Involvement of spinal release of α-neo-endorphin on the antinociceptive effect of TAPA

The antinociceptive effect of i.t.-administered Tyr-d-Arg-Phe-β-Ala (TAPA), an N-terminal tetrapeptide analog of dermorphin, was characterized in ddY mice. In the mouse tail-flick test, TAPA administered i.t. produced a potent antinociception. The antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with the κ-opioid receptor antagonist nor-binaltorphimine, as well as by the μ-opioid receptor antagonist β-funaltrexamine and the μ₁-opioid receptor antagonist naloxonazine. TAPA-induced antinociception was also significantly suppressed by co-administration of the μ₁-opioid receptor antagonist Tyr-d-Pro-Phe-Phe-NH₂ (d-Pro²-endomorphin-2) but not by co-administration of the μ₂-opioid receptor antagonists Tyr-d-Pro-Trp-Phe-NH₂ (d-Pro²-endomorphin-1) and Tyr-d-Pro-Trp-Gly-NH₂ (d-Pro²-Tyr-W-MIF-1). In CXBK mice whose μ₁-opioid receptors were naturally reduced, the antinociceptive effect of TAPA was markedly suppressed compared to the parental strain C57BL/6ByJ mice. Moreover, the antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with antiserum against the endogenous κ-opioid peptide α-neo-endorphin but not antisera against other endogenous opioid peptides. In prodynorphin-deficient mice, the antinociceptive effect of TAPA was significantly reduced compared to wild-type mice. These results suggest that the spinal antinociception induced by TAPA is mediated in part through the release of α-neo-endorphin in the spinal cord via activation of spinal μ₁-opioid receptors.     

δ-opioid supraspinal antinociception in mice is mediated by G13 transducer proteins

The intracerebroventricular (i.c.v.) injection to mice of antisera directed against different sequences G_i3α, impaired the antinociception produced by the selective ligands of δ opioid receptors DPDPE and [D-Ala²]-Deltorphin II, when studied 24 h later in the tail-flick test. Likewise, the potency of the μ/δ ligands DADLE, etorphine and β-endorphin-(1–31) was also reduced. Antinociception due to the μ-agonists morphine and DAMGO was slightly altered by this treatment. The selective δ antagonist ICI 174864 significantly reduced the antinociceptive activity of these opioids to the same extent observed after giving anti-G_i3α antisera. In animals treated with the antisera, ICI 174864 failed to reduce the antinociceptive effect that remained. It is concluded that G_i3 is the type of transducer protein regulated by δ opioid receptors to produce supraspinal antinociception in mice.     

Intrathecal substance P augments morphine-induced antinociception: Possible relevance in the production of substance P N-terminal fragments

The present study sought to examine the mechanism of substance P to modulate the antinociceptive action of intrathecal (i.t.) morphine in paw-licking/biting response evoked by subcutaneous injection of capsaicin into the plantar surface of the hindpaw in mice. The i.t. injection of morphine inhibited capsaicin-induced licking/biting response in a dose-dependent manner. Substance P (25 and 50 pmol) injected i.t. alone did not alter capsaicin-induced nociception, whereas substance P at a higher dose of 100 pmol significantly reduced the capsaicin response. Western blots showed the constitutive expression of endopeptidase-24.11 in the dorsal and ventral parts of lumbar spinal cord of mice. The N-terminal fragment of substance P (1–7), which is known as a major product of substance P by endopeptidase-24.11, was more effective than substance P on capsaicin-induced nociception. Combination treatment with substance P (50 pmol) and morphine at a subthreshold dose enhanced the antinociceptive effect of morphine. The enhanced effect of the combination of substance P with morphine was reduced significantly by co-administration of phosphoramidon, an inhibitor of endopeptidase-24.11. Administration of d-isomer of substance P (1–7), [d-Pro², d-Phe⁷]substance P (1–7), an inhibitor of [³H] substance P (1–7) binding, or antisera against substance P (1–7) reversed the enhanced antinociceptive effect by co-administration of substance P and morphine. Taken together these data suggest that morphine-induced antinociception may be enhanced through substance P (1–7) formed by the enzymatic degradation of i.t. injected substance P in the spinal cord.     

An examination of the relationship between μ-opioid antinociceptive efficacy and G-protein coupling using pertussis and cholera toxins

The hypothesis that μ-opioid agonists having low antinociceptive efficacy might be more susceptible to interference with G-protein coupling than μ-opioid agonists having higher antinocicep-tive efficacy was tested. Supraspinal antinociceptive efficacy for the three μ-opioid agonists morphine, [D-Ala², NMePhe⁴, Gly⁵-ol]-enkephalin (DAMGO) and sufentanil in the mouse 55 °C warm-water tail-flick test was evaluated 18–24 h after intracerebroventricular (i.c.v.) administration of β-funaltrexamine (β-FNA). The β-FNA pretreatment (0.2–2.0 nmol) attenuated antinociception in the order morphine > DAMGO > sufentanil, consistent with previous reports of their relative antinociceptive efficacy. The association of efficacy with G-protein coupling was then assessed by determining sensitivity to i.c.v. (0.1–3.0 μg) pertussis toxin (PTX) or cholera toxin (CTX). The effect of PTX on equiantinociceptive doses was in the inverse order of agonist efficacy. CTX augmented sufentanil-induced antinociception. Morphine- and DAMGO-induced antinociception were unaffected by CTX. These data suggest that: (i) highly efficacious μ agonists (viz., sufentanil) couple more efficiently to PTX-sensitive inhibitory G_i-proteins than do agonists of lower efficacy (viz., morphine, DAMGO) and (ii) highly efficacious μ agonists have greater capacity to utilize CTX-sensitive stimulatory G_s-proteins than do μ-agonists with lower efficacy.     

Comparative measurement of spinal CSF microdialysate concentrations and concomitant antinociception of morphine and morphine-6beta-glucuronide in rats

Morphine-6beta-glucuronide (M6G) is well known as a potent active metabolite in humans. To clarify concentration-antinociceptive effect relationships for morphine and M6G, we evaluated comparatively the pharmacokinetics and antinociceptive effects of morphine and M6G. The spinal CSF concentration and antinociception were simultaneously measured by using the combination of a microdialysis method and the formalin test in conscious rats after the s.c. administration of morphine (0.3-3 mg/kg) and M6G (0.1-3 mg/kg). The plasma concentration of M6G after s.c. administration was higher than that of morphine, as shown by the 2.1 times greater value of area under the concentration-time curve (AUC(plasma)). The spinal CSF concentrations of morphine and M6G increased dose-dependently. The AUC(CSF) of M6G was 1.6-1.8 times higher than that of morphine at each dose. Administration of morphine and M6G dose-dependently suppressed the flinching behavior induced by formalin injection. The ED(50) values for M6G were 3 times lower than those of morphine, although the spinal CSF concentration versus antinociceptive effect curves of morphine and M6G were very similar, with similar EC(50) values. These results suggest that the antinociceptive potencies of morphine and M6G, evaluated by simultaneous measurements of spinal CSF drug concentration and antinociception, are equivalent. Simultaneous measurement of spinal CSF concentration and antinociception by using microdialysis should be useful for elucidating the relationship between pharmacokinetics and pharmacodynamics of various opioids.     

Antibodies raised against the N-terminal sequence of δ opioid receptors blocjed δ-mediated supraspinal antinociception in mice

A polyclonal antiserum directed against the first 16 aminoacids of the N-terminal sequence of the murine δ opioid receptor was raised in rabbits. The intracerebroventricular (i.c.v.) injection to mice of the anti δ receptor IgGs impaired the antinociception produced by DPDPE, [D-Ala²]- Deltorphin II, DADLE and β-endorphin-(1–31) when studied 24 h later in the tail-flick test. Antinociception produced by morphine and DAMGO was fully expressed in mice undergoing this treatment. The selective δ antagonist ICI 174864 (0.8 nmols/mouse, i.c.v.) significantly reduced the antinociceptive activity of opioids to the extent observed after giving the antibodies. ICI 174864 did not decrease further the antinociception that remained after the anti δ receptor serum. The specific binding displayed by 3 nM [³H]-DPDPE was reduced in membranes pre-incubated with the antiserum, whereas no change could be detected for 0.6 nM [³H]-DAMGO labelling μ receptors. This experimental approach revealed the δ component of opioid-evoked supraspinal antinociception in mice.     

Streptozotocin-induced diabetes selectively enhances antinociception mediated by δ1-but not δ2-opioid receptors

We assessed the effect of diabetes on antinociception produced by intracerebroventricular injection of δ-opioid receptor agonists [D-Pen^2,5]enkephalin (DPDPE) and [D-Ala²]deltorphin II. The antinociceptive effect of DPDPE (10 nmol), administered i.c.v., was significantly greater in diabetic mice than in non-diabetic mice. The antinociceptive effect of i.c.v. DPDPE was significantly reduced in both diabetic and non-diabetic mice following pretreatment with 7-benzylidenenaltrexone (BNTX), a selective δ₁-opioid receptor antagonist, but not with naltriben (NTB), a selective δ₂- opioid receptor antagonist. There were no significant differences in the anticiceptive effect of [D-Ala²]deltorphin II (3 nmol, i.c.v.) in diabetic and non-diabetic mice. Furthermore, the antinociceptive effect of i.c.v. [D-Ala²]deltorphin II was significantly reduced in both diabetic and non-diabetic mice following pretreatment with NTB, but not with BNTX. In conclusion, mice with diabetes are selectively hyper-responsive to supraspinal δ₁-opioid receptor-mediated antinociception, but are normally responsive to activation of δ₂-opiod receptors.     

Role of pertussis toxin-sensitive G-protein, K+ channels, and voltage-gated Ca2+ channels in the antinociceptive effect of inosine

Inosine is the first metabolite of adenosine. It exerts an antinociceptive effect by activating the adenosine A₁ and A_2A receptors. We have previously demonstrated that inosine exhibits antinociceptive properties in acute and chronic mice models of nociception. The aim of this study was to investigate the involvement of pertussis toxin-sensitive G-protein-coupled receptors, as well as K⁺ and Ca²⁺ channels, in the antinociception promoted by inosine in the formalin test. Mice were pretreated with pertussis toxin (2.5 μg/site, i.t., an inactivator of G_i/0 protein); after 7 days, they received inosine (10 mg/kg, i.p.) or morphine (2.5 mg/kg, s.c., used as positive control) immediately before the formalin test. Another group of animals received tetraethylammonium (TEA) or 4-aminopyridine (4-AP) (1 μg/site, i.t., a non-specific voltage-gated K⁺ channel blockers), apamin (50 ng/site, i.t., a small conductance Ca²⁺-activated K⁺ channel blocker), charybdotoxin (250 pg/site, i.t., a large-conductance Ca²⁺-activated K⁺ channel blocker), glibenclamide (100 μg/site, i.t., an ATP-sensitive K⁺ channel blocker) or CaCl₂ (200 nmol/site, i.t.). Afterwards, the mice received inosine (10 mg/kg, i.p.), diclofenac (10 mg/kg, i.p., a positive control), or morphine (2.5 mg/kg, s.c., a positive control) immediately before the formalin test. The antinociceptive effect of inosine was reversed by the pre-administration of pertussis toxin (2.5 μg/site, i.t.), TEA, 4-aminopyridine, charybdotoxin, glibenclamide, and CaCl₂, but not apamin. Further, all K⁺ channel blockers and CaCl₂ reversed the antinociception induced by diclofenac and morphine, respectively. Taken together, these data suggest that the antinociceptive effect of inosine is mediated, in part, by pertussis toxin-sensitive G-protein coupled receptors and the subsequent activation of voltage gated K⁺ channel, large conductance Ca²⁺-activated and ATP-sensitive K⁺ channels or inactivation of voltage-gated Ca²⁺ channels. Finally, small conductance Ca²⁺-activated K⁺ channels are not involved in the antinociceptive effect of inosine.     

Decreased spinal morphine/clonidine antinociceptive synergism in morphine-tolerant mice

The antinociceptive interactions between spinally administered opioids and the alpha₂ agonist clonidine were examined in placebo and morphine pellet-implanted mice using the tail flick test. In placebo pellet-implanted animals, coadministered morphine and clonidine produced a synergistic antinociceptive effect. In mice implanted with morphine pellets, the synergism decreased to an additive interaction. The interactions between clonidine and the mu agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO), the delta agonist D-Pen²-D-Pen⁵-Enkephalin (DPDPE), and the kappa agonist U50-488H were also synergistic in placebo animals. In morphine pellet treated mice the DPDPE/clonidine interaction decreased to an antagonistic interaction, the DAMGO/clonidine remained synergistic and the U50-488H/clonidine interaction decreased to additive. These results support the proposal that the morphine spinal/supraspinal synergism depends upon the interaction between spinal opioid and alpha₂ receptors and a decrease in this interaction is a mechanism involved in development of tolerance to morphine. In addition, delta and kappa receptors appeared to be more involved in the morphine/clonidine decreased interaction than did mu opioid receptors.     

Possible involvement of mu1-opioid receptors in the fentanyl- or morphine-induced antinociception at supraspinal and spinal sites

Fentanyl has been shown to be a potent analgesic with a lower propensity to produce tolerance and physical dependence in the clinical setting. The present study was designed to investigate the mechanisms of fentanyl- or morphine-induced antinociception at both supraspinal and spinal sites. In the mouse tail-flick test, the antinociceptive effects induced by both fentanyl and morphine were blocked by either the mu1-opioid receptor antagonist naloxonazine or the mu1/mu2-opioid receptor antagonist beta-funaltrexamine (beta-FNA) after s.c., i.c.v. or i.t. injection. In contrast, both fentanyl and morphine given i.c.v. or i.t. failed to produce antinociception in mu1-deficient CXBK mice. These findings indicate that like morphine, the antinociception induced by fentanyl may be mediated predominantly through mu1-opioid receptors at both supraspinal and spinal sites in mice. We also determined the ED50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl- or morphine-induced antinociception in mice. The ED50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl-induced antinociception were 73.7, 18.5 and 1.2-fold lower than that of morphine, respectively. The present data clearly suggest the usefulness of peripheral treatment with fentanyl for the control of pain.     

Modulation of acute morphine tolerance by corticotropin-releasing factor and dynorphin A in the mouse spinal cord.

Previously, we have demonstrated that intrathecally (i.t.) administered corticotropin-releasing factor (CRF) in mice produces stimulus-specific antinociception and modulation of morphine-induced antinociception by mechanisms involving spinal kappa opioid receptors. Recently, we also have found that CRF releases immunoreactive dynorphin A, a putative endogenous kappa opioid receptor agonist, from superfused mice spinal cords in vitro. Dynorphin A administered intracerebroventricularlly (i.c.v.) to mice has been shown to modulate the expression of morphine tolerance. In the present study, the possible modulatory effects of i.t. administered CRF as well as dynorphin A on morphine tolerance were studied in an acute tolerance model. Subcutaneous administration of 100 mg/kg of morphine sulfate (MS) to mice caused an acute tolerance to morphine-induced antinociception. The antinociceptive ED50 of MS was increased from 4.4 mg/kg (naive mice) to 17.9 mg/kg (4 hours after the injection of 100 mg/kg MS). To study the modulatory effects of spinally administered CRF and dynorphin A on the expression of morphine tolerance, CRF and dynorphin A were injected i.t. at 15 min and 5 min, respectively, before testing the tolerant mice by the tail-flick assay. The antinociceptive ED50 of MS in tolerant mice was decreased to 8.8 mg/kg and 7.1 mg/kg, respectively, after i.t. administration of CRF (0.1 nmol) and dynorphin A (0.2 nmol). In contrast, 0.5 nmol of alpha-helical CRF (9-41), a CRF antagonist and 0.4 nmol of norbinaltorphimine, a highly selective kappa opioid receptor antagonist, when administered i.t. at 15 min before the tail-flick test in tolerant mice, increased the antinociceptive ED50 of MS to 56.6 mg/kg and 88.8 mg/kg, respectively. These data confirmed the modulatory effect of dynorphin A on morphine tolerance and suggested that CRF, which releases dynorphin A in several central nervous system regions, also plays a modulatory role in the expression of morphine tolerance.     

Environmental and intraperitoneal ethanol influences morphine antinociceptive effect in mice

The ability of acute environmental or intraperitoneal (i.p.) ethanol to influence morphine antinociceptive effect was studied in mice. In order to induce tolerance to morphine analgesia, mice received daily injections of 10 mg/Kg morphine over a period of 10 days. Mice were divided into three groups: i.p. ethanol (E), environmental ethanol (E*), and control saline (M). During the induction of tolerance these groups were treated identically except on days 1 and 11. On these days, 10 minutes prior to morphine injection, mice received either i.p. ethanol (1g/Kg), environmental ethanol (a bottle of 10% ethanol placed next to the animals cage during the experiments), or an equivalent volume of saline. Analgesia was assessed using a standard hot plate protocol and dose-response cumulative curves for morphine analgesia were obtained on days 1 and 11. On day 1, both the i.p. and environmental administration of ethanol showed similar morphine-potentiation effects [Mean Effective Dose: ED50 (M1)=4.5 mg/kg; ED50 (E1)=2.4 mg/kg; ED50 (E*1)=2.1 mg/kg]. On day 11, control group mice showed a reduction of morphine analgesia at test [ED50 (M11)=14.1 mg/kg]. Mice receiving i.p. and environmental ethanol again showed a leftward shift in dose-response cumulative curves for morphine antinociception with respect to controls [ED50 (E11)=9.1 mg/kg; ED50 (E*11)=4.7 mg/kg]. I.p. ethanol administration at non-antinociceptive doses enhances the morphine antinociception effect similarly in tolerant and non-tolerant (naive) mice. The presence of environmental ethanol can also induce a similar pattern of increase in morphine antinociception effect.     

Antinociceptive effects of spinally administered nociceptin/orphanin FQ and its N-terminal fragments on capsaicin-induced nociception

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the N/OFQ peptide (NOP) receptors, has been shown to be metabolized into some fragments. We examined to determine whether intrathecal (i.t.) N/OFQ (1-13), (1-11) and (1-7) have antinociceptive activity in the pain-related behavior after intraplantar injection of capsaicin. The i.t. administration of N/OFQ (0.3-1.2 nmol) produced an appreciable and dose-dependent inhibition of capsaicin-induced paw-licking/biting response. The N-terminal fragments of N/OFQ, (1-13) and (1-11), were antinociceptive with a potency lower than N/OFQ. Calculated ID₅₀ values (nmol, i.t.) were 0.83 for N/OFQ, 2.5 for N/OFQ (1-13) and 4.75 for N/OFQ (1-11), respectively. The time-course effect revealed that the antinociceptive effects of these N-terminal fragments lasted longer than those of N/OFQ. Removal of amino acids down to N/OFQ (1-7) led to be less potent than N/OFQ and its fragments, (1-13) and (1-11). Antinociception induced by N/OFQ or N/OFQ (1-13) was reversed significantly by i.t. co-injection of [Nphe¹]N/OFQ (1-13)NH₂, a peptidergic antagonist for NOP receptors, whereas i.t. injection of the antagonist did not interfere with the action of N/OFQ (1-11) and (1-7). Pretreatment with the opioid receptor antagonist naloxone hydrochloride did not affect the antinociception induced by N/OFQ and its N-terminal fragments. These results suggest that N-terminal fragments of N/OFQ are active metabolites and may modulate the antinociceptive effect of N/OFQ in the spinal cord. The results also indicate that N/OFQ (1-13) still possess antinociceptive activity through NOP receptors.     

Involvement of spinal kappa opioid receptors in the antagonistic effect of dynorphins on morphine antinociception.

The modulatory effects of intrathecally (i.t.) administered dynorphin A(1-17) and dynorphin A(1-13) on morphine antinociception have been studied previously in rats by other investigators. However, both potentiating and attenuating effects have been reported. In this study, the modulatory effects of i.t. administered dynorphin A(1-17) as well as the smaller fragment, dynorphin A(1-8), were studied in mice. In addition, nor-binaltorphimine (nor-BNI), a highly selective kappa opioid receptor antagonist, and naltrindole (NTI), a highly selective delta opioid receptor antagonist, were used to characterize the possible involvement of spinal kappa and delta opioid receptors in the modulatory effects of the dynorphins. Dynorphin A(1-17) and dynorphin A(1-8) administered i.t. at doses that did not alter tail-flick latencies, were both able to antagonize in a dose-dependent manner, the antinociceptive action of s.c. administered morphine sulfate. The antinociceptive ED50 of morphine sulfate was increased 3.9- and 5.3-fold by 0.4 nmol/mouse of dynorphin A(1-17) and dynorphin A(1-8), respectively. Injections of 0.4 and 0.8 nmol/mouse of nor-BNI i.t., but not its inactive enantiomer (+)-1-nor-BNI, inhibited dose-dependently the antagonistic effects of the dynorphins. These doses of nor-BNI alone did not affect the antinociceptive action of morphine sulfate. Intrathecal administration of 5 nmol/mouse of NTI also did not affect the modulatory effects of dynorphins. These observations that dynorphins exert their antagonistic effects on morphine-induced antinociception stereoselectively through spinal kappa opioid receptors may suggest a coupling between spinal kappa and mu opioid receptors.     

The acute antinociceptive effect of hyperbaric oxygen is not accompanied by an increase in markers of oxidative stress

Aims

Exposure to hyperbaric oxygen (HBO₂) causes an antinociceptive response in mice. However, breathing oxygen (O₂) at an elevated pressure can potentially cause oxygen toxicity. The aim of this study was to identify the determinants of HBO₂ antinociception and the toxicity profile of HBO₂.

Main methods

Male NIH Swiss mice were assessed for acute antinociceptive responsiveness under room air or 100% O₂ at 1.0 or 3.5 atmospheres absolute (ATA), using the acetic acid-induced abdominal constriction test. For the oxygen toxicity test, mice were exposed to 3.5 ATA oxygen for 11 min, 60 min, and 60 min daily for 2 days (120 min) or 60 min daily for 4 days (240 min), then assessed by analyzing the levels of two oxidative stress markers, MDA (malondialdehyde) and protein carbonyl in brain, spinal cord and lung.

Key findings

Only the combination of 100% O₂ and 3.5 ATA caused significant antinociception. The antinociceptive effect of 100% O₂ was pressure-dependent up to 3.5 ATA. In the oxygen toxicity test, mice exposed to HBO₂ for different time intervals had levels of brain, spinal cord and lung MDA and protein carbonyl that were comparable to that of control animals exposed to room air.

Significance

Treatment with 100% O₂ evokes a pressure-dependent antinociceptive effect. Since there was no significant increase in levels of the oxidative stress markers in the tested tissues, it is concluded that HBO₂ at 3.5 ATA produces antinociception in the absence of oxidative stress in mice.     

Synthesis of iboga-like isoquinuclidines: Dual opioid receptors agonists having antinociceptive properties

Some novel iboga-analogues consisting of benzofuran moiety and dehydroisoquinuclidine ring connected by –CH₂–, (CH₂)₂ and (CH₂)₃ linkers have been synthesized with the view to develop potential antinociceptive drugs. The compounds 14 and 21 showed binding at the μ-opioid receptor (MOR), while the compound 11a exhibited dual affinities at both MOR and κ-opioid receptor (KOR). MAP kinase activation indicated all three compounds have opioid agonistic properties. The presence of a double bond and endo-methylcarboxylate group in the dehydroisoquinuclidine ring and the benzofuran and methylene spacer appeared to be essential for opioid receptor binding. Further studies demonstrated 11a caused significant antinociception in mice in the hot-plate test which was comparable to that produced by morphine. The compound 11a was also found to be nontremorigenic unlike various iboga congeners. This study identifies a new pharmacophore which may lead to the development of suitable substitute of morphine in the treatment of pain.     

Role of the delta-opioid receptor in (1DMe)NPYF mediated antinociception

A selective delta-opioid antagonist, naltrindole, was used to study the role of the delta-opioid receptor in the antinociceptive actions of a synthetic NPFF analog, (1DMe)NPYF. I.t. (1DMe)NPYF (5 nmol) produced antinociception in the tail flick test and (1DMe)NPYF (0.5 nmol) potentiated the antinociceptive effect of i.t. morphine 7.8 nmol. (1DMe)NPYF (5 nmol) had an antihyperalgesic effect in carrageenan inflammation and it significantly reduced mechanical allodynia in the spinal nerve ligation model. All these effects were prevented or significantly reduced by pretreatment with naltrindole (28 nmol) (P < 0.01-0.001). These data suggest that activation of spinal delta-opioid receptors plays an important role in mediating the spinal antinociceptive effects of (1DMe)NPYF.     

Effect of Bacopasides on acquisition and expression of morphine tolerance

Opioids are extensively used for the management of both chronic malignant and non malignant pains. One major serious limitation associated with chronic use of opioids is the development of tolerance to its analgesic effect. The effect of Bacopa monnieri, a renowned ayurvedic medicine for acquisition and expression of morphine tolerance in mice, was investigated. Bacopa monnieri, n-Butanol fraction was analyzed on High performance liquid chromatography (HPLC), for Bacopaside A major components i.e. Bacoside A₃, Bacopaside ll and Bacosaponin C. Antinociceptive effect of n-Butanol extract of Bacopa monnieri (n Bt-ext BM) (5, 10 and 15 mg/kg) was assessed on hot plate. Effect of different doses of n Bt-ext BM on morphine antinociception was also assessed. n Bt-ext BM was also screened for development of tolerance to antinociceptive effect of Bacopa monnieri by administering 15 mg/kg n Bt-ext BM for seven days. Tolerance to morphine analgesia was induced in mice by administering intraperitoneally (I.P.) 20 mg/kg morphine twice daily for five days. Acute and Chronic administration of 5, 10 and 15 mg/kg n Bt-ext BM significantly reduced both expression and development of tolerance to morphine analgesia in mice. Additionally, Bacopa monnieri was found to enhance antinociceptive effect of morphine in intolerant animals. However, no tolerance to Bacopa monnieri antinociceptive effect was observed in seven days treatment schedule. These findings indicate effectiveness of Bacopa monnieri for management of morphine tolerance.     

Antinociceptive effects of [D-Ala2]deltorphin II, a highly selective delta agonist in vivo

The present study has characterized the antinociceptive actions of [D-Ala2]deltorphin II following intracerebroventricular (i.c.v.) administration in the mouse tail-flick test. [D-Ala2]deltorphin II produced dose- and time-related antinociception, with maximal effects at +10 min and significant antinociception which lasted for 40-60 min. [D-Ala2]deltorphin II was 13-fold more potent than i.c.v. [D-Pen2, D-Pen5]enkephalin (DPDPE), a second highly selective delta agonist, and approximately equipotent with i.c.v. morphine in producing antinociception. The antinociceptive effects of i.c.v. [D-Ala2]deltorphin II and DPDPE, but not those of morphine, were antagonized by the selective delta antagonist, ICI 174,864. In contrast, pretreatment with the non-equilibrium mu antagonist, beta-funaltrexamine blocked morphine antinociception, but failed to antagonize [D-Ala2]deltorphin II and DPDPE antinociception. These data indicate that [D-Ala2]deltorphin II produced its antinociceptive effects at a supraspinal delta receptor. [D-Ala2]deltorphin II appears to be the most appropriate delta opioid agonist currently available for studies in vivo and support the involvement of delta receptors in supraspinal antinociception.     

Dissimilar efficacy of opioids to produce μ-mediated analgesia: Role of Gx/z and G12 transducer proteins

Intracerebroventricular (i.c.v.) administration to mice of IgGs raised against α subunits of G₁₂ or G_x/z transducer proteins lessened the activation of low Km GTPase induced by morphine, DAMGO and DADLE in P₂ membranes from mouse periaqueductal grey matter (PAG). In mice injected with anti G₁₂α, DADLE, DPDPE and [D-Ala²] Deltorphin II, but not β-endorphin-(1-31), antagonized the analgesic activity of morphine. Conversely, following anti G_x/zα, morphine antagonized the antinociceptive potency of DADLE. It is concluded that opioids display diverse efficacy at μ-G_i2 and μ-G_x/z complexes to produce supraspinal analgesia in mice.