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.     

Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both mu- and epsilon-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [Xu J. Y. et al. (1992) J. Pharmac. exp. Ther. 263, 246-252]. Since the opioid epsilon-receptor-mediated antinociception induced by beta-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of delta-opioid receptors in the spinal cord, the present studies were designed to determine if beta-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5 to 20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 microg), but not with antiserum against [Leu]enkephalin (10 microg) or dynorphin A (1-13) (10 microg). Desensitization of delta-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 microg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 microg) or dynorphin A (1-17) (0.1 microg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the epsilon-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin, which subsequently stimulates delta-opioid receptors in the spinal cord.     

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.     

Logistic regression analysis of twin data: estimation of parameters of the multifactorial liability-threshold model

This study with the rat evaluated the contribution of omega-conotoxin GVIA-(omega-CgTx) and verapamil-sensitive Ca2+ channels in behavioural, antinociceptive and thermoregulatory responses to intracerebroventricular (i.c.v.) injection of [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) and dynorphin A-(1-17), which are selective agonists for putative mu, delta and kappa-opioid receptors, respectively. The rats treated with omega-CgTx (8-32 pmol i.c.v.) showed transient, dose-dependent shaking behaviour, hyperalgesia and hypothermia which gradually disappeared within 4 h. The behaviour of the rats was normal by 24 h. Histological examination of brain sections showed morphological alterations of neurons in the hippocampus, medial-basal hypothalamus and pyriform cortex. antinociception, catalepsy and thermoregulatory responses elicited by DAMGO (0.4 and 2.0 nmol) were significantly prolonged and potentiated by verapamil (20 pmol i.c.v. 15 min before) or omega-CgTx (8 pmol 24 h before). Antinociception and hypothermia induced by DPDPE were antagonized by verapamil and omega-CgTx, whereas only omega-CgTx prevented the behavioural arousal observed after DPDPE. Similarly, hypothermia induced by dynorphin A-(1-17) (5.0 nmol) and by the kappa-opioid receptor agonist U50,488H (215 nmol) was antagonized by the two Ca2+ channel blockers but only omega-CgTx prevented the barrel rolling and bizarre postures caused by the opioid peptide.     

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.     

Gabapentin enhances the antinociceptive effects of spinal morphine in the rat tail-flick test

The antinociceptive effects of the combination of spinal morphine and gabapentin were evaluated in the tail-flick test in rats. The intrathecal coadministration of a subantinociceptive dose of morphine at 0.2 microgram and gabapentin at 300 micrograms produced significant antinociception. Pretreatment with spinal gabapentin at 300 micrograms shifted the dose-response curve of spinal morphine to the left with a decrease in morphine ED50 value from 1.06 micrograms to 0.34 microgram. The antinociceptive effects produced by the combination of a subantinociceptive dose of morphine and gabapentin were reversed by spinal naloxone at 30 micrograms but were not reversed by spinal bicuculline at 0.3 microgram. Furthermore, the concurrent administration of spinal naloxone at 30 micrograms with the combination of morphine and gabapentin blocked antinociception, while the concurrent administration of spinal bicuculline at 0.3 microgram failed to prevent antinociception. These results indicate that the combination of spinal gabapentin and morphine produces an enhancement of antinociception that appears to involve the spinal mu opioid receptors. Furthermore, repeated administration of gabapentin for 3 days did not affect the enhancing effect of gabapentin on the antinociceptive effect of morphine, indicating that tolerance did not develop to gabapentin's ability to enhance morphine antinociception.     

delta Opioid receptor subtypes activate inositol-signaling pathways in the production of antinociception

To analyze the selectivity of delta receptor subtypes to regulate different classes of G proteins, the expression of the alpha-subunits of Gi2, Gi3, Go1, Go2, Gq and G11 transducer proteins was reduced by administration of oligodeoxynucleotides (ODNs) complementary to sequences in their respective mRNAs. Mice receiving antisense ODNs to Gi2 alpha, Gi3 alpha, Go2 alpha and G11 alpha subunits showed an impaired antinociceptive response to all the delta agonists evaluated. An ODN to Go1 alpha specifically blocked the antinociceptive effect of the agonist of delta-1 receptors, [D-Pen2,5]enkephalin (DPDPE), without altering the activity of [D-Ala2]deltorphin II or [D-Ser2]-Leu-enkephalin-Thr (DSLET). In mice treated with an ODN to Gq alpha, the effects of the agonists of delta-2-opioid receptors were reduced, but not those of DPDPE. Thus, Go1 proteins are selectively linked to delta-1-mediated analgesia, and Gq proteins are related to delta-2-evoked antinociception. After impairing the synthesis of Go1 alpha subunits, DPDPE exhibited an antagonistic activity on the antinociception produced by [D-Ala2]deltorphin II. After treatment with ODNs complementary to sequences in Gq alpha or PLC-beta 1 mRNAs, the analgesic capacity of [D-Ala2]deltorphin II was diminished. However, the delta-2-agonist did not alter the antinociceptive activity of DPDPE. An ODN complementary to nucleotides 7 to 26 of the murine delta receptor reduced the analgesic potency of [D-Ala2]deltorphin II, but not that observed for DPDPE. In these mice, [D-Ala2]deltorphin II did not antagonize the effect of DPDPE. These results suggest the existence of different molecular forms of the delta opioid receptor, and the involvement of inositol-signaling pathways in the supraspinal antinociceptive effects of delta agonists.     

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.     

Family history as a risk factor for ulcerative colitis-associated colon cancer in cotton-top tamarin

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.     

Differential antagonism by MK-801 against antinociception induced by opioid receptor agonists administered supraspinally in mice

Various doses of MK-801 ((+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate), a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.001-1 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. MK-801 (0.001-1 microgram i.c.v.) dose dependently attenuated the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered morphine (1 microgram), [D-Pen2, D-Pen5]enkephalin (DPDPE; 10 micrograms), and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeoce tamide ) 60 micrograms). However, the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered beta-endorphin (1 microgram) was not changed by i.c.v. administered MK-801. Our results indicate that, at the supraspinal level, NMDA receptors are involved in the production of antinociception induced by supraspinally administered morphine, DPDPE, and U50,488H but not beta-endorphin.     

Pretreatment with protein kinase C activator phorbol 12,13-dibutyrate attenuates the antinociception induced by mu- but not epsilon-opioid receptor agonist in the mouse

The effects of pretreatment with a protein kinase C activator, phorbol 12,13-dibutyrate, on antinociception induced by i.c.v.-administered mu-opioid receptor agonist (D-Ala2, NMePhe4, Gly(ol)5) enkephalin (DAMGO) or morphine and epsilon-opioid receptor agonist beta-endorphin were studied in male ICR mice. The tail-flick responses were used for antinociceptive tests. I.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 30 or 60 but not 10 min attenuated antinociception induced by i.c.v.-administered DAMGO. I.c.v. pretreatment with phorbol 12,13-dibutyrate (10 and 50 pmol) for 60 min caused a dose-dependent attenuation of DAMGO (19.5 pmol)- or morphine (6.0 nmol)-induced antinociception. The dose-response curve for DAMGO-induced antinociception was shifted to the right by 7.3-fold by i.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 60 min. However, the i.c.v.-administered beta-endorphin-induced antinociception was not affected by the same pretreatment with phorbol 12,13-dibutyrate. The attenuation of i.c.v.-administered DAMGO- and morphine-induced antinociception by phorbol 12,13-dibutyrate was reversed by concomitant i.c.v. pretreatment with a selective protein kinase C inhibitor calphostin C. These results suggest that activation of protein kinase C by phorbol 12,13-dibutyrate leads to the desensitization of mu-, but not epsilon-opioid receptor-mediated antinociception. These findings also provide additional evidence for differential intracellular modulation on antinociceptive action of mu- and epsilon-opioid receptor agonists.     

Experimental reproduction of bovine fetal Neospora infection and death with a bovine Neospora isolate

It is well established that alpha 2-adrenoceptor agonists have sedative and antinociceptive properties. In the current behavioral study we tried to find out if the alpha 2-adrenergic sedative and antinociceptive effects can be dissociated. We tested the hypothesis that alpha 2-adrenergic sedation is mediated by the locus coeruleus (LC) and antinociception by spinal alpha 2-adrenoceptors. Also, we addressed the possibility that intracerebral injection of an alpha 2-agonist might produce its antinociceptive effect by an action directly at the spinal cord. Medetomidine, an alpha 2-adrenergic agonist, or atipamezole, an alpha 2-adrenergic antagonist, were microinjected bilaterally into the LC through chronic cannulae in unanesthetized Han-Wistar rats. The effect on locomotor activity (/vigilance), tail-flick and hot-plate response, and on formalin-induced pain behavior was determined. Medetomidine microinjected into the LC (1-10 micrograms/cannula) produced dose-dependently hypolocomotion (/sedation), increase of response latencies in the hot-plate and the tail-flick tests, and a decrease in the formalin-induced pain behavior. Hypolocomotion (/sedation) was obtained at a lower medetomidine dose (1 microgram/cannula) than antinociception (3-10 micrograms/cannula). The lowest medetomidine dose used (1 microgram/cannula), which induced significant hypolocomotion (/sedation), produced either no antinociception (hot-plate and tail-flick tests) or even a slight hyperalgesia (formalin test). The hypolocomotion (/sedation) but not antinociception (tail-flick test) induced by systemic administration of medetomidine (100 micrograms/kg s.c.) could be reversed by atipamezole (10 micrograms/cannula) microinjected into the LC. Only a high systemic dose of atipamezole (1 mg/kg s.c.) reversed the antinociceptive effects of medetomidine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Facilitation of fear extinction by midbrain periaqueductal gray infusions of rb101(s), an inhibitor of enkephalin-degrading enzymes.

μ-Opioid receptors in the ventrolateral quadrant midbrain periaqueductal gray (vIPAG) contribute to extinction of conditioned fear. The present experiment studied whether fear extinction could be facilitated by infusions of a peptidase inhibitor that reduces catabolism of vIPAG enkephalins. Rats were trained to fear an auditory conditioned stimulus. Fear was then extinguished. Extinction training was preceded by infusions of vehicle or RB101(S), an inhibitor of enkephalin catabolising enzymes. RB101(S) dose dependently facilitated extinction as indexed by performance during extinction and on a drug-free test. This facilitation was not observed when RB101(S) was infused outside the vIPAG. These results confirm that vIPAG endogenous opioids contribute to fear extinction and show that extinction can be facilitated by manipulations that increase vIPAG opioid neuromodulation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Antinociception produced by an ascending spino-supraspinal pathway

Studies in mice and rats have shown that antinociception produced by intrathecal (i.t.) administration of opioids can be partially inhibited by intracerebroventricular (i.c.v.) administration of naloxone. In this study we tested the hypothesis that this inhibition by i.c.v. naloxone results from antagonism of supraspinal endogenous opioid-mediated antinociception produced by the action of i.t. opioids on an ascending antinociceptive pathway. In rats lightly anesthetized with urethane/alpha-chloralose, i.t. DAMGO, i.t. lidocaine, or spinal transection at T5-T6 all attenuated the trigeminal jaw opening reflex (JOR) (i.e., were antinociceptive), an effect that was antagonized in each case by i.c.v. naloxone. These findings support the suggestion that there exists a pathway that ascends from the spinal cord to a supraspinal site that tonically inhibits antinociception mediated by supraspinal opioids. When activity in this ascending pathway is suppressed (e.g., by i.t. opioids or local anesthetics or by spinal cord transection), antinociception mediated by supraspinal opioids is disinhibited. To determine the supraspinal site(s) at which endogenous opioid-dependent antinociception is evoked by i.t. opioids, we microinjected naloxone methiodide into several supraspinal sites. Microinjection of naloxone methiodide into nucleus accumbens but not into the rostral ventral medulla (RVM) or the periaqueductal gray matter (PAG) antagonized the suppression of the JOR produced by i.t. DAMGO or lidocaine. The possibility that this ascending pathway may represent a source of spinal input to mesolimbic circuitry involved in setting the state of sensorimotor reactivity to noxious stimuli is discussed.     

Continuous assay of proteases using a microtiter plate fluorescence reader

The present study was designed to determine if spinal calcium channels, calmodulin, and calcium/calmodulin-dependent protein kinase II were involved in the production of antinociception induced by cold water swimming stress (CWSS). The effects of intrathecal (i.t.) injection of nimodipine, omega-conotoxin GVIA, calmidazolium, or (S)-5-isoquinolinesulfonic acid, 4-[2-[(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperaz inyl)-propyl]phenyl ester (KN-62) on CWSS-induced antinociception were studied in ICR mice. The antinociception was assessed by the tail-flick test. CWSS produced inhibition of the tail-flick response. Various doses of nimodipine (10-40 ng), omega-conotoxin GVIA (5-40 ng), calmidazolium (10-40 ng), or KN-62 (5-40 ng) injected i.t. alone did not show any antinociceptive effect in the tail-flick test. I.t. pretreatment with omega-conotoxin GVIA, calmidazolium, or KN-62 dose dependently attenuated the CWSS-induced inhibition of the tail-flick response. However, i.t. pretreatment with nimodipine did not affect the inhibition of the tail-flick response induced by CWSS. Our results suggest that spinal N-type calcium channel, calmodulin and calcium/calmodulin-dependent protein kinase II may be involved in the production of antinociception induced by CWSS. On the other hand, CWSS-induced antinociception appears not to be mediated via the spinal L-type calcium channel.     

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.     

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.     

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.     

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.     

Involvement of dynorphin B in the antinociceptive effects of the cannabinoid CP55,940 in the spinal cord

Intrathecal administration of delta 9-tetrahydrocannabinol (delta 9-THC) but not the cannabinoid agonist CP55,940 enhances the antinociception produced by morphine. In addition, CP55,940- and delta 9-THC-induced antinociception is blocked by the kappa opioid antagonist norbinaltorphimine, and both cannabinoids are cross-tolerant to kappa agonists but do not act directly at the kappa receptor. Previous work in our laboratory has implicated dynorphins in the antinociceptive effects of delta 9-THC and its enhancement of morphine-induced antinociception. The goal of the present study was to evaluate the role of dynorphins in the antinociceptive effects of CP55,940 at the spinal level. Pretreatment of mice with antisera to dynorphin A(1-17), dynorphin A(1-8) or alpha-neoendorphin, all of which have been shown to retain specificity for blockade of their respective peptide in vivo, blocked the antinociceptive effects of delta 9-THC but not CP55,940. Dynorphin B produced antinociceptive effects on intrathecal administration to mice. Like CP55,940, dynorphin B failed to enhance the antinociceptive effects of morphine, whereas dynorphin A(1-17) and alpha-neoendorphin enhanced the antinociceptive effects of morphine. Using spinal catheterization of the rat, CP55,940 administration was shown to produce a significant release of dynorphin B concurrent with the production of antinociception. Our data suggest that CP55,940 induces a release of spinal dynorphin B that contributes at least in part to its antinociceptive effects in the spinal cord.