Antinociceptive effect of dihydroetorphine in diabetic mice

The antinociceptive potency of dihydroetorphine in diabetic mice was examined. Subcutaneous administration of dihydroetorphine produced a dose-dependent antinociception in both non-diabetic and diabetic mice. The antinociceptive potency of s.c. dihydroetorphine was less in diabetic mice than in non-diabetic mice. The antinociception induced by i.c.v. dihydroetorphine (0.02 microgram) was also significantly less in diabetic mice than in non-diabetic mice. The antinociceptive effects of dihydroetorphine (10 micrograms/kg i.p.) in both diabetic and non-diabetic mice were significantly antagonized by s.c. administration of beta-funaltrexamine, a selective mu-opioid receptor antagonist. Furthermore, the antinociceptive effect of dihydroetorphine (10 micrograms/kg i.p.) in non-diabetic mice, but not in diabetic mice, was also significantly antagonized by naloxonazine, a selective mu 1-opioid receptor antagonist. The time course and the potency of the antinociceptive effect of dihydroetorphine (10 micrograms/kg i.p.) in diabetic mice were similar to those in naloxonazine-treated non-diabetic mice. Naltrindole, a selective delta-opioid receptor antagonist, or nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had no significant effect on the antinociceptive effect of dihydroetorphine (10 micrograms/kg i.p.) in both diabetic and non-diabetic mice. These results suggest that dihydroetorphine produces an antinociceptive effect through the activation of both mu 1- and mu 2-opioid receptors in mice. Furthermore, the reduction in dihydroetorphine-induced antinociception in diabetic mice, as compared with non-diabetic mice, may be due to the hyporesponsive to supraspinal mu 1-opioid receptor-mediated antinociception in diabetic mice.     

Streptozotocin-induced diabetes selectively enhances antinociception mediated by delta 1- but not delta 2-opioid receptors

We assessed the effect of diabetes on antinociception produced by intracerebroventricular injection of delta-opioid receptor agonists [D-Pen2,5]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 delta 1-opioid receptor antagonist, but not with naltriben (NTB), a selective delta 2-opioid receptor antagonist. There were no significant differences in the antinociceptive 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 delta 1-opioid receptor-mediated antinociception, but are normally responsive to activation of delta 2-opioid receptors.     

Modification of morphine-induced place preference by diabetes

The effects of diabetes on morphine-induced place preference in mice were examined. Morphine caused dose-related place preference in both diabetic and non-diabetic mice. This morphine-induced place preference in diabetic mice was greater than that in non-diabetic mice. The morphine (5 mg/kg)-induced place preference in both diabetic and non-diabetic mice was significantly antagonized by pretreatment with beta-funaltrexamine, a selective mu-opioid receptor antagonist, but not with naloxonazine, a selective mu1-opioid receptor antagonist. The morphine (5 mg/kg)-induced place preference in non-diabetic mice was attenuated by pretreatment with either naltriben, a selective delta2-opioid receptor antagonist, or 7-benzylidenenaltrexone. a selective delta1-opioid receptor antagonist. Moreover, the morphine (10 mg/kg)-induced place preference in non-diabetic mice was antagonized by pretreatment with 7-benzylidenenaltrexone (0.7 mg/kg). Although 7-benzylidenenaltrexone had no effect on the place preference induced by 5 mg/kg morphine in diabetic mice, it reduced the place preference induced by 3 mg/kg morphine. Furthermore, the morphine (5 mg/kg)-induced place preference in diabetic mice was significantly antagonized by co-pretreatment with beta-funaltrexamine (10 mg/kg) and 7-benzylidenenaltrexone (0.7 mg/kg). 2-Methyl-4a alpha-(3-hydroxyphenyl)- 1,2,3,4,4a,5,12,12a alpha-octahydroquinolino[2,3,3-g]isoquinoline (TAN-67), a non-peptide delta-opioid receptor agonist, produced place preference in diabetic, but not in non-diabetic mice. These results support the hypothesis that the morphine-induced place preference is mainly mediated through the activation of the mu2-opioid receptor. Furthermore, the enhancement of the morphine-induced place preference in diabetic mice may be due to the up-regulation of delta-opioid receptor-mediated functions.     

Competitive and non-competitive NMDA antagonists block the development of antinociceptive tolerance to morphine, but not to selective mu or delta opioid agonists in mice

N-Methyl-D-aspartate (NMDA) receptor antagonists have been shown to block the development of antinociceptive tolerance to morphine. Assessment of the effects of NMDA antagonists on development of antinociceptive tolerance to selective opioid mu (mu) and delta (delta) agonists, however, has not been reported. In these experiments, selective mu and delta receptor agonists, and morphine, were repeatedly administered to mice either supraspinally (i.c.v.) or systemically (s.c.), alone or after pretreatment with systemic NMDA antagonists. Antinociception was evaluated using a warm-water tail-flick test. Repeated i.c.v. injections of mu agonists including morphine, fentanyl, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) or [D-Ala2, Glu4]deltorphin, a delta agonist, or s.c. injections of morphine or fentanyl, produced antinociceptive tolerance as shown by a significant rightward displacement of the agonist dose-response curves compared to controls. Single injections or repeated administration of MK801 (a non-competitive NMDA antagonist) or LY235959 (a competitive NMDA antagonist) at the doses employed in this study did not produce behavioral toxicity, antinociception or alter the acute antinociceptive effects of the tested opioid agonists. Consistent with previous reports, pretreatment with MK801 or LY235959 (30 min prior to agonist administration throughout the tolerance regimen) prevented the development of antinociceptive tolerance to i.c.v. or s.c. morphine. Neither NMDA antagonist, however, affected the development of antinociceptive tolerance to i.c.v. fentanyl, DAMGO, or [D-Ala2, Glu4]deltorphin. Additionally, MK801 pretreatment did not affect the development of antinociceptive tolerance to i.c.v. PL017 or to s.c. fentanyl. Further, MK801 pretreatment also did not affect the development of tolerance to the antinociception resulting from a cold-water swim-stress episode, previously shown to be a delta-opioid mediated effect. These data lead to the suggestion that the mechanisms of tolerance to receptor selective mu and delta opioids may be regulated differently from those associated with morphine. Additionally, these findings emphasize that conclusions reached with studies employing morphine cannot always be extended to 'opiates' in general.     

Antinociceptive evaluation of 14 beta-(bromoacetamido)-7,8-dihydro- N(cyclopropylmethyl)-normorphinone in mice

This study evaluated the supraspinal opioid effects of 14 beta-(bromoacetamido)-7,8-dihydro-N(cyclopropylmethyl)-normorphinone+ ++ (N-CPM-H2BAMO) in the mouse acetic acid-induced writhing and tail-flick assays. In the writhing test, N-CPM-H2BAMO produced a time- and dose-dependent antinociception after i.c.v. administration, with a 50% antinociceptive response being obtained with 0.28 (0.19-0.39) nmol when given 10 min before testing. The antinociceptive effect of N-CPM-H2BAMO was antagonized in a dose-dependent manner by the kappa-selective opioid receptor antagonist, nor-binaltorphimine. In the mouse tail-flick assay, N-CPM-H2BAMO failed to produce any antinociception after i.c.v. administration. N-CPM-H2BAMO produced a dose-dependent antagonism of morphine-induced antinociception but not antinociception induced by the delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin. Nor-binaltorphimine (0.3 nmol) at dose that completely antagonized N-CPM-H2BAMO-induced antinociception in the writhing assay did not prevent the antagonistic effect of N-CPM-H2BAMO on morphine-induced antinociception. Therefore, these data indicate that N-CPM-H2BAMO produces antinociception by acting at supraspinal kappa-opioid receptors in the writhing assay, and also acts as a mu-opioid receptor antagonist.     

The competence-related abilities of women criminal defendants

Delta9-tetrahydrocannabinol (delta9-THC) elicits antinociception in rodents through the central CB1 cannabinoid receptor subtype. In addition. Delta9-THC stimulates the release of dynorphin-related peptides leading to kappa-opioid spinal antinociception. In this work we describe the effect of a mixture of thiorphan (a neutral endopeptidase EC3.4.24.11 inhibitor) and bestatin (an aminopeptidase inhibitor), administered i.c.v., on the antinociceptive effect of peripherally administered delta9-THC in mice. As in the case of morphine or DAMGO ([D-Ala2.N-Me-Phe4,Gly-ol]enkephalin), a mu-selective opioid receptor agonist, the mixture of enkephalin-degrading enzyme inhibitors also enhanced the antinociceptive effect of delta9-THC. This effect was blocked by the CB1 cannabinoid receptor antagonist, SR-141,716-A, as well as by naloxone. The kappa-opioid receptor antagonist nor-binaltorphimine, administered i.t., also antagonized the effect of this combination. Similar results were obtained with the mu-opioid receptor antagonist beta-funaltrexamine after i.c.v. administration. These results demonstrate the involvement of both mu-opioid supraspinal and kappa-opioid spinal receptors in the interaction of both opioid and cannabinoid systems regulating nociception in mice.     

Effect of 7-nitroindazole on tolerance to morphine, U-50,488H and [D-Pen2, D-Pen5] enkephalin in mice

The effects of 7-nitroindazole (7-NI), an inhibitor of the neuronal nitric oxide synthase (nNOS) which does not increase blood pressure, on tolerance to the antinociceptive activity of mu-(morphine), kappa-(U-50,488H) and delta-([D-Pen2, D-Pen5]enkephalin, DPDPE) opioid receptor agonists were determined in mice. Male Swiss-Webster mice were made tolerant by twice daily injections of morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 micrograms/mouse, i.c.v.) for 4 days. When tested on day 5, tolerance to their antinociceptive activity was evidenced by decreased response in chronic drug treated mice in comparison to vehicle-injected mice. Concurrent administration of 7-NI (20, 40 or 80 mg/kg, i.p.) with DPDPE did not modify the development of tolerance to the antinociceptive action of DPDPE. However, 7-NI (40 or 80 mg/kg, i.p.) inhibited the development of tolerance to the antinociceptive activity of morphine and U-50,488H but the lower dose of 7-NI (20 mg/kg, i.p.) was not effective. Chronic administration of 7-NI by itself did not modify the acute response to morphine, U-50,488H or DPDPE. It is concluded that a specific inhibitor of nNOS can inhibit tolerance to the antinociceptive activity of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding

1. In rats, the interaction between the mu-opioid agonist dermorphin and the delta-opioid agonist [D-Ala2, Glu4]deltorphin was studied in binding experiments to delta-opioid receptors and in the antinociceptive test to radiant heat. 2. When injected i.c.v., doses of [D-Ala2, Glu4]deltorphin higher than 20 nmol produced antinociception in the rat tail-flick test to radiant heat. Lower doses were inactive. None of the doses tested elicited the maximum achievable response. This partial antinociception was accomplished with an in vivo occupancy of more than 97% of brain delta-opioid receptors and of 17% of mu-opioid receptors. Naloxone (0.1 mg kg-1, s.c.), and naloxonazine (10 mg kg-1, i.v., 24 h before), but not the selective delta-opioid antagonist naltrindole, antagonized the antinociception. 3. In vitro competitive inhibition studies in rat brain membranes showed that [D-Ala2, Glu4]deltorphin displaced [3H]-naltrindole from two delta-binding sites of high and low affinity. The addition of 100 microM Gpp[NH]p produced a three fold increase in the [D-Ala2, Glu4]deltorphin Ki value for both binding sites. The addition of 10 nM dermorphin increased the Ki value of the delta-agonist for the high affinity site five times. When Gpp[NH]p was added to the incubation medium together with 10 nM dermorphin, the high affinity Ki of the delta-agonist increased 15 times. 4. Co-administration into the rat brain ventricles of subanalgesic doses of dermorphin and [D-Ala2, Glu4]deltorphin resulted in synergistic antinociceptive responses. 5. Pretreatment with naloxone or with the non-equilibrium mu-antagonists naloxonazine and beta-funaltrexamine completely abolished the antinociceptive response of the mu-delta agonist combinations. 6. Pretreatment with the delta-opioid antagonists naltrindole and DALCE reduced the antinociceptive response of the dermorphin-[D-Ala2, Glu4]deltorphin combinations to a value near that observed after the mu-agonist alone. At the dosage used, naltrindole occupied more than 98% of brain delta-opioid receptors without affecting mu-opioid-receptors. 7. These data suggest that in the rat tail-flick test to radiant heat, mu- and delta-opioid agonists co-operate positively in evoking an antinociceptive response. Although interactions between different opioid pathways cannot be excluded, in vitro binding results indicate that this co-operative antinociception is probably mediated by co-activation of the delta-opioid receptors at the cellular level by the mu- and delta-agonist.     

Modulation of cocaine-induced motor activity in the rat by opioid receptor agonists

Selective opioid-receptor agonists were tested in combination with cocaine to determine the effect on the motor activity of rats. Cocaine produced dose-dependent increases in locomotor activity (distance traveled). The cocaine-induced increase in locomotor activity was potentiated by the selective delta-opioid receptor agonist [D-Pen2-D-Pen5]enkephalin (DPDPE). This potentiation was blocked by the general opioid receptor antagonist naltrexone, as well as by the selective opioid receptor antagonists beta-FNA (mu-opioid receptor) and naltrindole (delta-opioid receptor). DPDPE also potentiated the increase in locomotor activity produced by the selective dopamine reuptake inhibitor GBR12909, but not that produced by the direct dopamine receptor agonist apomorphine. Cocaine-induced motor activity was potentiated by the activation of central delta-opioid receptors. The synergistic effect seen with delta-opioid receptor activation may involve a mu-opioid receptor component, and is probably mediated via a dopaminergic pathway.     

Rational drug design and synthesis of a highly selective nonpeptide delta-opioid agonist, (4aS*,12aR*)-4a-(3-hydroxyphenyl)-2-methyl- 1,2,3,4,4a,5,12,12a-octahydropyrido[3,4-b]acridine (TAN-67)

We designed highly selective non-peptide agonists for the delta-opioid receptor. On the basis of the "message-address" concept in this field and the accessory site hypothesis, a novel class of heterocycle-fused octahydroisoquinoline derivatives were synthesized. One of these compounds [(4aS*,12aR*)-4a-(3-hydroxyphenyl)-2-methyl-1,2,3,4,4a,5,12, 12a -octahydropyrido[3,4-b]acridine, TAN-67 (2)] showed high selectivity for the delta-opioid receptor (Ki = 1.12 nM) in guinea-pig cerebrum with a 2070-fold lower affinity for the mu-opioid receptor and a 1600-fold lower affinity for the kappa-opioid receptor. TAN-67 was a potent delta-opioid receptor agonist with an IC50 value of 6.61 nM in the mouse vas deferens assay that was reversed by naltrindole (NTI) (Ke = 0.21). Moreover, TAN-67 was shown to have antinociceptive activity following subcutaneous administration in the mouse acetic acid abdominal constriction assay that was antagonized by NTI (delta 1- and delta 2-antagonist) and 7-benzylidinenaltrexone (delta 1-antagonist), but not by naltriben (delta 2-antagonist). This systemically applicable non-peptide agonist will be useful for elucidating the pharmacological properties of the delta-opioid receptor.     

Possible role of nitric oxide in the antinociceptive action of intraventricular bradykinin in mice

The i.c.v. administration of bradykinin (4, 8 and 16 micrograms) induced antinociception in mice which was resistant to naloxone; furthermore, the induction of tolerance to morphine by a single s.c. injection (100 mg/kg, 24 h before test doses of the peptide) did not affect antinociception. Since bradykinin is known to increase nitric oxide (NO) in peripheral tissues, we studied the possibility that its antinociceptive action may be related to NO effects in the central nervous system. Bradykinin effects were antagonized by previous treatment with NG-nitro-L-arginine or concomitant i.c.v. administration of bradykinin and methylene blue. The immediate precursor of NO, L-arginine, which by itself produces analgesia, also reduced bradykinin effects; moreover, tolerance to L-arginine significantly decreased the response to the peptide. These results suggest that NO is involved in antinociception induced by i.c.v. administration of bradykinin.     

ACEA-1328, an NMDA receptor antagonist, increases the potency of morphine and U50,488H in the tail flick test in mice

The effect of 5-nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1328), a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was examined on opioid-induced antinociception in the tail flick test. Swiss Webster mice were injected with ACEA-1328 either alone or in combination with morphine or (+/-)-trans-U-50488 methanesulfonate (U50,488H), a mu- and a kappa-opioid receptor agonist, respectively, and tested for antinociception. Systemic administration of ACEA-1328 alone increased the tail flick latencies with an ED50 of approximately 45 mg kg-1. Concurrent administration of ACEA-1328 with morphine, or U50,488H, at doses that did not affect tail flick latencies, potentiated the antinociceptive effect of the opioid analgesics and vice versa. Naloxone, an opioid receptor antagonist, while not modifying the effect of ACEA-1328, did block the augmentation, suggesting that opioid receptors might be involved in the latter effect. 5-Aza-7-chloro-4-hydroxy-3-(m-phenoxyphenyl)quinoline-2(1H)-one (ACEA-0762), a selective NMDA receptor/glycine site antagonist, also showed enhancement of the antinociceptive effect of morphine and U50,488H. However, concurrent administration of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol[f]quinoxaline (NBQX), a selective non-NMDA receptor antagonist, with morphine did not alter the antinociceptive potency of the opioid analgesic. Overall, the data suggest that ACEA-1328 may increase the potency of the opioid analgesics by antagonising the glycine site associated with the NMDA receptor.     

Antidepressant-like effects of CCK(B) receptor antagonists: involvement of the opioid system

RB 101 (N-[(R,S)-2-benzyl-3-[(S)-2-amino-4-methylthiobutyldithio]-1-oxopr opyl]-L -phenylalaninebenzyl ester), a systemically active inhibitor of enkep halin catabolism, has been shown to elicit antidepressant-like effects in mice, both in the forced-swimming and in the conditioned suppression of the mobility tests. The same type of response has been also observed following administration of the cholecystokinin CCK(B) receptor antagonist L-365,260 ((3R)-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin -3-yl)-3 -methylphenylurea). In terestingly, the delta-opioid receptor antagonist naltrindole (17-cyclopropylmethyl-6,7-dehydro-4,5alpha-epoxy-3,14-dihydroxy-6, 7,2'-3'-indolomorphinan) blocks the effect of both RB 101 and L-365,260 in the conditioned suppression of the motility test. In this work we have investigated the involvement of the opioid system in the antidepressant response to the CCK(B) receptor antagonist L-365,260 in the forced-swimming test in mice. The effect of L-365,260 was decreased by the delta-opioid receptor antagonist naltrindole. Furthermore, the CCK(B) receptor agonist, BC 264 (Boc-Tyr(OSO3H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH2), blocked the antidepressant-like effect of RB 101 while CCK-8 (H-Asp-Tyr(OSO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2) enhanced the effect of this drug, probably through stimulation of central CCK(A) receptors, since the CCK(A) receptor antagonist devazepide ((3S)-(-)-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-1H-indole-2 -carboxamide) abolished the CCK-8-induced potentiation of the RB 101 effect. In addition, RB 101 enhanced the effect of L-365,260. Such an effect was blocked by the delta-opioid receptor antagonist naltrindole. These data further support the involvement of opioid receptors in the antidepressant-type effect induced by CCK(B) receptor blockers and support the hypothesis of a regulatory role of CCK in the activity of the endogenous opioid system. As in other experimental paradigms, CCK(A) and CCK(B) receptor stimulation appears to have opposite effects in modulating opioidergic activity.     

Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Selective in vivo binding of [3H]naltriben to delta-opioid receptors in mouse brain

Naltriben (NTB) is a selective antagonist for the putative delta2-opioid receptor. We have determined the regional kinetics and pharmacological profile of [3H]naltriben in vivo in mouse brain. After i.v. administration to CD1 mice, [3H]naltriben uptake and retention were high in striatum, cortical regions and olfactory tubercles, and low in superior colliculi and cerebellum. Robust rank order correlation was found between [3H]naltriben uptake in discrete brain regions and prior delta-opioid receptor binding determinations in vitro and in vivo. [3H]Naltriben binding in vivo was saturable, and was blocked by the delta-opioid receptor antagonist naltrindole, but not by the mu-opioid receptor antagonist cyprodime or the K-opioid receptor agonist (trans)-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]ben zeneacetamide mesylate (U50,488H). (E)-7-Benzylidenenaltrexone (BNTX), a selective antagonist for the putative delta1-opioid receptor, was 9.6- to 12.9-fold less potent than naltriben as an inhibitor of [3H]naltriben binding. Thus, the sites labeled by [3H]naltriben in vivo may correspond to the delta2-opioid receptor subtype. Such assignment is not definitive, particularly considering the 4-fold higher brain uptake of naltriben as compared to (E)-7-benzylidenenaltrexone. Moreover, the regional distribution of [3H]naltriben in brains from CXB-7/BY (CXBK) mice, a strain that shows supraspinal delta1- but not delta2-opioid receptor agonist effects, was quite similar to that found for CD1 mice.     

Mechanisms of central antitussives

This paper provides an overview of our current understanding of the central mechanisms of cough and antitussives. Systemic administration of 8-OH-DPAT at doses of 0.1 and 0.3 mg/kg, i.p. markedly reduced the number of coughs in rats in a dose-dependent manner. The antitussive effect of 8-OH-DPAT, dihydrocodeine and dextromethorphan significantly was reduced by pretreatment with methysergide, but not ketanserin. Therefore, it is possible to speculate that the 5-HT1 receptors, in particular the 5-HT1A receptors, may be more important than others with respect to the effect of antitussive drugs. DAMGO, a selective mu-opioid receptor agonist, and U-50,488H, a highly selective kappa-opioid receptor agonist, have potent antitussive effects when administered either i.c. or i.p. However, we did not observe a cough-depressant effect of DPDPE, a selective delta-opioid receptor agonist. These results indicate that the antitussive effects of opioids are mediated predominantly by mu- and kappa-opioid receptors. On the other hand, naloxonazine, a selective mu 1-opioid receptor antagonist, had no effect on the antitussive effects associated with i.c.v. DAMGO. These results indicate that mu 2-rather than mu 1-opioid receptors are involved in mu-opioid receptor-induced antitussive effects. Antitussive effects of dextromethorphan and noscapine were significantly and dose-dependently reduced by pretreatment with rimcazole, a specific antagonist of sigma sites. However, rimcazole did not have a significant effect on the antitussive effect of morphine. These results suggest that sigma sites may be involved in the antitussive mechanism of non-narcotic antitussive drugs.     

Antitussive effect of dihydroetorphine in mice

The present study examined the opioid receptors involved in the antitussive effect of dihydroetorphine in mice. Dihydroetorphine suppressed coughs dose dependently at doses between 0.1-1 micrograms/kg i.p. Blockade of mu-opioid receptors by pretreatment with beta-funaltrexamine significantly reduced the antitussive effect of dihydroetorphine. Furthermore, the antitussive effect of dihydroetorphine was also antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. However, pretreatment with naltrindole, a delta-opioid receptor antagonist, did not affect the antitussive effect of dihydroetorphine. These results indicate that the antitussive effect of dihydroetorphine is mediated by the activation of mu-opioid receptors and of kappa-opioid receptors, but not delta-opioid receptors.     

Treatment with antisense oligodeoxynucleotide to the opioid delta receptor selectively inhibits delta 2-agonist antinociception

Using approaches emphasizing differential antagonism of receptor selective agonists and cross-tolerance paradigms, evidence in vivo has suggested the existence of subtypes of opioid delta receptors, which have been termed delta 1 and delta 2. Recent work has elucidated the structure of an opioid delta receptor. The present investigation attempted to continue to test the hypothesis of subtypes of delta receptors and to correlate the cloned delta receptor with the existing pharmacological classification. Synthetic oligodeoxynucleotides (oligos) complementary to the 5' end of the cloned delta receptor coding region (antisense) or its corresponding sequence (sense) were given by intracerebroventricular (i.c.v.) administration to mice, twice-daily for 3 days and antinociceptive responses to selective agonists at putative delta 1 and delta 2 receptors were subsequently determined. Treatment with antisense, but not sense, oligo significantly inhibited the response to [D-Ala2,Glu4]deltorphin (delta 2 agonist), but not to [D-Pen2,D-Pen5]enkephalin (DPDPE, delta 1 agonist). Further, subsequent administration of DPDPE elicited a full antinociceptive response in the same antisense oligo treated mice which did not show a significant response to [D-Ala2,Glu4]deltorphin while antisense oligo treated mice which responded to DPDPE did not show antinociception when tested subsequently with [D-Ala2,Glu4]deltorphin. The data suggest that the cloned delta receptor corresponds to that pharmacologically classified as delta 2 and continue to support the concept of subtypes of opioid delta receptors.     

Role of intracellular calcium on the modulation of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes

The role of intracellular calcium in the modifications of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes was examined. Naloxone-precipitated withdrawal jumping was significantly less in morphine-dependent diabetic mice than in morphine-dependent non-diabetic mice. Intracerebroventricular (i.c.v. ) pretreatment with ryanodine attenuated naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice. However, naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice was not affected by i.c.v. pretreatment with ryanodine. Moreover, i.c.v. pretreatment with thapsigargin, a Ca2+-ATPase inhibitor, enhanced naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice. The noradrenaline (NA) turnover in the frontal cortex in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was significantly increased by naloxone injection. Naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was blocked by i.c.v. pretreatment with ryanodine. In contrast to ryanodine, thapsigargin enhanced naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice. These results suggest that increased intracellular calcium augmented naloxone-precipitated withdrawal jumping and the turnover rate of NA in the frontal cortex in morphine-dependent non-diabetic mice. Furthermore, it seems likely that the attenuation of naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice may be due, in part, to the dysfunction of intracellular calcium store.     

Intrathecally administered baclofen for treatment of children with spasticity of cerebral origin

To determine the relative importance of CCK-A, CCK-B, and opioid receptors in mediating the antinociceptive actions of cholecystokinin, we evaluated the actions of selective agonists and antagonists in the mouse hot plate assay. The agonists used were CCK (1-30 nmol i.c.v.), a CCK-A receptor agonist (SNF9019; 0.3-10 nmol i.c.v.), and a CCK-B receptor agonist (SNF9007; 0.3-10 nmol i.c.v.). The antagonists used were the CCK-A receptor antagonist, L364,718 (12.5 nmol i.c.v.), CCK-B receptor antagonist, L365,260 (2.5-25 nmol i.c.v.), and the nonselective opioid receptor antagonist naloxone (1 mg/kg s.c.). CCK and its receptor-selective analogues, SNF9019 and SNF9007, resulted in antinociception that was blocked by naloxone, but was not antagonized by L364,718 or L365,260. In contrast, in positive control experiments, the inhibitory effects of CCK, SNF9019, and SNF9007 on gastrointestinal propulsion in mice were antagonized by identical i.c.v. doses of L364,718 and L365,260. We conclude that centrally administered CCK produces antinociception in the mouse hot plate assay via opioid receptors, but independent of CCK-A or CCK-B receptors. It is necessary to speculate that other CCK receptors, not antagonized by currently available selective antagonists, may exist.