Peripheral non-opioid analgesic effects of kyotorphin in mice

Blockade of analgesic effects following systemic administration of N-methyl-kyotorphin, NMYR and arginine in mice deficient of preproenkephalin or proopiomelanocortin gene

Kyotorphin is a unique biologically active neuropeptide (l-tyrosine-l-arginine), which is reported to have opioid-like analgesic actions through a release of Met-enkephalin from the brain slices. N-methyl-l-tyrosine-l-arginine (NMYR), an enzymatically stable mimetic of kyotorphin, successfully caused potent analgesic effects in thermal and mechanical nociception tests in mice when it was given through systemic routes. NMYR analgesia was abolished in μ-opioid receptor-deficient (MOP-KO) mice, and by intracerebroventricular (i.c.v.) injection of naloxone and of N-methyl l-leucine-l-arginine (NMLR), a kyotorphin receptor antagonist. In the Ca²⁺-mobilization assay using CHO cells expressing Gα_qi5 and hMOPr or hDOPr, however, the addition of kyotorphin neither activated MOPr-mechanisms, nor affected the concentration-dependent activation of DAMGO- or Met-Enkephalin-induced MOPr activation, and Met-enkephalin-induced DOPr activation. NMYR-analgesia was significantly attenuated in preproenkephalin (PENK)- or proopioimelanocortin (POMC)-KO mice. The systemic administration of arginine, which is reported to elevate the level of endogenous kyotorphin selectively in midbrain and medulla oblongata, pain-related brain regions, caused significant analgesia, and the analgesia was reversed by i.c.v. injection of NMLR or naloxone. In addition, PENK- and POMC-KO mice also attenuated the arginine-induced analgesia. All these findings suggest that NMYR and arginine activate brain kyotorphin receptor in direct and indirect manner, respectively and both compounds indirectly cause the opioid-like analgesia through the action of endogenous opioid peptides.

Pharmacological Potential of the Endogenous Dipeptide Kyotorphin and Selected Derivatives

The endogenous peptide kyotorphin (KTP) has been extensively studied since it was discovered in 1979. The dipeptide is distributed unevenly over the brain but the majority is concentrated in the cerebral cortex. The putative KTP receptor has not been identified yet. As many other neuropeptides, KTP clearance is mediated by extracellular peptidases and peptide transporters. From the wide spectrum of biological activity of KTP, analgesia was by far the most studied. The mechanism of action is still unclear, but researchers agree that KTP induces Met-enkephalins release. More recently, KTP was proposed as biomarker of Alzheimer disease. Despite all that, KTP limited pharmacological value prompted researchers to develop derivatives more lipophilic and therefore more prone to cross the blood–brain barrier (BBB), and also more resistant to enzymatic degradation. Conjugation of KTP with functional molecules, such as ibuprofen, generated a new class of compounds with additional biological properties. Moreover, the safety profile of these derivatives compared to opioids and their efficacy as neuroprotective agents greatly increases their pharmacological value.

Endothelium-Mediated Action of Analogues of the Endogenous Neuropeptide Kyotorphin (Tyrosil-Arginine): Mechanistic Insights from Permeation and Effects on Microcirculation

Kyotorphin (KTP) is an endogenous peptide with analgesic properties when administered into the central nervous system (CNS). Its amidated form (l-Tyr-l-Arg-NH2; KTP-NH2) has improved analgesic efficacy after systemic administration, suggesting blood-brain barrier (BBB) crossing. KTP-NH2 also has anti-inflammatory action impacting on microcirculation. In this work, selected derivatives of KTP-NH2 were synthesized to improve lipophilicity and resistance to enzymatic degradation while introducing only minor changes in the chemical structure: N-terminal methylation and/or use of d amino acid residues. Intravital microscopy data show that KTP-NH2 having a d-Tyr residue, KTP-NH2-DL, efficiently decreases the number of leukocyte rolling in a murine model of inflammation induced by bacterial lipopolysaccharide (LPS): down to 46% after 30 min with 96 μM KTP-NH2-DL. The same molecule has lower ability to permeate membranes (relative permeability of 0.38) and no significant activity in a behavioral test which evaluates thermal nociception (hot-plate test). On the contrary, methylated isomers at 96 μM increase leukocyte rolling up to nearly 5-fold after 30 min, suggesting a proinflammatory activity. They have maximal ability to permeate membranes (relative permeability of 0.8) and induce long-lasting antinociception.

Neuropeptide Kyotorphin (Tyrosyl-Arginine) has Decreased Levels in the Cerebro-Spinal Fluid of Alzheimer's Disease Patients: Potential Diagnostic and Pharmacological Implications

In Alzheimer’s disease (AD), besides the characteristic deterioration of memory, studies also point to a higher pain tolerance in spite of sensibility preservation. A change in the normal tau protein phosphorylation is also characteristic of AD, which contributes to the pathogenesis of the disease and is useful in early diagnosis. Kyotorphin (KTP) is an endogenous analgesic dipeptide (Tyr-Arg) for which there is evidence of eventual neuroprotective and neuromodulatory properties. The objective of this work was to study the possible correlation between KTP and phosphorylated tau protein (p-tau) levels in cerebro-spinal fluid (CSF) samples of AD patients. CSF samples were collected from 25 AD patients and 13 age-matched controls (N), where p-tau and KTP levels were measured. We found a statistically significant difference between p-tau/KTP values in AD and N groups with an inverse correlation between p-tau and KTP values in AD samples. These results suggest that in the future KTP may be a candidate biomarker for neurodegeneration and may be a lead compound to be used pharmacologically for neuroprotection.

The Analgesic Activity of Bestatin as a Potent APN Inhibitor

Bestatin, a small molecular weight dipeptide, is a potent inhibitor of various aminopeptidases as well as LTA4 hydrolase. Various physiological functions of Bestatin have been identified, viz.: (1) an immunomodifier for enhancing the proliferation of normal human bone marrow granulocyte-macrophage progenitor cells to form CFU-GM colonies; Bestatin exerts a direct stimulating effect on lymphocytes via its fixation on the cell surface and an indirect effect on monocytes via aminopeptidase B inhibition of tuftsin catabolism; (2) an immunorestorator and curative or preventive agent for spontaneous tumor; Bestatin alone or its combination with chemicals can prolongate the disease-free interval and survival period in adult acute or chronic leukemia, therefore, it was primarily marketed in 1987 in Japan as an anticancer drug and servers as the only marketed inhibitor of Aminopeptidase N (APN/CD13) to cure leukemia to date; (3) a pan-hematopoietic stimulator and restorator; Bestatin promotes granulocytopoiesis and thrombocytopoiesis in vitro and restores them in myelo-hypoplastic men; (4) an inhibitor of several natural opioid peptides. Based on the knowledge that APN can cleave several bioactive neuropeptides such as Met-enkaphalins, Leu-enkaphalins, beta-Endorphin, and so on, the anti-aminopeptidase action of Bestatin also allows it to protect endopeptides against their catabolism, exhibiting analgesic activity. Although many scientific studies and great accomplishments have been achieved in this field, a large amount of problems are unsolved. This article reviews the promising results obtained for future development of the analgesic activity of Bestatin that can be of vital interest in a number of severe and chronic pain syndromes.

[Functional arginine-containing amino acid sequences in peptides and proteins]

L-arginine is a source of nitrogen oxide and plays a great role in a number of other biochemical processes. Functions and prospects for practical application of five groups of arginine-containing amino acid sequences and synthetic polyarginine sequences are considered. The physiological characteristics of well-known arginine-containing peptides, such as RGD peptides, kyotorphin, and tuftsin, are described in detail. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2008, vol. 34, no. 2; see also http://www.maik.ru

Chiral Recognition ofD-Kyotorphin by Lipidic Membranes: Relevance Toward Improved Analgesic Efficiency

D-Kyotorphin (D-KTP), the most potent isomer of the endorphin-like dipeptide kyotorphin (KTP), is a good drug candidate for the treatment of chronic pain and is thought to be involved in receptor-mediated processes. According to the "membrane catalysis" model, ligands interact with membrane lipids to attain high local concentrations in the receptor vicinity and to adopt the necessary conformation for docking. Therefore, the interaction and recognition of D-KTP by membranes is potentially important to its increased analgesic effect. In spite of the neutral net charge of D-KTP at pH 7.4, fluorescence spectroscopy reveals that the interaction with large unilamellar vesicles is more extensive than was observed for KTP. The tyrosine residue interacts extensively with rigid membranes, with a location and well-defined orientation in the bilayer. This suggests not only that D-KTP meets the structural constraints needed for receptor-ligand interaction in a manner similar to that of KTP, but also that the stronger membrane interaction and ability to discriminate rigid membrane domains might contribute to its improved analgesic effect.

NSAID zaltoprofen possesses novel anti-nociceptive mechanism through blockage of B2-type bradykinin receptor in nerve endings

Zaltoprofen, a propionic acid derivative of non-steroidal anti-inflammatory drugs (NSAIDs), was shown to have more powerful inhibitory effects to bradykinin (BK)-nociception than other NSAIDs. However, the molecular mechanisms underlying this potent analgesia are not yet fully understood. Here we attempted to clarify the molecular mechanism underlying zaltoprofen-induced analgesia on BK-induced nociception by a novel algogenic-induced paw flexion (APF) test in mice. The intraplantar (i.pl.) injection of zaltoprofen at 1nmol showed strong analgesic action on BK (i.pl.)-induced nociceptive flexor responses, whereas loxoprofen or its active metabolite loxoprofen-SRS did not. Zaltoprofen also inhibited the nociception induced by [Tyr8]-BK, a specific agonist of B2-type BK receptor, but did not affect the nociception by [Lys-des-Arg9]-BK, a specific agonist of B1-type BK receptor. However, zaltoprofen did not affect the substance P-induced nociception, which is mediated by common post-receptor signaling through nociceptive fibers with BK-ones. All these results suggest that NSAID zaltoprofen possesses novel anti-nociceptive mechanism, which inhibits B2-type BK receptor function in nerve endings.

Conformational and Orientational Guidance of the Analgesic Dipeptide Kyotorphin Induced by Lipidic Membranes: Putative Correlation toward Receptor Docking

The analgesic dipeptide kyotorphin (L-Tyr-L-Arg) and an acylated kyotorphin derivative were studied by a combination of theoretical (molecular dynamics simulation and quantum mechanics methods) and experimental (fluorescence and infrared spectroscopies) approaches both in solution and in model systems of membranes. At biological pH the peptides have a neutral net charge. Nevertheless, their phenolic rings interact with phospholipid molecules (partition coefficient varies from 6 x 10(2) to 2 x 10(4), depending on the lipid and pH used) despite being exposed to the aqueous bulk medium. The lowest energy transition dipole moment is displaced from the normal to the lipid bilayer by 20 degrees on average. The observed extensive interaction, pK(a), precise location, and well-defined orientation in membranes combined with the ability to discriminate rigid raftlike membrane domains suggest that kyotorphin meets the structural constraints needed for receptor-ligand interaction. The acylated kyotorphin derivative mimics kyotorphin properties and represents a promising way for entrapment in a drug carrier and transport across the blood-brain barrier.

Molecular mechanisms of neuropathic pain–phenotypic switch and initiation mechanisms

Many known painkillers are not always effective in the therapy of chronic neuropathic pain manifested by hyperalgesia and tactile allodynia. The mechanisms underlying neuropathic pain appear to be complicated and to differ from acute and inflammatory pain. Recent advances in pain research provide us with a clear picture for the molecular mechanisms of acute pain, and substantial information is available concerning the plasticity that occurs under conditions of neuropathic pain. The most important changes responsible for the mechanisms of neuropathic pain are found in the altered gene/protein expression in primary sensory neurons. After damage to peripheral sensory fibers, up-regulated expression of the Ca(v)alpha(2)delta-(1) channel subunit, the Na(v)1.3 sodium channel, and bradykinin (BK) B1 and capsaicin TRPV1 receptors in myelinated neurons contribute to hyperalgesia; while the down-regulation of the Na(v)1.8 sodium channel, B2 receptor, substance P (SP), and even mu-opioid receptors in unmyelinated neurons is responsible for the phenotypic switch in pain transmission. Clarification of the molecular mechanisms for such complicated plasticity would be extremely valuable when considering the therapeutic design of pain relieving drugs. Although many reports deal with the changes in expression of key molecules related to neuropathic pain, the initiation and the mechanisms that follow remain to be determined. The current study using lysophosphatidic acid (LPA) receptor knockout mice revealed that LPA produced by nerve injury initiates neuropathic pain and demyelination following partial sciatic nerve ligation (PSNL). A single injection of LPA was found to mimic PSNL in terms of neuropathic pain and its underlying mechanisms. This discovery may lead to the subsequent discovery of LPA-induced secondary genes, which would be therapeutic targets for neuropathic pain.

Contributions of electromigration and electroosmosis to peptide iontophoresis across intact and impaired skin

d-(Arg)-Kyotorphin iontophoresis was investigated across intact and impaired skins in vitro. Iontophoretic flux increased from 68+/-12 to 538+/-116 nmol cm(-2) h(-1) when the peptide concentration in the anodal compartment was raised from 5 to 40 mM. Electromigration was the principal transport mechanism, accounting for approximately 70% of total peptide delivery. Reducing the number of competing ions in the formulation significantly increased iontophoretic flux but did not affect convective solvent flow. The latter was independent of peptide concentration indicating that skin permselectivity was not modified by kyotorphin transport. Total iontophoretic flux was unaffected when the stratum corneum was removed by tape-stripping (146+/-34 versus 150+/-26 nmol cm(-2) h(-1)). However, the contributions of the different transport mechanisms were significantly altered: (i) electromigration decreased, as more of the charge was carried by anions from the sub-dermal milieu; (ii) electroosmosis was absent; and (iii) passive permeation increased significantly. Transport rates across intact and impaired skin barriers were statistically indistinguishable when the donor electrolyte composition was modified; increased competition from anions was mitigated by the decreased Na+ levels in the formulation. Removal of Cl- ions from the receiver phase further increased peptide delivery, and also increased anodal electroosmosis.

Cutting edge: nociceptin stimulates neutrophil chemotaxis and recruitment: inhibition by aspirin-triggered-15-epi-lipoxin A4

The nociceptin receptor (Noci-R) is a G protein-coupled receptor present in neural tissues and its activation by nociceptin is involved in the processing of pain signals. Here, we report that Noci-R is present and functional on peripheral blood polymorphonuclear leukocytes (PMN). Human PMN express mRNA for Noci-R, its nucleotide sequence determined, and specific binding with [(125)I]-labeled nociceptin gave an apparent K(d) approximately 1.5 nM for this PMN opioid receptor. Nociceptin evoked PMN chemotaxis with maximal activity at 100 pM, without intracellular Ca(2+) mobilization. When injected in murine air pouches, nociceptin elicited leukocyte infiltration in a concentration-dependent fashion. Nociceptin-stimulated PMN infiltration was inhibited by treating mice with a synthetic analog of the aspirin-triggered lipid mediator 15-epi-lipoxin A(4). The present results identify nociceptin as a potent chemoattractant and provide a novel link between the neural and immune systems that are blocked by aspirin-triggered lipid mediators and may be relevant in neurogenic inflammation.

Complete inhibition of purinoceptor agonist-induced nociception by spinorphin, but not by morphine

We found that spinorphin, a novel neuropeptide showed analgesia in a different manner compared with morphine. By measuring flexor responses induced by the intraplanter injection of substances, the presence of three different types of sensory neurons were demonstrated. Although spinorphin completely blocked 2-metylthioadenosine (2-MeS ATP, a P2X(3) agonist)-induced responses, morphine did not. On the other hand, morphine-induced blockade of bradykinin (BK, a B(2)-receptor agonist)-responses was attenuated by pertussis toxin (PTX) treatment, whereas that of spinorphin was not. Thus it is suggested that spinorphin has a spectrum of analgesia which covers the blockade of nociception insensitive to morphine.

An enzymatically stable kyotorphin analog induces pain in subattomol doses

Intraplantar injection of the enzymatically stable, N-methylated kyotorphin analog Tyr(NMe)-Arg-OH produced marked and sharp nociceptive flexor responses in a dose-dependent manner. A significant response was observed with this compound at a dose of 0. 01 amol (6000 molecules). Tyr(NMe)-Arg-OH-nociception was completely blocked by the kyotorphin antagonist leucyl-arginine and its enzymatically stable, N-methylated analog, as well as by CP-99994, a specific neurokinin 1 antagonist. These findings suggest that the nociceptive effect produced by Tyr(NMe)-Arg-OH in subattomol doses occurs via specific interaction with the kyotorphin receptor and that the extraordinary potency observed may result from amplification through local substance P release.

Enhanced nociception by exogenous and endogenous substance P given into the spinal cord in mice lacking NR(2)A/epsilon(1), an NMDA receptor subunit

In capsaicin-pretreated mice, the nociceptive responses induced by intrathecally (i.t.) administered substance P (SP) were enhanced by N-methyl-D-aspartate (NMDA)-type receptor antagonists, dizocilpine (MK801) and D-2-amino-5-phosphonopentanoate (D-AP5) in a dose-dependent manner. Similar enhancement of SP-induced nociception was also observed in mice lacking the NMDA-type glutamate receptor NR2A/epsilon(1) subunit gene (GluRepsilon(1)(-/-) mice). On the other hand, GluRepsilon(1)(-/-) mice showed a marked enhancement of the peripheral nociceptive responses induced by intraplantar (i.pl.) injection of SP and bradykinin (BK). As the nociceptive responses to SP and BK (i.pl.) were both antagonized by CP-99994, an neurokinin(1) (NK(1)) antagonist (i.t.), these results suggest that GluRepsilon(1) receptor may play an inhibitory role in the downstream mechanisms of primary nociceptive SP neurones, possibly through activation of unidentified inhibitory neurones.

Lysophosphatidic acid-induced, pertussis toxin-sensitive nociception through a substance P release from peripheral nerve endings in mice

The intraplantar injection of lysophosphatidic acid (LPA) at doses of 0.1-100 pmol into the hind limb of mice showed dose-dependent nociceptive flexor responses. Repeated challenges of LPA at 100 pmol every 5 min showed constant responses at least for 30 min. The prior application of pertussis toxin (PTX) at a dose of 10 ng markedly reduced the following LPA (100 pmol) actions. In addition, the intraplantar application of CP-99994 (1 pmol), a substance P (NK1) receptor antagonist, but not CP-100263 (1 pmol), an inactive derivative, also markedly reduced the LPA responses. These findings suggest that LPA has a nociception-producing activity on sensory neurons through G(i/o) activation and substance P release from nociceptor endings.

Low dose of kyotorphin (tyrosine-arginine) induces nociceptive responses through a substance P release from nociceptor endings

The intraplantar injection of kyotorphin (Kyo) elicited nociceptive flexor responses in mice in a dose-dependent manner between 0.1 and 100 fmol. These actions were completely blocked by substance P (NK1) receptor antagonists, such as CP-96345 and CP-99994, but not by their inactive derivatives, CP-96344 or CP-100263, nor by MEN-10376, an NK2 antagonist. Kyo-responses were also abolished by the local pretreatment with capsaicin to deplete substance P from nociceptor endings, and in tachykinin 1 gene K/O mice. These findings suggest that Kyo indirectly stimulates nociceptor endings through a local substance P release.

Peripheral morphine analgesia resistant to tolerance in chronic morphine-treated mice

Intraplantarly (i.pl.)-injected morphine showed a peripheral analgesia in experiments to assess the blockade of bradykinin (i.pl.)-induced nociceptive flexor response in mice. The peripheral morphine analgesia in mice which developed central analgesic tolerance to chronic morphine (10 mg/kg s.c., 5 days), was equivalent to that in vehicle-treated mice in any doses between 0.1 and 1 nmol (i.pl.). These findings suggest that morphine tolerance in the central analgesia may use unique mechanisms deficient in the peripheral nervous system.

In vivo molecular signal transduction of peripheral mechanisms of pain

Although we have obtained a number of pharmacological tools and mutant mice lacking specific genes related to the pain, the distinct molecular basis of the pain-producing mechanism has remained to be fully clarified since we have been using conventional paradigms of the nociception test that may drive multiple endogenous molecules affecting nociception at the same time. Here, I will introduce a new paradigm of the nociception test. In this test, we focused on polymodal C-fibers by measuring nociceptive flexor responses induced by the peripheral application of a single species of nociceptive molecule. In addition, we identified the site of drug actions on nociceptor endings by the fact that the nociception was abolished by the intrathecal pretreatment with antisense oligodeoxynucleotide for receptors. Throughout experiments using this paradigm of the nociception test, it was firstly revealed that substance P, a major neurotransmitter of polymodal C-fibers, directly stimulates nociceptor endings through activation of Gq/11 and phospholipase C, followed by Ca2+ influx through plasma membrane-bound inositol trisphosphate receptors, and that bradykinin and histamine, both endogenous representative pain-producing substances, share this mechanism. Another unique mechanism is through Gi-coupled receptors such as receptors for nociceptin (orphanin FQ) or kyotorphin (tyrosine-arginine). The latter mechanism was found to be mediated through a substance P release from nociceptor endings. Future studies including some modifications of this paradigm should be also clinically useful for neuropathic pain research as well as understanding of pain physiology.

In vivo signal transduction of tetrodotoxin-sensitive nociceptive responses by substance P given into the planta of the mouse hind limb

1. We developed a simple and sensitive peripheral analgesic test in mice. 2. Substance P (SP) given into the planta (i.pl.) of the mouse hind limb produced a flexor response. The flexor response was dependent on SP doses (0.1-100 pmol, i.pl.). When SP (10 pmol) was given every 5 min, there were stable flexor responses. These nociceptive responses were completely abolished by CP-96,345, a neurokinin 1 receptor antagonist. 3. SP-induced responses were also blocked by several signal transduction-related compounds, such as tetrodotoxin, EGTA, and U73122, a selective phospholipase C inhibitor. 4. These findings suggest that SP depolarizes peripheral nerve endings, possibly through inositol trisphosphate (Ins P3)-gated Ca2+ influx, followed by induction of action potentials in the peripheral axons of primary afferent neurons.

Nociceptin/orphanin FQ-induced nociceptive responses through substance P release from peripheral nerve endings in mice

We have studied the in vivo signaling mechanisms involved in nociceptin/orphanin FQ (Noci)-induced pain responses by using a flexor-reflex paradigm. Noci was 10,000 times more potent than substance P (SP) in eliciting flexor responses after intraplantar injection into the hind limb of mice, but the action of Noci seems to be mediated by SP. Mice pretreated with an NK1 tachykinin receptor antagonist or capsaicin, or mice with a targeted disruption of the tachykinin 1 gene no longer respond to Noci. The action of Noci appears to be mediated by the Noci receptor, a pertussis toxin-sensitive G protein-coupled receptor that stimulates inositol trisphosphate receptor and Ca2+ influx. These findings suggest that Noci indirectly stimulates nerve endings of nociceptive primary afferent neurons through a local SP release.