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

Morphine ameliorates pentylenetetrazole-induced locomotor pattern in zebrafish embryos; mechanism involving regulation of opioid receptors, suppression of oxidative stress, and inflammation in epileptogenesis

Zebrafish (Danio rerio) is becoming an increasingly important model in epilepsy research. Pentylenetetrazole (PTZ) is a convulsant agent that induces epileptic seizure-like state in zebrafish and zebrafish embryos and is most commonly used in antiepileptic drug discovery research to evaluate seizure mechanisms. Classical antiepileptic drugs, such as valproic acid (VPA) reduce PTZ-induced epileptiform activities. Opioid system has been suggested to play a role in epileptogenesis. The aim of our study is to determine the effects of morphine in PTZ-induced epilepsy model in zebrafish embryos by evaluating locomotor activity and parameters related to oxidant-antioxidant status, inflammation, and cholinergic system as well as markers of neuronal activity c-fos, bdnf, and opioid receptors. Zebrafish embryos at 72 hpf were exposed to PTZ (20 mM), VPA (1 mM), and Morphine (MOR) (100 µM). MOR and VPA pretreated groups were treated with either MOR (MOR + PTZ) or VPA (VPA + PTZ) for 20 min before PTZ expoure. Locomotor activity was quantified as total distance moved (mm), average speed (mm/sec) and exploration rate (%) and analyzed using ToxTrac tracking programme. Oxidant-antioxidant system parameters, acetylcholinesterase activity, and sialic acid leves were evaluated using spectrophotometric methods. The expression of c-fos, bdnf, oprm1, and oprd1 were evaluated by RT-PCR. MOR pretreatment ameliorated PTZ-induced locomotor pattern as evidenced by improved average speed, exploration rate and distance traveled. We report the restoration of inflammatory and oxidant-antioxidant system parameters, c-fos, bdnf, and opioid receptor oprm1 as the possible mechanisms involved in the ameliorative effect of MOR against PTZ-induced epileptogenic process in zebrafish embryos.

The use of zebrafish as a non-traditional model organism in translational pain research: the knowns and the unknowns

The ability of the nervous system to detect a wide range of noxious stimuli is crucial to avoid life-threatening injury and to trigger protective behavioral and physiological responses. Pain represents a complex phenomenon, including nociception associated with cognitive and emotional processing. Animal experimental models have been developed to understand the mechanisms involved in pain response, as well as to discover novel pharmacological and non-pharmacological anti-pain therapies. Due to the genetic tractability, similar physiology, low cost, and rich behavioral repertoire, the zebrafish (Danio rerio) has been considered a powerful aquatic model for modeling pain responses. Here, we summarize the molecular machinery of zebrafish to recognize painful stimuli, as well as emphasize how zebrafish-based pain models have been successfully used to understand specific molecular, physiological, and behavioral changes following different algogens and/or noxious stimuli (e.g., acetic acid, formalin, histamine, Complete Freund's Adjuvant, cinnamaldehyde, allyl isothiocyanate, and fin clipping). We also discuss recent advances in zebrafish-based studies and outline the potential advantages and limitations of the existing models to examine the mechanisms underlying pain responses from an evolutionary and translational perspective. Finally, we outline how zebrafish models can represent emergent tools to explore pain behaviors and pain-related mood disorders, as well as to facilitate analgesic therapy screening in translational pain research.

The stress - Reproductive axis in fish: The involvement of functional neuroanatomical systems in the brain

The neuroendocrine-stress axis of nonmammalian species is evolutionarily conserved, which makes them useful to serve as important model systems for elucidating the function of the vertebrate stress response. The involvement of hypothalamo-pituitary-adrenal (HPA) axis hormones in regulation of stress and reproduction is well described in different vertebrates. However, the stress response is a complex process, which appears to be controlled by a number of neurochemicals in association with hypothalamo-pituitary-interrenal (HPI) axis or independent of HPI axis in fish. In recent years, the participation of neurohormones other than HPI axis in regulation of stress and reproduction is gaining more attention. This review mainly focuses on the involvement of functional neuroanatomical systems such as the catecholaminergic neurotransmitter dopamine (DA) and opioid peptides in regulation of the stress-reproductive axis in fish. Occurrences of DA and opioid peptides like β-endorphin, enkephalins, dynorphin, and endomorphins have been demonstrated in fish brain, and diverse roles such as pain modulation, social behaviour and reproduction are implicated for these hormones. Neuroanatomical studies using retrograde tracing, immunohistochemical staining and lesion methods have demonstrated that the neurons originating in the preoptic region and the nucleus lateralis tuberis directly innervate the pituitary gland and, therefore, the hypophysiotrophic role of these hormones. In addition, heightened synthetic and secretory activity of the opioidergic and the dopaminergic neurons in hypothalamic areas of the brain during stress exposure suggest potentially intricate relationship with the stress-reproductive axis in fish. Current evidence in early vertebrates like fish provides a novel insight into the underlying neuroendocrine mechanisms as additional pathways along the stress-reproductive axis that seem to be conserved during the course of evolution.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Naloxone prolongs abdominal constriction writhing-like behavior in a zebrafish-based pain model

The ability to detect noxious stimuli is essential to survival. However, pathological pain is maladaptive and severely debilitating. Endogenous and exogenous opioids modulate pain responses via opioid receptors, reducing pain sensibility. Due to the high genetic and physiological similarities to rodents and humans, the zebrafish is a valuable tool to assess pain responses and the underlying mechanisms involved in nociception. Although morphine attenuates pain-like responses of zebrafish, there are no data showing if the antagonism of opioid receptors prolongs pain duration in the absence of an exogenous opioid. Here, we investigated whether a common opioid antagonist naloxone affects the abdominal constriction writhing-like response, recently characterized as a zebrafish-based pain behavior. Animals were injected intraperitoneally with acetic acid (5.0%), naloxone (1.25 mg/kg; 2.5 mg/kg; 5.0 mg/kg) or acetic acid with naloxone to investigate the changes in their body curvature for 1 h. Acetic acid elicited a robust pain-like response in zebrafish, as assessed by aberrant abdominal body curvature, while no effects were observed following PBS injection. Although naloxone alone did not alter the frequency and duration of this behavior, it dose-dependently prolonged acetic acid-induced abdominal curvature response. Besides reinforcing the use of the abdominal writhing-like phenotype as a behavioral endpoint to measure acute pain responses in zebrafish models, our novel data suggest a putative role of endogenous opioids in modulating the recovery from pain stimulation in zebrafish.

Opioid Neurobiology, Neurogenetics and Neuropharmacology in Zebrafish

Despite the high prevalence of medicinal use and abuse of opioids, their neurobiology and mechanisms of action are not fully understood. Experimental (animal) models are critical for improving our understanding of opioid effects in vivo. As zebrafish (Danio rerio) are increasingly utilized as a powerful model organism in neuroscience research, mounting evidence suggests these fish as a useful tool to study opioid neurobiology. Here, we discuss the zebrafish opioid system with specific focus on opioid gene expression, existing genetic models, as well as its pharmacological and developmental regulation. As many human brain diseases involve pain and aberrant reward, we also summarize zebrafish models relevant to opioid regulation of pain and addiction, including evidence of functional interplay between the opioid system and central dopaminergic and other neurotransmitter mechanisms. Additionally, we critically evaluate the limitations of zebrafish models for translational opioid research and emphasize their developing utility for improving our understanding of evolutionarily conserved mechanisms of pain-related, addictive, affective and other behaviors, as well as for fostering opioid-related drug discovery.

Generation and Characterization of Antibodies against Opioid Receptors from Zebrafish

The opioid system is well conserved among species and plays a critical role in pain and addiction systems. The use of zebrafish as an experimental model to study development and genetics is extraordinary and has been proven to be relevant for the study of different diseases. The main drawback to its use for the analysis of different pathologies is the lack of protein tools. Antibodies that work in other models are not suitable for zebrafish due to the low degree of homology that exists among the opioid receptor protein sequences in different species. Here we report the successful generation and characterization of antibodies against the mu, delta 1 and delta 2 opioid receptors in zebrafish. The antibodies obtained, which are specific for each receptor due to the use of the C-terminus as antigens, work for Western blotting and immunohistochemistry. In addition, the antibodies against mu and delta 1 opioid receptors, but not those against delta 2, are able to immunoprecipitate the corresponding receptor from zebrafish lysates. The development of opioid receptor antibodies is an asset to the further study of the endogenous opioid system in zebrafish.

Morphine delays neural stem cells differentiation by facilitating Nestin overexpression

BACKGROUND

Morphine is used as an analgesic although it causes important secondary effects. These effects are triggered by several mechanisms leading to the dysregulation of gene expression. Here we aimed to study these alterations on neural stem cells (NSC) during CNS development.

METHODS

AB strain and tg nestin:GFP zebrafish embryos, zebrafish primary neuron culture and mouse embryonic stem cells were used to assess the effect of morphine by qPCR, time lapse microscopy and western blot. ChIP-qPCR and bisulfite conversion assay were performed to determine the changes exerted by morphine in a Nestin candidate enhancer.

RESULTS

Morphine increases GFP in nestin:GFP embryos and overexpresses the NSC marker Nestin. Morphine also exerts a hyperacetylation effect on H3K27 and decreases DNA methylation within a region located 18 Kb upstream nestin transcription starting site. Here, a binding site for the transcription factor complex Sox2/Oct4/Nanog was predicted. These factors are also upregulated by morphine. Besides, morphine increases the histone acetyl transferase p300. The inhibition of p300 activity decreases Nestin.

CONCLUSIONS

Morphine facilitates Nestin increase by several mechanisms which include hyperacetylation of H3K27, decreased DNA methylation and the overexpression of the transcription factors sox2, oct4 and nanog. It has also been demonstrated that nestin levels depend on p300 activity. The facilitated Nestin expression delays the normal differentiation of neural stem cells.

GENERAL SIGNIFICANCE

The present work provides novel evidence of the effects induced by morphine in the normal differentiation of NSCs, altering Nestin through changes on p300, H3K27ac, DNA methylation and Oct4, Sox2, and Nanog.

Neuroendocrine-immune interaction: Evolutionarily conserved mechanisms that maintain allostasis in an ever-changing environment

It has now become accepted that the immune system and neuroendocrine system form an integrated part of our physiology. Immunological defense mechanisms act in concert with physiological processes like growth and reproduction, energy intake and metabolism, as well as neuronal development. Not only are psychological and environmental stressors communicated to the immune system, but also, vice versa, the immune response and adaptation to a current pathogen challenge are communicated to the entire body, including the brain, to evoke adaptive responses (e.g., fever, sickness behavior) that ensure allocation of energy to fight the pathogen. This phenomenon is evolutionarily conserved. Hence it is both interesting and important to consider the evolutionary history of this bi-directional neuroendocrine-immune communication to reveal phylogenetically ancient or relatively recently acquired mechanisms. Indeed, such considerations have already disclosed an extensive "common vocabulary" of information pathways as well as molecules and their receptors used by both the neuroendocrine and immune systems. This review focuses on the principal mechanisms of bi-directional communication and the evidence for evolutionary conservation of the important physiological pathways involved.

Single Amino Acid Variation Underlies Species-Specific Sensitivity to Amphibian Skin-Derived Opioid-like Peptides

It has been suggested that the evolution of vertebrate opioid receptors (ORs) follow a vector of increased functionality. Here, we test this idea by comparing human and frog ORs. Interestingly, some of the most potent opioid peptides known have been isolated from amphibian skin secretions. Here we show that such peptides (dermorphin and deltorphin) are highly potent in the human receptors and inactive in frog ORs. The molecular basis for the insensitivity of the frog ORs to these peptides was studied using chimeras and molecular modeling. The insensitivity of the delta OR (DOR) to deltorphin was due to variation of a single amino acid, Trp7.35, which is a leucine in mammalian DORs. Notably, Trp7.35 is completely conserved in all known DOR sequences from lamprey, fish, and amphibians. The deltorphin-insensitive phenotype was verified in fish. Our results provide a molecular explanation for the species selectivity of skin-derived opioid peptides.

Modulation of the Interaction between a Peptide Ligand and a G Protein-Coupled Receptor by Halogen Atoms

Systematic halogenation of two native opioid peptides has shown that halogen atoms can modulate peptide-receptor interactions in different manners. First, halogens may produce a steric hindrance that reduces the binding of the peptide to the receptor. Second, chlorine, bromine, or iodine may improve peptide binding if their positive σ-hole forms a halogen bond interaction with negatively charged atoms of the protein. Lastly, the negative electrostatic potential of fluorine can interact with positively charged atoms of the protein to improve peptide binding.

Opioids potentiate electrical transmission at mixed synapses on the Mauthner cell

Opioid receptors were shown to modulate a variety of cellular processes in the vertebrate central nervous system, including synaptic transmission. While the effects of opioid receptors on chemically mediated transmission have been extensively investigated, little is known of their actions on gap junction-mediated electrical synapses. Here we report that pharmacological activation of mu-opioid receptors led to a long-term enhancement of electrical (and glutamatergic) transmission at identifiable mixed synapses on the goldfish Mauthner cells. The effect also required activation of both dopamine D1/5 receptors and postsynaptic cAMP-dependent protein kinase A, suggesting that opioid-evoked actions are mediated indirectly via the release of dopamine from varicosities known to be located in the vicinity of the synaptic contacts. Moreover, inhibitory inputs situated in the immediate vicinity of these excitatory synapses on the lateral dendrite of the Mauthner cell were not affected by activation of mu-opioid receptors, indicating that their actions are restricted to electrical and glutamatergic transmissions co-existing at mixed contacts. Thus, as their chemical counterparts, electrical synapses can be a target for the modulatory actions of the opioid system. Because gap junctions at these mixed synapses are formed by fish homologs of the neuronal connexin 36, which is widespread in mammalian brain, it is likely that this regulatory property applies to electrical synapses elsewhere as well.

Bioinformatics and evolution of vertebrate nociceptin and opioid receptors

G protein-coupled receptors (GPCRs) are ancestrally related membrane proteins on cells that mediate the pharmacological effect of most drugs and neurotransmitters. GPCRs are the largest group of membrane receptor proteins encoded in the human genome. One of the most famous types of GPCRs is the opioid receptors. Opioid family receptors consist of four closely related proteins expressed in all vertebrate brains and spinal cords examined to date. The three classical types of opioid receptors shown unequivocally to mediate analgesia in animal models and in humans are the mu- (MOR), delta- (DOR), and kappa-(KOR) opioid receptor proteins. The fourth and most recent member of the opioid receptor family discovered is the nociceptin or orphanin FQ receptor (ORL). The role of ORL and its ligands in producing analgesia is not as clear, with both analgesic and hyperalgesic effects reported. All four opioid family receptor genes were cloned from expressed mRNA in a number of vertebrate species, and there are enough sequences presently available to carry out bioinformatic analysis. This chapter presents the results of a comparative analysis of vertebrate opioid receptors using pharmacological studies, bioinformatics, and the latest data from human whole-genome studies. Results confirm our initial hypotheses that the four opioid receptor genes most likely arose by whole-genome duplication, that there is an evolutionary vector of opioid receptor type divergence in sequence and function, and that the hMOR gene shows evidence of positive selection or adaptive evolution in Homo sapiens.

Opioid receptors: methods for detection and their modes of actions in the eye

This study provides evidence for the presence of opioid-receptors in the retina, optic nerve, and optic nerve head astrocytes. These receptors were measured by more than one technique including Western blotting, immunohistochemistry, and functional assays such as scotopic electroretinogram (ERG) and Pattern ERG. I also have provided evidence in recently published work from my laboratory that opioid receptors, more specifically δ-opioid receptors, play crucial roles in retina neuroprotection against ischemic and glaucomatous injuries. This chapter provides detailed procedures to measure opioid receptor activation and their roles in retina neuroprotection using functional assays such as scotopic ERG and pattern ERG.

Cocaine modulates the expression of opioid receptors and miR-let-7d in zebrafish embryos

Prenatal exposure to cocaine, in mammals, has been shown to interfere with the expression of opioid receptors, which can have repercussions in its activity. Likewise, microRNAs, such as let-7, have been shown to regulate the expression of opioid receptors and hence their functions in mammals and in vitro experiments. In light of this, using the zebrafish embryos as a model our aim here was to evaluate the actions of cocaine in the expression of opioid receptors and let-7d miRNA during embryogenesis. In order to determine the effects produced by cocaine on the opioid receptors (zfmor, zfdor1 and zfdor2) and let-7d miRNA (dre-let-7d) and its precursors (dre-let-7d-1 and dre-let-7d-2), embryos were exposed to 1.5 µM cocaine hydrochloride (HCl). Our results revealed that cocaine upregulated dre-let-7d and its precursors, and also increased the expression of zfmor, zfdor1 and zfdor2 during early developmental stages and decreased them in late embryonic stages. The changes observed in the expression of opioid receptors might occur through dre-let-7d, since DNA sequences and the morpholinos of opioid receptors microinjections altered the expression of dre-let-7d and its precursors. Likewise, opioid receptors and dre-let-7d showed similar distributions in the central nervous system (CNS) and at the periphery, pointing to a possible interrelationship between them.In conclusion, the silencing and overexpression of opioid receptors altered the expression of dre-let-7d, which points to the notion that cocaine via dre-let-7 can modulate the expression of opioid receptors. Our study provides new insights into the actions of cocaine during zebrafish embryogenesis, indicating a role of miRNAs, let-7d, in development and its relationship with gene expression of opioid receptors, related to pain and addiction process.

Morphine and microRNA Activity: Is There a Relation with Addiction?

When we talk about drug addiction, we are really dealing with an extremely complex system in which there still remain many unknowns and where many empty spaces or missing links are still present. Recent studies have identified changes in the expression profiles of several specific miRNAs which affect the interactions between these molecules and their targets in various illnesses, including addiction, and which may serve as valuable targets for more efficient therapies. In this review, we summarize results which clearly demonstrate that several morphine-related miRNAs have roles in the mechanisms that define addiction. In this regard, morphine has been shown to have an important role in the regulation of different miRNAs, such as miR-let-7 [which works as a mediator of the movement of the mu opioid receptor (MOR) mRNA into P-bodies, leading to translational repression], miR-23b (involved in linking MOR expression and morphine treatment at the post-transcriptional level), and miR-190 (a key post-transcriptional repressor of neurogenic differentiation, NeuroD). Fentanyl increases NeuroD levels by reducing the amount of miR-190, but morphine does not affect the levels of NeuroD. We also discuss the relationship between morphine, miRNAs, and the immune system, based on the discovery that morphine treatment of monocytes led to a decrease in several anti-HIV miRNAs (mir-28, 125b, 150, and 382). This review is centered on miR-133b and its possible involvement in addiction through the effects of morphine. We establish the importance of miR-133b as a regulatory factor by summarizing its activity in different pathological processes, especially cancer. Using the zebrafish as a research model, we discuss the relationship between mir-133b, the dopaminergic system, and morphine, considering: (1) that morphine modulates the expression of miR-133b and of its target transcript Pitx3, (2) the role of the zebrafish mu opioid receptor (zfMOR) in morphine-induced regulation of miR-133b, which depends on ERK1/2, (3) that morphine regulates miR-133b in hippocampal neurons, and (4) the role of delta opioid receptors in morphine-induced regulation of miR-133b. We conclude that the control of miR-133b levels may be a mechanism for the development of addiction to morphine, or other drugs of abuse that increase dopaminergic levels in the extracellular space. These results show that miR-133b is a possible new target for the design of new treatments against addictive disorders.

Dynorphin regulates the phagocytic activity of splenic phagocytes in wall lizards: involvement of a κ-opioid receptor-coupled adenylate-cyclase-cAMP-PKA pathway

This in vitro study of the wall lizard Hemidactylus flaviviridis demonstrates the role of the opioid peptide dynorphin A((1-17)) [dyn A((1-17))] in the regulation of the phagocytic activity of splenic phagocytes. Dyn A((1-17)) in a concentration-dependent manner inhibited the phagocytic activity, and the maximum inhibition was recorded at a concentration of 10(-9) mol l(-1). To explore the receptor-mediated effect of dyn A((1-17)), cells were treated simultaneously with the non-selective opioid receptor blocker naltrexone and dyn A((1-17)). Naltrexone completely blocked the inhibitory effect of dyn A((1-17)) on phagocytosis. Moreover, the involvement of selective opioid receptors was investigated using selective opioid receptor antagonists. CTAP and naltrindole, selective μ- and δ-opioid receptor blockers, respectively, failed to block the inhibitory effect of dyn A((1-17)) on phagocytosis. However, the selective κ-opioid receptor blocker NorBNI completely antagonized the inhibitory effect of dyn A((1-17)). Regarding the κ-opioid receptor-coupled downstream signaling cascade, the adenylate cyclase (AC) inhibitor SQ 22536 and protein kinase A (PKA) inhibitor H-89 decreased the inhibitory effect of dyn A((1-17)) on phagocytosis. Furthermore, treatment with dyn A((1-17)) caused an increase in intracellular cAMP content in splenic phagocytes. Thus, it can be concluded that, in H. flaviviridis, dyn A((1-17)) negatively regulates the phagocytic activity of splenic phagocytes by acting through κ-opioid receptors that are coupled with the AC-cAMP-PKA signal transduction mechanism.

Neuroendocrine-immune interaction in fish: differential regulation of phagocyte activity by neuroendocrine factors

Coping with physical, chemical and biological disturbances depends on an extensive repertoire of physiological, endocrinological and immunological responses. Fish provide intriguing models to study bi-directional interaction between the neuroendocrine and the immune systems. Macrophages and granulocytes are the main actors in the first and rapid innate immune response. They are resident in different organs and are moreover rapidly recruited and activated upon infection. They act in response to recognition of pathogen-associated molecular patterns (PAMPs) via a repertoire of surface and intracellular receptors by inducing a plethora of defense reactions aiming to eradicate the pathogen. Subsequent production of inflammatory mediators stimulates other leukocytes required to develop an adaptive and specific antibody response. The type of phagocyte reaction will therefore depend on their differentiation state, specific receptor repertoire and their specific location. Apart from these pathogen induced responses, immune reactivity may be modulated by neuroendocrine factors. Over the last years we extensively studied changes in carp stress axis activity and the effect of its end-products on the immune system in an acute stress paradigm. We focus on specific neuroendocrine receptors on leukocytes and their effect on crucial phagocyte activities. We performed identification and functional analyses of different glucocorticoid, opioid and adrenergic receptors on carp phagocytes. Results show that their ligands of neuroendocrine origin may have substantial impact on specific phagocyte functions in a differential way. Inflammatory and microbicidal responses fight pathogens but may be detrimental to the host tissue. Neuroendocrine modulation may regulate inflammation to reach an optimum defense while preventing excessive host cell damage.

β-Endorphin inhibits phagocytic activity of lizard splenic phagocytes through μ receptor-coupled adenylate cyclase-protein kinase A signaling pathway

The receptor-coupled intracellular signaling mechanism of endogenous opioid peptide β-endorphin (β-end) is explored for the first time in ectothermic vertebrates using wall lizard as a model. β-End inhibited the percentage phagocytosis and phagocytic index of lizard splenic phagocytes in a dose-dependent manner. The inhibitory effect of β-end on phagocytosis was completely antagonized by non-selective opioid receptor antagonist naltrexone and also by selective μ-receptor antagonist CTAP. However, selective antagonists for other opioid receptors like NTI for δ-receptor and NorBNI for κ-receptor did not alter the effect of β-end on phagocytosis. This suggests that β-end mediated its inhibitory effect on phagocytic activity of splenic phagocytes exclusively through μ opioid receptors. The μ opioid receptor-coupled downstream signaling cascade was subsequently explored using inhibitors of adenylate cyclase (SQ 22536) and protein kinase A (H-89). Both SQ 22536 and H-89 abolished the inhibitory effect of β-end on phagocytosis in a concentration-related manner. Implication of cAMP as second messenger was corroborated by cAMP assay where an increase in intracellular cAMP level was observed in response to β-end treatment. It can be concluded that β-end downregulated the phagocytic activity of lizard splenic phagocytes through μ opioid receptor-coupled adenylate cyclase-cAMP-protein kinase A pathway.

A Novel Behavioral Fish Model of Nociception for Testing Analgesics

Pain is a major symptom in many medical conditions, and often interferes significantly with a person's quality of life. Although a priority topic in medical research for many years, there are still few analgesic drugs approved for clinical use. One reason is the lack of appropriate animal models that faithfully represent relevant hallmarks associated with human pain. Here we propose zebrafish (Danio rerio) as a novel short-term behavioral model of nociception, and analyse its sensitivity and robustness. Firstly, we injected two different doses of acetic acid as the noxious stimulus. We studied individual locomotor responses of fish to a threshold level of nociception using two recording systems: a video tracking system and an electric biosensor (the MOBS system). We showed that an injection dose of 10% acetic acid resulted in a change in behavior that could be used to study nociception. Secondly, we validated our behavioral model by investigating the effect of the analgesic morphine. In time-course studies, first we looked at the dose-response relationship of morphine and then tested whether the effect of morphine could be modulated by naloxone, an opioid antagonist. Our results suggest that a change in behavioral responses of zebrafish to acetic acid is a reasonable model to test analgesics. The response scales with stimulus intensity, is attenuated by morphine, and the analgesic effect of morphine is blocked with naloxone. The change in behavior of zebrafish associated with the noxious stimulus can be monitored with an electric biosensor that measures changes in water impedance.

Morphine regulates dopaminergic neuron differentiation via miR-133b

Morphine is one of the analgesics used most to treat chronic pain, although its long-term administration produces tolerance and dependence through neuronal plasticity. The ability of morphine to regulate neuron differentiation in vivo has been reported. However, the detailed mechanisms have not yet been elucidated because of the inability to separate maternal influences from embryonic events. Using zebrafish embryos as the model, we demonstrate that morphine decreases miR-133b expression, hence increasing the expression of its target, Pitx3, a transcription factor that activates tyrosine hydroxylase and dopamine transporter. Using a specific morpholino to knock down the zebrafish μ-opioid receptor (zfMOR) in the embryos and selective mitogen-activated protein kinase inhibitors, we demonstrate that the morphine-induced miR-133b decrease in zebrafish embryos is mediated by zfMOR activation of extracellular signal-regulated kinase 1/2. A parallel morphine-induced down-regulation of miR-133b was observed in the immature but not in mature rat hippocampal neurons. Our results indicate for the first time that zebrafish embryos express a functional μ-opioid receptor and that zebrafish serves as an excellent model to investigate the roles of microRNA in neuronal development affected by long-term morphine exposure.

Mu opioid receptor from the zebrafish exhibits functional characteristics as those of mammalian mu opioid receptor

The functional characterization of the mu opioid receptor from the zebrafish (zfMOR) is reported here. After transfection in HEK-293 cell line, using both peptidergic and nonpeptidergic opioid ligands in the competition and saturation-binding experiments, in addition to the functional assays of (35)S-GTPgammaS-binding assays and intracellular 3'-5'-cyclic adenosine monophosphate (cAMP) level determinations, we demonstrate that zfMOR exhibits a pharmacological profile similar to that of the mammalian MOR. Besides, the internalization process of zfMOR after opiate agonist treatment (morphine, DAMGO, etorphine) resembles the pattern observed for its mammalian counterpart. These similarities suggest that the zebrafish is a good model for the study of the opioid effects in development.

Delta opioid receptor-mediated immunoregulatory role of methionine-enkephalin in freshwater teleost Channa punctatus (Bloch.)

The immunoregulatory role of methionine-enkephalin (Met-enk) is well studied in mammals, but has not been explored in ectotherms despite the fact that this peptide is highly conserved in vertebrates. The present study demonstrates the diverse effects of Met-enk depending on its concentration and specific function of splenic phagocytes in the freshwater fish Channa punctatus. Although Met-enk increased both phagocytic as well as respiratory burst activity, the concentration-related response was opposite to each other. It had the maximum stimulatory effect on phagocytosis at 10(-9)M, while the same concentration was least effective in increasing superoxide production. Similarly, Met-enk at concentrations lower or higher than 10(-9)M was either ineffective or less effective in case of phagocytosis, while highly effective in stimulating superoxide production. On the other hand, concentration-independent inhibitory effect of Met-enk was observed in case of nitrite production. Nonetheless, Met-enk regulated all the functions of phagocyte through opioid receptors since non-specific opioid receptor antagonist naltrexone completely blocked the effect of Met-enk on phagocytosis, superoxide and nitrite production by splenic phagocytes of C. punctatus. Among selective opioid receptor antagonists, delta-opioid receptor antagonist naltrindole completely antagonized the effect of Met-enk on phagocytosis, superoxide and nitrite production, while mu- and kappa-opioid receptor antagonist, CTAP and norbinaltorphimine, respectively, were ineffective in influencing any of the functions. This suggests that Met-enk modulates splenic phagocyte functions in the fish C. punctatus via delta-opioid receptor. This is further substantiated by using highly selective delta-opioid receptor agonist, SNC80.

Cloning of opioid receptors in common carp (Cyprinus carpio L.) and their involvement in regulation of stress and immune response

In mammals opiate alkaloids and endogenous opioid peptides exert their physiological and pharmacological actions through opioid receptors (MOR, DOR and KOR) expressed not only on neuroendocrine cells but also on leukocytes. Therefore, opioids can modulate the immune response. We cloned and sequenced all three classical opioid receptors (MOR, DOR and KOR) in common carp, and studied changes in their expression during stress and immune responses. Messenger RNA of opioid receptors was constitutively expressed in brain areas, specially in the preoptic nucleus NPO (homologous to mammalian hypothalamus). After exposure to prolonged restraint stress, mRNA levels of MOR and DOR decreased in the NPO and in the head kidney. Increased expression of all studied opioid receptors was observed in the pituitary pars distalis (containing ACTH-producing cells). In immune organs, constitutive but lower expression of opioid receptor genes was observed. During in vivo zymosan-induced peritonitis or after in vitro LPS-induced stimulation, when pro-inflammatory functions are activated, expression of the OR genes in leukocytes was concomitantly up-regulated. Additionally, specific agonists of opioid receptors especially reduced leukocyte migratory properties, manifested by reduced chemotaxis and down-regulated expression of chemokine receptors. Our data indicate an evolutionary conserved role for the opioid system in maintaining a dynamic equilibrium while coping with stress and/or infection.

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.

A pharmacological comparison of the cloned frog and human mu opioid receptors reveals differences in opioid affinity and function

This study presents a direct comparison of the ligand binding and signaling profiles of a mammalian and non-mammalian mu opioid receptor. Opioid ligand binding and agonist potencies were determined for an amphibian (Rana pipiens) mu opioid receptor (rpMOR) and the human mu opioid receptor (hMOR) in transfected, intact Chinese hamster ovary (CHO) cells. Identical conditions were employed such that statistically meaningful differences between the two receptors could be determined. Identifying these differences is an important first step in understanding how evolutionary changes affect ligand binding and signaling in vertebrate opioid receptors. As expected, the rank of opioid ligand affinity for rpMOR and hMOR was consistent with the ligands' previously characterized type-selectivity. However, most of the opioid ligands tested had significant differences in affinity for rpMOR and hMOR. For example, the mu-selective agonist, DAMGO ([d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin), had a 10.9-fold greater affinity (K(i)) for hMOR (K(i)=268 nM) than rpMOR (K(i)=2914 nM). In addition, differences in signaling between these receptors were found by measuring inhibition of cAMP accumulation by morphine or DAMGO. DAMGO was significantly more potent (13.6-fold) in CHO cells expressing hMOR versus those expressing rpMOR. In addition, a significantly greater maximal inhibition was elicited by both opioid agonists in cells expressing hMOR. In summary, this study supports an ongoing effort to better understand how vertebrate evolution has shaped opioid receptor properties and function.

Evolution of vertebrate opioid receptors

The opioid peptides and receptors have prominent roles in pain transmission and reward mechanisms in mammals. The evolution of the opioid receptors has so far been little studied, with only a few reports on species other than tetrapods. We have investigated species representing a broader range of vertebrates and found that the four opioid receptor types (delta, kappa, mu, and NOP) are present in most of the species. The gene relationships were deduced by using both phylogenetic analyses and chromosomal location relative to 20 neighboring gene families in databases of assembled genomes. The combined results show that the vertebrate opioid receptor gene family arose by quadruplication of a large chromosomal block containing at least 14 other gene families. The quadruplication seems to coincide with, and, therefore, probably resulted from, the two proposed genome duplications in early vertebrate evolution. We conclude that the quartet of opioid receptors was already present at the origin of jawed vertebrates approximately 450 million years ago. A few additional opioid receptor gene duplications have occurred in bony fishes. Interestingly, the ancestral receptor gene duplications coincide with the origin of the four opioid peptide precursor genes. Thus, the complete vertebrate opioid system was already established in the first jawed vertebrates.

beta-Endorphin regulates diverse functions of splenic phagocytes through different opioid receptors in freshwater fish Channa punctatus (Bloch): an in vitro study

In this in vitro study, the role of beta-endorphin in the control of phagocytic and cytotoxic activities of fish splenic phagocytes was investigated. Further, the involvement of specific opioid receptor was explored. beta-Endorphin stimulated phagocytosis, whereas inhibited nitric oxide production as assessed by nitrite release. However, it had concentration-related biphasic effects on superoxide production, stimulatory at low and inhibitory at high concentration. Naltrexone, non-selective opioid receptor antagonist, antagonized the effect of beta-endorphin on phagocyte functions. Moreover, CTAP, selective mu-receptor antagonist, completely blocked the effect of beta-endorphin on phagocytosis and nitrite release. With regard to superoxide production, CTAP blocked the stimulatory effect of beta-endorphin at low concentration, while the inhibitory effect at high concentration was completely antagonized by selective delta-receptor antagonist, NTI. In conclusion, beta-endorphin acting via mu-receptor stimulated phagocytosis and inhibited nitric oxide production, while its biphasic effect on superoxide production seems to be mediated by mu- and delta-receptors.

Endogenous heptapeptide Met-enkephalin-Gly-Tyr binds differentially to duplicate delta opioid receptors from zebrafish

Met-enkephalin-Gly-Tyr (MEGY) is an endogenous peptide that binds to opioid sites in zebrafish and in rat brain homogenates. The aim of this work is to characterize the binding profile of this opioid ligand on two duplicate delta receptors from zebrafish, ZFOR1 and ZFOR4. Our results show that, while ZFOR1 presents one single binding site for [(3)H]-MEGY (K(D)=4.0+/-0.4 nM), the experimental data from ZFOR4 fit better to the two-site binding model (K(D1)=0.8+/-0.2 nM and K(D2)=30.2+/-10.2 nM). Two other MEGY synthetic analogues, (D-Ala(2))-MEGY and (D-Ala(2), Val(5))-MEGY were also prepared and tested, together with the original peptide MEGY and other opioid ligands, in competition binding assays. While these peptides presented K(i) values on the nanomolar range when using [(3)H]-MEGY as radioligand, these parameters were two orders higher in competition binding assays with the antagonist [(3)H]-diprenorphine. Functional [(35)S]GTPgammaS stimulation analysis has revealed that these two receptors can be activated by several opioid agonists. Our results prove that although the MEGY peptide acts as an agonist on ZFOR1 and ZFOR4, there are subtle pharmacological differences between these two delta opioid receptors from zebrafish.

Developmental expression and distribution of opioid receptors in zebrafish

Zebrafish is a novel experimental model that has been used in developmental studies as well as in the study of pathological processes involved in human diseases. It has been demonstrated that the endogenous opioid system is involved in developmental mechanisms. We have studied the relationship between the different embryonic stages and opioid receptor expression for the four known opioid receptors in zebrafish (mu, delta 1, delta 2 and kappa). The mu opioid receptor is detected at higher levels than the other opioid receptors before the midblastula transition and during the segmentation period. The delta duplicate 2 exhibits only one peak of expression at 21 h postfertilization (hpf), when the motor nervous system is forming. The kappa receptor is expressed at very low levels. In situ hybridization studies at 24 hpf show that the opioid receptors are widely distributed in zebrafish CNS and at 48 hpf their localization is detected in more defined structures. Our results support specific implications of the opioid receptors in developmental processes such as morphogenesis of the CNS, neurogenesis, neuroprotection and development of neuromuscular and digestive system. Pain-related alterations can be a consequence of changes in the endogenous opioid system during development, hence we provide important information that might help to solve pain-related pathological situations.

A choice behavior for morphine reveals experience-dependent drug preference and underlying neural substrates in developing larval zebrafish

Transparent larval zebrafish offer the opportunity to unravel genetic and neuronal networks underlying behavior in a developing system. In this study, we developed a choice chamber paradigm to measure reward-associated behavior in larval zebrafish. In the chamber where larval zebrafish have a choice of spending their time in either a water- or morphine-containing compartment, larvae that have previously experienced morphine spend significantly more time in the compartment containing morphine. This behavior can be attentuated by pre-treatment with antagonists of the opioid receptor or the dopamine receptor, and furthermore, is impaired in the too few mutant, which has a genetic deficiency in the production of specific groups of dopaminergic and serotonergic neurons in the ventral forebrain. These results uncover a choice behavior for an addictive substance in larval zebrafish that is mediated through central opioid and monoaminergic neurotransmitter systems.

Beta-endorphin alters electrical activity of gonadotropin releasing hormone neurons located in the terminal nerve of the teleost medaka (Oryzias latipes)

Endogenous opioid peptides (EOPs) are an important class of modulators of the hypothalamo-pituitary axis; treatment with opiates leads to inhibition of GnRH and LH secretion and suppression of reproductive functions. However, little work has been done to investigate the effect of opiates on the electrical activity of GnRH neurons, which ultimately controls GnRH secretion. The purpose of the present study was to investigate the effects of the EOP beta-endorphin on electrical activity of GnRH neurons located in the terminal nerve (TN) associated with the olfactory bulb. We used an excised intact brain preparation from transgenic medaka in which green fluorescent protein (GFP) is genetically expressed in TN-GnRH neurons. These GFP-expressing neurons were then targeted for whole-cell current clamp recordings. Treatment with beta-endorphin led to changes in several characteristics of electrical activity, including depolarization of membrane potential and a decrease in spike amplitude--similar to that observed in response to depolarizing high K(+) treatment. This finding suggests a model in which beta-endorphin depolarizes membrane potential leading to Na(+)-channel inactivation, and subsequent suppression of action-potential amplitude. On the other hand, beta-endorphin had no effect on membrane potential in synaptically isolated GnRH neurons. These results suggest that beta-endorphin is acting indirectly on TN-GnRH neurons to inhibit action potential firing.

Characterization of cannabinoid-binding sites in zebrafish brain

We present here the pharmacological characterization of cannabinoid-binding sites in zebrafish brain homogenates using radiolabeled binding techniques. The nonselective agonist [3H]-CP55940 binds with high affinity (KD = 0.50+/-0.06 nM and a Bmax = 1047+/-36.01 fmol/mg protein), displaying one binding site. The slightly CB2 selective agonist [3H]-WIN55212-2 also binds with high affinity to zebrafish brain membranes displaying two different binding sites with affinities KD1 = 0.35+/-0.09 nM and KD2 = 105.81+/-66.36 nM. Competition binding assays using [3H]-WIN55212-2 and several unlabeled ligands were performed. WIN55212-2 significantly displaced the tritiated ligand binding showing the two binding sites observed with its tritiated homologous, while the slightly selective CB1 cannabinoid ligand HU-210, the nonselective cannabinoid ligand CP55940 and the endogenous cannabinoid ligand anandamide presented one binding site. Also, the functionality of these cannabinoid sites was analyzed using the known [35S]GTPgammaS assay. All the agonist used presented an agonist profile and the rank order for potency was HU-210 > WIN55212-2 > CP55940 >anandamide. Our results provide evidence that, although some of the typical cannabinoid ligands for mammalian receptors do not fully recognize the cannabinoid-binding sites in zebrafish brain, the activity of the endogenous zebrafish cannabinoid system might not significantly differ from that displayed by the cannabinoid system described in other species. Hence the study of zebrafish cannabinoid activity may contribute to an understanding of the endogenous cannabinoid system in higher vertebrates.

Identification of dynorphin a from zebrafish: a comparative study with mammalian dynorphin A

We report the cloning and molecular characterization of the zfPDYN. The complete open reading frame for this propeptide is comprised in two exons that are localized on chromosome 23. zfPDYN cDNA codes for a polypeptide of 252 amino acids that contains the consensus sequences for four opioid peptides: an Ile-enkephalin, the neo-endorphins, dynorphin A and dynorphin B. Upon comparison between zebrafish (zfDYN A) and mammalian dynorphin A (mDYN A) it has been stated that these two peptides only differ in two amino acids: the Leu(5) is replaced by Met(5) and the Lys(13) by Arg(13). Taking into consideration that mDYN A is able to bind to the three mammalian opioid receptors, we have compared the pharmacological profile of zfDYN A and mDYN A on the zebrafish opioid receptors. By means of radioligand binding techniques, we have established that these two dynorphins bind and activate all of the cloned opioid receptors from zebrafish (delta-, mu- and kappa-like), although with different affinities. zfDYN A and mDYN A displace [(3)H]-diprenorphine binding with K(i) values on the nanomolar range, showing greater affinity for zebrafish opioid receptor (ZFOR) 3 (kappa) receptor. ZFOR1 (delta) and ZFOR4 (delta) present higher affinity for zfDYN A than for mDYN A, while the opposing behavior is observed in ZFOR2 (mu). Functional [(35)S]guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) stimulation experiments indicate that these two peptides fully activate the zebrafish opioid receptors, although the mean effective dose (EC(50)) values obtained for ZFOR2 and ZFOR3 receptors are lower than those seen for ZFOR1 and ZFOR4. A comparative study indicates that mammalian and zebrafish opioid receptors might bind their corresponding dynorphin A in a similar fashion, hence suggesting an important role of the opioid system through the vertebrate evolution.

Dissociation of food and opiate preference by a genetic mutation in zebrafish

Both natural rewards and addictive substances have the ability to reinforce behaviors. It has been unclear whether identical neural pathways mediate the actions of both. In addition, little is known about these behaviors and the underlying neural mechanisms in a genetically tractable vertebrate, the zebrafish Danio rerio. Using a conditioned place preference paradigm, we demonstrate that wildtype zebrafish exhibit a robust preference for food as well as the opiate drug morphine that can be blocked by the opioid receptor antagonist naloxone. Moreover, we show that the too few mutant, which disrupts a conserved zinc finger-containing gene and exhibits a reduction of selective groups of dopaminergic and serotonergic neurons in the basal diencephalon, displays normal food preference but shows no preference for morphine. Pretreatment with dopamine receptor antagonists abolishes morphine preference in the wildtype. These studies demonstrate that zebrafish display measurable preference behavior for reward and show that the preference for natural reward and addictive drug is dissociable by a single-gene mutation that alters subregions of brain monoamine neurotransmitter systems. Future genetic analysis in zebrafish shall uncover further molecular and cellular mechanisms underlying the formation and function of neural circuitry that regulate opiate and food preference behavior.

New kappa opioid receptor from zebrafish Danio rerio

A cDNA that encodes a kappa opioid receptor like from zebrafish (ZFOR3) has been cloned and characterized. The encoded protein is 377 residues long and presents 70% identity with the mammalian kappa receptors, although less homology is found in the amino- and carboxyl-terminus as well as in the extracellular loops. In situ hybridization studies have revealed that ZFOR3 mRNA is highly expressed in particular brain areas that coincide with the expression of the kappa opioid receptor in other species. When ZFOR3 is stably expressed in HEK293 cells, [(3)H]-diprenorphine binds with high affinity (K(D)=1.05+/-0.26 nM), being this value on the same range as those reported for mammalian kappa opioid receptors. On the other hand, the selective agonist for mammalian kappa receptors U69,593 does not bind to ZFOR3. [(3)H]-diprenorphine binding is readily displaced by the peptidic ligand dynorphin A and by the non-endogenous compounds bremazocine, naloxone and morphine, although with different affinities. Our results demonstrate that ZFOR3 is a unique model to study the kappa opioid receptor functionality.

Delta and mu opioid receptors from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa: cloning, heterologous expression, and pharmacological characterization

Two full-length cDNAs, encoding delta (delta) and mu (mu) opioid receptors, were cloned from the brain of the rough-skinned newt Taricha granulosa, complementing previous work from our laboratory describing the cloning of newt brain kappa (kappa) and ORL1 opioid receptors. The newt delta receptor shares 82% amino acid sequence identity with a frog delta receptor and lower (68-70%) identity with orthologous receptors cloned from mammals and zebrafish. The newt mu receptor shares 79% sequence identity with a frog mu receptor, 72% identity with mammalian mu receptors, and 66-69% identity with mu receptors cloned from teleost fishes. Membranes isolated from COS-7 cells transiently expressing the newt delta receptor possessed a single, high-affinity (Kd = 2.4 nM) binding site for the nonselective opioid antagonist [3H]naloxone. In competition binding assays, the newt delta receptor displayed highest affinity for Met-enkephalin, relatively low affinity for Leu-enkephalin, beta-endorphin, and [D-penicillamine, D-penicillamine] enkephalin (DPDPE) (a delta-selective agonist in mammals), and very low affinity for mu-, kappa-, or ORL1-selective agonists. COS-7 cells expressing the newt mu receptor also possessed a high-affinity (Kd = 0.44 nM) naloxone-binding site that showed highest affinity for beta-endorphin, moderate-to-low affinity for Met-enkephalin and Leu-enkephalin and DAMGO (a mu-selective agonist in mammals), and very low affinity for DPDPE and kappa- or ORL1-selective agonists. COS-7 cells expressing either receptor type (delta or mu) showed very high affinity (Kd = 0.1-0.3 nM) for the nonselective opioid antagonist diprenorphine. Taricha granulosa expresses the same four subtypes (delta, mu, kappa, and ORL1) of opioid receptors found in other vertebrate classes, but ligand selectivity appears less stringent in the newt than has been documented in mammals.

Characterization of Opioid-Binding Sites in Zebrafish Brain

The pharmacological profile of opioid-binding sites in zebrafish brain homogenates has been studied using radiolabeled binding techniques. The nonselective antagonist [(3)H]diprenorphine binds with high affinity (K(D) = 0.27 +/- 0.08 nM and a B(max) = 212 +/- 14.3 fmol/mg protein), displaying two different binding sites with affinities of K(D1) = 0.08 +/- 0.02 nM and K(D2) = 17.8 +/- 9.18 nM. The nonselective agonist [(3)H]bremazocine also binds with high affinity to zebrafish brain membranes but only displays one single binding site with a K(D) = 1.1 +/- 0.09 nM and a B(max) = 705 +/- 19.3 fmol/mg protein. Competition binding assays using [(3)H]diprenorphine and several unlabeled ligands were performed. The synthetic selective agonists for mammalian opioid receptors DPDPE ([DPen(2),D-Pen(5)]-enkephalin), DAMGO ([D-Ala(2),NMe-Phe(4),Gly(5)-ol]-enkephalin), and U69,593 [(5alpha,7alpha,8beta)-(+)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide] failed to effectively displace [(3)H]diprenorphine binding, whereas nonselective ligands and the endogenous opioid peptides such as dynorphin A showed good affinities in the nanomolar range, although several of the endogenous peptides only displaced approximately 50% of the specifically bound [(3)H]diprenorphine. Our results provide evidence that, although the selective synthetic compounds for mammalian receptors do not fully recognize the opioid-binding sites in zebrafish brain, the activity of the endogenous zebrafish opioid system might not significantly differ from that displayed by the mammalian opioid system. Hence, the study of zebrafish opioid activity may contribute to an understanding of endogenous opioid systems in higher vertebrates.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

Opioid research in amphibians: an alternative pain model yielding insights on the evolution of opioid receptors

This review summarizes the work from our laboratory investigating mechanisms of opioid analgesia using the Northern grass frog, Rana pipiens. Over the last dozen years, we have accumulated data on the characterization of behavioral effects after opioid administration on radioligand binding by using opioid agonist and antagonist ligands in amphibian brain and spinal cord homogenates, and by cloning and sequencing opioid-like receptor cDNA from amphibian central nervous system (CNS) tissues. The relative analgesic potency of mu, delta, and kappa opioids is highly correlated between frogs and other mammals, including humans. Radioligand binding studies using selective opioid agonists show a similar selectivity profile in amphibians and mammals. In contrast, opioid antagonists that are highly selective for mammalian mu, delta, and kappa opioid receptors were not selective in behavioral and binding studies in amphibians. Three opioid-like receptor cDNAs were cloned and sequenced from amphibian brain tissues and are orthologs to mammalian mu, delta, and kappa opioid receptors. Bioinformatics analysis of the three types of opioid receptor cDNAs from all vertebrate species with full datasets gave a pattern of the molecular evolution of opioid receptors marked by the divergence of mu, delta, and kappa opioid receptor sequences during vertebrate evolution. This divergence in receptor amino acid sequence in later-evolved vertebrates underlies the hypothesis that opioid receptors are more type-selective in mammals than in nonmammalian vertebrates. The apparent order of receptor type evolution is kappa, then delta, and, most recently, the mu opioid receptor. Finally, novel bioinformatics analyses suggest that conserved extracellular receptor domains determine the type selectivity of vertebrate opioid receptors.

Identification of duplicated fourth alpha2-adrenergic receptor subtype by cloning and mapping of five receptor genes in zebrafish

The alpha(2)-adrenergic receptors (alpha(2)-ARs) belong to the large family of rhodopsinlike G-protein-coupled receptors that share a common structure of seven transmembrane (TM) alpha-helices. The aims of this study were (1) to determine the number of alpha(2)-AR genes in a teleost fish, the zebrafish (Danio rerio), (2) to study the gene duplication events that generated the alpha(2)-AR subtypes, and (3) to study changes in receptor structure that have occurred since the divergence of the mammalian and fish lineages. Here, we report the cloning and chromosomal mapping of fish orthologs for all three mammalian alpha(2)-ARs. In addition, we identified a fourth alpha(2)-AR subtype with two duplicates in zebrafish. Chromosomal mapping showed that the zebrafish alpha(2)-AR genes are located within conserved chromosomal segments, consistent with the origin of the four alpha(2)-AR subtypes by two rounds of chromosome or block duplication before the divergence of the ray fin fish and tetrapod lineages. Thus, the fourth subtype has apparently been present in the common ancestor of vertebrates but has been deleted or is yet to be identified in mammals. The overall percentage identity between the fish and mammalian orthologs is 53% to 67%, and in the TM regions 80% to 87%. These values are clearly lower than what is observed between mammalian orthologs. Still, all of the residues thought to be important for alpha(2)-adrenergic ligand binding are conserved across species and subtypes, and even the most divergent regions of the fish receptors show clear "molecular fingerprints" typical for orthologs of a given subtype.

Characterization of mu, kappa, and delta opioid binding in amphibian whole brain tissue homogenates

Opioid agonists produce analgesia in mammals through the activation of mu, kappa, or delta opioid receptors. Previous behavioral and binding studies from our laboratory using an amphibian model suggested that mu, kappa, or delta opioid agonists may activate a single type of opioid receptor in the grass frog, Rana pipiens. In the present study, kinetic, saturation, and competitive binding profiles for three opioid radioligands, [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin) (mu-selective), [(3)H]U65953 [(5 alpha, 7 alpha,8 beta)-(+)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide] (kappa-selective), and [(3)H]DPDPE ([D-Pen(2),D-Pen(5)]-enkephalin) (delta-selective) were determined using frog whole brain homogenates. Kinetic analyses and experimentally derived values from saturation experiments gave affinity constants (K(D)) in the low nanomolar range. The density of opioid binding sites (B(max)) was 224.4, 118.6, and 268.9 fmol/mg for mu, kappa, and delta opioid radioligands, respectively. The affinity values did not significantly differ among the three opioid radioligands, but the kappa radioligand bound to significantly fewer sites than did the mu or delta radioligands. K(i) values for unlabeled mu, kappa, and delta competitors, including highly selective opioid antagonists, were consistent with each radioligand selectivity profile. The present data suggest that mu, kappa, and delta opioid radioligands bind to distinct opioid receptors in amphibians that are surprisingly similar to those found in mammalian brain.