ET – phone the pain clinic

Inhibition of endothelin A receptor by a novel, selective receptor antagonist enhances morphine-induced analgesia: Possible functional interaction of dimerized endothelin A and μ-opioid receptors

BACKGROUND

The misuse of opioids has led to an epidemic in recent times. The endothelin A receptor (ETAR) has recently attracted attention as a novel therapeutic target to enhance opioid analgesia. We hypothesized that endothelin A receptors may affect pain mechanisms by heterodimerization with μ opioid receptors. We examined the mechanisms of ETAR-mediated pain and the potential therapeutic effects of an ETAR antagonist, Compound-E, as an agent for analgesia.

METHODS

Real-time in vitro effect of Compound-E on morphine response was assessed in HEK293 cells expressing both endothelin A and μ opioid receptors through CellKey™ and cADDis cAMP assays. Endothelin A/μ opioid receptor dimerization was assessed by immunoprecipitation and live cell imaging. The in vivo effect of Compound-E was evaluated using a morphine analgesia mouse model that observed escape response behavior, body temperature, and locomotor activity.

RESULTS

In CellKey™ and cAMP assays, pretreatment of cells with endothelin-1 attenuated morphine-induced responses. These responses were improved by Compound-E, but not by BQ-123 nor by bosentan, an ETAR and endothelin B receptor antagonist. Dimerization of ETARs and μ opioid receptors was confirmed by Western blot and total internal reflection fluorescence microscopy in live cells. In vivo, Compound-E potentiated and prolonged the analgesic effects of morphine, enhanced hypothermia, and increased locomotor activity compared to morphine alone.

CONCLUSION

The results suggest that attenuation by endothelin-1 of morphine analgesia may be caused by dimerization of Endothelin A/μ opioid receptors. The novel ETAR antagonist Compound-E could be an effective adjunct to reduce opioid use.

Evidence for the endothelin system as an emerging therapeutic target for the treatment of chronic pain

Many people worldwide suffer from pain and a portion of these sufferers are diagnosed with a chronic pain condition. The management of chronic pain continues to be a challenge, and despite taking prescribed medication for pain, patients continue to have pain of moderate severity. Current pain therapies are often inadequate, with side effects that limit medication adherence. There is a need to identify novel therapeutic targets for the management of chronic pain. One potential candidate for the treatment of chronic pain is therapies aimed at modulating the vasoactive peptide endothelin-1. In addition to vasoactive properties, endothelin-1 has been implicated in pain transmission in both humans and animal models of nociception. Endothelin-1 directly activates nociceptors and potentiates the effect of other algogens, including capsaicin, formalin, and arachidonic acid. In addition, endothelin-1 has been shown to be involved in inflammatory pain, cancer pain, neuropathic pain, diabetic neuropathy, and pain associated with sickle cell disease. Therefore, endothelin-1 may prove a novel therapeutic target for the relief of many types of chronic pain.

Endothelin potentiates TRPV1 via ETA receptor-mediated activation of protein kinase C

Background

Endothelin-1 (ET-1) both stimulates nociceptors and sensitizes them to noxious stimuli, an effect probably mediated by the ET_A receptor (ET_AR) expressed in sensory neurons. The cellular mechanisms of this ET-1-mediated effect are only poorly understood. TRPV1, the heat-, pH- and capsaicin-sensitive cation channel already known to be modulated by a number of cellular mediators released in response to noxious stimuli and during inflammation, is a potential target for the action of ET-1.

Results

We studied the effects of ET-1 on TRPV1 in sensory neurons from the dorsal root ganglion (DRG) and in HEK293 cells coexpressing TRPV1 and the ET_AR. Specific ¹²⁵I-ET-1 binding sites (817 ± 92 fmol/mg) were detected in membrane preparations of DRG with an ET_AR/ET_BR ratio of 60:40. In an immunofluorescence analysis, coexpression of TRPV1 and the ET_AR was found in a subpopulation of primary sensory neurons. ET-1 strongly potentiated capsaicin-induced TRPV1 currents in some neurons, and in HEK293 cells co-expressing TRPV1 and the ET_AR. Weaker potentiation was observed in HEK293 cells coexpressing TRPV1 and the ET_BR. ET_AR activation also increased responses to low pH and heat. In HEK293 cells, strong potentiation of TRPV1 like that induced by ET-1 via the ET_AR could be induced by PKC activation, but not with activators of the adenylyl cyclase or the PKA pathway. Furthermore, inhibition of PKC with bisindolylmaleimide X (BIM X) or mutation of the PKC phosphorylation site S800 completely prevented ET_AR-mediated potentiation.

Conclusion

We conclude that ET-1 potentiates TRPV1 by a PKC-dependent mechanism and that this could play a major role in the algogenic and hyperalgesic effects of ET-1 described in previous studies.

Inflammatory mediators modulating the transient receptor potential vanilloid 1 receptor: therapeutic targets to treat inflammatory and neuropathic pain

The transient receptor potential vanilloid 1 receptor (TRPV1) plays an important role in inflammatory heat hyperalgesia. TRPV1 is a non-selective cation channel gated by noxious heat, protons and capsaicin, thus being regarded as a polymodal molecular integrator in nociception. Abundant evidence has demonstrated that TRPV1 is also modulated by numerous inflammatory mediators, including growth factors, neurotransmitters, peptides or small proteins, lipids, chemokines and cytokines. By activating multiple protein kinases to increase the phosphorylation of TRPV1, pronociceptive inflammatory mediators sensitise the TRPV1 response to noxious heat, protons and capsaicin, thus augmenting thermal hyperalgesia. In contrast, by inhibiting protein kinases or other mechanisms, antinociceptive inflammatory mediators suppress the response of TRPV1 to these stimuli, thus damping thermal hyperalgesia. The positive modulation of TRPV1 by inflammatory mediators may constitute a novel mechanism underlying sustained inflammatory or neuropathic pain. Blocking pronociceptive inflammatory mediator-exerted sensitising effects or boosting antinociceptive inflammatory mediator-induced suppressing effects on TRPV1 should be considered as sources of novel potential therapies to more effectively treat chronic pain conditions.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

Differential effects of endothelin on activation of renal mechanosensory nerves: stimulatory in high-sodium diet and inhibitory in low-sodium diet

Activation of renal mechanosensory nerves is enhanced by high and suppressed by low sodium dietary intake. Afferent renal denervation results in salt-sensitive hypertension, suggesting that activation of the afferent renal nerves contributes to water and sodium balance. Another model of salt-sensitive hypertension is the endothelin B receptor (ETBR)-deficient rat. ET and its receptors are present in sensory nerves. Therefore, we examined whether ET receptor blockade altered the responsiveness of the renal sensory nerves. In anesthetized rats fed high-sodium diet, renal pelvic administration of the ETBR antagonist BQ-788 reduced the afferent renal nerve activity (ARNA) response to increasing renal pelvic pressure 7.5 mmHg from 26+/-3 to 9+/-3% and the PGE2-mediated renal pelvic release of substance P from 9+/-1 to 3+/-1 pg/min. Conversely, in rats fed low-sodium diet, renal pelvic administration of the ETAR antagonist BQ-123 enhanced the ARNA response to increased renal pelvic pressure from 9+/-2 to 23+/-6% and the PGE2-mediated renal pelvic release of substance P from 0+/-0 to 6+/-1 pg/min. Adding the ETAR antagonist to ETBR-blocked renal pelvises restored the responsiveness of renal sensory nerves in rats fed a high-sodium diet. Adding the ETBR antagonist to ETAR-blocked pelvises suppressed the responsiveness of the renal sensory nerves in rats fed a low-sodium diet. In conclusion, activation of ETBR and ETAR contributes to the enhanced and suppressed responsiveness of renal sensory nerves in conditions of high- and low-sodium dietary intake, respectively. Impaired renorenal reflexes may contribute to the salt-sensitive hypertension in the ETBR-deficient rat.

[Mechanisms in the development of pain. Key issue in the periphery]

The interaction between neurons, tissue cells and inflammatory cells is of major importance for the development of pain in the periphery. In this interaction a variety of inhibitory and activating circuits has been identified in recent years. In addition to the receptors for classical inflammatory mediators on the sensory terminals like bradykinin, axonal ion channels have been identified as major modulators of pain and sensitization apart from their traditional role in conduction of action potentials. Sensitization of spinal nociceptive processing is crucial for the expansion of pain beyond the initially injured site and contributes to chronic pain. Learning processes and extinction of aversive memory are of major importance for the development, but also the therapy of chronic pain states.