Prodynorphin-derived Opioid Peptides Research Articles

Social Defeat Stress-Induced Behavioral Responses are Mediated by the Endogenous Kappa Opioid System

Previous studies have demonstrated that repeated forced-swim stress-induced behaviors (including analgesia, immobility, and increased drug reward) were mediated by the release of endogenous prodynorphin-derived opioid peptides and subsequent activation of the kappa opioid receptor (KOR). We tested the generality of these effects using a different type of stressful situation: repeated social defeat. C57Bl/6 mice subjected to social defeat stress (SDS) over 3 days showed a characteristic stress-induced immobility and defeated-postural response, as well as stress-induced analgesia (SIA). Daily pretreatment with the KOR antagonist nor-binaltorphimine (nor-BNI, 10 mg/kg, i.p.) blocked the SIA and significantly reduced the stress-induced immobility on the second and third days of SDS exposure. In contrast, prodynorphin gene-disrupted mice showed no significant increase in immobility, socially defeated postures, or SIA following repeated exposure to SDS. Since both stress and the kappa opioid system can modulate the response to drugs of abuse, we tested the effects of SDS on cocaine-conditioned place preference (CPP). SDS-exposed mice conditioned with cocaine (15 mg/kg, s.c.) showed significant potentiation of place-preference for the drug-paired chamber over the responses of unstressed mice. Nor-BNI pretreatment blocked stress-induced potentiation of cocaine-CPP. Consistent with this result, mice lacking the prodynorphin gene did not show stress-induced potentiation of cocaine-CPP, whereas wild-type littermates did. The findings suggest that chronic SDS may activate the kappa opioid system to produce analgesia, immobility, social defeat postures, and resulting in a potentiation of the acute rewarding properties of cocaine.

Neuropathic pain activates the endogenous kappa opioid system in mouse spinal cord and induces opioid receptor tolerance.

Release of endogenous dynorphin opioids within the spinal cord after partial sciatic nerve ligation (pSNL) is known to contribute to the neuropathic pain processes. Using a phosphoselective antibody [kappa opioid receptor (KOR-P)] able to detect the serine 369 phosphorylated form of the KOR, we determined possible sites of dynorphin action within the spinal cord after pSNL. KOR-P immunoreactivity (IR) was markedly increased in the L4-L5 spinal dorsal horn of wild-type C57BL/6 mice (7-21 d) after lesion, but not in mice pretreated with the KOR antagonist nor-binaltorphimine (norBNI). In addition, knock-out mice lacking prodynorphin, KOR, or G-protein receptor kinase 3 (GRK3) did not show significant increases in KOR-P IR after pSNL. KOR-P IR was colocalized in both GABAergic neurons and GFAP-positive astrocytes in both ipsilateral and contralateral spinal dorsal horn. Consistent with sustained opioid release, KOR knock-out mice developed significantly increased tactile allodynia and thermal hyperalgesia in both the early (first week) and late (third week) interval after lesion. Similarly, mice pretreated with norBNI showed enhanced hyperalgesia and allodynia during the 3 weeks after pSNL. Because sustained activation of opioid receptors might induce tolerance, we measured the antinociceptive effect of the kappa agonist U50,488 using radiant heat applied to the ipsilateral hindpaw, and we found that agonist potency was significantly decreased 7 d after pSNL. In contrast, neither prodynorphin nor GRK3 knock-out mice showed U50,488 tolerance after pSNL. These findings suggest that pSNL induced a sustained release of endogenous prodynorphin-derived opioid peptides that activated an anti-nociceptive KOR system in mouse spinal cord. Thus, endogenous dynorphin had both pronociceptive and antinociceptive actions after nerve injury and induced GRK3-mediated opioid tolerance.

Brain dynorphin and enkephalin systems in Fischer and Lewis rats: effects of morphine tolerance and withdrawal

Lewis rats are more likely to self-administer various drugs of abuse than Fischer rats. Here these two strains of rats were compared with regard to basal brain opioid peptide levels and the response to chronic morphine treatment and to naloxone-precipitated withdrawal. Lewis rats had lower basal dynorphin peptides in the substantia nogra, striatum (not Leu-enkephalinArg 6) and VTA (not dynorphin B) and the pituitary gland. Leu-enkephalinArg 6 levels were also lower in these structures (with the exception of striatum which had higher levels) and in the nucleus accumbens. There were also strain differences in the response to chronic morphine treatment; in the nucleus accumbens, morphine treatment increased dynorphin A levels in Fischer rats only, in the ventral tegmental area effects were opposite with increased dynorphin levels in Fischer and decreased levels in Lewis rats, in the hippocampus dynorphin levels were markedly reduced in Lewis rats only. In Fischer rats, chronic morphine strongly affected peptide levels in the substantia nigra and striatum, whereas Lewis rats responded less in these areas. Leu-enkephalin, which derives from both prodynorphin and proenkephalin, and Met-enkephalin, which derives from proenkephalin, were effected by chronic morphine mainly in Fischer rats, increasing levels in most of the brain areas examined. The results in this study show (1) strain differences in basal levels of prodynorphin-derived opioid peptides, (2) the prodynorphin system to be differently influenced by morphine in Lewis rats than in Fischer rats and 3) the proenkephalin system to be influenced by chronic morphine in brain areas related to reward processes only in Fischer rats.

Endogenous dynorphins inhibit excitatory neurotransmission and block LTP induction in the hippocampus.

Although anatomical and neurochemical studies suggest that endogenous opioids act as neurotransmitters, their roles in normal and pathophysiological regulation of synaptic transmission are not defined. Here we examine the actions of prodynorphin-derived opioid peptides in the guinea-pig hippocampus and show that physiological stimulation of the dynorphin-containing dentate granule cells can release endogenous dynorphins, which then activate kappa 1 opioid receptors present in the molecular layer of the dentate gyrus. Activation of kappa 1 receptors by either pharmacologically applied agonist or endogenously released peptide reduces excitatory transmission in the dentate gyrus, as shown by a reduction in the excitatory postsynaptic currents evoked by stimulation of the perforant path, a principal excitatory afferent. In addition, released dynorphin peptides were found to block the induction of long-term potentiation (LTP) at the granule cell-perforant path synapse. The results indicate that endogenous dynorphins function in this hippocampal circuit as retrograde, inhibitory neurotransmitters.

Modification of the enkephalin "message" with an artificial polycationic C-terminus

The C-terminal "address" sequences of prodynorphin-derived opioid peptides contain an unusually high proportion of basic residues, which are known to be crucial for conferring high activity and selectivity for kappa-opioid receptors. In an effort to investigate the possibility that the polycationic "tails" may be involved in a coulombic interaction with a complementary polyanionic receptor domain, we attached a series of achiral peptide-like cationic fragments to the C-terminus of the opioid peptide "message", Tyr-Gly-Gly-Phe. Binding of the various compounds to opioid receptor types in guinea pig brain membranes was weak, and the pharmacologic activities in the guinea pig ileum were marginal. These results indicate either that the chosen ligand design does not satisfy the structural requirements of the hypothesized coulombic interaction or that the latter is a minor criterion governing receptor recognition.

Purification and characterization of endoproteases from human choroid plexus cleaving prodynorphin-derived opioid peptides

An endoprotease converting the dynorphins and α-neoendorphin has been purified to apparent homogeneity from soluble extracts of human choroid plexus. The purified enzyme was stained as a single band after sodium dodecyl sulphate polyacrylamide gel electrophoresis with an apparent molecular weight of around 54,000 Da. The enzyme potently cleaves dynorphin A, dynorphin B and α-neoendorphin at consecutive pairs of basic amino acid residues generating Leu-Enk-Arg 6, but it is less active on other neuropeptides containing dibasic stretches. It is optimally active at neutral pH, sensitive to EDTA and slightly affected by the serine protease inhibitors DFP and PMSF. A similar membrane-bound enzyme present in the same tissue was solubilized with 0.5% Triton X-100 and isolated with the same purification procedure. This latter enzyme showed almost identical properties with the soluble peptidase, except for a slightly higher molecular weight.

Steady-state levels of pro-dynorphin-related end-products from the brain of the amphibian, Xenopus laevis

Steady-state analyses of prodynorphin-derived opioid peptides were conducted on acid extracts of the brain of the frog, Xenopus laevis. Radioimmunoassays specific for dynorphin A(1–17), dynorphin A(1–8), α-neoendorphin and dynorphin B coupled with gel filtration chromatography and reverse phase high performance liquid chromatography were used. The major prodynorphin-related end-product detected was α-neoendorphin. Leu-enkephalin was also detected. Since the Xenopus proenkephalin precursor does not contain the Leu-enkephalin sequence, these data suggest that some of the prodynorphin-related end-products had been cleaved to yield Leu-enkephalin.

Hindlimb paralytic effects of prodynorphin-derived peptides following spinal subarachnoid injection in rats

Dynorphin A-(1–17) acts through non-opioid mechanisms to produce dose-related neurological deficits following injection into the lumbar spinal subarachnoid space in rats. Hindlimb motor function was examined following subarachnoid injection of dynorphin A fragments and other opioid peptides derived from prodynorphin to establish: (1) which portion(s) of the dynorphin A molecule cause hindlimb motor dysfunction, and (2) whether these paralytic actions are shared by other opioids (dynorphin B, α-neo-endorphin, and β-neo-endorphin) derived from the same promolecule. To minimize the influence of enzymatic inactivation on relative bioactivities, peptides were coinjected with a combination of peptidase inhibitors previously shown to enhance the actions of dynorphin A fragments in vitro. Dynorphin A-(1–17) and -(2–17) produced dose-related neurological deficits with equal potencies and durations. Although without effect when injected alone, dynorphin A-(1–8), -(1–7) and -(3–8) caused transient motor dysfunction when co-injected with peptidase inhibitors. In contrast, dynorphin A-(1–6), -(1–5) and -(6–17) did not disrupt hindlimb motor function with or without peptidase inhibition. Dynorphin B, α-neo-endorphin also caused hindlimb dysfunction which was potentiated by peptidase inhibition. These deficits appeared to result from non-opioid actions of these three peptides, since they were not blocked by the opioid antagonist naloxone. Thus, the paralytic effects of dynorphin A: (1) result from non-opioid actions involving the 3–7 or 3–8 positions of the molecule, and (2) are shared by other prodynorphin-derived opioid peptides.

Co-localization of proenkephalin- and prodynorphin-derived opioid peptides in laminae IV/V spinal neurons revealed in arthritic rats

By the use of highly selective antisera and an immunohistochemical technique the possible coexistence of proenkephalin- (PRO-ENK)- and prodynorphin (PRO-DYN)-derived peptides was examined in 4- to 6-μm-thick serial sections of the L 4–L 5 segments of the spinal cord of non-colchicine-treated polyarthritic rats. In control, non-colchicine treated animals, virtually no cell bodies stained for the PRO-ENK-derived peptides, heptapeptide (MRF) and octapeptide (MRGL), nor for the PRO-DYN-derived peptides, dynorphin A (DYN) and α-neoendorphin (NEO). In contrast, in polyarthritic rats, numerous large (15–30 μm) multipolar neurons could be visualized with each antiserum in laminae IV/V. Alternate staining of adjacent sections with either anti-MRF or anti-MRGL antisera, followed by either anti-DYN or anti-NEO antisera, revealed a clear coexistence of PRO-ENK and PRO-DYN peptides. It was possible to demonstrate co-localization of all 4 opioids in a single cell. It appeared that all cells staining for PRO-ENK peptides in laminae IV/V also stained for PRO-DYN peptides.

Relative contents and concomitant release of prodynorphin/neoendorphin-derived peptides in rat hippocampus.

The contents and molecular forms of five different prodynorphin-derived opioid peptides were compared in extracts of rat hippocampus by radioimmunoassay after C18-HPLC resolution. Dynorphin (Dyn) A(1-17) immunoreactivity (ir) and Dyn B-ir were heterogeneous in form; Dyn A(1-8)-ir, alpha-neoendorphin (alpha neo)-ir and beta-neoendorphin (beta neo)-ir each eluted as single homogeneous peaks of immunoreactivity. The fraction of immunoreactivity having the same retention as the appropriate synthetic standard was used to estimate the actual hippocampal content of each peptide. Comparison of these values showed that the concentrations of Dyn B, alpha neo, and Dyn A(1-8) were nearly equal, whereas both Dyn A(1-17) and beta neo were 1/5th to 1/10th the value of the other three. Calcium-dependent K+-stimulated release of these prodynorphin-derived opioids from hippocampal slices was detected. The stimulated rates of release were highest for Dyn B-ir followed by alpha neo-ir, then beta neo-ir and Dyn A(1-8)-ir with Dyn A(1-17)-ir lowest. The relative rates of stimulated release were in agreement with the relative proportions of peptide present within the tissue. This evidence of the presence and release of these opioid peptides considerably strengthens the hypothesis that this family of endogenous opioids plays a neurotransmitter role in the hippocampus.