Pharmacological mechanisms involved in the antinociceptive effects of dexmedetomidine in mice

Dexmedetomidine (DEX) is a α₂‐adrenoceptor (α₂‐AR) agonist used as an anesthetic adjuvant and as sedative in critical care settings. Typically, α₂‐AR agonists release nitric oxide (NO) and subsequently activate NO‐GMPc pathway and have been implicated with antinociception. In this study, we investigate the pharmacological mechanisms involved in the antinociceptive effects of DEX, using an acetic acid‐induced writhing assay in mice. Saline or DEX (1, 2, 5, or 10 μg/kg) was intravenously injected 5 min before ip administration of acetic acid and the resulting abdominal constrictions were then counted for 10 min. To investigate the possible mechanisms related to antinociceptive effect of DEX (10 μg/kg), the animals were also pretreated with one of the following drugs: 7‐nitroindazole (7‐NI; 30 mg/kg ip); 1H‐[1,2,4] oxadiazole [4,3‐a] quinoxaline‐1‐one (ODQ; 2.5 mg/kg, ip); yohimbine (YOH; 1 mg/kg, ip); atropine (ATRO; 2 mg/kg, ip); glibenclamide (GLIB; 1 mg/kg, i.p.) and naloxone (NAL; 0.2 mg/kg, ip). A rotarod and open‐field performance test were performed with DEX at 10 μg/kg dose. DEX demonstrated its potent antinociceptive effect in a dose‐dependent manner. The pretreatment with 7‐NI, ODQ, GLIB, ATRO, and YOH significantly reduced the antinociceptive affects of DEX. However, NAL showed no effecting DEX‐induced antinociception. The rotarod and open‐field tests confirmed there is no detectable sedation or even significant motor impairment with DEX at 10 μg/kg dose. Our results suggest that the α₂‐AR and NO‐GMPc pathways play important roles in the systemic antinociceptive effect of DEX in a murine model of inflammatory pain. Furthermore, the antinociceptive effect exerted by DEX appears to be dependent on K_ATP channels, independent of opioid receptor activity.     

Central administration of GPR55 receptor agonist and antagonist modulates anxiety‐related behaviors in rats

G‐protein‐coupled receptor 55 (GPR55) has been proposed as an atypical cannabinoid receptor, which is activated by lysophosphatidylinositols and some synthetic or endogenous cannabinoid molecules. The exact role of GPR55 receptors in the central nervous system especially in anxiety needs to be evaluated. In this study, the effects of intracerebroventricular (i.c.v.) administration of agonist and antagonist of GPR55 receptor on anxiety‐related behaviors in rats were investigated. Here, O‐1602 (GPR55 agonist) at the doses of 0.2, 1, and 5 μg/rat increased %OAT and %OAE but not the locomotor activity, showing an anxiolytic response, whereas i.c.v. injection of ML193 (GPR55 antagonist) at the doses of 0.1 and 1 μg/rat increased anxiety‐like behaviors while causing locomotor impairment. The antagonistic effect of ML193 on the anxiolytic‐like effect of O‐1602 was also evaluated. The results showed that ML193 decreased the anxiolytic‐like effect of O‐1602. Based on these results, it may be concluded that central GPR55 may have a role in modulation of anxiety‐like behaviors in rats. Further experiments are needed to elucidate the exact role of these receptors in anxiety.     

Buprenorphine blocks epsilon- and micro-opioid receptor-mediated antinociception in the mouse

Antagonistic properties of buprenorphine for epsilon- and micro -opioid receptors were characterized in beta-endorphin- and [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO)-induced antinociception, respectively, with the tail-flick test in male ICR mice. epsilon-Opioid receptor agonist beta-endorphin (0.1-1 micro g), micro -opioid receptor agonist DAMGO (0.5-20 ng), or buprenorphine (0.1-20 micro g) administered i.c.v. dose dependently produced antinociception. The antinociception induced by 10 micro g of buprenorphine given i.c.v. was completely blocked by the pretreatment with beta-funaltrexamine (beta-FNA) (0.3 micro g i.c.v.), indicating that the buprenophine-induced antinociception is mediated by the stimulation of the micro -opioid receptor. The antinociceptive effects induced by beta-endorphin (1 micro g i.c.v.) and DAMGO (16 ng i.c.v.) were dose dependently blocked by pretreatment with smaller doses of buprenorphine (0.001-1 micro g i.c.v.), but not by a higher dose of buprenorphine (10 micro g i.c.v.). beta-FNA at a dose (0.3 micro g i.c.v.) that strongly attenuated DAMGO-induced antinociception had no effect on the antinociception produced by beta-endorphin (1 micro g i.c.v.). However, pretreatment with buprenorphine (0.1-10 micro g) in mice pretreated with this same dose of beta-FNA was effective in blocking beta-endorphin-induced antinociception. beta-FNA was 226-fold more effective at antagonizing the antinociception induced by DAMGO (16 ng i.c.v.) than by beta-endorphin (1 micro g i.c.v.). The antinociception induced by delta-opioid receptor agonist [d-Ala2]deltorphin II (10 micro g i.c.v.) or kappa1-opioid receptor agonist trans-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)benzeneacetamine methanesulfonate salt [(-)-U50,488H] (75 micro g i.c.v.) was not affected by pretreatment with buprenorphine (0.1-1.0 micro g i.c.v.). It is concluded that buprenorphine, at small doses, blocks epsilon-opioid receptor-mediated beta-endorphin-induced antinociception and micro -opioid receptor-mediated DAMGO-induced antinociception, and at high doses produces a micro -opioid receptor-mediated antinociception.     

Activation of central GABAB receptors offsets the cyclosporine counteraction of endotoxic cardiovascular outcomes in conscious rats

We have previously shown that cyclosporine (CSA) counteracts cardiovascular manifestations induced by endotoxemia (lipopolysaccharide, LPS) such as hypotension and cardiac autonomic dysfunction in conscious rats. In this study, we investigated whether the facilitation of central γ‐amino butyric acid (GABA) neurotransmission blunts these favorable influences of CSA. The LPS‐CSA interaction was determined in the absence and presence of drugs that activate GABA_A or GABA_B receptors or elevate synaptic GABA levels in the central nervous system. The consequent i.v. administration of CSA (10 mg/kg) blunted the LPS‐evoked hypotension, tachycardia, and reductions in time‐ and frequency‐domain indices of heart rate variability (measures of cardiac autonomic control) evoked by LPS (10 mg/kg i.v.). The ability of CSA to reverse the LPS effects disappeared in rats treated intracisternally (i.c.) with baclofen (selective GABA_B agonist, 2 μg/rat) but not muscimol (selective GABA_A agonist, 1 μg/rat), indicating a preferential compromising action for central GABA_B receptors on the advantageous effects of CSA. Moreover, the improvement by CSA of LPS‐evoked cardiovascular derangements was also eliminated after concurrent i.c. administration of vigabatrin (GABA transaminase inhibitor, 200 μg/rat) or tiagabine (GABA reuptake inhibitor, 100 μg/rat). These results demonstrate that the activation of central GABA_B receptors either directly via baclofen or indirectly following interventions that boost GABA levels in central synapses counterbalances the rectifying action of CSA on endotoxemia.     

The central analgesia induced by antimigraine drugs is independent from Gi proteins: superiority of a fixed combination of indomethacin, prochlorperazine and caffeine, compared to sumatriptan, in an in vivo model

A hypofunctionality of Gi proteins has been found in migraine patients. The fixed combination of indomethacin, prochlorperazine and caffeine (Indoprocaf) is a drug of well-established use in the acute treatment of migraine and tension-type headache. The aim of this study was to investigate if Indoprocaf was able to exert its central antinociceptive action when Gi proteins activity is abolished by pertussis toxin (PTX), compared to its single active ingredients and to sumatriptan. The mice model of abdominal constriction test induced by an i.p. injection of a 0.6% solution of acetic acid was used. The study showed that Indoprocaf (a fixed combination of indomethacin 1 mg/kg, prochlorperazine 1 mg/kg and caffeine 3 mg/kg, s.c.) and sumatriptan (20 mg/kg, s.c.) exert their central antinociceptive action independently from the Gi proteins. In addition, the antinociceptive efficacy of Indoprocaf in this study was statistically superior to that of sumatriptan. This study also showed that the single active ingredients of Indoprocaf, indomethacin (1 mg/kg, s.c.), prochlorperazine (1 mg/kg, s.c.) and caffeine (3 mg/kg, s.c.), were able to exert their central antinociceptive action independently from the Gi proteins. However, Indoprocaf at analgesic doses was able to abolish almost completely the abdominal constrictions, with a statistically higher efficacy compared to the single active ingredients, showing an important synergic effect of Indoprocaf. This synergic effect was evident not only when Gi proteins activity was abolished by PTX, but also under control condition, when Gi proteins were active. This study suggests that the central antinociceptive action induced by antimigraine drugs is independent from Gi proteins.     

Exenatide suppresses 1,2-dimethylhydrazine-induced colon cancer in diabetic mice: Effect on tumor angiogenesis and cell proliferation

Colon cancer is the third leading cause of cancer mortality worldwide, which results from interactions of different factors. It is frequently a pathological consequence of persistent inflammation. Diabetes affects several cancers and is positively correlated with the incidence of colon cancer. This study aimed to study the effect of exenatide in ameliorating inflammation, angiogenesis and cell proliferation in 1,2-dimethyl hydrazine (DMH) induced colorectal carcinoma in diabetic mice. Mice were randomly allocated into six groups, 8 mice each. Group 1: vehicle control group. Group 2: diabetic control group. Group 3: DMH control group: diabetic mice treated with DMH (20 mg/kg/week, s.c.) for 15 week. Group 4: DMH-cisplatin group: mice received cisplatin (4 mg/kg/week, i.p.). Groups 5 & 6: DMH-exenatide (10 and 20 μg/kg) group: mice received exenatide (10 or 20 μg/kg/day, s.c.), respectively. The present results highlighted an increase in angiogenic markers and cell proliferation in the DMH-diabetic group in comparison with the control group with greater expression of endothelial marker (CD₃₄) and Ki-67 in colon tissue. Monotherapy with cisplatin or exenatide (10 and 20 μg/kg) downregulated these markers to different extents. The current results provided evidence that exenatide represents a promising chemopreventive effect against DMH-induced colon carcinogenesis in diabetic mice, at least in part, attributed to its anti-angiogenic and anti-proliferative mechanisms.     

Bioguidage search of active compounds from Waltheria indica L. (Malvaceae) used for asthma and inflammation treatment in Burkina Faso

Waltheria indica is used in traditional pharmacopeia in Burkina Faso for the treatment of asthma and conditions of inflammation. To evaluate its pharmacological properties and isolate the active compounds, a study through a bioguided phytochemical approach was conducted. This search was guided by a two‐level investigation. First, we evaluated the impact of various fractions on the activity of enzymes involved in smooth muscle contraction (PDE4A1α) and inflammatory processes (PLA₂, 5‐LOX). Second, we investigated the inhibitory effect of fractions on isolated rat trachea. The initial hydroalcoholic extract from roots of W. indica (HA), n‐hexane fraction (F₁), dichloromethane fraction (F₂), ethyl acetate fraction (F₃), residuary fraction (F₄) reduced enzyme activity of PDE4A1α (inhibition of 22–42% at 50 μg/mL), 5‐LOX (60–80% at 10 μg/mL), and PLA₂ (42–94% at 100 μg/mL). On isolated rat trachea, only HA, F₃, and fractions obtained from F₃ by chromatography on silica gel column, using dichloromethane/methanol, dose dependently inhibited contraction induced by acetylcholine. IC₅₀ was 1051 μg/mL for HA and comprised between 181 and 477 μg/mL for F₃ and its fractions. The most active fractions were purified and led to the identification of (‐)‐epicatechin. (‐)‐epicatechin from W. indica dose dependently inhibited PLA₂ (IC₅₀ = 154.7 μm ) and 5‐LOX (IC₅₀ = 15.8 μm ). In conclusion, both inhibition of PDE4A1α, 5‐LOX, and PLA₂ activities and rat trachea relaxation by W. indica validate its use in traditional management of asthma and other conditions of inflammation. These effects should be, at least in part, attributed to the presence of (‐)‐epicatechin in roots of W. indica.     

Role of Na(+), K(+)-ATPase in morphine-induced antinociception

We evaluated the modulation by Na+,K+-ATPase inhibitors of morphine-induced antinociception in the tail-flick test and [3H]naloxone binding to forebrain membranes. The antinociception induced by morphine (1-32 mg/kg, s.c.) in mice was dose-dependently antagonized by ouabain (1-10 ng/mouse, i.c.v.), which produced a significant shift to the right of the morphine dose-response curve. The i.c.v. administration of three Na+,K+-ATPase inhibitors (ouabain at 0.1-100, digoxin at 1-1000, and digitoxin at 10-10000 ng/mouse) dose-dependently antagonized the antinociceptive effect of morphine (4 mg/kg, s.c.) in mice, with the following order of potency: ouabain > digoxin > digitoxin. This effect cannot be explained by any interaction at opioid receptors, since none of these Na+,K+-ATPase inhibitors displaced [3H]naloxone from its binding sites, whereas naloxone did so in a concentration-dependent manner. The antinociception induced by morphine (5 mg/kg, s.c.) in rats was antagonized by the i.c.v. administration of ouabain at 10 ng/rat, whereas it was not significantly modified by intrathecally administered ouabain (10 and 100 ng/rat). These results suggest that the activation of Na+,K+-ATPase plays a role in the supraspinal, but not spinal, antinociceptive effect of morphine.     

Incomplete, asymmetric, and route-dependent cross-tolerance between oxycodone and morphine in the Dark Agouti rat

Our previous studies indicate that oxycodone is a putative kappa-opioid agonist, whereas morphine is a well documented micro-opioid agonist. Because there is limited information regarding the development of tolerance to oxycodone, this study was designed to 1) document the development of tolerance to the antinociceptive effects of chronically infused i.v. oxycodone relative to that for i. v. morphine and 2) quantify the degree of antinociceptive cross-tolerance between morphine and oxycodone in adult male Dark Agouti (DA) rats. Antinociceptive testing was performed using the tail-flick latency test. Complete antinociceptive tolerance was achieved in 48 to 84 h after chronic infusion of equi-antinociceptive doses of i.v. oxycodone (2.5 mg/24 h and 5 mg/24 h) and i.v. morphine (10 mg/24 h and 20 mg/24 h, respectively). Dose-response curves for bolus doses of i.v. and i.c.v. morphine and oxycodone were produced in naive, morphine-tolerant, and oxycodone-tolerant rats. Consistent with our previous findings that oxycodone and morphine produce their intrinsic antinociceptive effects through distinctly different opioid receptor populations, there was no discernible cross-tolerance when i.c.v. oxycodone was given to morphine-tolerant rats. Similarly, only a low degree of cross-tolerance (approximately 24%) was observed after i.v. oxycodone administration to morphine-tolerant rats. By contrast, both i.v. and i.c.v. morphine showed a high degree of cross-tolerance (approximately 71% and approximately 54%, respectively) in rats rendered tolerant to oxycodone. Taken together, these findings suggest that, after parenteral but not supraspinal administration, oxycodone is metabolized to a mu-opioid agonist metabolite, thereby explaining asymmetric and incomplete cross-tolerance between oxycodone and morphine.     

Analgesic and toxic effects of ABT-594 resemble epibatidine and nicotine in rats

The present study directly compared the antinociceptive and toxic effects of the neuronal nicotinic receptor agonist ABT-594 ((R)-5-(2-azetidinylmethoxy)-2-chloropyridine) with (-)-nicotine and (+)-epibatidine. Like (-)-nicotine (0.8 and 1.6 mg/kg s.c.) and (+)-epibatidine (0.005 and 0.01 mg/kg s.c.), ABT-594 (0.05 and 0.1 mg/kg s.c.) increased response latencies in the hot-plate test in rats, indicating that it has antinociceptive activity. In contrast to (-)-nicotine and (+)-epibatidine, ABT-594 did not cause rotarod impairment at antinociceptive doses but did cause hypothermia and life-threatening adverse effects including seizures. ABT-594 (0.01 and 0.1 mg/kg i.v.) also produced a dose-dependent increase in blood pressure resembling that observed with (-)-nicotine (0.03, 0.1 and 0. 03 mg/kg i.v.) and (+)-epibatidine (0.001 and 0.003 mg/kg i.v.). Both the antinociceptive and toxic effects (convulsions and hypertension) were abolished by pretreatment with the brain penetrant neuronal nAChR antagonist mecamylamine (1 mg/kg s.c.; i.v. for cardiovascular studies), demonstrating that these actions of ABT-594 were mediated via activation of neuronal nicotinic receptors. Continuous infusion of ABT-594 (0.2 mg/kg per day s.c.) to rats for 7 days followed by challenge with mecamylamine (1 mg/kg i.p.) induced a nicotine-like abstinence syndrome suggesting that ABT-594 has nicotine-like dependence liability. These findings indicate that the acute safety profile of ABT-594 is not significantly improved over other nicotinic analgesics.     

Opioid agonist and antagonist antinociceptive properties of [D-Ala2,Leu5,Cys6]enkephalin: selective actions at the deltanoncomplexed site

The present study used the irreversibly binding enkephalin analog, [D-Ala2,Leu5,Cys6]enkephalin (DALCE) in an effort to determine whether selective agonist and antagonist properties could be demonstrated at hypothesized types of opioid delta receptors previously termed the deltanoncomplexed and the deltacomplexed sites. These putative subtypes of delta receptors have been functionally distinguished on the basis of involvement (i.e., deltacomplexed) in the modulation of mu-mediated effects such as antinociception. Intracerebroventricular administration of DALCE or the reference delta and mu agonists, [D-Pen2,D-Pen5]enkephalin (DPDPE) and morphine, to mice all produced antinociception in the warm-water tail-flick test in a dose- and time-related manner. Maximal effects with DALCE were seen at +10 min and significant antinociception could be detected for approximately 1 hr; DALCE was 3- and 90-fold more potent than i.c.v. morphine and DPDPE, respectively. The antinociceptive effects of i.c.v. DALCE and DPDPE, but not those of morphine, were antagonized by the selective delta antagonist, N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH, suggesting that the antinociception associated with the peptides was mediated through a delta receptor. DALCE pretreatment up to 24 hr before testing, a time at which this compound did not produce antinociception, significantly blocked the i.c.v. DPDPE antinociceptive effect as well as that of DALCE itself, but not that of morphine, suggesting long-lasting DALCE antagonism at a delta receptor. Modulation of morphine antinociception was demonstrated with subeffective doses of i.c.v. DPDPE or [Met5]enkephalin, but not with subeffective doses of i.c.v. DALCE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ascending Nociceptive Control Contributes to the Antinociceptive Effect of Acupuncture in a Rat Model of Acute Pain

Acupuncture-induced analgesia depends on the activation of endogenous pain modulation pathways. In this study, we asked whether ascending nociceptive control (ANC), a form of pain-induced analgesia, contributes to the antinociceptive effect of acupuncture. To answer this question, we tested the ability of procedures that block ANC-induced analgesia, at peripheral, spinal, nucleus accumbens and rostral ventral medulla levels, to block acupuncture-induced analgesia. Acupuncture at ST36 (Zusanli), a widely used acupoint located in the hind limb, induced potent heterosegmental antinociception in the orofacial formalin test. The magnitude of this antinociceptive effect was similar to that induced by an intraplantar injection of capsaicin, a procedure classically used to activate ANC. The antinociceptive effect of acupuncture was blocked by sciatic C-fibers depletion (1% perineural capsaicin), spinal administration of a μ-opioid (Cys2,Tyr3,Orn5,Pen7amide, .2 μg) or of a GABA_A (bicuculline, .3 μg) receptor antagonist, intra–nucleus accumbens administration of a μ-opioid receptor antagonist (Cys2,Tyr3,Orn5,Pen7amide, 1 μg), or intrarostral ventral medulla administration of a nicotinic acetylcholine receptor antagonist (mecamylamine, .6 μg). In addition, acupuncture at ST36 and/or upper lip formalin induced c-Fos expression in the nucleus accumbens and in rostral ventral medulla. On the basis of these results, we propose that ANC contributes to the antinociceptive effect of acupuncture.PerspectiveThis article presents a novel mechanism of acupuncture analgesia, contributing to the understanding of its scientific basis. Because ANC is a pain modulation pathway activated by peripheral noxious stimulation that ascends to supraspinal regions, it could be the link between acupoint stimulation and the central mechanisms underlying acupuncture analgesia.     

Extremely long-lasting antagonistic actions of nor-binaltorphimine (nor-BNI) in the mouse tail-flick test.

The duration of antagonistic action of nor-binaltorphimine (nor-BNI), a kappa antagonist, of antinociception resulting from selective opioid agonists, was examined using the mouse tail-flick assay as the endpoint. Nor-BNI (1 nmol, i.c.v. at -20 min) antagonized equiantinociceptive doses of the opioid kappa agonists (5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro (4,5)dec-8-yl) benzeneacetamide (U69,593) (70 nmol i.c.v.) or bremazocine (25 nmol i.c.v.), but did not antagonize antinociception produced by the mu opioid-selective [D-Ala2, NMePhe4, Gly-ol]enkephalin or the delta opioid-selective [D-Pen2, D-Pen5]enkephalin. Pretreatment with nor-BNI (1 nmol i.c.v.) antagonized the antinociceptive effects of U69,593 and bremazocine for up to 28 days. At all pretreatment times, the antinociceptive dose-response lines for these kappa agonists were displaced to the right to various degrees in a parallel fashion; an increasing rightward displacement of the U69,593 and bremazocine antinociceptive dose-response lines was observed at 1 and 3 days after a single nor-BNI pretreatment, with a gradual return toward the control level at later times after pretreatment. Increasing the dose of nor-BNI to 10 nmol produced only a transient blockade of equiantinociceptive doses of the mu selective agonist [D-Ala2, NMePhe4, Gly-ol]enkephalin and the delta selective agonist [D-Pen2, D-Pen5]enkephalin (at 20-30 min post-nor-BNI pretreatment).(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphine and ABT-594 (a nicotinic acetylcholine agonist) exert centrally mediated antinociception in the rat cyclophosphamide cystitis model of visceral pain.

A visceral pain model incorporating use of cyclophosphamide (CP) to induce bladder inflammation has been described. CP treatment in rats produces changes in behavior (abnormal postures and eye closure) and respiration rate indicative of visceral pain. We characterized the dose-dependency and progression of CP-induced cystitis pain after intraperitoneal (i.p.) CP. The behavioral and respiration rate changes were ameliorated by systemic morphine and ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine], a neuronal nicotinic acetylcholine receptor agonist, in a manner reversible by naloxone and mecamylamine, respectively. Sites of antinociceptive actions of morphine and ABT-594 were investigated using systemic, intrathecal (i.t.), or intracerebroventricular (i.c.v.) administration of blood-brain barrier impenetrant antagonists. Naloxone methiodide produced a complete antagonism of morphine antinociception after i.c.v. but not i.p. or i.t. administration. Chlorisondamine blocked ABT-594 antinociception after i.c.v. but not i.p. administration. Further pharmacological characterization of behavioral and respiration changes in CP-cystitis was performed using standard analgesics. The alpha(2)-adrenoceptor agonist clonidine produced a weak attenuation of CP-pain behavior. NSAIDs (ibuprofen, acetaminophen, and celecoxib) and anticonvulsants (gabapentin and lamotrigine) were without effect. These results demonstrate that morphine and ABT-594 produce antinociception in CP-cystitis by a predominantly supraspinal site of action, and that mechanisms producing robust centrally-mediated antinociception could be beneficial in cystitis pain. PERSPECTIVE: In this article, potential antinociceptive effects of a variety of pharmacological agents were evaluated in a rat cystitis pain model. Morphine and a nicotinic acetylcholine receptor agonist ABT-594 were found to exert potent antinociception in this model. Findings presented here aid identification of agents to treat cystitis pain in the clinic.     

Dissociation of opioid antinociception and central gastrointestinal propulsion in the mouse: studies with naloxonazine

The effect of pretreatment with naloxonazine on opioid-mediated antinociception against a thermal stimulus (55 degrees C warm-water tail-flick test) and inhibition of gastrointestinal transit at supraspinal and spinal levels was studied in unanesthetized mice. The mu-selective agonist [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAGO), the delta-selective agonist [D-Pen2, D-Pen5]enkephalin (DPDPE) and the reference mu-acting agonist morphine, all produced antinociception after either i.c.v. or intrathecal(ly) (i.t.) administration. Morphine and DAGO, but not DPDPE, inhibited gastrointestinal transit after i.c.v. administration, whereas all three agonists slowed gut propulsion when given i.t. A single s.c. naloxonazine pretreatment, 35 mg/kg given 24 hr earlier, failed to displace the dose-response line for i.c.v. DPDPE antinociception but produced a marked rightward displacement of the i.c.v. morphine and DAGO dose-response lines for antinociception. In contrast, naloxonazine (35 mg/kg) pretreatment did not alter the antinociceptive effects of i.t. morphine, DAGO or DPDPE. The effects of naloxonazine pretreatment on inhibition of gut propulsion were the converse of those observed for antinociception at supraspinal and spinal sites; naloxonazine had no effect on the antitransit properties of i.c.v. morphine and DAGO but inhibited the antitransit properties of all three agonists when they were given i.t. These results support the view that opioids may produce their supraspinal antitransit effects at a receptor different from that mediating antinociception; morphine and DAGO mediate their antitransit effects at a naloxonazine-insensitive site, whereas their antinociceptive effects are produced at the naloxonazine-sensitive receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antinociceptive and pharmacological effects of metanicotine, a selective nicotinic agonist.

Metanicotine [N-methyl-4-(3-pyridinyl)-3-butene-1-amine], a novel neuronal nicotinic agonist, was found to bind with high affinity (K(i) = 24 nM) to rat brain [(3)H]nicotine binding sites and it generalized to nicotine in a dose-dependent manner in the drug discrimination procedure. Metanicotine produced significant antinociceptive effects in mice and rats subjected to either acute thermal (tail-flick), mechanical (paw-pressure), chemical (para-phenylquinone), persistent (Formalin), and chronic (arthritis) pain stimuli. Metanicotine was about 5-fold less potent than nicotine in the tail-flick test after s.c administration, but slightly more potent after central administration. Its duration of action was longer than that of nicotine. Nicotinic antagonists, mecamylamine and dihydro-beta-erythroidine, blocked metanicotine-induced antinociception in the different pain models. However, the antinociceptive effect was not affected by pretreatment with either naloxone or by atropine, confirming that metanicotine exerts its antinociceptive effect via nicotinic rather than either opioid or muscarinic mechanisms. In contrast to nicotine, antinociceptive effects of metanicotine were observed at doses that had virtually no effect on spontaneous activity and body temperature in mice. These data indicate that metanicotine is a centrally acting neuronal nicotinic agonist with preferential antinociceptive effects in animals. Thus, metanicotine and related nicotinic agonists may have great potential for development as a new class of analgesics.     

Sigma-1 receptors are essential for capsaicin-induced mechanical hypersensitivity: Studies with selective sigma-1 ligands and sigma-1 knockout mice

We evaluated the role of σ₁ receptors on capsaicin-induced mechanical hypersensitivity and on nociceptive pain induced by punctate mechanical stimuli, using wild-type and σ₁ receptor knockout (σ₁-KO) mice and selective σ₁ receptor-acting drugs. Mutation in σ₁-KO mice was confirmed by PCR analysis of genomic DNA and, at the protein level, by [³H](+)-pentazocine binding assays. Both wild-type and σ₁-KO mice not treated with capsaicin showed similar responses to different intensities of mechanical stimuli (0.05–8 g force), ranging from innocuous to noxious, applied to the hind paw. This indicates that σ₁ gene inactivation does not modify the perception of punctate mechanical stimuli. The intraplantar (i.pl.) administration of capsaicin induced dose-dependent mechanical allodynia in wild-type mice (markedly reducing both the threshold force necessary to induce paw withdrawal and the latency to paw withdrawal induced by a given force). In contrast, capsaicin was completely unable to induce mechanical hypersensitivity in σ₁-KO mice. The high-affinity and selective σ₁ antagonists BD-1063, BD-1047 and NE-100, administered subcutaneously (s.c.), dose-dependently inhibited mechanical allodynia induced by capsaicin (1 μg,i.pl.), yielding ED₅₀ (mg/kg) values of 15.80 ± 0.93, 29.31 ± 1.65 and 40.74 ± 7.20, respectively. The effects of the σ₁ antagonists were reversed by the σ₁ agonist PRE-084 (32 mg/kg, s.c.). None of the drugs tested modified the responses induced by a painful mechanical punctate stimulus (4 g force) in nonsensitized animals. These results suggest that σ₁ receptors are essential for capsaicin-induced mechanical hypersensitivity, but are not involved in mechanical nociceptive pain.     

Methadone Antinociception Is Dependent on Peripheral Opioid Receptors

Morphine and methadone are both high-affinity, potent μ-opioid peptide (MOP) receptor analgesics. In this report, we compared the antinociceptive potencies of these 2 drugs when administered subcutaneously (s.c.), intrathecally (i.t.), or intracerebroventricularly (i.c.v.) in both rat and mouse, using the tail-flick assay. We found that both morphine and methadone were potently antinociceptive when the drugs were administered s.c., showing comparable AD50 values in both species. However, the antinociception produced by methadone, when it was administered centrally, was much weaker than that produced by centrally administered morphine. Specifically, the AD50 value for methadone antinociception was more than 30-fold higher at both the i.t. and i.c.v. sites in mouse and not measurable in rat. Naloxone methiodide (NLX-M), a peripherally restricted antagonist, was used to further examine the relative contribution of central versus peripheral sites to morphine and methadone antinociception. NLX-M, when administered s.c., blocked the antinociceptive effect of either systemically or centrally administered methadone but had little effect on the antinociception produced by centrally administered morphine. Furthermore, centrally administered NLX-M significantly blocked antinociception produced by centrally administered morphine but not that produced by centrally administered methadone. Together, these results suggest that methadone antinociception is significantly dependent on an action of the drug at peripheral sites and could provide novel insight into the neural mechanisms that distinguish morphine versus methadone antinociception.PerspectiveMethadone is often used as an alternative for pain management. The present study shows that a peripheral action plays a crucial role in methadone antinociception. This finding could have significant clinical relevance for the use of methadone versus morphine for the treatment of certain types of pain.     

Discovery of 2-(3,4-dichlorophenoxy)-N-(2-morpholin-4-ylethyl)acetamide: A selective σ1 receptor ligand with antinociceptive effect

Compound 2-(3,4-dichlorophenoxy)-N-(2-morpholin-4-ylethyl)acetamide (1) was designed, prepared and the in vitro binding evaluation against σ₁ and σ₂ receptors was measured. Compound 1 showed high σ₁ receptor affinity (K_i = 42 nM) and it was 36-times more selective for σ₁ than σ₂ receptor. Also, it was performed a molecular docking of compound 1 into the ligand binding pocket homology model of σ₁ receptor, showing a salt bridge between the ionized morpholine ring and Asp126, as well as important short contacts with residues Tyr120, His154 and Trp164. Ligand efficiency indexes and predicted toxicity analysis revealed an excellent intrinsic quality of 1. The antinociceptive effect of compound 1 was determined using the formalin test. The ipsilateral local peripheral (10–300 μg/paw) and intrathecal (100 μg/rat) administration of 1 produced a reduction in formalin-induced nociception. The in vivo results indicated that 1 may be effective in treating inflammatory pain.     

Examination of the interaction between peripheral lumiracoxib and opioids on the 1% formalin test in rats

It has been shown that the association of non‐steroidal anti‐inflammatory drugs (NSAIDs) with opioid analgesic agents can increase their antinociceptive activity, allowing the use of lower doses and thus limiting side effects. Therefore, the aim of the present study was to examine the possible pharmacological interaction between lumiracoxib and codeine or nalbuphine at the local peripheral level in the rat using the 1% formalin test and isobolographic analysis. Lumiracoxib, codeine, nalbuphine or fixed‐dose ratios lumiracoxib—codeine or lumiracoxib—nalbuphine combinations were administrated locally in the formalin‐injured paw and the antinociceptive effect was evaluated using the 1% formalin test. All treatments produced a dose‐dependent antinociceptive effect. ED₄₀ values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical ED₄₀'s for the lumiracoxib—codeine and lumiracoxib—nalbuphine combinations were 423.4±31.3μg/paw and 310.9±24.2μg/paw, respectively, being significantly higher than the actually observed experimental ED₄₀ values, 233.2±30.9μg/paw and 132.7±11.6μg/paw, respectively. These results correspond to a synergistic interaction between lumiracoxib and opioids at the local peripheral level, potency being about two times higher with regard to that expected from the addition of the effects of the individual drugs. Data suggest that low doses of the lumiracoxib—opioids combination can interact synergistically at the peripheral level and therefore this drug association may represent a therapeutic advantage for the clinical treatment of inflammatory pain.