Binding and metabolism studies with [3H](D-Ala2,Leu5)enkephalinamide and [3H](D-Ala2,Pro5)enkephalinamide: evidence for a selective interaction of (D-Ala2,Pro5)enkephalinamide with mu-receptors

Withdrawal responses of guinea-pig isolated ileum following brief exposure to opiates and opioid peptides

Withdrawal contractures following brief exposure of guinea-pig ileum to enkephalin analogues, dynorphin A-(1-13) and beta-endorphin and the opiate drugs morphine, ketocyclazocine, buprenorphine and MR2034 were investigated. Following 2 min contact with the ileum a withdrawal contracture was induced by washout of (Met5)- and (Leu5)-enkephalin and by several enkephalin analogues but not by washout of (D-Ala2,D-Leu5)-enkephalinamide or any of the other drugs tested. Addition of naloxone precipitated withdrawal to all opioids tested except the kappa receptor preferential drugs ketocyclazocine and MR2034 and the mu receptor partial agonist, buprenorphine. The heights of the withdrawal contractures to enkephalin analogues were found to be dependent on the concentration of agonist and of naloxone, and on the duration of the contact period of opioid with the ileum. The naloxone-precipitated withdrawal responses to morphine, dynorphin A-(1-13) and the washout withdrawal response to (Met5)-enkephalin were inhibited by substance P antagonists thus supporting the previous proposal that substance P mediates the opiate withdrawal response. This study has shown that mu receptor agonists produced dependence in the guinea-pig ileum revealed by a withdrawal contracture on addition of naloxone, whereas dependence on kappa receptor agonists could not be revealed with naloxone. Since the guinea-pig ileum preparation has millimicron and kappa receptors it was concluded that the endogenous opioid peptides, enkephalins, beta-endorphin and dynorphin A-(1-13), all induced dependence by acting on mu receptors in this preparation.

Distinct subtypes of the opioid receptor with allosteric interactions in brain membranes

The binding isotherms of opioid receptors in rat brain membranes with [3H]D-Ala2-D-Leu5-enkephalin ([3H]DADLE), [3H]dihydromorphine ([3H]DHM), and [3H]etorphine were analysed to show the effects of Mg2+, Na+, and guanine nucleotides. Four opioid receptor subtypes of delta, kappa, mu 1, and mu 2 specificities were differentiated, where necessary with the aid of specific displacing ligands. Both a guanine nucleotide [guanosine-5'-(beta, gamma-imido)triphosphate] and the cations (Na+, Mg2+) affect the affinity state of all four subtypes of the receptor. The opioid binding behaviour is found on detailed inspection to be complex, with cases of "half-of-the-sites" reactivity and of cooperativity. By their behaviour under the various ionic conditions noted, it was concluded that these subtypes are distinct, without the need to assume interconvertibility by such agents. The evidence suggests that the formation of heterologous kappa-delta or mu 1-mu 2 receptor complexes is required for stabilization of the high-affinity conformational state of the receptor. Important effects of cations in increasing the binding and regulating the equilibria of receptor association-dissociation were observed when these studies were conducted, not in the Tris-HCl buffer commonly used in opioid binding assays, but in N-tris[hydroxymethyl]-methyl-2-aminoethanesulphonate (K+) buffer (TES-KOH; 10 mM, pH 7.5): it was found that ionic species of Tris can substitute for divalent cations. Dithiothreitol effects on agonist binding in the presence and absence of the cations suggested that those cation effects involve the exchange of -SH/-SS- bonds between receptor subunits. All of the behaviour is interpreted in terms of a model involving association-dissociation equilibria of homologous and/or heterologous receptor subunits of an oligomeric opioid receptor structure.

Effects of deprolorphin, a casomorphin analog, on hippocampal CA1 field potentials in vitro

The effects of infusion of low concentrations of the synthetic opioid peptide D-Pro4-beta-casomorphin-5(deprolorphin) on electrical field responses in the in vitro hippocampal slice preparation of mice were investigated. Deprolorphin (0.01-10 microM) causes a large enhancement of the population spike (PS) and appearance of additional spikes of CA1 pyramidal cells to Schaffer-commissural stimulation, which were partially antagonized by the opiate receptor antagonist naloxone. It is likely that this analgesic peptide in the hippocampus acts through mu-receptors and neuronal mechanisms already described for morphine and enkephalin analogs.

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.

Differential interaction of opiates to multiple "kappa" binding sites in the guinea-pig lumbo-sacral spinal cord

Binding characteristics of [3H]-etorphine and [3H]-ethylketocyclazocine are different in the lumbo-sacral spinal cord of guinea-pig. [3H]-etorphine binds to a single class of high affinity sites whereas [3H]-ethylketocyclazocine interacts with two components, a high affinity and a low affinity components. In the presence of 5 microM (D-Ala2, D-Leu5) enkephalin (DAL), the total high affinity sites can be resolved in two classes of sites, DAL sensitive sites (DALs sites) and DAL insensitive sites (DALI sites). In these conditions, [3H]-etorphine binding is completely abolished, and the binding capacity and properties of [3H]-etorphine correspond to the DALs sites. Pharmacological investigations indicated that DALI sites represent the kappa sites, whereas DALs sites closely correspond to benzomorphan sites described in rat brain and spinal cord. From these results, a new subclassification of "kappa" sites is proposed.

Characterization of [3H]-etorphine binding in guinea-pig striatum after blockade of mu and delta sites

The guinea-pig striatum contains an apparent homogenous population of [3H]-etorphine high affinity sites (KD = 0.56 +/- 0.12 nM; Bmax = 267 +/- 47 fmoles/mg protein). The specific binding is completely abolished by 5 microM (D-Ala2, D-Leu5) enkephalin whereas an important residual binding is still present after the blockade of mu and delta sites. The binding properties of these residual sites are very similar to those of the benzomorphan sites characterized in rat brain and spinal cord. From the different binding properties of kappa and benzomorphan sites, the subdivision into kappa1 (kappa sites) and kappa2 (benzomorphan sites) is discussed.

The effects of receptor selective opioid peptides on morphine-induced analgesia

Utilizing the mouse tail-flick assay, four opioid peptides, which have been reported to be selective for either mu- or delta-opioid receptors, were examined for their analgesic potency and for their ability to modify morphine-induced analgesia. [D-Ala2,D-Leu5]enkephalin and [D-Ser2,Thr6]leucine-enkephalin, putative delta-receptor selective peptides, produced a potent analgesic response and at subanalgesic doses potentiated morphine-induced analgesia. Morphiceptin and [D-Ala2,Pro5]enkephalinamide, putative mu-receptor selective peptides, were similarly found to produce analgesia. However, in contrast to the delta-receptor selective peptides, three mu-receptor selective peptides were unable to alter the potency of morphine. Thus, it would appear that the potentiation of morphine analgesia is a unique property of delta-receptor selective peptides.