Opioid receptor signalling mechanisms

Sympathetic denervation-induced changes in G protein expression in enteric neurons of the guinea pig colon

Chronic sympathetic denervation entails subsensitivity to alpha(2)-adrenoceptor agonists and supersensitivity to kappa- and mu-opioid receptor agonists modulating cholinergic neurons in the guinea pig colon. A possible role for signal transduction G proteins in contributing to development of these sensitivity changes was investigated. Pertussis toxin (PTX), a blocker of the G(i/o)-type family of G proteins significantly reduced the inhibitory effects of UK14,304 (alpha(2)-adrenoceptor agonist), U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on acetylcholine (ACh) overflow in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. In this experimental condition, immunoblot analysis revealed reduced levels of G(alphao), G(alphai2), G(alphai3) and G(beta) in myenteric plexus synaptosomes. On reverse, synaptosomal levels of G(alphai1) and G(alphaz), a PTX-insensitive G-protein, increased after chronic ablation of the sympathetic pathways. These data suggest that changes in the function and expression of inhibitory G proteins coupled to alpha(2)-adrenoceptors, kappa- and mu-opioid receptors occur in the myenteric plexus of the guinea pig colon after chronic sympathetic denervation. The possibility that regulation of G proteins represents one of the biochemical mechanisms at the basis of the changes in sensitivity of enteric cholinergic neurons to alpha(2)-adrenoceptor, kappa- and mu-opioid receptor agonists is discussed.

Chronic morphine-induced changes in mu-opioid receptors and G proteins of different subcellular loci in rat brain

Prolonged exposure to opioid agonists can induce adaptive changes resulting in tolerance and dependence. Here, rats were rendered tolerant by subcutaneous injections of increasing doses of morphine from 10 to 60 mg/kg for 3, 5, or 10 consecutive days. Binding parameters of the mu-opioid receptor in subcellular fractions were measured with [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin). Although the density of surface mu-sites did not change after the 5-day morphine treatment, up-regulation of synaptic plasma membrane binding was detected after the 10-day drug administration. In contrast, the number of mu-binding sites in a light vesicle or microsomal fraction (MI) was elevated by 68 and 30% after 5 and 10 days of morphine exposure, respectively. The up-regulated MI mu-sites displayed enhanced coupling to G proteins compared with those detected in saline-treated controls. Pertussis toxin catalyzed ADP ribosylation, and Western blotting with specific antisera was used to quantitate chronic morphine-induced changes in levels of various G protein alpha-subunits. Morphine treatment of 5 days and longer induced significant increases in levels of Galpha(o), Galpha(i1), and Galpha(i2) in MI fractions that are part of an adaptation process. Up-regulation of intracellular mu-sites may be the result of post-translational changes and in part de novo synthesis. The results provide the first evidence that distinct regulation of intracellular mu-opioid receptor G protein coupling and G protein levels may accompany the development of morphine tolerance.

Homo- and hetero-oligomeric interactions between G-protein-coupled receptors in living cells monitored by two variants of bioluminescence resonance energy transfer (BRET): hetero-oligomers between receptor subtypes form more efficiently than between less closely related sequences

Homo- and hetero-oligomerization of G-protein-coupled receptors (GPCRs) were examined in HEK-293 cells using two variants of bioluminescence resonance energy transfer (BRET). BRET(2) (a variant of BRET) offers greatly improved separation of the emission spectra of the donor and acceptor moieties compared with traditional BRET. Previously recorded homo-oligomerization of the human delta-opioid receptor was confirmed using BRET(2). Homo-oligomerization of the kappa-opioid receptor was observed using both BRET techniques. Both homo- and hetero-oligomers, containing both delta- and kappa-opioid receptors, were unaffected by the presence of receptor ligands. BRET detection of opioid receptor homo- and hetero-oligomers required expression of 50,000-100,000 copies of the receptor energy acceptor construct per cell. The effectiveness of delta-kappa-opioid receptor hetero-oligomer formation was as great as for homomeric interactions. The capacity of the two opioid receptors to form oligomeric complexes with the beta(2)-adrenoceptor was also assessed. Although such interactions were detected, at least 250,000 copies per cell of the energy acceptor were required. Requirement for high levels of receptor expression was equally pronounced in attempts to measure hetero-oligomer formation between the kappa-opioid receptor and the thyrotropin-releasing hormone receptor-1. These studies indicate that constitutively formed homo- and hetero-oligomers of opioid receptor subtypes can be detected in living cells containing less than 100,000 copies of the receptors. However, although hetero-oligomeric interactions between certain less closely related GPCRs can be detected, they appear to be of lower affinity than homo- or hetero-oligomers containing closely related sequences. Interactions recorded between certain GPCR family members in heterologous expression systems are likely to be artefacts of extreme levels of overexpression.

Neuroprotective role of delta-opioid receptors in cortical neurons

We recently demonstrated that delta-opioid receptor (DOR) activation protects cortical neurons against glutamate-induced injury. Because glutamate is a mediator of hypoxic injury in neurons, we hypothesized that DOR is involved in neuroprotection during O2 deprivation and that its activation/inhibition may alter neuronal susceptibility to hypoxic stress. In this work, we tested the effect of opioid receptor activation and inhibition on cultured cortical neurons in hypoxia (1% O2). Cell injury was assessed by lactate dehydrogenase release, morphology-based quantification, and live/dead staining. Our results show that 1) immature neurons (days 4 and 6) were not significantly injured by hypoxia until 72 h of exposure, whereas day 8 neurons were injured after only 24-h hypoxia; 2) DOR inhibition (naltrindole) caused neuronal injury in both day 4 and day 8 normoxic cultures and further augmented hypoxic injury in these neurons; 3) DOR activation ([D-Ala2,D-Leu5]enkephalin) reduced neuronal injury in day 8 cultures after 24 h of normoxic or hypoxic exposure and attenuated naltrindole-induced injury with prolonged exposure; and 4) mu- or kappa-opioid receptor inhibition (beta-funaltrexamine or nor-binaltorphimine) had little effect on neurons in either normoxic or hypoxic conditions. Collectively, these data suggest that DOR plays a crucial role in neuroprotection in normoxic and hypoxic environments.

Withdrawal from dependence upon butorphanol uniquely increases kappa(1)-opioid receptor binding in the rat brain

Changes in kappa(1)-opioid receptor binding have been implicated in the development of dependence upon and withdrawal from butorphanol. Autoradiographic characterization of binding for brain kappa(1)-([3H]CI-977), mu-([3H]DAMGO), and delta-([3H]DPDPE) opioid receptors was performed in rats undergoing naloxone-precipitated withdrawal from dependence upon butorphanol or morphine. Dependence was induced by a 72h i.c.v. infusion with either butorphanol or morphine (26nmol/microl/h). Withdrawal was subsequently precipitated by i.c.v. challenge with naloxone (48 nmol/5 microl/rat), administered 2h following cessation of butorphanol or morphine infusion. During withdrawal from butorphanol, but not morphine, kappa(1)-opioid receptor binding was increased significantly in the frontal cortex, posterior basolateral amygdaloid nucleus, dorsomedial hypothalamus, hippocampus, posterior paraventricular thalamic nucleus, ventral tegmental area and locus coeruleus. In contrast, mu-opioid receptor binding decreased in these brain regions in naloxone-precipitated withdrawal from morphine, but not butorphanol, while binding for delta-opioid receptors was altered in both withdrawal groups. The brain kappa(1)-opioid receptor appears to be more directly involved in the development of physical dependence upon, and the expression of withdrawal from, butorphanol, as opposed to the prototypical opioid analgesic, morphine.

Agonists activate Gi1 alpha or Gi2 alpha fused to the human mu opioid receptor differently

As preferential coupling of opioid receptor to various inhibitory Galpha subunits is still under debate, we have investigated the selectivity of the human mu opioid receptor fused to a pertussis toxin insensitive C351I Gi1 alpha or C352I Gi2 alpha in stably transfected HEK 293 cells. Overall agonist binding affinities were increased for both fusion constructs when compared to the wild type receptor. [35 S]GTPgammaS binding was performed on pertussis toxin treated cells to monitor coupling efficiency of the fusion constructs. Upon agonist addition hMOR-C351I Gi1 a exhibited an activation profile similar to the non-fused receptor while hMOR-C352I Gi2 alpha was poorly activated. Interestingly no correlation could be drawn between agonist binding affinity and efficacy. Upon agonist addition, forskolin-stimulated cAMP production, as measured using a reporter gene assay, was inhibited by signals transduced via the fused Gi1 alpha and Gi2 alpha mainly. In contrast both fusion constructs were able to initiate ERK-MAPK phosphorylation via coupling to endogenous G proteins only. In conclusion our data indicate that hMOR couples more efficiently to Gi1 alpha than Gi2 alpha and that the coupling efficacy is clearly agonist-dependent.

Potentiation of the excitatory action of NMDA in ventrolateral periaqueductal gray by the mu-opioid receptor agonist, DAMGO

Several lines of evidence have suggested that mu-opioids, generally regarded as inhibitory, also have effects that stimulate neural activity. To look for possible excitatory opioid action in the rat periaqueductal gray (PAG), we first re-examined data from a previous study and found that met-enkephalin could evoke a delayed, sluggish excitation, suggestive of modulation by the opioid on the action of certain excitants. This observation, coupled with other studies that show mu-opioids can modulate NMDA receptor activation, prompted us to perform extracellular recording of the responses of single ventrolateral PAG (vlPAG) neurons in brain slices to DAMGO, a mu-opioid, and to NMDA. When applied alone, DAMGO at nM concentrations, like met-enkephalin, often evoked the delayed excitation and occasionally an inhibition. When applied after a brief exposure to NMDA, DAMGO at doses as low as 0.1 nM potentiated the excitation produced by a subsequent pulse of NMDA. This occurred, depending on cell type, in 23-100% of vlPAG neurons. The potentiating action of DAMGO was blocked by naloxone, suggesting it was mediated by mu-opioid receptors. Characterization of these mu-opioid actions revealed that the potentiation and the delayed excitation, unlike the inhibition, was not blocked by another opioid antagonist, nalmefene, nor by an inhibitor of the G protein of the G(i) class, N-ethylmaleimide. Moreover, the potentiating action was distinct from the inhibition in that it was: (a) enhanced by repeated opioid applications, (b) exhibited low effective doses, (c) had a long time course (minutes to develop and last tens of minutes) and (d) was present in distinct though overlapping cell populations. These data reveal an unconventional action of opioids in PAG neurons, that is, a potentiation of excitation produced by NMDA. This effect appeared mechanistically distinct from opioid inhibition or disinhibition and may be related to established examples of direct opioid excitation. These observations may help understanding behaviorally important mechanisms linked to acute and chronic opioid functions in the vlPAG.

MAP kinase activation by mu opioid receptor in cord blood CD34(+)CD38(-) cells

OBJECTIVE

Opioid receptor expression and function traditionally have been studied in neuronal cells and recently in mature lymphoid cells; however, little is known about their possible functions in hematopoietic stem cells (CD34(+) cells). We studied the expression of the mu receptor on CD34(+) cells and assessed the signal transduction cascade it induces.

MATERIALS AND METHODS

Mu-receptor expression on cord blood (CB) and peripheral blood (PB) CD34(+) cells was studied by microarrays, immunostaining, and fluorescence-activated cell sorting analysis. Signal transduction by the mu receptor was studied through Western blots and kinase assay of enkephalin-activated CB CD34(+) cells. Apoptotic, differentiation, and proliferation responses following mu-receptor activatioSn were studied by annexin V assay and inverted microscopy.

RESULTS

A prominent difference in gene expression, in favor of CB compared to PB CD34(+) cells, was observed in the mu-receptor gene. This receptor was mainly expressed on the CB CD34(+)CD38(-) subpopulation. A MAP kinase signal transduction cascade was shown to be induced through activation of this receptor by enkephalin or morphine.

CONCLUSIONS

We showed for the first time that the mu receptor is expressed on immature CB stem cells and that its activation by enkephalin or morphine induces a MAP kinase signal transduction cascade. Because the MAP kinase cascade is known to elicit proliferation and differentiation responses, these findings suggest a possible role of endogenous enkephalins in hematopoietic stem cell proliferation and differentiation and may lead to therapeutic applications of opiates in CB stem cell expansion and neuronal differentiation.

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.

Nociceptin/orphanin FQ and opioid receptor-like receptor mRNA expression in dopamine systems

Although nociceptin/orphanin FQ (N/OFQ) influences dopamine (DA) neuronal activity, it is not known whether N/OFQ acts directly on DA neurons, indirectly by means of local circuitry, or both. We used two parallel approaches, dual in situ hybridization (ISH) and neurotoxic lesions of DA neurons by using 6-hydroxydopamine (6-OHDA), to ascertain whether N/OFQ and the N/OFQ receptor (NOP) mRNA are expressed in DA neurons in the ventral tegmental area (VTA) and substantia nigra compacta (SNc). In the VTA and SNc, small populations (approximately 6-10%) of N/OFQ-containing neurons coexpressed mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis. Similarly, very few (1-2%) TH-positive neurons contained N/OFQ mRNA signal. A majority of NOP-positive neurons (approximately 75%) expressed TH mRNA and roughly half of the TH-containing neurons expressed NOP mRNA. Many N/OFQ neurons (approximately 50-60%) expressed glutamic acid decarboxylase 65 and 67 mRNAs, markers for gamma-aminobutyric acid (GABA) neurons. In the 6-OHDA lesion studies, NOP mRNA levels were nearly 80 and 85% lower in the VTA and SNc, respectively, on the lesioned side. These lesions appear to lead to compensatory changes, with N/OFQ mRNA levels approximately 60% and 300% higher in the VTA and SNc, respectively, after 6-OHDA lesions. Finally, N/OFQ-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate levels were decreased in the VTA and SNc but not the prefrontal cortex after 6-OHDA lesions. Accordingly, it appears that N/OFQ mRNA was found largely on nondopaminergic (i.e., GABA) neurons, whereas NOP mRNA was located on DA neurons. N/OFQ is in a position to influence DA neuronal activity by means of the NOP located on DA neurons.

Molecular components of tolerance to opiates in single hippocampal neurons

We examined the effect of acute and chronic opioid treatment on synaptic transmission and mu-opioid receptor (MOR) endocytosis in cultures of naïve rat hippocampal neurons. Opioid agonists that activate MOR inhibited synaptic transmission at inhibitory but not excitatory autapses. [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), morphine, and methadone were all effective at blocking inhibitory transmission. These same drugs also reduced the amplitude of voltage-dependent Ca(2+) currents in inhibitory but not excitatory neurons. Chronic treatment with all three opioids reduced the subsequent effects of a challenge with either the same drug or one of the others in individual autaptic neurons. Chronic treatment with DAMGO or methadone produced internalization of enhanced yellow fluorescent protein-tagged MOR expressed in hippocampal neurons within hours, whereas morphine produced internalization much more slowly, even when accompanied by overexpression of beta-arrestin-2. We conclude that DAMGO, methadone, and morphine all produce tolerance in single hippocampal neurons. Morphine-induced tolerance does not necessarily seem to involve receptor endocytosis.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Differential prevention of morphine amnesia by antisense oligodeoxynucleotides directed against various Gi-protein alpha subunits

The effect of the i.c.v. administration of pertussis toxin (PTX) and antisense oligodeoxynucleotide directed against the alpha subunit of different Gi-proteins (anti-Gialpha1, anti-Gialpha2, anti-Gialpha3) on amnesia induced by morphine was evaluated in the mouse passive avoidance test. The administration of morphine (6 - 10 mg kg(-1) i.p.) immediately after the training session produced amnesia that was prevented by PTX (0.25 microg per mouse i.c.v.) administered 7 days before the passive avoidance test. Anti-Gialpha1 (6.25 microg per mouse i.c.v.) and anti-Gialpha3 (12.5 microg per mouse i.c.v.), administered 18 and 24 h before the training session, prevented the morphine amnesia. By contrast, pretreatment with anti-Gialpha2 (3.12 - 25 microg per mouse i.c.v.) never modified the impairment of memory processes induced by morphine. At the highest effective doses, none of the compounds used impaired motor coordination, as revealed by the rota rod test, nor modified spontaneous motility and inspection activity, as revealed by the hole board test. These results suggest the important role played by Gi1 and Gi3 protein subtypes in the transduction mechanism involved in the impairment of memory processes produced by morphine.

Acute opioid receptor desensitization and tolerance: is there a link?

1. Morphine, used long-term for the treatment of pain, results in drug tolerance. The therapeutic benefits, as well as side effects, of morphine are mediated predominantly via activation of mu-opioid receptors. Although the underlying mechanisms for opioid tolerance remains unclear, early adaptive processes, such as acute receptor desensitization and receptor downregulation, have been suggested to be crucial to the development of opioid tolerance. 2. Other neuroadaptations resulting from chronic opioid use include upregulation of the cAMP pathway, an increase in the cAMP response element-binding protein and Fos-related antigens. However, the connection between upregulation of these cellular elements and the mechanism behind the behavioural phenomenon remains unclear. 3. Acute receptor desensitization is thought to occur via uncoupling of the receptor and G-protein, which is followed by internalization of the receptor from the cell membrane. This process occurs after a few minutes of agonist exposure. Receptor-G-protein uncoupling is mediated via phosphorylation of putative sites on the intracellular loops of activated receptors. 4. Acute desensitization and downregulation of receptors both result in a reduction of agonist efficacy. These events occur early in the cascade of cellular adaptation; however, it is uncertain whether these processes contribute to the long-term changes in receptor sensitivity that occur after repeated exposure to opioids. 5. Acute desensitization may, in fact, be a protective mechanism whereby cells adapt to avoid the development of physiological drug tolerance by rapidly attenuating receptor-mediated signalling. Those drugs that do not cause receptor internalization, such as morphine, may have higher propensities to develop tolerance.

The human delta opioid receptor activates G(i1)alpha more efficiently than G(o1)alpha

To assess the relative capacity of the human delta opioid receptor to activate closely related G proteins, fusion proteins were constructed in which the alpha-subunits of either G(i1) or G(o1), containing point mutations to render them insensitive to the actions of pertussis toxin, were linked in-frame with the C-terminus of the receptor. Following transient and stable expression in HEK 293 cells, both constructs bound the antagonist [(3)H]naltrindole with high affinity. D-ala(2),D-leu(5) Enkephalin effectively inhibited forskolin-stimulated adenylyl cyclase activity in intact cells in a concentration-dependent, but pertussis toxin-insensitive, manner. The high-affinity GTPase activity of both constructs was also stimulated by D-ala(2),D-leu(5) enkephalin with similar potency. However, enzyme kinetic analysis of agonist stimulation of GTPase activity demonstrated that the GTP turnover number produced in response to D-ala(2),D-leu(5) enkephalin was more than three times greater for G(i1)alpha than for G(o1)alpha. As the effect of agonist in both cases was to increase V:(max) without increasing the observed K:(m) for GTP, this is consistent with receptor promoting greater guanine nucleotide exchange, and thus activation, of G(i1)alpha compared with G(o1)alpha. An equivalent fusion protein between the human mu opioid receptor-1 and G(i1)alpha produced a similar D-ala(2),D-leu(5) enkephalin-induced GTP turnover number as the delta opioid receptor-G(i1)alpha fusion construct, consistent with agonist occupation of these two opioid receptor subtypes being equally efficiently coupled to activation of G(i1)alpha.

Opioid-induced adenylyl cyclase supersensitization in human embryonic kidney 293 cells requires pertussis toxin-sensitive G proteins other than G(i1) and G(i3)

Chronic activation of opioid receptors in cultured mammalian cells is known to induce adenylyl cyclase (AC) supersensitization via the pertussis toxin-sensitive G(i/o) proteins. To examine the role of G(i1) and G(i3) in opioid-induced AC supersensitization, pertussis toxin-resistant mutants of Galpha(i1) and Galpha(i3) (Galpha(i1)CG and Galpha(i3)CG) were stably co-expressed with different opioid receptors (mu, delta or kappa) in human embryonic kidney (HEK 293) cells. Although the opioid receptors were capable of inhibiting AC via Galpha(i1)CG and Galpha(i3)CG in pertussis toxin-treated cells, AC supersensitization induced by chronic opioid treatment remained sensitive to pertussis toxin. Our results demonstrated that despite their ability to interact with opioid receptors, the pertussis toxin-sensitive G(i1) and G(i3) proteins on their own are incapable of supporting opioid-induced AC supersensitization.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Opioid receptor and peptide gene polymorphisms: potential implications for addictions

Addictions to drugs of abuse and alcohol have been shown by studies of genetic epidemiology to have both a heritable and an environmental basis, with these factors influencing addiction to different substances to a different extent. In the search for specific alleles of specific genes that may contribute to the development of the addictions, many researchers have focused on the endogenous opioid system, which mediates a diverse array of neurological, physiological, and behavioral functions. The endogenous opioid system is also centrally important in mediating the effects of drugs of abuse and alcohol. Polymorphisms, including single nucleotide polymorphisms, have been identified in genes of the endogenous opioid receptors and peptides. A number of recent genetic association studies and a few studies of potential function provide clues as to which genes and which alleles may have implications for human physiology and pathophysiology, including the addictions.

A core-promoter region functions bi-directionally for human opioid-receptor-like gene ORL1 and its 5'-adjacent gene GAIP

We report the genomic structure and functional activities of the promoter regions of the human opioid-receptor-like gene ORL1 and its 5'-adjacent gene GAIP (G alpha interacting protein). The transcription and alternative splicing of human ORL1 are controlled by two alternate promoters, located approximately 10 kb apart. The two promoter regions lack a TATA-box and are GC rich. Promoter 1A initiates, from a single transcription start point (TSP), two transcripts: one consisting of exons 1A, 1B, 2, etc., the other without exon 1B. A potential ATG codon upstream of the initiation codon of ORL1 starts a new open-reading frame encoding a theoretical polypeptide of 205 amino acid residues. The promoter 1B transcribes, from multiple TSPs, only one mRNA starting with exon 1B. Two different repeat sequence polymorphisms are found in the ORL1 promoter regions. Luciferase reporter gene assays with promoter regions and a series of deletion mutants have mapped the core-promoter 1A and 1B within two short fragments. DNA sequencing and a database search reveal that the human GAIP gene is located upstream of ORL1 and is oriented in the opposite direction. The transcription and alternative splicing of GAIP are also under the control of alternate promoters. The first exons of ORL1 and GAIP are separated by only 83 bp. This 83 bp fragment, together with short surrounding sequences from both first exons, functions bi-directionally as a core-promoter for both genes. The transcription and alternative splicing of human ORL1 and GAIP are cell-type specific. While GAIP is expressed in both NT2 precursor cells and differentiated NT2 neuronal cells, ORL1 is only expressed in differentiated NT2 neurons. Since ORL1 is a G protein-coupled receptor and GAIP interacts with G protein alpha subunits, their physical linkage in the genome and co-operative transcriptional regulation may play a significant role in ORL1 receptor signal transduction.

Interaction of local anaesthetics with recombinant mu, kappa, and delta-opioid receptors expressed in Chinese hamster ovary cells

Local anaesthetics potentiate epidural or intrathecal opioid analgesia via a poorly defined mechanism. In this study, we have examined the interaction of local anaesthetics (lidocaine, bupivacaine and its optical isomers, tetracaine, procaine and prilocaine) with recombinant mu-, kappa-, and delta-opioid receptors expressed in Chinese hamster ovary cells (CHO-mu, kappa, and delta, respectively). Lidocaine produced a concentration-dependent displacement of radiolabelled opioid antagonist [3H]diprenorphine ([3H]DPN) binding with the following rank order of inhibitor constant (Ki): kappa (210 microM) > mu (552 microM) > delta (1810 microM). Procaine, prilocaine, tetracaine and bupivacaine also displaced [3H]DPN binding in CHO-mu with Ki values of 244, 204, 43 and 161 microM respectively. Lidocaine produced a concentration-dependent and naloxone-insensitive inhibition of cAMP formation in all cell lines including untransfected cells. Concentration producing 50% inhibition of maximum was mu, 1.32 mM; kappa, 2.41 mM; delta, 1.27 mM; untransfected, 2.78 mM. When lidocaine (300 microM) was co-incubated with spiradoline (kappa-selective) and [D-Ala2, MePhe4, Gly(ol)5] enkephalin (DAMGO mu-selective) in CHO-kappa and mu cells we did not observe an additive interaction for cAMP formation. In contrast, there was an apparent inhibitory action of the combination at the kappa receptor. This study suggests that clinical concentrations of local anaesthetics interact with mu and kappa but not delta opioid receptors. As there was no synergism between local anaesthetics and opioids we suggest that the interaction of these agents in the clinical setting does not occur at the cellular level.

Evidence for transduction of mu but not kappa opioid modulation of extracellular signal-regulated kinase activity by G(z) and G(12) proteins

Chronic treatment with micro or kappa opioid agonists (>/=2 h) inhibits EGF-induced ERK activation in opioid receptor overexpressing COS-7 cells. Although acute mu and kappa opioids activate ERK via a pertussis toxin-sensitive G protein, pertussis toxin insensitivity of the chronic mu (but not kappa) action was observed. Here, we tested several pertussis toxin-insensitive G proteins as candidates to transduce acute and/or chronic opioid modulation of ERK. Overexpressed Galpha(z) (but not Galpha(12)) transduced acute mu (but not kappa) ERK activation in pertussis toxin-treated COS-7 cells. Chronic mu (but not kappa) inhibited EGF stimulation of ERK in pertussis toxin-treated cells overexpressing Galpha(z) or Galpha(12). Transfection of Galpha(13) or Galpha(q) blocked inhibition under the same conditions. Overexpressed interfering and non-interfering Galpha(z) mutants differentially affected mu inhibition of ERK consistent with G(z) transduction. In this and prior studies, Galpha(z) and Galpha(12) immunoreactivity were detected in untransfected COS-7 cells, suggesting that these G proteins may be endogenous mediators of chronic mu inhibitory actions on ERK.

mu-Opioid receptors often colocalize with the substance P receptor (NK1) in the trigeminal dorsal horn

Substance P (SP) is a peptide that is present in unmyelinated primary afferents to the dorsal horn and is released in response to painful or noxious stimuli. Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through modulation of responses to SP. MOR ligands may either inhibit the release of SP or reduce the excitatory responses of second-order neurons to SP. We examined potential functional sites for interactions between SP and MOR with dual electron microscopic immmunocytochemical localization of the SP receptor (NK1) and MOR in rat trigeminal dorsal horn. We also examined the relationship between SP-containing profiles and NK1-bearing profiles. We found that 56% of SP-immunoreactive terminals contact NK1 dendrites, whereas 34% of NK1-immunoreactive dendrites receive SP afferents. This result indicates that there is not a significant mismatch between sites of SP release and available NK1 receptors, although receptive neurons may contain receptors at sites distant from the peptide release site. With regard to opioid receptors, we found that many MOR-immunoreactive dendrites also contain NK1 (32%), whereas a smaller proportion of NK1-immunoreactive dendrites contain MOR (17%). Few NK1 dendrites (2%) were contacted by MOR-immunoreactive afferents. These results provide the first direct evidence that MORs are on the same neurons as NK1 receptors, suggesting that MOR ligands directly modulate SP-induced nociceptive responses primarily at postsynaptic sites, rather than through inhibition of SP release from primary afferents. This colocalization of NK1 and MORs has significant implications for the development of pain therapies targeted at these nociceptive neurons.