Flurbiprofen inhibits capsaicin induced calcitonin gene related peptide release from rat spinal cord via an endocannabinoid dependent mechanism

Potential of Endocannabinoids to Control Bladder Pain

Bladder-related pain is one of the most common forms of visceral pain, and visceral pain is among the most common complaints for which patients seek physician consultation. Despite extensive studies of visceral innervation and treatment of visceral pain, opioids remain a mainstay for management of bladder pain. Side effects associated with opioid therapy can profoundly diminish quality of life, and improved options for treatment of bladder pain remain a high priority. Endocannabinoids, primarily anandamide (AEA) and 2-arachidonoylglycerol (2-AG), are endogenously-produced fatty acid ethanolamides with that induce analgesia. Animal experiments have demonstrated that inhibition of enzymes that degrade AEA or 2-AG have the potential to prevent development of visceral and somatic pain. Although experimental results in animal models have been promising, clinical application of this approach has proven difficult. In addition to fatty acid amide hydrolase (FAAH; degrades AEA) and monacylglycerol lipase (MAGL; degrades 2-AG), cyclooxygenase (COX) acts to metabolize endocannabinoids. Another potential limitation of this strategy is that AEA activates pro-nociceptive transient receptor potential vanilloid 1 (TRPV1) channels. Dual inhibitors of FAAH and TRPV1 or FAAH and COX have been synthesized and are currently undergoing preclinical testing for efficacy in providing analgesia. Local inhibition of FAAH or MAGL within the bladder may be viable options to reduce pain associated with cystitis with fewer systemic side effects, but this has not been explored. Further investigation is required before manipulation of the endocannabinoid system can be proven as an efficacious alternative for management of bladder pain.

The cannabinoid system and pain

Chronic pain states are highly prevalent and yet poorly controlled by currently available analgesics, representing an enormous clinical, societal, and economic burden. Existing pain medications have significant limitations and adverse effects including tolerance, dependence, gastrointestinal dysfunction, cognitive impairment, and a narrow therapeutic window, making the search for novel analgesics ever more important. In this article, we review the role of an important endogenous pain control system, the endocannabinoid (EC) system, in the sensory, emotional, and cognitive aspects of pain. Herein, we briefly cover the discovery of the EC system and its role in pain processing pathways, before concentrating on three areas of current major interest in EC pain research; 1. Pharmacological enhancement of endocannabinoid activity (via blockade of EC metabolism or allosteric modulation of CB1receptors); 2. The EC System and stress-induced modulation of pain; and 3. The EC system & medial prefrontal cortex (mPFC) dysfunction in pain states. Whilst we focus predominantly on the preclinical data, we also include extensive discussion of recent clinical failures of endocannabinoid-related therapies, the future potential of these approaches, and important directions for future research on the EC system and pain. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Dual-Acting Compounds Targeting Endocannabinoid and Endovanilloid Systems-A Novel Treatment Option for Chronic Pain Management

Compared with acute pain that arises suddenly in response to a specific injury and is usually treatable, chronic pain persists over time, and is often resistant to medical treatment. Because of the heterogeneity of chronic pain origins, satisfactory therapies for its treatment are lacking, leading to an urgent need for the development of new treatments. The leading approach in drug design is selective compounds, though they are often less effective and require chronic dosing with many side effects. Herein, we review novel approaches to drug design for the treatment of chronic pain represented by dual-acting compounds, which operate at more than one biological target. A number of studies suggest the involvement of the cannabinoid and vanilloid receptors in pain. Interestingly cannabinoid system is in interrelation with other systems that comprise lipid mediators: prostaglandins, produced by COX enzyme. Therefore, in the present review, we summarize the role of dual-acting molecules (FAAH/TRPV1 and FAAH/COX-2 inhibitors) that interact with endocannabinoid and endovanillinoid systems and act as analgesics by elevating the endogenously produced endocannabinoids and dampening the production of pro-inflammatory prostaglandins. The plasticity of the endocannabinoid system (ECS) and the ability of a single chemical entity to exert an activity on two receptor systems has been developed and extensively investigated. Here, we review up-to-date pharmacological studies on compounds interacting with FAAH enzyme together with TRPV1 receptor or COX-2 enzyme respectively. Multi-target pharmacological intervention for treating pain may lead to the development of original and efficient treatments.

Effects of intrathecal ketorolac on human experimental pain

BACKGROUND

Nonsteroidal antiinflammatory drugs, the most commonly used analgesics, reduce pain not only by inhibiting cyclooxygenase at peripheral sites of inflammation but also by potentially inhibiting cyclooxygenase in the central nervous system, especially the spinal cord. Animal studies suggest that products of cyclooxygenase in the spinal cord do not alter pain responses to acute noxious stimuli but reduce pain and sensitization after peripheral inflammation. We used a spinal injection of small doses of the cyclooxygenase inhibitor ketorolac to survey the role of spinal cyclooxygenase in human experimental pain and hypersensitivity states.

METHODS

After regulatory agency approval and informed consent, we examined the effect of 2.0 mg intrathecal ketorolac in 41 healthy volunteers to acute noxious thermal stimuli in normal skin and to mechanical stimuli in skin sensitized by topical capsaicin or ultraviolet burn. We also examined the effect of intravenous ketorolac.

RESULTS

Intrathecal ketorolac reduced hypersensitivity when it was induced by a combination of ultraviolet burn plus intermittent heat and, according to one of the two analytical strategies, when it was induced by ultraviolet burn alone.

CONCLUSIONS

These data suggest a more limited role for spinal cord cyclooxygenase in human pain states than predicted by studies in animals.

Calcitonin gene-related peptide8-37 antagonizes capsaicin-induced vasodilation in the skin: evaluation of a human in vivo pharmacodynamic model

The purpose of this study was to identify the mediators involved in capsaicin-induced vasodilation in the human skin and to evaluate a pharmacodynamic model for the early clinical evaluation of calcitonin gene-related peptide (CGRP) receptor antagonists. Dermal blood flow (DBF) response of the forearm skin to topically applied capsaicin was measured using laser Doppler perfusion imaging in 22 subjects. The effect of intra-arterially administered CGRP(8-37) (1200 ng . min(-1) . dl(-1) forearm), indomethacin (5 mug . min(-1) . dl(-1) forearm), and N(G)-monomethyl-l-arginine (l-NMMA; 0.2 mg . min(-1) dl(-1) forearm), and orally administered aprepitant (375 mg) on capsaicin-induced dermal vasodilation was assessed. Furthermore, the diurnal variation of the DBF response to capsaicin was studied. CGRP(8-37) inhibited the capsaicin-induced DBF increase: 217(145, 290)% in infused versus 370 (254, 486)% in the noninfused arm [mean (95% CI); p = 0.004]. In contrast, indomethacin, l-NMMA, aprepitant, and the time of assessment did not affect the DBF response to capsaicin. Thus, capsaicin-induced vasodilation in the human forearm skin is largely mediated by CGRP, but not by vasodilating prostaglandins, nitric oxide, or substance P. The response to capsaicin does not display a circadian rhythm. A pharmacodynamic model is proposed to evaluate CGRP receptor antagonists in humans in vivo.

Endocannabinoids and related compounds: walking back and forth between plant natural products and animal physiology

Cannabis sativa has been known, used, and misused by mankind for centuries, and yet only over the last two decades has research stemming from the chemical constituents specific to this plant, the cannabinoids, started to provide fundamental insights into animal physiology and pathology, resulting in the development of new therapeutics. The discovery of the endocannabinoid system, and its targeting with two new pharmaceutical preparations now on the market in several countries, represent the most recent example of how studies on medicinal plants and on the mechanism of their biological effects can reveal, through a chain of breakthroughs, new systems of endogenous signals and physiological phenomena that can become the source of novel strategies for unmet therapeutic challenges.

The contribution of cyclooxygenase-2 to endocannabinoid metabolism and action

The development of sensitive analytical methods for measurement of endocannabinoids, their metabolites, and related lipids, has underlined the complexity of the endocannabinoid system. A case can be made for an 'endocannabinoid soup' (akin to the inflammatory soup) whereby the net effect of a pathological state and/or a pharmacological intervention on this system is the result not only of changes in endocannabinoid levels but also of their metabolites and related compounds that affect their function. With respect to the metabolism of anandamide and 2-arachidonoylglycerol, the main hydrolytic enzymes involved are fatty acid amide hydrolase and monoacylglycerol lipase. However, other pathways can come into play when these are blocked. Cyclooxygenase-2 derived metabolites of anandamide and 2-arachidonoylglycerol have a number of properties, including effects upon cell viability, contraction of the cat iris sphincter (an effect mediated by a novel receptor), mobilization of calcium and modulation of synaptic transmission. Nonsteroidal anti-inflammatory agents, whose primary mode of action is the inhibition of cyclooxygenase, can also interact with the endocannabinoid system both in vitro and in vivo. Other enzymes, such as the lipoxygenase and cytochrome P450 oxidative enzymes, can also metabolize endocannabinoids and produce biologically active compounds. It is concluded that sensitive analytical methods, which allow for measurement of endocannabinoids and related lipids, should provide vital information as to the importance of these alternative metabolic pathways when the primary hydrolytic endocannabinoid metabolizing enzymes are inhibited.

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.

Synergistic antinociceptive effects of anandamide, an endocannabinoid, and nonsteroidal anti-inflammatory drugs in peripheral tissue: a role for endogenous fatty-acid ethanolamides?

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit fatty-acid amide hydrolase (FAAH), the enzyme responsible for the metabolism of anandamide, an endocannabinoid. It has been suggested that the mechanisms of action of NSAIDs could be due to inhibition of cyclooxygenase (COX) and also to an increase in endocannabinoid concentrations. In a previous study we have demonstrated that the local analgesic interaction between anandamide and ibuprofen (a non-specific COX inhibitor) was synergistic for the acute and inflammatory phases of the formalin test. To test this hypothesis further, we repeated similar experiments with rofecoxib (a selective COX-2 inhibitor) and also measured the local concentrations of anandamide, and of two fatty-acid amides, oleoylethanolamide and palmitoylethanolamide. We established the ED(50) for anandamide (34.52 pmol+/-17.26) and rofecoxib (381.72 pmol+/-190.86) and showed that the analgesic effect of the combination was synergistic. We also found that paw tissue levels of anandamide, oleoylethanolamide and palmitoylethanolamide were significantly higher when anandamide was combined with NSAIDs and that this effect was greater with rofecoxib. In conclusion, local injection of anandamide or rofecoxib was antinociceptive in a test of acute and inflammatory pain and the combination of anandamide with rofecoxib was synergistic. Finally, locally injected anandamide with either NSAID (ibuprofen or rofecoxib) generates higher amount of fatty-acid ethanolamides. The exact comprehension of the mechanisms involved needs further investigation.

Spinal modulation of calcitonin gene-related peptide by endocannabinoids in the development of opioid physical dependence

Studies implicate endocannabinoids in the acute and chronic actions of opioid drugs, including the genesis of physical dependence. Previous evidence suggests that spinal release of calcitonin gene-related peptide (CGRP) and activation of its receptors contribute to opioid physical dependence. The release of CGRP at the spinal level is modulated by cannabinoid (CB1)-receptors. Thus, this study examined whether CB1-receptor activity mediates changes in CGRP underlying development of opioid physical dependence. Systemic morphine administration for 5-days elevated CGRP-immunoreactivity in the rat spinal dorsal horn. In situ hybridization of dorsal root ganglion (DRG) neurons revealed an increase in CGRP mRNA during initial (day 1-3) but not later phase (day 4-5) of morphine treatment. CGRP-immunoreactivity in DRG neurons, however, was increased in the later phase of morphine treatment. Naloxone challenge to morphine-treated animals precipitated an intense withdrawal syndrome that depleted CGRP-immunoreactivity and increased Fos expression in the dorsal horn. The Fos-response primarily occurred in neurons that expressed CGRP receptor component protein (RCP) suggesting CGRP activity contributes to neuronal activation during precipitated withdrawal. Spinal slices obtained from morphine-treated animals showed higher levels of CGRP release than from saline controls. Intrathecal co-administration of CB1-receptor antagonists, AM-251 or SR141716A, with daily morphine attenuated the behavioral manifestations of withdrawal. Treatment with AM-251 also reduced the depletion of CGRP, suppressed Fos-induction, and prevented the increase in capsaicin-evoked spinal CGRP release. Altogether, this study suggests that endocannabinoid activity, expressed via CB1-receptors, contributes to the induction of opioid physical dependence through spinal modulation of CGRP.

Local interactions between anandamide, an endocannabinoid, and ibuprofen, a nonsteroidal anti-inflammatory drug, in acute and inflammatory pain

Anandamide, an endocannabinoid, is degraded by the enzyme fatty acid amide hydrolase which can be inhibited by nonsteroidal anti-inflammatory drugs (NSAIDs). The present work was designed to study the peripheral interactions between anandamide and ibuprofen (a non-specific cyclooxygenase inhibitor) in the rat formalin test. We first determined the ED50 for anandamide (0.018 microg +/- 0.009), ibuprofen (0.18 microg +/- 0.09), and their combination (0.006 microg +/- 0.002). Drugs were given 15 min before a 2.5% formalin injection into the dorsal surface of the right hind paw. Results were analyzed using isobolographic analysis. The antinociceptive interaction between anandamide and ibuprofen was synergistic. To further investigate the mechanisms by which the combination of anandamide with ibuprofen produced their antinociceptive effects, we used specific antagonists for the cannabinoid CB1 (AM251; 80 microg) and CB2 (AM630; 25 microg) receptors. We demonstrated that the antinociceptive effects of ibuprofen were not antagonized by either AM251 or AM630 and that those of anandamide were antagonized by AM251 but not by AM630. The synergistic antinociceptive effects of the combination of anandamide with ibuprofen were completely antagonized by AM251 but only partially inhibited by AM630. In conclusion, locally (hind paw) injected anandamide, ibuprofen or combination thereof decreased pain behavior in the formalin test. The combination of anandamide with ibuprofen produced synergistic antinociceptive effects involving both cannabinoid CB1 and CB2 receptors. Comprehension of the mechanisms involved needs further investigation.

Systemic and Intrathecal Effects of a Novel Series of Phospholipase A2 Inhibitors on Hyperalgesia and Spinal Prostaglandin E2 Release

Phospholipase A(2) (PLA(2)) forms are expressed in spinal cord, and inhibiting spinal PLA(2) induces a potent antihyperalgesia. Here, we examined the antihyperalgesic effects after systemic and i.t. delivery of four compounds constructed with a common motif consisting of a 2-oxoamide with a hydrocarbon tail and a four-carbon tether. These molecules were characterized for their ability to block group IVA calcium-dependent PLA(2) (cPLA(2)) and group VIA calcium-independent PLA(2) (iPLA(2)) in inhibition assays using human recombinant enzyme. The rank ordering of potency in blocking group IVA cPLA(2) was AX048 (ethyl 4-[(2-oxohexadecanoyl)amino]butanoate), AX006 (4-[(2-oxohexadecanoyl)amino]butanoic acid), and AX057 (tert-butyl 4-[(2-oxohexadecanoyl)amino]butanoate) > AX010 (methyl 4-[(2-oxohexadecanoyl)amino]butanoate) and for inhibiting group VIA iPLA(2) was AX048, AX057 > AX006, and AX010. No agent altered recombinant cyclooxygenase activity. In vivo, i.t. (30 mug) and systemic (0.2-3 mg/kg i.p.) AX048 blocked carrageenan hyperalgesia and after systemic delivery in a model of spinally mediated hyperalgesia induced by i.t. substance P (SP). The other agents were without activity. In rats prepared with lumbar i.t. loop dialysis catheters, SP evoked spinal prostaglandin E(2) (PGE(2)) release. AX048 alone inhibited PGE(2) release. Intrathecal SR141617, a cannabinoid CB1 inhibitor at doses that blocked the effects of i.t. anandamide had no effect upon i.t. AX048. These results suggest that AX048 is the first systemically bioavailable compound with a significant affinity for group IVA cPLA(2), which produces a potent antihyperalgesia. The other agents, although demonstrating enzymatic activity in cell-free assays, appear unable to gain access to the intracellular PLA(2) toward which their action is targeted.

The spinal basis of opioid tolerance and physical dependence: Involvement of calcitonin gene-related peptide, substance P, and arachidonic acid-derived metabolites

Chronic opioid use in the management of pain is limited by development of analgesic tolerance and physical dependence. The mechanisms underlying tolerance-dependence are not entirely clear, however, recent evidence suggests that spinal adaptations leading to increased activity of sensory neuropeptides (calcitonin gene-related peptide (CGRP), substance P) and their downstream signaling messengers derived from metabolism of arachidonic acid: prostaglandins (PG), lipoxygenase (LOX) metabolites, and endocannabinoids, plays an important role in this phenomenon. In this communication we review the evidence implicating these factors in the induction and expression of opioid tolerance and physical dependence at the spinal level.

Differential effects of endocannabinoids on [(3)H]-GABA uptake in the rat globus pallidus

In the globus pallidus, cannabinoid CB(1) receptors are localized pre-synaptically on GABAergic neurons. We assessed the influence of the endocannabinoids, anandamide, 2-arachidonoyl-glycerol (2-AG) and noladin ether, on the uptake of [(3)H]-GABA in pallidal slices from rat. Both 2-AG and noladin ether increased [(3)H]-GABA uptake (by 40.8 +/- 8.0% and 38.4 +/- 12.5%). The effect of 2-AG was blocked by the cannabinoid CB(1) receptor antagonist AM 251. In contrast, neither anandamide nor the agonist WIN 55,212-2 had an effect on [(3)H]-GABA uptake. Different roles might be played by different endocannabinoids, both physiologically and in basal ganglia disorders, such as Parkinson's disease.

Possible involvement of the endocannabinoid system in the actions of three clinically used drugs

There is considerable interest in developing new therapies that are based on compounds that modulate the endocannabinoid system to treat several disease states (e.g.pain, neurodegeneration and cancer). However, recent evidence from studies in experimental animals has suggested that three clinically used drugs, the anaesthetic agent propofol and the non-steroidal anti-inflammatory drugs indomethacin and flurbiprofen (when given spinally), activate cannabinoid receptors as an important part of their actions. In this article, the findings of these studies are summarized and possible spectra of actions of these drugs is discussed.