The spinal phospholipase-cyclooxygenase-prostanoid cascade in nociceptive processing

Design, Synthesis, and Biological Evaluation of 6-Substituted-3-(4-methanesulfonylphenyl)-4-phenylpyran-2-ones: A Novel Class of Diarylheterocyclic Selective Cyclooxygenase-2 Inhibitors

A group of 6-alkyl (alkoxy or alkylthio)-4-aryl-3-(4-methanesulfonylphenyl)pyran-2-ones (14a-v), possessing either a H or F substituent at the para-position of the C-4 phenyl ring, were designed for evaluation as selective cyclooxygenase-2 (COX-2) inhibitors with in vivo antiinflammatory-analgesic activities. Although 6-ethylthio-3-(4-methanesulfonylphenyl)-4-phenylpyran-2-one (14s) exhibited a very high in vitro COX-2 inhibitory potency (IC(50) = 0.0032 muM) and COX-2 selectivity (SI > 120 000), 14s exhibited moderate antiinflammatory activity compared to celecoxib in a carrageenan-induced rat paw edema assay. In contrast, the less potent (IC(50) = 0.10 muM), and less selective (SI = 2880) COX-2 inhibitor 6-ethoxy-3-(4-methanesulfonylphenyl)-4-phenylpyran-2-one (14i) exhibited good antiinflammatory activity where a 1 mg/kg oral dose reduced inflammation 32 and 67% at 3 and 5 h postdrug administration relative to the reference drug celecoxib where a 50 mg/kg oral dose reduced inflammation by 79 and 58% at the respective 3 and 5 h time periods. Molecular modeling studies, where 14i was docked in the active site of both COX-1 and COX-2, reveals that the C-6 ethoxy substituent orients the pyran-2-one ring to position the SO(2)Me pharmacophore in the vicinity of the secondary pocket in COX-2. The absence of this COX-2 secondary pocket in the COX-1 binding site is due to the presence of the bulky Ile(523) in COX-1 such that access to the amino acid residues (Ile(517), Phe(518), Gln(192), and His(90)), which line the COX-2 secondary pocket with which the SO(2)Me pharmacophore could interact, is hindered. The six-membered pyran-2-one ring system is a suitable central template to design selective COX-2 inhibitors.

Nicotinic acetylcholine receptor distribution in relation to spinal neurotransmission pathways

Neuronal nicotinic receptors (nAChR) are pentameric assemblies of subunits of a gene family where specified combinations of alpha and beta subunits form functional receptors. To extend our understanding of the role of spinal nAChR in the processing of sensory stimuli and regulation of autonomic and motor responses, we initiated investigations to localize nAChR subunit expression within discrete spinal regions and cell types. High-affinity epibatidine binding was present in the superficial dorsal and ventral horns, the mediolateral and central canal regions. RT-PCR identified transcripts for alpha3, alpha4, alpha5, beta2, and beta4 in both spinal cord parenchyma and dorsal root ganglia (DRG). Our affinity-purified antibodies against alpha3, alpha4, alpha5, beta2, and beta4 subunits identified specific protein bands of appropriate molecular mass (preadsorbed with the respective antigens) in specific tissues and cells that express nicotinic receptors, including the spinal cord and DRG neurons. Having established the absence of crossreactivity with related subunits, specific fluorescence labeling of nerve terminals and cell bodies was achieved and correlated with the distribution of defined marker proteins and nicotinic receptor binding sites determined autoradiographically. Our findings indicate that alpha3, alpha4, alpha5, beta2, and beta4 subunits are all expressed on primary afferents (IB4-positive terminals) in the spinal cord. The predominant presynaptic (synaptophysin colocalization) labeling is in the superficial layer of the dorsal horn. These receptor subunits, except for beta4, are also present in postsynaptic autonomic (anti-bNOS-positive) and somatic motor neurons (anti-VAChT-positive). The alpha3, alpha5, and beta2 subunits showed additional staining in glial (anti-GFAP-positive) cells. These studies reveal a dense and distinguishable distribution of nAChR subunits in the spinal cord and point toward future therapeutic targeting for specific spinal actions.

Up-regulation of fatty acid metabolizing-enzymes mRNA in rat spinal cord during persistent peripheral local inflammation

Persistent peripheral inflammation is associated with repetitive painful inputs into the spinal cord, leading to a chronic pain state. Related dramatic changes occur in the central nervous system (CNS) including central sensitization, which results in hyperalgesia. This neural plasticity involves in part fatty acids as functional and structural compounds. We hypothesized that central modification of fatty acids metabolism might occur after prolonged peripheral noxious stimulation. In the present study, the regulation of genes involved in fatty acids metabolism in the rat CNS was investigated during a chronic pain state. Using semiquantitative RT-PCR, we explored in the neuraxis the mRNA expression of brain acyl-CoA synthetases (ACS) and acyl-CoA oxidase (ACO), which are major fatty acid-metabolizing enzymes, following complete Freund's adjuvant (CFA) injection into a hind paw. Similar spinal up-regulation of the isoforms ACS2, ACS3, ACS4, and of ACO was detected early after 30 min, reaching a maximal after 6 h post-injection. Other peaks were also observed after 4 and 21 days post-inoculation, corresponding to the acute and chronic inflammation, respectively. Induction occurred only in the lumbar spinal cord ipsilaterally to the inflamed paw and was completely inhibited by a local anaesthesia of the sciatic nerve, suggesting a neural transmission of the inducing signal. Moreover, intrathecal injection of MK801, a noncompetitive NMDA antagonist, partially prevented these inductions, highlighting the involvement of the neurotransmitter glutamate in the central ACS and ACO up-regulation. These findings suggest that the fatty metabolism is stimulated in the CNS during a chronic pain state.

Gastrin and EGF synergistically induce cyclooxygenase-2 expression in Swiss 3T3 fibroblasts that express the CCK2 receptor

Over-expression of cyclooxygenase-2 (COX-2) has been demonstrated to be tumorigenic in transgenic mice. Chronic treatment with NSAIDs is chemoprotective for colorectal cancer. Gastrin is a growth factor for gastric mucosa and has been shown to promote proliferation of colorectal cells. Recent studies suggest that COX-2 expression levels could mediate the growth effects of gastrin. Here, we report that gastrin increased PGE2 secretion in Swiss 3T3 cells expressing the CCK2 receptor. Gastrin dose dependently induced COX-2 protein levels in a time dependent manner. COX-2 mRNA levels were rapidly induced by a dose dependent increase in gastrin. Prior treatment of the cells with the CCK2 receptor specific antagonist, L365,260, inhibited gastrin-induced COX-2 protein and mRNA expression. Pretreatment with L364,714, the CCK1 receptor specific antagonist did not block COX-2 induction by gastrin. Inhibition of de novo protein synthesis by cycloheximide did not block COX-2 mRNA induction by gastrin. Also, gastrin-dependent COX-2 expression did not require PKC activity, activation of ERK, or transactivation of EGFR. However, co-stimulation with EGF and gastrin synergistically induced COX-2 protein and mRNA expression and PGE2 secretion. Measurements of COX-2 mRNA stability and COX-2 gene transcription reveal that EGF significantly increased the half-life of COX-2 mRNA with only a slight increase in the COX-2 transcription rate. Conversely, gastrin significantly increased COX-2 gene transcription rates but did not enhance COX-2 mRNA stability.

Activation of p38 mitogen-activated protein kinase in spinal microglia is a critical link in inflammation-induced spinal pain processing

We examined the effect of p38 mitogen-activated protein kinase (MAPK) inhibitors in models of nociception and correlated this effect with localization and expression levels of p38 MAPK in spinal cord. There was a rapid increase in phosphorylated p38 MAPK in spinal cord following intrathecal administration of substance P or intradermal injection of formalin. Immunocytochemistry revealed that phosphorylated p38 MAPK-immunoreactive cells were predominantly present in laminae I-IV of the dorsal horn. Double-staining with markers for neurons, microglia, astrocytes and oligodendrocytes unexpectedly revealed co-localization with microglia but not with neurons or other glia. Pretreatment with p38 MAPK inhibitors (SB20358 or SD-282) had no effect on acute thermal thresholds. However, they attenuated hyperalgesia in several nociceptive models associated with spinal sensitization including direct spinal activation (intrathecal substance P) and peripheral tissue inflammation (intraplantar formalin or carrageenan). Spinal sensitization, manifested by enhanced expression of cyclo-oxygenase-2 and inflammation-induced appearance of Fos-positive neurons, was blocked by pretreatment, but not post-treatment, with p38 MAPK inhibitors. Taken together, these results indicate that spinal p38 MAPK is involved in inflammation-induced pain and that activated spinal microglia play a direct role in spinal nociceptive processing.

Hydrochloric acid induced changes in macrophage migration inhibitory factor in the bladder, peripheral and central nervous system of the rat

PURPOSE

We established the presence of the proinflammatory cytokine macrophage migration inhibitory factor (MIF) in the bladder and in nervous system structures innervating the bladder, and evaluated changes in MIF and cyclooxygenase-2 (COX-2) protein levels and expression following chemical cystitis.

MATERIALS AND METHODS

Male Sprague-Dawley rats were anesthetized and a catheter was introduced into the bladder dome. Cystitis was induced by infusing 0.4 N HCl into the bladder. Control rats received a similar volume of saline. Two hours later the bladder, major pelvic ganglia (MPG), L6/S1 dorsal root ganglia (DRG) and L6/S1 spinal cord were removed and assayed for MIF and COX-2 protein, and mRNA using Western blot and quantitative reverse transcriptase-polymerase chain reaction techniques.

RESULTS

Immunohistochemistry showed MIF located mainly in the urothelium of saline treated rats. Instillation of HCl into the bladder resulted in marked epithelial denudation, moderate edema and vasodilatation in the submucosa. MIF protein levels decreased but MIF mRNA expression remained unchanged in bladders treated with HCl compared with controls. However, MIF protein and mRNA levels increased in the MPG, L6/S1 DRG and L6/S1 spinal cord of HCl treated animals. COX-2 protein was not detected in the bladder, DRG or MPG of saline-treated rats. However, a small amount was present in the L6/S1 cord. On the other hand, HCl treated rats showed marked increases in COX-2 protein levels in all tissues examined. Similarly although cox-2 mRNA was constitutively expressed in all tissues examined, expression increased following HCl treatment.

CONCLUSIONS

Chemical cystitis induced by intravesical HCl in rats increases the protein levels and mRNA expression of MIF and COX-2 in central and peripheral nervous system tissues that are involved in innervating the bladder. This finding suggests that MIF may be involved in bladder inflammation and may have a role in the peripheral and central nervous system pathways that regulate bladder reflexes in response to bladder inflammation.

Intrathecally applied flurbiprofen produces an endocannabinoid-dependent antinociception in the rat formalin test

It is generally accepted that the phospholipase-A2-cyclooxygenase-prostanoids-cascade mediates spinal sensitization and hyperalgesia. However, some observations are not in line with this hypothesis. The aim of the present work was to investigate whether different components of this cascade exhibit nociceptive or antinociceptive effects in the rat formalin test. Intrathecal (i.th.) injection of prostaglandin E2 (PGE2) induced a dose-dependent antinociceptive effect on the formalin-induced nociception. Furthermore, thimerosal, which inhibits the reacylation of arachidonic acid thereby enhancing arachidonic acid levels, had an antinociceptive effect rather than the expected pronociceptive effect when given i.th. While the phospholipase A2 inhibitor methyl arachidonyl fluorophosphonate (MAFP; i.th.) had a significant antinociceptive effect, its analogue palmitoyl trifluoromethyl ketone (PTFMK; i.th.) had no significant effect on the formalin-induced nociception. However, MAFP, but not PTFMK, showed a cannabinoid CB1 agonistic effect as shown by the inhibition of electrically evoked contractions of the vas deferens isolated from CB1 wild-type mice but not of that from CB1 knockout mice. The antinociceptive effect of MAFP was completely reversed by the CB1 receptor antagonist AM-251 (i.th.), thus attributing such effect to its CB1 agonistic effect. Moreover, the antinociceptive effect of the cyclooxygenase inhibitor, flurbiprofen (i.th.) was reversed by the co-administration of AM-251, but not by PGE2. Finally. the combination of phenylmethylsulfonyl fluoride (PMSF; intraperitoneal), which inhibits the degradation of anandamide through the inhibition of fatty acid amidohydrolase, with thimerosal (i.th.) produced a profound CB1-dependent antinociception. The present results show that endocannabinoids play a major role in mediating flurbiprofen-induced antinociception at the spinal level.

Selective inhibitors of cyclo-oxygenase-2 (COX-2) induce hypoalgesia in a rat paw model of inflammation

1. It is well-established that inhibitors of cyclo-oxygenase (COX) and hence of prostaglandin (PG) biosynthesis reverse inflammatory hyperalgesia and oedema in both human and animal models of inflammatory pain. 2. Paw oedema and hyperalgesia in rats were induced by injecting carrageenan (250 micro g paw(-1)) into a hindpaw. Both inflammatory responses were followed for 24 h after the injection, measuring hyperalgesia by decreased pain threshold in the paws and oedema by plethysmography. 3. Three selective inhibitors of cyclo-oxygenase-2 (COX-2), celecoxib, rofecoxib and SC 236, given systemically in a range of doses, before the inflammatory stimulus, abolished carrageenan-induced hyperalgesia with little reduction of oedema. These inhibitors also induced hypoalgesia, increasing nociceptive thresholds in the inflamed paw above normal, non-inflamed levels. This hypoalgesia was lost at the higher doses of the selective inhibitors, although hyperalgesia was still prevented. 4. In paws injected with saline only, celecoxib, given at the dose inducing the maximum hypoalgesia after carrageenan, did not alter the nociceptive thresholds. 5. Two non-selective inhibitors of COX-2, indomethacin and piroxicam, abolished hyperalgesia and reduced oedema but did not induce hypoalgesia. 6. Celecoxib given locally into the paw also abolished inflammatory hyperalgesia and induced hypoalgesia without reducing oedema. 7. We conclude that hypoalgesia is expressed only over a critical range of COX-2 inhibition and that concomitant inhibition of COX-1 prevents expression of hypoalgesia, although hyperalgesia is still prevented. 8 Our results suggest a novel anti-nociceptive pathway mediating hypoalgesia, involving COX-2 selectively and having a clear peripheral component. This peripheral component can be further explored for therapeutic purposes.

Current treatment options for acute pain

The pain that accompanies surgical procedures remains prevalent and is an aspect of the perioperative experience that generates the greatest concern for patients about to undergo surgery. There is also a growing recognition of the extent that acute painful experiences can lead to longer-term painful consequences, even when tissue healing appears to be complete. The neurobiologic basis of this has been partially elucidated. The key observations are that multiple sites and multiple receptors collectively contribute, and that noxious stimuli initiate a cascade of events that sensitise the nervous system so that subsequent noxious stimuli are perceived with greater intensity and even previously non-painful stimuli can be painful. Incorporating these observations into effective perioperative regimens designed to limit acute pain and its consequences leads to a multimodal pre-emptive approach to acute pain management. Acute perioperative pain is an ideal setting for the use of pre-emptive analgesic techniques because the timing of noxious stimuli is known in advance and surgical sensitisation of the nervous system is ongoing despite adequate levels of general anaesthesia with volatile anaesthetics. The relevant neurobiology of pain, reviewed in this article, is the basis for advocating an aggressive, multimodal, pre-emptive approach to acute pain therapy throughout the entire perioperative period. A growing body of outcome studies demonstrates the long-term efficacy of this approach.

Can we conquer pain?

Pain can be an adaptive sensation, an early warning to protect the body from tissue injury. By the introduction of hypersensitivity to normally innocuous stimuli, pain may also aid in repair after tissue damage. Pain can also be maladaptive, reflecting pathological function of the nervous system. Multiple molecular and cellular mechanisms operate alone and in combination within the peripheral and central nervous systems to produce the different forms of pain. Elucidation of these mechanisms is key to the development of treatments that specifically target underlying causes rather than just symptoms. This new approach promises to revolutionize pain diagnosis and management.

Phase I safety assessment of intrathecal ketorolac

Spinal prostaglandin synthesis has been implicated in acute pain processes and in generation and maintenance of central sensitization, and intrathecal injection of cyclo-oxygenase (COX) inhibitors produce antinociception and reduce hypersensitivity in animals. We herein report a Phase I safety assessment of intrathecal injection of the COX inhibitor, ketorolac, in healthy volunteers, and demonstrate no serious side effects. Preclinical studies suggest a major site of action of COX inhibitors for analgesia lies in the central nervous system, especially the spinal cord. For example, COX isoenzymes are expressed in the spinal cord, acute noxious stimuli and inflammation increase spinal prostaglandin production, and spinally administered prostaglandins excite dorsal horn projection neurons, induce release of excitatory neurotransmitters, and cause nociceptive behavior. Intrathecal injection of COX inhibitors increases thermal and mechanical withdrawal threshold in animals with inflammation or nerve injury at doses several 100-fold less than those required systemically. Following pre-clinical neurotoxicity screening and regulatory agency approval, we examined the safety of intrathecal injection of a preservative-free formulation of the COX inhibitor, ketorolac. In an open label, dose-escalating design, 20 healthy volunteers received intrathecal ketorolac, 0.25, 0.5, 1, or 2mg (n=5 per group). Ketorolac did not alter blood pressure, although there was small (10-12%), dose-independent reduction in heart rate for the first hour after injection when data from all subjects were pooled. Ketorolac did not affect sensory or motor function or deep tendon reflexes, and there were no subjective sensations, neurologic or otherwise, reported by the volunteers. Ketorolac did not reduce pain report to heat stimuli applied to the lateral calf. One subject had a mild headache 24h after study, resolving the next day. There were no long-term side effects 6 months after study. These data suggest that intrathecal ketorolac does not produce a high incidence of serious adverse events, and they support further investigation for analgesia.