Analytical characterization and comparison of the blood-brain barrier permeability of eight opioid peptides

Biomimetic Dual-Target Theranostic Nanovaccine Enables Magnetic Resonance Imaging and Chemo/Chemodynamic/Immune Therapy of Glioma

Development of theranostic nanomedicines to tackle glioma remains to be challenging. Here, we present an advanced blood-brain barrier (BBB)-crossing nanovaccine based on cancer cell membrane-camouflaged poly(N-vinylcaprolactam) (PVCL) nanogels (NGs) incorporated with MnO2 and doxorubicin (DOX). We show that the disulfide bond-cross-linked redox-responsive PVCL NGs can be functionalized with dermorphin and imiquimod R837 through cell membrane functionalization. The formed functionalized PVCL NGs having a size of 220 nm are stable, can deplete glutathione, and responsively release both Mn2+ and DOX under the simulated tumor microenvironment to exert the chemo/chemodynamic therapy mediated by DOX and Mn2+, respectively. The combined therapy induces tumor immunogenic cell death to maturate dendritic cells (DCs) and activate tumor-killing T cells. Further, the nanovaccine composed of cancer cell membranes as tumor antigens, R837 as an adjuvant with abilities of DC maturation and macrophages M1 repolarization, and MnO2 with Mn2+-mediated stimulator of interferon gene activation of tumor cells can effectively act on both targets of tumor cells and immune cells. With the dermorphin-mediated BBB crossing, cell membrane-mediated homologous tumor targeting, and Mn2+-facilitated magnetic resonance (MR) imaging property, the designed NG-based theranostic nanovaccine enables MR imaging and combination chemo-, chemodynamic-, and imnune therapy of orthotopic glioma with a significantly decreased recurrence rate.

Analgesic tolerance and cross-tolerance to the bifunctional opioid/neuropeptide FF receptors agonist EN-9 and μ-opioid receptor ligands at the supraspinal level in mice

The chimeric peptide EN-9 was reported as a κ-opioid/neuropeptide FF receptors bifunctional agonist that modulated chronic pain with no tolerance. Many lines of evidence have shown that the effect of the κ-opioid receptor is mediated by not only its specific activation but also downstream events participation, especially interaction with the μ-opioid receptor pathway in antinociception and tolerance on most occasions. The present study investigated the acute and chronic cross-tolerance of EN-9 with μ-opioid receptor agonist EM-2, DAMGO, and morphine after intracerebroventricularly (i.c.v) injection in the mouse tail-flick test. In the acute tolerance test, EN-9 showed symmetrical acute cross-tolerance to DAMGO but no cross-tolerance to EM2. In the chronic tolerance test, EN-9 had no tolerance after eight days of repeated administration. However, EN-9 illustrated complete cross-tolerance to morphine and symmetrical cross-tolerance to EM2. In addition, inhibition of NPFF receptor could induce the tolerance development of EN-9. These findings indicated that supraspinal EN-9-induced antinociception contains additional components, which are mediated by the downstream μ-opioid receptor pathway both in acute and chronic treatment, whereas the subtypes of μ-opioid receptor or NPFF system pathway involved in antinociceptive effects induced by EN-9 are complex. Identifying the receptor mechanism could help design preferable bifunctional opioid compounds.

Mu-opioid receptor-dependent transformation of respiratory motor pattern in neonates in vitro

Endogenous opioid peptides activating mu-opioid receptors (MORs) are part of an intricate neuromodulatory system that coordinates and optimizes respiratory motor output to maintain blood-gas homeostasis. MOR activation is typically associated with respiratory depression but also has excitatory effects on breathing and respiratory neurons. We hypothesized that low level MOR activation induces excitatory effects on the respiratory motor pattern. Thus, low concentrations of an MOR agonist drug (DAMGO, 10–200 nM) were bath-applied to neonatal rat brainstem-spinal cord preparations while recording inspiratory-related motor output on cervical spinal roots (C4-C5). Bath-applied DAMGO (50–200 nM) increased inspiratory motor burst amplitude by 40–60% during (and shortly following) drug application with decreased burst frequency and minute activity. Reciprocal changes in inspiratory burst amplitude and frequency were balanced such that 20 min after DAMGO (50–200 nM) application, minute activity was unaltered compared to pre-DAMGO levels. The DAMGO-induced inspiratory burst amplitude increase did not require crossed cervical spinal pathways, was expressed on thoracic ventral spinal roots (T4-T8) and remained unaltered by riluzole pretreatment (blocks persistent sodium currents associated with gasping). Split-bath experiments showed that the inspiratory burst amplitude increase was induced only when DAMGO was bath-applied to the brainstem and not the spinal cord. Thus, MOR activation in neonates induces a respiratory burst amplitude increase via brainstem-specific mechanisms. The burst amplitude increase counteracts the expected MOR-dependent frequency depression and may represent a new mechanism by which MOR activation influences respiratory motor output.

Topical treatment with a mu opioid receptor agonist alleviates corneal allodynia and corneal nerve sensitization in mice

Corneal pain is considered to be a core symptom of ocular surface disruption and inflammation. The management of this debilitating condition is still a therapeutic challenge. Recent evidence supports a role of the opioid system in the management of corneal nociception. However, the functional involvement of the mu opioid receptor (MOR) underlying this analgesic effect is not known. We first investigated the expression of the MOR in corneal nerve fibers and trigeminal ganglion (TG) neurons in control mice and a mouse model of corneal inflammatory pain. We then evaluated the anti-nociceptive and electrophysiological effects of DAMGO ([D-Ala²,N-Me-Phe⁴,Gly⁵-ol] enkephalin), a MOR-selective ligand. MOR immunoreactivity was detected in corneal nerve fibers and primary afferent neurons of the ophthalmic branch of the TG of naive mice. MOR expression was significantly higher in both structures under conditions of inflammatory corneal pain. Topical ocular administration of DAMGO strongly reduced both the mechanical (von Frey) and chemical (capsaicin) corneal hypersensitivity associated with inflammatory ocular pain. Repeated instillations of DAMGO also markedly reversed the elevated spontaneous activity of the ciliary nerve and responsiveness of corneal polymodal nociceptors that were observed in mice with corneal pain. Finally, these DAMGO-induced behavioral and electrophysiological responses were totally blunted by the topical application of naloxone methiodide, an opioid receptor antagonist. Overall, these results provide evidence that topical pharmacological MOR activation may constitute a therapeutic target for the treatment of corneal pain and improve corneal nerve function to alleviate chronic pain.

[Endomorphins: structure, localization, immunoregulatory activity]

Endomorphins – endogenous tetrapeptides with the highest affinity for the µ-opioid receptor. Currently, two tetrapeptides that differ in one amino acid residue have been isolated and characterized. The structure of endomorphins differs from the structure of members of three main families of opioid peptides: endorphins, enkephalins, and dynorphins, which contain the same N-terminal sequence. In the central nervous system, endomorphins are distributed everywhere, where they are primarily responsible for antinociception. Distribution of endomorphins in the immune system, similar to that of other opioid peptides, has allowed to suggest their active participation in the processes of immune regulation. This review summarizes modern views on the structure of endomorphins, their localization, possible intracellular mechanisms of signal transmission and their effects on the processes of activation, proliferation and differentiation of cells of innate and adaptive immunity. Endomorphins actively modulate the functions of the cells of the immune system. Peptides predominantly suppress adaptive immunity reactions. There effects on the functions of innate immunity cells (granulocytes, macrophages, monocytes, dendritic cells) depending on the conditions and can have either an inhibitory or stimulating orientation. Thus, endomorphins can be promising compounds that can effectively regulate both nociceptive signals and processes in the immune system.

In Vitro Potency and Preclinical Pharmacokinetic Comparison of All-D-Enantiomeric Peptides Developed for the Treatment of Alzheimer's Disease

Diffusible amyloid-β (Aβ) oligomers are currently presumed to be the most cytotoxic Aβ assembly and held responsible to trigger the pathogenesis of Alzheimer’s disease (AD). Thus, Aβ oligomers are a prominent target in AD drug development. Previously, we reported on our solely D-enantiomeric peptide D3 and its derivatives as AD drug candidates. Here, we compare one of the most promising D3 derivatives, ANK6, with its tandem version (tANK6), and its head-to-tail cyclized isoform (cANK6r). In vitro tests investigating the D-peptides’ potencies to inhibit Aβ aggregation, eliminate Aβ oligomers, and reduce Aβ-induced cytotoxicity revealed that all three D-peptides efficiently target Aβ. Subsequent preclinical pharmacokinetic studies of the three all-D-peptides in wildtype mice showed promising blood-brain barrier permeability with cANK6r yielding the highest levels in brain. The peptides’ potencies to lower Aβ toxicity and their remarkable brain/plasma ratios make them promising AD drug candidates.

Comparison of blood-brain barrier penetration efficiencies between linear and cyclic all-d-enantiomeric peptides developed for the treatment of Alzheimer's disease

Alzheimer's disease (AD), until now, is an incurable progressive neurodegenerative disease. To target toxic amyloid β oligomers in AD patients' brains and to convert them into non-toxic aggregation-incompetent species, we designed peptides consisting solely of d-enantiomeric amino acid residues. The original lead compound was named D3 and several D3 derivatives were designed to enhance beneficial properties. Here, we compare four d-peptides concerning their efficiencies to pass the blood-brain barrier (BBB). We demonstrate that the d-peptides' concentrations in murine brain directly correlate with concentrations in cerebrospinal fluid. The cyclic d-enantiomeric peptide cRD2D3 is characterized by the highest efficiency to pass the BBB. For in total three cyclic peptides we show that administration of cyclic peptides resulted in up to tenfold higher peak concentrations in brain as compared to their linear equivalents which have partially been characterized before (Jiang et al., 2015; Leithold et al., 2016a). These results suggest that cyclic peptides pass the murine BBB more efficiently than their linear equivalents. cRD2D3's proteolytic stability, oral bioavailability, long duration of action and its favorable brain/plasma ratio reveal that it may become a suitable drug for long-term AD-treatment from a pharmacokinetic point of view.

Physico-chemical characterization of formulations containing endomorphin-2 derivatives

In this study semisolid formulations containing AcYPFF (N-acetyl-Tyr-Pro-Phe-Phe-NH₂) tetrapeptide were obtained and characterized in terms of rheology, stability by multiple light scattering and particle size distribution by laser diffraction. Additionally, the release studies of tetrapeptide from formulations obtained were performed. The influence of different factors such as semisolid and membrane type on tetrapeptide release rate was examined. The release experiments of tetrapeptide modified with palmitoyl group (PalmYPFF) were also carried out. The results proved that formulation type and its rheological properties strongly determined the permeation process of the tetrapeptide. The fastest release of tetrapeptide was observed from hydrogel that had the lowest viscosity. The kinetic data of tetrapeptide released from oil-in-water (o/w) and water-in-oil (w/o) emulsions prepared at elevated temperature showed good fit to the Higuchi equation, whereas when AcYPFF was released from oil-in-water (o/w) emulsion prepared with the addition of auto-emulsifier high linearity with Korsmeyer–Peppas model was observed. While when tetrapeptide was released from Hydrogel the most suitable model was the first-order kinetics. It was suggested that mechanism that led to the release of tetrapeptide from all formulations was non-Fickian diffusion transport. The presence of palmitoyl group changed the solubility of tetrapeptide both in formulation and receptor fluid and thus the release rate of active compound was modified.

N-Cadherin is Involved in Neuronal Activity-Dependent Regulation of Myelinating Capacity of Zebrafish Individual Oligodendrocytes In Vivo

Stimulating neuronal activity increases myelin sheath formation by individual oligodendrocytes, but how myelination is regulated by neuronal activity in vivo is still not fully understood. While in vitro studies have revealed the important role of N-cadherin in myelination, our understanding in vivo remains quite limited. To obtain the role of N-cadherin during activity-dependent regulation of myelinating capacity of individual oligodendrocytes, we successfully built an in vivo dynamic imaging model of the Mauthner cell at the subcellular structure level in the zebrafish central nervous system. Enhanced green fluorescent protein (EGFP)-tagged N-cadherin was used to visualize the stable accumulations and mobile transports of N-cadherin by single-cell electroporation at the single-cell level. We found that pentylenetetrazol (PTZ) significantly enhanced the accumulation of N-cadherin in Mauthner axons, a response that was paralleled by enhanced sheath number per oligodendrocytes. By offsetting this phenotype using oligopeptide (AHAVD) which blocks the function of N-cadherin, we showed that PTZ regulates myelination in an N-cadherin-dependent manner. What is more, we further suggested that PTZ influences N-cadherin and myelination via a cAMP pathway. Consequently, our data indicated that N-cadherin is involved in neuronal activity-dependent regulation of myelinating capacity of zebrafish individual oligodendrocytes in vivo.

A Cyclic Tetrapeptide ("Cyclodal") and Its Mirror-Image Isomer Are Both High-Affinity μ Opioid Receptor Antagonists

Head-to-tail cyclization of the μ opioid receptor (MOR) agonist [Dmt¹]DALDA (H-Dmt-d-Arg-Phe-Lys-NH₂ (9; Dmt = 2',6'-dimethyltyrosine) resulted in a highly active, selective MOR antagonist, c[-d-Arg-Phe-Lys-Dmt-] (1) ("cyclodal"), with subnanomolar binding affinity. A docking study of cyclodal using the crystal structure of MOR in the inactive form showed a unique binding mode with the two basic residues of the ligand forming salt bridges with the Asp¹²⁷ and Glu²²⁹ receptor residues. Cyclodal showed high plasma stability and was able to cross the blood-brain barrier to reverse morphine-induced, centrally mediated analgesia when given intravenously. Surprisingly, the mirror-image isomer (optical antipode) of cyclodal, c[-Arg-d-Phe-d-Lys-d-Dmt-] (2), also turned out to be a selective MOR antagonist with 1 nM binding affinity, and thus, these two compounds represent the first example of mirror image opioid receptor ligands with both optical antipodes having high binding affinity. Reduction of the Lys-Dmt peptide bond in cyclodal resulted in an analogue, c[-d-Arg-Phe-LysΨ[CH₂NH]Dmt-] (8), with MOR agonist activity.

Blood-brain barrier transport kinetics of the neuromedin peptides NMU, NMN, NMB and NT

The neuromedin peptides are peripherally and centrally produced, but until now, it is generally believed that they only function as locally acting compounds without any quantitative knowledge about their blood-brain barrier (BBB) passage. Here, we characterize the transport kinetics of four neuromedins (NMU, NMN, NMB and NT) across the BBB, as well as their metabolization profile, and evaluate if they can act as endocrine hormones. Using the in vivo mouse model, multiple time regression (MTR), capillary depletion (CD) and brain efflux studies were performed. Data was fitted using linear (NMU, NT and NMB) or biphasic modeling (NMU and NMN). Three of the four investigated peptides, i.e. NMU, NT and NMN, showed a significant influx into the brain with unidirectional influx rate constants of 1.31 and 0.75 μL/(g × min) for NMU and NT respectively and initial influx constants (K1) of 72.14 and 7.55 μL/(g × min) and net influx constants (K) of 1.28 and 1.36 × 10(-16) μL/(g×min) for NMU and NMN respectively. The influx of NMB was negligible. Only NMN and NT showed a significant efflux out of the brain with an efflux constant (kout) of 0.042 min(-1) and 0.053 min(-1) respectively. Our results indicate that locally produced neuromedin peptides and/or fragments can be transported through the whole body, including passing the BBB, and taken up by different organs/tissues, supporting the idea that the neuromedins could have a much bigger role in the regulation of biological processes than currently assumed.

Ring size in cyclic endomorphin-2 analogs modulates receptor binding affinity and selectivity

The study reports the solid-phase synthesis and biological evaluation of a series of new side chain-to-side chain cyclized opioid peptide analogs of the general structure Tyr-[D-Xaa-Phe-Phe-Asp]NH2, where Xaa = Lys (1), Orn (2), Dab (3), or Dap (4) (Dab = 2,4-diaminobutyric acid, Dap = 2,3-diaminopropionic acid), containing 17- to 14-membered rings. The influence of the ring size on binding to the MOP, DOP and KOP opioid receptors was studied. In general, the reduction of the size of the macrocyclic ring increased the selectivity for the MOP receptor. The cyclopeptide incorporating Xaa = Lys displayed subnanomolar MOP affinity but modest selectivity over the KOP receptor, while the analog with the Orn residue showed increased affinity and selectivity for MOP. The analog with Dab was a weak MOP agonist and did not bind to the other two opioid receptors. Finally, the peptide with Xaa = Dap was completely MOP receptor-selective with subnanomolar affinity. Interestingly, the deletion of one Phe residue from 1 led to the 14-membered Tyr-c[D-Lys-Phe-Asp]NH2 (5), a potent and selective MOP receptor ligand. The in vitro potencies of the new analogs were determined in a calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. A good correlation between binding and the functional test results was observed. The influence of the ring size, solid support and the N-terminal protecting group on the formation of cyclodimers was studied.

Quorum Sensing Peptides Selectively Penetrate the Blood-Brain Barrier

Bacteria communicate with each other by the use of signaling molecules, a process called 'quorum sensing'. One group of quorum sensing molecules includes the oligopeptides, which are mainly produced by Gram-positive bacteria. Recently, these quorum sensing peptides were found to biologically influence mammalian cells, promoting i.a. metastasis of cancer cells. Moreover, it was found that bacteria can influence different central nervous system related disorders as well, e.g. anxiety, depression and autism. Research currently focuses on the role of bacterial metabolites in this bacteria-brain interaction, with the role of the quorum sensing peptides not yet known. Here, three chemically diverse quorum sensing peptides were investigated for their brain influx (multiple time regression technique) and efflux properties in an in vivo mouse model (ICR-CD-1) to determine blood-brain transfer properties: PhrCACET1 demonstrated comparatively a very high initial influx into the mouse brain (Kin = 20.87 μl/(g×min)), while brain penetrabilities of BIP-2 and PhrANTH2 were found to be low (Kin = 2.68 μl/(g×min)) and very low (Kin = 0.18 μl/(g×min)), respectively. All three quorum sensing peptides were metabolically stable in plasma (in vitro) during the experimental time frame and no significant brain efflux was observed. Initial tissue distribution data showed remarkably high liver accumulation of BIP-2 as well. Our results thus support the potential role of some quorum sensing peptides in different neurological disorders, thereby enlarging our knowledge about the microbiome-brain axis.

Pronociceptive and Antinociceptive Effects of Buprenorphine in the Spinal Cord Dorsal Horn Cover a Dose Range of Four Orders of Magnitude

Due to its distinct pharmacological profile and lower incidence of adverse events compared with other opioids, buprenorphine is considered a safe option for pain and substitution therapy. However, despite its wide clinical use, little is known about the synaptic effects of buprenorphine in nociceptive pathways. Here, we demonstrate dose-dependent, bimodal effects of buprenorphine on transmission at C-fiber synapses in rat spinal cord dorsal horn in vivo. At an analgesically active dose of 1500 μg·kg(-1), buprenorphine reduced the strength of spinal C-fiber synapses. This depression required activation of spinal opioid receptors, putatively μ1-opioid receptors, as indicated by its sensitivity to spinal naloxone and to the selective μ1-opioid receptor antagonist naloxonazine. In contrast, a 15,000-fold lower dose of buprenorphine (0.1 μg·kg(-1)), which caused thermal and mechanical hyperalgesia in behaving animals, induced an enhancement of transmission at spinal C-fiber synapses. The ultra-low-dose buprenorphine-induced synaptic facilitation was mediated by supraspinal naloxonazine-insensitive, but CTOP-sensitive μ-opioid receptors, descending serotonergic pathways, and activation of spinal glial cells. Selective inhibition of spinal 5-hydroxytryptamine-2 receptors (5-HT2Rs), putatively located on spinal astrocytes, abolished both the induction of synaptic facilitation and the hyperalgesia elicited by ultra-low-dose buprenorphine. Our study revealed that buprenorphine mediates its modulatory effects on transmission at spinal C-fiber synapses by dose dependently acting on distinct μ-opioid receptor subtypes located at different levels of the neuraxis.

Cyclic endomorphin analogs in targeting opioid receptors to achieve pain relief

Endomorphins, the endogenous ligands of the µ-opioid receptor, are attractive candidates for opioid-based pain-relieving agents. These tetrapeptides, with their remarkable affinity for the µ-opioid receptor, display favorable antinociceptive activity when injected directly into the brain of experimental animals. However, the application of endomorphins as clinical analgesics has been impeded by their instability in body fluids and inability to reach the brain after systemic administration. Among numerous modifications of the endomorphin structure aimed at improving their pharmacological properties, cyclization can be viewed as an interesting option. Here, we have summarized recent advances in obtaining endomorphin-based cyclic peptide analogs.

In vitro and in vivo efficacy of a potent opioid receptor agonist, biphalin, compared to subtype-selective opioid receptor agonists for stroke treatment

To meet the challenge of identification of new treatments for stroke, this study was designed to evaluate a potent, nonselective opioid receptor (OR) agonist, biphalin, in comparison to subtype selective OR agonists, as a potential neuroprotective drug candidate using in vitro and in vivo models of ischemic stroke. Our in vitro approach included mouse primary neuronal cells that were challenged with glutamate and hypoxic/aglycemic (H/A) conditions. We observed that 10nM biphalin, exerted a statistically significant neuroprotective effect after glutamate challenge, compared to all selective opioid agonists, according to lactate dehydrogenase (LDH) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Moreover, 10nM biphalin provided superior neuroprotection after H/A-reoxygenation compared to selective opioid agonists in all cases. Our in vitro investigations were supported by in vivo studies which indicate that the nonselective opioid agonist, biphalin, achieves enhanced neuroprotective potency compared to any of the selective opioid agonists, evidenced by reduced edema and infarct ratios. Reduction of edema and infarction was accompanied by neurological improvement of the animals in two independent behavioral tests. Collectively these data strongly suggest that concurrent agonist stimulation of mu, kappa and delta ORs with biphalin is neuroprotective and superior to neuroprotection by activation of any single OR subtype.

Blood-brain transfer and antinociception of linear and cyclic N-methyl-guanidine and thiourea-enkephalins

Enkephalins are active in regulation of nociception in the body and are key in development of new synthetic peptide analogs that target centrally located opioid receptors. In this study, we investigated the in vivo blood-brain barrier (BBB) penetration behavior and antinociceptive activity of two cyclic enkephalin analogs with a thiourea (CycS) or a N-methyl-guanidine bridge (CycNMe), and their linear counterparts (LinS and LinNMe) in mice, as well as their in vitro metabolic stability. (125)I-LinS had the highest blood-brain clearance (K1=3.46μL/gmin), followed by (125)I-LinNMe, (125)I-CycNMe, and (125)I-CycS (K1=1.64, 0.31, and 0.11μL/gmin, respectively). Also, these peptides had a high metabolic stability (t1/2>1h) in mouse serum and brain homogenate, and half-inhibition constant (Ki) values in the nanomolar range with predominantly μ-opioid receptor selectivity. The positively charged NMe-enkephalins showed a higher antinociceptive activity (LinNMe: 298% and CycNMe: 205%), expressed as molar-dose normalized area under the curve (AUC) relative to morphine, than the neutral S-enkephalins (CycS: 122% and LinS: 130%).

Synthesis of tripeptides containing D-Trp substituted at the indole ring, assessment of opioid receptor binding and in vivo central antinociception

The noncationizable tripeptide Ac-D-Trp-Phe-GlyNH2 was recently proposed as a novel minimal recognition motif for μ-opioid receptor. The introduction of different substituents (methyl, halogens, nitro, etc.) at the indole of D-Trp significantly influenced receptor affinities and resulted in serum stability and in a measurable effect on central antinociception in mice after ip administration.

Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression

BACKGROUND

Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression.

METHODS

Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy.

RESULTS

Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH.

CONCLUSIONS

Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Design of brain imaging agents for positron emission tomography: do large bioconjugates provide an opportunity for in vivo brain imaging?

The development of brain imaging agents for positron emission tomography and other in vivo imaging modalities mostly relies on small compounds of low MW as a result of the restricted transport of larger molecules, such as peptides and proteins, across the blood–brain barrier. Besides passive transport, only a few active carrier mechanisms, such as glucose transporters and amino acid transporters, have so far been exploited to mediate the accumulation of imaging probes in the brain. An important question for the future is whether some of the abundant active carrier systems located at the blood–brain barrier can be used to shuttle potential, but non-crossing, imaging agents into the brain. What are the biological and chemical constrictions toward such bioconjugates and is it worthwhile to persue such a delivery strategy?

Role of endogenous opioid peptides in the infarct size-limiting effect of adaptation to chronic continuous hypoxia

AIMS

The objective of this study was to examine the involvement of endogenous opioid peptides and opioid receptor (OR) subtypes in the cardioprotective effect of adaptation to chronic hypoxia in rats.

MAIN METHODS

Rats were exposed to continuous normobaric hypoxia (CNH; 12% oxygen) for 3 weeks. Myocardial ischemia was induced by 20-min coronary artery occlusion followed by 3-h reperfusion in anesthetized open-chest animals. Various OR antagonists were administered to rats prior to ischemia. The size of myocardial infarction and the incidence of ischemic ventricular arrhythmias were assessed. Myocardial and plasma concentrations of opioid peptides (met-enkephalin, β-endorphin, and endomorphins) were determined.

KEY FINDINGS

Adaptation to CNH significantly increased myocardial and plasma concentrations of opioids, potentiated their further elevation by ischemia/reperfusion, and reduced myocardial infarct size, but it did not affect the incidence of ischemic arrhythmias. The infarct size-limiting effect of CNH was abolished by OR antagonists naltrexone (non-selective), naloxone methiodide (non-selective peripherally acting), TIPP[ψ] (δ-OR), naltriben (δ2-OR), or CTAP (μ-OR), while BNTX (δ1-OR) and nor-binaltorphimine (κ-OR) had no effect.

SIGNIFICANCE

The results suggest that the infarct size-limiting effect afforded by adaptation to CNH is mediated by activation of peripheral δ2- and μ-ORs by elevated levels of endogenous opioid peptides.

μ-Opioid blockade reduces ethanol effects on intake and behavior of the infant rat during short-term but not long-term social isolation

Numerous findings in adult and infant rats have shown that the endogenous opioid system is involved in control of ethanol consumption and its reinforcing effects. Opioid systems are also involved in reactivity to social isolation with several factors (age, duration, and type of isolation) affecting this modulation. The present study investigated the effects of a selective mu-opioid antagonist CTOP (0, 0.1, 0.5mg/kg), ethanol (0, 0.5 g/kg), and the interaction of the two drugs on the behavioral consequences of two types of social isolation given to preweanling rats: 1) short-term social isolation from littermates (STSI, duration 8 min) and 2) relatively long-term (5h) isolation (LTSI) from the dam and littermates. Voluntary intake of saccharin, locomotion, rearing activity, paw licking, and grooming were assessed during an 8-min. intake test. Thermal nociceptive reactivity was also measured before and after the testing session with normalized differences in pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). The results indicate that pharmacological blockade of mu-opioid receptors by CTOP substantially attenuated ethanol's anxiolytic effects on the developing rat's reactions to social isolation. Some of these stress-attenuating effects of CTOP were observed only in animals exposed to short-term isolation (STSI) but not in pups isolated for 5h (LTSI). Ethanol selectively increased saccharin intake during STSI in females and CTOP blocked this effect. Ethanol decreased the magnitude of analgesia associated with STSI but had no effect on pain reactivity during LTSI. CTOP by itself did not affect IIA or saccharin intake in sober animals. The findings of the present experiments indicate that the anxiolytic effects of 0.5 g/kg ethanol on pups exposed to STSI are modulated by endogenous opioid activity.

Variation of the net charge, lipophilicity, and side chain flexibility in Dmt(1)-DALDA: Effect on Opioid Activity and Biodistribution

The influence of the side chain charges of the second and fourth amino acid residues in the peptidic μ opioid lead agonist Dmt-d-Arg-Phe-Lys-NH(2) ([Dmt(1)]-DALDA) was examined. Additionally, to increase the overall lipophilicity of [Dmt(1)]-DALDA and to investigate the Phe(3) side chain flexibility, the final amide bond was N-methylated and Phe(3) was replaced by a constrained aminobenzazepine analogue. The in vitro receptor binding and activity of the peptides, as well as their in vivo transport (brain in- and efflux and tissue biodistribution) and antinociceptive properties after peripheral administration (ip and sc) in mice were determined. The structural modifications result in significant shifts of receptor binding, activity, and transport properties. Strikingly, while [Dmt(1)]-DALDA and its N-methyl analogue, Dmt-d-Arg-Phe-NMeLys-NH(2), showed a long-lasting antinociceptive effect (>7 h), the peptides with d-Cit(2) generate potent antinociception more rapidly (maximal effect at 1h postinjection) but also lose their analgesic activity faster when compared to [Dmt(1)]-DALDA and [Dmt(1),NMeLys(4)]-DALDA.

Enhanced brain delivery of the opioid peptide DAMGO in glutathione pegylated liposomes: a microdialysis study

Glutathione PEGylated (GSH-PEG) liposomes were evaluated for their ability to enhance and prolong blood-to-brain drug delivery of the opioid peptide DAMGO (H-Tyr-d-Ala-Gly-MePhe-Gly-ol). An intravenous loading dose of DAMGO followed by a 2 h constant rate infusion was administered to rats, and after a washout period of 1 h, GSH-PEG liposomal DAMGO was administered using a similar dosing regimen. DAMGO and GSH-PEG liposomal DAMGO were also administered as a 10 min infusion to compare the disposition of the two formulations. Microdialysis made it possible to determine free DAMGO in brain and plasma, while the GSH-PEG liposomal encapsulated DAMGO was measured with regular plasma sampling. The antinociceptive effect of DAMGO was determined with the tail-flick method. All samples were analyzed using liquid chromatography-tandem mass spectrometry. The short infusion of DAMGO resulted in a fast decline of the peptide concentration in plasma with a half-life of 9.2 ± 2.1 min. Encapsulation in GSH-PEG liposomes prolonged the half-life to 6.9 ± 2.3 h. Free DAMGO entered the brain to a limited extent with a steady state ratio between unbound drug concentrations in brain interstitial fluid and in blood (Kp,uu) of 0.09 ± 0.04. GSH-PEG liposomes significantly increased the brain exposure of DAMGO to a Kp,uu of 0.21 ± 0.17 (p < 0.05). By monitoring the released, active substance in both blood and brain interstitial fluid over time, we were able to demonstrate that GSH-PEG liposomes offer a promising platform for enhancing and prolonging the delivery of drugs to the brain.

Opioid Peptides: Potential for Drug Development

Opioid receptors are important targets for the treatment of pain and potentially for other disease states (e.g. mood disorders and drug abuse) as well. Significant recent advances have been made in identifying opioid peptide analogs that exhibit promising in vivo activity for treatment of these maladies. This review focuses on the development and evaluation of opioid peptide analogs demonstrating activity after systemic administration, and recent clinical evaluations of opioid peptides for possible therapeutic use.

Brainpeps: the blood-brain barrier peptide database

Peptides are able to cross the blood-brain barrier (BBB) through various mechanisms, opening new diagnostic and therapeutic avenues. However, their BBB transport data are scattered in the literature over different disciplines, using different methodologies reporting different influx or efflux aspects. Therefore, a comprehensive BBB peptide database (Brainpeps) was constructed to collect the BBB data available in the literature. Brainpeps currently contains BBB transport information with positive as well as negative results. The database is a useful tool to prioritize peptide choices for evaluating different BBB responses or studying quantitative structure-property (BBB behaviour) relationships of peptides. Because a multitude of methods have been used to assess the BBB behaviour of compounds, we classified these methods and their responses. Moreover, the relationships between the different BBB transport methods have been clarified and visualized.

Low Molecular Weight Opioid Peptide Esters Could be Developed as a New Class of Analgesics

Low molecular weight opioid peptide esters (OPE) could become a class of analgesics with different side effect profiles than current opiates. OPE may have sufficient plasma stability to cross the blood brain barrier (BBB), undergo ester hydrolysis and produce analgesia. OPE of dipeptides, tyr-pro and tyr-gly conjugated to ethanol have a structure similar to the anesthestic agent, etomidate. Based upon the analgesic activity of dipeptide opioids, Lipinski's criteria, and permeability of select GABA esters to cross the BBB, opioid peptides (OP) conjugated to ethanol, cholesterol or 3-glucose are lead recommendations. Preliminary animal data suggests that tyr-pro-ethyl ester crosses the BBB and unexpectedly produces hyperalgesia. Currently, there are no approved OP analgesics available for clinical use. Clinical trials of good manufacturing practice OP administered to patients suffering from chronic pain with indwelling intrathecal pumps could resolve the issue that OP may be superior to opiates and may redirect research.

Peripheral antinociceptive effects of the cyclic endomorphin-1 analog c[YpwFG] in a mouse visceral pain model

We previously described a novel cyclic endomorphin-1 analog c[Tyr-D-Pro-D-Trp-Phe-Gly] (c[YpwFG]), acting as a mu-opioid receptor (MOR) agonist. This study reports that c[YpwFG] is more lipophilic and resistant to enzymatic hydrolysis than endomorphin-1 and produces preemptive antinociception in a mouse visceral pain model when injected intraperitoneally (i.p.) or subcutaneously (s.c.) before 0.6% acetic acid, employed to evoke abdominal writhing (i.p. ED(50)=1.24 mg/kg; s.c. ED(50)=2.13 mg/kg). This effect is reversed by the selective MOR antagonist β-funaltrexamine and by a high dose of the mu(1)-opioid receptor-selective antagonist naloxonazine. Conversely, the kappa-opioid receptor antagonist nor-binaltorphimine and the delta-opioid receptor antagonist naltrindole are ineffective. c[YpwFG] produces antinociception when injected i.p. after acetic acid (ED(50)=4.80 mg/kg), and only at a dose of 20mg/kg did it elicit a moderate antinociceptive response in the mouse, evaluated by the tail flick assay. Administration of a lower dose of c[YpwFG] (10mg/kg i.p.) apparently produces a considerable part of antinociception on acetic acid-induced writhes through peripheral opioid receptors as this action is fully prevented by i.p. naloxone methiodide, which does not readily cross the blood-brain barrier; whereas this opioid antagonist injected intracerebroventricularly (i.c.v.) is not effective. Antinociception produced by a higher dose of c[YpwFG] (20mg/kg i.p.) is partially reversed by naloxone methiodide i.c.v. administered. Thus, only at the dose of 20mg/kg c[YpwFG] can produce antinociception through both peripheral and central opioid receptors. In conclusion, c[YpwFG] displays sufficient metabolic stability to be effective after peripheral administration and demonstrates the therapeutic potential of endomorphin derivatives as novel analgesic agents to control visceral pain.

Synthesis and biological evaluation of novel peripherally active morphiceptin analogs

Morphiceptin (Tyr-Pro-Phe-Pro-NH(2)), a tetrapeptide present in the enzymatic digest of bovine beta-casein, is a selective ligand of the mu-opioid receptor. In the present study, we describe the synthesis of a series of novel morphiceptin analogs modified in positions 1-3. Two of the obtained analogs, [Dmt(1), D-Ala(2), D-1-Nal(3)]morphiceptin and [Dmt(1), D-NMeAla(2), D-1-Nal(3)]morphiceptin (Dmt-2',6'-dimethyltyrosine and d-1-Nal-3-(1-naphthyl)-D-alanine)) displayed very high mu-receptor affinity, resistance to enzymatic degradation, and remarkable supraspinally mediated analgesia, as shown in the hot-plate test after intracerebroventricular but not intravenous administration, which indicated that they could not cross the blood-brain barrier. Therefore, these two analogs were further tested in vitro and in vivo towards their possible peripheral analgesic activity and inhibitory effect on gastrointestinal (GI) motility. We report that both peptides showed strong antinociceptive effect in the writhing test after intraperitoneal administration, inhibited smooth muscle contractility in vitro and GI motility in vivo. Taken together, these findings indicate that the novel morphiceptin analogs which induce peripheral, but not central antinociception, inhibit GI transit, and possess exceptional metabolic stability, may provide an interesting approach to the development of peripherally restricted agents for the treatment of GI motility disorders, such as diarrhea or diarrhea-predominant irritable bowel syndrome.