Molecular biology of the opioid receptors: structures, functions and distributions

Novel fluoroalkyl derivatives of selective kappa opioid receptor antagonist JDTic: Design, synthesis, pharmacology and molecular modeling studies

Novel N- and O-fluoroalkyl derivatives of the highly potent KOR antagonist JDTic were designed and synthesized. Their opioid receptor properties were compared in both in vitro binding assays and modeling approach. All compounds displayed nanomolar affinities for KOR. The fluoropropyl derivatives were more active than their fluoroethyl analogues. N-Fluoroalkylation was preferable to O-alkylation to keep a selective KOR binding. Compared to JDTic, the N-fluoropropyl derivative 2 bound to KOR with an only 4-fold lower affinity and a higher selectivity relative to MOR and DOR [Ki(κ) = 1.6 nM; Ki(μ)/Ki(κ) = 12; Ki(δ)/Ki(κ) = 159 for 2versus Ki(κ) = 0.42 nM; Ki(μ)/Ki(κ) = 9; Ki(δ)/Ki(κ) = 85 for JDTic]. Modeling studies based on the crystal structure of the JDTic/KOR complex revealed that fluorine atom in ligand 2 was involved in specific KOR binding. Ligand 2 was concluded to merit further development for KOR exploration.

New type of BACE1 siRNA delivery to cells

BACKGROUND

Small interfering RNA (siRNA) gene therapy is a new molecular approach in the search for an efficient therapy for Alzheimer disease (AD), based on the principle of RNA interference. Reducing BACE activity can have great therapeutic potential for the treatment of AD. In this study, receptor-mediated delivery was used to deliver opioid peptide-conjugated BACE 1 to INR-32 human neuroblastoma cells.

MATERIAL AND METHODS

An INR-32 human neuroblastoma cell line was stably transfected to express the APP cDNA coding fragment containing the predicted sites for cleavage by α, β, or γ-secretase. This was then treated with BACE 1 siRNA to silence BACE gene expression. BACE gene transcription and translation was determined using BACE-1 siRNA cross-linked with opioid peptide, together with RT-PCR, Western blot analysis, and ELISA.

RESULTS

Receptor-mediated delivery was used to introduce BACE1 siRNA to the APP - INR 32 human neuroblastoma cells. Decreased BACE mRNA and protein expression were observed after the cells were transfected with BACE1 siRNA.

CONCLUSIONS

Delivery of BACE1 siRNA appears to specifically reduce the cleavage of APP by inhibiting BACE1 activity.

Synthesis and κ-opioid receptor activity of furan-substituted salvinorin A analogues

The neoclerodane diterpene salvinorin A, found in the leaves of Salvia divinorum, is a potent κ-opioid receptor agonist, making it an attractive scaffold for development into a treatment for substance abuse. Although several successful semisynthetic studies have been performed to elucidate structure-activity relationships, the lack of analogues with substitutions to the furan ring of salvinorin A has prevented a thorough understanding of its role in binding to the κ-opioid receptor. Herein we report the synthesis of several salvinorin A derivatives with modified furan rings. Evaluation of these compounds in a functional assay indicated that sterically less demanding substitutions are preferred, suggesting the furan ring is bound in a congested portion of the binding pocket. The most potent of the analogues successfully reduced drug-seeking behavior in an animal model of drug-relapse without producing the sedation observed with other κ-opioid agonists.

κ-Opioid receptors are not necessary for the antidepressant treatment of neuropathic pain

BACKGROUND AND PURPOSE

Tricyclic antidepressants are used clinically as first-line treatments for neuropathic pain. Opioid receptors participate in this pain-relieving action, and preclinical studies in receptor-deficient mice have highlighted a critical role for δ-, but not μ-opioid receptors. In this study, we investigated whether κ-opioid (KOP) receptors have a role in the antiallodynic action of tricyclic antidepressants.

EXPERIMENTAL APPROACH

We used a model of neuropathic pain induced by unilateral sciatic nerve cuffing. In this model, the mechanical allodynia was evaluated using von Frey filaments. Experiments were conducted in C57BL/6J mice, and in KOP receptor-deficient mice and their wild-type littermates. The tricyclic antidepressant nortriptyline (5 mg · kg(-1)) was delivered twice a day for over 2 weeks. Agonists and antagonists of opioid receptors were used to test the selectivity of the KOP receptor antagonist norbinaltorphimine (nor-BNI) in mice with neuropathic pain.

KEY RESULTS

After 12 days of treatment, nortriptyline relieved neuropathic allodynia in both wild-type and KOP receptor-deficient mice. Surprisingly, acute nor-BNI reversed the effect of nortriptyline in both wild-type and KOP receptor-deficient mice. Further experiments showed that nor-BNI action was selective for KOP receptors at a late time-point after its administration (8 h), but not at an early time-point, when it may also interact with δ-opioid (DOP) receptors.

CONCLUSIONS AND IMPLICATIONS

KOP receptors are not necessary for the effect of a tricyclic antidepressant against neuropathic allodynia. These findings together with previous data indicate that the DOP receptor is the only opioid receptor that is necessary for the antiallodynic action of antidepressants.

Anti-nociception mediated by a κ opioid receptor agonist is blocked by a δ receptor agonist

BACKGROUND AND PURPOSE

The opioid receptor family comprises four structurally homologous but functionally distinct sub-groups, the μ (MOP), δ (DOP), κ (KOP) and nociceptin (NOP) receptors. As most opioid agonists are selective but not specific, a broad spectrum of behaviours due to activation of different opioid receptors is expected. In this study, we examine whether other opioid receptor systems influenced KOP-mediated antinociception.

EXPERIMENTAL APPROACH

We used a tail withdrawal assay in C57Bl/6 mice to assay the antinociceptive effect of systemically administered opioid agonists with varying selectivity at KOP receptors. Pharmacological and genetic approaches were used to analyse the interactions of the other opioid receptors in modulating KOP-mediated antinociception.

KEY RESULTS

Etorphine, a potent agonist at all four opioid receptors, was not anti-nociceptive in MOP knockout (KO) mice, although etorphine is an efficacious KOP receptor agonist and specific KOP receptor agonists remain analgesic in MOP KO mice. As KOP receptor agonists are aversive, we considered KOP-mediated antinociception might be a form of stress-induced analgesia that is blocked by the anxiolytic effects of DOP receptor agonists. In support of this hypothesis, pretreatment with the DOP antagonist, naltrindole (10 mg·kg(-1) ), unmasked etorphine (3 mg·kg(-1) ) antinociception in MOP KO mice. Further, in wild-type mice, KOP-mediated antinociception by systemic U50,488H (10 mg·kg(-1) ) was blocked by pretreatment with the DOP agonist SNC80 (5 mg·kg(-1) ) and diazepam (1 mg·kg(-1) ).

CONCLUSIONS AND IMPLICATIONS

Systemic DOP receptor agonists blocked systemic KOP antinociception, and these results identify DOP receptor agonists as potential agents for reversing stress-driven addictive and depressive behaviours mediated through KOP receptor activation.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

An Improved Antagonist Radiotracer for the κ-Opioid Receptor: Synthesis and Characterization of (11)C-LY2459989

The κ-opioid receptors (KORs) are implicated in several neuropsychiatric diseases and addictive disorders. PET with radioligands provides a means to image the KOR in vivo and investigate its function in health and disease. The purpose of this study was to develop the selective KOR antagonist (11)C-LY2459989 as a PET radioligand and characterize its imaging performance in nonhuman primates.

METHODS

LY2459989 was synthesized and assayed for in vitro binding to opioid receptors. Ex vivo studies in rodents were conducted to assess its potential as a tracer candidate. (11)C-LY2459989 was synthesized by reaction of its iodophenyl precursor with (11)C-cyanide, followed by partial hydrolysis of the resulting (11)C-cyanophenyl intermediate. Imaging experiments with (11)C-LY2459989 were performed in rhesus monkeys with arterial input function measurement. Imaging data were analyzed with kinetic models to derive in vivo binding parameters.

RESULTS

LY2459989 is a full antagonist with high binding affinity and selectivity for KOR (0.18, 7.68, and 91.3 nM, respectively, for κ, μ, and δ receptors). Ex vivo studies in rats indicated LY2459989 as an appropriate tracer candidate with high specific binding signals and confirmed its KOR binding selectivity in vivo. (11)C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, (11)C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, (11)C-LY2459989 displayed fast uptake kinetics in the brain and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity in the brain. Further, specific binding of (11)C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential values derived from the multilinear analysis-1 (MA1) method, as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, and 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus.

CONCLUSION

The novel PET radioligand (11)C-LY2459989 displayed favorable pharmacokinetic properties, a specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR.

Organ-specific microcirculatory mass transport of oxycodone in humans: clinical implications for therapeutic use

OBJECTIVES

To begin to address the problem of heterogeneity of distribution of oxycodone (OC) in humans, we developed an organ-specific microcirculatory capillary-tissue exchange 2-compartment model for studying regional OC mass transport.

MATERIALS AND METHODS

The model was developed in silico. It quantifies OC's organ-specific mass transport rates, clearances and recycling, and it considers the effects of blood flow on OC's convective and diffusive transport.

RESULTS

What is new is the finding that OC undergoes local recycling at the level of organ-specific capillary-tissue exchange units in humans. Results indicate recycled OC occurs in sufficient amounts to function as a reusable source of circulating OC; which has important implications for OC dosing. Results show the brain, which is central to OC effects only receives about 8% of OC delivered to all organs via the microcirculation. This suggests that differential regulation of receptor binding, trafficking, internalization, or desensitization in the brain likely plays a dominant role in OC's central analgesic effects.

DISCUSSION

Organ-specific OC mass transport kinetics provide new information for OC dosing in pain management. The model promotes patient safety in opioid prescribing because it allows predictions to be made about the relative contribution that OC recycling makes to circulating OC levels. The model indicates that pharmacologic modulation of the microcirculation may give way to site-specific delivery of opioids in the future. Our study demonstrates that translation of bench in silico research data into clinical practice, although still challenging, is feasible and can assist in OC dose regimen design for patient safety.

Pharmacogenetics of addiction therapy

Drug addiction is a serious relapsing disease that has high costs to society and to the individual addicts. Treatment of these addictions is still in its nascency, with only a few examples of successful therapies. Therapeutic response depends upon genetic, biological, social, and environmental components. A role for genetic makeup in the response to treatment has been shown for several addiction pharmacotherapies. For several addiction pharmacotherapies, response to treatment varies based on individual genetic makeup. In this chapter, we discuss the role of genetics in pharmacotherapies, specifically for cocaine, alcohol, and opioid dependences. The elucidation of the role of genetics should aid in the development of new treatments and increase the efficacy of existing treatments.

Development of functionally selective, small molecule agonists at kappa opioid receptors

The kappa opioid receptor (KOR) is widely expressed in the CNS and can serve as a means to modulate pain perception, stress responses, and affective reward states. Therefore, the KOR has become a prominent drug discovery target toward treating pain, depression, and drug addiction. Agonists at KOR can promote G protein coupling and βarrestin2 recruitment as well as multiple downstream signaling pathways, including ERK1/2 MAPK activation. It has been suggested that the physiological effects of KOR activation result from different signaling cascades, with analgesia being G protein-mediated and dysphoria being mediated through βarrestin2 recruitment. Dysphoria associated with KOR activation limits the therapeutic potential in the use of KOR agonists as analgesics; therefore, it may be beneficial to develop KOR agonists that are biased toward G protein coupling and away from βarrestin2 recruitment. Here, we describe two classes of biased KOR agonists that potently activate G protein coupling but weakly recruit βarrestin2. These potent and functionally selective small molecule compounds may prove to be useful tools for refining the therapeutic potential of KOR-directed signaling in vivo.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

The opioid placebo analgesia is mediated exclusively through μ-opioid receptor in rat

Placebo analgesia is one of the most robust and best-studied placebo effects. Recent researches suggest that placebo analgesia activated the μ-opioid receptor signalling in the human brain. However, whether other opioid receptors are involved in the placebo analgesia remains unclear. We have previously evoked placebo responses in mice (Guo et al. 2010, 2011) and these mice may serve as a model for investigating placebo analgesia. In the present study, we tried to explore the site of action and types of opioid receptors involved in placebo response. Male Sprague-Dawley rats were trained with 10 mg/kg morphine for 4 d to establish the placebo analgesia model. This placebo analgesia can be blocked by injection of 5 mg/kg dose naloxone or by microinjection with naloxone (1, 3 or 10 μg/rat) into rostral anterior cingulate cortex (rACC). Then, animals were tested after intra-rACC microinjection of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP, a selective μ-opioid receptor antagonist) or naltrindole (NTI, a highly selective δ-opioid receptor antagonist) or nor-binaltorphimine (nor-BNI, a highly selective κ-opioid receptor antagonist). Our results showed that CTOP, but not NTI or nor-BNI, could reduce the pain threshold in placebo analgesia rats. It may be concluded that rACC is the key brain region involved in placebo analgesia and the opioid placebo analgesia is mediated exclusively through μ-opioid receptor in rat.

Unraveling the central proopiomelanocortin neural circuits

Central proopiomelanocortin (POMC) neurons form a potent anorexigenic network, but our understanding of the integration of this hypothalamic circuit throughout the central nervous system (CNS) remains incomplete. POMC neurons extend projections along the rostrocaudal axis of the brain, and can signal with both POMC-derived peptides and fast amino acid neurotransmitters. Although recent experimental advances in circuit-level manipulation have been applied to POMC neurons, many pivotal questions still remain: how and where do POMC neurons integrate metabolic information? Under what conditions do POMC neurons release bioactive molecules throughout the CNS? Are GABA and glutamate or neuropeptides released from POMC neurons more crucial for modulating feeding and metabolism? Resolving the exact stoichiometry of signals evoked from POMC neurons under different metabolic conditions therefore remains an ongoing endeavor. In this review, we analyze the anatomical atlas of this network juxtaposed to the physiological signaling of POMC neurons both in vitro and in vivo. We also consider novel genetic tools to further characterize the function of the POMC circuit in vivo. Our goal is to synthesize a global view of the POMC network, and to highlight gaps that require further research to expand our knowledge on how these neurons modulate energy balance.

Synthesis and evaluation of 11C-LY2795050 as a κ-opioid receptor antagonist radiotracer for PET imaging

Kappa-opioid receptors (KOR) are believed to be involved in the pathophysiology of depression, anxiety disorders, drug abuse, and alcoholism. To date, only 1 tracer, the KOR agonist (11)C-GR103545, has been reported to be able to image KOR in primates. The goal of the present study was to synthesize the selective KOR antagonist (11)C-LY2795050 and evaluate its potential as a PET tracer to image KOR in vivo.

METHODS

The in vitro binding affinity of LY2795050 was measured in radioligand competition binding assays. Ex vivo experiments were conducted using microdosing of the unlabeled ligand in Sprague-Dawley rats and in wild-type and KOR knockout mice, to assess the ligand's potential as a tracer candidate. Imaging experiments with (11)C-LY2795050 in monkeys were performed on the Focus-220 scanner with arterial blood input function measurement. Binding parameters were determined with kinetic modeling analysis.

RESULTS

LY2795050 displays full antagonist activity and high binding affinity and selectivity for KOR. Microdosing studies in rodents and ex vivo analysis of tissue concentrations with liquid chromatography-tandem mass spectrometry identified LY2795050 as an appropriate tracer candidate able to provide specific binding signals in vivo. (11)C-LY2795050 was prepared in an average yield of 12% and greater than 99% radiochemical purity. In rhesus monkeys, (11)C-LY2795050 displayed a moderate rate of peripheral metabolism, with approximately 40% of parent compound remaining at 30 min after injection. In the brain, (11)C-LY2795050 displayed fast uptake kinetics (regional activity peak times of <20 min) and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity. Further, specific binding of (11)C-LY2795050 was reduced by the selective KOR antagonist LY2456302 in a dose-dependent manner.

CONCLUSION

(11)C-LY2795050 displayed favorable pharmacokinetic properties and binding profiles in vivo and therefore is a suitable ligand for imaging the KOR in primates. This newly developed KOR antagonist tracer has since been advanced to PET imaging of KOR in humans and constitutes the first successful KOR antagonist radiotracer.

The neurobiology of opiate motivation

Opiates are a highly addictive class of drugs that have been reported to possess both dopamine-dependent and dopamine-independent rewarding properties. The search for how, if at all, these distinct mechanisms of motivation are related is of great interest in drug addiction research. Recent electrophysiological, molecular, and behavioral work has greatly improved our understanding of this process. In particular, the signaling properties of GABA(A) receptors located on GABA neurons in the ventral tegmental area (VTA) appear to be crucial to understanding the interplay between dopamine-dependent and dopamine-independent mechanisms of opiate motivation.

14-Alkoxy- and 14-acyloxypyridomorphinans: μ agonist/δ antagonist opioid analgesics with diminished tolerance and dependence side effects

In the search for opioid ligands with mixed functional activity, a series of 5'-(4-chlorophenyl)-4,5α-epoxypyridomorphinans possessing alkoxy or acyloxy groups at C-14 was synthesized and evaluated. In this series, the affinity and functional activity of the ligands were found to be influenced by the nature of the substituent at C-14 as well as by the substituent at N-17. Whereas the incorporation of a 3-phenylpropoxy group at C-14 on N-methylpyridomorhinan gave a dual MOR agonist/DOR agonist 17h, its incorporation on N-cyclopropylmethylpyridomorphinan gave a MOR agonist/DOR antagonist 17d. Interestingly, 17d, in contrast to 17h, did not produce tolerance or dependence effects upon prolonged treatment in cells expressing MOR and DOR. Moreover, 17d displayed greatly diminished analgesic tolerance as compared to morphine upon repeated administration, thus supporting the hypothesis that ligands with MOR agonist/DOR antagonist functional activity could emerge as novel analgesics devoid of tolerance, dependence, and related side effects.

Effects of morphine on the differentiation and survival of developing pyramidal neurons during the brain growth spurt

Although morphine is frequently administered to treat procedural pain in neonates and young children, little is known about the effects of this drug on developing neural circuitry during the brain growth spurt. Here we systematically explored the impact of morphine on neuronal survival and differentiation during the peak synaptogenic period. By focusing on the rat medial prefrontal cortex, we show that single bolus ip injections of morphine, although it induces deep sedation and analgesia, do not entrain apoptosis in this cortical region either at postnatal day 7 or at postnatal day 15. Iontophoretic single cell injections of Lucifer Yellow followed by semiautomatic neuronal arbor tracing revealed that repeated daily administration of this drug between postnatal days 7 and 15 or 15 and 20 did not interfere with dendritic development of layer 5 pyramidal neurons. Confocal microscopic analysis of dendritic spines at the aforementioned distinct stages of the brain growth spurt demonstrated that neither single bolus nor repeated administration of morphine affected the density of these postsynaptic structures. Altogether, these preclinical rodent experimental observations argue against overt neurotoxic effects of morphine exposure during the brain growth spurt.

Preservation of retina ganglion cell function by morphine in a chronic ocular-hypertensive rat model

PURPOSE

The current study examined if opioid-receptor-activation by morphine can improve retinal function and retinal ganglion cell (RGC) integrity in a chronic glaucoma rat model.

METHODS

IOP was raised in Brown Norway rats by injecting hypertonic saline into the limbal venous system. Rats were treated daily with 1 mg/kg morphine for 28 days at 24-hour intervals; animals were examined for changes in IOP by a TonoLab tonometer. Pattern-ERG (PERG) was obtained in response to contrast-reversal of patterned visual stimuli. RGCs were visualized by fluorogold retrograde-labeling. Changes in the expression pattern of TNF-α and caspases were measured by Western blotting.

RESULTS

A significant IOP elevation was seen as early as 7 days, and maintained for up to 8 weeks, after surgery. PERG amplitudes were significantly reduced in ocular-hypertensive eyes (15.84±0.74 μvolts) when compared with normal eyes (19±0.86 μvolts). PERG deficits in hypertensive eyes were reversed by morphine treatment (18.23±0.78 μvolts; P<0.05). In untreated rats, a 24% reduction in labeled RGCs was measured in the hypertensive eye compared with the normal eye. This reduction in RGC labeling was significantly ameliorated in the presence of morphine. In retinal samples, TNF-α, caspase-8, and caspase-3 expressions were significantly upregulated in ocular hypertensive eyes, but completely inhibited in the morphine-treated animals.

CONCLUSIONS

These data provide evidence that activation of opioid receptors can provide significant improvement in PERG and RGC integrity against glaucomatous injury. Mechanistic data provide clues that activation of one or more opioid receptors can reduce glaucomatous-injury via suppression of TNF-α and caspase activation.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Salvinorin A has antiinflammatory and antinociceptive effects in experimental models of colitis in mice mediated by KOR and CB1 receptors

BACKGROUND

Salvinorin A (SA) has a potent inhibitory action on mouse gastrointestinal (GI) motility and ion transport, mediated primarily by kappa-opioid receptors (KOR). The aim of the present study was to characterize possible antiinflammatory and antinociceptive effects of SA in the GI tract of mice.

METHODS

Colonic damage scores and myeloperoxidase activity were determined after intraperitoneal (i.p.), intracolonic (i.c.), and oral (p.o.) administration of SA using the trinitrobenzene sulfonic acid (TNBS) and dextran sodium sulfate (DSS) models of colitis in mice. Additionally, KOR, cannabinoid (CB)1, and CB2 western blot analysis of colon samples was performed. The antinociceptive effect of SA was examined based on the number of behavioral responses to i.c. instillation of mustard oil (MO).

RESULTS

The i.p. (3 mg/kg, twice daily) and p.o. (10 mg/kg, twice daily) administration of SA significantly attenuated TNBS and DSS colitis in mice. The effect of SA was blocked by KOR antagonist nor-binaltorphimine (10 mg/kg, i.p.). Western blot analysis showed no influence of SA on KOR, CB1, or CB2 levels. SA (3 mg/kg, i.p. and 10 mg/kg, i.c.) significantly decreased the number of pain responses after i.c. instillation of MO in the vehicle- and TNBS-treated mice. The antinociceptive action of SA was blocked by KOR and CB1 antagonists. The analgesic effect of i.c. SA was more potent in TNBS-treated mice compared to controls.

CONCLUSIONS

Our results suggest that the drugs based on the structure of SA have the potential to become valuable antiinflammatory or analgesic therapeutics for the treatment of GI diseases.

Delta opioid receptors contribute to the cardiorespiratory effects of biphalin in anesthetized rats

Biphalin expresses almost equal affinity for μ- and δ-opioid receptors. The aim of this study was to delineate a possible role of δ-opioid receptors in the cardio-respiratory effects of systemic injection of biphalin in anesthetized, spontaneously breathing rats. In control animals, an intravenous bolus of biphalin (0.3 μmol/kg) evoked apnea, followed by a decreased breathing rate and increased tidal volume, hypotension and bradycardia. Blockade of δ-opioid receptors with naltrindole (4.2 μmol/kg) significantly reduced the duration of apnea, slowdown of respiration, immediate post-challenge hypotension and bradycardia induced by biphalin administration. These results indicate that the activation of δ-opioid receptors adds to the depressive response produced by biphalin.

Opioid receptor activation: suppression of ischemia/reperfusion-induced production of TNF-α in the retina

PURPOSE

The detrimental role of TNF-α in ischemia-induced tissue damage is known. The authors study examined whether opioid receptor activation alters TNF-α levels in the postischemic retina.

METHODS

Retinal ischemia was induced by raising the intraocular pressure above systolic blood pressure (155-160 mm Hg) for 45 minutes. Rats were pretreated with the opioid receptor agonist morphine (1 mg/kg; intraperitoneally) before injury. Selected animals were pretreated with the opioid antagonist naloxone (3 mg/kg; intraperitoneally). Human optic nerve head (ONH) astrocytes and rat microglial cells were treated with morphine (0.1-1 μM) for 24 hours and then treated with 10 μg/mL or 30 ng/mL lipopolysaccharide (LPS), respectively. TNF-α was measured by ELISA. Opioid receptor subtypes in astrocytes and microglia were determined by Western blot analysis.

RESULTS

There was a time-dependent increase in TNF-α production; the maximum production occurred at 4 hours after ischemia and localized to the inner retinal regions. Ischemia-induced TNF-α production was significantly inhibited by morphine. In astrocytes and microglia, LPS triggered a robust increase in the release of TNF-α, which was significantly inhibited (P < 0.05) by morphine. Naloxone reversed the morphine-induced suppression of TNF-α production in vivo and in vitro. Both ONH astrocytes and microglial cells expressed δ-, κ-, and μ-opioid receptor subtypes.

CONCLUSIONS

These data provide evidence that the production of TNF-α after ischemia/reperfusion injury is an early event and that opioid receptor activation reduces the production of TNF-α. Immunohistochemistry data and in vitro studies provide evidence that ONH astrocytes and microglial cells are the primary sources for the TNF-α production under ischemic/inflammatory conditions. Activation of one or more opioid receptors can reduce ischemic/reperfusion injury by the suppression of TNF-α production.

Morphine induces μ opioid receptor endocytosis in guinea pig enteric neurons following prolonged receptor activation

BACKGROUND & AIMS

The μ opioid receptor (μOR) undergoes rapid endocytosis after acute stimulation with opioids and most opiates, but not with morphine. We investigated whether prolonged activation of μOR affects morphine's ability to induce receptor endocytosis in enteric neurons.

METHODS

We compared the effects of morphine, a poor μOR-internalizing opiate, and (D-Ala2,MePhe4,Gly-ol5) enkephalin (DAMGO), a potent μOR-internalizing agonist, on μOR trafficking in enteric neurons and on the expression of dynamin and β-arrestin immunoreactivity in the ileum of guinea pigs rendered tolerant by chronic administration of morphine.

RESULTS

Morphine (100 μmol/L) strongly induced endocytosis of μOR in tolerant but not naive neurons (55.7% ± 9.3% vs 24.2% ± 7.3%; P < .001) whereas DAMGO (10 μmol/L) strongly induced internalization of μOR in neurons from tolerant and naive animals (63.6% ± 8.4% and 66.5% ± 3.6%). Morphine- or DAMGO-induced μOR endocytosis resulted from direct interactions between the ligand and the μOR because endocytosis was not affected by tetrodotoxin, a blocker of endogenous neurotransmitter release. Ligand-induced μOR internalization was inhibited by pretreatment with the dynamin inhibitor, dynasore. Chronic morphine administration resulted in a significant increase and translocation of dynamin immunoreactivity from the intracellular pool to the plasma membrane, but did not affect β-arrestin immunoreactivity.

CONCLUSIONS

Chronic activation of μORs increases the ability of morphine to induce μOR endocytosis in enteric neurons, which depends on the level and cellular localization of dynamin, a regulatory protein that has an important role in receptor-mediated signal transduction in cells.

Opioid-receptor gene expression and localization in cancer cells

This study examined the presence and cellular localization of three types of opioid receptors (MOR, DOR and KOR) in five human cancer cell lines: MCF-7, MDA-MB-231, HT-29, MGH-U1 and SH-SY5Y. Expression levels of opioid receptors were measured quantitatively using real-time PCR, and the localizations of the receptors in the cells were determined by immunocytochemistry. All three types of opioid receptors were present in each of the five cell lines examined. However, three of the cell lines (MCF-7, HT-29 and SH-SY5Y) showed significantly higher levels of MOR mRNA and protein than the other two types of receptors (DOR and KOR). Immunocytochemistry revealed that MOR, DOR and KOR receptors were predominantly present on the surface of cell membranes, although these receptors were also occasionally present in the cell cytoplasm.

UMB-3, a novel rabbit monoclonal antibody, for assessing μ-opioid receptor expression in mouse, rat and human formalin-fixed and paraffin-embedded tissues

BACKGROUND

The immunohistochemical localization of the μ-opioid receptor (MOR, MOP) has been studied in detail in mouse and rat brain using a variety of polyclonal antibodies. However, biochemical analysis of the MOR signaling complex in vivo has been hampered by the lack of suitable monoclonal antibodies for efficient immunoprecipitation of the receptor protein from native sources. Moreover, previous immunohistochemical investigations were restricted to frozen sections from perfusion-fixed rodent brain, largely due to the limited availability of MOR antibodies that effectively stain paraffin-embedded tissues.

METHODS

Here, we extensively characterized the novel rabbit monoclonal anti-MOR antibody UMB-3 using transfected cells and MOR-deficient mice. UMB-3 was also subjected to a comparative immunohistochemical study of formalin-fixed, paraffin-embedded mouse and rat organ samples as well as human normal and neoplastic tissues.

RESULTS

Specificity of UMB-3 was demonstrated by detection of a broad band migrating at M(r) 70,000-80,000 in immunoprecipitates from crude brain homogenates of MOR+/+ mice but not of MOR⁻/⁻ mice; cell surface staining of MOR-transfected cells; translocation of MOR receptor immunostaining after agonist exposure; distinct immunostaining of neuronal cell bodies and fibers in MOR-expressing brain regions; absence of staining in MOR-deficient mice; and abolition of tissue immunostaining by preadsorption of UMB-3 with its immunizing peptide.

CONCLUSIONS

The rabbit monoclonal antibody UMB-3 is an excellent tool for immunoprecipitation of MOR from native sources as well as for immunohistochemical staining of MOR in paraffin-embedded tissue samples of rodent and human origin.

G-protein-independent modulation of P-type calcium channels by mu-opioids in Purkinje neurons of rat

P-type calcium channels play a key role in the synaptic transmission between mammalian central neurons since a major part of calcium entering pre-synaptic terminals is delivered via these channels. Using conventional whole-cell patch clamp techniques we have studied the effect of mu-opioids on P-type calcium channels in acutely isolated Purkinje neurons from rat cerebellum. The selective mu-opioid agonist DAMGO (10nM) produced a small, but consistent facilitation of current through P-type calcium channels (10+/-1%, n=27, p<0.001). The effect of DAMGO was rapid (less than 10s) and fully reversible. This effect was both concentration and voltage-dependent. The EC(50) for the effect of DAMGO was 1.3+/-0.4nM and the saturating concentration was 100nM. The endogenous selective agonist of mu-opioid receptors, endomorphin-1 demonstrated similar action. Intracellular perfusion of Purkinje neurons with GTPgammaS (0.5mM) or GDPbetaS (0.5mM), as well as strong depolarizing pre-pulses (+50mV), did not eliminate facilitatory action of DAMGO on P-channels indicating that this effect is not mediated by G-proteins. Furthermore, the effect of DAMGO was preserved in the presence of a non-specific inhibitor of PKA and PKC (H7, 10microM) inside the cell. DAMGO-induced facilitation of P-current was almost completely abolished by the selective mu-opioid antagonist CTOP (100nM). These observations indicate that mu-type opioid receptors modulate P-type calcium channels in Purkinje neurons via G-protein-independent mechanism.

Age-dependent effects of kappa-opioid receptor stimulation on cocaine-induced stereotyped behaviors and dopamine overflow in the caudate-putamen: an in vivo microdialysis study

kappa-Opioid receptor stimulation attenuates psychostimulant-induced increases in extracellular dopamine in the caudate-putamen (CPu) and nucleus accumbens of adult rats, while reducing cocaine-induced locomotor activity and stereotyped behaviors. Because kappa-opioid receptor agonists (e.g., U50,488 or U69,593) often affect the behavior of preweanling rats in a paradoxical manner, the purpose of the present study was to determine whether kappa-opioid receptor stimulation differentially affects dopaminergic functioning in the CPu depending on age. In vivo microdialysis was used to determine whether U50,488 (5 mg/kg) attenuates cocaine-induced dopamine overflow in the dorsal CPu on postnatal day (PD) 17 and PD 85. In the microinjection experiment, cocaine-induced stereotyped behaviors were assessed in adult and preweanling rats after bilateral infusions of vehicle or U50,488 (1.6 or 6.4 microg per side) into the CPu. Results showed that U50,488 attenuated the cocaine-induced increases in CPu dopamine overflow on PD 85, while the same dose of U50,488 did not alter dopamine dialysate levels on PD 17. Cocaine also increased stereotyped behaviors (repetitive motor movements, behavioral intensity scores, and discrete behaviors) at both ages, but adult rats appeared to exhibit more intense stereotypic responses than the younger animals. Consistent with the microdialysis findings, bilateral infusions of U50,488 into the dorsal CPu decreased the cocaine-induced stereotypies of adult rats, while leaving the behaviors of preweanling rats unaffected. These results suggest that the neural mechanisms underlying kappa-opioid/dopamine interactions in the CPu are not fully mature during the preweanling period. This lack of functional maturity may explain why kappa-opioid receptor agonists frequently induce different behavioral effects in young and adult rats.

Mu and kappa opioid receptor expression in the mediobasal hypothalamus and effectiveness of selective antagonists on prolactin release during lactation

Endogenous opioid peptides are involved in prolactin release during lactation, in part by decreasing tuberoinfundibular dopaminergic (TIDA) neuronal activity. Both mu (mu) and kappa (kappa) opioid receptors have a role in the suckling-induced prolactin rise after 4-5 h up deprivation. The aim of this study was to investigate effects of mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), and kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI), on prolactin secretion and TIDA neuronal activity in lactating rats after 18 h pup deprivation. After 4 h separation from pups, the suckling-induced prolactin rise was abolished by 16 microg nor-BNI and 5 microg beta-FNA, coincident with increased dihydroxyphenylacetic acid (DOPAC):dopamine ratio in the stalk-median eminence (SME). However, after 18 h pups separation, these same doses of nor-BNI and beta-FNA did not alter the prolactin surge or DOPAC:dopamine ratios in the SME. Higher doses of nor-BNI (32 microg) and beta-FNA (10 microg) were required to inhibit suckling-induced prolactin secretion. beta-FNA (10 microg) increased the DOPAC:dopamine ratio in the SME, whereas nor-BNI (32 microg) treatment had no effect. The mu and kappa opioid receptor mRNA levels in the mediobasal hypothalamus were similar to suckled control rats after 4 h pup deprivation, but increased 1.4-fold after 18 h pup deprivation. These data support involvement of endogenous opioidergic systems in the suckling-induced prolactin rise after a prolonged (18 h) period of pup deprivation, as well as the shorter (4 h) pup deprivation period previously reported. Suppression of TIDA neuronal activity likely played a part in mu opioid receptor input to the suckling-induced prolactin rise after both 4 h and 18 h separation, whereas non-dopaminergic input was implicated with kappa opioid receptors after 18 h pup deprivation. Increased mu and kappa opioid receptors gene expression in the mediobasal hypothalamus may contribute to reduced effectiveness of opioid receptor antagonists to block suckling-induced prolactin release after 18 h pup deprivation.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

High-level production and characterization of a G-protein coupled receptor signaling complex

Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.

14-O-Heterocyclic-substituted naltrexone derivatives as non-peptide mu opioid receptor selective antagonists: design, synthesis, and biological studies

Mu opioid receptor antagonists have clinical utility and are important research tools. To develop non-peptide and highly selective mu opioid receptor antagonist, a series of 14-O-heterocyclic-substituted naltrexone derivatives were designed, synthesized, and evaluated. These compounds showed subnanomolar-to-nanomolar binding affinity for the mu opioid receptor. Among them, compound 1 exhibited the highest selectivity for the mu opioid receptor over the delta and kappa receptors. These results implicated an alternative 'address' domain in the extracellular loops of the mu opioid receptor.

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.

Opioid-induced regulation of µ-opioid receptor gene expression in the MCF-7 breast cancer cell line

The aim of the study was to investigate the presence of opioid receptor types in human breast adenocarcinoma MCF-7 cells and to characterize the changes in MOR expression induced by opioid agonist and antagonist treatment. We have shown that all three types of opioid receptors, but predominantly MOR, are expressed in MCF-7 cells. Selective MOR agonists, morphine, endomorphin-1, and endomorphin-2 downregulated MOR mRNA levels in a concentration- and time-dependent manner, but the effect produced by endomorphins was much stronger. Downregulation was blocked by the opioid antagonist naloxone. Naloxone alone produced a slight increase in MOR gene expression. Immunoblotting with antiserum against MOR-1 confirmed these results at the protein level. The results of our study indicate that, in MCF-7 cells, MOR gene expression is downregulated by opioid agonists and upregulated by opioid antagonists. We propose that the opioid-induced regulation of MOR mRNA expression is mediated by reduced binding of the transcription factors NFκB and AP-1 to the promoter region on the MOR gene.