mu-Opioid receptor stimulation of inositol (1,4,5)trisphosphate formation via a pertussis toxin-sensitive G protein

D,L-Methadone causes leukemic cell apoptosis via an OPRM1-triggered increase in IP3R-mediated ER Ca2+ release and decrease in Ca2+ efflux, elevating [Ca2+]i

The search continues for improved therapy for acute lymphoblastic leukemia (aLL), the most common malignancy in children. Recently, d,l-methadone was put forth as sensitizer for aLL chemotherapy. However, the specific target of d,l-methadone in leukemic cells and the mechanism by which it induces leukemic cell apoptosis remain to be defined. Here, we demonstrate that d,l-methadone induces leukemic cell apoptosis through activation of the mu1 subtype of opioid receptors (OPRM1). d,l-Methadone evokes IP3R-mediated ER Ca²⁺ release that is inhibited by OPRM1 loss. In addition, the rate of Ca²⁺ extrusion following d,l-methadone treatment is reduced, but is accelerated by loss of OPRM1. These d,l-methadone effects cause a lethal rise in [Ca²⁺]_i that is again inhibited by OPRM1 loss, which then prevents d,l-methadone-induced apoptosis that is associated with activation of calpain-1, truncation of Bid, cytochrome C release, and proteolysis of caspase-3/12. Chelating intracellular Ca²⁺ with BAPTA-AM reverses d,l-methadone-induced apoptosis, establishing a link between the rise in [Ca²⁺]_i and d,l-methadone-induced apoptosis. Altogether, our findings point to OPRM1 as a specific target of d,l-methadone in leukemic cells, and that OPRM1 activation by d,l-methadone disrupts IP3R-mediated ER Ca²⁺ release and rate of Ca²⁺ efflux, causing a rise in [Ca²⁺]_i that upregulates the calpain-1-Bid-cytochrome C-caspase-3/12 apoptotic pathway.

Opioid-Mediated Astrocyte-Neuron Signaling in the Nucleus Accumbens

Major hallmarks of astrocyte physiology are the elevation of intracellular calcium in response to neurotransmitters and the release of neuroactive substances (gliotransmitters) that modulate neuronal activity. While μ-opioid receptor expression has been identified in astrocytes of the nucleus accumbens, the functional consequences on astrocyte–neuron communication remains largely unknown. The present study has investigated the astrocyte responsiveness to μ-opioid signaling and the regulation of gliotransmission in the nucleus accumbens. Through the combination of calcium imaging and whole-cell patch clamp electrophysiology in brain slices, we have found that μ-opioid receptor activation in astrocytes elevates astrocyte cytoplasmic calcium and stimulates the release of the gliotransmitter glutamate, which evokes slow inward currents through the activation of neuronal N-methyl-D-aspartate (NMDA) receptors. These results indicate the existence of molecular mechanisms underlying opioid-mediated astrocyte–neuron signaling in the nucleus accumbens.

Rubiscolin-6 activates opioid receptors to enhance glucose uptake in skeletal muscle

Rubiscolin-6 is an opioid peptide derived from plant ribulose bisphosphate carboxylase/oxygenase (Rubisco). It has been demonstrated that opioid receptors could control glucose homeostasis in skeletal muscle independent of insulin action. Therefore, Rubiscolin-6 may be involved in the control of glucose metabolism. In the present study, we investigated the effect of rubiscolin-6 on glucose uptake in skeletal muscle. Rubiscolin-6-induced glucose uptake was measured using the fluorescent indicator 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxyglucose (2-NBDG) in L6 and C2C12 cell lines. The protein expressions of glucose transporter 4 (GLUT4) and AMP-activated protein kinase (AMPK) in L6 cells were observed by Western blotting. The in vivo effects of rubiscolin-6 were characterized in streptozotocin (STZ)-induced diabetic rats. Rubiscolin-6 induced a concentration-dependent increase in glucose uptake levels. The increase of phospho-AMPK (pAMPK) and GLUT4 expressions were also observed in L6 and C2C12 cells. Effects of rubiscolin-6 were blocked by opioid receptor antagonists and/or associated signals inhibitors. Moreover, Rubiscolin-6 produced a dose-dependent reduction of blood glucose and increased GLUT4 expression in STZ-induced diabetic rats. In conclusion, rubiscolin-6 increases glucose uptake, potentially via an activation of AMPK to enhance GLUT4 translocation after binding to opioid receptors in skeletal muscle.

Measurement of inositol(1,4,5)trisphosphate using a stereospecific radioreceptor mass assay

Inositol(1,4,5)trisphosphate [Ins(1,4,5)P(3)] is an important second messenger that activates its cognate Ins(1,4,5)P(3) receptor to release Ca(2+) from intracellular stores. The assay described in this chapter uses the Ins(1,4,5)P(3) receptor (essentially as a binding protein) to measure the biologically active trisphosphate (specifically from other trisphosphates). The binding protein (Ins(1,4,5)P3 receptor) is prepared from Bovine adrenal glands and this is mixed with [(3)H]-labeled and unlabeled (generated from biological samples or standards) Ins(1,4,5)P(3). Using the same principles as for radioimmunoassay/ELISA the mass of Ins(1,4,5)P(3) in biological samples can be estimated.

Opioid receptor subtypes: fact or artifact?

There is a vast amount of pharmacological evidence favouring the existence of multiple subtypes of opioid receptors. In addition to the primary classification of µ (mu: MOP), δ (delta: DOP), κ (kappa: KOP) receptors, and the nociceptin/orphanin FQ peptide receptor (NOP), various groups have further classified the pharmacological µ into µ(1-3), the δ into δ(1-2)/δ(complexed/non-complexed), and the κ into κ(1-3). From an anaesthetic perspective, the suggestions that µ(1) produced analgesia and µ(2) produced respiratory depression are particularly important. However, subsequent to the formal identification of the primary opioid receptors (MOP/DOP/KOP/NOP) by cloning and the use of this information to produce knockout animals, evidence for these additional subtypes is lacking. Indeed, knockout of a single gene (and hence receptor) results in a loss of all function associated with that receptor. In the case of MOP knockout, analgesia and respiratory depression is lost. This suggests that further sub-classification of the primary types is unwise. So how can the wealth of pharmacological data be reconciled with new molecular information? In addition to some simple misclassification (κ(3) is probably NOP), there are several possibilities which include: (i) alternate splicing of a common gene product, (ii) receptor dimerization, (iii) interaction of a common gene product with other receptors/signalling molecules, or (iv) a combination of (i)-(iii). Assigning variations in ligand activity (pharmacological subtypes) to one or more of these molecular suggestions represents an interesting challenge for future opioid research.

Mice lacking rhes show altered morphine analgesia, tolerance, and dependence

Rhes, the Ras Homolog Enriched in Striatum, is an intermediate-size GTP binding protein. Although its full functions are not yet known, it has been shown to affect signaling and behaviors mediated by G protein-coupled receptors. Here we have tested whether Rhes affects behaviors mediated by opioid receptors. Wild type and rhes-deficient mice were administered morphine and tested for analgesia in formalin and tail flick tests. Rhes⁻/⁻ mice showed significantly enhanced analgesia in both tests relative to rhes+/+ mice. Furthermore, rhes⁻/⁻ mice did not display tolerance to repeated morphine administration and displayed significantly less withdrawal than rhes+/+ mice. These findings indicate that Rhes is involved in behaviors mediated by mu opioid receptors and in the adaptive response to repeated morphine administration.

Involvement of PLC-beta3 in the effect of morphine on memory retrieval in passive avoidance task

Phospholipase C (PLC) is one signalling effector enzyme whose activity is directly modulated by opioids. Several physiological studies have implicated PLC-linked pathways in in-vivo pain regulation and opioid tolerance. Co-administration of PLC-beta(2/3) activity blocker M119 with morphine resulted in a dramatic increase in morphine-induced amnesic effect in mice, proving a role for beta subunit of PLC enzyme in these processes. Administration of morphine to mice at amnesic dose increased PLC-beta(3) activity, with respect to basal value, in the membrane-soluble material from anterior cortex and hippocampal formation in brain areas. PLC-beta(3) appears to be simultaneously implicated in both analgesic and amnesic effects induced by administration of morphine to mice suggesting a commonality in the molecular mechanisms of morphine-induced analgesia and memory impairment.

mu-Opioid agonists inhibit the enhanced intracellular Ca(2+) responses in inflammatory activated astrocytes co-cultured with brain endothelial cells

In order to imitate the in vivo situation with constituents from the blood-brain barrier, astrocytes from newborn rat cerebral cortex were co-cultured with adult rat brain microvascular endothelial cells. These astrocytes exhibited a morphologically differentiated appearance with long processes. 5-HT, synthetic mu-, delta- or kappa-opioid agonists, and the endogenous opioids endomorphin-1, beta-endorphin, and dynorphin induced higher Ca(2+) amplitudes and/or more Ca(2+) transients in these cells than in astrocytes in monoculture, as a sign of more developed signal transduction systems. Furthermore, stimulation of the co-cultured astrocytes with 5-HT generated a pronounced increase in intracellular Ca(2+) release in the presence of the inflammatory or pain mediating activators substance P, calcitonin gene-related peptide (CGRP), lipopolysaccharide (LPS), or leptin. These Ca(2+) responses were restored by opioids so that the delta- and kappa-opioid receptor agonists reduced the number of Ca(2+) transients elicited after incubation in substance P+CGRP or leptin, while the mu- and delta-opioid receptor agonists attenuated the Ca(2+) amplitudes elicited in the presence of LPS or leptin. In LPS treated co-cultured astrocytes the mu-opioid receptor antagonist naloxone attenuated not only the endomorphin-1, but also the 5-HT evoked Ca(2+) transients. These results suggest that opioids, especially mu-opioid agonists, play a role in the control of neuroinflammatory activity in astrocytes and that naloxone, in addition to its interaction with mu-opioid receptors, also may act through some binding site on astrocytes, other than the classical opioid receptor.

Adenylyl cyclase type 5 (AC5) is an essential mediator of morphine action

Opioid drugs produce their pharmacological effects by activating inhibitory guanine nucleotide-binding regulatory protein-linked mu, delta, and kappa opioid receptors. One major effector for these receptors is adenylyl cyclase, which is inhibited upon receptor activation. However, little is known about which of the ten known forms of adenylyl cyclase are involved in mediating opioid actions. Here we show that all of the major behavioral effects of morphine, including locomotor activation, analgesia, tolerance, reward, and physical dependence and withdrawal symptoms, are attenuated in mice lacking adenylyl cyclase type 5 (AC5), a form of adenylyl cyclase that is highly enriched in striatum. Furthermore, the behavioral effects of selective mu or delta opioid receptor agonists are lost in AC5-/- mice, whereas the behavioral effects of selective kappa opioid receptor agonists are unaffected. These behavioral data are consistent with the observation that the ability of a mu or delta opioid receptor agonist to suppress adenylyl cyclase activity was absent in striatum of AC5-/- mice. Together, these results establish AC5 as an important component of mu and delta opioid receptor signal transduction mechanisms in vivo and provide further support for the importance of the cAMP pathway as a critical mediator of opioid action.

Modulation of Na, K-ATPase activity by prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin

Adenylyl cyclase is activated by prostaglandin E and inhibited by mu-opioids. Since cAMP-related events influence the activity of the Na Pump and its biochemical correlate Na,K-ATPase in many systems, we tested the hypothesis that prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), a mu-opioid agonist, have opposing actions on Na,K-ATPase activity. Studies were conducted with alamethicin-permeabilized SH-SY5Y human neuroblastoma cells. Prostaglandin E1 (1 microM) transiently inhibited Na,K-ATPase activity for 10-15 min. A direct activator of protein kinase A, 8-Br-cAMP (150 and 500 microM), also inhibited, but more rapidly and for a shorter duration. Both DAMGO (1 microM) and Rp-adenosine 3',5'-cyclic monophosphorothioate (500 microM), a protein kinase A-inhibitor, reversed the inhibitory effect of prostaglandin E1. DAMGO alone (1 microM) stimulated Na,K-ATPase activity up to nearly three-fold control activity. The stimulatory action of DAMGO was blocked by cyclosporine A (2 microM), an inhibitor of calcineurin, and was dependent on Ca2+ entry through nifedipine-sensitive Ca2+ channels. In the presence of 1 mM EGTA, DAMGO inhibited Na,K-ATPase activity. DAMGO-induced inhibition was blocked by the inositol 1,4,5-trisphosphate receptor antagonist xestospongin C (1 microM). Na,K-ATPase is poised to modulate neuronal excitability through its roles in maintaining the membrane potential and transmembrane ion gradients. The differential effects of prostaglandin E1 and opioids on Na,K-ATPase activity may be related to their actions in hyperalgesia.

Phospholipase Cbeta1 modulates pain sensitivity, opioid antinociception and opioid tolerance formation

Phospholipase C (PLC) activity has been implicated in multiple opioid-induced sequelae. The relevance of PLC-linked pathways to opioid actions is isoform-specific. Chronic morphine augments PLCbeta1 signaling while diminishing that of PLCbeta3. This suggests that PLCbeta1 makes an important contribution to opioid tolerance formation (PNAS 100: 13686-1369, 2003). In the present study, PLCbeta1 knockout animals (-/-) were used to assess the relevance of PLCbeta1 to pain thresholds, morphine antinociception and analgesic tolerance formation. Response latencies to thermal nociceptive stimuli were markedly diminished in -/- animals relative to their wild-type (+/+) and heterozygous (+/-) counterparts; thermal nociceptive thresholds obtained in +/+ and +/- mice did not differ. This suggests that the contribution of PLCbeta1 to thermal pain thresholds requires a critical concentration of PLCbeta1 protein. PLCbeta1 genotype also influenced acute and chronic responsiveness to morphine. Analgesic dose responsiveness and the magnitude of analgesic tolerance formation to morphine were significantly attenuated in -/- vs. +/+ animals. Notably, in contrast to thermal nociceptive thresholds, acute and chronic morphine responsiveness differed significantly only between +/+ and -/- genotypes and not between -/- vs. +/- groups. These data suggest that whereas the contribution of PLCbeta1 to thermal nociceptive response thresholds requires a critical concentration of PLCbeta1 protein, its participation in morphine analgesic and tolerance-producing mechanisms is graded. Importantly, GTPgammaS binding studies revealed that there is no detectable diminution in functional opioid receptors in spinal tissue from -/- animals. This underscores the importance of PLCbeta1 to morphine sequelae that are initiated downstream from the opioid receptor.

Opioid elevation of intracellular free calcium: possible mechanisms and physiological relevance

Opioid receptors are seven transmembrane domain Gi/G0 protein-coupled receptors, the activation of which stimulates a variety of intracellular signalling mechanisms including activation of inwardly rectifying potassium channels, and inhibition of both voltage-operated N-type Ca2+ channels and adenylyl cyclase activity. It is now apparent that like many other Gi/G0-coupled receptors, opioid receptor activation can significantly elevate intracellular free Ca2+ ([Ca2+]i), although the mechanism underlying this phenomenon is not well understood. In some cases opioid receptor activation alone appears to elevate [Ca2+]i, but in many cases it requires concomitant activation of Gq-coupled receptors, which themselves stimulate Ca2+ release from intracellular stores via the inositol phosphate pathway. Given the number of Ca2+-sensitive processes known to occur in cells, there are therefore a myriad of situations in which opioid receptor-mediated elevations of [Ca2+](i) may be important. Here, we review the literature documenting opioid receptor-mediated elevations of [Ca2+]i, discussing both the possible mechanisms underlying this phenomenon and its potential physiological relevance.

Ultra-low dose naltrexone potentiates the anticonvulsant effect of low dose morphine on clonic seizures

Significant potentiation of analgesic effects of opioids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well understood. The bimodal modulation of pentylenetetrazole-induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms involved in opioid anti- (morphine at 0.5-3 mg/kg) versus pro-convulsant (20-100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg-10 ng/kg) significantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antagonism blocked this effect. Moreover, inhibition of opioid-induced excitatory signaling by naltrexone (1 ng/kg) unmasked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrexone could not increase the maximal anticonvulsant effect of morphine (1-3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while being completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids.

Effect of the A118G polymorphism on binding affinity, potency and agonist-mediated endocytosis, desensitization, and resensitization of the human mu-opioid receptor

The most prevalent single-nucleotide polymorphism (SNP) A118G in the human mu-opioid receptor gene predicts an amino acid change from an asparagine residue to an aspartatic residue in amino acid position 40. This N40D mutation, which has been implicated in the development of opioid addiction, was previously reported to result in an increased beta-endorphin binding affinity and a decreased potency of morphine-6-glucuronide. Therefore, in the present study we have investigated whether this mutation might affect the binding affinity, potency, and/or the agonist-induced desensitization, internalization and resensitization of the human mu-opioid receptor stably expressed in human embryonic kidney 293 cells. With the exception of a reduced expression level of N40D compared to human mu-opioid receptor (hMOR) in HEK293 cells, our analyses revealed no marked functional differences between N40D and wild-type receptor. Morphine, morphine-6-glucuronide and beta-endorphin revealed similar binding affinities and potencies for both receptors. Both the N40D-variant receptor and hMOR exhibited robust receptor internalization in the presence of the opioid peptide [d-Ala(2),N-MePhe(4),Glyol(5)]enkephalin (DAMGO) and beta-endorphin but not in response to morphine or morphine-6-glucuronide. After prolonged treatment with morphine, morphine-6-glucuronide or beta-endorphin both receptors showed similiar desensitization time courses. In addition, the receptor resensitization rates were nearly identical for both receptor types.

Reciprocal modulation of phospholipase Cbeta isoforms: adaptation to chronic morphine

Phosphoinositide turnover and calcium mobilization are fundamental determinants of acute and chronic opioid effects. Phosphoinositide-specific phospholipase C (PLC) are key signaling enzymes that play a pivotal role in mediating opioid modulation of inositol trisphosphate production and cytosolic calcium distribution, substrates for many acute and chronic opioid effects. Notably, phosphorylation of the beta isoforms of PLC, by kinases that are up-regulated after chronic morphine, is a potent modality for their regulation. Direct assessment of PLCbeta1 and PLCbeta3 phosphorylation in the guinea pig longitudinal muscle myenteric plexus tissue revealed substantial alterations after the induction of opioid tolerance. Notably, the direction of this modulation is isoform-specific. Phosphorylation of PLCbeta1 is significantly reduced, whereas that of PLCbeta3 is substantially augmented, changes not accompanied by altered content of PLCbeta1 or PLCbeta3 protein. In contrast to chronic morphine, acute morphine treatment of opioid naïve longitudinal muscle myenteric plexus tissue attenuates PLCbeta3 phosphorylation, an effect also manifested by endogenous opioids that is reflected by the ability of acute naloxone to substantially augment PLCbeta3 phosphorylation. This indicates that PLCbeta phosphorylation is dynamically regulated. PLCbeta1 and PLCbeta3 activities are negatively modulated by phosphorylation. Thus, their concomitant reciprocal phosphorylation would alter the relative contribution of these isoforms to PLC/Ca2+ signaling, a significant shift in light of their differential regulatory characteristics. Reciprocal modulation of the phosphorylation (activity) of two isoforms within the same subclass of signaling enzyme, proteins that have a high degree of structural similarity and subserve the same biological function, represents an adaptation modality to chronic morphine that has heretofore not been recognized.

Disruption of the type 1 inositol 1,4,5-trisphosphate receptor gene suppresses the morphine-induced antinociception in the mouse

The present study was designed to investigate whether endoplasmic inositol 1,4,5-trisphosphate (IP3) receptor-mediated intracellular signaling pathway could be involved in the morphine-induced antinociception in the mouse. An intracerebroventricular (i.c.v.) pretreatment with xestospongin C (10-100 pmol per mouse, i.c.v.), a membrane permeable and selective antagonist for IP3 receptor, produced a dose-dependent reduction in the supraspinal antinociceptive effect produced by i.c.v. administration of morphine (0.3-10 nmol). In addition, the dose-response curve for morphine-induced antinociception was significantly shifted to the right by i.c.v. pretreatment with xestospongin C at 100 pmol. In the present study, we confirmed that the IP3R1 mRNA in opisthonos (opt) heterozygote mice was approximately 50% reduced as compared to that in wild-type mice. Under these conditions, a significant antinociception produced by subcutaneous (s.c.) injection of morphine (5 mg/kg) observed in wild-type mice was markedly reduced in opt heterozygote mice. These findings suggest that IP3 receptor-, especially type 1 IP3 receptor-mediated intracellular signaling pathway may be implicated in the expression of antinociceptive effect induced by morphine in mice.

Alterations in brain metabolism induced by chronic morphine treatment: NMR studies in rat CNS

High-resolution (500 MHz) multiresonance/multinuclear proton (1H) nuclear magnetic resonance (NMR) spectroscopy was used to detect metabolic changes and cellular injury in the rat brain stem and spinal cord following chronic morphine treatment. Compensatory changes were observed in glycine, glutamate, and inositols in the brain stem, but not the spinal cord, of chronic morphine-treated rats. In spinal cord, increases were detected in lactate and N-acetyl-aspartate (NAA), suggesting that there is anaerobic glycolysis, plasma membrane damage, and altered pH preferentially in the spinal cord of chronic morphine-treated rats.

ERK1/2 activation in rat ventral tegmental area by the mu-opioid agonist fentanyl: an in vitro study

Opioid receptors in the ventral tegmental area, predominantly the mu-opioid receptors, have been suggested to modulate reinforcement sensitivity for both opioid and non-opioid drugs of abuse. The present study was conducted to study signal transduction proteins, which may mediate the functioning of mu-opioid receptors in the neurons of the ventral tegmental area. Therefore, brain slices of the ventral tegmental area were exposed in vitro to the specific mu-opioid agonist fentanyl and immunohistochemically stained for four different activated proteins using phospho-specific antibodies. Fentanyl dose-dependently activated extracellular signal-regulated protein in brain slices of the ventral tegmental area. This activation was reversible with naloxone. Furthermore, naloxone itself also activated extracellular signal-regulated protein kinase. Under the present conditions fentanyl did not affect extracellular signal-regulated protein kinase 1 and 2, Stat and cyclic AMP-response element-binding protein activity. The direct activation of extracellular signal-regulated protein kinase in ventral tegmental area slices by the mu-opioid agonist fentanyl may suggest a role of extracellular signal-regulated protein kinase in reward processes.

Kappa-opioid receptor-mediated enhancement of the hyperpolarization-activated current (I(h)) through mobilization of intracellular calcium in rat nucleus raphe magnus

The hyperpolarization-activated current (Ih) is important in the control of resting membrane potential, in the regulation of network firing pattern and in the modulation of presynaptic transmitter release in central neurons. Recent studies on native and cloned Ih channels have demonstrated that the Ih channel is commonly modulated by cAMP through a positive shift in its voltage dependence without a change in its maximum current. The present study demonstrates that activation of kappa-opioid receptors enhances Ih by increasing its maximum current in brainstem neurons in the nucleus raphe magnus. Agents that interfere with the release of intracellular calcium from calcium stores altered the maximum Ih and significantly attenuated the kappa-receptor-mediated enhancement of Ih. These results suggest that kappa-opioid receptors enhance the maximum Ih by mobilizing intracellular calcium from calcium stores. This provides a physiological function for kappa-receptor-stimulated calcium release and may suggest another Ih-regulating mechanism by intracellular calcium in central neurons.

The role of spinal cholecystokinin in chronic pain states

It is well established that cholecystokinin (CCK) reduces the antinociceptive effect of opioids. The level of CCK and CCK receptors, as well as CKK release, exhibits considerable plasticity after nerve injury and inflammation, conditions known to be associated with chronic pain. Such altered CCK release coupled in some situation with changes in CCK receptor levels may underlie the clinical phenomenon of varying opioid sensitivity in different clinical pain conditions. In particular, neuropathic pain after injury to the peripheral and central nervous system does not respond well to opioids, which is likely to be caused by increased activity in the endogenous CCK system. CCK receptor antagonists may thus be useful as analgesics in combination with opioids to treat neuropathic pain.

Attenuation of morphine tolerance after antisense oligonucleotide knock-down of spinal mGluR1

1. Chronic systemic treatment of rats with morphine leads to the development of opioid tolerance. This study was designed to examine the effects of intrathecal (i.t.) infusion of a metabotropic glutamate receptor 1 (mGluR1) antisense oligonucleotide, concomitant with chronic morphine treatment, on the development of tolerance to morphine's antinociceptive effects. 2. All rats received chronic (6 day) s.c. administration of morphine to induce opioid tolerance. Additionally, rats were treated with either mGluR1 antisense (AS), missense (MIS) or artificial cerebrospinal fluid (ACSF) by i.t. infusion via chronically implanted i.t. catheters connected to osmotic mini-pumps. The effects of acute i.t. or s.c. morphine on tail-flick latencies were assessed prior to and following chronic s.c. morphine treatment for all chronic i.t. infusion groups. mGluR1 protein level in the spinal cord was determined by Western blot analysis for all treatments, assessing the efficiency of knock-down with AS treatment. 3. Acute i.t. morphine dose-dependently produced antinociception in the tail-flick test in naïve rats. Systemic morphine-treated rats administered i.t. ACSF or MIS developed tolerance to i.t. morphine. Chronic i.t. infusion with mGluR1 AS significantly reduced the development of tolerance to i.t. morphine. 4. In contrast to i.t. morphine, tolerance developed to the antinociceptive effects of s.c. morphine, in all i.t. infusion groups, including the mGluR1 AS group. 5. The spinal mGluR1 protein level was dramatically decreased after mGluR1 AS infusion when compared to control animals (naïve and ACSF-treated animals). 6. These findings suggest that the spinal mGluR1 is involved in the development of tolerance to the antinociceptive effects of morphine. Selective blockade of mGluR1 may be beneficial in preventing the development of opioid analgesic tolerance.

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.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Cyclic AMP regulates the calcium transients released from IP(3)-sensitive stores by activation of rat kappa-opioid receptors expressed in CHO cells

We analyzed intracellular Ca(2+)and cAMP levels in Chinese hamster ovary cells expressing a cloned rat kappa opioid receptor (CHO-kappa cells). Although expression of kappa(kappa)-opioid receptors was confirmed with a fluorescent dynorphin analog in almost all CHO-kappa cells, the kappa-specific agonists, U50488H or U69593, induced a Ca(2+) transient only in 35% of the cells. The Ca(2+) response occurred in all-or-none fashion and the half-maximal dosage of U50488H (812.1nM) was higher than that (3.2nM) to inhibit forskolin-stimulated cAMP. The kappa-receptors coupled to G(i/o)proteins since pertussis toxin significantly reduced the U50488H actions on intracellular Ca(2+) and cAMP. The Ca(2+) transient originates from IP(3)-sensitive internal stores since the Ca(2+) response was blocked by a PLC inhibitor (U73122) or by thapsigargin depletion of internal stores while removal of extracellular Ca(2+) had no effect. Interestingly, application of dibutyryl cAMP (+ 56.2%) or 8-bromo-cAMP (+ 174.7%) significantly increased the occurrence of U50488H-induced Ca(2+) mobilization while protein kinase A (PKA) inhibitors, Rp-cAMP (-32.3%) or myr-psi PKA (-73.9%) significantly reduced the response. Therefore, it was concluded that cAMP and PKA activity can regulate the Ca(2+) mobilization. These results suggest that the kappa receptor-linked cAMP cascade regulates the occurrence of kappa-opioid-mediated Ca(2+) mobilization.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

Calcium and the anaesthetist

Calcium plays a central role in a large number of physiological actions that are essential for life. It is important therefore that the anaesthetist understands calcium pathophysiology. In this review, the physiology, regulation, clinical features, causes and treatment of alterations in circulating calcium will be discussed. In addition, the effects that acid-base status, massive blood transfusion and cardiopulmonary bypass may have on circulating calcium will be highlighted. Finally, the role that calcium plays in ischaemic/reperfusion injury and myocardial stunning will be summarised.

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolefance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly Gialpha inhibitory to Gbetagamma stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.

Opioid enhancement of calcium oscillations and burst events involving NMDA receptors and L-type calcium channels in cultured hippocampal neurons

Opioid receptor agonists are known to alter the activity of membrane ionic conductances and receptor-activated channels in CNS neurons and, via these mechanisms, to modulate neuronal excitability and synaptic transmission. In neuronal-like cell lines opioids also have been reported to induce intracellular Ca(2+) signals and to alter Ca(2+) signals evoked by membrane depolarization; these effects on intracellular Ca(2+) may provide an additional mechanism through which opioids modulate neuronal activity. However, opioid effects on resting or stimulated intracellular Ca(2+) levels have not been demonstrated in native CNS neurons. Thus, we investigated opioid effects on intracellular Ca(2+) in cultured rat hippocampal neurons by using fura-2-based microscopic Ca(2+) imaging. The opioid receptor agonist D-Ala(2)-N-Me-Phe(4),Gly-ol(5)-enkephalin (DAMGO; 1 microM) dramatically increased the amplitude of spontaneous intracellular Ca(2+) oscillations in the hippocampal neurons, with synchronization of the Ca(2+) oscillations across neurons in a given field. The effects of DAMGO were blocked by the opioid receptor antagonist naloxone (1 microM) and were dependent on functional NMDA receptors and L-type Ca(2+) channels. In parallel whole-cell recordings, DAMGO enhanced spontaneous, synaptically driven NMDA receptor-mediated burst events, depolarizing responses to exogenous NMDA and current-evoked Ca(2+) spikes. These results show that the activation of opioid receptors can augment several components of neuronal Ca(2+) signaling pathways significantly and, as a consequence, enhance intracellular Ca(2+) signals. These results provide evidence of a novel neuronal mechanism of opioid action on CNS neuronal networks that may contribute to both short- and long-term effects of opioids.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

Potentiation of transmitter release from NMB human neuroblastoma cells by kappa-opioids is mediated by N-type voltage-dependent calcium channels

The selective kappa-opioid agonist trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzenacetamidemethansulfonate (U50,488) potentiates both basal and depolarization-evoked [3H]dopamine release from NMB cells. The potentiation of dopamine release by U50,488 is mediated by N-type voltage-dependent calcium channels since it is blocked by omega-conotoxin, and is resistant to pertussis toxin (PTX)-treatment. When the stimulation of release by U50,488 is blocked by the N-channel antagonist omega-conotoxin, an inhibitory effect on dopamine release is revealed, suggesting that stimulatory and inhibitory effects of U50,488 are exerted in parallel.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

Genetic alteration of phospholipase C beta3 expression modulates behavioral and cellular responses to mu opioids

Morphine and other micro opioids regulate a number of intracellular signaling pathways, including the one mediated by phospholipase C (PLC). By studying PLC beta3-deficient mice, we have established a strong link between PLC and mu opioid-mediated responses at both the behavioral and cellular levels. Mice lacking PLC beta3, when compared with the wild type, exhibited up to a 10-fold decrease in the ED(50) value for morphine in producing antinociception. The reduced ED(50) value was unlikely a result of changes in opioid receptor number or affinity because no differences were found in whole-brain B(max) and K(d) values for mu, kappa, and delta opioid receptors between wild-type and PLC beta3-null mice. We also found that opioid regulation of voltage-sensitive Ca(2+) channels in primary sensory neurons (dorsal root ganglion) was different between the two genotypes. Consistent with the behavioral findings, the specific mu agonist [D-Ala(2),(Me)Phe(4),Gly(ol)(5)]enkephalin (DAMGO) induced a greater whole-cell current reduction in a greater proportion of neurons isolated from the PLC beta3-null mice than from the wild type. In addition, reconstitution of recombinant PLC protein back into PLC beta3-deficient dorsal root ganglion neurons reduced DAMGO responses to those of wild-type neurons. In neurons of both genotypes, activation of protein kinase C with phorbol esters markedly reduced DAMGO-mediated Ca(2+) current reduction. These data demonstrate that PLC beta3 constitutes a significant pathway involved in negative modulation of mu opioid responses, perhaps via protein kinase C, and suggests the possibility that differences in opioid sensitivity among individuals could be, in part, because of genetic factors.

Stimulation of mu- and delta-opioid receptors enhances phosphoinositide metabolism in mouse spinal cord: evidence for subtypes of delta-receptors

The accumulation of inositol phosphates (IPs) induced by agonist-activated opioid receptors was analysed in mouse spinal cord slices pre-labelled with myo-[3H]inositol. Agonists showing selectivity to mu-opioid receptors, morphine and [D-Ala2,MePhe4, Gly(ol)5]enkephalin (DAMGO), promoted concentration-dependent increases in the formation of IPs. The activation of delta-opioid receptors by the selective agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II produced similar increases in phosphoinositide (PI) metabolism. Pre-treatment of the slices with pertussis toxin (PTX) blocked the effect of opioid agonists on IP production. The involvement of Gi/Go-protein (guanine nucleotide-binding protein) classes in this opioid effect is therefore suggested. The activity of the opioid agonists was reduced by the opioid antagonists naltrexone and naloxone. The antagonist at delta1-receptors, 7-benzylidenenaltrexone (BNTX), exhibited greater potency than the antagonists at delta2-receptors, naltriben methanesulphonate (NTB) or naltrindrole 5'-isothiocyanate (NT II), in reducing the activating effect of DPDPE on phosphoinositide metabolism. Conversely, NTB and NT II were more potent antagonists of the activity of [D-Ala2]deltorphin II than BNTX. This work demonstrates the coupling of spinal mu- and delta-opioid receptors to phospholipase C and the generation of IPs. It also provides biochemical evidence for pharmacological subtypes of delta-opioid receptors in the activation of this signalling pathway.

Met-enkephalin-induced mobilization of intracellular Ca2+ in rat intracardiac ganglion neurones

The effects of Met-enkephalin on Ca2+-dependent K+ channel activity were investigated using the cell-attached patch recording technique on isolated parasympathetic neurones of rat intracardiac ganglia. Large-conductance, Ca2+-dependent K+ channels (BK(Ca)) were examined as an assay of agonist-induced changes in the intracellular free calcium ion concentration ([Ca2+]i). These BK(Ca) channels had a conductance of approximately 200 pS and were charybdotoxin- and voltage-sensitive. Caffeine (5 mM), used as a control, evoked a large increase in BK(Ca) channel activity, which was inhibited by 10 microM ryanodine. Met-enkephalin (10 microM) evoked a similar increase in BK(Ca) channel activity, which was dependent on the presence of extracellular Ca2+ and inhibited by either ryanodine (10 microM) or naloxone (1 microM). In Fura-2-loaded intracardiac neurones, Met-enkephalin evoked a transient increase in [Ca2+]i. Met-enkephalin-induced mobilization of intracellular Ca+ may play a role in neuronal excitability and firing behaviour in mammalian intracardiac ganglia.

Nonselective coupling of the human mu-opioid receptor to multiple inhibitory G-protein isoforms

The human mu-opioid receptor was expressed in Saccharomyces cerevisiae. Binding of [3H]diprenorphine to yeast spheroplasts was specific and saturable (Kd = 1 nm, Bmax = 0.2-1 pmol x mg-1 of membrane proteins). Inhibition of [3H]diprenorphine binding by antagonists and agonists with varying opioid selectivities (mu, delta and kappa) occurred with the same order of potency as in mammalian tissues. Affinities of antagonists were the same with yeast spheroplasts as in reference tissues whereas those of agonists, except etorphine and buprenorphine, were 10-fold to 100-fold lower. Addition of heterotrimeric Gi,o-proteins purified from bovine brain shifted the mu-opioid receptor into a high-affinity state for agonists. Using individually purified Galpha-subunits re-associated with betagamma-dimers, we showed that alphao1, alphao2, alphai1, alphai2 and alphai3 reconstituted high-affinity agonist binding with equal efficiency. This suggests that the structural determinants of the mu-opioid receptor responsible for G-protein coupling are not able to confer a high degree of specificity towards any member of the Gi,o family. The selective effects of opioid observed in specialized tissues upon opioid stimulation may be a result of regulation of G-protein activity by cell-specific factors which should conveniently be analysed using the reconstitution assay described here.

Enkephalin receptors and receptor-mediated signal transduction in cultured human lymphocytes

Enkephalins are opioid peptides that bridge the neuroendocrine and immune systems. Using flow cytometry and a fluorescein conjugate of the endogenous pentapeptide methionine-enkephalin (ME), we have identified enkephalin receptors on cultured human lymphocytes. Cell surface recognition sites that bound ME with high affinity and specificity were detected for NALM 6 (pre-B acute lymphoblastic leukemia) and Jurkat (T lymphoma) cells. Brain-like enkephalin receptors were measured for these lymphocytes using conventional radioligand-receptor assays and the highly delta opioid receptor-selective enkephalin analog [3H]DPDPE. Upon activation, the lymphocyte enkephalin receptors transmitted signals that enhanced the accumulation of intracellular cAMP. These studies provide evidence that cultured human lymphocytes of the B (NALM 6 cells) and T (Jurkat cells) lineages express functional enkephalin receptors and suggest that such receptors may be instrumental in the lymphocyte response to opioid peptides and alkaloids.

Mu-opioid and GABA(B) receptors modulate different types of Ca2+ currents in rat nodose ganglion neurons

Whole-cell patch-clamp recordings were obtained from nodose ganglion neurons acutely dissociated from 10-30-day-old rats to characterize the Ca2+ channel types that are modulated by GABA(B) and mu-opioid receptors. Five components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nifedipine, omega-agatoxin IVA and omega-conotoxin MVIIC. Administration of the mu-opioid agonist H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol (0.3-1 mM) or the GABA(B) agonist baclofen in saturating concentrations suppressed high-threshold Ca2+ currents by 49.9+/-2.4% (n=69) and 18.7+/-2.1% (n=35), respectively. The inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol exceeded that by baclofen in virtually all neurons that responded to both agonists (67%), and occlusion experiments revealed that responses to mu-opioid and GABA(B) receptor activation were not linearly additive. In addition, administration of staurosporine, a non-selective inhibitor of protein kinase A and C, did not affect the inhibitory responses to either agonist or prevent the occlusion of baclofen-induced current inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol. Blockade of N-type channels by omega-conotoxin GVIA eliminated current suppression by baclofen in all cells tested (n=11). Mu-opioid-induced inhibition in current was abolished by omega-conotoxin GVIA in 12 of 30 neurons tested, but was only partially reduced in the remaining 18 neurons. In the latter cells administration of omega-agatoxin IVA reduced, but did not eliminate the mu-opioid sensitive current component that persisted after blockade of N-type channels. This residual component of mu-opioid-sensitive current was blocked completely by omega-conotoxin MVIIC in nine neurons, whereas responses to H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol were still recorded in the remaining cells after administration of these Ca2+ channel toxins and nifedipine. Dihydropyridine-sensitive (L-type) current was not affected by activation of mu-opioid or GABA(B) receptors in any of the neurons. These data indicate that in nodose ganglion neurons mu-opioid receptors are negatively coupled to N-, P- and Q-type channels as well as to a fourth, unidentified toxin-resistant Ca2+ channel. In contrast, GABA(B) receptors are coupled only to N-type channels. Furthermore, the results do not support a role for either protein kinase C or A in the modulatory pathway(s) coupling mu-opioid and GABA(B) receptors to Ca2+ channels, but rather lend credence to the notion that the signalling mechanisms utilized by these two receptors might simply compete for inhibitory control of a common pool of N-type channels.

Quantitative immunolocalization of mu opioid receptors: regulation by naltrexone

The present study utilized a newly developed quantitative immunohistochemical assay to measure changes in mu opioid receptor abundance following chronic administration of the opioid receptor antagonist naltrexone. These data were compared with those obtained from mu receptor radioligand binding on adjacent tissue sections, in order to determine whether the characteristic antagonist-induced increase in radioligand binding is due to an increase in the total number of mu receptors and/or to an increase in the proportion of receptors that are in an active binding conformation in the absence of a change in the total number of receptors. Adult male Sprague-Dawley rats were administered naltrexone, 7-8 mg/kg per day, or saline continuously for seven days by osmotic minipumps, after which time their brains were processed for immunohistochemistry and receptor autoradiography on adjacent fresh frozen tissue sections. Semiquantitative immunohistochemistry was performed using a radiolabelled secondary antibody for autoradiographic determination and a set of radioactive standards. Results demonstrate an overall concordance between the distribution of mu opioid receptors as measured by the two different methods with a few exceptions. Following naltrexone administration, mu receptor immunoreactivity was significantly higher in the amygdala, thalamus, hippocampus, and interpeduncular nucleus as compared with the saline-treated control animals. [3H]D-Ala2,N-Me-Phe4,Gly-ol5-enkephalin binding to mu opioid receptors was significantly higher in the globus pallidus, amygdala, thalamus, hypothalamus, hippocampus, substantia nigra, ventral tegmental area, central gray, and interpeduncular nucleus of the naltrexone-treated rats. These findings indicate that in some brain regions chronic naltrexone exposure increases the total number of mu opioid receptors, while in other regions there is an increase in the percent of active receptors without an observable change in the total number of receptors. Quantitative receptor immunodetection together with ligand autoradiography provides a new approach for investigating the regulation of mu opioid receptors on tissue sections.

Delta-opioid receptors on astroglial cells in primary culture: mobilization of intracellular free calcium via a pertussis sensitive G protein

Astrocytes in primary culture from rat cerebral cortex were probed concerning the expression of delta-opioid receptors and their coupling to changes in intracellular free calcium concentrations ([Ca2+]i). Fluo-3 or fura-2 based microspectrofluorometry was used for [Ca2+]i measurements on single astrocytes in a mixed astroglial-neuronal culture. Application of the selective delta-opioid receptor agonist, [D-Pen2, D-Pen5]-enkephalin (DPDPE), at concentrations ranging from 10 nM to 100 microM, induced concentration-dependent increases in [Ca2+]i (EC50 = 114 nM). The responses could be divided into two phases, with an initial spike in [Ca2+]i followed by either oscillations or a sustained elevation of [Ca2+]i. These effects were blocked by the selective delta-opioid receptor antagonist ICI 174864 (10 microM). The expression of delta-opioid receptors on astroglial cells was further verified immunohistochemically, using specific antibodies, and by Western blot analyses. Pre-treatment of the cells with pertussis toxin (100 ng/ml, 24 h) blocked the effects of delta-opioid receptor activation, consistent with a Gi- or Go-mediated response. The sustained elevation of [Ca2+]i was not observed in low extracellular Ca2+ and was partly blocked by nifedipine (1 microM), indicating the involvement of L-type Ca2+ channels. Stimulating neurons with DPDPE resulted in a decrease in [Ca2+]i, which may be consistent with the closure of the plasma membrane Ca2+ channels on these cells. The current results suggest a role for astrocytes in the response of the brain to delta-opioid peptides and that these opioid effects in part involve altered astrocytic intracellular Ca2+ homeostasis.

Perinatal exposure to morphine disrupts brain norepinephrine, ovarian cyclicity, and sexual receptivity in rats

The effect of perinatal exposure to morphine on the development of catecholaminergic and reproductive function in female rats was investigated. Adult rats received morphine intraperitoneally daily for 40 days. The dose of morphine was progressively increased at 10-day intervals from 5, 7.5, 10 to 15 mg/kg body weight until day 40. The rats were mated between days 38 and 45. Administration of morphine at dose rates of 20 and 30 mg/kg continued during pregnancy. The dose was increased to 40 mg/kg for 10 days postpartum. Results showed that morphine disrupted ovarian cyclicity in 52% of the females. Amongst the remaining females, 43% became pregnant when mated. In the female offspring born to such dams, sexual maturation was delayed and body weight was reduced until weaning. At adulthood, lordosis behavior was inhibited when the female offspring were tested against stimulus males. Plasma estradiol and ovarian estradiol and progesterone levels were reduced. Norepinephrine concentration in the hypothalamus was reduced, whereas it remained unchanged in the amygdala. Dopamine concentrations in both hypothalamus and amygdala were not influenced by perinatal morphine exposure. These results suggest that chronic morphine treatment during perinatal life selectively influences the development of noradrenergic mechanisms in the rat brain and this may in turn be responsible for reduced reproductive activity.

Opioids potentiate transmitter release from SK-N-SH human neuroblastoma cells by modulating N-type calcium channels

Opioids induce dual (inhibitory and excitatory) regulation of depolarization-evoked [3H]dopamine release in SK-N-SH cells through either mu or delta receptors. The potentiation of dopamine release by opioid agonists is mediated by N-type voltage-dependent calcium channels and does not involve Gi/Go proteins. Removal of the excitatory opioid effect by blockade with omega-conotoxin, an N-channel antagonist, reveals the inhibitory effect of opioids on release, thus suggesting that both modulatory effects of opioids are exerted in parallel.

G proteins and opioid receptor-mediated signalling

Most opioid receptor-mediated functions appear to be mediated through G protein interactions, therefore an understanding of opioid signalling requires knowledge of those interactions. This review chronicles the studies examining these interactions for all the opioid receptor subtypes, both in vivo and in vitro.

Mechanisms of actions of opioids and non-steroidal anti-inflammatory drugs

Opioids and non-steroidal anti-inflammatory drugs (NSAIDs) are the commonest drugs used to treat pain. Opioids mimic the actions of endogenous opioid peptides by interacting with mu, delta or kappa opioid receptors. The opioid receptors are coupled to G1 proteins and the actions of the opioids are mainly inhibitory. They close N-type voltage-operated calcium channels and open calcium-dependent inwardly-rectifying potassium channels. This results in hyperpolarization and a reduction in neuronal excitability. They also decrease intracellular cAMP which modulates the release of nociceptive neurotransmitters (e.g. substance P). Inhibition of prostaglandin synthesis by cyclooxygenase is the principal mode of the analgesic and anti-inflammatory actions of NSAIDs. Cyclo-oxygenase is inhibited irreversibly by aspirin and reversibly by other NSAIDs. The widespread inhibition of cyclo-oxygenase is responsible for many of the adverse effects of these drugs. NSAIDs also reduce prostaglandin production within the CNS. This is the main action of paracetamol.

The effects of recombinant rat mu-opioid receptor activation in CHO cells on phospholipase C, [Ca2+]i and adenylyl cyclase

1. The rat mu-opioid receptor has recently been cloned yet its second messenger coupling remains unclear. The endogenous mu-opioid receptor in SH-SY5Y cells couples to phospholipase C (PLC), increases [Ca2+]i and inhibits adenylyl cyclase (AC). We have examined the effects of mu-opioid agonists on inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), [Ca2+]i and adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation in Chinese hamster ovarian (CHO) cells transfected with the cloned mu-opioid receptor. 2. Opioid receptor binding was assessed with [3H]-diprenorphine ([3H]-DPN) as a radiolabel. Ins(1,4,5)P3 and cyclic AMP were measured by specific radioreceptor assays. [Ca2+]i was measured fluorimetrically with Fura-2. 3. Scatchard analysis of [3H]-DPN binding revealed that the Bmax varied between passages. Fentanyl (10 pM 1 microM) dose-dependently displaced [3H]-DPN, yielding a curve which had a Hill slope of less than unity (0.6 +/- 0.1), and was best fit to a two site model, with pK1 values (% of sites) of 9.97 +/- 0.4 (27 +/- 4.8%) and 7.68 +/- 0.07 (73 +/- 4.8%). In the presence of GppNHp (100 microM) and Na+ (100 mM), the curve was shifted to the right and became steeper (Hill slope = 0.9 +/- 0.1) with a pK1 value of 6.76 +/- 0.04. 4. Fentanyl (0.1 nM-1 microM) had no effect on basal, but dose-dependently inhibited forskolin (1 microM)-stimulated, cyclic AMP formation (pIC50 -7.42 +/- 0.23), in a pertussis toxin (PTX; 100 ng ml-1 for 24 h)-sensitive and naloxone-reversible manner (K1 = 1.7 nM). Morphine (1 microM) and [D-Ala2, MePhe4, gly(ol)5]-enkephalin (DAMGO, 1 microM) also inhibited forskolin (1 microM)-stimulated cyclic AMP formation, whilst [D-Pen2, D-Pen5], enkephalin (DPDPE, 1 microM) did not. 5. Fentanyl (0.1 nM-10 microM) caused a naloxone (1 microM)-reversible, dose-dependent stimulation of Ins(1,4,5)P3 formation, with a pEC50 of 7.95 +/- 0.15 (n-5), PTX (100 ng ml-1 for 24 h) abolished, whilst Ni2 (2.5 mM) inhibited (by 52%), the fentanyl-induced Ins(1,4,5)P3 response. Morphine (1 microM) and DAMGO (1 microM), but not DPDPE (1 microM), also stimulated Ins(1,4,5)P3 formation. Fentanyl (1 microM) also caused an increase in [Ca2+]i (80 +/- 16.4 nM, n-6), reaching a maximum at 26.8 +/- 2.5 s. The increase in [Ca2+]i remained elevated until sampling ended (200 s) and was essentially abolished by the addition of naloxone (1 microM). Pre-incubation with naloxone (1 microM, 3 min) completely abolished fentanyl-induced increases in [Ca2+]i. 6. In conclusion, the cloned mu-opioid receptor when expressed in CHO cells stimulates PLC and inhibits AC, both effects being mediated by a PTX-sensitive G-protein. In addition, the receptor couples to an increase in [Ca2+]i. These findings are consistent with the previously described effector-second messenger coupling of the endogenous mu-opioid receptor.

Pertussis toxin, but not tyrosine kinase inhibitors, abolishes effects of U-50488H on [Ca2+]i in myocytes

The effects of 10(-5) M trans-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeacetamidel (U-50488H), a kappa-opioid receptor agonist, on cytosolic Ca2+ concentration ([Ca2+]i) and the [Ca2+]i transient in quiescent and electrically stimulated rat ventricular myocytes, respectively, were determined after the cells had been pretreated with pertussis toxin (PTX) or a tyrosine kinase inhibitor (genistein or tyrphostin). The [Ca2+]i was determined with a spectrofluorometric method, with fura 2 as Ca2+ indicator. U-50488H at 10(-5) M itself induced a [Ca2+]i transient in the quiescent cells but inhibited the [Ca2+]i transient in electrically stimulated cells. The effects of 10(-5) M U-50488H on [Ca2+]i, which were blocked by a selective kappa-opioid receptor antagonist, nor-binaltorphimine (10(-6) M), were abolished after pretreatment with PTX (1 microg/ml) for 24 h, but not with genistein (10(-4) M) or tyrphostin (5 x 10(-5) M) for 30 min. 1-[6-[[(17b)-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexy l]-1H-pyrrole-2,5-dione (U-73122), an inhibitor of phospholipase C, at 10(-5) M, but not its inactive structural isomer 1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexy l]-2,5-pyrrolidinedione (U-73343), also blocked the Ca2+ responses to U-50488H. The results indicate that activation of phospholipase C on kappa-opioid receptor stimulation is via PTX-sensitive G proteins but does not involve protein tyrosine phosphorylation.

Pretreatment with protein kinase C activator phorbol 12,13-dibutyrate attenuates the antinociception induced by mu- but not epsilon-opioid receptor agonist in the mouse

The effects of pretreatment with a protein kinase C activator, phorbol 12,13-dibutyrate, on antinociception induced by i.c.v.-administered mu-opioid receptor agonist (D-Ala2, NMePhe4, Gly(ol)5) enkephalin (DAMGO) or morphine and epsilon-opioid receptor agonist beta-endorphin were studied in male ICR mice. The tail-flick responses were used for antinociceptive tests. I.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 30 or 60 but not 10 min attenuated antinociception induced by i.c.v.-administered DAMGO. I.c.v. pretreatment with phorbol 12,13-dibutyrate (10 and 50 pmol) for 60 min caused a dose-dependent attenuation of DAMGO (19.5 pmol)- or morphine (6.0 nmol)-induced antinociception. The dose-response curve for DAMGO-induced antinociception was shifted to the right by 7.3-fold by i.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 60 min. However, the i.c.v.-administered beta-endorphin-induced antinociception was not affected by the same pretreatment with phorbol 12,13-dibutyrate. The attenuation of i.c.v.-administered DAMGO- and morphine-induced antinociception by phorbol 12,13-dibutyrate was reversed by concomitant i.c.v. pretreatment with a selective protein kinase C inhibitor calphostin C. These results suggest that activation of protein kinase C by phorbol 12,13-dibutyrate leads to the desensitization of mu-, but not epsilon-opioid receptor-mediated antinociception. These findings also provide additional evidence for differential intracellular modulation on antinociceptive action of mu- and epsilon-opioid receptor agonists.