Central glucocorticoid receptors modulate the expression of spinal cannabinoid receptors induced by chronic morphine exposure

Cannabinoid CB2 receptors in primary sensory neurons are implicated in CB2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB2 agonists show therapeutic efficacy without unwanted CB1-mediated side effects. The G protein-biased CB2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks development of morphine tolerance in this model. However, the cell types involved in this phenomenon are unknown and whether this therapeutic profile is observed in female mice has never been investigated. We used conditional deletion of CB2 receptors to determine the cell population(s) mediating the anti-allodynic and morphine-sparing effects of CB2 agonists. Anti-allodynic effects of structurally distinct CB2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB2f/f mice and in mice lacking CB2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1CRE/+; CB2f/f), but were absent in mice lacking CB2 receptors in peripheral sensory neurons (AdvillinCRE/+; CB2f/f). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male, but not female, mice. LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the development of morphine tolerance in male CB2f/f and CX3CR1CRE/+; CB2f/f mice with established paclitaxel-induced neuropathy but was absent in male (or female) AdvillinCRE/+; CB2f/f mice. Co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed morphine tolerance in paclitaxel-treated male CB2f/f mice, but not AdvillinCRE/+; CB2f/f mice of either sex. LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical or cold allodynia in either CB2f/f or CX3CR1CRE/+; CB2f/f mice of either sex. Our findings have potential clinical implications.

Cannabinoid CB 2 receptors in primary sensory neurons are implicated in CB 2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB 2 agonists show therapeutic efficacy without the unwanted side effects commonly associated with direct activation of CB 1 receptors. The G protein-biased CB 2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks the development of morphine tolerance in this model. However, the specific cell types involved in this phenomenon have never been investigated and whether this therapeutic profile is observed in female mice remains poorly understood. We used conditional deletion of CB 2 receptors from specific cell populations to determine the population(s) mediating the anti-allodynic and morphine-sparing effects of CB 2 agonists. Anti-allodynic effects of structurally distinct CB 2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB 2 f/f mice of either sex. The anti-allodynic effect of the CB 2 agonists were absent in conditional knockout (KO) mice lacking CB 2 receptors in peripheral sensory neurons (Advillin CRE/+ ; CB 2 f/f ) but preserved in mice lacking CB 2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1 CRE/+ ; CB 2 f/f ). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male mice but absent in female mice of any genotype. In mice with established paclitaxel-induced neuropathy, prior LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the subsequent development of morphine tolerance in male CB 2 f/f mice but was absent in male (or female) Advillin CRE/+ ; CB 2 f/f mice. LY2828360-induced sparing of morphine tolerance was preserved in male CX3CR1 CRE/+ ; CB 2 f/f mice, but this effect was not observed in female CX3CR1 CRE/+ ; CB 2 f/f mice. Similarly, co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed tolerance to the anti-allodynic efficacy of morphine in paclitaxel-treated male CB 2 f/f mice, but this effect was absent in female CB 2 f/f mice and Advillin CRE/+ ; CB 2 f/f mice of either sex. Additionally, LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical and cold allodynia in either CB 2 f/f or CX3CR1 CRE/+ ; CB 2 f/f mice of either sex. Our studies reveal that CB 2 receptors in primary sensory neurons are required for the anti-allodynic effects of CB 2 agonists in a mouse model of paclitaxel-induced neuropathic nociception. We also find that CB 2 agonists acting on primary sensory neurons produce a sexually-dimorphic sparing of morphine tolerance in males, but not female, paclitaxel-treated mice.

Cannabinoid Receptor Type 2 Agonist Reduces Morphine Tolerance via Mitogen Activated Protein Kinase Phosphatase Induction and Mitogen Activated Protein Kinase Dephosphorylation

Morphine is an opioid drug often used in treating moderate to severe pain. However, morphine tolerance in patients limits its used in clinical settings. Our previous study showed that a cannabinoid type 2 (CB2) receptor agonist attenuated morphine tolerance. However, the exact mechanism by which CB2 agonists reduce morphine tolerance remains unclear. In this study, we investigated the effect of mitogen activated protein kinase (MAPK) and mitogen activated protein kinase phosphatases 1 and 3 (MKP-1 and MKP-3) on the regulation of morphine tolerance by CB2 receptor agonist. Chronic morphine treatments for 7 days reduced the protein expression of MKP-1 and MKP-3 in the spinal cord and increased the phosphorylation of p38, ERK1/2 and the level of proinflammatory mediator, such as IL-1β, IL-6 and TNF-α. Coadministration of CB2 receptor agonist AM1241 alleviated the inhibition of MKP-1 and MKP-3 by chronic morphine administration and reduced the expression of phosphorylated MAPK and proinflammatory factors. The effect of the CB2 receptor agonist on morphine-induced downregulation of MKP-1 and MKP-3 was reversed by the MKP-1 and MKP-3 antagonist triptolide. Our findings suggested that CB2 receptor agonist may induce the expression of MKP-1 and MKP-3 to promote MAPK dephosphorylation and reduce the production of downstream cytokine, thereby reducing morphine tolerance. This finding suggested that MKPs may serve as a new target for alleviating morphine tolerance.

Effects of inflammatory pain on CB1 receptor in the midbrain periaqueductal gray

Western blot and GTPγS analyses reveal inflammatory pain–induced adaptations in the midbrain periaqueductal gray, which is critically involved in descending pain modulation. Pain upregulates the expression of the CB1 receptor and increases G-protein coupling in the periaqueductal gray.

Introduction:

The periaqueductal gray (PAG) mediates the antinociceptive properties of analgesics, including opioids and cannabinoids. Administration of either opioids or cannabinoids into the PAG induces antinociception. However, most studies characterizing the antinociceptive properties of cannabinoids in the PAG have been conducted in naive animals. Few studies have reported on the role of CB1 receptors in the PAG during conditions which would prompt the administration of analgesics, namely, during pain states.

Objectives:

To examine inflammatory pain-induced changes in CB1 receptor expression and function in the midbrain periaqueductal gray.

Methods:

In this study, we used the Complete Freund Adjuvant model to characterize CB1 receptor expression and G-protein coupling during persistent inflammatory pain.

Results:

Inflammatory pain induced an upregulation in the expression of synaptic CB1 receptors in the PAG. Despite this pain-induced change in CB1 expression, there was no corresponding upregulation of CB1 mRNA after the induction of inflammatory pain, suggesting a pain-induced recruitment of CB1 receptors to the synaptic sites within PAG neurons or increased coupling efficiency between the receptor and effector systems. Inflammatory pain also enhanced ventrolateral PAG CB1 receptor activity, as there was an increase in CP55,940-stimulated G-protein activation compared with pain-naïve control animals.

Conclusion:

These findings complement a growing body of evidence which demonstrate pain-induced changes in brain regions that are responsible for both the analgesic and rewarding properties of analgesic pharmacotherapies. Because much of our understanding of the pharmacology of cannabinoids is based on studies which use largely pain-naïve male animals, this work fills in important gaps in the knowledge base by incorporating pain-induced adaptations and cannabinoid pharmacology in females.

Antinociception mechanisms of action of cannabinoid-based medicine: an overview for anesthesiologists and pain physicians

Cannabinoid-based medications possess unique multimodal analgesic mechanisms of action, modulating diverse pain targets. Cannabinoids are classified based on their origin into three categories: endocannabinoids (present endogenously in human tissues), phytocannabinoids (plant derived) and synthetic cannabinoids (pharmaceutical). Cannabinoids exert an analgesic effect, peculiarly in hyperalgesia, neuropathic pain and inflammatory states. Endocannabinoids are released on demand from postsynaptic terminals and travels retrograde to stimulate cannabinoids receptors on presynaptic terminals, inhibiting the release of excitatory neurotransmitters. Cannabinoids (endogenous and phytocannabinoids) produce analgesia by interacting with cannabinoids receptors type 1 and 2 (CB1 and CB2), as well as putative non-CB1/CB2 receptors; G protein-coupled receptor 55, and transient receptor potential vanilloid type-1. Moreover, they modulate multiple peripheral, spinal and supraspinal nociception pathways. Cannabinoids-opioids cross-modulation and synergy contribute significantly to tolerance and antinociceptive effects of cannabinoids. This narrative review evaluates cannabinoids' diverse mechanisms of action as it pertains to nociception modulation relevant to the practice of anesthesiologists and pain medicine physicians.

Withametelin: a biologically active withanolide in cancer, inflammation, pain and depression

Withanolides are natural medicinal agents whose safety and therapeutic profiles make them valuable to mankind. Among multiple withanolides, withametelin is underexplored. The present study was aimed to create a general biological profile of isolated withametelin from Datura innoxia Mill. targeting different biological models. In-silico studies include drug-likeliness, pharmacokinetics, toxicity, molecular targets and cytotoxicity to cancer cell lines predictions. In silico directed preliminary in-vitro evaluation comprised of cancer/normal cell cytotoxicity, DPPH and protein kinase inhibition assays while in-vivo bioactivities include antiinflammatory, analgesic, antidepressant and anticoagulant assays. Pharmacological findings were strengthened by molecular docking studies to check interactions with various proteins and to propose the future path of studies. Results indicated compliance with Lipinski drug-likeliness rule (score −0.55). ADMET prediction showed strong plasma protein binding, GI absorption (Caco-2 cells permeability = 46.74 nm/s), blood brain barrier penetration (Cbrain/Cblood = 0.31), efflux by P-glycoprotein, metabolism by CYP1A2, CYP2C19 and CYP3A4, medium hERG inhibition and non-carcinogenicity in rodents. Predicted molecular targets included mainly receptors (glucocorticoid, kappa opioid, delta opioid, adrenergic and dopamine), oxidoreductase (arachidonate 5-lipoxygenase and cyclooxygenase-2), enzymes (HMG-CoA reductase) and kinase (NFκb). Withametelin was more cytotoxic to cancer cells (DU145 IC₅₀ 7.67 ± 0.54 µM) than normal lymphocytes (IC₅₀ 33.55 ± 1.31 µM). It also showed good antioxidant and protein kinase inhibition potentials. Furthermore, withametelin (20 mg/kg) significantly reduced inflammatory paw edema (68.94 ± 5.55%), heat-induced pain (78.94 ± 6.87%) and immobility time (50%) in animals. Molecular docking showed hydrogen bonding interactions (binding energies: −11.3 to −7.8 kcal/mol) with arachidonate 5 lipoxygenase, NFκb and glucocorticoid receptor. Withametelin has potential for advance investigations for its cytotoxic, anti-inflammatory, analgesic and antidepressant activities.

Effects of Cannabinoid Type 2 Receptor Agonist AM1241 on Morphine-Induced Antinociception, Acute and Chronic Tolerance, and Dependence in Mice

Morphine is a potent opioid analgesic used to alleviate moderate or severe pain, but the development of drug tolerance and dependence limits its use in pain management. Previous studies showed that cannabinoid type 2 (CB₂) receptor ligands may modulate opioid effects. However, there is no report of the effect of CB₂ receptor agonist on acute morphine tolerance and physical dependence. We therefore investigated the effect of a CB₂ receptor agonist (AM1241) on morphine-induced morphine tolerance and physical dependence in mice. Repeated coadministration of AM1241 (1 or 3mg/kg intraperitoneally) and morphine (10mg/kg subcutaneously) for 7days increased the mechanical paw withdrawal threshold in mice as measured by the von Frey filament test, and 3mg/kg AM1241 in combination with morphine increased the thermal paw withdrawal latency as measured by the hot-plate test. Combination with 3mg/kg AM1241 and morphine increased acute morphine antinociception. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced acute morphine tolerance, and 3mg/kg AM1241 reduced chronic morphine tolerance. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced naloxone-precipitated withdrawal jumping, but not diarrhea. Coadministration of AM1241 and morphine did not inhibit spontaneous locomotor activity. Pretreatment with 3mg/kg AM1241 decreased the chronic morphine-induced Iba1 expression in spinal cord. Coadministration of AM1241 (3 mg/kg) reduced the production of interleukin-1β, tumor necrosis factor-α, and interleukin-6 induced by long-term and acute morphine treatment. Our findings suggest that the coadministration of the CB₂ receptor agonist and morphine could increase morphine antinociception and reduce morphine tolerance and physical dependence in mice.

PERSPECTIVE

The combination of a CB₂ agonist and morphine may provide a new strategy for better treatment of acute and chronic pain and prevention of opioid tolerance and dependence. This finding may also provide a clue for the treatment of opioid tolerance and dependence in clinics.

Slowly Signaling G Protein-Biased CB2 Cannabinoid Receptor Agonist LY2828360 Suppresses Neuropathic Pain with Sustained Efficacy and Attenuates Morphine Tolerance and Dependence

The CB₂ cannabinoid agonist LY2828360 lacked both toxicity and efficacy in a clinical trial for osteoarthritis. Whether LY2828360 suppresses neuropathic pain has not been reported, and its signaling profile is unknown. In vitro, LY2828360 was a slowly acting but efficacious G protein-biased CB₂ agonist, inhibiting cAMP accumulation and activating extracellular signal-regulated kinase 1/2 signaling while failing to recruit arrestin, activate inositol phosphate signaling, or internalize CB2 receptors. In wild-type (WT) mice, LY2828360 (3 mg/kg per day i.p. × 12 days) suppressed chemotherapy-induced neuropathic pain produced by paclitaxel without producing tolerance. Antiallodynic efficacy of LY2828360 was absent in CB₂ knockout (KO) mice. Morphine (10 mg/kg per day i.p. × 12 days) tolerance developed in CB₂KO mice but not in WT mice with a history of LY2828360 treatment (3 mg/kg per day i.p. × 12 days). LY2828360-induced antiallodynic efficacy was preserved in WT mice previously rendered tolerant to morphine (10 mg/kg per day i.p. × 12 days), but it was absent in morphine-tolerant CB₂KO mice. Coadministration of LY2828360 (0.1 mg/kg per day i.p. × 12 days) with morphine (10 mg/kg per day × 12 days) blocked morphine tolerance in WT but not in CB₂KO mice. WT mice that received LY2828360 coadministered with morphine exhibited a trend (P = 0.055) toward fewer naloxone-precipitated jumps compared with CB₂KO mice. In conclusion, LY2828360 is a slowly signaling, G protein-biased CB₂ agonist that attenuates chemotherapy-induced neuropathic pain without producing tolerance and may prolong effective opioid analgesia while reducing opioid dependence. LY2828360 may be useful as a first-line treatment in chemotherapy-induced neuropathic pain and may be highly efficacious in neuropathic pain states that are refractive to opioid analgesics.

Low-Dose Cannabinoid Type 2 Receptor Agonist Attenuates Tolerance to Repeated Morphine Administration via Regulating μ-Opioid Receptor Expression in Walker 256 Tumor-Bearing Rats

BACKGROUND

Morphine is widely used in patients with moderate and severe cancer pain, whereas the development of drug tolerance remains a major problem associated with opioid use. Previous studies have shown that cannabinoid type 2 (CB2) receptor agonists induce morphine analgesia, attenuate morphine tolerance in normal and neuropathic pain animals, induce transcription of the μ-opioid receptor (MOR) gene in Jurkat T cells, and increase morphine analgesia in cancer pain animals. However, no studies of the effects of CB2 receptor agonists on morphine tolerance in cancer pain have been performed. Therefore, we investigated the effect of repeated intrathecal (IT) injection of the low-dose CB2 receptor agonist AM1241 on the development of morphine tolerance in walker 256 tumor-bearing rats. We also tested the influence of the CB2 receptor agonist AM1241 on MOR protein and messenger ribonucleic acid (mRNA) expression in the rat spinal cord and dorsal root ganglia (DRG).

METHODS

Walker 256 cells were implanted into the plantar region of each rat's right hindpaw. Tumor-bearing rats received IT injection of the CB2 receptor agonist AM1241 or antagonist AM630 with or without morphine subcutaneously twice daily for 8 days. Rats receiving drug vehicle only served as the control group. Mechanical paw withdrawal threshold and thermal paw withdrawal latency were assessed by a von Frey test and hot plate test 30 minutes after drug administration every day. MOR protein and mRNA expression in the spinal cord and DRG were detected after the last day (day 8) of drug administration via Western blot and real-time reverse transcription polymerase chain reaction. The data were analyzed via analysis of variance followed by Student t test with Bonferroni correction for multiple comparisons.

RESULTS

Repeated morphine treatments reduced the mechanical withdrawal threshold and thermal latency. Coadministration of a nonanalgetic dose of the CB2 receptor agonist AM1241 with morphine significantly inhibited the development of morphine tolerance and increased the MOR protein expression in the spinal cord and DRG and mRNA expression in the spinal cord in tumor-bearing rats.

CONCLUSIONS

Our findings indicate that IT injection of a nonanalgetic dose of a CB2 receptor agonist increased the analgesia effect and alleviated tolerance to morphine in tumor-bearing rats, potentially by regulating MOR expression in the spinal cord and DRG. This receptor may be a new target for prevention of the development of opioid tolerance in cancer pain.

Attenuation of morphine antinociceptive tolerance by cannabinoid CB1 and CB2 receptor antagonists

Cannabinoid CB1 and CB2 receptor antagonists may be useful for their potential to increase or prolong opioid analgesia while attenuating the development of opioid tolerance. The aim of this study was to investigate the effects of AM251 (a selective CB1 antagonist) and JTE907 (a selective CB2 antagonist) on morphine analgesia and tolerance in rats. Adult male Wistar albino rats weighing 205-225 g were used in these experiments. To constitute morphine tolerance, we used a 3 day cumulative dosing regimen. After the last dose of morphine was injected on day 4, morphine tolerance was evaluated by analgesia tests. The analgesic effects of morphine (5 mg/kg), ACEA (a CB1 receptor agonist, 5 mg/kg), JWH-015 (a CB2 receptor agonist, 5 mg/kg), AM251 (1 mg/kg) and JTE907 (5 mg/kg) were considered at 30-min intervals (0, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests. Our findings indicate that ACEA and JWH907 significantly increased morphine analgesia and morphine antinociceptive tolerance in the analgesia tests. In contrast, the data suggested that AM251 and JTE907 significantly attenuated the expression of morphine tolerance. In conclusion, we observed that co-injection of AM251 and JTE907 with morphine attenuated expression of tolerance to morphine analgesic effects and decreased the morphine analgesia.

Opioid receptor heteromers in analgesia

Opiates such as morphine and fentanyl, a major class of analgesics used in the clinical management of pain, exert their effects through the activation of opioid receptors. Opioids are among the most commonly prescribed and frequently abused drugs in the USA; however, the prolonged use of opiates often leads to the development of tolerance and addiction. Although blockade of opioid receptors with antagonists such as naltrexone and naloxone can lessen addictive impulses and facilitate recovery from overdose, systemic disruption of endogenous opioid receptor signalling through the use of these antagonistic drugs can have severe side effects. In the light of these challenges, current efforts have focused on identifying new therapeutic targets that selectively and specifically modulate opioid receptor signalling and function so as to achieve analgesia without the adverse effects associated with chronic opiate use. We have previously reported that opioid receptors interact with each other to form heteromeric complexes and that these interactions affect morphine signalling. Since chronic morphine administration leads to an enhanced level of these heteromers, these opioid receptor heteromeric complexes represent novel therapeutic targets for the treatment of pain and opiate addiction. In this review, we discuss the role of heteromeric opioid receptor complexes with a focus on mu opioid receptor (MOR) and delta opioid receptor (DOR) heteromers. We also highlight the evidence for altered pharmacological properties of opioid ligands and changes in ligand function resulting from the heteromer formation.

Repeated morphine treatment-mediated hyperalgesia, allodynia and spinal glial activation are blocked by co-administration of a selective cannabinoid receptor type-2 agonist

Spinal glial activation has been implicated in sustained morphine-mediated paradoxical pain sensitization. Since activation of glial CB2 cannabinoid receptors attenuates spinal glial activation in neuropathies, we hypothesized that CB2 agonists may also attenuate sustained morphine-mediated spinal glial activation and pain sensitization. Our data indicate that co-administration of a CB2-selective agonist (AM 1241) attenuates morphine (intraperitoneal; twice daily; 6 days)-mediated thermal hyperalgesia and tactile allodynia in rats. A CB2 (AM 630) but not a CB1 (AM 251) antagonist mitigated this effect. AM 1241 co-treatment also attenuated spinal astrocyte and microglial marker and pro-inflammatory mediator (IL-1β, TNFα) immunoreactivities in morphine-treated rats, suggesting that CB2 agonists may be useful to prevent the neuroinflammatory consequences of sustained morphine treatment.

Advances in the field of cannabinoid--opioid cross-talk

A remarkable amount of literature has been generated demonstrating the functional similarities between the endogenous opioid and cannabinoid systems. Anatomical, biochemical and molecular data support the existence of reciprocal interactions between these two systems related to several pharmacological responses including reward, cognitive effects, and the development of tolerance and dependence. However, the assessment of the bidirectionality of these effects has been difficult due to their variety and complexity. Reciprocal interactions have been well established for the development of physical dependence. Cross-tolerance and cross-sensitization, although not always bidirectional, are also supported by a number of evidence, while less data have been gathered regarding the relationship of these systems in cognition and emotion. Nevertheless, the most recent advances in cannabinoid-opioid cross-modulation have been made in the area of drug craving and relapse processes. The present review is focused on the latest developments in the cannabinoid-opioid cross-modulation of their behavioural effects and the possible neurobiological substrates involved.

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.

Involvement of cannabinoid (CB1)-receptors in the development and maintenance of opioid tolerance

Sustained exposure to opioid agonists such as morphine increases levels of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn, a response implicated in the development of opioid tolerance and physical dependence. Recent evidence suggests that both the opioid-induced increase in CGRP and the development of opioid physical dependence are suppressed by blockade of spinal cannabinoid (CB1)-receptors. The present study examined whether CB1-receptor activity also has a role in the development of opioid tolerance. In rats implanted with spinal catheters, repeated acute injections of morphine (15 microg) delivered over 4 h resulted in a rapid decline of thermal and mechanical antinociception and a significant loss of analgesic potency, reflecting development of acute opioid tolerance. In another set of experiments, chronic administration of spinal morphine (15 microg) once daily for 5 days produced a similar loss of analgesic effect and a marked increase in CGRP-immunoreactivity in the superficial laminae of the dorsal horn. Consistent with the in vivo findings, primary cultures of adult dorsal root ganglion (DRG) neurons exposed to morphine for 5 days showed a significant increase in the number of CGRP-immunoreactive neurons. Co-administration of acute or chronic morphine with a CB1-receptor antagonist/inverse agonist, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM-251), inhibited the development of both acute and chronic analgesic tolerance. In animals already exhibiting tolerance to morphine, intervention with AM-251 restored morphine analgesic potency. Co-administration with AM-251 attenuated the morphine-induced increase in CGRP-immunoreactivity in the spinal cord and in DRG cultured neurons. Collectively, the results of this study suggest that activity of endocannabinoids, mediated via CB1-receptors, contributes to both the development and maintenance of opioid tolerance by influencing the opioid-induced increase in spinal CGRP.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).