Naloxone blocks the beneficial effects of aqueous extract of Murraya koenigii (L.) Spreng leaves in models of pain

AIM

This study investigated the antinociceptive effects of aqueous extract of Murraya koenigii (AEMK) leaves (200, 400 and 800 mg/kg, orally) on animal models of acute and persistent pain and its modulation by naloxone.

MATERIALS AND METHODS

Antinociceptive effects were assessed using tail-flick, hot plate and formalin tests in mice. To differentiate between central and peripheral antinociceptive effect of AEMK, naloxone (2 mg/kg) was administered along with the 800 mg/kg dose of extract. Morphine was used as a standard drug.

RESULTS

AEMK and morphine significantly increased the tail-flick latency (tfl) and paw licking/jumping latency in tail-flick and hot plate tests, respectively, in comparison to control. Also, in both the tests AEMK and morphine significantly increased the AUC0-120 min. In formalin test, AEMK (400 mg/kg and 800 mg/kg) and morphine significantly reduced licking time in both early and late phases in comparison to control.

CONCLUSIONS

Thus, in all three pain models AEMK showed antinociceptive effect, which was blocked by naloxone suggesting the involvement of opioidergic central mechanism.

Evaluation of anti-nociceptive, anti-inflammatory and antipyretic activities of Artemisia scoparia hydromethanolic extract

ETHNOPHARMACOLOGICAL RELEVANCE

Artemisia scoparia (redstem wormwood) locally known as jhahoo or jaukay, is traditionally used in pain, inflammation and febrile conditions. So far, little or no scientific work has been reported to validate its folk uses in the alleviation of pain, fever and inflammation. The present study was designed to explore the analgesic, anti-inflammatory and antipyretic effects of the Artemisia scoparia hydromethanolic extract (ASHME), and to validate its traditional use in Asia.

MATERIALS AND METHODS

This study made use of thermal (hot plate induced) and chemical (acetic acid induced) nociception models in mice. In addition, the mechanism of antinociception in hot plate test was further evaluated in the presence of caffeine (10mg/kg), naloxone (2mg/kg) and monosodium glutamate (1g/kg). While carrageenan induced rat paw edema and yeast induced mouse pyrexia models were used to test the anti-inflammatory and antipyretic activities.

RESULTS

Administration of single intraperitoneal doses (400mg/kg and 800 mg/kg) of ASHME significantly reduced the carrageenan induced paw edema in rats (P<0.05, P<0.001) by 54% and 74%, increased the thermal nociception time in the hot plate test up to 2- and 2.5-fold (P<0.01, P<0.001), inhibited the acetic acid induced writhings in mice by 41.12% and 61.53% (P<0.001), and attenuated the yeast induced pyrexia in mice by nearly 74% and 90% respectively (P<0.01, P<0.001). Caffeine (10mg/kg), naloxone (2mg/kg) and monosodium glutamate (1g/kg) significantly (P<0.001) abolished the anti-nociceptive response of ASHME (400mg/kg).

CONCLUSION

These findings suggest that the Artemisia scoparia hydromethanolic extract of ASHME possesses anti-nociceptive, anti-inflammatory and antipyretic potentials, which support its use, for the said conditions, in traditional medicine and should be further exploited for its use in clinical medicine.

Physiological basis for inhibition of morphine and improgan antinociception by CC12, a P450 epoxygenase inhibitor

Many analgesic drugs, including μ-opioids, cannabinoids, and the novel nonopioid analgesic improgan, produce antinociception by actions in the rostral ventromedial medulla (RVM). There they activate pain-inhibiting neurons, termed "OFF-cells," defined by a nociceptive reflex-related pause in activity. Based on recent functional evidence that neuronal P450 epoxygenases are important for the central antinociceptive actions of morphine and improgan, we explored the convergence of opioid and nonopioid analgesic drug actions in RVM by studying the effects of the P450 epoxygenase inhibitor CC12 on the analgesic drug-induced activation of these OFF-cells and on behavioral antinociception. In rats lightly anesthetized with isoflurane, we recorded the effects of intraventricular morphine and improgan, with and without CC12 pretreatment, on tail flick latency and activity of identified RVM neurons: OFF-cells, ON-cells (pronociceptive neurons), and neutral cells (unresponsive to analgesic drugs). CC12 pretreatment preserved reflex-related changes in OFF-cell firing and blocked the analgesic actions of both drugs, without interfering with the increase in spontaneous firing induced by improgan or morphine. CC12 blocked suppression of evoked ON-cell firing by improgan, but not morphine. CC12 pretreatment had no effect by itself on RVM neurons or behavior. These data show that the epoxygenase inhibitor CC12 works downstream from receptors for both μ-opioid and improgan, at the inhibitory input mediating the OFF-cell pause. This circuit-level analysis thus provides a cellular basis for the convergence of opioid and nonopioid analgesic actions in the RVM. A presynaptic P450 epoxygenase may therefore be an important target for development of clinically useful nonopioid analgesic drugs.

In vivo characterization of (-)(-)MCL-144 and (+)(-)MCL-193: isomeric, bivalent ligands with mu/kappa agonist properties

Once opioid receptor dimers were postulated, a goal has been to synthesize and screen novel opioids, with the hope of furthering our knowledge of the structure-activity relationship of opioid ligands with the opioid receptors. The aim of the current study was to address whether two isomeric bivalent ligands would have pharmacological differences after central administration, in vivo. The two compounds, (-) bis(N-cyclobutylmethyl-morphinan-3-yl) sebacoylate dihydrochloride (MCL-144) and 1-((+)N-cyclobutylmethylmorphinan-3-yl)-10-((-) N-cyclobutylmethylmorphinan-3-yl)sebacolyate (MCL-193) are each linked by a 10-carbon chain ester. The active (-) enantiomer for both ligands is 3-hydroxy-N-cyclobutylmethyl morphinan ((-)MCL-101), a N-cyclobutylmethyl analogue of cyclorphan (J Med Chem 43:114-122, 2000). MCL-144 contains two active levo rotatory (-)(-) pharmacophores, while MCL-193 contains one active (-) and one inactive (+) pharmacophore of MCL-101. In vitro analysis demonstrated that all three compounds, (-)(-)MCL-144, (+)(-)MCL-193 and (-)MCL-101 were kappa agonists and mu partial agonists. (-)(-)MCL-144 and (-)MCL-101 had much higher affinity for both the mu and kappa opioid receptors compared to (+)(-)MCL-193. In vivo, (-)(-)MCL-144 and (+)(-)MCL-193 produced full dose-response curves, in the 55 degrees C tail-flick test, with each compound having an ED(50) value of 3.0 nmol after intracerebroventricular (i.c.v.) administration. The analgesic properties of both compounds were antagonized by the mu-selective antagonist, beta-funaltrexamine and the kappa-selective antagonist nor-binaltorphimine. Concomitant, i.c.v., administration of either (-)(-)MCL-144 or (+)(-)MCL-193 with morphine, did not significantly antagonize morphine-induced antinociception at any dose tested. In antinociceptive tests, (-)(-)MCL-144 and (+)(-)MCL-193 had the same pharmacological properties, demonstrating that having two active pharmacophores separated by a 10-carbon spacer group did not increase the antinociceptive efficacy of the compound. Additionally, it was also of interest to compare (-)(-)MCL-145 and (-)(-)MCL-144, as the only difference between these bivalent ligands is the spacer region connecting the two pharmacophores, yet (-)(-)MCL-145 produced an ED(50) value 10-fold lower than (-)(-)MCL-144 (ED(50) values = 0.3 nmol and 3.0 nmol, respectively).

Enterostatin (APGPR) suppresses the analgesic activity of morphine by a CCK-dependent mechanism

Enterostatin (APGPR) found in the gastrointestinal tract and brain is an anorectic pentapeptide. We found that APGPR inhibited morphine-induced analgesia after intracerebroventricular administration in mice at a dose of 10nmol/mouse. The anti-analgesic effect of APGPR was inhibited by pretreatment with lorglumide and LY225910, antagonists for cholecystokinin 1 (CCK1) and cholecystokinin 2 (CCK2) receptors, respectively. The anti-analgesic effect of APGPR may be mediated by CCK release, since APGPR does not have affinity for CCK receptors.

[Analgesic effect of interleukin-2 in mouse models of spared nerve injury]

OBJECTIVE

To investigate the analgesic effect of interleukin-2 (IL-2) in mice with spared nerve injury (SNI).

METHOD

IL-2 was intraperitoneally injected in mice with induced SNI, and von Frey Filaments test and cold plate test were carried out to accesses the analgesic effects of IL-2 and the effect of naloxone in antagonizing the effects of IL-2.

RESULTS

IL-2 produced analgesic effects against hyperalgesia and allodynia in mouse models of SNI, and the effect of IL-2 lasted for more than 24 h, showing a double-peak pattern in its action with the two peaks occurring at 30 and 105 min, respectively. The effect of IL-2 could be significantly antagonized by naloxone.

CONCLUSIONS

IL-2 has long-lasting analgesic effects in mouse models of SNI model, showing a double-peak pattern of its action. The analgesic effect of IL-2 is probably mediated by opiate receptor.

Antagonism of antinociceptive effect of hydro-ethanolic extract of Hypericum perforatum Linn. by a non selective opioid receptor antagonist, naloxone

Hydro-ethanolic crude extract of Hypericum perforatum Linn. family hypericaceae (St. John's Wort) aerial parts (Hp. Cr) was studied for its possible antinociceptive effect against acetic acid-induced abdominal constriction assay in mice. Hp. Cr (10-20 mg kg(-1)), opium (10-30 mg kg(-1)), morphine (0.75-3.0 mg kg(-1)) and aspirin (50-100 mg kg(-1)) showed dose-dependent antinociceptive effect. In animals treated with naloxone (0.5 mg kg(-1)), the antinociceptive effect of Hp. Cr was significantly reduced similar to that of opium, while effect of aspirin remained unchanged. These results suggest that the antinociceptive effect of Hypericum perforatum may be mediated through activation of opioid receptors.

Activation of opioid receptor like-1 receptor in the spinal cord produces sex-specific antinociception in the rat: estrogen attenuates antinociception in the female, whereas testosterone is required for the expression of antinociception in the male

Sex-related differences in the perception and modulation of pain have been reported. The present study is the first to investigate systematically whether activation of opioid receptor-like 1 receptor (ORL1) by orphanin FQ (OFQ) produces sex-specific modulation of spinal nociception and whether estrogen or testosterone contributes to these differences using the rat as an experimental animal. Two behavioral models, the NMDA and heat-induced nociceptive tests, were used to examine sex-specific modulation of spinal nociception. Intrathecal microinjection of OFQ in male, ovariectomized (OVX), and diestrous rats produced a significant antinociceptive effect on both tests. However, OFQ failed to produce antinociception in proestrous rats, the phase of the estrous cycle with the highest levels of circulating estradiol, and produced a dose-dependent effect in OVX females treated with 1 ng to 100 microg of estradiol. The antinociceptive effects of OFQ were dose dependent in male and OVX animals and were reversibly antagonized by UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ(1-13)-NH2), an ORL1 receptor-selective antagonist. Interestingly, OFQ was ineffective in gonadectomized (GDX) males, whereas testosterone replacement restored the antinociceptive effect of OFQ in GDX males. We conclude that OFQ produces sex-specific modulation of spinal nociception; estrogen attenuates antinociception in the female in parallel with normal cycling of estrogen levels, and testosterone is required for the expression of antinociception in the male; thus, the sensitivity of the male to the antinociceptive effects of OFQ is not simply attributable to the intrinsically low estrogen levels in these animals.

Involvement of mu- and delta-opioid receptors in the antinociceptive effects induced by AMPA receptor antagonist in the spinal cord of rats

The present study was performed to explore the involvement of opioid receptors in the antinociception induced by a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist in rats. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by hot plate test and the Randall Selitto Test. Intrathecal injection of 20 nmol of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) disodium, a competitive AMPA receptor antagonist, increased significantly the HWLs to both thermal and mechanical stimulation in rats. The increased HWLs induced by NBQX were dose-dependently attenuated by the opioid receptor antagonist naloxone, while naloxone itself had no marked influences on the HWL of rats. Furthermore, the increased HWLs induced by NBQX were inhibited by the mu-opioid antagonist beta-funaltrexamine (beta-FNA) or the delta-opioid antagonist naltrindole, but not by the kappa-opioid antagonist nor-binaltorphimine (nor-BNI). The results suggest that mu- and delta-opioid receptors, not kappa-opioid receptor, are involved in the antinociception induced by AMPA antagonist in the spinal cord of rats.

The Roles of the Opioidergic System and Nitric Oxide in the Analgesic Effect of Venlafaxine

The noradrenalin and serotonin re-uptake inhibitor venlafaxine has an analgesic effect that is independent of its antidepressant activity; however, the mechanism of this effect remains to be elucidated. This study was performed to investigate the possible roles of the opioidergic system and nitric oxide (NO) pathway in the analgesic effect of venlafaxine. Eighty Wistar rats of both sexes were allocated to 10 groups. The hot plate test was used to assess the antinociceptive/analgesic effect. The temperature of the hot plate was adjusted to 52.5+/-1 degrees C, the cut-off period was set to be 50 sec; licking of the hind paw was used as a sign of pain perception. Venlafaxine alone (25 mg/kg) showed marked analgesic activity (p<0.05). N-omega-nitro-L-arginine (L-NOARG) alone (20 mg/kg) and naloxone alone (2 mg/kg and 4 mg/kg) showed no analgesic activity (p>0.05). Coadministration of low-dose naloxone (2 mg/kg) and both doses of L-NOARG (20 and 40 mg/kg) with venlafaxine (25 mg/kg) did not modify the analgesic effect but high-dose naloxone (4 mg/kg) decreased it significantly (p<0.05). In conclusion, these results suggest that the opioidergic system but not the NO pathway has a role in the analgesic effect of venlafaxine.

The analgesic effects of intrathecal xylazine and detomidine in sheep and their antagonism with systemic atipamezole

OBJECTIVE

To evaluate the analgesic and adverse side effects of intrathecal (IT) xylazine (XYL) and detomidine (DET) and the subsequent effects of two doses of intravenous (IV) atipamezole (ATI).

STUDY DESIGN

Prospective, randomized, cross-over.

ANIMALS

Five adult healthy female sheep with mean body mass of 55 +/- 2.3 kg. Material and methods Each sheep underwent four treatments: 1) 50 microg kg(-1) XYL IT and 5 microg kg(-1) ATI IV, 2) 50 microg kg(-1) XYL IT and 2.5 microg kg(-1) ATI IV, 3) 10 microg kg(-1) DET IT and 5 microg kg(-1) ATI IV, 4) 10 microg kg(-1) DET IT and 2.5 microg kg(-1) ATI IV. Pain threshold (TH) was tested by applying pulsed and stepwise incremental direct current to the skin overlying the pastern. The current at the point of foot lift was recorded as the TH. Heart rate (HR), mean arterial pressure, arterial oxygen (PO(2)) and carbon dioxide (PCO(2)) tensions were monitored. Outcomes were derived as differences between baseline assessment and measurements after treatment. Two-way anova was used to analyse drug effects, treatment differences between groups were examined with an F-test or Wilcoxon's rank sum test in case of non-parametric data distribution. p was set at 0.05.

RESULTS

Both drugs increased the pain TH, caused small increases in PCO(2), and small decreases in HR, the latter was only significant for XYL recipients. Xylazine produced a significantly higher TH, more rapidly and for longer than DET. Atipamezole only significantly affected PaCO(2) in the XYL group 2. The pain TH was not affected in either group after IV ATI.

CONCLUSIONS

At the doses used, IT XYL, and to a lesser extent DET, induced pastern analgesia. Atipamezole 5 microg kg(-1) IV antagonized some side effects without affecting analgesia.

CLINICAL RELEVANCE

Intrathecal XYL may be useful as an analgesic in sheep. Its safety is increased because IV ATI antagonizes side effects, but not analgesia.

In vivo characterization of a novel inhibitor of CNS nicotinic receptors

There are multiple types of nicotine acetylcholine receptors (nAChR) in the brain associated with synaptic function, signal processing, or cell survival. The therapeutic targeting of nicotinic receptors in the brain will benefit from the identification of drugs, which may be selective for their ability to activate or inhibit a limited range of these receptor subtypes. We previously identified a family of bis-tetramethylpiperidine compounds as selective inhibitors of neuronal-type nicotinic receptors. In the present study we describe the in vivo effects and properties of 2,2,6,6-tetramethylpiperidin-4-yl heptanoate (TMPH), a novel inhibitor of neuronal nicotinic receptors. Delivered systemically, this drug can block central nervous system effects of nicotine, indicating that this drug is able to cross the blood-brain barrier and access sites in the brain. Unlike the prototype CNS-active nicotinic inhibitor, mecamylamine, TMPH blocked some but not all of the CNS effects of nicotine, indicating that it has a unique selectivity for specific receptor subtypes in the brain. The nAChR subtypes that mediate the locomotor effects and hypothermic effects of nicotine appear to be less sensitive to TMPH than those which mediate analgetic effects and discriminative stimuli. These results indicate that TMPH may possess unique selectivity for specific nicotinic receptor subtypes.

6-(2-Phenylethyl)nicotine: A novel nicotinic cholinergic receptor ligand

6-(2-Phenylethyl)nicotine (1b; K(i)=15 nM) was unexpectedly found to bind at alpha4beta2 nicotinic cholinergic (nACh) receptors. Although this compound failed to produce nicotine-like agonist action in several functional assays, 1b antagonized the antinociceptive effects of nicotine (mouse tail-flick assay) in a dose-dependent fashion when administered via an intrathecal route.

Study of the involvement of K+ channels in the peripheral antinociception of the kappa-opioid receptor agonist bremazocine

The involvement of the nitric oxide (NO)/cyclic GMP pathway in the molecular mechanisms of antinociceptive drugs like morphine has been previously shown by our group. Additionally, it is known that the desensitisation of nociceptors by K(+) channel opening should be the final target for several analgesic drugs including nitric oxide donors and exogenous micro-opioid receptor agonists. In our previous study, we demonstrated that bremazocine, a kappa-opioid receptor agonist, induces peripheral antinociception by activating nitric oxide/cyclic GMP pathway. In the current study, we assessed whether bremazocine is capable to activate K(+) channels eliciting antinociception. Bremazocine (20, 40 and 50 microg) dose-dependently reversed the hyperalgesia induced in the rat paw by local injection of carrageenan (250 microg) or prostaglandin E(2) (2 microg), measured by the paw pressure test. Using the selective kappa-opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 200 microg/paw), it was confirmed that bremazocine (50 microg/paw) acts specifically on the kappa-opioid receptors present at peripheral sites. Prior treatment with the ATP-sensitive K(+) channel blockers glibenclamide (40, 80 and 160 microg) and tolbutamide (40, 80 and 160 microg) did not antagonise the antinociceptive effect of bremazocine (50 microg). The same results were obtained when we used prostaglandin E(2) (2 microg) as the hyperalgesic stimulus. The supposed participation of other types of K(+) channels was tested using the Ca(2+)-activated K(+) channel blockers dequalinium (12.5, 25 and 50 microg) and charybdotoxin (0.5, 1 and 2 microg) and different types of the non-selective K(+) channel blockers tetraethylammonium (25, 50 and 100 microg) and 4-aminopyridine (10, 25 and 50 microg). None of the K(+) channel blockers reversed the antinociceptive effect of bremazocine. On the basis of these results, we suggest that K(+) channels are not involved in the peripheral antinociceptive effect of bremazocine, although this opioid receptor agonist induces nitric oxide/cGMP pathway activation.

Alpha 2-adrenoceptors and 5-HT receptors mediate the antinociceptive effect of new pyrazolines, but not of dipyrone

In this study, we investigated whether spinal noradrenergic and serotonergic systems are involved in the antinociception induced by the novel pyrazolines 3-methyl- and 3-phenyl-5-hydroxy-5-trichloromethyl-4,5-dihydro-1H-1-pyrazole-1-carboxyamide (MPCA and PPCA, respectively), and the pyrazolinone dipyrone in the acetic acid writhing (stretching) test in mice. Intrathecal (i.t.) administration of methysergide (3 and 10 microg) and yohimbine (3 microg), but not of prazosin (0.3 and 1 microg) prevented the antinociceptive action of MPCA and PPCA (500 micromol/kg, s.c.). Dipyrone-induced antinociception (500 micromol/kg, s.c.) was not affected by methysergide or adrenoceptor antagonists. These results suggest that spinal 5-HT receptors and alpha2-adrenoceptors are involved in the antinociception induced by MPCA and PPCA, but not in that elicited by dipyrone.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Nociceptin/orphanin FQ prevents the antinociceptive action of paracetamol on the rat hot plate test

Nociceptin/orphanin FQ (N/OFQ) is involved in many behavioural patterns; in particular, it exerts a modulating effect on nociception. Like other proposed antiopiates, nociceptin/orphanin FQ has been shown to have analgesic, hyperalgesic as well as antianalgesic properties. Among the various effects proposed on nociceptive sensitivity at supraspinal level, the antagonistic activity toward morphine analgesia seems to be of interest. Therefore, we decided to investigate whether nociceptin/orphanin FQ and [Arg14, Lys15] nociceptin/orphanin FQ (R-K, a nociceptin analogue) can have the same effect on the analgesia produced by nonopioid analgesics. In this study, we examined the antianalgesic effect of nociceptin/orphanin FQ and its analogue R-K on paracetamol-induced analgesia and evaluated by means of the hot plate test in rats. Nociceptin/orphanin FQ was intracerebroventricularly administered, and, after 5 min, a dose of 400 mg/kg paracetamol was injected intraperitoneally, 30 min before the hot plate test. Nociceptin/orphanin FQ and R-K showed a dose-dependent antagonism on the antinociceptive effect of paracetamol, and the activity of both drugs was significantly reduced by the antagonist [Nphe1] Arg14, Lys15-N/OFQ-NH2 (UFP-101). These data indicate that nociceptin/orphanin FQ and R-K have an antianalgesic effect on the analgesia produced by a nonopioid analgesic drug, like paracetamol, that seems to develop within the brain.

Mechanisms of Analgesic Action of Neurotropin on Chronic Pain in Adjuvant-Induced Arthritic Rat: Roles of Descending Noradrenergic and Serotonergic Systems

Neurotropin((R)), a non-protein extract from the inflamed skin of rabbits inoculated with vaccinia virus, has been clinically used as an analgesic drug for treatment of chronic pain. In this study, we investigated the analgesic mechanisms of Neurotropin in the adjuvant-induced arthritic rat, a chronic pain model with inflammation. Neurotropin caused dose-dependent inhibition of hyperalgesia in the adjuvant-induced arthritic rat after single intravenous (10 - 100 NU/kg) and oral (30 - 200 NU/kg) administration. The analgesic effect of Neurotropin (intravenous 100 NU/kg and oral 200 NU/kg) was significantly inhibited by intrathecal injections of the alpha(2)-adrenoceptor antagonist yohimbine (30 nmol/animal) and the selective 5-HT(3) serotonin receptor antagonist MDL72222 (30 nmol/animal), and slightly inhibited by the non-selective serotonin receptor antagonist methysergide (100 nmol/animal). The results suggest that the analgesic action of Neurotropin is at least in part due to the enhancement of noradrenergic and serotonergic descending pain inhibitory pathways. Neurotropin may be useful for the clinical management of chronic pain diseases such as a rheumatoid arthritis and osteoarthritis.

Pharmacologically selective block of mu opioid antinociception by peptide nucleic acid antisense in absence of detectable ex vivo knockdown

The goal of this study was to determine the neuroanatomical extent of mu opioid receptor knockdown in central nervous system (CNS) following intracerebroventricular (i.c.v.) administration of peptide nucleic acid antisense. Rats received subchronic i.c.v. injections of anti-mu opioid receptor antisense, mismatch or vehicle, and were tested for paw pressure latency following i.c.v. mu opioid receptor agonist ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin; DAMGO) or delta opioid receptor agonist ((+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide; SNC80). The anti-mu opioid receptor antisense (but not mismatch) sequence abolished DAMGO-induced antinociception with no reduction in the delta opioid receptor-mediated response. In contrast, postmortem receptor autoradiographic analysis of CNS areas revealed no change in mu opioid receptor functional response ([35S]GTPgammaS assay) or receptor labelling ([125I]FK-33824 and mu opioid receptor immunoautoradiography). These results provide further evidence for antisense-induced knockdown at the behavioural level in the absence of clear changes at the tissue level.

Acetyl-L-carnitine requires phospholipase C-IP3 pathway activation to induce antinociception

The cellular events involved in acetyl-L-carnitine (ALCAR) analgesia were investigated in the mouse hot plate test. I.c.v. pretreatment with aODNs against the alpha subunit of G(q) and G(11) proteins prevented the analgesia induced by ALCAR (100 mg kg(-1) s.c. twice daily for 7 days). Administration of the phospholipase C (PLC) inhibitors U-73122 and neomycin, as well as the injection of an aODN complementary to the sequence of PLCbeta(1), antagonized the increase of the pain threshold induced by ALCAR. Pretreatment with U-73343, an analogue of U-73112 inactive on PLC, did not modify ALCAR analgesic effect. In mice undergoing treatment with LiCl, which impairs phosphatidylinositol synthesis, or pretreatment with TMB-8, a blocker of Ca(++) release from intracellular stores, the antinociception induced by ALCAR was dose-dependently antagonized. I.c.v. treatment with heparin, an IP(3) receptor antagonist, prevented the increase of pain threshold induced by the investigated compound, analgesia that was restored by co-administration of D-myo-inositol. On the other hand, i.c.v. pretreatment with the selective protein kinase C (PKC) inhibitors calphostin C and cheleritryne, resulted in a dose-dependent potentiation of ALCAR antinociception. The administration of PKC activators, such as PMA and PDBu, dose-dependently prevented the ALCAR-induced increase of pain threshold. Neither aODNs nor pharmacological treatments produced any behavioral impairment of mice as revealed by the rota-rod and hole board tests. These results indicate that central ALCAR analgesia in mice requires the activation of the PLC-IP(3) pathway. By contrast, the simultaneous activation of PKC may represent a pathway of negative modulation of ALCAR antinociception.

Antagonism of nitrous oxide antinociception in mice by antisense oligodeoxynucleotide directed against neuronal nitric oxide synthase enzyme

Nitric oxide (NO) has been implicated in the antinociceptive effect of nitrous oxide (N2O) in mice. This study was conducted to determine the sensitivity of N2O-induced antinociception to antagonism by an antisense oligodeoxynucleotide (AS-ODN) against neuronal nitric oxide synthase (nNOS). The AS-ODN significantly antagonized the antinociceptive effect of N2O in the abdominal constriction test, but a mismatch ODN was without effect. This result implicates the specific involvement of nNOS in N2O-induced antinociception.

Leucopyrokinin (LPK) analog [d-Ala(5)]-[2-8]-LPK inhibits LPK-induced analgesia in rats

It has been previously found in our laboratory that insect neuropeptide leucopyrokinin and [2-8]-leucopyrokinin, a truncated analog without the first aminoacid of leucopyrokinin peptide chain exert an antinociceptive effect in rats. The present study confirmed our previous results, and moreover it has been found that [d-Ala(5)]-[2-8]-leucopyrokinin, an analog of leucopyrokinin antagonized the antinociceptive effect of leucopyrokinin and of [2-8]-leucopyrokinin. We conclude that this synthetic analog is a probable leucopyrokinin antagonist.

CB(1) receptor mediation of cannabinoid behavioral effects in male and female rats

RATIONALE

Cannabinoids have been shown to produce greater behavioral effects in female than male rats. Although central nervous system CB(1) receptors are known to mediate cannabinoid-induced behavioral effects in male rats, it is not known whether the same is true for females.

OBJECTIVE

To determine if cannabinoid-induced antinociception and catalepsy are similarly mediated by central CB(1) receptors in male and female rats.

METHODS

The ability of SR141716A, a CB(1) receptor selective antagonist, administered ICV (1-1000 microg) or IT (1-600 microg) to block 10 mg/kg IP delta(9)-THC-induced antinociception (paw pressure) and catalepsy (bar test), was compared in male and female rats.

RESULTS

Delta(9)-THC alone produced slightly greater antinociception, and significantly greater catalepsy in females than males. When administered ICV, SR141716A partially antagonized delta(9)-THC-induced antinociception in both females and males. IT SR141716A also antagonized delta(9)-THC-induced antinociception in both sexes; it was slightly more potent in males but equally effective in males and females. SR141716A antagonized delta(9)-THC-induced catalepsy in a similar manner in males and females when given ICV or IT.

CONCLUSIONS

These results confirm that delta(9)-THC-induced behavioral effects are mediated by central CB(1) receptors in male and female rats.

Effect of K+ channel modulators on the antiallodynic effect of gabapentin

The effect of K+ channel inhibitors on the antiallodynic activity induced by spinal gabapentin was assessed in rats. Ligation of L5 and L6 spinal nerves made the rats allodynic, whereas that intrathecal administration of gabapentin (25-200 microg) reduced tactile allodynia in a dose-dependent manner. Spinal pretreatment with glibenclamide (12.5-50 microg, ATP-sensitive K+ channel inhibitor), charybdotoxin (0.01-1 ng) or apamin (0.1-3 ng, large-and small-conductance Ca2+-activated K+ channel blockers, respectively), but not margatoxin (0.01-10 ng, voltage-dependent K+ channel inhibitor), significantly prevented gabapentin-induced antiallodynia. Pinacidil (1-30 microg, K+ channel opener) significantly reduced nerve ligation-induced allodynia. Intrathecal glibenclamide (50 microg), charybdotoxin (1 ng) and apamin (3 ng), but not margatoxin (10 ng), significantly reduced pinacidil-induced antiallodynia. K+ channel inhibitors alone did not modify allodynia produced by spinal nerve ligation. Results suggest that gabapentin and pinacidil may activate Ca2+-activated and ATP-sensitive K+ channels in order to produce part of its spinal antiallodynic effect in the Chung model.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

Magnesium chloride and ruthenium red attenuate the antiallodynic effect of intrathecal gabapentin in a rat model of postoperative pain

BACKGROUND

Gabapentin, a gamma-aminobutyric acid analog anticonvulsant, has been shown to possess antinociceptive effects in animal models and clinical trials. An endogenous binding site of [3H]gabapentin has been revealed to be the alpha(2)delta subunit of voltage-dependent Ca2+ channels. Magnesium chloride, ruthenium red, and spermine have been shown to modulate [3H]gabapentin binding to this binding site in vitro. In this study, the authors examined whether intrathecal magnesium chloride, ruthenium red, or spermine could affect the antiallodynic effect of intrathecal gabapentin in a rat model of postoperative pain.

METHODS

Under isoflurane anesthesia, male Sprague-Dawley rats received an incision over the plantar surface of the right hind paw to produce punctate mechanical allodynia. Withdrawal thresholds to von Frey filament stimulation near the incision site were measured before incision, 2 h after incision, and every 30 min after intrathecal coadministration of gabapentin with normal saline or different doses of magnesium chloride, ruthenium red, or spermine for 2 h.

RESULTS

Intrathecal gabapentin (30, 100, 200 microg) dose-dependently reduced incision-induced allodynia. Hexahydrated magnesium chloride (5, 10, 20 microg) and ruthenium red (0.2, 2, 20 ng) noncompetitively inhibited the antiallodynic effect of gabapentin. Spermine at doses not inducing motor weakness (30, 60 microg) did not affect the antiallodynic effect of gabapentin. The antiallodynic effect of intrathecal morphine (1.5 microg) was not affected by hexahydrated magnesium chloride (20 microg), ruthenium red (20 ng), or spermine (60 microg).

CONCLUSIONS

These results provide behavioral evidence to support that the alpha(2)delta subunit of Ca2+ channels may be involved in the antiallodynic action of intrathecal gabapentin in the postoperative pain model.

Intrathecal Treatment With 6-Hydroxydopamine or 5,7-Dihydroxytryptamine Blocks the Antinociception Induced by Endomorphin-1 and Endomorphin-2 Given Intracerebroventricularly in the Mouse

The involvement of spinopetal noradrenergic and serotonergic systems in antinociception induced by endomorphin-1 (EM-1) and endomorphin-2 (EM-2) given supraspinally or spinally were investigated in male CD-1 mice. Groups of mice were pretreated intrathecally (i.t.) with 6-hydroxydopamine (6-OHDA, 20 microg) or 5,7-dihydroxytryptamine (5,7-DHT, 50 microg) for 3 days before intracerebroventricular (i.c.v.) or i.t. injection of different doses of EM-1 or EM-2, and the tail-flick response was measured for antinociceptive effects. I.t. pretreatment with 6-OHDA for 3 days, which markedly depleted noradrenaline (NA) contents by more than 90%, but not serotonin (5-HT) in the spinal cord, completely abolished the antinociception induced by i.c.v.-administered EM-1 or EM-2. Intrathecal pretreatment with 5,7-DHT for 3 days, which markedly reduced 5-HT contents by more than 92%, but only reduced NA by 14 - 25% in the spinal cord, also markedly attenuated the antinociception induced by i.c.v.-administered EM-1 or EM-2. However, the antinociception induced by i.t.-administered EM-1 or EM-2 was not affected in either 6-OHDA or 5,7-DHT pretreated mice. It is concluded that NA and 5-HT in the spinal cord are involved in the antinociception induced by supraspinally, but not spinally administered EM-1 and EM-2.

Attenuation of nicotine-induced antinociception, rewarding effects, and dependence in mu-opioid receptor knock-out mice

The involvement of mu-opioid receptors in different behavioral responses elicited by nicotine was explored by using mu-opioid receptor knock-out mice. The acute antinociceptive responses induced by nicotine in the tail-immersion and hot-plate tests were reduced in the mutant mice, whereas no difference between genotypes was observed in the locomotor responses. The rewarding effects induced by nicotine were then investigated using the conditioning place-preference paradigm. Nicotine produced rewarding responses in wild-type mice but failed to produce place preference in knock-out mice, indicating the inability of this drug to induce rewarding effects in the absence of mu-opioid receptors. Finally, the somatic expression of the nicotine withdrawal syndrome, precipitated in dependent mice by the injection of mecamylamine, was evaluated. Nicotine withdrawal was significantly attenuated in knock-out mutants when compared with wild-type mice. In summary, the present results show that mu-opioid receptors are involved in the rewarding responses induced by nicotine and participate in its antinociceptive responses and the expression of nicotine physical dependence.

Inhibition of improgan antinociception by the cannabinoid (CB)(1) antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A): lack of obligatory role for endocannabinoids acting at CB(1) receptors

Improgan, a nonopioid antinociceptive agent, activates descending, pain-relieving mechanisms in the brain stem, but the receptor for this compound has not been identified. Because cannabinoids also activate nonopioid analgesia by a brain stem action, experiments were performed to assess the significance of cannabinoid mechanisms in improgan antinociception. The cannabinoid CB(1) antagonist N-(piperidin-1-yl)-5-(4-chloro phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) induced dose-dependent inhibition of improgan antinociception on the tail-flick test after i.c.v. administration in rats. The same treatments yielded comparable inhibition of cannabinoid [R-(+)-(2,3-dihydro-5-methyl-3-[(4-mor pholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl)methanone monomethanesulfonate, WIN 55,212-2] analgesia. Inhibition of improgan and WIN 55,212-2 antinociception by SR141716A was also observed in Swiss-Webster mice. Radioligand binding studies showed no appreciable affinity of improgan on rat brain, mouse brain, and human recombinant CB(1) receptors, ruling out a direct action at these sites. To test the hypothesis that CB(1) receptors indirectly participate in improgan signaling, the effects of improgan were assessed in mice with a null mutation of the CB(1) gene with and without SR141716A pretreatment. Surprisingly, improgan induced complete antinociception in both CB(1) (-/-) and wild-type control [CB(1) (+/+)] mice. Furthermore, SR141716A inhibited improgan antinociception in CB(1) (+/+) mice, but not in CB(1) (-/-) mice. Taken together, the results show that SR141716A reduces improgan antinociception, but neither cannabinoids nor CB(1) receptors seem to play an obligatory role in improgan signaling. Present and previous studies suggest that Delta(9)-tetrahydrocannabinol may act at both CB(1) and other receptors to relieve pain, but no evidence was found indicating that improgan uses either of these mechanisms. SR141716A will facilitate the study of improgan-like analgesics.

(−)6-n-Propylnicotine antagonizes the antinociceptive effects of (−)Nicotine

Several 6-alkyl analogues of nicotine were examined in radioligand binding and in vivo functional assays. Although (-)6-ethylnicotine (3) binds with high affinity at nACh receptors (Ki=5.6 nM) and produces nicotine-like actions, its n-propyl homologue (-)4 (Ki=22 nM) failed to produce such effects. In fact, (-)4 antagonized the antinociceptive effects of (-)nicotine in the tail-flick assay in mice, but not the spontaneous activity or discriminative stimulus effects of (-)nicotine. Compound (-)4 appears to selectively antagonize only one of the three effects examined and is an interesting cholinergic agent for subsequent investigation.

Antagonism of the antinociceptive and discriminative stimulus effects of heroin and morphine by 3-methoxynaltrexone and naltrexone in rhesus monkeys

It has been suggested that heroin and morphine may act on different opioid receptor populations in rodents. In support of this hypothesis, the opioid antagonist 3-methoxynaltrexone was reported to be more potent as an antagonist of the antinociceptive effects of heroin than of morphine in mice and rats. To assess the generality of this finding across species and experimental endpoints, the present study compared the potencies of naltrexone and 3-methoxynaltrexone as antagonists of heroin and morphine in two behavioral assays in rhesus monkeys. In the thermal nociception study, tail-withdrawal latencies were measured from water heated to 50 degrees C. In the heroin discrimination study, monkeys were trained to discriminate 0.1 mg/kg heroin from saline in a two-key, food-reinforced drug discrimination procedure, and percentage of heroin-appropriate responding and response rates were measured. Both heroin and morphine produced dose-dependent antinociception, increases in percentage of heroin-appropriate responding, and decreases in response rates. Heroin was approximately 20-fold more potent than morphine. Naltrexone (0.032-0.1 mg/kg) was equipotent in antagonizing all effects of heroin and morphine (pA(2) values = 7.90-8.22). 3-Methoxynaltrexone (0.1-3.2 mg/kg) was also equipotent in antagonizing the antinociceptive, discriminative stimulus, and rate-suppressant effects of heroin and morphine; however, 3-methoxynaltrexone was approximately 100-fold less potent than naltrexone (pA(2)/pK(B) values = 5.96-6.36). These results suggest that heroin and morphine act on pharmacologically similar populations of opioid receptors in rhesus monkeys, and also indicate that 3-methoxynaltrexone does not differentially antagonize the effects of heroin and morphine in rhesus monkeys.

Differential antagonism of endomorphin-1 and endomorphin-2 supraspinal antinociception by naloxonazine and 3-methylnaltrexone

To determine if different subtypes of mu-opioid receptors were involved in antinociception induced by endomorphin-1 and endomorphin-2, the effect of pretreatment with various mu-opioid receptor antagonists beta-funaltrexamine, naloxonazine and 3-methylnaltrexone on the inhibition of the paw-withdrawal induced by endomorphin-1 and endomorphin-2 given intracerebroventricularly (i.c.v.) were studied in ddY male mice. The inhibition of the paw-withdrawal induced by i.c.v. administration of endomorphin-1, endomorphin-2 or DAMGO was completely blocked by the pretreatment with a selective mu-opioid receptor antagonist beta-funaltrexamine (40 mg/kg), indicating that the antinociception induced by all these peptides are mediated by the stimulation of mu-opioid receptors. However, naloxonazine, a mu1-opioid receptor antagonist pretreated s.c. for 24h was more effective in blocking the antinociception induced by endomorphin-2, than by endomorphin-1 or DAMGO given i.c.v. Pretreatment with a selective morphine-6 beta-glucuronide blocker 3-methylnaltrexone 0.25mg/kg given s.c. for 25 min or co-administration of 3-methylnaltrexone 2.5 ng given i.c.v. effectively attenuated the antinociception induced by endomorphin-2 given i.c.v. and co-administration of 3-methylnaltrexone shifted the dose-response curves for endomorphin-2 induced antinociception to the right by 4-fold. The administration of 3-methylnaltrexone did not affect the antinociception induced by endomorphin-1 or DAMGO given i.c.v. Our results indicate that the antinociception induced by endomorphin-2 is mediated by the stimulation of subtypes of mu-opioid receptor, which is different from that of mu-opioid receptor subtype stimulation by endomorphin-1 and DAMGO.

Blockade of the antinociceptive effect of spinally administered kyotorphin by naltrindole in mice

We investigated the role of spinal opioid receptors in the antinociceptive effect of kyotorphin (Tyr-Arg, KTP) by using an in vivo mice tail-pinch test and an in vitro opioid receptor binding assays. Intrathecal administration of KTP produced a dose-dependent antinociceptive effect with an ED(50) value of 24 microg/mouse. This antinociception, which was reversed by the KTP antagonist Leu-Arg, was completely blocked by naltrindole but not by naloxonazine, beta-funaltrexamine, or nor-binaltorphimine. The results from the binding study in vitro indicated that KTP bound to spinal KTP receptors but not to any opioid receptors in the mouse spinal cord. These results suggest that KTP-induced antinociception is mediated by binding to KTP receptors followed by an indirect activation of the delta-opioid receptors in the spinal cord.

Identification and characterization of novel mammalian neuropeptide FF-like peptides that attenuate morphine-induced antinociception

The two mammalian neuropeptides NPFF and NPAF have been shown to have important roles in nociception, anxiety, learning and memory, and cardiovascular reflex. Two receptors (FF1 and FF2) have been molecularly identified for NPFF and NPAF. We have now characterized a novel gene designated NPVF that encodes two neuropeptides highly similar to NPFF. NPVF mRNA was detected specifically in a region between the dorsomedial and ventromedial hypothalamic nuclei. NPVF-derived peptides displayed higher affinity for FF1 than NPFF-derived peptides, but showed poor agonist activity for FF2. Following intracerebral ventricular administration, a NPVF-derived peptide blocked morphine-induced analgesia more potently than NPFF in both acute and inflammatory models of pain. In situ hybridization analysis revealed distinct expression patterns of FF1 and FF2 in the rat central nervous system. FF1 was broadly distributed, with the highest levels found in specific regions of the limbic system and the brainstem where NPVF-producing neurons were shown to project. FF2, in contrast, was mostly expressed in the spinal cord and some regions of the thalamus. These results indicate that the endogenous ligands for FF1 and FF2 are NPVF- and NPFF-derived peptides, respectively, and suggest that the NPVF/FF1 system may be an important part of endogenous anti-opioid mechanism.

Involvement of beta2-adrenergic and mu-opioid receptors in antinociception produced by intracerebroventricular administration of alpha,beta-methylene-ATP

The present study examined what kind of receptors are involved in the antinociception produced by intracerebroventricular (i.c.v.) administration of a,beta-methylene-ATP using antagonists at adrenergic, serotonin or opioid receptors. Antinociceptive effect of alpha,beta-methylene-ATP (10 nmol/rat) was significantly attenuated by subcutaneous pretreatment with propranolol and naloxone, but not phentolamine or methysergide, at a dose of 10 mg/kg. I.c.v. pretreatment with propranolol (100 nmol/rat), butoxamine (100 nmol/rat), ICI-I 18,551 (100 nmol/rat) and naloxone (30 nmol/rat) significantly attenuated the antinociceptive effect of alpha,beta-methylene-ATP. However, i.c.v. pretreatment with atenolol (100 nmol/rat), naltrindole (30 nmol/rat) or nor-binaltorphimine (30 nmol/rat) did not show any significant effects. These results suggest that supraspinal beta2-adrenergic and mu-opioid receptors are involved in the antinociceptive effect of i.c.v. administered alpha,beta-methylene-ATP.

The effects of chronic morphine treatment on neurotensin-induced antinociception

Previous studies have shown that the opioid antagonist naloxone does not alter neurotensin (NT)-induced antinociception. In the present studies, tolerance to morphine in mice significantly attenuated NT-induced antinociception, but not NT-induced hypothermia. In addition, centrally administered NT inhibited naloxone-precipitated jumping in morphine-dependent mice. These results indicate complex interactions between NT-induced antinociception and opioid systems.

Designing of an orally active complement C3a agonist peptide with anti-analgesic and anti-amnesic activity

Complement C3a is an anti-opioid peptide, having anti-analgesic and anti-amnesic effects after intracerebroventricular administration. However, the peptide is inactive after oral administration. Orally active C3a agonist peptide was designed based on the structure of oryzatensin, a C3a agonist peptide derived from rice albumin. Tyr-Pro-Leu-Pro-Arg, a pentapeptide at the carboxyl terminus of oryzatensin is the minimally essential structure for exerting C3a activity. Due to the affinity for mu-opioid receptor, both oryzatensin and Tyr-Pro-Leu-Pro-Arg showed analgesia after intracerebroventricular administration in mice which was blocked by the opioid antagonist naloxone. Tyr-Pro-Leu-Pro-Arg lost opioid activity by substitution the amino terminus tyrosine with other hydrophobic residues. Among the newly designed peptides, Trp-Pro-Leu-Pro-Arg was found to possess the strongest C3a activity. The peptide antagonized morphine-induced analgesia at 300 mg/kg after oral administration and also improved scopolamine- and ischemia-induced amnesia in a step-through passive avoidance test.

Preliminary studies on the analgesic activity of latex of Calotropris procera

In this study we have evaluated the analgesic activity of dry latex (DL) of Calotropis procera. A single oral dose of DL ranging from 165 to 830 mg/kg produced a significant dose dependent analgesic effect against acetic acid induced writhings. The effect of DL at a dose of 415 mg/kg was more pronounced as compared to a 100 mg/kg oral dose of aspirin. On the other hand DL (830 mg/kg) produced marginal analgesia in a tail-flick model which was comparable to aspirin. The analgesic effect of DL was delayed by 1 h by naloxone at a dose of 0. 5 mg/kg, i.p., which completely blocked the analgesic effect of morphine (10 mg/kg, i.p.). However, the effect of aspirin was not blocked by naloxone. The 830 mg/kg oral dose of DL did not produce toxic effects in mice and the LD(50) was found to be 3 g/kg.

Selective and segmentally restricted antinociception induced by MPV-2426, a novel alpha-2-adrenoceptor agonist, following intrathecal administration in the rat

BACKGROUND

MPV-2426 (radolmidine) is a novel alpha-2-adrenoceptor agonist developed for spinal pain therapy. In the present study we determined the segmental distribution and selectivity of the antinociceptive effect induced by MPV-2426 following i.t. administration in rats.

METHODS

The experiments were performed in lightly anesthetized rats with an i.t. catheter for administration of drugs into the lumbar spinal cord level. To determine segmental distribution of antinociception, the withdrawal latency of the tail and forepaw from a hot water bath was measured. To determine selectivity of reflex modulation, the effect of i.t. MPV-2426 on the innocuous H-reflex was determined.

RESULTS

In the hot water immersion test MPV-2426 produced a dose-dependent (0.3-3.0 microg) prolongation of tail withdrawal latency whereas the effect on forepaw withdrawal latency was short of significance. Dexmedetomidine, the reference alpha-2-adrenoceptor agonist, produced a significant dose-related prolongation of both the tail and the forepaw withdrawal (0.3 and 1.0 microg). MPV-2426 (1.0 and 3.0 microg) produced no significant change in the amplitude of the H-reflex or M-response induced by electrical stimulation of the tibial nerve, nor any change in the modulation of the H-reflex amplitude induced by conditioning sural nerve stimulation. The antinociception induced by MPV-2426 was completely reversed by atipamezole (1 mg/kg s.c.), an alpha-2-adrenoceptor antagonist.

CONCLUSION

MPV-2426 produces a selective and segmentally more restricted antinociceptive effect than dexmedetomidine following i.t. administration. The antinoception induced by MPV-2426 is due to action on spinal alpha-2-adrenoceptors.

Distinct domains of IFNalpha mediate immune and analgesic effects respectively

Interferon-alpha (IFNalpha) is not only an immunoregulatory factor, but is also an analgesic molecule. The analgesic effect of IFNalpha was mediated by mu opioid receptor. After the 129th Tyr residue of human IFNalpha was mutated to Ser, the antiviral activity almost disappeared, but there still remained a strong analgesic activity that could be blocked by naloxone. These results indicate that there exist distinct domains in the IFNalpha molecule, which mediate immune and analgesic effects respectively, and suggest that there are different receptor mechanisms inducing immune and analgesic effects of IFNalpha. However, although the antiviral activity of IFNalpha decreased to 34.1% of wild type IFNalpha after the 122nd Tyr residue was changed to Ser, the analgesic activity of this mutant was lost completely. There were significant cross reactivities between INFalpha and anti-opioid sera. These studies show strong structural and functional similarities between INFalpha and opioid peptides, and inferred that the analgesic domain locates around the 122nd Tyr residue of IFNalpha molecule in tertiary structure.

Antinociceptive effect of the essential oil from Cymbopogon citratus in mice

The essential oil (EO) from leaves of Cymbopogon citratus increased the reaction time to thermal stimuli both after oral (25 mg/kg) and intraperitoneal (25-100 mg/kg) administration. EO (50-200 mg/kg, p.o. or i.p.) strongly inhibited the acetic acid-induced writhings in mice. In the formalin test, EO (50 and 200 mg/kg, i.p.) inhibited preferentially the second phase of the response, causing inhibitions of 100 and 48% at 200 mg/kg, i.p. and 100 mg/kg, p.o., respectively. On the other hand, the opioid antagonist naloxone blocked the central antinociceptive effect of EO, suggesting that EO acts both at peripheral and central levels.

Dissociable effects of the kappa-opioid receptor agonists bremazocine, U69593, and U50488H on locomotor activity and long-term behavioral sensitization induced by amphetamine and cocaine

RATIONALE

Mesolimbic dopaminergic neurotransmission plays a critical role in the locomotor effects of psychostimulant drugs, but a general involvement in the induction of long-term psychostimulant sensitization is questionable. By influencing dopaminergic neurotransmission, opioid drugs can alter the behavioral effects of psychostimulants.

OBJECTIVES

The effects of the kappa-opioid receptor agonists bremazocine, U69593, and U50488H on the locomotor stimulant and the long-term sensitizing effects of amphetamine and cocaine were investigated in rats. Unlike U69593 and U50488H, bremazocine is also an antagonist at mu- and delta-opioid receptors, as well as an agonist at a subtype of delta-opioid receptors inhibiting dopamine D1 receptor-stimulated adenylate cyclase.

METHODS

Bremazocine, U69593, and U50488H were administered prior to amphetamine and cocaine, and locomotor activity was measured. In separate studies, the opioids were co-administered with amphetamine and cocaine for 5 days, and locomotor sensitization was assessed 3 weeks post-treatment.

RESULTS

Bremazocine and U69593 attenuated the psychomotor stimulant effects of amphetamine and cocaine. U50488H attenuated the locomotor effect of cocaine and biphasically affected amphetamine-induced locomotion, i.e., suppression followed by stimulation. Bremazocine prevented the development of amphetamine-induced but not cocaine-induced long-term sensitization. Neither U69593 nor U50448H affected the induction of long-term amphetamine or cocaine sensitization.

CONCLUSIONS

In agreement with previous studies, the present data suggest that differential mechanisms underlie the acute stimulant versus the long-term sensitizing effects of psychostimulants, and the induction of long-term sensitization by amphetamine versus cocaine. Stimulation of kappa-opioid receptors does not seem to block the induction of long-term psychostimulant sensitization. Thus, bremazocine is likely to block the induction of amphetamine sensitization through a non-kappa-opioid receptor mechanism. We suggest that this effect of bremazocine is the result of its unique agonist action at a subtype of delta-opioid receptors, thereby acting as a functional dopamine D1 receptor antagonist. This would be consistent with the literature showing that the induction of long-term amphetamine sensitization depends on the activation of dopamine D1 receptors. In addition, the present data are in keeping with studies showing that dopamine neurotransmission is not critical for the induction of long-term cocaine sensitization.

Antiallodynic effect of intrathecal gabapentin and its interaction with clonidine in a rat model of postoperative pain

BACKGROUND

Systemic administration of gabapentin was shown previously to attenuate mechanical allodynia in a rat model of postoperative pain. Because intrathecal administration of gabapentin is effective in other hypersensitivity states, the authors tested its effect in the postoperative model, its interaction with another antiallodynic agent (clonidine), and a possible mechanism of gabapentin action (entry into sites of action via an L-amino acid transporter).

METHODS

Male Sprague-Dawley rats were anesthetized with halothane, and an incision of the plantaris muscle of right hind paw induced punctate mechanical allodynia. Withdrawal threshold to von Frey filament application near the incision site was determined before and 2 h after surgery. Then, an intrathecal injection was performed and thresholds were determined every 30 min for 3 h thereafter.

RESULTS

Paw incision induced a mechanical hypersensitivity (mechanical threshold > 25 g before incision and < 5 g after). Intrathecal gabapentin dose-dependently (10-100 microg) reduced mechanical allodynia. Intrathecal injection of an inhibitor of L-amino acid transporters or a competitor for this transporter, L-leucine, did not reverse the intrathecal effect of gabapentin. The ED50 of intrathecal gabapentin, clonidine, and their combination were 51, 31, and 9 microg, respectively, and isobolographic analysis showed synergy between gabapentin and clonidine.

CONCLUSIONS

Intrathecal gabapentin is effective against tactile allodynia that occurs after paw incision, and interacts synergistically with clonidine. Unlike results in vitro, gabapentin does not obligatorily need to enter cells via the L-amino acid transporter mechanism to achieve its effects in vivo.

Postnatal naltrindole treatments induce behavioural modifications in preweanling rats

To investigate the physiological role of the delta-opioid receptor during the preweanling period, we have studied the effects of chronic (daily injections from birth to postnatal day 19) and acute treatments with the selective delta-antagonist naltrindole (1 mg/kg), on behavioural and nociceptive responses in 20-day old male rats. Behavioural testing was performed using an open field paradigm. Acute naltrindole induced significant decreases in external and total ambulation (horizontal activity) and rearing behaviour (vertical activity), as well as a significant increase in grooming frequency. In animals chronically treated with naltrindole there was an increase in total ambulation one day after the discontinuation of the treatment. In a test of nociception (tail immersion) no significant effect of chronic naltrindole treatment on baseline latencies or of acute naltrindole on latency quotients (post-treatment latency/pre-treatment latency) were found. However, chronic naltrindole administration significantly decreased the latency quotients. The results show that the delta-opioid receptor participates in the tonic regulation of motor activity during the preweanling period and might be involved in certain aspects of stress responsiveness.

Effects of 2-arachidonylglycerol, an endogenous cannabinoid, on neuronal activity in rat hippocampal slices

The monoacylglycerol 2-arachidonylglycerol is an endogenous ligand of cannabinoid receptors. We examined whether 2-arachidonylglycerol can influence excessive neuronal activity by investigating stimulation-induced population spikes and epileptiform activity in rat hippocampal slices. For this purpose, the effects of 2-arachidonylglycerol were compared with those of the synthetic cannabinoid agonist WIN 55,212-2. At concentrations of 10-50 microM, 2-arachidonylglycerol attenuated the amplitude of the orthodromic population spike and the slope of the field excitatory postsynaptic potential (field EPSP). However, the effect of the synthetic cannabinoid WIN 55,212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolol[ 1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone; 0.1 microM and 1 microM) was significantly higher than that of the endogenous ligand. At a concentration of 1 microM, WIN 55,212-2 completely suppressed the field EPSP. However, none of the investigated compounds did affect the presynaptic fiber spike of the afferents. The CB1 receptor antagonist SR 141716 (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorphenyl)-4-methyl-3- pyrazole-carboxamide) blocked the inhibition evoked by the cannabinoids. Both 2-arachidonylglycerol (30 microM) and WIN 55,212-2 (100 nM) shifted the input-output curve of the postsynaptic spike and the field EPSP to the right and increased the magnitude of paired-pulse facilitation, indicating a presynaptic mechanism of action. 2-Arachidonylglycerol and WIN 55,212-2 attenuated the frequency of spontaneously occurring epileptiform burst discharges in CA3 elicited by omission of Mg2+ and elevation of K+ to 8 mM. The antiepileptiform effect of these cannabinoids was blocked by SR 141716. In conclusion, 2-arachidonylglycerol seems to limit neuronal excitability via cannabinoid receptors of the CB1 type. By acting predominantly at a presynaptic site, it is capable of reducing excitatory neurotransmission, a mechanism which might be involved in the prevention of excessive excitability leading to epileptiform activity.

Analgesic effects of adenosine in syndrome X are counteracted by theophylline: a double-blind placebo-controlled study

It has been proposed that adenosine mediates ischaemic pain in humans. Patients with cardiac Syndrome X are hypersensitive to potential pain stimuli, including adenosine. On the other hand, recent findings suggest that low-dose adenosine infusion may have analgesic effects. Our aim was to test two hypotheses: (1) that the analgesic effect of adenosine is peripheral in origin, and (2) that part of the hypersensitivity to pain of patients with cardiac Syndrome X results from a disturbed mechanism of adenosine analgesia. A total of 12 female Syndrome X patients and eight healthy age-matched female controls were studied in a randomized, double-blind and placebo-controlled study. Adenosine (70 microg/min) or placebo was infused into the forearm via an intra-arterial catheter. After 15 min of infusion, a tourniquet on the upper arm was inflated to 225 mmHg to ensure arterial occlusion. The patient then carried out dynamic handgrip work at 60 Hz. Pain or discomfort in the forearm was estimated continuously according to the Borg CR-10 scale. After the first test, theophylline was infused for 10 min intravenously at a dose of 5 mg/kg body weight. The ischaemic forearm test was then repeated. On a second occasion, the procedure was repeated with the opposite treatment (adenosine/placebo). Only six of 12 Syndrome X patients completed the protocol because of pain during the catheterization procedure or an inability to establish an intra-arterial line. The time to onset of pain in the working, ischaemic forearm was greater for subjects treated with adenosine than for those treated with placebo, both in those Syndrome X patients who tolerated catheterization (49+/-27 s compared with 32+/-18 s; P<0.03) and in healthy controls (40+/-19 s compared with 16+/-8 s; P<0.02). The time to maximum pain, limiting ischaemic work, was also greater with adenosine pretreatment both in Syndrome X patients (137+/-28 s compared with 106+/-28 s; P<0.03) and in healthy controls (109+/-31 compared with 82+/-18 s; P<0.01). After infusion of theophylline there was no difference between adenosine and placebo in either group. Intra-arterially infused adenosine had similar peripheral analgesic effects on experimentally induced muscular ischaemia in those female Syndrome X patients who tolerated intra-arterial catheterization and in healthy controls. Thus adenosine analgesia is counteracted by theophylline, suggesting that the effect is mediated by membrane-bound peripheral adenosine receptors.

Antisense oligodeoxynucleotide targeting distinct exons of the cloned mu-opioid receptor distinguish between endomorphin-1 and morphine supraspinal antinociception in mice

Antisense oligodeoxynucleotides (ODN) were used to investigate the supraspinal antinociceptive effects of endomorphin-1, an endogenous peptide whose analgesic profile suggests that it is a ligand at the mu-opioid receptor. To selectively restrict the expression of this receptor, five ODN targeting distinct exons of the gene sequence were injected subchronically by the intracerebroventricular route (i.c.v.) into mice. The antinociception induced by endomorphin-1 was greatly reduced in animals receiving the ODN directed to nucleotides 677-697, which code for a sequence located on the second extracellular loop of the mu receptor. ODN-mu(un), one of the two antisense ODN directed to exon 1, also impaired endomorphin-1 antinociception. ODN targeting exons 2 and 4 were totally inactive. In contrast, all five ODN blocked the antinociception induced by morphine and beta-casomorphin. The analgesic potency of endomorphin-1, morphine, and beta-casomorphin remained unaltered by administration of an ODN to nucleotides 29-46 of the murine delta-opioid receptor gene sequence of a random-sequence ODN. This suggest the existence of diverse molecular forms for the mu-opioid receptor that mediate the antinociceptive effects of endomorphin-1 and morphine/beta-casomorphin.

Activation of peripheral kappa opioid receptors inhibits capsaicin-induced thermal nociception in rhesus monkeys

8-Methyl-N-vanillyl-6-nonenamide (capsaicin) was locally applied in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, which was manifested as reduced tail-withdrawal latencies in normally innocuous 46 degrees C water. Coadministration of three kappa opioid ligands, U50,488 (3.2-100 microgram), bremazocine (0.1-3.2 microgram), and dynorphin A(1-13) (3.2-100 microgram), with capsaicin in the tail dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by a small dose of an opioid antagonist, quadazocine; (0.32 mg), applied in the tail; however, this dose of quadazocine injected s.c. in the back did not antagonize local U50,488. Comparing the relative potency of either agonist or antagonist after local and systemic administration confirmed that the site of action of locally applied kappa opioid agonists is in the tail. In addition, local nor-binaltorphimine (0.32 mg) and oxilorphan (0.1-10 microgram) antagonist studies raised the possibility of kappa opioid receptor subtypes in the periphery, which indicated that U50,488 produced local antinociception by acting on kappa1 receptors, but bremazocine acted probably on non-kappa1 receptors. These results provide functional evidence that activation of peripheral kappa opioid receptors can diminish capsaicin-induced allodynia in primates. This experimental pain model is a useful tool for evaluating peripherally antinociceptive actions of kappa agonists without central side effects and suggests new approaches for opioid pain management.

Spinal administration of selective opioid antagonists in amphibians: evidence for an opioid unireceptor

In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.

Opioid activity of sendide, a tachykinin NK1 receptor antagonist

Sendide, a tachykinin NK1 receptor antagonist, was tested for antagonism against scratching, biting and licking responses elicited by intrathecal (i.t.) injections of various tachykinin receptor agonists, N-methyl-D-aspartate (NMDA), somatostatin and bombesin, in mice. Tachykinin NK1 receptor agonists, substance P, physalaemin and septide, produced a characteristic behavioural response, consisting of scratching, biting and licking. The substance P-induced response was reduced by small doses (0.0625-1.0 pmol) of sendide in a dose-dependent manner. The behavioural response elicited by other tachykinin NK1 receptor agonists, physalaemin and septide, was also reduced significantly by a small dose (1.0 pmol) of sendide. The inhibitory effect of sendide (1.0 pmol) was not affected by pretreatment with the opioid receptor antagonist, naloxone, at doses up to 4.0 mg/kg. Higher doses of sendide were needed to reduce the behavioural response to neurokinin A, a tachykinin NK2 receptor agonist, neurokinin B, a tachykinin NK3 receptor agonist and eledoisin, a tachykinin NK2/NK3 receptor agonist. Pretreatment with naloxone (2.0 mg/kg, i.p.) significantly antagonized sendide (1024 pmol)-induced inhibition of the behavioural responses to neurokinin A, neurokinin B and eledoisin. The behaviours elicited by i.t. injection of NMDA, somatostatin or bombesin were also reduced by a higher dose (1024 pmol) of sendide and this sendide effect was reversed by naloxone. These findings suggest that sendide at higher doses may possess opioid activity in addition to an antagonistic action at tachykinin NK1 receptors in the spinal cord.