Interaction of co-expressed mu- and delta-opioid receptors in transfected rat pituitary GH(3) cells

Predicting therapeutic and side effects from drug binding affinities to human proteome structures

Evaluation of the binding affinities of drugs to proteins is a crucial process for identifying drug pharmacological actions, but it requires three dimensional structures of proteins. Herein, we propose novel computational methods to predict the therapeutic indications and side effects of drug candidate compounds from the binding affinities to human protein structures on a proteome-wide scale. Large-scale docking simulations were performed for 7,582 drugs with 19,135 protein structures revealed by AlphaFold (including experimentally unresolved proteins), and machine learning models on the proteome-wide binding affinity score (PBAS) profiles were constructed. We demonstrated the usefulness of the method for predicting the therapeutic indications for 559 diseases and side effects for 285 toxicities. The method enabled to predict drug indications for which the related protein structures had not been experimentally determined and to successfully extract proteins eliciting the side effects. The proposed method will be useful in various applications in drug discovery.

Biased Agonism as an Emerging Strategy in the Search for Better Opioid Analgesics

Morphine and related drugs that act through activating opioid receptors are the most effective analgesics for the relief of severe pain. They have been used for decades, despite the range of unwanted side effects that they produce, as no alternative has been found so far. The major goal of opioid research is to understand the mechanism of action of opioid receptor agonists and to improve the therapeutic utility of opioid drugs. In the search for safer and more potent analgesics, analogs with mixed opioid receptor profile gained a lot of interest. However, recently the concept of biased agonism, that highlights the fact that some ligands are able to differentially activate receptor downstream pathways, became a new approach in the design of novel drug candidates for clinical application. In this review, we summarize current knowledge on the development of opioid ligands of peptide and nonpeptide structure, showing how much opioid pharmacology evolved in recent years.

In vitro and in vivo activity of cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH2, a mu opioid receptor agonist biased toward β-arrestin

Morphine and related drugs, which are the most effective analgesics for the relief of severe pain, act through activating opioid receptors. The endogenous ligands of these receptors are opioid peptides which cannot be used as antinociceptive agents due to their low bioactivity and stability in biological fluids. The major goal of opioid research is to understand the mechanism of action of opioid receptor agonists in order to improve therapeutic utility of opioids. Analgesic effects of morphine are mediated mostly through activation of the mu opioid receptor. However, in the search for safer and more effective drug candidates, analogs with mixed opioid receptor profile gained a lot of interest. Recently, the concept of biased agonists able to differentially activate GPCR downstream pathways, became a new approach in the design of novel drug candidates. It is hypothesized that compounds promoting G-protein signaling may produce analgesia while β-arrestin recruitment may be responsible for opioid side effects. In this report we showed that replacement of the tyrosine residue in the mu-selective ligand Tyr-c[d-Lys-Phe-Asp]NH₂ with 2',6'-dimethyltyrosine (Dmt) produced a cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH₂ with mu/delta opioid receptor agonist profile. This analog showed improved antinociception in the hot-plate test, probably due to the simultaneous activation of mu and delta receptors but also significantly inhibited the gastrointestinal transit. Using the bioluminescence resonance energy transfer (BRET) assay it was shown that this analog was a mu receptor agonist biased toward β-arrestin. β-Arrestin-dependent signaling is most likely responsible for the observed inhibition of gastrointestinal motility exerted by the novel cyclopeptide.

Allosterism within δ Opioid-κ Opioid Receptor Heteromers in Peripheral Sensory Neurons: Regulation of κ Opioid Agonist Efficacy

There is abundant evidence for formation of G protein-coupled receptor heteromers in heterologous expression systems, but little is known of the function of heteromers in native systems. Heteromers of δ and κ opioid receptors (DOR-KOR heteromers) have been identified in native systems. We previously reported that activation of DOR-KOR heteromers expressed by rat pain-sensing neurons (nociceptors) produces robust, peripherally mediated antinociception. Moreover, DOR agonist potency and efficacy is regulated by KOR antagonists via allosteric interactions within the DOR-KOR heteromer in a ligand-dependent manner. Here we assessed the reciprocal regulation of KOR agonist function by DOR antagonists in adult rat nociceptors in culture and in a behavioral assay of nociception. Naltrindole enhanced the potency of the KOR agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-pyrrolidin-1-ylethyl]acetamide (ICI-199441) 10- to 20-fold, but did not alter responses to 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50488). By contrast, the potency of U50488 was enhanced 20-fold by 7-benzylidenenaltrexone. The efficacy of 6'-guanidinonaltrindole (6'-GNTI) to inhibit nociceptors was blocked by small interfering RNA knockdown of DOR or KOR. Replacing 6'-GNTI occupancy of DOR with either naltrindole or 7-benzylidenenaltrexone abolished 6'-GNTI efficacy. Further, peptides derived from DOR transmembrane segment 1 fused to the cell membrane-penetrating HIV transactivator of transcription peptide also blocked 6'-GNTI-mediated responses ex vivo and in vivo, suggesting that 6'-GNTI efficacy in nociceptors is due to its positive allosteric regulation of KOR via occupancy of DOR in a DOR-KOR heteromer. Together, these results provide evidence for the existence of functional DOR-KOR heteromers in rat peripheral sensory neurons and that reciprocal, ligand-dependent allosteric interactions occur between the DOR and KOR protomers.

The design of multitarget ligands for chronic and neuropathic pain

BACKGROUND

Monodrug therapy has been used with success to fight various pathologies. When one medicine fails, co-administration of two or more drugs at the same time may be successfully applied in the treatment of infections, hypertension, HIV and in many other fields.

DISCUSSION

This approach has some weakness related to the pharmacokinetic of the two different substances administered, side effects, possible drug-drug interaction. Bivalent ligand approach would maintain the strength of the multidrug therapy (synergistic effect, lower doses, and little side effects) and overcome the weakness of a co-administration.

CONCLUSION

In this review we have described the state-of-the-art of the multitarget approach for the control of pain. Several approaches adopted by different research groups and future perspectives have been discussed.

Making structural sense of dimerization interfaces of delta opioid receptor homodimers

Opioid receptors, like other members of the G protein-coupled receptor (GPCR) family, have been shown to associate to form dimers and/or oligomers at the plasma membrane. Whether this association is stable or transient is not known. Recent compelling evidence suggests that at least some GPCRs rapidly associate and dissociate. We have recently calculated binding affinities from free energy estimates to predict transient association between mouse delta opioid receptor (DOR) protomers at a symmetric interface involving the fourth transmembrane (TM4) helix (herein termed “4” dimer). Here we present disulfide cross-linking experiments with DOR constructs with cysteines substituted at the extracellular ends of TM4 or TM5 that confirm the formation of DOR complexes involving these helices. Our results are consistent with the involvement of TM4 and/or TM5 at the DOR homodimer interface, but possibly with differing association propensities. Coarse-grained (CG) well-tempered metadynamics simulations of two different dimeric arrangements of DOR involving TM4 alone or with TM5 (herein termed “4/5” dimer) in an explicit lipid−water environment confirmed the presence of two structurally and energetically similar configurations of the 4 dimer, as previously assessed by umbrella sampling calculations, and revealed a single energetic minimum of the 4/5 dimer. Additional CG umbrella sampling simulations of the 4/5 dimer indicated that the strength of association between DOR protomers varies depending on the protein region at the interface, with the 4 dimer being more stable than the 4/5 dimer.

Purification and functional reconstitution of monomeric mu-opioid receptors: allosteric modulation of agonist binding by Gi2

Despite extensive characterization of the mu-opioid receptor (MOR), the biochemical properties of the isolated receptor remain unclear. In light of recent reports, we proposed that the monomeric form of MOR can activate G proteins and be subject to allosteric regulation. A mu-opioid receptor fused to yellow fluorescent protein (YMOR) was constructed and expressed in insect cells. YMOR binds ligands with high affinity, displays agonist-stimulated [(35)S]guanosine 5'-(gamma-thio)triphosphate binding to Galpha(i), and is allosterically regulated by coupled G(i) protein heterotrimer both in insect cell membranes and as purified protein reconstituted into a phospholipid bilayer in the form of high density lipoprotein particles. Single-particle imaging of fluorescently labeled receptor indicates that the reconstituted YMOR is monomeric. Moreover, single-molecule imaging of a Cy3-labeled agonist, [Lys(7), Cys(8)]dermorphin, illustrates a novel method for studying G protein-coupled receptor-ligand binding and suggests that one molecule of agonist binds per monomeric YMOR. Together these data support the notion that oligomerization of the mu-opioid receptor is not required for agonist and antagonist binding and that the monomeric receptor is the minimal functional unit in regard to G protein activation and strong allosteric regulation of agonist binding by G proteins.

Modulation between high- and low-frequency transcutaneous electric nerve stimulation delays the development of analgesic tolerance in arthritic rats

OBJECTIVE

To investigate whether repeated administration of modulating frequency transcutaneous electric nerve stimulation (TENS) prevents development of analgesic tolerance.

DESIGN

Knee joint inflammation (3% carrageenan and kaolin) was induced in rats. Either mixed or alternating frequency was administered daily (20min) for 2 weeks to the inflamed knee under light halothane anesthesia (1%-2%).

SETTING

Laboratory.

ANIMALS

Adult male Sprague-Dawley rats (N=36).

INTERVENTION

Mixed- (4Hz and 100Hz) or alternating- (4Hz on 1 day; 100Hz on the next day) frequency TENS at sensory intensity and 100micros pulse duration.

MAIN OUTCOME MEASURES

Paw and joint withdrawal thresholds to mechanical stimuli were assessed before induction of inflammation, and before and after daily application of TENS.

RESULTS

The reduced paw and joint withdrawal thresholds that occur 24 hours after the induction of inflammation were significantly reversed by the first administration of TENS when compared with sham treatment or to the condition before TENS treatment, which was observed through day 9. By the tenth day, repeated daily administration of either mixed- or alternating-frequency TENS did not reverse the decreased paw and joint withdrawal thresholds.

CONCLUSIONS

These data suggest that repeated administration of modulating frequency TENS leads to a development of opioid tolerance. However, this tolerance effect is delayed by approximately 5 days compared with administration of low- or high-frequency TENS independently. Clinically, we can infer that a treatment schedule of repeated daily TENS administration will result in a tolerance effect. Moreover, modulating low and high frequency TENS seems to produce a better analgesic effect and tolerance is slower to develop.

?-Opioid Receptor Ligands Lack Receptor Subtype Selectivity in the Aequorin Luminescence-based Calcium Assay

The aim of the present study was to characterize the binding selectivity of the mu-opioid receptor ligands, endomorphin-1, endomorphin-2, and DAMGO, in the in vitro functional assay, based on the changes in intracellular calcium levels. For the experiments Chinese hamster ovary cells, stably expressing human mu-receptor, were used. The mu-agonist-induced calcium responses were significantly inhibited by naloxone, an opioid antagonist with high preference for the mu-opioid receptors. Naloxonazine, a mu1-non-peptide antagonist, inhibited the effect of all tested mu-agonists. However, there was no significant difference in the antagonist effect of naloxonazine on the calcium response induced by mu1- (endomorphin-2) and mu2-agonists (endomorphin-1, DAMGO). [D-Pro2]endomorphin-1 and [D-Pro2]endomorphin-2, putative peptide mu2- and mu1-antagonists, respectively, which had been shown in vivo to inhibit the antinociception induced by mu-agonists, produced no inhibitory effect in our in vitro experiments. Our results demonstrated that there is only one population of the mu-opioid receptors expressed in the Chinese hamster ovary cells. We suggest that the mu-opioid receptors form a homogenous population in the in vitro systems. However, the existence of mu-receptor subtypes in vivo is still pharmacologically possible.

Clioquinol and docosahexaenoic acid act synergistically to kill tumor cells

Clioquinol, an 8-hydroxyquinoline derivative (5-chloro-7-iodo-8-hydroxyquinoline) with antimicrobial properties, has recently been found to have cytotoxic activity towards human cancer cell lines at concentrations achieved by oral administration. This study was initiated to determine whether clioquinol could potentiate the antitumor effects of two drugs, doxorubicin and docosahexaenoic acid (DHA), believed to act in part via the generation of reactant oxidant species. At low micromolar concentrations, clioquinol had little effect upon cell viability and did not potentiate doxorubicin's cytotoxicity. Clioquinol significantly enhanced DHA's cytotoxic effects, an interaction that was shown to be synergistic by isobolographic analysis. Clioquinol exhibited a synergistic interaction with DHA in reducing nuclear factor-kappaB activity and inducing apoptosis, and the combination reduced the level of several molecules that promote cell survival, including Akt, p65, and Bcl-2. Interestingly, clioquinol neither induced lipid peroxidation itself nor increased peroxidation brought about by the addition of DHA. However, when cells were pretreated with antioxidant vitamin E, the synergism of clioquinol and DHA was blocked, indicating the essential role of lipid peroxidation for their action. These findings reveal a novel antitumor drug combination that synergistically targets major cell survival signaling pathways.

Immunohistochemical labeling of the mu opioid receptor carboxy terminal splice variant mMOR-1B4 in the mouse central nervous system

The mu opioid receptor gene Oprm is alternatively spliced into many variants, providing for the multiplicity of mu opioid receptor subtypes. One of the mouse variants, mMOR-1B4, is unique in that it displays high affinity towards a wide range of mu opioid receptor antagonists, but poor affinity towards most classical mu opioid agonists. The present study examined the immunohistochemical distribution of the mMOR-1B4 variant in mouse brain and spinal cord. mMOR-1B4-like immunoreactivity (mMOR-1B4-LI) was enriched in many regions of the brain, spinal cord and in the dorsal root ganglia. Some of the structures showing prominent mMOR-1B4-LI include the olfactory bulb, cerebral cortex, bed nucleus of stria terminalis, hippocampus, habenular nucleus, amygdala, thalamus, hypothalamus, medium eminence, substantia nigra, ventral tegmental area, oculomotor nucleus, red nucleus, raphe nuclei, periaqueductal gray, locus coeruleus, trigeminal nucleus, reticular formation, area postrema and Purkinje cell layer and deep nuclei of cerebellum. mMOR-1B4-LI was present in afferent neurons of the dorsal root ganglia and their projecting fibers into the superficial laminae of the spinal dorsal horn. Some motor neurons in the anterior horn of the spinal cord also were immunopositive. The overall distribution of mMOR-1B4-LI in the central nervous system is distinguishable from previously characterized variants such as MOR-1-LI, MOR-1C-LI and exon-11-LI. These studies provide evidence for the region- and neuron-specific processing of the Oprm gene and support the possibility of functional differences among the variants.

Simultaneous activation of the delta opioid receptor (deltaOR)/sensory neuron-specific receptor-4 (SNSR-4) hetero-oligomer by the mixed bivalent agonist bovine adrenal medulla peptide 22 activates SNSR-4 but inhibits deltaOR signaling

Hetero-oligomerization among G protein-coupled receptors has been proposed to contribute to signal integration. Because sensory neuron-specific receptors (SNSRs) and the opioid receptors (OR) share a common ligand, the bovine adrenal medulla peptide (BAM) 22, and have opposite effects on pain modulation, we investigated the possible consequences of deltaOR/SNSR-4 hetero-oligomerization on the signaling properties of both receptor subtypes. Bioluminescence resonance energy transfer revealed that the human deltaOR has similar propensity to homo-oligomerize and to form hetero-oligomers with human SNSR-4 when coexpressed in human embryonic kidney 293 cells. The hetero-oligomerization leads to a receptor form displaying unique functional properties. Individual activation of either deltaOR or SNSR-4 in cells coexpressing the two receptors led to the modulation of their respective signaling pathways; inhibition of adenylyl cyclase and activation of phospholipase C, respectively. In contrast, the deltaOR/SNSR-4 bivalent agonist BAM22, which could activate each receptor expressed individually, fully activated the SNSR-4-dependent phospholipase C but did not promote deltaOR-mediated inhibition of adenylyl cyclase in deltaOR/SNSR-4-coexpressing cells. Likewise, concomitant activation of the deltaOR/SNSR-4 hetero-oligomer by selective deltaOR and SNSR-4 agonists promoted SNSR-4 but not deltaOR signaling, revealing an agonist-dependent dominant-negative effect of SNSR-4 on deltaOR signaling. Furthermore, the deltaOR selective antagonist naltrexone trans-inhibited the SNSR-4-promoted phospholipase C activation mediated by BAM22 but not by the SNSR-4-selective agonists, suggesting a bivalent binding mode of BAM22 to the deltaOR/SNSR-4 hetero-oligomer. The observation that BAM22 inhibited the Leu-enkephalin-promoted cAMP inhibition in rat dorsal root ganglia neurons supports the potential physiological implication of such regulatory mechanism.

Opioid ligands with mixed mu/delta opioid receptor interactions: an emerging approach to novel analgesics

Opioids are widely used in the treatment of severe pain. The clinical use of the opioids is limited by serious side effects such as respiratory depression, constipation, development of tolerance, and physical dependence and addiction liabilities. Most of the currently available opioid analgesics exert their analgesic and adverse effects primarily through the opioid mu receptors. A large number of biochemical and pharmacological studies and studies using genetically modified animals have provided convincing evidence regarding the existence of modulatory interactions between opioid mu and delta receptors. Several studies indicate that delta receptor agonists as well as delta receptor antagonists can provide beneficial modulation to the pharmacological effects of mu agonists. For example, delta agonists can enhance the analgesic potency and efficacy of mu agonists, and delta antagonists can prevent or diminish the development of tolerance and physical dependence by mu agonists. On the basis of these observations, the development of new opioid ligands possessing mixed mu agonist/delta agonist profile and mixed mu agonist/delta antagonist profile has emerged as a promising new approach to analgesic drug development. A brief overview of mu-delta interactions and recent developments in identification of ligands possessing mixed mu agonist/delta agonist and mu agonist/delta antagonist activities is provided in this report.

A role for the distal carboxyl tails in generating the novel pharmacology and G protein activation profile of mu and delta opioid receptor hetero-oligomers

Opioid receptor pharmacology in vivo has predicted a greater number of receptor subtypes than explained by the profiles of the three cloned opioid receptors, and the functional dependence of the receptors on each other shown in gene-deleted animal models remains unexplained. One mechanism for such findings is the generation of novel signaling complexes by receptor hetero-oligomerization, which we previously showed results in significantly different pharmacology for mu and delta receptor hetero-oligomers compared with the individual receptors. In the present study, we show that deltorphin-II is a fully functional agonist of the mu-delta heteromer, which induced desensitization and inhibited adenylyl cyclase through a pertussis toxin-insensitive G protein. Activation of the mu-delta receptor heteromer resulted in preferential activation of Galpha(z), illustrated by incorporation of GTPgamma(35)S, whereas activation of the individually expressed mu and delta receptors preferentially activated Galpha(i). The unique pharmacology of the mu-delta heteromer was dependent on the reciprocal involvement of the distal carboxyl tails of both receptors, so that truncation of the distal mu receptor carboxyl tail modified the delta-selective ligand-binding pocket, and truncation of the delta receptor distal carboxyl tail modified the mu-selective binding pocket. The distal carboxyl tails of both receptors also had a significant role in receptor interaction, as evidenced by the reduced ability to co-immunoprecipitate when the carboxyl tails were truncated. The interaction between mu and delta receptors occurred constitutively when the receptors were co-expressed, but did not occur when receptor expression was temporally separated, indicating that the hetero-oligomers were generated by a co-translational mechanism.

Induction of δ Opioid Receptor Function by Up-Regulation of Membrane Receptors in Mouse Primary Afferent Neurons

It is not clear whether primary afferent neurons express functional cell-surface opioid receptors. We examined delta receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after receptor up-regulation. [D-Ala2,Phe4,Gly5-ol]-enkephalin (DAMGO), [D-Ala2,D-Leu5]-enkephalin (DADLE), trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzene-acetamide methanesulfonate (U-50,488H; 1 microM), and baclofen (50 microM) inhibited Ca2+ currents, whereas the -selective ligands [D-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 microM) did not. The effect of DADLE (1 microM) was blocked by the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the -antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating mu receptors. Despite a lack of functional delta receptors, flow cytometry revealed cell-surface receptors. We used this approach to identify conditions that up-regulate receptors, including mu receptor gene deletion in dorsal root ganglion neurons of mu-/- mice and 18-h incubation of mu+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface delta receptors was up-regulated to 149 +/- 9 and 139 +/- 5%, respectively; furthermore, DPDPE and deltorphin II (1 microM) inhibited Ca2+ currents in both cases. Viral replacement of mu receptors in mu-/- neurons reduced delta receptor expression to mu+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished receptor coupling. Our observations suggest that receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which mu receptors predominate. However, up-regulation of cell-surface delta receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional delta receptor expression may reveal a novel target for analgesic therapy.

Heterodimerization of μ- and δ-Opioid Receptors Occurs at the Cell Surface Only and Requires Receptor-G Protein Interactions

Homo- and heterodimerization of the opioid receptors with functional consequences were reported previously. However, the exact nature of these putative dimers has not been identified. In current studies, the nature of the heterodimers was investigated by producing the phenotypes of the 1:1 heterodimers formed between the constitutively expressed mu-opioid receptor (MOR) and the ponasterone A-induced expression of delta-opioid receptor (DOR) in EcR293 cells. By examining the trafficking of the cell surface-located MOR and DOR, we determined that these two receptors endocytosed independently. Using cell surface expression-deficient mutants of MOR and DOR, we observed that the corresponding wild types of these receptors could not rescue the cell surface expression of the mutants, whereas the antagonist naloxone could. Furthermore, studies with constitutive or agonist-induced receptor internalization also indicated that MOR and DOR endocytosed independently and could not "drag in" the corresponding wild types or endocytosis-deficient mutants. Additionally, the heterodimer phenotypes could be eliminated by the pretreatment of the EcR293 cells with pertussis toxin and could be modulated by the deletion of the RRITR sequence in the third intracellular loop that is involved in the receptor-G protein interaction and activation. These data suggest that MOR and DOR heterodimerize only at the cell surface and that the oligomers of opioid receptors and heterotrimeric G protein are the bases for the observed MOR-DOR heterodimer phenotypes.

From inhibition to excitation: Functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.

High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.

Coexpression of delta-opioid receptors with micro receptors in GH3 cells changes the functional response to micro agonists from inhibitory to excitatory

GH3 cells show spontaneous activity characterized by bursts of action potentials and oscillations in [Ca 2+]i. This activity is modulated by the activation of exogenously expressed opioid receptors. In GH3 cells expressing only micro receptors (GH3MOR cells), the micro receptor-specific ligand [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) inhibited spontaneous Ca 2+ signaling by the inhibition of voltage-gated Ca 2+ channels, activation of inward-rectifying K+ channels, and inhibition of adenylyl cyclase. In contrast, in cells expressing both micro and delta receptors (GH3MORDOR cells), DAMGO had an excitatory effect on Ca 2+ signaling that was mediated by phospholipase C and release of Ca 2+ from intracellular stores. The excitatory effect of DAMGO was also inhibited by pretreatment with pertussis toxin. Despite the excitatory effect on Ca 2+ signaling, DAMGO inhibited Ca 2+ channels and activated inward-rectifying K+ channels in GH3MORDOR cells, although to a lesser extent than in GH3MOR cells. Long-term treatment with the delta receptor-specific ligand [D-Pen2,D-Pen5]-enkephalin reduced the excitatory effect of DAMGO in the majority of GH3MORDOR cells and restored the inhibitory response to DAMGO in some cells. The inhibitory effect of somatostatin on Ca 2+ signaling was not different in GH3MORDOR versus GH3MOR cells. These results indicate that interaction between micro- and delta-opioid receptors causes a change in the functional response to micro ligands, possibly by the formation of a micro/delta heterodimer with distinct functional properties.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Mu and Delta opioid receptors activate the same G proteins in human neuroblastoma SH-SY5Y cells

1. There is evidence for interactions between mu and delta opioid systems both in vitro and in vivo. This work examines the hypothesis that interaction between these two receptors can occur intracellularly at the level of G protein in human neuroblastoma SH-SY5Y cells. 2. The [(35)S]GTP gamma S binding assay was used to measure G protein activation following agonist occupation of opioid receptors. The agonists DAMGO (EC(50), 45 nM) and SNC80 (EC(50), 32 nM) were found to be completely selective for stimulation of [(35)S]-GTP gamma S binding through mu and delta opioid receptors respectively. Maximal stimulation of [(35)S]-GTP gamma S binding produced by SNC80 was 57% of that seen with DAMGO. When combined with a maximally effective concentration of DAMGO, SNC80 caused no additional [(35)S]-GTP gamma S binding. This effect was also seen when measured at the level of adenylyl cyclase. 3. Receptor activation increased the dissociation of pre-bound [(35)S]-GTP gamma S. In addition, the delta agonist SNC80 promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using the mu agonist DAMGO. Conversely, DAMGO promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using SNC80. 4. Tolerance to DAMGO and SNC80 in membranes from cells exposed to agonist for 18 h was homologous and there was no evidence for alteration in G protein activity. 5. The findings support the hypothesis that mu- and delta-opioid receptors share a common G protein pool, possibly through a close organization of the two receptors and G protein at the plasma membrane.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.