The opiate system in invertebrates

Caenorhabditis elegans for opioid addiction research

The problem of drug addiction has become a profound societal problem worldwide. A better understanding of the neurobiological basis of addiction and the discovery of more effective treatments are needed. Recent studies have shown that many mechanisms that underlie addiction exist in more primitive organisms, including the nematode Caenorhabditis elegans (C. elegans). C. elegans is also hypothesized to possess a functional opioid-like system, including the endogenous opioid-like peptide NLP-24 and opioid-like receptor NPR-17. Opioids, such as morphine, are thought to cause addiction-like behavior by activating dopamine nerves in C. elegans via the opioid-like system. Accumulating evidence suggests that C. elegans is an excellent animal model for identifying molecular mechanisms of addiction.

Food preference-based screening method for identification of effectors of substance use disorders using Caenorhabditis elegans

Substance use disorder (SUD) affects over 48 million Americans aged 12 and over. Thus, identifying novel chemicals contributing to SUD will be critical for developing efficient prevention and mitigation strategies. Considering the complexity of the actions and effects of these substances on human behavior, a high-throughput platform using a living organism is ideal. We developed a quick and easy screening assay using Caenorhabditis elegans. C. elegans prefers high-quality food (Escherichia coli HB101) over low-quality food (Bacillus megaterium), with a food preference index of approximately 0.2, defined as the difference in the number of worms at E. coli HB101 and B. megaterium over the total worm number. The food preference index was significantly increased by loperamide, a μ-opioid receptor (MOPR) agonist, and decreased by naloxone, a MOPR antagonist. These changes depended on npr-17, a C. elegans homolog of opioid receptors. In addition, the food preference index was significantly increased by arachidonyl-2'-chloroethylamide, a cannabinoid 1 receptor (CB1R) agonist, and decreased by rimonabant, a CB1R inverse agonist. These changes depended on npr-19, a homolog of CB1R. These results suggest that the conserved opioid and endocannabinoid systems modulate the food preference behaviors of C. elegans. Finally, the humanoid C. elegans strains where npr-17 was replaced with human MOPR and where npr-19 was replaced with human CB1R phenocopied the changes in food preference by the drug treatment. Together, the current results show that this method can be used to rapidly screen the potential effectors of MOPR and CB1R to yield results highly translatable to humans.

Caenorhabditis Elegans Exhibits Morphine Addiction-like Behavior via the Opioid-like Receptor NPR-17

Addiction has become a profound societal problem worldwide, and few effective treatments are available. The nematode Caenorhabditis elegans (C. elegans) is an excellent invertebrate model to study neurobiological disease states. C. elegans reportedly developed a preference for cues that had previously been paired with addictive drugs, similar to place conditioning findings in rodents. Moreover, several recent studies discovered and reported the existence of an opioid-like system in C. elegans. Still unclear, however, is whether C. elegans exhibits addictive-like behaviors for opioids, such as morphine. In the present study, we found that C. elegans exhibited dose-dependent preference for morphine using the conditioned chemosensory-cue preference (CCP) test. This preference was blocked by co-treatment with the opioid receptor antagonist naloxone. C. elegans also exhibited aversion to naloxone-precipitated withdrawal from chronic morphine exposure. The expression of morphine-induced CCP and morphine withdrawal were abolished in worms that lacked the opioid-like receptor NPR-17. Dopamine-deficient mutant (cat-2 (e1112)) worms also did not exhibit morphine-induced CCP. These results indicate that the addictive function of the opioid system exists in C. elegans, which may serve as a useful model of opioid addiction.

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.

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

Drosophila reward system - A summary of current knowledge

The fruit fly Drosophila melanogaster brain is the most extensively investigated model of a reward system in insects. Drosophila can discriminate between rewarding and punishing environmental stimuli and consequently undergo associative learning. Functional models, especially those modelling mushroom bodies, are constantly being developed using newly discovered information, adding to the complexity of creating a simple model of the reward system. This review aims to clarify whether its reward system also includes a hedonic component. Neurochemical systems that mediate the 'wanting' component of reward in the Drosophila brain are well documented, however, the systems that mediate the pleasure component of reward in mammals, including those involving the endogenous opioid and endocannabinoid systems, are unlikely to be present in insects. The mushroom body components exhibit differential developmental age and different functional processes. We propose a hypothetical hierarchy of the levels of reinforcement processing in response to particular stimuli, and the parallel processes that take place concurrently. The possible presence of activity-silencing and meta-satiety inducing levels in Drosophila should be further investigated.

Why Protect Decapod Crustaceans Used as Models in Biomedical Research and in Ecotoxicology? Ethical and Legislative Considerations

Simple Summary

Current European legislation that protects animals used for scientific purposes excludes decapod crustaceans (for example, lobster, crab and crayfish) on the grounds that they are non-sentient and, therefore, incapable of suffering. However, recent work suggests that this view requires substantial revision. Our current understanding of the nervous systems and behavior of decapods suggests an urgent need to amend and update all relevant legislation. This paper examines recent experiments that suggest sentience and how that work has changed current opinion. It reflects on the use of decapods as models in biomedical research and in ecotoxicology, and it recommends that these animals should be included in the European protection legislation.

Abstract

Decapod crustaceans are widely used as experimental models, due to their biology, their sensitivity to pollutants and/or their convenience of collection and use. Decapods have been viewed as being non-sentient, and are not covered by current legislation from the European Parliament. However, recent studies suggest it is likely that they experience pain and may have the capacity to suffer. Accordingly, there is ethical concern regarding their continued use in research in the absence of protective measures. We argue that their welfare should be taken into account and included in ethical review processes that include the assessment of welfare and the minimization or alleviation of potential pain. We review the current use of these animals in research and the recent experiments that suggest sentience in this group. We also review recent changes in the views of scientists, veterinary scientists and animal charity groups, and their conclusion that these animals are likely to be sentient, and that changes in legislation are needed to protect them. A precautionary approach should be adopted to safeguard these animals from possible pain and suffering. Finally, we recommend that decapods be included in the European legislation concerning the welfare of animals used in experimentation.

Provocation of life functions at a unicellular eukaryote level by extremely low doses of mammalian hormones: Evidences of hormesis

Hormones, characteristic to higher ranked animals, are synthesized, stored, and secreted by unicellular eukaryote animals. The unicells also have receptors for recognizing these materials and transmit the message into the cells for provoking response. The hormones are effective in very low concentrations (down to 10-21 M) and opposite effects of lower and higher concentrations can be observed. However, sometimes linear concentration effects can be found, which means that hormesis exists, nevertheless uncertain, as it is in the phase of formation (evolutionary experimentation). Hormesis, by transformation (fixation) of cytoplasmic receptor-like membrane components to receptors in the presence of the given hormone, likely helps the development of unicellular endocrine character and by this the evolution of endocrine system. The effect by extremely low concentrations of hormones had been forced by the watery way of unicellular life, which could establish the physiological concentrations of hormones in the blood of higher ranked animals. This means that hormetic low doses are the normal, effective concentrations and the high concentrations are artificial, consequently could be dangerous.

An opioid-like system regulating feeding behavior in C. elegans

Neuropeptides are essential for the regulation of appetite. Here we show that neuropeptides could regulate feeding in mutants that lack neurotransmission from the motor neurons that stimulate feeding muscles. We identified nlp-24 by an RNAi screen of 115 neuropeptide genes, testing whether they affected growth. NLP-24 peptides have a conserved YGGXX sequence, similar to mammalian opioid neuropeptides. In addition, morphine and naloxone respectively stimulated and inhibited feeding in starved worms, but not in worms lacking NPR-17, which encodes a protein with sequence similarity to opioid receptors. Opioid agonists activated heterologously expressed NPR-17, as did at least one NLP-24 peptide. Worms lacking the ASI neurons, which express npr-17, did not response to naloxone. Thus, we suggest that Caenorhabditis elegans has an endogenous opioid system that acts through NPR-17, and that opioids regulate feeding via ASI neurons. Together, these results suggest C. elegans may be the first genetically tractable invertebrate opioid model.

DOI:http://dx.doi.org/10.7554/eLife.06683.001

When and how much an animal eats is controlled by a complex web of signals that are produced by the animal's body and brain. Molecules called opioid neuropeptides are among these signals, and act to control eating in mammals by binding to receptors in the brain and body. These receptors can also bind to similar molecules called opiates (such as morphine); opiates are amongst the oldest drugs used by humans and have diverse effects ranging from pain relief to addiction. While the activities of opiates and opioid neuropeptides have been studied in mammals, relatively little is known about opioid signaling in simpler animals.

The mechanisms behind many biological processes have been investigated using a worm called C. elegans as a model system because it has a simple body plan and its genes can be altered easily. The feeding behavior of C. elegans is no exception. This worm feeds by contracting and relaxing its pharyngeal muscle to move food into its gut. When the worms sense that food is available, this ‘pharyngeal pumping’ is regulated by one type of nerve cell. Slow pharyngeal pumping also continues in starved worms when food is not available, possibly to encourage them to eat new potential sources of food. However, this slow pumping does not require the same type of nerve cell.

Cheong et al. hypothesized that the slow pumping in starved worms might depend on neuropeptide signaling instead, and have now tested this idea using engineered worms that made lower levels of a number of these molecules. The experiments uncovered a molecule called NLP-24 that promotes the slow pharyngeal pumping. This molecule is similar to opioid neuropeptides found in mammals. Worms that made less NLP-24 than normal grew more slowly; this suggests that they had problems feeding. Moreover, the levels of NLP-24 were found to increase in normal worms soon after they were deprived of food. Further experiments revealed the identity of the receptor for this molecule, which is also similar to mammalian opioid receptors.

The discovery that opioid signaling is involved in C. elegans' feeding behavior may well, in future, also help to identify new molecular players involved in opioid signaling. Further studies might also help the search for ways to reduce the problematic side-effects that limit the usefulness of opiate drugs as medicines.

DOI:http://dx.doi.org/10.7554/eLife.06683.002

Invertebrate models in addiction research

While drug addiction is a uniquely human problem, most research examining the biological mechanisms of the transition from substance use to addiction is conducted with vertebrate animal models. Many other fields of neuroscience have greatly benefitted from contributions from simple and manipulable invertebrate model systems. However, the potential of invertebrate research has yet to be fully capitalised on in the field of addiction neuroscience. This may be because of the complexity of addiction and the clinical imperative of addiction research. We argue that the homocentric diagnostic criteria of addiction are no more a hindrance to the use of invertebrate models than they are to vertebrate models. We highlight the strengths of the diversity of different invertebrate model systems in terms of neuroanatomy and molecular machinery, and stress that working with a range of different models will aid in understanding addiction and not be a disadvantage. Finally, we discuss the specific advantages of utilising invertebrate animals for addiction research and highlight key areas in which invertebrates are suited for making unique and meaningful contributions to this field.

Exposure to exogenous enkephalins disrupts reproductive development in the Eastern lubber grasshopper, Romalea microptera (Insecta: Orthoptera)

Enkephalins play a major role in reproductive physiology in crustaceans; however their role in reproductive development in insects is largely unknown. We investigated the effect of exposure to exogenous leucine-enkephalin (Leu-Enk), methionine-enkephalin (Met-Enk), and the opioid antagonist naloxone on gonad development in the Eastern lubber grasshopper, Romalea microptera. Injection of either Leu-Enk or naloxone alone significantly increased the testicular index and testicular follicular diameter in males, and the ovarian index, oocyte length, and oocyte diameter in females. In contrast, injection of Met-Enk inhibited all measures of reproductive development in both sexes. Surprisingly, co-injection of naloxone with either enkephalin enhanced the effect associated with administration of the enkephalin alone. This study clearly demonstrates the ability of enkephalins to disrupt insect sexual development and also suggests the existence of conserved enkephaline-dependent regulatory mechanisms in insects and crustaceans.

Urotensin II in invertebrates: from structure to function in Aplysia californica

Neuropeptides are ancient signaling molecules that are involved in many aspects of organism homeostasis and function. Urotensin II (UII), a peptide with a range of hormonal functions, previously has been reported exclusively in vertebrates. Here, we provide the first direct evidence that UII-like peptides are also present in an invertebrate, specifically, the marine mollusk Aplysia californica. The presence of UII in the central nervous system (CNS) of Aplysia implies a more ancient gene lineage than vertebrates. Using representational difference analysis, we identified an mRNA of a protein precursor that encodes a predicted neuropeptide, we named Aplysia urotensin II (apUII), with a sequence and structural similarity to vertebrate UII. With in-situ hybridization and immunohistochemistry, we mapped the expression of apUII mRNA and its prohormone in the CNS and localized apUII-like immunoreactivity to buccal sensory neurons and cerebral A-cluster neurons. Mass spectrometry performed on individual isolated neurons, and tandem mass spectrometry on fractionated peptide extracts, allowed us to define the posttranslational processing of the apUII neuropeptide precursor and confirm the highly conserved cyclic nature of the mature neuropeptide apUII. Electrophysiological analysis of the central effects of a synthetic apUII suggests it plays a role in satiety and/or aversive signaling in feeding behaviors. Finding the homologue of vertebrate UII in the numerically small CNS of an invertebrate animal model is important for gaining insights into the molecular mechanisms and pathways mediating the bioactivity of UII in the higher metazoan.

Expression of the mu opioid receptor in Drosophila and its effects on trehalose and glycogen when expressed by the AKH neuroendocrine cells

We made Drosophila which express the mu opioid receptor under control of UAS in order to inactivate neurons or neuroendocrine cells expressing this receptor with opioid agonists. However, while exposing flies expressing the mu opioid receptor in the SIFamide neurons to opioid agonists was expected to induce male-male courtship behavior, this did not occur. Furthermore, flies which expressed the mu opioid receptor in the AKH or corazonin endocrine cells increased rather than decreased trehalose levels and this was independent of opioid agonists. When the mu opioid receptor is expressed in AKH endocrine cells whole body glycogen also increases, which is no longer the case if the expression of the AKH gene is suppressed by RNAi. It appears that mu opioid receptors expressed in AKH or corazonin endocrine cells are constitutively active and facilitate release of neurohormones. The simultaneous increase in both glycogen and trehalose in these flies suggested that they consumed more food. Indeed, when normally fed males are offered sucrose, those that express this receptor in AKH cells consumed more sucrose, suggesting that AKH increases the motivation to feed. These pharmacological effects of the mu opioid receptor are not limited to neuroendocrine cells; expressing it in the fat body also leads to an increase in trehalose. Thus in Drosophila the mu opioid receptors appear to change the base line activity in the cells in which it is expressed, not unlike to what has been found in transgenic mice expressing receptors activated solely by synthetic ligands with significant constitutive activity.

The effect of opioids and their antagonists on the nocifensive response of Caenorhabditis elegans to noxious thermal stimuli

Opiates modulate nociception in vertebrates. This has also been demonstrated in a number of invertebrate models. Herein, the effect of the opiate morphine and opioid neuropeptides Endomorphin 1 and 2 on the thermal avoidance (Tav) behavior of Caenorhabditis elegans is explored. Adult wild-type C. elegans N2 were collected from NGM plates using M9 buffer and exposed to morphine and endomorphine 1 and 2 in concentrations between 10(-8) and 10(-4) M (2.5 pmol/mg to 25 nmol/mg) for 30 min and tested for Tav. The opioid receptor antagonists Naloxone and CTOP were tested in combination with the drugs. Forty-seven percentage of the morphine exposed worms exhibited a class I response versus 76% of the control group (P < 0.001). Endomorphin 1 and 2 also caused a statistically significant reduction in class I responses, 36 and 39%, respectively. These effects were reversed with Naloxone and CTOP. Thermonocifensive behavior in C. elegans is modulated by opioids.

Effect of opioid compounds on feeding and activity of the cockroach, Periplaneta americana

Opioid peptides have been implicated in regulation of feeding in invertebrates. Studies have suggested that receptors for opioids are present in cockroaches and that these receptors play roles in affecting both behaviour and feeding. We examined the effect of micro, delta, and kappa opioid receptor agonists and antagonists on feeding, mass changes and activity in the cockroach, Periplaneta americana. The kappa antagonist, nor-binaltorphimine, significantly increased food intake, while naltrexone (general antagonist) and naloxonazine (micro antagonist) both reduced feeding. A large mass loss was observed in cockroaches treated with nor-binaltorphimine, despite the increased food intake. Males did not lose as much mass during the 3h as females, although drug treatment did have some effect on the loss. Time of activity (%) was not influenced by any drug. Water loss experiments suggested that nor-binaltorphimine increased water loss, accounting for the mass loss despite the increased feeding. We suggest that two populations of opioid receptors are present as previously reported, with one affecting feeding and the other involved with evaporative water loss.

Behavioral and neuronal attributes of short- and long-term habituation in the crab Chasmagnathus

Investigations using invertebrate species have led to a considerable progress in our understanding of the mechanisms underlying learning and memory. In this review we describe the main behavioral and neuronal findings obtained by studying the habituation of the escape response to a visual danger stimulus in the crab Chasmagnathus granulatus. Massed training with brief intertrial intervals lead to a rapid reduction of the escape response that recovers after a short term. Conversely, few trials of spaced training renders a slower escape reduction that endures for many days. As predicted by Wagner's associative theory of habituation, long-term habituation in the crab proved to be determined by an association between the contextual environment of the training and the unconditioned stimulus. By performing intracellular recordings in the brain of the intact animal at the same time it was learning, we identified a group of neurons that remarkably reflects the short- and long-term behavioral changes. Thus, the visual memory abilities of crabs, their relatively simple and accessible nervous system, and the recording stability that can be achieved with their neurons provide an opportunity for uncovering neurophysiological and molecular events that occur in identifiable neurons during learning.

Selective involvement of opioids in the mechanisms of synapse-specific plasticity in the common snail during the acquisition of sensitization

Studies on defensive behavior command neurons LP11 and RP11 in semi-intact common snail preparations were performed to investigate the effects of the opioid peptide met-enkephalin and the opioid antagonist naloxone on the effects of nociceptive sensitization. Application of nociceptive stimuli to the snail's head elicited marked reversible membrane depolarization along with depression of neuron responses to sensory stimulation during the short-term stage of sensitization and facilitation of responses in the long-term stage. Met-enkephalin at a dose of 10 microM but not at a dose of 0.1 microM partially suppressed responses to nociceptive stimuli. Acquisition of sensitization during exposure to met-enkephalin at doses of 10 and 0.1 microM led to complete suppression of the facilitation of responses to tactile stimulation of the head. Facilitation of responses to chemical stimulation of the head and tactile stimulation of the foot in these conditions was similar to that of neurons in control sensitized animals. Acquisition of sensitization during exposure to met-enkephalin and/or naloxone elicited selective suppression of facilitation of responses to chemical stimulation of the head but had no effect on facilitation of responses to tactile stimulation of the head and foot. Met-enkephalin and naloxone had no effect on the depression of neuron responses evoked by sensory stimulation in the short-term stage of sensitization. It is suggested that during the acquisition of sensitization in the common snail, opioids are involved in controlling the mechanism of nociception and in the mechanisms of selective induction of long-term plasticity of the synaptic inputs to command neurons activated by tactile and chemical stimulation of the animal's head.

The selective action of opioid peptides on excitability and the various sensory inputs of defensive behavior command neurons LPl1 and RPl1 of the common snail

The nature of the effects of opioid peptides on the properties of electrogenic membranes and the responses of defensive behavior command neurons LPl1 and RPl1, evoked by sensory stimuli of different modalities and application sites was studied in semi-intact preparations from common snails. Application of met-enkephalin (10 microM) to the snail CNS produced increases in membrane excitability along with facilitation of responses to application of dilute quinine solution to the animal's head and depression of responses to tactile stimulation of the head. Met-enkephalin (0.1 microM) produced only depression of responses to tactile stimulation of the head. Application of leu-enkephalin (10 microM) was accompanied by depression of responses to tactile stimulation of the head. Membrane excitability and responses to chemical sensory stimulation during application showed no change during application of this peptide. These effects of both peptides appeared 10-20 min from the start of application and lasted 15-30 min after washing was started. In addition, facilitation of the responses of neurons to chemical sensory stimulation was seen 30-50 min after the start of leu-enkephalin application. The responses of neurons to tactile stimulation of the snail's foot were not altered by application of peptides. The neuronal effects of peptides were suppressed by simultaneous application of naloxone (50 microM). Thus, we observed the selective action of opioid peptides on the synaptic plasticity of neurons LPl1 and RPl1, both in relation to the location of sensory stimulation and in relation to sensory modality.

Analyzing the evolution of the opioid/orphanin gene family

Advances in molecular biology have made it possible to rapidly obtain the amino acid sequence of neuropeptide precursors-either by cloning and sequencing the cDNA that encodes the precursor, or by reconstructing the arrangement of exons and introns in a neuropeptide-coding gene through genomic approaches. The databases generated from these molecular approaches have been used to design probes to identify the cells that express the gene, or to ascertain the rate of expression of the gene, and even to predict the post-translational modifications that can generate functional neuropeptides from a biologically inert precursor. Although the power of these approaches is substantial, it is appreciated that a gene sequence or an mRNA sequence reflects the potential products that may be assembled in a secretory cell. To understand the functional capabilities of the secretory cell, the molecular genetics approaches must be combined with procedures that actually characterize the end-products generated by the secretory cell. Recent advances in two-dimensional gel electrophoresis and mass spectrometry now make it possible to analyze neuropeptides from a relatively small amount of tissue. These procedures can reveal novel end-products, tissue-specific endoproteolytic cleavage events, and developmental shifts in post-translational processing schemes. A gene family that illustrates all of these processes and the advantages of combining genomics with proteomics is the opioid/orphanin gene family.

Effects of opiate ligands on intraspecific aggression in crickets

In the cricket Gryllus bimaculatus, the opiate antagonist naloxone, 30 microg in 50 microl into hemolymph per animal, caused a release of intraspecific aggression in male-losers and in females. Naloxone had no significant effect on the aggression of winners and isolated males. The agonist of mu-opiate receptors DAGO, 45 microg, suppressed aggression in male winners and decreased the percentage and duration of contact fights between previously isolated males. The results suggest that, during social contacts, the activity of the endogenous opioid system may contribute to suppressing aggression in subordinate males, as well as in females.

Enkephalinergic control of the secretory activity of neurons producing stereoisomers of crustacean hyperglycemic hormone in the eyestalk of the crayfish Orconectes limosus

A subgroup of neurons in the classical X organ sinus gland neuroendocrine system of the crayfish (Orconectes limosus) eyestalk produces two chiral forms of the crustacean hyperglycemic hormone (CHH) in two different types of neurons: CHH in 22 cells and D Phe(3) CHH in eight cells. Previous reports have demonstrated that release of CHH from the sinus gland is inhibited by enkephalins. Here, we have addressed the questions of 1) whether this inhibition affects one or both types of CHH neurons, 2) where the site of enkephalinergic control of CHH and/or D Phe(3) CHH is, and 3) whether the inhibitory effect is due to direct or indirect interactions of enkephalinergic neurons with CHH cells. In vitro incubations of neurosecretory complexes followed by immunoassays of CHH isoforms indicated that both methionine and leucine enkephalins inhibit release of the two CHH isoforms from crayfish eyestalks, by a receptor mediated process. Whole mount double or triple immunofluorescence labelings combined with confocal microscopy revealed enkephalin immunostaining in all neuropils of the eyestalk, except in the sinus gland. Virtual thin confocal sections showed many close appositions between terminals of enkephalinergic neurons and dendritic arborizations of specific CHH immunoreactive cells in the medulla terminalis neuropil. This provides the first evidence for direct inputs from enkephalinergic neurons into dendrites of both CHH cell types, which suggests that enkephalins inhibit release of both CHH isoforms via synaptic contacts.

ZFOR2, a new opioid receptor-like gene from the teleost zebrafish (Danio rerio)

A new opioid receptor-like (ZFOR2) has been cloned and characterized in an anamniote vertebrate, the teleost zebrafish (Danio rerio). ZFOR2 encodes a 384-amino-acid protein with seven potential transmembrane domains, and its predicted amino acid sequence presents an overall 74% degree of identity to mammalian mu opioid receptors. Its inclusion in a dendrogram generated from the alignment of the opioid receptor's protein sequences, confirms its classification as a mu opioid receptor. Divergences in sequence are greater in the regions corresponding to extracellular loops, suggesting possible differences in ligand selectivity with respect to the classical mu opioid receptors. The genomic structure of ZFOR2 is also highly conserved throughout the phylogenetic scale, supporting the origin of opioid receptors early in evolution. Nevertheless, ZFOR2 lacks the fourth exon found in human and rodent mu opioid receptors, that is known to be involved in desensibilization and internalization processes.

Ultra-wideband electromagnetic pulses and morphine-induced changes in nociception and activity in mice

Mice were exposed to ultra-wideband (UWB) electromagnetic pulses averaging 99-105 kV/m peak amplitude, 0.97-1.03 ns duration, and 155-174 ps rise time, after intraperitoneal administration of saline or morphine sulfate. They were then tested for thermal nociception on a 50 degrees C surface and for spontaneous locomotor activity and its time profile over 5 min. Analysis of results showed no effect of UWB exposure on nociception and activity measures in CF-1 mice after 15-, 30-, or 45-min exposure to pulses at 600/s or after 30-min exposure to UWB pulses at 60/s. Similarly, no effect was seen in C57BL/6 mice after 30-min exposure to pulses at 60/s or 600/s. Although trends in morphine-modified measures seen with UWB pulse repetition frequency could be expected because of increased levels of low-frequency energy, no significant change was seen in normal or morphine-modified nociception or activity after UWB exposure. This indicated lack of effect of the UWB pulses used in these experiments on nervous system components, including endogenous opioids, involved in these behaviors.

Analgesic effects of a specific pulsed magnetic field in the land snail, Cepaea nemoralis: consequences of repeated exposures, relations to tolerance and cross-tolerance with DPDPE

It has been demonstrated previously that a short acute exposure to a specific extremely low frequency pulsed magnetic field (Cnp) can induce significant partly opioid-mediated analgesia in the land snail, Cepaea nemoralis. Here, this Cnp-induced analgesia is examined for the development of tolerance to daily repeated acute exposures of 15 or 30 min duration. Acute cross-tolerance to the delta opioid receptor directed agonist DPDPE, [D-Pen2, D-Pen5]enkephalin, was also found. Before (pre-exposure) and after (0, 15, 30 and 60 min) exposure to either a sham or Cnp magnetic field, snails were tested for an aversive reaction to a warmed surface (40 degrees C), and the latency time to the aversive reaction was recorded. Snails that were exposed to the Cnp showed a significant increase in the latency time (F1.55 = 2856.4; p < 0.001; Eta2 = 0.95), which may be interpreted as an induction of analgesia. During the daily (9 day) repeated acute exposures, the induction of analgesic response was significantly reduced, but not ablated. Altering the environmental conditions of the Cnp exposure restored a significant proportion of the partly developed tolerance, consistent with previous reports of environmental specificity in the development of opioid tolerance. These findings suggest that the partial development of tolerance to the opioid-mediated portion of Cnp-induced analgesia may be countered by altering the specific environmental Cnp exposure conditions.

Delta and kappa opioid receptors in eyestalk ganglia of a crustacean

Crustacean eyestalk ganglia are part of the protocerebrum and have been demonstrated to produce numerous neurohormones. 3H(2-D-Pen, 5-D-Pen)-enkephalin, 3H-(-)-ethylketocyclazocine and 3H(D-Ala2-NMePhe-Glyol5)-enkephalin were used as ligands for opioid receptors on neuronal membrane preparations of eyestalk ganglia under consideration of their stereospecific binding properties. In context with saturation binding isotherms, association and dissociation plots, we demonstrate here two opioid receptors; a delta-type receptor with high affinity (Bmax 68.5 fmol/mg protein, Kd = 4.0 nM) and low affinity (Bmax 493 fmol/mg, Kd = 83.6 nM) and a second receptor of kappa-type with Bmax of 3.1 pmol/mg protein and Kd = 68.6 nM.

Pulsed magnetic field induced "analgesia" in the land snail, Cepaea nemoralis, and the effects of mu, delta, and kappa opioid receptor agonists/antagonists

A brief exposure to a pulsed magnetic field (Cnp: patent pending) had significant antinociceptive or "analgesic" effects in the land snail, Cepaea nemoralis, as evidenced by an increase in the latency of response to a warmed (40 degrees C) surface. This analgesia was in part opioid mediated being significantly reduced, but not eliminated: by the prototypic opiate antagonist, naloxone; the mu (mu) opioid receptor directed antagonists, naloxazine or beta-funaltrexamine, and the delta (delta) opioid receptor directed antagonists, naltrindole-5'-isothiocyanate or ICI 174,864. However the Cnp induced analgesia was unaffected by the kappa (kappa) opioid receptor directed antagonist, nor-binaltorphimine. The delta 1 and delta 2 opioid receptor directed agonists, (DPDPE, [D-Pen2,D-Pen5]enkephalin), (deltorphin, [D-Ala2,Glu4]), respectively, also had significant differential analgesic effects, supporting a functional delta opioid receptor mediated enkephalinergic mechanism in Cepaea. These results suggest that this specific pulsed magnetic field (Cnp) elicits significant analgesic effects through mechanisms that, in part, involve delta and, to a lesser extent mu opioid receptors.

The NMDA receptor antagonist, NPC 12626, reduces the pronociceptive effects of orphanin FQ and kappa opiate antinociception in the land snail, Cepaea nemoralis

The heptadecapeptide, orphanin FQ or nociceptin (Phe-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), originally isolated from rat brain has been identified as an endogenous ligand for the orphan opioid-like receptor. Although orphanin FQ shares some sequence and structural homology with kappa-opioid peptides, it has been speculated to exert its effects through novel nonopioid mechanisms. Kappa opioids have also been suggested to have nonopioid actions in rodents involving the N-methyl-D-aspartate (NMDA) receptor. The present study examined the effects of the competitive NMDA antagonist, NPC 12626, on the antinociceptive effects of the specific kappa-opiate receptor agonist, U69,593, and the pronociceptive effects of orphanin FQ in an invertebrate system, the land snail, Cepaea nemoralis. NPC 12626 had no effect on the basal nociceptive sensitivity of snails, as measured by the latency of response to a thermal (40 degrees C) surface. As reported for rodents, NPC 12626 dose-dependently reduced U69,593-induced antinociception in a manner comparable to that produced by the specific kappa-opiate antagonist, nor-binaltorphimine, while slightly enhancing the antinociceptive effects of the predominately mu-opiate agonist, morphine. Similarly, NPC 12626 dose-dependently reduced the pronociceptive effects of orphanin FQ. These findings with the snail, Cepaea, indicate that NMDA systems/receptors are associated with the mediation of the nociceptive effects of both kappa opioids and orphanin FQ. They suggest an early evolutionary development and phylogenetic continuity of NMDA opioid and related neuropeptide interactions in the mediation of nociception.

Identification and localization of a [Met5]-enkephalin-like peptide in the mollusc, Lymnaea stagnalis

The goal of this study was to determine whether [Met5]-enkephalin, or an analog, is present in identified neurons in the central nervous system (CNS) of the freshwater snail, Lymnaea stagnalis. High performance liquid chromatography and radioimmunoassay of CNS tissue homogenates revealed both [Met5]-enkephalin and oxidized [Met5]-enkephalin. NO [Leu5]-enkephalin, [Met5]-enkephalin-Arg6-Phe7 or [Met5]-enkephalin-Arg6-Gly7-Leu8 were detected. Quantification of [Met5]-enkephalin, by radioimmunoassay, revealed that the Lymnaea CNS contains approximately 2.2 fmol/CNS (undigested tissue) and 4.5 fmol/CNS (tissue enzymatically digested with trypsin and carboxypeptidase B). The increased amount of [Met5]-enkephalin following tissue digestion indicates the presence of as yet unidentified extended forms of [Met5]-enkephalin in Lymnaea. Using indirect immunocytochemistry, a [Met5]-enkephalin-like peptide was localized to individual cells and cell clusters within the CNS, as well as to fibers in the atrium of the heart. A neuronal map depicting [Met5]-enkephalin-like immunoreactive cells was produced. Among the immunoreactive neurons were four identified, well-characterized, giant cells: VD1, RPD2, LB1 and RB1. Identifiable [Met5]-enkephalin-like immunoreactive neurons were characterized electrophysiologically and morphologically. Additionally, neurons VD1 and RPD2 were confirmed to be immunoreactive to Lymnaea alpha-peptide. The lack of both cross reactivity and sequence homology between alpha-peptide and [Met5]-enkephalin suggests that a [Met5]-enkephalin-like peptide and alpha-peptide are co-localized within these neurons.

In vivo assessment of opioid agonists and antagonists on ovarian maturation in the red swamp crayfish, Procambarus clarkii

The possible involvement of an endogenous opioid system in the regulation of ovarian development in the red swamp crayfish, Procambarus clarkii, was investigated in vivo. Injections of the opioid, methionine (Met) enkephalin, into females significantly slowed ovarian maturation in a dose-dependent manner. In contrast, injection of the general opioid antagonist, naloxone, produced dose-dependent ovarian maturation. Furthermore, the highly selective delta opioid agonist, DADLE, also inhibited ovarian maturation, whereas a highly selective delta receptor antagonist, ICI-174,864, stimulated ovarian maturation. In view of these results and the fact that Met-enkephalin is a relatively selective delta receptor agonist, we hypothesize that in the crayfish the inhibitory effect of Met-enkephalin involves this type of binding site. Furthermore, it is hypothesized that the inhibitory action of these opioids is due to either (a) stimulation of release of the gonad-inhibiting hormone, (b) inhibition of release of the gonad-stimulating hormone or (c) both (a) and (b).

Opioid receptor types for endogenous enkephalin in the thoracic ganglion of the crab, Carcinus maenas

In crustaceans, the endogenous opioid peptides, enkephalins, are known to be concentrated in the thoracic ganglion, although they have been demonstrated in all parts of the nervous system. Bmax and Kd measurements have been obtained for the binding of ligands used to characterize delta- and kappa-type opioid receptors in vertebrates. High- and low affinity binding of [3H] [2-D-Pen5-D Pen] enkephalin ([3H]DPDPE) has been measured with a Kd = 9.2 +/- 2.4 nM, Bmax = 153 fmol/mg, and Kd = 243 +/- 27 nM, Bmax = 1.785 pmol/mg, respectively. In addition a kappa-type receptor with Kd 85.5 +/- 12.6 nM and Bmax = 21.138 pmol/mg protein has been recorded. Binding characteristics of several ligands were monitored. Electrophoretic studies of affinity chromatographically purified receptor fractions revealed a molecular mass of 60 kDa. Isoelectric focusing showed a specific binding of [3H]DPDPE to thoracic ganglion membranes at a pl of 5.5.