Morphine withdrawal in vitro: potentiation of agonist-dependent polyphosphoinositide breakdown

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.

Mapping of a quantitative trait locus for morphine withdrawal severity

Chronic morphine exposure results in physical dependence, manifested by physical symptoms during naloxone-precipitated withdrawal. Jumping frequency is widely considered the most sensitive and reliable index of withdrawal intensity in mice. Inbred mouse strains surveyed for naloxone-precipitated withdrawal display large and significant strain differences in jumping frequency, including an approximately tenfold difference between C57BL/6 and 129P3 mice. In the present study, (B6 x 129)F2 hybrid mice were given daily morphine injections for four days using an escalating dosing schedule, and naloxone-precipitated withdrawal on day 5 was measured. A full-genome scan for linkage to phenotypic data was performed using polymorphic microsatellite markers. Significant linkage was observed between withdrawal jumping frequencies and a 28 cM-wide region of Chromosome 1 (32-60 cM; peak at 51 cM), accounting for 20% of the overall phenotypic variance. Two other suggestive QTLs were found, on Chromosomes 5 and 10, and an additive model fitting all three loci accounted for 43% of the total variance. F2 mice were also assessed for changes in morphine analgesic potency using the tail-withdrawal test in dose-response studies on days 1 and 4. No linkage was observed between Chromosomes 1, 5, and 10 and morphine analgesic tolerance, suggestive of genetic dissociation of naloxone-precipitated withdrawal from morphine and chronic morphine intake per se. The significant quantitative trait locus for naloxone-precipitated withdrawal severity in morphine-dependent mice, which we name Depmq1, may prove to be of considerable heuristic value once the underlying gene or genes are identified.

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.

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.

Modification of the expression of naloxone-precipitated withdrawal signs in morphine-dependent mice by diabetes: possible involvement of protein kinase C

The involvement of cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC) in the modulation of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes was examined. Naloxone-precipitated withdrawal jumps were significantly less in morphine-dependent diabetic mice than in morphine-dependent non-diabetic mice. I.c.v. pretreatment with either calphostin C, a PKC inhibitor, or KT-5720, a PKA inhibitor, attenuated naloxone-precipitated withdrawal jumps in morphine-dependent non-diabetic mice. However, naloxone-precipitated withdrawal jumps in morphine-dependent diabetic mice were not attenuated by i.c.v. pretreatment with either calphostin C or KT5720. Moreover, i.c.v. pretreatment with phorbol-12,13-dibutyrate (PDBu), a PKC activator, attenuated naloxone-precipitated withdrawal jumps in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice. The noradrenaline (NA) turnover in the frontal cortex in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was significantly increased 5 min after administration of naloxone. Naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was blocked by i.c.v. pretreatment with either calphostin C or KT5720 1 hr before naloxone challenge and blocked by PDBu 1 hr before the last injection of morphine. These results suggest that the co-activation of PKC and PKA is needed to elicit naloxone-precipitated withdrawal jumps and enhancement of turnover rate of NA in the frontal cortex in morphine-dependent non-diabetic mice. Furthermore, the attenuation of naloxone-precipitated withdrawal jumps in morphine-dependent diabetic mice may be due, in part, to the desensitization of mu-opioid receptors by the activation of PKC.

Role of intracellular calcium on the modulation of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes

The role of intracellular calcium in the modifications of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes was examined. Naloxone-precipitated withdrawal jumping was significantly less in morphine-dependent diabetic mice than in morphine-dependent non-diabetic mice. Intracerebroventricular (i.c.v. ) pretreatment with ryanodine attenuated naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice. However, naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice was not affected by i.c.v. pretreatment with ryanodine. Moreover, i.c.v. pretreatment with thapsigargin, a Ca2+-ATPase inhibitor, enhanced naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice. The noradrenaline (NA) turnover in the frontal cortex in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was significantly increased by naloxone injection. Naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was blocked by i.c.v. pretreatment with ryanodine. In contrast to ryanodine, thapsigargin enhanced naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice. These results suggest that increased intracellular calcium augmented naloxone-precipitated withdrawal jumping and the turnover rate of NA in the frontal cortex in morphine-dependent non-diabetic mice. Furthermore, it seems likely that the attenuation of naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice may be due, in part, to the dysfunction of intracellular calcium store.

Attenuation of precipitated morphine withdrawal symptoms by acute i.c.v. administration of a group II mGluR agonist

1. We previously showed that chronic i.c.v. antagonism of metabotropic glutamate receptors (mGluRs) concurrently with s.c. morphine significantly attenuated precipitated withdrawal symptoms. Conversely, acute i.c.v. injection of a selective group II mGluR antagonist just before the precipitation of withdrawal exacerbated abstinence symptoms. 2. In the present study, we showed that acute i.c.v. administration of the non-selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), as well as the group II selective agonist (2S,1'R,2'R,3'R)-2-(2'.3'-dicarboxycyclopropyl)glycine (DCG-IV), significantly attenuated the severity of precipitated withdrawal symptoms. 3. From these results we hypothesize that chronic opioid treatment may indirectly induce a desensitization of group II mGluRs, which contributes to the development of dependence.

Pharmacological characterization of metabotropic glutamate receptors coupled to phospholipase D in the rat hippocampus

1. Phospholipase D (PLD) is the key enzyme in a signal transduction pathway leading to the formation of the second messengers phosphatidic acid and diacylglycerol. In order to define the pharmacological profile of PLD-coupled metabotropic glutamate receptors (mGluRs), PLD activity was measured in slices of adult rat brain in the presence of mGluR agonists or antagonists. Activation of the phospholipase C (PLC) pathway by the same agents was also examined. 2. The mGluR-selective agonist (1S,3R)-l-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced a concentration-dependent (10-300 microM) activation of PLD in the hippocampus, neocortex, and striatum, but not in the cerebellum. The effect was particularly evident in hippocampal slices, which were thus used for all subsequent experiments. 3. The rank order of potencies for agonists stimulating the PLD response was: quisqualate > ibotenate > (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine > (1S,3R)-ACPD > L-cysteine sulphinic acid > L-aspartate > L-glutamate. L-(+)-2-Amino-4-phosphonobutyric acid and the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate failed to activate PLD. (RS)-3,5-dihydroxyphenylglycine (100300 microM), an agonist of mGluRs of the first group, stimulated PLC but inhibited the PLD response elicited by 100 microM (1S,3R)-ACPD. 4. (+)-alpha-Methyl-4-carboxyphenylglycine (0.1-1 mM), a competitive antagonist of mGluRs of the first and second group, elicited a significant PLD response. L-(+)-2-Amino-3-phosphonopropionic acid (1 mM), an antagonist of mGluRs of the first group, inhibited the 100 microM (1S,3R)-ACPD-induced PLC response but produced a robust stimulation of PLD. 5. 12-O-Tetradecanoylphorbol 13-acetic acid and phorbol 12,13-dibutyrate (PDBu), activators of protein kinase C, at 1 microM had a stimulatory effect on mGluRs linked to PLD but depressed (1S,3R)-ACPD-induced phosphoinositide hydrolysis. The protein kinase C inhibitor, staurosporine (1 and 10 microM) reduced PLD activation induced by 1 microM PDBu but not by 100 microM (1S,3R)-ACPD. 6. Our results suggest that PLD-linked mGluRs in rat hippocampus may be distinct from any known mGluR subtype coupled to PLC or adenylyl cyclase. Moreover, they indicate that independent mGluRs coupled to the PLC and PLD pathways exist and that mGluR agonists can stimulate PLD through a PKC-independent mechanism.

Chronic inhibition of intracellular Ca2+ release or protein kinase C activation significantly reduces the development of morphine dependence

We have previously shown that chronic antagonism of metabotropic glutamate receptors in the brain attenuates naloxone-precipitated withdrawal symptoms in rats treated chronically with subcutaneous (s.c.) morphine. Several subtypes of metabotropic glutamate receptors are directly linked, through a guanine nucleotide regulatory protein, to the phosphatidylinositol (p.i.) second messenger system. In the present investigation, we assessed the effect of inhibiting the products of p.i. hydrolysis on the development of opioid dependence. Thus, concurrently with subcutaneous morphine, we infused intracerebroventricularly (i.c.v.) in rats, various doses of chelerythrine, which selectively inhibits the activation of protein kinase C, and thapsigargin, which inhibits the release of intracellular Ca2+ when given chronically. Both chelerythrine and thapsigargin reduced the severity of naloxone-precipitated abstinence symptoms when infused i.c.v. at a dose of 10 nmol/day. A single injection of either chelerythrine or thapsigargin immediately prior to the precipitation of withdrawal failed to decrease the severity of abstinence symptoms. Our results suggest that by chronically inhibiting activity of the phosphatidylinositol system, the development of morphine dependence can be attenuated.

Inhibition of the morphine withdrawal syndrome and the development of physical dependence by lithium in mice

Due to the claim that lithium (Li+) reduces morphine self-administration in dependent rats, the effects of acute and chronic Li+ treatments on naloxone-precipitated withdrawal syndrome and physical dependence development to morphine in mice chronically treated with morphine, were evaluated. Morphine dependency was induced by the ingestion of morphine through drinking water in increasing doses for 10 days. Physical dependence to morphine was observed by precipitating an abstinence syndrome with naloxone (2 mg/kg, i.p.). In the acute experiments, Li+ (1 and 10 mg/kg, i.p.) was administered 1 hr prior to challenge with naloxone to morphine-dependent mice whereas for chronic studies, mice received morphine concomitant with Li+ (1200 mg/l) as drinking fluid for 10 days. Results obtained indicate that acute Li+ administration significantly reduced the withdrawal signs, and we were unable to induce some degree of morphine dependency in co-administration of Li+ to mice receiving chronic morphine treatment as compared to chronic morphine administration alone. The present study revealed that even in mice with very much lower serum Li+ levels than the commonly accepted therapeutic range there was a significant reduction in the withdrawal signs. It has been shown that Li+ and morphine have diverse effects on the transmembrane signal control systems. The interaction of Li+ and morphine might be through these systems.

The effect of lithium on morphine-induced analgesia in mice

1. The effects of acute and chronic lithium (Li+) treatments on the antinociception caused by morphine were studied in mice using the tail-flick test. 2. Subcutaneous injection of morphine (10 mg/kg) caused significant antinociception. 3. Acute Li+ administration (0.05, 0.1, 0.3, 1, 5 and 10 mg/kg, i.p.) alone had no significant antinociceptive effect but changed morphine analgesia; low doses of Li+ (0.1, 0.3 and 1 mg/kg) were found to decrease the antinociception induced by morphine whereas higher doses of the drug (10 mg/kg) potentiated this effect. 4. The 6 day administration of Li+ with a serum level of 0.528 mM decreased the antinociceptive effect of morphine. 5. The effect of Li+ on morphine-induced analgesia persisted for 96 hr in spite of the fact that Li+ drinking was discontinued (the serum Li+ level decreased from 0.528 to 0.022 mM). 6. It has been reported that Li+ might change both the binding of opioids to their receptors and biosynthesis or release of endogenous opioids. There is also a considerable body of evidence which indicates that both Li+ and morphine affect phosphoinositide turnover, intracellular calcium content and cyclic AMP level. The interaction of two drugs may conceivably take place through these systems. 7. These data suggest that the biological effects of Li+ may exist at very much lower serum Li+ levels than the commonly accepted therapeutic range.

The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

Rat brain slices were used to study the effects of different metabotropic glutamate receptor ligands on (i) the depolarization (30 mM KCl)-induced outflow of previously taken up D-[3H]aspartate; (ii) the inhibition of forskolin (30 microM)-induced cyclic AMP accumulation; and (iii) the hydrolysis of phosphoinositides. In addition, the localization of mRNAs coding for different metabotropic glutamate receptor subtypes was detected using in situ hybridization. (1S-3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (30-300 microM), a non selective metabotropic glutamate receptor agonist, significantly increased the KCl-induced output of radioactivity from cortical slices, whereas it inhibited the output from striatal slices. Conversely, (1S,3S,4S)-carboxycyclopropylglycine (0.1-1 microM), a relatively selective agonist of the mGluR2 metabotropic glutamate receptor subtype, had an inhibitory effect on the output of D-[3H]aspartate from both cortical and striatal slices and proved to be the most potent metabotropic glutamate receptor agonist in inhibiting cyclic AMP accumulation, but not in stimulating phosphoinositide hydrolysis. Since 2-amino-4-phosphonobutyrate (a mGluR4, mGluR6 and mGluR7 agonist) was not active in any of the assays tested, we hypothesized that the mGluR2 subtype could be involved in these events. Accordingly, mGluR2 mRNA expression was abundant in cortical neurons projecting to the striatum. Our experiments suggest that the stimulation of metabotropic glutamate receptors may either decrease or increase transmitter release depending on the subtype that prevails in the region under study.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Definition of a pharmacophore for the metabotropic glutamate receptors negatively linked to adenylyl cyclase

(2S,3S,4S)-alpha-Carboxycyclopropylglycine (L-CCG I) and trans-1-amino-(1S,3R)-cyclopentanedicarboxylic acid ((1S,3R)-ACPD), partially constrained L-glutamate analogs known to be agonists at the metabotropic glutamate receptors (mGluRs) adenylyl cyclase coupled, have been submitted to conformational analysis and the data obtained utilized to define a pharmacophore which takes into account the location of hydrogen bonding donating sites of the receptor. This pharmacophore has been utilized to define the agonist mGluRs decreases cAMP bioactive conformation of L-Glu.

Isradipine is able to separate morphine-induced analgesia and place conditioning

The effect of isradipine, a dihydropyridine calcium antagonist, on morphine-induced place preference and analgesia in rats and mice was studied. Isradipine (0.6-5.0 mg/kg s.c.) inhibited an acquisition of morphine-induced place preference in rats and mice in a dose-related manner. Isradipine did not affect or strengthen morphine-induced analgesia as measured by tail-clip and hot-plate tests in mice and tail-clip and tail-flick tests in rats. The results suggest that analgesic and reinforcing effects of morphine might be pharmacologically separated by isradipine.

Alpha 1-adrenoreceptor-mediated increase in acetylcholine release in brain slices during morphine tolerance

Norepinephrine, clonidine, and phenylephrine increased the electrically evoked release of endogenous acetylcholine in cortical slices taken from morphine-tolerant guinea pigs. This effect was alpha 1-adrenoreceptor mediated and was opposite to the alpha 2-adrenoreceptor-mediated inhibition of acetylcholine release, normally elicited by norepinephrine and clonidine. In the presence of prazosin, clonidine recovered its normal inhibitory properties, suggesting that morphine tolerance induced the appearance of an alpha 1-adrenoreceptor-mediated response that overshadowed, but did not cancel, the still present alpha 2-adrenoreceptor inhibitory control. The attempt to prove the presence of alpha-adrenoreceptors on the nerve endings by testing the effect of norepinephrine in synaptosomal preparations (preloaded with [3H]choline and depolarized with KCl and veratridine) was unsuccessful. Therefore the problem of the exact location of this excitatory input remains to be solved. These results confirm previous findings reporting the increase in cortical acetylcholine release induced by the alpha-adrenoreceptor agonists in morphine-tolerant, freely moving guinea pigs and demonstrate that opiate tolerance inverts the direction of the noradrenergic modulation even in the isolated intracortical cholinergic structures.