Opioid peptides in the nervous system of Aplysia: a combined biochemical, immunocytochemical, and electrophysiological study

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.

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

Important insights into the selection pressures and core molecular modules contributing to the evolution of pain-related processes have come from studies of nociceptive systems in several molluscan and arthropod species. These phyla, and the chordates that include humans, last shared a common ancestor approximately 550 million years ago. Since then, animals in these phyla have continued to be subject to traumatic injury, often from predators, which has led to similar adaptive behaviors (e.g., withdrawal, escape, recuperative behavior) and physiological responses to injury in each group. Comparisons across these taxa provide clues about the contributions of convergent evolution and of conservation of ancient adaptive mechanisms to general nociceptive and pain-related functions. Primary nociceptors have been investigated extensively in a few molluscan and arthropod species, with studies of long-lasting nociceptive sensitization in the gastropod, Aplysia, and the insect, Drosophila, being especially fruitful. In Aplysia, nociceptive sensitization has been investigated as a model for aversive memory and for hyperalgesia. Neuromodulator-induced, activity-dependent, and axotomy-induced plasticity mechanisms have been defined in synapses, cell bodies, and axons of Aplysia primary nociceptors. Studies of nociceptive sensitization in Drosophila larvae have revealed numerous molecular contributors in primary nociceptors and interacting cells. Interestingly, molecular contributors examined thus far in Aplysia and Drosophila are largely different, but both sets overlap extensively with those in mammalian pain-related pathways. In contrast to results from Aplysia and Drosophila, nociceptive sensitization examined in moth larvae (Manduca) disclosed central hyperactivity but no obvious peripheral sensitization of nociceptive responses. Squid (Doryteuthis) show injury-induced sensitization manifested as behavioral hypersensitivity to tactile and especially visual stimuli, and as hypersensitivity and spontaneous activity in nociceptor terminals. Temporary blockade of nociceptor activity during injury subsequently increased mortality when injured squid were exposed to fish predators, providing the first demonstration in any animal of the adaptiveness of nociceptive sensitization. Immediate responses to noxious stimulation and nociceptive sensitization have also been examined behaviorally and physiologically in a snail (Helix), octopus (Adopus), crayfish (Astacus), hermit crab (Pagurus), and shore crab (Hemigrapsus). Molluscs and arthropods have systems that suppress nociceptive responses, but whether opioid systems play antinociceptive roles in these phyla is uncertain.

Effects of leucine-enkephalin on catalase activity and hydrogen peroxide levels in the haemolymph of the Pacific Oyster (Crassostrea gigas)

The nervous and immune systems of invertebrates can exchange information through neuropeptides. Furthermore, some opioid peptides can function as endogenous immune system messengers and participate in the regulation of the immune responses. The present study was designed to investigate the effects of leucine-enkephalin (L-ENK) on the activity of catalase (CAT) and hydrogen peroxide (H₂O₂) content in the haemolymph of the Pacific Oyster (Crassostrea gigas). The CAT activity and H₂O₂ content were investigated after the haemolymph of the species was exposed to 1, 5, and 50 μg/mL of L-ENK. The results indicate that the intracellular and extracellular CAT activity was increased with increasing concentration of L-ENK, while the intracellular and extracellular H₂O₂ content was decreased with increasing concentration of L-ENK. L-ENK may regulate the intracellular and extracellular CAT activity and H₂O₂ content via binding with opioid neuropeptide receptors on immunocytes of the oysters. The data strongly suggests an involvement of opioid peptides in the regulation of the antioxidant defence systems of Crassostrea gigas.

Evidence for the involvement of an opioid system in sciatic nerve of Rana ridibunda

The effect of opioid peptide, D-alanine2-leucine-enkephalin and opioid homolog peptide, des-tyrosine-methionine-enkephalin in concentrations of 1 x 10(-6) and 1 x 10(-5) M was investigated on the action potential parameters of frog sciatic nerve. Des-tyrosine-methionine-enkephalin was used as the control to prove the opioid action of the peptide. The effects of both peptides were examined by means of the extracellular electrophysiological technique. The isolated sciatic nerves were stimulated by single square pulses each of which lasted for 0.5 ms at supramaximal strength. Effect of each single dose of peptides at 0 min was compared with the remaining time segments. Both peptides produced changes on action potential of Rana ridibunda sciatic nerve when compared with untreated nerves. D-alanine2-leucine-enkephalin decreased significantly the amplitude at about 34-83%, the area at about 34-92%. The same concentrations of this peptide decreased significantly the conduction velocity around 35-78%. In contrast, des-tyrosine-methionine-enkephalin reduced the action potential amplitude between 8% and 80%. The same concentrations of this peptide decreased significantly the area at about 12-76% and the conduction velocity around 42-70%. The depression of both peptides in action potential parameters was partially blocked by 1 x 10(-6) M naloxone.

The Aplysia mytilus inhibitory peptide-related peptides: identification, cloning, processing, distribution, and action

Neuropeptides are a ubiquitous class of signaling molecules. In our attempt to understand the generation of feeding behavior in Aplysia, we have sought to identify and fully characterize the neuropeptides operating in this system. Preliminary evidence indicated that Mytilus inhibitory peptide (MIP)-like peptides are present and operating in the circuitry that generates feeding in Aplysia. MIPs were originally isolated from the bivalve mollusc Mytilus edulis, and related peptides have been identified in other invertebrate species, but no precursor has been identified. In this study, we describe the isolation and characterization of novel Aplysia MIP-related peptides (AMRPs) and their precursor. Several AMRPs appear to have some structural and functional features similar to vertebrate opioid peptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to confirm that all 14 AMRPs predicted by the precursor are processed in isolated neurons. Northern analysis, whole-mount in situ hybridization, and immunohistochemistry are used to map the abundant expression of these peptides in the CNS and peripheral tissues such as the digestive tract, vasculature, and the reproductive organs. Physiological studies demonstrate that the rank order of the inhibitory actions of these peptides is different for three target muscles. These results underscore the importance of using a multidisciplinary approach to identifying and characterizing the actions of neuropeptides in an effort to gain understanding of their role in systems of interest. The widespread distribution of the AMRPs indicates that they may be operating in many different systems of Aplysia.

Molecular evolution of the opioid/orphanin gene family

Gene duplication is a recurring theme in the evolution of vertebrate polypeptide hormones and neuropeptides. These duplication events can lead to the formation of gene families in which divergence of function is the usual outcome. In the case of the opioid/orphanin family of genes, duplication events have proceeded along two paths: (a) an apparent duplication of function as seen in the analgesic activity of Proenkephalin and Prodynorphin end-products; and (b) divergence of function as seen in the nociceptic activity of Proorphanin end-products or the melanocortin (color change and chronic stress regulation) activity of Proopiomelanocortin end-products. Although genes coding for Proopiomelanocortin, Proenkephalin, Prodynorphin, and Proorphanin have been extensively studied in mammals, the distribution and radiation of these genes in nonmammalian vertebrates is less well understood. This review will present the hypothesis that the radiation of the opioid/orphanin gene family is the result of the duplication and divergence of the Proenkephalin gene during the radiation of the chordates. To evaluate the Proenkephalin gene duplication hypothesis, a 3'RACE procedure was used to screen for the presence of Prodynorphin-related, Proenkephalin-related, and Proorphanin-related cDNAs expressed in the brains of nonmammalian vertebrates.

Opioid receptors from a lower vertebrate (Catostomus commersoni): sequence, pharmacology, coupling to a G-protein-gated inward-rectifying potassium channel (GIRK1), and evolution

The molecular evolution of the opioid receptor family has been studied by isolating cDNAs that encode six distinct opioid receptor-like proteins from a lower vertebrate, the teleost fish Catostomus commersoni. One of these, which has been obtained in full-length form, encodes a 383-amino acid protein that exhibits greatest sequence similarity to mammalian mu-opioid receptors; the corresponding gene is expressed predominantly in brain and pituitary. Transfection of the teleost cDNA into HEK 293 cells resulted in the appearance of a receptor having high affinity for the mu-selective agonist [D-Ala2, MePhe4-Gly-ol5]enkephalin (DAMGO) (Kd = 0.63 +/- 0.15 nM) and for the nonselective antagonist naloxone (Kd = 3.1 +/- 1.3 nM). The receptor had negligible affinity for U50488 and [D-Pen2, D-Pen5]enkephalin (DPDPE), which are kappa- and delta-opioid receptor selective agonists, respectively. Stimulation of transfected cells with 1 microM DAMGO lowered forskolin-induced cAMP levels, an effect that could be reversed by naloxone. Experiments in Xenopus oocytes have demonstrated that the fish opioid receptor can, in an agonist-dependent fashion, activate a coexpressed mouse G-protein-gated inward-rectifying potassium channel (GIRK1). The identification of six distinct fish opioid receptor-like proteins suggests that additional mammalian opioid receptors remain to be identified at the molecular level. Furthermore, our data indicate that the mu-opioid receptor arose very early in evolution, perhaps before the appearance of vertebrates, and that the pharmacological and functional properties of this receptor have been conserved over a period of approximately 400 million years implying that it fulfills an important physiological role.

Inhibition of the Met-enkephalin-induced K+ current in B-cluster neurons of Aplysia by nitric oxide donor

The effects of sodium nitroprusside (SNP), a nitric oxide (NO) donor, on a methionine-enkephalin (Met-E)-induced K+ current recorded from B-cluster neurons in Aplysia cerebral ganglion were investigated with voltage-clamp and pressure ejection techniques. Bath-applied SNP (10-25 microM) reduced the Met-E-induced K+ current in the neurons without affecting the resting membrane conductance and holding current. The inhibitory effects of SNP were reversible. Pretreatment with methylene blue (10 microM), a non-specific inhibitor of guanylate cyclase, and hemoglobin (50 microM), a NO scavenger, decreased the SNP-induced inhibition of the Met-E-induced current. Intracellular injection of 1 mM guanosine 3',5'-cyclic monophosphate (cGMP) or bath-applied 3-isobutyl-1-methylxanthine (IBMX; 50 microM), a nonspecific phosphodiesterase inhibitor, inhibited the Met-E-induced current. Furthermore, 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM), a more specific inhibitor of NO-stimulated guanylate cyclase, decreased the SNP-induced inhibition of the Met-E-induced current. These results suggest that SNP induces suppression of the Met-E-induced K+ current recorded from B-cluster neurons of Aplysia cerebral ganglion via stimulation of cGMP formation.