Metkephamid (Tyr-D-ala-Gly-Phe-N(Me)Met-NH2), a potent opioid peptide: receptor binding and analgesic properties

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

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.

Design, synthesis, and biological evaluation of novel bifunctional C-terminal-modified peptides for delta/mu opioid receptor agonists and neurokinin-1 receptor antagonists

A series of bifunctional peptides that act as agonists for delta and mu opioid receptors with delta selectivity and as antagonist for neurokinin-1 (NK1) receptors were designed and synthesized for potential application as analgesics in various pain states. The peptides were characterized using radioligand binding assays and functional assays using cell membrane and animal tissue. Optimization was performed on the fifth residue which serves as an address moiety for both receptor recognitions. It had critical effects on both activities at delta/mu opioid receptors and NK1 receptors. Among the synthesized peptides, H-Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF3) 2 (5) and H-Tyr-D-Ala-Gly-Phe-Nle-Pro-Leu-Trp-O-3,5-Bzl(CF3)2 (7) had excellent agonist activity for both delta opioid and mu opioid receptors and excellent antagonist activity for NK1 receptors. These results indicate that the rational design of multifunctional ligands with opioid agonist and neurokinin-1 antagonist activities can be accomplished and may provide a new tool for treatment of chronic and several pain states.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Kappa 3 opiate receptor binding in the mouse and rat

Previous work examining the binding of [3H]naloxone benzoylhydrazone (NalBzoH) in calf brain has identified a novel binding site, kappa 3. In mice and rats NalBzoH elicits an analgesic response which can clearly be differentiated from classical mu, delta or kappa 1 mechanisms and which is pharmacologically consistent with a kappa 3 receptor mechanism of action. In the current studies we demonstrate the presence of kappa 3 sites in both mouse and rat brains. The selectivity of the kappa 3 sites for opioids clearly discriminated it from traditional mu, kappa 1, kappa 2 or delta receptors. In the rat, the density of kappa 3 sites increased 2.5-fold from age 2 days to 21 days, after which it remained relatively stable. Among a number of brain regions in the rat, the density of kappa 3 sites varied dramatically. Highest levels were present in the hypothalamus, thalamus, striatum and midbrain with very low levels in the cerebellum. Intermediate levels were present in cortex, brain stem and spinal cord. Together, these studies support the presence of kappa 3 receptors in both mouse and rat brain which are very similar to those previously reported in calf brain.

Brain passage of BUBU, a highly selective and potent agonist for delta opioid receptors: in vivo binding and mu versus delta receptors occupancy

The peptidase-resistance and bioavailability of BUBU [H-Tyr-D.Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)-OH], a highly selective and potent agonist of the delta opioid receptor, have been investigated in vitro and in vivo. In vitro at 37 degrees C, the peptide was fully resistant to degradation by rat serum and strongly resistant to degradation by rat brain membranes. In vivo 0.065% of the dose of [3H]BUBU injected intravenously to the mouse was present 15 min later in the brain. The percentage determined for [3H]DAGO [H-Tyr-D.Ala-Gly-(NMe)Phe-Gly-ol], a selective ligand for mu sites, was 0.038%. Specific binding to mouse brain membranes, determined after intracerebroventricular injection of [3H]BUBU, was saturable and a high affinity (KDapp = 25 pmol) was evaluated for the delta-agonist. Competition experiments showed that BUBU is a selective ligand for delta receptors in vivo. Comparison of the analgesic potency (hot plate test) of ICV or IV administered increasing doses of BUBU and DAGO with their in vivo binding properties supports the preferential involvement of mu receptors in supraspinal analgesia. BUBU also induced an increase in spontaneous locomotion after IV administration at a dose lower than that which produced analgesia. The quantitative results obtained in the present study demonstrate that BUBU and DAGO could be used to characterize the pharmacological responses induced by selective stimulation of delta and mu receptors after systemic administration.

Multiple mu opiate receptors

In addition to morphine-selective mu 2 and enkephalin-preferring delta sites, recent evidence supports the presence within the central nervous system of a common site with very high affinity for both enkephalins and opiates termed the mu 1 site. This concept of a common, very high affinity site for multiple neurotransmitters is a unique concept in neuropharmacology, differing from classical transmitter systems which possess multiple receptor classes for a single transmitter. This review will address both the biochemical and pharmacological evidence supporting the existence of this site.

Central nervous system effects of peptides, 1980-1985: a cross-listing of peptides and their central actions from the first six years of the journal Peptides

A tabular synopsis is presented for articles concerned with the effects of peptides on the central nervous system that appeared in the journal Peptides from 1980-1985. A table arranged alphabetically by peptide and one arranged by effects, both listing routes of injection, species, direction of change, and qualifying notes, provides easy cross-referencing of peptides and their effects. Over 80 peptides and over 135 effects are listed. The list of peptides includes, but is not limited to: ACTH, angiotensin, bombesin, bradykinin, calcitonin, casomorphin, CCK, ceruletide, CGRP, CRF, dermorphin, DSIP, dynorphin, endorphins, enkephalins, GRF, gastrin, LHRH, litorin, metkephamid, MIF-l, motilin, MSH, NPY, NT, oxytocin, ranatensin, sauvagine, substances P and K, somatostatin, TRH, VIP, vasopressin, and vasotocin. The list of effects includes, but is not limited to: aggression, alcohol, analgesia, attention, avoidance, behavior, cardiovascular regulation, catalepsy, conditioned behavior, convulsions, dopamine binding and metabolism, discrimination, drinking, EEG, exploration, feeding, fever, gastric secretion, GI motility, grooming, learning, locomotor behavior, mating, memory, neuronal activity, open field, operant behavior, rearing, respiration, satiety, scratching, seizure, sleep, stereotypy, temperature, thermoregulation and tolerance.

Mu and delta, but not kappa, opioid agonists induce contractions of the canine small intestine in vivo

Extraluminal strain gage transducers were sutured along the transverse axis of the duodenum in order to monitor circular muscle contractile activity in the pentobarbital anesthetized dog. Administration by intravenous bolus of a variety of mu- and delta-directed opioid ligands resulted in a dose-dependent increase in duodenal contractile activity. In contrast, all kappa-directed ligands were devoid of stimulatory activity. Naloxone reversed the effects of normorphine and [Met5]enkephalin but was 20 times more effective against normorphine than [Met5]enkephalin. Based on the inactivity of all kappa ligands examined and the differential potency of naloxone against [Met5]enkephalin and normorphine, we suggest that this model may be useful in the classification of opioid ligands as to their receptor selectivity in vivo. Further, these data indicate that the stimulation of duodenal contractile activity is not mediated by enteric kappa receptors.

Spinal and supraspinal opioid analgesia in the mouse: the role of subpopulations of opioid binding sites

The selective in vivo blockade of high affinity (mu1) opioid binding sites in mice by naloxazone reduced the analgesic potency of opiates and opioid peptides, evidenced by a shift of their analgesic ED50 values. However, the extent of these shifts varied significantly between a series of opioid drugs, ranging from 12-fold for morphine to 4-fold for D-Ala2-D-Leu5-enkephalin. These findings suggested that analgesia in naloxazone-treated animals is mediated through a different subpopulation of receptors than in normal controls. Correlating these analgesic shifts for a series of opioids with their affinity for different [3H]opioid binding sites suggested an analgesic role for delta sites. Additional studies in mice with spinal transections suggested that mu1 analgesia was primarily supraspinal.

Biochemical and pharmacological evidence for opioid receptor multiplicity in the central nervous system

Evidence from a variety of experimental models has suggested the existence of mu 1, mu 2 and delta binding sites for morphine and the enkephalins in the central nervous system. Additional biochemical experiments now support this concept of a common high affinity site for opiates and opioid peptides. Mu sites have now been implicated in a number of pharmacological actions, including supraspinal analgesia, prolactin release, and catalepsy, but not in others (spinal analgesia, respiratory depression, and the guinea pig ileum). The hypothesis of mu 1 sites was supported by the unique opioid meptazinol, which selectively bound to mu 1 sites. As expected from its mu 1 binding selectivity, its analgesic actions in the mouse, localized supraspinally, were antagonized by the selective mu 1 antagonist naloxonazine and it had no respiratory depressant actions. Other binding studies suggested the presence of discrete SKF10,047-selective (KD approximately 5 nM) binding sites in rat brain which differed from both kappa sites and the previously reported PCP-binding sigma sites. Additional binding and autoradiographical studies have also implied the presence of beta-endorphin, or epsilon, sites in the CNS.