A peptide-like substance from pituitary that acts like morphine. 2. Purification and properties

The role of the opioid peptide dynorphin during the seasonal and gonadotropin-induced ovarian recrudescence in the gecko

The aim of the current investigation was to elucidate the influence of the opioid peptide dynorphin-A (DYN) on the reproductive axis during breeding and non-breeding phases of the ovarian cycle in the gecko Hemidactylus frenatus. During the recrudescence phase, administration of a high dose of DYN (10 µg/0.1 ml saline) caused a significant reduction in the numbers of oogonia and primary oocytes in the germinal bed, compared to those of the initial controls or experimental controls. Administration of a low (2 µg DYN/0.1 ml saline) or high dose of DYN did not affect the follicular development up to stage IV, but there were no stage V (vitellogenic) follicles in the ovary in contrast to their presence in the experimental controls. Furthermore, there was a significant reduction in gonadotropin-releasing hormone-immunoreactive (GnRH-ir) content in the median eminence (ME) and pars distalis of the pituitary gland (PD) in low or high doses of DYN-treated lizards. During the regression phase, treatment with follicle-stimulating hormone (FSH) resulted in the appearance of stage IV and V follicles, in contrast to their absence in initial controls and treatment controls. However, treatment with 10 µg DYN + FSH did not promote the development of these follicles. In addition, in vitro treatment of DYN significantly inhibited ovarian levels of estradiol. Collectively, these findings reveal an inhibitory influence of DYN on the seasonal ovarian recrudescence, possibly mediated through the suppression of GnRH release into the ME and PD and directly at the level of the ovary by impairment in steroidogenesis and vitellogenic follicular growth in the gecko.

Biased agonism in peptide-GPCRs: A structural perspective

G protein-coupled receptors (GPCRs) are dynamic membrane receptors that transduce extracellular signals to the cell interior by forming a ligand-receptor-effector (ternary) complex that functions via allosterism. Peptides constitute an important class of ligands that interact with their cognate GPCRs (peptide-GPCRs) to form the ternary complex. "Biased agonism", a therapeutically relevant phenomenon exhibited by GPCRs owing to their allosteric nature, has also been observed in peptide-GPCRs, leading to the development of selective therapeutics with fewer side effects. In this review, we have focused on the structural basis of signalling bias at peptide-GPCRs of classes A and B, and reviewed the therapeutic relevance of bias at peptide-GPCRs, with the hope of contributing to the discovery of novel biased peptide drugs.

Opiorphin: an endogenous human peptide with intriguing application in diverse range of pathologies

Mammalian zinc ectopeptidases have significant functions in deactivating neurological and hormonal peptide signals on the cell surface. The identification of Opiorphin, a physiological inhibitor of zinc ectopeptidases that inactivate enkephalin, has revealed its strong analgesic effects in both chemical and mechanical pain models. Opiorphin achieves this by increasing the transmission of endogenous opioids, which are dependent on the body's own opioid system. The function of opiorphin is closely linked to the rat sialorphin peptide, which inhibits pain perception by enhancing the activity of naturally occurring enkephalinergic pathways that depend on μ- and δ-opioid receptors. Opiorphin is highly intriguing in terms of its physiological implications within the endogenous opioidergic pathways, particularly in its ability to regulate mood-related states and pain perception. Opiorphin can induce antidepressant-like effects by influencing the levels of naturally occurring enkephalin, which are released in response to specific physical and/or psychological stimuli. This effect is achieved through the modulation of delta-opioid receptor-dependent pathways. Furthermore, research has demonstrated that opiorphin's impact on the cardiovascular system is facilitated by the renin-angiotensin system (RAS), sympathetic ganglia, and adrenal medulla, rather than the opioid system. Hence, opiorphin shows great potential as a solitary candidate for the treatment of several illnesses such as neurodegeneration, pain, and mood disorders.

Challenges and new opportunities for detecting endogenous opioid peptides in reward

The endogenous opioid peptide system, comprised of enkephalins, endorphins, dynorphins, and nociceptin, is a highly complex neurobiological system. Opioid peptides are derived from four precursor molecules and undergo several processing events yielding over 20 unique opioid peptides. This diversity together with low in vivo concentration and complex processing and release dynamics has challenged research into each peptide's unique function. Despite the subsequent challenges in detecting and quantifying opioid peptides in vivo, researchers have pioneered several techniques to directly or indirectly assay the roles of opioid peptides during behavioral manipulations. In this review, we describe the limitations of the traditional techniques used to study the role of endogenous opioid peptides in food and drug reward and bring focus to the wealth of new techniques to measure endogenous opioid peptides in reward processing.

Exploring Pharmacological Functions of Alternatively Spliced Variants of the Mu Opioid Receptor Gene, Oprm1, via Gene-Targeted Animal Models

The mu opioid receptor has a distinct place in the opioid receptor family, since it mediates the actions of most opioids used clinically (e.g., morphine and fentanyl), as well as drugs of abuse (e.g., heroin). The single-copy mu opioid receptor gene, OPRM1, goes through extensive alternative pre-mRNA splicing to generate numerous splice variants that are conserved from rodents to humans. These OPRM1 splice variants can be classified into three structurally distinct types: (1) full-length 7 transmembrane (TM) carboxyl (C)-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Distinct pharmacological functions of these splice variants have been demonstrated by both in vitro and in vivo studies, particularly by using several unique gene-targeted mouse models. These studies provide new insights into our understanding of the complex actions of mu opioids with regard to OPRM1 alternative splicing. This review provides an overview of the studies that used these gene-targeted mouse models for exploring the functional importance of Oprm1 splice variants.

Pleiotropic Effects of Kappa Opioid Receptor-Related Ligands in Non-human Primates

The kappa opioid receptor (KOR)-related ligands have been demonstrated in preclinical studies for several therapeutic potentials. This chapter highlights (1) how non-human primates (NHP) studies facilitate the research and development of ligands targeting the KOR, (2) effects of the endogenous opioid peptide, dynorphin A-(1-17), and its analogs in NHP, and (3) pleiotropic effects and therapeutic applications of KOR-related ligands. In particular, synthetic ligands targeting the KOR have been extensively studied in NHP in three therapeutic areas, i.e., the treatment for itch, pain, and substance use disorders. As the KORs are widely expressed in the peripheral and central nervous systems, pleiotropic effects of KOR-related ligands, such as discriminative stimulus effects, neuroendocrine effects (e.g., prolactin release and stimulation of hypothalamic-pituitary-adrenal axis), and diuresis, in NHP are discussed. Centrally acting KOR agonists are known to produce adverse effects including dysphoria, hallucination, and sedation. Nonetheless, with strategic advances in medicinal chemistry, three classes of KOR-related agonists, i.e., peripherally restricted KOR agonists, mixed KOR/mu opioid receptor partial agonists, and G protein-biased KOR agonists, warrant additional NHP studies to improve our understanding of their functional efficacy, selectivity, and tolerability. Pharmacological studies in NHP which carry high translational significance will facilitate future development of KOR-based medications.

Endogenous Opioid Peptides and Alternatively Spliced Mu Opioid Receptor Seven Transmembrane Carboxyl-Terminal Variants

There exist three main types of endogenous opioid peptides, enkephalins, dynorphins and β-endorphin, all of which are derived from their precursors. These endogenous opioid peptides act through opioid receptors, including mu opioid receptor (MOR), delta opioid receptor (DOR) and kappa opioid receptor (KOR), and play important roles not only in analgesia, but also many other biological processes such as reward, stress response, feeding and emotion. The MOR gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms. One type of these splice variants, the full-length 7 transmembrane (TM) Carboxyl (C)-terminal variants, has the same receptor structures but contains different intracellular C-terminal tails. The pharmacological functions of several endogenous opioid peptides through the mouse, rat and human OPRM1 7TM C-terminal variants have been considerably investigated together with various mu opioid ligands. The current review focuses on the studies of these endogenous opioid peptides and summarizes the results from early pharmacological studies, including receptor binding affinity and G protein activation, and recent studies of β-arrestin2 recruitment and biased signaling, aiming to provide new insights into the mechanisms and functions of endogenous opioid peptides, which are mediated through the OPRM1 7TM C-terminal splice variants.

A Concise Review of Concepts in Opioid Pharmacology up to the Discovery of Endogenous Opioids

This brief review covers concepts in opioid pharmacology that were promoted during the period leading up to the establishment of the International Narcotics Research Conference (INRC) in the early 1970s and the discovery of endogenous opioid peptides in 1975. The founders of INRC, meeting together during the International Union of Pharmacology meeting in Basel in 1969, recognized that the time was ripe for the creation of an international society that would provide a venue for the discussion of research across disciplines in this rapidly expanding area of science. The emphasis here is on studies leading to the demonstration that specific receptors for morphine-like analgesics exist, the search for endogenous ligands for these receptors, and early attempts to elucidate the mechanisms underlying opiate drug tolerance, dependence, and addiction. SIGNIFICANCE STATEMENT: Research on opioids in the 20th century was driven by the search for nonaddicting analgesics. This review discusses the development of the "analgesic" receptor concept, the demonstration that such receptors existed, and the search for an endogenous ligand. Conceptual models were proposed to explain tolerance to the actions of opiate drugs and the development of dependence and addiction. This review explains these models and indicates how they foreshadowed more recent discoveries on the acute and chronic actions of opiate drugs.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

Reciprocal Evolution of Opiate Science from Medical and Cultural Perspectives

Over the course of human history, it has been common to use plants for medicinal purposes, such as for providing relief from particular maladies and self-medication. Opium represents one longstanding remedy that has been used to address a range of medical conditions, alleviating discomfort often in ways that have proven pleasurable. Opium is a combination of compounds obtained from the mature fruit of opium poppy, papaver somniferum. Morphine and its biosynthetic precursors thebaine and codeine constitute the main bioactive opiate alkaloids contained in opium. Opium usage in ancient cultures is well documented, as is its major extract morphine. The presence of endogenous opiate alkaloids and opioid peptides in animals owe their discovery to their consistent actions at particular concentrations via stereo select receptors. In vitro expression of morphine within a microbiological industrial setting underscores the role it plays as a multi-purpose pharmacological agent, as well as reinforcing why it can also lead to long-term social dependence. Furthermore, it clearly establishes a reciprocal effect of human intelligence on modifying evolutionary processes in papaver somniferum and related plant species.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Dynorphin--still an extraordinarily potent opioid peptide

This issue of Molecular Pharmacology is dedicated to Dr. Avram Goldstein, the journal's founding editor and one of the leaders in the development of modern pharmacology. This article focuses on his contributions to the discovery of the dynorphins and evidence that members of this family of opioid peptides are endogenous agonists for the kappa opioid receptor. In his original publication describing the purification and sequencing of dynorphin A, Avram described this peptide as "extraordinarily potent" ("dyn" from the Greek, dynamis = power and "orphin" for endogenous morphine peptide). The name originally referred to its high affinity and great potency in the bioassay that was used to follow its activity during purification, but the name has come to have a second meaning: studies of its physiologic function in brain continue to provide powerful insights to the molecular mechanisms controlling mood disorders and drug addiction. During the 30 years since its discovery, we have learned that the dynorphin peptides are released in brain during stress exposure. After they are released, they activate kappa opioid receptors distributed throughout the brain and spinal cord, where they trigger cellular responses resulting in different stress responses: analgesia, dysphoria-like behaviors, anxiety-like responses, and increased addiction behaviors in experimental animals. Avram predicted that a detailed molecular analysis of opiate drug actions would someday lead to better treatments for drug addiction, and he would be gratified to know that subsequent studies enabled by his discovery of the dynorphins resulted in insights that hold great promise for new treatments for addiction and depressive disorders.

Suicide neurobiology

In this review, we examine the history of the neurobiology of suicide, as well as the genetics, molecular and neurochemical findings in suicide research. Our analysis begins with a summary of family, twin, and adoption studies, which provide support for the investigation of genetic variation in suicide risk. This leads to an overview of neurochemical findings restricted to neurotransmitters and their receptors, including recent findings in whole genome gene expression studies. Next, we look at recent studies investigating lipid metabolism, cell signalling with a particular emphasis on growth factors, stress systems with a focus on the role of polyamines, and finally, glial cell pathology in suicide. We conclude with a description of new ideas to study the neurobiology of suicide, including subject-specific analysis, protein modification assessment, neuroarchitecture studies, and study design strategies to investigate the complex suicide phenotype.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

Prohormones of beta-melanotropin (beta-melanocyte-stimulating hormone, beta-MSH) and corticotropin (adrenocorticotropic hormone, ACTH): structure and activation

It is proposed that all peptide hormones and releasing factors are biosynthesized in the form of precursor molecules which are biologically inactive. Enzymic activation may take place by hydrolytic cleavage to release a terminal COOH group or by transmidation to form a COOH-terminal amide. Studies with pituitary prohormones and hormones are providing data that support this hypothesis. Evidence has been obtained that the 91 residue beta-lipotropin (beta-LPH) is the prohormone of beta-melanotropin (beta-MSH). The specificity of the pituitary enzymes involved in release of the hormone was demonstrated by the isolation of five constituent fragments of LPH, which were obtained in homogeneous form from the pituitary gland of the pig. The enzymes have specificities similar to trypsin and carboxypeptidase B; carboxypeptidase A and aminopeptidase activities do not appear to be involved. Mild digestion of beta-LPH by trypsin in vitro has confirmed the susceptibility of the peptide bond on the carboxy side of the paired basic residues at positions 59 and 60, adjacent to the COOH-terminus of beta-MSH, and tryptic digestion of a model peptide demonstrated the same specificity. The paired basic residues at positions 39 and 40 adjacent to the NH2-terminus of beta-MSH were more resistant to tryptic attack, both in LPH and in a model peptide. In the gland it is apparent that LPH is cleaved on the carboxy side of the paired lysyl residues at positions 39 and 40, whereas in the synthetic peptide cleavage takes place in between these residues. The activating enzyme may differ from trypsin; alternatively, explanation may be found in the conformation of the prohormone. Prediction of secondary indicates that both pairs of basic residues lie adjacent to beta-bends on the surface of the molecule and occupy sites accessible to enzymic attack. It seems likely that alpha-MSH and corticotropin (ACTH) share a common pro hormone. The release of ACTH could involve cleavage of a -Gly-Ser- bond in the prohormone to expose the NH2-terminus of the hormone. With alpha-MSH, a concerted acetylation and cleavage may take place to form the N-acetylserine residue; the COOH-terminus may be released as an amide by direct transamidation of a -Val-Gly- bond in the prohormone. Release of either hormone would be accompanied by the release of contiguous fragments of the prohormone. We have isolated two novel polypeptides from pig pituitary in substantial quantity and have determined the primary structures. They may represent fragments of a prohormone to alpha-MSH or ACTH.

Development of highly potent and selective dynorphin A analogues as new medicines

Dynorphin A (Dyn A), a 17 amino acid peptide H-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-OH, is a potent opioid peptide which interacts preferentially with kappa-opioid receptors. Research in the development of selective and potent opioid peptide ligands for the kappa-receptor is important in mediating analgesia. Several cyclic disulphide bridge-containing peptide analogues of Dyn A, which were conformationally constrained in the putative message or address segment of the opioid ligand, were designed, synthesized and assayed. To further investigate the conformational and topographical requirements for the residues in positions 5 and 11 of these analogues, a systematic series of Dyn A(1-11)-NH2 cyclic analogues incorporating the sulphydryl-containing amino acids L- and D-Cys and L- and D-Pen in positions 5 and 11 were synthesized and assayed. Cyclic lactam peptide analogues were also synthesized and assayed. Several of these cyclic analogues, retained the same affinity and selectivity (vs. the mu- and delta-receptors) as the parent Dyn A(1-11)-NH2 peptide in the guinea-pig brain (GPB), but exhibited a much lower activity in the guinea-pig ileum (GPI), thus leading to centrally vs. peripherally selective peptides. Studies of the structure-activity relationship of Dyn A peptide provide new insights into the importance of each amino acid residue (and their configurations) in Dyn A analogues for high potency and good selectivity at kappa-opioid receptors. We report herein the progress towards the development of Dyn A peptide ligands, which can act as agonists or antagonists at cell surface receptors that modulate cell function and animal behaviour using various approaches to rational peptide ligand-based drug design.

Structure-activity relationships of dynorphin analogs substituted in positions 2 and 3

Following up on the observation that the dynorphin analog [Pro(3)]Dyn A(1-11)-NH(2) 2 possesses high affinity and selectivity for the kappa opioid receptor, a number of related peptides were prepared and characterized by radioligand binding and [(35)S]GTPgammaS assays. While incorporation of 2-azetidine carboxylic acid in position 3 led to the equally potent analog 3, the corresponding analog containing piperidine-2-carboxylic acid showed a nearly 90-fold reduction in kappa affinity. Differential preferred bond angles phi in the three building blocks might account for these observations. Compounds 2 and 3 were kappa antagonists with IC(50) values of 380 and 350 nM, respectively. The Sar(3) analog 7 and the Sar(2) analog 8 were kappa agonists, with greater selectivity than Dyn A(1-11)-NH(2) 1. In view of their high kappa affinities (8: K(i) = 1.5 nM; 2: K(i) = 2.4 nM), the new analogs were surprisingly weak kappa agonists or antagonists, e.g., the EC(50) value for the agonist 8 was 280 nM. Different kappa receptor subtypes in binding vs functional assays can not account for these results, since both assays were performed using the same membrane preparation.

Neuropeptides in hippocampus and cortex in transgenic mice overexpressing V717F beta-amyloid precursor protein--initial observations

Immunohistochemistry was used to analyse 18- and 26-month-old transgenic mice overexpressing the human beta-amyloid precursor protein under the platelet-derived growth factor-beta promoter with regard to presence and distribution of neuropeptides. In addition, antisera/antibodies to tyrosine hydroxylase, acetylcholinesterase, amyloid peptide, glial fibrillary acidic protein and microglial marker OX42 were used. These mice have been reported to exhibit extensive amyloid plaques in the hippocampus and cortex [Masliah et al. (1996) J. Neurosci. 16, 5795-5811]. The most pronounced changes were related to neuropeptides, whereas differences between wild-type and transgenic mice were less prominent with regard to tyrosine hydroxylase and acetylcholinesterase. The main findings were of two types; (i) involvement of peptide-containing neurites in amyloid beta-peptide positive plaques, and (ii) more generalized changes in peptide levels in specific layers, neuron populations and/or subregions in the hippocampal formation and ventral cortices. In contrast, the parietal and auditory cortices were comparatively less affected. The peptide immunoreactivities most strongly involved, both in plaques and in the generalized changes, were galanin, neuropeptide Y, cholecystokinin and enkephalin. This study shows that there is considerable variation both with regard to plaque load and peptide expression even among homozygotes of the same age. The most pronounced changes, predominantly increased peptide levels, were observed in two 26-month-old homozygous mice, for example, galanin-, enkephalin- and cholecystokinin-like immunoreactivities in stratum lacunosum moleculare, and galanin, neuropeptide Y, enkephalin and dynorphin in mossy fibers. Many peptides also showed elevated levels in the ventral cortices. However, decreases were also observed. Thus, galanin-like immunoreactivity could not any longer be detected in the diffusely distributed (presumably noradrenergic) fiber network in all hippocampal and cortical layers, and dynorphin-like immunoreactivity was decreased in stratum moleculare, cholecystokinin-like immunoreactivity in mossy fibers and substance P-like immunoreactivity in fibers around granule cells. The significance of generalized peptide changes is at present unclear. For example, the increase in the mainly inhibitory peptides galanin, neuropeptide Y, enkephalin and dynorphin and the decrease in the mainly excitatory peptide cholecystokinin in mossy fibers (and of substance P fibers around granule cells) indicate a shift in balance towards inhibition of the input to the CA3 pyramidal cell layer. Moreover, it may be speculated that the increase in levels of some of the peptides represents a reaction to nerve injury with the aim to counteract, in different ways, the consequences of injury, for example by exerting trophic actions. Further studies will be needed to establish to what extent these changes are typical for Alzheimer mouse models in general or are associated with the V717F mutation and/or the platelet-derived growth factor-beta promoter.

Molecular dynamics simulations predict a tilted orientation for the helical region of dynorphin A(1-17) in dimyristoylphosphatidylcholine bilayers

The structural properties of the endogenous opioid peptide dynorphin A(1-17) (DynA), a potential analgesic, were studied with molecular dynamics simulations in dimyristoylphosphatidylcholine bilayers. Starting with the known NMR structure of the peptide in dodecylphosphocholine micelles, the N-terminal helical segment of DynA (encompassing residues 1-10) was initially inserted in the bilayer in a perpendicular orientation with respect to the membrane plane. Parallel simulations were carried out from two starting structures, systems A and B, that differ by 4 A in the vertical positioning of the peptide helix. The complex consisted of approximately 26,400 atoms (dynorphin + 86 lipids + approximately 5300 waters). After >2 ns of simulation, which included >1 ns of equilibration, the orientation of the helical segment of DynA had undergone a transition from parallel to tilted with respect to the bilayer normal in both the A and B systems. When the helix axis achieved a approximately 50 degrees angle with the bilayer normal, it remained stable for the next 1 ns of simulation. The two simulations with different starting points converged to the same final structure, with the helix inserted in the bilayer throughout the simulations. Analysis shows that the tilted orientation adopted by the N-terminal helix is due to specific interactions of residues in the DynA sequence with phospholipid headgroups, water, and the hydrocarbon chains. Key elements are the "snorkel model"-type interactions of arginine side chains, the stabilization of the N-terminal hydrophobic sequence in the lipid environment, and the specific interactions of the first residue, Tyr. Water penetration within the bilayer is facilitated by the immersed DynA, but it is not uniform around the surface of the helix. Many water molecules surround the arginine side chains, while water penetration near the helical surface formed by hydrophobic residues is negligible. A mechanism of receptor interaction is proposed for DynA, involving the tilted orientation observed from these simulations of the peptide in the lipid bilayer.

Endomorphin-1 and endomorphin-2, endogenous ligands for the mu-opioid receptor, inhibit electrical activity of rat rostral ventrolateral medulla neurons in vitro

The classic opioid peptide, enkephalin, and the novel member of the opioid family, nociceptin/orphanin FQ, inhibit the spontaneous electrical activity of neurons recorded from the rostral ventrolateral medulla, presumably cardiovascular neurons. In this study, the putative effects of endomorphin-1 and endomorphin-2, the newly discovered endogenous ligands for the micro-opioid receptor, on the electrical activity of rostral ventrolateral medulla neurons were investigated in rat brain slices in vitro. Like enkephalin and nociceptin, perfusion of endomorphin-1 or endomorphin-2 profoundly inhibited spontaneous discharges of 43% and 38% of the medullary neurons, respectively. No excitatory response to perfusion of either endomorphin was found in all neurons surveyed. Both endomorphins produced concentration-dependent inhibition. However, endomorphin-1 was more potent than endomorphin-2 for production of the inhibition, as demonstrated by the greater and longer suppression induced by endomorphin-1 than that induced by endomorphin-2 at the same concentration. Among the four opioid agonists tested, EC50 values (in nM) were 3.17 (endomorphin-1), 3.02 (nociceptin), 10.1 (endomorphin-2) and 150.0 (enkephalin). The non-selective opioid receptor antagonist, naloxone, blocked the inhibitory responses of the neurons to endomorphin-1, endomorphin-2 and enkephalin, but not to nociceptin. The selective mu antagonist, beta-funaltrexamine, prevented the neuronal inhibition induced by endomorphins, but not by enkephalin and nociceptin. Neither naloxone nor beta-funaltrexamine alone had a significant effect on the firing rate of the neurons. These results demonstrate that endomorphin-1 and, to a lesser extent, endomorphin-2 exert an inhibitory modulation of the electrical activity of rostral ventrolateral medulla neurons, which is mediated through the stimulation of mu-opioid receptors.

Proopiomelanocortin (POMC) mRNA expression: distribution and region-specific down-regulation by chronic morphine in female guinea pig hypothalamus

There is compelling evidence that endogenous opioid peptides are regulated by exogenous opiates. Our previous studies have shown that the mu-opioid receptor protein and mRNA are down-regulated in the mediobasal hypothalamus of the female guinea pig following chronic morphine treatment. In addition, electrophysiological studies have shown that hypothalamic beta-endorphin (beta-EP) neurons express mu-opioid receptors that are uncoupled and down-regulated following chronic morphine treatment. Currently, we tested the hypothesis that chronic morphine, which produces down-regulation of mu-opioid receptors, causes a down-regulation of pro-opiomelanocortin (POMC, the precursor of beta-EP) mRNA expression in female guinea pig hypothalamus. Female guinea pigs were ovariectomized and implanted subcutaneously (s.c.) with 4 x 75 mg pellets for 2 days plus six more pellets of either morphine (n = 6) or placebo (n = 6) for another 5 days. Animals were sacrificed between 1000 and 1100 h on day 7. The expression of POMC mRNA were investigated using in situ hybridization histochemistry with a guinea pig specific 35S-labeled cRNA probe in hypothalamic tissue sections. POMC mRNA was localized to the arcuate nucleus (Arc) and median eminence (ME) of the medial basal hypothalamus. The distribution pattern was the same in both morphine and placebo control animals. However, the density of silver grains was less in morphine treated animals versus placebo control animals. Overall, the level of POMC mRNA was decreased by 22% in the Arc of morphine-treated guinea pigs as compared with the placebo controls (p < 0.05). This decrease in POMC mRNA expression was even greater in the caudal Arc (28%, p < 0.01) in morphine-treated animals. These results suggested that the biosynthetic activity of POMC neurons is down-regulated with chronic exposure to morphine.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Structure-activity relationship studies of dynorphin A and related peptides

An up-to-date review is presented covering all the available information concerning the isolation, discovery, synthesis, conformation, receptor binding characteristics, pharmacological properties and SAR studies of dynorphin A and related peptides. The potential of dynorphin A and its analogs has yet to be fully realized.

The distribution of methionine-enkephalin and leucine-enkephalin in the brain and peripheral tissues. 1977

A method is described for the rapid extraction of opioid peptides from the brain and other tissues. The method is based on acid extraction of tissues followed by adsorption of the extract onto Amberlite XAD-2 resin. Elution with methanol separates the enkephalins and α-endorphin from β-endorphin.

Over 90% of the opioid peptide activity isolated from brain and gut of several species by our method was due to methionine- and leucine-enkephalin. In contrast, the major opioid peptide activity recovered from the pituitary was due to peptides of much greater mol. wt. than the enkephalins.

An opioid peptide with properties unlike those of the known endorphins or enkephalins was present in brain extracts. This peptide, termed ∈-endorphin, has an apparent mol. wt. of 700 to 1200; it constituted between 5 to 10% of the total opioid activity in our extracts.

A differential assay of methionine- and leucine-enkephalin was made either by destroying methionine-enkephalin activity with cyanogen bromide or by separating the peptides by thin layer chromatography.

The ratio of methionine-enkephalin to leucine-enkephalin varied greatly in different brain regions. The highest proportions of leucine-enkephalin were found in the cerebral cortex and hippocampus.

Formaldehyde perfusion and fixation of the brain in vivo had no significant effect on the brain content of enkephalin, indicating that proteolytic breakdown is not a major problem in the extraction of these peptides.

It is suggested that the enkephalins may have a neurotransmitter role in both brain and peripheral tissues and that methionine- and leucine-enkephalin may subserve separate neuronal functions.

The role of endozepine in the regulation of steroid synthesis

Endozepine has recently been isolated from various steroid-forming organs. The following article explores the role of endozepine in the regulation of steroid synthesis. Steroid hormone synthesis from cholesterol begins in the inner mitochondrial membrane, where cytochrome P450 converts cholesterol to pregnenolone. Scientists thought that ACTH would stimulate this conversion, but experiments showed no such stimulation. However, addition of aminoglutethimide to block side-chain cleavage caused the expected reaction of ACTH to take place. Next the role of protein synthesis on the actions of ACTH was explored. Then endozepine was isolated from bovine fasciculata based on stimulation of pregnenolone production by freshly prepared mitochondria. After further experimentation it was concluded that endozepine is a peptide with at least two groups of actions: It binds GABAA receptors in the central nervous system, and it increases the mitochondrial synthesis of pregnenolone.

In vitro stability of some reduced peptide bond pseudopeptide analogues of dynorphin A

Eight analogues of DYN A(1-11)-NH2 incorporating the nonhydrolyzable psi [CH2-NH] peptide bond surrogate were tested for their in vitro enzymatic stability in mouse brain homogenates. Results show that the Leu(5)-Arg6 and to a lesser extent the Arg(7)-Ile8 and Ile(8)-Arg9 peptide bonds are the more susceptible to enzymatic cleavage in the native peptide. (Leu5 psi[CH(2)-NH]Arg6)DYN A(1-11)-NH2 exhibits an almost complete resistance to enzymatic cleavage with a half-life greater than 500 min in brain, compared to 42 min for the standard peptide, DYN A(1-11)-NH2.

The opiate system in invertebrates

The presence in diverse species of a similar mode of communication, that of a soluble messenger binding to a receptor, raises the question as to whether the specific components of this system are equally widespread. Do invertebrates use the same hormones and receptors as vertebrates do? Invertebrates ranging from unicellular organisms to insects have been shown to contain opiate-like peptides and binding sites, and they exhibit biological responses to opiates. However, critical genetic data are lacking. It is not known how signal systems arise phylogenetically, but it is conceivable that signal molecules that are already present cause the formation of their own receptors from membrane proteins.

Synthesis and opioid activity of 2-substituted dynorphin A-(1-13) amide analogues

A series of 2-substituted dynorphin A-(1-13) amide (Dyn A-(1-13)NH2) analogues was prepared by solid phase peptide synthesis and evaluated for opioid receptor affinities in radioligand binding assays and for opioid activity in the guinea pig ileum (GPI) assay. Amino acid substitution at the 2 position produced marked differences in both opioid receptor affinities and potency in the GPI assay; Ki values for the analogues in the radioligand binding assays and IC50 values in the GPI assay varied over three to four orders of magnitude. The parent peptide, Dyn A-(1-13)NH2, exhibited the greatest affinity and selectivity for kappa receptors and was the most potent peptide examined in the GPI assay. The most important determinant of opioid receptor selectivity and opioid potency for the synthetic analogues was the stereochemistry of the amino acid at the 2 position. Except for [D-Lys2]Dyn A-(1-13)NH2 in the kappa receptor binding assay, the analogues containing a D-amino acid at position 2 were much more potent in all of the assays than their corresponding isomers containing an L-amino acid at this position. The L-amino acid-substituted analogues generally retained some selectivity for kappa opioid receptors. The more potent derivatives with a D-amino acid in position 2, however, preferentially interacted with mu opioid receptors. Introduction of a positively charged amino acid into the 2 position generally decreased opioid receptor affinities and potency in the GPI assay.

Dealing with sadness, madness and hostility. New psychotropic drug remedies for the future

The objective of this article is to present an overview of new forms of psychotropic drug therapy that may be expected to play a role in psychiatric practice in the 1990s. In predicting these future developments, three lines of approach have been followed. Firstly, progress in elucidating basic neuronal mechanisms is described. The radioligand receptor binding technique has proved to be an especially powerful tool in the search for novel psychoactive compounds. Secondly, those mental health problems most likely to undergo intensive study are discussed. It is likely that special attention will be devoted to organic mental disorders related to aging (dementia) or chronic exposure to toxic substances. In addition, research will be aimed at explaining and reducing the occurrence of auto-aggressive and hetero-aggressive behaviour. Thirdly, the types of newly designed agents and treatment strategies currently under investigation are outlined. In particular, the development of pharmacological agents that interfere with serotonergic molecular mechanisms has opened the way to improving existing psychotropic drugs, to inventing drugs that achieve known clinical effects via different mechanisms of action, and even to discovering entirely new categories of psychotropic drugs.

Spinal somatostatin superfusion in vivo affects activity of cat nociceptive dorsal horn neurons: comparison with spinal morphine

A controversy exists concerning the role of the neuropeptide somatostatin for the transmission or inhibition of nociceptive information in the spinal cord. To better correlate electrophysiological effects of somatostatin at single cell level with results obtained with intrathecal injections of somatostatin in behaving animals and human pain patients we applied somatostatin to the spinal cord by controlled superfusion of the recording segment in vivo. The hypothesis of an opioid link and possible neurotoxic effects of somatostatin were also addressed. In cats deeply anaesthetized with pentobarbitone, halothane and nitrous oxide, extracellular recordings were made from 27 neurons located in laminae I-VI. All neurons responded to both innocuous mechanical and noxious radiant heat stimuli applied to the glabrous skin of the ipsilateral hindpaw. The dorsal surface of the spinal cord was superfused at the recording segment by means of a Perspex chamber (7 x 7 mm). Somatostatin superfusions at 1.2 microM had no effect on responses to noxious heat. Responses were, however, depressed by somatostatin at 61 microM to 59.7 +/- 5.1% of control and by somatostatin at 1.53 mM to 39.9 +/- 9.5% of control. This inhibition was not antagonized by the mu-opiate antagonist naloxone applied to the spinal cord at concentrations of 2.7 mM, either together with somatostatin, or after the inhibition by somatostatin had fully developed. Neuronal responses were linear functions of the skin temperatures for stimulation intensities between 42 degrees C and 52 degrees C. The slopes of these stimulus response functions were reduced during somatostatin superfusion at 61 microM to 46.8 +/- 9.3% of control, without changing the temperature thresholds for responding (42.5 +/- 0.6 degrees C). Somatostatin superfusion at 61 microM had no effect on the number of action potentials evoked by innocuous skin brushing, or by electrical stimulation of primary afferent A-fibres in cutaneous nerves. The amplitude of intraspinally recorded field potentials evoked by these electrical nerve stimuli was also unaffected by somatostatin. The inhibition of nociceptive spinal dorsal horn neurons by spinally administered morphine was assessed in eight experiments. Morphine reduced noxious heat-evoked responses to 42.1 +/- 9.6% of control at 0.3 mM and to 51.8 +/- 6.9% of control at 3.0 mM. The slopes of the stimulus-response functions were reduced by morphine at 0.3 mM to 53.1 +/- 11.3% of control, without changing the temperature thresholds (42.7 degrees C). Naloxone superfusion (2.7 mM) reliably antagonized the inhibition by morphine. Brush-evoked responses were not, or much less, affected by spinal morphine.(ABSTRACT TRUNCATED AT 400 WORDS)

Sex-specific effects of met-enkephalin treatment on vasopressin immunoreactivity in the rat supraoptic nucleus

The supraoptic nucleus of male and female rats treated with met-enkephalin or naloxone and met-enkephalin was examined with light microscopical immunocytochemistry for Arginine-vasopressin. Both genders exhibited the same distribution of immunostained magnocellular neurons. Met-enkephalin treatment caused an increase in number of immunostained vasopressin neurons. This effect was more pronounced in females than in males. Naloxone treatment diminished immunoreactive cytoplasmic vasopressin in males more effectively than in females. In enkephalin-treated animals numerous vasopressin immunoreactive varicosities appeared within the supraoptic nucleus, but were mostly absent in naloxone-treated animals and in controls. Our results indicate that met-enkephalin treatment either stimulates vasopressin synthesis or inhibits secretion. It is likely that steroid hormones mediate the action of enkephalin on vasopressin secretion in a specific manner.

Somatostatin blocks a calcium current in rat sympathetic ganglion neurones

1. The effects of somatostatin and somatostatin analogues on a Ca2+ current from acutely isolated and short-term (24-48 h) cultured adult rat superior cervical ganglion (SCG) neurones were studied using the whole-cell variant of the patch-clamp technique. 2. [D-Trp8]Somatostatin (SOM) produced a rapid, reversible and concentration-dependent reduction of the Ca2+ current. Ca2+ current amplitude was reduced over the voltage range -15 to +40 mV with the greatest reduction occurring where the amplitude was maximal (ca +10 mV). In the presence of SOM, the Ca2+ current rising phase was slower and biphasic at potentials between 0 and +40 mV. 3. Application of 0.1 microM-SOM for greater than 10 s resulted in a desensitization of the response. During a 4 min application of 0.1 microM-SOM, Ca2+ current amplitude returned to about 90% of control. A second application of 0.1 microM-SOM produced less block than the initial application. 4. Concentration-response curves for SOM, somatostatin-14 (SOM-14) and somatostatin-28 (SOM-28) were fitted to a single-site binding isotherm. The concentrations producing half-maximal block and the maximal attainable blocks of the Ca2+ current for SOM, SOM-14 and SOM-28 were 3.3, 5.4 and 35 nM, respectively and 55, 51 and 54%, respectively. SOM-14 and SOM-28 slowed the Ca2+ current rising phase in a manner similar to that of SOM. Somatostatin-28 had no effect on the Ca2+ current at 1 microM. 5. The magnitude of the Ca2+ current block produced by 0.1 microM-SOM was not significantly altered in the presence of 1 microM-idazoxan, atropine, naloxone or the somatostatin antagonist aminoheptanoyl-Phe-D-Trp-Lys-O-benzyl-Thr. 6. Internal dialysis with solutions containing 500 microM-guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5'-O-(3-thiotriphosphate)(GTP-gamma-S) decreased the Ca2+ current amplitude by 36 and 41%, respectively, and induced a biphasic rising phase in the Ca2+ current. Under these conditions, application of 0.1 microM-SOM produced significantly less block of Ca2+ current amplitude (7.1 and 14.7%, respectively) when compared with controls. 7. Internal dialysis with solutions containing 500 microM-guanosine-5'-O-(2-thiodiphosphate)(GDP-beta-S) had no significant effect on either the Ca2+ current amplitude or block produced by 0.1 microM-SOM. 8. Internal dialysis with solutions containing 500 microM-cyclic adenosine 3',5'-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the Ca2+ current block produced by 0.1 microM-SOM or the Ca2+ current amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of centrally administered encephalinamides on regional cerebral blood flow in the dog

(D-ala2)-met5-encephalinamide (AM encephalinamide) and (D-ala2)-leu5-encephalinamide (AL encephalinamide) were administered into the cisterna magna in anesthetized dogs to determine whether these opiates effected the neurohypophyseal circulation differently than the circulation of other brain areas. At the beginning of the experimental protocol, animals were given either mock cerebral spinal fluid (CSF) or 5 or 25 mg of AM encephalinamide or 5 mg of AL encephalinamide in equal volumes of mock CSF into the cisterna magna. By 60 min after intracisternal injection, radiolabeled AM encephalinamide distributed throughout the brain with the highest concentration being in the area of the brainstem. Sixty minutes after intracisternal injection, heart rate was decreased 29.0 +/- 5.1%, 41.3 +/- 4.4%, and 36.0 +/- 3.6%, and MABP was decreased 25.2 +/- 8.0%, 26.4 +/- 2.4%, and 32.3 +/- 2.6% in animals treated with AL encephalinamide (5 mg), AM encephalinamide (5 mg), and AM encephalinamide (25 mg), respectively. Neither AL encephalinamide or AM encephalinamide altered CBF or CMRO2 when compared with animals treated with mock CSF, whereas both AL encephalinamide and AM encephalinamide reduced neurohypophyseal blood flow by 30 min (43 +/- 11%, AL encephalinamide; 35 +/- 7%, AM encephalinamide, 5 mg; 46 +/- 8%, AM encephalinamide, 25 mg); the reduction was sustained throughout the 60-min protocol (34 +/- 10%, AL encephalinamide; 37 +/- 3%, AM encephalinamide, 5 mg; 38 +/- 4% AM encephalinamide, 25 mg). Plasma arginine vasopressin was transiently elevated 15 (326 +/- 75%, AL encephalinamide; 323 +/- 109%, AM encephalinamide, 25 mg) and 30 min (271 +/- 68%, AL encephalinamide; 368 +/- 136%, AM encephalinamide, 25 mg) in animals treated with AL encephalinamide or AM encephalinamide (25 mg). Intravenous naloxone administered at the end of the 60-min encephalinamide protocol was associated with a rise toward control values in heart rate and MABP in the AL encephalinamide group and in heart rate, MABP, and neurohypophyseal blood flow in both the AM encephalinamide 5 mg and 25 mg groups. These data suggest that encephalinamides may play a role in the regulation of neurohypophyseal blood flow through their actions on opiate receptors.

The binding of opioid peptides to the Mcg light chain dimer: flexible keys and adjustable locks

Enkephalins and beta-casomorphins (opioid peptides) were found to bind in a variety of conformations to a human light chain (Bence-Jones) dimer from a patient (Mcg) with amyloidosis. The peptides were diffused into crystals of the protein and their positions, relative occupancies and modes of binding were determined at 2.7 A resolution by difference Fourier analyses. Collectively, the opioid peptides occupied practically all of the available space in the concave, internal parts of the binding region, as well as flat or convex external surfaces around the rim of the binding cavity. Suitable ligands ranged in size from four to seven residues. As many as five residues could be accommodated inside the binding region, and there was space for at least four residues on the external surfaces. External binding was influenced by solvent effects and local packing interactions among adjacent protein molecules in the crystal lattice. In the enkephalin series the presence of amino-terminal tyrosine was necessary, but not sufficient for binding. [Met]-enkephalin, a pentapeptide, showed two different modes of binding in overlapping subsites. In one subsite, preferred over the second in a ratio of 1.3:1.0, the side chain of amino-terminal tyrosine penetrated through the floor of the main cavity to lodge in the deep binding pocket about 20 A from the entrance. The remainder of the peptide spanned the length of the main cavity in an extended conformation. In the second subsite the amino end was restricted to the main cavity and the peptide backbone turned abruptly upward at residue 3 to interact with external surfaces. An (Arg-6, Phe-7) heptapeptide extension of [Met]-enkephalin entered the deep pocket and assumed an extended conformation in the main cavity like the pentapeptide. Its last two residues flattened against the external surfaces. [Leu]-enkephalin and its analogues displayed a combination of internal and external binding like [Met]-enkephalin in its secondary subsite. Enkephalin analogues with D-amino acids in position 2 generally adopted conformations which were more convoluted than those in the L-isomers. Moreover, external interactions tended to be more prominent in the D-derivatives. The beta-casomorphin-7 heptapeptide penetrated into the deep pocket and traversed the main cavity in as extended a conformation as the presence of two proline residues would permit. On removal of the ligand there was an unexpected hysteresis effect involving permanent structural alterations in the walls of the binding region. beta-casomorphins-4 and -5 were bound in the main cavity with the carboxyl ends protruding from the entrance.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulatory peptide receptors: visualization by autoradiography

The receptors for regulatory peptides have been extensively characterized using radioligand binding techniques. By combining these binding techniques with autoradiography it is possible to visualize at the light and electron microscopic levels the anatomical and cellular localization of these receptors. In this review we discuss the procedures used to label peptide receptors for autoradiography and the peculiarities of peptides as ligands. The utilization of autoradiography in mapping peptide receptors in brain and peripheral tissues, some of the new insights revealed by these studies particularly the problem of 'mismatch' between endogenous peptides and receptors, the existence of multiple receptors for a given peptide family and the use of peptide receptor autoradiography in human tissues are also reviewed.

Opioid peptides in blood and cerebrospinal fluid during acute stress

The opioid peptides beta-endorphin and [met]enkephalin are present in the peripheral circulation. Plasma beta-endorphin originates from the pituitary gland and its cosecretion with ACTH is stimulated by a variety of noxious stimuli. Although the adrenal medulla contains high concentrations of [met]enkephalin-containing polypeptides which are costored with catecholamines, and although the adrenal gland appears to secrete [met]enkephalin into the adrenal vein, the relative adrenal contribution to plasma [met]enkephalin appears to be negligible. Plasma concentrations of immunoreactive [met]enkephalin may be increased by insulin and by endotoxic shock, but they are not significantly altered by acute haemorrhagic stress nor by surgical stress. Thus blood plasma concentrations of beta-endorphin, but not of [met]enkephalin, are generally increased during acute stress. The physiological significance of endogenous opioids in the circulation is not known. It is unlikely that transient increases in the concentrations of opioid peptides in peripherally circulating blood modulate nociception, since the peptides do not enter ventricular cerebrospinal fluid in detectable amounts under these conditions. Recent evidence has raised the possibility that circulating opioids may be involved in regulating blood glucose and in activating the immune system. It is also possible that circulating beta-endorphin and related polypeptides have non-opioid actions on a variety of peripheral tissues.

beta-Endorphin-like immunoreactivity in the brain of the lizard, Lacerta muralis

Using immunocytochemical methods, a beta-endorphin-like immunoreactive substance was identified in the brain of the lizard Lacerta muralis. beta-Endorphin-like neurons were observed in the dorsal posterior part of the paraventricular nucleus and in the caudal region of the nucleus ventromedialis hypothalami. beta-Endorphin-like immunoreactive fibers were also detected in the median eminence. Another cell group displaying beta-endorphin-like immunoreactivity was found in both subdivisions of the oculomotor nucleus and in the periaqueductal gray of the mesencephalon. In addition, a beta-endorphin-like immunoreaction was observed in the perikarya of the Purkinje cells and in their axonal processes. These patterns of immunoreactivity were completely abolished when a specific antiserum was absorbed with its corresponding antigen or with beta-lipotropin. These control tests suggest that the immunoreaction might correspond to a beta-endorphin- or lipotropin-like reaction. The results are discussed in relation to the possibility that a beta-endorphin-like peptide may be involved in hypophysial regulation or neuromodulator activity in the brain of the lizard L. muralis.

Distribution of dynorphin and enkephalin peptides in the rat brain

The neuroanatomical distribution of dynorphin B-like immunoreactivity (DYN-B) was studied in the adult male and female albino rat. The distribution of DYN B in colchicine- and noncolchicine-treated animals was also compared to that of another opioid peptide derived from the prodynorphin precursor dynorphin A (1-8) (DYN 1-8), and an opioid peptide derived from the proenkephalin precursor met-enkephalin-arg-gly-leu (MERGL). DYN B cell bodies were present in nonpyramidal cells of neo- and allocortices, medium-sized cells of the caudate-putamen, nucleus accumbens, lateral part of the central nucleus of the amygdala, bed nucleus of the stria terminalis, preoptic area, and in sectors of nearly every hypothalamic nucleus and area, medial pretectal area, and nucleus of the optic tract, periaqueductal gray, raphe nuclei, cuneiform nucleus, sagulum, retrorubral nucleus, peripeduncular nucleus, lateral terminal nucleus, pedunculopontine nucleus, mesencephalic trigeminal nucleus, parabigeminal nucleus, dorsal nucleus of the lateral lemniscus, lateral superior olivary nucleus, superior paraolivary nucleus, medial superior olivary nucleus, ventral nucleus of the trapezoid body, lateral dorsal tegmental nucleus, accessory trigeminal nucleus, solitary nucleus, nucleus ambiguus, paratrigeminal nucleus, area postrema, lateral reticular nucleus, and ventrolateral region of the reticular formation. Fiber systems are present that conform to many of the known output systems of these nuclei, including major descending pathways (e.g., striatonigral, striatopallidal, reticulospinal, hypothalamospinal pathways), short projection systems (e.g., mossy fibers in hippocampus, hypothalamo-hypophyseal pathways), and local circuit pathways (e.g., in cortex, hypothalamus). The distribution of MERGL was, with a few notable exceptions, in the same nuclei as DYN B. From these neuroanatomical data, it appears that the dynorphin and enkephalin peptides are strategically located in brain regions that regulate extrapyramidal motor function, cardiovascular and water balance systems, eating, sensory processing, and pain perception.

D-phenylalanine: a putative enkephalinase inhibitor studied in a primate acute pain model

D-Phenylalanine, along with morphine, acetylsalicylic acid and zomepirac sodium were evaluated for their antinociceptive actions in monkeys (M. fascicularis) trained to autoregulate nociceptive stimulation using a discrete-trials, aversive-threshold paradigm. Morphine sulfate produced dose-related increases in aversive threshold which were reversible after administration of naloxone (12.5 or 25 micrograms/kg i.m.). D-Phenylalanine (500 mg/kg p.o.) produced a small increase in aversive threshold which was not statistically significant and not naloxone reversible. Acetylsalicylic acid (200 mg/kg p.o.) but not zomepirac sodium (200 mg/kg p.o.) in combination with D-phenylalanine (500 mg/kg) produced a small statistically significant increase in aversive threshold. Our results argue against the hypothesis that D-phenylalanine is responsible for increasing aversive thresholds via opiate receptor mechanisms involving increased activity of enkephalins at synaptic loci. Previous studies by others in rats and mice showed that D-phenylalanine and acetylsalicylic acid produced increases in nociceptive thresholds which were naloxone reversible. Our failure to find opiate receptor mediated analgesia in a primate model with demonstrated opiate receptor selectivity and sensitivity is discussed in terms of previous basic and clinical research indicating an analgesic role for D-phenylalanine. Possible species difference in drug action is discussed in terms of inhibition by D-phenylalanine of carboxy-peptidase-like enkephalin processing enzymes as well as inhibition of carboxypeptidase-like enkephalin degrading enzymes.

Opiate dependence and withdrawal--a new synthesis?

There is a growing body of evidence to suggest that adrenocorticotropin (ACTH) may have a physiological role as an endogenous contra-opioid agonist. In addition to having appreciable affinity for opiate receptors and inducing many behavioural and intracellular effects opposite to those observed following opioid administration, ACTH may interact with endorphins in a mutually antagonistic manner. On the basis of these data a model of opiate dependence is proposed whereby several aspects of the opiate abstinence syndrome may be attributed to the excitatory actions of ACTH acting at opiate receptors. Thus, it may be predicted that opiate antagonist administration during primary abstinence should significantly attenuate many aspects of this behavioural syndrome. The present study was conducted in order to investigate this hypothesis. Results indicated that whilst naloxone (1.5 mg/kg) exerted little influence in non-dependent animals, it significantly attenuated abstinence-exacerbated grooming, body shaking, teeth chattering and sneezing, in addition to completely antagonizing withdrawal hyperalgesia in post-dependent animals. These data are consistent with the proposed existence of an endogenous contra-opioid ligand, the antagonism of which markedly reduces the severity of the morphine withdrawal syndrome.