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.     

Involvement of Cholinergic and Opioid Receptor Mechanisms in Nicotine-Induced Antinociception

Abstract: In this work we have studied the influences of nicotinic agents on the antinociception of morphine in formalin test. Nicotine (0.001-0.1 mg/kg) induced antinociception in mice in a dose-dependent manner in the early phase of formalin test, and also potentiated the morphine effect. The nicotinic receptor antagonist, mecamylamine (0.5 mg/kg), but not hexamethonium decreased the antinociception induced by nicotine (0.1 mg/kg) in both phases. The muscarinic receptor antagonist atropine (5 and 10 mg/kg) also decreased the response of nicotine. Mecamylamine, hexamethonium or atropine did not alter morphine antinociceptive response, while naloxone decreased responses induced by nicotine or morphine. The antagonists by themselves did not elicit any response in formalin test, however, high doses of mecamylamine tend to increase pain response. It is concluded that central cholinergic and opioid receptor mechanisms may be involved in nicotine-induced antinociception.     

Yohimbine produces antinociception in the formalin test in rats: involvement of serotonin1A receptors

Rationale: Previous studies have suggested that the α₂-adrenergic receptor antagonist yohimbine produced antinociceptive effects in the formalin test in rats. However, yohimbine is also an agonist at serotonin (5-HT)_1A receptors, suggesting the possibility that the antinociceptive effects of yohimbine might be mediated via these receptors. Objective: The purpose of the present studies was to evaluate the potential role of 5-HT_1A receptors in mediating the antinociceptive effects of yohimbine. Methods: The antinociceptive effects of yohimbine were evaluated using the formalin test in rats. Results: Yohimbine (2.5–10 mg/kg s.c.) produced dose-related antinociception during both phase I and phase II of the formalin test, and was approximately equipotent and equiefficacious to morphine. The selective 5-HT_1A receptor antagonist WAY 100,635 (0.03–3.0 mg/kg s.c.) produced a partial reversal of yohimbine. In comparison, the selective 5-HT_1A receptor agonist (±)8-hydroxy- dipropylaminotetralin HBr (8OH-DPAT; 1.0 mg/kg s.c.) also produced a dose-related antinociception in the formalin test, although 8OH-DPAT was completely reversed by WAY 100,635 (3.0 mg/kg s.c.). The antinociceptive effects of yohimbine were not antagonized by the 5-HT_1B/1D antagonist GR 127935 (1.0 mg/kg and 3.0 mg/kg s.c.), the 5-HT₂ antagonist LY53857 (1.0 mg/kg s.c.), or the 5-HT₃ antagonist zatosetron (3.0 mg/kg s.c.). Conclusions: The present results demonstrate that yohimbine produces a dose-related antinociception in the formalin test in rats which is mediated in part by the agonistic actions at 5-HT_1A receptors. Received: 10 September 1999 / Final version: 5 November 1999     

Modification of formalin-induced nociception by different histamine receptor agonists and antagonists.

The present study evaluated the effects of different histamine receptor agonists and antagonists on the nociceptive response in the mouse formalin test. Intracerebroventricular (20-40 microg/mouse i.c.v.) or subcutaneous (1-10 mg/kg s.c.) injection of HTMT (H(1) receptor agonist) elicited a dose-related hyperalgesia in the early and late phases. Conversely, intraperitoneal (20 and 30 mg/kg i.p.) injection of dexchlorpheniramine (H(1) receptor antagonist) was antinociceptive in both phases. At a dose ineffective per se, dexchlorpheniramine (10 mg/kg i.p.) antagonized the hyperalgesia induced by HTMT (40 mug/mouse i.c.v. or 10 mg/kg s.c.). Dimaprit (H(2) receptor agonist, 30 mg/kg i.p.) and ranitidine (H(2) receptor antagonist, 20 and 40 mg/kg i.p.) reduced the nociceptive responses in the early and late phases. No significant change in the antinociceptive activity was found following the combination of dimaprit (30 mg/kg i.p.) with ranitidine (10 mg/kg i.p.). The antinociceptive effect of dimaprit (30 mg/kg i.p.) was prevented by naloxone (5 mg/kg i.p.) in the early phase or by imetit (H(3) receptor agonist, 25 mg/kg i.p.) in both early and late phases. The histamine H(3) receptor agonist imetit was hyperalgesic following i.p. administration of 50 mg/kg. Imetit-induced hyperalgesia was completely prevented by treatment with a dose ineffective per se of thioperamide (H(3) receptor antagonist, 5 mg/kg i.p.). The results suggest that histamine H(1) and H(3) receptor activations increase sensitivity to nociceptive stimulus in the formalin test.     

Adenosine receptor agonists or antagonists alter antinociception, but did not show an interaction with imipramine-induced antinociception in the formalin test in mice.

In this study, the antinociceptive effect of imipramine and adenosine agents, and interactions between imipramine with adenosine drugs in mice in the formalin test, have been investigated. Intraperitoneal administration of different doses of imipramine (10, 20, 30 and 40 mg/kg) induced a dose dependent antinociception in mice, in both the first and second phases of the formalin test. The adenosine A(1) receptor agonists, R-(N(6)-phenylisopropyl)-adenosine (0.015, 0.03 and 0.1 mg/kg) and 5'-N-ethylcarboxamide adenosine (0.001, 0.005, 0.01 mg/kg), but not 2-chloroadenosine (0.1 and 0.5 mg/kg), and the adenosine receptor antagonist, 8-phenyltheophylline (0.1, 0.5 and 1 mg/kg), but not 1,3-dipropyl-7-methyl-xanthine (0.5 and 5 mg/kg), also produced an antinociceptive response. Lower dose of the adenosine receptor antagonist theophylline induced antinociception, while a higher dose of the drug caused hyperalgesia. Theophylline reduced the response induced by imipramine. It is concluded that adenosine systems are not involved in imipramine responses in the formalin test.     

Antinociception in rat by sarpogrelate, a selective 5-HT(2A) receptor antagonist, is peripheral

The antinociceptive effect of sarpogrelate, a new selective 5-hydroxytriptamine (5-HT)(2A) receptor antagonist, in the formalin test was examined in rats. Sarpogrelate was administered intraperitoneally, locally (subcutaneously at the formalin test site) or intrathecally 10 min before formalin injection. Intraperitoneal (1-100 mg/kg) and local (0.01-1 mg) administration of sarpogrelate suppressed flinching behavior in both phases 1 (0-9 min) and 2 (10-60 min) in a dose-dependent manner. Intraperitoneal (100 mg/kg) and local (1 mg) injection 7 min after formalin injection reduced phase 2 flinches to the same degree as with the pre-treatment. Intrathecal administration (1-100 microg) showed no antinociceptive action, and facilitated phase 2 flinches at 10 microg. The plasma concentration of sarpogrelate after local administration of 1 mg was lower than after intraperitoneal administration of 10 mg/kg, although local administration produced more potent antinociception. The data imply that the antinociceptive effect of sarpogrelate results mainly from an action at peripheral sites.     

Mazindol and lidocaine are antinociceptives in the mouse formalin model: involvement of dopamine receptor

The antinociceptive potential of mazindol, an anorectic drug, and lidocaine, an amide-type local anesthetic, were investigated in the mouse formalin test with concurrent motor function assessment. In addition, the role of dopamine and opioid receptors in mediation of the antinociceptive action of these drugs was examined. The i.p. injection of mazindol (1.25–10 mg/kg) and lidocaine (10–30 mg/kg) induced significant antinociceptive responses in both phases of the test. Cocaine (20 mg/kg, i.p.), used as positive control, also inhibited the pain responses caused by formalin. Haloperidol (0.2 mg/kg, i.p.), and sulpiride (5 mg/kg, i.p.), a dopamine D₂ receptor antagonist, reduced the antinociceptive actions of mazindol and cocaine, while SCH 23390, R(+)-7-chloro 8-hydroxy-3methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3 benzazepine (0.03 mg/kg, i.p.), a dopamine D₁ receptor antagonist, did not affect these responses. Only the antinociception associated with mazindol was reversed by naloxone (2 mg/kg, i.p.). The same pretreatments failed to modify lidocaine-induced antinociception. The drug conditions used in this study did not reveal any motor impairment in the rotarod test. These observations suggest an involvement of dopaminergic mechanisms, mainly via dopamine D₂ receptors, in the antinociceptive action of mazindol in the formalin test, but the nature of mechanisms involved in the lidocaine responses remains unsolved.     

Profile of spinal and supra-spinal antinociception of (-)-linalool

We previously reported that administration of (-)-linalool, the naturally occurring enantiomer in essential oils, induced a significant reduction in carrageenin-induced oedema and in acetic acid-induced writhing. The latter effect was completely antagonised by the muscarinic receptor antagonist atropine and by the opioid receptor antagonist naloxone. To further characterise the antinociceptive profile of (-)-linalool, we studied its effect in the hot plate and the formalin in tests. In addition, to determine the possible involvement of the cholinergic, opioidergic and dopaminergic systems, we tested the effects of atropine, pirenzepine, a muscarinic M1 receptor antagonist, naloxone, sulpiride, a dopamine D2 receptor antagonist and (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH-23390), a dopamine D1 receptor antagonist on (-)-linalool-induced antinociception. Moreover, since K(+) channels seem to play an important role in the mechanisms of pain modulation, we examined the effect of glibenclamide, an ATP-sensitive K(+) channel inhibitor on (-)-linalool-induced antinociception. The administration of (-)-linalool (100 and 150 mg/kg, s.c.) increased the reaction time in the hot-plate test. Moreover, (-)-linalool (50 and 100 mg/kg) produced a significant reduction in the early acute phase of the formalin model, but not in the late tonic phase. The highest dose (150 mg/kg) caused a significant antinociceptive effect on both phases. The antinociceptive effects of (-)-linalool were decreased by pre-treatment with atropine, naloxone, sulpiride and glibenclamide but not by pirenzepine and SCH-23390. These results are in agreement with the demonstrated pharmacological properties of linalool, mainly its cholinergic, local anaesthetic activity and its ability to block NMDA receptors. Furthermore, a key role seems to be played by K(+) channels, whose opening might be the consequence of a stimulation of muscarinic M2, opioid or dopamine D2 receptors.     

Interaction of angiotensin II and adenosine A1 and A2A receptor ligands on the writhing test in mice

The effects of adenosine A1 and A2A receptor agonists and antagonists administered intraperitoneally (i.p.) and their interaction with angiotensin II (Ang II) administered intracerebroventricularly (i.c.v.) were studied in mice using the acetic acid-induced abdominal constriction test. Ang II (0.1 microg/mouse) induced antinociception in this model. The adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 0.05, 0.25 and 0.5 mg/kg) also showed a well-developed antinociceptive effect. Ang II (0.1 microg/mouse) administered 5 min before CPA (0.25 mg/kg) decreased the number of writhes, i.e., it enhanced the antinociceptive effect of CPA. Losartan, an AT1 receptor antagonist (25 microg/mouse i.c.v.), enhanced the antinociceptive effect of CPA, while the AT2 receptor antagonist 1-[-4-(dimethylamino)-3-methylphenylmethyl]-5-diphenylacetyl)-4,5,6,7-tetrahydro 1H-4-imidazol [4,5c]pyridine-6 carboxylic acid, ditrifluoroacetate, dihydrate (PD 123319; 10 microg/mouse) had less effect. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.1 mg/kg), an adenosine A1 receptor antagonist, exhibited a pronociceptive effect and did not change the antinociceptive effect of Ang II. The adenosine A2A receptor agonist PD-125944 (DPMA; 0.1, 0.5 and 1 mg/kg) showed pronounced antinociceptive effect. Ang II (0.1 microg/mouse) did not significantly influence the antinociceptive effect of DPMA (0.1 mg/kg). The A2A receptor antagonist 3,7-dimethyl-1-propargilxanthine (DMPX; 0.1 mg/kg) had no effect on the number of writhes and did not influence the effect of Ang II. These data indicate that the antinociceptive effect of Ang II interacts with that produced by adenosine A1 receptor agonist.     

Differential effects of K+ channel blockers on antinociception induced by alpha 2-adrenoceptor, GABAB and kappa-opioid receptor agonists.

1. The effects of several K+ channel blockers (sulphonylureas, 4-aminopyridine and tetraethylammonium) on the antinociception induced by clonidine, baclofen and U50,488H were evaluated by use of a tail flick test in mice. 2. Clonidine (0.125-2 mg kg-1, s.c.) induced a dose-dependent antinociceptive effect. The ATP-dependent K+ (KATP) channel blocker gliquidone (4-8 micrograms/mouse, i.c.v.) produced a dose-dependent displacement to the right of the clonidine dose-response line, but neither 4-aminopyridine (4-AP) (25-250 ng/mouse, i.c.v.) nor tetraethylammonium (TEA) (10-20 micrograms/mouse, i.c.v.) significantly modified clonidine-induced antinociception. 3. The order of potency of sulphonylureas in antagonizing clonidine-induced antinociception was gliquidone > glipizide > glibenclamide > tolbutamide, which is the same order of potency as these drugs block KATP channels in neurones of the CNS. 4. Baclofen (2-16 mg kg-1, s.c.) also induced a dose-dependent antinociceptive effect. Both 4-AP (2.5-25 ng/mouse, i.c.v.) and TEA (10-20 micrograms/mouse, i.c.v.) dose-dependently antagonized baclofen antinociception, producing a displacement to the right of the baclofen dose-response line. However, gliquidone (8-16 micrograms/mouse, i.c.v.) did not significantly modify the baclofen effect. 5. None of the K+ channel blockers tested (gliquidone, 8-16 micrograms/mouse; 4-AP, 25-250 ng/mouse and TEA, 10-20 micrograms/mouse, i.c.v.), significantly modified the antinociception induced by U50,488H (8 mg kg-1, s.c.). 6. These results suggest that the opening of K+ channels is involved in the antinociceptive effect of alpha 2 and GABAB, but not kappa-opioid, receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

The involvement of central cholinergic system in (+)-matrine-induced antinociception in mice

The antinociceptive effect of (+)-matrine was examined in mice by writhing, tail-pressure and hot-plate tests. (+)-Matrine (5, 10 and 20 mg/kg s.c.) produced antinociception in a dose-dependent manner. In hot-plate test, the antinociception produced by (+)-matrine (10 mg/kg s.c.) was attenuated by muscarinic receptor antagonists atropine (5 mg/kg i.p.) and pirenzepine (0.1 mug/mouse i.c.v.) and acetylcholine depletor hemicholinium-3 (HC-3) (1 mug/mouse i.c.v.), but not by opioid receptor antagonist naloxone (2 mg/kg i.p.), dopamine D(2) receptor agonist (-)-quinpirole (0.1 mg/kg i.p.) or catecholamine depletor reserpine (2.5 mg/kg i.p.). Radioligand binding assay demonstrated that (+)-matrine had no affinity for mu-, kappa- or delta-opioid receptors in a wide concentration range (1 x 10(-11)-1 x 10(-3) M). The results suggest that (+)-matrine exerts its antinociceptive effect through multiple mechanism(s) such as increasing cholinergic activation in the CNS rather than acting on opioid receptors directly.     

Effect of diabetes on pinacidil-induced antinociception in mice

The antinociceptive effects of pinacidil, an adenosine triphosphate (ATP)-sensitive K(+)i (K(ATP)) channel opener, were examined using the tail-flick test in non-diabetic and diabetic mice. Pinacidil i.c.v. produced dose-dependent antinociception in both non-diabetic and diabetic mice. There was no significant difference between the antinociceptive effect of i.c.v. pinacidil in non-diabetic mice and diabetic mice. The i.t. administration of pinacidil also produced dose-dependent antinociception in both non-diabetic and diabetic mice, however, the antinociceptive effect of i.t. pinacidil in diabetic mice was significantly greater than that in non-diabetic mice. The antinociceptive effect of i.c.v. or i.t. pinacidil was significantly antagonized by i.c.v. or i.t. glibenclamide, a K(ATP) channel blocker in both non-diabetic and diabetic mice. In non-diabetic mice, the antinociceptive effect of i.c.v. or i.t. administration of pinacidil was significantly antagonized by beta-funaltrexamine, a mu-opioid receptor antagonist, 7-benzylidenenaltrexone, a delta1-opioid receptor antagonist, naltriben, a delta2-opioid receptor antagonist, and nor-binaltorphimine, a kappa-opioid receptor antagonist. In diabetic mice, the antinociceptive effect of i.c.v. pinacidil was significantly reduced by 7-benzylidenenaltrexone, naltriben, and nor-binaltorphimine. However, beta-funaltrexamine had no effect on antinociception induced by i.c.v. pinacidil in diabetic mice. On the other hand, the antinociceptive effect of i.t. pinacidil was significantly antagonized by beta-funaltrexamine, 7-benzylidenenaltrexone, naltriben, and nor-binaltorphimine in diabetic mice. These results indicated that pinacidil produced antinociception through the release of opioid peptides acting at mu-, delta- and kappa-opioid receptors in surpraspinal and spinal cord of non-diabetic mice. On the other hand, in diabetic mice, the antinociception-induced by pinacidil was mediated through the release of opioid peptides acting at delta- and kappa-opioid receptors supraspinally, whereas pinacidil produced antinociception through the release of opioid peptides acting at mu-, delta-, and kappa-opioid receptors spinally.     

Ginsenosides that produce differential antinociception in mice.

Ginsenoside Rc, Rd, and Re induced antinociception in writhing and formalin tests among five representative ginsenosides: Rb1, Rc, Rd, Re, and Rg1. However, these ginsenosides had no effect in the tail-flick test. The antinociceptive effects induced by three ginsenosides were dose dependent. ED50 was 20.5 (7.3-57.4 mg/kg) for Rc, 17 (11.0-27.6 mg/kg) for Rd, and 3.5 (1-12 mg/ kg) for Re in the writhing test and 62 (42-90 mg/kg) for Rc, 45 (20.5-99.0 mg/kg) for Rd, and 82 (48-139 mg/kg) for Re in the second phase of the formalin test. The antinociceptive effects were not blocked by the opioid receptor antagonist naloxone in the writhing and formalin tests. These three ginsenosides did not affect motor function. Ginsenoside Rc and Rd induced hypothermia for 30 to 60 min, and ginsenoside Rc induced hyperthemia after 150 min of treatment at doses of 100 mg/kg. These results suggest that ginsenosides such as Rc, Rd, or Re inhibit mainly chemogenic pain rather than thermal pain by the nonopioid system in mice.     

TRK-820, a selective kappa-opioid agonist, produces potent antinociception in cynomolgus monkeys

TRK-820 ((-)-17-cyclopropylmethyl-3,14b-dihydroxy-4,5a-epoxy-6b-[N-methyl-trans-3-(3-furyl)acrylamide]morphinan hydrochloride) has been shown to be a potent opioid kappa-receptor agonist with pharmacological properties different from those produced by kappa1-opioid receptor agonists in rodents. To ascertain whether or not these properties of TRK-820 would be extended to primates, the antinociceptive effect of TRK-820 was evaluated in cynomolgus monkeys by the hot-water tail-withdrawal procedure. TRK-820 given intramuscularly (i.m.) produced a potent antinociceptive effect that was 295- and 495-fold more potent than morphine with the 50 degrees C and 55 degrees C hot-water tests, respectively, and 40-fold more potent than U-50,488H and 1,000-fold more potent than pentazocine in the 50 degrees C hot-water test. The duration of antinociceptive effects of TRK-820 treatment (0.01 and 0.03 mg/kg, i.m.) lasted more than 6 h, which was much longer than those of U-50,488H. The antinociception produced by the higher dose (0.03 mg/kg, i.m.) of TRK-820 was not inhibited by nor-binaltorphimine (3.2 and 10 mg/kg, s.c.) or by naloxone (0.1 mg/kg, s.c.), although the antinociception induced by a lower dose of TRK-820 (0.01 mg/kg, i.m.) was inhibited by nor-binaltorphimine (10 mg/kg, s.c.). The same doses of nor-binaltorphimine and naloxone effectively inhibited the antinociception induced by the higher doses of U-50,488H (1.0 mg/kg, i.m.) and morphine (10 mg/kg, i.m.), respectively. These results indicate that the antinociception induced by TRK-820 is less sensitive to nor-binaltorphimine and suggest that it is mediated by the stimulation of a subtype of kappa-opioid receptor different from the kappa-opioid receptor in cynomolgus monkeys.     

Differential modulation of kappa and mu opioid antinociception by the glycine/NMDA receptor agonist D-serine.

D-Serine, a selective agonist for the strychnine-insensitive glycine allosteric site associated with the NMDA receptor-ion channel complex, was found to modulate differentially the antinociception produced by kappa and mu-opioid receptor agonists in the rat formalin test. D-Serine (100 micrograms, i.c.v.) attenuated the antinociception produced by the selective kappa-opioid agonist, enadoline (0.003-0.1 mg kg-1, s.c.) against the tonic, but not acute, phase of the formalin response. Conversely, D-serine potentiated the antinociception produced by morphine (0.3-10 mg kg-1, s.c.) against both the acute and tonic phases. These results demonstrate an important interaction between the opioid and NMDA/glycine systems in the control of nociceptive information possibly at different levels of the neuraxis.     

Antinociceptive properties of the ethanolic extract and of the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in mice

The present study examined the antinociceptive effects of the ethanolic extract (EE) and of the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in chemical and thermal behavioural models of pain in mice. The EE (10-1000 mg/kg) given orally (p.o.), 1 h prior to testing, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 131.9 mg/kg. In the formalin test, the EE (10-300 mg/kg, p.o.) also caused significant inhibition of both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking, however, it was more potent and efficacious in relation to the late phase of the formalin test, with mean ID50 values for the neurogenic and the inflammatory phases of approximately 300 and 88.8 mg/kg, respectively. The EE (10-1000 mg/kg, p.o.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 160.5 and 38.3 mg/kg, respectively. Furthermore, the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene (1-30 mg/kg), given p.o., 1 h prior to testing, also produced dose-related inhibition of glutamate-induced pain, with a mean ID50 value of 5.6 mg/kg. When assessed in a thermal model of pain, the EE (10-300 mg/kg, p.o.) and fentanyl (100 microg/kg, s.c.) caused a significant and marked increase in the latency response on the hot-plate test (50 degrees C). The antinociception caused by EE (100 mg/kg, p.o.) in the glutamate test was significantly attenuated by intraperitoneal (i.p.) treatment of mice with naloxone (opioid receptor antagonist, 1 mg/kg), pindolol (a 5-HT 1A/1B receptor/beta adrenoceptor antagonist, 1 mg/kg), WAY100635 (a 5-HT 1A receptor antagonist, 0.7 mg/kg) or ketanserin (a 5-HT 2A receptor antagonist, 0.3 mg/kg). In contrast, EE (100 mg/kg, p.o.) antinociception was affected neither by L-arginine (precursor of nitric oxide, 600 mg/kg) nor by ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg) i.p. treatment. It was not associated with non-specific effects such as muscle relaxation or sedation. Together, these results indicate that EE produces dose-related antinociception in several models of chemical and thermal pain through mechanisms that involve an interaction with opioid and serotonergic (i.e., through 5-HT 1A/1B and 5-HT 2A receptors) systems.     

Anxiety does not affect the antinociceptive effect of Delta 9-THC in mice: participation of cannabinoid and opioid receptors

Cannabinoid receptor agonists significantly inhibit nociceptive responses in a large number of animal models. The present study examined whether mice displaying different basal levels of anxiety in the plus-maze test of anxiety might differ in terms of responsiveness to the antinociceptive effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC). Further, the involvement of the cannabinoid and/or opioid receptors in Delta(9)-THC-induced antinociception was investigated by using SR 141716A and naloxone, respectively, cannabinoid and opioid receptor antagonists. Delta(9)-THC-induced antinociception was evaluated in the formalin test that involves a biphasic response with an early and a late phase of high paw-licking activity. This characteristic biphasic response was observed in all control animals selected as "anxious" and "nonanxious." Delta(9)-THC (0.5-5 mg/kg i.p.) caused a dose-dependent antinociceptive effect in both groups of mice during the early and late phases. This response was fully reversed by SR 141716A (1 mg/kg i.p.) and partially reversed by naloxone (2 mg/kg i.p.). These findings suggest that mice selected for differences in anxiety-related behavior show similar responses to the antinociceptive action of Delta(9)-THC and that this action involves predominantly cannabinoid mechanisms.     

Potentiation of adenosine A1 receptor agonist CPA-induced antinociception by paeoniflorin in mice

The effect of paeoniflorin (PF), a major constituent isolated from Paeony radix, on N6-Cyclopentyladenosine (CPA), a selective adenosine A1 receptor (A1 receptor) agonist, induced antinociception was examined in mice. In the tail-pressure test, CPA (0.05, 0.1, 0.2 mg/kg, s.c.) could induce antinociception in a dose-dependent manner. PF (5, 10, 20 mg/kg, s.c.) alone failed to exhibit any antinociceptive effect in mice; however, pretreatment of PF (20 mg/kg, s.c.) could significantly enhance CPA-induced antinociception. Additionally, pretreatment of 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.25 mg/kg, s.c.), a selective A1 receptor antagonist, could antagonize the antinociceptive effect of combining CPA with PF. Furthermore, in the competitive binding experiments, PF did not displace the binding of [3H]-8-Cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX) but displaced that of [3H]-2-Chloro-N6-cyclopentyladenosine ([3H]-CCPA, a selective A1 receptor agonist) to the membrane preparation of rat cerebral cortex. These results suggested that PF might selectively increase the binding and antinociceptive effect of CPA by binding with A1 receptor.     

Dopamine receptor mechanism(s) and antinociception and tolerance induced by swim stress in formalin test

In the present study, involvement of D1 and D2 dopamine receptors in the antinociception and tolerance induced by water swim stress in the formalin test has been investigated. Water swim stress at 20 degrees C temperature induced antinociception in both phases of the formalin test. Intraperitoneal administration of the D2 dopamine receptor antagonist, sulpiride (25 and 50 mg/kg) reduced swim stress-induced antinociception in the second phase of the formalin test. A higher dose of the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, intraperitoneal) also reduced swim stress-induced antinociception in both phases of the test. Exposure to 3 min water swimming stress, once daily for 3 days, induced tolerance to swim stress-induced antinociception in the second phase of the formalin test. Administration of sulpiride (12.5, 25 and 50 mg/kg), during exposure to water swimming stress (once daily for 3 days), decreased tolerance in the second phase, whereas the antagonist (12.5 and 50 mg/kg) increased pain scores in the first phase of the formalin test. Sulpiride (25 mg/kg) treatment however, once daily for 3 days with no water swimming stress, did not alter swim stress-induced antinociception (0.5, 1 and 3 min tests). Similarly, repeated treatment with SCH23390 (0.05 mg/kg) and water swimming stress did not alter tolerance induced by water swimming stress. Repeated administration of the antagonist in the absence of water swimming stress also did not change swim stress-induced antinociception. The results may indicate a possible involvement of both dopamine D1 and D2 receptors in the antinociception induced by swim stress and D2 receptor mechanism in the tolerance induced by repeated swim stress.     

Endogenous opioid mechanisms partially mediate P2X3/P2X2/3-related antinociception in rat models of inflammatory and chemogenic pain but not neuropathic pain

P2X3/P2X2/3 receptors have emerged as important components of nociception. However, there is limited information regarding the neurochemical systems that are affected by antagonism of the P2X3/P2X2/3 receptor and that ultimately contribute to the ensuing antinociception. In order to determine if the endogenous opioid system is involved in this antinociception, naloxone was administered just prior to the injection of a selective P2X3/P2X2/3 receptor antagonist, A-317491, in rat models of neuropathic, chemogenic, and inflammatory pain. Naloxone (1-10 mg kg(-1), i.p.), dose-dependently reduced the antinociceptive effects of A-317491 (1-300 micromol kg(-1), s.c.) in the CFA model of thermal hyperalgesia and the formalin model of chemogenic pain (2nd phase), but not in the L5-L6 spinal nerve ligation model of neuropathic allodynia. In comparison experiments, the same doses of naloxone blocked or attenuated the actions of morphine (2 or 8 mg kg(-1), s.c.) in each of these behavioral models. Injection of a peripheral opioid antagonist, naloxone methiodide (10 mg kg(-1), i.p.), did not affect A-317491-induced antinociception in the CFA and formalin assays, suggesting that the opioid component of this antinociception occurred within the CNS. Furthermore, this utilization of the central opioid system could be initiated by antagonism of spinal P2X3/P2X2/3 receptors since the antinociceptive actions of intrathecally delivered A-317491 (30 nmol) in the formalin model were reduced by both intrathecally (10-50 nmol) and systemically (10 mg kg(-1), i.p.) administered naloxone. This utilization of the opioid system was not specific to A-317491 since suramin-, a nonselective P2X receptor antagonist, induced antinociception was also attenuated by naloxone. In in vitro studies, A-317491 (3-100 microM) did not produce any agonist response at delta opioid receptors expressed in NG108-15 cells. A-317491 had been previously shown to be inactive at the kappa and mu opioid receptors. Furthermore, naloxone, at concentrations up to 1 mM, did not compete for [3H] A-317491 binding in 1321N1 cells expressing human P2X3 receptors. Taken together, these results indicate that antagonism of spinal P2X3/P2X2/3 receptors results in an indirect activation of the opioid system to alleviate inflammatory hyperalgesia and chemogenic nociception.