"DAKLI": a multipurpose ligand with high affinity and selectivity for dynorphin (kappa opioid) binding sites

6-N,N-dimethylamino-2,3-naphthalimide: a new environment-sensitive fluorescent probe in delta- and mu-selective opioid peptides

A new environment-sensitive fluorophore, 6-N,N-(dimethylamino)-2,3-naphthalimide (6DMN) was introduced in the delta-selective opioid peptide agonist H-Dmt-Tic-Glu-NH(2) and in the mu-selective opioid peptide agonist endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH(2)). Environment-sensitive fluorophores are a special class of chromophores that generally exhibit a low quantum yield in aqueous solution but become highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. New fluorescent delta-selective irreversible antagonists (H-Dmt-Tic-Glu-NH-(CH(2))(5)-CO-Dap(6DMN)-NH(2) (1) and H-Dmt-Tic-Glu-Dap(6DMN)-NH(2) (2)) were identified as potential fluorescent probes showing good properties for use in studies of distribution and internalization of delta receptors by confocal laser scanning microscopy.

Highly selective fluorescent analogue of the potent delta-opioid receptor antagonist Dmt-Tic

A fluorescent tripeptide probe derived by coupling fluorescein to H-Dmt-Tic-Glu-NH2 was developed to interact with delta-opioid receptors with high affinity (Ki = 0.035 nM) and selectivity (Ki(mu)/Ki(delta) = 4371). It acts as an irreversible delta-opioid receptor antagonist, and binding to NG108-15 cells is blocked by the standard nonpeptidic delta-opioid receptor antagonist naltrindole. This probe should prove useful in the study of the distribution of delta-opioid receptors in tissues and the internalization of opioid peptides during signal transduction.

A solid-phase synthetic strategy for labeled peptides: synthesis of a biotinylated derivative of the delta opioid receptor antagonist TIPP (Tyr-Tic-Phe-Phe-OH)

A general solid-phase synthetic strategy for labeled peptides was developed and used to prepare a biotinylated derivative of the delta opioid receptor antagonist TIPP (Tyr-Tic-Phe-Phe-OH). A monoprotected hydrophilic diamine linker was attached to an aldehyde-containing solid-phase resin by reductive amination, followed by introduction of biotin and peptide synthesis to yield Tyr-Tic-Phe-Phe-Asp-NH(CH(2)CH(2)O)(2)CH(2)CH(2)NH-biotin (2). The high delta receptor affinity and selectivity of 2 demonstrate the applicability of this design approach for labeled peptide derivatives.

Extended TIP(P) analogues as precursors for labeled delta-opioid receptor ligands

Tyr-Tic-Phe-Phe-OH (TIPP) and the shorter Tyr-Tic-Phe-OH (TIP) peptides are potent and highly selective antagonists at the delta-opioid receptor and, therefore, are ideal candidates for the attachment of labels to assist in the study of delta-opioid receptors. Peptides extended at the C-terminus with residues which can be used as handles for further modification and/or labeling (i.e. Asx, Glx, and Lys) were synthesized. The TIPP-D/L-Asx/Glx derivatives exhibited similar delta-receptor affinity to TIPP (K(i) = 5-10 nM vs K(i) = 6 nM), and neither the location of the carboxylic acid moiety nor the stereochemistry of the C-terminal residue significantly affected the delta-receptor affinity of these derivatives. Extension of TIPP with an additional residue did not increase mu-receptor affinity, even though the position of the acidic group, which imparts delta-receptor selectivity to TIPP, was shifted relative to the carboxylic acid moiety of TIPP. The delta-receptor affinities of the TIP-D/L-Asx/Glx derivatives were found to be influenced mainly by the position of the carboxylic acid function rather than the stereochemistry of the C-terminal residue. TIP(P)-D/L-Lys(Ac)-OH derivatives exhibited moderate delta-receptor affinity (K(i)(delta) = 16-28 nM). The most potent compounds found in the extended TIP(P) series were TIPP-D-Gln-OH and TIP-D-Gln-OH (K(i)(delta) = 5 nM) which had similar affinities to TIPP.

Binding and internalization of fluorescent opioid peptide conjugates in living cells

The dynamics of agonist-stimulated opioid receptor internalization and trafficking have been difficult to study in living cells in part because the available probes were inadequate. To overcome this obstacle, six new fluorescent opioid peptides were developed. Dermorphin (DERM), deltorphin (DELT), TIPP, and endomorphin were conjugated to BODIPY TR or Alexa Fluor 488, two fluorescent dyes with distinct hydrophobic properties. In membrane binding assays the fluorescent conjugates DERM-A488 or -BTR, DELT-A488 or -BTR, and TIPP-A488 displayed good binding affinity and selectivity for mu- and delta-opioid receptor subtypes. Furthermore, the fluorescent conjugates of dermorphin and deltorphin were biologically active as demonstrated by their ability to hyperpolarize locus coeruleus neurons (DERM-A488 or -BTR) and inhibit calcium currents in NG108-15 (DELT-A488). Both of these responses were antagonized by naloxone. In conjunction with confocal fluorescent microscopy the trafficking of these fluorescent ligands was monitored in real-time. The internalization of these ligands by mu- and delta-opioid receptors was found to be naloxone-sensitive and temperature-dependent. Interestingly, once these ligands were internalized the fluorescent puncta that formed became distributed in one of two patterns. In Chinese hamster ovary cells heterologously expressing either mu- or delta-opioid receptors the intracellular puncta were concentrated in the perinuclear region of the cell, whereas they were distributed throughout the cytoplasm in cells derived from either NG108-15 or SH-SY5Y cells. In summary, we have demonstrated that these novel, fluorescent opioid peptide conjugates permit real-time visual tracking of receptor-ligand complexes, including their internalization and trafficking, in living cells.

Receptor-induced internalization of selective peptidic mu and delta opioid ligands

The binding and internalization of radioiodinated and fluorescent mu and delta opioid peptides in mammalian cells were quantitatively studied by biochemical techniques and directly visualized by confocal microscopy. The labeled peptides were prepared by inserting either a 125I-Bolton-Hunter group or a fluorescent probe into the C-terminal part of 5-aminopentylamide derivatives of deltorphin-I and [Lys7]dermorphin. The purified derivatives kept most of their specificity and selectivity toward delta and mu opioid receptors, respectively. Biochemical and confocal microscopy data showed that both mu and delta opioid peptides were internalized in mammalian cells transfected with the corresponding opioid receptor according to a receptor-mediated mechanism. The internalization process was time- and temperature-dependent and was completely blocked by the endocytosis inhibitor phenylarsine oxyde. Internalization of both delta and mu ligands occurred from a single large cap at one pole of the cell, indicating that polymerization of ligand-receptor complexes preceeded internalization. Finally, green and red fluorescent analogues of deltorphin-I and [Lys7]dermorphin, respectively, were found to internalize through partly distinct endocytic pathways in cells co-transfected with mu and delta receptors, suggesting that each of these receptors interacts with distinct proteins mediating intracellular sorting and trafficking.

Identification of kappa opioid receptors in the immune system by indirect immunofluorescence

A method to visualize the kappa opioid receptor is described that uses a high-affinity fluorescein-conjugated opioid ligand and indirect immunofluorescence with the phycoerythrin fluorophore to amplify the signal. The mouse thymoma cell line R1E/TL8x.1.G1.OUAr.1 (R1EGO), which expresses the kappa 1 but not mu or delta opioid receptors, was used as a positive control for fluorescence labeling. A fluorescein isothiocyanate-conjugated arylacetamide (FITC-AA) compound displaying high affinity for the kappa opioid receptor was synthesized. R1EGO cells were incubated with FITC-AA, in the absence or presence of the kappa-selective opioid antagonist nor-binaltorphimine (nor-BNI) as a competitor. By using fluorescence microscopy and flow cytometry, incubation of R1EGO cells with FITC-AA alone was not sufficient for the detection of specific staining of the kappa opioid receptor. To amplify the FITC-AA fluorescence, the fluorescein served as a hapten for subsequent antibody detection. R1EGO cells were incubated with FITC-AA, followed by biotinylated rabbit anti-fluorescein IgG and extravidin-conjugated R-phycoerythrin. By using this approach, R1EGO cells were stained with phycoerythrin-amplified FITC-AA, and the staining was displaced with nor-BNI. Flow cytometry showed that titrations of both FITC-AA and nor-BNI produced saturable concentration-dependent changes in the median phycoerythrin fluorescence intensity, with optimal staining at 30 microM FITC-AA. Up to 80% of the fluorescence above background was inhibited by nor-BNI. Freshly isolated thymocytes from C57BL/6ByJ mice also showed nor-BNI-sensitive staining with the FITC-AA amplification. This sensitive method of indirect phycoerythrin immunofluorescence can be used to amplify any fluorescein-conjugated opioid ligand for the detection of opioid receptors.

Microinjection of dynorphin into the supraoptic and paraventricular nuclei produces antidiuretic effects through vasopressin release

The mechanisms for the antidiuretic effects of dynorphin (DYN), an endogenous kappa-agonist, microinjected into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei were investigated. DYN decreased the urine outflow rate dose-dependently from 5 to 20 nmol in the SON and PVN, and it increased vasopressin release. Microinjection of des-Tyr-DYN (a non-opioid peptide) into the SON produced antidiuretic effects with similar potency to that of the DYN-induced effects. However, in the PVN, the effects of des-Tyr-DYN were very markedly weaker than those of DYN. The DYN-induced antidiureses in the SON were partially inhibited by phenoxybenzamine, timolol and atropine, but not by naloxone. Those in the PVN were partially inhibited by naloxone, timolol and atropine, but not by phenoxybenzamine. Synthetic specific kappa-agonists, U50, 488H and Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro- Arg-Leu-Arg-Gly 5-aminopentylamide (DAKLI), microinjected into the PVN also produced antidiuretic effects in a dose-dependent manner. The order of antidiuretic potency was DAKLI > DYN > U50,488H, which was the same as that of kappa-receptor binding affinity. The DAKLI-induced antidiureses in the PVN were not inhibited by naloxone. These results suggested that DYN caused antidiureses by vasopressin release, through adrenergic and cholinergic mechanisms in the SON and PVN. Only the DYN-induced effects in the PVN were mediated, at least partially, through opioid receptors, perhaps the kappa-subtype.

Pharmacologic effects in vivo in brain by vector-mediated peptide drug delivery

Pharmacologic effects in brain caused by systemic administration of neuropeptides are prevented by poor transport of the peptide through the brain vascular endothelium, which comprises the blood-brain barrier in vivo. In the present study, successful application of a chimeric peptide approach to enhance drug delivery through the blood-brain barrier for the purpose of achieving a central nervous system pharmacologic effect is described. The chimeric peptide was formed by linkage of a potent vasoactive intestinal peptide (VIP) analogue, which had been monobiotinylated, to a drug transport vector. The vector consisted of a covalent conjugate of avidin and the OX26 monoclonal antibody to the transferrin receptor. Owing to the high concentration of transferrin receptors on brain capillary endothelia, OX26 targets brain and undergoes receptor-mediated transcytosis through the blood-brain barrier. Systemic infusion of low doses (12 micrograms/kg) of the VIP chimeric peptide in rats resulted in an in vivo central nervous system pharmacologic effect: a 65% increase in cerebral blood flow. Biotinylated VIP analogue without the brain transport vector was ineffective.

Recent developments in peptide drug delivery to the brain

Peptide-based therapeutics are highly water-soluble compounds that do not readily enter brain from blood owing to poor transport through the brain capillary endothelial wall, i.e., the blood-brain barrier (BBB). Strategies available for peptide drug delivery to brain include: (a) neurosurgical-based (intraventricular drug infusion, hyperosmotic opening of the BBB); (b) pharmacological-based (peptide lipidization, liposomes); and (c) physiological-based (biochemical opening of the BBB, chimeric peptides). Chimeric peptides are formed by the covalent coupling of a pharmaceutical peptide (that is normally not transported through the BBB) to a brain transport vector that undergoes absorptive-mediated or receptor-mediated transcytosis through the BBB. The most efficient brain transport vector known to date is a monoclonal antibody to the transferrin receptor, and this vector achieves a brain volume of distribution approximately 18-fold greater than the plasma space by 5 hr after a single intravenous injection of antibody. The chimeric peptides are formed generally with chemical-based linkers. However, avidin/biotin-based linkers allow for high yield coupling of drug to vector, and for the release of biologically-active peptide following cleavage of the chimeric peptide linker. These strategies may also be used for the delivery of antisense oligonucleotide-based therapeutics to brain. In conclusion, the development of efficacious neuropharmaceuticals in the future will require the development of both drug delivery and drug discovery strategies that operate in parallel.

Expression cloning of cDNA encoding a seven-helix receptor from human placenta with affinity for opioid ligands

Here we report the expression cloning of cDNA encoding a putative opioid receptor from a human placenta cDNA library. Placental opioid receptors are of the kappa type. As the dynorphin opioid peptides are kappa-selective, a dynorphin ligand was used in an affinity-enrichment (panning) procedure to select transiently transfected COS-7 cells expressing kappa receptor binding sites. The cloned cDNA encodes a 440-residue protein of the seven-helix guanine nucleotide-binding protein (G-protein)-coupled receptor family. Ligand binding reveals a stereospecific site with typical opioid properties, which binds peptide and nonpeptide opioids with moderate affinity (Kd approximately 100 nM) and which lacks the expected kappa selectivity. The deduced transmembrane domain is 93% identical to the homologous region of the human neuromedin K (neurokinin B) receptor, but the N-terminal and C-terminal sequences have many dissimilarities. The expressed receptor binds opioid ligands but not tachykinins; and under the same conditions, a cloned rat neuromedin K receptor binds tachykinins but not opioids.

Pharmacological and molecular properties of opioid binding sites synthesized in a cell-free translation system

Cell-free translation of mRNA, extracted from NG108-15 cells, was used to examine some properties of the opioid binding sites synthesized in vitro. A monoclonal antiidiotype antibody directed against the delta opioid receptor immunoprecipitated a major band of Mr 51,000. Translational immunoassays of poly[A]+RNA, size fractionated by methylmercury agarose gel electrophoresis, demonstrated that the 51,000 Mr protein specifically immunoprecipitated by the anti-opioid receptor antiidiotype antibodies was coded by a transcript which length was in the 6 to 8 kb range. Displacement binding studies of tritiated ligands (either bremazocine or delta or mu selective peptides) with type selective opioid ligands showed that only one type of opioid binding site was synthesized in vitro. Although the pharmacological profiles of ligands binding to NG108-15 cells were characteristic of the delta receptor type, the de novo synthesized opioid binding site had lost its delta selectivity and showed equal affinity for both the mu and delta but not for the kappa ligands. Similar to our finding using the immunoprecipitation system, size fractionation of the NG108-15 poly[A]+RNA demonstrated that the transcript coding for the "mu-delta" binding site had a length of 6,500 to 7,500 nucleotides.

Chimeric opioid peptides: tools for identifying opioid receptor types

We synthesized several chimeric peptides in which the N-terminal nine residues of dynorphin-32, a peptide selective for the kappa opioid receptor, were replaced by opioid peptides selective for other opioid receptor types. Each chimeric peptide retained the high affinity and type selectivity characteristic of its N-terminal sequence. The common C-terminal two-thirds of the chimeric peptides served as an epitope recognized by the same monoclonal antibody. When bound to receptors on a cell surface or membrane preparation, these peptides could still bind specifically to the monoclonal antibody. These chimeric peptides should be useful for isolating mu, delta, and kappa opioid receptors and for identifying opioid receptors on transfected cells in expression cloning procedures. The general approach using chimeric peptides should be applicable to other peptide receptors.