Role Of Delta Opioid Receptors Research Articles

Opioid drug seeking after early-life adversity: a role for delta opioid receptors

Opioid use disorder (OUD) is associated with a history of early-life adversity (ELA), an association that is particularly strong in women. In a rodent model, we previously found that ELA enhances risk for opioid addiction selectively in females, but the mechanisms for this effect are unclear. Here, we show that ELA robustly alters cFos responses to opioid drugs in females’ nucleus accumbens (NAc) and basolateral amygdala (BLA), but not elsewhere. We further identify delta opioid receptors (DOR), which mature in the first week of life and thus later than kappa or mu opioid receptors, as a potential mediator of ELA's impacts on reward circuit functions. Accordingly, DOR mRNA in NAc was persistently reduced in adult females with ELA history. Moreover, pharmacological stimulation of NAc DORs increased opioid demand in control females (recapitulating the ELA phenotype), while blocking DORs in ELA females conversely reduced high-effort drug consumption, simulating the control rearing phenotype. These findings support a role for NAc DORs in mediating ELA-induced opioid vulnerability. In contrast, BLA neurons expressing DOR protein do not overlap heroin- responsive cells in ELA rats, arguing against a direct relationship of BLA DORs to heroin's addiction-relevant actions in the brain. Together, these results suggest a novel and selective role for NAc DORs in contributing to enduring, ELA-provoked vulnerability to OUD.

δ-Opioid receptor activation ameliorates lipopolysaccharide-induced inflammation and apoptosis by inhibiting the MAPK/caspase-3 pathway in BV2 microglial cells.

Delta-opioid receptor (DOR) is widely distributed in the central nervous system, and its activation protects against ischaemic/hypoxic brain injury. However, the role of DOR in microglia in ischaemic stroke has not yet been fully investigated. We found that DOR was expressed in both human and mouse cerebral microglia, besides, it was upregulated in activated BV2 microglial cells by immunofluorescence staining and Western blot. DOR activation by the specific agonist TAN-67 significantly enhanced BV2 microglial cell viability and reduced apoptosis, as evidenced by decreased cleaved caspase-3 levels and TdT-mediated aUTP-X nick end labelling (TUNEL) staining after LPS stimulation. Furthermore, activation of DOR significantly inhibited inducible nitric oxide synthase (iNOS) production and dose-dependently inhibited the mRNA and protein expression levels of other pro-inflammatory cytokines, including IL-1β and IL-6, whereas it increased the expression of the anti-inflammatory cytokine IL-10 in LPS-stimulated BV2 microglial cells; these effects were correlated with diminished phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. Moreover, these effects could be reversed by the DOR antagonist naltrindole. DOR activation can activate microglia to switch to the beneficial phenotype and inhibit LPS-induced inflammation and apoptosis via the mitogen-activated protein kinase (MAPK)/caspase-3 pathway in BV2 microglial cells. This study provides new insight into neuroprotection against and treatment of ischaemic stroke.

A54 DELTA OPIOID RECEPTOR SIGNALING MEDIATES OPIOID INDUCED TOLERANCE AND HYPERALGESIA IN COLONIC SENSORY NERVES

Abstract Background While opioid drugs are widely used for treating abdominal pain, prolonged exposure to opioids can induce tolerance and paradoxically increase pain. We previously showed in colonic afferent nerves that chronic morphine treatment causes tolerance and paradoxical hyperalgesia, however the mechanisms are unknown. Aims To evaluate the role of delta opioid receptor (DOR) signaling in opioid induced tolerance and hyperalgesia in colonic nerves during chronic morphine exposure. Methods C57BL/6 mice were injected (i.p.) twice daily for 7 days with escalating doses of morphine (10, 20, 30 mg/kg for the first 3 days, then 40mg/kg on 4th to 7th day); a subset of mice were also injected with 2.5 mg/kg naltrindole (NTI), a DOR antagonist. The analgesic response in vivo was monitored daily using the tail-flick test. To assess the effect of chronic morphine exposure, mice were euthanized on day 8, and the dorsal root ganglia (DRGs) and colons were harvested. Isolated DRG neurons were incubated with 1μM morphine (30 min) and neuronal excitability was determined by measuring the rheobase (amount of current required to elicit an action potential) using perforated patch clamp. To assess changes in colonic afferent nerve excitability, ex vivo afferent nerve recordings were obtained from flat-sheet colon preparations to determine the effects of 1μM morphine (10 min perfusion) on mechanosensitivity by probing with 1g von Frey hair. Results The tail flick test showed that the DOR antagonist NTI inhibited (30%) the maximal antinociceptive effect of morphine and reduced morphine tolerance. In patch clamp recordings, 1μM morphine paradoxically increased the excitability of small DRG neurons from morphine mice (rheobase decreased 28%; p<0.05, 2-way ANOVA), whereas this excitatory effect was absent on neurons from NTI+morphine mice. Acute application of 1μM morphine had no effect on afferent nerve responses to probing in morphine mice (14.9 vs 16.7 Hz; p=ns, paired t test, n=15) whereas it inhibited the afferent nerve response to probing in NTI+morphine mice (15.2 vs 9.9 Hz; p<0.001, paired t test, n=16). Furthermore, most single units from morphine mice had an increased response to probing following acute morphine application (7/15 vs 2/16 units excited), whereas most units form NTI-morphine mice were inhibited (14/16 vs. 3/15 units inhibited, P<0.001, Chi-square). Conclusions These findings suggest that DOR signaling plays a significant role in the development of opioid tolerance and hyperalgesia in colonic afferent nerves induced by chronic morphine treatment. Thus, antagonists of DOR may help to mitigate these side effects induced by opioids. Funding Agencies CCC

Neuroprotective role of delta opioid receptors in hypoxic preconditioning

Background/aimThe purpose of the present study was to explore the neuroprotective role of delta opioid receptors (DOR) in the rat cortex in hypoxic preconditioning.Materials and methodsRats were randomly divided into 8 groups: control (C), sham (S), hypoxic preconditioning (PC), severe hypoxia (SH), PC + SH, PC + SH + Saline (PS), PC + SH + DPDPE (DPDPE, selective DOR agonist), PC + SH + NT (NT, Naltrindole, selective DOR antagonist). Drugs were administered intracerebroventrically. Twenty four h after the end of 3 consecutive days of PC (10% O2, 2 h/day), the rats were subjected to severe hypoxia (7% O2 for 3 h). Bcl-2 and cyt-c were measured by western blot, and caspase-3 was observed immunohistochemically.ResultsBcl-2 expressions in the PC group were higher than in control, SH, and PC + SH groups. Even though there were no significant differences between the groups in terms of cyt-c levels, caspase-3 immunoreactivity of cortical neurons and glial cells in the severe hypoxia and NT groups were higher than in the control, sham, and hypoxic preconditioning groups. DPDPE administration diminished caspase-3 immunoreactivity compared with all of the severe hypoxia groups.ConclusionsThese results suggest that cortical cells are resistant to apoptosis via increased expression of Bcl-2 and decreased immunoreactivity of caspase-3 in the cortex, and that DOR is involved in neuroprotection induced by hypoxic preconditioning via the caspase-3 pathway in cortical neurons.

The delta opioid receptor agonist KNT-127 in the prelimbic medial prefrontal cortex attenuates veratrine-induced anxiety-like behaviors in mice

We previously reported that systemic administration of the selective delta opioid receptor (DOP) agonist KNT-127 produces potent anxiolytic-like effects in rats. Although a higher distribution pattern of DOPs was reported in the prelimbic medial prefrontal cortex (PL-PFC) of rodents, the role of DOPs in PL-PFC and in anxiolytic-like effects have not been well examined. Recently, we demonstrated that activation of PL-PFC with the sodium channel activator veratrine increases glutamatergic neurotransmission and produces anxiety-like behaviors in mice. Therefore, we investigated the effects of co-perfusion with KNT-127 in PL-PFC on veratrine-induced anxiety-like behaviors in mice. We also simultaneously measured extracellular glutamate and GABA levels. In addition, we assessed the effect of KNT-127 on the expression of c-Fos in sub-regions of the amygdala.Extracellular glutamate levels were measured in seven-week-old male C57BL/6N mice using an in vivo microdialysis-HPLC/ECD system, and behaviors were assessed simultaneously in an open field test. Basal levels of glutamate were measured by collecting samples every 10min for 60min. The drug-containing medium was perfused for 30min, and the open field test was performed during the last 10min of drug perfusion. After drug treatments, the perfusion was switched from drug-containing medium to control medium without drugs and samples were collected for another 90min.KNT-127 co-perfusion completely diminished veratrine-induced anxiety-like behaviors and attenuated the veratrine-induced increase in extracellular glutamate levels in PL-PFC. Interestingly, KNT-127 perfusion alone in PL-PFC did not affect anxiety-like behaviors. Local perfusion of veratrine in PL-PFC induced c-Fos immunoreactivity in sub-regions of amygdala. Co-perfusion of KNT-127 diminished c-Fos expression.Here we demonstrate that the DOP agonist KNT-127 in PL-PFC attenuates veratrine-induced anxiety-like behaviors in mice. These effects may be caused by the presynaptic suppression of activated glutamatergic transmission in PL-PFC, which projects to sub-regions of the amygdala. We propose that compounds like KNT-127, which inhibit glutamatergic transmission in PL-PFC, are candidates for novel anxiolytics.

Localization of the delta opioid receptor and corticotropin-releasing factor in the amygdalar complex: role in anxiety.

It is well established that central nervous system norepinephrine (NE) and corticotropin-releasing factor (CRF) systems are important mediators of behavioral responses to stressors. More recent studies have defined a role for delta opioid receptors (DOPR) in maintaining emotional valence including anxiety. The amygdala plays an important role in processing emotional stimuli, and has been implicated in the development of anxiety disorders. Activation of DOPR or inhibition of CRF in the amygdala reduces baseline and stress-induced anxiety-like responses. It is not known whether CRF- and DOPR-containing amygdalar neurons interact or whether they are regulated by NE afferents. Therefore, this study sought to better define interactions between the CRF, DOPR and NE systems in the basolateral (BLA) and central nucleus of the amygdala (CeA) of the male rat using anatomical and functional approaches. Irrespective of the amygdalar subregion, dual immunofluorescence microscopy showed that DOPR was present in CRF-containing neurons. Immunoelectron microscopy confirmed that DOPR was localized to both dendritic processes and axon terminals in the BLA and CeA. Semi-quantitative dual immunoelectron microscopy analysis of gold-silver labeling for DOPR and immunoperoxidase labeling for CRF revealed that 55% of the CRF neurons analyzed contained DOPR in the BLA while 67% of the CRF neurons analyzed contained DOPR in the CeA. Furthermore, approximately 41% of DOPR-labeled axon terminals targeted BLA neurons that expressed CRF while 29% of DOPR-labeled axon terminals targeted CeA neurons that expressed CRF. Triple label immunofluorescence microscopy revealed that DOPR and CRF were co-localized in common cellular profiles that were in close proximity to NE-containing fibers in both subregions. These anatomical results indicate significant interactions between DOPR and CRF in this critical limbic region and reveal that NE is poised to regulate these peptidergic systems in the amygdala. Functional studies were performed to determine if activation of DOPR could inhibit the anxiety produced by elevation of NE in the amygdala using the pharmacological stressor yohimbine. Administration of the DOPR agonist, SNC80, significantly attenuated elevated anxiogenic behaviors produced by yohimbine as measured in the rat on the elevated zero maze. Taken together, results from this study demonstrate the convergence of three important systems, NE, CRF, and DOPR, in the amygdala and provide insight into their functional role in modulating stress and anxiety responses.

(364) Morphine tolerance during inflammatory pain: role of delta opioid receptors in the midbrain periaqueductal gray

(364) Morphine tolerance during inflammatory pain: role of delta opioid receptors in the midbrain periaqueductal gray

Delta-opioid receptor blockade in the ventral pallidum increases perceived palatability and consumption of saccharin solution in rats

The ventral pallidum (VP) is involved in ingestive behaviour. It receives dense GABAergic projections from the nucleus accumbens. GABAergic terminals in the VP co-express enkephalin, an endogenous ligand of delta-opioid receptors. The role of the delta-opioid receptors in the VP in the context of ingestive behaviour remains unclear, in contrast to the well-understood involvement of the mu-opioid receptors. We used the single-bottle test to examine the effects of VP microinjections of the delta-opioid receptor antagonist naltrindole on consumption of a saccharin solution. Naltrindole injections significantly increased the intake of saccharin, but not water, during a 2-h test session. We also investigated perceived palatability of saccharin using a taste reactivity test. The drug treatments increased ingestive responses to intraorally infused saccharin. Further experimentation explored the role of VP delta-opioid receptors in behavioural responses to saccharin that were previously paired with malaise upon the retrieval of conditioned taste aversion (CTA). Naltrindole-injected rats exhibited longer latency for the first occurrence of aversive responses than vehicle-injected control rats. However, there was no between-group difference in total aversive responses. These results suggest that naltrindole injections into the VP induce an enhancement of perceived palatability of a normally preferred saccharin solution, and thereby facilitate consumption of the solution. On the other hand, delayed aversive responses to the conditioned aversive saccharin suggest that the delta-opioid receptors in the VP mediate the initiation of aversive taste reactivity responses to the conditioned stimulus upon CTA retrieval.

Cues predicting drug or food reward restore morphine-induced place conditioning in mice lacking delta opioid receptors

The exact role of delta opioid receptors in drug-induced conditioned place preference (CPP) remains debated. Under classical experimental conditions, morphine-induced CPP is decreased in mice lacking delta opioid receptors (Oprd1 (-/-)). Morphine self-administration, however, is maintained, suggesting that drug-context association rather than drug reward is deficient in these animals. This study further examined the role of delta opioid receptors in mediating drug-cue associations, which are necessary for the expression of morphine-induced CPP. We first identified experimental conditions under which Oprd1 (-/-) mice are able to express CPP to morphine (5, 10 or 20 mg/kg) in a drug-free state and observed that, in this paradigm, CPP was dependent on circadian time conditions. We then took advantage of this particularity to assess the ability of various cues (internal or discrete), predicting either drug or food reward, to restore CPP induced by morphine (10 mg/kg) in Oprd1 (-/-) mice in conditions under which they normally fail to express CPP. We found that presentation of circadian, drug or auditory cues, predicting morphine or food reward, restored morphine CPP in Oprd1 (-/-) mice, which then performed as well as control mice. This study reveals that, in contrast to spatial cues, internal or discrete morphine-predicting stimuli permit full expression of morphine CPP in Oprd1 (-/-) mice. Delta receptors, therefore, appear to play a crucial role in modulating spatial contextual cue-related responses. This activity may be critical when context gains control over behavior, as is the case for context-induced relapse in drug abuse.

A role for delta opioid receptors in the central nucleus of the amygdala in anxiety-like behaviors

Compounds acting on delta opioid receptors (DOR) modulate anxiety-like behaviors, yet the site of action underlying this effect is unknown. DOR mRNA and protein are expressed in the central nucleus of the amygdala, a region that plays an important role in processing fear, stress, and anxiety. We hypothesized that this brain region may contribute to the modulation of anxiety by DOR drugs. The present study investigated the role of DOR in the central amygdala in anxiety-like behaviors. The selective DOR agonist [D-Pen 2,5]-enkephalin (DPDPE) or antagonist naltrindole was bilaterally microinjected into the central nucleus of the amygdala of adult male Sprague Dawley rats and anxiety-like behaviors were assessed using the elevated plus maze. The effects of DOR agonists on heightened anxiety produced by stress were also investigated. Rats injected with DPDPE into the central nucleus of the amygdala demonstrated less anxiety-like behavior, as evidenced by significantly greater number of open-arm entries and time spent in the open arms than controls. Naltrindole administered alone did not affect the duration or number of entries onto the open arms; however, naltrindole pre-treatment blocked the anxiolytic effects produced by DPDPE. Systemic administration of the selective DOR agonist, SNC80, or microinjection of DPDPE into the central amygdala prior to a swim stress blocked the anxiogenic effect produced by the swim stress. These findings provide direct evidence that activation of DOR in the central amygdala reduces anxiety-like behavior and suggest that DOR in this area are important for regulating anxious states.

Role of the delta -opioid receptor in the regulation of mu -opioid receptor function during morphine tolerance

Morphine tolerance involves numerous and complex mechanisms. The heterodimerization of mu and delta opioid receptors put forward recently is one of the important mechanisms which cause the development of opiate tolerance. Once transported to the membrane of neural cells, delta opioid receptors (DOR) combine with mu opioid receptors (MOR) to form MOR-DOR heterodimers,leading to the formation of new signaling pathway. This novel mechanism plays the critical role of DOR in the development of morphine tolerance. This review summarizes the regulation of DOR on MOR function during morphine tolerance in light of the recent findings about the regulation of DOR membrane transport and the new complex: MOR-DOR heterodimer. Key words: Morphine tolerance; MOR-DOR heterodimerization; MOR gene knockout; β-arrestin; Signaling regulation

Peripheral nerve injury induces DOR trafficking to cell surface in dorsal horn neurons

Neuropathic (NP) pain remains a significant challenge for clinicians as it is typically refractory to traditional analgesics. However, research in ours and other laboratories increasingly suggests a therapeutic role of delta opioid receptor (DOR) agonists in treating NP pain. Spinal administration of a selective DOR agonist, attenuated nerve injury‐induced allodynia and hyperalgesia with enhanced antinociceptive effects in NP rats compared with controls. This functional enhancement was not due to increased DOR biosynthesis, as DOR protein did not change. In the present study, we examined DOR subcellular localization by electron microscopy immunohistochemistry to determine whether NP injury produces a redistribution of DORs from internal stores to the cell surface. Quantification of immunopositive dendrites in the dorsal horn revealed a bilateral increase in plasma membrane‐associated DORs within lamina V neurons of NP rats compared to shams. The recruitment of DORs to neuronal plasma membranes may represent a compensatory mechanism by which neurons may sustain an inhibitory tone during chronic pain states, and in turn, indicate a viable drug target. (CIHR, J.P. Bickell Foundation, CFI/OIT, CRC)

Withdrawal from chronic administration of cocaine decreases delta opioid receptor signaling and increases anxiety- and depression-like behaviors in the rat

Chronic administration of cocaine has been shown to attenuate the functional capacity of delta opioid receptors to inhibit adenylyl cyclase activity. Abuse and withdrawal from cocaine in humans is associated with increases in anxiety and depression. Since recent research supports the role of delta opioid receptors in anxiety- and depression-like behaviors in rodents, we hypothesized that functional desensitization of delta opioid receptors contributes to anxiety- and depression-like behavioral phenotypes following short-term withdrawal from chronic administration of cocaine. To test this hypothesis, delta opioid receptor signaling and behaviors were evaluated 24 h after 14 days of binge-pattern cocaine administration (15 mg/kg three times daily at 1 h intervals) in male Sprague–Dawley rats. Results showed that the inhibition of adenylyl cyclase by delta opioid receptor agonists was attenuated in the frontal cortex, nucleus accumbens and caudate putamen 24 h after cessation of cocaine administration. One day withdrawal from chronic administration of cocaine resulted in increased anxiety- and depression-like behaviors as measured by the elevated plus maze and the forced swim test respectively, and no change in locomotor activity. The anxiety- and depression-like behaviors were dose-dependently reduced by acute administration of the selective delta opioid receptor agonist, SNC80. These results demonstrate that early withdrawal from cocaine resulted in increased anxiety and depression, which accompanies the desensitization of delta opioid receptor function. Furthermore, cocaine-induced anxiety- and depression-like behaviors were reversible by the delta opioid receptor agonist SNC80.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

The anxiolytic effects of benzodiazepines appear to involve opioid processes in the amygdala. In previous experiments, overexpression of enkephalin in the amygdala enhanced the anxiolytic actions of the benzodiazepine agonist diazepam in the elevated plus maze. The effects of systemically administered diazepam are also blocked by injections of naltrexone into the central nucleus of the amygdala. The current studies investigated the role of delta opioid receptors in the anxiety-related effects of diazepam. Three days following bilateral stereotaxic injections of viral vectors containing cDNA encoding proenkephalin or β-galactosidase (control vector), the delta opioid receptor antagonist naltrindole (10 mg/kg, s.c.) attenuated the enhanced anxiolytic effects of 1–2 mg/kg diazepam in rats overexpressing preproenkephalin in the amygdala. Despite this effect, naltrindole failed to attenuate the anxiolytic action of higher diazepam doses (3 mg/kg) in animals with normal amygdalar enkephalin expression. Similarly, the mu opioid receptor antagonist, β-funaltrexamine (20 mg/kg, s.c.), had no effect on the anxiolytic effect of diazepam alone. These data support a role for delta opioid receptors in the opioid-enhanced anxiolytic effects of diazepam.

Brain delta 2 opioid receptors mediate SNC-80-evoked hypothermia in rats

Despite insights into an increasingly significant role for delta opioid receptors in thermoregulation, it is unclear whether delta receptors located in the brain or periphery play the more critical role in body temperature regulation. Moreover, it is not entirely clear which delta receptor phenotype, delta 1 or delta 2, mediates the hypothermic actions of delta agonists. Because SNC-80 distributes into central and peripheral compartments and produces rapid hypothermia following systemic injection, the nonpeptide delta agonist is particularly useful in discriminating the site of action of delta receptor-mediated hypothermia. To determine the locus and phenotype of delta receptor which mediates SNC-80-induced hypothermia, we injected SNC-80 and phenotype selective delta antagonists to male Sprague–Dawley rats. SNC-80 (10–50 mg/kg, im) evoked hypothermia that peaked 30 min post-injection. Naltrexone (5 mg/kg, sc), an opioid antagonist, or naltrindole (5 mg/kg, sc), a delta antagonist, blocked the hypothermic response to SNC-80 (35 mg/kg, im). The hypothermia caused by SNC-80 (35 mg/kg, im) was blocked by a delta 2 antagonist, naltriben (2.5 mg/kg, sc), but was not affected by BNTX (5 and 10 mg/kg, sc), a delta 1 antagonist. The administration of naltriben (10 μg/rat, icv) 30 min before SNC-80 (35 mg/kg, im) prevented SNC-80-evoked hypothermia. In contrast, methylnaltrexone (5 mg/kg, sc), a peripherally restricted opioid antagonist, did not affect the hypothermia caused by SNC-80. The present data demonstrate that selective activation of brain delta 2 receptors is a major mechanism of SNC-80-evoked hypothermia in rats.

Physiological control of emotion-related behaviors by endogenous enkephalins involves essentially the delta opioid receptors

The endogenous pentapeptide enkephalins bind to the mu and delta opioid receptors, with a slightly higher affinity for the latter. It remains a controversy regarding the respective physiological role of mu and delta opioid receptors in the control of emotion and motivation. One of the difficulties to investigate this problem is the low tonic extracellular release of enkephalins in various brain structures. To overcome this problem the synaptic levels of these pentapeptides were enhanced by inhibition of enzymes involved in their catabolism with the selective inhibitor H 3N-CH(CH 2-CH 2-S-CH 3)-CH 2-S-S-CH 2-CH(CH 2ϕ)-CONH-CH(CH 2ϕ)-COOCH 2ϕ (RB101). This compound was shown to increase the extracellular levels and lifetime of endogenous enkephalins. Similar responses were obtained in wild-type and mu opioid receptor knockout mice following RB 101 administration in behavioral tests measuring locomotor activity, anxiety (elevated O-maze), and motivation (forced swim test and conditioned suppression of motility). In contrast, RB 101 led to antinociceptive responses only in wild-type animals using hot plate and tail immersion tests. These results clearly demonstrate the critical role of delta opioid receptors activated by the endogenous opioid peptides, in the physiological control of emotion- and motivation-related behaviors. In contrast, antinociceptive modulation, at least with respect to thermal nociceptive stimuli, involves enkephalin-activated mu opioid receptors. These findings could open new perspectives in the treatment of mood disorders using either inhibitors of enkephalin catabolism or delta opioid agonists.

Effect of protein kinases on lactate dehydrogenase activity in cortical neurons during hypoxia

Our previous work shows that delta-opioid receptor (DOR) protects cortical neurons from hypoxic insults. Since an enhanced anaerobic capacity is important for neurons to adapt to the reduction of oxidative phosphorylation, we asked whether DOR plays a role in neuronal regulation of anaerobic capacity, thus protecting neurons from O 2 deprivation. Indeed, there is evidence suggesting that DOR may regulate protein kinase A (PKA) and C (PKC), which are involved in regulation of lactate dehydrogenase (LDH). However, little is known regarding the role of DOR and protein kinases in the regulation of glycolytic and related enzymes. As a first step, the present studies were performed in primary cultures of rat cortical neurons to clarify two issues: (1) Are protein kinases involved in the regulation of LDH activity in hypoxia? and (2) Does DOR affect LDH activity in hypoxic neurons? The results showed that PKC activation yielded substantial increases in normoxic LDH activity and significantly augmented LDH activity in hypoxic neurons, while PKC inhibition decreased LDH activity in both normoxic and hypoxic neurons. PKA activation significantly increased LDH activity in normoxic neurons and further elevated LDH activity in hypoxic neurons. However, PKA inhibition did not decrease in LDH activity in either normoxic or hypoxic neurons. Although DOR inhibition slightly reduced LDH activity in normoxia, DOR activation or inactivation did not alter LDH activity in hypoxic neurons. These data suggest that in cortical neurons, (i) PKC up-regulates LDH activity and plays an important role in its up-regulation during hypoxia; (ii) PKA is less likely involved in the regulation of LDH activity during hypoxia although its stimulation may slightly increase LDH activity and (iii) DOR does not contribute to LDH activity up-regulation during hypoxia.

Opioid action: Anti-hyperalgesic effects of delta opioid receptor agonists in a model of neuropathic pain

Pharmacological evidence supports the involvement of the Delta opioid receptor in antinociception. However, its role in the treatment of neuropathic (NP) pain remains largely unknown. In the present study, we examined the role of delta opioid receptors in an animal model of peripheral nerve injury (PNI). Rats (225-250 g) underwent chronic constriction of the sciatic nerve in accordance with the Mosconi/Kruger model. A cold water test was employed to assess cold hyperalgesia prior to and at various intervals following peripheral nerve injury. Neuropathic rats developed cold hyperalgesia as early as two days following surgery and this sensitivity persisted for the two week testing period. Sham animals did not respond in the testing paradigm employed. The effects of a selective delta opioid receptor agonist were assessed on day 14 following PNI. Intrathecal administration of [D-Ala2]-Deltorphin II (30ug), produced a significant increase in the latency to respond to the noxious cold compared to pre-drug values. Western blotting experiments on dorsal spinal cord segments of day 14 NP rats revealed significantly higher total delta opioid receptor protein levels ipsilateral to nerve injury compared to the contralateral side. These data suggest that delta opioid receptor agonists are a potential therapeutic target for drug development in treating neuropathic pain.

Delta-opioid receptor activation mimics ischemic preconditioning in the canine heart.

The role of delta-opioid receptors in mediating ischemic preconditioning (IPC) in rats, rabbits, and pigs has been well-established; however, no studies have been performed in dogs. Therefore, the purpose of the present study was to determine if activation of delta-opioid receptors can mimic the cardioprotective effects of IPC in the canine heart and to determine if a nonselective opioid receptor antagonist could block IPC. All dogs were subjected to 60 minutes of left anterior descending (LAD) coronary artery occlusion and 3 hours of reperfusion. Ischemic preconditioning was produced by one 5-minute period of ischemia 10 minutes before LAD coronary artery occlusion. Infarct size (IS) expressed as a percent of the area at risk (AAR; IS/AAR) was determined by triphenyltetrazolium staining. Two selective delta-opioid receptor (DOR) agonists, TAN-67 and BW373U86, were administered by intracoronary infusion for 30 minutes before LAD occlusion and the opioid receptor antagonist naloxone was administered 30 minutes before IPC. Both TAN-67 and BW373U86 produced significant reductions in IS/AAR similar to that of IPC (control: 28+/-2.1; TAN: 12.3+/-2.2; IPC: 9.3+/-3.0: BW: 11.7+/-2.6). Naloxone attenuated the effect of IPC (control: 28+/-2.1; naloxone: 18.2+/-4.5). These results suggest that opioid receptors are important in IPC in dogs, and stimulation of delta-opioid receptors with selective agonists can mimic the cardioprotective effects of IPC and may have therapeutic potential.

Involvement of delta-opioid receptors in pentylenetetrazol kindling development and kindling-related processes in rats.

The role of delta-opioid receptors on the development of kindling induced by the convulsant pentylenetetrazol (37.5 mg kg(-1) i.p.) was investigated in rats. Besides the seizure development, the kindling induced enhancement of glutamate binding and the kindling-induced learning deficit were examined. A clear depression of kindling development by blocking of delta-opioid receptors by intracerebroventricular administration of naltrindole (10 nmol/5 microl) was found. In an acute convulsion test performed 8 days after kindling completion, animals pretreated with naltrindole during kindling induction showed lower seizure stages compared to saline-pretreated kindled animals. The kindling-induced increase in hippocampal glutamate binding was completely prevented by naltrindole, whereas the kindling-induced learning deficit was not influenced. The learning performance of control animals pretreated with naltrindole was very low. It was hypothesized that the various consequences of kindling induction could be influenced separately. Summarizing the results, an involvement of the delta-opioid system in mechanisms underlying chemical kindling was clearly demonstrated. Interactions of endogenous opioid systems with glutamatergic transmission were suggested.