Δ-opioid Receptor Stimulation Research Articles

Shifting motivational states: The effects of nucleus accumbens dopamine and opioid receptor activation on a modified effort-based choice task

The nucleus accumbens (NAc) is critical for regulating the appetitive and consummatory phases of motivated behavior. These experiments examined the effects of dopamine and opioid receptor manipulations within the NAc during an effort-based choice task that allowed for simultaneous assessment of both phases of motivation. Male Sprague-Dawley rats received bilateral guide cannulas targeting the NAc core and were tested in 1-hr sessions with free access to rat chow and the choice to work for sugar pellets on a progressive ratio 2 (PR2) reinforcement schedule. Individual groups of rats were tested following stimulation or blockade of NAc D1-like or D2-like receptors, stimulation of μ-, δ-, or κ-opioid receptors, or antagonism of opioid receptors. Behavior was examined under ad libitum conditions and following 23-h food restriction. NAc blockade of the D1-like receptors or stimulation of the D2 receptor reduced break point for earning sugar pellets; D2 receptor stimulation also modestly lowered chow intake. NAc μ-opioid receptor stimulation increased intake of the freely-available chow while simultaneously reducing break point for the sugar pellets. In non-restricted conditions, δ-opioid receptor stimulation increased both food intake and breakpoint. There were no effects of stimulating NAc D1 or κ receptors, nor did blocking D2 or opioid receptors affect task behavior. These data support prior literature linking dopamine to appetitive motivational processes, and suggest that μ- and δ-opioid receptors affect food-directed motivation differentially. Specifically, μ-opioid receptors shifted behavior towards consumption, and δ-opioid receptor enhanced both sugar-seeking and consumption of the pabulum chow when animals were not food restricted.

Activation of HIF-1α by δ-Opioid Receptors Induces COX-2 Expression in Breast Cancer Cells and Leads to Paracrine Activation of Vascular Endothelial Cells.

Opioids promote tumor angiogenesis in mammary malignancies, but the underlying signaling mechanism is largely unknown. The current study investigated the hypothesis that stimulation of δ-opioid receptors (DOR) in breast cancer (BCa) cells activates the hypoxia-inducible factor 1α (HIF-1α), which triggers synthesis and release of diverse angiogenic factors. Immunoblotting revealed that incubation of human MCF-7 and T47D breast cancer cells with the DOR agonist d-Ala2,d-Leu5-enkephalin (DADLE) resulted in a transient accumulation and thus activation of HIF-1α DADLE-induced HIF-1α activation preceded PI3K/Akt stimulation and was blocked by the DOR antagonist naltrindole and naloxone, pertussis toxin, different phosphoinositide 3-kinase (PI3K) inhibitors, and the Akt inhibitor Akti-1/2. Whereas DADLE exposure had no effect on the expression and secretion of vascular endothelial growth factor (VEGF) in BCa cells, an increased abundance of cyclooxygenase-2 (COX-2) and release of prostaglandin E2 (PGE2) was detected. DADLE-induced COX-2 expression was also observed in three-dimensional cultured MCF-7 cells and impaired by PI3K/Akt inhibitors and the HIF-1α inhibitor echinomycin. Supernatant from DADLE-treated MCF-7 cells triggered sprouting of endothelial (END) cells, which was blocked when MCF-7 cells were pretreated with echinomycin or the COX-2 inhibitor celecoxib. Also no sprouting was observed when END cells were exposed to the PGE2 receptor antagonist PF-04418948. The findings together indicate that DOR stimulation in BCa cells leads to PI3K/Akt-dependent HIF-1α activation and COX-2 expression, which trigger END cell sprouting by paracrine activation of PGE2 receptors. These findings provide a potential mechanism of opioid-driven tumor angiogenesis and thus therapeutic targets to combat the tumor-angiogenic opioid effect. SIGNIFICANCE STATEMENT: Opioids are indispensable analgesics for treating cancer-related pain. However, opioids were found to promote tumor growth and metastasis, which questions the use of these potent pain-relieving drugs in cancer patients. Enhanced tumor vascularization after opioid treatment implies that tumor progression results from angiogenic opioid effects. Thus, understanding the signaling mechanism of opioid-driven tumor angiogenesis helps to identify therapeutic targets to combat these undesired tumor effects. The present study reveals that stimulation of δ-opioid receptors in breast cancer cells leads to an activation of HIF-1α and expression of COX-2 via PI3K/Akt stimulation, which results in a paracrine activation of vascular endothelial cells by prostaglandin E2 receptors.

The influence of β-endorphin on the functional activity of the isolated heart of rat. The role of δ-opioid receptors

The purpose of research is to learn the influence of β-endorphin on the functional activity indicators of the isolated heart of rat and to evaluate the role of opioid receptors. Materials and methods. The research was performed on isolated hearts of males nonlinear white rats. Perfusion of the heart was performed on the Langendorff System at constant flow. Register indicators are volume rate of coronary perfusion, perfusion pressure, the pressure in the left ventricle, pulse pressure, heart rate, speed of changing the pressure developed by the left ventricle. Additionally was calculated integral indicator minute performance of the left ventricle corresponding to the area under the curve of change of pressure in the left ventricle per a minute, index of contractile function and the integral indicator of the average performance of the left ventricle per one contraction. Results and discussion. It was identified depressant effect of β-endorphin in slightly declining trend to decrease of myocardial contractility indicators in a concentration range from 6.3x10 -10 to 6.3x10 -18 M. In the presence of a blocker of δ-opioid receptor naltrindole inhibitory effect of β-endorphin identified in isolated use, practically was not manifested. Thus, the peripheral δ-opioid receptors of cardiomyocytes mediate the decrease of myocardial contractility caused by β-endorphin using. Conclusions. β-endorphin in physiological concentrations has a depressing effect on functional parameters of the isolated heart of rat. Stimulation of δ-opioid receptors is one of the mechanisms leading to formation of the negative inotropic effect of β-endorphin as a component of local cardiac activity regulation.

Peripheral δ-opioid receptors attenuate the exercise pressor reflex

In rats with ligated femoral arteries, the exercise pressor reflex is exaggerated, an effect that is attenuated by stimulation of peripheral μ-opioid receptors on group IV metabosensitive afferents. In contrast, δ-opioid receptors are expressed mostly on group III mechanosensitive afferents, a finding that prompted us to determine whether stimulation of these opioid receptors could also attenuate the exaggerated exercise pressor reflex in "ligated" rats. We found femoral arterial injection of [D-Pen2,D-Pen5]enkephalin (DPDPE; 1.0 μg), a δ-opioid agonist, significantly attenuated the pressor and cardioaccelerator components of the exercise pressor reflex evoked by hindlimb muscle contraction in both rats with ligated and patent femoral arteries. DPDPE significantly decreased the pressor responses to muscle mechanoreflex activation, evoked by tendon stretch, in ligated rats only. DPDPE (1.0 μg) had no effect in either group on the pressor and cardioaccelerator responses to capsaicin (0.2 μg), which primarily stimulates group IV afferents. DPDPE (1.0 μg) had no effect on the pressor and cardioaccelerator responses to lactic acid (24 mM), which stimulates group III and IV afferents, in rats with patent femoral arteries but significantly decreased the pressor response in ligated rats. Western blots revealed the amount of protein comprising the δ-opioid receptor was greater in dorsal root ganglia innervating hindlimbs with ligated femoral arteries than in dorsal root ganglia innervating hindlimbs with patent femoral arteries. Our findings support the hypothesis that stimulation of δ-opioid receptors on group III afferents attenuated the exercise pressor reflex.

Pro‐apoptotic properties of morphine in neuroblastoma × glioma NG108‐15 hybrid cells: modulation by yohimbine

Short-term incubation with pharmacologically relevant concentrations of morphine has been shown to transiently affect the metabolism and redox status of NG108-15 cells through δ-opioid receptor stimulation, but apparently did not provoke cell death. The present work tries to determine if incubation with morphine at longer time intervals (24 h) provokes apoptosis and/or necrosis, as it has been described in other cell lines. We have also checked the potential modulatory role of yohimbine on these effects, on the basis of the previously described interactions between this drug and opioid receptor ligands. Incubation with morphine 0.1 and 10 μM provoked the appearance of images compatible with apoptosis (bebbling, pyknotic cells with cytoplasmic and nuclear condensation) and necrosis (cells swollen with vacuolated cytoplasm lacking cell processes) that could be observed directly and/or after staining with methylene blue, crystal violet and propidium iodide/4',6-diamidino-2-phenylindole (IP/DAPI). Quantification of apoptosis by activation of caspases 3 and 7 and DNA fragmentation with the Tunel assay revealed a modest but significant increase after incubation with the two concentrations of morphine used. Co-incubation with 10 μM yohimbine prevented all these effects of the opioid. The results extend previous findings of a yohimbine-sensitive, neurotoxic effect of morphine on NG108-15 cells.

δ‐Opioid receptors stimulate GLUT1‐mediated glucose uptake through Src‐ and IGF‐1 receptor‐dependent activation of PI3‐kinase signalling in CHO cells

Although opioids have been reported to affect glucose homeostasis, relatively little is known on the role of δ-opioid receptors. We have investigated the regulation of glucose transport by human δ-opioid receptors expressed in Chinese hamster ovary cells. The uptake of [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose in response to δ-opioid receptor ligands and the expression of GLUT1, GLUT3 and GLUT4 glucose transporters were examined. Moreover, the effects of intracellular signal transduction inhibitors on δ-opioid receptor-regulated [(3)H]-2-deoxy-D-glucose uptake and protein phosphorylation were investigated. Activation of δ-opioid receptors rapidly stimulated [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose uptakes, which were blocked by the GLUT inhibitors cytochalasin B and phloretin. The stimulation of [(3)H]-2-deoxy-D-glucose uptake that occurred without a change in plasma membrane GLUT1 - required the coupling to G(i) /G(o) proteins - was independent of cAMP and extracellular signal-regulated protein kinases, and was suppressed by blockade of Src and insulin-like growth factor-1 receptor (IGF-1R) tyrosine kinases. Inhibition of phosphatidylinositol 3-kinase (PI3K) by wortmannin or LY294002 and by PI3Kα, but not γ, isoform-selective inhibitors greatly reduced the δ-opioid receptor stimulation of glucose uptake. Moreover, the response was attenuated by overexpressing a dominant-negative kinase-deficient Akt form and by chemical inhibition of Akt. Stimulation of δ-opioid receptors increased protein kinase Cζ/λ (PKCζ/λ) phosphorylation and a selective PKCζ/λ inhibitor slightly reduced opioid stimulation of glucose uptake. δ-Opioid receptors stimulated glucose transport probably by enhancing GLUT1 intrinsic activity through a signalling cascade involving G(i)/G(o), Src, IGF-1R, PI3Kα, Akt and, to a minor extent, PKCζ/λ. This effect may contribute to the opioid regulation of glucose homeostasis in physio-pathological conditions.

Dark chocolate receptors: epicatechin-induced cardiac protection is dependent on δ-opioid receptor stimulation

Epicatechin, a flavonoid, is a well-known antioxidant linked to a variety of protective effects in both humans and animals. In particular, its role in protection against cardiovascular disease has been demonstrated by epidemiologic studies. Low-dose epicatechin, which does not have significant antioxidant activity, is also protective; however, the mechanism by which low-dose epicatechin induces this effect is unknown. Our laboratory tested the hypothesis that low-dose epicatechin mediates cardiac protection via opioid receptor activation. C57BL/6 mice were randomly assigned to 1 of 10 groups: control, epicatechin, naloxone (nonselective opioid receptor antagonist), epicatechin + naloxone, naltrindole (δ-specific opioid receptor antagonist), epicatechin + naltrindole, norbinaltorphimine (nor-BNI, κ-specific opioid receptor antagonist), epicatechin + nor-BNI, 5-hydroxydecanoic acid [5-HD, ATP-sensitive potassium channel antagonist], and epicatechin + 5-HD. Epicatechin (1 mg/kg) or other inhibitors (5 mg/kg) were administered by oral gavage or intraperitoneal injection, respectively, daily for 10 days. Mice were subjected to 30 min coronary artery occlusion followed by 2 h of reperfusion, and infarct size was determined via planimetry. Whole heart homogenates were assayed for downstream opioid receptor signaling targets. Infarct size was significantly reduced in epicatechin- and epicatechin + nor-BNI-treated mice compared with control mice. This protection was blocked by naloxone, naltrindole, and 5-HD. Epicatechin and epicatechin + nor-BNI increased the phosphorylation of Src, Akt, and IκBα, while simultaneously decreasing the expression of c-Jun NH(2)-terminal kinase and caspase-activated DNase. All signaling effects are consistent with opioid receptor stimulation and subsequent cardiac protection. Naloxone, naltrindole, and 5-HD attenuated these effects. In conclusion, epicatechin acts via opioid receptors and more specifically through the δ-opioid receptor to produce cardiac protection from ischemia-reperfusion injury.

Influence of glial cells in the dopamine releasing effect resulting from the stimulation of striatal δ-opioid receptors

Recent data indicate that striatal dopamine release induced by stimulation of δ-opioid receptors is a consequence of glutamate release. However, glial cells, which mainly support glutamate uptake and are involved in glutamate signaling and potentially express δ-opioid receptors, could participate to this effect. The present study investigates the contribution of glial cells in the releasing effects of [ d-Pen2, d-Pen5]-enkephalin (DPDPE) by using the gliotoxin l-α-aminoadipate ( l-αAA). Initially, we evaluated the early influence of l-αAA local infusion (10 μg/μL) on dialysate levels of glutamate and dopamine under basal or DPDPE treatment conditions. l-αAA produced a significant increase of glutamate and dopamine in dialysates (+76% and +50% respectively) and the concomitant infusion of DPDPE (10 μM) significantly enhanced this effect in an additive manner (+110% and +44% respectively). Secondly, we assessed the DPDPE effects on striatal glutamate and dopamine dialysate levels, 2 days after an intra-striatal injection of l-αAA which produced destruction of glial cells. This lesion, decreasing the basal glutamate dialysate level as well as its tissue content (by 55% and 36% respectively), prevented the increase in glutamate and dopamine extracellular levels induced by DPDPE. This result confirmed that the DPDPE-induced dopamine release requires an initial glutamate release. However, this effect could reflect a major disruption of glutamatergic transmission caused by the toxin, as suggested by the local infusion of glutamine (2.5 mM) which, in lesioned rats, prevented the decrease in the basal extracellular content of glutamate and restored the DPDPE-induced increase in glutamate and dopamine dialysate levels. Therefore, these results indicate that, although glial cells are essential to maintain functional glutamatergic neurotransmission, they are not directly involved in the process by which stimulation of striatal δ-opioid receptors induces extracellular glutamate release and, consecutively, dopamine release.

Survivin mediates the anti-apoptotic effect of δ-opioid receptor stimulation in cardiomyocytes

Survivin is known to be essential for cell division and to inhibit apoptosis during embryonic development and in adult cancerous tissues. However, the cardiovascular role of survivin is unknown. We observed that in cardiomyocytes cultured under conditions of serum and glucose deprivation (DEPV), the levels of survivin, Bcl-2 and extracellular signal-regulated kinase (ERK) were positively correlated with the anti-apoptotic action of a delta-opioid receptor agonist, [D-Ala2, D-Leu5]-enkephalin acetate (DADLE). By contrast, Bax translocation, mitochondrial membrane damage and reactive oxygen species (ROS) production were inversely correlated with the changes of survivin and Bcl-2. The use of RNA interference (RNAi) targeting survivin increased DEPV-induced cardiomyocyte apoptosis, whereas the anti-apoptotic effect of DADLE was blunted by survivin RNAi. Moreover, survivin transfection and overexpression provided protection against DEPV-induced cardiomyocyte apoptosis. Inhibition of ERK prevented the DADLE-induced decrease in apoptosis and Bax translocation, and increase in survivin and Bcl-2. DADLE-induced increase in survivin was also blunted by phosphoinositol 3-kinase (PI 3-kinase) inhibition. In conclusion, the present study provides the first direct evidence of an anti-apoptotic role of survivin mediating the anti-apoptotic effect of delta-opioid receptor activation in cardiomyocytes. ERK and PI 3-kinase were found to be upstream regulators of survivin. Mitochondrial membranes as well as ROS, Bcl-2 and Bax were also involved in this anti-apoptotic action.

Role of the phosphatidylinositol-specific phospholipase C pathway in δ-opioid receptor-mediated antinociception in the mouse spinal cord

Stimulation of δ-opioid receptors has been shown to activate phospholipase C via the activation of G-proteins in vitro. The present study was designed to determine, with the tail-flick method, whether the stimulatory effect of δ-opioid receptor agonists on phospholipase C and inositol lipid turnover participates in the mechanisms of the δ-opioid receptor-mediated antinociception in the mouse spinal cord. Intrathecal pretreatment with the phospholipase C inhibitors neomycin and U73122, which produced no changes in the basal tail-flick latencies when they were injected alone, significantly attenuated the antinociception induced by intrathecal administration of the selective δ-opioid receptor agonist [ d-Ala 2]deltorphin II in mice. The selective phosphatidylinositol-specific phospholipase C inhibitor ET-18-OCH 3 inhibited the antinociception induced by intrathecal administration of [ d-Ala 2]deltorphin II in a dose-dependent manner. In mice undergoing treatment with LiCl, which impairs phosphatidylinositol synthesis, the antinociception induced by intrathecal administration of [ d-Ala 2]deltorphin II was significantly reduced. Co-administration of d-myo-inositol-1,4,5-trisphosphate restored the [ d-Ala 2]deltorphin II-induced antinociception in LiCl-pretreated mice. On the other hand, intrathecal pretreatment with the selective protein kinase C inhibitor calphostin C, but not the protein kinase A inhibitor KT5720, resulted in a dose-dependent enhancement of the [ d-Ala 2]deltorphin II-induced antinociception. These results indicate a potential role for the phospholipase C–inositol-1,4,5-trisphosphate pathway in the expression of δ-opioid receptor-mediated antinociception in the mouse spinal cord. Furthermore, activation of protein kinase C by the stimulation of δ-opioid receptors may constitute a significant pathway involved in negative modulation of spinal δ-opioid receptor-mediated antinociception.

Regulation of the glial glutamate transporter GLT-1 by glutamate and δ-opioid receptor stimulation

The excitatory effect of presynaptically released glutamate is tightly regulated and terminated by high affinity sodium-dependent glutamate transporters. The regulation of the glial glutamate transporter GLT-1 is potentially important in synaptic modulation. Using astroglial cultures prepared from the rat cerebral cortex, we found that the δ-opioid receptor agonist [ d-pen 2, d-pen 5]-enkephalin decreases and glutamate increases the expression of the GLT-1 transporter mRNA. Corresponding changes in the uptake kinetics were found after incubation for 48 h with the respective agonists when glial glutamate uptake was measured in primary astroglial cultures. The data suggest that long-term receptor activation induces alterations in glial glutamate uptake properties.

The new heparins

The motivational properties of the non-peptide δ-opioid receptor agonists BW373U86 and SNC 80 were investigated using the place-conditioning paradigm. BW373U86 (0.5–1.0 mg/kg s.c.) and SNC 80 (1.25–5.0 mg/kg s.c.) elicited significant preference for the drug-paired compartment, in a dose-related fashion. Naltrindole (5.0 mg/kg s.c.) pretreatment, while failing to modify preference when given alone, completely prevented place-preference induced by BW373U86 (1.0 mg/kg s.c.) and SNC 80 (1.25 mg/kg s.c.). The dopamine D1 receptor antagonist SCH23390, given at doses that do not affect place-preference (0.012 mg/kg s.c.), completely prevented the place-preference induced by BW373U86 and SNC 80. At the doses effective in eliciting place-preference, BW373U86 and SNC 80 failed to modify extracellular dopamine in the medial nucleus accumbens, while in the dorso-lateral caudate-putamen BW373U86 (1.0 and 2.5 mg/kg s.c.) reduced extracellular dopamine, and this effect was prevented by naltrindole (5.0 mg/kg s.c.). SNC 80, only at the dose of 5 mg/kg s.c., significantly reduced extracellular DA in the dorso-lateral caudate-putamen. The results indicate that stimulation of δ-opioid receptors has incentive properties that might be related to an indirect amplification of post-synaptic dopamine transmission.

Overview

The unconditioned behavioural effects of two non-peptide δ-opioid receptor agonists, BW 373U86 and SNC 80, were studied in the intact rat. BW 373U86 (0.1–2.5 mg/kg s.c.) and SNC 80 (2.5–10 mg/kg s.c.) dose-dependently elicited locomotion, rearing, stereotyped sniffing, licking and gnawing. These effects were abolished by pretreatment with the δ-opioid receptor antagonist naltrindole (5.0 mg/kg s.c.). In view of the phenomenological similarities between this syndrome and that elicited by dopamine-receptor agonists, the role played by dopamine receptors was investigated. The specific dopamine D1 receptor antagonist SCH 23390 and the specific dopamine D2/D3 receptor antagonist raclopride reduced or even abolished the behavioural stimulation induced by lower doses of BW 373U86 and SNC 80. When higher doses of BW 373U86 were used (2.5 mg/kg), however, raclopride, even at high cataleptic doses (6.0 mg/kg), only partly prevented the behavioural stimulation induced by the δ-opioid receptor agonist. The behavioural stimulation remaining after high doses of raclopride was abolished by the administration of SCH 23390. These results show that δ-opioid receptor stimulation elicits dopamine-dependent behavioural activation in the rat that depends on dopamine receptors, particularly of the D1 subtype.

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 μ- and ϵ-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [29]Since the opioid ϵ-receptor-mediated antinociception induced by β-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of δ-opioid receptors in the spinal cord, the present studies were designed to determine if β-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 5–20 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 μg), but not with antiserum against [Leu]enkephalin (10 μg) or dynorphin A (1–13) (10 μg). Desensitization of δ-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 μg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 μg) or dynorphin A (1–17) (0.1 μg) 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 ϵ-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin which subsequently stimulates δ-opioid receptors in the spinal cord.

Antisense oligodeoxynucleotide to a δ-opioid receptor blocks the antinociception induced by cold water swimming

The type of opioid receptors in the spinal cord involved in antinociception induced by cold water swimming (CWS) was studied in male ICR mice. Mice were submitted to CWS for 3 min and antinociception was measured 7 min after the CWS by the tail-flick test. Intrathecal (i.t.) injection of naltrindole (NTI, 5 μg) which blocked i.t. DPDPE ([ d-Pen 2, d-Pen 5]enkephalin, 5 μg)-induced antinociception, blocked the CWS-induced antinociception. On the other hand, i.t. injection of CTOP ( d-Phe-Cys-Try- d-Try-Orn-Thr-Pen-Thr-NH 2, 50 ng) and norbinaltorphimine (nor-BNI, 5 μg) which blocked i.t. DMAGO ([ d-Ala 2,NHPhe 4,Gly-ol]enkephalin, 10 ng)- and U50,488H ( trans(±)-3,4-dichloro- N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide, 75 μg)-induced antinociception, respectively, did not block CWS-induced antinociception. Intrathecal pretreatment with antisense oligodeoxynucleotide to δ-opioid receptor mRNA (δ-AS oligo, 163 pmol) once a day for 1 to 3 days caused a time-dependent attenuation of CWS-induced antinociception. δ-AS at doses from 1.6 to 163 pmol pretreated i.t. for 3 days caused a dose-dependent blockade of CWS-induced antinociception. However, i.t. pretreatment with mismatch oligodeoxynucleotide (MM oligo, 163 pmol) for 3 days did not affect the CWS-induced antinociception. The results indicate that CWS-induced antinociception is mediated by the stimulation of δ-opioid receptors in the spinal cord.

CCK A, but not CCK B, agonists suppress the hyperlocomotion induced by endogenous enkephalins, protected from enzymatic degradation by systemic RB 101

Interactions between CCKergic and enkephalinergic systems were studied in mice using behavioral responses measured in Animex. The hyperlocomotion induced by 5 mg/kg of RB 101, a mixed inhibitor of enkephalin-degrading enzymes able to cross the blood-brain barrier, was previously shown to be mediated by δ-opioid receptor stimulation. The IP administration of a CCK A agonist, Boc-Tyr-Lys-(CONH- o-tolyl)-Asp-Phe- NH 2 (0.1, 1, 10 μg/ kg), suppressed the hyperlocomotion produced by IV injection of 5 mg/kg of RB 101. The effect of the CCK A agonist was suppressed by a selective CCK A antagonist, devazepide, injected IP at doses of 20 and 200 μg/kg and was potentiated by the selective δ-opioid antagonist naltrindole at the doses of 0.03 mg/kg. IP injection of the selective CCK B agonist BC 264 (0.1–1 mg/kg) did not modify the RB 101-induced hyperlocomotor effect. These results reinforce the observed physiological antagonism between the endogenous CCK and opioid systems but are at variance with the responses measured in stressful conditions. It is concluded that CCK A, but not CCK B, receptor activation counteracts the opioid-related hyperlocomotion.

Antisense oligodeoxynucleotide to a δ-opioid receptor given intrathecally blocks i.c.v. administered β-endorphin-induced antinociception in the mouse

The antinociception induced by supraspinally administered β-endorphin is mediated by the release of Met-enkephalin and subsequent stimulation of δ-opioid receptors from the spinal cord in mice. Repeated intrathecal administration of an antisense oligodeoxynucleotide against δ-opioid receptors selectively attenuated i.c.v. administered β-endorphin-induced antinociception without any effect on the antinociception induced by μ-opioid receptor agonists, morphine and DAMGO, or κ-opioid receptor agonists, U50,488H. A random sequence oligodeoxynucleotide was inactive against β-endorphin-induced antinociception. The study confirms previous findings that the antinociception induced by β-endorphin is mediated by the stimulation of the δ-opioid receptors in the spinal cord.