Antinociceptive effects of herkinorin, a MOP receptor agonist derived from salvinorin A in the formalin test in rats: new concepts in mu opioid receptor pharmacology: from a symposium on new concepts in mu-opioid pharmacology

Herkinorin ameliorates neuronal damage in a pentylenetetrazol-induced epilepsy rat model through altering microglial and astrocytic activation by inhibiting PARP1 and NF-κB

BACKGROUND

Altered astrocytic and microglial functions have been shown to mediate inflammation and oxidative stress in epilepsy. Herkinorin, a novel mu opioid receptor (MOR) agonist, has a neuroprotective role in ischemic brain injury. In this report, we sought to explore the effects and mechanism of herkinorin in the treatment of epilepsy and neuronal damage.

METHODS

SH-SY5Y cells were treated with pentylenetetrazol (PTZ) and herkinorin. The viability, reactive oxygen species (ROS) release, and apoptosis of the cells were detected. A rat epilepsy model was induced via PTZ injection, and herkinorin was used for pretreatment. Immunofluorescence staining and immunohistochemistry were used to observe neuronal damage and microglial and astrocyte activation in the hippocampal CA1/3 region. Western blotting was used to determine the expression profiles of PARP1 and NF-κB.

RESULTS

PTZ substantially facilitated SH-SY5Y cell apoptosis, induced oxidative stress and promoted NLRP3-ASC-Caspase-1 inflammasome activation. Herkinorin attenuated SH-SY5Y cell damage mediated by PTZ and suppressed PARP1 and NF-κB. The activation of PARP1 by lipopolysaccharide (LPS) aggravated SH-SY5Y cell injury, and herkinorin treatment reversed these LPS-mediated effects. In in vivo experiments, herkinorin hampered epileptic seizures in rats and weakened PTZ-induced neuronal damage in the hippocampus. Moreover, herkinorin reduced PTZ-induced neuroinflammation, resulting in "M1" to "M2" polarization of microglia and "A1" to "A2" polarization of astrocytes. Moreover, herkinorin inhibited the expression of PARP1 and NF-κB phosphorylation in the hippocampus.

CONCLUSION

Herkinorin ameliorates PTZ-induced neuroinflammation in epileptic rats by inhibiting PARP1 and NF-κB and regulating microglial and astrocytic activation.

Advances in cannabinoid receptors pharmacology: from receptor structural insights to ligand discovery

The medicinal and recreational uses of Cannabis sativa have been recognized for thousands of years. Today, cannabis-derived medicines are used to treat a variety of conditions, including chronic pain, epilepsy, multiple sclerosis, and chemotherapy-induced nausea. However, cannabis use disorder (CUD) has become the third most prevalent substance use disorder globally. Cannabinoid receptors are the primary targets that mediate the effects of cannabis and its analogs. Despite their importance, the mechanisms of modulation and the full therapeutic potential of cannabinoid receptors remain unclear, hindering the development of the next generation of cannabinoid-based drugs. This review summarizes the discovery and medicinal potential of phytocannabinoids and explores the distribution, signaling pathways, and functional roles of cannabinoid receptors. It also discusses classical cannabinoid drugs, as well as agonists, antagonists, and inverse agonists, which serve as key therapeutic agents. Recent advancements in the development of allosteric drugs are highlighted, with a focus on positive and negative allosteric modulators (PAMs and NAMs) that target CB1 and CB2 receptors. The identification of multiple allosteric sites on the CB1 receptor and the structural basis for allosteric modulation are emphasized, along with the structure-based discovery of ago-BAMs for CB1. This review concludes by examining the future potential of allosteric modulators in cannabinoid drug development, noting that ongoing progress in cannabinoid-derived drugs continues to open new avenues for therapeutic use and paves the way for future research into their full medicinal potential.

Emerging Psychotropic Drug for the Treatment of Trigeminal Pain: Salvinorin A

Trigeminal neuralgia (TN) is chronic pain caused by damage to the somatosensorial system on the trigeminal nerve or its branches, which involves peripheral and central dysfunction pain pathways. Trigeminal pain triggers disruptive pain in regions of the face, including within and around the mouth. Besides clinical experiences, translating the language of suffering into scientific terminology presents substantial challenges. Due to the complex and multifactorial pathophysiology underlying trigeminal pain, elucidating its social impact presents significant difficulties. Carbamazepine and oxcarbazepine are first-line treatments for TN, achieving approximately 50% pain reduction in 60-70% of treated patients. However, their efficacy is often limited by common side effects, such as dizziness, vertigo, nausea, seizures, and cognitive symptoms. In some cases, patients experience severe side effects, including myelosuppression, hyponatremia, hormonal imbalances, liver toxicity, suicidal ideation, teratogenicity, and other adverse reactions. Given these clinical limitations, the search for new painkiller candidates continues. Hence, we focused this review on salvinorin A (SalA), a natural agonist of κ-opioid receptors (KORs), which demonstrated anti-nociceptive, anti-inflammatory, and anti-neuropathic properties in various experimental models of the spinal sensory system. Furthermore, preclinical evidence indicates that SalA does not induce dependence and demonstrates a favorable toxicological and safety profile in comparison with currently marketed opioid drugs. We propose Salvinorin A as a promising candidate for treating trigeminal neuralgia, offering the potential for reduced adverse effects.

A systematic review on the kappa opioid receptor and its ligands: New directions for the treatment of pain, anxiety, depression, and drug abuse

Kappa opioid receptor (KOR) is a member of the opioid receptor system, the G protein-coupled receptors that are expressed throughout the peripheral and central nervous systems and play crucial roles in the modulation of antinociception and a variety of behavioral states like anxiety, depression, and drug abuse. KOR agonists are known to produce potent analgesic effects and have been used clinically for the treatment of pain, while KOR antagonists have shown efficacy in the treatment of anxiety and depression. This review summarizes the history, design strategy, discovery, and development of KOR ligands. KOR agonists are classified as non-biased, G protein-biased, and β-arrestin recruitment-biased, according to their degrees of bias. The mechanisms and associated effects of the G protein signaling pathway and β-arrestin recruitment signaling pathway are also discussed. Meanwhile, KOR antagonists are classified as long-acting and short-acting, based on their half-lives. In addition, we have special sections for mixed KOR agonists and selective peripheral KOR agonists. The mechanisms of action and pharmacokinetic, pharmacodynamic, and behavioral studies for each of these categories are also discussed in this review.

Analgesic Opioid Ligand Discovery Based on Nonmorphinan Scaffolds Derived from Natural Sources

Strong opioid analgesics, including morphine, are the mainstays for treating moderate to severe acute pain and alleviating chronic cancer pain. However, opioid-related adverse effects, including nausea or vomiting, sedation, respiratory depression, constipation, pruritus (itch), analgesic tolerance, and addiction and abuse liability, are problematic. In addition, the use of opioids to relieve chronic noncancer pain is controversial due to the "opioid crisis" characterized by opioid misuse or abuse and escalating unintentional death rates due to respiratory depression. Hence, considerable research internationally has been aimed at the "Holy Grail" of the opioid analgesic field, namely the discovery of novel and safer opioid analgesics with improved opioid-related adverse effects. In this Perspective, medicinal chemistry strategies are addressed, where structurally diverse nonmorphinan-based opioid ligands derived from natural sources were deployed as lead molecules. The current state of play, clinical or experimental status, and novel opioid ligand discovery approaches are elaborated in the context of retaining analgesia with improved safety and reduced adverse effects, especially addiction liability.

Delineating the Ligand-Receptor Interactions That Lead to Biased Signaling at the μ-Opioid Receptor

Biased agonists, which selectively stimulate certain signaling pathways controlled by a G protein-coupled receptor (GPCR), hold great promise as drugs that maximize efficacy while minimizing dangerous side effects. Biased agonists of the μ-opioid receptor (μOR) are of particular interest as a means to achieve analgesia through G protein signaling without dose-limiting side effects such as respiratory depression and constipation. Rational structure-based design of biased agonists remains highly challenging, however, because the ligand-mediated interactions that are key to activation of each signaling pathway remain unclear. We identify several compounds for which the R- and S-enantiomers have distinct bias profiles at the μOR. These compounds serve as excellent comparative tools to study bias because the identical physicochemical properties of enantiomer pairs ensure that differences in bias profiles are due to differences in interactions with the μOR binding pocket. Atomic-level simulations of compounds at μOR indicate that R- and S-enantiomers adopt different poses that form distinct interactions with the binding pocket. A handful of specific interactions with highly conserved binding pocket residues appear to be responsible for substantial differences in arrestin recruitment between enantiomers. Our results offer guidance for rational design of biased agonists at μOR and possibly at related GPCRs.

Comprehensive overview of biased pharmacology at the opioid receptors: biased ligands and bias factors

One of the main challenges in contemporary medicinal chemistry is the development of safer analgesics, used in the treatment of pain. Currently, moderate to severe pain is still treated with the "gold standard" opioids whose long-term often leads to severe side effects. With the discovery of biased agonism, the importance of this area of pharmacology has grown exponentially over the past decade. Of these side effects, tolerance, opioid misuse, physical dependence and substance use disorder (SUD) stand out, since these have led to many deaths over the past decades in both USA and Europe. New therapeutic molecules that induce a biased response at the opioid receptors (MOR, DOR, KOR and NOP receptor) are able to circumvent these side effects and, consequently, serve as more advantageous therapies with great promise. The concept of biased signaling extends far beyond the already sizeable field of GPCR pharmacology and covering everything would be vastly outside the scope of this review which consequently covers the biased ligands acting at the opioid family of receptors. The limitation of quantifying bias, however, makes this a controversial subject, where it is dependent on the reference ligand, the equation or the assay used for the quantification. Hence, the major issue in the field of biased ligands remains the translation of the in vitro profiles of biased signaling, with corresponding bias factors to in vivo profiles showing the presence or the lack of specific side effects. This review comprises a comprehensive overview of biased ligands in addition to their bias factors at individual members of the opioid family of receptors, as well as bifunctional ligands.

Natural Products for the Treatment of Pain: Chemistry and Pharmacology of Salvinorin A, Mitragynine, and Collybolide

Pain remains a very pervasive problem throughout medicine. Classical pain management is achieved through the use of opiates belonging to the mu opioid receptor (MOR) class, which have significant side effects that hinder their utility. Pharmacologists have been trying to develop opioids devoid of side effects since the isolation of morphine from papaver somniferum, more commonly known as opium by Sertürner in 1804. The natural products salvinorin A, mitragynine, and collybolide represent three nonmorphinan natural product-based targets, which are potent selective agonists of opioid receptors, and emerging next-generation analgesics. In this work, we review the phytochemistry and medicinal chemistry efforts on these templates and their effects on affinity, selectivity, analgesic actions, and a myriad of other opioid-receptor-related behavioral effects.

Addressing opioid tolerance and opioid-induced hypersensitivity: Recent developments and future therapeutic strategies

Opioids are a commonly prescribed and efficacious medication for the treatment of chronic pain but major side effects such as addiction, respiratory depression, analgesic tolerance, and paradoxical pain hypersensitivity make them inadequate and unsafe for patients requiring long-term pain management. This review summarizes recent advances in our understanding of the outcomes of chronic opioid administration to lay the foundation for the development of novel pharmacological strategies that attenuate opioid tolerance and hypersensitivity; the two main physiological mechanisms underlying the inadequacies of current therapeutic strategies. We also explore mechanistic similarities between the development of neuropathic pain states, opioid tolerance, and hypersensitivity which may explain opioids' lack of efficacy in certain patients. The findings challenge the current direction of analgesic research in developing non-opioid alternatives and we suggest that improving opioids, rather than replacing them, will be a fruitful avenue for future research.

Herkinorin negatively regulates NLRP3 inflammasome to alleviate neuronal ischemic injury through activating Mu opioid receptor and inhibiting the NF-κB pathway

Herkinorin is a novel opioid receptor agonist. Activation of opioid receptors, a member of G protein coupled receptors (GPCRs), may play an important role in Herkinorin neuroprotection. GPCRs may modulate NOD-like receptor protein 3 (NLRP3)-mediated inflammatory responses in the mechanisms of inflammation-associated disease and pathological processes. In this study, we investigated the effects of Herkinorin on NLRP3 and the underlying receptor and molecular mechanisms in oxygen-glucose deprivation/reperfusion (OGD/R)-treated rat cortex neurons. First, Western blot analysis showed that Herkinorin can inhibit the activation of NLRP3 and Caspase-1, decrease the expression of interleukin (IL)-1β, and decrease the secretion of IL-6 and tumour necrosis factor α detected by enzyme-linked immunosorbent assay in OGD/R-treated neurons. Then we found that Herkinorin downregulated NLRP3 levels by inhibiting the activation of nuclear factor kappa B (NF-κB) pathway, reducing the phosphorylation level of p65 and IκBα in OGD/R-treated neurons (p < .05 or .01, n = 3 per group). Instead, both the mu opioid receptor (MOR) inhibitor, β-funaltrexamine, and MOR knockdown reversed the effects of Herkinorin on NLRP3 (p < .05 or .01, n = 3 per group). Further, we found that the level of β-arrestin2 decreased in the cell membrane and increased in the cytoplasm after Herkinorin pretreatment in OGD/R-treated neurons. In co-immunoprecipitation experiments, Herkinorin increased the binding of IκBα with β-arrestin2, decreased the ubiquitination level of IκBα, and β-arrestin2 knockdown reversed the effects of Herkinorin on IκBα in OGD/R-treated neurons (p < .05 or .01, n = 3 per group). Our data demonstrated that Herkinorin negatively regulated NLRP3 inflammasome to alleviate neuronal ischemic injury through inhibiting NF-κB pathway mediated primarily by MOR activation. Inhibition of the NF-κB pathway by Herkinorin may be achieved by decreasing the ubiquitination level of IκBα, in which β-arrestin2 may play an important role.

Neo-clerodane diterpenic influence in the antinociceptive and anti-inflammatory properties of Salvia circinnata Cav

ETHNOPHARMACOLOGICAL RELEVANCE

Mexico is considered an ancestral center of diversity of Salvia species, however many of them lack scientific information. Salvia circinnata Cav. (syn. Salvia amarissima Ortega) is an endemic species used in traditional medicine to treat disorders attributed to a cold state like anxiety in the central nervous system, as well as gastrointestinal ailments and pain relief.

AIM OF THE STUDY

To give preclinical evidence about the pharmacological properties of this species by investigating its antinociceptive and anti-inflammatory effects, the chemical nature of at least one metabolite, and a possible mechanism of action and adverse effects, using different experimental models of pain.

MATERIAL AND METHODS

Different crude extracts of Salvia circinnata Cav. aerial parts were prepared using increasing polarity and evaluated in the formalin test in mice. This screening allowed to select and evaluate an ethyl acetate extract (EtOAc), as the most bioactive extract, and a metabolite. Antinociceptive and anti-inflammatory activities were confirmed using the plantar test and carrageenan-induced edema. The antinociceptive effects of the extracts were compared to that observed with morphine (1 mg/kg), tramadol (20 mg/kg) or indomethacin (20 mg/kg) as reference drugs. Participation of opioids and TRPV1 receptors was investigated, as well as acute toxicity and adverse effects of sedation and gastric damage.

RESULTS

EtOAc (0.1-10 mg/kg) of S. circinnata Cav. showed a dose-dependent and significant antinociceptive activity, associated in part with the presence of a neo-clerodane glycoside amarisolide A (0.01-1 mg/kg), in the neurogenic and inflammatory phases of the formalin test. Central action of both treatments was corroborated in the plantar test, whereas anti-inflammatory effects were confirmed with the extract (1 and 10 mg/kg) and amarisolide A (1 mg/kg) in the carrageenan-induced edema test. An opioid mechanism in both treatments, and the TRPV1 receptor modulation in the extract were involved. No acute toxicity and adverse effects were noticed with the extract and pure compound in comparison to the reference drugs.

CONCLUSION

These results provide preclinical evidence of the ethnopharmacological antinociceptive S. circinnata Cav. properties, in which the neo-clerodane diterpene glycoside amarisolide A was partially responsible involving the participation of the opioid receptors, while TRPV1 receptor modulation was implicated in the anti-inflammatory activity may be because of the presence of other constituents. This information supports the use of this species in folk medicine for pain therapy.

Toward Directing Opioid Receptor Signaling to Refine Opioid Therapeutics

The mu opioid receptor (MOR) is a diversely regulated target for the alleviation of pain in the clinical setting. However, untoward side effects such as tolerance, dependence, respiratory suppression, constipation, and abuse liability detract from the general activation of these receptors. Studies in genetically modified rodent models suggest that activating G protein signaling pathways while avoiding phosphorylation of the receptor or recruitment of β-arrestin scaffolding proteins could preserve the analgesic properties of MOR agonists while avoiding certain side effects. With the development of novel MOR "biased" agonists, which lead to preferential activation of G protein pathways over receptor phosphorylation, internalization, or interaction with other effectors, this hypothesis can be tested in a native, physiological setting. Overall, it is clear that the MOR is not a simple on-off switch and that the diverse means by which the receptor can be regulated may present an opportunity to refine therapeutics for the treatment of pain.

The G-protein-biased agents PZM21 and TRV130 are partial agonists of μ-opioid receptor-mediated signalling to ion channels

BACKGROUND AND PURPOSE

Opioids remain the most efficient medications against severe pain; they act on receptors that couple to heterotrimeric G-proteins in the G_αi/o family. Opioids exert many of their acute effects through modulating ion channels via G_βγ subunits. Many of their side effects are attributed to β-arrestin recruitment. Several biased agonists that do not recruit β-arrestins, but activate G-protein-dependent pathways, have recently been developed. While these compounds have been proposed to be full agonists of G-protein signalling in several high throughput pharmacological assays, their effects were not studied on ion channel targets.

EXPERIMENTAL APPROACH

Here, we used patch-clamp electrophysiology and Ca²⁺ imaging to test the effects of TRV130, PZM21, and herkinorin, three G-protein-biased agonists of μ-opioid receptors, on ion channel targets of G_αi/o /G_βγ signalling. We also studied G-protein dissociation using a FRET-based assay.

KEY RESULTS

All three biased agonists induced smaller activation of G-protein-coupled inwardly rectifying K⁺ channels (K_ir 3.2) and smaller inhibition of transient receptor potential melastatin (TRPM3) channels than the full μ receptor agonist DAMGO. Co-application of TRV130 or PZM21, but not herkinorin, alleviated the effects of DAMGO on both channels. PZM21 and TRV130 also decreased the effect of morphine on K_ir 3.2 channels. The Ca_V 2.2 channel was also inhibited less by PZM21 and TRV130 than by DAMGO. We also found that TRV130, PZM21, and herkinorin were less effective than DAMGO at inducing dissociation of the G_αi /G_βγ complex.

CONCLUSION AND IMPLICATIONS

TRV130, PZM21, and potentially herkinorin are partial agonists of μ receptors.

Salvia divinorum: from recreational hallucinogenic use to analgesic and anti-inflammatory action

Salvia divinorum is a herbal plant native to the southwest region of Mexico. Traditional preparations of this plant have been used in illness treatments that converge with inflammatory conditions and pain. Currently, S. divinorum extracts have become popular in several countries as a recreational drug due to its hallucinogenic effects. Its main active component is a diterpene named salvinorin A (SA), a potent naturally occurring hallucinogen with a great affinity to the κ opioid receptors and with allosteric modulation of cannabinoid type 1 receptors. Recent biochemical research has revealed the mechanism of action of the anti-inflammatory and analgesic effect of SA at the cellular and molecular level. Nevertheless, because of their short-lasting and hallucinogenic effect, the research has focused on discovering a new analogue of SA that is able to induce analgesia and reduce inflammation with a long-lasting effect but without the hallucinatory component. In this review, we explore the role of S. divinorum, SA and its analogues. We focus mainly on their analgesic and anti-inflammatory roles but also mention their psychoactive properties.

Current strategies toward safer mu opioid receptor drugs for pain management

INTRODUCTION

Pain relief remains a major public health challenge. The most efficient available painkillers are opioids targeting the mu opioid receptor (MOR). MORs are expressed in the areas of the brain [including pain and respiratory centers] that are important for processing reward and aversion. Thus, MOR activation efficiently alleviates severe pain, but the concomitant reward and respiratory depressant effects pose a threat; patients taking opioids potentially develop opioid addiction and high risk for overdose. Areas covered: Ongoing efforts to generate safer opioid analgesics are reviewed here. The design of biased compounds that trigger MOR induced G protein over β-arrestin signaling, peripheral opioids, drugs targeting MORs in heteromers and drugs enhancing endogenous opioid activity are discussed. Expert opinion: There is evidence that throttling MOR signaling may lead to an era of opioids that are truly efficient painkillers with lower side effects and risk of overdose. However, few of the drugs derived from the advanced approaches outlined here, are getting approval by regulatory committees for use in clinical settings. Thus, there is an urgent need to (i) better clarify mechanisms underlying the hazardous physiological effects of MOR activation, and (ii) fully validate the safety of these new MOR-based therapies.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

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.

Insights from molecular dynamics simulations to exploit new trends for the development of improved opioid drugs

Having accidental deaths from opioid overdoses almost quadrupled over the past fifteen years, there is a strong need to develop new, non-addictive medications for chronic pain to stop one of the deadliest epidemics in American history. Given their potentially fewer on-target overdosing risks and other adverse effects compared to classical opioid drugs, attention has recently shifted to opioid allosteric modulators and G protein-biased opioid agonists as likely drug candidates to prevent and/or reverse opioid overdoses. Understanding how these molecules bind and activate their receptors at an atomistic level is key to developing them into effective new therapeutics, and molecular dynamics-based strategies are contributing tremendously to this understanding.

Src Kinase Inhibition Attenuates Morphine Tolerance without Affecting Reinforcement or Psychomotor Stimulation

BACKGROUND

Prolonged opioid administration leads to tolerance characterized by reduced analgesic potency. Pain management is additionally compromised by the hedonic effects of opioids, the cause of their misuse. The multifunctional protein β-arrestin2 regulates the hedonic effects of morphine and participates in tolerance. These actions might reflect µ opioid receptor up-regulation through reduced endocytosis. β-Arrestin2 also recruits kinases to µ receptors. We explored the role of Src kinase in morphine analgesic tolerance, locomotor stimulation, and reinforcement in C57BL/6 mice.

METHODS

Analgesic (tail withdrawal latency; percentage of maximum possible effect, n = 8 to 16), locomotor (distance traveled, n = 7 to 8), and reinforcing (conditioned place preference, n = 7 to 8) effects of morphine were compared in wild-type, µ, µ, and β-arrestin2 mice. The influence of c-Src inhibitors dasatinib (n = 8) and PP2 (n = 12) was examined.

RESULTS

Analgesia in morphine-treated wild-type mice exhibited tolerance, declining by day 10 to a median of 62% maximum possible effect (interquartile range, 29 to 92%). Tolerance was absent from mice receiving dasatinib. Tolerance was enhanced in µ mice (34% maximum possible effect; interquartile range, 5 to 52% on day 5); dasatinib attenuated tolerance (100% maximum possible effect; interquartile range, 68 to 100%), as did PP2 (91% maximum possible effect; interquartile range, 78 to 100%). By contrast, c-Src inhibition affected neither morphine-evoked locomotor stimulation nor reinforcement. Remarkably, dasatinib not only attenuated tolerance but also reversed established tolerance in µ mice.

CONCLUSIONS

The ability of c-Src inhibitors to inhibit tolerance, thereby restoring analgesia, without altering the hedonic effect of morphine, makes c-Src inhibitors promising candidates as adjuncts to opioid analgesics.

Mu-Opioid receptor biased ligands: A safer and painless discovery of analgesics?

Biased activation of G-protein-coupled receptors (GPCRs) is shifting drug discovery efforts and appears promising for the development of safer drugs. The most effective analgesics to treat acute pain are agonists of the μ opioid receptor (μ-OR), a member of the GPCR superfamily. However, the analgesic use of opioid drugs, such as morphine, is hindered by adverse effects. Only a few μ-OR agonists have been reported to selectively activate the G_i over β-arrestin signaling pathway, resulting in lower gastrointestinal dysfunction and respiratory suppression. Here, we discuss the strategies that led to the development of biased μ-OR agonists, and potential areas for improvement, with an emphasis on structural aspects of the ligand-receptor recognition process.

New psychoactive substances of natural origin: A brief review

Plant-based drugs of abuse are as old as recorded human history. Although traditional addictive substances, such as opium, cannabis and coca, have been controlled by the United Nations anti-drug conventions, many, if not most, natural plants with addictive or abuse liability remain elusive. Therefore, the United Nations Office on Drugs and Crime (UNODC) has warned the emerging threat from new psychoactive substances (NPS), which are mostly derived or modified from the constituents of natural origin. For example, synthetic cannabinoids and synthetic cathinones are derived from the cannabis and khat plant, respectively. In this review, we briefly discussed the chemistry, pharmacology and toxicology of five common NPS of natural origin, i.e., khat, kratom, salvia, magic mushroom and mandrake. Through the review, we hope that professionals and general public alike can pay more attention to the potential problems caused by natural NPS, and suitable control measures will be taken.

Effects of acute and repeated treatment with the biased mu opioid receptor agonist TRV130 (oliceridine) on measures of antinociception, gastrointestinal function, and abuse liability in rodents

RATIONALE

TRV130 (oliceridine; N-[(3-methoxythiophen-2-yl)methyl]-2-[(9 R)-9-pyridin-2-yl-6-oxaspiro[4.5]decan-9-yl]ethanamine) is a novel mu opioid receptor (MOR) agonist that preferentially activates G-protein versus β-arrestin signaling pathways coupled to MORs. Prevailing evidence suggests that TRV130 and other G-protein-biased MOR agonists may produce therapeutic analgesic effects with reduced adverse effects compared to existing MOR agonists.

OBJECTIVES

This study compared the effects of acute and repeated TRV130 administration on measures of antinociception, gastrointestinal function, and abuse liability in rodents. We hypothesized that TRV130 would produce robust and sustained antinociception and abuse-related effects during repeated treatment, but that tolerance would develop to gastrointestinal inhibition.

METHODS

Antinociception was assessed using a warm-water tail-withdrawal procedure in mice. Gastrointestinal function was assessed in mice using an in vivo measure of fecal output and in vitro assays of colonic propulsion and of colon and ileum circular muscle contraction. Abuse liability was assessed in rats using an intracranial self-stimulation (ICSS) procedure. (+)-TRV130 was administered with acute and repeated dosing regimens, and (-)-TRV130 was also examined in the ICSS procedure to assess stereoselectivity.

RESULTS

Acute (+)-TRV130 treatment produced robust antinociception, complete inhibition of gastrointestinal function, and weak abuse-related effects. Repeated (+)-TRV130 treatment failed to produce tolerance to antinociception or gastrointestinal inhibition, and abuse-related effects were enhanced by repeated treatment. Effects of acute and repeated (+)-TRV130 in these procedures resemble effects of morphine, with the exception that TRV130 antinociception was more resistant to tolerance. (-)-TRV130 was inactive.

CONCLUSIONS

These results suggest that TRV130 retains undesirable constipating and abuse-related effects during repeated treatment despite its bias for G-protein signaling.

The analgesic and anti-inflammatory effects of Salvinorin A analogue β-tetrahydropyran Salvinorin B in mice

BACKGROUND

Drugs activating the mu opioid receptor are routinely used to treat severe acute and chronic pain. Unfortunately, side effects including nausea, constipation, respiratory depression, addiction and tolerance can limit clinical utility. In contrast, kappa opioid receptor (KOPr) agonists, such as Salvinorin A (SalA), have analgesic properties with little potential for abuse.

METHODS

We evaluated SalA and the novel analogue β-tetrahydropyran Salvinorin B (β-THP SalB) for the ability to modulate pain and inflammation in vivo. The hot water tail-withdrawal assay, intradermal formalin-induced inflammatory pain and paclitaxel-induced neuropathic pain models were used to evaluate analgesic properties in mice. Tissue infiltration of inflammatory cells was measured by histology and flow cytometry.

RESULTS

β-tetrahydropyran Salvinorin B produced a longer duration of action in the tail-withdrawal assay compared to the parent compound SalA, and, like SalA and U50,488, β-THP SalB is a full agonist at the KOPr. In the formalin-induced inflammatory pain model, β-THP SalB and SalA significantly reduced pain score, paw oedema and limited the infiltration of neutrophils into the inflamed tissue. β-THP SalB and SalA supressed both mechanical and cold allodynia in the paclitaxel-induced neuropathic pain model, in a dose-dependent manner.

CONCLUSIONS

Structural modification of SalA at the C-2 position alters its analgesic potency and efficacy in vivo. Substitution with a tetrahydropyran group at C-2 produced potent analgesic and anti-inflammatory effects, including a reduction in paclitaxel-induced neuropathic pain. This study highlights the potential for KOPr agonists as analgesics with anti-inflammatory action and little risk of abuse.

SIGNIFICANCE

Salvinorin A and the novel analogue β-THP Salvinorin B show analgesic effects in the tail-withdrawal and formalin assays. They reduce oedema and decrease neutrophil infiltration into inflamed tissue, and suppress mechanical and cold allodynia in paclitaxel-induced neuropathic pain.

Synthetic Studies of Neoclerodane Diterpenes from Salvia divinorum: Identification of a Potent and Centrally Acting μ Opioid Analgesic with Reduced Abuse Liability

Opioids are widely used to treat millions suffering from pain, but their analgesic utility is limited due to associated side effects. Herein we report the development and evaluation of a chemical probe exhibiting analgesia and reduced opioid-induced side effects. This compound, kurkinorin (5), is a potent and selective μ-opioid receptor (MOR) agonist (EC₅₀ = 1.2 nM, >8000 μ/κ selectivity). 5 is a biased activator of MOR-induced G-protein signaling over β-arrestin-2 recruitment. Metadynamics simulations of 5's binding to a MOR crystal structure suggest energetically preferred binding modes that differ from crystallographic ligands. In vivo studies with 5 demonstrate centrally mediated antinociception, significantly reduced rewarding effects, tolerance, and sedation. We propose that this novel MOR agonist may represent a valuable tool in distinguishing the pathways involved in MOR-induced analgesia from its side effects.

Biased mu-opioid receptor ligands: a promising new generation of pain therapeutics

Opioid chemistry and biology occupy a pivotal place in the history of pharmacology and medicine. Morphine offers unmatched efficacy in alleviating acute pain, but is also associated with a host of adverse side effects. The advent of biased agonism at G protein-coupled receptors has expanded our understanding of intracellular signaling and highlighted the concept that certain ligands are able to differentially modulate downstream pathways. The ability to target one pathway over another has allowed for the development of biased ligands with robust clinical efficacy and fewer adverse events. In this review we summarize these concepts with an emphasis on biased mu opioid receptor pharmacology and highlight how far opioid pharmacology has evolved.

cPKCγ membrane translocation is involved in herkinorin‑induced neuroprotection against cerebral ischemia/reperfusion injury in mice

Herkinorin is an opiate analgesic with limited adverse effects, functioning as a primary selective atypical opioid µ agonist. The present study aimed to identify whether herkinorin has a positive effect on ischemic/reperfusion (I/R) injury. Adult male C57BL/6 mice were randomly divided into five groups: i) Naïve, ii) sham, iii) I/R, iv) I/R with dimethyl sulfoxide (I/R+D) and v) I/R with herkinorin (I/R+H). The I/R injury model was induced by occluding the middle cerebral artery for 1 h followed by 24 h or 7 days of reperfusion. Neurobehavioral scores and sensorimotor functions were examined 24 h and 7 days following reperfusion. In addition, infarct volumes were examined at these time points using a 2,3,5‑triphenyltetrazolium chloride assay. Herkinorin treatment improved neurobehavioral and sensorimotor functional recovery from I/R‑induced brain injury. There was a significant decrease in infarct volume in the I/R+H group at 24 h or 7 days following reperfusion compared with the I/R and I/R+D groups. Western blotting suggested that the decrease in conventional protein kinase C γ (cPKCγ) membrane translocation in the peri‑infarct region may be attenuated by herkinorin pretreatment. These results indicated that herkinorin may be beneficial in I/R‑induced mouse brain injury, and this may be attributed to the membrane translocation of cPKCγ following activation.

How Oliceridine (TRV-130) Binds and Stabilizes a μ-Opioid Receptor Conformational State That Selectively Triggers G Protein Signaling Pathways

Substantial attention has recently been devoted to G protein-biased agonism of the μ-opioid receptor (MOR) as an ideal new mechanism for the design of analgesics devoid of serious side effects. However, designing opioids with appropriate efficacy and bias is challenging because it requires an understanding of the ligand binding process and of the allosteric modulation of the receptor. Here, we investigated these phenomena for TRV-130, a G protein-biased MOR small-molecule agonist that has been shown to exert analgesia with less respiratory depression and constipation than morphine and that is currently being evaluated in human clinical trials for acute pain management. Specifically, we carried out multimicrosecond, all-atom molecular dynamics (MD) simulations of the binding of this ligand to the activated MOR crystal structure. Analysis of >50 μs of these MD simulations provides insights into the energetically preferred binding pathway of TRV-130 and its stable pose at the orthosteric binding site of MOR. Information transfer from the TRV-130 binding pocket to the intracellular region of the receptor was also analyzed, and was compared to a similar analysis carried out on the receptor bound to the classical unbiased agonist morphine. Taken together, these studies lead to a series of testable hypotheses of ligand-receptor interactions that are expected to inform the structure-based design of improved opioid analgesics.

Distribution and trafficking of the μ-opioid receptor in enteric neurons of the guinea pig

The μ-opioid receptor (MOR) is a major regulator of gastrointestinal motility and secretion and mediates opiate-induced bowel dysfunction. Although MOR is of physiological and therapeutic importance to gut function, the cellular and subcellular distribution and regulation of MOR within the enteric nervous system are largely undefined. Herein, we defined the neurochemical coding of MOR-expressing neurons in the guinea pig gut and examined the effects of opioids on MOR trafficking and regulation. MOR expression was restricted to subsets of enteric neurons. In the stomach MOR was mainly localized to nitrergic neurons (∼88%), with some overlap with neuropeptide Y (NPY) and no expression by cholinergic neurons. These neurons are likely to have inhibitory motor and secretomotor functions. MOR was restricted to noncholinergic secretomotor neurons (VIP-positive) of the ileum and distal colon submucosal plexus. MOR was mainly detected in nitrergic neurons of the colon (nitric oxide synthase positive, 87%), with some overlap with choline acetyltransferase (ChAT). No expression of MOR by intrinsic sensory neurons was detected. [d-Ala(2), MePhe(4), Gly(ol)(5)]enkephalin (DAMGO), morphiceptin, and loperamide induced MOR endocytosis in myenteric neurons. After stimulation with DAMGO and morphiceptin, MOR recycled, whereas MOR was retained within endosomes following loperamide treatment. Herkinorin or the δ-opioid receptor agonist [d-Ala(2), d-Leu(5)]enkephalin (DADLE) did not evoke MOR endocytosis. In summary, we have identified the neurochemical coding of MOR-positive enteric neurons and have demonstrated differential trafficking of MOR in these neurons in response to established and putative MOR agonists.

Synthetic and Receptor Signaling Explorations of the Mitragyna Alkaloids: Mitragynine as an Atypical Molecular Framework for Opioid Receptor Modulators

Mu-opioid receptor agonists represent mainstays of pain management. However, the therapeutic use of these agents is associated with serious side effects, including potentially lethal respiratory depression. Accordingly, there is a longstanding interest in the development of new opioid analgesics with improved therapeutic profiles. The alkaloids of the Southeast Asian plant Mitragyna speciosa, represented by the prototypical member mitragynine, are an unusual class of opioid receptor modulators with distinct pharmacological properties. Here we describe the first receptor-level functional characterization of mitragynine and related natural alkaloids at the human mu-, kappa-, and delta-opioid receptors. These results show that mitragynine and the oxidized analogue 7-hydroxymitragynine, are partial agonists of the human mu-opioid receptor and competitive antagonists at the kappa- and delta-opioid receptors. We also show that mitragynine and 7-hydroxymitragynine are G-protein-biased agonists of the mu-opioid receptor, which do not recruit β-arrestin following receptor activation. Therefore, the Mitragyna alkaloid scaffold represents a novel framework for the development of functionally biased opioid modulators, which may exhibit improved therapeutic profiles. Also presented is an enantioselective total synthesis of both (-)-mitragynine and its unnatural enantiomer, (+)-mitragynine, employing a proline-catalyzed Mannich-Michael reaction sequence as the key transformation. Pharmacological evaluation of (+)-mitragynine revealed its much weaker opioid activity. Likewise, the intermediates and chemical transformations developed in the total synthesis allowed the elucidation of previously unexplored structure-activity relationships (SAR) within the Mitragyna scaffold. Molecular docking studies, in combination with the observed chemical SAR, suggest that Mitragyna alkaloids adopt a binding pose at the mu-opioid receptor that is distinct from that of classical opioids.

The hallucinogenic diterpene salvinorin A inhibits leukotriene synthesis in experimental models of inflammation

Leukotrienes (LTs) are lipid mediators derived from arachidonic acid (AA) involved in a number of autoimmune/inflammatory disorders including asthma, allergic rhinitis and cardiovascular diseases. Salvinorin A (SA), a diterpene isolated from the hallucinogenic plant Salvia divinorum, is a well-established analgesic compound, but its anti-inflammatory properties are under-researched and its effects on LT production is unknown to date. Here, we studied the possible effect of SA on LT production and verified its actions on experimental models of inflammation in which LTs play a prominent role. Peritoneal macrophages (PM) stimulated by calcium ionophore A23187 were chosen as in vitro system to evaluate the effect of SA on LT production. Zymosan-induced peritonitis in mice and carrageenan-induced pleurisy in rats were selected as LT-related models to evaluate the effect of SA on inflammation as well as on LT biosynthesis. SA inhibited, in a concentration-dependent manner, A23187-induced LTB4 biosynthesis in isolated PM. In zymosan-induced peritonitis, SA inhibited cell infiltration, myeloperoxidase activity, vascular permeability and LTC4 production in the peritoneal cavity without decreasing the production of prostaglandin E2. In carrageenan-induced pleurisy in rats, a more sophisticated model of acute inflammation related to LTs, SA significantly inhibited LTB4 production in the inflammatory exudates, along with reducing the phlogistic process in the lung. In conclusion, SA inhibited LT production and it was effective in experimental models of inflammation in which LTs play a pivotal role. SA might be considered as a lead compound for the development of drugs useful in LTs-related diseases.

Biased agonism: An emerging paradigm in GPCR drug discovery

G protein coupled receptors have historically been one of the most druggable classes of cellular proteins. The members of this large receptor gene family couple to primary effectors, G proteins, that have built in mechanisms for regeneration and amplification of signaling with each engagement of receptor and ligand, a kinetic event in itself. In recent years GPCRs, have been found to interact with arrestin proteins to initiate signal propagation in the absence of G protein interactions. This pinnacle observation has changed a previously held notion of the linear spectrum of GPCR efficacy and uncovered a new paradigm in GPCR research and drug discovery that relies on multidimensionality of GPCR signaling. Ligands were found that selectively confer activity in one pathway over another, and this phenomenon has been referred to as 'biased agonism' or 'functional selectivity'. While great strides in the understanding of this phenomenon have been made in recent years, two critical questions still dominate the field: How can we rationally design biased GPCR ligands, and ultimately, which physiological responses are due to G protein versus arrestin interactions? This review will discuss the current understanding of some of the key aspects of biased signaling that are related to these questions, including mechanistic insights in the nature of biased signaling and methods for measuring ligand bias, as well as relevant examples of drug discovery applications and medicinal chemistry strategies that highlight the challenges and opportunities in this rapidly evolving field.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

Anti-Inflammatory and Antihyperalgesic Activities of Ethanolic Extract and Fruticulin A from Salvia lachnostachys Leaves in Mice

The anti-inflammatory and analgesic effects of the ethanolic extract (SLEE) and fruticulin A from the leaves of Salvia lachnostachys were evaluated in mice, using experimental models of inflammation (paw oedema and pleurisy induced by carrageenan injection) and hyperalgesia (electronic Von Frey). Oral administration of SLEE (30, 100, and 300 mg/kg) and fruticulin A (0.3 and 3.0 mg/kg) decreased the total leucocytes number in pleural lavage, protein extravasation, and paw oedema. SLEE (100 mg/kg) and fruticulin A (3 mg/kg) also exhibited antihyperalgesic activity in carrageenan induced mechanical hyperalgesia. In addition, fruticulin A (3 mg/kg) prevented mechanical hyperalgesia, inhibiting TNF but not L-DOPA-induced mechanical hyperalgesia. In conclusion, SLEE and fruticulin A display anti-inflammatory and analgesic properties. Therefore, fruticulin A is at least partially responsible for the activity observed in the ethanolic extract of Salvia lachnostachys.

Efficacy and ligand bias at the μ-opioid receptor

In order to describe drug action at a GPCR, a full understanding of the pharmacological terms affinity, efficacy and potency is necessary. This is true whether comparing the ability of different agonists to produce a measurable response in a cell or tissue, or determining the relative ability of an agonist to activate a single receptor subtype and produce multiple responses. There is a great deal of interest in the μ-opioid receptor (MOP receptor) and the ligands that act at this GPCR not only because of the clinically important analgesic effects produced by MOP agonists but also because of their liability to induce adverse effects such as respiratory depression and dependence. Our understanding of the mechanisms underlying these effects, as well as the ability to develop new, more effective MOP receptor drugs, depends upon the accurate determination of the efficacy with which these ligands induce coupling of MOP receptors to downstream signalling events. In this review, which is written with the minimum of mathematical content, the basic meaning of terms including efficacy, intrinsic activity and intrinsic efficacy is discussed, along with their relevance to the field of MOP receptor pharmacology, and in particular in relation to biased agonism at this important GPCR.

Opioid receptor interacting proteins and the control of opioid signaling

Opioid receptors are seven-transmembrane domain receptors that couple to intracellular signaling molecules by activating heterotrimeric G proteins. However, the receptor and G protein do not function in isolation but their activities are modulated by several accessory and scaffolding proteins. Examples include arrestins, kinases, and regulators of G protein signaling proteins. Accessory proteins contribute to the observed potency and efficacy of agonists, but also to the direction of signaling and the phenomenon of biased agonism. This review will present current knowledge of such proteins and how they may provide targets for future drug design.

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.

β-arrestins: regulatory role and therapeutic potential in opioid and cannabinoid receptor-mediated analgesia

Pain is a complex disorder with neurochemical and psychological components contributing to the severity, the persistence, and the difficulty in adequately treating the condition. Opioid and cannabinoids are two classes of analgesics that have been used to treat pain for centuries and are arguably the oldest of "pharmacological" interventions used by man. Unfortunately, they also produce several adverse side effects that can complicate pain management. Opioids and cannabinoids act at G protein-coupled receptors (GPCRs), and much of their effects are mediated by the mu-opioid receptor (MOR) and cannabinoid CB1 receptor (CB1R), respectively. These receptors couple to intracellular second messengers and regulatory proteins to impart their biological effects. In this chapter, we review the role of the intracellular regulatory proteins, β-arrestins, in modulating MOR and CB1R and how they influence the analgesic and side-effect profiles of opioid and cannabinoid drugs in vivo. This review of the literature suggests that the development of opioid and cannabinoid agonists that bias MOR and CB1R toward G protein signaling cascades and away from β-arrestin interactions may provide a novel mechanism by which to produce analgesia with less severe adverse effects.

Functional selectivity of GPCR signaling in animals

At one time, G protein-coupled receptors were envisioned to simply relay either inhibitory or stimulatory binary signals through engaging particular G proteins. These receptors are now viewed as complex, multidimensional triggers of a variety of potential signaling cascades. This review will showcase current attempts to elucidate biased signaling and functional selectivity in tissues and organs as well as in the whole animal. In addition, it will emphasize the challenges that are inherent in attributing bias in a living system as well as offer opinions as to the manner in which these problems may be approached.

Studies toward bivalent κ opioids derived from salvinorin A: heteromethylation of the furan ring reduces affinity

The recent crystal structure of the κ-opioid receptor (κ-OR) revealed, unexpectedly, that the antagonist JDTic is a bivalent ligand: in addition to the orthosteric pocket occupied by morphinans, JDTic also occupies a distinct (allotopic) pocket. Mutagenesis data suggest that salvinorin A (1) also binds to this allotopic pocket, adjacent to the aspartate residue that anchors the basic nitrogen atom of classical opiates (Asp138). It has been suggested that an H-bond donor appended to 1 might interact with Asp138, increasing affinity. Such a bivalent ligand might also possess altered functional selectivity. Based on modeling and known N-furanylmethyl opioid antagonists, we appended H-bond donors to the furan ring of 1. (Dimethylamino)methyl groups at C-15 or C-16 abolished affinity for κ-OR. Hydroxymethylation at C-16 was tolerated, but 15,16-bis-hydroxymethylation was not. Since allosteric modulators may go undetected in binding assays, we also tested these and other low-affinity derivatives of 1 for allosteric modulation of dynorphin A in the [³⁵S]GTPγS assay. No modulation was detected. As an alternative attachment point for bivalent derivatives, we prepared the 2-(hydroxyethoxy)methyl ether, which retained high affinity for κ-OR. We discuss alternative design strategies for linked, fused or merged bivalent derivatives of 1.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Neoclerodanes as atypical opioid receptor ligands

The neoclerodane diterpene salvinorin A is the major active component of the hallucinogenic mint plant Salvia divinorum Epling and Játiva (Lamiaceae). Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of opioid receptors, the site of action of morphine and related analogues, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable because (1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist and (2) its effects are not mediated through the previously investigated targets of psychotomimetics. This Perspective outlines our research program, illustrating a new direction to the development of tools to further elucidate the biological mechanisms of drug tolerance and dependence. The information gained from these efforts is expected to facilitate the design of novel agents to treat pain, drug abuse, and other central nervous system disorders.

Ligand-directed signalling within the opioid receptor family

The classic model of GPCR activation proposed that all agonists induce the same active receptor conformation. However, research over the last decade has shown that GPCRs exist in multiple conformations, and that agonists can stabilize different active states. The distinct receptor conformations induced by ligands result in distinct receptor-effector complexes, which produce varying levels of activation or inhibition of subsequent signalling cascades. This concept, referred to as ligand-directed signalling or biased agonism has important biological and therapeutic implications. Opioid receptors are G(i/o) GPCRs and regulate a number of important physiological functions, including pain, reward, mood, stress, gastrointestinal transport and respiration. A number of in vitro studies have shown biased agonism at the three opioid receptors (µ, δ and κ); however, in vivo consequences of this phenomenon have only recently been demonstrated. For the µ and δ opioid receptors, the majority of reported ligand selective behavioural effects are observed as differential adaptations to repeated drug administration. In terms of the κ opioid receptor, clear links between ligand-selective signalling events and specific in vivo responses have been recently characterized. Drugs for all three receptors are either already used or are being developed for clinical applications. There is clearly a need to better characterize the specific events that occur following agonist stimulation and how these relate to in vivo responses. This understanding could eventually lead to the development of tailor-made pharmacotherapies where advantageous drug effects can be selectively targeted over adverse effects.

Herkinorin dilates cerebral vessels via kappa opioid receptor and cyclic adenosine monophosphate (cAMP) in a piglet model

Since herkinorin is the first non-opioid mu agonist derived from salvinorin A that has the ability to induce cerebral vascular dilatation, we hypothesized that herkinorin could have similar vascular dilatation effect via the mu and kappa opioid receptors and the cAMP pathway. The binding affinities of herkinorin to kappa and mu opioid receptors were determined by in-vitro competition binding assays. The cerebral arteries were monitored in piglets equipped with a closed cranial window and the artery responses were recorded before and every 30s after injection of artificial cerebrospinal fluid (CSF) in the presence or absence of the investigated drugs: herkinorion, norbinaltorphimine (NTP), a kappa opioid receptor antagonist, β-funaltrexamine (β-FNA), a mu opioid receptor antagonist, or Rp-8-Br-cAMPS (Rp-cAMPS), an inhibitor of protein kinase A (PKA). CSF samples were collected before and 10 min after herkinorin and NTP administration for the measurement of cAMP levels. Data were analyzed by repeated-measures analysis of variance. Our results show that herkinorin binds to both kappa and mu opioid receptors. Its vasodilation effect is totally abolished by NTP, but is not affected by β-FNA. The levels of cAMP in the CSF elevate after herkinorin administration, but are abolished with NTP administration. The cerebral vasodilative effect of herkinorin is also blunted by Rp-cAMPS. In conclusion, as a non-opioid kappa and mu opioid receptor agonist, herkinorin exhibits cerebral vascular dilatation effect. The dilatation is mediated though the kappa opioid receptor rather than the mu opioid receptor. cAMP signaling also plays an important role in this process.