The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh ...insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.
Hypoxic/ischemic disruption of ionic homeostasis is a critical trigger of neuronal injury/death in the brain. There is, however, no promising strategy against such pathophysiologic change to protect ...the brain from hypoxic/ischemic injury. Here, we present a novel finding that activation of δ-opioid receptors (DOR) reduced anoxic Na⁺ influx in the mouse cortex, which was completely blocked by DOR antagonism with naltrindole. Furthermore, we co-expressed DOR and Na⁺ channels in Xenopus oocytes and showed that DOR expression and activation indeed play an inhibitory role in Na⁺ channel regulation by decreasing the amplitude of sodium currents and increasing activation threshold of Na⁺ channels. Our results suggest that DOR protects from anoxic disruption of Na⁺ homeostasis via Na⁺ channel regulation. These data may potentially have significant impacts on understanding the intrinsic mechanism of neuronal responses to stress and provide clues for better solutions of hypoxic/ischemic encephalopathy, and for the exploration of acupuncture mechanism since acupuncture activates opioid system.
Activation of delta-opioid receptors (DOR) is neuroprotective against hypoxic/ischemic injury in the cortex, which is at least partially related to its action against hypoxic/ischemic disruption of ...ionic homeostasis that triggers neuronal injury. Na+ influx through TTX-sensitive voltage-gated Na+ channels may be a main mechanism for hypoxia-induced disruption of K+ homeostasis, with DOR activation attenuating the disruption of ionic homeostasis by targeting voltage-gated Na+ channels. In the present study we examined the role of DOR in the regulation of Na+ influx in anoxia and simulated ischemia (oxygen-glucose deprivation) as well as the effect of DOR activation on the Na+ influx induced by a Na+ channel opener without anoxic/ischemic stress and explored a potential PKC mechanism underlying the DOR action. We directly measured extracellular Na+ activity in mouse cortical slices with Na+ selective electrodes and found that (1) anoxia-induced Na+ influx occurred mainly through TTX-sensitive Na+ channels; (2) DOR activation inhibited the anoxia/ischemia-induced Na+ influx; (3) veratridine, a Na+ channel opener, enhanced the anoxia-induced Na+ influx; this could be attenuated by DOR activation; (4) DOR activation did not reduce the anoxia-induced Na+ influx in the presence of chelerythrine, a broad-spectrum PKC blocker; and (5) DOR effects were blocked by PKCβII peptide inhibitor, and PKCθ pseudosubstrate inhibitor, respectively. We conclude that DOR activation inhibits anoxia-induced Na+ influx through Na+ channels via PKC (especially PKCβII and PKCθ isoforms) dependent mechanisms in the cortex.
► Anoxic Na+ influx occurred mainly through TTX-sensitive Na+ channels. ► DOR activation inhibited Na+ influx induced by anoxia or Na+ channel opener. ► The DOR effect is dependent on PKC, especially PKCβII and PKCθ, pathway.
Although endomorphins-1 (EM-1; H-Tyr-Pro-Phe-Trp-NH(2)) and -2 (EM-2; H-Tyr-Pro-Phe-Phe-NH(2)) are primarily considered agonists for the μ-opioid receptor (MOR), systematic alterations to specific ...residues provided antagonists and ligands with mixed μ/δ-opioid properties, suitable for application to health-related topics. While the application of endomorphins as antinociceptive agents and numerous biological endpoints were experimentally delineated in laboratory animals and in vitro, clinical use is currently absent. However, structural alterations provide enhanced stability; formation of MOR antagonists or mixed and dual μ/δ-acting ligands could find considerable therapeutic potential.
This review attempts to succinctly provide insight on the development and bioactivity of endomorphin analogues during the past decade. Rational design approaches will focus on the engineering of endomorphin agonists, antagonists and mixed ligands for their application as a multi-target ligand.
Aside from alleviating pain, EM analogues open new horizons in the treatment of medical syndromes involving neural reward mechanisms and extraneural regulation effects on homeostasis. Highly selective MOR antagonists may be promising to reduce inflammation, attenuate addiction to drugs and excess consumption of high-caloric food, ameliorate alcoholism, affect the immune system and combat opioid bowel dysfunction.
Nociceptin (NC) and some of its fragments as well as nociceptin-(1−13)-peptide amide NC(1−13)-NH2 and a series of its analogues were prepared and tested in the mouse vas deferens in an attempt to ...identify the sequences involved in the activation (message) and in the binding (address) of nociceptin to its receptor. The NC receptor that inhibits the electrically evoked twitches of the mouse vas deferens was demonstrated to be distinct from the δ opioid receptor, since naloxone and Dmt-Tic-OH (a selective δ opioid receptor antagonist) block the δ opioid receptor but have no effect on the nociceptin receptor. Results from structure−activity experiments suggest that (a) the entire sequence of NC may not be required for full biological activities, since NC(1−13)-NH2 is as active as NC; (b) fragments of NC have however to be amidated as in NC(1−13)-NH2 in order to be protected from degradation by proteases; (c) cationic residues (as Arg8,12, Lys9,13) appear to play a functional role, since their replacement with Ala in the sequence of NC(1−13)-NH2 leads to inactivity; (d) the N-terminal tetrapeptide Phe-Gly-Gly-Phe is essential for activity: its full length and flexibility appear to be required for NC receptor activation and/or occupation; (e) Phe4 and not Phe appears to be the residue involved in receptor activation, since the replacement of Phe1 with Leu has no effect, while that of Phe4 leads to inactivity. Results summarized in this paper indicate that the structural requirements of NC for occupation and activation of its receptor are different from that of opioids, particularly δ agonists.
Knockout and pharmacological studies have shown that delta opioid peptide (DOP) receptor signalling regulates emotional responses. In the present study, the in vitro and in vivo pharmacological ...profile of the DOP ligand, H-Dmt-Tic-NH-CH(CH
2-COOH)-Bid (UFP-512) was investigated. In receptor binding experiments performed on membranes of CHO cells expressing the human recombinant opioid receptors, UFP-512 displayed very high affinity (p
K
i
10.20) and selectivity (>150-fold) for DOP sites. In functional studies (
35SGTPγS binding in CHO
hDOP membranes and electrically stimulated mouse vas deferens) UFP-512 behaved as a DOP selective full agonist showing potency values more than 100-fold higher than DPDPE. In vivo, in the mouse forced swimming test, UFP-512 reduced immobility time both after intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) administration. Similar effects were recorded in rats. Moreover, UFP-512 evoked anxiolytic-like effects in the mouse elevated plus maze and light–dark aversion assays. All these in vivo actions of UFP-512 were fully prevented by the selective DOP antagonist naltrindole (3
mg/kg, s.c.). In conclusion, the present findings demonstrate that UFP-512 behaves as a highly potent and selective agonist at DOP receptors and corroborate the proposal that the selective activation of DOP receptors elicits robust anxiolytic- and antidepressant-like effects in rodents.
Endomorphin 1 (Endo-1
=
Tyr-Pro-Trp-Phe-NH
2), an endogenous opioid with high affinity and selectivity for μ-opioid receptors, mediates acute and neuropathic pain in rodents. To overcome metabolic ...instability and poor membrane permeability, the N- and C-termini of Endo-1 were modified by lipoamino acids (Laa) and/or sugars, and 2′,6′-dimethyltyrosine (Dmt) replacement of Tyr. Analogues were assessed for μ-opioid receptor affinity, inhibition of cAMP accumulation, enzymatic stability, and permeability across Caco-2 cell monolayers. C-Terminus modification decreased receptor affinity, while N-terminus C8-Laa improved stability and permeability with slight change in receptor affinity. Dmt provided a promising lead compound: C8Laa-Dmt1-Endo-1 is nine times more stable (
t
1/2
=
43.5
min), >8-fold more permeable in Caco-2 cell monolayers, and exhibits 140-fold greater μ-opioid receptor affinity (
K
iμ
=
0.08
nM).
Analogues of endomorphin (Dmt-Pro-Xaa-Xaa-NH2) modified at position 4 or at positions 4 and 3, and tripeptides (Dmt-Pro-Xaa-NH2) modified at position 3, with various phenylalanine analogues (Xaa=Trp, ...1-Nal, 2-Nal, Tmp, Dmp, Dmt) were synthesized and their effects on in vitro opioid activity were investigated. Most of the peptides exhibited high μ-opioid (MOP) receptor binding affinity (KiMOP=0.13–0.81nM), modest MOP-selectivity (Kiδ-opioid (DOP)/KiMOP=3.5–316), and potent functional MOP agonism (GPI, IC50=0.274–249nM) without DOP and κ-opioid (KOP) receptor agonism. Among them, compounds 7 (Dmt-Pro-Tmp-Tmp-NH2) and 9 (Dmt-Pro-1-Nal-NH2) were opioids with potent mixed MOP receptor agonism/DOP receptor antagonism and devoid of β-arrestin2 recruitment activity. They may offer a unique template for the discovery of potent analgesics that produce less respiratory depression, less gastrointestinal dysfunction and that have a lower propensity to induce tolerance and dependence compared with morphine.
To improve the structure−activity studies of the lead δ opioid agonist H-Dmt-Tic-Asp*-Bid, we synthesized and pharmacologically characterized a series of analogues in which the side chain next to ...1H-benzimidazole-2-yl (Bid) was substituted by those endowed with different chemical properties. Interesting results were obtained: (1) only Gly, Ala, and Asp resulted in δ agonism, (2) Phe yielded δ antagonism, (3) and all other residues except Glu (devoid of any activity) gave μ agonism.
PDF
