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

Opioid Peptides and Their Receptors in Chickens: Structure, Functionality, and Tissue Distribution

Opioid peptides, derived from PENK, POMC, PDYN and PNOC precursors, together with their receptors (DOR, MOR, KOR and ORL1), constitute the opioid system and are suggested to participate in multiple physiological/pathological processes in vertebrates. However, the question whether an opioid system exists and functions in non-mammalian vertebrates including birds remains largely unknown. Here, we cloned genes encoding opioid system from the chicken brain and examined their functionality and tissue expression. As in mammals, 6 opioid peptides encoded by PENK (Met-enkephalin and Leu-enkephalin), POMC (β-endorphin), PDYN (dynorphin-A and dynorphin-B) and PNOC (nociceptin) precursors and four opioid receptors were found to be highly conserved in chickens. Using pGL3-CRE-luciferase and pGL4-SRE-luciferase reporter systems, we demonstrated that chicken opioid receptors (cDOR, cMOR, cKOR and cORL1) expressed in CHO cells, could be differentially activated by chicken opioid peptides, and resulted in the inhibition of cAMP/PKA and activation of MAPK/ERK signaling pathways. cDOR is potently activated by Met-enkephalin and Leu-enkephalin, and cKOR is potently activated by dynorphin-A, dynorphin-B and nociceptin, whereas cORL1 is specifically activated by nociceptin. Unlike cDOR, cKOR and cORL1, cMOR is moderately/weakly activated by enkephalins and other opioid peptides. These findings suggest the ligand-receptor pair in chicken opioid system is similar, but not identical to, that in mammals. Quantitative real-time PCR revealed that the opioid system is mainly expressed in chicken central nervous system including the hypothalamus. Collectively, our data will help to facilitate the better understanding of the conserved roles of opioid system across vertebrates.

The evolution of vertebrate opioid receptors

The proteins that mediate the analgesic and other effects of opioid drugs and endogenous opioid peptides are known as opioid receptors. Opioid receptors consist of a family of four closely-related proteins belonging to the large superfamily of G-protein coupled receptors. The three types of opioid receptors shown unequivocally to mediate analgesia in animal models are the mu (MOR), delta (DOR), and kappa (KOR) opioid receptor proteins. The role of the fourth member of the opioid receptor family, the nociceptin or orphanin FQ receptor (ORL), is not as clear as hyperalgesia, analgesia, and no effect was reported after administration of ORL agonists. There are now cDNA sequences for all four types of opioid receptors that are expressed in the brain of six species from three different classes of vertebrates. This review presents a comparative analysis of vertebrate opioid receptors using bioinformatics and data from recent human genome studies. Results indicate that opioid receptors arose by gene duplication, that there is a vector of opioid receptor divergence, and that MOR shows evidence of rapid evolution.

Use of preproenkephalin knockout mice and selective inhibitors of enkephalinases to investigate the role of enkephalins in various behaviours

RATIONALE

The most simple and efficient method to study the physiological role of enkephalins is to increase the lifetime of these endogenous opioid peptides by inhibiting their inactivating enzymes. Enkephalins are degraded by the concomitant action of two metallopeptidases: neutral endopeptidase (NEP, EC3.4.21.11) and aminopeptidase N (APN, EC3.4.11.2), both enzymes releasing inactive metabolites.

OBJECTIVES

Potent dual inhibitors have been developed, such as RB101. However, NEP and APN have a broad specificity and can cleave various peptides in vitro. Therefore, it was essential to investigate the specific involvement of enkephalins in the various pharmacological responses induced by dual inhibitors.

MATERIALS AND METHODS

We compared the pharmacological responses induced by RB101 in wild-type and preproenkephalin-deficient mice (Penk1-/-) using several behavioural assays.

RESULTS

In all the tests used (hot plate test, force swim test, castor-oil-induced diarrhoea), RB101 induced strong effects in wild-type animals, whereas slight effects were observed in Penk1-/- animals. These residual effects are blocked by pre-administration of the opioid antagonist naloxone, supporting the involvement of the opioid receptors in the responses observed.

CONCLUSIONS

The pharmacological effects induced by dual inhibitors acting on both NEP and APN are mainly due to the protection of the endogenous enkephalins at supraspinal and peripheral levels. It could be speculated that the residual effects observed in Penk1-/- mice after RB101 administration could be due to the direct action of other opioid peptides or through an indirect effect involving the protection of other peptide substrates of NEP or APN, as substance P or angiotensin.

Trends in the Evolution of the Proenkephalin and Prodynorphin Genes in Gnathostomes

The opioid/orphanin gene family provides a model system for analyzing the outcomes of genome duplication events. Recent studies on the proenkephalin gene provide additional evidence that the organizational plan for this gene has been conserved throughout the extensive radiation of the gnathostome vertebrates. However, an analysis of the amino acid sequence of proenkephalin from the zebrafish, Danio rerio, suggests that novel forms of this opioid precursor may be evolving in teleosts. Analyses of sarcopterygian prodynorphin sequences revealed a proenkephalin signature in prodynorphin. Current studies on the opioid/orphanin gene family point to the duplication events that shaped this family occurring prior to the radiation of the gnathostomes.

In the african lungfish Met-enkephalin and leu-enkephalin are derived from separate genes: cloning of a proenkephalin cDNA

A full-length proenkephalin cDNA (accession number: AF232670) was cloned from an African lungfish (Protopterus annectens) brain cDNA library. The 1,351-bp African lungfish proenkephalin contains an open reading frame that codes 266 amino acids and a stop codon. Within the sequence of lungfish proenkephalin there are 5 pentapeptide opioid sequences (all YGGFM), 1 octapeptide opioid sequence (YGGFMRSL) and 1 heptapeptide opioid sequence (YGGFMGY). A Leu-enkephalin sequence was conspicuously absent in lungfish proenkephalin. These results, coupled with observations on the organization of amphibian proenkephalin and mammalian proenkephalin, indicate that among the Sarcopterygii (lobed finned fish and tetrapods), the appearance of a Leu-enkephalin sequence in proenkephalin may have evolved in either the ancestral amniotes or the ancestral mammals, but not earlier in sarcopterygian evolution. Furthermore, the detection of neurons in the lungfish CNS that are only immunopositive for Met-enkephalin, coupled with earlier anatomical studies on the presence of neurons in the lungfish CNS that are only immunopositive for Leu-enkephalin, indicates that a Leu-enkephalin-coding opioid gene must be present in the CNS of the lungfish. This gene may be the lungfish form of prodynorphin. Given the phylogenetic position of the lungfish in vertebrate evolution, the putative Leu-enkephalin-coding gene must have evolved in the ancestral sarcopterygian vertebrates, or in the ancestral gnathostomes. The apparent slow rate of lungfish evolution makes these organisms interesting models for investigating the evolution of the opioid/orphanin gene family.

Deciphering the origin of Met-enkephalin and Leu-enkephalin in Lobe-finned fish: cloning of australian lungfish proenkephalin

The previous detection of Met-enkephalin and Leu-enkephalin in the CNS of the Australian lungfish, Neoceratodus forsteri, in a molar ratio comparable to mammals suggested that the lungfish proenkephalin precursor should contain the sequences of both Met-enkephalin and Leu-enkephalin as seen for mammalian proenkephalin. However, the cloning of a full-length proenkephalin cDNA from the CNS of the Australian lungfish indicates that the organization of this precursor is more similar to amphibian proenkephalin than mammalian proenkephalin. The Australian lungfish cDNA is 1284 nucleotides in length and the open reading frame (267 amino acids) contains seven opioid sequences (GenBank #AF232671). There are five copies of the Met-enkephalin sequence flanked by sets of paired basic amino acid proteolytic cleavage sites and two C-terminally extended forms of Met-enkephalin: YGGFMRSL and YGGFMGY. As seen for amphibians, no Leu-enkephalin sequence was detected in the Australian lungfish proenkephalin cDNA. The fact that Leu-enkephalin has been identified by radioimmunoassay and HPLC analysis in the CNS of the Australian lungfish indicates that a Leu-enkephalin-coding gene, distinct from proenkephalin, must be expressed in lungfish. Potential candidates may include a prodynorphin- or other opioid-like gene. Furthermore, the absence of a Leu-enkephalin sequence in lungfish and amphibian proenkephalin would suggest that the mutations that yielded this opioid sequence in tetrapod proenkephalin occurred at some point in the radiation of the amniote vertebrates.

Organization of proenkephalin in amphibians: cloning of a proenkephalin cDNA from the brain of the anuran amphibian, Spea multiplicatus

Cloning of a proenkephalin cDNA from the pelobatid anuran amphibian, Spea multiplicatus, provides additional evidence that Leu-enkephalin, although present in the brain of anuran amphibians, is not encoded by the proenkephalin gene. The S. multiplicatus proenkephalin cDNA is 1375 nucleotides in length, and the open reading frame contains the sequences of seven opioid sequences. There are five copies of the Met-enkephalin sequence, as well as an octapeptide opioid sequence (YGGFMRNY) and a heptapeptide opioid sequence (YGGFMRF). In the proenkephalin sequence of S. multiplicatus the penultimate opioid is a Met-enkephalin sequence rather than the Leu-enkephalin present in mammalian sequences. The same order of opioid sequences also is observed for the proenkephalin sequence of the pipid anuran amphibian, Xenopus laevis. Hence, from a phylogenetic standpoint the organization of tetrapod proenkephalin has been remarkably conserved. What remains to be resolved is whether the Leu-enkephalin sequence found in mammalian proenkephalin is an ancestral trait or a derived trait for the tetrapods. Unlike the proenkephalin precursor of X. laevis, all of the opioid sequences in the S. multiplicatus proenkephalin cDNA are flanked by paired-basic amino acid proteolytic cleavage sites. In this regard the proenkephalin sequence for S. multiplicatus is more similar to mammalian proenkephalins than the proenkephalin sequence of X. laevis. However, a comparison of the proenkephalin sequences in human, X. laevis, and S. multiplicatus revealed several conserved features in the evolution of the tetrapod proenkephalin gene. By contrast, a comparison of tetrapod proenkephalin sequences with the partial sequence of a sturgeon proenkephalin cDNA indicates that the position occupied by the penultimate opioid sequence in vertebrate proenkephalins may be a highly variable locus in this gene.

Multiple opioid receptor-like genes are identified in diverse vertebrate phyla

Polymerase chain reaction was used to determine whether opioid receptor-like sequences are present in species from the protostome and deuterostome branches of the metazoan kingdom. Multiple opioid receptor-like sequences were found in all vertebrates, but no specific fragments were obtained from any invertebrates. Delta, mu, kappa and ORL-1 receptors were identified from bovine DNA, and three different opioid receptor-like fragments were identified from the other vertebrates analyzed. The data suggest that the opioid receptor gene family has been highly conserved during vertebrate evolution and that, even in the primitive jawless fish, multiple members of the opioid receptor family appear to be present.

Purification, sequence analysis, and cellular localization of a prodynorphin-derived peptide related to the alpha-neo-endorphin in the rhynchobdellid leech Theromyzon tessulatum

Cells immunoreactive to an antiserum specifically directed against vertebrate alpha-Neo-endorphin (alpha-NE) were detected in the internal wall of anterior and posterior suckers of the rhynchobdellid leech Theromyzon tessulatum. These cells have morphological and ultrastructural characteristics close to the "releasing gland cells" of adhesive organs. The epitope recognized by anti-alpha-NE was contained in granules having a diameter of 0.2-0.3 microm. Previous works involving the brain of this leech demonstrate the existence of approximately 14 neurons immunoreactive to the anti-alpha-NE. Following an extensive purification including high pressure gel permeation and reversed-phase high performance liquid chromatography, epitopes contained in both suckers and central nervous system were isolated. Purity of the isolated peptides was controlled by capillary electrophoresis. Their sequences were determined by a combination of automated Edman degradation, electrospray mass spectrometry measurement, and coelution experiments in reversed-phase high performance liquid chromatography with synthetic alpha-NE. The results demonstrate that epitopes recognized by the anti-alpha-NE in the suckers and the central nervous system are identical to vertebrate alpha-NE (YGGFLRKYPK). This finding constitutes the first biochemical characterization of a prodynorphin-derived peptide in invertebrates. Moreover the isolation of this peptide in the annelida establishes the very ancient phylogenetic origin of alpha-NE as well as its conservation in evolution.

Isolation and structural characterization of enkephalins in the brain of the rhynchobdellid leech Theromyzon tessulatum

This paper reports the purification of four peptides related to enkephalins from the brain of the leech Theromyzon tessulatum. After reverse-phase HPLC purification, the sequence of the enkephalins (YGGFM, YGGFL, FM, FL) was established by a combination of automated Edman degradation, electrospray mass spectrometry measurement, and co-elution experiments in reverse-phase HPLC with synthetic peptides. ELISA titrations performed on each purified peptide indicated that the major amount was borne by the leucine-enkephalin. The ratio of leucine-enkephalin and methionine-enkephalin of 2:1 is in line with previous immunocytochemical data obtained on T. tessulatum brains. The presence of enkephalins in T. tessulatum, an animal belonging to the oldest group of coelomate metazoans (the Annelida) establishes the very ancient phylogenetic origin of opioids and their conservation in the course of evolution.

Immunocytochemical evidence for the presence of Met-enkephalin and Leu-enkephalin in distinct neurons in the brain of the elasmobranch fish Scyliorhinus canicula

Immunohistochemical methods have been used to investigate the distribution of various opioid peptides derived from mammalian proenkephalin in the central nervous system of Scyliorhinus canicula. The results indicate that both Leu- and Met-enkephalin-immunoreactive peptides are present in the dogfish brain. In contrast, enkephalin forms similar to Met-enkephalin-Arg-Phe or Met-enkephalin-Arg-Gly-Leu, and mammalian alpha-neo-endorphin, dynorphin A (1-8), dynorphin A (1-13), and dynorphin A (1-17) were not detected. Met- and Leu-enkephalin immunoreactivities were found in distinct neurons of the telencephalon and hypothalamus. In particular, cell bodies reacting only with the Met-enkephalin antiserum were localized in the preoptic nucleus and in the suprachiasmatic region of the hypothalamus. Conversely, cell bodies reacting only with the Leu-enkephalin antiserum were localized in the pallium and the nucleus lobi lateralis hypothalami. Several areas of the telencephalon and diencephalon exhibited both Met- and Leu-enkephalin-like immunoreactivity, but the two immunoreactive peptides were clearly contained in distinct perikarya. The overall distribution of Met-enkephalin-immunoreactive elements in the dogfish exhibited similarities to the distribution of proenkephalin-derived peptides previously reported for the brain of tetrapods. The fact that Met- and Leu-enkephalin-like peptides were detected in distinct neurons, together with the absence of dynorphin-related peptides, suggests the existence of a novel Leu-enkephalin-containing precursor in the dogfish brain.

Detection of prodynorphin end products in lizard, turtle, and alligator brain extracts

Heterologous radioimmunoassays (RIAs) for the mammalian prodynorphin end products, alpha-neo-endorphin, dynorphin A(1-17), dynorphin A(1-8), and dynorphin B(1-13) were used to screen brain extracts obtained from representatives of the major surviving orders of reptiles: Chelonia (Pseudemys scripta), Squamata (Anolis carolinensis), and Crocodylia (Alligator mississippiensis). Methanol/acid extracts of whole brains obtained from each species were separately fractionated by gel filtration chromatography and reversed-phase HPLC. In all three species, an immunoreactive form of alpha-neo-endorphin was detected with the same retention time as synthetic mammalian alpha-neo-endorphin following reversed-phase HPLC analysis. In all three species, reversed-phase HPLC analysis revealed a novel form of dynorphin B(1-13)-related immunoreactivity. With the available immunological probes, dynorphin A products were only detected in the Anolis brain extracts. Both dynorphin A(1-17) and dynorphin A(1-8) were detected in this species.

Occurrence of immunoreactive Met- and Leu-enkephalin-like peptides in the ovary of the green frog, Rana esculenta

In the present study, we have localized for the first time Met- and Leu-enkephalin-like material in the ovary of the anuran, Rana esculenta, using the indirect immunofluorescence method. The ovaries were sampled during the main representative phases of the annual reproductive cycle of the frog, living in a mountain pond (Colfiorito, Umbria at 820 m a.s.l.). Strong immunoreactivity to Met- and Leu-enkephalin antisera was observed in the follicle cells of the granulosa layer of vitellogenic oocytes; moreover, during this phase, immunofluorescent materials were also radially localized in the cytoplasm and in the perinuclear zone. The mature oocytes showed Met- and Leu-enkephalin-like immunostaining in the thecal layer and in several granules scattered in the peripheral zone of the yolk. The different pattern of Leu- and Met-enkephalin-like immunoreactivity in the frog ovary parallels and complements the changes occurring in the reproductive (May) and in the vitellogenetic (September) phases during the ovarian cycle. Consequently, these findings strongly support the hypothesis for a local synthesis of these peptides in the ovary and suggest their possible involvement in the control of ovarian function.

The phylogeny of Met-enkephalin and Leu-enkephalin: studies on the holostean fish Lepisosteus platyrhincus and the Australian lungfish, Neoceratodus forsteri

Acid extracts of the brain of the holostean fish Lepisosteus platyrhincus and the forebrain of the dipnoan fish Neoceratodus forsteri were separately fractionated by Sephadex G-50 column chromatography. For both species, Met-enkephalin-related immunoreactivity was detected coeluting with the total volume internal standard. Higher-molecular-weight Met-enkephalin-containing immunoreactive peaks were not detected in these chromatographs. Furthermore, immunoreactive forms with antigenic determinants identical to mammalian dynorphin A(1-17), dynorphin A(1-8), alpha-neo-endorphin, or dynorphin B(1-13) were not detected in either species. Reverse-phase HPLC analysis of enkephalin-sized immunoreactive material indicated the presence of authentic Met-enkephalin and Leu-enkephalin in the extracts of both species. In the brain of L. platyrhincus the molar ratio of Met-enkephalin to Leu-enkephalin was approximately 3:1, whereas, the molar ratio of these enkephalins in the forebrain of N. forsteri was approximately 5:1 [corrected]. C-terminally extended forms of Met-enkephalin were also detected in the extracts of both species. These results suggest that the ancestral proenkephalin gene of both actinopterygian and sarcopterygian fish contained both the Met-enkephalin and Leu-enkephalin sequences.

Detection of Met-enkephalin in the CNS of the teleosts, Anguilla rostrata and Oncorhynchus kisutch

Acid extracts of the brains of the American eel, Anguilla rostrata, and the coho salmon, Oncorhynchus kisutch, were screened for enkephalin-related products and dynorphin-related products. Following Sephadex G-50 column chromatography, a peak of Met-enkephalin-related immunoreactivity was detected near the total volume of the column for both species. No higher molecular weight forms of Met-enkephalin-related material were detected, nor were any immunoreactive forms with antigenic determinants similar to mammalian dynorphin A(1-17), dynorphin A(1-8), dynorphin B(1-13) or alpha-neo-endorphin detected for either species. The enkephalin-sized immunoreactivity was further analyzed by reverse phase HPLC. For both species, a peak of authentic Met-enkephalin was detected. However, Leu-enkephalin, Met-enkephalin-RGL and Met-enkephalin-RF were not detected by RIA in either species. In addition, no novel C-terminally extended forms of Met-enkephalin were detected in either species. Finally, opiate receptor binding activity was only found associated with the peak of immunoreactive Met-enkephalin.

Evidence for enkephalin- and endorphin-immunoreactive cells in the anterior pituitary of the axolotl Ambystoma mexicanum

An immunohistochemical study of opioid peptides in the hypophysis of the axolotl, Ambystoma mexicanum, was carried out with antisera against leu-enkephalin, beta-endorphin, met-enkephalin, and dynorphin A (1-8). We found leu-enkephalin immunoreactivity in some fibers of the neural lobe and the median eminence. In contrast to previous reports on mammals and other vertebrates, we found leu-enkephalin immunoreactivity in many cells scattered throughout the anterior lobe. As in other vertebrates, the beta-endorphin immunoreactivity was present in all the cells of the intermediate lobe and in a few cells of the anterior lobe. Met-enkephalin and dynorphin A (1-8) immunoreactivities were only present in the neural lobe and the median eminence. The presence of leu-enkephalin and beta-endorphin in the anterior lobe suggests that these peptides could be acting as hormones released from the hypophysis of the unmetamorphosed amphibian.

Detection of Met-enkephalin and Leu-enkephalin in the brain of the hagfish, Eptatretus stouti, and the lamprey, Petromyzon marinus

Acid extracts of the brain of the pacific hagfish, Eptatretus stouti, and the marine lamprey, Petromyzon marinus, were each fractionated by gel filtration chromatography and aliquots of column fractions were screened with radioimmunoassays (RIAs) specific for pro-dynorphin-related end products and for pro-enkephalin-related end products. Only pro-enkephalin-related immunoreactive forms were detected. The enkephalin-sized immunoreactive material, isolated for each species, was separately fractionated by reverse-phase high-performance liquid chromatography (HPLC). Aliquots of column fractions were screened with RIAs specific for Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Phe, and Met-enkephalin-Arg-Gly-Leu. In the hagfish brain, immunoreactive forms with the same retention times as synthetic Met-enkephalin and Leu-enkephalin were detected in a ratio of approximately 2:1. In addition, an immunoreactive form was detected with the Met-enkephalin-Arg-Phe-specific RIA. This form had the same chromatographic properties as synthetic Met-enkephalin-Arg-Phe. Analyses with the Met-enkephalin-Arg-Gly-Leu RIA were negative. HPLC analysis of the lamprey enkephalin-related material revealed the presence of authentic Met-enkaphalin and Leu-enkephalin in a molar ratio of 3:1. C-terminally extended forms of Met-enkaphalin were not detected in the lamprey extracts. Collectively these observations indicate that pro-enkephalin-related opioid peptides are present in the brain of cyclostomes.

Effects of sodium and temperature on naloxone binding in brain tissues of a urodele amphibian

1. Partially purified brain membranes obtained from male rough-skinned newts (Taricha granulosa) were used to determine the effects of NaCl and temperature on the specific binding of the opioid receptor antagonist [3H]naloxone. 2. The addition of NaCl to the incubation medium at concentrations up to 400 mM produced a dose-related increase of the specific binding of [3H]naloxone. 3. The addition of other salts to the incubation medium had less pronounced effects: KCl and MgCl2 slightly increased and decreased, respectively, the specific binding of naloxone, and CaCl2 had no effect. 4. Results of an equilibrium saturation experiment showed that the addition of 200 mM NaCl resulted in over a 10-fold increase in the number of high affinity (KD = 0.61 nM) binding sites for naloxone, with no changes in the number of low affinity (KD = 21.8 nM) binding sites. 5. Changes in NaCl concentrations did not significantly affect either dissociation constant. 6. The binding of [3H]naloxone was temperature-dependent; it increased when the incubation temperatures were elevated from 0 degree C to 37 degrees C. 7. Results obtained for this urodele amphibian are compared with those available for other vertebrate species.

Detection of Met-enkephalin and Leu-enkephalin in the posterior pituitary of the holostean fish, Amia calva

Immunohistochemical analysis of the pituitary of the holostean fish, Amia calva, indicated that enkephalin-related immunoreactivity was restricted to the pars nervosa, and was not detected in other regions of the pituitary. Fractionation of acid extracts of posterior pituitaries by reverse phase HPLC followed by RIA analysis indicated the presence of immunoreactive Met-enkephalin and Leu-enkephalin. No immunoreactive forms were detected with RIAs specific for either Met-enkephalin-RF or Met-enkephalin-RGL. The molar ratio of Met- to Leu-enkephalin in this terminal field was 3:1 (n = 4). HPLC fractions were also digested with trypsin and carboxypeptidase B to test for C-terminally extended forms of Met-enkephalin. A novel modified form of Met-enkephalin was detected. Extracts of the posterior pituitary, forebrain, midbrain, hypothalamus and hindbrain were screened with RIAs specific for the Pro-dynorphin end products, alpha-neo-endorphin, dynorphin A(1-17), dynorphin A(1-8) and dynorphin B(1-13). The results of these analyses were negative. Collectively, these data suggest that a Pro-enkephalin-like molecule is present in holostean fish. The holostean enkephalin precursor contains at least Met-enkephalin and Leu-enkephalin. However, Pro-dynorphin-related end products with antigenic determinants similar to mammalian dynorphin A(1-17), dynorphin A(1-8), dynorphin B(1-13) and alpha-neo-endorphin could not be detected in the brain or pituitary of this species.