Mechanisms of L-NG-nitro arginine methyl ester-induced antinociception in mice: a role for serotonergic and adrenergic neurons.

1. The mechanisms involved in the antinociceptive action of L-NG-nitro arginine methyl ester (L-NAME) were investigated in mice. 2. Intraperitoneal administration of L-NAME produced a dose-dependent antinociception in the tail-flick, hot-plate and phenyl-p-quinone-induced writhing tests. 3. Pretreatment with the catecholamine depletors 6-hydroxydopamine (5 micrograms i.c.v.) or reserpine (5 mg/kg i.p.) or the serotonin synthesis inhibitor, p-chlorophenylalanine methyl ester (200 mg/kg i.p. on 2 consecutive days) resulted in a significant decrease in the antinociceptive effect of L-NAME. 4. Similarly, pretreatment with the selective alpha 1-adrenoceptor antagonist prazonin (2.5 mg/kg, i.p.), or the non-selective alpha-adrenoceptor blocker, phentolamine (5 mg/kg, i.p.) antagonized the antinociceptive effect of L-NAME. 5. However, the administration of the selective alpha 2-adrenoceptor antagonist, idazoxan (2.5 mg/kg i.p.) was without effect. 6. Likewise, pretreatment with the serotonin 5-HT2 receptor blocker, ketanserin (1 mg/kg, i.p.), the D2 dopamine receptor antagonist (+/-) sulpiride (30 mg/kg, i.p.) or the opioid antagonist naloxone (5 mg/kg, i.p.) did not inhibit the antinociceptive effect of L-NAME. 7. These results suggest that L-NAME produces antinociception in the mouse probably by an action on adrenergic and serotonergic synapses.     

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.     

Antinociceptive effects of the novel spirocyclopiperazinium salt compound LXM-10 in mice

The compound LXM-10 (2,4-dimethyl-9-beta-phenylethyl-3-oxo-6, 9-diazaspiro [5.5]undecane chloride) is a new spirocyclopiperazinium salt compound. This is the first article to evaluate its antinociceptive effect in the abdominal constriction test induced by acetic acid and the hot-plate test. In the abdominal constriction test, LXM-10 had a significant dose-response effect, and the maximal inhibition ratio was 79.2%. In the hot-plate test, LXM-10 had significant dose-response and time-response effects. The antinociceptive effect began at 1.0 h, peaked at 2.0 h, and persisted 3.0 h after s.c. administration. The hot-plate latency was increased by 126.8% at the dose of 12.0 mg/kg. The antinociceptive effect of LXM-10 was blocked by mecamylamine (a central and peripheral neuronal nicotinic acetylcholine receptor antagonist, 0.25, 0.5, 1.0 mg/kg, i.p.), hexamethonium (a peripheral neuronal nicotinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.), atropine (a central and peripheral muscarinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.), and atropine methylnitrate (a peripheral muscarinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.) in a dose-dependent fashion. In contrast, the effect was not blocked by naloxone (a non-selective opioid receptor antagonist, 2.0 mg/kg, i.p.) or yohimbine (a alpha(2)-adrenergic receptor antagonist, 1.0, 2.5, 5.0 mg/kg, i.p.) in the hot-plate test. Therefore, the antinociceptive effects of LXM-10 involve the peripheral neuronal nicotinic and muscarinic acetylcholine receptors; they are not related to opioid receptors or alpha(2)-adrenergic receptors. LXM-10 did not affect motor coordination, spontaneous activity, or body temperature. These findings with LXM-10 suggest that spirocyclopiperazinium derivatives could provide insight on new analgesics.     

Antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, in mice

The antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, were examined in mice. The p.o. administration of N. sativa oil (50-400 mg/kg) dose-dependently suppressed the nociceptive response in the hot-plate test, tail-pinch test, acetic acid-induced writhing test and in the early phase of the formalin test. The systemic administration (2.5-10 mg/kg, p.o. and 1-6 mg/kg, i.p.) and the i.c.v. injection (1-4 microgram/mouse) of thymoquinone attenuated the nociceptive response in not only the early phase but also the late phase of the formalin test. Naloxone injected s.c. (1 mg/kg) significantly blocked N. sativa oil- and thymoquinone-induced antinociception in the early phase of the formalin test. Moreover, the i.c.v. injection of naloxone (10 microgram/mouse), the mu(1)-opioid receptor antagonist, naloxonazine (1-5 microgram/mouse), or the kappa-opioid receptor antagonist, nor-binaltorphimine (1-5 microgram/mouse), significantly reversed thymoquinone-induced antinociception in the early phase but not the late phase of the formalin test, whereas the delta-opioid receptor antagonist, naltrindole (1-5 ng/mouse, i.c.v.), had no effect on either phase. The antinociceptive effect of morphine was significantly reduced in thymoquinone- and N. sativa oil-tolerant mice, but not vice versa. These results suggest that N. sativa oil and thymoquinone produce antinociceptive effects through indirect activation of the supraspinal mu(1)- and kappa-opioid receptor subtypes.     

Opioid, cannabinoid CB1 and NOP receptors do not mediate APAP-induced hypothermia in rats

Acetaminophen (APAP) produces antinociception and hypothermia. Because the antinociceptive effect in rats is partially dependent on opioid and cannabinoid CB₁ receptor activation, we determined if activation of these receptors also contributes to the hypothermic effect of APAP. Rats injected with APAP (100, 250, 375 or 500 mg/kg, i.p.) displayed dose-related hypothermia. For combined administration, the hypothermic effect of APAP (400 mg/kg, i.p.) was not altered by pretreatment with: naltrexone (10 mg/kg, s.c.), a non-selective opioid antagonist; naltrindole (1 mg/kg, s.c.), a delta opioid antagonist; nor-binaltorphimine (10 mg/kg, i.p.), a kappa opioid antagonist; SR 141716A (3 mg/kg, i.m.), a cannabinoid CB₁ receptor antagonist; or JTC-801(1 mg/kg, i.p.), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist. The demonstration that APAP produces hypothermia independent of opioid, cannabinoid CB₁ or NOP receptor activation is contrary to its antinociceptive effect, which requires opioid and cannabinoid CB₁ receptor activation.     

Role of muscarinic receptor subtypes in central antinociception.

1. The ability to modify the pain threshold by the two M1-muscarinic agonists: McN-A-343 and AF-102B and by the specific M2-agonist arecaidine was examined in mice and rats by using three different noxious stimuli: chemical (writhing test), thermic (hot-plate test) and mechanical (paw pressure test). 2. In the mouse hot-plate test McN-A-343 (20-50 micrograms per mouse i.c.v.) and AF-102B (1-10 mg kg-1 i.p.) produced significant antinociception which was prevented by atropine (1 microgram per mouse i.c.v.) and by the two selective M1 antagonists: pirenzepine (0.01 micrograms per mouse i.c.v.) and dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by the specific M2-antagonist AFDX-116 (0.1 micrograms per mouse i.c.v.), naloxone (1 mg kg-1 i.p.) or by the acetylcholine (ACh) depletor hemicholinium-3 (HC-3) (1 micrograms per mouse i.c.v.). McN-A-343 and AF-102B were able to increase the pain threshold also in the mouse acetic acid writhing test and in rat paw pressure test. These antinociceptive effects were completely prevented by dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by AFDX-116 (0.1 microgram per mouse or rat i.c.v.). 3. In contrast with the M1-agonists, the M2-agonist arecaidine (0.1-2 micrograms per mouse or rat i.c.v.) did not induce antinociception in all three analgesic tests. However, arecaidine, at the same i.c.v. doses, was able to reduce the pain threshold in the hot-plate and paw pressure tests.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antinociceptive activity of 3-O-beta-d-glucopyranosyl-23,24-dihydrocucurbitacin F from Hintonia standleyana (Rubiaceae)

Oral administration of a MeOH-CH(2)Cl(2) (1:1) extract of the stem bark of Hintonia standleyana (HSE) produced a dose-dependent antinociceptive effect when tested in mice using the writhing (150-750 mg/kg) and the hot-plate (150-600 mg/kg) models. From the active extract 3-O-beta-d-glucopyranosyl-23,24-dihydrocucurbitacin F (GDHCF), 5-O-[beta-d-apiofuranosyl-(1-->6)-beta-d-glucopyranosyl]-7-methoxy-3',4'-dihydroxy-4-phenylcoumarin (AG4-PC) and desoxycordifolinic acid (DCA) were isolated. GDHCF (10-100 mg/kg, p.o.) significantly reduced acetic acid-induced abdominal contortions and increased the hot-plate latency in comparison to vehicle-treated mice. Metamizol (50-100 mg/kg) and morphine (2.5-5 mg/kg) were used as positive controls, respectively. GDHCF-induced antinociception was partially blocked by naloxone (1 mg/kg, i.p.), l-NAME (150 mg/kg, i.p.) and glibenclamide (10mg/kg, i.p.) suggesting that its pharmacological effect could be due to the activation of the nitric oxide pathway, followed by the opening of the ATP-sensitive K(+) channels, as well as an activation of the opioid receptors.     

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.     

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.     

L-arginine exerts a dual role in nociceptive processing in the brain: involvement of the kyotorphin-Met-enkephalin pathway and NO-cyclic GMP pathway.

1. Intracerebroventricular (i.c.v.) administration of L-arginine (L-Arg), at 10-100 micrograms per mouse, produced antinociception in mice, as assessed by the tail flick test; this antinociception was reversed by pretreatment (s.c.) with naltrindole (NTI), a delta-selective opioid antagonist, and by co-administered L-leucyl-L-arginine (Leu-Arg), a kyotorphin (endogenous Met-enkephalin releaser) receptor antagonist. 2. L-NG-nitroarginine methyl ester (L-NAME), a NO synthase inhibitor, but not D-NG-nitroarginine methyl ester, given i.c.v. at 3-10 micrograms per mouse, exhibited antinociceptive activity that was resistant to naloxone (s.c.), NTI (s.c.) and Leu-Arg (i.c.v.). 3. The L-NAME (i.c.v.)-induced antinociception was not reversed by L-Arg (i.c.v.), which was antinociceptive by itself, but was abolished by combined injection of L-Arg plus Leu-Arg (i.c.v.) or by L-Arg (i.c.v.) after NTI (s.c.). 4. Methylene blue (MB), a soluble guanylate cyclase inhibitor, at 0.1-1 microgram per mouse, produced antinociception by i.c.v. administration. The antinociception induced by MB (i.c.v.) or L-NAME (i.c.v.) was reversed by co-administered dibutyryl cyclic GMP. 5. These findings suggest that L-Arg plays a dual role in nociceptive processing in the brain, being antinociceptive via the kyotorphin-Met-enkephalin pathway and nociceptive via the NO-cyclic GMP pathway.     

The involvement of endogenous opioid mechanisms in the antinociceptive effects induced by antidepressant drugs, desipramine and trimipramine

The present study was designed to investigate the involvement of endogenous opioid systems in the antinociception induced by the antidepressant drugs, desipramine and trimipramine. For this purpose, the antinociceptive effects of desipramine (7.5 and 15.0 mg/kg i.p.) and trimipramine (5.0 and 10.0 mg/kg i.p.) were compared to that induced by morphine (0.2 and 2.0 mg/kg i.p.) in the tail-clip model in mice. Naloxone (0.3 and 3.0 mg/kg i.p.), a non-specific opioid receptor antagonist, inhibited morphine-induced antinociception in mice, whereas the antinociceptive effects of antidepressant drugs were found to be resistant to naloxone blockade to some extent, since only the higher concentration of naloxone (3.0 mg/kg i.p.) caused significant inhibition of the effects of antidepressant drugs. In contrast, naltrindole (1.0 mg/kg i.p.), a specific delta-receptor antagonist, inhibited antinociception induced by desipramine and trimipramine in this test, while it inhibited the antinociceptive effect of morphine only partly. None of the opioid antagonists produced a significant effect in the tail-clip experiment when they were injected alone. Based on these findings, we concluded that endogenous opioids are involved in the antinociceptive effects of the antidepressant drugs using different mechanisms.     

Implication of the descending dynorphinergic neuron projecting to the spinal cord in the (+)-matrine- and (+)-allomatrine-induced antinociceptive effects

We previously reported that either (+)-matrine (matridin-15-one) or (+)-allomatrine (the C-6 epimer of matrine)-induced antinociceptive effect was attenuated by s.c. pretreatment with a kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI), indicating the critical role of KORs in antinociceptive effects induced by these alkaloids. In the present study, we found that i.c.v. administration of either (+)-matrine- or (+)-allomatrine induced antinociceptive effects in the mouse tail-flick and warm-plate test, whereas these alkaloids when given spinally failed to induce antinociception. In the guanosine-5'-O-(3-[(35)S]thio)trisphosphate ([(35)S]GTPgammaS) binding assay, we demonstrated that neither (+)-matrine nor (+)-allomatrine produced the stimulation of [(35)S]GTPgammaS binding in the membranes of the spinal cord, indicating that (+)-matrine- and (+)-allomatrine-induced supraspinal antinociceptive actions was not due to a direct stimulation of KORs by these alkaloids. Therefore, we next investigated the involvement of dynorphin A (1-17) release at the spinal or supraspinal site in (+)-matrine- or (+)-allomatrine-induced antinociception. The i.c.v. pretreatment with an antiserum against dynorphin A (1-17) could not affect the antinociceptive effect induced by s.c. treatment of (+)-matrine. In contrast, the s.c.-administered (+)-matrine- and (+)-allomatrine-induced antinociceptive effect was significantly attenuated by i.t. pretreatment of an antiserum against dynorphin A (1-17). The present data suggest that either (+)-matrine or (+)-allomatrine when given i.c.v. may stimulate the descending dynorphinergic neuron, resulting in the stimulation of KORs in the spinal cord, and this phenomenon in turn produces the antinociception in mice.     

Desensitization of mu-opioid receptors does not modify the analgesia induced by an enkephalinase inhibitor.

Acetorphan, an enkephalinase inhibitor, or morphine was injected in mice which had received saline or morphine (32 mg/kg s.c. twice a day on 8 consecutive days) chronically. In the hot-plate test, the analgesia (increase in jump latency) induced by morphine (2 mg/kg i.p.) or by the mu selective opioid agonist, [D-Ala2,N-Me-Phe4, Gly5-ol]enkephalin (DAGO) (1.5, 3 or 6 ng/mouse i.c.v.), was significant in the saline group but was strongly decreased in morphine-pretreated mice. In contrast the analgesic effect of acetorphan (5 mg/kg i.v.) or of the delta selective opioid agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.75, 1.5 or 3 micrograms/mouse i.c.v.) was similar in both groups. These results suggest that the enkephalins protected by acetorphan act on the delta receptor site to produce antinociception.     

Participation Of Central and Peripheral κ1 and κ2 Opioid Receptors In Arrhythmogenesis

1. The kappa 1 and kappa 2 opioid receptor agonists U-62066 (8 mg/kg, i.p.) and (-)-bremazocine (0.7 mg/kg, i.v.), respectively, both exhibit anti-arrhythmic properties against adrenaline-induced dysrhythmias in rats. 2. In contrast, (+)-bremazocine has no effect on adrenaline-induced dysrhythmias. 3. The kappa 1 opioid receptor agonists U-50488 (110 nmol) and [D-Ala2]-dynorphin A (20 nmol) and the kappa 2 opioid receptor agonist (-)-bremazocine (30 nmol) exhibit pro-arrhythmic properties following intracerebroventricular administration. 4. Prior administration of the kappa opioid receptor antagonist nor-binaltorphimine doses i.c.v. (14 nmol), i.p. (10 mg/kg), completely abolishes the pro-arrhythmic (BNI, i.c.v., 14 nmol) as well as anti-arrhythmic (BNI, 10 mg/kg, i.p.) effects of the kappa opioid receptor agonists. 5. Neither hexamethonium (10 mg/kg, i.v.) nor atropine (1 mg/kg, i.v.) have any effect on the anti-arrhythmic actions of the kappa 1 opioid receptor agonist U-62066 following systemic administration. 6. It is suggested that the anti-arrhythmic effects of U-62066 and (-)-bremazocine are associated with the activation of peripheral kappa opioid receptors and do not depend on the activation of kappa opioid receptors in the autonomic nervous system.     

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.     

Buspirone-induced antinociception is mediated by <Emphasis Type="SmallCaps">l</Emphasis>-type calcium channels and calcium/caffeine-sensitive pools in mice

Rationale. Previous studies have shown that buspirone, a partial 5-HT_1A receptor agonist, produces antinociceptive effects in rats and mice; Ca²⁺ plays a critical role as a second messenger in mediating nociceptive transmission. 5-HT_1A receptors have been proven to be coupled functionally with various types of Ca²⁺ channels in neurons, including N-, P/Q-, T-, or L-type. It was of interest to investigate the involvement of extracellular/intracellular Ca²⁺ in buspirone-induced antinociception. Objectives. To determine whether central serotonergic pathways participate in the antinociceptive processes of buspirone, and investigate the involvement of Ca²⁺ mechanisms, particularly L-voltage-gated Ca²⁺ channels and Ca²⁺/caffeine-sensitive pools, in buspirone-induced antinociception. Methods. Antinociception was assessed using the hot-plate test (55°C, hind-paw licking latency) in mice treated with either buspirone (1.25–20 mg/kg i.p.) alone or the combination of buspirone and fluoxetine (2.5–10 mg/kg i.p.), 5-HTP (25 mg/kg i.p.), nimodipine (2.5–10 mg/kg i.p.), nifedipine (2.5–10 mg/kg i.p.), CaCl₂ (25–200 nmol per mouse i.c.v.), EGTA (5–30 nmol per mouse i.c.v.), or ryanodine (0.25–2 nmol per mouse i.c.v.). Results. Buspirone dose dependently increased the licking latency in the hot-plate test in mice. This effect of buspirone was enhanced by fluoxetine, 5-HTP, nimodipine, and nifedipine. Interestingly, central administration of Ca²⁺ reversed the antinociceptive effects of buspirone. In contrast to these, ryanodine or EGTA administered centrally potentiated buspirone-induced antinociception. Conclusions. Decreasing neuronal Ca²⁺ levels potentiated buspirone-induced antinociception; conversely, increasing intracellular Ca²⁺ abolished the antinociceptive effects of buspirone. These results suggest that Ca²⁺ influx from extracellular fluid and release of Ca²⁺ from Ca²⁺/caffeine-sensitive microsomal pools may be involved in buspirone-induced antinociception. Electronic Publication     

Antinociception induced by systemic administration of local anaesthetics depends on a central cholinergic mechanism.

1 The antinociceptive effects of systemically-administered procaine, lignocaine and bupivacaine were examined in mice and rats by using the hot-plate, writhing and tail flick tests. 2 In both species all three local anaesthetics produced significant antinociception which was prevented by atropine (5 mg kg-1, i.p.) and by hemicholinium-3 (1 microgram per mouse, i.c.v.), but not by naloxone (3 mg kg-1, i.p.), alpha-methyl-p-tyrosine (100 mg kg-1, s.c.), reserpine (2 mg kg-1, i.p.) or atropine methylbromide (5.5 mg kg-1, i.p.). 3 Atropine (5 mg kg-1, i.p.) which totally antagonized oxotremorine (40 micrograms kg-1, s.c.) antinociception did not modify morphine (5 mg kg-1, s.c.) or baclofen (4 mg kg-1, s.c.) antinociception. On the other hand, hemicholinium, which antagonized local anaesthetic antinociception, did not prevent oxotremorine, morphine or baclofen antinociception. 4 Intracerebroventricular injection in mice of procaine (200 micrograms), lignocaine (150 microgram) and bupivacaine (25 micrograms), doses which were largely ineffective by parenteral routes, induced an antinociception whose intensity equalled that obtainable subcutaneously. Moreover, the i.c.v. injection of antinociceptive doses did not impair performance on the rota-rod test. 5 Concentrations below 10(-10) M of procaine, lignocaine and bupivacaine did not evoke any response on the isolated longitudinal muscle strip of guinea-pig ileum, or modify acetylcholine (ACh)-induced contractions. On the other hand, they always increased electrically-evoked twitches. 6 The same concentrations of local anaesthetics which induced antinociception did not inhibit acetylcholinesterase (AChE) in vitro. 7 On the basis of the above findings and the existing literature, a facilitation of cholinergic transmission by the local anaesthetics is postulated; this could be due to blockade of presynaptic muscarinic receptors.     

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.     

Antinociceptive effect of novel trihalomethyl-substituted pyrazoline methyl esters in formalin and hot-plate tests in mice

The aim of the present study was to evaluate the antinociceptive potential of four novel pyrazoline methyl ester compounds on chemical and thermal models of pain in mice. The following 5-trihalomethylated-4,5-dihydro-1H-pyrazole methyl ester compounds were tested: 3-methyl-5-trifluoromethyl-(MPF3), 4-methyl-5-trifluoromethyl-(MPF4), 3-methyl-5-trichloromethyl-(MPCl3) and 4-methyl-5-trichloromethyl-(MPCl4). MPF3, MPF4, MPCl3 and MPCl4 (0.03-1.0 mmol/kg) given intraperitoneally decreased neurogenic and inflammatory phases of nociception in the formalin test. Moreover, MPF3, MPF4, MPCl3, MPCl4 (0.1-1.0 mmol/kg) and dipyrone (1.5 mmol/kg) also produced a dose-dependent antinociceptive effect in the hot-plate test. However, MPF3, MPF4, MPCl3 and MPCl4 did not impair motor coordination in the rotarod test or spontaneous locomotion in the open field test. The antinociceptive effect of MPF4 (1.0 mmol/kg, i.p.) was reversed by the opioid receptor antagonist naloxone (2 mg/kg, i.p.), but not by the alpha(2)-adrenergic receptor antagonist yohimbine (0.15 mg/kg, i.p.) or by p-chlorophenylalanine ethyl ester (PCPA, 300 mg/kg, i.p.) treatment. In contrast to morphine (5 mg/kg, i.p.), MPF4 given daily for up to 8 days did not generate a tolerance to its antinociceptive effect. However, similar to morphine (11 mg/kg, i.p.), MPF4 reduced gastrointestinal transit in mice. Taken together these results demonstrate that these novel pyrazoline methyl esters tested may be promising prototypes of additional mild analgesics.     

Opioid pharmacology of the antinociceptive effects of loperamide in mice

Loperamide (0.1-3.2mg/kg i.p.) produced dose-dependent and complete suppression of writhing in the acetic acid-induced writhing assay in mice. Naltrexone (NTX; 0.1-10.0mg/kg s.c.) and its N-methylated derivative quaternary naltrexone (QNTX; 1.0 and 10.0mg/kg s.c.) were roughly equipotent in antagonizing the antinociceptive effects of loperamide. In contrast, NTX was approximately 100-fold more potent than QNTX in antagonizing the antinociceptive effects of the classical mu agonist morphine. Furthermore, the antinociceptive effects of loperamide were not antagonized by central administration of the selective mu antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP; 300ng i.c.v.), or by systemic administration of either the kappa selective antagonist nor-binaltorphimine (nor-BNI; 32.0mg/kg s.c.), or the delta antagonist naltrindole (NTI; 10.0mg/kg s.c.). These doses of CTAP, nor-BNI and NTI were effective antagonists of morphine, the kappa agonist U69,593 and the delta agonist BW 373U86 [(+/-)-4-((R*)-a-((2S*5R*)-4-allyl-2,5-dimethyl-1- piperazinal)-3-hydroxybenzyl)-N, N-diethylbenzamide dihydrochloride], respectively. These results indicate that the antinociceptive effects of loperamide in mice are mediated, at least in part, by opioid receptors; however, these receptors are distinct from the opioid receptors mediating the effects of morphine, U69,593 and BW 373U86. These results are consistent with the hypothesis that loperamide produces its antinociceptive effects by acting, at least in part, at peripheral opioid receptors.