Contribution of mu and delta opioid receptors to the pharmacological profile of kappa opioid receptor subtypes

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.

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.

A Review of the Therapeutic Potential of Recently Developed G Protein-Biased Kappa Agonists

Between 2000 and 2005 several studies revealed that morphine is more potent and exhibits fewer side effects in beta-arrestin 2 knockout mice. These findings spurred efforts to develop opioids that signal primarily via G protein activation and do not, or only very weakly, recruit beta-arrestin. Development of such molecules targeting the mu opioid receptor initially outpaced those targeting the kappa, delta and nociceptin opioid receptors, with the G protein-biased mu opioid agonist oliceridine/TRV130 having completed phase III clinical trials with improved therapeutic window to treat moderate-to-severe acute pain. Recently however, there has been a sharp increase in the development of novel G protein-biased kappa agonists. It is hypothesized that G protein-biased kappa agonists can reduce pain and itch, but exhibit fewer side effects, such as anhedonia and psychosis, that have thus far limited the clinical development of unbiased kappa opioid agonists. Here we summarize recently discovered G protein-biased kappa agonists, comparing structures, degree of signal bias and preclinical effects. We specifically reviewed nalfurafine, 22-thiocyanatosalvinorin A (RB-64), mesyl-salvinorin B, 2-(4-(furan-2-ylmethyl)-5-((4-methyl-3-(trifluoromethyl)benzyl)thio)-4H-1,2,4-triazol-3-yl)pyridine (triazole 1.1), 3-(2-((cyclopropylmethyl)(phenethyl)amino)ethyl)phenol (HS666), N-n-butyl-N-phenylethyl-N-3-hydroxyphenylethyl-amine (compound 5/BPHA), 6-guanidinonaltrindole (6′GNTI), and collybolide. These agonists encompass a variety of chemical scaffolds and range in both their potency and efficacy in terms of G protein signaling and beta-arrestin recruitment. Thus unsurprisingly, the behavioral responses reported for these agonists are not uniform. Yet, it is our conclusion that the kappa opioid field will benefit tremendously from future studies that compare several biased agonists and correlate the degree of signaling bias to a particular pharmacological response.

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).

Role of endogenous opioid peptides in the infarct size-limiting effect of adaptation to chronic continuous hypoxia

AIMS

The objective of this study was to examine the involvement of endogenous opioid peptides and opioid receptor (OR) subtypes in the cardioprotective effect of adaptation to chronic hypoxia in rats.

MAIN METHODS

Rats were exposed to continuous normobaric hypoxia (CNH; 12% oxygen) for 3 weeks. Myocardial ischemia was induced by 20-min coronary artery occlusion followed by 3-h reperfusion in anesthetized open-chest animals. Various OR antagonists were administered to rats prior to ischemia. The size of myocardial infarction and the incidence of ischemic ventricular arrhythmias were assessed. Myocardial and plasma concentrations of opioid peptides (met-enkephalin, β-endorphin, and endomorphins) were determined.

KEY FINDINGS

Adaptation to CNH significantly increased myocardial and plasma concentrations of opioids, potentiated their further elevation by ischemia/reperfusion, and reduced myocardial infarct size, but it did not affect the incidence of ischemic arrhythmias. The infarct size-limiting effect of CNH was abolished by OR antagonists naltrexone (non-selective), naloxone methiodide (non-selective peripherally acting), TIPP[ψ] (δ-OR), naltriben (δ2-OR), or CTAP (μ-OR), while BNTX (δ1-OR) and nor-binaltorphimine (κ-OR) had no effect.

SIGNIFICANCE

The results suggest that the infarct size-limiting effect afforded by adaptation to CNH is mediated by activation of peripheral δ2- and μ-ORs by elevated levels of endogenous opioid peptides.