The distribution of methionine-enkephalin and leucine-enkephalin in the brain and peripheral tissues

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Functional Characterization and Molecular Marker Development of the Proenkephalin as Biomarker of Food Addiction in Food Habit Domestication of Mandarin Fish (Siniperca Chuatsi)

Proenkephalin (PENK), as the precursor of endogenous opioid enkephalin (ENK), is widely present in the nervous system and plays an important role in animal food addiction and rewarding behavior. In our study, we intend to study the functional characterization and molecular marker development of the penk gene related to food habit domestication of mandarin fish. We found that the penk gene of mandarin fish had three types of endogenous opioid peptide sequences. Compared with other tissues, penk mRNA was highly expressed in the whole brain. Intracerebroventricular (ICV) injection of lysine or methionine significantly increased the expression of penk mRNA. The expression of penk mRNA in the brain of mandarin fish that could be easily domesticated from eating live prey fish to artificial diets was significantly higher than those that could not. After feeding with high-carbohydrate artificial diets, the expression of penk mRNA showed no significant difference between mandarin fish with hypophagia and those that still ate normally. A total of four single nucleotide polymorphisms (SNP) loci related to easy domestication toward eating artificial diets were screened from the mandarin fish population. Additionally, the TT genotype at one of the loci was significantly correlated with the food habit domestication of mandarin fish.

Plasma Pro-Enkephalin A and Ischemic Stroke Risk: The Reasons for Geographic and Racial Differences in Stroke Cohort

OBJECTIVES

The opioid neuropeptide pro-enkephalin A (PENK-A) may be a circulating marker of cardiovascular risk, with prior findings relevant to heart failure, kidney disease, and vascular dementia. Despite these findings, the association of PENK-A with ischemic stroke is unknown, so we examined this association in a prospective cohort study and analyzed differences by race and sex.

MATERIALS AND METHODS

The REasons for Geographic and Racial Differences in Stroke study (REGARDS) is a prospective cohort study of 30,239 Black and White adults. Plasma PENK-A was measured in 473 participants that developed first-time ischemic stroke over 5.9 years and 899 randomly selected participants. Cox models adjusted for demographics and stroke risk factors were used to calculate hazard ratios (HRs) of stroke by baseline PENK-A.

RESULTS

PENK-A was higher with increasing age, female sex, White race, lower body mass index, and antihypertensive medication use. Each SD higher increment of PENK-A was associated with an adjusted HR of 1.20 (95% CI 1.01-1.42) for stroke, with minimal confounding by stroke risk factors. Spline plots suggested a U-shaped relationship, particularly in White men, with an adjusted HR 3.88 (95% CI 1.94-7.77) for the 95^th versus 50^th percentile of PENK-A in White men.

CONCLUSIONS

Higher baseline plasma PENK-A was independently associated with future stroke risk in REGARDS. This association was most apparent among White men. There was little confounding by established stroke risk factors, suggesting a possible causal role in stroke etiology. Further research is needed to understand the role of endogenous opioids in stroke pathogenesis.

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Presynaptic Inhibitory Effects of Acetylcholine in the Hippocampus: A 40-Year Evolution of a Serendipitous Finding

Cholinergic regulation of hippocampal circuit activity has been an active area of neurophysiological research for decades. The prominent cholinergic innervation of intrinsic hippocampal circuitry, potent effects of cholinomimetic drugs, and behavioral responses to cholinergic modulation of hippocampal circuitry have driven investigators to discover diverse cellular actions of acetylcholine in distinct sites within hippocampal circuitry. Further research has illuminated how these actions organize circuit activity to optimize encoding of new information, promote consolidation, and coordinate this with recall of prior memories. The development of the hippocampal slice preparation was a major advance that accelerated knowledge of how hippocampal circuits functioned and how acetylcholine modulated these circuits. Using this preparation in the early 1980s, we made a serendipitous finding of a novel presynaptic inhibitory effect of acetylcholine on Schaffer collaterals, the projections from CA3 pyramidal neurons to dendrites of CA1 pyramidal cells. We characterized this effect at cellular and pharmacological levels, published the findings in the first volume of the Journal of Neuroscience, and proceeded to pursue other scientific directions. We were surprised and thrilled to see that, nearly 40 years later, the paper is still being cited and downloaded because the data became an integral piece of the foundation of the science of cholinergic regulation of hippocampal function in learning and memory. This Progressions article is a story of how single laboratory findings evolve through time to be confirmed, challenged, and reinterpreted by other laboratories to eventually become part of the basis of fundamental concepts related to important brain functions.

Electroosmotic Perfusion-Microdialysis Probe Created by Direct Laser Writing for Quantitative Assessment of Leucine Enkephalin Hydrolysis by Insulin-Regulated Aminopeptidase in Vivo

There are many processes that actively alter the concentrations of solutes in the extracellular space. Enzymatic reactions, either by soluble enzymes or membrane-bound ectoenzymes, and uptake or clearance are two such processes. Investigations of ectoenzymatic reactions in vivo is challenging, particularly in the brain. Studies using microdialysis have revealed some qualitative information about what enzymes may be present, but microdialysis is a sampling technique so it is not designed to control conditions such as a substrate concentration outside the probe. Micropush-pull perfusion has been used to determine which nitric oxide synthase enzymes are active in discrete regions of the rat retina. Ectopeptidases are a particularly important class of ectoenzymes. As far as it is known, the extracellular activity of active peptides in the brain is controlled by ectopeptidases. To understand ectopeptidase activity, we developed a physical probe and an accompanying method. The probe has a two-channel source that supplies substrate or substrate plus inhibitor using electroosmotic perfusion (EOP). It also has a microdialysis probe to collect products and unreacted substrate. The method provides quantitative estimates of substrate-to-product conversion and the influence of inhibitors on this process. The quantitative estimates are made possible by including a d-amino acid-containing peptide analog of the substrate in the substrate-containing solution infused. Quantitative analysis of substrate, substrate analog, and products is carried out by quantitative, online capillary liquid chromatography-tandem mass spectrometry. The electroosmotic perfusion-microdialysis probe and associated method were used to determine the effect of the selective inhibitor HFI-419 on insulin-regulated aminopeptidase (EC 3.4.11.3) in the rat neocortex.

Five Decades of Research on Opioid Peptides: Current Knowledge and Unanswered Questions

In the mid-1970s, an intense race to identify endogenous substances that activated the same receptors as opiates resulted in the identification of the first endogenous opioid peptides. Since then, >20 peptides with opioid receptor activity have been discovered, all of which are generated from three precursors, proenkephalin, prodynorphin, and proopiomelanocortin, by sequential proteolytic processing by prohormone convertases and carboxypeptidase E. Each of these peptides binds to all three of the opioid receptor types (μ, δ, or κ), albeit with differing affinities. Peptides derived from proenkephalin and prodynorphin are broadly distributed in the brain, and mRNA encoding all three precursors are highly expressed in some peripheral tissues. Various approaches have been used to explore the functions of the opioid peptides in specific behaviors and brain circuits. These methods include directly administering the peptides ex vivo (i.e., to excised tissue) or in vivo (in animals), using antagonists of opioid receptors to infer endogenous peptide activity, and genetic knockout of opioid peptide precursors. Collectively, these studies add to our current understanding of the function of endogenous opioids, especially when similar results are found using different approaches. We briefly review the history of identification of opioid peptides, highlight the major findings, address several myths that are widely accepted but not supported by recent data, and discuss unanswered questions and future directions for research. SIGNIFICANCE STATEMENT: Activation of the opioid receptors by opiates and synthetic drugs leads to central and peripheral biological effects, including analgesia and respiratory depression, but these may not be the primary functions of the endogenous opioid peptides. Instead, the opioid peptides play complex and overlapping roles in a variety of systems, including reward pathways, and an important direction for research is the delineation of the role of individual peptides.

A Survey of Molecular Imaging of Opioid Receptors

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as μ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-¹¹C]morphine, -codeine and -heroin did not show significant binding in vivo. [¹¹C]Diprenorphine ([¹¹C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the μ-preferring agonist [¹¹C]carfentanil ([¹¹C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [¹¹C]DPN or [¹¹C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate μ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of μORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [¹¹C]GR103545 is validated for studies of κORs. Structures such as [¹¹C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

Opioid-Induced Foregut Dysfunction

The impact of opioid use on the lower gastrointestinal tract is well described, but recent opioid crisis has caused increased awareness of the detrimental effects of these drugs on esophageal and gastroduodenal motility. Opioid use has been associated with increased incidence of spastic esophageal motility disorders and gastroduodenal dysfunction. Opioid receptors are present with high abundance in the myenteric and submucosal plexus of the enteric nervous system. Activation of these receptors leads to suppressed excitability of the inhibitory musculomotor neurons and unchecked tonic contraction of the autogenic musculature (such as the lower esophageal sphincter and the pylorus).

Plant- and Nutraceutical-based Approach for the Management of Diabetes and its Neurological Complications: A Narrative Review

Diabetes is an important metabolic disease affecting many organs and systems in the body. The nervous system is one of the body systems affected by diabetes and neuropathic complications are troublesome in diabetic patients with many consequences. As diabetes has deleterious influences almost on bodily systems, an integrative approach seems to be necessary accepting the body as a whole and integrating body systems with lifestyle and living environment. Like some traditional health systems such as Ayurveda, integrative approach includes additional modalities to overcome both diabetes and diabetic complications. In general, these modalities consist of nutraceuticals and plant products. Prebiotics and probiotics are two types of nutraceuticals having active ingredients, such as antioxidants, nutrient factors, microorganisms, etc. Many plants are indicated for the cure of diabetes. All of these may be employed in the prevention and in the non-pharmacological management of mildto- moderate diabetes. Severe diabetes should require appropriate drug selection. Being complementary, prebiotics, probiotics, plants and exercise may be additive for the drug therapy of diabetes. Similarly, there are complementary approaches to prevent and cure neurological and/or behavioral manifestations of diabetes, which may be included in therapy and prevention plans. A scheme is given for the prevention and therapy of comorbid depression, which is one of the most common behavioral complications of diabetes. Within this scheme, the main criterion for the selection of modalities is the severity of diseases, so that personalized management may be developed for diabetic patients using prebiotics and probiotics in their diets, plants and drugs avoiding possible interactions.

The Mechanisms Involved in Morphine Addiction: An Overview

Opioid use disorder is classified as a chronic recurrent disease of the central nervous system (CNS) which leads to personality disorders, co-morbidities and premature death. It develops as a result of long-term administration of various abused substances, along with morphine. The pharmacological action of morphine is associated with its stimulation of opioid receptors. Opioid receptors are a group of G protein-coupled receptors and activation of these receptors by ligands induces significant molecular changes inside the cell, such as an inhibition of adenylate cyclase activity, activation of potassium channels and reductions of calcium conductance. Recent data indicate that other signalling pathways also may be involved in morphine activity. Among these are phospholipase C, mitogen-activated kinases (MAP kinases) or β-arrestin. The present review focuses on major mechanisms which currently are considered as essential in morphine activity and dependence and may be important for further studies.

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

Untangling the complexity of opioid receptor function

Mu opioid receptor agonists are among the most powerful analgesic medications but also among the most addictive. The current opioid crisis has energized a quest to develop opioid analgesics that are devoid of untoward effects. Since their discovery in the 1970's, there have been major advances in our understanding of the endogenous opioid systems that these drugs target. Yet many questions remain and the development of non-addictive opioid analgesics has not been achieved. However, access to new molecular, genetic and computational tools have begun to elucidate the structural dynamics of opioid receptors, the scaffolding that links them to intracellular signaling cascades, their cellular trafficking and the distinct ways that various opioid drugs modify them. This mini-review highlights some of the chemical and pharmacological findings and new perspectives that have arisen from studies using these tools. They reveal multiple layers of complexity of opioid receptor function, including a spatiotemporal specificity in opioid receptor-induced cellular signaling, ligand-directed biased signaling, allosteric modulation of ligand interactions, heterodimerization of different opioid receptors, and the existence of slice variants with different ligand specificity. By untangling these layers, basic research into the chemistry and pharmacology of opioid receptors is guiding the way towards deciphering the mysteries of tolerance and physical dependence that have plagued the field and is providing a platform for the development of more effective and safer opioids.

Acupuncture and Neural Mechanism in the Management of Low Back Pain-An Update

Within the last 10 years, the percentage of low back pain (LBP) prevalence increased by 18%. The management and high cost of LBP put a tremendous burden on the healthcare system. Many risk factors have been identified, such as lifestyle, trauma, degeneration, postural impairment, and occupational related factors; however, as high as 95% of the cases of LBP are non-specific. Currently, LBP is treated pharmacologically. Approximately 25 to 30% of the patients develop serious side effects, such as drowsiness and drug addiction. Spinal surgery often does not result in a massive improvement of pain relief. Therefore, complementary approaches are being integrated into the rehabilitation programs. These include chiropractic therapy, physiotherapy, massage, exercise, herbal medicine and acupuncture. Acupuncture for LBP is one of the most commonly used non-pharmacological pain-relieving techniques. This is due to its low adverse effects and cost-effectiveness. Currently, many randomized controlled trials and clinical research studies have produced promising results. In this article, the causes and incidence of LBP on global health care are reviewed. The importance of treatment by acupuncture is considered. The efforts to reveal the link between acupuncture points and anatomical features and the neurological mechanisms that lead to acupuncture-induced analgesic effect are reviewed.

On-Column Dimethylation with Capillary Liquid Chromatography-Tandem Mass Spectrometry for Online Determination of Neuropeptides in Rat Brain Microdialysate

We have developed a method for online collection and quantitation of neuropeptides in rat brain microdialysates using on-column dimethylation with capillary liquid chromatography-tandem mass spectrometry (cLC-MS2). This method addresses a number of the challenges of quantifying neuropeptides with cLC-MS. It is also a completely automated and robust method for the preparation of stable isotope labeled-peptide internal standards to correct for matrix effects and thus ensure accurate quantitation. Originally developed for tissue-derived proteomics samples ( Raijmakers et al. Mol. Cell. Proteomics 2008 , 7 , 1755 - 1762 ), the efficacy of on-column dimethylation for native peptides in microdialysate has not been demonstrated until now. We have modified the process to make it more amenable to the time scale of microdialysis sampling and to reduce the accumulation of nonvolatile contaminants on the column and, thus, loss of sensitivity. By decreasing labeling time, we have a temporal resolution of 1 h from sample loading to elution and our peptide detection limits are in the low pM range for 5 μL injections of microdialysate. We have demonstrated the effectiveness of this method by quantifying basal and potassium stimulated concentrations of the neuropeptides leu-enkephalin and met-enkephalin in the rat hippocampus. To our knowledge, this is the first report of quantitation of these peptides in the hippocampus using MS.

Effect of methylnaltrexone and naloxone on esophageal motor function in man

BACKGROUND

Endogenous opioids (EO) acting on μ-opiod receptors in central and enteric nervous system (ENS) control gastrointestinal motility but it is still unclear whether EO in ENS may control esophageal function in man, thus we will study the effects of methylnaltrexone (MNTX), a peripherally selective, and naloxone (NA), a non-selective μ-opiod receptor antagonist, on esophageal motility in healthy subjects.

METHODS

Fifteen HV (6 M; 34.1 ± 0.6 years; BMI: 22.1 ± 0.1 kg/m² ) underwent three esophageal high-resolution manometry impedance (HRiM) studies with 10 saline swallows administered every 30 minutes: drug was administered after 30 minutes (MNTX subcutaneously/NA or saline intravenously), a solid meal after 90 minutes; measurements continued for 120 minutes postprandially.

KEY RESULTS

Methylnaltrexone did not significantly decrease the upper esophageal sphincter (UES) percentage of relaxation preprandially (72.5 ± 5 vs 66.9 ± 4.6 and 73 ± 3.8%, ANOVA between placebo, MNTX and NA, P=NS) and postprandially (60 minutes: 68.2 ± 5.6 vs 61 ± 5.5 and 67.1 ± 5.6%; 120 minutes: 68 ± 5.9 vs 59.3 ± 5.2 and 67.7 ± 4.7%; ANOVA between placebo, MNTX and NA, P=NS). MNTX and NA did not significantly alter preprandial and postprandial LES resting pressures and integrated relaxation pressure (ANOVA between placebo, MNTX and NA, all P=NS). Peak front velocity and distal contractile integral were not altered pre- and postprandially by MNTX and NA (ANOVA between placebo, MNTX and NA, P=NS). Transient lower esophageal sphincter relaxations (TLESRs') number was not altered by MNTX and NA (ANOVA between placebo, MNTX and NA, all P=NS).

CONCLUSIONS AND INFERENCES

The peripheral selective and non-selective μ-opioid receptor antagonists MNTX and NA, respectively, do not alter TLESRs occurrence and esophageal peristalsis.

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

The Useage of Opioids and their Adverse Effects in Gastrointestinal Practice: A Review

Opium is one of the oldest herbal medicines currently used as an analgesic, sedative and antidiarrheal treatment. The effects of opium are principally mediated by the μ-, κ- and δ-opioid receptors. Opioid substances consist of all natural and synthetic alkaloids that are derived from opium. Most of their effects on gastrointestinal motility and secretion result from suppression of neural activity. Inhibition of gastric emptying, increase in sphincter tone, changes in motor patterns, and blockage of peristalsis result from opioid use. Common adverse effects of opioid administration include sedation, dizziness, nausea, vomiting, constipation, dependency and tolerance, and respiratory depression. The most common adverse effect of opioid use is constipation. Although stool softeners are frequently used to decrease opioid-induced bowel dysfunction, however they are not efficacious. Possibly, the use of specific opioid receptor antagonists is a more suitable approach. Opioid antagonists, both central and peripheral, could affect gastrointestinal function and visceromotor sensitivity, which suggests an important role for endogenous opioid peptides in the control of gastrointestinal physiology. Underlying diseases or medications known to influence the central nervous system (CNS) often accelerate the opioid's adverse effects. However, changing the opioid and/or route of administration could also decrease their adverse effects. Appropriate patient selection, patient education and discussion regarding potential adverse effects may assist physicians in maximizing the effectiveness of opioids, while reducing the number and severity of adverse effects.

Cardiorenal Effects of Kappa Opioid Peptides During Ontogeny

This review focuses on the physiological roles for kappa opioid receptors (KORs) in adult animals and humans, as well as in the developing newborn animal. Our recent findings have provided new information that under physiological conditions in conscious newborn animals, activation of KORs with the selective agonist, U-50488H, results in an aquaresis, as previously observed in adult animals and humans. In addition, we have shown in conscious lambs that KORs modulate systemic and renal haemodynamics as well as the arterial baroreflex control of heart rate, providing a previously unidentified role for KORs.

Effects and underlying mechanisms of human opiorphin on colonic motility and nociception in mice

In the present study, we investigated the effects of human opiorphin on colonic motility and nociception in mice. In in vitro bioassay, opiorphin (10(-6) to 10(-4)M) caused colonic contraction in a concentration-dependent manner, which was completely blocked by naloxone and partially attenuated by beta-funaltrexamine and naltrindole. Moreover, opiorphin (10(-4)M) significantly enhanced the contractile response induced by Met-enkephalin. The data suggested that the effect of opiorphin on colonic contraction may be due to the protection of enkephalins. In in vivo bioassay, intracerebroventricular (i.c.v.) administration of opiorphin (1.25-10 microg/kg) dose- and time-dependently induced potent analgesic effect (ED(50)=3.22 microg/kg). This effect was fully blocked by naloxone and significantly inhibited by co-injection (i.c.v.) with beta-funaltrexamine or naltrindole, but not by nor-binaltorphimine, indicating the involvement of both mu- and delta-opioid receptors in the analgesic response evoked by opiorphin. In addition, i.c.v. administration of 5 microg/kg opiorphin produced the comparative effect as 10 microg/kg morphine on the analgesia, suggesting that opiorphin displayed more potent analgesic effect than that induced by morphine.

Opioid receptors in the gastrointestinal tract

Opium is arguably one of the oldest herbal medicines, being used as analgesic, sedative and antidiarrheal drug for thousands of years. These effects mirror the actions of the endogenous opioid system and are mediated by the principal mu-, kappa- and delta-opioid receptors. In the gut, met-enkephalin, leu-enkephalin, beta-endorphin and dynorphin occur in both neurons and endocrine cells. When released, opioid peptides activate opioid receptors on the enteric circuitry controlling motility and secretion. As a result, inhibition of gastric emptying, increase in sphincter tone, induction of stationary motor patterns and blockade of peristalsis ensue. Together with inhibition of ion and fluid secretion, these effects cause constipation, one of the most frequent and troublesome adverse reactions of opioid analgesic therapy. Although laxatives are most frequently used to ameliorate opioid-induced bowel dysfunction, their efficacy is unsatisfactory. Specific antagonism of peripheral opioid receptors is a more rational approach. This goal is addressed by the use of opioid receptor antagonists with limited absorption such as oral prolonged-release naloxone and opioid receptor antagonists that do not penetrate the blood-brain barrier such as methylnaltrexone and alvimopan. Preliminary evidence indicates that peripherally restricted opioid receptor antagonists may act as prokinetic drugs in their own right.

Prolonged postoperative ileus-definition, risk factors, and predictors after surgery

BACKGROUND

Postoperative ileus (POI) remains an inevitable consequence of abdominal surgery. Although the pathogenesis of delayed gastrointestinal transit in the postoperative period has been the subject of considerable study, a clinically useful definition of what constitutes a pathologically prolonged ileus has yet to be established. The objectives of this study were to describe a definition for an abnormally prolonged ileus and to identify risk factors and predictors of prolonged ileus in patients undergoing abdominal surgery.

MATERIALS AND METHODS

Over a 12-month period 88 patients who had abdominal surgery were retrospectively reviewed. The association of clinical factors with the duration of POI was examined with statistical tests.

RESULTS

The mean time to commencing the consumption of unrestricted clear fluids after surgery was 2.3 +/- SD 1.6 days. The median duration of POI was 5 days (median 6 days), with an interquartile range of 3-6 days. Univariate regression analysis demonstrated significant correlations between duration of POI and estimated blood loss (EBL), total surgical time, and total opiate dose (TOD) (p = 0.009, p = 0.045, and p = 0.041, respectively). Multiple regression analysis identified EBL and TOD as independent predictors of duration of POI.

CONCLUSIONS

We have identified two risk factors (EBL and TOD) that are independently associated with duration of POI. Our data suggest that with the definition of abnormal prolonged postoperative ileus as the number of days above the 3rd quartile, an ileus greater than 6 days serves as a better clinical definition of prolonged POI than 3 days, the measure that has previously been suggested.

Reversible lipidization for the oral delivery of leu-enkephalin

The endogenous opioid peptide leu-enkephalin (ENK) was chemically modified by a method known as reversible aqueous lipidization (REAL) with a novel amine-reacting lipophilic dimethylmaleic anhydride analog, 3,4-bis(decylthiomethyl)-2,5-furandione. The binding affinity of the product, REAL-ENK, to opioid receptors was greatly reduced. This prodrug was stable in neutral and basic phosphate buffers but underwent rapid hydrolysis under acidic conditions in the presence of 50% acetonitrile. It also showed increased stability toward enzymatic degradations in various tissue preparations. The half-lives of REAL-ENK in mouse small intestinal mucosal homogenate and liver homogenate were 12 and 80 min, representing a 12- and 32-fold increase over those of ENK itself. In contrast to ENK (t(1/2) 6.7 min), REAL-ENK was stable in mouse plasma. More importantly, REAL-ENK produced significant and sustained antinociception mediated by peripheral opioid receptors in a rodent inflammatory pain model. Pharmacokinetic studies employing a radioimmunoassay (RIA) demonstrated that significantly higher and sustained plasma peptide levels were detected up to 24 h following the oral administration of REAL-ENK in normal mice. The peak concentration and area under the curve of oral REAL-ENK were 4.4 and 21 times higher than that of oral ENK. Our results indicate that like its disulfide-based counterpart, amine-based REAL may be an enabling technology which can be applied to enhance metabolic stability, increase oral absorption, and preserve and possibly prolong the pharmacological activity of peptide drugs.

Neural circuitry of stress-induced insomnia in rats

Sleep architecture is often disturbed after a stressful event; nevertheless, little is known about the brain circuitry responsible for the sleep perturbations induced by stress. We exposed rats to a psychological stressor (cage exchange) that initially causes an acute stress response, but several hours later generates a pattern of sleep disturbances similar to that observed in stress-induced insomnia in humans: increased sleep latency, decreased non-REM (nREM) and REM sleep, increased fragmentation, and high-frequency EEG activity during nREM sleep. We examined the pattern of Fos expression to identify the brain circuitry activated, and found increased Fos in the cerebral cortex, limbic system, and parts of the arousal and autonomic systems. Surprisingly, there was simultaneous activation of the sleep-promoting areas, most likely driven by ongoing circadian and homeostatic pressure. The activity in the cerebral cortex and arousal system while sleeping generates a novel intermediate state characterized by EEG high-frequency activity, distinctive of waking, during nREM sleep. Inactivation of discrete limbic and arousal regions allowed the recovery of specific sleep components and altered the Fos pattern, suggesting a hierarchical organization of limbic areas that in turn activate the arousal system and subsequently the cerebral cortex, generating the high-frequency activity. This high-frequency activity during nREM was eliminated in the stressed rats after inactivating parts of the arousal system. These results suggest that shutting down the residual activity of the limbic-arousal system might be a better approach to treat stress-induced insomnia, rather than potentiation of the sleep system, which remains fully active.

Prolonged postoperative ileus-definition, risk factors, and predictors after surgery

BACKGROUND

Postoperative ileus (POI) remains an inevitable consequence of abdominal surgery. Although the pathogenesis of delayed gastrointestinal transit in the postoperative period has been the subject of considerable study, a clinically useful definition of what constitutes a pathologically prolonged ileus has yet to be established. The objectives of this study were to describe a definition for an abnormally prolonged ileus and to identify risk factors and predictors of prolonged ileus in patients undergoing abdominal surgery.

MATERIALS AND METHODS

Over a 12-month period 88 patients who had abdominal surgery were retrospectively reviewed. The association of clinical factors with the duration of POI was examined with statistical tests.

RESULTS

The mean time to commencing the consumption of unrestricted clear fluids after surgery was 2.3 +/- SD 1.6 days. The median duration of POI was 5 days (median 6 days), with an interquartile range of 3-6 days. Univariate regression analysis demonstrated significant correlations between duration of POI and estimated blood loss (EBL), total surgical time, and total opiate dose (TOD) (p = 0.009, p = 0.045, and p = 0.041, respectively). Multiple regression analysis identified EBL and TOD as independent predictors of duration of POI.

CONCLUSIONS

We have identified two risk factors (EBL and TOD) that are independently associated with duration of POI. Our data suggest that with the definition of abnormal prolonged postoperative ileus as the number of days above the 3rd quartile, an ileus greater than 6 days serves as a better clinical definition of prolonged POI than 3 days, the measure that has previously been suggested.

Proteases for processing proneuropeptides into peptide neurotransmitters and hormones

Peptide neurotransmitters and peptide hormones, collectively known as neuropeptides, are required for cell-cell communication in neurotransmission and for regulation of endocrine functions. Neuropeptides are synthesized from protein precursors (termed proneuropeptides or prohormones) that require proteolytic processing primarily within secretory vesicles that store and secrete the mature neuropeptides to control target cellular and organ systems. This review describes interdisciplinary strategies that have elucidated two primary protease pathways for prohormone processing consisting of the cysteine protease pathway mediated by secretory vesicle cathepsin L and the well-known subtilisin-like proprotein convertase pathway that together support neuropeptide biosynthesis. Importantly, this review discusses important areas of current and future biomedical neuropeptide research with respect to biological regulation, inhibitors, structural features of proneuropeptide and protease interactions, and peptidomics combined with proteomics for systems biological approaches. Future studies that gain in-depth understanding of protease mechanisms for generating active neuropeptides will be instrumental for translational research to develop pharmacological strategies for regulation of neuropeptide functions. Pharmacological applications for neuropeptide research may provide valuable therapeutics in health and disease.

Renal responses to the kappa-opioid-receptor agonist U-50488H in conscious lambs

In adult animals and humans, activation of kappa-opioid receptors results in a diuresis. The aim of the present study was to investigate whether kappa-opioids are also diuretic early in life and whether this is altered during postnatal maturation. Therefore, the renal effects of the kappa-opioid-receptor agonist U-50488H were measured in two separate age groups of conscious lambs at two stages of postnatal maturation (approximately 1 wk and approximately 6 wk) under physiological conditions. To evaluate whether the renal responses to U-50488H resulted from receptor-dependent effects, responses to U-50488H were also tested in the presence of the specific kappa-opioid-receptor antagonist 5'-guanidinonaltrindole (GNTI). Urinary flow rate, free water clearance, and electrolyte excretions and clearances were measured for 30 min before and for 90 min after intravenous injection of U-50488H or vehicle. An increase in urinary flow rate accompanied by an increase in free water clearance occurred in response to administration of U-50488H but not vehicle. There were no effects of U-50488H on electrolyte excretions or clearances at either 1 or 6 wk of postnatal life. Although there were no effects of GNTI on any of the measured or calculated variables, the aforementioned diuretic response to U-50488H was abolished by pretreatment with GNTI in both age groups. We conclude that kappa-opioid receptors are diuretic early in life and that this response does not appear to be altered as postnatal maturation proceeds. Therefore, these data provide evidence that activation of kappa-opioid receptors early in life may lead to alterations in fluid balance.

Opioid peptides as possible neuromodulators in the frog peripheral nerve system

Sciatic nerves of the frog Rana ridibunda were examined for the effects of applied opioid peptide, methionine-enkephalin, synthetic enkephalin analogue, leucine-enkephalin-NH(2) and opiate antagonist, naloxone. The effect of both peptides in concentrations of 1x10(-6) and 1x10(-5)M or naloxone in 1x10(-6)M was investigated on the action potential parameters using electrophysiological techniques. The isolated nerves were stimulated by single square pulses each of which lasted for 0.5ms at supramaximal strength. Effect of each single dose of peptides at 0min was compared with the remaining time segments. Both peptides produced changes in action potential of nerve when compared with untreated nerves. Methionine-enkephalin in both concentrations reduced the amplitude between 7% and 41% and conduction velocity at about 26-61%. This peptide in the same concentrations prolonged the duration around 12-53% and increased the stimulating voltage at about 9-50%. In contrast, leucine-enkephalin-NH(2) in both concentrations caused a decrease in amplitude between 13% and 48% and in conduction velocity around 20-50%. The same concentrations of this peptide prolonged the duration at about 3-33% and increased the stimulating voltage at about 10-56%, but naloxone in 1x10(-6)M antagonized the responses of both peptides over 75%. The results indicate that both opioid peptides produce changes in action potential parameters in frog peripheral nerve system and these changes are partially reversed by naloxone.

Interface of physical and emotional stress regulation through the endogenous opioid system and mu-opioid receptors

Unraveling the pathways and neurobiological mechanisms that underlie the regulation of physical and emotional stress responses in humans is of critical importance to understand vulnerability and resiliency factors to the development of a number of complex physical and psychopathological states. Dysregulation of central stress response circuits have been implicated in the establishment of conditions as diverse as persistent pain, mood and personality disorders and substance abuse and dependence. The present review examines the contribution of the endogenous opioid system and mu-opioid receptors to the modulation and adaptation of the organism to challenges, such as sustained pain and negative emotional states, which threaten its internal homeostasis. Data accumulated in animal models, and more recently in humans, point to this neurotransmitter system as a critical modulator of the transition from acute (warning signals) to sustained (stressor) environmental adversity. The existence of pathways and regulatory mechanisms common to the regulation of both physical and emotional states transcend classical categorical disease classifications, and point to the need to utilize dimensional, "symptom"-related approximations to their study. Possible future areas of study at the interface of "mind" (cognitive-emotional) and "body" (physical) functions are delineated in this context.

Hypotensive effect of novel chimeric peptides of met-enkephalin and FMRFa

Endogenous opioid peptides like endomorphins, met-enkephalin and NPFF/FMRFamide family of neuropeptides, besides playing a role in modulation of antinociception, also affect cardiovascular system. Based on MERF, which consists of overlapping sequences of FMRFa and met-enkephalin, two chimeric peptides YGGFMKKKFMRFamide (YFa) and [D-Ala2] YAGFMKKKFMRFamide ([D-Ala2] YFa) were designed and synthesized. In this study, effect of YFa and [D-Ala2] YFa on arterial blood pressure and heart rate was evaluated in anaesthetized rats. Both YFa and [D-Ala2] YFa showed a dose-dependent fall in mean arterial pressure in dose-range of 13-78 micromol/kg. After naloxone treatment (5 mg/kg), vasodepressor effect of [D-Ala2] YFa and YFa was only partially blocked as compared to met-enkephalin. Partial blockade of vasodepressive effect of YFa and [D-Ala2] YFa by naloxone may be attributed to interaction of these chimeric peptides with receptors other than naloxone-sensitive receptors such as anti-opioid receptors, adrenergic receptors and D-analogue receptors.

Myocardial and peripheral concentrations of beta-endorphin before and following myocardial ischemia and reperfusion during coronary angioplasty

There is substantial evidence indicating that endogenous opioid peptides are involved in the pathophysiology of myocardial ischemia and reperfusion. We measured the myocardial and peripheral concentrations of beta-endorphin before and following myocardial ischemia and reperfusion during coronary angioplasty. The results indicate that in patients with coronary artery disease, there was an augmented myocardial concentration of beta-endorphin. Moreover, there was an increased peripheral concentration of beta-endorphin following myocardial ischemia and reperfusion. The data support the previous notion that endogenous opioid peptides are involved in the pathophysiology of ischemic heart disease.

Evidence for the involvement of an opioid system in sciatic nerve of Rana ridibunda

The effect of opioid peptide, D-alanine2-leucine-enkephalin and opioid homolog peptide, des-tyrosine-methionine-enkephalin in concentrations of 1 x 10(-6) and 1 x 10(-5) M was investigated on the action potential parameters of frog sciatic nerve. Des-tyrosine-methionine-enkephalin was used as the control to prove the opioid action of the peptide. The effects of both peptides were examined by means of the extracellular electrophysiological technique. The isolated sciatic nerves were stimulated by single square pulses each of which lasted for 0.5 ms at supramaximal strength. Effect of each single dose of peptides at 0 min was compared with the remaining time segments. Both peptides produced changes on action potential of Rana ridibunda sciatic nerve when compared with untreated nerves. D-alanine2-leucine-enkephalin decreased significantly the amplitude at about 34-83%, the area at about 34-92%. The same concentrations of this peptide decreased significantly the conduction velocity around 35-78%. In contrast, des-tyrosine-methionine-enkephalin reduced the action potential amplitude between 8% and 80%. The same concentrations of this peptide decreased significantly the area at about 12-76% and the conduction velocity around 42-70%. The depression of both peptides in action potential parameters was partially blocked by 1 x 10(-6) M naloxone.

Bimodal delta-opioid receptors regulate vagal bradycardia in canine sinoatrial node

Methionine-enkephalin-arginine-phenylalanine (MEAP) introduced into the interstitium of the canine sinoatrial (SA) node by microdialysis interrupts vagal bradycardia. In contrast, raising endogenous MEAP by occluding the SA node artery improves vagal bradycardia. Both are blocked by the same delta-selective antagonist, naltrindole. We tested the hypothesis that vagal responses to intranodal enkephalin are bimodal and that the polarity of the response is both dose- and opioid receptor subtype dependent. Ultralow doses of MEAP were introduced into the canine SA node by microdialysis. Heart rate frequency responses were constructed by stimulating the right vagus nerve at 1, 2, and 3 Hz. Ultralow MEAP infusions produced a 50-100% increase in bradycardia during vagal stimulation. Maximal improvement was observed at a dose rate of 500 fmol/min with an ED50 near 50 fmol/min. Vagal improvement was returned to control when MEAP was combined with the delta-antagonist naltrindole. The dose of naltrindole (500 fmol/min) was previously determined as ineffective vs. the vagolytic effect of higher dose MEAP. When MEAP was later reintroduced in the same animals at nanomoles per minute, a clear vagolytic response was observed. The delta1-selective antagonist 7-benzylidenenaltrexone (BNTX) reversed the vagal improvement with an ED50 near 1 x 10-21 mol/min, whereas the delta2-antagonist naltriben had no effect through 10-9 mol/min. Finally, the improved vagal bradycardia previously associated with nodal artery occlusion and endogenous MEAP was blocked by the selective delta1-antagonist BNTX. These data support the hypothesis that opioid effects within the SA node are bimodal in character, that low doses are vagotonic, acting on delta1-receptors, and that higher doses are vagolytic, acting on delta2-receptors.

Distribution of leucine-enkephalin in bone and joint tissues

The distribution and concentration of leu-enkephalin in periosteum, cortical bone, bone marrow and synovial membrane of normal rats were analysed. Periosteum, cortical bone and bone marrow of the rat femurs were collected as well as the ankles. The distribution of leu-enkephalin was analysed by immunoelectron microscopy and the concentration of leu-enkephalin was measured with radioimmunoassay. Immunoelectron microscopy showed that leu-enkephalin is abundant in monocytes of bone marrow, nerve fibers and endothelial cells in the periosteum and also in macrophage-like-cells of the synovial membrane. The concentration of leu-enkephalin measured by RIA showed highest concentration in bone marrow followed by periosteum and cortical bone. The study supports that leu-enkephalin is present and can be quantified in different compartments of bone and joint tissues suggesting that leu-enkephalin may be involved in the physiological regulation of nociception and immunoregulation.

Clonidine toxicity revisited

The incidence of clonidine overdose is increasing, yet there is a paucity of new information regarding treatment options for clonidine toxicity. Reported treatment approaches vary widely, demonstrating the lack of science on which current treatment is based. Available research needs to be reassessed. Neurotransmitters, receptors, endogenous opioids, and baseline sympathetic tone determine the clinical response to clonidine as well as the potential response to drug therapy following clonidine overdose. This article reviews aspects of clonidine toxicity that need to be further investigated. Multicenter research trials will be required to evaluate new treatment options.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Release of [Met5]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat

Effects of intraventricular injection of endomorphin-1, endomorphin-2 and beta-endorphin on the release of immunoreactive [Met(5)]enkephalin from the spinal cord were studied in pentobarbital anesthetized rats. Intraventricular injection of endomorphin-2, but not endomorphin-1, caused an increased release of immunoreactive [Met(5)]enkephalin in the spinal perfusates. Beta-endorphin given intraventricularly also increased the release of immunoreactive [Met(5)]enkephalin in an amount 15-fold higher than that produced by endomorphin-2. The increase of the release of immunoreactive [Met(5)]enkephalin induced by endomorphin-2 was blocked by mu-opioid receptor antagonist CTOP. Our result suggests that endomorphin-2 stimulates another subtype of mu-opioid receptor different from that acted by endomorphin-1 at the supraspinal site and subsequently increases the release of [Met(5)]enkephalin from the spinal cord.

Lung opioid receptors: pharmacology and possible target for nebulized morphine in dyspnea

Opioid receptors are located throughout the respiratory tract. Yet, these have received relatively scant attention compared to other opioid receptors. The most abundant sites within the respiratory tract appear localized within the alveolar walls, other sites appear to line the smooth muscle within the trachea and main bronchi near the lumen. There is about 100-times greater [3H]morphine binding density within the bronchioles and lobes than in the main bronchi or trachea. In addition to the usual mu, delta and kappa types of opioid receptors, 'non-conventional' opioid binding sites have been suggested, although the function of these or of the other opioid receptors in the pulmonary tract is not known. However, they might explain the otherwise counterintuitive apparent utility of morphine treatment of dyspnea. Dyspnea is a common and distressing symptom in terminally-ill cancer patients and patients with chronic lung disease. It results from multiple causes, is difficult to treat and is a significant precipitating factor for late-stage hospital or hospice admissions. Nebulized morphine or other opioids have been reported to have beneficial effect, but the mechanism by which opioids might produce this seemingly contradictory effect is not clear. We review here lung opioid receptor distribution, pharmacology and possible clinical relevance in the treatment of dyspnea.

Identification of opioid receptors in the sympathetic and parasympathetic nerves of guinea-pig atria

The opioid receptor subtypes of autonomic nerves of guinea-pig atria were elucidated by monitoring the effects of selective opioid receptor agonists on the negative and positive inotropic responses associated with the stimulation of the parasympathetic and sympathetic nerves, respectively. The positive inotropic effect, evoked by electrical field stimulation (2 Hz) was strongly reduced by the selective OP2-opioid receptor agonists U-50488 and U-69593, but partly by the OP3-opioid receptor agonist morphine. This effect of U-50488 and U-69593 were reversed by the selective OP2-opioid receptor antagonist nor-BNI. The effect of morphine was partly reversed by naloxone, whereas OP1-opioid receptor agonists, BW373 U86 and DPDPE, were ineffective. On the other hand, the negative inotropic response to electrical field stimulation was not affected by opioid receptor agonists. These results suggest that the noradrenaline release from cardiac sympathetic nerves of guinea-pig could be modulated, mainly by the OP2-opioid receptor, however, the acetylcholine release from cardiac parasympathetic nerves is not modulated by opioid receptors.