Coexistence of kappa- and delta-opioid receptors in rat spinal cord axons

Antibodies to probe endogenous G protein-coupled receptor heteromer expression, regulation, and function

Over the last decade an increasing number of studies have focused on the ability of G protein-coupled receptors to form heteromers and explored how receptor heteromerization modulates the binding, signaling and trafficking properties of individual receptors. Most of these studies were carried out in heterologous cells expressing epitope tagged receptors. Very little information is available about the in vivo physiological role of G protein-coupled receptor heteromers due to a lack of tools to detect their presence in endogenous tissue. Recent advances such as the generation of mouse models expressing fluorescently labeled receptors, of TAT based peptides that can disrupt a given heteromer pair, or of heteromer-selective antibodies that recognize the heteromer in endogenous tissue have begun to elucidate the physiological and pathological roles of receptor heteromers. In this review we have focused on heteromer-selective antibodies and describe how a subtractive immunization strategy can be successfully used to generate antibodies that selectively recognize a desired heteromer pair. We also describe the uses of these antibodies to detect the presence of heteromers, to study their properties in endogenous tissues, and to monitor changes in heteromer levels under pathological conditions. Together, these findings suggest that G protein-coupled receptor heteromers represent unique targets for the development of drugs with reduced side-effects.

Nociceptive-induced myocardial remote conditioning is mediated by neuronal gamma protein kinase C

Deciphering the remote conditioning molecular mechanism may provide targets to develop therapeutics that can broaden the clinical application. To further investigate this, we tested whether two protein kinase C (PKC) isozymes, the ubiquitously expressed epsilon PKC (εPKC) and the neuronal-specific gamma PKC (γPKC), mediate nociceptive-induced remote myocardial conditioning. Male Sprague-Dawley rats were used for both in vivo and ex vivo myocardial ischemia-reperfusion protocols. For the in vivo studies, using a surgical abdominal incision for comparison, applying only to the abdomen either bradykinin or the εPKC activator (ψεRACK) reduced myocardial infarct size (45 ± 1, 44 ± 2 %, respectively, vs. incision: 43 ± 2 %, and control: 63 ± 2 %, P < 0.001). Western blot showed only εPKC, and not γPKC, is highly expressed in the myocardium. However, applying a selective γPKC inhibitor (γV5-3) to the abdominal skin blocked remote protection by any of these strategies. Using an ex vivo isolated heart model without an intact nervous system, only selective εPKC activation, unlike a selective classical PKC isozyme activator (activating α, β, βII, and γ), reduced myocardial injury. Importantly, the classical PKC isozyme activator given to the abdomen in vivo (with an intact nervous system including γPKC) during myocardial ischemia reduced infarct size as effectively as an abdominal incision or ψεRACK (45 ± 1 vs. 45 ± 2 and 47 ± 1 %, respectively). The classical PKC activator-induced protection was also blocked by spinal cord surgical transection. These findings identified potential remote conditioning mimetics, with these strategies effective even during myocardial ischemia. A novel mechanism of nociceptive-induced remote conditioning, involving γPKC, was also identified.

Twin and triplet drugs in opioid research

Twin and triplet drugs are defined as compounds that contain respectively two and three pharmacophore components exerting pharmacological effects in a molecule. The twin drug bearing the same pharmacophores is a "symmetrical twin drug", whereas that possessing different pharmacophores is a "nonsymmetrical twin drug." In general, the symmetrical twin drug is expected to produce more potent and/or selective pharmacological effects, whereas the nonsymmetrical twin drug is anticipated to show both pharmacological activities stemming from the individual pharmacophores (dual action). On the other hand, nonsymmetrical triplet drugs, which have two of the same pharmacophores and one different moiety, are expected to elicit both increased pharmacological action and dual action. The two identical portions could bind the same receptor sites simultaneously while the third portion could bind a different receptor site or enzyme. This review will mainly focus on the twin and triplet drugs with an evaluation of their in vivo pharmacological effects, and will also include a description of their pharmacology and synthesis.

N-naphthoyl-beta-naltrexamine (NNTA), a highly selective and potent activator of μ/kappa-opioid heteromers

Numerous G protein-coupled receptors (GPCRs) have been shown to form heteromeric receptors in cell-based assays. Among the many heteromers reported in the opioid receptor family are μ/κ, κ/δ, and μ/δ. However, the in vivo physiological and behavioral relevance for the proposed heteromers have not yet been established. Here we report a unique example of a ligand, N-naphthoyl-β-naltrexamine (NNTA) that selectively activates heteromeric μ/κ-opioid receptors in HEK-293 cells and induces potent antinociception in mice. NNTA was an exceptionally potent agonist in cells expressing μ/κ-opioid receptors. Intriguingly, it was found to be a potent antagonist in cells expressing only μ-receptors. In the mouse tail-flick assay, intrathecal (i.t.) NNTA produced antinociception that was ~100-fold greater than by intracerebroventricular (i.c.v.) administration. The κ-antagonist, norBNI, decreased the i.t. potency, and the activity was virtually abolished in μ-opioid receptor knockout mice. No tolerance was induced i.t., but marginal tolerance (3-fold) was observed via the i.c.v. route. Moreover, NNTA produced neither significant physical dependence nor place preference in the ED50 dose range. Taken together, this work provides an important pharmacologic tool for investigating the in vivo functional relevance of heteromeric μ/κ-opioid receptors and suggests an approach to potent analgesics with fewer deleterious side effects.

A κ Opioid Pharmacophore Becomes a Spinally Selective κ-δ Agonist When Modified with a Basic Extender Arm

We have explored the concept of a molecular extender arm attached to a κ opioid agonist pharmacophore 3 (ICI-199,441) in an effort to potentially interact with a complementary group on a neighboring opioid receptor. The molecular arm containing a terminal amine group was lengthened incrementally from 11 up to 18 atoms. Increasing the number of atoms in the arm produced virtually no change in the mouse intracerebroventricular (i.c.v.) antinociceptive potency. In contrast, the intrathecal (i.t.) potency of 6 (KDA-16) with a 16-atom arm was dramatically increased, as reflected by its antinociceptive i.c.v./i.t. ED50 ratio of ∼130. Further lengthening led to a decreased ED50 ratio. In vivo selective antagonist studies of KDA-16 revealed that κ and δ opioid receptors were responsible for the greatly enhanced i.t. potency. Calcium release experiments in HEK-293 cells suggested that KDA-16 selectively activate κ-δ heteromers. These data are consistent with the reported possible presence of heteromeric κ-δ opioid receptors in mouse spinal cord but not in the brain. The use of a molecular extender arm may be useful for developing spinally selective analgesics.

G protein-coupled receptor dimers: functional consequences, disease states and drug targets

With an ever-expanding need for reliable therapeutic agents that are highly effective and exhibit minimal deleterious side effects, a greater understanding of the mechanisms underlying G protein-coupled receptor (GPCR) regulation is fundamental. GPCRs comprise more than 30% of all therapeutic drug targets and it is likely that this will only increase as more orphan GPCRs are identified. The past decade has seen a dramatic shift in the prevailing concept of how GPCRs function, in particular the growing acceptance that GPCRs are capable of interacting with one another at a molecular level to form complexes, with significantly different pharmacological properties to their monomeric selves. While the ability of like-receptors to associate and form homodimers raises some interesting mechanistic issues, the possibility that unlike-receptors could heterodimerise in certain tissue types, producing a functionally unique signalling complex that binds specific ligands, provides an invaluable opportunity to refine and redefine pharmacological interventions with greater specificity and efficacy.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Effects of mu- and kappa-2 opioid receptor agonists on pain and rearing behaviors

BACKGROUND

Management of pain involves a balance between inhibition of pain and minimization of side effects; therefore, in developing new analgesic compounds, one must consider the effects of treatment on both pain processing and behavior. The purpose of this study was to evaluate the effects of the mu and kappa-2 opioid receptor agonists on general and pain behavioral outcomes.

METHODS

As a general behavioral assessment, we modified the cylinder rearing assay and recorded the number and duration of rearing events. Thermal sensitivity was evaluated using either a reflexive measure of hindpaw withdrawal latency to a radiant heat source or using an orofacial operant thermal assay. Acetic acid-induced visceral pain and capsaicin-induced neurogenic inflammatory pain were used as painful stimuli. The mu-opioid receptor agonist, morphine or the kappa-2 receptor agonist GR89696 was administered 30 min prior to testing. A general linear model repeated measures analysis was completed for baseline session comparisons and an analysis of variance was used to evaluate the effects of treatment on each outcome measure (SPSS Inc). When significant differences were found, post-hoc comparisons were made using the Tukey honestly significant difference test. *P < 0.05 was considered significant in all instances.

RESULTS

We found that morphine and GR89,696 dose-dependently decreased the number of reaching events and rearing duration. Rearing behavior was not affected at 0.5 mg/kg for morphine, 1.25 x 10-4 mg/kg for GR89,696. Hindpaw thermal sensitivity was significantly increased only at the highest doses for each drug. At the highest dose that did not significantly influence rearing behavior, we found that visceral and neurogenic inflammatory pain was not affected following GR89,696 administration and morphine was only partially effective for blocking visceral pain.

CONCLUSION

This study demonstrated that high levels of the opioids produced significant untoward effects and made distinguishing an analgesic versus a more general effect more difficult. Quantification of rearing behavior in conjunction with standard analgesic assays can help in gaining a better appreciation of true analgesic efficacy of experimental drugs.

Comparison of opioid receptor distributions in the rat central nervous system

The opioid receptors, mu, delta and kappa, conduct the major pharmacological effects of opioid drugs, and exhibit intriguing functional relationships and interactions in the CNS. Previously established hypotheses regarding the mechanisms underlying these phenomena specify theoretical patterns of relative cellular localisation for the different receptor types. In this study, we have used double-label immunohistochemistry to compare the cellular distributions of delta and kappa receptors with those of mu receptors in the rat CNS. Regions of established significance in opioid addiction were examined. Extensive mu/delta co-localisation was observed in neuron-like cells in several regions. mu and kappa receptors were also often co-localised in neuron-like cell bodies in several regions. However, intense kappa immunoreactivity (ir) also appeared in a separate, morphologically distinct population of cells that did not express mu receptors. These small, ovoid cells were often closely apposed against the larger, mu-ir cell bodies. Such cellular appositions were seen in several regions, but were particularly common in the medial thalamus, the periaqueductal grey and brainstem regions. These findings support proposals that functional similarities, synergy and cooperativity between mu and delta receptors arise from widespread co-expression by cells and intracellular molecular interactions. Although co-expression of mu and kappa receptors was also detected, the appearance of a separate population of kappa-expressing cells supports proposals that the contrasting and functionally antagonistic properties of mu and kappa receptors are due to expression in physiologically distinct cell types. Greater understanding of opioid receptor interaction mechanisms may provide possibilities for therapeutic intervention in opioid addiction and other conditions.

Role of delta-opioid receptor subtypes in anxiety-related behaviors in the elevated plus-maze in rats

RATIONALE

Recent studies have shown that endogenous opioid systems are associated with the regulation of emotional responses. In particular, it has been reported that delta-opioid receptors act naturally to inhibit stress and anxiety.

OBJECTIVE

The present study was designed to examine the possible involvement of opioid delta-receptor subtypes in the anxiety-related behavior in the elevated-plus-maze test.

METHODS

Six-week-old male Lewis rats were used. The total numbers of visits to the closed and open arms and the cumulative time spent and visits in the open arms were determined. Plasma corticosterone levels were measured by enzyme immunoassay.

RESULTS

Naltrindole (NTI), a delta-opioid receptor antagonist (3 mg/kg s.c.), induced a significant decrease in the percentages of time spent and visits in the open arms. Naltriben (NTB), a delta2-opioid receptor antagonist (3 mg/kg s.c.), but not 7-benzylidenenaltrexone, a delta1-opioid receptor antagonist, produced similar anxiety-related behaviors in the elevated plus-maze. These effects of NTI and NTB were antagonized by pretreatment with (+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), a delta-opioid receptor agonist. Furthermore, after exposure to the elevated plus-maze, the maximal increase in the plasma corticosterone level in NTI-treated rats was clearly higher than that in vehicle-treated rats. However, when NTI and SNC80 were coadministered, higher levels of plasma corticosterone were not seen after exposure to the elevated plus-maze.

CONCLUSION

These results suggest that endogenous delta2-opioid-receptor-mediated systems are involved in the regulation of anxiety-related behaviors and might play a physiologically important role in the regulation of adrenocortical activity.

Interaction of Bivalent Ligand KDN21 with Heterodimeric δ-κ Opioid Receptors in Human Embryonic Kidney 293 Cells

KDN21 is a bivalent ligand that contains delta and kappa opioid antagonist pharmacophores linked through a 21-atom spacer. It has been reported that KDN21 bridges delta and kappa receptors that are organized as heterodimers. We have shown previously that when using [(3)H]diprenorphine as radioligand, KDN21 displayed greatly enhanced affinity in this series for coexpressed delta and kappa opioid receptors (CDK). The present study used in vitro expression systems to investigate interactions of members of the KDN series with delta-kappa heterodimers through competition binding using selective ligands and the mitogen-activated protein kinase (MAPK) assay. In this regard, the use of the selective radioligands [(3)H]naltrindole and [(3)H]norbinaltorphimine (nor-BNI) in competition binding studies revealed that KDN21 has much higher affinity than other KDN members for CDK and bound to CDK more selectively relative to mixed delta and kappa opioid receptors or singly expressed delta and kappa opioid receptors. Other experiments revealed that the binding of naltrindole to delta opioid receptors could increase the binding of nor-BNI to kappa opioid receptors and vice versa, suggesting reciprocal allosteric modulation of receptors in the heterodimer. Regarding the selectivity of KDN21 for phenotypic delta and kappa opioid receptors, we investigated the effect of KDN21 on the activation of MAPKs [extracellular signal-regulated kinases 1 and 2 (ERK1/2)] by delta- or kappa-selective agonists. KDN21 inhibited the activation of ERK1/2 by [D-Pen(2),D-Pen(5)]-enkephalin (delta(1)) and bremazocine (kappa(2)) but had no effect on the activation by deltorphin II (delta(2)) and (+)-(5alpha,7alpha,8beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]benzeneacetamide (U69593, kappa(1)). 7-Benzylidenenaltrexone (delta(1)) and bremazocine (kappa(2)) significantly reduced the binding of KDN21 to CDK, whereas naltriben (delta(2)) and U69593 produced no such change. Taken together, these data support the idea that the organization of delta and kappa receptors as heterodimers gives rise to delta(1) and kappa(2) phenotypes.

A heterodimer-selective agonist shows in vivo relevance of G protein-coupled receptor dimers

There has been much speculation regarding the functional relevance of G protein-coupled receptor heterodimers, primarily because demonstrating their existence in vivo has proven to be a considerable challenge. Here we show that the opioid agonist ligand 6'-guanidinonaltrindole (6'-GNTI) has the unique property of selectively activating only opioid receptor heterodimers but not homomers. Importantly, 6'-GNTI is an analgesic, thereby demonstrating that opioid receptor heterodimers are indeed functionally relevant in vivo. However, 6'-GNTI induces analgesia only when it is administered in the spinal cord but not in the brain, suggesting that the organization of heterodimers is tissue-specific. This study demonstrates a proof of concept for tissue-selective drug targeting based on G protein-coupled receptor heterodimerization. Importantly, targeting opioid heterodimers could provide an approach toward the design of analgesic drugs with reduced side effects.

A bivalent ligand (KDN-21) reveals spinal delta and kappa opioid receptors are organized as heterodimers that give rise to delta(1) and kappa(2) phenotypes. Selective targeting of delta-kappa heterodimers

In view of recent pharmacological studies suggesting the existence of delta-kappa opioid receptor heterodimers/oligomers in the spinal cord, we have synthesized and evaluated (intrathecally in mice) a series of bivalent ligands (KDN series) containing kappa and delta antagonist pharmacophores. Pharmacological and binding data have provided evidence for the bridging of spinal delta-kappa receptor heterodimers by KDN-21 and for their identification as delta(1) and kappa(2). The selectivity profile of KDN-21 and the apparent absence of coupled delta(1)-kappa(2) phenotypes in the brain suggest a new approach for targeting receptors.

Characterization of Specific Opioid Binding Sites in Neural Membranes from the Myenteric Plexus of Porcine Small Intestine

Delta- and kappa-opioid receptors (OPRs), but not micro-OPRs, are expressed in the myenteric plexus of the porcine distal small intestine. In a subpopulation of myenteric neurons, delta- and kappa-OPRs seem to be colocalized and may functionally interact. In this study, radioligand binding was used to characterize myenteric OPR populations in detail. The nonselective OPR antagonist [3H]diprenorphine bound to a single, high-affinity site in myenteric neural membrane homogenates. Naloxone displaced 65 and 59% of [3H]diprenorphine binding from this site in Na(+)-free Tris and Krebs-HEPES buffers, respectively. Naltrexone-derived delta- and kappa-OPR antagonists, including naltriben, 7-benzylidenenaltrexone, nor-binaltorphimine, and 5'-guanidinonaltrindole, displaced [3H]diprenorphine from two distinct binding sites to levels similar to that of naloxone. The selective delta-OPR ligands Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (TIPP), [D-Pen2,D-Pen5]enkephalin (DPDPE), [D-Ala2, Glu4]deltorphin II, and (+)-4-[(alphaR)-alpha((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl-3-methoxybenzyl)-N,N-diethylbenzamide (SNC-80) and the kappa-OPR agonist (D-(5alpha,7alpha,8beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxoaspiro-(4,5)dec-8-yl) benzeneacetamide (U-69,593) displaced [3H]diprenorphine from three independent binding sites; these included high-affinity delta- and kappa-OPR sites, and a residual binding site. Residual [3H]diprenorphine binding was displaced by the selective kappa-OPR antagonist nor-binaltorphimine after saturation of delta and kappa sites, respectively, with DPDPE and U-69,593. The residual binding site displayed low affinity for delta- and kappa-OPR agonists and TIPP, as well as moderate affinity for naltrexone-derived ligands, properties reminiscent of delta-/kappa-OPR heterodimers.

Identity of the putative delta1-opioid receptor as a delta-kappa heteromer in the mouse spinal cord

In view of the co-localization of spinal delta- and kappa-opioid receptors, we have investigated the interaction of selective opioid receptor agonists and antagonists in the spinal cord of mice in order to determine if these receptors are organized as heteromers. The finding that norbinaltorphimine (kappa) antagonized [D-Pen(2,5)]enkephalin (delta(1)), but not deltorphin II (delta(2)), strongly suggests that the putative delta(1)-subtype is a delta-kappa heteromer. Studies with selective opioid receptor (ant)agonists support this conclusion.

Analgesia from a peripherally active kappa-opioid receptor agonist in patients with chronic pancreatitis

Preclinical studies suggest that visceral afferents constitutively express kappa-opioid receptors (KORs) and that noxious visceral stimuli can be inhibited at a peripheral site by KOR activation. To test the relevance of these observations to humans, we infused, in a randomized, double blind manner, a peripherally selective KOR agonist (ADL 10-0101) or placebo into six patients with chronic pancreatitis and ongoing abdominal pain despite mu-opioid agonist therapy. Pain was assessed using a pain magnitude estimate, an open ended scale of each patient's choosing and compared to their rating of pain from a 1.6 cm(2) thermode applied to the skin and heated to 49 degrees C for 5s. Normalizing pain scores to this rating as 100, pain prior to study drug treatment was 4070, and was unaffected by placebo infusion in the two individuals receiving this therapy. In contrast, ADL 10-0101 infusion reduced pain score from 63+/-7.6 (mean+/-SE) prior to infusion to 23+/-15 4h after infusion (P<0.05 vs. baseline). One patient receiving placebo and one receiving ADL 10-0101 experienced a mild headache during the study. One patient receiving ADL 10-0101 experienced restlessness and another had assymptomatic transient dysrhythmia upon standing after the 4h study. Neither of the treatments affected blood pressure, heart rate, respiratory rate, or oxyhemoglobin saturation, and no patient experienced nausea during the study. These limited data support the hypothesis that human visceral afferents express KOR and that peripherally restricted KOR agonists produce analgesia in patients with chronic visceral pain.