Postnatal development of delta-opioid receptor subtypes in mice

The Delta Opioid Receptor in Pain Control

Nowadays, the delta opioid receptor (DOPr) represents a promising target for the treatment of chronic pain and emotional disorders. Despite the fact that they produce limited antinociceptive effects in healthy animals and in most acute pain models, DOPr agonists have shown efficacy in various chronic pain models. In this chapter, we review the progresses that have been made over the last decades in understanding the role played by DOPr in the control of pain. More specifically, the distribution of DOPr within the central nervous system and along pain pathways is presented. We also summarize the literature supporting a role for DOPr in acute, tonic, and chronic pain models, as well as the mechanisms regulating its activity under specific conditions. Finally, novel compounds that have make their way to clinical trials are discussed.

Vas deferens response to selective opioid receptor agonists in adult mice is impaired following postnatal repeated mild stress

Mild stress repeatedly applied to neonatal rodents induces alterations of central nervous system functions, persisting up to the adult age. Most alterations may be mediated through hormones and neuromediators active on the autonomic nervous system, therefore we tested the efficacy of selective opioid receptor agonists on the vas deferens of adult mice that, as neonates, had undergone daily mild stress until weaning (brief isolation and solvent injection). We found in the adult mouse (90 days old) decreased sensitivity of vas deferens to selective mu-, delta- and kappa-opioid receptor agonist drugs. The neonatal administration of an antisense oligodeoxynucleotide adrenocorticotropin-synthesis-inhibitor partly prevented these effects.

Respiratory function in adult mice lacking the mu-opioid receptor: role of delta-receptors

Mice lacking the mu-opioid receptor (MOR) provide a unique model to determine whether opioid receptors are functionally interactive. Recent results have shown that respiratory depression produced by delta-opioid receptor agonists is suppressed in mice lacking the mu-opioid receptor. Here we investigated the involvement of mu- and delta-opioid receptors in the control of ventilation and mu/delta receptor interactions in brainstem rhythm-generating structures. Unrestrained MOR-/- and wild-type mice showed similar ventilatory patterns at rest and similar chemosensory responses to hyperoxia (100% O2), hypoxia (10% O2) or hypercapnia (5%CO2-95%O2). Blockade of delta-opioid receptors with naltrindole affected neither the ventilatory patterns nor the ventilatory responses to hypoxia in MOR-/- and wild-type mice. In-vitro, respiratory neurons were recorded in the pre-Bötzinger complex of thick brainstem slices of MOR-/- and wild-type young adult mice. Respiratory frequency was not significantly different between these two groups. The delta2 receptor agonist deltorphin II (0.1-1.0 microM) decreased respiratory frequency in both groups whereas doses of the delta1 receptor agonist enkephalin[D-Pen2,5] (0.1-1.0 microM) which were ineffective in wild-type mice significantly decreased respiratory frequency in MOR-/- mice. We conclude that deletion of the mu-opioid receptor gene has no significant effect on ensuing respiratory rhythm generation, ventilatory pattern, or chemosensory control. In MOR-/- mice, the loss of respiratory-depressant effects of delta2-opioid receptor agonists previously observed in vivo does not result from a blunted response of delta receptors in brainstem rhythm-generating structures. These structures show an unaltered response to delta2-receptor agonists and an augmented response to delta1-receptor agonists.

delta-Opioid modulation of social interactions in juvenile mice weaned at different ages

The environmental stimulus of weaning has been shown to affect both the developmental expression of social behavior and the maturation of the opioid delta-receptors' subpopulation in altricial rodents. The aim of this study was to address both these issues by using the social interaction paradigm. Separate groups of male and female mice were randomly assigned to three different weaning ages -- early (Wean-15), regular (Wean-20), and delayed (Wean-25) -- and assessed when 30 days old under intraperitoneal administration of the selective delta-opioid agonist SNC80 (0, 0.1, or 0.3 mg/kg). Wean-15 male and female subjects were much more involved in investigating the partner as well as the cage environment compared to the regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehavioral changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-induced increase in the expression of investigative and affiliative social interactions was further magnified by early weaning. A delayed weaning time was instead associated with reduced sensitivity to the drug, which suggests a delayed maturation of the system. As a whole, the present results indicate that the time of weaning is able to markedly affect the expression of social interactions of adolescent mice by possibly exerting a direct modulatory role on the development of the still plastic delta-opioid system.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

Differential knockdown of delta-opioid receptor subtypes in the rat brain by antisense oligodeoxynucleotides targeting mRNA

Two antisense oligodeoxynucleotides (A-ODN), targeting delta-opioid receptor mRNA (DOR) and two mismatch ODN sequences (mODN) were continuously infused for 24 days into the lateral brain ventricles of Wistar rats. The density of delta-opioid receptors in rat brain homogenates was measured by saturation binding experiments using four selective ligands, two agonists ([D-Ala2, Glu4]-deltorphin and DPDPE) and two antagonists (Dmt-Tic-OH and naltrindole), and by immunoblotting SDS solubilized receptor protein. In brain membranes of mODN or saline-infused rats, the rank order of delta-opioid receptor density, calculated by Bmax values of the four delta-opioid receptor ligands, was: [D-Ala2, Glu4]deltorphin approximately Dmt-Tic-OH approximately naltrindole (86-118 fmo/mg protein) > DPDPE (73.6+/-6.3 fmol/mg protein). At the end of the 24 day infusion of A-ODN targeting DOR nucleotide sequence 280299 (A-ODN280-299), the Bmax of DPDPE (62.4+/-3.2 fmol/mg protein) was significantly higher than that of Dmt-Tic-OH (31.5+/-3.9 fmol/mg protein). Moreover, both the Kd value for DPDPE saturation binding and the Ki value for Dmt-Tic-OH displacement by DPDPE were halved. In contrast, an A-ODN treatment targeting exon 3 (A-ODN741-760) decreased the specific binding of [D-Ala2, Glu4]deltorphin and Dmt-Tic-OH significantly less (67%-81%) than the binding of DPDPE (53%), without changes in DPDPE Ki and KD values. No A-ODN treatment modified the specific binding of the micro-opioid agonist DAMGO and of the k-selective opioid receptor ligand U69593. On the Western blot of solubilized striatum proteins, A-ODN(280-299) and A-ODN(741-760) downregulated the levels of the DOR protein, whereas the corresponding mODN were inactive. The 24-day infusion of A-ODN(280-299) inhibited the rat locomotor response to [D-Ala2, Glu4]deltorphin but not to DPDPE. Intracerebroventricular (i.c.v.) infusion of A-ODN(741-760) reduced the locomotor responses to both delta-opioid receptor agonists, whereas mODN infusion never affected agonist potencies. In conclusion, these results demonstrate that 24-day continuous i.c.v. infusion of A-ODN targeting the nucleotide sequence 280-299 of DOR can differentially knockdown delta1 and delta2 binding sites in the rat brain.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.