Blockade of morphine supersensitivity by an antisense oligodeoxynucleotide targeting the delta opioid receptor (DOR-1)

An antisense oligodeoxynucleotide (ODN) targeting 20 bases of the coding sequence of the cloned delta opioid receptor (DOR-1), a mismatched ODN (different from the antisense ODN at 4 bases) or saline was administered to 3 groups of CD-1 mice implanted with naltrexone pellets (7.5 mg) for 7 days. Morphine supersensitivity (i.e., increased potency as defined by decreased morphine ED₅₀ values) was observed 24 h after pellet removal (day 8) in mice treated with saline or mismatch ODN, but not in antisense ODN treated mice. Antisense ODN alone had no effect on basal nociceptive thresholds or morphine analgesia but reduced the analgesic potency of the delta₂ opioid agonist [D-Ala²]deltorphin II. These data suggest that the delta₂ opioid receptor system participates in the adaptive changes contributing to increased morphine potency following chronic naltrexone treatment.     

家兔伏核—杏仁核神经通路在吗啡镇痛中的作用

用辐射热照射家兔鼻嘴侧部皮肤,测量其躲避反应潜伏期作为痛反应阈,简称痛阈。通过预先埋植的慢性套管向伏核或杏仁核内进行注射,结果表明:(1)在家兔的伏核内微量注射吗啡可产生镇痛作用,该作用可被杏仁核内注射纳洛酮所削弱,并有量效依从关系;在杏仁核内注射甲啡肽抗血清(ME AS)或β-內啡肽抗血清(β-EP AS)亦可削弱上述镇痛作用;(2)在杏仁核内微量注射吗啡可产生镇痛作用,此作用不能被伏核内注射纳洛酮所阻断;(3)在伏核内注射吗啡所产生的镇痛作用可被同一部位注射γ-氨基丁酸(GAEA)受体阻断剂氯甲基荷包牡丹碱所增强,被 GABA 受体激动剂异鹅羔胺所削弱。上述结果提示:在家兔脑内从伏核到杏仁核可能存在一条与镇痛有关的神经通路,伏核内的阿片样物质及杏仁核内的甲啡肽,β-内啡肽可能参与镇痛信息的传递,而伏核内的 GABA 可能有对抗吗啡镇痛的作用。     

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.     

Potential hypoglycemic effects of Chlorella in streptozotocin-induced diabetic mice

Chlorella, a type of unicellular fresh water algae, has been a popular foodstuff in Japan and Taiwan. Chlorella has been shown to produce hypoglycemic effects in alloxan-induced diabetic animals. However, there are no other reports of the effects of this substance in other diabetic animal models. Here we have used streptozocin (STZ)-induced diabetic mice to study the thypoglycemic effects of Chlorella. Diabetes was induced in ICR strain mice by the i.p. injection of STZ. Vehicle-treated ICR mice were used as normal control animals and glibenclamide was used as a positive drug control. The effects of Chlorella on basal blood glucose, exogenous insulin sensitivity test and plasma insulin levels were measured. In normal mice Chlorella produced a transient hypoglycemic effect at 90 min after acute administration; whereas glibenclamide produced a more sustained hypoglycemic effect between 90 min and 180 min after acute administration. Chlorella did not affect the basal blood glucose level in STZ mice. However, Chlorella enhanced and prolonged the hypoglycemic effects of injected insulin in STZ mice for a further 60 min compared to the normal vehicle-treated group. Plasma insulin levels were increased in normal mice after treatment with glibenclamide, whereas Chlorella had no such effect. The current results indicate that Chlorella enhances the hypoglycemic effects of exogenous insulin at a dose which does not produce hypoglycemia in STZ mice, suggesting that insulin sensitivity is increased in these mice.     

Antianalgesic action of dynorphin A mediated by spinal cholecystokinin

Previous work indicates that the antianalgesic action of pentobarbital and neurotensin administered intracerebroventricularly in mice arises from activation of a descending system to release cholecystokinin (CCK) in the spinal cord where CCK is known to antagonize morphine analgesia. Spinal dynorphin, like CCK, has an antianalgesic action against intrathecally administered morphine. This dynorphin action is indirect; even though it is initiated in the spinal cord, it requires the involvement of an ascending pathway to the brain and a descending pathway to the spinal cord where an antianalgesic mediator works. The aim of the present investigation was to determine if the antianalgesic action of intrathecal dynorphin A involved spinal CCK. All drugs were administered intrathecally to mice in the tail flick test. Morphine analgesia was inhibited by dynorphin as shown by a rightward shift of the morphine dose-response curve. The effect of dynorphin was eliminated by administration of the CCK receptor antagonists lorglumide and PD135 158. One hour pretreatment with CCK antiserum also eliminated the action of dynorphin. On the other hand, the antianalgesic action of CCK was not affected by dynorphin antiserum. Thus, CCK did not release dynorphin. Both CCK and dynorphin were antianalgesic against DSLET but not DPDPE, delta 2 and delta 1 opioid receptor peptide agonists, respectively. The results suggest that the antianalgesic action of dynorphin occurred through an indirect mechanism ultimately dependent on the action of spinal CCK.     

Sea-anemone toxin ATX-II elicits A-fiber-dependent pain and enhances resurgent and persistent sodium currents in large sensory neurons

Background

Despite decades of intense research efforts, actions of acute opioids are not fully understood. Increasing evidence suggests that in addition to well-documented antinociceptive effects opioids also produce paradoxical hyperalgesic and excitatory effects on neurons. However, most studies focus on the pronociceptive actions of chronic opioid exposure. Matrix metalloproteinase 9 (MMP-9) plays an important role in neuroinflammation and neuropathic pain development. We examined MMP-9 expression and localization in dorsal root ganglia (DRGs) after acute morphine treatment and, furthermore, the role of MMP-9 in modulating acute morphine-induced analgesia and hyperalgesia in mice.

Results

Subcutaneous morphine induced a marked up-regulation of MMP-9 protein in DRGs but not spinal cords. Morphine also increased MMP-9 activity and mRNA expression in DRGs. MMP-9 up-regulation peaked at 2 h but returned to the baseline after 24 h. In DRG tissue sections, MMP-9 is expressed in small and medium-sized neurons that co-express mu opioid receptors (MOR). In DRG cultures, MOR agonists morphine, DAMGO, and remifentanil each increased MMP-9 expression in neurons, whereas the opioid receptor antagonist naloxone and the MOR-selective antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) suppressed morphine-induced MMP-9 expression. Notably, subcutaneous morphine-induced analgesia was enhanced and prolonged in Mmp9 knockout mice and also potentiated in wild-type mice receiving intrathecal injection of MMP-9 inhibitors. Consistently, intrathecal injection of specific siRNA targeting MMP-9 reduced MMP-9 expression in DRGs and enhanced and prolonged morphine analgesia. Subcutaneous morphine also produced heat hyperalgesia at 24 h, but this opioid-induced hyperalgesia was not enhanced after MMP-9 deletion or inhibition.

Conclusions

Transient MMP-9 up-regulation in DRG neurons can mask opioid analgesia, without modulating opioid-induced hyperalgesia. Distinct molecular mechanisms (MMP-9 dependent and independent) control acute opioid-induced pronociceptive actions (anti-analgesia in the first several hours and hyperalgesia after 24 h). Targeting MMP-9 may improve acute opioid analgesia.     

NMDA receptor antagonism disrupts the development of morphine analgesic tolerance in male, but not female C57BL/6J mice

Multiple studies demonstrate that coadministration of N-methyl-D-aspartate (NMDA) receptor antagonists with the opioid agonist morphine attenuates the development of analgesic tolerance. Sex differences in the effects of noncompetitive, but not competitive NMDA receptor antagonists on acute morphine analgesia, have been reported in mice, yet the role of sex in modulation of morphine tolerance by NMDA receptor antagonists has yet to be addressed. Therefore, we tested whether there is a sex difference in the effect of NMDA receptor antagonists on the development of morphine analgesic tolerance in C57BL/6J mice. Acutely, at a dose required to affect morphine tolerance in male mice, the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) prolonged morphine analgesia similarly in both sexes in the hot plate and tail withdrawal assays. In the hot plate assay, coadministration of MK-801 or the competitive antagonist 3-(2-carboxpiperazin-4-yl)propyl-1-phosphanoic acid (CPP) with morphine attenuated the development of tolerance in male mice, while having no effect in females. Like normal and sham females, ovariectomized mice were similarly insensitive to the attenuation of morphine tolerance by MK-801 in the hot plate assay. Surprisingly, in the tail withdrawal assay, MK-801 facilitated the development of morphine-induced hyperalgesia and tolerance in males but not females. The results demonstrate that male mice are more sensitive to modulation of nociception and morphine analgesia after repeated coadministration of NMDA receptor antagonists. Furthermore, the underlying mechanisms are likely to be different from those mediating the sex difference in the modulation of acute morphine analgesia that has previously been reported.     

Ageing,opioid analgesia and the pineal gland

The effects of ageing on day-night rhythms of analgesia was examined with young (1–2 months), mature (8–12 months) and old (20–30 months) mice. Significant age-related declines were observed both in the absolute levels and diel rhythms of morphine analgesia, with the most pronounced changes occuring at night. Administration of the pineal hormone, melatonin, augmented day-time levels of analgesia in all age classes and reversed the age-related decline in nocturnal morphine analgesia in old mice. Inhibition of pineal function in young mice by either exposure to light pulses or treatment with benserazide mimicked the effects of ageing on nocturnal morphine analgesia. These findings suggest that the pineal gland and melatonin are involved in modulating diel rhythms of analgesia and have an influential role on age-related changes in opioid responses.     

Central administration of neuropeptide FF and related peptides attenuate systemic morphine analgesia in mice

Neuropeptide FF (NPFF) belongs to an opioid-modulating peptide family. NPFF has been reported to play important roles in the control of pain and analgesia through interactions with the opioid system. However, very few studies examined the effect of supraspinal NPFF system on analgesia induced by opiates administered at the peripheral level. In the present study, intracerebroventricular (i.c.v.) injection of NPFF (1, 3 and 10 nmol) dose-dependently inhibited systemic morphine (0.12 mg, i.p.) analgesia in the mouse tail flick test. Similarly, i.c.v. administration of dNPA and NPVF, two agonists highly selective for NPFF(2) and NPFF(1) receptors, respectively, decreased analgesia induced by i.p. morphine in mice. Furthermore, these anti-opioid activities of NPFF and related peptides were blocked by pretreatment with the NPFF receptors selective antagonist RF9 (10 nmol, i.c.v.). These results demonstrate that activation of central NPFF(1) and NPFF(2) receptors has the similar anti-opioid actions on the antinociceptive effect of systemic morphine.     

Opioid control of the ruminant stomach motility: functional importance of mu, kappa and delta receptors

In sheep, the subcutaneous (SC) or intracerebroventricular (ICV) administration of the mu-type opioid agonists, fentanyl and morphine, evokes a blockade of the cyclic contractions of the reticulum. A similar inhibition of forestomach motility was recorded following the administration of the two enkephalin analogs, D-Ala2-Met5-enkephalinamide (DAMA) and D-Ala2-D-Leu5-enkephalin (DADLE) which are mixed mu - delta opioid agonists. In contrast, the reticular contractions were enhanced by the SC or ICV administration of the kappa type agonist, ethylketazocine (EKC) and U - 50 488 H. The proximal duodenum motor activity was transiently increased resulting in the occurrence of a phase III-like activity by these opioid agonists, regardless of the subtypes. The effects of the opioid agonists on reticular motility were prevented by the injection of naloxone but not by the quaternary parent compound methylnaloxone which does not cross the blood-brain barrier. The duodenal motor effects elicited by the opioid agonists were antagonized by both naloxone and methylnaloxone. The results suggest that the inhibition of the ruminant stomach motility is centrally mediated by mu - delta type opioid agonists and are consistent with opposite effects from kappa type opioid agonists. The stimulatory effect of peptide and non-peptide opioid agonists on the duodenum may result in part from direct opioid receptor-mediated actions on smooth muscle.     

The occurrence of cross-tolerance between morphine and ethyl-ketocyclazocine using the tail-flick test: Lack of effect of diazepam,phenobarbital, or amphetamine

Experiments were designed to test for short-term tolerance to morphine and ethyl-ketocyclazocine (EKC), mu and kappa agonists, respectively, and cross-tolerance between the two drugs. Mice were primed with one of the drugs, using doses that did not affect the tail-flick response when tested at a time 1 or 3 hours later, when the same or alternate test drug was administered. All animals were injected with the priming drug IP. In one series of experiments, the test drugs were given SC, and in the other, the test drugs were injected ICV under brief halothane anesthesia. Priming with morphine (30 or 100 mg/kg) significantly raised the ED₅₀ for ICV morphine. Priming with EKC (2 or 6 mg/kg) similarly elevated the ED₅₀'s for SC and ICV EKC. Symmetrical cross-tolerance was produced in experiments where the test drugs were administered SC when tested at 3 hrs. The effects of priming with EKC on morphine analgesia was evident when the interval between priming and test drugs was 1 hour. When the test drugs were given ICV, cross-tolerance was also symmetrical: priming with EKC significantly raised the ED₅₀ for morphine and priming with morphine raised the ED₅₀ for EKC when tested at 3 hrs. These data suggest that both agonists act on a common site to produce analgesia as similar pA₂ values for naloxone antagonism were determined. The occurrence of short-term tolerance and cross-tolerance to the opiates was unaltered by chronic pretreatment with diazepam, phenobarbital, or amphetamine.     

Confirmation of Superoxide Generation via Xanthine Oxidase in Streptozotocin-induced Diabetic Mice

Reactive oxygen species (ROS) may play key roles in vascular inflammation and atherogenesis in patients with diabetes. In this study, xanthine oxidase (XO) system was examined as a potential source of superoxide in mice with streptozotocin (STZ)-induced experimental diabetes. Plasma XO activity increased 3-fold in diabetic mice (50±33?μU/ml) 2 weeks after the onset of diabetes, as compared with non-diabetic control mice (15±6?μU/ml). In vivo superoxide generation in diabetic mice was evaluated by an in vivo electron spin resonance (ESR)/spin probe method. Superoxide generation was significantly enhanced in diabetic mice, and the enhancement was restored by the administration of superoxide dismutase (SOD) and 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), which was reported to scavenge superoxide. Pretreatment of diabetic mice with XO inhibitors, allopurinol and its active metabolite oxipurinol, normalized the increased superoxide generation. In addition, there was a correlation (r=0.78) between the level of plasma XO activity and the relative degree of superoxide generation in diabetic and non-diabetic mice. Hence, the results of this study strongly suggest that superoxide should be generated through the increased XO seen in the diabetic model mice, which may be involved in the pathogenesis of diabetic vascular complications.     

Spinal and Peripheral Mechanisms Involved in the Enhancement of Morphine Analgesia in Acutely Inflamed Mice

The analgesic effect induced by opiates is often potentiated during experimental inflammatory processes. We describe here that lower doses of systemic morphine are necessary to increase thermal withdrawal latencies measured in both hind paws of mice acutely inflamed with carrageenan than in healthy ones. This bilateral potentiation seems mediated through spinal opioid receptors since it is inhibited by the intrathecal (i.t.), but not intraplantar (i.pl.) administration of the opioid receptor antagonist naloxone-methiodide, and also appears when morphine is i.t. administered. Furthermore, the i.pl. administration of the nitric oxide (NO) synthase inhibitor, l-NMMA, or the K_ATP⁺-channel blocker, glibenclamide, to carrageenan-inflamed mice inhibits the enhanced effect of systemic morphine in the paw that receives the injection of the drug, without affecting the potentiation observed in the contralateral one. The i.pl. administration of l-NMMA also partially antagonised the analgesic effect induced by i.t. morphine in inflamed mice. Finally, the increased analgesic effect evoked by the i.pl. administration of the NO donor SIN-1 either in the inflamed or in the contralateral paw of carrageenan-inflamed mice suggests that enhanced responsiveness to the peripheral analgesic effect of NO may be also underlying the bilateral potentiation of morphine-induced analgesia in acutely inflamed mice.     

Lack of antinociceptive cross-tolerance between intracerebroventricularly administered beta-endorphin and morphine or DPDPE in mice

Antinociceptive tolerance and cross-tolerance to intracerebroventricular (i.c.v.) beta-endorphin, morphine, and DPDPE (D-Pen2-D-Pen5-enkephalin) induced by a prior i.c.v. administration of beta-endorphin, morphine and DPDPE, respectively, were studied in mice. Acute tolerance was induced by i.c.v. pretreatment with beta-endorphin (0.58 nmol), morphine (6 nmol) and DPDPE (31 nmol) for 120, 180 and 75 min, respectively. Various doses of beta-endorphin, morphine or DPDPE were then injected. The tail-flick and hot-plate tests were used as antinociceptive tests. Pretreatment of mice with beta-endorphin i.c.v. reduced inhibition of the tail-flick and hot-plate responses to i.c.v. administered beta-endorphin, but not morphine and DPDPE. Pretreatment of mice with morphine i.c.v. reduced inhibition of the tail-flick and hot-plate responses to morphine but not beta-endorphin. Pretreatment of mice with DPDPE reduced inhibition of the tail-flick and hot-plate responses to DPDPE but not beta-endorphin. The results indicate that one injection of beta-endorphin, morphine or DPDPE induces acute antinociceptive tolerance to its own distinctive opioid receptor and does not induce cross-tolerance to other opioid agonists with different opioid receptor specificities. The data support the hypothesis that beta-endorphin, morphine and DPDPE produce antinociception by stimulating specific epsilon, mu- and delta-opioid receptors, respectively.     

Confirmation of superoxide generation via xanthine oxidase in streptozotocin-induced diabetic mice

Reactive oxygen species (ROS) may play key roles in vascular inflammation and atherogenesis in patients with diabetes. In this study, xanthine oxidase (XO) system was examined as a potential source of superoxide in mice with streptozotocin (STZ)-induced experimental diabetes. Plasma XO activity increased 3-fold in diabetic mice (50±33 μU/ml) 2 weeks after the onset of diabetes, as compared with non-diabetic control mice (15±6 μU/ml). In vivo superoxide generation in diabetic mice was evaluated by an in vivo electron spin resonance (ESR)/spin probe method. Superoxide generation was significantly enhanced in diabetic mice, and the enhancement was restored by the administration of superoxide dismutase (SOD) and 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), which was reported to scavenge superoxide. Pretreatment of diabetic mice with XO inhibitors, allopurinol and its active metabolite oxipurinol, normalized the increased superoxide generation. In addition, there was a correlation (r=0.78) between the level of plasma XO activity and the relative degree of superoxide generation in diabetic and non-diabetic mice. Hence, the results of this study strongly suggest that superoxide should be generated through the increased XO seen in the diabetic model mice, which may be involved in the pathogenesis of diabetic vascular complications.     

Sex chromosome complement affects nociception in tests of acute and chronic exposure to morphine in mice

We tested the role of sex chromosome complement and gonadal hormones in sex differences in several different paradigms measuring nociception and opioid analgesia using "four core genotypes" C57BL/6J mice. The genotypes include XX and XY gonadal males, and XX and XY gonadal females. Adult mice were gonadectomized and tested 3-4 weeks later, so that differences between sexes (mice with testes vs. ovaries) were attributable mainly to organizational effects of gonadal hormones, whereas differences between XX and XY mice were attributable to their complement of sex chromosomes. In Experiment 1 (hotplate test of acute morphine analgesia), XX mice of both gonadal sexes had significantly shorter hotplate baseline latencies prior to morphine than XY mice. In Experiment 2 (test of development of tolerance to morphine), mice were injected twice daily with 10 mg/kg morphine or saline for 6 days. Saline or the competitive NMDA antagonist CPP (3-(2-carboxypiperazin-4yl) propyl-1-phosphonic acid) (10 mg/kg) was co-injected. On day 7, mice were tested for hotplate latencies before and after administration of a challenge dose of morphine (10 mg/kg). XX mice showed shorter hotplate latencies than XY mice at baseline, and the XX-XY difference was greater following morphine. In Experiment 3, mice were injected with morphine (10 mg/kg) or saline, 15 min before intraplantar injection of formalin (5%/25 microl). XX mice licked their hindpaw more than XY mice within 5 min of formalin injection. The results indicate that X- or Y-linked genes have direct effects, not mediated by gonadal secretions, on sex differences in two different types of acute nociception.     

Effects of sunitinib on immunoreactivity of vimentin,E-cadherin and S100 in kidneys of streptozotocin induced diabetic mice

Diabetes mellitus (DM) affects many organs including kidney. Tyrosine kinase can cause hypoglycemia and sunitinib is an inhibitor of tyrosine kinase. We investigated the possible effects of sunitinib on the kidney of streptozotocin (STZ) induced type 1 diabetic mice. We used 28 CD 1 type male mice divided into four groups of seven. Type 1 diabetes was induced by injection of STZ. Group 1 was the untreated control. Group 2 comprised non-diabetic mice + sunitinib. Both groups 1 and 2 exhibited normal blood glucose levels. Group 3 comprised STZ treated diabetic mice + saline. Group 4 were diabetic mice + sunitinib treatment. Kidneys were removed after 8 weeks. The immunoreactivities of vimentin, E-cadherin and S100 were assessed. Immunostaining of vimentin, E-cadherin and S100 was located in both the glomeruli and tubules of the kidney. We found that the number of vimentin and E-cadherin positive glomeruli and tubules were increased after sunitinib treatment compared to saline treated diabetic mice. The number of vimentin labeled tubules was decreased in the sunitinib treated group compared to diabetic + saline groups. Differences in the number of S100 positive tubules and glomeruli between groups 3 and 4 were not statistically significant. The effect of sunitinib on experimental diabetic mice appears to be related to levels of vimentin, E-cadherin and S100 in the glomeruli and tubules of the kidney, and sunitinib may protect against renal damage from DM.     

Footshock induced analgesia in mice: its reversal by naloxone and cross tolerance with morphine

Using the abdominal constriction response as the criterion for analgesia, mice tested immediately after a period of footshock showed a significant analgesic response compared with non-footshocked controls. Footshock induced analgesia could not be elicited in mice that had been made tolerant to morphine or in mice that had been pretreated with the narcotic antagonist naloxone. It is concluded that footshock induced analgesia in the mouse is due to the release of endogenous opioid peptides.     

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.