-specific reduction in BDNF (embryonic BDNF inducible knockout mice). Our data show a remarkable sensitivity of hippocampal 5-HT1A receptors to mild stress and a deficiency in BDNF. Other 5-HT(1A) receptor populations, specifically in frontal cortex and dorsal raphe, were resistant to the combined detrimental effects of mild stress and reductions in BDNF expression. Decreases in hippocampal 5-HT(1A) receptor function induced by mild stress in BDNF-deficient mice were prevented by administration of the selective serotonin reuptake inhibitor fluoxetine, which increased activation of TrkB, the high affinity receptor for BDNF, in wild-type and BDNF(+/-) mice. In hippocampal cultures, BDNF increased the capacity of 5-HT(1A) receptors to activate G proteins, an effect eliminated by the knockout of TrkB, confirming TrkB activation increases 5-HT(1A) receptor function. The mechanisms underlying the sensitivity of hippocampal 5-HT(1A) receptors to mild stress and decreased BDNF expression remain to be elucidated and may have important implications for the emotional and cognitive impairments associated with stress-related mental illness.

Trigeminal satellite cells modulate neuronal responses to triptans: relevance for migraine therapy

In the present paper, we have further developed an in vitro model to study neuronal–glial interaction at trigeminal level by characterizing the effects of conditioned medium (CM) collected from activated primary cultures of satellite glial cells (SGCs) on calcitonin gene-related peptide (CGRP) release from rat trigeminal neurons. Moreover, we investigated whether such release is inhibited by a clinically relevant anti-migraine drug, sumatriptan. CM effects were tested on trigeminal neuronal cultures in different conditions of activation and at different time points. Long-term exposures of trigeminal neurons to CM increased directly neuronal CGRP release, which was further enhanced by the exposure to capsaicin. In this framework, the anti-migraine drug sumatriptan was able to inhibit the evoked CGRP release from naïve trigeminal neuron cultures, as well as from trigeminal cultures pre-exposed for 30 min to CM. On the contrary, sumatriptan failed to inhibit evoked CGRP release from trigeminal neurons after prolonged (4 and 8 h) pre-exposures to CM. These findings were confirmed in co-culture experiments (neurons and SGCs), where activation of SGCs or a bradykinin priming were used. Our data demonstrate that SGCs activation could influence neuronal excitability, and that this event affects the neuronal responses to triptans.

Allosteric interactions between δ and κ opioid receptors in peripheral sensory neurons

The peripheral δ opioid receptor (DOR) is an attractive target for analgesic drug development. There is evidence that DOR can form heteromers with the κ-opioid receptor (KOR). As drug targets, heteromeric receptors offer an additional level of selectivity and, because of allosteric interactions between protomers, functionality. Here we report that selective KOR antagonists differentially altered the potency and/or efficacy of DOR agonists in primary cultures of adult rat peripheral sensory neurons and in a rat behavioral model of thermal allodynia. In vitro, the KOR antagonist nor-binaltorphimine (nor-BNI) enhanced the potency of [D-Pen(2,5)]-enkephalin (DPDPE), decreased the potency of [D-Ala(2),D-Leu(5)]-enkephalin (DADLE), and decreased the potency and efficacy of 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80) to inhibit prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity. In vivo, nor-BNI enhanced the effect of DPDPE and decreased the effect of SNC80 to inhibit PGE(2)-stimulated thermal allodynia. In contrast to nor-BNI, the KOR antagonist 5'-guanidinonaltrindole (5'-GNTI) reduced the response of DPDPE both in cultured neurons and in vivo. Evidence for DOR-KOR heteromers in peripheral sensory neurons included coimmunoprecipitation of DOR with KOR, a DOR-KOR heteromer selective antibody augmented the antinociceptive effect of DPDPE in vivo, and the DOR-KOR heteromer agonist 6'-GNTI inhibited adenylyl cyclase activity in vitro as well as PGE(2)-stimulated thermal allodynia in vivo. Taken together, these data suggest that DOR-KOR heteromers exist in rat primary sensory neurons and that KOR antagonists can act as modulators of DOR agonist responses most likely through allosteric interactions between the protomers of the DOR-KOR heteromer.

Pharmacology of peripheral opioid receptors

PURPOSE OF REVIEW

Since the detection of morphine by the pharmacologist Friedrich Sertürner in 1806, opioids have been used as potent centrally acting analgesics. In addition to the central site of action, peripheral endogenous opioid analgesic systems have been extensively studied, especially in the past two decades. This review is not only mentioned to give a brief summary in this well investigated field of peripheral opioid receptors, but also to highlight the role of peripheral opioid receptors in other physiological and pathophysiological conditions.

RECENT FINDINGS

A number of studies, which initially focused on nociception, also revealed an important role of the peripheral opioid receptor system in opioid-induced bowel dysfunction and pruritus, as well as in wound healing, cardioprotection, and the analgesic effects of celecoxib.

SUMMARY

Efforts continue to develop opioid analgesics unable to cross the blood-brain barrier, which act only peripherally in low doses, thus providing adequate analgesia without central and systemic side-effects.The awareness of the influence of peripheral opioid receptors beyond nociception may also have therapeutic ramifications on the other fields mentioned above. For example, the treatment of opioid-induced bowel dysfunction by methylnaltrexone is one of the major findings in the previous years.

Update on pruritic mechanisms of hypertrophic scars in postburn patients: the potential role of opioids and their receptors

Although itching (or pruritus) in a scar is a very common and distressing symptom and is increasingly being recognized as a significant obstacle in burn rehabilitation, the exact mechanisms underlying this symptom have not been elucidated; hence, a reliable therapy has not been established. Recent findings have suggested that itching caused by inflammatory dermatosis can be reduced by antihistamines, but histamine antagonists cannot block all types of pruritus (eg, neuropathic itch). This indicates the presence of a histamine-independent pathway for itch. Itch or pruritus may also be evoked by direct activation of opioid receptors, which have recently been identified in the skin. This article aims to assess the current state of knowledge regarding the role of opioids in the generation of itch in hypertrophic scars in postburn patients. To this end, the authors have reviewed the relevant literature and present some clinical data. The authors hope that this review will form the basis for future research to elucidate the mechanism and treatment of itch.

Regulation of κ-opioid receptor signaling in peripheral sensory neurons in vitro and in vivo

There is considerable interest in understanding the regulation of peripheral opioid receptors to avoid central nervous system side effects associated with systemically administered opioid analgesics. Here, we investigated the regulation of the κ-opioid receptor (KOR) on rat primary sensory neurons in vitro and in a rat model of thermal allodynia. Under basal conditions, application of the KOR agonist trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide hydrochloride hydrate (U50488) did not inhibit adenylyl cyclase (AC) activity nor release of calcitonin gene-related peptide (CGRP) in vitro and did not inhibit thermal allodynia in vivo. However, after 15-min pretreatment with bradykinin (BK), U50488 became capable of inhibiting AC activity, CGRP release, and thermal allodynia. Inhibition of AC by 5-hydroxytryptamine 1 or neuropeptide Y(1) receptor agonists and stimulation of extracellular signal-regulated kinase activity by U50488 did not require BK pretreatment. The effect of U50488 in BK-primed tissue was blocked by the KOR antagonist nor-binaltorphimine both in vitro and in vivo. The effect of BK in vitro was blocked by either indomethacin or bisindolylmaleimide, suggesting that an arachidonic acid (AA) metabolite and protein kinase C (PKC) activation mediate BK-induced regulation of the KOR system. Furthermore, the effect of U50488 in BK-treated tissue was blocked by a soluble integrin-blocking peptide (GRGDSP), but not the inactive reverse sequence peptide (GDGRSP), suggesting that, in addition to AA and PKC, RGD-binding integrins participate in the regulation of KOR signaling in response to U50488. Understanding the mechanisms by which peripheral KOR agonist efficacy is regulated may lead to improved pharmacotherapy for the treatment of pain with reduced adverse effects.

Metallopeptidase inhibition potentiates bradykinin-induced hyperalgesia

The neuropeptide bradykinin (BK) sensitizes nociceptor activation following its release in response to inflammatory injury. Thereafter, the bioactivity of bradykinin is controlled by the enzymatic activities of circulating peptidases. One such enzyme, the metalloendopeptidase EC3.4.24.15 (EP24.15), is co-expressed with bradykinin receptors in primary afferent neurons. In this study, using approaches encompassing pharmacology, biochemistry, cell biology, and behavioral animal models, we identified a crucial role for EP24.15 and the closely related EP24.16 in modulating bradykinin-mediated hyperalgesia. Pharmacological analyses indicated that EP24.15 and EP24.16 inhibition significantly enhances bradykinin type-2 receptor activation by bradykinin in primary trigeminal ganglia cultures. In addition, bradykinin-induced sensitization of TRPV1 activation was increased in the presence of the EP24.15/16 inhibitor JA-2. Furthermore, behavioral analyses illustrated a significant dose-response relationship between JA-2 and bradykinin-mediated thermal hyperalgesia. These results indicate an important physiological role for the metallopeptidases EP24.15 and EP24.16 in regulating bradykinin-mediated sensitization of primary afferent nociceptors.

Co-administration of δ- and μ-opioid receptor agonists promotes peripheral opioid receptor function

Enhancement of peripheral opioid analgesia following tissue injury or inflammation in animal models is well-documented, but clinical results of peripheral opioid therapy remain inconsistent. Previous studies in the central nervous system have shown that co-administration of μ- and δ-opioid receptor agonists can enhance analgesic outcomes; however, less is known about the functional consequences of opioid receptor interactions in the periphery. The present study examines the effects of intraplantar injection of the μ- and δ-opioid receptor agonists, morphine and deltorphin, alone and in combination on behavioral tests of nociception in naïve rats and on potassium-evoked release of CGRP from sciatic nerves of naïve rats. Neither drug alone affected nociceptive behaviors or CGRP release. Two separate measures of mechanical nociceptive sensitivity remained unchanged after co-administration of the two drugs. In contrast, when deltorphin was co-injected with morphine, dose-dependent and peripherally restricted increases in paw withdrawal latencies to radiant heat were observed. Similarly, concentration-dependent inhibition of CGRP release was observed when deltorphin and morphine were administered in sequence prior to potassium stimulation. However, no inhibition was observed when morphine was administered prior to deltorphin. All combined opioid effects were blocked by co-application of antagonists. Deltorphin exposure also enhanced the in vivo and in vitro effects of another μ-opioid receptor agonist, DAMGO. Together, these results suggest that under normal conditions, δ-opioid receptor agonists enhance the effect of μ-opioid receptor agonists in the periphery, and local co-administration of δ- and μ-opioid receptor agonists may improve results of peripheral opioid therapy for the treatment of pain.

17beta-estradiol rapidly enhances bradykinin signaling in primary sensory neurons in vitro and in vivo

Many studies have demonstrated that premenopausal women are at increased risk for various pain disorders. Pain-sensing neurons, termed "nociceptors," in the trigeminal ganglia (TG) and dorsal root ganglia (DRG) express receptors for inflammatory mediators and noxious physical stimuli and transmit signals for central processing of pain sensation. Estrogen receptors (ERs) are also expressed on nociceptors in the TG and DRG, and there is ample literature to suggest that activation of ERs can influence pain mechanisms. However, the mechanism for ER modulation of nociceptor activity is incompletely understood. The aim of this study was to characterize the effect of 17β-estradiol (17β-E(2)) on signaling of the inflammatory mediator bradykinin (BK) in primary cultures of rat sensory neurons and a behavioral model of thermal allodynia in rats. Here, we show that exposure to 17β-E(2) rapidly (within 15 min) enhanced responses to BK in vitro and in vivo. The 17β-E(2)-mediated enhancement of BK signaling was not blocked by the transcription inhibitor anisomycin and was mediated by a membrane-associated ER. The effect of 17β-E(2) to enhance BK responses required activation of β1-containing, RGD-binding integrins. These data show that 17β-E(2) rapidly enhances inflammatory mediator responses both in vitro and in vivo and suggest that 17β-E(2) acting at primary sensory pain neurons may participate in regulating the sensitivity of women to painful stimuli.

Opioid receptors and opioid peptide-producing leukocytes in inflammatory pain--basic and therapeutic aspects

This review summarizes recent findings on neuro-immune mechanisms underlying opioid-mediated inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms by immune cell-derived opioid peptides. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generators of impulses relaying nociceptive information towards the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. This is in part achieved by endogenously released immune cell-derived opioid peptides within inflamed tissue. In addition, exogenous opioid receptor ligands that selectively modulate primary afferent function and do not cross the blood-brain barrier, avoid centrally mediated untoward side effects of conventional analgesics (e.g., opioids, anticonvulsants). This article discusses peripheral opioid receptors and their signaling pathways, opioid peptide-producing/secreting inflammatory cells and arising therapeutic perspectives.

Peripheral antinociceptive effects of low doses of naloxone in an in vivo and in vitro model of trigeminal nociception

Naloxone has been used to antagonize opioid effects for many years, even though at low doses it can exert antinociceptive effects. This 'paradoxical' analgesia has been detected after systemic administration of naloxone given alone or in combination with opioid drugs. In the present study, we investigated possible peripheral antinociceptive effects of low doses of naloxone using both an in vivo and in vitro model of trigeminal nociception. Low doses of naloxone injected locally into the rat wiskerpad elicited antinociceptive activity in the rat orofacial formalin test. The block of primary afferents with local administration of capsaicin suggested that naloxone acts both directly on sensory neurons and indirectly, by modulating the inflammatory component of the second phase of formalin test. Naloxone analgesia is maintained in rats made tolerant to the mu-receptor agonist DAMGO, suggesting the involvement of delta- and kappa-opioid receptors. Subsequently, the effects of very low doses of naloxone were tested in primary cultures of rat trigeminal neurons activated with bradykinin, in order to elucidate the mechanisms of action underlying naloxone antinociceptive effects. Naloxone inhibited bradykinin-evoked CGRP release in two different experimental paradigms, i.e. primed and unprimed cultures, acting at the level of delta- and kappa-opioids receptors. These results suggest that low doses of naloxone can directly modulate the activation of the trigeminal neurons by modulating the activity of specific opioid receptors, and this effect may be clinically relevant in combined therapies where an increased analgesic effect is sought through the potentiation of peripheral mechanisms.

Capsaicin-evoked iCGRP release from human dental pulp: a model system for the study of peripheral neuropeptide secretion in normal healthy tissue

The mechanisms underlying trigeminal pain conditions are incompletely understood. In vitro animal studies have elucidated various targets for pharmacological intervention; however, a lack of clinical models that allow evaluation of viable innervated human tissue has impeded successful translation of many preclinical findings into clinical therapeutics. Therefore, we developed and characterized an in vitro method that evaluates the responsiveness of isolated human nociceptors by measuring basal and stimulated release of neuropeptides from collected dental pulp biopsies. Informed consent was obtained from patients presenting for extraction of normal wisdom teeth. Patients were anesthetized using nerve block injection, teeth were extracted and bisected, and pulp was removed and superfused in vitro. Basal and capsaicin-evoked peripheral release of immunoreactive calcitonin gene-related peptide (iCGRP) was analyzed by enzyme immunoassay. The presence of nociceptive markers within neurons of the dental pulp was characterized using confocal microscopy. Capsaicin increased the release of iCGRP from dental pulp biopsies in a concentration-dependent manner. Stimulated release was dependent on extracellular calcium, reversed by a TRPV1 receptor antagonist, and desensitized acutely (tachyphylaxis) and pharmacologically by pretreatment with capsaicin. Superfusion with phorbol 12-myristate 13-acetate (PMA) increased basal and stimulated release, whereas PGE2 augmented only basal release. Compared with vehicle treatment, pretreatment with PGE2 induced competence for DAMGO to inhibit capsaicin-stimulated iCGRP release, similar to observations in animal models where inflammatory mediators induce competence for opioid inhibition. These results indicate that the release of iCGRP from human dental pulp provides a novel tool to determine the effects of pharmacological compounds on human nociceptor sensitivity.

Buprenorphine inhibits bradykinin-induced release of calcitonin gene-related peptide from rat trigeminal neurons via both mu-opioid and nociceptin/orphanin peptide receptors

In this study we used the dual opioid and nociceptin/orphanin peptide (NOP) agonist buprenorphine to investigate the relative contributions of opioid and NOP systems in regulating bradykinin-stimulated calcitonin-gene related peptide (CGRP) release from primary cultures of neonatal rat trigeminal neurons. We found that: bradykinin stimulates CGRP secretion either by a direct effect or after applying so-called "bradykinin-priming" protocol. In both cases, buprenorphine was able to inhibit bradykinin-stimulated CGRP secretion; however, inhibition was mediated by NOP receptors when buprenorphine was added to the incubation medium along with bradykinin, whereas it appeared to be mediated by mu-opioid receptors in bradykinin priming experiments. Bradykinin treatments also caused an increase in neuronal prostaglandin production; prostanoids appeared to be involved in the stimulatory effects of bradykinin as well as in buprenorphine inhibition, through apparently unrelated mechanisms.

Opioids and sensory nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Rapid pain modulation with nuclear receptor ligands

We discuss and present new data regarding the physiological and molecular mechanisms of nuclear receptor activation in pain control, with a particular emphasis on non-genomic effects of ligands at peroxisome proliferator-activated receptor (PPAR), GPR30, and classical estrogen receptors. PPARalpha agonists rapidly reduce both acute and chronic pain in a number of pain assays. These effects precede transcriptional anti-inflammatory actions, and are mediated in part by IK(ca) and BK(ca) channels on DRG neurons. In contrast to the peripheral site of action of PPARalpha ligands, the dorsal horn supports the expression of PPARgamma. Intrathecal administration of PPARgamma ligands rapidly (< or =5 min) attenuated mechanical and thermal hypersensitivity associated with nerve injury in a dose-dependent manner that could be blocked with PPARgamma antagonists. By contrast, a PPARgamma antagonist itself rapidly increased the mechanical allodynia associated with nerve injury. These data suggest that ligand-dependent, non-genomic activation of spinal PPARgamma decreases behavioral signs of inflammatory and neuropathic pain. We also report that the GPR30 is expressed on cultured sensory neurons, that activation of the receptor elicits signaling to increase calcium accumulation. This signaling may contribute to increased neuronal sensitivity as treatment with the GPR30 agonist induces hyperalgesia. Finally, application of the membrane-impermeable 17beta-E(2)-BSA rapidly (within 15 min) enhanced BK-stimulated inositol phosphate (IP) accumulation and PGE(2)-mediated cAMP accumulation in trigeminal ganglion cultures. We conclude that nuclear receptor ligands may operate through rapid, non-genomic mechanisms to modulate inflammatory and neuropathic pain.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Peripheral delta opioid receptors require priming for functional competence in vivo

Although centrally acting opioid analgesics produce profound antinociception under basal conditions, the antinociceptive properties of peripherally restricted opioid analgesics are generally only detectable after inflammation or injection of inflammatory mediators. Despite considerable research, the cellular mechanisms regulating the functional competence of peripheral opioid receptor systems for inhibition of nociception remain unclear. Recent work has demonstrated that brief pre-treatment (priming) with bradykinin, arachidonic acid, protease-activated receptor-2 agonists, or direct activators of protein kinase C (PKC) are capable of inducing the functional competence of the opioid receptor system in cultures of primary sensory neurons in vitro. Here we report that the peripheral delta opioid receptor system also requires PKC-dependent priming to inhibit prostaglandin E(2) (PGE(2))-induced thermal allodynia in the rat. Peripheral hindpaw injection of [D-Pen(2,5)]-enkephalin (DPDPE), a selective delta opioid receptor agonist, did not alter PGE(2)-induced thermal allodynia. However, following priming (15 min) with bradykinin or arachidonic acid, DPDPE produced a significant reduction in allodynia that was antagonist reversible, peripherally restricted, and exhibited a typical dose-response relationship. Furthermore, the bradykinin priming effect was blocked by the PKC inhibitors, bisindolylmaleimide I and chelerythrine. Collectively, these data support prior in vitro findings that, although present on primary sensory neurons, peripheral opioid receptor systems are functionally inactive under basal conditions and require activation of a PKC- and arachidonic acid-dependent signaling pathway to develop functional competence in vivo.