Opioid receptor binding properties of analgesic analogues of cholecystokinin octapeptide

Multitarget ligands that comprise opioid/nonopioid pharmacophores for pain management: Current state of the science

Chronic pain, which affects more than one-third of the world's population, represents one of the greatest medical challenges of the 21st century, yet its effective management remains sub-optimal. The 'gold standard' for the treatment of moderate to severe pain consists of opioid ligands, such as morphine and fentanyl, that target the µ-opioid receptor (MOP). Paradoxically, these opioids also cause serious side effects, including constipation, respiratory depression, tolerance, and addiction. In addition, the development of opioid-use disorders, such as opioid diversion, misuse, and abuse, has led to the current opioid crisis, with dramatic increases in addiction, overdoses, and ultimately deaths. As pain is a complex, multidimensional experience involving a variety of pathways and mediators, dual or multitarget ligands that can bind to more than one receptor and exert complementary analgesic effects, represent a promising avenue for pain relief. Indeed, unlike monomodal therapeutic approaches, the modulation of several endogenous nociceptive systems can often result in an additive or even synergistic effect, thereby improving the analgesic-to-side-effect ratio. Here, we provide a comprehensive overview of research efforts towards the development of dual- or multi-targeting opioid/nonopioid hybrid ligands for effective and safer pain management. We reflect on the underpinning discovery rationale by discussing the design, medicinal chemistry, and in vivo pharmacological effects of multitarget antinociceptive compounds.

Novel peptide ligands with dual acting pharmacophores designed for the pathophysiology of neuropathic pain

The conventional design of high affinity drugs targeted to a single molecule has not resulted in clinically useful therapies for pain relief. Recent reviews have suggested that newly designed analgesic drugs should incorporate multiple targets. The distributions of cholecystokinin (CCK) and CCK receptors in the central nervous system (CNS) overlap significantly with endogenous opioid systems and can be dually targeted. CCK has been shown to act as an endogenous "anti-analgesic" peptide and neuropathic pain conditions promote endogenous CCK release in CNS regions of pain modulation. Administration of CCK into nuclei of the rostral ventromedial medulla induces pronociceptive behaviors in rats. RSA 504 and RSA 601 are novel bifunctional compounds developed to target neuropathic pain by simultaneously acting as agonists at two distinct opioid receptors and antagonizing CCK receptors in the CNS. RSA 504 and RSA 601 demonstrate agonist activity in vitro and antihypersensitivity to mechanical and thermal stimuli in vivo using the spinal nerve ligation model of neuropathic pain. Intrathecal administration of RSA 504 and RSA 601 did not demonstrate antinociceptive tolerance over 7 days of administration and did not display motor impairment or sedation using a rotarod. These are the first behavioral studies that demonstrate how multi-targeted molecule design can address the pathology of neuropathic pain. These compounds with δ and μ opioid agonist activity and CCK antagonist activity within one molecule offer a novel approach with efficacy for neuropathic pain while lacking the side effects typically caused by conventional opioid therapies.

Design, synthesis, and biological evaluation of novel bifunctional C-terminal-modified peptides for delta/mu opioid receptor agonists and neurokinin-1 receptor antagonists

A series of bifunctional peptides that act as agonists for delta and mu opioid receptors with delta selectivity and as antagonist for neurokinin-1 (NK1) receptors were designed and synthesized for potential application as analgesics in various pain states. The peptides were characterized using radioligand binding assays and functional assays using cell membrane and animal tissue. Optimization was performed on the fifth residue which serves as an address moiety for both receptor recognitions. It had critical effects on both activities at delta/mu opioid receptors and NK1 receptors. Among the synthesized peptides, H-Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF3) 2 (5) and H-Tyr-D-Ala-Gly-Phe-Nle-Pro-Leu-Trp-O-3,5-Bzl(CF3)2 (7) had excellent agonist activity for both delta opioid and mu opioid receptors and excellent antagonist activity for NK1 receptors. These results indicate that the rational design of multifunctional ligands with opioid agonist and neurokinin-1 antagonist activities can be accomplished and may provide a new tool for treatment of chronic and several pain states.

New paradigms and tools in drug design for pain and addiction

New modalities providing safe and effective treatment of pain, especially prolonged pathological pain, have not appeared despite much effort. In this mini-review/overview we suggest that new paradigms of drug design are required to counter the underlying changes that occur in the nervous system that may elicit chronic pain states. We illustrate this approach with the example of designing, in a single ligand, molecules that have agonist activity at mu and delta opioid receptors and antagonist activities at cholecystokinin (CCK) receptors. Our findings thus far provide evidence in support of this new approach to drug design. We also report on a new biophysical method, plasmon waveguide resonance (PWR) spectroscopy, which can provide new insights into information transduction in G-protein coupled receptors (GPCRs) as illustrated by the delta opioid receptor.

Structure-activity relationships of bifunctional peptides based on overlapping pharmacophores at opioid and cholecystokinin receptors

Cholecystokinin (CCK) has been identified as a pronociceptive endogenous peptide which also possesses antiopioid actions. CCK may be upregulated in conditions of chronic pain or during sustained morphine administration resulting in attenuation of opioid-mediated pain relief. These complex interactions between opioids and endogenous CCK receptor systems have suggested the need for a new paradigm in drug design for some states of chronic pain. In these circumstances the rational design of potential drugs for the treatment of these conditions must be based on one ligand for multiple targets. We have designed a single peptide which can interact with delta and mu opioid receptors as agonists and with CCK receptors as antagonists. The ligands were designed based on a model of overlapping pharmacophores of opioid and CCK peptide ligands, which incorporates opioid pharmacophores at the N-terminal and CCK tetrapeptide pharmacophores at the C-terminal of the designed ligands. We measured binding and activities of our bifunctional peptides at opioid and CCK receptors. Compound 11 (Tyr-d-Ala-Gly-d-Trp-NMeNle-Asp-Phe-NH(2)) demonstrated opioid agonist properties at delta and mu receptors (IC(50) = 63 +/- 27 nM and 150 +/- 65 nM, respectively in MVD and GPI tissue assays) and high binding affinity at CCK-1 and CCK-2 receptors (K(i) = 320 and 1.5 nM, respectively). Compound 9 (Tyr-d-Nle-Gly-Trp-Nle-Asp-Phe-NH(2)) displayed potent agonist activity at delta and mu receptors (IC(50) = 23 +/-10 nM and 210 +/- 52 nM, respectively in MVD and GPI tissue assays), with a balanced binding affinity for CCK-1 and CCK-2 receptors (K(i) = 9.6 and 15 nM, respectively). These results provide evidence supporting the concept that opioid and CCK receptors have overlapping pharmacophores required for binding affinity and biological activity and that designing overlapping pharmacophores of two peptides into a single peptide is a valid drug design approach.

Design and synthesis of novel hydrazide-linked bifunctional peptides as delta/mu opioid receptor agonists and CCK-1/CCK-2 receptor antagonists

A series of hydrazide-linked bifunctional peptides designed to act as agonists for delta/mu opioid receptors and antagonists for CCK-1/CCK-2 receptors was prepared and tested for binding to both opioid and CCK receptors and in functional assays. SAR studies in the CCK region examined the structural requirements for the side chain groups at positions 1', 2', and 4' and for the N-terminal protecting group, which are related to interactions not only with CCK, but also with opioid receptors. Most peptide ligands that showed high binding affinities (0.1-10 nM) for both delta and mu opioid receptors generally showed lower binding affinities (micromolar range) at CCK-1 and CCK-2 receptors, but were potent CCK receptor antagonists in the GPI/LMMP assay (up to Ke = 6.5 nM). The results indicate that it is reasonable to design chimeric bifunctional peptide ligands for different G-protein coupled receptors in a single molecule.

Design of novel peptide ligands which have opioid agonist activity and CCK antagonist activity for the treatment of pain

Disease states such as neuropathic pain offer special challenges in drug design due to the system changes which accompany these diseases. In this manuscript we provide an example of a new approach to drug design in which we have modified a potent and selective peptide ligand for the CCK-2 receptor to a peptide which has potent agonist binding affinity and bioactivity at delta and mu opioid receptors, and simultaneous antagonist activity at CCK receptors. De novo design based on the concept of overlapping pharmacophores was a central hypothesis of this design, and led to compounds such as H-Tyr-DPhe-Gly-DTrp-NMeNle-Asp-Phe-NH(2) (i.e., RSA 601) which have the designed properties.

Cholecystokinin and morphine-induced hypothermia

The effects of cholecystokinin-8 sulfate (CCK-8), cholecystokinin-8 unsulfate (CCK-8U), cholecystokinin-4 (CCK-4), caerulein and morphine on mice core body temperature have been studied in the present work. Subcutaneous injection of different doses of caerulein (0.05, 0.1 and 0.5 mg/kg), CCK-8 (0.05, 0.1 and 0.25 mg/kg) and morphine (10, 20 and 30 mg/kg) induced hypothermia. CCK-8U and CCK-4 did not elicit any response. The hypothermic response induced by caerulein, a CCK-related decapeptide but not morphine was decreased by selective CCK(A) receptor antagonist MK-329. However, the hypothermia induced by morphine but not caerulein was reduced by opioid antagonist naloxone. When morphine plus caerulein was administered a higher hypothermia was induced. Pretreatment of animals with L-365 260, a selective CCK(B) receptor antagonist did not alter the hypothermia induced by the drugs. The response induced by combination of the both drugs was decreased by MK-329. Administration of CCK antagonists MK-329 and L-365 260 to mice did not exert any effect on temperature. It is concluded that the CCK(A) receptor mechanism may be involved in the hypothermic effect of CCK agonists or morphine, while opioid receptor mechanism is not involved in CCK receptor agonists' response.

Cholecystokinin receptor mechanism(s) and morphine tolerance in mice

In a previous work, the effects of cholecystokinin receptor agonists on tolerance to morphine antinociception were evaluated. In the present study, the influence of cholecystokinin antagonists on the inhibition of tolerance to morphine antinociception by cholecystokinin agonists has been investigated. Maximum tolerance to morphine antinociception was obtained by morphine administration (50 mg/kg) to mice once daily for 4 days. The cholecystokinin receptor agonists caerulein (0.005 mg/kg) or cholecystokinin-8 (0.01 mg/kg) but not unsulfated cholecystokinin-8 (0.01 mg/kg) decreased the development of tolerance to morphine (9 mg/kg). The cholecystokininA receptor antagonist MK-329 (1 mg/kg) or the cholecystokininB receptor antagonist L-365,260 (0.25, 0.5 and 1 mg/kg) also diminished the tolerance to morphine antinociception. When animals were challenged with different doses of MK-329 (0.25, 0.5 and 1 mg/kg) against cholecystokinin-8 (0.01 mg/kg), caerulein (0.005 mg/kg) or unsulfated cholecystokinin-8 (0.01 mg/kg) on day 4 in tolerant mice, different response were obtained. Higher doses of MK-329 (1 mg/kg) caused a small decrease in attenuation of the morphine tolerance induced by cholecystokinin-8 and caerulein. Low doses of L-365, 260 diminished the effect of cholecystokinin-8 on morphine tolerance. Conversely high doses of the drug potentiated the response of caerulein (0.005 mg/kg). When animals were treated with MK-329 or L-365,260 before unsulfated cholecystokinin-8, reduction of the tolerance to morphine antinociception was obtained. These data indicate that both cholecystokinin receptors may modulate morphine tolerance.

Effects of caerulein and CCK antagonists on tolerance induced to morphine antinociception in mice

Different groups of mice received one daily dose (50 mg/kg) of morphine subcutaneously (SC) for 3, 4 or 5 days to develop tolerance to the opioid. The antinociceptive response of morphine (9 mg/kg) was tested in the hot-plate test 24 h after the last dose of the drug. Tolerance to morphine was obtained in all groups. The group of mice that received morphine for 4 days was employed for the rest of the experiments. Pretreatment of animals with a single dose of caerulein (0.025, 0.05, and 0.1 mg/kg, SC) 30 min prior to receiving morphine (50 mg/kg; during the development of tolerance to the opioid) on day 1, 2, 3, 4 or 5 of morphine administration potentiate antinociception induced by morphine (test dose of 9 mg/kg). The dose of 0.05 mg/kg of caerulein, used 30 min before morphine administration on day 3, was also used to evaluate the effects of antagonists on caerulein-induced decrease in tolerance. The selective cholecystokinin (CCK) receptor antagonists, MK-329 [1-methyl-3-(2 indoloyl)amino-5-phenyl-3H-1,4-benzodiazepin-2-one; 0.25 and 0.5 mg/kg] or L-365,260 [3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H- 1,4-benzodiazepin-3-yl)-N-(3-methyl-phenyl)urea: 0.25 and 0.5 mg/kg] decreased potentiation of morphine response induced by caerulein. MK-329 or L-365,260, when were injected 35 min before morphine injection during the development of tolerance and on day 3, decreased the tolerance to morphine. A single administration of MK-329 or L-365,260 (in the absence of caerulein) 35 min and 48 h before the test dose of morphine (9 mg/kg) potentiated the antinociception of morphine in nontolerant animals. In conclusion, CCK mechanism(s) may interact with morphine tolerance.

Effects of mu- and delta-opioid-receptor antagonists on the stimulus properties of cholecystokinin

Melton and Riley recently reported that the relatively selective mu-opioid-antagonist naloxone potentiated the stimulus properties of the gut peptide cholecystokinin (CCK). To assess whether such opioid potentiation is limited to activity at the mu-receptor subtype, in the present experiment the effects of the highly selective delta-antagonist naltrindole on CCK's stimulus properties were examined. Because in the initial report of naloxone's potentiation of CCK a relatively high, nonphysiologic dose of CCK (i.e., 13 micrograms/kg) was used as the training drug, in the current analysis subjects were trained to discriminate 5.6 micrograms/kg CCK from its vehicle and the assessments and comparisons of the effects of naloxone and naltrindole were based on this dose. Specifically, rats were administered 5.6 micrograms/kg CCK before saccharin-LiCl pairings and the CCK vehicle before saccharin alone. With such training, they rapidly acquired the drug discrimination, avoiding saccharin consumption when it was preceded by CCK and consuming the same saccharin solution when it was preceded by its vehicle. In subsequent generalization tests, doses of CCK that were ineffective in suppressing saccharin consumption (i.e., did not substitute for the training dose of CCK) did result in the suppression of saccharin consumption when combined with doses of the mu antagonist naloxone that alone had no effect on saccharin intake. On the other hand, the highly selective delta-opioid-receptor antagonist naltrindole was ineffective in potentiating the effects of CCK. Specifically, when naltrindole was combined with ineffective doses of CCK, subjects drank at control levels. The ability of naloxone to potentiate CCK's stimulus effects is consistent with a range of other demonstrations of the role of the mu-opioid-receptor subtype in CCK-opioid interactions, although the specific basis for the interaction remains unknown. Given recent findings on the effects of delta agonists and antagonists on CCK-induced activity, the failure of naltrindole to potentiate CCK's stimulus effects may be due to the absence of delta activity within this preparation, rather than the absence of delta mediation of CCK-opioid interactions in general.

Caerulein may potentiate morphine-induced antinociception by cholecystokinin-A and/or cholecystokinin-B receptor mechanisms

1. The effects of a cholecystokinin agonist and antagonist on morphine antinociception in the tail-flick test have been evaluated. 2. The administration of different doses of caerulein (0.01, 0.05 and 0.1 mg/kg) 30 min prior to morphine (1.5, 3 and 6 mg/kg) increased the antinociception induced by morphine in mice. 3. In animals pretreated with cholecystokinin antagonists MK-329 (0.125 and 0.25 mg/kg) and L-365,260 (0.125 and 0.25 mg/kg), the antinociceptive effect of morphine was not changed. However, high doses (0.5 mg/kg) of each antagonist potentiated the morphine response. 4. Low doses of cholecystokinin antagonists (0.125 and 0.25 mg/kg), that did not cause antinociception, when employed in combination with caerulein (0.05 mg/kg) decreased the response of morphine plus caerulein. 5. It is concluded that the cholecystokinin agonist caerulein potentiated the morphine response by stimulation of cholecystokinin-A and/or cholecystokinin-B receptors.

Cholecystokinin-induced antinociception is not blocked by CCK-A or CCK-B receptor antagonists

To determine the relative importance of CCK-A, CCK-B, and opioid receptors in mediating the antinociceptive actions of cholecystokinin, we evaluated the actions of selective agonists and antagonists in the mouse hot plate assay. The agonists used were CCK (1-30 nmol i.c.v.), a CCK-A receptor agonist (SNF9019; 0.3-10 nmol i.c.v.), and a CCK-B receptor agonist (SNF9007; 0.3-10 nmol i.c.v.). The antagonists used were the CCK-A receptor antagonist, L364,718 (12.5 nmol i.c.v.), CCK-B receptor antagonist, L365,260 (2.5-25 nmol i.c.v.), and the nonselective opioid receptor antagonist naloxone (1 mg/kg s.c.). CCK and its receptor-selective analogues, SNF9019 and SNF9007, resulted in antinociception that was blocked by naloxone, but was not antagonized by L364,718 or L365,260. In contrast, in positive control experiments, the inhibitory effects of CCK, SNF9019, and SNF9007 on gastrointestinal propulsion in mice were antagonized by identical i.c.v. doses of L364,718 and L365,260. We conclude that centrally administered CCK produces antinociception in the mouse hot plate assay via opioid receptors, but independent of CCK-A or CCK-B receptors. It is necessary to speculate that other CCK receptors, not antagonized by currently available selective antagonists, may exist.

The use of topographical constraints in receptor mapping: investigation of the topographical requirements of the tryptophan 30 residue for receptor binding of Asp-Tyr-D-Phe-Gly-Trp-(N-Me)Nle-Asp-Phe-NH2 (SNF 9007), a cholecystokinin (26-33) analogue that binds to both CCK-B and delta-opioid receptors

The cholecystokinin (26-33) [CCK (26-33)] octapeptide analog Asp-Tyr-D-Phe-Gly-Trp(N-Me)-Nle-Asp-Phe-NH2 (SNF 9007) is a potent and selective ligand for both the CCK-B and delta-opioid receptors. Pharmacological studies of SNF 9007 suggest a relationship between the ligand requirements of CCK-B and delta-opioid receptors, which further implies a possible structural relationship between these receptors. We have utilized topographical constrainment of the important Trp30 residue to investigate structural features of SNF 9007 that would distinguish between binding requirements in this region for the CCK-B and delta-opioid receptors. Thus, the four optically pure isomers of beta-MeTrp were substituted for L-Trp30 of SNF 9007. Receptor binding results suggest that the preferred topography of the Trp30 residue for CCK-B receptor binding may be the 2S,3S (erythro-L) configuration whereas for the delta-opioid receptor it may be the 2S,3R (threo-L) configuration. Molecular modeling studies of these ligands further support the recently revised receptor-bound model for CCK-B octapeptide ligands (Kolodziej et al. J. Med. Chem. 1995, 38, 137-149) and are in good agreement with the DPDPE-delta opioid receptor "template" model (Nikiforovich et al. Biopolymers 1991, 31, 941-955).

Inhibition of adenylyl cyclase activity by the cholecystokinin analog SNF 9007 in neuroblastoma x glioma NG108-15 hybrid cells

The effect of the cholecystokininB (CCKB) receptor-selective cholecystokinin octapeptide (CCK-8) analog SNF 9007 on forskolin-stimulated adenylyl cyclase activity in NG108-15 hybrid cells was measured. The activity of SNF 9007 was compared to the delta opioid agonists D-Pen2-D-Pen5-enkephalin (DPDPE, delta 1 receptor-selective) and Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2, (D-Ala2-deltorphin II, delta 2-receptor-selective) because SNF 9007 binds with moderate affinity to delta opioid receptors. SNF 9007 inhibited forskolin-stimulated adenylyl cyclase activity with efficacy similar to DPDPE. IC50 determinations showed that D-Ala2-deltorphin II was the most potent, followed by DPDPE, then SNF 9007 (IC50 values = 0.013, 0.21 and 4.8 microM, respectively). CCK-8 had no effect on adenylyl cyclase activity. The delta 1 receptor-selective antagonist 7-benzylidenenaltrexone hydrochloride (BNTX, 10 nM) had no effect on the activity of any of these agonists, but the delta 2 receptor-selective antagonist naltriben methanesulfonate (NTB, 10 nM) increased IC50 values of all the agonists. Combinations of BNTX and NTB (10 nM each) increased the D-Ala2-deltorphin II IC50 value 12-fold, the DPDPE IC50 value 18-fold and the SNF 9007 IC50 value 26-fold. The effect of the combined delta antagonists on SNF 9007 activity was different from the effect on DPDPE or D-Ala2-deltorphin II activity. These data suggest that the interaction of the CCK-8 analog SNF 9007 with opioid receptors in NG108-15 hybrid cells is different from the interaction of opioid peptides with these receptors.

Effects of cholecystokinin receptor agonist and antagonists on morphine dependence in mice

In the present study, the effect of cholecystokinin agonists and antagonists on dependence to morphine in mice has been investigated. Mice were treated subcutaneously with morphine (50, 50 and 75 mg/kg) three times daily for 2-4 days, and a last dose of morphine (50 mg/kg) was administered on day 3, 4 or 5. Withdrawal syndrome (jumping) was precipitated by naloxone (2.5, 5 and 10 mg/kg) which was administered intraperitoneally 2 hr after the last dose of morphine. To study the effects of cholecystokinin receptor agonists or antagonists, 10 injection of morphine (3 administrations each day) for dependence and a dose of 5 mg/kg of naloxone for withdrawal induction were employed. Cholecystokinin-8 (0.001-0.01 mg/kg), low doses of the cholecystokinin agonists caerulein (0.00001 and 0.0001 mg/kg) and, unsulfated cholecystokinin (but not high doses) as well as the antagonists MK-329 (0.5-1 mg/kg) and L-365,260 (0.5-1 mg/kg) elicit reduction of the nalaxone-induced jumping. The inhibition of jumping induced by caerulein was reduced with the selective cholecystokinin antagonists MK-329 and L-365,260. It is concluded that cholecystokinin mechanism(s) may be involved in morphine dependence, that the agonists may act on a presynaptic receptors and that the antagonists may work on postsynaptic receptors.

Cholecystokinin-A specific antagonism of KSG-504 to cholecystokinin receptor binding and pancreatic secretion in mammals

The effects of KSG-504 ((S)-arginium (R)-4-[-N-(3-methoxypropyl)-N-pentylcarbamoyl]-5-(2- naphthylsulfonyl) pentanoate monohydrate), a new cholecystokinin (CCK)-receptor antagonist, on 125I-CCK-8 binding to rat pancreatic, canine gallbladder and guinea pig cerebrocortical membranes and the pancreatic amylase release from isolated rat acini stimulated by several kinds of secretagogues, including CCK, were investigated. The 125I-CCK-8 saturation experiment showed that pancreatic, gallbladder and cerebrocortical CCK receptors had a single high affinity binding component with dissociation constants (Kd) of 0.18, 0.31 and 0.88 nM, respectively. The maximum numbers of specific binding sites (Bmax) in these membranes were 1012, 52 and 20 fmol/mg protein, respectively. KSG-504 and CCK-8 displaced specific 125I-CCK-8 binding to CCK receptors in all membrane preparations in a competitive manner. The affinity of KSG-504 for pancreatic (Ki = 173 nM) and gallbladder (Ki = 283 nM) CCK receptors were > 3 orders of magnitude higher than its affinity for cerebrocortical CCK receptors. KSG-504 also inhibited 125I-gastrin-I binding to guinea pig gastric glands, but the IC50 value (18.2 microM) was apparently much higher. CCK-8-stimulated amylase release from isolated pancreatic acini of rats was antagonized by KSG-504 in a concentration-dependent manner. KSG-504 did not affect amylase release stimulated by secretagogues such as gastrin-releasing peptide, carbachol, vasoactive intestinal peptide and A23187. These results indicate that KSG-504 acts as a CCK-A-receptor-specific antagonist in the pancreas and gallbladder.

Conformationally readdressed CCK-B/delta-opioid peptide ligands

The sequence of a cholecystokinin (CCK) related peptide was modified to obtain analogues, which interact selectively either with CCK-B, or with delta-opioid receptors. Two kinds of peptides were designed, namely, the cyclic peptides of the H-Tyr-cyclo (D-Pen-Gly-Trp-L/D-3-transmercaptoproline)-Asp-Phe-NH2 sequence (compounds 1a and 1b, respectively), and the linear peptides of the H-Tyr-D-Val-Gly-Trp-L/D-3-trans-methylmercaptoproline-Asp-Phe- NH2 sequence (compounds 2a and 2b, respectively). The only difference between the chemical structures of the linear analogues compared to the cyclic ones is that one covalent bond has been eliminated and a sulfur atom is replaced by a methyl group. Molecular modeling showed that, among low-energy conformers of cyclic compounds 1, there are three-dimensional structures compatible to the model for delta-receptor-bound conformer, suggested earlier [G. V. Nikiforovich, V.J. Hruby, O. Prakash, and C.A. Gehrig (1991) Biopolymers, vol. 31, pp. 941-955]. Results of binding assays fully supported the rationale for the design of compounds 1 and 2. The cyclic analogue 1a has Ki values of 4.5 and > 5000 nM at delta- and mu-opioid receptors, respectively; and IC50 values of 1.6 and > 10,000 nM for CCK-A and CCK-B receptors, respectively. The results of this study demonstrate a possibility to redirect a peptide sequence that interacts with one type of receptors (CCK-B receptors) toward interaction with another type (delta-opioid receptors) belonging to a different physiological system. This redirection could be performed by changing the conformational properties of the peptide with very minimal changes in its chemical structure.

Spinal opioid systems in inflammation

Until recently, basic science studies, both behavioural and electrophysiological, have concentrated on the antinociceptive actions of opioids primarily gauged against acute nociceptive responses. However, of more relevance to clinical situations are the actions of opioids in more persistent/prolonged pain states. This review sets out to examine the central actions of opioids against nociception of inflammatory origins. The first section deals with the response of the endogenous opioid system to the development of an inflammatory state and the second examines the ability of exogenous opioids to modulate inflammatory nociception. There are complex changes in the roles of endogenous opioids, in particular dynorphin, at the spinal level after inflammation although the physiological consequences remain unclear. With regard to exogenous opioids, the effectiveness of spinal morphine is rapidly enhanced after inflammation, likely to be due to changes in the interaction between the peptide cholecystokinin and the mu opioid receptor. The ability of inflammatory processes to alter both endogenous opioids and morphine analgesia at the spinal level illustrates the considerable degree of plasticity observed in opioid function.

CCK receptor activation may prevent tolerance to morphine in mice

Different groups of mice were treated with morphine (50 mg/kg s.c.) once daily for 1, 2, 3, 4 or 5 days, in order to develop tolerance to the drug. The antinociceptive effect of morphine (9 mg/kg s.c.) was tested 24 h after each dose of the drug administration. Tolerance to morphine reached its peak on the 4th day. Daily pretreatment of animals for a period of 4 days with different doses of cholecystokinin octapeptide (CCK-8; 0.001, 0.01, 0.05 and 0.1 mg/kg s.c.), caerulein (0.0001, 0.001, 0.005 and 0.01 mg/kg s.c.) but not unsulfated cholecystokinin octapeptide (0.001, 0.01 or 0.1 mg/kg s.c.) 30 min before daily administration of morphine (50 mg/kg s.c.) prevented the development of tolerance. A group of animals received a single dose of caerulein (0.005 mg/kg), CCK-8 (0.01 mg/kg) or unsulfated CCK-8 (0.01 mg/kg) 30 min before morphine injection (50 mg/kg s.c.) on the 3rd or 4th day. In these animals, which were tested for antinociception on the 5th day, tolerance to the drug (3, 6 and 9 mg/kg s.c.) was also decreased by caerulein, CCK-8 but not unsulfated CCK-8. In a group of mice in which peptides were administered 30 min prior to the doses of morphine (3, 6 or 9 mg/kg s.c.) on the 5th day, similar results were obtained. The results of the present study indicate that activation of both CCKB and CCKA receptors may prevent the development of tolerance to morphine, and the sulfate group in the CCK-8 molecule may be essential for the tolerance inhibition.

CCK antagonists: pharmacology and therapeutic interest

CCK was first identified and characterized in the digestive tract where it is known to be a factor involved in the control of gut motility. Later, CCK and CCK receptors were identified in regions of the central nervous system that are associated with the control of emotion, motivation and sensory processing. The recent discovery and development of CCK-receptor antagonists having selective affinity for either CCKA or CCKB receptors has led to a better understanding of the functional role of CCK and its binding sites in the brain and periphery. Some of these compounds are being examined in man for their therapeutic usefulness in mental as well as in digestive disorders. This review will highlight the results from both basic and clinical investigations that have examined the effects of selective CCK receptor ligands. The focus will be on the central nervous system pharmacology of CCK antagonists and the involvement of CCK in gastrointestinal and colonic motility.

An assessment of the interaction between cholecystokinin and the opiates within a drug discrimination procedure

Recently, cholecystokinin (CCK) has been reported to antagonize a variety of opiate-induced effects, including nociception, body shaking, thermoregulation, and locomotion. Consistent with these results, a number of CCK antagonists potentiate the opiates in a range of behavioral and physiological assessments. The present study further examined the interaction between CCK and the opiates within the conditioned taste aversion baseline of drug discrimination learning, a design that utilizes the stimulus properties of the drug to control consummatory behavior. Specifically, animals injected with CCK prior to saccharin-LiCl pairings and the CCK vehicle prior to saccharin alone rapidly acquired the CCK-vehicle discrimination, avoiding saccharin consumption following the administration of CCK and consuming the same saccharin solution following the vehicle. Although the stimulus properties of CCK did not generalize to either naloxone or diprenorphine, morphine blocked and naloxone potentiated CCK's stimulus effects. These data are thus consistent with a physiological (rather than a pharmacological) interaction between CCK and the opiates.