TENA, a selective kappa opioid receptor antagonist

A number of opioid antagonists (TENA, naloxone, Mr 2266, WIN 44441) were evaluated for their selectivity in antagonizing the effect of mu, kappa, and delta agonists in the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations. Among these four antagonists, TENA was the most potent and the only ligand which was selective for kappa receptors. In this regard TENA was approximately 27-times more effective in antagonizing the kappa agonist, U-50488H, relative to the mu agonist, morphine, and it was about 5-times more effective against ethylketazocine (EK) relative to morphine. At the same concentration (20 nM) TENA did not significantly antagonize the delta agonist, [D-Ala2,D-Ala5]enkephalin (DADLE), in the MVD. Also, TENA was more effective than naloxone, EK, or U-50488H in protecting kappa receptors from irreversible blockage by beta-CNA. The results of this study indicate that TENA is the most selective kappa antagonist yet reported.     

Characterization of opioid receptors on isolated canine gallbladder smooth muscle cells

Smooth muscle cells were isolated from the fundus of the canine gallbladder and examined for the presence of opioid receptors. The cells contracted in a concentration-dependent manner in response to three opioid peptides (Met-enkephalin, dynorphin1-13 and Leu-enkephalin), which are known derivatives of opioid precursors present in myenteric neurons of the gut. The order of potency was Met-enkephalin greater than dynorphin1-13 greater than Leu-enkephalin. The contractile response to opioid agonists was selectively inhibited by opioid antagonists (naloxone and Mr2266) but not by muscarinic, CCK/gastrin or tachykinin antagonists. Equivalent responses to the three opioid peptides exhibited differential sensitivity to preferential antagonists of mu (naloxone) and kappa (Mr2266) opioid receptors consistent with the presence of the three main types of opioid receptors (mu, delta and kappa) on canine gallbladder muscle cells.     

Effects of ethanol on deep pain evoked by formalin injected in TMJ of rat

It has been reported that ethanol can alter nociceptive sensitivity from superficial tissues, such as skin and subcutaneous region. However, the influence of ethanol on deep pain conditions is not understood. The aim of this study was to demonstrate the acute, chronic and ethanol withdrawal effects on nociceptive behavioral responses induced by the injection of formalin into the temporomandibular joint (TMJ) region of rats. In experiment 1, rats were injected with ethanol (2,5 g/Kg, i.p.) or an equal volume of saline 15 min before the administration of formalin (1.5%) into the TMJ. Rats pretreated with ethanol showed a decrease in nociceptive behavioral responses. In experiment 2, rats were given an ethanol solution (6.5%) or tap water to drink for 4 and 10 days. On day 4, the animals (ethanol group) showed amounts of analgesia when submitted to the TMJ formalin test. Tolerance to the antinociceptive effects was observed on day 10. Behavioral hyperalgesia was verified 12 hr after withdrawal in another group that drank ethanol for 10 days. These results show that ethanol can affect the nociceptive responses related to deep pain evoked by the TMJ formalin test.     

Antinociceptive activity of a 3(2H)-pyridazinone derivative in mice.

The antinociceptive activity of a 3(2H)-pyridazinone derivative (18a) was investigated in mice. 18a administered at doses which did not change either motor coordination or locomotor activity was able to induce antinociceptive effects in four nociceptive tests, the hot plate test, the tail flick test, the writhing test, and the formalin test. In the hot plate and tail flick test, 18a-induced antinociception was observed both after intraperitoneal administration and after intracerebroventricular injection thus indicating 18a has a central site of action. The pretreatment with the opioid antagonist naloxone, the alpha2-antagonist yohimbine or the GABA(B) antagonist CGP 35348 did not change 18a-induced antinociception in the hot plate test and in the tail flick test. Pretreatment with nicotinic antagonist mecamylamine did not change 18a effects either. A reversion of the 18a effects was observed after pretreatment with the muscarinic antagonists atropine and pirenzepine. Binding experiments revealed that 18a binds to muscarinic receptors, suggesting that 18a antinociception is mediated by central muscarinic receptors. The above findings together with the lack of parasympathomimetic cholinergic side effects indicate useful clinical application for this compound.     

Anatomy and physiology of a nociceptive modulatory system

Although efferent control of sensory transmission is a well-established concept, a specific network for nociceptive modulation has only recently been discovered. This network includes interconnected components at midbrain, medullary and spinal levels. At the midbrain level, electrical stimulation of the periaqueductal grey (p.a.g.) inhibits spinal neurons that respond to noxious stimuli as well as nociceptor-induced reflexes and escape behaviour in a variety of species. Midbrain stimulation also produces analgesia in patients with clinically significant pain. The rostral ventral medulla (r.v.m.) has similar behavioural and physiological effects and mediates midbrain antinociceptive actions at the level of the spinal cord. Endorphins are present at all levels of this nociceptive modulating network. Opiate microinjections at p.a.g., r.v.m. or spinal levels produce analgesia, presumably by mimicking the actions of the endorphins. The nociceptive modulatory system is diffusely organized, highly interconnected and appears to act as a unit whether activated by opiates or electrical stimulation. There are two classes of r.v.m. neurons the activity of which is correlated with the occurrence of reflexes induced by noxious stimulation. One class (the on-cell) accelerates, the other class (the off-cell) pauses just before tail flick. Both classes project to the spinal cord and are excited by electrical stimulation of the midbrain. However, when morphine is injected either systemically or into the p.a.g., the off-cell is excited and the on-cell stops firing. The off-cell is probably the r.v.m. output cell that inhibits nociceptive transmission at the level of the spinal cord. The function of the on-cell is not clear. The nociceptive modulatory system can be activated by a variety of stressful environmental factors, which are often, but not necessarily, noxious. The idea that the system acts as a simple negative feedback circuit is not consistent with its known properties.     

P2X receptor antagonists for pain management: examination of binding and physicochemical properties

Enhanced sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful are hallmark sensory perturbations associated with chronic pain. It is now appreciated that ATP, through its actions as an excitatory neurotransmitter, plays a prominent role in the initiation and maintenance of chronic pain states. Mechanistically, the ability of ATP to drive nociceptive sensitivity is mediated through direct interactions at neuronal P2X3 and P2X2/3 receptors. Extracellular ATP also activates P2X4, P2X7, and several P2Y receptors on glial cells within the spinal cord, which leads to a heightened state of neural-glial cell interaction in ongoing pain states. Following the molecular identification of the P2 receptor superfamilies, selective small molecule antagonists for several P2 receptor subtypes were identified, which have been useful for investigating the role of specific P2X receptors in preclinical chronic pain models. More recently, several P2X receptor antagonists have advanced into clinical trials for inflammation and pain. The development of orally bioavailable blockers for ion channels, including the P2X receptors, has been traditionally difficult due to the necessity of combining requirements for target potency and selectivity with suitable absorption distribution, metabolism, and elimination properties. Recent studies on the physicochemical properties of marketed orally bioavailable drugs, have identified several parameters that appear critical for increasing the probability of achieving suitable bioavailability, central nervous system exposure, and acceptable safety necessary for clinical efficacy. This review provides an overview of the antinociceptive pharmacology of P2X receptor antagonists and the chemical diversity and drug-like properties for emerging antagonists of P2X3, P2X2/3, P2X4, and P2X7 receptors.

     

P2X receptor antagonists for pain management: examination of binding and physicochemical properties

Enhanced sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful are hallmark sensory perturbations associated with chronic pain. It is now appreciated that ATP, through its actions as an excitatory neurotransmitter, plays a prominent role in the initiation and maintenance of chronic pain states. Mechanistically, the ability of ATP to drive nociceptive sensitivity is mediated through direct interactions at neuronal P2X3 and P2X2/3 receptors. Extracellular ATP also activates P2X4, P2X7, and several P2Y receptors on glial cells within the spinal cord, which leads to a heightened state of neural-glial cell interaction in ongoing pain states. Following the molecular identification of the P2 receptor superfamilies, selective small molecule antagonists for several P2 receptor subtypes were identified, which have been useful for investigating the role of specific P2X receptors in preclinical chronic pain models. More recently, several P2X receptor antagonists have advanced into clinical trials for inflammation and pain. The development of orally bioavailable blockers for ion channels, including the P2X receptors, has been traditionally difficult due to the necessity of combining requirements for target potency and selectivity with suitable absorption distribution, metabolism, and elimination properties. Recent studies on the physicochemical properties of marketed orally bioavailable drugs, have identified several parameters that appear critical for increasing the probability of achieving suitable bioavailability, central nervous system exposure, and acceptable safety necessary for clinical efficacy. This review provides an overview of the antinociceptive pharmacology of P2X receptor antagonists and the chemical diversity and drug-like properties for emerging antagonists of P2X3, P2X2/3, P2X4, and P2X7 receptors.     

Antinociceptive properties of the 5-HT3 receptor antagonist in the model of inflammatory pain in rats of different age with prenatal deficit of serotonine and under stress

The role of peripheral 5-HT3 receptors in the nociceptive behavioral response and the effect of the 5-HT3 antagonist ondansetron on indices of acute and tonic pain were investigated in the formalin test in 25- and 90-day-old Wistar male rats. The experimental rats were prenatally exposed to 5-HT depletion (a single injection ofparachlorophenilalanine 400 mg/kg/2 ml, i. p.; ICN, USA to the dams on day 9 of pregnancy) and to stress (dams immobilization during the last week of pregnancy). Antinociceptive effects of ondansetron in the rats with both prenatal 5-HT deficiency and stress (experimental rats) and prenatal injection of saline solution and stress (control rats) were more obvious in the younger animals. Prenatal 5-HT deficiency attenuated the antinociceptive effect of ondansetron in licking patterns in the younder age group in acute pain, and in adults--in tonic pain. Thus, the data obtained in the rats with prenatal 5-HT deficiency and stress indicate involvement of 5-HT3 receptors in mediation of prolonged pain in the formalin test, and antinociceptive effect of ondansetron which is attenuated in animals with prenatal 5-HT deficiency and specifically depends on rat's age.     

Effect of diphenylhydramine on the Tyr-MIF-1 antinociception in rats.

Tyr-MIF-1 is a representative of the MIF's family of endogenous peptides. It has been isolated from bovine hypothalamus and human parietal cortex that suggests its involvement in nociception. Tyr-MIF-1 can bind to the mu-receptors as well as to its specific non-opiate receptors in the brain. Data in the literature rise the idea that histamine (HA), a well known nociceptive agent, and Tyr-MIF-1 might have a common pathway in their effects on nociception. We tested that possibility by investigation of the combined action of diphenhydramine (DPH, an H (1) -antagonist) and Tyr-MIF-1 on nociception. The changes in the nociceptive effects were examined in the male Wistar rats by the Randall-Sellito paw-pressure (PP) and the tail-flick (TF) tests. Tyr-MIF-1 in a dose of 1 mg/kg exerted strong naloxone-reversible analgesic effects. DPH (100 microg/kg, i.p.) had an antinociceptive action, too. The co-administration of Tyr-MIF-1 and DPH enhanced the antinociceptive effect, as compared to DPH (PP) and to TYR-MIF-1 alone (TF). These effects were reversed when methylene blue (MB, 500 microg/rat) was applied 1h before the combination. However, naloxone (1 mg/kg, i.p.) only slightly affected the antinociceptive effect of DPH and TYR-MIF-1, compared to that of MB. The results obtained confirmed the hypothesis that cyclic nucleotides are involved in the realization of nociceptive effects of both HA and Tyr-MIF-1.     

Effects of intracerebroventricular administration of opiate receptor antagonists on the suppressed pulsatile LH release during acute fasting in ovariectomized estradiol-treated rats.

The involvement of specific opiate receptors in the suppression of LH release during acute fasting in ovariectomized estradiol-treated rats was examined by intracerebroventricular (i.c.v.) administration of opiate receptor antagonists that exert a specificity directed mainly, although not absolutely, towards the delta-, kappa- or mu-opiate receptors. Fasting for 48 h significantly decreased mean plasma LH levels in estradiol-treated animals by increasing sensitivity to the negative feedback effect of estradiol. Injecting i.c.v. the mu-opiate receptor antagonist naloxone (10 or 100 nmol in 2 microliters of saline) blocked the inhibitory effect of fasting on pulsatile LH release and reinstated LH pulses. On the other hand, i.c.v. administration of the same dosages of a delta-opiate receptor antagonist ICI 174,864 or a kappa-opiate receptor antagonist WIN 44441-3 did not have any effect. These results suggest that the increased sensitivity of the LH-releasing mechanism to the negative feedback effect of estradiol during fasting involves the endogenous opioids mainly through the selective activation of the mu-opiate receptors.     

Chiral arylpyrrolidinols: preparation and biological profile

The preparation and biological evaluation of a new class of arylpyrrolidinols is reported. The antinociceptive activity was evaluated in vivo with the hot plate test (HPT) and formalin test (FT), excluding any involvement on motor coordination with the rota-rod test (RRT). The nociceptive behavior in the late phase of FT (representative of chronic pain) suggests an involvement of the antiinflammatory process and it is clearly influenced by the stereochemical features, being the eutomer of phenylpyrrolidinols, the (2R,3S) enantiomer. Despite this, a specific mechanism of action is not yet clarified.     

Spinal and peripheral mechanisms of cannabinoid antinociception: behavioral,neurophysiological and neuroanatomical perspectives

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation. This review examines behavioral, neurophysiological and neuroanatomical evidence supporting a role for cannabinoids in suppressing nociceptive transmission at spinal and peripheral levels. The development of subtype-selective competitive antagonists and high-affinity agonists provides the pharmacological tools required to study cannabinoid antinociceptive mechanisms. These studies provide insight into the functional roles of cannabinoid receptor subtypes, CB1 and CB2, in cannabinoid antinociceptive mechanisms as revealed in animal models of acute and persistent (somatic inflammatory, visceral inflammatory, neuropathic) pain. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing.     

The Opioid Interactions of the Antipsychotic Medications Risperidone and Amisulpride in Mice and Their Potential Use in the Treatment of Other Non-Psychotic Medical Conditions

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. The opioid epidemic in the USA has highlighted the need for alternative treatments for pain. Following reports on the opioid interactions of various antipsychotic medications, we speculated that the involvement of the opioid system in some of the antipsychotics’ mechanism of action may suggest their potential use in the treatment of pain. Risperidone is a neuroleptic with a potent dopamine D2 and serotonin 5-HT2 receptor-blocking activity as well as a high affinity for adrenergic and histamine H1 receptors. Amisulpride is a neuroleptic which selectively blocks dopamine D2 and D3 receptors. Both had a potent antinociceptive effect on ICR mice tested with a tail flick assay. That effect on both medications was antagonized by naloxone, indicating that at least some of the antinociceptive effects were mediated by an opioid mechanism of action. Further investigation found that β-Funaltrexamine hydrochloride (β-FNA), naloxonazine, and nor-Binaltorphimine dihydrochloride (nor-BNI) reversed the antinociceptive effect of both risperidone and amisulpride. Naltrindole at a dose that blocked [D-Pen2,D-Pen5]enkephalin (DPDPE, δ analgesia) blocked notably amisplride effect and only partially reversed that of risperidone. Risperidone induced an antinociceptive effect, implying involvement of μ and κ-opioid and δ-opioid mechanisms. Amisulpride-induced antinociception was mediated through selective involvement of all three opioid receptor subtypes. These findings emphasize the need for clinical trials to assess the possibility of extending the spectrum of medications available for the treatment of pain.

     

Antinociceptive effects of two deltorphins analogs in the tail-immersion test in rats

The antinociceptive effects of analogs of deltorphins: cyclo(Nδ,Nδ-carbonyl-d-Orn2, Orn4)deltorphin (DEL-6) and deltorphin II N-(ureidoethyl)amide (DK-4) after intracerebroventricular (i.c.v.) administration were investigated in the tail-immersion test in rats. Morphine, the most commonly used μ-opioid receptors (MOR) agonist, was employed as a reference compound. The contribution of the MOR, δ-(DOR) and κ-opioid receptors (KOR) in antinociceptive effects of the deltorphins analogs was studies using selective antagonists of these receptors. The results indicated that DK-4 (5, 10 and 20 nmol) and DEL-6 (5, 10 and 20 nmol) were the most effective in alleviating thermal pain at the dose of 20 nmol. The antinociceptive potency of DEL-6 at the dose of 20 nmol was approximately equal but DK-4 at the dose of 20 nmol was less effective than morphine at the dose of 13 nmol. DOR antagonist – naltrindole (NTI, 5 nmol) very strongly and, to the lower extent MOR antagonist – β-funaltrexamine (β-FNA, 5 nmol), inhibited antinociceptive effect of DK-4 (20 nmol). In turn, β-FNA was more potent than NTI in inhibition of the antinociceptive effects of DEL-6. Co-administration of DEL-6 and morphine at doses of 5 nmol, which do not produce measurable antinociception, generated additive antinociceptive effect. Chronic intraperitoneal (i.p.) injection of morphine (9 days) displayed a marked analgesic tolerance to the challenge dose of morphine and a slight cross-tolerance to challenge doses of DEL-6 and DK-4, given i.c.v. These findings indicate that the new deltorphin analogs recruit DOR and MOR to attenuate the nociceptive response to acute thermal stimuli.     

The spinal antinociceptive effect of FR140423 in mice involvement of the descending noradrenergic and serotonergic systems.

In this study, we investigated the role of descending monoaminergic systems in the antinociceptive activity of FR140423, 3-(difluoromethyl)-1-(4-methoxyphenyl)-5-[4-(methyl sulfinyl)phenyl]pyrazole, by using the tail-pinch test in mice and various monoamine depletors. The antinociceptive effects of FR140423 given p.o. and i.t. with ED50 values of 22 mg/kg and 3.5 microg/mouse, respectively, in the tail-pinch test were antagonized by reserpine, 6-hydroxydopamine plus nomifensine, and p-chlorophenylalanine treatment, whereas the antinociceptive action of morphine in this assay was attenuated by reserpine and 6-hydroxydopamine plus nomifensine but not by p-chlorophenylalanine treatment. These results suggest that both descending noradrenergic and serotonergic systems are involved in the antinociceptive activity of spinally and systemically administered FR140423 against mechanical noxious stimuli. The mechanisms underlying the suppressive action of FR140423 on the nociceptive response may differ from those of morphine in mice.     

Effects of transcranial laser irradiation in near infrared range on antinociceptive reactions in mice after administration of diazepam, clopheline and morphine]

The experiments were carried out on white mice whose brain was irradiated transcranially with laser light in infrared range. Exposure to irradiation was 20 min. In one group of animals only laser light was used, in others laser was combined with morphine (3mg/kg), clonidine (0.5 mg/kg), and diazepam (1 mg/kg) injected intraperitoneally. The nociceptive reactions were studied with the help of "tail-flick" and "hot-plate" tests. It was found that laser light did not modify significantly the results of both tests. Moreover, it didn't influence the antinociceptive properties of morphine, clonidine and diazepam in the "hot-plate" test. In the "tail-flick" test laser light did not affect the action of clonidine, but provided naloxone-independent antinociceptive reaction with diazepam and increased the antinociceptive effect of morphine. Laser irradiation of the brain did not cause any significant morphological changes. These results suggest the possibility of modulating antinociceptive actions of morphine and diazepam by laser irradiation of the brain.     

Antinociceptive activity of Sesbania sesban (Linn) wood extracts, a preliminary study

The wood of the plant Sesbania sesban, is reported to have antinociceptive activity. To validate its folk use in the treatment of pain, wood was extracted successively with petroleum ether, chloroform, ethyl acetate, ethanol, and water to produce respective extracts. The extracts (50 and 100 mg/kg, ip) were screened for antinociceptive activity using hot plate test and acetic acid-induced writhing test in mice. Petroleum ether, chloroform, and ethyl acetate extracts showed significant and dose-dependent activity in both the tests. In order to find out the involvement of opioid receptors, effect of naloxone (1 mg/kg, sc) on the action of extracts was checked in hot plate test. Petroleum ether, chloroform, and ethyl acetate extracts showed significant and dose dependant antinociceptive activity. The antinociceptive action of the extracts was blocked by naloxone, suggesting involvement of opioid receptors in the action.     

Antinociceptive role of oxytocin in the nucleus raphe magnus of rats, an involvement of mu-opioid receptor

Recent studies showed that oxytocin plays an important role in nociceptive modulation in the central nervous system. The present study was undertaken to investigate the role of oxytocin in antinociception in the nucleus raphe magnus (NRM) of rats and the possible interaction between oxytocin and the opioid systems. Intra-NRM injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulation in rats. The antinociceptive effect of oxytocin was significantly attenuated by subsequent intra-NRM injection of the oxytocin antagonist 1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin. Intra-NRM injection of naloxone dose-dependently antagonized the increased HWLs induced by preceding intra-NRM injection of oxytocin, indicating an involvement of opioid receptors in oxytocin-induced antinociception in the NRM of rats. Furthermore, the antinociceptive effect of oxytocin was dose-dependently attenuated by subsequent intra-NRM injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA), but not by the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) or the delta-opioid antagonist naltrindole. The results demonstrated that oxytocin plays an antinociceptive role in the NRM of rats through activating the oxytocin receptor. Moreover, mu-opioid receptors, not kappa and delta receptors, are involved in the oxytocin-induced antinociception in the NRM of rats.     

Stereospecific opiate receptors in the actions of thyrotropin releasing hormone and morphine on gastrointestinal transit

Studies in these laboratories have shown that morphine and thyrotropin releasing hormone (TRH) inhibit gastrointestinal transit in the mouse. Administration of morphine sulfate (5 mg/kg, s.c.) or TRH (10 microgram i.c.v.) to mice inhibited gastrointestinal transit as measured by the charcoal meal test. In order to determine whether the effects of TRH and morphine were mediated via stereospecific opiate receptors, the effects of two stereoisomers of an antagonist, (-) alpha -5,9-diethyl-2'-hydroxy-2-(3-furylmethyl)6,7-benzomorphan (MR2266), the active isomer and (+) alpha-5,9-diethyl-2'-hydroxy-2-(3-furylmethyl)6,7-benzomorphan (MR 2267), the inactive isomer, on morphine and TRH induced changes in gastrointestinal transit were determined. Morphine and THR induced inhibition of gastrointestinal transit was antagonized by MR 2266 (1 and 3 mg/kg, s.c.) but was unaffected by MR 2267. These studies provide evidence for the involvement of opiate receptors in the actions of morphine and TRH on gastrointestinal transit, and further suggest that the receptors are stereospecific in nature.     

SR142801 behaves as a tachykinin NK-3 receptor agonist on a spinal nociceptive reflex in the rat

Effects of two commonly used tachykinin NK-3 receptor antagonists (SR 142801 and R820) intrathecally (i.t.) administered were assessed in the rat tail-flick test. SR142801 and its (R)-enantiomer SR142806 (1.3, 6.5 and 65 nmol) were found as potent as senktide and [MePhe7]NKB (NK-3 selective agonists) to induce transient antinociceptive effects. Naloxone (10 microg) and R820 (6.5 nmol) blocked reversibly the responses to 6.5 nmol senktide, [MePhe7]NKB, SR142801 and SR142806 when administered i.t. 15 min earlier. However, the antinociceptive responses induced by SR142801 and SR142806 were not affected by i.t. pretreatments with NK-1 (6.5 nmol SR140333) and NK-2 (6.5 nmol SR48968) receptor antagonists. In control experiments, the NK-1 and NK-2 antagonists prevented the hyperalgesic effects to NK-1 ([Sar9,Met(O2)11]SP) and NK-2 ([beta-Ala8] NKA(4-10)) receptor agonists (6.5 nmol i.t.), respectively. R820 had no direct effect on nociceptive threshold and failed to alter angiotensin II-induced antinociception. The data suggest that the antinociceptive effect of SR142801 is due to an agonist effect at NK-3 receptor in the rat spinal cord that involves a local opioid mechanism. These results can be best explained by the existence of inter-species NK-3 receptor subtypes.