Differential sensitivity of models of antinociception in the rat, mouse and guinea-pig to mu- and kappa-opioid receptor agonists

NCP, a Dual Kappa and Mu Opioid Receptor Agonist, Is a Potent Analgesic Against Inflammatory Pain without Reinforcing or Aversive Properties

While agonists of μ (MOR) and κ (KOR) opioid receptors have analgesic effects, they produce euphoria and dysphoria, respectively. Other side effects include respiratory depression and addiction for MOR agonists and sedation for KOR agonists. We reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-{[4'-(2'-cyanopyridyl)]carboxamido}morphinan (NCP) displayed potent KOR full agonist and MOR partial agonist activities (58%) with 6.5x KOR-over-MOR selectivity in vitro. Herein, we characterized pharmacological effects of NCP in rodents. In mice, NCP exerted analgesic effects against inflammatory pain in both the formalin test and the acetic acid writhing test, with A50 values of 47.6 and 14.4 μg/kg (s.c.), respectively. The analgesic effects in the acetic acid writhing test were mediated by the KOR. NCP at doses much higher than those effective in reducing inflammatory pain did not produce antinociception in the hot plate and tail flick tests, inhibit compound 48/80-induced scratching, cause conditioned place aversion (CPA) or preference, impair rotarod performance, inhibit locomotor activity, cause respiratory depression, or precipitate morphine withdrawal. However, NCP (10∼100 μg/kg) inhibited gastrointestinal transit with a maximum of ∼40% inhibition. In MOR knockout mice, NCP caused CPA, demonstrating that its lack of CPA is due to combined actions on the MOR and KOR. Following subcutanous injection, NCP penetrated into the mouse brain. In rats trained to self-administer heroin, NCP (1∼320 μg/kg/infusion) did not function as a reinforcer. Thus, NCP produces potent analgesic effects via KOR without side effects except constipation. Therefore, dual full KOR/partial MOR agonists with moderate KOR-over-MOR selectivity may be promising as nonaddictive analgesics for inflammatory pain. SIGNIFICANCE STATEMENT: Developing nonaddictive analgesics is crucial for reducing opioid overdose deaths, minimizing drug misuse, and promoting safer pain management practices. Herein, the pharmacology of a potential nonaddictive analgesic, NCP, is reported. NCP has full KOR agonist/partial MOR agonist activities with a 6.5x selectivity for KOR over MOR. Unlike MOR agonists, analgesic doses of NCP do not lead to self-administration or respiratory depression. Furthermore, NCP does not produce aversion, hypolocomotion, or motor incoordination, side effects typically associated with KOR activation.

Signaling underlying kappa opioid receptor-mediated behaviors in rodents

Kappa opioid receptor (KOR) agonists are potentially useful as analgesic and anti-pruritic agents, for prevention and treatment of substance use disorders, and for treatment of demyelinating diseases. However, side effects of KOR agonists, including psychotomimesis, dysphoria, and sedation, have caused early termination of clinical trials. Understanding the signaling mechanisms underlying the beneficial therapeutic effects and the adverse side effects may help in the development of KOR agonist compounds. In this review, we summarize the current knowledge in this regard in five sections. First, studies conducted on mutant mouse lines (GRK3-/-, p38alpha MAPK-/-, β-arrestin2-/-, phosphorylation-deficient KOR) are summarized. In addition, the abilities of four distinct KOR agonists, which have analgesic and anti-pruritic effects with different side effect profiles, to cause KOR phosphorylation are discussed. Second, investigations on the KOR agonist nalfurafine, both in vitro and in vivo are reviewed. Nalfurafine was the first KOR full agonist approved for clinical use and in the therapeutic dose range it did not produce significant side effects associated with typical KOR agonists. Third, large-scale high-throughput phosphoproteomic studies without a priori hypotheses are described. These studies have revealed that KOR-mediated side effects are associated with many signaling pathways. Fourth, several novel G protein-biased KOR agonists that have been characterized for in vitro biochemical properties and agonist biases and in vivo behavior effects are described. Lastly, possible mechanisms underlying KOR-mediated CPA, hypolocomotion and motor incoordination are discussed. Overall, it is agreed upon that the analgesic and anti-pruritic effects of KOR agonists are mediated via G protein signaling. However, there is no consensus on the mechanisms underlying their side effects. GRK3, p38 MAPK, β-arrestin2, mTOR pathway, CB1 cannabinoid receptor and protein kinase C have been implicated in one side effect or another. For drug discovery, after initial in vitro characterization, in vivo pharmacological characterizations in various behavior tests are still the most crucial steps and dose separation between beneficial therapeutic effects and adverse side effects are the critical determinant for the compounds to be moved forward for clinical development.

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Amniotic-fluid ingestion enhances central δ-opioid-induced hypoalgesia in rats in the cold-water tail-flick assay in a repeated-measures design

Placental Opioid Enhancing Factor (POEF) is found in amniotic fluid (AF) and placenta. When ingested, it enhances opioid-mediated pain relief. Our laboratory has shown that ingestion of AF specifically enhances the hypoalgesia associated with δ-opioid receptor activation in the brain. The specific biochemical compound in AF responsible for the enhancement of δ-opioid activity is of great interest as an analgesic adjunct for pain but is unknown at this time. Research efforts to isolate and characterize this biochemical compound are hampered by the lack of an algesiometric assay that allows repeated measurement of pain threshold and repeated exposure to δ-opioid receptor activation. The cold water tail-flick assay (CWTF) may be a sensitive and reliable pain threshold test of (a) all species of opioids that is (b) not subject to repeated-testing effects. Therefore the CWTF test is potentially ideal for the study of δ opioid systems in a repeated measures design. Here, we confirm these attributes of the CWTF test, and determined that (a) there are no repeated-exposure effects associated with the CWTF assay; (b) there are no repeated-exposure effects associated with repeated central injections of DPDPE ([D-Pen2,D-Pen5]-Enkephalin, a selective δ-opioid agonist) as measured by the CWTF assay; and (c) ingestion of AF in conjunction with a central injection of DPDPE produced the same hypoalgesic enhancement as previously found using another assay.

Pharmacokinetics of buprenorphine after intravenous and oral transmucosal administration in guinea pigs (Cavia porcellus)

OBJECTIVE To determine pharmacokinetics and sedative effects of buprenorphine after IV and oral transmucosal (OTM) administration in guinea pigs. ANIMALS 14 male guinea pigs (6 adults for preliminary experiment; eight 8 to 11-week-old animals for primary study). PROCEDURES A preliminary experiment was conducted to determine an appropriate buprenorphine dose. In the primary study, buprenorphine (0.2 mg/kg) was administered IV or OTM, and blood samples were obtained. The pH of the oral cavity was measured before OTM administration. Sedation was scored for 6 hours on a scale of 0 to 3 (0 = no sedation and 3 = heavy sedation). After a 7-day washout period, procedures were repeated in a crossover manner. Plasma buprenorphine concentration was quantified, and data were analyzed with a noncompartmental pharmacokinetic approach. RESULTS Mean peak plasma buprenorphine concentrations were 46.7 and 2.4 ng/mL after IV and OTM administration, respectively. Mean time to maximum plasma buprenorphine concentration was 1.5 and 71.2 minutes, and mean terminal half-life was 184.9 and 173.0 minutes for IV and OTM administration, respectively. There was a range of sedation effects (0 to 2) for both routes of administration, which resolved within the 6-hour time frame. CONCLUSIONS AND CLINICAL RELEVANCE On the basis of pharmacokinetic parameters for this study, buprenorphine at 0.2 mg/kg may be administered IV every 7 hours or OTM every 4 hours to maintain a target plasma concentration of 1 ng/mL. Further studies are needed to evaluate administration of multiple doses and sedative effects in guinea pigs with signs of pain.

Evidence for the involvement of TNF-α, IL-1β and IL-10 in the antinociceptive and anti-inflammatory effects of indole-3-guanylhydrazone hydrochloride, an aromatic aminoguanidine, in rodents

BACKGROUND

Indole-3-guanylhydrazone hydrochloride (LQM01) is a new derivative of aminoguanidine hydrochloride, an aromatic aminoguanidine.

METHODS

Mice were treated with LQM01 (5, 10, 25 or 50 mg/kg, i.p.), vehicle (0.9% saline i.p.) or a standard drug. The mice were subjected to carrageenan-induced pleurisy, abdominal writhing induced by acetic acid, the formalin test and the hot-plate test. The model of non-inflammatory chronic muscle pain induced by saline acid was also used. Mice from the chronic protocol were assessed for withdrawal threshold, muscle strength and motor coordination. LQM01 or vehicle treated mice were evaluated for Fos protein.

RESULTS

LQM01 inhibits TNF-α and IL-1β production, as well as leukocyte recruitment during inflammation process. The level of IL-10 in LQM01-treated mice increased in pleural fluid. In addition, LQM01 decreased the nociceptive behavior in the acetic acid induced writhing test, the formalin test (both phases) and increased latency time on the hot-plate. LQM01 treatment also decreased mechanical hyperalgesia in mice with chronic muscle pain, with no changes in muscle strength and motor coordination. LQM01 reduced the number of Fos positive cells in the superficial dorsal horn. This compound exhibited antioxidant properties in in vitro assays.

CONCLUSIONS

LQM01 has an outstanding anti-inflammatory and analgesic profile, probably mediated through a reduction in proinflammatory cytokines release, increase in IL-10 production and reduction in neuron activity in the dorsal horn of the spinal cord in mice.

GENERAL SIGNIFICANCE

Beneficial effects of LQM01 suggest that it has some important clinical features and can play a role in the management of 'dysfunctional pain' and inflammatory diseases.

Pharmacokinetics and Paw Withdrawal Pressure in Female Guinea Pigs (Cavia porcellus) Treated with Sustained-Release Buprenorphine and Buprenorphine Hydrochloride

Providing appropriate analgesia is essential in minimizing pain and maintaining optimal animal care and welfare in laboratory animals. Guinea pigs are common animal models in biomedical research, often requiring analgesic support. Here we evaluated the pharmacokinetics and efficacy of a sustained-release formulation of buprenorphine (Bup-SR) in this species. Guinea pigs (n = 7 each group) received either Bup-HCl (0.05 mg/kg BID for 3 d) or Bup-SR (0.3 mg/kg once). Plasma collection and measurement of paw-withdrawal pressure (PWP) was conducted at 0, 1, 3, 6, 12, 26, 48, and 72 h after treatment. Plasma levels of Bup-HCl peaked at 2331 pg/mL at 1 h after administration and declined to 165 pg/mL by 12 h. Plasma concentrations of Bup-SR peaked at 1344 pg/mL at 26 h after administration and declined to 429 pg/mL by 48 h. The PWP of the Bup-HCltreated guinea pigs peaked at 674 g at 1 h and declined to 402 g at 6 h, whereas that of Bup-SRtreated guinea pigs at 1 h was 361 g, 555 g at 6 h (significantly higher than that after Bup-HCl), and peaked at 680 g at 12 h. The PWP of both treatments was similar from 24 to 72 h and ranged from 348 to 450 g. The plasma concentration and PWP showed good correlation. These results suggest that Bup-SR provides consistent analgesia equivalent to that of Bup-HCl for a prolonged period of time and that Bup-SR is an alternative method of analgesia in guinea pigs.

Analgesic Potential of Essential Oils

Pain is an unpleasant sensation associated with a wide range of injuries and diseases, and affects approximately 20% of adults in the world. The discovery of new and more effective drugs that can relieve pain is an important research goal in both the pharmaceutical industry and academia. This review describes studies involving antinociceptive activity of essential oils from 31 plant species. Botanical aspects of aromatic plants, mechanisms of action in pain models and chemical composition profiles of the essential oils are discussed. The data obtained in these studies demonstrate the analgesic potential of this group of natural products for therapeutic purposes.

Dioscorea Extract (DA-9801) Modulates Markers of Peripheral Neuropathy in Type 2 Diabetic db/db Mice

The purpose of this study was to investigate the therapeutic effects of DA-9801, an optimized extract of Dioscorea species, on diabetic peripheral neuropathy in a type 2 diabetic animal model. In this study, db/db mice were treated with DA-9801 (30 and 100 mg/kg, daily, p.o.) for 12 weeks. DA-9801 reduced the blood glucose levels and increased the withdrawal latencies in hot plate tests. Moreover, it prevented nerve damage based on increased nerve conduction velocity and ultrastructural changes. Decrease of nerve growth factor (NGF) may have a detrimental effect on diabetic neuropathy. We previously reported NGF regulatory properties of the Dioscorea genus. In this study, DA-9801 induced NGF production in rat primary astrocytes. In addition, it increased NGF levels in the sciatic nerve and the plasma of type 2 diabetic animals. DA-9801 also increased neurite outgrowth and mRNA expression of Tieg1/Klf10, an NGF target gene, in PC12 cells. These results demonstrated the attenuation of diabetic peripheral neuropathy by oral treatment with DA-9801 via NGF regulation. DA-9801 is currently being evaluated in a phase II clinical study.

Evaluation of the anti-inflammatory and antinociceptive effects of the essential oil from leaves of Xylopia laevigata in experimental models

Xylopia laevigata (Annonaceae) is a medicinal plant used in folk medicine to treat pain and inflammation. Thus, we investigated the possible antioxidant, antinociceptive, and anti-inflammatory effects of X. laevigata leaf essential oil (EOX) in animal models. Our EOX sample showed the presence of γ-muurolene (17.78%), δ-cadinene (12.23%), bicyclogermacrene (7.77%), and α-copaene (7.17%) as main compounds. EOX presented a strong antioxidant potential according to the DPPH, TBARS, and nitrite production tests. Additionally, pretreatment with EOX, in mice, also significantly produced (P < 0.05 or P < 0.001) antinociceptive effect by reduction of nociceptive behavior (in formalin and writhing tests). The EOX showed c-Fos label in the olfactory bulb, piriform cortex, and periaqueductal gray. Acute administration of EOX exhibited a significant (P < 0.01 or P < 0.001) anti-inflammatory profile in the carrageenan-induced peritonitis and by the carrageenan-induced hindpaw edema tests in mice. Our results provide evidence for the use of X. laevigata by traditional medicine practitioners in the management of pain and inflammatory disorders.

Bioassay-guided evaluation of Dioscorea villosa - an acute and subchronic toxicity, antinociceptive and anti-inflammatory approach

BACKGROUND

Dioscorea villosa (DV) has been used in Brazil as an alternative medicine to attenuate menopause symptoms, as well as for the treatment of joint pain and rheumatoid arthritis. In spite of the popular use of DV for the treatment of various disorders, there are limited scientific data regarding safety aspects of this herb. In this regard, we carried out to evaluated both antinociceptive and anti-inflammatory activities in experimental models and assess the toxic effects of the acute (single dose) and subchronic (30 days) oral administration of dry extract of Dioscorea villosa in rodents.

METHODS

The LC analyses were performed to assess the presence of the diosgenin in samples of DV. The antinociceptive study of DV was performed using models of acetic acid-induced writhing and formalin-induced pain in mice. The anti-inflammatory study was accomplished by leukocyte migration to the peritoneal cavity. A dry extract of DV was tested at doses of 100, 200 and 400 mg/kg (per os or p.o.). The toxicological properties of the dry extract were evaluated by toxicity assays of acute (5 g/kg, single dose) and subchronic (1 g/kg/day, 30 days) treatment. Haematological, biochemical, and histopathological parameters were studied. The results are expressed as mean ± S.D., and statistical analysis of the data were performed with the Student's t-test or one-way analysis of variance (ANOVA) followed by Tukey's test. In all cases differences were considered significant if p < 0.05.

RESULTS

HPLC-DAD analysis of the extract from DV revealed the presence of diosgenin as the major compound. Doses of 200 and 400 mg⁄kg significantly reduced the amount of acetic acid-induced writhing in relation to the vehicle (p < 0.0001). In the first phase, using the formalin-induced neurogenic pain test, only the 400 mg/kg dose of DV showed significant inhibition of neurogenic pain (p < 0.001). In the second phase, 200 and 400 mg/kg of DV showed significant inhibition of inflammatory pain (p < 0.0001). Significant inhibition of leukocyte migration was observed with doses of 100 (p < 0.001), 200 (p < 0.01) and 400 mg/kg (p < 0.01). Haematological, biochemical and histopathological data obtained in both acute and subchronic toxicological assays revealed only unremarkable changes, which are unlikely to indicate DV toxicity with oral administration.

CONCLUSION

We found that DV possesses antinociceptive and anti-inflammatory properties in rodent models. In addition, no acute or subchronic toxicity was evident when the herbal extract was administered orally. These results supporting the folkloric usage of the plant to treat various inflammatory diseases.

Arcuate Src activation-induced phosphorylation of NR2B NMDA subunit contributes to inflammatory pain in rats

The tyrosine kinases of Src family play an important role in the central sensitization following peripheral inflammation. However, whether the Src family in the arcuate nucleus (ARC) of mediobasal hypothalamus is involved in central sensitization remains unknown. The aim of this study was to investigate the role and mechanisms of tyrosine kinases of Src family in N-methyl-d-aspartate (NMDA) receptor activity in the ARC following peripheral inflammation. Peripheral inflammation was induced by unilateral injection of complete Freund's adjuvant (CFA) into rat hindpaw. The neuronal activities of the ARC were recorded using electrophysiological field recording from the in vitro mediobasal hypothalamic slices from control and CFA rats. Expression of total and phosphorylated Src and NR2B subunit protein was analyzed by Western blot and immuoprecipitation. Our results showed that CFA injection resulted in an increase in mechanical and thermal sensitivity, which was partially blocked by neonatal monosodium glutamate treatment. CFA injection also enhanced spontaneous firings of ARC neurons, which were reversed by the NMDA receptor NR2B subunit specific antagonist Ro25–6981 and by PP2, an Src family tyrosine kinase inhibitor. In addition, peripheral inflammation enhanced Src phosphorylation and NMDA receptor NR2B subunit phosphorylation without alteration of total NR2B subunit expression in the ARC. Peripheral inflammation also increased the association of NR2B protein with p-Src protein in the ARC. Administration of PP2 blocked the upregulation of NR2B phosphorylation induced by CFA injection. Taken together, our present results suggest that the arcuate Src activation-induced tyrosine phosphorylation of NR2B NMDA subunit may contribute to inflammatory pain.

Sex difference in κ-opioid receptor (KOPR)-mediated behaviors, brain region KOPR level and KOPR-mediated guanosine 5'-O-(3-[35S]thiotriphosphate) binding in the guinea pig

We examined whether sex differences in κ-opioid receptor (KOPR) pharmacology exist in guinea pigs, which are more similar to humans in the expression level and distribution of KOPR in the brain than rats and mice. The KOPR agonist trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)benzeneacetamide methanesulfonate (U50,488H) produced a dose-dependent increase in abnormal postures and immobility with more effects in males than females. Males also showed more U50,488H-induced antinociception in the paw pressure test than females. Pretreatment with the KOPR antagonist norbinaltorphimine blocked U50,488H-induced abnormal body postures and antinociception. In contrast, inhibition of cocaine-induced hyperambulation by U50,488H was more effective in females than males. Thus, sex differences in the effects of U50,488H are endpoint-dependent. We then examined whether sex differences in KOPR levels and KOPR-mediated G protein activation in brain regions may contribute to the observed differences using quantitative in vitro autoradiography of [(3)H](5a,7a,8b)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)1-oxaspiro(4,5)dec-8-yl)benzeacetamide ([(3)H]U69,593) binding to the KOPR and U50,488H-stimulated guanosine 5'-O-(3-[(35)S]thiotriphosphate ([(35)S]GTPγS) binding. Compared with females, males exhibited more [(3)H]U69,593 binding in the deep layers of somatosensory and insular cortices, claustrum, endopiriform nucleus, periaqueductal gray, and substantial nigra. Concomitantly, U50,488H-stimulated [(35)S]GTPγS binding was greater in males than females in the superficial and deep layers of somatosensory and insular cortices, caudate putamen, claustrum, medial geniculate nucleus, and cerebellum. In contrast, compared with males, females showed more U50,488H-stimulated [(35)S]GTPγS binding in the dentate gyrus and a trend of higher [(35)S]GTPγS binding in the hypothalamus. These data demonstrate that males and females differ in KOPR expression and KOPR-mediated G protein activation in distinct brain regions, which may contribute to the observed sex differences in KOPR-mediated pharmacology.

Phytochemical characterization and antinociceptive effect of Lippia gracilis Schauer

Lippia gracilis Schauer is an aromatic plant widely found in Northeastern Brazil. The leaf infusions or decoctions and alcoholic macerate are used for some inflammatory diseases and headache. This paper reports the isolation of naringenin by semi-preparative liquid chromatography from the methanolic extract of L. gracilis (ELg) and the evaluation of the analgesic and anti-inflammatory activities of this extract by measuring nociception through acetic acid, formalin, and hot-plate tests in carrageenan-induced inflammation in mice. Following oral administration, ELg (100, 200, and 400 mg/kg) significantly reduced the number of writhes in the writhing test and the time of paw licks in both phases of the formalin test when compared to the control group animals. Mice treated with ELg did not exhibit any behavioral alteration during the hot plate and rota-rod tests, suggesting non-participation of the supraspinal components in the modulation of pain by ELg and no motor abnormality. The oral administration of 400 mg/kg of ELg produced an anti-inflammatory effect on peritonitis induced by carrageenan. These effects can be associated with a decrease of inflammatory mediator synthesis by compounds of ELg, such as naringenin, which has anti-inflammatory action as already described.

Antioxidant, antinociceptive, and anti-inflammatory properties of the ethanolic extract of Combretum duarteanum in rodents

The antioxidant, antinociceptive, and anti-inflammatory activities of the ethanolic extract from leaves of Combretum duarteanum (EEC) were assessed in rodents through in vitro tests. The antioxidant activity was investigated by using thiobarbituric acid reactive species (TBARS), hydroxyl radical-scavenging, and scavenging activity of nitric oxide assays. The antinociceptive activity was investigated by using acetic acid-induced writhing, formalin, and hot-plate tests in mice. The anti-inflammatory activity was assessed in rats by using the carrageenan-induced hind-paw edema test and arachidonic acid-induced paw edema test. EEC possesses a strong antioxidant potential according to the TBARS, nitric oxide, and hydroxyl radical-scavenging assays; it also presented scavenger activity in all in vitro tests. After intraperitoneal injection, EEC (100, 200, and 400 mg/kg) significantly reduced the number of writhes (38.1%, 90.6%, and 97.8%, respectively) in a writhing test and the number of paw licks during phase 1 (30.5% and 69.5%, higher doses) and phase 2 (38.1%, 90.6%, and 97.8%, all doses) of a formalin test when compared with the control group. Naloxone (1.5 mg/kg, intraperitoneally) antagonized the antinociceptive action of EEC (400 mg/kg), and this finding suggests participation of the opioid system. Administration of 200 and 400 mg/kg (intraperitoneally) of EEC exhibited an anti-inflammatory activity in the carrageenin test, which was based on interference with prostaglandin synthesis. This finding was confirmed by the arachidonic acid test. Together, these results indicate that properties of EEC might be further explored in the search for newer tools to treat painful inflammatory conditions, including those related to pro-oxidant states.

Antioxidant and antinociceptive effects of Citrus limon essential oil in mice

The antioxidant and antinociceptive activities of Citrus limon essential oil (EO) were assessed in mice or in vitro tests. EO possesses a strong antioxidant potential according to the scavenging assays. Moreover, it presented scavenger activity against all in vitro tests. Orally, EO (50, 100, and 150 mg/kg) significantly reduced the number of writhes, and, at highest doses, it reduced the number of paw licks. Whereas naloxone antagonized the antinociceptive action of EO (highest doses), this suggested, at least, the participation of the opioid system. Further studies currently in progress will enable us to understand the action mechanisms of EO.

Antinociceptive effect of citronellal in mice

Citronellal is a monoterpene reported to be a major component of the essential oils in various aromatic species of plants. The present study evaluated the central nervous system depressant and antinociceptive properties of citronellal through behavioral experimental models. Following intraperitoneal injection, citronellal induced the reduction of spontaneous activity, ataxia, analgesia, and sedation. In pentobarbital-induced hypnosis, CTL (citronellal) at 50, 100, and 200 mg/kg (i.p.) significantly increased sleeping time (88.0 +/- 11.4, 100.2 +/- 16.4, and 119.5 +/- 20.9 min) when compared to vehicle solution injections (43.0 +/- 6.1). Citronellal (100 and 200 mg/kg, i.p.) significantly reduced the number of writhes (66.4 and 81.9%) in a writhing test and the number of paw licks during phase 1 (47.0 and 66.8%) and phase 2 (71.1 and 79.2%) of a formalin test when compared to control group animals. In addition, the results of a hot plate test showed central analgesic properties for citronellal (p < 0.05). These results indicate depressant, hypnotic, and antinociceptive properties of this monoterpene.

2-Methoxymethyl-salvinorin B is a potent kappa opioid receptor agonist with longer lasting action in vivo than salvinorin A

Salvinorin (Sal) A is a naturally occurring, selective kappa opioid receptor (KOPR) agonist with a short duration of action in vivo. Pharmacological properties of a C(2) derivative, 2-methoxymethyl (MOM)-Sal B, were characterized. MOM-Sal B bound to KOPR with high selectivity and displayed approximately 3-fold higher affinity than U50,488H [(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate] and Sal A. It acted as a full agonist at KOPR in guanosine 5'-O-(3-[(35)S]thio)triphosphate binding and was approximately 5- and approximately 7-fold more potent than U50,488H and Sal A, respectively. In Chinese hamster ovary cells stably expressing KOPR, all three kappa agonists internalized or down-regulated KOPR to similar extents, with MOM-Sal B being the most potent. In mice, MOM-Sal B (0.05-1 mg/kg s.c.) caused immediate and dose-dependent immobility lasting approximately 3 h, which was blocked by norbinaltorphimine. In contrast, ambulation in a Y-maze was increased when rats received MOM-Sal B (1-5 mg/kg s.c.). In addition, MOM-Sal B (0.5-5 mg/kg i.p.) produced antinociception (hot-plate test) and hypothermia in a dose-dependent manner in rats. MOM-Sal B was more potent than U50,488H in both tests and more efficacious than U50,488H in the hot-plate test. These latter two in vivo effects were blocked by norbinaltorphimine, indicating KOPR-mediated actions. Sal A at 10 mg/kg elicited neither antinociception nor hypothermia 30 min after administration to rats. In summary, MOM-Sal B is a potent and efficacious KOPR agonist with longer lasting in vivo effects than Sal A.

Synthesis and Analgesic Activities of Endomorphin-2 and Its Analogues

Endomorphin-2 (1; H-Tyr-Pro-Phe-Phe-NH2; EM2) and its novel cyclic asparagine (cycloAsn) analogues, H-Tyr-cAsn(CHPh)-Phe-Phe-NH2 (2) and H-Tyr-cAsn(CHMe2)-Phe-Phe-NH2 (3), were synthesized via liquid-phase synthesis. The structures of the products and intermediates were characterized by IR, 1H-NMR, MS, and HR-MS analyses. The antinociceptive activity of EM2 and its cyclic asparagine analogues were assessed in AcOH-induced abdominal constriction tests in mice with i.p. injection. The results show that the antinociceptive activities of EM2 and its cyclic asparagine analogue 2 were higher than those of aspirine and meperidine. Analogue 2 was observed to be a stronger analgesic with dose-dependence than EM2. The test mice did not show any tendency to be addicted while administrated of analogue 2 repeatedly and regularly.

Buprenorphine: considerations for pain management

New effective analgesics are needed for the treatment of pain. Buprenorphine, a partial mu-opioid agonist which has been in clinical use for over 25 years, has been found to be amenable to new formulation technology based on its physiochemical and pharmacological profile. Buprenorphine is marketed as parenteral, sublingual, and transdermal formulations. Unlike full mu-opioid agonists, at higher doses, buprenorphine's physiological and subjective effects, including euphoria, reach a plateau. This ceiling may limit the abuse potential and may result in a wider safety margin. Buprenorphine has been used for the treatment of acute and chronic pain, as a supplement to anesthesia, and for behavioral and psychiatric disorders including treatment for opioid addiction. Prolonged use of buprenorphine can result in physical dependence. However, withdrawal symptoms appear to be mild to moderate in intensity compared with those of full mu agonists. Overdoses have primarily involved buprenorphine taken in combination with other central nervous system depressants.

Antinociceptive properties of acetylenic thiophene and furan derivatives: Evidence for the mechanism of action

The aim of the present study was to evaluate the antinociceptive potential of the acetylenic thiophene and furan derivatives: 3-(furan-2-il) prop-2-yn-1-ol 1, 1-(thiofen-2-il) pent-1yn-3-ol 2 and 4-(thiofen-2-il)-2-metilbut-3-yn-2-ol 3 on three different pain models in mice. The pain models evaluated were the acetic acid-induced writhing, capsaicin-induced pain and the tail immersion test. The possible mechanisms involved in the antinociceptive effect of these compounds were also investigated. Thus, the acetylenic thiophene and furan derivatives presented antinociceptive effect in the pain tests caused by chemical agents. Statistical analysis showed that compounds 1 and 3 increased the latency for tail withdrawal in the tail immersion test (phasic pain). Besides, the role of the opioidergic, muscarinic cholinergic and dopaminergic systems in the acetic acid-induced writhing was examined. The antinociceptive effect of compounds 2 and 3 was prevented by pretreatment with naloxone (1 mg/kg, s.c), but not by atropine (5 mg/kg, s.c) or metoclopramide (1 mg/kg, s.c). Neither naloxone nor metoclopramide prevented the antinociceptive effect caused by compound 1, while the pretreatment with atropine antagonized the antinociceptive action of this compound. The compounds 1-3 used in this study did not reveal any motor impairment to mice in the open field. The results suggest that compounds 2 and 3 induced antinociception in the abdominal writhing test and that their effects are mediated by opiodergic receptors, while the antinociceptive effect of compound 1 may involve muscarinic cholinergic receptors.

Antinociception by a peripherally administered novel endomorphin-1 analogue containing beta-proline

We previously described a novel endomorphin-1 analogue (Tyr-L-beta-Pro-Trp-Phe-NH(2); Endo1-beta-Pro) more resistant to enzymatic hydrolysis than endomorphin-1 that acts as a mu-opioid receptor agonist. In this study we report that Endo1-beta-Pro, s.c. injected in the mouse, is an effective antinociceptive agent in the tail flick (ED(50)=9.2 mg/kg) and acetic acid-induced abdominal constriction (ED(50)=1.2 mg/kg) tests. Moreover, s.c. Endo1-beta-Pro significantly decreases, in the mouse, the gastrointestinal propulsion measured as transit of an orally administered charcoal meal (ED(50)=10.0 mg/kg). Subcutaneous beta-funaltrexamine or a high dose of the mu(1)-opioid receptor-selective antagonist naloxonazine (50 mg/kg) prevents the antinociceptive and antitransit action of Endo1-beta-Pro; moreover, these effects are partially blocked by i.c.v. naloxone or by i.p. naloxone methiodide, this latter does not readily cross the blood-brain barrier. On the contrary, the kappa-opioid receptor antagonist nor-binaltorphimine or the delta-opioid receptor antagonist naltrindole are ineffective Thus, Endo1-beta-Pro may act, preferentially, through central and peripheral mu(2)-opioid receptors to produce antinociception and to inhibit gastrointestinal transit. Endo1-beta-Pro is among the first endomorphin-1 analogues showing antinociceptive activity after systemic administration. This compound will be extremely useful for exploring the pharmacological profile of endomorphins in vivo and confirms the potential therapeutic interest of endomorphin derivatives as novel analgesic agents.

Sex-related differences in mechanical nociception and antinociception produced by mu- and kappa-opioid receptor agonists in rats

Previous studies indicate that in antinociceptive procedures employing thermal, chemical and electrical stimuli, opioids are generally more potent in male than female rodents. The purpose of the present study was to examine nociception and opioid antinociception in male and female rats using a mechanical nociceptive stimulus. Results indicated that males had a higher threshold for nociception, and in tests in which a constant pressure was applied to the hindpaw, the paw withdrawal latencies were consistently longer in males. Opioids with activity at the mu receptor, including levorphanol, morphine, dezocine, buprenorphine, butorphanol and nalbuphine, were generally more potent and/or effective in males. In contrast, sex differences were not consistently observed with the kappa-opioid receptor agonists spiradoline, (5,7,8b)-N-methyl-N[2-1(1-pyrrolidinyl),1-oxaspiro[4,5]dec-8-yl benzeneacetamide (U69593), trans-(+/-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50488), enadoline, ethylketocyclazocine, and nalorphine. These findings suggest that males and females differ in their responsiveness to mechanical nociception and that sex differences in sensitivity to kappa-, but not mu-, opioid receptor agonists are specific to certain nociceptive stimulus modalities.

Sex differences in opioid antinociception: kappa and 'mixed action' agonists

A number of investigators have shown that male animals are more sensitive than females to the antinociceptive effects of mu-opioid agonists. The present study was conducted to examine sex differences in opioid antinociception in the rat using agonists known to differ in selectivity for and efficacy at kappa- versus mu-receptors. Dose- and time-effect curves were obtained for s.c. U69593, U50488, ethylketazocine, (-)-bremazocine, (-)-pentazocine, butorphanol and nalbuphine on the 50 or 54 degrees C hotplate and warm water tail withdrawal assays; spontaneous locomotor activity was measured 32-52 min post-injection in the same rats. On the hotplate assay, only butorphanol (54 degrees C) and nalbuphine (50 degrees C) were significantly more potent in males than females. On the tail withdrawal assay, all agonists were significantly more potent or efficacious in males than females at one or both temperatures. In contrast, no agonist was consistently more potent in one sex or the other in decreasing locomotor activity. Estrous stage in female rats only slightly influenced opioid effects, accounting for an average of 2.6% of the variance in females' antinociceptive and locomotor responses to drug (50 degrees C experiment). These results suggest that (1) sex differences in antinociceptive effects of opioids are not mu-receptor-dependent, as they may occur with opioids known to have significant kappa-receptor-mediated activity; (2) the mechanisms underlying sex differences in kappa-opioid antinociception may be primarily spinal rather than supraspinal; (3) sex differences in antinociceptive effects of opioid agonists are not secondary to sex differences in their sedative effects.

Effects of yohimbine on the antinociceptive and place conditioning effects of opioid agonists in rodents

1. The pharmacological modulation of opioid actions by drugs acting on heterologous mechanisms could be useful to overcome some of the main problems associated with the use of opiate agonists. Based on previous findings on the interactions between yohimbine and opioid drugs, we have further studied the effects of yohimbine on the antinociceptive and positive-negative reinforcing effects of morphine (mu opioid receptor-preferring agonist), U-50,488 (kappa agonist) and SNC80 (delta agonist). 2. Pretreatment with yohimbine completely blocked the antinociception provided by the three opioid agonists in the mouse tail-immersion test. 3. A similar blockade of SNC80 and U-50,488-induced antinociception was observed with yohimbine in the mouse hot plate test at the same doses. In this paradigm, the effect of the kappa agonist was very slight and the actions of yohimbine rather variable. 4. In place conditioning experiments with SD (Sprague -- Dawley) male rats, yohimbine alone was inactive but it limited the preference induced by morphine and SNC80 and the aversive effect of U-50,488. Impaired novelty preference was also observed with the combination of yohimbine and U-50,488. 5. It is concluded that yohimbine tends to limit opioid antinociception and the addictive potential of mu and delta opioid agonists. More selective drugs could help to understand the mechanisms involved in these actions.

Nonpeptide analogues of dynorphin A(1-8): design, synthesis, and pharmacological evaluation of kappa-selective agonists

Two novel series of kappa opioid receptor agonist analogues of MPCB-GRRI and MPCB-RRI, hybrid ligands of MPCB ((-)-cis-N-(2-phenyl-2-carbomethoxy)cyclopropylmethyl-N-normetazocine ) and of the C-terminal fragments of dynorphin A(1-8), have been synthesized. The critical functional groups of the peptide fragments of hybrid compounds were maintained, and the binding affinities and selectivities for compounds 1-40 to mu, delta, and kappa opioid receptors were analyzed. Compounds 15 and 16, MPCB-Gly-Leu-NH-(CH(2))(n)()-NH-C(=NH)-C(4)H(9) (n = 5, 6), displayed high affinity and selectivity for kappa opioid receptors (K(i)(kappa) = 6.7 and 5.3 nM, K(i)(mu)/K(i)(kappa) = 375 and 408, and K(i)(delta)/K(i)(kappa) = 408 and 424, respectively). Since kappa agonists may also cause psychotomimetic effects by interaction with sigma sites, binding assays to sigma(1) sites were performed where compounds 15 and 16 showed negligible affinity (K(i) > 10 000). Compounds 15 and 16 were further characterized in vivo and showed potent antinociceptive activity in mouse abdominal constriction tests (ED(50) = 0.88 and 1.1 mg/kg, respectively), fully prevented by nor-BNI. Thus, these novel analogues open an exciting avenue for the design of peptidomimetics of dynorphin A(1-8).

Antinociceptive effect of amikacin and its interaction with morphine and naloxone

Amikacin sulphate (30 mg kg(-1)) administered either intraperitoneally (i.p.) or subcutaneously (s.c.) produced antinociceptive effect in BALB/c mice in the acetic acid writhing test which is employed as an inflammatory pain model. The lack of difference between two routes with regard to antinociceptive potency was taken as evidence for the absence of a local effect. Amikacin sulphate-induced antinociception seems unlikely to be due to non-specific behaviour alteration, since this drug, at a dose range of 15-100 mg kg(-1)did not affect motor coordination of mice in rot-a-rod test. Morphine (1 mg kg(-1)) also caused antinociception when administered i.p. or s.c. but the effect was greater with the latter route. At the i.p. site; the concurrent use of amikacin and morphine produced more remarkable antinociception compared to their individual usages. Besides, naloxone (2 mg kg(-1)) significantly decreased antinociceptive effect of amikacin but itself also exerted antinociception. At present, we have no plausible explanation for these findings at the i.p. site.

U-50,488 and the kappa receptor: a personalized account covering the period 1973 to 1990

All clinically significant analgesics for severe pain derive from the morphine model. Morphine has provided a fertile area for medicinal chemistry research and received an additional stimulus in the 1970s with the appearance of the opioid receptors. The background for the birth of U-50,488 is described herein. It occurred before the discovery of the kappa receptor and, thus, U-50,488 was classified originally as a non-mu compound, and only later as a kappa agonist. U-50,488 provided a succinct template for structural modifications and they are described for the period up to 1990. A description of the structural classes of kappa agonists is provided including a summary of kappa recognition sites based on known agonists.

The role of angiotensin II and of its receptor subtypes in the acetic acid-induced abdominal constriction test

The effects of angiotensin II (AngII), the AngII analogues saralasin--[Sar1, Ala8]AngII, sarmesin--[Sar1Tyr(Me)4]AngII, the nonpeptide AngII receptor antagonists DuP753 (losartan) (for AT1 receptor subtype) and PD123319 (for AT2 receptor subtype), as well as combinations of AngII and each of its analogues and receptor antagonists, administered intracerebroventricularly (ICV), were studied on mice using the acetic acid-induced abdominal constrictions test (acetic acid 1% intraperitoneally, IP). The abdominal constrictions were counted at 5-min intervals for 30 min. AngII at doses of 0.05, 0.1, and 1 microg exerted a dose-dependent antinociceptive effect. Saralasin, sarmesin, losartan, and PD123319 exhibited a dose-dependent effect on nociception: they either increased or decreased it. PD123319 antagonized the antinociceptive effect of AnglI while losartan was ineffective. The importance of AT2 receptor subtype for the nociception reducing effect of AngII is considered.

The involvement of opioidergic and noradrenergic mechanisms in nefopam antinociception

Nefopam is a clinically effective analgesic agent used to control mild to moderate pain, whose mechanism of action is unknown. We have investigated the antinociceptive activity of nefopam in the mouse abdominal constriction assay and tail immersion test (48 degrees C). Nefopam was found to possess a high degree of potency against acetic acid-induced visceral nociception (ED50 2.5 mg kg(-1)). In the presence of the opioid receptor antagonists, naloxone or naltrindole, the resulting nefopam dose-response relationships were shifted to the right. Naloxone or naltrindole had no effect upon aspirin (ED50 32.1 mg kg(-1)) or clonidine (ED50 0.061 mg kg(-1)) induced antinociception. Acetorphan (10 mg kg(-1); s.c.), an inhibitor of neutral endopeptidase (EC 3.4.24.11) was able to potentiate nefopam's antinociceptive activity (ED50 1.5 mg kg(-1)). The alpha2-adrenoceptor antagonist, 2-[2-(2-methoxy-1,4-benzodioxanyl)]imidazoline hydrochloride (RX821002; 1 mg kg(-1); s.c.), shifted the dose-response curves for clonidine (ED50 7.1 mg kg(-1)) and nefopam (ED50 5.3 mg kg(-1)) to the right in this assay. Additionally, centrally administered RX821002 (1 microg/5 microl/animal; i.c.v.) reduced both clonidine (ED50 7.2 mg kg(-1)) and nefopam's ED50 15.5 mg kg(-1)) efficacy in the abdominal constriction assay. Nefopam (3 and 7.5 mg kg(-1); s.c.) produced significant antinociceptive effect in the thermal assay. Aspirin and RX821002 were devoid of any significant activity in the tail immersion test. Nefopam was shown to possess RX821002-reversible antinociceptive activity in both the tail immersion test and the abdominal constriction assay. These data suggest the involvement of an opioidergic and noradrenergic component to nefopam's antinociceptive activity in the mouse abdominal constriction assay and tail immersion test. However, the present results are unable to determine if the opioidergic component of nefopam antinociception is through a direct and/or indirect activation of opioid receptors.

Effect of analgesics on audible and ultrasonic pain-induced vocalization in the rat

Brief electrical pulses applied to the rat tail elicit a complex vocal response which includes audible (peeps, chatters) and ultrasonic (USV) components. Aspirin and amitriptyline had no effect on the vocal responses. Morphine showed a dose-dependent and naloxone reversible antinociceptive effect on the 1st and 2nd audible peeps by decreasing their intensity (evaluated by their envelopes which correspond to the outer bounds of the soundwave amplitude plotted as a function of time), with ED50 values of 1.96 mg/kg and 0.36 mg/kg i.v. respectively. Paracetamol significantly reduced only the intensity of the second peep at the dose of 200 mg/kg iv. Chatter intensity was decreased by doses of 1 and 3 mg/kg i.v. of morphine which would suggest an effect on emotional components of pain. The intensity of USV was affected by morphine injection although the variations observed were non-significant. These data clearly implicate a specific role for the opioid analgesics in modifying the vocal pain related behaviors.

The involvement of the opioidergic system in the antinociceptive mechanism of action of antidepressant compounds

1. Debate exists as to the nature of antidepressant-induced antinociception. It is unclear whether antidepressants are inherently antinociceptive, are able to potentiate opioid antinociception or both. We have used the acetic acid induced abdominal constriction assay in mice to investigate antidepressant-induced antinociception. 2. All the antidepressants tested (s.c.) produced dose-dependent protection against acetic acid-induced abdominal constriction. Similarly, morphine and aspirin were also effective antinociceptive agents in this nociceptive assay. 3. Opioid antagonists, naloxone (0.5 mg kg(-1), s.c.) and naltrindole (1 mg kg(-1), s.c.), shifted the dose-response relationships to the right for each of the antidepressant agents (dothiepin, amitriptyline, sibutramine, (+)-oxaprotiline and paroxetine). In this context the naloxone dose-ratios were 1.95, 3.90, 2.32, 4.50 and 2.65, with naltrindole dose-ratios of 4.36, 17.00, 4.28, 11.48 and 2.65 were obtained, respectively. Naloxone also shifted the morphine dose-response relationship to the right, by a factor of 2.62, whilst naltrindole had no effect upon morphine antinociception. Aspirin antinociception remained unaffected by both opioid antagonists. 4. The enkephalin catabolism inhibitor acetorphan, by itself, produced no activity in this test at a dose of 10 mg kg(-1) (s.c.). However, at higher doses, acetorphan produced a linear dose-response relationship against acetic acid-induced abdominal constriction. 5. When acetorphan was administered before either the antidepressants or morphine, there was a clear potentiation of the antidepressant- or morphine-induced antinociception. However, acetorphan had no effect on aspirin antinociception. 6. Since neither of the opioid antagonists were able to attenuate, nor was acetorphan able to potentiate, aspirin antinociception, we concluded that the mechanism of antidepressant-induced antinociception is different from that of the non-steroidal anti-inflammatory drugs. 7. These data are consistent with the view that antidepressants may induce endogenous opioid peptide release, as shown by the acetorphan study. In this context, the ability of naltrindole to displace the antidepressant dose-response relationship to the right without affecting morphine antinociception, implicates the delta-opioid receptor and endogenous opioid peptides in antidepressant-induced antinociception.

Analgesic effects of amlodipine and its interaction with morphine and ketorolac-induced analgesia

1. The antinociceptive effects of amlodipine, administered subcutaneously (s.c.), intracerebroventricularly (i.c.v.) and intrathecally (i.t.) were examined with the acetic acid writhing and tail-flick tests in mice. Amlodipine was also tested in combination with morphine and ketorolac. Isobolographic analyses were used to define the nature of functional interactions between amlodipine and morphine or ketorolac. 2. The s.c. (0.1, 1.25, 2.5, 5 and 10 mg/kg), i.c.v. (2.5, 5, 10 and 20 micrograms/mice) and i.t. (2.5, 5, 10 and 20 micrograms/mice) administration of amlodipine exhibited a dose-dependent antinociceptive effect in the writhing test but had no effect on the tail-flick latency. Isobolographic analyses revealed an additive interaction between amlodipine and morphine or ketorolac in the writhing test. 3. These results suggest that amlodipine induces antinociception and increases antinociceptive action of morphine and ketorolac, possibly through a decrease in cellular calcium availability.

Tolerance and cross tolerance to the accuracy- and rate-decreasing effects of mu opioids in rats responding under a fixed-consecutive-number schedule

The purpose of the present investigation was to examine the development of tolerance to the effects of morphine and other mu opioids in rats responding under a fixed-consecutive-number (FCN) schedule of food presentation. To this end, five rats were trained under an FCN schedule and subsequently tested with a variety of mu opioids both before and during chronic exposure to a 0.4 mg/ml morphine drinking solution. Morphine, fentanyl, buprenorphine, butorphanol and nalbuphine produced dose-dependent decreases in both accuracy and response rate when tested prior to the chronic regimen. In most instances, doses of these drugs that decreased accuracy also decreased response rate. During chronic treatment, tolerance developed to the effects of morphine and cross-tolerance was conferred to the effects of fentanyl, buprenorphine and butorphanol. A greater degree of tolerance was conferred to the effects of butorphanol than to the other opioids examined, and the degree of tolerance conferred to butorphanol's rate-decreasing effects was greater than the degree of tolerance conferred to its accuracy-decreasing effects. Doses of naloxone that had no effect prior to morphine treatment produced large decreases in accuracy and response rate when tested during the chronic regimen. In contrast to the other opioids examined, the potency of nalbuphine was not altered by chronic morphine administration. These data emphasize the importance of both pharmacological and procedural variables in the development of tolerance and cross tolerance to the behavioral effects of opioids.

Aspartic acid conjugates of 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1(3-aminophenyl)-2-(1-pyrrolidi nyl) ethyl]acetamide: kappa opioid receptor agonists with limited access to the central nervous system

Aspartic acid conjugates of 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-aminophenyl)-2-(1-pyrrol idinyl) ethyl]acetamide (5) were synthesized and evaluated in mice for antinociceptive activity by intravenous and intracerebroventricular routes of administration. The intravenously-administered alpha-conjugate of L-Asp (2), its D-Asp diastereomer (3), and the beta -conjugate of L-Asp (4) were found to be 11-, 31-, and 40-fold, respectively, less effective than the parent ligand 1 (ICI 199,441) in producing central nervous system mediated antinociception in the mouse abdominal stretch assay. In addition, iv-administered 2 and 3 were found to also produce potent antinociception in the tonic phase of the mouse formalin assay, which is a model of tonic rather than acute pain. This study suggests that the attachment of a zwitterionic moiety to a position in the molecule that exhibits bulk tolerance is a viable strategy for the design of peripherally-selective and peripherally-active opioids.

Tolerance-like phenomenon of nifedipine-induced antinociception

We confirmed that repeated injection of nifedipine evokes an acute tolerance in rats. Nociceptive response was assessed by the tail-flick test. Rats, randomly divided into 3 groups, were injected once a week for 1 month with nifedipine intraperitoneally at the doses of: (A) 2 mg/kg; (B) 10 mg/kg; and (C) 10 mg/kg at the first injection and followed by dimethyl sulfoxide in the subsequent injections. After one month, group A and C were injected with 10 mg/kg and group B with 30 mg/kg nifedipine. In group A, 2 mg/kg of nifedipine did not change the TF latency but its repeated injections prevented the antinociceptive effect of 10 mg/kg nifedipine. In group B, only the first dose of 10 mg/kg and the last dose of 30 mg/kg produced a significant antinociception. In group C, the first but not the last dose of 10 mg/kg nifedipine, produced a significant antinociception. Our data suggest that a single dose of 10 mg/kg nifedipine or repeated doses of 2 mg/kg produced a tolerance-like phenomenon in nifedipine-induced antinociception.

Effects of the delta opioid against BW373U86 in pigeons trained to discriminate fentanyl, bremazocine and water in a three-choice drug discrimination procedure

The delta opioid agonist BW373U86 was examined alone and in combination with mu agonists in pigeons trained to discriminate the mu agonist fentanyl (0.056 mg/kg), the kappa agonist bremazocine (0.017 mg/kg), and distilled water in a three-choice drug discrimination procedure. BW373U86 (0.01-10 mg/kg) produced a dose-dependent increase in fentanyl-appropriate responding and complete generalization to fentanyl in four of five subjects. BW373U86 did not elicit bremazocine-appropriate responding in any of the subjects. Fentanyl-appropriate responding elicited by BW373U86 was antagonized by the delta selective antagonist naltrindole (0.1-10 mg/kg) but not by the mu selective antagonist naloxone (0.1-30.0 mg/kg). When BW373U86 was administered in combination with the mu agonists fentanyl, morphine and nalbuphine, a low dose of BW373U86 (0.01 mg/kg) that elicited primarily water-appropriate responding when administered alone did not produce a significant change in the ED50 values for fentanyl, morphine or nalbuphine. Higher doses of BW373U86 (0.1-1.0 mg/kg) increased levels of fentanyl-appropriate responding elicited by low doses of fentanyl, morphine and nalbuphine to levels similar to those produced by BW373U86 alone. These results indicate that BW373U86 shares discriminative stimulus properties with the mu agonist fentanyl in pigeons, possibly by acting at delta opioid receptors. However, BW373U86 does not potentiate the discriminative stimulus effects of mu agonists or share discriminative stimulus effects with the kappa agonist bremazocine.

Kappa-opioid receptor modulation of the release of substance P in the dorsal horn

Substance P (SP), a member of the tachykinin peptide family, has been found in high concentrations in the superficial laminae of the dorsal horn and it is thought to play a major role in the transmission of nociceptive information. Dynorphin(1-8), an opioid peptide with high selectivity for the kappa-opioid receptor subtype, is also found in the dorsal horn of the spinal cord. The aim of this study was to determine the effect of dynorphin(1-8) on the release of SP-like-immunoreactivity (SPLI) in the dorsal horn before and during the activation of peripheral nociceptors by a thermal stimulus. A push-pull canula was used to perfuse the dorsal horn of non-anesthetized decerebrate/spinal transected rats and the collected perfusates were assayed for SPLI by using radioimmunoassay. Dynorphin(1-8) applied to the spinal cord at a concentration of 1 microM elicited a 27 +/- 8% decrease in the basal release of SPLI and prevented the increase in the release of SPLI evoked by the application of a noxious thermal stimulus to the ipsilateral hind paw and lower limb. The effect of dynorphin(1-8) was reversed by 2 microM of nor-binaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist. Application of nor-BNI alone to the perfusate resulted in a 62 +/- 23% increase in the basal release of SPLI. In conclusion, dynorphin(1-8) reduces the basal release of SPLI and prevents the increase in the release of SPLI elicited by the application of a noxious cutaneous thermal stimulus. This effect is mediated through the kappa-opioid receptor, which appears to tonically regulate the release of SPLI in the dorsal horn.

Spinal mu and delta, but not kappa, opioid-receptor agonists attenuate responses to noxious colorectal distension in the rat

The antinociceptive efficacy of different opioid-receptor agonists following their intrathecal (i.t.) administration was examined in awake, unanesthetized rats in a model of visceral pain. Cumulative i.t. doses of the mu-preferring opioid-receptor agonist morphine produced dose-dependent attenuation of the change (increase) in mean arterial pressure (delta MAP) and elevation of the visceromotor threshold to colorectal distension (CRD). Similar dose-dependent antinociceptive effects were produced after i.t. administration of the mu opioid-receptor-selective agonist DAMPGO. Morphine and DAMPGO were equipotent against the delta MAP to phasic CRD (80 mm Hg, 20 sec), but DAMPGO was more than 6 times more potent than morphine in elevating the visceromotor threshold to an incrementing CRD. Intrathecal administration of the delta opioid-receptor-selective agonist DPDPE produced, like morphine and DAMPGO, a dose-dependent attenuation of the delta MAP to CRD; DPDPE was one-tenth as potent as morphine or DAMPGO. DPDPE also dose-dependently elevated the visceromotor threshold to CRD, but its efficacy was only half that of morphine or DAMPGO. The kappa opioid-receptor-selective agonist U 50488H was without antinociceptive efficacy after i.t. administration, but did attenuate responses to CRD after systemic administration. The antinociceptive effects produced by morphine and DAMPGO were antagonized by i.t. pretreatment with naloxone and the effects produced by DPDPE were antagonized by i.t. pretreatment with the delta opioid-receptor-selective antagonist naltrindole. These data indicate that local mu and delta, but not kappa, opioid receptors can modulate visceral nociceptive transmission in the spinal cord.

CCB, a novel specific kappa opioid agonist, which discriminates between opioid and sigma 1 recognition sites

CCB, 6,11-Dimethyl-1,2,3,4,5,6-hexahydro-3-([2'-methoxycarbonyl-2'-(4- chlorophenyl)cyclopropyl]methyl)-2,6-methano-3-benzazocin-8-ol, displays specificity and very high affinity for kappa opioid receptor types (Ki = 0.41 +/- 0.19 nM). In contrast to other kappa opioid agonists, CCB is also selective with respect to sigma 1 sites (Ki = 1,050 +/- 55 nM). CCB displays antinociceptive and sedative effects in the mouse comparable to those of U50,488H and morphine. Naltrexone fully antagonizes these effects. The sedative effects of CCB are blocked in mice pretreated with naltrexone or nor-BNI. CCB and U50,488H produce a superimposable diuresis in rats. Naltrexone and nor-BNI, both are effective in antagonizing the effect. CCB does not produce any stereotyped behavior or ataxia in the behavioral assay in doses up to 40 mg/kg s.c. These findings suggest that CCB might be a useful tool to investigate the physiological role of kappa opioid receptors.

An innovative cold tail-flick test: the cold ethanol tail-flick test

An innovative antinociceptive test, the cold ethanol tail-flick test (CET), was developed for evaluating the actions of opioid analgesics. To select an optimal operation temperature range for the CET, temperatures from -5 degrees C to -30 degrees C were screened. After screening, temperatures ranging between -20 degrees C and -30 degrees C were both strong and effective enough to act as a noxious cold stimulus. In the following study, -20 degrees C was selected as the cold stimulus for the CET. The sensitivity and specificity of this test were challenged by opioid analgesics: an agonist (morphine) and two agonist-antagonists (buprenorphine and nalbuphine), two tranquilizers (droperidol and diazepam), and four nonopioid analgesics (acetaminophen, aspirin, indomethacin, and ketoprofen). The sensitivity of the CET was also compared with the assays using heat (radiant heat and hot water). The AD50 values determined by the CET for morphine, buprenorphine, and nalbuphine were 0.16 mg/kg, 0.22 micrograms/kg, and 0.19 mg/kg, respectively. Naloxone, an opioid antagonist, blocked the antinociceptive effects of these opioids which were determined by the CET. Furthermore, the tranquilizers and nonopioid analgesics did not show any activity in the CET. Our results show that not only can the CET assess the antinociceptive activity of both opioid agonist and mixed agonist-antagonist, it also possess the characteristics of sensitivity, specificity, simplicity, and reproducibility.

Opioid peptide receptor studies. 3. Interaction of opioid peptides and other drugs with four subtypes of the kappa 2 receptor in guinea pig brain

Using guinea pig, rat, and human brain membranes depleted of mu and delta receptors by pretreatment with the site-directed acylating agents BIT (mu selective) and FIT (delta selective), previous studies from our laboratory resolved two subtypes of the kappa 2 binding site, termed kappa 2a and kappa 2b. In more recent studies, we used 6 beta-[125Iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) to characterize multiple kappa 2 binding sites in rat brain. The results indicated that [125I]IOXY, like [3H]bremazocine, selectively labels kappa 2 binding sites in rat brain membranes pretreated with BIT and FIT. In the rat brain, using 100 nM [D-Ala2-MePhe4,Gly-ol5]enkephalin to block [125I]IOXY binding to the kappa 2b site, we resolved two subtypes of the kappa 2a binding site. In the present study we examined the binding of [125I]IOXY to the kappa 2 receptors of guinea pig brain. As observed in rat brain, [125I]IOXY, under appropriate assay conditions, selectively labels kappa 2 binding sites. Quantitative binding studies readily demonstrated the presence of kappa 2a and kappa 2b binding sites. The kappa 2a binding sites were selectively assayed using 5 microM [Leu5]enkephalin to block [125I]IOXY binding to the kappa 2b sites, and kappa 2b sites were selectively assayed using 5 microM (-)-(1S,2S)-U50,488 to block [125I]IOXY binding to the kappa 2a sites. Under these conditions, two subtypes of the kappa 2a site were resolved with high (kappa 2a-1) and low (kappa 2a-2) affinity for nor-BNI (Ki values = 0.88 and 476 nM) and CI977 (Ki values = 17.5 and 95,098 nM). Similarly, two subtypes of the kappa 2b site were observed with high (kappa 2b-1) and low (kappa 2b-2) affinity for [D-Ala2-MePhe4,Gly-ol5]enkephalin (DAMGO) (Ki values = 97 and 12,321 nM) and alpha-neoendorphin (Ki values = 33 and 5308 nM). Two-site models were also resolved in the presence of 100 microM 5'-guanylyimidodiphosphate (GppNHp). We carried out detailed ligand selectivity analysis of the multiple kappa 2 binding sites. Most test agents were either nonselective or selective for the kappa 2a-1 site. Nalbuphine was moderately selective for the kappa 2a-2 site. Similarly, although most test agents were either nonselective or selective for the kappa 2b-1 site, butorphanol, and the delta antagonists naltrindole, naltriben, and 7-benzylidene-7-dehydronaltrexone were moderately selective for the kappa 2b-2 site.(ABSTRACT TRUNCATED AT 400 WORDS)