The Antinociceptive Effects of Local Injections of Propofol in Rats Are Mediated in Part by Cannabinoid CB1 and CB2 Receptors

The Basic Science of Cannabinoids

The cannabis plant has been used for centuries to manage the symptoms of various ailments including pain. Hundreds of chemical compounds have been identified and isolated from the plant and elicit a variety of physiological responses by binding to specific receptors and interacting with numerous other proteins. In addition, the body makes its own cannabinoid-like compounds that are integrally involved in modulating normal and pathophysiological processes. As the legal cannabis landscape continues to evolve within the United States and throughout the world, it is important to understand the rich science behind the effects of the plant and the implications for providers and patients. This narrative review aims to provide an overview of the basic science of the cannabinoids by describing the discovery and function of the endocannabinoid system, pharmacology of cannabinoids, and areas for future research and therapeutic development as they relate to perioperative and chronic pain medicine.

Total intravenous anesthesia versus inhalation anesthesia: how do outcomes compare?

RECENT FINDINGS

Surgical procedures that involve general anesthesia are performed with either volatile anesthetics or propofol-based total intravenous anesthesia. Both techniques are safe and provide appropriate conditions for surgery. Despite being a well established anesthetic, the use of propofol-based total intravenous anesthesia (TIVA) remains low. Possible explanations include the perceived increase risk of awareness, lack of target controlled infusion devices, increased turnover time for device set up and individual preference.

SUMMARY

There are some scenarios where patients could potentially benefit from propofol-based TIVA rather than a volatile anesthetic (e.g. postoperative nausea and vomiting) and some other clinical scenarios where the use of propofol-based anesthesia remains controversial since the strength of the evidence remains low.

PURPOSE

In this review we will summarize the clinical evidence comparing the effect of propofol-based TIVA and volatile anesthetic on postoperative outcomes such as postoperative nausea and vomiting, postoperative pain, quality of recovery, postoperative cognitive dysfunction and cancer outcomes.

Effects of intraoperative propofol-based total intravenous anesthesia on postoperative pain in spine surgery: Comparison with desflurane anesthesia - a randomised trial

BACKGROUND

As reported, patients experience less postoperative pain after propofol-based total intravenous anesthesia (TIVA). In the present study, we investigated the postoperative analgesic effects between propofol-based TIVA and desflurane anesthesia after spine surgery.

METHODS

Sixty patients were included who received (surgical time >180 minutes) lumbar spine surgery. Patients were randomly assigned to receive either TIVA (with target-controlled infusion) with propofol/fentanyl-based anesthesia (TIVA group) or desflurane/fentanyl-based anesthesia (DES group), titrated to maintain Bispectral Index values between 45 and 55. All patients received patient-controlled analgesia (PCA) with fentanyl for postoperative pain relief. Numeric pain rating scale (NRS) pain scores, postoperative fentanyl consumption, postoperative rescue tramadol use, and fentanyl-related side effects were recorded.

RESULTS

The TIVA group patients reported lower NRS pain scores during coughing on postoperative day 1 but not day 2 and 3 (P = .002, P = .133, P = .161, respectively). Less fentanyl consumption was observed on postoperative days 1 and 2, but not on day 3 (375 μg vs 485 μg, P = .032, 414 μg vs 572 μg, P = .033, and 421 μg vs 479 μg, P = .209, respectively), less cumulative fentanyl consumption at postoperative 48 hours (790 μg vs 1057 μg, P = .004) and 72 hours (1210 μg vs 1536 μg, P = .004), and total fentanyl consumption (1393 μg vs 1704 μg, P = .007) when compared with the DES group. No difference was found in rescue tramadol use and fentanyl-related side effects.

CONCLUSION

Patients anesthetized with propofol-based TIVA reported less pain during coughing and consumed less daily and total PCA fentanyl after lumbar spine surgery.

Effects of propofol anesthesia versus sevoflurane anesthesia on postoperative pain after radical gastrectomy: a randomized controlled trial

Purpose

After a radical gastrectomy, patients may experience severe pain. Some studies have reported that the use of propofol significantly reduced postoperative pain, while others have argued that this effect was not significant. Thus, we aimed to assess whether anesthesia with propofol could help to reduce pain after an open radical gastrectomy procedure.

Patients and methods

Sixty patients who were scheduled to undergo a laparotomy for radical gastrectomy were randomly assigned to either the propofol or sevoflurane group (n=30 each). A target-controlled infusion of propofol or inhalation of sevoflurane, titrated to bispectral index of 40–60, was maintained. All patients were administered a standardized multimodal analgesic plan, including intraoperative dexmedetomidine, dexamethasone, and postoperative flurbiprofen axetil, as well as patient-controlled fentanyl. Hemodynamics, pain scores, fentanyl consumption, adverse events, and the incidence of chronic pain 1 month and 3 months following hospital discharge were recorded.

Results

The intensity of postoperative pain was relatively low to moderate in all the patients. The propofol group showed lower pain scores, at rest and while coughing, up to 48 h postoperatively compared to the sevoflurane group (P<0.05). Cumulative fentanyl consumption 0–24 h after surgery was lower for the propofol group (364.4 ± 139.1 vs. 529.3 ± 237.9 µg; P=0.002). However, for fentanyl consumption 0–48 h, the difference between the two groups was not significant (710.9 ± 312.8 vs. 850.9 ± 292.0 µg; P=0.078). There were no differences in the incidences of adverse events or chronic pain between the groups.

Conclusions

Overall, the multimodal analgesic approach reduced postoperative pain after an open radical gastrectomy procedure in all patients anesthetized with either propofol or sevoflurane. Furthermore, our results indicated better analgesic outcome for the propofol group, especially in the early postoperative period.

Sedation Analgesia and Neuromuscular Blockade in Pediatric Critical Care: Overview and Current Landscape

Sedation is a mainstay of therapy for critically ill children. Although necessary in the care of the critically ill child, sedative drugs are associated with adverse effects, such as disruption of circadian rhythm, altered sleep, delirium, potential neurotoxicity, and immunosuppression. Optimal approaches to the sedation of the critically ill child should include identification of sedation targets and sedation interruptions, allowing for a more individualized approach to sedation. Further research is needed to better understand the relationship between critical illness and sedation pharmacokinetics and pharmacodynamics, the impact of sedation on immune function, and the genetic implications on drug disposition and response.

Activation of Endocannabinoid Receptor 2 as a Mechanism of Propofol Pretreatment-Induced Cardioprotection against Ischemia-Reperfusion Injury in Rats

Propofol pretreatment before reperfusion, or propofol conditioning, has been shown to be cardioprotective, while its mechanism is unclear. The current study investigated the roles of endocannabinoid signaling in propofol cardioprotection in an in vivo model of myocardial ischemia/reperfusion (I/R) injury and in in vitro primary cardiomyocyte hypoxia/reoxygenation (H/R) injury. The results showed that propofol conditioning increased both serum and cell culture media concentrations of endocannabinoids including anandamide (AEA) and 2-arachidonoylglycerol (2-AG) detected by LC-MS/MS. The reductions of myocardial infarct size in vivo and cardiomyocyte apoptosis and death in vitro were accompanied with attenuations of oxidative injuries manifested as decreased reactive oxygen species (ROS), malonaldehyde (MDA), and MPO (myeloperoxidase) and increased superoxide dismutase (SOD) production. These effects were mimicked by either URB597, a selective endocannabinoids degradation inhibitor, or VDM11, a selective endocannabinoids reuptake inhibitor. In vivo study further validated that the cardioprotective and antioxidative effects of propofol were reversed by selective CB2 receptor antagonist AM630 but not CB1 receptor antagonist AM251. We concluded that enhancing endogenous endocannabinoid release and subsequent activation of CB2 receptor signaling represent a major mechanism whereby propofol conditioning confers antioxidative and cardioprotective effects against myocardial I/R injury.

Subhypnotic doses of propofol impair spatial memory retrieval in rats

Abundant evidence indicates that propofol profoundly affects memory processes, although its specific effects on memory retrieval have not been clarified. A recent study has indicated that hippocampal glycogen synthase kinase-3β (GSK-3β) activity affects memory. Constitutively active GSK-3β is required for memory retrieval, and propofol has been shown to inhibit GSK-3β. Thus, the present study examined whether propofol affects memory retrieval, and, if so, whether that effect is mediated through altered GSK-3β activity. Adult Sprague-Dawley rats were trained on a Morris water maze task (eight acquisition trials in one session) and subjected under the influence of a subhypnotic dose of propofol to a 24-hour probe trial memory retrieval test. The results showed that rats receiving pretest propofol (25 mg/kg) spent significantly less time in the target quadrant but showed no change in locomotor activity compared with those in the control group. Memory retrieval was accompanied by reduced phosphorylation of the serine-9 residue of GSK-3β in the hippocampus, whereas phosphorylation of the tyrosine-216 residue was unaffected. However, propofol blocked this retrieval-associated serine-9 phosphorylation. These findings suggest that subhypnotic propofol administration impairs memory retrieval and that the amnestic effects of propofol may be mediated by attenuated GSK-3β signaling in the hippocampus.

Effects of Intra-Operative Total Intravenous Anaesthesia with Propofol versus Inhalational Anaesthesia with Sevoflurane on Post-Operative Pain in Liver Surgery: A Retrospective Case-Control Study

Background

Patients receiving total intravenous anesthesia (TIVA) with propofol have been shown to experience less postoperative pain. We evaluated the post-operative analgesic effects of propofol compared with sevoflurane maintenance of anesthesia in liver surgery. This study was registered at ClinicalTrials.gov (NCT02179437).

Methods

In this retrospective study, records of patients who underwent liver surgery between 2010 and 2013 were reviewed. Ninety-five patients anesthetized with propofol TIVA were matched with 95 patients anesthetized with sevoflurane. Numeric pain rating scale (NRS) pain scores, postoperative morphine consumption, side effects and patients’ satisfaction with pain relief were evaluated.

Results

The TIVA group reported lower NRS pain scores during coughing on postoperative days 1 and 2 but not 3 (p = 0.0127, p = 0.0472, p = 0.4556 respectively). They also consumed significantly less daily (p = 0.001 on day 1, p = 0.0231 on day 2, p = 0.0004 on day 3), accumulative (p = 0.001 on day 1, p<0.0001 on day 2 and p = 0.0064 on day 3) and total morphine (p = 0.03) when compared with the sevoflurane group. There were no differences in total duration of intravenous patient controlled analgesia (PCA) morphine use and patient satisfaction. No difference was found in reported side effects.

Conclusion

Patients anesthetized with propofol TIVA reported less pain during coughing and consumed less daily, accumulative and total morphine after liver surgery.

Inhibition of acid-sensing ion channel currents by propofol in rat dorsal root ganglion neurons

Acid-sensing ion channels (ASICs), part of the epithelial sodium channel/degenerin family, are activated by extracellular protons. The ASICs play a significant role in the acidosis-mediated perception of pain. The anaesthetic agent propofol also exerts antinociceptive effects, but the underlying mechanisms for this effect are not clear. We used whole-cell patch clamping to investigate the effect of propofol on proton-gated currents in: (i) rat dorsal root ganglion (DRG) neurons; and (ii) HEK293 cells transfected with either ASIC1a or ASIC3. Propofol inhibited the amplitude of proton-gated currents in DRG neurons, but did not change the sensitivity of ASICs to H(+). Notably, propofol altered acid-evoked excitability of rat DRG neurons and decreased the number of action potentials induced by acid stimuli. In addition, we demonstrated that propofol inhibited ASICs by directly binding with these channels in HEK293 cells. These results suggest that propofol inhibits proton-gated currents in DRG neurons and that inhibition of proton-gated currents explains, in part, the antinociceptive effects of propofol in primary afferent neurons.

Synergistic activation of lipopolysaccharide-stimulated glial cells by propofol

Despite the extensive use of propofol in general anesthetic procedures, the effects of propofol on glial cell were not completely understood. In lipopolysaccharide (LPS)-stimulated rat primary astrocytes and BV2 microglial cell lines, co-treatment of propofol synergistically induced inflammatory activation as evidenced by the increased production of NO, ROS and expression of iNOS, MMP-9 and several cytokines. Propofol augmented the activation of JNK and p38 MAPKs induced by LPS and the synergistic activation of glial cells by propofol was prevented by pretreatment of JNK and p38 inhibitors. When we treated BV2 cell culture supernatants treated with LPS plus propofol on cultured rat primary neuron, it induced a significant neuronal cell death. The results suggest that the repeated use of propofol in immunologically challenged situation may induce glial activation in brain.

Paradoxic effects of propofol on visceral pain induced by various TRPV1 agonists

Intraperitoneal injection of propofol inhibits subsequent acetic acid-induced writhing response in mice. Propofol increases the sensitivity of dorsal root ganglion neurons to capsaicin through transient receptor potential ankyrin subtype-1 (TRPA1) and protein kinase Cε (PKCε)-mediated phosphorylation of transient receptor potential vanilloid subtype-1 (TRPV1). Intraperitoneal co-injection of propofol may increase visceral nociception induced by TRPV1 agonists via sensitization of TRPV1. Therefore, we investigated the effects of intraperitoneal co-injection of propofol on nociception induced by acetic acid and capsaicin. The number of writhing movements induced by acetic acid or nociception time by capsaicin with or without propofol were counted. Neonatal capsaicin-treated mice were also used to demonstrate the role of TRPV1 in the effects of propofol on nociception, induced by TRPV1 agonists. Co-injection of propofol resulted in a pronociceptive effect on the writhing response induced by acetic acid, while the same dose of propofol ameliorated the response to capsaicin. The writhing response to intraperitoneal acetic acid was sharply inhibited following neonatal treatment with capsaicin. Co-injection with propofol reduced the number of writhing movements induced by acetic acid in neonatal capsaicin-treated mice. These results suggest that propofol binds to TRPV1 at the capsaicin-binding pocket.

Cannabinoids and muscular pain. Effectiveness of the local administration in rat

BACKGROUND

Pain associated with musculoskeletal disorders can be difficult to control and the incorporation of new approaches for its treatment is an interesting challenge. Activation of cannabinoid (CB) receptors decreases nociceptive transmission in acute, inflammatory and neuropathic pain states; however, although the use of cannabis derivatives has been recently accepted as a useful alternative for the treatment of spasticity and pain in patients with multiple sclerosis, the effects of CB receptor agonists in muscular pain have hardly been studied.

METHODS

Here, we characterized the antinociceptive effect of non selective and selective CB agonists by systemic and local administration, in two muscular models of pain, masseter and gastrocnemius, induced by hypertonic saline (HS) injection. Drugs used were: the non-selective agonist WIN 55,212-2 and two selective agonists, ACEA (CB 1) and JWH 015 (CB 2); AM 251 (CB 1) and AM 630 (CB 2) were used as selective antagonists.

RESULTS

In the masseter pain model, both systemic (intraperitoneal) and local (intramuscular) administration of CB 1 and CB 2 agonists reduced the nociceptive behaviour induced by HS, whereas in the gastrocnemius model the local administration was more effective than systemic.

CONCLUSIONS

Our results provide evidence that both, CB 1 and CB 2 receptors can contribute to muscular antinociception and, interestingly, suggest that the local administration of CB agonists could be a new and useful pharmacological strategy in the treatment of muscular pain, avoiding adverse effects induced by systemic administration.

The antinociceptive and antihyperalgesic effects of topical propofol on dorsal horn neurons in the rat

BACKGROUND

Propofol (2,6-diisopropylphenol) is an IV anesthetic used for general anesthesia. Recent evidence suggests that propofol-anesthetized patients experience less postoperative pain, and that propofol has analgesic properties when applied topically. We presently investigated the antinociceptive effects of topical propofol using behavioral and single-unit electrophysiological methods in rats.

METHODS

In behavioral experiments with rats, we assessed the effect of topical hindpaw application of propofol (1%-25%) on heat and mechanically evoked paw withdrawals. In electrophysiological experiments, we recorded from lumbar dorsal horn wide dynamic range (WDR)-type neurons in pentobarbital-anesthetized rats. We assessed the effect of topical application of propofol to the ipsilateral hindpaw on neuronal responses elicited by noxious heat, cold, and mechanical stimuli. We additionally tested whether propofol blocks heat sensitization of paw withdrawals and WDR neuronal responses induced by topical application of allyl isothiocyanate (AITC; mustard oil).

RESULTS

Topical application of propofol (1%-25%) significantly increased the mean latency of the thermally evoked hindpaw withdrawal reflex on the treated (but not opposite) side in a concentration-dependent manner, with no effect on mechanically evoked hindpaw withdrawal thresholds. Propofol also prevented shortening of paw withdrawal latency induced by AITC. In electrophysiological experiments, topical application of 10% and 25% propofol, but not 1% propofol or vehicle (10% intralipid), to the ipsilateral hindpaw significantly attenuated the magnitude of responses of WDR neurons to noxious heating of glabrous hindpaw skin with no significant change in thermal thresholds. Maximal suppression of noxious heat-evoked responses was achieved 15 minutes after application followed by recovery to the pre-propofol baseline by 30 minutes. Responses to skin cooling or graded mechanical stimuli were not significantly affected by any concentration of propofol. Topical application of AITC enhanced the noxious heat-evoked response of dorsal horn neurons. This enhancement of heat-evoked responses was attenuated when 10% propofol was applied topically after application of AITC.

CONCLUSIONS

The results indicate that topical propofol inhibits responses of WDR neurons to noxious heat consistent with analgesia, and reduced AITC sensitization of WDR neurons consistent with an antihyperalgesic effect. These results are consistent with clinical studies demonstrating reduced postoperative pain in surgical patients anesthetized with propofol. The mechanism of analgesic action of topical propofol is not clear, but may involve desensitization of TRPV1 or TRPA1 receptors expressed in peripheral nociceptive nerve endings, engagement of endocannabinoids, or activation of peripheral γ-aminobutyric acid A receptors.

Changes of blood endocannabinoids during anaesthesia: a special case for fatty acid amide hydrolase inhibition by propofol?

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Available data from animal studies suggest that the narcotic drug propofol interacts with the endocannabinoid system. Inhibition of enzymatic degradation of anandamide could explain some of the characteristics of propofol. Direct measurements have not been reported yet in humans.

WHAT THIS STUDY ADDS

• Propofol does not change the time course of anandamide plasma concentrations during anaesthesia. Furthermore, propofol does not inhibit fatty acid amide hydrolase activity ex vivo or in vitro. Thus, specific characteristics of the narcotic drug propofol cannot be explained by peripheral inhibition of anandamide degradation in humans.

AIMS

The aim of our study was to describe the time course of endocannabinoids during different anaesthesia protocols in more detail, and to challenge the hypothesis that propofol acts as a FAAH inhibitor.

METHODS

Endocannabinoids were measured during the first hour of anaesthesia in 14 women and 14 men undergoing general anaesthesia with propofol and in 14 women and 14 men receiving thiopental/sevoflurane. We also incubated whole human blood samples ex vivo with propofol and the known FAAH inhibitor oloxa and determined FAAH enzyme kinetics.

RESULTS

Plasma anandamide decreased similarly with propofol and thiopental/sevoflurane anaesthesia, and reached a nadir after 10 min. Areas under the curve for anandamide (mean and 95% CI) were 53.3 (47.4, 59.2) nmol l(-1) 60 min with propofol and 48.5 (43.1, 53.8) nmol l(-1) 60 min with thiopental/sevoflurane (P= NS). Anandamide and propofol plasma concentrations were not correlated at any time point. Ex vivo FAAH activity was not inhibited by propofol. Enzyme kinetics (mean ± SD) of recombinant human FAAH were K(m) = 16.9 ± 8.8 µmol l(-1) and V(max) = 44.6 ± 15.8 nmol mg(-1) min(-1) FAAH without, and K(m) = 16.6 ± 4.0 µmol l(-1) and V(max) = 44.0 ± 7.6 nmol mg( 1 ) min(-1) FAAH with 50 µmol l(-1) propofol (P= NS for both).

CONCLUSIONS

Our findings challenge the idea that propofol anaesthesia and also propofol addiction are directly mediated by FAAH inhibition, but we cannot exclude other indirect actions on cannabinoid receptors.

Sevoflurane versus propofol anesthesia in patients undergoing lumbar spondylodesis: a randomized trial

BACKGROUND

Spondylodesis is a procedure aiming at providing stability in one or more spinal segments. The aim of our study was to compare sevoflurane and propofol as induction and maintenance agents, focusing on hemodynamic stability, recovery characteristics, postoperative nausea and vomiting, and pain intensity.

MATERIALS AND METHODS

Seventy patients, with a physical status according to American Society of Anesthesiologists (ASA) I-II, 50-72 y old, undergoing selective lumbar spondylodesis were enrolled.

RESULTS

There was no statistically significant difference between groups in overall mean hemodynamic parameters. No differences in fluid administration and vasoactive substances used were noted. Postoperatively, there was a significant difference in overall mean visual analog score at rest and at cough, with the sevoflurane group showing lower values. No differences in the incidence of nausea, vomiting, shivering, postoperative sedation scores, and orientation to place were revealed. Orientation to time exhibited a statistically significant difference at the time just after transfer to the post-anesthesia care unit, where more patients of the sevoflurane group seemed to be well oriented.

CONCLUSIONS

Sevoflurane and propofol anesthesia for lumbar spondylodesis surgery provide safe and comparable results.

Peripheral antinociceptive effect of anandamide and drugs that affect the endocannabinoid system on the formalin test in normal and streptozotocin-diabetic rats

Diabetes is often associated with painful neuropathy. The current treatments are symptomatic and ineffective. Cannabinoids have been proposed as promising drugs for chronic pain treatment and its antinociceptive effect has already been related in nerve injury models of neuropathic pain, but little has been investigated in painful diabetic neuropathy models. Thus, the current study aims to investigate the potential antinociceptive effect of drugs that alter endocannabinoid system when injected subcutaneously into the dorsal surface of the ipsilateral hind paw in chemical hyperalgesia induced by formalin in both normoglycemic (Ngl) and streptozotocin-diabetic (Dbt) rats. Diabetic rats exhibited exaggerated flinching behaviors during first and second phases of the formalin test, indicating the presence of hyperalgesia. AM404, an anandamide (AEA) re-uptake inhibitor, AEA (an agonist of CB1/CB2 receptors) or ACEA (a selective CB1 receptor agonist) induced antinociception in both phases of formalin test in Ngl and Dbt rats. In both groups, the antinociceptive effect of ACEA was prevented by AM251, a CB1 inverse agonist while the antinociceptive effect of AEA was prevented by AM251 or AM630, a CB2 receptor antagonist. In Ngl rats, the antinociceptive effect of AM404 was prevented by AM251 or capsazepine only during first phase of the formalin test while in Dbt rats, this effect was blocked by pretreatment with AM251 (both phases) or AM630 (second phase). Taken together, these results demonstrated broad-spectrum antinociceptive properties of cannabinoids in a model of painful diabetic neuropathy. Peripheral activation of both cannabinoid receptors seems to mediate the antinociceptive effect of exogenous or endogenous anandamide.

Propofol reduces early post-operative pain after gynecological laparoscopy

BACKGROUND

There is some evidence that propofol may reduce post-operative pain. However, the results on the analgesic effects of propofol are inconsistent. Thus, we hypothesized that propofol reduces acute pain if confounding factors like opioids are avoided.

METHODS

In this prospective, randomized, subject- and assessor-blind, parallel-group, head-to-head comparative study, 90 American Society of Anesthesiologists I or II females underwent elective laparoscopies were randomized to receive either propofol-based (PR), or sevoflurane-based (SR), or sevoflurane-propofol-based anesthesia (SPR). Pain score at rest assessed by a numerical rating scale at 0.5 h after surgery was the primary outcome. The secondary outcomes included pain score at 1 and 24 h post-operatively, duration of post-anesthesia care units stay (PACU), incidence of post-operative nausea and vomiting, incidence of shivering, and post-operative quality of recovery score (QoR-40) within the first 24 h post-operatively.

RESULTS

No patients received rescue analgesia. The pain score at 0.5 h post-operatively was less in group PR when compared with group SR (0.7 ± 1.4 vs. 2.1 ± 1.8; P = 0.010) or group SPR (0.7 ± 1.4 vs. 2.1 ± 2.2; P = 0.008). Group PR was also associated with shorter PACU stay than group SR (21.8 ± 5.7 vs. 26.2 ± 6.9; P = 0.050) or group SPR (21.8 ± 5.7 vs. 27.8 ± 8.9; P = 0.005). Intraoperative bispectral index values, hemodynamic values and post-operative QoR-40 scores did not differ among the three groups.

CONCLUSIONS

Propofol anesthesia was associated with significantly less pain at 0.5 and 1 h after surgery in patients undergoing gynecological laparoscopies with planned opioid-free post-operative analgesia.

Peripheral antinociceptive effects of inhibitors of monoacylglycerol lipase in a rat model of inflammatory pain

BACKGROUND AND PURPOSE The endocannabinoid 2-arachidonoylglycerol (2-AG) is degraded primarily by monoacylglycerol lipase (MGL). We compared peripheral antinociceptive effects of JZL184, a novel irreversible MGL inhibitor, with the reversible MGL-preferring inhibitor URB602 and exogenous 2-AG in rats. EXPERIMENTAL APPROACH Nociception in the formalin test was assessed in groups receiving dorsal paw injections of vehicle, JZL184 (0.001-300 µg), URB602 (0.001-600 µg), 2-AG (ED(50)), 2-AG + JZL184 (at their ED(50)), 2-AG + URB602 (at their ED(50)), AM251 (80 µg), AM251 + JZL184 (10 µg), AM630 (25 µg) or AM630 + JZL184 (10 µg). Effects of MGL inhibitors on endocannabinoid accumulation and on activities of endocannabinoid-metabolizing enzymes were assessed. KEY RESULTS Intra-paw administration of JZL184, URB602 and 2-AG suppressed early and late phases of formalin pain. JZL184 and URB602 acted through a common mechanism. JZL184 (ED(50) Phase 1: 0.06 ± 0.028; Phase 2: 0.03 ± 0.011 µg) produced greater antinociception than URB602 (ED(50) Phase 1: 120 ± 51.3; Phase 2: 66 ± 23.9 µg) or 2-AG. Both MGL inhibitors produced additive antinociceptive effects when combined with 2-AG. Antinociceptive effects of JZL184, like those of URB602, were blocked by cannabinoid receptor 1 (CB(1)) and cannabinoid receptor 2 (CB(2)) antagonists. JZL184 suppressed MGL but not fatty-acid amide hydrolase or N-arachidonoyl-phosphatidylethanolamine phospholipase D activities ex vivo. URB602 increased hind paw 2-AG without altering anandamide levels. CONCLUSIONS AND IMPLICATIONS MGL inhibitors suppressed formalin-induced pain through peripheral CB(1) and CB(2) receptor mechanisms. MGL inhibition increased paw skin 2-AG accumulation to mediate these effects. MGL represents a target for the treatment of inflammatory pain.

The abused inhalant toluene differentially modulates excitatory and inhibitory synaptic transmission in deep-layer neurons of the medial prefrontal cortex

Volatile organic solvents such as toluene are voluntarily inhaled for their intoxicating effects. Solvent use is especially prevalent among adolescents, and is associated with deficits in a wide range of cognitive tasks including attention, behavioral control, and risk assessment. Despite these findings, little is known about the effects of toluene on brain areas mediating these behaviors. In this study, whole-cell patch-clamp recordings were used to determine the effect toluene on neurons within the medial PFC, a region critically involved in cognitive function. Toluene had no effect on measures of intrinsic excitability, but enhanced stimulus-evoked γ-amino butyric acid A-mediated inhibitory postsynaptic currents (IPSCs). In the presence of tetrodotoxin (TTX) to block action potentials, toluene increased the frequency and amplitude of miniature IPSCs. In contrast, toluene induced a delayed but persistent decrease in evoked or spontaneous AMPA-mediated excitatory postsynaptic currents (EPSCs). This effect was prevented by an intracellular calcium chelator or by the ryanodine receptor and SERCA inhibitors, dantrolene or thapsigargin, respectively, suggesting that toluene may mobilize intracellular calcium pools. The toluene-induced reduction in AMPA EPSCs was also prevented by a cannabinoid receptor (CB1R) antagonist, and was occluded by the CB1 agonist WIN 55,212-2 that itself induced a profound decrease in AMPA-mediated EPSCs. Toluene had no effect on the frequency or amplitude of miniature EPSCs recorded in the presence of TTX. Finally, toluene dose-dependently inhibited N-methyl-D-aspartate (NMDA)-mediated EPSCs and the magnitude and reversibility of this effect was CB1R sensitive indicating both direct and indirect actions of toluene on NMDA-mediated responses. Together, these results suggest that the effect of toluene on cognitive behaviors may result from its action on inhibitory and excitatory synaptic transmission of PFC neurons.

The endocannabinoid system and pain

The therapeutic potential of cannabinoids has been the topic of extensive investigation following the discovery of cannabinoid receptors and their endogenous ligands. Cannabinoid receptors and their endogenous ligands are present at supraspinal, spinal and peripheral levels. Cannabinoids suppress behavioral responses to noxious stimulation and suppress nociceptive processing through activation of cannabinoid CB(1) and CB(2) receptor subtypes. Endocannabinoids, the brain's own cannabis-like substances, share the same molecular target as Delta(9)-tetrahydrocannabinol, the main psychoactive component in cannabis. Endocannabinoids serve as synaptic circuit breakers and regulate multiple physiological and pathological conditions, e.g. regulation of food intake, immunomodulation, inflammation, analgesia, cancer, addictive behavior, epilepsy and others. This review will focus on uncovering the roles of anandamide and 2-arachidonoylglycerol, the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of anandamide and 2-arachidonoylglycerol, released under physiological conditions, in modulating nociceptive responding at different levels of the neuraxis will be emphasized in this review. Effects of modulation of endocannabinoid levels through inhibition of endocannabinoid hydrolysis and uptake is also compared with effects of exogenous administration of synthetic endocannabinoids in acute, inflammatory and neuropathic pain models. Finally, the therapeutic potential of the endocannabinoid signaling system is discussed in the context of identifying novel pharmacotherapies for the treatment of pain.

Adjuvant topical therapy with a cannabinoid receptor agonist in facial postherpetic neuralgia

BACKGROUND

Postherpetic neuralgia is a frequent adverse event in herpes zoster patients and difficult to treat. Conventional analgetic therapy often fails to reduce the burning pain transmitted by unmyelinated nerve fibers. These nerves express cannabinoid receptors which exert a role in modulation of nociceptive symptoms. Therefore, topical therapy with cannabinoid receptor agonist seems likely to suppress local burning pain.

PATIENTS AND METHODS

In an open-labeled trial, 8 patients with facial postherpetic neuralgia received a cream containing the cannabinoid receptor agonist N-palmitoylethanolamine. The course of symptoms was scored with the visual analog scale.

RESULTS

5 of 8 patients (62.5 %) experienced a mean pain reduction of 87.8 %. Therapy was tolerated by all patients. No unpleasant sensations or adverse events occurred.

CONCLUSIONS

Topical cannabinoid receptor agonists are an effective and well-tolerated adjuvant therapy option in postherpetic neuralgia.