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

Genetic and functional analysis of a Pacific hagfish opioid system

The actions of endogenous opioids and nociceptin/orphanin FQ are mediated by four homologous G protein-coupled receptors that constitute the opioid receptor family. However, little is known about opioid systems in cyclostomes (living jawless fish) and how opioid systems might have evolved from invertebrates. Here, we leveraged de novo transcriptome and low-coverage whole-genome assembly in the Pacific hagfish (Eptatretus stoutii) to identify and characterize the first full-length coding sequence for a functional opioid receptor in a cyclostome. Additionally, we define two novel endogenous opioid precursors in this species that predict several novel opioid peptides. Bioinformatic analysis shows no closely related opioid receptor genes in invertebrates with regard either to the genomic organization or to conserved opioid receptor-specific sequences that are common in all vertebrates. Furthermore, no proteins analogous to vertebrate opioid precursors could be identified by genomic searches despite previous claims of protein or RNA-derived sequences in several invertebrate species. The presence of an expressed orthologous receptor and opioid precursors in the Pacific hagfish confirms that a functional opioid system was likely present in the common ancestor of all extant vertebrates some 550 million years ago, earlier than all previous authenticated accounts. We discuss the premise that the cyclostome and vertebrate opioid systems evolved from invertebrate systems concerned with antimicrobial defense and speculate that the high concentrations of opioid precursors in tissues such as the testes, gut, and activated immune cells are key remnants of this evolutionary role.

Observations on the evolution of the melanocortin receptor gene family: distinctive features of the melanocortin-2 receptor

The melanocortin receptors (MCRs) are a gene family in the rhodopsin class of G protein-coupled receptors. Based on the analysis of several metazoan genome databases it appears that the MCRs are only found in chordates. The presence of five genes in the family (i.e., mc1r, mc2r, mc3r, mc4r, mc5r) in representatives of the tetrapods indicates that the gene family is the result of two genome duplication events and one local gene duplication event during the evolution of the chordates. The MCRs are activated by melanocortin ligands (i.e., ACTH, α-MSH, β-MSH, γ-MSH, δ-MSH) which are all derived from the polypeptide hormone/neuropeptide precursor, POMC, and as a result the functional evolution of the MCRs is intimately associated with the co-evolution of POMC endocrine and neuronal circuits. This review will consider the origin of the MCRs, and discuss the evolutionary relationship between MC2R, MC5R, and MC4R. In addition, this review will analyze the functional evolution of the mc2r gene in light of the co-evolution of the MRAP (Melanocortin-2 Receptor Accessory Protein) gene family.

Observations on the radiation of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes: phylogeny of the opioid/orphanin gene family and the 2R hypothesis

At the close of the Devonian Period the rapid decline in the diversity of the lobe-finned fishes was countered by the emergence and diversification of the ray-finned fishes and the cartilaginous fishes that now dominate marine and freshwater ecosystems. All of these jawed vertebrates were derived from the ancestral gnathostomes; a chordate lineage that had experienced two genome duplication events during the evolution of the phylum. This review analyzes trends in the phylogeny of the opioid/orphanin gene family (four prohormone/neuropeptide precursor-coding genes) in the major classes of gnathostomes that survived the extinction events at the close of the Devonian Period and focuses on some features of this gene family that appear to set the cartilaginous fishes (class Chondrichthyes) apart from class Sarcopterygii (lobe-finned fishes and tetrapods) and class Actinopterygii (the ray-finned fishes).

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Immunoreactive endozepine-like peptides in the brain and pituitary of the Atlantic hagfish, Myxine glutinosa

Endozepines are a family of peptides capable of displacing benzodiazepines from their specific binding sites, to which belong the diazepam-binding inhibitor and the octadecaneuropeptide (ODN). This paper reports the distribution of ODN-related peptides, investigated for the first time by immunocytochemistry, in different brain and pituitary regions of the Atlantic hagfish, Myxine glutinosa. Immunoreactive ODN-like material was found in the telencephalon at the level of bundles of different olfactory nerve fibres. Moreover, at the level of the pallium, immunoreactive multipolar neurons were observed in the pars parvocellularis of the stratum griseum superficialis. Similar immunopositive nerve cell bodies were found in the nucleus medialis of the central prosencephalic complex. In the mesencephalon, few immunoreactive neurons lining and contacting the mesencephalic ventricle were detected; such nerve cells could be involved in the regulation of cerebrospinal fluid homeostasis. Dorsally in the mesencephalon, numerous ODN-containing cell bodies were present in the area praetectalis. The rhomboencephalon was immunostained only in the octavolateral area and in the nucleus motorius magnocellularis of the trigeminal nerve. Furthermore, ODN immunoreactivity was also present in the nerve cells of ganglia of the ophthalmic division of the trigeminal nerve complex. The immunocytochemical patterns described here in the brain of M. glutinosa suggest an involvement of ODN-like peptides as neuromodulators in sensory pathways, such as olfactory and visual. Finally, ODN-like substances were localized in discrete populations of adenohypophysial cells and in tanycytes lining the neurohypophyseal walls, suggesting for endozepines a paracrine and/or endocrine control of pituitary hormones release and a neurohormone role respectively. These results could give new insights into the chemioarchitecture of the brain of myxinoids.

Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

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.

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.

High-performance liquid chromatographic determination of intrathecally administered [D-Ala2-D-Leu5]-enkephalin concentrations in canine cerebrospinal fluid

A rapid and useful method for high-performance liquid chromatographic analysis of exogenous [D-Ala2-D-Leu5]-enkephalin (DADLE) in cerebrospinal fluid (CSF) is described. CSF (0.5 ml) samples were filtered using a 0.22-micron Co-Star filter. Chromatography was performed on a mu Bondapak C18 column using a mobile phase of A, 0.05 M sodium phosphate (monobasic, pH 6.0) and B, 60% acetonitrile in 0.05 M sodium phosphate (pH 6.0) with a flow-rate of 1 ml/min. Absorbance at 210 nm was measured. The procedure produced a linear curve for the concentration range 1-10 micrograms/ml. The development of the assay produced rapid, repeatable and accurate results for CSF analysis of DADLE at concentrations achieved with therapeutic administration of the peptide. This method could also be used in the future for analysis of compounds like DADLE.

Detection of Met-enkephalin in the pars intermedia of the lampreys, Ichthyomyzon castaneus and Petromyzon marinus

Acid extracts of the brain and pars intermedia of the chestnut lamprey, Ichthyomyzon castaneus, were fractionated by a combination of gel filtration chromatography and reversed-phase HPLC, and screened with RIAs specific for Met-enkephalin and Leu-enkephalin, respectively. In the brain extract, both Met-enkephalin and Leu-enkephalin were detected in a molar ratio of approximately 4:1. These results would suggest that these two enkephalins were derived from a lamprey Proenkephalin precursor. However, reversed-phase HPLC analysis of the pars intermedia of this species revealed the presence of Met-enkephalin, but not Leu-enkephalin. Analysis of the pars intermedia of the marine lamprey, Petromyzon marinus, also indicated the presence of Met-enkephalin but not Leu-enkephalin. These results would suggest that the Met-enkephalin present in the pars intermedia of lampreys may not be derived from Proenkephalin, but may originate from another opioid precursor. This possibility and alternative hypotheses to explain these observations are discussed.

Immunoreactive Met-enkephalin-like material in the testis of Rana esculenta: identification and localization

Methionine-enkephalin (Met-Enk) has been detected in the testis of the frog, Rana esculenta, using a reverse-phase high-performance liquid chromatography system coupled with a specific radioimmunoassay. By means of immunocytochemical techniques Met-Enk positive cells have been localized in interstitial and germinal compartments. Particularly, spermatogonia, spermatocytes, and spermatozoa were stained in seminiferous tubules, and numerous interstitial cells showed strong cytoplasmic immunoreactivity in summer animals. Variations in the concentration of Met-Enk immunoreactive material occurred during the annual cycle. Our data show that Met-Enk is present in testes of nonmammalian vertebrate species. These results suggest that autocrine and/or paracrine mechanisms may regulate testicular activity in amphibians.

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.