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

Erythroxylum cuneatum prevented cellular adaptation in morphine-induced neuroblastoma cells

BACKGROUND

Chronic morphine stimulates prolonged stimulation of opioid receptors, especially µ-opioid subtype (MOR), which in turn signals cellular adaptation. However, the sudden termination of morphine after chronic intake causes withdrawal syndrome.

OBJECTIVES

Hence, this study was designed to find an alternative treatment for the morphine withdrawal using the alkaloid leaf extract of Erythroxylum cuneatum (E. cuneatum), done on morphine-exposed neuroblastoma cell lines.

METHODS

SK-N-SH, a commercialised neuroblastoma cell line, was used in two separate study designs; the antagonistic and pre-treatment of morphine. The antagonistic treatment was conducted through concurrent exposure of the cells to morphine and E. cuneatum or morphine and methadone for 24 h. The pre-treatment design was carried out by exposing the cells to morphine for 24 h, followed by 24 h exposures to E. cuneatum or methadone. The cytosolic fraction was collected and run for protein expression involved in cellular adaptation; mitogen-activated protein (MAP)/extracellular signal-regulated (ERK) kinase 1/2 (MEK 1/2), extracellular signal-regulated kinase 2 (ERK 2), cAMP-dependent protein kinase (PKA) and protein kinases C (PKC).

RESULTS

The antagonistic treatment showed the normal level of MEK 1/2, ERK 2, PKA and PKC by the combination treatment of morphine and E. cuneatum, comparable to the combination of morphine and methadone. Neuroblastoma cells exposed to morphine pre-treatment expressed a high level of MEK 1/2, ERK 2, PKA and PKC, while the treatments with E. cuneatum and methadone normalised the expression of the cellular adaptation proteins.

CONCLUSION

E. cuneatum exerted anti-addiction properties by lowering the levels of cellular adaptation proteins, and its effects are comparable to that of methadone (an established anti-addiction drug).

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Restoration of Cyclo-Gly-Pro-induced salivary hyposecretion and submandibular composition by naloxone in mice

Cyclo-Gly-Pro (CGP) attenuates nociception, however its effects on salivary glands remain unclear. In this study, we investigated the acute effects of CGP on salivary flow and composition, and on the submandibular gland composition, compared with morphine. Besides, we characterized the effects of naloxone (a non-selective opioid receptor antagonist) on CGP- and morphine-induced salivary and glandular alterations in mice. After that, in silico analyses were performed to predict the interaction between CGP and opioid receptors. Morphine and CGP significantly reduced salivary flow and total protein concentration of saliva and naloxone restored them to the physiological levels. Morphine and CGP also reduced several infrared vibrational modes (Amide I, 1687-1594cm^-1; Amide II, 1594-1494cm^-1; CH₂/CH₃, 1488-1433cm^-1; C = O, 1432-1365cm^-1; PO₂ asymmetric, 1290-1185cm^-1; PO₂ symmetric, 1135-999cm^-1) and naloxone reverted these alterations. The in silico docking analysis demonstrated the interaction of polar contacts between the CGP and opioid receptor Cys219 residue. Altogether, we showed that salivary hypofunction and glandular changes elicited by CGP may occur through opioid receptor suggesting that the blockage of opioid receptors in superior cervical and submandibular ganglions may be a possible strategy to restore salivary secretion while maintaining antinociceptive action due its effects on the central nervous system.

Hydrogen sulfide inhibits opioid withdrawal-induced pain sensitization in rats by down-regulation of spinal calcitonin gene-related peptide expression in the spine

Hyperalgesia often occurs in opioid-induced withdrawal syndrome. In the present study, we found that three hourly injections of DAMGO (a μ-opioid receptor agonist) followed by naloxone administration at the fourth hour significantly decreased rat paw nociceptive threshold, indicating the induction of withdrawal hyperalgesia. Application of NaHS (a hydrogen sulfide donor) together with each injection of DAMGO attenuated naloxone-precipitated withdrawal hyperalgesia. RT-PCR and Western blot analysis showed that NaHS significantly reversed the gene and protein expression of up-regulated spinal calcitonin gene-related peptide (CGRP) in naloxone-treated animals. NaHS also inhibited naloxone-induced cAMP rebound and cAMP response element-binding protein (CREB) phosphorylation in rat spinal cord. In SH-SY5Y neuronal cells, NaHS inhibited forskolin-stimulated cAMP production and adenylate cyclase (AC) activity. Moreover, NaHS pre-treatment suppressed naloxone-stimulated activation of protein kinase C (PKC) α, Raf-1, and extracellular signal-regulated kinase (ERK) 1/2 in rat spinal cord. Our data suggest that H2S prevents the development of opioid withdrawal-induced hyperalgesia via suppression of synthesis of CGRP in spine through inhibition of AC/cAMP and PKC/Raf-1/ERK pathways.

Inhibition of Gβγ-subunit signaling potentiates morphine-induced antinociception but not respiratory depression, constipation, locomotion, and reward

Inhibition of Gβγ-subunit signaling to phospholipase C β3 has been shown to potentiate morphine-mediated antinociception while attenuating the development of tolerance and dependence in mice. The objective of this study was to determine the effect of Gβγ-subunit inhibition on antinociception and other pharmacological effects, such as respiratory depression, constipation, and hyperlocomotion, mediated by the μ-opioid receptor. The Gβγ-subunit inhibitor, gallein, was administered to C57BL/6J mice by intraperitoneal injection before morphine, and data were compared with mice treated with vehicle, morphine, or gallein alone. Morphine-induced antinociception was measured using the 55°C warm-water tail-withdrawal test. Pretreatment with gallein produced a dose-dependent potentiation of morphine-mediated antinociception, producing up to a 10-fold leftward shift in the morphine dose-response curve and extending the duration of antinociception induced by a single dose of morphine. Gallein pretreatment also prevented acute antinociceptive tolerance induced by morphine. In contrast, the dose-dependent respiratory depression and hyperlocomotion induced by morphine were not potentiated by gallein pretreatment. Similarly, gallein pretreatment did not potentiate morphine-conditioned place preference responses or morphine-induced constipation, as measured as a reduction in excreta. These results suggest that selectively inhibiting Gβγ-mediated signaling may selectively increase μ-opioid receptor-mediated antinociception without matching increases in adverse physiological effects.

Differential modulation of drug-induced structural and functional plasticity of dendritic spines

Drug-induced plasticity of excitatory synapses has been proposed to be the cellular mechanism underlying the aberrant learning associated with addiction. Exposure to various drugs of abuse causes both morphological plasticity of dendritic spines and functional plasticity of excitatory synaptic transmission. Chronic activation of μ-opioid receptors (MOR) in cultured hippocampal neurons causes two forms of synaptic plasticity: loss of dendritic spines and loss of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. With use of live imaging, patch-clamp electrophysiology, and immunocytochemistry, the present study reveals that these two forms of synaptic plasticity are mediated by separate, but interactive, intracellular signaling cascades. The inhibition of Ca(2+)/calmodulin-dependent protein kinase II with 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62) blocks MOR-mediated structural plasticity of dendritic spines, but not MOR-mediated cellular redistribution of GluR1 and GluR2 AMPA receptor subunits. In contrast, the inhibition of calcineurin with tacrolimus (FK506) blocks both cellular processes. These findings support the idea that drug-induced structural and functional plasticity of dendritic spines is mediated by divergent, but interactive, signaling pathways.

Protein kinase C phosphorylates the cAMP response element binding protein in the hypothalamic paraventricular nucleus during morphine withdrawal

BACKGROUND AND PURPOSE

Exposure to drugs of abuse or stress results in adaptation in the brain involving changes in gene expression and transcription factors. Morphine withdrawal modulates gene expression through various second-messenger signal transduction systems. Here, we investigated changes in activation of the transcription factor, cAMP-response element binding protein (CREB), in the hypothalamic paraventricular nucleus (PVN) and the kinases that may mediate the morphine withdrawal-triggered activation of CREB and the response of the hypothalamic-pituitary-adrenocortical (HPA) axis after naloxone-induced morphine withdrawal.

EXPERIMENTAL APPROACH

The effects of morphine dependence and withdrawal, phosphorylated CREB (pCREB), corticotrophin-releasing factor (CRF) expression in the PVN and HPA axis activity were measured using immunoblotting, immunohistochemistry and radioimmunoassay in controls and in morphine-dependent rats, withdrawn with naloxone and pretreated with vehicle, calphostin C, chelerythrine (inhibitors of protein kinase C (PKC) or SL-327 [inhibitor of extracellular signal regulated kinase (ERK) kinase]. In addition, changes in PKCα and PKCγ immunoreactivity were measured after 60 min of withdrawal.

KEY RESULTS

In morphine-withdrawn rats, pCREB immunoreactivity was increased within CRF immunoreactive neurons in the PVN and plasma corticosterone levels were raised. SL-327, at doses that reduced the augmented pERK levels in the PVN, did not attenuate the rise in pCREB immunoreactivity or plasma corticosterone secretion. In contrast, PKC inhibition reduced the withdrawal-triggered rise in pCREB, pERK1/2 and corticosterone secretion.

CONCLUSIONS AND IMPLICATIONS

PKC mediated, in part, both CREB activation and the HPA response to morphine withdrawal. The ERK kinase/ERK pathway might not be necessary for either activation of CREB or HPA axis hyperactivity.

Morphine induces AMPA receptor internalization in primary hippocampal neurons via calcineurin-dependent dephosphorylation of GluR1 subunits

Chronic morphine treatment resulting in the alteration of postsynaptic levels of AMPA receptors, thereby modulating synaptic strength, has been reported. However, the mechanism underlying such drug-induced synaptic modification has not been resolved. By monitoring the GluR1 trafficking in primary hippocampal neurons using the pHluorin-GluR1 imaging and biotinylation studies, we observed that prolonged morphine exposure significantly induced loss of synaptic and extrasynaptic GluR1 by internalization. The morphine-induced GluR1 endocytosis was independent of neural network activities or NMDA receptor activities, as neither blocking the sodium channels with tetrodotoxin nor NMDA receptors with dl-APV altered the effects of morphine. Instead, morphine-induced GluR1 endocytosis is attributed to a change in the phosphorylation state of the GluR1 at Ser(845) as morphine significantly decreased the dephosphorylation of GluR1 at this site. Such changes in Ser(845) phosphorylation required morphine-induced activation of calcineurin, based on the observations that a calcineurin inhibitor, FK506, completely abrogated the dephosphorylation, and morphine treatment led to an increase in calcineurin enzymatic activity, even in the presence of dl-APV. Importantly, pretreatment with FK506 and overexpression of the GluR1 mutants, S845D (phospho-mimic) or S845A (phospho-blocking) attenuated the morphine-induced GluR1 endocytosis. Therefore, the calcineurin-mediated GluR1-S845 dephosphorylation is critical for the morphine-induced changes in the postsynaptic AMPA receptor level. Together, these findings reveal a novel molecular mechanism for opioid-induced neuronal adaptation and/or synaptic impairment.

A novel Gbetagamma-subunit inhibitor selectively modulates mu-opioid-dependent antinociception and attenuates acute morphine-induced antinociceptive tolerance and dependence

The Gbetagamma subunit has been implicated in many downstream signaling events associated with opioids. We previously demonstrated that a small molecule inhibitor of Gbetagamma-subunit-dependent phospholipase (PLC) activation potentiated morphine-induced analgesia (Bonacci et al., 2006). Here, we demonstrate that this inhibitor, M119 (cyclohexanecarboxylic acid [2-(4,5,6-trihydroxy-3-oxo-3H-xanthen-9-yl)-(9Cl)]), is selective for mu-opioid receptor-dependent analgesia and has additional efficacy in mouse models of acute tolerance and dependence. When administered by an intracerebroventricular injection in mice, M119 caused 10-fold and sevenfold increases in the potencies of morphine and the mu-selective peptide, DAMGO, respectively. M119 had little or no effect on analgesia induced by the kappa agonist U50,488 or delta agonists DPDPE or Deltorphin II. Similar results were obtained in vitro, as only activation of the mu-opioid receptor stimulated PLC activation, whereas no effect was seen with the kappa- and delta-opioid receptors. M119 inhibited mu-receptor-dependent PLC activation. In studies to further explore the in vivo efficacy of M119, systemic administration M119 also resulted in a fourfold shift increase in potency of systemically administered morphine. Of particular interest, M119 was also able to attenuate acute, antinociceptive tolerance and dependence in mice treated concomitantly with both M119 and morphine. These studies suggest that small organic molecules, such as M119, that specifically regulate Gbetagamma subunit signaling may have important therapeutic applications in enhancing opioid analgesia, while attenuating the development of tolerance and dependence.

Development of morphine induced tolerance and withdrawal symptoms is attenuated by lamotrigine and magnesium sulfate in mice

The goal of this study was to evaluate the effects of lamotrigine and magnesium sulfate on morphine induced tolerance and withdrawal symptoms in mice. Different groups of mice were received morphine (30 mg kg(-1), s.c.) or morphine (30 mg kg(-1), s.c.)+lamotrigine (10, 20, 30 or 40 mg kg(-1), i.p.) or morphine (30 mg kg(-1), s.c.) + magnesium sulfate (20, 40 or 60 mg kg(-1), i.p.) or morphine (30 mg kg(-1), s.c.) + [lamotrigine (10 mg kg(-1), i.p.) + magnesium sulfate (20mg kg(-1), i.p.)] daily for 4 days. Tolerance was assessed using hot plate after administration of a test dose of morphine (9 mg kg(-1), i.p.) on fifth day. Withdrawal zsymptoms (Jumping and Rearing) were assessed by administration of naloxone (5 mg kg(-1), i.p.) 2 h after the last dose of morphine in fourth day. It was found that administration of lamotrigine or magnesium sulfate or their combination decreased the morphine induced tolerance and withdrawal symptoms. From these results it is concluded that lamotrigine and magnesium sulfate alone or in combination could prevent the development of morphine tolerance and withdrawal symptoms. Glutamate release inhibitory effect of lamotrigine and its possible mechanism and property of magnesium, blocking the N-Methyl-D-Aspartate (NMDA) receptor calcium channel, is probably its mechanism on preventing morphine induced tolerance and dependence.

Pre-treatment with a PKC or PKA inhibitor prevents the development of morphine tolerance but not physical dependence in mice

We previously demonstrated that intracerebroventricular (i.c.v.) administration of protein kinase C (PKC) or protein kinase A (PKA) inhibitors reversed morphine antinociceptive tolerance in 3-day morphine-pelleted mice. The present study aimed at evaluating whether pre-treating mice with a PKC or PKA inhibitor prior to pellet implantation would prevent the development of morphine tolerance and physical dependence. Antinociception was assessed using the warm-water tail immersion test and physical dependence was evaluated by quantifying/scoring naloxone-precipitated withdrawal signs. While drug-naïve mice pelleted with a 75 mg morphine pellet for 3 days developed a 5.8-fold tolerance to morphine antinociception, mice pre-treated i.c.v. with the PKC inhibitors bisindolylmaleimide I, Go-7874 or Go-6976, or with the myristoylated PKA inhibitor, PKI-(14-22)-amide failed to develop any tolerance to morphine antinociception. Experiments were also conducted to determine whether morphine-pelleted mice were physically dependent when pre-treated with PKC or PKA inhibitors. The same inhibitor doses that prevented morphine tolerance were evaluated in other mice injected s.c. with naloxone and tested for precipitated withdrawal. The pre-treatment with PKC or PKA inhibitors failed to attenuate or block the signs of morphine withdrawal including jumping, wet-dog shakes, rearing, forepaw tremor, increased locomotion, grooming, diarrhea, tachypnea and ptosis. These data suggest that elevations in the activity of PKC and PKA in the brain are critical to the development of morphine tolerance. However, it appears that tolerance can be dissociated from physical dependence, indicating a role for PKC and PKA to affect antinociception but not those signs mediated through the complex physiological processes of withdrawal.

Role of spinal metabotropic glutamate receptor subtype 5 in the development of tolerance to morphine-induced antinociception in rat

Prolonged intrathecal (i.t.) administration of morphine results in tolerance to morphine-induced antinociception. We found that co-administration of selective metabotropic glutamate receptor subtype 5 antagonist MPEP with morphine could suppress the loss of morphine-induced antinociception and inhibit the development of tolerance to morphine-induced antinociceptive effect. Whereas, the specific metabotropic glutamate receptor subtype 5 agonist CHPG does the opposite. As the activation of NMDA receptor after chronic morphine administration has been verified, we suppose there is an enhanced activation of mGluR5 during the development of tolerance to morphine-induced antinociception. Activation of mGluR5 may mobilize the release of intracellular Ca(2+) and activate PKC, leading to morphine-induced antinociception suppression. We conclude that mGluR5 contributes to the development of tolerance to morphine-induced antinociception after chronic morphine exposure.

Regulation of neuronal PLCgamma by chronic morphine

Alterations in neurotrophic signaling pathways may contribute to the changes in the mesolimbic dopamine system induced by chronic morphine exposure. In a rat model of morphine dependence, we previously identified increased levels of phospholipase C gamma-1 (PLCgamma1) immunoreactivity specifically within the ventral tegmental area (VTA) following chronic morphine treatment. Using an antibody specific for the tyrosine-phosphorylated, activated form of PLCgamma1, we now show that chronic morphine also significantly upregulates PLCgamma1 activity in the VTA, as well as in the nucleus accumbens and hippocampus, regions which are also implicated in the reinforcing properties of morphine. In contrast, no increase in PLCgamma1 activity was found in the substantia nigra or dorsal striatum. HSV-mediated overexpression of PLCgamma1 in PC12 cells induced ERK activation via a mechanism dependent, in part, on both MAP-ERK kinase (MEK) and protein kinase C. PLCgamma1 overexpression in the VTA similarly induced ERK activation in the VTA in vivo. As chronic morphine treatment has been shown to increase ERK activity within the VTA, the current results suggest that increased PLCgamma1 activity may be an upstream mediator of this effect.

Role of PKC in regulation of Fos and TH expression after naloxone induced morphine withdrawal in the heart

We previously demonstrated that morphine withdrawal induced hyperactivity of the heart by activation of noradrenergic pathways innervating the left and right ventricle, as evaluated by noradrenaline (NA) turnover and Fos expression. The present study was designed to investigate the role of protein kinase C (PKC) in this process, by estimating whether pharmacological inhibition of PKC would attenuate morphine withdrawal induced Fos expression and changes in tyrosine hydroxylase (TH) immunoreactivity levels and NA turnover in the left and right ventricle. Dependence on morphine was induced on day 8 by an injection of naloxone. Morphine withdrawal induced Fos expression and increased TH levels and NA turnover in the right and left ventricle. Infusion of calphostin C, a selective PKC inhibitor, did not modify the morphine withdrawal-induced increase in NA turnover and TH levels. However, this inhibitor produced a reduction in the morphine withdrawal-induced Fos expression. The results of the present study provide new information on the mechanisms that underlie morphine withdrawal-induced up-regulation of Fos expression in the heart and suggest that TH is not a target of PKC during morphine withdrawal at heart levels.

Activity of adenylyl cyclase and protein kinase A contributes to morphine-induced spinal apoptosis

Our previous study has shown that chronic morphine exposure induces neuronal apoptosis within the spinal cord dorsal horn; however, the mechanisms of morphine-induced apoptosis remain unclear. Here we examined whether adenylyl cyclase (AC) and protein kinase A (PKA) would play a role in this process. Intrathecal morphine regimen (10 microg, twice daily x 7 days) that resulted in antinociceptive tolerance induced spinal apoptosis as revealed by in situ terminal deoxynucleotidyl transferase (TdT)-UTP-biotin nick end labeling (TUNEL). The TUNEL-positive cells were detected primarily in the superficial laminae of the spinal cord dorsal horn, which was associated with an increase in the expression of activated caspase-3 and mitogen-activated protein kinase (MAPK) within the same spinal region. Co-administration of morphine with a broad AC inhibitor (ddA), a PKA inhibitor (H89), or a MAPK inhibitor (PD98059) substantially reduced the number of TUNEL-positive cells, as compared with the morphine alone group. The results indicate that the spinal AC and PKA pathway through intracellular MAPK may be contributory to the cellular mechanisms of morphine-induced apoptosis.

Alterations in protein kinase A and different protein kinase C isoforms in the heart during morphine withdrawal

The present study was designed to investigate the possible changes of protein kinase A (PKA) and different isoforms of protein kinase C (PKC): PKC alpha, PKC delta and PKC zeta after naloxone induced morphine withdrawal in the heart. Male rats were implanted with placebo (naïve) or morphine (tolerant/dependent) pellets for 7 days. On day 8 rats received saline s.c. or naloxone (5 mg/kg s.c.). The protein levels of PKA, PKC delta and PKC zeta were significantly up-regulated in the heart from morphine withdrawal rats. By contrast, morphine withdrawal induced down-regulation of PKC alpha. These results suggest that both PKA and PKC may be involved in the cardiac adaptive changes observed during morphine withdrawal.

Role of PKC-α,γ isoforms in regulation of c-Fos and TH expression after naloxone-induced morphine withdrawal in the hypothalamic PVN and medulla oblongata catecholaminergic cell groups

We previously demonstrated that morphine withdrawal induced hyperactivity of the hypothalamus-pituitary-adrenocortical axis by activation of noradrenergic pathways innervating the hypothalamic paraventricular nucleus (PVN), as evaluated by Fos expression and corticosterone release. The present study was designed to investigate the role of protein kinase C (PKC) in this process by estimating changes in PKCalpha and PKCgamma immunoreactivity, and whether pharmacological inhibition of PKC would attenuate morphine withdrawal-induced c-Fos expression and changes in tyrosine hydroxylase (TH) immunoreactivity levels in the PVN and nucleus tractus solitarius/ ventrolateral medulla (NTS/VLM). Dependence on morphine was induced in rats by 7 day s.c. implantation of morphine pellets. Morphine withdrawal was induced on day 8 by an injection of naloxone. The protein levels of PKCalpha and gamma were significantly down-regulated in the PVN and NTS/VLM from the morphine-withdrawn rats. Morphine withdrawal induced c-Fos expression in the PVN and NTS/VLM, indicating an activation of neurons in those nuclei. TH immunoreactivity was increased in the NTS/VLM after induction of morphine withdrawal, whereas there was a decrease in TH levels in the PVN. Infusion of calphostin C, a selective protein kinase C inhibitor, produced a reduction in the morphine withdrawal-induced c-Fos expression. Additionally, the changes in TH levels in the PVN and NTS/VLM were significantly modified by calphostin C. The present results suggest that activated PKC in the PVN and catecholaminergic brainstem cell groups may be critical for the activation of the hypothalamic-pituitary adrenocortical axis in response to morphine withdrawal.

Increased glutamate synaptic transmission in the nucleus raphe magnus neurons from morphine-tolerant rats

Currently, opioid-based drugs are the most effective pain relievers that are widely used in the treatment of pain. However, the analgesic efficacy of opioids is significantly limited by the development of tolerance after repeated opioid administration. Glutamate receptors have been reported to critically participate in the development and maintenance of opioid tolerance, but the underlying mechanisms remain unclear. Using whole-cell voltage-clamp recordings in brainstem slices, the present study investigated chronic morphine-induced adaptations in glutamatergic synaptic transmission in neurons of the nucleus raphe magnus (NRM), a key supraspinal relay for pain modulation and opioid analgesia. Chronic morphine significantly increased glutamate synaptic transmission exclusively in one class of NRM cells that contains μ-opioid receptors in a morphine-tolerant state. The adenylyl cyclase activator forskolin and the cAMP analog 8-bromo-cAMP mimicked the chronic morphine effect in control neurons and their potency in enhancing the glutamate synaptic current was significantly increased in neurons from morphine-tolerant rats. MDL12330a, an adenylyl cyclase inhibitor, and H89, a protein kinase A (PKA) inhibitor, reversed the increase in glutamate synaptic transmission induced by chronic morphine. In addition, PMA, a phorbol ester activator of protein kinase C (PKC), also showed an increased potency in enhancing the glutamate synaptic current in these morphine-tolerant cells. The PKC inhibitor GF109203X attenuated the chronic morphine effect. Taken together, these results suggest that chronic morphine increases presynaptic glutamate release in μ receptor-containing NRM neurons in a morphine-tolerant state, and that the increased glutamate synaptic transmission appears to involve an upregulation of both the cAMP/PKA pathway and the PKC pathway. This glutamate-mediated activation of these NRM neurons that are thought to facilitate spinal pain transmission may contribute to the reduced opioid analgesia during opioid tolerance.

Inhibition of SNAP-25 Phosphorylation at Ser187 Is Involved in Chronic Morphine-induced Down-regulation of SNARE Complex Formation

Opiate abuse has been shown to cause adaptive changes in presynaptic release and protein phosphorylation-mediated synaptic plasticity, but the underlying mechanisms remain unclear. Neuronal SNARE proteins serve as important regulatory molecules underlying neural plasticity in view of their major role in the process of neurotransmitter release. In the present study, the expression of SNAP-25, a t-SNARE protein essential for vesicle release, was found to be dramatically regulated in hippocampus after chronic morphine treatment, which was visualized with two-dimensional gel electrophoresis. The spots of SNAP-25 in the gel were shifted along the dimension of isoelectric point, indicating a likely change of the post-transcriptional modification. Immunoblotting analysis with specific antibody to Ser187, a protein kinase C (PKC) phosphorylation site of SNAP-25, revealed that the specific phosphorylation was correspondingly decreased, which was correlated with morphine-induced inhibition of PKC activity. Moreover, the level of ternary complex of SNARE proteins in either synaptosomes or PC12 cells was significantly reduced after chronic morphine treatment. This suggests a causal relationship between the inhibition of PKC-dependent SNAP-25 phosphorylation and the down-regulation of SNARE complex formation after chronic morphine treatment. Further analysis of SNARE complex formed by transfection of the wild-type or Ser187 mutants of SNAP-25 showed that only wild-type-formed complex was inhibited by morphine treatment. Thus, these results indicate that chronic morphine treatment inhibits phosphorylation of SNAP-25 at Ser187 and leads to a down-regulation of SNARE complex formation, which presents a potential molecular mechanism for the alteration of exocytotic process and neural plasticity during opiate abuse.

Morphine actions in the rat forebrain: role of protein kinase C

Acute administration of morphine induces expression of the immediate-early gene (IEG) c-Fos in dorsomedial striatum, portions of cerebral cortex, and in several midline-intralaminar thalamic nuclei, partly via a trans-synaptic mechanism that involves activation of glutamate receptors. Because activation of protein kinase C (PKC) may occur following the activation of glutamate receptors, we determined whether pharmacological inhibition of PKC would attenuate morphine-induced c-Fos expression, and whether acute administration of morphine would induce translocation of PKC. The selective PKC antagonist NPC 15437 given 30 min prior to morphine significantly decreased morphine-induced c-Fos expression in striatum and cingulate cortex, but not in centrolateral thalamus. In another experiment, rats were given an acute dose of morphine, and immunocytochemical analysis was performed for the betaI and betaII isoforms of PKC. Morphine induced a rapid and transient translocation of PKC betaII, but not betaI, from perinuclear spots to plasma membrane in numerous cortical and striatal neurons. Prior administration of naloxone blocked this response. Ultrastructural studies confirmed translocation from Golgi apparatus to plasma membrane 15 min after morphine injection. Double immunocytochemistry at the light microscopic level demonstrated co-localization of translocated PKC betaII and c-Fos in some cortical neurons 90 min after morphine injection. These results support a role for PKC, especially PKC betaII, in the rapid effects of morphine on the brain.

The expression of a high level of morphine antinociceptive tolerance in mice involves both PKC and PKA

We have previously reported that intracerebroventricular (i.c.v.) injection of either a PKC or PKA inhibitor completely reversed the expression of 5- to 8-fold morphine antinociceptive tolerance. We developed a model of 45-fold morphine tolerance that included a 75-mg morphine pellet and twice daily morphine injections. PKC inhibitor doses of bisindolylmaleimide I and Gö-7874 that completely reversed 8-fold tolerance only partly reversed the 45-fold level of antinociceptive tolerance. A component of tolerance was resistant to PKC inhibition, since even higher inhibitor doses failed to further reverse the high level of morphine tolerance. In addition, the 45-fold tolerance was only partly reversed by the PKA inhibitor KT-5720 at a dose previously cited by others to reverse 5-fold tolerance. Another PKA inhibitor 4-cyano-3-methylisoquinoline only partly reversed the morphine tolerance as well. In other experiments PKC and PKA inhibitors were co-administered together to determine their effectiveness for completely reversing the 45-fold level of morphine tolerance. Co-administering either bisindolylmaleimide I with KT-5720, or Gö-7874 with KT-5720, completely reversed the high level of tolerance. The high level of morphine tolerance was also completely reversed by co-administering Gö-7874 with 4-cyano-3-methylisoquinoline. Thus, high levels of morphine tolerance may reflect increases in protein phosphorylation by the terminal kinases of both the adenylyl cyclase and phosphatidylinositol cascades in brain and spinal cord areas critical to the expression of antinociception.

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.

Prolonged reversal of morphine tolerance with no reversal of dependence by protein kinase C inhibitors

The phosphatidylinositol (PI) cascade plays a pivotal role in mediating behavioral tolerance to the antinociceptive effects of morphine. Earlier we reported that antinociceptive tolerance was completely reversed 30 min after the administration of inhibitors of each step in the PI cascade. The aim of this study was to determine whether injection of a single dose of protein kinase C (PKC) inhibitor would elicit a prolonged reversal of morphine tolerance for up to 24 h. Three days after implantation of placebo- or 75-mg morphine pellets, mice received intracerebroventricular (i.c.v.) injections of vehicle or PKC inhibitor drug. Morphine challenge doses were then administered 4, 8 and 24 h later to test for tolerance reversal. In non-tolerant mice, Gö-7874 and sangivamycin had no effect on the potency of morphine. However, Gö-7874 and sangivamycin significantly reversed morphine tolerance at 4, 8 and 24 h. In addition, the role of PKC in morphine physical dependence was determined. Gö-7874 and sangivamycin by themselves did not precipitate spontaneous morphine withdrawal. Therefore, experiments were conducted to determine whether the PKC inhibitors would block naloxone-precipitated withdrawal. However, neither a 30-min nor a 24-h pretreatment with Gö-7874 or sangivamycin blocked naloxone withdrawal. Our results along with other publications indicate that PKC is a pivotal kinase essential for maintaining animals in an opioid tolerant state. Finally, the use of persistent PKC inhibitors that lasted for 24 h demonstrated that the neuronal systems in these animals did not adapt by increasing the activity of other protein kinase cascades to re-establish morphine tolerance.

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.

The neurobiology of opiate tolerance, dependence and sensitization: mechanisms of NMDA receptor-dependent synaptic plasticity

Long-term administration of opiates leads to changes in the effects of these drugs, including tolerance, sensitization and physical dependence. There is, as yet, incomplete understanding of the neural mechanisms that underlie these phenomena. Tolerance, sensitization and physical dependence can be considered adaptive processes similar to other experience-dependent changes in the brain, such as learning and neural development. There is considerable evidence demonstrating that N-methyl-D-aspartate (NMDA) receptors and downstream signaling cascades may have an important role in different forms of experience-dependent changes in the brain and behavior. This review will explore evidence indicating that NMDA receptors and downstream messengers may be involved in opiate tolerance, sensitization and physical dependence. This evidence has been used to develop a cellular model of NMDA receptor/opiate interactions. According to this model, mu opioid receptor stimulation leads to a protein kinase C-mediated activation of NMDA receptors. Activation of NMDA receptors leads to influx of calcium and activation of calcium-dependent processes. These calcium-dependent processes have the ability to produce critical changes in opioid-responsive neurons, including inhibition of opioid receptor/second messenger coupling. This model is similar to cellular models of learning and neural development in which NMDA receptors have a central role. Together, the evidence suggests that the mechanisms that underlie changes in the brain and behavior produced by long-term opiate use may be similar to other central nervous system adaptations. The experimental findings and the resulting model may have implications for the treatment of pain and addiction.

Protection by nitric oxide of morphine-induced inhibition of rat submandibular gland function

The effects of morphine, l -arginine (nitric oxide precursor) and l -NAME (nitric oxide synthesis inhibitor ) and their concurrent therapy on rat submandibular secretory function were studied. Pure submandibular saliva was collected intraorally by micro polyethylene cannula from anaesthetized rats using pilocarpine as secretagogue. Single intraperitoneal injection of morphine (6 mg kg(-1)) to rats induced significant (P< 0.01) inhibition of salivary flow rate (28%), total protein (12%) and calcium concentrations (27%). Sodium output was increased (23%, P< 0.01). Single intraperitoneal administration of l -arginine (100 mg kg(-1)) and l -NAME (10 mg kg(-1)) affected salivary gland function. Saliva flow rate was reduced by l -NAME (23%, P< 0.01). The total protein concentration of saliva was increased by l -arginine (21%, P< 0.05) and decreased by l -NAME (19%, P< 0.01). Calcium concentration of saliva was increased by l -arginine (25%, P< 0.01) and reduced by l -NAME (21%, P< 0.01). In combination treatment, l -arginine prevented (P< 0.01) morphine-induced reduction of flow rate while l -NAME potentiated it (P< 0.01). The secretion of total protein and calcium were influenced in a similar trend by concurrent therapy. l -NAME potentiated morphine-induced decrease of total protein and calcium concentrations (P< 0.01) while l -arginine restored (P< 0.01) them to levels close to control and morphine groups respectively. It is concluded that morphine inhibits salivary gland function and nitric oxide (NO) plays a positive role in this system. Also it is confirmed that morphine inhibitory effects on submandibular function are somewhat restored by l -arginine and expanded by l -NAME. The modulatory effect of the l -arginine/NO system on salivary gland function is suggested.

Reduced development of tolerance to the analgesic effects of morphine and clonidine in PKC gamma mutant mice

A variety of second messenger systems have been implicated in the intracellular mechanisms of tolerance development to the analgesic actions of morphine, a mu opioid, and clonidine, an alpha-2 adrenergic receptor agonist. Here, we studied mice that carry a null mutation in the gene encoding a neuronal specific isoform of protein kinase C (PKC), namely, PKC gamma. We used the tail-flick test to construct dose-response curves before and 4 days after chronic morphine (75-mg pellets, subcutaneously (s.c.)) or clonidine treatment (0.3mg/kg, s.c., twice daily). Baseline tail-flick latencies did not differ in PKC gamma mutant and wild-type mice (3-4s). Both morphine and clonidine produced a dose-dependent suppression of the tail-flick response with an ED(50) (effective dose resulting in a 50% reduction of the control response) value (2.0mg/kg for morphine and 0.1mg/kg for clonidine) that was similar for naive mutant and wild-type mice. In contrast, after 4 days of drug delivery, mutant mice showed significantly less rightward shift in the dose-response curve to morphine (six-fold for wild-type and three-fold for mutant mice) and to clonidine (five-fold for wild-type and no shift for the mutant mice). These results indicate that PKC gamma contributes to the development of tolerance to the analgesic effects of both morphine and clonidine. Chronic morphine treatment can also result in sensitization of spinal cord neurons and increased pain behaviors following a noxious insult. To assess the contribution of PKC gamma to this process, we studied the responses of wild-type and mutant mice to an intraplantar injection of formalin (a model of persistent pain) following chronic morphine treatment. Although morphine tolerance increased formalin-evoked persistent pain behavior and Fos-LI in wild-type mice, there was no difference between placebo- and morphine-treated mutant mice, suggesting that PKC gamma also contributes to chronic morphine-induced changes in nociceptive processing.

Characterization of the signal transduction pathways mediating morphine withdrawal-stimulated c-fos expression in hypothalamic nuclei

The transcription factor, Fos, is considered as a functional marker of activated neurons. We have shown previously that acute administration of morphine induces the expression of Fos in hypothalamic nuclei associated with control of the hypothalamus-pituitary-adrenocortex axis, such as the paraventricular nucleus and the supraoptic nucleus. In the current study, we examined the role of protein kinase A, protein kinase C and Ca2+ entry through L-type Ca2+ channels in naloxone-precipitated Fos expression in the paraventricular and supraoptic nuclei. After 7 days of morphine treatment, we did not observe any modification in Fos production. However, when opioid withdrawal was precipitated with naloxone a dramatic increase in Fos immunoreactivity was observed in the parvocellular division of the paraventricular nucleus and in the supraoptic nucleus. Chronic co-administration of chelerythrine (a selective protein kinase C inhibitor acting at its catalytic domain) with morphine did not affect the increase in Fos expression observed in nuclei from morphine withdrawn rats. In addition, infusion of calphostin C (another protein kinase C inhibitor, which interacts with its regulatory domain) did not modify the morphine withdrawal-induced expression of Fos. In contrast, when the selective protein kinase A inhibitor, N-(2'guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004), was infused it greatly diminished the increased Fos production observed in morphine-withdrawn rats. Furthermore, chronic infusion of the selective L-type Ca2+ channel antagonist, nimodipine, significantly inhibited the enhancement of Fos induction in the paraventricular and supraoptic nuclei from morphine-withdrawn animals. Taken together, these data might indicate that protein kinase A activity is necessary for the expression of Fos during morphine withdrawal and that an up-regulated Ca2+ system might contribute to the activation of Fos. The present findings suggest that protein kinase A and Ca2+ influx through L-type Ca2+ channels might contribute to the activation of neuroendocrine cells in the paraventricular and supraoptic nuclei.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.

Anxiolytic 2,3-benzodiazepines, their specific binding to the basal ganglia

Over the past 20 years, several members of the 2,3-benzodiazepine family have been synthesized. Some of these compounds--tofisopam (Grandaxin), girisopam, nerisopam--exert significant anxiolytic and antipsychotic activities. Sites where actions of 2,3-benzodiazepines are mediated differ from those of 1,4-benzodiazepines. Binding of 2,3-benzodiazepines to neuronal cells in the central nervous system shows a unique and specific distribution pattern: their binding sites are located exclusively to the basal ganglia. Chemical lesioning of the striato-pallido-nigral system, surgical transections of the striato nigral pathway and the activation of c-fos expression in the basal ganglia after application of 2,3-benzodiazepines suggest that these compounds mainly bind to projecting neurons of the striatum. The binding sites are transported from the striatum to the substantia nigra and the entopeduncular nucleus. Recent studies on mechanism of action of 2,3-benzodiazepines indicate their possible role in opioid signal transduction since 2,3-benzodiazepines augment the agonist potency of morphine to induce catalepsy and analgesia, and their action is diminished in morphine tolerant animals. The possible biochemical target of 2,3-benzodiazepines is an alteration in the phosphorylation of protein(s) important in the signal transduction process. Agents affecting emotional responses evoked by endogenous opioids without danger of tolerance and dependence may represent a new therapeutic tool in the treatment of addiction and affective disorders.

Involvement of phospholipid signal transduction pathways in morphine tolerance in mice

1. Opioid tolerance involves an alteration in the activity of intracellular kinases such as cyclic AMP-dependent protein kinase (PKA). Drugs that inhibit PKA reverse morphine antinociceptive tolerance. The hypothesis was tested that phospholipid pathways are also altered in morphine tolerance. Inhibitors of the phosphatidylinositol and phosphatidylcholine pathways were injected i.c.v. in an attempt to acutely reverse morphine antinociceptive tolerance. 2. Seventy-two hours after implantation of placebo or 75 mg morphine pellets, mice injected i.c.v. with inhibitor drug were challenged with morphine s.c. for generation of dose-response curves in the tail-flick test. Placebo pellet-implanted mice received doses of inhibitor drug having no effect on morphine's potency, in order to test for tolerance reversal in morphine pellet-implanted mice. Injection of the phosphatidylinositol-specific phospholipase C inhibitor ET-18-OCH3 significantly reversed tolerance, indicating a potential role for inositol 1,4,5-trisphosphate (IP3) and protein kinase C (PKC) in tolerance. Alternatively, phosphatidylcholine-specific phospholipase C increases the production of diacylglycerol and activation of PKC, without concomitant production of IP3. D609, an inhibitor of phosphatidylserine-specific phospholipase C, also reversed tolerance. Heparin is an IP3 receptor antagonist. Injection of low molecular weight heparin also reversed tolerance. PKC was also examined with three structurally dissimilar inhibitors. Bisindolylmaleimide I, Go-7874, and sangivamycin significantly reversed tolerance. 3. Chronic opioid exposure leads to changes in phospholipid metabolism that have a direct role in maintaining a state of tolerance. Evidence is accumulating that opioid tolerance disrupts the homeostatic balance of several important signal transduction pathways.

Alteration of spinal protein kinase C expression and kinetics in morphine, but not clonidine, tolerance

Antinociceptive synergism between spinally administered morphine and clonidine decreases to an additive interaction in morphine- and clonidine-tolerant mice. Spinally administered protein kinase C (PKC) inhibitors also decrease the synergism to addition. To determine whether chronic morphine or clonidine treatment alters spinal PKC activity, the present studies measured PKC activity and expression of PKC isoform proteins in spinal cord cytosol and membrane fractions. Mice were treated for 4 days with either placebo pellets, morphine pellets, s.c. saline, or s.c. clonidine. Morphine pellet-implanted mice were tolerant to morphine-induced tail flick antinociception, but not cross-tolerant to clonidine. Clonidine-pretreated mice were tolerant to clonidine, but not cross-tolerant to morphine. Induction of morphine tolerance produced a 2-fold lower Km value for PKC (8.24 +/- 1.67 microM in placebo pellet vs 4.43 +/- 1.24 microM in morphine pellet) in cytosol, but not membrane fractions from spinal cord. Vmax values were not different. No difference in Km or Vmax values was found between proteins from saline- and clonidine-pretreated animals. Immunoreactive cPKCalpha, betaI, and gamma isoforms decreased 14, 26, and 17%, respectively, in cytosol from morphine-tolerant animals. No difference in PKC isoforms was found in the membranes or in fractions from clonidine-tolerant mice. Morphine tolerance, but not clonidine tolerance, enhanced PKC activity while decreasing protein expression.

Attenuation of delta opioid receptor-mediated signaling by kainic acid in neural cells: involvement of protein kinase C and intracellular Ca2+

The potential modulation of opioid receptor signaling by kainic acid (KA) has been investigated in neuroblastoma x glioma NG 108-15 hybrid cells and neuroblastoma SK-N-SH cells. Acute incubation of KA significantly attenuated delta opioid receptor (DOR) signaling induced by the DOR agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE), as measured by activation of G proteins and inhibition of cAMP accumulation. The attenuation by KA was time- and dose-dependent and could be blocked by antagonists of kainate/AMPA receptors, suggesting possible mediation through kainate/AMPA receptors. KA attenuation of DPDPE-stimulated G protein activation was reversed by inhibitors of protein kinase C or by removal of both extracellular Ca2+ and intracellular Ca2+. In contrast, NMDA attenuation of DPDPE-stimulated G protein activation was independent of intracellular Ca2+, indicating that different mechanism(s) may underlie the modulation effect of KA and NMDA. This notion was further supported by the results that KA did not alter nociceptin/orphanin FQ-stimulated G protein activation in NG 108-15 cells but NMDA did. In addition, pretreatment of NG 108-15 cells with antagonists of kainate/AMPA receptors blocked the acute desensitization of DOR signaling. These data provide evidence that KA may be involved in the modulation of opioid receptor signal transduction.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: I. PKC translocation to the membrane of SH-SY5Y neuroblastoma cells is induced by mu-opioid agonists

Agonist-induced down-regulation of opioid receptors appears to require the phosphorylation of the receptor protein. However, the identities of the specific protein kinases that perform this task remain uncertain. Protein kinase C (PKC) has been shown to catalyze the phosphorylation of several G protein-coupled receptors and potentiate their desensitization toward agonists. However, it is unknown whether opioid receptor agonists induce PKC activation under physiological conditions. Using cultured SH-SY5Y neuroblastoma cells, which naturally express mu- and delta-opioid receptors, we investigated whether mu-opioid receptor agonists can activate PKC by measuring enzyme translocation to the membrane fraction. PKC translocation and opioid receptor densities were simultaneously measured by 3H-phorbol ester and [3H]diprenorphine binding, respectively, to correlate alterations in PKC localization with changes in receptor binding sites. We observed that mu-opioid agonists have a dual effect on membrane PKC density depending on the period of drug exposure. Exposure for 2-6 h to [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin or morphine promotes the translocation of PKC from the cytosol to the plasma membrane. Longer periods of opioid exposure (>12 h) produce a decrease in membrane-bound PKC density to a level well below basal. A significant decrease in [3H]diprenorphine binding sites is first observed at 2 h and continues to decline through the last time point measured (48 h). The opioid receptor antagonist naloxone attenuated both opioid-mediated PKC translocation and receptor down-regulation. These results demonstrate that opioids are capable of activating PKC, as evidenced by enhanced translocation of the enzyme to the cell membrane, and this finding suggests that PKC may have a physiological role in opioid receptor plasticity.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: II. Activation and involvement of the alpha, epsilon, and zeta isoforms of PKC

Phosphorylation of specific amino acid residues is believed to be crucial for the agonist-induced regulation of several G protein-coupled receptors. This is especially true for the three types of opioid receptors (mu, delta, and kappa), which contain consensus sites for phosphorylation by numerous protein kinases. Protein kinase C (PKC) has been shown to catalyze the in vitro phosphorylation of mu- and delta-opioid receptors and to potentiate agonist-induced receptor desensitization. In this series of experiments, we continue our investigation of how opioid-activated PKC contributes to homologous receptor down-regulation and then expand our focus to include the exploration of the mechanism(s) by which mu-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells. [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin (DAMGO)-induced PKC translocation follows a time-dependent and biphasic pattern beginning 2 h after opioid addition, when a pronounced translocation of PKC to the plasma membrane occurs. When opioid exposure is lengthened to >12 h, both cytosolic and particulate PKC levels drop significantly below those of control-treated cells in a process we termed "reverse translocation." The opioid receptor antagonist naloxone, the PKC inhibitor chelerythrine, and the L-type calcium channel antagonist nimodipine attenuated opioid-mediated effects on PKC and mu-receptor down-regulation, suggesting that this is a process partially regulated by Ca2+-dependent PKC isoforms. However, chronic exposure to phorbol ester, which depletes the cells of diacylglycerol (DAG) and Ca2+-sensitive PKC isoforms, before DAMGO exposure, had no effect on opioid receptor down-regulation. In addition to expressing conventional (PKC-alpha) and novel (PKC-epsilon) isoforms, SH-SY5Y cells also contain a DAG- and Ca2+-independent, atypical PKC isozyme (PKC-zeta), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-zeta is similarly sensitive to activation by mu-opioids. PKC-zeta translocates from the cytosol to the membrane with kinetics similar to those of PKC-alpha and epsilon in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by mu-opioid agonists is involved in the processes that result in mu-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.

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.

Nociceptin/orphanin FQ stimulates extracellular acidification and desensitization of the response involves protein kinase C

A Chinese hamster ovary (CHO) cell line, CHO-ORL1, stably expressing human opioid receptor-like receptor 1 (ORL1) has been used to determine ORL1-mediated signaling events using microphysiometry. Nociceptin/orphanin FQ (N/OFQ), a specific endogenous agonist of ORL1, induced an increase in extracellular acidification rate (ECAR) in CHO-ORL1 cells. The ECAR response stimulated by N/OFQ was concentration-dependent and pertussis toxin-sensitive. Repeated exposures of the cells to N/OFQ caused desensitization of ORL1. The ECAR response was recovered at the half-life of approximately 12 min after the initial challenge. Pretreatment with inhibitor of cAMP-dependent kinase did not affect desensitization of ORL1. However, specific inhibitors for protein kinase C almost abolished N/OFQ-induced desensitization of extracellular acidification responsiveness, indicating the involvement of protein kinase C in the process.

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.

Activation of N-methyl-D-aspartate receptor attenuates acute responsiveness of delta-opioid receptors

Coadministration of antagonists of N-methyl-D-aspartate (NMDA) receptor and opioids has been shown to prevent development of opiate tolerance in animal and clinical studies, but its cellular and molecular mechanisms are not understood. In this study, the effect of NMDA on delta-opioid receptor (DOR)-mediated signal transduction was investigated in neuroblastoma x glioma NG108-15 cells that functionally express both DOR and NMDA receptors. Acute incubation of NG108-15 cells with NMDA, a specific agonist of NMDA receptor, significantly attenuated the ability of DOR agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) to inhibit forskolin-stimulated cAMP production. The attenuation caused by NMDA was dose-dependent, and the EC50 of DPDPE increased 100-fold (from 4.6 nM to 500 nM) after NMDA treatment. The NMDA effect on responsiveness of delta-opioid receptors to DPDPE could be blocked by ketamine, a NMDA receptor-specific antagonist. This NMDA attenuation effect on DOR activity was also observed in neuronal primary cell cultures from fetal mouse brain but not in the Chinese hamster ovary cell line stably transfected with DOR alone. Interestingly, NMDA pretreatment reduced the cellular response to epinephrine but not to that of prostaglandin E1 in NG108-15 cells, which suggests differential modulation of NMDA on different G protein-coupled receptors. Pretreatment of NG108-15 cells with ketamine along with DPDPE greatly attenuated DPDPE-induced acute desensitization of DOR. Furthermore, the specific inhibitors of protein kinase C, either chelerythrine chloride or Go 6979, effectively blocked the NMDA effect, which indicates the involvement of protein kinase C in the process. In conclusion, the activation of NMDA receptors can attenuate acute responsiveness of DOR in neuronal cells, whereas its blockage leads to reduction of DOR desensitization. These results have thus provided an insight into cross-talk between NMDA and opioid signal transduction.