Opioid peptides: simultaneous delta agonism and mu antagonism in somatostatin analogues

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

Opioid drugs, including the newly developed peptides, should penetrate the blood-brain barrier (BBB) for pain management activity. Although BBB transport is fragmentarily described for some mu-opioid peptides, a complete and comparative overview is currently lacking. In this study, the BBB transport of eight opioid peptides (EM-1, EM-2, CTAP, CTOP, DAMGO, dermorphin, TAPP and TAPS) is described and compared. In addition, the metabolic stability in plasma and brain was evaluated. The highest influx rate was obtained for dermorphin (K(in)=2.18 microl/(g x min)), followed by smaller rates for EM-1, EM-2 and TAPP (K(in)=1.06-1.14 microl/(g x min)). Negligible influx was observed for DAMGO, CTOP and TAPS (K(in)=0.18-0.40 microl/(g x min)) and no influx for CTAP. Capillary depletion revealed that all peptides reached brain parenchyma for over 75%. Efflux was shown for TAPP (t(1/2)=2.82 min) and to a lesser extent for EM-1, EM-2 and DAMGO (t(1/2)=10.66-21.98 min), while no significant efflux was observed for the other peptides. All peptides were stable in mouse plasma and brain, with generally higher stability in brain, except for EM-1 and EM-2 which showed plasma half-life stabilities of a few minutes only.

Endogenous opioids: role in prostaglandin-dependent and -independent fever

This study evaluated the participation of μ-opioid-receptor activation in body temperature (Tb) during normal and febrile conditions (including activation of heat conservation mechanisms) and in different pathways of LPS-induced fever. The intracerebroventricular treatment of male Wistar rats with the selective opioid μ-receptor-antagonist cyclic d-Phe-Cys-Try-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 0.1–1.0 μg) reduced fever induced by LPS (5.0 μg/kg) but did not change Tb at ambient temperatures of either 20°C or 28°C. The subcutaneous, intracerebroventricular, and intrahypothalamic injection of morphine (1.0–10.0 mg/kg, 3.0–30.0 μg, and 1–100 ng, respectively) produced a dose-dependent increase in Tb. Intracerebroventricular morphine also produced a peripheral vasoconstriction. Both effects were abolished by CTAP. CTAP (1.0 μg icv) reduced the fever induced by intracerebroventricular administration of TNF-α (250 ng), IL-6 (300 ng), CRF (2.5 μg), endothelin-1 (1.0 pmol), and macrophage inflammatory protein (500 pg) and the first phase of the fever induced by PGF2α (500.0 ng) but not the fever induced by IL-1β (3.12 ng) or PGE2 (125.0 ng) or the second phase of the fever induced by PGF2α. Morphine-induced fever was not modified by the cyclooxygenase (COX) inhibitor indomethacin (2.0 mg/kg). In addition, morphine injection did not induce the expression of COX-2 in the hypothalamus, and CTAP did not modify PGE2 levels in cerebrospinal fluid or COX-2 expression in the hypothalamus after LPS injection. In conclusion, our results suggest that LPS and endogenous pyrogens (except IL-1β and prostaglandins) recruit the opioid system to cause a μ-receptor-mediated fever.

Differential effects of CB1 and opioid agonists on two populations of adult rat dorsal root ganglion neurons

Inhibition of primary afferent neurons contributes to the antihyperalgesic effects of opioid and CB1 receptor agonists. Two bioassays were used to compare the effects of the CB1 receptor agonist CP 55,940 and morphine on dissociated adult rat DRG neurons. Both agonists inhibited the increase in free intracellular Ca2+ concentration evoked by depolarization; however, effects of CP 55,940 occurred primarily in large neurons (cell area, >800 microm2), whereas morphine inhibited the response in smaller neurons. Cotreatment with selective blockers of L-, N-, and P/Q-type voltage-dependent Ca2+ channels indicated that CB1 receptors on DRG neurons couple solely with N-type channels but opioid receptors couple with multiple subtypes. Experiments with selective agonists and antagonists of opioid receptors indicated that mu and delta, but not kappa, receptors contributed to the inhibitory effect of morphine on voltage-dependent Ca2+ influx. Because Ca2+ channels underlie release of transmitters from neurons, the effects of opioid agonists and CP 55,940 on depolarization-evoked release of calcitonin gene-related peptide (CGRP) were compared. Morphine inhibited release through delta receptors but CP 55,940 had no effect. Colocalization of CGRP with delta-opioid but not mu-opioid or CB1 receptor immunoreactivity in superficial laminae of the dorsal horn of the spinal cord was consistent with the data for agonist inhibition of peptide release. Therefore, CB1 and opioid agonists couple with different voltage-dependent Ca2+ channels in different populations of DRG neurons. Furthermore, differences occur in the distribution of receptors between the cell body and terminals of DRG neurons. The complementary action of CB1 and opioid receptor agonists on populations of DRG neurons provides a rationale for their combined use in modulation of somatosensory input to the spinal cord.

Dual effects of DAMGO [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin and CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) on adenylyl cyclase activity: implications for mu-opioid receptor Gs coupling

The mu-opioid receptor (MOR) couples to multiple G proteins, of which coupling to Gs has long been debated. As expected, in opioid naive Chinese hamster ovary cells expressing recombinant MOR, the predominant action of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) is inhibitory. However, inactivation of Gi/Go proteins via pertussis toxin (PTX) unmasks its ability to facilitate forskolin activation of adenylyl cyclase (AC) activity. Tolerance develops to this effect of DAMGO, which can also be attenuated by cholera toxin (CTX). The latter suggests G mediation. D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), previously considered to be a neutral MOR antagonist, also produces a facilitation of forskolin (FSK) activation of AC that is augmented by chronic morphine. Facilitative effects of CTAP in naive as well as its augmentation in tolerant membranes are both substantially reduced by CTX. This suggests that not only Gs mediation but also G(salpha)-linked signaling is critical to the chronic morphine-induced enhanced facilitative action of CTAP. Interestingly, the (augmented) CTAP facilitation of FSK-stimulated AC activity that is observed in opioid tolerant (but not in naive) membranes is also sensitive to PTX. This can best be explained by postulating the involvement of Gi-derived G(betagamma), which would stimulate type 2 ACs, conditional on the presence of activated G(salpha). The emergence of a G(betagamma) dimension of AC stimulation by CTAP after chronic morphine could explain its ability to augment the stimulatory action of CTAP on AC. These results support putative MOR coupling to Gs and underscore the multifaceted nature and plasticity of MOR G protein coupling.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

The discovery of endogenous opioid peptides 25 years ago opened up a new chapter in efforts to understand the origins and control of pain, its relationships to other biological functions, including inflammatory and other immune responses, and the relationships of opioid peptides and their receptors to a variety of undesirable or toxic side effects often associated with the nonpeptide opiates such as morphine including addiction, constipation, a variety of neural toxicities, tolerance, and respiratory depression. For these investigations the need for potent and highly receptor selective agonists and antagonists has been crucial since they in principle allow one to distinguish unequivocally the roles of the different opioid receptors (mu, delta, and kappa) in the various biological and pathological roles of the opioid peptides and their receptors. Conformational and topographical constraint of the linear natural endogenous opioid peptides has played a major role in developing peptide ligands with high selectivity for mu, delta, and kappa receptors, and in understanding the conformational, topographical, and stereoelectronic structural requirements of the opioid peptides for their interactions with opioid receptors. In turn, this had led to insights into the three-dimensional pharmacophore for opioid receptors. In this article we review and discuss some of the developments that have led to potent, selective, and stable peptide and peptidomimetic ligands that are highly potent and selective, and that have delta agonist, mu antagonist, and kappa agonist biological activities (other authors in this issue will discuss the development of other types of activities and selectivities). These have led to ligands that provide unique insight into opioid pharmacophores and the critical roles opioid ligands and receptor scan play in pain, addiction, and other human maladies.

Novel ligands lacking a positive charge for the delta- and mu-opioid receptors

Recently we reported using minilibraries to replace Lys(9) [somatostatin (SRIF) numbering] of the potent somatostatin agonist L-363,301 (c[-Pro-Phe-D-Trp-Lys-Thr-Phe-]) to generate the potent neurokinin receptor (NK-1) antagonist c[-Pro-Phe-D-Trp-p-F-Phe-Thr-Phe-]. This novel cyclic hexapeptide did not bind the SRIF receptor. Thus, a single mutation converted L-363,301, a SRIF agonist with potency ca. 2-8 times the potency of SRIF in laboratory animals,(24) into a selective NK-1 receptor antagonist with an IC(50) of 2 nM in vitro. During the screening of the same libraries for ligands of the delta-opioid receptor, we identified four compounds (1-4) which represent a new class of delta-opioid antagonists, some of which were also NK-1 receptor antagonists. The most potent delta-opioid antagonist, c[-Pro-1-Nal-D-Trp-Tyr-Thr-Phe-] (2), showed a K(e) value of 128 nM in the mouse vas deferens assay and a delta-receptor binding affinity constant of 152 nM in the rat brain membrane binding assay. These results are of interest because they represent a novel class of delta-opioid antagonists and, like two previously reported delta-opioid antagonists, they lack a positive charge. To examine further the requirement for a positive charge in the delta-opioid ligands, we prepared two analogues of the beta-casomorphin-derived mixed mu-agonist/delta-antagonist, H-Dmt-c[-D-Orn-2-Nal-D-Pro-Gly-] (7), in which we eliminated the positive charge either through formylation of the primary amino group (5) or by the deletion of this N-terminal amino group (6). These latter compounds proved to be delta-opioid antagonists with K(e) values in the 16-120 nM range, as well as fairly potent mu-opioid antagonists (K(e) approximately 200 nM). These six compounds provide the most convincing evidence to date that there is no requirement for a positive charge in mu- and delta-opioid receptor antagonists. In addition, cyclic hexapeptide 4 lacks a phenolic hydroxyl group. Taken together, these data suggest that the prevailing assumptions about delta- and mu-opioid receptor binding need revision and that the receptors for these opioid ligands have much in common with the NK-1 and somatostatin receptors.

Opiate aromatic pharmacophore structure-activity relationships in CTAP analogues determined by topographical bias, two-dimensional NMR, and biological activity assays

Topographically constrained analogues of the highly mu-opioid-receptor-selective antagonist CTAP (H-D-Phe-c[Cys-Tyr-D-Trp-Arg-Thr-Pen]-Thr-NH(2), 1) were prepared by solid-phase peptide synthesis. Replacement of the D-Phe residue with conformationally biased beta-methyl derivatives of phenylalanine or tryptophan (2R,3R; 2R,3S; 2S,3R; 2S,3S) yielded peptides that displayed widely varying types of biological activities. In an effort to correlate the observed biological activities of these analogues with their structures, two-dimensional (1)H NMR and molecular modeling was performed. Unlike the parent (1), which is essentially a pure mu antagonist with weak delta agonist activities in the MVD bioassay, the diastereomeric beta-MePhe(1)-containing peptides exhibited simultaneous delta agonism and mu antagonism by the (2R,3R)-containing isomer 2; mu antagonism by the (2R,3S)-containing isomer 3; weak mu agonism by the (2S,3R)-containing isomer 4; and delta agonism by the (2S,3S)-containing isomer 5. Incorporation of beta-MeTrp isomers into position 1 led to peptides that were mu antagonists (2R,3R), 8; (2R,3S), 9, or essentially inactive (<10%) in the MVD and GPI assays (2S,3R), 10; (2S,3S), 11. Interestingly, in vivo antinociceptive activity was predicted by neither MVD nor GPI bioactivity. When D-Trp was incorporated in position 1, the result (7) is a partial, yet relatively potent mu agonist which also displayed weak delta agonist activity. Molecular modeling based on 2D NMR revealed that low energy conformers of peptides with similar biological activities had similar aromatic pharmacophore orientations and interaromatic distances. Peptides that exhibit mu antagonism have interaromatic distances of 7.0-7.9 A and have their amino terminal aromatic moiety pointing in a direction opposite to the direction that the amino terminus points. Peptides with delta opioid activity displayed an interaromatic distance of <7 A and had their amino terminal aromatic moiety pointing in the same direction as the amino terminus.

Side chain methyl substitution in the delta-opioid receptor antagonist TIPP has an important effect on the activity profile

The delta-opioid antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP-OH) or its C-terminal amide analogue was systematically modified topologically by substitution of each amino acid residue by all stereoisomers of the corresponding beta-methyl amino acid. The potency and selectivity (delta- vs mu- and kappa-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potency were assayed in the mouse vas deferens and in the guinea pig ileum. In the TIPP analogues containing L-beta-methyl amino acids the influence on delta-receptor affinity and on delta-antagonist potency is limited, the [(2S,3R)-beta-MePhe3]TIPP-OH analogue being among the most potent delta-antagonists reported. In the D-beta-methyl amino acid series, the [D-beta-MeTic2] analogues are delta-selective antagonists whereas [D-Tic2]TIPP-NH2 is a delta-agonist. NMR studies did not indicate any influence of the beta-methyl substituent on the conformation of the Tic residue. The [(2R,3S)-beta-MePhe3]TIPP-NH2 is a potent delta-agonist, its C-terminal carboxylic acid analogue being more delta-selective but displaying partial agonism in both the delta- and mu-bioassay. These results constitute further examples of a profound influence of beta-methyl substitution on the potency, selectivity, and signal transduction properties of a peptide.