Spinal glucocorticoid receptors contribute to the development of morphine tolerance in rats

Effects of Corticosterone on the Excitability of Glutamatergic and GABAergic Neurons of the Adolescent Mouse Superficial Dorsal Horn

Stress evokes age-dependent effects on pain sensitivity and commonly occurs during adolescence. However, the mechanisms linking adolescent stress and pain remain poorly understood, in part due to a lack of information regarding how stress hormones modulate the function of nociceptive circuits in the adolescent CNS. Here we investigate the short- and long-term effects of corticosterone (CORT) on the excitability of GABAergic and presumed glutamatergic neurons of the spinal superficial dorsal horn (SDH) in Gad1-GFP mice at postnatal days (P)21-P34. In situ hybridization revealed that glutamatergic SDH neurons expressed significantly higher mRNA levels of both glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) compared to adjacent GABAergic neurons. The incubation of spinal cord slices with CORT (90 min) evoked select long-term changes in spontaneous synaptic transmission across both cell types in a sex-dependent manner, without altering the intrinsic firing of either Gad1-GFP+ or GFP- neurons. Meanwhile, the acute bath application of CORT significantly decreased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs), as well as the frequency of miniature inhibitory postsynaptic currents (mIPSCs), in both cell types leading to a net reduction in the balance of spontaneous excitation vs. inhibition (E:I ratio). This CORT-induced reduction in the E:I ratio was not prevented by selective antagonists of either GR (mifepristone) or MR (eplerenone), although eplerenone blocked the effect on mEPSC amplitude. Collectively, these data suggest that corticosterone modulates synaptic function within the adolescent SDH which could influence the overall excitability and output of the spinal nociceptive network.

Astroglial Knockout of Glucocorticoid Receptor Attenuates Morphine Withdrawal Symptoms, but Not Antinociception and Tolerance in Mice

The development of tolerance and drug dependence limit the clinical application of opioids for the treatment of severe pain. Glucocorticoid receptors (GRs) are among molecular substrates involved in these processes. Most studies focus on the role of neuronal GR, while the involvement of GR on glial cells is not fully understood. To address this issue, we used a transgenic model of conditional GR knockout mice, targeted to connexin 30-expressing astrocytes, treated with repeated doses of morphine. We observed no difference between control mice and astrocytic GR knockouts in the development of antinociceptive tolerance. Nevertheless, when animals were subjected to precipitated withdrawal, knockouts presented some attenuated symptoms, including jumping. Taken together, our data suggest that hippocampal and spinal astrocytic GRs appear to be involved in opioid withdrawal, and drugs targeting the GR may relieve some symptoms of morphine withdrawal without influencing its antinociceptive properties.

Disability-specific genes GRIN1, GRIN2 and CNR1 show injury-dependent protein expression in the lumbar spinal cord of CCI rats

The sensory changes triggered by peripheral nerve injury result from functional changes in both neurons and glia in the dorsal horn of the spinal cord. Whether the disrupted affective-motivational states often comorbid with injury-evoked changes in sensation are driven directly by these functional changes is a question only recently investigated. Using a combination of GeneChip microarrays and RT-PCR techniques we identified differences in mRNA expression unique to rats with sustained changes to their social behaviour following sciatic nerve chronic constriction injury (CCI). Amongst these changes were the mRNAs encoding several of the NMDA subunits and the CB1 receptor. However, as protein translation is not a necessary consequence of the upregulation or downregulation of genes we decided to evaluate the functional significance of our initial observations using immunohistochemical detection of their translated protein products to determine their location and abundance in the lumbar spinal cord. Spinal cord tissue from rats with ('Affected'), and without ('Unaffected') changes in social behaviour after CCI was compared with tissue from uninjured controls. The expression of NMDA-1 (NR1) subunit, NMDA-2D subunit, Cannabinoid Receptor 1 (CB1), Glucocorticoid Receptor (GR) and Glial Fibrillary Acidic Protein (GFAP) immunoreactivities was quantified for these rats and revealed that nerve injury increased the expression of NMDA-2D, CB1 and GFAP immunoreactivity compared to uninjured controls. However, these changes were not specific to rats whose social behaviours were 'Affected' or 'Unaffected' by the nerve injury. Our data thus suggest that the development and expression of changes in social behaviour seen in a proportion of rats following CCI are unlikely to be directly related to the spinal changes in NMDA-2D, CB1 and GFAP expression induced by the nerve injury.

Spinal glucocorticoid receptor‑regulated chronic morphine tolerance may be through extracellular signal‑regulated kinase 1/2

Opioid use has been limited in the treatment of chronic pain due to their side effects, including analgesic tolerance. Previous studies demonstrated that glucocorticoid receptors (GRs) may be involved in the development of chronic morphine tolerance; however, the mechanism remains unknown. It was hypothesized that the expression of spinal phosphorylated mitogen‑activated protein kinase [MAPK; phosphorylated extracellular signal‑regulated kinase (ERK)] is regulated through the spinal GRs, following chronic treatment with morphine. In the first experiment, the experimental rats were randomly divided into four groups: Control, morphine, morphine+GR antagonist mifepristone (RU38486) and morphine+GR agonist dexamethasone (Dex). Each group was treated with continuous intrathecal (IT) injection of the drugs for 6 days. The expression of GRs and MAPK 3/1 (p‑ERK 1/2) in the spinal dorsal horn was detected by western blot analysis and immunofluorescence staining. In the second experiment, the MAPK inhibitor PD98059 was added and the rats were randomly divided into four groups: Control, morphine, PD98059+morphine and PD98059+morphine+Dex. The continuous IT injection lasted for 7 days in each group. For all experiments, the tail flick test was conducted 30 min following administration every day to assess the thermal hyperalgesia of the rats. The experimental results demonstrated that there was a co‑existence of GRs and p‑ERK 1/2 in the spinal cord dorsal horn by double immunofluorescence staining. The GR antagonist RU38486 attenuated the morphine analgesia tolerance by inhibiting the expression of GR and increasing the expression of p‑ERK. The MAPK inhibitor PD98059 increased the effect of morphine tolerance and prolonged the duration of morphine tolerance. The present results suggest that spinal GRs may serve an important role in the development of morphine tolerance through the ERK signaling pathway.

Endoplasmic Reticulum Stress in Spinal Cord Contributes to the Development of Morphine Tolerance

Morphine tolerance remains an intractable problem, which hinders its prolonged use in clinical practice. Endoplasmic reticulum (ER) stress has been proved to play a fundamental role in the pathogenesis of Alzheimer's disease, diabetes, atherosclerosis, cancer, etc. In this study, we provide the first direct evidence that ER stress may be a significant driver of morphine tolerance. Binding immunoglobulin protein (BiP), the ER stress marker, was significantly upregulated in neurons in spinal dorsal horn in rats being treated with morphine for 7 days. Additionally, chronic morphine treatment resulted in the activation of three arms of unfolded protein response (UPR): inositol-requiring enzyme 1/X-box binding protein 1 (IRE1/XBP1), protein kinase RNA-like ER kinase/eukaryotic initiation factor 2 subunit alpha (PERK/eIF2α), and activating transcription factor 6 (ATF6). More importantly, inhibiting either one of the three cascades could attenuate the development of morphine tolerance. Taken together, our results suggest that ER stress in spinal cord might contribute to the development of morphine tolerance. These findings implicate a potential clinical strategy for preventing morphine tolerance and may contribute to expanding the morphine usage in clinic.

Behavioral and transcriptional patterns of protracted opioid self-administration in mice

Chronic exposure to opioids induces adaptations in brain function that lead to the formation of the behavioral and physiological symptoms of drug dependence and addiction. Animal models commonly used to test these symptoms typically last less than two weeks, which is presumably too short to observe the alterations in the brain that accompany drug addiction. Here, we analyzed the phenotypic and molecular effects of nearly lifelong morphine or saccharin intake in C57BL/6J mice. We used multiple paradigms to evaluate the symptoms of compulsive drug intake: a progressive ratio schedule, intermittent access and a schedule involving a risk of punishment were programmed into an automated IntelliCage system. Gene expression profiles were evaluated in the striatum using whole-genome microarrays and further validated using quantitative polymerase chain reaction in the striatum and the prefrontal cortex. Mice voluntary self-administering morphine showed addiction-related behavioral pattern that included: higher motivation to work for a drug reward, increased reward seeking and increased craving. The analysis of molecular changes revealed a tolerance effect in the transcriptional response to morphine injection (20 mg/kg, ip), as well as some long-lasting alterations in gene expression profiles between the analyzed groups of animals. Interestingly, among the morphine-drinking animals, certain transcriptional profiles were found to be associated with alterations in behavior. In conclusion, our model represents a novel approach for investigating the behavioral and molecular mechanisms underlying opioid addiction. Prolonged morphine intake caused adaptive processes in the brain that manifested as altered behavior and transcriptional sensitivity to opioids.

The effect of preoperative dexamethasone on pain 1 year after lumbar disc surgery: a follow-up study

Background

It has been hypothesized that dexamethasone can inhibit persistent postoperative pain, but data on humans is lacking and results from animal studies are conflicting. We explored the effect of 16 mg dexamethasone IV administered preoperatively on persistent pain 1 year after lumbar discectomy.

Methods

This is a prospective 1-year follow-up on a single-centre, randomized, and blinded trial exploring the analgesic effect of 16 mg IV dexamethasone or placebo after lumbar discectomy. One year follow-up was a written questionnaire including back and leg pain (VAS 0–100 mm), Short Form 36 survey (SF-36), EuroQol 5D (EQ-5D), OSWESTRY Low Back Pain Questionnaire, duration of sick leave, working capability, contentment with surgical result.

Results

Response rate was 71% (55 patients) in the dexamethasone group, 58% (44 patients) in the placebo group. Leg pain (VAS) was significantly lower in the placebo group compared to the dexamethasone group: 17 (95% CI 10–26) vs 26 (95% CI 19–33) mm, respectively (mean difference 9 mm (95% CI −1 to 0), (P = 0.03). No difference regarding back pain. The placebo group reported significantly more improvement of leg pain and were significantly more satisfied with the surgical result. Patients in the dexamethasone group reported significantly higher pain levels in EQ-5D- and Oswestry questionnaires. No difference in the SF-36 survey or daily analgesic consumption.

Conclusions

We found significantly higher pain levels in the dexamethasone group compared to placebo 1 year after lumbar disc surgery.

Trial registration

Clinicaltrials.gov (NCT01953978). Registered 26 Sep 2013.

Highlights in opioid agonists and antagonists

This review highlights new insights in to opioid agonists and antagonists, focusing on their mechanism of action with spinal and systemic administration, chronic use and main adverse effects. Short-cuts on some opioid agonists and antagonists of clinical interest are also presented, revealing potential clinical implications and future clinical directions as part of multimodal analgesia.

The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception

BACKGROUND

Spironolactone, a commonly used mineralocorticoid receptor antagonist, has been reported to potentiate the effect of morphine in the rat. The aim of this study was to investigate the effects of spironolactone on morphine antinociception and tissue distribution.

METHODS

The effects of spironolactone on acute morphine-induced antinociception, induction of morphine tolerance and established morphine tolerance were studied with tail-flick and hot plate tests in male Sprague-Dawley rats. Serum, brain, and liver morphine and its metabolite concentrations were quantified using high-pressure liquid chromatography-tandem mass spectrometry. Spironolactone was also administered with the peripherally acting, P-glycoprotein (P-gp) substrate loperamide to test whether spironolactone allows loperamide to pass the blood-brain barrier.

RESULTS

Spironolactone (50 mg/kg, i.p.) had no antinociceptive effects of its own, but it enhanced the antinociceptive effect of morphine in both thermal tests. Two doses of spironolactone enhanced the maximum possible effect (MPE) from 19.5% to 100% in the hot plate test 90 min after administration of 4 mg/kg morphine. Morphine concentrations in the brain were increased fourfold at 90 min by spironolactone. Spironolactone did not inhibit the formation of morphine-3-glucuronide. Acute spironolactone restored morphine antinociception in morphine-tolerant rats but did not inhibit the development of tolerance. The peripherally restricted opioid, loperamide (10 mg/kg), had no antinociceptive effects when administered alone, but co-administration with spironolactone produced a 40% MPE in the hot plate test.

CONCLUSIONS

Spironolactone has no antinociceptive effects in thermal models of pain, but it enhances the antinociceptive effects of morphine mainly by increasing morphine central nervous system concentrations, probably by inhibiting P-gp.

Intraperitoneal hydrocortisone for pain relief after laparoscopic cholecystectomy

Background:

Laparoscopic cholecystectomy is associated with shorter hospital stay and less pain in comparison to open surgery. The aim of this study was to evaluate the effect of intraperitoneal hydrocortisone on pain relief following laparoscopic cholecystectomy.

Methods:

Sixty two patients were enrolled in a double-blind, randomized clinical trial. Patients randomly received intraperitoneal instillation of either 250 ml normal saline (n=31) or 100 mg hydrocortisone in 250 ml normal saline (n=31) before insufflation of CO₂ into the peritoneum. Abdominal and shoulder pain were evaluated using VAS after surgery and at 6, 12, and 24 hours postoperatively. The patients were also followed for postoperative analgesic requirements, nausea and vomiting, and return of bowel function.

Results:

Sixty patients completed the study. Patients in the hydrocortisone group had significantly lower abdominal and shoulder pain scores (10.95 vs 12.95; P<0.01). The patients were similar regarding analgesic requirements in the recovery room. However, those in the hydrocortisone group required less meperidine than the saline group (151.66 (±49.9) mg vs 61.66 (±38.69) mg; P=0.00). The patients were similar with respect to return of bowel function, nausea and vomiting. No adverse reaction was observed in either group.

Conclusion:

Intraperitoneal administration of hydrocortisone can significantly decrease pain and analgesic requirements after laparoscopic cholecystectomy with no adverse effects.

Epigenetics and the transition from acute to chronic pain

OBJECTIVE

The objective of this study was to review the epigenetic modifications involved in the transition from acute to chronic pain and to identify potential targets for the development of novel, individualized pain therapeutics.

BACKGROUND

Epigenetics is the study of heritable modifications in gene expression and phenotype that do not require a change in genetic sequence to manifest their effects. Environmental toxins, medications, diet, and psychological stresses can alter epigenetic processes such as DNA methylation, histone acetylation, and RNA interference. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, steroid responsiveness, and opioid sensitivity, they are likely key factors in the development of chronic pain. Although our knowledge of the human genetic code and disease-associated polymorphisms has grown significantly in the past decade, we have not yet been able to elucidate the mechanisms that lead to the development of persistent pain after nerve injury or surgery.

DESIGN

This is a focused literature review of epigenetic science and its relationship to chronic pain.

RESULTS

Significant laboratory and clinical data support the notion that epigenetic modifications are affected by the environment and lead to differential gene expression. Similar to mechanisms involved in the development of cancer, neurodegenerative disease, and inflammatory disorders, the literature endorses an important potential role for epigenetics in chronic pain.

CONCLUSIONS

Epigenetic analysis may identify mechanisms critical to the development of chronic pain after injury, and may provide new pathways and target mechanisms for future drug development and individualized medicine.

Examining the role of mu opioid receptor endocytosis in the beneficial and side-effects of prolonged opioid use: from a symposium on new concepts in mu-opioid pharmacology

Opioid drugs remain the gold standard for the treatment of severe pain, both acute/post-surgical and chronic. However, the utility of opioid drugs for the treatment of chronic pain is compromised by the development of analgesic tolerance which, in turn, leads to dose-escalation and increased likelihood of dangerous side effects, including dependence. Consequently, there remains resistance among clinicians and the general population to using opiates for pain management because of risk of "addiction." These fears are not unwarranted. More than 2.5 million people begin abusing opioid painkillers each year, and prescription opioid abuse is now the second most common type of illegal drug use after marijuana. Some abusers become dependent due to recreational use of prescription painkillers. However, many abusers are among the 40 million people suffering from chronic pain, and developed dependence while using the drugs for legitimate purposes. Both of these trends highlight the need to develop opioid therapeutics with a reduced liability to cause tolerance, dependence and addiction. Identifying the ideal properties of opioid drugs that would retain analgesia but reduce these side-effects has been a goal of my laboratory for more than a decade. During this time, we have proposed the novel hypothesis that opioid drugs that promote desensitization, endocytosis and recycling of the mu-opioid-receptor (MOR) will retain analgesic efficacy, but will have a reduced liability to cause tolerance, dependence and addiction. We have generated substantial data, both pharmacological and genetic to suggest that our hypothesis is a valid one. These data are summarized in this review.

Gene knockdown with lentiviral vector-mediated intrathecal RNA interference of protein kinase C gamma reverses chronic morphine tolerance in rats

BACKGROUND

Although morphine is a widely used opioid analgesic, morphine tolerance (MT) has limited the use of the drug because it creates the necessity for high doses. Protein kinase C (PKC), especially the PKCγ isoform, is considered to play a key role in the development of MT. Because RNA interference provides a powerful method for the investigation of gene function, and lentiviral delivery systems have been approved for human use, this present study examined rats tolerant to morphine to determine whether an intrathecal injection of a lentiviral vector of PKCγ short hairpin RNA (LV-shPKCγ) down-regulated the expression of the PKCγ gene and reversed MT.

METHODS

MT was induced by intrathecal morphine (10 µg b.i.d.) for six consecutive days. A lentiviral-mediated short hairpin RNA (shRNA) system was synthesized to deliver the PKCγ shRNAs to the spinal cord of the rats with MT. Mechanical and thermal paw withdrawal threshold were assessed to determine the analgesic effects of morphine. Expression of PKCγ mRNA and protein was determined by reverse transcriptase-polymerase chain reaction and western blotting analysis, respectively.

RESULTS

The chronic administration of morphine induced a stabilized analgesic tolerance. A single injection of LV-shPKCγ significantly reversed morphine antinociceptive tolerance. Compared to the control group, PKCγ mRNA and protein levels were dramatically down-regulated in the LV-shPKCγ group.

CONCLUSIONS

A single injection of LV-shPKCγ reversed MT by reducing the expression of PKCγ in the spinal cord. These findings indicate that the use of LV-shPKCγ might be a potential strategy for therapy in MT.

Biomarkers of morphine tolerance and dependence are prevented by morphine-induced endocytosis of a mutant mu-opioid receptor

Growing evidence shows that trafficking of the mu-opioid receptor (MOR) is a critical process in functional recovery from desensitization following activation and plays important roles in morphine tolerance and dependence largely because of the failure of morphine to promote such trafficking. However, morphine tolerance and dependence are believed to be mediated by multiple mechanisms, including well-documented biochemical changes in cAMP activity, N-methyl-D-aspartate receptors (NMDARs), glucocorticoid receptors (GRs), and c-fos. Here, we assess the consequences of promoting morphine-induced endocytosis on these biochemical changes utilizing a knock-in mouse model, RMOR, in which MORs undergo morphine-induced endocytosis. Chronic morphine treatment of wild-type (WT) mice promoted superactivation of adenylyl cyclase, alterations in NMDARs, and up-regulation of GR and c-fos in distinct brain regions. Notably, none of these biochemical changes occurred in the RMOR-knock-in mice. Together, these data demonstrate that morphine tolerance and dependence are mediated by multiple biochemical mechanisms and that MOR endocytosis plays a critical role in each of these mechanisms.

Effects of environmental enrichment on sensitivity to mu, kappa, and mixed-action opioids in female rats

Several studies report that environmental enrichment enhances sensitivity to opioid receptor agonists in male rats. Very few studies have examined the effects of enrichment in female rats, and thus it is not clear whether females are similarly sensitive to these effects. Consequently, the purpose of the present study was to examine the effects of environmental enrichment on sensitivity to representative mu, kappa, and mixed-action opioids in female rats. Following a protocol established in males, females were obtained at weaning and randomly assigned to two groups immediately upon arrival: isolated rats were housed individually with no visual or tactile contact with other rats; enriched rats were housed in groups of four in large cages and given various novel objects on a regular basis. After 6 weeks under these conditions, the antinociceptive effects of mu (morphine, levorphanol), kappa (spiradoline, U69,593), and mixed-action (buprenorphine, butorphanol) opioids were examined in a warm-water, tail-withdrawal procedure. All the opioids examined produced dose-dependent increases in antinociception; however, no differences in opioid sensitivity were observed between the two groups. To determine whether these findings were consistent across behavioral endpoints, the antidiuretic effects of representative mu opioids, and the diuretic effects of representative kappa opioids, were examined in female rats reared under isolated or enriched conditions for 10 weeks. Similar to that seen in the antinociceptive experiment, no significant differences in opioid sensitivity were observed between groups. These data indicate that environmental enrichment does not alter sensitivity to the effects of opioid receptor agonists in female rats, and suggest that females may respond differently to environmental enrichment than males.

Morphine effects on striatal transcriptome in mice

Global transcriptional analysis of mouse striata following acute and chronic exposure to morphine reveals multiple physiological factors which may affect opioid-related phenotypes and implicates a number of gene networks, including glucocorticoid receptor regulated genes, in the response to this opioid.

Background

Chronic opiate use produces molecular and cellular adaptations in the nervous system that lead to tolerance, physical dependence, and addiction. Genome-wide comparison of morphine-induced changes in brain transcription of mouse strains with different opioid-related phenotypes provides an opportunity to discover the relationship between gene expression and behavioral response to the drug.

Results

Here, we analyzed the effects of single and repeated morphine administrations in selected inbred mouse strains (129P3/J, DBA/2J, C57BL/6J, and SWR/J). Using microarray-based gene expression profiling in striatum, we found 618 (false discovery rate < 1%) morphine-responsive transcripts. Through ontologic classification, we linked particular sets of genes to biologic functions, including metabolism, transmission of nerve impulse, and cell-cell signaling. We identified numerous novel morphine-regulated genes (for instance, Olig2 and Camk1g), and a number of transcripts with strain-specific changes in expression (for instance, Hspa1a and Fzd2). Moreover, transcriptional activation of a pattern of co-expressed genes (for instance, Tsc22d3 and Nfkbia) was identified as being mediated via the glucocorticoid receptor (GR). Further studies revealed that blockade of the GR altered morphine-induced locomotor activity and development of physical dependence.

Conclusion

Our results indicate that there are differences between strains in the magnitude of transcriptional response to acute morphine treatment and in the degree of tolerance in gene expression observed after chronic morphine treatment. Using whole-genome transcriptional analysis of morphine effects in the striatum, we were able to reveal multiple physiological factors that may influence opioid-related phenotypes and to relate particular gene networks to this complex trait. The results also suggest the possible involvement of GR-regulated genes in mediating behavioral response to morphine.

Combination of dexamethasone and etanercept reduces secondary damage in experimental spinal cord trauma

The aim of our study was to evaluate the therapeutic efficacy of combination therapy with etanercept and dexamethasone (DEX) in vivo in experimental murine model of spinal cord trauma, which was induced by the application of vascular clips (force of 24 g) to the dura via a four-level T5-T8 laminectomy. Spinal cord injury in mice resulted in severe trauma characterized by edema, neutrophil infiltration, and cytokine production followed by recruitment of other inflammatory cells, production of inflammation mediators, tissue damage, apoptosis and disease. Treatment of the mice with etanercept (1.25 mg/kg) and DEX (0.025 mg/kg) when administered as a combination therapy but not as a single treatment significantly reduced the degree of (1) spinal cord inflammation and tissue injury (histological score), (2) infiltration of neutrophils (MPO evaluation), (3) inducible nitric oxide synthase, nitrotyrosine, and cytokines expression (tumor necrosis factor-alpha and interleukin-1 beta), (4) and apoptosis (Terminal deoxynucleotidyltransferase-mediated UTP end labeling staining, Fas-ligand expression and Bax and Bcl-2 expression). In a separate set of experiments we have also clearly demonstrated that the combination therapy significantly ameliorated the recovery of limb function (evaluated by motor recovery score). Taken together, our results clearly demonstrate for the first time that strategies targeting multiple proinflammatory pathways may be more effective than a single effector molecule for the treatment of spinal cord trauma.

Morphine withdrawal modifies antinociceptive effects of acute morphine in rats

Repeated opioid use is known to cause tolerance of antinociceptive effects. Whether opioid abstinence modifies antinociceptive effects is unknown. Here we reported that morphine withdrawal for 18 h and 4 days after repeated morphine treatment largely reduced tail-flick latencies compared with control, while the rats showed severe withdrawal syndromes. However, the latencies and withdrawal syndromes were restored to control level at 20 days withdrawal. Similarly, antinociceptive effects of acute morphine were decreased at 18 h and further decreased at 4 days but restored to control level at 20 days withdrawal. Behavioral stress that was given to the rats at 18 h withdrawal further reduced tail-flick latencies and antinociceptive effects. Conversely, the glucocorticoid receptor antagonist RU38486 increased tail-flick latencies and antinociceptive effects at 4 days withdrawal. These results suggest that morphine withdrawal could evoke behavioral stress to modify antinociceptive effects, implicating a significant influence of opioid abstinence on chronic pain treatment.

Expression of spinal NMDA receptor and PKCgamma after chronic morphine is regulated by spinal glucocorticoid receptor

Spinal NMDA receptor (NMDAR), protein kinase C (PKC), and glucocorticoid receptor (GR) have all been implicated in the mechanisms of morphine tolerance; however, how these cellular elements interact after chronic morphine exposure remains unclear. Here we show that the expression of spinal NMDAR and PKCgamma after chronic morphine is regulated by spinal GR through a cAMP response element-binding protein (CREB)-dependent pathway. Chronic morphine (10 microg, i.t.; twice daily for 6 d) induced a time-dependent upregulation of GR, the NR1 subunit of NMDAR, and PKCgamma within the rat's spinal cord dorsal horn. This NR1 and PKCgamma upregulation was significantly diminished by intrathecal coadministration of morphine with the GR antagonist RU38486 or a GR antisense oligodeoxynucleotide. Intrathecal coadministration of morphine with an adenylyl cyclase inhibitor (2',5'-dideoxyadenosine) or a protein kinase A inhibitor (H89) also significantly attenuated morphine-induced NR1 and PKCgamma expression, whereas intrathecal treatment with an adenylyl cyclase activator (forskolin) alone mimicked morphine-induced expression of GR, NR1, and PKCgamma. Moreover, the expression of phosphorylated CREB was upregulated within the spinal cord dorsal horn after chronic morphine, and a CREB antisense oligodeoxynucleotide coadministered intrathecally with morphine prevented the upregulation of GR, NR1, and PKCgamma. These results indicate that spinal GR through the cAMP-CREB pathway played a significant role in NMDAR and PKCgamma expression after chronic morphine exposure. The data suggest that genomic interaction among spinal GR, NMDAR, and PKCgamma may be an important mechanism that contributes to the development of morphine tolerance.

Activation of p38 mitogen-activated protein kinase in spinal microglia mediates morphine antinociceptive tolerance

Compelling evidence has suggested that spinal glial cells were activated by chronic morphine treatment and involved in the development of morphine tolerance. However, the mechanisms of glial activation were still largely unknown in morphine tolerance. In present study, we investigated the role of p38 mitogen-activated protein kinase (p38 MAPK) in the spinal cord in the development of chronic morphine antinociceptive tolerance. We found that intrathecal administration of morphine (15 microg) daily for 7 consecutive days significantly induced an increase in number of phospho-p38 (p-p38) immunoreactive cells in the spinal cord compared with chronic saline or acute morphine treated rats. Double immunofluorescence staining revealed that p-p38 immunoreactivity was exclusively restricted in the activated spinal microglia, not in astrocytes or neurons. Repeated intrathecal administration of 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) (10 microg or 2 microg), a specific p38 inhibitor, 30 min before each morphine injection for 7 consecutive days significantly attenuated tolerance to morphine analgesia assessed by tail flick test. However, a single intrathecal administration of SB203580 (10 microg) did not antagonize the established tolerance to morphine analgesia. Taken together, these findings suggested that p38 MAPK activation in the spinal microglia was involved in the development of morphine antinociceptive tolerance. Inhibition of p38 MAPK by SB203580 in the spinal cord attenuated but not reversed the tolerance to morphine analgesia. The present study provides the first evidence that p38 activation in spinal microglia played an important role in the development of tolerance to morphine analgesia.