Ciproxifan在小鼠痛觉传导调节机制中的作用

目的:研究组胺H3受体拮抗剂ciproxifan(CPF)在小鼠痛觉传导调节过程中的作用及其机制.方法:用3种不同的小鼠痛觉模型(热板法、扭体法和福尔马林实验)观察CPF的镇痛作用.同时用特异性组胺脱羧酶(HDC)抑制药α-氟甲基组胺酸(α-FMH),观察组胺在CPF发挥镇痛效应过程中所起的作用.在福尔马林致痛模型中,还测定了小鼠脑、脊髓和血清中一氧化氮(NO)和前列腺素E2(PGE2)的含量.结果:热板实验中,CPF 1 mg%.皮下注射福尔马林能引起2个时相(Ⅰ相、Ⅱ相)的痛反应.这种由福尔马林引起的2个时相的痛反应均可明显被CPF 0.3, 1, 3 mg*kg-1抑制. 在3种致痛模型中,CPF的镇痛效应均可被α-FMH 50 mg*kg-1逆转.使用福尔马林后,小鼠脑和脊髓中NO和PGE2水平升高,而CPF能明显抑制这种升高作用,该抑制作用不被α-FMH所拮抗.但CPF对血清中NO和PGE2的浓度没有影响.结论:组胺H3受体拮抗药CPF对多种性质刺激引起的疼痛均有镇痛作用,对福尔马林引起的炎性疼痛和非炎性疼痛都有效.CPF的这种镇痛作用可能与其促进组胺释放有关;同时脑和脊髓中的NO和PGE2可能参与了CPF的镇痛作用.     

The mu-opioid receptor agonist morphine, but not agonists at delta- or kappa-opioid receptors, induces peripheral antinociception mediated by cannabinoid receptors

BACKGROUND AND PURPOSE: Although participation of opioids in antinociception induced by cannabinoids has been documented, there is little information regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids could be involved in peripheral antinociception induced by activation of mu-, delta- and kappa-opioid receptors. EXPERIMENTAL APPROACH: Nociceptive thresholds to mechanical stimulation of rat paws treated with intraplantar prostaglandin E2 (PGE2, 2 microg) to induce hyperalgesia were measured 3 h after injection using an algesimetric apparatus. Opioid agonists morphine (200 microg), (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) (80 microg), bremazocine (50 microg); cannabinoid receptor antagonists N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) (20-80 microg), 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl) methanone (AM630) (12.5-100 microg); and an inhibitor of methyl arachidonyl fluorophosphonate (MAFP) (1-4 microg) were also injected in the paw. KEY RESULTS: The CB1-selective cannabinoid receptor antagonist AM251 completely reversed the peripheral antinociception induced by morphine in a dose-dependent manner. In contrast, the CB2-selective cannabinoid receptor antagonist AM630 elicited partial antagonism of this effect. In addition, the administration of the fatty acid amide hydrolase inhibitor, MAFP, enhanced the antinociception induced by morphine. The cannabinoid receptor antagonists AM251 and AM630 did not modify the antinociceptive effect of SNC80 or bremazocine. The antagonists alone did not cause any hyperalgesic or antinociceptive effect. CONCLUSIONS AND IMPLICATIONS: Our results provide evidence for the involvement of endocannabinoids, in the peripheral antinociception induced by the mu-opioid receptor agonist morphine. The release of cannabinoids appears not to be involved in the peripheral antinociceptive effect induced by kappa- and delta-opioid receptor agonists.     

在小鼠痛觉传导调节机制中的作用(英文)

目的 :研究组胺H3受体拮抗剂ciproxifan(CPF)在小鼠痛觉传导调节过程中的作用及其机制。方法 :用 3种不同的小鼠痛觉模型 (热板法、扭体法和福尔马林实验 )观察CPF的镇痛作用。同时用特异性组胺脱羧酶 (HDC)抑制药α 氟甲基组胺酸 (α FMH) ,观察组胺在CPF发挥镇痛效应过程中所起的作用。在福尔马林致痛模型中 ,还测定了小鼠脑、脊髓和血清中一氧化氮 (NO)和前列腺素E2(PGE2 )的含量。结果 :热板实验中 ,CPF 1mg·kg- 1和 3mg·kg- 1能明显延长小鼠的痛反应时间 ,其镇痛作用从用药后 2 0min开始 ,可持续 6 0min以上。扭体实验中 ,CPF 1mg·kg- 1可明显抑制小鼠的扭体次数 ,最高抑制率达 4 9.85 %。皮下注射福尔马林能引起 2个时相 (Ⅰ相、Ⅱ相 )的痛反应。这种由福尔马林引起的 2个时相的痛反应均可明显被CPF 0 .3,1,3mg·kg- 1抑制。在 3种致痛模型中 ,CPF的镇痛效应均可被α FMH 5 0mg·kg- 1逆转。使用福尔马林后 ,小鼠脑和脊髓中NO和PGE2 水平升高 ,而CPF能明显抑制这种升高作用 ,该抑制作用不被α FMH所拮抗。但CPF对血清中NO和PGE2 的浓度没有影响。结论 :组胺H3受体拮抗药CPF对多种性质刺激引起的疼痛均有镇痛作用 ,对福尔马林引起的炎性疼痛和非炎性疼痛都有效。CPF的这种镇痛作用可能与其促进?     

Antiobesity effects of chronic cannabinoid CB1 receptor antagonist treatment in diet-induced obese mice

We determined the effect of a cannabinoid CB1 receptor antagonist (AM-251; N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide) on food intake, body weight and adipose tissue mass in Western diet-induced obese (DIO) mice using a chronic, interrupted, oral dosing paradigm. The dosing paradigm was 2 weeks on treatment (treatment 1), 2 weeks off-treatment, followed by 2 weeks on treatment (treatment 2). During treatment 1 and treatment 2, food intake and body weight were reduced after a single dose. At 30 mg/kg/day, anorectic efficacy was maintained through 12 days (treatment 1) and 7 days (treatment 2). Body weight of AM-251-treated mice remained less than vehicle-treated mice throughout treatment 1 and treatment 2. Administration of AM-251 reduced inguinal subcutaneous, retroperitoneal and mesenteric adipose tissue mass. Antiobesity effects of AM-251 were lost during the off-treatment period, and hyperphagia was observed in treated animals. With re-initiation of AM-251 treatment, mice again responded to the effects of the compound. These results support the hypothesis that chronic treatment of obese individuals with cannabinoid CB1 receptor antagonists is a viable pharmacologic approach to sustained weight loss.     

Nitric oxide modulates spinal antinociceptive effect of clonidine but not that of baclofen in the formalin test in rats.

The aim of present study was to determine the influence of nitric oxide (NO) synthesis on intrathecal (i.t.) clonidine or baclofen antinociception in the formalin test. Formalin injection into the hindpaw of a rat induces a biphasic response in pain-related behaviours, such that C-fiber activation (acute pain) during phase 1 triggers a state of spinal sensitization characterized by longer lasting phase 2 (tonic pain). Intrathecal clonidine and baclofen, at doses without effect upon motor performance, produced a dose-dependent inhibition of both phases of the formalin test. Potency of both drugs, defined by ID50 for phase 2 of the formalin test, was 3.5 and 0.6 nmol, respectively. Intrathecal coadministration of L-arginine, substrate of NO synthase (NOS) or NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), dose-dependently reduced or potentiated, respectively, the antinociceptive effect of clonidine but not that of baclofen in the formalin test. The importance of NO formation in the antinociceptive effect of clonidine is further supported by the observation that neither D-arginine nor D-NAME were able to modify clonidine antinociception. These results suggest that the NO synthesis plays a modulatory role in the antinociceptive effect of clonidine, while the mechanism underlying the baclofen-induced antinociception seems to be different.     

Possible antinociceptive mechanisms of opioid receptor antagonists in the mouse formalin test

It has been reported that opioid receptor antagonist can induce antinociception in several nociceptive tests. In the intraplantar formalin pain model, however, opioid antagonist-induced antinociception, as well as its underlying mechanism, has not been well characterized. Therefore, in the mouse formalin test, we attempted to characterize the site of action and the possible opioid receptor subtypes. We found that naltrexone (a nonselective opioid antagonist) injected intraperitoneally (i.p., 1-20 mg/kg), intrathecally (i.t., 0.1-10 microg) and intracerebroventricularly (i.c.v., 0.1-10 microg) phase. Administration of beta-funaltrexamine (beta-FNA, 10-40 mg/kg i.p., 1.25-5 microg it or i.c.v.), naltrindole (1-10 mg/kg i.p., 1.25-5 microg it or i.c.v.) and nor-binaltorphimine (nor-BNI, 1-10 mg/kg i.p., 10-40 microg it or i.c.v.), which are selective mu-, delta- and kappa-opioid antagonists, respectively, also produced antinociception during the second phase. Additionally, we examined the involvement of the descending monoaminergic systems in the naltrexone-induced antinociception in the formalin test. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, a serotonergic neurotoxin, 20 microg i.t.), but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, a noradrenergic neurotoxin, 20 microg i.t.), reversed the naltrexone-induced antinociception during the second phase. Our results suggest that blockade of supraspinally or spinally located opioid receptors may play roles in the regulation of antinociception during the tonic painful stage. In addition, opioid receptors localized at the neuroterminal of the descending serotonergic, but not noradrenergic, inhibitory system in the spinal cord appear to be involved in opioid antagonist-induced antinociception during the second tonic phase of the formalin test.     

CB1 receptor activation in the basolateral amygdala produces antinociception in animal models of acute and tonic nociception

1. Recent studies have suggested that the basolateral nucleus of the amygdala (BLA) participates in the processing of pain information, especially noxious somatic information. Cannabinoid receptors or CB1 mRNA are expressed more in the BLA than in other nuclei of the amygdala. Thus, the aim of the present study was to examine whether CB1 receptors in the BLA may be involved in modulating acute and/or tonic nociceptive processing. 2. Adult rats were exposed to intra-BLA microinjection of the cannabinoid receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo [1,2,3,-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate [WIN 55,212-2 (1, 2.5, 5 or 10 microg/side)] and subjected to the tail flick and formalin tests. 3. The rats demonstrated a dose-dependent increase in latency to withdraw from a thermal noxious stimulus in the tail flick test and a decrease in formalin-induced pain behaviours. The antinociceptive effects of the CB1 receptor agonist WIN 55,212-2 (10 microg/side) in both tests were attenuated in the presence of the selective CB1 receptor antagonist, N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3- carboxamide (AM251; 0.55 ng/side). Administration of the CB1 receptor antagonist AM251 (0.55, 5.5, or 55.5 ng/side) alone did not alter the nociceptive thresholds in either test. Bilateral microinjection of the selective CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethyl bicyclo [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528; 1 microg/side) had no effect on the antinociception produced by WIN 55,212-2, suggesting that the antinociceptive actions of WIN 55,212-2 are mediated by CB1 receptors. 4. The findings suggest the existence of a CB1-mediated inhibitory system in the BLA that, when activated, can diminish responsivity to acute and tonic noxious stimuli, but that normally has no tonic effect on the response threshold of these stimuli.     

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.     

Analgesic actions of N-arachidonoyl-serotonin, a fatty acid amide hydrolase inhibitor with antagonistic activity at vanilloid TRPV1 receptors

BACKGROUND AND PURPOSE: N-arachidonoyl-serotonin (AA-5-HT) is an inhibitor of fatty acid amide hydrolase (FAAH)-catalysed hydrolysis of the endocannabinoid/ endovanilloid compound, anandamide (AEA). We investigated if AA-5-HT antagonizes the transient receptor potential vanilloid-1 (TRPV1) channel and, as FAAH and TRPV1 are targets for analgesic compounds, if it exerts analgesia in rodent models of hyperalgesia. EXPERIMENTAL APPROACH: AA-5-HT was tested in vitro, on HEK-293 cells overexpressing the human or the rat recombinant TRPV1 receptor, and in vivo, in rats and mice treated with formalin and in rats with chronic constriction injury of the sciatic nerve. The levels of the endocannabinoids, AEA and 2-arachidonoylglycerol, in supraspinal (periaqueductal grey, rostral ventromedial medulla), spinal or peripheral (skin) tissues were measured. KEY RESULTS: AA-5-HT behaved as an antagonist at both rat and human TRPV1 receptors (IC(50)=37-40 nM against 100 nM capsaicin). It exerted strong analgesic activity in all pain models used here. This activity was partly due to FAAH inhibition, elevation of AEA tissue levels and indirect activation of cannabinoid CB(1) receptors, as it was reversed by AM251, a CB(1) antagonist. AA-5-HT also appeared to act either via activation/desensitization of TRPV1, following elevation of AEA, or as a direct TRPV1 antagonist, as suggested by the fact that its effects were either reversed by capsazepine and 5'-iodo-resiniferatoxin, two TRPV1 antagonists, or mimicked by these compounds administered alone. CONCLUSIONS AND IMPLICATIONS: Possibly due to its dual activity as a FAAH inhibitor and TRPV1 antagonist, AA-5-HT was highly effective against both acute and chronic peripheral pain.     

Sildenafil produces antinociception and increases morphine antinociception in the formalin test

The antinociceptive activity of an inhibitor of phosphodiesterase 5 alone or combined with morphine was assessed in the formalin test. Local administration of 1-[4-ethoxy-3-(6, 7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo [3, 4-d]pyrimidin-5-yl)phenylsulfonyl]-4-methyl piperazine (sildenafil, inhibitor of phosphodiesterase 5) produced a dose-dependent antinociceptive effect in the second phase of the formalin test in female Wistar rats. In contrast, morphine produced antinociception in both phases. Sildenafil significantly increased the morphine-induced antinociception. The antinociception produced by the drugs alone or combined was due to a local action, as its administration in the contralateral paw was ineffective. Pretreatment of the paws with N(G)-L-nitro-arginine methyl ester (L-NAME, nitric oxide (NO) synthesis inhibitor), 1H-[1,2, 4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor) or naloxone blocked the effect of the combination. Results suggest that opioid receptors, NO and cyclic GMP are relevant in the combination-induced antinociception. In conclusion, sildenafil produced antinociception and increased that produced by morphine, probably through the inhibition of cyclic GMP degradation.     

Antinociceptive effects of methysergide in various pain models

Several studies have demonstrated that the nonselective opioid receptor antagonist naloxone produces a paradoxical antinociception in the formalin test. The opioid system is related to the serotonergic system for producing antinociception at the spinal level. Here we also asked whether systemic (i.p.) and intrathecal (i.t.) administrations of a nonselective serotonergic antagonist, methysergide, might produce paradoxical antinociception similar to naloxone in the mouse formalin test. A diluted formalin solution was injected into the mouse plantar region of the hind paw and the duration of licking responses was measured at periods of 0-5 min (1st phase) and 20-40 min (2nd phase) after formalin injection. Methysergide administered i.p. and i.t. showed an attenuated licking duration only in the 2nd phase. The effect observed in the 2nd phase was reversed in the 5,7-dihydroxytriptamine, but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine pretreated group of mice, suggesting that descending serotonergic, but not noradrenergic, systems are involved in the methysergide antinociception. To further investigate the mechanism by which methysergide inhibited the nociceptive behaviors induced by formalin, the antinociceptive effect of methysergide was also tested in substance P (i.t.) and excitatory amino acids (i.t.), such as glutamate, N-methyl-D-aspartic acid, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainic acid, which are major components in the formalin-induced nociceptive transmission in the spinal cord pain models. The duration of nociceptive behaviors shown in these models was significantly shortened by i.p. and i.t. administration of methysergide. These results suggest that methysergide also produces a paradoxical antinociception in various pain models including the formalin test, similar to the results of naloxone.     

Coronary vasodilator effects of endogenous cannabinoids in vasopressin-preconstricted unpaced rat isolated hearts

The mechanisms by which cannabinoids alter coronary vascular tone and cardiac performance are controversial. We investigated the effects of various cannabinoids in spontaneously beating Langendorff-perfused rat hearts. Bolus injections of anandamide (0.1-1 micromol) caused no change in coronary flow (CF) or left ventricular systolic pressure (LVSP). In hearts preperfused with vasopressin to induce vasoconstrictor tone, anandamide or the selective CB1 receptor agonist ACEA (1-100 nmol) dose-dependently increased CF by up to 267% and LVSP by 20 mm Hg. The metabolically stable endocannabinoid derivatives, R-methanandamide and noladin ether, displayed similar effects. In contrast, Delta-THC (10-100 nmol), the major psychoactive ingredient of cannabis, strongly decreased CF and LVSP. The CB2 receptor agonist JWH-133 (10-100 nmol) elicited vasodilator and positive inotropic effects only at higher doses. The CB1 antagonists SR141716A and AM-251 as well as the potassium channel inhibitors tetraethylammonium and iberiotoxin blocked the anandamide-induced increases in CF and LVSP, whereas the CB2 antagonist SR144528 and the putative "CB3 antagonist" O-1918 did not have an inhibitory effect. Immunohistochemistry revealed the presence of cardiac CB1 but no CB2 receptors. Anandamide and 2-arachidonoylglycerol were detected in heart tissue. However, combined application of fatty acid amidohydrolase inhibitors and the transport inhibitor AM-404 to augment tissue levels of endocannabinoids was without effect on CF or LVSP. We conclude that in the rat isolated heart with reestablished vasoconstrictor tone, cannabinoids including anandamide elicit coronary vasodilation and a secondary increase in contractility via CB1 receptors and potassium channels.     

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.     

Cannabinoids acting on CB1 receptors decrease contractile performance in human atrial muscle

Cannabinoids elicit hypotension mainly via activated CB(1) receptors and show complex cardiovascular actions. Effects on human heart muscle have not been studied yet. Isolated human atrial heart muscle preparations were stimulated by electrical field with 1 Hz to contract isometrically at optimal length and were challenged with the endogenous cannabinoid arachidonyl ethanolamide (anandamide), the metabolically stable analogue R-methanandamide, and the potent synthetic CB(1) receptor agonist HU-210. Anandamide dose-dependently decreased systolic force (82.2 +/- 4.8% and 60.8 +/- 6.8% of maximal systolic force for 0.1 and 1 microM, respectively, P < 0.05). The selective CB(1) receptor antagonist AM-251 (1 microM, P < 0.05), but not the CB(2) receptor antagonist, AM-630 (1 microM), the nitric oxide synthase inhibitor N omega-nitro-l-arginine methyl ester (l-NAME) (500 microM), or the cyclooxygenase inhibitor indomethacin (100 microM), prevented the effect. Contrary to indomethacin, l-NAME alone showed negative inotropic effects (72.1 +/- 3.54%, P < 0.001). The R-methanandamide (1 microM: 50.4 +/- 3.5%, P < 0.001) and HU-210 (1 microM: 60.1 +/- 3.8%, P < 0.001) had similar negative inotropic effects. The existence of CB(1) receptors on heart muscle was verified using Western blot analysis and immunofluorescence staining. The conclusion is that anandamide, R-methanandamide, and HU-210 decrease contractile performance in human atrial muscle via CB(1) receptors.     

Roles of serotonin receptor subtypes for the antinociception of 5-HT in the spinal cord of rats

The contribution of 5-HT (5-hydroxytryptamine) receptor subtypes to the antinociception produced by intrathecal 5-HT in the formalin test was investigated in rats. Intrathecal 5-HT suppressed both phases of behaviors produced by 5% formalin, and this was blocked by antagonists for 5-HT(1B) (3-[3-(Dimethylamino)propyl]-4-hy-droxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride, GR 55562), 5-HT(2C) (N-ormethylclozapine/8-Chloro-11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine, D-MC), 5-HT3 (1-Methyl-N-(8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-1H-indazole-3-carboxamide maleate, LY-278,584) and 5-HT4 receptors (4-Amino-5-chloro-2-metho-xy-benzoic acid 2-(diethylamino)ethyl ester hydrochloride, SDZ-205,557), but not the 5-HT(1D) receptor antagonist 3-[4-(4-Chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL 15572). The 5-HT(1A) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635) decreased only the second phase antinociception of 5-HT. Intrathecal administration of agonists for 5-HT(1A) (3-(N,N-Dipropylaminoethyl)-1H-indole-5-carboxamide maleate, Dipropyl-5CT), 5-HT(1B) (7-Trifluoromethyl-4(4-met-hyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline maleate, CGS-12066A), 5-HT(2C) (6-Ch-loro-2-(1-piperazinyl)pyrazine hydrochloride, MK 212), 5-HT3 (N-(3-Chlorophenyl)imidodicarbonimidic diamide hydrochloride, m-CPBG) and 5-HT4 receptors (2-[1-(4-Piperonyl)piperazinyl]benzothiazole, BZTZ) suppressed both phases of the formalin response. The results of the present study indicate that spinal 5-HT(1B,) 5-HT(2C,) 5-HT3 and 5-HT4 receptors, but not the 5-HT(1D) receptor, mediate antinociception produced by 5-HT in the formalin test. The relevance of the 5-HT(1A) receptor is less clear because of the different effects of antagonist and agonist.     

Peripheral and spinal mechanisms of antinociceptive action of lumiracoxib

The possible participation of the nitric oxide (NO)-cyclic GMP-K(+) channel pathway, serotonergic or opioidergic system on lumiracoxib-induced local or intrathecal antinociception was assessed in the formalin test. Local or intrathecal administration of lumiracoxib dose-dependently produced antinociception in the second phase of the test. Moreover, local or intrathecal pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, NO synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor), glibenclamide (ATP-sensitive K(+) channel blocker), charybdotoxin and apamin (large- and small-conductance Ca(2+)-activated-K(+) channel blockers, respectively) or margatoxin (voltage-dependent K(+) channel blocker), but not N(G)-D-nitro-arginine methyl ester (D-NAME) or vehicle, significantly prevented lumiracoxib-induced antinociception. The intrathecal injection of methiothepin (serotonin receptor antagonist) reduced lumiracoxib-induced intrathecal antinociception. Local peripheral or intrathecal naloxone did not modify either local or intrathecal lumiracoxib-induced antinociception. Results suggest that lumiracoxib activates the NO-cyclic GMP-K(+) channels to produce local and intrathecal antinociception. Data also suggest that lumiracoxib activates the intrathecal serotonergic system, but not opioid receptors either at peripheral or spinal sites.     

Adenosine receptor mediates nicotine-induced antinociception in formalin test.

In this study, the effect of adenosine receptor agents on nicotine induced antinociception, in formalin test, has been investigated. Intraperitoneal (i.p.) administration of different doses of nicotine (0.1, 1, 10 and 100 microgkg(-1)) induced a dose-dependent antinociception in mice, in the both first and second phases of the test. Adenosine receptor antagonist, theophylline (5, 10, 20 and 80 mgkg(-1), i.p.) also induced antinociception in the both phases, while a dose of the drug (40 mgkg(-1), i.p.) did not induce any response. Theophylline reduced antinociception induced by nicotine in both phases of formalin test. The A(2) receptor agonist, 5'-N-ethylcarboxamide adenosine (NECA; 1 and 5 microgkg(-1), i.p.) also produced antinociception, which was reversed with different doses of theophylline (5, 10, 20 and 40 mgkg(-1), i.p.). But administration of the adenosine receptor agonist, NECA did not potentiate the response of nicotine. It is concluded that adenosine system may be involved in modulation of antinociception induced by nicotine.     

Centrally mediated antinociceptive effects of cannabinoid receptor ligands in rat models of nociception

The endogenous nonapeptide hemopressin (HE) demonstrates potent block of the cannabinoid subtype-1 (CB1) receptor in vitro and robust antinociception in vivo. The current study evaluated the effects of centrally administered HE in mechanistically distinct pre-clinical rat models of pain—the hot plate test and the hind paw formalin test. The non-subtype selective CB receptor agonist WIN 55,212-2 was tested concurrently as a positive control. In the hot plate test, neither intrathecal (i.t.) HE nor WIN 55,212-2 significantly altered the latency to respond to noxious heat. By contrast, i.t. HE and WIN 55,212-2 significantly reduced pain-related behaviors in the formalin test. Possible HE functionality as a CB1 receptor antagonist at the spinal level was evaluated in the formalin test. Intrathecal pretreatment with HE did not attenuate the antinociceptive effect of i.t. WIN 55,212-2. However, pretreatment with the CB1 receptor antagonist rimonabant did; i.t. rimonabant pretreatment was not antinociceptive. Potential supraspinal antinociceptive activity of HE was also evaluated. Whereas intracerebroventricular (i.c.v.) injection of WIN 55,212-2 reduced pain-related behaviors in the formalin test, interestingly, i.c.v. HE increased behaviors. In the current study, an antinociceptive effect with the CB receptor ligand HE was obtained under the specific condition of tissue injury and not in the uninjured state. Thus, HE could be a useful analgesic peptide with a novel spinal mechanism of action.     

2-Arachidonoylglycerol and anandamide oppositely modulate norepinephrine release from the rat heart sympathetic nerves.

Anandamide (10(-7) and 10(-6) M) as well as a synthetic cannabinoid HU210 (10(-8) to 10(-6) M) suppressed the norepinephrine release evoked by perivascular nerve stimulation (PNS) of the rat heart Langendorff's preparation. The effects of HU210 and the lower dose of anandamide were completely blocked by the cannabinoid CB1-receptor antagonist AM251, while that of anandamide at 10(-6) M was partly mediated by arachidonate-derived metabolites. 2-Arachidonoylglycerol (2-AG), at 10(-6) M in the presence of DFP and indomethacin, increased PNS-evoked norepinephrine release, which was completely blocked by AM251. The present results suggest that the two endocannabinoids may oppositely participate in the CB1-receptor-mediated modulation of sympathetic norepinephrine release.     

AM-251 and rimonabant act as direct antagonists at mu-opioid receptors: implications for opioid/cannabinoid interaction studies

Mu-opioid and CB1-cannabinoid agonists produce analgesia; however, adverse effects limit use of drugs in both classes. Additive or synergistic effects resulting from concurrent administration of low doses of mu- and CB1-agonists may produce analgesia with fewer side effects. Synergism potentially results from interaction between mu-opioid receptors (MORs) and CB1 receptors (CB1Rs). AM-251 and rimonabant are CB1R antagonist/inverse agonists employed to validate opioid–cannabinoid interactions, presumed to act selectively at CB1Rs. Therefore, the potential for direct action of these antagonists at MORs is rarely considered. This study determined if AM-251 and/or rimonabant directly bind and modulate the function of MORs. Surprisingly, AM-251 and rimonabant, but not a third CB1R inverse agonist AM-281, bind with mid-nanomolar affinity to human MORs with a rank order of affinity (K_i) of AM-251 (251 nM) > rimonabant (652 nM) > AM281 (2135 nM). AM-251 and rimonabant, but not AM-281, also competitively antagonize morphine induced G-protein activation in CHO-hMOR cell homogenates (K_b = 719 or 1310 nM, respectively). AM-251 and rimonabant block morphine inhibition of cAMP production, while only AM-251 elicits cAMP rebound in CHO-hMOR cells chronically exposed to morphine. AM-251 and rimonabant (10 mg/kg) attenuate morphine analgesia, whereas the same dose of AM-281 produces little effect. Therefore, in addition to high CB1R affinity, AM-251 and rimonabant bind to MORs with mid-nanomolar affinity and at higher doses may affect morphine analgesia via direct antagonism at MORs. Such CB1-independent actions of these antagonists may contribute to reported inconsistencies when CB1/MOR interactions are examined via pharmacological methods in CB1-knockout versus wild-type mice.