Endogenous opiates and behavior: 2022.

This paper is the forty-fifth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2022 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Characterization of Pyrrolidinyl-hexahydro-pyranopiperazines as a Novel Kappa Opioid Receptor Agonist Scaffold.

The kappa agonist structure-activity relationship around the novel, pyrrolidinyl substituted pyranopiperazine scaffold was developed. More specifically, the dichloroPhenylAcetamide-Pyrrolidinyl-PyranoPiperazine (PAPPP) core A was the focus of our work. The modulation of kappa receptor potency/G-protein activation and arrestin recruitment with respect to changes of the piperazine R group in A was demonstrated. Reduced ß2-arrestin recruitment and differential G-protein bias were observed for select analogues. To better understand the subtlety in receptor signaling, analogues were profiled as the resolved enantiomers. To determine in vivo target engagement, a subset of compounds was tested in mice for stimulation of serum prolactin, a neuroendocrine biomarker of KOR-agonist effects. Additional in vivo characterization included measurement of potential unwanted effects of kappa receptor activation such as sedation. These studies demonstrate a novel kappa receptor agonist scaffold with potential for G-protein signaling bias to probe in vivo pharmacology.

Profile of a short-acting κ-antagonist, LY2795050, on self-grooming behaviors, forced swim test and locomotor activity: sex comparison in mice.

BACKGROUND: Novel short-acting κ(kappa)-opioid receptor selective antagonists are translational tools to examine the impact of the κ-receptor/dynorphin system in assays related to central nervous system dysfunction (e.g., substance use disorders, anhedonia and depression). The effects of such compounds have been compared in males and females under very limited conditions. AIMS: The goal of this study was to examine potential sex differences in the effects of a κ-agonist and a short-acting κ-antagonist in an ethologically relevant test of anhedonia, the "splash test" of self-grooming, and also in the forced swim test and in locomotor activity. METHODS: We examined the dose-dependence of grooming deficits caused by the κ-agonist U50,488 (0.1-3.2 mg/kg intraperitoneal (i.p.)) in gonadally intact adult male and female C57BL/6J mice. We then compared the effects of the short-acting κ-antagonist LY2795050 ((3-chloro-4-(4-(((2S)-2-pyridin-3-ylpyrrolidin-1-yl)methyl) phenoxy)benzamide)); 0.032-0.1 mg/kg i.p.) in blocking grooming deficits caused by U50,488 (3.2 mg/kg). The effects of LY2795050 were also studied in the forced swim test (FST). The effects of LY2795050 in blocking the locomotor depressant effects of U50,488 (10 mg/kg) were also studied. RESULTS: U50,488 produced dose-dependent grooming deficits in male and female mice, and LY2795050 prevented these effects. In contrast, LY2795050 decreased immobility in the FST in males at a dose of 0.1 mg/kg, but not in females, up to a dose of 0.32 mg/kg. Also, LY2795050 (0.32 mg/kg) prevented and also reversed the locomotor-depressant effects of U50,488 (10 mg/kg), in males and females. CONCLUSIONS: This study further implicates the κ-receptor system in ethologically relevant aspects of anhedonia, and confirms sexual dimorphism in some behavioral effects of novel κ-antagonists.

Quantification of kappa opioid receptor ligand potency, efficacy and desensitization using a real-time membrane potential assay.

We explored the utility of the real-time FLIPR Membrane Potential (FMP) assay as a method to assess kappa opioid receptor (KOR)-induced hyperpolarization. The FMP Blue dye was used to measure fluorescent signals reflecting changes in membrane potential in KOR expressing CHO (CHO-KOR) cells. Treatment of CHO-KOR cells with kappa agonists U50,488 or dynorphin [Dyn (1-13)NH2] produced rapid and concentration-dependent decreases in FMP Blue fluorescence reflecting membrane hyperpolarization. Both the nonselective opioid antagonist naloxone and the κ-selective antagonists nor-binaltorphimine (nor-BNI) and zyklophin produced rightward shifts in the U50,488 concentration-response curves, consistent with competitive antagonism of the KOR mediated response. The decrease in fluorescent emission produced by U50,488 was blocked by overnight pertussis toxin pretreatment, indicating the requirement for PTX-sensitive G proteins in the KOR mediated response. We directly compared the potency of U50,488 and Dyn (1-13)NH2 in the FMP and [35S]GTPγS binding assays, and found that both were approximately 10 times more potent in the cellular fluorescence assay. The maximum responses of both U50,488 and Dyn (1-13)NH2 declined following repeated additions, reflecting receptor desensitization. We assessed the efficacy and potency of structurally distinct KOR small molecule and peptide ligands. The FMP assay reliably detected both partial agonists and stereoselectivity. Using KOR-selective peptides with varying efficacies, we found that the FMP assay allowed high throughput quantification of peptide efficacy. These data demonstrate that the FMP assay is a sensitive method for assessing κ-opioid receptor induced hyperpolarization, and represents a useful approach for quantification of potency, efficacy and desensitization of KOR ligands.

Multifunctional Enkephalin Analogs with a New Biological Profile: MOR/DOR Agonism and KOR Antagonism.

In our previous studies, we developed a series of mixed MOR/DOR agonists that are enkephalin-like tetrapeptide analogs with an N-phenyl-N-piperidin-4-ylpropionamide (Ppp) moiety at the C-terminus. Further SAR study on the analogs, initiated by the findings from off-target screening, resulted in the discovery of LYS744 (6, Dmt-DNle-Gly-Phe(p-Cl)-Ppp), a multifunctional ligand with MOR/DOR agonist and KOR antagonist activity (GTPγS assay: IC50 = 52 nM, Imax = 122% cf. IC50 = 59 nM, Imax = 100% for naloxone) with nanomolar range of binding affinity (Ki = 1.3 nM cf. Ki = 2.4 nM for salvinorin A). Based on its unique biological profile, 6 is considered to possess high therapeutic potential for the treatment of chronic pain by modulating pathological KOR activation while retaining analgesic efficacy attributed to its MOR/DOR agonist activity.

Implications of Chronic Opioid Therapy on Perioperative Complications and Long-Term Surgical Recovery.

With chronic opioid use becoming an increasingly common occurrenceamong the general population, perioperative specialties must adapt to the physiologic changes caused by long-term opioids. However, data on the clinicalanesthetics implications of long-term opioid use is scarce. This review intends to survey the literature addressing the molecular mechanisms of long-term opioid use as well as their interaction with various organ systems.

Prodynorphin and kappa opioid receptor mRNA expression in the brain relates to social status and behavior in male European starlings.

Numerous animal species display behavioral changes in response to changes in social status or territory possession. For example, in male European starlings only males that acquire nesting sites display high rates of sexual and agonistic behavior. Past studies show that mu and delta opioid receptors regulate behaviors associated with social ascension or defeat. Opioids also act at kappa receptors, with dynorphin binding with the highest affinity; however, the role of these opioids in social behavior has not been well studied. We observed flocks of male starlings during the breeding season and ran quantitative real-time polymerase chain reaction (qPCR) to measure expression of kappa opioid receptors (OPRK1) and prodynorphin (PDYN) in brain regions involved in social behavior and motivation (ventral tegmental area [VTA], medial preoptic nucleus [mPOA]) and vocal behavior (Area X). Males with nesting territories displayed more sexual/agonistic behavior than males without nesting territories. They also had lower OPRK1 expression in VTA and mPOA. OPRK1 expression in VTA correlated negatively with sexual/agonistic behaviors, consistent with past studies showing kappa receptors in VTA to inhibit sociosexual behaviors. PDYN in mPOA correlated negatively with a measure of nesting behavior that may also reflect sexual motivation. PDYN in Area X related positively to song. Distinct patterns of OPRK1 and PDYN expression in VTA, mPOA, and Area X related to gonad volume, suggesting that breeding condition may modify (or be modified by) OPRK1 and PDYN expression. Studies are now needed to further characterize the role of OPRK1 and PDYN in status-appropriate social behaviors.

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice.

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

Emerging treatments in neurogastroenterology: eluxadoline - a new therapeutic option for diarrhea-predominant IBS.

BACKGROUND: Irritable bowel syndrome (IBS) is the most common functional gastrointestinal disorder worldwide. The global prevalence of IBS is estimated to be as high as 15%. For many patients, IBS is a chronic disorder which can significantly reduce quality of life. Just as important as the effects on any one individual, IBS also places a significant impact on the population as a whole with its negative effects on the health care system. Irritable bowel syndrome is categorized into one of three main categories: IBS with diarrhea, IBS with constipation, and IBS with mixed bowel habits. Patients with diarrhea-predominant IBS (IBS-D) comprise a substantial proportion of the overall IBS population. A number of therapeutic options exist to treat the symptoms of abdominal pain, bloating, diarrhea, and fecal urgency, including non-pharmacologic therapies such as dietary changes and probiotics, or pharmacologic therapies such as loperamide and alosetron. However, many patients have persistent symptoms despite these therapies. This unmet need led to the development of eluxadoline, a mu-opioid receptor agonist/delta-opioid receptor antagonist/kappa-receptor agonist. Approved by the FDA in May 2015, this medication shows promise in the treatment of diarrhea-predominant IBS for both men and women. PURPOSE: This monograph will briefly review the impact of IBS, discuss current treatments for IBS-D, and then focus on the pharmacology, clinical efficacy and safety of eluxadoline. Potential mechanisms related to rare events of acute pancreatitis or elevated liver tests will be discussed.

Buprenorphine-Clinically useful but often misunderstood.

Background There are a number of false myths about buprenorphine based on unconfirmed animal data, even from isolated animal organs, and early clinical research. These myths came into textbooks on pharmacology and pain about 30 years ago and have been difficult to eradicate. Animal models of pain and pain relief are notoriously unreliable as predictors of human clinical effects. The fact is that in clinical practice there is NO bell-shaped dose-response curve, there is NO plateau on the dose-response curve, and there is NO antagonist effect from buprenorphine on other mu-opioid agonists. Methods This narrative, topical review of relevant research publications evaluates new knowledge on the pharmacodynamics and pharmacokinetics of buprenorphine of importance in clinical practice. Results Buprenorphine is a potent opioid analgesic acting on all four opioid receptors: it is an agonist on the mu-, the delta, and the ORL-1 receptors. It is an antagonist at the kappa-receptor. Buprenorphine has a number of active metabolites with different effects on the four opioid receptors; all except the norbup-3-glu are analgesic. Buprenorphine itself is not a respiratory depressant or sedative, but some of its active metabolites are. Buprenorphine and its active metabolites are not excreted by the kidney. Therefore buprenorphine may be used in patients with advanced renal failure. Buprenorphine has a slow onset and a long offset. These properties are advantageous, except sometimes when treating severe acute pain. Its agonist effect on the ORL-1 receptor reduces reward-effects and slows the development of tolerance to the analgesic effects. Buprenorphine inhibits voltage-gated sodium-channels and enhances and prolongs peripheral nerve blocks. Its ORL-1 -effect at the spinal cord may do the same. Buprenorphine is well suited for treatment of chronic pain, especially chronic neuropathic pain and cancer pain. The beneficial effects as a co-medication during treatment of the opioid-abuse disease are due to its slow onset (less "kick-effect"). Its prolonged offset-time reduces the likelihood of acute withdrawal problems and reduces the "craving" of opioids. Adverse effects Buprenorphine, being a mu-agonist, may induce or maintain opioid addiction. Illegally obtained high-dose transmucosal buprenorphine, intended for treatment of addiction, is dissolved and injected by opioid abusers. This is an increasing problem in some countries. Conclusions Buprenorphine's unusual pharmacodynamics and pharmacokinetics make it an ideal opioid for treatment of most chronic pain conditions where opioid therapy is indicated. Implications Buprenorphine is a well studied and often misunderstood analgesic opioid drug. The evidence base predicts that it will be an increasingly important alternative for treatment of chronic pain conditions caused by cancer and non-cancer diseases. It will continue to be an attractive alternative to methadone for opioid abuse rehabilitation.

Kappa opioid receptor antagonist and N-methyl-D-aspartate receptor antagonist affect dynorphin-induced spinal cord electrophysiologic impairment.

The latencies of motor- and somatosensory-evoked potentials were prolonged to different degrees, and wave amplitude was obviously decreased, after injection of dynorphin into the rat subarachnoid cavity. The wave amplitude and latencies of motor- and somatosensory-evoked potentials were significantly recovered at 7 and 14 days after combined injection of dynorphin and either the kappa opioid receptor antagonist nor-binaltorphimine or the N-methyl-D-aspartate receptor antagonist MK-801. The wave amplitude and latency were similar in rats after combined injection of dynorphin and nor-binaltorphimine or MK-801. These results suggest that intrathecal injection of dynorphin causes damage to spinal cord function. Prevention of N-methyl-D-aspartate receptor or kappa receptor activation lessened the injury to spinal cord function induced by dynorphin.