Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Systemic and Peripheral Mechanisms of Cortical Stimulation-Induced Analgesia and Refractoriness in a Rat Model of Neuropathic Pain

Epidural motor cortex stimulation (MCS) is an effective treatment for refractory neuropathic pain; however, some individuals are unresponsive. In this study, we correlated the effectiveness of MCS and refractoriness with the expression of cytokines, neurotrophins, and nociceptive mediators in the dorsal root ganglion (DRG), sciatic nerve, and plasma of rats with sciatic neuropathy. MCS inhibited hyperalgesia and allodynia in two-thirds of the animals (responsive group), and one-third did not respond (refractory group). Chronic constriction injury (CCI) increased IL-1β in the nerve and DRG, inhibited IL-4, IL-10, and IL-17A in the nerve, decreased β-endorphin, and enhanced substance P in the plasma, compared to the control. Responsive animals showed decreased NGF and increased IL-6 in the nerve, accompanied by restoration of local IL-10 and IL-17A and systemic β-endorphin. Refractory animals showed increased TNF-α and decreased IFNγ in the nerve, along with decreased TNF-α and IL-17A in the DRG, maintaining low levels of systemic β-endorphin. Our findings suggest that the effectiveness of MCS depends on local control of inflammatory and neurotrophic changes, accompanied by recovery of the opioidergic system observed in neuropathic conditions. So, understanding the refractoriness to MCS may guide an improvement in the efficacy of the technique, thus benefiting patients with persistent neuropathic pain.

αN-Acetyl β-Endorphin Is an Endogenous Ligand of σ1Rs That Regulates Mu-Opioid Receptor Signaling by Exchanging G Proteins for σ2Rs in σ1R Oligomers

The opioid peptide β-endorphin coexists in the pituitary and brain in its ^αN-acetylated form, which does not bind to opioid receptors. We now report that these neuropeptides exhibited opposite effects in in vivo paradigms, in which ligands of the sigma type 1 receptor (σ1R) displayed positive effects. Thus, ^αN-acetyl β-Endorphin reduced vascular infarct caused by permanent unilateral middle cerebral artery occlusion and diminished the incidence of N-methyl-D-aspartate acid-promoted convulsive syndrome and mechanical allodynia caused by unilateral chronic constriction of the sciatic nerve. Moreover, ^αN-acetyl β-Endorphin reduced the analgesia of morphine, β-Endorphin and clonidine but enhanced that of DAMGO. All these effects were counteracted by β-Endorphin and absent in σ1R^-/- mice. We observed that σ1Rs negatively regulate mu-opioid receptor (MOR)-mediated morphine analgesia by binding and sequestering G proteins. In this scenario, β-Endorphin promoted the exchange of σ2Rs by G proteins at σ1R oligomers and increased the regulation of G proteins by MORs. The opposite was observed for the ^αN-acetyl derivative, as σ1R oligomerization decreased and σ2R binding was favored, which displaced G proteins; thus, MOR-regulated transduction was reduced. Our findings suggest that the pharmacological β-Endorphin-specific epsilon receptor is a σ1R-regulated MOR and that β-Endorphin and ^αN-acetyl β-Endorphin are endogenous ligands of σ1R.

Diels-Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels-Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

Atypical opioid receptors: unconventional biology and therapeutic opportunities

Endogenous opioid peptides and prescription opioid drugs modulate pain, anxiety and stress by activating four opioid receptors, namely μ (mu, MOP), δ (delta, DOP), κ (kappa, KOP) and the nociceptin/orphanin FQ receptor (NOP). Interestingly, several other receptors are also activated by endogenous opioid peptides and influence opioid-driven signaling and biology. However, they do not meet the criteria to be recognized as classical opioid receptors, as they are phylogenetically distant from them and are insensitive to classical non-selective opioid receptor antagonists (e.g. naloxone). Nevertheless, accumulating reports suggest that these receptors may be interesting alternative targets, especially for the development of safer analgesics. Five of these opioid peptide-binding receptors belong to the family of G protein-coupled receptors (GPCRs)-two are members of the Mas-related G protein-coupled receptor X family (MrgX1, MrgX2), two of the bradykinin receptor family (B_1, B₂), and one is an atypical chemokine receptor (ACKR3). Additionally, the ion channel N-methyl-d-aspartate receptors (NMDARs) are also activated by opioid peptides. In this review, we recapitulate the implication of these alternative receptors in opioid-related disorders and discuss their unconventional biology, with members displaying signaling to scavenging properties. We provide an overview of their established and emerging roles and pharmacology in the context of pain management, as well as their clinical relevance as alternative targets to overcome the hurdles of chronic opioid use. Given the involvement of these receptors in a wide variety of functions, including inflammation, chemotaxis, anaphylaxis or synaptic transmission and plasticity, we also discuss the challenges associated with the modulation of both their canonical and opioid-driven signaling.

Stress and opioids: role of opioids in modulating stress-related behavior and effect of stress on morphine conditioned place preference

Research studies have defined the important role of endogenous opioids in modulating stress-associated behavior. The release of β-endorphins in the amygdala in response to stress helps to cope with a stressor by inhibiting the over-activation of HPA axis. Administration of mu opioid agonists reduces the risk of developing post-traumatic stress disorder (PTSD) following a traumatic event by inhibiting fear-related memory consolidation. Similarly, the release of endogenous enkephalin and nociceptin in the basolateral amygdala and the nucleus accumbens tends to produce the anti-stress effects. An increase in dynorphin levels during prolonged exposure to stress may produce learned helplessness, dysphoria and depression. Stress also influences morphine-induced conditioned place preference (CPP) depending upon the intensity and duration of the stressor. Acute stress inhibits morphine CPP, while chronic stress potentiates CPP. The development of dysphoria due to increased dynorphin levels may contribute to chronic stress-induced potentiation of morphine CPP. The activation of ERK/cyclic AMP responsive element-binding (CREB) signaling in the mesocorticolimbic area, glucocorticoid receptors in the basolateral amygdala, and norepinephrine and galanin system in the nucleus accumbens may decrease the acute stress-induced inhibition of morphine CPP. The increase in dopamine levels in the nucleus accumbens and augmentation of GABAergic transmission in the median prefrontal cortex may contribute in potentiating morphine CPP. Stress exposure reinstates the extinct morphine CPP by activating the orexin receptors in the nucleus accumbens, decreasing the oxytocin levels in the lateral septum and amygdala, and altering the GABAergic transmission (activation of GABAA and inactivation of GABAB receptors). The present review describes these varied interactions between opioids and stress along with the possible mechanism.

A computational drug-target network for yuanhu zhitong prescription

Yuanhu Zhitong prescription (YZP) is a typical and relatively simple traditional Chinese medicine (TCM), widely used in the clinical treatment of headache, gastralgia, and dysmenorrhea. However, the underlying molecular mechanism of action of YZP is not clear. In this study, based on the previous chemical and metabolite analysis, a complex approach including the prediction of the structure of metabolite, high-throughput in silico screening, and network reconstruction and analysis was developed to obtain a computational drug-target network for YZP. This was followed by a functional and pathway analysis by ClueGO to determine some of the pharmacologic activities. Further, two new pharmacologic actions, antidepressant and antianxiety, of YZP were validated by animal experiments using zebrafish and mice models. The forced swimming test and the tail suspension test demonstrated that YZP at the doses of 4 mg/kg and 8 mg/kg had better antidepressive activity when compared with the control group. The anxiolytic activity experiment showed that YZP at the doses of 100 mg/L, 150 mg/L, and 200 mg/L had significant decrease in diving compared to controls. These results not only shed light on the better understanding of the molecular mechanisms of YZP for curing diseases, but also provide some evidence for exploring the classic TCM formulas for new clinical application.

The Maillard reaction induced modifications of endogenous opioid peptide enkephalin

Nonenzymatic glycation (Maillard reaction) is a posttranslational modification of peptides and proteins by sugars, which, after a cascade of reactions, leads to the formation of a complex family of irreversibly changed advanced glycation end products (AGE) implicated in the pathogenesis of human diseases. Last reversible intermediates of this reaction are Amadori/Heyns compounds formed in glucose/fructose induced modification of peptides. The stability of these compounds determines the further course of the reaction.To provide information concerning the preparation of model systems as well as the fate of glycated opioid peptides introduced in the human circulation, the enzymatic (80 % human serum) and chemical (PBS) stability of Amadori and Heyns compounds related to the endogenous opioid pentapeptides leucine- and methionine-enkephalin (Tyr-Gly-Gly-Phe-Leu/Met) were investigated.

Acupuncture May Stimulate Anticancer Immunity via Activation of Natural Killer Cells

This article presents the hypothesis that acupuncture enhances anticancer immune functions by stimulating natural killer (NK) cells. It provides background information on acupuncture, summarizes the current scientific understanding of the mechanisms through which NK cells act to eliminate cancer cells, and reviews evidence that acupuncture is associated with increases in NK cell quantity and function in both animals and humans. The key contribution of this article involves the use of cellular immunology and molecular biological theory to interpret and synthesize evidence from disparate animal and human studies in formulating the 'acupuncture immuno-enhancement hypothesis': clinicians may use acupuncture to promote the induction and secretion of NK-cell activating cytokines that engage specific NK cell receptors that endogenously enhance anticancer immune function.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Níveis de beta-endorfina em resposta ao exercício e no sobretreinamento

O sobretreinamento (ST) é um fenômeno esportivo complexo e multifatorial; e atualmente não existe nenhum marcador independente que possa diagnosticá-lo. Interessantemente, alguns sintomas do ST apresentam relação com os efeitos da b-endorfina (b-end1-31). Alguns de seus efeitos são importantes para o treinamento, como analgesia, maior tolerância ao lactato e euforia do exercício. Esses efeitos podem ser revertidos por destreinamento ou por ST, ocasionando diminuição no desempenho, redução da tolerância à carga e depressão. O exercício físico é o principal estímulo da b-end1-31, pois sua secreção é volume/intensidade dependente, tanto para exercícios aeróbios quanto anaeróbios. No entanto, o treinamento excessivo pode diminuir suas concentrações, alterando assim seus efeitos benéficos para o treinamento. Portanto, a b-end1-31 poderia ser utilizada como um marcador adicional de ST, principalmente porque seus efeitos apresentam extensa relação com os sintomas do ST.

Antinociception produced by 14,15-epoxyeicosatrienoic acid is mediated by the activation of beta-endorphin and met-enkephalin in the rat ventrolateral periaqueductal gray

Cytochrome P450 genes catalyze formation of epoxyeicosatrienoic acids (EETs) from arachidonic acid. The effects of 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET microinjected into the ventrolateral periaqueductal gray (vlPAG) on the thermally produced tail-flick response were studied in male Sprague-Dawley rats. 14,15-EET microinjected into vlPAG (3-156 pmol) dose-dependently inhibited the tail-flick response (ED50 = 32.5 pmol). In contrast, 5,6-EET, 8,9-EET, and 11,12-EET at a dose of 156 pmol were not active when injected into the vlPAG. 14,15-EET failed to displace the radiobinding of [3H][D-Ala2,NHPe4, Gly-ol5]enkephalin (mu-opioid receptor ligand) or [3H]naltrindole (delta-opioid receptor ligand) in crude membrane fractions of rat brain. Tail-flick inhibition produced by 14,15-EET from vlPAG was blocked by intra-vlPAG pretreatment with antiserum against beta-endorphin or Met-enkephalin or the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) or the delta-opioid receptor antagonist naltrindole but not with dynorphin A[1-17] antiserum or the kappa-opioid receptor antagonist nor-binaltorphimine. In addition, tail-flick inhibition produced by 14,15-EET treatment was blocked by intrathecal pretreatment with Met-enkephalin antiserum, naltrindole, or CTOP but not with beta-endorphin antiserum. It is concluded that 1) 14,15-EET itself does not have any affinity for mu- or delta-opioid receptors and 2) 14,15-EET activates beta-endorphin and Met-enkephalin, which subsequently act on mu- and delta-opioid receptors to produce antinociception.

Opioids

Opioids are the most effective and widely used drugs in the treatment of severe pain. They act through G protein-coupled receptors. Four families of endogenous ligands (opioid peptides) are known. The standard exogenous opioid analgesic is morphine. Opioid agonists can activate central and peripheral opioid receptors. Three classes of opioid receptors (mu, delta, kappa) have been identified. Multiple pathways ofopioid receptor signaling (e.g., G(i/o) coupling, cAMP inhibition, Ca++ channel inhibition) have been described. The differential regulation of effectors, preclinical pharmacology, clinical applications, and side effects will be reviewed in this chapter.

Formalin pretreatment attenuates tail-flick inhibition induced by β-endorphin administered intracerebroventricularly or intrathecally in mice

We examined the effect of the subcutaneous (s.c.) pretreatment of formalin into both hind paws of mice on the antinociception induced by the intracerebroventricularly (i.c.v.) or intrathecally (i.t.) administration of beta-endorphin using the tail-flick test. Pretreatment with formalin (5%) for 5 h had no affect on the i.c.v. administered beta-endorphin-induced tail-flick response. However, pretreatment with formalin for 40 h attenuated the tail-flick inhibition induced by i.c.v. administered beta-endorphin. This antinociceptive tolerance to i.c.v. beta-endorphin continued up to 1 week, but to a lesser extent. Pretreatment with formalin for 5 and 40 h significantly reduced the i.t. beta-endorphin-induced inhibition of the tail-flick response, which continued up to 1 week. The s.c. formalin treatment increased the hypothalamic pro-opiomelanocortin (POMC) mRNA level at 2 h, but this returned to the basal level after 40 h. Our results suggest that the increase in the POMC mRNA level in the hypothalamus appears to be involved in the supraspinal or spinal beta-endorphin-induced antinociceptive tolerance in formalin-induced inflammatory pain.

Rational drug design and synthesis of a selective ε opioid receptor antagonist on the basis of the accessory site concept

To newly synthesize a selective opioid receptor antagonist, 17-(cyclopropylmethyl)-4,5 alpha-epoxy-6 beta,21-epoxymethano-3-hydroxy-6,14-endoethenomorphinan-7 alpha-(N-phenethyl)carboxamide was first designed from an opioid receptor agonist TAN-821 on the basis of the accessory site concept. The designed compound antagonized the agonistic effects induced by an opioid receptor agonist beta-endorphin on the rat vas deference test. Moreover, the designed compound blocked the antinociception induced by beta-endorphin given intracerebroventricularly.

No evidence for G-protein-coupled epsilon receptor in the brain of triple opioid receptor knockout mouse

Pharmacological approaches have defined the epsilon receptor as a beta-endorphin-preferring opioid receptor, described in rat vas deferens and in brain of several species. Only three opioid receptors-mu, delta and kappa-have been cloned and the existence of this additional subtype as a distinct protein remains controversial. Recently, the mouse brain epsilon receptor was detected in a G protein activation assay, as mediating residual beta-endorphin activity following pharmacological blockade of mu, delta and kappa receptors. To clarify whether this site is independent from mu, delta and kappa receptors, we performed beta-endorphin-induced [(35)S]GTPgammaS binding using mice lacking these three receptors (triple knockout mice). We tested both pons-medulla and whole brain preparations. beta-Endorphin strongly stimulated [(35)S]GTPgammaS binding in wild-type membranes but had no detectable effect in membranes from triple knockout mice. We conclude that the brain epsilon site involves mu, delta and/or kappa receptors, possibly coupled to nonclassical G proteins.

Role of micro-opioid receptors in formalin-induced pain behavior in mice

Intraplantar formalin injection is widely used as an experimental model of tonic pain. We investigated the role of endogenous micro-opioid receptor mechanisms in formalin-induced nocifensive behavior in mice. The flinching response induced by formalin (2%, 20 microl) was studied in mice with normal (wild type, n = 8) and absent (homozygous micro-opioid receptor knockout, n = 8) micro-opioid receptor levels. The flinch responses were counted every 5 min for 60 min post-formalin injection. Lumbar spinal cord (L4, 5) was harvested 2 h post-formalin injection to examine c-Fos expression using immunohistochemistry. The effects of naloxone (5 mg/kg, sc) administered 30 min before the intraplantar formalin injection on the flinching response of wild-type mice (n = 7) were also recorded. The second-phase formalin response (10-60 min after formalin) was higher in homozygous micro-opioid receptor knockout mice compared to the wild-type mice (P < 0.01). Naloxone administration in wild-type mice before formalin injection resulted in pain behavior similar to that observed in homozygous micro-opioid receptor knockout mice (P > 0.05). The c-Fos expression induced by formalin injection in the knockout mice was not different from that observed in wild-type mice. Our results suggest that the endogenous micro-opioid system is activated by intraplantar formalin injection and exerts a tonic inhibitory effect on the pain behavior. These results suggest an important modulatory role of endogenous micro-opioid receptor mechanisms in tonic pain states.

Involvement of kappa-opioid receptors in peripheral response to nerve stimulation in kappa-opioid receptor knockout mice

1 The present study aimed to evaluate the role of kappa-opioid receptors at two peripheral sites, the vas deferens and the proximal colon, in kappa-opioid receptor knockout mice. We investigated the role of the kappa-opioid receptor in the vas deferens twitch response and in the colonic "off-contraction", a rebound contractile response which follows the inhibitory response to low frequencies stimulation (10, 20, 30 Hz) and which has been suggested to "locally" reproduce the contractile component of the peristaltic reflex. 2 Transmural stimulation of the vas deferens at lower frequencies (10 Hz, 10 V, 1 ms pulse trains lasting 0.5 s) evoked a contractile response that was significantly higher in the preparations from knockout mice because of lack of kappa-opioid receptors than in wild type mice. A selective kappa-opioid receptor agonist, U-50,488H, induced a dose-dependent inhibition of the electrically stimulated contraction in vas deferens. The percentages of reduction of the twitch response were significantly lower in knockout mice than in wild type mice after treatment with U-50,488H. The reduction of twitch response caused by U-50,488H was not reversed by administration of nor-binaltorphimine (nor-BNI) (5 x 10-6 m), a selective kappa-opioid receptor antagonist, in preparations from both knockout mice and wild type mice. U-50,488H has no effect on postsynaptic adrenergic receptors, as its administration did not affect the direct contractile response to noradrenaline. 3 Transmural stimulation (5 Hz, 20 V, 2 ms pulse trains lasting 30 s) induced inhibition of spontaneous activity of colonic strips during the period of stimulation, followed by an "off-contraction" after the cessation of stimulation. The statistical evaluation of the "off-contraction" responses between the two strains showed no significant difference. The off-contraction, measured in specimens from knockout mice, was inhibited concentration-dependently by U-50,488H (P < 0.01) and significantly less than from wild type mice. 4 The effect of U-50,488H was not reversed by administration of nor-BNI (5 x 10-6 m), either in preparations from knockout mice or from wild type mice. 5 Our data may suggest that kappa-opioid receptors are involved in some peripheral responses to the nerve stimulation, as indicated by the effect of U-50,488H, a selective kappa-opioid receptor agonist. However, the involvement of kappa-opioid receptor was also present, although less apparent, in kappa -opioid receptor knockout mice, suggesting either that this drug acts not only on kappa-opioid receptors but also on other receptor sites, such as kappa-like receptors. An alternative interpretation can be related to a sodium channel blocking action of U-50,488H, which could explain the inhibitory effects of twitch response still present but less evident in knockout strain and the lack of effect of the antagonist nor-BNI.

The antinociceptive properties of endomorphin-1 and endomorphin-2 in the mouse

Two highly selective mu-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2), have been identified and postulated to be endogenous mu-opioid receptor ligands. The present minireview describes the antinociceptive properties with the tail-flick test of these two ligands given intracerebroventricularly (i.c.v.) and intrathecally (i.t.) in ICR mice. EM-1 or EM-2 given i.c.v. or i.t. dose-dependently produce antinociception. These antinociceptive effects induced by EM-1 and EM-2 given i.c.v. or i.t. are selectively mediated by the stimulation of mu-, but not delta- or kappa-opioid receptors. Like other mu-opioid agonists morphine and DAMGO ([D-Ala2,NMePhe4,Gly5-ol]enkephalin), EM-1 and EM-2 given i.c.v. activate descending pain controls by the releases of noradrenaline and 5-HT and subsequently act on alpha2-adrenoceptors and 5-HT receptors, respectively, in the spinal cord to produce antinociception. However, the antinociception induced by EM-2 given i.c.v. or i.t. also contain an additional component, which is mediated by the release of dynorphin A(1-17) acting on kappa-opioid receptors at the supraspinal and spinal sites. In addition, the antinociception induced by EM-2 given i.c.v. contains another component, which is mediated by the release of Met-enkephalin acting on delta2-opioid receptors in the spinal cord. It is proposed that there are two subtypes of mu-opioid receptors,which are involved in EM-1- and EM-2-induced antinociception. One subtype of mu-opioid receptors is stimulated by EM-1, EM-2 and other mu-opioid agonists morphine and DAMGO; and another subtype of mu-opioid is sorely stimulated by EM-2 and is involved in the releases of dynorphin A(1-17) and Met-enkephalin for the production of antinociception.

Exploring the opioid system by gene knockout

The endogenous opioid system consists of three opioid peptide precursor genes encoding enkephalins (preproenkephalin, Penk), dynorphins (preprodynorphin, Pdyn) and beta-endorphin (betaend), proopiomelanocortin (POMC) and three receptor genes encoding mu-opiod receptor (MOR), delta-opiod receptor (DOR) and kappa-opiod receptor (KOR). In the past years, all six genes have been inactivated in mice by homologous recombination. The analysis of spontaneous behavior in mutant mice has demonstrated significant and distinct roles of each gene in modulating locomotion, pain perception and emotional behaviors. The observation of opposing phenotypes of MOR- and DOR-deficient mice in several behaviors highlights unexpected roles for DOR to be further explored genetically and using more specific delta compounds. The analysis of responses of mutant mice to exogenous opiates has definitely clarified the essential role of MOR in both morphine analgesia and addiction, and demonstrated that DOR and KOR remain promising targets for pain treatment. These studies also show that prototypic DOR agonists partially require MOR for their biological activity and provide some support for the postulated mu-delta interactions in vivo. Finally, data confirm and define a role for several genes of the opioid system in responses to other drugs of abuse, and the triple opioid receptor knockout mutant allows exploring non-classical opioid pharmacology. In summary, the study of null mutant mice has extended our previous knowledge of the opioid system by identifying the molecular players in opioid pharmacology and physiology. Future studies should involve parallel behavioral analysis of mice lacking receptors and peptides and will benefit from more sophisticated gene targeting approaches, including site-directed and anatomically-restricted mutations.

Opioid receptor genes inactivated in mice: the highlights

The opioid system controls nociception, stress responses, and addictive behaviors. Exogenous alkaloid opiates and endogenous opioid peptides stimulate mu-, delta- and kappa-opioid receptors, whose activities have long been analyzed by pharmacological tools. Mice lacking opioid receptor and opioid peptide precursor genes have now been produced by gene targeting. Behavioral analysis of mutant animals in the absence of drug has highlighted a distinct role of opioid receptors or peptides in nociception and revealed an important role for delta receptors in emotional behaviors. The examination of responses to drugs has clarified involvement of each receptor as molecular targets for exogenous opiates in vivo. Those data have also demonstrated the critical role of mu-receptor in cannabinoid and alcohol reinforcement and confirmed the involvement of kappa receptor in several dysphoric responses. Ongoing studies therefore help in understanding the molecular basis of opioid-controlled behaviors and will contribute to the development of novel therapeutics for pain, anxiety, and drug abuse.

Evidence for epsilon-opioid receptor-mediated beta-endorphin-induced analgesia

Among the opioid receptors, which have been pharmacologically classified as mu, delta, kappa and epsilon, the existence of the epsilon receptor has been controversial, and this receptor is generally not recognized as a member of the opioid peptide receptor family because it has not been precisely characterized. However, results from pharmacological, physiological and opioid receptor binding studies clearly indicate the presence of epsilon-opioid receptors, which are distinct from mu-, delta- or kappa-opioid receptors. This putative epsilon-opioid receptor is stimulated supraspinally by the endogenous opioid peptide beta-endorphin, which induces the release of Met-enkephalin, which, in turn, acts on spinal delta2-opioid receptors to produce antinociception. In this article, this beta-endorphin-sensitive epsilon-opioid receptor-mediated descending pain control system, which is distinct from that activated by the mu-opioid receptor agonist morphine, is described and the physiological role of the beta-endorphin-mediated system in pain control activated by cold-water swimming and intraplantar injection of formalin is discussed.

Synthetic beta-endorphin-like peptide immunorphin binds to non-opioid receptors for beta-endorphin on T lymphocytes

The synthetic decapeptide H-SLTCLVKGFY-OH (termed immunorphin) corresponding to the sequence 364-373 of the CH3 domain of human immunoglobulin G heavy chain was found to compete with [125I]beta-endorphin for high-affinity receptors on T lymphocytes from the blood of healthy donors (K(i) = 0.6 nM). Besides immunorphin, its synthetic fragments H-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 15 nM), H-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 8.0 nM), H-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 3.4 nM), H-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 2.2 nM), H-Leu-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 1.0 nM) possessed the ability to inhibit specific binding of [125I]beta-endorphin to T lymphocytes. Tests of the specificity of the receptors revealed that they are not sensitive to naloxone and Met-enkephalin, i.e. they are not opioid receptors. K(d) values characterizing the specific binding of 125I- labeled immunorphin and its fragment H-Val-Lys-Gly-Phe-Tyr-OH to the receptors have been determined to be 7.4 nM and 36.3 nM, respectively.

The effect of a paracetamol and morphine combination on dynorphin A levels in the rat brain

The purpose of this study was to find out whether the combination of inactive doses of paracetamol (PARA) and morphine was able to change dynorphin (DYN) A levels, evaluated by radioimmunoassay, and whether naloxone or [(-)-2-(3 furylmethyl)-normetazocine] (MR 2266), a kappa-opioid antagonist, modifies or prevents the activity of this combination on nociception and on DYN levels. The work was suggested by our previous findings which demonstrated that inactive doses of PARA and morphine, when given in combination, share an antinociceptive effect, and that PARA, at antinociceptive doses, decreases DYN levels in the frontal cortex, thus indicating a selective action within the CNS. Our present results demonstrate that the combination of inactive doses of PARA (100 mg/kg) and morphine (3 mg/kg) is just as effective in decreasing the levels of DYN A as full antinociceptive doses of PARA or morphine alone in the frontal cortex of the rat. The values, expressed in pmol/g tissue, were: control = 2.83 +/- 0.20; paracetamol (100) = 2.60 +/- 0.23; morphine (3) = 2.73 +/- 0.24; paracetamol + morphine = 1.34 + 0.16 (P < 0.05). The decrease was partially antagonised by MR 2266, but not by naloxone, suggesting that the activity of PARA and morphine in combination on DYN A levels could be mediated, at least in part, through kappa-receptors, although other systems may be involved. On the other hand, both naloxone and MR 2266 prevented the antinociceptive effect of the combination in the hot plate test. All our experimental data suggest that PARA and morphine in combination exert their antinociceptive effect through the opioidergic system, which in turn may cause a decrease in DYN levels in the CNS of the rat.

Disparate spinal and supraspinal opioid antinociceptive responses in beta-endorphin-deficient mutant mice

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter beta-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, beta-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of beta-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose-response curves were obtained in beta-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the micro-opioid receptor-selective agonists, morphine and D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of mu, delta, or kappa opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas. Thus we report that the absence of a putative endogenous ligand for the mu-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.

A biochemical model of peripheral tinnitus

Subjective tinnitus may be defined as the perceptual correlate of altered spontaneous neural activity occurring in the absence of an externally evoking auditory stimulus. Tinnitus can be caused or exacerbated by one or more of five forms of stress. We propose and provide evidence supporting a model that explains, but is not limited to, peripheral (cochlear) tinnitus. In this model, naturally occurring opioid dynorphins are released from lateral efferent axons into the synaptic region beneath the cochlear inner hair cells during stressful episodes. In the presence of dynorphins, the excitatory neurotransmitter glutamate, released by inner hair cells in response to stimuli or (spontaneously) in silence, is enhanced at cochlear N-methyl-D-aspartate (NMDA) receptors. This results in altered neural excitability and/or an altered discharge spectrum in (modiolar-oriented) type I neurons normally characterized by low rates of spontaneous discharge and relatively poor thresholds. It is also possible that chronic exposure to dynorphins leads to auditory neural excitotoxicity via the same receptor mechanism. Finally, the proposed excitatory interactions of dynorphins and glutamate at NMDA receptors need not be restricted to the auditory periphery.

Regulations of opioid dependence by opioid receptor types

Three major types of opioid receptors, designated mu, delta, and kappa, are widely expressed in the CNS. Development of selective receptor ligands and recent cloning of each receptor have contributed greatly to our increasing knowledge of the neuropharmacological profile of each opioid receptor type. It is of interest to note that they include noncompetitive and allosteric interactions among their types. This review focuses on the functional interaction among these opioid receptor types that contribute to opioid dependence. Various studies provide arguments to support substantial roles for mu-opioid receptors and the possible involvement of delta-opioid receptors in the development of physical and psychological dependence on morphine. Noradrenergic transmission originating in the locus coeruleus is most likely to play the primary causal role in the expression of physical dependence on morphine. In contrast, many studies have pointed to the mesolimbic dopaminergic pathway projecting from the ventral tegmental area to the nucleus accumbens as a critical site for the initiation of psychological dependence on opioids. It is noteworthy as the broad existence of opposing interactions between mu/delta- and kappa-receptors in the brain. The activation of kappa-receptors leads to the suppression of unpleasant mu/delta-mediated side effects such as the rewarding effect. Considering the functional interaction among opioid receptor types, the co-administration of morphine-like compounds with kappa-receptor agonists may constitute a preferable and superior approach to the treatment of pain with fewer side effects.

Endogenous opioids and reward

The discovery of endogenous opioids has markedly influenced the research on the biology of addiction and reward brain processes. Evidence has been presented that these brain substances modulate brain stimulation reward, self-administration of different drugs of abuse, sexual behaviour and social behaviour. There appears to be two different domains in which endogenous opioids, present in separate and distinct brain regions, are involved. One is related to the modulation of incentive motivational processes and the other to the performance of certain behaviours. It is concluded that endogenous opioids may play a role in the vulnerability to certain diseases, such as addiction and autism, but also when the disease is present, such as alcoholism.

G protein activation by endomorphins in the mouse periaqueductal gray matter

The midbrain periaqueductal gray matter (PAG) is an important brain region for the coordination of mu-opioid-induced pharmacological actions. The present study was designed to determine whether newly isolated mu-opioid peptide endomorphins can activate G proteins through mu-opioid receptors in the PAG by monitoring the binding to membranes of the non-hydrolyzable analog of GTP, guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS). An autoradiographic [(35)S]GTPgammaS binding study showed that both endomorphin-1 and -2 produced similar anatomical distributions of activated G proteins in the mouse midbrain region. In the mouse PAG, endomorphin-1 and -2 at concentrations from 0.001 to 10 microM increased [(35)S]GTPgammaS binding in a concentration-dependent manner and reached a maximal stimulation of 74.6+/-3.8 and 72.3+/-4.0%, respectively, at 10 microM. In contrast, the synthetic selective mu-opioid receptor agonist [D-Ala(2),NHPhe(4), Gly-ol]enkephalin (DAMGO) had a much greater efficacy and produced a 112.6+/-5.1% increase of the maximal stimulation. The receptor specificity of endomorphin-stimulated [(35)S]GTPgammaS binding was verified by coincubating membranes with endomorphins in the presence of specific mu-, delta- or kappa-opioid receptor antagonists. Coincubation with selective mu-opioid receptor antagonists beta-funaltrexamine or D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP) blocked both endomorphin-1 and-2-stimulated [(35)S]GTPgammaS binding. In contrast, neither delta- nor kappa-opioid receptor antagonist had any effect on the [(35)S]GTPgammaS binding stimulated by either endomorphin-1 or -2. These findings indicate that both endomorphin-1 and -2 increase [(35)S]GTPgammaS binding by selectively stimulating mu-opioid receptors with intrinsic activity less than that of DAMGO and suggest that these new endogenous ligands might be partial agonists for mu-opioid receptors in the mouse PAG.

Region-dependent G-protein activation by mu-, delta 1- and delta 2-opioid receptor agonists in the brain: comparison between the midbrain and forebrain

The ability of selective mu- ([D-Ala2, NHPhe4, Gly-ol]enkephalin: DAMGO), delta1- ([D-Pen2, Pen5]enkephalin: DPDPE) and delta2- ([D-Ala2]deltorphin II: DELT II) opioid receptor agonists to activate G-proteins in the midbrain and forebrain of mice and rats was examined by monitoring the binding of guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS). The levels of [35S]GTPgammaS binding stimulated by DAMGO in the mouse and rat midbrain were significantly greater than those by DPDPE or DELT II. However, relatively lower levels of stimulation of [35S]GTPgammaS binding by all of the agonists than would have been predicted from the receptor densities were observed in either the limbic forebrain or striatum of mice and rats. The effects of DAMGO, DPDPE and DELT II in all three regions were completely reversed by selective mu-, delta1- and delta2-antagonists, respectively. The results indicate that the levels of mu-, delta1- and delta2-opioid receptor agonist-induced G-protein activation in the midbrain are in good agreement with the previously determined distribution densities of each receptor type. Furthermore, the discrepancies observed in the forebrain might reflect differential catalytic efficiencies of receptor-G-protein coupling.

Identification of the G-protein-coupled ORL1 receptor in the mouse spinal cord by [35S]-GTPgammaS binding and immunohistochemistry

1 Although the ORL1 receptor is clearly located within the spinal cord, the functional signalling mechanism of the ORL1 receptor in the spinal cord has not been clearly documented. The present study was then to investigate the guanine nucleotide binding protein (G-protein) activation mediated through by the ORL1 receptor in the mouse spinal cord, measuring the modulation of guanosine-5'-o-(3-[35S]-thio) triphosphate ([35S]-GTPgammaS) binding by the putative endogenous ligand nociceptin, also referred as orphanin FQ. We also studied the anatomical distribution of nociceptin-like immunoreactivity and nociceptin-stimulated [35S]-GTPgammaS autoradiography in the spinal cord. 2 Immunohistochemical staining of mouse spinal cord sections revealed a dense plexus of nociceptin-like immunoreactive fibres in the superficial layers of the dorsal horn throughout the entire length of the spinal cord. In addition, networks of fibres were seen projecting from the lateral border of the dorsal horn to the lateral grey matter and around the central canal. 3 In vitro [35S]-GTPgammaS autoradiography showed high levels of nociceptin-stimulated [35S]-GTPgammaS binding in the superficial layers of the mouse dorsal horn and around the central canal, corresponding to the areas where nociceptin-like immunoreactive fibres were concentrated. 4 In [35S]-GTPgammaS membrane assay, nociceptin increased [35S]-GTPgammaS binding of mouse spinal cord membranes in a concentration-dependent and saturable manner, affording maximal stimulation of 64.1+/-2.4%. This effect was markedly inhibited by the specific ORL1 receptor antagonist [Phe1Psi (CH2-NH) Gly2] nociceptin (1 - 13) NH2. None of the mu-, delta-, and kappa-opioid and other G-protein-coupled receptor antagonists had a significant effect on basal or nociceptin-stimulated [35S]-GTPgammaS binding. 5 These findings suggest that nociceptin-containing fibres terminate in the superficial layers of the dorsal horn and the central canal and that nociceptin released in these areas may selectively stimulate the ORL1 receptor to activate G-protein. Furthermore, the unique pattern of G-protein activation in the present study provide additional evidence that nociceptin is distinct from the mu-, delta- or kappa-opioid system.

Behavioral pharmacology of neuropeptides related to melanocortins and the neurohypophyseal hormones

Neuropeptides are peptides which affect the nervous system. They are derived from large precursor molecules. These are converted to neurohormones, neuropeptides of the "first generation", which can be further converted to neuropeptides of the "second generation". This review is a brief survey of the nervous system effects of neuropeptides derived from pro-opiomelanocortin (POMC) and the neurohypophyseal hormones. Processing of these molecules results in neuropeptides of the first and second generation which have similar, different, more selective or even opposite effects. Among those are effects on learning and memory processes, grooming, stretching and yawning, social, sexual and rewarded behavior, aging and nerve regeneration, thermoregulation, pain, sensitivity to seizures, and cardiovascular control. Results of animal studies as well as those of clinical studies suggest that these neuropeptides may be beneficial in aging, neuropathy, memory disturbances and schizophrenia. Most of these nervous system effects in animal studies were found before receptors in the nervous system for the various neuropeptides were detected. G-protein-coupled receptors for the neuropeptides of the "first generation", i.e., melanocortin receptors, opioid receptors, and neurohypophyseal hormone receptors have been found, in contrast to the receptors for neuropeptides of the "second generation", although there are indications that G-protein coupled receptors for these may be present in the brain.

Pain management and weaning from narcotics and sedatives

Over the last 10 years, there has been a fundamental change in physicians' attitudes toward analgesia and sedation in pediatrics. In this time, basic and clinical research have provided a wealth of information. In this paper we review important advances registered in the past year, including new molecular and physiological mechanisms of antinociception and sedation, behavioral and psychoemotional implications of pain, and advances in the clinical practice of pediatric analgesia and sedation. Fortunately, the attitude of physicians toward these matters has changed significantly and much more attention is now paid to the alleviation of pain and provision of adequate sedation. However, there remains, according to most estimates, incongruity between these advances and what is practiced clinically.

Actions of NMDA and cholinergic receptor antagonists in the rostral ventromedial medulla upon beta-endorphin analgesia elicited from the ventrolateral periaqueductal gray

Analgesia elicited by morphine in the ventrolateral periaqueductal gray is mediated in part by NMDA and cholinergic receptors in the rostral ventromedial medulla because selective receptor antagonists applied to the latter structure reduced morphine analgesia elicited from the former structure. Previous studies have demonstrated that morphine and beta-endorphin employ different anatomical and neurochemical pathways in exerting their supraspinal analgesic effects. The present study evaluated whether pretreatment with either competitive (AP7, 3-10 microg) or non-competitive (MK-801, 3-10 microg) NMDA antagonists, or muscarinic (scopolamine, 5 microg) or nicotinic (mecamylamine, 1 microg) cholinergic antagonists administered into the rostral ventromedial medulla altered beta-endorphin (15 microg) analgesia elicited from the ventrolateral periaqueductal gray as measured by the tail-flick and jump tests in rats. Whereas AP7 produced minimal (11%) and transient (30 min) reductions in beta-endorphin analgesia on the jump test, MK-801 produced minimal (9%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Whereas mecamylamine failed to reduce beta-endorphin analgesia on either measure, scopolamine produced small (23%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Each of these antagonists administered into the rostral ventromedial medulla at comparable or lower doses virtually eliminated morphine analgesia elicited from the ventrolateral periaqueductal gray. The opioid mediation of beta-endorphin analgesia in the ventrolateral periaqueductal gray was confirmed by its sensitivity to naltrexone (1-20 microg) pretreatment into the same structure. These data provide further evidence for dissociations between the descending neuroanatomical and neurochemical circuitry mediating the supraspinal analgesic responses induced by morphine and beta-endorphin, and indicate that the latter response is mediated by either non-cholinergic and non-NMDA synapses within the rostral ventromedial medulla, and/or by brainstem sites outside of the rostral ventromedial medulla.

Absence of G-protein activation by mu-opioid receptor agonists in the spinal cord of mu-opioid receptor knockout mice

1. The ability of mu-opioid receptor agonists to activate G-proteins in the spinal cord of mu-opioid receptor knockout mice was examined by monitoring the binding to membranes of the non-hydrolyzable analogue of GTP, guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS). 2. In the receptor binding study, Scatchard analysis of [3H][D-Ala2,NHPhe4,Gly-ol]enkephalin ([3H]DAMGO; mu-opioid receptor ligand) binding revealed that the heterozygous mu-knockout mice displayed approximately 40% reduction in the number of mu-receptors as compared to the wild-type mice. The homozygous mu-knockout mice showed no detectable mu-binding sites. 3. The newly isolated mu-opioid peptides endomorphin-1 and -2, the synthetic selective mu-opioid receptor agonist DAMGO and the prototype of mu-opioid receptor agonist morphine each produced concentration-dependent increases in [35S]GTPgammaS binding in wild-type mice. This stimulation was reduced by 55-70% of the wild-type level in heterozygous, and virtually eliminated in homozygous knockout mice. 4. No differences in the [35S]GTPgammaS binding stimulated by specific delta1- ([D-Pen2,5]enkephalin), delta2-([D-Ala2]deltorphin II) or kappa1-(U50,488H) opioid receptor agonists were noted in mice of any of the three genotypes. 5. The data clearly indicate that mu-opioid receptor gene products play a key role in G-protein activation by endomorphins, DAMGO and morphine in the mouse spinal cord. They support the idea that mu-opioid receptor densities could be rate-limiting steps in the G-protein activation by mu-opioid receptor agonists in the spinal cord. These thus indicate a limited physiological mu-receptor reserve. Furthermore, little change in delta1-, delta2- or kappa1-opioid receptor-G-protein complex appears to accompany mu-opioid receptor gene deletions in this region.

Characterization of endomorphin-1 and -2 on [35S]GTPgammaS binding in the mouse spinal cord

In the present study, G-protein activation by newly-isolated opioid peptides, endomorphin-1 and -2, was examined in the mouse spinal cord by monitoring the binding of the non-hydrolyzable analog of GTP, guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS). Both endomorphin-1 and -2 increased [35S]GTPgammaS binding to mouse spinal cord membranes in a concentration-dependent and saturable manner and reached a maximal stimulation of 57.3+/-5.0 and 60.2+/-3.2%, respectively, at 10 microM. In contrast, the synthetic selective micro-opioid receptor agonist [D-Ala2,NHPhe4,Gly-ol]enkephalin (DAMGO) had a much greater efficacy and produced 103.4+/-5.4% of the maximal stimulation. The receptor specificity of endomorphin-stimulated [35S]GTPgammaS binding was verified by co-incubating membranes with endomorphins in the presence of specific micro-(beta-funaltrexamine and D-Phe-Cys-D-Tyr-Om-Thr-Pen-Thr-NH2 (CTOP)), delta-(naltrindole) or K-(nor-binaltorphimine) opioid receptor antagonists. Co-incubation with either beta-funaltrexamine or CTOP blocked both endomorphin-1- and-2-stimulated [35S]GTPgammaS binding in a concentration-dependent manner, whereas neither naltrindole nor nor-binaltorphimine had any effect on the [35S]GTPgammaS binding stimulated by either endomorphin-1 or -2. The data presented indicate that either endomorphin-1 or -2 activate G-proteins by specific stimulation of micro-opioid receptors, and may act as partial agonists with moderate catalytic efficacies in the mouse spinal cord.