Functional coupling of a mu opioid receptor to G proteins and adenylyl cyclase: modulation by chronic morphine treatment

Human hair pigmentation--biological aspects

Skin and hair colour contribute significantly to our overall visual appearance and to social/sexual communication. Despite their shared origins in the embryologic neural crest, the hair follicle and epidermal pigmentary units occupy distinct, although open, cutaneous compartments. They can be distinguished principally on the basis of the former's stringent coupling to the hair growth cycle compared with the latter's continuous melanogenesis. The biosynthesis of melanin and its subsequent transfer from melanocyte to hair bulb keratinocytes depend on the availability of melanin precursors and on a raft of signal transduction pathways that are both highly complex and commonly redundant. These signalling pathways can be both dependent and independent of receptors, act through auto-, para- or intracrine mechanisms and can be modified by hormonal signals. Despite many shared features, follicular melanocytes appear to be more sensitive than epidermal melanocytes to ageing influences. This can be seen most dramatically in hair greying/canities and this is likely to reflect significant differences in the epidermal and follicular microenvironments. The hair follicle pigmentary unit may also serve as an important environmental sensor, whereby hair pigment contributes to the rapid excretion of heavy metals, chemicals and toxins from the body by their selective binding to melanin; rendering the hair fibre a useful barometer of exposures. The recent availability of advanced cell culture methodologies for isolated hair follicle melanocytes and for intact anagen hair follicle organ culture should provide the research tools necessary to elucidate the regulatory mechanisms of hair follicle pigmentation. In the longer term, it may be feasible to develop hair colour modifiers of a biological nature to accompany those based on chemicals.

Genetic mutations that prevent pain: implications for future pain medication

Part of the interindividual variability in pain therapy has been associated with genetic polymorphisms. Several genetic variants prevent or at least decrease pain in their carriers as compared with carriers of the respective wild-type or common alleles by impeding the generation, transmission and processing of nociceptive information or by increasing the local availability of active analgesics or their pharmacodynamic effects. Complete prevention of pain has so far been seen in six distinct rare hereditary syndromes, namely the ‘channelopathy-associated insensitivity to pain’, caused by 13 currently identified variants in the SCN9A gene coding for the α-subunit of the voltage-gated sodium channel, and five maladies belonging to the hereditary sensory and autonomic neuropathy (HSAN) I–V syndromes, caused by various mutations in several genes. Reduced pain in the average population has been associated with frequent variants in the µ-opioid receptor gene (OPRM1), catechol-O-methyltransferase gene (COMT), guanosine triphosphate cyclohydrolase 1/dopa-responsive dystonia gene (GCH1), transient receptor potential cation channel, subfamily V, member 1 gene (TRPV1) or the melanocortin-1 receptor gene (MC1R). Duplications/amplifications of the cytochrome P450 2D6 (CYP2D6) gene leading to increased enzyme function may cause intense opioid effects of codeine up to toxicity. The COMT V158M variant has been associated with decreased morphine requirements for analgesia. Inactivating MC1R variants have been associated with increased opioid analgesia of the µ-opioid receptor agonist morphine-6-glucuronide and, in women only, of κ-opioid agonists. Finally, variants in the P-glycoprotein gene (ABCB1) conferring decreased transporter function have been associated with increased respiratory depressive effects of fentanyl. In summary, a finite number of genetic variants that prevent pain by decreasing nociception or increasing analgesia have been identified. Given the complex biological and psychological nature of pain, we will see in the near future how much of the interindividual variance in pain and analgesia is due to identifiable genetic causes, and to what extent genetics enters clinical pain therapy.

Raf-1 kinase mediates adenylyl cyclase sensitization by TNF-alpha in human airway smooth muscle cells

Tumor necrosis factor (TNF)-alpha is a potent inflammatory cytokine implicated in the exacerbation of asthma. Chronic exposure to TNF-alpha has been reported to induce G protein-coupled receptor desensitization, but adenylyl cyclase sensitization, in airway smooth muscle cells by an unknown mechanism. Cyclic AMP, which is synthesized by adenylyl cyclases in response to G protein-coupled receptor signals, is an important second messenger involved in the regulation of the airway muscle proliferation, migration, and tone. In other cell types, TNF-alpha receptors transactivate the EGF receptor, which activates raf-1 kinase. Further studies in transfected cells show that raf-1 kinase can phosphorylate and activate some isoforms of adenylyl cyclase. Cultured human airway smooth muscle cells were treated with TNF-alpha in the presence or absence of inhibitors of prostaglandin signaling, protein kinases, or G(i) proteins. TNF-alpha caused a significant dose- (1-10 ng/ml) and time-dependent (24 and 48 h) increase in forskolin-stimulated adenylyl cyclase activity, which was abrogated by pretreatment with GW5074 (a raf-1 kinase inhibitor), was partially inhibited by an EGF receptor inhibitor, but was unaffected by pertussis toxin. TNF-alpha also increased phosphorylation of Ser(338) on raf-1 kinase, indicative of activation. IL-1beta and EGF sensitization of adenylyl cyclase activity was also sensitive to raf-1 kinase inhibition by GW5074. Taken together, these studies link two signaling pathways not previously characterized in human airway smooth muscle cells: TNF-alpha transactivation of the EGF receptor, with subsequent raf-1 kinase-mediated activation of adenylyl cyclase.

G protein signaling mediates developmental processes and pathogenesis of Alternaria alternata

A G protein alpha subunit gene (AGA1) has been cloned and characterized from a toxigenic and necrotrophic Alternaria alternata pathogen. Targeted disruption of AGA1 in the apple pathotype of A. alternata gave rise to mutants that differed in colony and conidial morphology as well as sporulation. The conidia of wild type and deltaAGA1 mutants showed equal germination on cellulose membranes. However, wild-type germ tubes formed readily from different points around the conidia, grew randomly, and were often branched, whereas those of the mutants formed only at one or both ends of the conidia and tended to grow in straight paths. Targeted disruption of AGA1 also resulted in reduction of pathogenicity on apple leaves, although the mutant produced host-specific AM-toxin, a fungal secondary metabolite associated with pathogenicity of the pathogen, at levels similar to the wild-type strain. Measurement of the intracellular cAMP levels of the mutant revealed that it was consistently higher than that of the wild type, indicating that AGA1 negatively regulates cAMP levels similar to mammalian Galphai systems. These results indicate that the signal transduction pathway represented by AGA1 appears to be involved in developmental pathways leading to sporulation and pathogenesis of A. alternata.

Morphine Sulfate Attenuates Hemorrhagic Shock- Induced Hyperpermeability

Morphine sulfate is often administered for patients requiring surgical intervention for the control of hemorrhage. Recent data implicate morphine as an immune modulator that affects white blood cells and increases infection rates. In addition, morphine releases histamine, an inflammatory mediator that increases microvascular permeability. Both of these actions of morphine could aggravate the inflammatory progress after hemorrhagic shock. In this study, we evaluated the role of morphine sulfate on microvascular permeability and its effects on leukocyte adherence after hemorrhagic shock. After a control period, blood was withdrawn to reduce the mean arterial blood pressure to 40 mm Hg for 1 h in urethane-anesthetized Sprague-Dawley rats. Mesenteric postcapillary venules in a transilluminated segment of small intestine were examined to quantify changes in permeability and leukocyte adherence. The rats received an IV injection of fluorescein isothiocyanate-bovine albumin during the control period. The fluorescent light intensity emitted from the fluorescein isothiocyanate-bovine albumin was recorded with digital microscopy within the lumen of the microvasculature and compared with the intensity of light in the extraluminal space over time. These images were downloaded to a computerized image analysis program that quantitates changes in light intensity. This change in light intensity represents albumin extravasation. In addition, bright-field images were recorded on compact disk for playback to determine leukocyte adherence. Leukocytes stationary for more than 30 s or longer in a 100-micron segment of venule was considered adherent. Our results demonstrated a marked increase in fluorescein isothiocyanate-bovine albumin leakage into the extravascular space after hemorrhagic shock. Hemorrhagic shock was also associated with an increase in leukocytes adhering to the postcapillary venular endothelium. Morphine sulfate 10 microg/kg given before the shock period, attenuated both the hyperpermeability (P < 0.05) and the increase in leukocyte adherence (P < 0.05) after hemorrhagic shock. These results suggest that instead of aggravating the inflammatory response after hemorrhagic shock, morphine may provide protection to the microvasculature.

Opioid abuse and brain gene expression

Opiate addiction is a central nervous system disorder of unknown mechanism. Neuronal basis of positive reinforcement, which is essential to the action of opioids, relies on activation of dopaminergic neurons resulting in an increased dopamine release in the mesolimbic brain structures. Certain aspects of opioid dependence and withdrawal syndrome are also related to the activity of noradrenergic and serotonergic systems, as well as to both excitatory and inhibitory amino acid and peptidergic systems. The latter pathways have been recently proven to be involved both in the development of dependence and in counteracting the states related to relapse. An important role in neurochemical mechanisms of opioid reward, dependence and vulnerability to addiction has been ascribed to endogenous opioid peptides, particularly those acting via the mu- and kappa-opioid receptors. Opiate abuse leads to adaptive reactions in the nervous system which occur at the cellular and molecular levels. Recent research indicates that intracellular mechanisms of signal transmission-from the receptor, through G proteins, cyclic AMP, MAP kinases to transcription factors--also play an important role in opioid tolerance and dependence. The latter link in this chain of reactions may modify synthesis of target genes and in this manner, it may be responsible for opiate-induced long-lasting neural plasticity.

The mu opioid receptor: from molecular cloning to functional studies

Opioids have been used and abused by humans for centuries. The mu opioid receptor represents the high affinity binding site for opioid narcotics with high abuse liability such as morphine, codeine and fentanyl. Heroin (diacetylmorphine), a semi-synthetic derivative of morphine, crosses the blood-brain barrier more readily than morphine due to its increased hydrophobicity. Once in the brain heroin is hydrolyzed to morphine, which acts at the mu opioid receptor and results in euphoria, thus conferring the reinforcing properties of heroin. Using molecular biology techniques, the mu opioid receptors from several species have been cloned. This article reviews recent progress in this area, with respect to the two major cellular functions of the mu opioid receptor: reduction of intracellular cAMP concentration by inhibiting adenylyl cyclase activity, and inhibition of neuronal firing by modulating membrane ion channels.

MAP kinase activation by mu opioid receptor involves phosphatidylinositol 3-kinase but not the cAMP/PKA pathway

The involvement of protein kinases was studied in mu opioid receptor activation of mitogen-activated protein (MAP) kinase using cells transfected with the receptor clone. The cAMP/protein kinase A (PKA) pathway is known to be the major biochemical pathway for mu opioid receptor signaling. However, our data showed that stimulating adenylyl cyclase or activating PKA had no effect on mu receptor enhancement of MAP kinase activity, suggesting that the cAMP/PKA pathway is not involved in mediating the mu receptor activation of MAP kinase. Inhibition of phosphatidylinositol (PI) 3-kinase reduced mu receptor enhancement of MAP kinase activity, suggesting PI 3-kinase involvement. Together, these results show that cross-talk between the mu opioid receptor and the MAP kinase cascade is not mediated by the cAMP/PKA pathway, but involves PI 3-kinase.

Endomorphins fully activate a cloned human mu opioid receptor

Endomorphins were recently identified as endogenous ligands with high selectivity for mu opioid receptors. We have characterized the ability of endomorphins to bind to and functionally activate the cloned human mu opioid receptor. Both endomorphin-1 and endomorphin-2 exhibited binding selectivity for the mu opioid receptor over the delta and kappa opioid receptors. Both agonists inhibited forskolin-stimulated increase of cAMP in a dose-dependent fashion. When the mu opioid receptor was coexpressed in Xenopus oocytes with G protein-activated K+ channels, application of either endomorphin activated an inward K+ current. This activation was dose-dependent and blocked by naloxone. Both endomorphins acted as full agonists with efficacy similar to that of [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO). These data indicate that endomorphins act as full agonists at the human mu opioid receptor, capable of stimulating the receptor to inhibit the cAMP/adenylyl cyclase pathway and activate G-protein-activated inwardly rectifying potassium (GIRK) channels.

Single-nucleotide polymorphism in the human mu opioid receptor gene alters beta-endorphin binding and activity: possible implications for opiate addiction

Opioid drugs play important roles in the clinical management of pain, as well as in the development and treatment of drug abuse. The mu opioid receptor is the primary site of action for the most commonly used opioids, including morphine, heroin, fentanyl, and methadone. By sequencing DNA from 113 former heroin addicts in methadone maintenance and 39 individuals with no history of drug or alcohol abuse or dependence, we have identified five different single-nucleotide polymorphisms (SNPs) in the coding region of the mu opioid receptor gene. The most prevalent SNP is a nucleotide substitution at position 118 (A118G), predicting an amino acid change at a putative N-glycosylation site. This SNP displays an allelic frequency of approximately 10% in our study population. Significant differences in allele distribution were observed among ethnic groups studied. The variant receptor resulting from the A118G SNP did not show altered binding affinities for most opioid peptides and alkaloids tested. However, the A118G variant receptor binds beta-endorphin, an endogenous opioid that activates the mu opioid receptor, approximately three times more tightly than the most common allelic form of the receptor. Furthermore, beta-endorphin is approximately three times more potent at the A118G variant receptor than at the most common allelic form in agonist-induced activation of G protein-coupled potassium channels. These results show that SNPs in the mu opioid receptor gene can alter binding and signal transduction in the resulting receptor and may have implications for normal physiology, therapeutics, and vulnerability to develop or protection from diverse diseases including the addictive diseases.

Pineal opioid receptors and analgesic action of melatonin

Physicians have noted since antiquity that their patients complained of less pain and required fewer analgesics at night times. In most species, including the humans, the circulating levels of melatonin, a substance with analgesic and hypnotic properties, exhibit a pronounced circadian rhythm with serum levels being high at night and very low during day times. Moreover, melatonin exhibits maximal analgesic effects at night, pinealectomy abolishes the analgesic effects of melatonin, and mu opioid receptor antagonists disrupt the day-night rhythm of nociception. It is believed that melatonin, with its sedative and analgesic effects, is capable of providing a pain free sleep so that the body may recuperate and restore itself to function again at its peak capacity. Moreover, in conditions when pain is associated with extensive tissue injury, melatonin's ability to scavenge free radicals and abort oxidative stress is yet another beneficial effect to be realized. Since melatonin may behave as a mixed opioid receptor agonist-antagonist, it is doubtful that a physician simply could potentiate the analgesic efficacy of narcotics such as morphine by coadministering melatonin. Therefore, future research may synthesize highly efficacious melatonin analogues capable of providing maximum analgesia and hopefully being devoid of addiction liability now associated with currently available narcotics.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Protein kinase modulation of mu opioid receptor signalling

The mu opioid receptor mediates the pain-relieving effects of opioid narcotics. Cellular signalling of the mu receptor involves G-proteins, and the effectors include adenylyl cyclase and ion channels. Studies in which the cloned mu opioid receptor is used for cellular expression show that the receptor function is modulated by multiple protein kinases.