Activation of mu opioid receptors inhibits transient high- and low-threshold Ca2+ currents, but spares a sustained current

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

Conotoxin contulakin-G engages a neurotensin receptor 2/R-type calcium channel (Cav2.3) pathway to mediate spinal antinociception

ABSTRACT

Intrathecal application of contulakin-G (CGX), a conotoxin peptide and a neurotensin analogue, has been demonstrated to be safe and potentially analgesic in humans. However, the mechanism of action for CGX analgesia is unknown. We hypothesized that spinal application of CGX produces antinociception through activation of the presynaptic neurotensin receptor (NTSR)2. In this study, we assessed the mechanisms of CGX antinociception in rodent models of inflammatory and neuropathic pain. Intrathecal administration of CGX, dose dependently, inhibited thermal and mechanical hypersensitivities in rodents of both sexes. Pharmacological and clustered regularly interspaced short palindromic repeats/Cas9 editing of NTSR2 reversed CGX-induced antinociception without affecting morphine analgesia. Electrophysiological and gene editing approaches demonstrated that CGX inhibition was dependent on the R-type voltage-gated calcium channel (Cav2.3) in sensory neurons. Anatomical studies demonstrated coexpression of NTSR2 and Cav2.3 in dorsal root ganglion neurons. Finally, synaptic fractionation and slice electrophysiology recordings confirmed a predominantly presynaptic effect. Together, these data reveal a nonopioid pathway engaged by a human-tested drug to produce antinociception.

Dose-dependent consequences of sub-chronic fentanyl exposure on neuron and glial co-cultures

Fentanyl is one of the most common opioid analgesics administered to patients undergoing surgery or for chronic pain management. While the side effects of chronic fentanyl abuse are recognized (e.g., addiction, tolerance, impairment of cognitive functions, and inhibit nociception, arousal, and respiration), it remains poorly understood what and how changes in brain activity from chronic fentanyl use influences the respective behavioral outcome. Here, we examined the functional and molecular changes to cortical neural network activity following sub-chronic exposure to two fentanyl concentrations, a low (0.01 μM) and high (10 μM) dose. Primary rat co-cultures, containing cortical neurons, astrocytes, and oligodendrocyte precursor cells, were seeded in wells on either a 6-well multi-electrode array (MEA, for electrophysiology) or a 96-well tissue culture plate (for serial endpoint bulk RNA sequencing analysis). Once networks matured (at 28 days in vitro), co-cultures were treated with 0.01 or 10 μM of fentanyl for 4 days and monitored daily. Only high dose exposure to fentanyl resulted in a decline in features of spiking and bursting activity as early as 30 min post-exposure and sustained for 4 days in cultures. Transcriptomic analysis of the complex cultures after 4 days of fentanyl exposure revealed that both the low and high dose induced gene expression changes involved in synaptic transmission, inflammation, and organization of the extracellular matrix. Collectively, the findings of this in vitro study suggest that while neuroadaptive changes to neural network activity at a systems level was detected only at the high dose of fentanyl, transcriptomic changes were also detected at the low dose conditions, suggesting that fentanyl rapidly elicits changes in plasticity.

Opioid Receptor Regulation of Neuronal Voltage-Gated Calcium Channels

Neuronal voltage-gated calcium channels play a pivotal role in the conversion of electrical signals into calcium entry into nerve endings that is required for the release of neurotransmitters. They are under the control of a number of cellular signaling pathways that serve to fine tune synaptic activities, including G-protein coupled receptors (GPCRs) and the opioid system. Besides modulating channel activity via activation of second messengers, GPCRs also physically associate with calcium channels to regulate their function and expression at the plasma membrane. In this mini review, we discuss the mechanisms by which calcium channels are regulated by classical opioid and nociceptin receptors. We highlight the importance of this regulation in the control of neuronal functions and their implication in the development of disease conditions. Finally, we present recent literature concerning the use of novel μ-opioid receptor/nociceptin receptor modulators and discuss their use as potential drug candidates for the treatment of pain.

Critical Assessment of G Protein-Biased Agonism at the μ-Opioid Receptor

G protein-biased agonists of the μ-opioid receptor (MOPr) have been proposed as an improved class of opioid analgesics. Recent studies have been unable to reproduce the original experiments in the β-arrestin2-knockout mouse that led to this proposal, and alternative genetic models do not support the G protein-biased MOPr agonist hypothesis. Furthermore, assessment of putatively biased ligands has been confounded by several factors, including assay amplification. As such, the extent to which current lead compounds represent mechanistically novel, extremely G protein-biased agonists is in question, as is the underlying assumption that β-arrestin2 mediates deleterious opioid effects. Addressing these current challenges represents a pressing issue to successfully advance drug development at this receptor and improve upon current opioid analgesics.

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

The "Culture" of Pain Control: A Review of Opioid-Induced Dysbiosis (OID) in Antinociceptive Tolerance

It is increasingly recognized that chronic opioid use leads to maladaptive changes in the composition and localization of gut bacteria. Recently, this "opioid-induced dysbiosis" (OID) has been linked to antinociceptive tolerance development in preclinical models and may therefore identify promising targets for new opioid-sparing strategies. Such developments are critical to curb dose escalations in the clinical setting and combat the ongoing opioid epidemic. In this article, we review the existing literature that pertains to OID, including the current evidence regarding its qualitative nature, influence on antinociceptive tolerance, and future prospects. PERSPECTIVE: This article reviews the current literature on OID of gut bacteria, including its qualitative nature, influence on antinociceptive tolerance, and future prospects. This work may help identify targets for new opioid-sparing strategies.

Pain Vision System for Evaluating Chronic Pain: A Comparison with VAS Scoring

Objective. To study the value of pain vision system in quantitative evaluation of chronic pain from the perspective of neuroelectrophysiology, as compared with VAS (visual analog scale). Methods. Seventy-one patients with chronic pain were randomly selected to receive pain treatments from August 1, 2018, to September 30, 2018, in the Pain Department of Li Zhuang Tongji Hospital. Among all patients, 26 had cervical spondylosis and 45 had lumbar disc herniation. Pain vision and visual analog scoring (VAS) were used to evaluate the degree of pain before and after treatment. The correlation between the pain vision system’s pain ratio (pain ratio) and the VAS score was analyzed. Results. Pain ratio and VAS scores were linearly correlated before and after treatment in patients with chronic pain (Pearson coefficient was 0.730 before treatment and 0.449 after treatment; P<0.001). Conclusions. The pain vision system can evaluate chronic pain well and quantify it in the form of pain degree, which is helpful for objective quantitative analysis of chronic pain. This trial is registered with ChiCTR1900026331.

Opioid Receptors in Immune and Glial Cells-Implications for Pain Control

Opioid receptors comprise μ (MOP), δ (DOP), κ (KOP), and nociceptin/orphanin FQ (NOP) receptors. Opioids are agonists of MOP, DOP, and KOP receptors, whereas nociceptin/orphanin FQ (N/OFQ) is an agonist of NOP receptors. Activation of all four opioid receptors in neurons can induce analgesia in animal models, but the most clinically relevant are MOP receptor agonists (e.g., morphine, fentanyl). Opioids can also affect the function of immune cells, and their actions in relation to immunosuppression and infections have been widely discussed. Here, we analyze the expression and the role of opioid receptors in peripheral immune cells and glia in the modulation of pain. All four opioid receptors have been identified at the mRNA and protein levels in immune cells (lymphocytes, granulocytes, monocytes, macrophages) in humans, rhesus monkeys, rats or mice. Activation of leukocyte MOP, DOP, and KOP receptors was recently reported to attenuate pain after nerve injury in mice. This involved intracellular Ca²⁺-regulated release of opioid peptides from immune cells, which subsequently activated MOP, DOP, and KOP receptors on peripheral neurons. There is no evidence of pain modulation by leukocyte NOP receptors. More good quality studies are needed to verify the presence of DOP, KOP, and NOP receptors in native glia. Although still questioned, MOP receptors might be expressed in brain or spinal cord microglia and astrocytes in humans, mice, and rats. Morphine acting at spinal cord microglia is often reported to induce hyperalgesia in rodents. However, most studies used animals without pathological pain and/or unconventional paradigms (e.g., high or ultra-low doses, pain assessment after abrupt discontinuation of chronic morphine treatment). Therefore, the opioid-induced hyperalgesia can be viewed in the context of dependence/withdrawal rather than pain management, in line with clinical reports. There is convincing evidence of analgesic effects mediated by immune cell-derived opioid peptides in animal models and in humans. Together, MOP, DOP, and KOP receptors, and opioid peptides in immune cells can ameliorate pathological pain. The relevance of NOP receptors and N/OFQ in leukocytes, and of all opioid receptors, opioid peptides and N/OFQ in native glia for pain control is yet to be clarified.

Proteomic profiling reveals Arl6ip-1 as a candidate target in cancer-induced bone pain rat model after oxycodone treatment

Treatment of cancer-induced bone pain (CIBP) is challenging in clinics. Oxycodone is used to treat CIBP. However, the lack of understanding of the mechanism of CIBP limits the application of oxycodone. In this study, proteomic profiling of oxycodone-treated spinal dorsal cord of rats with CIBP was performed. Briefly, a total of 3519 proteins were identified in the Sham group; 3505 proteins in the CIBP group; and 3530 proteins in the CIBP-OXY treatment group. The 2-fold cut-off value was used as the differential protein standard for abundance reduction or increase (p < 0.05). Significant differences were found in the abundance of 16 proteins between Sham and CIBP group; 11 proteins in the CIBP group had increased abundance while 5 proteins had reduced abundance. Furthermore, fifteen proteins with differential abundance were identified between the CIBP group and the OXY group. Compared with the CIBP group, there were six increased abundances and nine reduced abundances in the OXY group. In addition, a reduced expression of ADP-ribosylation factor-like 6 binding factor 1 (Arl6ip-1), an endoplasmic reticulum protein that has an important role in cell conduction and material transport, was found in the CIBP group compared with the Sham group. Its expression increased after the administration of OXY. Proteomics results were further verified by Western-blot. Fluorescent staining revealed that Arl6ip-1 co-localized with spinal dorsal horn neurons, but not with astrocytes or microglia. Based on the observed results, we believe that Arl6ip-1 may be a potential drug target for OXY treatment of CIBP rats.

Emerging Insights into Mu Opioid Pharmacology

Opioid analgesics, most of which act through mu opioid receptors, have long represented valuable therapeutic agents to treat severe pain. Concerted drug development efforts for over a 100 years have resulted in a large variety of opioid analgesics used in the clinic, but all of them continue to exhibit the side effects, especially respiratory depression, that have long plagued the use of morphine. The recent explosion in fatalities resulting from overdose of prescription and synthetic opioids has dramatically increased the need for safer analgesics, but recent developments in mu receptor research have provided new strategies to develop such drugs. This chapter reviews recent advances in developing novel opioid analgesics from an understanding of mu receptor structure and function. This includes a summary of the mechanism of agonist binding deduced from the crystal structure of mu receptors. It will also highlight the development of novel agonist mechanisms, including biased agonists, bivalent ligands, and allosteric modulators of mu receptor function, and describe how receptor phosphorylation modulates these pathways. Finally, it will summarize research on the alternative pre-mRNA splicing mechanisms that produces a multiplicity of mu receptor isoforms. Many of these isoforms exhibit different pharmacological specificities and brain circuitry localization, thus providing an opportunity to develop novel drugs with increased therapeutic windows.

Fluoxetine increases analgesic effects of morphine, prevents development of morphine tolerance and dependence through the modulation of L-type calcium channels expression in mice

Here, we aimed to investigate the effects of fluoxetine on morphine-induced analgesia, as well as preventive effects of it on morphine induced tolerance and dependence in mice. We also elucidate the involvement of L-type Ca²⁺ channels in these phenomena. To induce morphine tolerance, mice were treated with morphine (50 mg/kg) for 3 consecutive days. To evaluate the involvement of the calcium channel in the effects of fluoxetine (5, 20 mg/kg), combination ineffective doses of the two L-type calcium channel blockers, nimodipine (5 mg/kg) or diltiazem (20 mg/kg) with flouxetine were used with each morphine dose. Nociceptive behavior was evaluated using hot-plate test, while physical dependence assessed by naloxone-precipitated withdrawal on the fourth day of experiment. The expression of Cav1.2 and Cav1.3 subunits of the L-type calcium channels in cortex and mesolimbic tissues were measured using western immunoassay. Results showed that co-administration of fluoxetine (20 mg/kg) with morphine increased its acute analgesia effect and prevented the induction of morphine antinociceptive tolerance and physical dependence in mice. Moreover, these effects was potentiated by pre-treatment with diltiazem or nimodipine. Results also showed up-regulation of the Cav1.3 and Cav1.2 expression in the cerebral cortex and mesolimbic regions through the development of morphine dependence. Moreover, chronic administration of fluoxetine with morphine reduced the observed up-regulation of Cav1.3 and Cav1.2 expression in cortex and mesolimbic tissues. Our data indicated that co-administering of fluoxetine with morphine could potentiate the antinociceptive effect of morphine, prevent morphine analgesia tolerance and attenuated the morphine withdrawal signs during induction phases. Moreover, we also pointed out for the first time the role of L-type Ca²⁺ channel channels in the modulatory effects of fluoxetine on the morphine-related effects.

Co-expression of μ and δ opioid receptors by mouse colonic nociceptors

BACKGROUND AND PURPOSE

To better understand opioid signalling in visceral nociceptors, we examined the expression and selective activation of μ and δ opioid receptors by dorsal root ganglia (DRG) neurons innervating the mouse colon.

EXPERIMENTAL APPROACH

DRG neurons projecting to the colon were identified by retrograde tracing. δ receptor-GFP reporter mice, in situ hybridization, single-cell RT-PCR and μ receptor-specific antibodies were used to characterize expression of μ and δ receptors. Voltage-gated Ca²⁺ currents and neuronal excitability were recorded in small diameter nociceptive neurons (capacitance <30 pF) by patch clamp and ex vivo single-unit afferent recordings were obtained from the colon.

KEY RESULTS

In situ hybridization of oprm1 expression in Fast Blue-labelled DRG neurons was observed in 61% of neurons. μ and δ receptors were expressed by 36-46% of colon DRG neurons, and co-expressed by ~25% of neurons. μ and δ receptor agonists inhibited Ca²⁺ currents in DRG, effects blocked by opioid antagonists. One or both agonists inhibited action potential firing by colonic afferent endings. Incubation of neurons with supernatants from inflamed colon segments inhibited Ca²⁺ currents and neuronal excitability. Antagonists of μ, but not δ receptors, inhibited the effects of these supernatant on Ca²⁺ currents, whereas both antagonists inhibited their actions on neuronal excitability.

CONCLUSIONS AND IMPLICATIONS

A significant number of small diameter colonic nociceptors co-express μ and δ receptors and are inhibited by agonists and endogenous opioids in inflamed tissues. Thus, opioids that act at μ or δ receptors, or their heterodimers may be effective in treating visceral pain.

Tolerance to Morphine-Induced Inhibition of TTX-R Sodium Channels in Dorsal Root Ganglia Neurons Is Modulated by Gut-Derived Mediators

In the clinical setting, analgesic tolerance is a primary driver of diminished pain control and opioid dose escalations. Integral to this process are primary afferent sensory neurons, the first-order components of nociceptive sensation. Here, we characterize the factors modulating morphine action and tolerance in mouse small diameter dorsal root ganglia (DRG) neurons. We demonstrate that acute morphine inactivates tetrodotoxin-resistant (TTX-R) Na+ channels in these cells. Chronic exposure resulted in tolerance to this effect, which was prevented by treatment with oral vancomycin. Using colonic supernatants, we further show that mediators in the gut microenvironment of mice with chronic morphine exposure can induce tolerance and hyperexcitability in naive DRG neurons. Tolerance (but not hyperexcitability) in this paradigm was mitigated by oral vancomycin treatment. These findings collectively suggest that gastrointestinal microbiota modulate the development of morphine tolerance (but not hyperexcitability) in nociceptive primary afferent neurons, through a mechanism involving TTX-R Na+ channels.

The Peptide PnPP-19, a Spider Toxin Derivative, Activates μ-Opioid Receptors and Modulates Calcium Channels

The synthetic peptide PnPP-19 comprehends 19 amino acid residues and it represents part of the primary structure of the toxin δ-CNTX-Pn1c (PnTx2-6), isolated from the venom of the spider Phoneutria nigriventer. Behavioural tests suggest that PnPP-19 induces antinociception by activation of CB1, μ and δ opioid receptors. Since the peripheral and central antinociception induced by PnPP-19 involves opioid activation, the aim of this work was to identify whether this synthetic peptide could directly activate opioid receptors and investigate the subtype selectivity for μ-, δ- and/or κ-opioid receptors. Furthermore, we also studied the modulation of calcium influx driven by PnPP-19 in dorsal root ganglion neurons, and analyzed whether this modulation was opioid-mediated. PnPP-19 selectively activates μ-opioid receptors inducing indirectly inhibition of calcium channels and hereby impairing calcium influx in dorsal root ganglion (DRG) neurons. Interestingly, notwithstanding the activation of opioid receptors, PnPP-19 does not induce β-arrestin2 recruitment. PnPP-19 is the first spider toxin derivative that, among opioid receptors, selectively activates μ-opioid receptors. The lack of β-arrestin2 recruitment highlights its potential for the design of new improved opioid agonists.

Yawning-Its anatomy, chemistry, role, and pathological considerations

Yawning is a clinical sign of the activity of various supra- and infratentorial brain regions including the putative brainstem motor pattern, hypothalamic paraventricular nucleus, probably the insula and limbic structures that are interconnected via a fiber network. This interaction can be seen in analogy to other cerebral functions arising from a network or zone such as language. Within this network, yawning fulfills its function in a stereotype, reflex-like manner; a phylogenetically old function, preserved across species barriers, with the purpose of arousal, communication, and maybe other functions including respiration. Abnormal yawning with ≥3 yawns/15min without obvious cause arises from lesions of brain areas involved in the yawning zone, its trajectories causing a disconnection syndrome, or from alteration of network activity by physical or metabolic etiologies including medication.

Potential role of CXCL10/CXCR3 signaling in the development of morphine tolerance in periaqueductal gray

Tolerance to morphine antinociception hinders its long-term use in clinical practice. Interaction between neuron and microglia has been proved to play critical role in the mechanism of morphine tolerance, while CXCL10/CXCR3 signaling has been implicated in neuron-glia signaling and morphine analgesia. This study aims to investigate whether CXCL10/CXCR3 signaling in periaqueductal gray (PAG) contributes to the development of morphine tolerance by modulating neuron-microglia interaction. The results showed that the expressions of CXCR3 and CXCL10 were gradually increased in parallel with repeated morphine administration and activation of microglia. CXCR3 was co-localized with neuronal marker NeuN, while CXCL10 was derived from microglia. Microglia inhibitor minocycline significantly attenuated the expression of CXCL10, besides, both minocycline and CXCR3 inhibitor alleviated the development of morphine tolerance. Taken together, our study provided the evidence that CXCL10/CXCR3 signaling in PAG is involved in the development of morphine analgesic tolerance via neuron-microglia interaction.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

Targeting inflammation as a treatment modality for neuropathic pain in spinal cord injury: a randomized clinical trial

Background

The purpose of the present study was to examine the effectiveness of an anti-inflammatory intervention as a treatment for neuropathic pain following spinal cord injury (SCI).

Methods

This randomized, parallel-group, controlled clinical trial (NCT02099890) examined 20 participants with varying levels and severities of SCI, randomized (3:2) to either a 12-week anti-inflammatory diet, or control group. Outcome measures consisted of self-determined indices of pain as assessed using the neuropathic pain questionnaire (NPQ) and markers of inflammation as assessed by various pro- and anti-inflammatory cytokines, as well as the eicosanoids PGE2 and LTB4.

Results

A significant group × time interaction was found for sensory pain scores (p < 0.01). A Mann-Whitney test revealed that the change scores (3-month baseline) were significantly different between groups for IFN-y (U = 13.0, p = 0.01), IL-1β (U = 14.0, p = 0.01), and IL-2 (U = 12.0, p = 0.01). A Friedman test revealed the treatment group had a significant reduction in IFN-y (x² = 8.67, p = 0.01), IL-1β (x² = 17.78, p < 0.01), IL-6 (x² = 6.17, p < 0.05), while the control group showed no significant change in any inflammatory mediator. A stepwise backward elimination multiple regression analysis showed that the change in sensory neuropathic pain was a function of the change in the proinflammatory cytokines IL-2 and IFN-y, as well as the eicosanoid PGE2 (R = 0.689, R² = 0.474).

Conclusions

Overall, the results of the study demonstrate the efficacy of targeting inflammation as a means of treating neuropathic pain in SCI, with a potential mechanism relating to the reduction in proinflammatory cytokines and PGE2.

Trial registration

ClinicalTrials.gov, NCT02099890

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0625-4) contains supplementary material, which is available to authorized users.

Remifentanil alters sensory neuromodulation of swallowing in healthy volunteers: quantification by a novel pressure-impedance analysis

Exposure to remifentanil contributes to an increased risk of pulmonary aspiration, likely through reduced pharyngeal contractile vigor and diminished bolus propulsion during swallowing. We employed a novel high-resolution pressure-flow analysis to quantify the biomechanical changes across the upper esophageal sphincter (UES). Eleven healthy young (23.3 ± 3.1 yr old) participants (7 men and 4 women) received remifentanil via intravenous target-controlled infusion with an effect-site concentration of 3 ng/ml. Before and 30 min following commencement of remifentanil administration, participants performed ten 10-ml saline swallows while pharyngoesophageal manometry and electrical impedance data were recorded using a 4.2-mm-diameter catheter housing 36 circumferential pressure sensors. Remifentanil significantly shortened the duration of UES opening (P < 0.001) and increased residual UES pressure (P = 0.003). At the level of the hypopharynx, remifentanil significantly shortened the latency from maximum bolus distension to peak contraction (P = 0.004) and significantly increased intrabolus distension pressure (P = 0.024). Novel mechanical states analysis revealed that the latencies between the different phases of the stereotypical UES relaxation sequence were shortened by remifentanil. Reduced duration of bolus flow during shortened UES opening, in concert with increased hypopharyngeal distension pressures, is mechanically consistent with increased flow resistance due to a more rapid bolus flow rate. These biomechanical changes are congruent with modification of the physiological neuroregulatory mechanism governing accommodation to bolus volume.

Structural and functional interactions between six-transmembrane μ-opioid receptors and β2-adrenoreceptors modulate opioid signaling

The primary molecular target for clinically used opioids is the μ-opioid receptor (MOR). Besides the major seven-transmembrane (7TM) receptors, the MOR gene codes for alternatively spliced six-transmembrane (6TM) isoforms, the biological and clinical significance of which remains unclear. Here, we show that the otherwise exclusively intracellular localized 6TM-MOR translocates to the plasma membrane upon coexpression with β₂-adrenergic receptors (β₂-ARs) through an interaction with the fifth and sixth helices of β₂-AR. Coexpression of the two receptors in BE(2)-C neuroblastoma cells potentiates calcium responses to a 6TM-MOR ligand, and this calcium response is completely blocked by a selective β₂-antagonist in BE(2)-C cells, and in trigeminal and dorsal root ganglia. Co-administration of 6TM-MOR and β₂-AR ligands leads to substantial analgesic synergy and completely reverses opioid-induced hyperalgesia in rodent behavioral models. Together, our results provide evidence that the heterodimerization of 6TM-MOR with β₂-AR underlies a molecular mechanism for 6TM cellular signaling, presenting a unique functional responses to opioids. This signaling pathway may contribute to the hyperalgesic effects of opioids that can be efficiently blocked by β₂-AR antagonists, providing a new avenue for opioid therapy.

The pharmacology of nociceptor priming

Nociceptors and neurons in the central nervous system (CNS) that receive nociceptive input show remarkable plasticity in response to injury. This plasticity is thought to underlie the development of chronic pain states. Hence, further understanding of the molecular mechanisms driving and maintaining this plasticity has the potential to lead to novel therapeutic approaches for the treatment of chronic pain states. An important concept in pain plasticity is the presence and persistence of "hyperalgesic priming." This priming arises from an initial injury and results in a remarkable susceptibility to normally subthreshold noxious inputs causing a prolonged pain state in primed animals. Here we describe our current understanding of how this priming is manifested through changes in signaling in the primary nociceptor as well as through memory like alterations at CNS synapses. Moreover, we discuss how commonly utilized analgesics, such as opioids, enhance priming therefore potentially contributing to the development of persistent pain states. Finally we highlight where these priming models draw parallels to common human chronic pain conditions. Collectively, these advances in our understanding of pain plasticity reveal a variety of targets for therapeutic intervention with the potential to reverse rather than palliate chronic pain states.

Commonalities between pain and memory mechanisms and their meaning for understanding chronic pain

Pain sensing neurons in the periphery (called nociceptors) and the central neurons that receive their projections show remarkable plasticity following injury. This plasticity results in amplification of pain signaling that is now understood to be crucial for the recovery and survival of organisms following injury. These same plasticity mechanisms may drive a transition to a nonadaptive chronic pain state if they fail to resolve following the termination of the healing process. Remarkable advances have been achieved in the past two decades in understanding the molecular mechanisms that underlie pain plasticity following injury. The mechanisms bear a striking resemblance to molecular mechanisms involved in learning and memory processes in other brain regions, including the hippocampus and cerebral cortex. Here those mechanisms, their commonalities and subtle differences, will be highlighted and their role in causing chronic pain will be discussed. Arising from these data is the striking argument that chronic pain is a disease of the nervous system, which distinguishes this phenomena from acute pain that is frequently a symptom alerting the organism to injury. This argument has important implications for the development of disease modifying therapeutics.

The μ-opioid receptor: an electrophysiologist's perspective from the sharp end

Morphine, the prototypical opioid analgesic drug, produces its behavioural effects primarily through activation of μ-opioid receptors expressed in neurones of the central and peripheral nervous systems. This perspective provides a historical view of how, over the past 40 years, the use of electrophysiological recording techniques has helped to reveal the molecular mechanisms by which acute and chronic activation of μ-opioid receptors by morphine and other opioid drugs modify neuronal function.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Activation of μ-opioid receptors inhibits calcium-currents in the vestibular afferent neurons of the rat through a cAMP dependent mechanism

Opioid receptors are expressed in the vestibular endorgans (afferent neurons and hair cells) and are activated by the efferent system, which modulates the discharge of action potentials in vestibular afferent neurons (VANs). In mammals, VANs mainly express the μ opioid-receptor, but the function of this receptors activation and the cellular mechanisms by which they exert their actions in these neurons are poorly studied. To determine the actions of μ opioid receptor (MOR) and cell signaling mechanisms in VANs, we made perforated patch-clamp recordings of VANs that were obtained from postnatal days 7 to 10 (P7–10) rats and then maintained in primary culture. The MOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) inhibited the total voltage-gated outward current; this effect was prevented by the perfusion of a Ca²⁺-free extracellular solution. We then studied the voltage-gated calcium current (I_ca) and found that DAMGO Met-enkephalin or endomorphin-1 inhibited the I_Ca in a dose-response fashion. The effects of DAMGO were prevented by the MOR antagonist (CTAP) or by pertussis toxin (PTX). The use of specific calcium channel blockers showed that MOR activation inhibited T-, L- and N-type I_Ca. The use of various enzyme activators and inhibitors and of cAMP analogs allowed us to demonstrate that the MOR acts through a cAMP dependent signaling mechanism. In current clamp experiments, MOR activation increased the duration and decreased the amplitude of the action potentials and modulated the discharge produced by current injection. Pre-incubation with PTX occluded MOR activation effect. We conclude that MOR activation inhibits the T-, L- and N-type I_Ca through activation of a Gα_i/o protein that involves a decrease in AC-cAMP-PKA activity. The modulation of I_Ca may have an impact on the synaptic integration, excitability, and neurotransmitter release from VANs.

Highlights in opioid agonists and antagonists

This review highlights new insights in to opioid agonists and antagonists, focusing on their mechanism of action with spinal and systemic administration, chronic use and main adverse effects. Short-cuts on some opioid agonists and antagonists of clinical interest are also presented, revealing potential clinical implications and future clinical directions as part of multimodal analgesia.

The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K⁺ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.

GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

The antinociceptive activity of intrathecally administered amiloride and its interactions with morphine and clonidine in rats

In this study, we aimed to evaluate the antinociceptive interaction between intrathecally administered amiloride and morphine or clonidine. Using rats chronically implanted with lumbar intrathecal catheters, we examined the ability of intrathecal amiloride, morphine, clonidine, and mixtures of amiloride-morphine and amiloride-clonidine to alter tail-flick latency. To characterize any interactions, isobolographic analysis was performed. The effects of pretreatment with intrathecally administered naloxone or yohimbine were tested. Intrathecal administration of amiloride (25-150 μg), morphine (.25-10 μg), or clonidine (.5-10 μg) alone produced significant dose-dependent antinociception in the tail-flick test. The median effective dose (ED(50)) values for intrathecally administered amiloride, morphine, and clonidine were 120.5 μg, 5.0 μg, and 4.4 μg, respectively. Isobolographic analysis exhibited a synergistic interaction after coadministration of amiloride-morphine and amiloride-clonidine. Intrathecal pretreatment with naloxone (10 μg) completely blocked the antinociceptive effects of morphine and the amiloride-morphine mixture. Intrathecal pretreatment with yohimbine (20 μg) completely blocked the antinociceptive effect of clonidine and antagonized the effect of the amiloride-clonidine mixture. There was no motor dysfunction or significant change in blood pressure or heart rate after the intrathecal administration of amiloride, amiloride-morphine, and amiloride-clonidine. The synergistic effect observed after the coadministration of amiloride and morphine or clonidine suggests a functional interaction among calcium channels, μ-receptors and α(2)-receptors at the spinal cord level of the nociceptive processing system.

PERSPECTIVE

Although intrathecal morphine and clonidine produces pronounced analgesia, antinociceptive doses of intrathecal morphine and clonidine produce several side effects, including hypotension, bradycardia, sedation, and tolerance. This article presents antinociceptive synergistic interaction between amiloride and morphine, amiloride, and clonidine on thermal nociceptive tests in the rat.

Regulation of spinal substance p release by intrathecal calcium channel blockade

BACKGROUND

The authors investigated the role of different voltage-sensitive calcium channels expressed at presynaptic afferent terminals in substance P release and on nociceptive behavior evoked by intraplantar formalin by examining the effects of intrathecally delivered N- (ziconotide), T- (mibefradil), and L-type voltage-sensitive calcium channel blockers (diltiazem and verapamil).

METHODS

Rats received intrathecal pretreatment with saline or doses of morphine, ziconotide, mibefradil, diltiazem, or verapamil. The effect of these injections upon flinching evoked by intraplantar formalin (5%, 50 μl) was quantified. To assess substance P release, the incidence of neurokinin-1 receptor internalization in the ipsilateral and contralateral lamina I was determined in immunofluorescent-stained tissues.

RESULTS

Intrathecal morphine (20 μg), ziconotide (0.3, 0.6, and 1 μg), mibefradil (100 μg, but not 50 μg), diltiazem (500 μg, but not 300 μg), and verapamil (200 μg, but not 50 and 100 μg) reduced paw flinching in phase 2 compared with vehicle control (P < 0.05), with no effect on phase 1. Ziconotide (0.3, 0.6, and 1 μg) and morphine (20 μg) significantly inhibited neurokinin-1 receptor internalization (P < 0.05), but mibefradil, diltiazem, and verapamil at the highest doses had no effect.

CONCLUSION

These results emphasize the role in vivo of N-type but not T- and L-type voltage-sensitive calcium channel blockers in mediating the stimulus-evoked substance P release from small primary afferents and suggest that T- and L-type voltage-sensitive calcium channel blockers exert antihyperalgesic effects by an action on other populations of afferents or mechanisms involving postsynaptic excitability.

Dissociation of μ- and δ-opioid inhibition of glutamatergic synaptic transmission in superficial dorsal horn

BACKGROUND

There is anatomical and behavioural evidence that μ- and δ-opioid receptors modulate distinct nociceptive modalities within the superficial dorsal horn. The aim of the present study was to examine whether μ- and δ-opioid receptor activation differentially modulates TRP sensitive inputs to neurons within the superficial dorsal horn. To do this, whole cell patch clamp recordings were made from lamina I - II neurons in rat spinal cord slices in vitro to examine the effect of opioids on TRP agonist-enhanced glutamatergic spontaneous miniature excitatory postsynaptic currents (EPSCs).

RESULTS

Under basal conditions the μ-opioid agonist DAMGO (3 μM) reduced the rate of miniature EPSCs in 68% of neurons, while the δ- and κ-opioid agonists deltorphin-II (300 nM) and U69593 (300 nM) did so in 13 - 17% of neurons tested. The TRP agonists menthol (400 μM) and icilin (100 μM) both produced a Ca2+-dependent increase in miniature EPSC rate which was unaffected by the voltage dependent calcium channel (VDCC) blocker Cd2+. The proportion of neurons in which deltorphin-II reduced the miniature EPSC rate was enhanced in the presence of icilin (83%), but not menthol (0%). By contrast, the proportion of DAMGO and U69593 responders was unaltered in the presence of menthol (57%, 0%), or icilin (57%, 17%).

CONCLUSIONS

These findings demonstrate that δ-opioid receptor activation selectively inhibits inputs activated by icilin, whereas μ-opioid receptor activation has a more widespread effect on synaptic inputs to neurons in the superficial dorsal horn. These findings suggest that δ-opioids may provide a novel analgesic approach for specific, TRPA1-like mediated pain modalities.

A novel alternatively spliced isoform of the mu-opioid receptor: functional antagonism

Background

Opioids are the most widely used analgesics for the treatment of clinical pain. They produce their therapeutic effects by binding to μ-opioid receptors (MORs), which are 7 transmembrane domain (7TM) G-protein-coupled receptors (GPCRs), and inhibiting cellular activity. However, the analgesic efficacy of opioids is compromised by side-effects such as analgesic tolerance, dependence and opioid-induced hyperalgesia (OIH). In contrast to opioid analgesia these side effects are associated with cellular excitation. Several hypotheses have been advanced to explain these phenomena, yet the molecular mechanisms underlying tolerance and OIH remain poorly understood.

Results

We recently discovered a new human alternatively spliced isoform of MOR (MOR1K) that is missing the N-terminal extracellular and first transmembrane domains, resulting in a 6TM GPCR variant. To characterize the pattern of cellular transduction pathways activated by this human MOR1K isoform, we conducted a series of pharmacological and molecular experiments. Results show that stimulation of MOR1K with morphine leads to excitatory cellular effects. In contrast to stimulation of MOR1, stimulation of MOR1K leads to increased Ca²⁺ levels as well as increased nitric oxide (NO) release. Immunoprecipitation experiments further reveal that unlike MOR1, which couples to the inhibitory Gα_i/o complex, MOR1K couples to the stimulatory Gα_s complex.

Conclusion

The major MOR1 and the alternative MOR1K isoforms mediate opposite cellular effects in response to morphine, with MOR1K driving excitatory processes. These findings warrant further investigations that examine animal and human MORK1 expression and function following chronic exposure to opioids, which may identify MOR1K as a novel target for the development of new clinically effective classes of opioids that have high analgesic efficacy with diminished ability to produce tolerance, OIH, and other unwanted side-effects.

Involvement of PLC-beta3 in the effect of morphine on memory retrieval in passive avoidance task

Phospholipase C (PLC) is one signalling effector enzyme whose activity is directly modulated by opioids. Several physiological studies have implicated PLC-linked pathways in in-vivo pain regulation and opioid tolerance. Co-administration of PLC-beta(2/3) activity blocker M119 with morphine resulted in a dramatic increase in morphine-induced amnesic effect in mice, proving a role for beta subunit of PLC enzyme in these processes. Administration of morphine to mice at amnesic dose increased PLC-beta(3) activity, with respect to basal value, in the membrane-soluble material from anterior cortex and hippocampal formation in brain areas. PLC-beta(3) appears to be simultaneously implicated in both analgesic and amnesic effects induced by administration of morphine to mice suggesting a commonality in the molecular mechanisms of morphine-induced analgesia and memory impairment.

Gi- and Gq-coupled ADP (P2Y) receptors act in opposition to modulate nociceptive signaling and inflammatory pain behavior

BACKGROUND

Investigations of nucleotide signaling in nociception to date have focused on actions of adenosine triphosphate (ATP). Both ATP-gated ion channels (P2X receptors) and G protein-coupled (P2Y) receptors contribute to nociceptive signaling in peripheral sensory neurons. In addition, several studies have implicated the Gq-coupled adenosine diphosphate (ADP) receptor P2Y1 in sensory transduction. In this study, we examined the expression and function of P2Y1 and the Gi-coupled receptors P2Y12, P2Y13 and P2Y14 in sensory neurons to determine their contribution to nociception.

RESULTS

We detected mRNA and protein for ADP receptors P2Y12 and P2Y13 in mouse dorsal root ganglia (DRG). P2Y14, a homologous Gi-coupled nucleotide receptor, is also expressed in DRG. Immunohistochemical analysis of receptor distribution indicated that these receptors are widely expressed in nociceptive neurons. Using ratiometric calcium imaging, we found that ADP evokes increases in intracellular calcium in isolated DRG neurons and also produces a pertussis toxin-sensitive inhibition of depolarization-evoked calcium transients. The inhibitory effect of ADP was unaltered in the presence of the selective P2Y1 antagonist MRS2179 and in neurons isolated from P2Y1 knockout mice, whereas ADP-evoked calcium transients were greatly reduced. Analysis of behavioral responses to noxious heat before and after inflammatory injury (injection of complete Freund's adjuvant into the hindpaw) revealed that P2Y1 is required for the full expression of inflammatory hyperalgesia, whereas local injection of agonists for Gi-coupled P2Y receptors reduced hyperalgesia.

CONCLUSIONS

We report that Gi-coupled P2Y receptors are widely expressed in peripheral sensory neurons. Agonists for these receptors inhibit nociceptive signaling in isolated neurons and reduce behavioral hyperalgesia in vivo. Anti-nociceptive actions of these receptors appear to be antagonized by the Gq-coupled ADP receptor, P2Y1, which is required for the full expression of inflammatory hyperalgesia. We propose that nociceptor sensitivity is modulated by the integration of nucleotide signaling through Gq- and Gi-coupled P2Y receptors, and this balance is altered in response to inflammatory injury. Taken together, our data suggest that Gi-coupled P2Y receptors are broadly expressed in nociceptors, inhibit nociceptive signaling in vivo, and represent potential targets for the development of novel analgesic drugs.

Opiate-induced suppression of rat hypoglossal motoneuron activity and its reversal by ampakine therapy

Background

Hypoglossal (XII) motoneurons innervate tongue muscles and are vital for maintaining upper-airway patency during inspiration. Depression of XII nerve activity by opioid analgesics is a significant clinical problem, but underlying mechanisms are poorly understood. Currently there are no suitable pharmacological approaches to counter opiate-induced suppression of XII nerve activity while maintaining analgesia. Ampakines accentuate α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor responses. The AMPA family of glutamate receptors mediate excitatory transmission to XII motoneurons. Therefore the objectives were to determine whether the depressant actions of μ-opioid receptor activation on inspiratory activity includes a direct inhibitory action at the inspiratory premotoneuron to XII motoneuron synapse, and to identify underlying mechanism(s). We then examined whether ampakines counteract opioid-induced depression of XII motoneuron activity.

Methodology/Principal Findings

A medullary slice preparation from neonatal rat that produces inspiratory-related output in vitro was used. Measurements of inspiratory burst amplitude and frequency were made from XII nerve roots. Whole-cell patch recordings from XII motoneurons were used to measure membrane currents and synaptic events. Application of the μ-opioid receptor agonist, DAMGO, to the XII nucleus depressed the output of inspiratory XII motoneurons via presynaptic inhibition of excitatory glutamatergic transmission. Ampakines (CX614 and CX717) alleviated DAMGO-induced depression of XII MN activity through postsynaptic actions on XII motoneurons.

Conclusions/Significance

The inspiratory-depressant actions of opioid analgesics include presynaptic inhibition of XII motoneuron output. Ampakines counteract μ-opioid receptor-mediated depression of XII motoneuron inspiratory activity. These results suggest that ampakines may be beneficial in countering opiate-induced suppression of XII motoneuron activity and resultant impairment of airway patency.

Abnormal calcium homeostasis in peripheral neuropathies

Abnormal neuronal calcium (Ca2+) homeostasis has been implicated in numerous diseases of the nervous system. The pathogenesis of two increasingly common disorders of the peripheral nervous system, namely neuropathic pain and diabetic polyneuropathy, has been associated with aberrant Ca2+ channel expression and function. Here we review the current state of knowledge regarding the role of Ca2+ dyshomeostasis and associated mitochondrial dysfunction in painful and diabetic neuropathies. The central impact of both alterations of Ca2+ signalling at the plasma membrane and also intracellular Ca2+ handling on sensory neurone function is discussed and related to abnormal endoplasmic reticulum performance. We also present new data highlighting sub-optimal axonal Ca2+ signalling in diabetic neuropathy and discuss the putative role for this abnormality in the induction of axonal degeneration in peripheral neuropathies. The accumulating evidence implicating Ca2+ dysregulation in both painful and degenerative neuropathies, along with recent advances in understanding of regional variations in Ca2+ channel and pump structures, makes modulation of neuronal Ca2+ handling an increasingly viable approach for therapeutic interventions against the painful and degenerative aspects of many peripheral neuropathies.

Supraspinal Gbetagamma-dependent stimulation of PLCbeta originating from G inhibitory protein-mu opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in micro-opioid receptor (microOR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to microOR coupling, is presently unknown. A pronounced coupling of microOR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective microOR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) administered at the PAG site, showing that the low dose morphine effect is triggered by microOR activated G inhibitory protein at supraspinal level. When Gbetagamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits Gbetagamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with Gbetagamma in PAG. Although opioid administration induces a shift in microOR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.

Morphine-induced early delays in wound closure: involvement of sensory neuropeptides and modification of neurokinin receptor expression

Dose-limiting side effects of centrally acting opioid drugs have led to the use of topical opioids to reduce the pain associated with chronic cutaneous wounds. However, previous studies indicate that topical morphine application impairs wound healing. This study was designed to elucidate the mechanisms by which morphine delays wound closure. Rats were depleted of sensory neuropeptides by treatment with capsaicin, and full-thickness 4-mm diameter wounds were excised from the intrascapular region. Wounds were treated topically twice daily with 5mM morphine sulfate, 1mM substance P, 1mM neurokinin A, or 5mM morphine combined with 1mM substance P or neurokinin A and wound areas assessed. During closure, wound tissue was taken 1, 3, 5, and 8 days post-wounding from control and morphine-treated rats and immunostained for neurokinin receptors and markers for macrophages, myofibroblasts, and vasculature. Results obtained from capsaicin-treated animals demonstrated a significant delay in the early stages of wound contraction that was reversed by neuropeptide application. Treatment of capsaicin-treated rats with topical morphine did not further delay wound closure, suggesting that topical opioids impair wound closure via the inhibition of peripheral neuropeptide release into the healing wound. Morphine application altered neurokinin-1 and neurokinin-2 receptor expression in inflammatory and parenchymal cells essential for wound healing in a cell-specific manner, demonstrating a direct effect of morphine on neurokinin receptor regulation within an array of cells involved in wound healing. These data provide evidence indicating a potentially detrimental effect of topical morphine application on the dynamic wound healing process.

Opioidergic and dopaminergic modulation of respiration

Opioids, dopamine and their receptors are present in many regions of the bulbar respiratory network. The physiological importance of endogenous opioids to respiratory control has not been explicitly demonstrated. Nonetheless, studies of opioidergic respiratory mechanisms are important because synthetic opiate drugs have respiratory side effects that in some situations pose health risks and limit their therapeutic usefulness. They can depress breathing depth and rate, blunt respiratory responsiveness to CO2 and hypoxia, increase upper airway resistance and reduce pulmonary compliance. The opiate respiratory disturbances are mainly due to agonist activation of mu- and delta-subtypes of receptor and involve specific types of respiratory-related neurons in the ventrolateral medulla and the dorsolateral pons. Endogenous dopaminergic modulation in the CNS and carotid bodies enhances CO2-dependent respiratory drive and depresses hypoxic drive. In the CNS, synthetic agonists with selectivity for D1-and D4-types of receptor slow respiratory rhythm, whereas D2-selective agonists modulate acute and chronic responses to hypoxia. D1-receptor agonists also act centrally to increase respiratory responsiveness to CO2, and counteract opiate blunting of CO2-dependent respiratory drive and depression of breathing. Cellular targets and intracellular mechanisms responsible for opioidergic and dopaminergic respiratory effects for the most part remain to be determined.

Temporal effects of topical morphine application on cutaneous wound healing

BACKGROUND

Studies have shown that topical administration of exogenous opioid drugs impairs wound healing by inhibiting the peripheral release of neuropeptides, thereby inhibiting neurogenic inflammation. This delay is immediate and peaks during the first days of wound closure. This study examined the effects of topical morphine treatment in a cutaneous wound healing model in the rat.

METHODS

Full-thickness 4-mm-diameter wounds were placed on the periscapular region of rats that subsequently received twice-daily topical applications of IntraSite Gel (Smith+Nephew, Hull, United Kingdom) alone or gel infused with 5 mm morphine sulfate on days 0-3 or 4-10 postwounding or throughout the time course. Wound tissue was taken on days 1, 3, 5, 8, and 18 postwounding and immunostained for myofibroblast and macrophage markers or stained with hematoxylin and eosin.

RESULTS

Delays in wound closure observed during morphine application on days 0-3 postwounding mimicked those seen in wounds treated with morphine throughout the entire healing process. However, no significant delays in closure were seen in wounds treated with morphine beginning on day 4 postwounding. Treatment of wounds with morphine significantly reduced the number of myofibroblasts and macrophages in the closing wound. In addition, morphine application resulted in decreases in skin thickness and an increase in residual scar tissue in healed skin.

CONCLUSIONS

These findings demonstrate the time-dependent and persistent nature of the detrimental effects of topical morphine on cutaneous wound healing. The data identify specific limitations that could be ameliorated to optimize topical opioid administration as an analgesic therapeutic strategy in the treatment of painful cutaneous wounds.

Distinct inhibition of voltage-activated Ca2+ channels by delta-opioid agonists in dorsal root ganglion neurons devoid of functional T-type Ca2+ currents

Both mu- and delta-opioid agonists selectively inhibit nociception but have little effect on other sensory modalities. Voltage-activated Ca(2+) channels in the primary sensory neurons are important for the regulation of nociceptive transmission. In this study, we determined the effect of delta-opioid agonists on voltage-activated Ca(2+) channel currents (I(Ca)) in small-diameter rat dorsal root ganglion (DRG) neurons that do and do not bind isolectin B(4) (IB(4)). The delta-opioid agonists [d-Pen(2),d-Pen(5)]-enkephalin (DPDPE) and deltorphin II produced a greater inhibition of high voltage-activated I(Ca) in IB(4)-negative than IB(4)-positive neurons. Furthermore, DPDPE produced a greater inhibition of N-, P/Q-, and L-type I(Ca) in IB(4)-negative than IB(4)-positive neurons. However, DPDPE had no significant effect on the R-type I(Ca) in either type of cells. We were surprised to find that DPDPE failed to inhibit either the T-type or high voltage-activated I(Ca) in all the DRG neurons with T-type I(Ca). Double immunofluorescence labeling showed that the majority of the delta-opioid receptor-immunoreactive DRG neurons had IB(4) labeling, while all DRG neurons immunoreactive to delta-opioid receptors exhibited Cav(3.2) immunoreactivity. Additionally, DPDPE significantly inhibited high voltage-activated I(Ca) in Tyrode's or N-methyl-d-glucamine solution but not in tetraethylammonium solution. This study provides new information that delta-opioid agonists have a distinct effect on voltage-activated Ca(2+) channels in different phenotypes of primary sensory neurons. High voltage-activated Ca(2+) channels are more sensitive to inhibition by delta-opioid agonists in IB(4)-negative than IB(4)-positive neurons, and this opioid effect is restricted to DRG neurons devoid of functional T-type Ca(2+) currents.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Amelioration of pain and histopathologic joint abnormalities in the Col1-IL-1beta(XAT) mouse model of arthritis by intraarticular induction of mu-opioid receptor into the temporomandibular joint

OBJECTIVE

To evaluate opioid receptor function as a basis for novel antinociceptive therapy in arthritis.

METHODS

We induced human mu-opioid receptor (HuMOR) expression in arthritic joints of mice, using the feline immunodeficiency virus (FIV) vector, which is capable of stably transducing dividing, growth-arrested, and terminally differentiated cells. Male and female Col1-IL-1beta(XAT)-transgenic mice developed on a C57BL/6J background and wild-type littermates were studied.

RESULTS

A single injection of FIV(HuMOR) into the temporomandibular joints of Col1-IL-1beta(XAT)-transgenic mice 1 week prior to induction of arthritis prevented the development of orofacial pain and joint dysfunction, and reduced the degree of histopathologic abnormality in the joint. In addition, FIV(HuMOR) prevented the attendant sensitization of trigeminal sensory neurons and activation of astroglia in brainstem trigeminal sensory nuclei. These effects were mediated by the transduction of primary sensory neurons via transport of FIV vectors from peripheral nerve endings to sensory ganglia, as evidenced by HuMOR expression in neuronal cell bodies located in the trigeminal ganglia, as well as in their proximal and distal nerve branches located in the main sensory and subnucleus caudalis of the brainstem and joints, respectively. The presence of MOR ligands predominantly in the descending trigeminal nucleus suggested that the observed antinociception occurred at the subnucleus caudalis. Articular chondrocytes and meniscal tissue were also infected by FIV(HuMOR), which presumably exerted an antiinflammatory effect on cartilage.

CONCLUSION

Our results indicate that prophylactic therapy with MOR overexpression in joints can successfully prevent the development of pain, dysfunction, and histopathologic abnormalities in the joints in arthritis. These findings may provide a basis for the future development of spatiotemporally controlled antinociceptive and antiinflammatory therapy for arthritis.

Delay of cutaneous wound closure by morphine via local blockade of peripheral tachykinin release

Topically applied morphine is routinely used to alleviate pain in cutaneous wounds such as burns and pressure sores. Evidence suggests the topical administration of exogenous opioid drugs may impair wound closure. This study examined the effects of topical morphine on a standardized model of cutaneous wound healing in the rat. Full-thickness 4mm diameter circular skin flaps were excised from the intrascapular region of male Sprague-Dawley rats. IntraSite Gel infused with either morphine-sulfate, neurokinin-1 (NK-1) or neurokinin-2 (NK-2) receptor antagonists, substance P (SP), neurokinin A (NKA), SP+morphine-sulfate, or NKA+morphine-sulfate was applied to the wound twice daily. Results demonstrated a significant overall delay in the time course of wound contraction in morphine-treated animals when compared with gel-only treated controls. The delay in wound contraction seen in morphine-treated animals increased in a concentration-dependent manner. Topical application of NK-1 or NK-2 receptor antagonists mimicked the effects of morphine in delaying wound closure, suggesting topical opioids impair wound closure via the inhibition of SP and NKA release peripherally into the healing wound. Additionally, no significant delays in closure were seen in rats receiving morphine combined with SP or NKA, demonstrating the ability of each neuropeptide to attenuate the effects of morphine in delaying wound closure and restore normal wound closure rates. The combination of SP or NKA and morphine-sulfate for wound therapy may provide local analgesia while maintaining normal closure rates.

Direct Inhibition of Ih by Analgesic Loperamide in Rat DRG Neurons

Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels are responsible for the functional hyperpolarization-activated current ( Ih) in dorsal root ganglion (DRG) neurons, playing an important role in pain processing. We found that the known analgesic loperamide inhibited Ih channels in rat DRG neurons. Loperamide blocked Ih in a concentration-dependent manner, with an IC50 = 4.9 ± 0.6 and 11.0 ± 0.5 μM for large- and small-diameter neurons, respectively. Loperamide-induced Ih inhibition was unrelated to the activation of opioid receptors and was reversible, voltage-dependent, use-independent, and was associated with a negative shift of V1/2 for Ih steady-state activation. Loperamide block of Ih was voltage-dependent, gradually decreasing at more hyperpolarized membrane voltages from 89% at –60 mV to 4% at –120 mV in the presence of 3.7 μM loperamide. The voltage sensitivity of block can be explained by a loperamide-induced shift in the steady-state activation of Ih. Inclusion of 10 μM loperamide into the recording pipette did not affect Ih voltage for half-maximal activation, activation kinetics, and the peak current amplitude, whereas concurrent application of equimolar external loperamide produced a rapid, reversible Ih inhibition. The observed loperamide-induced Ih inhibition was not caused by the activation of peripheral opioid receptors because the broad-spectrum opioid receptor antagonist naloxone did not reverse Ih inhibition. Therefore we suggest that loperamide inhibits Ih by direct binding to the extracellular region of the channel. Because Ih channels are involved in pain processing, loperamide-induced inhibition of Ih channels could provide an additional molecular mechanism for its analgesic action.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.