Dynorphin is a specific endogenous ligand of the kappa opioid receptor

In the guinea pig ileum myenteric plexus--longitudinal muscle preparation, dynorphin-(1--13) and the prototypical kappa agonist ethylketocyclazocine had equally poor sensitivity to naloxone antagonism and showed selective cross protection in receptor inactivation experiments with the alkylating antagonist beta-chlornaltrexamine. In binding assays with membranes from guinea pig brain, ethylketocyclazocine and dynorphin-(1--13) amide were more potent in displacing tritium-labeled ethylketocyclazocine than in displacing typical mu and delta opioid receptor ligands. In the two preparations studied, the dynorphin receptor appears to be the same as the kappa opioid receptor.

Binaltorphimine and nor-binaltorphimine, potent and selective kappa-opioid receptor antagonists

The opioid antagonist activities of two bivalent ligands, BNI and nor-BNI, have been evaluated in smooth muscle preparations and in mice. Both ligands are highly potent and selective as kappa opioid receptor antagonists, with relatively feeble blocking activity at mu and delta opioid receptors. BNI and nor-BNI represent the first highly selective kappa opioid receptor antagonists and should be of great utility as molecular probes for identifying the interaction of agonist ligands with kappa opioid receptors in vitro and in vivo.

Characterization of the kappa-subtype of the opiate receptor in the guinea-pig brain

1. In homogenates of guinea-pig brain, the characteristics of the binding of [3H]-ethylketazocine, an agonist for the putative kappa-receptor, were determined by estimation of the affinity and capacity of binding, by competitive inhibition for the binding site by unlabelled ligands and by selective protection of the binding site from alkylation by phenoxybenzamine. 2. At 25 degrees C the maximum number of binding sites for [3H]-ethylketazocine was about 14 pmol/g fresh brain, of which about 50% were high affinity sites. 3. In competition experiments, the high affinity binding of [3H]-ethylketazocine to the kappa-binding site was readily displaced by several kappa-agonists but not by the selective mu-ligand, D-Ala2, MePhe4, Gly-ol5-enkephalin or the selective delta ligand, D-Ala2, D-Leu5-enkephalin. In contrast, the kappa-agonists tested so far exhibit a high degree of cross-reactivity with the mu-binding site but somewhat less with the sigma-binding site. Similar specificities were observed in protection experiments. 4. The approximate proportions of the three subtypes of opiate receptor in the guinea-pig brain are 25% mu-binding sites, 45% delta-binding sites and 30% kappa-binding sites. 5. The endogenous opioids, Met-enkephalin, Leu-enkephalin and porcine beta-endorphin have only a low affinity for the kappa-binding site.

Properties of a selective kappa agonist, U-50,488H

U-50,488H has been shown to be a naloxone antagonizable analgesic in rodents. However, the dose of naloxone needed for antagonism is higher than it is for morphine. U-50,488H does not produce physical dependence; however it does produce tolerance upon chronic administration. U-50,488H is cross tolerant with bremazocine but not with morphine. Monkeys trained to discriminate ethylketocyclazocine (EKC) from saline show a complete generalization to U-50,488H but not to morphine. The evaluation of U-50,488H in 3H-EKC site-selective binding indicated that U-50,488H has a high affinity for the kappa receptor (Ki = 114 nM) and a low affinity for the mu receptor (Ki = 6100 nM). The ratio of Ku/Kk was 0.08 for morphine, 0.4 for dynorphin, and 53.5 for U-50,488H. The data suggest that U-50,488H is a selective agonist at the opioid kappa receptor.

Increased food and water intake produced in rats by opiate receptor agonists

It has been suggested that endogenous opiate mechanisms may be involved in the physiological control of food and water intake. Support for this hypothesis has been obtained from studies of the effects of narcotic antagonists which reduce feeding and drinking, but it is also necessary to show that food and water intake can be facilitated by opiate agonists. In the present study the food intake of freely-feeding rats was increased by subcutaneous injections of morphine, a stabilised enkephalin and analogue (RX 783030), and ethylketocyclazocine. Water intake was also increased but this effect was more variable than the increased eating. The increased food intake produced by the putative mu receptor agonists morphine and RX 783030 was blocked by a dose of naloxone which did not affect the facilitation of eating produced by ethylketocyclazocine, which may act at a separate population of receptors known as kappa receptors. These data are consistent with the possibility that opiate receptors are involved in the control of feeding and drinking.

Naloxone-inaccessible sigma receptor in rat central nervous system

It has been postulated that the psychotomimetic effects of opiates of the benzomorphan series are due to their activity at the sigma receptor. Therefore, the binding of (+/-)-[3H]ethylketocyclazocine ( [3H]EKC), a benzomorphan, to synaptosomal membranes of rat central nervous tissue was studied. Surprisingly, high concentrations of naloxone, a mu, delta, and kappa receptor antagonist, only inhibited about 80% of the specifically bound [3H]EKC in the spinal cord. This suggested that the remaining 20% of the binding sites were not mu, delta, or kappa. The Scatchard plot of the binding of [3H]EKC was nonlinear but became linear in the presence of naloxone (1 microM), suggesting a single class of naloxone-inaccessible receptor sites. This biochemically readily distinguishable receptor type bound the dextrorotatory isomer of EKC stereoselectively. The sigma agonist N-allylnormetazocine [(+)-SKF 10,047] stereoselectively competed with the binding of [3H]EKC to this naloxone-inaccessible binding site. A number of opiates that have psychotomimetic activity also competed for binding to this binding site. This binding site is designated as sigma binding site according to the nomenclature originally suggested by Martin et al. [Martin, W. R., Eades, C. G., Thompson, J. A., Huppler, R. E. & Gilbert, P. E. (1976) J. Pharmacol. Exp. Ther. 197, 517-532]. The drug selectivity and regional distribution of this sigma binding site in the rat central nervous system are different from that of the mu and delta opioid receptors and phencyclidine receptors. The concentration of the sigma binding site is highest in the spinal cord, pons and medulla, and cerebellum.

In vivo evidence for multiple opiate receptors mediating analgesia in the rat spinal cord

In rats implanted with chronic catheters in the spinal subarachnoid space, intrathecal injections of SKF 10047 and dynorphin did not produce any elevation of the nociceptive threshold as defined by hot-plate and tail-flick tests. In contrast, intrathecal ethylketocyclazocine (EKC) and (D-Ala2,D-Leu5)-enkephalin (DADL) administration resulted in a dose-dependent antinociceptive effect which was reversible with intraperitoneal naloxone. Calculation of the Schild dose-ratio plots for the data derived from systemically administered naloxone reveals a slope of--1 and a calculated pA2 value of 6.8 for EKC and 6.2 for DADL. Also, animals made tolerant to systemic morphine showed a diminished analgesic response to intrathecal morphine and EKC when compared to naive animals. There was, however, no significant change in the dose response curve of intrathecal DADL. Thus, these experiments suggest that in addition to mu receptors a separate subpopulation of delta but not kappa or sigma receptors are involved with spinally mediated analgesia.

Evidence for multiple "Kappa" binding sites by use of opioid peptides in the guinea-pig lumbo-sacral spinal cord

Binding properties of [3H]-etorphine and [3H]-ethylketocyclazocine have been studied in the lumbo-sacral spinal cord of guinea-pig which does not contain mu or delta binding sites. [3H]-etorphine binds to a single class of high affinity sites, whereas [3H]-ethylketocyclazocine interacts with a high and a low affinity component. Using a discriminative procedure, 5 microM (D-Ala2, D-Leu5) enkephalin (DAL), the high affinity component of [3H]-ethylketocyclazocine can be resolved in two classes of sites, (D-Ala2, D-Leu5) enkephalin sensitive sites (DALS sites) and (D-Ala2, D-Leu5) enkephalin insensitive sites (DALI sites). In these conditions, there is a total loss of [3H]-etorphine sites, whose binding capacity and properties strictly correspond to the DALS sites labelled by [3H]-ethylketocyclazocine. Pharmacological investigations indicate that DALI sites for which dynorphin (1 leads to 17) is the best ligand, can be related to kappa sites previously described in guinea-pig brain, whereas DALS sites for which (Arg6, Phe7) Met-enkephalin possesses a good affinity, closely correspond to benzomorphan sites recently characterized in rat brain and spinal cord. [3H]-ethylketocyclazocine interacts additionally with "non opiate" low affinity sites, for which only benzomorphan drugs exhibit a good affinity, whereas morphine, naloxone, phencyclidine or endogenous opioid peptides do not present any affinity for them. On the basis of these data, a new subdivision of "kappa" sites is discussed.

Evidence that dynorphin-(1-13) acts as an agonist on opioid kappa-receptors

The study concerned the opioid-receptor subtype on which dynorphin-(1-13) acts in in vitro isolated preparations. The potency of dynorphin-(1-13) relative to that of ethylketocyclazocine (Mr 2266), a representative kappa-receptor agonist, in inhibiting the electrically evoked contractions of the guinea-pig ileum was found to be similar to that found with either mouse was deferens or rabbit ileum. Moreover, Mr 2266 was found to be several-fold more effective than naloxone to antagonize the agonist actions of both kappa-receptor agonists such as ethylketocyclazocine, ketocyclazocine and bremazocine, and dynorphin-(1-13) either in the guinea-pig ileum, mouse vas deferens, or in rabbit ileum. Additionally, dynorphin-(1-13) was found to have a significant inhibitory action on the rabbit vas deferens which had been shown to contain kappa-receptors exclusively. The data indicate that dynorphin-(1-13) acts as an endogenous agonist on kappa-receptors.

Binaltorphimine-related bivalent ligands and their kappa opioid receptor antagonist selectivity

In an effort to develop selective antagonists for kappa opioid receptors, bivalent ligands that contain opioid antagonist pharmacophores derived from naltrexone or other morphinans were synthesized and tested on the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations. The minimum requirements for kappa selectivity are at least one free phenolic OH group and one N-cyclopropyl or N-ally substituent. Several compounds (3, 8, 10) with kappa selectivity as good as or better than norbinaltorphimine (nor-BNI, 2) were discovered. The structure-activity relationship revealed that the pyrrole ring functions strictly as a spacer and does not contribute to kappa selectivity. The pharmacologic data suggest that only one antagonist pharmacophore may be required for kappa selectivity and that the other morphinan portion of the molecule confers selectivity by interacting with a unique subsite proximal to the antagonist pharmacophore recognition locus.

Opiates inhibit the discharges of fine afferent units from inflamed knee joint of the cat

The spontaneous discharges in 15 out of 19 small-diameter afferent units from inflamed knee joints of anaesthetized cats were significantly inhibited by one or several opiates (morphine in the dose range 1.0-5.0 mg/kg; gly-ol 0.5-5.0 mg/kg; U50488 1.0-10.0 mg/kg; ethylketocyclazacine 0.5-4.0 mg/kg administered by close arterial injection into the joint). In the majority of cases a subsequent injection of naloxone (1 mg/kg i.a.) significantly reversed this effect. These data provide an electrophysiological demonstration that opiates may act on opiate receptors located at peripheral sites of primary afferent fibres and hence exert a peripheral 'analgesic' effect.

Endogenous ligands for sigma opioid receptors in the brain ("sigmaphin"): evidence from binding assays

Two endogenous ligands which interact preferentially with the sigma opioid receptors were identified from the guinea-pig brain extract in a Sephadex G-50 fractionation. These two ligands inhibited more potently the binding of [3H]SKF-10047 to sigma opioid receptors than [3H]naloxone to mu opioid receptors, [3H]ethylketocyclazocine to kappa opioid receptors and [3H]DADLE to delta opioid receptors. In the phencyclidine receptor assay, these two ligands were almost inactive. Incubation of these ligands with trypsin destroyed at least 50% of the activities in the sigma opioid receptor assay. Both ligands inhibited the sigma binding in a dose-dependent manner. The inhibition could be eliminated when the two ligands were removed from incubation media by extensive washings. It is therefore concluded that sigma opioid receptors are not phencyclidine receptors and that endogenous ligands for sigma opioid receptors may exist in the brain.

Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

Evidence from studies which utilise either opiate receptor agonists and antagonists strongly indicate a role for endorphinergic mechanisms in the control of feeding responses. Two means by which these compounds may exert an effect on feeding can be singled-out. Firstly, emerging evidence suggests that the process of achieving satiety (terminating a meal, or choice of a commodity) may be accelerated following treatments with opiate receptor antagonists. Secondly, the preference for highly palatable solutions (sweet solutions have received most attention) in two-bottle tests is blocked after injection of opiate receptor antagonists. This finding has been interpreted in terms of the abolition of the reward or incentive quality associated with the particularly attractive flavour. These two mechanisms of action may represent two aspects of a single, fundamental process. Following an introduction to rat urination model of in vivo kappa agonist activity, the consistent effect of several kappa agonists (including the highly selective U-50,488H) to stimulate food consumption is described. Recognising that members of the dynorphin group of endogenous opioid peptides are kappa receptor ligands, some with a high degree of selectivity, and the evidence the dynorphins and neo-endorphins produce hyperphagia in rats is particularly interesting. Such lines of evidence lead to the hypothesis that peptides of the dynorphin group may act endogenously to promote the expression of normal feeding behaviour.

(+)-[3H]SKF 10,047, (+)-[3H]ethylketocyclazocine, mu, kappa, delta and phencyclidine binding sites in guinea pig brain membranes

Binding of the opiates (+)-[3H]SKF 10,047 [N-allylnormetazocine; (+)-[3H]SKF] and (+)-[3H]ethylketocyclazocine [(+)-[3H]EKC] were compared to mu, kappa and delta and phencyclidine (PCP) receptor binding in guinea pig brain membranes. (+)-[3H]SKF and (+)-[3H]EKC binding were not blocked by naloxone, and had different drug selectivity compared to mu, kappa and delta binding sites. The number of binding sites, drug selectivity and region distribution in brain were similar for (+)-[3H]SKF and (+)-[3H]EKC. Sigma opiates that are associated with psychotomimetic activities, such as pentazocine, cyclazocine, SKF 10,047 and bremazocine, were potent inhibitors of (+)-[3H]SKF and (+)-[3H]EKC binding. Haloperidol was the most potent inhibitor of (+)-[3H]SKF binding. Haloperidol and sigma opiates demonstrated biphasic displacement of [3H]PCP binding, suggesting that [3H]PCP labelled two sites. PCP had a similar affinity for both (+)-[3H]SKF and [3H]PCP binding sites in the presence of 100 mM NaCl. The highest concentrations of (+)-[3H]SKF and (+)-[3H]EKC bindings sites were in the hypothalamus, anterior pituitary, midbrain, pons and medulla.

The epileptogenic spectrum of opiate agonists

The present authors gave mu, delta, kappa, epsilon and sigma opiate receptor agonists intracerebroventricularly to rats both singly and in combination while monitoring the electroencephalogram from cortical and depth electrodes. Dose-response curves were plotted with naloxone against the changes produced by each agonist, and the effect of a number of anticonvulsant drugs on agonist-induced seizures was ascertained. Each opiate agonist produced a different seizure pattern with a different naloxone dose-response curve and anticonvulsant profile. The order of convulsive potency was epsilon greater than delta greater than mu greater than sigma much greater than kappa. Petit mal-like seizure activity was unique to the delta agonist, leucine-enkephalin, while only the mu agonist, morphine produced generalized convulsive seizures. These experiments raise the possibility that opiate systems in the brain may be involved in the pathogenesis of a wide spectrum of seizure disorders.

Evidence for an endogenous peptide ligand for the phencyclidine receptor

Porcine brain contained an active factor that competed with [3H]-phencyclidine (PCP) for binding to rat brain membranes. On reverse phase high pressure liquid chromatography, the active material eluted between 38-42% acetonitrile. Gel filtration chromatography of the factor predicted a molecular weight of approximately 3000 daltons. The endogenous substance appeared to be selective for PCP receptors as it did not interact with either benzodiazepine, neurotensin, nor with mu, delta, or kappa opioid receptors. The active material showed a heterogenous distribution in brain, with highest concentrations found in hippocampus and cortex. It is likely to be a small peptide since various proteases eliminated or markedly reduced the potency of the compound in a [3H]-PCP binding assay. The material also possessed PCP-like activity in two bioassays. Like PCP, it induced contralateral rotational behavior after unilateral intranigral injection and depressed spontaneous cell activity after iontophoretic micropressure application in hippocampus and cerebral cortex. Thus, this small peptide is likely to be an endogenous ligand for the PCP receptor.

Effects of selective and non-selective kappa-opioid receptor agonists on cutaneous C-fibre-evoked responses of rat dorsal horn neurones

We have studied the effects of 3 putative kappa-opioid receptor agonists, U50488H, ethylketocyclazocine (EKC) and dynorphin A1-13 (DYN) on the processing of nociceptive information in the dorsal horn of the rat under halothane anaesthesia. Extracellular single unit recordings were made from convergent or multireceptive lumbar dorsal horn neurones, which could be excited by impulses in A beta and C fibre afferents following transcutaneous electrical stimulation of their ipsilateral hind paw receptive fields and also by noxious and innocuous natural stimuli. Agonists were applied directly onto the surface of the spinal cord. DYN and U50488H consistently produced both a facilitation and inhibition of the C-fibre evoked nociceptive responses of individual cells, these dual effects being relatively insensitive to naloxone antagonism and cancelled each other for the whole population of cells. A beta fibre-evoked responses were little altered. In contrast, EKC consistently depressed C-fibre transmission in a dose-dependent, naloxone reversible manner, analogous to, but considerably less potent than intrathecal morphine under identical experimental conditions. Agonist-induced effects on neuronal responses to natural stimulation (noxious pinch and innocuous prod) were consistent with the changes observed with the electrically evoked responses. The present results therefore indicate that EKC probably exerts its spinal antinociceptive activity in the rat spinal cord in a manner akin to mu-receptor activation. Results with U50488H and DYN indicate that -opioids can excite and inhibit individual neurones but produce no overall change on the whole population, so differing from effects mediated by the other opiate receptors.

Interaction of opiates with opioid binding sites in the bovine adrenal medulla: II. Interaction with kappa sites

In this study we examined the interaction of opiates with kappa binding sites in the bovine adrenal medulla. [3H]Ethylketocyclazocine (EKC), [3H]etorphine, and [3H]bremazocine stereoselective bindings were used to assay these interactions. The kappa sites were found to be heterogeneous: [3H]bremazocine identified with high affinity all subtypes of these sites. [3H]EKC, in the presence of saturating concentrations of [D-Ala2, D-Leu5]-enkephalin (DADLE) (5 microM), was used to identify kappa 1 sites, on which dynorphin A (1-13) bound with high affinity. Either [3H]EKC or [3H]etorphine in the presence of 5 microM DADLE identified the kappa 2 subtype. This subtype was found to interact with beta-endorphin and especially with the octapeptide Met5-enkephalyl-Arg6-Gly7-Leu8. Furthermore, [3H]etorphine identified in the bovine adrenal medulla a third high-affinity component, in the presence of 5 microM DADLE. This residual interaction was found to be equally stereoselective and presenting kappa selectivity. Met5-enkephalyl-Arg6-Phe7 interacted preferentially with this site. The three kappa subtypes interacted differentially with monovalent (Na+, K+, and Li+) and divalent (Ca2+, Mg2+, and Mn2+) ions by modification of the apparent concentration of the accessible sites and/or by changes of the apparent KD for radioligands. Modifying agents (proteolytic enzymes, thiol-modifying reagents, and A2-phospholipase) produced different effects on each subtype of the kappa site, suggesting a different protein (or protein-lipid?) composition.

Autoradiography of [3H]U-69593 binding sites in rat brain: evidence for kappa opioid receptor subtypes

In vitro quantitative autoradiography was used to determine the distribution of [3H]U-69593 binding sites in rat brain. The highest density of binding was found in areas of the tel- and diencephalon while certain areas of the mes- and metencephalon were moderately labeled. The distribution of [3H]U-69593 binding sites corresponds closely (but not precisely) to the distribution of sites labeled by [3H]EKC and [3H]bremazocine at room temperature but differs substantially from the distribution of sites labeled by [3H]EKC at 4 degrees C (as described by others). These anatomical findings support previous biochemical evidence indicating that [3H]U-69593 selectively labels a kappa opioid receptor subtype with characteristics that differ from the kappa receptors labeled by [3H]EKC at 4 degrees C.

Strain differences in opiate receptors in mouse brain

Various opiate ligands were bound to brain membranes of mice of the Recombinant Inbred System. The specific binding of low levels of [3H]naloxone, [3H]dihydromorphine and [3H]ethylketocyclazocine was disturbed in a similar fashion among the inbred strains, and in a pattern different from that observed for [3H](D-Ala2,D-Leu5)-enkephalin. The results indicate that the inbred strains differ in mu- and delta-type binding and support the concept of multiple opiate receptors in mouse brain.

Comparison of sigma- and kappa-opiate receptor ligands as excitatory amino acid antagonists

Using the technique of microelectrophoresis in pentobarbitone-anaesthetized cats and rats, the effects of benzomorphans, with known actions at sigma- and kappa- opioid receptors, were tested on responses of spinal neurones to amino acids and acetylcholine. The racemic mixture and both enantiomers of the sigma opiate receptor agonist, N-allylnormetazocine (SKF 10, 047), and the dissociative anaesthetic, ketamine, reduced or abolished excitation evoked by N-methyl-aspartate (NMA) with only small and variable effects on responses to quisqualate or kainate. (+)-SKF 10, 047 was 1.2 +/- 0.7 times more potent than the (-)-enantiomer in antagonizing NMA. On Renshaw cells, (+)-SKF 10, 047 enhanced responses to acetylcholine whereas the (-) enantiomer produced only a small reduction. The kappa- opiate receptor agonist, ethylketocyclazocine, had no selective effects on responses to amino acids or to acetylcholine. We conclude that actions at sigma- but not kappa-, opiate receptors are responsible for the NMA antagonism observed with benzomorphans.